Formation and Analysis of CoreâShell Fine ... - Technische Chemie

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Formation and Analysis of Core−Shell Fine Structures in Pt Bimetallic Nanoparticle Fuel Cell Electrocatalysts Marc Heggen,*,† Mehtap Oezaslan,‡ Lothar Houben,† and Peter Strasser‡ †

Ernst Ruska-Center for Microscopy and Spectroscopy with Electrons, Forschungszentrum Juelich GmbH, 52425 Juelich, Germany The Electrochemical Energy, Catalysis, and Materials Science Laboratory, Department of Chemistry, Technical University Berlin, 10623 Berlin, Germany

‡

S Supporting Information *

ABSTRACT: An Ångstrom-scale structural and compositional investigation of a dealloyed Pt−Co core−shell nanoparticle fuel cell catalyst with characteristic diameter of 10−15 nm in an early stage of its life cycle reveals unusual selforganized compositional subsurface fine structure, that is, subsequent shells of Co depletion and enrichment. The origin of the unusual structure is rationalized by interplay of Co dissolution, Pt surface diffusion, and an inverse Kirkendall effect. A detailed picture about the chemical composition of the surface and subsurface provides a fundamental insight into the catalytically active structure of bimetallic electrocatalysts.

1. INTRODUCTION Bimetallic nanoparticles play an important role in many areas of science and technology, such as magnetic recording, biomedicine, sensing, or, in particular, chemical catalysis.1−9 The atomic neighborhood of two dissimilar metals modifies their monometallic behavior and can give rise to desired new characteristics. For instance, in heterogeneous gas phase catalysis or electrocatalysis, bimetallic alloys often exhibit significantly improved catalytic activities due to short-range electronic charge transfer between dissimilar metal atoms (ligand effects10−13) or due to altered lattice constants (geometric effects10,14). This is why the knowledge of the compositional structure is of utmost importance to understand catalyst reactivity. A prominent example where geometric strain effects control surface reactivity of bimetallic particles is dealloyed Pt alloy nanoparticle electrocatalysts,1,15,16 which show high catalytic activity for the electroreduction of molecular oxygen (oxygen reduction reaction, ORR, O2 + 4H+ + 4e− → 2H2O) at the cathode of hydrogen fuel cells.17−22 Dealloyed nanoporous noble metal catalysts form during the selective electrochemical surface dissolution of a less noble metal M from a homogeneous noble metal-poor alloy precursor.23−29 The rapid dissolution of M from the surface causes the formation of a Pt-rich, 3−5 atomic layer shell, surrounding a M-rich particle core (single core−shell structure).30 Lattice mismatches between M-rich cores and Pt shells modify the electronic structure of Pt and ultimately account for significantly altered catalytic activity.14 Recent experiments1,31 with dealloyed Pt nanoparticles indeed confirmed that the compressive surface strain weakens the chemisorption of oxygen, which improves the catalytic rate of the ORR. This insight underscores how the © 2012 American Chemical Society

detailed compositional and structural arrangement of Pt and M atoms within dealloyed core−shell particles critically controls their activity. Very recently, the dependence of morphology and intraparticle composition on the initial particle size of dealloyed Pt− Co and Pt−Cu alloy nanoparticle catalysts was examined.32 This study indicates the existence of a characteristic diameter of about 10−15 nm, which discriminates distinct morphological regimes. Smaller particles usually show a simple core−shell structure. However, above this characteristic diameter, larger particles show clearly an irregular multiple core structure. Yet, the morphological fine structure of these highly complex dealloyed bimetallic particles and their process of formation have not been studied in detail to date and are hence poorly understood. In the present study, we performed an Ångstrom-scale structural and compositional investigation of a dealloyed Pt− Co core−shell nanoparticle fuel cell catalyst with a characteristic diameter of 10−15 nm using aberration-corrected high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The nanoparticles reveal a complex fine structure, that is, subsequent shells of Co depletion and enrichment. The formation of this complex structure is rationalized by interplay of Co dissolution, Pt surface diffusion, and an inverse Kirkendall effect. Received: May 2, 2012 Revised: August 10, 2012 Published: August 13, 2012 19073

dx.doi.org/10.1021/jp306426a | J. Phys. Chem. C 2012, 116, 19073−19083

The Journal of Physical Chemistry C

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Figure 1. (a) Differences in CV profiles for Pt−Co catalyst before (solid blue line) and after (solid black line) electrochemical activation by dealloying in 0.1 M HClO4 as compared with that for pure Pt/HSAC (dotted lines). (b) Polarization curves for dealloyed Pt−Co catalyst, pure Pt catalyst, and heat-treated HT-Pt catalyst.

