Template-Assisted Synthesis and Characterization of Passivated Nickel Nanoparticles
© The Author(s) 2010
Received: 30 December 2009
Accepted: 15 March 2010
Published: 2 April 2010
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© The Author(s) 2010
Received: 30 December 2009
Accepted: 15 March 2010
Published: 2 April 2010
Potential applications of nickel nanoparticles demand the synthesis of self-protected nickel nanoparticles by different synthesis techniques. A novel and simple technique for the synthesis of self-protected nickel nanoparticles is realized by the inter-matrix synthesis of nickel nanoparticles by cation exchange reduction in two types of resins. Two different polymer templates namely strongly acidic cation exchange resins and weakly acidic cation exchange resins provided with cation exchange sites which can anchor metal cations by the ion exchange process are used. The nickel ions which are held at the cation exchange sites by ion fixation can be subsequently reduced to metal nanoparticles by using sodium borohydride as the reducing agent. The composites are cycled repeating the loading reduction cycle involved in the synthesis procedure. X-Ray Diffraction, Scanning Electron Microscopy, Transmission Electron microscopy, Energy Dispersive Spectrum, and Inductively Coupled Plasma Analysis are effectively utilized to investigate the different structural characteristics of the nanocomposites. The hysteresis loop parameters namely saturation magnetization and coercivity are measured using Vibrating Sample Magnetometer. The thermomagnetization study is also conducted to evaluate the Curie temperature values of the composites. The effect of cycling on the structural and magnetic characteristics of the two composites are dealt in detail. A comparison between the different characteristics of the two nanocomposites is also provided.
Metal nanoparticles are of great interest because they exhibit interesting optical, electronic, magnetic, and chemical properties. They find potential applications in various optoelectronic devices, as catalysts in chemical reactions and also as biosensors [1–4]. Synthesis of metal nanoparticles either in the form of independent entities or in matrices thus assume significance and are of interest to chemists and physicists alike. Preparation of nanoparticles of Fe/Ni/Co is not very easy and hence novel methods and alternate routes are normally scouted for. The large surface area of unprotected metal nanoparticle is prone to oxidation and thus conventional methods for the synthesis of metal nanoparticles are not feasible. Self-protected metal particles embedded in host matrices are thus a viable alternative. Stabilization of metal nanoparticles by employing capping agents or coating with surfactants are usually adopted to [5, 6]. The fabrication of polymer-stabilized metal nanoparticles is a promising solution to the metal nanoparticle instability and thus they attract the attention of material scientists and technologists [7, 8]. The areas of practical applications of metal–polymer composite are in spin-polarized devices, sensors , and carriers for drug delivery  and in catalysis .
A wide variety of methods are adopted for the fabrication of metal–polymer composites which include both physical and chemical techniques. Examples of physical methods are cryo chemical deposition of metals on polymeric supports and simultaneous plasma-induced polymerization and metal evaporation techniques . The chemical methods mainly include the reduction of metal inside the polymer i.e. the intermatrix synthesis of these composites [13, 14].
Mesoporous ion exchange resins were employed to prepare polystyrene γ-Fe2O3 nanocomposites with magnetic functionality [15, 16]. The size of the magnetic oxide can be predetermined depending on the choice of the particular resin which is again graded according to the channels in the porous resin. Thus, a judicious choice of the polymer matrix determines the size of the oxide particle. This study caught the imagination of many researchers and various metal oxide polymer nanocomposites were prepared [17–19]. The availability of various ion exchange resins commercially were an added attraction to these researchers. However, for the fabrication of metal–polymer composites, a different route has to be adopted. For example, mesoporous ion exchange resins can be a template matrix where suitable metal ions are anchored to the functional resins followed by their subsequent reduction inside the polymer network. This method is generally known as the ion exchange reduction process.
Nickel nanoparticles embedded in a polymer matrix are important not only from a commercial point of view but also from a fundamental perspective. They are ideal templates for studying the size effects on the magnetic properties. The optical properties of these particles at the nanolevel also assume significance. The interaction between metal nanoparticles embedded in a polymer matrix can also be an interesting topic of investigation. The method of ion exchange has been employed for the incorporation of metal oxide nanoparticles in the host matrix [15, 16]. However, reports employing the method of ion exchange resins for the preparation of passivated magnetic metal particles are not very common.
Nickel polymer nanocomposites can be synthesized by the method of reduction using two different templates namely, strongly acidic cation exchange resin and weakly acidic cation exchange resins. Since both the strong and weak resins are characterized by their channels and pores, respectively, the overall properties of the composite need not be identical vis a vis the nature of the embedded nano nickel inside the matrix, the impurity phase, etc. This investigation is an attempt to synthesize nickel nanocomposites using two different matrices having different functional groups and different structures and to study their structural and magnetic properties with a view to optimize the method of synthesis by cycling to increase the net magnetization of the nanocomposite. The exact determination of the amount of nickel on the composite is also important. Therefore, compositional analysis using techniques like Inductively Coupled Plasma Analysis (ICP)/Energy Dispersive X-ray Spectrum Analysis (EDS) enables one to determine the exact composition of nickel in the synthesized nanocomposites. The morphological and structural aspects are investigated using X-Ray diffraction, Transmission Electron Microscopy, and Scanning Electron Microscopy. The effect of cycling on the magnetic properties of these composites form another objective. Hence, a complete study on the nanocomposites is undertaken in the present investigation. The motivation of the present study is not only to synthesize self protected nickel nanoparticles in a porous network having two structures, but also to investigate how the structural and magnetic properties differ in these two matrices. The different type of interactions between metal nanoparticles trapped in a dense matrix and porous matrix can be quite interesting.
