Shape Stability of Octahedral PtNiNanocatalysts for Electrochemical OxygenReduction Reaction Studied by in situTransmission Electron MicroscopyMartin Gocyla,†,⊥ Stefanie Kuehl,‡,⊥ Meital Shviro,† Henner Heyen,‡ Soeren Selve,§

Rafal E. Dunin-Borkowski,† Marc Heggen,*,† and Peter Strasser*,‡

†Ernst Ruska-Centre for Microscopy and Spectroscopy with Electrons, Forschungszentrum Julich GmbH, 52425 Julich, Germany‡Department of Chemistry, Chemical Engineering Division, Technical University Berlin, 10623 Berlin, Germany§ZELMI−Zentraleinrichtung fur Elektronenmikroskopie, Technical University Berlin, 10623 Berlin, Germany

*S Supporting Information

ABSTRACT: Octahedral faceted nanoparticles are highly attractive fuel cell catalysts as aresult of their activity for the oxygen reduction reaction (ORR). However, their surfacecompositional and morphological stability currently limits their long-term performancein real membrane electrode assemblies. Here, we perform in situ heating ofcompositionally segregated PtNi1.5 octahedral nanoparticles inside a transmissionelectron microscope, in order to study their compositional and morphological changes.The starting PtNi1.5 octahedra have Pt-rich edges and concave Ni-rich {111} facets. Wereveal a morphological evolution sequence, which involves transformation from concaveoctahedra to particles with atomically flat {100} and {111} facets, ideally representingtruncated octahedra or cuboctahedra. The flat {100} and {111} facets are thought to comprise a thin Pt layer with a Ni-richsubsurface, which may boost catalytic activity. However, the transformation to truncated octahedra/cuboctahedra alsodecreases the area of the highly active {111} facets. The morphological and surface compositional evolution, therefore,results in a compromise between catalytic activity and morphological stability. Our findings are important for the design ofmore stable faceted PtNi nanoparticles with high activities for the ORR.

KEYWORDS: in situ TEM, PtNi octahedra, PtNi cuboctahedra, surface segregation, oxygen reduction reaction

The future supply of renewable energy depends stronglyon the development of novel high-performancecatalysts for energy conversion and storage applica-

tions. In proton exchange membrane fuel cells recentdevelopments of electrocatalysts used at the cathode site forthe oxygen reduction reaction (ORR) have shown, for instance,that alloying Pt with a second metal can lead to animprovement in catalytic activity.1,2 The discovery of highlyactive {111} surfaces in the Pt3Ni system for the ORR3 was thestarting signal to search for polymetallic octahedral materialswith highly improved performance in the field of electro-catalysis.4 Whereas the exceptionally high activities arecontinuously being improved,5 the durability of octahedralmaterials still remains an issue. With regard to the rationaldesign of advanced catalyst materials, previous work hasfocused on the mechanisms of growth6−8 and degradation9 ofPtNi octahedra. Degradation under electrochemical conditionswas, for instance, described in terms of a loss of octahedralshape due to dissolution of Ni from the {111} facets9 ormigration of Pt atoms at the surface.10 Several attempts havebeen made to suppress this degradation process. For example,

Park et al.11 covered PtNi octahedral nanoparticles (NPs)chemically with an ultrathin Pt layer to prevent the dissolutionof Ni. Other groups doped the PtNi octahedral surface by athird metal to successfully prevent degradation of theoctahedral shape. For example Huang et al.5 used Mo, whileBeermann et al.10 used Rh as a third element. Theydemonstrated that Rh doping of PtNi octahedra leads to areduction in Pt surface migration and, thus, to the retention ofoctahedral shape.10 Furthermore, Cao et al.12 providedtheoretical calculations of the beneficial behavior of a thirdmetal. They reported that a third metal could reduce theequilibrium concentration of Ni surface atoms and, thus, limitthe rate at which Ni atoms are dissolved from the particles.Moreover, Cao et al.12 predicted the stabilization of low-coordinated sites by introducing a third metal, e.g., Mo.

