An electrochemical strategy to incorporate nitrogen in nanostructured TiO2 thin films:: modification of bandgap and photoelectrochemical properties

We describe a simple and facile electrochemical method to introduce anionic dopants into TiO2. N-doped thin films with chemical composition TiO2-xNx, up to x = 0.23, were fabricated by anodic oxidation of a pure titanium sheet in electrolyte solutions containing ammonium ions, nitrate ions and fluoride ions enabling simultaneous nanostructuring and doping of the growing anodic oxide. Analysis by x-ray photoelectron spectroscopy (XPS) indicates that nitrogen atoms substitute for oxygen sites within the TiO2. F atoms were present in the amorphous, as-anodized samples but were resubstituted by O atoms upon annealing in oxygen at temperatures higher than 600 degrees C. For nitrogen doped films UV-vis spectroscopy indicates a shift in the primary absorption threshold as well as significant optical absorption in the visible wavelength range from 400 to 530 nm. The concentration of the incorporated anionic dopants, and the morphology of the doped thin film are strong functions of electrolyte chemistry and anodization time. Longer anodization periods resulted in a well-developed nanotube-array structure but smaller amounts of incorporated nitrogen. XPS depth profiling reveals the nitrogen doping to be inhomogeneous, with maximum nitrogen incorporation occurring near the oxide-electrolyte interface at the surface of the anodized film.