(470a) Electrochemical Investigation of Transport through Self-Assembled Nanoporous Silica Thin Films

Nanoporous thin films with well-defined pore size and geometry are of interest for many applications including electrochemical sensors and the fabrication of nanostructured films for photovoltaic and thermoelectrics devices. For these applications, solution phase species must be able to enter the nanopores and transport through the film thickness to react at the electrode surface. Here, we report the results of systematic studies of transport through well-ordered nanoporous films and the accessibility of solution phase species to an electrode surface as a function of nanopore topology, orientation, and order. In particular, care has been taken to account for diffusion length scales to be able to precisely quantify the accessible area of the substrate. Nanoporous silica films with p6mm (2D hexagonal), Im3m (body centered cubic), R3m (rhombohedral), Pm3n, and Ia3d symmetry have been synthesized on FTO (F-doped tin oxide) by evaporation induced self-assembly. The films have been calcined and characterized by FESEM, TEM, and GISAXS. Depending on the synthesis, the pore size of the films is from 2 nm up to 4 nm. Cyclic voltammetry and electrochemical impedance spectroscopy were used to characterize the response to a variety of redox couples over a range of pH values. Results show that domain boundaries can play a dominate role in transport and that more disordered films yield better access to the substrate. However, for highly ordered and oriented films, only the Ia3d and Im3m structures have facile transport to the substrate. Other topologies effectively block the electrode surface. Results of the effects of probe molecule size, probe charge, wall charge will also be discussed along with the results of rotated disk electrode studies that quantify the diffusion coefficient in the films.