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Cite this: Chem. Commun., 2014, 50, 12726 Received 22nd July 2014, Accepted 1st September 2014
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Phase-dependent photocatalytic H2 evolution of copper zinc tin sulfide under visible light† Zhi-Xian Chang,ab Wen-Hui Zhou,*a Dong-Xing Kou,a Zheng-Ji Zhoua and Si-Xin Wu*a
DOI: 10.1039/c4cc05654j www.rsc.org/chemcomm
CZTS exhibited apparently phase-dependent photocatalytic H2 evolution under visible light. Possible factors for the phase-dependent photocatalytic activity of CZTS were discussed in detail.
The increasing energy crisis and environmental problems force us to explore the utilization of clean, abundant and maintainable solar energy. Photocatalytic H2 evolution from water is one of the most promising approaches to convert solar energy to green chemical energy. Since Fujishima and Honda found that light irradiation could drive H2 evolution on the TiO2 photoanode,1 numerous semiconductor photocatalysts were developed for H2 evolution from water, such as SrTiO3, NaNbO3, Ga2O3, CdS and ZnO:GaN etc.2 However, most of these photocatalysts are of wide band gaps, and only absorb UV and partial visible illumination, thus cause low efficient utilization of solar irradiation. Multicomponent chalcogenide semiconductors, namely Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 and Cu2ZnSn(S1 xSex)4, recently received increasing attention, because of their applications as candidate materials for absorbing layers3 and counter electrodes4 in photovoltaic devices. Recently, a graphene-like ultrathin kesterite CZTS was claimed to possess activity to generate H2 from water with a H2 evolution rate higher than those of commercial CdS and N-doped TiO2 nanoparticles under UV-Vis irradiation.5 This finding undoubtedly opens up a new application of CZTS in photocatalytic fields. Tsang used the nano-sized metallic component to modify wurtzite CZTS and thus promoted H2 evolution significantly. The Au–CZTS core–shell nanoparticles showed an enhanced H2 evolution rate, which was about 6.4 and 2.5 as high as that of CZTS nanoplates and nanorods.6 By forming CZTS– metal (Au or Pt) heterostructured nanoparticles, the enhanced a
The Key Laboratory for Special Functional Materials of MOE, Henan University, Kaifeng 475004, China. E-mail: [email protected], [email protected]; Fax: +86-371-23881358; Tel: +86-371-23881358 b Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc05654j
12726 | Chem. Commun., 2014, 50, 12726--12729
photocatalytic activity towards degradation of rhodamine B and H2 evolution by water splitting was observed when compared to pure CZTS.7 In the above studies, it was noted that the photocatalytic activity of CZTS was related to not only the morphologies (nanosheets, nanoplates, nanorods and nanoparticles) but also the crystalline phases (kesterite and wurtzite), which display more crucial roles. Generally, the crystalline phase determines the electric structure and surface atomic configuration, thus significantly influences the photocatalytic activity of a photocatalyst. To date, the photocatalytic activities of various photocatalysts with different crystalline phases have been widely studied on NaNbO3, CdS, Ga2O3, TiO2, AgGaO2 etc.2b–d,8 A preliminary comparison of the photocatalytic properties of wurtzite and kesterite CZTS has been provided by Cabot.7 However, the comparison is not totally fair due to the large differences in particle size between wurtzite and kesterite CZTS. Here, we are particularly interested in the influence of crystalline phases of CZTS on the photocatalytic H2 evolution performance. Wurtzite CZTS was firstly synthesized via a facile one-pot method. By using a simple thermal process, mixed wurtzite–kesterite and pure kesterite CZTS were obtained. CZTS has been found to exhibit phase-dependent photocatalytic H2 evolution under visible light. Possible factors for the phasedependent photocatalytic activity of CZTS were discussed. As shown in Fig. 1a, the as-synthesized CZTS shows spindle shape with an average diameter of 25 5 nm and a length of 100 15 nm. The high-resolution TEM image of single spindle indicates highly crystalline structure of CZTS (Fig. 1b). The lattice fringe with an interplanar spacing of 0.32 nm is ascribed to (002) planes of wurtzite CZTS, indicating the growth along the [001] direction.3e The XRD pattern (Fig. 1c) confirms the wurtzite structure of as-synthesized CZTS. The diffraction peaks at 2y = 26.9, 28.3, 30.4, 39.3, 47.4, 51.3 and 56.4 can be indexed to (100), (002), (101), (102), (110), (103) and (112) planes, matching well with the previously reported hexagonal wurtzite CZTS.9 The Raman peak (Fig. 1d) at 335 cm 1 corresponds well to the single phase of wurtzite CZTS. And we cannot probe the presence of other impurities such as ZnS, SnS and Cu2S.10
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Fig. 1 (a) TEM image, (b) HRTEM image, (c) XRD pattern and (d) Raman shift of as-synthesized wurtzite CZTS nanospindles.
