ISSN 0020-1685, Inorganic Materials, 2009, Vol. 45, No. 2, pp. 193–197. © Pleiades Publishing, Ltd., 2009. Original Russian Text © Shancheng Yan, Yuping Zhang, Yong Zhang, and Zhongdang Xiao, 2009, published in Neorganicheskie Materialy, 2009, Vol. 45, No. 2, pp. 231–234.
Synthesis of Silver Sulfide Nanowires in Ethylene Glycol through a Sacrificial Templating Route Shancheng Yan, Yuping Zhang, Yong Zhang, and Zhongdang Xiao* State Key Laboratory of Bioelectronics, School of Biological Science & Medical Engineering, Southeast University, Nanjing, China *e-mail:
[email protected] Received March 26, 2008
Abstract—In this study, Ag2S nanowires were prepared in ethylene glycol using single-crystal silver nanowires as a sacrificial templating and choosing suitable sulfur sources for the sulfuration reaction. X-ray powder diffraction and a transmission electron microscope equipped with an energy dispersive X-ray analysis were used to characterize the products. The results indicated Ag2S nanowires with diameters of about one hundred nanometers and lengths up to several micrometers could be obtained through this method. The selected area electron diffraction pattern and high-resolution transmission electron microscope imaging indicated that the Ag2S nanowires thus formed were crystalline. DOI: 10.1134/S0020168509020150
INTRODUCTION Silver sulfide has been known to be a mixed ionic– electronic conductor at high temperature (above 200°C). It has three different forms. α-Ag2S is a monoclinic phase stable up to 178°C. α-Ag2S transforms to body-centered cubic β-Ag2S above 178°C. Above 600°C, the body-centered cubic phase transforms to a face-centered cubic γ-Ag2S phase [1, 2]. Among these three forms, bulk α-Ag2S is an important semiconductor with a band gap of approximately 1 eV at room temperature and has a relatively high absorption coefficient (about 104 cm–1) [3]. Bulk α-Ag2S also has good photoelectric and thermoelectric properties, which has been widely applied in optical and electronic devices, such as photoconducting cells, photovoltaic cells, and so on [4–10]. Ag2S nanoparticles have been synthesized and studied widely [11–17], and their application potential in photography and luminescent devices has been recognized. Very recently, Ag2S nanowires and nanowire arrays have also been successfully prepared through the gas–solid reaction route and anodic aluminum oxide (AAO) template method [18–20]. Nevertheless, only a few reports on the synthesis of Ag2S nanowires via solution-phase routes can be found in the literature owing to its low solubility (Ksp = 6.3 × 10–50) [21, 22]. Herein, α-Ag2S nanowires have been prepared in ethylene glycol through a sacrificial templating route. To our knowledge, there is no report of synthesis of silver sulfide nanowires in ethylene glycol solution using
single-crystal silver as sacrificial templating up to now. The results demonstrated that suitable sulfur sources and solvent played crucial roles in the formation of the α-Ag2S nanowires. The formation mechanism of the Ag2S nanowires has also been investigated. EXPERIMENTAL All the chemicals were of analytical grade and used without further purification. Our synthesis began with single-crystal nanowires of silver, which served as templates to obtain Ag2S nanowires. The details of synthesis of silver nanowire were described by Xia et al. [23]. In a typical experimental procedure for the synthesis of Ag2S nanowires, 0.2 mmol of the as-prepared Ag nanowires was dispersed by sonication in 20 ml of ethylene glycol, in which 0.4 mmol thiacetamide had already been dissolved. The round flask was refluxed and kept at 80°C for 12 h. The obtained black solid product was collected by centrifuging the mixture and then washed with absolute ethanol several times. Finally, the products were dried in vacuum at room temperature for future characterization. We characterized the samples using transmission electron microscope (TEM) (Tecnai G2 S-TWIN) equipped with an energy dispersive x-rays analysis (EDXA) using an accelerating voltage of 200 kV, x-ray powder diffraction (XRD SHIMADZU CuKα, λ = 0.1548 nm), and UV-vis absorption spectroscopy
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Fig. 1. (a) XRD pattern of the as-prepared Ag nanowires and (b) TEM image of the as-prepared Ag nanowire and its SAED pattern (inset).
Fig. 2. (a) TEM image of the as-prepared Ag2S nanowire and its SAED pattern (inset) and (b) HRTEM image of the left product.
(UV-3150 SHIMADZU). Selected-area electron diffraction (SAED) patterns were obtained on TEM.
