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September 29, 2015

C 2015 American Chemical Society

Carbon Monoxide-Induced Stabilityand Atomic Segregation Phenomenain Shape-Selected Octahedral PtNiNanoparticlesMahdi Ahmadi,† Chunhua Cui,‡ Hemma Mistry,†,§ Peter Strasser,‡ and Beatriz Roldan Cuenya*,§

†Department of Physics, University of Central Florida, Orlando, Florida 32816, United States, ‡Department of Chemistry, Chemical Engineering Division,Technical University Berlin, Berlin, Germany, and §Department of Physics, Ruhr-University Bochum, 44780 Bochum, 10623 Berlin, Germany

Bimetallic nanoparticle (NP) catalystshave been extensively investigatedin the past,1�5 due to the many ad-

vantages they could offer as compared totheir monometallic counterparts. Differentfactors such as synergetic effects betweenelements, a change in the electronic struc-ture, and strain effects have been heldresponsible for their high efficiency andtunable catalytic performance.4,6�8 Two ofthemost widely usedmetals in gaseous andliquid phase catalytic reactions are Pt andNi. In addition, a bimetallic combination ofsuch metals as an alloy or core�shell struc-ture has also been shown to display superiorcatalytic performance for reactions such asethanol reforming,9 methane andmethanol

reformingwith CO and CO2,10�13 oxygen re-

duction reactions,4,14�17 CO oxidation,15,18�23

and methanol electro-oxidation.8,24,25

The spatial atomic distribution in bime-tallic catalysts plays a pivotal role in theirelectronic properties, in particular, in theshift of the d-band center, which is knownto affect the binding energy of reactants tothe NP surface.7,26,27 Therefore, understand-ing the structure and chemical compositionof bimetallic NPs is of utmost importance forthe rational design of highly efficient bime-tallic catalysts. For example, Gauthier et al.observed higher binding energy of CO on Ptatoms in the PtCo(111) systemby increasingthe number of nearest neighbor Co atoms,and they explained this behavior through a

* Address correspondence toBeatriz.Roldan@rub.de.

Received for review March 25, 2015and accepted September 29, 2015.

Published online10.1021/acsnano.5b01807

ABSTRACT The chemical and morphological stability of size- and

shape-selected octahedral PtNi nanoparticles (NP) were investigated after

different annealing treatments up to a maximum temperature of 700 �Cin a vacuum and under 1 bar of CO. Atomic force microscopy was used to

examine the mobility of the NPs and their stability against coarsening, and

X-ray photoelectron spectroscopy to study the surface composition,

chemical state of Pt and Ni in the NPs, and thermally and CO-induced

atomic segregation trends. Exposing the samples to 1 bar of CO at room

temperature before annealing in a vacuum was found to be effective at

enhancing the stability of the NPs against coarsening. In contrast,

significant coarsening was observed when the sample was annealed in

1 bar of CO, most likely as a result of Ni(CO)4 formation and their enhanced mobility on the support surface. Sample exposure to CO at room temperature

prior to annealing led to the segregation of Pt to the NP surface. Nevertheless, oxidic PtOx and NiOx species still remained at the NP surface, and, irrespective

of the initial sample pretreatment, Ni surface segregation was observed upon annealing in a vacuum at moderate temperature (T < 300 �C). Interestingly,a distinct atomic segregation trend was detected between 300 and 500 �C for the sample pre-exposed to CO; namely, Ni surface segregation was partiallyhindered. This might be attributed to the higher bonding energy of CO to Pt as compared to Ni. Annealing in the presence of 1 bar CO also resulted in the

initial surface segregation of Ni (T< 400 �C) as long as PtOx and NiOx species were available on the surface as a result of the higher affinity of Ni for oxygen.Above 500 �C, and regardless of the sample pretreatment, the diffusion of Pt atoms to the NP surface and the formation of a Ni�Pt alloy are observed.

