DOI: 10.1002/cctc.201300235

Ni-Catalyzed Growth of Graphene Layers during ThermalAnnealing: Implications for the Synthesis of Carbon-Supported Pt�Ni Fuel-Cell CatalystsLin Gan, Stefan Rudi, Chunhua Cui, and Peter Strasser*[a]

Introduction

Pt-alloy nanoparticles (NPs) that contain 3d transition metals(such as Fe, Co, Ni, and Cu) that are typically supported ona high-surface-area carbon support represent a class of highlyactive electrocatalysts for the cathodic oxygen-reduction reac-tion (ORR) in fuel cells.[1] To achieve high activities, thermal an-nealing at elevated temperatures either during or after thesynthesis of the catalysts is generally required. Such annealingcan increase the extent of alloying,[1c, 2] form a new Pt-skin sur-face,[3] or induce the transformation from a disordered phaseinto an ordered intermetallic compound;[4] all of these structur-al features can result in higher intrinsic activities and/or stabili-ties. Despite these benefits, thermal annealing at high temper-atures also resulted in the sintering of the nanoparticles, thusleading to a decrease in the active surface area and, hence,low overall mass activities,[2b, 5] which hindered their practicalapplications. To further advance the synthesis and applicationof highly active and robust ORR catalysts, understanding thesintering of carbon-supported Pt alloy NPs during thermal an-nealing is crucial, although it is remains largely un-addressed.

Nevertheless, there have been a number of studies on thecatalyst-sintering mechanisms for monometallic catalysts (e.g. ,Pd and Pt) that are dispersed over oxide supports.[6] In princi-

ple, the driving force for particle sintering is the high surfaceenergy of NPs compared to the bulk surface, although Ostwaldripening and particle migration/coalescence are the two mainmechanisms. The former process involves the diffusion ofatomic species from smaller particles towards larger ones, ac-cording to the Gibbs–Thomson effect,[6a,c] and the latter pro-cess involves the migration of nanoparticles over the supportand the subsequent coalescence of neighboring particles. Bothof these mechanisms can be regarded as mass-transport overthe catalyst support and the support can also impact the sin-tering process, owing to metal–support interactions.[7] Basically,the knowledge of the sintering mechanisms that has been re-ported for oxide-supported monometallic catalysts can be ap-plied to carbon-supported Pt- and Pt-alloy catalysts. However,the influence of different (carbon) supports, as well as the exis-tence of a second metal in the Pt-alloy NPs, on their sinteringmechanisms still needs to be studied.

Herein, we investigate the sintering effect during the ther-mal annealing of a series of carbon-supported Pt1–xNix NPs,which are a class of highly active and stable ORR catalysts, inparticular at higher Ni content, as recently reported.[8] By usinghigh-resolution transmission electron microscopy (HRTEM), weuncover an unexpected solid-state transformation of thecarbon support into graphene-like multilayers, catalyzed bythe Ni-richer alloy NPs. The resultant instability of the carbonsupport in turn promotes the particle migration and coales-cence, thereby leading to more-significant particle coarseningin the Ni-richer alloy catalysts. Our results demonstrate uniquemetal–support interactions during the thermal annealing, thusproviding important implications for the synthesis of carbon-supported Pt-alloy fuel-cell catalysts.

[a] Dr. L. Gan, S. Rudi, Dr. C. H. Cui, Prof. Dr. P. StrasserThe Electrochemical Energy, Catalysis andMaterials Science LaboratoryTC03, Institute of ChemistryTechnical University BerlinStrasse des 17. Juni 124, Berlin (Germany)Fax: (+ 49) (030) 314 22261E-mail : pstrasser@tu-berlin.de

