Two-step nitridation of photocatalytic TiO2 films by low energy ion irradiation

Nitridation of TiO₂ films is performed by the simultaneous irradiation of low-energy N₂⁺ and H₂⁺ ions under substrate-heating condition. Spectroscopic observations of the resultant films clarify the formation of nitrogen-substituted TiO₂ (TiO_2−xN_x) 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.