Article pubs.acs.org/Langmuir
Neuron-Shaped Gold Nanocrystals and Two-Dimensional Dendritic Gold Nanowires Fabricated by Use of a Long-Chain Amidoamine Derivative Yoshiro Imura,†,‡ Ayumi Maezawa,† Clara Morita,†,‡ and Takeshi Kawai*,† †
Department of Industrial Chemistry, Tokyo University of Science, 1-3 Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan The Japan Society for the Promotion of Science, 8 Ichibancho, Chiyoda-ku, Tokyo 102-8471, Japan
‡
S Supporting Information *
ABSTRACT: We report the synthesis of two-dimensional (2D) dendritic Au nanowires (DNWs) with diameters of 100−200 nm in an aqueous solution of long-chain amidoamine derivative (C18AA), which acted as both capping and reducing agent, and the preparation of large 2D DNWs with diameters of 400−700 nm by seeded growth of the original DNWs. The seeded growth method in the presence of C18AA enables the fabrication of novel neuron-shaped Au nanostructures consisting of two DNWs dangling from both ends of an ultrathin Au nanowire.
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INTRODUCTION Morphological control of metal nanomaterials is important in the development of modern material chemistry, because the physical and chemical properties can be easily and widely tuned by tailoring the particle size and shape.1,2 Recently, there have been many reports on the fabrication of various-shaped metal nanomaterials, such as cubes,3 plates,4 rods,5−8 and wires.9−15 The general approach to the shape control of metal nanomaterials is the conventional seeded growth method: seed nanoparticles (NPs) with specific shapes and/or particular crystal facets are grown to obtain the desired nanostructure shape. In the seeded growth method, capping agents are typically used to control the growth rate of a specific crystal facet, due to selective adsorption on specific crystal facets of metal NPs. Tsuji et al.16−18 have successfully synthesized cubic, triangular-bipyramidal, and rod-shaped core−shell nanocrystals (Au@Ag) using octahedral, triangular plate, and decahedral seeds, respectively, in the presence of a poly(vinylpyrrolidone) (PVP) as a capping agent. The presence of PVP is important to achieve shape control, because PVP is preferentially adsorbed on the (100) and (110) crystal facets of Ag, which consequently promotes growth of the (111) crystal facet.19−23 This inhomogeneous growth rate leads to the formation of various-shaped nanomaterials rather than spherical NPs.2 Among the uniquely shaped Au nanomaterials, one- and twodimensional (1D and 2D) nanomaterials such as straight ultrathin Au nanowires2,10−12 (Au NWs) and highly branched Au dendritic nanowiress (Au DNWs)1,9 have attracted considerable interest, due to their distinctive electrical, optical, and magnetic properties, high catalytic activity, and potential application in nanodevices.1 Furthermore, there have been many recent reports24,25 on the applications of 1D and 2D Au © 2012 American Chemical Society
nanomaterials as platforms for surface-enhanced Raman scattering. Previous reports26−31 showed that a long-chain amidoamine derivative (C18AA, Figure 1) can selectively adsorb on the
Figure 1. Molecular structure of C18AA.
(100) and (110) facets of Au surfaces, rather than on the (111) facet. It was also demonstrated that ultrathin Au NWs with diameters less than 2 nm can be prepared on a lamellar structure in an organogel of C18AA by exploiting its selective adsorption for a specific gold surface,26 and that both ends of the ultrathin Au NWs consisted of (111) crystal facets.27,28 Here, we report the preparation of 2D Au DNWs with diameters of 100−200 nm in an aqueous solution of C18AA, which can then be used as seeds to synthesize larger 2D Au DNWs with diameters of 400−700 nm (Figure 2a). We also demonstrate that the seeded growth method can be applied to Received: February 23, 2012 Revised: September 26, 2012 Published: September 27, 2012 14998
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X-ray Diffraction Measurements. Two-dimensional Au DNWs were synthesized by adding HAuCl4 (1 wt % aqueous solution, 6.0 g) to C18AA (2 wt % aqueous solution, 30 g) and heating the mixture at 55 °C for 8 h without stirring. Two-dimensional Au DNWs in water (30 g) were centrifuged at 10 000 rpm for 30 min to remove excess C18AA, and the resulting precipitate was used for X-ray diffraction (XRD) analysis. XRD patterns were recorded on a Rigaku UltimaIV diffractometer. Transmission Electron Microscopic Measurements. Standard and high-resolution transmission electron microscopy (TEM and HRTEM) were performed on a Hitachi H9500 instrument equipped with an energy-dispersive X-ray spectrometer (EDX) operated at 200 kV.
