Sol-gel chemical principles

The sol-gel synthesis can be obtained from metallo-organic compounds such as alkoxides used as precursors. Alkoxides with a M(OR)n structure, where M is a metal or a metalloid and R is an alkyl group (R = CH3, C2H5, …). For example, tetraethylorthosilicate (TEOS), Si(OC2H5)4, is commonly used in the sol–gel synthesis of silica and glasses. Such chemicals are dispersed in a solvent (usually organic, ethyl alcohol) and react in an pH controlled medium, according to the following steps [1],[2]:

  • Hydrolysis of the alkoxide: It is initiated by the addition of water to the silane solution under acidic, neutral or basic conditions. During the hydrolysis reaction a water molecule replaces the alkoxi (nOR) group by hydroxyl (nOH). As a result of the hydrolysis of the silicon alkoxyde precursor, hydroxylated product (silanol groups) and the corresponding alcohol are generated. The reactive bond M–OH, which is necessary for the continuation of the reaction, is formed during this step.

M(OR)n + xH2O ⇌ M(OR)n-x(OH)x + xROH

  • This reaction is followed by the condensation between silanol groups (SinOH), forming siloxanic bonds (SinOnSi) and generating water or alcohol molecules as secondary products. In most of the cases, the condensation process starts before the hydrolysis is complete. The condensation can be produced between an unhydroxylated alkoxide group and a hydroxyl group (alcohol condensation or alcoxolation) or between two hydroxyl groups (water condensation or oxolation), which eliminates the solvent, and making possible the formation of a colloidal mixture known as sol.

Oxolation, which corresponds to a dehydration (the leaving group is H2O)

oxolation

Alcoxolation, which corresponds to a dealcoholation (the leaving group is ROH)

oxolation1.png

Fig. Sol-gel reactions [2]

When the condensation reactions are complete, each oxide ion is coordinated with four ions from the metal or metalloid, forming a homogeneous net. Depending on the chemical nature of the precursors, the final material contains one or several metal elements.

  • Polycondensation between sols or additional networking, resulting in a porous and three-dimensional (3D) crosslinked network. In this situation, the viscosity of the solution is gradually increased, and as result, the sol becomes interconnected to form a rigid and porous network known as gel.

[1] Rivero Fuente, P.J., Contribution to the development of functional nanostructured coatings based on silver nanoparticles, in Departamento de Ingeniería Eléctrica y Electrónica. 2014: Pamplona.

[2] Celzard, A. and J. Marêché, Applications of the sol-gel process using well-tested recipes. Journal of chemical education, 2002. 79(7): p. 854.

© Nadetech Innovations 2017

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