24. Visible-Light-Responsive Photocatalysts and Photoelectrodes Using WO₃ Semiconductors for Degradation of Organics and Water Splitting

Visible-light-responsive photocatalysts have been extensively investigated for indoor or in-vehicle applications. Conventional TiO₂ photocatalysts are responsive in the UV region and hence cannot be utilized under fluorescent light through acrylic plate, LED light, or sunlight through UV-cut glass, as the intensity of UV is negligible. However, there are large total photon numbers in the visible region from these light sources, and therefore the activity of visible-light-responsive photocatalysts is high compared with that of TiO₂. Tungsten oxide (WO₃) is a visible-light-responsive photocatalyst, which absorbs light up to ca. 480 nm. Compared with mixed metal oxides and doped oxides, WO₃ is easy to prepare, modify, and coat onto substrates. The absorption coefficient of WO₃ is very high due to the direct photon transition, and the amount of absorbed photons under fluorescent and sunlight is 10 and 3 times higher than that of TiO₂, respectively. WO₃ also is non-toxic and is stable in acidic and oxidative conditions. Considering these advantages, WO₃ semiconductors are of interest for further studies for use as photocatalysts. Here, the development of various WO₃ photocatalysts for degradation of various organic compounds and for water splitting is reported. Moreover, WO₃ particles have been used to fabricate porous and nanocrystalline photoelectrodes for water splitting. The charge separation of e⁻ and h⁻ occurs in the semiconductor nanoparticles; therefore, the nanocrystalline photoelectrodes are called “photocatalytic photoelectrodes.” The development of various WO₃ nanocrystalline photoelectrodes with other semiconductors such as BiVO₄ is also reported.