Large-Scale and Template-Free Growth of Free-Standing Single

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DOI: 10.1021/cg900217t

Large-Scale and Template-Free Growth of Free-Standing SingleCrystalline Dendritic Ag/Pd Alloy Nanostructure Arrays

2009, Vol. 9 4351–4355

Dawei Wang,†,‡ Tao Li,†,‡ Yang Liu,†,‡ Jianshe Huang,†,‡ and Tianyan You*,† †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, People’s Republic of China, and ‡Graduate School of the Chinese Academy of Sciences, Beijing 100039, People’s Republic of China Received February 20, 2009; Revised Manuscript Received July 26, 2009

ABSTRACT: Large-scale arrays consist of dendritic single-crystalline Ag/Pd alloy nanostructures are synthesized for the first time. A simple galvanic replacement reaction is introduced to grow these arrays directly on Ag substrates. The morphology of the products strongly depended on the reaction temperature and the concentration of H2PdCl4 solution. The mechanism of the formation of alloy and the dendritic morphology has been discussed. These alloy arrays exhibit high surface-enhanced Raman scattering (SERS) activity and may have potential applications in investigation of “in situ” Pd catalytic reactions using SERS. Moreover, electrocatalytic measurements suggest that the obtained dendritic Ag/Pd alloy nanostructures exhibit electrocatalytic activity toward the oxidation of formic acid.

Introduction Anisotropic nanostructures have been attracting growing interest because of their unusual chemical and physical properties.1 Because dendrites are hierarchical structures with enhanced surface areas, dendritic metal nanomaterials have potential applications in surface-enhanced Raman scattering (SERS), catalysis, biosensing, and fabrication of superhydrophobic surfaces.2-5 Recently, the synthesis of Au,2 Ag,3 Pd,4 and Cu5 dendritic nanostructures has been reported. Concerning novel technologies based on nanoscale machines and devices, scientists not only need to prepare anisotropic nanomaterials but also try to organize them into well-defined arrays on various substrates.6 Arrays consisting of nanowires, nanotubes and nanobelts have been developed;7 however, it is still a great challenge to fabricate well-aligned arrays with dendritic nanostrucutres.8 Thanks to their composition-dependent properties, alloy nanomaterials have been used in many important technological areas, for example, optical, catalytic, electronic, magnetic, and even medical applications.9 Up to now, most studies on alloy nanomaterials have been limited to spherical ones. Although extensive researches have focused on shape-controlled synthesis of monometallic nanoparticles, there are few reports on synthesis of anisotropic alloy nanostructures with high crystallinity, for example, dendritic alloy nanostructures,10,2a especially using them as building blocks to fabricate 3D arrays. Herein, for the first time, single-crystalline Ag/Pd alloy dendritic nanostructures are synthesized. More importantly, these alloy dendritic nanostructures can be aligned into largescale vertical arrays. A simple galvanic replacement reaction is introduced to grow these arrays directly on Ag substrates. Galvanic replacement reaction is a type of electroless deposition in which metal ions in an aqueous solution are reduced by electrons arising from the substrate.11 Very recently, it has been proved to be an attractive approach for the growth of

nanostructures directly on metal or semiconductor substrates.12,2b,3a Compared with the most common technique used to fabricate nanoarrays, template synthesis,13 the present method can avoid the expensive raw materials for the template device and sophisticated process in template removal.14 These alloy arrays exhibit high surface-enhanced Raman scattering (SERS) activity and may have potential applications in investigation of “in situ” Pd catalytic reactions using SERS. Moreover, electrocatalytic measurements suggest that the obtained dendritic Ag/Pd alloy nanostructures exhibit electrocatalytic activity toward the oxidation of formic acid. Experimental Procedures

*Corresponding author. E-mail: [email protected]. Tel: þ86-43185262964. Fax: þ86-431-85262964.

