  In the present work, extended Argon ion bombardment was used to modify
anatase TiO2 nanotube (NTs) layers. In situ X-ray photoelectron spectroscopy
revealed a high content of sub-oxide formed (Ti3+, Ti2+ states) in the nanotube
layers. These samples were tested for their ability to show open-circuit
photocatalytic hydrogen evolution. We found for UV (cw laser 325nm)
illumination as well as for AM1.5 conditions, a strongly enhanced H2 evolution
activity in absence of the usual noble metal-nanoparticle decoration. We
conclude therefore that these sub-oxide states can play an important role in
the activation of TiO2 nanotubes - namely as a H2 evolution co-catalyst.
