Cryst. Res. Technol. 45, No. 11, 1189 – 1193 (2010) / DOI 10.1002/crat.201000256

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Controllable synthesis, characterization and optical properties of

Ag@AgCl coaxial core-shell nanocables

Fei Li*, Xueqin Liu, Yuliang Yuan, Jianfang Wu, and Zhen Li

Faculty of Materials Sci. and Chemical Eng., China University of Geosciences, Wuhan 430074, P. R. China

Received 1 May 2010, revised 2 September 2010, accepted 21 September 2010

Published online 8 October 2010

Key words Ag@AgCl core-shell, nanocable, X-ray diffraction, scanning electron microscopy.

Core-shell structures often exhibit improved physical and chemical properties. Developing a relatively

general, facile, and low temperature synthetic approach for core-shell structures with complex compositions is

still a particularly challenging work. Here we report a general chemical conversion route to prepare high

quality Ag@AgCl coaxial core-shell nanocables via the redox reaction between Ag nanowires and FeCl3 in

solution. The powder X-ray diffraction of the Ag@AgCl coaxial core-shell nanocables shows additional

diffraction peaks corresponding to AgCl crystals apart from the signals from the Ag nanowire cores. Scanning

electron microscopy and transmission electron microscopy images of the Ag@AgCl coaxial core-shell

nanocables reveal that the Ag nanowires are coated with AgCl nanoparticles. The effect of the molar ratio of

Fe:Ag on the morphology and optical absorption of the Ag@AgCl coaxial core-shell nanocables is

systematically investigated. The result shows that the optical absorption of Ag nanowires decreases gradually

and that of AgCl nanoparticles improves gradually with the increase of the molar ratio of Fe:Ag. The

formation process of the Ag@AgCl coaxial core-shell nanocables has been discussed in detail. The present

chemical conversion approach is expected to be employed in a broad range of applications to fabricate

innovative core-shell structures with different compositions and shapes for unique properties.

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction

A significant step after the remarkable success in growing single-component nanocrystals is the preparation of nanostructures that are composed with discrete domains of different materials. The attraction of multicomponent nanostructures is that multiple functions can be integrated into one system for specific applications [1-3]. Moreover, the interactions between nanoscale-spaced components can greatly improve the overall application performance of the nanostructured system and even generate new synergetic properties. Among them, core-shell structured nanomaterials have attracted particular research attention in recent years because of the great potential in protection, modification, and functionalization of the core particles with suitable shell materials to achieve specific physical, chemical, and biological performance [4-8]. Recently, one-dimensional (1-D) coaxial core-shell nanowires have become of particular interest because their functions could be further tuned or enhanced by fabricating the core and shell with different materials [9-11].

Ag nanostructures have drawn much attention due to their unique electrical, optical and thermal properties, as well as their potential applications in microelectronics, optoelectronic devices and surface-enhanced Raman scattering [12-17]. Ag nanowire-based coaxial core-shell nanocables are attracting more and more interest due to their tuned and multi-functional properties. For example, González-Elipe et al. [18] fabricated Ag@TiO2 core-shell nanofibers by a plasma deposition method. Li et al. [19] prepared Ag@polypyrrole core-shell nanocables in aqueous solution at room temperature through a redox reaction between silver nitrite and pyrrole, using poly(vinylpyrrolidone) (PVP) as assistant agent. Xie et al. [20] reported a new method for the fabrication of Ag@PPy coaxial nanocables based on ion adsorption on the nanowires' surface. Ghoshal et al. [21] prepared Ag@Si coaxial core-shell nanocables by a vapor-liquid-solid technique. Ag@AgCl coaxial core-shell nanocables are a new kind of Ag nanowire-based heterostructure, having great potential in the fields of plasmonic photocatalysts and reference electrode [22,23].

