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Composition- and Aspect-Ratio-Dependent Electrocatalytic Performances of One-Dimensional Aligned Pt–Ni Nanostructures Ping Wu, Hua Zhang, Yingdan Qian, Yaojuan Hu, Hui Zhang, and Chenxin Cai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp4068087 • Publication Date (Web): 23 Aug 2013 Downloaded from http://pubs.acs.org on August 24, 2013

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The Journal of Physical Chemistry

Composition- and Aspect-Ratio-Dependent Electrocatalytic Performances of One-Dimensional Aligned Pt–Ni Nanostructures Ping Wu,a Hua Zhang,a Yingdan Qian,a Yaojuan Hu,b Hui Zhang,a Chenxin Caia* a

Jiangsu Key Laboratory of New Power Batteries, College of Chemistry and Materials Science,

Nanjing Normal University, Nanjing 210097, P. R. China. b

School of Biochemical and Environmental Engineering, Nanjing Xiaozhuang University, Nanjing

211171, P. R. China. * Corresponding author, E-mail: [email protected] (C. Cai) RECEIVED DATE:

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ABSTRACT: This work presents the synthesis of 1D aligned Pt–Ni nanostructures with ultrahigh aspect-ratio (>1000) based on the anodic aluminum oxide (AAO) template and their composition- and aspect-ratio-dependent catalytic performances to methanol oxidation reaction (MOR). The 1D aligned Pt–Ni nanostructures was electrochemically deposited in the pores of the AAO template, and the diameter and the length of the synthesized nanostructures are comparable with the diameter of pores and the thickness of the used AAO. The aspect-ratio of the 1D aligned nanostructures can be tuned merely by altering AAO templates with the appropriate aspect-ratios. Voltammetric results show that the catalytic activities (both mass activities and specific activities) of the 1D aligned Pt–Ni nanostructures for MOR are composition-dependent, and the highest electrocatalytic activity exhibits at a Pt/Ni molar ratio of 1:1 (PtNi). The mechanism of the promoting effect of Ni on Pt is explained based on modification of the electronic characteristics of the surface Pt atoms (Pt 4f) by Ni atoms due to the shift in electron transfer from Ni to Pt. Moreover, the catalytic activities of the 1D aligned PtNi nanostructures are in an aspect-ratio-dependent manner and increase in the order 0D PtNi nanoparticles, 1D aligned PtNi nanostructures at the aspect-ratio of ~200, 500, and 1300 due to the preferential exposure of certain crystal facets and less surface defects of the 1D aligned nanostructures. This work is believed to open the new and exciting possibilities for enhancing the performance of fuel cell catalysts. Our synthesized high aspect-ratio 1D aligned nanostructures may also be useful as sensors, and in other electrochemical applications. Keywords: Pt–Ni nanocatalysts; Methanol oxidation; Electrocatalysis; Fuel cell.


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1. INTRODUCTION Designing and synthesizing effective Pt and Pt-based electrocatalysts for the cathodic oxygen reduction reaction (ORR) and/or anodic alcohol oxidation reaction (AOR) has been a key scientific objective because there is an increased need for practical, cost-effective viable fuel cell devices and metal-air batteries for further energy applications.1-12 The recent search for advanced catalysts has led to some exciting advances in our understanding of the catalytic nature, allowing more rational tuning of the catalytic properties via controlled synthesis. Highly active catalysts have been prepared by rationally manipulating the chemical composition of the Pt-based catalysts via alloying Pt with other transition metals. For example, Pt-based binary and ternary metallic catalysts have been synthesized and studied for enhancing the catalytic activity, improving the stability, and reducing the use of Pt.1-3,9,10,13-20 The catalytic characteristics of the Pt-based catalysts can be further activated when the catalysts are prepared in a controlled size, shape, and morphology.3,11,12,21-28 Although tangible gains have been achieved in the continued development of the nanocatalysts, generating viable, permanent solutions to many inherent performance issues (for example, the specific activity of commercial Pt catalysts is considerably lower than that of bulk Pt, and significant overpotentials are often required to achieve suitable kinetics for practical use)3 continues to remain elusive. Most of these previous synthesized Pt and Pt-based catalysts, however, have been limited to nanoparticles (zero-dimension, 0D) that could be obtained by using a variety of chemical procedures.7,9,13,16,18 In contrast to those 0D nanoparticles, one-dimensional (1D) nanostructures, particularly those composed of noble metals, have recently become the focus of significant attention and effort as potentially effective materials in a broad range of applications because they exhibit additional advantages associated with their anisotropies, unique structures, or surface properties.3,4,11,29-34 1D nanostructure is expected to be an exciting and promising structural paradigm, which is inherently beneficial to efficient ORR and AOR performances and may solve many of the inherent technological shortfalls associated with commercial Pt catalyst (Pt nanoparticles).35,36 For example, the anisotropic structure of 1D noble metal nanostructures leads to the preferential surface exposure of smooth, defect3 ACS Paragon Plus Environment


