ABSTRACT
Solar light is the most abundant source of energy (15,000 GW annual) on Earth. The estimated practical solar power that is received by Earth is 40 times greater than the present global energy need (Fujishima and Honda 1972). Articial photosynthesis (AP) is a key strategy for conversion of solar energy into chemically stored energy, that is, hydrogen. Hydrogen, being a clean source of energy, is predicted to be the “fuel of the future” (Dicks 1996; Armor 1999). Water proved itself as a carbon-free renewable energy source for AP and attracted immense attention from researchers. But splitting of water needs a smart material (semiconductor/photocatalyst) to reduce aqueous protons to H2 fuel and oxidize water to make O2. Both redox reactions are multielectron processes that need two and four electrons for reduction and oxidation of water to molecular hydrogen and oxygen, respectively. Understanding how this two-electron or four-electron reaction works in detail is important for the development of improved robust catalysts made of Earth-abundant materials. In this chapter, we try to unveil the mechanistic part of water splitting by identifying the intermediates in the aforementioned processes and their changes in physical and chemical behavior that belong to different catalytic sites. Knowledge of the structure and kinetics of surface intermediates will enable the design of improved metal oxide materials for more efcient water cleavage for hydrogen and oxygen generation. Mimicking natural photosynthesis, recent efforts have also provided insights into photocatalytic systems for oxygen generation (Sirjoosingh and Hammes-Schiffer 2011; Iyer et al. 2012). Periodic work has been conducted on several photocatalysts, although, to date, no published work dealing with photoreactions (with proof) is known. Before discussing some of these results, it is worthwhile to give a brief introduction on the surfaces of the photocatalysts. This will be followed by the brief introduction of the chemical and physical processes considered to be involved in photocatalytic reactions over a semiconductor.
