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Simple, One-Step Synthesis of Gold Nanowires in Aqueous Solution Krasimir Vasilev,† Tao Zhu,‡ Michael Wilms,§ Graeme Gillies,| Ingo Lieberwirth, Silvia Mittler,⊥ Wolfgang Knoll, and Maximilian Kreiter* Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany Received August 29, 2005. In Final Form: September 28, 2005 A simple procedure to synthesize gold nanowires based on the reduction of hydrogen tetrachloroaurate by 2-mercaptosuccinic acid in aqueous solution is presented. This procedure requires no additional capping or reduction agent and produces wires with an apparent curly morphology several micrometers in length with diameters as thin as 15 nm. Some of the wires produced end in a ribbonlike structure, finally terminated by a flat triangular prism. Investigations by scanning electron microscopy, transmission electron microscopy (bright and dark field), scanning transmission electron microscopy, and atomic force microscopy as well as conductivity measurements indicate fully connected, polycrystalline gold objects.
Introduction Considerable effort has been concentrated on the structuring of material on a nanometer scale. These activities were motivated, on one hand, by the expectation that the classical “top down” strategies in microelectronics will have to be complemented by a “bottom up” strategy to further decrease feature sizes. On the other hand, structure at the nanometer scale can drastically alter material properties, which, beyond being interesting from a fundamental point of view, opens the way to tailor superior materials for application.1-4 Metallic wires with diameters in the range of a few nanometers5 are important and necessary structures in electronic circuits of future nanodevices. In particular, gold and silver play an important role as a result of their high electrical conductivity and chemical inertness. Previously, gold nanorods with aspect ratios as large as 20 have been synthesized in solution6-8 while various templating strategies were required for higher aspect ratios. Reports of wire formation on lithographically prepared templates,9-11 as * Corresponding author. Phone: +49-6131-379163. Fax: +496131-379100. E-mail:
[email protected]. † Present address: Ian Wark Research Institute, University of South Australia, Adelaide, South Australia 5095, Australia. ‡ Present address: College of Chemistry, Peking University, Beijing 100871, China. § Present address: Leibniz-Institut fu ¨ r Polymerforschung, Dresden e.V., Hohe Straβe 6, 01069 Dresden, Germany. | Present address: Department of Inorganic, Analytical and Applied Chemistry, University of Geneva, Switzerland. ⊥ Present address: Department of Physics and Astronomy, University of Western Ontario, London, Canada. (1) See, for example, Nature 2000, 406, 1021 (special issue). (2) Edelstein, A. S.; Cammarata, R. C. Nanomaterials: Synthesis, Properties, and Applications; Institute of Physics: Philadelphia, PA, 1996. (3) Klabunde, K. J. Nanoscale Materials in Chemistry; WileyInterscience: New York, 2001. (4) Hu, J.; Ouyang, M.; Yang, P.; Lieber, C. M. Nature 1999, 399, 48. (5) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (6) Busbee, B. D.; Obare, O. S.; Murphy, J. C. Adv. Mater. 2003, 15, 414. (7) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (8) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (9) Bietsch, A.; Michel, B. App. Phys. Lett. 2002, 80, 3346. (10) Dumpich, G.; Friedrichowski, S.; Lohau, J. J. Phys. Soc. Jpn. 2000, 69, 99. (11) Schopfer, F.; Bauerle, C.; Rabaud, W.; Saminadayar, L. Phys. Rev. Lett. 2003, 90, 056801.
