Properties and performance of photocatalytic CeO2, TiO2, and CeO2-TiO2 layered thin films

CeO2, TiO2, and CeO2-TiO2 layered thin films of different compositions were deposited on polished fused silica glass substrates using spin coating and then annealed at 550 degrees C for 1 h. Poorly crystalline fluorite-type CeO2 and anatase-type TiO2 were the only phases detected. The valence effects for Ce are attributed to intervalence charge transfer (IVCT) while those for TiO2 are attributed to simultaneous Ti4+ -> Ti3+ redox and IVCT. Contamination of the grain boundaries by Si from the substrate is likely to have contributed to blockage of surface-active sites. While the roughnesses of the former thin films exhibited the expected direct correlation with the grain size, those of the latter two types of thin films showed a converse relation, suggesting that the grain boundaries of the layered TiO2 thin films included a recrystallized CeO2-TiO2 invariant point liquid and/or a CeO2-TiO2-SiO2 glass. These would have derived from the other sources of grain boundary contamination, which are Si contamination from the substrate and/or exsolved CeO2 beyond the solubility limit of TiO2. The UV-Vis spectra demonstrated two principal types of curves, with the CeO2 and higher TiO2 concentration giving typical spectra with sharp absorption edges while those of the lower TiO2 concentration indicated highly defective nanostructures with diffuse absorption edges. These data support the conclusion that CeO2 initially decreases the E-g by enhancing nucleation and recrystallization through defect formation but subsequently increases the E-g through supersaturation and consequent lattice distortion. The CeO2 thin films exhibited much worse photocatalytic performances than those of the TiO2 thin films, although no significant differences between the TiO2 thin films and the CeO2-TiO2 layered thin films were apparent, which probably resulted from the competing effects of blockage of the active sites on the grain boundaries and the extent of recrystallization of the thin film phases, the latter of which is more significant.