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Solar‐Triggered Engineered 2D‐Materials for Environmental Remediation: Status and Future Insights

Solar‐Triggered Engineered 2D‐Materials for Environmental Remediation: Status and Future Insights IntroductionModern society critically demands the availability of chemical supplies and sustainable energy sources that are extremely important for daily life's convenience, stability, and transportation. At the global level, energy production based on fossil fuels currently constitutes 85% of the total energy requirements. Rapid progress and development of novel technologies have encouraged the research community to develop environmentally friendly and sustainable methods for energy production. Global warming has drastically increased in the past few years, resulting from the ever‐growing pollution content in environment due to excessive use of fossil fuels.[1–4] Due to increased population growth, industrialization and modernization have been enhanced rapidly worldwide, which is believed to escalate the energy demands by 2050.[5–8] Moreover, fossil fuels like natural gases, coal, and petroleum are rapidly consumed since the world relies on these fuels to meet energy demands. Consequently, several harmful gases are released into the atmosphere, causing severe environmental pollution. Hence, developing innovative technologies and methodologies in engineering and science is highly eminent to pursue a safer and sustainable future.[9–13] On the other hand, many industries also utilize fossil fuels for chemical production by organically or inorganically transforming them at elevated temperatures. Apart from the fact that the current fossil fuel reserves are most probably sufficient to meet the energy demands for three to four generations, the unsustainability and low cost of such fuels are harmful, and releasing of hazardous gases like CO2 is causing the greenhouse effect.[14,15] Several technical issues are involved regarding developing chemical systems to reduce the harmful impact of fossil fuel‐based energy production on the environment. Sustainable and renewable methods such as hydrogen‐based electrocatalytic and photocatalytic energy production seem to be effective solution.[14,16,17]Conversely, environmental pollution and growing industrial activities are consistently causing a severe increase in pure water shortage worldwide. Due to this problem, water pollution is now declared a global challenge to public health.[18,19] According to a report from World Health Organization (WHO), millions of children (age limit < 5 years) die annually from diarrhea caused by using polluted water and an unhygienic environment.[20] Human activities, along with natural pollutants, have resulted in intensified water pollution. Different lethal diseases have emerged due to common pollutants like volatile organic compounds, pharmaceuticals, pathogenic microorganisms, pesticides, metalloids, and heavy metals.[21–23] Apart from the underdeveloped countries around the world, several developed countries like the USA also suffer from water pollution. Several treatment techniques have been conventionally employed for water purification, including screening, centrifugation, coagulation, electrolysis, flocculation, and other aerobic/anaerobic methods. However, considerable work and cost are required to operate the technologies mentioned above.[24] Furthermore, considering the energy crisis we face, sustainable and green energy‐based strategies must be designed for effective water treatment. In this regard, using abundantly available green solar energy to design solar‐inspired designs for water purification can prove to be highly effective, mainly categorized into photocatalytic degradation, photothermal evaporation, and photoinduced antimicrobial treatment.[25,26]Several new developments have been introduced in water treatment technologies after the beginning of nanoscience and nanotechnology in the 21st century. Several nanosystems, including 0D,[27,28] 1D,[29,30] 2D,[31,32] 3D nanostructures,[33] and their functional composites[34,35] have been introduced. Among these nanostructures, ultrathin structure and the highest surface‐to‐volume ratio of 2D material make them potential candidates for offering ultrahigh efficiency, ultrafast processing, and ultralow material uses. The sudden growth of 2D family with distinct morphological features after the discovery of graphene unlocked new prospects in water treatment.[36] Gradual increase in bandgap energies and shifting of absorption region can be observed from graphene to molybdenum disulfide (MoS2). Several innovative water treatment systems with outstanding separation, adsorptive and catalytic performances have been developed by controlling the structure, thickness, and size of these 2D materials. Such systems mainly include graphene membranes,[37] oil adsorbents,[38] ultrafast photocatalysts,[39] and various smart/self‐healing structures.[40]Although many materials have been extensively studied and significant developments have been made in the field of photocatalysis. However, this process has low efficiency and deprived stability, which fall well short of the requirements for practical application. Three critical steps are involved during a typical photocatalytic process: 1) light‐harvesting for exciton pairs generation, 2) separating and transferring the photogenic charges from bulk to surface, and 3) initiation of redox reactions through charges at surface or the corresponding active sites.[41] 2D materials can considerably enhance charge separation, light absorption, and interfacial redox reactions.[42] Unlike their analogous bulk forms, 2D materials offer overwhelming advantages due to their unique atomic properties. The generation of light‐induced charge carriers is effectively boosted accredited to ultrathin nature and large surface area of 2D materials. Furthermore, the possibility of recombination is effectively reduced due to a decrease in diffusion length for charge carriers in ultrathin structures. In addition, the redox reactions are boosted due to the availability of numerous active sites in 2D materials enriched with coordination‐unsaturated atoms at their surface.[43] However, the performance of these 2D materials has not yet reached the level of practical application demands. Researchers have adopted various surface and interfacial engineering strategies to enhance their photocatalytic potential further.Producing defects in semiconductors is an effective method of tuning and improving the characteristics of 2D materials.[44] Apart from the most frequently studied defects in photocatalytic materials, i.e., oxygen vacancies, other defects like metal vacancies have also been explored recently. However, they are relatively difficult to handle. Using 2D materials in photocatalysis has made it possible for researchers to intentionally produce various types of defects with a reasonable degree of ease. Because of their thin structures, even a tiny amount of generated defects can produce the considerable potential to tailor a photocatalyst's bandgap, spin nature, carrier concentrations, etc. Moreover, it is much easier to produce defects in 2D materials to generate different phenomena than their bulk counterparts. Additionally, creating defects on the surface of bulky materials results in defects inside the bulk, which is highly detrimental to photocatalysis since these defects might function as recombination centers within the structure.[45] The researchers have emphasized the importance of defects in light‐driven catalysis to design better and more robust photocatalytic systems. Several modification techniques, including vacancy engineering, hybridization, morphological and structural control, and elemental doping, have been devised to improve the properties of 2D materials.[46,47] We believe that an inspiring and compact review regarding this field is hence much necessary to encourage further exploring of defective 2D materials for various photocatalytic applications.This feature review summarizes exciting developments in defective 2D materials and their potential application in photocatalytic environmental remediation. Some of the most significant and recent accomplishments in surface and interface engineering of 2D materials, for instance, vacancy associates, anion–cation vacancies, disorders, pits, and distortions, are presented. Furthermore, various approaches for the controlled formation of these defects, such as ball milling, vacuum activation, and chemical reduction, are presented. In addition, different hybridization engineering strategies and essential parameters for surface and interfacial engineering have also been stated. Lastly, the crucial effects of defects (both surface and interfacial) on the performance of these photocatalysts in pollutant removal, CO2 reduction, water oxidation, hydrogen evolution, organic synthesis, and nitrogen fixation have been summarized.Supremacy of 2D Materials2D materials with single or few atom thicknesses (>5 nm) have high surface atom ratio, large surface area, and intrinsic quantum confined electrons that exhibit extraordinary optical, mechanical, and electronic properties and have great potential for the research of transistors, catalysis, optoelectronic, conversion, and the energy storage devices.[48–53] These materials present unique physiochemical behavior like electronic anisotropy, high surface activity, planar conductivity, and tunable energy structure.[54–58] The decrease in thickness of bulk substances to the atomic level, atomic structures will go through the apparent distinctions, including length and angle of bonds, coordination number, and formation of surface defects and disordering of surface atoms. As a result, 2D materials exhibit bulk properties and new features.[51,59–62] Semiconductor photocatalytic materials have gained interest as they give promising solutions for environmental pollution and energy storage. These materials split the water into hydrogen and oxygen, eliminating pollutants and reducing CO2 by solar light as an external driving force.[15,63–66] Light absorption, migration, separation of charges, and surface redox reactions are the major steps for photocatalysis. After irradiation exposure, the photocatalysts absorb the light and produce electron and hole pairs in the conduction and valance bond. These photogenerated electron–hole pairs diffuse to the surface of a material and then migrate to the active sites before the surface reactions. The recombination of charges happens during this; surface atomic, crystal structure, particle size, and crystallinity affect the separation efficiency. At the end, target molecules adsorb on the material's surface and undergo charge injection and desorption to make final products.[59,67–70] Now, many semiconductors exist for photocatalysis with tunable electronic and crystal structures.Although remarkable achievements are done to optimize the photocatalysis process, many photocatalysts show relatively low performance, which depends on the rational design of such materials. 2D materials have awakened a new aspect of this field because of the appropriate band structure. The 2D configuration can harvest more ultraviolet–visible radiations and have a large specific surface area. However, the absorption of photons is very limited in bulk materials due to the reflection and transmission of grain boundaries. Additionally, as atomic thickness decreases the migration distance, charge carriers quickly move to the surface area in 2D materials. It reduces the recombination possibility and enhances photocatalysis. Lastly, unique 2D structures with a high surface‐atom ratio render many active sites for accelerating the reaction processes. Also, atomic escape energies become relatively small due to the decrease in thickness. Surface defects play a role in target molecule adsorption, building strong interplay, super activation process, and charge transfer. These features help photocatalysts display various features, and several scientific reports have been done.[71–74] It is urgent and desirable to present an inclusive review on this field to encourage further developments in this niche.Computational Screening of 2D Materials for PhotocatalysisThermodynamic StabilityThermodynamic stability is an essential property of materials, and during the search for an effective photocatalytic material, this property should be checked first. In 2D materials, when two layers are brought towards each other, the energy of dispersion interaction continuously decreases, which shows that 2D materials are metastable and have no actual thermodynamic ground state. Therefore, the lack of 3D competing phases makes the dispersion energy stabilize bulk layered material instead of individual layers. However, 2D materials can make themselves kinetically stable, i.e., in the presence of different absorb rates, solvents or substrates like GaN, 2D silica AIN (aluminum nitride), materials like MoS2,[75] free‐standing graphene[76] or BN can be stabilized.[77,78] The difference in the free energy of bulk materials and 2D materials having the same composition gives the value of thermal stability, given below (Equation 1).1ΔEf=E2DN2D−E3DN3D\[\begin{array}{*{20}{c}}{\Delta {E_{\rm{f}}} = \frac{{{E_{2{\rm{D}}}}}}{{{N_{2{\rm{D}}}}}} - \frac{{{E_{3{\rm{D}}}}}}{{{N_{3{\rm{D}}}}}}}\end{array}\]2D materials must be stably suspended nanosheets in an aqueous solution. In Figure 1, blue bars illustrate the energy of formation for some experimentally synthesized, and red bars present the theoretically predicted 2D materials. Here, N3D and N2D are the numbers of atoms in corresponding unit cells, and E3D and E2D are the energies of bulk and single‐layered materials. With weak van‐der Waals (vdW) layered bulk species, 2D materials have low formation energies and therefore are extracted as free‐standing single‐layer flakes.[79,80] Several examples include graphene,[81] MoS2,[82] GaSe,[83] SnS2,[84] BN,[85] MoSe2,[85] WSe2,[85,86] SnSe,[84] and NbTe2.[85] On the other hand, 2D materials having formation energy of more than 200 meV/atom, like 2D group III–V elements,[87] 2D oxides,[88] and silicone,[80] are not primarily synthesized without a stable substrate.[89] Moreover, it is unlikely to achieve suspended flakes (freestanding) of 2D materials with a higher energy of formation.[90,91] It is depicted in Figure 1 that only some materials, such as ZnO,[92] AIN,[78] and silicone,[93] with higher formation energy, have been prepared on the surface of the substrate and not extracted in freestanding state.1FigureFormation energies of some experimentally synthesized (blue bars) and theoretically predicted but experimentally unsynthesized 2D materials (red bars). The materials in the yellow‐shaded region, with ΔEf > 0.2 eV per atom, have only been extracted as single‐layer or few‐layer nanosheets on suitable substrates and are yet to be obtained in a free‐standing or suspended form. Reproduced with permission.[94] Copyright 2015, American Chemical Society.Electronic StructureFor the selection of materials to be potential photocatalysts, 2D materials need a prediction of their band edge position and bandgap (electronic properties). A commonly used method for determining the electronic properties is DFT, as it exhibits an excellent relation between the computational cost and accuracy which depend upon the exchange‐correlation function. Bandgaps of the semilocal and local approximations to exchange‐correlation function underestimate the fundamental gap due to less discontinuity of function than the number of electrons, and there is no clarity about the physical meaning of orbitals (unoccupied).[95] Heyd‐Scuseria‐Ernzerhof (HSE06)[96,97] and PBE0[98] are hybrid functional, which are computationally expensive but very effective in circumventing the bandgap issue of density‐functional theory (DFT).[96,97] It incorporates a part of an exact exchange which provides better results agreeing with the experimentally measured bandgaps.[99] The band structures of quasiparticles, such as electron–electron interactions, can be calculated by a theoretically correct approach named GW approximation as for 3D objects, it exhibits better bandgap results; for 2D species, lack of experimental data has hampered the comparisons.[100] Both techniques GW and hybrid functionals, determine the fundamental bandgap, but exciton effects should be considered to determine the optical bandgap. The difference between electronic and optical bandgaps is mainly ignored due to the small exciton energies of 3D semiconductors. However, this difference is significant in the case of organic semiconductors and 2D materials. In these materials, high binding energies are ascribed to the reduction of coulomb screening. Bethe–Salpeter equation (BSE) is utilized to estimate the exciton energies and optical bandgap. For instance, an optical bandgap for MoS2 of 1.84 eV is obtained if the GW electronic bandgap of 2.8 eV is corrected by the BSE exciton binding energy of 0.96 eV,[101–103] which is in good agreement with the experimentally observed optical bandgap of 1.9 eV.[75] In a vacuum, DFT techniques are used to obtain the intrinsic band edge locations by aligning electrostatic potential.[79] Like bandgap energy, band alignments also depend upon exchange‐correlation functions. Hybrid functional mainly presents enhanced accuracy for semilocal functionals. ECBm/VBMDFT can be fixed by quasiparticle GW energy contributions and δECBm/VBM, give correct exchange‐correlation functional independent band‐edge positions (Equation 2).[104,105]2ECBm/VBMQP=ECBm/VBMDFT+δECBm/VBM\[E_{{\rm{CBm/VBM}}}^{{\rm{QP}}} = E_{{\rm{CBm/VBM}}}^{{\rm{DFT}}} + \delta {E_{{\rm{CBm/VBM}}}}\]Optical AbsorptionThe ability of a particular material to capture sunlight plays a vital role in splitting water molecules. Capturing the visible part of the light spectrum is preferable because it contains about 40% of the energy compared to the ultraviolent part, which only captures 5%. Optical absorption is obtained from the imaginary part for the dielectric function expression using GW or DFT approximations. Standard DFT wave functions and GW calculations are used to provide input of quasiparticle energies to BSE.[106,107] 2D materials show better efficiency for photocatalytic water splitting because of their significant absorption. BSE describes the optical spectrum and excitonic energies of 2D materials, such as the spectrum of MoS2,[102,108] and SnS2 depicts the domination of excitonic states.[109] The imaginary part for permittivity of SnS2 was determined from BSE and random phase approximation. This phase could not describe the electron and hole pairs; BSE spectrum shows three peaks below 3.2 eV (low energy range). The first peak (at 2.75 eV) is ascribed to SnS2 optical bandgap, which is in agreement with the experimental value of 2.55 eV determined by UV spectroscopy and second peak corresponds to a different exciton and the third one (at 3.16 eV) is due to direct quasiparticle bandgap determined by GW method. The exciton binding energy differs between the third and the first peak for SnS2. The BSE value (0.41 eV) is close to bulk SnS2 (0.4 eV), which is comparable to the exciton binding energy WS2 (0.6 eV)[102] and MoS2 (0.96 eV).[103]Classification of 2D PhotocatalystsTo date, a significant amount of 2D nanosheets have been synthesized by various chemical and/or physical methods, which are mainly divided into two types, layered and non‐layer structural materials. Concerning layered materials, the in‐plane layer is formed by connecting the in‐plane atoms by strong chemical bonding. However, the weak vdW interaction is essential in stacking these monolayers to form bulk crystals.[110,111] The representative layered material is graphite crystal, stacked by many graphene layers via weak vdW force. In addition, nitrides (such as g‐C3N4, h‐BN, GaN, and Ca2N), black phosphorus (BP), Xenes, transition metal dichalcogenides (TMDs), transition metal oxides (MOs) are also layered materials, as shown in Figure 2.[110]2FigureClassification of 2D photocatalysts. Reproduced with permission.[110] Copyright 2020, MDPI.In recent years, material and engineering technology advances have enabled tremendous progress in catalysis, sustainable energy generation, and photocatalytic degradation. Novel spectroscopy and nanofabrication techniques provided the properties of primary materials and emerged their functionalities through configuration and structure engineering. It enables significant progress in multicomponent industrial catalysts,[112] superior chemical processing, electrocatalysis,[113] and photocatalysis.[114] Today's environmental growth and technology will require a diverse collection of materials, some of which are rare and unequally distributed on the planet, indicating economic feasibility and a possible challenge to their supply. Ironically, sustainability and the dangers involved with materials are often overlooked in academic research. Nevertheless, a fundamental point of view is the material's durability for the ultimate goal of green energy synthesis. Indeed, given the near‐limitless supply of renewable energy (wind and solar energy), the materials used to convert it to actual energy (electrical) are obtained infrequently. Remarkably, the precious metals group, gallium, rare earth elements, aluminum cobalt, and a wide range of other elements[115] are crucial components of industrial catalysts that are widely used. If reliance on those materials can be reduced by substitution in future, they can be recycled more efficiently, avoiding monetary disruptions and accumulating reserves competitiveness.[116] The following properties should be present in these materials: Strengthening the catalyst's durability via material design (confinement and post modifications), minimizing the loading of noble metals while maintaining superior activity by maximizing available active surface area, replacing primary materials for more affordable and readily available alternatives (base metals and carbocatalysis), improvements in the synthesis and elimination of catalysts (green chemistry based catalyst recycle) and toxicological and environmental impact assessment of catalyst products.Another critical characteristic is the catalyst's nanostructure, which is used to analyze and compare various catalysts to determine their activity propensity. In general, the electronic structure and interfacial degree (smaller‐sized materials have more atoms at the support tip) of photocatalysts are highly affected by their size (<10 nm). As a result, controlling the size and structure of photocatalytic semiconductor materials can result in unexpected and significant changes in their properties.[117] As a suitable candidate for photocatalysis, the most extensively studied 2D‐layered materials‐based photocatalysts include MOs, bismuth‐based materials, metal hydroxides, metal composite oxides, metal chalcogenides, and metal‐free photocatalytic materials. Therefore, techniques for the large‐scale synthesis of 2D materials will be required for industrialization. Moreover, since specific applications of these materials are strongly dependent on characteristics such as their morphology and quality, mass‐production techniques should also be developed to accommodate such requirements (Figure 3).[118] Fabricating 2D materials with controllable edge morphology, several layers, and a degree of crystallinity are critical for their use in high‐activity catalytic applications. As a result, this section provides a concise overview of the fundamental properties of these 2D materials used in photocatalysis and brief descriptions of the synthesis strategies for 2D materials.[119]3Figure2D films and heterostructures require high crystal quality and homogeneous thickness for applications for various applications. Reproduced with permission.[118] Copyright 2022, Nature Publishing Group.Carbon‐Based PhotocatalystsTypical carbon nanostructures are 0D (fullerenes and carbon quantum dots abbreviated as QDs), 1D (nanotubes of carbon), and 2D (graphene and its derivatives). The 2D materials can be thought of as derivatives of graphene sheets that are one atom thick. Since we are only interested in 2D materials, other structures (3D, 1D, and 0D) are beyond the reach of this feasibility study. The crystalline structures of graphene were revealed after the advent of X‐ray diffraction methods in the 1930s. Following that, in 1947, the theoretical analysis proposed that isolated layers of graphene would exhibit distinct electrical features. Currently, these forecasts are still confirmed to be accurate. Mechanical exfoliation of graphite results in the formation of single graphene layers. Initially, this was shown by the micromechanical cleavage of graphite by an adhesive tape, which is well known as the “scotch tape method” recorded by Novoselov and Geim in 2004 that later earned them the Noble Prize in Physics in 2010.[120] Chemical vapor deposition techniques have been designed and are currently used to mass‐produce graphene with minimal defect count.[121] Chemical exfoliation of bulk graphite can yield graphene layers. These methods are economical since they start with low‐cost raw materials that easily mix with different chemical treatments to produce a variety of graphene and derivative materials, including graphene oxide (GO) and reduced graphene oxide (RGO).[122] Their surface chemistry and structural properties make them an ideal platform for stabilizing photocatalytic processes. For example, Ton et al. fabricated visible‐light active TiO2/graphene nanocomposites with a significantly narrowed energy gap that demonstrated superior photocatalytic potential against various dyes.[123] Chen and co‐workers produced visible light‐active TiO2/GO composites having an energy gap of less than 2.43 eV for methyl orange degradation.[124] The improved photocatalytic activity of TiO2/graphene photocatalyst has been inferred to be due to close coupling between TiO2 and graphene or GO that enables charge transfer across the interface and prevents excitons from recombining. Also, GO‐formed p‐ or n‐type junction in the prepared composite serves as a sensitizer, improving the light‐driven activity of nanocomposites in the visible range.Graphitic carbon nitride (g‐C3N4) is another carbon‐based 2D material with remarkable physical, chemical, and electronic properties that are being widely used as a metal‐free photocatalyst to degrade several dyes[125] such as Rhodamine B (RhB),[126] methyl orange,[127] methylene blue (MB),[128] and so on.[129] However, bare g‐C3N4 has poor dislocation conductivity, a small actual surface region, and a large recombination rate, which contribute to low photocatalytic efficiency.[130,131] Furthermore, g‐C3N4 in bulk has a layered 2D structure and an acceptable bandgap (≈2.7 eV) to absorb visible light effectively. The g‐C3N4 is acquired from delamination of its bulk form, typically formed by polycondensation or bulk reaction of N2‐rich precursors. Ailan et al. investigated a straightforward top‐down approach, adopting the oxidation etching approach to form g‐C3N4 nanosheets from bulk g‐C3N4 in air at elevated temperatures.[132] The g‐C3N4 nanosheets obtained with specific surface area of 306 m2 g−1 were approximately 2 nm thin, more significant than the thickness of the bulk process. The quantum confinement effect enhances the ability of excitons to pass in the plane direction and increases their lifespan. Consequently, the photocatalytic efficiency of prepared nanosheets for H2 processing was significantly improved.Liu et al. demonstrated that N‐deficient carbon nitride could be formed post‐treatment via molten salt. In this study, bulk carbon nitride was ground with KCl and LiCl in an agate mortar.[133] Next, it was heated in air at 550 °C and naturally cooled to ambient temperature to synthesize N‐deficient carbon nitride.[133] Besides, N‐deficient carbon nitride can achieve synthesis based on the thermal treatment with a suitable gas etching agent for assisting in the selective removal of set atoms.[134–136] As opposed to direct clear thermal treating process, the gas etching approaches defect type in a controlled manner. Specifically, Li et al. synthesized carbon‐abundant carbon nitride nanosheet with nitrogen vacancies via thermally treating carbon nitride in an N2 atmosphere (Figure 4a).[136] Che et al. initially reported ultrathin carbon nitride synthesized based on the hydrogen bond intercalating influence exerted by NO3−.[137] Notably, the nitrogen vacancies strength exhibited by the ultrathin carbon nitride can receive the regulation from NO3− concentration at layer of insertion. Figure 4b presents the probable forming process regarding nitrogen defects. To investigate the ultrafast deflagration performance to the formation of defect‐modified g‐C3N4, flame images were captured by high‐speed camera, as shown in Figure 4c, the whole process only lasts for 5 s.[138] Shen et al.[139] performed synergy of dopants and defects in graphitic carbon nitride with exceptionally modulated band structures for efficient photocatalytic oxygen evolution. In this study, boron dopants and nitrogen defects were simultaneously introduced into g‐C3N4 via simply calcining the mixture of g‐C3N4 and sodium borohydride (NaBH4) in a nitrogen atmosphere at different temperatures (see Figure 4d). The obtained boron‐doped and N‐deficient g‐C3N4 was denoted as BHx (x = 300, 350, 400, 450, and 500), where x represents the calcination temperature. As depicted in Figure 4d, during the calcination process, active hydrogen and boron released from NaBH4 would react with the carbon and nitrogen atoms in the framework of g‐C3N4 and produce ammonia and alkanes gases, then BHx was finally obtained, in which amino (NH2) was decomposed and cyano (NC) was introduced by breaking CNC bonds, along with the doping of boron atoms at carbon sites.4Figurea) The process for the preparation of carbon‐rich g‐C3N4 nanosheets through successively thermally treating carbon nitride in different atmosphere. Reproduced with permission.