Strategic Design of Heterojunction CdS Photocatalysts for Solar Hydrogen

A number of photocatalytic semiconductors that can produce hydrogen from water via sunlight or visible light are known. Among them, CdS may be the most appropriate because of its narrow bandgap (~2.5 eV) and suitable conduction band (−0.75 V vs. NHE) and valence band (+1.75 V vs. NHE) levels. Although photocorrosion still limits its widespread applicability, CdS is very useful as a model semiconductor for producing solar hydrogen. To improve the photocatalytic activity of CdS, charge separation and charge injection should be simultaneously considered. The simplest and most effective way to enhance charge separation is to couple with noncorrosive, wide bandgap oxide semiconductors such as TiO₂. TiO₂ plays a dual role in hybrids as it supports CdS and prevents its aggregation and enhances charge separation by forming a potential gradient at the interface of CdS and TiO₂. Importantly, the morphologies of CdS and/or TiO₂ greatly influence the overall photocatalytic activity of the hybrids. This chapter will briefly describe the importance of the hybrid configuration in terms of charge separation and morphological effects will be discussed in detail (e.g., CdS bulk/TiO₂ nanoparticles, CdS nanoparticles/TiO₂ bulk, CdS nanoparticles/TiO₂ nanosheets, and CdS nanowires/TiO₂ nanoparticles). Charge injection can also be improved by coupling with hydrogen-evolution catalysts (e.g., Pt-group metals). Pt nanoparticles are often deposited on CdS or CdS/TiO₂. However, the effects of Pt are diverse and often contradictory. The inconsistencies are most likely related to chemical interactions at the CdS/Pt interface. Instead of expensive Pt-group metals, inexpensive carbon-based materials such as carbon blacks, activated carbons (AC), carbon nanofibers, single- and multiwalled carbon nanotubes, graphite, graphite oxides, and reduced graphene oxides can be utilized. Carbon-based materials are very attractive because of their unique physicochemical properties such as thermal conductivity, electrical resistivity, BET surface area, and sp valence hybrid configuration. Nevertheless, the following questions still remain: Which physicochemical property of carbon-based materials is the primary factor in the catalysis of solar hydrogen in water? Why do carbon-based materials show different catalytic effects? To what extent can the carbon-based materials enhance the production of solar hydrogen? This contribution will address these questions and discuss the diverse effects of carbon-based materials.