Abstract
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.
Highlights
Hydrogen is currently attracting a great deal of attention as a generation energy source and a potential alternative to fossil fuels, which are the main cause of global warming
Among numerous hydrogen production processes including classical reforming from fossil fuels and biomass, a direct photolysis from pure water using solar energy has emerged as a potential way to achieve eco-friendly hydrogen production
The superior photocatalytic activity of the Au-CdS yolkshell structure was due to the synergistic effect of radiative relaxation of the plasmon energy in the Au core and multiple reflections of the incident light within the voids of the yolk-shell structure. These findings demonstrate that surface plasmon resonance of the metal scitation.org/journal/apm core is an additional significant factor to promote the photocatalytic hydrogen evolution
Summary
Hydrogen is currently attracting a great deal of attention as a generation energy source and a potential alternative to fossil fuels, which are the main cause of global warming. Kamat and co-workers observed much faster kinetics of electron transfer to Pt in CdSe/Pt than in bare CdSe nanoparticles by monitoring bleaching recovery with methyl viologen.18 In these experiments, exciton transfer can be artificially regulated by carefully controlled decoration of a metal cocatalyst on the semiconductor. An intermediate (adsorbed proton in this case) of the photocatalytic reaction, which is very important to determine catalytic activity in multistep reactions, is modulated at the same time In this semiconductor-metal hybrid catalyst system, the control of morphology with a direct junction between different domains is very important. We revisit how the intermediates of photocatalytic hydrogen evolution correlate with the morphology of semiconductor and metal domains This can provide essential information on how to modulate exciton pathways by regulating the catalyst morphology for an efficient photocatalytic hydrogen evolution reaction
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