Elemental analysis by ICP-AES indicated a Cu/Zn/Sn metal ratio of 1.68 : 1.13 : 1, which is slightly Cu-poor and Zn-rich. This composition is consciously adopted, because it makes contribution to single CZTS and is close to the best value for CZTS-based solar cells.11 The mixed wurtzite–kesterite and pure kesterite CZTS were obtained by a thermal process. Fig. 2 shows the crystallographic phases of as-obtained CZTS determined by XRD. The XRD patterns suggest that wurtzite CZTS undergoes gradual phase transition to kesterite CZTS upon increasing the annealed temperature from 350 1C to 550 1C. As the temperature increases, the intensities of wurtzite (100), (101), (102) and (103) decrease gradually. Meanwhile, the small peak at 2y = 33.0, which can be indexed to the (200) plane of kesterite CZTS (JCPDS no. 26-0575), appears and continually increases. At 550 1C, the diffraction peaks are completely indexed to kesterite CZTS. The co-existing peaks for wurtzite [(100), (101), (102) and (103)] and kesterite (200) suggested that the samples were of the wurtzite–kesterite mixed-phase. Fig. 3 shows the TEM images of as-obtained CZTS with different phase composition. Apparently, thermal pretreatment
Fig. 2 XRD patterns of different samples prepared by annealed as-synthesized wurtzite CZTS at different temperatures.
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Fig. 3 TEM images of as-obtained CZTS at (a) 350 1C, (b) 450 1C and (d) 550 1C (c), which are recorded from the sample annealed at 450 1C, show the contact boundary between two wurtzite nanospindles.
at 350 1C has no influence on the morphology, as well as the crystalline phase of CZTS (Fig. 1a and 3a). The sample annealed at 450 1C shows some irregular grains, but also some crystallites similar to the original nanospindles (Fig. 3b). Fig. 3c shows that two crystal lattices with different orientations integrate together [wurtzite CZTS, d(002) spacing of 0.32 nm3e,9a and kesterite CZTS, d(112) spacing of 0.31 nm12]. The samples annealed at 550 1C display larger grains (Fig. 3d). These results suggest that the nanospindles undergo the grain growth of the neighbour boundary during the thermal process. In the wurtzite nanospindles, the sidewalls are formed by {100} planes, which are expected to be relatively stable. The phase transition from wurtzite to kesterite could be seen as shift of sulphur atoms on {100} planes, which results in the formation of instable atoms.13 These instable atoms might act as the active sites, which unavoidably brings the boundary growth of the newly formed crystal to form large grains. It is reported that the kesterite surfaces are {112} planes preferentially.14 Due to the different orientation of wurtzite {100} and kesterite {112}, the shape of wurtzite nanospindles will not be preserved. Thus, the shape of CZTS changed from nanospindles into the irregular large grains. Based on this observation, the wurtzite CZTS was expected to be very suitable for the fabrication of large grain thin films. UV-Visible absorption spectra of various samples are shown in Fig. S1 (ESI†). All samples exhibit broad absorbance in the UV-Vis region. From the inset in Fig. S1 (ESI†), Eg for wurtzite CZTS is determined to be 1.43 eV, which is consistent with that of the reported value.3e The Eg values for different mixed wurtzite–kesterite and pure kesterite CZTS are calculated to be 1.38, 1.39 and 1.39 eV, respectively, which are slightly lower than that of wurtzite CZTS. It has been reported that the Eg values of wurtzite Cu2ZnSn(S1 xSex)4 alloys are always higher than those of kesterite ones, despite that their differences are
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Fig. 4 (a) H2 evolution rates of various annealed CZTS, and (b) three consecutive 12 h cycles of H2 evolution of the single-phase kesterite CZTS (CZTS550).
