In Situ Liquid Cell Electron Microscopy of the Solution Growth of Au

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In Situ Liquid Cell Electron Microscopy of the Solution Growth of Au−Pd Core−Shell Nanostructures K. L. Jungjohann,† S. Bliznakov,‡ P. W. Sutter,† E. A. Stach,† and E. A. Sutter*,† †

Center for Functional Nanomaterials and ‡Department of Chemistry, Brookhaven National Laboratory, Upton, New York 11973, United States S Supporting Information *

ABSTRACT: Using in situ liquid cell electron microscopy we investigate Pd growth in dilute aqueous Pd salt solutions containing Au nanoparticle seeds. Au−Pd core−shell nanostructures are formed via deposition of Pd0, generated by the reduction of chloropalladate complexes by radicals, such as hydrated electrons (eaq−) induced by the electron beam in the solution. The size and shape of the Au seeds determine the morphology of the Pd shells, via preferential Pd incorporation in low-coordination sites and avoidance of extended facets. Analysis of the Pd incorporation on Au particles at different distances from a focused electron beam provides a quantitative picture of the growth process and shows that the growth is limited by the diffusion of eaq− in the solution. KEYWORDS: Liquid cell TEM, in situ TEM, gold−palladium core−shell nanoparticles, seeded growth of Pd in solution, diffusion of aqueous electron

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solution.31,32 Au−Pd core−shell nanoparticles have been formed in solution by sonochemical irradiation,26,31 radiolytic reduction,27,33,34 Keggin ions,35 hydrogen gas reduction,36 alcoholic reduction,32 hydroxylamine hydrochloride reduction,37 and hydrothermal treatment.27 Recently, exquisite control over the morphology of Au−Pd core−shell nanocrystals has been achieved by a seed-mediated synthesis approach in which spherical Au seeds were incorporated in Pd salt solution and the morphology of the Pd shell could be controlled by manipulating the kinetics of the reaction.38 Typically, the progress of growth and morphology of the shell structure is analyzed after the reaction is complete, or by quenching the reaction at various time durations followed by TEM imaging.38,39 Studies at the solid−gas/vacuum interface have demonstrated the power of in situ observations for providing a fundamental understanding of microscopic mechanisms,40−43 which in turn can accelerate the discovery of routes toward engineered structures with desired properties. While the deposition of Pd on extended single crystalline Au surfaces has been studied in solution using scanning tunneling microscopy,44−46 the observation of the solution growth of Pd on nanometer-sized Au particles and formation of Au−Pd core−shell nanostructures requires a different approach and can only be accomplished by liquid cell (S)TEM.

lectron microscopy of processes in liquids has emerged as an active field of research that provides real-time observations of the synthesis of nanostructures in solution,1−5 electrochemical deposition,6−8 nanoparticle dynamics,9,10 and assembly11 with subnanometer resolution.12,13 In solution synthesis experiments the electron beam has been used both for imaging and as means of reducing a precursor species in solution, a strategy that has allowed the nucleation and growth of a wide variety of metals and compounds (Cu,7 Pt,1 Pd,3 Ag,4,14 Pt3Fe,5 PbS,2 iron oxyhydroxide,12 etc.) to be followed in real time. An important class of solution growth processes that have not been addressed by real-time (scanning) transmission electron microscopy ((S)TEM) is the deposition of one material on seeds of a different one, which underlies the formation of core−shell nanostructures. Composite heterostructures, such as core−shell nanoparticles, are of interest due to the promise of enhanced and tunable functional properties, for example, increased catalytic activity and selectivity in a number of chemical reactions. Au−Pd bimetallic nanoparticles and core−shell nanostructures, in particular, demonstrate higher catalytic activity compared to monometallic catalysts in a variety of reactions.15−30 Bimetallic nanoparticles can be synthesized using either simultaneous or successive addition of the metal precursors to form alloy or core−shell structures.26 Au−Pd core−shell nanoparticles can be formed in either process due to the higher ionization potential of Au: Aucontaining ionic species are reduced prior to Pd ions, which causes the formation of a Au core followed by Pd deposition on the surface even if both types of ions are initially present in the © XXXX American Chemical Society

