Background

Iron oxides exist in different forms in nature, magnetite, maghemite, and hematite [1] Maghemite is ferrimagnetic at room temperature but maghemite nanoparticles smaller than 10 nm are superparamagnetic at room temperature, maghemite is unstable at high temperatures, and loses its susceptibility with time [2, 3].

Among the researchers working in the field of nanotechnology, magnetic nanoparticles have attracted intense experimental activities because of their potential applications in a numerous different industries such as storage of information [4], ferrofluid [5], audio and video recording [6], bioprocess [7] gas sensor [8–13], refrigeration systems [14], information storage [15], medical applications [16], magnetic resonance imaging [17], catalyst [18, 19], magnetooptic [20], and removal of heavy metals [38, 39].

We have various chemistry methods for iron oxide nanoparticles preparation, including gas phase methods (reduction, hydrolysis, disproportionation, oxidation, or other reactions to precipitate solid products from the gas phase), that depend on thermal decomposition [21], liquid phase methods, two-phase methods, sol–gel methods, high-pressure hydrothermal methods [22], various methods have been reported for the synthesis of maghemite nanoparticles, such as co-precipitation [23–25], sol–gel synthesis [26–28], micro emulsion [29], flow injection synthesis [30], hydrothermal synthesis [31, 32], flame spray pyrolysis [33], decomposition of organic precursors at high temperatures, and the oxidation of magnetite nanoparticles [31, 34].

Among various chemical methods for synthesis of different types of metal oxides, co-precipitation process has several advantages over other methods including, good homogeneity, low cost, high purity of product and not requiring organic solvents and heat treatment. Recently, co-precipitation method has been developed for preparation of magnetite nanoparticles using metallorganic precursors. In this paper, we report the synthesis and characterization of the γ-Fe2O3 nanoparticles by a chemical co-precipitation technique of ferric and ferrous ions. In this method, unlike previous methods of producing maghemite nanoparticles, we do not need expensive equipments, organic solvents and Hydrochloric acid which is a corrosive substance in the industry and create corrosion in equipment. Also, production time, compared with the previous method is faster and has the ability to produce on an industrial scale.

Methods

To prepare Iron oxide nanoparticles, especially maghemite (γ-Fe2O3), ferric chloride (FeCl3, 99 %), ferrous chloride tetra hydrate (FeCl2·4H2O, 98 %), hydrochloric acid (HCl, 37 %), ammonium hydroxide (NH4OH, 25–30 % of ammonia), de-ionized water, and ethanol (CH3CH2OH, 99.93 %) were used in the experiments. All the reagents used were of analytical grade. The synthesis was as follows:

FeCl3 and FeCl2·4H2O were dissolved in a 2 M hydrochloric acid to form a solution with the concentration of 1 M for FeCl3 and 2 M for FeCl2·4H2O. The NH3·H2O solution (0.8 M) was dropped to this solution with vigorous stirring at room temperature for 80 min. The final pH was 8.7.

FeCl3 and FeCl2·4H2O were dissolved in a de-ionized water to form a solution with the concentration of 1 M for FeCl3 and 2 M for FeCl2·4H2O. The NH3·H2O solution (0.8 M) was dropped to this solution with vigorous stirring at room temperature for 40 min. The final pH was 8.3.

The brown precipitate was then collected by filtration and rinsed three times with deionized water and ethanol. Finally, the washed precipitate was dried at room temperature.

The crystallographic structure of the as-synthesized iron oxide nanoparticles was characterized by high-resolution XRD analysis (Philips, X, pert-MPD) using Cu Kα (λ = 1.54 Å) radiation and the crystallite size was estimated using Scherrer’s formula.

