Twinned Structure and Growth of V-Shaped Silver Nanowires

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Twinned Structure and Growth of V-Shaped Silver Nanowires Generated by a Polyol-Thermal Approach X. C. Jiang,*,† S. X. Xiong,† Z. A. Tian,†,‡ C. Y. Chen,† W. M. Chen,† and A. B. Yu† † ‡

School of Materials Science and Engineering, The University of New South Wales, Sydney, NSW 2052, Australia School of Physics & Microelectronic Science, Hunan University, Changsha 410082, China

bS Supporting Information ABSTRACT: This study demonstrates the details on the twinned structure and growth process of V-shaped silver nanowires with different bending angles (e.g., 90°, 120°, and 135°) confirmed by high-resolution transmission electron microscopy (HRTEM). These nanowires could be synthesized by a facile but effective polyol-thermal reaction method in autoclaves (160-180 °C). The nearly uniform-size silver nanowires show an average diameter of ∼45 nm and length up to tens of micrometers. The microstructure and optical properties of the silver nanowires were characterized by various advanced techniques, including TEM, HRTEM, scanning electron microscopy (SEM), and ultraviolet-visible (UV-vis) spectroscopy. The twinned structure can occur in both silver spherical particles and nanowires, confirmed by HRTEM analysis and also simulated by molecular dynamics methods. The growth of V-shaped nanowires by two possible means was particularly investigated: (i) crystal lattice match-induced end-to-end or end-to-side fusion of two nanowires, and (ii) twinned crystal plane-induced growth. Such structural and mechanistic understanding of silver crystals would be useful for the shape, size, and property control of functional nanoparticles.

1. INTRODUCTION Silver nanoparticles have received considerable attention because of their fascinating optical, electronic, and physicochemical properties.1-7 Research has been directed toward not only the development of synthesis, growth, and mechanistic understanding but also the exploration of potential applications in many areas, such as near-field optical probes, optical sensors, surface enhanced Raman spectroscopy (SERS), and biomedical labeling.8-20 It is well-known that the properties and applications of metal nanoparticles are heavily affected by the particle characteristics, i.e., morphology, size, and size distribution. The shape/size control of metal nanoparticles is critical and has attracted considerable attention in the past.8-20 Of the achieved silver nanoparticles so far, one-dimensional (1D) silver nanostructures (e.g., rods and wires) have been extensively studied because of their anisotropic geometry and because the ends can be used to generate generating maximum electromagnetic-field enhancement in SERS. For example, the array of silver nanowires formed by the Langmuir-Blodgett technique can be used for SERS measurements of dye molecules because of the rod coupling and the specific end effect.7 Many attempts have been made to control the formation and growth of 1D silver nanowires, including physical and chemical techniques. Nanolithography, a commonly used physical technique using beams of electrons, ions, or photons (UV, X-ray) offers the highest degree of flexibility and control over the nanodot size, r 2011 American Chemical Society

shape, and arrangement. Electron beam-induced lithography (EBID) is a fast, one-step physical technique used for preparation of nanostructures, particularly for fabrication of metal lines, twoand three-dimensional (2D/3D) structures, and submicrometer devices with diverse applications such as microelectronics, nanophysics, and molecular biology.21-28 Compared to the above-mentioned physical techniques, various chemical techniques have been reported in the past, such as photochemical, electrochemical, template-directed, and wetchemical methods. They are much more successful and show some unique features in preparation, e.g., simple operation condition, diverse choices in solvents, controllable shape and size. Examples of such chemical techniques have been investigated including the so-called `soft’ template surfactant-directed synthesis of 1D metallic (gold and silver) nanostructures29,30 and a DNA molecule template for gold or silver particle aggregates in conductive wires;31-33 other examples have also been demonstrated including the so-called `hard’ template methods such as an electrochemical method for silver nanowires using an anodic aluminum oxide (AAO) template,34-36 a polycarbonate template for silver nanowires in the ionic liquid [EMIm]PF6,37 carbon nanotubes,38,39 and TiO2 film40,41 for 1D silver nanowires. Received: November 4, 2010 Revised: December 13, 2010 Published: January 18, 2011 1800