KEYWORDS: CO . XPS . AFM . atomic segregation . Pt . Ni . nanoparticles

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shift in the d-band center toward the Fermi level.28

Such a shift was also held responsible for the higheractivity of Ni�Pt�Pt(111) as compared to Pt�Ni�Pt(111) for the reforming of oxygenates.29�31 In con-trast, for hydrogenation reactions, a d-band centerfurther away from the Fermi level was reported to leadto higher hydrogenation activity.32�34

Strain effects are also important in understandingthe behavior of bimetallic catalysts. For example, lowerbinding energy of oxygen to Pt(shell)Ni(core) as com-pared to Pt(111) was shown and assigned to thecompression of the Pt lattice in the former structure.Nevertheless, by increasing the Ni concentration onthe NP surface, an increase in the binding energy of O2

has been reported.35 In contrast, in the same PtNibimetallic system, the binding energy of CO wasreported to first decrease with increasing Ni concen-tration, but was found to increase again for very largeNi surface contents upon segregation.36,37

Although the initial atomic distribution of the differ-ent metallic components in bimetallic nanocatalystsis of importance, structural changes during catalystoperation conditions are key to understanding thecatalyst reactivity. The surface energy, atomic size,chemical ordering energy, composition, size of theNPs, and the adsorption enthalpy of adsorbatesare all key factors affecting the segregation and struc-ture of nanoscale catalysts.12,20,28,38�50 The presentwork addresses the role of adsorbates such as CO onthe structural and chemical stability of shape-selectedoctahedral PtNi NPs at different temperatures.Carbon monoxide is one of most commonly used

and produced molecules in catalytic reactions. There-fore, understanding CO-induced structural changes innanoscale catalysts is valuable for designing catalystswith higher stability, yield, and selectivity. Mayrhoferet al. have reported CO-induced segregation of Ptatoms to the surface of PtxCoy alloys through bothgas phase annealing and electrochemical CO potentialcycling in CO-saturated electrolyte and explained thisbehavior based on the higher adsorption enthalpy ofCO on Pt as compared to Co.38 Tenny et al. observed Ptsegregation to the surface of bimetallic PtAu NPs uponCO exposure at room temperature.51 The surface com-position of Ni/Cu(100) was studied during the hydro-genation of CO2, and Cu segregation to the surfacewas observed. However, by adding CO to the reactantstream, such segregation was hindered due to thehigher binding energy of CO to Ni as compared toCu.11,12,40 A dissimilar trendwas observed in the case ofCo/Cu(100) because of the lower binding energy of COto Co as compared to Ni.11,12,52 Additionally, Ni surfacesegregation in NixCu1�x alloys in the presence of COandO2was predicted theoretically.

40 An increase in thesurface concentration of Ruwas theoretically predictedafter exposing Pt/Ru(0001) samples to 1 bar of CO.43

Surface segregation of Pd and Cu in AuPd(100) single

crystals and CuPt near-surface alloys was observed inthe presence of CO.39,53,54 Interestingly, a negativesegregation energy for Ni atoms dissolved in Pt waspredicted in the presence of CO, which leads to apreference of Ni to stay in the interior of bimetallicNiPt NPs.40

Because of the importance of the PtNi system as abimetallic catalyst and CO as a reactant, we presenthere an investigation of the stability and atomic seg-regation phenomena in size- and shape-controlledPtNi NPs supported on highly oriented pyrolyticgraphite (HOPG) supports in the presence of CO. Theex situ NP preparation method and initially oxidizednature of the as-prepared NPsmake this study relevantfor industrial applications. Since the segregation phe-nomena also depend on the crystal orientation,55,56 inorder to reduce the complexity of the multiple param-eters simultaneously affecting segregation phenom-ena at the nanoscale, wehave prepared octahedral NPswith just (111) facets.The NP surface composition and their coarsening

behavior were examined using X-ray photoelectronspectroscopy (XPS) and atomic force microscopy(AFM). The shape of the as-prepared NPs was resolvedusing transmission electron microscopy (TEM).

