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CdS composite photocatalyst for H2 evolution under visible light irradiation
Catalysis Communications 40 (2013) 51–54
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Short Communication
A novel noble metal-free ZnS–WS2/CdS composite photocatalyst for H2 evolution under visible light irradiation Guoping Chen a,b, Fan Li a,b, Yuzun Fan c, Yanhong Luo a,b, Dongmei Li a,b,⁎, Qingbo Meng a,b,⁎⁎ a b c
Key Laboratory for Renewable Energy, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, PR China
a r t i c l e
i n f o
Article history: Received 12 April 2013 Received in revised form 23 May 2013 Accepted 24 May 2013 Available online 4 June 2013 Keywords: ZnS–WS2/CdS H2 evolution Synergistic effect ZnS passivation Noble metal-free
a b s t r a c t A novel ZnS–WS2/CdS composite photocatalyst for H2 evolution under visible light irradiation has been successfully synthesized. The loading amounts of WS2 and ZnS on the CdS surface were optimized. A H2 evolution rate up to 1224 μmol·h−1 has been achieved for the 9.6 mol% ZnS–1 mol% WS2/CdS photocatalyst. The enhancement of the photocatalytic activity is attributed to the synergistic effect of WS2 as co-catalyst and ZnS as passivation layer. © 2013 The Authors. Published by Elsevier B.V. Open access under CC BY license.
1. Introduction Photocatalytic H2 generation from water has attracted increasing attention recently [1–3]. In order to utilize the abundant visible light, developing visible light driven photocatalysts is indispensable. Sulfide photocatalysts (such as CuInS2 [4], CdS [5], ZnIn2S4 [6], etc.) are generally attractive candidates due to their suitable band gaps and good catalytic activities for H2 evolution. In particular, CdS is widely studied as an important sulfide for its suitable band positions as well as its low cost. However, its photocatalytic efficiency is extremely low due to severe recombination, and it's not stable under irradiation due to photocorrosion. Generally, the above problems can be greatly solved by loading noble metals or their oxides (such as Pt, Pd, RuO2, etc.) as co-catalysts, which facilitate the separation of photo-generated electrons and holes and afford the redox reaction sites [7–9]. However, the noble metals are not appropriate for practical application. In order to reduce the cost, some metal sulfides as co-catalysts, such as MoS2 [10], WS2 [11] and NiS [12], have been recently reported to replace noble metals. In
the meantime, for the CdS nanoparticles, some photo-generated carriers are easily captured by its surface states, surface passivation is thus supposed to be a good modification method to restrain this surface trapping. In quantum-dot sensitized solar cells, ZnS passivation layer has been widely used to reduce electron losses of quantum dot sensitizers. That is to say, some surface states of CdS can be passivated by ZnS with higher conduction band and lower valence band, which is further favorable for the photocatalytic reaction [13–15]. Herein, we report a novel noble metal-free ZnS–WS2/CdS composite photocatalyst for H2 evolution, which is obtained by our recently developed ball-milling (BM) combined calcination method for WS2 loading [16] and subsequently hydrothermal treatment for ZnS loading on CdS. A H2 evolution rate up to 1224 μmol·h−1 has been achieved for the 9.6 mol% ZnS–1 mol% WS2/CdS photocatalyst. The synergistic effect of ZnS as passivation layer and WS2 as co-catalyst is supposed to contribute to the increase of the photocatalytic activity. 2. Experimental 2.1. Photocatalyst preparation
⁎ Correspondence to: D. Li, Key Laboratory for Renewable Energy, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China. Tel./fax: +86 10 82649242. ⁎⁎ Correspondence to: Q. Meng, Key Laboratory for Renewable Energy, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China. Tel./fax: +86 10 82649242. E-mail addresses: [email protected] (D. Li), [email protected] (Q. Meng). 1566-7367 © 2013 The Authors. Published by Elsevier B.V. Open access under CC BY license. http://dx.doi.org/10.1016/j.catcom.2013.05.025
CdS powder was from Beijing Shuanghuanweiye Reagent Co., Ltd. Zn(Ac)2·2H2O, thiourea and lactic acid were from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were analytical grade (AR) and used directly except for the CdS which was thermally treated in advance [16–18]. The (NH4)2WS4 was synthesized according to the literature (details in Supporting Information) [19].
