Introduction

Metallic Cu nanoparticles are attractive materials fundamentally and industrially because they give unique properties over catalysis, electronics, and photoelectronics [1–6], and their cost is low compared with other metallic materials such as gold and silver. The practical use of Cu nanoparticles faces at the main difficulty that arises from their instability toward oxidation in air. In addition, colloidal instability of nanoparticles brings about aggregation of the nanoparticles, which spoils the unique properties of nanoparticles. Various methods for stabilizing the Cu nanoparticles have been reported. Several researchers have proposed the use of nonaqueous liquid such as organic compounds as solvent or dispersing media, which does not dissolve oxygen gas and prevents it from contacting Cu nanoparticles, so that the Cu surface oxidation would be minimized [7–11]. Though their methods could avoid oxidation of Cu nanoparticles, harm and hazard of organic compounds are matter of concern.

Protection of Cu nanoparticles with a stabilizer such as surfactant can be given as another stabilization method, and various methods using the stabilizer have been previously reported [4, 12–15]. The nanoparticles are surrounded with the stabilizer by hydrophilic interaction between the nanoparticles and the stabilizer and/or coordination of functional groups of the stabilizer to metal atoms. However, the stabilizer may not prevent oxidation and aggregation of nanoparticles enough because of their molecular motion.

Coating of metallic nanoparticles can also be another approach for the stabilization because physical barrier of coating materials will prevent the nanoparticles from contacting oxygen molecules and aggregating and control interparticle interactions. Several works on coating with silica have been reported [16–22]. The silica coating has the following advantages: exceptional stability of aqueous dispersions and easy surface modification that allows the preparation of nonaqueous colloids. We have developed a technique for silica coating of Co nanoparticles prepared through reduction of cobalt inorganic salts in aqueous solution [23]. The nanoparticles were chemically stable even after being heated at high temperature in air. In addition, we also performed silica coating of metallic Cu nanoparticles prepared from aqueous solutions of inorganic salts, and oxidation of the nanoparticles did not progress even 1 month after preparation [24].

Polymers have been also used for coating metallic nanoparticles. Mandal et al. fabricated polymethylmethacrylate-coated Au nanoparticles by living radical polymerization [25]. The polymerization was confined on the Au particle surface because of Br termination of the Au particles. Kang and Taton reported a study on polystyrene-block-poly(acrylic acid) coating of Au nanoparticles using interaction between amphiphile of the block copolymer and nanoparticle surface [26]. Structure of the coated particles was varied with a relative ratio of the amount of polymer to available surface area of Au particles. Kim and Lee proposed a method for hydrogel coating of Au nanoparticles by surfactant-free emulsion polymerization of N-isopropylacrylamide and acrylic acid [27]. The produced particles revealed a response to temperature and pH. Quaroni and Chumanov prepared polystyrene/methacrylate-coated Ag nanoparticles via emulsion polymerization [28]. The obtained particles could adsorb protein (bovine serum albumin) due to carboxylate functionality derived from methacrylic acid monomers. Apart from these polymers, electro-conducting polymers such as polypyrrole (PPy) and polyaniline (PA) have also been used as shell materials. In the electro-conducting polymers, charge transfer might be expected at the interface, and the optical functions of both materials should be quite different from the corresponding bulk materials because the conducing polymers provide an effective route for the flow of electronic charges [29, 30]. Liu and Chuang synthesized Au nanoparticles coated with PPy by a simple mixing of pyrrole (Py) monomer and AuCl −4 -containing aqueous solution, in which polymerization of PPy took place through reduction of AuCl −4 to elemental Au [31]. Ye and Lu fabricated PPy-coated Ag nanoparticles with polymerization of PPy using FeCl3 as an initiator [32]. Optical properties of the produced particles were investigated on the basis of the classical Mie theory. Du et al. proposed a method for preparing Ag nanoparticles coated with PA with a reaction between AgNO3 and aniline [33]. According to a series of comparative experiments, it was found that both the reaction temperature and AgNO3 concentration had a profound effect on macrostructure of particles.

In the present paper, we propose an alternative method for polymer coating of Cu nanoparticles, which stabilizes Cu nanoparticles chemically. Cu ions were reduced with hydrazine in the presence of citric acid and CTAB to generate Cu metallic nanoparticles, which were PPy-coated with polymerization of Py in the presence of Cu nanoparticles.

Experimental

Chemicals

Copper chloride dihydrate (CuCl2·2H2O; >99%) was used as copper precursors. Cetyltrimethylammonium bromide (CTAB; 99%) and citric acid monohydrate (>99.5%) were used as stabilizers. Hydrazine monohydrate (>98.0%) was used as a reducing reagent. Py (>99%) and hydrogen peroxide (H2O2; 30.0–35.5%) were used for PPy coating. HCl solution (35.0–37.0%) was used for adjusting pH of solution in the PPy coating. All chemicals were purchased from Kanto Chemical and were used as received. Water that was ion-exchanged and distilled with Shimadzu SWAC-500 was used in all the preparations and was deaerated by bubbling with N2 gas for 30 min prior to preparation of aqueous solutions of CuCl2 and hydrazine.

