Length-Scale Modulated and Electrocatalytic Activity Enhanced

Feb 28, 2011 - In the present paper, we have investigated the dealloying of Pt- and/or Pd-doped Al2Au intermetallic compounds and the formation of ult...
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Length-Scale Modulated and Electrocatalytic Activity Enhanced Nanoporous Gold by Doping Xiaoguang Wang,† Jan Frenzel,‡ Weimin Wang,† Hong Ji,† Zhen Qi,† Zhonghua Zhang,*,† and Gunther Eggeler‡ †

Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (MOE), School of Materials Science and Engineering, Shandong University, Jingshi Road 17923, Jinan 250061, People's Republic of China ‡ Institut f€ur Werkstoffe, Ruhr Universit€at Bochum, Bochum 44780, Germany

bS Supporting Information ABSTRACT: In the present paper, we have investigated the dealloying of Pt- and/or Pd-doped Al2Au intermetallic compounds and the formation of ultrafine nanoporous Au (np-Au) alloys through a chemical dealloying strategy. The microstructural characterization confirms that these doping atoms enter into crystal lattices of the precursors and then transmit into the as-obtained npAu, both existing in the form of solid solutions. When dealloying in the 20 wt % NaOH solution is performed, a certain amount of Pt and/or Pd addition shows a superior refining effect and the ligament/channel sizes of the as-doped np-Au can be facilely modulated below 10 nm. When dealloying in the 5 wt % HCl solution is performed, however, the anticoarsening capacity of Pt doping is more remarkable compared with that of Pd doping. In addition, the amount of doping also has an important influence on the ligament resistance to coarsening. Apart from causing the refinement of ligaments/channels, the introduction of Pt and/or Pd into np-Au has generated novel bi- or trimetallic functionalized nanoporous alloys. These as-doped np-Au alloys with an appropriate amount of Pt and/or Pd exhibit excellent electrocatalytic activities toward methanol and formic acid oxidation and will find promising applications in the catalysis-related areas.

1. INTRODUCTION Porous materials have recently been attracting considerable attention because of a wide range of applications in catalysis, sensors, heat exchangers, supercapacitors, and so on.1-4 Among the varieties of porous candidates, nanoporous gold (np-Au) is of special interest due to its unique mechanical, physical, and chemical properties associated with a three-dimensional bicontinuous interpenetrating ligament-channel structure with a length scale in nanometers.5-7 With the progress of research toward npAu, it has been experimentally proved that the size of ligaments/ channels has a significant influence on the associated properties of np-Au.8 However, the commonly as-obtained np-Au exhibits a characteristic length scale of ligaments/channels located in several tens of nanometers, especially under free corrosion conditions (for example, the ligaments/channels of the np-Au dealloyed from the Au32Ag68 alloy in nitric acid are in the scope of 2040 nm when performing at room temperature.).5,9 It is known that the coarser structure of np-Au is generally obtained through heating treatment and can be modulated to several hundreds of nanometers.8 However, it is difficult to obtain the ultrafine np-Au with a length scale of ligaments/channels less than 10 nm at room temperature because of fast diffusion of gold atoms along alloy/ electrolyte interfaces.10 At present, some research groups have reported that the npAu with an ultrafine microstructure, especially with ligaments/ r 2011 American Chemical Society

channels less than 10 nm, reveals lots of peculiar physical, chemical, and mechanical properties, such as strong surfaceenhanced Raman spectroscopy, improved effective Young’s modulus, excellent catalytic activity, etc.8,11,12 Thus, more and more attention has been paid to explore suitable strategies to design ideal np-Au with ultrafine ligaments/channels. Among them, low-temperature and anodic-potential-modulated dealloying techniques are usually applied to fabricate this ultrafine np-Au with ligaments/channels below 10 nm. Qian and Chen13 have found that the average nanopore can be reduced to ∼7 nm when performing dealloying of Ag-Au alloys at -20 C for 4 h, and even to a smaller value (∼5 or ∼3 nm) within a shorter dealloying time (1 h or 10 min). In general, the dealloying manipulated at a rigorous low temperature (down to minus tens of degrees centigrade) is complicated and expensive. On the one hand, the dealloying time should be accurately controlled to avoid coarsening as soon as possible. On the other hand, the rather slow reaction kinetics at such a low temperature will unavoidably result in a long reaction time for full dealloying, especially for ribbon-like and bulk precursors. Obviously, the low-temperature method is merely suitable for ultrathin film Received: October 19, 2010 Revised: February 4, 2011 Published: February 28, 2011 4456