always small (less than 0.1 eV).3f The relatively larger Eg values for the wurtzite phase could be explained by the symmetry of valence band maximum (VBM) and conduction band minimum (CBM) states.15 Fig. 4a summarizes the H2 evolution rates of various CZTS samples. The H2 evolution rate of wurtzite CZTS is 21.2 mmol g 1 h 1. The photocatalytic activity for H2 evolution was enhanced with the increase of kesterite CZTS. The H2 evolution rate of pure kesterite CZTS reaches 54.8 mmol g 1 h 1, which is almost 3 times as high as that of original wurtzite CZTS nanospindles. The stability of photocatalytic activity of kesterite CZTS was also examined. The H2 evolution of kesterite CZTS increases linearly without obvious change in three cycles of 12 hours (Fig. 4b), indicating the stable photocatalytic activity of kesterite CZTS. For further investigation, the specific surface areas for all samples were determined to be 39.6, 25.8, 19.5 and 16.2 m2 g 1 by the BET method. The sharp decrease of surface areas also suggests the growth of wurtzite CZTS during the thermal process. Correlation between the specific surface area and photocatalytic activity indicated that the photocatalytic activity of CZTS is probably dominated by the phase composition. Generally, different phases possess different electric structure (band structure) and surface atomic configuration. The band structures for wurtzite and kesterite CZTS have been calculated and schemed by Zhao and Chen, respectively.16Apparently, both wurtzite and kesterite CZTS are direct band gap semiconductors (both VBM and CBM are located at the gamma point). By carefully comparing VBs and CBs of both phases, it can be seen that the VB and CB (E, 2 to 6 eV) of kesterite are more dispersive than those of wurtzite. This phenomenon means that the photogenerated holes and electrons in kesterite CZTS have smaller effective masses, and therefore higher hole and electron migration abilities.2b,17 Higher hole and electron migration abilities including that kesterite CZTS show higher photocatalytic H2 evolution performance. Except for electric structure, the surface atomic configuration of CZTS might also play a significant role in H2 evolution. The dominant exposed facets of wurtzite spindles are {100} sidewalls, while kesterite CZTS is preferentially of the {112} facet. The surface atomic configurations for wurtzite {100} and kesterite {112} have been previously constructed by Unold.13 It is shown that the wurtzite {100} sidewalls are of a cation–anion parallel alignment surface. While, kesterite {112} are of the alternate
12728 | Chem. Commun., 2014, 50, 12726--12729
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cation layer and anion layer, exhibiting charge imbalance. Therefore, the kesterite {112} facets are polar and not stable compared with wurtzite {100} facets.14 They could be stabilized by absorbing the cations and/or the anions which could compensate the charge imbalance. When it was used for H2 evolution in the Na2S–Na2SO3 system, much more H+ and S2 would be absorbed, subsequently enhanced H2 evolution. This phenomenon has been observed as S2 capping CZTS or CuIn1 xGaxS2 nanocrystals which exhibited higher H2 evolution activity than organic-ligand capping ones.7,18 Moreover, during the annealing process, wurtzite {100} slightly changed into kesterite {112} accompanied by some instable atom formation on the surface, which could also enhance the adsorption of H+ and S2 . Based on the above discussion, kesterite CZTS obviously exhibits superior H2 evolution performance to that of wurtzite CZTS. In other words, CZTS exhibited apparently phasedependent photocatalytic H2 evolution. In conclusion, we have demonstrated that CZTS exhibited apparently phase-dependent photocatalytic H2 evolution under visible light. Kesterite CZTS exhibits superior H2 evolution performance than that of wurtzite CZTS, which could be explained by different electric structure (band structure) and surface atomic configuration of wurtzite and kesterite CZTS. We believe that the annealed process will also enhance the photocatalytic activities of other chalcogenide-based compounds, like metastable ZnS, CuInS2 and CZTSe etc. Moreover, the growth of grain boundaries during the annealed process allows us to design new approaches to fabricate CZTS thin films with large grains. This work is supported by the National Natural Science Foundation of China (21203053, 21271064 and 61306016), the Joint Talent Cultivation Funds of NSFC-HN (U1204214) and the Program for Changjiang Scholars and Innovative Research Team in University (PCS IRT1126).
