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Preparation and Characterization of Copper Nanoparticles via the Liquid Phase Plasma Method Heon Lee1, Sung Hoon Park1, Seong-Gyu Seo2, Sun-Jae Kim3, Sang-Chai Kim4, Young-Kwon Park5 and Sang-Chul Jung1* 1
Dept. of Environmental Engineering, Sunchon National University, Sunchon, Jeonnam 540-742, Republic of Korea; 2Dept. of Civil & Environmental Engineering, Chonnam National University, Yosu 550-749, Republic of Korea; 3Faculty of Nanotechnology and Advanced Materials Eng., Sejong University, Seoul 143-747, Republic of Korea; 4Dept. of Environmental Eduactaion, Mokpo National University, Muan, Jeonnam 534-729, Republic of Korea; 5School of Environmental Engineering, University of Seoul, Seoul 130-743, Republic of Korea Abstract: Polycrystalline copper nanoparticles were synthesized from copper chloride dihydrate solution using the liquid phase plasma reduction method. A bipolar pulsed power supply with tungsten electrodes was used to generate discharge in the aqueous solutions. While large size of dendrite-shaped copper nanoparticles were mostly observed in the initial stage and particle size decreased with discharge time. The particles were dispersed with less and less small particles by the addition of CTAB and anisotropic shapes nanoparticles were mostly observed at long time plasma-treated with high concentration of surfactant. Many spots could be seen in the selected area diffraction pattern (SADP) for polycrystalline particles.
Keywords: Bipolar pulsed discharge, copper, liquid phase plasma, nanoparticle, surfactant. 1. INTRODUCTION In the past few years, metal nanoparticles have been intensively prepared and characterized for their extensive applications in catalysis, electrooptical devices, electronic devices, imaging materials [1-5]. Among various metal particles, copper nanoparticles have attracted considerable attention because of their catalytic, optical, and electrical conducting properties. Fabrication of nanoparticles has become one of the important topics in nanotechnology [6, 7]. Accordingly, for that purpose it is very important to be able to control the particle size, shape and size distribution of metal nanoparticles. In recent years, several methods were reported to synthesize copper nanoparticles: thermal decomposition method [8], pulsed sonoelectrochemical synthesis [9], laser ablation method [10], radiation method [11-13], microemulsion technique [14, 15], polyol method [16], and chemical reduction [17, 18]. Recently, synthesis of nanoparticles using liquid phase plasma (LPP), in which metal ions are reduced to zero-valent metal particles in a bipolar pulsed electrical discharge system, has been reported [19-21]. The so-called “LPP reduction method” is reportedly an attractive method for nanoparticle synthesis in that the composition, size and morphology of the particles synthesized can be controlled easily. The LPP reduction method has been applied successfully to the synthesis of gold [19, 20] and silver [21] nanoparticles. Therefore, the present study was motivated by the expectation that the LPP reduction method can also be used to synthesize copper nanoparticles. The effects of the concentrations of copper salt and surfactant and discharge time on the size and morphology of the nanoparticles generated were investigated. 2. EXPERIMENTAL DETAILS Fig. (1) shows the schematic of the liquid-phase bipolar pulsed electrical plasma discharge system used in this study. Pulsed electrical discharge was generated in liquid by a high-frequency bipolar pulse power supply (Nano technology lnc., NTI-500W). One can *Address correspondence to this author at the Dept. of Environmental Engineering, Sunchon National University, Sunchon, Jeonnam 540-742, Republic of Korea; E-mail:
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Fig. (1). Schematic of the experimental system with liquid-phase bipolar pulsed electrical discharge.
