RhPt Flowerlike Bimetallic Nanocrystals with Tunable Composition as

May 12, 2014 - nanocrystals, commercial Pt black, and commercial Ru50Pt50/C recorded in 0.1 M HClO4 solution. The specific current was normalized by t...
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RhPt Flowerlike Bimetallic Nanocrystals with Tunable Composition as Superior Electrocatalysts for Methanol Oxidation Qiang Yuan,*,† Da-Bing Huang,† Hong-Hui Wang,‡ and Zhi-You Zhou*,‡ †

Department of Chemistry, College of Chemistry and Chemical Engineering, Guizhou University, Guiyang, Guizhou Province 550025, PR China ‡ State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, PR China S Supporting Information *

ABSTRACT: For the first time, composition-tunable, high-yield, RhPt flowerlike bimetallic nanocrystals were successfully synthesized through an aqueous solution approach. The electrocatalytic activity of these RhPt nanoalloys toward methanol oxidation was investigated and compared to the activity of commercial Pt black and commercial Ru50Pt50/C. The RhPt flowerlike bimetallic nanoallys have shown composition-dependent and superior catalytic properties relative to those of commercial Pt black and commercial Ru50Pt50/C. The peak current density and mass current value of Rh19Pt81 nanoalloys are 0.75 mA cm−2 and 0.12 mA μg−1, respectively. For commercial Pt black, they are 0.48 mA cm−2 and 0.074 mA μg−1, and for commercial Ru50Pt50/C, they are 0.28 mA cm−2 and 0.10 mA μg−1. Moreover, the chronoamperometric measurements show that the RhPt flowerlike nanoalloys have excellent stability over commercial Pt black and commercial Ru50Pt50/C.

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INTRODUCTION Methanol, with a high energy density (gravimetric energy density of 22 650 kJ/kg and volumetric energy density of 17 900 kJ/L)1 and easier and safer storage than hydrogen, is a promising choice for the development of miniaturized, highenergy-density fuel cells for portable applications. However, there are two major barriers that must be conquered before the commercialization of low-temperature direct methanol fuel cells (DMFCs). Namely, the catalysts used in DMFC have to meet two fundamental requirements: high catalytic performance and longevity in an acidic environment. As a matter of fact, the catalytic performance of the catalysts has been proven to depend intrinsically on the elementary steps composing them, such as size, shape, and composition.2−4 For example, PdPt and PtCu bimetallic nanocrystals exhibit composition-/shapedependent catalytic properties in an oxygen reduction reaction (ORR) and in an alcohol/formic acid oxidation reaction.5−8 We have also shown that PdPt nanocubes exhibit compositiondependent and high electrochemical activity toward formic acid oxidation compared to commercial Pd black.9As a consequence, along with the size and shape of nanocrystals, the composition of nanocrystals also plays a vital role in terms of the catalytic activity. Furthermore, Pt is the best catalyst of all pure metals for low-temperature DMFC. Thus, these factors have made the synthesis of Pt-based bimetallic nanocrystals the focus of increasing attention in low-temperature DMFC research because of their superior catalytic performance relative to that of monometallic Pt nanocrystals.10−17 For instance, PtZn nanocrystals11 synthesized in an oleylamine, oleic acid, and © 2014 American Chemical Society

benzyl ether solution with Pt(acac)2 and Zn(acac)2 as precursors under an N2 atmosphere at 330 or 350 °C show enhanced catalytic activity for the methanol oxidation reaction (MOR). Multiply twinned dumbbell-shaped Rh@Pt nanostructures17 synthesized by a two-step, seed-mediated approach also displays superior catalytic activity for MOR. Despite all of the above-mentioned achievements for MOR, there is a limited successful report for MOR with the simultaneous decrease in the peak potential and the increase in peak current while preserving the long durability. Furthermore, to the best of our knowledge, the direct synthesis of RhPt flowerlike bimetallic nanoalloys has not been reported. In this paper, for the first time, we introduce a facile aqueous solution approach to directly prepare high-yield (Figure S1), composition-tunable RhPt bimetallic nanoalloys while preserving the same shape and size, which is very important in studying the composition-dependent catalytic performance. Moreover, compared to commercial Pt black, these RhPt flowerlike bimetallic nanocrystals exhibit superior catalytic activity (as evidenced by both the increase in current density and the decrease in oxidation potential) for MOR while maintaining their durability. Received: March 31, 2014 Revised: May 9, 2014 Published: May 12, 2014 5711

