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Synthesis of Flexible, Ultrathin Gold Nanowires in Organic Media Nicola´s Pazos-Pe´rez,† Dmitry Baranov,† Stephan Irsen,‡ Michael Hilgendorff,† Luis M. Liz-Marza´n,*,§ and Michael Giersig*,| Center of AdVanced European Studies and Research (CAESAR), Department of Nanoparticle Technology and Electron Microscopy Group, 53175 Bonn, Germany, Departamento de Quı´mica Fı´sica and Unidad Asociada CSIC - UniVersidade de Vigo, 36310 Vigo, Spain, and Helmholtz-Zentrum Berlin fu¨r Materialien und Energie GmbH, 14109 Berlin, Germany ReceiVed May 30, 2008. ReVised Manuscript ReceiVed July 2, 2008 Gold nanoparticles are very interesting because of their potential applications in microelectronics, optical devices, analytical detection schemes, and biomedicine. Though shape control has been achieved in several polar solvents, the capability to prepare organosols containing elongated gold nanoparticles has been very limited. In this work we report a novel, simplified method to produce long, thin gold nanowires in an organic solvent (oleylamine), which can be readily redispersed into nonpolar organic solvents. These wires have a characteristic flexible, hairy morphology arising from a small thickness (10 nm diameter) or polycrystalline wires.55,56 Recently, Halder and Ravishankar reported18 a rather complicated, multistep method to produce single-crystalline gold wires in toluene using a mixture of oleic acid, oleylamine, and ascorbic acid, which required hightemperature treatment at some intermediate steps. There is thus a need to develop simpler methods to produce high-quality gold nanowires in solution.57 In this article, we present a simplified procedure for the production of long, ultrathin single-crystal gold nanowires in organic solvent. These wires display a characteristic hairy morphology, indicating a relatively high flexibility, which originates from their very low thickness (down to ∼2 nm for the thinnest wires) and very high aspect ratios (up to ∼2500!). In this process, which can be scaled up easily, oleylamine (OA) is used as both a solvent and shape-directing agent. Additionally, through modification of the growth conditions, nanowires of various lengths can be obtained. The resulting nanostructures were characterized by high-resolution transmission electron microscopy (HRTEM), UV-visible spectroscopy, and energydispersive X-ray (EDX) elemental analysis. As compared to a similar previous report,18 the use of a single surfactant (oleylamine), which is simultaneously used as a reaction medium and reducing agent, largely simplifies the system. Additionally, the reaction temperature is kept near room temperature (∼30-40 °C) and a higher yield and better control of the nanowires’ dimensions can be achieved. (37) Patolsky, F.; Lieber, C. M. Mater. Today 2005, 8, 20. (38) Whang, D.; Jin, S.; Wu, Y.; Lieber, C. M. Nano Lett. 2003, 3, 1255. (39) Tang, Z. Y.; Kotov, N. A.; Giersig, M. Science 2002, 297, 237. (40) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (41) Gudiksen, M. S.; Lauhon, L. J.; Wang, J.; Smith, D. C.; Lieber, C. M. Nature 2002, 415, 617. (42) Rodrigues, V.; Fuhrer, T.; Ugarte, D. Phys. ReV. Lett. 2000, 85, 4124. (43) Xia, Y. N.; Yang, P. D.; Sun, Y. G.; Wu, Y. Y.; Mayers, B.; Gates, B.; Yin, Y. D.; Kim, F.; Yan, Y. Q. AdV. Mater. 2003, 15, 353. (44) Rao, C. N. R.; Deepak, F. L.; Gundiah, G.; Govindaraj, A. Prog. Solid State Chem. 2003, 31, 5. (45) Wang, D.; Qian, F.; Yang, C.; Zhong, Z. H.; Lieber, C. M. Nano Lett. 2004, 4, 871. (46) Rodrigues, V.; Ugarte, D. Mater. Sci. Eng., B 2002, 96, 188. (47) Coura, P. Z.; Legoas, S. B.; Moreira, A. S.; Sato, F.; Rodrigues, V.; Dantas, S. O.; Ugarte, D.; Galvao, D. S. Nano Lett. 2004, 4, 1187. (48) Sun, Y. G.; Mayers, B.; Xia, Y. N. Nano Lett. 2003, 3, 675. (49) Liu, Z. P.; Li, S.; Yang, Y.; Peng, S.; Hu, Z. K.; Qian, Y. T. AdV. Mater. 2003, 15, 1946. (50) Mo, M. S.; Zeng, J. H.; Liu, X. M.; Yu, W. C.; Zhang, S. Y.; Qian, Y. T. AdV. Mater. 2002, 14, 1658. (51) Huang, T. K.; Cheng, T. H.; Yen, M. Y.; Hsiao, W. H.; Wang, L. S.; Chen, F. R.; Kai, J. J.; Lee, C. Y.; Chiu, H. T. Langmuir 2007, 23, 5722. (52) Martin, C. R. Chem. Mater. 1996, 8, 1739. (53) Hou, S. F.; Wang, J. H.; Martin, C. R. Nano Lett. 2005, 5, 231. (54) Zhang, J. L.; Du, J. M.; Han, B. X.; Liu, Z. M.; Jiang, T.; Zhang, Z. F. Angew. Chem., Int. Ed. 2006, 45, 1116. (55) Maddanimath, T.; Kumar, A.; Arcy-Gall, J.; Ganesan, P. G.; Vijayamohanan, K.; Ramanath, G. Chem. Commun. 2005, 1435. (56) Ramanath, G.; Arcy-Gall, J.; Maddanimath, T.; Ellis, A. V.; Ganesan, P. G.; Goswami, R.; Kumar, A.; Vijayamohanan, K. Langmuir 2004, 20, 5583. (57) After submission of this manuscript, two independent, similar synthesis of ultrathin gold nanowires were reported: (a) Wang, C.; Hu, Y.; Lieber, C. M.; Sun, S. J. Am. Chem. Soc. DOI: 10.1021/ja803408f. (b) Lu, X.; Yavuz, M. S.; Tuan, H.; Korgel, B. A.; Xia, Y. J. Am. Chem. Soc. DOI: 10.1021/ja803343.
