Single Ag Nanoparticle Electro-oxidation: Potential-Dependent

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials

Single Ag Nanoparticle Electro-Oxidation: Potential-Dependent Current Traces and Potential-Independent Electron Transfer Kinetic Wei Ma, Hui Ma, Zhe-Yao Yang, and Yitao Long J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.8b00386 • Publication Date (Web): 05 Mar 2018 Downloaded from http://pubs.acs.org on March 5, 2018

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The Journal of Physical Chemistry Letters

Single Ag Nanoparticle Electro-Oxidation: Potential-Dependent Current Traces and PotentialIndependent Electron Transfer Kinetic Wei Ma, Hui Ma, Zhe-Yao Yang and Yi-Tao Long* Key Laboratory for Advanced Materials & Department of Chemistry, East China University of Science and Technology, Shanghai 200237, P. R. China. AUTHOR INFORMATION Corresponding Author *[email protected]

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ABSTRACT Potential-dependent current traces were firstly observed for the same sized nanoparticles (NPs) during the dynamic electro-oxidation process of single AgNPs. In this work, we demonstrated that the motion trajectories of NPs, coupled with electrochemical kinetics parameters, qualitatively predicted from the series of the experimentally observed current traces obtained single AgNPs collision behaviors. Based on the Poisson-Boltzmann equation for a general electrochemical reaction, a rate constant of Ag oxidation could be further estimated to be 1 x 10-6 mol/cm-2·s-1 for electron transfer between AgNPs and the Au electrode by comparing the experimental results. Our method provided a meaningful attempt to test electron transfer kinetics and motion behaviors of single NPs using the high-resolution electrochemical signal.

TOC GRAPHICS

KEYWORDS Single nanoparticle, Stochastic collision measurement, Silver oxidation, Electrochemical kinetics, Reaction mechanism


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Research of single entity electrochemistry (SEC) is receiving increased interests in the last decade because of its extremely high sensitivity and the single entity properties available that are averaged in ensemble measurements.1-3 Stochastic collision electrochemistry, a convenient and powerful analysis method in this field, has widespread applicability in areas ranging from electrocatalytic amplification4-7 and direct electrochemical oxidation of individual metal nanoparticles (NPs)8-10 to soft particles11-13 and biologically relevant detection.14,15 Typical current signals were observed when single entities colliding with the ultramicroelectrode (UME) surface and undergoing electrochemical reaction. The shape and statistical properties of these signals have been used for analyzing properties and behaviors of individual entities.16,17 The widely studied SEC system is that of single AgNP electro-oxidation using stochastic collision amperometry.7-11,14 Considering the destructive process of Ag oxidation, the charge associated with each colliding AgNP can be used for estimating AgNP size.8 Recently, our group reported the size-discriminated current traces of single AgNPs, that are largely different from that in the previous report, owing to the dynamic motion during AgNP dissolution process.16 Almost at the same time, similar multi-collision behaviors were also observed for the electro-oxidation of single AgNPs by Unwin et al18 and White and Zhang et al,19 demonstrating electrochemical reaction of individual NPs at the UME surface is a highly dynamic process. With the going deep of the SEC research, the corresponding collision dynamics of individual NPs have been investigated by using the combined simulated and experimental results.16,20 Collision dynamics during the electrochemical oxidation of single AgNPs has been reported by White and Zhang to further quantitatively simulate the random-walk motion path taken by the NPs for multiple current peak. However, to date, the quantitative electron transfer kinetics at a single NP level still needs the further exploration.21,22 In this work, we investigated the


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electrochemical oxidation behaviours and the corresponding mechanism of individual AgNPs as a function of the applied potential (Figure 1). We further determined the potential-independent electron transfer kinetics using the Poisson-Boltzmann equation by comparing the predicted motion trajectories with the experimentally observed current traces.

Figure 1. (a) Experimental setup of single AgNPs collision. (b) Schematic depiction of individual AgNPs Collision. The tunnelling region is marked with blue colour. Three typical current patterns (right, black line) relate to three scenarios of the motion trajectories of AgNPs (red dash line). The electrochemical dissolution of individual AgNPs was carried out at a 12.5 µm diameter Au UME in 20 mM PB solution containing 200 pM dispersed AgNPs (diameter of 20 nm). Figure 2a showed 10 s amperometric traces at different applied potentials. The obvious current spikes were observed for the electrochemical oxidation of AgNPs at potentials from +600 to +300 mV vs Ag/AgCl wire (Ag/AgCl QRE, +172 mV vs SCE based on a cyclic voltammogram of Fe(CN)63/Fe(CN)64-, Figure S1). An important feature of these transients is the current amplitude gradually decreased from 194.9 ± 3.8 to 107.2 ± 1.2 pA as the negative shift of applied potential (Figure 2a and Figure S2). Notably, the duration and shape of these current traces significantly


