17158
2007, 111, 17158-17162 Published on Web 10/25/2007
General Method for Extended Metal Nanowire Synthesis: Ethanol Induced Self-Assembly Jianping Xie,† Qingbo Zhang,§ Jim Yang Lee,*,†,§ and Daniel I. C. Wang†,‡ Singapore-MIT Alliance, 4 Engineering DriVe 3, National UniVersity of Singapore, Singapore 117576, Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts AVe., Cambridge, Massachusetts 02139, and Department of Chemical and Biomolecular Engineering, National UniVersity of Singapore, 10 Kent Ridge Crescent, Singapore 119260 ReceiVed: August 24, 2007; In Final Form: September 21, 2007
A new template-free method to form extended nanowires from the spontaneous self-assembly and fusion of discrete metal nanoparticles has been developed. The key strategy was the use of a weak polar solvent (ethanol) to suitably destabilize the metal nanoparticles in a hydrosol. The number of nanoparticles that were assembled and fused into an extended crystalline structure was found to depend on the dielectric constant of the solvent system. Three distinctive fusion modes consistent with HRTEM analysis of the products were hypothesized, which could satisfactorily explain the formation and the structures of the nanowires. This method of nanowire synthesis is simple, scalable, and capable of producing nanowires with virtually “clean” surfaces. The synthesis method can also be extended to other metals, such as palladium and platinum.
Rod- and wire-like nanocrystals of metals with tunable optical and electronic properties are valuable to the fabrication of a new generation of electronic, sensing, and photonic devices.1 Most metal nanorods and nanowires synthesized to date were based on template-assisted methodologies where “hard templates” such as wires,2 tubes,3 pores,4 and step-edges5 or “soft templates” such as organic surfactants,6 block copolymers,7 and biomacromolecules including DNA8 and viruses9 were used to constraint the growth of the metal into desirable geometrical forms. Template-assisted syntheses are multistep operations and require substantial investments in the cost of the templates. “Soft templates”, in particular, are known to be difficult to remove from the nanoparticle surface, often requiring multiple washings under harsh conditions. There is therefore a strong motivation to develop template-free alternatives for the synthesis of nanorods/nanowires. Low-dimensional nanomaterials may be assembled into extended structures by nanoparticle-nanoparticle interactions. This has been demonstrated in recent years where large crystalline structures were formed by the self-assembly of primary nanoparticles under the attractive forces of dipoledipole or van der Waals’ interactions.1b,10 A good example is the formation of an anisotropic chain-like structure of maghemite (γ-Fe2O3) nanoparticles in terrestrial magnetotactic bacteria11 by the cooperative alignment of the permanent magnetic dipole moments of individual particles. While dipole-dipole or van der Waals’ interactions also operate in semiconductor and metal nanoparticles, strong electrostatic or steric repulsion from adsorbed stabilizers on the nanoparticle surface in a stable * To whom correspondence should be addressed. E-mail: cheleejy@ nus.edu.sg. † Singapore-MIT Alliance. ‡ Massachusetts Institute of Technology. § Department of Chemical and Biomolecular Engineering, National University of Singapore.
