Correlation between microstructure and optical properties of nano-crystalline TiO2 thin films prepared by sol-gel dip coating

Titanium dioxide thin films have been prepared from tetrabutyl-orthotitanate solution and methanol as a solvent by sol-gel dip coating technique. TiO2 thin films prepared using a sol-gel process have been analyzed for different annealing temperatures. Structural properties in terms of crystal structure were investigated by Raman spectroscopy. The surface morphology and composition of the films were investigated by atomic force microscopy (AFM). The optical transmittance and reflectance spectra of TiO2 thin films deposited on silicon substrate were also determined. Spectroscopic ellipsometry study was used to determine the annealing temperature effect on the optical properties and the optical gap of the TiO2 thin films. The results show that the TiO2 thin films crystallize in anatase phase between 400 and 800 degrees C, and into the anatase-rutile phase at 1000 degrees C, and further into the rutile phase at 1200 degrees C. We have found that the films consist of titanium dioxide nano-crystals. The AFM surface morphology results indicate that the particle size increases from 5 to 41 nmby increasing the annealing temperature. The TiO2 thin films have high transparency in the visible range. For annealing temperatures between 1000 and 1400 degrees C, the transmittance of the films was reduced significantly in the wavelength range of 300-800nm due to the change of crystallite phase and composition in the films. We have demonstrated as well the decrease of the optical band gap with the increase of the annealing temperature. (C) 2010 Elsevier B. V. All rights reserved.