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IntroductionNitrogen coordination transition metal single atom catalysts supported on carbon matrix (SACs‐NC) feature the maximum atomic utilization and unique electronic structure,[1–3] and show great potential in fields of energy conversion and chemical synthesis.[4–12] In order to achieve its promising application, splendid efforts had been made in developing synthetic strategy for well‐defined SACs‐NC, especially the strategy for large‐scale preparation, but still challenging.[13–19] Of the various pioneering innovations, the strategy combining metal ion impregnation and annealing (MIA) is considered as the approach that most commonly used and most amenable to scale‐up due to its easy operation and low cost.[20–27] To further optimize the MIA strategy, the formation process of SACs‐NC had also been investigated. Traditionally, metal ions are thought to suffer from the adsorption, reduction, followed by anchoring on defects of supports, that is the metal ions directly transformed into SACs without additional steps (Scheme 1A).[21,28–31] However, it is still fuzzy whether a metal single atom directly forms via metal ions with one‐ or multiple‐step process. The knowledge gap generally generated undesired metal clusters or nanoparticles (NPs), particularly at elevated temperature or introducing high metal contents.[1,33–36] This observation not only enables the loss of active sites, but also causes problems in accurately modulating coordination environment.[20,37–42] A molecular level understanding that how metal ions evolved into SACs‐NC could provide important guidance to circumvent the metal sintering, in particular when scale‐up preparation is practiced. However, the dynamic evolution process of metal atomization from ions remains obscure and is rarely discussed.1SchemeSchematic illustration of SACs formation process. A) traditional ion‐to‐SA one‐step process, B) practical ion‐NP‐SA two‐step process.In this work, we disclosed an unconventional two‐step process for the formation of SACs‐NC under the MIA approach based on the experimental investigation and DFT calculation. The metal ions initially underwent a sintering process into metal NPs, followed by the conversion of metal NPs into metal single atoms (Fe, Co, Ni, Cu) at higher temperature (Scheme 1B). We select nitrogen doped carbon (NC) derived by ZIF‐8 or guanine as the substrate materials to trap metal atoms. According to the operando TEM and XRD results, the obvious Cu NPs can be observed at the temperature range from 500 to 600 °C. Then, the Cu NPs gradually disappeared and transformed into metal single atoms at higher temperature (700–800 °C). Interestingly, when the pure carbon (XC‐72 and graphene) was adopted to replace NC as substrate materials, the Cu NPs would only grow steadily into larger Cu NPs as the temperature increased to 800 °C. This observation indicates the formation and enlargement of metal NPs can be attributed to the reducing capacity of carbon, while the nitrogen defect sites was closely related with transforming metal NPs into single atoms. Furthermore, based on the ascertained mechanism, we develop a two‐step pyrolysis strategy to prepare Cu SACs, which exhibited excellent ORR performance.Results and DiscussionCu Ion‐NP‐SA Evolution ProcessA classical dual solvent ions adsorption‐pyrolysis strategy was used to prepare transition metal single atom catalysts (Fe, Co, Ni, Cu SACs, Figure S1, Supporting Information), in which the ZIF‐8 nanocrystal with the size around 200–300 nm was selected as the precursors of NC supports (Figure S2, Supporting Information). Initially, the ion‐NP‐SA conversion process was discovered on Cu SACs. We adopted in situ ETEM techniques to