New insight in the role of modifying ligands in the sol-gel processing of metal alkoxide precursors: A possibility to approach new classes of materials


Kessler, Vadim G. and Spijksma, Gerald I. and Seisenbaeva, Gulaim A. and Hakansson, Sebastian and Blank, Dave H.A. and Bouwmeester, Henny J.M. (2006) New insight in the role of modifying ligands in the sol-gel processing of metal alkoxide precursors: A possibility to approach new classes of materials. Journal of Sol-Gel Science and Technology, 40 (2-3). pp. 163-179. ISSN 0928-0707

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Abstract:This paper summarizes recent literature data
and presents new experimental data on the mechanisms of
chemical modification, hydrolysis and polycondensation
of the alkoxides and demonstrates possibilities to approach
new classes of materials, exploiting these mechanisms. Low
reactivity of silicon alkoxides is improved by either basic
catalysis exploiting an SN2 mechanism or acidic catalysis
facilitating a proton-assisted SN1 mechanism as well as by
modification with chelating ligands. Metal alkoxides are
much stronger Lewis bases compared to silicon alkoxides
and the acidity of water is strong enough to achieve their
rapid hydrolysis via proton-assisted SN1 pathway even in the
absence of additional catalysts. Introduction of the modifying
chelating ligands is leading generally to increased charge
distribution in the precursor molecules.Modifying chelating
ligands are also appreciably smaller than the alkoxide ligands
they replace. The modification with chelating ligands is thus
facilitating the kinetics of hydrolysis and polycondensation.
The size and shape of the primary particles formed in sol-gel
treatment of metal alkoxides are defined not by kinetic
factors in their hydrolysis and polycondensation but by
the interactions on the phase boundary, which is in its turn
directed by the ligand properties. The products of the fast
hydrolysis and condensation sequence consist of micelles
templated by self-assembly of ligands (mainly oxo-species).
This concept provides explanations for commonly observed
material properties and allows for the development of new
strategies for the preparation of materials. We discuss the
formation of inverted micelles, obtained by the appropriate
choice of solvents, which allows for the formation of
hollow spheres. The modifying β-diketonate ligands act as
the surfactant and form an interface between the hollow
sphere and the solvent. Retention of ligands inside the
gel particles is possible only if ligands possessing both
chelating and bridging properties are applied. Application
of such ligands, for example, diethanolamine, permits to
prepare new transition metal oxide based microporous
Item Type:Article
Copyright:© 2006 Springer Science + Business Media
Science and Technology (TNW)
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