Synthesis of novel magnetic spheres by electroless nickel coating of polymer spheres

Synthesis of novel magnetic spheres by electroless nickel coating of polymer spheres

Surface & Coatings Technology 200 (2005) 2531 – 2536 www.elsevier.com/locate/surfcoat Synthesis of novel magnetic spheres by electroless nickel coati...

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Surface & Coatings Technology 200 (2005) 2531 – 2536 www.elsevier.com/locate/surfcoat

Synthesis of novel magnetic spheres by electroless nickel coating of polymer spheres Hong-xia Guoa,*, Zhen-ping Qinb, Jie Weic, Chang-xi Qind a Department of Chemical Engineering, Tsinghua University, Beijing 100084, PR China College of Environmental and Energy Engineering, Beijing University of Technology, Beijing 100022, PR China c School of Materials Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, PR China d Institute of China Lucky Film Corp., Baoding 071054, PR China b

Received 8 June 2004; accepted in revised form 10 April 2005 Available online 3 June 2005

Abstract A grid of micrometer-sized core-shell particles was fabricated by electroless plating technique. The core-shell particles consist of a PMMA core covered with a shell of nickel – phosphorus alloy component. The morphology and structure of the as-synthesized products were characterized by OM, TEM, XRD and EDS. The magnetic property of the particles was measured by an Alternating Gradient Magnetometer (AGM). The results showed that the less uniform coverage of magnetic shell was formed as nickel – phosphorus other than pure nickel component during electroless procedure. The as-prepared particles exhibited magnetism and showed magnetic response by being formed into chain structures under an external magnetic field. D 2005 Elsevier B.V. All rights reserved. Keywords: Magnetic spheres; Electroless; Coating

1. Introductions The control and selective surface modification of colloidal particles allows the fabrication of composite materials with tailored and unique properties for various applications in the areas of coatings, electronics, photonics, catalysis, sensing and separations [1 –5]. Composite particles that contain an inner core covered by a shell (core-shell particles) exhibit significant different properties from those of the core itself (for example, surface chemical composition, increased stability, higher surface area, as well as distinct magnetic and optical properties) [6,7]. The surface properties are governed by the characteristics of the shell coating. The surface engineering of particles is to produce core-shell particles, where the core consists of a solid or liquid coated by a number of methods. Recent efforts to produce uniformly coated colloidal particles in solution have relied on a number of different * Corresponding author. Tel.: +86 19 62782432; fax: +86 10 62770304. E-mail address: [email protected] (H. Guo). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.04.021

approaches [8– 15]. Inorganic and hybrid coatings (or shells) on polymeric and inorganic particles on both nanometer and micrometer scale have been commonly prepared by precipitation of the coating materials onto the cores or by direct surface reactions utilizing specific functional groups on the core to induce coating [8 –11]. Alternatively, particles can be coated by layer-by-layer deposition [12 – 15] of charged species utilizing electrostatic interactions between alternately deposited, oppositely charged species. Colloidal particles with magnetic properties have become increasingly important both technologically and for fundamental studies due to the tunable anisotropic interaction they exhibit. Also, they have many potential applications like magnetic recording media, ferrofluid technology, magnetocaloric refrigeration, biomedicine, and catalysis [16 – 19]. Furthermore, magnetic colloidal suspensions have been used as building blocks for the formation of ordered patterns via manipulation using magnetic fields [20,21]. Recently, reports have been published on the deposition of magnetite nanoparticles on submicrometer colloid particles using layerby-layer (LbL) assembly technique [20,22]. However, the

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limitation of the LbL strategy is the time-consuming sequential polyelectrolyte deposition cycles and purification steps. We have introduced the metallic nickel component into porous silica cores using electroless plating to form composite particles with response to external electric and magnetic fields [23]. In the present work, we describe the electroless plating procedure to deposit nickel nanoparticles onto micrometer polymethylmethacrylate (PMMA) spheres, which has recently been used for decoration of carbon nanotubes [24,25], but has not been implemented on polymer spheres to our knowledge. This will result in imparting a magnetic function to the polymer particles, which have less density than inorganic ones [26]. Electroless plating method is used, as the surface of the core spheres need not be charged; also, the polymer spheres can be ultrasonically dispersed in plating solution. The purpose of choosing PMMA polymer sphere as a core material lies in the advantages, i.e., the facile synthesis of monodisperse PMMA particles and the probability as a sacrificial template to be removed to produce the hollow nickel capsules, which will extend its applications.