Thermal annealing is an important and widely adopted stepduring the synthesis of Pt bimetallic fuel-cell catalysts, al-though it faces the inevitable drawback of particle sintering.Understanding this sintering mechanism is important for thefuture development of highly active and robust fuel-cell cata-lysts. Herein, we studied the particle sintering during the ther-mal annealing of carbon-supported Pt1–xNix (PtNi, PtNi3, andPtNi5) nanoparticles, a reported recently class of highly activefuel-cell catalysts. By using high-resolution transmission elec-tron microscopy, we found that annealing at an intermediatetemperature (400 8C) effectively increased the extent of alloy-

ing without particle sintering; however, high-temperature an-nealing (800 8C) caused severe particle sintering, which, unex-pectedly, was strongly dependent on the composition of thealloy, thus showing that a higher Ni content resulted ina higher extent of particle sintering. This result can be ascribedto the solid-state transformation of the carbon support intographene layers, catalyzed by Ni-richer catalyst, which, in turn,promoted particle migration/coalescence and, hence, more-sig-nificant sintering. Therefore, our results provide important in-sight for the synthesis of carbon-supported Pt-alloy fuel-cellcatalysts.

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Results and Discussion

First, we synthesized monodispersed Pt1–xNix (PtNi, PtNi3, andPtNi5) NPs by using a well-developed low-temperature organic-phase-reduction method,[8a, 9] which were further dispersedover a high-surface-area carbon support (Vulcan XC). Then, theobtained supported Pt1–xNix catalysts were annealed in 4 vol. %H2/Ar at 400 8C (denoted as Pt1–xNix-400) or 800 8C (denoted asPt1–xNix-800) for 4 h. X-ray diffraction (XRD) analysis (Figure 1 a)

clearly shows that annealing at 400 8C leads to a substantialshift of the 111 diffraction peaks towards the Ni 111 peak. Thisshift can be ascribed to the existence of unalloyed amorphousNi oxide in the as-prepared NPs, which was reduced and al-loyed with Pt at high temperatures. At 800 8C, the extent of al-loying modestly increased further and the concomitant much-shaper diffraction peaks manifest significant growth in the cat-alyst NPs.

According to the thermodynamic phase diagram, both PtNiand PtNi3 alloys show order/disorder transformations at about700 8C. For the catalysts that were annealed at 400 8C, we didnot see the formation of ordered intermetallic compounds. At800 8C, a weak reflection at about 348 (the 110 diffractionpeak) indicated the slight formation of an ordered intermetallicPtNi phase in the PtNi-800 catalyst (Figure 1 b, enlarged fromFigure 1 a), but still no ordering effect was found in PtNi3-800

catalyst. This result suggests that an ordering effect could bequite difficult in the Pt�Ni nanoparticles or even does not existat higher Ni content.

To gain further insight into the catalyst-sintering process, weperformed TEM investigations on the Pt1–xNix catalysts thatwere annealed at different temperatures (Figure 2 andFigure 3). Consistent with the XRD results, the Pt1–xNix catalyststhat were annealed at 400 8C showed no clear particle growth

Figure 1. a) XRD patterns of as-prepared Pt�Ni and the Pt�Ni catalysts afterannealing at different temperatures; b) XRD patterns of the PtNi-400, PtNi-800, and PtNi3-800 catalysts, which show a weak ordering effect in the PtNi-800 catalyst, whereas the others remain in a disordered FCC phase.

Figure 2. TEM images of a, b) non-annealed PtNi3 and PtNi catalysts, respec-tively; c, d) PtNi3 and PtNi catalysts after annealing in H2/Ar at 400 8C, respec-tively; and e, f) PtNi3 and PtNi catalysts after annealing in H2 at 800 8C, re-spectively.

Figure 3. TEM images of the PtNi5 catalyst after annealing in H2/Ar at a) 400or b) 800 8C.

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(Figure 2 c, d and Figure 3 a), whereas, at 800 8C, the Pt1–xNix cat-alysts (Figure 2 e, f and Figure 3 b) showed clearly lager sizes.Unexpectedly, we found that the extent of particle sintering in-creased with increasing Ni content in the alloy NPs, thus show-ing much-larger particle sizes in the PtNi3-800 and PtNi5-800catalyst compared to those in the PtNi-800 catalyst. More inter-estingly, accompanying the particle growth, we also observedclear structural changes in the carbon support in the PtNi3-800and PtNi5-800 catalysts (Figure 2 e and Figure 3 b), thus show-ing the formation of graphene multi-layers (Figure 2 e, inset)surrounding the catalytic NPs. In contrast, no clear structuralchange on the carbon support was found in the PtNi-800 cata-lyst.

