Single-Crystalline Pb Nanowires Grown by Galvanic Displacement

Apr 12, 2007 - Single-crystalline Pb nanowires (NWs) have been grown with galvanic displacement method on the zinc foil using Zn as a reducing agent a...
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J. Phys. Chem. C 2007, 111, 6215-6219

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Single-Crystalline Pb Nanowires Grown by Galvanic Displacement Reactions of Pb Ions on Zinc Foils and Their Superconducting Properties Chiu-Yen Wang, Ming-Yen Lu, Huai-Chung Chen, and Lih-Juann Chen* Department of Materials Science and Engineering, National Tsing Hua UniVersity, Hsinchu 300, Taiwan ReceiVed: December 17, 2006; In Final Form: February 13, 2007

Single-crystalline Pb nanowires (NWs) have been grown with galvanic displacement method on the zinc foil using Zn as a reducing agent at room temperature. The Pb NWs are hundreds of micrometers long with an average diameter of 80 nm. Heterostructured metal/semiconductor Pb/ZnOx core-shell nanocables were byproducts of the reactions. ZnOx shells served as a protective layer but can be stripped readily with a dilute HCl etching. The inner Pb cores were melted and leaked out and polycrystalline ZnO nanotubes were obtained after annealing at 400 °C. The superconducting transition temperature was measured to be about 7 K. The cathodoluminescence of ZnO nanotubes presents two emission bands around 380 and 590 nm. The work represents for the first time the achievement to produce the metallic NWs with the efficient and low-cost galvanic displacement method.

Introduction Galvanic displacement is an efficient fabrication process applicable to a large number of systems since positive redox potential leads to spontaneous reaction.1 It needs simple apparatus, generates little waste, and works at room temperature compared to standard evaporation techniques.2 Most of the previous studies involving galvanic displacement reactions have focused on noble metals (Au, Ag, Pt, and Pd) on different substrates.3.4 Galvanic displacement method has been previously used to produce Ag nanoinukshuks and Au, Pd, and Pt nanoparticles on various patterned semiconducting and metallic substrates.3,5 In addition, Ag nanocubes were transformed into Pd-Ag and Pt-Ag nanoboxes.6 On the other hand, it has been recognized that one of the drawbacks of galvanic displacement is the lacking of control to prepare isolated metal nanostructures of various shapes and sizes.3 Only with elaborate lithography techniques, polycrystalline nanowires were prepared.5 Recently, shape- and size-controlled syntheses of metal nanoparticles on Cu foils have been achieved and were exploited to site-selectively deposit Au and Pt nanoparticles onto carbon nanotubes.7 In this communication, we show that single-crystalline Pb nanowires can be synthesized under appropriate conditions by the simple and cost-effective method. Pb nanowires were previously prepared by electrodeposition on nanoporous templates.8-11 However, the method requires applying electric field to reduce the metallic ion solutions and there is a need to remove the template by heating or chemical etching. The lengths and widths of these nanowires are also severely restricted by the dimensions of pores in the template. Up to 250 µm single-crystalline Pb nanowires can be prepared by hydrothermal reaction decomposition of the metal precursor containing the polymer as a capping reagent.12,13 The hydrothermal method, however, suffers from the requirement of hightemperature (>198 °C and up to 350 °C) processing and the need of a polymer to control the morphological evolution of * To whom correspondence should be addressed. Phone: 886-35731166. Fax: 886-35718328. E-mail: [email protected].

Pb nanowires. In the present work, long and narrow singlecrystalline Pb nanowires were synthesized using Zn foil as a reducing agent for the Pb ions at room temperature without the need of external potentials. In addition, no surfactant or polymer was used as the capping reagent to confine the morphology. The method can be extended to other systems with positive redox potential to prepare nanowires. Therefore, the growth of the Pb nanowires by galvanic displacement at room temperature opens up a new and facile route to synthesize 1D nanostructures efficiently. The initial products of the reaction were one-dimensional (1D) Pb-ZnOx core-shell nanostructures. The ZnOx shell can be readily stripped with a dilute HCl etching and serves as a protective layer for the Pb core. Furthermore, ZnO nanotubes could be obtained by annealing at 400 °C. Hierarchical nanostructures are important since they are a step ahead of simple nanowires and nanotubes as the building blocks of nanodevices. Nanocables of a diverse variety of core-shell materials have been synthesized. Notable examples include different combinations of metallic, semiconducting, and insulating materials. The nanocables have been prepared by thermal evaporation (ZnO/ZnS, Zn/ZnO, Si/CdSe, CdS/Si, ZnO/ ZnGa2O4),14-19 solvothermal method (Ag/SiO2, Au/SiO2, and Se/CdSe),20-22 and two-step electrodeposition on porous template (Co/Ge and Au/Ni nanocables).23,24 Measurements for Pb nanowires indicate a transition to the superconducting state around 7 K. Cathodoluminescence spectrum has been commonly used to determine semiconductor band gap and defect states. In the work, cathodoluminescence (CL) of ZnO nanotubes presents two emission bands, around 380 and 590 nm, attributed to zinc oxide band gap and oxygen defects, respectively.14 Lead is known to possess the highest transition temperature (∼7 K) among common metals and is the dominating superconductor for electronic applications.12 ZnO is an important semiconductor material as the building blocks for the fabrication of nanoscale electronic and optoelectronic devices.25 The integration of superconducting nanostructures and semiconductors shall be important for technological applications.