Although there are some reports on the Ag-based coaxial core-shell nanocables, their synthesis remains a great challenge for material scientists. In the present work, we report a feasible and effective solution route to synthesize Ag@AgCl coaxial core-shell nanocables via a controllable redox reaction between Ag nanowires ____________________

* Corresponding author: e-mail: alexfly2002@sina.com

1190 Fei Li et al.: Ag@AgCl coaxial core-shell nanocables

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

and FeCl3 in solution at room temperature. The effect of the molar ratio of Fe:Ag on the formation and optical absorption properties of the Ag@AgCl coaxial core-shell nanocables was investigated systematically. The formation process of the Ag@AgCl coaxial core-shell nanocables was discussed in detail. We have further expected that the present chemical conversion approach can be used in a broad range of applications to fabricate innovative core-shell structures with various compositions.

2 Experimental

All chemicals were of analytical reagent grade and purchased from Shanghai Chemical Reagents Company, China, and used without further purification. The Ag nanowires were prepared by a modified polyol process according to [24]. In a typical synthesis, 1.5 mg of NaCl and 0.88 g of poly(vinylpyrrolidone) (PVP, MW≈30000) were added into 20 ml ethylene glycol (EG) solution, then, the solution was added drop by drop into 20 ml of a magnetically stirred EG solution of AgNO3 (0.34 g) for 10 min. Subsequently, the mixed solution was transferred into a teflon-lined autoclave 60 ml in volume, sealed and maintained at 160 °C for 2 h, and then cooled naturally to room temperature. Afterwards, the precipitates were centrifuged and washed with distilled water and ethanol three times. Finally, the products were dried under vacuum at 60 °C for 3 h, and Ag nanowires were obtained.

In the typical synthesis of Ag@AgCl coaxial core-shell nanocables, the as-synthesized Ag nanowires (0.108 g, 1 mmol) were added into 30 ml ethanol. A certain amount of FeCl3 (the molar ratio of Fe:Ag=1:20, 1:10, 1:5, 2:5, 4:5, 6:5, respectively) was added into 20 ml ethanol. Then, the whole FeCl3 ethanol solution was slowly added dropwise to the Ag nanowire solution. The resulting mixture was maintained at room temperature for 2 h and its color became brown gradually. Vigorous stirring was carried out throughout the synthesis. Finally, the products were centrifuged and washed with ethanol and dried in vacuum at 60 °C for 3 h to obtain the Ag@AgCl coaxial core-shell nanocables.

The X-ray diffraction (XRD) patterns of the products were carried out with a Dmax-3β diffractometer with nickel-filtered Cu Kα radiation (λ= 1.54178 Å). Morphologies of the products were determined by field emission scanning electron microscopy (FESEM; JEOL-6300F). Transmission electron microscope (TEM), high-resolution transmission electron microscope (HRTEM) images and chemical composition analysis were performed by Tecnai F20 transmission electron microscope and energy-dispersive X-ray spectroscopy (EDS). The ultraviolet-visible (UV-Vis) spectra were measured at room temperature with a Lambda 35 spectrophotometer.

3 Results and discussion

Figure 1 shows the XRD pattern and SEM images of the Ag nanowires. All the peaks in figure 1a can be identified as cubic Ag with a lattice constant a=4.0861 Å, corresponding to the literature data of JCPDS 04-0783. As shown in the SEM images in figure 1b and c, uniform Ag nanowires with an average diameter of about 100 nm have been fabricated by the polyol process. The mean length of the Ag nanowires is about 10 μm.

Figure 2 displays the XRD patterns of the Ag@AgCl core-shell nanocables synthesized at various molar ratio of Fe:Ag. Apart from the peaks of metallic Ag crystals, additional peaks could be identified as cubic AgCl (JCPDS 31-1238) after the addition of FeCl3

