Au–Pd Alloy and Core–Shell Nanostructures: One-Pot Coreduction

Article pubs.acs.org/Langmuir

Au−Pd Alloy and Core−Shell Nanostructures: One-Pot Coreduction Preparation, Formation Mechanism, and Electrochemical Properties Long Kuai, Xue Yu, Shaozhen Wang, Yan Sang, and Baoyou Geng* College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Laboratory of Molecular-Based Materials, Anhui Normal University, Wuhu 241000, P. R. China S Supporting Information *

ABSTRACT: It is a known fact that Pd-based bimetallic nanostructures possess unique properties and excellent catalytic performance. In this work, the Au−Pd alloy and core−shell nanostructures have been prepared by a simple one-pot hydrothermal coreduction route, and their formation process and mechanism are discussed in detail. A reducing capacity-induced controlled reducing mechanism is proposed for the formation process of Au−Pd bimetallic nanostructures. CTAB plays a key role in the formation of alloy Au−Pd nanostructures. When CTAB is absent, the products are typical core−shell nanostructures. Moreover, the as-prepared nanostructures exhibit excellent electrocatalytic ORR performance in alkaline media, especially for Au−Pd alloy nanostructures. The overpotential of oxygen reduction gets reduced significantly, and the peak potential is positive-shifted by 44 and 34 mV in comparison with the core−shell ones and Pd/C catalyst, respectively. Thus, the controllable preparation and excellent electrocatalytic properties will make them become a potentially cheaper Pd-based cathodic electrocatalyst for DAFCs in alkaline media.

1. INTRODUCTION In recent decades, noble metal nanostructures have received great attention for their unique properties and catalytic performance.1 The extensive research of Pt is a case in point due to its exceptional electrocatalytic performance in the direct alcohol fuel cells (DAFCs).1b−e However, the high cost and low content in the Earth have limited its practical applications. It is desirable to investigate new low-cost electrocatalysts with high performance to replace the role of Pt. Recently, Pd-based nanostructures have been of great research interest for their high catalytic performance.2 Besides, Pd is at least 50 times more abundant than Pt in the Earth. As a result, developing Pdbased electrocatalysts is an effective approach to drive the practical applications of DAFCs. Extensive research indicates that Pd-based bimetallic nanostructures exhibit unique properties, enhanced catalytic performance, and electrocatalytic activity, which display some advantages over that of Pt.3 Especially, the Au−Pd bimetallic system, including core−shell,4 alloy,5 etc., has drawn wide attention in the field of surface-enhanced Raman scattering (SERS),6 organic catalysis,6a,7 DAFCs electrocatalysis,8 and so forth. For example, Wang and co-workers prepared various Au@Pd core−shell nanostructures by crystal epitaxial growth technology.9 They exhibit superior catalytic performance in Suzuki coupling reaction. In addition, Han et al. reported octapodal Au−Pd bimetallic nanoparticles, which have higher catalytic activity for the electro-oxidation of ethanol.10 Hence, it is desirable to fabricate Au−Pd bimetallic nanostructures for the electrocatalysis of DAFCs. However, the preparation is still © 2012 American Chemical Society

a challenge although the Au−Pd nanostructures have attracted intensive research interests. Typically, the Au−Pd core−shell nanostructures are always obtained by two-step epitaxial growth approach. As a result, the large-scale preparation is limited, and the preparation process is always costly and time-consuming. Besides, the polycrystalline Au−Pd alloy nanostructures based on self-assembling are rarely obtained although this structure is very beneficial to catalysis.11 Thus, it is necessary to further develop the facile one-pot method to prepare Au−Pd bimetallic nanostructures with excellent properties. In this work, we successfully fabricate Au−Pd core−shell and polycrystalline alloy bimetallic nanostructures through a facile one-pot hydrothermal coreduction route. HAuCl4 and H2PdCl4 are herein used as the raw materials. Polyvinylpyrrolidone (PVP) is used as the reductant12 for the preparation of Au−Pd core−shell nanostructures. Moreover, with the addition of cetyltrimethylammonium bromide (CTAB), polycrystalline alloy Au−Pd nanostructures are prepared. The electrochemical properties of the as-prepared two kinds of Au−Pd bimetallic nanostructures are investigated by catalyzing the oxygen reduction reaction (ORR) in alkaline media, and it turns out that the both bimetallic nanostructures exhibit good catalytic ORR activity. Notably, polycrystalline Au−Pd alloy nanostructures possess superior catalytic performance to Au−Pd core− shell nanostructures. The peak potential of O2 reduction is Received: February 25, 2012 Revised: April 13, 2012 Published: April 13, 2012 7168

