Polyol Synthesis of Uniform Silver Nanowires: A Plausible Growth

Jun 10, 2003 - Nasir M. Abbasi , Li Wang , Haojie Yu , Zain-ul-Abdin , Muhammad Akram , Hamad Khalid , Chen Yongshen , Ruoli Sun , Muhammad Saleem , Z...
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NANO LETTERS

Polyol Synthesis of Uniform Silver Nanowires: A Plausible Growth Mechanism and the Supporting Evidence

2003 Vol. 3, No. 7 955-960

Yugang Sun, Brian Mayers, Thurston Herricks, and Younan Xia* Department of Chemistry, UniVersity of Washington, Seattle, Washington 98195-1700 Received May 15, 2003

ABSTRACT We have recently demonstrated an approach based on the polyol process for the large-scale synthesis of silver nanowires with uniform diameters (see Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. Sun, Y.; Xia, Y. Adv. Mater. 2002, 14, 833. Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736). Although the capability and feasibility of this method have been successfully illustrated with the production of silver nanowires 30−60 nm in diameter and 1−50 µm in length, the growth mechanism of this process is yet to be elucidated. Here we report some progress on this matter: First, electron microscopy studies on microtomed samples indicated that the cross sections of such silver nanowires had a pentagonal shape, together with a 5-fold twinned crystal structure. Second, the side surfaces (bounded by {100} facets) and the ends (bounded by {111} facets) of each nanowire were shown to have significant difference in reactivity toward dithoil molecules, with the side surfaces being completely passivated by poly(vinyl pyrrolidone) (PVP) and the ends being partially passivated (or essentially uncovered) by PVP. This result implied that the PVP macromolecules interacted more strongly with the {100} planes than with the {111} planes of silver. On the basis of these new results, we proposed that each silver nanowire evolved from a multiply twinned nanoparticle (MTP) of silver with the assistance of PVP at the initial stage of the Ostwald ripening process. The anisotropic growth was maintained by selectively covering the {100} facets with PVP while leaving the {111} facets largely uncovered by PVP and thus highly reactive.

Solution-phase syntheses may represent the most promising route to nanostructures in terms of cost, throughput, and the potential for high-volume production.1 The formation of 1D nanostructures (such as nanowires) in an isotropic liquid phase is relatively simple and straightforward if the solid material has a highly anisotropic crystal lattice. As demonstrated with molybdenum chalcogenides and trigonal-phase chalcogens (selenium and tellurium) as two examples, uniform nanowires could be easily grown from their solutions with lengths of up to ∼50 µm.2 The growth of metal nanowires from isotropic solutions has been a more challenging task because almost all metals are crystallized in the highly symmetric cubic lattices. For this system, anisotropic confinements have to be applied to induce and maintain 1D growth. Confinements that have been widely explored include physical templates (such as channels in porous materials and mesostructures self-assembled from various molecular species) and surface-capping reagents.3 A number of capping reagents have been examined to control the growth rates of metal surfaces kinetically and thus achieve 1D growth. For example, Wang et al. have demonstrated the synthesis of gold nanorods by using * To whom correspondence should be addressed. E-mail: xia@chem. washington.edu. 10.1021/nl034312m CCC: $25.00 Published on Web 06/10/2003

© 2003 American Chemical Society

cetyltrimethylammonium bromide (CTAB) and another much more hydrophobic cationic surfactant (e.g., tetraoctylammonium bromide or TOAB) as the capping reagents.4 This method was later exploited by El-Sayed et al. to generate gold nanorods with well-controlled aspect ratios and thus surface plasmon resonance properties.5 It was also extended by Murphy et al. to the synthesis of gold and silver nanorods with relatively high aspect ratios by adding seeds to the synthetic sytem.6 In a typical procedure, gold or silver nanoparticles 3-5 nm in diameter were added as seeds to a solution containing CTAB, together with a metal precursor such as HAuCl4 or AgNO3. When a weak reducing agent such as ascorbic acid was introduced, the seeds could serve as nucleation sites for the growth of nanorods under the confinement of CTAB. More recently, this synthetic route was combined with a photochemical process by Esumi et al. and Yang et al. to prepare uniform gold nanorods with controllable aspect ratios.7 In general, the 1D nanostructures synthesized with CTAB as the capping reagent are twinned in crystal structure. On the basis of their electron diffraction and high-resolution transmission electron microscopy (HRTEM) studies, Murphy and Mann et al.8 and Gai et al.9 have proposed a mechanism in which gold nanorods were assumed to evolve from multiply twinned particles (MTPs) with a