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The Journal of Physical Chemistry C precursors, such as commercial Ag-Au leaves with a thickness of ∼100 nm. Xu and Ding12 have employed a potential-modified dealloying technique by etching a commercial Ag-Au leaf in concentrated nitric acid and have fabricated np-Au with ligaments/channels of less than 6 nm. Simultaneously, Snyder and Erlebacher14 have found that ultrafine ligaments/channels of about 4-6 nm can be obtained through electrochemically dealloying Ag-Au in a neutral AgNO3 solution rather than conventional acidic electrolytes. Besides, they have revealed that, when adding a small amount of Pt into the traditional Ag-Au precursor, the as-obtained np-Au has ultrafine ligaments/channels of ∼4 nm using the same dealloying strategy.15 However, a large amount of undealloyed Ag residual is inevitable.14,15 Jin et al.16 and Xu et al.,17 hereafter, have successfully fabricated ultrafine nanoporous Au-Pt alloys with ligaments/channels of less than 10 nm by dealloying Ag-Au-Pt and Cu-Au-Pt precursors in a potential-controlled strategy, respectively. Apart from those mentioned above, little literature is available involving the size modulation and functionalization of np-Au as well as nanoporous alloys until now. In our former work, we have successfully fabricated np-Au with continuous and bimodal ligament/channel structures by dealloying another important alloy system, Al-Au.18,19 From the viewpoint of formation mechanism, the Al-Au system, especially the Al2Au intermetallic compound, involves a more complicated dealloying process as well as nanoporosity evolution relative to the prototype Ag-Au solid solution.19 From the viewpoint of industrial applications, the selection of Al rather than Ag offers some advantages: First, Al is much cheaper than Ag. Second, Al is more active than Ag not only in the electroless process but also under potential-controlled conditions, and thereby, the selective corrosion of Al largely shortens the dealloying time even if the precursors are designed as ribbons or bulk. These obvious advantages are suitable for mass production and make it possible to meet the requirement of industrial applications. Taking into account the potential prospect of the Al-Au system, the research regarding the length scale modulation and functional design of np-Au is still of great significance. In the present paper, the doping element M (M = Pt and/or Pd) has been introduced into a single Al2Au intermetallic compound precursor, and the influence of elemental doping on nanoporosity evolution has been investigated in a chemical dealloying mode. In addition, it is reasonable to assume that the inherent character of the doping element should be inherited to the as-obtained np-Au. As is known to all, the noble metal Pt or Pd is widely used in the field of fuel cells as an electrocatalyst and exhibits excellent catalytic activities toward the oxidation of small organic molecules, such as methanol, ethanol, formic acid, etc.20,21 Moreover, it has been recently found that the synergy between the platinum group metals (Pt, Pd, etc.) and Au has a significant improvement on the electrocatalytic process, either changing the reaction pathway or accelerating the reaction kinetics.22,23 Thus, the electrocatalytic activity of these np-Au(M) alloys toward small organic molecules (methanol and formic acid) has also been probed in the present work.

2. EXPERIMENTAL SECTION 2.1. Preparation of Precursors and Dealloying Procedure. The initial prealloyed precursor (Al66Au34, atm. %) was designed according to the nominal composition of a single-phase Al2Au intermetallic compound, and then the platinum group elements

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Figure 1. XRD patterns of (a) the rapidly solidified Al66Au27.2Pt6.8 alloy and (b) the corresponding as-dealloyed np-Au80Pt20 alloy.

(Pt and/or Pd) were substituted for small amounts of Au. The asdoped precursors (nominal compositions: Al66Au27.2Pt6.8, Al66Au27.2Pd6.8, Al66Au30.6Pd3.4, Al66Au32.3Pd1.7, and Al66Au27.2Pd3.4Pt3.4) were prepared by melting corresponding pure metals (Al, Au, Pt, and Pd, 99.9 wt % purity) in a quartz crucible using a high-frequency induction heating apparatus. The postalloyed ingots were then melt-spun onto a copper roller with a diameter 0.35 m at a speed of 1000 rpm in a controlled argon atmosphere. The ribbons obtained were typically 20-50 μm in thickness, 24 mm in width, and several centimeters in length. The as-doped alloy ribbons were dealloyed in a 20 wt % NaOH solution first at room temperature until no obvious bubbles emerged, and then at 90 ( 5 C to further leach out the residual Al in the samples. In some cases, the dealloying process was also performed in a 5 wt % HCl aqueous solution for comparison. Finally, the as-doped npAu(Pt), np-Au(Pd), and np-Au(PdPt) samples were obtained. 2.2. Microstructural Characterization. The phases present in the as-doped precursor alloys (Al66Au27.2Pt6.8, Al66Au27.2Pd6.8, Al66Au30.6Pd3.4, Al66Au32.3Pd1.7, and Al66Au27.2Pd3.4Pt3.4) and as-dealloyed samples (np-Au(Pt), np-Au(Pd), and np-Au(PdPt)) were identified using an X-ray diffractometer (XRD, Hitachi Rigaku D/max-RB) with Cu KR radiation. The microstructure and chemical composition of the as-dealloyed samples were 4457

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Figure 2. (a) SEM and (b) TEM images showing the microstructure of np-Au80Pt20 through dealloying the rapidly solidified Al66Au27.2Pt6.8 alloy in the 20 wt % NaOH solution. (c) A typical EDX spectrum. The inset in (b) shows the corresponding SAED pattern of the fcc Au(Pt) [110] zone axis.

