Regulation of electron-hole recombination kinetics on uniform metal-semiconductor nanostructures for photocatalytic hydrogen evolution

Photocatalytic hydrogen evolution has garnered considerable attention as a potential technology for the conversion of solar energy to chemical energy to replace fossil fuels with the development of hydrogen energy infrastructure. Semiconductors have been intensively studied as photocatalysts due to their tunable bandgap, eco-friendly reaction mechanism, photochemical stability, and ease of reusability. To achieve highly efficient photocatalysts, regulation of exctions, which are photoinduced electrons and holes in photocatalysts, is necessary. Semiconductor nanoparticles have been applied in this purpose because of their confined exciton pathways and differentiated catalytic characteristics depending on their size, shape, and morphology. In addition, metal cocatalysts have been decorated with semiconductor nanoparticles because the metal cocatalyst not only provides efficient shuttling of photoinduced electrons and proper reaction sites for the hydrogen evolution but also controls exciton pathways via fast electron transfer kinetics from semiconductor to metal. This research update reviews recent advances in representative metal-semiconductor hybrid nanostructures of core-shell and tipped nanorods for photocatalysts with a focus on the exciton pathways. The metal at semiconductor core-shell nanostructures has shown extraordinary photocatalytic stability via passivation of the metal by a semiconductor. In photocatalytic hydrogen evolution, the semiconductor shell hinders electron transfer to water. Hence, various core-shell related metal-semiconductor nanostructures such as yolk-shell, core-island shell, and double shell hollow structures have been proposed in efforts to overcome the electron transfer barrier to water. Metal tipped nanorods are another versatile nanostructure to control and monitor exciton pathways. The correlation between exciton pathways and photocatalytic efficiencies was demonstrated by monitoring metal tipped semiconductor nanorods with different composition, morphology, and surface structure. The insights reported here suggest a rational and versatile design strategy of metal-semiconductor hybrid nanostructures for developing highly efficient photocatalysts for hydrogen evolution.