Notes and references 1 A. Fujishima and K. Honda, Nature, 1972, 238, 37–38. 2 (a) S. X. Ouyang, H. Tong, N. Umezawa, J. Y. Cao, P. Li, Y. P. Bi, Y. J. Zhang and J. H. Ye, J. Am. Chem. Soc., 2012, 134, 1974–1977; (b) P. Li, S. X. Ouyang, G. C. Xi, T. Kako and J. H. Ye, J. Phys. Chem. C, 2012, 116, 7621–7628; (c) X. Wang, Q. Xu, M. R. Li, S. Shen, X. L. Wang, Y. C. Wang, Z. C. Feng, J. Y. Shi, H. X. Han and C. Li, Angew. Chem., Int. Ed., 2012, 51, 13089–13092; (d) K. Maeda, T. Takata, M. Hara, N. Saito, Y. Inoue, H. Kobayashi and K. Domen, J. Am. Chem. Soc., 2005, 127, 8286–8287; (e) N. Z. Bao, L. M. Shen, T. Takata, K. Domen, A. Gupta, K. Yanagisawa and C. A. Grimes, J. Phys. Chem. C, 2007, 111, 17527–17534. 3 (a) C. K. Miskin, W. C. Yang, C. J. Hages, N. J. Carter, C. S. Joglekar, E. A. Stach and R. Agrawal, Prog. Photovoltaics, 2014, DOI: 10.1002/ pip.2472; (b) D. A. R. Barkhouse, O. Gunawan, T. Gokmen, T. K. Todorov and D. B. Mitzi, Prog. Photovoltaics, 2012, 20, 6–11; (c) W. Wang, M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Y. Zhu and D. B. Mitzi, Adv. Energy Mater., 2014, 4, DOI: 10.1002/ aenm.201301465; (d) H. Azimi, Y. Hou and C. J. Brabec, Energy Environ. Sci., 2014, 7, 1829–1849; (e) A. Singh, H. Geaney, F. Laffir and K. M. Ryan, J. Am. Chem. Soc., 2012, 134, 2910–2913; ( f ) F. J. Fan, L. Wu, M. Gong, G. Y. Liu, Y. X. Wang, S. H. Yu, S. Y. Chen, L. W. Wang and X. G. Gong, ACS Nano, 2013, 7, 1454–1463. 4 (a) X. K. Xin, M. He, W. Han, J. Jung and Z. Q. Lin, Angew. Chem., Int. Ed., 2011, 50, 11739–11742; (b) Y. B. Cao, Y. J. Xiao, J. Y. Jung, H. D. Um, S. W. Jee, H. M. Choi, J. H. Bang and J. H. Lee, ACS Appl. Mater. Interfaces, 2013, 5, 479–484; (c) S. J. Yuan, Z. J. Zhou, Z. L. Hou, W. H. Zhou, R. Y. Yao, Y. Zhao and S. X. Wu, Chem. – Eur. J., 2013, 19, 10107–10110.
