Development of photocatalytic hybrid nanostructures and their applications for energy generation reactions

Abstract

Against the deficiency of petroleum resources, efficient generation and management of alternative energy resources is one of the most urgent topics that human has to resolve right now. Photocatalytic energy generation has attracted a huge attention as an ideal reaction due to the use of renewable solar energy. Water splitting using $TiO_2$ electrodes was devised by Honda and Fujishima in the early 1970s. However, photocatalysts, developed thus far, commonly use ultraviolet light owing to their large band gap energies, which lose the visible and near-IR regions that majorly dominate the solar spectrum. In addition, there are still few papers are reported using well defined nanostructures for photocatalytic reactions. In part 2, we synthesized metal-CdS hybrid hollow nanocubes with well-defined structures and focused on the photocatalytic effect of co-catalysts with various metal species (Pt, Pd and Au). The reaction tendencies along the co-catalyst species were investigated in detail by photocatalytic H2 generation and photoelectrochemical analysis. In the stability test, Pt/CdS double shell hollow nanocatalysts exhibited excellent durability maintained up to 10-cycles. To enhance the catalytic reactivity and stability in general, this result is one of the best approach to consider the optimal structure or composition of photocatalysts. In part 3 and 4, we synthesized ZnO-$Cu_2 O$ hybrid nanoparticles with well-defined morphologies for the photocatalytic $CO_2$ reduction. Photochemical $CO_2$ reduction reactions were conducted using the ZnO-$Cu_2 O$ catalysts both in an aqueous phase and in a gas phase condition. The catalysts showed high reactivity and remarkable selectivity compared to the photocatalysts reported thus far. The detailed mechanism was also carefully investigated. It will be invaluable to design and synthesize well-defined nanostructures for other materials useful for photochemical reactions. It will also help to understand the mechanisms and key factors to achieve maximum catalytic performances. Furthermore, this result will help to approach practical processes of $CO_2$ management in industrial scales.