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Solar light-driven photocatalytic hydrogen evolution over ZnIn2S4 loaded with transition-metal sulfides

Shaohua Shen12, Xiaobo Chen2, Feng Ren2, Coleman X Kronawitter2, Samuel S Mao2* and Liejin Guo1*

Author affiliations

1 State Key Laboratory of Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi 710049, China

2 Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA

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Citation and License

Nanoscale Research Letters 2011, 6:290  doi:10.1186/1556-276X-6-290


The electronic version of this article is the complete one and can be found online at: http://www.nanoscalereslett.com/content/6/1/290


Received:6 October 2010
Accepted:5 April 2011
Published:5 April 2011

© 2011 Shen et al; licensee Springer.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

A series of Pt-loaded MS/ZnIn2S4 (MS = transition-metal sulfide: Ag2S, SnS, CoS, CuS, NiS, and MnS) photocatalysts was investigated to show various photocatalytic activities depending on different transition-metal sulfides. Thereinto, CoS, NiS, or MnS-loading lowered down the photocatalytic activity of ZnIn2S4, while Ag2S, SnS, or CuS loading enhanced the photocatalytic activity. After loading 1.0 wt.% CuS together with 1.0 wt.% Pt on ZnIn2S4, the activity for H2 evolution was increased by up to 1.6 times, compared to the ZnIn2S4 only loaded with 1.0 wt.% Pt. Here, transition-metal sulfides such as CuS, together with Pt, acted as the dual co-catalysts for the improved photocatalytic performance. This study indicated that the application of transition-metal sulfides as effective co-catalysts opened up a new way to design and prepare high-efficiency and low-cost photocatalysts for solar-hydrogen conversion.

Introduction

Since the discovery of photo-induced water splitting on TiO2 electrodes [1], solar-driven photocatalytic hydrogen production from water using a semiconductor catalyst has attracted a tremendous amount of interest [2,3]. To efficiently utilize solar energy, numerous attempts have been made in recent years to realize different visible light-active photocatalysts [4-8]. Among them, sulfides, especially CdS-based photocatalysts with narrow band gaps, proved to be good candidates for photocatalytic hydrogen evolution from water under visible light irradiation [9-12]. However, CdS itself is not stable for water splitting, and Cd2+ is hazardous to environment and human health. A number of nontoxic multicomponent sulfides without Cd2+ ions have been developed to show comparable photocatalytic efficiency for hydrogen evolution [13-17]. In our previous work [18-22], hydrothermally synthesized ZnIn2S4 was found to have photocatalytic and photoelectrochemical properties that made it a good candidate for hydrogen production from water under visible light irradiation. On the other hand, a solid co-catalyst, typically noble metal (e.g., Pt, Ru, Rh) [23] or transition-metal oxide (e.g., NiO [24], Rh2-yCryO3 [25], RuO2 [26], IrO2[27]), loaded on the surface of the base photocatalyst can be beneficial to photocatalytic H2 and/or O2 evolution for water splitting [25]. Nevertheless, there have been only a limited number of studies in which metal sulfides acted as co-catalysts to enhance the activity of a semiconducting photocatalyst [28-30]. For instance, Li and co-workers observed that dual co-catalysts consisting of noble metals (Pt, Pd) and noble-metal sulfides (PdS, Ru2S3, Rh2S3) played a crucial role in achieving very high efficiency for hydrogen evolution over CdS photocatalyst [29,30]. In this study, a series of transition-metal sulfides (MS: Ag2S, SnS, CoS, CuS, NiS, and MnS) were deposited on hydrothermally synthesized ZnIn2S4 by a simple precipitation process. The photocatalytic activities for hydrogen evolution over these MS/ZnIn2S4 products were investigated. We demonstrated that transition-metal sulfides, such as CuS, combined with Pt could act as dual co-catalysts for improving photocatalytic activity for hydrogen evolution from a Na2SO3/Na2S aqueous solution under simulated sunlight.

