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Structural Characteristics and Growth of Pentagonal Silver Nanorods Prepared by a Surfactant Method Chaoying Ni,*,† Puthusserickal A. Hassan,‡ and Eric W. Kaler§ Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, Novel Materials & Structural Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India, and Department of Chemical Engineering, University of Delaware, Newark, Delaware 19716 Received December 22, 2004. In Final Form: January 30, 2005 The crystal structure and growth mechanism of silver nanorods prepared by a seed-mediated surfactant method using the cationic surfactant cetyltrimethylammonium tosylate (CTAT) and its wormlike micelles are characterized by conventional and high-resolution transmission electron microscopy. Depending on the nanorod orientations, two types of electron diffraction patterns are obtained from a truncated decahedral structure consisting of five crystal units packing along {111} twining planes with five {111} planes on each end and five circumferential {001} side surfaces parallel to a 〈110〉 longitudinal direction. High-resolution images of the nanorods and the corresponding Fourier transform patterns confirm the results from the morphological and diffraction analyses. The silver nanorods grow only from multiply twinned decahedral seeds, and the high selectivity of surfactant attachment results in a barrier to the transfer of silver atoms from the solution to the circumferential {100} planes. Blockage of circumferential growth causes the aspect ratio of the rod to grow.
Introduction Controlling the growth habit of nanoparticles has become a new challenge to achieving or optimizing electronic, optical, or other properties imparted by the characteristic structure, geometry, and size of nanoparticles. Of particular interest are the production and characterization of fcc metal nanorods.1-6 In a HRTEM study of Au nanorods synthesized electrochemically using surfactant micelles as a capping material, Wang et al.1 found that short gold nanorods with aspect ratios of 3-7 were single crystals enclosed mainly by {100} and {110} facets with a [001] axial growth direction, together with long gold nanorods of aspect ratios 20-35 that contained single twinning. In addition, their surfaces were predominantly {111} and {110}, and their axial growth direction was 〈112〉. A third component, spherical particles with mass equivalent to the short rods, was found to be truncated octahedra, icosahedra, or decahedra with {111} and {100} facets and multiple twinning. In another observation of Au nanorods prepared by a seed-mediated sequential growth process, Johnson et al.2 suggested that the Au nanorod had a penta-twinned structure with five {111} twin boundaries arranged radially to the 〈110〉 direction of elongation and 10 {111} end faces. The side faces were believed to be either {100} or {110}, or both. Using a gas-phase aggregation technique, Nepijko et al.3 * Corresponding author. Phone: (302) 831-2318. Fax: (302) 8314545. E-mail:
[email protected]. † Department of Materials Science and Engineering, University of Delaware. ‡ Bhabha Atomic Research Centre. § Department of Chemical Engineering, University of Delaware. (1) Wang, Z. L.; Mohamed, M. B.; Link, S.; El-Sayed, M. A. Surf. Sci. 1999, 440, L809. (2) Johnson, C. J.; Dujardin, E.; Davis, S. A.; Murphy, C. J.; Mann, S. J. Mater. Chem. 2002, 12, 1765. (3) Nepijko, S. A.; Levlev, D. N.; Schulze, W.; Urban, J.; Ertl, G. ChemPhysChem 2000, 3, 140. (4) Sun, Y.; Mayers, B.; Herricks, T.; Xia, Y. Nano Lett. 2003, 3, 955. (5) Urban, J. Cryst. Res. Technol. 1998, 33, 1009. (6) Urban, J.; Sack-Kongehl, H.; Weiss, K.; Lisiecki, I.; Pileni, M.-P. Cryst. Res. Technol. 2000, 35, 731.
