Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Degradation of Methylene Blue by Hot Electrons Transfer in SnSe

Degradation of Methylene Blue by Hot Electrons Transfer in SnSe IntroductionThe thermoelectric, piezoelectric, and photoelectric effects are widely present in nature and can convert the thermal, mechanical and optical energy into electrical power by producing hole‐electron pairs and built‐in electric filed.[1–5] The hole‐electron pairs generated can also accelerate chemical reactions under certain conditions, and the corresponding catalysis effects of thermoelectric, piezoelectric, and photoelectric have received widespread attention.[6–11] It has been a long‐vision goal to explore an environmentally friendly, non‐polluting, and efficient way to catalyze the degradation of organic matter.[12,13] Fundamentally, the degradation of organic matter is accompanied by the breaking of chemical bonds, which requires activation of free energy. When the activation free energy is changed, the rate of organic degradation can be accelerated.[14] In general, piezoelectric, photoelectric, and thermoelectric catalysis can all accelerate catalytic reactions by providing additional electron‐hole pairs to change the activation free energy.[15–18] However, piezoelectrics and photocatalysis require more demanding external conditions, so their wide industrial use is limited. Instead, there are temperature differences almost everywhere, and thermoelectric materials provide the feasibility of using this waste heat from differences in temperature.[19] Therefore, thermoelectric materials can be selected as thermal catalysts to degrade dye‐contained wastewater, and those with high Seebeck coefficient should be selected to generate high voltage and thus achieve high thermal catalytic efficiency. In this work, SnSe was selected as a thermal catalyst to degrade wastewater.SnSe, a tin‐based chalcogenide compound,[20] has attracted a lot of attention because it does not contain heavy metal ions and has excellent thermoelectric properties.[21–24] Both the p‐type and n‐type SnSe crystals have high carrier mobility and excellent thermoelectric properties.[25,26] Heat transfer in the p‐type and n‐type SnSe crystals is almost the same, but the transmission direction is different. Without doping, SnSe samples are n‐type semiconductors with low conductivity when Sn is compensated; When Sn vacancy occurs, SnSe is the most commonly p‐type.[27–29] It has a low carrier concentration at room temperature. As the temperature increases, the temperature difference between the dye wastewater solution and the external environment increases.[30] The charge carrier concentration increases with more electrons and holes produced. In addition, if the thermal energy is equal to or greater than the bandgap of SnSe, electrons may be lifted from the valence band to the conduction band and leave holes in the valence band. Since textile wastewater containing methylene blue usually has the characteristics of high temperature (50–80 °C), SnSe can use this characteristic to decompose dye molecules into harmless products.[15,31] Therefore, SnSe is very suitable for thermal degradation of wastewater.In this study, we collected the UV absorption spectra of methylene blue, SnSe, and their mixtures respectively at different timings to study the thermal degradation efficiency of methylene blue under SnSe catalysis at different temperatures (0, 25, 50, 75, 100 °C). Methylene blue has strong UV absorption characteristics, so it is convenient to detect the concentration of methylene blue by intensity of UV absorption spectra. If the intensity of UV absorption spectra decreased, it means that the methylene blue was degraded. The results show that the degradation efficiency is greatly increased at higher temperatures with the help of SnSe, and the degradation effect of SnSe to methylene blue is highest at 100 °C. Further temperature difference thermoelectric measurements and first‐principles calculations of SnSe show that the potential gradient caused by the temperature difference in SnSe forms a cathode and anode in solution, which catalyzes the degradation of methylene blue. Therefore, SnSe has great application prospects in the field of wastewater degradation.Results and DiscussionDue to the excellent thermoelectric properties of SnSe, hole‐electron pairs are separated in SnSe along the temperature gradient, resulting in a large number of electrons at the edge and a large number of holes at the other edge. The combination of H2O and heated electrons produce the hydroxyl radical,[30] which is a key factor to decompose methylene blue by extracting H from organic compounds and eventually forming CO2, H2O, NH4+, and SO42−. Hydroxyl radicals can strongly destroy the structure of methylene blue (as shown in Figure 1).1FigureThe schematic of the degradation process of methylene blue by the thermocatalyst of SnSe. Built‐in electric field of SnSe is induced by temperature difference. Carriers accumulated at the cathode react with water and produce the hydroxyl radical, which can degrade the methylene into a series of inorganic substances.The crystal structure of SnSe was studied by XRD and Raman spectra. Figure 2a shows the XRD pattern of SnSe, its peak shape, and peak position are relatively obvious, and there are almost no stray peaks and all strong peaks correspond to the (400) crystal plane, which belongs to the orthogonal structural phase with the space group Pnma, XRD characterization showed that no other impurities were produced in the stripped sample, and no new phase was generated. It shows that the stripped snse sample has higher purity, better crystallinity, better conductivity, and better thermoelectric properties, which can further enhance the catalytic degradation effect of SnSe on methylene blue. The peaks are indexed as (201), (400), (311), and (203) for the angular positions at 25.3°, 31.3°, 37.7°, and 64.7°, respectively verified by jcpd#32‐1382.