Received: April 20, 2013 Revised: May 10, 2013

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Figure 1. Pd growth on seed 5 nm Au nanoparticles. (a) Initial dark-field STEM image of an ensemble of 5 nm Au nanoparticles in 10 μM aqueous PdCl2 solution, electron dose per image: 150 e−/Å2. (a−g) Time lapse series following the growth of Pd on the Au seeds over 92.4 s. (h) Highresolution Z-contrast image of a core−shell Au−Pd nanoparticle formed during the Pd growth. (i) Images series showing the growth on an individual particle marked by a white frame in a (scale bar: 5 nm).

nucleation of Pd nanoparticles (Figure S1), consistent with previous reports.3 Time-lapse dark-field STEM imaging shows an initially homogeneous and uniform solution, in which nanoparticles with brighter contrast nucleate and grow. In darkfield (or Z-contrast) STEM, the intensity is related to the atomic number (Z) and thickness of the specimen; that is, the bright contrast of the observed particles indicates that they are composed of a heavy element. Postgrowth energy-dispersive Xray spectroscopy (EDS) analysis confirms that they consist entirely of Pd. Even a brief exposure of the solution to the electron beam results in the nucleation of nanoparticles; further exposure increases their size, and movement in the liquid leads to the clustering of small Pd nanocrystals. In parts of the liquid cell far outside the field of view (FOV), which have not previously been exposed to electrons, the solution remains free of nanoparticles, but their observation by STEM triggers the same nucleation and growth process as described above. Thus, we conclude that (i) direct electron irradiation causes the reduction of a metal chloro-complex [PdCl4]2− to Pd0, while remote areas of the solution remain unaffected; and (ii) the resulting supersaturation of Pd0 causes Pd nanoparticle nucleation and growth in the solution, which is readily imaged in real time by STEM. The introduction of Au nanoparticles into the solution completely changes the growth characteristics and leads to a rich set of core−shell heterostructures. Figures 1−4 show darkfield STEM images from time-lapse series that follow the Pd growth when Au particles of 5 nm, 15 nm, and 30 nm diameter (initial Au nanoparticle structure shown in Figures 3 and S2), respectively, are present in the solution. Figure 1a shows an image of the initial density and distribution of 5 nm Au

Here we use liquid STEM to investigate in situ the growth of Pd in solutions containing Au nanoparticle seeds. The real-time observations allow us (i) to follow the evolution of the growing material and the formation of heterostructures; (ii) to directly identify different growth processes, that is, growth in the solution, on the seed particle surface, or combinations of the two; (iii) to determine growth rates at different surface sites on nanoparticles; and (iv) to avoid uncontrolled modifications that may accompany the removal from the native growth solution, dispersal on a support, drying, etc., required for imaging in vacuum. High-energy electron beams affect aqueous solutions containing metal ions or charged complexes through the generation of hydrated electrons and other radicals. Hydrated electrons, for instance, are strong reducing agents that react with complex ions of most metals at a diffusion-controlled rate. Their transfer from solution reduces the metal ions to zerovalent metal atoms or atom clusters.47−49 Control over the amount of reducing species produced by the electron beam has been attempted by adjusting imaging parameters, such as the beam current and scan rate, and the solution volume.14 We have used this reduction mechanism to perform real-time electron microscopy of Pd growth in PdCl2 aqueous solution, generally used for electrochemical Pd deposition on different substrates44−46,50,51 and for colloidal growth of Pd.38,52 The prevailing Pd complexes in such solutions are metal−chloro complexes, that is, tetrachloropalladate ions [PdCl4]2‑ which can be easily reduced by capture of two electrons: [PdCl4]2− + 2e− ⇌ Pd + 4Cl−. Electron exposure during STEM imaging of the liquid cell (10 μM PdCl2 precursor solution; imaging parameters: see Methods) results in the homogeneous B

