Preparation of silver nanoparticles by pulse sonoelectrochemical method and studying their

characteristics

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2009 J. Phys.: Conf. Ser. 187 012077

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Preparation of silver nanoparticles by pulse sonoelectrochemical method and studying their characteristics

Le Van Vu, Nguyen Ngoc Long, Sai Cong Doanh and Bui Quoc Trung

Faculty of Physics, Hanoi University of Science, 334 NguyenTrai, Hanoi, Vietnam

E-mail: levanvu@hus.edu.vn

Abstract. Silver nanoparticles with spheres, rods have been prepared by a pulse sonoelectrochemical technique from an aqueous solution of AgNO3 in the presence of sodium dodecyl sulfate C12H25NaO4S -(SDS). The as-prepared silver nanoparticles are characterized by electron microscopy (TEM, SEM), powder X-ray diffraction (XRD), and UV-vis absorption spectrum. It was found that the concentration of AgNO3 and SDS affects the shape of the nanoparticles. The crystal size could be varied from 5 nm up to 200 nm by controlling the various electrodeposition and sonic parameters.

Keywords: Silver nanoparticles, sonoelectrochemical method, metallic nanoparticles, surface plasmon resonance absorption.

1. Introduction In recent years, metallic nanoparticles are a strong research focus because of their unique functional properties which lead to varied application in the areas of plasmonics [1], surface enhanced Raman scattering [2], catalysis, medicine and biological sensing [3, 4] It is well-known that catalytic reactivity depends on the size and shape of the metal nanoparticles and that the synthesis of well-controlled shapes and the size of particles could be critical for their applications [5]. The developed methods for nanoparticle fabrication include chemical reduction [6], seed-mediated growth procedure [7, 8], sonochemical reduction [9], microwave-assisted synthesis [11], laser ablation, metal evaporation [12], etc.

Sonochemistry is a very useful method that has been known since the 1930s and has increased in popularity over the past decade. The effects of ultrasonic radiation on chemical reactions are due to the very high temperatures and pressures which develop in and around collapsing bubbles. However, the potential benefits of combining sonochemistry with electrochemistry have become increasingly studied. Some of these beneficial effects include acceleration of mass transport, cleaning and degassing of the electrode surface, and increased reaction rates. Reisse and co-workers have described a novel device for the production of metal powders using pulsed sonoelectrochemical reduction [9, 10]. This device exposes only the flat circular area at the end of the sonic tip to the electrodeposition solution. The exposed area acts as both cathode and ultrasound emitter. The metals were obtained as chemically pure, fine crystalline powders with a high surface area giving an average particle size of 100 nm. The present understanding of the sonoelectrochemical mechanism describes an electrochemical deposition of a thin layer on the sonoelectrode during the electrical pulse. The thin layer is then destroyed by the coming sonic pulse, to form jet-suspended nanoparticles into the

APCTP–ASEAN Workshop on Advanced Materials Science and Nanotechnology (AMSN08) IOP PublishingJournal of Physics: Conference Series 187 (2009) 012077 doi:10.1088/1742-6596/187/1/012077

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solution. Thus, the electrochemical and sonic parameters, including the pulse duration, can be used to control the particles’ parameters.

In the present paper, we report the use of the sonoelectrochemical method for the preparation of silver nanopaticles with different shapes including spherical and rodlike nanoparticles. The sonoelectrochemical device employed is different from that described by Reisse and co-workers. A titanium horn acted as the ultrasound emitter only in the experimental setup. This experiment is based on the electroreduction of AgNO3 in aqueous solution in the presence of SDS.

