First principles investigation of GaNbO₄ as a photocatalytic material

We have performed first principles density functional total energy calculations on pure and doped GaNbO₄ to investigate its applicability as a photo catalyst. Pure GaNbO₄ is an indirect, wide band gap semiconductor similar to the widely investigated TiO₂ which is known to be a photo catalyst in UV light [K. Yang et. al. Chem. Mater. 20, 6528 (2008)]. S atom doping of TiO₂ reduces the band gap [F. Tian et. al. J. Phys. Chem. B 110, 17866 (2006)], and increases its efficiency in the visible light range. It has been experimentally reported that S doping of GaNbO₄ at the O site, decreases its photo catalytic efficiency. Our band structure calculations show that both pure and doped GaNbO₄ have indirect band gaps and S atom doping reduces the band gap in agreement with experiments. The decrease in the band gap is due to the lowering of the conduction band minimum towards the Fermi level. An unequal reduction in the band gap was observed at the four inequivalent O sites chosen for S doping. This suggests that the photo catalytic activity varies with the dopant site.