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nanoparticles in the solution. Figures 1b−g (and movie S1) show the evolution of the growth initiated by reduction of the Pd precursor by the electron beam. The analysis of the images yields two findings: (i) with each time step, the number of the nanoparticles in the field of view (FOV, 877 nm × 877 nm, of which 1/4 is shown in Figure 1a−g) either remains the same or decreases [overall from ∼294 (Figure 1a) to ∼202 (Figure 1e)], the decrease being mostly due to displacement/jumps of Au nanoparticles outside the FOV10 and jumps resulting in attachment of particles and (ii) the nanoparticle size increases. The size increase is not uniform across the FOV but is most pronounced near the center and falls off toward the periphery (see below). The constant (or decreasing) number of nanoparticles indicates that homogeneous nucleation and growth of Pd nanoparticles does not occur when Au particles of sufficient density are present in the solution. Instead, Pd is deposited exclusively on the surface of the Au nanoparticles, causing the observed progressive increase in size with elapsed time of electron exposure. Figure 1i follows the deposition of Pd on one of the Au particles (marked in Figure 1a) and shows its growth from 5−6 to 22.8 nm diameter, corresponding to the formation of a Pd shell with ∼8 nm thickness during ∼90 s total exposure to the scanning beam. The Au nanoparticle, clearly distinguished with its brighter Z-contrast at the center of the darker Pd shell, plays the role of a seed particle templating the growth of Pd, which transforms it into a Au−Pd core−shell nanostructure. The morphology of the Au−Pd core−shell structures was investigated ex situ via high-resolution Z-contrast imaging (Figure 1h), which shows a continuous shell with homogeneous contrast encapsulating the 5 nm Au nanoparticle. Lattice planes with spacing ∼0.225 nm, indexed to the (111) lattice spacing of Pd, are clearly resolved throughout the shell, indicating templated epitaxial growth of Pd to form a shell on the surface of the Au core. Figure 2a−d (and Movie S2) follows the growth of Pd on larger (∼15 nm) Au nanoparticles. Similar to the solution with 5 nm Au seeds, the distribution and number of nanoparticles in this group remain unchanged over time. Pd growth retains some of the overall characteristics of the growth on 5 nm Au seeds: the growth is again preferentially associated with the Au nanoparticles without additional homogeneous nucleation and growth of Pd particles in the solution, and the amount of Pd deposited on the Au seeds increases with time. The morphology of the growing Pd is, however, very different. In contrast to the uniform epitaxial Pd shells that form on the 5 nm Au seeds, the growth on the 15 nm Au particles is nonuniform. Already during the initial frame (8.4 s) of electron exposure we see preferential deposition of Pd at ridges and vertices of the Au particles, which then transforms into small, attached Pd filaments. High-resolution Z-contrast images (Figure 2a, inset and S3) of particles taken during the initial stages of Pd growth clearly show the formation of a thin shell over the entire nanoparticle surface and locally thicker Pd at the vertices. With longer imaging and beam exposure the accelerated growth at the vertices continues, more filaments develop at the corners, their thickness increases, the Pd growth becomes dendritic and results in formation of flower-like Au− Pd core−shell nanoparticles (Figures 2c,d and S4) with polycrystalline “petals”. Importantly, Pd deposition on larger planar Au facets is much slower, so that the thin epitaxial (∼1 nm) shell formed originally (Figure S3) is retained. The facets therefore remain largely excluded from the growth process even at high Pd coverage.

Figure 2. Pd growth on 15 nm seed Au nanoparticles. (a) Initial darkfield STEM image of an ensemble of 15 nm Au nanoparticles in 10 μM aqueous PdCl2 solution, electron dose per image: 626 e−/Å2. (a−d) Time lapse series following the growth of Pd on the Au seeds over 33.6 s. The inset in a shows a high-resolution Z-contrast image of one individual particle, comparable to the one selected with the frame in a (scale bar: 5 nm).

