Study of time effect on the optical properties of spin-coated CeO2–TiO2 thin films

CeO2–TiO2 thin films have been deposited onto glass substrates by spin coating method with 2500, 3000, and 4000 rpm spin rates using sol–gel process. The time effect on the optical properties of the films has been investigated by evaluating the variation of spectral behavior over a period of about 11 weeks. In the first week of aging, the optical indices of the films show observable variation. After four weeks, the changes reduce, and these parameters become constant approximately after six weeks. After 11 weeks of aging it was found that the refractive index of the CeO2–TiO2 thin films is 1.91±0.03, 1.98±0.04, and 2.10±0.02, and extinction coefficient is 2.8±0.2×10−3, 3.3±0.2×10−3 and 3.8±0.3×10−3 for spinning rate of 2500, 3000, and 4000 rpm, respectively. For the same spinning rates, the film thickness are 261±1, 229±1, and 204±1 nm. Optical band gaps of the films, which are estimated from the absorption coefficients, have not changed. The optical band gap is independent of the spin speed and for the direct and indirect transitions the band gaps are 3.30±0.06, and 2.92±0.05 eV, respectively.