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The WS2/CdS photocatalysts with different WS2 loading amounts (denoted as x mol% WS2/CdS) were prepared by our recently developed ball-milling combined calcination method (details in Supporting Information) [16]. ZnS was then loaded on the 1 mol% WS2/CdS as follows: the slurry containing a proper amount of Zn(Ac)2·2H2O, 10 mmol of thiourea, 0.3 g of as-prepared 1 mol% WS2/CdS (treated at 723 K) and 35 mL deionized H2O was ultra-sonicated for 1 h, then transferred into a Teflon-lined autoclave and maintained at 393 K for 5 h. The ultimate solids (denoted as y mol% ZnS–1 mol% WS2/CdS) were collected by centrifugation and washing with distilled water for several times. Here, x or y means the loading molar quantity of ZnS or WS2 per 100 mol CdS. For comparison, the 1 mol% WS2–9.6 mol% ZnS/CdS photocatalyst and the 9.6 mol% ZnS/CdS photocatalyst were also prepared by the same method. 2.2. Characterization The surface morphologies were obtained by a scanning electron microscope (SEM, XL30 S-FEG, FEI) and a transmission electron microscope (TEM, JEM-2010, JEOL Ltd.). The X-ray diffraction (XRD) patterns were collected by a rotating-anode diffractometer with Cu Kα radiation (M18X-AHF, MacScience). The X-ray photoelectron spectra (XPS) were recorded on an ULVAC-PHI X-ray photoelectron spectrometer (PHI Quantera SXM) using Al Kα radiation. The thermal decomposition behavior was studied by a differential thermal analysis/thermogravimetric analysis instrument (DTA/TGA, NETZSCH STA 449F3) in N2 atmosphere with the heating rate of 10 K/min. The photoluminescence spectra were recorded on an FLS 920 fluorescence spectrometer (Edinburgh Instruments). 2.3. Photocatalytic performance The reaction was performed in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. 0.1 g photocatalyst powder was dispersed in a 150 mL aqueous solution containing 15 mL lactic acid as the sacrificial reagent [12]. The suspension was irradiated by a 300 W Xenon lamp equipped with an optical cut-off filter (λ > 420 nm). The temperature of the reaction solution was maintained at 293 ± 1.5 K by a cooling liquid flow. The evolved H2 amount was determined by an on-line gas chromatography. The photocatalytic activities were compared by the average H2 evolution rate in the first 5 h. 3. Results and discussion The XRD patterns of the WS2/CdS photocatalysts treated at different temperatures indicate that CdS always keeps the hexagonal phase during the thermal treatment (see Fig. 1(a)), whereas diffraction peaks assigned to WS2 are not observed due to low loading amount. The TG–DSC curve of (NH4)2WS4 and XRD patterns of different temperatures treated
(NH4)2WS4 confirm that (NH4)2WS4 is completely decomposed into WS2 above 673 K (see Fig. S1 and Fig. 1(b)). This is also validated by XPS spectra (see Fig. 1(c)). The W4f peaks at 35.7 and 37.9 eV are assigned to W6+ while the peaks at 32.6 and 34.8 eV can be assigned to W4+ in WS2 [20]. For 1 mol% WS2/CdS photocatalysts, the influence of calcination temperature on H2 evolution rate was firstly investigated. As illustrated in Fig. 2(a), with the temperature increasing, the H2 evolution rate of 1 mol% WS2/CdS increases gradually, reaching the maximum value of 780 μmol·h−1 at 723 K. However, further increasing the temperature will decrease the H2 evolution rate instead, which may be ascribed to the decrease of the surface areas. From SEM images of 1 mol% WS2/CdS samples calcined at different temperatures, we can see that with the temperature increasing, the particle sizes increase gradually. It is supposed that some small particles grow together into larger ones, as shown in Fig. S2. Fig. 2(b) shows the H2 evolution rates of WS2/CdS photocatalysts with different WS2 amounts. The H2 evolution rate of bare CdS is only 85 μmol·h−1. After loading 0.25 mol% WS2, the H2 evolution rate increases sharply to 262 μmol·h−1. With 1 mol% WS2 loaded, the H2 evolution rate reaches the maximum of 780 μmol·h−1, which is much higher than 488 μmol·h−1 for 0.2 wt.