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The Journal of Physical Chemistry C A polyol-mediated synthesis, as a solvothermal reduction method, has been widely studied, which could lead to the kinetics or thermodynamics growth depending on experimental parameters.42-45 Tang et al.46 demonstrated the effect of gas bubbling for the synthesis of silver nanostructures from a mixture of AgNO3/NaCl/poly(vinylpyrrolidone) (PVP) in ethylene glycol (EG). Monodispersed silver nanowires were synthesized under oil-bath heating in high yield (>90% without isolation) while bubbling air through a reagent solution from ca. 20 °C to boiling point of EG (∼197 °C) for ∼20 min. Ni et al.47 reported the EG-PVP/NaCl method to synthesize Ag nanowires with different aspect ratios at 170 °C. Tsuji et al.48 demonstrated microwave (MW)-polyol synthesis experiments, by focusing on the influence of Pt catalysts and Cl- anions on the formation of the 1D Ag nanowires. The autoclave, specifically designed for the synthesis of silver nanoparticles under a relatively high pressure, has been widely used in the past. For example, Qian et al.49,50 reported the hydrothermal synthesis of silver nanowires prepared by the interaction of AgNO3 and glucose in the presence of poly(vinyl alcohol) (PVA). Nanowires and microfibers of silver have been synthesized by the interaction of silver nitrate and starch employing a hydrothermal route in an autoclave at 170 °C for 24 h.51 Lu et al.52 synthesized silver nanoplates (20-100 nm in edge length) in DMF or ethanol in the presence of PVP as a capping agent through a solvothermal process in autoclave(s). Tian et al.53 reported that a direct solvothermal treatment of an ethanol solution of AgNO3 and dodecanethiol resulted in the formation of Ag nanoparticles (5-100 nm). Wani et al.54 demonstrated that silver nanoparticles (5 nm) have been successfully synthesized by a simple and modified ethanol-thermal method with NaBH4 as reducing agent. You et al.55 reported that singlecrystalline silver microplates with average edge length of about 1.5 μm and thickness of 100 nm have been synthesized by an extraction-solvothermal method. Yang et al.56 reported that silver nanoparticles with different structural architectures, including nanorods, triangular plates, hexagonal plates, nanocubes, and polyhedrons, could be synthesized in a solvothermal process by using DMF or ethanol as a solvent. Liu and co-workers reported that they prepared multitwinned nanorods and single crystalline nanohexapods (a = 9.6193 Å) using PVP-H (MW ≈ 1 300 000) and PVP-L (MW ≈ 30 000) in ethylene glycol solution heated at around 160 °C, respectively.57 Despite some success, limitations still exist in many chemical methods. Many of them are empirical in nature and applicable to various specific system(s) or critical experimental conditions, and no single approach could be developed as a general synthesis route in view of its potential as an efficient, cost-saving, and largescale production approach for shape/size-controlled silver nanostructures. Moreover, the formation and growth of the anisotropic morphology of silver nanoparticles, particularly for the V-shaped silver nanowires, are not adequately understood at the moment. To develop facile but efficient synthesis strategies for high yield, large-scale, and shape/size-controlled silver nanoparticles with desirable functional properties and to fully understand their formation and growth mechanism is still a challenging task. In this study, we develop a facile but effective polyol-thermal synthesis method to prepare silver nanowires with uniform size in diameter, in high yield (>98%), and to be reproducible, controllable, and have potential for large scale production. The function of various experimental parameters (e.g., temperature, time, concentration) in the formation and growth of silver

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nanoparticles are identified by advanced experimental techniques including transmission electron microscopy (TEM) and highresolution TEM (HRTEM), scanning electron microscopy (SEM), and UV-vis spectrometry. Finally, the formation and growth processes of the V-shaped silver nanowires with different bending angles are discussed.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Silver nitrate (AgNO 3, 99.9%), ethylene glycol (EG, 99.9%), and various surfactants such as poly(vinylpyrrolidone) (PVP, MW = 55 000), sodium bis(2-ethylhexyl)sulfosuccinate (NaAOT, 99%), sodium dodecyl sulfate (SDS, AR grade), cetyltrimethylammonium bromide (CTAB, AR grade), melamine (AR grade), and thiol (AR grade) were all purchased from Sigma-Aldrich and used as received without further treatment. All the solutions were freshly made for the synthesis of silver nanoparticles. Ultrapure water was used in all the synthesis processes. 2.2. Synthesis of Silver Nanowires. The experimental procedure was performed as follows. In a typical synthesis, 10 mL of EG solution containing a small amount of PVP (MW= 55 000) (0.050 g) was stirred vigorously until all the PVP dissolved completely and the solution became transparent. Second, 0.02548 g of AgNO3 (∼0.15 mmol) was dissolved in 5 mL of EG solution, followed by vigorous stirring until the solution became homogeneous. Finally, the two solutions were put into a 25-mL vial and mixed homogeneously by stirring, followed by placing the vial into a Teflon-lined stainless steel autoclave (50 mL capacity). The autoclave was heated in an oven and maintained at a temperature range of 80 to 180 °C for different times (30 min to a few hours) for optimization of products. The color of the reaction system changed from light yellow to brown, purple, green, or gray, depending on the particle shape, size, and size distribution. To optimize parameters for uniform size silver nanowires, the parametric variables (e.g., temperature, concentration, time, and ratio of PVP to silver nitrate) and the possible fusion growth by using two short silver rods were investigated in the reaction system and are summarized in the Supporting Information (Figures S1-7). 2.3. Characterization. Transmission electron microscopy (TEM) images were conducted with a JEOL1400 operated at 100 kV. The crystalline structure of the silver nanowires was further characterized by high resolution transmission electron microscopy (HRTEM), Philips CM200, at an accelerating voltage of 200 kV. The specimen was prepared by dropping the particle suspension onto copper grids covered with amorphous carbon and air-drying naturally. The morphology and size observations were also performed by scanning electron microscopy (SEM). The specimen was prepared by dropping the particle suspension onto a conductive substrate and drying naturally in air. The UV-vis absorption spectrum was obtained on a CARY 5G UV-visible spectrophotometer (Varian) with a 1-cm quartz cell.