[136] Copyright 2017, John Wiley & Sons, Ltd. b) The stripping process and defect generation mechanism of ultrathin carbon nitride nanosheets, Reproduced with permission.[137] Copyright 2019, Elsevier B.V. c) Preparation process of defect‐modified carbon nitride samples: calcination for the mixture of dicyandiamide (DICY) and NaN3. Reproduced with permission.[138] Copyright 2019, Elsevier B.V. d) Schematic illustration of the preparation process of BHx (boron‐doped and nitrogen‐deficient g‐C3N4, top left) and the proposed structural changes in the heptazine units of carbon nitride as induced by NaBH4 thermal treatment (bottom left). Reproduced with permission.[139] Copyright 2019, John Wiley & Sons, Ltd. e) Schematic representation of the morphological evolution of porous carbon nitride microtubules modified by nitrogen vacancies. Reproduced with permission.[140] Copyright 2019, American Chemical Society. f) Schematic illustration for the synthetic process of nanocage‐like carbon nitride. Reproduced with permission.[141] Copyright 2019, The Royal Society of Chemistry.In another work Wang et al.[140] prepared a series of g‐C3N4 microtubes with tunable N‐vacancy concentrations and porous wall structures were synthesized by an in situ soft‐chemical method (Figure 4e). The novel synthesis involved calcining N‐deficient rod like precursors, which were synthesized by the self‐conversion of monomeric melamine for different hydrothermal treatment times. The morphological evolution of the porous tubular architecture is discussed in detail, aided by time‐resolved hydrothermal experiments. The prepared porous g‐C3N4 microtubes exhibited clearly improved photocatalytic activity for nitric oxides degradation compared to bulk g‐C3N4. The effects of N‐vacancies on O2 and nitric oxide adsorption activation, electron capture, and electronic structure and the effect of the tubular structure on oriented electron transfer were systematically investigated through experimental and computational studies, which led to the proposal of a mechanism for the enhanced nitric oxide removal activity.[140] Template‐free synthesis of nanocage‐like g‐C3N4 with high surface area and nitrogen defects for enhanced photocatalytic H2 activity as shown in Figure 4f.[141]Recently, new exfoliation techniques (liquid phase) to synthesize g‐C3N4 nanostructures from their bulk counterparts have also been designed. For instance, Yang's group synthesized freestanding g‐C3N4 nanostructured materials by exfoliating g‐C3N4 powder in isopropanol; these nanostructures demonstrated high visible light‐derived photocatalytic effectiveness for H2 production. The H2 evolution performance of exfoliated nanostructures was significantly greater than that of their nonexfoliated counterparts by a factor of >17 and significantly more significant relative to previous g‐C3N4 nanostructures by a factor of >8.[142] Besides its unique energy band structure, g‐C3N4 is responsible for photocatalysis applications like H2 generation, CO2 reduction, water purification, and decontamination.Recently, g‐C3N4‐based heterogeneous catalysts are also being investigated as a tool for improving photocatalytic properties.[143,144] Hou et al. synthesized g‐C3N4/N2‐doped graphene/MoS2 ternary heterojunctions, demonstrating excellent photocatalytic efficiency under visible light to remove Cr(VI) in water.[145] It may be due to augmented light absorption, charge separation at the interface, or efficient charge transfer. Han et al. successfully inserted Co3O4 into g‐C3N4 to entrap the photogenic holes in the product, resulting in efficient methylene orange degradation.[146] Some studies have demonstrated the high photocatalytic behavior of silver or gold nanoparticles decorated g‐C3N4 for the decomposition of methylene orange. It was due to the surface plasma resonance phenomenon and synergistic influence of the catalyst's electron sink action with gold or silver nanoparticles.[147,148] Numerous pollutants have been degraded using g‐C3N4/carbon composites. Typically, g‐C3N4/GO is used to degrade RhB and 2,4‐dichlorophenol,[149] g‐C3N4/CNT is used to degrade MB,[150] and g‐C3N4/graphene is used to degrade RhB.[151] It is primarily due to the following factors: first, carbon materials may act as charge transfer and acceptor medium, significantly increasing the exciton separation ability. Second, carbon products can act as cocatalysts, increasing the available photocatalytic active sites. Thirdly, black carbon is more active at absorbing light. However, the loading amount must be carefully monitored since an abundance of black carbon may negatively impact light shielding. In recent years, g‐C3N4 photocatalysts with Z‐scheme heterojunctions have attracted much because of their unique charge transfer procedures to degrade organic contaminants that can significantly improve photocatalytic performance.[152] The benefits of Z‐scheme photocatalysts include cost savings, increased charge separation ability, elimination of light shielding, and optimization of redox potential.[153] Lu et al. identified a g‐C3N4/Ag/MoS2 ternary composite that degraded more efficiently than either MoS2 or g‐C3N4 alone.[154]Metal CompositesMetal composite oxides exhibited photocatalytic advantages, mainly formed with ultrathin thickness.[155] HNbWO6 nanosheets have been extracted via the dispersion of HNbWO6•1•5H2O in an aqueous medium of tri ethanolamine, steady with acid/base effect and exfoliation supported by ion intercalation procedure. Atomic force microscopy (AFM) findings indicated that the prepared samples’ thickness was between 1.8 and 2.0 nm, which is consistent with the single‐layer reputation. Synthesized nanosheets exhibited a high capacity for photocatalytic H2 production (158.9 µmol h−1). Additionally, metal composite oxides were synthesized through ion exchange reactions using ultrathin precursors. SnNb2O6 nanosheets, for example, are synthesized using K4Nb6O17 nanosheets and SnCl2 as precursors.[156] It was retained in SnNb2O6 with a 3 nm thickness through K4Nb6O17 ultrathin thickness, as verified by AFM analysis. In contrast to bulk SnNb2O6, its nanosheets had a higher bandgap and a lower CB (conduction band) potential, indicating a favorable reduction capacity for photocatalytic H2 evolution. Additionally, the charge transfer efficacy of SnNb2O6 nanosheets was improved due to their ultrathin thickness. Additional analysis revealed an exceptional H2 evolution performance over SnNb2O6 nanosheets under visible light, which was approximately 14 times greater than the bulk structures.2D Metal OxidesNumerous MOs have large bandgaps, which provide attractive energy levels for redox reactions but frequently suffer from poor electron conductivity and reduced photocatalytic frequencies. Due to difficulty of accelerating the transfer of photogenic electrons in pure MOs, reducing the migration direction will be a more efficient way to boost photocatalysis. To do this, we can shrink the third dimension while extending the scale of the remaining two dimensions, resulting in a thin assembly with a large surface fraction. This 2D structure minimizes the distance between bulk and surface active sites for electron migration and maintains a high specific surface area. Certainly, fabricating MOs based on 2D materials is a cost‐effective method for optimizing surface area and charge transfer, thus achieving a proficient photocatalytic efficiency.[42] Because the bulk of MO lacks layered architectures, only a few 2D MOs were first recognized for photocatalysis. Moreover, with new synthetic approaches and procedures, 2D nanosheets like TiO2, Fe2O3, Cu2O, ZnO, WO3, SnO, In2O3, CeO2, and HNb3O8 were developed and used as photocatalyst.[157,158]Because of the simple nonlayered structure, several 2D MOs are challenging to shape using the straightforward ultrasonic exfoliation technique. As a result, several other techniques were used to monitor the shape of such materials. For instance, a lamellar organic/inorganic hybrid policy has been suggested to fabricate TiO2 nanosheets.[157] The solvothermal method was used to produce lamellar TiO2 octylamine hybrid precursors using Ti isopropoxide (Ti source), octylamine (capping reagent), and 2‐phenyl ethanol (solvent) (Figure 5a–c).[42,159] The powder obtained from the ultrasound‐based exfoliation was washed to remove octylamine and obtained smooth, TiO2 nanosheets 91.66 nm). Additionally, Bi2W2O9‐shaped exfoliated single‐crystalline WO3 nanosheets. Due to the layered composition of Bi2W2O9 ([Bi2O2]2+ and [W2O7]2− layers), layers of WO3 were obtained by carefully etching the [Bi2O2]2+ layers with acids, and stabilized layers were attained using a surfactant (tetrabutylammonium hydroxide). Attributed to the quantum confinement effect, the exfoliated WO3 nanosheets had a higher bandgap than bulk WO3. Besides an exfoliation process, wet chemical techniques were used to achieve the direct preparation production of MOs nanosheets. Several MOs have been developed using a self‐assembly technique involving ethylene glycol (co‐surfactant) and polyethylene oxidepolypropylene oxide‐polyethylene.[42] These MOs include TiO2, Fe3O4, Co3O4, ZnO, MnO2, and WO3. Furthermore, perovskite oxides have recently experienced a renaissance because of their improved efficiency in photocatalysis and solar cell applications. Numerous stratiform perovskites can be easily exfoliated to produce their layered perovskite nanosheets.5FigureFlakes of ultrathin TiO2 : a) Synthesis scheme, Reproduced with permission.[157] Copyright 2016, Elsevier B.V. b) AFM image, c) AFM images of height profiles in (b) Reproduced with permission.[42] Copyright 2018 John Wiley & Sons, Ltd. d) Exfoliation of titanate crystals of the lepidocrocite type into TiO2 nanosheets is depicted schematically. Reproduced with permission.[162] Copyright 2014, American Chemical Society.Various MOs, such as SnO2, WO3, TiO2, Fe2O3, and ZnO, have been broadly examined as photocatalysts over the last four decades.[160,161] TiO2 was the most studied because of its high stability, adequate electronic structure, biocompatibility, and superior light absorption properties. The 2D nanostructures of TiO2 nanosheets obtained by exfoliating layered titanate have attracted interest for their potential use as photocatalysts.[162] These nanosheets exhibit semiconductor disposition just like their bulk counterparts and contain anatase and rutile phases but with somewhat higher bandgap due to size quantization. Ti0.91O20.36 nanosheets, for example, demonstrated an energy gap value of 3.8 eV, which was more significant when compared to anatase titania (3.2 eV).[163] Top‐down multistep access was well known during the exfoliation and intercalation of MOs to form nanosheets.[164] For instance, layered titanates were initially formed in the case of TiO2 nanosheets through a traditional solid‐state reaction between an alkali metal carbonates solution and TiO2 at high temperature (Figure 5d). Numerous layered MOs, including titanoniobate (Ti5NbO14, Ti2NbO7, and TiNbO5), perovskite oxides (KLnNb2O7 and RbLnTa2O7), HCa2–xSrxNb3, HNb3O8, and WO3 were fabricated via similar solid‐state reactions and wet‐chemical exfoliation approaches.[155,165,166] Zhou and co‐workers recently developed freestanding single‐layer Bi2WO6 nanosheets from cetyltrimethylammonium bromide (CTAB) using a wet chemical method. Several active sites were generated on the surface of single‐layer nanosheets since Bi atoms were not saturated on single layer, which directly produced h+s upon light irradiation. Quick exciton separation at a highly photoactive surface revealed that single‐layer Bi2WO6 has outstanding photocatalytic efficiency for photodegradation of RhB.[167] Titanoniobate nanosheets have improved photocatalytic efficiency in eliminating organic pollutants.[168] Using a simple wet chemical technique, Tae and co‐workers recently investigated the development of multiple titanate nanosheets having diamond‐like shapes with a typical lateral size of 30 nm.[169]Metal ChalcogenidesTransition metal chalcogenides (TMCs) are materials with the chemical symbol MnXm, (M stands for a transition metal while X stands for a chalcogenide). These materials have been identified with group IV (Zr, Hf, and Ti), group VI (W and Mo), and group X (Pt and Pd), including Nb, Re, and Ta.[170] Layered TMCs constitute a structure of 2D layers lying on top of one another and are often presented stoichiometrically as MX2 (Figure 6c). In a typical arrangement, each layer with a thickness of three atoms comprises a central metal atom; in contrast, the chalcogenide atoms are tightly bound above and below. Furthermore, these metal/chalcogen atoms are covalently bonded with a strong interaction while mild London dispersion forces keep the layers together. Consequently, the weakly bonded layers can easily be separated (exfoliated) in TMCs that initially contributed to their applications as high‐efficiency lubricants.6Figurea) Crystal structures of MoS2 with different polymorphisms including 1H, 1T, 1T0, 2H, and 3R structures. Reproduced with permission.[183] Copyright 2015, The Royal Society of Chemistry. b) Crystal structures of MXenes in different layer arrangements by a chemical formula of Mn+1Xn, where n is 1, 2, or 3 (M2X, M3X2, or M4X3), “M” represents the early transition metals (Ti, V, Cr, and Nb, etc.), and “X” is carbon and/or nitrogen. Reproduced with permission.[184] Copyright 2019, American Chemical Society.TMCs have recently gained popularity as a graphene substitute due to their special physicochemical features that other 2D materials lack or cannot achieve. For instance, complex engineering of energy gaps is needed to allow graphene to be used as a transistor. Additionally, TMCs exhibit a broad range of electronic properties, including insulators, semiconductors, metals, and semimetals, with a high range of charge mobilities and direct/indirect bandgap based on their configuration. As a result, TMCs exhibit a host of highly favorable properties relating to charge transfer, magnetism, ion, small molecule intercalation, catalytic and optical properties. TMCs nanosheets perform a variety of roles in photoelectrocatalytic and photocatalysis applications. Mainly, they serve as photosensitizers by enhancing their ability to harvest photons in the visible zone of the electromagnetic spectrum as charge carriers and separators due to their adequate bandgap. Due to their unique electronic structure, metal chalcogenides generally exhibited a relatively large light absorption field, indicating they are potential candidates for various photocatalytic applications.Several 2D TMCs have been developed recently, including SnS2, SnS, MoS2, CdS, SnSe, ZnIn2S4, In2S3, and ZnSe.[171] Due to the critical layer structure of these 2D TMCs, synthetic techniques usually focus on exfoliation. For instance, bulk SnS2 in formamide refluxing is required to break the weak vdWs interactions among the 2D layers to obtain hexagonal SnS2 single‐layers.[172] Due to their favorable electronic properties, various TMCs nanosheets, such as MoSe2, MoS2, WSe2, WS2, TiS2, and SnS2, have recently emerged as potential 2D materials for photocatalysis applications.[173–176] An illustration of 2D MoS2 with 2H and 1T phases is given in Figure 6 possesses numerous active sites and a large surface area, allows for extensive interaction and effective reactions. As a result, MoS2‐based heterostructures and composites have been demonstrated as promising materials, opening up new avenues for photocatalysis applications. The direct removal of organic pollutants and elimination of heavy metals have been thoroughly studied using photocatalytic activity based on MoS2 materials.[177,178] Zhou et al. used primitive hydrothermal methods to fabricate a TiO2‐MoS2 monolayer hybrid photocatalytic activity with a 3D‐layered structure.[179] The 3D‐layered structure comprises a TiO2 nanobelt core and a MoS2 nanosheet shell (referred to as TiO2@MoS2). It has a greater potential for adsorption and superior photocatalytic ability to degrade organic dyes. The alignment of energy levels between MoS2 and TiO2 is ideal for efficiently prohibiting photogenic exciton recombination and better charge transfer. Cheon et al. synthesized disk‐shaped ZrS2 nanosheets with lateral dimensions of 20 to 60 nm through a reaction between CS2 and ZrCl4 in oleylamine.[180] Soon, this process was generalized to include different transition metal selenide and sulfide nanosheets. In another study, MoS2 nanosheets were synthesized using a solvothermal approach using the precursors’ thiourea and (NH4)6Mo7O24•4H2O.[181] Wang et al. showed that the ultraviolet light irradiation of Bi2O2CO3/MoS2 composites significantly photodegraded RhB. This influence was ascribed to the synergy between MoS2 and Bi2O2CO3.[182] Zhang et al. showed that vertically aligned MoS2 layers exhibit high photocatalytic activity and can effectively degrade organic compounds in contaminated water.[3]One more TMCs material, WS2, had already shown a rare and valuable photocatalyst that operated across the entire solar spectrum. Sang et al. announced the discovery of WS2 nanosheets with enhanced photocatalytic performance for UV to NIR (near‐infrared) regions to degrade organic pollutants.[175] The methylene orange solution was nearly fully degraded in 100 min when exposed to UV light, approximately 90% of methylene orange was degraded in 300 min when exposed to visible irradiation, and approximately 80% degradation of methylene orange was recorded in 300 min when exposed to NIR light. Additionally, a solution of RhB dye was degraded using a WS2 photocatalyst under NIR light. After 5 h of exposure, 60% degradation of RhB was evaluated, showing that WS2 is an active photocatalyst in the NIR region. Several top‐down approaches to formulating few‐layers or single‐layer metal chalcogenide nanosheets have been identified, including liquid‐phase ultrasonic exfoliation, mechanical exfoliation, and lithium intercalation exfoliation.[170] Additionally, bottom‐up approaches like chemical processing and chemical vapor deposition offered potentially effective solutions for fabricating metal chalcogenide nanosheets, such as exfoliation techniques.2D MXenesRecent years have seen a boom in studies involving 2D MXenes in photocatalysis and the discovery of novel MXenes materials. While MXenes are incompetent for photocatalysis, they have potential to play a significant role in cocatalysts owing to the metallic conductivity, layered architecture, abundance of hydrophilic active sites, and stable functionalized capacity.[185] Yury's group first defined MXene as a novel 2D family member in 2011. It comprises early transition metal nitrides and carbides, while M in the formula denotes a transition metal, including Nb, Ta, or Ti, and X denotes carbon and N2. In a typical synthesis process, elective etching of group IIIA/IVA elemental layer (A layer) from the bulk Mn+1AXn is carried out to obtain MXenes.[186] This novel synthetic method provides MXene with abundant fluorine and hydroxyl functional groups on the soil, which is advantageous for further surface modification. In 2011, Naguib and co‐workers treated Ti3AlC2 for 2 h at room temperature in hydrofluoric acid; as a result, the exfoliated 2D Ti3C2 layers were initially obtained through the selective etching of Al atoms from a ternary precursor of carbide Ti3AlC2. Because of exceptional electronic properties, large specific surface area, and hydrophilicity, this 2D material has been used in a wide variety of applications since then. Over the last decade, 20 different MXene materials have been synthesized, namely Ti3C2, TiNbC, Ti2C, V2C, Nb4C3, and Mo2C.[187–190]MXenes have also drawn the interest of several researchers for photocatalytic CO2 reduction, H2 evolution, removal of contaminants, and N2 fixation. MXenes are used as a platform for one or more catalyst materials to increase photocatalysis. A Schottky barrier is formed among the semiconductor catalyst and MXene, which prevents photoinduced charge carriers from recombining. Additionally, the high basic surface area of MXenes produces an abundance of photocatalytic active sites.[191,192] Additionally, MXenes exhibit remarkable properties such as high electrical conductivity, chemical stability, environmental friendliness, hydrophilicity, and many surface functional sites that allow them to be used as extremely powerful adsorbents for a wide range of molecules and ions. Due to their increased electroconductivity, MXenes can significantly minimize hole and electron recombination. It is advantageous for the purification of toxic pollutants. When two cationic dyes (acid blue 80 and MB) were combined with Ti3C2Tx under UV irradiation for 5 h, degradation up to 62% and 81% was achieved, respectively.[192] It is hypothesized that the development of TiO2 and/or TiH4O4 on surface of Ti3C2Tx, when exposed to ultraviolet light, enhances photocatalytic performance.[193,194]Wang et al.[195] designed an aqueous droplet light heating system along with a thorough mathematical procedure, which combined leads to a precise determination of internal light‐to‐heat conversion efficiency of various nanomaterials (Figure 7a). To precisely evaluate the light‐to‐heat conversion efficiency of Ti3C2 MXene, droplet‐based light absorption and heat measurement system was carefully established based on literature with certain modification and the system setup is schematically presented in Figure 7b. The internal light‐to‐heat conversion efficiency of MXene, more specifically Ti3C2, was measured to be 100%, indicating a perfect energy conversion. Furthermore, a self‐floating MXene thin membrane was prepared by simple vacuum filtration. In the presence of a rationally chosen heat barrier, it produced a light‐to‐water‐evaporation efficiency of 84% under one sun irradiation, which is among the state of art energy efficiency for similar photothermal evaporation system. The outstanding internal light‐to‐heat conversion efficiency and great light‐to‐water evaporation efficiency reported in this work suggest that MXene is an auspicious light‐to‐heat conversion material and thus deserves more research attention toward practical applications. The laser light is partially adsorbed by MXene existing in the optical path of the laser beam inside the droplet (Figure 7c), which is a circular column with length of 2.6 mm and diameter of 0.85 mm.[195]7Figurea) Schematic illustration of light‐to‐heat conversion by MXene Ti3C2. b) Experimental setup for droplet‐based light‐to‐heat conversion experiment. c) Schematic of droplet with laser irradiation. Reproduced with permission.[195] Copyright 2017, American Chemical Society. Schematic illustration of d) the fabrication process of the hydrophobic d‐Ti3C2 membrane and e) hydrophobic d‐Ti3C2 membrane‐based solar desalination device. Schematic illustration of the solar desalination process of f) hydrophilic and g) hydrophobic membranes. h) Solar thermal conversion efficiency, i) long‐term real‐time seawater weight loss through the hydrophobic d‐Ti3C2 membrane evaporation with 10 mg loading under one sun. Reproduced with permission.[196] Copyright 2018, The Royal Society of Chemistry.Recently Que et al.[196] fabricated a hydrophobic surface‐enabled salt‐blocking d‐Ti3C2 MXene membrane, which contains a salt‐blocking d‐Ti3C2 nanosheet layer modified by trimethoxy(1H,1H,2H,2H‐perfluorodecyl)silane (PFDTMS) for sunlight harvesting and a piece of commercial filter membrane for water supply, was fabricated for efficient and long‐term stable solar desalination (Figure 7d). The whole solar steam generation device consists of three components: a hydrophobic d‐Ti3C2 membrane on a filter membrane as a solar absorber, vapor evaporator, and salt blocker, a piece of commercial polystyrene foam as a thermal insulator and floater, and nonwoven fabric as the water path which pumps water by the capillary effect (Figure 7e). The schematic illustration of the evaporation process for the hydrophilic and hydrophobic d‐Ti3C2 membranes is shown in Figure 7f,g, respectively. Seawater infiltrates the membrane through the capillary effect for the hydrophilic membrane, and the salt continues crystallizing on the membrane surface with water evaporation. The hydrophobic membrane with 10 mg d‐Ti3C2 loading mass delivers a water evaporation rate of 1.31 kg m−2 h−1, which is 3.12 times that of pristine seawater (0.42 kg m−2 h−1). Besides, the natural evaporation rate of the device in the dark shows a relatively low value of 0.19 kg m−2 h−1, which is attributed to the hydrophobic feature of the membrane. Therefore, the corresponding solar steam conversion efficiency is up to 71% under only one sun, as shown in Figure 7h. The solar steam generation performance can be maintained well for at least 10 cycles, with each cycle sustaining for over 1 h. The evaporation rate can be maintained without decay for over 200 h, as shown in Figure 7i.[196]Another research group used a simple hydrothermal oxidation process to fabricate the (001) TiO2/Ti3C2 combination. In this method, 2D Ti3C2Tx MXene was used to incorporate TiO2, while Ti atoms on Ti3C2 acted as nucleation sites. Thus, the atomic level interfacial heterojunction within TiO2 and Ti3C2 layers facilitated the reduction of defect‐induced excitonic recombination. The most abundant reactive species across the light‐induced oxidation of MO are radical hydroxyl ions (OH). Additionally, the photodegradation of MB has been investigated using TiO2–carbon composites synthesized via ball grinding of 2D Ti2CTx.[197] MXene groups have been identified as promising materials to fabricate hybrid carbon/transition oxide. By significantly reducing the recombination of photogenic excitons, carbon nanocomposites will significantly enhance TiO2's photocatalytic potential.[198,199] To increase the photocatalytic performance of metal sulfides, a ternary Ti3C2‐OH/ln2S3/CdS photocatalytic device was formulated through a straightforward hydrothermal synthesis process.[200] The fabricated samples had a spherical shape with a high specific surface area of 4‐TIC (Ti3C2‐OH/ln2S3/CdS with 4% Ti3C2‐OH), which provided extra active sites and facilitated the adsorption of dye molecules. Additionally, the strong relation between the three materials and superior conductivity of Ti3C2‐OH maintained the composites’ vigorous degradation performance. Lu et al. defined the disintegration of MXene mixtures to produce TiO2‐Ti3C2‐CuO composites.[201]Due to their superior electrical features, hydrophilic surface terminations, and large specific surface area, MXenes are widely applied in photocatalytic water splitting.[202] However, since the water redox potentials of the majority of MXenes exceed their band edges, they cannot be explicitly utilized as photocatalysts for water splitting to create H2. Zr2CO2 and Hf2CO2 are exceptions since they possess ideal band edge positions and outstanding optical absorption in UV–vis region, enabling them to be utilized for photocatalytic water splitting to create H2. Thus, for photocatalytic H2 generation, MXenes primarily play cocatalysts’ role, assisting photocatalysts in increasing photocatalytic performance. In N2 fixation, Liu et al. developed and produced the first Ti3C2Tx‐derived TiO2@C/g‐C3N4 composites, demonstrating exceptional photocatalysts N2 reduction performance (250.6 mmol g−1 h−1). The heterostructure of type II formed by TiO2 and g‐C3N4 aided in the effective separation and migration of photogenic excitons. Furthermore, Ti3+ formed during thermal treatment of Ti3C2Tx could serve as photoinduced electron captors and as activation/adsorption sites for N2.[203] Effective harvesting of NIR light is yet a significant problem in photocatalysis; however, Ti3C2Tx/TiO2 catalyst was prepared recently by heating Ti3C2Tx in a muffle furnace which demonstrated impressive photocatalytic behavior when exposed to both full‐spectrum and monochromatic 740 nm light. The increased efficiency may be attributed to the NIR absorption through the plasmonic Ti3C2Tx process and activation of N2 via oxygen vacancies (VO) formed on TiO2.[204]Metal/Layered Double HydroxidesDue to their superior physical and chemical properties, metal hydroxides or layered double hydroxides (LDHs)‐based materials provide an innovative route for the design, optimization, and mechanistic study of photocatalyzed contaminant degradations. These properties provide a unique layered structure, bandgap engineering, large surface area, better anion exchange strength, and wide‐range light absorption. Besides this, their chemical, architectural, and electronic properties can easily be tailored to a particular reaction and working condition.[37] Most research on LDHs‐based photocatalysts has been devoted to controlling their morphological properties and preparing novel nanostructured complexes through methods (elemental doping, loading of functional groups).[205,206] Lately, a significant increase has been noted in the use of LDHs for applications such as photocatalytic oxidative removal of organic compounds, CO2 elimination, water‐gas shift reaction, and other significant chemical transformations.[207–209]LDHs are anionic clays characterized by their layered structures and a particular formula of [M2+1‐x M3+x(OH)2]x+(An−x/n)·yH2O, M3+ represents a trivalent metal ion including Fe3, Cr3+, or Al3+ while M2+ stands for a divalent metal ion like Zn2+, Mg2+, or Cu2+; each metal cation on the terrace is connected to one or more OH groups, forming the backbone of LDHs.