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

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.