decahedral shape. However, these studies did not provide any evidence to confirm that the cross sections of these nanorods indeed exhibited a pentagonal shape. Also, the role played by CTAB (or mesostructures self-assembled from these molecules) in inducing and maintaining 1D growth was not clear. We have recently demonstrated a different approach based on the polyol process for the large-scale synthesis of uniform silver nanowires.10 It involved the reduction of silver nitrate by ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP). The key to the formation of uniform silver nanowires was believed to be the use of PVP as a polymer capping reagent and the introduction of a seeding step. When silver nitrate was reduced in the presence of seeds (Pt or Ag particles of a few nanometers), silver nanoparticles with a bimodal size distribution were generated in the reaction mixture via heterogeneous and homogeneous nucleation processes, respectively. In the following step, silver nanoparticles with larger sizes were able to grow at the expense of smaller ones through Ostwald ripening. In the early stage of the ripening process, the majority of larger silver particles could be directed to grow into nanorods with uniform diameters, which could then grow continuously into uniform nanowires of up to 50 µm in length. Although the silver nanostructures involved in all steps of this process have been confirmed by electron microscopy and spectroscopic (based on surface plasmon resonance absorption) measurements, the growth mechanism is still elusive. For example, it is not clear how the silver nanorods are evolved from nanoparticles at the initial stage of the Ostwald ripening process. The exact role played by PVP in the growth process also remains to be elucidated. The aim of this paper is to address these two issues by providing some new experimental evidence and thus to shed light on the growth mechanism. Figure 1 shows a schematic illustration of the mechanism that we propose to account for the evolution of silver into 1D nanostructures at the initial stage of the Ostwald ripening. This mechanism resembles the one proposed by Murphy and Mann et al. for growing gold nanorods with CTAB as the capping reagent.8 Two components are critical to this growth mechanism: the MTP with the decahedral shape and PVP. The MTP has 5-fold symmetry, with its surface bounded by ten {111} facets. In terms of surface-energy minimization, it is favorable to form such a twinned nanostructure once the particle size has reached a critical value.11 A set of five twin boundaries are required to generate the decahedral particle because it is impossible to fill the space of an object of 5-fold symmetry with only a single-crystalline lattice. The twin boundary is believed to originate from an angular gap or wedge Volterra disclination (with an angle of ∼7.5°).11 Previous work on the synthesis of silver colloids suggested that the formation of decahedral MTPs was, in particular, popular in solution-based methods such as the polyol process.12 Our TEM studies (Figure 2, the first piece of evidence) confirmed that some of the silver nanoparticles indeed had decahedral topology (with the twin boundaries fanning out in a 5-fold symmetry) before silver nanorods appeared in the reaction solution. Because a twin boundary 956

Figure 1. Schematic illustration of the mechanism proposed to account for the growth of silver nanowires with pentagonal cross sections: (A) Evolution of a nanorod from a multiply twinned nanoparticle (MTP) of silver under the confinement of five twin planes and with the assistance of PVP. The ends of this nanorod are terminated by {111} facets, and the side surfaces are bounded by {100} facets. The strong interaction between PVP and the {100} facets is indicated with a dark-gray color, and the weak interaction with the {111} facets is marked by a light-blue color. The red lines on the end surfaces represent the twin boundaries that can serve as active sites for the addition of silver atoms. The plane marked in red shows one of the five twin planes that can serve as the internal confinement for the evolution of nanorods from MTP. (B) Schematic model illustrating the diffusion of silver atoms toward the two ends of a nanorod, with the side surfaces completely passivated by PVP. This drawing shows a projection perpendicular to one of the five side facets of a nanorod, and the arrows represent the diffusion fluxes of silver atoms.

