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. From its initial use to describe hydrolysis and condensation processes, the term 'sol-gel' is now used for a diverse range of chemistries.