3. RESULTS AND DISCUSSION 3.1. Synthesis and Structure of Silver Nanowires. To characterize the morphology and microstructure of silver particles, TEM (HRTEM) microscopy was used. Figure 1A shows a typical TEM image of the generated silver nanowires through a polyol-thermal approach heated at 180 °C for 2 h in the 1801

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The Journal of Physical Chemistry C presence of PVP molecules. After careful inspection, two kinds of silver nanowires can be observed: straightforward and V-shaped. These silver nanowires are nearly uniform in diameter at ∼40 nm, and the lengths range from hundreds of nanometers to tens of micrometers. The nanowires can generally be fabricated through an ethylene glycol-thermal approach in either an open-air reaction system or in a sealed autoclave. Compared to an open-air reaction system, the use of an autoclave shows some advantages in the preparation of silver nanowires, such as convenient operation, easily controllable, reproducible, high yield (>98%), potential for large scale production, and lowered content of unwanted spherical particles. With close inspection, the ends of the rods or wires are usually tapering tips (Figure 1). This is commonly observed in face-centered-cubic (fcc) metallic 1D nanostructures (silver, gold nanowires).42-45 3.2. Twinned Silver Nanowires. The crystallographic structure of one tip of an individual nanorod was identified using HRTEM. Figure 2A shows the HRTEM image of a twinned structure at the end of a nanowire. Three twinned sections (T1, T2, and T3) show a similar lattice fringe distance of ∼2.32 Å, assigned to fcc Ag{111} planes. Upon careful measurement, there is a twisting angle (∼12.5°) in the {111} planes between T1 T2 and T2 T3 sections, suggesting that the twinned silver crystal exists. Again, the T2 section shows a relative weak contrast in {111} planes compared to those shown in T1 and T3 sections under the same electron beam direction, further indicating that the nanorod is a twinned crystal structure. Moreover, the lattice fringes with a distance of ∼2.03 Å observed parallel to the

Figure 1. TEM image showing silver nanowires with bending structures obtained by the polyol-thermal method in an autoclave(s).

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growth direction of the silver nanowires could be assigned to {002} planes of the fcc silver crystal. The corresponding selected area electron diffraction (SAED) was recorded and is shown in Figure 2B. The diffraction pattern indicates that the silver nanowire is crystallized but is not a single crystal because the diffraction spots could not be assigned to any particular simple pattern associated with the fcc silver crystal. Among them, two (or more) sets of diffraction spots exist in the SAED pattern based on the contrast and distance to the beam center. Two sets could be indexed as (00-2), (1-1-1), and (1-11) planes marked in red and green circles, respectively.58-60 This fact suggests that the obtained silver nanowires are of multiply twinned structure. The longitudinal direction of the nanowires is [110], which is commonly observed in the twinned metal crystals. In addition, a few weak diffraction spots observed are not attributed to the basic diffraction of silver but indexed as the double diffractions.61 This is in agreement with the observations for twinned crystals of gold,61 copper,62 and silver nanorods.63 To further understand the growth of Ag nanowires, more details of the microstructure were investigated by using HRTEM image in Figure 3, in which the crystal lattice fringes of one part of an individual nanowire are clearly observed with a spacing distance of ∼2.03 and 2.35 Å, corresponding to {002} and {111} planes of fcc silver nanostructures (a = 4.0862 Å), respectively. The growth direction is [110]. This is different from that of a silver nanohexapod with a = 9.6193 Å, reported by Liu et al.,57 who used PVP in different molecular weights to control the silver particle growth in a polyol process. In our proposed approach, the formation and growth of silver nanowires may be caused by the deposition of silver atoms on {111} planes (Figure 3A). This is because the stronger surface interaction of PVP molecules with Ag{100} planes can lead kinetic growth to 1D nanostructure.42-45 Generally, for the fcc metals (e.g., Au, Ag, and Cu), the {111} facets are the most stable among the three (111, 110 and 100), the {100} facets are the next stable, and the {110} are the least stable. The growth rate of {110} facets should be much faster than those of the other two facets in the absence of surfactant(s) or polymer(s) which can affect the aspect ratios (longitudinal to transversal) of nanowires. The free energy minimization could result in atomic preferential growth if the PVP strongly attached on the {100} facets. The twinned fcc-Ag nanowires have been suggested to arise from selective interaction between the PVP molecules and Ag{100} planes. In comparison, the interaction between PVP and the {111} facets should be much weaker to