[210] An− is a charge‐balancing anion that is stable only under primary conditions and can take on the chemical identities of organic, inorganic, or complex heteropoly anions. Due to the simplicity of the cation guideline, the desired bandgap was developed in metal hydroxides by incorporating specific photoactive metal cations. Thus, the 2D metal hydroxide structure demonstrated significant potential for photocatalysis. For instance, 2D ZnAl‐LDHs were synthesized through a reverse micelle approach to be applied as photocatalysts in CO2 conversion.[211] Due to the ultrathin thickness of the ZnAl‐LDH nanosheets, Vo was developed, resulting in the generation of Zn vacancy complexes. The Zn vacancy complexes can act as electrostatic traps for CO2 photoreduction. As a result, ZnAl‐LDH nanosheets demonstrated ominously enhanced photocatalytic potential for CO2 reduction compared to bulk ZnAl‐LDH. Tian et al. prepared oriented CuCr‐LDHs films (16.5 m thick) on a Cu substrate employing electrophoretic deposition. They discovered that these CuCr‐LDHs films effectively catalyze the visible light photocatalytic decomposition of 2,4,6‐trichlorophenol, RhB, and Congo red.[212] The extraordinary degradation efficiency was due to the high crystallinity, the inclusion of meso‐ and microporous nanostructures, and the large surface area. Numerous new ultrathin metal hydroxides or LDHs, including ZnTi, and NiTi‐LDH, have demonstrated exceptional efficiency in various photocatalytic applications.[213]Recently, LDHs research has expanded beyond the basic dicationic MgAl‐LDHs to tricationic or tetracationic multiphased species, including the MgFeTi‐LDHs, MgZnAlFe‐LDHs, and post‐calcination multiphase MOs‐LDHs, most of which have shown encouraging photocatalytic properties.[209,214,215] Additionally, by loading, intercalation, doping, and chemical modification, LDHs may shape novel layered or nanocomposite photocatalysts with other materials. Ju et al. calcined ZnAlTi‐LDO (ZnAlTi layered double oxide) to 500 °C to obtain ZnAlTi‐LDO nanoparticles, which were then strategically designed to form nanocomplexes with C60 and AgCl for visible‐light photodegradation of bisphenol A at room temperature.[216] The LDO's photocatalytic efficiency was found to be highly susceptible to the pH of the reaction media. They discovered that a mildly acidic environment (pH = 5) promotes photoexcitation of dissolved oxygen by e− and the formation of free radicals such as •O2‐ and •OH; whereas, a strong pH under simple conditions results in a cathodic displacement of the valence band (VB), reducing photocatalytic efficiency.Bismuth‐Based MaterialsBi‐based semiconductor photocatalysts have generated considerable interest owing to their unusual optical, electrical, and structural properties, as well as their exceptional photocatalytic redox behavior and stability. As compared to TiO2 electronic band configuration, the VB of Bi‐based materials is composed of hybridized orbitals of well‐dispersed Bi (6s) and O (2p)Bi 6s orbital will both accelerate and narrow the mobilities of photogenic charge carriers.[217] In Bi (III), the Bi 6s can hybridize with O 2p orbitals to develop narrowed VB and bandgap for visible light absorption. Numerous Bi‐based photocatalysts were synthesized and studied, including Bi2O3, BiOX (X = I, Cl and Br), BiVO4, BiPO4, Bi2MO6 (M = Cr, W and Mo), Bi2O2CO3, BiOIO3, Bi4NO8Cl, Bi2O2(OH)(NO3), (N = Ta and Nb), Bi2O2[BO2(OH)] and Bi4B2O9 etc.Zhang's group developed BiOCl monocrystalline nanosheets with 001 and 010 facets using a mild hydrothermal process involving pH adjustment.[218] BiOCl nanosheets (001) exhibited greater photo‐oxidation behavior for methylene orange degradation when illuminated with UV irradiation. At the same time, BiOCl with 010 facets exhibited superior degradation efficiency when illuminated with visible irradiation leading to indirect photosensitization of dye. The Bi2WO6 nanosheets with a single layer are synthesized using the surfactant cetyltrimethoxysilane. CTAB assisted the hydrothermal technique and the single‐layer Bi2WO6 was decorated with Br ions provided by CTAB and formed Coulomb repulsions, delaying the Bi2WO6 stacking.[167] Additionally, long chains of CTA+ ions with hydrophobic nature on the surface of Bi2WO6 acted as an additional surface repulsion, preventing the growth of crystals along the c‐axis. Numerous coordinative unsaturated Bi atoms were revealed and function as active sites at Bi2WO6 nanosheet. Following light irradiation, holes were formed in [BiO]+ as e−s in [WO4]2‐. Sandwich [BiO]+‐[WO4]2−‐[BiO]+ substructures, similar to heterojunctions, provide an effective interface for space charge separation (Figure 8a–d). As a result, Bi2WO6 exhibited greatly improved photocatalytic performance for ozone removal in visible light. Wang's group fabricated BiOCl microflowers consisting of porous nanosheets with approximately 100% revealing [001] facets, demonstrating exceptional photoactivity to degrade various dyes, such as RhB, MB, and methylene orange.[219] At the terminal of [001] facets, there is a high concentration of O atoms, which favors the adsorption of cationic dye and collection of photogenic e− injected from an excited dye. It was established that dye photosensitization of BiOCl is a feasible method for increasing its absorption of visible light. As a result, it is suitable for photocatalytic degradation when exposed to visible irradiation. Additionally, the porous BiOCl (001) is hoped to be used in dye‐sensitized solar cells.8Figurea) Crystal structure of Bi2WO6, b) CTAB synthesis of a single layer of Bi2WO6, c) by CTAB support TEM/HRTEM of Bi2WO6 formed, d) CTAB‐based AFM investigation of single‐layerBi2WO6, Reproduced with permission.[42] Copyright 2018, John Wiley & Sons, Ltd. Comparison of photocatalytic degradation of e) MB, f) methylene orange, and g) phenol over Bi2O3, Bi2O3/Bi2O4‐x, and Bi2O4‐x, respectively, Reproduced with permission.[220] Copyright 2008, American Chemical Society.Fornasiero and co‐workers have investigated a Bi2O3/Bi2O4‐x composite as a possible photocatalyst (Figure 8e–g). As a result, they used UV–vis light in Bi2O3 to induce surface changes, resulting in the formation of a Bi2O3/Bi2O4‐x nanocomposite arrangement. Thus, such surface modifications significantly improve the photocatalytic efficiency of MB.[220] Zhang et al. used a homogeneous doping approach to synthesize Bi3O4Cl nanosheets having a large exposure percentage of the [001] or [010] facets. The rates of H2 evolution for [010] and [001] BiOCl nanosheets were 0.42 and 0.24 mM h−1 g−1, respectively, with 3% NiOx filling. The findings suggest that 010‐BiOCl nanosheets have a significantly higher charge carrier separation performance and photoabsorption capability due to the facet dependency of homogeneous carbon doping.[221] Composite photocatalyst schemes incorporate a wide range of heterostructures for improving overall photocatalytic performance. This technique enables the tuning of band structures, the promotion of carrier transition, the enhancement of operation and selectivity, and the manipulation of other factors necessary for achieving outstanding photocatalytic efficiency.Additionally, hybrid systems provide suitable templates for in‐depth studies of the critical interfacial effects of catalysis. 2D heterostructures have attracted substantial attention among the numerous composite photocatalysts because of many advantageous characteristics.[222,223] The contact area is expanded by using the wide specific area of 2D heterostructures, enabling intimate interaction and effective transfer of charges at the interface. Owing to this behavior, 2D hybrid systems have enormous potential for achieving superior photocatalytic potentials.[185,224] The heterojunction interfacial effect will significantly increase the lifetime of photogenic excitons in catalysts by effective separation that directly/indirectly leads to redox reactions such as photocatalytic organic degradation and H2 generation. Numerous attempts were made to engineer 2D components or reinforce the interfacial force to shape capable 2D photocatalysts.[225] The 2D/2D heterojunctions have many catalytic advantages, including increased active sites due to larger specific surface area/interface regions and ultrathin structure. Charges quickly move due to the 2D components’ low specific resistance and shorter traveling path in one dimension. Transparency as a result of the ultrathin structure aids in light harvesting.Graphene, in particular, is an excellent medium to fabricate 2D composite photocatalysts. Besides its high electron conductivity, graphene's ample surface functional groups enable it to integrate easily with 2D semiconductors. The compact interaction enables the swift transfer of excitons from 2D semiconductors to graphene, facilitating the photocatalytic phase.[226,227] She et al. created an effective 2D α‐Fe2O3/g‐C3N4 photocatalytic water‐splitting catalyst. Due to their strong interface, electrons in CB of α‐Fe2O3 can swiftly move to VB of g‐C3N4 and recombine to photoinduced holes in VB of g‐C3N4. Consequently, electron excitation in g‐C3N4 was most effective, and photoinduced electron recombination was firmly suppressed.[228] Zhu et al. synthesized another 2D/2D composite photocatalyst by utilizing g‐C3N4. In contrast to the 2D α‐Fe2O3/g‐C3N4 structure, Type‐I heterojunction was fabricated by a hybrid structure.[229] The photoexcited e− in g‐C3N4 traveled to CB of BP when exposed to visible light, which combined with the e− produced in BP. The e− was captured collectively in the interfacial PN coordinate bond between g‐C3N4 and BP, extending the lifetime of the electrons. Correspondingly, the Janus bilayer junction was fabricated using 1T MoS2 and single‐layer BiOCl, demonstrating enhanced visible‐light hydrogen evolution behavior.[230] The key to this Vo‐oriented assembly lies in the metallic characteristic of MoS2 monolayer and the asymmetric structure of 1L‐BOC composed of only (Cl2) layers and oxygen‐deficient (Bi12O17) layers. The enormous difference in electrostatic potential between the (Cl2) and (Bi12O17) sides produced an internal electric field vertically aligned from the latter to the former in this one‐of‐a‐kind configuration. Internal electric fields eased the movement of photoinduced charge carriers to the (Bi12O17) and (Cl2) faces, respectively. As a result of the transfer of electrons to monolayers of MoS2 via BiS bonds, light‐induced H2 evolution was achieved.Engineering ProtocolsDefects were shown to be important across photocatalytic processes. The material's intrinsic properties, including carrier concentration, coordination number, electrical conductivity, electronic structure, and microstructure, could be altered by engineering various defects. Apart from doping, defect engineering has always had a significant impact on 2D materials, as demonstrated in the case of photocatalysis. Because of the atomic‐scale thin nature of 2D materials, even a very minute content of defects can exert huge effects on basic properties. Due to the lower escape energy of atoms in 2D materials, a wide range of various types of defects can be generated in contrast to the bulk materials. Thus, surface defects such as anion and cation vacancies, pits, associated vacancies, and distortions can be easily created to optimize the electronic properties of 2D materials. Herein, various surface defects in 2D photocatalysts and their beneficial properties are summarized.Defect Engineering in 2D PhotocatalystsAnion VacanciesDue to the low formation energy of VO, they have been considered more prevalent and extensively studied defects in transition‐MOs.[231] Due to the atomic thickness of the VO, the electronic structure and physiochemical properties of 2D materials are being effectively customized, affecting photocatalytic efficiency.[232] Along with altering the carrier concentration and electronic structure, engineered VO can aid in molecule activation, including O2, N2, and CO2, and enhanced photocatalytic efficiency. Zhang and co‐workers discovered that the VO in BiOBr contains clustered electrons for back‐donation, which can cause changes in the adsorbed N2 molecule, lengthening the bond between N‐atoms from 1.078 to 1.133 for free molecular nitrogen.[233] The N2 molecule can also be effectively reduced to NH3 using electrons transmitted through the interface from its excited BiOBr. Comparable to N2 activation, it has been shown that VO in ZnAl‐LDH is conducive to CO2 activation.[234] An increase in the density of VO was observed upon gradual decrement in the thickness of prepared samples (210 to 2.7 nm) due to the emergence of various unsaturated coordinate Zn ions adjacent to the Vo.Kong's group adopted the plasma engraving method to produce defects of VO and Ti3+ on TiO2 nanosheet's surface.[235] The electronic configuration of TiO2 nanosheets undergoes considerable variation due to the manufactured defects, with bandgap decreasing (3.13–2.88 eV), along with upshifting of CB and VB edges forming a defective state in the forbidden gap. Due to the formation of this defective state, the H2 generation activity reached dramatically twofold compared to pure TiO2. Besides facilitating the forming of intermediate bands in the bandgap, the VO in WO3 atomic layers promotes the adsorption and activation of CO2 into radical COOH• species.[236] The critical function of VO in WO3 layers enables the creation of more CO and O2 in the infrared region.Lei and co‐workers achieved totally controlled formation of Vo‐rich and Vo‐deficient In2O3 nanosheets by rapidly heating In(OH)3 nanosheets in the presence of oxygen or air.[237] They highlight the first synthesis of ultrathin cubic‐In(OH)3 sheets through a mesoscopic‐assembly strategy and hence realize the fabrication of 5 atom thick In2O3 porous sheets with VO via a fast‐heating strategy. The number of oleate ions and their peculiar arrangement play a crucial role in forming In(OH)3 2D sheets (Figure 9a). Initially, three oleate ions interact with one In3+ ion to form an In–oleate complex via electrostatic interaction. The homogeneously dispersed oleate ions on the surface of In3+ ions lead the complex to take on a hexagonal mesostructure, in which all the In3+ ions are uniformly separated by one oleate ion. Meanwhile, the corresponding small‐angle XRD pattern in Figure 9b demonstrates the presence of a hexagonal mesophase with a = 27.7 Å, which relatively consists with the length of one oleate ion. Additionally, a strong VO signal at g = 2.004 was detected in ESR (electron spin resonance spectroscopy) spectra, indicating that the Vo‐rich In2O3 contains the largest VO. The Vo innovation significantly altered the electronic structure of In2O3 nanosheets with high Vo material. As shown by DRS (Diffuse reflectance spectroscopy) and XPS examination, Vo‐doped In2O3 sample exhibited a narrower energy gap, and an upshift was observed in VB tip. DFT calculations demonstrated that large density of states (DOS) was produced at valence band maxima. A new concentration of defects revealed that Vo‐rich In2O3 nanosheets were more abundant than VO‐poor In2O3 nanosheets. As a result, the VO‐doped In2O3 produced a stronger electric field and a higher carrier level. Irradiation further excited the electrons into CB. Thus, Vo‐rich In2O3 nanosheets outperformed VO‐weak In2O3 nanosheets and bulk In2O3 by 2.5 and 15 times, respectively, for H2O oxidation. These findings substantiated the efficacy of anion vacancy in electronic configuration engineering. The TEM image in Figure 9c clearly shows their sheet‐like morphology, while the HRTEM in Figure 9d demonstrates their high orientation along the [01‐1] projection. The AFM image and the corresponding height profiles in Figure 9e,f show their average thickness of ca. 0.9 nm, which agrees well with the 0.88 nm thickness of a 5 atom thick In2O3 slab along the [01‐1] direction. The TEM and AFM images in Figure 9c,e reveals the presence of abundant pores in the ultrathin In2O3 sheets, verified by the height profiles in Figure 9e. Evidently, as displayed by the calculated DOS in Figure 9h,i, the perfect 5 atom thick In2O3 slab shows an increased DOS at the conduction band edge compared with the bulk counterpart, which indicates that more carriers can be effectively transferred to the conduction band minimum of the atomically thin perfect In2O3 sheets. In addition, the calculated results in Figure 9g,h reveal that the presence of VO endows the In2O3 2D structure with increased DOS at valence band maxima, which ensures a higher carrier concentration and hence increases the electric field in the space charge regions, thus achieving enhanced carrier separation.9Figurea) Under specific conditions, atomically thin In2O3 porous nanosheets with VO‐rich/VO‐poor, b) Small‐angle XRD patterns for an as‐synthesized precursor that is time dependent. c–f) Characterization of VO‐rich atomically thin In2O3 porous nanosheets produced by fast thermal heat treatment of In(OH)3 nanosheets in air, c,d) TEM/HRTEM image, e) AFM analysis, f) O 1s XPS spectra, g) ESR spectrum, h) simulated DOS of five‐atom‐thickness In2O3 slab with O2 defect, i) ideal five atom‐thickness In2O3 slab. Reproduced with permission.[237] Copyright 2014, American Chemical Society.Cation VacanciesCation vacancies impart a similar moderating impact on the electronic, physical and chemical features of metallic compounds, just like anion vacancies, leading to their diverse electronic and orbital distributions.[238] Compared to anion vacancies, metal cation vacancies are more difficult to engineer and control due to their high forming energy, making it much more difficult to decide their work.[231] Scientists have created many photocatalytic materials with cation vacancies and are investigating their effect on efficiency of the photocatalysts.[239–241] The 2D atomic layers, both with and without constrained cationic vacancies, can become excellent models for elucidating the structure–activity relationship in considerable detail. Vanadium vacancies (Vv) have been generated in BiVO4 unit cell nanosheets (1.28 nm) in a variety of amounts (Figure 10). To detect the types and quantities of defects in the synthetic o‐BiVO4 atomic layers, positron annihilation spectrometry (PAS) was performed and the results were shown in Figure 10c–e. As revealed by the positron lifetime spectra in Figure 10c, the orthorhombic‐BiVO4 (o‐BiVO4) atomic layers exhibited three‐lifetime components, with the two longer life components (τ2, τ3) could be ascribed to the large voids and the interface present in the samples. Jiao et al. synthesized dense ZnIn2S4 layers (unit cell) with extensive or limited Zn vacancies using a simple hydrothermal method and temperature variance.[242] The TEM images revealed a sheet‐like morphology, while the AFM parameters indicated that the layers of ZnIn2S4 formed were unit‐cell thick along the direction of c‐axis.10Figurea,b) TEM and AFM analysis of Vv‐rich o‐BiVO4 with one‐unit‐cell thickness, c–e) defects study of Vv‐rich and Vv‐poor o‐BiVO4 atomic layers, c) positron duration spectra, d,e) scheme of entrapped positrons, f,g) V defects in o‐BiVO4 single unit cell layer slab and pure o‐BiVO4 single unit cell layer slab calculated by DOS, g) along [001] direction. Reproduced with permission.[245] Copyright 2017, American Chemical Society.The Zn vacancies‐poor and rich characteristics of the investigated products were evaluated using zeta‐potentials, EPR (Electron paramagnetic resonance spectroscopy), and PAS, demonstrating that ZnIn2S4 layers with distinct Zn vacancy concentrations were successfully prepared, providing two ideal models for examining the relation among Zn vacancies and photocatalysts actions.[242] Due to the high Zn vacancies, ZnIn2S4 layers exhibited significantly improved excitons separation performance, confirmed by PL (photoluminescence) analysis, surface photovoltage, and ultrafast transient absorption. Additionally, the plentiful Zn vacancies boosted the light‐harvesting from 440 to NIR region, along with superior CO2 adsorption and hydrophilicity. The distribution of charges in space near the CB's edge has been measured using DFT. The Zn vacancy is observed to increase the charge density of neighboring sulfur atoms, implying that electrons are being excited to CB more quickly.[242] Song et al. discovered that Ti vacancies in single‐layer H1.07Ti1.73O4•H2O nanosheets would cause the development of numerous radical O species that interact with water molecules through H2 bonds to form surface coordination. Consequently, a 10.5‐fold increase in photocatalytic efficiency for H2 evolution can be achieved compared to its layered equivalent.[243,244]Associated VacanciesAlong with monatomic vacancies, associated vacancies have the potential to significantly alter the physical and chemical properties of semiconductors via multiatomic vacancy coupling. A published study revealed that high‐energy facet treatment facilitates surface defects forming like associated vacancies. Atoms lost from the surface not only add mono vacancies but thereby associated vacancies as well. Due to the multi‐atomic coupling of vacancies, these defects will intensely engineer the electronic properties and result in extraordinary electronic output. For example, by morphologically manipulating Bi2WO6 nanosheets to form peony‐like aggregations and nano‐bipyramids, the exposed (100) and (113) facets can also be obtained.[246] As verified by XPS, PAS, and theoretical measurements, the “Bi‐O” associated vacancies are present in the exposed high‐energy (100) facets of Bi2WO6 nano‐bipyramids. The formed “Bi‐O” associated vacancies lead to bandgap narrowing and increasing the exciton separation efficiency that, in turn, enhances the photocatalytic potential to degrade dyes. Conversely, due to Bi2WO6 complex structures, along with crystal facet, grain boundary, morphology, capping agents, and a direct relationship with vacancy associates and photocatalytic behavior can be difficult to establish.[246]The 2D materials have been an excellent model for investigating simple structure–activity correlations since it allows for the careful introduction of defects on its surface while being consistent with other configurations. For example, dimension engineering was used to produce triple vacancies of VBi′′′VO••VBi′′′ in BiOCl nanosheets.[247] The PAS was used to verify the corresponding triple vacancy VBi′′′VO••VBi′′′ (Figure 11). When the outer Bi atoms surface is revealed in the BiOCl lattice, they can easily break out and form a vacancy. Since the thickness had been reduced to the atomic level, oxygen atoms associated with Bi atoms present inside an internal layer can escape more easily. Thus, controlled defects in BiOCl nanoplates were isolated, while in BiOCl nanosheets, their corresponding vacancy VBi′′′VO••VBi′′′ was modified. Different defect forms undoubtedly impact the electronic structure, which ensures improved RhB adsorption due to the additional ‐ve charge. Due to the change in defect forms, ultrathin structures exhibited upshifting of CB and VB potentials that supported the mobility of charge carriers, allowing enhanced exciton separation. As a result, BiOCl nanosheets demonstrated significant solar photocatalytic behavior for pollutant elimination.11Figurea) TEM image of BiOCl nanosheet, b) Positron lifetime spectrum of BiOCl nanosheets and BiOCl nanoplates, respectively, c,d) Schematic representations of trapped positrons of VBi′′′ defect and VBi′′′VO••VBi′′′ vacancy associates, respectively. Reproduced with permission.[247] Copyright 2013, American Chemical Society.Distortions and PitsApart from atom‐different vacancy‐related defects, many other types of lattice defects, such as lattice dislocations, distortion, and disorders, can also have significant regulation effects on materials’ electron configuration and physicochemical properties.[248] As compounds are limited to atomic thickness, a significant amount of breaking of interatomic bonds occurs, resulting in relatively active and wish‐bonding surface atoms. As a result, 2D crystals would have high specific surface energy, making the structure extremely unstable. In order to achieve a much more stable thermodynamical state, crystals tend to gain minimum surface energy values in general. Surface distortions can effectively decrease the surface energy of 2D materials and boost their structural stability. Several lattice parameters, such as interatomic distance, bond angle, coordination number, and bond length, are affected by the generation of surface distortions.[249] Consequently, the electronic structure of the crystals is undoubtedly disturbed due to such changes, which tailor the photocatalytic activity. For instance, X‐ray absorption fine structure spectroscopy (XAFS) results validated a notable change in the local arrangement of atoms in single‐layer ZnSe in contrast to bulk structure.[250] The significant peaks observed at 2.11 and 3.63 Å for bulk ZnSe are ascribed to the closest Zn–Se, and Zn–Zn coordinates, respectively (Figure 12). However, a shift in Zn–Se peak to 2.17 Å with reduced intensity and a significant decrease in Zn–Zn peak intensity were observed for single‐layer ZnSe.12FigureXAFS measurements and calculated DOS from Synchrotron radiation. a) Zn and Se K‐edge extended XAFS oscillation function kχ(k) and b) Fourier transforms for ZnSe single layers, ZnSe‐pa (pa: n‐propylamine) single‐layers, and bulk ZnSe, respectively; the red, blue, and black lines correspond to ZnSe single layers, ZnSe‐pa single layers, and bulk ZnSe, respectively, c) The black, blue, and olive lines in the estimated DOS represent the total, Se sp, and Zn sp state, respectively; the calculated bandgaps for ZnSe single layers and bulk ZnSe were 1.25 and 1.07 eV, respectively. d) ZnSe single‐layer structural model viewed in the (110) plane. Reproduced with permission.[250] Copyright 2012, Nature Publishing Group.In addition, SeSe bond lengths were extended from 4.012 Å to 4.11 Å for bulk and single‐layer structures, and the bond angle between Zn–Se–Zn surface atoms was also compressed by 7.0° as well, in single‐layer ZnSe in contrast to the bulk structure. These results suggested a significant disturbance in the local atomic arrangements arising from the surface distortions. Apart from that, the structural stability of single‐layer ZnSe structures was greatly improved due to reduced surface energy from the distorted surface structure. Additionally, the charge migration ability in single‐layer structures is improved significantly, attributable to higher DOS near CB edge, arising from surface distortions. Benefiting from such outstanding properties, a very stable and efficient water photo‐oxidation performance was realized for single‐layer ZnSe. Similar findings have also been reported for surface distortions in other 2D NiTi‐LDH, SnS, and SnS2 systems.[84,213,251] Zhao et al. reported that in contrast to Ti4+ in bulk NiTi‐LDH, very thin nanosheets (2 nm) of NiTi‐LDH can acquire Ti3+ with lower coordination numbers, which was verified from X‐ray absorption near‐edge structure (XANES) spectra as well.[213] The electronic configuration of thin NiTi‐LDH nanosheets was effectively tailored due to immense structural distortions. Hence, a superior photocatalytic response can be achieved for O2 evolution under visible irradiation. As a result, the photocatalytic oxygen evolution behavior under visible light can be improved.Strategies for Tuning Defect CreationSurface defects can efficiently modify materials’ local atomic and electronic structure, electrical conductivities, and optical properties, further affecting physical and chemical features and light‐driven catalytic efficiency. Hence, it is necessary to describe efficient methods for regulating the formation of defects and the intrinsic mechanism for defect formation.Chemical Reduction TreatmentsChemical reduction is a powerful technique for engineering surface defects into semiconductors.[252,253] It can be used with reducing reagents such as NaBH4, CaH2, and N2H4 or with reducing solvents such as ethylene glycol and glycerol. By chemical reduction with NaBH4, Bi et al. synthesized defective K4Nb6O17 nanosheets (Figure 13a). The high reducibility of NaBH4 reacts with the oxygen atoms in the lattice K4Nb6O17 nanosheets during the reaction, leaving VO on the surface.[254] This VO may be used for bandgap narrowing via lowering CB edge, thereby increasing light absorption. Simultaneously, this will create a barrier for surface electrons, promoting charge separation and increasing H2 evolution behavior. As MO is prepared using reducing solvents, the oxygen atoms are often lost, leaving VO in the lattice. For instance, during the BiOCl nanosheet preparation at 160 °C, highly reducing ethylene glycol can interact with O2‐ at (001) facet much more quickly, forming VO.[255] Not only do the shaped VO expand the absorption edge of light spectrum, but they also allow the efficient capturing of photogenic electrons and molecular O2 to form radical superoxide species.13Figurea) Photogenerated carriers migration routes in defective K4Nb6O17 bulk and nanosheets, with comparison of H2 evolution activity between defective K4Nb6O17 nanosheets. Reproduced with permission.[254] Copyright 2014, John Wiley & Sons, Ltd. b) There are two energy barriers; the first one (0.51 eV) is due to the SH bond breaking, and the second one (0.22 eV) is due to SC bond breaking. The sulfur vacancies in the initial state are illustrated by dashed. The inset shows the chemical structure of MPS. c) As‐exfoliated and d) top‐side treated monolayer MoS2 sample, showing the significant reduction of sulfur vacancy by MPS treatment. Red arrows highlight the sulfur vacancies. The overlaid blue and yellow symbols mark the position of Mo and S atoms, respectively. Scale bar, 1 nm. e) Typical σ‐Vg characteristics for as‐exfoliated (black), top‐side‐treated (blue), and DS‐treated (red) monolayer MoS2 at T = 300 K. f) µ‐T characteristics for the three devices at n = 7.1 × 1012 cm−2. Solid lines are the best theoretical fittings. The dashed red line shows T−0.72 scaling. Arrhenius plot of σ (symbols) and theoretical fittings (lines) for the g) as‐exfoliated, h) top‐side‐treated MoS2. Reproduced with permission.