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free, low-energy facets.37 These intrinsic structural and electronic properties are collectively expected to culminate in a weakened interaction with adsorbed oxygen species and to delay passivation of the nanostructure’s surface to higher potentials, thereby effectively reducing the overpotential necessary for high kinetics.35 Furthermore, the relatively low defect density associated with 1D structure also represents a promising route toward achieving increased durability because these structures are significantly less susceptible to both dissolution and ripening.38,39 Although 1D Pt nanostructures (nanowires and nanotubes) have been synthesized by templating against channels in porous materials, self-assembled structures of surfactant, and Ag or Se nanowires, or by growing directly on nanosphere of a carbon black for catalyzing ORR and AOR,31,40,41 there are several issues still remaining to be disclosed. For example, those synthesized Pt nanowires and nanotubes usually have a low aspect-ratio and cannot be used for evaluating the aspect-ratio-dependent catalytic performances. The presence of carbon degrades durability and leads to a fast and significant loss of electrochemical surface area (ECSA) during fuel cell operation.5,42 Thus, there has been considerable interest in the development of 1D nanostructure having a high aspect-ratio for fuel cell catalysts. To develop such catalysts, several characteristics have to be taken into consideration for their synthesis and assembly. For fuel cell catalysis, high surface area, multicomponent 1D Pt-based alloy is particularly promising for increasing the activity and utilization of precious metals. This work presents the synthesis of 1D aligned Pt–Ni nanostructures with ultrahigh aspect-ratio (>1000) based on the anodic aluminum oxide (AAO) template and their composition- and aspect-ratiodependent catalytic performances to methanol oxidation reaction (MOR). Since its discovery about two decades ago,43 AAO has been widely used as template for synthesizing various 1D nanostructures because it provides a rigid matrix with well-aligned pores, whose mean diameter and the space between the interpores can be easily controlled by adjusting the anodization conditions.44-51 However, the common used AAO is produced by two-step anodization procedures at a constant voltage (usually at 0– 5 °C and 30–230 V) and has a low aspect-ratio (less than 300),46,48 limiting its use for preparing the high aspect-ratio 1D nanostructures. Recently, we reported a two-step anodization approach under ACS Paragon Plus Environment

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galvanostatic conditions for the preparation of an AAO template with an ultrahigh aspect-ratio of more than 1000.52 To explore the application of the prepared ultrahigh aspect-ratio AAO, we here first synthesized 1D aligned Pt–Ni nanostructures with the aspect-ratio of more than 1000 using the prepared AAO as template. We then studied the composition-dependent electrocatalytic features of the synthesized 1D aligned Pt–Ni catalyst toward MOR. We finally compared the electrocatalytic performances of the catalyst with different aspect-ratios (0D Pt–Ni nanoparticles, 1D aligned Pt–Ni nanocatalysts with aspect-ratio of ~1300, 500, and 200, respectively), and highlighted the aspect-ratiodependent electrocatalytic activity of the 1D aligned nanostructures. The reason we select the Pt–Ni nanostructures as our studied model is that Pt–Ni alloys have recently emerged as an attractive and active bimetallic nanocatalyst for achieving both high activity and stability while reducing the amount of Pt used, and thus represent so far one of the most promising catalysts for ORR and MOR in fuel cells.9,53-55 This work is believed to open a new and exciting possibility for enhancing the performance of fuel cell catalysts, and our synthesized high aspect-ratio 1D aligned nanostructures may also be useful as sensors and in other electrochemical applications. 2. EXPERIMENTAL SECTION 2.1. Synthesizing 1D Aligned Pt–Ni Nanostructures. For synthesizing the 1D aligned Pt–Ni nanostructures, AAO membranes with different aspect-ratios (~1300, 500, and 200) were fabricated by two-step galvanostatic anodization process in 0.3 M oxalic acid solutions using our recently reported procedures52 (step a in Figure 1). Briefly, a highly purity of aluminum foil (99.999%, Goodfellow) was annealed at 450 °C for 2 h to eliminate the internal stress, then ultrasonicated in acetone, ethanol, and double-distilled water, respectively, for ~30 min. Prior to anodization, the aluminum foil was first electropolished for 20 min under a constant DC voltage of 7 V at 10–15 °C in a solution of ethanol/perchloric acid (5:1, v/v). Afterward, the aluminum foil was anodized in an oxalic acid solution (0.3 M) at 0 °C and constant current densities of 8 mA/cm2 for 6 h. After this was done, the alumina film formed was stripped in a chromic acid/phosphate acid solution (42 mL H3PO4 and 15.3 g Cr2O3 in ACS Paragon Plus Environment

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1 L solution) for 4 h at ~60 °C. The aluminum foil exhibited regular pits after removal of the alumina (Figure S1), the sample was anodized again under the same conditions (0 °C and 8 mA/cm2) for 18 h. This anodic process produced an AAO membrane having a thickness of ~200 µm and pore diameter of ~150 nm (the aspect-ratio is more than 1300). The pores as formed were uniform and almost in a regular hexagonal arrangement, the inner wall of the pores is smooth, vertical, and parallel to each other (Figure S2). The AAO membrane was peeled off aluminum substrate by a lift-off process that involved the selective etching of the unoxidized aluminum in saturated HgCl2 aqueous solution. The barrier layer at the bottom of AAO membrane was dissolved in a 5% (in mass ratio) of H3PO4 solution at 30 °C for 5– 10 min. Note that increasing the anodization time to more than 18 h results the decrease of the thickness of AAO membrane because AAO formation is dynamically balanced between the electrochemical formation process and the dissolution process in acidic media, long time anodization also results more formed AAO to be dissolved, leading to the decrease of the thickness of AAO membrane. Aluminum

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Figure 1. Illustration of the preparation of the AAO membrane with ultrahigh aspect-ratio and the synthesis of the 1D aligned Pt–Ni nanostructures using the prepared AAO membrane as a template.

