Dealloying of Noble-Metal Alloy Nanoparticles - American Chemical

Apr 1, 2014 - Ira A. Fulton School of Engineering, Arizona State University, Tempe, Arizona 85287 United States. ‡. Department of Materials Science ...
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Dealloying of Noble-Metal Alloy Nanoparticles Xiaoqian Li,† Qing Chen,† Ian McCue,‡ Joshua Snyder,§ Peter Crozier,† Jonah Erlebacher,‡ and Karl Sieradzki*,† †

Ira A. Fulton School of Engineering, Arizona State University, Tempe, Arizona 85287 United States Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland 21218, United States § Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States ‡

S Supporting Information *

ABSTRACT: Dealloying is currently used to tailor the morphology and composition of nanoparticles and bulk solids for a variety of applications including catalysis, energy storage, sensing, actuation, supercapacitors, and radiation damage resistant materials. The known morphologies, which evolve on dealloying of nanoparticles, include core−shell, hollow core− shell, and porous nanoparticles. Here we present results examining the fixed voltage dealloying of AgAu alloy particles in the size range of 2−6 and 20−55 nm. High-angle annular dark-field scanning transmission electron microcopy, energy dispersive, and electron energy loss spectroscopy are used to characterize the size, morphology, and composition of the dealloyed nanoparticles. Our results demonstrate that above the potential corresponding to Ag+/Ag equilibrium only core−shell structures evolve in the 2−6 nm diameter particles. Dealloying of the 20−55 nm particles results and in the formation of porous structures analogous to the behavior observed for the corresponding bulk alloy. A statistical analysis that includes the composition and particle size distributions characterizing the larger particles demonstrates that the formation of porous nanoparticles occurs at a well-defined thermodynamic critical potential. KEYWORDS: Nanoparticle, dealloying, nanoporous, noble metal, core−shell, scanning transmission electron microcopy

D

face-centered cubic crystal structures9) because at this composition there are continuous conduits or active paths of Ag available to the invading electrolyte and so no solid-state transport should be necessary to support selective dissolution. However, the diameter of these conduits must be large enough so as to allow for the creation of enough volume for the electrolyte to penetrate the evolving nanoporous structure and in the case of AgAu alloys this occurs at about 55 at% Ag. We refer to this active path selective dissolution process as percolation dissolution. The thermodynamic discussion that follows below regarding alloy nanoparticles should be appropriate for particle sizes that retain the same crystal structure as the bulk planar metal counterpart. For close-packed metals (atomic volume ∼ 14 Å3), this corresponds to a particle diameter of about 3 nm, equivalent to a spherical particle containing ∼1000 atoms. Smaller particles are known to display quantum size effects,10,11 oscillatory behaviors,12 entropically driven phase transformations,13 and so forth, as a function of the number of atoms defining the cluster size. Thermodynamics of Dealloying. For a macroscopic binary ApB1−p solid solution alloy selective dissolution of the less-noble A component can occur as a surface or bulk dealloying process. Surface dealloying can occur at any alloy composition, p, at a potential above the equilibrium potential,

ealloying, the selective dissolution of elemental components from an alloy is currently being used to tailor the morphology and composition gradient of noble-metal alloy nanoparticles for catalytic applications.1−3 Selective dissolution of the less-noble component from these particles typically results in either the formation of a core−shell structure with a conformal noble-metal shell encasing an alloy core or a porous noble-metal rich nanoparticle. Each one of these modifications has been reported to yield a considerable enhancement in catalytic activity. The focus of our work has been to examine the composition and morphology changes occurring in nanoparticles as a function of electrochemical potential and to compare this to that observed in the dealloying of the macroscopic bulk alloy counterpart displaying a planar surface to the electrolyte. We have chosen AgAu alloys as a model system primarily because the great majority of recent experimental and computational work on dealloying has been performed for this system.4−6 At high enough electrochemical potentials, the ambient temperature dealloying of bulk planar macroscopic noble metal alloys such as AgAu results in the formation of solid-void bicontinuous morphologies. Because the ambient temperature solid state diffusivity of silver is too low by many orders of magnitude7 to support dealloying there is a lower compositional bound for dealloying in such systems often termed the “parting limit” or “dealloying threshold”. In AgAu alloys, this lower bound is ∼55 at% Ag.8 One might expect that ambient temperature dealloying should be possible for Ag atom fractions just above the site percolation threshold (∼20% for © 2014 American Chemical Society

