Facet-Dependent Deposition of Highly Strained Alloyed Shells on

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Facet-Dependent Deposition of Highly Strained Alloyed Shells on Intermetallic Nanoparticles for Enhanced Electrocatalysis Chenyu Wang,† Xiahan Sang,‡ Jocelyn T. L. Gamler,† Dennis P. Chen,† Raymond R. Unocic,‡ and Sara E. Skrabalak*,† †

Department of Chemistry, Indiana University, Bloomington, 800 East Kirkwood Avenue, Bloomington, Indiana 47405, United States Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, One Bethel Valley Road, Oak Ridge, Tennessee 37831 United States

‡

S Supporting Information *

ABSTRACT: Surface strains can enhance the performance of platinum-based core@shell electrocatalysts for the oxygen reduction reaction (ORR). Bimetallic core@shell nanoparticles (NPs) are widely studied nanocatalysts but often have limited lattice mismatch and surface compositions; investigations of core@ shell NPs with greater compositional complexity and lattice misfit are in their infancy. Here, a new class of multimetallic NPs composed of intermetallic cores and random alloy shells is reported. Specifically, face-centered cubic Pt−Cu random alloy shells were deposited on PdCu B2 intermetallic seeds in a facetdependent manner, giving rise to faceted core@shell NPs with highly strained surfaces. High-resolution transmission electron microscopy revealed orientation-dependent surface strains, where the compressive strains were greater on Pt−Cu {200} than {111} facets. These core@shell NPs provide higher specific area and mass activities for the ORR when compared to conventional Pt−Cu NPs. Moreover, these intermetallic@random alloy NPs displayed high endurance, undergoing 10,000 cycles with only a slight decay in activity and no apparent structural changes. KEYWORDS: Pt-M nanoparticles, core−shell, lattice misfit, surface strains, oxygen reduction reaction, seeded growth

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metallic surfaces are less studied, but these results highlight the potential of these types of structures.16−18 Inspired by our recent success with the synthesis of monodisperse PdCu B2 intermetallic NPs,19 here we employ them as core templates for deposition of Pt−Cu random alloy shells. Pt−Cu alloyed surfaces were selected on account of their superior catalytic reactivity for ORR, with a Pt-to-Cu ratio of 2:1 being targeted as Pt is the major component responsible for catalyzing the ORR and the Cu component modifies the electronic structure of Pt through heterometallic bonding interactions.20,21 The conventional representations for the Pt− Cu random alloy and PdCu B2 phases are fcc (Fm3̅m; a = b = c = ∼3.82 Å at a Pt/Cu ratio of 2:1) and CsCl-type (Pm3̅m; a = b = c = 2.98 Å), respectively.22 However, from comparing of the basis vector of their primitive cells, the lattice mismatch can be as high as ∼5.7% when the shell has a composition of Pt:Cu = 2:1 (the targeted composition of this work). This hierarchical nanostructure with highly strained surfaces was achieved through the use of seed-mediated coreduction. Facet-dependent deposition on the PdCu B2 seeds gave rise to orientationdependent surface strains, whereby compressive strains are

roton exchange membrane fuel cells (PEMFCs) are being considered as a platform for clean and efficient power, yet they are not widely deployed in part because of the cost and limited lifetime of the Pt catalysts used at the cathode.1,2 Investigations have focused on the development of novel nanostructured electrocatalysts to enhance the durability and reduce the cost of these catalysts.3,4 For example, Pt-skin structures are promising candidates, decreasing the use of Pt and boosting the catalytic efficiency for the oxygen reduction reaction (ORR) compared to conventional Pt NP catalysts.5,6 The study of these structures identified ligand effects and strain as levers to tune the surface electronic structure and the adsorption behavior of oxygenated intermediates during ORR, toward high performance.7 Two approaches include (1) alloying Pt with late transition metals (e.g., Fe, Co, Ni, and Cu) and (2) straining the Pt-skin with core materials of different lattices.8,9 Both methods can effectively weaken the adsorption of oxide species, as pure Pt surfaces bind them too strongly.10 Considering these developments, alloyed Pt−M shells strained through a core@shell NP architecture should be ideal PEMFC catalysts. Seed-mediated synthesis is an effective tool to prepare core@shell NPs.11,12 Nevertheless, the state-ofthe-art studies are mostly confined to bimetallic systems with minimal lattice mismatch, such as [email protected]−15 Structurally intricate multimetallic catalysts with highly strained multi© XXXX American Chemical Society