The aspect-ratio of the AAO membrane can be tuned by altering the anodization time in the second anodization process. For example, tuning the anodization time to 10 and 4 h (keeping the temperature at ACS Paragon Plus Environment

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0 °C and current density at 8 mA/cm2), respectively, we have prepared the AAO membrane with a thickness of ~70 and 35 µm, respectively. However, the pore diameters are ~150 nm, almost independent on the anodization time (Figure S3). Therefore, the AAO membrane has an aspect-ratio of ~500 and 200, respectively. To facilitate electrodeposition, one side of the AAO membrane was coated with a layer of platinum film with thickness of ~10 nm by high-vacuum evaporation to serve as a working electrode (step b in Figure 1). The electrical contact was made to the Pt-coated AAO membrane using copper wire via carbon conductive epoxy (Kunmin Institute of Noble Metals, China). Then both the coated Pt film and the copper wire were coated with a layer of insulating epoxy to avoid contact of solution with them. After that, the working electrode was subjected to electrodepositing the Pt–Ni nanostructures in solution of 10 mM PtCl62– and 5 mM Ni2+ at a constant potential of –0.55 V (vs. saturated calomel electrode, SCE) (step c in Figure 1). The electrodeposition occurred in the pore of the AAO membrane results the formation of the 1D aligned Pt–Ni nanostructures (PtNi0.5). Note that extra care had to be taken, and the electrodepositing current had to be monitored in order to obtain the 1D aligned Pt–Ni nanostructures without overgrowing the Pt–Ni film on the surface of the AAO membrane. Usually, we terminated the electrodeposition process once we observed a sudden change of the current in current-time profile because this sudden change is an indication of growing the 1D Pt–Ni nanostructures out of the pores of the AAO membrane and starting to form the Pt–Ni film on the surface of AAO membrane. SEM image (top-view) shows that almost every pore is filled with 1D Pt–Ni nanostructures and the top surface of the nanostructures is flush with that of AAO membrane (Figure S4). Using the similar procedures, we have synthesized the 1D aligned Pt–Ni nanostructures with the Pt/Ni ratios of 1:1 (PtNi) and 1:2 (PtNi2), respectively, by controlling the amount of the metal precursors in solution. For comparison, we also synthesized the 1D aligned Pt nanostructures. By changing the AAO membrane with the different aspect-ratios, we have prepared the 1D aligned Pt–Ni nanostructures with the aspect-ratio of ~1300, 500, and 200, respectively.


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2.2. Electrochemical Measurements. The electrochemical measurements were performed with an Autolab PGSTAT302N electrochemical station (Metrohm) at ambient temperature (22 ± 2 °C). A conventional three-electrode system was used with the 1D aligned Pt–Ni nanostructures as a working electrode, a Pt sheet (~1 × 1 cm) and a saturated calomel electrode (SCE) as the counter electrode and the reference electrode, respectively. After etching away the AAO membrane using 6 M NaOH (step d in Figure 1), MOR in 0.5 M H2SO4 solution was studied as model reaction system to characterize the electrocatalytic characteristics of the 1D aligned Pt–Ni nanostructure electrode with various molar ratios of Pt/Ni and different aspect-ratios (step e in Figure 1). The electrolyte solution was purged with highpurity nitrogen for at least 30 min prior to each electrochemical measurement, and the nitrogen environment was then kept over the solution to prevent the solution from oxygen. 2.3. Instruments. Scanning electron microscopic (SEM) images were recorded using a JSM–7600F field-emitting scanning electron microscope. The transmission electron microscopic (TEM) and highresolution TEM (HRTEM) images, energy-dispersive spectroscopy (EDS), and element analysis mapping were carried out on a JEM–2100F transmission electron microscope. The compositions of the prepared Pt–Ni nanostructures were evaluated by inductively coupled plasma-atomic emission spectroscopy (ICP–AES, Prodigy, Teledyne Leeman Labs) measurements and EDS analysis, respectively. The EDS was recorded with an Oxford Link ISIS energy-dispersive spectrometer fixed on the microscope. The crystalline structures of the prepared nanostructures were analyzed by X–ray diffraction (XRD, Rigaku/Max–3A X–ray diffractometer) with Cu Kα radiation (λ = 0.15418 nm). X-ray photoelectron spectroscopy (XPS) was recorded on an ESCALAB 250 XPS spectrometer (VG Scientifics) using the monochromatic Al Kα line at 1486.6 eV. The binding energies were calibrated with respect to the C (1s) peak at 284.6 eV. The peak was analyzed based on curve–fitting with a mixed Gaussian-Lorentzian line shape using the XPS PEAK program (version 4.0). 3. RESULTS AND DISCUSSION


