Phase mixing in Fe/TiO2 thin films prepared by ion beam-induced chemical vapour deposition: optical and structural properties

Transparent Fe/TiO2 thin films with different Fe/Ti atomic ratios (0.07, 0.18 and 0.73) have been prepared at room temperature by ion beam-induced chemical vapour deposition (IBICVD). These thin films can be of interest as optical filters and for photo-catalytic applications. Ion beam bombardment of the growing thin films made the original samples amorphous, presenting complete mixing of the iron and titanium oxide phases. The original samples were annealed in air (T=473–873 K) and then structurally characterised by X-ray diffraction (XRD) and Fourier-transform infrared spectroscopy (FT-IR). It is shown that FT-IR spectra can be used, on a fingerprint basis, to ascertain the structure of TiO2 thin films. Anatase (pure TiO2 sample), mixtures of anatase and rutile (sample with a Fe/Ti ratio of 0.07), or rutile phases (samples with Fe/Ti ratios higher than 0.18) were obtained by annealing at T>673 K. Iron-containing samples present a shift towards the visible in the absorption edge of their UV-vis absorption spectra. The magnitude of this shift increases with the amount of iron in the films and with the annealing temperature. Both effects, rutilisation and the increase in the magnitude of the shift, are attributed to the presence of iron in the films and discussed in terms of the solubility/segregation of the Fe2O3 and TiO2 phases.