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Nanomaterials 2017, 7(8), 220; doi:10.3390/nano7080220

Article
Preparation of Magnetic Nanoparticles via a Chemically Induced Transition: Role of Treating Solution’s Temperature
Ting Zhang 1, Xiangshen Meng 1, Zhenghong He 1, Yueqiang Lin 1, Xiaodong Liu 1, Decai Li 2, Jian Li 1,*Orcid and Xiaoyan Qiu 1,*
1
School of Physical Science and Technology, Southwest University, Chongqing 400715, China
2
State Key Laboratory of Tribology, Tsinghua University, Beijing 100084, China
*
Correspondence:
Received: 14 June 2017 / Accepted: 3 August 2017 / Published: 12 August 2017

Abstract

:
Using FeOOH/Mg(OH)2 as precursor and FeCl2 as the treating solution, we prepared γ-Fe2O3 based nanoparticles. The FeCl2 treating solution catalyzes the chemical reactions, dismutation and oxygenation, leading to the formation of products FeCl3 and Fe2O3, respectively. The treating solution (FeCl2) accelerates dehydration of the FeOOH compound in the precursor and transforms it into the initial seed crystallite γ-Fe2O3. Fe2O3 grows epitaxially on the initial seed crystallite γ-Fe2O3. The epitaxial layer has a magnetically silent surface, which does not have any magnetization contribution toward the breaking of crystal symmetry. FeCl3 would be absorbed to form the FeCl3·6H2O surface layer outside the particles to form γ-Fe2O3/FeCl3·6H2O nanoparticles. When the treating solution’s temperature is below 70 °C, the dehydration reaction of FeOOH is incomplete and the as-prepared samples are a mixture of both FeOOH and γ-Fe2O3/FeCl3·6H2O nanoparticles. As the treating solution’s temperature increases from 70 to 90 °C, the contents of both FeCl3·6H2O and the epitaxial Fe2O3 increased in totality.
Keywords:
γ-Fe2O3 nanoparticle; FeCl2 solution; temperature; magnetization

1. Introduction

Nanotechnology involves the study of matter whose dimensions approximately range between 1 and 100 nm [1]. Nanoparticles are typically defined as tiny solids, whose dimensions do not exceed 100 nm in all the three directions [2]. Magnetic nanoparticles have attracted a lot of interest in the community of researchers, because these tiny particles are extremely useful models for understanding the fundamental aspects of magnetic ordering phenomena in magnetic materials with small dimensions. The findings of these research studies can be used to develop novel technological applications [3,4,5]. In most studies of magnetic nanoparticles, scientists have tried to develop novel synthesis methods [2]. Liquid phase synthesis is one of the most common methods to produce inorganic nanoparticles. Many oxide nanoparticles, including ferrite particles, can be synthesized by co-precipitation. The chemical reactions involved in the synthesis of oxide nanoparticles can be classified into two categories: (i) oxide nanoparticles produced directly and (ii) production of a precursor that is then subjected to further processing, such as drying, calcination, etc. [6]. During the chemical reaction, a new phase is formed that is later subjected to further processing, such as calcination or annealing.
Presently, γ-Fe2O3(maghemite) particles are one of the commonly used ferric oxide particles for their simple synthesis procedures and chemical stability [7]. Maghemite exhibits ferrimagnetic behavior at temperatures lower than 1000 K. Furthermore, it is found in corrosion products, but also in several useful compounds, including proteins [8]. It has many industrial applications: as a drug delivery agent; in nuclear magnetic resonance imaging; in magnetic data storage applications; etc. [7,8,9]. Previous studies have described many novel methods for the preparation of γ-Fe2O3 magnetic nanoparticles, including co-precipitation, gas-phase reaction, direct thermal decomposition, thermal decomposition/oxidation, sonochemical synthesis, microemulsion reaction, hydrothermal synthesis, vaporization-condensation, and sol-gel approach [10,11,12,13,14,15,16,17,18]. In general, the preparation of γ-Fe2O3 by FeOOH transformation is a complex process [19,20] that can be summarized as follows:
α ( γ ) FeOOH  dehydration  α ( γ ) - F e 2 O 3  reduction  F e 3 O 4  oxidation  γ - F e 2 O 3
We have found a new route to synthesize γ-Fe2O3 magnetic nanoparticles. In this method, we synthesize the precursor FeOOH/Mg(OH)2 by conducting a chemical co-precipitation method. The resultant hydroxide precursor FeOOH/Mg(OH)2 is subsequently treated in the liquid phase with FeCl2 solution [21]. During the treatment, Mg(OH)2 compound dissolves and the FeOOH undergoes dehydration and transforms into γ-Fe2O3 nanoparticles:
FeOOH / Mg ( OH ) 2 in Fecl 2 solution Δ γ - F e 2 O 3 + H 2 O + M g 2 + + O H -
This method is known as chemically induced transition (CIT) [22,23]. Under boiling conditions, we could synthesize γ-Fe2O3 nanoparticles coated by FeCl3·6H2O by ensuring that the concentration of the FeCl2 solution was in the range of 0.06–0.25 M [23]. In this experimental study, we adjust the temperature of the treating solution and investigate whether magnetization is dependent on the temperature, and the relevance between magnetization and components.

