Photo-induced hydrophilicity of TiO2−xNx thin films on PET plates

TiO2−xNx thin films were deposited on PET (polyethylene terephthalate) plates by sputtering a TiN target in a N2/O2 plasma and without heating. X-ray photoemission spectroscopy (XPS) was used to investigate the N 1s, Ti 2p core levels and the nitrogen composition in the TiO2−xNx films. The results indicate that Ti–O–N bonds are formed in the thin films. Two nitrogen states, substitution and interstitial nitrogen atoms, were attributed to peaks at 396 and 399 eV, respectively. It was observed that the nitrogen atoms occupy both the substitutive and interstitial sites in respective of the nitrogen content in the thin films. UV–VIS absorption spectroscopy of PET coated thin films shows a significant shift of the absorption edge to lower energy in the visible-light region. UV and visible-light irradiation are used to activate PET coated thin films for the development of hydrophilicity. The photo-induced surface wettability conversion reaction of the thin films has been investigated by means of water contact angle measurement. PET plates coated with TiO2−xNx thin films are found to exhibit lower water contact angle than non-coated plates when the surface is illuminated with UV and visible light. The effects of nitrogen doping on photo-generated hydrophilicity of the thin films are investigated in this work.