Experimental section

All chemicals are of analytical grade and used as received without further purification. ZnIn2S4 products were prepared by a cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal synthetic method as described in our previous studies [18,19]. For the synthesis of MS/ZnIn2S4 (MS = Ag2S, SnS, CoS, CuS, NiS, and MnS), 0.1 g of prepared ZnIn2S4 was dispersed in 20 mL of distilled water and ultrasonicated for 20 min. Under stirring, a desired amount of 0.1 M AgNO3 (J.T.Baker Chemical Co., Phillipsburg, NJ, USA), SnCl2 (Sigma-Aldrich, Milwaukee, WI, USA), Co(NO3)2 (Aldrich), Cu(NO3)2 (Fluka Chemical Company, Buchs, Switzerland), Ni(NO3)2 (Fluka), or Mn(CH3COO)2 (Alfa-Aesar, Ward Hill, MA, USA) aqueous solution was dropped into the above suspension, followed by a drop-wise addition of 0.1 M Na2S·9H2O (Sigma-Aldrich) aqueous solution, containing double excess of S2- relative to the amount of metal ions. The resulting suspension was stirred for another 20 min, then the MS/ZnIn2S4 precipitate was collected by centrifugation and washed with distilled water for several times, and dried overnight at 65°C. The weight contents of transition-metal sulfides (MS) in these MS/ZnIn2S4 products were controlled at 0.5% to approximately 2.0%.

X-ray diffraction patterns were obtained from a PANalytical X'pert diffractometer (PANalytical, Almelo, The Netherlands) using Ni-filtered Cu Kα irradiation (wavelength 1.5406 Å). UV-visible absorption spectra were determined with a Varian Cary 50 UV spectrophotometer (Varian Inc, Cary, NC, USA) with MgO as reference. Morphology inspection was performed with a high-resolution scanning electron microscope (SEM, Hitachi S-4300, Tokyo, Japan). Transmission electron microscopy (TEM) study was carried out on a JEOL JEM 2010 instrument (JEOL Ltd., Tokyo, Japan). The X-ray photoelectron spectroscopy (XPS) measurements were conducted on a Kratos spectrometer (AXIS Ultra DLD, Shimadzu/Kratos Analytical, Hadano, Kanagawa, Japan) with monochromatic Al Kα radiation (hν = 1,486.69 eV) and with a concentric hemispherical analyzer. Elemental Analysis was conducted on the Bruker S4 PIONEER X-ray fluorescence spectrum (XRF, Bruker AXS GmbH, Karlsruhe, Germany) using Rh target and 4-kW-maximum power.

Photocatalytic hydrogen evolution was performed in a side-window reaction cell. A 300-W solar simulator (AM 1.5; Newport Corporation, Irvine, CA, USA) was used as the light source. The amount of hydrogen evolved was determined using a gas chromatograph (CP-4900 Micro-GC, thermal conductivity detector, Ar carrier; Varian Inc., Palo Alto, CA, USA). In all experiments, 100 mL of deionized water containing 0.05 g of catalyst and 0.25 M Na2SO3/0.35 M Na2S mixed sacrificial agent was added into the reaction cell. Here, sacrificial agent was used to scavenge photo-generated holes. Argon gas was purged through the reaction cell for 30 min before reaction to remove air. Pt (1.0 wt.%) as a co-catalyst for the promotion of hydrogen evolution was deposited in situ on the photocatalyst from the precursor of H2PtCl6·xH2O (Aldrich; 99.9%). In all cases, the reproducibility of the measurements was within ± 10%. Control experiments showed no appreciable H2 evolution without solar light irradiation or photocatalyst.

Results and discussion

The ZnIn2S4 products prepared by the CTAB-assisted hydrothermal method possessed a hexagonal structure and morphology of microspheres comprising of numerous petals, and showed an absorption edge at about 510 nm (Additional file 1, Figure S1-3). Compared to pure ZnIn2S4, the obtained MS/ZnIn2S4 (MS = metal sulfide: Ag2S, SnS, CoS, CuS, NiS, and MnS) displayed different absorption profiles (Additional file 1, Figure S4), with enhanced absorption in the visible light region from 550 to 800 nm. Such additional broad band (λ > 550 nm) can be assigned to the absorption of transition-metal sulfides.

