Facile Synthesis of Ultrathin Au Nanorods by Aging the AuCl

Aug 6, 2008 - Despite plenty of reports on the preparation of Au nanorods, it remains challenging to grow uniform Au nanorods with diameters below 5 n...
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NANO LETTERS

Facile Synthesis of Ultrathin Au Nanorods by Aging the AuCl(oleylamine) Complex with Amorphous Fe Nanoparticles in Chloroform

2008 Vol. 8, No. 9 3052-3055

Zhengquan Li,† Jing Tao,‡ Xianmao Lu,† Yimei Zhu,‡ and Younan Xia*,† Department of Biomedical Engineering, Washington UniVersity, Saint Louis, Missouri 63130, and Center for Functional Nanomaterials, BrookhaVen National Laboratory, Upton, New York 11973 Received June 15, 2008; Revised Manuscript Received July 23, 2008

ABSTRACT Despite plenty of reports on the preparation of Au nanorods, it remains challenging to grow uniform Au nanorods with diameters below 5 nm. In this communication, we demonstrate the facile synthesis of ultrathin Au nanorods with a uniform diameter of 2 nm and an average aspect ratio of 30. The synthesis involves the room-temperature aging of a mixture of the [AuCl(oleylamine)] complex with amorphous Fe nanoparticles in chloroform. Analysis of the growth mechanism indicates that Au nanoparticles with a high density of defects were formed at early stages, followed by etching and redeposition process that gradually led to the growth of ultrathin Au nanorods along the 〈111〉 direction. This growth mechanism is different from the mechanism recently reported for ultrathin Au nanowires (ref 26), where the [AuCl(oleylamine)] complex is assembled into polymer chains followed by reduction to form wires, although the template effect of oleylamine for the formation of ultrathin Au nanorods cannot be completely ruled out.

One-dimensional (1-D) metallic nanostructures have attracted considerable attention for several decades because of their unique electronic, magnetic, and optical properties.1,2 Among various 1-D metallic nanostructures, Au nanorods are of particular importance owing to their tunable surface plasmon resonance properties.1 Because of the chemical inertness, good biocompatibility, and facile bioconjugation of Au, the nanorods are expected to play a critical role in biomedical research including optical imaging and photothermal treatment.3-5 In addition, Au nanorods may find use as interconnects in fabricating nanoelectronic devices owning to the good electrical and thermal conductance of Au.6,7 A variety of methods, such as seed-mediated growth, photochemical reduction, template-directed synthesis, and electrochemical deposition, have been developed to synthesize Au nanorods.8-11 However, there is only very limited success with regard to the control of their diameters, especially on the scale below 5 nm. It is generally accepted that observation and utilization of the size-dependent properties of metallic nanostructures require one to control their diameters in the range of 1-5 nm.12-14 For example, Au * Corresponding author. E-mail: [email protected]. † Washington University. ‡ Brookhaven National Laboratory. 10.1021/nl8017127 CCC: $40.75 Published on Web 08/06/2008

 2008 American Chemical Society

nanoparticles with a diameter of 3.5 nm are no longer “noble” but can act as effective catalysts for room-temperature oxidation of CO.15 Quantized transport can only be observed for metallic nanowires with diameters close to 1 nm.16 It is expected that development of Au nanorods with diameters in the size-dependent region can bring in new physical and chemical properties, and may lead to the fabrication of new types of nanoscale devices and catalysts.17,18 In this communication, we demonstrate the facile synthesis of ultrathin Au nanorods with a uniform diameter of 2 nm by reducing a complex of AuCl and oleylamine (OLA) with amorphous Fe nanoparticles in chloroform at room temperature. The [AuCl(OLA)] complex was prepared by codissolving AuCl and OLA in chloroform (see Supporting Information).19 The Fe nanoparticles of 6-9 nm in diameter were prepared via thermal decomposition of Fe(CO)5 with OLA added as a surfactant.20,21 In a typical synthesis, 1.5 mL of the [AuCl(OLA)] complex in chloroform (0.01 M) was added dropwise into 5 mL chloroform, which contained freshly prepared Fe nanoparticles under argon flow. The molar ratio of Au to Fe was 3:1. After aging the mixture at room temperature for 6-8 days, ultrathin Au nanorods of ∼2 nm in diameter were formed in the solution. The

Figure 1. (A,B) TEM images and (C) HRTEM image of Au nanorods at different magnifications. The sample was prepared by aging a mixture of amorphous Fe nanoparticles and the [AuCl(OLA)] complex in chloroform for 7 days. (D) EDX analysis of the Au nanorods. The Cu and Si signals were from the copper grid and TEM chamber, respectively.

