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ZnS heterojunction
Catalysis Communications 85 (2016) 39–43
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Visible light photocatalytic H2-production activity of epitaxial Cu2ZnSnS4/ZnS heterojunction Fan Jiang, Bao Pan, Daotong You, Yangen Zhou, Xuxu Wang, Wenyue Su ⁎ State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350002, PR China
a r t i c l e
i n f o
Article history: Received 11 April 2016 Received in revised form 18 July 2016 Accepted 21 July 2016 Available online 22 July 2016 Keywords: Cu2ZnSnS4/ZnS Photocatalysis H2 evolution Lattice matching
a b s t r a c t Novel visible light driven photocatalyst Cu2ZnSnS4/ZnS composites were prepared by two-step hydrothermal method. The two semiconductors in the composites are lattice-matched and form close contact between them. With 0.1% Cu2ZnSnS4 grew on ZnS surface, the composite exhibits a high and stable visible light photocatalytic H2 generation of 432 μmolg−1 h−1. This excellent visible light activity could be attributed to the enhancement of visible light absorption by surfacial modification of ZnS with CZTS and the photoinduced interfacial charge transfer from the valence band of ZnS to Cu2ZnSnS4 in the close contact interface. © 2016 Elsevier B.V. All rights reserved.
1. Introduction As a potential answer to the global energy crisis and environmental pollution, hydrogen production from renewable energy sources has attracted great attention. Photocatalytic water-splitting has been considered as one of the most ideal approaches for clean, economical, environmentally friendly production of hydrogen. Hundreds of active photocatalysts for splitting water have been reported in the past four decades [1–3]. However, most of these photocatalysts can solely absorb the UV light, which accounts for only 4% of the total sunlight, and thus greatly restricts their practical applications [4,5]. The efficiency of solar hydrogen production can be improved by enhancing visible light absorption and photogenerated charge transfer [6–8]. Metal sulfides have been intensively studied in photocatalysis because of their suitable bandgap and catalytic functions [9–12]. The well-known II–VI group transition metal sulfides ZnS has the highly negative reduction potentials of excited electrons under irradiation. However, it only responds to UV light due to its wide band gap (3.6– 3.8 eV). Many methods have been applied to improve the visible light driven H2 evolution activity, such as doping transition-metal ions(i.e., Cu2 +, Ni2 +, and Pb2 +) [13–15], decorating plasmonic nanoparticles on surface [16,17], composting suitable narrow bandgap semiconductors [18,19]. Developing low-toxic and cost-effective ZnS-based photocatalysts with high visible light driven H2 production activities is still a challenge.
⁎ Corresponding author. E-mail address: [email protected] (W. Su).
http://dx.doi.org/10.1016/j.catcom.2016.07.017 1566-7367/© 2016 Elsevier B.V. All rights reserved.
Recently, kesterite Cu2ZnSnS4 (CZTS) has been studied widely as a most valuable absorber candidate for low cost solar cell, which has a direct band gap energy of (1.0–1.5 eV) and high absorption coefficient [20, 21]. As an environmental friendly and earth-abundant material, CZTS has great potential applications in photodegradation of pollutions and photocatalytic production of hydrogen and other value-added chemical [22,23]. In this work, we reported the fabrication of CZTS-ZnS composites by two step hydrothermal method for the first time. Their highly visible light-driven photocatalytic H2-production activity were evaluated in aqueous solutions containing Na2S and Na2SO3 without Pt cocatalyst. The origin for their visible light induced response and enhanced visible light H2-production activity were explained by the photoinduced interfacial charge transfer (IFCT) from the valence band of ZnS to Cu2ZnSnS4. In the photoinduced IFCT process, the closed contact between the two lattice-matched materials CZTS and ZnS surface is considered to play the critical role.