2. Experimental

2.1. Preparation Using Chemicals

From China National Medicines Corporation Ltd. (Shanghai, China), we purchased the following analytical reagent (AR) grade chemicals: FeCl3·6H2O, Mg(OH)2·6H2O, NaOH, FeCl2·4H2O and acetone. Since these reagents were of AR quality, we used them without performing further purification. We used only distilled water for performing the preparations of solutions in the experiment.
While performing this CIT method, we categorically divided the preparation process of nanoparticles into two steps: (i) we carried out the well-known method of co-precipitation to synthesize a precursor based on FeOOH; the precursor was wrapped with Mg(OH)2. The synthesis of this precursor has been described in detail elsewhere [21]; (ii) we added 5 g of the dried precursor to 400 mL of 0.25 M FeCl2 solution. The pH of resultant solution was about 6. Then, the resultant solution was heated to a certain temperature, and then it was refluxed for 30 min in air. After completing the process of heating, we were able to obtain nanoparticles gradually in the form of a precipitate. Finally, we washed the precipitate with acetone and air-dried it in the laboratory. We obtained the samples (1)–(5) by adjusting the temperature of the treating solution to the following respective values: 40, 60, 70, 80, and 90 °C.

2.2. Characterization

For precursor and samples (1)–(5), we measured the curves of specific magnetization (σ) against field strength (H) using vibrating sample magnetometer (VSM) (HH-15, Nanjing University Instrument Plant, Nanjing, China). After obtaining the measured results of VSM, we performed transmission electron microscopy (TEM) (TEM-2100F, Tokyo, Japan) on all the samples; however, we record particle morphologies only in the following typical sample (1) (treated solution temperature: 40 °C), sample (3) (treated solution temperature: 70 °C), and sample (5) (treated solution temperature: 90 °C), according to the results measured by VSM. Then, we analyzed the crystal structure of samples by X-ray diffraction (XRD) (D/Max-Rc, Rigaku, Tokyo, Japan). We analyzed the bulk chemical species by performing energy disperse X-ray spectroscopy (EDS) in a scanning electron microscopy (SEM) (Quanta-200, FEI, Hillsboro, OR, USA). Finally, we analyzed surface chemical compositions of samples by performing X-ray photoelectron spectroscopy (XPS) (ESCALAB 250 xi, Thermo Fisher Scientific, Waltham, MA, USA).

