Preparation of magnetic nanoparticles via chemically induced transition

Abstract

Using an FeOOH/Mg(OH)₂ precursor, maghemite-based magnetic nanoparticles can be prepared by a chemically induced transition in an Iron(II) chloride (FeCl₂) treating solution. FeCl₂ solutions of various concentrations were used to investigate the dependence of sample components and magnetization on the treating solution. The bulk chemical species, crystal structures, surface chemical components, morphologies, and specific magnetizations of the samples were characterized. When the concentration of FeCl₂ solution was in a moderate range of 0.060–0.250 M, maghemite nanoparticles coated by hydromolysite, that is, maghemite/hydromolysite nanoparticles, could be prepared. At lower concentrations, below 0.030 M, the samples contained maghemite/hydromolysite and magnesium oxide nanoparticles, and at higher concentrations, up to 1.000 M, the samples contained maghemite/hydromolysite and hydromolysite nanoparticles. The molar and mass percentages of each phase were estimated for each sample. The apparent magnetization behavior of the samples, which exhibited a non-monotonic variation with increasing concentration of FeCl₂ solution, is explained from the variation of mass percentage of the maghemite phase with concentration.

Keywords

Magnetic nanoparticles concentration component magnetization

Introduction

Nanoscale materials having dimensions of roughly 1–100 nm can display different physical and chemical properties from those of their bulk counterparts. Nanoparticles are typically defined as solids measuring less than 100 nm in all three dimensions.¹ Magnetic nanoparticles have attracted increasing interest as particles in this size range and may allow investigation of fundamental aspects of magnetic ordering phenomena in magnetic materials with reduced dimensions, leading to new technological applications.^2
–4 Studies of magnetic nanoparticles have focused on the development of novel synthesis methods.¹ The synthesis of nanoparticles is a complex process, and hence there is a wide range of techniques available for producing different kinds of nanoparticles.⁵ Liquid-phase synthesis remains one of the most common methods to obtain inorganic nanoparticles. The synthesis of many oxide nanoparticles, including ferrite particles, can be achieved by the co-precipitation method. Reactions for the synthesis of oxide nanoparticles can be classified into two categories: oxide nanoparticles produced directly and production of a precursor that is then subjected to further processing (drying, calcination, and so on).⁶

Iron (Fe) oxides, including maghemite (γ-Fe₂O₃), magnetite, hematite, akaganetite, and goethite, have attracted enormous attention due to their interesting electrical,⁷ magnetic,⁸ and catalytic⁹ properties. They show a wide variety of potential applications in various fields as electro-optical materials,¹⁰ sorbents,¹¹ ion exchanges,¹² magnetic resonance imaging,¹³ catalysis,¹⁴ biotechnology,¹⁵ magneto-optical effects,¹⁶ the removal of toxic metals ions,¹⁷ and so on. In the past decade, the synthesis of magnetic Fe oxide nanoparticles has been intensively developed not only because of its fundamental scientific interest but also for its many technological applications.¹⁸ Many methods have been presented for the preparation of γ-Fe₂O₃ nanoparticles, such as co-precipitation, gas-phase reaction, thermal decomposition, sonochemical synthesis, microemulsion techniques, hydrothermal synthesis,^19

–25 and so on. Generally, the preparation of γ-Fe₂O₃ particles by FeOOH transformation is a complex process, including dehydration, reduction, and oxidation.²⁶ Recently, we proposed a method of liquid-phase synthesis to produce γ-Fe₂O₃-based magnetic nanoparticles, which involved preparing an FeOOH/Mg(OH)₂ precursor having loose aggregation without regular edges and treating the hydroxide precursors in an Iron(II) chloride (FeCl₂) solution at 100°C.²⁷ For the preparation, FeOOH species dehydrated to transform it into the γ-Fe₂O₃ nanoparticles and Mg(OH)₂ dissolved to act as a precipitation agent. This method is referred to as chemically induced transition (CIT),²⁸ which is a novel reaction on a nanoscale and may provide a new route for the preparation of oxide nanoparticles. In the work presented here, we investigate the components and magnetization of the nanoparticles as a function of the concentration of FeCl₂ treating solution used.