IR spectra were recorded on a Bruker tensor 27 FTIR spectrometer with RTDLATGS detector, in the range of 400–4,000 cm−1 with a spectral resolution of 4 cm−1 in transmittance mode. The surface morphology of the powders was observed by the TEM (CM-120 PW6031/10). The magnetic properties of the as-synthesized nanopowders were analyzed by a vibrating sample magnetometer (VSM), in the Development Center of Kashan University (Kashan, Iran).

Results and discussion

The crystalline structure of the nanoparticles was characterized by X-ray diffraction (XRD, PHILIPS, X’pert-MPD system) using Cu Kα (λ = 1.54 A°) radiation and the crystallite size was estimated using Scherrer’s formula. The results of analysis material by X-ray diffraction are shown in Fig. 1, which can be indexed as the primitive cubic system by comparison with data from γ-Fe2O3 (JCPDS No. 39-1346) [28]. The diffraction peaks at 2θ = 18.30°, 30.20°, 35.45°, 43.32°, 53.81°, 57.22°, 62.98°, 74.54°, correspond to (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), (4 4 0) and (5 3 3), respectively. No impurity peak indicates that the product is pure. According to the Scherrer’s equation, the average crystallite size of the product was calculated to be about 8–12 nm.

Fig. 1
figure 1

X-ray diffraction patterns of γ-Fe2O3 nanoparticles by chemical precipitation method at room temperature

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Figure 2 shows the FTIR spectrum of the sample, the absorption peak at 587 cm−1 identified vibration of γFe-O [35, 36], and the other peaks at pure maghemite are 454, 632, 795, 892 cm−1. The two absorption peaks at 3,170 and 3,408 cm−1 arising from the O–H group stretching band and the absorption peak at 1,629 cm−1 correspond to hydroxyl group (O–H) of water [34]. There is a tiny dip in the spectra at 2,362 cm−1 due to the presence of atmospheric CO2 [35]. The FTIR spectra show no impurities such as chloride and ammonium groups in the sample.

Fig. 2
figure 2

FT-infrared spectra of γ-Fe2O3 nanoparticles performed by 200 mg KBr

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The size and shape of maghemite nanoparticles were investigated using TEM device. Figure 3 shows the TEM images and distribution curve of maghemite nanoparticles for (a) the sample that 2 M hydrochloric acid was used in process and (b) is for the sample that deionized water was used in process. Figure 4 shows the distribution of iron oxide nanoparticles for the sample (a) and sample (b). The results show that the size of nanoparticles is decreased, and their homogeneity is increased using deionized water in process in comparison with the process that 2 M hydrochloric acid was used. The shape of nanoparticles was spherical and the average size of them was 8 nm for first sample and 12 nm for second sample.

Fig. 3
figure 3

TEM images of γ-Fe2O3 nanoparticles for a the sample that 2 M hydrochloric acid was used in process and b the sample that deionized water was used in process

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Fig. 4
figure 4

Particle size distributions measured from TEM images for a the sample that 2 M hydrochloric acid was used in process and b the sample that deionized water was used in process

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Figure 5 shows a magnetization hysteresis loop for the prepared γ-Fe2O3 nanoparticle powder at room temperature. The values of saturation magnetization and coercivity were Ms = 19.2 emu g−1, Hs = 7.7 Oe for first sample and were Ms = 50 emu g−1, Hs = 50 Oe for second sample. By increasing saturation magnetization factor, the size of the nanoparticle is decreased [37].

Fig. 5
figure 5

Magnetization hysteresis loop for as-prepared powder

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Conclusions

In summary, γ-Fe2O3 nanoparticles have been prepared by chemical precipitation method at room temperature. The procedure in the present study offers very important advantageous features for preparation of maghemite nanoparticles. The synthetic process is economical, able to control the size of nanoparticles and production scale up and do not need heating stage and complex equipment. The XRD patterns indicated that as-synthesized iron oxide nanoparticles were maghemite. According to the TEM image, the particle size was around 9 nm, and particle shape was almost a sphere. The saturation magnetization of the iron oxide nanopowders was 50 emu g−1.