Received: December 29, 2017Accepted: May 25, 2018Published: May 25, 2018

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© 2018 American Chemical Society 5306 DOI: 10.1021/acsnano.7b09202ACS Nano 2018, 12, 5306−5311

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Another concept for improving the stability of octahedralNPs is post-annealing of PtNi octahedra, as suggested andinvestigated by Ahmadi et al.13 By using in situ transmissionelectron microscopy (TEM), Pan et al.14 and Gan et al.15

applied thermal annealing to Pt-rich PtNi octahedra. Pan et al.14

observed a redistribution of Ni content close to the surface,toward more alloyed octahedra, while Gan et al. followedmorphological changes, observing the formation of PtNioctahedra with a thin Pt shell on the {111} facets. In bothcases, a highly active and stable catalyst was predicted for thiskind of octahedra.In the present work, in contrast to Pan et al.14 and Gan et

al.,15 we investigate the still poorly understood morphologicalchanges and transformation mechanisms of Ni-rich PtNixoctahedral NPs. As a result of their low Pt content, they offerlarge Pt mass-based activity benefits. This study involvesmorphological investigations performed using in situ TEM togain insight into structure−stability relationships. On the basisof previous work,13 we expose octahedral PtNi particlessupported on carbon to thermal annealing under vacuum atdifferent temperatures.

RESULTS AND DISCUSSION

The Ni-rich octahedral PtNi NPs that are studied here have ageneral composition of 40 at. % Pt and 60 at. % Ni, asdetermined by inductively coupled plasma-optical emissionspectroscopy (ICP-OES).Figure 1A shows a representative high-angle annular dark

field scanning TEM (HAADF STEM) image of the PtNi1.5octahedral NPs, which have a vertex-to-vertex size along ⟨100⟩of between 8 and 9 nm. In addition to octahedral NPs, sphericalNPs with a diameter of between 2 and 3 nm are found. Figure1B shows an HAADF STEM image of an octahedral NPoriented close to a ⟨110⟩ direction. The image shows a strip ofbright contrast in the middle, concave facets, and roundedvertices. Figure 1C shows two orthogonal bright stripes on anHAADF STEM image of an octahedral NP oriented close to a⟨100⟩ direction. The corresponding EDX composition map inFigure 1D demonstrates the presence of a Pt-rich skeleton,which is consistent with the bright HAADF STEM contrastvisible in Figure 1C. Furthermore, the presence of partly Ni-rich facets (white arrows in Figure 1D), as well as partly emptyfacets (gray arrow in Figure 1D), is evident. In the neighboringspherical particle at the bottom left of the octahedron, the EDXmap shows primarily the presence of Pt. This observation isconsistent with previous work,6,9 which described the rapidgrowth of a Pt-rich hexapod from a spherical Pt nucleus along⟨100⟩ directions and the delayed deposition of Ni on concave{111} sites.

The morphological stability of the PtNi1.5 octahedral NPsupon annealing was studied using in situ TEM. Heat treatmentunder vacuum was applied in two different setups, referred to aslow magnification TEM (LMTEM) and high-resolution TEM(HRTEM).Figure 2 shows an LMTEM image series of PtNi1.5

octahedral NPs examined under in situ thermal heatingconditions from room temperature (RT = 23 °C) to 600 °C,using 100 °C stepwise heating (a continuation to 800 °C isincluded in the Supporting Information (SI) Figure S1). Twoareas are marked with green and yellow circles, showing thesame NPs at different temperatures. (Figure S2 in the SI showsan additional image series recorded during the same heatingexperiment.)As described above, octahedral NPs with a vertex-to-vertex

size of 8−9 nm along ⟨100⟩ and 2−3 nm spherical NPs areobserved at RT. Thermal annealing at 200 °C was not found tolead to significant changes in the LMTEM images, but only to aslight decrease in the vertex-to-vertex size of the octahedra(Figure 2B). At 300 and 400 °C (Figure 2C and D), significantmorphological changes are visible in both areas, with thevertices of the octahedra becoming rounded. At 500 and 600°C (Figure 2E and F), the octahedral shape is lost and theparticles become progressively more spherical in shape.Furthermore, former neighboring octahedra coalesce, includingthose enclosed by the yellow circle (see arrow in Figure 2D andE), whereas the small spherical NPs beside them remainunchanged. Heating to 700 and 800 °C does not induce furthermorphological changes (Figure S1 in the SI).At higher temperatures, coalescence leads to sintering of the

octahedral NPs (see arrow in Figure 2F and Figure S1 in theSI), whereas the smaller NPs are maintained. The observedcoalescence and sintering of neighboring octahedral NPs wasobserved previously by Gan et al.15 for Pt-rich octahedra andwas explained in terms of enhanced surface diffusion, whichminimizes surface curvature.In order to study the influence of electron beam irradiation

