Abstract
Electrochemical interfaces used for sensing, (electro)catalysis, and energy storage are usually nanostructured to expose particular surface sites, but probing the intrinsic activity of these sites is often beyond current experimental capability. Herein, it is demonstrated how a simple meniscus imaging probe of just 30 nm in size can be deployed for direct electrochemical and topographical imaging of electrocatalytic materials at the nanoscale. Spatially resolved topographical and electrochemical data are collected synchronously to create topographical images in which step-height features as small as 2 nm are easily resolved and potential-resolved electrochemical activity movies composed of hundreds of images are obtained in a matter of minutes. The technique has been benchmarked by investigating the hydrogen evolution reaction on molybdenum disulfide, where it is shown that the basal plane possesses uniform activity, while surface defects (i.e., few to multilayer step edges) give rise to a morphology-dependent (i.e., height-dependent) enhancement in catalytic activity. The technique was then used to investigate the electro-oxidation of hydrazine at the surface of electrodeposited Au nanoparticles (AuNPs) supported on glassy carbon, where subnanoentity (i.e., sub-AuNP) reactivity mapping has been demonstrated. We show, for the first time, that electrochemical reaction rates vary significantly across an individual AuNP surface and that these single entities cannot be considered as uniformly active. The work herein provides a road map for future studies in electrochemical science, in which the activity of nanostructured materials can be viewed as quantitative movies, readily obtained, to reveal active sites directly and unambiguously.
Highlights
Techniques that can resolve nanoscale structure−activity at complex electrochemical interfaces and ensembles are much needed in order to understand the behavior of functional nanostructured electrodes[1,2] that have applications ranging fromcatalysis and energy storage[3−6] to biomedical and environmental sensing.[7,8] In this work, we demonstrate how a simple meniscus imaging probe, based on a single-channeled nanopipet, can be used to carry out synchronous electrochemical/topographical imaging with high spatial resolution to provide unprecedented views of electrocatalytic processes in action
A single-channel nanopipet probe [transmission electron microscopy (TEM) image shown inset in Figure 1a] with surface current positional feedback was employed. This instrumental set up is greatly simplified compared to the conventional dual-channel scanning electrochemical cell microscopy (SECCM) set up,[27,28,39] only requiring x−y−z piezoelectric positioners, a current follower, a waveform generator, and a data acquisition system (FPGA card )
SECCM has been operated in the voltammetric “hopping” mode regime,[36−38] where the nanopipet probe was approached to the surface of interest at a series of predefined locations in a grid, and, upon each meniscus landing, a voltammetric experiment was carried out, building up a dynamic electrochemical “map” of the substrate
Summary
Techniques that can resolve nanoscale structure−activity at complex electrochemical interfaces and ensembles are much needed in order to understand the behavior of functional nanostructured electrodes[1,2] that have applications ranging from (electro)catalysis and energy storage[3−6] to biomedical and environmental sensing.[7,8] In this work, we demonstrate how a simple meniscus imaging probe, based on a single-channeled nanopipet (inner diameter, d ≈ 30 nm), can be used to carry out synchronous electrochemical/topographical imaging with high spatial resolution to provide unprecedented views of electrocatalytic processes in action. (a, b) Topographical and (c) spatially resolved electrochemical maps (2500 pixels over a 2.5 × 2.5 μm scan area, 400 pixels μm−2) obtained with the voltammetric hopping mode SECCM protocol (Figure 1), visualizing HER activity on a cleaved MoS2 surface. It is worth noting that the fact that an area of basal plane is observable between the two closely spaced but large multilayer steps (i.e., see the blue trace in Figure S7) in the activity map shown in Figure 3c again indicates that the meniscus (droplet) cell is stable in these experiments, with minimal distortion or spreading over the relatively large surface features. After selecting a suitable area to study (marked as a red box in Figure S9), a voltammetric hopping mode SECCM was carried out (scan rate of 1 V s−1) to construct spatially resolved synchronous topographical and electrochemical maps, as shown in parts b and c, respectively, of Figure 4. It should be noted that there might be further scope to improve the spatial resolution achievable with SECCM, with single-barreled quartz laser pulled nanopipets of sub-10 nm diameter being reported elsewhere, such probes have never been used for imaging of any kind.[47]
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