[32] The crystal structure was further confirmed by HR‐TEM shown in Figure 2e. In order to study the atomic vibration mode and crystalline quality of SnSe, we measured the Raman spectra of SnSe. Peaks at 69, and 152 cm−1 originate from out‐of‐plane vibration, while the peaks at 98 and 181 cm−1 originate from in‐plane vibration. This is consistent with the previous literature.[33,34] The observation of the vibration modes proves the high crystallinity of SnSe.2Figurea) X‐ray diffraction (XRD) pattern of the SnSe. b) Raman spectra of SnSe sample. c) The EDS spectrum of the SnSe sample. d) TEM image of a SnSe crystal. e) HRTEM image of SnSe crystal. f) selected area electron diffraction (SAED) micrograph of SnSe.Figure 2d,e shows the TEM image of the SnSe film with low and high magnification. Figure 2d shows the monolayer structure of the SnSe crystal. We can see that the van der Waals force leads to the formation of ultra‐thin SnSe aggregates. From the high‐resolution TEM image, we can derive that the crystal plane spacing is 0.28 nm, which corresponds to the (400) plane. No other phases were found in HRTEM images, indicating that the stripped SnSe is high quality without obvious oxidation behavior, and good crystallinity, which further proves the characterization results of XRD The corresponding SAED pattern in Figure 2f clearly shows the high crystallinity of the crystal, which is consistent with the results of XRD and TEM. According to the EDS spectrum shown in Figure 2c, the selenium content is ≈1.5:1, showing that SnSe tends to be selenium rich at room temperature, Sn vacancy appears, which is a p‐type SnSe, consistent with the results of previous reports,[34] since SnSe tends to be selenium rich at low temperature (T < 600 K), while at high temperature (T > 600 K), low selenium component is preferred, that is, its actual stoichiometry changes with temperature.[35,36]Before the thermal degradation test, the stability of the methylene blue solution at different temperatures was studied. Figure 3a shows the UV absorption spectra of methylene blue after different periods of time at different temperatures, and its UV absorption curve stays almost the same. UV absorption spectra at other temperatures are shown in Figure S1a–e (Supporting Information). As shown in Figure 3a, the UV absorption peak of methylene blue does not change over time at different temperatures. At 660 nm, there is almost no difference in absorbance, which shows that the self‐degradation rate of methylene blue remains unchanged.3Figurea) UV–vis absorption spectra of methylene blue at different times at different temperatures (0, 25, 50, 75, and 100 °C). b) Variation of the UV absorption peak of methylene blue and SnSe at different temperatures(0, 25, 50, 75, and 100 °C). c) Measured the relationship between voltage and temperature, the red line shows the linear fit.An estimation of the optical bandgap can be calculated by the following equations.[37,38]1A=K(hν−Eg)m/2hν\[\begin{array}{*{20}{c}}{A = \frac{{K{{(h\nu - {E_g})}^{m/2}}}}{{h\nu }}}\end{array}\]Where A is the absorbance, K is a constant, and m = 1 stands for direct transition and m = 2 for indirect transition. The (Ahν)2∼hν curves for the products were drawn in Figure S3a (Supporting Information). The bandgap of as‐prepared SnSe is estimated to be 0.61 eV from the UV–vis absorption spectra.In order to further verify the effect of temperature on the degradation of methylene blue by SnSe, we measured the UV absorption spectra of the mixed solution of SnSe and methylene blue at different temperatures. Figure 3b showed the UV absorption spectra of the mixed solution of methylene blue and SnSe at different temperatures. The UV absorption spectra at other temperatures are shown in Figure S2a–e (Supporting Information). It is apparent that the UV absorption peak of methylene blue and SnSe mixed solutions gradually decreased with the change of time, while the peak decreased more dramatically with the increase of temperature, which showed that SnSe had the degradation effect on methylene blue. The higher the temperature, the more obvious the degradation effect. The degradation efficiency was the best at 100 °C.Further research findings this enhanced degradation efficiency with temperature arise from the thermoelectric effect of SnSe. The solution is stirred throughout the methylene blue degradation test, which will allow the temperature of the water in the solution to converge quickly. The extremely low thermal conductivity of SnSe makes the temperature rise of the SnSe nanoparticles unable to keep up with the temperature rise of the solution. This results in a temperature difference on the SnSe nanoparticles. Similar thermocatalytic processes and methods in solution have also been reported in ref. [16]. The higher the temperature difference, the more the concentration of electron‐hole pairs and hydroxyl radicals on the catalyst surface. The faster methylene blue structure is destroyed, so the degradation rate is the fastest at 100 °C. The relation between temperature and voltage potential at the edge of SnSe provides a direct evidence for the temperature‐dependent separated carriers. As illustrated in Figure 3c, we measured the SnSe thermoelectric potential at different temperatures. Considering the difficulty of directly measuring the voltage of SnSe in the nanoscales. We used a SnSe block with a size of 1 cm for the thermoelectric test. Two thin copper electrodes were placed on each side of SnSe block along the b‐axis. One of the electrodes is placed on the hot plate and the other electrode is away from the hot plate. While heating the hot plate, a thermal gradient will emerge in the SnSe block. Therefore, a strong electric potential will appear at both ends of SnSe, due to its excellent thermoelectric properties. Voltage measurement by connecting an oscilloscope. In Figure 3c, it is obvious that the thermoelectric voltage potential increases linearly as the