Chemicals and Materials. Palladium(II) chloride (PdCl2, anhydrous, 59% as Pd) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). Silver wires (0.25 mm in diameter, 99.99%) were purchased from Alfa Aesar and treated by sonication in acetone and water successively before use. Formic acid (HCOOH, 88%) was obtained from Chemical Reagent Company of Shanghai (China). Doubly-distilled water was used throughout the experiments. Ten mM H2PdCl4 solution was prepared by dissolving 0.1773 g of PdCl2 in 10 mL of 0.2 M HCl solution and further diluting to 100 mL with double-distilled water. Synthesis of Dendritic Ag/Pd Alloy Nanostructure Arrays. Onetenth of a milliliter of a 10 mM H2PdCl4 solution was first added to 0.9 mL of doubly-distilled water to obtain a 1 mM H2PdCl4 solution in a 5 mL centrifuge tube. The synthesis of dendritic Ag/Pd alloy nanostructures was achieved simply by immersing a silver wire into H2PdCl4 solution. The solution was heated at 50 C for 5 min before silver wire was immersed. The reaction was allowed to proceed for 1 h at 50 C. After the reaction, the silver wire was washed with water thoroughly for SEM characterization. The products were separated from silver wire by ultrasonic treatment in water and then washed for TEM and XRD characterization. Characterization. SEM images were taken using FEI XL30 ESEM FEG scanning electron microscopy operating at 25 kV. TEM and selected area electron diffraction studies were performed on a Tecnai G2 20 S-TWIN TEM. X-ray powder diffraction (XRD) investigations were performed using a Rigaku D/MAX-2500 instrument (Cu KR1 radiation) operated at 50 kV and 250 mA over a range of 30-90 by step scanning with a step size of 0.02. SERS Measurement. Raman spectra were taken using a Renishaw 2000 confocal Raman spectrophotometer (Gloucestershire, UK)

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Figure 2. High-magnification SEM image of the dendritic nanostructure.

Figure 1. SEM images of the silver wire surface (a) before and after reaction with H2PdCl4 solution at 50 C for 1 h: (b) original state, (c) after scraping, and (d) after pressing under external force. equipped by an Arþ ion laser at excitation wavelength of 514 nm. The as-prepared samples were dipped into a solution with 1  10-7 M R6G with stirring for 10 min, rinsed with deionized water, and dried with high-purity flowing nitrogen before Raman examination. Electrochemical Measurement. Electrochemical measurements were performed with a CHI 832 electrochemical workstation (Shanghai, China). A conventional three-electrode configuration was employed throughout electrochemical measurement. Ag/Pd dendritic nanostructure-modified Au electrode was prepared as follows: Ag/Pd dendritic nanostructures were dispersed in water from Ag wire by ultrasonic treatment an then cast on Au electrode (1 mm in diameter) and allowed to dry in a desiccator at room temperature. A Pt wire served as the counter electrode and an Ag/ AgCl (saturated KCl solution) electrode was used as the reference electrode. The electrolyte used was 0.2 M HCOOH þ 0.5 M H2SO4 solution.

Figure 3. XRD pattern of the dendrites separated from the Ag wire via ultrasonic treatment (values of pure metals are marked with vertical lines).

Results and Discussion In this study, Ag wire with a smooth surface (Figure 1a) was used as substrate to reduce H2PdCl4 solution. Because the standard reduction potential of the Pd2þ/Pd pair (0.83 V versus the standard hydrogen electrode, or SHE) is higher than that of the Agþ/Ag pair (0.80 V versus SHE), galvanic replacement reaction occurred as follows: 2AgðsÞþPdCl4 2 - ðaqÞ f PdðsÞþ2AgClðsÞþ2Cl - ðaqÞ Figure 1b shows the SEM image of the surface of the Ag wire after its reaction with PdCl42- ions for 1 h. It can be clearly seen that vertically aligned arrays formed on the surface of the silver wire. These arrays can be scraped off from the Ag substrate (Figure 1c). They can also lie down and form patterns aligned in a certain direction under external force (Figure 1d). Images c and d in Figure 1 indicate that the arrays are made up of uniform dendrites in a length of 10-20 μm. Figure 2 shows a typical high-magnification SEM image of the dendrites structures, which suggests that the hierarchical dendrite morphology consists of two groups of parallel branches grown on a central trunk, and each branch consists of two groups of secondary branches grown in a similar manner. Such morphology with high surface area is unstable at high temperature. The detail morphology becomes obscure at ∼300 C and the whole dendrite melts at ∼400 C (see Figure S1 in the Supporting Information). X-ray diffraction (XRD) pattern of the dendrites is shown in Figure 3. The diffraction peaks can be indexed to a facecentered cubic (fcc) lattice and are positioned between the

Figure 4. EDS pattern and elemental mapping of these dendrites (Si peaks correspond to the substrates).

reflections of pure Ag and pure Pd, and no peak splitting is observed, which strongly supports the formation of a homogeneous Ag/Pd alloy.15 The peak position is close to that of Ag, indicating a high Ag ratio in the alloy. The molar composition of Ag and Pd in the alloy is 71 and 29%, respectively, as calculated using Vegard’s law16 and averaged for all five diffraction peaks. Corresponding EDS spectrum and EDS mapping of the dendrites (Figure 4) show that the composition is almost quantitatively consistent with the value calculated using Vegard’s law, and that Ag and Pd elements are uniformly distributed over the entire structure. Moreover, the elemental mapping of Ag is brighter than that of Pd, which confirms the formation of homogeneous alloy with a higher Ag content. On the other hand, when we characterized the arrays without washing treatment after the reaction, we found

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Figure 5. (a) TEM image of a single dendrite. The inset is the ED pattern taken from the whole dendrite with the electron beam perpendicular to its basal plane. (b) HRTEM image of the marked region. Figure 7. SEM images of the samples obtained at different temperatures: (a) 25 and (b-d) 90 C.