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3.1. Synthesizing the 1D Aligned Pt–Ni Nanostructures. When the Pt film-coated AAO membrane was biased with a constant potential of –0.55 V (vs. SCE) in a solution containing PtCl62– (10 mM) and Ni2+ ions (5 mM), the precursors could be reduced and deposited in the pores of the AAO membrane, producing the 1D aligned Pt–Ni nanostructures. After etching away AAO membrane, we can observe that the Pt–Ni nanostructures are well-aligned and uniform in diameter (Figure 2a, b, and c). The crosssectional SEM images show that the Pt–Ni nanostructure has a diameter of ~150 nm (Figure 2b) and a length of ~200 µm (Figure 2c), and thus the aspect-ratio is more than 1300. The diameter of the nanostructure agrees well with that viewed from TEM image of the single Pt–Ni nanostructure (Figure 2e). TEM image also indicates that Pt–Ni nanostructure is remarkably uniform. The diameter and the length of the Pt–Ni nanostructures are comparable with the diameter of pores and the thickness of AAO membrane used as template (Figure S2). Note that the highly aligned Pt–Ni nanostructures can be produced on a large scale as viewed from the SEM image at low magnification (Figure 2d). These results imply that our AAO membrane can be used as template for synthesizing the 1D aligned nanostructures with ultrahigh aspect-ratio. Due to its low redox potential as compared to Pt, Ni-loss may occur during the reduction of Ni2+ and PtCl62– ions. Therefore, the compositions of the prepared Pt–Ni nanostructures were estimated by EDS analysis and ICP–AES measurement. The typical EDS results reveal the presence of Pt and Ni, and no other metallic element is detectable (Figure 2g. Noted that EDS has been recorded in different areas, the EDS depicted here represents only one of the typical results), implying that the electrodeposited nanostructures are Pt–Ni. The atomic ratio between Pt and Ni obtained by EDS and ICP–AES measurements is ~1:0.49, which is very close to the nominal values (1:0.5), suggesting that the molar ratio of Pt/Ni in the nanostructures is almost the same as that of precursors in the deposition solution. HRTEM image recorded for the PtNi0.5 shows the dominant formation of face-centered cubic (fcc) (111) lattice image (Figure 2f). The lattice spacing is observed to be 0.223 nm, which is in good agreement with that obtained from d–spacing calculation from an XRD pattern (discussed below). However, this value is slightly lower than that of Pt (111) plane (0.227 nm)6,17 because of a contraction ACS Paragon Plus Environment

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of the lattice upon substitution of Pt with Ni atom56 (Ni atoms are 11% smaller relative to Pt atoms). The selected-area electron diffraction (SAED) pattern (inset in Figure 2e) of the synthesized nanostructure shows concentric rings, composed of bright discrete diffraction spots, indicating that the Pt–Ni nanostructures are polycrystalline with high degree of crystallinity.

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Figure 2. (a–d) SEM image of the 1D aligned PtNi0.5 nanostructures. (a) and (d) are the top-view images, (b) and (c) are the cross-sectional images. (e) and (f) are TEM (e) and HRTEM images (f), respectively, of the nanostructures. (g) and (h) are EDS analysis (g) and XRD patterns (h), respectively, of the synthesized PtNi0.5 nanostructures. The images were recorded after etching away AAO membrane. The inset in (e) shows the typical SAED patterns of the PtNi0.5 nanostructures.

XRD patterns of the prepared Pt–Ni nanostructures demonstrate the characteristic peaks of the Pt fcc structure (such as 40.5, 46.9, 68.4, and 82.4° correspond to the plane of (111), (200), (220), and (311), respectively), no characteristic peaks of Ni are detected (Figure 2h), indicative of the formation of alloy of Pt and Ni. These characteristic peaks shift slightly to higher angles as compared to those of Pt ACS Paragon Plus Environment

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(JCPDS 04-0802), thereby indicating a contraction of the lattice upon substitution of Pt with Ni. This result is in good agreement with that obtained from HRTEM analysis. The values of d–spacing, and lattice parameter (a) of Pt in the PtNi0.5 were calculated to be 0.223 and 0.3869 nm (the value of d was calculated using Bragg’s equation: 2dsinθ = nλ, and the value of a was calculated using the equation dhkl = a / (h2 + k2 + l2)1/2 of fcc system), respectively, showing a decrease in these parameters in the Pt–Ni nanostructures in comparison with those in Pt (~0.226 and 0.3921 nm, respectively). These results suggest the replacement of Pt atoms by Ni atoms, further prove the formation of the Pt–Ni alloys. Additional evidences of the formation of the Pt–Ni alloys come from the element analysis mapping, which reveals the uniform distribution of Pt and Ni throughout the entire PtNi0.5 nanostructures without agglomeration (Figure S5). Both XRD and HRTEM results indicate that the geometric environment of the Pt atoms in 1D Pt–Ni nanostructures is different from that of pure Pt because of formation of alloy with Ni. The change in geometric or atomic configuration may affect the electronic structures of Pt,57 leading to an enhanced electrocatalytic activity for MOR (see below). By altering the amount of the metal precursors, we have synthesized 1D aligned Pt–Ni nanostructures with Pt/Ni ratio of 1:1 (PtNi) and 1:2 (PtNi2). EDS analysis and ICP–AES measurement confirmed that the atomic ratios of Pt/Ni in the nanostructures were almost same as those of PtCl62–/Ni2+ in solution. The values of d–spacing, and lattice parameter (a) of Pt were ~0.222 and 0.3852 nm, respectively, in PtNi, and ~0.221 and 0.3848 nm, respectively, in PtNi2 nanostructures, showing the formation of Pt–Ni alloys. These results further indicate that the compositions of the prepared 1D aligned Pt–Ni nanostructures can be tuned by adjusting the concentrations of the PtCl62– and Ni2+ ions. By analogy, we have synthesized 1D aligned Pt–Ni nanostructures (using PtNi as an example) having a length of ~70 and 35 µm (Figure 3c, g), respectively, using appropriate thickness of AAO membranes as templates. The synthesized PtNi nanostructures are well-aligned and uniform (Figure 3a, b, e, and f). The diameters of PtNi is ~150 nm (Figure 3b, f), and thus the aspect-ratio is ~500 and 200, respectively. The diameter and the length of the PtNi nanostructures agree well with the diameter of pores and the thickness of AAO membrane we used (Figure S3). Similar to the Pt–Ni nanostructures with a aspectACS Paragon Plus Environment

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ratio over 1000, the PtNi with aspect-ratios of ~500 and 200 can also be produced on a large scale (Figure 3d, h), implying the aspect-ratio (diameter and length) control of the 1D aligned Pt–Ni nanostructures is achievable merely by using templates of the appropriate aspect-ratio. a

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Figure 3. SEM images of the 1D aligned PtNi nanostructures having a length of ~70 (a, b, and c) and 35 µm (d, e, and f). (a), (b), (d), and (e) are the cross-sectional images, (c) and (f) are the top-view images. The PtNi nanostructures were synthesized using AAO as template by electrodeposition approach in the solution of 10 mM PtCl62– and 10 mM Ni2+ ions under a constant potential of –0.55 V. The images were recorded after etching away AAO membrane.