10.1021/jp068662j CCC: $37.00 © 2007 American Chemical Society Published on Web 04/12/2007

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Figure 1. SEM images of Pb nanowires produced from reactions in 1 mM Pb(OAc)2 solutions for (a) 3 min, (b) 5 min, and (c) 10 min. a and c are of high and low magnifications to highlight the intersecting nanoplates and extended lengths, respectively. (d) XRD spectra indicating the formation of metallic Pb in the as-deposited sample and only polycrystalline ZnO remaining after heating at 400 °C, which is a temperature higher than the melting temperature of Pb, for 30 min.

Experimental Section Materials. Lead acetate trihydrate (99.99%, Aldrich) and zinc foil (99.999%, Aldrich) were used as received without further purification. Synthesis. Pb nanowires were prepared on zinc substrate by using 10 mL of Pb(OAc)2 solution in a 20 mL vial. After the growth, the zinc foil was cleaned with deionized water and was dried with N2 gas. Characterization. The morphologies, chemical compositions, structures, and CL of the reaction products were characterized by field-emission scanning electron microscopy (FESEM, JEOL JSM-6500F), transmission electron microscopy (TEM, JEOL JEM-2010), and powder X-ray diffractometry (XRD, Shimadzu SRD-6000, at a scanning rate of 2° per minute in the range from 30° to 70°). The TEM is equipped with an energydispersive X-ray spectrometer (EDS). Magnetic measurement was performed using a superconducting quantum interference device (SQUID) extractive magnetometer (Quantum Design, MPMS-5). Results and Discussion A previous report showed that from open-cell potential (OCP), which is a potential measured across the reference electrodes of a cell when the circuit is open, that is, producing no net currents, measurements, the nucleation of Ag on oxidized Ge substrate surface was found to take place in the first 100 s.3 For reduction of Pb by Zn foil in a solution containing 1 mM Pb(OAc)2, the nucleation time was found to be about 50 s from the OCP measurements (shown in Supporting Information S1). SEM examinations provided the consistent results. For 30 s reactions in a solution containing 1 mM Pb(OAc)2, the major products are intersecting nanoplates with 10-20 nm in thickness. For the 3 min reactions, several nanowires were extended from intersecting nanoplates (roots). The Pb nanowires (NWs) grew longer as the reaction time was extended to 10 min.

Figure 1a-c shows SEM images of Pb NWs prepared in the Pb(OAc)2 solution following 3, 5, and 10 min reactions using the zinc foil as the reducing agent. For lead acetate concentration of 1 mM, the reduced Pb formed a high density of nanowires with lengths and average diameter of hundreds of micrometers and 80 nm, respectively. The Pb nanowires were covered with a thin layer of ZnOx because of the oxidation of Zn during the galvanic displacement reaction. The Pb core and ZnOx shell nanostructures are herein called nanocables. With higher Pb(OAc)2 concentration (10 mM), the reduced products include intersecting nanoplates (200-500 nm in thickness) and cottonlike structures. When the Pb(OAc)2 concentration was as low as 0.1 mM, only folded nonplanar plates were formed. One millimolar of Pb ions appears to be the optimum concentration at which reduced Pb on the surface combines with oxidized Zn to form Pb core and ZnOx shell nanostructures. Figure 1d contains XRD spectra indicating the formation of Pb nanowires and after heating at 400 °C, which is a temperature higher than the melting temperature of Pb, for 30 min. XRD spectrum for the latter reveals the presence of polycrystalline ZnO with no trace of Pb. Bright-field TEM images of Pb nanowires are shown in Figure 2a. From the analysis of electron diffraction pattern and the EDS (Figure 2b), it is concluded that the 1D nanostructure consists of a single-crystalline Pb nanowire core and a thin amorphous ZnOx shell. The thicknesses of the shell of Pb/ZnOx nanocables are as thin as 10-20 nm. The ZnOx shell can be readily stripped by dipping into a dilute HCl (HCl:H2O ) 1:10) solution. Examples are shown in Figure 2c. Comparing the image in 2c with the inset electron diffraction pattern reveals that the growth direction of Pb nanowire is in [100] direction. Because of the relatively low melting point of Pb (∼327 °C), the cores of Pb/ZnOx nanocables could be readily removed (by heating at ∼400 °C for a few min) and could consolidate the ZnOx into ZnO. As a result, ZnO nanotubes were obtained. The short segments of Pb NWs were selected for TEM examinations

