UTILIZATION OF SUNLIGHT USING STRATIFIED PHOTOCATALYSTS

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The application of photocatalytic reaction to generate hydrogen as a source of energy is being given higher priority in photochemical research. However, the prevention of re-association of the excited electron and the hole within the catalyst has been considered difficult and considerable researches are in progress. We had succeeded to synthesize the ZnS photocatalyst with high-efficiency of hydrogen generation under UV-light irradiation. The stratified-ZnS is synthesized by a novel wet-reaction process from ZnO or Zn(OH)_2 as starting materials and has the "stratified" structure composed by nano-crystals.In this project, we had researched the relationship between the stratified structure and the photocatalytic activity. It was found that the stratified-ZnS formed capsule with a thin layer by ultra fine particles and maintained a potential gradient within this layer, from the results of the morphology observation by transmission electron microscope and the analysis of elemental distribution in the semiconductor particle by energy dispersive x-ray spectroscopy. We can consider that the effective separation of the photoexcited electron and the hole was due to the potential gradient created by the stratified structure.We also applied the synthesis process of stratified material to other compound semiconductor materials considered they can use visible region of sunlight. As the results of experiments, we found the stratified-CdS was able to synthesize from CdO or Cd(OH)_2 as starting materials by the same process in ZnS with optimized conditions. These stratified-CdS particles have photoactivity toward visible light less than 510 nm in wavelength and can generate about 7 litters/hour to 1 m^2 of sunlight irradiation. As the next step for practical application of the hydrogen generation system using the sunlight, we will carry out a research about a sulfur-recycle process by using biochemical or hydrothermal reaction, and so on.