Nitridation of TiO2 films is performed by the simultaneous irradiation of low-energy N-2(+) and H-2(+) ions under substrate-heating condition. Spectroscopic observations of the resultant films clarify the formation of nitrogen-substituted TiO2 (TiO2-xNx) with large N fractions and the agglomeration of undesirable oxynitride species attributed to the deep states in the band gap. We find that the addition of a thin TiO2 cap layer on the ion-irradiated films improves the nitrogen bonding structure and distribution near the surfaces, leading to a good photocatalytic performance even in the visible region. (C) 2007 Elsevier B.V. All rights reserved.