X-Ray diffraction patterns of the samples were recorded using an X-Ray Powder Diffractometer (Rigaku Dmax—C) using Cu-Kα radiation (λ = 1.5405 Å). The diffraction patterns were taken in the range from 2θ = 35° to 110°. Lattice parameter was calculated assuming cubic symmetry. The average crystallite size was estimated by using Debye Scherer’s formula. The particle size was also determined by subjecting the samples to Transmission electron microscopy (Joel JEM-2200 FS). Energy Dispersive X-ray Spectra (EDS) was also obtained. Thermo Electron Corporation, IRIS INTRPID II XSP model ICP was used for elemental analysis. Magnetic measurements were performed using a vibrating sample magnetometer (model EG & G PAR 4500) under an applied magnetic field of 15kOe. High resolution Scanning Electron Microscopy was employed to check the morphology of the samples (JSM-6335 FESEM).
Although there is an increase in the crystalline behavior of the composites (SAC), the particle size of the nanoparticles incorporated in the matrix do not undergo any change with cycling. Hence, it is to be presumed that the nickel nanoparticles are trapped in the polymer matrix as soon as they are formed and further growth of nanoparticles is inhibited. After each cycle, it is the concentration of nickel nanoparticles in the matrix which is increasing. The improved crystallinity of the composites is manifested in the XRD pattern. The absence of any oxide phase in all the cycled samples of SAC confirms the formation of self protected metal nanoparticles. On the other hand, the presence of Nickel oxide in WAC composites points toward the existence of nickel oxide layer on the nickel particles.
Magnetic parameters of SAC–Ni and WAC–Ni nanocomposites
M s (emu/g) (300 K)
M s (emu/g) (100 K)
H c (Oe) (300 K)
H c (Oe) (100 K)
T c K
The composites are showing saturation property at the 2nd cycle itself. This makes clear the formation of pure nickel nanoparticles in the resins. With cycling, the saturation magnetization values are showing an increasing trend. It is the increase in metal loading after each cycling that resulted in the increased M s values. The increase in M s is found to be slow at higher cycles in both the composites. The coercivity values of the composites are found to be around 100 Oe. The coercivity values show a little variation after the second cycle in SAC composites while a clear variation is observed in WAC composite (insets of Figs. 11, 12). Accordingly, the formation of self-protected elementary nanoparticles of nickel can be assured in the samples on SAC. In the case of WAC composites, the formation of an oxide layer over the nanoparticles is expected.
In both the composites, the cycling enhances the magnetization values. The magnetization value of the composite is entirely due to the magnetic nickel nanoparticles in the matrix. The M s in bulk nickel is 55 emu/g and the effective M s values of the nickel nanoparticles embedded in the matrix can be estimated from the percentage of Nickel content estimated from ICP analysis of these composites. It can be seen that the maximum cycled sample has an effective magnetization (M s at 100 K/Nickel content) of 47.39 emu/g for SAC-16 and 24.44 emu/g for WAC-8. The decrease in M s compared with the bulk might be due to the decrease in particle size and the accompanied increase in surface area. The presence of nickel oxide along with nickel also could be a contributing factor for the enhanced reduction of nickel nanoparticles embedded in the WAC-resin . Reduction in M s in Ni nanoparticles also could be due to the presence of amorphous nickel and the non magnetic or weakly magnetic interfaces .
Nickel–polystyrene nanocomposites are synthesized by the intermatrix ion exchange synthesis where we have used strongly acidic cationic Exchange Resin (SAC) and weakly acidic cationic Exchange Resin (WAC) with cationic exchange sites as the parent matrices. The sequential loading of the cationic exchange sites with metal ions and their subsequent reduction using Sodium borohydride resulted in Ni–Polystyrene nanocomposites. The crystallinity and magnetic characteristics are modified by repeating the loading reduction cycle. The effect of cycling on the structural and magnetic properties of the composites is also investigated. The XRD patterns of the cycled samples confirmed that the there is no particle growth with cycling. These investigations indicate that SAC composites contain phase pure nickel nanoparticles trapped in the interstitial channels of the polystyrene matrix and their further growth is inhibited. On the other hand, WAC composites contain two distinct phases of Nickel and Nickel oxide. Comparison of the structural and magnetic properties of the two types of composites showed that the SAC resin composites are better in structural as well as magnetic properties compared to WAC resin composites. These interesting attributes of the magnetic nanocomposites can be tailored for promising applications. Moreover, optical and electrical characterization of these composites can be promising areas of research for device applications.
EVG acknowledges Cochin University of Science and Technology for the Research Fellowship and STIC, CUSAT for the ICP measurements. KAM thanks University Grant Commission, Government of India for the financial assistance received under UGC minor project. GS acknowledges Department of Collegiate Education, Govt. of Kerala. Al–Omari would like to thank the Sultan Qaboos University for the support under Grant number IG-SCI-PHYS-09-01. MRA acknowledges Kerala State Council for Science, Technology and Environment (C.O. No. (T)/159/SRS/2004/CSTE dated: 25-09-2004), Kerala, India, for the financial assistance.
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