Transient Electrocatalytic Water Oxidation in Single-Nanoparticle

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Transient Electrocatalytic Water Oxidation in Single Nanoparticle Collision Fan Zhang, Peter A. Defnet, Yunshan Fan, Rui Hao, and Bo Zhang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b00576 • Publication Date (Web): 26 Feb 2018 Downloaded from http://pubs.acs.org on February 26, 2018

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The Journal of Physical Chemistry

Transient Electrocatalytic Water Oxidation in Single Nanoparticle Collision

†

Fan Zhang,† Peter A. Defnet,† Yunshan Fan,† Rui Hao,† and Bo Zhang†* Department of Chemistry, University of Washington, Seattle, WA 98195-1700 United States

Corresponding authors: [email protected]

Submitted to J. Phys. Chem. C.

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Abstract. We report the observation of electrocatalytic water oxidation during transient collision and oxidation of single Ag nanoparticles (NPs) on a gold ultramicroelectrode (UME). Collision and oxidation of single Ag NPs is studied under higher positive potentials (≥1 V versus Ag/AgCl electrode) on a gold UME than previously used. The use of higher potentials allows us to observe several unique features not seen at lower potentials (0.6−0.8 V) including a greatly increased collision frequency and the appearance of sub-collision peaks with a long current tail. Moreover, the average charge transferred in a collision event is 3 to 5 times more than that estimated from a complete NP oxidation. Our results suggest that the enhanced charge transfer is due to coupled water oxidation catalyzed by Ag oxide transiently formed upon NP collision. A pH-dependence study was further performed to reveal a characteristic feature of water oxidation, and the formation of silver oxide was directly confirmed by single-NP fluorescence microscopy. This study offers a deeper insight into the collision electrochemistry of single metal NPs and provides a new method for studying the electrocatalytic activity of silver oxide toward water oxidation on an electrode surface.


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Introduction Nanoparticles (NP) of controlled materials, composition, size, and shape find wide application in many different areas, including electrocatalyzed water splitting1,2, fuel cells3,4, photoelectrochemistry5-7, and biosensors8,9. Single-particle collision electrochemistry has been a topic of extensive interest in the last decade because of its unique ability to study individual NPs of interest without ensemble averaging. Understanding electrochemical processes at nanoscale with sufficient time resolution is one of the key challenges in this research field. Compared to traditional ensemble studies, electrochemical methods of single NPs enable one to probe electrocatalytic property at a single-particle level and at ultrashort timescale, e.g., µs, unraveling key information hidden in ensemble measurements10,11. Among previous methods applied in single-NP studies, stochastic collision electrochemistry of single NPs has great advantages of high temporal resolution and sensitivity. Largely depending on the relative electroactivity of the NPs and the substrate electrode, previous research in this field has used three different strategies. First, as reported by Lemay and coworkers12, one can observe transient current decays when electro-inactive particles adsorb on an ultramicroelectrode (UME) blocking redox diffusion. Second, Bard’s group reported that a signal enhancement can be expected if an electrocatalytic NP collides on an inert electrode and catalyzes a redox reaction, such as the oxidation of hydrazine or water1315

. Third, an increasing current response can also be observed due to a NP itself being oxidized or reduced

during collision16,17. These methods all give single-particle sensitivity and have now been extended to studying a wide variety of NPs including metals (Ag, Au, Pt)18-20, oxides (ZnO, IrOx)21,22 and even small molecules23-25. A metal NP, such as silver, can be oxidized when it collides on the surface of an UME held at an anodic potential, resulting in a transient current signal whose charge reflects the fraction of the NP oxidized during the collision. These single-NP collision events can be a single current spike for complete NP oxidization leading to an effective method for particle size analysis.16 Such responses are usually seen for NPs smaller than 20 nm in diameter or in the presence of chloride17,26. Additionally, the detection signal can include several smaller sub-collision peaks, reported by us and others using Ag NPs on gold or 3 ACS Paragon Plus Environment


