Photoelectrochemical and Structural Properties of TiO2 and N-Doped TiO2 Thin Films Synthesized Using Pulsed Direct Current Plasma-Activated Chemical Vapor Deposition

Amorphous TiO 2 thin films were synthesized using a pulsed direct current plasma deposition technique. The films were prepared in the discharges of Ar, Ar + O 2, and Ar + N 2 . The original and annealed samples were characterized using X-ray diffraction, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy, ultraviolet and visible spectroscopy (UV−vis), and electrochemistry. The presence of oxygen was found to be essential in the annealing medium for the formation of polycrystalline films. By optimizing the experimental conditions, we obtained TiO 2 films that showed 80% incident photon conversion efficiency for splitting of water near 300 nm. Films containing up to 12 atom % N were obtained when mixtures of Ar and N 2 were used as the plasma source gas. Although there is evidence for the presence of Ti−N bonds in the amorphous film, the annealing in the presence of oxygen to obtain polycrystalline films led to decomposition of these bonds. The resulting polycrystalline films contained 1−2 atom % of N (XPS peak at 399.5 ± 0.5 eV) and showed visible light absorption. However, in contrast to recent reports in the literature for powdered materials with similar XPS and UV−vis characteristics oxidation of water or formic acid could not be achieved using these films under visible light illumination. There is evidence that the holes generated in the occupied N 2p midgap levels are recombining efficiently at the carbon-impurity sites.