Synthesis of Gold Nanosheets through Thermolysis of Mixtures of ...

98 Journal of the Chinese Chemical Society, 2009, 56, 98-106
Synthesis of Gold Nanosheets through Thermolysis of Mixtures of Long
Chain 1-Alkylimidazole and Hydrogen Tetrachloroaurate(III)
Shih-Jen Hsu ( ) and Ivan J. B. Lin* ( )
Department of Chemistry, National Dong Hwa University, Shoufeng, Hualien 974, Taiwan, R.O.C.
Micron-sized gold nanosheets were produced through thermolysis of a mixture composed of 1-octadecanylimidazole
(C 18 -im) and HAuCl 4 in a molar ratio of 4:1 at 200 o C for 1 h. Effects of the molar ratio
of [C 18 -im]/[HAuCl 4 ], the reaction temperature, and the N-alkylimidazole chain length were studied. Adjusting
the molar ratio of [C 18 -im]/[HAuCl 4 ] can tune the morphology and size of the nanostructures; the
effect of reaction temperature is minimum; while using long chain imidazole tends to favor the formation
of nanosheets, using 1-methylimidazole (C 1 -im) produces micron-sized polyhedra. The growth mechanism
of these nanostructures was proposed. C 18 -im functions both as a templating and capping agent and
favors the growth of nanosheets. On the other hand, C 1 -im functions only as a capping agent and thus favors
the formation of polyhedra, especially octahedra.
Keywords: Gold nanosheets; Thermolysis; N-Alkylimidazole stabilizer.
INTRODUCTION
Gold nanostructures have been widely studied because
of their unique size and shape dependent properties,
which are potentially useful in electronics, catalysis and
especially optics. 1 For example, while a single surface
plasmon resonance (SPR) absorption band is usually observed
for spherical nanoparticles (NPs), more than one
band can be observed for non-spherical AuNPs. Various
chemical and physical methods have been used to synthesize
gold-nanostructured prisms, 2 rods, 1a,3 wires, 4 disks, 5
cubes, 2a,6,7 boxes, 7 rings, 8 sheets 9 and even branched multipods.
1c,10 2D Au nanosheets, in particular, have been attracting
special attention, because they may offer new possibilities
both for research and technological applications 11-15
in surface-enhanced Raman scattering, gas sensing, infrared-absorbent
optical coatings, and inducing hyperthermia
in tumors.
A polyol process, in which a diol or polyol functions
as a solvent and reducing agent, has previously been used
to produce Au nanosheets in high yield. 4,5f,16 Studies have
also been conducted on the reduction of aqueous HAuCl 4
by reducing agents, such as amines, 17 oxalic acid, 18 and citrate,
19 to produce Au nanosheets. Separate studies on the
biological synthesis of Au nanosheets with seaweed and
lemongrass as reductants have also been conducted. 13
Structured ionic liquids and self-assembled lyotropic liquid
crystals, presumably serving as templates, have recently
provided a further alternative approach to the synthesis of
Au nanosheets. 20
In this paper, we report a new finding, in which a simple
mixture of 1-alkylimidazole and HAuCl 4 produces different
nano- to micrometer-sized Au crystals under thermal
treatment. In brief, heating a mixture of N-long chain imidazole
(C n -im, C n =C n H 2n+1 , n = 16, 18) and HAuCl 4 at 200
o C produced hexagonal, triangular and truncated triangular
Au nanosheets with sizes of several to tens of micrometers,
and with yields of greater than 85%. When 1-methylimidazole
was used, polyhedra, particularly octahedra (around
30%), were obtained. To our knowledge, the use of this
simple technique to control and synthesize gold nanostructures
in gram quantity has not yet been reported.