Received: May 27, 2017 Revised: August 13, 2017

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Figure 1. (a) Low-magnification and (b) high-magnification TEM images of PdCu B2@Pt−Cu NPs, showing a highly uniform and well-defined cuboctahedral morphology; (c) spherical aberration (Cs)-corrected STEM image taken from a single NP along the PdCu B2 [111] axis (confirmed by the fast Fourier transform (FFT) in the inset), showing the contrast in brightness and difference of lattice spacing between core and shell; (d) HAADF-STEM image of five PdCu B2@Pt−Cu NPs and the corresponding EDX mapping of Pd, Cu, and Pt, confirming a core@shell structure.

minimal on Pt−Cu {111} planes but significant on {200} facets. With such strains, the core@shell PdCu B2@Pt−Cu NPs can deliver specific and mass activities for ORR that are greater than Pt−Cu reference nanocatalysts. Moreover, such nanostructures show high durability. Monodisperse PdCu B2 intermetallic seeds were prepared by slightly modifying a protocol developed by our group (Figure S1).19 Then, seed-mediated coreduction was carried out to deposit Pt−Cu shells. Specifically, platinum(II) bromide (PtBr2) and copper(II) acetylacetonate (Cu(acac)2) were used as metal precursors and coreduced with 1,2-hexadecanediol (HDD) in a mixture of oleylamine (OAm) and 1octadecene (ODE) as surfactants and solvents (see Supporting Information for full details). Figure 1a−d shows electron microscopy characterization of the core@shell PdCu B2@Pt− Cu NPs with thin shells. Specifically, Figure 1a,b shows transmission electron microscopy (TEM) images of product at different magnifications, indicating a high yield of cuboctahedral core@shell NPs with a monodisperse size distribution of 12.6 ± 0.7 nm. The XRD pattern of this sample indicates that there are two phases, the ordered intermetallic B2 phase of PdCu and a fcc phase consistent with a Pt−Cu alloy phase (Figure S2). Figure 1c shows a spherical aberration (Cs)-corrected highangle annular dark-field scanning TEM (HAADF-STEM) image taken from a single NP that is oriented with the electron beam along the PdCu B2 [111] direction (inset of Figure 1c).

A core@shell architecture is evident from the high contrast between the shell (brighter) and the core (darker) and can be attributed to the difference in atomic number between Pt and Pd. Moreover, energy-dispersive X-ray spectroscopy (EDS) mapping (Figure 1d) and line scan analysis (Figure S3) support the formation of a core@shell structure. The d-spacing for the shell and core are measured as 0.218 and 0.212 nm, which correspond to the values for the Pt−Cu {111} (with approximately a 2:1 Pt/Cu ratio) and PdCu B2 {110} planes, respectively.22 Figure 1c also indicates that the Pt−Cu shell is ∼4 atomic layers in thickness based on contrast differences and that the Pt−Cu deposition was facet-dependent in nature. Such growth can be attributed to the different crystalline structures for the core and shell and relatively small discrepancy in lattice spacing between Pt−Cu {111} and PdCu B2 {110} facets, as discussed more fully later. The compositions for the PdCu B2 seeds and PdCu B2@Pt−Cu NPs were evaluated by inductively coupled plasma mass spectrometry (ICP-MS) and found to be Pd/Cu = 54:46 and Pt/Pd/Cu = 19:39:42 atomic %, respectively, which suggests a surface composition of Pt/Cu of about 2:1 in the core@shell product. Such well-defined nanostructures can be a paradigm for investigating the surface strains of Pt alloys on top of intermetallic cores as well as for understanding facet-dependent deposition processes at the nanoscale. In many seeded syntheses, the shell material follows the crystallographic B

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Figure 2. (a,d,g) Aberration-corrected STEM images along PdCu B2 [111], [100], and [110] zone axes, respectively; the orientations are confirmed by the FFT pattern in each inset. (b,e,h) Atomic resolution images cropped and enlarged image from the boxed area in (a,d,g), respectively; the annotations indicate the angle−plane relationship to determine the facets of core and shell. (c,f,i) The 2D atomic models showing the core−shell interface for (b,e,h), respectively; annotations on the left side indicate the angle−plane relationship, while the annotations on the right side are the theoretical spacings of the repeating units for PdCu B2 core and Pt−Cu shell with the arrows indicating the directions of biaxial strains.

