A new route to obtain high-yield multiple-shaped gold nanoparticles in aqueous solution using microwave irradiation

Article abstract of DOI:10.1021/ic8004135

A highly effective, very fast microwave method is described to synthesize shape controlled gold nanoparticles in the presence of 2,7-dihydroxy naphthalene (2,7-DHN) as a new reducing agent under microwave heating for 60-90 s. The growth of the particles with different shapes (spherical, polygonal, rod, and triangular/prisms) was directed by the surfactant to metal ion molar ratios and the concentration of 2,7-DHN. The evolved nanorods and nanoprisms are fairly small in diameter, and the particle size and shape were successfully tuned just by varying the molar ratios of the reactants. The process presented here can be extended to the synthesis of other nanomaterials with desired size and shape and might find a variety of applications in wide areas, such as catalysis, clinical and diagnostic medicine, and nanoelectronics.

Full text of DOI:10.1021/ic8004135

Inorg. Chem. 2008, 47, 6344-6352
A New Route to Obtain High-Yield Multiple-Shaped Gold Nanoparticles
in Aqueous Solution using Microwave Irradiation
Subrata Kundu,* Luohan Peng, and Hong Liang*
Materials Science and Mechanical Engineering, Texas A & M UniVersity, College Station,
Texas 77843-3123
Received March 6, 2008
A highly effective, very fast microwave method is described to synthesize shape controlled gold nanoparticles in
the presence of 2,7-dihydroxy naphthalene (2,7-DHN) as a new reducing agent under microwave heating for 60-90
s. The growth of the particles with different shapes (spherical, polygonal, rod, and triangular/prisms) was directed
by the surfactant to metal ion molar ratios and the concentration of 2,7-DHN. The evolved nanorods and nanoprisms
are fairly small in diameter, and the particle size and shape were successfully tuned just by varying the molar
ratios of the reactants. The process presented here can be extended to the synthesis of other nanomaterials with
desired size and shape and might find a variety of applications in wide areas, such as catalysis, clinical and
diagnostic medicine, and nanoelectronics.
1
3–16
Introduction
processes developed for the synthesis of gold NPs.
Among those, the chemical reduction remains the most
popular. As NPs tend to be unstable in a solution, special
precautions have to be taken to avoid their agglomeration
and/or precipitation.
In the past few years, nanomaterials research has attracted
enormous attention because of the variety of unique spec-
troscopic, electronic, and chemical properties that arises
from the nanoparticles (NPs) small size and large surface to
volume ratio. These NPs have found wide applications in
1
2
3
There are reports on the development of simple ways to
synthesize size- and shape-controlled nanomaterials, such as
nanocubes, nanorods, nanowires, nanodisks, nanotapes,
4
5
6
7
the fields of physics, chemistry, biology, medicine,
8
1
3–15
materials science, and other interdisciplinary fields in
engineering. Among a wide variety of NPs studied, gold is
of particular interest because of its fascinating properties and
potential applications in catalysis, drug delivery, opto-
electronics, and magnetic devices. There are many
nanobelts, nanotetrapods, among others.
Methods in-
1
3–16
clude chemical, physical, biological, and photoreduction.
Shape controlled gold NPs have been synthesized in the
9
10
17
presence of different beta-diketones as chelating ligands.
1
1
12
However, all these above-mentioned methods were found
to be time-consuming, and particles were found in general
to be large with uncontrollable size distribution. The
microwave chemistry has recently been reported to be
effective in making size and shape controlled metallic NPs.
The microwave irradiation (MWI) has a rapid heating
capacity with a high penetration power. The importance of
the microwave heating lies in the fact that it can heat a
substance uniformly and generate homogeneous nucleation
*
To whom correspondence should be addressed. E-mail: skundu@
tamu.edu (S.K), hliang@tamu.edu (H.L). Phone: 979-862-2578. Fax: 979-
45-3081.
8
(
(
(
(
(
(
1) Henglein, A. J. Phys. Chem. 1993, 97, 5457.
2) Kundu, S.; Liang, H. AdV. Mater. 2008, 20, 826.
3) El-Sayed, M. A. Acc. Chem. Res. 2001, 34, 257.
4) Bae, S.; Lee, S. W.; Takemura, Y. Appl. Phys. Lett. 2006, 89, 252506.
5) Daniel, M.; Astruc, D. Chem. ReV. 2004, 104, 293.
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1
4, 517.
(
(
(
7) Haes, A. J.; Van Duyne, R. P. J. Am. Chem. Soc. 2002, 124, 10596.
