CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 11 2422-2426
Communications Formation of Flexible Ag/C Coaxial Nanocables through a Novel Solution Process Weizhi Wang, Shenglin Xiong, Liyong Chen, Baojuan Xi, Hongyang Zhou, and Zude Zhang* Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, People’s Republic of China ReceiVed February 10, 2006; ReVised Manuscript ReceiVed September 23, 2006
ABSTRACT: A novel polymer-assisted solution approach is designed for the synthesis of flexible Ag/C coaxial nanocables via one-step reduction and carbonization under hydrothermal conditions. In the process, glucose acts as reducing agent and carbon source, and CCl4 and poly(ethylene glycol) (PEG) are used in the system to control the reaction process and assist the oriented growth of the silver nanowire core. In the past decade, one-dimensional (1-D) nanostructures of metals, such as nanorods, nanowires, nanotubes, and nanobelts, have been an active research area due to their distinctive geometries and novel chemical, physical, and mechanical properties different from the bulk counterparts.1 Among these 1-D nanostructures, metal nanowires have attracted particular interest because they are expected to play an important role in fabricating nanoscale electronic, optoelectronic, and magnetic devices, which also provides an ideal model system to experimentally investigate physical phenomena such as quantized conductance and localization effects.2 Silver, as an important metallic material with the highest electrical and thermal conductivity, has great potential applications and may be further enhanced by fabricating silver with 1-D nanostructures. Therefore, the preparation of silver nanowires has now received much attention. Template-directed synthesis is the most widely used method for generating silver nanowires that involve either chemical or electrochemical depositions.3 Many other methods have also been developed for the syntheses of silver nanowires, such as electrochemical techniques and polymer-directed synthesis.4,5 On the nanometer scale, however, the metallic materials are very sensitive to air and moisture, which degrades the performance of the nanodevices.6 Coaxial nanocables, a new kind of 1-D nanocomposite of nanowires (core) wrapped with one or more outer layers (shell), have emerged recently and attracted much intensive investigations. 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.7 Some synthetic methods have been reported to prepare coaxial nanocables based on high-temperature approaches, including a laser ablation method,8 a high-temperature route,9 or a carbothermal reduction method.10 These methods, however, are mostly complicated and time-consuming. Recently, several solution-based methods have emerged to generate polymer/ polymer, semiconductor/polymer, and metal/polymer nanocables at relatively low temperature.11 Nevertheless, there have been only a few reports on the solution synthesis of silver nanocables up till now.12
In this paper, we design a novel polymer-assisted solution process to prepare flexible silver/carbon (Ag/C) coaxial nanocables via onestep reduction and carbonization under mild hydrothermal conditions. In the process, glucose acts as reducing agent and carbon source, and CCl4 and poly(ethylene glycol) (PEG) are used in the system to control the reaction process and assist the oriented growth of the silver nanowires core. As is known, glucose is a typical soft reducer and is widely used to produce mirrors by silver mirror reaction. In addition, the glucose solution heated in autoclaves to 160-180 °C, which is higher than the normal glycosidation temperature, will lead to aromatization and carbonization.13 For these reasons, we select glucose both as the carbon source to form a carbonaceous shell and as the reducing agent, and silver nitrate acts as the silver source to get the silver nanowire core. Silver nitrate is soluble and easily dissociates in water, and the concentration of free Ag+ ions is high in silver nitrate solution. Therefore, the reaction that glucose reduces silver nitrate occurs so quickly in autoclaves at 160-180 °C that as-prepared abundant silver nuclei have no time to anisotropically grow to a uniform 1-D nanostructure, thus leading to the formation of unshaped silver nanoparticles instead of silver nanowires.14 In the solution-phase synthesis of nanomaterials, a slow reaction rate is favorable for separating the growth step from the nucleation step,15 and controlling the rate of nucleation and growth could conceivably modulate the size and shape of the final products.16 For this reason, the rate of reaction and nucleation was slowed down via controlling the concentration of Ag+ ions in our designed system to help the oriented growth of the silver nanowire core. When CCl4 mixes with much water and is heated in sealed vessel, it decomposes into carbon dioxide and chlorine hydride as described by reaction 1:17
* Corresponding author. Telephone: 86-551-3607752. E-mail: zzd@ ustc.edu.cn.
