Electrochemical Synthesis of Individual Core@Shell and Hollow Ag

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Electrochemical Synthesis of Individual Core@Shell and Hollow Ag/AgS Nanoparticles 2

Donald A. Robinson, and Henry S. White Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.9b02144 • Publication Date (Web): 23 Jul 2019 Downloaded from pubs.acs.org on July 24, 2019

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Electrochemical Synthesis of Individual Core@Shell and Hollow Ag/Ag2S Nanoparticles Donald A. Robinson and Henry S. White.* Department of Chemistry, University of Utah, Salt Lake City, Utah 84112, United States.

Keywords. nanoparticle collision, electrochemistry, phase transformation, growth kinetics Abstract. This letter presents an electrochemical methodology for structure-tunable synthesis, characterization, and kinetic monitoring of metal-semiconductor phase transformations at individual Ag nanoparticles. In the presence of HS- in aqueous solution, the stochastic collision and adsorption of Ag nanoparticles at a Au microelectrode initiates the partial anodic transformation of Ag to Ag2S at each particle. A single continuous current transient is observed for each Ag nanoparticle reacted. The characteristic shapes of the transients are distinct from previously reported amperometric recordings of electrochemical reactions involving single nanoparticles, and are highly uniform at a constant applied potential. The average maximum current increases while event duration decreases as a function of increasing potential. Independent of applied potential, the electrochemical transformation event abruptly stops after converting ~80% of the Ag in the nanoparticle to Ag2S, a self-terminating process that does not occur for bulk Ag electrodes under similar conditions. The resulting products are a mixture of core@shell Ag@Ag2S nanoparticles with and without voids in the core, as characterized by transmission electron microscopy (TEM) and energy-dispersive x-ray spectroscopy (EDX). Both the frequency and size of voids increase at more positive potentials. The average size of the core@shell nanoparticles determined by coulometric analysis of the current transients agrees well with TEM measurements. In this letter, we demonstrate the versatility of single-nanoparticle electrochemistry to simultaneously drive and monitor the kinetics of solid-state phase transformations at the nanoscale, using as a model system the anodic transformation of metallic silver to semiconducting silver sulfide, Ag2S. 2Ag + HS- + OH- ↔ Ag2S + H2O + 2e-

(1)

Ag2S is a highly insoluble material (pKsp ≈ 53.5),1 thus making the reaction a suitable model system for studying electrochemical metal-semiconductor transformations at individual nanoparticles (NPs). Metal/metal sulfide hybrid nanostructures and hollow metal sulfide NPs are a widely researched class of materials due to their optical2-5 and electrochemical6-16 properties. Target applications include solar-driven fuel production,12 electrochemical energy storage,14-16 biosensing,17 drug delivery,3 photothermal 1 ACS Paragon Plus Environment