[256] Copyright 2014, Nature Publishing Group.Chemical therapies are often used to modify the surface of objects. Chemical additives can be used to enhance doping efficiency while causing less harm to the components. Chemical therapies are also used to control optoelectronic interface trap states. The sulfur vacancies in MoS2 have been filled using thiol chemistry, where chemical reactions dominate the curing process.[256] The monolayer MoS2 samples studied here are obtained by mechanical exfoliation from bulk crystals. As demonstrated in earlier works, a high density of sulfur vacancies exists in as‐exfoliated MoS2.[257,258] These defects, which can act as catalytic sites for hydrodesulfurization reactions, are chemically reactive. Therefore, it is possible to repair the sulfur vacancies by thiol chemistry. For two reasons, they choose a specific molecule trimethoxysilane (MPS, Figure 13b, inset). i) The SC bond in MPS is weaker than other thiol molecules like dodecanethiol due to the acidic nature of CH3O groups, leading to a low energy barrier for the reaction. ii) The trimethoxysilane groups in MPS react with the SiO2 substrate to form a self‐assembled monolayer.The reaction kinetics of sulfur vacancies and MPS by DFT is shown in Figure 13b. The MPS and generated sulfur vacancies undergo a two‐step reaction with energy barriers of 0.22 and 0.51 eV, respectively. Therefore, the sulfur vacancies in Figure 13c,d are believed to be intrinsic rather than induced by electron irradiation. High‐temperature annealing can also be used to solve the low energy barrier. The concentration of sulfur vacancies is decreased fourfold after MPS therapy. As a result, the monolayer MoS2 field‐effect transistor exhibited superior mobilities (>80 cm2 V−1 s−1) even at room temperature. It is significantly greater than the sample size as packed. Another approach for defect healing is the sulfur vacancy self‐healing technique by utilizing poly(4‐styrenesulfonate) (PSS) therapy.[259] The healing mechanism of PSS (acting as a catalyst) can be explained as the hydrogenation of PSS guiding sulfur adatom clusters onto the as‐grown MoS2 to fill the vacancies. Thus, electronic density of healed MoS2 is reduced by a factor of 643, resulting in the fabrication of a lateral homojunction with a complete rectifying response. The efficiency of homojunction was significantly improved due to the elimination of lattice defect‐induced local fields. Surprisingly, the three types of devices exhibit very different behavior (Figure 13e–h).Ball MillingBall‐milling may be used to break the surface of materials and introduce defects. As graphite is ball‐milled, the particle size of the graphite decreases, and more edges/defects are revealed, which would be useful for maximizing the catalytic activity.[260] Zhu et al. discovered that during the ball‐milling process, various defects in BiPO4, such as VBi and VO, were created.[261] However, since the generated vacancies/defects were in bulk form, it prevented the isolation of photogenerated charges, and the photocatalytic activity was decreased. In 2D crystals, surface defects would predominate due to the extremely thin (atomic‐scale) structure having numerous exposed atoms at the surface. Even so, this technique aims to introduce a variety of surface defects into 2D materials, thus increasing photocatalytic efficiency. While the defect concentration can be adjusted by adjusting the ball milling time and strength, the type of defects is difficult to monitor.Vacuum ActivationAdditionally, vacuum activation is widely used for moderating defects into 2D materials. Xing's group presented an easy and inexpensive vacuum‐activated low‐temperature process for TiO2 modification.[262] Without altering the crystal structure or crystallinity of TiO2, Ti3+ and Vo can be added. As TiO2 is heated to a sufficient temperature in a vacuum, the O atoms on the surface lack any external pressure restraint and appear to discharge from the surface. As a consequence of the increasing temperature and lengthening of time, Vo and Ti3+ developed. By stretching the light‐harvesting range and creating defective states to catch photogenic charge carriers, the vacuum‐activated process will enhance photocatalytic operation for contaminant elimination and H2 production. Since this technique falls into very mild treatment methods, generated defects can eventually vanish during the photocatalytic phase. However, these defects can be regenerated again by repeating the procedure and can be applied to several MOs, including ZnO, WO3, and MoO3. In addition, the VO concentrations can be effectively controlled by controlling the temperature and treatment time during the treatment. Moreover, this method had difficulty realizing other types of vacancies like S, B, or metal vacancies.Hybridization Engineering2D materials have an extremely high specific surface area, which enhances the importance of the surface state relative to the bulk inside. Charge carriers produced by photons are dispersed at the surface to participate in the oxidation/reduction reactions. Thus, the hybridization of surfaces to boost the effective ingestion of excitons is enviable in the absence of a 2D structure. In this section, consistent with surface hybridization, various 2D hybridization techniques with robust case studies are added, including QDs/2D materials, single atoms/2D materials, molecular/2D materials, and layered 2D/2D hybridization.Single Atoms/2D Materials HybridizationIn order to boost photocatalytic performance, it is possible that nanoparticles could be reduced to single atoms. However, the fraction of monoatomic with unsaturated coordination bonds is maximized, enabling a high surface effect.[263] Zhang et al. pioneering work on monoatomic‐based catalysis attracted attention in the photocatalysis domain. The monoatomic‐dependent photocatalyst was focused on the dispersion or coordination of secluded monoatoms on the surface of the support material. Monoatomic‐based strategies may enhance photocatalytic behavior and provide another method for adjusting selectivity. Additionally, active single atoms, chemical bonding between single atoms, and 2D materials‐based supports have developed into a robust and straightforward charge transfer process. Thus, building a single atom/2D materials hybridization is highly desirable to achieve a superior photocatalytic response (Figure 14a).[264] Via calcination, protonation, and coupled exfoliation, single Rh atoms have been scattered on uniform TiO2 nanosheets (Figure 14b).[265] The water dissociation at the surface, as in states 2 or 4 (Figure 14c), is not the final state (generation of H2) but only a reaction intermediate state. At least one or more transition states may influence the overall rate in the full reaction pathway to the final state. Hence, an Rh‐doped titania nanosheet was prepared. The photocatalytic activity for hydrogen production from this nanosheet was 10 times that for an undoped nanosheet. The presence of single Rh atoms substituting for Ti4+ in the nanosheet lattice was confirmed by TEM observation. It indicates that single Rh atoms can act as reaction centers for the photocatalytic reaction. Thus, crystal sites containing a single transition metal atom in the photocatalyst can act as cocatalysts. First‐principles modeling methods support the experimental observations. These results will be useful in better understanding the role of the cocatalyst and the mechanism and will provide new insight for the design of advanced photocatalysts for water splitting. On the other hand, simulations reveal a metastable dissociatively adsorbed state for both the doped and the undoped system. The atomic configuration of the dissociated states is similar for both cases (Figure 14c,d), with an OH fragment adsorbed atop a (now seven‐fold coordinated) Ti atom and the H atom adsorbed atop a surface bridging oxygen. Despite the similar geometries, the total energy cost for dissociation is 0.85 eV in the undoped system but only 0.48 eV when dissociated near the Rh atom. The different energy cost of dissociation, 0.85 eV versus 0.48 eV, suggests that more dissociated water molecules can be found when the nanosheets are doped. Based on the Boltzmann factor based on this difference, approximately six times more dissociated water molecules on the Rh‐doped systems can be expected than the undoped ones. Additionally, HAADF‐STEM techniques are employed to confirm the defects of 2D photocatalyst. In the HAADF‐STEM image, distinct brightest spots indicated Rh atoms, while moderate brightness spots indicated Ti atoms (Figure 14e,f). In addition, the Extended X‐ray absorption fine structure (EXAFS) analysis revealed that Rh atoms in prepared samples exhibited a chemical environment similar to that of Rh2O3, which exhibited the bonding with O atoms and undergoing oxidation. Guo and co‐workers investigated using single Pt atoms as cocatalysts to enhance the hydrogen generation behavior of C3N4 nanosheets under irradiation.[266] To form Pt single atoms/C3N4, a basic liquid phase reaction with H2PtCl6 and C3N4 was used in conjunction with low‐temperature annealing. HAADF‐STEM technique was used to determine the dispersion and structure of Pt. Specific, transparent spots have been observed to be uniformly scattered on graphitic‐C3N4 sheets, with 99.4% of Pt having a size greater than 0.2 nm, indicating that Pt exists entirely as monoatomic (Figure 14e–j). As the doping concentration of Pt exceeded 0.38%, the dispersion of Pt atoms became denser, and numerous nanoclusters were formed. The local atomic configuration of the Pt/C3N4 has been investigated using extended EXAFS spectroscopy. The coordination number was evaluated to be approximately 5 for the Pt atoms, which confirmed the decoration of monoatomic on the top of g‐C3N4 surface having a bandgap value of 2.03 eV. The photocatalytic H2 evolution behavior was significantly enhanced regarding the fabrication of a single Pt atom/C3N4 structure. Pt/C3N4 (0.16 wt% Pt loading) reached a production rate of nearly 318 µmol h−1, about 50 times that of pure C3N4. Concurrently, prepared structures demonstrated remarkable stability during the photocatalytic H2 generation after several cycles. The desirable quality of ultrafast transient absorption spectroscopy, surface trap states of C3N4 was largely altered by the secluded single Pt atom, which increases the exciton life period and increases the possibility of e− engaging in H+ reduction.14Figurea) Merging single‐atom‐dispersed silver and carbon nitride to a joint electronic system via copolymerization with silver tricyanomethanide. Reproduced with permission.[264] Copyright 2016, American Chemical Society. b) Illustration of photocatalytic reaction centers in 2D titanium oxide crystals. c) Charge density and dissociation energetics for undoped and Rh‐doped titania nanosheet. d) Top view images for charge density of nanosheet during water dissociation process for undoped and Rh‐doped nanosheets. e) HAADF‐STEM (200 kV) image of Rh (x = 0.026)‐doped Ti1.82‐xRhxO4 nanosheet. (f) Magnified HAADF‐STEM image (80 kV) of Rh (x = 0.026)‐doped nanosheet. Reproduced with permission.[265] Copyright 2015, American Chemical Society. g) Pt‐CN HAADF‐STEM picture. The size distribution of the bright spots is seen in the inset. h) Pt L3‐edge EXAFS oscillations of Pt‐CN, K2PtCl6, and Pt foil Fourier transform. i) Pt‐CN schematic models. j) H2 generating activity of g‐C3N4 and Pt‐CN photocatalysts. Reproduced with permission.[267] Copyright 2016, John Wiley & Sons, Ltd.The observation that secluded metal atoms possess high surface energy was shown, as well as the possibility that these atoms cooperate closely with the supports surface. The hybridization energy scheme can become a local minimum by interacting with affected metal atoms with available defects on the support surface. Hence, these atoms could be secured and maintained in their stable state. Surface defects are more likely to form in 2D materials due to the extremely high specific surface area and minute atomic flee radiation. Thus, a monoatomic‐anchored surface defect rich 2D structure can be constructed to enhance photocatalytic behavior.[42]Quantum Dots/2D Materials HybridizationNanoparticles have dangling bonds between their coordinated unsaturated surface atoms. In order to reduce the size of nanoparticles any further, a higher proportion of atoms at the surface must be achieved in comparison to total atoms, and their average binding energy should be more significant. Thus, if the size of nanoparticles can be used to monitor the QDs and adjust the 2D materials, it is possible to produce strong interfacial coupling between them. Additionally, due to the small size of QDs, they may exhibit a strong dispersion on 2D materials, enhancing the photocatalytic properties. To improve Ag's operational effectiveness, Ag QDs with a diameter greater than 5 nm have been developed.[268] Upon irradiation with visible light, photocatalytic performance to degrade tetracycline hydrochloride (TCH), ciprofloxacin (CIP), and RhB was greatly enhanced following hybridization with BiOBr nanosheets. The study revealed that tuned Ag QDs could effectively activate molecular O2 with the help of excited electrons, which decreases upon exposure to irradiation. Ag QDs will act as adsorption, charge separation, and reaction centers simultaneously, resulting in increased photocatalytic behavior. In order to reduce the service life of noble metals, metal‐free or non‐noble metal QDs are used instead. For example, N‐doped carbon QDs (N‐CQDs) of 3 nm diameter have been fabricated via a hydrothermal route and then adjusted on the atomically thin surface of BiOI nanosheets.[269] AFM images show that the average thickness of BiOI is about 0.9 nm, suggesting the single‐layer configuration. After the introduction of N‐CQDs, the N‐CQDs/BiOI materials display a greatly prolonged lifetime of photogenerated charge carriers, as proved by time‐resolved transient photoluminescence decay and instantaneous photocurrent. The atomically thin structure of BiOI ensures the strikingly fast bulk charge diffusion to the surface, and the modified N‐CQDs with conjugated π structure can effectively promote the surface charge separation, resulting in a longer carrier lifetime. As a result, the concentration of active species and photocatalytic performance of the N‐CQDs/BiOI photocatalyst were expressively increased.Likewise, many other systems involving QD/2D materials hybridization that can be used to improve photocatalytic performance, including NiS2, Zn–Ag–In–S, and CdSe QDs, have been explored.[270] Kang and co‐workers have utilized carbon nanodots (CDots) as a chemical catalyst to boost photocatalysts H2O splitting through C3N4 significantly (Figure 15a–d).[271] They systematically confirmed that water‐splitting photocatalysis by CDots‐C3N4 indeed proceeds via the stepwise 2e−/2e− two‐step process, in which H2O oxidation to H2O2 is the first and rate‐limiting step, followed by the second and fast step of H2O2 disproportionation to O2, which CDots chemically catalyze. Additionally, CQDs have also been involved in the disintegration of H2O2 and O2 evolution. As a result, exceptional photocatalytic H2O splitting efficacy can be achieved, with a solar to H2 conversion performance of 2.0% and healthy stability of 200 days. Apart from the above results, several other systems regarding QD/2D material hybridization to promote photocatalytic performance, such as CdSe QDs, Zn–Ag–In–S QDs, NiS2 QDs, and so on.[270,272,273] Recently, Ning et al.[272] developed a kind of cadmium‐free Zn–Ag–In–S (ZAIS) colloidal QDs that shows remarkably photocatalytic efficiency in the visible (Figure 15h). More importantly, a nanocomposite based on the combination of 0D ZAIS colloidal QDs and 2D MoS2 nanosheet is developed. This can leverage the strong light harvesting capability of colloidal QDs and catalytic performance of MoS2 simultaneously. As a result, an excellent external quantum efficiency of 40.8% at 400 nm is achieved for colloidal QD‐based hydrogen generation catalyst. This work presents a new platform for the development of high‐efficiency photocatalyst based on 0D–2D nanocomposite. The morphology of the as‐produced ZAIS‐2 (higher Ag amount compared to pristine sample) QDs‐MoS2 nanocomposites using TEM (Figure 15e–g) was analyzed. ZAIS‐2 QDs (35 nm) are uniformly distributed on the surface of MoS2 nanosheet (Figure 15e,f). HRTEM images of ZAIS‐2 QDs show a lattice fringe with a spacing of 0.23 nm (Figure 15g), corresponding to the (111) facet. The HRTEM image of 2D MoS2 nanosheet displays the expected hexagonal MoS2 nanostructure with a lattice spacing of 0.28 nm, belonging to (100) facet of MoS2.[272] These above results undoubtedly demonstrated the superiority of QDs modification, and the QDs/2D configuration may be an effective alternative structure to achieve high‐efficiency photocatalytic behavior.15Figurea) TEM image of the CDots‐C3N4 composite and inset: magnified TEM image of the CDots‐C3N4 region. b) UV–vis absorption spectra of C3N4 and CDots‐C3N4 catalysts. c) Typical time course of H2 and O2 production from water under visible light irradiation catalyzed by CDots‐C3N4. d) Wavelength‐dependent quantum efficiency (red dots) of water splitting by CDots‐C3N4. Reproduced with permission.[271] Copyright 2015, American Association for the Advancement of Science. Structures of ZAIS‐2 QDs‐2D MoS2 nanocomposites. e,f) TEM images of ZAIS‐2 QDs‐MoS2 nanocomposites. Almost all QDs are homogeneously dispersed on MoS2 nanosheet. g) A magnified TEM image of ZAIS‐2 QDs located on single MoS2 nanosheet with HRTEM images of MoS2 nanosheet (very right side). The lattice fringes of 0.28 nm are indexed to the (100) planes. h) Schematic of ZAIS‐2 QDs‐2D MoS2 nanocomposite. Reproduced with permission.[272] Copyright 2017, John Wiley & Sons, Ltd.Molecular/2D Materials HybridizationBecause a single isolated atom is being used to engineer electronic structure, single molecular structures may also tune the electronic features by serving as a cocatalyst to improve photocatalytic action. Xia and co‐workers synthesized H2O soluble molecular cocatalyst trifluoroacetic acid (TFA) to improve the H2 evolution potential of K4Nb6O17.[269] Utilizing the reversible redox couple TFA/TFA including the highly active intermolecular radical responses, the TFA molecule acted as a solid hole supplier, efficiently transporting the photogenerated h+ and resulting in increased charge separation potency. The TFA improvement consistently increased H2 generation, and the highest rate reached 6344 µmol g−1 h−1 for the sample with 25:6 molar ratio. The maximal generation rate was ≈32 times greater as compared to bare K4Nb6O17, definitely indicating the effect of molecular cocatalyst.Furthermore, molecular cocatalyst soluble in water can provide a good amount of available area to the photocatalyst by diffusing in the solution. Using C3N4 subnanopores, molecular TiO2 has been included in C3N4 catalyst via an easy polycondensation of TiO2 ion precursors and DICY.[274] TEM study obtained and verified the morphology of prepared TiO2‐C3N4 nanosheets having a thickness of approximately 3 nm. TiO2 originated from elemental mapping and HAADF‐STEM to be reliably dispersed with an isolated format on C3N4 frame. As demonstrated in Figure 16a, controllable subnanopore engineering in 2D g‐C3N4 using molecular titanium‐oxide incorporation (TiO‐g‐C3N4) was achieved by a simple “bottom‐up” polycondensation of precursors which contain DICY and titanium oxide ions. In detail, TiCl4 and DICY were first dissolved in a cooled ammonium chloride solution, forming a colorless and transparent aqueous solution. During the dissolving process, TiCl4 was rapidly hydrolyzed to TiO2+.[275] The corresponding TEM image in Figure 16b also verifies the clean ultrathin nanosheet with a sheet‐like morphology as that of 2D g‐C3N4. The AFM image reveals that the thickness of the TiO–CN2 nanosheet (one of the TiO‐g‐C3N4 samples with higher molecular titanium‐oxide incorporation) is about 3–3.3 nm. Furthermore, EDX mapping analyses were performed to identify the distribution of molecular titanium‐oxide in the matrix of 2D g‐C3N4.[275]16Figurea) The synthesis of TiO‐g‐C3N4 from “bottom‐up” polycondensation of the intended precursors is depicted in this diagram, b) TEM, c) HAADF‐STEM, d) AFM image of TiO‐CN2 nanosheet, e) 2D g‐C3N4 and TiO‐CN2 nanosheets XRD patterns, f) The solid‐state 13C NMR spectra of 2D g‐C3N4 and TiO‐CN2, g) IR spectra of 2D g‐C3N4 and TiO‐CN2. Reproduced with permission.[275] Copyright 2016, The Royal Society of Chemistry.The HAADF‐STEM image of TiO–CN2 nanosheet also confirms that titanium‐oxide was homogeneously dispersed in the framework of 2D g‐C3N4, as shown in Figure 16c. Moreover, at the edge of the TiO‐2D g‐C3N4 nanosheet where the yellow circles are noted, it can be identified that the element Ti exists as an isolated atom with atomic size, suggesting that an isolated titanium‐oxide species was coordinated in the subnanopores of 2D g‐C3N4. All of the above characterization results demonstrate that 2D TiO‐g‐C3N4 was successfully obtained by subnanopore engineering using homogeneous titanium‐oxide incorporation in the framework of 2D g‐C3N4. The XRD results clearly illustrate that molecular titanium‐oxide incorporation in 2D g‐C3N4 does not destroy the primary building structure of the CN framework.[275] The solid‐state 13C NMR spectra also confirmed this result in Figure 16f. The two prominent peaks in the solid‐state 13C NMR spectra of 2D g‐C3N4 and TiO–CN2 are similar. The first peak at = 164.7 ppm is assigned to the C atoms in CN2(NHX) and the second peak at = 164.7 ppm is attributed to the C atoms in CN3 (other sample with more higher TiO content). Also, the structure of 2D TiO‐g‐C3N4 was further characterized using FTIR spectra (Figure 16g).[275] Upon TiO2 molecule incorporation, prepared photocatalysts showed bandgap narrowing compared to pure C3N4 with reduced CB position. It results from further Ti–O electron presence in the lattice and increases π–e− delocalization in the conjugated structure. Furthermore, the electronic structure of hybrid catalysts can also facilitate the isolation of charge carriers. Thus, TiO2–C3N4 improved the light‐induced degradation performance and generated more •OH radicals for pollutant removal.[275] While 2D structure allows fast transfer of charge carriers in bulk, deficiency of charge separation sites for surface charging would also ruin overall photocatalytic operation. A hybridization technique is necessary to facilitate charge separation at surface and, more importantly, the transfer of holes. Moreover, using water‐soluble molecular materials as a homogeneous cocatalyst can significantly improve photocatalytic efficiency. So, perhaps evolved molecular cocatalyst strategies can potentially separate photogenerated carriers and thus enhance photocatalytic efficiency.2D/2D Stackings HybridizationBuilding 2D–2D stacks to boost the photocatalytic potential is a widely applied process. Lattice mismatch has been significantly reduced due to the comparable layered structures of 2D materials. Zhang and co‐workers studied single‐layer Bi12O17Cl2 with surface Vo through an exfoliation technique based on Li intercalation.[230] The key to this VO‐oriented assembly lies in the metallic characteristic of 1L (monolayer)‐MoS2 and the asymmetric structure of 1L‐Bi12O17Cl2 composed of only (Cl2) layers and oxygen‐deficient (Bi12O17) layers. TEM image (Figure 17a) of Bi12O17Cl2‐MoS2 revealed a 2D heterostructured bilayer composed of many small nanosheets tightly wrapped on a large nanosheet, while the observed transparent nature indicated their ultrathin structures. Elemental mapping images (Figure 17b–e) coupling with XPS (Figure 17f). S K‐edge XANES (Figure 17g) revealed that the MoS2 monolayers in the bilayer showed a distorted 1T metallic phase. 3D topographic AFM images (Figure 17h,i) and their corresponding height profiles (Figure 17j,k) demonstrated that the average thickness values of the small‐sized and large‐sized nanosheets were 0.686 and 0.717 nm, respectively, well‐matching with the theoretical ones of the MoS2 and Bi12O17Cl2 monolayers as shown in Figure 17l, which further evidenced the assembly of MoS2 on Bi12O17Cl2. More interestingly, they found that all the MoS2 sheets were anchored on the same surface in Bi12O17Cl2, which was further evidenced by their side‐view TEM image (Figure 17m). These observations demonstrate the occurrence of an oriented assembly. As 1L‐Bi12O17Cl2 has an asymmetric structure consisting of (Cl2) and VO of (Bi12O17) end‐faces might initiate oxygen‐deficient (Bi12O17) end‐faces and their assembly. It could thus deduce that 1L‐MoS2 were anchored selectively on the (Bi12O17) end‐faces of 1L‐Bi12O17Cl2. The HAADF‐STEM image (Figure 17n) and energy loss spectroscopy (EELS) elemental maps (Figure 17o–s) of their cross‐sectional atomic structures provided direct, atomic‐resolution shreds of evidence that this oriented assembly resulted in 2D Janus bilayer junctions of (Cl2)‐(Bi12O17)‐(MoS2).[230]17Figurea) Top‐view TEM image, b–e) images of elemental mapping, f) XPS spectra, h,i) AFM images, m) side‐view TEM image, n) HAADF‐STEM picture at atomic resolution, and o–s) Bi12O17Cl2‐MoS2 EELS elemental maps in (a), (h), (i), (m), and (n), the scale bars are 500 nm, 1 mm, 500 nm, 10 nm, and 5 nm, respectively, g) Bi12O17Cl2‐MoS2, 1L‐MoS2, and bulk MoS2 S K‐edge XANES spectra, j) Comparison of the average thicknesses of 1L‐Bi12O17Cl2 and 1L‐MoS2 in Bi12O17Cl2‐MoS2 along the lines in (i), (k). The error bars in (k) show the standard deviation of over 100 AFM measurements. l) Monolayer thicknesses of MoS2 and Bi12O17Cl2 monolayers, in theory. Reproduced with permission.[230] Copyright 2016, Nature Publishing Group.Since the concentration of charges surrounding (Bi12O17)2+ layer was larger in contrast to (Cl2)2− layer, photogenic excitons strived to (Bi12O17)2+ and (Cl2)2− end faces when exposed to light. The photogenic electrons traveled between the single layers of MoS2 via BiS bonds, which allowed improved separation of excitons (ultralong carrier life of 3446 ns), as confirmed by transient absorption analysis. Using atomic size thickness, effective direct interface charging isolation, and ample active sites in MoS2, the fabricated hybrid bilayers showed superior visible light‐derived H2 evolution efficiency. Furthermore, using a hole‐donating agent such as ascorbic acid can significantly improve the H2 production rate up to 33 mmol h−1 g−1. Other than that, many 2D stacking studies, including MoS2/C3N4, C3N4/Bi4O5I2, Fe2O3/C3N4, MoS2/TiO2, NiO/Ca2Nb3O10, SnS2/C3N4, MoS2/CdS, ZnIn2S4/MoSe2 and ZnCr‐LDH/layered titanate have been reported.[276–279] These hybridized 2D/2D structures have many overwhelming advantages. For example, they offer a significant amount of available area at the 2D–2D interface and effectively decrease the barriers for electron migration with cocatalysts help. Consequently, they can immensely boost the interfacial charge transfer in the photocatalysts by allowing quantum tunneling phenomenon. In addition, these hybrid materials can significantly improve the light‐harvesting property of the photocatalyst by easing the light‐blocking phenomenon in cocatalysts as well. However, there is a dire need to alter the 2D components to strengthen the interfacial force between the layers to fabricate 2D/2D hybridized photocatalysts with superior activities.3D/2D HybridizationInstead of the beneficial effect of the 3D/2D heterostructure system, this binary system still greatly suffered from the inefficient photo charge carrier migration between the neighboring photocatalyst, consequently promoting the photo charge carrier recombination.[280–282] Thus, ensuring the smooth photo charge carrier migration between the neighboring photocatalyst in the binary g‐C3N4/BiVO4 heterostructure system is paramount to warrant better photocatalytic activity. Recently, the addition of carbonaceous materials, particularly RGO has triggered widespread interest in photocatalytic application owing to its beneficial effect as an electron shuttle for photo charge carrier migration across the heterostructure interface, thus alleviating the existing problem within the binary heterostructure system.[283] Equally important is the fact that green hydrogen production remains a boundless challenge hitherto and limited study has been focused on utilizing lake water as a source of hydrogen. Until now, most photocatalytic or photoelectrocatalytic hydrogen production was only employed as a chemical‐based electrolyte solution with sacrificial reagents in which their environmentally friendly approach can be questioned.Additionally, despite many photocatalyst studies that have made epigrammatic progress, there is still limited analysis provided by the current literature for the bifunctional application of the as‐developed photocatalysts for hydrogen generation and photodegradation studies. To date, the discharge of antibiotics to the aquatic environment stemming from the pharmaceutical industries has jeopardized the aquatic environment. Hence, it is of utmost significance that needs to be addressed and solved before it becomes irrevocable.[280] Samsudin and Sufian[280] fabricated 2D/3D g‐C3N4/BiVO4 photocatalyst decorated with RGO for boosted photoelectrocatalytic hydrogen production from natural lake water and photocatalytic degradation of antibiotics. The morphological structure of the 2D/3D g‐C3N4/BiVO4 decorated with RGO at different amounts of RGO loading was examined using FESEM analysis as shown in Figure 18. All of the composite heterostructure samples revealed the typical flower‐type structure in a micro‐size range, corresponding to 3D BiVO418FigureFESEM images of the 2D/3D g‐C3N4/BiVO4 decorated with RGO at different amount of RGO loading along with the elemental mapping of the 1.2 wt% RGO@g‐C3N4/BiVO4 sample. Reproduced with permission.