temperature difference increases. This directly indicates that temperature differences can produce carrier accumulation at the edges of SnSe materials, which assists the degeneration of methylene blue. In fact, in this measurement, the actual temperature difference is smaller than the predicted temperature difference because the heat at the cold end does not conduct away quickly. This can lead to an underestimation of the measured thermoelectric performance. In addition, the thermoelectric effect of copper contributes to the measurement results, but considering the extremely low thermoelectric coefficient of copper, it can be essentially considered that the measured electric potential is all derived from the SnSe thermoelectric effect.Although the massive SnSe can also degrade methylene blue, theoretically speaking, the stripped micro‐layered SnSe has a better degradation effect on methylene blue.[39] Compared with commercial bulk SnSe, the ultra‐thin SnSe obtained after stripping has unique electronic structure and physical properties. It has smaller specific surface area and better catalytic degradation effect. However, it is still difficult to measure the thermoelectric properties of micro‐state SnSe at present, so we choose to measure its thermoelectric properties with bulk SnSe.As illustrated in Figure 3b, with the temperature difference increasing, the reduction of UV absorption is increased. At 100 °C, the UV absorption rapidly decreased by 15.0% in half an hour, which indicates that the methylene blue is decomposed at a higher temperature. It is worth noting that the UV absorption basically stopped decreasing after 30 min at 100 °C, The main possibility stems from the fact that the solution and SnSe nanoparticles tend to the same temperature and the temperature gradient of SnSe decreases over time, thus the number of separated hole electron pairs is also reduced. This result is also can be corroborated with the catalytic efficiency tests in fixed temperature solutions in ref. [16]. However, the decrease in the concentration of methylene blue in the solution due to the reduced opportunity for contact could also cause a decrease in degradation efficiency since the thermoelectric effect of SnSe plays the pivotal role in controlling the degradation of methylene blue.To further explore the mechanism of SnSe hot carriers‐assisted organic degradation, we performed Density functional theory (DFT) to calculate the distribution of hot carriers at different temperatures in SnSe. Microscopically, temperature affects the intensity of the thermal motion of carriers in a material, at higher temperatures, carriers have a higher probability to occupy higher energy orbitals, which is consistent with the Fermi–Dirac distribution2f(ε)=1exp[(ε−εf)/kT]+1\[\begin{array}{*{20}{c}}{f(\varepsilon ) = \frac{1}{{\exp [(\varepsilon - {\varepsilon _f}){\rm{/}}kT] + 1}}}\end{array}\]where 𝜀 is energy, 𝜀f is the fermi level. Considering that SnSe is a natural p‐type, we set the Fermi energy level to −0.2 eV. The number of holes is calculated by the Multiplication of DOS and the Fermi–Dirac distribution3N=∫−∞∞f(ε)·g(ε)dε\[\begin{array}{*{20}{c}}{N = \mathop \smallint \limits_{ - \infty }^\infty f\left( \varepsilon \right)\cdotg\left( \varepsilon \right)d\varepsilon }\end{array}\]where g(𝜀) is DOS. The calculated band structure and density of states (DOS) are illustrated in Figure 4a. The calculation results of Equation 3 are shown in Figure 4b and Figure S4 (Supporting Information). It is very clear that there are a greater number of carriers distributed in the high‐energy region at 375 K compared to 275 K, labeled in the diagram with high‐energy excess carriers. The energy distribution of carriers at other temperatures is represented in Figure S3 (Supporting Information), it can be observed that as the temperature gradually rises, more and more hot carriers are distributed in the high‐energy region. This result coincides well with our experiment.4Figurea) The band structure of SnSe and density of states distribution. b) Carrier distribution at different temperatures, the excess high energy carriers at 375 K is illustrated.ConclusionsIn general, we demonstrated that SnSe has an excellent thermocatalytic effect and can effectively accelerate the degradation of methylene blue. By performing the UV absorption measurement, we found that, with increasing temperature, the degradation efficiency of methylene blue was significantly improved by the catalytic effect of SnSe. In contrast, there was no significant degradation of the methylene blue solution without SnSe at different temperatures. Further thermoelectric effect tests and DFT calculations showed that the temperature difference caused a built‐in electric field in SnSe, forming cathodes and anodes at the ends of SnSe particles. This provided a large number of high‐energy carriers for the degradation of methylene blue, thus accelerating the degradation of methylene blue at higher temperatures. Our work provides a new direction toward environmentally friendly organic degradation and wastewater treatment with thermocatalytics.Experimental SectionChemical and MaterialsSnSe was purchased from alpha Elsa chemical (China) Co., Ltd. (Shanghai, China), methylene blue was purchased from Ron reagent (Shanghai, China), and N‐methylpyrrolidone (NMP) was purchased from sigma Aldrich (Shanghai) Trading Co., Ltd. (Shanghai, China). Ultrapure water was obtained through Millipore pure water filters (Millipore Q, Billerica, USA).Liquid Phase Stripping of SnseSnSe (0.04 g ) was weighed into a glass bottle and 10 mL N‐methylpyrrolidone (NMP) was added, the bottle mouth was sealed and the bottle was sonicated in the ice water mixture for 6 h, then it was put into a centrifuge (3000 r, 10 min), the supernatant was taken and continued to centrifuge it (8000 r, 10 min), the precipitated water was taken out, and it was washed twice to obtain the stripped SnSe.UV Absorption Spectrum TestThe stripped