Fabrication of α-S/CdS Composites for Photocatalytic Hydrogen Production Under Visible Light
Authors: Akiyama Satoshi , Nagata Morio 
Published: December 2020

Energy issue is the one of the environmental problems. In particular, dependence on fossil fuels such as oil, coal, and natural gas should be resolved due to depletion and greenhouse gas emission such as CO2 after energy conversion. Hydrogen is attracting attention as one of the energies as a substitute for fossil fuels because of not emission CO2, abundant resources, and high energy efficiency. However, in the steam reforming, the current main method of hydrogen generation, fossil fuels are used as the raw material, so CO2 is emitted. Therefore, establishment of environmentally friendly generation method is desired.
Photocatalysts are substances that absorb light energy and proceed various chemical reactions to proceed. One of the functions of photocatalyst is the generation of hydrogen from water. If the light energy can be supplemented by sunlight, it will be possible to generate hydrogen from natural energy, which is a very attractive hydrogen generation method. Therefore, ideal photocatalysts should at least have the characteristics of absorbing visible light, which accounts for the majority of solar energy, suitable band edges for targeted reactions, good stability in water, and low cost.
α-sulfur (α-S) crystals of cyclo-octasulfur (S8) are a visible-light-responsive elemental photocatalyst. Although, ionic sulfur is widely used in sulfide photocatalysts or as a dopant in oxide photocatalysts, the possibility of using elemental sulfur as a photocatalyst has been rarely reported. The α-S crystals were found to have the ability not only to generate・OH radicals to degradation of organic dye but also to generate hydrogen by split water in a photoelectrochemical process under both UV−vis and visible-light irradiation. Although the hydrogen production efficiency was low because of fast charge recombination, poor hydrophilicity and large particle size of the α-S crystals studied, there is great potential for increasing the activity with the assistance of known strategies such as surface modification, nanoscaling, doping, and coupling with other photocatalysts.
In this study, we focused on coupling with cadmium sulfide (CdS) to improve the hydrogen production efficiency because CdS is one of the popular visible-light-responsive photocatalyst, hydrophilicity, and has the band structure suitable for charge separation to α-S.
α-S is synthesized by the decomposition of sodium thiosulfate in acetic acid solution. CdS is synthesized by the simple method of adding sodium sulfide (Na2S) to a cadmium nitrate solution and drying overnight. In addition, α-S/CdS composite is synthesized by replacing Na2S used in CdS synthesis process with potassium sulfide (K2S). The sample of α-S8/CdS composite were named X:Y. (X and Y are molar ratio of Na2S and K2S )
The identification and structure of the prepared sample were analyzed by XRD and IR. Absorbed light wavelength was measured by UV-vis DRS. Surface condition was observed by SEM and TEM. The hydrogen generation experiment was conducted in a closed circulation system using the reaction solution with sulfide sacrificial reagent, and 300 W Xe lamp was used with a cut filter to irradiate visible light. (λ≧420nm)
From the hydrogen generation experiment, 30:70 sample showed about 1300 and 2.6 times higher hydrogen generation efficiency than those of pure α-S and CdS. Based on the measured band gaps and the flat band potentials of the pure materials, a mechanism for electron and hole separation in α-S/CdS is proposed as shown in below figure. The valence band level of CdS is higher than the corresponding bands in α-S, so oxidation reaction of sulfur compounds is carried out at valence band of CdS. In addition, the level of the conduction band of α-S is lower than that of CdS, so hydrogen production reaction is carried out at conduction band of α-S.
Through this study, we succeeded in preparing a new composite photocatalyst of α-S/CdS by a simple method and confirming improve of hydrogen production efficiency of α-S by coupling with other photocatalysts. CdS exhibits decomposition sulfur compounds such as hydrogen sulfide, so we aim for hydrogen sulfide decomposition in tandem with hydrogen generation in this composite photocatalysts. Furthermore, using this model, we expect that even more composite photocatalysts with even higher efficiencies will be discovered.

Figure 1