during heating, additional images of NP agglomerates wererecorded only at the beginning and after heating to 400 °C(Figure S3 in the SI). The same shape changes of theoctahedral NPs were observed as on the stepwise beamirradiated octahedra, suggesting that electron beam inducedeffects can be neglected here.In addition to the stepwise experiment, a continuous in situ

heating experiment was performed (see the SI, page 4, forexperimental details), supporting the findings from the stepwiseheating experiment. The resulting image sequences (seesupporting movie files at two different magnifications) indicatethat all of the morphological changes occur gradually, starting at

Figure 1. HAADF STEM images and EDX composition maps of PtNi1.5 octahedral nanoparticles in the as-prepared state. (A) OverviewHAADF STEM image. (B, C) High-resolution HAADF STEM images of PtNi octahedral nanoparticles oriented close to (B) ⟨110⟩ and (C)⟨100⟩. (D) Pt (red) and Ni (green) EDX composition map of the same octahedral nanoparticle as in (C). The arrows indicate Ni-rich facets(white) and a partly empty facet (gray), respectively.

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a temperature of between 250 and 300 °C, with rounding of thevertices taking place before the octahedral shape is lost between450 and 500 °C.Figure 3 shows an HRTEM image series of PtNi1.5 octahedral

NPs examined under in situ thermal heating conditions from 50to 700 °C. Here, we follow the structural evolution of threerepresentative octahedra (marked by black, blue, and red circlesin Figure 3). At 50 °C, the octahedra have slightly roundedvertices (Figure 3A), corresponding to an energetically morestable shape than sharp vertices.16 The vertex-to-vertex sizes ofthe octahedra along ⟨100⟩ are 8.6 nm (black circle), 8.7 nm(red circle), and 7.9 nm (blue circle). Heating to 150 °C doesnot result in significant morphological changes (Figure 3B).Further heating to 200 °C leads to the formation of octahedrawith truncated vertices and less concave facets (Figure 3C).This structural evolution can be explained in terms of the

diffusion of Pt atoms from the vertices to the {111} facets. Suchreconstruction of facets in bimetallic nanoparticles has beenhypothesized in previous studies.15 An increase in temperatureto 250 and 300 °C (Figure 3D and E) leads to truncation of therounded vertices and, finally, to the formation of atomically flat{100} facets in addition to the {111} facets (see Figure 3K andL). The formation of {100} facets in addition to the {111}facets leads to a truncation of the octahedra. Ideally this processmay lead to the formation of regular truncated octahedra orcuboctahedra. For simplicity, we will use the term cuboctahedrafor {100}-truncated octahedral particles in the further course ofthis article, although the particles may not possess perfectcuboctahedral shape. This transformation corresponds as wellto the decrease in vertex-to-vertex size along ⟨100⟩ observedusing LMTEM. Further heating to 350 and 400 °C (Figure 3Fand G) probably leads to further Pt atom surface diffusion andto the transformation of flat facets to more convex shapes byinterdiffusion. Well-faceted cuboctahedra, gradually developinto round NPs. This process continues at increasingtemperature up to 500 °C (Figure 3H). Although the formationof several spherical shaped NPs is observed, some flat {111}surfaces are visible, especially for the cuboctahedra enclosed bythe blue circle in Figure 3. Heating up to 700 °C results in

sintering of the NPs, in good agreement with the LMTEMresults.The observed formation of such cuboctahedral particles with

almost atomically flat {100} facets represents an additionalintermediate NP morphology. The formation of flat {100}facets in addition to {111} facets due to thermal annealing wasnot reported to occur in Pt-rich PtNi NPs by Gan et al.15 Thetransformation from concave octahedra to cuboctahedra couldin fact be energetically favorable due to the presence of a {100}facet reconstruction, as pointed out in the theoretical work ofWang et al.17 Furthermore, previous theoretical18 andexperimental3,19 works clarified how the reactivity toward theORR varies over different single-crystal surfaces of Pt and Pt3Nicatalysts. Pt3Ni alloy {111} and {100} surfaces were thenpredicted to be catalytically more active for ORR than pure Ptsurfaces. Furthermore, Pt3Ni {111} surfaces have been reportedto be superior to Pt3Ni {100} surfaces.3