8) Niemeyer, C. M.; Doz., P. Angew. Chem., Int. Ed. 2001, 40, 4128.
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Chem. Res. 2001, 34, 181.
(13) Murphy, C. J.; Jana, N. R. AdV. Mater. 2002, 14, 80.
(14) Sun, Y.; Mayers, B.; Xia, Y. AdV. Mater. 2003, 15, 641.
(15) Kundu, S.; Panigrahi, S.; Praharaj, S.; Basu, S.; Ghosh, S. K.; Pal,
A.; Pal, T. Nanotechnology 2007, 18, 75712.
(
10) Neuberger, T.; Bch o¨ pf, S.; Hofmann, H.; Hofmann, M.; Rechenberg,
B. J. Magn. Magn. Mater. 2005, 293, 483.
(16) Pol, V. G.; Srivastava, D. N.; Palchik, V.; Slifkin, M. A.; Weiss, A. M.;
Gedanken, A. Langmuir 2002, 18, 3352.
(
11) Taleb, A.; Petit, C.; Pileni, M. P. J. Phys. Chem. B 1998, 102, 2214.
(12) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science 2000,
(17) Kundu, S.; Pal, A.; Ghosh, S. K.; Nath, S.; Panigrahi, S.; Praharaj,
S.; Pal, T. Inorg. Chem. 2004, 43, 5489.
2
87, 1989.
6344 Inorganic Chemistry, Vol. 47, No. 14, 2008
10.1021/ic8004135 CCC: $40.75  2008 American Chemical Society
Published on Web 06/18/2008

Gold Nanoparticles
sites as compared to those of conventional heating. The MW
heating method has been used to synthesize spherical NPs
1
8
19
18
20
of metals like Au, Ag, Pd, Pt, and so forth and
2
1
semiconductor nanorods and wires at a high speed. There
is no report for the controlled synthesis of nanorods and
nanoprisms using the MW heating method. Murphy et al.
synthesized gold nanorods using a seed mediated approach.
Mirkin et al. synthesized silver nanoprisms by irradiating
with UV-light for 70 h. Shastry et al. synthesized large
size (0.05-1.8 µm) gold nanoprisms by extracting a lem-
ongrass plant through conventional heating methods. Re-
13
2
2
2
3
2
4
cently, Chung et al. synthesized 98 ( 17 nm gold
nanoprisms by seed mediated methods. Using this method,
the yields of nanoprisms was very low, ∼45 ( 5%. As seen,
all the above-mentioned methods for making shape controlled
NPs require multiple steps, high temperature, long time, or
results in mixed particles of different shapes with lower
yields.
In this present research, we developed a process using a
microwave to synthesize shape controlled (spherical, po-
lygonal, rods, and prisms) gold NPs in a reaction time of
less than 90 s in the presence of a cationic surfactant. The
reduction of gold salt was done in CTAB (cetyl trimethyl
ammonium bromide) micellar media in the presence of
alkaline 2,7-dihydroxy naphthalene (2,7-DHN) as a new
reducing agent. This method generates exclusively spherical,
polygonal, rods, and triangular gold NPs within a short time.
The particle size and shape can be tuned just by varying the
metal salt to surfactant molar ratio and by changing the
concentration of 2,7-DHN and the MW heating time. To our
best knowledge, shape controlled gold NPs synthesized
within 90 s have not been reported. The yield of the NPs
like nanorods (aspect ratio ∼ 12) and nanoprisms (size ∼
Figure 1. (a) UV-visible absorption spectra at various stages of gold NPs
synthesis. A, absorption spectra of gold chloride (HAuCl4) solution in water;
B, absorption spectra of HAuCl4 in CTAB micellar solution; C, Surface
Plasmon Resonance (SPR) band for spherical gold NPs. Inset shows the
image of orange color CTAB-Au(III) complex and pink color spherical
gold NPs solution. (b) UV-visible absorption spectra of four different gold
NPs solution having different shapes. A, SPR band of spherical gold NPs;
B, SPR band of mixed shaped gold NPs; C, SPR band of gold nanorods;
and D, SPR of gold nanoprisms. All the spectra were recorded after 90 s of
MW heating. Inset shows four different color gold NPs solution.