deposited Ag nanoclusters on the {100} planes of freshly cleaved NaCl. A small fraction of Ag rods was observed in addition to mostly isometric particles, and the rods were up to 70 nm long with an aspect ratio of about 10. Based on electron diffraction, HETEM imaging, and calculation of the corresponding power spectra (PS), a truncated decahedral nanorod model was proposed. The truncated decahedral nanorod structure was further observed in a study by Sun et al.4 for the Ag nanowires prepared in a process involving ethylene glycol and poly(vinyl pyrrolidone) (PVP). In parallel to the experimental work on TEM characterization of various fcc metal nanoclusters and nanorods, efforts were made by Urban et al.5,6 on computer modeling of HRTEM. Data were generated for a truncated decahedral atomic model that was surrounded by 10 {111} planes and five {100} planes in the middle. Calculated results were found to have a close fit with the experimental data collected for Cu nanorods prepared by reduction of Cu(AOT)2 in Cu(AOT)2-isooctane-NaCl-water colloidal self-assemblies with hydrazine as a reduction agent.7 These recent results suggest that fcc metal nanoseeds or nanoclusters can grow isometrically and, if conditions permit, some of the nanoparticles may grow into nanorods. In terms of crystal structure, some of the fcc metal nanorods are single crystals with surface facets of low Miller indices, while others are composed of multiple crystal units. In addition, for a given fcc crystal, different structural details are reported. Aside from possible experimental errors, the apparent disagreement is probably due to the fact that the nanorod structure can depend on kinetics of growth and may vary as a function of growth conditions. In the wet chemistry synthesis of nanorods and nanotubes, the roles of surfactants and the consequent micelles are widely reported.7-15 Although there is not yet direct (7) Lisiecki, I.; Filankembo, A.; Sack-Kongehl, H.; Weiss, K.; Pileni, M.-P.; Urban, J. Phys. Rev. B 2000, 61, 4968. (8) Pileni, M. P. Langmuir 1997, 13, 3266. (9) Li, M.; Schnablegger, H.; Mann, S. Nature 1999, 402, 393.
10.1021/la046807c CCC: $30.25 © 2005 American Chemical Society Published on Web 03/11/2005
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Figure 1. (a) Typical TEM micrograph showing a mixture of Ag nanorods and isometric nanoparticles: 〈001〉 single crystals, a, b, h; 〈110〉 decahedral particles, e, f, g; 〈100〉 decahedral particles, c; decahedral nanorod, d. Scale bar ) 20 nm; (b) particle e; (c) particle d.
Figure 2. (a) Two Ag nanorod orientations on TEM grid, and (b) schematic diagram of truncated decahedral structure of (i) [001] orientation and (ii) [11 h 0] orientation.
in-situ cryo-TEM determination of the surfactant and micelle mechanisms in promoting nanorod or nanotube formation, rodlike micelles are suspected to provide templates for directing or constraining the growth of nanoparticles.10-12 However, such templating mechanism cannot satisfactorily explain the cases when the nanoparticles or nanoseeds under consideration become larger than the diameter of the micelles. Here, we report a TEM characterization of Ag nanorods prepared by a seed-mediated surfactant method. A growth mechanism is proposed linking both the structural characteristics of the nanorods and the roles of cetyltrimethylammonium tosylate (CTAT) surfactants.
of surfactant CTAT. Ag seed solution was prepared involving silver dodecyl sulfate (AgDS) and borohydride (BH4). 5 mL of 0.25 mM AgDS and 150 µL of 10 mM BH4 were added and shaken gently to mix, and they were kept at 35 °C for 2 h. For nanorod growth, an aqueous solution of CTAT doped with 0.25 mM AgDS was used, and CTAT concentration was varied from 0.2% to 1%. Varied amounts of seed solution and 0.1 M ascorbic acid stock solution were added. For TEM analysis, excess surfactant was removed after the synthesis. The initial solution was further diluted, and a drop of the solution was placed on TEM Lacey carbon grid for drying in ambient environment. TEM observation was performed using a JEM-2010F transmission electron microscope operated at 200 kV.
