Redox Titrations via Surface Interrogation Scanning Electrochemical Microscopy at an Extended Semiconducting Surface for the Quantification of Photogenerated Adsorbed Intermediates

Abstract

We introduce a methodology based on electrochemical simulations and the identification of the relevant experimental factors that enable the use of micro- and nano- redox titrations via Surface Interrogation Scanning Electrochemical Microscopy (SI-SECM) on an extended substrate electrode geometry. Specifically, we address the quantification of adsorbed intermediates so that information such as the local surface coverage can be conveniently extracted from SI-SECM transients despite diffusional broadening. Using a time normalization parameter ρ, we studied the time evolution of the SI-SECM signal and its impact on the titrated surface below the SECM tip. We identified the experimental and time conditions on which diffusional broadening dominates the signal. Our simulated results suggest that, while this effect decreases the spatial resolution of the SI-SECM process, it enhances significantly the measured signal. It is also a strong function of the SECM tip size, where quantification enhancements on nano-electrodes may facilitate their practical experimental deployment. A versatile parameter, ρ, not only allowed to understand practical enhancements, but also to guide the fitting of experimental results to theory. SI-SECM was used to quantify photogenerated intermediates of the photoassisted water oxidation reaction on lightly n-doped SrTiO3 using the ferri/ferrocyanide system as redox mediator. Agreement between simulated and experimental transients was excellent, and fitting allowed us to determine the potential-dependent surface coverage of hydroxyl radical intermediate, which ranged from 0.04 to 0.50C/m2. The analysis and experimental demonstration introduced here move SI-SECM forward as a quantitative platform to study (photo) electrocatalytic samples without needing to constrain the size of the interrogated electrode. Furthermore, they establish SI-SECM as an analytical tool for mechanism-focused study of adsorbed intermediates and photocatalytic processes that occur on semiconducting and insulating surfaces, where quantification of surface reaction intermediates with view towards the nano-scale remains challenging for other electrochemical and spectroscopic approaches.