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therapy,18, 19 and nanothermometry in biological media.20 Ag/Ag2S is one of the most highly researched metal/metal sulfide nanocomposites for such diverse applications. Core@shell Ag@Ag2S NPs are also known to be less toxic to the environment than Ag NPs.21 The most popular approach to studying electrochemical reactions at individual nanoparticles involves the detection of stochastic nanoparticle collisions at a microelectrode.22-26 If the nanoparticle itself is composed of an electroactive material, the size and composition of the NPs can be determined by oxidizing or reducing the materials compromising the particles when they collide with an inert microelectrode.26 The charge obtained from integration of a current transient is a direct measure of the amount oxidized or reduced.27-29 The size of the reactant particle can then be determined if the reaction occurs to completion and the particle’s morphology/density are known. This characterization method has been demonstrated for Ag,26 Au,30 Fe3O4,31 core@shell Ag@Au,32 and metal alloy33, 34 NPs. The reverse reaction is also possible to achieve electrodeposition of NPs, as monitored individually, from femtomolar concentration of metal ions35-37 or upon collision of water-in-oil microemulsion droplets38, 39 or inverse micelles40 containing metal ions. Alternate approaches to studying single-NP electrodeposition have relied on nanoelectrodes, either fabricated by laser pulling41, 42 or created using scanning electrochemical cell microscopy (SECCM) techniques.43, 44 While a few examples in the recent literature have reported electrochemical measurements of surface reactions at individual particles45, 46 and partial/full conversion of NPs to a new solid phase,47-49 the electrochemical transformation of core@shell nanoparticles has been limited to ensemble studies.50-52 Our electrochemical methodology employs a Au microelectrode to drive the anodic sulfidation of Ag NPs upon collision, resulting in either core@shell Ag@Ag2S NPs or void-containing Ag/Ag2S NPs (Figure 1a). Experiments were performed in a deaerated solution containing 1 mM Na2S, 10 mM NaOH, and 9 pM Ag NPs (70±8 nm diameter). Detailed experimental procedures are provided in Section S1 of the Supporting Information. More than 99.99% of the sulfide in solution is in the form of bisulfide ions (HS-) at the experimental pH (~12), given the dissociation constant of H2S (pKa1 = 7.0, pKa2 > 17).53, 54 The anodic transformation of Ag to Ag2S initiates when a single NP stochastically collides and adsorbs if the applied electrode potential is positive versus the redox potential of Ag/Ag2S. The net reaction in eq 1 can be broken down into two steps: the electron transfer reaction at the Ag/Ag2S interface (eq 2) and the chemical reaction at the Ag2S/solution interface (eq 3).55 2(Ag ↔ Ag+ + e-)

(2)

2Ag+ + HS- + OH- ↔ Ag2S + H2O

(3)

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Ag2S is an n-type semiconductor with mixed electronic/ionic conduction properties; both electrons and Ag+ ions are the mobile current carriers in the material.56-60 At room temperature, the electronic conductivity is poor and ionic conduction dominates the current-voltage relationship.56, 59 The rate of the net reaction (eq 1) is thus controlled by two transport processes; the transport of HS- to the Ag2S/solution interface, and the transport of Ag+ across the Ag2S film.61 Figure 1 shows the amperometric recording of individual Ag NP electrochemical sulfidation events at a constant potential of 0.1 V vs Ag/Ag2S. All current transients shown in Figure 1 share a characteristic shape. The current rises rapidly upon collision, followed by a slower nonlinear increase to reach a current maximum. The current then decays until a sudden sharp decrease to the baseline, signaling the termination of the reaction. Because the current does not rapidly decay after the first abrupt increase in current when the particle adsorbs, we can conclude that Ag2S nucleation is not instantaneous, but may follow a progressive nucleation model62, 63 whereby the number of nucleation sites at the Ag NP increases as a function of time. The subsequent decay in the current may be the sole result of decreasing nucleation rate as a function of time, but may also be influenced by the decreasing ion conductance across the Ag2S film with increasing film thickness.61 Each single-nanoparticle current-time response in Figure 1 is continuous until reaction termination. We do not observe multipeak i-t events, which would signal the stepwise anodic dissolution of a Ag NP colliding multiple times at a Au electrode; such behavior has been reported for the partial oxidation of Ag nanoparticles at neutral pH without sulfide.27-29, 64, 65 In the present system, however, the formation of insoluble Ag2S prevents dissolution and may promote particle adsorption due to a strong interaction between Au and Ag2S. This explanation is supported by multiple reports from Yang and coworkers demonstrating evidence of a strong Au/Ag2S interaction during the synthesis of core@shell Au@Ag2S NPs and Au-Ag2S heterodimers.66-68 The Au electrode surface is terminated with an adlayer of S at the potentials investigated in this study69 (see Section S2 of the Supporting Information), which may also play a key role in NP adsorption. To support the sticking hypothesis, cyclic voltammetry was performed at Ag NPs dropcast on a Au microelectrode (Section S2 of Supporting Information, Figure S4). From coulometric analysis of the voltammogram, we obtain a charge of 24 nC for Ag2S formation and 23 nC for Ag2S reduction back to Ag, signifying that the reaction is reversible (eq 1) and NPs remain immobilized at the electrode surface after sulfidation. We also find no discernible difference between the Ag/Ag2S redox potential for 70 nm diameter Ag NPs (-0.74 V vs Ag/AgCl) and bulk Ag, consistent with the experimental findings of Zamborini for Ag NPs larger than 40 nm diameter.70 Although we cannot definitively rule out the possibility that the nanoparticle undergoes multiple sequential collisions with the