characterized using a scanning electron microscope (SEM, LEO 1530 VP), equipped with an energy-dispersive X-ray (EDX) analyzer. Transmission electron microscopy (TEM) images and selected-area electron diffraction (SAED) patterns were obtained using a Philips CM 20 microscope and a high-resolution FEI Tecnai G2 microscope. TEM specimens were prepared using a Gatan ion mill at 5 kV. Fast Fourier transform (FFT) patterns were obtained from the corresponding HRTEM images using a Gatan software. 2.3. Electrochemical Measurements. The electrochemical measurements were performed in a standard three-electrode cell using a LK 2005A potentiostat. The fabrication technique of the modified electrodes with these as-dealloyed samples is described below: 2 mg of fine ground as-dealloyed samples, 4 mg of Vulcan XC-72 carbon powders, 300 μL of isopropanol, and 100 μL of Nafion solution (0.5 wt %) were ultrasonically mixed. The homogeneously mixed catalyst ink (5 μL) was placed on a freshly polished glassy carbon (GC) electrode with a diameter of 4 mm. These electrodes were used as the working electrodes. The counter electrode was a bright Pt plate, and a saturated calomel electrode (SCE) or a Hg/HgO (1.0 M KOH) electrode (MMO) was used as the reference electrode, depending on the experimental requirements. Voltammetric behavior was characterized in a 0.5 M H2SO4 solution deaerated with N2. The electrocatalytic activity measurements were carried out in formic acid (0.5 M H2SO4 þ 0.5 M HCOOH) and methanol (1.0 M KOH þ 1.0 M CH3OH) solutions. The corresponding catalytic current was normalized by the metal loadings per cm2 GC electrode surface. All electrochemical experiments were performed at ambient temperature (∼25 C).

3. RESULTS AND DISCUSSION 3.1. Phase Analysis of Precursors and Corresponding AsDealloyed Samples. Figure 1 shows the XRD patterns of the

ternary Al66Au27.2Pt6.8 alloy and corresponding as-dealloyed sample in the 20 wt % NaOH solution. By comparison with

the reference profile of Al2Au (PDF No. 17-0877), it is clear that the Al66Au27.2Pt6.8 precursor alloy consists of a single-phase intermetallic compound with an Al2Au-type structure, as shown in Figure 1a. In addition, no diffraction peaks referring to the involved metal elements, such as Al, Au, and Pt, can be identified in the rapidly solidified precursor. According to our previous studies, the binary Al66.6Au33.4 precursor is composed of a singlephase Al2Au (CaF2-type) intermetallic compound.18 Therefore, it is reasonable to assume that Pt exists in a solid solution form partially substituting for Au in the Al2Au lattice, and this phase in the Al66Au27.2Pt6.8 precursor can be denoted as Al2(Au,Pt) (Figure S1 in the Supporting Information). As smaller Pt atoms replace bigger Au atoms in the Al2Au crystal cells, the as-doped Al2(Au,Pt) crystal cells contract. According to Bragg’s law, λ = 2d sin θ, as the crystal cell contracts, that is, the value of d decreases, the values of the angles θ and 2θ of the fundamental reflections increase at a given radiation wavelength λ. This is exactly what is observed from Figure 1a, where the Al2(Au,Pt) reflections shift slightly toward larger 2θ values in comparison with those of the pure Al2Au. The lattice parameter of Al2(Au,Pt) was calculated to be 5.973 Å, smaller than that of pure Al2Au (aAl2Au = 5.997 Å). After dealloying, the as-dealloyed sample reveals a single fcc Au phase, which is consistent with the standard pattern of fcc Au (PDF No. 04-0784), as shown in Figure 1b. However, all the diffraction peaks for the (111), (200), (220), and (311) reflections show broad profiles and the peak positions slightly shift to those of fcc Pt (PDF No. 04-0802), indicating the formation of a Au(Pt) solid solution. The stabilities of bimetallic architectures are usually dictated by the thermodynamic miscibility of the two metals. According to the binary phase diagram, Au and Pt are immiscible over a wide composition and temperature range. In addition, theoretical calculations have also suggested that Pt and Au prefer core-shell or core-shell-like structures due to their thermodynamic immiscibility,24 and yet it is well recognized that the dealloying process involves the dissolution of the less noble element (here, Al) and the surface diffusion/rearrangement of the more noble element (here, Au and Pt) at the nanoscale.5,25 4458

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Figure 3. (a) SEM and (b) TEM images showing the microstructure of np-Au80Pd20 through dealloying the rapidly solidified Al66Au27.2Pd6.8 alloy in the 20 wt % NaOH solution. (c) A typical EDX spectrum. The inset in (b) shows the corresponding SAED pattern of the fcc Au(Pd) [100] zone axis.