orientation of the underlying seed to minimize interfacial energy.23,24 Even in NP systems with large lattice mismatch, the deposited layers share the same Miller indices with the core surfaces, though conformal shell deposition may be limited due to the buildup of strain.25 Thus, the core@shell interface of the PdCu B2@Pt−Cu NPs was examined using aberrationcorrected HAADF-STEM by acquiring images along the PdCu B2 [111], [100], and [110] zone axes, respectively. When the NP was projected along the PdCu B2 [111] direction, the atomic arrangement of the Pt−Cu shell indicates that the surface projection is along the [110] zone axis (Figure 2a,b). Considering the angle-to-plane relationships in cubic lattice systems, one can deduce that Pt−Cu {111} facets were deposited on the PdCu B2 {110} facets, as shown by a structural model in Figure 2c. The difference in d-spacing for PdCu B2 {110} and Pt−Cu {111} is minimal, as shown in Figure 2c, inferring a small interfacial energy along this direction. Figure 2d displays a NP with its PdCu B2 core projected along the [100] direction. The same analysis indicates that the Pt−Cu shell takes a [110] projection, with {200} facets preferentially deposited on the PdCu B2 {100} planes (Figure 2e,f). As shown in Figure 2f, the stacking sequence for the

PdCu B2 core resembles that for the Pt−Cu {200} shell. Finally, when analyzed from the PdCu B2 [110] projection (Figure 2g), one can observe that Pt−Cu {200} were deposited on top of PdCu B2 {100} and the projection of the shell is [100], which has a closer stacking periodicity to the core (Figure 2h,i). Taken together, this analysis shows that the facetdependent deposition of the random alloy Pt−Cu shell diminishes interfacial energy by adopting lattice spacings and atomic stacking sequences as similar to the core as possible to maintain a coherent interface. This unique core@shell structure leads to orientationdependent surface strains. Minimal compressive strain is imposed on the shell along the Pt−Cu ⟨111⟩ axes, as the theoretical mismatch in lattice spacing between the Pt−Cu {111} and PdCu B2 {110} planes is relatively small (∼4.5%; Figure 2c). Slight elastic deformation of the Pt−Cu lattices along the ⟨111⟩ directions can be viewed in Figure 2a. By contrast, the lattice strains on Pt−Cu {200} facets are considerable and complicated due to the larger misfit of lattice spacing relative to the PdCu B2 core. Such lattice mismatch leads to a biaxial strain on a coherent interface. As can be seen from Figure 2f, the lateral spacing of the smallest repeating unit C

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Figure 3. (a−e) TEM images of PdCu B2@Pt−Cu NPs with increased shell thickness, which were synthesized under a molar input ratio (PtCu/ PdCu seeds) of (a) 1:5, (b) 1:4, (c) 1:3, (d) 2:5, and (e) 1:2, respectively.

shell thickness can be increased by increasing the relative amount of depositing metal to seeds. Interestingly, this increase in shell thickness is accompanied by changes in NP morphology, which can be ascribed to the interplay of surface energy and capping agents. When the shell is relatively thin (∼0.8 nm), it tends to be more close to the profile of the core (Figure 3a,b). When the shell is thicker (∼2 nm), nanocubic particles are achieved (Figure 3c−e and Figures S6 and S7). With increasing particle size, some sample monodispersity is also lost. Therefore, the cuboctaherdal PdCu B2@Pt−Cu NPs with highly strained surfaces were evaluated as catalysts for the ORR (those in Figure 1). Core@shell NPs with compressively strained Pt surfaces have downshifted Pt d-band centers, which have been shown to boost catalysis for the ORR.28 This model for enhanced performance is supported both by experiment and modeling on close-packed Pt surfaces. Recently, tensile strains on Pt(110) facets were found to also increase the activity for the ORR and that low-coordinated surface atoms can be activated by large tensile strains.29 Thus, the biaxial stains in the Pt−Cu shells that arise from their large structural and lattice misfit with PdCu B2 should be beneficial for ORR catalyst development. Moreover, the alloyed Pt−Cu surface is a highly efficient electrocatalyst due to the ligand and electronic effects of Cu.6 The electrochemical properties of the PdCu B2@Pt−Cu NPs were studied and benchmarked against Pt−Cu polyhedral NPs (Figure S8) of the same Pt/Cu composition (2:1). We note that shape-defined Pt−Cu NPs may perform better than polyhedra, but this comparison was selected to verify the contributions of core@shell architecture and strained surface to catalyst performance. Before electrochemical evaluation, the NPs were uniformly deposited on a commercial carbon (EC600JD, Ketjen) support (named as PdCu B2@Pt−Cu/C and