Figure 2. Typical TEM image of a sample that was prepared from a reaction solution prior to the appearance of silver nanorods. The arrow marks a decahedral MTP of silver characterized by a 5-fold twin structure.

represents the highest-energy site on the surface of an MTP, it helps to attract silver atoms to diffuse toward its vicinity from the solution during the Ostwald ripening process. The crystallization of silver atoms on the twin boundaries thus leads to the uniaxial elongation of an MTP into a rod-shaped nanostructure under the confinement of twin planes (labeled in red). Because the twin planes do not twist or bend during the entire growth process, we expected to observe five straight edges along the longitudinal axis of each individual Nano Lett., Vol. 3, No. 7, 2003

Figure 3. (A) Selected-area electron diffraction pattern taken from an individual nanowire by directing the electron beam perpendicular to one of the five side surfaces. It corresponds to a superposition of square [001] (marked in blue) and rectangular [11h2h] (marked in red) zone patterns of face-centered cubic silver, together with the double-diffraction reflections. The (220) and (2h2h0) reflections resulted in the same diffraction spots in the pattern. (B) HRTEM image taken from the end of a nanowire, showing the existence of a twin plane along the longitudinal axis. (C) TEM images taken from a microtomed sample of nanowires. (D) TEM image of the product prepared in an early stage of the wire growth process. The arrows indicate the twin planes in the middle of each nanorod. (E) TEM image taken from a long silver nanowire, suggesting that there was no twisting or bending for the twin planes (indicated by the arrow).

nanowire. The newly formed side surfaces, {100} facets, must be stabilized through chemical interactions with the oxygen (and/or nitrogen) atoms of the pyrrolidone units of PVP.13 In comparison, the interaction between PVP and the {111} facets should be much weaker to enable the two ends of the nanorod to grow continuously throughout Ostwald ripening. Once the rod-shaped structure has been formed, it can readily grow into a longer nanowire because its side surfaces are tightly passivated by PVP and its ends are largely uncovered and remain to be attractive (or reactive) toward new silver atoms. This mechanism is consistent with our previous spectroscopic observation of the growth of silver nanowires, where the growth rate of silver nanorods was greatly accelerated once the aspect ratio of silver nanorods had reached a critical value. (See, for example, Figure 3B in ref 10c.) Figure 1B shows how silver atoms are continuously transported to the ends of a silver nanorod through diffusion, indicating that silver atoms always prefer to diffuse to the ends of a nanorod because of their high chemical potential and reactivity. As a result, the pentagonal cross section, the straightness of five side edges, and the flatness of five side surfaces are all kept throughout the growth process. The second piece of evidence that supports the above mechanism came directly from our electron diffraction and electron microscopy studies on the silver nanowires. Figure 3A shows the typical selected-area electron diffraction (SAED) pattern recorded from an individual silver nanowire Nano Lett., Vol. 3, No. 7, 2003