Figure 2. (A) HRTEM image showing a tip of silver nanowire with twinned structures (T1-T2-T3). (B) The corresponding selected area electron diffraction (SAED) pattern with one set of assigned planes {00-2}, {1-1-1}, and {1-11} of face-center-cubic (fcc) silver crystals. 1802


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Figure 3. (A) HRTEM image showing the middle part of an individual silver nanowire with crystalline lattice {111} and {002} planes and the growth direction of [110]. (B) The corresponding electron diffraction pattern with one set of assigned planes (1-11), (002), and (-111). (C) The fast-Fouriertransformation (FFT) image of lattice fringes in the nanowire.

enable the two ends of the nanorod to grow continuously throughout Ostwald ripening. An anisotropic growth of the nanorod is achieved by deposition of Ag atoms onto the pyramidal tip terminated by {111} planes (Figure 2A) through a kinetics balance. Once the rod-shaped structure is 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 reactive toward the newly formed silver atoms. The growth rate of silver nanowires was accelerated once the aspect ratio of silver nanorods had reached a critical value.42-45,60,64 To better understand the growth process, the growth of silver nanoparicles at a high temperature of 180 °C and the effect of purity of PVP on particle shape were investigated (see Figures S5 and S6 in Supporting Information). The corresponding SAED pattern of the nanowire is shown in Figure 3B, indicating that a part of the silver nanowires is crystallized as a single crystal in the fcc structure, although the whole nanowire is multiply crystallized. One set of diffraction spots can be indexed as (1-11), (002), and (-111) planes. Because of the existence of a rotation angle during the picture taken in the microscopy, a proper angle needs to be adjusted to match the lattice fringes in HRTEM with the SAED pattern. However, the fast-Fourier transformation (FFT) process on the silver nanowire could avoid the misfit problem caused by TEM machine and directly reflect the crystalline planes and their growth orientation. Figure 3C shows the FFT image that is obtained directly from one part of the HRTEM image (Figure 3A), which is useful to understand the SAED pattern of the crystal structure (Figure 3B). 3.3. V-Shaped Silver Nanowires. Through carefule inspection, there are some V-shaped Ag nanowires formed in the product while using the polyol-thermal approach in the presence of PVP as a capping agent in the autoclaves (as circled or marked in Figure 1). The bent-shaped nanowires display different twisting angles (e.g., ∼120°, ∼135°). Why such nanowires are

formed and grow to be V-shaped structures needs to answered. Several representative examples were investigated in detail and are discussed in the following text. Figure 4 shows the TEM image of a V-shaped nanowire with a bending angle around 90° (Figure 4A). Apparently, a thin amorphous layer on the silver wires was formed, probably caused by the strong attachment of PVP molecules. Similar phenomena can be observed in Figures 2A and 3A. The amorphous thin layer of PVP molecules could benefit the formation of silver nanowires by strongly attaching to the Ag{100} and {110} rather than {111} facets. In addition, such a thin layer could protect Ag wires from possible surface etching by oxygen in air or in solution.42-45,60,64 The SAED pattern was recorded from the joint part and is shown in the inset of Figure 4A. Two or more sets of diffraction spots can be observed, and one set could be indexed as (1-11), (002), (020), (220), and (311) planes. Clearly, Figure 4B shows the HRTEM image of the V-shaped silver structure, in which a lattice spacing of 2.30 Å and 2.03 Å could be indexed as {111} and {002} planes, respectively, in the two directions of the silver nanowire(s). The formation of such V-shaped structures is probably caused by sharing a twinned crystal plane. The dot-circled part shows the boundary of the two directions; however, it is difficult to clearly identify the coemployed crystal plane. Fortunately, the square-framed part in the top of Figure 4B clearly shows the {111} planes sharing a boundary, and the symmetric {111} planes with an angle of ∼141° (=2 70.5°) is estimated according to the HRTEM image, which is in agreement with 5-fold multiply twinned planes (MTP) in a silver nanowire and a nanoparticle.42-45,60,64 For the 5-fold twinned silver nanowires, Wiley et al.64 demonstrated that these nanowires are produced by a polyol process with the side surfaces being bounded by the {100} facets, and the end surfaces being bounded by the {111} facets. They also described that two dominant factors are critical to this growth mechanism: the multiply twinned nanopaticles with a decahedral shape at the initial stage (discussed later in this work) and PVP as 1803



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Figure 4. (A) TEM image of a V-shaped nanowire with a bending angle of ∼90° and the corresponding SAED pattern (inset of TEM image). (B) The corresponding HRTEM image shows the joint section caused by the (111) lattice match in the two directions.

