Black TiO2 nanotubes: cocatalyst-free open-circuit hydrogen generation.

Here we report that TiO2 nanotube (NT) arrays, converted by a high pressure H2 treatment to anatase-like "black titania", show a high open-circuit photocatalytic hydrogen production rate without the presence of a cocatalyst. Tubes converted to black titania using classic reduction treatments (e.g., atmospheric pressure H2/Ar annealing) do not show this effect. The main difference caused by the high H2 pressure annealing is the resulting room-temperature stable, isolated Ti(3+) defect-structure created in the anatase nanotubes, as evident from electron spin resonance (ESR) investigations. This feature, absent for conventional reduction, seems thus to be responsible for activating intrinsic, cocatalytic centers that enable the observed high open-circuit hydrogen generation.