SnSe was put into a glass bottle, 10 mL methylene blue was added and heated at different temperatures. Ice was added to water to form an ice water mixture, and it reached 0 °C. Then the glass bottle was put into the ice water mixture, and the UV absorption spectrum of SnSe and methylene blue mixed solution was measured at different times (0, 15, 30, 45, and 60 min). A water bath was used to heat at 25 and 50 °C, and then the glass bottle was put into a water bath pan to measure the UV absorption spectrum of SnSe and methylene blue mixed solution at different times. In order to avoid water volatilization at high temperatures and difficulty in controlling temperature, an oil bath was used for heating at 75 and 100 °C, and then the glass bottle was put into a water bath pan to measure the UV absorption spectrum of SnSe and methylene blue mixed solution at different times.DFT CalculationThe DFT was performed to calculate the energy distribution of hot carriers in SnSe. In calculating the DOS and band structure of SnSe, a 3 × 4 × 3 Monkhorst–Pack mesh grid was used to sample the Brillouin zone. The generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) was used for the exchange‐correlation potential.[40–42] The plane‐wave basis cutoff energy was 350 eV. SCF (Self Consistent Field) energy tolerance was 2 × 10−6 eV per atom. The first principle calculations gave the detailed distribution of occupied states of SnSe in energy space so that the specific energy distribution of carriers at different temperatures was calculated in the main text.AcknowledgementsL.F. and X.S. contributed equally to this work. This work is supported by the National Science Foundations of China (No. 62274093, No. 61991431), the Excellent Youth Foundation of Jiangsu Scientific Committee (BK20211538), and the National Basic Research Program of China (2018YFA0209100).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available in the supplementary material of this article.X.‐L. Shi, J. Zou, Z.‐G. Chen, Chem. Rev. 2020, 120, 7399.R. L. Woolley, Nature 1954, 174, 566.Y. Yan, L. D. Geng, H. Liu, H. Leng, X. Li, Y. U. Wang, S. Priya, Nat. Commun. 2022, 13, 3565.C. Covaci, A. Gontean, Sensors 2020, 20, 3512.F. Teng, K. Hu, W. Ouyang, X. Fang, Adv. Mater. 2018, 30, 1706262.B. Anasori, M. R. Lukatskaya, Y. Gogotsi, Nat. Rev. Mater. 2017, 2, 16098.A. Achour, J. Liu, P. Peng, C. Shaw, Z. Huang, ACS Catal. 2018, 8, 10164.C. Xu, P. Ravi Anusuyadevi, C. Aymonier, R. Luque, S. Marre, Chem. Soc. Rev. 2019, 48, 3868.S. Li, Z. Zhao, J. Zhao, Z. Zhang, X. Li, J. Zhang, ACS Appl. Nano Mater. 2020, 3, 1063.M. B. Starr, X. Wang, Sci. Rep. 2013, 3, 2160.M. Wang, B. Wang, F. Huang, Z. Lin, Angew. Chem. Int. Ed. 2019, 58, 7526.F. Keppler, R. Eiden, V. Niedan, J. Pracht, H. F. Schöler, Nature 2000, 403, 298.B. Wild, N. Shakhova, O. Dudarev, A. Ruban, D. Kosmach, V. Tumskoy, T. Tesi, H. Grimm, I. Nybom, F. Matsubara, H. Alexanderson, M. Jakobsson, A. Mazurov, I. Semiletov, Ö. Gustafsson, Nat. Commun. 2022, 13, 5057.Y. Jiao, Y. Qiu, L. Zhang, W.‐G. Liu, H. Mao, H. Chen, Y. Feng, K. Cai, D. Shen, B. Song, X.‐Y. Chen, X. Li, X. Zhao, R. M. Young, C. L. Stern, M. R. Wasielewski, R. D. Astumian, W. A. Goddard, J. F. Stoddart, Nature 2022, 603, 265.Y. Zhao, C. Deng, D. Tang, L. Ding, Y. Zhang, H. Sheng, H. Ji, W. Song, W. Ma, C. Chen, J. Zhao, Nat. Catal. 2021, 4, 684.Y.‐J. Lin, I. Khan, S. Saha, C.‐C. Wu, S. R. Barman, F.‐C. Kao, Z.‐H. Lin, Nat. Commun. 2021, 12, 180.A. Achour, K. Chen, M. J. Reece, Z. Huang, Adv. Energy Mater. 2018, 8, 1701430.Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov, T. F. Jaramillo, Science 2017, 355, aad4998.H. Li, J. Gong, J. C. Li, X. Zhang, C. Tang, H. Yao, Q. Ding, Chin. Chem. Lett. 2020, 31, 2275.X. Z. Li, J. Xia, L. Wang, Y. Y. Gu, H. Q. Cheng, X. M. Meng, Nanoscale 2017, 9, 14558.M. Jin, Z. Chen, X. Tan, H. Shao, G. Liu, H. Hu, J. Xu, B. Yu, H. Shen, J. Xu, H. Jiang, Y. Pei, J. Jiang, ACS Energy Lett. 2018, 3, 689.L.‐D. Zhao, C. Chang, G. Tan, M. G. Kanatzidis, Energy Environ. Sci. 2016, 9, 3044.Z.‐G. Chen, X. Shi, L.‐D. Zhao, J. Zou, Prog. Mater. Sci. 2018, 97, 283.L. D. Zhao, S. H. Lo, Y. Zhang, H. Sun, G. Tan, C. Uher, C. Wolverton, V. P. Dravid, M. G. Kanatzidis, Nature 2014, 508, 373.C. Chang, G. Tan, J. He, M. G. Kanatzidis, L.‐D. Zhao, Chem. Mater. 2018, 30, 7355.Y. Wang, W.‐D. Liu, X.‐L. Shi, M. Hong, L.‐J. Wang, M. Li, H. Wang, J. Zou, Z.‐G. Chen, Chem. Eng. J. 2020, 391, 123513.P. P. Shang, J. Dong, J. Pei, F. H. Sun, Y. Pan, H. Tang, B. P. Zhang, L. D. Zhao, J. F. Li, Research 2019, 2019, 9253132.X. Wang, J. Xu, G. Liu, Y. Fu, Z. Liu, X. Tan, H. Shao, H. Jiang, T. Tan, J. Jiang, Appl. Phys. Lett. 2016, 108, 083902.L. Su, T. Hong, D. Wang, S. Wang, B. Qin, M. Zhang, X. Gao, C. Chang, L.‐D. Zhao, Mater. Today Phys. 2021, 20, 1004255.Y. Zhiqiang, S. S. C. Chuang, Appl Catal B 2008, 83, 277.X. Luo, S. Zhang, X. Lin, J. Hazard. Mater. 2013, 260, 112.W. J. Baumgardner, J. J. Choi, Y. F. Lim, T. Hanrath, J. Am. Chem. Soc. 2010, 132, 9519.L. Hao, Y. Du, Z. Wang, Y. Wu, H. Xu, S. Dong, H. Liu, Y. Liu, Q. Xue, Z. Han, K. Yan, M. Dong, Nanoscale 2020, 12, 7358.Y. Huang, L. Li, Y. H. Lin, C. W. Nan, J. Phys. Chem. C 2017, 121, 17530.X. Gong, Y. Wang, Q. Hong, J. Liu, C. Yang, H. Zou, Y. Zhou, D. Huang, H. Wu, Z. Zhou, B. Zhang, X. Zhou, Spectrochim. Acta, Part A 2022, 265, 120375.K. Patel, G. Solanki, K. Patel, V. Pathak, P. Chauhan, Eur. Phys. J. B 2019, 92, 200.S. Yan, K. Shen, Y. Zhang, Y. Zhang, Z. Xiao, J. Nanosci. Nanotechnol. 2009, 9, 4886.G. H. Chandra, J. N. Kumar, N. M. Rao, S. Uthanna, J. Cryst. Growth 2007, 306, 68.L. Li, Z. Chen, Y. Hu, X. Wang, T. Zhang, W. Chen, Q. Wang, J. Am. Chem. Soc. 2013, 135, 1213.G. Kresse, J. Furthmüller, Phys. Rev. B 1996, 54, 11169.G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758.J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Advanced Materials Interfaces Wiley

Degradation of Methylene Blue by Hot Electrons Transfer in SnSe

Loading next page...
