Effects of Temperature on the Raman Spectra and Dispersed Oxides

An investigation has been conducted of the effects of temperature on the intensities and positions of Raman bands of bulk and supported metal oxides. The relationship of band intensity to optical absorption of the sample at frequencies close to that of the laser used for Raman spectroscopy was also investigated. It was observed that bulk oxides exhibit an increase in optical absorption with increasing temperature due to a loss of oxygen and the consequent formation of nonstoichiometric oxides and stoichiometric suboxides. When the increase in optical absorption is significant at or near the frequency of the laser used for Raman spectroscopy, this gives rise to a loss in Raman signal intensity due to a decrease in the sampling depth. The closer the absorption band edge of the oxide is to the laser frequency and the weaker the metal−oxygen bond in the oxide, the more severe will be the effects of temperature on the intensity of the Raman spectrum. For dispersed metal oxides, the influence of temperature on Raman band intensity becomes less severe as the volume fraction of oxide lowers. Temperature also affects the position of the Raman bands for both bulk and supported metal oxides. With increasing temperature, all Raman bands shift to lower frequencies. This effect is attributable to thermal expansion and changes in the population of the vibrational energy levels with increasing temperature.