3.2. Composition-Dependent Electrocatalytic Characteristics of the 1D aligned Pt–Ni Nanostructures. Having synthesized and characterized the 1D aligned Pt–Ni nanostructures with ultrahigh aspect-ratio, we study the composition-dependent electrocatalytic activity of the nanostructures (using the aspect-ratio of ~1300 as an example) for MOR in an acidic solution (0.5 M H2SO4) as a model reaction using voltammetry (CV). CV of the 1D aligned Pt–Ni nanostructures in 0.5 M H2SO4 solution showed typical Pt characteristics (Figure S6) and served to evaluate ECSAs based on the hydrogen adsorption/desorption charges, assuming 210 µC/cmPt2.6,56 ECSAs were estimated to be ~70, 100, and 80 m2/gPt for the PtNi0.5, PtNi, and PtNi2, respectively, which are ~1.2, 1.8, and 1.3 times, respectively, that of 1D aligned Pt nanostructures (~60 m2/gPt). Note that the deposited platinum film on ACS Paragon Plus Environment

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the back side of AAO membrane should not contribute to the ECSAs of the Pt–Ni nanostructures because it has been coated with a layer of insulating epoxy to prevent solution contacting with it. Such a high ECSA value makes the 1D aligned Pt–Ni nanostructures not only have large number of electrochemically active sites regarding per gram of the catalyst but also be easily accessible for transferring the electrons to and from the electrocatalyst surface. These features may significantly

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Figure 4. Cyclic (A and B), and linear-sweep voltammetric (C), and amperometric responses (D) for the oxidation of 0.5 M methanol in H2SO4 solution (0.5 M) catalyzed by 1D aligned Pt nanostructures (a), and Pt–Ni nanostructures at the composition of PtNi0.5 (b), PtNi2 (c), and PtNi (d). The currents in (A and C) were normalized with the mass of Pt loading on electrode surface; the current in (B) was normalized with ECSAs. The cyclic and linear-sweep voltammograms were recorded at a 50 mV/s. The current-time profiles in (D) were recorded under a constant potential of 0.64 V (vs SCE). The current densities (representing mass activities) have been normalized with the highest current densities (in mA/cmPt2) obtained at the 1D aligned Pt (a), PtNi0.5 (b), PtNi2 (c), and PtNi (d) nanostructures, respectively.

Figure 4 compares the electrocatalytic performances of the 1D aligned Pt, PtNi0.5, PtNi, and PtNi2 nanostructures recorded in a H2SO4 solution (0.5 M) with 0.5 M methanol, in which both mass activities


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(currents were normalized by the loaded Pt mass, Figure 4A) and specific activities (currents were normalized by ECSAs, Figure 4B) were compared. The peak current density (at ~0.64 V, in the positive direction sweep) of the 1D aligned PtNi0.5, PtNi, and PtNi2 is ~263 (curve b in Figure 4A), 657 (curve d in Figure 4A), and 368 mA/mgPt (curve c in Figure 4A), respectively, which is ~2.2, 5.4, and 3.0 times that of the 1D aligned Pt nanostructures (~122 mA/mgPt, curve a in Figure 4A). After normalized by ECSAs, the catalytic peak current density of the aligned Pt–Ni nanostructures still shows a value that is ~1.9 (0.52 mA/cmPt2 for PtNi0.5, curve b in Figure 4B), 3.3 (0.89 mA/cmPt2 for PtNi, curve d in Figure 4B), and 2.3 (0.63 mA/cmPt2 for PtNi2, curve c in Figure 4B) times that of the 1D aligned Pt nanostructures (0.27 mA/cmPt2, curve a in Figure 4B). The ratio of the forward anodic peak current (if) to the backward anodic peak current (ib) is an important index and can be used for evaluating the poison tolerance of Pt-based catalysts in MORs.58 A higher if/ib value indicates more effective removal of the poisoning species on the catalyst surface, which means that methanol can be oxidized much more efficiently. The if/ib of the 1D aligned PtNi0.5, PtNi, and PtNi2 is ~1.6, 1.4, and 1.4, respectively. All these values are much higher than that of 1D aligned Pt nanostructures (~0.93), suggesting that the 1D aligned Pt–Ni nanostructure surface has less carbonaceous species accumulation and therefore has a good poison tolerance. The onset potential of MOR catalyzed by the 1D aligned PtNi0.5, PtNi, and PtNi2 is ~275 (curve b in Figure 4C), 200 (curve d in Figure 4C), and 240 mV (curve c in Figure 4C), respectively, which has a ~125, 200, and 160 mV, respectively, negative-shift compared with that 1D aligned Pt nanostructures (~400 mV, curve a in Figure 4C), implying that the MOR is easier to undergo on the surface of the Pt– Ni nanostructure electrocatalysts. Furthermore, at any given oxidation current, the oxidation potential on the Pt–Ni nanostructures decreases in the order PtNi2, PtNi0.5, and PtNi (curves b–d in Figure 4C). They are certainly lower than that on Pt nanostructures (curve a in Figure 4C), indicating higher catalytic performance of the 1D aligned Pt–Ni nanostructures for MORs. The composition-dependent stability of 1D aligned Pt–Ni nanostructures was evaluated by chronoamperometric (CA) measurements under a constant potential of 0.64 V and compared with that ACS Paragon Plus Environment

