Suppression of photocatalytic efficiency in highly N-doped anatase films

We report the role of N in epitaxial films of anatase TiO2. The films were artificially grown with a two-step temperature-tuned epitaxy which utilized the high-temperature cubic phase of LaAlO3 substrates. The preparation of highly crystallized anatase with various N concentrations (C-N <= 3.85 at. %) allowed us to identify the optimum dopant concentration (C-N=1-2 at. %). At higher doping levels, N is found to be difficult to substitute for O having been predicted to contribute to the band-gap narrowing, giving rise to the undesirable deep-level defects. In addition, a study by x-ray and Raman spectroscopy revealed that the growth of anatase became more difficult, and the stable phase was shifted to rutile at the higher N concentrations.