Received: January 29, 2014 Revised: March 10, 2014 Published: April 1, 2014 2569

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E̅equil(p), of A in the alloy determined by the activity of A, aA, in the alloy Eequil ̅ (p) = E ̅ An+ /A −

⎛ RT ⎞ ⎜ ⎟ln a ⎝ nF ⎠ A

the alloy in equilibrium with An+ cations in the electrolyte is given by16 Eequil(p , r ) = Eequil ̅ (p) − [γAlloy⟨Ω⟩ + fAlloy (Ω̂A − ⟨Ω⟩)]

(1)

×

where E̅A /A is the equilibrium potential of elemental A that depends on the concentration of dissolved An+ cations in the electrolyte, R is the gas constant, and F is the Faraday constant. (We use the E̅ notation to indicate that we are referring to the behavior of a bulk electrode presenting a planar surface to the electrolyte.) On an atomic scale surface dealloying can occur by a process such as kink or adatom site dissolution that does not result in the evolution of new surface area such as that occurring from vacancy formation owing to the dissolution of an A atom from a terrace site. In this respect, surface dealloying of a solid alloy is similar to that occurring in a liquid alloy. In the case of bulk dealloying, the situation is more complicated as the process depends upon the solid-state transport processes available to support dealloying. Under conditions for which a mono- and/or divacancy volume diffusion process is operative,14 bulk dealloying can occur at any composition at a potential above E̅ equil(p) however, the dealloyed morphology will depend on both the rate of dealloying and the composition of the alloy. When p is below the site percolation threshold for the crystal structure, bulk dealloying can only occur if volume diffusion processes are significant. Because such a process is isomorphic to electrodeposition affected by fluid-phase mass transport, a process that can result in dendritic morphologies, symmetry suggests that similar “void dendrite” forms should evolve under solid-state mass transport limited dealloying.15 Additionally, such an alloy diffusion process may result in an internal void morphology owing to the Kirkendall effect. At alloy compositions higher than the dealloying threshold, percolation dissolution becomes feasible. Obviously, a dealloying threshold only exists under conditions for which solid-state transport occurs at rates negligible in comparison to dealloying rates. At ambient temperatures, noble-metal alloys show well-defined dealloying thresholds or parting limits. The process of percolation dissolution results in a B-rich solid/void bicontinuous morphology. The electrochemical parameter signifying the onset of percolation dissolution is called the critical potential. Above the critical potential dealloying results in the evolution of nanoporous morphologies and below the critical potential only superficial dealloying occurs resulting in a near conformal layer of the more-noble component and “passivation”. The initial stage of porosity formation involves the injection of “cylindrical” pits or holes into the surface of radius, ξ = (1 + p)a/(1 − p) where a is the nearest-neighbor spacing. The dealloying critical potential is given by6 n+

Ecrit ̅ (p) = Eequil ̅ (p) +

(3)

where γAlloy is the alloy/electrolyte interfacial free energy, fAlloy is the alloy/electrolyte interface stress, ⟨Ω⟩ is the average molar volume of the alloy, Ω̂A is the partial molar volume of A in the alloy, and r is the particle radius. Here we have assumed that both the interface energy and stress are isotropic.17 This potential defines the onset of surface dealloying of an alloy nanoparticle. Generally γAlloy and fAlloy are unknown quantities and while in principle amenable to first-principles based calculations the required number of atoms for solid solution alloys makes such a calculation prohibitive. Nevertheless, by considering alloys dilute in the A component we can take γAlloy and fAlloy equal to that of the pure B/electrolyte interface. In this case for dilute (p ≈ 0.01 or less) AgAu nanoparticles and for r = 1.5 nm, we estimate Eequil(p,r) − E̅equil(p) to be ∼−150 mV. Similar estimates for Eequil(p,r) − E̅ equil(p) in the case of 3 nm diameter Cu−Au and Cu−Pt nanoparticles yield −70 and −145 mV, respectively. In the case of AgAu alloys, owing to the almost identical atomic volumes of these components, the surface stress term is negligible. The situation for the “bulk dealloying” of nanoparticles depends on whether solid-state transport processes can support the imposed rate of dealloying. If so, then dealloying will initiate at Eequil(p,r). Otherwise, dealloying can only occur by a percolation dissolution process and the corresponding critical potential is given by Ecrit(p , r ) = Ecrit ̅ (p) − [γAlloy⟨Ω⟩ + fAlloy (Ω̂A − ⟨Ω⟩)] ×