2. EXPERIMENTAL SECTION 2.1. Bimetallic Alloy Nanoparticle Synthesis. The Pt− Co alloy electrocatalyst was synthesized via liquid metal impregnation, freeze drying, and subsequent reductive annealing.16,33 Commercial 28.2 wt % Pt nanoparticles supported on high surface area carbon (HSAC) (part no. TEC10E30E, supplied by TKK, Japan) were mixed with an appropriate stoichiometric amount of solid cobalt precursor salt Co(NO3)2·6H2O (Alfa Aesar, #010694). The impregnated and freeze-dried powder was annealed in a tube furnace (supplied by Carbolite, Germany) at 800 °C for 7 h in 4 vol. % H2/96 vol. % Ar flow (quality of 5.0, supplied by AirLiquid) with 100 mL min−1. The electrochemical experiments were performed using a rotating disk electrode (RDE) technique with a homemade three compartment electrochemical cell, electrode rotator (supplied by PINE Instruments, United States), and potentiostat (supplied by BioLogic Inc., France). The catalyst suspension was prepared by mixing of alloy catalyst powder, water, 2-propanol, and Nafion solution. An aliquot from the sonicated catalyst suspension was pipetted onto a polished and cleaned surface of a commercial glassy carbon (GC) of a RDE (supplied by PINE Instruments) and dried at 60 °C for 10 min in air, resulting in a thin, homogeneous, catalytic film as the working electrode. Cyclic voltammogram (CV) measurements were conducted in a potential range between 0.06 and 1.00 V/ RHE in deaerated 0.1 M HClO4 electrolyte at room temperature. The electrochemical dealloying process consisted of three scans with 100 mV s−1 followed by 200 scans with 500 mV s−1 and finally three scans with 100 mV s−1. After the catalyst was electrochemically dealloyed, the electrolyte was purged with oxygen for 15 min. The polarization curves were recorded by sweeping the potential anodically from 0.06 V/ RHE to open circuit potential (around 1.1 V/RHE) with a scan rate of 5 mV s−1 and 1600 rpm (rotations per minute). All activities were corrected with respect to the mass transport diffusion-limiting current in a voltage range between 0.2 and 0.5 V/RHE and compared at 0.9 V/RHE. 2.2. High-Resolution Electron Microscopy and EELS. Dealloyed Pt−Co nanoparticles were prepared for highresolution STEM in the following way: Nanoparticles were immersed in water−2-propanol solution and dispersed ultrasonically. The procedure of washing and centrifugation of the sample was repeated several times to reduce particle agglomeration and to dissolve the Nafion polymer, which was

used in an earlier stage of the electrode preparation. Finally, the suspension was deposited onto a holey carbon film-coated aluminum grid and dried on air. Scanning transmission electron micrographs were acquired in a probe-corrected FEI TITAN 80−300 employing a HAADF detector. “Z contrast” conditions were achieved using a probe semiangle of 25 mrad and a detector inner collection angle of 70 mrad. Under these conditions, the image intensity was about proportional to Z1.6 to Z1.8 for sufficiently thin objects.34 EELS spectra were recorded with a Gatan image filter (GIF) Tridiem 866ERS with a collection angle of 18 mrad. The energy resolution of the system under these conditions was about 1.2 eV measured as the full width at half-maximum of the zero loss peak. Compositional profiles were taken by collecting 20 individual EELS spectra along a line across the electrochemically treated bimetallic particles. The curves were normalized with respect to the elemental scattering crosssections so that their intensity can be directly related to the number of atoms crossed by the beam or, equivalently, to the projected thickness of the respective element.