2. Experimental section 2.1. Procedures 2.1.1. Preparation of PMMA polymer spheres Monodispersed PMMA polymer spheres were prepared by dispersion polymerization, which comprises the 2,2-

Table 1 Bath composition and operating conditions of electroless nickel coating Chemicals

Concentration (molIl 1)

NiSO4I6H2O Na3C6H5O7I1.5H2O NaH2PO2I2H2O NH4Ac Pb(NO3)2 pH Bath temperature

0.114 0.054 0.220 0.32 7.0  10 3 7.2 35 -C

azobis(isobutyronitrile) (AIBN) catalyzed polymerization of methyl methacrylate (MMA) in a water – ethanol solution under N2 atmosphere in the presence of polyvinylpyrrolidone (PVP, K-30) as the stabilizer. Typically, 58.46 ml distilled water and 21.54 ml ethanol were mixed into 250-ml three-necked glass vessel equipped with condenser, stirrer, nitrogen inlet. After adding 20 ml of MMA, 0.08 g of AIBN, 0.2 g of PVP into the solution, the vessel was immersed in a thermostated water bath and stirred with the speed of 400 rpm under nitrogen. The polymerization was carried out at 60 -C for 24 h. Then the obtained spheres were centrifuged. The supernatant was then decanted and the remaining precipitate was repeatedly washed in the distilled water and dried in vacuo at ambient temperature overnight. Therefore, PMMA particles are 1.88 Am in diameter obtained. FT-IR analysis of the particle showed the typical spectra of PMMA polymer.

Scheme 1. Schematic diagram of nickel electroless plating: (a) PMMA particles were pretreated by acid SnCl2 solution; (b) the Sn2+-treated PMMA particles were activated by PdCl2 solution; (c) electroless plating procedure.

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Fig. 1. The optical microscope of the (a) PMMA particles and (b) PMMA/Ni particles.

2.1.2. Nickel nanoparticles coating on PMMA polymer spheres Nickel was coated onto PMMA polymer beads by electroless plating. The procedure used is illustrated in Scheme 1. The known amount of parent PMMA particles were first ‘‘sensitized’’ using an acid SnCl2 solution (0.1 M SnCl2/0.1 M HCl) ultrasonically for 40 min. This resulted in adsorption of Sn2+ ions onto all the surface of the parent spheres. Then the mixture was washed with deionized water. The supernatant was discarded centrifugally. After removal of the excess Sn2+ ions from solution through cleaning, the Sn2+ ion sensitized particles were then immersed into a palladium chloride (PdCl2) hydrochloride acid (1.4  10 3 M PdCl2/0.25 M HCl) solution. A surface redox reaction occurred, involving Sn2+ reduction of the Pd2+ to Pd and the oxidation of Sn2+ to Sn4+. This step resulted in creating nanoscopic metallic Pd particles as catalytic sites on the PMMA particles surfaces. The Pd-modified particles were rinsed again using de-ionized water and introduced into an electrolessplating solution bath. The composition of the plating solution and the reaction condition are given in Table 1. The Pd catalytic sites created on the polymer sphere

surfaces allow the second surface redox reaction to begin as follows [27]:  þ Ni2þ þ 2H2 PO 2 þ H2 O ¼ Ni þ 2H3 PO3 þ 2H  H2 PO 2 þ H2 O ¼ H3 PO3 þ H2

The first ions of nickel, which were reduced, aggregate on the polymer particle surface to form the nuclei. The onset of this process was indicated by the appearance of hydrogen bubbles. The deposition process can continue after the catalytic sites are coated with nickel because it is also a catalyst for the reaction. Subsequently, nickel deposition continues automatically. Finally, the product was separated by placing a magnet under the bottom of the bath and washed twice with de-ionized water. 2.2. Characterization Transmission electron microscopy (TEM) of the specimens was carried out on a JEM-200 CX (Japan) instrument using standard techniques. Scanning electron microscopy (SEM) images were obtained on a Hitachi-570 scanning electron microscope operating at 20 kV with Au sprayed

Fig. 2. The TEM images of (a) PMMA particles and (b) PMMA/Ni particles.

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Fig. 3. The diameter distribution of (a) PMMA particles and (b) PMMA/Ni particles.

prior to examination. The distribution in diameters of all samples was performed by a Zetasizer 3000HS instrument (Malvern Instruments Ltd, Malvern UK). The optical images of the samples were observed by an Alphaphot-2 YS2-H (Nikon) optical microscope. The X-ray diffraction (XRD) patterns of the samples were performed at room temperature with a Cu Ka X-ray source using a D/MAX-gA instrument (Japan). The element analyses were carried out on a energy-dispersive spectroscopy (EDS) instrument (LINK-ISIS, Oxford). The magnetic measurement of the as-synthesized products set in a resin was performed on an Alternating Gradient Magnetometer (AGM, MicroMagTM 2900, USA) at room temperature, and the hysteresis loops were recorded infields up to 5 kOe.

