Journal of Colloid and Interface Science 430 (2014) 47–55

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Journal of Colloid and Interface Science www.elsevier.com/locate/jcis

Electroless nickel plating on polymer particles Syuji Fujii a,⇑, Hiroyuki Hamasaki a, Hiroaki Takeoka a, Takaaki Tsuruoka b, Kensuke Akamatsu b, Yoshinobu Nakamura a,c a

Department of Applied Chemistry, Faculty of Engineering, Osaka Institute of Technology, 5-16-1 Ohmiya, Asahi-ku, Osaka 535-8585, Japan Department of Nanobiochemistry, Frontiers of Innovative Research in Science and Technology (FIRST), Konan University, 7-1-20 Minatojimaminami, Chuo-ku, Kobe 650-0047, Japan c Nanomaterials Microdevices Research Center, Osaka Institute of Technology, Japan b

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Article history: Received 31 March 2014 Accepted 22 May 2014 Available online 29 May 2014 Keywords: Electroless plating Nickel Polymer particles Dispersibility Nanocomposite Core/shell morphology

a b s t r a c t Near-monodisperse, micrometer-sized polypyrrole–palladium (PPy–Pd) nanocomposite-coated polystyrene (PS) particles have been coated with Ni overlayers by electroless plating in aqueous media. Good control of the Ni loading was achieved for 1.0 lm diameter PPy–Pd nanocomposite-coated PS particles and particles of up to 20 lm in diameter could also be efficiently coated with the Ni. Laser diffraction particle size analysis studies of dilute aqueous suspensions indicated that an additional water-soluble colloidal stabilizer, poly(N-vinyl pyrrolidone), in the electroless plating reaction media was crucial to obtain colloidally stable Ni-coated composite particles. Elemental microanalysis indicated that the Ni loading could be controlled between 61 and 78 wt% for the 1.0 lm-sized particles. Scanning/transmission electron microscopy studies revealed that the particle surface had a flaked morphology after Ni coating. Spherical capsules were obtained after extraction of the PS component from the Ni-coated composite particles, which indicated that the shell became rigid after Ni coating. X-ray diffraction confirmed the production of elemental Ni and X-ray photoelectron spectroscopy studies indicated the existence of elemental Ni on the surface of the composite particles. Ó 2014 Elsevier Inc. All rights reserved.

1. Introduction Electroless plating is a method for the deposition of metals such as nickel and copper onto an insulating substrate (e.g., plastics) via catalyzed chemical reduction of solution-phase metal ions at the substrate surface [1]. In order to form interconnects in printed circuit boards, electroless nickel plating has been widely utilized [2]. In contrast to electroplating where an applied current is needed to reduce a high-oxidation-state metal precursor, the basis of electroless plating is an autocatalytic redox reaction [3]. The important material prerequisite for electroless plating is the presence of an appropriate catalytic surface. There are only a limited number of reports regarding electroless nickel plating on colloidal polymer

Abbreviations: PPy, polypyrrole; Pd, palladium; PS, polystyrene; SEM, scanning electron microscopy; TEM, transmission electron microscopy; EDX, energy dispersive X-ray spectroscopy; XPS, X-ray photoelectron spectroscopy; XRD, X-ray diffractometry; TGA, thermogravimetric analysis; PdCl2, palladium(II) chloride; PNVP, poly(N-vinyl pyrrolidone); NaH2PO2H2O, sodium phosphinate monohydrate; IPA, isopropanol; AIBN, 2,20 -azobis(isobutyronitrile); NaCl, sodium chloride; Dv, volume-average diameter; Dn, number-average diameter; Cv, coefficient of variation. ⇑ Corresponding author. E-mail address: [email protected] (S. Fujii). http://dx.doi.org/10.1016/j.jcis.2014.05.041 0021-9797/Ó 2014 Elsevier Inc. All rights reserved.

particles, although nickel-coated polymer particles have found applications as effective additives for electromagnetic absorbing materials and conducting spacers for electronic devices [4–6]. Tierno and Goedel successfully prepared poly(methyl methacrylate) particles coated by various Ni-based metallic shells (NiP, NiFeP and CoNiP) [4]. Their synthesis method consisted of nine steps including purification procedures: activation of seed particles using HCl and then SnCl2 followed by PdCl2 and electroless Ni plating, with annealing and purification between each step. Sanles-Sobrido et al. fabricated Ni-coated submicrometer-sized polystyrene (PS) particles by the electroless plating method [5]. Their synthesis procedure involved thirteen steps including purifications: functionalization of the PS seed particles with four polyelectrolyte layers, subsequent attachment of Pt nanoparticles onto their surface, and reduction of Ni2+ ions on the surface favored by the Pt nanoparticles that behave as catalytic centers. Li et al. succeeded in preparation of PS/polypyrrole (PPy)/Ni composite particles by activation-electroless plating technology [6]. Their synthesis method was simple, but still required six steps including purifications: coating PS seed particles with a copolymer consisting of pyrrole and N-2-carboxyethyl pyrrole, absorption of Pd2+ followed by reduction to form Pd nanoparticles on the surface, and electroless plating. In the studies mentioned above, little char-