2. Experimental 2.1. Prepare of photocatalyts 2.1.1. Preparation of ZnS All reagents are of analytical purity and used without further purification. ZnS is prepared by hydrothermal method. 10 mmol Zn(AC)2 and 50 mmol thiourea were dissolved in 70 mL of H2O under vigorous stirring. After 30 min, the mixture was transferred into 100 mL Teflon-lined stainless steel autoclave and maintained at 170 °C for 5 h. After cooling,
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F. Jiang et al. / Catalysis Communications 85 (2016) 39–43
the products were sequentially washed with distilled water and absolute ethanol several times and then dried under vacuum.
photodiode. The apparent quantum yield (AQY) was calculated according to the following equations:
2.1.2. Preparation of Cu2ZnSnS4/ZnS 3 mmol of the as-prepared ZnS was dispersed into 60 mL of ethylene glycol, then CuCl2, ZnCl2, SnCl2, and TAA were added according to the stoichiometric ratio and loading amount of CZTS. The mixture was poured into autoclaves (100 mL) and keep at 180 °C for 15 h. After cooling, the product was filtered and washed with absolute ethanol and deionized water for several times, and dried under 60 °C. Five samples with different CZTS loading (0%,0.05%,0.1%,1% and 10%) were prepared and recorded as n% CZTS-ZnS, where n = precursor molar ratios. CZTS was synthesized by the same method without ZnS.
numberof reactedelectrons 100 numberof incidentphotons numberofevolved H2 molecules 2 ¼ 100 numberofincidentphotons
2.2. Characterization The XRD was measured on a Bruker D8 Advance X-ray diffractometer. SEM images were obtained on a Nova NanoSEM 230 microscopy (FEI Corp). TEM images were using TEM-JEOL JEM 2010F microscope. UV–vis diffuse reflection spectra were tested on a Varian Cary 500 Scan UV–vis-NIR spectrophotometer. STEM-mapping was conducted on FEI Tecnai F20. XPS measurements were carried on an ESCALAB 250 photoelectron spectroscope system. The Brunauer–Emmett–Teller (BET) specific surface area were measured by ASAP2020M. Photocurrents were measured on an electrochemical analyzer (Zahner, Germany). The working electrode was made by dip-coating catalyst slurry (10 mg mL−1 in EtOH) on FTO glass, and followed by air-drying. 0.2 M Na2SO4 was used as electrolyte. 2.3. Photocatalytic reactions Photocatalytic H2 production performances were evaluated with a gas-closed circulated system accompanied with a top-irradiation Pyrex cell. 300-W Xe lamp (CHF-XM300) with cut-off filter (λ ≥ 420 nm) was used as visible light source. 0.05 g photocatalyst was added to 100 mL aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3 as sacrificial reagents. The amount of H2 was detected every 2 h by an on-line gas chromatograph (GC-8A). The incident light intensity of visible light was tested using Spectroradiometer ILT950. The total number of incident photons was tested by calibrated silicon
AQYð%Þ ¼
3. Result and discussion 3.1. XRD Fig. 1 shows the XRD patterns of the as-prepared catalysts. The ZnS sample shows three characteristic peaks located at 29.0°, 48.3° and 57.4°, which are indexed to the (111), (220) and (311) diffraction planes of ZnS, respectively (JCPDS 79-0043), consistent with the previous report [24]. The CZTS shows three peaks located at 28.5°, 47.3° and 56.1° which are indexed to the (112), (220) and (312) diffraction planes of CZTS, respectively (JCPDS 26–0575). Interestingly, the XRD peaks of ZnS are very close to that of CZTS, indicating their crystal structures are almost the same, just as shown in Fig. 1b (lattice parameters: for ZnS, a = b = c = 5.41 Å; for CZTS, a = b = 5.43 Å, c = 10.85 Å). It suggests that nearly defect-free interfaces can be formed using the two lattice-matched semiconductors, which could greatly enhance charge separation in photocatalytic reaction. As expected, all the CZTS-ZnS composites exhibit almost the same XRD peaks as ZnS and CZTS. 3.2. Morphology and microstructure The morphologies of the as-synthesized samples were characterized by SEM. As shown in Fig. S1a, spheres with size of about several micrometers are observed for the ZnS sample. For the 0.1% CZTS/ZnS composite, the monodisperse ZnS spheres exhibit no obvious change, except that many tiny CZTS nanosheets uniformly inlayed into the spherical surface (Fig. 2a), suggesting the strong adhesion between the CTZS and the ZnS [25]. When the CZTS loading content is up to 1%, the ZnS spheres are overgrown with the tiny CZTS nanosheets, and thus many small aggregates of CZTS appear without attaching to the ZnS spheres (Fig. S1c). Further increasing the CZTS to 10%, there are more aggregates of CZTS presented to encapsulate the ZnS spheres (Fig. S1d) [26].
Fig. 1. (a) XRD patterns of ZnS, CZTS and CZTS-ZnS samples with different molar ratios; (b) the crystal structures of ZnS and CZTS.