by aligning the electron beam perpendicular to one of the five side surfaces. This pattern indicated that each silver nanowire was not a single crystal because the diffraction spots could not be assigned to any particular simple pattern associated with face-centered cubic silver. As shown in Figure 3A, this pattern contained an interpenetrated set of two individual diffraction patterns, with the one in square symmetry corresponding to the [001] zone axis and the other one in rectangular symmetry corresponding to the [11h2h] zone axis. The mutual orientation of these two zones induced the generation of double diffraction, which accounted for the remaining spots. These assignments were also consistent with the results obtained for multiply twinned nanorods of gold and copper having the same pentagonal symmetry.8,14 The twin lamellae on the surface could be easily resolved from the HRTEM image shown in Figure 3B. This image was recorded from the end of a silver nanowire, showing a twin plane oriented parallel to its longitudinal axis, as indicated by the arrow. Figure 3C shows the TEM image taken from a microtomed sample of silver nanowires.15 Most of the cross sections shown in this image exhibited a pentagonal shape (or D5h symmetry). The nonpentagonal cross sections could be attributed to the random orientations of silver nanowires relative to the edge of the microtome knife. The coexistence of a small number of silver colloidal particles in this sample might be another reason for the observation of nonpentagonal cross sections. The inset gives the TEM image of a pentagonal cross section (at a slightly higher magnification), clearly revealing the 5-fold contrast for the twin boundaries. The stark contrast across each twin plane confirmed that each silver nanowire contained five single-crystalline subunits. Note that each nanowire had five equivalently flat side surfaces. No matter which one of them lay against the surface of the TEM grid during sample preparation, the TEM image always displayed a twin boundary in the middle of a nanowire (Figure 3B and D). In particular, the TEM images shown in Figure 3D and E clearly showed that the twin boundary (or twin plane) was straight and continuous along the entire longitudinal direction of each nanorod or nanowire. It is believed that the existence of such 5-fold twinned structures provides the internal confinement necessary for the processing of silver nanoparticles into nanorods or nanowires. As the third piece of evidence, our SEM studies further confirmed that silver nanowires synthesized using the polyol method had pentagonal cross sections. Figure 4A shows the SEM image taken from a sample of silver nanowires after it had been sonicated for 1 h. During sample preparation, some wires were broken and happened to be oriented perpendicular to the substrate. The cross sections of these broken nanowires displayed a pentagonal shape, and the ends of the unbroken ones exhibited a rounded, pyramidal profile. It is also worth pointing out that the nanowire had five straight side edges parallel to the longitudinal axis and five flat side surfaces (as indicated by the arrows). More interestingly, the pentagonal cross sections of these nanowires could also be preserved in a template-directed process where gold nano957

Figure 4. SEM images taken from (A) silver nanowires and (B) gold nanotubes that had been broken to show their cross sections through sonication. The gold nanotubes were prepared by reacting the silver nanowires with an aqueous HAuCl4 solution.

tubes were formed via the galvanic replacement reaction between silver nanowires and an aqueous HAuCl4 solution.16 Figure 4B shows the SEM image taken from a typical sample that had also been sonicated for a few minutes to break some of the gold nanotubes. As indicated by an arrow, the morphology of these gold nanotubes was similar to that of the silver nanowires, with each one of them having a pentagonal cross section, five straight side edges along its longitudinal axis, and five flat side surfaces. This observation also supports our statement in a previous publication with regard to the epitaxial relationship between the silver surface and the gold sheath.17 It has been suggested that atoms on different crystallographic facets might have different interaction strengths with a polymeric or surfactant capping reagent, leading to the anisotropic growth of a solid material.18 For the silver/ PVP system, it has long been suspected that PVP might interact more strongly with the {100} facets (i.e., the side surfaces of a silver nanowire) than with the {111} facets (i.e., the ends of a silver nanowire). Such a difference in interaction strength is now verified, for the first time, by 958