3. RESULTS AND DISCUSSION 3.1. Electrochemical Treatment and ORR Activities of Dealloyed Pt−Co Alloy Nanoparticles. CV experiments were performed to monitor the features of the electrochemical dissolution of the less noble metal cobalt and to generate the dealloyed Pt−Co nanoparticle electrocatalyst with improved activity for the ORR. Figure 1a shows the differences in CV profiles of Pt−Co catalyst before (solid blue line) and after (black solid lines) the electrochemical activation process by dealloying in perchloric acid. After dealloying, the CV profile for Pt−Co clearly resembles the voltammetric characteristics of pure Pt nanoparticles (denoted with dots in Figure 1a), corresponding to the underpotentially hydrogen ad/desorption regime (0.06−0.40 V/RHE), the capacitive double layer regime (0.40−0.65 V/ RHE), and the formation of Pt(hydr)oxide species (0.7−1.0 V/ RHE). Only the first CV profile (solid blue line) exhibits an additional broad anodic peak between 0.45 and 0.70 V/RHE, indicating the dissolution of the underpotentially deposited cobalt from the surface of Pt−Co alloy nanoparticles. Furthermore, the CV profile of dealloyed Pt−Co signifies a clear onset shift of the anodic potential at 0.8−0.9 V/RHE for the formation of Pt(hydr)oxide as compared to that for pure Pt. 19074



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The shift to higher potentials indicates an improvement of activity due to the reduced formation of oxygenated surface species on the dealloyed Pt-enriched surface. The increase of the activity was also corroborated by measuring the polarization curves for dealloyed Pt−Co catalyst. Pure Pt/HSAC catalyst and heat-treated Pt/HSAC (referred to as HT-Pt), which was prepared by thermal annealing of Pt/HSAC with the same temperature−time protocol for Pt−Co,20 were taken for the comparison of the ORR activity. All polarization curves show similar behavior. Only the mixed kinetic-diffusion control region for dealloyed Pt−Co moved to more positive potentials as compared to those for pure Pt and HT-Pt catalysts, highlighting a strong improvement of ORR activity. The activity benefits of dealloyed Pt−Co catalyst for ORR are 3−4fold higher than those for pure Pt and 4-fold higher than those for heat-treated HT-Pt catalyst at comparable particle sizes (see Table 1). The improved ORR activity for dealloyed Pt−Co Table 1. Comparison of Mean Particle Size, Pt-Based Mass (jmass), and Pt-Based Surface Area-Specific (jspecific) ORR Activities for the Dealloyed Pt−Co Catalyst, Commercial Pt Catalyst, and Heat-Treated HT-Pt Catalyst catalyst

mean particle size (nm)a

jmass (A mgPt−1) at 0.9 V/RHE

jspecific (μA cmPt−1) at 0.9 V/RHE

Pt−Co HT-Pt Pt

4.0 ± 1.0 3.6 ± 0.8 2.3 ± 0.7

0.38 ± 0.05 0.08 ± 0.01 0.14 ± 0.01

804 ± 146 215 ± 12 179 ± 4

a

The initial mean particle size was determined from TEM images by counting of more than 400 particles.

catalyst is related to the structural and compositional changes by electrochemical dealloying, which will be discussed in the following chapters. 3.2. Structural Investigation of a Dealloyed Pt−Co Alloy Nanoparticle Ensemble. Figure 2a shows an HAADF electron micrograph of dealloyed Pt−Co nanoparticles of various sizes. It can be noted that particles larger than about 10 nm show complex contrast features. Because of the “Z contrast” conditions, Pt (Z = 78) shows a bright contrast, whereas Corich regions (Z = 27) display a dark contrast. The size distribution of the dealloyed particles was measured on more than 1200 individual particles analyzing HAADF micrographs. The histogram in Figure 2b represents the particle size distribution. The particles are polydisperse, and the fraction of the particles larger than 10 nm is about 8.5% of the total number of particles but represents about 95 wt %. 3.3. Detailed Microstructural Investigation of a Dealloyed Pt−Co Core−Shell Particle. We selected a number of particles in the 10−15 nm size range and performed a detailed HAADF STEM and EELS analysis. Figure 3a shows a high-resolution aberration-corrected HAADF electron micrograph of a representative dealloyed Pt−Co nanoparticle. Inside the particle, a slightly off-center oval region of about 6.5 nm × 7.5 nm has darker contrast than portions of the shell resembling the two-layer single core−shell arrangement as previously reported in dealloyed Pt bimetallic nanoparticles with 2−8 nm1 and several dark contrast features in the shell region, which indicate a more complex structure (see red areas in Figure 3b). To correlate the contrast features toward chemical information, we performed EELS profiles by scanning across the nanoparticle. The integrated EELS intensities are presented in Figure 3c,d for Pt (black) and Co (red), respectively (see the