Additional file 1. Figures S1, S2, S3, S4 and S5.

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We investigated the photocatalytic activity for hydrogen evolution over MS/ZnIn2S4 (MS = metal sulfide: Ag2S, SnS, CoS, CuS, NiS, and MnS). Photocatalytic activities for hydrogen evolution over MS/ZnIn2S4 were evaluated by loading 1 wt.% Pt as co-catalyst. Figure 1 shows the average rates of H2 evolution over Pt-loaded MS/ZnIn2S4 photocatalysts under simulated solar irradiation in the initial 20-h period. The Pt-ZnIn2S4 showed a photocatalytic activity for H2 production at the rate of 126.7 μmol·h-1, which is comparable to reported values in previous literatures [18-20]. The hydrogen production rates of Pt-MS/ZnIn2S4 photocatalysts varied with different kinds of loaded transition-metal sulfides. The Pt-MS/ZnIn2S4 (MS = Ag2S, SnS, and CuS) photocatalysts displayed enhanced activities for hydrogen evolution under solar irradiation. In particular, the H2 evolution rate greatly increased to 200 μmol·h-1 after loading 1.0 wt.% of CuS on ZnIn2S4. In this CuS/ZnIn2S4 sample, the formation of CuS (copper monosulfide) could be evidenced by XPS analysis results shown in Figure S5 (Additional file 1). The survey scan spectrum (Figure S5A of Additional file 1) indicated the presence of Cu, Zn, In, and S in the sample [21,31]. The binding energies shown in Figure S5E (Additional file 1) for Cu 2p3/2 and Cu 2p1/2 were 952.5 and 932.5 eV, respectively, which are close to the reported value of Cu2+[31]. The actual molar ratio of Cu:Zn:In:S was 0.011:0.2:0.39:1.01 as confirmed by XRF analysis result, with weight content of CuS calculated to be 1.15 wt.%, which is quite close to the proposed stoichiometric ratio. The photocatalytic activities for hydrogen evolution over Pt-MS/ZnIn2S4 (MS = Ag2S, SnS, and CuS) in the initial 20-h period were measured to increase in the order of SnS <Ag2S <CuS. Generally, these transition-metal sulfides (SnS, Ag2S, and CuS) alone are not photocatalytically active for H2 evolution, as no H2 was detected when they were used as the catalysts. Thus, the improvement of photocatalytic performances of Pt-MS/ZnIn2S4 (MS = Ag2S, SnS, and CuS) can be related to the enhanced separation of photo-generated electrons and holes induced by the hybridization of MS with ZnIn2S4. In this photocatalysis system, transition-metal sulfides (MS = Ag2S, SnS, and CuS) combined with noble-metal Pt acted as dual co-catalysts for photocatalytic hydrogen evolution. However, when transition-metal sulfides (MS = CoS, NiS, and MnS) were loaded on ZnIn2S4, the rates of H2 evolution over Pt-MS/ZnIn2S4 (MS = CoS, NiS, and MnS) were sharply decreased. Instead of the role as effective co-catalysts, these transition-metal sulfides (i.e., CoS, NiS, and MnS) may work as the recombination center of photo-generated electron-hole pairs, which lowered the photocatalytic activity for hydrogen evolution over Pt-MS/ZnIn2S4 (MS = CoS, NiS, and MnS). Further investigation is needed and also under way to provide enough supporting information to evidence the negative effects of CoS, NiS, and MnS, although main attention has focused on the more effective co-catalysts such as Ag2S, SnS, and CuS in the following discussion.

thumbnailFigure 1. Average rates of H2 evolution. The average rates of H2 evolution over Pt-loaded MS/ZnIn2S4 (MS = metal sulfide: Ag2S, SnS, CoS, CuS, NiS, and MnS) under solar light irradiation in the initial 20-h period.