[280] Copyright 2020, Elsevier B.V.Surface and Interface EngineeringIn current history, photocatalytic activity has focused on issues related to structural engineering by clarifying photocatalytic processes and developing strategies for detailed synthesis and comprehensive characterizations. This structure creation's goal is relatively simple: boost each charge kinetics stage. Structural engineering mainly involves energy band, surface, and interface engineering to optimize the effects of selected primary measures.[284] The aim of energy band engineering (EBE) is to optimize the photocatalysts’ photon‐harvesting efficiency, thus promoting exciton pair production during the first step. The primary objective of interface engineering is to prevent negative e−–h+ recombination to increase the number of e−s and h+s reaching the surface for redox reactions. Certainly, accumulating enough e−s and h+s on the surface might not guarantee the high current effectiveness of a light‐driven catalytic mechanism. Surface engineering has proved to be a flexible way to improve adsorption properties and reaction species activation capabilities. The surface e−s or h+s are more easily involved in a redox reaction.[285] The EBE has made great efforts over the past decade, including developing narrow and wide bandgap semiconductors doping.[286] As we can determine from the research, this issue received only a few review articles, although the EBE was extensively summarized.[287]However, research focusing on surface and interface engineering showed the significance of their activities in optimizing photoconversion efficiency. Surface engineering, vacancy engineering, element doping, surface heterojunction formation and facet‐dependent site control have proven effective in tuning photocatalyst properties.[288–291] The surface atoms, however, only comprise a tiny fraction of the bulk photocatalysts, typically producing an inconspicuous impact to improve their photoconversion efficiency. 2D structures, with thin thickness, massive lateral scale, and plentiful exposed surface atoms with controlled facets, establish perfect platforms for atomic surface engineering. First, the surface atoms in 2D materials can almost reflect the entire material's overall physical and chemical composition and include all details about the atomic structure. Second, 2D surface atoms are vulnerable to escape from the crystal lattice, thereby promoting vacancy engineering. Thirdly, surface atoms of 2D materials typically have low coordination numbers that can be considered alteration sites, facilitating surface modification and hybrid structure building. Meanwhile, the interface between two elements is the site for transition and differentiation in hybrid systems. The reliability of transferring charges through the interface is crucial to avoiding the recombination of charges. For example, the electric field generated inside the interfaces may drive the charge carriers to be spatially divided into various components. This spatial isolation limits bare material load recombination.[292] Because the photogenic excitons must pass through the interface in a typical catalytic process, several parameters such as interface compositions, zones, faults, electronic binding, facets, and band bending seem to have a major impact on charging and separation efficiency.[293]This section described a series of important surface and interface engineering parameters that compensate for or influence load kinetics and overall photocatalytic efficiency. These variables can help researchers develop more effective and stable 2D photocatalytic remediation systems. At the start of sections, specific surface and interface engineering rules will be explained. Then relevant parameters that define or affect charging kinetics performance will be elaborated. Along with the parameter‐efficiency relationship, the architecture rules will direct us to maneuver the charging kinetics by selecting or adjusting parameters. It is the fundamental challenge of surface and interface engineering.Design Rules and Parameters for Surface EngineeringDesigning RulesPhotocatalysts’ morphological architecture has some general guidelines. Initially, the surface of the engineered part must be on that a catalytic reaction takes place. Numerous exposed components in hybrid systems may not react. For example, solid semiconductors in Z‐scheme photocatalysts merely contribute to transferring and recombining the charges.[294] Thus, the catalytic efficiency cannot be changed by changing these components’ surface parameters (SPs).Secondly, different reactions can affect different SPs. Therefore, rationally tuning suitable SPs is critical for behavior improvement, which should have been focused on a thorough understanding of surface kinetics. For example, tuning catalyst surface pore sizes may influence large molecules’ transport. While adjusting the pore size can be an effective technique for enhancing the photocatalytic abilities to remove pollutants, this technique is unsuitable for H2O splitting applications since the size of water molecules is far less than the dye molecules.[295]Thirdly, modifications to particular SPs will modify band composition and light‐harvesting semiconductors absorption. For instance, it was argued that surface vacancies and compositions play an essential part in extending the solar absorption spectrum of semiconductors. In contrast, structures having textured surfaces have better light‐reflecting properties that can help with better absorption of photons.[247,296] In such conditions, the role of SPs in improving photoconversion efficiency becomes much more difficult. In comparison, cocatalyst surface engineering is pretty simple, as light absorption does not change.Fourthly, consider relations among various SPs; for instance, surface pore adjustment eventually contributes to surface area variation. Eventually, correlations between SPs and interface structures should be recognized, as interface structures significantly affect the catalytic efficiency surface. Under the above rules, surface design should be designed by optimizing a few essential factors: surface compositions, vacancies, pores, facets, areas, surface conditions, phases, and band bending.ParametersAccording to the rules above, surface design can be performed by selecting or optimizing important parameters, including surface compositions, phases, facets, areas, pores, vacancies, surface state, and band bending. Three surface engineering methods were used to increase the catalytic activity and selectivity of photocatalysis: 1) modify these SPs to provide a surface that is much more active and promote superior adsorption and activation capabilities for specific species; 2) Control the SPs to encourage an abundant number of charge carriers to enter catalyst’ surface reaction sites, which dramatically increase the diffusion rate by reducing the diffusion distance for charge carriers; and 3) enhancing the redox reactions occurring at the catalyst's surface. After that, we will demonstrate a series of SPs, which can influence the photocatalytic process across the following three techniques.Parameters—Surface CompositionsSurface composition is essential for catalytic reactions as it critically affects catalyst surface adsorption and activation activity. However, the surface composition is often associated with semiconductor light absorption, making it a sophisticated element in efficient tuning. Thus, a better method for tuning catalytic efficiency is adjusting cocatalyst surface compositions. Using nanosheets of Pt‐doped TiO2 (Pt as cocatalysts) for photocatalytic reduction of CO2 in water, H2 will become the primary product given that the surface of Pt cocatalysts plays an important role in H2O activation.[297] Furthermore, the selectivity for the reduction of CO2 was improved by changing the surface composition of Pt cocatalyst through the slight coating of Cu2O (Figure 19a). Pt core transferred TiO2 photogenerated e−s to Cu2O shell in this process, and Cu2O shell will serve as an active site for CH4 and CO generation. Apart from improved activation of reactants, core–shell photocatalysts with cocatalysts have also been utilized for back reactions suppression during photocatalysis. For example, a significant back reaction between photocatalytic H2 evolution reaction with the surrounding O2 to produce H2O was observed for Rh as a cocatalyst. In order to overcome this issue, core–shell structures of Rh‐Cr2O3 cocatalysts were fabricated, which offered different active sites for H2‐generation through Cr2O3 surface. In contrast, Rh facilitated the transfer of photogenic electrons toward Cr2O3.[298] The Cr2O3 surface effectively prohibited the production of water through back reaction. Additionally, varying surface compositions can play several crucial roles in surface engineering. One typical case is the photocatalytic splitting of H2O over LaMg1/3Ta2/3O2N along with RhCrOy cocatalyst to reduce H+.[299] Authors reported that the reduction of O2 occurred on LaMg1/3Ta2/3O2N surface in opposition with oxidation of H2O while the accrued photogenic h+s oxidized nitrogen species to N2. In order to avoid O2 reduction, LaMg1/3Ta2/3O2N was coated with amorphous TiOXH to be served as a selective pervasion layer (Figure 19b). After the selective infusion of O2 and H2 generated at the composite and coating interface, into the atmosphere, the reverse flow of O2 into the coated layer was prohibited due to higher O2 pressure in the layer, hence rendering the oxygen reduction. In some other cases, Ta3N5 nanosheets synthesized via the thermal oxidation of Ta foils exhibited very poor performance in the photoelectrochemical (PEC) splitting of water due to intense recombination of photogenic excitons in surface passivation layer (Figure 19c).[300] By exfoliating the surface passivation layer thermally or mechanically, Ta3N5 photocurrent was greatly improved.19FigureSchematic representation of essential parameters in surface engineering for photocatalysis: a) To boost photocatalytic CO2 reduction, a Pt cocatalyst is selectively coated with Cu2O, Reproduced with permission.[297] Copyright 2013, John Wiley & Sons, Ltd. (b) To suppress the oxygen reduction reaction (ORR) reaction, a TiOXH‐coated LaMg1/3Ta2/3O2N catalyst was used as a selective permeation layer. Reproduced with permission.[299] Copyright 2015, John Wiley & Sons, Ltd. c) after exfoliation of the surface passivation layer, a process for improving photocurrent. Reproduced with permission.[300] Copyright 2013, John Wiley & Sons, Ltd. (d) C3N4 nanosheets operate as cocatalysts in a photocatalytic CO2 reduction reaction with Pd nanocubes and nanotetrahedrons, Reproduced with permission.[301] Copyright 2014, The Royal Society of Chemistry. e) Internal electric fields produce the difference in charge diffusion distance between BOC‐001 and BOC‐010, Reproduced with permission.[218] Copyright 2012, American Chemical Society. f) Between the (010) and (110) facets of a BiVO4 crystal, there is spatial charge separation. Reproduced with permission.[315] Copyright 2013, Springer Nature, g) TiO2 with metallic and semiconducting MoS2 nanosheets as cocatalysts: charge transfer characteristics. Reproduced with permission.[225] Copyright 2015, Springer Nature. h) 2D nanostructures have advantages of large surface area and short charge‐diffusion distance to surface. Reproduced with permission.[304] Copyright 2015, American Scientific Publishers, i) CO2 reduction via photocatalysis using porous Ga2O3 as a photocatalyst. Reproduced with permission.[306] Copyright 2012, The Royal Society of Chemistry. j) In surface and bulk defects, the behavior of photogenerated electrons and holes is studied, Reproduced with permission.[309] Copyright 2013, The Royal Society of Chemistry. k) Surface band bending in both upwards and downwards semiconductors. Reproduced with permission.[312] Copyright 2015, The Royal Society of Chemistry.Parameters—Surface FacetsDue to atomic arrangement variations, surface facets also play a crucial role in tuning the adsorption and activation capabilities. In light‐harvesting semiconductors and cocatalysts, variation in exposed facets can contribute to distinct photocatalytic behavior and selectivity.[301,302] In a typical case, they have investigated photocatalytic CO2 reduction in the presence of H2O using C3N4 nanosheets and Pd nanocrystals as light‐harvesting semiconductors and reduction cocatalysts, respectively (Figure 19d).[301] In a typical scenario, photocatalytic CO2 reduction was examined using Pd reducing cocatalyst in C3N4 nanosheets. Additionally, H2O was also reduced to H2 during the process over Pd (100) nanocubes, whereas the reduction of CO2 in water was mainly carried over Pd (111) nanotetrahedrons. The theoretical calculations also proposed lower potentials for CO2 activation along with superior adsorption capabilities for Pd (111) and higher potentials for H2O adsorption on Pd (100). Apart from the adsorption and activation of reacting species, surface facets affect photoconversion efficiency with other impacts. For instance, semiconductor facets perpendicular to internal electric field orientation will be even more active in photocatalysis relative to facets parallel to direction. For example, BiOCl‐001 nanosheets displayed better photocatalytic activity than BiOCl‐010 nanosheets.[218] Since the separation and transport of photogenic charge carriers are greatly facilitated by the internal electric field induced in the crystal, a relatively short diffusion length is offered by the small [001]‐facet in BOC‐001 (Figure 19e). Another crucial role played by the surface facets is the spatial exciton separation in semiconductors which can accumulate photogenic charge carriers on various facets to offer varying selectivity and catalytic activity. A characteristic instance is the (110) and (010) facets of BiVO4 crystal. Studies found that the (110) and (010) facets can be decorated selectively with oxidation and reduction, respectively, owing to the spatial separation of charges. The spatial effect produces a marginal difference in the energy levels of CB and VB between (110) and (010) facets, resulting in separate deposition of e− and h+ on (010) and (110) facets (Figure 19f).Parameters—Surface PhaseThe surface phase is another critical surface design parameter. A semiconductors bandgap is considered to be closely associated with its surface phase.[303] In addition, phase of cocatalysts is also critical to their performance as active sites and carrier transporters. The literature showed TiO2's superior photocatalytic H2 output capacity where semiconducting (2H) and metallic (1T) nanosheets of MoS2 have been employed as cocatalysts.[225] TiO2‐incorporated MoS2 nanosheets showed slightly better output rates of photocatalytic H2 relative to TiO2‐decorated 2H MoS2 nanosheets. MoS2 cocatalyst surface phases impact photoconversion efficiency from two distinct angles. Initially, active H2 evolution sites have been found only on the edges of 2H MoS2 sample, whereas the 1T MoS2 samples offered active sites on the basal planes as well. Second, the charge mobilities in 1T sample were greater as compared to the 2H nanosheets. Higher and shorter diffusion rates distance mean that more photogenerated e−s in TiO2 reach 1T MoS2 reaction sites and involve in the reactions (Figure 19g).Parameters—Surface AreaA larger surface area is known to allow higher photocatalytic activity because atoms only present at the surface of the photocatalyst take part in the redox reactions. Raising the surface‐to‐volume ratio allows additional active sites for the adsorption of surrounding species to react with each other. For example, newly invented 2D photocatalysts proved more efficient than their 3D counterparts (Figure 19h), partly attributed to an increased surface area.[304,305] In this scenario, g‐C3N4 nanosheets had a considerably greater surface area (384 m2g−1), resulting in a ninefold improvement in H2 generation performance compared to their parent g‐C3N4.[142] A major advantage of 2D photocatalysts has been the minimized distance from bulk to a surface (Figure 19h).[304] The minimal nanosheet thickness shortens path to reaction sites, decreasing likelihood of recombination loss across charge transfer.Parameters—Surface PoresSurface pore structures improve the surface‐to‐volume ratio, leading to better reactant capturing and adsorption. For example, in photocatalytic activity, CO2 reduction with H2O, and porous Ga2O3 showed a 400% higher conversion rate than commercial Ga2O3 nanoparticles.[306] The superior reduction potential was ascribed to the better potential (300%) for CO2 adsorption offered by greater surface area (200%) of porous Ga2O3 (Figure 19i). In another study, Cu3(BTC)2 MOF (Metal‐organic framework) along with TiO2 were utilized to fabricate core–shell nanostructures for light‐derived reduction of CO2. Since CO2 could penetrate through porous TiO2 quite easily and get trapped within the pores of MOF, the selectivity and photoactivity of the prepared core–shell photocatalyst were highly enhanced. The trapped CO2 in the MOF received photogenic electrons from TiO2 and was reduced to CH4 effectively.[307]Parameters—Surface VacanciesFor surface catalytic reactions, adsorption and activation processes frequently occur in coordinately unsaturated, thermodynamically unstable locations. Site bonds are prone to absorb reactants and charge carriers, and therefore, surface vacancy forms and numbers may influence catalytic activity and selectivity. Kong et al. and Yan's group showed that TiO2's photocatalytic activity is significantly improved after an increment in the concentration ratio of surface defects to bulk defects.[308,309] Normally, photogenerated e−s or h+s retained by large defects cannot continue to proceed toward surface reactions and serve as new recombination centers for the photogenic charge carriers. Conversely, photogenerated e− or h+ trapped by surface defects could participate in the redox reactions with the adsorbed species to facilitate catalytic reactions (Figure 19j).Parameters—Surface Band BendingIn semiconductors, tailoring various SPs can change the redox potentials of excitons since the surface variations cause band bending in semiconductors surface owing to disturbed bonding networks within the crystal structure.[310] For example, a downward and upward surface band bending was observed in p‐type and n‐type GaN semiconductors, respectively. This is attributable to the generation of defects and the insertion of dopants.[311] The downward bending facilitated the photogenic e−s to move below for reduction reactions, while h+ were driven up for oxidation reactions due to the upward bending of surface bands (Figure 19k).[312] Consequently, lower O2 and higher H2 evolution were observed for the p‐type GaN nanowires in light‐driven H2O splitting, in contrast to n‐type GaN.[311]In the structure of a typical semiconductor, band bending (downward/upward) would generate extra barriers for either redox reactions (oxidation or reduction) since they proceed simultaneously on a single surface, limiting the overall performance. However, suppose the active sites for both reactions can be separated spatially by utilizing various facets or components. In that case, the surface band bending can effectively boost photocatalytic performance. Semiconductors with polar surfaces, for instance, can offer facets with opposing charges on two sides by distinctively terminating the surface bonds. Hence improving the redox reactions activity separately.[313] On the other hand, various dopants have been utilized to regulate the Fermi level to tailor the surface band bending in non‐polar semiconductors. For example, the downward band bending in p‐type GaN has been effectively controlled by tailoring doping levels of Mg‐dopant to boost the water oxidation potential.[311] The reaction conditions, like the nature of electrolyte and the interfacial contact with catalyst's surface, can also induce surface band bending in semiconductors.[313] In addition, particles’ size plays a crucial role in tailoring the surface band bending as well; reducing the size of particles up to two times the width of surface space charge region results in incomplete relaxing of surface band bending to the bulk level, which is highly undesirable for surface reactions. From this behavior, it can also be inferred that all SPs are interconnected and depict interplay effects.[285,314]Design Rules and Parameters for Interface EngineeringRulesLikewise, to surface design, many principles must be explained for photocatalyst interface design. First, the interface between two elements seems to be where carriers migrate. Occasionally, hybrid structure interfaces do not use e−s or h+s transportation. Tuning interface parameters do not change catalytic efficiency. Magnetic semiconducting nanoparticls core–shell configurations collapse into this process, in which the magnetic core also contributes to magnetic separation after using a photocatalyst.[316] Other than this, the roles of interfaces in their designed systems should be fully understood by dynamic charge kinetics models before interface engineering. For instance, interfacial defects are mostly e−–h+ recombination centers, thereby preventing interface charging. Consequently, removing interfacial defects would mostly boost photoconversion efficiency. Thus the interfacial defects are necessary to modulate the exciton recombination in solid‐state semiconductor–semiconductor Z‐scheme architectures.[317] Two‐part job functions should be evaluated when an interface is designed to transfer e−s or h+s.[318] Generally, when two components interact, the role of the component to take e−s (or have h+s) must be higher compared to the other components beyond the interface (i.e., one to have e−s). Moreover, transition directions will rely on particular kinetic charging models on photoexcitation. The job functions must also satisfy the specifications of the respective models. After this, consider the relationship between various interface parameters. Interfacial compositions and facets, for example, significantly affect interfacial defect formation. In particular, close compositions and minor lattice mismatches for two components will minimize the likelihood of interface defects.ParametersTwo mutually exclusive interface engineering approaches have been used for charge transport in PC. One will create an interface that provides a driving force for excitons’ symmetric movement to separate them for surface reactions. The second approach would optimize the charge transfer efficiency by adjusting different interfacial parameters to mitigate the losses generated from excitonic recombination. If design rules are defined, parameters along with interface compositions, regions, defects, facets, electronic binding, and band bending should be customized smoothly.Unlike the advantageous effects of surface defects, interface defects chiefly behave as e–h recombination sites and affect the transfer of photogenic charge carriers through the interfaces. Interfacial defects may be reduced by generating a single‐crystal interface between two components. These interfacial defects in Cu2O–Pd nanosheets are crucial to charge separation efficiency where a Schottky junction is formed. The interfacial defects in the widely employed Pd‐decorated Cu2O structure prevent the interfacial charge transition between Cu2O and Pd.[293] Pd–Cu2O core–shell architectures have been established with superior interfaces for single‐crystal structures to mitigate them. Conversely, delivering the h+s trapped in Pd cores across the Cu2O shells was challenging for surface reactions. A new Cu2OPd–graphene stack configuration was created to prevent this unfavorable condition by contacting Pd with Cu2O and GR. The single‐crystal Cu2O–Pd interface avoids the defects in this structure, and the Pd–graphene interface provides transfer channels to the h+s for extraction, leading to increased output of photocatalytic H2 (Figure 20d).20FigureThe important parameters in photocatalysis interface engineering are illustrated in the diagram: a) the hydrophilic interface modification to enhance O2 water oxidation by increasing the interfacial contact between Ta3N5 and CoOx., Reproduced with permission.[323] Copyright 2015, John Wiley & Sons, Ltd. b) To enhance charge transfer between Fe2O3 and BiV1‐xMoxO4, graphene was used as a conductive interlayer. Reproduced with permission.[324] Copyright 2012, American Chemical Society. c) The advantages of 2D layered stack structures in both large interfacial area and short charge‐diffusion distance from interface to surface. Reproduced with permission.[312] Copyright 2015, The Royal Society of Chemistry. d) Pd‐decorated Cu2O, Pd‐Cu2O core–shell, and Cu2O‐Pd‐rGO stack structures all show charge transfer. Reproduced with permission.[293] Copyright 2014, John Wiley & Sons, Ltd. e) BiOCl(001)‐Pd and BiOCl(110)‐Pd surfaces have different interfacial barrier layer thicknesses. Reproduced with permission.[322] Copyright 2015, John Wiley & Sons, Ltd. f) Electron density contour maps for the bottom of CB of K2La2Ti3O10 In Ni(111)‐K2La2Ti3O10(101) and Ni(111)‐K2La2Ti3O10(002) interfaces of electron density contour maps for bottom of CB of K2La2Ti3O10. Reproduced with permission.[325] Copyright 2007, Chemical Society of Japan. g) In a semiconductor‐based junction, interfacial band bending occurs, Reproduced with permission.[312] Copyright 2015, The Royal Society of Chemistry.Another critical parameter in interface engineering is interfacial engineering. A wide interfacial region could theoretically have enough channels for effective charge transfer. In contrast to other structures, 2D layered stack systems had the highest interfacial region and the shortest distance between interface and surface for charge transfer.[319] Interfacial electron transfer values of 1.15108, 3.47108, and 1.06109 s−1 were estimated in a study for 0D–2D, 1D–2D, and 2D–2D TiO2–graphene nanosheets, respectively, emphasizing critical position of interfacial region in charge transfer.[320] Transfer of charges through the interfaces is often based on components’ facets in interaction with one another. For example, TiO2 (100)–graphene interfaces exhibited a superior charge transfer rate in contrast to TiO2 (101)–graphene and TiO2 (001)–graphene interfaces, allowing them to produce more photocatalytic H2.[321] The discrepancy was most likely due to formation of TiC bonds between (100) facets of TiO2 and graphene, whereas the (001) (101) facets have been bound to graphene by TiOC bonds. Similarly, Pd nanocubes have been decorated on various facets of BiOCl nanoplates, exhibiting a broad range of interfacial hole‐trapping capabilities through a Schottky junction.[322] The photocatalytic O2 evolution efficiency of BiOCl (110)–Pd was found to be significantly higher than that of BiOCl (001)–Pd, owing to the Schottky barrier being more easily formed in BiOCl (110)–Pd. BiOCl (110)–Pd (100) had a slightly thinner interfacial boundary layer than BiOCl (001)–Pd (100). As a result, charge recombination was inhibited more efficiently in BiOCl (110)–Pd (Figure 20e).Similar to surface engineering, interfacial composition is the first critical parameter in interface engineering. The structure is related to constituents’ light absorption and surface reaction capabilities. Consequently, interface structure tuning is often achieved by building new interfaces by adding novel components in the composite structures. The primary function of the added component is to generate a new interface for promoting charge conversion since it was never implicated in light‐harvesting or redox reactions. A typical example is the interface engineering of Ta3N5 semiconductor and CoOx cocatalyst for water oxidation.[323] Because of the difficulties of intimate interaction between the hydrophilic and hydrophobic surfaces of CoOx and Ta3N5, respectively, the surface of Ta3N5 was coated with magnesia nanolayer to turn hydrophobic into hydrophilic surface. Apart from enhancing the interfacial interaction of CoOx and Ta3N5, it also has a passivation effect, lowering Ta3N5 defect density. Greatly enhanced interfacial charge transfer and increased water oxidation performance (Figure 20a). Conductive interlayers can be added to improve transferring interfacial charge. For context, Fe2O3/rGO/BiV1‐xMoxO4 core–shell nanorods surpassed Fe2O3/BiV1‐xMoxO4 in PEC water splitting.[324] Performance improvement can be attributed to the electron‐conducting properties of RGO nanosheet, which facilitate charge transfer between Fe2O3 and BiV1‐xMoxO4 (Figure 20b). The electron density contour maps below the K2La2Ti3O10 CB are given in Figure 20f. Apparently, the Ni3d+Ti3d hybrid orbitals spread from the interface region to the Ni bulk at Ni(111)–K2La2Ti3O10(101) interface, while the electron density was only localized within K2La2Ti3O10 for Ni(111)–K2La2Ti3O10 (002) interface. Hence, the transfer of photogenic e−s to Ni cocatalyst is much easier at the (111)Ni–(101)K2La2Ti3O10 interface. Also, other interfacial parameters like defects and interfacial compositions played a crucial role in the electronic coupling at the interface.[285]As far as the charging kinetics is concerned, interfacial band bending could occur to achieve an equilibrium of e−s Fermi distributions between the two components, which is necessary for effective diffusion of charge carriers through the interface. Like surface band bending, the e−s can move downward with more +ve CB while h+s can move upward with more ‐ve VB along with the interfacial band bending (Figure 20g).