Single-Crystalline Pb Nanowires

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Figure 3. In-situ TEM images of a Pb/ZnOx nanocable (a) as-prepared and (b, c) annealed at 700 K with a heating rate 50 °C/min. The elapsed times are marked on the micrographs.

Figure 2. (a) Bright-field TEM image of Pb/ZnOx nanocables with the high-magnification image of a nanocable as the inset. (b) EDS taken from Pb/ZnOx; a nanocable showing that the nanocable is composed of Pb, O, and Zn. The Cu peak came from the TEM grid. (c) TEM image showing the Pb NWs after the ZnOx shells were stripped with a dilute HCl dip. The inset electron diffraction pattern in c reveals that the growth direction of Pb nanowire is in [100] direction.

since the longer ones tended to tangle together without the protection of ZnOx shells. On the other hand, the Pb/ZnOx nanocables were found to be rather stable after exposure in the air for days. The ZnOx shells can therefore serve as protective layers for the Pb cores during the storage in the ambient. In-situ TEM observations were carried out to address the important question of where does the Pb go when the cables were heated above the melting temperature? Figure 3 a-c is a series of TEM images clipped from video recordings of the heating event. It is apparent that the melted Pb leaked out of the shell during the heating. Figure 4 a and b shows bright-field and high-resolution TEM (HRTEM) images of a ZnO nanotube after the removal of Pb core. The contrast clearly depicts a tubular morphology. From the HRTEM images, EDS (Figure 4c), and electron diffraction ring patterns (not shown), the nanotubes were identified to be composed of polycrystalline ZnO grains. A previous study on the Pb NWs produced by the hydrothermal method has shown that the NWs grow out of the tapered corners of platelike structures. It was concluded that the plates served as the roots for the growth of uniform NWs during the reaction.13 From the analysis of nanostructures in the present study, a similar mechanism is likely to be operating. The growth of Pb/ZnOx nanocables includes several steps: (1) conversion of Pb ions into Pb atoms, (2) growth of lead atoms into

Figure 4. (a, b) Bright-field and high-resolution TEM images of a sample with the removal of Pb core through thermal evaporation. (c) EDS taken from a ZnO nanotube showing that that the nanotube is composed of O and Zn. The Cu peak came from the TEM grid.

nanoparticles and then into nanoplates (roots), (3) nucleation and growth of Pb NWs extended from the corners of roots, and (4) Zn atoms were oxidized to form ZnOx at the surface of Pb nanowires. For galvanic displacement processed in the absence of an external reducing agent, the reducing electrons are derived from the bonding electrons of the substrate lattice. The electrons acted as the reducing agent for Pb2+ ions in solution. This led to the continuing supply of Pb atoms and concomitant oxidation of Zn(s) that would not inhibit further metal deposition. The growth direction and cross section of the Pb NWs produced by galvanic displacement reactions are [001] and circular, respectively. In contrast, [011] growth direction and rectangular cross section were found for Pb NWs synthesized by hydrothermal reactions. The proposed mechanism for the formation of the Pb/ZnOx nanocables and ZnO nanotubes is illustrated schematically in Figure 5. In the present work, several important findings led to new physical insights of the galvanic displacement reaction. It is

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Figure 5. Proposed growth mechanism for the growth of the Pb/ZnOx nanocables and ZnO nanotubles. (a) Pb atoms accumulate to form Pb nanoplates, (b) growth of Pb NWs extended from the nanoplates and the deposition of ZnOx on the Pb NWs, (c) evaporation of the Pb core resulting in the formation of ZnO nanotubes, and (d) the spontaneous redox reaction of Pb ions on the zinc foil.

