Spectroscopic studies of interfacial structures of CeO_2-TiO_2 mixed oxides

摘要

We have investigated the geometric and electronic structures of the cerium oxide (CeO_2)-titanium dioxide (TiO_2) mixed oxides with various Ce/TiO_2 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 TiO_2 from the anatase phase to the rutile phase. XRD peaks of TiO_2 anatase attenuate continuously with the increasing amount of CeO_2 in the mixed oxide, but the XRD peaks of cubic CeO_2 appear only after the weight ratio of Ce/TiO_2 reaches 0.50. The average crystalline sizes of TiO_2 anatase and cubic CeO_2 in CeO_2-TiO_2 mixed oxides are smaller than those in the corresponding individual TiO_2 anatase and cubic CeO_2. Raman spectroscopy excited by the 514.5 nm laser detects CeO_2 after the weight ratio of Ce/TiO_2 reaches 0.70 whereas Raman spectroscopy excited by the 325 nm laser detects CeO_2 after the weight ratio of Ce/TiO_2 reaches 0.90. XPS results demonstrate that Ti exists in the form of Ti~(4+) in the CeO_2-TiO_2 mixed oxide. Ce is completely in the form of Ce~(3+) in the mixed oxides with a 0.05 weight ratio of Ce/ TiO_2. With the increasing weight ratio of Ce/TiO_2, Ce~(4+) dominates. On basis of these results, we proposed that CeO_2 initially nucleates at the defects (oxygen vacancies) within TiO_2 anatase, forming an interface bridged with oxygen between CeO_2 and TiO_2 anatase. At the interface, Ce species cannot substitute Ti~(4+) in the lattice of TiO_2 anatase whereas Ti~(4+) can substitute Ce~(4+) in the lattice of cubic CeO_2. The decreasing concentration of oxygen vacancies, the Ti-O-Ce interface, and the decreasing average crystalline size of TiO_2 anatase act to inhibit the phase transformation of TiO_2 anatase. With the increasing amounts of CeO_2, the CeO_2 clusters continuously grow and form cubic CeO_2 nanocrystals. Spectroscopic results strongly demonstrate that the surface region of CeO_2-TiO_2 mixed oxide is enriched with TiO_2.