nanorods were then collected by adding an equal volume of acetone into the reaction solution, followed by centrifugation at 13 000 rpm for 20 min. Figure 1A shows a typical transmission electron microscopy (TEM) image of the Au nanorods obtained by aging the mixture of Fe nanoparticles and [AuCl(OLA)] complex in chloroform for 7 days. Figure 1B shows a TEM image at a higher magnification, indicating that these nanorods had an average diameter of 2 ( 0.2 nm, together with an average aspect ratio of ∼30. These ultrathin nanorods were sensitive to the electron beam and could slowly melt upon exposure to the electron beam (Supporting Information Figure S1). High-resolution TEM (HRTEM) image reveals that the majority of these nanorods are single-crystal (Figure 1C), and only a few of them have twin defects along the nanorods (Figure S2). The fringe spacing of 0.23 nm matches the interplanar distance between the {111} planes of facecentered cubic (fcc) gold. Selected area electron diffraction pattern recorded from a number of nanorods also confirms their high crystallinity, where the ring could be indexed to the (111) diffraction from gold (Figure S3). By using a focused electron probe about 2-3 nm in size, energy dispersive X-ray (EDX) experiments were carried out in a JEOL 3000F TEM. The EDX analysis (Figure 1D) of these nanorods suggests that they were purely composed of Au without detectable Fe. The iron nanoparticles prepared via thermal decomposition of Fe(CO)5 in the presence of OLA had a spherical morphology, while the final product after aging with the [AuCl(OLA)] complex for 6-8 days mainly contained Au nanorods. To investigate the growth mechanism for the ultrathin Au nanorods, we monitored the growth process by taking samples out of the reaction mixture after it had been aged for different periods of time. Figure 2A shows a TEM image of the Fe nanoparticles, which had diameters in the Nano Lett., Vol. 8, No. 9, 2008

Figure 2. Typical TEM images of intermediate products obtained from the same synthesis but at different aging times, showing the evolution from twinned Au nanoparticles into Au nanorods. (A) t ) 0 (iron nanoparticles covered by native oxides); (B) t ) 12 h; (C) t ) 48 h; (D) t ) 96 h; (E) t ) 144 h; and (F) t ) 196 h. After 144 h, no further morphological change was observed.

range of 6-9 nm. Note that the actual size of these Fe nanoparticles might be slightly smaller than the size measured from the TEM image because the surface of these nanoparticles was inevitably oxidized during TEM sample preparation. No obvious change was observed within 3 h after the addition of the [AuCl(OLA)] complex into the suspension of Fe nanoparticles, indicating that the reaction between the Fe nanoparticles and the [AuCl(OLA)] complex was relatively slow. The slow reaction can be attributed to the formation of a thin layer of iron oxide on the Fe nanoparticles before they were mixed with the [AuCl(OLA)] complex. After the mixture had been aged for 12 h, many irregularly shaped nanoparticles started to appear (Figure 2B). EDX analysis of these nanoparticles showed that they were made of pure gold (Figure S4). The dimensions of these gold nanoparticles were larger than those of the Fe nanoparticles. This might be related to the stoichiometry of the reaction between Fe and [AuCl(OLA)]: to oxide each Fe(0) atom, three [AuCl(OLA)] will be consumed to give one Fe(III) ion and three Au(0) atoms.22 If all of the Au(0) atoms were used to form one Au nanoparticle, it should be larger than the Fe nanoparticle. More significantly, we found that these Au nanoparticles were unstable in the solution. After 48 h, some of them were elongated into nanoparticles with a dumbbell shape (i.e., a structure consisting of a thin rod with two enlarged ends, Figure 2C). After the solution had been 3053

Figure 3. HRTEM images of typical Au nanoparticles obtained at the early and later stages of an aging process, showing the change of crystallinity accompanying the shape evolution. (A) A heavily defected Au nanoparticle obtained at t ) 12 h and (B) the end of a dumbbell-shaped Au nanoparticle obtained at t ) 96 h. All of the arrows are along the 〈111〉 direction.

