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Preparation of nanocrystalline silver by the method of liquid-solid arc discharge combined with hydrothermal treatment

Preparation of nanocrystalline silver by the method of liquid-solid arc discharge combined with hydrothermal treatment

Materials Research Bulletin, Vol. 34, Nos. 10/11, pp. 1683–1688, 1999 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 00...

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Materials Research Bulletin, Vol. 34, Nos. 10/11, pp. 1683–1688, 1999 Copyright © 2000 Elsevier Science Ltd Printed in the USA. All rights reserved 0025-5408/99/$–see front matter

PII S0025-5408(99)00169-5

PREPARATION OF NANOCRYSTALLINE SILVER BY THE METHOD OF LIQUID-SOLID ARC DISCHARGE COMBINED WITH HYDROTHERMAL TREATMENT

Y. Zhou1,2, H.J. Liu1,2, S.H. Yu1,2, Z.Y. Chen1,2*, Y.R. Zhu1, and W.Q. Jiang1,2 1 Department of Chemistry, University of Science and Technology of China, Hefei, Anhui, 230026, P.R. China 2 Structure Research Laboratory, University of Science and Technology of China, Hefei, Anhui, 230026, P.R. China (Refereed) (Received September 11, 1998; Accepted November 23, 1998)

ABSTRACT A technique of liquid–solid arc discharge combined with hydrothermal treatment in the presence of surfactant has been successfully developed to prepare nanocrystalline noble metallic Ag with an average particle size of about 10 nm. The products were characterized by adsorption spectroscopy, X-ray diffraction (XRD), transmission electron microscopy (TEM), and microscope photodensitometer measurements. The influence of preparation conditions on the size of Ag particles was investigated. The appearance of Ag nanowires in the process was observed. © 2000 Elsevier Science Ltd KEYWORDS: A. nanostructures, A. metals, C. electron microscopy, C. X-ray diffraction INTRODUCTION Research on ultrafine metal particles has been fairly active in recent years because of their great importance not only in theoretical research, but also in a wide range of potential applications [1–3]. Up until now, many methods have been developed to prepare ultrafine metal particles [4 – 6]. Unlike gas-arc discharge, which should be operated under the condition of high-purity inert atmosphere with complex apparatus, liquid-solid arc discharge can

*To whom correspondence should be addressed. E-mail: [email protected] 1683

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FIG. 1 The experimental apparatus of the method of liquid–solid arc discharge.

be performed not only conveniently under ambient pressure at room temperature, but also without costly instruments. Therefore, there is good prospect for application of liquid–solid arc discharge in preparing nano-metal particles. However, since the metal particles produced by liquid-solid arc discharge are in the cluster state, further treatment is needed to form ultrafine powders. In this paper, we report the preparation of nanocrystalline silver powders by the method of the liquid–solid arc discharge combined with hydrothermal treatment. EXPERIMENTAL The experimental apparatus of the method of liquid–solid arc discharge is described in Figure 1. High-purity silver filaments (diameter 1.5 mm) were used as two electrodes. One of the electrodes was dipped into a 0.1 mol/L NaNO3 solution containing an appropriate amount of polyvinylalcohol (PVA) as surfactant. The end of the another silver electrode was momentarily brought into contact with the surface of the above NaNO3 solution while a certain voltage (⬃150 V) was used between the two electrodes, using an ac step-down circuit. It formed the instantaneous circulation between the two electrodes and an arc discharge spark on the surface of the latter silver electrode. This resulted in a continuous dissolution of the silver into the solution due to the great exothermic during the arc discharge. Colloidal silver was produced. For the exothermic, the solution was cooled with water during the arc discharge. The colloidal solution of silver obtained was put into a Teflon-lined stainless-steel autoclave and heated in an oven at various temperatures ranging from 130 –200°C for different periods of time. After cooling to room temperature, the silver powders obtained were washed with distilled water and absolute alcohol and dried at 60°C for 2 h. UV-vis absorption spectra were recorded with a Shimadzu UV-200 spectrophotometer. Powder X-ray diffraction (XRD) patterns were recorded at a scanning rate of 0.02°s⫺1 in a 2␪ range from 20° to 70°, with a Rigaku D/max ␥A X-ray diffractometer using graphite monochromatized Cu K␣ radiation (␭ ⫽ 0.154178 nm). The average particle sizes of products were calculated by the Scherrer formula. TEM images were taken with a Hitachi H-800 transmission electron microscope, using an accelerating voltage of 200 kV. Particle size and distribution were measured with a Perkin-Elmer microphotodensitometer.

