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Scheme 8.

The metal species bearing a terminal hydroxy group react with other species in the solution to form oligomeric structures linked by oxo- or hydroxo- ligands, which can be transformed into polymeric gels by aging. The polymerization reactions and the subsequent drying and thermal treatments result in the elimination of water (oxolation) and alcohol (alcoholation) to finally produce a solid material (Scheme 9). These modified molecular entities arrange themselves via hydrolysis and cross-condensation reactions to form oxo- or hydroxo-macromolecular networks, thus favoring the gelation process.

Oxolation

Alcoholation

Scheme 9.

The stage where the molecular structure of the precursor is no longer reversible is called a sol-gel transition. The gel is an elastic solid filling the same volume as the solution; it shrinks upon drying to give a rigid oxide skeleton with interconnected porosity that can be tailored in terms of size, shape, and volume. The natural drying of gels results in xerogels, whereas removal of the solvent under hypercritical conditions (without disturbing the gel structure) produces aerogels [334]. A schematic representation of the various steps of the sol-gel processing is given in Figure 7.

The partially hydrolyzed alkoxides can be viewed as molecular building blocks for the development of sol (par-ticulate) or gel (polymeric), which can be used to obtain films by spin- or dip-coating and spray methods. Modifying the alkoxide precursors with less hydrolyzable or polymer-izable groups can chemically control the condensation reactions [37, 335, 336]. Finally, the chemical reactivity of metal alkoxides toward nucleophilic reactions mainly depends on the strength of the nucleophile, the electrophilic character of the metal atom (an electronegativity-related property characterized by the positive charge on the metal atom), and its ability to increase the coordination number N: the degree of unsaturation at the metal center can be simply expressed by the difference N — Z, where N is the coordination number usually found in the oxide and Z is the oxidation state. A comparison of these values for some tetravalent metal alkoxides is made in Table 6. The above facts underline the critical role of the precursor chemistry when materials are

Xerogel film

Hydrolysis

Xerogel film

Dense film

Hydrolysis

Heat g Aerogel

Uniform particles m

Figure 7. Various steps of sol-gel processing starting from a molecular alkoxide.

Dense film

Heat g Aerogel

Uniform particles m

Figure 7. Various steps of sol-gel processing starting from a molecular alkoxide.

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