aged for 96 h, the number of the dumbbell-shaped nanoparticles increased significantly (Figure 2D). The compositions of these nanoparticles were analyzed by EDX (Figure S4), and the spectra taken from the tips and middle portion of the particles suggested that they were all made of pure gold. As the aging proceeded to 144 h, all of the dumbbell-shaped nanoparticles had evolved into thin nanorods with a uniform diameter of ∼2 nm (Figure 2E). After this point, the Au nanorods did not exhibit any further morphological change even after aged for 196 h (Figure 2F), indicating that the ultrathin nanorods were stable in solution once they had been formed. To further support this claim, we checked the sample aged in the solution at room temperature for two weeks and did not find significant change in size or morphology for the ultrathin Au nanorods. On the basis of these observations, it is clear that the ultrathin Au nanorods were evolved from the unstable Au nanoparticles initially derived from the reaction between the Fe nanoparticles and the [AuCl(OLA)] complex. As shown in Figure 3A, the Au nanoparticles formed at early stages exhibited a multiply twinned structure rich of twins and other types of defects. The high density of defects could be ascribed to the amorphous nature of the Fe nanoparticles which probably served as templates (or at least nucleation sites) for the formation of Au nanoparticles during the galvanic replacement reaction.21 It has been demonstrated that multiply twinned metal nanoparticles are highly susceptible to oxidative etching (as caused by the oxygen from air) because of their high density of defects and thus high 3054

Figure 4. TEM images of the samples prepared with different molar ratios between the [AuCl(OLA)] complex and the Fe nanoparticles: (A) [AuCl(OLA)]/Fe ) 1.5 and (B) [AuCl(OLA)]/Fe ) 6. The diameters of the Au nanorods in both samples were nearly identical to those shown in Figure 1, regardless of the byproduct which were Fe and Au nanoparticles, respectively.

reactivity.23,24 In addition, oxidative etching caused by Fe(III) species might also be involved, as it has been reported for the formation of Pt nanowires.25 We believe that a similar oxidative etching was likely involved in the growth of Au nanorods. During this process, the defected regions of Au nanoparticles formed in the initial stage would be preferentially etched and oxidized to Au(I) or Au(III) at a later stage of the synthesis. These Au ions were reduced again by Fe(0) or Fe(II) and the newly formed Au(0) atoms were deposited epitaxially onto the defect-free regions on the same or different Au nanoparticles. With the aid of OLA, the growth was confined to the 〈111〉 direction. Continuous etching and deposition eventually reshaped the nanoparticle into a nanorod, which should be more stable than the original twinned particle due to reduction of defect density. We also characterized the dumbbell-shaped nanoparticles by HRTEM. Figure 3B shows a typical image, where the well-resolved and continuous fringes along the 〈111〉 direction indicate single crystallinity for the dumbbell-shaped nanoparticle. Although a detailed mechanism for the formation of Au nanorods is yet to be fully resolved, it is clear that the evolution from defect-rich Au nanoparticles to single-crystal Au nanorods serves as the major driving force. Control experiments with different molar ratios between [AuCl(OLA)] complex and iron nanoparticles were also carried out (Figure 4). By reducing the amount of [AuCl(OLA)] solution to half (i.e., [AuCl(OLA)]/Fe ) 1.5), the final product contained Au nanorods and some unconsumed Nano Lett., Vol. 8, No. 9, 2008

Fe nanoparticles (Figure 4A). If the amount of [AuCl(OLA)] solution was doubled (i.e., [AuCl(OLA)]/Fe ) 6), both Au nanorods and nanoparticles were observed in the final product (Figure 4B). A few of the Au nanorods could grow into longer lengths, but these nanorods were still ∼2 nm in diameter. Overall, we did not observe any significant change to the diameter of the Au nanorods when different molar ratios of [AuCl(OLA)] to Fe were involved. The key to initiate the growth of ultrathin Au nanorods was to generate Au nanoparticles rich of defects in the initial stage. Therefore, variation of the molar ratio between reactants should only change the number of defect-enriched Au nanoparticles rather than the morphology and dimensions of the final products. These results also suggest that ultrathin Au nanorods could be readily obtained under a broad range of different conditions, making this simple method more attractive for practical use. It is critical to use chloroform as the solvent in order to obtain Au nanorods in high yields. When dimethylformamide (DMF) was used as a solvent, DMF could directly reduce the [AuCl(OLA)] complex to form irregularly shaped Au powders in a few minutes. Ultrathin Au nanorods could be obtained in dichloromethane (DCM), albeit the yield was not as high as that in chloroform, presumably because the [AuCl(OLA)] complex was less stable in DCM than in chloroform. When hexane was used as a solvent, the [AuCl(OLA)] complex could form polymer chains via the aurophilic interaction. In this case, ultrathin Au nanowires with a diameter of 1.8 nm and with a length up to several micrometers were recently synthesized by reducing the polymer strands with Ag nanoparticles or OLA.26 It is worth pointing out that we did not observe this kind of polymer when the [AuCl(OLA)] complex was dispersed in chloroform. As a matter of fact, Au nanoparticles with a uniform size were obtained when a chloroform solution of the [AuCl(OLA)] complex was heated to 60 °C.19 In summary, ultrathin Au nanorods with a diameter of 2 nm have been synthesized by aging a mixture of amorphous Fe nanoparticles and the [AuCl(OLA)] complex in chloroform at room temperature. The use of amorphous Fe nanoparticles as a reductant could lead to the formation of Au nanoparticles rich of defects. These Au nanoparticles could gradually evolve into ultrathin Au nanorods in the solution, as driven by etching and deposition taking place at different sites of the nanoparticle (similar to an internal Ostwald ripening process). The formation of ultrathin Au