attaching gold nanoparticles to the surfaces of a silver nanowire through the dithiol linkage. In a typical experiment, the as-obtained suspension of silver nanowires was diluted with ethanol (1:10 V/V), followed by the addition (120 µL to 1 mL of a silver nanowire suspension) of a very dilute (0.5 µM) solution of 1,12-dodecanedithiol in ethanol and incubation for 1 h. Gold nanoparticles (synthesized by reducing HAuCl4 with sodium citrate19) were then added to the dithiol-modified silver nanowires, and the mixture was held at room temperature for 10 h under gentle magnetic stirring. After removing the free gold nanoparticles via centrifugation, the silver nanowires were collected for SEM imaging. As shown in Figure 5, most of the ends of the nanowires were decorated with gold nanoparticles (which appeared as bright dots), and the side surfaces remained smooth without any attachment of gold nanoparticles. In addition, the gold nanoparticles were also able to adsorb selectively onto the fresh surfaces created when a nanowire was broken. These observations implied that the interaction between PVP and the side surfaces of silver nanowires was too strong to be replaced with the dithiol molecules when the concentration of dithiol was sufficiently low. In comparison, the interaction between PVP and the ends of each nanowire was much weaker, and the dithiol molecules could selectively adsorb onto these surfaces, resulting in the attachment of gold nanoparticles. This study also provides another piece of evidence to support the mechanism illustrated in Figure 1: PVP is able to inhibit the growth of {100} facets by covering them heavily through a strong chemical adsorption process. In comparison, the {111} facts were largely uncovered by PVP and thus remained to be highly reactive toward the continuous addition of silver atoms. It is worth mentioning that the final morphology of silver nanostructures synthesized using the polyol process was highly dependent on the concentration of PVP added to the reaction solution. When the concentration of PVP was relatively high (e.g., with the molar ratio between PVP and silver nitrate being larger than 10), only silver nanoparticles with quasi-spherical shapes were obtained as the product. The absence of nanowires in the final product could be attributed to two possibilities: (i) a high concentration of PVP was not favorable for the formation of MTPs with the decahedral shape and (ii) a high concentration of PVP might lead to the formation of a thick coating over the entire surface of an MTP, including the twin boundaries. In the later case, the selectivity in interaction between PVP and various crystallographic planes was lost, and thus no anisotropic growth could be induced. However, needlelike structures with rough surfaces were produced when the molar ratio between PVP and silver nitrate was relatively low (e.g., less than 1). Under this condition, PVP was not be able to form a continuous layer to passivate the side surfaces of individual nanowires completely, resulting in loose control over the growth of silver nanostructures in the lateral directions. In addition to the concentration of PVP, the reaction temperature was also found to play an important role in controlling the morphology of silver nanostructures. No Nano Lett., Vol. 3, No. 7, 2003

Figure 5. SEM image of several silver nanowires after they had been reacted with 1,12-dodecanedithiol, followed by incubation with gold nanoparticles for 10 h. The bright dots at the ends and broken sites of the nanowires represent gold nanoparticles that had been attached through the dithiol linkage. Silver nanorods with relatively low aspect ratios were selected for this study to show both ends of each nanorod in the same image.

nanowire could be formed when the temperature was lower than 110 °C. Our TEM studies indicated that the products obtained at 100 °C were composed of silver nanoparticles with different morphologies and sizes. Some of these nanoparticles could be transformed into nanorods/nanowires after the solution (prepared at 100 °C) was treated at 160 °C for 1 h. This observation implies that the twin boundaries of MTPs could not be activated or that the {100} facets could not be generated until the temperature was sufficiently high. It is well known that the melting point of metallic nanoparticles is much lower than that of the bulk material.20 As a result, the twin boundaries of MTPs might be slightly melted at relatively high temperatures, and thus their reactivity might be enhanced as compared to that of the solid state. During the growth process, the slightly melted twin regions on the end surfaces of a silver nanowire could serve as intermediate phases to facilitate the transport of silver atoms from the solution phase to the growing surfaces. In this regard, the mechanism of our polyol process seems to be similar to that of the solution-liquid-solid (SLS) methods demonstrated for the synthesis of highly crystalline nanowires from III-V semiconductors and silicon.21 In summary, the present work shows that the silver nanowires synthesized using the PVP-mediated polyol process have a pentagonal cross section together with a 5-fold twin structure. It is believed that such nanowires are evolved from decahedral MTPs of silver with the help of PVP, a polymer capping reagent capable of effectively covering and thus stabilizing the newly formed {100} facets rather than the {111} facets. The pentagonal symmetry of the twinned structure and the use of PVP with a proper concentration both play important roles in confining (internally and externally) the growth of silver nanowires to the 1D mode. On the basis of this argument, the yield for the formation of silver nanowires should be largely determined by the concentration of decahedral MTPs generated in the early stage of the Ostwald ripening process, which can be Nano Lett., Vol. 3, No. 7, 2003