CCl4 + 2H2O + 4AgNO3 f CO2 + 4HNO3 + 4AgCl (2)
CCl4 + 2H2O f CO2 + 4HCl
(1)
It is known that chlorine hydride reacts rapidly with silver nitrate to produce silver chloride colloid. So the total reaction process of CCl4 with silver nitrate could be expressed as the following reaction 2:
10.1021/cg060068b CCC: $33.50 © 2006 American Chemical Society Published on Web 10/18/2006
Communications Scheme 1.
Crystal Growth & Design, Vol. 6, No. 11, 2006 2423
Schematic Illustration of the Ag/C Nanocables Formation Processa
a Steps: (a) the mixture of CCl and PEG is mixed with AgNO aqueous 4 3 solution to form a homogeneous clear solution, and Ag+ ions are transferred from the water to the surface of PEG-CCl4; (b) Ag+ ions react with in situ generated chlorine hydride via reaction 1 to form silver chloride particles; (c) silver nanowire cores have formed via slowing down the reaction rate of reaction 3 and under the assistance of PEG molecules; (d) further growth of Ag and carbonization of glucose result in the formation of Ag/C coaxial nanocables.
Figure 1. XRD patterns of the Ag/C coaxial nanocables obtained at 180 °C for 24 h.
Silver chloride dissociates in water releasing Ag+ ions. Because the solubility of silver chloride in water is very low, the concentration of Ag+ ions is lowered in solution. Therefore, when CCl4 is added in the reaction system, the concentration of Ag+ ions could be effectively controlled. As a result, the reduction of Ag+ ions by glucose and the formation of silver nuclei are both slowed down, and the reaction can be formulated as shown in eq 3. Such a slow rate of reaction and nucleation is favorable for the 1-D orientation growth of silver nanowires.
2Ag+ + H2O + CH2OH-(CHOH)4-CHO f 2Ag + 2H+ + CH2OH-(CHOH)4-COOH (3) What’s more, many recent studies have confirmed that the selective interaction of the appropriate capping molecules on a facet of the first-formed nanoparticles is the key to anisotropic growth of nanomaterials.18 In this work, surfactant PEG was introduced to assist steadily oriented growth of the silver nanowire core in the synthesis system of Ag/C nanocables. The PEG molecule with a uniform and ordered chain structure is easily adsorbed onto the surfaces of particles,19 which can effectively prevent agglomeration of silver nanoparticles formed at the initial stage of reaction and lead to preferential growth of distinct crystal faces of face-centered cubic silver through selective adsorption processes. The roles of PEG are similar to those of poly(vinyl pyrrolidone) played in the syntheses of Ag and Pb nanowires.20 Therefore, it is expected that the 1-D silver nanowires will be obtained via slowing down the reaction rate of glucose with Ag+ ions under the assistance of PEG molecules. Carbonization of glucose takes place at 160-180 °C and leads to deposition of carbonaceous products around the silver nanowire surface to form a carbonaceous shell, resulting in the onestep formation of the Ag/C nanocables. Further consideration of the water-PEG-CCl4 system indicated more interesting characteristics. CCl4 is insoluble in water and can exist only as oil droplets in water. In the field of colloidal chemistry, PEG as a non-ionic polymer has hydrophilic -O- and hydrophobic -CH2-CH2- on the long chains. As the surfactant, PEG can enwrap the CCl4 oil phase to form micelles, and then CCl4 will be homogeneously and stably dispersed in water. On the
Figure 2. TEM images of the as-prepared Ag/C coaxial nanocables: (a) general view of the nanocables; (b) TEM image of an individual nanocable; (c) selected area electron diffraction (SAED) pattern of the nanocables; (d) TEM image of the nanocable with growth direction [110]; (e) HRTEM image taken from the area enclosed by a square in panel d. The arrows on the figure denote the normal directions of the two planes and the growth direction of nanowires (black and white arrow, respectively), and the corresponding FFT of the HRTEM is shown in the inset.