the surface modifier. In terms of surface-energy minimization, it is favorable to form such a twinned nanostructure once the particle size has reached a critical value.65 The twin boundary is believed to originate from an angular gap or wedge Volterra disclination (with an angle of ∼7.5°).65 The growth of gold nanorods was claimed to follow the same mechanism.66 As further confirmation, a multiple V-shaped nanowire was characterized. As shown in the TEM image in Figure 5A, one individual nanowire possesses multiple V-shaped structures with different angles (V1, V2, and V3). The corresponding SAED pattern is shown in Figure 5B, in which the diffraction spots are difficult to be indexed because of the complicated bending structure. The V1 section was checked by HRTEM and is shown in Figure 5C, where the lattice fringes are clearly observed in two directions. The lattice spacing of 2.03 Å and 2.32 Å could be indexed as the fcc-Ag {002} and {111} planes, respectively. The boundary (marked in a blue line) between the two groups' {111} planes is clearly observed, and the angle between the two groups' {111} planes is around 120°. An atomic model simulated by molecular dynamics (MD) is shown in Figure 5D. The structure is obtained from a freezing nanoparticle at 273 K without any relaxation, which resulted from a rapid cooling simulation for a silver droplet from 1273 K at a cooling rate of 1011 K/s. All other conditions are the same as those described in the previous study.66 The twinned {111} planes have a twinning angle of 120° in the simulated model (Figure 5D), which is in good agreement with our experimental observations shown in Figure 5C. Similarly, V2 and V3 sections with bending structure were also investigated. The HRTEM images for these two V-shaped sections are shown in Figure 5, parts E and F, respectively. Both of the V-shaped sections also result from the twinned {111} planes of the fcc silver crystal, as marked in blue lines (Figure 5E) and by the dot-circled section (Figure 5F). The corresponding MD-simulated atomic model (inset of Figure 5E) is obtained, which would support our experimental observations. In general, the V-shaped nanowires probably result from the twinned fcc silver crystals in this study. The formation of such bending structures has also been studied in the past. For example, Gao and co-workers58 reported the bending angles of silver nanowires in a range of 90-170°. They also speculated on a self-assembly growth of the Z-shaped

rods based on the formation of straight nanowires, namely, two nanowires that can connect closely in an end-to-end manner but are not fused to each other thoroughly. There is an obvious coherent interface between the two nanowires. Liu et al.57 observed that the turning angles are much more abundant in the range of 110-150°. They demonstrated that a V-shaped nanorod can be grown by elongation of two opposite tetragonal tips of a decahedral 5-fold-twinned seed. They proposed another explanation for such a structure: the capping mode. The PVP molecules can direct an isotropic packing for spherical particles with low packing density or anisotropic growth for the fcc-Ag nanowires with a close packing. Zhang et al.59 presented a V-shaped nanorod where the twinning area is connected by two nanowires and bonded with {111} facets. Such a joint may be caused by two reasons: (i) a new cyclic 5-fold twinned structure formed on the terminated {111} planes that results in the V-shaped nanowire; (ii) two neighboring 5-fold twinned nuclei bond together by the twinning of {111} planes and further grow into two nanowires independently. Both cases support that the intrinsic factor of the cyclic 5-fold twinned structure determines the diameter of the nanowires. 3.4. Fusion Growth of Silver Nanowires. The abovementioned HRTEM images (Figures 4 and 5) confirm that the 5-fold twinned structure on the terminated {111} planes could result in the V-shaped nanowire, in good agreement with the first reason proposed by Zhang et al.59 The second reason, fusion growth on the V-shaped silver nanostructures, was hardly supported. Such structures were observed in our experiments and discussed in detail in the following text. Figure 6A shows the TEM image of two nanowires that fuse via an end-to-end process. A small turning angle of ∼35° between the two nanowires was estimated. The corresponding SAED pattern recorded from the joint of two nanowires is shown in the inset of Figure 6B, in which the diffraction spots could be indexed as two sets of crystal planes originated from two nanowires. One set can be indexed as (-1-11), (00-2), and (-11-1) planes, marked with a red circle. Another one marked with a green frame can be indexed as the same but rotating an angle of ∼35°, corresponding to the fused two nanowires (Figure 6A). As further confirmation, the HRTEM image is shown in Figure 6B. The dash circled section clearly shows the 1804



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Figure 5. (A) TEM image of an individual silver nanowire with several bending structures (V1, V2, and V3) with different angles. (B) The corresponding SAED pattern of this nanowire. (C) HRTEM image showing the joint section of V1 with bent structure caused by the twinned (111) planes with lattice match. (D) The silver atomic symmetry scheme simulated by molecular dynamics (MD), in which the two twinned fcc Ag(111) planes share an atomic edge and match with the experimental observation. (E) The HRTEM image showing the joint section of V2 with bent structure caused by the twinned (111) planes with lattice match, where the corresponding silver atomic symmetry scheme simulated by MD is shown in the inset of the HRTEM image. (F) The HRTEM image showing the joint section of V3 with bent structure caused by the twinned planes.