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3. RESULTS AND DISCUSSION Through the hydrothermal coreduction route, we successfully obtain the Au−Pd polycrystalline alloy and core−shell nanostructures by controlling the introduction of CTAB. When CTAB is introduced, uniform spherical polycrystalline alloy nanostructures are obtained (Figure S1a,b). Figure 1a,b

significantly positive-shifted by 44 mV, and the electrocatalytic stability is apparently better. Therefore, they may become a potentially Pd-based cathodic catalyst for DAFCs because of the facile preparation and excellent electrocatalytic properties.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Hydrogen tetrachloroaurate(III) hydrate (HAuCl4), tetrachloropalladate acid (H2PdCl4), potassium hydroxide (KOH), cetyltrimethylammonium bromide (CTAB), sodium borohydride (NaBH4), and ammonia (NH3·H2O) were obtained from Shanghai Reagents Co. (Shanghai, China), and polyvinylpyrrolidone (PVP, Mw = 360 000) was purchased from China Institute of New Chemical Reagents (Shanghai, China). Commercial carbon (Vucan XC-72, Cabot) and Nafion were bought from YiBang/RuiBang New Power Sources Technology Co. Ltd. All the reagents were used as received without any further purification, and all the chemicals are analytical grade. 2.2. Preparation of Au−Pd Alloy Bimetallic Nanostructures. 0.2 g of PVP and 0.2 g of CTAB were added in order into 10 mL of 0.7 mM HAuCl4 and 0.9 mM H2PdCl4 mixture aqueous solution under vigorous stirring, and then 5 mL of 2.8% NH3·H2O was added. After stirring for 10 min, the solution was transferred into a 25 mL Teflonlined stainless-steel autoclave for 12 h at 180 °C. When cooled down at room temperature, the product was collected by centrifugation and washed with deionized water and absolute ethanol three times. 2.3. Preparation of Au−Pd Core−Shell Bimetallic Nanostructures. 0.2 g of PVP was added into 10 mL of 0.7 mM HAuCl4 and 0.9 mM H2PdCl4 mixture aqueous solution under vigorous stirring, and then 5 mL of 2.8% NH3·H2O was added. After stirring for 10 min, the solution was transferred into a 25 mL Teflon-lined stainless-steel autoclave for 12 h at 180 °C. When cooled down to room temperature, the product was collected by centrifugation and washed with deionized water and absolute ethanol three times, respectively. 2.4. Preparation of Pd/C Catalyst. 2 mg of commercial carbon (Vucan XC-72, Cabot) was dispersed in 10 mL of deionized water, and 2 mL of 4.5 mM H2PdCl4 solution was added. After stirring for 10 min, 5 mg of NaBH4 was added and kept stirring for 30 min. The product was collected by centrifugation and washed with deionized water and absolute ethanol 1−2 times, respectively. 2.5. Characterizations. The samples were characterized by scanning electron microscopy (SEM; Hitachi S-4800, Japan) and transmission electron microscopy (TEM; Tecnai G2 20 S-TWIN, Holland). The HAADF-STEM image and elemental mappings are performed by using a JEM-2100F (JEOL, Japan) equipped with energy dispersive spectrometer (EDS) analyses. The samples for above characterizations are prepared by dropping their dilute solution on a carbon-coated copper grid and dried at room temperature. The electrochemical characterization was carried out using a CHI660C electrochemical workstation (Shanghai Chenhua Apparatus Co., China). 2.6. Electrocatalytic Measurements. To prepare the working electrode, 1 mg of the as-prepared Au−Pd alloy, core−shell products, and Pd/C catalyst was dispersed into 1 mL of absolute ethanol, respectively, and then 15 μL of as-prepared catalysts colloid was dropped onto a glassy carbon electrode (3 mm diameter) which was polished with 0.3 and 0.05 μm γ-Al2O3 in turn before using. Finally, 10 μL of Nafion (0.05 wt %) solution was pipetted onto the catalyst film and then dried at room temperature. All the electrochemical measurements were performed using a CHI660C electrochemical workstation at a scanning rate of 50 mV/s at room temperature in a standard three-electrode cell at room temperature, in which a Pt wire was used as the counter electrode, Ag/ AgCl (saturated KCl) electrode was used as the reference, and the modified glassy carbon electrode was used as the working electrode. The electrochemical measurements for ORR were performed in 1 M O2-saturated KOH solution. The current density is normalized by the electrode area.

