Mo doped TiO2 thin films

Abstract

TiO2 thin films doped with 0.2 to 2.0 mol% Mo were spin coated on glass substrates and annealed in air at 450°C for 2 hours. The effects of varying the doping concentration on the mineralogical, microstructural, morphological, optical, and photocatalytic properties were determined using X-ray diffraction, scanning electron microscopy and atomic force microscopy, focused ion beam milling, UV–vis spectrophotometry, and methylene blue and methyl orange tests, respectively. The distribution of Mo was determined by energy dispersive spectroscopy. All of the films consisted of the polymorph anatase. The grain sizes were in the range 25–41 nm and the crystallite sizes were in the rage 2.3–4.2 nm. The thicknesses of all films were ~270 nm. Both grain size and crystallite size increased with doping level, where the former reached a maximum at the doping level of 1.0 mol% Mo and the latter maximised at 0.8 mol% Mo. The range of 0.8%–1.0% mol% was identified as the solubility limit of Mo in anatase. The absorption spectra indicated a red shift and the optical indirect band gaps of the films were calculated and found to decrease from 3.46 eV to 3.30 eV, with two tight bands of values, the lower corresponding to increasing Mo solubility and the higher corresponding to saturated solution. The methylene blue and methyl orange tests showed that doping did not have a beneficial effect on the photodecomposition. In fact, the degree of photoreduction was inversely proportional to the level of doping for both organic dyes. This was attributed to a combination of lattice distortion and a reduction in the surface actives sites associated with increasing crystallite size.