Experimental Section
Nanorods with aspect ratios 5:1-20:1 were synthesized in a wide range of synthesis conditions including various surfactant and seed concentrations. However, synthesis of nanorods alone was not possible, and the fact that particles form in mixed shapes suggests that those silver seeds that can grow preferentially along their longitudinal axes possess a unique crystal configuration to favor an axial growth. Figure 1a shows a typical nanoparticle dispersion containing both nanorods and isometric nanoparticles. A few nanoparticles in Figure 1a possess the geometry
Silver nanoparticles were synthesized by a seed growth approach where nanoseeds were preformed and then acted as nuclei for subsequent growth in the presence of varying amount (10) Pileni, M.-P.; Ninham, B. W. Adv. Mater. 1999, 11, 1358. (11) Xiong, Y.; Xie, Y.; Yang, J.; Zhang, R.; Wu, C.; Du, G. J. Mater. Chem. 2002, 12, 3712. (12) Jinxin, G.; Bender, C. M.; Murphy, C. J. Langmuir 2003, 19, 9065. (13) Murphy, C. J.; Jana, N. R. Adv. Mater. 2002, 14, 80. (14) Sun, Y.; Xia, Y. Adv. Mater. 2002, 14, 833. (15) Lv, R.; Cao, C.; Guo, Y.; Zhu, H. J. Mater. Sci. 2004, 39, 1575.
Results and Discussion
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Figure 3. (a) Electron diffraction pattern of Ag nanorod with zero tilt: dashed line, zone [001] from unit T1, and solid line, zone [11 h 2] from units T3 and T4; (b) electron diffraction pattern from [11 h 0] of T2 and T5 of Ag nanorod with ∼18° tilt; (c) orientation diagram of the truncated decahedral nanorods with respect to the incident beams.
consistent with the Ag fcc crystal structure. Among them are 〈001〉 single crystals (labeled a, b, h), 〈110〉 decahedral particles (labeled e, f, g), and a 〈100〉 decahedral particle (labeled c). Particles e, f, and g contain a characteristic 5-fold symmetry, as is more clearly shown in Figure 1b for particle e. Figure 1c is a magnified view of particle d, and this image is more likely a projection of a nanorod with a longitudinal axis located slightly off the incident electron beam. It appears to have a 5-fold symmetry with facets surrounding the rod edge. As a general observation, Ag nanorods are always seen to have ends consisting of facets, suggesting a truncated decahedral structure. In preparing carbon film supported TEM specimens, Ag nanorods may settle at different orientations. Two nanorod orientations on TEM Lacey carbon grids are shown in Figure 2a, where the left nanorod lies on the carbon film and the right nanorod sits at the edge of a hole. The morphology of both nanorods is consistent with the truncated decahedral structure and is further illustrated in Figure 2b where the nanorod on the right rotates about 18° with respect to its longitudinal axis. Electron diffraction patterns and HRTEM images were collected to determine the crystallography of silver nanorods. Two typical electron diffraction patterns from nanorods are shown in Figure 3a and b. Both patterns deviate from any single-crystal patterns. Instead, the pattern in Figure 3a is found to be a superimposition of two fcc patterns from zone [001] and zone [11 h 2], together with some double diffractions, and the pattern in Figure 3b is indexed as from a zone close to [11 h 0]. The diagram in Figure 3c shows the typical orientations of truncated decahedral nanorods with respect to the incident beam at 0° tilt (left) and at 18° tilt (right). The 0° tilt configuration can lead to a superimposed diffraction pattern as shown in Figure 3a, where the [001] diffraction corresponds to
the scattering from T1 and the [11 h 2] diffraction is from T3 and T4. If the nanorod is slightly tilted in its lateral direction for about 18°, the units T2 and T5 then orient toward [11 h 0] and [1 h 10], respectively, and a diffraction pattern like Figure 3b can be generated. HRTEM lattice imaging further confirms the nanorod structure and orientation characterized by conventional TEM techniques. Figure 4 shows a typical lattice image from a nanorod with 18° tilt and its corresponding fast Fourier transform (FFT) pattern. In addition to {111} lattice fringes from T2 and T5, some modulated strips are visible due to double diffractions arising from the overlap of twined units and are further reflected as double spots in the corresponding FFT pattern. This FFT pattern is in close agreement with the experimental data shown in Figure 3b. The above TEM study indicates that seed-mediated nanorods grow from multiply twinned decahedral clusters along 〈110〉 direction and are surrounded by five {111} facets on each end and five {001} side surfaces. A model for the growth is proposed as shown in Figure 5, where the five {111} atomic planes on each end are active locations to receive Ag atoms from the solution. It is clear that the structural features of the truncated decahedron in Figure 5 can promote a preferential growth along the rod longitudinal axis. The end {111} planes have the lowest surface energy (weak bonding between interplanar atoms), and the attachment of Ag atoms from solution to the surface is relatively easier. The edges surrounding rod tip and the tip self can be sites for easy nucleation of atomic steps, which further allow rapid attachment of Ag atoms for growth. In addition, the defect energy that exists along the {111} twinning planes between five crystal units limits the lateral growth as it would significantly increase the faulty area, and hence
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Figure 4. (a) A typical nanorod high-resolution TEM image (arrows indicate the longitudinal direction) and (b) its corresponding [11 h 0]/[1 h 10] FFT pattern.