Figure 1. SEM (a), TEM (b), HAADF-STEM (c) and HAADFSTEM-EDS mapping (d) images of Au−Pd alloy nanostructures. SEM (e) and TEM (f) images of Au−Pd core−shell nanostructures.

displays the typical SEM and TEM images of the as-prepared Au−Pd polycrystalline alloy nanostructures. The overall size is about 200 nm. According to the TEM image, it can be obviously found that the whole nanostructures are assembled by small nanoparticles. To further confirm the alloy nanostructures, the HAADF-STEM image and elements mapping analysis are carried out . As shown in Figure 1c, the HAADF-STEM image apparently reveals the uniform element distribution of Au and Pd. It is further illustrated by elements mapping analysis (Figure 1d and Figure S1c,d), which presents that Au and Pd atoms almost distribute in the same position and form alloy nanostructures. In addition, we find that the surface (about 10 nm) of the alloy nanostructures is Pd-enrich (Figure 1d). Meanwhile, the Au−Pd core−shell nanostructures are fabricated when CTAB is absent. As shown in Figure 1f, the products are typical core−shell nanostructures. Their size is much larger than that of Au−Pd alloy bimetallic nanostructures. The whole size is about 300 nm. Specifically, the size of Au core is about 180 nm, and the thickness of Pd shell is about 60 nm. As for the formation process of the two kinds of Au−Pd bimetallic nanostructures, we propose a reducing capacityinduced controlled reducing mechanism which is illustrated in Scheme 1. Herein, CTAB plays a key role in the formation of 7169

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The mentioned two reaction processes also can be confirmed by the experiments and previous reports. Han et al. found that H2PdCl4 can be well reduced with only CTAB as the reductant even at the temperature of as low as 90 °C in the hydrothermal conditions.4b However, we find that H2PdCl4 cannot be reduced at 160 °C when only PVP is used as the reductant. Moreover, when the experiments are repeated at 160 °C while keeping other conditions unchanged, we find that the products obtained without CTAB are smooth nanoparticles whose size is very close to that of Au core (Figure S3). In addition, we investigate the growth process of the two kinds of Au−Pd bimetallic nanostructures by imaging the products obtained at different reaction times. Typically at 1 h, we find that the products obtained without CTAB are smooth nanoparticles and the size is close to that of Au core (Figure 2a). However, the products with CTAB are small nanoparticles with the tendency to congregate with each other (Figure 2b). When the reaction further proceeds to 4 h, the products obtained without CTAB are also smooth nanoparticles only with change in size (Figure 2c). Nevertheless, the morphology of the products obtained with CTAB is typically assembled by small nanoparticles (Figure 2d), which is similar to the final Au−Pd alloy nanostructures. As shown in Scheme 2, the possible reducing mechanisms of PVP and CTAB are displayed. The reducing capacity of PVP is assigned to the hydroxyl groups at the end of PVP molecules, which has been well discussed by Prof. Xia and co-workers with the help of the 13C NMR spectrum.12a However, it is the ammonium that determines the reducing capacity of CTAB. Very recently, some ammonium molecules, such as poly(diallyldimethylammonium chloride)13 and CTAB,14 have been found to have reducing capacity in hydrothermal synthesis