⎛ 2 ⎞ ⎜ ⎟ ⎝ nFr ⎠

(4)

Because the maximum values of γAlloy and fAlloy are ∼2 and 6 J m−2 respectively,18 alloy particles larger than ∼10 nm in diameter behave as essentially bulk samples so that eqs 2 and 4 yield virtually identical results. Preparation of Nanoparticles and Experimental Protocols. In order to examine the effect of particle size on dealloying we fabricated AgAu nanoparticles in the size range of 2−6 and 20−55 nm. Adenosine triphosphate (ATP) coated alloy nanoparticles of nominal diameter and composition of 4 nm and 23 atom % Au were synthesized by coreduction of the corresponding salts in aqueous solvent following protocols developed by Buttry19 and larger citrate-coated nanoparticles of nominal diameter and composition of 40 nm and 27 atom % Au were synthesized following a procedure reported by Link et al.20 (see Methods). In ancillary experiments conducted on bulk alloys we found that these coatings had no significant effects on either the dealloying critical potential or the resultant morphology (Supporting Information, Figures S1 and S2). The dealloying behavior of these nanoparticles was examined as a function of potential (i.e., fixed voltage dealloying) and the resulting composition, morphology, and particle size were characterized by scanning transmission electron microcopy (STEM), energy dispersive X-ray spectroscopy (EDS), and electron energy loss spectroscopy (EELS) (see Methods). All potentials in the manuscript are referred to the normal hydrogen electrode (NHE). The standard potential corre-

4γB/elec Ω nFξ

⎛ 2 ⎞ ⎜ ⎟ ⎝ nFr ⎠

(2)

where, γB/elec is the B/electrolyte interfacial free energy and Ω is the molar volume of component A. This equation has been verified in the case of AgAu alloys and to a lesser extent for Cu−Pt alloys as well as various parametrizations in Kinetic Monte Carlo simulations of dealloying.6 For a ApB1−p spherical nanoparticle of radius r, the activity of the A-component is a function of both composition and particle size. Accordingly, the equilibrium potential, Eequil(p,r), of A in 2570

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sponding to dissolution of a pure Ag electrode is 0.425 V (NHE). Dealloying of Nominally 4 nm Diameter Ag0.77Au0.23 Particles. The mean particle diameter and composition of the ATP-coated alloy nanoparticles was 3.8 nm and 23 atom % Au (Figure S3, Supporting Information, particle size, and composition distributions). Ancillary experiments were performed in order to determine suitable dealloying times. For example, we found that there were no observable differences in the final composition for sample sets dealloyed at 0.90 V for different periods of time (30 s to 10 min) indicating that 30 s was sufficient for dealloying at this potential. The average composition of ∼100 particles in each sample set dealloyed is plotted versus the dealloying potential in Figure 1. It has been

Figure 2. Aberration-corrected STEM images of nominally 4 nm diameter Ag0.77Au0.23 nanoparticles. (A) BF and (B) HAADF images of ATP-coated particles prior to dealloying. (C) BF and (D) HAADF images of ATP-coated particles dealloyed at 1.4 V. Individual atoms in the vicinity of the particles can be seen in the HAADF image. Figure 1. Composition of ∼4 nm dealloyed Ag0.77Au0.23 particles and estimated number of dealloyed layers versus dealloying potential. The resulting structure is that of an alloy core and a Au shell. For each potential, ∼100 spectra were taken and the standard deviations of the compositions is indicated by the error bars in the figure.

