Molecular Transport into Mesostructured Silica Thin Films: Electrochemical Monitoring and Comparison between p6m, P63/mmc, and Pm3n Structures

Abstract

Ellipso-porosimetry and electrochemical techniques were used to characterize porosity and molecular transport into mesostructured porous silica thin films displaying various structures. The films have been prepared by the evaporation-induced self-assembly (EISA) method using cetyltrimethylammonium bromide (CTAB) as the template. According to previous investigations, the structure of the film can be controlled by the fine adjustment of the CTAB concentration and relative humidity during the dip-coating process. Films displaying p6m (2D hexagonal), P63/mmc (3D hexagonal), and Pm3n (cubic) nanoporous structures have been obtained (as revealed by XRD and TEM) and characterized after template removal by ethanol washing. Characterization of the film porosity has been performed on the basis of water adsorption−desorption cycles and ellipso-porosimetry measurements. They revealed significant contraction of the mesoporous structure when contacting water molecules, leading to a decrease in the pore diameter for both 3D hexagonal and cubic mesostructures and to an important damaging of the 2D hexagonal mesostructure. Another mesoporous silica film prepared with a block copolymer (F127), displaying a cubic structure (Im3m), was used for comparison purposes. Electrochemical investigations by cyclic voltammetry and wall-jet electrochemistry were performed using electrochemical probes displaying various charges and sizes (I-, Fe(CN)63-, Ru(bpy)32+, FcMeOH). The organization of the porous network constituting the silica film and its stability in aqueous medium were found to have a profound effect on its permeability properties, and the following sequence was observed by cyclic voltammetry: Pm3n > P63/mmc ≈ Im3m > P6m. The electrochemical responses were also dramatically influenced by the charge and to a lesser extent the size of the molecular probe. Positively charged species (Ru(bpy)33+/2+ and oxidized FcMeOH) were likely to accumulate in the film, whereas negatively charged species (Fe(CN)63-, I-) could be totally or partially excluded, leading to either preconcentration or permselective behaviors. Quantitative evaluation of the permeability (PDf, where P is the partition coefficient and Df the apparent diffusion coefficient in the film) of the different molecular probes in thin films displaying a cubic mesostructure was achieved using the wall-jet electrochemistry technique. The obtained values (in the 1 × 10-8 to 1 × 10-7 cm2 s-1 range) were discussed with respect to the probe size and charge. The results presented in this article can be useful in the various fields that promote the use of mesoporous silica layers for sensing, membranes, chromatography, catalysis, and electrochemical-cell-based applications.