Thereby, the nanoscale alloying along the alloy/solution interface should contribute to the formation of the Au(Pt) solid solution alloy during dealloying. Similarly, in three Pd-doped ternary Al66Au34-xPdx (x = 1.7, 3.4, 6.8) precursors and a Pd/Pt-codoped quaternary Al66Au27.2Pd3.4Pt3.4 precursor, all these starting alloys also reveal a singlephase Al2Au-type (Al2(Au,Pd), Al2(Au,Pt,Pd)) structure (Figure S2a-d) in the Supporting Information). Moreover, it is clear that no other reflections can be observed, indicating that Pd or Pd/Pt atoms exist in the form of solid solution substituting for Au sites in the Al2Au lattice. Simultaneously, all diffraction peaks shift to higher angles compared with the reference profile of Al2Au, especially for the high-index reflections. It further confirms the lattice contraction of Al2Au due to the introduction of Pd or Pd/Pt atoms. After dealloying in the 20 wt % NaOH solution, likewise, only one set of broad fcc diffraction peaks can be observed, which can be assigned to the (111), (200), (220), and (311) reflections (Figure S3 in the Supporting Information). All diffraction peaks are close to those of Au, but slightly shift to higher angles, also demonstrating the formation of Au(Pd) and Au(Pd,Pt) single-phase solid solutions. The present results indicate that these Al2Au-type phases in the precursors can be fully dealloyed in the NaOH solution and contribute to the formation of stable Au(Pt), Au(Pd), and Au(Pt,Pd) solid solution alloys. In addition, according to the XRD results (Figure 1b and S3 (Supporting Information)) and the empirical Vegard’s law, the compositions of the Au(Pt), Au(Pd), and Au(Pt,Pd) solid solutions were calculated and are listed in Table S1 (Supporting Information). It is obvious that the calculated compositions are well consistent with the nominal compositions of the as-dealloyed samples (Table S1, Supporting Information). 3.2. Influence of Dopants and Dealloying Solutions on the Microstructure of Au-Based Nanoporous Alloys. Figure 2 shows the microstructure of the as-obtained np-Au(Pt) through dealloying the Al66Au27.2Pt6.8 alloy in the 20 wt % NaOH solution.

As shown in Figure 2a, the as-dealloyed sample exhibits an open, bicontinuous interpenetrating ligament-channel structure with a ligament size significantly smaller than that of traditional np-Au. In addition, a typical TEM image provides more details for this kind of Pt-doped np-Au(Pt) (Figure 2b). The sharp contrast between dark skeletons (ligaments) and inner bright regions (channels) further confirms the formation of a three-dimensional bicontinuous nanoporous structure. With an average of more than 50 ligaments in a randomly selected area, the length scale of ligaments/channels is in the range of 3.5 ( 1.0 nm. The SAED pattern of the fcc Au(Pt) [110] zone axis confirms the singlecrystalline nature of the np-Au80Pt20 across the whole selected area, which was 200 nm in diameter (inset of Figure 2b). The chemical composition of the as-dealloyed sample was determined to be 75.84 atm. % Au, 20.53 atm. % Pt, and 3.63 atm. % Al, and a typical EDX spectrum is shown in Figure 2c. The EDX analysis demonstrates that the atomic ratio of Au/Pt is close to 4:1 in the as-dealloyed sample, consistent with that in the initial Al66Au27.2Pt6.8 precursor. Thereafter, the as-dealloyed sample is designated as np-Au80Pt20 for simplicity in the light of the Au/Pt atomic ratio. Similarly, the np-Au(Pd) and np-Au(Pt,Pd) samples can also be designated as np-Au80Pd20, np-Au90Pd10, npAu95Pd5, and np-Au80Pt10Pd10. Figure 3 shows the microstructure of the as-obtained npAu80Pd20 through dealloying the Al66Au27.2Pd6.8 alloy in the 20 wt % NaOH solution. It is obvious that the ultrafine bicontinuous interpenetrating ligament-channel structure can also be obtained when doping with the same amount of Pd instead of Pt (Figure 3a). The detailed microstructure can be clearly observed in the TEM image (Figure 3b). The average length scale of ligaments/channels is located in 5.0 ( 1.0 nm. Similarly, the SEAD pattern of the fcc Au(Pd) [100] zone axis verifies the single-crystalline nature of the np-Au80Pd20 across the neighboring ligaments in the selected area (inset of Figure 3b). As expected, the EDX result shows that the atomic ratio of Au/Pd 4459

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Figure 4. (a) SEM, (b) TEM, and (c) HRTEM images showing the microstructure of np-Au80Pd10Pt10 through dealloying the rapidly solidified Al66Au27.2Pd3.4Pt3.4 alloy in the 20 wt % NaOH solution. (d) A typical EDX spectrum. The insets in (b) and (c) show the corresponding SAED and FFT patterns, respectively.

is close to that in the initial Al66Au27.2Pd6.8 precursor and only several percent of residual Al can be detected (Figure 3c). Figure 4 shows the microstructure of the as-obtaied np-Au80Pd10Pt10 through dealloying the quaternary Al66Au27.2Pd3.4Pt3.4 alloy in the 20 wt % NaOH solution. Although the profile revealed by the SEM image is vague, the typical bicontinuous interpenetrating ligament-channel structure can be confirmed on the basis of TEM observation (Figure 4a,b). The length scale of ligaments/channels is modulated to 3.7 ( 1.0 nm in the npAu80Pd10Pt10. The corresponding SEAD pattern of the fcc Au(Pt, Pd) [100] zone axis indicates the single-crystalline nature of the np-Au80Pd10Pt10 (inset of Figure 4b). Figure 4c shows a typical HRTEM image for this structure, and the ultrafine ligaments/channels can be further confirmed. Moreover, lattice fringes can be seen extending throughout all the ligaments, indicating a single-crystalline structure across the whole frame of observation. The corresponding FFT pattern also confirms the