RESULTS AND DISCUSSION

Figure 1 shows a TEM image of our octahedralPt50Ni50 NPs. According to TEM the NPs are ∼9.0 (1.1 nm in diameter and have a narrow size distri-bution.57

Ex situAFM images of three initially identical Pt50Ni50NP samples acquired at 25 �C are displayed in Figure 2.Images a�d correspond to sample S1 and wereacquired after annealing under vacuum for 20 min atthe indicated temperatures. Images e�h were ob-tained from sample S2 after in situ exposure to 1 barof CO at 25 �C followed by sample annealing during

Figure 1. TEM image of octahedral Pt0.5Ni0.5 NPs.

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20 min under vacuum at the indicated temperatures,and i�l after annealing in 1 bar of CO for 10 min at thegiven temperatures. In the as-prepared samples the NPsare homogeneously dispersed over the entire substratesurface. In all samples, the decoration of the HOPG stepedges by the NPs at moderate temperature indicatestheir high mobility on the support surface.The average NP size as a function of temperature is

shown in Figure 3. A minimum of ∼150 NPs has beenconsidered for each data point. In order to bettercompare the evolution of the size of the CO-exposedand vacuum-treated samples, the NP sizes arenormalized by the initial (as-prepared) average size.The progressive decrease in the NP height of ∼15%

observed after CO dosing and subsequent annealingin a vacuum (S2) could be explained by the reductionof the initially oxidized NPs and removal of oxygenfrom the NP structure, which is corroborated by theXPS data shown in the next section. In contrast,vacuum annealing without any CO pre-exposure(S1) led to a slight decrease (∼15%) in NP height atmoderate temperature (<200 �C), while significantcoarsening (∼45% increase in height) was seen afterannealing above 250 �C. On the other hand, whenthe NPs were annealed in 1 bar of CO (S3), drasticcoarsening (over 20 times the initial size) was foundto take place even at moderate temperature (150�200 �C). The high coarsening rate of the NPs in S3maybe explained on the basis of the formation of Ni(CO)4species, their diffusion on the graphite surface, andsubsequent decomposition. In particular, Shen et al.showed a high Ni carbonyl decomposition rate attemperatures above 130 �C.58,59 Although nickel carbo-nyl formation might take place even at room tempera-ture, the presence of NiOx species on the NPs in S2 attemperatures below100 �C is expected to hinder such aprocess and therefore, contrary to the case of sampleS3, no low-temperature coarsening is expected for S2,which was only pre-exposed to CO at 25 �C.XPS spectra of Pt-4f (a�c) and Ni-2p (d�f) core-level

regions of samples S1, S2, and S3 are shown in Figure 4.Additional XPS spectra (and the correspondinganalysis) of sample S2 before CO exposure but afterannealing are shown in Figure S1. As described above,two initially identical samples were prepared. SampleS1 was annealed under vacuum in the absence of any

Figure 3. Normalized height of Pt0.5Ni0.5 NPs after a thermaltreatment under vacuum (squares, S1), after 1 bar COdosing at 25 �C followed by annealing under vacuum(circles, S2), and after annealing in 1 bar of CO (triangles,S3). Normalized height for S3 is shown on the right axis, andfor samples S1 and S2 on the left axis.

Figure 2. AFM images of octahedral Pt0.5Ni0.5 NPs supported on HOPG acquired ex situ (air) at room temperature (RT) afterannealing in UHV (top row), after dosing 1 bar of CO at 25 �C and subsequent annealing in UHV (middle row), and afterannealing in 1 bar of CO (bottom row). All images display a 1 � 1 μm sample region.