carbon fiber microelectrodes (CFEs)26-28. This multipeak phenomenon was explained by repeated collision and partial oxidation of the same NP at the electrode/solution interface26,27, and is supported by our recent study using ultrathin nanoband electrodes29. Most collision experiments of Ag NPs reported in previous studies were performed at lower positive potentials, e.g., 0.3-0.8 V vs Ag/AgCl, where only silver oxidation is expected. Kwon and coworkers recently reported collision electrochemistry of Ag NPs in 15 mM hydrazine at potentials as high as 1.3 V30. They observed blip-type current responses with peak currents as large as 40 nA and integrated peak charges that are three orders of magnitude higher than expected from a complete NP oxidation. They suggested that Ag NP oxidation was coupled with the oxidation of hydrazine. Although it is reasonable to believe that hydrazine oxidation may contribute in the anodic current signal, the large peak amplitudes and peak area cannot be explained by the limited hydrazine concentration. A simple calculation reveals that the diffusion-limited hydrazine current on a 30-nm-diameter particle is ~1.06 nA assuming a full 4-eoxidation, which is far less than their reported 40 nA current.

Figure 1. (a) A cartoon showing collision and oxidation of a single silver NP on a gold UME at higher positive potentials. The particle is negatively charged prior to collision. Ag is oxidized to Ag+ shown as small red dots and Ag2O shown as the gray layer. Water is oxidized on the surface of the NP generating protons and molecular oxygen due to the electrocatalytic effect of the Ag2O. (b) A typical collision peak showing a large anodic peak at the beginning due to silver oxidation. The current decays to a finite value 4 ACS Paragon Plus Environment

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defined as iτ within ~1 ms, which then undergoes a slower decay to the baseline current. The duration of the decay period (current tail) is defined as τ. In this study we describe transient formation of silver oxide and its role in electrocatalytic water oxidation during collision and oxidation of silver NPs. We observe large amperometric collision peaks on a gold UME at 1.0 and 1.1 V vs Ag/AgCl with the total transferred charge significant greater than that predicted from a complete NP oxidation. These amperometric peaks show characteristic long tails following their initial sharp current spikes, which are somewhat similar to that observed in IrOx NPcatalyzed water oxidation15,22. Inspired by the fact that silver oxide can have electrocatalytic effects toward water oxidation31-34, we propose a mechanism for Ag NP collision at higher positive potentials as shown in Figure 1a. When a Ag NP collides on an UME held at a sufficiently positive potential, it is partially oxidized generating both freely diffusing Ag+ ions and a layer of Ag2O on the NP surface. Figure 1b gives a typical anodic current peak of a 40-nm Ag NP on a gold UME at 1.1 V vs Ag/AgCl. A sharp current spike is seen corresponding to the initial collisional contact. The Ag2O layer generated on or around the NP catalyzes electrochemical oxidation of water showing as a slow current decay following the initial ~1-ms-long Ag oxidation spike. Due to its relatively low chemical stability in neutral and acidic environments, Ag2O may dissolve quickly into the nearby solution within 10s of ms to more than 2 s depending on the size of the initial Ag particle and the amount of Ag2O formed during particle collision. The anodic current slowly returns to baseline. We systematically characterized the transient collision electrochemistry of Ag NPs of three different sizes, 40, 60, and 110 nm in diameters with gold and carbon UMEs at potentials from 0.6 V to 1.1 V vs Ag/AgCl. Our results have shown significant differences between potentials below and above 0.8 V. At lower potentials, collision events contain multiple amperometric subpeaks characteristic of multipeak collision behavior as reported previously.26,27 The oxidation current quickly increases as a Ag particle collides on the electrode and the current falls back to the baseline within ~1 ms as it leaves the surface. Partial oxidation was still observed, in good accordance with previous works. At higher potentials (e.g., 1.1 V), however, the Ag oxidation current does not immediately fall back to the baseline.