Figure 5. Growth of Ag nanorod from preferential transferring of Ag atoms to the end {111} facets.
the energy in the system, whereas the longitudinal growth causes relatively smaller increment of the faulty area and the growth is favored. It is also plausible that the unique truncated decahedral nanorod surface planes can couple the influence from CTAT surfactants. CTAT is known to form long wormlike micelles in water even at low surfactant concentrations. In this study, CTAT concentration was varied from 0.2% to 1%, and with an increase in CTAT concentration, an increased number of silver nanorods were observed. When the seed concentration was varied from 1.25 to 6.25 µM at a given surfactant concentration, a decrease of seed concentration increased the aspect ratio of the rods and led to the formation of nanowires (aspect ratio > 100:1). In each case, the final nanorod diameters range from 30 to 50 nm and are generally larger than the diameter of the micelles. This shows that the effect of the micelles to template the nanorod growth does not occur in this process. Instead, we suspect that the nanorod anisotropic growth is promoted by a selective pining process involving individual surfactant molecules. In an fcc lattice, {111} planes have the highest atomic density and less open sites to which the hydrophobic ends of the CTAT surfactants can attach. On the other hand, {100} planes have lower atomic density and the centers of octahedral interstitial sites of the lattice locate right on the plane, offering more open sites to pin surfactant molecules. The high surfactant coverage on the {100} planes, as illustrated in Figure 6, constitutes a barrier to further lateral attachment of Ag atoms. The increased formation of nanorods at increased surfactant concentration and the formation of larger aspect ratio nanowires at decreased seed concentration imply that more effective surfactant barrier on the {100} planes is established under these conditions largely due to increased availability of surfactant molecules when an equilibrium between the
Figure 6. Preferential attachment of CTAT to the lateral {100} planes of the Ag nanorod.
wormlike micelles and the true surfactant concentration in the solution is maintained. Conclusions Silver nanoparticles were synthesized by a seedmediated method involving CTAT surfactants and micelles. A TEM structural characterization was performed to include morphological observation, election diffraction, and high-resolution imaging techniques. Conventional TEM imaging reveals that in a range of synthesizing conditions, the nanoparticles consist of a mixture of isometric single crystals and nanorods of various aspect ratios. All of the nanorods are found to have facets on both ends. There are two general types of electron diffraction patterns obtained from nanorods, both of which deviate from any regular fcc single crystal patterns. Instead, they are indexed as from a truncated decahedral structure consisting of five crystal units separated by {111} twining planes. Results from high-resolution imaging are consistent with both the morphological observation and the diffraction analysis. TEM characterization also suggests that Ag nanorods grow only from multiply twinned decahedral seeds. Increasing CTAT surfactant concentration or reducing the seed concentration is found to have similar favorable effects on the formation of nanorods or nanowires. The CTAT surfactants, instead of the micelles, are believed to be an active component in promoting high aspect ratio growth. Preferential growth is explained in terms of the unique decahedral nanorod tip geometry, the defect energy existed between five crystal twining units, and the formation of CTAT surfactant barrier surrounding the nanorod side planes. LA046807C