F. Jiang et al. / Catalysis Communications 85 (2016) 39–43
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Fig. 2. (a) SEM images, (b) HRTEM of the 0.1% CZTS-ZnS sample.
Therefore, when the CZTS loading amount is as low as 0.1%, the CZTS could grow very well on the ZnS spherical surface with good contact. To further confirm the interface between ZnS and CZTS, the 0.1% CZTS-ZnS composite was characterized by TEM. As shown in Fig. 2b, CZTS is attached on the surface of ZnS, which results in intimate interfacial contact. The TEM images confirm the formation of intimate contact between CZTS and ZnS, which leads to much more efficient electron transfer between two phases. The lattice spacing of the ZnS structure are ca. 0.307 nm and ca. 0.266 nm, corresponding to the (111) and (200) plane of cubic ZnS, respectively. Meanwhile, the lattice spacing of the CZTS structure is ca. 0.312 nm, corresponding to the (112) plane of Tetragonal CZTS. The obtained EDS patterns corresponding to Fig. S1f shows that the composite are composed of Zn, S, Sn and Cu elements (Part of Cu component arise from TEM grid), and the ratios of all elements calculated from the EDS are listed in Table S1. The mapping results of the 0.1% CZTS-ZnS composite (Fig. S2) show that CZTS tiny sheets are evenly distributed on the surface of ZnS sphere. 3.3. XPS X-ray photoelectron spectrometry (XPS) was used to further investigate the surface chemical compositions of the 0.1% CZTS-ZnS composite. The high-resolution XPS spectrum of Zn, Cu, Sn and S are shown in Fig. S3. For Zn 2p, the binding energies of Zn 2p1/2 and Zn 2p3/2 peaks are present at 1046.3 and 1023.2 eV, respectively, corresponding to Zn2+. The binding energies of S 2p peaks appear at 161.4 eV, which is in good accordance with those reported for S in sulfide phases. For Cu 2p, the peak at 932.8 eV is assigned to Cu+. The result implies that the Cu2+ of the starting precursor was reduced in the synthesis process. In the high-resolution XPS spectrum of Sn 3d, peak at 486.7 eV is attributed to Sn 3d3/2. All these results indicate that the CZTS are formed in the as-synthesized sample, which is consistent with the XRD and SEM results.
dispersed on the ZnS spheres and photoinduced charge transfer occurs at the close contact interface between the two lattice-matched semiconductors. 3.5. Photocatalytic activity and stability Fig. 3 shows the visible-light photocatalytic H2 generation performances of the as-prepared samples. The blank ZnS sample shows weak photocatalytic activity under visible light, which can be due to the presence of some defects on the ZnS [27]. With the introduction of sensitizer CZTS, the photocatalytic activity of the composites increases remarkably, and reaches a maximum H2-production rate of 432 μmolg−1 h−1 at the content of 0.1% CZTS with 0.62% apparent quantum yield (AQY). While the BET specific surface area of 0.1% CZTS-ZnS is much less than that of ZnS sample (Fig. S5), indicating that the enhanced photocatalytic activity does not depend on the specific surface area but result from the effective separation of the photogenerated electrons and holes. Further increasing the content of CZTS induces the H2 production activity decrease. When the CZTS content is up to 10%, the H2-production rate drops to 64 μmolg−1 h−1, which is still a little higher than that of the ZnS. In contrast, the physical mixture of ZnS and CZTS (sample ZnS + CZTS) exhibits only slight improvement in H2 production rate than that of the ZnS sample, which may be attributed to the lack of close contact between CZTS and ZnS, as the intimate contact is crucial for the electron transfer across interface [28,29]. In addition, no H2 was detected when the CZTS alone was used as the catalyst for 6 h irradiation, suggesting that the CZTS is not active for photocatalytic H2 evolution. The action spectrum of the 0.1% CZTS-ZnS sample (Fig. S6) show the trend of H2 evolution matched well with its light absorption
3.4. Optical property Fig. S4 shows the UV–vis diffuse reflection spectra of the as-prepared samples. The ZnS sample shows the absorption edge at 370 nm and the corresponding bandgap energy is estimated to be 3.23 eV according to the Kubelka-Munk method. It should be noted that the ZnS sample has very weak absorption in the visible light region, which can be attributed to the presence of some defects on the ZnS [27]. For the CZTS-ZnS samples, as the loading content of CZTS increase, the absorption at visible light region increases remarkably. This suggests that the CZTS could be used as a sensitizer to enhance the visible light absorption of widebandgap semiconductor. For comparison, the absorption spectrum of the mechanical mixture of CZTS and ZnS (“ZnS + CZTS” sample with 0.1% CZTS) was also measured. Interestingly, the 0.1% CZTS-ZnS sample shows weaker absorption at visible light region than the ZnS + CZTS. Moreover, the curve slopes in the range of ca. 380–500 nm are different between the ZnS + CZTS and the 0.1% CZTS-ZnS. The phenomena can be ascribed to that for the 0.1% CZTS-ZnS composite, the CZTS is highly
Fig. 3. The hydrogen evolution rate of different samples.