Therefore, a transformation from a concave octahedron to acuboctahedron, leading to a decrease of the exposed {111}facet area due to the formation of {100} facets, would on theone hand lower the surface ORR activity. On the other hand,the formation of a larger ratio of perfectly flat Pt-rich {111}surfaces with Ni-rich subsurfaces, like probably formed in ourexperiments, could improve the activity due to a change inreaction mechanism, as was suggested by DFT calculations ofDuan and co-workers.18,20 The benefit of the observedtransformation to cuboctahedral NPs is also explained in thetheoretical work of Mahata et al.,21 based on a detailedinvestigation of the surface energy and compressive strain. Forcuboctahedra they found a shift in the d-band center positiontoward the Fermi level due to the presence of {111} and {100}facets. This stabilizes the intermediates due to a stronginteraction between cuboctahedron and intermediates and,hence, might lead to an improved ORR activity.However, the stability of such cuboctahedral NPs is expected

to be more important than their influence on the activity itself.Previous work has shown that the leaching of Ni-enrichedfacets under electrochemical conditions leads to a loss incatalytic ORR activity.9 Cuboctahedra, in which Ni is protectedin the centers of the {111} facets by a thin layer of Pt, should be

Figure 2. (A−F) LMTEM image series of PtNi1.5 octahedral NPs under in situ thermal heating conditions from 23 °C (RT) to 600 °C. Greenand yellow circles mark identical NPs at different temperatures. The arrows indicate coalescence and sintering of two NPs.

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more resistant to Ni leaching and could be stronger candidatesfor long-term catalysts. Such cuboctahedra could provide anoptimal compromise between high activity and long-termmorphological stability.Based on our HRTEM results, Figure 4A−E illustrate a

proposed morphological evolution sequence for the trans-formation of concave Ni-rich octahedra during in situ heating,alongside representative HRTEM images (F−M). Themorphological evolution sequence starts with an initiallyconcave octahedron that has Ni-rich facets (Figure 4A), asobserved for the as-prepared NPs (Figure 1D). Upon thermalannealing, Pt diffuses from the edges toward the {111} facetsand forms octahedra with partly filled {111} facets (Figure 4B),as observed in our in situ HRTEM results (Figures 3C and 4G).Further Pt diffusion leads to the formation of a truncatedoctahedron with flat {111} facets and additional flat {100}facets, as observed at 250 °C (Figure 3D). Continuation of thisprocess leads to further truncation, an increase in {100} surface

area, and, thereby, the formation of cuboctahedra, which mostprobably have a Pt shell (Figure 4C). Further heating at highertemperature leads to partly rounded-off facets (Figure 4D) andfinally to the formation of spherical NPs (Figure 4E).The proposed Pt and Ni distribution in the morphological

evolution sequence (Figure 4) is in accordance with HAADFSTEM images (see SI, Figure S4). A change in Z-contrast wasobserved upon heating, which can be correlated to a strongerintermixing of Pt at the facets. Furthermore, EDX maps of heat-treated NPs (see SI, Figure S5) clearly show a Pt surfaceenrichment at 300 °C, which is in accordance with theproposed mechanism.

CONCLUSION

In conclusion, we have performed in situ heating of octahedralPtNi1.5 NPs with Pt-rich edges and concave Ni-rich facets in theTEM. Our observations, which were performed between RTand 800 °C, reveal a morphological evolution sequence in the

Figure 3. HRTEM image series of PtNi1.5 octahedral NPs examined under in situ thermal heating conditions from 50 to 700 °C (A−I). Red,blue, and black circles indicate the same NPs at different temperatures. (K and L) Magnified images of cuboctahedra from images E (bluecircle) and F (red circle), respectively. Additionally, the {100} facets of the cuboctahedron as well as its crystallographic directions are markedin (K). (J) Additional selected cuboctahedron.

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transformation of concave octahedra during thermal annealingunder inert conditions (vacuum). The following states areobserved: concave octahedra; octahedra with partly flat {111}surfaces; cuboctahedra; cuboctahedra with a partial loss of{111} facets; spherical NPs (Figure 4). Starting from concaveoctahedra, heating results in the diffusion of Pt atoms, leadingfirst to the formation of almost atomically flat cuboctahedralfacets and, upon further heating, to the formation of sphericalNPs. Our results reveal that the flat {111} surfaces of thecuboctahedra are stable over a wide temperature range. Thedevelopment of an understanding and control over this processprovides synthetic possibilities for tuning the stability−activityrelationships of octahedral NPs in fuel cell electrocatalysis. Theability to achieve a balance between Pt shell thickness, Nisubsurface enrichment, and an optimized ratio between {111}and {100} facets promises to lead to long-term stable and activecatalysts.