Instead, the anodic current quickly decays to a finite value, iτ, after the first sub-collision peak, which then slowly decays back to baseline after tens of ms or even several seconds. The average charge transferred in a particle collision event including the current decay period exceeds that estimated from a complete particle oxidation indicating that other species are also oxidized during the particle collision event. Our results suggest that water is electrocatalytically oxidized during particle collision. This hypothesis is further supported by studies of the effects of pH and chloride ions. Single-particle fluorescence microscopy was also used to verify formation of silver oxide during single collision events.

Experimental Section Chemicals and Solutions. Silver nitrate (AgNO3, Merck), trisodium citrate (Fisher Scientific), citric acid (Sigma-Aldrich), ferrocenemethanol (FcMeOH, Sigma-Aldrich), potassium nitrate (KNO3, SigmaAldrich), sodium sulfate (Na2SO4, Fisher Scientific), and potassium chloride (KCl, Fisher Scientific) were all used as received from the manufacturers. Ultrapure water (>18 MΩ·cm) was obtained through a Barnstead Nanopure water purification system and used for all aqueous solutions. Citrate-capped 60 nm and 110 nm Ag NPs used in collision experiments were prepared following previous work27,35 (see Supplementary Information, Figures S1 and S2, for more information on NP characterization). 40 nm Ag NPs were purchased from Nanocomposix. The NP solution for collision experiments was prepared by combining the stock NP solution, 100 mM KNO3, and 10 mM trisodium citrate in a 2:1:3 ratio by volume, resulting in an approximate NP concentration of 30 pM (40 nm), 75 pM (60 nm) and 8 pM (110 nm), and concentrations of 18 mM KNO3 and 7 mM trisodium citrate. A Thermo Scientific Orion 2-Star Benchtop pH Meter confirms the pH to be 7.8. This solution was used throughout this work, unless otherwise stated. A pH 5.2 solution was made by mixing Ag NP solution, 100 mM KNO3, 10 mM citric acid, and 10 mM trisodium citrate in a 2:1:1:2 ratio by volume. To adjust the pH to be 9.7, a small amount of 1 M KOH solution was added into pH 7.8 solution. Solutions with pH 7.8 and 5.2 were found to give stable NP recording over one hour with


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unchanged frequency and amplitude. For pH 9.7 solution, the experiment period was limited within 30 min. Electrode Fabrication and Characterization. A 12.7-µm-diameter Au disk electrode was fabricated and used as the working UME electrode27. Before Ag NP collision measurements, the UME was first polished on a sand paper and in slurry of 50 nm Al2O3 NPs followed by a 5-min sonication in ultrapure water. Cyclic voltammetry (CV) was further employed to characterize the electrodes to ensure they were properly fabricated (Figure S3). Although the CV curve show some hysteresis, it’s possibly caused by shallow recession of electrodes36,37, which would not influence further application in NP collision. The sigmodal shape indicates diffusion-limited steady-state electrochemical behavior. Carbon fiber microelectrodes of 5 µm in diameters were made according to previous work38. Before use, they were cut by a clean surgical knife and rinsed with DI water. Ag NP’s were drop-casted on Au by dropping 40 or 60 nm Ag NP stock solution on electrode. The electrode was allowed to dry in air before applying another drop. The procedure was repeated a total of 5 times to reach a large enough NPs coverage. Electrochemical Measurements. NP collisions were recorded using an Axopatch 200B patch-clamp amplifier (Molecular Devices) interfaced to a PC through a Digidata 1440A digitizer (Molecular Devices). The Axopatch was used in V-clamp mode with whole cell β = 1 and the low-pass filter set to 5 kHz. Amperometric traces were recorded with a 100 kHz sampling rate. Current spikes were analyzed using pClamp 10.4 Clampfit software (Molecular Devices). All electrochemical experiments were performed using a two-electrode setup placed in a lab-built Faraday cage. A piece of Ag/AgCl wire was used as the counter/quasi-reference electrode (QRE) in all experiments. Total-Internal Reflection Fluorescence (TIRF) Microscopy. TIRF imaging of single Ag NPs was performed on a custom-modified Olympus IX70 inverted microscope configured for TIRF using an Olympus Apo N 60× 1.49 NA objective and a 532-nm green laser (CrystaLaser) emitting at 10 mW. An additional 1.5× magnification on the microscope was used. The fluorescence images were optically filtered with an ET610/75m emission filter (Chroma Technology) and acquired on an Andor iXon3 EMCCD camera cooled to -85 °C. Images were recorded with an exposure time of 50 ms (frame rate of 7 ACS Paragon Plus Environment


