Electrocatalytic Reduction of Benzyl Bromide during Single Ag Nanoparticle Collisions.

Previous reports of the electrocatalytic activity of Ag nanoparticles (AgNPs) toward the reduction of organic halides have been limited to measurements of immobilized nanoparticle ensembles. Here, we have investigated the electrochemical reduction of benzyl bromide (PhCH2Br) occurring at single AgNPs (4.2 to 37 nm radius) in methanol, where the effects of nanoparticle size on catalytic behavior can be more thoroughly examined and rigorously quantified. AgNP collisions at a 6.3 µm radius Au ultramicroelectrode (UME) result in measurable electrocatalytic amplification currents from the reduction of PhCH2Br, where collision events are indicated by a sudden step increase in the reduction current recorded in the current-time trace. The dependence of the height of these steps on the applied potential allowed for an analysis of reaction kinetics based on the Butler-Volmer model, resulting in an estimation of the standard rate constant (k0) as a function of AgNP size. Measured values of k0 range from 4.0 × 10-4 to 8.0 × 10-4 cm/s on AgNPs with radii of 14, 29, and 37 nm, whereas k0 was found to be 6.2 × 10-4 cm/s at a 12.3 µm radius Ag disk UME. The results indicate that the kinetics of PhCH2Br reduction are independent of AgNP size and are similar to the reaction kinetics observed at a Ag UME. The frequency of observed particle collisions was found to be dependent on particle size, where 14 nm radius AgNPs resulted in the highest-frequency collisions. The potential- and size-dependent interactions of AgNPs with the Au UME are discussed in terms of the DLVO theory.

Critical Nucleus Size and Activation Energy of Ag Nucleation by Electrochemical Observation of Isolated Nucleation Events.

The induction times for electrodeposition of individual Ag nanoparticles on Pt nanodisk electrodes in acetonitrile were used to determine the critical nucleus size and activation energy barrier associated with the formation of Ag nuclei. Induction times for the nucleation and growth of a single Ag nanoparticle were determined following the application of a potential step to reduce Ag+ at overpotentials, η, ranging from -130 to -70 mV. Sufficiently small Pt electrodes (5.1 × 10-10-2.6 × 10-11 cm2) were used to ensure that the detection of a single Ag nucleation event occurred during the experimental observation time (150 ms-1000 s). Multiple measurements of Ag nucleation induction times were recorded to determine nucleation rates as a function of η using cumulative probability theory. Both classical nucleation theory (CNT) and the atomistic theory of electrochemical nucleation were employed to analyze experimental nucleation rates, without a requisite knowledge of the nucleus geometry or surface free energy. Using the CNT, the number of atoms comprising the critical size nucleus, Nc, was estimated to be 1-9 atoms for η ranging from -130 to -70 mV, in good agreement with 1-5 atoms obtained using atomistic theory. The experimental nucleation rates were also used to determine the activation energy barriers for nucleation from the CNT, which varied from 3.31 ± 0.05 to 13 ± 1 kT over the same range of η.

Single Ag nanoparticle collisions within a dual-electrode micro-gap cell.

An adjustable width (between 600 nm and 20 µm) gap between two Au microelectrodes is used to probe the electrodissolution dynamics of single Ag nanoparticles. One Au microelectrode is used to drive the oxidation and subsequent dissolution of a single Ag nanoparticle, which displays a multi-peak oxidation behavior, while a second Au microelectrode is used to collect the Ag+ that is produced. Careful analysis of the high temporal resolution current-time traces reveals capacitive coupling between electrodes due to the sudden injection of Ag+ ions into the gap between the electrodes. The current-time traces allow measurement of the effect of citrate concentration on the electrodissolution dynamics of a single Ag nanoparticle, which reveals that the presence of 2 mM citrate significantly slows down the release of Ag+. Intriguingly, these experiments also reveal that only a portion (ca. 50%) of the oxidized Ag nanoparticle is released as free Ag+ regardless of citrate concentration.

Collision Dynamics during the Electrooxidation of Individual Silver Nanoparticles.

Recent high-bandwidth recordings of the oxidation and dissolution of 35 nm radius Ag nanoparticles at a Au microelectrode show that these nanoparticles undergo multiple collisions with the electrode, generating multiple electrochemical current peaks. In the time interval between observed current peaks, the nanoparticles diffuse in the solution near the electrolyte/electrode interface. Here, we demonstrate that simulations of random nanoparticle motion, coupled with electrochemical kinetic parameters, quantitatively reproduce the experimentally observed multicurrent peak behavior. Simulations of particle diffusion are based on the nanoparticle-mass-based thermal nanoparticle velocity and the Einstein diffusion relations, while the electron-transfer rate is informed by the literature exchange current density for the Ag/Ag+ redox system. Simulations indicate that tens to thousands of particle-electrode collisions, each lasting ∼6 ns or less (currently unobservable on accessible experimental time scales), contribute to each experimentally observed current peak. The simulation provides a means to estimate the instantaneous current density during a collision (∼500-1000 A/cm2), from which we estimate a rate constant between ∼5 and 10 cm/s for the electron transfer between Ag nanoparticles and the Au electrode. This extracted rate constant is approximately equal to the thermal collisional velocity of the Ag nanoparticle (4.6 cm/s), the latter defining the theoretical upper limit of the electron-transfer rate constant. Our results suggest that only ∼1% of the surface atoms on the Ag nanoparticles are oxidized per instantaneous collision. The combined simulated and experimental results underscore the roles of Brownian motion and collision frequency in the interpretation of heterogeneous electron-transfer reactions involving nanoparticles.

Observation of Multipeak Collision Behavior during the Electro-Oxidation of Single Ag Nanoparticles.

The dynamic collision behavior of the electro-oxidation of single Ag nanoparticles is observed at Au microelectrodes using stochastic single-nanoparticle collision amperometry. Results show that an Ag nanoparticle collision/oxidation event typically consists of a series of 1 to ∼10 discrete "sub-events" over an ∼20 ms interval. Results also show that the Ag nanoparticles typically undergo only partial oxidation prior to diffusing away from the Au electrode into the bulk solution. Both behaviors are characterized and shown to exist under a variety of experimental conditions. These previously unreported behaviors suggest that nanoparticle collision and electro-dissolution is a highly dynamic process driven by fast particle-electrode interactions and nanoparticle diffusion.