6
5 nm) is very high (>85%), and the present approach is
straightforward, simple, cost-effective, and less time-
consuming.
equipped with 1 cm quartz cuvette holder for liquid samples. A
high resolution-transmission electron microscope (HR-TEM) (ZEOL
ZEM 2010) was used at an accelerating voltage of 200 kV. The
energy dispersive X-ray spectrum (EDS) was recorded with the
Oxford Instruments INCA energy system connected with the TEM.
The X-ray diffraction (XRD) analysis was done with a scanning
rate 0.020 s- in the 2θ range 20-80° using a Rigeku Dmax γA
X-ray diffractometer with Cu KR radiation (λ ) 0.154178). Atomic
force microscopy (AFM) images were generated with a multimode
atomic force microscope (AFM, Pacific Nanotechnology Inc.). Field
emission scanning electron microscopy (FE-SEM) images were
obtained using a Zeiss SEM ultra 60. A domestic microwave (MW)
oven (Gold star company, EM-Z200S, 1000 W, 60 Hz) was used
for MW irradiation for the entire synthesis.
Experimental Details
Reagents. 2,7-Dihydroxy naphthalene (2,7-DHN), 2-naphthol (2-
N), and 1,2-dihydroxy naphthalene (1,2-DHN) were purchased from
Sigma-Aldrich and were recrystallized in hot water. Cetyltrimethyl
ammonium bromide (CTAB, 99%), sodium lauryl sulfate (SDS),
1
4 2
and hydrogen tetrachloro aurate, trihydrate (HAuCl , 3H O, 99.9%)
were purchased all from Sigma-Aldrich and were used without
further purification. Other chemicals like polyallylamine hydro-
chloride (PAH, Polymer Scientific Products, Ltd.) and sodium
hydroxide (NaOH, Sigma-Aldrich) were used as received. Ultra
pure distilled (UPD) water was used for the entire synthesis.
Instruments. UV-visible absorption spectra were recorded in
the Hitachi (model U-4100) UV-vis-NIR spectrophotometer
Preparation of Multiple Shapes Gold Nanoparticles. Spherical
(
(
(
18) Harpeness, R.; Gedanken, A. Langmuir 2004, 20, 3431.
-2
gold NPs were prepared by mixing 4 mL of CTAB (10 M)
solution with 200 µL of (10- M) aqueous Au (III) solution. After
that 2 mL of aqueous 2,7-DHN solution was added to the reaction
mixture such that the final concentration became 3.17 × 10- M.
Finally 100 µL of (1 M) NaOH was added, and the mixture was
stirred for 30 s and then irradiated by MW for 90 s with an
intermittent pause after every 10 s to cool the reaction vessel. For
the synthesis of anisotropic gold NPs we vary the concentration of
2,7-DHN and NaOH keeping the other concentration fixed. The
19) Pastoriza-Santos, I.; Liz-Marz a´ n, L. Langmuir 2002, 18, 2888.
2
20) Chen, W.; Zhao, J.; Lee, J. Y.; Liu, Z. Mater. Chem. Phys. 2005, 91,
1
24.
(
(
(
(
21) Panda, A. B.; Glaspell, G. P.; El-Shall, M. S. J. Am. Chem. Soc. 2006,
3
1
28, 2790.
22) Jin, R.; Cao, Y.; Mirkin, C. A.; Kelly, K. L.; Schatz, G. C.; Zheng,
J. G. Science 2001, 294, 1901.
23) Sankar, S. S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmed, A.; Sastry,
M. Nat. Mater. 2004, 3, 482.
24) Ha, T. H.; Koo, H.-J.; Chung, B. H. J. Phys. Chem. C 2007, 111,
1
123.
Inorganic Chemistry, Vol. 47, No. 14, 2008 6345

Kundu et al.
Figure 2. TEM image of different shaped gold NPs synthesized after 90 s of MW exposure. (A) Image of spherical gold NPs; (B) low magnified images
of mixed (mixture of different shapes) shaped gold NPs; C and D show the images of gold nanorods at different magnification. The inset of A and B shows
the corresponding electron diffraction of the particles and a higher magnified image.
-
3
final concentrations of 2,7-DHN and NaOH were 1.05 × 10
M
a native oxide layer. The chip was rinsed thoroughly with ethanol
and piranha (30% H and 70% H SO ) followed by final cleaning
-
2
and 1.05 × 10 M, respectively. For rod shaped particles the final
O
2 2
2
4
concentrations of CTAB, Au(III), 2,7-DHN, and NaOH were 7.54
with ethanol and acetone. The chip was then air-dried and dipped
in 0.1 wt % solution of PSS overnight followed by a vigorous wash
to remove excess PSS leaving a monolayer. The sample was then
blow-air-dried and placed on a gold NPs solution for 30 min to
deposit gold nanoprisms.