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gas dosing, and XPS spectra were acquired after eachannealing step, Figure 4a,d (S1). Sample S2 was firstannealed under vacuum and subsequently exposed to1 bar CO at room temperature, Figure 4b,e (S2), aftereach annealing step. Sample S3 was annealed in thepresence of 1 bar CO, and XPS spectra were acquiredafter each annealing step, Figure 4c,f (S3). The Pt-4fcore-level region was deconvoluted by three doubletscorresponding to metallic Pt (4f7/2, 71.1 eV), Pt

2þ (4f7/2,71.8 eV), and Pt4þ (4f7/2, 73.4 eV).

60�62 The Ni-2p regionwas also fitted with two doublets assigned to metallicNi (2p3/2, 852.2 eV), Ni2þ (2p3/2, 856.3 eV), and satelliteNi features (861�863.5 eV).63�65 The Ni-2p region ofsample S3 was also fitted with an additional doubletcorresponding to Ni(CO)x species (Ni-2p3/2, 854 eV).

66 Itis worth mentioning that a binding energy increase ofonly ∼0.2 eV is expected for Pt and Ni upon formationof the Ni�Pt alloy,67 which is close to the limit of ourexperimental resolution for these kinds of NP samples,where, even in the metallic state for monometallicsystems, positive binding energy shifts are observeddue to initial- and final-state effects.The phase content of the different metallic and

oxidic Pt and Ni species extracted from the XPS spectrain Figure 4 is displayed in Figure 5. The Ni atoms in thefresh samples are mostly oxidized (∼98%), with Pt alsopartially oxidized (∼40�50%). Annealing up to 700 �Cin UHV results in the partial reduction of Ni2þ

(∼50�55%) for samples S1 and S2. With increasingannealing temperature, a decrease in the PtO2 andparallel increase in the PtO signal are observed,

indicating the stepwise decomposition of the oxidizedPt species. Annealing in 1 bar CO also resulted in thepartial reduction of the NPs, with ∼10�15% PtOx and∼30% NiOx remaining at 700 �C. Furthermore, theformation of nickel carbonyl species was also observedbetween 300 and 500 �C. The reduction rate of PtOx

and NiOx was not significantly affected by room-tem-perature CO exposure (see Figure S2 and Table S1).However, annealing in 1 bar CO (S3) led to a strongerdecrease in the content of Ni2þ species, with ∼30% at700 �C for S3 and ∼50% for S1 (vacuum annealed) at700 �C.In order to gain insight into atomic segregation

phenomena, the Ni/Pt ratio was obtained from theXPS data, Figure 6. Two competitive causes are respon-sible for the atomic segregation trends observed: (i) thehigher binding energy of CO to Pt as compared to Niand larger size of the Pt atoms leading to Pt surfacesegregation, (ii) the larger oxygen affinity of Ni ascompared to Pt (oxygen is initially present in theex situ as-prepared samples due to air exposure afterpreparation) leading to Ni surface segregation.An increase in the Ni to Pt atomic ratio was obtained

initially for S2 up to 300�C, for S3 up to 400�C, and for S1up to 500�C.20 This phenomenonmay be explained onthe basis of the initial presence of PtOx species on thesurface of the as-preparedNPs and consequent surfacesegregation of Ni atoms and Pt�Ni exchange as aresult of the higher oxygen affinity for Ni as comparedto Pt. Between 300 and 500 �C, however, the Ni to Ptratio in sample S2 did not change significantly, while

Figure 4. XPS spectra from the (a�c) Pt-4f and (d�f) Ni-2p core-level regions of Pt0.5Ni0.5 NPs supported on HOPG acquired at25 �C as prepared (fresh) and after annealing in UHV (S1) (a, d) and after exposure to 1 bar CO at 25 �C before each annealingtreatment and subsequent annealing in UHV (b, e) (S2) and after annealing in 1-bar CO (c, f) (S3).