well as in organic and inorganic nanoporous media,12-15 in cylindrical core-shell polymer brushes or on DNA,16,17 and by organizing gold nanoparticles in one-dimensional arrays,18,19 have appeared in the literature. Xia et al.20,21 demonstrated the synthesis of silver and lead nanowires by a high-yield solution-phase method, leading to nanowires with typical diameters of 30-40 nm and lengths up to 50 µm. They pointed out that a simple one-step solution-phase approach offers a high flexibility together with cost effectiveness for future applications. For example, the availability of nanowires in large quantity would be of great importance in the electronic industry as fillers in polymer-metal composites as the load of metal will be greatly reduced if high aspect ratio nanowires were used instead of particles.22 Pei et al.23 demonstrated a solution-phase synthesis of gold nanowires, synthesizing a two-dimensional network using sodium citrate as a reduction agent. In this paper, a simple solution-phase synthesis yielding gold nanowires with no apparent upper limit in length is described. Hydrogen tetrachloroaurate (HAuCl4) has been reduced in aqueous solution by 2-mercaptosuccinic acid (MSA) without additional capping agents. The structure and morphology of the wires are ascertained by a variety of microscopic techniques. Experimental Section Materials. HAuCl4 (99.9%) was purchased from Aldrich, MSA (99%) was purchased from Arcos Organics, and NaOH (analytical purity >99%) was from WTL Laborbedarf GmbH, Kastellaun. Milli-Q water with resistivity > 18.2 MΩ‚cm was used. All glassware was cleaned using aqua regia and subsequently rinsed with copious amounts of Milli-Q water. (12) Wang, Z.; Su, Y.-K.; Li, H.-L. Appl. Phys. A 2002, 74, 563. (13) Wirtz, M.; Martin, C. R. Adv. Mater. 2003, 15, 455. (14) Yu, S. F.; Li, N. C.; Wharton, J.; Martin, C. R. Nano Lett. 2003, 3, 815. (15) Han, Y. J.; Kim, J. M.; Stucky, G. D. Chem. Mater. 2000, 12, 2068. (16) Djalali, R.; Li, S.-Y.; Schmidt, M. Macromolecules 2002, 35, 4282. (17) Harnack, O.; Ford, W. E.; Yasuda, A.; Wessels, J. M. Nano Lett. 2002, 2, 919. (18) Teranishi, T.; Sugawara, A.; Shimizu, T.; Miyake, M. J. Am. Chem. Soc. 2002, 124, 4210. (19) Yin, Y.; Lu, Y.; Gates, B.; Xia, Y. J. Am. Chem. Soc. 2001, 123, 8718. (20) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. (21) Wang, Y.; Herrick, T.; Xia, Y. Nano Lett. 2003, 3, 1163. (22) Carmona, F.; Barreau, F.; Delhaes, P.; Canet, R. J. Phys. Lett. 1980, 41, L531. (23) Pei, L.; Mori, K.; Adachi, M. Langmuir 2004, 20, 7837.
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Figure 2. Transmission electron micrographs of two representative gold nanowires. AFM measurements were performed on a Nanoscope IV Multimode scanning probe microscope with a Si3N4 tip, using the “tapping mode”.
Results
Figure 1. Scanning electron micrographs of the gold nanowires. Parts a and b show wires deposited by dip-coating onto a silicon wafer premodified by 3-APTES. (c) Wires deposited on a silicon wafer by drying a droplet of the reaction mixture. Synthesis. A total of 100 mL of a 0.01 wt % solution of HAuCl4 in Milli-Q water was heated to 100 °C in a 250 mL, round-bottom, two-neck flask equipped with a coiled condenser. A total of 2.5 mL of an aqueous 10-3 mol/L solution of MSA was pre-neutralized by a stoichiometric amount of NaOH and added to the boiling solution using a 25 mL buret, leading to a molar ratio of 10 to 1 between HAuCl4 and MSA. The reaction mixture was stirred at 700 rpm, and within 10 min of adding the MSA, the mixture changed from pale yellow to an almost gray color. The mixture was kept at boiling temperature and stirred for 10 more min. Subsequently, the heater was removed and the solution was left under stirring to cool to ambient temperature. Different molar ratios were obtained accordingly by adjusting the amount of MSA solution added. Sample Preparation. Samples for scanning electron microscopy (SEM; Figure 1a,b) and atomic force microscopy (AFM; Figure 6) were prepared by immersion of