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Correlation Between Surface Chemistry and Electrocatalytic Properties of Monodisperse Pt x Ni 1 − x Nanoparticles

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Chao Wang , Miaofang Chi , Guofeng Wang , Dennis van der Vliet , Dongguo Li ,

Karren More , Hsien-Hau Wang , John A. Schlueter , Nenad M. Markovic , and Vojislav R. Stamenkovic *

Monodisperse and homogeneous Pt x Ni 1 − x alloy nanoparticles of various compositions are synthesized via an organic solution approach in order to reveal the correlation between surface chemistry and their electrocatalytic properties. Atomic-level microscopic analysis of the compositional profi le and modeling of nanoparticle structure are combined to follow the dependence of Ni dissolution on the initial alloy composition and formation of the Pt-skeleton nanostructures. The developed approach and acquired knowledge about surface structure-property correlation can be further generalized and applied towards the design of advanced functional nanomaterials.

1. Introduction

The need for noble metal Pt as a catalyst for the oxygen reduc-tion reaction (ORR) in polymer electrolyte membrane fuel cells limits the large-scale application of this technology. [ 1 ] In general, there are two different approaches to overcome this limitation: (i) Using a non-precious metal instead of Pt, which however has not achieved in neither activity nor stability to the same level as Pt-based materials under fuel cell conditions; [ 2 ] (ii) Alloying Pt with other transition metals, relying on the fi ne tuning of Pt electronic properties to substantially improve its catalytic performance and thus reduce the amount of noble metal needed. [ 3 ] Our previous work has demonstrated remarkable catalytic activity enhance-ment (factor of 4–10 relative to Pt) in bimetallic PtM (M = Fe, Co, and Ni) alloys toward the ORR on extended surfaces. The mecha-nism of the observed enhancement was ascribed to the reduced

© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, WeinheimAdv. Funct. Mater. 2011, 21, 147–152

DOI: 10.1002/adfm.201001138

Dr. C. Wang , D. van der Vliet , D. Li , H.-H. Wang , J. A. Schlueter , N. M. Markovic , V. R. Stamenkovic Materials Science DivisionArgonne National LaboratoryArgonne, IL 60559, USA E-mail: vrstamenkovic@anl.gov Dr. M. Chi , K. More Division of Material Science and TechnologyOak Ridge National LaboratoryOak Ridge, TN 37831, USA Dr. G. Wang Department of Mechanical EngineeringIndiana University-Purdue University IndianapolisIndianapolis, IN 46202, USA

adsorption of oxygenated spectator species (e.g., OH − ), which was caused by modifi ed electronic properties of Pt in the nanoseg-regated near-surface region. [ 4 ] Meanwhile, many studies in the literature have been devoted to pursuing the synthesis of Pt-based alloy nanocatalysts with characteristics that mimic the advanced electronic, struc-tural and ultimately catalytic properties that have been established on extended surfaces. Usually the catalysts are synthesized by con-ventional impregnation method in the form of Pt-bimetallic nanoparticles (NPs) dis-persed in high-surface-area carbon matrix. [ 5 ] lloying elements, [ 5 b] particle size [ 5 a] and pre-

Topics like nature of a

treatment conditions [ 5 e, 5 f ] have been examined. However, another important factor, the dependence of electrocatalytic properties on alloy composition, has not been systematically addressed even though it is known that the ratio between alloying components plays an important role in defi ning the physical and chemical properties of alloy materials. [ 6 ] In addition to the recent reports on bimetallic nanocatalysts, [ 5 g, 7 ] the development of advanced synthetic routes are still demanded in order to provide fi ne con-trol over critical parameters such as particle size and composition profi le at nanoscale for Pt-bimetallic alloy catalysts. Considering that catalytic processes are taking place at the surfaces, a system-atic study of the surface chemistry and catalytic properties of the alloy catalysts with various compositions would be an important supplement to elucidate the underlying structure/composition-property correlation in these systems.

Here we report the synthesis of monodisperse Pt x Ni 1– x NPs with controlled compositions and their application as electrocatalysts for the ORR. We have applied state-of-the-art microscopy techniques with atomic-resolution to investigate the nanostructure evolution in the electrochemical environments. We found that the dissolu-tion of the 3 d transition element to a large extent depended on the initial composition of the alloy NPs, which in turn determined the catalytic performance of the alloy catalysts. The observed composi-tion effect and its infl uence on the surface chemistry of the alloy catalysts were further postulated via theoretical modeling of the NP composition profi le by Monte Carlo simulation.