2. Experimental The schema of the experimental setup assembly for these experiments is shown in figure 1a. It contains an electrochemical cell (cathode, anode, and electrolyte), an ultrasonic pulse generator (sonicator), a direct-current power source, and a synchronizer. The synchronizer is used to drive the durations of ultrasonic and electric pulses. Each sonoelectrochemical cycle lasts 0.9 s, consisting of a 300 ms electric current pulse of 40 mA, a 300 ms sonic pulse immediately following the electric pulse, and a 300 ms rest (figure 1b). The catholic deposition of the silver was done on stainless steel foil. The other stainless steel foil served as the anode. A Vibrate 700 W was used as the sonicator. The power source was limited to 20 V. The experimental cell had dimensions as follows: The effective surface area of the stainless steel foil was 2 cm2. The distance between the anode and the cathode was set to 2 mm. In all of the experiments, the amount of electrolyte was 50 cm3. The temperature was stabilized by a water bath and kept in the temperature range 25-28°C. The vessel was cover in by using aluminum paper. The usual value of electric current density was 20 mA/cm2. The appropriate ultrasonic power density was about 100 W/cm2.

Figure 1a. The schema of the experimental setup.

Figure 1b. The sonoelectric cycle: electric current pulse of 0.3 s duration; sonic pulse of 0.3 s duration; the total cycle is 0.9 s long.

Quantities of 1, 5, and 20 g/L AgNO3 with sodium dodecyl sulfate C12H25NaO4S-SDS (Merck)

were mixed in an aqueous solution under a N2 atmosphere. The deposition was carried out by using different reaction times. Oxygen was removed from the water by nitrogen bubbling, and the electrolyte was mixed under a nitrogen atmosphere. A complexing agent, sodium dodecyl sulfate C12H25NaO4S-SDS, was added at the 40 g/L level to avoid aggregation. The products were washed with doubly distilled water and collected by using a centrifuge (Hettich Universal 320, 9000 RPM, 30 minutes). The silver particles were characterized by transmission electron microscopy (TEM) and by scanning electron microscopy (SEM), powder X-ray diffraction (XRD) using Cu-Kα radiation, and UV-vis absorption spectrum. The TEM studies were performed with a JEOL, JEM-1010 microscope. The TEM provides quantitative information for each size distribution calculation using the Image J 1.37 v

APCTP–ASEAN Workshop on Advanced Materials Science and Nanotechnology (AMSN08) IOP PublishingJournal of Physics: Conference Series 187 (2009) 012077 doi:10.1088/1742-6596/187/1/012077

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software.

3. Results and discussion Figure 2a presents a typical TEM image of the product obtained by the sonoelectrochemical method with 1 g/L AgNO3 and 40 g/L SDS. The deposition was carried out for 5 minutes. It shows that silver nanoparticles are spherical and well-dispersed. The particle size distribution measured directly from TEM micrographs is shown as a histogram in figure 3. The average size of particles is about 8 nm. One can readily see that, at the initial time interval, up to about 15 minutes, single crystallites grow. The mean size of the crystallites increases from 8 nm after 5 minutes of sonoelectrochemical processing to 20 nm after 15 minutes (figure 2) and the aggregation is observed if the reaction is carried out for longer periods of time.

a)

b)

Figure 2. TEM pictures of silver particles obtained by pulsed sonoelectrochemical treatment of different durations for 1 g/L AgNO3 and 40 g/L SDS solution: a) 5 minutes, b) 15 minutes.

Figure 3. Size distribution measured from TEM micrographs in figure 2a.

Experimental results indicate that the concentration of AgNO3 plays a significant role in the

formation and growth of the silver nanoparticles. The TEM image of the products of the 5 g/L AgNO3 electrolysis shows a nanorod structure with a width of 10-20 nm in diameter, the reaction was carried out for 5 minutes (figure 4a). Figure 4b shows TEM image of the product obtained with sonoelectrochemical processing duration of 15 minutes. The nanorods grow longer and have higher density. A detailed observation of the image shows that many convex areas appeared on the surface of the silver nanorods. These convex areas may further develop into dendrites. It is possible that the excess of silver in the solution favors the aggregation and grows into the dendritic structure of Ag clusters.

