Molecular Transport through TiO2Mesoporous Thin Films: Correlation with the Partially Blocked Electrode Model

Abstract

Mesoporous titania thin film (MTTF)-based electrodes are vastly popular due to their versatility and their potential use as sensors, catalysts, and photovoltaic devices. For these applications, understanding of diffusion through the pores along the thickness of the films is critical. Cyclic voltammetry with the use of a freely diffusing redox probe is a promising and simple strategy for the characterization of this kind of modified electrodes. However, the application of a mesoporous insulating material on a flat electrode greatly modifies the diffusion regimes in the vicinity of the reaction zone. This makes the use of traditional tools for data interpretation, such as the Randles-Ševčı́k equation, insufficient. In this work, the application of a model previously developed for partially covered electrodes (Matsuda et al.,J. Electroanal. Chem. 1979 , 101 (1), 29-38) is proposed to interpret experimental results. An excellent agreement between experimental and simulated voltammograms was achieved for MTTFs with different pore sizes and pore arrays. This analysis can be attributed to a lack of uniformity along the films related to differences in pore-to-pore connectivity within their thickness. At the same time, it is revealed that pore and neck sizes are determinant for diffusion within these materials. Thus, two dimensions govern the electrochemical results: nanopore sizes and arrays and microregions with different connectivities along the MTTF.