EXPERIMENTAL SECTION
Reagents and Materials
Compounds 1-methylimidazole (C 1 -im) and HAuCl 4
were purchased from Aldrich. Compounds 1-octadecanylimidazole
(C 18 -im), 1-hexadecanylimidazole (C 16 -im) and
1,3-octadecanylimidazolium chloride [(C 16 H 33 ) 2 -im][Cl]
were prepared by the known method. 21
Synthesis of Au(III) Complexes
Preparation of [C 18 -imH][AuCl 4 ].[C 18 -imH]Cl (200
mg, 0.56 mmol) and [HAuCl 4 ] (190 mg, 0.56 mmol) in eth-
* Corresponding author. Tel: +886-3-863-3599; Fax: +886-3-863-3570; E-mail: ijblin@mail.ndhu.edu.tw

Fabrication of Gold Nanosheets through Thermolysis J. Chin. Chem. Soc., Vol. 56, No. 1, 2009 99
anol (50 mL) were stirred for 2 h. The volume of the resultant
solution was reduced to 5 mL by rotary evaporator. The
precipitate was filtered and washed two times with ethanol
(5 mL), the solid was collected and dried under vacuum.
The yield was 65%. 1 H NMR (ppm, d 6 -DMSO): 0.83 (t,
3 J = 7 Hz, 3H, CH 3 ), 1.19-1.35 (m, 30H, CH 2 ), 1.77 (m,
2H, CH 2 ), 4.15 (t, 3 J = 7 Hz, 2H, CH 2 ), 7.67 (m, 1H, CH),
7.75 (m, 1H, CH), 9.11 (s, 1H, CH). Anal. Calcd. for
C 21 H 41 N 2 AuCl 4 : C, 38.20; H, 6.26; N, 4.24. Found: C,
38.65; H, 6.44; N, 4.38%.
Preparation of (C 18 -im)AuCl 3·C 18 H 37 -im (200 mg,
0.62 mmol) and [HAuCl 4 ] (220 mg, 0.65 mmol) in ethanol
(50 mL) were stirred for 2 h. After which, the solvent was
removed by rotary evaporator. The residue was dissolved
in CH 2 Cl 2 (20 mL), to which silica gel (5 g) was added and
stirred for 30 min. This was then filtered and the filtrate
was collected and dried under vacuum to give the product
with yield of 55%. 1 H NMR (ppm, d 6 -DMSO): 0.83 (t,
3 J = 7 Hz, 3H, CH 3 ), 1.19-1.35 (m, 30H, CH 2 ), 1.95 (m,
2H, CH 2 ), 4.15 (t, 3 J = 7 Hz, 2H, CH 2 ), 7.65 (m, 1H, CH),
7.70 (m, 1H, CH), 9.08 (s, 1H, CH). Anal. Calcd. for
C 21 H 40 N 2 AuCl 3 : C, 40.43; H, 6.46; N, 4.49. Found: C,
40.75; H, 6.55; N, 4.35%.
Synthesis of Au Crystals
To prepare precursor of [C 18 -im]/[HAuCl 4 ] (4:1),
C 18 -im (0.38 g, 1.2 10 -3 mmol) and HAuCl 4 (0.10 g, 3.0
10 -4 mmol) were mixed in ethanol (50 mL) and the mixture
was stirred for 1 h. The solvent was then removed by rotary
evaporator. The residue was collected and dried under vacuum.
To form Au nanostructures, the resultant mixture was
heated at 200 o C for 1 h in a sealed glass tube. These thermal
treated solid products were collected and washed several
times with ethanol. Similar procedures were applicable
for other ratios and imidazoles. Samples with HAuCl 4 up to
1.0 g worked equally well.
Instrumentation
X-ray diffraction analysis of the samples was carried
out using an X-ray diffractometer (XRD D8 Advanced,
Bruker and XRD Rigaku D/max-2500, Japan) with Cu K
radiation. The morphology of the as-prepared products was
characterized by scanning electron microscopy (SEM,
JMF-6500F. JOEL) and transmission electron microscopy
(TEM, JEOL JEM-3010, operating voltage of 300 kV). A
ZEISS Axioplan2 imaging polarizing microscope equipped
with a Mettler FP 82 hot stage and a Mettler FP 90 central
processor was used to examine and follow the growth of
the nanomaterials. The 1 H NMR spectra were recorded on a
Bruker AC-F300 spectrometer. Infrared (IR) spectra of the
samples were measured on a Perkin-Elmer Fourier transform
spectrophotometer (model spectrum one) with a LITA
(lithium tantalite) midinfrared detector.