19.806 Hz). An amplifier gain of 300 as well as a preamplifier gain of 5.1 was used. The voltage function was generated on a 273A potentiostat (Princeton Applied Research) and applied across the working electrode (20-nm-thick Au film on a glass slide) and the Pt quasi reference electrode (QRE).

Results and Discussion Nanoparticle Collision at Higher Potentials. Figures 2a-d show representative current-time traces of 40nm Ag NPs on a 12.7-µm-diameter gold UME where different potentials from +0.6 to +1.1 V vs. Ag/AgCl were used. In all cases, large oxidative current spikes can be seen on top of the baseline current indicating effective detection of particle collision events. A close inspection of the current spikes reveals several key differences between low (i.e., 0.6 V to 0.8 V) and high potentials (i.e., 1.0 V to 1.1 V). First, an extra slow current decay is observed on each particle collision event at higher potentials shown as an upward shift to the baseline current in the current traces. At lower potentials, normal multi-peak collision events are observed where each sub-collision spike increases and quickly returns to the baseline within ~1 ms27. At higher potentials, however, while one can still see sharp current spikes corresponding to subcollision events, the faradaic current does not immediately drop back to the baseline after the first subcollision spike for most of the collision events (77% or more; see Table 1). Instead, it slowly decays back to the baseline taking a much longer time between 20 to 150 ms. All the subsequent sub-peaks following the first sub-peak are positioned on top of this slowly decaying current. For easy comparison, we presented two representative single-particle collision events for each voltage condition in the right column of Figures 2a-d. These results indicate that additional oxidation reactions are coupled with Ag NP oxidation on UME at higher potentials.


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Figure 2. Representative current-time traces of 40 nm Ag NPs on a 12.7-µm Au UME at 0.6 (a), 0.8 (b), 1.0 (c), and 1.1 V (d) vs. Ag/AgCl. Right column shows details of representative peaks. Red dashed lines act as guide. Conditions: 30 pM Ag NPs, 7 mM trisodium citrate, 18 mM potassium nitrate, pH = 7.8. (eh) Histograms of total transferred charge during one collision event corresponding to (a) to (d): (e) 0.6 V. (f) 0.8 V. (g) 1.0 V. (h) 1.1 V. The blue dashed lines represent average charge transfer in each condition and black dashed lines represent the full amount of charge obtainable for a 40-nm NP (320 fC). 9 ACS Paragon Plus Environment


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Table 1: Statistics for collision events of 40 nm Ag NPs on a gold UME at different potentialsa Potential/V

Number of events

Q/fC

SD Q/fC

iτ/pA

SD iτ/pA

τ/ms

0.6

129

94

74

Ratio of peaks with iτ ~0

0.8

172

103

85

~0

1.0

262

514

516

141

80

63

77%

1.1

94

1114

910

235

111

102

~100%

a

pH of Ag NP solution was 7.8. iτ is defined as in Figure 1b. τ is duration of the tail current. SD is standard deviation. About 1/3 and 1/10 of all peaks at 1.0 V and 1.1 V were randomly selected in statistics. The ratio applies for all data at high potentials throughout the text. To avoid bias, peaks with medium height were chosen.