[285,312] Moreover, the direction of charge carrier's migration is highly dependent on the band bending orientation, which is based on the work functions of coupled materials. Conversely, the charge transfer efficiency is highly affected by the degree of band bending, and speaking thermodynamically, larger band bending results in superior charge transfer.Theoretical InsightBased on computational studies investigating the bandgap and band edge criterion, scores of 2D materials have been predicted to be capable of spontaneous water splitting.[94] Figure 21 illustrates the band edge alignment for several families of 2D materials. Figure 21a shows the prediction by Zhuang et al. that the single‐layer group‐III monochalcogenides, GaS, GaSe, GaTe, InS, InSe, and InTe are suitable candidates for spontaneous photocatalytic water splitting.[326] Another class of 2D materials that have received much attention for its potential in photocatalysis is the transition metal dichalcogenides.[327–330] Figure 21b illustrates that the band edge positions of the single‐layer TMDs like CrS2, MoS2, WS2, PtS2, and PtSe2 make them suitable for photocatalytic splitting of water.[327,328] Further, studies on the electronic structure of vacancies and edges of MoS2 show that these defects can provide catalytically active sites.[326,331,332]21FigureBand‐edge positions of 2D materials compared to redox potentials of water: a) group‐III monochalcogenides. Reproduced with permission.[79] Copyright 2013, American Chemical Society. b) TMDs. Reproduced with permission.[328] Copyright 2013, American Chemical Society, and c) metal phosphide trichalcogenides relative to the vacuum level. Reproduced with permission.[333] Copyright 2014, AIP Publishing LLC.Liu et al. predicted that single‐layer metal‐phosphorus‐trichalcogenides, MPX3 (M = Zn, Mg, Ag0.5Sc0.5, Ag0.5In0.5, and X = S, Se) exhibit the intrinsic electronic properties suitable for spontaneous photocatalytic water splitting, see Figure 21c.[333] Single layer α‐MNX (M = Zr, Hf; X = Cl, Br, and I) and β‐MNX (M = Zr, Hf; X = Cl, Br) have been shown to be yet another class of 2D materials suitable for photocatalytic water splitting.[326] Furthermore, single‐layer bismuth oxyhalides including BiOCl, BiOBr, and BiOI are suggested to exhibit photocatalytic activity for water splitting.[334]DFT‐based calculations are a powerful tool for materials design.[335–341] As known, the traditional local density approximation and generalized gradient approximation functionals usually underestimate the bandgaps of semiconductors.[341,342] In contrast, the HSE06 hybrid functional usually predicts more accurate results of bandgaps concerning experimental results.[97,343] However, HSE06 functional uses the one‐particle approximation in band energies’ calculations, and there are still systematic errors compared with experimental data.[344] Hence, many investigations have been done to explore the accuracy of the bandgaps and band edge positions of semiconductors calculated by HSE06 hybrid functional.[345–347] It is found that HSE06 hybrid functional shows sizable errors in the ionization potential and electron affinity but is much better in MPS and is weaker than other thiol at predicting relative band positions due to error cancellation.[348] Most theoretical results of electronic structures discussed here are calculated by HSE06 hybrid functional. The optical absorption spectra are simulated by converting the complex dielectric function to the absorption coefficient α abs (ω) according to the relation (Equation 3):[341,349]3αabs(ω)=2ω(ε12(ω)+ε22(ω)−1ε1(ω))12\[{\alpha _{{\rm{abs}}}}\left( \omega \right) = \sqrt 2 \omega {\left( {\sqrt {\varepsilon _1^2\left( \omega \right) + \varepsilon _2^2\left( \omega \right)} - 1{\varepsilon _1}\left( \omega \right)} \right)^{\frac{1}{2}}}\]where ε1(ω) and ε2(ω) are the real and imaginary parts of frequency‐dependent complex dielectric function ε(ω), respectively, due to the tensor nature of the dielectric function, ε1(ω) and ε2(ω) are averaged over three polarization vectors (along x‐, y‐, and z‐directions). In addition, the light adsorption calculated with HSE06 method can only serve as a guide in searching photocatalysts under one‐particle approximation. An accurate light absorption spectrum will be obtained by taking excitonic effects into account, for example, the GW plus BSE approach.[341,350,351]Environmental RemediationAs a result of the above, 2D materials demonstrated enormous photocatalytic benefits due to their microstructure, bandgap, electronic configuration, and surface composition. Additionally, as previously discussed, engineered defects can be used to modulate light absorption, electrical conductivity, electronic structure, carrier concentration, and interfacial catalysis mechanism, indicating an immense potential for enhancing photocatalytic action for various applications. The advanced photocatalytic activity in water oxidation, H2 processing, CO2 reduction, N2 fixation, organic synthesis, H2O2 production, and contaminants removal over defective 2D materials have been explored in this section (also see Table 1).1TablePerformance of various 2D materials in environmental remediation (Abbreviations: TEOA, triethanolamine; HIMD, Hole‐in‐microdisk; DDE, dichlorodiphenyldichloroethylene: MBP, methylparaben: TCH,tetracycline hydrochloride)ApplicationMaterialSynthesisEngineering toolReaction conditionPerformanceRefs.H2 evolutionZnIn2S4 nanosheetsSolvothermalSulfur vacancies100 mL 20 vol% TEOA with 3 wt% Pt, 300 W Xe lamp (> 420 nm)1504.9 µmol g−1 h−1[352]2D ZnIn2S4/2D g‐C3N4SolvothermalHeterojunction100 mL 20 vol% TEOA with 3 wt% Pt, 300 W Xe lamp (> 420 nm)6095.1 µmol g−1 h−1[352]ZnIn2S4 nanosheetsHydrothermalO‐doping200 mL H2O containing 0.25 m Na2SO3 and 0.35 m Na2S, 300 W Xe lamp (>420 nm)2120 µmol −1 h−1[353]ZnIn2S4 nanosheetsHydrothermalPristine sample200 mL H2O containing 0.25 m Na2SO3and 0.35 m Na2S, 300 W Xe lamp (>420 nm)471.11 µmol g−1 h−1[353]ZnIn2S4 nanosheetsSolvothermalMonolayer100 mL H2O containing 10 mL TEOA, 300 W Xe lamp (>400 nm)1.723 mmol g−1 h−1[354]ZnIn2S4 nanosheetsSolvothermalBilayer100 mL H2O containing 10 mL TEOA, 300 W Xe lamp (>400 nm)0.799 mmol g−1 h−1[354]ZnIn2S4 nanosheetsSolvothermalMonolayer + sulfur vacancies100 mL H2O containing 10 mL TEOA, 300 W Xe lamp (>400 nm)13.478 mmol g−1 h−1[354]ZnIn2S4 nanosheetsHydrothermalPristine sample100 mL H2O containing 0.25 m Na2S and 0.25 m Na2SO3 with 2 wt% Pt, A 300 W Xe lamp (>420 nm)263.9 µmol g−1 h−1[355]0D AgIn5S8/2D ZnIn2S4HydrothermalHeterostructure100 mL H2O containing 0.25 m Na2S and 0.25 m Na2SO3 with 2 wt% Pt, A 300 W Xe lamp (>420 nm)949.9 µmol g−1 h−1[355]Monolayer Ti3C2Tx /TiO2ImpregnationFabrication of composite40 mL solution of DI H2O containing 25% methanol, 200 W Hg lamp (285–325 nm)2.65 mmol g−1 h−1[356]Ti3C2/g‐C3N4Electrostatic self‐assemblyHeterojunctionH2PtCl6 with 3.0 wt% Pt and 40 mL DI H2O with 10% TEOA72.3 µmol g−1 h−1[357]CdS‐MoS2‐MZeneHydrothermalFabrication of composite50 mL solution containing 0.25 m sodium sulfide and 0.35 m sodium sulfite, 300 W Xe lamp (420 nm)9679 µmol g−1 h−1[358]g‐C3N4/ Ti3C2/PtPhotodepositionHeterostructureAqueous solution (50 mL) containing TEOA (10 vol%), 300 W Xe lamp (HSXF/UV 300)5.1 mmol g−1 h−1[359]Nb2O5/C/Nb2CN/AFabrication of compositeMixed solution of deionized water/methanol (3:1,v/v, 40 mL) with the use of methanol,200 W Hg lamp (285–325 nm)7.81 µmol g−1 h−1[360]RGO/TiO2HydrothermalFabrication of compositeH2PtCl6 with 1.0 wt% Pt, solution of 25 mL methanol and 75 mL H2O, 500 W Xe lamp43.8 µmol g−1 h−1[361]N‐doped MoS2/S‐doped g‐C3N4One‐step thermal polycondensation(S‐scheme) heterojunction100 mL aqueous solution containing 10 mL TEOA, PLS‐SXE300/300UV Xe lamp658.5 µmol g−1 h−1[362]CO2 reductionTiO2 nanosheetsHydrothermalPristine sampleA mixture of pure CO2 gas and H2O vapor, 500 W Xe arc lamp (> 400 nm)CH4, 1.643 µmol g−1 h−1[363]TiO2 nanosheetsHydrothermalSurface acidification by H2SO4A mixture of pure CO2 gas and H2O vapor, 500 W Xe arc lamp (> 400 nm)CH4, 1.907 µmol g−1 h−1[363]TiO2 nanosheetsSolvothermalPristine sample100 mL H2O, 300 W Xe arc lampCO, 0.15 µmol g−1 h−1[364]TiO2 nanosheetsIn situ ion exchange methodTreated by Lewis base [WO4]2−100 mL H2O, 300 W Xe arc lampCO, 3.05 µmol g−1 h−1[364]WO3 nanosheetsSolvothermalPristine sample0.2 mL H2O, 40 W silicon nitride lamp (0.8–17 µm)No detectable product[365]WO3 nanosheetsSolvothermalPoor Vo0.2 mL H2O, 40 W silicon nitride lamp (0.8–17 µm)CO, 6 µmol g−1 h−1[365]WO3 nanosheetsSolvothermalRich Vo0.2 mL H2O, 40 W silicon nitride lamp (0.8–17 µm)CO, 2.75 µmol g−1 h−1[365]Sr2Bi2Nb2TiO12 nanosheetsSolvothermalPristine sample5 mL 4 m H2SO4 with 1.3 g NaHCO3,300 W Xe lamp (simulated solar light)]CO, 2.62 µmol g−1 h−1[366]Sr2Bi2Nb2TiO12 nanosheetsHydrothermalRich Vo5 mL 4 m H2SO4 with 1.3 g NaHCO3, 300 W Xe lamp (simulated solar light)CO, 17.11 µmol g−1 h−1[366]CuIn5S8 nanosheetsPristine sampleCO2 reduction2 mL H2O, PLS‐SXE 300/300UV Xe lamp (standard AM 1.5G filter and >420 nm)CO, 1.3 µmol g−1 h−1; CH4, 1.6 µmol g−1 h−1[367]CuIn5S8 nanosheetsHydrothermalSulfur vacancies2 mL H2O, PLS‐SXE300/300UV Xe lamp (standard AM 1.5G filter and >20 nm)CH4, 8.7 µmol g−1 h−1[367]ZnIn2S4 nanosheetsHydrothermalZn vacancies2 mL H2O, PLS‐SXE300/300UV Xe lamp (standard AM 1.5G filter)CO, 33.2 µmol g−1 h−1[242]ZnIn2S4 nanosheetsHydrothermalPristine sample2 mL H2O, PLS‐SXE300/300UV Xe lamp (standard AM 1.5G filter)CO, 9.22 µmol g−1 h−1[242]HIMD Cs2PbBr6/RGOSurfactant‐mediated antisolvent precipitationPristine sampleMixture of 5 mL ethylacetate and 5 µL H2O, PLS‐SXE300D/300 W Xe lamp (> 420 nm)CO, 5.5 µmol g−1 for 3 hCH4, 5.3 µmol g−1 for 3 h[368]HIMD Cs2PbBr6/RGOSurfactant‐mediated antisolvent precipitationHybridizationMixture of 5 mL ethyl acetate and 5 5 µL H2O, PLS‐SXE300D/300 W Xe lamp (>420 nm)CO, 13.2 µmol g−1 for 3 hCH4, 9.3 µmol g−1 for 3 h[368]PbBiO2Br/GOHydrothermalPristine sample300 mL of 1 n aqueous NaOH containing Ar and CO2, 8 W Xe lamp (Philips T5)CH4, 1.193 µmol g−1 h−1[369]PbBiO2Br/GOHydrothermalHeterojunction300 mL of 1 N aqueous NaOH containing Ar and CO2, 8 W Xe lamp (Philips T5)CH4, 2.836 µmol g−1 h−1[369]CsPbBr3/MoS2 nanosheetself‐assemblyHeterostructuresMixture of 5 mL of ethyl acetate and 20 µL of mL DI water, (PLSSXE300/300 UV) 300 W Xe lampCH4, 12.8 µmol g−1 h−1CO, 25.0 µmol g−1 h−1[370]BiOI/MoS2/CdS microspheresSolvothermalHeterostructureH2O, (PLSSXE300/300 UV)/300 W Xe lamp (400 nm <λ < 700 nm)CH4, 54.7 µmol g−1 h−1CO, 74.9 µmol g−1 h−1[371]Ti3C2(OH)2/g‐C3N4N/AFabrication of compositePLSSXE300/300 W Xe lamp (420 nm)CO, 11.21 µmol g−1 h−1[372]Porous g‐C3N4 nanosheetsSelf‐assembly and alkali assistedCyano modificationN2 and CO2 carrier gas, 0.5 mL DI H2O, Xe lamp (Labsolar 6A, perfectlight)CH4, 0.6 µmol g−1 h−1CO, 13.7 µmol g−1 h−1[373]g‐C3N4 nanosheetsSelf‐assemblyN vacancies0.5 mL DI H2O, 300 W Xe lamp 300–100 nm (Labsolar 6A, perfect light)CO, 6.61 µmol g−1 h−1[374]Flower‐like SnO2/Ag/ MoS2 nanocompositeHydrothermalElectronic band alignmentN/ACH4, 20 µmol g−1 h−1CO, 9 µmol g−1 h−1[375]Removal of pollutantsFlower‐like SnO2/Ag/ MoS2 nanocompositeHydrothermalElectronic band alignmentN/AMB, 100% within 12 minRhB, 100% within 14 minmethylene orange, 100% within 22 min[375]TiO2/GO/CuFe2O4Ball millingHeterostructureAn aqueous solution of test chlorinated pesticides,150 W xenon arc lamp (λ = 365 nm) (CHF‐XQ‐500 W)DDE, 96.5%[376]Cellulose/GO/TiO2 hydrogelgreen, simple, and one‐stepGreen hydrogelMB (200 mL) in a thick film of 120 × 80 × 0.5 mm3,125 W mercury lamp (Vilber Laurmat, λ = 312 nm)MB, 93% within 120 min[377]Cu‐ZnO/S‐g‐C3N4 (CZS) nanocompositesCo‐precipitationHeterojunctionMB under sunlightMB, 100% within 60 min[378]C3N4‐doped Fe@Co3O4Co‐precipitationNanocomposite3 mL of dye was syringed out at regular intervals of time, 400 W Mercury lamp (400–700 nm)MB, 99% within 40–45 min[379]MoS2/MoSe2 nanocompositesone‐step microwaveHeterojunction20 mL of MB dye solution having 0.1 g L−1 sample, visible light sourceMB, 99% with rate constant 0.033 min−1[380]MoS2/NiS2 nanosheetHydrothermalNanocomposite fabrication2 mg dye in 100 mL DI water containing as a prepared sample, 40 W xenon light source (MAX‐303, Asahi Spectra)RhB, 90.61% within 32 min[381]TiO2/MoS2 burst tube compositesHydrothermalHeterostructure100 mg photocatalyst mixed with 60 mL MB solution, 360 W Hg lampMB, 94.1% within 30 min[382]C3N4/CS‐doped CdSCo‐precipitationChitosan and C3N4‐dopingSynthesized samples (10 mg) were added to 30 mL of MB standard solution, 400 W Hg lampMB, 99.75%[383]AgCl/Ag3PO4/g‐C3N4 nanocompositeSedimentation precipitation and ion exchangeHeterojunction50 mg photocatalyst was dispersed in 100 mL 20 mg L−1 MB and MPB solution, 300 W metal‐halide lamp (PLS‐SXE300) (< 400 nm)MB and MBP, 100% within 20 min[384]Nitrogen FixationPhosphorus‐doped Bi2WO6 monolayerHydrothermalVo50 mL mixed solution containing 40 mg L‐1 K2Cr2O7 and 10 mg L‐1 TCH,300 W Xe lamp (> 420 nm)73.6 µmol g−1 h−1[385]P‐1 T‐MoS2@N‐g‐C3N4Hydrothermal and annealingComposite100 mL DI H2O with 20 vol% methanol solution, 500 W Xe lamp with an AM 1.5G filter is used as the light source (1100 nm >λ > 350 nm),689.76 µmol g−1 h−1[386]Pt/N‐MoS2 microspheresPhoto ultrasonic reductionUltrasound irradiation50 mL methanol solution, 300 W Xe lamp (> 420 nm)121.2 µmol g−1 h−1[387]N‐ MoS2 microspheresHydrothermalN doping5 mL methanol into 45 mL DI H2O, 300 W Xe lamp (> 420 nm)101.2 µmol g−1 h−1[388]Z‐type g‐C3N4/BiOCl compositeHydrothermalTernary composite fabrication50 mL methanol‐H2O, 25 °C, 300 W Xe lamp1773.8 µmol g−1 h−1[389]g‐C3N4Defect dopandN defect and B dopant100 mL Na2SO3 solution with N2 blowing, 300 W Xe lamp (> 420 nm)435.28 µmol g−1 h−1[390]g‐C3N4 nanosheet/FeOClCalcination Hydrothermalcomposite fabricationMixture of 2 mL H2O and 40 mL ethanol, s500 W Xe lamp2600.4 µmol g−1 h−1[391]Black phosphorous nanosheet/CdSOil bath heatingcomposite fabrication250 mL methanol–H2O, 300 W Xe lamp (> 420 nm)137.46 µmol g−1 h−1[392]H2O OxidationPhotocatalytic water splitting was already observed as a potentially game‐changing strategy for producing safe and renewable hydrogen, with the largest energy density, has been identified as a possible energy carrier for storing energy from the sun in chemical bond energy between two H atoms. Due to the slow dynamics of the four‐hole, half‐reaction mechanism in water oxidation, it was critical for optimum splitting performance. As a result, increasing the efficiencies for light‐derived H2O oxidation application by reasonable photocatalyst structure design is highly desirable. To design vis‐light active semiconductors for H2O splitting, the bandgap and band positions must be optimized, charge separation must be adequate, charge movement must be easy, and the semiconductor must be durable in aqueous solutions. 2D architecture with a high defect density may represent an ideal structure for increasing O2 generation activity.Additionally, an appealing design strategy for meeting these criteria is to combine defective 2D materials (i.e., graphene, MoS2, and g‐C3N4) with suitable semiconductors. The photocatalytic separation of water into its components is observed as the Holy Grail of chemistry because it requires merely renewable energy sources, photocatalysts as a medium, and H2O as a reaction source. Although a significant move forward has been made, the efficacy of water splitting is still limited in the majority of photocatalytic methods. Generally, H2O oxidation is a slow and inefficient process in photocatalytic H2O splitting schemes due to the complex four h+s redox method. As a result, it is critical to propose a photocatalyst with a robust solar H2O oxidation system. Given this, Di et al. engineered rational pit defects in 2D BiOCl nanosheets by partially digging pits on previously prepared BiOCl nanosheets with ethylene glycol.[393] TEM and STEM images clearly showed the engineered pit defects on the (001) exposed facet (Figure 22).22Figurea,b) TEM images and c) HAADF‐STEM images BiOCl nanosheets with defects, d) DOS diagrams of BiOCl (001) and (110) faces from first‐principles simulations, e) charge migration between 001 and 110 facets is shown in this diagram, f) photogenerated charges are separated and transferred in this schematic illustration, Reproduced with permission.[393] Copyright 2017, The Royal Society of Chemistry.According to DFT calculations, photogenerated e−s will move to gravitate toward the (001) facet in BiOCl, while h+s will migrate in the (110) direction. Because O2 production is an absolute hole participating reaction, this will appear on the (110) facet, even though the h+s long migratory path to the (110) facet and the h+s distance towards the facet will eventually include significant e–h recombination. Furthermore, the introduced pit defects reduced the migration distance of h+s, thus increasing its h+s utilization. According to the DFT measurement, the engineered pit defects often marginally improve DOS at CB and VB edges, thus raising carrier concentration and facilitating electron excitation. Additionally, the abundance of unsatisfied chemical bonds accompanying defects created a favorable chemical atmosphere for reaction molecules to chemisorb and fostered photocatalytic water oxidation reactions. So, the pit‐rich BiOCl nanosheet could generate O2 at a rate of 56.85 mol g−1 h−1, between 3 to 8 times faster than the BiOCl nanosheet and bulk BiOCl. In another study, ultrasonic exfoliation of lamellar hybrid intermediate (Zn2Se2)(propylamine) resulted in the formation of 4 atomic thin freestanding single‐layers ZnSe.[250] Although the size of ZnSe had been atomically reduced, the local atomic structure had undergone remarkable improvements. Simultaneously, the SeSe bond lengths increased from 4.012 to 4.11. These findings established surface distortion in the single‐layer structure, which decreased surface energy and exemplary stability of fabricated structures’ single‐layers. Additionally, surface deformation can increase DOS at the CB tip, ensuring an even higher charge carrier transfer rate. ZnSe single‐layer exhibit high light‐harvesting, improved exciton separation, and lower resistance to charge carriers due to their single‐layer configuration with surface defects. As a result, single‐layers ZnSe nanosheets demonstrated a 195‐fold increase in photoconversion efficiency for H2O oxidation following Xe lamp irradiation relative to bulk ZnSe.Correspondingly, defects engineered in other 2D photocatalysts, including Vo confined in In2O3, pits formed in WO3 nanosheet, or surface distortions formed in ZnSe, SnS2, and SnS nanosheet, will show superior photocatalytic water oxidation behaviors.[237,251,394] Liu et al. developed a variety of pore structures in WO3 nanosheets using a rapid heating technique on previously exfoliated WO3•2H2O nanosheets.[395] Given that the photogenerated h+s migration direction was along 001 facets in X‐direction in W‐O‐W chains, the photogenerated h+s almost certainly experienced many charge carrier recombination, severely impairing damaging photoconversion efficiency. The pores formed effectively shorten the diffusion path of h+s and promote H2O oxidation to form O2 at the WO3 surface. Besides that, an abundance of dangling bonds along the pore environment provided favorable conditions for facile chemisorption of molecular reactions, which increased O2 evolution kinetics. Photocatalytic H2O oxidation efficiency was increased by 18 times when pore‐rich WO3. Moreover, nanosheets were compared to bulk WO3. It demonstrates an essential technique for increasing conversion efficiency with a 2D structure for photocatalytic H2O oxidation.H2 EvolutionAlthough photocatalytic solar energy conversion to hydrogen fuels is an ideal approach for future energy sustainably, the relatively low energy conversion efficiency still greatly limits its potential practical applications. The photocatalytic hydrogen production efficiency can be significantly improved by virtue of 2D configuration coupled with abundant surface defects.[46,396] Peng et al.[397] recently performed vacancy‐induced 2H@1T MoS2 phase incorporation on ZnIn2S4 to boost photocatalytic hydrogen evolution. They discover the synergistic regulations of both structural and electronic benefits by introducing sulfur vacancies in a 1T‐MoS2 nanosheet host to prompt the transformation of the surrounding 1T‐MoS2 local lattice into a 2H phase, leading to dramatically enhanced photocatalytic hydrogen evolution activity. Multiple in situ spectroscopic and microscopic characterizations combined with theoretical calculations demonstrated that in plane sulfur vacancies as active sites could activate the proton. At the same time, the 2H@1T‐MoS2 phase incorporation can effectively regulate the electronic structure and further improve the conductivity. Therefore, the optimized ZnIn2S4@MoS2 photocatalyst achieves a high photocatalytic hydrogen evolution activity of 23233 µmol g−1 with an apparent quantum yield of ≈5.09%.[46] The PEC measurements were performed to investigate the promoted interfacial charge separation. As presented in Figure 23a, the smaller arc radius of electrochemical impedance spectroscopy (EIS) Nyquist plot of ZnIn2S4@MoS2 (4.8 at%) electrode than that of bare ZnIn2S4 showed a much lower interfacial charge transfer resistance. Through fitting the EIS by equivalent circuit, charge transfer resistance (Rct) of ZnIn2S4@MoS2 electrode (18.9 kΩ) was much smaller than that of pristine ZnIn2S4 (62.6 kΩ), suggesting an improved charge transfer efficiency in the presence of 2H@1T‐MoS2 phase‐incorporation.[398,399] In addition, ZnIn2S4@MoS2 (4.8 at%) electrode also showed an enhanced transient photocurrent response, compared with pristine ZnIn2S4 counterparts under the identical conditions, which further confirmed the favored interfacial charge transfer (inset, Figure 23a). To further investigate the impact of 2H@1T‐MoS2 phase incorporation on charge separation and transfer property, Mott‐Schottky measurement was employed (Figure 23b). The flat‐band potential of ZnIn2S4@MoS2 was measured to be ‐0.80 V versus normal hydrogen electrode (NHE), which indicated the MoS2 loading had little influence on the conduction band edge of n‐type ZnIn2S4.[400,401] The result also implied that the intrinsic sulfur vacancy defects of ZnIn2S4 were homogeneously distributed in the surface region of ZnIn2S4.23Figurea) EIS Nyquist plots and the transient photocurrent responses of pristine ZnIn2S4 and ZnIn2S4@MoS2 (4.8 at%) (inset, a), b) Mott‐Schottky plots of ZnIn2S4 and ZnIn2S4@MoS2 (4.8 at%) in the dark, c) in situ DRIFT (diffuse reflectance infrared Fourier transform spectroscopy) spectra of water on the ZnIn2S4@MoS2 (4.8 at%) surface with an increasing water amount (water‐saturated flow under He for 2 h) and d) with irradiation under 300 Xe lamp for 2 h, and e) proposed mechanism for photocatalytic H2 production in the ZnIn2S4@MoS2 system under visible‐light irradiation. Reproduced with permission.[397] Copyright 2021, Elsevier B.V.Zhang et al.[402] developed a smart strategy to position MoS2 QDs at the sulfur vacancies on a Zn facet in monolayered ZnIn2S4 (Vs‐M‐ZnIn2S4) to craft a 2D atomic‐level heterostructure (MoS2QDs@Vs‐M‐ZnIn2S4) as shown in Figure 24a. The electronic structure calculations indicated that the positive charge density of the Zn atom around the sulfur vacancy was more intensive than other Zn atoms. The sulfur vacancy confined in monolayered ZnIn2S4 established an important link between the electronic manipulation and activities of ZnIn2S4. The sulfur vacancy acted as electron traps, prevented vertical transmission of electrons, and enriched electrons onto the Zn facet. The sulfur vacancy‐induced atomic‐level heterostructure sewed up vacancy structures of Vs‐M‐ZnIn2S4, resulting in a highly efficient interface with low edge contact resistance. Photogenerated electrons could quickly migrate to MoS2 QDs through the intimate ZnS bond interfaces. As a result, MoS2 QDs@Vs‐M‐ZnIn2S4 showed a high photocatalytic hydrogenation activity of 6.884 mmol g−1 h−1, 11 times higher than 0.623 mmol g−1 h−1 for bulk ZnIn2S4, and the apparent quantum efficiency reached as high as 63.87% (Figure 24b,c).[402]24Figurea) Formation mechanism of MoS2QDs@Vs‐M‐ZnIn2S4. Nitrogen adsorption‐desorption isotherms of b) bulk ZnIn2S4 and c) Vs‐M‐ZnIn2S4 (insets are the corresponding pore size distribution curves). Reproduced with permission.[402] Copyright 2018, American Chemical Society. d) TEM image of defective carbon nitride nanosheets, e) EPR spectra, f) bandgap structures, and g) photocatalytic H2 evolution performance, Reproduced with permission.[403] Copyright 2015, John Wiley & Sons, Ltd.The holey carbon nitride nanosheets with carbon vacancies have been controlled and prepared by NH3 treating of bulk counterpart at 510 °C (Figure 24d–g).[403] The formed defect‐rich ultrathin structure can effectively tune the electronic structure with reduced bandgap and upshifted CB and VB positions. At the same time, a higher donor density and remarkably increased charge separation efficiency can be acquired in defective carbon nitride nanosheets. Compared with bulk carbon nitride, the defective carbon nitride nanosheets can display roughly 20 times improved photocatalytic hydrogen production activity under visible light (λ > 420 nm) irradiation, reaching 82.9 µmol h−1, with no decrease in the hydrogen production rate during a 9 h measurement. In another case, by the cooperative utilization of ultrathin structure and surface VO, the photocatalytic hydrogen production rate of defective K4Nb6O17 nanosheets can be significantly improved to 1661 µmol g−1 h−1, about 7 and 21 times higher than defect‐free K4Nb6O17 ultrathin nanosheets and defect‐free bulk K4Nb6O17, respectively.[254] This hydrogen production rate is the optimal report for defective 2D materials under visible light. The sulfur vacancies can also observe a similar function in a ZnIn2S4 monolayer to build MoS2 quantum dot/ZnIn2S4 monolayer heterostructure with outstanding photocatalytic hydrogen production performance.[402]Wang and co‐workers investigated the photocatalytic H2 evolution of few‐layered MoS2 to C3N4 nanosheets,[404] and C3N4 was absorbed in (NH4)2MoS4 during the formation phase. At 350 °C, sulfidation was carried out using an H2O solution, followed by sulfidation using H2S gas. C3N4 and MoS2 have similar layer formation that reduces lattice mismatch and aid in the planar MoS2 slab growth when the surface is C3N4. In response, an inorganic‐organic 2D–2D stacking was established using G‐like thin layer heterojunctions. As a result of the few‐layered MoS2 nanosheets, numerous H2 evolution sites have been established. Additionally, due to the tunneling effect in thin interfacial layers of MoS2, these dispersed layers on the C3N4 nanosheets could be more effective than multilayer MoS2. Based on the charge distinction between the help and the number of H2 evolving sites affected by few‐layer MoS2 nanosheets, it was determined that the developed MoS2/C3N4 2D junctions used to have superior H2 generation activity than a pristine sample. Creating a direct Z‐scheme heterostructure between g‐C3N4 and other semiconductors is a cost‐effective way to maximize photocatalytic efficiency. A clear explanation is that the direct Z‐Scheme photocatalyst 2D/2D Fe2O3/g‐C3N4 generates H2 at a rate 13 times that of g‐C3N4.[405] Because of the difference in work functions between g‐C3N4 (4.18 eV) and Fe2O3 (4.34 eV), electrons will switch from g‐C3N4 to Fe2O3 at the intimate 2D/2D interface. Thus, at the Fe2O3/g‐C3N4 interface, a built‐in electric field is created, which becomes advantageous for photoinduced charge carrier transfer and separation.Additionally, a direct Z‐scheme system that relies on the band structures of Fe2O3 and g‐C3N4 has been established. After all, e−s formed in the CB of Fe2O3 will pass through the intimate 2D/2D interface to the VB of g‐C3N4 and recombine with the h+s through d–p conjugation, hence impeding inhibiting photogenerated charge carrier recombination. As an effect, photoinduced e−s and h+s accumulate in g‐C3N4 in CB and Fe2O3 in VB, respectively. This direct Z‐scheme method will not only this direct Z‐scheme method increase exciton separation performance but also generates a major driving force for the light‐driven splitting of water, thus increasing the ability of g‐C3N4.Lei et al.[406] developed a proof‐of‐concept strategy to enable visible‐light photocatalytic activity of wide bandgap Ca2Nb3O10 monolayer nanosheet by incorporating RGO nanosheet as a photosensitizer. The Ca2Nb3O10 monolayer/RGO 2D–2D nanohybrids exhibit vastly elevated performance in photocatalytic H2 evolution with a H2 production rate of 820.76 µmol h−1 g−1 and TCH degradation reactions under the visible light irradiation. The combined experimental and theoretical results demonstrate that the electrons generated from the photoexcited RGO transfer to the Ca2Nb3O10 monolayer and then participate in photocatalytic reactions. The constructed RGO sensitized monolayer perovskite photocatalyst nanohybrids are demonstrated as an efficient photosensitizer for enhancing visible light harvesting of wide bandgap semiconductor in solar‐energy conversion, and elaborated in details of the charge transfer process of this type of nanohybrid photocatalyst (Figure 25).25Figurea) Schematic drawing of the formation process of CNOMS (Ca2Nb3O10 monolayer nanosheet)/RGO nanohybrids, b) AFM image of CNOMS, c,d) TEM and SEM images of CNOMS, e) TEM image of RGO nanosheets, f,g) TEM and HRTEM images of CNOMS/RGO nanohybrids, h) HAADF‐STEM images of CNOMS/RGO nanohybrids and the corresponding EDX mapping of Ca, Nb, O, and C elements. i) Schematic illustrations are showing the photocatalytic process and the electrons transfer in the CNOMS/RGO nanohybrids. j) Cycling stability test of CNOMS/RGO nanohybrids under photocatalytic H2 production. Reproduced with permission.