Size-dependent properties ofCeO2−ynanoparticles as studied by Raman scattering

Abstract

The combined effects of strain and phonon confinement are seen to explain why the Raman peak near 464 cm Ϫ1 in CeO 2Ϫy nanoparticles shifts to progressively lower energies and the lineshape of this feature gets progressively broader and asymmetric ͑on the low-energy side͒ as the particle size gets smaller. The increasing lattice constant measured for decreasing particle size explains this Raman shift well. The linewidth change is fairly well explained by the inhomogenous strain broadening associated with the small dispersion in particle size and by phonon confinement. The spectra are also likely to be directly affected by the presence of oxygen vacancies. Comparison of the temperature dependence of the Raman lineshape in the nanoparticles and the bulk shows that phonon coupling is no faster in the nanoparticles, so size-dependent phonon coupling does not contribute to the large nanoparticle peak red shifts and broadening at room temperature. Irreversible thermally induced changes are observed in the Raman peak position of the nanoparticles.

Synthesis, Characterization, and Photocatalytic Properties of Flower-like Mn-doped Ceria

“…The UV-vis spectral changes of RhB solution over 1% Mn doped CeO 2 samples during the photodegradation are shown in Figure 6a, clearly show that the characteristic absorption peaks corresponding to RhB decrease rapidly as the exposure time increases, indicating the decomposition of RhB and the significant reduction in the RhB concentration. was previously explained by phonon confinement model 31,32 . Another reason of shifting and broadening may be the increase in oxygen vacancies, which is related to structural defects derived from partially incorporation of manganese into CeO 2 lattice.…”

Role of Hydroxyl Groups in the Preferential Oxidation of CO over Copper Oxide–Cerium Oxide Catalysts

“…25,26 The position of this band provides information about the introduction of foreign cations into the ceria lattice. The position of the F 2g band in pure ceria is usually ca 464 cm -1 , 20 and the position obtained for all CuO/Ce 0.8 X 0.2 O δ catalysts is well below this value (see data in Table 1).…”

Techniques for the Study of the Structural Properties

“…Application of equation (5.4.1) to any given Raman line of the perfect crystal results, as experimentally observed, [60][61][62][63][64] in frequency shifts and asymmetric broadenings that increase with decreasing the particle size of the nanocrystal, and of a sign and magnitude that depends on the form and dispersion of the ω(q) function. Thus, a negative/positive dispersion in ω values as q increases is expected to produce asymmetric low/high frequency broadening and a red/blue shift of the peak.…”

“…The model employed for analyzing the features observed in the Raman spectra of nanocrystalline materials is the so called "phonon confinement" (or "spatial correlation") model. 63 Within such model, nanocrystalline materials can be considered as an intermediate case between that of a perfect infinite crystal and that of an amorphous material. Thus, in a perfect crystal, conservation of phonon momentum (q) during phonon creation or decay upon interaction with the external radiation field requires that, in first-order Raman scattering, only optic phonons near the center of the Brillouin zone (q ≈ 0) are involved.…”

“…In the intermediate case of nanocrystalline materials, due to the uncertainty principle, a range of q vectors of the order ∆q ≈ 1/L (where L is the particle size) can be involved in the Raman process. [60][61][62][63] The approach employed in the phonon confinement model follows from the work by Richter et al, 60 which shows that the relaxation of the conservation of phonon momentum selection rule arises from substitution of the phonon wave function in an infinite crystal by a wave function in which the phonon is confined in the volume of the crystallite. In their approach, which is employed to explain the first-order Raman spectrum of microcrystalline spherical silicon particles, it is used a Gaussian function to impose the phonon localization.…”