 
/lp/wiley/degradation-of-methylene-blue-by-hot-electrons-transfer-in-snse-4rP1BY8axR
Publisher
Wiley
Copyright
© 2023 Wiley‐VCH GmbH
eISSN
2196-7350
DOI
10.1002/admi.202202207
Publisher site
See Article on Publisher Site

Abstract

IntroductionThe thermoelectric, piezoelectric, and photoelectric effects are widely present in nature and can convert the thermal, mechanical and optical energy into electrical power by producing hole‐electron pairs and built‐in electric filed.[1–5] The hole‐electron pairs generated can also accelerate chemical reactions under certain conditions, and the corresponding catalysis effects of thermoelectric, piezoelectric, and photoelectric have received widespread attention.[6–11] It has been a long‐vision goal to explore an environmentally friendly, non‐polluting, and efficient way to catalyze the degradation of organic matter.[12,13] Fundamentally, the degradation of organic matter is accompanied by the breaking of chemical bonds, which requires activation of free energy. When the activation free energy is changed, the rate of organic degradation can be accelerated.[14] In general, piezoelectric, photoelectric, and thermoelectric catalysis can all accelerate catalytic reactions by providing additional electron‐hole pairs to change the activation free energy.[15–18] However, piezoelectrics and photocatalysis require more demanding external conditions, so their wide industrial use is limited. Instead, there are temperature differences almost everywhere, and thermoelectric materials provide the feasibility of using this waste heat from differences in temperature.[19] Therefore, thermoelectric materials can be selected as thermal catalysts to degrade dye‐contained wastewater, and those with high Seebeck coefficient should be selected to generate high voltage and thus achieve high thermal catalytic efficiency. In this work, SnSe was selected as a thermal catalyst to degrade wastewater.SnSe, a tin‐based chalcogenide compound,[20] has attracted a lot of attention because it does not contain heavy metal ions and has excellent thermoelectric properties.[21–24] Both the p‐type and n‐type SnSe crystals have high carrier mobility and excellent thermoelectric properties.[25,26] Heat transfer in the p‐type and n‐type SnSe crystals is almost the same, but the transmission direction is different. Without doping, SnSe samples are n‐type semiconductors with low conductivity when Sn is compensated; When Sn vacancy occurs, SnSe is the most commonly p‐type.[27–29] It has a low carrier concentration at room temperature. As the temperature increases, the temperature difference between the dye wastewater solution and the external environment increases.[30] The charge carrier concentration increases with more electrons and holes produced. In addition, if the thermal energy is equal to or greater than the bandgap of SnSe, electrons may be lifted from the valence band to the conduction band and leave holes in the valence band. Since textile wastewater containing methylene blue usually has the characteristics of high temperature (50–80 °C), SnSe can use this characteristic to decompose dye molecules into harmless products.[15,31] Therefore, SnSe is very suitable for thermal degradation of wastewater.In this study, we collected the UV absorption spectra of methylene blue, SnSe, and their mixtures respectively at different timings to study the thermal degradation efficiency of methylene blue under SnSe catalysis at different temperatures (0, 25, 50, 75, 100 °C). Methylene blue has strong UV absorption characteristics, so it is convenient to detect the concentration of methylene blue by intensity of UV absorption spectra. If the intensity of UV absorption spectra decreased, it means that the methylene blue was degraded. The results show that the degradation efficiency is greatly increased at higher temperatures with the help of SnSe, and the degradation effect of SnSe to methylene blue is highest at 100 °C. Further temperature difference thermoelectric measurements and first‐principles calculations of SnSe show that the potential gradient caused by the temperature difference in SnSe forms a cathode and anode in solution, which catalyzes the degradation of methylene blue. Therefore, SnSe has great application prospects in the field of wastewater degradation.Results and DiscussionDue to the excellent thermoelectric properties of SnSe, hole‐electron pairs are separated in SnSe along the temperature gradient, resulting in a large number of electrons at the edge and a large number of holes at the other edge. The combination of H2O and heated electrons produce the hydroxyl radical,[30] which is a key factor to decompose methylene blue by extracting H from organic compounds and eventually forming CO2, H2O, NH4+, and SO42−. Hydroxyl radicals can strongly destroy the structure of methylene blue (as shown in Figure 1).1FigureThe schematic of the degradation process of methylene blue by the thermocatalyst of SnSe. Built‐in electric field of SnSe is induced by temperature difference. Carriers accumulated at the cathode react with water and produce the hydroxyl radical, which can degrade the methylene into a series of inorganic substances.The crystal structure of SnSe was studied by XRD and Raman spectra. Figure 2a shows the XRD pattern of SnSe, its peak shape, and peak position are relatively obvious, and there are almost no stray peaks and all strong peaks correspond to the (400) crystal plane, which belongs to the orthogonal structural phase with the space group Pnma, XRD characterization showed that no other impurities were produced in the stripped sample, and no new phase was generated. It shows that the stripped snse sample has higher purity, better crystallinity, better conductivity, and better thermoelectric properties, which can further enhance the catalytic degradation effect of SnSe on methylene blue. The peaks are indexed as (201), (400), (311), and (203) for the angular positions at 25.3°, 31.3°, 37.7°, and 64.7°, respectively verified by jcpd#32‐1382.