Experiments

Chemicals

FeCl₂.6H₂O(AR), Mg(NO₃)₂.6H₂O(AR), NaOH(AR), FeCl₂.4H₂O(AR), and acetone(AR) were all purchased from China National Medicines Corporation Ltd (Shanghai, China), and they were used as received without further purification. Distilled water was used throughout the experiments.

Preparation

The preparation of the nanoparticles by a CIT was divided into two steps. First, a precursor based on FeOOH wrapped with Mg(OH)₂ was synthesized using the well-known co-precipitation method, which has been described in detail elsewhere.²⁴ Second, 5 g of the dried precursor was added to various FeCl₂ solutions using the concentrations of 0.010, 0.030, 0.060, 0.125, 0.250, and 1.000 M to obtain 400 mL of solution (samples 1–6, respectively). The mixing solutions, whose pH values were about 6, were then heated to boiling for 30 min in atmosphere, and the nanoparticles gradually precipitated out after the heating was completed. Finally, these samples were washed with acetone and allowed to dry at laboratory atmosphere. It was noted that sample 6 was significantly more hydrophilic than the other samples.

Characterization

The bulk chemical species were analyzed by energy disperse X-ray spectroscopy (EDS) using an EDS spectrometer on a scanning electron microscope (Quanta-200, Hillsboro, Oregon State, USA) at 25 kV. The crystal structures were measured by X-ray diffractometry (XRD; D/Max-RC, Japan) with Cu Kα radiation. Sample’s morphologies and microstructure were observed by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM; JEM-2100F, Japan). Surface chemical compositions were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi; Boston, Massachusetts, USA) with Al Kα radiation. The specific magnetization curves of the precursor and a series of samples were measured by a vibrating sample magnetometer (HH-15; China) using go-and-return magnetic field cycles at room temperature.

Results and analysis

EDS analysis indicated that all the samples contained oxygen (O), Fe, and chlorine (Cl) elements. In addition, there was magnesium (Mg) in samples 1 and 2. For quantitative analysis, many zones in each sample were probed to average the content of each element. Typical EDS spectra and the zone probed are shown in Figure 1. The quantitative results are shown in Table 1. The ratio of Cl to Fe increased monotonically with increasing concentration of FeCl₂ solution, which means the Cl-containing phase increased with increasing concentration.

Figure 1.

Typical (a) EDS spectra and (b) zone probed for the samples. EDS: energy disperse X-ray spectroscopy.

Table 1.

Atomic percentages of the samples from EDS.

Samples	O	Fe	Cl	Mg	Cl/Fe(× 10⁻²)
1	60.00	35.53	0.50	3.97	1.14
2	63.31	33.63	0.54	2.52	1.61
3	69.36	29.80	0.84		2.82
4	64.17	34.67	1.16		3.35
5	60.48	37.93	1.59		4.19
6	52.49	37.27	10.24		27.47

EDS: energy disperse X-ray spectroscopy; O: oxygen; Fe: iron; Mg: magnesium; Cl: chlorine.

The XRD results (Figure 2) showed that these samples contained mainly γ-Fe₂O₃ (JCPDS file 39-1346). In addition, there could be trace of hydromolysite (FeCl₃·6H₂O; JCPDS file 33-0645), whose diffraction peaks of ( $\bar{2}$ 01) (2θ = 19.429) and (202) (2θ = 36.774) overlapped with that of (111) (2θ = 18.384) and (222) (2θ = 37.249) of the γ-Fe₂O₃ due to the diffraction peaks broadening, respectively. Consequently, the peaks of both ( $\bar{2}$ 01) and (202) planes of FeCl₃·6H₂O could appear through the strength are very weak. Samples 1 and 2 also had magnesium oxide (MgO; JCPDS file 30-0794). The intensity of diffraction peaks corresponding to the γ-Fe₂O₃ phase varies non-monotonically with increasing concentration of FeCl₂ treating solution, and sample 3 showed the strongest response.

