Electrochemically assisted self-assembly of ordered and functionalized mesoporous silica films: impact of the electrode geometry and size on film formation and properties

Abstract

Surfactant-templated mesoporous silica thin films can be deposited onto solid electrode surfaces by electrochemically assisted self-assembly (EASA). The method involves a cathodically triggered self-assembly of cationic surfactants (cetyltrimethyl ammonium bromide, CTAB) and local pH increase leading to the polycondensation of silica precursors (i.e., tetraethoxysilane, alone or in the presence of (3-mercaptopropyl)trimethoxysilane (MPTMS)) and concomitant growth of the ordered mesoporous silica or organosilica film. The present work shows that the EASA method can be applied to film deposition on electrode supports of various morphologies, geometries and sizes (large and flat discs or non-flat streaked supports, i.e., gold CD-trodes, as well as several kinds of ultramicroelectrodes, including carbon fibers, platinum wires, and platinum microdiscs). Galvanostatic conditions were mainly preferred to potentiostatic conditions to avoid problems related to various overpotentials and surface areas experienced with the various working electrodes used here. The results indicate that film deposition was possible on each electrode support but also that both the film formation and properties were dependent on the experimental conditions for EASA. For example, passing from large electrodes to ultramicroelectrodes required the application of larger current densities to ensure film deposition, which can be due to faster loss of the hydroxyl species in solution in the case of radial or spherical diffusion, in comparison to the linear. Highly porous deposits were obtained after template removal, as ascertained by cyclic voltammetry using Ru(NH₃)₆³⁺ as a redox probe. The advantage of better signal-to-background current ratios for ultramicroelectrodes relative to the macroscopic ones was maintained after film deposition, also resulting in higher sensitivity when used in conditions of preconcentration electroanalysis (using silver(I) or mercury(II) as a probe being accumulated by complexation to MPTMS-based films).