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electrode over timescales shorter than the temporal resolution of the amplifier (~100 μs),71 the observed continuity of the current transients (Figure 1) indicates NP adsorption upon collision.

Figure 1. (a) Schematic of Ag NP adsorption and transformation to a core@shell Ag@Ag2S NP at a Au electrode; the relative lengths of the HS- transport vectors (black arrows) representing the HS- flux profile based on theory.72 (b) Current monitored over a 420 s time window at a 12.5 μm diameter Au microdisk electrode, E = 0.1 V vs Ag/Ag2S, in a deaerated solution containing 9 pM of ~70 nm diameter Ag NPs, 1 mM Na2S, and 10 mM NaOH. Also included are current-time traces for two controls excluding HS- or Ag NPs, as labelled in the figure. (c) Expanded view of the first 60 s section of part b, as indicated by red dashed box. The 60 s section is shown as 5 stacked traces, each of 12 s duration and stacked in chronological order from top to bottom. (d) Expanded view of the last three single-NP electrochemical events in part c, as indicated by the blue dashed box. All data in parts b-d were originally recorded with a low-pass 3-pole


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Bessel 10 kHz filter at 50 kHz sampling, then post-filtered at 100 Hz and sampling rate reduced to 500 Hz. See Supporting Information, Section S1, for details on data acquisition and processing.71 Figure 2 shows the effect of electrode potential on the shape, amplitudes, and durations of the current transients. The average maximum current, imax, increases with overpotential, as expected, while the average duration, Δt, decreases (Figure 2c). In this potential range, the average imax does not reach a diffusion-limited plateau. Assuming the Ag core remains in direct contact with Au throughout the course of the reaction, the current is therefore influenced by both Ag+ ion transport across the Ag2S film and HStransport to the Ag2S/solution interface. At low overpotentials (0.05 and 0.10 V in Figure 2b), the current decreases after reaching a maximum. The crest shape of the transients changes from a parabola to a halfparabola at potentials positive of ~0.15 V. This kinetic transition is also evident from the apparent prewave in the potential dependence plot of the average imax (Figure 2c), with a half-wave potential between 0.10 and 0.15 V. Above 0.15 V, we suggest that the Ag+ transport rate through the Ag2S film becomes sufficiently fast in comparison to HS- transport to the Ag2S/solution interface. Thus, the current continuously increases, presumably due to progressive nucleation/growth as proposed above, until the reaction abruptly self-terminates.


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Figure 2. Current-time responses for Ag2S nanoshell formation at Ag NPs (70 nm average diameter) upon electrode collision at varied potentials vs Ag/Ag2S as indicated in top labels: (a) multiple NP collisions over 50 s intervals, and (b) representative i-t responses for individual NP reactions. Solution conditions, acquisition settings, and post-processing for i-t traces in parts a and b of Figure 2 are identical to those described in Figure 1 caption. Additional examples of observed current transients are provided in Section S3 of the Supporting Information. (c) Average maximum current, imax, (black plot), average i-t event duration, Δt, (blue plot) and (d) average charge from integration of current vs time events, Q, obtained from the analysis of multiple i-t transients representing anodic formation of Ag2S at individual Ag NPs, plotted as a function of potential. Dashed red line in part d represents the calculated faradaic charge for complete oxidation of a 70 nm Ag particle (1.7 pC). Numbers of particles analyzed for each potential are shown below the scatter points in part d, and also pertain to the total number of particles analyzed in part c. (e) 6 ACS Paragon Plus Environment