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SYNTHESIS OF RHPT FLOWERLIKE NANOCRYSTALS

rhodium (0.220 nm, JCPDS 65-2866). In addition, these (111) lattice fringes are not continuous, their orientations are varied, and the crystal boundaries between the spherical particles can be clearly observed, demonstrating that the flowerlike particles are polycrystalline and formed through the reaction-limited aggregation (RLA) of spherical nanoparticles (Figure S3).18 Moreover, the combination of CTAC and formaldehyde is essential to the synthesis of the flowerlike RhPt nanocrystals. RhPt flowerlike structure cannot be achieved by individually using CTAC or formaldehyde (Figure S4). The results of energy-dispersive X-ray spectroscopy (EDX) show that the flowerlike nanocrystals are made of Rh and Pt (Figure S5). The composition of the as-synthesized flowerlike nanocrystals is determined by the quantitative analysis of the EDX. The atomic ratios of Rh and Pt are 9.7 and 90.3 atom %, 19 and 81 atom %, and 33 and 67 atom %, respectively, which are very close to the precursor ratio of Rh and Pt, 10 and 90 atom %, 20 and 80 atom %, and 30 and 70 atom % (composition, σ < 5%). The elemental distribution of Rh and Pt in the flowerlike nanocrystal was measured by EDX line-scan analysis. The compositional line profiles of Rh and Pt on flowerlike nanocrystals are consecutive variation without any segregation of each component, which indicates that the flowerlike nanocrystal is indeed a RhPt alloy. And the signals of Rh and Pt fluctuate along the scanning direction, which implies that the Rh and Pt atoms randomly distribute in the flowerlike bimetallic alloys. The alloyed structure is also supported by the results of X-ray diffraction (XRD) spectra (Figure 2g). All the peaks of XRD are located between the typical fcc structure of rhodium and platinum, and the peaks shift to a high angle with increasing Rh content (Figure 2g,h), thus implying the presence of only RhPt alloy crystals. Furthermore, the morphology and size of the RhPt bimetallic nanocrystals are not varied with the change in the ratio of Rh and Pt (Figure S2), which is a key factor in investigating the compositiondependent property. Electrochemical characterization was carried out in a standard three-electrode cell at a temperature of about 6.0 ± 1.0 °C. The working electrode was a glassy carbon electrode loaded with RhPt nanocrystals, commercial Pt black, or commercial Ru50Pt50/C. (The average size of Ru50Pt50 is about 4.5 nm, as shown in Figure S6.) The electrode was immersed in a nitrogen-saturated solution, and the potential was scanned from −0.25 to 1.05 V (vs Ag/AgCl) at a scan rate of 50 mV s−1 to obtain cyclic voltammograms (CVs). Figure 3a shows the CVs of Rh9.7Pt90.3, Rh19Pt81, Rh33Pt67 bimetallic nanocrystals, commercial Pt black, and commercial Ru50Pt50/C recorded in 0.1 M HClO4 solution. The specific current was normalized by the electrochemical surface area (ECSA) that was calculated by measuring the charge collected in the hydrogen adsorption−desorption region and assuming a value of 210 μC cm−2. The significantly large current observed on Ru50Pt50/C comes from the double-layer charging of the carbon black support. The RhPt alloys show a similar CV shape to commercial Pt black. However, the hydrogen oxidation peak at −0.02 V disappeared in the CVs of RhPt alloys compared to the CV of commercial Pt black (Figure S7), which is the same as the CV of commercial Ru50Pt50/C. More importantly, both the onset potentials of oxygen adsorption in the forward scan and peak potentials of oxygen desorption in the backward scan on the RhPt alloys shift to low potential relative to that on the commercial Pt black. And the negative-shift values increase with increasing Rh content. This result indicates electron interaction