To quantitatively analyze the particle sintering, we per-formed statistical measurements of the sizes of over 100 NPs indifferent catalysts, from which the particle-size distributionsand the average sizes were obtained, as shown in Figure 4.Thus, we unambiguously confirmed that all of the catalysts

that were annealed at 400 8C showed similar size distributionsto the non-annealed catalysts, whereas, at 800 8C, the particlesizes become much larger. Attractively, at 800 8C, with in-creased Ni content from PtNi to PtNi5, the particle sizesbecame larger and larger and the size distribution becamebroader and broader.

From these above results, we found a good correlation be-tween the particle sintering and the structural changes of thecarbon support (i.e. , the formation of graphene layers), both ofwhich became more significant in the annealed catalysts with

higher Ni content. We consider that the graphene layers mustgrow from the carbon support through a solid-state transfor-mation, in which the surface Ni atoms in Ni-rich particles playa catalytic role. Indeed, the solid-state transformation of amor-phous carbon into graphene layers on pure 3d transitionmetals (Fe, Co, Ni, etc.) has recently been reported.[10] It wasfound that the amorphous carbon was dissolved in the metalcrystals at temperatures above 600 8C, followed by the nuclea-tion and growth of graphene layers on the metal surface.Based on this argument, a higher Ni content in the Pt�Ni cata-lysts would provide more nucleation sites and, hence, acceler-ate the solid-state transformation of the Vulcan carbon supportinto graphene layers. A direct consequence of this property isthe structural instability of the support during the thermal an-nealing, which could, in turn, promote the migration and coa-lescence of the NPs. This result nicely explains why a highercontent of Ni results in a higher degree of catalyst sintering inthe Pt�Ni catalysts.

To gain further insight into the catalytic role of Ni-rich alloyNPs on the formation of graphene multilayers, Figure 5 showsseveral typical HRTEM images of the graphene-like multi-layersthat were formed in the PtNi3-800 catalyst. The graphenelayers generally exhibited epitaxial growth on the surface ofthe PtNi3 NPs, which eventually resulted in the encapsulationof some NPs. It is also interesting that the epitaxial growth ofgraphene multi-layers also induced a shape change of some ofthe NPs into cubes or elongated cuboids (e.g. , Figure 5 b).Such a change in particle shape is considered to be character-istic of strong metal–support interactions.[7]

Owing to the importance of thermal annealing in the syn-thesis of Pt-alloy fuel-cell catalysts, in particular the most activePt�Ni alloys as reported recently,[3b,c, 8a] our results highlighta challenging issue in controlling the particle size during the

Figure 4. Particle-size distributions and average particle size, <d> , of thePtxNi1–x catalyst before and after annealing at 400 and 800 8C, which showthat a higher Ni content results in more-significant particle coarsening.

Figure 5. HRTEM image of the PtNi3-800 catalyst, which shows the formationof graphene multilayers on the PtNi3 NPs.

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annealing process. We have uncovered a detrimental effect onthe catalytic reactions between Ni-rich Pt-alloy NPs and carbonsupports with an amorphous structure or low crystallinityduring high-temperature annealing. In this context, we suggestthat, the extent of particle sintering could be decreased by an-nealing at lower temperatures, by limiting the content of tran-sition metals in the Pt alloys, or by engineering the supportstructure (for instance, the use of highly graphitized carbonnanostructures.[11]

Conclusions

We have studied how thermal annealing at elevated tempera-tures influences the crystal structure and particle sintering ofmonodispersed Pt1–xNix catalysts over a Vulcan XC carbon sup-port, which a recently reported class of highly active fuel-cellcatalysts. Whereas annealing at 400 8C clearly increased theextent of alloying without particle-size growth, clear particlesintering was observed at 800 8C. Unexpectedly, higher Ni con-tent resulted in more-significant particle sintering, which couldbe ascribed to a solid-state transformation of the carbon sup-port into graphene multi-layers catalyzed by Ni-richer particles,thereby leading to instability of the carbon support that, inturn, promotes the coalescence of Ni-richer NPs. These resultsprovide important implications in terms of the synthesis ofcarbon-supported Pt-alloy catalysts (in particular, the mostpromising Pt�Ni alloy catalysts, as recently reported).