10.1021/jp0768120 CCC: $37.00
colloid can reduce the attraction between nanoparticles and inhibit the self-assembly process. Recently, Tang et al.12 developed a strategy to reduce interparticle repulsion through the partial removal of surface stabilizers, and CdTe nanowires were successfully formed from the self-assembly and fusion of primary CdTe nanoparticles. A similar self-assembly and fusion process may also occur in noble metal nanoparticles, if the strong electrostatic or steric repulsion between nanoparticles can be suitably lowered. Herein, we present a new method to fabricate extended metal nanowires from the spontaneous self-assembly and fusion of discrete metal nanoparticles. The key strategy is the introduction of a weak polarity solvent (ethanol) to modulate the electrostatic repulsions between metal nanoparticles in an aqueous solution. This method of nanowire synthesis is simple and scalable and can produce extended nanowires with nearly “clean” surfaces. The number of nanoparticles that are fused in an extended crystal can be controlled to some extent by the ethanol to water ratio (by volume, modulation of the dielectric constant of the solvent mixture). The self-assembly and fusion modes were examined experimentally by high-resolution TEM and discussed. In a typical synthesis, HAuCl4 aqueous solution (25 µL, 20 mM), sodium citrate aqueous solution (25 µL, 0.1 M), ethanol (3 mL), and ice-cold NaBH4 aqueous solution (100 µL, 0.1M) were added sequentially to water (2 mL) under vigorous stirring. The color of the mixture changed from light yellow to pink within seconds of NaBH4 addition and was light black in approximately 10 s. The solution was stirred for another 10 min, and the product was recovered by centrifugation at low speeds (3000 rpm for 10 min). Figure 1A is a typical TEM image of the Au nanocrystals in the precipitate, showing an extensive network of Au nanowires approximately 5 nm in diameters as the primary product. The high-resolution image in the inset shows changes in the orientation of the lattice fringes and randomly interspersed single-crystalline and poly crystalline © 2007 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17159
Figure 1. TEM images of extended nanowires of (A) Au, (B) Pd, and (C) Pt. Inset are high-resolution TEM images showing changes in the orientation of lattice fringes along the length of the nanowires, in particular the single-crystalline (SC), polycrystalline (PC) sections, and “amorphous” sections (AM) (D) UV-vis spectra of gold nanospheres (spectrum 1) and extended nanowires (spectrum 2). The inset shows a digital picture of the colloidal solution of gold nanospheres (1) and nanowires (2).
sections along the length of the extended nanowire. There are also sections where lattice fringes were absent (see inset in Figure 1A) and indicative of a more disordered “amorphous” gold structure. This particular method of synthesis of nanowires in ethanol-water mixtures could also be applied to other noble metals, for example palladium (Figure 1B) and platinum (Figure 1C). In contrast to the characteristic red color of a solution of spherical Au nanoparticles, the solution of extended Au nanowires was light black in color, as shown in the inset of Figure 1D. Although the absorption spectrum of Au nanospheres (spectrum 1) is dominated by a single surface plasmon resonance (SPR) band at ca. 506 nm, the absorption spectrum of extended Au nanowires (spectrum 2) shows very broad absorption in the near-IR region, typical of the longitudinal plasmon resonance of nanorods/wires,13 in addition to the SPR absorption at ca. 508 nm. Variations in the length and morphology (e.g., loops and junctions) of the nanowires in the extended network could give rise to the observed band broadening of the longitudinal plasmon resonance. In order to understand the nanowire growth process, we isolated the intermediate product formed 5 s after the NaBH4 addition when the solution was pink in color. We found the presence of mainly spherical nanoparticles (3-5 nm) and some peanut-shaped nanocrystals (Figure S1) apparently formed upon the aggregation and fusion of two Au nanoparticles. The peanutshaped structure is a simpler form of the pearl-necklace (extended peanut-shaped) intermediates observed by Tang et al. in their CdTe system.12 A control experiment in which the addition sequence of ethanol and sodium borohydride was
reversed so as to preform the Au nanoparticles before ethanol addition, also led to similar nanowire structures (Figure S2). This is an indication that the formation of extended nanowires did not proceed through “point-initiated” crystal growth (general seeded process), where one seed served as the nucleus for subsequent growth by atom addition,6 but rather by the selfassembly and fusion (recrystallization) of multiple nanoparticles into a single extended crystal. Ethanol played a crucial role in this process. A control experiment without ethanol only produced Au nanospheres (3-5 nm, Figure S3). It is well-known that Au nanospheres synthesized in the presence of citrate acquire a negative surface charge because of citrate ion adsorption. The strong electrostatic repulsion between similarly charged nanoparticles has been used advantageously to inhibit particle aggregation in