Recent Publications Feed (ADMIN)

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.

The properties of steels and other alloys are often tailored to suit specific applications through the manipulation of microstructure (e.g., grain structure). Such microscopic heterogeneities are also known to modulate corrosion susceptibility/resistance, but the exact dependency remains unclear, largely due to the challenge of probing and correlating local electrochemistry and structure at complex (alloy) surfaces. Herein, high-resolution scanning electrochemical cell microscopy (SECCM) is employed to perform spatially-resolved potentiodynamic polarisation measurements, which, when correlated to co-located structural information from electron backscatter diffraction (EBSD), analytical scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM), reveal the relationship between anodic metal (iron) dissolution and the crystallographic orientation of low carbon steel in aqueous sulfuric acid (pH 2.3). Considering hundreds of individual measurements made on each of the low-index planes of body-centred cubic (bcc) low carbon steel, the rate of iron dissolution, and thus overall corrosion susceptibility, increases in the order (101) < (111) < (100). These results are rationalized by complementary density functional theory (DFT) calculations, where the experimental rate of iron dissolution correlates with the energy required to remove (and ionise) one iron atom at the surface of a lattice, calculated for each low-index orientation. Overall, this study further demonstrates how nanometre-resolved electrochemical techniques such as SECCM can be effectively utilised to vastly improve the understanding of structure–function in corrosion science, particularly when combined with complementary, co-located structural characterisation (EBSD, STEM etc.) and computational analysis (DFT).

Layered transition metal dichalcogenides (TMDs), such as molybdenum disulfide (MoS2) and tungsten disulfide (WS2) have attracted considerable interest as alternatives to platinum in hydrogen evolution reaction (HER) electrocatalysis. It is generally accepted that the edge planes of 2H phase MS2 (where M = Mo or W) are catalytically active, while the basal planes are said to be “catalytically inert”, which has inspired the rational design/synthesis of defect-rich nanomaterials with an abundance of exposed edge sites. The intrinsic electrochemical properties of pristine MoS2/WS2 crystals has been largely overlooked in this materials-driven approach. Herein, nanometer-resolved measurements using scanning electrochemical cell microscopy (SECCM) reveal electrochemical activity at the basal plane, including spatial variations attributed to the localized folding of the surface (e.g., mechanical strain) or variations in electronic structure (e.g., defect density) throughout the crystal. Such effects are particularly evident in synthetic WS2 compared to natural crystal of MoS2. Catalytic activity for the HER is greatly enhanced at macroscopic surface defects on both materials, measured directly where the active edge plane is exposed (e.g., crevices, holes, cracks, etc.) with single-layer sensitivity. Aging the crystals under ambient conditions (i.e., exposed to the ambient atmosphere for 30 days) substantially decreases the HER activities of MoS2 and WS2, attributable to the presence of adventitious adsorbates or surface oxidation, which particularly affects at the active edge plane. Overall, this work presents previously unseen electrochemical phenomena at TMD electrodes, highlighting how subtle changes in sample source, structure, and history can alter the catalytic activity drastically, and emphasizing the care that must be taken when interpreting conventional macroscopic electrochemical data. This study further demonstrates the advantage of probe-based electrochemical mapping for establishing structure-function relationships in electromaterials science.