Figure 2.

XRD patterns for the samples with (hkl), (hkl)_Mg, and (hkl)_Cl corresponding to γ-Fe₂O₃, MgO, and FeCl₃.6H₂O, respectively. (?)_Mg means that the index corresponding the peak of MgO is undetermined in JCPDS file 30-0794. XRD: X-ray diffractometry; γ-Fe₂O₃: maghemite; MgO: magnesium oxide; FeCl₃.6H₂O: hydromolysite.

Typical TEM images of the samples are shown in Figure 3. Particles in samples 1–5 are mostly spherical, with size ranging from 4 nm to 20 nm. Samples 1 and 2 show a mixture of large (arrow A) and small (arrow B) particles. Sample 6 shows both rod-shaped (inset) and spherical particles. HRTEM indicates that the spherical and rod-shaped particles in sample 6 are γ-Fe₂O₃ and FeCl₃·6H₂O single crystalline grains, respectively, as shown in Figure 4. Also, as shown in Figure 4, it can be seen from HRTEM images that the both γ-Fe₂O₃ and FeCl₃·6H₂O particles contained a coating layer, whose thickness is about 0.8 nm. The fast Fourier transform pattern of the spherical particles (inset, Figure 4(a)) justifies the crystalline plans displayed on the XRD pattern.

Figure 3.

TEM images for the samples. TEM: transmission electron microscopy.

Figure 4.

HRTEM image of (a) spherical particle and (b) rod-shaped particles in sample (6). The corresponding spacing of 0.250 nm is indexed to the (311) plane of γ-Fe₂O₃ and that of 0.443 nm is indexed to the (11) plane of FeCl₃·6H₂O, respectively. Inset exhibits the FFT pattern of the image. HRTEM: high resolution transmission electron microscopy; FFT: fast Fourier transform; γ-Fe₂O₃: maghemite; FeCl₃.6H₂O: hydromolysite.

Combining the results from EDS, XRD, and HRTEM, we conclude there could be two Fe-containing phases, γ-Fe₂O₃ and FeCl₃·6H₂O in all samples and an additional MgO phase in samples 1 and 2. To study the surface characteristics of the particles, XPS analysis was carried out, which indicated the existence of the same chemical species for each samples as EDS. It is known that XPS spectra can be used to determine the oxidation state and the environment of Fe.^29
–31 For samples 1–5, the measured binding energy of O1s was approximately 530 eV, which agreed with binding energy of O1s line for ferric oxide.³¹ The binding energy of O1s for sample 6 was 531.7 eV, which was significantly larger than that of the other samples and meant having two components. The Fe 2p_3/2 peaks for all the samples were approximately 710.5 eV, which could be assigned to Fe³⁺ oxidation sample. Accordingly, the Fe 2p_3/2 peaks were resolved into two peaks for each of the samples, and the O1s peak was resolved into two peaks for sample 6, as shown in Figure 5, in which the solid lines are experimental results and dashed lines are resolved results. Table 2 summarizes the results extracted from the XPS analysis. These results confirmed the presence of γ-Fe₂O₃ and FeCl₃·6H₂O in all samples, MgO in samples 1 and 2, and molecular water in sample 6. In addition, the O1s peak corresponding to crystal water (P1 peak) in sample 6 is very strong, whereas that in other samples is difficult to distinguish. This means that the FeCl₃·6H₂O phase in sample 6 could be far more than that in other samples. The signal collection depth in the XPS experiments is 3λ, where λ = 1.12 nm for Fe 2p electrons.³² Thus, combining the results of HRTEM, it can be judged that for spherical particles contain γ-Fe₂O₃ and FeCl₃·6H₂O, γ-Fe₂O₃ is the core, and FeCl₃·6H₂O is the coating due to the depth of the Fe 2p electrons measured in XPS (approximately 3.4 nm) being larger than the thickness of the coating layer (approximately 0.8 nm).

Figure 5.