joint of two nanowires. The end of the bottom nanorod seems to embed into the end of the top one through a crystal lattice match of the terminated {111} planes. Such a fusion at the end(s) could minimize the surface energy at the end(s) of silver nanowires/ rods. That is, the lattice match provides the possibility for fusion growth of two nanowires, and the {111} surface energy

minimization accelerates the fusion process, as discussed in the previous study.59 However, we found that the V-shaped fusion growth starting from two short nanorods is not available under the reported conditions. This is probably because the cooling to room temperature could inactivate or weaken the reactivity on the rod ends capped by PVP molecules, so two short rods were more difficult to fuse together 1805



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Figure 6. (A) TEM image showing the fused growth of two silver nanowires with an angle of 35° (marked in green and red lines). (B) The corresponding HRTEM image showing the fused section by an end-to-end approach due to the crystalline lattice match, and the SAED pattern with one set of diffraction planes (marked in red circles): (-1-11), (00-2), and (-11-1).

Figure 7. (A) TEM image showing a V-shaped fusion of two silver nanowires via an end-to-side approach and the corresponding SAED pattern (inset of TEM image). (B) The corresponding HRTEM image of the joint section, probably caused by the crystalline lattice match.

compared to those directly reacted in a high-temperature system (see Supporting Information, Figure S7). Figure 7 shows the TEM image of two nanowires that fuse together via an end-to-side process, marked as a dot circled section. The corresponding SAED pattern from the joint section was recorded and is shown in the inset of Figure 7A. One set of diffraction spots corresponding to the crystalline planes parallel to the nanowire marked by a red arrow could be indexed as {-200}, {-1-11}, and {-11-1} planes, respectively. Figure 7B shows the HRTEM image of the end-to-side fusion section of the two nanowires. The {002} planes nearly parallel to each other in the two sections sharing the boundary (dot circled area) were

clearly observed, although there is a small twisting angle between the lattice fringes marked by green-blue and blue-red lines. Nonetheless, the lattice match in the crystal {002} planes benefits the end-to-side fusion of two nanowires under the reported conditions. Both end-to-end and end-to-side fusion processes of silver nanowires (Figures 6 and 7) could happen at a relatively high temperature. This implies that the twin boundaries of MTPs could not be activated or that the {200} facets could not be generated until the temperature was sufficiently high (e.g., 180 °C). It is well-known that the melting point of metallic nanoparticles is much lower than that of the bulk material.68 As a result, the twin 1806



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Figure 8. (A) HRTEM image showing 5-fold symmetry twinned structure of a silver nanoparticle, in which 5 {111} planes share five edges as noted by arrows. (B) The MD-simulated scheme of such a 5-fold symmetry twinned structure where the five (111) facets (gray atoms) share five edges (coffeecolored atoms). (C) The corresponding SAED pattern of the 5-fold symmetry twinned structure. (D) The FFT image directly obtained from the HRTEM image showing the twinned silver planes, in which the angle of L1 L2 is ∼70.5° for (1-11) and (-111), and L2 L3 is ∼54.7° for (-111) and (002) planes, respectively.

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 or side 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.60 In this regard, the mechanism of our proposed polyol-thermal process seems to be the same as that of the solution-liquid-solid (SLS) methods for the synthesis of highly crystalline nanowires from III-V semiconductors and silicon.69-71 Another explanation for the fusion growth of silver nanowires was provided by Liz-Marzan et al., who demonstrated the aggregative mechanism of silver nanoparticles into wires through oriented aggregation of precursor nanoparticles (icosahedral and cuboctahedral Ag nanoparticles with multiple facets). These particles could self-assemble to aligned stripes and then fuse with each other with twisted structures and eventually yield single crystalline nanowires. In addition, they found that the nanoparticle aggregation only takes place through preferred facets of the original nanocrystals which are not perfectly

isotropic but have well-defined facets that may easily have both different polarizability and reactivity. Different reactivity on the crystallographic surfaces may give rise to the nanoparticleto-nanowire transition. This fusion aggregation is probably thermodynamically favorable due to the decrease in overall surface. Similarly, Tang et al.72a reported the formation of CdTe nanowires fused by spherical nanoparticles after removal of surface ligands, and Weller et al.72b reported the formation of ZnO nanorods from nanodots. Other examples involving spherical particle fusion have also been reported by Penn et al.73 who presented a mechanism for dislocation formation that may operate during early growth which involves attachment between two or more nanoparticles. Further studies need to be performed by in situ observation techniques for understanding the fusion mechanism. Such a fusion mechanism can be observed in other morphologies of nanoparticles. For example, Mirkin et al.74,75 demonstrated that the bimodal growth process of silver nanoprisms occurs through an edge-selective particle fusion mechanism, with four `small’ prisms coming together in stepwise fusion to form a `big’ prism through a photoinduced approach. Under such 1807