2. Results and Discussion

Organic solvothermal synthesis has been demonstrated as a robust method for preparing monodisperse alloy NPs with size

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Figure 1 . Representative TEM (a,b) and HRTEM (c,d) images of 5 nm Pt 3 Ni and PtNi NPs, respectively. e) XRD patterns of Pt x Ni 1– x alloy NPs of various compositions, with the positions of (111) peak labeled by a dashed line.

control and homogeneous compositions which have shown great potential in catalytic applications, [ 1 c, 8 ] and therefore becomes a more advantageous way in tailoring the properties of alloy catalysts than conventional impregnation methods. In this work we have applied an organic solvothermal synthesis modifi ed from previous reports [ 8 , 9 ] to control the composition of monodisperse Pt x Ni 1– x NPs. The particle size was adjusted to be around the optimal value ( ∼ 5 nm) for catalytic perform-ance as reported in our recent work. [ 8 ] Platinum acetylac-etonate, Pt(acac) 2 and Nickel acetate, Ni(ac) 2 were reduced by 1,2-tetradecanediol in the presence of oleic acid and oleylamine as surfactants (see the Experimental Section for details). By adjusting the ratio between Pt and Ni precursors we were able to obtain Pt x Ni 1– x NPs with different compositions. Figure 1 a and 1 b show representative transmission electron microscopy (TEM) images of Pt 3 Ni and PtNi (Pt/Ni = 1/1) NPs obtained with the ratios of 1:1.5 and 1:2 between Pt(acac) 2 and Ni(ac) 2 respectively, while the atomic ratios between Pt and Ni in the NPs were characterized by Energy-dispersive X-ray spectros-copy (EDX) (Figure S1 in the Supporting Information). Adding more Ni(ac) 2 can produce Ni-rich NPs, with 1:3.7 for PtNi 2 and 1:5 for PtNi 3 (Figure S2). In all cases monodisperse NPs were obtained and the particle size was confi rmed to be ∼ 5 nm.

HRTEM images of the NPs show lattice fringes with inter-fringe distance measured to be 0.22 ∼ 0.24 nm, corresponding to (111) planes of Pt-Ni alloy in face-centered cubic (fcc) phase (Figure 1 c, 1 d, S2b, and S2d). The crystal structure was further verifi ed by X-ray diffraction (XRD) patterns of the as-synthesized Pt x Ni 1 − x NPs (Figure 1 e and Figure S3). [ 10 ] As the Ni ratio in the NPs increases, the main peaks slightly shift toward high angle (as labeled for (111) peak in Figure 1 e), indicating a decrease in lattice constant caused by the replacement of Pt with Ni in the crystal lattice. [ 11 ]

The as-synthesized NPs were supported on carbon black (Tanaka, ∼ 900 m 2 g − 1 ) via a colloidal-deposition approach [ 6 c] and NP surfactant was removed by heating the catalyst in an oxygen atmosphere. [ 12 ] After surfactant removal, the same set of analytical tools was applied to verify the particle

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properties, and we did not observe any signifi cant change in particle size or crystal structure, e.g., the particles remained in the fcc phase (Figure S4). Electrochemical properties of the NPs were characterized by rotational disk electrode (RDE) method (see the Experimental Section for details) with the results summarized in Figure 2 . Cyclic voltam-metries (CVs) show typical Pt-like H upd features, while the oxidation peaks in the anodic scan ( ∼ 0.9 V) for the Pt x Ni 1– x catalysts have a positive shift compared to the state-of-the-art Pt/carbon catalyst (6 nm, Tanaka) (Figure 2 a). A similar pat-tern is also observed for the reduction peak in the cathodic scan ( ∼ 0.8 V). These differences have been assigned to the reduced adsorption strength of oxygenated species on these alloy surfaces, and more fundamentally, due to the downshift of the surface d band center in the presence of subsurface Ni. [ 13 ] The altered (reduced) surface coverage of oxides led to a substantial enhancement in the ORR catalytic activity for Pt x Ni 1– x catalysts, as evidenced by the positive shift of half-wave potentials in the polarization curves versus Pt/carbon (Figure 2 b). At 0.9 V, the Pt x Ni 1 − x /carbon catalysts show about 3 ∼ 4 fold enhancement in specifi c activity compared with the Pt/carbon benchmark (Figure 2 c). Among the alloy catalysts, the ones with intermediate Pt/Ni ratio, i.e. PtNi and PtNi 2 , exhibited higher activity than the others, reaching 3.9 and 3.6 mA cm − 2 for PtNi and PtNi 2 , against 3 and 3.3 mA cm − 2 for Pt 3 Ni and PtNi 3 , respectively. The sur-face area calculated from H upd was almost the same for all the four catalysts, as refl ected by the similar H upd peaks ( E < 0.4 V) in the CVs. This confi rms that suffi cient control over experimental parameters such as particle size, alloy homoge-neity, Pt and total metal loading has been achieved, which is critical for systematic catalytic study of such high-surface-area catalysts. As a result of the NPs composition variation, a vol-cano curve was obtained for the mass activity dependence on the alloy composition of the catalyst (Figure 2 e).

