Abstract
AbstractThe requirement to separate topographical effects from surface electrochemistry information is a major limitation of scanning electrochemical microscopy (SECM). With many applications of SECM involving the study of (semi)conducting electrode surfaces, the hybridisation of SECM with scanning tunnelling microscopy (STM) or a surface conductance probe would provide the ultimate topographical imaging capability to SECM, but previous attempts are limited. Here, the conversion of a general scanning electrochemical probe microscopy (SEPM) platform to facilitate contact electrical conductance (C)‐ and electron tunnelling (T)‐SECM measurements is considered. Measurements in air under ambient conditions with a Pt/Ir wire tip are used to assess the performance of the piezoelectric positioning system. A hopping‐mode imaging protocol is implemented, whereby the tip approaches the surface at each pixel until a desired current magnitude is exceeded, and the corresponding z position (surface height) is recorded at a set of predefined xy coordinates in the plane of the surface. At slow tip approach rates, the current shows an exponential dependence on tip‐substrate distance, as expected for electron tunnelling. For measurements in electrochemical environments, in order to overcome well‐known problems with leakage currents at coated‐wire tips used for electrochemical STM, Pt‐sensitised carbon nanoelectrodes are used as tips. The hydrogen evolution reaction on 2D Au nanocrystals serves as an exemplar system for the successful simultaneous mapping of topography and electrochemical activity.
Highlights
The requirement to separate topographical effects from surface electrochemistry information is a major limitation of scanning electrochemical microscopy (SECM)
For measurements in electrochemical environments, in order to overcome well-known problems with leakage currents at coated-wire tips used for electrochemical shows an exponential depend17 microscopy (STM), Ptsensitised carbon nanoelectrodes are used as tips
To demonstrate the C- and T-SECM setup, we consider the hydrogen evolution reaction (HER) on 2D Au nanocrystals supported on a glassy carbon (GC) electrode, for which we have recently reported nanoscale scanning electrochemical cell microscopy (SECCM) topography-activity measurements.[39]
Summary
Given that coated wire tips[40,41] can show parasitic currents, we instead used nanopipette-based SECM probes for measurements in electrolyte solution. The final step of fabrication of SECM tips involved the potentiodynamic electrodeposition of Pt onto the carbon nanoelectrode, by adapting a method previously reported.[44] This step served 2 purposes to: (i) sensitise the tip so that it could detect electrocatalytic processes ( the detection of H2 by electro-oxidation); and (ii) ensure that the metal electrode protruded from the end of the quartz nanopipette, essential for the tunnelling and conductance current measurements. We found that damage to the nanopipette during the carbon deposition process sometimes resulted in Pt deposition in locations other than the end of the probe Such electrodes had non-ideal geometries, and could not be used for imaging, but still displayed the sigmoidal voltammetry profile expected for steady-state diffusion to a nanoelectrode (Supporting Information, Figure S3). The GC was washed with water to remove any salt residue before experiments
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