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

Abstract

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 N₂ gas as the precursor and carrier gas respectively, 600 °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.