XPS spectra of the (a) O 1s and (b) Fe 2p_3/2. XPS: X-ray photoelectron spectroscopy; O: oxygen; Fe: iron.

Table 2.

Binding energies from XPS (electronvolt) for the elements in the samples.^a

Samples	O 1s		Fe 2p_3/2		Cl 2p_3/2	Mg 1s
Samples	P1	P2	P1	P2	Cl 2p_3/2	Mg 1s
1		530.14	711.30	709.90	199.23	1303.62
2		530.04	711.30	709.90	198.23	1303.87
3		529.88	711.20	710.00	198.79
4		530.17	711.30	709.90	198.42
5		530.08	711.20	710.00	198.24
6	532.00	530.50	711.30	709.90	198.90
Fe₂O₃		529.5		709.9
FeCl₃			711.3		199.0
MgO		530.2				1303.9
H₂O	532.8

^aData for Fe₂O₃, FeCl₃, MgO, and H₂O are sourced from the NIST online database for XPS (www.nist.gov).

XPS: X-ray photoelectron spectroscopy; O: oxygen; Fe: iron; Mg: magnesium; Cl: chlorine.

Figure 6 shows the specific magnetization curves of the precursor and the samples. The precursor was paramagnetic, whereas the samples prepared using the FeCl₂ treating solution exhibited a superparamagnetic-like transition having no clear remanence or coercivity. The specific magnetization of the samples varied non-monotonically, similar to the γ-Fe₂O₃ diffraction peak intensity, with increasing concentration of FeCl₂ treating solution, that is, magnetization increased as the concentration of the treating solution increased from 0.010 M to 0.060 M and then decreased with increasing concentration from 0.060 M to 1.000 M. The specific saturation magnetization (σ_s ) of as-prepared samples was obtained from σ versus 1/H data at high field.³³ The values of σ_s are listed in Table 3.

Figure 6.

Specific magnetization curves for the precursor and samples.

Table 3.

Specific saturation magnetization, σ_s , molar percentage, y_i , and mass percentage, z_i , of the phase for the samples.

Samples	σ_s (emu/g)	Molar percentage (y)	Mass percentage (z)
Samples	σ_s (emu/g)	γ-Fe₂O₃/FeCl₃·6H₂O/MgO	γ-Fe₂O₃/FeCl₃·6H₂O/MgO
1	31.1	81.04/0.76/18.20	93.23/1.48/5.29
2	37.0	86.10/0.93/12.97	94.67/1.73/3.60
3	66.9	98.14/1.86	96.89/3.10
4	59.9	97.79/2.21	96.31/3.68
5	50.5	97.24/2.76	95.41/4.51
6	24.7	83.22/16.78	74.56/25.44

γ-Fe₂O₃: maghemite; FeCl₃·6H₂O: hydromolysite; MgO: magnesium oxide.

Discussion

According to the experimental results and analysis, it was determined that when the precursor based on FeOOH/Mg(OH)₂ was treated in the FeCl₂ solution, the FeOOH species was transformed into γ-Fe₂O₃ crystallites by dehydration, and the Mg(OH)₂ species was dissolved to either assist the precipitation of nanoparticles or to form MgO particles. During the precipitation, both Fe³⁺ resulted from the oxidized Fe²⁺ and Cl⁻ were absorbed on the γ-Fe₂O₃ crystallites to form γ-Fe₂O₃-coated FeCl₃·6H₂O (γ-Fe₂O₃/FeCl₃·6H₂O) nanoparticles, and single FeCl₃·6H₂O nanorods could also be formed. The characteristics of the as-prepared sample components depended on the concentration of the FeCl₂ treating solution used. In a moderate concentration range from 0.060 M to 0.250 M, the samples were pure γ-Fe₂O₃/FeCl₃·6H₂O nanoparticles, in which γ-Fe₂O₃ crystallites were coated by FeCl₃·6H₂O.²⁸ With concentrations less than 0.060 M, the dissolved Mg partially formed MgO, such that the sample contained spherical γ-Fe₂O₃/FeCl₃·6H₂O nanoparticles and a few MgO nanoparticles. For high concentrations, the samples contained spherical γ-Fe₂O₃/FeCl₃·6H₂O nanoparticles and rod-shaped FeCl₃·6H₂O nanoparticles, which shown that in addition to Fe³⁺ and Cl⁻ adsorbing on the γ-Fe₂O₃ crystallites to form FeCl₃·6H₂O coating, excessive Fe³⁺ and Cl⁻ would crystallize to form FeCl₃·6H₂O nanorod. On the surface of the rod-shaped particles, there exists a thin amorphous layer (see HTRM image), which could result from crystal symmetry breaking at the surface.^1,34 Furthermore, for concentrations exceeding 0.060 M, the samples contained no MgO species. This indicates that the acidity of the treating solution, which increased with increasing FeCl₂ concentration, could make MgO formation difficult.