is weak. Figure 3A shows a dealloyed AgAu particle and the inset image is the overlying Ag EELS map and HAADF image. The center pink area is where the Ag EELS signal and HAADF image overlap. Figure 3B shows an EELS line scan on another dealloyed particle demonstrating a core−shell structure with almost no Ag in the shell. This was the only morphology observed in dealloying of the ∼4 nm diameter Ag0.77 Au0.23 particles. Dealloying of Nominally 40 nm Diameter Ag0.73Au0.27 Particles. The initial composition and particle size distribution for the citrate-coated nanoparticles is shown in Figure 4. The mean particle size and composition of these particles was 41 nm and 27 atom % Au. Ancillary experiments were performed in order to determine suitable dealloying times for potentials ranging from 0.50−1.30 V. NHE, such that the integrity of Ccoated Au TEM grid (on which the particles were held) was preserved (see Methods). We found that the integrity of grid was preserved at 1.30 V NHE for hold times up to 30 min. As described in more detail below, dealloying experiments performed at 0.90 V over times ranging from 2 min to 5 h demonstrated that just 2 min of dealloying was sufficient to result in porosity formation in 100% of the particles in these sample sets. At the lower dealloying voltages, sample sets were dealloyed for 5−6 h. Table 1 shows the dealloying times for each of the potentials examined. HAADF images of the dealloyed ∼40 nm diameter Ag0.73Au0.27 citrate-coated nanoparticles is shown in Figure 5. For particles dealloyed at potentials above 0.84 V, almost all the particles showed a porous bicontinuous structure. For the sample set dealloyed at 0.74 V, 35% of ∼200 of particles examined were porous while many particles showed some degree of superficial dealloying and surface roughing as seen in Figure 5. For the sample set dealloyed at 0.64 V, only ∼3% of

reported that Ag is enriched on the surface of AgAu alloys and the atomic percent of Ag on the surface for a Ag77Au23 alloy may be ∼100%.21 Assuming that these nanoparticles originally contained a top surface layer of essentially pure Ag and a second layer Ag composition of 54% (yielding an average of 77 atom % Ag in the first two layers) the resulting number of dealloyed layers in the 4 nm diameter Ag0.77Au0.23 nanoparticles versus the potential was estimated and is shown on the secondary y-axis of Figure 1. The Ag loss for particles dealloyed at 0.10 V is less than expected for complete first layer surface dealloying, which may have resulted from the chemical dissolution of silver oxide. For the particles dealloyed at 0.30 V, the final mean Au composition was ∼37 atom % that corresponds to only first layer dealloying. We observed that to 1.40 V, the Au composition is 65 atom %, which corresponds to dealloying of only the first 3−4 atomic layers of the particle.22 The atomic structure and composition of the particles dealloyed at 1.40 V were examined in the aberration-corrected JEOL ARM200F STEM at 80 kV. No porous particles were observed for dealloying treatments at any potential for the ∼4 nm particles. High-angle annular dark-field (HAADF) images of AgAu particles dealloyed at 1.40 V are compared with the original AuAg particles in Figure 2. The dealloyed particles show a rougher and more diffuse surface structure in that there were more isolated atoms surrounding these particles than the undealloyed particles. The composition distribution in individual AgAu particles dealloyed at 1.40 V was examined by a combination of HAADF imaging and Ag EELS mapping, because the EELS signal for Au 2571

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Figure 3. Composition distribution within 4.8 nm diameter Ag0.77Au0.23 nanoparticles dealloyed at 1.4 V. (A) Aberration corrected HAADF image and corresponding EELS map; blue dark field image intensity, Red, Ag EELS intensity. (B) EELS line scan of another particle within the same sample set through the center of the particle; green, intensity profile of the HAADF image; red, EELS line profile of Ag. This particle shows a core− shell structure and has almost no Ag in the shell.