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of 1D aligned Pt nanostructures. The polarization currents decrease rapidly at the initial stage (Figure 4D). However, the anodic currents of the Pt–Ni nanostructures (curves b, c, and d) are higher than that of Pt nanostructures (curve a) during the entire time range, indicative of high electrocatalytic stability of the Pt–Ni nanostructures in comparison with the Pt nanostructures. The oxidation currents at a time of 2000 s remain ~3, 10, 18, and 35% of their respective initial value for the 1D aligned Pt (curve a), PtNi0.5 (curve b), PtNi2 (curve c), PtNi nanostructures (curve d), respectively, showing a compositiondependent stability. These results demonstrate that the electrocatalytic activity of the 1D aligned Pt–Ni nanostructures is composition-dependent. Increasing Ni content from 0.5 to 1 in the nanostructure can enhance its electrocatalytic activity, which reaches its highest value at a Pt/Ni molar ratio of 1:1. Further increasing the Ni content, for example 1:2 significantly decreases the electrocatalytic activity. (A)

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The Ni content-dependent catalytic characteristics of the Pt-Ni nanostructures for MOR can be understood based on the modification of the electronic properties of Pt by Ni in the nanostructures. XPS measurements show that Pt has a various states in the prepared Pt–Ni nanostructures as analyzed by curve-fitting with a mixed Gaussian-Lorentzian line shape (Figure 5A). The 4f core-level spectrum consists of two peaks for metallic platinum at ~70.7 (Pt 4f7/2) and 74.1 eV (Pt 4f5/2), with two more peaks at ~71.4 and 75.1 eV, which could be assigned to Pt(II) species as in PtO and Pt(OH)2,59 respectively. A comparison of the relative intensities of those components (metallic Pt, PtO, and ACS Paragon Plus Environment

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Pt(OH)2) shows that the Pt state in the Pt–Ni nanostructures is predominately metallic Pt, which can provide more suitable sites for methanol decomposition than Pt(II). An analysis of the Pt 4f peaks with respect to the compositions of Pt–Ni nanostructures shows that the Pt 4f peaks shift to a lower binding energy (Figure 5B), suggesting that the electronic structure of the Pt is modified by alloying with the Ni component. The position of Pt 4f peak in PtNi0.5, PtNi, and PtNi2 has ~0.2 (curve b in Figure 5B), 0.7 (curve c in Figure 5B), and 0.5 eV (curve d in Figure 5B), respectively, negative shifts in comparison with that in Pt nanostructures (curve a in Figure 5B), implying that the largest modification occurs at the Pt/Ni molar ratio of 1:1. The change in the binding energy of metal core level reflects the shift of its d-band center relative Fermi level.60 The negative shift of binding energy represents the downshift of d-band center because shifting the d-band center of surface Pt is known to be accompanied by a change in the surface core-level shift in the same direction.61,62 The downshift of d-band center in Pt–Ni nanostructures, in comparison with that in Pt, is in good agreement with that predicted by Hammer-Nørskov reactivity model,60,61,63-66 which concludes that when a metal with larger lattice constants (Pt in this case, its lattice parameter is 0.3921 nm) ) is alloyed with metal with smaller lattice constants (Ni, its lattice parameter is 0.352 nm), the d-band center will shift down, resulting from a combination of d-band bandwidth changes upon alloying, followed by d-band shifts to maintain a constant local d-band filling (note that the lattice parameters of PtNi0.5, PtNi, and PtNi2 are 0.3869, 0.3852, and 0.3848 nm, respectively. Please refer to the Section 3.1 for details). According to Hammer-Nørskov model,60,61,63,64,66 there is close correlation between the energy center of the valence d-band center of a metal surface and its ability to form chemisorption bonds. As the d-band center shifts up, a distinctive antibonding state appears above the Fermi level. The antibonding states above the Fermi level are empty, and the bond becomes increasingly stronger as their numbers increase. Thus, strong bonding occurs if the antibonding states are shifted up though the Fermi level (and become empty), and weak bonding occurs if antibonding states are shifted down through the Fermi level (and become filled).67 The downshifted d-band center in the Pt–Ni nanostructures lowers the density of state on the Fermi level and reduces the Pt–COads bond energy,68,69 and weakens the chemical adsorption of ACS Paragon Plus Environment

