Size stabilization of nanoparticles by polysaccharides: Effectiveness in the wet and curing steps
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Starch suspension proves to be a useful matrix for the hydrolytic route to metal oxide nanoparticles, due to its size-stabilization effect, which works also at high temperatures. To understand the type of interaction between the organic part and the oxide particles, various parameters, such as viscosity, temperature, degree of polymerization, and organic/inorganic kinds of dispersant, are tested through x-ray diffraction (XRD), transmission electron microscopy (TEM), solid-state nuclear magnetic resonance (NMR), and thermogravimetric mass spectra (TG–MS) analyses of the obtained SnO2 nanopowders. Results highlight the unique role of starch compared with other hydrophilic dispersants that do not ensure effective size stabilization on curing up to 600 °C. The proof comes from the study of pyrolysis of the residual organic groups surrounding the particles. They are chelating carboxylic species that prevent the coalescence among metal oxide nanoparticles.
I. INTRODUCTION
Chemical processes taking place in a solution phase are currently explored in the synthesis of inorganic oxides, because they join both the advantage of starting from a homogeneous mixture of reactants and the accurate control of parameters, such as temperature, pH, and concentration.1–3 The use of alkoxides and of salts, as precursors suitable for hydrolysis and condensation, promotes the preparation of materials with a defined shape, such as films,4–7 coating layers,8,9 nanotubes,10,11 and self-carrying membranes.12 This approach—namely, the sol-gel process13,14 and the hydrolytic route15—is the simplest one to obtain homogeneous suspensions of nanoparticles, which may be stabilized in dimension by such size stabilizers as capping agents or templates.16–18 We recently reported that starch may be successfully used in the hydrolytic route as an additive to obtain oxides and mixtures of oxides in the form of nanocrystalline powders.19 The polysaccharide presence offers many advantages, such as high availability at low cost, clean easy biodegradation to soluble glucosidic units, and original particle shape maintenance during heat treatment, owing to the possible coordination of glucosidic moieties with particle surface. Moreover, the organic hydrophilic envelope of the particles facilitates the formation of colloidal suspensions with conve-
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Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2007.0416 3344
http://journals.cambridge.org
J. Mater. Res., Vol. 22, No. 12, Dec 2007 Downloaded: 11 Mar 2015
nient binders, which are usually used to obtain bulk materials through various methods, such as microscreen printing. In the proposed approach, starch exerts two important functions: (i) the stabilization of primary crystallization nuclei versus size increase, either by mass addition of molecular species from the solution or by coalescence between particles, and (ii) the preservation of nanometric dimensions during heat treatment. Thus, after the proof of the general usefulness of starch, it has been de
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