Photoelectrochemical and water photoelectrolysis properties of ordered TiO2 nanotubes fabricated by Ti anodization in fluoride-free HCl electrolytes

Described is the synthesis of TiO2 nanotube array films by anodization of Ti foil in HCl electrolytes containing different H 2O2 concentrations. Highly ordered nanotube arrays up to 860 nm in length, 15 nm inner pore diameter, and 10 nm wall thickness were obtained for one hour anodizations using a 0.5 M HCl aqueous electrolyte containing 0.1-0.5 M H2O2 concentrations for anodization potentials between 10-23 V. The use of ethylene glycol as the electrolyte medium significantly alters the anodization kinetics and resulting film morphologies; nanotube bundles several microns in length achieved for anodization potentials between 8 V and 18 V in only a few minutes. The nanotube arrays obtained from the ethylene glycol electrolytes show relatively higher photocurrents, ≈0.8 mA cm-2 under AM 1.5. Under 100 mW cm-2 AM 1.5 illumination a 500 °C annealed 1 cm2 nanotube array sample, obtained by anodization of a Ti foil sample in ethylene glycol + 0.5 M HCl + 0.4 M H2O2 electrolyte, demonstrates a hydrogen evolution rate of approximately 391 μL h-1 by water photoelectrolysis, time-power normalized evolution rate of 3.9 mL W-1 h-1, with water splitting confirmed by the 2: 1 ratio of evolved hydrogen to oxygen. © 2008 The Royal Society of Chemistry.