Experimental Section

Monodisperse PtNi, PtNi3, and PtNi5 NPs were synthesized by or-ganic-phase reduction of the metal precursors with oleic acid andoleylamine as surfactants.[8a, 9b] The compositions of the nanoparti-cles were confirmed to be the same as the nominal ratios of theprecursors by using &&energy-dispersive X-ray&& (EDX) and in-ductively coupled plasma-atomic emission spectroscopy. Thesenanoparticles were further dispersed over a high-surface-areacarbon support (Vulcan XC-72) and heated in air at 180 8C for 1 hto remove residual surfactant. The obtained carbon-supported Pt�Ni catalysts were further annealed in 4 vol. % H2 in Ar at 400 8C or800 8C for 4 h.

X-ray diffraction patterns were recorded on a D8 Advance diffrac-tometer (Bruker) that was equipped with a Lynx Eye Detector anda KFL Cu 2K X-ray tube. TEM measurements were performed ona FEI Tecnai G2 20 S-TWIN transmission electron microscope(200 kV) that was equipped with a LaB6 cathode. Particle sizeswere manually measured by using Image J software. The measure-ments were based on several TEM images that were recorded atdifferent places on the TEM sample; for each image, all of the dis-cernible particles were measured. Then, the obtained particle sizeswere inputted into Originlab software to perform the frequencyanalysis.

Acknowledgements

We thank the Zentraleinrichtung f�r Elektronenmikroskopie(Zelmi) of the Technical University Berlin for their support with

the TEM and EDX analysis. This work was supported by a USDOE EERE award (DE-EE0000458) through a subcontract withGeneral Motors. P.S. acknowledges financial support from theCluster of Excellence in Catalysis (UniCat), which is funded by theDFG and managed by the TU Berlin.

Keywords: alloys · fuel cells · nickel · platinum · solid-statereactions

[1] a) H. A. Gasteiger, N. M. Markovic, Science 2009, 324, 48 – 49; b) M. K.Debe, Nature 2012, 486, 43 – 51; c) P. Strasser, S. Koh, T. Anniyev, J. Gree-ley, K. More, C. F. Yu, Z. C. Liu, S. Kaya, D. Nordlund, H. Ogasawara, M. F.Toney, A. Nilsson, Nat. Chem. 2010, 2, 454 – 460; d) P. Strasser, Rev.Chem. Eng. 2009, 25, 255 – 295; e) L. Gan, R. Yu, J. Luo, Z. Y. Cheng, J.Zhu, J. Phys. Chem. Lett. 2012, 3, 934 – 938.

[2] a) Z. Liu, C. Yu, I. Rusakova, D. Huang, P. Strasser, Top. Catal. 2008, 49,241 – 250; b) B. N. Wanjala, B. Fang, J. Luo, Y. S. Chen, J. Yin, M. H. Enge-hard, R. Loukrakpam, C. J. Zhong, J. Am. Chem. Soc. 2011, 133, 12714 –12727.

[3] a) V. R. Stamenkovic, B. S. Mun, M. Arenz, K. J. J. Mayrhofer, C. A. Lucas,G. F. Wang, P. N. Ross, N. M. Markovic, Nat. Mater. 2007, 6, 241 – 247;b) V. R. Stamenkovic, B. Fowler, B. S. Mun, G. F. Wang, P. N. Ross, C. A.Lucas, N. M. Markovic, Science 2007, 315, 493 – 497; c) C. Wang, M. F. Chi,D. G. Li, D. Strmcnik, D. van der Vliett, G. F. Wang, V. Komanicky, K. C.Chang, A. P. Paulikas, D. Tripkovic, J. Pearson, K. L. More, N. M. Markovic,V. R. Stamenkovic, J. Am. Chem. Soc. 2011, 133, 14396 – 14403; d) S.Chen, P. J. Ferreira, W. C. Sheng, N. Yabuuchi, L. F. Allard, Y. Shao-Horn, J.Am. Chem. Soc. 2008, 130, 13818 – 13819.

