Shape and Composition-Controlled Platinum Alloy Nanocrystals

Jan 4, 2011 - changing from the (100) to (111) Pt3Ni single crystal surfaces.10. Therefore ... show the d-spacing for (200) planes changed from 0.190 ...
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LETTER pubs.acs.org/NanoLett

Shape and Composition-Controlled Platinum Alloy Nanocrystals Using Carbon Monoxide as Reducing Agent Jianbo Wu, Adam Gross, and Hong Yang* Department of Chemical Engineering, University of Rochester, Gavett Hall 206, Rochester, New York 14627, United States

bS Supporting Information ABSTRACT: The shape of metal alloy nanocrystals plays an important role in catalytic performances. Many methods developed so far in controlling the morphologies of nanocrystals are however limited by the synthesis that is often material and shape specific. Here we show using a gas reducing agent in liquid solution (GRAILS) method, different Pt alloy (Pt-M, M = Co, Fe, Ni, Pd) nanocrystals with cubic and octahedral morphologies can be prepared under the same kind of reducing reaction condition. A broad range of compositions can also be obtained for these Pt alloy nanocrystals. Thus, this GRAILS method is a general approach to the preparation of uniform shape and composition-controlled Pt alloy nanocrystals. The area-specific oxygen reduction reaction (ORR) activities of Pt3Ni catalysts at 0.9 V are 0.85 mA/cm2Pt for the nanocubes, and 1.26 mA/cm2Pt for the nanooctahedra. The ORR mass activity of the octahedral Pt3Ni catalyst reaches 0.44 A/mgPt. KEYWORDS: Pt alloy, nanocrystal, carbon monoxide, cube, octahedron, oxygen reduction reaction

reducing agents.28-31 Shape controlled Pt3M (M = Fe, Co, Ni) nanocrystals were recently made using W(CO)6.12,32 In the synthetic mixtures that involve metal carbonyl compounds, CO gas may be released.33-35 Such CO gas is generally thought to bind to the metal surfaces and inhibit the metal addition to the nanocrystals.4 Here we present the use of gas reducing agent in liquid solution (GRAILS) approach to the preparation of a range of well-defined Pt alloy nanocrystals using a general reduction protocol. Instead of functioning as the growth inhibition agent, CO gas is intentionally introduced and used as the reductant. Our study indicates that the GRAILS method can be used to synthesize both cubic and octahedral nanocrystals of Pt-M (M = Co, Fe, Ni, Pd) alloys, which represent the two ends of the growth morphologies of platonic solids.36 Furthermore, this GRAILS method allows for the production of Pt alloy nanocrystals at a broad composition range without losing the shape control. Noticeably, all these composition, size, and shape controls are achieved under the same reducing reaction conditions with CO as the reductant. An ORR specific activity of 1.26 mA/cm2Pt, which is a five-time improvement over the reference Pt/C catalyst, was obtained with the octahedral Pt3Ni nanocrystals. Figure 1 shows transmission electron microscope (TEM) micrographs of representative cubic Pt-Ni alloys made using the GRAILS method (see Supporting Information for details of the synthesis). The nanocrystals were uniform and the typical edge length was between 10 and 20 nm (Figure 1a-c, Supporting Information Figure S1). Scanning electron microscope-energy

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etal alloy has been the subject of widespread research for applications as catalysts in fuel cell, battery, petrochemical, pharmaceutical, and other chemical conversion systems.1-9 Since each crystallographic plane provides different surface atomic and electronic structures, the shape of nanocrystals can greatly affect the physical and chemical properties, such as catalytic activity and selectivity.10-15 An over 7-fold improvement in area-specific activity in the oxygen reduction reaction (ORR) has been achieved by changing from the (100) to (111) Pt3Ni single crystal surfaces.10 Therefore, morphology control over Pt alloy nanocrystals has great technological implications.3-5 Nanomaterials are generated by both physical and chemical approaches.16-21 Among the various methods, solution phase synthesis is preferable for controlling the size and shape of nanocrystals of metal alloys.3,4,16 The synthesis typically employs the reduction of metal precursors in the presence of surface capping agents. The choice of reducing agents in the methods developed so far has largely been based on soluble solid or liquid molecules. One drawback of those synthetic methods is the reaction mixtures and conditions used for controlling the shape of a material often have to be changed when a different material or morphology is desirable. While gas reducing agents are much less commonly used for shape control of metal nanocrystals in nonaqueous solutions,13,22-25 metal nanocubes were made previously from metal-organic precursor using hydrogen gas as the reducing agent.3,4,26 Uniform Pt nanocubes have recently been prepared using CO gas as the reducing agent.27 These studies suggest that reducing gas can be used as a reductant in controlling the shapes of metal nanocrystals. Previously, a few shape-controlled Pt alloy nanocrystals, such as Pt-Fe and Pt-Co, have been made using diols and other r 2011 American Chemical Society