Figure 2. (a) High-resolution HAADF micrograph of dealloyed Pt− Co particle ensemble providing “Z contrast” conditions. Particles >10 nm show complex contrast features (white arrows). Smaller particles have simple contrast features (gray arrows). (b) Histogram showing the size distribution of dealloyed particles.

Supporting Information). First, a vertical EELS line scan through the particle of Figure 3a is presented in Figure 3c. The direction of the scan is illustrated in Figure 3b (arrow “c”). It shows a steep increase of the Pt signal at the edges (0−4 and 11.5−14 nm) and a distinctly defined minimum at the center of the dealloyed particle at 8 nm. In between, two maxima in the Pt signal are discernible at 4 and 11.5 nm. Meanwhile, the Co signal peak in the center at 8 nm drops off toward the edges of the particle. A smaller local maximum was observed at 2.5 nm, clearly corresponding to a dark spot at the bottom of the dealloyed nanoparticle (Figure 3a and red area in Figure 3b). Such small Co-rich “satellite cores” within the Pt-rich shell are a frequently recurring structural motif. The EELS spectrum profiles in Figure 3d show an asymmetric Pt signal with maximum concentration at 4 and 10 nm; the corresponding Co signals show a Co-rich core at around 8 nm and a satellite core at 3 nm, corresponding to the dark feature at the upper left of the micrograph. To strengthen our conclusions on the compositional structure, we performed theoretical thickness-projection modeling of different Pt−Co core−shell model architectures 19075



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Figure 3. (a) Ångstrom-scale resolution HAADF micrograph providing “Z contrast” conditions of a dealloyed Pt−Co bimetallic nanoparticle showing a complex core−shell structure. At the upper part of the nanoparticle, individual atomic columns are resolved. It is oriented along a (1̅10) zone axis of the face-centered cubic structure. (b) As a guide to the eye, dark contrast features that represent Co-enriched areas (red areas) are highlighted (see the text for details). Dashed lines represent neighboring particles that are not in focus. Black arrows indicate the trace and direction of EELS line scans. (c and d) EELS intensity profiles across the particle for Pt (black squares) and Co (red circles), respectively. The EELS intensity profile in panel c represents the scan along arrow “c”, and panel d represents the scan along arrow “d”, respectively. The EELS profiles are normalized with the elemental scattering factor and hence represent thickness-projected compositions.

Figure 4. Calculated EELS line profiles considering the projected thickness of Pt and Co for different core−shell structure arrangements. (a) A symmetric line profile for a simple Pt−Co core−shell particle of 14 nm with a 7 nm core diameter where the core is centered along the scan direction. (b) An asymmetric Pt line profile of a simple Pt−Co core−shell particle of 14 nm, resulting in the shift of the core of 7 nm diameter from the particle center along the scan direction. (c) Line profile for a core−shell particle with an additional Co-rich nucleus with 1.5 nm diameter.

