Raman spectra of La2O3 dispersed on γ-Al2O3

Raman spectra have been obtained for La2O3 dispersed on Al2O3, but only after La2O3 coverages reach one theoretical monolayer or higher. Sharp peaks at 104, 191, and 408 cm(-1) were observed, which are in excellent agreement with previous studies of bulk La2O3; however, two additional broader, but intense bands not previously reported were also observed at 322 and 935 cm(-1). These latter two bands disappear at temperatures of 450 K or higher, but they returned with full intensity upon cooling to 300 K. A sample of bulk La2O3 (a-form) with a low surface/volume ratio also exhibited weak peaks at these latter two positions. Raman spectra taken with different exciting laser lines showed that the bands at 322 and 935 cm(-1) can be attributed to a laser-excited luminescence. It is inferred that these luminescence bands are evidence for the presence of oxygen anion vacancies. After exposure to NO at 300 K, two peaks developed at 747 and 1047 cm(-1) which can be assigned to a surface nitrate species. (C) 1998 Academic Press.