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F. Jiang et al. / Catalysis Communications 85 (2016) 39–43
characteristics, indicating the reaction was induced by light excitation of the composite. Moreover, the repeated experiment is performed over the 0.1% CZTS-ZnS sample and the results show (Fig. S7) that no significant reduction in the activities after the six cycle runs, suggesting the good stability of 0.1% CZTS-ZnS sample. 3.6. Photocurrent responses and photocatalytic mechanism As shown in Fig. 4a, the photocurrent responses of the 0.1% CZTSZnS, ZnS and CZTS samples were measured. It is obvious that the 0.1% CZTS-ZnS sample shows much more photocurrent than the ZnS and CZTS. Here, it should be noted that CZTS as a p-type semiconductor produces cathodic photocurrent. The photocurrent of the CZTS is the smallest. These results are in good agreement with the photocatalytic H2 production performances. It indicates that the composition of the CZTS nanosheets on the ZnS spheres can efficiently enhance the photogenerated charge separation. Based on above results, the mechanism for the excellent visible light activity of CTZS/ZnS composites is proposed in Fig. 4b. Under visible light illumination, the electrons are excited from the valence band of ZnS to the conduction band of CZTS [30]. Then the photoinduced electrons in the CZTS could effectively reduce protons to produce H2 molecules. Meanwhile, the holes in the valence band of ZnS could be scavenged by the sacrificial agents. Therefore, it is reasonable to assume that the absorption from ca. 380 to 500 nm can be ascribed to this interfacial charge transfer (IFCT). Moreover, the close contact interface between the lattice-matched components facilitates the IFCE process for high activity [29]. When the content of CZTS is up to higher than 0.1%, excessive CZTS nanoparticles without attaching to the ZnS spheres will shield the incident light and decrease the photocatalytic H2-production activity. For the pure CZTS, the trapping of valence band (VB) holes into surface states induces efficient hole-electron recombination,
resulting in no visible light activity [31]. This photoinduced interfacial charge transfer (IFCT) has also been proposed as the cause for the visible light activities of Cu(II)-TiO2, Cu(II)-WO3, CuS/ZnS [13,32,33]. 4. Conclusion In summary, a series of CZTS/ZnS composites with visible light photocatalytic H2 evolution activities were successfully synthesized via two step hydrothermal method. After growing the tiny CZTS nanosheets on the ZnS spheres, the product CZTS/ZnS composites show strong absorption in the visible light region and excellent visible light photocatalytic H2 evolution activity. The optimal CZTS content is determined to be 0.1%, and the corresponding H2 evolution rate is 432 μmolg−1 h−1. It is believed that visible light irradiation induces the direct IFCT from the valence band of ZnS to CZTS, which results in efficient charge separation. The close contact between the two lattice-matched semiconductors plays the critical role for the photoinduced IFCT. This finding suggests that Cu2ZnSnS4/ZnS composites are a promising photocatalyst for hydrogen evolution. Moreover, the lattice-matched semiconductors complex method may provide a guiding principle for developing visible light driven photocatalysts. Acknowledgments This work was financially supported by the NSFC (Grants Nos. U1305242, 21373050), the National Key Basic Research Program of China (973 Program: 2013CB632405, 2014CB239303, 2014CB260410 and 2014BAC13B03). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2016.07.017. References
Fig. 4. (a) The transient photocurrent response of samples under intermittent irradiation with an applied potential of 0.1 V; (b) Schematic illustration of the proposed mechanism for photocatalytic H2 production over CZTS/ZnS.