Figure 6. SEM images showing the surface of silver wire at the early stages of formation of dendrites arrays at 50 C (reaction time: (a) 10 s and (b) 3 min, respectively).

that the AgCl nanoparticles were generated on the surface of the arrays and were separate from the alloy, which can be easily washed off (see Figure S2 in the Supporting Information). TEM image and SAED pattern of an individual Ag/Pd dendrite are shown in Figure 5a. The ED pattern shows clear spots index to the [011] zone axis of fcc crystal, indicating even though as alloy, the whole dendrite is single-crystalline with the trunk grown along the Æ111æ direction and two group of branches grown along the Æ100æ and Æ111æ directions, respectively. A typical HRTEM image recorded from one Æ111æ -oriented branch is shown in Figure 5b. The d-spacing of adjacent lattice planes is determined to be 0.233 nm, which corresponds to the mean value of the (111) planes of fcc Ag (0.236 nm) and Pd (0.225 nm) and is in good agreement with d111 value calculated from XRD results (0.2336 nm, Figure 3). The growth of the arrays is monitored by SEM with controlling the reaction time. As shown in Figure 6a, at the beginning of reaction (10 s), densely packed nucleation sites less than 100 nm formed on the Ag surface. The subsequent growth process would preferentially take place on the preformed nuclei possibly because of their relatively high activation energy. Therefore, these nuclei vertically grew up to dendritic nanostructures during the growth process. As shown in Figure 6b, many free-standing small dendrites appeared as soon as 3 min of reaction. When the reaction time was prolonged to 1 h, large-scale vertically aligned dendritic nanostructure arrays formed as the final product (Figure 1b). The morphology of the products strongly depended on the reaction temperature. For example, at 25 C reaction temperature, the product mainly consisted of irregularly shaped dendrites, with a very low yield of perfectly shaped ones (Figure 7a). Interestingly, when the temperature was increased to 90 C, arrays consisting of several clusters were observed on the surface of silver wire (Figure 7b,c). Each

cluster was made up of many independently aligned dendrites with their tips bended and assembling together (Figure 7d). The concentration of H2PdCl4 solution also showed a considerable effect on the formation of dendritic arrays. When the H2PdCl4 concentration decreased from 1 mM to 0.1 mM or increased to 4 mM, similar to the products obtained at 25 C, the amount of well-defined dendrites remarkably decreased (see Figure S3 in the Supporting Information). Dendritic growth has been generally studied in solidification of alloys. It was documented that the nonequilibrium conditions at the liquid-solid interface (e.g., temperature gradient and solute concentration) provide a driving force for the dendritic growth.17 In this study, we fixed the temperature during the growth of the Ag/Pd arrays, thus there is no temperature gradient in the system. Therefore, under the constant temperature condition, the nonequilibrium concentration distribution of H2PdCl4 at the liquid-solid interface determines the morphology of the grown nanoarrays. It was proved by a control experiment taken under stirring for creating a current equilibrium condition. As a result, no dendrites formed in the final products (see Figure S4 in the Supporting Information). Furthermore, such nonequilibrium concentration at the liquid-solid interface is affected by varying the temperature. In other words, the temperaturerelated nature of the morphology presented in Figure 7 results from the temperature-dependent diffusion rate of H2PdCl4. The Pd-Ag system is a well-known binary alloy.18 Xia’s group has used nanosized Ag as template to react with Na2PdCl4 solution.19 Because Ag atoms have a strong tendency to diffuse from bulk to surface when they alloy with Pd, hollow nanostructure with Ag/Pd alloy walls formed.20 Though Ag wire was used here, the process of the formation of Ag/Pd alloy is considered to be similar to their experiments. In addition, according to the Vacancy diffusion theory,21 the existence of vacancies can facilitate the diffusion of atoms. Here, galvanic replacement reaction creates the vacancies: as defined by the stoichiometric relationship between Ag and H2PdCl4, two Ag atoms will be consumed, whereas only one Pd atom formed in the galvanic replacement reaction, leaving vacancies in the crystal lattice of Ag wire. In assistant of these vacancies, the Ag atoms diffuse from the interior to the surface more readily to form an alloy with Pd atoms resulted from the galvanic replacement reaction. Large numbers of Ag atoms are consumed during the galvanic replacement reaction (Ag atoms turn to Agþ) and alloying (Ag atoms diffuse to the

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Figure 8. SERS spectra of R6G on different substrates: (a) dendritic Ag/Pd alloy arrays obtained at 50 C, (b) flat surface of Ag wire before reaction with H2PdCl4 solution.