Scheme 1. Formation Process of Au−Pd Core−Shell and Alloy Bimetallic Nanostructures

the two kinds of Au−Pd bimetallic nanostructures. The reducing ability of PVP molecular is rather poor which makes the reduction of Pd(II) (PdCl42−/Pd0 = 0.6 vs RHE (reversible hydrogen electrode)) more difficult than that of Au(IV) (AuCl4−/Au0 = 1.02 vs RHE). As a consequence, HAuCl4 is first reduced by PVP, and H2PdCl4 is reduced subsequently. Because HAuCl4 and H2PdCl4 are reduced in order, the core− shell nanostructures bimetallic nanostructures form naturally. Nevertheless, the reaction process gets changed when CTAB is introduced into the reaction system. The reducing capacity of CTAB is much stronger than that of PVP in the hydrothermal conditions (which will be discussed later), so that the Pd(II) and Au(IV) can be reduced at the same time. As a result, the Au 0 and Pd 0 are combined together and the alloy nanostructures are formed. Furthermore, when CTAB acts as the reductant, PVP plays a common role in controlling the morphology of the products instead of the reductant. As vividly shown in Figure S2, many disordered nanoparticles were obtained in the final products without PVP.

Figure 2. The obtained products with the reaction time of 1 h without (a) and with 0.2 g of CTAB (b), 4 h without (c) and with 0.2 g of CTAB (d) at 180 °C. 7170

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carried out in the same condition, and the corresponding results have been shown in Figure 3a,b and summarized in Table 1. Obviously, the Au−Pd alloy nanostructures exhibit the

Scheme 2. Reducing Mechanisms of PVP (a) and CTAB (b) in Hydrothermal Conditions

Table 1. The Mentioned Electrocatalysts and Their Corresponding Catalyzing ORR Performance electrocatalytic ORR performance sample Au−Pd alloy Au−Pd core− shell Pd/C

conditions. Moreover, Han et al. also found the nitroso group on the cetyltrimethylammonium chloride (CTAC)-stabilized Au@Pd core−shell nanoparticles by FT-IR spectra. Herein, the nitroso group is produced from the N-oxidation of CTAC.4b However, the reducing capacity of PVP is lower than that of ammonium, although it also can be found that there is N atom in the PVP molecule. We suppose that the N atom of PVP is in the condition of stable 5-membered ring structure, and high energy is needed to open the ring. Moreover, there is a carbonyl (−CO) beside N atom. We know that −CO is a strong electron-withdrawing group, so that the electron cloud of N atom is away from N atom and the reducing capacity decreased significantly. As a result, the N atom of PVP does not attend the reaction in the hydrothermal conditions. On the contrary, the N atom of CTAB is in the straight-chain hydrocarbons, and it is reaction-active in the hydrothermal conditions because high energy is not needed. In addition, the reducing capacity of −OH is much lower than that of ammonium, so the two as-prepared Au−Pd bimetallic nanostructures are obtained with the control of CTAB. In this work, their electrocatalytic properties for oxygen reduction reaction (ORR) have been investigated in alkaline media. Figure 3a exhibits their typical cyclic voltammograms (CVs) of the as-prepared Au−Pd polycrystalline alloy (red line) and core−shell (green line) nanostructures in O2-saturated 1 M KOH solution. The oxygen reduction signal can be obviously found. For Au−Pd polycrystalline alloy nanostructures, the peak potential of electrocatalytic ORR is at −0.342 V versus (vs) Ag/AgCl (saturated by KCl), and the peak current density is 0.281 mA cm−2. And the oxygen reduction peak is located at −0.386 V for Au−Pd core−shell nanoparticles. For comparison, the performance of Pd/C catalyst (blue line) is also