refer to our previous paper published recently [21], in which the same system was used to synthesize silver nanoparticles, for the explanation of the bipolar pulsed electrical plasma discharge system and the detailed experimental procedures. The only difference between the previous and this studies in terms of the system setting was that the interelectrode gap was 0.3 mm in this study, whereas it was 0.2 mm in the previous study. The applied voltage, pulse width, and frequency were 250 V, 5 s, and 30 kHz, respectively. Fig. (2) shows a snapshot of the tungsten electrodes in the LPP reactor with the plasma off (2a) and on (2b). When the plasma was on, glare appeared between the electrodes, above which bubbles were observed. 300 ml of 2-mM CuCl2 solution was prepared by dissolving copper chloride tetrahydrate (CuCl22H2O, Junsei chemical Co. Ltd) in ultrapure water (Daejung Chemicals & metals Co., Ltd). The LPP process was used to fabricate Cu nanoparticles from the solutions containing Cu ions. To prevent coagulation among the Cu nanoparticles, cetyltrimethylammonium bromide (CTAB, CH3 (CH2)15N(CH3)3Br) was used as a surfactant. Transmission Electron
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Fig. (2). Snapshot of the tungsten electrodes with (a) plasma off and (b) plasma on in the LPP reactor.
Microscopy (TEM, Tecnai20) was used to observe the morphology of the copper nanoparticles generated. 3. RESULTS AND DISCUSSION 3.1. Characterization of the Optical Emission Spectra Fig. (3) shows the optical emission spectra measured by AvaSpec-3648 Fiber optical spectrometer (Avantes) during the electrical discharge with ultrapure water (a) and CuCl2 solution (b). The discharge voltage, frequency, and pulse width were set to 250 V, 30 kHz, and 5 s, respectively. It is expected that many kinds of chemically active species would be generated in LPP by the electrical discharge system. Therefore, it is necessary to identify those active species and to investigate their effects. Excited states of atomic copper (324.8, 510.8, 515.5, 522.1, 570.0 and 578.5 nm), atomic hydrogen (656.3 and 486.1 nm), atomic oxygen (777 and 844 nm), and hydroxyl radical (283 and 309 nm) were observed. Fig. (3) suggests that copper nanoparticles were produced by the reaction between CuCl2 and the activated radicals generated in LPP.
Fig. (4). TEM images of Cu nanoparticles synthesized using electrical discharge in aqueous solution with different discharge times of a) 5 min, b) 10 min, c) 20 min and d) 30 min (Scale bar size: 50 nm).
nanoparticles of about 1 ~ 5 nm size appeared (4c). When the discharge time was 30 min (4d), spherical nanoparticles with the size of 5 ~ 15 nm were observed. Fig. (5) shows high-resolution TEM images and SADP of copper nanoparticles. The reaction solution with the initial CuCl2 concentration of 2 mM was plasma-treated for 30 min with 50% CTAB added. The images clearly indicated the formation of a roundedshape phase with the lattice fringes. The measured distance of 1.85 Å between the adjacent lattice fringes can be. The SADP shows that diffraction points corresponding to the crystal direction of the lattice are observed at various directions indicating polycrystalline structure. In the meantime, the diffraction points did not comprise in complete circles, which indicates that nanoparticles are mixed with much larger particles. An EDX spectrum was obtained from the same copper nanoparticle sample as that used for the TEM analysis shown in Fig. (5). On top of the peaks for copper, peaks for tungsten were observed, which is attributed to the employment of tungsten electrodes. The atomic %’s of copper and tungsten contained in the nanoparticles were 97.23 % and 2.77 %, respectively.
Fig. (3). Spatially and temporally integrated emission spectra for the discharge with ultrapure water (a) and CuCl2 aqueous solution (b).
3.2. Preparation of Copper Nanoparticles The TEM images of the copper nanoparticles produced with different discharge times are compared in Fig. (4). The initial CuCl2 concentration was 2 mM, with 50% CTAB added. When the discharge time was 5 min (4a), dendrite-shaped nanoparticles with the bulk size of about 70 nm were observed. When the discharge time was 10 min (4b), the size of the dendrite-shaped nanoparticles became smaller, whereas the particle number increased. On the other hand, the size of the nanoparticles synthesized by a discharge running for 20 min significantly decreased, and well dispersed Fig. (5). TEM images Cu nanoparticles and corresponding SADP.