% Pt/CdS photocatalyst [16,17]. However, further increasing the WS2 amount results in the decrease of the H2 evolution rate, possibly due to the introduction of recombination sites and the light shading effect of WS2 [16]. In order to further improve the photocatalytic activity, ZnS was loaded on WS2/CdS photocatalysts. As shown in Fig. 2(c), with the increase of the ZnS amount, the H2 evolution rate increases firstly, reaching the maximum of 1224 μmol·h−1 for 9.6 mol%, then decreases gradually due to the incident light hinder and the introduction of recombination centers by excessive ZnS. For comparison, a H2 evolution rate of 104 μmol·h−1 is obtained for the 9.6 mol% ZnS/CdS photocatalyst, slightly higher than that of the bare CdS whereas far lower than the ZnS–WS2/CdS photocatalysts. Obviously, the loadings of WS2 and sequent ZnS are simultaneously beneficial to improve the photocatalytic activity. Fig. 3(a) and (b) clearly shows that WS2 with a typical layered structure is deposited on the surface of CdS, depicted by the black circles, of which the lattice spacing of the (002) plane is 6.18 Å. As shown in Fig. S3, some small ZnS particles are dispersed at the CdS nanoparticle surface after hydrothermal treatment. The HRTEM images (Fig. 3(c) and (d)) demonstrate the small particles are ZnS for the lattice spacing of 3.3 Å is (100) planes (depicted by the white circles). As we know, the photoluminescence (PL) spectroscopy is an effective way to investigate the photo-generated charge transfer and recombination processes. Herein, at the excitation wavelength of 400 nm, the PL spectra are presented in Fig. 4. The CdS sample displays the highest emission band intensity centered at 700 nm, which is ascribed to the trap-related states that generally serve as recombination centers. As
Fig. 1. (a) X-ray diffraction patterns of 1 mol% WS2/CdS photocatalysts calcined at different temperatures, along with thermally treated CdS for comparison; (b) X-ray diffraction patterns of (NH4)2WS4, (NH4)2WS4 (BM), and WS2 prepared by calcining (NH4)2WS4 (BM) at different temperatures; and (c) XPS spectra of the W4f regions for 1 mol% WS2/CdS photocatalysts prepared at (I) 573 K, (II) 673 K, (III) 723 K, (IV) 773 K and (V) 9.6 mol% ZnS–1 mol% WS2/CdS photocatalyst.
G. Chen et al. / Catalysis Communications 40 (2013) 51–54
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Fig. 2. The H2 evolution rates of (a) 1 mol% WS2/CdS photocatalysts prepared at different temperatures; (b) WS2/CdS photocatalysts loaded with different amount of WS2 at 723 K; and (c) y mol% ZnS–1 mol%WS2/CdS photocatalysts loaded with different amount of ZnS. The composition of the photocatalyst system: 0.1 g photocatalyst dispersed in 150 mL solution containing 15 mL lactic acid as the sacrificial reagent; light source, xenon lamp (300 W) with a cut-off filter (λ > 420 nm).
expected, the PL intensity is dramatically quenched after the load of WS2, indicating that WS2 can efficiently capture electrons which is favorable for the photocatalytic reaction. After the load of ZnS, the PL intensity is even lower. Interestingly, the variation tendency between the PL intensity and ZnS amount is the same as that between the H2 evolution rate and the ZnS amount, for excessive ZnS will introduce recombination centers again. Because ZnS and CdS have similar crystal structures, ZnS is prone to grow on CdS to eliminate its interfacial defects. The higher conduction band and lower valence band of ZnS demonstrate the type I heterojunction of ZnS/CdS. This heterojunction can effectively passivate the electron loss at the CdS surface, as shown in Scheme 1, which has also been found in many quantum-dot sensitized solar cells [21,22]. Moreover, because of the presence of ZnS on the CdS surface, the ZnS–WS2/CdS photocatalyst exhibits good photostability, only a
slight drop of the evolved H2 amount was observed in the third 5 h reaction (details in Supporting Information). It should be noted that as we can see from the TEM image (Fig. 3), the ZnS layer is not compact and very thin, which is in agreement with the above hypothesis in some extent. By the way, the photocatalytic activity of the 1 mol% WS2– 9.6 mol% ZnS/CdS photocatalyst with an opposite deposition order of ZnS and WS2 only accounts for 66.3% of that of the 9.6 mol% ZnS– 1 mol% WS2/CdS photocatalyst, for firstly deposited ZnS decreases the electrons to react with the protons. The exact mechanism needs further investigation. 4. Conclusions In summary, a novel noble metal-free ZnS–WS2/CdS photocatalyst for photocatalytic H2 evolution under visible light irradiation was prepared
Fig. 3. HRTEM images of (a, b) 1 mol% WS2/CdS photocatalyst prepared at 723 K and (c, d) 9.6 mol% ZnS–1 ml% WS2/CdS photocatalytst. The black circles depict the WS2 while the white circles depict the ZnS.
