Insulating titanium oxynitride for visible light photocatalysis

Abstract: We propose a systematic approach to obtain various forms of insulating titanium oxynitrides Ti${n}$N${2}$O${2n-3}$ and we conduct a detailed study on its $n=2$ case, Ti${2}$N${2}$O. We study the energetics and the electronic structures of Ti${2}$N${2}$O and compare these results with those of pristine and nitrogen-doped TiO${2}$ within the framework of the density-functional theory (DFT) and the GW approximation. We find that Ti${2}$N${2}$O is semiconducting with the calculated band-gap of 1.81 eV, which is significantly smaller than those of pristine TiO${2}$ rutile (3.14 eV) or anatase (3.55 eV). Furthermore, the reduction of the band-gap of Ti${2}$N${2}$O is realized not by lowering of the conduction-band minimum but by rising the valence-band maximum. Therefore the proposed Ti${2}$N${2}$O has suitable band-edge alignment for water-splitting photocatalysis. Finally, total energy calculations indicate that Ti${2}$N${2}$O is potentially easier to synthesize than nitrogen-doped TiO${2}$. Based on these results, we propose Ti${2}$N${2}$O as a promising visible-light photocatalytic material.