Nanometric oxides from molecular precursors in the presence of starch: Coatings of glass with these oxides in silica sol
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Sn, Sn-Sb, Ti, Ce, Zr, Zn, In, In–Sn, and Fe oxides of nanometric size were obtained through the hydrolytic route, starting from molecular precursors in the presence of starch. Gels were treated with ␣-amylase to produce oxide suspensions and starch degradation. Solid oxide products resulted from reaction with H2O2, washing with H2O, and centrifugation of the suspensions. The nanometric size and morphology of crystallites were assessed by x-ray diffractometry, transmission electron microscopy, and, for SnO2, solid-state nuclear magnetic resonance. Product consolidation at 600 °C did not produce any noticeable increase in dimensions. Thermal analysis coupled with mass spectrometry of evolved gaseous species showed that glucoside residues remain chemically bonded to the nanoparticles, thus explaining the effective stabilization of crystallite dimensions. Aqueous suspensions of nanopowders were mixed with a silicon tetra-ethoxide ethanol solution and subjected to an ordinary sol-gel process. The resulting suspensions were used to obtain stable and homogeneous coatings on glass sheets. I. INTRODUCTION
The availability of materials on a nanometric scale is an attractive topic, which currently engages many research groups from academic and industrial institutions.1–5 This interest is justified by the peculiar features of nanomaterials.6 Heightened interactions with substrates, resulting from the high surface-to-volume ratio, lead to the magnified response of sensors7 and improved specific activity of catalysts.8 Objects composed of few thousand atoms may also be organized into unusual crystalline structures or present a unique local distribution of electrons. These properties confer particular magnetic and electric properties.9 The preparation of nanocrystalline powders requires the use of chemical precursors in solution, i.e., homogenous dispersion of the starting species, and leads to insoluble compounds. Two methods of arriving at this result are the hydrolytic route and the sol-gel process, involving hydrolysis and condensation of molecular precursors.10 The advance of these reactions leads to crystallization nuclei and subsequent crystal growth—by addition of molecular species or aggregation between particles—which must be quenched. These phenomena are well-known in colloidal science.11 Artifacts, such as the addition of surfactants or non-ionic polar stabilizers, a)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2006.0217 1726 J. Mater. Res., Vol. 21, No. 7, Jul 2006 http://journals.cambridge.org Downloaded: 06 Sep 2014
are currently used to maintain the nanometric dimensions of nuclei as soon as they are formed.12–14 On this subject, we recently reported that starch may be successfully used as a stabilizer of nanopowders of inorganic oxides, which are prepared from alkoxides or ordinary salts through hydrolysis, olation, and oxo-olation reactions.15 The advantage of starch stabilization of colloidal particles is due to biological degradation into small glucosi
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