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nanorods should enable further study of their unconventional size-dependent electronic, optical, and mechanical properties. Acknowledgment. This work was supported in part by two research grants from the NSF (DMR, 0451788 and 0804088). Supporting Information Available: Detailed descriptions of the experimental procedures, melting of a single nanorod under HRTEM, HRTEM image of a single nanorod with twin defects and EDX analysis of intermediate Au nanoparticles. This material is available free of charge via the Internet at http://pubs.acs.org. References (1) See, for example, Xia, Y; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y; Kim, F.; Yan, Y. AdV. Mater. 2003, 15, 353. (2) Chen, J.; Wiley, B.; Xia, Y. Langmuir 2007, 23, 4120. (3) Yu, C.; Irudayaraj, J. Anal. Chem. 2007, 79, 572. (4) Ding, H.; Yong, K. T.; Roy, I.; Pudavar, H. E.; Law, W. C.; Bergey, E. J.; Prasad, P. N. J. Phys. Chem. C 2007, 111, 12552. (5) Oyelere, A. K.; Chen, P. C.; Huang, X.; El-sayed, I. H.; El-Sayed, M. A. Bioconjugate Chem. 2007, 18, 1490. (6) Perez-Juste, J.; Pastoriza-Santos, I.; Liz-marzan, L. M.; Mulvaney, P. Coordin. Chem. ReV. 2005, 249, 1870. (7) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (8) Nikhil, R. J.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (9) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (10) Cepak, V. M.; Martin, C. R. J. Phys. Chem. B 1998, 102, 9985. (11) Yu, Y. Y.; Chang, S. S.; Lee, C. L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (12) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (13) Henglein, A. J. Phys. Chem. 1993, 97, 5457. (14) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (15) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (16) Ohnishi, H.; kondo, Y.; Takayanagi, K. Nature 1998, 395, 780. (17) Rodrigues, V.; Fuhrer, T.; Ugarte, D. Phys. ReV. Lett. 2000, 85, 4124. (18) Coura, P. Z.; Legoas, S. B.; Moreira, A. S.; Sato, F.; Rodrigues, V.; Dantas, S. Q.; Ugarte, D.; Galvao, D. S. Nano Lett. 2004, 4, 1187. (19) Lu, X.; Tuan, H.-Y.; Korgel, B. A.; Xia, Y. Chem. Eur. J. 2008, 14, 1584. (20) Shao, H.; Lee, H. S.; Huang, Y.; Ko, I. Y.; Kim, C. O. IEEE Trans. Magn. 2005, 41, 3388. (21) Peng, S.; Wang, C.; Xie, J.; Sun, S. J. Am. Chem. Soc. 2006, 128, 10676. (22) Handbook of Chemistry and Physics, 60th ed.; Weast, R. C., Ed.; CRC Press: Boca Raton, FL, 1980. (23) Wiley, B.; Herricks, T.; Sun, Y.; Xia, Y. Nano Lett. 2004, 4, 1733. (24) Xiong, Y.; Chen, J.; Wiley, B.; Xia, Y. J. Am. Chem. Soc. 2005, 127, 7332. (25) Chen, J.; Herricks, T.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 2589. (26) (a) Lu, X.; Yavuz, M. S.; Tuan, H.-Y.; Korgel, B. A.; Xia, Y. J. Am. Chem. Soc. 2008, 130, 8900. (b) Wang, C.; Hu, Y.; Lieber, C. M.; Sun, S. J. Am. Chem. Soc. 2008, 130, 8902. (c) Huo, Z.; Tsung, C. K.; Huang, W.; Zhang, X.; Yang, P. Nano Lett. 2008, 8, 2041.

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