Further insight can be obtained by considering mixtures of shape-controlled Au seed particles. Here, we consider two distinct shapes of 15 nm Au particles, multiply twinned icosahedral,53,54 and octahedral (Figure 3a),55,56 as well as

Figure 3. HRTEM images of (a, b) 15 nm and (c) 5 nm Au particles. (d, e) Time lapse series following the growth of Pd on 15 nm Au seeds with different morphology: (d) icosahedral and (e) triangular over 92.4 s (scale bar: 10 nm). The 5 nm Au seeds have homogeneous high curvature and expose a continuum of undercoordinated sites. The corners of the 15 nm particles provide similar curvature and hence mimic the morphology of the 5 nm particle surface. C

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single crystalline triangular (Figure 3b),57 and compare them with the multiply twinned 5 nm particles (Figure 3c and S2). Pd growth on two 15 nm Au particles representative of the two shape families is followed in the Z-contrast image series in Figure 3d (icosahedral) and Figure 3e (triangular). The morphology of Pd growth is controlled by the morphology of the Au particle. In multiply twinned 15 nm particles, which inherently contain more corners and asperities and smaller facets than the triangular particles, the growth involves the formation of a thin shell on the facets and thick dendritic chains at corners, turning the particles into flower-like core−shell nanostructures. The triangular particles consist of much larger facets and relatively isolated corner sites. Here, the growth proceeds only at the three corners, with large denuded zones between them. In both cases, our observations are consistent with Pd incorporation mainly at corner sites and Pd surface diffusion lengths that are small compared to the facet sizes of the 15 nm particles. Enhanced growth at corners is not surprising as the corners offer undercoordinated sites58,59 for facile incorporation of Pd, an effect that is being exploited to achieve shape-controlled nanoparticles.38 Further increasing the size of the Au particles to 30 nm (multiply twinned, icosahedral, Figure S2d) again induced dendritic Pd growth (Figure 4), similar to the triangular 15 nm

Au seed particles: formation of compact Pd nanoparticles in solutions without Au seeds; deposition of a uniformly wetting Pd shell on small (5 nm) spherical Au seed particles; partial wetting followed by local Pd incorporation at corners and vertices at intermediate size (15 nm); and dendritic Pd filaments anchored at low-coordination sites for large (30 nm) Au seeds. A comparison of Pd deposition on Au seeds with different size shows a substantially larger overall volume of Pd growth on small (5 nm) Au particles (Figure 5). On the other

Figure 5. Comparison of Pd growth on 5 and 15 nm Au seeds. (a, d) Starting dark-field STEM images of a 5 nm (a) and a 15 nm (c) Au nanoparticles in 10 μM aqueous PdCl2 solution (same scale). (b, e) The same two particles after Pd deposition (84 s total beam exposure). In b, a second particle moved into the FOV during the growth. (c, f) Schematic illustration of the Pd growth morphology for the two sizes of Au seed nanoparticles.

