ZnO nanomaterials: strategies for improvement of photocatalytic and photoelectrochemical activities

Abstract ZnO is a promising material for photoanodes and applications within photocatalysis, due to its controllable morphology, excellent stability, and high velocity (>100 cm2 V−1·s−1) for charge carrier migration. In addition, the deep lying valence band edge provides a high driving force for many oxidation reactions, including water oxidation. For a tailored artificial light such as UV light–emitting diodes, ZnO photocatalysis can be very effective while the relatively wide bandgap of ∼3.3 eV yields a limitation in utilizing the full potential of the solar spectrum for photocatalysis. A lot of effort has been made to enhance the photocatalytic (PC) activity of ZnO, either by extending the absorption into the visible range by doping or by more efficient use of the absorbed photons in the UV range. In our previous studies, we have demonstrated that the PC activity of ZnO nanocrystals could be enhanced via morphology tuning, the formation of a Schottky junction with Au or Ag nanoparticles, and the combination with narrow-bandgap semiconductors. We have also shown the photoelectrochemical activity of ZnO nanorod arrays can be improved through thermal treatment or being modified with a ZnS thin layer. Another strategy is to control the electronic properties in ZnO by quantum confinement, which provides tunability of the electronic levels and introduces the ability to target specific reactions at the expense of widening the bandgap. In this chapter, we succinctly present the current progress in ZnO photocatalysis, strategies to improve and control the PC activity, and bring up the present and future prospect of ZnO as a photocatalyst.