Figure 2 shows the reaction time depended H2 evolution over Pt-loaded MS/ZnIn2S4 (MS = Ag2S, SnS, and CuS) under solar irradiation. Pt-SnS/ZnIn2S4 and Pt-CuS/ZnIn2S4 exhibited stable activity in the period of 34-h experiment. However, the rate of H2 production over Pt-Ag2S/ZnIn2S4 had a significant drop after irradiation for approximately 20 h. This deactivation may result from gradual reduction of Ag2S particles loaded on the surface of ZnIn2S4 to metallic Ag by photo-generated electrons during the reaction. Similar deactivation of photocatalyst was previously observed for CdS modified with Ag2S [32]. However, this result is quite different from our previous report on Pt-Ag2S/CdS, in which the high dispersion of Ag2S in the nanostructure of CdS contributed to stable photocatalytic activity for hydrogen evolution [33]. Taking into account the reduction potential (vs. normal hydrogen electrode (NHE)) of Ag+/Ag (0.80 V), Cu2+/Cu (0.34 V), and Sn2+/Sn (-0.14 V), reduction of Ag2S by photo-generated electrons is easier than photoreduction of CuS and SnS. Therefore, Pt-MS/ZnIn2S4 (MS = SnS and CuS) turned to be more stable than Pt-Ag2S/ZnIn2S4 during the photocatalytic reaction for hydrogen evolution.

thumbnailFigure 2. Time courses of H2 evolution. The time courses of H2 evolution over Pt-loaded MS/ZnIn2S4 (MS = Ag2S, SnS, and CuS) under solar light irradiation.

Table 1 shows the dependence of photocatalytic activity for H2 evolution over Pt-loaded MS/ZnIn2S4 (MS = SnS and CuS) on the loading amount of transition-metal sulfides. With the increase of SnS-loading from 0 to 2.0 wt.%, the rate of H2 evolution over Pt-SnS/ZnIn2S4 does not show significant changes. In contrast, the photocatalytic performance of Pt-CuS/ZnIn2S4 depends strongly on the amount of CuS-loading, and the optimum loading amount of CuS is approximately1.0 wt.%. The progressive regression of photocatalytic activity observed with the amount of CuS increasing from 1.0 to 2.0 wt.% may be due to the fact that excess CuS particles loaded on the surface of ZnIn2S4 could act as the optical filter or charge recombination center instead of co-catalyst for charge separation [19,32].

Table 1. Average rates of H2 evolution over Pt-loaded MS/ZnIn2S4.

To visualize hybridization of CuS with ZnIn2S4, ZnIn2S4, and CuS/ZnIn2S4 photocatalysts were investigated by TEM. A representative TEM image of ZnIn2S4 is shown in Figure 3A, which shows the formation of microspheres, 1-2 μm in diameter and comprised of a circle of micro-petals. The ED pattern (inset of Figure 3A) substantiates that the ZnIn2S4 microsphere is of a hexagonal phase. The TEM image in Figure 3B shows that some nanoparticles are loaded on the surface of ZnIn2S4 microspheres. Such nanoparticles were confirmed by the ED pattern (inset in Figure 3B) to be CuS with typical orthorhombic structure. Thus, nanosized CuS particles dispersed on the ZnIn2S4 surface would act as the charge-transfer co-catalyst, together with photodeposited Pt particles. The Pt-CuS dual co-catalysts improved the charge separation and therefore increased the photocatalytic activity.

thumbnailFigure 3. TEM images (A) ZnIn2S4 and (B) CuS/ZnIn2S4.