-
3
-4
-3
-3
×
10 M, 4.71 × 10 M, 1.88 × 10 M, and 9.43 × 10 M,
respectively. For nanoprism synthesis, a mixer was prepared with
-2
4
2
mL of (0.1 M) CTAB, 320 µL of (10 M) Au (III) solution,
80 µL of (10 M) 2,7-DHN, and 10 µL of (1 M) NaOH. The
-
2
solution was stirred for 30 s and irradiated by MW for 90 s. For
all the above cases, the Au particles’ formation started just after
Results and Discussion
1
0-20 s of MW irradiation as observed from the color change of
the solution mixture and from the UV-visible spectrum. The
resulting solution was centrifuged at 3000 rpm for 10 min for the
removal of excess surfactants. The precipitate was redispersed in
DI water and centrifuged again at 2000 rpm for 5 min. This process
was repeated twice. Finally, the precipitate Au NPs was collected
and redispersed in water for characterization. The solution became
deep pink (for spherical), bluish pink (for mixed shapes), bluish
purple (for rod shapes), and pinkish red (nanoprisms) after
completion of the reaction. The color remained stable for at least
The Au NPs were synthesized by the reduction of Au(III)
ions in the presence of alkaline 2,7-DHN using CTAB as a
stabilizing agent in a 1000 W MW oven. Figure 1a shows
the UV-visible spectrum of the solution mixture at various
stages of the process. The light yellowish color of the
4
aqueous HAuCl solution exhibits an intense absorption band
at 292 nm (curve A, Figure 1a) because of a metal-ligand
-
25
charge transfer (MLCT) band from the AuCl
4
complex.
The light yellowish color of the aqueous HAuCl
4
solution
3
months in dark under ambient environment without change in
changes to deep orange having three characteristic absorption
peaks at 262 nm, 326 nm, and 402 nm with the addition of
CTAB with HAuCl (curve B, Figure 1a). This orange color
4
optical properties.
Preparation of Samples for HR-TEM, FE-SEM, and AFM
Analysis. The samples for TEM analysis were prepared by placing
a drop of fresh NPs solution on a carbon-coated copper grid
followed by slow evaporation of the solvent at ambient condition.
The FE-SEM and AFM studies were performed on a Si-chip with
is due to 1:1 complex formation of CTAB with aqueous
(25) Roy Meson, W.; Gray, H. B. Inorg. Chem. 1968, 7, 55.
6346 Inorganic Chemistry, Vol. 47, No. 14, 2008

Gold Nanoparticles
Figure 3. TEM images for the synthesis of gold nanoprisms under different MW exposure times. A, B, and C are the TEM images after 30 s, 45 s, and 60 s
MW exposure, respectively. D and E are the low and high magnified images after 90 s MW exposure. F is the image after 90 s MW exposure after 3 months
aging in air. The inset of C shows a corresponding higher magnified image of single particles.
HAuCl
4
solution. The three peaks are assigned for the
indicating the completion of the reaction. The inset of Figure
1a shows the image of brown complex and pink spherical
Au NPs solution. Addition of NaOH and 2,7-DHN in
different concentration with the reaction mixture (containing
-
vibrational frequencies of [AuBr
2
] (at 262 nm), Au(III) in
CTAB (at 326 nm), and the ion-pair formation of
+
-
+
[
CTA] [AuBr
4
]
(at 402 nm), where [CTA] ) cetyl
26,27
trimethyl ammonium ion.
Now, addition of alkaline 2,7-
4
HAuCl and CTAB) and subsequent microwave exposure
DHN with the orange complex and subsequent MW heating,
the color changed to pink with the appearance of a new peak
at 520 nm because of surface plasmon resonance (SPR) of
produced anisotropic Au NPs. In this case, the solution color
changed from pink to purple, bluish purple to pinkish red
depending upon the concentration of 2,7-DHN and NaOH.
Figure 1b shows the UV-visible spectrum of the different
gold NPs solution. In Figure 1b, curves A, B, C, and D
denote the SPR band for spherical, polygonal (mixed shaped),
rods, and triangular gold NPs respectively. The spectrum B
containing the mixture of anisotropic particles shows two
intense absorption peaks, one at ∼ 576 nm and another broad
peak >700 nm. The spectrum C, containing the nanorods,
shows two peaks, one at 530 nm (due to transverse SPR)
and another peak at 720 nm (due to longitudinal SPR). The
spectrum D, containing the nanoprisms, shows a broad peak,
28–31
spherical gold NPs (curve C, Figure 1a).