[406] Copyright 2018, John Wiley & Sons, Ltd.In another study, Kwon and co‐workers[407] reported a cathode based on WS2/p‐Si structure for light‐driven H2 evolution; with an increase in thickness, fabricated WS2 film exhibits a change in color from yellow to brown, and the absorbance of films increases as well. However, no shift in position of absorption peak was observed for the prepared thin films. The highest current density (8.375 mA cm−2 at 0 V) was achieved for 23 nm WS2/p‐Si sample with the incident photon to current conversion efficiency of 72%. This study validated that the compositing of conventional semiconductors with TMCs such as MoS2 and WS2 could effectively attain superior water‐splitting performance under light.Despite the benefit of the 2D/2D heterojunction, charge carrier recombination occurs in the interlaminar space region owing to the weak vdW force in the layers. As a result, it is desirable to prompt an inherent driving force in the 2D semiconductor plane to facilitate photogenerated charge carriers’ delocalization near photoexcited locations. The in plane heterostructure configuration generates a strong electric field within the system, guiding photogenic excitons to appropriate redox‐active sites. In‐plane heterojunctions between 2H and 1T′ MoS2 have been fabricated in the MoS2 monolayer and employed for photocatalytic H2 production.[408] Via thermal annealing of the MoS2 nanosheet, the ratio of different MoS2 phases can be regulated in a systematic manner. The in‐plane 1T′ MoS2/2H MoS2 annealed (60 °C) heterostructure showed maximum activity under visible light with an H2 generation rate of 1500 µmol h−1 g−1, and the MoS2/Al2O3‐60 °C heterostructure revealed high stability in 0.1 m lactic acid aqueous solution. Due to graphene's outstanding carrier mobility, ultrahigh optical transmittance, and high electrical conductivity is touted as a versatile photosensitizer for enhancing photocatalytic reactions when exposed to visible light.CO2 ReductionDue to the continued burning of fossil fuels and the exponential rise in CO2 levels in the environment over the past decades, the oil crisis and climate change have sparked widespread concern. As a result, natural oil exploitation misuse is a matter of priority. Using an effective semiconductor photocatalyst, oxidation of CO2 to chemical fuels like CO, CH4, CH3OH, CH2O2, and CH2O is a practicable way to mitigate the greenhouse effect and address the energy crisis.[409,410] However, converting CO2 to other products is highly complex and challenging since CO bond in CO2 has a high dissociation energy of 750 kJ mol−1 and requires the presence of several electrons. The formation of a CO2‐intermediate through single electron transmission to allow the formation of CO2 has been identified as rate‐limiting step in proton‐related reduction process. For initialization, a theoretical potential of ‐1.9 V versus NHE is needed, and a superior over potential is desired for actually exploited potentials. Since Inoue et al. in 1979 worked on the photoconversion of CO2 to usable fuels, numerous photocatalysts for CO2 reduction have been published.[411] Moreover, since CO2 has strong thermodynamic stability (G = ‐394.4 kJ mol−1), releasing CO2 to reactive carbon intermediates on the surface of photocatalysts is a major concern.[412] Thus, in addition to light absorption and the transfer and separation of photoinduced e−s and h+s, CO2 adsorption and activation are critical for photocatalytic CO2 reduction. Recent research demonstrates that defective 2D photocatalysts have a tremendous capacity for exhibiting exceptional CO2 photoreduction activity. For instance, by reducing the thickness of ZnAl‐LDH nanosheet, VO defects were added, creating Zn vacancy complexes.[234] The formed Zn vacancy complexes will act as traps for CO2 and H2O molecules, promoting charge separation and enhancing CO2 photoreduction activity to produce CO. Along with anion and cation vacancies, photocatalytic CO2 reduction may be beneficial. A lamellar hybrid intermediate approach has been used to form Bi2WO6 layers with a single unit cell thickness.[413]To produce oleate ions, sodium oleate formed an electrostatic bond with Bi3+. Then, lamellar Bi oleate complexes are formed by self‐assembling oleate ions in a tail‐to‐tail/head‐to‐head bilayer sequence to form a mesostructure. As Na2WO4 was injected and hydrothermally refined, Bi2WO6 was self‐exfoliated and formed into a single‐unit cell sheet. As synthesized, combined with a 300 W Xe lamp, a single‐unit Bi2WO6 layer is used as a photocatalyst for CO2 photoreduction. The Bi2WO6 powder was suspended in H2O along with an incredibly pure CO2 gas that constantly bubbled in the solution. In the synthesis of CH3OH, an average rate of 75 µmol g−1 h−1 was observed over single unit cell Bi2WO6 layers at over a 5 h period, nearly three and a half times faster than the values obtained with Bi2WO6 nanocrystals and bulk Bi2WO6, respectively. Engineered Zn vacancies in one‐unit‐cell ZnIn2S4 increased charge separation efficiency by allowing higher charge density and transport.[242] The defect‐mediated successful charge separation results in a CO formation rate of 33.2 µmol g−1 h−1 for the Zn vacancy‐rich ZnIn2S4 nanosheet, approximately 3.6 times that of Zn vacancy‐deficient ZnIn2S4 nanosheet. This other research[414] demonstrated the facile one‐step in situ hydrothermal syntheses of a 2D/2D g‐C3N4/NiAl‐LDH hybrid heterojunction. The negatively charged g‐C3N4 nanosheet can act as nucleation sites for the in situ growth of NiAl‐LDH nanosheets, forming an intimate interface between the g‐C3N4 and NiAl‐LDH nanosheet. The 2D/2D g‐C3N4/NiAl‐LDH exhibits a significantly higher CO evolution rate (8.2 µmol h−1 g−1) than pure g‐C3N4 (1.56 µmol h−1 g−1), NiAl‐LDH (0.92 µmol h−1 g−1), and a physical mixture of g‐C3N4 and NiAl‐LDH (2.84 µmol h−1 g−1). Moreover, the selectivity for CO is nearly 82% for the 2D/2D g‐C3N4/NiAl‐LDH (10%) photocatalyst. In addition, the 2D/2D g‐C3N4/NiAl‐LDH improved not only CO generation but also the evolution of H2 and O2.The CO2 adsorption capability of Bi2WO6/RGO/g‐C3N4 was significantly greater than that of g‐C3N4 and Bi2WO6, owing to the CO2 molecules’ delocalized π‐conjugated binding and the broad‐conjugated structure of RGO, which established the special π–π conjugation interaction. The increased CO2 adsorption capacity of the Bi2WO6/RGO/g‐C3N4 could benefit photocatalytic CO2 reduction. In addition, due to the electric field associated with the 2D/2D interfaces formed in the Bi2WO6/RGO/g‐C3N4 heterojunctions, the 2D/2D interface creation accelerates the migration of charge carriers. Moreover, since the Bi2WO6, RGO, and g‐C3N4 have an intimate interface, photoinduced e−s in Bi2WO6 (CB) will rapidly merge with h+s in g‐C3N4 (VB) through RGO redox mediator, resulting in the e−s aggregation in g‐C3N4 (CB) and h+s in Bi2WO6 (VB). Furthermore, after that, accumulated e−s in g‐C3N4 (CB) can be transferred to RGO, owing to RGO's excellent electron conductivity and storage power, resulting in an increased electron density on RGO surface. Thus, CO2 molecules can be reduced to CH4 and CO by accumulated e−s on the RGO surface, while the water molecules are being oxidized by h+s on Bi2WO6 (VB) to form O2 and protons. Formation of 2D/2D/2D Bi2WO6/RGO/g‐C3N4 hybrid Z‐scheme heterojunction enhances photocatalytic CO2 reduction efficiency and H2 and O2 generation with up to 92% selectivity for CO/CH4. Hou et al. used PO4 and VO to synergistically enhance the CO2 photoreduction activity of Bi2WO6 atomic layers.[232] Functionalized Bi2WO6 atomic layers exhibit a CH3OH formation average of 157 µmol g−1 h−1, which is over two and a half times that of Bi2WO6 atomic layers and bulk Bi2WO6, respectively. After 60 h of reaction time, there is no discernible loss of CO2 reduction activity over the functionalized Bi2WO6 atomic layers, implying the possibility of realistic solar fuel production.Besides charge carrier separation, improving the catalyst's CO2 adsorption capacity is critical to enhancing the photocatalytic CO2 reduction efficiency. Using Ti3C2 MXene's excellent electrical conductivity and excess of exposed metal sites, Ti3C2/Bi2WO6 nanosheet was synthesized by 2D Bi2WO6 on the surface of 2D Ti3C2.[415] The 2D/2D Ti3C2/Bi2WO6 matrix established a strong interface between Ti3C2 and Bi2WO6 nanosheet, and the O or OH group on the Ti3C2 surface aids Ti3C2 in capturing photoinduced e−s from Bi2WO6. When exposed to light, it gets excited from Bi2WO6's VB and then moves to its CB. Since Bi2WO6's CB potential exceeds Ti3C2's Fermi stage, photoinduced e−s will then be passed from Bi2WO6 to Ti3C2 via the 2D/2D interface. Additionally, increased specific surface area and Ti3C2/Bi2WO6 nanosheet pores promoted CO2 adsorption. CO2 molecules adsorbed on Ti3C2 surface will react to CH4 and CH3OH with photoinduced e−s. Additionally, the special photothermal conversion property of Ti3C2 will provide energy to activate the catalyst, enhancing photocatalytic CO2 performance. O2 can also be formed as an H2O by‐product during the photocatalytic CO2 reaction (Figure 26a). However, since this study did not analyze H2 processing, it is unclear if all photogenerated e−s are included in the CO2 reduction process. Ye et al. used a basic mechanical mixing technique to combine surface alkalinized Ti3C2 MXene as cocatalysts with commercially available P25, resulting in a significant increase in photocatalytic CO2RR.[416] After surface alkalinization, 5 wt% Ti3C2(OH)2‐doped P25 (5TC OH/P25) shows a large rise in CH4 release compared to unmodified 5TC/P25 (Figure 26b). The DFT study showed that CO2 adsorption energy on TCF (F termination) exceeded CO2 adsorption energy on TC‐OH (OH termination). As a response, CO2 molecules have been readily adsorbed on the TC‐OH wall, creating activated CO32−. Facilitating charging isolation, excellent electrical conductance, sufficient CO2 adsorption, and activation sites on alkalinized MXene contributed significantly to photocatalytic progress. These findings demonstrated MXene's crucial position as an effective metal‐free cocatalyst for synthetic photosynthesis.26Figurea) For CO2 to *CH4 and **H2O conversion, the lowest amount of energy paths (PBE/DFT‐D3 computations) were explored, catalyzed by Mo3C2. C, Mo, O, and H atoms are represented by grey, lilac, red, and white spheres, respectively. Reproduced with permission.[417] Copyright 2017 American Chemical Society. b) CO and CH4 evolution rates photo catalytically over P25, 5Pt/P25, 5TC/P25, and 5TCOH/P25, Reproduced with permission.[418] Copyright 2016, American Chemical Society. c–f) P25 for CH4 generation and photocatalytic CO2RR of c) TiO2/Ti3C2 (TT‐x) samples and images of TT550 obtained with d,e) FESEM, and f) TT650. Reproduced with permission.[233] Copyright 2015, American Chemical Society. g,h) Photoinduced e‐migration technique at g) Ti3C2/ Bi2WO6 heterointerface and h) photocatalytic activity of Ti3C2/Bi2WO6 at various Ti3C2/Bi2WO6 mass ratios (0%, 0.5%, 1–5%). Reproduced with permission.[415] Copyright 2018, John Wiley & Sons, Ltd.In a similar in situ study reported by Xu et al.,[419] conductive Ti3C2 was decorated with TiO2 nanoparticles via thermal annealing for fabricating TiO2/Ti3C2 composites to generate CH4 from CO2RR (Figure 26c). At high temperatures, oxidation of Ti3C2 occurred, removing F functional groups and attaching O functional groups. The rice crust morphology of TT550 and TT650 was observed to be completely different from Ti3C2 (Figure 26d–f). The e−s are efficiently transferred to TiO2 due to Ti3C2 conductivity, whereas the rice crust morphology offers abundant active sites to promote photocatalytic performance. However, straightforward evaluation of photocatalytic behavior for both TiO2/Ti3C2Tx systems was difficult because of the morphology difference, various surface modification by MXene, different synthetic routes, and different phases of TiO2. Recently, the same group has fabricated a hybrid ultrathin Ti3C2/Bi2WO6 2D/2D heterojunction (Figure 26g).[420] The separation and transport of photogenic charges are significantly improved due to intensive physical effects and electronic coupling. 2 wt% Ti3C2‐modified Bi2WO6 nanosheets record the highest CH4 release rate than other stoichiometry (Figure 26h). Moreover, large interfacial contact surfaces of intimate 2D/2D heterojunction offer larger contact areas and shorted diffusion lengths at the interfaces, which generate superior charge mobilities in contrast to 1D/2D and 0D/2D heterojunctions. Intensive research on 2D/2D heterointerfaces has engendered new potential in photocatalyst designs based on layered heterojunctions.N2 FixationAmmonia (NH3) is not only an essential chemical to provide a variety of important chemicals (fertilizers) but also an important energy carrier and fuel.[421,422] Currently, NH3 is primarily synthesized in industry through the well‐known Haber‐Bosch process that needs high pressure and temperature.[423] The photocatalytic NH3 synthesis from N2, H2O and daylight was fascinating since this reaction is being performed at room temperature and pressure. The theory of N2 fixation through photosynthesis is close to that of CO2 reduction. Under solar light, photogenerated e−s and h+s become excited, and afterward, h+s oxidize the H2O to form O2 and protons, while the e−s reduces the N2 and protons to NH3.[423–426] However, N2 fixation seems to be more difficult than CO2 reduction due to the higher dissociation energy of the NN triple bond (up to 962 kJ mol−1) and the poor binding force between molecular N2 and the catalyst surfaces.[427] Usually, N2 catalyst conversion is extremely harsh due to N2's poor affinity for solid‐state catalysts and high‐energy intermediates presence. The defect‐rich surface, which contains abundant electron donors and active catalytic sites, can encourage photocatalytic N2 fixation.[428] Latest studies show that 2D photocatalysts, including VO‐BiOBr, Bi5O7I, and MoS2, were promising materials for effective photocatalytic N2 fixation.[418,429,430] However, these 2D photocatalysts demonstrated low activity, indicating that further improvement in photoconversion efficiency is needed. Consequently, various faulty 2D photocatalysts and 2D heterojunction photocatalysts were formed for photocatalytic N2 fixation during the last few decades.Previous research has shown that with their excess localized e−s, VO can efficiently capture and activate inert N2 molecules as electron trap centers. The NN bond length of N2 can be prolonged to 1.133 Å via an end‐on configuration on the O vacancies in (001) facet exposed BiOBr.[430] This bond length is between the triple bond length (1.078 Å) of free molecular nitrogen and the double bond length (1.201 Å) of diazene, suggesting the effective activation of N2. Subsequently, e−s from the excited BiOBr's CB could be smoothly injected into N2's π‐antibonding orbitals, leading to photoreduction to ammonia. MoS2 nanosheets with sulfur vacancies were developed and subsequently used for N2 fixation when stimulated by VO‐motivated N2 activation.[430] MoS2 nanosheets formed NH3 at a rate of nearly 325 µmol g−1 after 10 h of simulated solar light irradiation. While commercially available bulk MoS2 is incapable of photocatalytic NH3 synthesis under identical test conditions, this demonstrates the exclusive MoS2 nanosheet advantage for N2 reduction. According to Mott‐Schottky spectra, CB locations of MoS2 nanosheets and bulk MoS2 have been predicted are ‐0.35 and ‐0.24 V, respectively, since they were located below the N2 thermodynamic reduction potentials by one or two e−s transition. As a result, it has been assumed that N2 reduction using MoS2 nanosheets involved multielectron coupled proton transfer. Owing to the n‐type semiconductor essence of MoS2 nanosheets, numerous free e−s exist, and these free e−s can combine with photogenerated excitons to form charge excitons (trions), which have been primarily found around Mo sites. The generated trions contain many e−s in a single‐bound state, which is advantageous for multi‐emigration reactions. If sulfur vacancies constrain N2, this was bound by trions by three Mo atoms following irradiation. A trion‐supported six‐electron reduction method is obtained as N2 is allowed by donating e‐s from its bonding orbitals and taking e‐s into its antibonding orbitals. Zhang et al. discovered that VO in CuCr‐LDH nanosheet could cause sheet distortions, dramatically increasing N2 chemisorption and favoring charge transfer from LDH to N2 (Figure 27).[234] As an outcome, CuCr‐LDH nanosheet exhibits outstanding photoreduction of N2 to NH3. Under UV–vis/sunlight illumination, optimized NH3 concentration will reach 184.8 or 142.9 µmol L−1 in water at 25 °C, respectively. It was the most active material for NH3 synthesis identified to date in pure water. After five consecutive cycles, no noticeable decrease in activity can be detected, indicating exceptional photocatalytic stability.27Figurea) photocatalytic N2 fixation process is illustrated in this diagram. Under UV–vis irradiation, yield of NH3 for several LDH photocatalysts throughout a 1 h test period: b) Illumination with visible light (wavelength > 400 nm), c) with water as a proton source, d) CuCr nanosheet catalyst cycling studies in the presence of water and N2 under visible light illumination, Reproduced with permission.[431] Copyright 2017, John Wiley & Sons, Ltd.In addition, creating a 2D/2D heterojunction is also a good route to optimizing 2D photocatalyst photoconversion efficiency for N2 conversion. For example, a 2D p–n heterojunction composed of AgCl/‐Bi2O3 nanosheet with a thickness of ≈2.7 nm has been synthesized and demonstrated good photocatalytic N2 fixation efficiency. ‐Bi2O3 was a p‐type semiconductor in the AgCl/‐Bi2O3 heterojunction, whereas AgCl was an n‐type semiconductor. Under sunlight, photogenerated e−s and h+s can be excited across AgCl and Bi2O3, and recombination of e−s and h+s can be hindered by creating an internal electric field within the space charge region. N2 has chemically adsorbed on the active sites considering the vast unique AgCl/Bi2O3 nanosheet surface areas and VO presence. Moreover, VO could insert photoinduced e−s directly into chemically adsorbed N2 molecules, weakening NN bond and activating N2. The active N2 thus reacted with the H+ present in the water to form NH3, which then get dissolved in it to form NH4+.[432] The 2D/2D g‐C3N4/RGO hybrid heterojunction catalyst produces 42.4 times more NH4+ than g‐C3N4 attributed to electrostatic reaction between g‐C3N4 and RGO.[433]H2O2 ProductionH2O2 (frequently used bleach and disinfectant) is a potential liquid fuel H2 substitute that exhibits improved applications in rocket engines, fuel cells, and other fields because of its high energy density (3.0 MJL−1). As H2O2 is more convenient for storing and transportation than compressed H2, it possesses great potential for future energy applications. Now, H2O2 is produced mainly by anthraquinone technique and has some technical limitations due to toxic byproducts and high energy consumption. So, it is essential to adopt effective and clean methods for its production. Its photocatalytic production using H2O and O2 has gained significant attention due to its clean, efficient, and safe process.[434] Photocatalytic consists of four electron reaction processes; H2O2 reduction or H2O oxidation process should be enhanced to improve selectivity. Like, Wei et al. improved the selectivity of H2O2 over g‐C3N4 by oxygen doping through the calcination of DICY and ammonium para tungstate.[435] AFM depicts that the nanosheet of g‐C3N4 is 2.1 nm thick, as presented in Figure 28a,b. UV–vis study demonstrated the redshift on the absorption spectra compared to the pristine g‐C3N4. XPS, FTIR, 13C‐NMR, and FTIR confirmed the existence of COC and OH groups in sample, which helped to optimize the O2 absorption. The quantum efficiency of enhanced catalyst is 28.5%, 3.5 time higher than pristine g‐C3N4, as shown in Figure 28c. The results for these catalysts showed good stability and no chance of deactivation over 20 h. DFT calculations and rotating disk electrode technique confirm oxygen doping which helps to generate 1,4‐endoperoxide intermediate. This is very important for enhancing the selectivity and efficiency of two‐electron ORR for producing H2O2 shown in Figure 28d. Moreover, H2O2 can be produced by modifying defects for g‐C3N4.[436] Xie et al. established a g‐C3N4 for maximizing solar energy conversion efficiency with two synergistic N defects. One is responsible for oxygen activation, and the other is important for separation and excitation of photogenerated charges.[436] The mixture of KOH and g‐C3N4 is heated to obtain the NHx, catalyst, and N2C vacancies. This enhanced sample demonstrated 152.6 µmol h−1 efficiency of H2O2 production, 15 times higher than pristine g‐C3N4.28Figurea,b) AFM images of g‐C3N4 and O‐doped g‐C3N4. c) Diffuse reflectance absorption spectra (left axis) and apparent quantum yield (apparent quantum yield, right axis) of samples. d) Koutecký–Levich plots the ORR data measured by a rotating disk electrode. Reproduced with permission.[424] Copyright 2019, John Wiley & Sons, Ltd.Further, DFT results demonstrated that the decreased bandgap for g‐C3N4 and promoted carrier separation is due to the newly generated NHx vacancy. The N vacancy is considered the real active site for oxygen reduction and activation. Single metal atomic catalysts having uniformly distributed reaction centers show brilliant optimization for oxygen reduction. Teng et al. established a single Sb‐doped g‐C3N4 photocatalyst and in the mixture of O2 and water, realized the photocatalytic preparation of H2O2 under visible light.[437] Sb has +3 oxidation state and 4d105s2 electronic configuration, which form reactive centers with electron–hole pairs.The O bond and the prevention of O2 bond from overreaction for generating H2O are presented. This report has stated a complete mechanism for producing H2O2 compared to the constructed catalyst shown in Figure 29. Photoelectrons and holes gathered on adjacent N and Sb atoms achieve effective carrier separation. Consequently, O2 molecules get adsorbed at the Sb site and react with photoexcited electrons to form µ‐peroxide under a two‐electron reduction. Meanwhile, gathered holes at N atoms of the melem units near Sb sites accelerate water kinetics (oxidation). Without any sacrificial agent, H2O2 photocatalytic production rate is about 12.4 mg L−1 in 2 h under visible light, 248 times more than the pristine g‐C3N4. The determined solar chemical conversion rate and quantum efficiencies are 0.61% and 17.6%, respectively, at 420 nm. Using catalysts with a single atom to regulate the path of the reaction will find a better design for developing improved catalysts for more comprehensive applications.29FigureMechanism of photocatalytic H2O2 production. The white, gray, blue, red, and magenta spheres refer to hydrogen, carbon, nitrogen, oxygen, and Sb atoms, respectively. After shining visible light, the photogenerated electrons are localized at the Sb sites (with a blue glow). In contrast, the photogenerated holes are localized at the N atoms at the melem units (with a red glow). Subsequently, the dissolved O2 molecules are adsorbed (orange arrows) onto the Sb sites and then reduced (blue arrows) via a 2e− transfer pathway by forming an electron µ‐peroxide as the intermediate. Simultaneously, water molecules are oxidized (pink arrows) to generate O2 by the highly concentrated holes on the melem units. Reproduced with permission.[437] Copyright 2021, Springer Nature Limited.AntimicrobialThe formation of reactive oxygen species (ROS) during PC will result in mineral, genetic, and protein leakage, resulting in cell death and a decrease in microbial load. Semiconductor photocatalysts based on MOs, for instance, TiO2, ZnO, MgO, and WO3, as well as nonmetal oxides, for example, g‐C3N4, multiwall carbon nanotube, and GO, have been formed and used to inactivate microbial species.[438] Photocatalysts with a high surface area have more reactive sites on the surface, enhanced charge transfer, and separation efficiencies, all of which contribute to increasing photocatalytic activity. These properties make 2D photocatalysts an appealing prospect for effective microbial species removal from water. Huang et al. demonstrated that the presence of mesoporous g‐C3N4, Escherichia coli K‐12 can be successfully eliminated, achieving 100% inactivation efficiency after 4 h of visible region radiation. Moreover, they stated that mesoporous g‐C3N4 has a surface area 20 times that of bulk g‐C3N4 and that photogenerated h+s on the g‐C3N4 surface will aid in bacterial inactivation.[439] Recently, Kang et al. demonstrated the manufacture of visible light active porous g‐C3N4 nanosheet using two distinct methods: alternative heating and cooling and bacterial‐inspired exfoliation. Porous g‐C3N4 nanosheet demonstrated superior water disinfection behavior when disinfecting E. coli due to its broad surface region, small bandgap, and improved transportation ability.[440] Rtimi et al. published a ground‐breaking finding for bacterial inactivation using Cu/TiO2 sputtered films. Also, in the dark, the films showed outstanding bactericidal properties. Cu ions have increasingly enhanced the inactivation process, even under low visible light irradiation conditions. The researchers monitor copper leaching and discover that the levels are only ppb, considered noncytotoxic by human standards.[441]Transition and noble metals are frequently used as dopants to increase the performance of photocatalytic disinfection in 2D materials. A transition metal could introduce an additional energy level into semiconductor material, facilitating electron–hole pair formation and broadening absorption toward the visible spectrum. Electrons migrating from one of these levels to CB require significantly less photon energy than e−s in original semiconductors. Many transition metal‐doped g‐C3N4 photocatalytic methods were investigated. However, none of them discusses its photocatalytic microbe's disinfection. The noble metals incorporation into g‐C3N4 photocatalysts may improve photocatalytic efficiency by generating charge carriers and extending the photocatalyst's spectral absorption into the visible light. Additionally, the metal species’ surface plasmon resonance (SPR) effect can result in charge carriers’ generation in g‐C3N4.[442,443] In addition, noble metals can act as an electron sink, facilitating the separation of photogenerated charge carriers and thereby increasing the photoconversion efficiency of g‐C3N4.[444] Specifically, photogenerated e−s is transferred from the g‐C3N4 (CB) to metal nanoparticles deposited on the g‐C3N4 surface, whereas the photogenerated hole remains on g‐C3N4. This results in a successful separation of photogenerated charge carriers and consequent improvement of the efficiency of photocatalytic disinfection by generating ROS. For instance, Xu et al. demonstrated efficacy against Staphylococcusaureus using Ag‐doped g‐C3N4, which was prepared in two distinct ways (hydrothermal treatment and photo‐assisted reduction). After 3 h of exposure, g‐C3N4 and Ag/g‐C3N4 composites inactivated almost 29.6 and 99.4% of bacterial cells, respectively.[445] Moreover, h+ and O2− species are significant for bacterial inactivation by photocatalysis by Ag/g‐C3N4 (Ag/PCNO). Additionally, the high efficiency of Ag/PCNO was attributed to the SPR effect of Ag nanoparticles and the synergistic effect of PCNO molecules. Besides Ag, the combination of both nanoparticles of Au and g‐C3N4 may provide exceptional peroxidase activity toward the breakdown of H2O2 to OH radicals and can effectively eradicate G+ and G‐ bacteria. In addition, it is effective at degrading existing DR biofilms and inhibiting the formation of new biofilms in vitro. Furthermore, g‐C3N4 is highly toxic to cancer cells.[446]In another study, Cao's group coated polydiallyldimethylammonium chloride (PDDA), which is a positively charged polyelectrolyte on cysteine (Cys)‐modified MoS2 nanosheets decorated with Ag+ for effective bactericidal (in vitro) and rapid wound healing (in vivo) applications.[447] Authors highlighted that rather than being in the nanoparticles form; the decorated Ag was essentially in an ionic state (Figure 30a) that can significantly decrease the Ag wastage and toxicity to living creatures. The prepared Ag+ modified nanosheets offered superior bactericidal performance in contrast to AgNO3 solution or Ag nanoparticles (Figure 30b,c). Zhang and co‐workers loaded TH, an antibiotic, on chitosan‐modified MoS2 nanosheets, as demonstrated in Figure 30d.[448]30FigureDrug cargos in MoS2 nanosheets for disinfection. a) PDDA‐Ag+‐Cys‐MoS2 dark‐field TEM image with corresponding EDS element mapping of Mo L‐edge, S K‐edge, and Ag L‐edge. Analyses of viability. b) E. coli and c) S. aureus treated with various Ag‐containing agents. Reproduced with permission.[447] Copyright 2017, American Chemical Society. d) Chitosan‐modified MoS2 nanosheets‐TCH scheme for anti‐biofilm applications. Reproduced with permission.[448] Copyright 2017, IOP Publishing.The hybridization of 2D materials with other semiconductor materials is an appealing strategy for increasing the photocatalytic efficacy of these materials. The primary benefits involve broadening absorption into the visible region, separating excitons effectively by shifting e−s from higher to lower CB and h+s from higher to lower VB, and inhibiting photo corrosion of semiconductor materials. By hydrothermal calcination, micron‐sized TiO2 spheres have been enfolded with a g‐C3N4 hybrid structure (g‐C3N4/TiO2), and the (g‐C3N4)/TiO2 hybrid material was able to properly inactivate E. coli within 180 min of exposure to visible light. Likewise, hydrothermal temperature changes can affect photocatalytic inactivation efficiency.