other hand, Ag+ can also be combined with the -O- of PEG to form a complex,21 probably leading to the transfer of Ag+ from the water to the surface of micelles where they could react easily with chlorine hydride released in situ by reaction 1 to produce silver chloride, and thus reaction 2 can help avoid heterogeneous processes in solution. The whole process can be illustrated in Scheme 1. Based on the above strategy, a polymer-assisted one-step reduction and carbonization route was developed to prepare coaxial Ag/C nanocables. A typical procedure follows: 0.05 mL of CCl4 was put into 20 mL of PEG-400 (average molecular weight 400) and stirred for about 10 min, and then 100 mL of 0.01 M AgNO3 aqueous solution was slowly added into the mixture under stirring until a homogeneous clear solution was formed. Then 2 mmol of
2424 Crystal Growth & Design, Vol. 6, No. 11, 2006
Communications
Figure 3. TEM images showing the various structural forms of the Ag/C coaxial nanocables: (a) open ended nanocable; (b) closed ended nanocable; (c) incompletely filled nanocable.
glucose was dissolved in the above solution under magnetic stirring. All the regents are commercially available, and analytical grades were used without further purification. The 45 mL resulting solution was then transferred into a Teflon-lined autoclave of 55 mL capacity, which was maintained at 180 °C for 24 h and then cooled to room temperature naturally. The products were collected by centrifugation, cleaned by three cycles of centrifugation/washing/ redispersion in distilled water and in anhydrous alcohol, and finally dried in a vacuum at 60 °C for 4 h. Figure 1 shows a typical X-ray diffraction (XRD) pattern of the as-prepared sample (supported on a silicon substrate), and the peaks can be indexed to diffraction from the (111), (200), (220), and (311) planes of face-centered cubic silver. The lattice constant calculated from this pattern was a ) 4.083 Å, which is in good agreement with the literature value of 4.086 Å (JCPDS Card File No. 04-0783). It is worth noting that the ratio between the intensities of the (220) and (111) diffraction peaks was lower than the conventional value (0.02 versus 0.25), indicating that the {111} planes of nanocables tended to be preferentially oriented (or textured) parallel to the surface of the supporting substrate.22 A general transmission electron microscopy (TEM) image in Figure 2a shows that the products are a composite comprised of a smooth core about 100-150 nm in diameter and a surrounding sheath about 20-60 nm in thickness, from which the contrast between the dark inner core and light sheath layer along the axis direction can be easily observed. The lengths of these nanocables are in the range of several to several tens of micrometers. A TEM image of an individual nanocable in Figure 2b shows a uniform and long silver nanowire in carbonic hull with the diameter about 100 and 30 nm, respectively. Figure 2c shows the corresponding selected-area electron diffraction (SAED) pattern of the nanocables. The SAED pattern consists of two sets of spots, and each set could be independently indexed revealing that the silver nanowire core should be a twin crystal, which is consistent with a previous report for silver nanowires.23 Previous studies have suggested a low threshold for twinning parallel to the {111} planes of a fcc metal such as silver or gold.24 These materials tend to grow as bicrystals twinned at the {111} planes. The structure of the as-obtained nanocables was investigated in more detail by high-resolution transmission electron microscopy (HRTEM). Figure 2e is the HRTEM image of the area enclosed by a square in Figure 2d. The two sets of regular spacing of the observed lattice planes are both 2.4 Å, which corresponds well to the literature value of 2.36 Å of Ag {111} planes. The two planes are (111) and (111h), and the corresponding fast Fourier transform (FFT) of the HRTEM is shown in the inset of Figure 2e. Based on the normal directions of the two planes (black arrows on the figure), we can know that the silver nanowire core grew along the [110] direction, as shown in Figure 2, panels e (white arrow) and d.
Figure 4. XPS spectrum of the Ag/C coaxial nanocables obtained at 180 °C for 24 h.
Figure 5. XRD patterns of the samples obtained at 180 °C for different stages of reaction time: (a) 6, (b) 12, (c) 16, and (d) 20 h.