A second potential-dependent feature is the detection frequency. The detection frequency is ~0.2 Hz at 0.6 and 0.8 V potentials. This frequency is lower than reported in our previous work, which could be due to different surface conditions of the gold UMEs. As a comparison, the detection frequencies are 2 Hz and 4.6 Hz on the same UME at 1.0 and 1.1 V, respectively. For 1.1 V, the detection frequency is close to the diffusion-limited collision rate of 4.8 Hz, which can be calculated based on the size and concentration of the NPs and the radius of the UME. These higher detection rates are possibly due to faster electrontransfer kinetics and the migration effect where water molecules are oxidized generating positively charged H+ species and attracting negatively-charged Ag NPs. To better understand this collision behavior, we analyzed >90 randomly selected collision events recorded at four potentials and plotted the transferred charge distribution (Figures 2e-h). For easy comparison, average charge transfer (Q) is represented by a blue dashed line in each figure and the exact values are listed in Table 1. The average charge was 94 fC at 0.6 V and 103 fC at 0.8 V, corresponding to ~30% oxidation of a 40 nm Ag NP (320 fC, 100% oxidation), in agreement with previous work26,29. While at higher potentials, the transferred charge was greatly enhanced to 514 fC at 1.0 V and 1114 fC at 1.1 V corresponding to 160% and 350% oxidation of a 40 nm Ag NP at these two potentials (Figures 2g,h). This result can be anticipated from the long tail following the first sub-peak. The parameters of the 10 ACS Paragon Plus Environment

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tails are listed in Table 1. A strong potential dependence is observed from the height (iτ) and duration (τ) of the tail current. Therefore, we can conclude that other redox reactions must take place at high potentials. The reduction potential for Ag2O(s) + 2H+ + 2e- ⇌ 2Ag(s) + H2O can be calculated by formula E(V) = 1.173 − 0.059 × pH39. In our case (pH = 7.8, no Cl- present), the value is estimated to be ~0.5 V versus Ag/AgCl. Therefore, when a Ag NP collides at Au UME under high positive potentials (≥1 V vs. Ag/AgCl), it is highly possible that a significant amount of the Ag is oxidized forming Ag2O. Other forms of silver oxide like AgO and Ag2O3 may coexist as well, judging from their standard reduction potential (about 0.7 V and 0.9 V vs. Ag/AgCl respectively in neutral solutions). It is difficult to determine the exact ratio of all silver oxide components. Since the reduction potentials of AgO and Ag2O3 are higher than Ag2O, we may assume Ag2O account for the majority of the oxidation products. For simplicity, “Ag2O” is used in the following text. Secondly, it was reported that silver oxide can catalyze water oxidation in aqueous solutions near neutral pH32,33. Considering the high abundance of water (i.e., ~56 M), we believe silver oxide catalyzed water oxidation is responsible for the long tails and the extra charges observed in the amperometric collision events. Water molecules on Ag2O surface probably undergo following oxidative processes: water dissociation, formation of adsorbed peroxide intermediates, and evolution of oxygen, similar to other metal oxides like IrOx and RuO2.40,41 The only other oxidizable species to be considered besides Ag NP itself is citrate acid present at 7 mM. One can estimate the maximum oxidation current from citrate oxidation on a 40-nm particle supported on a large flat surface:

iss = 4πln(2)nFDC*r

(1)

where n is the number of electrons transferred per redox molecule, F is the Faraday constant, D = 6.5 × 10-6 cm2/s is the diffusion coefficient of citrate ions42, C* is concentration of citrate ions, and r is the NP radius. The diffusion-limited current is ~76 pA assuming a 1-e- transfer oxidation process, which is far 11 ACS Paragon Plus Environment


less than the average tail current iτ (Table 1). Additionally, no significant changes to the average iτ were observed when the concentration of citrate ions was changed. These results indicate the long tail current is unlikely originated from citrate oxidation. Since H2O is present at 55.6 M and the tail current is observed at ~1.0 V vs Ag/AgCl or ~1.22 V vs NHE, which is close to the standard potential for water oxidation, we believe that the tail current is due to electrocatalytic oxidation of water molecules on Ag/Ag2O surfaces.

