Hierarchical Pore Systems
For diverse applications involving mass transport an ideal pore system would be build up in a “hierarchical” fashion: the small nanopores (3 nm – 100 nm, “mesopores”) are located in the walls of larger pores, ideally on the micrometer scale (1 micrometer = 0.000001 m), thus making the larger surface area readily accessible through such bigger pores. Our group tries to develop strategies to generate such “pore hierarchy” by self-assembly strategies, to establish methods to characterize such complex pore systems in the form of powders and also films and finally to study the influence of pore hierarchy on physico-chemical properties (e.g. by means of electrochemisty).
Pores with sizes on the nanometer scale (1 nanometer = 0.000000001 m) are supposed to be beneficial for a variety of applications (sensing, catalysis, etc.) owing to the high surface area. However, the transport through such small pores is highly aggravated, because the only transport mechanism is “diffusion”, which usually rather slow for such “mesopores”.
Within this topic we develop strategies to create such “hierarchical” pore architectures by sol-gel chemistry together with suitable template structures (see “nanocasting” for an introduction into sol-gel templating). Since in general sol-gel templating results in a 1:1 hard copy of a soft structure (e.g. lyotropic phase of block copolymers) into a metal oxide with corresponding porosity, the underlying challenge is to establish a solution-based “hierarchical” soft structure with corresponding hierarchy of micelles, polymer phases, etc. Therefore, our projects are also devoted to establish more general profound understanding of how polymeric structures self-assemble in concentrated solutions in the presence of other moieties.
For instance, one of the most successful pathways towards silica hierarchical pore structures is the procedure developed by Nakanishi, which we use in a modified procedure.