SDS molecules are used to disperse the silver nanoparticles and also to stabilize the formation of shaped silver nanoparticles. When the concentration of SDS is low, much larger silver particles are obtained. Figure 5 presents a typical TEM image of the product obtained by the sonoelectrochemical method with 10 g/L AgNO3 and 5 g/L SDS. It shows that the product consists of single crystallites with a rod structure having a width of 150-200 nm in diameter. In the current investigation, we have found that the concentration of AgNO3 and SDS affects a key factor in controlling the size and the

APCTP–ASEAN Workshop on Advanced Materials Science and Nanotechnology (AMSN08) IOP PublishingJournal of Physics: Conference Series 187 (2009) 012077 doi:10.1088/1742-6596/187/1/012077

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shape of Ag nanoparticles. During the reaction, the particles are not completely detached from the cathode by the ultrasonic waves, so that the particles formed in the next pulse can further grow on seed left on the electrode. This growth can promote the formation of different shaped silver particles.

Figure 4a. TEM picture of silver nanorods (5 g/L AgNO3; electrosonication time: 5 minutes).

Figure 4b. TEM picture of silver nanorods and dendritic nanoparticles (5 g/L AgNO3; electrosonication time: 15 minutes).

04-0783 (I) - Silver-3C, syn - Ag - Y: 4.01 % - d x by: 1.000 - WL: 1.54056File: Vu-Trung-CMS-mau Ag-AgNO3-H2O-SDS.raw - Type: 2Th/Th locked - Start: 25.000 ° - End: 70.000 ° - S

Lin

(Cps

)

0

100

200

300

400

500

2-Theta - Scale35 40 50 60 70

d=2.

3595

d=2.

0440

d=1.

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Figure 5. TEM picture of silver rods (10 g/L AgNO3 and 5 g/L SDS; electrosonication time: 15 minutes).

Figure 6. XRD pattern of the silver nanoparticles (5 g/L AgNO3; 40 g/L SDS; electrosonication time: 15 minutes).

A typical XRD pattern of the as-prepared silver nanoparticles is shown in the figure 6. All the

peaks in the XRD pattern can be indexed as a face centered cubic (fcc) structure (JCPDS, file no. 4-0783). XRD pattern shows the presence of the diffraction peaks corresponding to the (111), (200), and (220) planes.

The optical absorption spectra for silver nanoparticles prepared with 15 minutes electrosonication time are shown in figure 7. Aqueous solution of the silver spherical nanoparticles developed a surface plasmon resonance band centered at 401 nm that was characteristic for silver nanoparitcles with a dimension less than 20 nm (curve a). The curves b, c and d are the optical absorption spectra for aqueous solutions of silver rodlike nanoparticles. For the rodlike nanoparticles the absorption spectra exhibit two bands: one at 418 nm and the other at 811 nm (curve c). It is well-known that the size and shape of particles may influence the position and the width of the plasmon resonance absorption band. The red shift of plasmon resonance absorption band of silver particles could be attributed to the increasing size due to the particle aggregation as indicated by the electron microscopy observations. For anisotropic metallic nanoparticles such as nanorods and nanowires, the surface plasmon absorption spectra have two bands: one band at the short wavelength side due to the transverse oscillation of electrons, the other band at the long wavelength side due to the longitudinal oscillation of electrons [7].

APCTP–ASEAN Workshop on Advanced Materials Science and Nanotechnology (AMSN08) IOP PublishingJournal of Physics: Conference Series 187 (2009) 012077 doi:10.1088/1742-6596/187/1/012077

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Figure 7. The UV-vis absorption spectra of silver nanoparticles in aqueous solution with 15 minutes electrosonication time: a) 1 g/L AgNO3 and 40g/L SDS; b) 5g/L AgNO3 and 40g/L SDS, c) 5g/L AgNO3 and 20g/L SDS and d) 5g/L AgNO3 and 10g SDS).

3. Conclusion In summary, the preparation of spherical Ag nanoparticles and Ag nanorods was successfully developed by using a pulse sonoelectrochemical method. By changing the concentration of both AgNO3 and SDS we could control the size distribution and the shape of silver nanoparticles. This present technique can be applied to prepare other metal nanoparticles.

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Acknowledgements This work is supported by the Project QG.07.03, Key Project QGTD.08.05 of Vietnam National University and National Natural Science Foundation of Vietnam.

APCTP–ASEAN Workshop on Advanced Materials Science and Nanotechnology (AMSN08) IOP PublishingJournal of Physics: Conference Series 187 (2009) 012077 doi:10.1088/1742-6596/187/1/012077

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