However, the differences in catalytic activity among the Pt x Ni 1– x catalysts are not that signifi cant, considering the large varia-tion in alloy composition among the as-synthesized NPs. This

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Figure 2 . Electrochemical characterizations by RDE in 0.1 M HClO 4 of Pt x Ni 1– x nanoparticle catalysts compared to commercial Pt/carbon: a) cyclic voltammetries in argon purged electrolyte, b) polarization curves for the ORR and c) corresponding Tafel plots. The half-wave potentials in (b) are marked with a dashed line. d) Summary of the specifi c activities for the ORR at 0.9 V and the improvement factors versus the Pt/carbon. e) Mass activity and the ratio of Ni left after electrochemical characterizations for different alloy compositions.

inspired us to take a closer view of the surface chemistry of the NPs that were exposed to the electrochemical environment. EDX analysis was performed on the NPs with different compo-sitions after the ORR measurements. It was found that the alloy composition has substantially changed, with much less Ni in the catalysts than the as-synthesized Pt x Ni 1– x NPs (Figure S5), i.e, Pt x Ni 1– x → Pt y Ni 1- y , where x < y . The Pt/Ni ratio in Pt y Ni 1– y NPs after the electrochemical measurements and dissolution of Ni also possesses a volcano-like dependence on the initial alloy composition, coinciding with the trend in mass activity (Figure 2 e). For example, 27% Ni remains in the PtNi/carbon catalyst after the ORR, versus 11% for Pt 3 Ni and 13% for PtNi 3 , which is con-sistent with the previous studies on thin fi lms. [ 11 , 14 ] Meanwhile, no signifi cant change in size or morphology of the catalyst par-ticles can be seen after the measurements (Figure S6).

The loss of Ni is not out of expectation, since the surface Ni will be dissolved when the alloy NPs are exposed to the acidic electrolyte and potential cycling (between 0.05 and 1.1 V vs. RHE in our measurements). [ 5 g, 15 ] What is more intriguing is that the extent of Ni dissolution is likely to be regulated by the initial alloy composition and that the average amount of Ni sta-bilized in an alloy NP has a certain limitation ( < 30%). Hence the formed nanostructure after Ni dissolution would be more intrinsic for understanding the observed dependence of catalytic activity on alloy composition. For the latter reason, we carried out a more detailed characterization of the NP compositions at atomic scale by combining aberration-corrected high angle

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annular dark fi eld (HAADF) scanning transmission electron microscopy (STEM) imaging and EDX line-profi le analysis.

Figure 3 shows the results of compositional line-profi le anal-ysis for Pt y Ni 1- y NPs in the catalyst after electrochemical charac-terizations. The EDX measurements were carried out crossing individual particles with a beam size of ∼ 2 Å and an acquisition time of 2 seconds for each step. Initially the as-synthesized NPs had homogeneous distributions of alloy elements (Figure S7). After the electrochemical measurements, a Ni depleted Pt-Ni structure with a Pt rich surface layer was observed in all the NPs with different alloy compositions, i.e., Ni was not detected in the surface region compared to the bulk of the NPs. However, the thickness of the Pt-rich layer was found to be dependent on the initial particle composition, where the thickness increased with Ni content. As revealed by EDX line-profi le analysis, a Pt-rich layer with a thickness of > 1 nm (at half maximum) was formed for PtNi 3 , versus ∼ 0.5 nm for PtNi NPs. This trend was further confi rmed by EDX atomic-scale point-analyses for the particle center and near-surface regions respectively (Figures S8 and S9). Similar observations were also obtained in the recent report on dealloyed PtCu catalysts. [ 7 e]