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Experimental Section Oleylamine (OA, Aldrich, technical grade, 70%), chloroform (Roth, 99%), and tetrachloroauric acid (HAuCl4 · 3H2O, Fluka) were used as received without further purification. All glassware was washed with aqua regia prior to the experiments. UV-vis/near-IR spectroscopy (Varian, Cary 5000), transmission electron microscopy (TEM, Leo 922A EFTEM, operating at 200 KV), and high-resolution TEM (HRTEM, ZEISS Libra 200-CRISP, operating at 200 KV) combined with EDX detection were applied to characterize the optical response, composition, structure, and size distribution of the synthesized nanocrystals. For TEM investigations and further processing, the Au nanowire solution was mixed with chloroform (∼50 mL), centrifuged (8000 rpm, 20 min), the supernatant discarded, and the precipitate redispersed (via vortex stirring) in a nonpolar solvent such as toluene, hexane, or chloroform. A drop of particle solution was then placed on a carbon-coated Cu grid and dried at room temperature.
Results and Discussion Nanowire Formation. Ultrathin Au nanowires were synthesized using an approach derived from the method reported by Wang et al.58 for the production of FePt nanowires. In this synthesis, OA is considered to be a crucial component for onedirectional growth, thus leading to the formation of elongated structures. The method described here ensures the synthesis of Au nanowires with diameters of ∼1.6 nm and lengths ranging from 10 nm to g3.5 µm. Control over Au wire lengths was realized by tuning the OA/HAuCl4 volume ratio (0.2-2.2), the reaction time, and the addition of a second solvent. In a typical synthesis, HAuCl4 (20 mg) was dissolved in OA (8 mL) by vortex mixing or sonication at room temperature until the solution turned from pale yellow, the characteristic color of OA, to an intense orange color, which indicates complex formation between Au3+ and OA. Thereafter, the solution was left undisturbed for 24 h, during which time the solution color changed again gradually from orange to pale yellow, indicating the reduction of Au3+ to Au+.59 A white precipitate also originated from OA oxidation, which was redissolved by the addition of CHCl3 (7 mL) when the reaction was finished. The formation of this white precipitate depends on the degree of OA oxidation given by the HAuCl4/ OA ratio. The last step of the gold nanowire synthesis was carried out in a thermostatic bath at 35-40 °C to speed up the reaction, followed by aging for several days, depending on the reaction temperature (e.g., 5 days at 35 °C). The reaction was continuously monitored by UV-vis/NIR spectroscopy and TEM. Changes in the ratio of precursor to surfactant, the reaction temperature, and the addition of extra solvents (such as chloroform, toluene, or hexane) strongly affected the final particle size. Once the reaction was completed, the Au nanoparticle solution could be stored for long periods of time with no apparent changes. The TEM image in Figure 1a shows that the synthesis from HAuCl4/OA at 50 °C can lead to the formation of spheres and thick wires in addition to ultrathin wires. After decreasing the HAuCl4/OA ratio and the temperature to 35 °C, we found that the amount of nanomaterials of other morphologies (spheres and thick wires) could be considerably reduced, as shown in Figure 1b. TEM and HRTEM analyses of the Au nanowires confirmed that they are monodisperse and highly crystalline (Figure 2). It is clearly visible from Figure 2c that each nanowire is a single crystal showing the characteristic (111) lattice planes of the facecentered cubic (fcc) phase. A separation distance of 0.23 nm (58) Wang, C.; Hou, Y. L.; Kim, J. M.; Sun, S. H. Angew. Chem., Int. Ed. 2007, 46, 6333. (59) Halder, A.; Ravishankar, N. J. Phys. Chem. B 2006, 110, 6595.
