Analysis of Diffusion-Controlled Stochastic Events of Iridium Oxide Single Nanoparticle Collisions by Scanning Electrochemical Microscopy

Abstract

We investigated the electrochemical detection of single iridium oxide nanoparticle (IrO(x) NP) collisions at the NaBH(4)-treated Pt ultramicroelectrode (UME) in a scanning electrochemical microscope (SECM) over an insulating surface. The NP collision events were monitored by observing the electrocatalytic water oxidation reaction at potentials where it does not take place on the Pt UME. These collisions occurred stochastically, resulting in a transient response ("blip") for each collision. The frequency of the collisions is proportional to the flux of NPs to the UME tip, and thus equivalent to the SECM current. A plot of collision frequency versus distance followed the theoretical approach curve behavior for negative feedback for a high concentration of mediator, demonstrating that the collisions were diffusion-controlled and that single-particle measurements of mass transport are equivalent to ensemble ones. When the SECM was operated with a Pt substrate at the same potential as the tip, the behavior followed that expected of the shielding mode. These studies and additional ones result in a model where the IrO(x) NP collision on the Pt UME is adsorptive, with oxygen produced by the catalyzed water oxidation causing a current decay. This results in a blip current response, with the current decay diminished in the presence of the oxygen scavenger, sulfite ion. Random walk and theoretical bulk simulations agreed with the proposed mechanism of IrO(x) NP collision, adsorption, and subsequent deactivation.

Tracking motion trajectories of individual nanoparticles using time-resolved current traces

“…We observed the duration of rising part 1 (blue-filled areas) and the corresponding charge accounted for approximately 50% of the complete oxidation (Table 1), demonstrating that the timescale of a single encounter is sufficient for a 10 nm AgNP to oxidize completely. In the case of 20 nm AgNPs, an asymmetrical current trace that initially rises quickly and decays at a slower rate 14,31 was observed (Fig. 2b(ii)), yielding a current amplitude of 146 ± 2 pA and a longer duration of 0.5 ± 0.1 ms. We compared the duration and charge of rising part 1 with that of a single peak (Table 1), suggesting that the decay part makes a major contribution to the dissolution of 20 nm particles.…”

“…31 However, the current traces obtained for individual AgNPs began to flow and increasingly reach a peak and decay with different fluctuations. Having obtained the above results, we are in the position to establish a semi-quantitative dynamic Monte Carlo simulation to estimate the corresponding motion trajectories of AgNPs from the current traces.…”

“…31–33 The project-augmented wave method was used to represent the core–valence electron interaction. 39,40 To model the Au (111) surface of a Au electrode, a four-layer p (3 × 3) slab corresponding to 36 Au atoms and a vacuum layer of 25 Å was applied.…”

Review—Advances in Scanning Electrochemical Microscopy (SECM)

“…47 Downloaded on 2018-05-12 to IP H3095 nanoparticle (IrO x NP) collisions at a Pt UME by SECM over an insulating (a) and conducting (b) surface. 111 Individual NP collision events were monitored by observing the electrocatalytic water oxidation reaction at potentials where the reaction did not take place on the Pt UME. 112 These collisions occurred stochastically, resulting in a transient blip response for each collision.…”

Impact and oxidation of single silver nanoparticles at electrode surfaces: one shot versus multiple events

“…First, electrocatalytic current amplification, which relies on an electrochemical process that is catalysed by the NP under study, but not the collector electrode (over the applied potential range of interest). 5,7–14 The recorded current–time ( I – t ) transient can provide information on the interaction of the NP with the substrate (elastic collision, adsorption, etc. ) as well as the kinetics of the reaction under study.…”