[32] The crystal structure was further confirmed by HR‐TEM shown in Figure 2e. In order to study the atomic vibration mode and crystalline quality of SnSe, we measured the Raman spectra of SnSe. Peaks at 69, and 152 cm−1 originate from out‐of‐plane vibration, while the peaks at 98 and 181 cm−1 originate from in‐plane vibration. This is consistent with the previous literature.[33,34] The observation of the vibration modes proves the high crystallinity of SnSe.2Figurea) X‐ray diffraction (XRD) pattern of the SnSe. b) Raman spectra of SnSe sample. c) The EDS spectrum of the SnSe sample. d) TEM image of a SnSe crystal. e) HRTEM image of SnSe crystal. f) selected area electron diffraction (SAED) micrograph of SnSe.Figure 2d,e shows the TEM image of the SnSe film with low and high magnification. Figure 2d shows the monolayer structure of the SnSe crystal. We can see that the van der Waals force leads to the formation of ultra‐thin SnSe aggregates. From the high‐resolution TEM image, we can derive that the crystal plane spacing is 0.28 nm, which corresponds to the (400) plane. No other phases were found in HRTEM images, indicating that the stripped SnSe is high quality without obvious oxidation behavior, and good crystallinity, which further proves the characterization results of XRD The corresponding SAED pattern in Figure 2f clearly shows the high crystallinity of the crystal, which is consistent with the results of XRD and TEM. According to the EDS spectrum shown in Figure 2c, the selenium content is ≈1.5:1, showing that SnSe tends to be selenium rich at room temperature, Sn vacancy appears, which is a p‐type SnSe, consistent with the results of previous reports,[34] since SnSe tends to be selenium rich at low temperature (T < 600 K), while at high temperature (T > 600 K), low selenium component is preferred, that is, its actual stoichiometry changes with temperature.[35,36]Before the thermal degradation test, the stability of the methylene blue solution at different temperatures was studied. Figure 3a shows the UV absorption spectra of methylene blue after different periods of time at different temperatures, and its UV absorption curve stays almost the same. UV absorption spectra at other temperatures are shown in Figure S1a–e (Supporting Information). As shown in Figure 3a, the UV absorption peak of methylene blue does not change over time at different temperatures. At 660 nm, there is almost no difference in absorbance, which shows that the self‐degradation rate of methylene blue remains unchanged.3Figurea) UV–vis absorption spectra of methylene blue at different times at different temperatures (0, 25, 50, 75, and 100 °C). b) Variation of the UV absorption peak of methylene blue and SnSe at different temperatures(0, 25, 50, 75, and 100 °C). c) Measured the relationship between voltage and temperature, the red line shows the linear fit.An estimation of the optical bandgap can be calculated by the following equations.[37,38]1A=K(hν−Eg)m/2hν\[\begin{array}{*{20}{c}}{A = \frac{{K{{(h\nu - {E_g})}^{m/2}}}}{{h\nu }}}\end{array}\]Where A is the absorbance, K is a constant, and m = 1 stands for direct transition and m = 2 for indirect transition. The (Ahν)2∼hν curves for the products were drawn in Figure S3a (Supporting Information). The bandgap of as‐prepared SnSe is estimated to be 0.61 eV from the UV–vis absorption spectra.In order to further verify the effect of temperature on the degradation of methylene blue by SnSe, we measured the UV absorption spectra of the mixed solution of SnSe and methylene blue at different temperatures. Figure 3b showed the UV absorption spectra of the mixed solution of methylene blue and SnSe at different temperatures. The UV absorption spectra at other temperatures are shown in Figure S2a–e (Supporting Information). It is apparent that the UV absorption peak of methylene blue and SnSe mixed solutions gradually decreased with the change of time, while the peak decreased more dramatically with the increase of temperature, which showed that SnSe had the degradation effect on methylene blue. The higher the temperature, the more obvious the degradation effect. The degradation efficiency was the best at 100 °C.Further research findings this enhanced degradation efficiency with temperature arise from the thermoelectric effect of SnSe. The solution is stirred throughout the methylene blue degradation test, which will allow the temperature of the water in the solution to converge quickly. The extremely low thermal conductivity of SnSe makes the temperature rise of the SnSe nanoparticles unable to keep up with the temperature rise of the solution. This results in a temperature difference on the SnSe nanoparticles. Similar thermocatalytic processes and methods in solution have also been reported in ref. [16]. The higher the temperature difference, the more the concentration of electron‐hole pairs and hydroxyl radicals on the catalyst surface. The faster methylene blue structure is destroyed, so the degradation rate is the fastest at 100 °C. The relation between temperature and voltage potential at the edge of SnSe provides a direct evidence for the temperature‐dependent separated carriers. As illustrated in Figure 3c, we measured the SnSe thermoelectric potential at different temperatures. Considering the difficulty of directly measuring the voltage of SnSe in the nanoscales. We used a SnSe block with a size of 1 cm for the thermoelectric test. Two thin copper electrodes were placed on each side of SnSe block along the b‐axis. One of the electrodes is placed on the hot plate and the other electrode is away from the hot plate. While heating the hot plate, a thermal gradient will emerge in the SnSe block. Therefore, a strong electric potential will appear at both ends of SnSe, due to its excellent thermoelectric properties. Voltage measurement by connecting an oscilloscope. In Figure 3c, it is obvious that the thermoelectric voltage potential increases linearly as the temperature difference increases. This directly indicates that temperature differences can produce carrier accumulation at the edges of SnSe materials, which assists the degeneration of methylene blue. In fact, in this measurement, the actual temperature difference is smaller than the predicted temperature difference because the heat at the cold end does not conduct away quickly. This can lead to an underestimation of the measured thermoelectric performance. In addition, the thermoelectric effect of copper contributes to the measurement results, but considering the extremely low thermoelectric coefficient of copper, it can be essentially considered that the measured electric potential is all derived from the SnSe thermoelectric effect.Although the massive SnSe can also degrade methylene blue, theoretically speaking, the stripped micro‐layered SnSe has a better degradation effect on methylene blue.