Optical properties and X-ray photoelectron spectroscopic study of pure and Pb-doped TiO 2 thin films

Abstract

Pure and Pb-doped (5 and 10 mol %) titanium dioxide (TiO 2) 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 TiO 2 is doped with an increased amount of Pb, which indicates a decrease in the bandgap of TiO 2 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 Pb-doped film indicates the incorporation of Pb into the TiO 2 lattice. Except for the enrichment of Pb near the surface, Pb, 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 TiO 2 lattice as a tetravalent ion and therefore the formation of the material Pb x Ti 1− x O 2.