Figure 2a shows a single silver sulfide nanowire 90 nm in diameter and its corresponding SAED spots. Similar to the above observation, the transformed silver sulfide was a single-crystalline structure. The HRTEM image in Fig. 2b provides further insight into its structure. It is observed that the nanowire exhibits good crystalline and clear lattice fringes. The experimental lattice fringe spacing, 0.2615 nm calculated by the TEM software, corresponds to the unique 0.2664 nm separation between two (120) planes in bulk acanthite Ag2S crystallites [24]. We found an interesting phenomenon that the Ag2S nanowires are more unstable than nanowires of other semiconductor materials, such as ZnO, under high-energy electron beam radiation. From Fig. 2a, it is clear that, at the position irradiated with the electron beam (in the white circle), the diameter of the nanonanowire becomes thinner. This suggests that the Ag2S nanowires are easily etched away by high-energy
RESULTS AND DISCUSSION The XRD spectrum of the Ag nanowires prepared using Xia’s method is shown in Fig. 1a. It can be seen that there are four peaks in the XRD spectra of the Ag nanowires. They correspond to the (111), (200), (220), and (311) planes, respectively. All the intense XRD peaks can be indexed to the face-centered-cubic phase of silver (JCPDS 4-0783). The structure information for the Ag nanowires further came from TEM and SAED (Fig. 1b). As shown in Fig. 1b, Ag nanowires about 100 nm in diameter and up to several micrometers in length are observed. The SAED pattern inset of Fig. 1b taken from these Ag nanowires yields diffraction spots rather than rings, which are suggestive of the presence of single-crystal Ag.
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Fig. 4. UV-vis absorption spectra of the as-prepared products.
electrons [19]. It may be a useful method to fabricate nanoscale devices. The further EDXA analysis of Ag and Ag2S nanowires is shown in Fig. 3. In the EDXA spectrum, the peaks of Ag and S are pronounced and no other peaks are found except for those of Cu and C originating from the Cu microgrid with amorphous carbon used to support the Ag2S nanowires. The molar ratio of silver and sulfur was close to 2, calculated by EDXA software, which indicated that the Ag templates had been transformed into Ag2S. The UV-vis adsorption spectrum is one of the most widely used techniques for structural characterization of nanomaterials. We recorded the UV-vis absorption spectra of the Ag nanowire and Ag2S nanowire (Fig. 4). The variety of morphologies and sizes should be responsible for the broad peaks and the different features of the spectra in the UV-vis adsorption spectra. The peaks at 350 nm are ascribed to the transversal modes of the Ag nanowires with pentagonal cross secINORGANIC MATERIALS
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tions, corresponding to and out-of-plane resonance [25]. The peak at 470 nm is assigned to the in-plane quadrupole resonances of the [100] planes of the Ag nanowires. As compared to the direct band gap (1 eV) of bulk α-Ag2S [3], the blue shift to 365 nm can be found compared with the band gap of the characteristic absorption of bulk Ag2S probably due to the size effect, chemical modification, and amorphous effect, which is similar to the result of the Ag2S nanoparticles in the previous report [26]. Of course, the origin of the observed absorption peaks in the UV-vis region is still far from well understood now, and the more detailed research is needed to elucidate the interesting shapedependent optical properties. The growth mechanism of Ag nanowires has been extensively studied. The anisotropic growth of the Ag nanowires is believed to be the result of the stronger adsorption of PVP on the (100) side facets than the (111) ends [27, 28]. The possible formation mechanism of Ag2S nanowires was described as follow: TAA was
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comparatively unstable and slowly released S2– ions into the reaction solution. Then the template Ag nanowires were eroded with S2– ions in the solution. S2– ions instead of elemental S in bulk solution were found to play a key role in the formation of Ag2S nanowires. We once tried to use Na2S instead of TAA, but Ag2S nanowires could not be formed, indicating that the slow release of sulfide ions was essential for the formation of the Ag2S nanowires. It is well known that the diffusion rate of an ion is different when the diffusion medium is different. We used distilled water as the solvent instead of ethylene glycol. A nanowire structure also could not be obtained. This indicates that the reaction medium also plays a crucial role in such a sulfidation reaction. The present corrosion process of the Ag nanowire is different from the previous report on the corrosion of the Cu core and the Kirkendall-type diffusion process [29]. This corrosion process of the Ag nanowire is the same as a previous report on the corrosion of silicacoated silver nanoparticles with molecular iodine to form AgI nanoparticles linked with SiO2 instead of AgIcoated SiO2 capsules through a transport process from core to surface [30]. The exact mechanism for the formation of Ag2S nanowires is under way. CONCLUSIONS To sum up, Ag2S nanowires were prepared in ethylene glycol through a sacrificial templating route. The Ag2S nanowires had diameters of a few hundred nanometers and lengths up to several micrometers. HRTEM image further indicated that the formed Ag2S nanowires were crystalline. In principle, it can be extended to the synthesis of other semiconductor sulfide nanostructures. ACKNOWLEDGMENTS This work was financially supported by the FANEDD (no. 200053), NCET-04-0480, and the Scientific Research Foundation of Graduate School of Southeast University. REFERENCES 1. Schaaff, T.G. and Rodinone, A., Preparation and Characterization of Silver Sulfide Nanocrystals Generated From Silver(I)-Thiolate Polymers, J. Phys. Chem. B., 2003, vol. 107, no. 38, pp. 10416–10422. 2. Belman, N., Golan, Y., and Berman, A., Nanocrystalline Ag2S on Polydiacetylene Langmuir Films, Cryst. Growth Des., 2005, vol. 5, no. 2, pp. 439–443. 3. Dlala, H., Amlouk, M., Belgacem, S.J., et al., Intramuscular Pressure, Force and Blood Flow in Rabbit Tibialis Anterior Muscles During Single and Repetitive Contrac-
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