Nickel nanoparticles obtained by a modified polyol process: Synthesis, characterization, and magnetic properties

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Abstract

The synthesis of nickel nanoparticles using poly(N-vinilpyrrolidone) (PVP) as protective agent was studied. The nanoparticles were prepared in air according to a modified polyol route, using nickel chloride as precursor and sodium borohydride as reducing agent. Samples with different nickel/PVP ratio were obtained. The X-ray diffraction and transmission electron microscopy (TEM) measurements indicate the occurrence of face-centered cubic metallic nickel nanoparticles with a medium diameter of 3.8 nm and good size dispersion. Fourier transformed infrared spectroscopy (FT-IR) and X-ray photoelectron spectroscopy (XPS) data show an effective interaction between the nickel nanoparticles surface and the carboxyl oxygen atoms of PVP. Magnetic measurements show single-domain nonideal superparamagnetism behavior due to dipolar magnetic coupling between particles.

Graphical abstract

The picture represents a TEM image showing spherical PVP-coated Ni nanoparticles prepared by a modified polyol route.

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Introduction

Metal nanoparticles have attracted much interest in the last years due their unique properties and potential applications in several areas as microelectronics, optoelectronics, catalysis, photocatalysis, magnetic materials, information storage, among others [1], [2], [3], [4], [5], [6]. Specifically, magnetic nanoparticles have potential applications in ultra-high magnetic storage devices, ferrofluids, magnetic refrigeration systems, contrast agent in magnetic resonance imaging, magnetic carriers for drug targeting, catalysis, and a rich variety of novel phenomena derived from their collective interactions [7], [8].

Most physical and chemical properties of these nanoparticles (NPs) depend on their size and shape. The development of synthetic routes in order to obtain nonagglomerated nanoparticles with a well controlled mean size and a narrow size distribution is imperative [9], [10]. An important feature in the production of the NPs is the ability to keep them physically isolated one from another preventing irreversible aggregation. The stability of the NPs is commonly achieved using different protective molecules, which binds to the NPs surface avoiding their aggregation, making them “soluble” in given solvents [9], [10].

Several synthetic routes leading to metal nanoparticles have been recently reported. Among them, the so-called polyol process uses a poly-alcohol (ethylene glycol—EG, for example) as both solvent and reducing agent to produce nanoparticles from the metallic cation precursor [11], [12], [13]. In this process, the polyol itself can act as protective agent to avoid particles agglomeration and growth. Many different factors affect the characteristics of the NPs obtained by the polyol route, as the type of precursor, the temperature, the atmosphere, the presence of some protective compound (besides the polyol molecules), the utilization of an extra-reducing agent, among others.

A whole of metallic and bimetallic nanoparticles have been prepared by the polyol route such as Ag, Fe, Co, Pt, Ni, FePt, Pd, Ru, etc. [11], [12], [13], [14], [15], [16], [17], [18]. One of the earliest reports on the synthesis of nickel nanoparticles by the polyol route was done by Fievet et al. starting from Ni(OH)2 as precursor [11]. Many papers were published on Ni NPs preparation, but in comparison to other metals, few of them were related to the polyol route [19], [20], [21], [22]. The use of poly(N-vinilpyrrolidone) (PVP) as protective agent has been reported for Ni NPs obtained both by the polyol [22] and other synthetic routes [23]. In the absence of PVP or other protective agent, particles are generally obtained in the micrometer size range [11], [20], [22]. Wu and Chen [19] obtained nickel nanoparticles without protective agent, through the hydrazine reduction of nickel chloride in ethylene glycol in the presence of appropriate amount of NaOH.

This work reports the synthesis, characterization and study of different nickel nanoparticles obtained in air by a modified polyol route, starting from nickel chloride as precursor, sodium borohydride (NaBH4) as reducing and PVP as protective agent. To the best of our knowledge, this is the first report on the utilization of sodium borohydride on the polyol route to produce nickel nanoparticles.

Section snippets

Experimental

Ethylene glycol (Merck), PVP (Merck), NiCl2⋅6H2O (Merck), NaBH4 (Merck), acetone (Merck), and ethanol (Merck) were used as received.

Nickel nanoparticles were synthesized according to the following: 20.0 ml of a 2.0×10−4molL−1 solution of NiCl2⋅6H2O in ethylene glycol were added to a 125 ml round flask. A certain amount of PVP was added on this solution, and the system was maintained under magnetic stirring until the total dissolution of the PVP. In the sequence a condenser was adapted to the

Results and discussion

The formation of nickel nanoparticles in all experiments was noticeable by the dramatic color change of the system after the introduction of the reducing agent. The overall reaction proposed for this process should be sketched as2Ni2+(eg) + BH4(s) + 2H2O(eg) + 2nPVP(eg)  → 2Ni(PVP)n(s) + 2H2(g) + 4H+(eg) + BO2(eg), where (eg) represents the specie in the ethylene glycol solution. Water was provided by the hydrated salt used as precursor (NiCl2⋅6H2O). The analysis of the solution remaining

Conclusions

Stable PVP-protected nickel nanoparticles have been synthesized by a modified polyol route, using nickel chloride as precursor and sodium borohydride as reducing. Samples with different nickel/PVP ratio were obtained, and nanoparticles with diameter varying from 3.4 to 3.8 nm were obtained according the nickel/PVP ratio employed. Sample prepared without PVP were also obtained, and the average diameter size was 7.7 nm. The presence of PVP was considered essential to prevent the aggregation and

Acknowledgements

Authors thank CNPq, CT-ENERG/CNPq, PROCAD-CAPES, Rede Nacional de Nanotubos de Carbono, FAPESP (04/08524-0, 03/09933-8) and TWAS for financial support; Centro de Microscopia Eletrônica-UFPR and Dr. Marcela Mohallem Oliveira for the TEM images. G.G.C. thanks CAPES for the fellowship.

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