CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 6 1823–1826
Articles Fabrication of Ag/C Coaxial Nanocables with Cross-Linked Structure by SDS-Assisted Hydrothermal Approach Xu Chun Song,*,† Yang Zhao,‡ Yi Fan Zheng,§ E Yang,† Jian Fu,† and Yong He† Institute of Physical Chemistry, Department of Chemistry, Fujian Normal UniVersity, Fuzhou 350007, P. R. China, Department of Chemistry, Henan Normal UniVersity, Xinxiang 453007, P. R. China, and College of Chemical Engineering & Materials Science, Zhejiang UniVersity of Technology, Hangzhou, Zhejiang 310014, P. R. China ReceiVed August 12, 2007; ReVised Manuscript ReceiVed February 12, 2008
ABSTRACT: Ag/C nanocables with cross-linked structure were obtained through the reduction of Ag+ with glucose in the presence of sodium dodecyl sulfate (SDS) under hydrothermal conditions. In the process, glucose acts as reducing agent and carbon source, and the final morphology of the product was determined by the SDS concentration. The formation process of nanocables included two evolution stages: the synthesis of the Ag nanowires with cross-linked structure by the SDS assistant; the carbonization of glucose and the formation of an amorphous carbon layer on Ag nanowires surface. 1. Introduction Recently, much effort has been focused on the preparation of noble metal nanowires, e.g., Au, Ag, and Cu, because of their superior performance as nanocircuits, nanodevices, and nanosensors.1–3 Among all metals, Ag exhibits the highest thermal and electrical conductivity and has been extensively used in catalysis, electronics, photonics, photography, biological labeling, and surface-enhanced Raman scattering.4–7 When Ag takes on a 1D nanostructure, its performance in many aspects could be potentially enhanced. On the nanometer scale, however, the metallic materials are very sensitive to air and moisture, which degrades the performance of the nanodevices.8,9 As a new kind of nanostructure, coaxial nanocables are a good candidate for nanodevices and they have attracted a lot of attention.10–13 Functions and properties of nanocables would be further enhanced because metal nanowires would be protected from oxidation and corrosion by outer shell and core-sheath heterostructures that are formed.14–16 The study of Ag nanocables has attracted much attention. For example, Wang and co-workers have designed a polymerassisted solution process to prepare flexible silver/carbon (Ag/ C) coaxial nanocables via one-step reduction and carbonization under mild hydrothermal conditions.17 Adopting a microwaveassisted hydrothermal carbonization process, Ag/C nanocables with lengths ranging from 1 to 10 µm could be synthesized.18 Ag/SiO2 nanocables can be formed using a sol-gel method to * Corresponding author. Tel: 86-591-87441126. Fax: 86-591-83465376. E-mail:
[email protected]. † Fujian Normal University. ‡ Henan Normal University. § Zhejiang University of Technology.
Figure 1. XRD pattern of Ag/C coaxial nanocables with cross-linked structure.
coat Ag nanowires with amorphous silica.19 Recently, a new method to prepare flexible silver/cross-linked poly(vinyl alcohol) (PVA) nanocables via one-step in situ reduction of Ag+ and Ag+-catalyzed cross-linking of PVA chains under hydrothermal conditions was reported by Luo et al.20 According to this, a novel sodium dodecyl sulfate (SDS) assisted hydrothermal route to fabricate Ag/C nanocables with cross-linked structure was reported in this paper. In the process, glucose acts as reducing agent and carbon source, and SDS are used in the system to assist the oriented growth of the silver nanowires core and formation of Ag/C coaxial nanocables. 2. Experimental Section 2.1. Sample Preparation. All the chemicals were analytic grade reagents without further purification. The Ag/C coaxial nanocables
10.1021/cg700761e CCC: $40.75 2008 American Chemical Society Published on Web 05/10/2008
1824 Crystal Growth & Design, Vol. 8, No. 6, 2008
Song et al.
Figure 2. (a) SEM and (b) TEM images of Ag/C coaxial nanocables with cross-linked structure.
Figure 3. SEM and TEM images showing the various structural forms of the Ag/C coaxial nanocables: (a) (b) many-offset, (c) T-style, (d) Y-style, (e) open-ended nanocable, (f) closed-end nanocable. transmission electron microscopy (TEM, PHILIPS CM200, 200kV). The X-ray diffraction (XRD, Thermo ARL SCINTAG X’TRA with CuKa´ irradiation, λ ) 0.154056 nm.) was used to analysis the crystallinity. Energy-dispersive X-ray spectroscopy (EDS) is attached to the Hitachi S-4700.
