Molecular Transport through TiO₂ Mesoporous 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−S ̌evcǐ ́k 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.

Impact of Chemical Primers on the Growth, Structure, and Functional Properties of ZIF-8 Films

“…There is a strong deviation from expected linearity in the I vs ν 1/2 plots, as well as changes in the CV shape. This means that the diffusion regime changes with the scan rate used and that the film-modified substrates behave as partially blocked electrodes (PBE) . PBE refers to a situation in which the electrode geometrical area is not equivalent to the active area.…”

“…The results are consistent with the case in which an overlapped planar diffusion layer with the same geometrical area as the bare electrode is present. Since some variability in the electrochemical response of substrate is expected, and differences among D eff values may not be significant …”

“…Structural characterization was conducted by X-ray diffraction (XRD) and X-ray reflectivity (XRR), while atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) were employed for characterization of morphology and surface chemistry. Finally, functional properties related to permeability, in terms of molecular transport of redox probes through a film’s porosity, were probed via cyclic voltammetry, ,,, and the obtained results were analyzed using the partially blocked electrode model (PBE) …”

Nanoconfinement Effects on the Transport of Redox Probes in Ionic Liquid-Loaded Mesoporous Silica Thin Films

“…Positively charged and/or neutral redox species FDM + /FDM and DBD redox couples were selected as probe molecules. Because the silica surface is negatively charged, 65 electrostatic repulsion would hinder transport if an anionic redox species was used 66 and complicate the interpretation of the EIS data. FDM and DBD represent hydrophilic (log P = 1.0 as determined by Bergren et al 67 ) and hydrophobic (log P = 7.8 as determined by Molinspiration Cheminformatics software (http:// molinspiration.com)) redox probes.…”

Visible light induced RAFT for asymmetric functionalization of silica mesopores

“…Angelomé and coworkers 38 used CV measurements to analyze diffusion in mesoporous TiO 2 films, where they predicted the diffusion layer for [Fe(CN) 6 ] 3-/4- to be in a range of 20–200 µm using scan rates of 5–500 mVs −1 . This agrees with our observation that PMEP functionalization on the outer surface of a 500–600 nm mesoporous silicafilm significantly influences the detected pore accessibility.…”