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Monodisperse Pt3Co Nanoparticles as a Catalyst for the Oxygen Reduction Reaction: Size-Dependent Activity Chao Wang, Dennis van der Vliet, Kee-Chul Chang, Hoydoo You, Dusan Strmcnik, John A. Schlueter, Nenad M. Markovic, and Vojislav R. Stamenkovic* Materials Science DiVision, Argonne National Laboratory, Argonne, Illinois 60439 ReceiVed: August 06, 2009; ReVised Manuscript ReceiVed: September 18, 2009
Monodisperse Pt3Co nanoparticles with size controlled from 3 to 9 nm have been synthesized through an organic solvothermal approach and applied as electrocatalysts for the oxygen reduction reaction. Electrochemical study shows that the Pt3Co nanoparticles are highly active for the oxygen reduction reaction and the activity is size-dependent. The optimal size for maximal mass activity was established to be around 4.5 nm by balancing the electrochemically active surface area and specific activity. Alloy nanoparticles (NPs) have attracted increasing interest due to their superior performance in magnetic,1 optical,2 and catalytic3 applications. Particularly, Pt alloys with transition metals (PtM with M ) Fe, Co, Ni, etc.) have been found to be highly active for oxygen reduction, the troubled cathode reaction in fuel cells.3a-d,4 This has initiated a lot of effort in the synthesis of Pt-based alloy catalysts, which are usually in the form of PtM NPs dispersed in a high-surface-area carbon matrix. The approaches mostly include coprecipitation of metal salts in aqueous solution,5 impregnation of transition metals into a Pt/ carbon catalyst,6 and electrodeposition.7 Despite the progress in preparing various types of alloy catalyst, synthesis of catalysts with monodisperse and size-controlled alloy NPs is yet challenging in the literature. On the other hand, the particle size effect is known to play an important role in catalysis, particularly in the case of electrocatalysts comprising NPs. Not only the activity but also the reaction mechanism and selectivity have been reported to be dependent on the catalyst size.8 Contrary to the extensive study in the conventional Pt/carbon catalyst, size-dependent activity has not been well investigated for Pt alloy catalysts,9 which yet requires monodisperse alloy NPs of controlled size, composition, structure, and uniform shape.8d Here, we use Pt3Co as an example for systematic studies of size-dependent catalytic activity for the oxygen reduction reaction (ORR). Monodisperse Pt3Co NPs were synthesized through an organic solvothermal approach modified from previous publications,10 which has been demonstrated as a robust method for preparing monodisperse alloy NPs with size control and homogeneous compositions.1-3,11 Electrochemical properties were compared to the commercially available state-of-the-art Pt/carbon catalyst supplied by Tanaka. Platinum acetylacetonate, Pt(acac)2, was reduced by 1,2-tetradecanediol in the presence of 1-adamantanecarboxylic acid and a large excess of oleylamine, while Co was introduced by thermal decomposition of cobalt carbonyl, Co2(CO)8 (Figure 1a and also see the Supporting Information for details). Adding Co2(CO)8 at different temperatures gave Pt3Co NPs of various sizes. Figure 1b-e * To whom correspondence
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show representative TEM images of Pt3Co NPs of 3, 4.5, 6, and 9 nm obtained by adding Co2(CO)8 at 225, 200, 170, and 145 °C, respectively. The control of size in this case has been reported to be due to a balance between the rates of nucleation and growth.10b Energy-dispersive X-ray spectroscopy (EDX) analysis of the NPs shows that the atomic ratio between Co and Pt is equal to 1:3 (Figure S1, Supporting Information). Figure 2a shows X-ray diffraction (XRD) patterns of the assynthesized Pt3Co NPs. All of the XRD patterns correspond to a face-centered cubic (fcc) Pt3Co crystal.10 As the NP size increases, the XRD peaks become sharper, indicating the increase of crystalline size in the NPs. Crystalline sizes can further be calculated from the XRD patterns according to the Scherrer equation, as shown in Figure 2b. These sizes are quite close to those from TEM observations, implying the singlecrystal nature of individual NPs, which is also consistent with the HRTEM image analysis in the previous reports.10 The as-synthesized NPs were supported on carbon black (Tanaka, ∼900 m2/g) via a colloidal deposition approach12 by mixing the NPs and carbon in chloroform