Controlled deposition of Pt on Au nanorods and their ...

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Electrochemistry Communications 10 (2008) 961–964

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Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom

Controlled deposition of Pt on Au nanorods and their catalytic activity towards formic acid oxidation Shuangyin Wang a, Noel Kristian a, Sanping Jiang b,*, Xin Wang a,* a b

School of Chemical and Biomedical Engineering, Nanyang Technological University, Singapore 637 457, Singapore School of Mechanical and Aerospace Engineering, Nanyang Technological University, 50 Nanyang Drive, Singapore, 639798, Singapore

a r t i c l e

i n f o

Article history: Received 4 March 2008 Received in revised form 11 April 2008 Accepted 11 April 2008 Available online 27 May 2008 Keywords: Gold nanorods Platinum Formic acid electrooxidation Fuel cells

a b s t r a c t Platinum submonolayer decorated gold nanorods with controlled coverage were prepared by the addition of Au nanorods into the growth solution of Pt in the presence of NH2OH HCl as the growth agent. The properties of Au nanorods decorated by Pt submonolayer were investigated by various techniques including transimission electron microscopy, X-ray diffraction, and electrochemical methods. The Pt decorated Au nanorods on carbon black showed signiﬁcantly higher activity on formic acid electrooxidation than the conventional Pt/C catalysts. They showed different reaction path of formic acid electrooxidation by suppressing the formation of poisoning intermediate CO. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Recently, direct formic acid fuel cell (DFAFC) has been the focus of continuous efforts, because of its advantages over direct methanol fuel cell including higher theoretical open circuit potential and lower fuel crossover [1]. Platinum is a commonly used electrocatalyst in fuel cell applications. It has been reported that the electrooxidation of formic acid on Pt catalysts adopts a dual path mechanism, that is, a dehydrogenation path and a dehydration path, and the adsorption of the intermediate CO from the dehydration path signiﬁcantly hinders its activity [2–4]. Various efforts have been devoted to the mitigation of the CO poisoning phenomenon, such as by alloying Pt with other metals [5–7] or using a Pdbased catalysts [8,9]. New ideas are still needed to develop novel electrocatalysts with lower cost and higher activity for fuel cell applications. On the other hand, Gold nanorods have been extensively studied mostly due to their interesting optical properties during the past decade [10–13]. Pt covering Au and Pt–Au core– shell nanoparticle structure have been reported for catalytic applications. [14,15] However, very limited reports are available on Pt decorated Au nanorod structure and little attention has been paid to their application in fuel cells, especially for DFAFC. The Pt decorated or core–shell nanorod structure may exhibit different proper-

* Corresponding authors. E-mail addresses: [email protected] (S. Jiang), [email protected] (X. Wang). 1388-2481/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2008.04.018

ties from their individual component or nanoparticle counterpart, therefore is of considerable interest for fundamental study. Here, we report the controlled deposition of Pt on Au nanorod and their catalytic activity for formic acid electrooxidation. Au nanorods were prepared according to the previous report [16]. In order to deposit Pt submonolayer onto the Au nanorods, NH2OH HCl was carefully chosen as reducing agent, because it is well known that NH2OH HCl acts only as a growth agent in slightly acid environments without forming new nuclei [17]. Pt submonolayer or Pt shell on Au nanorods is realized by varying the molar ratios of Pt/Au. Subsequently, Pt submonolayer decorated Au nanorods on carbon black were used for the formic acid oxidation. They show signiﬁcantly higher activity than the conventional Pt/C electrocatalysts by suppressing the formation of the CO poisoning intermediate. 2. Experimental 2.1. Synthesis of Au nanorods In a typical procedur\e, the seed solution was prepared by mixing cetyltrimethylammonium bromide (CTAB) (5 ml, 200 mM) with HAuCl4 (5 ml, 0.5 mM) and then adding ice-cold NaBH4 (0.6 ml, 10 mM). For the growth of gold nanorods, 75 ml of 200 mM CTAB was mixed with 3 ml of 4 mM AgNO3 solution and 75 ml of 1 mM HAuCl4, and then 1.05 ml of 100 mM ascorbic acid was added. 0.18 ml of the seed solution was ﬁnally injected to initiate the growth of gold nanorods.

