A Study on Doped Heterojunctions in TiO₂ Nanotubes: An Efficient Photocatalyst for Solar Water Splitting

The two important factors that affect sunlight assisted water splitting ability of TiO₂ are its charge recombination and large band gap. We report the first demonstration of nitrogen doped triphase (anatase-rutile-brookite) TiO₂ nanotubes as sun light active photocatalyst for water splitting with high quantum efficiency. Nitrogen doped triphase TiO₂ nanotubes, corresponding to different nitrogen concentrations, are synthesized electrochemically. Increase in nitrogen concentration in triphase TiO₂ nanotubes is found to induce brookite to anatase phase transformation. The variation in density of intra-band states (Ti³⁺ and N 2p states) with increase in nitrogen doping are found to be critical in tuning the photocatalytic activity of TiO₂ nanotubes. The presence of bulk heterojunctions in single nanotube of different nitrogen doped TiO₂ samples is confirmed from HRTEM analysis. The most active nitrogen doped triphase TiO₂ nanotubes are found to be 12 times efficient compared to pristine triphase TiO₂, for solar hydrogen generation. The band alignment and charge transfer pathways in nitrogen doped TiO₂ 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.