Thermogravimetric analysis (TGA) is investigated for determination of the OH surface density (OH/ nm 2 ) and carbon content of silica and titania powders made by flame aerosol and sol-gel processes. It is shown that it is possible to distinguish between physically adsorbed and chemically bound water and to rapidly determine the OH surface density even of small powder samples (<0.2 g) by TGA calibrated with LiAlH 4 titration data. The high accuracy of the OH surface density determination by TGA is confirmed further with additional LiAlH4 titration data of silica powders and by comparison with the specifications of commercially available silica Aerosil and titania P25 powders. Furthermore, by connecting a CO2 sensor or a mass spectrometer to the TGA balance, it is possible to verify the carbon content and determine other components (organic residues) of the powders. Thereby, it is shown that flame-made powders have high purity while the preparation conditions of sol-gel powders greatly affect their purity. At a water-toprecursor ratio of 1000:1, no organic residues are detected in sol-gel powders, while lower ratios result in residues in the product powder from the precursor, solvent, or catalyst.
Gas phase nanoparticle production, manipulation and deposition is of primary importance for the synthesis of nanostructured materials and for the development of industrial processes based on nanotechnology. In this review we present and discuss this approach, introducing cluster sources, nanoparticle formation and growth mechanisms and the use of aerodynamic focusing methods that are coupled with supersonic expansions to obtain high intensity cluster beams with a control on nanoparticle mass and spatial distribution. The implication of this technique for the synthesis of nanostructured materials is also presented and applications are highlighted.
A systematic study of the shift and linewidth of the Eg Raman peak at 144 cm−1 of anatase TiO2 nanopowders, produced by a flame aerosol technique, is here presented. The analysis was performed as a function of the crystal domain size and of the degree of oxidation. In the nanopowders, a clear contribution of the stoichiometry defects to the peak shift was evidenced, while the peak width seems to be less affected by the oxygen content. The Raman peak behavior due to size reduction has been interpreted in the framework of a phonon quantum confinement model. A critical review of the different approaches to this model, adopted in the literature to explain the behavior of the anatase Raman spectra as a function of the domain size, is presented. In particular, the hypothesis of a three-dimensional isotropic model for the dispersion relations is discussed. This analysis evidences general limits in the application of the phonon confinement model to the study and characterization of nanoparticles and nanostructured materials, showing how an uncritical use of the confinement theory can yield distorted results
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