RESULTS AND DISCUSSION
Thermolysis of [C 18 -im]/[HAuCl 4 ] = 4:1 at 200 o Cfor
1 h in a sealed glass tube produced mostly shining Au platelets
in >85% yield. To monitor macroscopically the progress
of thermolysis, an optical microscope equipped with a
temperature-controlled hot stage was employed. Precursor
of [C 18 -im]/[HAuCl 4 ] = 4/1 sandwiched between two slide
glasses showed the melting began at 125 o C; upon continuedheatingto200
o C, small particles are formed at an early
stage; platelets are subsequently formed and grew (Fig. 1).
Since nanoscale particles could not be observed via optical
microscope, the growth of nanostructures was followed by
TEM. For a reaction time of 1 min, spherical Au NPs in
20-40 nm diameter were observed (Fig. 2a). Increasing the
duration of thermolysis to 3 min, worm-like nanostructures
of ca. 50~100 nm and nanoplates of 60-160 nm lateral
lengths were formed (Fig. 2b1-2). Reaction quenched after
30 min, produced platelets of ca. 500 nm lateral lengths
along with small amount of irregular faceted particles of ca.
100 nm (Fig. 2c). For a reaction duration of 60 min, SEM
image revealed that predominantly hexagonal and triangular
plates with lateral sizes of ca. 50 m and thickness of ca.
100 nm, together with ca. 1 m irregular faceted particles
are present (Fig. 2d). Although several processes have been
known for the preparation of large Au nanoplates, our work
presents an alternative simple method to fabricate Au nano-
Fig. 1. Monitoring the thermolysis of a mixture of
[C 18 -im]/[HAuCl 4 ] (4:1) at 200 o Conaglass
slide by POM for (a) 4 min, (b) 6 min, and (c) 8
min.

100 J. Chin. Chem. Soc., Vol. 56, No. 1, 2009 Hsu and Lin
Fig. 2. TEM and SEM image of [C 18 -im]/[HAuCl 4 ] (4/1) by thermolysis at 200 o C for (a) 1 min, (b1-2) 3 min, (c) 30 min, and
(d) 60 min (inset shows the side view image of a gold plate).
plates. Earlier, we reported the thermolysis of the Au(I)
complexes of benzimidazole/imidazole to produce Au
nanoparticles, in which the yield of platelets was ca. 55%. 22
XRD pattern of the platelets is given in Fig. 3, in
which the intensity of diffraction peak at 2 of ~38.2 o assigned
to the {111} lattice planes of face-centered cubic
(fcc) Au crystals is overwhelmingly high in comparison to
those of others. This result implies that the basal plane, i.e.,
the top plane of the microplates, is the {111} planes. 23 The
ED pattern obtained by focusing the electron beam onto the
triangular plate showed hexagonally arranged diffraction
spots, characteristic of single-crystalline (111)-oriented
gold crystals (Fig. 4). 24
Thermolysis at different [C 18 -im]/[HAuCl 4 ] ratios
was also studied. SEM images of products obtained for the
ratios of 12/1, 10/1, 8/1, 6/1, 2/1 and 1/1 are given in Fig. 5.
At higher ratios of 12/1, 10/1, 8/1 and 6/1, under similar
thermal treatments, platelets around 30 m together with a
small amount of irregular faceted particles in micrometer
and sub-micrometer were obtained (Fig. 5a-d). At the lower
ratios of 2/1 and 1/1, while micrometer-sized plates with
wider size distribution were still the dominant morphology,
Fig. 3. Powder X-ray diffraction pattern of gold
nanosheets obtained by heating the mixture of
[C 18 -im]/[HAuCl 4 ] (4:1) at 200 o Cfor1h.
Fig. 4. Typical TEM images of gold nanosheets, the representative images of (a) an individual triangular and (b) hexagonal
nanosheets. (c) The corresponding electron diffraction (ED) pattern of the triangular gold nanosheet lying flat on a
TEM grid.

Fabrication of Gold Nanosheets through Thermolysis J. Chin. Chem. Soc., Vol. 56, No. 1, 2009 101
a few long rods, belts, and highly faceted particles were
also observed (Fig. 5e-f). These results indicate that thermolysis
with [C 18 -im]/[HAuCl 4 ] ratios from 12/1 to 1/1,
gives predominantly Au platelets. As expected, diffractograms
show predominantly {111} reflections for these products.