Figure 3. CV curves at 50 mV/s of bare 12.7 µm Au UME (black) and 40 nm Ag NP-modified gold UME (red) measured in 7 mM trisodium citrate, 18 mM potassium nitrate aqueous solution. Scan direction is from −0.2 to 1.2V, back from 1.2 to −1.2 V and then to −0.2 V.

Characterization of Ag NP-Modified Au UMEs. To further prove the catalytic effect of nanometer-sized Ag2O toward water oxidation, cyclic voltammetry was applied to characterize Ag-NP-modified gold UMEs. Figure 3 shows CVs of a bare Au UME (black curve) and a Au UME modified by 40 nm Ag NPs (red curve) in a solution of 7 mM sodium citrate and 18 mM KNO3. A bare Au UME shows an anodic wave appearing at an onset potential of ~0.96 V, which can be attributed to the oxidation of water. Small 12 ACS Paragon Plus Environment

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waves located at +0.68 V and -0.04 V can be assigned to the oxidation of the gold electrode and the reduction of gold oxide, respectively43. Although sodium citrate can also be oxidized, the standard reduction potential is lower than observed waves in our CV curves according to previous work44. Compared to the bare gold, the Ag NP-modified UME shows a large anodic wave at about 0.21 V vs Ag/AgCl (red curve). This additional anodic current must be from the oxidation of silver NPs on gold45, with an oxidation potential even lower than the theoretical value (0.5 V vs Ag/AgCl)30,39, likely due to small particle size46,47. More importantly, the NP-modified gold UME exhibits much higher current due to water oxidation starting at ~0.83 V. The onset potential is also lowered by ~0.13 V compared with the bare Au electrode. These results suggest that Ag NPs can be oxidized on a Au electrode likely generating Ag2O which can catalyze water oxidation. Therefore, it is reasonable to believe that electrocatalytic water oxidation is coupled during amperometric detection of Ag NPs at higher potentials. In the backward scan, no reduction peak for silver oxide was observed, probably due to desorption and dissolution of Ag NPs after water oxidation reaction at higher potentials.



Figure 4. Representative current-time traces of 30 pM 40 nm Ag NPs at a 12.7 µm Au UME (a) 1.0 V, pH 5.2 (b) 0.6 V, pH 9.7. (c), (d) Histograms of transferred charge per collision event at 1.0 V (c) and 0.6 V (d) for all pH values.

pH-Dependent NP Collision. We run another key experiment to confirm silver oxide catalyzes water oxidation and to examine its pH dependence. First, according to thermodynamic Pourbaix diagrams40,48, which show the standard potential of oxygen evolution reaction (OER) at different pH, the equilibrium potential at room temperature is given by E (V) = 1.23 + (0.059/4) × log(pO2) − 0.059 × pH, relative to NHE, where pO2 is O2 fugacity41. Therefore, higher pH promotes OER by lowering the equilibrium potential. Second, since formation of Ag2O is dependent on the local presence of OH- in solution39, higher pH also promotes formation of Ag2O.


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Table 2: Statistics for collision events of 40 nm Ag NPs on a gold UME at different pH and potentials. Potential/V

