Electrochemically Induced Templated Sol-gel Deposition of Mesoporous Silica and Nanocomposites: The New Kid on the ECiD Block

A relatively new member of the group of electrochemically induced deposition is that of templated sol-gel deposition. Uniform and smooth mesoporous silica thin films were first fabricated by an ECiD process nicknamed electrochemically assisted self-assembly or EASA, introduced by Walcarius et al. The addition of cetyltrimethylammonium bromide or CTAB as organic template molecules to the tetraethyl orthosilicate (TEOS) sol-gel precursor solution resulted in silica thin films with highly ordered, hexagonal arrays of vertical mesopores of 2–3 nm in diameter. These films have been proposed for applications in electroanalysis and sensors where effects such as pre-concentration or perm selectivity have been exploited. Their synthesis combines the well-established field of templated sol-gel synthesis with an electrochemically induced deposition process. To prevent spontaneous gel formation, the silica sol-gel precursor solution has a mildly acidic pH which kinetically inhibits the gelation reaction. Only when a cathodic potential/current is applied to electrogenerate OH- ions from the pro-base (water mostly), the gelation reaction is catalyzed by the local increase in pH, resulting in the growth of silica gel films on the electrode. We will first review the principles of sol-gel reactions, before proceeding to details of ECiD of sol-gel films.