[39] Compared with commercial bulk SnSe, the ultra‐thin SnSe obtained after stripping has unique electronic structure and physical properties. It has smaller specific surface area and better catalytic degradation effect. However, it is still difficult to measure the thermoelectric properties of micro‐state SnSe at present, so we choose to measure its thermoelectric properties with bulk SnSe.As illustrated in Figure 3b, with the temperature difference increasing, the reduction of UV absorption is increased. At 100 °C, the UV absorption rapidly decreased by 15.0% in half an hour, which indicates that the methylene blue is decomposed at a higher temperature. It is worth noting that the UV absorption basically stopped decreasing after 30 min at 100 °C, The main possibility stems from the fact that the solution and SnSe nanoparticles tend to the same temperature and the temperature gradient of SnSe decreases over time, thus the number of separated hole electron pairs is also reduced. This result is also can be corroborated with the catalytic efficiency tests in fixed temperature solutions in ref. [16]. However, the decrease in the concentration of methylene blue in the solution due to the reduced opportunity for contact could also cause a decrease in degradation efficiency since the thermoelectric effect of SnSe plays the pivotal role in controlling the degradation of methylene blue.To further explore the mechanism of SnSe hot carriers‐assisted organic degradation, we performed Density functional theory (DFT) to calculate the distribution of hot carriers at different temperatures in SnSe. Microscopically, temperature affects the intensity of the thermal motion of carriers in a material, at higher temperatures, carriers have a higher probability to occupy higher energy orbitals, which is consistent with the Fermi–Dirac distribution2f(ε)=1exp[(ε−εf)/kT]+1\[\begin{array}{*{20}{c}}{f(\varepsilon ) = \frac{1}{{\exp [(\varepsilon - {\varepsilon _f}){\rm{/}}kT] + 1}}}\end{array}\]where 𝜀 is energy, 𝜀f is the fermi level. Considering that SnSe is a natural p‐type, we set the Fermi energy level to −0.2 eV. The number of holes is calculated by the Multiplication of DOS and the Fermi–Dirac distribution3N=∫−∞∞f(ε)·g(ε)dε\[\begin{array}{*{20}{c}}{N = \mathop \smallint \limits_{ - \infty }^\infty f\left( \varepsilon \right)\cdotg\left( \varepsilon \right)d\varepsilon }\end{array}\]where g(𝜀) is DOS. The calculated band structure and density of states (DOS) are illustrated in Figure 4a. The calculation results of Equation 3 are shown in Figure 4b and Figure S4 (Supporting Information). It is very clear that there are a greater number of carriers distributed in the high‐energy region at 375 K compared to 275 K, labeled in the diagram with high‐energy excess carriers. The energy distribution of carriers at other temperatures is represented in Figure S3 (Supporting Information), it can be observed that as the temperature gradually rises, more and more hot carriers are distributed in the high‐energy region. This result coincides well with our experiment.4Figurea) The band structure of SnSe and density of states distribution. b) Carrier distribution at different temperatures, the excess high energy carriers at 375 K is illustrated.ConclusionsIn general, we demonstrated that SnSe has an excellent thermocatalytic effect and can effectively accelerate the degradation of methylene blue. By performing the UV absorption measurement, we found that, with increasing temperature, the degradation efficiency of methylene blue was significantly improved by the catalytic effect of SnSe. In contrast, there was no significant degradation of the methylene blue solution without SnSe at different temperatures. Further thermoelectric effect tests and DFT calculations showed that the temperature difference caused a built‐in electric field in SnSe, forming cathodes and anodes at the ends of SnSe particles. This provided a large number of high‐energy carriers for the degradation of methylene blue, thus accelerating the degradation of methylene blue at higher temperatures. Our work provides a new direction toward environmentally friendly organic degradation and wastewater treatment with thermocatalytics.Experimental SectionChemical and MaterialsSnSe was purchased from alpha Elsa chemical (China) Co., Ltd. (Shanghai, China), methylene blue was purchased from Ron reagent (Shanghai, China), and N‐methylpyrrolidone (NMP) was purchased from sigma Aldrich (Shanghai) Trading Co., Ltd. (Shanghai, China). Ultrapure water was obtained through Millipore pure water filters (Millipore Q, Billerica, USA).Liquid Phase Stripping of SnseSnSe (0.04 g ) was weighed into a glass bottle and 10 mL N‐methylpyrrolidone (NMP) was added, the bottle mouth was sealed and the bottle was sonicated in the ice water mixture for 6 h, then it was put into a centrifuge (3000 r, 10 min), the supernatant was taken and continued to centrifuge it (8000 r, 10 min), the precipitated water was taken out, and it was washed twice to obtain the stripped SnSe.UV Absorption Spectrum TestThe stripped SnSe was put into a glass bottle, 10 mL methylene blue was added and heated at different temperatures. Ice