Synthesis and characterization of magnetite-maghemite nanoparticles in presence of polyethylene glycol obtained by mechanical milling

https://doi.org/10.1016/j.mseb.2020.114873Get rights and content

Highlights

  • Total conversión hematite-magnetite can be performed by soft and wet ball milling.

  • Ball milling in presence of PEG hepls to stabilize magnetite nanoparticles.

  • Ball milling in presence of PEG helps to reduce particle size distribution of MNPs.

Abstract

We present the synthesis and characterization of magnetite-maghemite nanoparticles obtained by mechanical milling of sub micrometric hematite in presence of polyethylene glycol (PEG) as stabilizing medium. Nanoparticles were obtained in a milling time of 24 h, being their aggregation and particle size distribution less than those obtained by us in a previous work employing a mixture of hematite and deionized water as precursor material. X-ray and room temperature Mössbauer measurements confirmed that milling products are composed only by the phases magnetite-maghemite. TEM micrographs showed quasi-spherical particles with low aggregation and mean diameter around 14 nm. Room temperature magnetization loops are consistent with ferrimagnetic multi domain nanoparticles, with saturation magnetization around 62 A m2 kg−1 and coercive field around 10.2 kA m−1. The results suggest that mechanical milling in presence of polyethylene glycol is a simple and efficient route to produce magnetite-maghemite nanoparticles with good magnetization, which can be useful for applications in nanotechnology.

Introduction

Wet methods for synthesis of magnetic particles, such as sol-gel, hydrothermal, coprecipitation, thermal decomposition, combustion solution and hydrogels have been widely studied by several researchers in order to obtain particles at the nanoscale, highly crystalline and with good magnetic properties [1]. Among magnetic materials, magnetite (Fe3O4) and maghemite (γ-Fe2O3) have attracted great attention due to their high magnetic response, biocompatibility [2], [3], [4], and because they are polarized spin materials, useful to develop spintronic devices [5], [6] and devices for ecology applications [7], [8]. Wet synthesis methods present some advantages over other methods, among them good control of particle size and crystallinity, simplicity and low cost of the synthesis process, however the obtaining of spinel iron oxide nanoparticles admits additional efforts in order to reach synthesis conditions friendly with the environment [9], [10], which allow obtaining products free of impurities, reproducible, scalable, with good magnetic response, good crystallinity, narrow particle size distribution, among other desirable characteristics. In this sense, mechanical milling is a promising route to obtain magnetite-maghemite nanoparticles. Several works have reported the conversion of hematite to magnetite by mechanical milling, by using different conditions, among them controlled atmosphere [11], [12], prolonged milling times [13] and use of organic solvents and extreme power of the mill [14]. In a previous work [15], we reported an efficient route to obtain magnetite-maghemite nanoparticles by the high-energy ball milling method, starting from sub-micrometric hematite mixed with deionized water and using some particular milling conditions. In that work, we obtained full conversion hematite to nano-spinel in a time of 24 h, by using air atmosphere and without an extreme demand of power of the mill, however we observed a considerable aggregation and particle size distribution in the final products. With the purpose of advance towards a green and scalable route to obtain magnetic nanoparticles with convenient magnetic and structural properties for applications as those mentioned before, we have introduced in our previous method a stabilizing medium, with biocompatible and biodegradable properties as the polyethylene glycol (PEG) [9], [10], [16], [17]. Polyethylene glycol has been reported as a convenient polymer to assist the particle size reduction, as well as prevent the aggregation of nanoparticles of silver [18] and Cu-W [19] obtained by mechanical milling. The inclusion of an aqueous solution of PEG 400 in the milling process of hematite allowed us obtain quasi spherical spinel nanoparticles, with less aggregation and narrower particle size distribution than that obtained by using hematite and deionized water as precursor material. Details of the synthesis process of magnetite-maghemite nanoparticles with the new method are described in the following sections of this paper.

Section snippets

Materials and methods

The milling process was carried out with a planetary ball mill FRITSCH, model Pulverisette 7 premium line. As milling bodies, we used two hardened steel vials with 80 cm3 volume, each one charged with 60 balls of hardened steel 3 mm nominal diameter. As precursor material we poured in each vial a mixture of 2.0 g of powder analytical grade hematite (α-Fe2O3) with mean particle size of 0.5 μm, supplied by Sigma Aldrich, and 5 g of 40% aqueous solution of analytical grade PEG 400 Da supplied by

The milling process

Fig. 1 presents a view of the materials, before and after the milling process. To have some evidence of the reproducibility of the products, we used the two vials of the mill, charged with the same precursor materials and milling bodies. The final products presented a black color and a viscous appearance, the latter conferred by the presence of the PEG.

X-ray diffraction measurements

Fig. 2 presents the X-ray diffraction patterns of the milling products obtained in the two vials. We applied the Rietveld method to fit the

Conclusions

We have obtained nanostructured particles of the spinel system magnetite-maghemite by the route of mechanical milling, taking as precursor materials a mixture of polyethylene glycol 400, deionized water and microstructured hematite. Complete conversion hematite to nano magnetite-maghemite is obtained after 24 h, by using non-controlled conditions, without the need of especial accessories for the milling bodies and operating the mill far below from its operation limits. Similar products were

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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