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Communication 5 L. Wang, W. Z. Wang and S. M. Sun, J. Mater. Chem., 2012, 22, 6553–6555. 6 E. Ha, L. Y. S. Lee, J. C. Wang, F. H. Li, K. Y. Wong and S. C. E. Tsang, Adv. Mater., 2014, 26, 3496–3500. ´n ˜ ez and A. Cabot, J. Am. Chem. 7 X. Yu, A. Shavel, X. An, Z. Luo, M. Iba Soc., 2014, 136, 9236–9239. 8 (a) J. Zhang, Q. Xu, Z. C. Feng, M. J. Li and C. Li, Angew. Chem., 2008, 120, 1790–1793; (b) S. X. Ouyang, N. Kikugawa, D. Chen, Z. G. Zou and J. H. Ye, J. Phys. Chem. C, 2009, 113, 1560–1566. 9 (a) X. T. Lu, Z. B. Zhuang, Q. Peng and Y. D. Li, Chem. Commun., 2011, 47, 3141–3143; (b) M. D. Regulacio, C. Ye, S. H. Lim, M. Bosman, E. Ye, S. Y. Chen, Q. H. Xu and M. Y. Han, Chem. – Eur. J., 2012, 18, 3127–3131. 10 J. M. R. Tan, Y. H. Lee, S. Pedireddy, T. Baikie, X. Y. Ling and L. H. Wong, J. Am. Chem. Soc., 2014, 136, 6684–6692. 11 (a) B. J. Stanbery, Crit. Rev. Solid State Mater. Sci., 2002, 27, 73–117; (b) H. Yoo and J. H. Kim, Sol. Energy Mater. Sol. Cells, 2011, 95(1), 239–244. 12 (a) C. Steinhagen, M. G. Panthani, V. Akhavan, B. Goodfellow, B. Koo and B. A. Korgel, J. Am. Chem. Soc., 2009, 131, 12554–12555;
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(b) S. C. Riha, B. A. Parkinson and A. L. Prieto, J. Am. Chem. Soc., 2011, 133, 15272–15275. R. Mainz, A. Singh, S. Levcenko, M. Klaus, C. Genzel, K. M. Ryan and T. Unold, Nat. Commun., 2013, 5, 3133, DOI: 10.1038/ncomms4133. P. Xu, S. Y. Chen, B. Huang, H. J. Xiang, X. G. Gong and S. H. Wei, Phys. Rev. B, 2013, 88, 045427. S. Y. Chen, A. Walsh, Y. Luo, J. H. Yang, X. G. Gong and S. H. Wei, Phys. Rev. B, 2010, 82, 195203. (a) Z. Zhao, C. Ma, Y. Cao, J. Yi, X. He and J. Qiu, Phys. Lett. A, 2013, 377, 417–422; (b) S. Chen, A. Walsh, X. G. Gong and S. H. Wei, Adv. Mater., 2013, 25, 1522–1539. (a) X. Ma, Y. Dai, M. Guo and B. Huang, ChemPhysChem, 2012, 13, 2304–2309; (b) H. Lin, C. P. Huang, W. Li, C. Ni, S. I. Shah and Y. H. Tseng, Appl. Catal., B, 2006, 68, 1–11; (c) J. Liu, S. Chen, Q. Liu, Y. Zhu and J. Zhang, Chem. Phys. Lett., 2013, 572, 101–105; (d) Z. Li, Y. Dai, X. Ma, Y. Zhu and B. Huang, Phys. Chem. Chem. Phys., 2014, 16, 3267–3273; (e) L. E. Brus, J. Chem. Phys., 1984, 80, 4403–4409. ˜ez and A. Cabot, J. Mater. Chem. A, X. Yu, X. An, A. Shavel, M. Iban 2014, 2, 12317–12322.
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Cite this: Chem. Commun., 2014, 50, 12726 Received 22nd July 2014, Accepted 1st September 2014
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Phase-dependent photocatalytic H2 evolution of copper zinc tin sulfide under visible light† Zhi-Xian Chang,ab Wen-Hui Zhou,*a Dong-Xing Kou,a Zheng-Ji Zhoua and Si-Xin Wu*a
DOI: 10.1039/c4cc05654j www.rsc.org/chemcomm
CZTS exhibited apparently phase-dependent photocatalytic H2 evolution under visible light. Possible factors for the phase-dependent photocatalytic activity of CZTS were discussed in detail.