More detailed TEM observations indicated that there were various forms of nanocable structures, which included open ended, closed ended, and incompletely filled nanocables, as shown in Figure 3. Further evidence for the surface composition of the nanocables was obtained from the X-ray photoelectron spectra (XPS). The results are shown in Figure 4. The spectrum of the products shows two very strong peaks at 284.64 and 532.54 eV corresponding to the C 1s and O 1s binding energies, respectively. However, the binding energy at 368.3 eV for Ag 3d is quite weak. These XPS
Communications
Figure 6. TEM images of samples obtained at 180 °C after different reaction times of (a) 6, (b) 12, (c) 16, and (d) 20 h.
results confirm that the nanocables are composed of inner silver nanowires and outer carbonaceous layers. The above analysis shows that experimental results are in good agreement with our expectation that the one-step synthesis of Ag/C nanocables was successful via a mild hydrothermal process. In order to demonstrate the growth mechanism of the Ag/C nanocables more clearly, the samples obtained at different stages were also examined by using XRD and TEM techniques. Figure 5a-d shows the XRD pattern of the products obtained after different reaction times (6, 12, 16, and 20 h). Most of the diffraction peaks can be indexed to silver chloride (JCPDS Card File No. 31-1238) after reaction for 6 h, and the diffraction peaks for silver is weak, which indicates that the main component of the sample is silver
Crystal Growth & Design, Vol. 6, No. 11, 2006 2425 chloride and only a little silver metal has been produced. The diffraction peaks of silver chloride gradually got weaker and weaker with the increase of reaction time, while those of silver evidently strengthened. After the reaction had proceeded for 20 h, the diffraction peaks of silver are stronger and only several very weak peaks of silver chloride are presented. The XRD patterns indicate that silver chloride is formed in the early stage and gradually reduced accompanied with continuous generation of silver metal through the reduction of glucose. Figure 6a shows the TEM image of the sample obtained after 6 h, which reveals that the product is composed of a large quantity of irregular particles with sizes of about 0.5-1.5 µm. Based on the XRD pattern of Figure 5a, we can conclude that these particles were silver chloride. When the time of hydrothermal process was prolonged to 12 h, wire-like products could be clearly observed in Figure 6b, which indicates that silver nanowire cores had been generated. However, there were still many particles in the products. After further increase of the reaction time to 16 h, the proportion of irregular particles decreased in the products, as shown in Figure 6c. After reaction for 20 h, the XRD pattern of Figure 5d had demonstrated that the products were almost all silver. As expected, the TEM image in Figure 6d reveals that the sample is mainly composed of wire-like products after 20 h. These studies confirm the evolution of products from silver chloride particles to nanocables over time in the reaction system. Hence, the designed polymerassisted reduction route for the one-step synthesis of Ag/C coaxial nanocables should be reasonable. Although the Ag/C nanocables are not slim in this work, they still show good flexibility. The low-magnification TEM image in Figure 7a shows the typical bending morphology of the nanocables without signs of breakage. High-magnification TEM images revealing more details of twisted Ag/C nanocables are shown in Figure 7b,c. These results are different from the previous hydrothermal synthetic routes,12a indicating better machinability for future nanodevice design.25 In summary, a novel polymer-assisted hydrothermal route has been designed for the one-step synthesis of Ag/C coaxial nanocables. The formation of cables comprises mainly two processes: (1) CCl4 reacts with silver nitrate to produce silver chloride particles for controlling the concentration of Ag+ ions in solution and (2) glucose reduces Ag+ ions into elemental silver and is carbonized to form a carbonaceous shell. In these processes, the presence of PEG is significant to assisting the anisotropic growth of silver nanowire cores and the formation of a homogeneous reaction system. The present Ag/C nanocables exhibit good flexibility, which could have potential applications in the future. Further work is under way.
Figure 7. (a) Low-magnification TEM image of Ag/C coaxial nanocable with the typical bending morphology and (b,c) high-magnification TEM images showing more details of twisted Ag/C nanocables.
2426 Crystal Growth & Design, Vol. 6, No. 11, 2006 Acknowledgment. We gratefully acknowledge the financial support from the National Natural Science Research Foundation of China. The authors also thank Dr. Yongchun Zhu and Dr. Xiong Wang for helpful discussions. Supporting Information Available: Characterization of the product. This material is available free of charge via the Internet at http://pubs.acs.org.
