Solvothermal preparation of single-crystalline gold nanorods in novel nonaqueous microemulsions

Article abstract of DOI:10.1246/cl.2005.730

Single-crystalline gold nanorods have been prepared by one-step, seedless solvothermal reduction of HAuCl₄·4H₂O in cetyl-trimethylammonium bromide (CTAB)/octane + butanol/formamide microemulsions. Formamide provides not only reductio

Full text of DOI:10.1246/cl.2005.730

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Chemistry Letters Vol.34, No.5 (2005)
Solvothermal Preparation of Single-crystalline Gold Nanorods
in Novel Nonaqueous Microemulsions
ꢀ
Jieming Cao, Xianjia Ma, Mingbo Zheng, Jinsong Liu, and Hongmei Ji
Nanomaterials Research Institute, College of Material Science and Engineering,
Nanjing University of Aeronautics and Astronautics, Nanjing 210016, P. R. China
(Received January 26, 2005; CL-050118)
Single-crystalline gold nanorods have been prepared by one-
spectively. After cooled to room temperature naturally, yellow
colloids were obtained. The products were separated by centrifu-
gation, washed with absolute ethanol several times, and dried at
.
step, seedless solvothermal reduction of HAuCl4 4H2O in cetyl-
trimethylammonium bromide (CTAB)/octane + butanol/form-
amide microemulsions. Formamide provides not only reduction
ꢁ
80 C in vacuum. The products were characterized by transmis-
3þ
but also sites occupied by Au ion through replacing ‘‘water
pools’’ in the traditional microemulsions.
sion electron microscopy (TEM), scanning electron microscopy
(SEM) and X-ray diffraction (XRD).
Figure 1 shows TEM images of the typical samples by the
ꢁ
solvothermal reduction at 180 C for 10 h. Nanorods were ob-
The properties and application of nanostructured materials
are strongly dependent on the size, morphology, and crystallin-
ity. Preparation of anisotropic gold nanorods and nanowires,
which show potential application in the optoelectronic field,
have been reported.1 Most of nanosized gold synthesis was
performed with the NaBH4 as reducing agent in the reverse mi-
served. Nanorod has the diameter of about 135 nm and a length
of ꢂ4 mm (shown in Figure 1a). Figure 1b shows a high magni-
fication TEM image of a randomly chosen individual Au nano-
rod with a diameter of about 130 nm and a length of ꢂ1 mm (as-
pect ratios is ca. 8). One can see that the nanorod has the smooth
edges, and its corresponding selected area electron diffraction
(SAED, shown in the inset of Figure 1b) indicates that the gold
nanorod is single crystalline. It can be indexed with the fcc sym-
metry. Some branch-like Au nanostructures were also observed
(shown in Figure 1c). It confirms the formation of Au micro-
emulsions in the reaction system and the diffusion of Au from
one droplet to another in the transient droplet dimers.
–3
4
,5
celle system, which may lead to too fast reduction rate. Polyol,
7
aspartate,6 N,N-dimethylformamide, dimyristoyl-L-phosphati-
8
9
10
dyl-DL-glycerol, metallic Ag, poly (ethylene oxide) blocks,
hydrazine,11 glucose,
12
13
meso-2,3-dimercaptosuccinic acid,
and sodium citrate were reportedly used as reducing agents
of Au to prepare metallic Au in the certain conditions such
as at room temperature, under the microwave irradiation, at re-
flux, by electrochemical process or photoirradiation.
14
3þ
Though microemulsions are widely used as microreactors or
nanoreactors to prepare nanomaterials, the microemulsions have
almost all limited their inside cores to water pools until now.
Self-assembly of surfactants in nonaqueous systems has recently
been a hot topic as both a basic investigation and an attractive
way to prepare water-free or water-sensitive materials. Forma-
mide has been investigated as a solvent for micelle formation
of surfactants, it has been found that micelle aggregates of some
ionic surfactants can be formed in formamide solvent.1 So we
try to expand the microemulsions from aqueous pools (water
cores) into nonaqueous pools (formamide cores). To our knowl-
edge, this nonaqueous microemulsion is the first time used as
nanoreactors for the synthesis of Au nanorods.
5
ꢁ
Figure 1. TEM images of the individual Au nanorod at 180 C
for 10 h (a). The high magnification with the inset is the SAED
pattern (b). Branch-like Au nanostructures (c).
Solvothermal reduction route is widely used to prepare nov-
el metal materials, because of its advantages such as high tem-
perature and high pressure. Until now, solvothermal reduction
route has rarely been reported to the synthesis of gold nanoma-
terials.17 Herein we report one-step seedless formamide-thermal
reduction route to anisotropic single crystalline gold nanorods of
We also investigated the effect of reaction temperature and
reaction time over the morphologies of the samples. By increas-
ing heating time up to 20 h at 180 C, nanorods with 230–480 nm
1
6
ꢁ
in diameters and 6 mm in lengths (aspect ratios are ca. 12–26)
were obtained (Figures 2a and 2b). Electron diffraction patterns
of these nanorods reveal the similar single crystalline structures.
Compared with the reported process to synthesize single-crystal-
.
high crystallinity by the reduction of HAuCl4 4H2O in CTAB/
octane + butanol/formamide microemulsions.
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8
In the glove-box filled with argon, the mixture of 1.09 g of
CTAB, 2.74 mL of butanol, 1.2 mL of formamide, and 24 mL
of octane was stirred for 10 min, and then 0.0245 g of
line Au nanorods, our method is a simple one-step process in-
ꢃ
stead of multi-processes: formation of AuCl4 –surfactant com-
plexes, photolysis of the complexes and the growth of Au parti-
cles, and the nanorods obtained in our experiments are larger in
size. The relative ratio of number of Au nanorods to total number
of Au nanostructures was estimated to be about 50%, the yield of
.
HAuCl4 4H2O was added and further stirred for 10 min. The so-
lution was transferred into a Teflon-lined stainless autoclave and
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ꢁ
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kept at 130 C for 20 h, 180 C for 10 h and 180 C for 20 h, re-
Copyright ꢀ 2005 The Chemical Society of Japan

Chemistry Letters Vol.34, No.5 (2005)
731
and even the supercritical condition), on one hand, can increase
the reduction ability of formamide and thus reach the suitable re-
action rate. On the other hand, it facilitates the opening of drop-
lets owing to a decrease in interfacial rigidity, enhanced inter-
droplet interaction, and rapid diffusion of Au from one droplet
to another in the transient droplet dimers. The increased reaction
ꢁ
time kept at 180 C has led to size-increased nanorods, which is
probably attributed to further and full growth of nanorods. The
ꢁ
low temperature (130 C, slightly higher than the boiling point
ꢁ
of the solution: 125 C) leads to the elongated spheric particles
instead of nanorods. This is probably due to the weak interdrop-
let interaction, and slow diffusion of Au from one droplet to
another in the transient droplet dimers.
In summary, single crystalline gold nanorods have been pre-
pared by solvothermal reduction method in novel nonaqueous
microemulsions. The novel solvothermal microemulsion system
can provide a useful tool for preparation of other nanomaterials
with different shapes, structures and crystallinity.
Figure 2. SEM images of the samples. (a) Several Au nano-
ꢁ
rods, (b) single Au nanorod obtained at 180 C for 20 h. (c) Au
ꢁ
nanoparticles at 130 C for 20 h.
This work was financially supported by the Natural Science
Foundation of Jiangsu Province (BK2002414).
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Published on the web (Advance View) April 22, 2005; DOI 10.1246/cl.2005.730