single-crystalline nature of the connecting ligaments in the npAu80Pd10Pt10 (inset of Figure 4c). The anticipated atomic ratio of Au/Pt/Pd in the np-Au80Pd10Pt10 sample has been confirmed by the corresponding EDX analysis, and a typical EDX spectrum is shown in Figure 4d. Erlebacher et al.5 have found a maximally unstable spatial period that scales as λ ¥ (Ds/V0)1/6, where λ is the characteristic spacing separating the evolution of gold clusters, Ds is the surface diffusion coefficient, and V0 is the velocity of a flat alloy surface with no gold accumulated upon it. Extrapolation to room temperature (298 K) from the literature data yields surface mass transfer diffusion coefficients of Pt, Pd, and Au of 3.6  10-22, 1.1  10-20, and 2.2  10-19 cm2/s in vacuum.8,26,27 Snyder et al.15 have argued that the additives possessing much slower surface diffusion rates will pin the mobile Au step edges and ultimately stabilize them, reducing the scale of porosity. Obviously, the variation of ligaments in the np-Au can be related to 4460

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Figure 5. (a) SEM and (b) TEM images showing the microstructure of np-Au80Pd10Pt10 through dealloying the rapidly solidified Al66Au27.2Pd3.4Pt3.4 alloy in the 5 wt % HCl solution. (c, d) Corresponding SAED patterns of regions A and B, respectively.

the diffusion of Au atoms during the dealloying process. Although the surface diffusion coefficients of elements in electrolyte are significantly larger than those in vacuum, it still can help us to understand their different anticoarsening effect by comparing the surface diffusion coefficients in vacuum.28 Moreover, the pinning effect has also been observed in the dealloying of the Mg-AgPd system, in which the slow surface diffusion of Pd retards the diffusion of Ag atoms, resulting in the formation of ultrafine ligaments/channels as small as ∼5 nm in the np-Ag80Pd20 alloy.29 Therefore, it is reasonable to assume that Pt and/or Pd atoms with lower surface diffusivities can agglomerate on the forepart of diffusive Au atoms so as to inhibit the continuity of self-assembled proliferation in the dealloying process and thus refine the ligament/channel sizes of the as-obtained np-Au alloys. On the other hand, these embedded Pt and/or Pd atoms in exposed terraces also enhance the stability of np-Au against coarsening in the electrolyte. As expected, by introduction of a certain amount of Pt, Pd, or Pt/Pd into the precursors, we have obtained the npAu alloys with ultrafine ligament/channel sizes. It should be emphasized that the dealloying solution has a significant influence on the morphology of np-Au as well as other nanoporous metals. The nondoped Al2Au precursor can be dealloyed to form the nanoporous structure with the length scale of 10-20 nm in the 20 wt % NaOH solution. However, the dealloying of Al2Au in the 5 wt % HCl solution will lead to the formation of a coarse nanoporous structure with the length scale of 60-80 nm.18,19 The halide ions, such as Cl-, are well known to enhance the surface diffusion of adatoms during dealloying and have a significant coarsening effect on the nanoporous structure. Newman and Sieradzki30 have reported that the coarsening process of np-Au is affected by the environment; in particular, adsorption of Cl- increases the diffusion rate. Hakamada and Mabuchi31 have also reported that np-Au can

even be coarsened into a skin-divided nanoporous prism microassembly when immersed in the concentrated HCl solution. Here, the refining effect of Pt and/or Pd doping on the ligaments of np-Au has also been investigated in the 5 wt % HCl solution. For the target precursors (Al66Au27.2Pt6.8 and Al66Au27.2Pd6.8), the microstructure of the as-dealloyed samples in the 5 wt % HCl solution is quite different from that in the 20 wt % NaOH solution (Figure S4 in the Supporting Information). The length scale of the np-Au80Pt20 dealloyed in the 5 wt % HCl solution is coarsened to 7.0 ( 1.0 nm (Figure S4a,b in the Supporting Information), double that dealloyed in the 20 wt % NaOH solution. Strikingly, the length scale of the np-Au80 Pd20 dealloyed in the 5 wt % HCl solution is even coarsened to 21.4 ( 5.0 nm (Figure S4c,d in the Supporting Information), which is ∼4 times that dealloyed in the 20 wt % NaOH solution. Of course, even if the np-Au80Pt20 and np-Au80Pd20 undergo coarsening, their single-crystalline characteristic is still preserved (the SAED patterns are shown in the insets of Figure S4b,d in the Supporting Information). It should be noted that the ligament size of np-Au80Pt20 and np-Au80Pd20 is much smaller than that of pure np-Au when performing dealloying in the 5 wt % HCl solution (60-80 nm). Moreover, even if dealloying is carried out in the HCl solution, the ultrafine ligament size (less than 10 nm) can also be acquired for the Pt-doped sample (np-Au80Pt20). According to the context, both Pt and Pd reveal an excellent refining effect on the porosity of np-Au in the NaOH solution and can reduce the ligament size to less than 10 nm (3.5 ( 1.0 and 5.0 ( 1.0 nm for the np-Au80Pt20 and np-Au80Pd20, respectively). However, the refining discrepancy between Pt and Pd is remarkable in these two adopted solutions. In the HCl solution, the anticoarsening capacity of Pt doping is much superior to that of Pd doping. It may be caused by the fact that surface diffusion of 4461

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Figure 7. CV profiles of (a) np-Au, (b) np-Au80Pt20, and (c) npAu80Pd10Pt10 in the 0.5 M H2SO4 solution. Scan rate: 50 mV s-1.