[449] A vertically aligned Z‐scheme heterojunction was designed hydrothermally, joining g‐C3N4 and TiO2 (anatase) with 001 facets. In comparison to fabricated bare g‐C3N4 and TiO2, coupled band structures in the composite lead to better photocatalytic bactericidal potential.[450]Visible light enabled importantly, bi‐based semiconductor materials gained significant interest in photocatalysis due to their extraordinary crystalline structures and optical absorption properties. Under visible light exposure, the photocatalytic disinfection activity of Bi2MoO6/g‐C3N4 nanosheet composites is enhanced. Moreover, photocatalytic disinfection activity of the Bi2MoO6/g‐C3N4 nanosheet has been enhanced at a Bi2MoO6 loading of 20%.[451] A more comprehensive effort was made to develop a new composite of AgBr‐Ag‐Bi2WO6 and to investigate the composites’ bactericidal properties when exposed to visible light. In comparison to other binary composites, the resulting composite demonstrated efficient inactivation. The TEM photos confirmed the bacterial cell destruction, which was confirmed again by releasing potassium ions.[452] Xia et al. synthesized a monoclinic dibismuth tetraoxide (mg‐C3N4/Bi2O4) heterojunction using a simple hydrothermal method. For 1.5 h of visible light penetration, the optimum ratio of g‐C3N4/Bi2O4 in the composite (1:0.5) was capable of inactivating 6 log10 CFU mL−1 of E. coli, E. coli K‐12 was much more effective relative to g‐C3N4 (1.5 log) and m‐Bi2O4 (4 log).[453] In particular, disinfection efficiency has not improved significantly with m‐Bi2O4 material, even though excessive m‐Bi2O4 can provide a recombination center for excitons, thus decreasing the ability to separate electron–hole pairs.Removal of PollutantsAny of the conventional waste treatment methods would trigger severe ecological problems. The oxidation of toxic organic compounds was a hot subject in medicine. Recent years were seen an increase in the study and novel technologies development, including photocatalytic water photodegradation of organic compounds dependent on effective solar energy conversion.[454] Semiconductor photocatalysis is considered one of the most powerful solutions to approach the severe environmental waste and energy crisis.[455,456] The use of photocatalysis tends to become a more attractive than conventional chemical oxidation approaches to decompose poisonous substances into harmless materials. Despite the benefits of large specific surface area, high adsorption potential of organic contaminants, and good light harvesting potential, 2D photocatalysts exhibit substantial application potential in pollutant removal. Moreover, photocatalytic behavior of 2D photocatalysts also desires is increased owing to rapid recombination of photogenerated e−s and h+s. The defective establishing and hybridized 2D photocatalyst is an efficient way to inhibit charge carrier recombination and to facilitate organic contaminants degradation.[42]Photocatalytic wastewater treatment using 2D materials is a well‐known and proven technique.[457] Long carbon chains act as a capping reagent in ionic liquids, regulating crystal growth along c‐axis. Sequentially, reaction pH was changed to 11, supplying OH− to replace Br and completing the dehalogenation process in the fabrication route of Bi4O5Br2. Thus, after 120 min of irradiation with visible light, all thickness and part‐engineered Bi4O5Br2 materials have been developed and used in photocatalytic antibiotic CIP and tetracycline degradation. After illumination, CIP (75%) is photodegraded via Bi4O5Br2, while CIP (51.4%) is photodegraded via BiOBr. Moreover, Bi4O5Br2 nanosheets demonstrated a 77.8% degradation rate for tetracycline after 60 min of irradiation, which would be significantly higher than the 31.7% degradation rate for BiOBr. The altered EBE of Bi4O5Br2 was confirmed to account for the enhanced photocatalytic behavior. Bi4O5Br2's more negative CB role facilitates the formation of more active O2•‐ organisms. Upshifting CB role and VB broadening would gain charge separation efficiency. Thus, the obtained Bi4O5Br2 nanosheets demonstrated improved pollutant removal efficiency. Two bulk g‐C3N4 thermal treatments form porous g‐C3N4 nanosheets with surface carbon defects.[458] The ultrathin structure and surface defects will accelerate carrier separation between the bulk and the surface in a bidirectional fashion. Consequently, when exposed to visible light, the defective porous ultrathin g‐C3N4 demonstrated a 25.7‐fold improvement in photocatalytic activity for RhB degradation elimination. Zhou et al. used primitive hydrothermal techniques to fabricate a TiO2–MoS2 monolayer hybrid photocatalyst with a 3D‐layered structure.[179] The 3D‐layered structure comprises a TiO2 nanobelt core and a MoS2 nanosheet shell (referred to as TiO2@MoS2). It seems to have a greater potential for adsorption and a higher photocatalytic ability for degrading organic dyes such as RhB. The energy levels between MoS2 and TiO2 are supposed to be ideal for charge transfer and recombination inhibition of photon‐induced e−s and h+s. Zhang and co‐workers discovered that VBi‐O′′′ would probably enhance the DOS at CB minimum, thus improving photon reaction and photoabsorption.[459] Simultaneously, VBi‐O′′′ defects in monolayer BiO2‐x can aid in more electron–hole pair separation. Consequently, defective monolayer BiO2‐x exhibits enhanced photocatalytic behavior when exposed to UV, visible, and NIR light for RhB and phenol removal. Mashtalir et al. degraded MB (a cationic dye) and acid blue 80 (AB80) (an anionic dye) using Ti3C2Tx (Figure 31a,b).[460] UV irradiation has been used to accelerate the decay of MB and AB80. In dark, MB concentration decreases due to MB's negatively charged adsorption on Ti3C2Tx surfaces. Following UV irradiation, a significant decrease in MB and AB80 concentrations of 81 and 62% have been observed in the suspended Ti3C2Tx presence. This was observed that over a long period of time, Ti3C2Tx oxidation to TiO2 in the dissolved O2 existence was evident, which merits extensive research in this area. Similar to G‐TiO2 nanocomposite, it is hypothesized that Ti3C2Tx‐assisted TiO2 could be a trigger, promoting more development in this direction. Peng's group[224] adopted a hydrothermal route to partially oxidize Ti3C2 to fabricate a composite comprising of TiO2 with exposed {001}‐facet and Ti3C2 (Figure 31c,d).31FigureMXenes and their hybrid nanocomposites for photocatalytic pollutant degradation: a,b) Time‐dependent a) MB level and b) AB80 (b) in Ti3C2Tx. Reproduced with permission.[460] Copyright 2014, The Royal Society of Chemistry. Preparation of nanohybrids (001)TiO2/Ti3C2: c) charge transfer process, d) bandgap transition. Reproduced with permission.[224] Copyright 2016, American Chemical Society.Coupling a 2D photocatalyst with a cocatalyst is a safe way to promote organic contaminant oxidation. Due to its wide surface area, high adsorption potential, superior electron conductivity, and high thermal stability, RGO was coupled with WO3 by in situ growth of WO3 rectangular sheets on RGO.[461] The 2D/2D RGO‐WO3 process utilizes photogenerated e−s to convert O2 to •O2‐, and the photocatalyst and water combine to form the •OH radicals. Despite the strong oxidizing properties, the •O2– and •OH radicals are used to degrade MB and RhB dyes. Compared to the hydroxyl, epoxy, and carboxylic functional groups, the RGO may interact directly with heterocyclic dye molecules. Additionally, interaction between the aromatic rings of the 2D/2D RGO‐WO3 and heterocyclic dye molecules promotes the formation of hydroxyl radicals, which enhances the dye molecules’ degradation efficiency. The photodegradation rate of MB and RhB reaches 32 and 85%, respectively. The increased degradation rate of RhB is due to its higher positive potential at the COOH group than MB, which strengthens the bond between •O2−/•OH and the RhB. The above results indicate that a 2D cocatalyst with high electron conductivity and appropriate functional groups can likely improve the photodegradation efficiency of organic pollutants. Wang et al. demonstrated the intense photodegradation of RhB by Bi2O2CO3/MoS2 composites through ultraviolet light irradiation.[182] This was due to the synergy effect between MoS2 and Bi2O2CO3 cocatalyst. The 2D/2D heterostructures (AgIO3/g‐C3N4) have been successfully developed for photocatalytic waste H2O treatment after vis light exposure by Li et al.[462] The g‐C3N4 nanosheet used as polymeric organic semiconductors has demonstrated superior visible light reaction. The photoconversion efficiency of AgIO3/g‐C3N4 heterostructures for organic dye degradation was extensively greater than single AgIO3/g‐C3N4 nanosheets. Significantly, the degradation reaction rate constant of RhB was ≈22.86 times that of the synthesized AgIO3/g‐C3N4 nanosheet sample in addition to heterostructures consisting of AgIO3 and bulk g‐C3N4 (AgIO3/g‐C3N4‐B). This demonstrates 2D materials’ role in the fabrication of heterojunction photocatalysts. The Z‐scheme method is considered a perfect tactic for photocatalysis over the type II heterojunction due to the dual advantages of high redox capability and effective charge separation.A basic reflux procedure has been used to fabricate a 2D/2D BiOBr/g‐C3N4 Z‐scheme heterojunction that has been used to photocatalyzed the RhB and bisphenol A degradation.[463] The photocatalytic degradation observed and confirmed the presence of h+, •O2, and •OH as oxidative species, with •OH being the most critical species, while h+ and •O2− had a minor contribution to the photocatalytic degradation reaction. Resulting in a significant gap in the BiOBr/g‐C3N4 configuration, excited e−s can pass from the g‐C3N4 (CB) to BiOBr (VB). At the same time, h+s can transfer from BiOBr (VB) to g‐C3N4 (CB) through 2D/2D interface, referred to as type II heterojunction. Even so, when the BiOBr/g‐C3N4 system flows through a type II heterojunction, no •O2− or •OH is generated due to a lack of appropriate reduction and oxidation potentials of the BiOBr and g‐C3N4, respectively. A Z‐scheme photocatalytic system for the BiOBr/g‐C3N4 system has been developed. Thus, photoinduced e−s may pass from BiOBr (CB) to g‐C3N4 (VB) and interact with h+s. The e−s in g‐C3N4's can react with O2 molecules to form •O2−, while the h+s in BiOBr (VB) can react with H2O molecules to form •OH. This electron transfer direction benefits charge carrier separation and improve the photoconversion efficiency of the photocatalyst. Liu et al. investigated the photodegradation of organic dyes using a 2D/2D nanocomposite photocatalyst (ZnO/MoS2) derived from P‐doped ZnO nanosheets with a large specific surface area and 2D MoS2.[464] ZnO/MoS2 heterostructures with varying concentrations of MoS2 (mass ratios of 0, 0.01, 0.1, and 1 wt%) and commercial P25 have been used as photocatalysts for photodegradation measurements conducted under similar experimental conditions. Comparative results indicate that ZnO/MoS2 with a low molecular weight of MoS2 (0.1 wt%) significantly improves photocatalytic activity compared to pristine ZnO nanosheets. As the molecular weight of MoS2 has been improved from 0.01 to 0.1 wt%, the photoconversion efficiency of ZnO/MoS2 has been further increased and was decreased after the molecular weight of MoS2 was increased to 1 wt%. A high concentration of MoS2 blocks the sunlight that is being used to force photodegradation of MB, lowering the photoconversion efficiency. As a result of the interfacial effect between ZnO and MoS2, photogenerated e−s will move from the ZnO‐CB to the MoS2‐CB, significantly improving carrier separation and, thus, catalytic activity. Increased transport and separation of photogenerated h+s and e−s induced by the interfacial effect can be formed by introducing MoS2 components into MoS2/ZnO heterostructures. After being transferred to the catalyst surface, the photogenerated charges react with O2 and H2O, generating numerous reactive radicals (OH and superoxide anion radicals) that degrade dye molecules. In contrast to the heterojunction effect, P‐loading induced defects in ZnO nanosheets promote photocatalytic activity by introducing an energy level difference between bandgaps.Organic SynthesisPhotocatalytic organic synthesis was proposed as a viable method for reviving chemical reactions sustainably.[465–467] The prospect of lowering energy costs associated with chemically formed, solar light‐driven chemical transformations with the assistance of semiconductors is enormous. Semiconductors may use solar light to generate excitons or heat carriers, which can be used to simulate chemical reactions at the catalyst's surface. Moreover, owing to the quick recombination of photoinduced charge carriers and the high oxidizing potential of the h+s, photoconversion efficiency and selectivity remain insufficient for large‐scale implementation of the photocatalytic organic synthesis process. At large scale, the photocatalytic conversion's functionality and selectivity remain challenging. Weak interaction between O2 molecules and photocatalyst surfaces, especially defect‐free surfaces, is a significant factor in the ineffectiveness of photocatalytic organic formations. In addition, there is the issue of low selectivity, which can be produced from photo‐generated h+s. Produced h+s have a high oxidizing potential and apply to nonselective over oxidation.[468] It was recently shown that defective 2D materials could selectively synthesize organic compounds under mild conditions.By engineering VO into WO3 nanosheet, O2 molecules are effectively activated into superoxide radicals, thus initiating organic aerobic coupling of amines to corresponding imines.[469] HAADF‐STEM showed the surface atomic structure of WO3 that demonstrated continuous and orderly lattice fringes in defect‐deficient WO3 nanosheet but minor lattice disorder and dislocations in defect‐rich WO3 nanosheet. This finding unquestionably established the presence of different defects in two samples. According to XAFS analysis, total coordination number of W atoms in defect‐deficient WO3 is 5.4, which is slightly less than expected value of 6, suggesting a local shortage of O atoms in defect‐deficient WO3 nanosheet. The slightly improved EPR signal strength at 2.002 g in defect‐rich WO3 also showed that VO were designed into the defect‐rich WO3 nanosheet. Based on first‐principles calculations, it was determined that chemisorbed O2 to a coordinatively unsaturated W site switches to a side‐on mode upon WO3 electron charging. The deficient WO3 then donates 0.72 e−s to the adsorbed O2, raising the length of the OO bond from 1.22 to 1.47, favoring the creation of O2•− species. The technique effectively converted sunlight into the aerobic coupling of amines to their corresponding imines, achieving a six times increase in kinetic rate over defect‐deficient WO3. Xiong et al.[469] activated the oxygen molecules to initiate the effective degradation of organic pollutants by generating VO into WO3 ultrathin nanosheets. In order to fabricate defect‐rich WO3 nanosheets, the primary product (WO3·H2O) was calcined at 673 K in an N2 environment, while atmospheric conditions were adopted during calcination to generate defect‐deficient nanosheets. HAADF‐STEM results revealed a much smoother and flat surface of defect‐rich nanosheets compared to defect‐deficient WO3 nanosheets. Moreover, controlled and continuous lattice fringes were simultaneously observed through atomic‐scale HAADF‐STEM images (Figure 32).32Figurea–f) Study of morphology, g) W L3 edge EXAFS spectra are Fourier transformed concerning commercial WO3, h) at room temperature, ESR spectra, i) VO positions in WO3 lattice are depicted in this scheme, j) after irradiation with >400 nm at 298 K, cyclic analysis for defect rich WO3 in catalytic aerobic coupling of benzylamine, k) diagram depicts entire light‐driven catalytic reaction pathway. Reproduced with permission.[469] Copyright 2016, American Chemical Society.Xie and co‐workers created VO in BiOBr nanoplates to enhance excitons dissociation and molecular O2 activation.[470] According to DFT calculations, excitons become unstable as they approach VO. Furthermore, femtosecond time‐resolved transient absorption spectroscopy showed that the BiOBr sample with Vo (BiOBr‐OV) had a slightly longer photoinduced electron recovery lifetime than the BiOBr sample, allowing charge carrier migration for photocatalytic reactions. Remarkably, the superoxide radical produced on BiOBr‐OV was highly efficient and selective in aerobic oxidative coupling of amines to imines. In contrast, the imine transformation efficiency was significantly lower in BiOBr. Li and co‐workers investigated the BiOCl nanosheet's colloidal structure.[471] Single crystalline BiOCl colloidal (BiOCl C‐UT) nanosheets with a thickness of almost 3.7 nm are produced via BiCl3 hydrolysis in octadecylamine solution and in situ H2O formation via reaction in oleylamine and oleate solution. BiOCl nanosheets are also developed hydrothermally and are referred to as BiOCl H‐UT nanosheets. The wettability of as‐synthesized BiOCl nanosheets was determined using surface H2O touch angle measurements. BiOCl C‐UT nanosheets exhibited an H2O contact angle of 116.3°, indicating that they are hydrophobic. Organic ligands seem to have capped the surface of BiOCl C‐UT nanosheets during colloidal development. BiOCl H‐UT nanosheets exhibited an H2O contact angle of 0°, indicating their super hydrophilic origin. Significant differences between BiOCl C‐UT nanosheets and BiOCl H‐UT nanosheets can significantly impact photocatalytic organic conversion.Additionally, there is a high concentration of VO on BiOCl C‐UT nanosheets, resulting in heavy absorption in the visible light spectrum. To take advantage of their hydrophobicity and enhanced light‐harvesting ability, BiOCl C‐UT nanosheets demonstrated significantly enhanced photocatalytic activity for converting N‐t‐butylbenzylamine to N‐t‐butylbenzylimine. The 78% conversion ratio was observed with BiOCl C‐UT nanosheets, while BiOCl H‐UT nanosheets converted at a rate of approximately 15% after 1 h of Xe lamp irradiation. Moreover, the BiOCl C‐UT nanosheet sample was extensively used to convert secondary amines to their respective imines, improving conversion selectivity and performance. The 2D photocatalysts were demonstrated to be an excellent alternative for photocatalytic organic formation. This approach can extend perceptions of organic conversion methods and pave the way for creating additional excellent organic transformation systems.Conclusion and PerspectiveEngineered 2D materials and their hybridizations are excellent materials for elementary photocatalytic processing and have many possible commercial uses. This systematic research focused on significant progress in applying 2D materials for photocatalytic solar conversion. This study summarizes the classification and controlled fabrication process of defective 2D photocatalysts. Following that, methods for modifying the electronic structure of 2D materials and their photocatalytic properties are discussed, including surface and interface engineering and doping. In addition, additional hybridizations of 2D features, including single atoms/2D materials, QDs/2D materials, molecular/2D materials, and 2D–2D stacking compounds, are provided which will further improve photocatalytic properties. Finally, several photocatalytic applications, including H2 evolution, H2O oxidation, CO2 reduction, N2 fixation, organic synthesis, antimicrobial activity, and contaminants degradation, were analyzed with a focus on observations into structure–performance relationships. These applications included H2O oxidation, H2 evolution, CO2 reduction, N2 fixation, H2O2 production, antimicrobial activity, and organic synthesis. Different surface defect forms, such as anion–cation vacancies, multivacancies, disorders, vacancy associates, pits, and distortions, have been used to adjust the microstructure, atom coordination number, electronic structure, carrier concentration, or electrical conductivity of 2D photocatalysts and thereby improve photocatalytic efficiency. Numerous effective methods for controlling defect formation have been described, including vacuum activation, chemical reduction, ball milling, and ultraviolet irradiation.Additionally, the critical functions of surface defects in improving photocatalytic activity are suggested, including lowering the molecular activation energy, acting as an active site for direct reaction, increasing light absorption and carrier concentration, and acting as charge separation centers to facilitate surface charge separation. Numerous studies outlined in this section revealed that defective 2D materials with special electronic structures are advantageous for improved photocatalytic efficiency in processing O2, H2, CO2, and N2 reduction to selective organic synthesis ammonium and pollutant elimination. Furthermore, owing to their special benefit, 2D heterojunctions such as Type I, Type II, Z‐scheme, and Schottky heterojunction exhibit high performance in photocatalysis. Separating charge carriers from exposed active sites is critical for optimizing the photocatalytic efficiency of 2D photocatalysts. Constructing 2D/2D heterojunctions is a viable method for promoting charge carrier separation and increasing the percentage of exposed active sites, significantly improving the photocatalytic activity of 2D photocatalysts.Despite rapid development in 2D materials for photocatalysis, this field faces many obstacles. Apart from the recent advances outlined here, research in this field is still in its infancy; issues and challenges in the design, synthesis, and application of defective 2D photocatalysts remain. Though various top‐down and bottom‐up approaches were used to synthesize 2D materials beyond graphene, large‐scale preparation of 2D materials remains difficult. The mass development of 2D materials with specified surface defects would be critical for photocatalytic applications. To investigate diverse and abundant synthetic strategies for defect‐rich 2D materials with atomic‐scale thickness on a large scale, more diverse and abundant synthetic strategies should be investigated. Second, in contrast to well‐developed 2D materials like hydroxides, MOs, and sulfides for photocatalysis. Some novel 2D materials along with layer oxyhalides (e.g., Bi4VO8Cl and FeOBr), multi‐metal chalcogenides (e.g., C3N and C2N) and thiophosphates (e.g., CoPS3) should be considered. 2D materials with an intrinsic non‐vdW layer structure provide, in particular, tremendous photocatalytic potential, as ample surface atoms between bonds contribute to the formation of an excellent chemical mechanism conducive to molecular reaction chemisorption and catalytic dynamics. Thirdly, many 2D materials, especially those with a defect‐rich architecture, would be unstable physicochemically. During the storage and photoreaction processes, isolated nanosheet can endure irreversible aggregation and structural disintegration, resulting in the loss of advanced structural characteristics. Moreover, along with surface VO, certain faults would be filled by ambient H2O or O2 during long‐term photocatalytic action, negating the microenvironment's distinct benefits.Thus, methods for stabilizing these defective 2D photocatalysts must be studied. Fourth, since photogenerated e−s and h+s recombine quickly, sacrificial reagents have been used in the majority of photocatalytic reactions, including H2, O2 evolution, and CO2 reduction, to achieve high‐efficiency half‐reactions. It has been shown that using sacrificial reagents enhances the photocatalytic action of photocatalysts. Moreover, as the sacrificial reagents are consumed, the photocatalytic activity reduces, which undermines the application of 2D photocatalysts. Although forming 2D/2D heterojunctions is a viable method of enhancing charge carrier separation, more efficient 2D/2D heterojunctions are mostly needed for useful applications. Fifth, owing to rapid recombination of photogenerated electron–hole pairs, in most photocatalytic reactions, sacrificial reagents could be deployed to accomplish high‐efficiency half‐reactions, including H2 evolution, O2 evolution, and CO2 reduction. It was shown that the use of sacrificial reagents enhances the photocatalytic action of photocatalysts. Even then, as the sacrificial reagents are absorbed, the photocatalytic activity reduces, which undermines the practical application of 2D photocatalysts. Sixth, a structure–activity relationship study in defective 2D photocatalysts is insufficient; additional research is required to establish an authentic relationship between surface defects, atomic thickness, and photocatalytic activity. Via sophisticated characterization techniques, in situ observation is needed to determine active sites and mechanisms involved during the photocatalytic phase. Ultimately, most tests involving defective 2D materials for photocatalysis remain in the manual trial‐and‐error stage. Numerous 2D materials with various components and defects can show a variety of photocatalytic properties that make them play an important role in a variety of photocatalytic applications. Bi2WO6 nanosheet, for instance, is an outstanding photocatalyst for CO2 reduction but is ineffective at producing H2. A mixture of ab initio DFT studies and systematic study of photocatalytic production involving different component or defect types can be used to effectively help in the detection of novel defect 2D photocatalysts from unexpected elemental combinations and defect shapes.To summarize, tremendous progress has been made in the last few years in design of 2D material‐based heterostructures, surface alteration, morphology regulation, and element doping. Therefore, the critical challenge in photocatalysis is improving applications such as CO2 reduction, organic pollutant elimination, and solar fuel processing. Most work remains to be performed in designing and improving better photocatalysts. It can precisely tune the composition, shape, reaction sites on the surface, and bandgap of 2D material photocatalysts to allow interfacial reactions, light trapping, and charge carrier isolation and conversion. In certain cases, hybrid 2D materials resulted in decreased crystallinity and increased cracks, which is detrimental to photocatalytic behavior and charge transfer. As a result, these are critical stages in creating more efficient photocatalysts, such as the investigation of novel doping techniques and the precise regulation of the structure, surface state, and dopant distribution. The properties of photocatalysts are critical for optimizing photocatalytic performance and enabling a future paradigm shift in design principles. Over the last decade, working on 2D materials has advanced at a breakneck rate. Photocatalysts based on 2D materials can be critical in solving the environmental and energy problems associated with photochemical conversion aided by sunlight.Furthermore, the majority of photocatalysis research is still in the manual trial‐and‐error stage, with many of the reaction mechanisms unclear. Certain underdeveloped, highly efficient 2D photocatalysts can be ignored due to the limited preparation processes. With the advent of DFT computing and machine learning, increased emphasis must be placed on developing more stable and efficient 2D photocatalysts and heterojunctions in two dimensions. Likewise, environmental considerations about the solar‐powered 2D device are important for commercial applications. However, no research is being conducted on this subject at the moment. Biocompatibility evidence for 2D components used in biomedical applications may be used to approximate their environmental impact. While stable 2D binary compounds such as MoS2 and MXenes are nontoxic, unstable MXenes such as tellurene are harmful. Further investigation of the 2D materials’ long‐term environmental impact is recommended.We see the future challenges for the computational screening of materials for photocatalytic applications, mostly in the description of solvation and kinetics. The active development of implicit solvation models and their implementation into widely used software packages is expected to lead to an efficient description of the effect of the electrolyte on electronic properties and stability, including corrosion. A challenge for solvation models and computational corrosion studies is overcoming the complexity of the interaction and chemical reactions between solvated species and 2D materials. Regarding kinetics, computational methods that describe nonadiabatic dynamics and can assess the rate of electron transfer reactions and exciton dynamics are still being developed and not yet routinely applied or available in community codes. The enormous progress in the fields of 2D materials and computational methods suggests that many of those challenges will be tackled in the near future.AcknowledgementsThe authors are thankful to HEC, Pakistan for data through digital library.Conflict of InterestThe authors declare no conflict of interest.K. Huang, Z. Li, J. Lin, G. Han, P. Huang, Chem. Soc. Rev. 2018, 47, 5109.K. Khan, A. K. Tareen, M. Aslam, Y. Zhang, R. Wang, Z. Ouyang, Z. Gou, H. Zhang, Nanoscale 2019, 11, 21622.Z. Xie, Y.‐P. Peng, L. Yu, C. Xing, M. Qiu, J. Hu, H. Zhang, Sol. RRL 2020, 4, 1900400.T. Liu, X. Liu, N. Graham, W. Yu, K. Sun, J. Membr. Sci. 2020, 593, 117431.K. Khan, A. K. Tareen, M. Aslam, R. Wang, Y. Zhang, A. Mahmood, Z. Ouyang, H. Zhang, Z. Guo, J. Mater. 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Solar‐Triggered Engineered 2D‐Materials for Environmental Remediation: Status and Future Insights

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