peak potential/V

peak current density/ mA cm−2

stable current density/ mA cm−2

−0.342 −0.386

0.281 0.285

0.080 0.051

−0.376

0.235

0.028

best electrocatalytic ORR performance. On the one hand, the current density of Au−Pd polycrystalline alloy nanostructures is about 1.2 times higher than that of the Pd/C catalyst. On the other hand, the overpotential of oxygen reduction gets reduced significantly, and the peak potential is apparently positiveshifted by 44 and 34 mV in comparison with the Au−Pd core− shell nanostructures and Pd/C catalysts, respectively. This indicates that O2 can be reduced more facilely. The electrocatalytic stability is also one of the important factors to evaluate the performance of a catalyst. Long-term chronoamperometric experiments are carried out to evaluate the stability of the as-prepared electrocatalysts for ORR. As shown in Figure 3b and Table 1, Au−Pd polycrystalline alloy nanostructures (red line) display best electrocatalytic stability for ORR. The current density reaches stable state after shorttime decay. However, the electrocatalytic stability of Au−Pd core−shell nanostructures (green line) and Pd/C (blue line) is poorer. When the endurability test is prolonged to 3600 s, it can be found that the current density for Au−Pd alloy still keeps an apparently higher scale, and it is about 1.6 times that of the Au−Pd core−shell nanostructures and 2.9 times that of Pd/C catalysts, respectively. Therefore, the Au−Pd alloy bimetallic nanostructures exhibit the best electrocatalytic ORR performance. From the above discussion, we can say that the enhanced electrocatalytic ORR performance of Au−Pd bimetallic nanostructures stems from their structures, especially for the polycrystalline alloy nanostructures. Two advantages of the obtained nanostructures are reasonable to the best electrocatalytic performance of Au−Pd alloy nanostructures. For one,

Figure 3. CVs (a) and current−time curves (b) of Au−Pd polycrystalline alloy (red line), core−shell nanostructures (green line), and Pd/C (blue line) electrocatalysts in O2-saturated 1 M KOH solution measured at room temperature, and the current−time curves are all recorded at the corresponding peak potential. 7171

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bimetallic nanostructures are better than the pure metal nanoparticles. As a result, Au−Pd alloy and core−shell nanostructures all show better electrocatalytic activity than Pd/C catalyst. Moreover, the alloy nanostructures may change the density of states of the d-band of pure Pd nanoparticles so that it is more beneficial to the combination with O2 molecular, which results in the higher catalytic performance.15 Therefore, the ORR peak potential gets positive-shifted by 44 mV in comparison with that of Au−Pd core−shell nanostructures. For another, the polycrystalline nanostructures are good for catalysis because of the larger specific surface and many active sites. And such nanostructures contribute greatly to enhancing the catalytic stability, so the Au−Pd alloy nanostructures exhibit the best electrocatalytic stability. In addition, we find that the surface of Au−Pd alloy nanostructures is Pd-rich. Similar to the surface Pt-rich catalysts,16 the surface is full of catalytic active composition, which is very beneficial to electrocatalytic ORR performance. Hence, the Au−Pd polycrystalline alloy nanostructures show the best electrocatalytic performance for ORR.

ASSOCIATED CONTENT

S Supporting Information *

Additional figures and figure captions. This material is available free of charge via the Internet at http://pubs.acs.org.

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4. CONCLUSIONS In summary, two kinds (alloy and core−shell) of Au−Pd bimetallic nanostructures have been prepared by a simple onepot hydrothermal route, and their formation process and mechanism are discussed in detail. The reduction capacity of PVP and CTAB is a key to forming core−shell and alloy nanostructures. Electrochemical investigation suggests that the as-prepared bimetallic nanostructures possess excellent electrocatalytic ORR performance, notably for Au−Pd alloy nanostructures, in which the peak potential is positive-shifted by 44 and 34 mV compared with Au−Pd core−shell nanostructures and Pd/C catalyst, respectively. In particular, the polycrystalline Au−Pd alloy nanostructures display superior catalytic performance to that of core−shell ones. Hence, the controllable preparation and good electrocatalytic performance may render them an excellent Pd-based cathodic catalyst for the DAFCs in the future.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Fax (+86)-553-3869303. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (20671003 and 20971003), the Key Project of Chinese Ministry of Education (209060), the Program for New Century Excellent Talents in University (NCET 11-0888), the Science and Technological Fund of Anhui Province for Outstanding Youth (10040606Y32), the Foundation of Key Project of Natural Science of Anhui Education Committee (KJ2012A143), and the Program for Innovative Research Team at Anhui Normal University. 7172

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