References (1) (a) Favier, F.; Walter, E. C.; Zach, M. P.; Benter, T.; Penner, R. M. Science 2001, 293, 2227. (b) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (2) (a) Zhang, Z.; Sun, X.; Dresselhaus, M. S.; Ying, J. Y. Phys. ReV. B 2000, 61, 4850. (b) Wang, Z. L. AdV. Mater. 2000, 12, 1295. (c) Sun, L.; Searson, P. C.; Chien, C. L. Appl. Phys. Lett. 2001, 79, 4429. (d) Cui, Y.; Wei, Q.; Park, H.; Lieber, C. M. Science 2001, 293, 1289. (e) Gillingham, D. M.; Linington, I.; Bland, J. A. C. J. Phys.: Condens. Mater. 2002, 14, L567. (f) Prokes, S. M.; Wang, K. L. MRS Bull. 1999, 24 (8), 13. (g) Bockrath, M.; Liang, W.; Bozovic, D.; Hafner, J. H.; Lieber, C. M.; Tinkham, M.; Park, H. Science 2001, 291, 283. (3) (a) Sloan, J.; Wright, D. M.; Woo, H. G.; Bailey, S.; Brown, G.; York, A. P. E.; Coleman, K. S.; Hutchison, J. L.; Green, M. L. H. Chem. Commun. 1999, 699. (b) Govindaraj, A.; Satishkumar, B. C.; Nath, M.; Rao, C. N. R. Chem. Mater. 2000, 12, 202. (c) Sauer, G.; Brehm, G.; Schneider, S.; Nielsch, K.; Wehrspohn, R. B.; Choi, J.; Hofmeister, H.; Go¨sele, U. J. Appl. Phys. 2002, 91, 3243. (d) Huang, M. H.; Choudrey, A.; Yang, P. D. Chem. Commun. 2000, 1063. (e) Braun, E.; Eichen, Y.; Sivan, U.; Ben-Yoseph, G. Nature 1998, 391, 775. (4) (a) Zhou, Y.; Yu, S. H.; Cui, X. P.; Wang, C. Y.; Chen, Z. Y. Chem. Mater. 1999, 11, 545. (b) Zhu, J. J.; Liu, S. W.; Palchik, O.; Koltypin, Y.; Gedanken, A. Langmuir 2000, 16, 6396. (5) (a) Sun, Y. G.; Gates, B.; Mayers, B.; Xia, Y. N. Nano Lett. 2002, 2, 165. (b) Zhang, D. B.; Qi, L. M.; Yang, J. H.; Ma, J. M.; Cheng, H. M.; Huang, L. Chem. Mater. 2004, 16, 872. (c) Bhattacharrya, S.; Saha, S. K.; Chakravorty, D. Appl. Phys. Lett. 2000, 76, 3896. (d) Liu, S. W.; Yue, J.; Gedanken, A. AdV. Mater. 2001, 13, 656. (e) Bhattacharrya, S.; Saha, S. K.; Chakravorty, D. Appl. Phys. Lett. 2000, 77, 3770. (f) Jana, N. R.; Gearheart, L.; Murphy, C. L. Chem. Commun. 2001, 617. (6) Cao, H.; Xu, Z.; Sang, H.; Sheng, D.; Tie, C. AdV. Mater. 2001, 13, 121. (7) (a) Lauhon, L. J.; Gudiksen, M. S.; Wang, D. L.; Lieber, C. M. Nature 2002, 420, 57. (b) Li, Q.; Wang, C. R. J. Am. Chem. Soc. 2003, 125, 9892. (c) Wu, Y. Y.; Yang, P. D. Appl. Phys. Lett. 2000, 77, 43. (d) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (8) (a) Morales, A. M.; Lieber, C. M. Science 1998, 279, 208. (b) Shi, W. S.; Peng, H. Y.; Xu, L.; Wang, N.; Tang, Y. H. H.; Lee, S. T. AdV. Mater. 2000, 12, 1927.
