Abstract
The characterization of electrocatalytic reactions at individual nanoparticles (NPs) is presently of considerable interest but very challenging. Herein, we demonstrate how simple-to-fabricate nanopipette probes with diameters of approximately 30 nm can be deployed in a scanning ion conductance microscopy (SICM) platform to simultaneously visualize electrochemical reactivity and topography with high spatial resolution at electrochemical interfaces. By employing a self-referencing hopping mode protocol, whereby the probe is brought from bulk solution to the near-surface at each pixel, and with potential-time control applied at the substrate, current measurements at the nanopipette can be made with high precision and resolution (30 nm resolution, 2600 pixels μm-2, <0.3 s pixel-1) to reveal a wealth of information on the substrate physicochemical properties. This methodology has been applied to image the electrocatalytic oxidation of borohydride at ensembles of AuNPs on a carbon fiber support in alkaline media, whereby the depletion of hydroxide ions and release of water during the reaction results in a detectable change in the ionic composition around the NPs. Through the use of finite element method simulations, these observations are validated and analyzed to reveal important information on heterogeneities in ion flux between the top of a NP and the gap at the NP-support contact, diffusional overlap and competition for reactant between neighboring NPs, and differences in NP activity. These studies highlight key issues that influence the behavior of NP assemblies at the single NP level and provide a platform for the use of SICM as an important tool for electrocatalysis studies.
Highlights
The widely used scanning electrochemical microscopy (SECM) technique has recently been applied to electrocatalytic nanomaterials adhered to an electrocatalytically inert support.[18−20] this technique usually operates in a constant plane scanning mode with no positional feedback of the probe with respect to the surface and no topographical information obtained
To overcome the lack of positional feedback, SECM has been successfully integrated with other scanning probe techniques such as atomic force microscopy (AFM),[26,27] scanning ion conductance microscopy (SICM),[28−30] and scanning tunneling microscopy (STM),[31,32] as well as through the use of dual redox mediators,[33] to enable electrochemical and topographical images to be obtained
SICM is a well-established contactless topographical probe imaging technique, capable of characterizing delicate samples with nanometer scale resolution, using nanoscale glass or quartz probes that can be made very and quickly using a laser capillary puller. These probes are filled with electrolyte, and a contacting electrode is inserted.[35−38] There has been significant progress to improve the time resolution of SICM through the use of high bandwidth electrometers and the introduction of versatile scanning regimes.[35,39−41] SICM has very recently expanded beyond its major use for topographical imaging to become a multifunctional tool capable of elucidating a variety of surface properties, beyond topography.[15,35−38,42] Notably, the ion conductance current is sensitive to changes in the local ionic atmosphere near surfaces induced by either surface charge[39,40,43] or as the result of electrochemical reactions.[44]
Summary
The study of catalytic nanomaterials has become an important research area, due to a range of significant real-world applications, as well as fundamental interest.[1−4] Numerous studies, using ensembles of catalytic nanoparticles (NPs), have revealed that changes in NP shape, size, and structure can significantly affect (electro)catalytic activity,[5−7] but investigations of activity at the individual NP level are challenging.[8,9] Among a rather limited set of tools that have been applied for single NP electrochemical characterization,[9−14] scanning electrochemical probe microscopy (SEPM) techniques are attractive and can be highly sensitive.[15]. SICM is a well-established contactless topographical probe imaging technique, capable of characterizing delicate samples with nanometer scale resolution, using nanoscale glass or quartz probes that can be made very and quickly using a laser capillary puller These probes are filled with electrolyte, and a contacting electrode is inserted.[35−38] There has been significant progress to improve the time resolution of SICM through the use of high bandwidth electrometers and the introduction of versatile scanning regimes.[35,39−41] SICM has very recently expanded beyond its major use for topographical imaging to become a multifunctional tool capable of elucidating a variety of surface properties, beyond topography.[15,35−38,42] Notably, the ion conductance current is sensitive to changes in the local ionic atmosphere near surfaces induced by either surface charge[39,40,43] or as the result of electrochemical reactions.[44] Since all electrochemical processes result in a change in ionic composition, SICM is potentially a very powerful general probe for visualizing nanoscale electrocatalysis
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