Molecular Transport into Mesostructured Silica Thin Films: Electrochemical Monitoring and Comparison between <i>p</i>6<i>m</i>, P6₃/<i>mmc</i>, and <i>Pm</i>3<i>n</i> Structures

Abstract

Ellipso-porosimetry and electrochemical techniques were used to characterize porosity and molecular\ntransport into mesostructured porous silica thin films displaying various structures. The films have been\nprepared by the evaporation-induced self-assembly (EISA) method using cetyltrimethylammonium bromide\n(CTAB) as the template. According to previous investigations, the structure of the film can be controlled\nby the fine adjustment of the CTAB concentration and relative humidity during the dip-coating process.\nFilms displaying &lt;i&gt;p&lt;/i&gt;6&lt;i&gt;m&lt;/i&gt; (2D hexagonal), &lt;i&gt;P&lt;/i&gt;6&lt;sub&gt;3&lt;/sub&gt;/&lt;i&gt;mmc&lt;/i&gt; (3D hexagonal), and &lt;i&gt;Pm&lt;/i&gt;3&lt;i&gt;n&lt;/i&gt; (cubic) nanoporous structures\nhave been obtained (as revealed by XRD and TEM) and characterized after template removal by ethanol\nwashing. 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\nmesoporous structure when contacting water molecules, leading to a decrease in the pore diameter for\nboth 3D hexagonal and cubic mesostructures and to an important damaging of the 2D hexagonal\nmesostructure. Another mesoporous silica film prepared with a block copolymer (F127), displaying a\ncubic structure (&lt;i&gt;Im&lt;/i&gt;3&lt;i&gt;m&lt;/i&gt;), was used for comparison purposes. Electrochemical investigations by cyclic\nvoltammetry and wall-jet electrochemistry were performed using electrochemical probes displaying various\ncharges and sizes (I&lt;sup&gt;-&lt;/sup&gt;, Fe(CN)&lt;sub&gt;6&lt;/sub&gt;&lt;sup&gt;3-&lt;/sup&gt;, Ru(bpy)&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;2+&lt;/sup&gt;, FcMeOH). The organization of the porous network\nconstituting the silica film and its stability in aqueous medium were found to have a profound effect on\nits permeability properties, and the following sequence was observed by cyclic voltammetry: &lt;i&gt;Pm&lt;/i&gt;3&lt;i&gt;n&lt;/i&gt; &gt;\n&lt;i&gt;P&lt;/i&gt;6&lt;sub&gt;3&lt;/sub&gt;/&lt;i&gt;mmc&lt;/i&gt; ≈ &lt;i&gt;Im&lt;/i&gt;3&lt;i&gt;m&lt;/i&gt; &gt; &lt;i&gt;P&lt;/i&gt;6&lt;i&gt;m&lt;/i&gt;. The electrochemical responses were also dramatically influenced by the charge\nand to a lesser extent the size of the molecular probe. Positively charged species (Ru(bpy)&lt;sub&gt;3&lt;/sub&gt;&lt;sup&gt;3+/2+&lt;/sup&gt; and\noxidized FcMeOH) were likely to accumulate in the film, whereas negatively charged species (Fe(CN)&lt;sub&gt;6&lt;/sub&gt;&lt;sup&gt;3-&lt;/sup&gt;,\nI&lt;sup&gt;-&lt;/sup&gt;) could be totally or partially excluded, leading to either preconcentration or permselective behaviors.\nQuantitative evaluation of the permeability (&lt;i&gt;PD&lt;/i&gt;&lt;sub&gt;f&lt;/sub&gt;, where &lt;i&gt;P&lt;/i&gt; is the partition coefficient and &lt;i&gt;D&lt;/i&gt;&lt;sub&gt;f&lt;/sub&gt; the apparent\ndiffusion coefficient in the film) of the different molecular probes in thin films displaying a cubic\nmesostructure was achieved using the wall-jet electrochemistry technique. The obtained values (in the 1\n× 10&lt;sup&gt;-8&lt;/sup&gt; to 1 × 10&lt;sup&gt;-7&lt;/sup&gt; cm&lt;sup&gt;2&lt;/sup&gt; s&lt;sup&gt;-1&lt;/sup&gt; range) were discussed with respect to the probe size and charge. The results\npresented in this article can be useful in the various fields that promote the use of mesoporous silica\nlayers for sensing, membranes, chromatography, catalysis, and electrochemical-cell-based applications.