For magnetic nanoparticle systems having many phases, the magnetization of the system is related to the ratios among the phases.^26,35 Samples 1 and 2 included γ-Fe₂O₃, FeCl₃·6H₂O, and MgO phases. Using the measured atomic percentages of Fe, Cl, and Mg (a _Fe, a _Cl, and a _Mg, respectively), the molar percentages of the γ-Fe₂O₃ phase, y_γ , FeCl₃·6H₂O phase, y _Cl, and MgO phase, y _Mg, could be estimated by

y_{γ} = \frac{\frac{(\frac{a_{Fe} - a_{Cl}}{3})}{2}}{\frac{(\frac{a_{Fe} - a_{Cl}}{3})}{\frac{2 + a_{Cl}}{3 + a_{Mg}}}} \times 100,

y_{Cl} = \frac{\frac{a_{Cl}}{3}}{\frac{(\frac{a_{Fe} - a_{Cl}}{3})}{\frac{2 + a_{Cl}}{3 + a_{Mg}}}} \times 100,

and

y_{Mg} = \frac{a_{Mg}}{\frac{(\frac{a_{Fe} - a_{Cl}}{3})}{\frac{2 + a_{Cl}}{3 + a_{Mg}}}} \times 100

Samples 3–6 have γ-Fe₂O₃ and FeCl₃·6H₂O phases. Using the measured atomic percentages of Fe and Cl (a _Fe and a _Cl, respectively), the molar percentages of y_γ and y _Cl are estimated by

y_{γ} = \frac{\frac{(\frac{a_{Fe} - a_{Cl}}{3})}{2}}{\frac{(\frac{a_{Fe} - a_{C l}}{3})}{\frac{2 + a_{C l}}{3}}} \times 100

and

y_{Cl} = \frac{\frac{a_{Cl}}{3}}{\frac{(\frac{a_{F e} - a_{C l}}{3})}{\frac{2 + a_{C l}}{3}}} \times 100

Thus, the molar percentages of y_i in the respective samples are estimated from the atomic percentages (a _Fe, a _Cl, and a _Mg) measured by EDS (see Table 1). These values are listed in Table 3. The mass percentage of each phase in a sample, z_i , is

z_{i} = \frac{y_{i} A_{i}}{\sum y_{i} A_{i}} \times 100

where A_i is the molar mass of the ith phase. The calculated z_i values are also listed in Table 3. The γ-Fe₂O₃ content changed non-monotonically with increasing concentration of FeCl₂ solution, similar to the XRD results. Also, it can be seen from Table 3 that the FeCl₃·6H₂O content increased monotonically with increasing concentration of FeCl₂ treating solution, which confirms that the FeCl₃ in the samples resulted from the treatment solution. From Table 3, the FeCl₃·6H₂O content in sample 6 is significantly larger than that in the other samples. The additional oxide has a binding energy approximately 532.00 eV (Table 2), which could be attributed to crystal water in the FeCl₃·6H₂O, which are hydrophilic. As a consequence, sample 6 was significantly more hydrophilic than the other samples since the FeCl₃·6H₂O content of the former was far more than that of the latter. Additionally, the FeCl₃·6H₂O content in samples 1–5 is much less than the γ-Fe₂O₃ content, and so the additional oxide is difficult to distinguish from the oxide of γ-Fe₂O₃ from the O spectrum of the XPS.

