Structural and electronic studies of selected titanium oxides and oxynitrides.

Abstract

Titanium dioxide (TiO2) is an abundant, chemically stable, non-toxic, and highly versatile material, with applications ranging from photovoltaics to catalysis. TiO2 rutile and anatase have band gaps of 3.0 eV and 3.2 eV, respectively, which is too large to absorb visible light, resulting in low photocatalytic efficiency. Efforts have been undertaken to generate a band gap suited for solar energy absorption in order to improve TiO2's photo-activity under visible light (400 nm to 700 nm). Nitrogen doping into TiO2 in particular has been able to narrow its band gap, resulting in an absorption tail in the visible-light region. However, TiO2 has limits to which it can be doped suggesting investigations of the oxygen-deficient corundum Ti2O3. Using the state-of-the-art density functional theory (DFT), in the Quantum ESPRESSO package, the properties of the oxides were studied and presented in this work. The structural and electrical properties of the oxides were computed using the generalized gradient approximation (GGA). Ti2O3 exhibited metallic properties, yet it has been reported to have semi-conducting characteristics experimentally leading an improved prediction of the bandgaps of the oxides using the DFT+U approach. On doping, the band gaps of N doped TiO2 structures were reduced as dopant concentration was increased. Mid gap states, having shallower energies in 4%N doping than 2%N cases, were observed in N: TiO2 structures. However, TinN2O2n-3, n=2, appeared to have a higher absorption threshold than other Ti-based oxides such as TiO2, N: TiO2, and Ti2O3. The most stable sample of the oxynitrides (Ti2N2O_P1) had a gap of 2.2 eV, this is clearly near the middle of visible light and did not have mid-gap states. This suggests that they are more efficient visible-light-driven materials for photocatalytic applications compared to TiO2, N: TiO2, and Ti2O3.