The increasing energy crisis and environmental problems force us to explore the utilization of clean, abundant and maintainable solar energy. Photocatalytic H2 evolution from water is one of the most promising approaches to convert solar energy to green chemical energy. Since Fujishima and Honda found that light irradiation could drive H2 evolution on the TiO2 photoanode,1 numerous semiconductor photocatalysts were developed for H2 evolution from water, such as SrTiO3, NaNbO3, Ga2O3, CdS and ZnO:GaN etc.2 However, most of these photocatalysts are of wide band gaps, and only absorb UV and partial visible illumination, thus cause low efficient utilization of solar irradiation. Multicomponent chalcogenide semiconductors, namely Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 and Cu2ZnSn(S1 xSex)4, recently received increasing attention, because of their applications as candidate materials for absorbing layers3 and counter electrodes4 in photovoltaic devices. Recently, a graphene-like ultrathin kesterite CZTS was claimed to possess activity to generate H2 from water with a H2 evolution rate higher than those of commercial CdS and N-doped TiO2 nanoparticles under UV-Vis irradiation.5 This finding undoubtedly opens up a new application of CZTS in photocatalytic fields. Tsang used the nano-sized metallic component to modify wurtzite CZTS and thus promoted H2 evolution significantly. The Au–CZTS core–shell nanoparticles showed an enhanced H2 evolution rate, which was about 6.4 and 2.5 as high as that of CZTS nanoplates and nanorods.6 By forming CZTS– metal (Au or Pt) heterostructured nanoparticles, the enhanced a
The Key Laboratory for Special Functional Materials of MOE, Henan University, Kaifeng 475004, China. E-mail: [email protected], [email protected]; Fax: +86-371-23881358; Tel: +86-371-23881358 b Institute of Environmental and Analytical Sciences, College of Chemistry and Chemical Engineering, Henan University, Kaifeng 475004, China † Electronic supplementary information (ESI) available. See DOI: 10.1039/ c4cc05654j
12726 | Chem. Commun., 2014, 50, 12726--12729
photocatalytic activity towards degradation of rhodamine B and H2 evolution by water splitting was observed when compared to pure CZTS.7 In the above studies, it was noted that the photocatalytic activity of CZTS was related to not only the morphologies (nanosheets, nanoplates, nanorods and nanoparticles) but also the crystalline phases (kesterite and wurtzite), which display more crucial roles. Generally, the crystalline phase determines the electric structure and surface atomic configuration, thus significantly influences the photocatalytic activity of a photocatalyst. To date, the photocatalytic activities of various photocatalysts with different crystalline phases have been widely studied on NaNbO3, CdS, Ga2O3, TiO2, AgGaO2 etc.2b–d,8 A preliminary comparison of the photocatalytic properties of wurtzite and kesterite CZTS has been provided by Cabot.7 However, the comparison is not totally fair due to the large differences in particle size between wurtzite and kesterite CZTS. Here, we are particularly interested in the influence of crystalline phases of CZTS on the photocatalytic H2 evolution performance. Wurtzite CZTS was firstly synthesized via a facile one-pot method. By using a simple thermal process, mixed wurtzite–kesterite and pure kesterite CZTS were obtained. CZTS has been found to exhibit phase-dependent photocatalytic H2 evolution under visible light. Possible factors for the phasedependent photocatalytic activity of CZTS were discussed. As shown in Fig. 1a, the as-synthesized CZTS shows spindle shape with an average diameter of 25 5 nm and a length of 100 15 nm. The high-resolution TEM image of single spindle indicates highly crystalline structure of CZTS (Fig. 1b). The lattice fringe with an interplanar spacing of 0.32 nm is ascribed to (002) planes of wurtzite CZTS, indicating the growth along the [001] direction.3e The XRD pattern (Fig. 1c) confirms the wurtzite structure of as-synthesized CZTS. The diffraction peaks at 2y = 26.9, 28.3, 30.4, 39.3, 47.4, 51.3 and 56.4 can be indexed to (100), (002), (101), (102), (110), (103) and (112) planes, matching well with the previously reported hexagonal wurtzite CZTS.9 The Raman peak (Fig. 1d) at 335 cm 1 corresponds well to the single phase of wurtzite CZTS. And we cannot probe the presence of other impurities such as ZnS, SnS and Cu2S.10
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Fig. 1 (a) TEM image, (b) HRTEM image, (c) XRD pattern and (d) Raman shift of as-synthesized wurtzite CZTS nanospindles.