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Figure 2. Schematic illustration of (a) large 2D dendritic Au NWs and (b) neuron-shaped Au NCs.
RESULTS AND DISCUSSION In the case of 2.0 wt % C18AA (i.e., [C18AA]/[HAuCl4] = 6.67), the solution changed from yellow to light brown (Figure 3), and Au DNWs comprising 4−7 nm branches were
fabricate neuron-shaped Au NWs consisting of two DNWs dangling from both ends of an ultrathin Au NW (Figure 2b).
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EXPERIMENTAL SECTION
Materials. Octadecylamine (Aldrich) was recrystallized from hexane. Methyl acrylate (Kanto Chemicals) was purified by distillation under reduced pressure. HAuCl4 (Nacalai Tesque), toluene (Kanto Chemicals), and ethylenediamine (Kanto Chemicals) were used without further purification. Synthesis of C18AA.26−31 Methyl acrylate (10.22 g, 0.12 mol) was added to 2.0 g (7.12 mmol) of octadecylamine in 15 mL of methanol. The solution was stirred at 40 °C for 3 days, and the solvents and excess methyl acrylate were then removed by rotary evaporation. 3-{[(2-Methoxycarbonyl)ethyl]octadecylamino}propionic acid methyl ester (C18ME) was obtained as a viscous liquid. C18ME (3.2 g) and ethylenediamine (17.8 g, 0.30 mol) were dissolved in 15 mL of methanol and the mixture was stirred for 1 week at room temperature. C18AA was obtained as a light yellow solid upon removal of the solvent and ethylenediamine by evaporation and freezedrying. The crude solid was recrystallized from a mixed solvent of toluene and methanol. Yield: 90%. Preparation of Dendritic Au Nanowires. a. Effect of [C18AA]/ [HAuCl4]. HAuCl4 (1 wt % aqueous solution, 1.0 g) was added to C18AA (0.5, 2.0, or 10 wt % aqueous solution; 4.0 g) and the mixture was heated at 55 °C for 8 h without stirring. The molar ratios of [C18AA]/[HAuCl4] for 0.5, 2.0, and 10 wt % C18AA were 1.67, 6.67, and 33.5, respectively. b. Effect of Synthesis Temperature. HAuCl4 (1 wt % aqueous solution, 1.0 g) was added to C18AA (2.0 wt % aqueous solution, 4.0 g). The mixture was left at room temperature (25 °C) for 3 days or heated at 70 °C for 8 h without stirring. c. Effect of Reducing Agent. After HAuCl4 (1 wt % aqueous solution, 1.0 g) was added to C18AA (2 wt % aqueous solution, 4.0 g) and the mixture was heated at 55 °C, NaBH4 (0.25 M aqueous solution, 100 μL) was quickly added and the mixture was heated at 55 °C for 8 h without stirring. Size and Morphology Control of Dendritic Au Nanowires. Two-dimensional Au DNWs as seeds were synthesized by adding HAuCl4 (1 wt % aqueous solution, 1.0 g) to C18AA (2 wt % aqueous solution, 4.0 g) and heating the mixture at 55 °C for 8 h without stirring. HAuCl4 (1 wt % aqueous solution, 0.375 g) was added to 2D Au DNWs dispersed in water (1.0 g) and C18AA aqueous solution (0.375 g). The mixture was then left for 24 h at room temperature. Preparation of Neuron-Shaped Au Nanocrystals. Ultrathin Au NWs were prepared as seeds according to a previously reported method: 2 wt % C18AA−toluene gel (10 g) was added to HAuCl4 (0.02 g) and the mixture was heated at 55 °C for 8 h.27 The ultrathin Au NWs dispersed in toluene (5 mL) were transferred to the aqueous phase by the addition of water (5.0 mL).27 C18AA (2 wt %)−water (4.0 g) and HAuCl4 (1 wt % aqueous solution, 1.0 g) were added to the resulting aqueous solution and the mixture was left for 3 days at room temperature (25 °C). The [C18AA]/[HAuCl4] was maintained at 6.67.