silicon wafers premodified with 3-aminopropyl-triethoxysilane (3APTES)24 into the reaction mixture (after cooling) for 2 h. Samples were then rinsed carefully with Milli-Q water to remove all weakly bound species. This procedure allows the electrostatic binding of negatively charged species from the reaction mixture onto the positively charged substrates. The sample shown in Figure 1c was prepared by drying a droplet of the reaction mixture on a silicon wafer under ambient conditions. For all transmission electron microscopy (TEM) observations, the reaction mixture was diluted 1:3 with Milli-Q water, and a droplet was placed on an amorphous carbon coated, copper-TEM grid. The major part of the liquid was blotted off with the help of filter paper, then the sample was dried under ambient conditions. Microscopy. The SEM images were recorded using a LEO 1530 field-emission scanning electron microscope (LEO Electron Microscopy, Ltd.). Bright field and dark field TEM and diffraction were measured with a Philips CM 12 operated at an acceleration voltage of 120 kV. High angle annular dark field scanning transmission electron microscopy (HAADF STEM) images were taken on a JEOL JEM 2100F, operated at an acceleration voltage of 200 kV. (24) Wang, J.; Zhu, T.; Song, J. Q.; Liu, Z. F. Thin Solid Films 1998, 327, 591.
The morphology of the reaction product was investigated by first selectively adsorbing some of its content onto silicon wafers modified with a monolayer of 3APTES.24 That is, once the reaction had completed and the mixture had cooled, modified silicon wafers were immersed into the mixture for 2 h. The positive charge arising from the 3APTES monolayer promotes electrostatic deposition of negatively charged species. For comparison, a droplet of the solution was dried on a Si wafer to view the entire solid-phase content. SEM (Figure 1) revealed single nanowires with lengths of several micrometers plus some particles with aspect ratios close to one with round, triangular, or polyhedral shapes. Because these species adsorb on a positively charged surface in solution they appear to be negatively charged. Some of the wires were found to end with a large ribbonlike structure terminated by thin polyhedral platelets, often triangles (compare Figure 1c). The polyhedral shape suggests monocrystallinity. The thin portions of the wires appear curly and twisted. The SEM micrograph in Figure 1c indicates that a large amount of nanowires are present in solution, suggesting a satisfying reaction yield. Again, most of the wires possess a crystalline plate at one end, which usually has a triangular shape, sometimes exceeding 100 nm in size. TEM of wire samples prepared as described above, Figure 2a, shows an object with strongly varying contrast along an apparently cylindrical wire with domains of alternating gray tones. This alternating contrast along the individual wire intimates a polycrystalline object. The wire is composed of individual crystallites having different crystalline orientations with regard to the incident electron beam. Accordingly, some of the crystallites are oriented such that Bragg or diffraction-contrast appears in the bright field micrograph and the respective crystallite areas appear darker. Furthermore, this object possesses an approximate diameter of 15 nm and appears curly with a loop extending away from the surface. A second TEM micrograph, Figure 2b, shows a nanowire extending from the corner of a triangularly shaped object. Here, the gradually varying contrast observed within the triangle can be explained in terms of a monocrystalline object with some internal tension. One can see a ribbon with decreasing width extending out of this triangle. This ribbon appears monocrystalline like the triangle while the thinner end appears to have a similar polycrystalline morphology as shown in Figure 2a. Complementary experimental
One-Step Synthesis of Gold Nanowires in Solution
Figure 3. Electron diffraction pattern of a gold nanowire (a) and the radial integration of this pattern (b).
