One-step room temperature synthesis of very small γ-Fe2O3 nanoparticles
Graphical abstract
Introduction
Iron oxide nanoparticles (NPs) have been the subject of many studies due to their great importance in several technological applications, which range from information data storage to biological and biomedical applications [1], [2], [3]. Of these oxides, maghemite (γ-Fe2O3) is one of the most used because its magnetic properties and chemical stability. In bulk, γ-Fe2O3 is ferrimagnetic at room temperature since its inverse spinel crystal structure has two sublattices where Fe(III) ions are in different oxygen environments. However, when γ-Fe2O3 NPs are smaller than 5 nm, the magnetic behavior at room temperature is superparamagnetic.
Diverse routes for synthesizing γ-Fe2O3 NPs are reported: sol-gel synthesis [4], electron beam deposition [5], mechanochemical synthesis [6] and pyrolysis [7], among others. However, the conventional method to obtain γ-Fe2O3 NPs is the oxidation of magnetite [8], [9] in where particles size exceeds 12 nm. In this way, it is desirable a direct method to synthesize smaller NPs.
Ferric salt hydrolysis generates an oxyhydroxide, akaganeite or two-line ferrihydrite, that depends strongly on the thermodynamics conditions (pH, temperature, concentration, stirring velocity, etc.) [10], [11], [12], [13]. Obtaining of two-line ferrihydrite is favored if synthesis, in basic medium, is done at room temperature and ferric solution aging is avoid [11], [12], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25].
This is relevant because the α-Fe2O3 or γ-Fe2O3 formation in aqueous medium depends on the oxyhydroxide precursor. If akaganeite is the precursor, α-Fe2O3 is obtained, but if two-line ferrihydrite is the precursor, is possible to stabilize γ-Fe2O3 [26], [27], [28]. The possibility to obtain γ-Fe2O3 from two-line ferrihydrite is based on structural similarity that may exist between these two phases. Nevertheless, there is a controversy about the existence of tetrahedral Fe (III) in two-line ferrihydrite [11], [29], [30], [31], [32], [33]. One of the aims of this work is to provide new facts to such discussion.
This paper gives evidences about the transformation of two-line ferrihydrite in maghemite, as well as support the hypothesis of the existence of Fe(III) tetrahedral in two-line ferrihydrite. Furthermore, a one-step synthesis of 3 nm maghemite nanoparticles, based on hydrolysis of iron (III) chloride at room temperature is reported.
Section snippets
Materials and methods
Ferric chloride solution (0.8 M) was slowly dropped in 40 ml of 28% ammonia solution until pH = 9 is reached. The aqueous solution was stirred at 500 rpm by 2 h. The solid precipitated was filtered and washed with distilled water in order to remove ammonium chloride. Finally, the product was spread onto a glass to improve drying and oxidation at room temperature. The final solid (we will name it “experimental sample”) was characterized by High Resolution Transmission Electron Microscopy (HR-TEM),
Results and discussion
The analysis of HR-TEM images shows that the experimental sample is composed by very tiny particles with small size distribution. A histogram was built analyzing NPs size in TEM images (Fig. 1), and as result, a mean diameter of 3.3 ± 0.7 nm was obtained. By the Fast Fourier Transformation (FFT) technique a common feature is observed in most of the FFT graphs corresponding to the nanoparticles with diameters less than 3 nm. Usual measured interplanar distances, d = 2.5 Å (plane (3 1 1)) and 2.2 Å (plane
Conclusion
Very small maghemite nanoparticles (∼3 nm) can be synthesized by co-precipitation in a one-step reaction (neutralization process of a ferric solution). The maghemite, as a final product, can be obtained only if the precursor has Fe(III) in tetrahedral interstices. HR-TEM, XAFS, and Mössbauer spectroscopy confirm that experimental sample is composed of nanoparticles that have the crystalline structure of maghemite, compatible with its small sizes. The corresponding electronic change of the
Acknowledgements
Agencies of Argentine (CONICET and ANPCyT (PICT 280-08 and CU1006)), Colombia (COLCIENCIAS), and Mexico (CONACyT), are acknowledged for financial support. National Synchrotron Light Laboratory (LNLS, CNPEM, Campinas, Brazil) is acknowledged for the use of the XAS beamline and HR-TEM (Electron Microscopy Laboratory, C2NANO, CNPEM, Campinas, Brazil). Also we thank R. Cohen and L. Nagamine (LMM, USP, Brazil) for Mössbauer measurements.
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