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it continued to increase when no CO had beendosed before annealing (S1). This is in agreement withearlier theoretical calculations predicting a negativeheat of segregation for Ni atoms dissolved in Pt inthe presence of CO. The CO environment favors thatNi stays subsurface, closer to the NP core. In otherwords, because of the higher chemisorption bondenergy of CO on Pt (134 kJ/mol) as compared to Ni(126 kJ/mol), Ni segregation to theNP surface becomesthermodynamically unfavorable.40 The lower coarsen-ing rate observed for the CO pre-exposed sample (S2)

as compared to the vacuum-annealed sample (S1)could be explained based on the distinct initial (pre-UHV annealing) atomic arrangement of Pt and Niinduced by the CO chemisorption. As was shown, theinitial CO exposure leads to Pt segregation to the NPsurface, which appears to contribute to the NP stabi-lization against low-temperature coarsening undervacuum. On the other hand, the drastic coarseningobserved in sample S3 after annealing in 1 bar of CO islikely related to the formation of Ni-carbonyl speciesupon moderate temperature annealing. Such volatilespecies are highly mobile on the graphite surfaceand might contribute to the increase in the NP sizevia adsorbate-driven coarsening. It is plausible thatNi(CO)x species transfer from small to larger NPs, givingrise to coarsened NPs with Ni-rich overlayers.58

Further annealing at higher temperature (T > 500 �C)causes the reduction of Ni species and back diffusionof Pt atoms and formation of a PtNi alloy. Sincethe desorption temperature of CO from Ni is less than500 �C, no difference between the CO-pre-exposedand vacuum-annealed samples is expected at highertemperature (T > 500 �C).68 It should be noted that theobservation of a larger Pt concentration on the surfaceof the CO-pre-exposed NPs at all temperaturesmight explain the higher stability of the CO-exposedsample (S2) as compared to the vacuum-annealedsample (S1) observed via AFM. Since Pt has a highermelting temperature than Ni, the larger concentrationof Pt on the surface might help to decrease thecoarsening process. The contrary would be the caseof S3 (annealed in 1 bar of CO), where the highercontent of Ni species on the NP surface, particularlyNi-carbonyl species, appears to favor low-temperaturecoarsening.

Figure 6. Normalized Ni to Pt atomic ratios extracted fromXPS measurements of Pt0.5Ni0.5 NPs in samples S1, S2, andS3 displayed after different annealing treatments and ac-quired at 25 �C. The Ni/Pt ratio for S3 is shown on the rightaxis, while the Ni/Pt ratio for all other data is shown on theleft axis. The insets showmodels describing the segregationof Pt andNi atoms in theseNPs in their as-prepared state andafter annealing from 25 to 500 �C under vacuum (S1), afterpre-exposure to CO at 25 �C and annealing at 500 �C undervacuum (S2), and after annealing at 400 �C in 1bar of CO. Thedata point labeled as a star corresponds to S2 before COexposure at 25 �C, and the rest of the data points labeled ascircles for S2were acquired after pre-exposure to COat 25 �Cand subsequent annealing at the temperatures indicated.

Figure 5. Relative content of Pt (a,b,c) andNi (d,e,f) species extracted from the analysis of XPS data acquired after annealing inUHV (S1) (a, d) and after exposure to 1 bar CO at room temperature and subsequent annealing in UHV (S2) (b, e) and afterannealing in 1 bar CO (c, f) (S3).

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In fact, a very drastic increase in the Ni/Pt ratio wasobserved below 400 �C for the sample annealed in CO(S3), Figure 6. This trend is explained on the basis of thehigher affinity of Ni for oxygen, leading to Ni surfacesegregation as long as oxide species are available onthe NP surface, but also based on the decrease in the Ptsignal expected due to the NP coarsening observed via