Received: November 24, 2010 Revised: December 21, 2010 Published: January 4, 2011 798

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Figure 1. (a-c) TEM and (d-f) HRTEM micrographs of cubic Pt-Ni alloy nanocrystals: (a,d) Pt3Ni, (b,e) PtNi, and (c,f) PtNi3.

well with the high-resolution TEM (HRTEM) micrographs, which show the d-spacing for (200) planes changed from 0.190 nm for Pt3Ni to 0.185 nm for PtNi3 nanocubes (Figure 1d-f). Noticeably, all three samples were made under identical CO reduction conditions with the same total molar amount of amine and acid capping agents. There are very few, if any, reported methods with which all three compositions of Pt-Ni alloy nanocubes have been made under the same reducing conditions. Besides Pt-Ni alloy, we were able to synthesize other Pt-M (M = Fe, Co, Pd) alloys also under the same reduction conditions using this GRAILS method (Figure 3). The edge lengths of these nanocubes were between 10 and 12 nm (Figure 3a-c). The SEM-EDX analysis indicates that compositions of these nanocubes were close to Pt3M (M = Fe, Co, Pd), the molar ratios between the two metal elements in their corresponding salt precursors (Supporting Information Figure S3). HRTEM micrographs show the (200) planes of fcc crystals, and the d-spacing for Pt3Fe nanocubes was 0.189 nm (Figure 3d). The d-spacings of (200) planes were 0.189 nm for Pt3Co and 0.196 nm for Pt3Pd alloy cubic nanocrystals (Figure 3e,f). The high-angle annular dark-field scanning transmission electron microscope (HAADFSTEM) micrographs and their corresponding elemental maps show that both Pt and the secondary metal (Fe, Co or Pd) were distributed evenly in each individual cubic nanoparticle (Figure 3g-i). PXRD patterns show that (200) diffractions was the strongest for these Pt-M alloy nanostructures, further indicating cube is the dominant morphology (Supporting Information Figure S4). Cube bound by the {100} facets represents one end of the platonic solids, while octahedron bound by {111} facets represents the other.36 By replacing oleic acid with an equal volume of diphenyl ether while keeping all other reaction conditions the same, we were able to obtain octahedral Pt3Ni and PtNi nanocrystals (Figure 4 and Supporting Information Figure S5). The average edge lengths were about 5 nm for these two octahedral nanocrystals. PtNi3 octahedral nanocrystals were produced when oleic acid (OA) was replaced by adamantaneacetic

Figure 2. XRD patterns of Pt-Ni alloy nanocubes. Color code for the reference X-ray diffraction lines: dark green, Pt, and purple, Ni.

dispersive X-ray (SEM-EDX) analysis indicates that the compositions were close to Pt3Ni, PtNi, and PtNi3 for those nanocubes shown in Figure 1a-c, respectively (Supporting Information Figure S2). The powder X-ray diffraction (PXRD) patterns of these cubes could be indexed to a face-centered-cubic (fcc) phase (Figure 2). Unlike the typical bulk samples whose diffractions from (111) plane are the strongest, the (200) diffractions of these nanocubes, which centered around 46° 2θ, were much stronger than those (111) diffractions appearing at 40-41° 2θ. The intensity ratio between these two peaks (I(200)/I(111)) was as high as 1.7, and much larger than that of the bulk Pt alloys (0.5). The (200) diffractions were much stronger than the (111) diffractions, which is consistent with a cubic morphology bound by the {100} facets. The (111) diffractions of the three cubic samples were 40.02° for Pt3Ni, 40.44° for PtNi, and 41.24° 2θ for PtNi3 alloy nanocubes. This observed shift corresponds to the change in composition based on the Vegard’s law, that is, the peak positions change monotonically from low angle for Pt rich alloy, to high angle for Ni rich alloy nanocubes. This observation agrees 799

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Figure 3. (a-c) TEM and (d-f) HRTEM micrographs of Pt alloy nanocubes: (a,d) Pt3Fe, (b,e) Pt3Co, and (c,f) Pt3Pd. (g-i) HAADF-STEM study showing the distribution of both Pt (blue) and M (M = Fe, purple; Co, green; and Pd, yellow) elements (scale bars in the STEM images represent 10 nm).

