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Templateless Room-Temperature Assembly of Nanowire Networks from Nanoparticles G. Ramanath,*,† J. D’Arcy-Gall,† T. Maddanimath,†,‡ A. V. Ellis,† P. G. Ganesan,† R. Goswami,† A. Kumar,† and K. Vijayamohanan‡ Department of Materials Science & Engineering, Rensselaer Polytechnic Institute, Troy, New York 12180, and National Chemical Laboratory, Pune, India Received January 26, 2004. In Final Form: March 23, 2004 We demonstrate a new, room-temperature approach to assemble two-dimensional and three-dimensional networks of gold nanowires by agitating nanoparticles in a toluene-aqueous mixture, without the use of templates. The nanowires have a uniform diameter of about 5 nm and consist of coalesced face-centered cubic nanocrystals. Toluene molecules passivate the gold surfaces during nanoparticle coalescence, rendering the nanowires hydrophobic and enabling their transfer into the toluene layer. Such templateless lowtemperature assembly of mesostructures from nanoscale building blocks open up new possibilities for creating porous self-supporting nanocatalysts, nanowires for device interconnection, and low-density highstrength nanofillers for composites.
Introduction One-dimensional nanostructures (e.g., nanowires and nanotubes) have unusual properties because of anisotropic confinement of electronic states and are attractive for potential applications such as switches, sensors, and interconnects for future device systems. Nanowires of several materials have been fabricated using a variety of approaches including chemical vapor deposition,1 laserassisted synthesis,2 electron beam lithography,3 electrochemical deposition,4 and chemical and structural templating.5 Recently, there has been a great deal of interest to assemble nanoscale building blocks into larger scale structures via templateless low-temperature wet-chemical approaches as a result of the enormous process flexibility offered by these methods to controllably create a variety of architectures.6 For example, nanowires can now be synthesized using micelles7 and from surfactant-stabilized biphasic microemulsions,8 stringing nanoparticles with molecular connectors,9 and room-temperature coalescence of molecularly linked nanoparticles.10 Here we report a simple, templateless room-temperature approach to synthesize interwoven mesoscale porous networks of gold nanowires by agitation of nanoparticles in biphasic mixtures. While previous works have reported * Corresponding author. E-mail:
[email protected]. † Rensselaer Polytechnic Institute. ‡ National Chemical Laboratory. (1) Huang, M. H.; Mao, S.; Feick, H.; Yan, H.; Wu, Y.; Kind, H.; Weber, E.; Russo, R.; Yang, P. Science 2001, 292, 1897-1899. (2) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617-620. (3) Durkan, C.; Welland, M. E. Phys. Rev. B 2000, 61, 14215-14218. (4) Li, C. Z.; Tao, N. J. Appl. Phys. Lett. 1998, 72, 894-896. (5) See for example Zhang, Y.; Dai, H. Appl. Phys. Lett. 2000, 77, 3015-3017. Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775-777. Natelson, D.; Willett, R. L.; West, K. W.; Pfeiffer, L. N. Appl. Phys. Lett. 2000, 77, 1991-1993. Sugawara, A.; Coyle, T.; Hembree, G. G.; Scheinfein, M. R. Appl. Phys. Lett. 1997, 70, 10431045. (6) Caswell, K. K.; Bender, C. M.; Murphy, C. J. Nano Lett. 2003, 3, 667-669. (7) Lisiecki, I.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1996, 100, 4160-4166. (8) Rees, G. D.; Evans-Gowing, R.; Hammond, S. J.; Robinson, B. H. Langmuir 1999, 15, 1993-2002. (9) Lu, Q.; Gao, F.; Zhao, D. Nano Lett. 2002, 2, 725-728. (10) Wang, X.; Li, Y. J. Am. Chem. Soc. 2002, 124, 2880-2881.
