Supported mesostructured thin films are of major importance for applications in optical, electrochemical and sensing devices. However, good performance is restricted to mesostructured phases ensuring good accessibility from the film surface, which would be straightforward with cylindrical pores oriented normal to the underlying support, but this remains challenging. Here, we demonstrate that electrochemistry is likely to induce self-assembly of surfactant-templated (organo)silica thin films on various conducting supports, homogeneously over wide areas. The method involves the application of a suitable cathodic potential to an electrode immersed in a surfactant-containing hydrolysed sol solution to generate the hydroxyl ions that are necessary to catalyse polycondensation of the precursors and self-assembly of hexagonally packed one-dimensional channels that grow perpendicularly to the electrode surface. The method is compatible with controlled and localized deposition on heterogeneous supports, opening the way to electrochemically driven nanolithography for designing complex patterns of widely accessible mesostructured materials.

Highly ordered and vertically oriented mesoporous silica films can be generated by electro-assisted self-assembly (EASA). The method involves the electrogeneration of hydroxide ions at an electrode surface immersed in an hydrolyzed sol solution (containing typically tetraethoxysilane, TEOS, and cetyltrimethylammonium bromide, CTAB) in order to catalyze polycondensation of the precursors and self-assembly of hexagonally packed one-dimensional channels that grow perpendicularly to the support. Vertically aligned mesostructures have been demonstrated by TEM imaging and by grazing incidence X-ray diffraction (GIXD), this latter technique enabling characterization of thin films directly on their underlying electrode surface. The influence of the electrosynthesis medium composition (precursor and surfactant concentrations, surfactant chain length) on the mesostructural order and film thickness has been thoroughly examined. It was shown that the highly ordered and oriented mesoporous silica films can be obtained over a wide composition of the starting sol (i.e., 10−200 mM CTAB and 50−350 mM TEOS) and that the lattice parameter can be moderately tuned by changing the chain length of the surfactant template. Thickness of these films can be accurately controlled by applying galvanostatic conditions and by varying the deposition time, which offer the versatility to be applied in the same way to electrodes of different nature without overpotential problems encountered in the potentiostatic mode. Thin mesoporous films are often covered with an additional byproduct made of particulate aggregates arising from bulk gelification at the electrode/solution interface. Getting aggregate-free thin films is possible by working in diluted solutions (i.e., [TEOS] < 125 mM and CTAB/TEOS ratio <0.32) and with a short deposition time (∼10 s). Voltammetric experiments carried out on these films deposited onto planar indium−tin-oxide electrodes, after template extraction, have revealed very sensitive responses to solution-phase redox probes as a result of fast mass transport from the external solution through the film to the electrode surface. Quantitative characterization of these mass transfer processes reveals that apparent diffusion coefficients as high as about 1 × 10−7 cm2 s−1 can be reached but great care should be taken in defining the film synthesis conditions that may lead to some additional limiting effects.

Five different ordered mesoporous silica samples displaying various pore sizes and structures (two small-pore MCM-41, two large-pore MCM-41, and one small-pore MCM-48) and one amorphous silica gel have been grafted with either aminopropyl or mercaptopropyl groups. The resulting aminopropyl-grafted silicas (APS) and mercaptopropyl-grafted silicas (MPS) have been studied in solution via protonation of APS and metal ion binding on both APS and MPS. Special attention was given to characterize the accessibility to the binding sites and to the speed at which the reactants are reaching these reactive centers inside the mesoporous materials. Results have been obtained from batch experiments, by monitoring the reactant depletion in suspensions containing APS or MPS particles, and discussed with respect to the structure and porosity of the organic−inorganic hybrids. As a general trend, both accessibility and rate of access to the reactive sites were higher with ordered mesoporous solids than with amorphous materials of comparable porosity (average pore size ∼60−70 Å). The ordered mesoporous structures of smaller pore size (∼35 Å) gave rise to the same performance as that of large-pore amorphous silica, only if pore blocking can be avoided during the grafting process; if not, the advantage of uniform pore structure over the corresponding amorphous material did not exist anymore:  a pore volume of at least 0.5 cm3 g-1 remaining upon grafting was necessary to keep this advantage. Increasing the amount of grafted moieties in the mesopores also led to restricted mass-transfer rates because of increasing steric hindrance. Moreover, protonation of mesoporous APS displaying uniform-sized channels was found to be dramatically slow at protonation levels higher than 50%, leading even to less-than-complete occupancy levels after 24 h of equilibration, most probably because of strong repulsive electrostatic interactions in such a confined medium. Applying a simplified diffusion model, the access rates of CuII in APS and HgII in MPS materials were quantified via calculation of apparent diffusion coefficients, D app, by appropriate fitting of kinetic curves. D app values were found to decrease slowly upon gradual completion of reaction because progressively less space was available for the ingress of reactants upon filling the mesostructures. This work would help at selecting the most appropriate conditions for target applications of grafted mesoporous solids in terms of capacity, accessibility, and especially, access rates to the active sites.