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oxygen-containing species such as COads on the surface of Pt–Ni nanostructures.59 Therefore, the electrocatalytic activity of the Pt–Ni nanostructures is improved in comparison with that of Pt itself. Moreover, the Pt–Ni nanostructures exhibit the lowest binding energy at the composition of PtNi, implying the lowest Pt–CO bond energy is reached at this composition. Thus, the electrocatalytic activity of PtNi is the highest among the prepared Pt–Ni nanostructures (PtNi0.5, PtNi, and PtNi2). 3.3. Aspect-Ratio-Dependent Electrocatalytic Characteristics of the Pt–Ni Nanostructures. We next studied the aspect-ratio-dependent electrocatalytic performances of the 1D aligned Pt–Ni nanostructures to MOR. Only results for PtNi composition (the Pt/Ni molar ratio of 1:1) at aspect-ratio of ~1300, 500, and 200 are presented, because this composition of the Pt–Ni nanostructures has been proved to be optimum in the prepared 1D aligned Pt–Ni nanostructures (section 3.2). To better compare the electrocatalytic performance of the Pt–Ni nanostructures at different aspect-ratios, we also synthesized the PtNi nanoparticles (the Pt/Ni molar ratio is 1:1) with a size of ~30–50 nm and recorded their electrocatalytic activity to MOR (the detailed procedures of the synthesis of the PtNi nanoparticles are presented in Supporting Information and their TEM images are depicted in Figure S7). We compare the ECSAs (Figure 6A), mass activities (in mA/mgPt, Figure 6B and Figure S8A), specific activities (in mA/cmPt2, Figure 6C and Figure S8B), the stability (Figure 6D), the value of the if/ib, and the onset potential (Figure S8C) for MOR at the 1D aligned PtNi nanostructures and PtNi nanoparticles. The ECSA value of the 1D aligned PtNi nanostructures at aspect-ratios of ~200, 500, and 1300 is ~93, 95, and 100 m2/gPt (Figure 6A), respectively, almost independent on the aspect-ratios. These values are ~20% lower than that of PtNi nanoparticles, probably due to the much smaller size of PtNi nanoparticles (~30–50 nm) in comparison with the diameter of the 1D PtNi nanostructures (~150 nm). From a practical viewpoint, the figure-of-merit of an electrocatalyst is its mass activity and specific activity.40 The latter, even for a unique element, such as Pt, may vary due to crystallographic orientation of the surface, crystallite size, shape, and morphology.16,18,27,40,70 Quite surprisingly, both mass activity and specific activity of the 1D aligned PtNi nanostructures are much higher than those of the PtNi nanoparticles (Figure 6B, C) in spite of a ~20% lower Pt area for the 1D aligned PtNi nanostructures ACS Paragon Plus Environment

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(Figure 6A). For example, the mass activities of the 1D aligned PtNi nanostructures at the aspect-ratio of ~1300, 500, and 200 are ~657, 394, and 136 mA/mgPt (Figure 6B, the cyclic voltammetric responses for the oxidation of 0.5 M methanol in 0.5 M H2SO4 solution catalyzed by PtNi nanoparticles and 1D aligned PtNi nanostructures at the aspect-ratio of ~1300, 500, and 200, respectively, are depicted in Figure S8), respectively, which are ~8.2-, 4.9-, and 1.7-fold, respectively, better than that of the PtNi nanoparticles (~80 mA/mgPt). Moreover, the activities of the 1D aligned PtNi nanostructures are significantly dependent on their aspect-ratios (Figure 6B, C).

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Figure 6. Comparison of the ECSA (A), mass activities (B), specific activities (C), and stability (D) for the 1D aligned PtNi nanostructures at the aspect-ratio of ~1300 (a), 500 (b), and 200 (c), and PtNi nanoparticles (d). The current-time profiles in panel D were recorded for oxidation 0.5 M methanol in a 0.5 M H2SO4 solution under a constant potential of 0.64 V (vs SCE). The presented current densities (mass activities) have been normalized with the highest current (in mA/cmPt2) obtained at the 1D aligned PtNi nanostructures at the aspect-ratio of ~1300 (a), 500 (b), and 200 (c), and PtNi nanoparticles (d), respectively. Note that the cyclic voltammetric responses for the oxidation of 0.5 M methanol in 0.5 M H2SO4 solution catalyzed by PtNi nanoparticles and 1D aligned PtNi nanostructures at the different aspect-ratios are depicted in Figure S8. For better comparison, the values mass and specific activities of the 1D aligned PtNi nanostructures at aspect-ratio of ~1300 in this Figure are same as those depicted in Figure 4 (curve d).


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The values if/ib of the 1D aligned PtNi nanostructures at the aspect-ratio of ~1300, 500, and 200 are ~1.4, 1.35, and 1.4, respectively, which is higher than that of the PtNi nanoparticles (~1.2), implying that MOR undergoes more efficiently on the surface of 1D aligned PtNi nanostructures than on PtNi nanoparticles due to effective removal of the poisoning species on the 1D aligned PtNi nanostructures. The onset potentials of MOR at the 1D aligned PtNi are ~200, 230, and 380 mV at the aspect-ratio of ~1300, 500, and 200 (Figure S8C), respectively, which has a ~210, 180, and 30 mV, respectively, negative-shift compared with that PtNi nanoparticles (~410 mV, Figure S8C), implying that the MOR is easier to undergo on the surface of the 1D aligned PtNi nanostructure electrocatalysts. The aspect-ratio-dependent stability of the 1D aligned PtNi nanostructures for MOR was evaluated by chronoamperometric measurement under a constant potential of 0.64 V and compared with that of the PtNi nanoparticles. The polarization currents at these nanostructures decrease rapidly at the initial stage (Figure 6D. Note that the presented currents have been normalized with the highest current (in mA/cmPt2) obtained at the 1D aligned PtNi nanostructures at the aspect-ratio of ~1300 (curve a), 500 (curve b), and 200 (curve c), and PtNi nanoparticles (curve d), respectively), which is due to the formation of intermediate species during methanol oxidation.71 However, the anodic current densities of the 1D aligned PtNi nanostructures (curves b, c, and d in Figure 6D) are higher than that of PtNi nanoparticles (curve a in Figure 6D), indicative of high electrocatalytic stability of the 1D aligned PtNi nanostructures in comparison with the PtNi nanoparticles. The oxidation current densities at a time of 2000 s remain ~6, 17, 20, and 35% of their respective initial value for the PtNi nanoparticles, and the 1D aligned PtNi nanostructures at the aspect-ratio of ~200, 500, and 1300 (Figure 6D), respectively, showing a aspectratio-dependent stability. These results again confirm the better tolerance to the intermediate species and superior electrocatalytic performance of the prepared 1D aligned PtNi nanostructures in MORs. The higher activity of the 1D aligned PtNi nanostructures as compared to PtNi nanoparticles might be due to the their inherent anisotropic morphology and unique structure,11 which make 1D aligned PtNi nanostructures preferentially expos certain crystal facets of the former3,40 and/or have less surface defects bearing, therefore, a closer resemblance to the surface of large single crystals that exhibits even ACS Paragon Plus Environment

