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

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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.
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