(Figure 4). Considering the basic principle that for thin specimens the EELS signal is directly proportional to the

amount of the respective element, they were determined by simple calculation of the projected thickness of Pt and Co for 19076



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Figure 5. High-resolution HAADF micrograph (a), schematic representation (b), and EELS intensity profile (c) of a dealloyed Pt−Co particle showing a complex compositional core−shell structure. The red and white region in panel b represents dark and bright contrast features in the HAADF micrograph. These contrast features correspond to the high Co composition in the core (1) and a subsequent depletion zone around the core (2). Red arrows highlight shoulder/plateau features of the Co signal (numbers see text).

show perfect compositional homogeneity without any local metal-enriched regions (Figure S3 in the Supporting Information). However, the simple model with a sharp compositional transition between the core and the shell, which was applied in Figure 4, is only a first approximation to the observed particle in Figure 3. To the outside of the particle, the Co EELS signals (Figure 3c and 3d) show a much smoother decrease as well as the presence of a small amount of Co in the shell. In addition, the measured Pt peaks are not as distinct as those in the calculated curves. These observations point toward a smooth continuous compositional transition rather than an abrupt one between core and shell. Interestingly, we also found that a number of dealloyed particles exhibit complex “shoulder”-like features in the Co EELS profiles. These features and their implication on the compositional fine structure of the dealloyed Pt−Co particles are investigated next. 3.4. Compositional Fine Structure Investigation. Figures 5a, 6a, 7a, and 9 show representative examples of dealloyed self-organized Pt−Co core−shell nanoparticles with a complex compositional fine structure. Figure 5a shows a HAADF micrograph of an ellipsoid core−shell particle. Around the dark core, a thin bright contrast region is visible (this contrast region is indicated by a white area in Figure 5b). An EELS line scan was performed across the particle (the direction of the arrow is seen in Figure 5b). The Co signal shows a center

different particle configurations. Figure 4a−c shows the line profiles of three Pt−Co core−shell particles with a 14 nm diameter and a 7 nm core diameter. In Figure 4a, the core is centered along the scan direction, which leads to a symmetric line profile featuring a pronounced valley of the Pt profile at the core region. In Figure 4b, the core is displaced from the center of the particle by 1 nm along the scan direction, which yields a nonsymmetric Pt line profile. This feature is experimentally observed in Figure 3d. In Figure 4c, besides the centered 7 nm diameter core, an additional Co-rich off-center core with a 1.5 nm diameter is located in the particle shell. The experimental Pt and Co EELS profiles (Figure 3c,d) are largely consistent with the calculated EELS intensities in Figure 4. A comparison of the calculated, the experimental EELS data, and the respective HAADF micrograph (Figure 3a) confirms that the circular dark shading at the particle center, as well as smaller additional dark spots at the bottom and upper left of the particle, represent Co-rich cores. As the dark spots in the HAADF micrograph are always associated with a distinct local increase in the Co concentration by EELS analysis, we found no evidence for voids, hollow spaces, or pores. Similar complex morphological features were also observed in dealloyed Pt−Cu nanoparticles (Figure S2 in the Supporting Information). These features are clearly the result of the electrochemical treatment, because initial nondealloyed Pt−Co and Pt−Cu nanoparticles 19077



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Figure 6. High-resolution HAADF micrograph (a), schematic representation (b), and EELS intensity profile (c) of a dealloyed Pt−Co particle. As in Figure 5, a bright ringlike contrast feature is around the core (white area in b), indicating a local Co depletion zone (2) and thus a Pt-enriched zone.

3.5. Calculation of a Multilayer Model Structure. The unusual radial variation of EELS and HAADF signals evidence a compositional profile across the nanoparticle that is inconsistent with a simple two-layer core−shell structure and hence is more complicated. To create a compositional structure model that describes the experimentally observed features, we computed EELS line profiles and a HAADF image (Figure 8). For this purpose, we used a multishell radial symmetric compositional model (Figure 8c) with a nonmonotonic elemental profile. Figure 8a shows a radial compositional profile, and Figure 8c depicts a central slice through a concentric spherical particle with the same radial profile. Using this compositional model, we obtain the calculated EELS profile shown in Figure 8b. Mathematically, the connection between the actual Co composition profile (Figure 8a) and the EELS intensity (Figure 8b) is given by the Abel transform.35 The calculated EELS profile exemplifies “shoulder” features (3) like they are observed in experiment, for example, in Figure 6. It is noteworthy to mention that the EELS profiles tend to look smoothed as compared with compositional profiles because the Abel transform is a line integral averaging over all shells along a line of sight. Although we chose to calculate an EELS curve with only slight “shoulder” features in the Co profile, the respective Co composition shows a distinct depletion [position (2) in Figure 8b] and subsequent enrichment zone (3). The Co composition shows a pronounced drop from the core to 30% at (2) and a

