Study of N-doped TiO2 thin films for photoelectrochemical hydrogen generation from water

The present work deals with nitrogen-doped stoichiometric TiO2:N and non-stoichiometric TiO2-x:N thin films deposited by means of dc-pulsed reactive sputtering for application as photoanodes for hydrogen generation from water, using solar energy. Stoichiometric thin films of TiO2 crystallize as a mixture of anatase and rutile while rutile phase predominates in TiO2:N at higher nitrogen flow rates as shown by X-ray diffraction at grazing incidence, XRD GID. Lack of bulk nitridation of stoichiometric TiO2:N is indicated by valence-to-core X-ray emission spectroscopy, XES, analysis. The energy band gap as well as flat band potential remain almost unaffected by increasing nitrogen flow rate in this case. In contrast to that, non-stoichiometric thin films of TiO2-x:N demonstrate systematic evolution of the structural, morphological, optical and photolectrochemical properties upon increasing level of nitrogen doping. Pronounced changes in the growth pattern of non-stoichiometric TiO2-x:N upon varied nitrogen flow rate, demonstrated by scanning electron microscopy, SEM, can be easily correlated with the crystallographic properties studied by XRD GID. Relative positions of K beta'' XES lines of the TiO2-x:N thin films, which depend strongly on the nature of the ligands and their local coordination, change with the increasing nitrogen flow. Doping of nonstoichiometric titanium dioxide with nitrogen shifts the absorption spectrum towards the visible range and increases considerably the flat band potential which is beneficial for water photolysis.