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2.2. Growth of Pt on Au nanorods Certain amount of 0.1% aqueous H2PtCl6 and 0.1% aqueous NH2OH HCl were mixed with 100 ml water and heated to 60 °C for 3 h. A ﬁvefold molar ratio of NH2OH HCl over H2PtCl6 was chosen to ensure the complete reduction of H2PtCl6. Then the as-synthesized Au nanorods were added and the temperature of the solution was kept at 60 °C for another 2 h. Then the obtained hydrosol was mixed with carbon black (XC-72) under sonication in a large amount of ethanol to remove the excess CTAB and to deposit the Pt/Au NRs on carbon black, while keeping the loading of Au nanorods on XC-72 at 30 wt%. The ﬁnal product was collected by the ﬁltration followed by the drying in the vacuum oven at 70 °C for 7 h. For comparison, 20 wt% Pt/C was prepared according to our previous report [15]. 2.3. Characterization The morphology was observed using JEOL 2010. An X-ray diffractometer was used for identiﬁcation of the crystalline structure. The electrochemical signals were recorded with Autolab PGSTAT302 potentiostat at room temperature. Electrode potentials were measured and reported against a saturated calomel electrode (SCE). The counter electrode was Pt wire. For the preparation of working electrode, the catalysts ink was produced by dispersing the obtained products in ethanol under sonication for 30 mins. The concentration of total catalysts was kept at 2 mg/ml. Ten microliter of the obtained suspension was dropped on the glassy carbon electrode (GCE, Pure Instrument, 4 mm in diameter). Thus the catalysts loading on GCE is 20 lg. After drying the catalyst ink at room temperature, 5 ll of 0.05% naﬁon solution in isopropanol (diluted from the commercial Naﬁon solution in isopropanol) was dropped on GCE to ﬁx the catalyst.

nanorods is due to the higher curvature on the end of the Au nanorods than that on the sides [18]. With the increase of the Pt/Au molar ratios, the surface of the Au nanorods became rougher (Fig. 1C, D, F, G), and when the ratio reached 1:2 (Fig. 1D and G), the clear shell of Pt can be observed around Au rods, that means Au nanorods probably were completely covered by Pt shell. These results suggest that Pt can be successfully grown onto Au nanorods by use of NH2OH HCl as the growth agent. As shown in Fig. 2, the XRD patterns of Pt/Au = 1:10, 1:6 and 1:2 with the submonolayer decoration structure resemble the Au spectra where all the (1 1 1), (2 0 0), (2 2 0) and (3 1 1) peaks can be observed. The absence of Pt peaks implies that the size of the Pt nanoislands on the surface of Au nanorods is smaller than the detecting capability of XRD (1 nm) or most of the Au surface is still exposed For Pt/Au = 1:10 and 1:6, this observation agrees well with TEM pictures (Fig. 1E and F). But for Pt/Au = 1:2, Pt peaks are still absent, even though the Pt shells could be observed from the TEM picture (Fig. 1G). This inconsistency between XRD and TEM results for Pt/Au = 1:2 sample denmonstrated that XRD technique is not a very precise tool to characterize the thin shell coated Au nanorods due to the limitation of the detecting capability. To characterize the submonolayer structure of Pt, electrochemical method was performed, which can detect only the surface atoms and is capable of differentiating surface Au and Pt atoms,

3. Results and discussion Fig. 1A shows the typical TEM image of as-prepared Au nanorods. They show smooth surface, with a mean aspect ratio of about 2.5. When Pt submonolayer or nanoislands were successfully decorated onto Au nanorods, it was expected that the surface of the Au nanorods would become rough. Fig. 1B and E show the TEM image of submonolayer Pt decorated Au nanorods with the molar ratio of 1:10, where we can see that most parts of the rod surface are still very smooth, with some small Pt nanoislands observed at the tips of Au nanorods. The preferential growth of Pt on the tips of Au

Fig. 2. XRD patterns of Au nanorods, Pt-Au nanorods with the molar ratio at 1:10, 1:6 and 1:2, and Pt nanoparticles deposited on carbon black.

Fig. 1. Typical TEM images of as-synthesized Au nanorods (A), Pt decorated Au nanorods with different molar ratios of Pt/Au = 1:10 (B), 1:6 (C) and 1:2 (D) and the magniﬁed TEM images of B, C, and D are E, F, and G, respectively.

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and thus is more accurate compared to XRD and TEM techniques. Fig. 3 shows the cyclic voltammograms (CVs) of the pure Au nanorods and Pt decorated Au nanorods with different molar ratios. It was observed that after the decoration of Au nanorods by Pt submonolayer, the well-deﬁned hydrogen adsorption–desorption and the Pt oxide reduction peaks show up and gradually increase in intensity with the increase of the molar ratios of Pt/Au from 1:10 to 1:2, which are characteristics of Pt-based electrodes. From the zoom in of the cathodic scan (Fig. 3B), the peak for the gold surface oxide reduction decrease. The Au oxide reduction peaks at 0.9 V still can be observed for the Pt decorated Au nanorods with a Pt:Au ratio from 1:10 to 1:6, but not for the Pt:Au = 1:2. The existence of the Au oxide reduction peak means the presence of exposed Au surface at these ratios and indicates Pt growth on Au is not purely epitaxial. It may grows as islands following Volmer-Weber mechanism, preferentially on the tip of Au nanorod. Further increase in the Pt ratio leads to the connection of these islands to form shell structure. The complete disappearance of the reduction peak at high ratio suggests that Au nanorods surface are completely coated by Pt shells. Here, we can say for Pt–Au nanorods with the molar ratio of 1:10 and 1:6, they both have a decorated structure, but for Pt:Au = 1:2 or above a core–shell structure develops. The as-prepared novel nanostructured materials with decorated structure were tested for their activity as electrocatalysts for formic acid oxidation. Fig. 4 shows the CVs of Au/C, Pt/C and Pt/Au nanorods with different molar ratios of 1:10, 1:6 (decorated struc-

