Design of complex materials with tailored porosity in the nanoscalem

M.C. FUERTES; G.J.A.A. SOLER ILLIA

Bariloche, Argentina

Workshop; Pan American Advanced Studies Institute on “Nano and Biotechnology”; 2006

National Science Foundation

In the lasts decades, the chemical synthesis routes have permited the development of complex materials with novel properties and features, opening the possibility to combine the advantages of both the inorganic and the organic worlds. Patterning of inorganic materials using polymers or surfactants (by self-assembly or controlled phase separation) can lead to the formation of a multiscale porous texture that mimics some hierarchical structures found in nature. Mesoporous oxide thin films present a great interest for their potential in various domains such as in optics, electronics, chemical sensing, catalysis, separation, etc. The template process, which combines the sol-gel chemistry and self-assembly of the organic counterparts is an interesting approach to tailor pore size and symmetry. In mesophase formation, the organisation process is governed by the competition of three simultaneous processes: the evaporation of the liquid phase, the self assembly of the organic micelles with inorganic entities, and the condensation rate of the inorganic precursors. Silica and transition metal (Ti, Zr) mesoporous oxides have been synthesised by following different chemical routes and can be obtained as xerogels, powders or films with high surface areas (200-1000 m2/g), a variety of mesostructures (2D- or 3D- hexagonal, cubic, local…), different pore diameters (3-12 nm) determined by the template and amorphous or nanocrystalline walls in the case or metal transition frameworks. A combination of sol-gel synthesis with micro phase segregation strategies has been used to prepare macroporous titania films with high surface area (80 m2/g) and hierarchically organized porosity. Polyethylene glycol (PEG of M = 2000) was used to induce a controlled spinodal phase separation of the solvent and a metal-oxo-polymer complex. The resulting titania-PEG hybrid materials present organization at two characteristic length scales, one in the macroporous regime (having a pore diameter from 0.1 to 1 microns) and the other in the mesoporous regime (locally organized pore arrays with an interpore separation of 3 to 5 nm). The macroporous morphology is formed when the transitional structure arising from the phase separation process is “frozen” as a permanent structure by the sol-gel transition. The macroporous texture can be tuned by changing the synthesis conditions (sol temperature and composition, withdrawal speed, relative humidity…). The association of PEG with the Ti-oxo species resulting from inorganic polymerisation gives rise to mesostructured walls, and mesopores develop upon film calcination at T > 250ºC. In consequence, double textured porous thin films (200-500 nm thickness) are obtained in only one step. The wall surface can be further modified by incorporation of organic functions in a subsequent step. The films thus produced can find potential applications as optical materials, sensors, depollutants, etc. The conjunction of meso and macroporous films synthesis strategies makes possible the production of interconnected multilayered structures, alternating porous films with different pore size and physicochemical properties. The construction of such tailored hierarchical meso/macroporous structures is a fundamental step in order to create multifunctional bioinspired devices that include the advantages of both porous systems, such as a very high specific surface, tailored pore size and controlled surface characteristics, and an open macroporosity, which permits matter transport and interaction with species such as biomolecules, membranes, cells. Successive dip-coating/stabilization/template extraction cycles permit to create multilayered thin films presenting tailored porosity in the z axis. Tridimentional architectures with porosity at different scales can be achieved by selectively etching a determined layer. The design and construction of meso/macroporous hierarchical structures is a fundamental step in order to create multifunctional devices that include the advantages of both systems, such as a very high specific surface, tailored pore size and controlled surface characteristics, and an open macroporosity, which permits matter transport and interaction with species such as biomolecules, membranes, cells. It is also possible to build optical materials stacking mesoporous films, in order to create an ordered structure whose dielectric constant can be described as an spatially periodic function. These photonic crystals are accessible to different organic functions; this feature can be used to tune the optical properties.