Universal synthesis method for mixed phase TiO2(B)/anatase TiO2 thin films on substrates via a modified low pressure chemical vapour deposition (LPCVD) route

A universal method for the synthesis of mixed phase TiO2 bronze (B)/anatase titania thin films by Low Pressure Chemical Vapour Deposition (LPCVD) onto any substrate is presented. General LPCVD conditions were titanium isopropoxide (TTIP) and N-2 gas as the precursor and carrier gas respectively, 600 degrees C nominal reaction temperature, and 15 min reaction time; a range of different substrates were investigated including: a silicon wafer, fused quartz, highly ordered pyrolytic graphite (HOPG) and pressed graphite flake (grafoil). X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, scanning and transmission electron microscopy were used to characterise the thin films which exhibited a columnar morphology together with smaller equi-axed particles. Pre-treatment of substrates by spraying with a Na-containing solution was found to encourage the crystallization of TiO2(B) during the LPCVD process. Increasing the concentration of Na in the pre-treatment process resulted in a higher proportion of TiO2(B) in the thin films up to an optimum condition of 0.75% w/v of Na. Na diffusion from the substrate surface into the adjacent TiO2 is the proposed mechanism for promoting TiO2(B) formation as opposed to the anatase phase with Density Functional Theory (DFT) modelling suggesting the presence of Na stabilises the TiO2(B) phase. Dye degradation tests indicate an increased photocatalytic activity for mixed phase anatase/TiO2(B) thin films.