Figure 3. Photographic images (a) before and (b) after the synthesis of 2D Au DNWs.
successfully obtained, as shown in Figure 4a. However, spherical and 2D platelike Au nanocrystals were obtained at lower and higher [C18AA]/[HAuCl4] ratios of 1.67 and 33.5, respectively (Figure 4b,c). The formation of 2D Au DNWs is a rare case, because it is common for three-dimensional (3D) DNWs to be obtained during the preparation of dendritic structures without use of 2D templates or reaction spaces. Preparation of 2D Au DNWs was attempted under various temperatures, and they were found to be produced in the temperature range of 25−70 °C (Figure S1, Supporting Information). The formation of Au DNWs was affected mainly by the [C18AA]/[HAuCl4] ratio and not by the reaction temperature. Interestingly, the formation of Au NPs as a byproduct was negligible, as confirmed by TEM observation. This result was also confirmed by the absence of the surface plasmon band of Au NPs at ca. 526 nm in the UV−vis spectrum (Figure 5). TEM-EDX measurements confirmed that 2D Au DNWs were composed of pure gold (Figure S2, Supporting Information). XRD peaks of 2D Au DNWs at 38.3°, 44.4°, 64.6°, 77.6°, and 81.5° were assigned to the (111), (200), (220), (311), and (222) diffractions of face-centered cubic Au, respectively (Figure 6).24 The appearance of (111), (200), (220), and (311) diffractions in the selected area electron 14999
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Figure 4. TEM images of Au nanomaterials synthesized at 55 °C with [C18AA]/[HAuCl4] ratios of (a) 6.67, (b) 1.67, and (c) 33.5 without NaBH4. (d) TEM image of Au NPs synthesized at 55 °C with a [C18AA]/[HAuCl4] ratio of 6.67 and with NaBH4 as a reducing agent.
Figure 5. UV−vis spectra of (a) Au NPs synthesized with a [C18AA]/ [HAuCl4] ratio of 1.67 and (b) 2D Au DNWs synthesized with a [C18AA]/[HAuCl4] ratio of 6.67. Figure 6. XRD pattern of 2D Au DNWs.
diffraction (SAED) pattern (Figure 7a) also confirmed that the Au DNWs consisted of face-centered cubic Au. Furthermore, the HR-TEM image of 2D Au DNWs shown in Figure 7b has fringes with a periodicity of 0.24 nm, which corresponds to the (111) lattice spacing observed along the long axis of the branch in the Au DNWs. HR-TEM observations of various branches also revealed that the growth of any Au DNWs was in the (111) crystal direction. We have previously demonstrated26 that C18AA is preferably adsorbed on (100) and (110) facets rather than on the (111) facet; therefore, the (100) and (110) crystal facets of Au are thought to be covered with a high-density monolayer of C18AA due to selective adsorption, whereas the (111) crystal facet is covered with a low-density monolayer. Thus, the growth rate of the
(111) facet is faster than the other facets, and consequently the branches of 2D Au DNWs grow in the (111) direction. The formation of 3D Au DNWs generally follows the diffusion-limited aggregation (DLA) mechanism under appropriate reaction conditions with kinetic control.1,32−34 Thus, the growth of Au DNWs in the present system is also probably kinetically controlled by a suitable reaction condition. The key to kinetic control is to maintain an extremely low concentration of Au atoms or Au nanocrystals (NCs), so that a slow growth rate is ensured. The slow growth process was confirmed by TEM observation at various reaction times. The average diameters of the 2D Au DNWs were 30−60 nm at 2 h, 80−160 15000
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Figure 7. (a) SAED pattern and (b) HR-TEM image of 2D Au DNWs synthesized at 55 °C with [C18AA]/[HAuCl4] = 6.67.
Figure 8. TEM images of Au DNWs grown with [C18AA]/[HAuCl4] ratios of (a, b) 6.67 and (c, d) 3.34 at 24 h.
nm at 6 h, 100−200 nm at 8 h, and 100−200 nm at 14 h of reaction time (Figure S3, Supporting Information). Therefore, C18AA acting as a weak reducing agent is crucial for the formation of Au DNWs in this work, because a weak reductant can supply low concentrations of Au atoms or Au NCs. The effect of reduction rate on the formation of Au DNWs was then examined by using NaBH4 as a strong reducing agent. After HAuCl4 was added to C18AA (2 wt %)−water, an aqueous solution of NaBH4 was quickly added to the solution. The color of the mixture immediately changed from yellow to dark red, where only Au NPs and no Au DNWs were formed, as confirmed by TEM measurements (Figure 4d). Thus, control
of the growth rate is essential for the preparation of Au DNWs in the present system. Figure 4a shows that the diameter of the Au DNWs was a few hundred nanometers; therefore, an attempt was made to prepare larger-size Au DNWs by changing the preparation conditions, such as the concentrations of HAuCl4 and C18AA. However, these attempts were ultimately unsuccessful. The Au DNWs shown in Figure 4a were therefore used as seeds to prepare larger-size Au 2D DNWs. An aqueous solution of 10 wt % C18AA (0.3 g) and 1 wt % HAuCl4 (0.375 g) was added to the seed solution (1 g), so that the [C18AA]/[HAuCl4] ratio was 6.67, and the mixture was left at room temperature for 24 15001
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Figure 9. TEM images of Au DNWs grown with [C18AA]/[HAuCl4] ratios of 2.78 at (a) 24 and (b) 6 h.