the aqueous solution.14 However, the charge on the Au nanoparticles can be altered by changing the polarity of the solvent. Since the polarity of ethanol (dielectric constant, ) 25.315) is significantly lower than that of water ( ) 80.1), the addition of ethanol to the aqueous solution should decrease the charge on the Au nanoparticles. It would then be possible for dipole-dipole or van der Waals’ interactions between individual Au nanoparticles to induce self-assembly and fusion, forming nanowires based on the anisotropy in these interactions. Similar extended Au nanowires were also formed using propanol ( ) 20.2) instead of ethanol (Figure S4). The citrate ions were found to be a nonessential component as extended nanowires were still formed in their absence (Figure S5). It has been reported that “uncapped” Au nanoparticles formed upon the NaBH4 reduction of gold precursor salts in aqueous solution, were actually
17160 J. Phys. Chem. C, Vol. 111, No. 46, 2007
Letters
Figure 2. TEM images of Au nanocrystals formed under different reaction conditions: (A) ethanol/water ) 0/5; (B) ethanol/water ) 2/3; (C) ethanol/water ) 1/1, and (D) ethanol/water ) 3/2. The inset in (A) is the HRTEM image of a typical Au nanosphere showing its single-crystalline TO geometry. Insets in (B and C) are histograms of the number of constituent nanoparticles in an extended nanocrystal.
stabilized by small negatively charged inorganic ions such as the oxidized form of borohydride (BO2-) and Cl-, and could be stored for weeks without changes.16 The cleaner surface of the “uncapped” Au nanowires synthesized without citrate presence may provide an easier platform for postsynthesis surface modifications with suitable functional ligands. The charge on the Au nanoparticles is dependent on the dielectric constant of the ethanol-water mixture; hence, the extent of aggregation or the number of nanoparticles in a fused particle should be modifiable by the ethanol to water ratio. Solvents with different ethanol to water ratios (Rethanol/water ) 0/5, 2/3, 1/1, 3/2, and 5/0; with corresponding dielectric constants17 of mixture ) 80, 58, 52, 47, and 25) were used to produce the extended Au nanocrystals, while keeping the other experimental conditions the same. Products were collected after 10 min of reaction, stabilized by adding an alkylamine (octadecylamine), and transferred to an organic phase (toluene) by a technique previously developed in our group.18 The treatment was used to stabilize the Au nanocrystals sufficiently enough for TEM characterization. The structure of the nanocrystals was not altered by this treatment, as confirmed by the similarity between the UV-vis spectra of Au nanocrystals in aqueous solution (untreated) and in toluene (treated). Figure 2 shows TEM images of Au nanocrystals formed under different ethanol-water ratios. Spherical Au nanoparticles with an average diameter of 3.7 nm (Figure 2A) were the sole product in the water-only system (R ) 0/5). The high-resolution image of a representative Au nanoparticle in Figure 2A inset shows that it was a single-crystal with truncated octahedral (TO) geometry. At an ethanol/water ratio of 2/3 (Figure 2B), peanut-shaped Au
nanocrystals with an aspect ratio of two (length/diameter ) ∼10/5 nm, henceforth labeled as nanopeanuts_2) were the main product (∼78% yield, from counting 500 particles, see histogram in the inset) with a minor presence of nanospheres (6%) and some nanopeanuts with higher aspect ratios. When the ethanol/ water ratio was increased to 1/1 (Figure 2C), the predominant product was nanopeanuts with an aspect ratio of three (∼68% yield, see the inset for histogram), with nanospheres and nanopeanuts of other aspect ratios making up the balance. Continued increase in the ethanol/water ratio to 3/2 produced nanowires with aspect ratios above three (Figure 2D). The nanowires could not be transferred into toluene; instead they clustered at the water-toluene interface. On the other hand Au nanoparticles quickly settled into a macroscopic black solid when pure ethanol was used as the solvent. It should be mentioned that some small segments were found in Figure 2D, which were absent in the TEM image of nanowires without alkylamine protection (Figure 1A). Although lattice fringes were periodically absent in the pristine citrate-capped nanowires (inset of Figure 1A), fewer fringe-free segments were found in the nanowires after alkylamine stabilization (Figure 2D and the corresponding HRTEM images (data not shown)). The fringefree segments should correspond to a highly disordered “amorphous” gold region. There are two possible explanations for the absence of small segments in the TEM image of the pristine nanowires. The first could be attributed to the lack of longterm stability of the citrate-capped nanowires which grew further during the sample preparation for TEM (the centrifugation process and/or drying process). This did not occur for nanowires under the strong protection of alkylamine molecules. The second
Letters
J. Phys. Chem. C, Vol. 111, No. 46, 2007 17161
Figure 3. HRTEM images of the three fusion modes for nanopeanuts_2: (A) (100) with (100), (B) (111) with (111), and (C) (100) with (111). (D and E) HRTEM images of nanopeanuts_3 and (F) nanowires. All scale bars are 5 nm.
