Effect of Indium Segregation on the Surface versus Bulk Chemistry for Indium-Doped TiO2

This work reports the effect of indium segregation on the surface versus bulk composition of indium (In)-doped TiO2. The studies are performed using proton-induced X-ray emission (PIXE), secondary-ion mass spectrometry (SIMS), X-ray photoelectron spectroscopy (XPS), and Rutherford backscattering spectroscopy (RBS). The results of XPS analysis indicate that annealing of In-doped TiO2 containing 0.3 atom % In at 1273 K in the gas phase of controlled oxygen activity [p(O2) = 75 kPa and 10 Pa] results in a surface enrichment of 2.95 and 2.61 atom % In, respectively. The obtained segregation data are considered in terms of the transport of indium ions from its titanium sites in the bulk phase to the surface where these ions are incorporated into interstitial sites. The effect of oxygen activity on the segregation-induced surface enrichment is considered in terms of the formation of a low-dimensional surface structure and a sublayer, which are charged negatively. The latter is formed as a result of strong interactions between titanium vacancies and interstitial indium ions, leading to the formation of defect complexes. The data obtained in this work may be used for engineering of TiO2-based semiconductors with enhanced performance in solar energy conversion. © 2012 American Chemical Societ