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Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short communication
Visible light photocatalytic H2-production activity of epitaxial Cu2ZnSnS4/ZnS heterojunction Fan Jiang, Bao Pan, Daotong You, Yangen Zhou, Xuxu Wang, Wenyue Su ⁎ State Key Laboratory of Photocatalysis on Energy and Environment, Fuzhou University, Fuzhou 350002, PR China
a r t i c l e
i n f o
Article history: Received 11 April 2016 Received in revised form 18 July 2016 Accepted 21 July 2016 Available online 22 July 2016 Keywords: Cu2ZnSnS4/ZnS Photocatalysis H2 evolution Lattice matching
a b s t r a c t Novel visible light driven photocatalyst Cu2ZnSnS4/ZnS composites were prepared by two-step hydrothermal method. The two semiconductors in the composites are lattice-matched and form close contact between them. With 0.1% Cu2ZnSnS4 grew on ZnS surface, the composite exhibits a high and stable visible light photocatalytic H2 generation of 432 μmolg−1 h−1. This excellent visible light activity could be attributed to the enhancement of visible light absorption by surfacial modification of ZnS with CZTS and the photoinduced interfacial charge transfer from the valence band of ZnS to Cu2ZnSnS4 in the close contact interface. © 2016 Elsevier B.V. All rights reserved.
1. Introduction As a potential answer to the global energy crisis and environmental pollution, hydrogen production from renewable energy sources has attracted great attention. Photocatalytic water-splitting has been considered as one of the most ideal approaches for clean, economical, environmentally friendly production of hydrogen. Hundreds of active photocatalysts for splitting water have been reported in the past four decades [1–3]. However, most of these photocatalysts can solely absorb the UV light, which accounts for only 4% of the total sunlight, and thus greatly restricts their practical applications [4,5]. The efficiency of solar hydrogen production can be improved by enhancing visible light absorption and photogenerated charge transfer [6–8]. Metal sulfides have been intensively studied in photocatalysis because of their suitable bandgap and catalytic functions [9–12]. The well-known II–VI group transition metal sulfides ZnS has the highly negative reduction potentials of excited electrons under irradiation. However, it only responds to UV light due to its wide band gap (3.6– 3.8 eV). Many methods have been applied to improve the visible light driven H2 evolution activity, such as doping transition-metal ions(i.e., Cu2 +, Ni2 +, and Pb2 +) [13–15], decorating plasmonic nanoparticles on surface [16,17], composting suitable narrow bandgap semiconductors [18,19]. Developing low-toxic and cost-effective ZnS-based photocatalysts with high visible light driven H2 production activities is still a challenge.
⁎ Corresponding author. E-mail address: [email protected] (W. Su).
http://dx.doi.org/10.1016/j.catcom.2016.07.017 1566-7367/© 2016 Elsevier B.V. All rights reserved.
Recently, kesterite Cu2ZnSnS4 (CZTS) has been studied widely as a most valuable absorber candidate for low cost solar cell, which has a direct band gap energy of (1.0–1.5 eV) and high absorption coefficient [20, 21]. As an environmental friendly and earth-abundant material, CZTS has great potential applications in photodegradation of pollutions and photocatalytic production of hydrogen and other value-added chemical [22,23]. In this work, we reported the fabrication of CZTS-ZnS composites by two step hydrothermal method for the first time. Their highly visible light-driven photocatalytic H2-production activity were evaluated in aqueous solutions containing Na2S and Na2SO3 without Pt cocatalyst. The origin for their visible light induced response and enhanced visible light H2-production activity were explained by the photoinduced interfacial charge transfer (IFCT) from the valence band of ZnS to Cu2ZnSnS4. In the photoinduced IFCT process, the closed contact between the two lattice-matched materials CZTS and ZnS surface is considered to play the critical role.
