Recently a great deal of attention has been focused on “black TiO2”, a modified TiO2 material that can absorb visible light much more efficiently than pristine TiO2. Black TiO2 consists of nanoparticles (NPs) with a crystalline TiO2 core covered by a highly reduced outer shell, whose chemical composition and atomic structure are not known in detail. To obtain insight into the stability and properties of these NPs, we have carried out first principles calculations on model structures consisting of reduced overlayers on the majority (101) surface of anatase, the TiO2 phase typically found in nanomaterials. The overlayers are formed by aggregation of extended defects known as crystallographic shear planes (CSPs), which are relatively frequent in TiO2 and other reducible oxides.
Our results show that formation of a reduced overlayer (“shell”) on the anatase surface is thermodynamically favorable under a wide range of experimental conditions. This shell has Ti2O3 stoichiometry and its structure is not the well-known corundum-like Ti2O3 phase (the mineral ”tistarite”), but a novel phase that has not yet been reported. DFT calculations with various exchange-correlation functionals predict that this new phase − denoted csp-Ti2O3, to distinguish it from standard corundum Ti2O3 − is very close in stability to corundum Ti2O3 and has a band gap of 1.2-1.8 eV, which is consistent with the absorption of black TiO2. These findings suggest a possible important role of csp-Ti2O3 in the properties black TiO2 nanomaterials. In a broader context, analogous structures could be relevant for describing the reduced surface region of nanomaterials of other metal oxides as well.