AFM. Since Pt is in the NP core and Ni on the surface,the thicker Ni overlayer present on the coarsened NPs(due to the interparticle migration of Ni-carbonylspecies and redeposition on the larger NPs) is expectedto lead to the apparent decrease of the Pt signal andincrease of the Ni/Pt ratio. The Pt signal is later on atleast partially recovered upon annealing at and above500 �C, indicating a reversible Pt surface segregationand the formation of a Pt�Ni alloy. It is worth mention-ing that a maximum Ni/Pt ratio was reached at 400 �Cfor S3, while it happened at 500 �C for S2. Thisdifference could be explained on the basis of thehigher reduction rate of PtOx and NiOx species onsample S3, since Ni surface segregation is expectedto stop when oxygen is not available at the NP surface.The insets in Figure 6 display the atomic distribution

of Pt, Ni, and O within the NPs in their as-preparedstate, aswell as after annealing under vacuumat 500 �Cbefore (S1) and after exposure to 1 bar of CO at 25 �C(S2) and after annealing in 1 bar CO (S3). On the basis ofan earlier TEM study,69 the as-prepared sample wasfound to contain Pt-rich edges and corners, while thefacets and NP core are Ni-rich. In addition, because ofthe ex situ preparationmethod, the NP ismostly oxidized.After annealing under vacuum at 500 �C (S1) a higherconcentrationofNi atoms is observedat theNP surface asa result of their atomic exchangewith Pt oxide species onthe surface. Furthermore, annealing at 500 �C in UHV ledto the partial reduction of the NPs. In contrast, when theNPsarepre-exposed toCOat25 �Cbeforeeachannealingstep up to 500 �C (S2), a higher concentration of Pt on theNP surface is detected as a result of stronger bondingenergy of CO to Pt as compared to Ni. It should be notedthat the octahedral shape of our NPs is a result of thekinetic preparation procedure, and it is not the moststable shape. Therefore, a change in theNP shape towarda spherical shape is expected at moderate temperature.Such shape change (octahedral to spherical) is alreadyindicated in themodel of S3, the sample annealed in1barof CO,where significant coarseningwas observed alreadyat temperatures as lowas 200 �C (Figure 2).Moreover, thepresence of a Ni-rich overlayer (Ni-carbonyl) and Pt-richcore extracted from the analysis of our XPS data is alsoillustrated in the model.

CONCLUSIONS

In this study, the effect of CO pre-exposure onthermally induced atomic segregation trends and itsconsequent effect on the stability of PtNi NPs wasexplored. Size- and shape-controlled Pt0.5Ni0.5 NPs sup-ported on HOPG were used to investigate atomic segre-gation phenomena in the presence of carbonmonoxide.For samples annealed under vacuum, an enhancedstability against particle coarsening was observed if thesamples were pre-exposed to CO at 25 �C before eachthermal treatment under vacuum (S2). This could beattributed to the higher concentration of Pt atoms onthe NP surface due to Pt surface segregation, as wasevidenced by XPS measurements. In contrast, whenthe samples were annealed in CO, drastic coarseningwas detected at moderate temperatures (<250 �C). In allcases, significant NP mobility and the decoration of stepedges of the HOPG support were seen.Complex and distinctly different segregation pat-

terns were observed for samples annealed undervacuum in the absence and presence of a CO-pretreatment at 25 �C. At moderate annealing tem-peratures under vacuum (<300 �C), the segregation ofNi atoms to theNP surfacewas observed for the pristine(S1), the CO-pre-exposed (S2), and 1 bar CO annealed(S3) samples. This segregation trend is explained bythe presence of oxygen species on the NP surface ofthe as-prepared samples and the higher affinity of Ni tooxygen. Between 300 and 500 �C, Ni surface segrega-tion continued to occur for the pristine vacuum-annealed sample (S1), while it ceased for the 25 �CCO-exposed sample (S2). This is likely assigned tothe negative heat of segregation of Ni dissolved in Ptand the preference of Ni to stay subsurface in thepresence of CO. A drastic increase in the Ni/Pt ratiowas observedwhen the sample was annealed at 400 �Cin the presence of 1 bar CO. This is explained on thebasis of a combinationof the formationof a thickNi-richoverlayer via the migration of volatile Ni(CO)x speciesfrom small to larger NPs as well as NP coarsening,leading to a suppression of the Pt signal within thecore of the larger NPs. After annealing at 500 �C, stronginterdiffusion and Pt�Ni alloy formation are detectedfor all samples via the strong decrease in the Ni to Ptratio measured.Since the composition of the NP surface plays a

pivotal role in the stability and reactivity of bimetallicnanoparticles, our findings are expected to be valuablefor the optimization of the performance of nanocata-lysts for reactions involving a CO environment.