surface) proceeds, resulting in the rough surface of Ag wire beneath these alloy arrays (Figure 1c). Ag/Pd alloy nanostructures have potential applications in the investigation of “in situ” Pd catalytic reactions and examining the compositional dependence of oxidation, adsorption, and ligand formation with surface-enhanced Raman scattering (SERS).22,23 As a preliminary test, the SERS on Ag/ Pd alloy array was investigated by using Rhodamine 6G (R6G) as probe molecular. Well-defined SERS spectra of 10-7 M R6G was obtained on dendritic Ag/Pd alloy array (Figure 8a), which was probably due to the high proportion of Ag in the alloy24 and enhanced surface area of the dendritc morphology. For comparison, we synthesized Ag dendritic nanostructure using similar galvanic replacement reaction between Cu foil and AgNO3 solution and measured its SRES spectra. The result indicated that Ag/Pd alloy dendritic nanostructure exhibited high SERS activity comparable with the Ag one (see Figure S5 in the Supporting Information). And the advantage of Ag/Pd alloy dendritic nanostructure is that it combines the SERS activity of Ag and the catalytic activity of Pd. Additionally, because no surfactants and capping legends were used in the whole synthesis process, the as-prepared Ag/Pd alloy arrays have a “clean surface”. Therefore, it is believed to provide the possibility to use SERS to investigate surface chemistry in a particular reaction. To study the electrochemical activity of the Ag/Pd alloy dendritic nanostructure, we used a Au electrode modified with these nanostructures as the working electrode for electrocatalytic oxidation of formic acid. In a control experiment, the Au electrode was measured under the same condition for comparison. As can be seen in Figure 9, the Au electrode (curve a) exhibits neglectable activity toward formic acid oxidation, whereas the Ag/Pd dendritic nanostructure-modified Au electrode (curve b) shows an oxidation peak at 0.193 V (versus Ag/AgCl), which might be related to the Pd component in the alloy. In addition, as the traditional application of Ag/Pd alloy, this dendrite Ag/Pd alloy arrays may also be exploited in producing pure H2,25 detection of H2,26 and the catalytic hydrogenation reaction.27 It is mentionable that such arrays can also be obtained using other bulk Ag substrate, for example Ag flake (see Figure S6 in the Supporting Information). Therefore, arrays on a certain formed Ag substrate can be fabricated to fulfill the special demand of the practical applications.

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Figure 9. Cyclic voltammograms of (a) Au electrode and (b) Ag/Pd dendritic nanostructure-modified Au electrode in 0.2 M HCOOH þ 0.5 M H2SO4 solution (scan rate 50 mV s-1).

Conclusion In summary, arrays consisting of dendritic single-crystalline Ag/Pd alloy nanostructures are large-scale synthesized for the first time. A simple galvanic replacement reaction is introduced to grow these arrays directly on Ag substrates. During the growth process, densely packed nucleation sites less than 100 nm formed on the Ag surface first, an then these nuclei further grew up to dendritic nanostructures. The nonequilibrium concentration distribution of H2PdCl4 at the liquid-solid interface determines the morphology of the grown nanoarrays. And such concentration is affected by varying the temperature. The galvanic replacement reaction assisted diffusion of Ag atoms is considered to be the reason for the formation of Ag/Pd alloy. These alloy arrays exhibit high surface-enhanced Raman scattering (SERS) activity and may have potential applications in investigating the surface chemistry in a particular reaction using SERS, for example, “in situ” Pd catalytic reactions. Moreover, electrocatalytic measurements suggested that the obtained dendritic Ag/Pd alloy nanostructures exhibited electrocatalytic activity toward the oxidation of formic acid, which might be related to the Pd component in the alloy. This synthetic strategy may also open new routes to synthesize alloy nanostructures directly on metal substrates. Acknowledgment. We gratefully acknowledge Prof. Z. Y. Tang and Prof. Z. X. Wang for the TEM measurements and helpful discussions. This work was supported by the National Natural Science Foundation of China (20605020 and 20875085), the Chinese Academy of Science (KJCX2-YWH11), and the Foundation of Distinguished Young Scholars of Jilin Province (20060112). Supporting Information Available: SEM image of the products obtained in H2PdCl4 solution with different concentrations, the Ag surface after reaction with H2PdCl4 solution under stirring, the morphology of Ag/Pd dendritic nanostrucutres at high temperature, Ag/Pd alloy array without washing treatment, dendritic Ag nanostructures synthesized by control experiment, and the arrays synthesized using Ag flake as substrate. SERS spectra of 10-7 M R6G on the Ag dendritic nanostructure and Ag/Pd dendritic nanoarray (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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