No 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), P6 3 /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 adsorptiondesorption 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) 6 3-, Ru(bpy) 3 2+ , 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 > P6 3 /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) 3 3+/2+ and oxidized FcMeOH) were likely to accumulate in the film, whereas negatively charged species (Fe(CN) 6 3-, I -) could be totally or partially excluded, leading to either preconcentration or permselective behaviors. Quantitative evaluation of the permeability (PD f , where P is the partition coefficient and D f 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 cm 2 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.

The speed at which selected reactants are reaching active sites grafted on porous silica gels of various nature and porosity was evaluated using some model systems:  protonation of aminopropyl-grafted silica (APS), mercury(II) binding on mercaptopropyl-grafted silica (MPS), and accumulation of copper(II) on APS. Data were obtained from batch experiments, by monitoring the consumption of reactant in solutions containing the solid phases as dispersed particles (average size:  60−150 μm). Various grafted solids were studied, with pore diameter ranging between 4 and 15 nm and organic group contents of typically 1.4−1.9 mmol g-1. Diffusion processes in the porous organically modified silicas were found to be dramatically restricted as compared to those observed in homogeneous solution (≈103−104 times slower). They were dependent on several factors such as the pore size of the material and the size of the reactant, the density of grafted organic sites, and the nature of the starting silica gel. Evaluation of apparent diffusion coefficients was achieved by applying a simplified model based on spherical diffusion. This has allowed us to point out a significant decrease in the access rates upon gradual completion of reactions:  as the reactant concentration in the vicinity of increasing amounts of grafted groups is raised progressively, there is less room available in the porous structure to enable the probe to reach rapidly the remaining active sites. The apparent diffusion coefficient was found to drop dramatically after typically 30−50% reaction completion, depending on the nature of the probe. This study allows highlighting the optimal conditions that should be required to ensure efficient application of grafted silica gels, that is, in the fields of catalysis or heavy metal extraction.

We have recently reported the possible fabrication of highly ordered mesoporous silica thin films with mesopore channels oriented perpendicularly to the underlying substrate, by means of novel electroassisted self-assembly (EASA) method (Nature Mater. 6, 602 (2007)). Such films deposited on an electrode surface can be of great interest in sensing applications if one could introduce organo-functional groups likely to interact with target analytes in a regular environment ensuring great accessibility and fast mass transfer rates. We demonstrate here that aminopropyl-functionalized mesoporous silica films can be electrogenerated in one step by the sol-gel co-condensation route using cetyltrimethylammonium bromide as template. The orientation of the pore is maintained up to 10% aminopropyltriethoxysilane precursor in the starting sol. The presence of amine functions into the film affects its permeability to external reagents, as studied using various redox probes (Ru(bpy)3(2+), FcEtOH, I(-), Fe(CN)6(3-)), and the lack of mesostructuration was found to hinder dramatically mass transport processes. When applied to the voltammetric detection of copper(II) subsequent to open-circuit accumulation, the response of the electrode was greatly affected by the functionalization level, the optimal sensor sensitivity being defined from the best compromise between an amount of amine groups high enough while maintaining mesostructural order.

No abstract