peak at the location of the core (1) as well as pronounced side peaks or “shoulders” (3) (Figure 5b and c). At the position (2), the Co EELS scan shows distinct minima, which indicate a local minimum of the Co concentration (see the Supporting Information). It is evidence that the local depletion of Co goes hand in hand with an enrichment of Pt in the region (2), which causes stronger high-angle scattering and hence a bright area in the HAADF STEM image (Figure 5a,b). The core−shell particle in Figure 6a also shows similar “shoulder” features in the Co EELS scan. The HAADF micrograph reveals that the dark shaded core region is surrounded by a bright region. Shown in Figure 6c, an EELS scan across the particle exhibits a central Co peak (1) and pronounced side Co peaks (3) (indicated by red arrows). Again, the location of the Co minimum and Pt maximum (2) corresponds to the bright region in the HAADF image. Figure 7a shows a particle with an elongated dark-shaded core region surrounded by a pronounced bright contrast region. We performed two EELS line scans across the particle, which are displayed in Figure 7c,d. Scanned along arrow “c”, the Co EELS signal (Figure 7c) shows a sharp peak between about 4 and 7 nm emanating from a Co plateau with “shoulders” between the positions 3 and 9 nm (denoted with red arrows). Scanned along arrow “d”, a broad central Co peak (Figure 7d) is visible with narrow shoulders, again between 3 and 9 nm (denoted with red arrows). 19078



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Figure 7. High-resolution HAADF micrograph (a), schematic representation (b), and EELS intensity profiles (c and d) of a dealloyed Pt−Co particle. The EELS intensity profile in panels c and d represent scans along the arrows “c” and “d”, respectively.

subsequent increase of Co to 45% at the core/shell interface (3). Stronger EELS features, like experimentally observed in Figures 5 and 6 lead to even more pronounced depletion and enrichment zones. The Co depletion zone is necessarily connected to a local enrichment of Pt [position (2) in Figure 8b], which causes a high “Z contrast” and creates a bright ring around the core in the simulated HAADF image [position (2) in Figure 8d]. Hence, the presence of a distinct bright ring around the core in the HAADF image of a radial-symmetric Pt−Co nanoparticle indicates the presence of a local Co depletion zone, that is, an additional shell with lower Co content than the neighboring shells. This bright ring around the core is evident in the experimentally observed HAADF images in Figures 5a, 6a, and 7a, although its shape is often more irregular. Figure 9 shows more examples of core−shell nanoparticles above and around 10 nm diameter. They all show core−shell structures with noncircular cores or small satellite cores. They also show the presence of bright ringlike contrast features around the central core (see arrows in Figure 9a for instance). Although these contrast features are often uneven or more pronounced on one side of the core, we will, as a first approximation, assume that these zones are spherical zones surrounding the core. The existence of adjacent spherical shells with a minimum and maximum in the Co composition has likely important implications on the strain distribution across the nanoparticle

surface and thus on the resulting catalytic activity. On the basis of Vegard's rule, Co-rich regions exhibit smaller cubic lattice constants as compared to Pt-rich regions. Because geometric strain is a long-range effect1,14,36,37 extending across up to 16 atomic Pt layers,1 the Co maxima just below the Pt shell can be stabilized or even enhanced the compressive surface strain and lower the chemisorption energy of adsorbates;10,14,37−39 this is beneficial for the intrinsic electrocatalytic activity of ORR12,13,40,41 and helps offset some of the Pt mass-based activity loss due to the larger particle size.5 Similarly, the Corich satellite cores help maintain compressive strain in nearby surface sections and plausibly account for the sustained high catalytic activity of particles with a size of up to 20 nm. 3.6. A Formation Model for Complex Core−Shell Structures with Co Enrichment and Depletion Zones. The formation of off-center Co-rich satellite cores near the surface of pore- and void-free dealloyed Pt-rich bimetallic nanoparticles, such as shown in Figures 3, 5−7, and 9, finds a plausible mechanistic origin in the interplay of (i) rapid electrochemical Co surface dissolution and (ii) terrace/step/ kink vacancy formation, (iii) followed by a much slower vacancy annihilation through Pt surface atom diffusion. Dealloying based on Co bulk diffusion across radii (>5 nm) is unlikely due to low diffusion coefficients and bulk vacancy concentrations in the particle core. Erlebacher et al.42 showed that these processes control the formation of dealloyed porous noble metals from bimetallic alloys. Electrochemical dissolution 19079