A more fundamental understanding of the alloy nanostruc-ture evolution subject to the Ni dissolution relies on atomistic modeling of the element distribution in the NPs under dif-ferent conditions. Figure 4 a shows the simulated NP structure change for a 5 nm PtNi particle before and after Ni dissolution from Monte Carlo simulations, while the results for various

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Figure 3 . Composition line-profi les crossing typical Pt x Ni 1– x NPs after the electrochemical characterization obtained by combinational HAADF-STEM and EDX analyses. Corresponding STEM images were presented in the Supporting Information (Figure S6).

Figure 4 . Monte Carlo simulation of the nanostructure evolution of PtNi NPs in the electrochemical environment. a) Initial and fi nal confi gura-tions of the PtNi NP. The simulated NP has a cubo-octahedral shape and contains 4033 atoms ( ∼ 5 nm) in the initial confi guration. In the fi nal con-fi guration of the PtNi NP, the Ni atoms exposed to the electrochemical environment have been dissolved, forming a Pt y Ni 1– y ( y > 0.5 here) NP. b) Calculated Ni concentration in the fi nal confi guration of the PtNi NP under the assumption that the dissolution of Ni atoms is driven by a chemical potential of Δ μ at T = 300 K. Symbol “x” in the fi gure marks the results from experiments.

compositions are summarized in Figure 4 b. Here the dissolu-tion process is simulated by removing from the particle sur-face Ni atoms with coordination number less than or equal to 9, and the process is repeated until no more Ni atoms on the surface satisfi ed the removal condition. Meanwhile, removing a Ni atom is regulated by the probability P = NS

NiNP

exp −�E−�:kBT )) ,

where NSNi is the number of Ni atoms on the surface, N P is the

total number of atoms in the particle, Δ μ chemical potential change, which is driving Ni dissolution, k B is the Boltzmann constant, and T is the temperature. The above probability was derived considering the chance of fi nding a Ni surface atom in the particle NS

Ni/NP and the chance of accepting the removal of the chosen surface Ni atom (given by a Boltzmann factor exp (− �E−�:

kBT ) for the energy change associated to the con-fi guration transformation in a grand canonical ensemble). We performed the Monte Carlo simulation up to 10 6 steps for each simulated particle, whose composition was thermodynamically stable at the end of the simulation and would not change with further simulations. When Δ μ = 0, there is no difference in chemical potential for dissolution of Ni atoms from the surface out to the solution, and hence the composition will not change. When Δ μ = ∞ , all Ni atoms will be dissolved if the initial Ni concentration in the PtNi NP is beyond 60 at.%, which is con-sistent with percolation theory. [ 16 ] However, in the real case the chemical potential change, Δ μ , should have a fi nite value for the dissolution process. Therefore, for a NP with random

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distribution of elements (a homogeneous alloy) the amount of dissolved Ni depends on the initial ratio of Ni. For instance (see Figure 4 b), if Δ μ = 5.4 eV, the fi nal Ni concentration is calcu-lated to be about 39 at.% in the case of PtNi, and 14 at.% in PtNi 3 , in agreement with the experimental results.

The element line-profi le analysis plus the simulation of acid treatment clearly depicted the correlation between Ni dissolu-tion and the initial composition of the bimetallic alloy NPs. It was revealed that a skeleton-like structure was formed in the near-surface region after Ni dissolution whereas the particle central region did not suffer much change (Figure 4 a). The thickness of such a skeleton structure depends on the extent of dissolution and thus on the initial alloy composition. For instance, the PtNi particle shows Pt-skeleton structure with a thickness of 2 ∼ 3 atomic layers, corresponding to the ∼ 0.5 nm Pt-rich layer observed in element line-profi le analysis, while the Ni dissolution process penetrated into the particle for more than 5 atomic layers in the case of PtNi 3 , giving the > 1 nm Pt-rich layer (Figure 3 ). The difference in the thickness of the Pt-skeleton structure could be one of the key factors governing the catalytic performance dependence on the alloy composition. [ 15 ]

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Further focused work on particle architecture and nanostruc-ture optimization is ongoing.