The effect of reaction temperature was also examined.
When samples with a molar ratio of 4:1 were heated at
175 o C (Fig. 6a), 200 o C (Fig. 1a and 1b) and 250 o C(Fig.
6b), it took around 2 h, 1 h and 25 min respectively to complete
the reaction (followed by optical microscopy). Despite
the difference in the growth rate, the major morphology
was still nanosheets with thickness of 80 ~ 140 nm.
The one carried out at 175 o C, had a slower growth rate, and
thus provided a better defined sheets and particles.
N-alkyl chain length of imidazole influenced the morphology
of Au crystals, as unveiled by the results of various
chain lengths at a fixed [C n -im]/[HAuCl 4 ] ratio of 4/1
given in Table 1. When n was 18, ca. 85% of platelets and
ca. 15% of non-plate-like particles were produced; decreasing
the chain length to n = 12, 6, and 1, the population
of platelets versus particles in percentage is changed to 60
versus 40, 30 versus 70, and 10 versus 90, respectively. It
appears that whereas long N-alkyl chain length of imida-
Fig. 5. SEM images of the precipitates obtained from mixture of [C 18 -im]/[HAuCl 4 ] with molar ratios of (a) 12/1, (b) 10/1,
(c) 8/1, (d) 6/1, (e) 2/1, and (f) 1/1 at 200 o Cfor1h.
Fig. 6. SEM images of the precipitates obtained at a 4:1 molar ratio of [C 18 -im]/[HAuCl 4 ] (a) at 175 o C for 2 h and (b) 250 o C
for 25 min.

102 J. Chin. Chem. Soc., Vol. 56, No. 1, 2009 Hsu and Lin
Table 1. Product distribution of micrometer-sized gold crystals obtained by using different alkyl chain
lengths (n) at a fixed [C n -im]/[HAuCl 4 ] molar ratio of 4/1 at 200 C for1h
Chain length
18 12 6 1
(n)
85/15 a 60/40 a 30/70 a 10/90 a
Plates: particles
(%)
a octahedra, pentagonal decahedra, and other ill-defined particles.
zole favors the formation of platelets, short chain imidazole
advances particle generation.
The imidazole N-alkyl chain length played an important
role in the control of morphology. Compounds with
chain lengths of n = 16 and 1 were studied for comparison.
For n = 16, thermolysis of [C 16 -im]/[HAuCl 4 ] = 4:1 at 200
o C for 1 h, produced similar distributions of morphology
and size to that of the analogous C 18 -im. For n = 1, images
of SEM obtained from the thermolysis of [C 1 -im]/[HAuCl 4 ]
with molar ratios of 18:1, 10:1, 6:1 and 4:1 at 200 o Cfor1h
is shown in Fig. 7. At the molar ratio of 18:1 (Fig. 7a),
many regular octahedra (around 30%), highly faceted particles,
and few nanosheets were obtained. Diffractogram of
this product, showed patterns characteristic of cubic Au
single crystals bounded by (111) planes, consistent with the
observed octahedral morphology. At the molar ratios of
10:1 (Fig. 7b), octahedral, pentagonal bipyramidal, highly
faceted and irregular shaped particles were observed with
size distribution of few microns. At the ratio of 6:1 (Fig.
7c), mostly irregularly faceted aggregates and some micron-sized
octahedra were produced. At the ratios of 4:1
(Fig. 7d) and 1:1 (not shown), mainly irregularly faceted
particles, few octahedra and around 20% distribution of
nanosheets were obtained. The sizes of the particles were in
few microns, while the sheets were in tens of microns.
What is the nature of the [C 18 -im]/[HAuCl 4 ] mixture?
We believed that when C 18 -im was mixed with acidic
HAuCl 4 , ionic [C 18 -im-H][AuCl 4 ] was presumably formed.
Free C 18 -im may also react with AuCl 4 - to produce (C 18 -
im)AuCl 3 and possibly some further substituted compounds.
The mixture may contain C 18 -im, (C 18 -im)AuCl 3 ,
[C 18 -imH][AuCl 4 ], [C 18 -imH]Cl and others. [C 18 -imH]
[AuCl 4 ]and(C 18 -im)AuCl 3 were separately prepared to
confirm their presences in the mixture. These possible reactions
can be represented by equations given below (eq.
