Semiconductor photocatalysis has been applied to various energy and environmental problems: it has been used for water splitting to produce hydrogen gas, CO₂ reduction, waste-water treatment, odor control, and synthesis of chemicals.^1–4  However, although there are numerous different semiconductors, only a few of them are suitable as practical photocatalysts. For a semiconductor to be an efficient photocatalyst, it should absorb as much solar energy as possible. Figure 5.1(a) shows the solar spectra and possible solar-energy absorption ranges of various semiconductors. Figure 5.1(b) shows the diffuse reflectance UV-visible spectra of some widely used semiconductors. Wide band-gap semiconductors such as TiO₂ can absorb only a small portion (i.e., the UV light region) of solar energy, while small band-gap semiconductors including Fe₂O₃, CdS, and Si can extend their absorption spectrum into the visible light region of solar energy. This absorption of solar energy is the first step of semiconductor photocatalysis to convert solar energy into chemical energy.