Self-organized arrays of single-metal catalyst particles in TiO2 cavities: A highly efficient photocatalytic system

Photocatalytic reactions are based on the interaction of light with a semiconductor that is immersed in a suitable reaction environment.1 The light, if of sufficient energy (>Eg), creates electrons and holes that then can be captured from the semiconductor surface by redox states in the environment, namely a liquid or gas. Hole transfer from the valence band to the environment may be exploited for oxidation reactions, and electron transfer from the conduction band may be used for reduction reactions. Essentially, this simple principle has an extremely large potential for applications.2–8 Currently, photocatalytic features are widely explored to tackle several contemporary global challenges, including pollutant degradation (hydrocarbons, CO2, NOx),9, 10 formation of innovative self-cleaning systems,11 or to create hydrogen from solar energy, which is currently at the forefront of interest.2, 12 In the most straightforward approach, a photocatalyst is simply immersed into suitable source of a renewable substance, such as water or ethanol, and both the oxidation and reduction reaction take place simultaneously on the illuminated semiconductor surface. In practice, the charge transfer from the conduction or valence band to the environment may be strongly kinetically hindered and the semiconductor needs to be decorated by suitable charge-transfer catalysts, such as noble metal particles, to reach reasonable conversion efficiencies.