Fig. 3. (A). CVs of Au/C, Pt/Au = 1:10, 1:6, 1:2 at the scan rate of 50 mV/s in 0.5 M H2SO4. The current are normalized by Au weight. (B) Zoom in of the cathodic scan.

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Fig. 4. CVs of Au/C, Pt/Au = 1:10, 1:6, 1:2, and Pt/C at the scan rate of 50 mV/s in 0.5 M H2SO4 + 0.5 M HCOOH. The currents are normalized by Pt weight.

ture) and 1:2 (core–shell structure) in 0.5 M H2SO4 + 0.5 M HCOOH. As shown, Au/C itself has negligible activity towards formic acid oxidation in acid medium. For the conventional Pt/C catalysts, two peaks were observed in the forward scan. The ﬁrst peak at 0.35 V on the Pt/C CVs is attributed to the direct oxidation of formic acid to form CO2 while the second peak at 0.65 V refers to the oxidation of the poisoning intermediate CO generated from the dissociative adsorption step. The intensity of these two peaks gives an indication along which path the reaction is proceeding. It was observed that Pt/Au nanorods = 1:2, which is of core–shell structure, showed slightly higher activity than the conventional pure Pt/C with similar peak shape. Apparently, the ﬁrst peak at 0.35 V became higher relative to the second peak at 0.65 V, compared to Pt/C. This indicates that the formic acid oxidation on the Pt/Au nanorods = 1:2 shifts towards the dehydrogenation branch. With the further decrease of the Pt/Au molar ratios to 1:6 and 1:10, the second peak completely vanished. This observation suggest that the dehydration branch to form CO poisoning intermediate is mostly suppressed on the Pt decorated Au with a Pt/Au ratio between 1:2 and the majority of the formic acid is oxidized via a dehydrogenation step. This catalytic behavior is very similar to that of Pd/C toward formic acid oxidation where Pd facilitates the formic acid oxidation via the dehydrogenation step without forming the strongly adsorbed intermediates CO. It is also worthy note that Pt decorated Au nanorods show the lower onset potential than the Pt/Au core–shell nanorods and Pt/C do. A lower onset potential on Pt decorated Au nanorods means that the formic acid is easier to oxidize. The forward peak current intensities increase with the decrease in the Au surface coverage by Pt, suggesting that the catalytic capabilities of the Pt atoms actually improved with the decrease in the coverage within the range studied. This observation is very similar to the result obtained with Pt decorated Au nanoparticles [15], where an ensemble effect is proposed for the explanation of the activity change. A lower activity towards formic acid oxidation was observed for the Pt decorated Au nanorods when compared with our previously reported Pt decorated Au nanoparticles catalysts [15]. This difference apparently is due to the bigger size of Au nanorods in the present work. The smaller size of Au cores provides higher surface area for the depositon of Pt submonolayer or nanoislands. It also has higher curvature and more defect sites. For the ﬁxed molar ratios of Pt/Au, smaller cores will then facilitate the formation of decorated Pt structure, which thus enhances the activity towards formic acid oxidation. However, if the sizes of Au nanorods and Au nanoparticles as the cores are comparable, the one-dimensional nanostructure of Pt decorated Au nanorods may provide certain

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advantage than the spherical structure. The systemic investigation on the control of size, shape and dimensions of Au cores on the catalytic activity is still ongoing. 4. Conclusion In summary, the hybrid of Pt submonolayer or nanoislands was successfully deposited on Au nanorods with the decorated structure or core–shell structure through seed-mediated growth method, and their excellent performance toward formic acid electrooxidation was demonstrated. The decorated structure and core–shell structure are well characterized with various techniques. The decorated nanorod catalyst shows signiﬁcant improvement of the activity toward formic acid oxidation, by suppressing the formation of the poisoning intermediate CO. Acknowledgment This work is supported by Academic research fund AcRF tier 1(RG40/05) and AcRF tier 2 (ARC11/06), Ministry of Education, Singapore.

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