UTILIZATION OF SUNLIGHT USING STRATIFIED PHOTOCATALYSTS

利用光催化反应制氢作为能源是光化学研究的重点。然而，如何防止被激发的电子与催化剂内部空穴的再结合一直被认为是困难的，目前正在进行大量的研究。在紫外光照射下，成功合成了产氢效率高的ZnS光催化剂。以ZnO或Zn(OH)_2为原料，采用新型湿法合成了层状zns，具有由纳米晶体组成的“层状”结构。在这个项目中，我们研究了层状结构和光催化活性之间的关系。透射电镜形貌观察和x射线能谱分析结果表明，层状zns由超细颗粒形成薄层，并在薄层内保持电位梯度。我们可以认为，光激发电子与空穴的有效分离是由于层状结构产生的势梯度。我们还将分层材料的合成工艺应用于其他化合物半导体材料，认为它们可以利用太阳光的可见区域。实验结果表明，在优化的条件下，以CdO或Cd(OH)_2为原料，在ZnS中以相同的工艺合成了层状cds。这些层状cds颗粒对波长小于510 nm的可见光具有光活性，并且可以产生约7升/小时至1 m^2的阳光照射。作为太阳能制氢系统实际应用的下一步，我们将开展生化或水热反应等硫磺回收工艺的研究。

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.