The formation
of a pink color started after 30 s of MW heating as observed
from the UV-visible spectrum, increased gradually with the
increase in microwave exposure, and saturated after 90 s
(
(
26) Chen, S.; Carroll, D. L. J. Phys. Chem. B 2004, 108, 5500.
27) Aguirre, C. M.; Kaspur, T. R.; Radloff, C.; Halas, N. J. Nano. Lett.
2
003, 3, 1707.
(28) Jana, N. R.; Gearheart, L.; Murphy, C. J. Langmuir 2001, 17, 6782.
(29) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389.
(30) Zhu, J. J.; Liu, S. W.; Palchik, O.; Koltypin, T.; Gedanken, A.
Langmuir 2000, 16, 6396.
(
31) Kundu, S.; Maheshwari, V.; Saraf, R. F. Langmuir 2008, 24, 551.
Inorganic Chemistry, Vol. 47, No. 14, 2008 6347

Kundu et al.
Figure 4. FESEM and TEM image of single gold nanoprisms synthesized after 90 s of MW exposure. (A) FESEM images of single gold nanoprisms and
(
B) TEM image of single gold nanoprisms. C is the electron diffraction pattern of single prisms obtained by TEM and shows that the particles are single
crystal.
at ∼ 630 nm. Similar types of absorption spectra for
2A shows the corresponding selected area electron diffraction
(SAED) pattern of the gold nano spheres indicating that the
particles are a single crystal. We studied the growth
mechanism of the particles’ formation with different MW
exposure time taking triangular particles as an example.
Figure 3 shows the TEM images of the Au nanoprisms at
various stages of the microwave heating. Figure 3A,B shows
the TEM image of the formation of Au nanoprisms after 30
and 45 s of MW heating, respectively. The particles are ∼40
1
3,28,29
23,24,32
nanorods
and nanoprisms
were also observed
by other researchers. The inset of the Figure 1b shows the
different-colored Au NPs solutions corresponding to curves
A, B, C, and D for different shapes, respectively. The
significant difference in the optical properties of the nanorods
and nanoprisms, as compared to the spherical particles, is
responsible for the broad peak >600 nm in the longer
wavelength side resulting from the anisotropic structure (rod,
3
3
wires, cubes, etc.). It is noticeable that the final product
(
10 nm in diameter having no defined shape and started to
has high yield (∼80-85% rods and ∼95% prisms), signifi-
grow into the triangular shape. Figure 3C shows the low
magnified image after 60 s of MW heating with particles
perfectly grown into triangular shapes. This is clearly seen
in the high magnification image of a single particle (inset of
Figure 3C). Figure 3D and 3E shows the different magnified
images of nanoprisms after 90 s of MW heating. The image
contains >95% of prisms with a few other shaped particles
having average particle size ∼65 ( 10 nm. Inside the
particles, the contrast changes might be due to the internal
stress of the on-growing crystal layers. The thickness of the
prisms is 3.5-4 nm calculated from the AFM height profile.
Figure 3F shows the TEM image of the gold nanoprisms
after aging for 3 months in air. The particles’ size and shape
were almost unchanged (similar to Figure 3D and 3E)
indicating the high stability of the nanoprism solution.
Figure 4 shows TEM and field emission scanning electron
microscopy (FESEM) images of a single nanoprism. Figure
1
3,23
cantly higher than the reported ones (∼45-50%).
Figure 2 shows the transmission electron microcscopy
TEM) images of various CTAB capped Au NPs prepared
(
with the above method. Figure 2A shows the TEM image
of the low magnified spherical Au nanoparticles correspond-
ing to curve C in Figure 1a. The average size of the particles
is 25 ( 2 nm. Figure 2B shows the mixture of different
shaped particles containing spheres (∼16%), triangles (∼22%),
rods (∼ 10%), hexagonal shapes (∼28%), and a few other
shaped particles. Figure 2C and 2D has the images of the
nanorods at different magnification. The average length of
the rods is around ∼300 nm and the width ∼25 nm
corresponding to an aspect ratio of ∼12. The inset of Figure
(
32) Xue, C.; Millstone, J. E.; Li, S.; Mirkin, C. A. Angew. Chem. 2007,
1
19, 8588.
(33) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. E. J. Am.
Chem. Soc. 2003, 125, 16186.
