Sol-gel engineering to tune structural colours

For several decades, the sol-gel process allows the fabrication of functional materials through the strong coupling between materials chemistry and advanced processing. This branch of material's science is generally associated with the formation of oxide and hybrid materials obtained through a transition between a sol and a gel. Engineering the sol-gel process to extend same principles to other classes of materials is an emerging and promising research line. These aspects will be illustrated in this article by reviewing the successful case of optical materials with tunable structural colours. It will be shown that sol-gel transition from molecular or colloidal building blocks results in optical materials with great diversity in structure and composition such as oxide, hybrid, or metal-organic frameworks. Applicative examples will be provided including photocatalytic, "invisible" and anti-reflective coatings, graded photonic mirrors and 2D photonic sensors.