Elemental analysis by ICP-AES indicated a Cu/Zn/Sn metal ratio of 1.68 : 1.13 : 1, which is slightly Cu-poor and Zn-rich. This composition is consciously adopted, because it makes contribution to single CZTS and is close to the best value for CZTS-based solar cells.11 The mixed wurtzite–kesterite and pure kesterite CZTS were obtained by a thermal process. Fig. 2 shows the crystallographic phases of as-obtained CZTS determined by XRD. The XRD patterns suggest that wurtzite CZTS undergoes gradual phase transition to kesterite CZTS upon increasing the annealed temperature from 350 1C to 550 1C. As the temperature increases, the intensities of wurtzite (100), (101), (102) and (103) decrease gradually. Meanwhile, the small peak at 2y = 33.0, which can be indexed to the (200) plane of kesterite CZTS (JCPDS no. 26-0575), appears and continually increases. At 550 1C, the diffraction peaks are completely indexed to kesterite CZTS. The co-existing peaks for wurtzite [(100), (101), (102) and (103)] and kesterite (200) suggested that the samples were of the wurtzite–kesterite mixed-phase. Fig. 3 shows the TEM images of as-obtained CZTS with different phase composition. Apparently, thermal pretreatment
Fig. 2 XRD patterns of different samples prepared by annealed as-synthesized wurtzite CZTS at different temperatures.
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Fig. 3 TEM images of as-obtained CZTS at (a) 350 1C, (b) 450 1C and (d) 550 1C (c), which are recorded from the sample annealed at 450 1C, show the contact boundary between two wurtzite nanospindles.
at 350 1C has no influence on the morphology, as well as the crystalline phase of CZTS (Fig. 1a and 3a). The sample annealed at 450 1C shows some irregular grains, but also some crystallites similar to the original nanospindles (Fig. 3b). Fig. 3c shows that two crystal lattices with different orientations integrate together [wurtzite CZTS, d(002) spacing of 0.32 nm3e,9a and kesterite CZTS, d(112) spacing of 0.31 nm12]. The samples annealed at 550 1C display larger grains (Fig. 3d). These results suggest that the nanospindles undergo the grain growth of the neighbour boundary during the thermal process. In the wurtzite nanospindles, the sidewalls are formed by {100} planes, which are expected to be relatively stable. The phase transition from wurtzite to kesterite could be seen as shift of sulphur atoms on {100} planes, which results in the formation of instable atoms.13 These instable atoms might act as the active sites, which unavoidably brings the boundary growth of the newly formed crystal to form large grains. It is reported that the kesterite surfaces are {112} planes preferentially.14 Due to the different orientation of wurtzite {100} and kesterite {112}, the shape of wurtzite nanospindles will not be preserved. Thus, the shape of CZTS changed from nanospindles into the irregular large grains. Based on this observation, the wurtzite CZTS was expected to be very suitable for the fabrication of large grain thin films. UV-Visible absorption spectra of various samples are shown in Fig. S1 (ESI†). All samples exhibit broad absorbance in the UV-Vis region. From the inset in Fig. S1 (ESI†), Eg for wurtzite CZTS is determined to be 1.43 eV, which is consistent with that of the reported value.3e The Eg values for different mixed wurtzite–kesterite and pure kesterite CZTS are calculated to be 1.38, 1.39 and 1.39 eV, respectively, which are slightly lower than that of wurtzite CZTS. It has been reported that the Eg values of wurtzite Cu2ZnSn(S1 xSex)4 alloys are always higher than those of kesterite ones, despite that their differences are
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Fig. 4 (a) H2 evolution rates of various annealed CZTS, and (b) three consecutive 12 h cycles of H2 evolution of the single-phase kesterite CZTS (CZTS550).