6348 Inorganic Chemistry, Vol. 47, No. 14, 2008

Gold Nanoparticles
Figure 5. AFM images of gold nanoprisms. Topographic (A) and 3-D (B) image of the triangular nanoparticle. The line profile shows that the width (C)
of a triangular nanoparticle is about 55-60 nm and the height (D) of a triangular nanoparticle is 3.5-4 nm.
4
A corresponds to the FESEM image, and 4B corresponds
mixture (pH∼ 7.5-8.5), the SPR band for gold nanoparticles
red-shifted, indicating the formation of anisotropic Au
nanoparticles. The facilitated formation of nanorods and
nanoprisms at higher pH values should be understood on
the grounds that the reduction power of 2,7-DHN can be
strengthened because of the elimination of protons produced
by reduction. With further increased pH, a blue solution was
seen with some aggregated particles. In this case there were
no clearly defined shaped particles, as shown in Figure 6.
This is probably due to the hydroxyl compound of gold or
to the TEM image of a single nanoprism. The side length of
the prism is the same for both techniques. Figure 4C shows
the selected area electron diffraction (SAED) pattern in the
corresponding area obtained from TEM. The SAED proves
that the particles are single crystalline in nature. The
diffraction spots exhibit the triangular shape. This type of
well defined shape effects as a result of nanoprisms has not
been reported before.
The AFM images of gold nanoprisms are shown in Figures
5
A and 5B. Figure 5A is the topographic image while 5B is
2
Au O in the highly alkaline condition. A similar observation
3
3
the 3-D mode. The line profile shows the width of the
particles, ∼ 55-60 nm (Figure 5C), and the height, ∼ 3.5-4
nm (Figure 5D). The AFM results perfectly match with the
results obtained from TEM and SEM studies.
The effects of NaOH concentration on NP synthesis was
examined in detail. The reducing capacity of any hydroxyl
compounds strongly depends on the pH of the reaction
medium. In our experiments, while in the presence of
microwave heating and the absence of NaOH (other param-
eters fixed), there were no gold nanoparticles formed. On
the contrary, the gold nanoparticles were formed with
addition of NaOH. With the increase in pH of the reaction
was reported by Chen et al. It was reported earlier that the
hydroxyl ion concentration directs the growth of metal
34,35
nanoparticles in different shapes.
The feasible formation
of a definite shape with an addition of small amount of NaOH
has also been observed in a previous report by Mirkin et
36
-
al. In our experiments, a proper OH concentration assists
the growth of the particles in selective crystallographic
(
(
(
34) Panigrahi, S.; Kundu, S.; Ghosh, S. K.; Nath, S.; Praharaj, S.; Basu,
S.; Pal, T. Polyhedron 2006, 25, 1263.
35) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001,
1
05, 4065.
36) Millstone, J. E.; Park, S.; Shuford, K. L.; Qin, L.; Schatz, G. C.; Mirkin,
C. A. J. Am. Chem. Soc. 2005, 127, 5312.
Inorganic Chemistry, Vol. 47, No. 14, 2008 6349

Kundu et al.
Figure 6. TEM image of gold NPs with higher concentration of NaOH (pH ∼ 10).
Table 1. Concentrations of the Reaction Variables, Average Size, and Shape of Gold NPs after 90 s MW Exposure
conc. of HAuCl4 conc. of
conc. of 2,
conc. of MW time
average size (final, nm),
solution (M) CTAB (M) 7-DHN (M) NaOH (M)
(s)
(λmax)
shape distribution
-
-
4
4
-3
-3
-3
-3
-2
-2
3
4
.17 × 10
.21 × 10
6.34 × 10
8.42 × 10
3.17 × 10
1.05 × 10
1.15 × 10
1.05 × 10
90
90
25 ( 2 (520)
50-120 (576 and >700)
100% spherical
spheres (∼16%), triangles (∼22%), rods (∼ 10%),
hexagonal shapes (∼ 28%), and few other
shaped particles
-
-
4
4
-3
-2
-3
-3
-4
4
6
.71 × 10
.94 × 10
7.54 × 10
8.68 × 10
1.88 × 10
9.43 × 10
1.73 × 10
90
90
aspect ratios ∼ 12 (530 and 720) ∼75-80% rods; and rest, spheres
-4
6.07 ×10
65 ( 10 (630) 95% prisms; and rest, hexagonal and other shapes
Scheme 1. Chemical Structure of 2,7-Dihydroxy Naphthalene
nanorods and other anisotropic particles because of the
(
(
2,7-DHN), 1,2-Dihydroxy Naphthalene (1,2-DHN), and 2-Naphthol
2-N)
28,35,37
formation of an elongated micellar template.
