Structural properties and photocatalytic behaviour of phosphate-modified nanocrystalline titania films

Abstract

A series of phosphate-modified titanium dioxide samples (P-TiO2) with varying P/Ti atomic ratio was prepared by sol–gel method. The influence of the surface-bound phosphate on the structural behaviour of the titania samples was studied by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and nitrogen sorption measurements. The ratio of the numbers of phosphorus and titanium atoms was compared by XPS and inductively coupled plasma atomic emission spectrometry (ICP-AES). In each of the samples the P/Ti ratio was greater than expected from the chemical composition or found by ICP-AES. Taking into account the surface sensitivity of the XPS one can conclude that the surface region is richer in phosphorus than the bulk phase. A practically constant P/Ti ratio was determined at different X-ray incident angles for the P-TiO2/0.05 sample, which means that the P-abundant layer is thicker than the escape depth of the P 2p electrons from the sample. The presence of phosphate retards the amorphous titania→anatase and anatase→rutile phase transformations during calcination. It was shown that the specific surface area of the P-TiO2 samples strongly depends on their phosphate content.The photocatalytic activity of the phosphate-modified samples was compared with that of Degussa P25 titanium dioxide by ethanol photooxidation in gas phase, with and without water vapor. The presence of water vapor increased the rate of ethanol degradation on P-TiO2. At high relative humidity (∼70%) the initial rate of CO2 formation on the phosphate-modified sample with optimal phosphate content was ∼3 times higher with respect to Degussa P25.