Figure 1. Synthesis scheme to form nanowire networks at room temperature. The reduction of a HAuCl4 aqueous solution (pale yellow) with NaBH4 results in a wine red color colloidal gold solution. The Au hydrosol is then mixed with toluene and agitated leading to the spontaneous formation of black-colored nanowire networks (depicted here in blue for clarity) at the aqueous-toluene interface.
the formation of one-dimensional11 and two-dimensional12 assemblies and liquidlike membranes comprising nanoparticles,13 here we demonstrate nanowire formation without the use of molecular capping agents. Extensions of similar strategies could offer potential ways to tailor the nanowire aspect ratio and network connectivity and open up new routes for forming porous mesoarchitectures14 (11) Wyrwa, D.; Beyer, N.; Schmid, G. Nano Lett. 2002, 2, 419-421. (12) Schwartz, H.; Harel, Y.; Efrima, S. Langmuir 2001, 17, 38843892. (13) Kumar, A.; Mandal, S.; Mathew, S. P.; Selvakannan, P. R.; Mandale, A. B.; Chaudhari, R. V.; Sastry, M. Langmuir 2002, 18, 64786483.
10.1021/la0497649 CCC: $27.50 © 2004 American Chemical Society Published on Web 05/21/2004
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Figure 2. Representative TEM micrographs and diffraction pattern obtained from gold nanowire networks dispersed on a carboncoated TEM grid, (a) a two-dimensional network of nanowires, (b) a higher magnification view from a different area, (c) a cocoonshaped three-dimensional nanocage comprised of interwoven Au nanowires, and (d) a selected-area electron diffraction pattern. (e) A lattice-resolution image of a single polycrystalline nanowire. Red arrows point to grain boundaries.
that could serve as self-supporting nanocatalysts,15 lowdensity high-strength foams for structural reinforcement,16 and interconnect wiring for devices. Experimental Section The scheme for forming nanowires is shown in Figure 1. In a typical experiment, gold hydrosol was synthesized by reducing 500 mL of 1 mM auric chloride (HAuCl4) with equal parts of 7.4 mM sodium borohydride (NaBH4), added drop-by-drop over 1 h at 5 °C. No organic capping agents were used, unlike conventional synthesis methods17 where short molecules (e.g., octanethiol) or polymers are added to transfer Au3+ ions into an organic solvent layer prior to reduction. The initially pale yellow solution transformed into a wine red color as a result of nanoparticle formation. The strong surface plasmon peak at 518 nm (see Figure 3) in as-prepared hydrosols indicates an average nanoparticle size of ∼5 nm.18 The hydrosol was mixed with toluene in a 1:2 ratio. Vigorous agitation of this mixture (e.g., with a magnetic stirrer at 600 rpm for 10 min) depletes the gold nanoparticles from the aqueous layer and results in the spontaneous formation of black-colored agglomerates that migrate to the liquid-liquid interface. We can also obtain similar results by agitating HAuCl4 and toluene during nanoparticle synthesis. These nanowires were characterized with various techniques such as transmission electron (14) Pileni, M. P. Nat. Mater. 2003, 2, 145-150. (15) Rolison, D. R. Science 2003, 299, 1698-1701. (16) Ko¨rner, C. R.; Singer, F. Adv. Eng. Mater. 2000, 2, 159-165. (17) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425-5429. (18) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 42124217.
microscopy (TEM), UV-vis spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and thermogravemetric analysis (TGA). FTIR measurements of the gold nanowire were carried out on a Perkin-Elmer, Paragon 1000 spectrometer in the transmission mode at an 80° incident beam angle to the surface normal. UVvis spectroscopy measurements of all the samples were performed in a Varian Cary 500 Scan UV-vis-NIR spectrophotometer operated at a resolution of 2 nm. Samples for TEM analysis were prepared by lifting the nanowire at the water-toluene interface on carbon-coated copper TEM grids and allowing to dry prior to measurement. TEM measurements were carried out in a CM 12 microscope operated at an accelerating voltage of 120 kV. Latticeresolution images of the nanowires were obtained in a JEOL 2010 instrument operated at 200 kV. To understand the strength of interaction of toluene with the nanowire, we carried out TGA measurements using a Mettler Toledo TGA/SDTA851e instrument by heating the nanowire samples from room temperature to 900 °C at 10 °C/min under 50 mL/min N2 flow.