Communications (9) Zhang, Y.; Suenaga, K.; Colliex, C.; Iijima, S. Science 1998, 281, 973. (10) (a) Meng, G. W.; Zhang, L. D.; Mo, C. M.; Zhang, S. Y.; Qin, Y.; Feng, S. P.; Li, H. J. J. Mater. Res. 1998, 13, 2533. (b) Zhang, L. D.; Meng, G. W.; Phillipp, F. Mater. Sci. Eng. A 1998, 286, 34. (11) (a) Jang, J.; Lim, B.; Lee, J.; Hycon, T. Chem. Commun. 2001, 83. (b) Obare, S. O.; Jana, N. R.; Murphy, C. J. Nano Lett. 2001, 1, 601. (c) Mayya, K. S.; Gittins, D. I.; Dibaj, A. M.; Caruso, F. Nano Lett. 2001, 1, 727. (d) Xie, Y.; Qiao, Z.; Chen, M.; Liu, X.; Qian, Y. AdV. Mater. 1999, 11, 1512. (12) (a) Yu, S. H.; Cui, X. J.; Li, L. L.; Li, K.; Yu, B.; Antonietti, M.; Colfen, H. AdV. Mater. 2004, 16, 1636. (b) Yin, Y. D.; Lu, Y.; Sun, Y. G.; Xia, Y. N. Nano Lett. 2002, 2, 427. (c) Wong, Y. H.; Li, Q. J. Mater. Chem. 2004, 14, 1413. (13) (a) Sakaki, T.; Shibata, M.; Miki, T.; Hirosue, H.; Hayashi, N. Bioresour. Technol. 1996, 58, 197. (b) Luijkx, G. C. A.; Rantwijk, F. van.; Bekkum, H. van.; Antal, M. J., Jr. Carbohydr. Res. 1995, 272, 191. (14) Sun, X. M.; Li, Y. D. Langmuir 2005, 21, 6019. (15) Liu, Z. P.; Li, S.; Yang, Y.; Peng, S.; Hu, Z.; Qian, Y. AdV. Mater. 2003, 15, 1946. (16) (a) Lee, S. M.; Jun, Y. W.; Cho, S. N.; Cheon, J. J. Am. Chem. Soc. 2002, 124, 11244. (b) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389. (c) Jana, N. R.; Gearheart, L.; Murphy, C. J. Chem. Commun. 2001, 7, 617. (17) Chen, N. L. Handbook of SolVents; Chemical Industry Press: Beijing, 1994, p 198. (18) (a) Cordente, N.; Respaud, M.; Senocq, F.; Casanove, M. J.; Amiens, C.; Chaudret, B. Nano Lett. 2001, 1, 565. (b) Peng, Z. A.; Peng, X. G. J. Am. Chem. Soc. 2001, 123, 1389. (c) Manna, L.; Scher, E. C.; Alivisatos, A. P. J. Am. Chem. Soc. 2000, 122, 12700. (19) Dobryszycki, J.; Biallozor, S. Corros. Sci. 2001, 43, 1309. (20) (a) Sun, Y. G.; Gates, B.; Mayers, B.; Xia, Y. N. Nano Lett. 2002, 2, 165. (b) Sun, Y. G.; Yin, Y. D.; Mayers, B.; Herricks, T.; Xia, Y. N. Chem. Mater. 2002, 14, 4736. (c) Wang, Y. L.; Herricks, T.; Xia, Y. N. Nano Lett. 2003, 3, 1163. (21) Kerker, M. J. Colloid Interface Sci. 1985, 105, 297. (22) Cullity, B. D.; Stock, S. R. Elements of X-Ray Diffraction, 3rd ed.; Prentice Hall: Upper Saddle River, NJ, 2001; pp 402-404. (23) Sun, Y.; Xia, Y. N. AdV. Mater. 2002, 14, 833. (24) (a) Bo¨gels, G.; Meekes, H.; Bennema, P.; Bollen, D. J. Phys. Chem. B 1999, 103, 7577. (b) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 2000, 104, 7867. (25) Jiang, P.; Li, S. Y.; Xie, S. S.; Gao, Y.; Song, L. Chem.sEur. J. 2004, 10, 4817.
CG060068B