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Acknowledgments The work was financially supported by NSFC (21173260 and 51072221), the MOST (“973”) (2012CB932903 and 2012CB932904) and the Knowledge Innovation Program of the Chinese Academy of Sciences. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.catcom.2013.05.025. References
Fig. 4. Photoluminescence spectra of CdS, 1 mol% WS2/CdS and ZnS–WS2/CdS photocatalysts with different loading amounts of ZnS.
Scheme 1. Proposed mechanism for photocatalytic H2 evolution on ZnS–WS2/CdS photocatalyst.
by a two-step method. WS2 was firstly loaded on the CdS surface by a simple ball-milling combined calcination method, and then ZnS was deposited on WS2/CdS by the hydrothermal method. The H2 evolution rate of 9.6 mol%ZnS–1 mol% WS2/CdS is 1224 μmol·h−1, much higher than that of 0.2 wt.% Pt/CdS. The synergistic effect of ZnS as passivation layer and WS2 as co-catalyst results in the high photocatalytic activity. Besides, the ZnS–WS2/CdS photocatalyst exhibits good anti-photocorrosion property.
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Contents lists available at SciVerse ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Short Communication
A novel noble metal-free ZnS–WS2/CdS composite photocatalyst for H2 evolution under visible light irradiation Guoping Chen a,b, Fan Li a,b, Yuzun Fan c, Yanhong Luo a,b, Dongmei Li a,b,⁎, Qingbo Meng a,b,⁎⁎ a b c
Key Laboratory for Renewable Energy, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry and Environment, Beihang University, Beijing 100191, PR China
a r t i c l e
i n f o
Article history: Received 12 April 2013 Received in revised form 23 May 2013 Accepted 24 May 2013 Available online 4 June 2013 Keywords: ZnS–WS2/CdS H2 evolution Synergistic effect ZnS passivation Noble metal-free
a b s t r a c t A novel ZnS–WS2/CdS composite photocatalyst for H2 evolution under visible light irradiation has been successfully synthesized. The loading amounts of WS2 and ZnS on the CdS surface were optimized. A H2 evolution rate up to 1224 μmol·h−1 has been achieved for the 9.6 mol% ZnS–1 mol% WS2/CdS photocatalyst. The enhancement of the photocatalytic activity is attributed to the synergistic effect of WS2 as co-catalyst and ZnS as passivation layer. © 2013 The Authors. Published by Elsevier B.V. Open access under CC BY license.
