Vapour phase approach for iron oxide nanoparticle synthesis from solid precursors

https://doi.org/10.1016/j.jssc.2013.01.037Get rights and content

Abstract

A new non-solution mediated approach to the synthesis of iron oxide nanoparticles directly from solid FeCl2 salt precursors has been developed. The method is rapid, simple and scalable. The structural properties and the phase of the resulting iron oxide particles has been determined by a range of methods including XRD, FT-IR and Mössbauer spectroscopy, and the phase is shown to be maghemite (γ-Fe2O3). The magnetic properties of the iron oxide particles have been measured using SQUID, confirming superparamagnetic behaviour of the powder and a saturation magnetization of 53.0 emu g−1 at 300 K. Aqueous dispersions at increasing concentrations were prepared and their heating rate under a 400 kHz alternating magnetic field measured. The specific absorption rate (SAR) of the iron oxide was found to be 84.8 W g−1, which makes the material suitable for the formulation of ferrofluids or ferrogels with RF heating properties.

Graphical Abstract

Superparamagnetic iron oxide nanoparticles obtained by a novel vapour phase approach.

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Highlights

► Novel vapour phase (non-solvent) approach for iron oxide nanoparticle synthesis. ► Attractive alternative approach to the present co-precipitation method. ► Better magnetic properties with high coercivity of nanoparticles. ► A high specific absorption rate (SAR) for hyperthermia applications.

Introduction

Due to biocompatibility and useful properties in a number of application areas, there has been a growing interest in the development of synthesis procedure to obtain iron oxide nanoparticles. Iron oxide nanoparticles are being extensively exploited in several scientific and technological applications such as magnetic storage media [1], targeted drug delivery [2], [3], magnetic resonance imaging (MRI) [3], [4], [5], [6], hyperthermia and cancer therapy [7], [8], [9], [10], [11], water remediation [12], magnetic ferrofluids [13], [14], magnetic refrigeration [15], catalysis [16], [17], magnetic sensors [18] etc. Depending upon the synthesis method and conditions, iron oxide can be obtained in one of several phases, such as FeO (Wustite), Fe3O4 (magnetite); α-Fe2O3 (hematite), γ-Fe2O3 (maghemite), ε-Fe2O3 and β-Fe2O3. Out of these iron oxides, magnetite and maghemite are the most commonly obtained phases by synthesis procedures carried out from room temperature till 100 °C.

The synthesis routes used to obtain iron oxide nanoparticles can be categorised as: (i) solid phase synthesis—combustion synthesis [19], annealing [20], solid state chemical reaction method [21], [22]; (ii) liquid phase synthesis—precipitation normal/reverse/co-precipitation [23], [24], hydrothermal reactions [25], sol–gel reactions [12], microemulsion methods—reverse/bicontinuous [26], [27], sonolysis [4], thermal decomposition of organometallic compounds [28], polyol methods [29], microwave synthesis [30]; (iii) gas phase synthesis—chemical vapour deposition [31], arc discharge [32], laser pyrolysis [33]. The methods differ not only in their underlying principle but also in the practicality and cost of the necessary equipment. Thus, co-precipitation is one of the most commonly used method, as it has the advantage of being a rapid and highly efficient (in terms of yield) chemical pathway to obtain iron oxide nanoparticles at benign conditions. In the co-precipitation method, iron oxides (either Fe3O4 or γ-Fe2O3) are usually prepared from a mixture of ferrous and ferric salts (typically, FeCl2 and FeCl3) dissolved in an aqueous medium according to the stoichiometric reactionFe2++2Fe3++8OHFe3O4+4H2Owhereby the addition of a base such as NH4OH is the source of the OH anions. The resulting phase depends on the stoichiometry of the precursors (Fe3+/Fe2+ ratio), the pH and the process conditions (oxidising atmosphere—air, or inert gas—N2).

Although maghemite has a lower saturation magnetization of 73.5 emu g−1 compared to magnetite (92 emu g−1), it is still being preferred due to its most stable form. Recently, Maity et al. [23] and Alibeigi et al. [24] have shown that the maghemite phase (γ-Fe2O3) is obtained directly when the co-precipitation is carried out under oxidative conditions (air) even if a molar ratio of Fe3+/Fe2+≤2:1 is used. Haneda and Morrish [34] have reported that at room temperature, 95% of Fe3O4 was converted to γ-Fe2O3 within 50 days. Alibeigi et al. [24] further reported that to obtain magnetite, reverse precipitation is the best option. However, these two phases are not easily differentiated by most of the present techniques like XRD, SQUID etc. Finding difficulty in differentiating between these two phases, some authors use the generic term “SPION” (super-paramagnetic iron oxide nanoparticles) to denote the resulting material. However, for the potential application of these iron oxide nanoparticles, especially in medical and water applications, one must know the exact phase of the formed iron oxide particles.

The present work has the following goals. First a new non-solution mediated approach to obtain iron oxide nanoparticles directly from the solid iron salt precursors will be described. Second, the structural properties and the phase of the resulting iron oxide particles will be determined by a range of methods including XRD, FT-IR and Mössbauer spectroscopy. Finally, the magnetic properties of the particles will be measured and the heating properties of their aqueous dispersions under a radiofrequency alternating magnetic field established.

Section snippets

Chemicals used

Iron (II) chloride tetrahydrate (FeCl2·4H2O) was purchased from Sigma Aldrich, ammonium hydroxide (NH4OH, 25–29%) and ethanol from Penta Chemicals. All reagents were used as received.

Particle synthesis

The uncoated iron oxide powder was synthesised using a single-step “vapour phase approach” as follows: 5 mL of concentrated NH4OH was added into a volumetric flask kept on a heating plate initially at room temperature. Simultaneously, 0.25 g of FeCl2·4H2O was placed onto a filter paper positioned on the surface of a

Structural characterisation

The size and morphology of the particles obtained directly from the solid powder precursors were studied by transmission electron microscopy (TEM). The TEM images of particles shown in Fig. 2a exhibit a high degree of agglomeration probably due to magneto dipole interactions. At a higher magnification shown in Fig. 2b we can observe that these aggregates consist of nanoparticles in the low 10’s nm size range despite being originally formed from much larger crystals of FeCl2.

Fig. 3 shows the XRD

Conclusions

A novel non-solvent based method for the synthesis of iron oxide nanoparticles has been developed and described in this work. The iron oxide powder obtained by this method was found to be in the maghemite phase and its radiofrequency heating properties (expressed by the SAR) were found to be comparable or superior to those of co-precipitation based nanoparticles. The SAR of co-precipitated iron oxide particles measured under identical conditions (same RF coil) were 17.8 W g−1 in the best case [38]

Acknowledgments

F. Stepanek and M. Singh would like to acknowledge support by the European Research Council (project 200580-Chobotix). The work of V. Prokopec was supported by Grant Agency of Czech Republic, grant No. P206-11-P405. We are thankful to Prof. Oldrich Schneeweiss from Institute of Physics of Materials, Brno for the Mössbauer spectroscopy study. The work of P. Svoboda was supported by Grant Agency of Czech Republic, grant no. P108-10-1006, experiments were performed in MLTL (http://mltl.eu/), which

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