ABSTRACT
Photocatalysis reactions occurring on a semiconductor nanomaterial’s surface have attracted intensive attention with the aim to utilize solar energy and thus address the increasing global concerns of environmental remediation and energy consumption. From the point of view of photochemistry, photocatalysis aims to enable or accelerate the specic reduction/oxidation reactions by the excited semiconductor. Typically, the electronic energy structure within a semiconductor consists of three distinguished regimes: conduction band (CB), valence band (VB), and the forbidden band (band gap, Eg). The semiconductor absorbs light and causes interband transitions if the energy of the incident photons matches or exceeds the band gap, subsequently exciting electrons from the VB into the CB in the femtosecond time scale and leaving holes in the VB. This stage is referred to as the semiconductor’s “photoexcited” state. Typically, the CB electrons can act as reductants with a chemical potential of +0.5 to −1.5 V vs. the normal hydrogen electrode (NHE), while the VB holes exhibit an oxidative potential of +1.0 to +3.5 V vs. NHE.1 The excited electrons and holes in a semiconductor migrate to the surface and can be trapped by the trapping sites there. These surface holes and electrons can oxidize and reduce surface-adsorbed species through interfacial charge transfer and surface reactions. Figure 11.1 illustrates the basic mechanism of a semiconductor photocatalytic process. During the migration process, recombination of photogenerated charge carriers may occur either in the bulk or on the surface by dissipating the energy as light or heat, thus suppressing the photocatalytic activity. It is noted that the recombination process is usually enhanced by impurities or defects in the crystal.