3. Results and discussions The technique for electroless metal plating is based on the use of a chemical reducing agent that permits the reduction of the metal from solution on the surface of the substrate. For this process, the beads surfaces need not be electronically conducting, while the kinetics of electron transfer should be slow enough to avoid the reduction of the metal ions and nucleation in solution. The surface acts then as a catalyst to ensure that reduction only takes place on the surface, so that the metal remains attached. The presence of the loaded nickel nanoparticles on the PMMA polymer spheres can be confirmed initially by the optical microscope. As can be observed from Fig. 1(a), the optical image of the prepared PMMA particles is of uniform size and transparent. While the optical image of the nickel loaded particles, PMMA/ Ni particles, in Fig. 1(b), clearly become opaquely black. It has been demonstrated that a uniform and relatively dense distribution of Pd catalysts on the surfaces of PMMA particle cores was crucial for the formation of continuous nickel shell particles. Therefore, the PMMA particles need to be pretreated by palladium chloride of

hydrochloride acid solution for more than 30 min, prior to electroless plating. In addition, the amount of nickel required to cover the surface depends on the surface areas of the particles. Therefore, for 0.5g of PMMA, the critical content of nickel salt was 500 ml of aqueous nickel sulphate with 0.086 molIl 1 concentration. At a lower concentration of the nickel salts, the nickel cannot cover the whole surface of the PMMA particles. The TEM images of the as-prepared PMMA polymer particles, shown in Fig. 2(a) are monodisperse and exhibit a smooth surface. The average diameter of the particles is 1.88 Am. After deposited by nickel nanoparticles, the TEM image in Fig. 2(b) showed that the particle surface has become much rough. The average diameter of the PMMA/Ni particles is 2.07 Am, and the corresponding SEM image in the inset of Fig. 2(b) showed that the nickel particles have been formed into less uniform coverage on the polymer particles. Also, as can be seen no smaller particle in Fig. 1(b) and Fig. 2(b), this indicated that no Ni2+ ions was reduced and nucleated in solution.

Fig. 4. The XRD pattern of (a) PMMA/Ni particles and (b) nickel particles.

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Fig. 5. The energy-dispersive spectroscopy of the PMMA/Ni particles.

The measured distribution in diameter of the particles is shown in Fig. 3. The polydispersity of the PMMA particles is 0.09 in Fig. 3(a). The distribution of PMMA/Ni particles in diameters with a polydispersity of about 0.26 shown in Fig. 3(b) has shifted to larger ones comparing with that of Fig. 3(a). This further suggested the deposition of nickel on particles. Fig. 4 showed the XRD pattern of the samples. The only broad peak around 2h of 40 –60- in curve (a) of Fig. 4 was assigned as a diffraction of the crystalline nickel – phosphorus alloy component. This peak is much different from that of the metallic nickel shown in curve (b) of Fig. 4, which showed two discrete sharp peaks at the same range of 2h. This broad peak of curve (a) indicated that the covered nickel – phosphorus (Ni –P) other than pure nickel component was formed during electroless procedure. EDS analysis of the PMMA/Ni particles, shown in Fig. 5, indicated that there were 95.85 wt.% of nickel and 4.15 wt.% of phosphorous in the deposited nickel – phosphorous component. The phosphorous derived from the reducing agent is considered to affect the magnetic property of the Ni –P coating. In order to gain high saturation magnetization of the magnetic PMMA/Ni microspheres and not to cause a

Fig. 7. The optical microscopy image of the particles structure under magnetic field (1500 G).

low rate of nickel deposit, the concentration of the reducing agent was limited to about 0.150– 0.250 molIl 1. Fig. 6 shows the magnetic hysteresis loops at room temperature of the sample. The coercivity Hc and saturation magnetization Ms of the sample were about 21.33 Oe and 184.1 Aemu, respectively. This result revealed the magnetism in the samples. Moreover, the behavior of the magnetic beads under external magnetic field was examined by optical microscopy. In a magnetic field of 1500 G, the particles dispersed in silicone oil suspensions can be aligned into chain structure as shown in Fig. 7. This revealed that the prepared magnetic particles showed good response to magnetic field. And the behavior of the particles could be manipulated by using external magnetic field.

4. Conclusions In conclusion, the novel PMMA/Ni magnetic particles were prepared by electroless plating technique. The resulting particles were of 2.07 Am average diameter. The coercivity Hc and saturation magnetization Ms of the assynthesized particles were about 21.33 Oe and 184.1 Aemu, respectively. The particles showed good response to the external magnetic field by being ordered into chain structures under an external magnetic field. This provided a new route to control and selective surface modification of colloidal particles.

Acknowledgements

Fig. 6. The hysteresis loops of the samples examined by AGM at room temperature.

The Major State Basic Research Development Program of China (973 Program) under Grant No. 2003CB615701 and the Research Program of Science and Technology Commission Foundation of Beijing City under Grant No. H030630080220 as well as the China Postdoctoral Science Foundation (20040350021) are all gratefully acknowledged.

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