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Histograms of Q distributions measured by single Ag NP electroanalysis (bottom black plot, charge measurements concatenated over all applied potential conditions) and calculated from TEM-measured Ag NP diameters assuming complete oxidation using eq. 4 (top red plot, calculated from Ag NP diameter histogram shown below). SD, standard deviation; SEM, standard error of the mean; N, number of measurements. The charge associated with the mass of Ag oxidized, Q, was measured by integration of individual Ag NP i-t events. As shown in Figure 2d, the average Q does not vary significantly with varied overpotential. ANOVA statistical analysis was used to confirm that at the 0.05 level, the mean Q values recorded over the overpotential range of 0.1 to 0.35 V do not vary significantly (see Supporting Information, section S4). The measured average Q for the 0.05 V overpotential is significantly larger than the other sample means, but this may be due to the much lower sample size (N = 30) and measurement accuracy for the more challenging current transients at lowest overpotential. Figure 2e shows the comparison of measured Q for concatenated data at all overpotentials to the calculated Q, assuming full Ag NP oxidation. The theoretical charge for full oxidation, QT, was calculated from the measured distribution of Ag NP diameters shown below by relating volume to charge via density of Ag using eq 4, assuming a spherical particle geometry.

QT 

n1 F  Ag πd 3 6 M Ag

(4)

Here, n1 = 1 electron, F is Faraday’s constant (96485 C mol-1), ρAg is the density of Ag (10.49 g cm-3), d is the measured Ag NP diameter, and MAg is the molar mass of Ag (107.87 g mol-1). The difference of two population means in Figure 2e was found to be significantly greater than zero according to a two-sample t-test (see Supporting Information, section S4). The measured average Q of ~1.3 pC is roughly 80% of the theoretical Q for full oxidation of a 70 nm diameter Ag NP (1.7 pC). Thus, regardless of applied potential or current density, the reaction terminates after ~80% of the Ag in the NP has transformed to Ag2S. In contrast, the growth of a Ag2S film at a bulk polycrystalline Ag electrode does not terminate after 2.8 hours of applying 0.3 V (see Section S5 of Supporting Information). The major difference between the bulk Ag electrode and collision experiments is that the former always maintains a constant potential at the Ag/Ag2S interface to drive the reaction. We propose the reaction terminates at the NP after breakage of Au/Ag contact due to the formation of an Ag2S interphase between Ag and Au. Because the electronic conductivity of room temperature Ag2S is poor,59 the potential then drops completely across the newly formed Ag2S interphase.


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The potential of the Ag/Ag2S interface then returns to equilibrium, thus terminating the oxidation reaction at the Ag/Ag2S interface (eq 3). To characterize the NP structure and composition with electron microscopy, we used a Au nanofilm-coated TEM grid as the working electrode instead of a Au microelectrode (Section S1 of the Supporting Information). TEM samples imaged after performing the sulfidation reaction were comprised of a mixture of core@shell Ag@Ag2S NPs with continuous solid Ag cores (Figure 3a) and Ag/Ag2S NPs containing a void region in each core (Figure 3b-c). (Additional images are provided in Section S6 of the Supporting Information). Of the NPs produced at a 0.1 V bias, 51% were observed to contain voids. At the higher potential (0.3 V), 86% contain voids. As shown in Figure 3d, the average diameter of the void regions is smaller for particles synthesized at 0.1 V (~30 nm) and larger at 0.3 V (~50 nm). Further characterization was conducted via EDX spectral imaging, as shown in Section S7 of the Supporting Information. Element mapping confirms that the core contains Ag and the shell contains S in core@shell Ag@Ag2S NPs synthesized at 0.1 V (Figure S15). Line scans across void-containing particles show that both Ag and S are largely concentrated in the shell encapsulating the voids (Figures S16 and S17).