Figure 6. SEM images of (a) the np-Au90Pd10 and (b) the np-Au95Pd5 through dealloying the rapidly solidified Al66Au30.6Pd3.4 and Al66Au32.3Pd1.7 alloys in the 20 wt % NaOH solution.

Pd atoms can be significantly enhanced in the HCl solution as compared with Pt atoms.18 Interestingly, when dealloying in the 5 wt % HCl solution for the quaternary Al66Au27.2Pd3.4Pt3.4 precursor is performed, the microstructure of the as-dealloyed np-Au80Pd10Pt10 evolves into two distinct regions: coarse regions with the ligament size of 13.0 ( 2.0 nm and fine regions with the ligament size of 7.0 ( 1.0 nm (marked by A and B in Figure 5a, respectively). The more detailed microstructure can be revealed in a typical TEM image, as shown in Figure 5b. There exists a compact adhesive interface between these two distinct regions, and a structural mutation occurs across this interface. In addition, the SAED patterns show that both these two regions exhibit a single-crystalline nature in the selected areas, but with different crystal orientations (Figure 5c,d). Even though the underlying reason is not known at present, we assume that the formation of this peculiar nanoporous structure might be paralleled with different surface diffusivities of the released Pt and Pd atoms during the dealloying process, especially in the Cl--containing solutions. Figure 6 shows the microstructure of the np-Au90Pd10 and npAu95Pd5 dealloyed from the Al66Au30.6Pd3.4 and Al66Au32.3Pd1.7 precursors in the 20 wt % NaOH solution. When the percent of Pd doping decreases to 10 atm. % (np-Au90Pd10), the refining effect is still conspicuous and contributes to the formation of ultrafine ligaments/channels in the scope of 6.8 ( 1.0 nm (Figure 6a). When the percent of Pd doping further decreases to 5 atm. % in the as-dealloyed sample (np-Au95Pd5), however, the refining effect on the ligament size is greatly inhibited. As shown in Figure 6b, the length scale of ligaments/channels in the as-dealloyed np-Au95Pd5 is coarsened to 15.0 ( 3.0 nm, comparable to that of the pure np-Au through dealloying Al2Au in the

NaOH solution.18 Except for the sort of dealloying solutions, the percent of doping atoms also plays a key role in the modulation of the ligament scale in the as-dealloyed samples. Too little percent of doping atoms (e5 atm. %) cannot achieve the ideal refining effect in the np-Au(Pt or Pd) alloys, and the optimum doping quantities should be located between 10 and 20 atm. % according to our experiments. 3.3. Electrocatalytic Activities of Au-Based Nanoporous Alloys. Nowadays, bi- or multicomponent metallic nanostructures constituting various combinations of noble metals have received increasing interest owing to their unique catalytic, optical, electronic, and magnetic properties that are distinct not only from the bulk metals but also from the corresponding monometallic counterparts.32,33 It should be emphasized that the addition of Pt and/or Pd into np-Au not only refines the nanoporous structure but also generates a type of Au-based alloy nanostructure. During the porosity evolution, the doping atoms (Pt and/or Pd) will be gradually embedded in exposed terraces to stabilize the step edges of the growing vacancy-island. It is reasonable to expect that the as-doped np-Au(Pt and/or Pd) alloys will exhibit extraordinary catalytic activities due to their inherent synergistic effect between Au and doping atoms.34,35 Moreover, the typical three-dimensional bicontinuous interpenetrating characteristic allows unblocked transport of medium molecules and electrons. Therefore, these as-doped np-Au(Pt and/or Pd) alloys are particularly desirable to catalysis and will be a kind of promising electrode material in the field of fuel cells. As shown in Figure 7, the influence of Pt and/or Pd doping on the electrochemical feature of np-Au was investigated by cyclic voltammograms (CVs) in the 0.5 M H2SO4 solution. In contrast to the silent feature of pure np-Au, the CVs for the np-Au80Pt20 and np-Au80Pd10Pt10 reveal hydrogen adsorption/desorption (under-potentially deposited hydrogen, Hupd) in the potential region between -0.2 and 0.0 V (vs SCE), indicative of the presence of Pt and/or Pd component on the ligament surface of the as-obtained np-Au alloys. In the reverse scan, the CV profile of pure np-Au only presents a single reduction peak located at ∼0.86 V (vs SCE) related to the reduction of Au oxides, whereas for the np-Au80Pt20, two well-separated peaks can be observed: the large one located at the high potential is ascribed to the reduction of Au oxides; the other small one placed at the lower potential (∼0.4 V (vs SCE)) should be attributed to the 4462

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Figure 8. CV curves of the np-Au, np-Au80Pt20, and np-Au80Pd10Pt10 in the (a) 0.5 M H2SO4 þ 0.5 M formic acid and (b) 1.0 M KOH þ 1.0 M methanol solutions. Scan rate: 50 mV s-1.

