Impact of adsorbed alkali ions on photoelectrochemical hydrogen production by titania nanotubes

While titania nanotubes are expected to have important applications in the photoelectrochemical production of hydrogen, it is found that surface-adsorbed ions on the nanotube surface, specifically alkali ions, have a significant impact on the photoelectrochemical water-splitting characteristics. Such surface-adsorbed alkali ions are invariably a result of the electrochemical fabrication process which uses an alkali-based electrolyte for the production of vertically aligned array of titania nanotubes. The presence of surface-adsorbed ions is revealed in synchrotron-based near edge X-ray absorption fine structure (NEXAFS) studies. Photoelectrochemical behavior of such surface-adsorbed titania nanotubes is found to be significantly altered when compared with pure titania nanotubes which may be explained in terms of a simple phenomenological model predicting modifications to the energy band structure of titania as a result of surface adsorption.