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Figure 3. Scanning transmission electron micrographs of (a) core@shell Ag@Ag2S NP and (b) hollow Ag/Ag2S NP synthesized at 0.1 V and (c) hollow Ag/Ag2S NP synthesized at 0.3 V. (d) Measurements of void diameters associated with hollow NPs, applied potentials as indicated in legend. Other experimental conditions consistent with those in Figure 1 caption.


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High-resolution TEM measurements were performed to determine the crystallinity of the Ag2S phase (Figure 4). The sections chosen for measuring interplanar lattice spacings are located in areas where the underlying broken Au film is not present to interfere with the measurements. The substrate material in these Au-free regions is likely composed of residual organic polymer that was used as a sacrificial layer in the fabrication of the film by the manufacturer.73 In the control experiment without any potential applied, we observe atomic lattice fringes corresponding to Ag only (Figure 4a). This is expected because the chemical sulfidation of Ag (i.e. silver tarnishing) requires O2 as an oxidizing agent,74, 75 the concentration of which is sufficiently minimized in the Ar-purged solutions to prevent Ag oxidation.

Figure 4. High resolution transmission electron micrographs of (a) Ag NP observed at open circuit, (b) core@shell Ag@Ag2S NP produced at 0.1 V, and (c) hollow Ag/Ag2S NP produced at 0.3 V after stochastic adsorption of a Ag NP (~70 nm diameter) on a Au nanofilm-coated TEM grid electrode. Solution conditions are similar to those described in Figure 1 caption. The core@shell NP in Figure 4b exhibits interplanar spacings consistent with Ag(111) in the core region (0.24 nm) and Ag2S (111) in the lighter contrast shell region (0.31 nm). The Ag core appears to have 10 ACS Paragon Plus Environment

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a polyhedral morphology. Interplanar lattice spacings for Ag and Ag2S were also observed in the shell of the hollow Ag/Ag2S NP in Figure 4c. In contrast to the core@shell Ag@Ag2S NP, the two phases appear to be more intermixed. The inner empty core has an icosahedral shape, as defined by the thin frame of dark contrast indicative of a more electron dense material, presumably metallic Ag. We also assign the lineshaped regions of dark contrast extending outward from corner and edge sites of the hollow Ag icosahedral frame, and along apparent grain boundaries of the Ag2S film, as evidence of metallic Ag. Because we do not observe such interfacial Ag phase segregation in the continuous Ag2S shells of core@shell Ag@Ag2S NPs (Figure 4b), we deduce that the tendency of Ag0 to migrate away from the core and toward the outer Ag2S shell correlates with the rate of void nucleation/growth. The observed dependence of nanovoid frequency and size on the applied potential can be explained based on models that predict void formation as the initiating process for anodic pitting corrosion at oxide-passivated electrodes.76-78 According to such models, the oxidation reaction at the metal/oxide interface is thought to produce metal vacancies in balance with metal ion products.77 For our system, the electron transfer reaction would be modified to include the production of metal vacancies (VAg) at the Ag/Ag2S interface (eq. 5). 2(Ag ↔ Ag+ + VAg + e-)

(5)