2. Experimental 2.1. Prepare of photocatalyts 2.1.1. Preparation of ZnS All reagents are of analytical purity and used without further purification. ZnS is prepared by hydrothermal method. 10 mmol Zn(AC)2 and 50 mmol thiourea were dissolved in 70 mL of H2O under vigorous stirring. After 30 min, the mixture was transferred into 100 mL Teflon-lined stainless steel autoclave and maintained at 170 °C for 5 h. After cooling,
40
F. Jiang et al. / Catalysis Communications 85 (2016) 39–43
the products were sequentially washed with distilled water and absolute ethanol several times and then dried under vacuum.
photodiode. The apparent quantum yield (AQY) was calculated according to the following equations:
2.1.2. Preparation of Cu2ZnSnS4/ZnS 3 mmol of the as-prepared ZnS was dispersed into 60 mL of ethylene glycol, then CuCl2, ZnCl2, SnCl2, and TAA were added according to the stoichiometric ratio and loading amount of CZTS. The mixture was poured into autoclaves (100 mL) and keep at 180 °C for 15 h. After cooling, the product was filtered and washed with absolute ethanol and deionized water for several times, and dried under 60 °C. Five samples with different CZTS loading (0%,0.05%,0.1%,1% and 10%) were prepared and recorded as n% CZTS-ZnS, where n = precursor molar ratios. CZTS was synthesized by the same method without ZnS.
numberof reactedelectrons 100 numberof incidentphotons numberofevolved H2 molecules 2 ¼ 100 numberofincidentphotons
2.2. Characterization The XRD was measured on a Bruker D8 Advance X-ray diffractometer. SEM images were obtained on a Nova NanoSEM 230 microscopy (FEI Corp). TEM images were using TEM-JEOL JEM 2010F microscope. UV–vis diffuse reflection spectra were tested on a Varian Cary 500 Scan UV–vis-NIR spectrophotometer. STEM-mapping was conducted on FEI Tecnai F20. XPS measurements were carried on an ESCALAB 250 photoelectron spectroscope system. The Brunauer–Emmett–Teller (BET) specific surface area were measured by ASAP2020M. Photocurrents were measured on an electrochemical analyzer (Zahner, Germany). The working electrode was made by dip-coating catalyst slurry (10 mg mL−1 in EtOH) on FTO glass, and followed by air-drying. 0.2 M Na2SO4 was used as electrolyte. 2.3. Photocatalytic reactions Photocatalytic H2 production performances were evaluated with a gas-closed circulated system accompanied with a top-irradiation Pyrex cell. 300-W Xe lamp (CHF-XM300) with cut-off filter (λ ≥ 420 nm) was used as visible light source. 0.05 g photocatalyst was added to 100 mL aqueous solution containing 0.1 M Na2S and 0.1 M Na2SO3 as sacrificial reagents. The amount of H2 was detected every 2 h by an on-line gas chromatograph (GC-8A). The incident light intensity of visible light was tested using Spectroradiometer ILT950. The total number of incident photons was tested by calibrated silicon
AQYð%Þ ¼
3. Result and discussion 3.1. XRD Fig. 1 shows the XRD patterns of the as-prepared catalysts. The ZnS sample shows three characteristic peaks located at 29.0°, 48.3° and 57.4°, which are indexed to the (111), (220) and (311) diffraction planes of ZnS, respectively (JCPDS 79-0043), consistent with the previous report [24]. The CZTS shows three peaks located at 28.5°, 47.3° and 56.1° which are indexed to the (112), (220) and (312) diffraction planes of CZTS, respectively (JCPDS 26–0575). Interestingly, the XRD peaks of ZnS are very close to that of CZTS, indicating their crystal structures are almost the same, just as shown in Fig. 1b (lattice parameters: for ZnS, a = b = c = 5.41 Å; for CZTS, a = b = 5.43 Å, c = 10.85 Å). It suggests that nearly defect-free interfaces can be formed using the two lattice-matched semiconductors, which could greatly enhance charge separation in photocatalytic reaction. As expected, all the CZTS-ZnS composites exhibit almost the same XRD peaks as ZnS and CZTS. 3.2. Morphology and microstructure The morphologies of the as-synthesized samples were characterized by SEM. As shown in Fig. S1a, spheres with size of about several micrometers are observed for the ZnS sample. For the 0.1% CZTS/ZnS composite, the monodisperse ZnS spheres exhibit no obvious change, except that many tiny CZTS nanosheets uniformly inlayed into the spherical surface (Fig. 2a), suggesting the strong adhesion between the CTZS and the ZnS [25]. When the CZTS loading content is up to 1%, the ZnS spheres are overgrown with the tiny CZTS nanosheets, and thus many small aggregates of CZTS appear without attaching to the ZnS spheres (Fig. S1c). Further increasing the CZTS to 10%, there are more aggregates of CZTS presented to encapsulate the ZnS spheres (Fig. S1d) [26].
