Colloidal sol-gel: A powerful low-temperature aqueous synthesis route of nanosized powders and suspensions

The colloidal sol-gel is a well-established method for the synthesis of nanoparticles with high reproducibility, homogeneity and low cost. It is a versatile route which allows synthesising many ceramic materials at the nanometric scale. The process starts with the hydrolysis of the metal precursor and ends with the formation of a colloidal suspension/sol with high stability. The particle size can be easily tuned by synthesis parameters such as temperature, metal precursor-water ratio and/or metal precursor-catalyst ratio. Depending on the applications, the sol can be directly used for the preparation of coatings or it can be dried to be used as powder. In this review, the fundamentals of the colloidal sol-gel process are provided, highlighting the great capacity of this synthesis route. Its use in combination with other synthesis methods and the influence of the parameters controlling the process is presented, together with applications and examples of the synthesised powders.