identify the atomization process at the temperature range from room temperature to 800 °C. As shown in Figure 1A and Figure S3 (Supporting Information), the tiny Cu clusters and NPs can be observed at 500 °C due to the reduction and aggregation of Cu ions. When temperature increased to 600 °C, the dispersed Cu clusters grew into larger NPs (Figure 1B). Subsequently, the number of Cu NPs gradually decreased at 700 °C (Figure 1C) and then completely disappeared at 800 °C (Figure 1D). For excluding the effects of consecutive electron beam at operando conditions, ex situ TEM tests were also conducted, as shown in Figure S4 (Supporting Information). The Fast Fourier Transform (FFT) images and 3D atom‐overlapping Gaussian‐function fitting mapping of the TEM images clearly demonstrated the formation of crystalline Cu at 500 and 600 °C (Figure S4A,B, Supporting Information). The crystalline structure evolution of Cu was further confirmed by the in situ XRD patterns of Cu/ZIF (Figure 1E). A diffraction peak at about 43° attributed to Cu (111) plane appeared at 500–700 °C and disappeared at 800 °C, in line with the in situ ETEM results and ex situ XRD patterns (Figure S5, Supporting Information). Note that the ZIF‐8 still maintained initial structure before 600 °C, as further verified by Raman spectra (Figure S6, Supporting Information). No significant difference was observed on Cu/ZIF between 600 and 800 °C in XPS spectra of C 1s and N 1s (Figure S7, Supporting Information) and EPR spectra (Figure S8, Supporting Information). Furthermore, the Cu element presented zero valent state at 500–700 °C and subsequently transformed into Cuδ+ (0 < δ < 2) at 800 °C (Figure 1F). These results significantly demonstrated the formation and transformation of Cu NPs under pyrolysis process.1FigureCu ion‐NP‐SA two‐step evolution process during annealing. A–D) In situ ETEM images of Cu/ZIF from 500 to 800 °C, dark field. E) In situ XRD patterns of Cu/ZIF from 200 to 800 °C. F) Cu 2p XPS spectra of Cu/ZIFs pyrolyzed at different temperatures. G) HAADF‐TEM image of Cu/ZIF‐800, coupled with EDS mappings. H) AC HAADF‐STEM image of Cu/ZIF‐800. Inset, corresponding intensity profile. I) Cu K edge X‐ray absorption near‐edge structure (XANES) spectrum of Cu/ZIF‐800. J) k3‐weighted χ(k) function of EXAFS spectrum of Cu/ZIF‐800.The resulted Cu/ZIF‐800 exhibited uniform element distribution of Cu, N, and C, according to EDS mapping analysis (Figure 1G). Aberration‐corrected HAADF‐STEM images showed that atomically dispersed Cu single atoms with a size of 1.24 Å can be discerned on supports (Figure 1H and Figure S9, Supporting Information). For further confirming the formation of Cu SACs, we conducted X‐ray absorption fine structure (XAFS) testing (Figure 1I,J). The white‐line intensity of Cu/ZIF‐800 located between Cu foil and CuO, indicating the partial oxidation state between Cu (0) and Cu (II). This observation was greatly consistent with the XPS analysis above. The XAFS curves at R‐space and corresponding fitting analysis showed that only one dominant peak at about 1.48 Å was detected, which can be attributed to the Cu‐N4 coordination structure (Figure 1J and Table S1, Supporting Information).