The effects of nonstoichiometry on optical properties of oxide nanopowders

In this paper we illustrate the change of optical properties of mechanically activated wurtzite ZnO powder and laser synthesized anatase TiO2 nanopowder due to the nonstoichiometry caused by mechanical activation and/or laser irradiation in vacuum. Both of the investigated materials are widely used in optoelectronics and the examination of their optical properties under different preparation and environmental conditions is of great practical interest

Use of Phonon Confinement Model in Simulation οf Raman Spectra of Nanostructured Materials

The simulation of the Raman spectra of nanostructured materials, where the effects of frequency shift and asymmetric broadening of the Raman modes play an important role, can be very useful in systematic characterization of these materials. Use of phonon confinement model for calculating Raman spectra of different nanomaterials is considered both from the viewpoint of different confinement function and the confinement strength, as well as the dimensionality of the confinement model. The phonon dispersion relations and the choice of their approximation are also studied. The influence of particle size distribution on the shape of the calculated spectra is discussed and contributions of Gaussian and asymmetric Gaussian distribution are compared. The effects of average and inhomogeneous strain on the behavior of simulated Raman spectra are also discussed. The results of the phonon confinement model are compared to the experimental spectra of $CeO_2$ and anatase $TiO_2$ nanopowders

Optical Characterization of Laser-Synthesized Anatase TiO2TiO_{2} Nanopowders by Spectroscopic Ellipsometry and Photoluminescence Measurements

Nanosized titania $(TiO_{2})$ is synthesized by laser-induced pyrolysis using $TiCl_{4}$ as a liquid precursor. X-ray diffraction and Raman scattering confirmed anatase structure of $TiO_{2}$ nanocrystals. The dielectric function ε(ω) of $TiO_{2}$ nanopowders has been determined by spectroscopic ellipsometry in the energy range from 1.5 to 6 eV at room temperature. The features observed in ε(ω) have been fitted to analytical line shapes by using the second derivatives of experimental spectra. The energies corresponding to different interband electronic transitions have been determined. Photoluminescence measurements have been carried out in vacuum for T = 20 K and T = 300 K. Under laser irradiation with sub-band gap photon energy, anatase nanocrystals have displayed strong visible photoluminescence emission. In this broad photoluminescence band different variations of line shape and position with excitation energy and temperature are observed for nanopowders with different crystallite size, pointing out to the various electronic transitions mediated by defect levels within the band gap

Characterization of anatase TiO2 nanopowder by variable-temperature Raman spectroscopy

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

Characterization of ZnSe Nanolayers by Spectroscopic Ellipsometry

Single layers of ZnSe with thicknesses of 30, 40, 50, 70 and 100 nm are deposited at room substrate temperature by thermal evaporation of ZnSe powder in vacuum. The layers surface morphology has been investigated by atomic force microscopy. Structural characterization by the Raman scattering measurement revealed the existence of randomly oriented crystalline ZnSe particles in all layers, and the presence of amorphous phase in layers thinner than 100 nm. The ellipsometric measurements were performed in the range from 1.5 to 5 eV at room temperature in air. To interpret the experimental results, the Bruggeman effective medium approximation of dielectric function of ZnSe layers has been used, representing the layers as different mixtures of crystalline ZnSe (c-ZnSe), amorphous ZnSe (a-ZnSe), and voids. The assumption of polycrystalline ZnSe layers modeled as mixture of porous c-ZnSe (with volume fraction of voids ≈ 0.17) and a-ZnSe gives the best fit of ellipsometric experimental data. Single layer thicknesses similar to those expected from preparation conditions have been obtained by this fitting procedure. It has been also found that decrease in the layer thickness causes an increase of the volume fraction of a-ZnSe. Thus, c-ZnSe/a-ZnSe ratio, porosity and layer thickness obtained by spectroscopic ellipsometry, provides useful information about crystallinity and micro-/nanostructure of ZnSe nanolayers

Low-Frequency Raman Spectroscopy of Pure and La-Doped TiO2TiO_2 Nanopowders Synthesized by Sol-Gel Method

Pure and La-doped titania $(TiO_2)$ nanopowders are synthesized by sol-gel technology. The crystallite sizes determined by X-ray diffraction measurements range from 10 to 15 nm. Dependence of structural and morphological characteristics of nanopowders on synthesis conditions and $La^{3+}$ content is investigated by the Raman spectroscopy. Very intensive modes observed in the Raman spectra of all nanopowder samples are assigned to anatase phase of $TiO_2$. Additional Raman modes of extremely low intensity can be related to the presence of a small amount of brookite amorphous phase in nanopowders, which is in accordance with the results of X-ray diffraction analysis. The particle size distribution in TiO_2 nanopowders was estimated from the low frequency Raman spectra, using the fact that the phonon modes in nanosized $TiO_2$ observed in the low frequency region (ω <40 $cm^{-1}$) can be well described by the elastic continuum model, assuming that nanoparticles are of perfect spherical shape and isotropic. The nanosized particle distribution obtained by this method is used for the calculation of the frequency and shape of the most intensive $E_g$ Raman mode in anatase $TiO_2$ by the phonon confinement model. The calculated broadening of this mode, associated with the particle size distribution, coincides well with the characteristics of $E_g$ mode observed in measured Raman spectra of $TiO_2$ nanopowders. This confirms the Raman spectroscopy method as a powerful tool for determination of particle size distribution in nanosized materials