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Figure 1. TEM images of Au nanowires prepared under different reaction conditions. (a) Ultrathin Au wires, together with spheres and thick wires obtained at high HAuCl4 concentration (50 °C). (b) Pure ultrathin Au wires obtained at lower HAuCl4 concentration (35 °C). (c) Higher magnification of the image in part b. (d, e) Ultrathin Au wires with two different lengths (∼50 and 300 nm) obtained through modification of the reaction conditions by means of varying the amount of the second solvent (addition of CHCl3 50%v and 33%v of the total volume for parts d and e, respectively); meanwhile, the other reaction parameters were constant.
could be calculated from the fast Fourier transformation (FFT) shown in the inset of Figure 2c. The wire diameter distribution shown in Figure 2e was obtained from a statistical evaluation of nanocrystals from several TEM images. These measurements confirm a relatively narrow distribution with an average wire diameter of 1.6 nm with a standard deviation of less than 10%. The composition of the nanowires was confirmed by EDX analysis (Figure 2d). The spectrum shows exclusively Au peaks apart from Cu signals of the TEM grids. UV-vis spectra were continuously monitored during the growth process. Figure 3 shows a spectral series corresponding to various times during the reaction, plotted in (a) two and (b) three dimensions. From this series, it can clearly be seen how the initial absorbance of the AuCl4 complex at 400 nm (absorbance 0.95) decreases with time, showing the reduction of Au3+ to Au+. A plateau is quickly reached, corresponding to nanowire formation. Then the absorbance increases again with time as a result of the further development of Au0 wires. The optical response of the Au nanowires does not exhibit plasmon resonances in the UV-vis region, corresponding to a colorless solution, which was expected for such thin wires (diameter ∼1.6 nm). We selected for Figure 3 a spectral series in which a plasmon resonance occurred at later stages of the reaction so as to exemplify the growth conditions at which spheroidal nanoparticles and thick
wires (Figure 1a) are formed at the end of the growth process. However, when only thin Au nanowires were present, no plasmon bands were identified. Growth Mechanism. The method used here for the production of Au nanowires is reminiscent of that reported by Wang et al.58 for FePt, for which oleylamine was claimed to direct anisotropic growth. During the formation of Au nanowires, chloroauric acid (containing Au3+ and thereby exhibiting a strong orange color) is reduced by oleylamine. During the reaction, a pale-yellow solution is observed first, revealing the reduction of Au3+ to Au+.59 Because the thin wires have no characteristic optical signature, a combined TEM study was performed by withdrawing small aliquots at different stages in their formation. The collected products were monitored by UV-vis spectrometry and TEM examination with no previous washing to ensure that nothing was lost during the centrifugation steps. Thus, there are no perturbations of the growth process due to reduced surfactant concentrations or promoted aggregation during centrifugation. This definitely affected the quality of the TEM images but allowed us to determine the shapes of the nanocrystals present in the initial stages of the process. These results are summarized in Figure 4, showing that small, short, “twisted” rodlike structures were present in the solution almost from the beginning of the
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Figure 2. (a-c) TEM images of Au nanowires at different magnifications (a, b) and an HRTEM image (c) showing the single-crystalline structure. (d) EDX spectrum of the same sample. (e) Histogram showing the narrow width distribution of the nanowires.