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higher specific activities.57 The synthesis of such 1D aligned PtNi nanostructures is thus an important step toward the fabrication of Pt-based alloy nanostructures that would retain the high activity. The larger aspect-ratio of the present PtNi nanostructures compared to that of PtNi nanoparticles is believed to open new and exciting possibilities for improving the performance of fuel cell at high current densities. Since 1D aligned PtNi nanostructures are stretching out from the surface, electroactive species (such as methanol, O2 etc) diffusion toward the catalyst surface might be improved. From a practical viewpoint, this orientation of the PtNi nanostructures with high aspect-ratio also allows larger quantities of the proton conducting ionomer to be applied: the optimum ionomer thickness is usually set by the height of electrocatalyst above the surface. More ionomer than this would cover the electrocatalyst surface and therefore decrease the ability of species to diffuse toward the catalyst surface.72 Yet, large quantities of proton conducting ionomer are desirable to improve the overall transport of protons through the electrodes.72 In fact, it has been predicted by a comprehensive model that ionomer contents of 50–70 wt% should be optimum, provided that the electrode porosity be not reduced by such high ionomer contents.73 Therefore, the high aspect-ratio of the 1D aligned PtNi nanostructures can be expected to significantly increase the optimum quantity of ionomer in the fuel cell cathode and anode., and thus improve the performances of the fuel cell. 4. CONCLUSIONS In conclusion, we have demonstrated the synthesis of the 1D aligned Pt–Ni nanostructures with ultrahigh aspect-ratios using AAO membrane as template. The aspect-ratio (diameter and length) of the 1D aligned nanostructures can be tuned merely by altering AAO templates of the appropriate aspectratio. The catalytic activities of the 1D aligned Pt–Ni nanostructures for MOR exhibit a compositionand aspect-ratio-dependent manner, and much higher that those of the 1D Pt nanostructures and 0D PtNi nanoparticles. The 1D aligned PtNi (Pt/Ni molar ratio of 1:1) shows the highest electrocatalytic activity, and it increases in the order 0D PtNi nanoparticles, 1D aligned PtNi nanostructures at the aspect-ratio of ~200, 500, and 1300. The use of the 1D aligned PtNi nanostructures as catalyst is believed to open the ACS Paragon Plus Environment

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new and exciting possibilities for enhancing the performance of fuel cell catalysts. Moreover, this aligned nanostructure may also be useful as sensors and in other electrochemical applications. ACKNOWLEGMENTS This work is supported by the NSFC (21175067, and 21273117), the Research Fund for the Doctoral Program of Higher Education of China (20103207110004), the NSF of Jiangsu Province (BK2011779), the Program for Outstanding Innovation Research Team of Universities in Jiangsu Province, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and Jiangsu Collaborative Innovation Center of Biomedical Functional Materials. Supporting Information Available The detailed procedures of synthesizing the 0D PtNi nanoparticles, and eight figures. This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Zhu, H.; Zhang, S.; Guo, S.; Su, D.; Sun, S. Synthetic Control of FePtM Nanorods (M = Cu, Ni) To Enhance the Oxygen Reduction Reaction. J. Am. Chem. Soc. 2013, 135, 7130-7133. (2) Liu, L.; Pippel, E. Low-Platinum-Content Quaternary PtCuCoNi Nanotubes with Markedly Enhanced Oxygen Reduction Activity. Angew. Chem. Int. Ed. 2011, 50, 2729-2733. (3) Koenigsmann, C.; Scofield, M. E.; Liu, H.; Wong, S. S. Designing Enhanced One-Dimensional Electrocatalysts for the Oxygen Reduction Reaction: Probing Size- and Composition-Dependent Electrocatalytic Behavior in Noble Metal Nanowires. J. Phys. Chem. Lett. 2012, 3, 3385-3398. (4) Guo, S.; Dong, S.; Wang, E. Ultralong Pt-on-Pd Bimetallic Nanowires with Nanoporous Surface: Nanodendritic Structure for Enhanced Electrocatalytic Activity. Chem. Commun. 2010, 46, 1869-1871. (5) Carmo, M.; Sekol, R. C.; Ding, S.; Kumar, G.; Schroers, J.; Taylor, A. D. Bulk Metallic Glass Nanowire Architecture for Electrochemical Applications. ACS Nano 2011, 5, 2979-2983. (6) Xia, B. Y.; Wu, H.; Yan, Y.; Lou, X. W.; Wang, X. Ultrathin and Ultralong Single-Crystal ACS Paragon Plus Environment

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1D aligned PtNi nanostructures

Composition- and aspect-ratio-dependent catalytic performances


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