Figure 4 illustrates the process of photo-generated charge transfer for photocatalytic hydrogen evolution over Pt-CuS/ZnIn2S4 in an aqueous solution containing Na2SO3/Na2S under simulated sunlight. Band gap excitation produces electron-hole pairs in ZnIn2S4 particles. The excited electrons are subsequently channeled to Pt sites, which reduce protons to generate hydrogen. On the other hand, the valence band potential of ZnIn2S4, deduced from the conduction band potential (0.29 V vs. NHE) [22] and the band gap energy (2.43 eV), is about 2.72 V vs. NHE, which is more positive than the OH-/O2 redox potential [4]. The valence band potential of CuS is less positive than the OH-/O2 redox potential [34]. Such a difference of valence band potentials makes it possible for the excited holes to transfer from ZnIn2S4 to CuS to react with Na2S/Na2SO3 electron donor in the solution. Therefore, Pt and CuS are supposed to act as the reduction and oxidation co-catalyst, respectively, which leads to more efficient charge separation, thus improves photocatalytic activity of Pt-CuS/ZnIn2S4. Similar benefits of dual co-catalysts on photocatalytic activity have been observed for CdS loaded with noble metals as reduction catalysts and noble-metal sulfides as oxidation catalysts [29,30]. It is noteworthy that replacing noble-metal sulfides (such as PdS) by transition-metal sulfides (such as CuS) as the co-catalysts would help lower the cost of photocatalysts for solar-hydrogen production. Moreover, seeking effective co-catalyst candidates could be expanded to other transition-metal sulfides such as FeS and SnS2, etc. Detailed research on this subject is still an ongoing progress in our group.

thumbnailFigure 4. Schematic illustration of photo-generated charge-transfer process for photocatalytic hydrogen evolution over Pt-CuS/ZnIn2S4. From an aqueous solution containing Na2SO3/Na2S under simulated solar light.

Conclusions

In summary, a series of Pt-loaded MS/ZnIn2S4 (MS = transition-metal sulfides: Ag2S, SnS, CoS, CuS, NiS, and MnS) photocatalysts were developed. It is found that Ag2S, SnS, and CuS could enhance the photocatalytic activity of hydrogen evolution over ZnIn2S4 to varying degrees, while SnS, CoS, and NiS would reduce the activity. Among them, the Pt-CuS/ZnIn2S4 photocatalyst exhibited the most efficient and stable activity for hydrogen evolution. This can be attributed to the fact that the dual co-catalysts of Pt and CuS entrapped photo-induced electrons and holes for reduction and oxidation reaction, respectively, improving charge separation and hence the photocatalytic activity. Application of transition-metal sulfides as co-catalysts opens up an opportunity toward realizing high-efficiency, low-cost photocatalysts for solar-hydrogen conversion.

Competing interests

The authors declare that they have no competing interests.

Authors' contributions

SS carried out experiments except SEM and TEM characterization, and drafted the manuscript. XC participated in the design of the study. FR performed the TEM characterization. CXK performed the SEM characterization and improved English writing. SSM provide financial support and participated in the design and coordination of this study. LG conceived of the study, and participated in its design and coordination. All authors read and approved the final manuscript.

Acknowledgements

The authors acknowledge the support by the National Basic Research Program of China (No. 2009CB220000), Natural Science Foundation of China (No. 50821064 and No. 90610022), and the U.S. Department of Energy. One of the authors (SS) was also supported by China Scholarship Council and the Fundamental Research Funds for the Central Universities (No. 08142004 and No. 08143019).

References

  1. Fujishima A, Honda K: Electrochemical photolysis of water at a semiconductor electrode.

    Nature (London) 1972, 238:37. Publisher Full Text OpenURL

  2. Mao SS, Chen X: Selected nanotechnologies for renewable energy applications.

    Int J Energy Res 2007, 31:619. Publisher Full Text OpenURL

  3. Chen X, Mao SS: Titanium dioxide nanomaterials: synthesis, properties, modifications, and applications. In Chem Rev. Volume 107. Washington, D.C.; 2007::2891. PubMed Abstract | Publisher Full Text OpenURL

  4. Kudo A, Miseki Y: Heterogeneous photocatalyst materials for water splitting.

    Chem Soc Rev 2009, 38:253. PubMed Abstract | Publisher Full Text OpenURL

  5. Chen X, Shen S, Guo L, Mao SS: Semiconductor-based photocatalytic hydrogen generation. In Chem Rev. Volume 110. Washington, D.C.; 2010::6503. PubMed Abstract | Publisher Full Text OpenURL

  6. Zou ZG, Ye JH, Sayama K, Arakawa H: Direct splitting of water under visible light irradiation with an oxide semiconductor photocatalyst.