hand, the maximum extent of deposited Pd (hPd, Figure 5b,e) is comparable in both cases. This implies that the growth rates of Pd in areas with high curvature are similar in both cases, both for the initial Pd/Au and the subsequent Pd/Pd deposition, whereas growth on extended facets is nearly entirely suppressed (Figure 5c,f). Such nonuniform deposition of Pd0 could, in itself, explain the changes in morphology from small, highly curved Au particles to larger Au seeds that are increasingly dominated by facets. Our in situ experiments on larger (30 nm) Au particles clearly identify a second contribution to the increasingly dendritic growth. Due to the selective incorporation of Pd into minority sites (ridges, vertices),59 larger Au nanoparticles are much less effective in removing Pd0 from the surrounding solution. Without an efficient sink and in the presence of an abundant reducing agent (e.g., eaq−), Pd0 supersaturations sufficient for homogeneous nucleation of small Pd clusters can build up in the solution (Figure 4). Instead of continuing to grow (as in solutions without Au seeds), the diffusion of these embryos leads to their incorporation into and sintering with Pd filaments on the surface of the Au nanoparticles (Figure 4b−d; arrows). Our in situ observations thus provide direct evidence for a competition between growth by monomer and cluster incorporation, which would be difficult to identify in conventional ex situ studies. The use of Au seed particles in a dilute Pd salt solution provides a system suitable for studying the role of the excitations by the electron beam. A well-defined distribution of seed particles facilitates the analysis of the growth rate, and the absence of homogeneous nucleation in the solution implies a simple growth mechanism, in which the reduced Pd0 is captured by the Au seeds. However, in experiments with a scanned electron beam (as discussed so far), the two primary

Figure 4. Pd growth on 30 nm seed Au nanoparticles. (a) Starting dark-field STEM image of a 30 nm Au nanoparticles in 50 μM aqueous PdCl2 solution, electron dose per image: 322 e−/Å2. (a−k) Time lapse series following the growth of Pd on the Au seeds over 84 s. The arrows in b−d indicate the homogeneous nucleation of Pd clusters in solution and their consecutive attachment.

Au seeds. A series of Z-contrast images (Figure 4a−k and Movie S3) follows the growth of Pd on the surface of a typical 30 nm Au particle. On these larger Au seeds, whose surface consists of well-defined extended Au facets, we see an even more pronounced effect of facet against corner growth of Pd. The growth now appears entirely limited to the vertices, whereas extended surface facets are completely avoided throughout the entire sequence. In contrast to the growth on 5 and 15 nm Au seeds, we also observe significant homogeneous nucleation of Pd clusters in the solution, followed by movement and attachment to the corner sites of the particles. One such event of cluster attachment into existing chains is indicated by arrows in Figure 4b−d. High-resolution Z-contrast images (Figure S5) of the interface between the Au and the Pd shows an agglomeration of crystalline Pd clusters attached at the corners of the Au particle, while a shell is not detectable on the facet surface. Further growth proceeds via expansion of the Pd chains probably mostly via incorporation of additional Pd clusters from the solution. Our in situ observations of the formation of Au−Pd core− shell structures from PdCl2 solution show a pronounced dependence of the growth mode on the size and shape of the D

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Figure 6. Electron beam effect on Pd growth on Au nanoparticles. (a−c) Pd growth on 15 nm Au seed particles, induced by cycles of local focused electron beam exposure (60 s exposure, 369 pA beam current, 0.2 nm probe size) at the center of the FOV (scale bar: 0.5 μm). (a) Au nanoparticles before first excitation; (b) after one exposure; (c) after five exposures. (d) Starting configuration: equally sized Au seed particles in the solution. (e) Electron exposure: Hydrated electrons diffusing from the excitation spot (concentration c(r)) reduce [PdCl4]2−, Pd0 is captured by the growing particles (surface area A(r)). (f) Simulation of the growth of the particles, assuming a growth rate proportional to the product of c(r) and A(r). In steady state: particle volume V(r) ∼ r−3. (g) Experimentally determined dependence of the particle volume as a function of the distance from the excitation: initial state (green); after 1 exposure (60 s, blue); after 10 excitations (60 s each, red).