Figure 4. Characterization results for the citrate-coated AgAu nanoparticles. (A) Particle size distribution with a mean value of 41 nm and (B) composition distribution with a mean value of 27 atom % Au. The red curve is the Gaussian fit to the composition data with a mean value of 27 atom % Au and standard deviation of 3.5 atom % Au.

have been reported and attributed to plastic processes.23 At high dealloying rates, the smallest ligaments in the as dealloyed structure will tend to collapse via plastic processes as these smallest diameter ligaments do not have time to be stabilized by a coarsening process. For the sample dealloyed at 1.30 V, the morphology of most particles as shown in Figure 5L show a smaller pore and ligament size than the particles dealloyed at lower potentials. As this voltage is within the gold oxidation region, the surface diffusivity is considerably lower,24 which results in a smaller final pore and ligament size. Dealloying Potential and Particle Size Effects on Morphology Evolution. We did not observe any porosity on dealloying of the 4 nm diameter particles. At potentials above the equilibrium potential of the particle, Eequil(p, r), dealloying occurs but porosity does not evolve. This behavior can be understood by the following considerations. A characteristic measure in bicontinuous nanostructures is the average ligament diameter, which in general depends on temperature, dealloying rate, and both alloy and electrolyte composition. At ambient temperature, for nanoporous Au this length scale stabilizes within hours. At potentials just above the critical potential, ancillary experiments have shown that this length scale is of order 8 nm, while at higher potentials where Au oxidation occurs this length scale can be as small as 4 nm. Because there

Table 1. Dealloying Times at the Indicated Potential in 1 M HClO4 for the ∼40 nm Citrated-Coated Ag73Au27 Nanoparticles dealloying potential (V vs NHE)

dealloying time

0.54 0.64 0.74 0.84 0.9 0.9 0.9 1.1 1.3

6h 6h 5h 30 min 2 min 30 min 5h 30 min 30 min

∼200 particles were porous. There were no porous particles observed for sample sets dealloyed at 0.54 V. The size distribution and percent of porous particles for sample sets dealloyed at different potentials are shown in Figure 6. There was almost no size change for particles dealloyed at 0.74 V or below. As the dealloying voltage increases, the average dealloyed particle size becomes smaller. This finding is similar to that observed for dealloying of bulk Ag0.75Au0.25 samples above 0.75 V where volume changes as large as 20% 2572

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Figure 5. Representative HAADF images of ∼40 nm diameter Ag0.73Au0.27 nanoparticles dealloyed at the indicated potential, time. (A) Undealloyed particle; (B) 0.54 V, 6 h; (C,D) 0.64 V, 6 h; (E,F) 0.74 V, 6 h; (G) 0.84 V, 30 min; (H) 0.94 V, 2 min; (I) 0.94 V, 30 min; (J) 0.94 V, 6 h; (K); 1.1 V, 30 min; (L) 1.3 V, 15 min.

must be ∼3−4 of such length scales defining a porous structure, a lower bound on the particle size that can support such a morphology is 12−24 nm. Thus, a 4 nm diameter AgAu alloy particle cannot support the evolution of a porous particle via an ambient temperature dealloying process. Because there is no porosity formation in these small particles a critical potential for these particles does not exist. The thermodynamic analysis predicts surface dealloying at a potential just above the equilibrium defined by eq 3 [note that the term (Ω̂A − ⟨Ω⟩) is negligible for a AgAu alloy]. Using γAlloy = 1.23 Jm−2 6, ⟨Ω⟩ = 4.1 × 10−5 m3/mol, p = 0.77 and r = 2 nm the equilibrium potential is ≅ 0.30 V, which closely corresponds to the observed value of surface dealloying in the 4 nm diameter alloy particles. The data shown in Figure 6, for the potential dependent fraction of porous ∼40 nm particles results from the compositional distribution (Figure 4B). Together, these distributions translate via eq 4 (using γAlloy = 1.23 J m−2) to critical potentials ranging from 0.590 V (14 atom % Au) to 0.960 V (35 atom % Au). In Figure 7, these probability

