Electrogeneration of highly methylated mesoporous silica thin films with vertically-aligned mesochannels and electrochemical monitoring of mass transport issues

In this work, we describe a versatile approach to generate highly methylated mesoporous silica films exhibiting hexagonally- packed one-dimensional mesochannels oriented uniquely perpendicular to the underlying support. It involves the electro-assisted self-assembly (EASA) of tetraethoxysilane (TEOS) and methyltriethoxysilane (MTES) with cetyltrimethylammonium bromide (CTAB) under potential control. Transmission electron microscopy and grazing-incidence X-ray diffraction indicate that high level of ordering and vertical alignment of mesopores can be maintained at rather high MTES contents (up to 50-60 mol% with respect to total precursors in the starting sol). The rate of film growth decreased concomitantly to increasing the MTES/TEOS ratio, resulting in thinner films for higher functionalization degrees. This also led to more hydrophobic films (after template extraction), which tended to become less permeable to aqueous phase solutes, as pointed out qualitatively and quantitatively by several electrochemical methods and various redox probes. Effective rate constants for electron transfer were found to be several orders of magnitude higher than for other thin silica films on electrodes, explained by preferential porosity orientation.