3. Results

Figure 1 illustrates the curves representing the plot of σ against H for various samples. The precursor was paramagnetic. In contrast, the as-prepared samples exhibited ferromagnetic transition because they were treated with FeCl2 solution. Furthermore, the specific magnetization of samples varied non-monotonically with an increase in the temperature of the treating solution: the magnetization (σ) values increased drastically as the temperature of treating solution (FeCl2) was increased from 40 to 70 °C, but then σ values of samples decreased slightly with a further increase in temperature from 70 °C to 90 °C of the treated solution. The specific saturation magnetization (σs) of the as-prepared samples was obtained from the plot of σ versus 1/H at high field strength [24]. For samples (1)–(5), the σs values are 42.96, 59.33, 70.51, 68.18, and 66.61 A·m2/kg, respectively.
By performing TEM on all the samples, we noted the following observations: the samples (1), (3), and (5) are mostly spherical nanoparticles, with sizes ranging from 2 to 30 nm. Figure 2 illustrates TEM images of the samples. In the case of Sample (1), TEM images clearly depict a small mixture (refer arrow A) and large (refer arrow B) particles. We performed statistical analysis of the results observed for samples (3) and (5) [25]. The histograms of the particle size are illustrated as the insets in Figure 2. Based on the statistical analysis, we inferred the particle size exhibited a log-normal form of distribution. Table 1 presents the median diameter, that is, the most probable value of the particle size dg, and the standard deviation lnσg. High resolution TEM measurements have been performed in some of the samples (see inset in Figure 2), confirming that the nanoparticles are crystallines.
As shown in Figure 3, XRD patterns reveal that the samples (1), (3), and (5) predominantly contained maghemite (γ-Fe2O3; JCPDS file 39-1346) with traces of hydromolysite (FeCl3·6H2O; JCPDS file 33-0645). In addition, sample (1) may contain some crystals of iron oxide hydroxide (FeOOH; JCPDS file 13-0157), whose diffraction peak in (211) plane (2θ = 35.264) overlapped with the diffraction peak of γ-Fe2O3 in (311) plane (2θ = 35.630). This phenomenon is attributed to the broadening of diffraction peaks. For samples (3) and (5), we used Scherrer’s formula to estimate the most probable grain size (dc) from the half-maximum width of (311) diffraction peak (β) [26,27]. The expression of Scherrer’s formula is as follows: dc = kλ/βcosθ, where k is the coefficient and equals to 0.89 [28], λ is the wavelength (Cu Kα wavelength is 0.1542 nm), and θ is the Bragg diffraction angle of (311) plane. Table 1 presents dc values of samples (3) and (5). These values indicate that dc value is almost same for both the samples (3) and (5).
By performing energy-dispersive X-ray spectroscopy (EDS), we found that all the three samples contained O, Fe, and Cl, but not Mg and Na. For quantitative analysis, many zones were probed to average the content of each element. Figure 4 illustrates images of typical EDS spectra. Table 2 summarizes the quantitative results of this experiment.
After comparing the results of samples analyzed by XRD and EDS techniques, we conclude that γ-Fe2O3 and FeCl3·6H2O may be the primary constituents in samples (1), (3), and (5); however, an additional FeOOH compound may be present in sample (1). To examine the surface characteristics of particles, we performed an XPS analysis on the samples. The results of the XPS analysis indicate that the chemical species detected in each sample were the same as those detected by EDS. Table 2 presents a quantitative analysis of results. For sample (1), O1s spectra can be resolved into two peaks: P1 and P2 (See Figure 5a). The P1 peak corresponds to O1s line in samples (3) and (5), which approximately appears at 529.3 eV. Thus, the P1 peak’s energy agreed with the binding energy of O1s in ferric oxide. The P2 peak appears at 530.66 eV, which is same as the binding energy of O1s in FeOOH. Furthermore, the Fe 2p3/2 spectra for sample (1), (3), and (5) can be resolved into two peaks: P1 and P2. As shown in Figure 5b, peak P1 corresponds to Fe 2p3/2 line of Fe2O3, while P2 peak corresponds to that of FeOOH and/or FeCl3. Table 3 summarizes the results obtained by performing XPS analysis on the samples. Thus, based on the binding energy data, we conclude that γ-Fe2O3, FeCl3·6H2O and FeOOH were present in sample (1), while γ-Fe2O3 and FeCl3·6H2O were present in samples (3) and (5).