Figure 4. (a-c) TEM and (d-f) HRTEM micrographs of octahedral Pt-Ni alloy nanocrystals: (a,d) Pt3Ni, (b,e) PtNi, and (c,f) PtNi3.

acid (AAA) and oleylamine (OAm) by octadecylamine (ODA) (Figure 4c and Supporting Information Figure S5c). The corresponding d-spacings for the (111) planes were found to be 0.219 nm for Pt3Ni, 0.216 nm for PtNi, and 0.212 nm for PtNi3 alloy octahedral nanocrystals (Figure 4d-f). The PXRD result is consistent with the composition based on the SEM-EDX analysis (Supporting Information Figure S5). PXRD patterns confirm all three octahedral nanocrystals had the fcc phase and the strongest

diffraction was from the (111) plane (Figure 5), consistent with the TEM observation that octahedron was the dominant morphology. The intensity ratios between these two main peaks (I(200)/I(111)) were 0.28 for Pt3Ni, 0.18 for PtNi, and 0.25 for PtNi3. All these values are lower than those from the typical bulk materials (0.53, from JCPDS database card no. 04-0802). This result shows the (111) diffraction was dominant for all three octahedral Pt-Ni nanocrystals. The composition-dependent 800

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Co(acac)2 was used instead of Ni(acac)2 (Supporting Information Figure S6). The d-spacing was 0.218 nm, which can be assigned to the (111) plane of fcc phase Pt3Co alloy. The size of the octahedral nanocrystals can be readily controlled. For example, the size of Pt3Ni octahedral nanocrystals increased from about 5 to 10 nm by using oleic acid in the reaction mixture (Supporting Information Figure S7). It has been known that CO adsorbs preferentially onto specific surfaces of various metals.37,38 Besides the effect of capping agents,39 this selective binding could help the preferred addition of metal atoms onto given facets of Pt alloy nanocrystals, because CO can undergo preferential oxidation to CO2 on selected sufaces.40 For the transition state, the absorbed CO molecules on the {100} surfaces are slightly more stretched and closer to the surface than those over the {111} surfaces.40 Adsorption of CO on Pt atoms is stronger on the {100} surfaces than the closely packed {111} surfaces. This variation could lead to the difference in the potential energy of the transition state for the adsorbed CO on these two surfaces.40-42 Since the oxidation produces gas phase byproducts, much less perturbation to the synthetic mixture is expected than those with soluble liquid or solid reductants. Thus, stable reduction and other local reaction environments should be maintained using the GRAILS approach. The shape-dependent catalytic properties, especially the ORR in the cathode of proton exchange membrane fuel cell (PEMFC), were studied using Pt3Ni nanocrystals both in about 10 nm (Figure 1a and Figure S7).1,10-12 The rotating disk electrode (RDE) polarization curves show that cubic and octahedral Pt3Ni catalysts had more positive onset potentials and thus were more active than Pt (Figure 6a).11 The area-specific ORR activities at 0.9 V were found to be 0.85 mA/cm2Pt for the cubic, and 1.26 mA/cm2Pt for the octahedral Pt3Ni catalysts (Figure 6b). The ORR activity increased with the change of shapes from cube to octahedron. The ORR specific activity of the octahedral Pt3Ni is a five-time improvement over that of the commonly used Pt/C (∼0.20 mA/cm2 Pt) and among the highest known values reported so far for shape-controlled Pt3Ni nanocrystals.11,12 The mass activity of this octahedral Pt3Ni catalyst reaches 0.44 A/mgPt (Figure 6c), which is about three times that of the reference Pt/C (0.14 A/mgPt). The octahedral particles outperformed the cubic in catalyzing the ORR, and should be able to attributed largely to the shape of the nanocrystals.10 In summary, the GRAILS approach has been developed and used to make a range of cubic and octahedral Pt alloy nanocrystals. All the Pt alloy nanocrystals can be prepared under the same reducing reaction condition. Compared with the other reducing agent, CO should be more suitable as a gas reductant for making metal alloy nanocrystals with controlled morphology because of its excellent selectivity in the adsorption on metal surfaces. Our results show that GRAILS method represents a general approach to the synthesis of shape and composition controlled metal alloy nanocrystals that can potentially be very important for the development of highly active and selective catalysts and electrocatalysts.1,5

