Interpretation of Raman Spectra of Oxide Materials: The Relevance of Absorption Effects

Abstract

When using Raman spectra for structural characterization of solid powder materials such as catalysts, the interpretation needs to take into account the possible absorption of radiation. For example, the reduction of oxide materials may result in new UV−vis absorption bands and therefore affect the Raman intensity. In resonance Raman spectroscopy, the absorption correction of Raman intensity based on the Kubelka−Munk theory is widely established. In contrast, in Raman spectroscopy, typically no absorption correction or normalization by a phonon mode is applied. Using a combined approach of Raman and UV−vis spectroscopy, this study examines the effect of absorption on Raman spectra as well as the usefulness of different absorption corrections for Raman spectroscopy. The results show that a change in absorption of about 10% may result in a decrease in Raman intensity of up to one-third. Corrections based on the Kubelka−Munk theory predict large intensity changes for small changes in the low-absorption region, resulting in the largest deviations among different absorption corrections. On the other hand, the commonly used normalization by a phonon mode shows increasing deviations with increasing spectral distance of the investigated feature from the position of the phonon mode, since it is not wavelength dependent. To reduce these discrepancies, we have developed a new wavelength-dependent absorption correction. As illustrated for ceria materials, the corrected Raman intensity may vary significantly from the raw data intensity. Consequently, for a profound interpretation of Raman spectra of solid powder materials, the combined use of Raman and UV−vis spectroscopy is recommended, allowing one to investigate and quantify the effect of absorption on Raman spectra, which may be of relevance even for Raman spectroscopy.

Raman spectroscopy as a powerful tool to characterize ceria-based catalysts

“…When interpreting Raman spectra of ceria based catalysts, one should take into account the possible changes of absorption coefficient since ceria is a reductive oxide leading to UV-vis absorption and strong decrease in Raman bands intensity. An interesting wavelengthdependent absorption correction has been recently developped to take into account absorption effects [114].…”

Dry reforming of glycerol over Rh-based ceria and zirconia catalysts: New insights on catalyst activity and stability

“…In addition to this intense feature, fresh RhCe catalyst also presented weaker and broader features at 240 and 571 cm −1 which can be assigned to oxygen vacancies and defects in the CeO 2 lattice leading to a sub-stoichiometric structure. These defects can either originate from Rh incorporation into the ceria lattice due to strong metal support interaction between Rh particles and the reducible CeO 2 lattice [70][71][72][73][74] or from the reduction of ceria with hydrogen which was used during the routine activation protocol of the fresh catalysts after synthesis. Moreover, another weak feature appearing at 167 cm −1 for the Raman spectrum of the fresh RhCe catalyst can be due to the defects in the ceria lattice or possibly to Rh-O-Ce and/or RhO x species [60][61][62]72,73].…”