Results and Discussion The nanowires obtained at the liquid-liquid interface were lifted on a carbon-coated grid and characterized by TEM. Figure 2 showing typical TEM images reveals that the agglomerated structures at the toluene-water interface are two-dimensional networks of gold nanowires with a uniform diameter of ∼5 nm (Figure 2a,b). In many cases, the two-dimensional networks fold into cocoon-shaped three-dimensional cagelike structures (Figure 2c) of sizes between hundreds of nanometers to several hundreds of micrometers. Electron diffraction patterns (e.g., Figure 2d) indicate that the nanowires are comprised of coalesced
Assembly of Nanowire Networks from Nanoparticles
Figure 3. UV-vis absorption spectra from (a) gold hydrosols before (black curve) and after (red curve) vigorous agitation with toluene and (b) gold nanowires (blue curve) drop-coated on a quartz slide. A reference spectrum from pure toluene (green curve) is also shown.
face-centered cubic nanocrystals. Figure 2e shows a highresolution lattice image of a single nanowire consisting of randomly oriented grains whose average diameter is identical to that of the nanoparticles in the hydrosol. The grain boundaries are at a 90° ((20°) angle to the nanowire axis (see arrows in Figure 2e) and are relatively flat compared to the spherical shape of the as-prepared nanoparticles observed by TEM. Flattening of the nanoparticles at the grain boundaries, with no observable separation between grains, indicates that the nanowires are not merely linear aggregates of nanoparticles but are melded through particle impingement and coalescence. Upon cessation of agitation of the hydrosol-toluene mixture, the nanowire networks migrate to the toluenewater interface rather than sinking to the bottom of the aqueous layer, indicating hydrophobic passivation of the nanowire surfaces. In contrast, agitating the hydrosol without toluene results in aggregated structures whose smallest feature sizes of ∼0.5-1 µm are more than 1000fold larger than that of the nanowires which precipitate to the container bottom in a lump. These aggregates also take 100-fold larger time frames (e.g., >10 h) to form compared to that taken for nanowires (∼5-15 min) to form from toluene-hydrosol mixtures, suggesting the participation of toluene in nanowire formation and passivation. The above results and inferences are consistent with the features of UV-vis spectra obtained from the hydrosols before and after nanowire formation (Figure 3a). The strong surface plasmon peak at 518 nm in as-prepared hydrosols indicates an average nanoparticle size of ∼5 nm.18 Remnant hydrosols after nanowire formation (and removal) show a significantly lower intensity plasmon peak which extends toward higher wavelengths because of nanoparticle depletion and coalescence. The additional peak observed at ∼260 nm (with a shoulder at ∼270 nm) is due to π-π* transitions in toluene dissolved in the hydrosol during agitation. Spectra obtained from the gold nanowires (see example in Figure 3b) vigorously washed in deionized water and dried in N2 show a very broad plasmon band with additional peaks in the 600-800 nm range arising from the longitudinal component due to the aspect-ratio increase. The merging of this peak with the transverse component in the range 580-640 nm is likely due to interconnected nanowires of different lengths,
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Figure 4. FTIR spectra obtained from nanowire networks before (red) and after (black) air-annealing at 300 °C in air for 1 h.