Low-Frequency Raman Spectroscopy of Pure and La-Doped TiO 2

Pure and La-doped titania $(TiO_2)$ nanopowders are synthesized by sol-gel technology. The crystallite sizes determined by X-ray diffraction measurements range from 10 to 15 nm. Dependence of structural and morphological characteristics of nanopowders on synthesis conditions and $La^{3+}$ content is investigated by the Raman spectroscopy. Very intensive modes observed in the Raman spectra of all nanopowder samples are assigned to anatase phase of $TiO_2$. Additional Raman modes of extremely low intensity can be related to the presence of a small amount of brookite amorphous phase in nanopowders, which is in accordance with the results of X-ray diffraction analysis. The particle size distribution in TiO_2 nanopowders was estimated from the low frequency Raman spectra, using the fact that the phonon modes in nanosized $TiO_2$ observed in the low frequency region (ω <40 $cm^{-1}$) can be well described by the elastic continuum model, assuming that nanoparticles are of perfect spherical shape and isotropic. The nanosized particle distribution obtained by this method is used for the calculation of the frequency and shape of the most intensive $E_g$ Raman mode in anatase $TiO_2$ by the phonon confinement model. The calculated broadening of this mode, associated with the particle size distribution, coincides well with the characteristics of $E_g$ mode observed in measured Raman spectra of $TiO_2$ nanopowders. This confirms the Raman spectroscopy method as a powerful tool for determination of particle size distribution in nanosized materials

Influence of some sol-gel synthesis parameters of mesoporous tio2 on photocatalytic degradation of pollutants

Titanium dioxide (TiO2) nanopowders were produced by sol-gel technique from tetrabutyl titanate as a precursor by varying some parameters of the sol-gel synthesis, such as temperature (500 and 550 degrees C) and the duration of calcination (1.5, 2 and 2.5 h). X-ray powder diffraction (XRPD) results have shown that all synthesized nanopowders were dominantly in the anatase phase, with the presence of a small amount of rutile in samples calcined at 550 degrees C. According to the results obtained by the Williamson-Hall method, the anatase crystallite size was increased with the duration of the calcination (from 24 to 29 nm in samples calcined at lower temperature, and from 30 to 35 nm in samples calcined at higher temperature). The analysis of the shift and line-width of the most intensive anatase Eg Raman mode confirmed the XRPD results. The analysis of pore structure from nitrogen sorption experimental data described all samples as mesoporous, with mean pore diameters in the range of 5-8 nm. Nanopowder properties have been related to the photocatalytic activity, tested in degradation of the textile dye (C.I. Reactive Orange 16), car-bofuran ray scattering

Photocatalytic degradation of alprazolam in water suspension of brookite type TiO2 nanopowders prepared using hydrothermal route

Two series of nanocrystalline brookite-type powders have been synthesized by using combined sal gel hydrothermal method with titanium tetrachloride (TiCI4) as a precursor and hydrothermal temperature and reaction time varied in the range of 120-200 degrees C and 12-48 h, respectively. The effects of chosen synthesis parameters on structural, morphological and optical properties of synthesized powders have been investigated by the XRPD, SEM, EDS and BET measurements, as well Raman spectroscopy and spectroscopic ellipsometry. The XRPD results have shown that pure brookite phase, with mean crystallite size of -33 nm, has been obtained only in the sample synthesized at 200 degrees C, after 24 h of hydrothermal process. In all other samples anatase phase also appears, whereas rutile and sodium titanate phases have been noticed in the samples synthesized at lower temperatures. The presence of different titania phases has also been confirmed and analyzed by Raman scattering measurements. The SEM measurements have shown spindle-like particles in brookite-rich samples synthesized at 200 degrees C, whereas BET measurements have detected mesoporous structure in these samples. The properties of synthesized powders have been correlated to their photocatalytic efficiency, tested in degradation of alprazolam, one of the 5th generation benzodiazepines. The sample consisted of pure brookite has shown the highest efficiency in the photodegradation of alprazolam, practically equal to the activity of Degussa P25

Ce1-xY(Nd)(x)O2-delta nanopowders: potential materials for intermediate temperature solid oxide fuel cells

Nanopowdered solid solution Ce1-xY(Nd)(x)O2-delta samples ( 0.1 LT = x LT = 0.25) were made by self-propagating room temperature (SPRT) synthesis. The first-order Raman spectra of Ce1-xY(Nd)(x)O2-delta 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 similar to 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 Ce4+ ions are replaced with trivalent cations (Y3+, Nd3+). The intensity of this mode increases with doping in both series of samples, indicating a change of O2- 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.International School and Workshop on Nanoscience and Nanotechnology, Nov 14-16, 2005, Monte Porzio Catone, Ital