Similarly,
in our case, the nanoprisms only formed at high concentration
-
2
of CTAB (9.6 × 10 M). In our previous studies, it was
1
5
reported that the gold nanocubes could be formed in
-
3
relatively low concentration of CTAB (6.3 × 10 M) in
the presence of UV light irradiation.
In our experiments an alkaline solution of 2,7-DHN was
used as a new reducing agent for the formation of gold NPs.
In the absence of 2,7-DHN while keeping other reaction
parameters the same, a turbid brown solution was formed
+
-
-
because of the formation of a [CTA] [AuBr
4
] or [AuBr
4
] /
-
[
AuBr
2
] complex. The variation of the 2,7-DHN concentra-
tion affected the formation of NPs as a result of the formation
of spherical or other shaped particles. The detailed reaction
conditions are listed in Table 1. We have studied reactions
with other hydroxyl compounds that have similarity with 2,7-
DHN including 1,2-dihydroxynaphthalene (1,2-DHN), 2-naph-
thol (2-N), and so forth (Scheme 1). The preliminary study
indicates that the formation of gold NPs and the detailed
results will be reported elsewhere.
Figure 7A and 7B presents the results obtained from EDS
and XRD pattern analysis, respectively. All the diffraction
peaks can be indexed to the characteristic peaks of gold NPs.
The Bragg reflection obtained from the gold nanoprisms
directions leading to the shape control. For the synthesis of
nanorods and nanoprisms we fixed the NaOH concentra-
-
3
-4
tion to 9.43 × 10 M and 1.73 × 10 M respectively
(
Table 1).
-
1
We have examined the CTAB concentration from 10
-
4
M to 10 M. In the absence of CTAB, the micron sized
gold particles are formed as observed from TEM (not shown
here) but precipitated immediately because of the absence
of any stabilizer in the aqueous solution. The different
concentrations of CTAB used for different shapes were
shown in Table 1. It was reported earlier that high concentra-
tion of CTAB (0.1 M) enhanced the formation of gold
(37) Tornbolm, M.; Henriksson, U. J. Phys. Chem. B 1997, 101, 6028.
6350 Inorganic Chemistry, Vol. 47, No. 14, 2008

Gold Nanoparticles
enabled the reduction of Au(III) to Au(0). When the
nucleation of Au(0) starts, the growth takes place in multiple
steps and produces different anisotropic particles. The
reduction of Au(III) to Au(0) leads to the formation of small
spherical nuclei. The nuclei subsequently form Au atoms and
assemble them together into a crystalline particle. The
surfactant CTAB molecule is attached to the surface of these
crystals and thus slows down the growth rate of the crystal
-2
facets. When the surfactant concentration is less (e10 M),
the interaction of the cationic parts with the specific surface
is weak. As a result, the growth of particles takes place in
all directions leading to the formation of spherical particles.
-1
With a high surfactant concentration (10 M or higher), the
cationic CTAB in aqueous solution formed polygonal mi-
cellar shapes (rod like and others) as reported by Murphy et
2
8,35
al.
The shape of the rod-like micelle promotes the
1
3
formation of rod like nanomaterials. Here, in our study,
we believe that at that particular concentration CTAB forms
the rod like “micellar template” and interacts strongly with
the growing crystal facets of gold and generates gold
nanorods. But for the nanoprism synthesis in other CTAB
concentrations, some trimeric clusters might be formed in
the solution having similarity with the formation of triangular
4
4
nanoplates of silver done by Xia’s group. Now during the
nucleation stage, the trimeric clusters generate triangular
nuclei. With the increasing MW time, the Au atoms added
more on the growing triangular nuclei, and they converted
to either the triangular shape or the truncated one depending
upon the ratio of {111} to {100} facets over the other side
facets. In our study we observed mostly triangular nano-
prisms as confirmed from TEM images in Figure 3D and
Figure 7. (A) EDS and (B) powder XRD pattern of the gold NPs.
3
F. The CTAB tends to attach to the lowest energy {111}
38
clearly corresponds to the fcc crystalline structure of gold.
facet and suppress the growth rate of this facet. The cationic
headgroup of CTAB interacts strongly with the {111} facets
of Au particles. The {111} facets of gold are more reactive
than the other facets like {110} or {100} in the presence of
bromide ion leading to the formation of nanoprisms. As a
result this stabilizes the formation of a bilayer structure due
to the van der Waals interaction between hydrophobic groups.