Figure 3. (a) 2D and (b) 3D plots of time-resolved UV-vis spectra during the formation of Au nanowires.
reaction. Control experiments were carried out to investigate whether Au+ remained in solution at selected reaction times by simply adding sodium borohydride and observing spectral changes through Au nanoparticle formation. These control experiments indicate that the formation of the Au wires takes place through a slow but continuous reduction of Au+. The gold salt is only partly reduced at the beginning of the reaction, forming small clusters on which Au+ is gradually reduced over time to form the final, thin wires. HRTEM analysis shows (Figure 2c), apart
from the single-crystalline fcc structure, that the [111] direction is perpendicular to the growth direction of the wires. All of these results suggest that oleylamine induces the 1D Au growth very likely by means of self-organization into elongated reverse micelles, inside of which small Au crystals are formed, as proposed by Wang et al.58 for a similar reaction that forms FePt nanowires. The main idea is that small Au nanocrystals have different surface energies on their individual crystal facets, resulting in different oleylamine packing densities at different
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Figure 4. TEM images of particles from aliquots taken at different times during the growth process: (a) 10, (b) 35, (c) 45, and (d) 50 h. The insets show the corresponding enlarged images.
surfaces, which guides the growth along the direction with lower packing density.58 As mentioned above (Figure 1a), thicker wires can also be formed when higher gold concentrations are used. Under these conditions, nucleation can continue after nanowire formation, yielding spheroidal gold nanoparticles (∼10 nm) that can merge into the thicker gold wires through oriented attachment in solution. This mechanism has also been proposed by Halder et al.18 for a similar Au nanowire system and previously for several other materials.39,60–63 Stability of Au Nanowires. The Au nanowires display longterm stability in solution, and no noticeable changes in TEM images obtained several months after synthesis are observed. However, during lengthy TEM examinations we often observed morphological changes upon exposure to high-energy beams. Figure 5 shows two examples in which nanowires were seen to split under the electron beam during HRTEM analysis. The structure of Au wires was affected by the high-energy electron beam, and prolonged exposure resulted in fractures, which, over time, can lead to complete conversion into spheres. This is probably related to relatively high localized temperatures on the wires due to electron absorption. A similar process has been reported for FePt nanowires upon thermal annealing.58,64
Conclusions In summary, we have demonstrated that ultrathin, singlecrystalline gold nanowires with a narrow diameter distribution (∼1.6 (60) Cho, K. S.; Talapin, D. V.; Gaschler, W.; Murray, C. B. J. Am. Chem. Soc. 2005, 127, 7140. (61) Penn, R. L.; Banfield, F. J. Am. Miner. 1998, 83, 1077. (62) Yu, J. H.; Joo, J.; Park, H. M.; Baik, S. I.; Kim, Y. W.; Kim, S. C.; Hyeon, T. J. Am. Chem. Soc. 2005, 127, 5662. (63) Giersig, M.; Pastoriza-Santos, I.; Liz-Marza´n, L. M. J. Mater. Chem. 2004, 14, 607.
Figure 5. TEM images showing nanowire degradation upon long exposure to the high-energy electron beam during HRTEM analysis.
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nm) and with various lengths ranging from ∼10 nm up to ∼4 µm can be produced in organic solvents in high yield. Nanowire growth might be related to the initial formation of small nanoparticles, followed by surfactant-directed reduction on the existing nuclei. Whereas the thin Au nanowires do not exhibit plasmons in the UV-vis region, solutions containing additionally thicker structures and spheres exhibit one (transverse) plasmon band at ∼530 nm. Au wires are extremely stable in solution, but local heating through electron irradiation strongly affects their structure, making them split into smaller wires and spheres.
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Acknowledgment. We thank Izabela Firkowska and Georgios Ctistis for useful comments. This work was supported by the Marie Curie Research Training Network SyntOrbMag (contract number MRTN-CT-2004-005567). L.M.L.-M. acknowledges funding from the Spanish MEC through project no. MAT2007-62696. LA801675D (64) Krakow, W.; Jose-Yamacan, M.; Aragon, J. L. Phys. ReV. B 1994, 49, 10591.
