Multifunctional Transition Metal Oxide Mesoporous Thin Films and Xerogels

PAULA C. ANGELOMÉ; GALO J. A. A. SOLER-ILLIA

Simposio; 4th International Mesostructured Materials Symposium; 2004

Mesoporous transition-metal oxides (MTMO) can be efficiently produced by Evaporation-Induced Self-Assembly (EISA) methods as xerogels, powders, films or fibers with high surface areas (200-500 m2/g), variety of mesostructures (2D- or 3D- hexagonal, cubic, local) and nanocrystalline walls. A further step is the modification of MTMO pore surface, to create spatially separated well-defined functionalities important for advanced applications (sensors, actuators, controlled delivery devices). The simplest approach is to post-functionalize an MTMO by grafting organic or organometallic groups. However, the routes to multifunctional MTMO are not straightforward, and particular attention has to be paid to aspects such as surface chemistry, diffusion within pores, etc. Another important issue is the integrity of modified MTMO under solvent flux (i.e, function leaching). In this work, titania and zirconia mesoporous films or xerogels with 2D-Hex (p6m) or Im3m cubic mesostructure were prepared by EISA, using poly(ethylene oxide)-based templates (Brij 56 or 58, F127 or P123). Control dense films were produced in the absence of templates. TiO2 and ZrO2 mesoporous nanocrystalline coatings obtained upon heating (T>400°C), present high organisation, excellent optical quality and thermal stability (Figure 1a). Organic molecules presenting complexing grafting groups (carboxylate, polyphenol, phosphate) and diverse functions (hydrophobic (alkyl or aryl chains), hydrophilic (amine, polyols, sulfonic acids), metallophilic (thiol)) were explored as surface modifiers. The incorporation of these functions to the mesoporous network was monitored by crossing FT-IR, EDS, XRF and chemical analysis. Leaching experiments in different solvents and conditions were performed in order to assess the anchoring of the grafted groups. FTIR or UV/Vis spectroscopy show dramatic differences for the incorporation of functions in mesoporous vs. non-mesoporous films or gels. UV/Vis spectra of TIRON (disodium 1,2 dihydroxy benzene, 3,5 disulfonate) substituted TiO2 cubic films show an important absorption at 450 nm (Figure 1b), attributable to a Ti-TIRON complex, not observed for non-templated films. The same differences in the organic uptake are observed for dihexadecyl phosphate (DHDP, Figure 1c); FTIR of substituted films show evidence of incorporated molecules grafted by complexation. Typically, 5-6 molecule/M% incorporation is observed, irrespectively of the grafting groups (phosphate, polyol, carboxylate); the adsorption kinetics was followed by FTIR and EDS. In most cases, the maximum incorporation occurs within 5 minutes suggesting fast molecule mobility and diffusion inside the film pore system. For DHDP, (Figure 1d) two steps are observed: 80% DHDP enters within 5 minutes; a gradual increase is further observed. However, longer times are required to homogeneously incorporate organic functions in xerogels. Leaching of DHDP in THF shows that ~20% of the incorporated DHDP is eliminated within 100 minutes (Figure 1e); this value is unchanged after several days, suggesting two different incorporation modes: strongly bound to the walls, or loosely incorporated within the pore. For carboxylic acids or polyphenols, leaching is pH-dependent. The accessibility of functionalised pores is confirmed by the presence of Hg (Figure 1f) in cysteine-substituted mesoporous titania films exposed to Hg(NO3)2 solutions. In summary, we demonstrate the possibility to obtain multifunctional MTMO, by anchoring organic groups to the pore walls. Most grafting groups (carboxylate, phosphonate, polyphenol) are stable towards leaching; in some cases, a fraction of the functions can be loosely bound and partial removal occurs. Substituted pores are accessible to other molecules (fluorescent probes) or ions (i.e. Hg(II)), opening the way for sensor or sorption applications.