techniques were employed to thoroughly investigate the structure of the wires, ribbons, and platelets because Figure 2, alone, would allow alternative interpretations of the observed contrast. The electron diffraction pattern, Figure 3a, was obtained from the same wire as that shown in Figure 2a. Radial integration of the diffraction pattern (Figure 3b) shows individual diffraction peaks corresponding to the (111), (200), (220), and (311) reflections of the gold lattice structure. The emergence of multiple peaks verifies the polycrystalline nature of the wires. To further investigate the structural nature of these nanowires, TEM dark field imaging was performed. This imaging method uses only a fraction of the diffracted electrons in the image formation. These images only show crystallites of a particular orientation with respect to the incident electron beam and the setting of the objective aperture of the TEM. By moving the objective aperture azimuthally around the central beam, different reflections (Figure 3a) and, therefore, different crystallites contribute to the image. A gallery of dark field images of a gold nanowire taken at different azimuthal positions of the objective aperture is shown in Figure 4. It is clearly visible that different parts of the wire appear bright while the angles change, thus supporting the interpretation that the wires are composed of crystals with different orientations, forming a fully connected, polycrystalline gold object. This observation is in agreement with the interpretation of the contrast observed in Figure 2a. Importantly, while the bright field transmission electron micrograph shown in Figure 2 could be explained as well in terms of gold grains fused together by some undefined reaction product, the dark field TEM clearly supports an interpretation of connected, differently oriented grains. The synthesized nanowires were also characterized with STEM HAADF.25 This technique is highly sensitive to (25) Otten, M. J. Electron. Mirosc. Technol. 1991, 17, 221-230.
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atomic number contrast, generated by variations in the total electron density, and was used to investigate the homogeneity of the wires with the results presented in Figure 5. The wire shows a constant signal, indicating homogeneous elemental composition. This unambiguously demonstrates that the nanowires are formed by tightly fused gold crystals. AFM (Figure 6) yields information about the threedimensional shape of the nanowires. The topographical image depicts a triangular structure which is flat within the experimental uncertainty. From the triangle a wire emerges which appears to be ribbonlike until its diameter falls below roughly 50 nm. The rest of the structure is thinner and can be regarded as a true wire, showing several loops extending away from the surface. Cross sections orthogonal to the wire yield apparent diameters between 15 and 40 nm. The AFM phase image (Figure 3b) shows a constant signal along the entire object, pointing toward a homogeneous surface chemistry. Additional information about the nature of the wires was obtained in a single-wire conductivity study, which is published elsewhere.26 The essential result is a specific conductivity of roughly 2 × 107 S/m, which is in agreement with that of bulk gold (4.3 × 107 S/m), thus giving the most convincing evidence that these structures are continuous gold structures and can be used as wires in electronic applications. In addition, it was found that the wires are flexible enough to be deformed without rupture, allowing for the fabrication of simple structures such as conductor path elements in nanoelectronic circuitry without breaking. Discussion From the experimental evidence given above, we conclude that the structures obtained in our synthesis are fully connected gold objects. On one hand, thin wires with diameters below about 40 nm that are polycrystalline but fully connected were observed. On the other hand, apparently monocrystalline triangular structures as well as ribbonlike wires with a diameter above about 40 nm are seen. They appear to be negatively charged, and their surface chemistry appears homogeneous within our experimental abilities. The formation of such structurally heterogeneous objects requires a highly complex reaction mechanism that we were unable to fully discern. Current literature provides pieces of information toward an understanding of the wire formation as will be discussed in the following. The chemical reaction path of the HAuCl4-MSA system is expected to be dependent on the initial molar ratios and the reaction parameters, such as solvent, pH, and temperature. When the amount of MSA is in excess compared to HAuCl4 and reacted in water at nearly neutral pH and room temperature, the main product is clusters of a Au(I)-MSA complex with 1 to 10 gold atoms with the MSA moderately oxidized to disulfide.27,28 Negishi and Tsukuda demonstrated that 2,3-dimercaptosuccinic acid can also reduce HAuCl4 to Au(0) at ambient temperatures with the formation of small gold nanoparticles ranging from 1 to 3 nm in size.29 It must be stressed that all synthesis was performed at 100 °C. Under these conditions, wires were formed only when the molar ratio between HAuCl4 (26) Wilms, M.; Conrad, J.; Vasilev, K.; Kreiter, M.; Wegner, G. Appl. Surf. Sci. 2004, 238, 490. (27) Brown, H.; Paton, M. D.; Smith, W. E. Inorg. Chim. Acta 1982, 66, L51. (28) Nomiya, K.; Yokoyama, H.; Nagano, H.; Oda, M.; Sakuma, S. Bull. Chem. Soc. Jpn. 1995, 68, 287. (29) Negishi, Y.; Tsukuda, T. J. Am. Chem. Soc. 2003, 125, 4046.