1. Introduction Photocatalytic H2 generation from water has attracted increasing attention recently [1–3]. In order to utilize the abundant visible light, developing visible light driven photocatalysts is indispensable. Sulfide photocatalysts (such as CuInS2 [4], CdS [5], ZnIn2S4 [6], etc.) are generally attractive candidates due to their suitable band gaps and good catalytic activities for H2 evolution. In particular, CdS is widely studied as an important sulfide for its suitable band positions as well as its low cost. However, its photocatalytic efficiency is extremely low due to severe recombination, and it's not stable under irradiation due to photocorrosion. Generally, the above problems can be greatly solved by loading noble metals or their oxides (such as Pt, Pd, RuO2, etc.) as co-catalysts, which facilitate the separation of photo-generated electrons and holes and afford the redox reaction sites [7–9]. However, the noble metals are not appropriate for practical application. In order to reduce the cost, some metal sulfides as co-catalysts, such as MoS2 [10], WS2 [11] and NiS [12], have been recently reported to replace noble metals. In
the meantime, for the CdS nanoparticles, some photo-generated carriers are easily captured by its surface states, surface passivation is thus supposed to be a good modification method to restrain this surface trapping. In quantum-dot sensitized solar cells, ZnS passivation layer has been widely used to reduce electron losses of quantum dot sensitizers. That is to say, some surface states of CdS can be passivated by ZnS with higher conduction band and lower valence band, which is further favorable for the photocatalytic reaction [13–15]. Herein, we report a novel noble metal-free ZnS–WS2/CdS composite photocatalyst for H2 evolution, which is obtained by our recently developed ball-milling (BM) combined calcination method for WS2 loading [16] and subsequently hydrothermal treatment for ZnS loading on CdS. A H2 evolution rate up to 1224 μmol·h−1 has been achieved for the 9.6 mol% ZnS–1 mol% WS2/CdS photocatalyst. The synergistic effect of ZnS as passivation layer and WS2 as co-catalyst is supposed to contribute to the increase of the photocatalytic activity. 2. Experimental 2.1. Photocatalyst preparation
⁎ Correspondence to: D. Li, Key Laboratory for Renewable Energy, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China. Tel./fax: +86 10 82649242. ⁎⁎ Correspondence to: Q. Meng, Key Laboratory for Renewable Energy, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China. Tel./fax: +86 10 82649242. E-mail addresses: [email protected] (D. Li), [email protected] (Q. Meng). 1566-7367 © 2013 The Authors. Published by Elsevier B.V. Open access under CC BY license. http://dx.doi.org/10.1016/j.catcom.2013.05.025
CdS powder was from Beijing Shuanghuanweiye Reagent Co., Ltd. Zn(Ac)2·2H2O, thiourea and lactic acid were from Sinopharm Chemical Reagent Co., Ltd. All the chemicals were analytical grade (AR) and used directly except for the CdS which was thermally treated in advance [16–18]. The (NH4)2WS4 was synthesized according to the literature (details in Supporting Information) [19].
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The WS2/CdS photocatalysts with different WS2 loading amounts (denoted as x mol% WS2/CdS) were prepared by our recently developed ball-milling combined calcination method (details in Supporting Information) [16]. ZnS was then loaded on the 1 mol% WS2/CdS as follows: the slurry containing a proper amount of Zn(Ac)2·2H2O, 10 mmol of thiourea, 0.3 g of as-prepared 1 mol% WS2/CdS (treated at 723 K) and 35 mL deionized H2O was ultra-sonicated for 1 h, then transferred into a Teflon-lined autoclave and maintained at 393 K for 5 h. The ultimate solids (denoted as y mol% ZnS–1 mol% WS2/CdS) were collected by centrifugation and washing with distilled water for several times. Here, x or y means the loading molar quantity of ZnS or WS2 per 100 mol CdS. For comparison, the 1 mol% WS2–9.6 mol% ZnS/CdS photocatalyst and the 9.6 mol% ZnS/CdS photocatalyst were also prepared by the same method. 2.2. Characterization The surface morphologies were obtained by a scanning electron microscope (SEM, XL30 S-FEG, FEI) and a transmission electron microscope (TEM, JEM-2010, JEOL Ltd.). The X-ray diffraction (XRD) patterns were collected by a rotating-anode diffractometer with Cu Kα radiation (M18X-AHF, MacScience). The X-ray photoelectron spectra (XPS) were recorded on an ULVAC-PHI X-ray photoelectron spectrometer (PHI Quantera SXM) using Al Kα radiation. The thermal decomposition behavior was studied by a differential thermal analysis/thermogravimetric analysis instrument (DTA/TGA, NETZSCH STA 449F3) in N2 atmosphere with the heating rate of 10 K/min. The photoluminescence spectra were recorded on an FLS 920 fluorescence spectrometer (Edinburgh Instruments). 