1-2).
C 18 -im + HAuCl 4 [C 18 -imH] + [AuCl 4 ] - (1)
C 18 -im + AuCl 4 - (C 18 -im)AuCl 3 +Cl -
+other(C 18 -im)-Au compounds (2)
Fig. 7. SEM images of the products synthesized by
heating [C 1 -im]/[HAuCl 4 ] at 200 o Cfor1hwith
molar ratios of (a) 18:1, (b) 10:1, (c) 6:1, and
(d) 4:1.
Infrared (IR) spectroscopy was employed to verify
the species presented in the mixture by monitoring the
N(sp 2 )-H stretching vibrations ( N-H , 3400-3000 cm -1 )
(Fig. 8). For neutral C 18 -im and (C 18 -im)AuCl 3 there was
no N(sp 2 )-H bond, therefore no corresponding stretching
frequency was expected; this was the case as shown in Fig.

Fabrication of Gold Nanosheets through Thermolysis J. Chin. Chem. Soc., Vol. 56, No. 1, 2009 103
8a and 8b respectively. On the other hand, ionic [C 18 -
imH]Cl and [C 18 -imH][AuCl 4 ] possessed N(sp 2 )-H bonds,
therefore N-H bands were expected. Indeed [C 18 -imH]Cl
and [C 18 -imH][AuCl 4 ] showed strong N-H at 3417 (Fig.
8c) and 3293 (Fig. 8d) cm -1 respectively. The difference
in these two N-H positions is likely caused by the difference
between the hydrogen bonding interactions of NH
ClAuCl 3 and NH Cl; the latter has a stronger hydrogen
bonding interaction. The sample prepared by mixing a 1:1
molarof[C 18 -im]/[HAuCl 4 ] showed N-H at 3296 cm -1 (Fig.
8e), suggesting the presence of [C 18 -imH][AuCl 4 ]. At the
ratio of 4:1 a broad N-H band at 3417 cm -1 was observed,
indicating the presence of [C 18 -imH]Cl. This observation
can be reasoned that at 4:1 molar ratio, C 18 -im may have
better chance to react with AuCl 4 - to form neutral (C 18 -
im)AuCl 3 (eq. 2), therefore more [C 18 -imH]Cl may be
formed.
The 1 H-NMR spectra of C 18 -im (Fig. 9a), (C 18 -
im)AuCl 3 (Fig. 9b), [C 18 -imH]Cl (Fig. 9c), [C 18 -imH]
[AuCl 4 ] (Fig. 9d), [C 18 -im]/[HAuCl 4 ] (1:1) (Fig. 9e), and
[C 18 -im]/[HAuCl 4 ] (4:1) (Fig. 9f) in d 6 -DMSO were examined.
Because the chemical shifts of C 2,4,5 -H protons are
sensitive to the surrounding environment, they were used
to monitor the formation of each individual species. The
neutral C 18 -im and (C 18 -im)AuCl 3 and the ionic [C 18 -
imH]Cl and [C 18 -imH][AuCl 4 ]hadC 2,4,5 protons at 7.58,
7.12, 6.85; 9.08, 7.75, 7.65; 9.21, 7.79, 7.67, and 9.11,
7.75, 7.67 ppm, respectively. For the 1:1 mixture the corresponding
chemical shifts were at 9.11, 7.77, and 7.68 ppm.
These values were close to that of the ionic [C 18 -imH]
[AuCl 4 ], consistent with the results from IR studies. For the
4 to 1 mixture, the values at 8.65, 7.56, and 7.40 ppm could
not be assigned to any of the species mentioned. An exchange
of imidazole among C 18 -im, (C 18 -im)AuCl 3 ,[C 18 -
imH][AuCl 4 ]and[C 18 -imH]Cl may however explain the
results.
Diffractograms of C 18 -im (Fig. 10a), (C 18 -im)AuCl 3
(Fig. 10b), [C 18 -imH]Cl (Fig. 10c), [C 18 -imH][AuCl 4 ](Fig.