The Journal of Physical Chemistry C conditions, they tend to stick together by their edges, followed by fusion and recrystallization driven by a certain force. The driving force is thought to be the strong dipole-dipole interaction between the triangular plates. After the close connection at the edges, the plates are fused and recrystallized again to form the final morphology. 3.5. Twinned Silver Nanoparticles. Despite parametric optimization of temperature (Figures S1, S2), concentration of silver nitrate (Figure S3), molar ratio of PVP to silver (Figure S4, S5), and reaction time (Figure S6), it is still difficult to obtain pure nanowires. Spheres or cubes are always observed in the product (Figure 2B and Figure S3B-D). Two possible reasons can explain the phenomena: first, at the initial stage, two or more kinds of silver clusters or nuclei with different shapes (e.g., plates, spheres, hexagonal, tetragonal, or octagonal) coexist in the reaction system, which may lead to the formation of versatile shapes. The ratio of different shapes is dependent on the amount of the silver clusters and nuclei with different geometries at the initial stage, as reported in the solvothermal method,42-45 hydrochemical method,76-83 ultrasonic-assisted method,84 and templating method.85,86 The assumption can be proved by the existence of Ag2þ and Ag2þ(citrate) clusters,81 or Ag42þ, Ag84þ, Ag3þ, or Ag3 clusters.87-89 Henglein et al.87,88 has shed some light on the nucleation process by controlling the generation of zerovalent atoms and thus their agglomeration into small clusters in a gamma-radiation-based synthesis. Growth of these clusters (a few atoms) into nuclei (hundreds and more atoms in a nucleus) and nanocrystals (thousands of atoms or more) likely occurred through a combination of aggregation and atomic addition. Xia et al.89 demonstrated that there exists a smaller cluster, Ag3þ or Ag3, in the nucleation stage. These trimeric clusters can serve as nuclei for the addition of newly formed silver atoms and eventually lead to the formation of triangular nanoplates, while the Ag2þ, Ag42þ, and Ag84þ might benefit the formation of spheres. The second reason is the coexistence of several morphologies probably caused by the essential crystalline silver fcc structure, which usually shows single-, twin-, or multiple-twinned planes. This could play an important role in the formation of nanoparticles with special geometries. The ratios of rods to spheres/nearsphere structures could be determined by the amount of single or twin structures formed at the initial nucleation stage of the reaction. The 5-fold twinned structure of silver nanoparticles was particularly investigated, as described in the following text. Figure 8A shows the HRTEM image of a multiply twinned decahedral particle with an imperfect 5-fold twinned structure. Clearly, the five crystallites bond together with the Ag{111} lattice planes, as separated and marked by dotted arrows. Two fcc silver domains share a hexagonal close-packed (hcp) strip which includes several layers of hcp atoms (where the sequence of {111} planes is ABABAB 3 3 3 ). The joint {111} lattice planes probably provide a way to release the strain, which would lead to the formation of larger nanoparticles if the aggregations happened in the incomplete 5-fold twinned particles.59 A complete 5-fold twinned decahedral particle was simulated by MD simulation and is shown in Figure 8B. However, the real particles always contain some atomic stacking faults leading to an incomplete 5-fold twinned structure. A set of five twin boundaries are usually 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. Figure 8C shows the SAED of the silver particle (A), and the diffraction spots could be indexed as

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{002}, {1-11}, and {-111} planes from the [1-1-1] zone axis of the fcc silver crystal. In the SAED pattern, some additional weak diffraction spots were observed, probably caused by the double diffraction.59 The corresponding FFT image of the 5-fold twinned particle is shown in Figure 8D, where the angles between {1-11} and {-111} planes and {1-11} and {002} planes are estimated to be 70.5° and 54.7°, respectively. This is useful for abetter understanding of the twinned crystal structure. Considering the formation mechanisms of the multiply twinned particles, several explanations have been demonstrated, such as stacking errors or faults, intrinsic equilibrium structures of the lower energy at smaller size, or layer-by-layer growth around the 5-fold symmetry axes. To date, most of the evidence is directed toward the mechanism of intrinsic equilibrium.65 Zhang et al.59 reported that the cyclic 5-fold twinned particles are bounded by the {111} planes, i.e., the twinned crystal of the mirror symmetry is due to the stacking faults on the {111} planes and intrinsic equilibrium structures of lower energy. However, the detailed formation mechanism for the 5-fold twinned structures is still not completely understood, and more work needs to be performed in this area.