beam effects(i) imaging by dark-field STEM, in which the moving electron beam continuously creates reducing species (hydrated electrons and other anionic radicals produced by the beam) at different positions along the scan; and (ii) stationary generation of reducing species for Pd growth from solution during the dwell time between scans (see Methods for details)are difficult to separate. We have therefore used the local excitation by a stationary, focused electron beam combined with STEM imaging with minimal additional electron dose to further examine the mechanism of solution growth of Pd on Au nanoparticles (Figure 6a−c). The growth rate clearly depends on the distance from the excitation, in a way that is consistent with the high-energy electrons producing radicals and a high concentration of hydrated electrons (eaq−). Previous experiments by pulse radiolysis and related methods have shown substantial diffusion coefficients of eaq− (D ∼ 4.9 × 10−5 cm2 s−1).48 The steady-state concentration profile as the eaq− diffuse outward from the excitation is given by the diffusion equation in spherical symmetry, ∂c ∂ ⎛ ∂c ⎞ = 0 = D∇2 c = D ⎜r 2 ⎟ ∂t ∂r ⎝ ∂r ⎠

c(r ) =

ca + cb r

(2)

where ca,b denote constants. The concentration of hydrated electrons thus falls off as c(r) ∼ r−1 with increasing distance from the excitation. We assume that the growth of the Pd shell on Au nanoparticle seeds is limited by the rate at which eaq− can reduce the [PdCl4]2− precursor in the solution (or on the surface of the particles)46 to generate Pd0. Thus, the growth rate scales with both the concentration of hydrated electrons and the surface area (A) of the nanoparticles: ∂V ∼ c(r ) × A(r ) ∂t

(3)

Starting with uniformly sized Au seed particles (for which A(r) = const., Figure 6d), the particles closer to the excitation spot experience faster growth due to the higher concentration of eaq−. The larger size (i.e., capture area, A) of particles near the excitation then further accelerates their growth rate compared to more distant particles. Ultimately, steady-state conditions are reached (Figure 6e), with an expected distribution of particle volumes as a function of distance from the excitation V(r) ∼ r−3 (Figure 6f). Figure 6g shows the analysis of our experiments, in which a STEM probe (focused electron beam) was kept stationary to

(1)

which is solved by E

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expose a 50 μM PdCl2 solution containing 15 nm Au nanoparticles. The experimental results, obtained by alternating 60 s local excitation and STEM imaging, are consistent with the scenario put forth above. Already after the first excitation, the initially uniformly sized Au seed particles show significant Pd growth at a rate that clearly depends on the distance r from the excitation spot. After the first excitation, the particle sizes obey a power law, V(r) ∼ r−2.3. With additional excitation/imaging sequences the exponent changes continually. After the tenth increment, steady-state conditions are reached and V(r) ∼ r−(3.0±0.1). Additional electron beam excitation further increases the volume of the individual core−shell particles, but the 1/r3 dependence is maintained. Equation 1 assumes the outward diffusion of eaq− from a nanoscale electron excitation region but neglects possible losses (e.g., via 2H2O + 2e− → H2 + 2HO−) due to a finite lifetime of the hydrated electrons. Our analysis shows a very good match with experiment to at least 0.5 μm distance from the excitation, suggesting a (3D) diffusion length S = (6Dt)1/2 > 0.5 μm of eaq−, that is, a lifetime t of ∼1 μs consistent with time-resolved optical measurements.60−62 In conclusion, real-time observations of electron beam induced templated solution growth of Pd show a strong dependence on the size and surface morphology of the Au seed nanoparticles. For 5 nm Au nanoparticles, continuous, uniform shells can be deposited to large thicknesses by Pd monomer incorporation from the solution. In larger (15 nm, 30 nm) particles we find accelerated growth at corners and asperities, while deposition in facets is suppressed. This nonuniform growth causes a strongly varying thickness of the Pd shell, and the slower depletion of Pd ions in the solution promotes competing growth processes, such as the homogeneous nucleation of small Pd clusters in solution and incorporation into the Au seeds. Our real-time observations rationalize the pathways toward different morphologies of Au−Pd core−shell nanoparticles: complete epitaxial Pd shells, partial Pd coverage, flower-like Pd morphologies, and growth of dendritic chains. While we have focused on the effect of different sizes and shapes of the Au seed particles and the formation of reducing species by beam-excitation of the solution, future work could address the possibility of changing the growth kinetics by tuning the concentration of the Pd precursor, the use of capping agents, and so forth, that is, avenues that are actively pursued for controlling the morphology and shape of colloidal core−shell nanoparticles.38,52,63 Materials and Methods. Solutions of palladium chloride (PdCl2, Fisher Scientific) were made by dissolving 88.65 mg of the salt in 300 μL HCl, which was then diluted with water to concentration between 0.01 mM and 0.1 mM. For seeded growth, Au nanoparticles (British Biocell International) with 5 nm, 15 nm, and 30 nm sizes and different densities (5.0 × 1013, 1.4 × 1012, and 2.0 × 1011 particles/mL, respectively) were introduced in the solutions. Observations of Pd growth in the liquid cell were carried out at different PdCl2 concentrations above 5 μM PdCl2, the threshold for observable Pd growth under the observation conditions used here (see below). Solutions with two different concentrations were used: 10 μM PdCl2 for unseeded growth and growth in the presence of 5 nm Au nanoparticles and 50 μM PdCl2 for growth on 15 and 30 nm Au nanoparticles. The liquid experiments were carried out in a Hummingbird Scientific holder with liquid cells consisting of two 30 nm thick SiN membrane windows with 50 × 50 μm window area. The spacing between the windows was controlled