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Figure 8. (a) Composition profile showing the Co core (1), subsequent Co depletion (2), and enrichment zones (3). (b) Corresponding computed EELS line profile showing “shoulder”-like features (2 and 3). (c) Multishell concentric compositional model according to the compositional profile shown in panel a. Red and gray colors represent the contents of Co and Pt, respectively. (d) Calculated HAADF image. The Co-depleted, that is, Ptenriched zone (2), creates a distinct bright ring around the dark-shaded core (1). The HAADF image was computed using the Abel transform and the Z1.7-weighted density of the corresponding element. In addition, a slight Gaussian blur filter was applied.

of the less noble metal atoms creates terrace vacancies and lowcoordinated mobile Pt surface atoms. The electric field, combined with surface adsorption of oxygenated (hydr)oxide species above 0.7 V/RHE,43−45 promotes rapid Co segregation toward the surface, which causes terrace vacancies to be injected in the second and third subsurface layer, while more Co is dissolved on the surface. Simultaneously, low-coordinated Pt atoms diffuse toward step edges and induce inhomogeneous Pt distributions on the surface. This leads to Rayleigh surface instabilities46 promoting the growth of stable ligaments and pores, thereby exposing more of the less noble component to (electro)chemical dissolution until all near-surface vacancies are annihilated by Pt surface diffusion. This mechanism explains the inhomogeneous loss of Co from nanoparticles below 20 nm diameter considered here, with one difference. Interfacial energies prevent the growth of terrace vacancies into stable pores and voids inside the nanoparticles. As a result, terrace vacancies and surface pits are eventually annealed out by Pt surface diffusion; therefore, the particles remain nonporous. Bulk regions near surface sections with fewer terrace vacancies remain less affected by dealloying, and this results in the observed Co-rich satellite subsurface regions.

The formation of near-spherical compositional shells with Co enrichment adjacent to Co depletion up to 3 nm deep inside the particle involves the combined processes of electrochemical Co dissolution and Pt diffusion on the surface;23,24 however, it requires an additional element of short-range subsurface Co segregation as well. Our proposed formation mechanism is illustrated in Figure 10a,b. Initially, potential cycling causes very rapid electric field-driven Co dissolution from the surface and subsurface. Co removal is accompanied by equally rapid injection of lattice vacancies in the opposite direction, that is, toward the surface and subsurface. As discussed earlier, vacancies are unable to form stable voids inside the nanocrystals due to prohibitively high surface energies;47 instead, because of Pt surface diffusion,23,24 subsurface vacancies are gradually annihilated (Figure 10b). As predicted by Yu et al.,48 this outward flux of vacancies is associated with the diffusion of the faster diffusing component, here Co, in the opposite direction, that is, toward the center of the particle, and hence uphill against the general radial Co concentration profile. This process is referred to in the literature as the “inverse Kirkendall effect”, because regular Kirkendall phenomena49−52observed in bimetallic diffusion upon thermal annealinghave vacancies diffusing toward high concentrations of the fast diffuser, that is, 19080



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Figure 9. Dealloyed Pt−Co nanoparticles show an almost core−shell structure. The presence of bright ringlike or irregular contrast features around the core indicating a local Co depletion zone is present in all images (for example, see the arrows).