EXPERIMENTAL SECTIONOctahedral Pt0.5Ni0.5 bimetallic NPswere prepared via a solvo-

thermal process at elevated pressure, with dimethylformamide

(DMF) serving as solvent, surfactant, and reducing agent.14,57,69

The as-prepared NP solution was 5-fold diluted with 2-propanol

and drop-coated on HOPG substrates after 10 min of sonication.

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The synthesis method used is surfactant free, and no residueswere detected in the samples from the solvent employed viaXPS. HOPG has been selected as substrate due to it low rough-ness, which makes it ideal for AFM imaging, high conductivity,which is required for further electrochemical applications, andits low interaction with the PtNi NPs, allowing us thus to ruleout the influence of NP�support interactions in the adsorbate(CO)-induced atomic segregation trends observed.In order to extract information about the chemical state and

surface composition of the NPs, XPS measurements were ac-quired using a monochromatic X-ray source (Al KR, 1486.6 eV)operating at 350 W and a hemispherical electron analyzer(Phoibos 100, SPECS GmbH). Three initially identical Pt50Ni50samples were prepared. One of the samples (S1) was stepwiseannealed under vacuum (∼1� 10�10mbar) at 150 �C, 300 �C, andup to 700 �C for 20 min at each temperature in 100 �C intervals.A second sample (S2) was exposed to 1 bar CO for 10min at roomtemperature (prior to each annealing step) and subsequentlystepwise annealed under vacuum under the same conditions asS1. For sample S2, after annealing under vacuum at a giventemperature, XPS spectra were taken before (S2-before) and afterCO dosing at 25 �C (S2). Sample exposure to COwas conducted ina high-pressure (HP) reaction cell (SPECS GmbH). For the thirdsample (S3), after annealing in 1 bar CO in a HP reaction cell, XPSspectrawere taken. After each exposure, theHP cell was evacuatedto∼1� 10�8mbar and the sample transferred in situdirectly to theXPS analysis chamberwithout exposure to air. All XPS spectra werealigned using the graphite C-1s peak at 284.3 eV as a reference.TEM images were obtained with a FEI TECNAI G2 20 S-TWIN

TEM operating at an accelerating voltage of 200 kV. To inves-tigate NP mobility and coarsening after different thermal andchemical treatments, AFM images were acquired ex situ at roomtemperature in tapping mode with a Nanoscope-III DigitalInstruments microscope. The AFM study was conducted onthree identical Pt50Ni50 samples. Two of them were treated asdescribed before for the XPS study (S1 and S2) with 50 �Ctemperature increment, and a third sample (S3) was annealedin 1 bar of CO from 50 to 200 �C in 50 �C intervals. A summary ofthe different treatments is shown in Scheme 1.

Conflict of Interest: The authors declare no competingfinancial interest.

Acknowledgment. Financial support from the U.S. NationalScience Foundation (NSF CHE-1213182) is greatly appreciated.Additional financial support was provided by the Cluster ofExcellence RESOLV (EXC 1069) at RUB funded by the DeutscheForschungsgemeinschaft. Partial financial support by theGerman Research Foundation (DFG) through grants STR596/4-1 (“Pt stability”) and STR 596/5-1 (“Shaped Pt bimetallics”)is gratefully acknowledged.

Supporting Information Available: The Supporting Informa-tion is available free of charge on the ACS Publications websiteat DOI: 10.1021/acsnano.5b01807.

Additional XPS data and related analysis (PDF)

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Scheme 1. Overview of the Different Treatments Under-went by the Pt0.5Ni0.5 NPs

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