Far infrared absorption by acoustic phonons in titanium dioxide nanopowders

We report spectral features of far infrared electromagnetic radiation absorption in anatase TiO2 nanopowders which we attribute to absorption by acoustic phonon modes of nanoparticles. The frequency of peak excess absorption above the background level corresponds to the predicted frequency of the dipolar acoustic phonon from continuum elastic theory. The intensity of the absorption cannot be accounted for in a continuum elastic dielectric description of the nanoparticle material. Quantum mechanical scale dependent effects must be considered. The absorption cross section is estimated from a simple mechanical phenomenological model. The results are in plausible agreement with the absorption being due to a sparse layer of charge on the nanoparticle surface.Comment: 8 pages, 5 figures, submitted to Journal of Nanoelectronics and Optoelectronic

Inelastic neutron scattering due to acoustic vibrations confined in nanoparticles: theory and experiment

The inelastic scattering of neutrons by nanoparticles due to acoustic vibrational modes (energy below 10 meV) confined in nanoparticles is calculated using the Zemach-Glauber formalism. Such vibrational modes are commonly observed by light scattering techniques (Brillouin or low-frequency Raman scattering). We also report high resolution inelastic neutron scattering measurements for anatase TiO2 nanoparticles in a loose powder. Factors enabling the observation of such vibrations are discussed. These include a narrow nanoparticle size distribution which minimizes inhomogeneous broadening of the spectrum and the presence of hydrogen atoms oscillating with the nanoparticle surfaces which enhances the number of scattered neutrons.Comment: 3 figures, 1 tabl

Synthèses de nanocristaux de TiO2 anatase à distribution de taille contrôlée. Influence de la taille des cristallites sur le spectre Raman et étude des propriétés de surface.

The determination of the size and the size distribution of anatase TiO2 nanopowders using Raman spectroscopy is presented. Several synthesis methods (soft chemistry, water-in-oil microemulsion, continuous hydrothermal synthesis) are used in order to control the size (from 3 to 20 nm), shape, phase and size distribution. The shift and width of the anatase Eg peak are often used to obtain the nanoparticles size. Homever, this peak is also sensitive to nonstoichiometry and others parameters. Low-frequency Raman scattering does not suffer from this problem. Size distibutions obtained by Raman spectroscopy and MET micrographs are compared. Finally, in situ Raman spectroscopy is used to study the surface adsorption influence on TiO2 nanocrystals.L'objet de cette thèse est l'étude par spectroscopie Raman de la taille et la distribution de taille de nanopoudres de TiO2 anatase. L'utilisation de plusieurs techniques de synthèse (chimie douce, microémulsion inverse, synthèse hydrothermale continu) a permis d'obtenir des matériaux dont la taille (de 3 à 20 nm), la forme, la phase sont parfaitement contrôlées et avec une distribution de taille resserrée. L'utilisation des modes optiques à travers le suivi de la raie Eg de l'anatase est la méthode la plus employée pour déterminer la taille des nanocristaux. Cependant, cette raie est entre autres sensible à la non-stoechiométrie. Afin de s'affranchir de ces effets, l'analyse des modes acoustique a été réalisée. Les distributions de taille obtenues par spectroscopie Raman sont comparées avec celles obtenues par MET. L'influence de l'état d'adsorption de la surface des nanocristaux de TiO2 a également été étudiée par spectroscopie Raman in situ

Low-frequency Raman characterization of size-controlled anatase TiO2 nanopowders prepared by continuous hydrothermal syntheses

7 pagesInternational audienceThe advantages of a recently presented continuous hydrothermal elaboration route for size-controlled anatase TiO2 nanoparticles are investigated. Nanopowders prepared by this route and using a soft chemistry route are characterized using X-ray diffraction, surface area measurements, high-resolution electron microscopy and Raman spectroscopy with an emphasis on the determination of the size distribution with low-frequency Raman measurements. The continuous hydrothermal route is shown to be more suitable for producing narrower size distributions

Thermal stability of FeAl intermetallics prepared by SHS Sintering

International audienc

Hydrogen trapping: Synergetic effects of inorganic additives with cobalt Sulfide absorbers and reactivity of cobalt polysulfide

International audienceThe biphasic product CoS2 + Co(OH)(2) obtained by oxidation of cobalt sulfide is known to trap hydrogen at room temperature and low pressure according to a balanced reduction equation. Adding various inorganic compounds to this original absorber induces their reduction by hydrogen in the same conditions at a significant rate: (i) excess cobalt hydroxide is reduced to metallic cobalt; (ii) nitrate ions are reduced to ammonia; (iii) sulfur and sodium thiosulfate are reduced to H2S or NaHS and Na2S, respectively. Without a hydrogen absorber these inorganic compounds are not reduced by H-2, suggesting synergetic effects involving H-2 and the hydrogen absorber. Amorphous cobalt polysulfide, CoS5, is also reduced by hydrogen at room temperature and releases H2S gas. In the presence of a base to neutralize H2S gas, the reaction rate is initially slower than with the CoS2 + Co(OH)(2) mixture due to the higher stability of polysulfide chains but the H-2 trapping yield is improved, making CoS5 a good candidate for H-2 trapping