[26,43] These results provided solid evidences in verifying the formation of atomically dispersed Cu single atoms. Together, the formation of Cu SACs was confirmed to go through ion‐NP‐SA two‐step process rather than direct ion‐to‐SA one‐step process. At higher temperature such as 1200 °C, Cu NPs can be formed again (Figure S10, Supporting Information), which might be associated with the elimination of N species (Figure S11, Supporting Information).Evolution Process of Fe, Co, and NiInspired by the observation above, we also probed the dynamic evolution process of other transition metals, such as Fe, Co, and Ni, to testify the universality of ion‐NP‐SA two‐step conversion rules. As shown in Figure 2A,E,I, the obvious Fe, Co, and Ni NPs was observed at 600 °C, as further attested by corresponding XRD patterns, SAED patterns and EDS mapping images (Figures S12–S14, Supporting Information). Then, the Fe, Co, and Ni NPs disappeared at 800 °C (Figure 2B,F,G). The as‐obtained Fe/ZIF‐800, Co/ZIF‐800, and Ni/ZIF‐800 showed uniformly dispersed Fe (Figure S15, Supporting Information), Co (Figure S16, Supporting Information), and Ni (Figure S17, Supporting Information) over whole architecture, confirming the existence of Fe, Co, and Ni in ultimate samples (pyrolysis at 800 °C). HAADF‐STEM images exhibited the Fe, Co, and Ni single atoms were homogeneously distributed on supports, indicating the transformation from NPs to SAs (Figure 2C,G,K and Figures S15–S17, Supporting Information). XAFS results demonstrated that only FeN, CoN, and NiN bonds can be discerned at R‐space, further verifying the formation of Fe, Co, and Ni SACs (Figure 2D,H,L and Figures S18–S20, Supporting Information).[29,44,45] These results indicated the two‐step ion‐NP‐SA evolution was not a unique observation but a universal phenomenon for MIA synthesis.2FigureEvolution process of Fe, Co, and Ni during annealing. A,E,I) TEM images of Fe/ZIF‐600, Co/ZIF‐600, and Ni/ZIF‐600. Inset, HRTEM images of the black spots. B,F,J) TEM images of Fe/ZIF‐800, Co/ZIF‐800, and Ni/ZIF‐800. Inset, corresponding HAADF‐TEM images. C,G,K) AC HAADF‐STEM images of Fe/ZIF‐800, Co/ZIF‐800, and Ni/ZIF‐800. Inset, intensity profiles of the bright spots. k3‐weighted χ(k) function of EXAFS spectrum of D) Fe/ZIF‐800, H) Co/ZIF‐800, and L) Ni/ZIF‐800.Mechanism Study Based on Carbon‐Based SupportsConsidering the generation of plentiful volatilization species during ZIF‐8 pyrolysis (Figure S11, Supporting Information), prefabricated graphene‐like N‐doped carbon (NG) derived by guanine at 1000 °C was adopted as supports to revalidate the ion‐NP‐SA conversion. This prefabricated support exhibited high thermal stability at high temperature, as shown in the TG analysis (Figure S21A, Supporting Information). As shown in Figure 3A–D and Figure S21 (Supporting Information), Cu ions still aggregated into Cu NPs at 600 °C (Cu/NG‐600), which was then transformed into Cu SAs at 800 °C (Cu/NG‐800), indicating the ion‐NP‐SA conversion was not determined by volatile species. Also, control experiments were carried out on N‐free carbon black XC‐72 (CB) and graphene (GP). Interestingly, the Cu ions were reduced to small Cu NPs at 600 °C and subsequently grew into larger Cu NPs at 800 °C (Figure 3E,F and Figure S22, Supporting Information), failing to transform Cu NPs to Cu SAs. On the basis of these results, one can reasonably speculate the formation and development of metal NPs can be attributed to the reducing capacity of carbon under thermal treatment, while the nitrogen defect sites play key effects on transforming metal NPs into SAs.3FigureTheoretical studies based on graphene‐like N‐doped carbon (NG) and carbon black (CB). A) TEM images of Cu/NG‐600 and Gu/NG‐800. B) XRD patterns of Cu/NG‐600 and Cu/NG‐800. C) Cu K edge XANES spectrum of Cu/NG‐800. D) k3‐weighted χ(k) function of EXAFS spectrum of Cu/NG‐800. E) TEM images of Cu/CB‐600 and Gu/CB‐800. F) XRD patterns of Cu/CB‐600 and Cu/CB‐800. G) Evolution model from Cu3 cluster to CuN4 coordination structure (top), and the calculated energy diagram (bottom). H) evolution model from 3Cu(NO3)2 to Cu3 cluster (top), and the calculated energy diagram (bottom).To further shed light on the mechanism, we constructed a typical Cu3 clusters to investigate the ion‐NP‐SA conversion process