solution, which confirms the formation of AgCl nanocrystals on the surface of the Ag nanowires. With the increase of the molar ratio of Fe:Ag, the peaks of the metallic Ag crystals were gradually reduced during the oxidation process and the peaks of AgCl crystals were significantly enhanced. When the molar ratio of Fe:Ag was increased up to 6:5, the Ag nanowires were etched completely and transformed into AgCl crystals entirely. Additionally, except for the metallic Ag and AgCl, no diffraction peaks corresponding to other impurities were detected, indicating that the obtained products in the whole growth process only consisted of metallic Ag and AgCl crystals, and the thickness of AgCl nanoshells could be easily adjusted. The SEM images of the Ag@AgCl coaxial core-shell nanocables that were obtained by mixing the Ag nanowires and FeCl3 in ethanol solution at different molar ratio of Fe:Ag are shown in figure 3. Obviously, the surfaces of the Ag nanowires were not smooth after the redox reaction comparing to the pure Ag nanowires. At the molar ratio of Fe:Ag=1:20, few very small AgCl nanocrystals (about 20 nm) were formed at the edges of Ag nanowires. With the gradual increase of the molar ratio of Fe:Ag (1:10, 1:5, 2:5, 4:5), more and more AgCl nanoparticles with a mean size of 40 nm were formed and Ag nanowires were gradually etched and replaced by AgCl thin nanoshells. Further increasing the molar ratio of Fe:Ag to 6:5, many AgCl nanorods were formed because the Ag nanowire were completely etched.

Cryst. Res. Technol. 45, No. 11 (2010) 1191

www.crt-journal.org © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Fig. 1 XRD pattern and SEM images of the Ag nanowires. (a) XRD pattern, (b, c) SEM images.

Fig. 2 XRD patterns of the Ag@AgCl coaxial core-shell

nanocables synthesized at different molar ratio of Fe:Ag. (a)

1:20, (b) 1:10, (c) 1:5, (d) 2:5, (e) 4:5, (f) 6:5.

(Online color at www.crt-journal.org)

Fig. 3 SEM images of the Ag@AgCl coaxial core-shell nanocables

synthesized at different molar ratio of Fe:Ag. (a, b) 1:20, (c, d) 1:10,

(e, f) 1:5, (g, h) 2:5, (i, j) 4:5, (k, l) 6:5.

Table 1 The elements and their weight percentage and atomic percentage.

Element Weight % Atomic %

ClK 1.40 4.20

AgK 98.60 95.80

Figure 4 shows the typical TEM images and EDS spectrum of the synthesized Ag@AgCl coaxial core-shell nanocables with the molar ratio of Fe:Ag=1:20. The TEM images in figure 4 confirm the results of the SEM images in figure 3a and b. It was obvious that the Ag nanowires were coated by AgCl nanoparticles. The average size of the AgCl nanoparticles was about 20 nm. The AgCl nanoparticle marked black in figure 4b is further observed in figure 4c. The HRTEM image reveals the lattice spacing of d = 0.318 nm, corresponding to the (111) planes of AgCl with cubic structure. This result is in good agreement with the XRD analysis. Figure 4d shows the EDS spectrum of the Ag@AgCl coaxial core-shell nanocables dispersed on amorphous

1192 Fei Li et al.: Ag@AgCl coaxial core-shell nanocables

© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.crt-journal.org

carbon-coated copper grids. It can be seen that silver, and chlorine elements are present in the samples. The detected carbon and copper elements come from the amorphous carbon-coated copper grids. Table 1 shows the chemical compostion of the Ag@AgCl coaxial core-shell nanocables. The weight percentage of Cl element is about 1.40% and the atomic percentage of Cl element is about 4.20%.

Fig. 4 TEM images and EDS spectrum of the Ag@AgCl coaxial core-shell nanocables with the molar ratio

of Fe:Ag=1:20. (a) low-magnification TEM image, (b) high-magnification TEM image, (c) HRTEM image

marked in (b), (d) EDS spectrum.

Fig. 5 Formation process of the Ag@AgCl coaxial core-shell

nanocables synthesized at different molar ratio of Fe:Ag. (Online

color at www.crt-journal.org)

Fig. 6 UV-Vis absorption spectra of the Ag

nanowires and the Ag@AgCl coaxial core-shell

nanocables synthesized at different molar ratio of

Fe:Ag. (a) Ag nanowires, (b) 1:20, (c) 1:10, (d) 1:5,

(e) 2:5, (f) 4:5, (g) 6:5. (Online color at www.crt-

journal.org)

During such coaxial core-shell nanocable synthesis, Ag nanowires not only serve as the precursors, but also act as templates. After the addition of FeCl3 solution, Ag nanowires react with FeCl3, which can be formulated as following: Ag+Fe3++Cl- →Fe2++AgCl↓ The process is a typical redox reaction and the half reactions are as follows:

Ag+Cl- →AgCl↓ (E0 AgCl/ Ag=+0.223V), Fe3+ + e- →Fe2+ (E0 Fe3+

/ Fe2+=+0.771V).

