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Wet chemical synthesis of silver nanorods and nanowires of controllable aspect

ratio

Nikhil R. Jana,* Latha Gearheart and Catherine J. Murphy*

Department of Chemistry and Biochemistry, University of South Carolina, 631 Sumter Street, Columbia, SC 29208,
USA. E-mail: murphy@mail.chem.sc.edu; jana@mail.chem.sc.edu

Received (in Cambridge, UK) 15th January 2001, Accepted 15th February 2001
First published as an Advance Article on the web 13th March 2001

Using a seed-mediated growth approach in a rodlike Transmission electron microscopy (TEM) was performed on
micellar media, silver nanorods of varied aspect ratio were centrifuged solutions that had additional long wavelength peaks
prepared from nearly spherical 4 nm silver nanoparticles. in their optical absorption spectra.** Figs. 1 and 2 show
micrographs of particles prepared from 4 nm seeds after shape
Published on 13 March 2001 on http://pubs.rsc.org | doi:10.1039/B100521I

The physical and photophysical properties of metals on the separation. The elongated rods shown in Fig. 1 were of uniform
nanometer scale are influenced by the shape of the nano- length (42 3 nm) and aspect ratio (3.5). Interestingly, these
particle.1 Well defined silver nanorods and nanowires are rods self-assemble in a manner resembling a two-dimensional
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desirable for their optical and electronic properties.2,3 However, smectic liquid crystal upon gradual solvent evaporation.
preparation of silver nanoparticles by chemical reduction Depending on seed concentration, rods of aspect ratio 1015
methods generally yields a wide range of sizes and morpholo- can also be seperated. Fig. 2 shows a micrograph of silver
gies.4 Silver nanorods and nanowires have been prepared by nanowires, 14 mm long with aspect ratio 50350, mostly
ultraviolet irradiationphotoreduction,5 solidliquid phase arc- separated from spherical side-products. EDAX analysis of the
discharge,6 a pulsed sonochemical method,7 templated by samples confirmed that the particles were silver (not silver
DNA,8 with a carbon nanotube template,9 in mesoporous oxide) with a considerable amount of CTAB still present.
silica,10 in polymer films,11 and in membrane templates.12 Here, The only difference between the preparation of nanorods and
a new strategy is applied. A preformed silver seed was used to the preparation of nanowires was the relative amount of NaOH
promote silver growth in solution, by chemical reduction of a in solution. For the nanorods, the pH of the reaction solution
silver salt. The presence of a rodlike micelle in solution was slightly higher than the pKa of the second proton of ascorbic
promoted silver rod formation. We have been able to reproduci- acid ( 11.8), suggesting that the ascorbate dianion is a
bly make silver nanorods of aspect ratio 2.515 (1015 nm short significant component of the solution. In the case of the
axes) and nanowires of 14 micrometer length with 1218 nm nanowires, the pH of the solution was slightly lower than this
short axes, and effectively separate the rods or wires from pKa, sugggesting that the monoanion of ascorbic acid (first
spheres and other shapes by centrifugation. Our method is not pKa 4.1) is predominant in solution. It is reasonable that silver
electrochemical and requires no nanoporous membrane, and ion complexes of these two different forms of the reducing
thus may be more amenable to large-scale preparation of these agent, in conjunction with their complexation with the cationic
materials. CTAB and silver seed in solution, are important in nanorod and
The Ag seeds, 4 nm in diameter on average, were prepared by nanowire formation. Mechanistic studies are in progress.
chemical reduction of AgNO3 by NaBH4 in the presence of Nonetheless, our new wet chemical synthetic method of silver
trisodium citrate to stabilize the nanoparticles. To make
nanorods and wires of varying aspect ratio, AgNO3 was
reduced by ascorbic acid in the presence of seed, the micellar
template cetyltrimethylammoniun bromide (CTAB), and
NaOH. The seed concentration and base concentration relative
to the Ag+ concentration are key to making larger aspect-ratio
nanomaterials. CTAB is also necessary to produce a high yield
of rods. Rods and wires can be separated from spheres by
centrifugation.
The electronic absorption spectra of silver nanorod solutions
show the conventional 400 nm peak observed for spherical
silver nanoparticles and another peak at longer wavelengths,
due to the longitudinal plasmon band of rod-shaped particles
(ESI).1a,c,1315 Decreasing the amount of seed in the nanorod
preparation led to a further red shift of longer-wavelength
longitudinal plasmon bands in the nanorod products, implying
that the silver rods increased in average aspect ratio as the seed
concentration decreased. Our optical data are in accord with
what others have observed for metallic nanorods for transverse
and longitudinal plasmon bands.1a,c,1315 In the absence of
CTAB, spheroidal nanorods (aspect ratio < 2.5) were unstable
and reverted to spheres (as judged by the disappearance of the
long-wavelength absorption band) within 10 min. In the
absence of seed, silver ion reduction by ascorbic acid in the
presence of CTAB yielded only a few rods, which varied in
aspect ratio.

Electronic supplementary information (ESI) available: UVVIS spectra Fig. 1 TEM image of shape-separated silver nanorods from a preparation
of silver nanorods. See http://www.rsc.org/suppdata/cc/b1/b100521i/ with 0.06 mL seed; scale bar = 100 nm.

DOI: 10.1039/b100521i Chem. Commun., 2001, 617618 617

This journal is The Royal Society of Chemistry 2001
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Next, 2.5 mL of the 4 nm seed solution was added. Finally, 0.5 mL of 1 M

NaOH was added. After adding the NaOH, the solution was gently shaken
just enough to mix the NaOH with the rest of the solution. A yellow color
appeared within 15 min.
Rods were concentrated and partially separated from spheres and
surfactant by centrifugation. For solutions containing 0.25 mL of seed or
more, 10 mL of solution was centrifuged at 6000 rpm for 30 min. The
supernatant, containing mostly small spheres and platelets, was removed
and the solid, containing some platelets and more rods with aspect ratio of
34, were redispersed in 0.5 mL of deionized water. For solutions
containing 0.125 mL or 0.06 mL of seed, 10 mL of the solution was
centrifuged at 2000 rpm for 6 min. The supernatant, containing short rods,
spheres, and platelets, was separated from the solid which contained rods of
aspect ratio 34 and a few larger rods (aspect ratio 1015). Wires were
partially separated from spheres and surfactant by centrifugation. For wires,
10 mL of the solution was centrifuged at 6000 rpm for 30 min. The
supernatant was removed and the precipitate, containing silver nanowires,
was redispersed in 0.5 mL of deionized water.
** All TEM grids were prepared from the solutions that were separated by
centrifugation. 1.5 mL of solution was added to each grid. The grids and a
small beaker of water were placed under a glass dish. The beaker of water
provided a water vapor atmosphere to allow for slow drying (12 h) of the
Published on 13 March 2001 on http://pubs.rsc.org | doi:10.1039/B100521I

rod or wire solutions on the grid.

1 (a) J. A. Creighton and D. G. Eadon, J. Chem. Soc., Faraday Trans.,

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Fig. 2 TEM image of shape-separated silver nanowires; scale bar = 100
3 J. T. Hu, T. W. Odom and C. M. Lieber, Acc. Chem. Res., 1999, 32, 435;
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618 Chem. Commun., 2001, 617618