Condensation of silica sol-gel precursors

Chapter 48 The Chemistry of Hydrolysis and Condensation of Silica Sol-Gel Precursors. 637... 

Starting with acid-catalyzed hydrolysis and condensation (step 1) of the silica precursor (tetraethoxysilane, TEOS), the viscous product (poly(ethoxy)siloxane, PES) is converted into silica hydrogel beads in a stirring process under basic conditions (step 2). Both the viscosity of the sol-gel derived PES and the stirring speed directly influence... 

Figure 8.14. Growth of silica colloids and their morphology during hydrolysis of precursor molecules and condensation in a sol-gel process.

We will give emphasis on silica-based materials, which have attracted great attention, owing to the versatile chemistry of silicon compoimds, the smooth hydrolysis-condensation of the alkoxy derivatives, and the stability of the silicon-carbon bonds. Moreover, it offers easy processing of the sol-gel materials to generate films or particles with controlled size [5]. Silica is produced upon simple hydrolysis-condensation of an alkoxide precursor to form a three-dimensional covalent network, the properties of which depend on the reaction conditions (e.g., nature of the alkoxide precursor, solvent, catalyst, concentration, and temperature) (Scheme 4.1). 

The existence of a number of metal alkoxides suitable as sol-gel precursors for many oxide powders, such as silica or titanium oxide, and the compositional variability available are the main advantages of sol-gel procedures. However, for metal alkoxides with low valence, such as Fe, the hydrolysis and condensation are too rapid to control particle size and shape. Usually, magnetite and ferrites are preferably obtained by hydrolysis of metal salts such as acetates or citrates in water, ethanol, or glycol. The counterions work as a pH buffer and as a component of the precursor complexes. The conditions need to be chosen carefully to yield a material with appropriate properties for each specific application. Disadvantages of the sol-gel methods include contamination from reaction by-products, as well as the need for posttreatment of the products. 

The Stober method is also known as a sol-gel method [44, 45], It was named after Stober who first reported the sol-gel synthesis of colloid silica particles in 1968 [45]. In a typical Stober method, silicon alkoxide precursors such as tetramethylorthosili-cate (TMOS) and tetraethylorthosihcate (TEOS), are hydrolyzed in a mixture of water and ethanol. This hydrolysis can be catalyzed by either an acid or a base. In sol-gel processes, an acidic catalyst is preferred to prepare gel structure and a basic catalyst is widely used to synthesize discrete silica nanoparticles. Usually ammonium hydroxide is used as the catalyst in a Stober synthesis. With vigorous stirring, condensation of hydrolyzed monomers is carried out for a certain reaction time period. The resultant silica particles have a nanometer to micrometer size range. 

In a tetralkoxysilane/alkyltrialkoxysilane system, the hydrolysis and condensation reactions proceed not only between similar precursor molecules but also between the two different precursor species. Figure 8.3 illustrates a simplified scheme of hydrolysis and condensafion reactions in a hybrid system containing TMOS and an alkyltrimethoxysilane. Notice the difference from the scheme in Figure 8.1 the alkyl group, represented by R, does not participate in the hydrolysis and condensation reactions that lead to the silica monolith. The sol-gel process kinetics in a hybrid sol-gel... 

Another challenge faced by sol-gel technologies involves controUing the dispersion of different metals within a mixed metal (e.g., sihcon and titanium) matrix. The solvolysis and condensation steps for metal alkoxide precursors involved in sol-gel reactions can be quite different from that of orthosilicates, which often leads to the loss of dispersion and formation of separate silica and other metal oxide domains [54]. 

The preferentially employed approach for the fabrication of inorganic (silica) monolithic materials is acid-catalyzed sol-gel process, which comprises hydrolysis of alkoxysilanes as well as silanol condensation under release of alcohol or water [84-86], whereas the most commonly used alkoxy-silane precursors are TMOS and tetraethoxysilane (TEOS). Beside these classical silanes, mixtures of polyethoxysiloxane, methyltriethoxysilane, aminopropyltriehtoxysilane, A-octyltriethoxysilane with TMOS and TEOS have been employed for monolith fabrication in various ratios [87]. Comparable to free radical polymerization of vinyl compounds (see Section 1.2.1.5), polycondensation reactions of silanes are exothermic, and the growing polymer species becomes insoluble and precipitates... 

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