The combinatorial atmospheric pressure chemical vapour deposition (cAPCVD) of a gradating substitutional/interstitial N-doped anatase TiO2 thin-film; UVA and visible light photocatalytic activities

Combinatorial atmospheric pressure chemical vapour deposition (cAPCVD) was used to synthesise a film with gradating substitutional (Ns) and interstitial (Ni) nitrogen dopant concentrations across an anatase TiO2 thin-film. A transition from predominantly Ns-doped, to Ns/Ni mixtures, to purely Ni-doped titania was observed by X-ray photoelectron spectroscopy (XPS) analysis of positions across the film. We believe this to be the first time that such a gradient from Ni to Ns-doping has been achieved by a CVD process in a single film. The film was characterized by X-ray diffraction, Raman, and atomic force microscopy. Film thicknesses and bandgap energies were calculated from Swanepoel and Tauc plot manipulations of transmittance spectra. The photocatalytic activity to UVA (365 nm = 1.75 mW cm−2) light was assessed by monitoring colour changes in digital images of an even layer of intelligent ink deposited by an aerosol-spray method and by UV–visible spectroscopic assessment of the degradation of an aqueous methylene blue dye. The photo-activity to visible light (indoor white light source) was assessed by monitoring the degradation of a stearic acid over-layer. This allowed for the systematic investigation on the effect of Ns and/or Ni-doping in anatase TiO2 and the dopant concentration on the thin-film's photocatalytic activity. The results indicated that Ns-doping of anatase titania causes detriment to the film's photocatalytic activity to UVA-light. It was also found that purely Ni-doped anatase made a marginally more active visible light photocatalyst than predominantly Ns-doping. The differences were related to increased recombination effects in Ns-doped systems versus Ni-doped systems. Films synthesized by the cAPCVD route analysed in conjunction with mapping analysis tools provide a shortcut to identifying numerous phases and compositions and their functional property relationships on a single film, offering a rapid method for analysis of phase-space.