Raman spectroscopy of molybdenum oxides Part I. Structural characterization of oxygen defects in MoO3−x by DR UV/VIS, Raman spectroscopy and X-ray diffraction

Abstract

MoO_3−x samples were prepared under different temperature and gas atmosphere conditions. The samples, which are all of the crystallographic MoO₃ structure, have been investigated by XRD, SEM, diffuse reflection (DR) UV/VIS and Raman spectroscopies to characterize their properties as a function of the different concentrations of oxygen vacancies. The different preparation conditions led to differences in the concentration of oxygen vacancies as well as primary crystallite sizes and morphologies. The UV/VIS spectra of the MoO_3−x samples were deconvoluted into Gaussian bands attributed to ligand to metal charge transfer (LMCT), d–d transitions, and intervalence charge transfer (IVCT) transitions of [Mo⁵⁺O₅] and [Mo⁵⁺O₆] defect centers. The IVCT band positions at 2 eV were used to determine the sample oxygen stoichiometries. A resonance Raman enhancement was observed for the MoO_3−x samples and explained by resonant coupling to the electronic absorption of the type [Mo⁵⁺O₅]–[Mo⁶⁺O₆] → [Mo⁶⁺O₅]–[Mo⁵⁺O₆] at 2.03 eV. A linear correlation was found between the Raman band intensity ratios I₂₈₅/I₂₉₅ and the sample oxygen/metal ratio, and hence this Raman band intensity ratio can be used to determine the oxygen stoichiometry of MoO_3−x. The integral intensity of the Raman band at 823 cm⁻¹ was also found to depend on the oxygen stoichiometry. These observed changes of the integral Raman intensities of the band at 823 cm⁻¹ are explained by the interplay of a resonant enhanced Raman scattering and changes of the positions of the IVCT transition to which the resonant Raman scattering is coupled. The intensity ratio of the translational Raman bands at 117 and 130 cm⁻¹, on the other hand, was shown to be a function of the primary crystallite size. The observed resonance Raman effect is discussed in the frame of in situ Raman characterization of operating Mo-based catalysts.