always small (less than 0.1 eV).3f The relatively larger Eg values for the wurtzite phase could be explained by the symmetry of valence band maximum (VBM) and conduction band minimum (CBM) states.15 Fig. 4a summarizes the H2 evolution rates of various CZTS samples. The H2 evolution rate of wurtzite CZTS is 21.2 mmol g 1 h 1. The photocatalytic activity for H2 evolution was enhanced with the increase of kesterite CZTS. The H2 evolution rate of pure kesterite CZTS reaches 54.8 mmol g 1 h 1, which is almost 3 times as high as that of original wurtzite CZTS nanospindles. The stability of photocatalytic activity of kesterite CZTS was also examined. The H2 evolution of kesterite CZTS increases linearly without obvious change in three cycles of 12 hours (Fig. 4b), indicating the stable photocatalytic activity of kesterite CZTS. For further investigation, the specific surface areas for all samples were determined to be 39.6, 25.8, 19.5 and 16.2 m2 g 1 by the BET method. The sharp decrease of surface areas also suggests the growth of wurtzite CZTS during the thermal process. Correlation between the specific surface area and photocatalytic activity indicated that the photocatalytic activity of CZTS is probably dominated by the phase composition. Generally, different phases possess different electric structure (band structure) and surface atomic configuration. The band structures for wurtzite and kesterite CZTS have been calculated and schemed by Zhao and Chen, respectively.16Apparently, both wurtzite and kesterite CZTS are direct band gap semiconductors (both VBM and CBM are located at the gamma point). By carefully comparing VBs and CBs of both phases, it can be seen that the VB and CB (E, 2 to 6 eV) of kesterite are more dispersive than those of wurtzite. This phenomenon means that the photogenerated holes and electrons in kesterite CZTS have smaller effective masses, and therefore higher hole and electron migration abilities.2b,17 Higher hole and electron migration abilities including that kesterite CZTS show higher photocatalytic H2 evolution performance. Except for electric structure, the surface atomic configuration of CZTS might also play a significant role in H2 evolution. The dominant exposed facets of wurtzite spindles are {100} sidewalls, while kesterite CZTS is preferentially of the {112} facet. The surface atomic configurations for wurtzite {100} and kesterite {112} have been previously constructed by Unold.13 It is shown that the wurtzite {100} sidewalls are of a cation–anion parallel alignment surface. While, kesterite {112} are of the alternate
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cation layer and anion layer, exhibiting charge imbalance. Therefore, the kesterite {112} facets are polar and not stable compared with wurtzite {100} facets.14 They could be stabilized by absorbing the cations and/or the anions which could compensate the charge imbalance. When it was used for H2 evolution in the Na2S–Na2SO3 system, much more H+ and S2 would be absorbed, subsequently enhanced H2 evolution. This phenomenon has been observed as S2 capping CZTS or CuIn1 xGaxS2 nanocrystals which exhibited higher H2 evolution activity than organic-ligand capping ones.7,18 Moreover, during the annealing process, wurtzite {100} slightly changed into kesterite {112} accompanied by some instable atom formation on the surface, which could also enhance the adsorption of H+ and S2 . Based on the above discussion, kesterite CZTS obviously exhibits superior H2 evolution performance to that of wurtzite CZTS. In other words, CZTS exhibited apparently phasedependent photocatalytic H2 evolution. In conclusion, we have demonstrated that CZTS exhibited apparently phase-dependent photocatalytic H2 evolution under visible light. Kesterite CZTS exhibits superior H2 evolution performance than that of wurtzite CZTS, which could be explained by different electric structure (band structure) and surface atomic configuration of wurtzite and kesterite CZTS. We believe that the annealed process will also enhance the photocatalytic activities of other chalcogenide-based compounds, like metastable ZnS, CuInS2 and CZTSe etc. Moreover, the growth of grain boundaries during the annealed process allows us to design new approaches to fabricate CZTS thin films with large grains. This work is supported by the National Natural Science Foundation of China (21203053, 21271064 and 61306016), the Joint Talent Cultivation Funds of NSFC-HN (U1204214) and the Program for Changjiang Scholars and Innovative Research Team in University (PCS IRT1126).
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