It is evident that the standard electrode potential of anode (Fe3+/Fe2+) is much high than that of cathode (AgCl/Ag), implying a great tendency toward the right-hand side of the above reaction according to the principle rule of redox reaction. Therefore, Ag nanowires lose their electrons and are oxidized to AgCl nuclei. The AgCl nuclei are further grown into AgCl nanoparticles. Because the extent of the etching of Ag nanowires is determined by the amount of FeCl3, FeCl3 has great effect on the composition and morphology of the Ag@AgCl coaxial core-shell nanocables. When the molar ratio of Fe:Ag was 1:20, few AgCl nanoparticles (about 20 nm) were formed only on some sites of Ag nanowires due to the less quantity of FeCl3. With the increase of the molar ratio of Fe:Ag (1:10, 1:5, 2:5, 4:5), because both Fe3+ and Cl- can continuously diffuse across the edges of Ag nanowires, Ag nanowires were gradually etched and replaced by AgCl thin nanoshells to form the Ag@AgCl coaxial core-shell nanocables. When the molar ratio of Fe:Ag was further increased to 6:5, Ag nanowires were transformed into AgCl nanowires completely because FeCl3 was excessive. The AgCl nanowires were constituted by large numbers of AgCl nanoparticles. Due to the strong stress between the AgCl nanoparticles, AgCl nanowires were splitted into AgCl nanorods to get the lowest surface energy. The whole transformation process from the Ag nanowires to the Ag@AgCl coaxial core-shell nanocables at various molar ratio of Fe:Ag is illustrated in figure 5.

Cryst. Res. Technol. 45, No. 11 (2010) 1193

www.crt-journal.org © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Figure 6 shows the UV-Vis absorption spectra of the Ag nanowires and the Ag@AgCl coaxial core-shell nanocables synthesized at different molar ratio of Fe:Ag. The absorption band at about 400 nm in figure 6a occurs due to the surface plasmon resonance in nano-silver, which have been widely investigated [25-28]. It can be clearly seen that the absorption of metallic Ag nanowires were gradually reduced after the redox process, and the absorption of AgCl nanoparticles were significantly enhanced. The UV absorption at about 270 nm is the characteristic absorption peak of AgCl nanocrystals [29]. When the molar ratio of Fe:Ag was increased to 6:5, the absorption of metallic Ag nanowires disappeared completely and the absorption of AgCl was the strongest, implying the complete transformation of the Ag to AgCl from another aspect.

4 Conclusion

In summary, a general, facile, and economic solution method via a redox reaction between Ag nanowires and FeCl3 solution is developed to fabricate Ag@AgCl coaxial core-shell nanocables. The XRD analysis, SEM and TEM images confirm the formation of the Ag@AgCl coaxial core-shell nanocables. The systematical study finds that the molar ratio of Fe:Ag has great effect on the formation process and optical absorptions of the Ag@AgCl coaxial core-shell nanocables. With the increase of the molar ratio of Fe:Ag, the optical absorption of Ag nanowires decreases gradually and disappears finally, and that of AgCl improves significantly. Such formation process and mechanism might be extended to other systems for the synthesis of coaxial core-shell nanocables and could be applied in many fields such as electronics, sensing, photonics and catalysis.

References

[1] J. Kim, J. E. Lee, S. H. Lee, J. H. Yu, J. H. Lee, T. G. Park, and T. Hyeon, Adv. Mater. 20, 478 (2008).

[2] J. Han, G. P. Song, and R. Guo, Chem. Mater. 19, 973 (2007).

[3] J. H. Gao, G. L. Liang, J. S. Cheung, Y. Pan, Y. Kuang, F. Zhao, B. Zhang, X. X. Zhang, E. X. Wu, and B. Xu, J.

Am. Chem. Soc. 130, 11828 (2008).

[4] A. Steinbru, A. Csaki, K. Ritter, M. Leich, J. M. Kohler, and W. Fritzsche, J. Nanopart. Res. 11, 623 (2009).