2.3. Photocatalytic performance The reaction was performed in a Pyrex reaction cell connected to a closed gas circulation and evacuation system. 0.1 g photocatalyst powder was dispersed in a 150 mL aqueous solution containing 15 mL lactic acid as the sacrificial reagent [12]. The suspension was irradiated by a 300 W Xenon lamp equipped with an optical cut-off filter (λ > 420 nm). The temperature of the reaction solution was maintained at 293 ± 1.5 K by a cooling liquid flow. The evolved H2 amount was determined by an on-line gas chromatography. The photocatalytic activities were compared by the average H2 evolution rate in the first 5 h. 3. Results and discussion The XRD patterns of the WS2/CdS photocatalysts treated at different temperatures indicate that CdS always keeps the hexagonal phase during the thermal treatment (see Fig. 1(a)), whereas diffraction peaks assigned to WS2 are not observed due to low loading amount. The TG–DSC curve of (NH4)2WS4 and XRD patterns of different temperatures treated
(NH4)2WS4 confirm that (NH4)2WS4 is completely decomposed into WS2 above 673 K (see Fig. S1 and Fig. 1(b)). This is also validated by XPS spectra (see Fig. 1(c)). The W4f peaks at 35.7 and 37.9 eV are assigned to W6+ while the peaks at 32.6 and 34.8 eV can be assigned to W4+ in WS2 [20]. For 1 mol% WS2/CdS photocatalysts, the influence of calcination temperature on H2 evolution rate was firstly investigated. As illustrated in Fig. 2(a), with the temperature increasing, the H2 evolution rate of 1 mol% WS2/CdS increases gradually, reaching the maximum value of 780 μmol·h−1 at 723 K. However, further increasing the temperature will decrease the H2 evolution rate instead, which may be ascribed to the decrease of the surface areas. From SEM images of 1 mol% WS2/CdS samples calcined at different temperatures, we can see that with the temperature increasing, the particle sizes increase gradually. It is supposed that some small particles grow together into larger ones, as shown in Fig. S2. Fig. 2(b) shows the H2 evolution rates of WS2/CdS photocatalysts with different WS2 amounts. The H2 evolution rate of bare CdS is only 85 μmol·h−1. After loading 0.25 mol% WS2, the H2 evolution rate increases sharply to 262 μmol·h−1. With 1 mol% WS2 loaded, the H2 evolution rate reaches the maximum of 780 μmol·h−1, which is much higher than 488 μmol·h−1 for 0.2 wt.% Pt/CdS photocatalyst [16,17]. However, further increasing the WS2 amount results in the decrease of the H2 evolution rate, possibly due to the introduction of recombination sites and the light shading effect of WS2 [16]. In order to further improve the photocatalytic activity, ZnS was loaded on WS2/CdS photocatalysts. As shown in Fig. 2(c), with the increase of the ZnS amount, the H2 evolution rate increases firstly, reaching the maximum of 1224 μmol·h−1 for 9.6 mol%, then decreases gradually due to the incident light hinder and the introduction of recombination centers by excessive ZnS. For comparison, a H2 evolution rate of 104 μmol·h−1 is obtained for the 9.6 mol% ZnS/CdS photocatalyst, slightly higher than that of the bare CdS whereas far lower than the ZnS–WS2/CdS photocatalysts. Obviously, the loadings of WS2 and sequent ZnS are simultaneously beneficial to improve the photocatalytic activity. Fig. 3(a) and (b) clearly shows that WS2 with a typical layered structure is deposited on the surface of CdS, depicted by the black circles, of which the lattice spacing of the (002) plane is 6.18 Å. As shown in Fig. S3, some small ZnS particles are dispersed at the CdS nanoparticle surface after hydrothermal treatment. The HRTEM images (Fig. 3(c) and (d)) demonstrate the small particles are ZnS for the lattice spacing of 3.3 Å is (100) planes (depicted by the white circles). As we know, the photoluminescence (PL) spectroscopy is an effective way to investigate the photo-generated charge transfer and recombination processes. Herein, at the excitation wavelength of 400 nm, the PL spectra are presented in Fig. 4. The CdS sample displays the highest emission band intensity centered at 700 nm, which is ascribed to the trap-related states that generally serve as recombination centers. As
Fig. 1. (a) X-ray diffraction patterns of 1 mol% WS2/CdS photocatalysts calcined at different temperatures, along with thermally treated CdS for comparison; (b) X-ray diffraction patterns of (NH4)2WS4, (NH4)2WS4 (BM), and WS2 prepared by calcining (NH4)2WS4 (BM) at different temperatures; and (c) XPS spectra of the W4f regions for 1 mol% WS2/CdS photocatalysts prepared at (I) 573 K, (II) 673 K, (III) 723 K, (IV) 773 K and (V) 9.6 mol% ZnS–1 mol% WS2/CdS photocatalyst.
