Synchronous Electrical Conductance- and Electron Tunnelling-Scanning Electrochemical Microscopy Measurements

The 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.