    Nature (London) 2001, 414:625. Publisher Full Text OpenURL

  7. Maeda K, Teramura K, Lu D, Takata T, Saito N, Inoue Y, Domen K: Photocatalyst releasing hydrogen from water.

    Nature (London) 2006, 440:295. Publisher Full Text OpenURL

  8. Maeda K, Domen K: New non-oxide photocatalysts designed for overall water splitting under visible light.

    J Phys Chem C 2007, 111:7851. Publisher Full Text OpenURL

  9. Xing C, Zhang Y, Yan W, Guo L: Band structure-controlled solid solution of Cd1-xZnxS photocatalyst for hydrogen production by water splitting.

    Int J Hydrogen Energy 2006, 31:2018. Publisher Full Text OpenURL

  10. Jing D, Guo L: A novel method for the preparation of a highly stable and active CdS photocatalyst with a special surface nanostructure.

    J Phys Chem B 2006, 110:11139. PubMed Abstract | Publisher Full Text OpenURL

  11. Shen S, Guo L: Growth of quantum-confined CdS nanoparticles inside Ti-MCM-41 as a visible light photocatalyst.

    Mater Res Bull 2008, 43:437. Publisher Full Text OpenURL

  12. Bao N, Shen L, Takata T, Domen K: Self-templated synthesis of nanoporous CdS nanostructures for highly efficient photocatalytic hydrogen production under visible light.

    Chem Mater 2008, 20:110. Publisher Full Text OpenURL

  13. Kaga H, Saito K, Kudo A: Solar hydrogen production over novel metal sulfide photocatalysts of AGa2In3S8 (A = Cu or Ag) with layered structures.

    Chem Commun 2010, 46:3779. Publisher Full Text OpenURL

  14. Jang JS, Choi SH, Shin N, Yu C, Lee JS: AgGaS2-type photocatalysts for hydrogen production under visible light: Effects of post-synthetic H2S treatment.

    J Solid State Chem 2007, 180:1110. Publisher Full Text OpenURL

  15. Tsuji I, Shimodaira Y, Kato H, Kobayashi H, Kudo A: Novel stannite-type complex sulfide photocatalysts AI2-Zn-AIV-S4 (AI = Cu and Ag; AIV = Sn and Ge) for hydrogen evolution under visible-light irradiation.

    Chem Mater 2010, 22:1402. Publisher Full Text OpenURL

  16. Tsuji I, Kato H, Kobayashi H, Kudo A: Photocatalytic H2 evolution reaction from aqueous solutions over band structure-controlled (AgIn)xZn2(1-x)S2 solid solution photocatalysts with visible-light response and their surface nanostructures.

    J Am Chem Soc 2004, 126:13406. PubMed Abstract | Publisher Full Text OpenURL

  17. Tsuji I, Kato H, Kudo A: Photocatalytic hydrogen evolution on ZnS-CuInS2-AgInS2 solid solution photocatalysts with wide visible light absorption bands.

    Chem Mater 2006, 18:1969. Publisher Full Text OpenURL

  18. Shen S, Zhao L, Guo L: Cetyltrimethylammoniumbromide (CTAB)-assisted hydrothermal synthesis of ZnIn2S4 as an efficient visible-light-driven photocatalyst for hydrogen production.

    Int J Hydrogen Energy 2008, 33:4501. Publisher Full Text OpenURL

  19. Shen S, Zhao L, Zhou Z, Guo L: Enhanced photocatalytic hydrogen evolution over Cu-doped ZnIn2S4 under visible light irradiation.

    J Phys Chem C 2008, 112:16148. Publisher Full Text OpenURL

  20. Shen S, Zhao L, Guo L: Crystallite, optical and photocatalytic properties of visible-light-driven ZnIn2S4 photocatalysts synthesized via a surfactant-assisted hydrothermal method.