Figure 6. (a) The temperature-dependent magnetization curve with an applied field of 100 T. (b) The cathodoluminescence spectra of the as-synthesized Pb/ZnOx nanocables and ZnO nanotubes.

likely that the galvanic displacement reaction converted Zn(s) to ZnOx and adhered on Pb nanowire to confine Pb(s) morphology as the Pb ions were reduced by active Zn substrate at the same time. The confinement facilitated the growth of straight Pb nanowires as well as more complex nanostructures such as Pb/ZnOx nanocables and ZnO nanotubes. Magnetization studies have proved to be a very informative method to characterize superconductors. Figure 6a shows the temperature-dependent magnetization curve of single-crystalline Pb nanowires with an applied field of 100 T. The temperaturedependent diamagnetism can be observed below about 7.0 K. The value is the same as that of bulk Pb, which is known to possess the highest transition temperature among common metals. The results indicate that the superconducting properties of Pb NWs are potentially useful for electronic applications.9 The cathodoluminescence of the galvanic displacement process products has been measured. A comparison of the CL spectra

recorded from the as-synthesized Pb/ZnOx nanocables and ZnO nanotubes is given in Figure 6b. The peak located at 380 nm corresponds to the 3.2 eV band gap transition of ZnO and the width of half-height is 18 nm. The broad peak at 590 nm is a resultant contribution from oxygen defects of ZnO. The defective nature of ZnO is rather common and may be improved by controlling the annealing temperature and ambient. The effects of the temperature, stirring, pH value, lead salt, and solvent on the galvanic displacement reactions were also investigated. The morphology of products changed from nanocables to pyramidal Pb nanostructures (shown in Supporting Information S2) for reactions at a higher temperature (70 °C). Lowering the temperature to 4 °C leads to a significant decrease in the deposition rate. Stirring the solution has a subtle effect on the deposition of Pb/ZnOx nanocables. A layered netlike morphology is observed on the zinc foil (shown in Supporting Information S3). Different pH values also dramatically influence the growth of the galvanic displacement reaction, and the Pb/ ZnOx nanocables formed only in solutions with pH values between 4.5 and 5.5. In lower pH solution, faceted Pb rods micrometers in length were formed (shown in Supporting Information S4). In higher pH solution, on the other hand, the Pb2+ will react with OH- that produce the Pb(OH)2 and intersecting nanoplates on zinc foil (shown in Supporting Information S5). Some of the other Pb2+ salts, including Pb(NO3)2 and Pb(ClO4)2‚3H2O, consistently yield Pb/ZnOx nanocables larger in diameter and more uniform in size (shown in Supporting Information S6). In addition, PbCO3 and PbSO4, although insoluble in water, also reacted with zinc foil. Lead microinukshuks were observed (shown in Supporting Information S7). Finally, effects of the solvents, including ethyl alcohol and acetone, have been assessed. The Pb/ZnOx nanocables were no longer formed. In contrast to the reactions in water, the resulting products were insoluble and inhibited further metal deposition. Conclusions In summary, we have demonstrated a fast and facile solution method in mild condition to prepare highly dense Pb NWs on zinc foil surface at room temperature by galvanic displacement. Heterostructured metal/semiconductor Pb/ZnOx core-shell nanocables were byproducts of the reactions. ZnOx shells can be stripped readily with a dilute HCl etching. The inner Pb cores were melted and leaked out and polycrystalline ZnO nanostubes were obtained after annealing at 400 °C. The growth mecha-

Single-Crystalline Pb Nanowires nisms of these nanostructures have been clarified. It is expected that this galvanic displacement process can be applied to other metals and metal oxides for the synthesis of nanowires and nanocable systems. Acknowledgment. The research was supported by the Republic of China National Science Council Grant No. NSC 94-2215-E-007-003 and the Ministry of Education Grant No. 91-E-FA04-1-4. Supporting Information Available: The OCP experiment process and results are shown as S1. The effects of the temperature, stirring, pH value, and lead salt on the galvanic displacement reactions are shown as S2-S7. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Niwa, D.; Homma, T.; Osaka, T. J. Phys. Chem. B 2004, 108, 9900. (2) Oskam, G.; Long, J. G.; Natarajan, A.; Searson, P. C. J. Phys. D: Appl. Phys. 1998, 31, 1927. (3) Azawa, M.; Cooper, A. M.; Malac, M.; Buriak,; J. M. Nano Lett. 2005, 5, 815. (4) Porter, L. A., Jr.; Choi, H. C.; Schmeltzer, J. M.; Ribbe, A. E.; L. Elliott, C. C.; Buriak, J. M. Nano Lett. 2002, 2, 1369. (5) Hormozi-Nezhad, M. R.; Azawa, M.; Porter, L. A., Jr.; Ribbe, A. E.; Buriak, J. M. Small 2005, 1, 1076.

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