Citric acid monohydrate (0.210 g) and CTAC (0.256 g) were added to 9.0 mL of superpure water and stirred for several minutes. Then 0.1 M RhCl3 and 0.1 M H2PtCl6 aqueous solutions were added in Rh/Pt ratios of 1:9, 2:8, and 3:7 (namely, 0.1 mL of 0.1 M RhCl3 and 0.9 mL of a 0.1 M H2PtCl6 aqueous solution; 0.2 mL of 0.1 M RhCl3 and 0.8 mL of a 0.1 M H2PtCl6 aqueous solution; and 0.3 mL of 0.1 M RhCl3 and 0.7 mL of a 0.1 M H2PtCl6 aqueous solution, respectively), after being stirred for several minutes, and then 60 μL of a formaldehyde solution was added. Finally, the resulting solution was transferred to a 15 mL Teflon-lined stainless steel autoclave. The sealed vessel was then held at 180 °C for 50 min before it was cooled to room temperature. The products were separated via three centrifugation/ washing cycles at 10 000 rpm for 15 min with superpure water. Finally, the collected product was vacuum dried for 3 days.

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RESULTS AND DISCUSSION RhPt flowerlike bimetallic nanocrystals were synthesized by the coreduction of H2PtCl6 and RhCl3 in the presence of citric acid, cetyltrimethylammonium chloride (CTAC), and formaldehyde through an aqueous solution approach in a Teflon-lined stainless steel autoclave at 180 °C for 50 min. Figure 1a and

Figure 1. TEM (a), HRTEM (b, c), and HADDF-STEM (d) images of as-synthesized Rh19Pt81 nanocrystals.

Figure S2b illustrate the representative transmission electron microscopy (TEM) images of as-synthesized Rh19Pt81 bimetallic nanocrystals prepared with the aqueous solution approach. As can be seen, the product consists of a large quantity of uniform particles with a flowerlike shape, and the average diameter of the particles is about 28.6 nm. The high-resolution TEM (HRTEM) images of a single particle are shown in Figure 1b,c. From the HRTEM images, we can clearly find that a flowerlike nanoparticle consists of dozens of smaller spherical particles and the size of a spherical particle is about 4.3 ± 0.3 nm. Moreover, some pores are observed between the spherical particles in the flowerlike particles from the high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image (Figure 1d and Figure S2d). Obvious lattice fringes appear on the surface of single spherical particles (Figure 1c). The interval between two lattice fringes is examined to be 0.222 nm, which is between the (111) lattice spacing of the platinum (0.227 nm, JCPDS 65-2868) and the 5712

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Figure 3. Cyclic voltammetric curves (CVs) of as-synthesized RhPt flowerlike nanoalloys, commercial Pt black, and commercial Ru50Pt50/ C. (a) Electrochemical characterization in 0.1 M HClO4 solution. (b, c) Specific activity and mass activity in 0.2 M CH3OH + 0.1 M HClO4 solution. (d) Current vs time curves of methanol oxidation on RhPt nanoalloys, commercial Pt black, and commercial Ru50Pt50/C (metal: 30 wt %) in 0.2 M CH3OH + 0.1 M HClO4 solution at 0.5 V for 1 h (black, commercial Pt black; red, Rh9.7Pt90.3; blue, Rh19Pt81; dark cyan, Rh33Pt67; and magenta, Ru50Pt50/C.). All data were obtained at a temperature of about 6.0 ± 1.0 °C.