reduction of Pt oxides, which are comparable to those reported in the literature.36,37 Jin et al.16 have argued that the double reduction peaks indicate a kind of heterogeneous surface structure composed of Au and Pt “surface phases”, which may derive from a possible phase separation with the surface layer of atoms.38 However, two separate reduction peaks have also been documented in homogeneous Au-Pt alloys. Unfortunately, it still remains controversial, and thus, the actual surface state of the Au-Pt ligament needs to be further explored in future work. It is interesting to note that the CV profile of the np-Au80Pd10Pt10 is similar to that of np-Au80Pt20. Apart from the affirmatory signal related to the reduction of Au oxides, there exists another reduction signal with the peak potential at 0.45 V (vs SCE), which positively shifts 0.05 V as compared with the reduction peak of Pt oxides. By comparing Pt and Pd redox profiles in the literature, it is reasonable to assume that the reduction signals of these two elements are merged together to form an individual broad contour owing to their small reduction potential gap in the 0.5 M H2SO4 solution.39 By virtue of the different electrochemical responses, it confirms that the doping atoms (Pt and/or Pd) have effectively modified the surface state of np-Au. In the following section, the electrocatalytic activities were examined in the formic acid (0.5 M H2SO4 þ 0.5 M HCOOH) and methanol (1.0 M KOH þ 1.0 M CH3OH) solutions, respectively. In general, gold is considered as a chemically inert metal in the acid circumstance. As shown in Figure 8a, the pure np-Au shows negligible catalytic activity toward the formic acid oxidation (FAO). Upon doping with a certain amount of Pt, however, the FAO catalytic activity is remarkable on the as-obtained np-

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Au80Pt20 alloy. The onset potential of FAO is around -0.1 V (vs SCE), and a single oxidation peak is located at ∼0.3 V (vs SCE) in both forward and backward scans. Moreover, the oxidation peak current reaches 281 mA 3 cm-2 3 (mg metal)-1 (Table 1). It is well known that dual parallel reaction paths have been considered to exist in the FAO process: a dehydrogenation step (direct path) to form CO2 and a dehydration one (indirect path) to form poisoning CO species, which are associated with the oxidation peaks located at different potentials.40 Obviously, the dehydration branch to form poisoning CO intermediates (∼0.7 V vs SCE) is mostly suppressed on this np-Au80Pt20 alloy and the majority of formic acid is oxidized via a dehydrogenation step. It is generally recognized that the direct oxidation of formic acid on the Pt surface (dehydrogenation) does not require the presence of continuous neighboring Pt sites, whereas the dissociative adsorption of formic acid to form CO requires at least two ensemble binding sites, which is the so-called ensemble effect.22 On the basis of the present Au/Pt atomic ratio, the surface Pt atoms will be effectively isolated by the neighboring Au atoms on a Au-rich surface layer. As expected, the as-obtained np-Au80Pt20 can undoubtedly enhance the FAO activity through the direct dehydrogenation path. When half the amount of Pt in npAu80Pt20 is replaced by Pd to form a typical np-Au80Pt10Pd10, the FAO activity can be further improved. For the npAu80Pd10Pt10, the onset potential of the FAO reaction is located at -0.15 V (vs SCE), negatively shifting 50 mV in comparison with that of np-Au80Pt20. Moreover, the oxidation peak current reaches 372 mA 3 cm-2 3 (mg metal)-1, larger than that of npAu80Pt20 (281 mA 3 cm-2 3 (mg metal)-1). Generally, the activity enhancement could be explained by geometric (lattice mismatch, strain, defects, and/or dislocations, etc.) effects and/or electronic (d-band shift) effects.41,42 For the core-shell structure, it is generally recognized that the strain effect between the alloy core and the noble shell always plays a key role in enhancing the activity. For example, the dealloyed PtxCu1-x catalyst with a Pt skin and a Cu/Pt core reveals an enhanced activity.43-45 In our present case, the Pt and/or Pd atoms are embedded into the Au-rich surface layer to form a bior trimetallic alloy state. Therefore, the electronic (d-band shift) effect should be the essential causation for the catalytic activity enhancement. Norskov et al.46,47 have proposed a model describing the surface electronic structure and reactivity of noble metals and have stated that the center of the d band, εd, is an important parameter characterizing the ability of the surface d electrons to participate in bonding to the adsorbates and the energy barrier for dissociation. In addition, the d-band center of a given metal atom will depend on the surroundings. When a metal atom with a small lattice constant is put as an impurity at the surface of a metal with a large lattice constant, the local d bandwidth decreases and the center of the d band shifts up in order to preserve the degree of d-band filling. It is reasonable to assume that the Pt and/or Pd (small lattice) alloying with Au (larger lattice) induces an appropriate shift of the d-band center, resulting in an enhanced catalytic activity. Of course, the underlying reason needs to be further explored on the basis of theoretical and experimental research in the following work. We have also examined the electrocatalytic activities of these bi- and trimetallic nanoporous alloys toward the methanol oxidation reaction (MOR) in an alkaline circumstance. For pure np-Au, it is generally recognized that methanol oxidation follows different reaction mechanisms: at the low-potential region (0.4 V), the MOR occurs on the oxidized surface by a six-electron reaction.39,48 For the CV curve of pure np-Au (Figure 8b), two oxidation processes can be observed in the forward scan; however, the highest catalytic current merely reaches ∼120 mA 3 cm-2 3 (mg metal)-1. The sharp contrast can be discerned between pure np-Au and these as-doped counterparts. For np-Au80Pt20, the oxidation current starts to rise at about -0.5 V (vs MMO), reaches the highest level (694 mA 3 cm-2 3 (mg metal)-1) at 0 V, and then gradually drops off. For the trimetallic np-Au80Pt10Pd10, it exhibits an even higher activity of 870 mA 3 cm-2 3 (mg metal)-1. It is clear that the introduction of Pt and Pt/Pd dopants into np-Au can induce an enhanced MOR activity in comparison with the pure np-Au. On the AuPt bimetallic surface, Pt atoms should play a key role in the enhancement of activity, whereas Au atoms may provide oxygenated species and remove the intermediate CO-like species. It is worth noting that Pd is also an oxophilic element with excellent catalytic activity for alcohol oxidation in an alkaline circumstance. Consequently, the activity of this trimetallic npAu80Pt10Pd10 alloy can be further enhanced. To better understand what kind of roles Pt and Pd play in this trimetallic catalyst, the electrocatalytic activities of three bimetallic Pd-doped samples (np-Au95Pd5, np-Au90Pd10, and np-Au80Pd20) were also evaluated (Figure S5 in the Supporting Information). For the lowest Pd doping, the MOR activity on the np-Au95Pd5 can reach 798 mA 3 cm-2 3 (mg metal)-1, higher than the activity on the pure np-Au and np-Au80Pt20. With further increasing the Pd doping, the activity increases correspondingly. For the npAu80Pd20, the activity reaches 2263 mA 3 cm-2 3 (mg metal)-1. The enhanced activity should result from the increasing active Pd sites on the ligament surface as well as their ultrafine porosity. The distinct CV profiles of np-Au(PtPd) and np-Au(Pd), especially in the backward scan, may indicate the synergy between Pt and Pd in the surrounding Au circumstance. However, the promoting effect of Pd doping is significantly better than that of Pt doping toward methanol oxidation in the alkaline solution. The underlying reason is perplexing and may be correlated to the different inherent d-band center locations, the resultant activation barriers, and chemisorption energies of different surface alloy states.

