Formation and stability of reduced TiOx layers on anatase TiO2(101): Identification of a novel Ti2O3 phase

We use density functional theory (DFT) calculations to investigate structural models consisting of anatase TiO2(101) slabs covered by reduced overlayers formed by (101) crystallographic shear planes (CSPs). Ab initio thermodynamics supports the stability of these structures under a wide range of experimental conditions. The overlayers are found to have Ti2O3 stoichiometry with a crystal structure different from the known corundumlike Ti2O3 (here denoted a alpha-Ti2O3) phase. DFT calculations predict this new "csp-Ti2O3" phase to be energetically close to alpha-Ti2O3 and to have also a similar band gap. These results suggest a possible role of the csp-Ti2O3 phase in the properties of black TiO2, a promising photocatalytic material made of nanoparticles with a crystalline TiO2 core and a highly reduced TiOx shell that is capable of absorbing the whole spectrum of visible light.