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S. Fujii et al. / Journal of Colloid and Interface Science 430 (2014) 47–55

Scheme 1. Electroless nickel plating on PPy–Pd nanocomposite-coated PS latex particles.

acterization of the particles in the dispersed state was conducted, and there were no data on colloidal stability. This is rather surprising, since nickel-coated particles can be used as a dispersion in many cases. Furthermore, there was no investigation on electrical conductivity of the Ni-coated particles. Recently, we reported a facile procedure for the one-step coating of polymer particles with a polypyrrole–palladium (PPy–Pd) nanocomposite by chemical oxidative seeded dispersion polymerization of pyrrole using palladium(II) chloride (PdCl2) as an oxidant in aqueous media [7–9]. Here, PdCl2 acted both as an oxidant and as a source of metal atoms, yielding conducting polymer–metal nanocomposites in one step. We have also shown that these composite particles functioned as an efficient catalyst for Suzuki– Miyaura cross coupling reactions and aerobic oxidative homocoupling reactions. Pd nanoparticles dispersed in PPy matrix worked as a catalyst for these organic reactions [7–10]. In this study, electroless nickel plating was conducted on PPy–Pd nanocomposite-coated polymer particles using the Pd nanoparticles in the shell as a catalyst (Scheme 1). The synthesis method proposed in this study only requires four steps including purification procedures, and is advantageous because production on an industrial scale is much more likely compared to previous routes. The resulting nickel-coated particles have been extensively characterized in terms of their colloidal stability, particle size, size distribution, morphology, surface and bulk compositions, and electrical conductivity using a wide range of analytical techniques such as optical microscopy, laser diffraction particle size analysis, scanning/transmission electron microscopy (SEM/TEM), energy dispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), X-ray diffractometry (XRD), elemental analysis, thermogravimetric analysis (TGA) and conductivity measurements.

apparatus (Advantec MFS RFD240NA: GA25A-0715) and was used for syntheses and purification of particles. PS particles with 20 lm diameters were purchased from Microbeads, Norway. Optical microscopy and laser diffraction particle analysis confirmed that a stable aqueous dispersion of seed PS particles was obtained simply by mixing the dried PS seed particles and water (Fig. S1). The anionic sodium dodecyl sulfate and cellulosic stabilizer absorbed on the particle surface gave high dispersion stability to the seed particles [11]. The volume-average diameter (Dv) of the PS seed particles was measured as 22 ± 5.6 lm by laser diffraction particle analysis. A typical SEM image of the PS seed particles is shown in the Supporting Information, from which the numberaverage diameter (Dn) and the coefficient of variation (Cv) were estimated to be 21 lm and 8.4%, respectively (over 100 particles were counted). 2.2. Synthesis of PS particles The dispersion polymerization of styrene was performed in the presence of the PNVP steric stabilizer in batch mode at 70 °C using an AIBN initiator as described previously [7]. A typical synthesis protocol was as follows. PNVP (2.0 g; 10 wt% based on styrene) was added to IPA (200 mL) in a round-bottomed 500 mL flask with a magnetic stirrer bar and stirred vigorously at 70 °C until the PNVP had dissolved completely, followed by degassing with a nitrogen purge. Polymerization commenced after the addition of 0.20 g of AIBN dissolved in 20.0 g of styrene. The reaction was allowed to proceed for 24 h with continuous stirring at 250 rpm under a nitrogen atmosphere. The latex particles were then purified by repeated centrifugation–redispersion cycles, replacing successive supernatants with deionized water. The resulting PNVP stabilized PS latex particles were subsequently used as seed particles for deposition of the PPy–Pd nanocomposite.

2. Experimental 2.3. Deposition of PPy–Pd nanocomposite onto PS particles 2.1. Materials Unless otherwise stated, all materials were guaranteed reagent grade. Poly(N-vinyl pyrrolidone) (PNVP, nominal molecular weight 360,000), palladium(II) chloride (PdCl2, 99.9%), indium shot (99.9999%) and sodium phosphinate monohydrate (NaH2PO2H2O, 82.0–86.5%) were obtained from Wako Chemicals. Isopropanol (IPA, SAJ first grade, P99.0%), 2,20 -azobis(isobutyronitrile) (AIBN, 98%), sodium chloride (NaCl, 99.5%), aluminum oxide (activated, basic, Brockmann l, standard grade, 150 mesh, 58 Å), tetrahydrofuran (THF, >99.0%), nickel(II) sulfate hexahydrate (P99.99% metals basis) and sodium acetate (99%) were obtained from Sigma–Aldrich and were used without further purification. Styrene (P99%) and pyrrole (98%) were also obtained from Sigma–Aldrich and purified by passing through a column of activated basic alumina prior to storage at 15 °C before use. Deionized water (