Relative photooxidation and photoreduction activities of the {100}, {101}, and {001} surfaces of anatase TiO2.

The photoredox ability of the TiO2 {100}, {101}, and {001} surfaces is investigated by examining the trapping energies, trapping sites, and relative oxidation and reduction potentials of simulated photogenerated holes and electrons in the form of more realistic polaronic states on the basis of density functional electronic structure calculations. Our results enable us to re-estimate their relative photooxidation ({100} > {101} > {001}) and photoreduction ({100} > {101} > {001}) activities, which rectify the conventional understanding. The dual functions of the surface under coordinated atoms acting as active adsorption sites for adsorbates and hindering the population of electrons to the outermost surface layer are identified, and the specific surface geometric structures also play an important role in trapping holes and electrons through the ease of lattice distortion. In addition, we attribute the commonly low photocatalytic performance of the {101} surface to the large and similar trapping energies and adjacent trapping sites for electrons and holes, which result in high electron-hole recombination rates. However, the large difference in trapping energies for electrons and holes on different surfaces allows us to spatially gather electrons and holes on different surfaces by artificially designing the exposing facets of nanocrystals without resorting to the energy band potential difference between surfaces, thus expanding the ideas to improve the photocatalytic properties of materials through the regulation of crystal facets. Our present work can provide a helpful message for the design of more reactive photocatalytic TiO2 nanocrystals and the fabrication of other reactive photocatalysts.