peak potential is about 110 mV, which indicates an increased output voltage for DMFC. In addition, the peak current densities of Rh9.7Pt90.3, Rh19Pt81, and Rh33Pt67 are 0.60, 0.75, and 0.68 mA cm−2, respectively, and that of commercial Pt black is 0.48 mA cm−2. The maximum increase in peak current density on RhPt flowerlike nanocrystals is 0.27 mA cm−2 compared to commercial Pt black. These results mean that the addition of Rh to Pt can enhance the activity of methanol oxidation with increasing peak current density and decreasing peak potential simultaneously. This behavior is very different from that of Ru-modified Pt, a well-known bifunctional electrocatalyst for methanol oxidation.21,22 In the latter case, the peak potential is negatively shifted, but the peak current density is decreased (e.g., from 0.68 on Pt(111) to 0.64 mA cm−2 on Ruad-Pt(111) at 25 °C in ref 22), which also can be seen in the commercial Ru50Pt50/C reported here. In the typical bifunctional system, the second metal (e.g., Ru) is inert or less active with respect to MOR, but it can produce oxygen species at low potential, thus enhancing MOR. However, at high potential (e.g., peak potential), oxygen species can also form easily on the Pt surface. In this case, the methanol oxidation current on the Pt alloy should be lower than that of pure Pt due to the small number of available Pt sites.22 The highest catalytic activity observed on Rh19Pt81 is attributed to a similar reason. Rh33Pt67 has too few Pt sites on the surface, and the Rh9.7Pt90.3 cannot produce enough oxygen species at low potential (Figure 3a). On the basis of the above discussion, we can conclude that the enhanced electrocatalytic activity of the RhPt flowerlike nanoalloys over pure Pt observed here may be ascribed to both bifunctional effect that were proved by the low onset potential for oxygen adsorption in Figure 3a and the electronic structure effect owing to the presence of Rh and Pt atoms on the surface and/or the grain boundary created by the flowerlike structure. The former factor results in the negative shift potential for

Figure 2. HADDF-STEM images and the corresponding EDX linescanning profiles of as-synthesized RhPt flowerlike bimetallic nanocrystals. (a, b) Rh9.7Pt90.3; (c, d) Rh19Pt81; (e, f) Rh33Pt67; (g, h) XRD patterns of RhPt flowerlike bimetallic nanocrystals (black, Rh33Pt67; red, Rh19Pt81; blue, Rh9.7Pt90.3; and green and wine stand for the standard peaks for Pt and Rh, respectively).

between Rh and Pt atoms among RhPt alloy samples. It is known that Rh has a high affinity for oxygen. The formation of oxygen species at low potential on RhPt will promote the oxidation of adsorbed CO, a notorious poisoning intermediate for MOR. The oxidation of CO needs surface oxygen species that can be generated only through H2O dissociation at relatively high potential on pure Pt, resulting in catalyst poisoning at low potential for MOR.26 The MOR was therefore used as a test reaction to characterize the electrocatalytic properties of the RhPt flowerlike bimetallic nanocrystals. Figure 3b shows CVs of RhPt alloys, commercial Pt black, and commercial Ru50Pt50/C in 0.2 M CH3OH + 0.1 M HClO4 solution at 50 mV s−1. The current has been normalized by the ECSA to obtain the current density (j). Thus, it can be directly compared for different samples. A well-defined peak that appears at about 0.5 V can be attributed to the methanol oxidation in both the forward and backward scans.19,20 In the forward scan, the peak potential of RhPt alloys is obviously lower than that of commercial Pt black. The peak potentials of Rh9.7Pt90.3, Rh19Pt81, and Rh33Pt67 are 0.57, 0.54, and 0.52 V, respectively, while the peak potential of commercial Pt black is 0.63 V. The maximum negative shift in 5713

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mV on Rh 19 Pt 81 over the Pt black. Moreover, the chronoamperometric measurements show that the RhPt flowerlike nanoalloys have excellent stability over commercial Pt black and commercial Ru50Pt50/C. After 1.0 h, the steadystate current density of Rh19Pt81 is 5.8 and 2.5 times that of commercial Pt black and commercial Ru50Pt50/C, respectively. This study demonstrates the successful tailoring of electrocatalytic properties of RhPt bimetallic nanoalloys by controlling their composition while maintaining the same morphology.