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Figure 4. Dark field TEM images of a gold nanowire taken by selecting reflections at different azimuthal angles of the objective aperture.
Figure 5. STEM HAADF dark field image of a gold nanowire.
and MSA was 10:1, whereas when higher MSA concentrations are employed gold nanoparticles are formed the size of which is strongly dependent on the concentrations of HAuCl4 and MSA.30 Elevated temperatures promote extensive oxidation of MSA; therefore, in contrast to earlier studies at room temperature, not only S-S formation but
also the cleavage of S-C and C-C bonds must be considered. It is anticipated that this extensive oxidation is especially important with the low MSA concentration used here. In all known syntheses of gold nanoparticles by chemical means there is a sufficient amount of capping agent in the reaction mixture which covers the surface of the particles and thus prevents aggregation and precipitation. In the synthesis of gold nanowires described in this paper the amount of MSA in the reaction mixture is very small (10 times smaller than the amount of HAuCl4). This may lead to insufficient passivation of the gold particles and, in turn, the formation of nanowires.23 The observation of two different morphologies (flat monocrystalline triangles and ribbons and also thin curly wires) implies different growth mechanisms. It seems that the monocrystalline portions grow by attachment of very small gold clusters which still have the ability to fully rearrange, aligning with the existing crystal lattice. The long curly wires, on the other hand, are polycrystalline, and their formation may involve fusion of larger gold nanoparticles.31 Which mechanism prevents simple aggregation and favors the formation of wires remains to be investigated.
Figure 6. AFM images displaying topology (a) and phase (b) of a single nanowire. The z scale in part a is 100 nm.
One-Step Synthesis of Gold Nanowires in Solution
Conclusion In conclusion, the synthesis of long gold nanowires in aqueous solution is demonstrated. The method is simple and does not require the use of any additional surfactants. The wires possess a curly structure, a length in the range of micrometers, and a diameter down to 15 nm. The SEM, TEM (dark and bright field), and HAADF STEM analyses demonstrate fully connected polycrystalline structures. The inherent curvature of these wires leads to a high flexibility which is a prerequisite for their manipulation. In addition, real “bottom up” wiring of electronic circuits will require the directed growth of the wires along nontrivial paths. For this goal, inherently curved wires (30) Vasilev, K.; Zhu, T.; Glasser, G.; Knoll, W.; Kreiter, M. In preparation. (31) Biggs, S.; Mulvaney, P.; Zukoski, C. F.; Grieser, F. J. Am. Chem. Soc. 1994, 116, 9150.
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which are still fully electrically conductive may prove technologically important because they should allow for wiring in nontrivial geometries despite being less welldefined than the straight single-crystalline wires obtained with other methods.20,21 Additionally, the good yield of the wires may be useful for conductive polymer composites in the electronic industry. Acknowledgment. We thank Gunnar Glasser for the scanning electron images, Ru¨diger Berger for the AFM images, and JEOL, Inc., for providing the possibility for HAADF STEM measurements on their microscope. We acknowledge financial support from the Bundesministerium fu¨r Bildung und Forschung, BMBF (03N8702 and 03N6500), from the German-Israeli Project on Futureoriented Topics (DIP, D3.1), and from the Deutsche Forschungsgemeinschaft, DFG (SPP 1072). LA052354F