Figure 5. PXRD patterns of octahedral Pt-Ni alloy nanocrystals. Color code for the reference X-ray diffraction lines: dark green, Pt; purple, Ni.

Figure 6. Comparison of electrocatalytic properties of cubic and octahedral Pt3Ni nanocrystals: (a) ORR polarization and CV (insert) curves, (b) area, and (c) mass specific activities.

’ ASSOCIATED CONTENT

bS

Supporting Information. Description of experimental details, EDX spectra, TEM images, and PXRD patterns of Pt alloy nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.

peak shift in 2θ angles was also observed. Similarly, Pt3Co octahedral nanocrystals were prepared under the same reduction condition used for making Pt3Ni octahedral nanocrystals, except 801

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’ AUTHOR INFORMATION

(26) Dumestre, F.; Chaudret, B.; Amiens, C.; Renaud, P.; Fejes, P. Science 2004, 303, 821–823. (27) Kang, Y.; Ye, X.; Murray, C. B. Angew. Chem., Int. Ed. 2010, 49, 6156–6159. (28) Wang, C.; Hou, Y. L.; Kim, J. M.; Sun, S. H. Angew. Chem., Int. Ed. 2007, 46, 6333–6335. (29) Chou, S. W.; Zhu, C. L.; Neeleshwar, S.; Chen, C. L.; Chen, Y. Y.; Chen, C. C. Chem. Mater. 2009, 21, 4955–4961. (30) (a) Chen, M.; Kim, J.; Liu, J. P.; Fan, H. Y.; Sun, S. H. J. Am. Chem. Soc. 2006, 128, 7132–7133. (b) Chen, M.; Pica, T.; Jiang, Y. B.; Li, P.; Yano, K.; Liu, J. P.; Datye, A. K.; Fan, H. Y. J. Am. Chem. Soc. 2007, 129, 6348–6349. (31) Teng, X. W.; Yang, H. Front. Chem. Eng. China 2010, 4, 45–51. (32) Zhang, J.; Fang, J. Y. J. Am. Chem. Soc. 2009, 131, 18543–18547. (33) Wang, C.; Daimon, H.; Lee, Y.; Kim, J.; Sun, S. J. Am. Chem. Soc. 2007, 129, 6974–6975. (34) Lim, S. I.; Ojea-Jimenez, I.; Varon, M.; Casals, E.; Arbiol, J.; Puntes, V. Nano Lett. 2010, 10, 964–973. (35) Kang, Y. J.; Murray, C. B. J. Am. Chem. Soc. 2010, 132, 7568– 7569. (36) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153–1175. (37) Herrero, E.; Franaszczuk, K.; Wieckowski, A. J. Phys. Chem. 1994, 98, 5074–5083. (38) Schauermann, S.; Hoffmann, J.; Johanek, V.; Hartmann, J.; Libuda, J.; Freund, H. J. Angew. Chem., Int. Ed. 2002, 41, 2532–2535. (39) Peng, Z. M.; You, H. J.; Yang, H. ACS Nano 2010, 4, 1501– 1510. (40) Eichler, A. Surf. Sci. 2002, 498, 314–320. (41) Zhang, C. J.; Hu, P. J. Am. Chem. Soc. 2001, 123, 1166–1172. (42) Somorjai, G. A.; Li, Y. Introduction to Surface Chemistry and Catalysis, 2nd ed.; John Wiley & Sons: Hoboken, NJ, 2010; Chapter 9, p 615.

Corresponding Author

*E-mail: [email protected]. Telephone: (585) 2752110. Fax: (585) 273-1348.

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