TiO2 nanomaterials are of great interest in many fields, ranging from energy conversion and storage to coatings and biomedical applications. In particular, recently many studies have focused on “black TiO2”, which consists of photocatalytically active core-shell nanoparticles (NPs) that absorb visible light much more efficiently than pristine TiO2. It is widely accepted that the characteristic properties of black TiO2 are due to strong reduction and structural changes in the outer shell of the NPs while the core maintains the crystal structure of conventional TiO2. However, understanding of the detailed chemical composition and atomic structure of the outer shell has remained limited. To obtain insight into the stability and properties of these NPs, we 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, 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, but a novel phase, denoted csp-Ti2O3, that has not yet been reported. csp-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, suggesting a possible role of this phase in the properties of black TiO2 nanomaterials.

Formation and structure of black TiO2: Insights from first principles simulations

256th National Meeting (2018)

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