MacDonald and coworkers proposed that when the rate of cation transport (and thus, metal hole creation) is much higher than the rate of vacancy submergence into the bulk, vacancies condense at the metal/oxide interface to form voids as intermediates to pit formation.76 At low cation transport rates, these vacancies or “metal holes” submerge into the bulk of the metal and do not lead to void formation.76, 77 For our system, at sufficiently low current densities, the inward flux of S2- is fast enough to prevent metal vacancy condensation and maintain Ag/Ag2S interfacial cohesion. Thus, the Ag/Ag2S interface moves inward towards the particle’s center as the reaction proceeds, producing a void-free Ag@Ag2S NP. However, at higher applied potentials, the current density is large enough for the rate of Ag+ ion transport to exceed the rate of metal vacancy dissipation into the bulk Ag core. The formation/growth of a stable void occurs at a location of the Ag/Ag2S interface where the metal vacancies reach a critical concentration, likely at corners and edges of the developing Ag/Ag2S interface where Ag atoms are most active for the oxidation reaction (eq 5). Thus, the observed variation of NP product structures and void sizes at a single applied potential is likely due to the morphological heterogeneity of the Ag NPs. The average Q measured from electrochemical collision experiments (Figure 2d) directly relates to the mass of Ag converted to Ag2S by Faraday’s law. The final volumes of both phases can then be calculated via their respective densities (ρAg = 10.49 g cm-3, ρAg2S = 7.23 g cm-3) and molar masses (MAg = 11 ACS Paragon Plus Environment

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107.87 g mol-1, MAg2S = 247.80 g mol-1). Assuming a spherical core@shell Ag@Ag2S structure, we can then determine the average core@shell dimensions. (See Section S8 of Supporting Information for analysis details). The average Q of 1.3 pC corresponds to a Ag core diameter of 43 nm, Ag2S shell thickness of 19 nm, and total NP diameter of 80 nm. Charge/size conversions were also applied in the analysis of hollow Ag/Ag2S NPs. However, because Faraday’s law only informs on the mass/volume of material reacted, we cannot depend on electrochemical measurements alone to determine the final average total diameter of the hollow NPs. We therefore incorporated the extra empty volume of the inner cores into the analysis using the average TEM-measured void diameters in Figure 3d. Figure 5 presents low-magnification STEM images and the analysis of measured total particle diameters for the initial Ag NPs (part a), as well as products from electrochemical experiments (parts c-d) and the control experiment without applied potential (part b). The size distribution for the initial Ag NPs (Figure 5a) is comparable to that of the control experiment (Figure 5b), as expected. Upon limiting our analysis to core@shell NPs without observable voids (red histogram in Figure 5c), we measure an average total diameter of 81.5 nm by TEM. This value is in good agreement with that determined from the average Q in Figure 2d (80.5 nm), signifying the accuracy of coulometric characterization. The green histograms in Figure 5c-d represent the TEM measurements of total particle diameters when the analysis is restricted to only void-containing particles. The average total diameter measured by TEM (84 nm) is slightly smaller than that determined from the void-corrected TEM/electrochemical analysis (86 nm) for both 0.1 V and 0.3 V potentials. Partial dissolution may account for the 2 nm discrepancy in diameters determined by the two different methods. Indeed, we occasionally observe EDX spectroscopic evidence of thin Ag2S coatings over the nanostructured Au film at areas encompassing hollow Ag/Ag2S NPs, exclusively. As shown by EDX element mapping in Figure S18 of the Supporting Information, an apparent film of Ag2S surrounds the particle, up to hundreds of nanometers from the particle’s surface. The maximum solubility of Ag2S (via formation of Ag(HS)2- complexes) occurs at the H2S pKa (7.02).79 Considering the poor pH buffering capacity of the 0.01 M NaOH + 1 mM Na2S solution, a proton concentration gradient likely forms due to the consumption of HS- and OH- ions according to eq 1, decreasing the pH at the Ag2S/solution interface. Thus, Ag(HS)2- species may form and diffuse a short distance into solution before precipitating as Ag2S at the Au surfaces in close proximity to the nanoparticle.