Number of events

pH

Q/fC

SD Q/fC

0.6

63

5.2

92

89

Ratio of peaks with iτ ~0

129

7.8

94

74

~0

183

9.7

201

157

73

36

8.4

85%

160

5.2

264

230

109

44

33

34%

262

7.8

514

516

141

80

63

77%

99

9.7

908

883

191

111

83

90%

1.0

iτ/pA

SD iτ/pA

τ/ms

We found that the probability of observing collision events with the long tail current on a gold electrode is significantly higher at higher solution pH. While 1.1 V is high enough for water oxidation for all of the pH we tested (Figure S4), greater differences can be seen at lower potentials. Figure 4a shows a typical i-t trace measured at 1.0 V and pH 5.2, which is quite different from Figure 2c, where NP collision events were collected at 1.0 V and neutral pH. Only 34% of events at pH 5.2 show tail current following the main peak, while the ratio is 77% for pH 7.8 (Table 2), suggesting water oxidation is suppressed in an acidic environment. Additionally, Figure 4b shows an i-t trace measured at pH 9.7 solution at 0.6 V. Most events (~85%) show a tail following the main peak, which is absent in pH 7.8 at 0.6 V (Figure 2a). The trend of observing more water oxidation signals with higher pH at lower potential can be well understood from the thermodynamic Pourbaix diagrams40,48. And it also aligns with the idea that a higher pH promotes silver oxide formation. Our results have also shown a strong pH dependence in the stability of formed Ag2O and the water oxidation current. In an acidic or neutral environment, Ag2O dissolves more readily than in alkaline solutions (e.g., pH 9.7)49. Our results show fast decay of water oxidation signal in pH 5.2 (33 ms at 1.0 V, Table 2). In alkaline solutions, Ag2O is more stable and can continue to catalyze water oxidation for as long as 83 ms before it completely dissolves. Table 2 shows that higher pH leads to enhanced iτ (from 109 to 191 pA for pH 5.2 to pH 9.7), since more Ag2O can be formed at higher pH. As a result, increased average charge transfer was observed from pH 5.2 to 7.8 at potentials between 0.6 V and 1.0 V, due to 15 ACS Paragon Plus Environment


more favorable water oxidation and enhanced stability of Ag2O, as shown in Figures 4c and 4d.

Figure 5. Current-time traces of 75 pM 60 nm Ag NPs (a) and 8 pM 110 nm Ag NPs (b) at a 12.7 µm Au UME. Conditions: 7 mM trisodium citrate, 18 mM potassium nitrate, pH = 7.8, and oxidation potential 1.1 V.

The Effect of Particle Size. To further examine the catalytic effect of Ag2O on NPs of other sizes, we performed collision experiments using 60 nm and 110 nm Ag NPs. Typical traces for both larger NP sizes at 1.1 V are shown in Figure 5 and the key parameters are summarized in Table S1. These two kinds of Ag NPs behave quite similarly to the 40 nm NPs, suggesting electrocatalytic water oxidation during particle collision and oxidation. Obvious tail current can be observed following every large spike in Figure 5. Tail current height (iτ) and duration (τ) show strong size dependence: larger NPs have increased tail current height and longer duration. Some of these collision events can last more than 2 s, implying that more Ag2O is formed to catalyze water oxidation. Importantly, collision behavior of 60 and 110 nm Ag NPs also show strong pH dependence. 16 ACS Paragon Plus Environment

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Table S2 listed key parameters from 60 nm NP at 0.6 V and 1.0 V. One can find that at higher pH, collision peaks are accompanied with longer duration and higher tail current, indicating enhanced water oxidation and oxide stability. The probability of signals coupled with water oxidation also increases with pH. These results are in good agreement with results of the 40 nm NPs supporting our hypothesis that Ag2O can catalyze water oxidation leading to enhanced charge transfer in transient collision signals.

Figure 6. Current-time traces of 30 pM 40 nm Ag NPs at a 5-µm carbon UME at 1.1 V. Conditions: 7 mM trisodium citrate, 18 mM potassium nitrate(a), and 7 mM trisodium citrate, 18 mM potassium chloride(b). For traces with KCl, details of peaks are shown below.