Preparation

Colloids of Cu nanoparticles were prepared by mixing of CuCl2 and hydrazine. Freshly prepared 10 ml of 2 M hydrazine aqueous solution containing 0.0005 M citric acid and 0.005 M CTAB was added to 10 ml of 0.02 M CuCl2 aqueous solution containing 0.0005 M citric acid and 0.005 M CTAB under vigorous stirring at room temperature. The reaction time was 3 h. A dark-red and muddy colloid solution was produced, as shown in Fig. 1a.

Fig. 1
figure 1

Colloid solutions of Cu nanoparticles (a) and Cu/PPy particles (b)

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PPy coating for the Cu nanoparticles was performed by polymerization of Py in the presence of the Cu nanoparticles. Ten microliters of the HCl solution was added to 10 ml of the Cu colloid. Then, 10 ml of 0.17 M Py solution and 0.32 ml of 12.4 M H2O2 was added to the colloid in turn. The reaction time for the PPy coating was 24 h. The obtained colloid solution is shown in Fig. 1b. A dark-red colloid solution was also produced.

Characterization

The PPy-coated Cu (Cu/PPy) nanoparticles were characterized by ultraviolet (UV)–visible (VIS) spectroscopy, X-ray diffractometry (XRD), and transmission electron microscopy (TEM). UV–VIS extinction spectra were measured with a Hitachi UV-3010 spectrophotometer using a quartz cuvette with a light-path length of 1 cm. Samples for UV–VIS spectroscopy were prepared by diluting as-prepared colloid solution with water by 2.5 times. An XRD chart was recorded with an X-ray diffractometer (Rigaku RAD-C) operated at 20 kV and 40 mA with CuKα radiation. Samples for XRD were prepared by centrifugation of the colloid solutions at 12,000 rpm for 1 h, removal of supernatant, and drying sediment. TEM was performed with a JEOL JEM-2000FX II microscope operated at 200 kV. Samples for TEM were prepared by dropping and evaporating the nanoparticle suspensions on a collodion-coated copper grid.

Results and discussion

Cu nanoparticles

Figure 2 (a) gives extinction spectrum of Cu nanoparticle colloid. The background intensity of the solution itself was as high as ca. 1, which was derived from the muddiness of the colloid solution. This result provided speculation that aggregates or flocculates, which scattered UV–VIS light, were produced in the solution. A peak was observed at 577 nm on the background. According to Refs. [12, 16, 34], the peak was attributed to surface plasmon resonance of Cu nanoparticles, which provided evidence for not only the production of aggregates or flocculates but also generation of Cu particles. Figure 3a shows TEM image of the Cu nanoparticles. Submicron-sized aggregates composed of particles irregular in shape were observed. This observation indicated that the production of aggregates resulted in the strong background intensity shown in Fig. 2 (a). Figure 4 (a) shows XRD pattern of the Cu nanoparticles. The pattern showed peaks at 43.3, 50.5, and 74.1 degree, which were attributed to those of metallic Cu (JCPDS card no. 4-0836). Besides the metallic Cu peaks, several peaks, which were assigned to Cu2O (JCPDS card no. 5-0667), were strongly detected at 36.7, 42.4, and 61.4 degree.

Fig. 2
figure 2

UV–VIS extinction spectra of colloid solutions of Cu nanoparticles (a) and Cu/PPy particles (b)

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Fig. 3
figure 3

TEM images of Cu nanoparticles (a) and Cu/PPy particles (b)

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Fig. 4
figure 4

XRD patterns of Cu nanoparticles (a) and Cu/PPy particles (b). (c) stands for a pattern of Cu/PPy particles, which was measured 50 days after preparation. Circles metallic Cu, triangles Cu2O

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Cu/PPy particles

Figure 2 (b) shows extinction spectrum of Cu/PPy particle colloid. Extinction of the spectrum gradually increased as the wavelength became shorter, and a peak due to the surface plasmon resonance was not observed. Figure 3b shows TEM image of the Cu/PPy particles. Darker and lighter parts of particles correspond to copper and PPy, respectively, since the electron density of copper is significantly higher. Cu nanoparticles were coated with PPy shell (high magnification images in Fig. 3b). Some Cu nanoparticles incorporated in large gel-like structure were also observed. The Cu particle size was 27.6 ± 11.1 nm. Figure 4 (b) shows XRD pattern of Cu/PPy particles. The pattern was measured a few days after preparation of the particles. The pattern gave clear peaks attributed to metallic Cu appeared at 43.3, 50.5, and 74.2 degree. The UV–VIS measurement (Fig. 2 (b)) could not provide evidence of production of metallic copper since no peak due to the surface plasmon resonance was observed. Strong scattering from the PPy shell possibly screened the surface plasmon peak. Figure 4 (c) shows XRD pattern of Cu/PPy particles, which was measured 50 days after preparation of the particles. The XRD pattern did not change even 50 days after preparation. This indicated that the PPy coating prevented the Cu nanoparticles from being oxidized even in air, compared with the uncoated Cu nanoparticles, which is evidence of the protection that the PPy exerts against oxidation of the Cu core. Thus, quite stable Cu nanoparticles could be fabricated with PPy coating.

Summary

A method for PPy coating of metallic Cu nanoparticles in aqueous solution was proposed. To coat the Cu nanoparticles with PPy, polymerization of Py was performed in the presence of the Cu nanoparticles, which were prepared using hydrazine, citric acid, and CTAB in water in air. The present method fabricated PPy-coated metallic Cu nanoparticles, which were chemically stable even in air, compared to uncoated Cu nanoparticles.