here the particle center. This kinetic segregation of Co is illustrated by the red arrows in Figure 10b; it continues as long as excess vacancies of the dealloying process are present in the subsurface of the particle. In effect, the Co segregation results in a nonmonotonic Co profile (solid blue curve in Figure 10b) with a Co minimum and maximum consistent with the experimentally observed profiles. The proposed formation mechanism involves Co diffusion only over very short distances of about 1 nm (according to the experimental EELS curves in Figures 5−7 and, as schematically shown, from region 5 to region 3 in Figure 10b). It seems plausible that this diffusion mechanism is considerably enhanced by the presence of subsurface excess vacancies injected during dealloying in comparison with diffusion with thermodynamic equilibrium vacancy concentration. Once the excess vacancies are annihilated, the microstructural state of the subsurface region becomes more stable and kinetically frozen. It is noted that the complex morphologies are relatively stable during electron microscopy and do not show significant microstructural and compositional changes after some minutes of electron beam irradiation (see the Supporting Information). Longer electron irradiation, however, which is required to

obtain EELS maps with sufficient signal-to-noise ratio, led to beam damage of the particles. Although the observed complex morphologies and compositional profiles do not necessarily represent thermodynamic final states, they are kinetically frozen metastable states. In a recent study, Xin et al.53 have presented a very detailed STEM/EELSbased degradation study of the end-of-life structure and morphology of a Pt75Co25 fuel cell catalysts after 30.000 voltage cycles. Their investigation revealed a wide range of complex core−shell morphologies including multiple core structures in particles of various sizes. Their data suggested a synergy between coalescence and Ostwald type particle coarsening. In contrast to Xin's study, we have focused on the structure and morphology of a dealloyed Pt−Co catalyst at the most active stage in its life cycle.

4. CONCLUSION We have uncovered the existence of the unusual compositional fine structure of dealloyed Pt bimetallic core−shell nanoparticle catalysts in the characteristic size regime of 10−15 nm by STEM and EELS. A detailed EELS compositional analysis revealed spherical 1−2 nm wide Co depletion and enrichment 19081



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size range. Our results reflect how important a detailed knowledge of the compositional structure of bimetallic nanoparticles is for a thorough understanding of their electrocatalytic activity.

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ASSOCIATED CONTENT

S Supporting Information *

Figures of raw EELS spectra, high-resolution HAADF micrograph of a dealloyed Pt−Cu nanoparticle, HAADF micrograph of Pt−Co and Pt−Cu nanoparticles, Co EELS scan of the particle in Figure 5c, and three HAADF-STEM images subsequently taken from the same particle. 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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REFERENCES

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Figure 10. (a and b) Illustration of the formation mechanism of the unusual compositional multishell nanoparticle structure. A quarter of a spherical cross-section through a dealloyed Pt−Co particle with a Corich core (red color near origin) and Pt-rich shell (gray color) is shown as the background in both figures. (a) The blue solid line schematically shows an early stage Co composition profile (Co concentration on vertical axis and distance from particle center on the x-axis) with monotonic negative slope as a consequence of rapid initial surface dealloying, Co bulk downhill diffusion (red arrow), and associated with vacancy injection into the particle (black arrow). Prohibitively high surface energies of nanoscale voids prevent aggregation of the injected vacancies. (b) Instead, Pt surface diffusion acts as a vacancy sink and causes vacancy back migration toward the particle surface (inverse Kirkendall effect) followed by subsequent vacancy ejection/ annihilation at the surface. The migration of vacancies is coupled to an opposite uphill diffusion of the faster diffusing Co atoms (red arrow) (inverse Kirkendall effect) inducing a sharp Co drop-off at position 4. The resulting Co nonmonotonic single-maximum compositional profile (solid blue curve) is consistent with the experimentally observed compositional multilayer structure involving adjacent Co maxima and minima.

zones in the shell regime inconsistent with conventional models of dissolution and diffusion during the dealloying of bimetallics.23−25,46,54 We rationalized the formation mechanism of the enrichment features through a combination of rapid selective Co surface electrochemical dissolution and vacancy injection, followed by an inverse Kirkendall effect of outward vacancy annihilation associated with opposite very short-range Co segregation. On the basis of our results, the conventional simple two-layer core−shell description of bimetallic Pt nanoparticle electrocatalysts is insufficient in the 10−15 nm 19082



Article

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