3. Results and Discussion
Figure 4. EDS patterns of Ag/C coaxial nanocables with cross-linked structure. with cross-linked structure were synthesized under hydrothermal conditions. Experimental details were as follows: SDS (2.5 g) was dissolved in 25 mL of distilled water and glucose (4 g) was added to it with vigorous stirring. When the solution clarified, 10 mL of aqueous solution containing 0.02 g of AgNO3 was added slowly to the above solution under continuous stirring. Immediately, a quantity of milk-white precipitation formed and the solution was transferred into a Teflon-lined stainless steel autoclave (50 mL) of 80% capacity of the total volume. The autoclave was sealed and maintained at 150 °C for 6 h. After the reaction was completed, the autoclave was allowed to cool to room temperature naturally. The solid black precipitate was filtered, washed several times with distilled water and anhydrous ethanol to remove impurities, and then dried at 60 °C in air. The obtained black powders were collected for the following characterization. 2.2. Characterization. The morphologies were characterized using scanning electron microscopy (SEM, Hitachi S-4700, 15kV) and
The phase and purity of the products were examined by XRD. Figure 1 shows a typical XRD pattern of the products. All the peaks can be indexed to diffraction from the (111), (200), (220), (311), and (222) planes of face-centered cubic silver (space group: Fm3m (225)). The lattice constant calculated from this pattern was a ) 4.085 Å, which is in good agreement with the reported data (a ) 4.086 Å, JCPDS No. 04-0783). No other phases were detected in Figure 1, which indicate that silver nitrate would be completely changed into Ag/C nanocables. The morphologies and microstructures of the as-prepared products were further surveyed by SEM and TEM. Figure 2a shows a typical SEM image of the as-prepared Ag/C nanocables with cross-linked structure. It is found that nanocables are interconnected with each other. These nanocables have diameters ranging from 300 to 600 nm and lengths of several micrometers. TEM image in Figure 2b shows that the products are a composite comprised of a smooth core about 80 nm in diameter and a surrounding sheath about 200 nm in thickness. It can be found very clearly from Figure 2b that the contrast between the dark inner core and light sheath layer along the axis direction. The typical high-resolution transmission electron microscopy (HRTEM) image of the coaxial nanocables is shown in the inset of Figure 2b. The lattice fringe spacing was measured to be 0.24 nm, which corresponds to the (111) plane of cubic
Ag/C Coaxial Nanocables by SDS-Assisted Hydrothermal Approach
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Figure 5. TEM images of the samples synthesized with different amounts of SDS: (a) 0 and (b) 1 g.
silver. More detailed SEM and TEM observations for various forms of nanocable structures obtained in the studies were shown in Figure 3, which included many-offset, T-style, Y-style, openended, and closed-end nanocables. The energy-dispersive X-ray (EDS) analysis was employed to determine the composition of products. As shown in Figure 4, the EDS results confirm that the obtained nanocables are composed of inner silver nanowires and outer carbonaceous layers. Surfactants are useful in controlling morphology of nanostructures because of their soft template effect, reproducibility, and simple maneuverability. Under our experimental conditions, the concentration of SDS is important in determining the final morphology of the product. Figure 5a is the TEM image of the sample obtained without SDS. It can be known that the products synthesized were only Ag microparticles with no surrounding sheath observed. The most probable reason is that silver particles would collide with each other and congregate into microparticles in the absence of SDS. With the addition of 1 g of SDS, the morphologies of synthesized sample shown in Figure 5b is a mixture of core-shell nanoparticles and nanocable. In addition, during the carbonization, SDS also plays an important role in the formation of Ag/C core-shell structured nanoparticles and nanocables. SDS molecules could be absorbed on the surface of Ag nanoparticles and nanowires and form a soft interface. It is known that glucose solution in autoclaves at high temperature would lead to aromatization and carbonization.21,22 Glucose will carbonize in this region and the obtained C will gradually deposit on the surface of the Ag nanoparticles or nanowires to form Ag/C core-shell structured nanoparticles and nanocables. With the addition of 2.5 g of SDS, Ag/C nanocables with cross-linked structure are the major product. According to the results, the proposed formation mechanism of Ag/C nanocables with cross-linked structure in this work could be illustrated as Scheme 1. It is the synergistic growth mechanism that controls the formation of nanocables with crosslinked structure. Clearly, SDS is responsible for both the formation of silver nanoparticles and further growth of silver nanowires with cross-linked structure stabilized by SDS. At the same time, the silver wires act as a backbone on which crosslinked carbon shell will form. As the SDS molecule is easily adsorbed onto the surfaces of particles, this can effectively prevent agglomeration of silver nanoparticles formed at the initial stage of reaction. It is helpful for Ag nanoparticles with oriented arrangement to form nanowires by the amalgamation of nanoparticles. Figure 6 shows the incompletely filled nanocable. It could be found that the Ag cores are composed by a series of discontinuous nanoparticles and nanowires, which suggests that the formation of Ag cores could experience a process including the formation of nanoparticles, oriented arrangement and amalgamation of nanoparticles. This result
Scheme 1. Schematic Illustration of the Ag/C Nanocables with Cross-Linked Structure Formation Process Stepsa
a (A) Formation of Ag nanoparticles by SDS assisted; (B) Ag nanoparticles with oriented arrangement in the presence of SDS adsorbed on the surface of particles; (C) formation of Ag nanowires by the amalgamation of nanoparticles; (D) carbonization of glucose, resulting in the formation of Ag/C coaxial nanocables with cross-linked structure.
Figure 6. TEM images of the Ag/C incompletely filled nanocables.
supports the above formation mechanism of Ag/C nanocables with cross-linked structure. 4. Conclusion In summary, a novel SDS-assisted hydrothermal one-step route has been designed for the synthesis of Ag/C coaxial nanocables with cross-linked structure. In these processes, the presence of SDS is significant to assisting the growth of silver cores and the formation of carbon sheath. The obtained Ag/C nanocables exhibit cross-linked structure, which could have potential applications in the future.
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