suspension, followed by sonication. Organic surfactants were removed by heat treatment of the NPs/carbon mixture in an oxygen atmosphere at 185 °C.13 The obtained catalyst was then dispersed in deionized water by vigorous sonication, and the formed suspension was dropped onto a glassy carbon (GC) electrode (6 mm in diameter). The ratio of Pt in the catalyst was tuned to 28%, and the loading of Pt on the GC electrode was controlled at 9 µg/cm2disk, except for 9 nm particles, which were at 12 µg/cm2disk in order to reach the diffusion limiting current.14 After drying under argon flow, the GC electrode was immersed into 0.1 M HClO4 for electrocatalytic measurements, which were carried out in a three-compartment electrochemical cell with Pt wire as the counter and Ag/AgCl as the reference electrode. All potentials in this report are given versus the reversible hydrogen electrode (RHE), and readout currents are corrected for the ohmic iR drop.15 The cyclic voltammogram (CV) was collected under Ar saturation with a scanning rate of 50 mV/s at 20 °C, and the ORR activity was measured by the rotational disk electrode (RDE) method with a scanning rate of 20 mV/s at 2009 American Chemical Society
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Figure 1. (a) Schematic illustration of the synthetic route for monodisperse Pt3Co NPs. (b-e) TEM images of as-synthesized 3, 4.5, 6, and 9 nm Pt3Co NPs.
60 °C. The electrochemical surface area of the catalyst was evaluated from under potentially deposited hydrogen (Hupd) and CO stripping charges (Figures S2 and S3, Supporting Information) and used to normalize the electrode current for calculation of the specific activity, which is given as the kinetic current density at 0.9 V (Figure S4). Figure 3a shows the voltammograms for Pt3Co/carbon catalysts with Pt3Co NPs of various sizes. As the size of NPs increases from 3 to 9 nm, the Hupd region (0.05 < E < 0.4 V versus RHE) shrinks, resulting in the decrease of the specific surface area from 692 to 277 cm2/mgPt (Figure 3b). Specific activities (at 0.9 V vs RHE) measured with a rotation speed of 1600 rpm and a scanning rate of 20 mV/s are also depicted in Figure 3b, showing an ascending trend as the NP size increases. The specific activity of 9 nm Pt3Co/carbon is over two times of that measured for 3 nm Pt3Co/carbon NPs. The two opposite trends in specific surface area and specific activity lead to a volcano-shape behavior in size-dependent mass activities, as shown in Figure 3c, and therefore, the maximum mass activity has been achieved by 4.5 nm Pt3Co NPs. Even though particle size effect for the Pt catalyst has been well documented in the literature8 and explained in terms of the surface geometry and associated electronic properties, disputations yet exist. For example, Watanabe et al. claim no
size effect observed in their combinational electrochemical and 195 Pt EC-NMR study.16 Despite a lack of consensus, it is generally accepted that the mechanism of the Pt size effect is fulfilled through enhanced adsorption of oxygenated species (O-, OHad-, etc.) in smaller particles due to the decrease of average coordination number8e and consequently more pronounced oxophilic behavior. Oxygenated species adsorbed on lowcoordinated Pt surface sites (steps, edges, kinks) inhibit the ORR.8b The first systematic study of bimetallic alloy particles presented here shows that the particle size effect is also reflected in the case of Pt3Co NPs. A careful analysis of the voltammograms presented in Figure 3a shows that both the oxidation peak (∼0.9 V) in the anodic scan and the reduction peak (∼0.8 V) in the cathodic scan exhibit a negative shift of ∼30 mV from 9 to 3 nm Pt3Co. Our experiments indicate that the smaller NPs are oxidized at a lower potential, which corresponds to enhanced adsorption of oxygenated species and thus decreased ORR activity. The results presented here show about three-fold enhancement in the specific activity for the ORR (Figure 4) between synthesized Pt3Co/carbon (6 nm) and commercially available Pt/carbon (6 nm) catalysts. The enhancement has been ascribed to the modification of the Pt surface electronic structure by alloying with 3d transition metals.17 The improvement factor is
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Figure 2. (a) XRD patterns of Pt3Co NPs of various sizes showing the typical peaks of Pt3Co crystal in fcc phase. (b) Crystalline sizes of Pt3Co NPs calculated from the XRD patterns according to the Scherrer equation.