3. Conclusion

In summary, monodisperse and homogeneous Pt-Ni alloy nanoparticles of various compositions were synthesized via an organic solution approach. This approach has been dem-onstrated to be advantageous in controlling particle size, alloy homogeneity and elemental composition compared to con-ventional impregnation method, which has enabled the con-sistent and systematic study of the correlation between surface chemistry and electrocatalytic properties of Pt-bimetallic alloy catalysts. By combining atomic-level microscopic analyses of compositional profi les and modeling of nanoparticle structure, we have further revealed the extent of Ni depletion for different Pt x Ni 1– x NPs and the formation of a Pt-skeleton structure in the near-surface region. The thickness of the Pt-skeleton layer, which likely determines the catalytic properties, was found to be dependent on the initial composition of Pt-bimetallic catalysts. The developed approach and knowledge about surface struc-ture-property correlation can be further generalized and applied towards the design of advanced functional nanomaterials.

4. Experimental Section Material Synthesis: Monodisperse Pt x Ni 1– x NPs were synthesized by

modifying previously reported methods. [ 9 , 17 ] In a typical synthesis of PtNi NPs, Ni(ac) 2 · 4H 2 O (0.67 mmol) was dissolved in diphenyl either (20 ml) in the presence of oleylamine (0.4 ml) and oleic acid (0.4 ml). 1,2-tetradodecanediol (0.33 mmol) was added to and the formed solution was heated to 80 ° C. After a transparent solution formed, the temperature was further raised to 200 ° C, where Pt(acac) 2 (0.33 mmol) dissolved in dichlorobenzene (1.5 ml) was injected. The solution was kept at this temperature for 1 hour and then cooled down to room temperature. Ethanol (60 ml) was added to precipitate the NPs and the product was collected by centrifuge (6500 rpm, 6 minutes). The obtained NPs were further washed by ethanol for two times and then dispersed in hexane. By changing the precursor ratio between Pt(acac) 2 and Ni(ac) 2 · 4H 2 O, the alloy NPs of other compositions can be obtained, with 1:1.5, 1:3.7 and 1:5 for Pt 3 Ni, PtNi 2 and PtNi 3 respectively. The organic surfactant was removed by heating the catalyst at ∼ 180 ° C in oxygen atmosphere.

Characterization: TEM images were collected on a Philips EM 30 (200 kV) equipped with EDX functionality. XRD patterns were collected on a Rigaku RTP 300 RC machine. STEM and EDX elemental analysis were carried out on JEOL 2200FS TEM/STEM equipped with a CEOS aberration (probe) corrector and a Bruker-AXS X-Flash 5030 silicon drift x-ray detector. The microscope was operated at 200kV, and the probe size was ∼ 0.7 Å for imaging and ∼ 2 Å for EDX analysis.

Electrochemical Studies: Electrochemical measurements were conducted by using a three-compartment electrochemical cell on a rotational disc electrode setup (Pine) with potentiostat (Autolab 302). A saturated Ag/AgCl electrode and a Pt wire were used as reference and counter electrodes, respectively. HClO 4 was used as electrolyte. All the potentials in this report are given versus reversible hydrogen electrode (RHE), and the readout currents are recorded with ohmic iR drop correction during the measurements. [ 18 ] Electrochemical surface area (ESA) of the catalyst was evaluated from under potentially deposited hydrogen (H upd ) region obtained by integration of voltammetry curve. ESA was used for normalization of kinetic current density, also known

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as specifi c activity. All cyclic voltammograms were recorded after 100 potential cycles and no change in voltammetric features was observed before and after electrochemical characterization, indicating rather stable behavior of the catalysts. The catalyst loading on glassy carbon electrode was controlled to be 12 μ g Pt per cm 2 disk in all cases.

Supporting Information Supporting Information is available from the Wiley Online Library or from the author.

Acknowledgements This work was conducted at Argonne National Laboratory, a U.S. Department of Energy, Offi ce of Science Laboratory, operated by UChicago Argonne, LLC, under contract no. DE-AC02–06CH11357. This research was sponsored by the U.S. Department of Energy, Offi ce of Energy Effi ciency and Renewable Energy, Fuel Cell Technologies Program. Microscopy research was conducted at the Electron Microscopy Center for Materials Research at Argonne, and ORNL’s SHaRE User Facility, sponsored by the Scientifi c User Facilities Division, Offi ce of Basic Energy Sciences, the U.S. Department of Energy.

Received: June 4, 2010 Revised: August 28, 2010

Published online: November 9, 2010

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