According to the TEM images in Figure 3, it is clear that
after a short time of MW heating small spherical particles
were formed. With the increased MW heating time, these
small particles grew into bigger particles in certain crystal-
lographic directions resulting in nanoprisms. The surfactant
CTAB was adsorbed on the particular faces of the gold
crystal and directed the 3D growth. Formation of well-
defined-shaped particles depends on the faceting tendency
of the stabilizing agent and on the growth kinetics (rate of
The peaks are assigned to the diffraction from the (111),
200), (220), and (311) planes of fcc gold, respectively. An
(
overwhelmingly strong diffraction peak located at 44.19° is
ascribed to the {111} facets of face centered cubic metal
structure. The normal orientation of the observed flat
nanoprisms was assigned as the {111} direction as hexagonal
patterns on the spectrum are lying flat on the planner surface.
The formation of anisotropic gold NPs initiated by the
reduction of Au(III) ions by the phenolic compound, 2,7-
DHN in alkaline medium, in presence of MW heating. The
use of these phenolic compounds for NPs synthesis is not
expected. It was reported that most phenolic compounds such
3
9
40
41
as benzophenone, dendrimer, polyvinyl alcohol, ascor-
4
2
43
bic acid, TX-100, and so forth underwent photolytic
cleavage to a phenoxy radical in the presence of UV light
or at high temperature. In the present work, it was assumed
that the radicals or the solvated electrons formed during the
MW heating of a solution mixture containing 2,7-DHN
45
Au(0) supply to the crystallographic planes). The long alkyl
chain of the CTAB molecule always anchors to the Au(0)
46
surface. In our research, NPs were found well separated
from each other. This is due to the CTAB adsorption on the
particle surface as a protective shell from aggregation.
In summary, we presented a new chemical route for the
synthesis of shape-controlled gold NPs in large scale. The
process was successful for making multiple shapes (spheres,
(
(
(
38) Guinier, A. X-Ray Diffraction,; W. H. Freeman: San Franscisco, CA,
1
963.
39) Sato, T.; Maeda, N.; Ohkoshi, H.; Yonezawa, Y. Bull. Chem. Soc.
Jpn. 1994, 67, 3165.
40) Esumi, K.; Suzuki, A.; Aihara, N.; Usui, K.; Torigoe, K. Langmuir
1
998, 14, 3157.
(
(
(
41) Henglein, A. Langmuir 1999, 15, 6738.
42) Pal, A.; Pal, T. J. Raman. Spectrosc. 1999, 30, 199.
43) Pal, A. Talanta 1998, 46, 583.
(44) Xiong, Y.; Washio, I.; Chen, J.; Sadilek, M.; Xia, Y. Angew. Chem.
2007, 119, 5005.
Inorganic Chemistry, Vol. 47, No. 14, 2008 6351

Kundu et al.
mixed shapes, rods, and prisms) in a short time (60-90 s)
under MW heating in the presence of CTAB and alkaline
Acknowledgment. This research was in part sponsored
by the NSF-0506082; the Department of Mechanical Engi-
neering, Texas A & M University; and the Texas Engineering
Experiments Station. Supports for TEM and EDS by Dr.
Zhiping Luo at the Microscopy Imaging Center (MIC), Texas
A & M University and for HR-SEM by Dr. Dwight
Romanovicz at the Biological Science Department, Univer-
sity of Texas, Austin were greatly appreciated.
2
,7-DHN. The synthesized particles were found to be stable
for at least 3 months. With the variation of surfactant to metal
ion molar ratio, the particles size and shape were successfully
modified. The approach developed here would lead to the
formation of other metallic (like Ag, Pd, Pt, etc.) and
semiconductor (CdS, CdSe, CdTe, etc.) particles with well-
defined shapes. There are promising applications for this in
many areas such as catalysis, clinical and diagnostic medi-
cine, and nanoelectronics, among others.
IC8004135
(
45) Petroski, J. M.; Wang, Z. L.; Green, T. C.; El-Sayed, M. A. J. Phys.
Chem. B 1998, 102, 3316.
(46) Praharaj, S.; Ghosh, S. K.; Nath, S.; Kundu, S.; Panigrahi, S.; Basu,
S.; Pal, T. J. Phys. Chem. B 2005, 109, 13166.
6352 Inorganic Chemistry, Vol. 47, No. 14, 2008