using 90 nm polystyrene beads. Bowing of the thin membrane allows a continuous range of liquid thicknesses to be established within the same cell (Figure S6). (S)TEM imaging was performed in a FEI Titan 80-300 environmental Cscorrected (for TEM mode) microscope operated at 300 kV. Low-loss electron energy loss spectra (EELS) were acquired on a Gatan Enfina spectrometer with an entrance aperture semiangle of 3.5 mrad. The local fluid thickness was calculated using low-loss EELS according to ref 13, as shown in Figure S6. STEM imaging was performed with ∼2 Å beam size and 0.36 nA beam current, measured in vacuum before introduction of the liquid cell. Typical conditions for the acquisition of growth series were electron dose per image of between 150 and 630 e/ Å2 and ∼200−300 nm fluid path length. Figure S7 shows the dependence of Pd growth on the electron dose at constant fluid thicknesses. Time-lapse STEM images of Pd solution growth were recorded with a frame interval of 8.4 ± 0.2 s and 5 μs dwell time. Between frames of the serial acquisition, the beam was left stationary at the center of the frame during the process time of the camera (∼3.16 s). Suitable serial acquisition imaging conditions were achieved at an appropriate distance from the area of interest, followed by blanking of the beam and translation to the area of interest, resulting in its electron exposure only upon the first frame. All images in the manuscript are unprocessed, as recorded. The background intensity gradient in the images from the top left to the bottom right is probably related to the variation in fluid thickness (see Figure S6). The images in Figure 4 are shown in false color. High-resolution STEM images of the Au−Pd core−shell structures were acquired ex situ on a Hitachi HD2700C 200 kV cold-field emission Cs-corrected STEM.

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ASSOCIATED CONTENT

* Supporting Information S

Supporting figures of Pd nucleation and growth, HR-TEM images of the seed Au nanoparticles, high-resolution STEM images showing the initial formation of a thin epitaxial Pd shell, the formation of flower-like Au−Pd core−shell nanostructures, and the absence of a complete Pd shell and preferential cluster attachement, fluid thickness variation in the liquid cell, dependence of Pd growth rate on 15 nm Au nanoparticles; and Movies S1−S3 showing the sequence of dark-field STEM images following Pd growth on seed 5, 15, and 30 nm Au nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors would like to thank Dr. S. Lymar for helpful discussions. This research has been carried out at the Center for Functional Nanomaterials, Brookhaven National Laboratory, which is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, under Contract No. DE-AC0298CH10886. F

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dx.doi.org/10.1021/nl4014277 | Nano Lett. XXXX, XXX, XXX−XXX