Figure 5. TGA mass-loss characteristics recorded from the toluene-passivated nanowires during heating from room temperature to 900 °C at 10 °C/min under 50 mL/min N2.
unlike those previously reported for well-dispersed nanowires.19 We attribute the 230-480 nm range peaks with a maximum at ∼350 nm to the π-π* transitions of adsorbed toluene. The red shifts of these peaks (∼75 nm for the strongest peak) with respect to the corresponding 260-nm peak seen in pure toluene suggests strong binding interaction between toluene and gold. FTIR spectra obtained from the as-prepared nanowire networks, after vigorous washing and sonication in deionized water and drying in N2, confirm toluene passivation on the nanowire surfaces (see Figure 4). The inplane skeletal sCdCs stretching modes and their overtones in the ∼1480-1945 cm-1 range20 and ring hydrogen rocking modes between ∼1000 and 1250 cm-1 (refs 21 and 22) are typical of aromatic hydrocarbons. The CsH deformation mode23 at ∼1445 cm-1 (δas CH3) and CsH stretching vibrations (νs and νas CH3) between 2800 and 3000 cm-1 also support the presence of toluene. The (19) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065-4067. (20) Manna, A.; Chen, P.; Akiyama, H.; Wei, T.; Tamda, K.; Knoll, W. Chem. Mater. 2003, 15, 20-28. (21) The Aldrich library of FT-IR spectra, 2nd ed.; Sigma-Aldrich: Milwaukee, 1997; Vol. 2, pp 1625-1626. (22) Cross, A. D. An introduction to practical infrared spectroscopy, 2nd ed.; Butterworths: London, 1964; p 63. (23) Hostetler, M. J.; Strokes, J. J.; Murray, R. W. Langmuir 1996, 12, 3604-3612.
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Figure 6. Bright-field TEM micrographs from Au nanowire networks obtained (a) in the as-prepared state and (b) after annealing the same sample-containing grid at 300 °C for 1 h in air. The insets show schematic sketches of the nanowire microstructures.
Figure 7. Bright-field TEM images obtained from Au nanostructures synthesized by agitation of 5-nm-diameter nanoparticles in (a) benzene and (b) xylene.
relatively low intensity of the skeletal modes observed in the nanowires, compared to that of pure toluene, suggests toluene immobilization on Au nanowires via π electron interactions. The in-plane sCdCs and CsH stretch modes and ring hydrogen rocking vibrations either strongly diminish in intensity or entirely disappear upon annealing the nanowires in air for 1 h at 300 °C due to toluene desorption. The annealed nanowires, however, show an increase in the CsH deformation mode intensity, which is most likely due to the aliphatic hydrocarbon residue formed through toluene decomposition on the nanowire surface. Toluene desorption and decomposition during annealing are corroborated by TGA mass loss characteristics of the gold nanowires (see Figure 5), which are similar to desorption of chemisorbed anthracene and benzene on gold.13 The onset of mass loss at ∼110 °Cs close to the boiling point of toluenesis due to vaporization of toluene. The 4.6 wt % loss between 110 and 765 °C corresponds to desorption and decomposition of chemisorbed toluene. We suggest that the aliphatic residue formed by toluene decomposition, as indicated by FTIR results, contributes to the mass loss in the 400-765 °C temperature range. Surface depassivation by toluene removal during annealing is accompanied by the coarsening of the nanowires as seen from TEM micrographs (see Figure 6), which show that the average diameter of annealed nanowires is nearly 65% higher than that of the as-prepared ones. Further studies are necessary to reveal
whether the simultaneity of nanowire coarsening and toluene desorption implies a correlation between the two processes. Replacing toluene with benzene or xylene in our synthesis process results in partially networked nanowires and agglomerated nanoparticles in the ∼5-20 nm size range (e.g., see Figure 7). These partially coalesced structures were transferred to the organic layer, indicating hydrophobic surface passivation as in the toluene case. Agitation of gold nanoparticles synthesized from HAuCl4 reduction by sodium citrate (C6H8O7Na) instead of NaBH4 does not result in any observable nanoparticle coalescence or nanowire formation. This is confirmed by identical UVvisible spectra (not shown) obtained from citrate-reduced hydrosols before and after agitation. The strong binding of C6H8O7- ions to the nanoparticle surface24 inhibits nanoparticle coalescence and toluene passivation. The salient features of nanowire formation via nanoparticle coalescence in biphasic media can be qualitatively understood based upon the following description. The nanoparticles used in our experiments possess a slight positive surface charge due to the presence of partially reduced HAuCl4 species25 and are weakly solvated by anionic species such as OH- and BO3- ions in the hydrosol. (24) Li, G.; Lauer, M.; Schulz, A.; Boettcher, C.; Li, F.; Fuhrhop, J.-H. Langmuir 2003, 19, 6483-6491. (25) Henglein, A. Langmuir 1999, 15, 6738-6744.