For the systems of particles containing many phases, the magnetization should be given by the weighted sum of the contribution from every phase. For samples 1 and 2, the specific magnetization, σ, is

σ = φ_{m, γ} σ_{γ} + φ_{m, Cl} σ_{Cl} + φ_{m, Mg} σ_{Mg}

whereas for samples 3–6, it is

σ = φ_{m, γ} σ_{γ} + φ_{m, Cl} σ_{Cl}

where σ_γ , σ _Cl, and σ _Mg are the specific magnetization of γ-Fe₂O₃, FeCl₃·6H₂O, and MgO phases, respectively, and φ_m,i=(z_i /100) is the mass fraction. Since FeCl₃·6H₂O and MgO are non-magnetic and z _Cl, z _Mg is much less than z_γ , equations (4) and (5) can be simplified as

σ \approx φ_{m, γ} σ_{γ}

Thus, the specific magnetization of the samples is determined by the mass fraction of the γ-Fe₂O₃ phase. From Table 3, φ_m,γ varies non-monotonically with increasing concentration of FeCl₂ solution, that is, φ_m,γ increases with increasing concentration from 0.010 M [sample (1)] to 0.060 M [sample (3)], then decreases as the concentration increases to 1.000 M [sample (6)], which corresponds with the measured σ_s from samples (1)–(6).

Conclusions

Using a CIT to prepare γ-Fe₂O₃-based magnetic nanoparticles, the components of the as-prepared products were found to depend on the concentration of the FeCl₂ treatment solution. When the FeCl₂ concentration was less than 0.060 M, the samples consisted of γ-Fe₂O₃/FeCl₃·6H₂O and spherical MgO nanoparticles. When the FeCl₂ concentration reached 1.000 M, the sample consisted of spherical γ-Fe₂O₃ and rod-shaped FeCl₃·6H₂O nanoparticles. In an appropriate range of FeCl₂ concentration in the treating solution, 0.060–0.250 M, the samples contained only spherical γ-Fe₂O₃/FeCl₃·6H₂O nanoparticles. The apparent magnetization was determined by the content of the γ-Fe₂O₃ phase presented in the sample. As the concentration of FeCl₂ solution increased, the consequent concentration of FeCl₃·6H₂O increased, whereas that of MgO decreased, such that the mass fraction of γ-Fe₂O₃ varied non-monotonically. Thus, the specific magnetization of the samples also exhibited non-monotonic variation with increasing concentration of FeCl₂ solution. For the γ-Fe₂O₃/FeCl₃·6H₂O magnetic nanoparticles, the FeCl₃·6H₂O inert surface helped improve their chemical stability and prevented aggregation. In the concentration range of 0.060–0.250 M, the samples contained single γ-Fe₂O₃/FeCl₃·6H₂O nanoparticles. The ratio of the γ-Fe₂O₃ to FeCl₃·6H₂O phase and hence the magnetization can therefore be modified by changing the concentration of FeCl₂ solution used. Such γ-Fe₂O₃-based nanopaticles having hydrophilic FeCl₃·6H₂O could be very suitable for the synthesis of the ionic ferrofluids.³⁶

Obviously, for the preparation of γ-Fe₂O₃-based magnetic nanoparticles by the CIT method, the dehydration of FeOOH into γ-Fe₂O₃ (2FeOOH→γ-Fe₂O₃ + H₂O) was induced by the Fe²⁺ in the FeCl₂ solution transforming into Fe³⁺. The relation between the acting energy of alkali-oxide dehydration and oxidation of ferrous ions may be very interesting as it indicates an unknown reaction process and a new route for preparing oxide nanoparticles. This mechanism will be further investigated.

Footnotes

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: Financial support for this work was provided by the Innovation Foundation Project of Southwest University, China (grant no. 1318001), and the National Science Foundation of China (grant no. 11274257).

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