Optical properties and X-ray photoelectron spectroscopic study of pure and Pb-doped TiO2 thin films

Pure and Pb-doped (5 and 10 mol %) titanium dioxide (TiO2) thin films have been deposited on single-crystal Si (100) and vitreous silica substrates by the sol-gel dip-coating method. Spectroscopic techniques, such as, UV-Vis transmission spectroscopy, photoluminescence (PL) spectroscopy, and X-ray photoelectron spectroscopy (XPS), together with X-ray diffraction (XRD) analysis and scanning electron microscopy, have been used to study the optical and structural properties of the films. A gradual shift of the transmission spectrum towards longer wavelengths has been observed as TiO2 is doped with an increased amount of Pb, which indicates a decrease in the bandgap of TiO2 upon Pb doping. The PL spectrum of the films, measured at liquid nitrogen temperature, also shows a gradual shift of the emission peak towards the longer wavelength region and supports the lowering of the bandgap with Pb doping. XRD results show that the films are polycrystalline, anatase (A) type, and oriented predominantly to the A(101) plane. A slight shift in the d-spacing for the ph-doped film indicates the incorporation of Pb into the TiO2 lattice. Except for the enrichment of Pb near the surface, ph, Ti and O are found to be distributed uniformly along the thickness of the film as observed by XPS, which is further supported by the energy dispersive X-ray analysis. Valence state analysis by XPS confirms the incorporation of Pb into the TiO2 lattice as a tetravalent ion and therefore the formation of the material PbxTi1-xO2. (C) 1998 Elsevier Science Ltd. All rights reserved.