Recent Publications Feed (ADMIN)

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.

Calcium carbonate crystallisation on surfaces has been studied extensively due to its prominence in biomineralisation, but the role of surface charge in nucleation and growth is not well understood. We have employed potential-controlled Highly Oriented Pyrolytic Graphite (HOPG) surfaces to demonstrate the significant impact of surface charge on calcium carbonate crystallisation: at negatively charged HOPG surfaces, calcite, aragonite and vaterite all nucleate from high energy positively-charged crystal faces, contrasting with the stable (104) calcite planes nucleated at positively charged surfaces. These observations are explained and rationalised.

Scanning ion conductance microscopy (SICM) is a nanopipette-based scanning probe microscopy technique that utilizes the ionic current flowing between an electrode inserted inside a nanopipette probe containing electrolyte solution and a second electrode placed in a bulk electrolyte bath, to provide information on a substrate of interest. For most applications to date, the composition and concentration of the electrolyte inside and outside the nanopipette is identical, but it is shown herein that it can be very beneficial to lift this restriction. In particular, an ionic concentration gradient at the end of the nanopipette, generates an ionic current with a greatly reduced electric field strength, with particular benefits for live cell imaging. This differential concentration mode of SICM (ΔC-SICM) also enhances surface charge measurements and provides a new way to carry out reaction mapping measurements at surfaces using the tip for simultaneous delivery and sensing of the reaction rate. Comprehensive finite element method (FEM) modeling has been undertaken to enhance understanding of SICM as an electrochemical cell and to enable the interpretation and optimization of experiments. It is shown that electroosmotic flow (EOF) has much more influence on the nanopipette response in the ΔC-SICM configuration compared to standard SICM modes. The general model presented advances previous treatments, and it provides a framework for quantitative SICM studies.