of the shell is smaller than that for the PdCu B2 layer underneath (theoretical mismatch ∼9.5%). A tensile strain on the shell is yielded to expand the lattice laterally (along the [11̅0] direction) to have matching spacing at the interface. To compensate, a compressive strain emerges vertically (along [001] direction) to decrease the spacing of deposited Pt−Cu {200} layers. The same scenario can be observed from Figure 2i, where tensile and compressive strains are applied along [001] and [010] axes, respectively. Such greater misfit and biaxial strains manifest in obvious inward lattice bending (Figure 2d,g). In addition to the elastic deformation of lattices, edge dislocations, as circled out in Figure S4, emerge in the Pt− Cu shell to release the high surface strains. The partial missing of a (111) plane suggests the existences of Frank partial dislocations.26 Well-defined NPs are required to investigate structure− property relationships. Therefore, experiments screened parameters for optimum surface structure and synthetic versatility. This work revealed that the surface of the PdCu B2 seeds plays a critical role in achieving NPs with extended facets. When spherical B2 NPs (Figure S5a) were used as seeds, the Pt−Cu shells took a faceted but irregular profile, as can be seen from Figure S5b. We surmise that the surfaces of the spherical NP seeds are composed of a large quantity of subfacets with different atomic arrangements. The deviation from one subfacet to another results in nonuniform growth of shells, as the surface atomic coordination influences the deposition behavior.27 In comparison, well-defined cuboctahedral shells with extended facets form (Figure S5c) when slightly faceted (pseudospherical) B2 seeds were employed (Figure S5d). In this case, deposition of Pt−Cu layers is preferentially along certain directions to minimize the interfacial energy created by the crystallographic mismatch. Furthermore, the D

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Figure 4. Electrocatalytic performances of PdCu B2@Pt−Cu/C and Pt−Cu/C catalysts: (a) ORR polarization curves with an inset of CVs; (b) specific and mass activities of each catalyst; (c) CO-stripping curves for each catalyst; (d) ORR polarization curves of PdCu B2@Pt−Cu/C before and after different potential cycles in durability test; the changes on specific and mass activities of the (e) PdCu B2@Pt−Cu/C catalyst and the (f) Pt−Cu/C catalyst before and after different potential cycles.

structures, as the CO-stripping peak potential of Pt catalysts was found to correlate directly with the Pt d-band center and binding energy.32,33 The PdCu B2@Pt−Cu/C displayed more negative CO-stripping peak potential compared with Pt−Cu/C (Figure 4c), indicating more facile removal of CO from the surface of PdCu B2@Pt−Cu/C electrocatalysts. The asymmetry of CO-stripping peak profile for PdCu B2@Pt−Cu/C could be a consequence of nonuniform strains and defects on the Pt− Cu shell. This behavior suggests a downshift of the d-band center of surface Pt on PdCu B2@Pt−Cu/C relative to that of Pt−Cu/C, which explains why PdCu B2@Pt−Cu/C outperformed Pt−Cu/C in catalyzing ORR. The degradation of core metals can severely jeopardize the durability of core@shell catalysts under harsh fuel-cell operation conditions. Therefore, core materials need to be carefully selected and designed to inhibit dissolution and to enhance durability. The electrochemical durability of PdCu B2@Pt−Cu/C and Pt−Cu/C were assessed by repeatedly sweeping at the potential range of 0.6−1.1 V (vs RHE) in 0.1 M HClO4 solution. Figure 4d shows the ORR polarization curves of the PdCu B2@Pt−Cu/C before and after 5,000 and 10,000 cycles. After 5,000 sweeping cycles, there was a slightly negative shift in ORR polarization curve, together with a 12.3% loss in activity (Figure 4e). There was negligible shift between the polarization curve after 5,000 sweeps and that after 10,000

