Optical Fluorescence Microscopy for Spatially Characterizing Electron Transfer across a Solid-Liquid Interface on Hetero
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Optical Fluorescence Microscopy for Spatially Characterizing Electron Transfer across a Solid-Liquid Interface on Heterogeneous Electrodes Eric Choudhary1, Jeyavel Velmurugan2, James M. Marr2, James A. Liddle1, and Veronika Szalai1 1
Center for Nanoscale Science and Technology, National Institute of Standards and Technology 100 Bureau Dr, Gaithersburg, MD 20899, U.S.A. 2
Maryland NanoCenter, University of Maryland, College Park, MD 20742, U.S.A.
ABSTRACT Heterogeneous catalytic materials and electrodes are used for (electro)chemical transformations, including those important for energy storage and utilization.1, 2 Due to the heterogeneous nature of these materials, activity measurements with sufficient spatial resolution are needed to obtain structure/activity correlations across the different surface features (exposed facets, step edges, lattice defects, grain boundaries, etc.). These measurements will help lead to an understanding of the underlying reaction mechanisms and enable engineering of more active materials. Because (electro)catalytic surfaces restructure with changing environments,1 it is important to perform measurements in operando. Sub-diffraction fluorescence microscopy is well suited for these requirements because it can operate in solution with resolution down to a few nm. We have applied sub-diffraction fluorescence microscopy to a thin cell containing an electrocatalyst and a solution containing the redox sensitive dye p-aminophenyl fluorescein to characterize reaction at the solid-liquid interface. Our chosen dye switches between a nonfluorescent reduced state and a one-electron oxidized bright state, a process that occurs at the electrode surface. This scheme is used to investigate the activity differences on the surface of polycrystalline Pt, in particular to differentiate reactivity at grain faces and grain boundaries. Ultimately, this method will be extended to study other dye systems and electrode materials. INTRODUCTION Background and Motivation Catalysts are critical to supporting modern society and are used in petroleum refining, air and water pollution removal, and electron transfer reactions for energy applications including solar fuels among others.3 Amongst all types of catalysts, heterogeneous catalysts, which include nanoparticles, are an important class. Due to the intrinsic hetereogeneous nature of these materials, there are many different surface features available as binding sites for chemical species. It has long been understood that only a minority of the available surface sites are responsible for catalytic activity.4 Further, different exposed surface facets or features can have preferential catalytic activity towards one transformation over another.5, 6 In order to develop new catalysts with higher activity, it is first necessary to quantify the desired activity at the different surface features of a catalyst. This was first done by studying single crystals with known surface facets exposed.2, 7 However, single-crytals are not very accurate model systems for the dispersi
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