reason could be attributed to a combination of amine etching and Ostwald ripening, which disintegrated the segments by delinking the highly disordered “amorphous” gold regions. The amine etching process is a working hypothesis which will be verified in our future work. To understand the aggregation and fusion of nanoparticles that led to nanowire formation, the crystal structures of nanopeanuts were examined by high-resolution TEM. Assuming the primary nanocrystals adopted the TO geometry with welldefined planar facets,19 the assembly and fusion of tiny TO crystals would give rise to the formation of a curvature, as corroborated by the formation of peanut-shaped nanocrystals. The TO crystals have six (100) planes and eight (111) planes. Three fusion modes are possible in a two nanoparticles assembly: (100) with (100), (111) with (111), and (100) with (111). All of these fusion modes were experimentally detected, as shown by the HRTEM images in Figure 3A-C. Singlecrystalline nanopeanuts were formed when two perfectly matched surfaces were fused together: a (100) plane with another (100) plane (Figure 3A) or a (111) plane with another (111) plane (Figure 3B). Unlike these perfect fusions, a (100) plane attached to a (111) plane could only generate a polycrystalline nanopeanut (Figure 3C) and lattice distortion could be clearly seen in the interfacial regions. The interparticle void could also be partially filled by depositing gold atoms or by the recrystallization of a highly active surface. These three fusion modes were also prevalent in the formation of nanowires and nanopeanuts with other aspect ratios, such as nanopeanuts_3 (Figure 3D,E) and extended nanowires (Figure 3F). In summary, extended Au nanowires were formed by a simple procedure which promoted the self-assembly and fusion of primary single crystalline nanoparticles at room temperature. This synthesis procedure could be extended to other noble metal nanoparticles. The number of nanoparticles which could be assembled into an extended crystal could be controlled by
adjusting the dielectric constant of the ethanol-water mixture. Three fusion modes were hypothesized and detected by HRTEM analysis, which could also rationalize the nanowire structures and their formation. The protocol and products are important not only because they provide a simple method of production for extended nanowires, but also because they highlighted and exemplified that the inter-nanoparticle interactions (dipoledipole or van der Waals’) could be used to advantage for the creation of extended metal nanostructures. Acknowledgment. J.X. acknowledges the Singapore-MIT Alliance (SMA) program for his research scholarship. Supporting Information Available: Experimental section and TEM/FESEM images. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. AdV. Mater. 2003, 15, 353. (b) Tang, Z.; Kotov, N. A. AdV. Mater. 