in line with that observed for extended surfaces,3d implying that the synthetic approach and treatment procedures developed here do produce a homogeneous alloy and highly active monodisperse catalysts with controllable size. Compared with Pt alloy catalysts prepared by conventional approaches such as impregnation and coprecipitation,4-6,8c the improved activity for Pt bimetallic NPs developed here is due to the unique chemical solution synthesis, which generates alloy nanoparticles with more homogeneous elemental distribution and better mixing of alloying components, which was found to be crucial in determining the electronic/adsorption/catalytic properties.3c,d In summary, we have synthesized monodisperse Pt3Co nanoparticles with the size controlled from 3 to 9 nm and applied them as electrocatalysts for the oxygen reduction reaction. The organic solvothermal approach has been proven to be a powerful method for synthesis of high-quality alloy nanoparticles with superior performance in catalyzing the cathodic fuel cell reaction. Systematic study of the Pt bimetallic alloy catalysts comprising nanoparticles of various sizes reveals that the ORR activity of Pt3Co is size-dependent and decreases with the particle size. By balancing the specific surface area and activity, the optimal size for the maximum in mass activity was established to be around 4.5 nm. In a quest to control the size, shape, and composition of nanoparticles, the strategy and trends reported in this study may be generalized to other systems and
Figure 3. (a) CVs of Pt3Co NPs of different sizes. (b) Specific activities at 0.9 V versus RHE measured with a scanning rate of 20 mV/s and a rotation speed of 1600 rpm (black) and specific surface areas (red) of Pt3Co/carbon catalysts. The error of the specific activities was estimated to be (10% according to three measurements for each sample. (c) Mass activities of Pt3Co/carbon catalysts.
utilized to guide the future development of advanced functional nanomaterials. Acknowledgment. The work was supported by the contract (DE-AC02-06CH11357) between the University of Chicago and Argonne, LLC, and the U.S. Department of Energy. The electron microscopy was accomplished at the Electron Microscopy Center for Materials Research at Argonne National Laboratory.
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Figure 4. Specific activity at 0.9 V versus RHE, 60 °C and 1600 rpm, for Pt3Co/carbon catalysts compared to that for 6 nm Pt/carbon catalyst.
Supporting Information Available: Material synthesis and characterization, electrochemistry study. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000, 287, 1989–1992. (b) Desvaux, C.; Amiens, C.; Fejes, P.; Renaud, P.; Respaud, M.; Lecante, P.; Snoeck, E.; Chaudret, B. Nat. Mater. 2005, 4, 750–753. (c) Bader, S. D. ReV. Mod. Phys. 2006, 78, 1–15. (d) Seo, W. S.; Lee, J. H.; Sun, X.; Suzuki, Y.; Mann, D.; Liu, Z.; Terashima, M.; Yang, P. C.; Mcconnell, M. V.; Nishimura, D. G.; Dai, H. Nat. Mater. 2006, 5, 971–976. (e) Wang, C.; Peng, S.; Lacroix, L.-M.; Sun, S. Nano Res. 2009, 2, 380–385. (2) (a) Link, S.; Wang, Z. L.; EI-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529–3533. (b) Cottancin, E.; Lerme, J.; Gaudry, M.; Pellarin, M.; Vialle, J.-L.; Broyer, M. Phys. ReV. B 2000, 62, 5179–5185. (c) Sun, Y.; Wiley, B.; Li, Z.-Y.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 9399–9406. (d) Wang, C.; Peng, S.; Chan, R.; Sun, S. Small 2009, 5, 567–571. (3) (a) Toda, T.; Igarashi, H.; Uchida, H.; Watanabe, M. J. Electrochem. Soc. 1999, 146, 3750–3756. (b) Greeley, J.; Mavrikakis, M. Nat. Mater. 2004, 3, 810–815. (c) Stamenkovic, V. R.; Fowler, B.; Mun, B. S.; Wang, G.; Ross, P. N.; Lucas, C. A.; Markovic, N. M. Science 2007, 315, 493– 497. (d) Stamenkovic, V. R.; Mun, B. S.; Arenz, J. J.; Mayrhofer, M.; Lucas, C. A.; Wang, G.; Ross, P. N.; Markovic, N. M. Nat. Mater. 2007, 6, 241–
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