on NC supports (Figure 3G,H). The formation of Cu3 clusters from Cu ions and transformation of Cu3 clusters into CuN4 moieties showed obvious exothermicity of 0.16 and 3.29 eV, respectively. This indicated the whole ion‐NP‐SA conversion was a thermo‐spontaneous process. On the other hand, the NP‐to‐SA process exhibited a larger barrier of 0.56 eV to overcome than that of ion‐to‐NP process (0.29 eV). In this case, the ion‐to‐NP conversion became primary at relatively low‐temperature (≈500–600 °C) and NP‐to‐SA conversion dominated at high temperature (≈700–800 °C).Application of Ion‐NP‐SA ProcessIn view of the discovery of ion‐NP‐SA process, the formation of SACs under MIA was originated from the thermal migration of metal atoms. Therefore, we designed a two‐step pyrolysis strategy to access SACs, as shown in Figure 4A. Firstly, excessive Cu precursors were impregnated on N‐doped carbon in order to generate plenty of Cu NPs at 900 °C (denoted as Cu NPs/NC), as characterized by TEM (Figure 4B) and XRD (Figure 4C). Subsequently, extra N‐doped carbon was mixed with Cu NPs/NC and then calcined at 900 °C. After 2 h under the thermal treatment, smaller Cu particles than that in Cu NPs/NC were observed (Cu NP/NC‐2 h, Figure S23, Supporting Information), indicating the dispersion of Cu particles during the two‐step pyrolysis. Figure 4D,E shows the TEM and XRD of the obtained sample after 4 h of calcination (Cu SACs/NC), in which Cu NPs were not observed. As shown in Figure 4F, heavier isolated Cu atoms can be observed in its aberration‐corrected HAADF‐STEM image. And, only the CuN bond was detected in its XAFS curve, which strongly proved the formation of Cu SACs (Figure 4G and Figure S24, Supporting Information). The unsaturated Cu–N coordination (≈CuN3) was demonstrated by the detailed fitting results of the Cu K edge EXAFS (Table S1, Supporting Information).4FigureApplication of the ion‐NP‐SA process. A) schematic illustration of two‐step pyrolysis strategy. B) TEM, SAED, and EDS mapping images of Cu NPs/NC. C) XRD pattern of Cu NPs/NC. D) TEM, SAED, and EDS mapping images of Cu SACs/NC. E) XRD pattern of Cu SACs/NC. F) AC HAADF‐STEM image of Cu SACs/NC. G) k3‐weighted χ(k) function of EXAFS spectrum of Cu SACs/NC. H) Polarization curves of Pt/C and Cu SACs/NC in ORR application. I) The i–t chronoamperometric response of Pt/C and Cu SACs/NC.The catalytic performance of Cu SACs/NC was evaluated in the oxygen reduction reaction (ORR). As a result, Cu SACs/NC exhibited excellent ORR activity with a half‐wave (E1/2) value of 0.87 V, 50 and 70 mV higher than that of Pt/C and Cu NPs/NC (Figure 4H and Figure S25, Supporting Information). The rotating ring‐disk electrode (RRDE) test revealed the near four electron ORR pathway and low H2O2 yields for Cu SACs/NC (Figure S26, Supporting Information). The robust stability of Cu SACs/NC was confirmed by the long‐term test (Figure 4I). Furthermore, the XRD pattern, TEM image and EXAFS spectrum of the used catalyst indicated the maintaining of the structure of N coordinated Cu SACs (Figure S27, Supporting Information). Based on the structure identification, contrast ORR experiment of Cu NP and previously splendid works,[46–48] the excellent ORR performance of Cu SACs/NC can be attributed to its unsaturated Cu‐N coordination single atom sites, which enables the modulation of ORR dynamics over Cu.ConclusionIn summary, we discovered the ion‐NP‐SA evolution process of transition metals (Fe, Co, Ni, Cu) during ions impregnation‐pyrolysis synthesis. The formation of SACs supported on NC was ascertained as a two‐step process, that was initially metal sintering into NPs (500–600 °C), followed by the conversion of NPs into metal single atoms (Fe, Co, Ni, Cu SAs) at higher temperature (700–800 °C). This discovery reveals the formation and transformation of metal NPs during the formation of SACs, which will provide valuable guidance to rationally develop synthesis strategies of SACs.Experimental SectionPreparation of M/ZIFs (M = Fe, Co, Ni, Cu)100 mg of ZIF‐8 powder was dispersed in 10 mL n‐hexane under ultrasound for 15 min at room temperature. Then, 50 µL of 0.4 m metal nitrate aqueous solution (Fe(NO3)3·9H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O or Cu(NO3)2·3H2O) was added into the above solution with stirring for 6 h. After centrifugation and stoving, the obtained powders were labeled as M/ZIF. The pyrolyzed M/ZIFs were denoted as M/ZIF‐X, where X corresponded to the pyrolysis temperatures.Preparation of NG and Cu/NGThe powder of guanine was calcined at 1000 °C for 2 h under argon atmosphere (denoted as NG). 100 mg of NG powder was dispersed in 10 mL n‐hexane under ultrasound for 15 min at room temperature. Then, 25 µL of 0.4 m Cu(NO3)2·3H2O aqueous solution was added into the above solution under ultrasound for 6 h. The mixed suspension was centrifuged and dried in vacuum at 80 °C. The obtained Cu/NG was calcined at 600 and 800 °C for 1 h under argon atmosphere, respectively (labeled as Cu/NG‐600 and Cu/NG‐800).Preparation of Cu/CB100 mg of XC‐72 (CB) powder was dispersed in 10 mL n‐hexane under ultrasound for 15 min at room temperature. Then, 25 µL of 0.4 m Cu(NO3)2·3H2O aqueous solution was added into the above solution under ultrasound for 6 h. The mixed suspension was centrifuged and dried in vacuum at 80 °C. Next, the Cu/CB was calcined at 600 and 800 °C for 1 h under argon atmosphere, respectively (labeled as Cu/CB‐600 and Cu/CB‐800).Preparation of Cu NPs/NC and Cu SACs/NCThe NC (N‐doped carbon) support was acquired by pyrolyzing ZIF‐8 at 900 °C for 1 h under argon atmosphere. Then, 0.4 m Cu(NO3)2·3H2O aqueous solution was added in NC support, to prepare the Cu/NC mixture with 15 wt% Cu loading. After impregnation and maturation, the prepared sample was dried at 60 °C under vacuum overnight, which was then calcined at 900 °C for 1 h under argon atmosphere and denoted as Cu NPs/NC. Next, 3 g NC powder and 1 g Cu NPs/NC were mixed and ground. This mixture was calcined at 900 °C for 4 h under argon atmosphere (labeled as Cu SACs/NC).AcknowledgementsZ.H. and Y.W. contributed equally to this work. This work was supported by the National Natural Science Foundation of China (51873134, 21902150), Natural Science Basic Research Program of Shaanxi (2022JM‐018), Natural Science Foundation of Shaanxi Province (2018JQ5058), and Scientific Research Program Funded by Shaanxi Provincial Education Department (20JK0945). The authors thank the photoemission endstations beamline 1W1B station in the Beijing Synchrotron Radiation Facility (BSRF), BL14W1 at the Shanghai Synchrotron Radiation Facility for help with the characterizations. This work is supported by a project of the DICP‐DENS laboratory for in‐situ microscopy applications.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.S. Ji, Y. Chen, X. Wang, Z. Zhang, D. Wang, Y. Li, Chem. Rev. 2020, 120, 11900.Y. Qu, Z. Li, W. Chen, Y. Lin, T. Yuan, Z. Yang, C. Zhao, J. Wang, C. Zhao, X. Wang, F. Zhou, Z. Zhuang, Y. Wu, Y. Li, Nat. Catal. 2018, 1, 781.J. Gu, M. Jian, L. Huang, Z. Sun, A. Li, Y. Pan, J. Yang, W. Wen, W. Zhou, Y. Lin, H.‐J. Wang, X. Liu, L. Wang, X. Shi, X. Huang, L. Cao, S. Chen, X. Zheng, H. Pan, J. Zhu, S. Wei, W.‐X. Li, J. Lu, Nat. Nanotechnol. 2021, 16, 1141.X. Wu, Q. Wang, S. Yang, J. 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Advanced Science – Wiley
Published: Apr 1, 2023
Keywords: nanoparticles; single atom catalysts; transition metals
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