    Mater Res Bull 2009, 44:100. Publisher Full Text OpenURL

  21. Shen S, Zhao L, Guo L: Morphology, structure and photocatalytic performance of ZnIn2S4 synthesized via a solvothermal/hydrothermal route in different solvents.

    J Phys Chem Solids 69:2426. Publisher Full Text OpenURL

  22. Li M, Su J, Guo L: Preparation and characterization of ZnIn2S4 thin films deposited by spray pyrolysis for hydrogen production.

    Int J Hydrogen Energy 2008, 33:2891. Publisher Full Text OpenURL

  23. Sasaki Y, Iwase A, Kato H, Kudo A: The effect of co-catalyst for Z-scheme photocatalysis systems with an Fe3+/Fe2+ electron mediator on overall water splitting under visible light irradiation.

    J Catal 2008, 259:133. Publisher Full Text OpenURL

  24. Tian M, Shangguan W, Yuan J, Jiang L, Chen M, Shi J, Ouyang Z, Wang S: K4Ce2M10O30 (M = Ta, Nb) as visible light-driven photocatalysts for hydrogen evolution from water decomposition.

    Appl Catal A: Gen 2006, 309:76. Publisher Full Text OpenURL

  25. Maeda K, Teramura K, Domen K: Development of cocatalysts for photocatalytic overall water splitting on (Ga1-xZnx)(N1-xOx) solid solution.

    Catal Surv Asia 2007, 11:145. Publisher Full Text OpenURL

  26. Yuan Y, Lv J, Jiang X, Li Z, Yu T, Zou Z, Ye J: Large impact of strontium substitution on photocatalytic water splitting activity of BaSnO3.

    Appl Phys Lett 2007, 91:094107. Publisher Full Text OpenURL

  27. Hara M, Waraksa CC, Lean JT, Lewis BA, Mallouk TE: photocatalytic water oxidation in a buffered Tris(2,2'-bipyridyl)ruthenium complex-colloidal IrO2 system.

    J Phys Chem A 2000, 104:5275. Publisher Full Text OpenURL

  28. Zong X, Yan H, Wu G, Ma G, Wen F, Wang L, Li C: Enhancement of photocatalytic H2 evolution on CdS by loading MoS2 as cocatalyst under visible light irradiation.

    J Am Chem Soc 2008, 130:7176. PubMed Abstract | Publisher Full Text OpenURL

  29. Ma G, Yan H, Shi J, Zong X, Lei Z, Li C: Direct splitting of H2S into H2 and S on CdS-based photocatalyst under visible light irradiation.

    J Catal 2008, 260:134. Publisher Full Text OpenURL

  30. Yan H, Yang J, Ma G, Wu G, Zong X, Lei Z, Shi J, Li C: Visible-light-driven hydrogen production with extremely high quantum efficiency on Pt-PdS/CdS photocatalyst.

    J Catal 2009, 266:165. Publisher Full Text OpenURL

  31. Li Y, Chen G, Wang Q, Wang X, Zhou A, Shen Z: Hierarchical ZnS-In2S3-CuS nanospheres with nanoporous structure: facile synthesis, growth mechanism, and excellent photocatalytic activity.

    Adv Funct Mater 2010, 20:3390. Publisher Full Text OpenURL

  32. Reber JF, Rusek M: Photochemical hydrogen production with platinized suspensions of cadmium sulfide and cadmium zinc sulfide modified by silver sulfide.

    J Phys Chem 1986, 90:824. Publisher Full Text OpenURL

  33. Shen S, Guo L, Chen X, Ren F, Mao SS: Effect of Ag2S on solar-driven photocatalytic hydrogen evolution of nanostructured CdS.

    Int J Hydrogen Energy 2010, 35:7110. Publisher Full Text OpenURL

  34. Xu Y, Schoonen MAA: The absolute energy positions of conduc-tion and valence bands of selected semiconducting minerals.

    Am Mineral 2000, 85:543. OpenURL