4. CONCLUSIONS We have demonstrated a facile and effective polyol-thermal method for the synthesis of silver nanowires. The formation and growth process of the V-shaped silver nanowires were detailed and discussed. A few interesting findings can be summarized below. (i) Compared to the air-open reaction system, the use of an autoclave shows some advantages in the preparation of silver nanowires by the polyol-thermal approach, such as convenient operation, easily controllable, reproducible, high yield (>98%) in nanowires, potential for large scale production, and lowered content of unwanted spherical particles. (ii) The silver nanowires generated in the proposed approach are multiply twinned structures but not a single crystal, which probably results from 5-fold symmetry twinned planes of the fcc silver crystal, confirmed by HRTEM and molecular dynamics (MD) simulation. A similar scenario is found for the silver nanospheres with a 5-fold twinned structure as well, and they are of incomplete 5-fold symmetry based on the HRTEM image and the MD simulation model. However, either spherical or nearly spherical particles have larger diameters than those of nanowires because of the possible aggregations in the incomplete 5-fold twinned particles. (iii) The V-shaped nanowires with different bending angles are commonly observed in this work. The details on the formation and growth of V-shape structure were identified by HRTEM. Two main reasons are demonstrated for such V-shaped structures: twinned planes and crystal lattice match (end-to-end and end-to-side approaches) to lower the surface energy at a relatively high temperature (e.g., 180 °C). (iv) The twinned silver crystal of the mirror symmetry is probably due to the stacking faults on the {111} planes and/or intrinsic equilibrium structure for lower energy. However, the detailed formation mechanism for the 5-fold twinned structures (silver, gold) is still not completely understood, and more work needs to be performed in this area. 1808


The Journal of Physical Chemistry C The findings reported in this paper should help to understand the formation and growth process to achieve better shape and size control of nanoparticles for desirable functional properties.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional experimental results including TEM images, UV-vis spectra of silver nanoparticles related to the optimization of experimental parameters such as temperature, concentration of silver ions, time, and molar ratio of silver to PVP, and the test of two short nanorods growing to be longer ones. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT We gratefully acknowledge the financial support of the Australia Research Council (ARC) through the ARC Centres of Excellence for Functional Nanomaterials and ARC projects. The authors acknowledge access to the UNSW and USyd node of the Australian Microscopy & Microanalysis Research Facility (AMMRF). ’ REFERENCES (1) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257–264. (2) Kamat, P. V. J. Phys. Chem. B 2002, 106, 7729–7744. (3) Brioude, A.; Jiang, X. C.; Pileni, M. P. J. Phys. Chem. B 2005, 109, 13138–13142. (4) Yin, Y.; Alivisatos, P. Nature 2005, 437, 664–670. (5) Bohren, C. F. Huffman, D. R. Absorption and scattering of light by small particles; Wiley: New York, 1983. (6) Bloemer, M. J.; Buncick, M. C.; Warmack, R. J.; Ferrell, T. L. J. Opt. Soc. Am. B 1998, 5, 2552–2559. (7) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R.; Sun, Y.; Xia, Y.; Yang, P. D. Nano Lett. 2003, 3, 1229–1233. (8) Shanmukh, S.; Jones, L.; Driskell, J.; Zhao, Y.-P.; Dulhy, R.; Tripp, R. A. Nano Lett. 2006, 6, 2630–2636. (9) McFarland, A. D.; Young, M. A.; Dieringer, J. A.; Van Duyne, R. P. J. Phys. Chem. B 2005, 109, 11279–11285. (10) Nicewarner-Pe~na, S. R.; Freeman, R. G.; Reiss, B. D.; He, L.; Pe~ na, D. J.; Walton Ian, D.; Cromer, R.; Keating, C. D.; Natan, M. J. Science 2001, 294, 137–141. (11) Kelly, K. L.; Coronado, E.; Zhao, L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668–677. (12) Nie, S.; Emory, S. R. Science 1997, 275, 1102–1106. (13) Hache, F.; Ricard, D.; Flytzanis, C. J. Opt. Soc. Am. B 1986, 3, 1647–1655. (14) Tokizaki, T.; Nakamura, A.; Kaneko, S.; Uchida, K.; Omi, S.; Tanji, H.; Asahara, Y. Appl. Phys. Lett. 1994, 64, 941–943. (15) West, R.; Wang, Y.; Goodson, T. J. Phys. Chem. B 2003, 107, 3419–3426. (16) Okada, N.; Hamanaka, Y.; Nakamura, A.; Pastoriza-Santos, I.; Liz-Marzan, L. M. J. Phys. Chem. B 2004, 108, 8751–8755. (17) Giersig, M.; Pastoriza-Santos, I.; Liz-Marzan, L. M. J. Mater. Chem. 2004, 14, 607–610. (18) Kowshik, M.; Ashtaputre, S.; Kharrazi, S.; Vogel, W.; Urban, J.; Kulkarni, S. K.; Paknikar, K. M. Nanotechnology 2003, 14, 95–100. (19) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515–7520.

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Twinned Structure and Growth of V-Shaped Silver Nanowires

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