Compensation of band-edge positions in titanium-doped Ta3N5 photoanode for enhanced water splitting performance: A first-principles insight

${\mathrm{Ta}}_{3}{\mathrm{N}}_{5}$ is a promising photoanode for solar water splitting. However, it often suffers from high onset potential for water oxidation, which may be partially ascribed to the oxygen impurities in ${\mathrm{Ta}}_{3}{\mathrm{N}}_{5}$. Oxygen impurities, which are always introduced into ${\mathrm{Ta}}_{3}{\mathrm{N}}_{5}$ during the preparation process, are difficult to remove due to the low formation energy of O-doped ${\mathrm{Ta}}_{3}{\mathrm{N}}_{5}$. The valence- and conduction-band-edge positions shift almost in parallel towards more positive potentials with addition of oxygen impurities, which may increase the onset potential for water oxidation. In this study, the hybrid-DFT (density functional theory) calculations of Ti-doped ${\mathrm{Ta}}_{3}{\mathrm{N}}_{5}$ show that as Ti doping concentration increases, both the valence- and conduction-band-edge positions of ${\mathrm{Ta}}_{3}{\mathrm{N}}_{5}$ move towards more negative potentials, which is opposite to the role of oxygen impurties. In the case of Ti-O codoped ${\mathrm{Ta}}_{3}{\mathrm{N}}_{5}$, Ti doping can compensate the effect of oxygen impurities and may reduce the onset potential for water oxidation. Defect formation energies reveal that Ti-O codoped ${\mathrm{Ta}}_{3}{\mathrm{N}}_{5}$ is thermodynamically stable. Therefore we propose that by controlling the amount of O and Ti, the band-edge positions can be modified to a proper level so that better photoelectrochemical performances for solar water splitting can be achieved.