Spectroscopic studies of interfacial structures of CeO2-TiO2 mixed oxides

We have investigated the geometric and electronic structures of the cerium oxide (CeO2)-titanium dioxide (TiO2) mixed oxides with various Ce/TiO2 weight ratios prepared by the sol-gel method in detail by means of X-ray diffraction (XRD), high-resolution X-ray photoelectron spectroscopy (XPS), Raman spectroscopy excited by 325 and 514.5 nm lasers, and scanning electron microscope (SEM). Existence of cerium effectively inhibits the phase transition of TiO2 from the anatase phase to the rutile phase. XRD peaks of TiO2 anatase attenuate continuously with the increasing amount of CeO2 in the mixed oxide, but the XRD peaks of cubic CeO2 appear only after the weight ratio of Ce/TiO2 reaches 0.50. The average crystalline sizes of TiO2 anatase and cubic CeO2 in CeO2-TiO2 mixed oxides are smaller than those in the corresponding individual TiO2 anatase and cubic CeO2. Raman spectroscopy excited by the 514.5 nm laser detects CeO2 after the weight ratio of Ce/TiO2 reaches 0.70 whereas Raman spectroscopy excited by the 325 nm laser detects CeO2 after the weight ratio of Ce/TiO2 reaches 0.90. XPS results demonstrate that Ti exists in the form of Ti4+ in the CeO2-TiO2 mixed oxide. Ce is completely in the form of Ce3+ in the mixed oxides with a 0.05 weight ratio of Ce/TiO2. With the increasing weight ratio of Ce/TiO2, Ce4+ dominates. On basis of these results, we proposed that CeO2 initially nucleates at the defects (oxygen vacancies) within TiO2 anatase, forming an interface bridged with oxygen between CeO2 and TiO2 anatase. At the interface, Ce species cannot substitute Ti4+ in the lattice of TiO2 anatase whereas Ti4+ can substitute Ce4+ in the lattice of cubic CeO2. The decreasing concentration of oxygen vacancies, the Ti-O-Ce interface, and the decreasing average crystalline size of TiO2 anatase act to inhibit the phase transformation of TiO2 anatase. With the increasing amounts of CeO2, the CeO2 clusters continuously grow and form cubic CeO2 nanocrystals. Spectroscopic results strongly demonstrate that the surface region of CeO2-TiO2 mixed oxide is enriched with TiO2.