MOR, and the latter factor leads to a higher peak current density on RhPt than on Pt. The enhancement effect produced by synergistic effects of different kinds of atoms or grain boundary on the catalyst surface has also been observed in other bimetallic alloys.6,23 Furthermore, relative to commercial Ru50Pt50/C, the peak current density on Rh19Pt81 is about 2.7 times that on commercial Ru50Pt50/C, which means that the catalytic activity of RhPt flowerlike alloys is much better than that of commercial Ru50Pt50/C. We also compared the CO stripping behaviors among Pt black, Rh19Pt81, and Pt50Ru50/C, and found the peak potentials of CO stripping to be 0.501, 0.446, and 0.385 V (Figure S8), respectively, i.e., with the same order as for MOR (Figure 3b). If the MOR current is normalized by a noble metal, then RhPt nanoalloys also have a higher mass activity than Pt black and Ru50Pt50/C (Figure 3c). For example, the mass current is about 0.12 mA μg−1 for Rh19Pt81 while only 0.073 mA μg−1 for commercial Pt black and 0.10 mA μg−1 for commercial Ru50Pt50/C. In the backward scan (Figure 3b), the current density peaks are located at around 0.43−0.45 V. The current density peak of RhPt alloys has a clear decline relative to that of commercial Pt black, and the greater the Rh composition in the RhPt nanoalloy, the more the peak current decreases. This behavior is similar to that of the PtRu alloy with respect to MOR. The decline of the backward scan peak on RhPt with respect to MOR is ascribed to the sluggish reduction of Rh oxide (Figure 3b) as PtRu catalysts.21,24,25 That is, the catalytic surface is passivated by inert Rh(Pt) oxide. We also observed that the MOR current on the commercial Ru50Pt50/C in the backward scan is very small, which is consistent with the results reported previously.26,27 As for practical applications in fuel cells, a higher steady-state current at a fixed potential is a more acceptable criterion for a good catalyst. Figure 3d shows current vs time curves of methanol oxidation at 0.50 V for 1 h. All curves show a similar declining trend. However, the RhPt flowerlike nanocrystals have much higher MOR electrocatalytic activities than do commercial Pt black and commercial Ru50Pt50/C. After 1.0 h, the current density of Rh19Pt81 is about 0.10 mA cm−2; nevertheless, it is only 0.017 mA cm−2 for commercial Pt black and 0.040 mA cm−2 for commercial Ru50Pt50/C. The steady-state current density of Rh19Pt81 is 5.8 and 2.5 times that of commercial Pt black and commercial Ru50Pt50/C, respectively, indicating high activity and stability of RhPt in MOR.

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ASSOCIATED CONTENT

S Supporting Information *

Experimental details, digital photographs, EDX profiles, STEM images, TEM images, and CV. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected].. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the NSFC (21361005 and 21373175), the Foundation for the Talents by the Guizhou University (X060025), the Natural Science Foundation of Guizhou Province (20072013), and Fundamental Research Funds for the Central Universities (2012121019). We also appreciate the useful discussion with Prof. Hua Chun Zen of National University Singapore about this work.

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REFERENCES

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CONCLUSIONS The composition-tunable, high-yield, RhPt flowerlike bimetallic nanocrystals were first successfully synthesized by the coreduction of H2PtCl6 and RhCl3 in the presence of citric acid, cetyltrimethylammonium chloride (CTAC), and formaldehyde through an aqueous solution approach. The electrocatalytic activity of these RhPt nanoalloys toward methanol oxidation was investigated and compared to the activity of commercial Pt black and commercial Ru50Pt50/C in 0.2 M CH3OH + 0.1 M HClO4 solution. The RhPt flowerlike bimetallic nanoalloys have shown composition-dependent and superior catalytic properties relative to commercial Pt black and commercial Ru50Pt50/C. The peak current density and mass current value on Rh19Pt81 nanoalloys are 0.75 mA cm−2 and 0.12 mA μg−1, respectively. For commercial Pt black, they are 0.48 mA cm−2 and 0.074 mA μg−1, and for commercial Ru50Pt50/C, they are 0.28 mA cm−2 and 0.10 mA μg−1. Correspondingly, the peak potential is decreased by about 90 5714

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