G. Chen et al. / Catalysis Communications 40 (2013) 51–54
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Fig. 2. The H2 evolution rates of (a) 1 mol% WS2/CdS photocatalysts prepared at different temperatures; (b) WS2/CdS photocatalysts loaded with different amount of WS2 at 723 K; and (c) y mol% ZnS–1 mol%WS2/CdS photocatalysts loaded with different amount of ZnS. The composition of the photocatalyst system: 0.1 g photocatalyst dispersed in 150 mL solution containing 15 mL lactic acid as the sacrificial reagent; light source, xenon lamp (300 W) with a cut-off filter (λ > 420 nm).
expected, the PL intensity is dramatically quenched after the load of WS2, indicating that WS2 can efficiently capture electrons which is favorable for the photocatalytic reaction. After the load of ZnS, the PL intensity is even lower. Interestingly, the variation tendency between the PL intensity and ZnS amount is the same as that between the H2 evolution rate and the ZnS amount, for excessive ZnS will introduce recombination centers again. Because ZnS and CdS have similar crystal structures, ZnS is prone to grow on CdS to eliminate its interfacial defects. The higher conduction band and lower valence band of ZnS demonstrate the type I heterojunction of ZnS/CdS. This heterojunction can effectively passivate the electron loss at the CdS surface, as shown in Scheme 1, which has also been found in many quantum-dot sensitized solar cells [21,22]. Moreover, because of the presence of ZnS on the CdS surface, the ZnS–WS2/CdS photocatalyst exhibits good photostability, only a
slight drop of the evolved H2 amount was observed in the third 5 h reaction (details in Supporting Information). It should be noted that as we can see from the TEM image (Fig. 3), the ZnS layer is not compact and very thin, which is in agreement with the above hypothesis in some extent. By the way, the photocatalytic activity of the 1 mol% WS2– 9.6 mol% ZnS/CdS photocatalyst with an opposite deposition order of ZnS and WS2 only accounts for 66.3% of that of the 9.6 mol% ZnS– 1 mol% WS2/CdS photocatalyst, for firstly deposited ZnS decreases the electrons to react with the protons. The exact mechanism needs further investigation. 4. Conclusions In summary, a novel noble metal-free ZnS–WS2/CdS photocatalyst for photocatalytic H2 evolution under visible light irradiation was prepared
Fig. 3. HRTEM images of (a, b) 1 mol% WS2/CdS photocatalyst prepared at 723 K and (c, d) 9.6 mol% ZnS–1 ml% WS2/CdS photocatalytst. The black circles depict the WS2 while the white circles depict the ZnS.
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Acknowledgments The work was financially supported by NSFC (21173260 and 51072221), the MOST (“973”) (2012CB932903 and 2012CB932904) and the Knowledge Innovation Program of the Chinese Academy of Sciences. Appendix A. Supplementary data Supplementary data to this article can be found online at http:// dx.doi.org/10.1016/j.catcom.2013.05.025. References
Fig. 4. Photoluminescence spectra of CdS, 1 mol% WS2/CdS and ZnS–WS2/CdS photocatalysts with different loading amounts of ZnS.
Scheme 1. Proposed mechanism for photocatalytic H2 evolution on ZnS–WS2/CdS photocatalyst.
by a two-step method. WS2 was firstly loaded on the CdS surface by a simple ball-milling combined calcination method, and then ZnS was deposited on WS2/CdS by the hydrothermal method. The H2 evolution rate of 9.6 mol%ZnS–1 mol% WS2/CdS is 1224 μmol·h−1, much higher than that of 0.2 wt.% Pt/CdS. The synergistic effect of ZnS as passivation layer and WS2 as co-catalyst results in the high photocatalytic activity. Besides, the ZnS–WS2/CdS photocatalyst exhibits good anti-photocorrosion property.
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