Figure 10. TEM images of neuron-shaped Au NCs synthesized from ultrathin Au NWs with diameters of (a, b) 2 and (c) 5 nm.
2.78, the structure was completely changed to 3D (Figure 9a). Formation of the 3D DNW structure is not caused by the aggregation of smaller 2D DNWs onto larger 2D DNWs, because the 3D portions were not disk-shaped but random- and fan-shaped.35 This conclusion was also confirmed by the observation of 3D branches everywhere in the TEM image (Figure 9b) of the Au DNWs at a reaction time of 6 h and with [C18AA]/[HAuCl4] = 2.78. The seeded growth method using the selective adsorption of C18AA is very useful for preferential growth of Au from the (111) crystal facet when an appropriate concentration of C18AA in water is used. Considering that both ends of the ultrathin Au NWs prepared in C18AA−toluene gel have (111)
h. Larger 2D Au DNWs with diameters of 400−700 nm were successfully obtained, as shown in Figure 8a,b. The morphology was very similar to that of the original seeds, and it was difficult to determine the connection between the seed and growth parts, which indicates that larger 2D Au DNWs were formed by growth from the (111) crystal facet of the ends of the Au DNW seeds; therefore, the seeded growth method proved to be very useful for the preparation of larger 2D Au DNWs. Interestingly, the molar ratio of [C18AA]/[HAuCl4] has a strong influence on the dimensionality of the Au DNWs; at a molar ratio of [C18AA]/[HAuCl4] = 3.34, the structure of the Au DNWs was partially changed from 2D to 3D, as shown in Figure 8c,d. At a lower molar ratio of [C18AA]/[HAuCl4] = 15002
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crystal facets, as previously reported,27,28 it was expected that application of the seeded growth method to ultrathin Au NWs would be useful to obtain a hybrid nanostructure of straight NWs and DNWs (Figure 2b). Ultrathin Au NWs with diameters of 2−5 nm were prepared according to a previously reported method; ultrathin Au NWs were synthesized by adding 2 wt % C18AA−toluene gel (10 g) to HAuCl4 (0.02 g) and heating the mixture at 55 °C for 8 h (Figure S4, Supporting Information). The ultrathin Au NWs were obtained by use of the C18AA organogel as a soft template.27 The ultrathin Au NWs dispersed in toluene were then transferred to the aqueous phase to obtain an aqueous dispersion.27 The morphology of the ultrathin Au NWs was not changed by the phase transfer method (Figure S5, Supporting Information).27 C18AA (2 wt %)−water (4.0 g) and HAuCl4 (1 wt % aqueous solution, 1 g) were added to the resulting aqueous dispersion of ultrathin Au NWs (1.0 g), and the mixed solution was left at room temperature (25 °C) for 3 days. The [C18AA]/[HAuCl4] ratio was maintained at 6.67. Novel neuron-shaped Au nanocrystals consisting of two DNWs dangling from both ends of the ultrathin Au NWs were successfully obtained, as shown in Figure 10. It was difficult to distinguish the connection point between the ultrathin 5 nm diameter Au NWs and the DNWs, as evidenced in Figure 10c. However, in the case of 2 nm diameter Au NWs, it was apparent that the diameter at the connection was gradually increased as DNW growth occurred from the straight ultrathin Au NWs (Figure 11). These results
Article
CONCLUSION Two-dimensional Au DNWs with diameters of 100−200 nm were successfully synthesized via the selective adsorption of C18AA in aqueous solution. In addition, the seeded growth method was used to produce larger 2D Au DNWs with diameters of 400−700 nm by use of smaller 100−200 nm diameter Au DNWs as seeds in the presence of C18AA. The seeded growth method can also be applied to fabricate a complicated nanostructure comprising two DNWs dangling from both ends of the (111) crystal facet of a straight ultrathin Au NW. This seeded growth method from a specific crystal facet of Au could lead to various applications, such as nanoconnectors and the fabrication of complex-shaped Au NCs.
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ASSOCIATED CONTENT
S Supporting Information *
Additional text and five figures showing synthesis of C18AA, EDX spectra of dendritic Au NWs, and TEM images of dendritic and ultrathin Au NWs. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone +81-3-5228-8312; fax +81-3-5261-4631; e-mail
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was partially supported by a Grant-in-Aid for Scientific Research B from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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REFERENCES
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Figure 11. TEM image of the connection point between a 2 nm diameter ultrathin Au NW and DNWs.
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