A Study on Doped Heterojunctions in TiO Nanotubes: An Efficient Photocatalyst for Solar Water Splitting.

The two important factors that affect sunlight assisted water splitting ability of TiO are its charge recombination and large band gap. We report the first demonstration of nitrogen doped triphase (anatase-rutile-brookite) TiO nanotubes as sun light active photocatalyst for water splitting with high quantum efficiency. Nitrogen doped triphase TiO nanotubes, corresponding to different nitrogen concentrations, are synthesized electrochemically. Increase in nitrogen concentration in triphase TiO nanotubes is found to induce brookite to anatase phase transformation. The variation in density of intra-band states (Ti and N 2p states) with increase in nitrogen doping are found to be critical in tuning the photocatalytic activity of TiO nanotubes. The presence of bulk heterojunctions in single nanotube of different nitrogen doped TiO samples is confirmed from HRTEM analysis. The most active nitrogen doped triphase TiO nanotubes are found to be 12 times efficient compared to pristine triphase TiO, for solar hydrogen generation. The band alignment and charge transfer pathways in nitrogen doped TiO with triphase heterojunctions are delineated. Bulk heterojunctions among the three phases present in the nanotubes with intra-band defect states is shown to enhance the photocatalytic activity tremendously. Our study also confirms the theory that three phase system is efficient in photocatalysis compared to two phase system.