Preparation and photoelectrochemical properties of nitrogen doped nanotubular TiO2 arrays

The effect of nitrogen doping by ammonilysis at different temperatures on the photoelectrochemical properties of nanotubular TiO2 arrays has been investigated. Nanotubular TiO2 arrays were prepared by anodization of titanium foil. Nitrogen was doped into TiO2 by NH3 thermal treatment. Undoped and nitrided nanotubular TiO2 arrays were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and photocurrent measurements under visible light. Two characteristic N 1s peaks are ascribed to interstitial and substitutional nitrogen doping, which are respectively the dominant states at low and high nitridation temperatures. Photocurrent results suggest that at low nitridation temperature the oxygen vacancies and interstitial nitrogen contribute to the improvement of the photocurrent density of nitrogen doped samples. However, at high nitridation temperature, the substitution of an oxygen atom by a nitrogen atom produces a proton, which attracts a photogenerated electron that is trapped by an oxygen vacancy to form a recombination center. Consequently, the photocurrent decreases. At the highest nitridation temperature TiN is formed whose presence results in very low photocurrents. The optimum conditions of nitrogen doping with respect to the photocurrent density appears to be nitridation at 500 °C resulting in a surface N content of 0.4 at%.