4. Discussion

Based on the experimental results, it is showed that the magnetization of as-prepared samples varied non-monotonically with an increase in the temperature of treated solution. Combining the results from XRD and XPS, we conclude there could be γ-Fe2O3 and FeCl3·6H2O phases in all samples and an addition FeOOH phase in sample (1), which is in agreement with our previous work [21]. In addition, it is noticed that the ferrite-like spinel structure, γ-Fe2O3 and Fe3O4, is difficult to discriminate by XRD due to peak broadening [29] or by XPS because the data are very close (the binding energy of Fe 2p3/2 in Fe3O4 is 710.8 eV). However, Fe3O4 is not stable and is sensitive to oxidation [9]. It was found that Fe3O4 nanocrystallites transformed into γ-Fe2O3 nanocrystallites using Fe(NO3)3 solution treatment [30]. Therefore, it is judged that the magnetic compound for the as-prepared samples is γ-Fe2O3, rather than Fe3O4. Furthermore, we demonstrated the following synthesis: the precursor of FeOOH was employed as FeOOH/Mg(OH)2, and the resultant complex phase was transformed into γ-Fe2O3 crystallites by dehydration. During this process, Mg(OH)2 species were dissolved in the reaction medium. Such a reaction takes place below the boiling point temperature of water. When the treating solution’s temperature is lower than 70 °C, for example 40 °C, the reaction does not reach completion, leading to the formation of only a few FeOOH nanoparticles. Thus, the compositions of as-prepared samples were as follows: sample (1) contained FeOOH nanoparticles along with γ-Fe2O3-coated FeCl3·6H2O (γ-Fe2O3/FeCl3·6H2O) nanoparticles, which correspond to the smaller and larger particles in sample (1) (See Figure 2). With a steady increase in temperature, this reaction progressed towards completion. At this stage, γ-Fe2O3 phase increased, but FeOOH phase decreased. Consequently, magnetization enhanced from samples (1) to (3).
When the temperature reached 70 °C and increased further, the as-prepared samples were obtained in the form of pure γ-Fe2O3/FeCl3·6H2O nanoparticles. Since the as-prepared samples (3), and (5) contained γ-Fe2O3 and FeCl3·6H2O phase, we infer that the magnetization of samples may be related to the ratio between the two phases [22]. It is noticed that though EDS measurements are usually not very sensitive to oxygen content, the ratio between Fe and Cl elements is independent on the oxygen content. So using the measured atomic percentages of Fe and Cl (aFe and aCl), the molar percentages of Fe2O3 (yFe) and FeCl3·6H2O compounds (yCl) could be estimated by the following formulae:
y Fe = ( a Fe - a Cl / 3 ) / 2 ( a Fe - a Cl / 3 ) / 2 + a Cl / 3 y Cl = a Cl / 3 ( a Fe - a Cl / 3 ) / 2 + a Cl / 3 .
Here, yi is the molar percentage of i compound, in samples (3) and (5), and it can be obtained from the values of aFe and aCl, which were previously measured by EDS and XPS analyses (see Table 2). The results of yi are enlisted in Table 4. As a consequence, the mass fraction percentage (zi) and the volume fraction percentage (ϕi) of each compound in respective samples can be deduced from the following formulae:
z i = y i A i y i A i × 100
and
ϕ i = z i / ρ i z i / ρ i × 100 .
Here, Ai and ρi are the molar mass and the density of i compound, respectively. Accordingly, zi and ϕi values of each compound in samples (3) and (5) were calculated using the values of yi, the molar mass and density of γ-Fe2O3 and FeCl3·6H2O (See Table 4). By referring to the data presented in Table 4, we infer that FeCl3·6H2O/Fe2O3 volume ratio (ϕCl/ϕFe) obtained for each sample by XPS was much larger than that obtained by EDS. It is well-known that EDS information is obtained from signal depths that largely exceed the dimensions of nanoparticles, whereas XPS information is obtained from the surface to a depth of approximately 3λ (λ = 1.27 nm for Fe2P electrons) [31,32]. As Figure 6a shows, EDS results depict an average ϕCl/ϕFe across the entire particle, whereas XPS results depict the ratio of nanoparticles’ surface: dx is the depth measured by XPS; dCl is the thickness of FeCl3·6H2O surface layer and dFe is Fe2O3 region probed. Thus, the difference in ϕCl/ϕFe values computed from EDS and XPS results indicates that FeCl3·6H2O is formed outside the Fe2O3 phase [33] in samples (3) and (5).
In this experiment, the measured results of XPS indicate that ϕCl/ϕFe value of sample (5) is greater than that of sample (3). As shown in Figure 6a, the depth probed by XPS (dx) can be regarded as constant, so this difference in ϕCl/ϕFe values indicates that FeCl3·6H2O surface layer (dCl) of sample (5) was thicker than that of sample (3). Therefore, FeCl3·6H2O content in as-prepared samples increased with an increase in the temperature of the treating solution. However, the measured result obtained by EDS is opposite to that obtained by XPS. Therefore, the ϕCl/ϕFe value obtained from the measured results of EDS is smaller for sample (5) than for sample (3). Thus, we conclude that Fe2O3 content in as-prepared samples would increase steadily with an increase in temperature. Let V F e ( 3 ) and V F e ( 5 ) represent Fe2O3 volume, while V C l ( 3 ) and V C l ( 5 ) represent FeCl3·6H2O volume in samples (3) and (5), respectively. Thus, we deduce the following expressions:
