Time-Resolved Detection and Analysis of Single Nanoparticle Electrocatalytic Impacts

Abstract

ABSTRACT: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 H2O2 at ruthenium oxide (RuOx) NPs as an example, the rise time of currenttime transients for NP impacts is consistent with a hydrodynamic trapping model for the arrival of a NP with a distancedependent 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 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.An important frontier in electrochemistry is measuring the behavior of individual nano-entities such as nanoparticles (NPs), nanowires and nanorods and relating this to other properties such as size, structure and electronic characteristics, so as to develop fundamental understanding and rational applications. 1-3 An interesting approach for observing the electrochemical properties of catalytic NPs is to monitor their impact (or landing) from solution onto a collector electrode, as introduced by Bard et al.,4,5 and developed by several groups. [6][7][8][9][10][11][12] In order to resolve such impacts, the use of a small-sized ultramicroelectrode (UME) is mandatory to reduce both background currents and the impact frequency. To enhance the impact signal to background current, electrode surfaces have been modified with Hg or Bi 7 and borondoped diamond 12 has also been used as an UME material. Alternatively, scanning electrochemical cell microscopy (SECCM) functioning as an ultramicro-electrochemical cell system offers particularly low background currents by reducing the area of the collector electrode, as well as offering the widest range of support electrodes. This is because the electrochemical cell is formed by meniscus confinement, rather than electrode encapsulation (Figure 1). 13 Despite these innovations, detailed analysis of the form of the current-time profile which is the primary signal for the landing (and detachment) of a single NP on an electrode has not yet been forthcoming, but would represent a huge advance towards understanding the impact process. Herein, we are able to analyze this process as never before and deduce key information on the NP arrival and release process from individual impact transients. Moreover, we show that impact frequencies can be orders of magnitude higher than expected based on single pass diffusion due to the repetitiv...

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

“…The repetitive engagement and detachment of NPs with a collector electrode has been previously demonstrated for ruthenium oxide NPs impacting on HOPG during catalytic amplification of hydrogen peroxide oxidation, and can be revealed provided that the time constant is sufficiently short. 14 Additionally, the ready detachment of Ag NPs during electrodeposition has also been shown. 40 …”

“…The first case represents a NP engaging quickly at the collector electrode and being held back as it attempts to disengage. 14 The second case embodies a NP gradually engaging with the electrode and sticking, and then quickly detaching after partial stripping. It should be noted that the charge consumed in both cases would represent the partial stripping of fractions of 18% and 22% of a particle of d = 100 nm.…”

“…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.…”

“…14 This is due to the confinement of the electrochemical cell within a meniscus formed by a micropipette and the collector electrode. Hence, the SECCM platform has been used successfully for single-entity electrochemical measurements such as monitoring the electrodeposition of single NPs, 39,40 electrochemical detection of single molecules 41 and, most recently, the time-resolved detection of single NP electrocatalytic impacts, 14 including surface oxide formation on noble material (Au) NPs. 42 An advantage of the SECCM approach is that there is no need for an encapsulated collector electrode, so that a wide range of electrode materials can be used for this purpose, including materials with low noise characteristics.…”

Transient Interactions of Agglomerates of Sensitized TiO2Nanoparticles in Colloidal Suspensions

“…In the formation of agglomerates one expects hundreds to thousands of discrete particles to make an 870 nm agglomerate, with a resulting lower concentration of agglomerates, and therefore, the frequency of 10 2 to 10 3 smaller, in line with the frequencies detected here. Finally, the frequency values calculated with Equations 4 or 5 do not take into account that a single NP or an agglomerate can undergo multiple collisions with the electrode 6 and therefore, the equation will correspond to the limit of the irreversible collisions for NPs.…”