The evolution of 'sol-gel' chemistry as a technique for materials synthesis

From its initial use to describe hydrolysis and condensation processes, the term 'sol-gel' is now used for a diverse range of chemistries. In fact, it is perhaps better defined more broadly as covering the synthesis of solid materials such as metal oxides from solution-state precursors. These can include metal alkoxides that crosslink to form metal-oxane gels, but also metal ion-chelate complexes or organic polymer gels containing metal species. What is important across all of these examples is how the choice of precursor can have a significant impact on the structure and composition of the solid product. In this review, we will attempt to classify different types of sol-gel precursor and how these can influence a sol-gel process, from self-assembly and ordering in the initial solution, to phase separation during the gelation process and finally to crystallographic transformations at high temperature.