’ CONCLUSIONS In summary, elemental doping has no influence on the phase constitution of rapidly solidified Al-Au-Pt, Al-Au-Pd, and Al-Au-Pt-Pd alloys, and all these precursor alloys are composed of a single Al2Au-type intermetallic compound (Al2(Au, Pt), Al2(Au,Pd), and Al2(Au,Pt,Pd)). Ultrafine nanoporous gold alloys with ligaments/channels of less than 10 nm can be facilely fabricated through dealloying these rapidly solidified Al2Aubased precursors under free corrosion conditions. When

dealloying in the 20 wt % NaOH solution is performed, a certain amount of Pt and/or Pd addition exhibits a superior refining effect and the length scale of ligaments/channels in the asobtained np-Au alloys can reach ∼3 nm for the Pt doping or Pt/Pd codoping. When dealloying in the 5 wt % HCl solution is performed, the anticoarsening capacity of Pt doping is more remarkable than that of Pd doping. In addition, the amount of doping can significantly affect the anticoarsening ability of ligaments/channels in the as-obtained np-Au alloys. Apart from the refining effect on ligaments/channels, the introduction of Pt and/or Pd into np-Au has generated novel bi/trimetallic nanoporous functionalized alloys. These as-doped np-Au alloys with an appropriate amount of Pt and/or Pd exhibit excellent electrocatalytic activities toward methanol and formic acid oxidation and will find promising applications in the catalysis-related areas. On the basis of our present findings, novel ultrafine nanoporous bi/trimetallic alloys can be fabricated and functionalized by alloy design of precursors and control over surface diffusion of morenoble atoms through elemental doping during dealloying.

’ ASSOCIATED CONTENT

bS

Supporting Information. Schematic illustration of the Al2(Au,Pt) crystal lattice, XRD patterns of the Pd- and Pt/Pddoped precursor alloys and corresponding as-dealloyed samples, SEM and TEM micrographs of np-Au80Pt20 and np-Au80Pd20 by dealloying in the 5 wt % HCl solution, and CV curves for catalytic activities of np-Au(Pd) alloys toward methanol oxidation in the alkaline solution. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully acknowledge financial support by the National Natural Science Foundation of China under Grant Nos. 50971079 and 50801031, Independent Innovation Foundation of Shandong University (2010JQ015), 2nd special support from China Postdoctoral Science Foundation (200902555), and 43rd China Postdoctoral Science Foundation. ’ REFERENCES (1) Zielasek, V.; Jurgens, B.; Schulz, C.; Biener, J.; Biener, M. M.; Hamza, A. V.; Baumer, M. Angew. Chem., Int. Ed. 2006, 45, 8241. (2) Yu, F.; Ahl, S.; Caminade, A. M.; Majoral, J. P.; Knoll, W.; Erlebacher, J. Anal. Chem. 2006, 78, 7346. (3) Huang, J. F.; Sun, I. W. Adv. Funct. Mater. 2005, 15, 989. (4) Cortie, M. B.; Maaroof, A. I.; Smith, G. B. Gold Bull. 2005, 38, 14. 4464

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