distributions have been parametrically combined to yield an experimental cumulative distribution function for the fraction of dealloyed porous particles as a function of potential. Additionally Figure 7 shows a theoretical cumulative distribution function derived from the Gaussian fit to the composition probability density data of Figure 4B and eq 4. The experimental and the theoretical distribution functions are in good agreement. The relatively small fraction of particles that show no porosity to potentials of 1.30 V is attributed to particles in the size distribution less than ∼25 nm in diameter (Figure 4A). This seems to be the lower cutoff in particle size for porosity evolution in AgAu nanoparticles under the conditions examined. The experimental sample size effects we report here are similar to results of our recent Kinetic Monte Carlo (KMC) simulations.25 This work measured the critical or porosity evolution potential for particle diameters of 4, 6, 10, and 17 nm, and for compositions of 65, 70, and 75 atom % Ag. Similar to what we report herein, particles below a certain size (4 nm) only underwent superficial dealloying and did not form 2573

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Figure 6. Size distributions showing the total fraction of citrate-coated AgAu nanoparticles (blue bars) and those that were porous (red bars) at different dealloying potentials. The mean particle size at each potential is indicated by ⟨ ⟩. (A) 0.54 V; ⟨41 nm⟩. Because there were no porous particles observed following dealloying at this potential, we compare this result to the size distribution prior to dealloying (black bars). (B) 0.64 V; ⟨41 nm⟩. (C) 0.74 V; ⟨40 nm⟩. (D) 0.90 V; ⟨35 nm⟩. (E) 1.10 V; ⟨34 nm⟩. (F) 1.30 V; ⟨33 nm⟩.

formation of hollow core−shell nanoparticles.1,26−28 This is typically envisioned to occur via a Kirkendall process2,26,27 although the details of exactly how this can occur remain obscure. The central issue in understanding this behavior is the following. Near the melting point of the corresponding bulk alloy the equilibrium vacancy concentration is of order 10−3. In a 5 nm diameter particle, this concentration would correspond to an order of a single vacancy. Thus, at ambient temperature it seems likely that there would be few vacancies to support solidstate diffusion. The Gibbs−Thomson melting point depression for a 5 nm Au particle is only ∼20%29 and this would correspond to an increase in the diffusion coefficient from ∼10−32 to ∼10−29 cm2 s−1 at room temperature.30 For this range in mobility, the time required for a solute atom to undergo a net displacement ∼2 nm would be greater than 1015 s. Another possibility is that dealloying of such nanoparticles may be envisioned to “inject” a nonequilibrium concentration of vacancies into the particle that may be available to support

porosity. In our experiments, we did not observe porosity formation on dealloying the ATP-coated particles (2−6.5 nm). The KMC study also reported an increase in critical potential with decreasing particle size and for Ag0.75Au0.25 nanoparticles the increase ranged from 25 mV for the 17 nm diameter particles to 100 mV for 8 nm diameter particles. This was attributed to the increasing fraction of corner and edge sites to total surface sites with decreasing particle size. The higher populations of these sites affect surface mobility and consequently can raise the critical potential for porosity formation. Because in the experiments reported here no porosity was observed for particles smaller than 25 nm, we would not have been able observe this particle size effect if it existed as 25 mV is within the uncertainty of our experimental measurement. In the case of fcc noble-metal alloy nanoparticles, ambient temperature solid-state diffusion has been discussed in some recent literature primarily with respect to dealloying and the 2574