Assembly of Nanowire Networks from Nanoparticles
Agitation of this hydrosol with an immiscible lowviscosity organic liquid26 forms a number of transient highmobility aqueous-organic liquid interfaces, which provide pathways for the hydrophobic passivation, transport, and impingement of the nanoparticles. Strong electrostatic attraction between the charged nanoparticles and the π electrons of the aromatic solvent molecules results in aromatic molecules displacing the weakly solvating anions on the gold surface. Hydrophobic passivation not only enables nanoparticle segregation to the aqueous-organic liquid interfaces but also facilitates coalescence by decreasing interparticle repulsion. Preventing hydrophobic passivation by capping nanoparticles with strongly binding molecules such as citrates turns off the mechanism for hydrophobic-passivation-mediated nanoparticle segregation and coalescence. The morphology of the coalesced nanoparticles is determined by the relative rates of nanoparticle impingement and surface passivation. The nanowire microstructure formed in toluene-aqueous mixtures suggests nanoparticle diffusion-limited impingement with a sticking probability close to unity and coalescence at a rate slightly faster than the rate of surface passivation by toluene. Lower coalescence rates result in partially networked structures such as those obtained in the case of benzene and xylene. This is consistent with the higher viscosity of both these liquids (0.65 and 0.81 cP, respectively)27 compared to that of toluene (0.59 cP).27 Our preliminary measurements28 showing that toluene concentration (26) Beek, W. J.; Muttzall, K. M. K.; van Heuven, J. W. Transport Phenomena, 2nd ed.; Wiley & Sons: West Sussex, U.K., 1999; pp 1016. (27) Lange’s Handbook of Chemistry, 15th ed.; McGraw-Hill: New York, 1999. (28) Maddanimath, T.; Kumar, A.; D’Arcy-Gall, J.; Vijayamohanan, K.; Ramanath, G. Unpublished results, 2004.
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strongly influences nanowire yield supports the view that impingement rate of the nanoparticles is a key factor in determining nanowire morphology. If surface passivation is very slow or absent (e.g., extreme case where there is no toluene, described earlier), we would observe a coarse morphology. Other factors that could influence the morphology of the coalesced structures include mutual solubility of the two liquids, their boundary layer dimensions and shape, steric hindrance of the solvent, and the strength of their bonding interactions with the nanoparticles. Quantitative modeling of the coalescence and passivation kinetics should shed further light on the morphological evolution of the coalesced nanoparticles. Summary We have demonstrated a new, simple, room-temperature strategy for assembling gold nanowire networks through coalescence of nanoparticles by agitation of gold hydrosol-toluene mixtures. The nanowires have a uniform diameter identical to that of the nanoparticles. During coalescence, toluene displaces the weak solvating agents and adsorbs onto the Au surface, enabling the transfer of the nanowire networks to the organic layer. Extending our methodology to other materials systems would open up the possibility of low-temperature synthesis of new mesoscale structures from nanounits, for applications such as self-supporting catalyst networks, nanowires for devices, and porous nanofillers for composites. Acknowledgment. We gratefully acknowledge a gift grant from Philip Morris USA through Rensselaer’s Nanotechnology Center, a seed grant from Rensselaer’s Nano Science and Engineering Center, and NSF CAREER Grant DMR 9984478. LA0497649
