Raman scattering spectroscopy has been used for the characterization of zinc oxide nanoparticles obtained by mechanical activation in a high-energy vibro-mill and planetary ball mill. Raman modes observed in spectra of nonactivated sample are assigned to Raman spectra of the ZnO monocrystal, while the spectra of mechanically activated samples point out to the structural and stoichiometric changes, depending on the milling time and the choice of equipment. Observed redshift and peak broadening of the E 2 high and E 1 (LO) first-order Raman modes are attributed to increased disorder induced by mechanical milling, followed by the effects of phonon confinement due to correlation length decrease. The additional modes identified in Raman spectra of activated ZnO samples are related to the surface optical phonon modes, due to the intrinsic surface defects and presence of ZrO 2 as extrinsic defects introduced by milling in zirconia vials.
Raman spectroscopy has been used for characterization of commercial nanosized TiO2 powder with declared grain size of 5 nm. The Raman spectra measured in Stokes and anti-Stokes regime confirm the anatase phase of TiO2 powder in temperature range 25-1173K. It is shown that phonon-confinement (due to small grain size) and nonstoichiometry (caused by laser irradiation in vacuum) have a great influence on blueshift and broadening of the main Eg Raman mode at low temperatures, while the influence of the strong anharmonic effect becomes dominant at higher temperatures. The phonon confinement effect decreases due to the crystallite growth at temperatures above 673K
The conventional Raman scattering spectroscopy is one of the most used and powerful techniques for characterization of nano-sized materials and structures. By proper analysis of optical mode shift and broadening in nanomaterials based on phonon confinement model, it is possible to deduce about the influence of various effects like particle size and size distribution, strain, change of phonon dispersion, substitutional effects, defect states and nonstoichiometry, electron-phonon coupling. We have demonstrated potentials of this technique in CeO2 and TiO2 nanocrystalline systems analyzing their optical phonon properties.
Nanopowdered solid solution Ce 1−x Y(Nd) x O 2−δ samples (0.1 x 0.25) were made by self-propagating room temperature (SPRT) synthesis. The first-order Raman spectra of Ce 1−x Y(Nd) x O 2−δ samples measured at room temperature exhibit three broad features: the main Raman active F 2g mode at about 450 cm −1 and two broad features at about 550 (545) and 600 cm −1 . The mode at ∼600 cm −1 was assigned to the intrinsic oxygen vacancies due to the nonstoichiometry of ceria nanopowders. The mode at about 550 (545) cm −1 was attributed to the oxygen vacancies introduced into the ceria lattice whenever Ce 4+ ions are replaced with trivalent cations (Y 3+ , Nd 3+ ). The intensity of this mode increases with doping in both series of samples, indicating a change of O 2− vacancy concentration. The mode frequency shifts in opposite direction in Y-and Nd-doped samples with doping level, suggesting that different types of defect space can occur in Y-and Nd-doped ceria nanopowders.
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