Nanostructured CeO2 Thin Films Prepared by the Sol-Gel Dip-Coating Method with Anomalous Behavior of Crystallite Size and Bandgap

Amorphous CeO2 thin films were deposited by a dip-coating method on Corning glass substrates and annealed for one hour at the temperatures (T) of 250, 450, and 550°C in air for crystallization. The precursor solution was prepared by dissolving cerium acetate in methanol, lactic acid, glycerol, and trimethylamine at 55°C. X-ray diffraction (XRD) patterns showed the cubic structure of CeO2. From XRD data and employing the Scherrer formula, the crystallite size (CS) was calculated to be within the 4.0±0.5 to 10±1 nm interval. SEM micrographs revealed cracks of the films annealed at 250 and 450°C, even though for 550°C, the film shows a homogeneous morphology free of cracks. CS increases (from 4.0 to 10 nm) and thickness decreases (from 217 to 182 nm) when T increases. The UV-vis spectra exhibited an average transmittance of 80% in the 300 to 2000 nm wavelength range. Also, from XRD, it was observed that the lattice shrinks and from transmittance that the bandgap energy increases with T. The Raman spectra exhibit 461 cm-1 assigned to F2g mode of the fluorite cubic structure, where F2g hardens when T increases as an effect of the shrinkage of the lattice.