Table 1.

Classification of five different types of “gels” essential to a material’s sol-gel synthesis [35].

Type of Gel	Bonding	Source	Gel Schematic
Colloidal	Particles connected by Van der Waals or hydrogen bonding	Metal oxides or hydroxide sols
Metal-oxane polymer	Inorganic polymers connected via covalent or intermolecular bonding	Hydrolysis or condensation of metal alkoxides, e.g., SiO2 from tetramethyl orthosilicate
Metal complex	Weakly interconnected metal complexes	Concentrated metal complex solution, e.g., aqueous metal citrate or ethanolic metal urea often form resins or glassy solids rather than gels
Polymer complex I in situ polymerizable complex (“Pechini” method)	Organic polymers interconnected by covalent or coordinate bonding	Polyesterification between polyhydroxy alcohol (e.g., ethylene glycol) and carboxylic acid with metal complex (e.g., metal-citrate)
Polymer complex II coordinating and crosslinking polymers	Organic polymers interconnected by coordinate and intermolecular bonding	Coordinating polymer (e.g., alginate) and metal salt solution (typically aqueous)