EXPERIMENTAL SECTIONSynthesis. According to Cui et al.,22 0.2 mmol of Pt(acac)2 and 1.4

mmol of Ni(acac)2 were dissolved in 50 mL of dimethylformamide byultrasonication for 5 min. The homogeneous solution was transferredinto a glass-lined stainless steel autoclave. The sealed autoclave washeated from RT to 130 °C within 30 min. The temperature was held42 h before it was cooled to RT. Afterward the particles weresupported on carbon (Vulcan XC72R) by 5 min of ultrasonication andaround 23 h of static conditions at RT. Finally, the PtNi/C waswashed with ethanol/water several times and treated with a freeze-drying method.Physical and Chemical Characterization. ICP-OES. ICP-OES

was used for elemental and compositional analysis using a Varian 715-ES spectrometer with a CCD detector. Standards with a knownconcentration were coanalyzed with the samples.Low-Magnification TEM. LMTEM was performed at a FEI Tecnai

G220 S-TWIN transmission electron microscope with a LaB6 cathodeoperated at an accelerating voltage of 200 kV (ZELMI Centrum,Technical University of Berlin).High-Resolution TEM. HRTEM was performed using a FEI Titan

80-300 TEM with a Cs corrector for the objective lens (CEOSGmbH).23 The microscope was operated at 300 kV.Scanning Transmission Electron Microscopy. STEM was

performed using a FEI Titan 80-200 (“ChemiSTEM”) electron

microscope with a Cs-probe corrector (CEOS GmbH) and anHAADF detector.24 The microscope was operated at 200 kV. In orderto achieve “Z-contrast” conditions, a probe semiangle of 25 mrad andan inner collection semiangle of the detector of 88 mrad were used.Compositional maps were obtained with energy-dispersive X-rayspectroscopy (EDX) using four large-solid-angle symmetrical Si driftdetectors. For EDX elemental mapping, Pt L and Ni K peaks wereused. The error of the EDX composition measurement is ±2 at. %.

in situ TEM. For in situ TEM experiments a heating holder(DENSsolutions B.V.) was applied. The catalyst was dispersed onto aMEMS chip equipped with four-point measurement for precisetemperature control and a distinct heating zone on a SiNx film andSiNx-coated windows for LMTEM and carbon-coated windows forHRTEM and STEM. In LMTEM the temperature was changedinstantly in steps of 100 °C and the specimen was annealed for about20 min at constant temperature (stepwise heating experiment; fordetails to continuous heating experiment see SI, page 4). In HRTEMand STEM the temperature was changed in steps of 50 °C and thespecimen was annealed for about 5−10 min at constant temperature.

ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsnano.7b09202.

Video showing image sequences of a continuous in situLMTEM heating experiment (AVI)Video showing image sequences of a continuous in situLMTEM heating experiment (AVI)Additional images of in situ LMTEM measurements,details of a continuous in situ LMTEM heatingexperiment, as well as additional STEM images (PDF)

AUTHOR INFORMATIONCorresponding Authors*E-mail: m.heggen@fz-juelich.de.*E-mail: pstrasser@tu-berlin.de.ORCIDPeter Strasser: 0000-0002-3884-436XAuthor Contributions⊥M. Gocyla and S. Kuehl contributed equally.

Figure 4. (A−E) Illustrations of proposed Pt (red) and Ni (green) distributions in PtNi1.5 octahedra that undergo transformations during insitu thermal heating. (F−J) Corresponding HRTEM image series of PtNi1.5 octahedral nanoparticles recorded during in situ thermal heatingfrom 50 to 500 °C.

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NotesThe authors declare no competing financial interest.

ACKNOWLEDGMENTSFinancial support was provided by German ResearchFoundation (DFG) grant STR 596/5-1 (“Shaped Pt Bimet-allics”) and HE 7192/1-1, as well as by the German Ministry ofEducation and Research (BMBF) via the project “LoPlaKats”(number 03SF0527A). M.S. thanks the Alexander vonHumboldt Foundation for financial support.

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