Preparation and Characterization of Copper Nanoparticles
3.3. Effect of Surfactant on Size Distribution of Copper Particles The TEM images of the copper nanoparticles produced with different surfactant (CTAB) doses are compared in Fig. (6). The surfactant/CuCl2 molar ratio was varied between 0 and 50%. The initial CuCl2 concentration was 2 mM and the discharge time was 30 min. When CTAB was not added, dendrite-shaped copper nanoparticles were observed (Fig. 6a). When 10% CTAB was added, similar dendrite-shaped nanoparticles with smaller size were observed (Fig. 6b). When 30% CTAB was added (Fig. 6c), welldispersed copper nanoparticles with the diameter of 10 ~ 50 nm were observed. When 50% CTAB was added (Fig. 6d), spherical nanoparticles with the diameter of 5~15 nm were observed. To sum up, relatively large aggregate particles were generated and the degree of dispersion was low when 10% or less CTAB was added, whereas well-dispersed nanoparticles were generated when 30% or more CTAB was added.
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Fig. (7). TEM images of wisker-shaped, tetragonal and pentagonal copper nanoparticles synthesized with electrical discharge in aqueous solution.
smaller in size. Therefore, the rate of reduction of [CuCl2] ion declines. Copper nanoparticles with anisotropic shapes form in the solution, which are derived from the dissolved [CuCl2] ion, and grow at a low rate, whereas spherical nanoparticles continue to dissolve in the solution. It is suggested that the dissolution of copper nanoparticles depends on their shape. 4. CONCLUSIONS Copper nanoparticles were synthesized using the LPP reduction method under a wide range of conditions. Following conclusions were deduced from the results of this study: 1) Excited states of atomic copper, atomic hydrogen, atomic oxygen, and hydroxyl radical were observed in the optical emission spectra measured during the electrical discharge. 2) While large size of dendrite-shaped copper nanoparticles were mostly observed in the initial stage and particle size decreased with discharge time. 3) The copper nanoparticles were well dispersed with less and less small particles by the addition of CTAB. 4) While spherical copper nanoparticles were mostly observed but anisotropic (wisker, tetragonal and pentagonal) shaped nanoparticles were often observed at long time plasmatreated with high concentration of surfactant. 5) The ED pattern shows that diffraction points corresponding to the crystal direction of the lattice are observed at various directions indicating polycrystalline structure.
Fig. (6). TEM images of Cu nanoparticles synthesized using electrical discharge in aqueous solution with different CTAB doses of a) 0 %, b) 10 %, c) 30 % and d) 50 % (Scale bar size: 50 nm).
Fig. (7) shows TEM images of wisker-shaped, tetragonal and pentagonal nanoparticles generated from the aqueous solution by LPP. While spherical copper nanoparticles were mostly observed but anisotropic shaped nanoparticles were often observed at long time plasma-treated with high concentration of surfactant. Spherical particles are generated when the surface energies of various crystal faces do not differ from one another, which is the case for very small particles. Surface energy stems from the breakage of interatomic bonds of the atoms located at the surface. When a particle is small, the value of surface energy does not depend significantly on the crystal orientation. As a particle grows, however, total surface energy can vary considerably depending on the crystal orientation. In the initial stage, [CuCl2] ion is reduced and dendrite-shaped nanoparticles are simultaneously synthesized by discharge in the solution. In the formation of nanoparticles during this stage, nucleation reaction drastically proceeds. These nanoparticles would be composed of nanoclusters with very small size. Then, the pH of the solution gradually decreases with increasing the discharge time under the influence of the discharge. Copper nanoparticles dissolve in the solution at low pH values, and the nanoparticles become
CONFLICT OF INTEREST None. ACKNOWLEDGEMENTS This research was financially supported by the Ministry of Knowledge Economy (MKE), Korea Institute for Advancement of Technology (KIAT) through the Inter-ER Cooperation Projects. REFERENCES [1]
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Received: April 30, 2012
Revised: February 14, 2013
Accepted: April 30, 2013
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