Fig. 1. (a) XRD patterns of ZnS, CZTS and CZTS-ZnS samples with different molar ratios; (b) the crystal structures of ZnS and CZTS.
F. Jiang et al. / Catalysis Communications 85 (2016) 39–43
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Fig. 2. (a) SEM images, (b) HRTEM of the 0.1% CZTS-ZnS sample.
Therefore, when the CZTS loading amount is as low as 0.1%, the CZTS could grow very well on the ZnS spherical surface with good contact. To further confirm the interface between ZnS and CZTS, the 0.1% CZTS-ZnS composite was characterized by TEM. As shown in Fig. 2b, CZTS is attached on the surface of ZnS, which results in intimate interfacial contact. The TEM images confirm the formation of intimate contact between CZTS and ZnS, which leads to much more efficient electron transfer between two phases. The lattice spacing of the ZnS structure are ca. 0.307 nm and ca. 0.266 nm, corresponding to the (111) and (200) plane of cubic ZnS, respectively. Meanwhile, the lattice spacing of the CZTS structure is ca. 0.312 nm, corresponding to the (112) plane of Tetragonal CZTS. The obtained EDS patterns corresponding to Fig. S1f shows that the composite are composed of Zn, S, Sn and Cu elements (Part of Cu component arise from TEM grid), and the ratios of all elements calculated from the EDS are listed in Table S1. The mapping results of the 0.1% CZTS-ZnS composite (Fig. S2) show that CZTS tiny sheets are evenly distributed on the surface of ZnS sphere. 3.3. XPS X-ray photoelectron spectrometry (XPS) was used to further investigate the surface chemical compositions of the 0.1% CZTS-ZnS composite. The high-resolution XPS spectrum of Zn, Cu, Sn and S are shown in Fig. S3. For Zn 2p, the binding energies of Zn 2p1/2 and Zn 2p3/2 peaks are present at 1046.3 and 1023.2 eV, respectively, corresponding to Zn2+. The binding energies of S 2p peaks appear at 161.4 eV, which is in good accordance with those reported for S in sulfide phases. For Cu 2p, the peak at 932.8 eV is assigned to Cu+. The result implies that the Cu2+ of the starting precursor was reduced in the synthesis process. In the high-resolution XPS spectrum of Sn 3d, peak at 486.7 eV is attributed to Sn 3d3/2. All these results indicate that the CZTS are formed in the as-synthesized sample, which is consistent with the XRD and SEM results.
dispersed on the ZnS spheres and photoinduced charge transfer occurs at the close contact interface between the two lattice-matched semiconductors. 3.5. Photocatalytic activity and stability Fig. 3 shows the visible-light photocatalytic H2 generation performances of the as-prepared samples. The blank ZnS sample shows weak photocatalytic activity under visible light, which can be due to the presence of some defects on the ZnS [27]. With the introduction of sensitizer CZTS, the photocatalytic activity of the composites increases remarkably, and reaches a maximum H2-production rate of 432 μmolg−1 h−1 at the content of 0.1% CZTS with 0.62% apparent quantum yield (AQY). While the BET specific surface area of 0.1% CZTS-ZnS is much less than that of ZnS sample (Fig. S5), indicating that the enhanced photocatalytic activity does not depend on the specific surface area but result from the effective separation of the photogenerated electrons and holes. Further increasing the content of CZTS induces the H2 production activity decrease. When the CZTS content is up to 10%, the H2-production rate drops to 64 μmolg−1 h−1, which is still a little higher than that of the ZnS. In contrast, the physical mixture of ZnS and CZTS (sample ZnS + CZTS) exhibits only slight improvement in H2 production rate than that of the ZnS sample, which may be attributed to the lack of close contact between CZTS and ZnS, as the intimate contact is crucial for the electron transfer across interface [28,29]. In addition, no H2 was detected when the CZTS alone was used as the catalyst for 6 h irradiation, suggesting that the CZTS is not active for photocatalytic H2 evolution. The action spectrum of the 0.1% CZTS-ZnS sample (Fig. S6) show the trend of H2 evolution matched well with its light absorption
3.4. Optical property Fig. S4 shows the UV–vis diffuse reflection spectra of the as-prepared samples. The ZnS sample shows the absorption edge at 370 nm and the corresponding bandgap energy is estimated to be 3.23 eV according to the Kubelka-Munk method. It should be noted that the ZnS sample has very weak absorption in the visible light region, which can be attributed to the presence of some defects on the ZnS [27]. For the CZTS-ZnS samples, as the loading content of CZTS increase, the absorption at visible light region increases remarkably. This suggests that the CZTS could be used as a sensitizer to enhance the visible light absorption of widebandgap semiconductor. For comparison, the absorption spectrum of the mechanical mixture of CZTS and ZnS (“ZnS + CZTS” sample with 0.1% CZTS) was also measured. Interestingly, the 0.1% CZTS-ZnS sample shows weaker absorption at visible light region than the ZnS + CZTS. Moreover, the curve slopes in the range of ca. 380–500 nm are different between the ZnS + CZTS and the 0.1% CZTS-ZnS. The phenomena can be ascribed to that for the 0.1% CZTS-ZnS composite, the CZTS is highly
Fig. 3. The hydrogen evolution rate of different samples.