2005, 17, 951. (c) Liz-Marzan, L. M. Langmuir 2006, 22, 32. (d) Eustis, S.; El-Sayed, M. A. Chem. Soc. ReV. 2006, 35, 209. (2) (a) Yin, Y.; Lu, Y.; Sun, Y.; Xia, Y. Nano Lett. 2002, 2, 427. (b) Mayya, K. S.; Gittins, D. I.; Dibaj, A. M.; Caruso, F. Nano Lett. 2001, 1, 727. (c) Sun, X.; Li, Y. AdV. Mater. 2005, 17, 2626. (3) (a) Ajayan, P. M.; Iijima, S. Nature 1993, 361, 333. (b) Zhang, Y.; Dai, H. Appl. Phys. Lett. 2000, 77, 3015. (c) Guan, L.; Shi, Z.; Li, H.; You, L.; Gu, Z. Chem. Commun. 2004, 1988. (4) (a) Kang, H.; Jun, Y.; Park, J.; Lee, K.; Cheon, J. Chem. Mater. 2000, 12, 3530. (b) Yang, C.; Sheu, H.; Chao, K. AdV. Funct. Mater. 2002, 12, 143. (5) (a) Zach, M. P.; Ng, K. H.; Penner, R. M. Science 2000, 290, 2120. (b) Young, N. P.; Palfreyman, J.; Li, Z. Small 2006, 2, 71. (c) Ji, X.; Banks, C. E.; Xi, W.; Wilkins, S. J.; Compton, R. G. J. Phys. Chem. B 2006, 110, 22306. (6) (a) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (b) Nikoobakht, B.; EI-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (c) Murphy, C. J.; Gole, A. M.; Hunyadi, S. E.; Orendorff, C. J. Inorg. Chem. 2006, 45, 7544.
17162 J. Phys. Chem. C, Vol. 111, No. 46, 2007 (7) (a) Zhang, D.; Qi, L.; Ma, J.; Cheng, H. Chem. Mater. 2001, 13, 2753. (b) Cornelissen, J. J. L. M.; Heerbeek, R.; Kamer, P. C. J.; Reek, J. N. H.; Sommerdijk, N. A. J. M.; Nolte, R. J. M. AdV. Mater. 2002, 14, 489. (8) (a) Keren, K.; Krueger, M.; Gilad, R.; Ben-Yoseph, G.; Sivan, U.; Braun, E. Science 2002, 297, 72. (b) Patolsky, F.; Weizmann, Y.; Lioubashevski, O.; Willner, I. Angew. Chem., Int. Ed. 2002, 41, 2323. (9) Knez, M.; Bittner, A. M.; Boes, F.; Wege, C.; Jeske, H.; Maiss, E.; Kern, K. Nano Lett. 2003, 3, 1079. (10) (a) Banfield, J. F.; Welch, S. A.; Zhang, H.; Ebert, T. T.; Penn, R. L. Science 2000, 289, 751. (b) Tanase, M.; Bauer, L. A.; Hultgren, A.; Silevitch, D. M.; Sun, L.; Reich, D. H.; Searson, P. C.; Meyer, G. J. Nano Lett. 2001, 1, 155. (c) Niederberger, M.; Colfen, H. Phys. Chem. Chem. Phys. 2006, 8, 3271. (11) Dunin-Borkowski, R. R.; McCartney, M. R.; Posfai, M.; Frankel, R. B.; Bazylinski, D. A.; Buseck, P. R. Eur. J. Mineral. 2001, 13, 671.
Letters (12) Tang, Z.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (13) (a) Link, S.; EI-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410. (b) Nikoobakht, B.; EI-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (14) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (15) CRC Handbook of Chemistry and Physics, 87th ed.; Electronic version via Chapman & Hall/CRC Press: Boca Raton, FL, 20062007. (16) Gole, A.; Murphy, C. J. Chem. Mater. 2004, 16, 3633. (17) The dielectric constant of the ethanol-water solvent was calculated based on the equation: (mixture) ) % water × water + % ethanol × ethanol. (18) Yang, J.; Lee, J. Y.; Too, H. P. J. Phys. Chem. B 2005, 109, 19208. (19) Whetten, R. L.; Khoury, J. T.; Alvarez, M. M.; Murthy, S.; Vezmar, I.; Wang, Z. L.; Stephens, P. W.; Cleveland, C. L.; Luedtke, W. D.; Landman, U. AdV. Mater. 1996, 8, 428.
