Photocatalysis and surface doping states of N-doped TiOx films prepared by reactive sputtering with dry air

Complex N (NO)-doped TiOx films on the glass substrate were prepared by radio-frequency (RF) magnetron reactive sputtering of Ti target in a mixed gas of argon and dry air with low oxygen concentrations. The surface doping states and energy band gap properties were studied by XPS and density-functional theory (DFT) applied to a 2 x 2 x 1 supercell of N-doped TiO2 with oxygen deficiency. Although all the films exhibit an anatase structure, the photocatalytic properties as well as other film properties (lattice parameters, grain sizes, introduced nitrogen contents, and optical properties) largely depend on the air flow ratios. The electronic bonding configurations at the nanoscale film surface are extremely important for photocatalysis, and there appears an optimal surface nitrogen amount incorporated in the anatase TiO2 lattice. Reduced Ti ions (and regions) at the nanoscale surface are proposed to play an important role by providing a local charge imbalance through the Schottky-barrier-like mechanism. Our DFT calculation shows the modified band calculation, especially for the film surface, involving both oxygen deficiency and N (NO) doping is important, due to the variation in the number and location of the impurity levels in the energy band gap. It is suggested that interstitial NOx (or substitutional NO) doping states with oxygen vacancy involving N-Ti-O or Ti-N-O bondings (linkages) are more effective on photocatalysis than the substitutional N doping states with oxygen vacancy. The seemingly desired impurity energy level(s) introduced in the electronic band gap does not necessarily improve photocatalysis, despite the desired optical properties observed, due to the active recombination sites newly produced and the complexity of the nanoscale surface science. (C) 2009 Elsevier B.V. All rights reserved.