10d), [C 18 -imH]/[AuCl 4 ] (1:1) (Fig. 10e), and [C 18 -im]/
[HAuCl 4 ] (4:1) (Fig. 10f) are given and compared. Diffractogram
of the 1:1 mixture (Fig. 10e) showed patterns
corresponding to that of [C 18 imH][AuCl 4 ] and (C 18 -
im)AuCl 3 ; the former had a much higher (001) peak intensity
than that of the latter (around 3:2). Diffractogram for
the sample with molar ratio of 4:1 is complicated. Although
we were unable to identify the species in the 4:1 mixture
from the diffractogram, the observation of reflections with
corresponding d-spacings of 28.0, 14.0, and 9.3 Å suggests
the formation of lamellar structure.
Fig. 8. Infrared spectra of (a) C 18 -im, (b) (C 18 -
im)AuCl 3 , (c) [C 18 -imH]Cl, (d) [C 18 -imH]
[AuCl 4 ], (e) [C 18 -im]/[HAuCl 4 ] (1:1), and (f)
[C 18 -im]/[HAuCl 4 ] (4:1). All spectra were obtained
in KBr pellets.
Fig. 9. The 1 H-NMR of (a) C 18 -im, (b) (C 18 im)AuCl 3 ,
(c) [C 18 -imH]Cl, (d) [C 18 -imH][AuCl 4 ], (e)
C 18 im/HAuCl 4 (1:1), and (f) C 18 im/HAuCl 4
(4:1) in d 6 -DMSO.

104 J. Chin. Chem. Soc., Vol. 56, No. 1, 2009 Hsu and Lin
will tend to self-assemble through chain-chain interactions
to form layered structure; the Au(III) ions either as (C 18 -
im)AuCl 3 or [C 18 -imH][AuCl 4 ] will be sandwiched between
these chain layers. Upon thermal treatment, Au atoms
can be produced from the “Au(III)” mixture presumably
through successive electron transfer from Cl - to
Au(III). These Au nuclei formed will be protected by the
long chain imidazoles. As capping agent C 18 -im appears to
bond preferentially on the closest packed {111} planes.
The templating effect of C 18 -im, due to the hydrophobic
chain-chain interactions, favors an extended growth along
{111} planes.
Although C 1 -im may also preferentially adsorb on the
{111} gold faces, the methyl group lacks the chain-chain
interaction to form layered structure (Scheme Ib). Without
the template effect, the {111} faces of different particles
may not align in a lamellar fashion. Therefore, random
growth of highly faceted polyhedra with mostly (111) faces
is favored.
Fig. 10. Powder X-ray diffraction patterns of (a)
C 18 -im, (b) (C 18 im)AuCl 3 ,(c)[C 18 -imH]Cl,
(d) [C 18 -imH][AuCl 4 ], (e) C 18 im/HAuCl 4
(1:1), and (f) C 18 im/HAuCl 4 (4:1).
Why the [C 18 -im]/[HAuCl 4 ] mixture produced nanosheets
preferentially? The possible steps in the formation
of Au nanosheets from these Au(III) mixtures are presented
in Scheme Ia. Compounds with long alkyl chains
CONCLUSIONS
In summary, a new method of controlling the fabrication
of micron-sized gold nanosheets and polyhedra are described.
This method is simple, no additional solvents,
templating and protecting agents are required. For the mixture
of [C 18 -im]/[HAuCl 4 ], formation of the gold nanosheets
is preferred. For the mixture of [C 1 -im]/[HAuCl 4 ]
faceted polyhedra, especially well-defined octahedra are
formed favorably. We propose that C 18 -im as a capping
Scheme I

Fabrication of Gold Nanosheets through Thermolysis J. Chin. Chem. Soc., Vol. 56, No. 1, 2009 105
agent adsorbs preferentially on the {111} planes of Au nuclei.
The long alkyl chain also allows C 18 -imtofunctionas
templating agent, which favors the formation of hexagonal,
triangular and truncated triangular nanosheets. On the
other hand, C 1 -im only functions as a capping agent, which
also adsorbs favorably on the Au (111) plane. Formation
of particles with mostly {111} facets are therefore preferred.
ACKNOWLEDGMENT
We thank the National Science Council of Taiwan for
financial support (NSC 96-2113-M-259-012) and National
Dong Hwa University Nanotechnology Research Center.
Received June 22, 2008.
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