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resulting in the evolution of closed porosity, yielding hollow core−shell nanoparticles. In conclusion, we have shown that during fixed voltage dealloying of AgAu nanoparticles one of two morphologies evolve. For nanoparticles larger in diameter than ∼25 nm and above the critical potential, we observe porous particles and below the critical potential only surface dealloying occurs resulting in core−shell structures. The critical potential for porosity evolution is well explained by the thermodynamics of dealloying. For particles smaller in diameter than ∼25 nm only core−shell structures evolve. Furthermore, under fixed voltage dealloying conditions, we did not observe hollow core−shell morphologies. This result is inconsistent with the operation of a Kirkendall mechanism and implies that voltage cycling is key to the evolution of this type of dealloyed morphology. Methods. Nanoparticle Synthesis. ATP coated ∼4 nm diameter alloy nanoparticles of nominal composition Ag0.75Au0.25 alloy were synthesized by coreduction of the corresponding salts in aqueous solvent following protocols developed by Buttry.19 Chloroauric acid (HAuCl4) and silver nitrate (AgNO3) were used as precursors of the particles. Sodium borohydride (NaBH4) was used as reducing agent and ATP served as a capping agent. The molar ratio of the precursors/capping agent/reducing agent was 1:1:16.7, which yielded a uniform particle size distribution. The molar concentrations of HAuCl4 and AgNO3 were calculated based on the solubility of AgCl (Ksp = 1.8 × 10−10) to avoid AgCl precipitation during synthesis. For Ag75Au25 particle, HAuCl4 (3.87 × 10−7 mol), AgNO3 (1.16 × 10−6 mol) and ATP (1.547 × 10−6 mol) were added to 100 mL nanopure water in a flask under bubbling N2, and the solution was stirred at room temperature for 15 min. Freshly prepared NaBH4 (5 mM, 5 mL) was then quickly added with continued stirring. The solution immediately changed to a light yellow color. Then the solution was continually stirred and bubbled for 3 h to allow for the growth of particles to completion. Because the concentration of the precursors was very low, the particle solution was then concentrated from 100 mL to ∼2 mL on a rotary evaporator. Citrate-coated ∼40 nm diameter alloy nanoparticles of nominal composition Ag0.75Au0.25 were synthesized by a method reported by Link et al.20 Briefly, HAuCl4 (3.87 × 10−7 mol) and AgNO3 (1.16 × 10−6 mol) were added to 100 mL nanopure water and the solution was stirred and boiled. Sodium citrate is added (1 mL, 1% by mass) to the boiling solution for the reduction of the gold and silver ions, which was stirred for 30 min. During stirring and boiling, the solution turned to a light yellow color. Upon reaction completion the solution was left to cool to room temperature. Finally, the particle solution was concentrated from 100 to ∼2 mL on a rotary evaporator. Ancillary dealloying experiments demonstrated that these as synthesized particles were not of sufficient composition homogeneity for our dealloying work, so they were homogenized at 400 °C in forming gas (5% H2/N2) for an hour. Nanoparticle Characterization. A Cu grid with a carbon film (Ted Pella Inc.) was plasma cleaned for 10 s to remove static electricity and then a 20 μL particle solution was dropped on the grid and air-dried. The size and composition distributions of the particles were characterized by STEM, EELS, and EDS see Supporting Information for details) using a JEOL 2010F and a JEOL ARM200F STEM. Typical probe currents used for EDS analysis were 0.5 or 1.0 nA. The imaging

Figure 7. Porous particle cumulative distribution functions as a function of potential. The red data points are derived from the data of Figure 6 and correspond to the cumulative fraction of porous particles measured at each of the potentials. The black curve is derived from the Gaussian fit to the composition data of Figure 4B and eq 4 using a value of γAu/elect equal to 1.23 J m−2.

diffusion via a vacancy mechanism, however, there is no evidence for such a process occurring in the bulk alloy counterpart. Vacancies generated by dealloying at the solid/ electrolyte interface are always incorporated into the solid surface at the solid/electrolyte interface as increased surface area. Even under conditions of solid-state mass transport supported dealloying the vacancies that form at the surface upon dissolution will stay at the evolving surface forming vacancy clusters or negative void-dendrite structures.15 Under conditions of mass transport supported dealloying there would be a Kirkendall effect resulting in a net vacancy flow toward the particle interior, however, the vacancies available to participate in this process would simply correspond to those that preexisted in the nanoparticle prior to dealloying or those that may be produced by dislocation climb processes during dealloying, should the particle contains dislocations. So, how do hollow core−shell nanoparticle structures evolve? First to our knowledge, these structures only evolve in high-rate (greater than 50−100 mV s−1) cyclic voltammetry under conditions for which the positive voltage limit results in the formation of an oxide surface that is then reduced on the negative sweep, that is, we know of no reports, including the work described here, showing the evolution of such a nanoparticle morphology on a single positive voltage sweep or under fixed voltage dealloying conditions. For example, in the case of oxygen reduction catalysts, Pt alloy nanoparticles are used and dealloying is performed in high-rate cyclic voltammetry over prescribed voltage limits between ∼0.05− 1.0 V (NHE). When elemental noble metals such as gold and platinum are electrochemically oxidized and reduced electrochemical scanning tunneling microscopy has shown that the surface undergoes a significant degree of roughening and reordering (alterations of the step and terrace structure) by a surface diffusion process.31 The same situation occurs in the case of noble metal alloys such as AgAu, NiPt, or CuPt. If 1.0 V is above the critical potential of the alloy, the particle will undergo percolation dissolution. Cycling will cause continued surface reordering through repetitive oxidation reduction cycles of the remaining noble metal component that can close-up the bicontinuous structure characteristic of percolation dissolution 2575

