Nanoscale Scanning Electrochemical Cell Microscopy and Correlative Surface Structural Analysis to Map Anodic and Cathodic Reactions on Polycrystalline Zn in Acid Media
Scanning electrochemical cell microscopy (SECCM) is used to map anodic and cathodic processes on polycrystalline zinc in 10 mM H2SO4, at the nanoscale. Electrochemical maps are correlated directly with structural data from electron backscatter diffraction applied to the same regions of the surface, and density functional theory (DFT) calculations are used to rationalize the data. Preliminary data on droplet stability with SECCM point measurements indicated that there was a significant spreading of the meniscus cell with an air atmosphere, attributed to changes in pH during the oxygen reduction reaction, compromising the lateral resolution of the SECCM measurement. Experiments with an argon atmosphere, as well as the application of a hydrophobic n-dodecane oil layer on the Zn interface, prevented spreading. Electrochemical maps of polycrystalline Zn surface under an Ar atmosphere indicated that the hydrogen evolution reaction (HER) and Zn electrodissolution on individual low-index grains decreased in the order DFT calculations revealed a correlation between experimental values of current associated with HER and Zn dissolution reactions and the predicted hydrogen adsorption and Zn dissolution energies on individual facets, respectively. This work further advances SECCM as a technique for probing electrified interfaces and demonstrates its applicability to reactive metals.
Investigation of sp2-carbon pattern geometry in boron doped diamond electrodes for the electrochemical quantification of hypochlorite at high concentrations
An electrochemical sensor that contains patterned regions of sp2 -carbon in a boron doped diamond (BDD) matrix is presented for the quantitative detection of hypochlorite (OCl- ) at high concentrations in the alkaline, chemically oxidizing environment associated with bleach. As BDD itself is unresponsive to OCl- reduction within the solvent window, by using a laser micromachining process it is possible to write robust electrochemically active regions of sp2 -carbon into the electrochemically inert sp3 BDD electrode. In this work, four different laser patterned BDD electrodes are examined and their response compared across a range of OCl- concentrations (0.02 M to 1.50 M). A single macro-spot (0.37 mm diameter disk) electrode and a closely spaced micro-spot (46 m diameter disk) hexagonal array electrode, containing the same surface area of sp2 -carbon, are shown to provide the most linear response towards OCl- reduction. Finite element modelling (FEM) is employed to better understand the electrochemical system, due to the complexity of the electrode geometry, as well as the need to include contributions from migration and Ohmic drop at these high concentrations. FEM data suggest that only a small percentage (~ 1×10-3 %) of the total laser-machined sp2 area is active towards the OCl- reduction process and that this process is kinetically very sluggish (~ keff = 1×10-12 cm s-1). The sensitivity at the micro-array electrode (-0.127 ± 0.004 mA M-1; R2 = 0.992) is higher than that at the single-spot (-0.075 ± 0.002 mA M-1; R2 = 0.996) due to the enhanced effect of transport to the edges of the micro-spots, shown via simulation. The electrodes returned a relatively stable response over a > three-month period of use in the OClsolutions, demonstrating these hybrid sp2 -BDD electrodes show promise for long-term monitoring applications in the harsh environments associated with bleaching applications.
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.
Nanoscale electrochemical visualization of grain-dependent anodic iron dissolution from low carbon steel
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.
High-Throughput Correlative Electrochemistry−Microscopy at a Transmission Electron Microscopy (TEM) Grid Electrode
As part of the revolution in electrochemical nanoscience, there is growing interest in using electrochemistry to create nanostructured materials, and to assess properties at the nanoscale. Herein, we present a platform that combines scanning electrochemical cell microscopy with ex-situ scanning transmission electron microscopy, to allow the ready creation of an array of nanostructures coupled with atomic-scale analysis. As an illustrative example, we explore the electrodeposition of Pt at carbon-coated transmission electron microscopy (TEM) grid supports, where in a single high-throughput experiment it is shown that Pt nanoparticle (PtNP) density increases and size polydispersity decreases with increasing overpotential (i.e., driving force). Furthermore, the coexistence of a range of nanostructures − from single atoms to aggregates of crystalline PtNPs − during the early stages of electrochemical nucleation and growth supports a non-classical aggregative growth mechanism. Beyond this exemplary system, the presented correlative electrochemistry−microscopy approach is generally applicable to solve the ubiquitous structure-function problems in electrochemical science and beyond, positioning it as a powerful plat-form for the rational design of functional nanomaterials.