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Figure 5. Scanning transmission electron micrographs (left) and histograms from measured diameters (right) for Ag NPs deposited by (a) dropcast on holey carbon support film from stock suspension and (b/c) adsorption at Au nanofilm-coated TEM grid electrode at (b) no potential applied and (c) 0.1 V vs Ag/Ag2S. Average diameters and standard deviations reported on each histogram. Solution conditions for parts b and c are similar to those described in Figure 1 caption. Dendritic surface structures in the background of parts b-d make up the broken Au nanofilm on the TEM substrate. Abrupt spikes in current are often observed to occur during termination of single-NP sulfidation events (see Figures 1-2 and Section S3 of the Supporting Information). We obtain an average charge of 40(±20) fC from integration of the end spikes associated with six representative events, which accounts for ~3% of the average total charge passed. (See left column of Figure S6 in the Supporting Information for the six transients analyzed). It is unclear whether the observed end spikes correspond to a sudden increase of the Ag oxidation rate during the final stage of particle sulfidation or to a double-layer charging process. Both possibilities may result from a structural transformation associated with reaction termination. Considering a double-layer charging process, we assume a typical differential capacitance of 20 μF/cm2 and the geometric surface area of a particle with 80 nm diameter (2×10-10 cm2), which leads to 13 ACS Paragon Plus Environment

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a single-NP capacitance of 4 fF. However, if the Ag2S film is porous and/or amorphous, the true Ag2S surface area may be much larger than the calculated geometric area. If we assume an electrostatic potential of 0.5 V, the Ag2S surface area is estimated from the measured charge (45 fC) to be ~20 times larger than the geometric area, which is reasonable if the film is highly porous. Thus, the current spikes could represent the “squeezing out” of ions from within the film as the Ag2S forms the crystalline phase we observe from TEM (Figure 4b-c). Different particles likely have different rates of crystallization and surface areas, which could explain why end spikes are not always observed. However, we note that on some occasions (< 5%), a more prominent peak of longer duration is observed towards the end of the event (e.g., Figure S6, bottom right corner). At this point, we cannot rule out the possibility that these current spikes result from some combination of Ag oxidation and double layer charging. We have shown that the electrochemical sulfidation of Ag NPs produces core@shell Ag@Ag2S and hollow Ag/Ag2S NPs. The NP transformation rate increases with overpotential. The structure of the NP product also depends on the potential applied, with a greater yield of hollow Ag/Ag2S NPs and larger void sizes produced at higher overpotentials. The single-NP reaction terminates after ~80% of the Ag in the particle has transformed to Ag2S. We propose the termination is brought about by disruption of intermetallic contact between the Ag core and Au electrode due to the formation of an electronically insulating Ag2S interphase. Our understanding of the complex solid-state electrochemical transformation described in this letter is limited because the structural characterization by ex-situ electron microscopy was constrained to reactant and product NPs. The implementation of an in-situ nanoscale structural characterization method is necessary to correlate kinetic information from the observed current transients to the structural evolution of intermediates. Elucidation of the phase transformation and voiding mechanisms might be possible by combining the electroanalytical NP collision technique with in-situ electron microscopy,80 superlocalization optical microscopy,48 and/or nanoscale spectroscopic methods.49 The electrochemically tunable synthetic methodology introduced in this letter could be adopted as an approach for preparing metal/semiconductor nanomaterials to function as water-splitting photoelectrodes12, 81 and energy storage materials.15, 16 However, such applications require developments in scaling up the electrochemical synthesis protocol.

ASSOCIATED CONTENT Supporting Information. Experimental details, supporting voltammetric measurements, supplemental examples of current transients, statistical analysis, coulometric analysis, electron microscopy, EDX


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microanalysis, and references. This material is available free of charge via the internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding author *Email: [email protected] ORCID Donald A. Robinson: 0000-0003-4531-0806 Henry S. White: 0000-0002-5053-0996 Notes. The authors declare no competing financial interests.

ACKNOWLEDGMENTS This work was funded by Air Force Office of Scientific Research MURI FA9550-14-1-0003. This work made use of University of Utah USTAR shared facilities support, in part, by the MRSEC Program of NSF under Award No. DMR-1121252. DAR acknowledges B. Van Devener for expert assistance and consultation in S/TEM imaging and EDX analysis. DAR also wishes to thank M. Edwards, K. Barman, and U. Zahra for helpful discussions.

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