Recent Publications Feed (ADMIN)

Two dimensional (2D) semiconductor materials, such as molybdenum disulfide (MoS₂) have attracted considerable interest in a range of chemical and electrochemical applications, for example, as an abundant and low-cost alternative electrocatalyst to platinum for the hydrogen evolution reaction (HER). While it has been proposed that the edge plane of MoS₂ possesses high catalytic activity for the HER relative to the “catalytically inert” basal plane, this conclusion has been drawn mainly from macroscale electrochemical (voltammetric) measurements, which reflect the “average” electrocatalytic behavior of complex electrode ensembles. In this work, we report the first spatially-resolved measurements of HER activity on natural crystals of molybdenite, achieved using voltammetric scanning electrochemical cell microscopy (SECCM), whereby pixel-resolved linear-sweep voltammogram (LSV) measurements have allowed the HER to be visualized at multiple different potentials to construct electrochemical flux movies with nanoscale resolution. Key features of the SECCM technique are that characteristic surface sites can be targeted and analyzed in detail and, further, that the electrocatalyst area is known with good precision (in contrast to many macroscale measurements on supported catalysts). Through correlation of the local voltammetric response with information from scanning electron microscopy (SEM) and atomic force microscopy (AFM) in a multi-microscopy approach, it is demonstrated unequivocally that while the basal plane of bulk MoS₂ (2H crystal phase) possesses significant activity, the HER is greatly facilitated at the edge plane (e.g., surface defects such as steps, edges or crevices). Semi-quantitative treatment of the voltammetric data reveals that the HER at the basal plane of MoS₂ has a Tafel slope and exchange current density (J₀) of ∼120 mV per decade and 2.5 × 10⁻⁶ A cm⁻² (comparable to polycrystalline Co, Ni, Cu and Au), respectively, while the edge plane has a comparable Tafel slope and a J₀ that is estimated to be more than an order-of-magnitude larger (∼1 × 10⁻⁴ A cm⁻²). Finally, by tracking the temporal evolution of water contact angle (WCA) after cleavage, it is shown that cathodic polarization has a ‘self-cleaning’ effect on the surface of MoS₂, consistent with the time-independent (i.e., time after cleavage) HER voltammetric response.

Although the dissolution kinetics of calcite in acid waters has been studied for more than a century, the process is not fully understood, and for particles and microcrystals the process is often assumed to be diffusion-controlled. Herein, the dissolution kinetics of calcite single microcrystals in aqueous solution (pH ca. 3) has been investigated for the first time by a combination of real-time optical microscopy coupled with numerical simulations. The small size and well-defined geometry of rhombohedral calcite single crystals enables the measurement of the dissolution rates of the individual crystal faces exposed to the solvent and an assessment of the relative importance of corners and edges compared to the {104} faces. Data are used to parameterise finite element method (FEM) models for the quantitative analysis of dissolution kinetics. The simulations provide an accurate determination of the near-interface concentration of solution species during dissolution, as well as concentration gradients. The intrinsic first-order dissolution rate constant for the attack of protons on the exposed {104} faces, k_surf = (6.4 ± 2.8) × 10⁻⁴ m s⁻¹, is in good agreement with previous microscopic and macroscopic measurements, corroborating the method. This study is a further demonstration of the power of simple in situ optical microscopy for quantitative interfacial (dissolution/growth) kinetic measurements, using a configuration of practical relevance for processes as diverse as the remediation of acid water and scale removal.

Fabrication of sub-micron (meso)pores in single crystal diamond membranes, which span the entirety of the membrane, is described for the first time, and the translocation properties of polymeric particles through the pore investigated. The pores are produced using a combination of laser micromachining to form the membrane and electron beam induced etching to form the pore. Single crystal diamond as the membrane material, has the advantages of chemical stability and durability, does not hydrate and swell, has outstanding electrical properties that facilitate fast, low noise current-time measurements and is optically transparent for combined optical-conductance sensing. The resulting pores are characterized individually using both conductance measurements, employing a microcapillary electrochemical setup, and electron microscopy. Proof-of-concept experiments to sense charged polystyrene particles as they are electrophoretically driven through a single diamond pore are performed, and the impact of this new pore material on particle translocation is explored. These findings reveal the potential of diamond as a platform for pore-based sensing technologies and pave the way for the fabrication of single nanopores which span the entirety of a diamond membrane.