(V) Electrochemical tuning of phase-junctions in TiO2 for enhanced H2 generation by solar water splitting

Hydrogen generation by water photolysis is an uphill (ΔG>0) multi-electron process, which is kinetically and stochastically hindered as simultaneous multi-photo absorption by a single particle is forbidden. However, in TiO₂ with anatase-rutile-brookite (ARB) hetero-junctions the multi-electron process is accelerated because of the cascade charge transfer through the sequential hetero-junctions. 2 & 3 phase TiO₂ nanotubes are synthesized by anodizing Ti foils in perchloric acid, by adjusting potential and electrolyte concentration. The presence of ARB phase junctions in each nanotube is confirmed using HR-TEM and XRD. The band gap is obtained from UV-vis spectroscopy. Water splitting is performed by irradiating, the photocatalyst dispersion in ethanol-aqueous solution (1:4), using a solar simulator under one sun. The generated hydrogen is measured using a gas chromatograph. The highest hydrogen evolution of 645 µmol/g is obtained for A:R:B (45:26:29).

The major drawback of TiO₂ is its visible light inactivity. It can be made visible light active by N-doping. N-doped ARB nanotubes are obtained by anodizing Ti in perchloric acid containing hydrazine hydrate. Nitrogen incorporation and visible light activity are confirmed by XPS and UV-vis spectroscopy. The hydrogen generation of N-ARB is 24 times of pristine ARB. Since the dopants often act as recombination centres affecting the photocatalytic efficiency, visible light active dopant free ARB are synthesized by annealing ARB in H₂. Visible light absorption and introduction of native defects are confirmed by UV-vis and XPS respectively. The hydrogen generation efficiency of the TiO₂ nanotubes having defective ARB hetero-junctions are found to be 40 times that of pristine ARB hetero-junctions.

Based on the band gap and energy gap values between the Fermi level and the valence band maxima of anatase, rutile and brookite, obtained from PL and UPS respectively, the band diagram and the charge transfer pathway for pristine, N-doped and defective ARB systems are delineated.