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Figure 2. Collision behaviors of individual AgNPs at various applied potential. (a) Amperometric i-t curve of 20 nm AgNPs collision at the potentials of + 300, + 400, + 500 and + 600 mV vs Ag/AgCl QRE. (b) Time-resolved current traces of individual AgNPs including a single peak (black), a spike with an undulating terrain and a cluster of current spikes (blue). (c) Ratio variation of event I (black) and event II (blue) at different potentials. (d) Size distributions of AgNPs from the integrated charge at different applied potentials. changed with different potentials, reflecting the variation of electrochemical oxidation behaviors in the NP-electrode collision processes. Three kinds of current patterns with sub-millisecond timescale were observed for the dissolution of 20 nm AgNPs, including a single peak, a spike with an undulating terrain and a cluster of current spikes, respectively (Figure 2b). A single peak is defined as event I (black mark), while the current traces of a spike with an undulating terrain and a close spaced cluster are defined as event II (blue marks). The variation tendency of the number ratio of event I decayed from 62.3 to 23.8% as the potential decreased from +600 to +300 mV vs Ag/AgCl QRE, while the opposite trend was identified for the ratio of event II (Figure 2c). Our result showed the size of the collided AgNPs from the integrated charge in a


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transient spike associate with each potential, exhibiting the size distributions of 16.8 ± 0.1, 15.2 ± 0.1, 14.7 ± 0.8, and 14.4 ± 0.1 nm at potential of +600, +500, +400, and +300 mV vs Ag/AgCl QRE (Figure 2d). Notably, the experimentally measured sizes are smaller than the dynamic light scattering measurements of 17.1 ± 0.9 nm (Figure S3) and gradually decreased with the negative shift of applied potential. Several key features can be seen in this experiment that (i) A 20 nm single AgNP was easier to undergo single encounter oxidation at a higher potential of +600 mV vs Ag/AgCl QRE; (ii) AgNPs could not be totally dissolved at lower potentials; (iii) Multi-peak collision behaviour elongated the duration of single AgNP dissolution. According to the previous literature, we inferred that the dynamic motion of NP on the electrode/solution interface resulted in different adopted potential during dynamic single AgNP dissolution process.16-18 Initial AgNPs collision on electrode was driven by stochastic diffusion and the electron transfer occurred in the tunnelling region. In the region near the electrode, the electric potential approximation exhibited a sharp decay as a function of distance. It follows Equation 1: = 4 / tanh tanh /4 1

where is the effective electric potential, is the applied potential, is Boltzmann constant,

is reciprocal of Debye length. Here, we assume that the boundary of tunnelling region is equivalent to the onset potential (+100 mV vs Ag/AgCl QRE, Figure S4). This means when an AgNP enters into the tunneling region on UME surface, particle is momentarily involved in the oxidation and dissolution via electron tunneling. Under typical condition, the electrochemical reaction rate of Ag dissolution depends on the electrical potential on the AgNP. In the case of

+600 mV vs Ag/AgCl QRE, a single spike was mostly observed with a duration of 0.27 ± 0.01 ms and the expected particle size with 16.2 ± 0.1 nm was estimated from the integrated charge,


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demonstrating that a single encounter is sufficient for a namely 20 nm AgNP to dissolve totally due to a high overpotential. However, the electro-oxidation rate decreased at lower potentials, resulting in the decrease in current amplitude for the same sized AgNPs. For the remaining AgNP due to incomplete dissolution, subsequent subcollision events are likely occurred, but affected by a change in the applied potential. Qualitatively, three scenarios of the motion trajectories of AgNPs were proposed as shown in Figure S5.

Figure 3. (a) Histogram of calculated k0 at the applied potential of +300 (pink), +400 (blue), +500 (red) and +600 mV (black). (b)Potential profile from the Poisson-Boltzmann equation at +300 (pink line), +400 (blue line), +500 (red line) and +600mV (black line). Single spike (c) and tailed peak (d) of current traces (black line) and the corresponding motion trajectories (red line) for electrochemical oxidation of individual AgNPs. Having obtained the above results, we further analyzed the current traces to extract the kinetic information for AgNP oxidation. In this system, we considered the overall electrochemical behaviours of single AgNPs at Au UME governed by an electrochemical oxidation rate of Ag (a) and a surface diffusion of Ag+ to bulk solution (b). Assuming the electrochemical dissolution of


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AgNPs that is initiated by irreversible adsorption when the particle touches the electrode, the timescale of spike for diffusion-limited process ( ) by solving diffusion equation can be expressed by Equation 2:20

where

! = 2 2ln 2$% &'

is the radius of AgNPs, is the number density, $% is Avogadro constant, D is

diffusion coefficient and C is the saturation concentration of released Ag+ surrounding the NP. The diffusion duration caused by a 20 nm AgNP calculates to less than 1 µs based on Equation 2. Since the diffusion rate of Ag+ is very fast, the resulting current trace of a AgNP is mainly controlled by kinetically-limited electrochemical oxidation process as previously reported.23 Following classical Butler-Volmer kinetics, the electrochemical rate constant ( increases exponentially with increasing overpotential by Equation 3: )*+

= ( ,- ./ 3

where is the standard heterogeneous electron transfer rate constant for AgNPs oxidation, 1 is transfer coefficient， is the number of electron transferred per Ag atom, 2 is Faraday constant,

3 is gas constant and is the absolute temperature. 4 is the overpotential as 4 = 56 − 5 , where

56 is the applied potential and 5 is the onset potential. For a one-step, one-electron oxidation reaction of AgNP, the dissolution rate of particle can be expressed by Equation 4:20 9 = 4 9 The radius of AgNP is considered to depend on the charge transferred during a collision event, and can be calculated as Equation 5:

> 3;< − < =: 5 42=


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where ; is the molar mass of Ag, is the density of Ag and

Single Ag Nanoparticle Electro-oxidation: Potential-Dependent

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