Combinatorial atmospheric pressure chemical vapour deposition (cAPCVD) of niobium doped anatase; effect of niobium on the conductivity and photocatalytic activity

Abstract

Combinatorial atmospheric pressure chemical vapour deposition (cAPCVD) was used to synthesise anatase thin-films with gradating substitutionally doped niobium content on a single film. This is the first time such a combinatorial system has been formed by CVD or other means. The film was characterised via X-ray diffraction (XRD) mapping, Raman spectroscopy, atomic force microscopy (AFM), wavelength dispersive X-ray analysis (WDX) and X-ray photoelectron spectroscopy (XPS) analysis. Film thicknesses and band-gap energies were derived from Swanepoel thickness analysis and Tauc plots of transmittance spectra. Water droplet contact angle measurements displayed a UVC induced movement to super hydrophilic behaviour by a photo-induced superhydrophilicity (PSH) mechanism, irrespective of niobium levels. The film's conductivity and photocatalytic activity to different light sources were mapped using a four-point probe and novel flat-bed digital scanning technique. This allowed for the systematic investigation on the effect of niobium doping. It was found that film growth rate in addition to niobium concentration directly influenced photocatalytic activity and electrical conductivity. A direct relationship between conductivity of these Nb-doped anatase thin-films and photocatalytic activity to both UVA and indoor lighting was also observed. Films synthesized by the cAPCVD route analysed in conjunction with mapping analysis tools provide a shortcut to investigating the effect of metal dopant on the functional properties for a wide range of phase space on a single film, enabling rapid analysis.