Electrogeneration of oriented and functionalized mesoporous films

Since their discovery in the early 1990s, mesoporous materials with a regular structure have continued to arouse the curiosity of the scientific community, as much for their applicative interest in numerous fields ranging from electronics to biology and medicine, via the environment, as for more fundamental questions linked to reactivity in confined environments or for the challenges associated with their preparation under well-controlled conditions. In this context, organized mesoporous silicas are particularly attractive, as they can be synthesized as organo-mineral hybrids, giving rise to functional materials potentially usable for many applications. While these solids exist in a variety of forms (powders, monoliths, etc.), a "thin film" configuration is often required when their use is envisaged in devices such as sensors or reactors.

Traditionally, organized mesoporous silica films have been produced using the sol-gel process, in the presence of surfactants or block copolymers, under conditions of evaporation-induced self-assembly (EISA). The EISA method not only produces materials with a very high specific surface area (up to 1,000 m ² /g or even more), but also organized porosity (in hexagonal, cubic or lamellar structure), and controllable porosity on the mesoporous scale (with a pore diameter typically between 2 and 10 nm, and even up to 20 nm in some cases), with a very high porosity of up to 50%. Mesoporous silicas can also be functionalized with organic groups, either by post-synthesis grafting or, in one step, by co-condensation with organosilane reagents. The resulting organo-mineral hybrids feature a large number of functional groups distributed within an organized mesostructure, and are therefore accessible for a variety of applications.

A major challenge in the field of mesoporous films is controlling the orientation of the mesostructure, and in particular access to channels oriented perpendicular to the support, an optimal configuration for many applications, as it allows easy unidirectional access of external reagents to the active surface (e.g. an electrode). To date, this vertical orientation of mesoporous channels remains very difficult to induce by the EISA method. This is why another, electrochemical approach has recently been proposed: electrochemically-assisted self-assembly (EASA). The EASA method combines potential-controlled surfactant self-assembly with electrochemically-induced sol-gel deposition. It has the advantage of enabling the formation of uniform films on non-planar substrates, which is very difficult with EISA. However, it has the disadvantage of being applicable only to electronically conductive substrates. Strategies for preparing these oriented films in functionalized form are...