Research on photocatalytic reactions by using semiconductor as a main element has been studied on-going worldwide due to its characteristics generating electrons and holes by absorbing sunlight and applying them to catalytic reactions. In addition, when materials are controlled to be in a nanoscale, new characteristics are emerged due to the quantum confinement effect. In this respect, researches on the synthesis of nanoparticles and their application for photocatalysis have been explosively increased in the 2000s. Among them, hybrid nanostructure has been spotlighted because it can not only combine individual characteristics, but also express new optical and electrical properties by synergetic effect. Therefore, in this research, mechanistic studies of photocatalytic small molecule activation using semiconductor-based hybrid nanocatalyst have been carried out based on engineering the elaborated design of hybrid nanosystems.
In Chapter 2, metal-semiconductor hybrid nanostructures with metal-tipped CdSe nanorods were synthesized, which were employed for observing photocatalytic hydrogen generation and exploring the mechanism. First, we developed a synthetic method asymmetrically attaching different metals, platinum and gold on both ends of CdSe nanorods, and applied them to hydrogen evolution by photocatalytic water splitting. Compared to nanostructures in which platinum or gold was symmetrically attached, the asymmetric structure exhibited a nearly twice activity. In order to clarify the mechanism, photoelectrochemical and transient absorption spectroscopy analyses were conducted. From these experiments, we found out that the electron-hole recombination rate between the interface of metal and semiconductor is highly dependent upon the type of metal. Second, we have found a correlation between photocatalytic hydrogen production and the composition of semiconductor alloy in metal-semiconductor hybrid structure. The composition, which was adjusted from ZnSe to CdSe, directly controlled the band gap of the semiconductor. The light absorbing efficiency and the overpotential for hydrogen reduction were also changed. In addition, electron mobility was dependent upon the atomic composition ratio. Therefore, in this study, it was found that the electron mobility as well as the light absorption efficiency played a crucial role in the catalytic reaction efficiency. Finally, we investigated the effect of the length of the semiconductor rods on hydrogen evolution. Since the moving distance of electrons varied according to the length of the semiconductor, it directly influenced on the catalytic activity. In fact, the catalytic tendency was varied according to the length of nanorods between metal single- and double-tipped CdSe nanorods. This was probably due to the difference in the distribution of holes depending on the number of metal sites. Based on these studies, it was expected that the efficiency could be maximized by designing a catalyst having suitable morphology and composition for desired reaction.
In Chapter 3, we conducted the photocatalytic $CO_2$ reduction using cobalt oxide and found the reaction mechanism. CoO nanoparticles were activated by the addition of N-bromosuccinimide (NBS), which carried out multiple actions of the surface oxidation to $Co_3O_4$, the removal of surfactants, and the coating of Br on the surface. As a result of detailed reaction mechanism analysis, it was found that surface activation had a decisive influence on the efficiency and selectivity of CO2 reduction. This surface activation is expected to be applicable not only for CoO but also for other metal oxides, which provide potential application in industry when its detailed reaction mechanism is closely understood.