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FIG. 2 UV-vis spectra of (a) the colloidal solution without hydrothermal treatment and (b) the suspension of silver obtained through hydrothermal treatment of the colloidal solution.

RESULTS AND DISCUSSION The UV-vis spectra of silver colloidal solution and suspension obtained through hyrothermal treatment of the colloidal solution at 130°C for 6 h are shown in Figure 2(a) and (b), respectively. The absorption peak for (a) is found at 302 nm and that for (b), at 400 nm. The red shift of absorption band indicates the growth of silver clusters to nanoparitcles via the hydrothermal treatment. The XRD pattern of silver particles obtained via the hydrothermal treatment at 130°C for 6 h is shown in Figure 3. The average particle size of the silver powders, according to the Scherrer formula, is about 10 nm. The morphology of the silver particles observed by TEM is given in Figure 4. This TEM image shows that the silver particles consisted of quasi-spherical crystallites. From Figure 5, one can see that the silver particle size distribution ranges from 4 to 20 nm; the largest

FIG. 3 X-ray diffraction pattern of silver particles.

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FIG. 4 TEM image of the silver particles.

percentage is about 47% in the range 8 –15 nm. The average particle size calculated from Figure 5 is 13 nm, which is in agreement with that estimated from XRD patterns. The influence of temperature and time on the average particle size of the product were studied; the results are summarized in Table 1. When the treatment time was 6 h at 140°C, the silver particle size was 11 nm. When the time was prolonged to 10 h and 18 h, the silver particle size increased slightly, to 12 nm and 15 nm, respectively. When the temperature of treatment was increased from 140 to 180°C or 200°C for 10 h, the silver particle size increased, respectively, from 12 to 25 or 30 nm. These results indicate that temperature, compared with treatment time, had a more considerable effect on particle size. Further experiments showed that when the treatment temperature was more than 200°C or the treatment time exceeded 10 h, particle size did not increase significantly. The optimum condition for producing nanocrystalline silver powders from the silver colloid was at 130°C for 6 h.

FIG. 5 Particle size distribution.

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TABLE 1 The Particle Sizes of Nanocrystalline Silver Prepared by Liquid-Solid Arc Discharge Combined with Hydrothermal Treatment Under Different Preparation Conditions Hydrothermal treatment Sample 1 2 3 4 5 6 7 8 9

Temperature (°C)

Time (h)

Average particle size (nm)

130 140 140 140 160 180 180 200 200

6 6 10 18 5 5 10 5 10

10 11 12 15 18 20 25 30 30

The colloidal solution of silver prepared with the method of liquid–solid arc discharge could be kept stable for several days without obvious precipitation. After being kept for more than one week, a black precipitate appeared. The precipitate was collected. XRD patterns showed that the particle size of the precipitated silver powders was 27 nm, according to the Scherrer formula. TEM observation confirmed that these powders consisted of spherical nanoparticles and nanowires. A typical TEM image of the silver nanowires is shown in Figure 6. This image reveals that there is a strong interaction among the silver particles prepared by the liquid–solid arc discharge method, during precipitation. From further study, it was found that the wires consisted of spherical particles, and some interfaces among the particles disappeared completely when they linked with each other to become a chain or wire. These results demonstrate that the present method may be used to prepare other noble metal nanowires.

FIG. 6 Typical TEM image of the silver nanowires.

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CONCLUSIONS Nanocrystalline silver and silver nanowires were successfully prepared by the method of liquid–solid arc discharge combined with a hydrothermal treatment. Hydrothermal treatment conditions have obvious influence on the particle size of the powders. The present method may be extended to prepare other nanocrystalline noble metals and alloys and to prepare other metal nanowires. ACKNOWLEDGMENTS This work was supported by the Chinese National Science Research Foundation. REFERENCES 1. 2. 3. 4. 5. 6.

H.D. Glicksman, in Metal Handbook, 9th ed., Vol. 7, p. 147, American Society for Metals, Metals Park, OH (1984). R.E. Cavicchi and R.H. Silsbee, Phys. Rev. Lett. 52, 1453 (1984). G. Fisher, Ceram. Ind. (Chicago) 120, 80 (1983). J.S. Bradley, G.B. Ansell, E.W. Hill, and M.E. Leonowice, J. Mol. Catal. 41, 59 (1981). N.Toshima and T.Takahashi, Bull. Soc. Chem. Jpn. 65, 400 (1992). Y.J. Zhu and Y.T. Qian, Mater. Sci. Eng. B 23, 116 (1994).