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characteristics, indicating the reaction was induced by light excitation of the composite. Moreover, the repeated experiment is performed over the 0.1% CZTS-ZnS sample and the results show (Fig. S7) that no significant reduction in the activities after the six cycle runs, suggesting the good stability of 0.1% CZTS-ZnS sample. 3.6. Photocurrent responses and photocatalytic mechanism As shown in Fig. 4a, the photocurrent responses of the 0.1% CZTSZnS, ZnS and CZTS samples were measured. It is obvious that the 0.1% CZTS-ZnS sample shows much more photocurrent than the ZnS and CZTS. Here, it should be noted that CZTS as a p-type semiconductor produces cathodic photocurrent. The photocurrent of the CZTS is the smallest. These results are in good agreement with the photocatalytic H2 production performances. It indicates that the composition of the CZTS nanosheets on the ZnS spheres can efficiently enhance the photogenerated charge separation. Based on above results, the mechanism for the excellent visible light activity of CTZS/ZnS composites is proposed in Fig. 4b. Under visible light illumination, the electrons are excited from the valence band of ZnS to the conduction band of CZTS [30]. Then the photoinduced electrons in the CZTS could effectively reduce protons to produce H2 molecules. Meanwhile, the holes in the valence band of ZnS could be scavenged by the sacrificial agents. Therefore, it is reasonable to assume that the absorption from ca. 380 to 500 nm can be ascribed to this interfacial charge transfer (IFCT). Moreover, the close contact interface between the lattice-matched components facilitates the IFCE process for high activity [29]. When the content of CZTS is up to higher than 0.1%, excessive CZTS nanoparticles without attaching to the ZnS spheres will shield the incident light and decrease the photocatalytic H2-production activity. For the pure CZTS, the trapping of valence band (VB) holes into surface states induces efficient hole-electron recombination,
resulting in no visible light activity [31]. This photoinduced interfacial charge transfer (IFCT) has also been proposed as the cause for the visible light activities of Cu(II)-TiO2, Cu(II)-WO3, CuS/ZnS [13,32,33]. 4. Conclusion In summary, a series of CZTS/ZnS composites with visible light photocatalytic H2 evolution activities were successfully synthesized via two step hydrothermal method. After growing the tiny CZTS nanosheets on the ZnS spheres, the product CZTS/ZnS composites show strong absorption in the visible light region and excellent visible light photocatalytic H2 evolution activity. The optimal CZTS content is determined to be 0.1%, and the corresponding H2 evolution rate is 432 μmolg−1 h−1. It is believed that visible light irradiation induces the direct IFCT from the valence band of ZnS to CZTS, which results in efficient charge separation. The close contact between the two lattice-matched semiconductors plays the critical role for the photoinduced IFCT. This finding suggests that Cu2ZnSnS4/ZnS composites are a promising photocatalyst for hydrogen evolution. Moreover, the lattice-matched semiconductors complex method may provide a guiding principle for developing visible light driven photocatalysts. Acknowledgments This work was financially supported by the NSFC (Grants Nos. U1305242, 21373050), the National Key Basic Research Program of China (973 Program: 2013CB632405, 2014CB239303, 2014CB260410 and 2014BAC13B03). Appendix A. Supplementary data Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.catcom.2016.07.017. References
Fig. 4. (a) The transient photocurrent response of samples under intermittent irradiation with an applied potential of 0.1 V; (b) Schematic illustration of the proposed mechanism for photocatalytic H2 production over CZTS/ZnS.
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