Recent Publications Feed (ADMIN)

Nanoporous platinum nanoparticles (NPs) have been proposed as promising electrocatalytic materials. Routes to produce them typically consist of chemical synthesis or selective dissolution of one component of a two-component mix. Here we show that by employing a pulsed laser heating approach during electrodeposition, whereby the electrode/electrolyte interface is continually heated and cooled, NPs with a nanoporous structure can be grown directly on the electrode (boron-doped diamond) surface. Transmission electron microscopy shows the NPs to be composed of loosely packed aggregates of much smaller crystalline particles of size 2–5 nm, with the porosity increasing with increasing deposition overpotential. In contrast, electrodeposition at room temperature (RT) results in particles which show a considerably more compact morphology and fewer higher index crystal facets, as revealed by electron diffraction techniques. Pulsed heating also offers a route toward controlling the monodispersity of the electrodeposited NPs. When applied to the oxidation of methanol, the laser-heated NPs show considerably higher catalytic current densities in comparison to RT-deposited particles. The highest catalytic activity is observed for the most porous NPs produced at the highest overpotential. Interestingly, the ratio of the forward oxidative current to the backward current is highest for those particles deposited under laser-heated conditions but with the smallest overpotential. This suggests that the most catalytically active NPs may also encourage binding of residual adsorbed carbon monoxide and that a compromise must be reached.

The characterization of electrocatalytic reactions at individual nanoparticles (NPs) is presently of considerable interest but very challenging. Herein, we demonstrate how simple-to-fabricate nanopipette probes with diameters of approximately 30 nm can be deployed in a scanning ion conductance microscopy (SICM) platform to simultaneously visualize electrochemical reactivity and topography with high spatial resolution at electrochemical interfaces. By employing a self-referencing hopping mode protocol, whereby the probe is brought from bulk solution to the near-surface at each pixel, and with potential-time control applied at the substrate, current measurements at the nanopipette can be made with high precision and resolution (30 nm resolution, 2600 pixels μm^–2, <0.3 s pixel⁻¹) to reveal a wealth of information on the substrate physicochemical properties. This methodology has been applied to image the electrocatalytic oxidation of borohydride at ensembles of AuNPs on a carbon fiber support in alkaline media, whereby the depletion of hydroxide ions and release of water during the reaction results in a detectable change in the ionic composition around the NPs. Through the use of finite element method simulations, these observations are validated and analyzed to reveal important information on heterogeneities in ion flux between the top of a NP and the gap at the NP-support contact, diffusional overlap and competition for reactant between neighboring NPs, and differences in NP activity. These studies highlight key issues that influence the behavior of NP assemblies at the single NP level and provide a platform for the use of SICM as an important tool for electrocatalysis studies.

The dissolution kinetics of individual microscale bicalutamide (BIC) form-I crystals are tracked over time using in situ atomic force microscopy (AFM), with the evolution of crystal morphology used to obtain quantitative data on dissolution kinetics via finite element method (FEM) modeling of the dissolution reaction-diffusion problem. Dissolution is found to involve pit formation and roughening on all dissolving surfaces of the BIC crystal, and this has a strong influence on the overall dissolution process and kinetics. While all of the exposed faces (100), {051}, and {1̅02} show dissolution kinetics that are largely surface-kinetic controlled, each face has an intrinsic dissolution characteristic that depends on the degree of hydrogen bonding with aqueous solution, with hydrogen bonding promoting faster dissolution. Moreover, as dissolution proceeds with pitting and roughening, the rate accelerates considerably, so that there is an increasing diffusion contribution. Such insight is important in understanding the oral administration of poorly soluble active pharmaceutical ingredients (APIs) in crystal form. Evidently, surface roughening and defects greatly enhance dissolution kinetics, but the evolving crystal topography during dissolution leads to complex time-dependent kinetics that are important for modeling and understanding API release rates.