On the apparent visible-light and enhanced UV-light photocatalytic activity of nitrogen-doped TiO₂ thin films

Nitrogen-doped titania (N—TiO₂) thin films were synthesized using atmospheric-pressure chemical vapor deposition (APCVD) using ammonia, tert-butylamine or benzylamine as the nitrogen source. The influence of these precursors on the structural, morphological and optical absorption properties of the films was studied using X-ray diffraction (XRD), Raman spectroscopy, Scanning electron microscopy (SEM) and UV/Vis spectroscopy. The chemical state and location of the nitrogen species in the films was investigated using X-ray photoelectron spectroscopy (XPS). The photocatalytic activity of films with similar structural properties was evaluated during degradation of stearic acid under UVA and visible light illumination. A previous study established a potential photosensitization mechanism involving surface N groups with binding energy of ∼400 eV, which would result in extrinsic enhanced UV activity of the N—TiO₂ films. Here, an empirical approach was adopted in order to establish correlation between structural features, nitrogen content and photocatalytic properties of these films. Within the thickness range considered, the photocatalytic activities of the undoped TiO₂ ilms were consistent with their diffraction features (peak intensities and sharpness). Nevertheless, the activities of the N—TiO₂ films did not follow the same trend but it was consistent with their nitrogen content. Further evidence is provided on the participation of nitrogen species on the enhanced UV activity of N—TiO₂ films and the impact of surface N—O groups such as N—O—Ti—O (or O—N—Ti—O) and bulk substitutional nitrogen groups is discussed. Discussion is also provided on the apparent visible light activity of the N—TiO₂ films