[4] a) J. Kim, Y. Lee, S. Sun, J. Am. Chem. Soc. 2010, 132, 4996 – 4997; b) D.Wang, H. L. Xin, R. Hovden, H. Wang, Y. Yu, D. A. Muller, F. J. DiSalvo,H. D. AbruÇa, Nat. Mater. 2012, 12, 81 – 87.

[5] C. Wang, G. F. Wang, D. van der Vliet, K. C. Chang, N. M. Markovic, V. R.Stamenkovic, Phys. Chem. Chem. Phys. 2010, 12, 6933 – 6939.

[6] a) C. T. Campbell, S. C. Parker, D. E. Starr, Science 2002, 298, 811 – 814;b) A. K. Datye, Q. Xu, K. C. Kharas, J. M. McCarty, Catal. Today 2006, 111,59 – 67; c) S. B. Simonsen, I. Chorkendorff, S. Dahl, M. Skoglundh, J. Se-hested, S. Helveg, J. Am. Chem. Soc. 2010, 132, 7968 – 7975; d) S. R.Challa, A. T. Delariva, T. W. Hansen, S. Helveg, J. Sehested, P. L. Hansen, F.Garzon, A. K. Datye, J. Am. Chem. Soc. 2011, 133, 20672 – 20675; e) S. B.Simonsen, I. Chorkendorff, S. Dahl, M. Skoglundh, J. Sehested, S.Helveg, J. Catal. 2011, 281, 147 – 155; f) Q. Xu, K. C. Kharas, B. J. Croley,A. K. Datye, ChemCatChem 2011, 3, 1004 – 1014.

[7] J. Liu, ChemCatChem 2011, 3, 934 – 948.[8] a) L. Gan, M. Heggen, S. Rudi, P. Strasser, Nano Lett. 2012, 12, 5423 –

5430; b) L. Gan, M. Heggen, R. O’Malley, B. Theobald, P. Strasser, NanoLett. 2013, 13, 1131 – 1138; c) C. Cui, L. Gan, H.-H. Li, S.-H. Yu, M.Heggen, P. Strasser, Nano Lett. 2012, 12, 5885 – 5889; d) C. Wang, M. F.Chi, G. F. Wang, D. van der Vliet, D. G. Li, K. More, H. H. Wang, J. A.Schlueter, N. M. Markovic, V. R. Stamenkovic, Adv. Funct. Mater. 2011, 21,147 – 152.

[9] a) K. Ahrenstorf, O. Albrecht, H. Heller, A. Kornowski, D. Gorlitz, H.Weller, Small 2007, 3, 271 – 274; b) S. Rudi, X. Tuaev, P. Strasser, Electro-catalysis 2012, 3, 265 – 273.

[10] a) J. A. Rodr�guez-Manzo, C. Pham-Huu, F. Banhart, ACS Nano 2011, 5,1529 – 1534; b) M. Zheng, K. Takei, B. Hsia, H. Fang, X. Zhang, N. Ferralis,H. Ko, Y.-L. Chueh, Y. Zhang, R. Maboudian, A. Javey, Appl. Phys. Lett.2010, 96, 063110 – 063113.

[11] a) F. Hasch�, M. Oezaslan, P. Strasser, Phys. Chem. Chem. Phys. 2010, 12,15251 – 15258; b) L. Gan, R. Lv, H. D. Du, B. H. Li, F. Y. Kang, Carbon 2009,47, 1833 – 1840; c) L. Gan, H. Du, B. Li, F. Kang, Chem. Commun. 2011,47, &&page numbers?&&.

Received: March 28, 2013Published online on && &&, 0000

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FULL PAPERS

L. Gan, S. Rudi, C. H. Cui, P. Strasser*

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Ni-Catalyzed Growth of GrapheneLayers during Thermal Annealing:Implications for the Synthesis ofCarbon-Supported Pt�Ni Fuel-CellCatalysts

Layer cake: The solid-state transforma-tion of a carbon support into graphene-like layers was achieved during the ther-mal annealing of carbon-supportedPtxNi1–x catalysts with higher Ni content.In turn, the resultant instability of thecarbon support promoted particle mi-gration and coalescence, thus leadingto more-significant particle sintering onNi-richer PtxNi1–x catalysts.

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