Structural design and kinetic study of metal chalcogenide-based hybrid nanostructures for photocatalytic hydrogen generation

Abstract

Research on photocatalytic hydrogen generation has been studied on-going worldwide due to its environmentally benign energy conversion process, which yields no undesirable by-products. Specifically, semiconductor-based nanoparticles undergo charge separation into electrons and holes under light irradiation. The generated photo-induced carriers participate in reduction and oxidation reactions. The integration of cocatalysts, including metals and metal oxides, can effectively decrease the energy barrier of the reaction and represent specific optical and electrical characteristics. In this research, we designed metal-semiconductor hybrid nanostructures for efficient photocatalytic hydrogen generation and conducted studies on the photophysical dynamics occurring within the catalysts. In Chapter 1, we introduced general concepts of photocatalytic hydrogen evolution and hybrid nanostructures. In Chapter 2, we conducted a study on the influence of the length of cadmium selenide nanorods and the distribution of platinum cocatalysts on the photocatalytic properties. The maximum catalytic activity was achieved when one cocatalyst was deposited per length of $15-20 nm$ in the photocatalytic hydrogen generation reaction. To elucidate this phenomenon, time-resolved spectroscopic measurements and kinetic analysis were performed. The results revealed that as the length of the nanorods increased, the electron transfer rate from cadmium selenide to platinum decreased, while the visible light absorption increased proportionally. This study contributes to providing insights into charge transport processes and optimal cocatalyst distribution in metal-semiconductor hybrid structures. Secondly, to examine the effect of the shape of platinum cocatalysts on the photocatalytic reactivity and photophysical changes, platinum nanoparticles were selectively deposited using surface-modifying ligands in cubic, round, and rough morphologies. The results showed that the rough form of cocatalyst exhibited the highest reactivity in the photocatalytic hydrogen generation reaction. Time-resolved spectroscopic measurements revealed that the charge separation state, where charges remained separated for a long time without recombination, varied depending on the shape of the platinum particles, indicating its influence on photocatalytic properties. Additionally, it was observed that the presence of a high-index surface on the rough surface of the cocatalyst affected the surface reactivity. This study contributes to the understanding that the shape control of metal cocatalysts in hybrid structures influences the crucial charge separation in photocatalysis. In Chapter 3, we conducted a study to maximize the photocatalytic activity. We controlled the metal cocatalyst in hetero-nanostructure of cadmium sulfide nanorods containing cadmium selenide quantum dots (CdSe@CdS), based on the research findings obtained in Chapter 2. The metal cocatalyst (Au@r-Pt) was synthesized of Au@Pt core-shell structure, and the platinum shell was coated in a rough form, aiming to enhance charge separation and surface reactivity. The optimized catalyst showed approximately five times higher photocatalytic activity compared to conventional platinum cocatalysts. Such catalyst design is expected to be applicable not only to metal chalcogenide nanorods but also to various materials such as metal oxides.