[5] M. Abdulla-Al-Mamun, Y. Kusumoto, and M. S. Islam, Chem. Lett. 38, 980 (2009).

[6] M. A. Lim, S. I. Seok, W. J. Chung, and S. I. Hong, Opt. Mater. 31, 201 (2008).

[7] B. Sreedhar, P. Radhika, B. Neelima, N. Hebalkar, and A. K. Mishra, Catal. Commun. 10, 39 (2008).

[8] O. Muehling, A. Seeboth, T. Haeusler, R. Ruhmann, E. Potechius, and R. Vetter, Sol. Energy Mater. Sol. Cells 93,

1510 (2009).

[9] H. Wang, C. W. Xu, F. L. Cheng, M. Zhang, S. Y. Wang, and S. P. Jiang, Electrochem. Commun. 10, 1575 (2008).

[10] C. Y. Li, X. H. Wang, G. Chen, L. He, and H. Can, Chem. Lett. 39, 64 (2010).

[11] Q. L. Bao, C. M. Li, L. Liao, H. B. Yang, W. Wang, C. Ke, Q. L. Song, H. F. Bao, T. Yu, K. P. Loh, and J. Guo,

Nanotechnology 20, 065203 (2009).

[12] Y. Liu, Y. Chu, L. K. Yang, D. X. Han, and Z. X. Lv, Mater. Res. Bull. 40, 1796 (2005).

[13] J. Xu, J. Hu, C. J. Peng, H. L. Liu, and Y. Hu, J. Colloid Interface Sci. 298, 689 (2006).

[14] D. D. Evanoff and G. Chumanov, Chem. Phys. Chem. 6, 1221 (2005).

[15] B. J. Wiley, Y. C. Chen, J. M. McLellan, Y. J. Xiong, Z. Y. Li, D. Ginger, and Y. N. Xia, Nano Lett. 7, 1032

(2007).

[16] X. M. Sun and Y. D. Li, Adv. Mater. 17, 2626 (2005).

[17] Y. R. Fang, H. Wei, F. Hao, P. Nordlander, and H. X. Xu, Nano Lett. 9, 2049 (2009).

[18] A. Borras, A. Barranco, F. Yubero, and A. R. Gonzalez-Elipe, Nanotechnology 17, 3518 (2006).

[19] A. H. Chen, H. Q. Wang, and X. Y. Li, Chem. Commun. 14, 1863 (2005).

[20] H. X. Xie, T. Qiu, and X. Y. Li, Chem. Res. Chin. Univ. 19, 1046 (2008).

[21] T. Ghoshal, S. Biswas, and S. Kar, J. Phys. Chem. C 112, 20138 (2008).

[22] P. Wang, B. B. Huang, X. Y. Qin, X. Y. Zhang, Y. Dai, J. Y. Wei, and M. H. Whangbo, Angew. Chem. Int. Edit.

47, 7931 (2008).

[23] Y. H. Heo, W. I. Park, J. S. Whang, and C. O. Park, Elec. Mater. Lett. 3, 33 (2007).

[24] W. C. Zhang, X. L. Wu, H. T. Chen, Y. J. Gao, J. Zhu, G. S. Huang, and P. K. Chu, Acta. Mater. 56, 2508 (2008).

[25] V. S. Gurin, V. P. Petranovskii, M. A. Hernandez, N. E. Bogdanchikova, and A. A. Alexeenko, Mater. Sci. Eng. A

391, 71 (2005).

[26] N. V. Loginova, A. A. Chernyavskaya, M. S. Parfenova, N. P. Osipovich, G. I. Polozov, Y. A. Fedutik, T. V.

Kovalchukv, and G. P. Shevchenko, Polyhedron 25, 1723 (2006).

[27] M. Darroudi, M. B. Ahmad, K. Shameli, A. H. Abdullah, and N. A. Ibrahim, Solid State Sci. 11, 1621 (2009).

[28] A. Zielińska, E. Skwarek, A. Zaleska, M. Gazda, and J. Hupka, Procedia Chem. 1, 1560 (2009).

[29] M. R. Song and Z. J. Zhang, Mater. Lett. 58, 2049 (2004).