The Effects of Electrode Material and Chloride Ions. Ag NP collision behavior at high voltages was further studied by changing electrode material and electrolyte composition. In the presence of chloride, Ag can be easily oxidized to AgCl17,50. The standard reduction potential for the reaction AgCl(s) + e- ⇌ Ag(s) + Cl- is 0 V vs Ag/AgCl by definition, much lower than Ag2O formation (0.5 V vs Ag/AgCl at pH



7.8). In a 18 mM Cl- solution, the Cl- concentration is several orders of magnitude higher than that of OH(6.3 × 10-4 mM) at pH 7.8, where OH- is necessary to form Ag2O51. As a result, the formation of AgCl outcompetes Ag2O formation in KCl-containing solutions. This creates a stark difference in the amperometric signals for NP collision in the presence and absence of KCl. Since the surface of a Au electrode may be etched by chloride (AuCl4- + 3e- ⇌ Au(s) + 4Cl-, 0.7 V vs Ag/AgCl), we used a carbon fiber microelectrode (CFE) instead, to examine the amperometric signal differences in electrolyte solutions containing KCl and KNO3. Figure 6a shows characteristic long tails observed from 40 nm Ag NP on a 5-µm disk CFE holding at 1.1 V in KNO3. Parameters of these events including charge, tail current height, and duration are listed in Table S3. These results indicate that both carbon and Au electrodes can support Ag2O formation, and yield electrocatalytic signal of water oxidation during particle collision. When 18 mM KCl is used, the collision signals dramatically change as shown in Figure 6b. Most collision events are single peaks without significant current tails, and the oxidation current spikes quickly return to baseline within 1 ms. This phenomenon is similar to previously reports on Ag NP collision in KCl at lower potentials, suggesting AgCl is the main oxidation product. The relatively small average charge transfer (56 fC, Table S3) corresponding to partial oxidation in KCl solutions implies that AgCl cannot catalyze water oxidation nearly as efficiently as Ag2O (808 fC, Table S3). A 1.0 V potential was also applied to compare electrolyte effects on CFEs, and the same trend was observed (Table S3). Ag NP detection with KCl results in much smaller current peaks and significantly reduced charge transfer than with KNO3. These results further confirm the formation of Ag2O and its catalytic effect for water oxidation during particle collision.


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Figure 7. Visualizing a 110 nm Ag nanoparticle collision and oxidation on gold film electrode recorded by TIRF. (a) Images from one Ag NP at different time. Scale bar represents 2 μm; (b) Time dependent fluorescence intensity corresponding to (a). Conditions: 23 pM Ag NPs, 7 mM trisodium citrate, 18 mM potassium nitrate at 0.9 V vs. Pt.

Fluorescence Imaging of Ag2O Formation. As described in a previous paper, Ag2O is highly fluorescent under intensive light illumination52 and the fluorescence is from small Ag clusters generated by photoreduction and photodissolution of Ag2O53,54. Following this idea, we attempted to use TIRF microscopy to directly observe the processes of Ag2O formation and dissolution on a gold electrode. In this experiment, a 20-nm-thick gold film on glass was used as the working electrode for fluorescence imaging. The output power of a 532-nm laser was limited to 10 mW. Figure 7a shows a 110 nm Ag NP approaching the gold surface at 0 s. The particle is visible due to weak fluorescence from the Ag NP itself. It diffuses near the gold surface in the first 0.86 s before it starts to interact with the gold surface and undergoes electro-oxidation at 0.9 V vs a Pt QRE. One can see the fluorescence signal becomes stronger and reaching a maximum at ~1.1 s. Thereafter the fluorescence signal becomes weaker and vanishes at 4.6 s. Since TIRF microscopy enables imaging of a space only ∼150 to 200 nm above the



electrode surface55, it is reasonable to attribute the fluorescence increase to Ag oxidation on gold. The intensity-time relationship is plotted in Figure 7b. The fluorescence lasts for ~3 s, in good accordance with timescale measured from electrochemical traces (Table S1), suggesting the nanosized Ag2O (110 nm) continues to catalyze water oxidation before it completely dissolves in 2 to 3 seconds.

Conclusions In summary, we studied transient collision electrochemistry of single Ag NPs of 40 nm, 60 nm, and 110 nm in diameters on gold UMEs at different potentials. In addition to the multi-peak collision behavior at lower potentials (

Transient Electrocatalytic Water Oxidation in Single-Nanoparticle

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