**2.1. Brief introduction of sol-gel process**

Sol-gel processes are versatile solution processes which have been widely applied to synthesize nano-particles, nano-composites, thin films, fibers and ceramics. They have shown consider‐ able advantages, including excellent chemical stoichiometry, compositional homogeneity and low crystallization temperature due to the mixing of liquid precursors on a molecular level [5]. The sol-gel process can be described as the creation of an oxide/chloride network by progres‐ sive condensation reactions of molecular precursors in a liquid medium.

The sols can be basically classified as inorganic sols and organic sols. The most widely used is the organic sols, which are generally fabricated by using solutions of metal or metalloid alkoxide precursors M(OR)n in organic solvents [11]. Typically, a sol-gel process comprises solution, gelation, drying and densification. The precursors are subjected to a series of hydrolysis and polycondensation reactions to form a colloidal suspension, namely sol. Further processing of the "sol" makes it possible to form materials in different shapes, such as spinning or dipping to produce coatings and films, drawing from a liquid to produce fibers, precipi‐ tating or spray pyrolysis to get ultrafine uniform powders and processing in a vessel to produce aerogels and monoliths.
