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Time-Resolved Detection and Analysis of Single Nanoparticle Electrocatalytic Impacts
Kang, Minkyung,Perry, David,Kim, Yang-Rae,Colburn, Alex W.,Lazenby, Robert A.,Unwin, Patrick R. American Chemical Society 2015 JOURNAL OF THE AMERICAN CHEMICAL SOCIETY - Vol.137 No.34
<P>There is considerable interest in understanding the interaction and activity of single entities, such as (electro)catalytic nanoparticles (NPs), with (electrode) surfaces. Through the use of a high bandwidth, high signal/noise measurement system, NP impacts on an electrode surface are detected and analyzed in unprecedented detail, revealing considerable new mechanistic information on the process. Taking the electrocatalytic oxidation of H<SUB>2</SUB>O<SUB>2</SUB> at ruthenium oxide (RuO<SUB><I>x</I></SUB>) NPs as an example, the rise time of current–time transients for NP impacts is consistent with a hydrodynamic trapping model for the arrival of a NP with a distance-dependent NP diffusion-coefficient. NP release from the electrode appears to be aided by propulsion from the electrocatalytic reaction at the NP. High-frequency NP impacts, orders of magnitude larger than can be accounted for by a single pass diffusive flux of NPs, are observed that indicate the repetitive trapping and release of an individual NP that has not been previously recognized. The experiments and models described could readily be applied to other systems and serve as a powerful platform for detailed analysis of NP impacts.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jacsat/2015/jacsat.2015.137.issue-34/jacs.5b05856/production/images/medium/ja-2015-05856p_0006.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/ja5b05856'>ACS Electronic Supporting Info</A></P>
Nanoscale Electrocatalysis of Hydrazine Electro-Oxidation at Blistered Graphite Electrodes
E, Sharel P.,Kim, Yang-Rae,Perry, David,Bentley, Cameron L.,Unwin, Patrick R. American Chemical Society 2016 ACS APPLIED MATERIALS & INTERFACES Vol.8 No.44
<P>There is great interest in finding and developing new, efficient, and more active electrocatalytic materials. Surface modification of highly oriented pyrolytic graphite, through the introduction of surface 'blisters', is demonstrated to result in an electrode material with greatly enhanced electrochemical activity. The increased electrochemical activity of these blisters, which are produced by electro-oxidation in HClO4, is revealed through the use of scanning electrochemical cell microscopy (SECCM), coupled with complementary techniques (optical microscopy, field emission scanning electron microscopy, Raman spectroscopy, and atomic force microscopy). The use of a linear sweep voltammetry (LSV)-SECCM scan regime allows for dynamic electrochemical mapping, where a voltammogram is produced at each pixel, from which movies consisting of spatial electrochemical currents, at a series of applied potentials, are produced. The measurements reveal significantly enhanced electrocatalytic activity at blisters when compared to the basal planes, with a significant cathodic shift in the onset potential of the hydrazine electro-oxidation reaction. The improved electrochemical activity of the hollow structure of blistered graphite could be explained by the increased adsorption of protonated hydrazine at oxygenated defect sites, the ease of ion solvent intercalation/deintercalation, and the reduced susceptibility to N-2 nanobubble attachment (as a product of the reaction). This study highlights the capability of electrochemistry to tailor the surface structure of graphite and presents a new electrocatalyst for hydrazine electro-oxidation.</P>
Kim, Yang-Rae,Lai, Stanley C. S.,McKelvey, Kim,Zhang, Guohui,Perry, David,Miller, Thomas S.,Unwin, Patrick R. American Chemical Society 2015 JOURNAL OF PHYSICAL CHEMISTRY C - Vol.119 No.30
<P>The mechanism and kinetics of the electrochemical nucleation and growth of palladium (Pd) nanoparticles (NPs) on carbon electrodes have been investigated using a microscale meniscus cell on both highly oriented pyrolytic graphite (HOPG) and a carbon-coated transmission electron microscopy (TEM) grid. Using a microscale meniscus cell, it is possible to monitor the initial stage of electrodeposition electrochemically, while the ability to measure directly on a TEM grid allows subsequent high-resolution microscopy characterization which provides detailed nanoscopic and kinetic information. TEM analysis clearly shows that Pd is electrodeposited in the form of NPs (approximately 1–2 nm diameter) that aggregate into extensive nanocrystal-type structures. This gives rise to a high NP density. This mechanism is shown to be consistent with double potential step chronoamperometry measurements on HOPG, where a forward step generates electrodeposited Pd and the reverse step oxidizes the surface of the electrodeposited Pd to Pd oxide. The charge passed in these transients can be used to estimate the amounts of NPs electrodeposited and their size. Good agreement is found between the electrochemically determined parameters and the microscopy measurements. A model for electrodeposition based on the nucleation of NPs that aggregate to form stable structures is proposed that is used to analyze data and extract kinetics. This simple model reveals considerable information on the NP nucleation rate, the importance of aggregation in the deposition process, and quantitative values for the aggregation rate.</P><P><B>Graphic Abstract</B> <IMG SRC='http://pubs.acs.org/appl/literatum/publisher/achs/journals/content/jpccck/2015/jpccck.2015.119.issue-30/acs.jpcc.5b03513/production/images/medium/jp-2015-03513y_0011.gif'></P><P><A href='http://pubs.acs.org/doi/suppl/10.1021/jp5b03513'>ACS Electronic Supporting Info</A></P>