maximized by optimizing the molar ratio between PVP and silver nitrate as well as the reaction temperature. Acknowledgment. This work has been supported in part by a Career Award from the National Science Foundation (DMR-9983893) and a Fellowship from the David and Lucile Packard Foundation. Y.X. is a Camille Dreyfus Teacher Scholar (2002) and an Alfred P. Sloan Research Fellow (2000). B.M. and T.H. thank the Center for Nanotechnology at the University of Washington for two IGERT Fellowship Awards supported by the NSF (DGE-9987620). We also thank Mrs. Stephanie Lara in the Department of Pathology at the University of Washington for preparing the microtomed samples. References (1) See, for example, (a) Alivisatos, A. P. Science 1996, 271, 933. (b) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80. (c) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257. (d) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng, J. G. Science 2001, 294, 1901. (e) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (2) (a) Golden, J. H.; DiSalvo, F. J.; Fre´chet, J. M. J.; Silcox, J.; Thomas, M.; Elman, J. Science 1996, 273, 782. (b) Song, J.; Messer, B.; Wu, Y.; Kind, H.; Yang, P. J. Am. Chem. Soc. 2001, 123, 9714. (c) Gates, B.; Yin, Y.; Xia, Y. J. Am. Chem. Soc. 2000, 122, 12582. (d) Mayers, B.; Xia, Y. J. Mater. Chem. 2002, 12, 1875. (3) See, for example, (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) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (c) Ahmadi, T. S.; Wang, Z. L.; Green, T. C.; Henglein, A.; El-Sayed, M. A. Science 1996, 272, 1924. (d) Peng, Z. A.; Peng, X. J. Am. Chem. Soc. 2002, 124, 3343. (e) Lee, S.-M.; Cho, S.-N.; Cheon, J. AdV. Mater. 2003, 15, 441. (4) Yu, Y.-Y.; Chang, S. S.; Lee, C.-L.; Wang, C. R. C. J. Phys. Chem. B 1997, 101, 6661. (5) Link, S.; Mohamed, M. B.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3073. (6) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (b) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 617. (7) (a) Kim, F.; Song, J.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316. (b) Kameo, A.; Suzuki, A.; Torigoe, K.; Esumi, K. J. Colloid Interface Sci. 2001, 241, 289. (8) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765. 959

(9) Gai, P. L.; Harmer, M. A. Nano Lett. 2002, 2, 771. (10) (a) Sun, Y.; Gates, B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. (b) Sun, Y.; Xia, Y. AdV. Mater. 2002, 14, 833. (c) Sun, Y.; Yin, Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736. (11) Marks, L. D. Rep. Prog. Phys. 1994, 57, 603. (12) Silvert, P.-Y.; Herrera-Urbina, R.; Tekaia-Elhsissen, K. J. Mater. Chem. 1997, 7, 293. (13) (a) Huang, H. H.; Ni, X. P.; Loy, G. L.; Chew, C. H.; Tan, K. L.; Loh, F. C.; Deng, J. F.; Xu, G. Q. Langmuir 1996, 12, 909. (b) Bonet, F.; Tekaia-Elhsissen, K.; Sarathy, K. V. Bull. Mater. Sci. 2000, 23, 165. (14) Lisiecki, I.; Filankembo, A.; Sack-Kongehl, H.; Weiss, K.; Pileni, M.-P.; Urban, J. Phys. ReV. B 2000, 61, 4968. (15) For the preparation of microtomed samples, the as-synthesized silver nanowires were dispersed in Spurr’s epoxy with a low viscosity (Polysciences, Warrington, PA) and were then allowed to polymerize for 48 h at 60-65 °C. A microtome (Reichert/Jung Ultra-cut E, Leica, Arcadia, CA) equipped with a diamond knife was used to cut the cured epoxy resin into slices with a thickness of less than 100 nm.

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Nano Lett., Vol. 3, No. 7, 2003