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mode for the observation of supported nanoparticles was highangle annular dark-field (HAADF) using an efficient, low-noise scintillator-photomultiplier detector. The initial composition and size of about 100 ATP- and 100 citrate-coated AgAu nanoparticles was characterized with EDS and STEM imaging. Nanoparticle Dealloying Protocols. Prior to examining the dealloying behavior of the particles, ancillary experiments were performed on citrate and ATP coated bulk Ag0.72Au0.28 alloy samples to assess coating effects on dealloying. Supporting Information Figures S1 and S2 compares the polarization and dealloyed morphologies of the coated planar samples and uncoated samples. The potential for porosity formation for all three samples is 1.15 ± 0.05 V (NHE) that is within 50 mV of that observed in our previous studies of noncoated Ag0.72Au0.28 alloy samples. All three samples show uniform nanoporous bicontinuous dealloyed morphologies with gold ligament diameters of 5 ± 2 nm. We conclude that these coatings had no significant effect on both the electrochemical aspects of dealloying as well as the dealloyed morphologies. A 20 μL particle solution of the ATP coated ∼4 nm diameter Ag77Au23 alloy nanoparticles nm was dropped on to a Ni TEM grid (Pacific Grid-Tech) and air-dried. The counter electrode was a PtIr wire. These sample sets were dealloyed in 0.1 M H2SO4 (GFS Chemicals, VERITAS DOUBLE DISTILLED). All potentials quoted are with respect to the NHE. Ni was chosen in order to avoid overlap of EDS peaks of the grid material with that of Ag or Au. We found that the Ni grid had suffcieient corrosion stability over the range of dealloying potentials and time periods examined for these sample sets. The Ni grid with particles was held on a piece of a Au thin film (Au on Mica) by Au wires and dipped in the electrolyte. The particles were dealloyed at fixed potential from 0.1 to 1.4 V. For dealloying at potentials above the open circuit potential (OCP) of the Au substrate (∼0.75 V), the dealloying time was 30 s and after that the sample was quickly washed in nanopure water. For dealloying at potentials lower than the OCP, to avoid OCP dealloying, the electrolyte was diluted gradually with nanopure water by a factor of 105 while the potential was maintained on the sample. Dealloying of the ∼40 nm diameter citrate-coated Ag0.73Au0.27 nanoparticles was performed in 1 M HClO4 (GFS Chemicals, VERITAS DOUBLE DISTILLED). All potentials quoted are with respect to the NHE. The counter electrode was a PtIr wire. We found that a Ni grid was not stable enough for dissolution for these sample sets. Instead, a C-coated Au grid was used and consequently only the Ag EDS signal was used in assessing the compositional changes in the nanoparticles. A 20 μL particle solution was dropped on to a Ccoated Au TEM grid and air-dried. The Au grid with particles was held on a Au thin film (Au on Mica) by Au wires and dipped in the electrolyte. The particles were dealloyed at fixed potential from 0.50 to 1.3 V. Ancillary experiments were performed in order to determine suitable dealloying times at each potential.

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J. E., P. C., and K. S. are grateful to the NSF for financial support under Program Number DMR-1003901 and Number DMR-0855969.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

Discussion of coating effects on dealloying, nanoparticle size and composition distribution and EDS and EELS characterizations. This material is available free of charge via the Internet at http://pubs.acs.org. 2576

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Nano Letters

Letter

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