was added to water to form an ice water mixture, and it reached 0 °C. Then the glass bottle was put into the ice water mixture, and the UV absorption spectrum of SnSe and methylene blue mixed solution was measured at different times (0, 15, 30, 45, and 60 min). A water bath was used to heat at 25 and 50 °C, and then the glass bottle was put into a water bath pan to measure the UV absorption spectrum of SnSe and methylene blue mixed solution at different times. In order to avoid water volatilization at high temperatures and difficulty in controlling temperature, an oil bath was used for heating at 75 and 100 °C, and then the glass bottle was put into a water bath pan to measure the UV absorption spectrum of SnSe and methylene blue mixed solution at different times.DFT CalculationThe DFT was performed to calculate the energy distribution of hot carriers in SnSe. In calculating the DOS and band structure of SnSe, a 3 × 4 × 3 Monkhorst–Pack mesh grid was used to sample the Brillouin zone. The generalized gradient approximation (GGA) of Perdew, Burke, and Ernzerhof (PBE) was used for the exchange‐correlation potential.[40–42] The plane‐wave basis cutoff energy was 350 eV. SCF (Self Consistent Field) energy tolerance was 2 × 10−6 eV per atom. The first principle calculations gave the detailed distribution of occupied states of SnSe in energy space so that the specific energy distribution of carriers at different temperatures was calculated in the main text.AcknowledgementsL.F. and X.S. contributed equally to this work. This work is supported by the National Science Foundations of China (No. 62274093, No. 61991431), the Excellent Youth Foundation of Jiangsu Scientific Committee (BK20211538), and the National Basic Research Program of China (2018YFA0209100).Conflict of InterestThe authors declare no conflict of interest.Data Availability StatementThe data that support the findings of this study are available in the supplementary material of this article.X.‐L. Shi, J. Zou, Z.‐G. Chen, Chem. Rev. 2020, 120, 7399.R. L. Woolley, Nature 1954, 174, 566.Y. Yan, L. D. Geng, H. Liu, H. Leng, X. Li, Y. U. Wang, S. Priya, Nat. Commun. 2022, 13, 3565.C. Covaci, A. Gontean, Sensors 2020, 20, 3512.F. Teng, K. Hu, W. Ouyang, X. Fang, Adv. Mater. 2018, 30, 1706262.B. Anasori, M. R. Lukatskaya, Y. Gogotsi, Nat. Rev. Mater. 2017, 2, 16098.A. Achour, J. Liu, P. Peng, C. Shaw, Z. Huang, ACS Catal. 2018, 8, 10164.C. Xu, P. Ravi Anusuyadevi, C. Aymonier, R. Luque, S. Marre, Chem. Soc. Rev. 2019, 48, 3868.S. Li, Z. Zhao, J. Zhao, Z. Zhang, X. Li, J. Zhang, ACS Appl. Nano Mater. 2020, 3, 1063.M. B. Starr, X. Wang, Sci. Rep. 2013, 3, 2160.M. Wang, B. Wang, F. Huang, Z. Lin, Angew. Chem. Int. Ed. 2019, 58, 7526.F. Keppler, R. Eiden, V. Niedan, J. Pracht, H. F. Schöler, Nature 2000, 403, 298.B. Wild, N. Shakhova, O. Dudarev, A. Ruban, D. Kosmach, V. Tumskoy, T. Tesi, H. Grimm, I. Nybom, F. Matsubara, H. Alexanderson, M. Jakobsson, A. Mazurov, I. Semiletov, Ö. Gustafsson, Nat. Commun. 2022, 13, 5057.Y. Jiao, Y. Qiu, L. Zhang, W.‐G. Liu, H. Mao, H. Chen, Y. Feng, K. Cai, D. Shen, B. Song, X.‐Y. Chen, X. Li, X. Zhao, R. M. Young, C. L. Stern, M. R. Wasielewski, R. D. Astumian, W. A. Goddard, J. F. Stoddart, Nature 2022, 603, 265.Y. Zhao, C. Deng, D. Tang, L. Ding, Y. Zhang, H. Sheng, H. Ji, W. Song, W. Ma, C. Chen, J. Zhao, Nat. Catal. 2021, 4, 684.Y.‐J. Lin, I. Khan, S. Saha, C.‐C. Wu, S. R. Barman, F.‐C. Kao, Z.‐H. Lin, Nat. Commun. 2021, 12, 180.A. Achour, K. Chen, M. J. Reece, Z. Huang, Adv. Energy Mater. 2018, 8, 1701430.Z. W. Seh, J. Kibsgaard, C. F. Dickens, I. Chorkendorff, J. K. Nørskov, T. F. Jaramillo, Science 2017, 355, aad4998.H. Li, J. Gong, J. C. Li, X. Zhang, C. Tang, H. Yao, Q. Ding, Chin. Chem. Lett. 2020, 31, 2275.X. Z. Li, J. Xia, L. Wang, Y. Y. Gu, H. Q. Cheng, X. M. Meng, Nanoscale 2017, 9, 14558.M. Jin, Z. Chen, X. Tan, H. Shao, G. Liu, H. Hu, J. Xu, B. Yu, H. Shen, J. Xu, H. Jiang, Y. Pei, J. Jiang, ACS Energy Lett. 2018, 3, 689.L.‐D. Zhao, C. Chang, G. Tan, M. G. Kanatzidis, Energy Environ. Sci. 2016, 9, 3044.Z.‐G. Chen, X. Shi, L.‐D. Zhao, J. Zou, Prog. Mater. Sci. 2018, 97, 283.L. D. Zhao, S. H. Lo, Y. Zhang, H. Sun, G. Tan, C. Uher, C. Wolverton, V. P. Dravid, M. G. Kanatzidis, Nature 2014, 508, 373.C. Chang, G. Tan, J. He, M. G. Kanatzidis, L.‐D. Zhao, Chem. Mater. 2018, 30, 7355.Y. Wang, W.‐D. Liu, X.‐L. Shi, M. Hong, L.‐J. Wang, M. Li, H. Wang, J. Zou, Z.‐G. Chen, Chem. Eng. J. 2020, 391, 123513.P. P. Shang, J. Dong, J. Pei, F. H. Sun, Y. Pan, H. Tang, B. P. Zhang, L. D. Zhao, J. F. Li, Research 2019, 2019, 9253132.X. Wang, J. Xu, G. Liu, Y. Fu, Z. Liu, X. Tan, H. Shao, H. Jiang, T. Tan, J. Jiang, Appl. Phys. Lett. 2016, 108, 083902.L. Su, T. Hong, D. Wang, S. Wang, B. Qin, M. Zhang, X. Gao, C. Chang, L.‐D. Zhao, Mater. Today Phys. 2021, 20, 1004255.Y. Zhiqiang, S. S. C. Chuang, Appl Catal B 2008, 83, 277.X. Luo, S. Zhang, X. Lin, J. Hazard. Mater. 2013, 260, 112.W. J. Baumgardner, J. J. Choi, Y. F. Lim, T. Hanrath, J. Am. Chem. Soc. 2010, 132, 9519.L. Hao, Y. Du, Z. Wang, Y. Wu, H. Xu, S. Dong, H. Liu, Y. Liu, Q. Xue, Z. Han, K. Yan, M. Dong, Nanoscale 2020, 12, 7358.Y. Huang, L. Li, Y. H. Lin, C. W. Nan, J. Phys. Chem. C 2017, 121, 17530.X. Gong, Y. Wang, Q. Hong, J. Liu, C. Yang, H. Zou, Y. Zhou, D. Huang, H. Wu, Z. Zhou, B. Zhang, X. Zhou, Spectrochim. Acta, Part A 2022, 265, 120375.K. Patel, G. Solanki, K. Patel, V. Pathak, P. Chauhan, Eur. Phys. J. B 2019, 92, 200.S. Yan, K. Shen, Y. Zhang, Y. Zhang, Z. Xiao, J. Nanosci. Nanotechnol. 2009, 9, 4886.G. H. Chandra, J. N. Kumar, N. M. Rao, S. Uthanna, J. Cryst. Growth 2007, 306, 68.L. Li, Z. Chen, Y. Hu, X. Wang, T. Zhang, W. Chen, Q. Wang, J. Am. Chem. Soc. 2013, 135, 1213.G. Kresse, J. Furthmüller, Phys. Rev. B 1996, 54, 11169.G. Kresse, D. Joubert, Phys. Rev. B 1999, 59, 1758.J. P. Perdew, K. Burke, M. Ernzerhof, Phys. Rev. Lett. 1996, 77, 3865.

Journal

Advanced Materials InterfacesWiley

Published: Apr 1, 2023

Keywords: degradation; methylene blue; SnSe; thermoelectric effect

References