Anisotropic diffusion in face-centered cubic opals

Diffusion in face-centered cubic (fcc) opals synthesized from 250 nm-diameter silica spheres was investigated by electrochemical methods and finite-element simulations. Opal modified electrodes (OME) ((111) opal surface orientation) were prepared by thermal evaporation of Au onto similar to1 mm-thick opals. Linear sweep voltammetry of Au OMEs in aqueous solutions containing an electroactive molecule and a supporting electrolyte (0.1 M Na2SO4) was used to determine molecular diffusion coefficients, D-fcc, within the opal. D-fcc is related to the diffusion coefficient of the molecule in free solution, D-sol, by the relationship D-fcc, = (epsilon/tau)D-sol, where epsilon is the interstitial volume fraction of a fcc opal (epsilon = 0.260 for an infinitely thick opal) and tau is the tortuosity; the tortuosity reflects the increased distance traversed by molecules as they diffuse through the curved interstitial spaces of the opal lattice, and is a function of both the direction of transport relative to the lattice and the number of layers of spheres in the opal lattice. Finite-element simulations are used to compute tau for transport orthogonal to the (111), (110), and (100) surface orientations for 1-7 layers of spheres. Values of tau = 1.9 +/- 0.7 and 3.1 +/- 1.2 were obtained from experiment for transport of Ru(NH)(6)(3+) and Fe(CN)(6)(4-) normal to the (111) surface, respectively, in reasonable agreement with a value of similar to3.0 obtained from the simulation.