Pt−Cu/C). As-prepared catalysts were treated with methanol/ acetic acid to remove surfactant. The inset of Figure 4a shows cyclic voltammograms (CVs) of the catalysts in Ar-purged 0.1 M HClO4 solution at a sweep rate of 50 mV/s. The PdCu B2@ Pt−Cu/C provided a greater electrochemically active surface area (ECSA) of 58.8 m2/gPt than Pt−Cu (34.2 m2/gPt), and this finding is consistent with the core@shell structure that positions a larger percentage of Pt atoms at the surface. The evaluation of electrocatalytic activities toward ORR were carried out in an O2-saturated 0.1 M HClO4 solution. As can be seen from Figure 4a, the polarization curve for PdCu B2@ Pt−Cu/C displayed a positive shift in half-wave potential (30 mV) when compared with that for Pt−Cu/C. The specific activity (SA) of PdCu B2@Pt−Cu/C, as shown in Figure 4b, was determined to be 4.33 mA/cm2 at 0.9 V versus the reversible hydrogen electrode (RHE), 4.6 times greater than that of Pt−Cu/C (0.77 mA/cm2). Under the same potential, the mass activity of PdCu B2@Pt−Cu/C was calculated as 2.55 A/mgPt, which is 8.7 higher than that of Pt−Cu/C (0.26 A/ mgPt). The theoretical d-band model predicts a downshift of the Pt d-band center could generate the most active Pt−M alloy catalyst; however, a direct measurement of the Pt d-band center under ORR conditions is not viable.30,31 We employed instead electrochemical CO-stripping to probe the surface electronic E

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ACKNOWLEDGMENTS We acknowledge financial support from the U.S. Department of Energy (Basic Energy Sciences) through an Early Career Award Grant (DE-SC0010489). Access to the X-ray powder diffractometer and the XPS was provided by NSF CHE CRIF-1048613 and DMR MRI-1126394, respectively. HAADFSTEM EDX mapping was conducted at Nanoscale Characterization Facility and Electron Microscopy Center at IU. Aberration-corrected STEM was conducted as part of a user proposal at Oak Ridge National Laboratory’s Center for Nanophase Materials Sciences (CNMS), a U.S. Department of Energy Office of Science User Facility (X.S. and R.R.U.). S.E.S is also supported through the Camille Dreyfus Teacher Scholar Program. D.P.C. is thankful for the financial support provided by the President’s Diversity Dissertation Fellowship.

sweeps and the loss in activity from 5,000 to 10,000 cycles was only 3.5%. The Pt−Cu/C, by contrast, showed consistent and considerable loss of activity after 5,000 sweeps and 10,000 sweeps (Figure 4f). The catalysts before and after the durability tests were characterized by TEM (Figure S9). There was no noticeable change of the morphology and core@shell architecture of the PdCu B2@Pt−Cu NPs after cycling (Figures S9a,c and S10) as revealed by TEM and elemental mapping. In contrast, a noticeable number of small particles appeared in the Pt−Cu/C sample, possibly due to partial dissolution during the test (Figure S9b,d). The high catalytic stability of PdCu B2@Pt−Cu/C may originate from the ordered intermetallic structure and robust bonding character in the PdCu B2 core, which afford resistance to dissolution in acidic environment. Additionally, a comparison of the catalytic performances was carried out between PdCu B2@Pt−Cu NPs and commercial Pt/C catalysts. As-prepared PdCu B2@Pt− Cu/C catalysts displayed enhanced activity and durability (Figure S11), showing the potential of these NPs to be nextgeneration cathode materials. In summary, we have demonstrated a novel core@shell catalyst for the ORR based on PdCu B2 intermetallic cores with highly strained Pt−Cu random alloy shells. Owing to the large structural and lattice misfit between the core and shell components, the Pt−Cu overlayers exhibit orientation-dependent surface strains in which minimal compressive strains are imposed on Pt−Cu {111} while significant strains arise on the Pt−Cu {200}. When benchmarked against the catalyst of pure Pt−Cu polyhedral NPs, the PdCu B2@Pt−Cu NPs exhibited substantial enhancement in both activity and durability toward ORR. These results provide a paradigm for designing core@ shell nanocatalysts with excellent activity and durability by selecting ordered intermetallic NPs as the substrate that not only offer extraordinary strains to the surface Pt but also inhibit the degradation in a corrosive environment. It is anticipated that seed-mediated coreduction can be applied to achieve related nanostructures for catalytic applications.

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S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.nanolett.7b02239.

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Materials and experimental methods, additional electron micrographs, XRD patterns, elemental analysis (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiahan Sang: 0000-0002-2861-6814 Sara E. Skrabalak: 0000-0002-1873-100X Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. F

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