V F e ( 5 ) = V F e ( 3 ) + Δ V F e V C l ( 5 ) = V C l ( 3 ) + Δ V C l
where ∆VFe and ∆VCl are incremental contents of Fe2O3 and FeCl3·6H2O in samples (5) and (3), respectively. The results measured by EDS help us deduce the following expression: ϕ F e ( 5 ) / ϕ C l ( 5 ) > ϕ F e ( 3 ) / ϕ C l ( 3 ) , where ϕ F e ( 5 ) , ϕ C l ( 5 ) , and ϕ F e ( 3 ) ϕ C l ( 3 ) are volume fraction percentages of Fe2O3 and FeCl3·6H2O phases in samples (5) and (3), respectively. Using the expression ϕ F e / ϕ C l = V F e / V C l , we proved that Δ V F e / Δ V C l > V F e ( 3 ) / V C l ( 3 ) ( = ϕ F e ( 3 ) / ϕ C l ( 3 ) ) . Experimental results indicate that the value of ϕ F e ( 3 ) / ϕ C l ( 3 ) is more than unity, so Δ V F e / Δ V C l is more than unity. Thus, compared with sample (3), the incremental content of Fe2O3 ( Δ V F e ) is more than the incremental content of FeCl3·6H2O ( Δ V C l ) for sample (5). Based on these results, we proposed the following process for the formation of nanoparticles:
First, FeOOH in the precursor was subjected to dehydration, which initially led to the seeds of γ-Fe2O3 crystals in the solution. This reaction was accelerated and completed due to the catalytic action of FeCl2 treating solution; the catalytic effect of this treating solution increased as its temperature was increased by heating. Simultaneously, some Fe2+ in the treating solution would undergo dismutation reaction as follows: 3Fe2+ → 2Fe3+ + Fe0 [34,35]. Then, the resultant Fe0 would be oxygenated to form iron oxide in the presence of atmospheric oxygen: 4Fe0 + 3O2 → 2Fe2O3. Thus, an epitaxial Fe2O3 layer was built on initial crystallites, and FeCl3·6H2O was adsorbed onto an epitaxial layer during the precipitation process. Consequently, we synthesized γ-Fe2O3 based nanoparticles coated with FeCl3·6H2O. Such a chemical reaction involving the steps of dismutation and oxygenation can be written as follows:
12 Fecl 2 + 3 O 2 Δ 8 Fecl 3 + 2 F e 2 O 3
A schematic model of particle structure is shown in Figure 6b. Obviously, this reaction (involving dismutation and oxygenation) would be enhanced by increasing the temperature of the treating solution. Consequently, both Fe2O3 and FeCl3·6H2O contents in as-prepared samples increased with temperature.
For the system of particles containing many phases, magnetization can be described as follows: M = σ<ρ>, where <ρ> is the average density of every sample, and it can be obtained as follows:
< ρ > = ϕ i ρ i ϕ i .
Herein, ϕi and ρi are volume fraction percentage and density of i phase, respectively. Thus, based on ϕFe and ϕCl values measured by EDS and the densities of γ-Fe2O3 and FeCl3·6H2O, 4.90 × 103 and 1.844 × 103 kg/m3, respectively, the ρ value was calculated. It was found to be 4.55 × 103 and 4.62 × 103 kg/m3 for samples (3) and (5), respectively. As a consequence, the saturation magnetization (Ms) can be obtained from σs and ρ, and it was computed to be 320.82 and 307.74 kA/m for samples (3) and (5), respectively. In addition, the magnetization can be determined as follows: M = (ϕFeMFe + ϕClMCl)/100, where MFe and MCl are the magnetization of γ-Fe2O3 and FeCl3·6H2O compounds, respectively. According to the definition of volume fraction percentage, ϕFe + ϕCl = 100, M can be written as follows:
M = 1 1 + ϕ C l / ϕ F e ( M F e M C l ) + M C l .
MFe and MCl are regarded as contents. Thus, using the relations MFe >> MCl and ϕFe >> ϕCl, the Formula (7) can be written simply as follows:
M = M F e 1 + ϕ C l / ϕ F e .
From Formula (8), we conclude that saturation magnetization (Ms) is inversely related to ϕCl/ϕFe. Therefore, the smaller the value of ϕCl/ϕFe, the stronger would be Ms. However, experimental results appear to be contradictory because Ms is lower for sample (5) than for sample (3), despite the fact that the ϕCl/ϕFe value of the former is smaller than the latter (see Table 4). This paradox means that the apparent magnetization of as-prepared sample could be not only related to chemical compounds but also to their effective magnetic compounds. We substantiate our claim in the following paragraph.
Surface magnetic properties become extremely important with a decrease in particle size, since a decrease in particle size leads to an increase in surface-to-volume ratio. The properties depend on the surface microstructure and the surrounding, e.g., generally because of variation in the local and exchange fields [34]. In magnetic nanoparticles, crystal symmetry breaking at the surface results in surface anisotropy. This phenomenon is more pronounced in ferrimagnets [36]. Many ramifications are associated with breaking of crystal symmetry at the surface of crystallites. One of the most important developments would be the occurrence of spin disorder in the surface layer [37,38]. With a thickness of 0.3–1.0 nm, the disordered surface layer is similar to a magnetic “dead layer” [29]. Experimental results indicate that the grain size (dc) of both the samples (5) and (3) were almost the same when we compared the measured results obtained by XRD; however, the physical size (dg) measured by TEM is greater for the former (sample 5) than for the latter (sample 3), while the Fe2O3 content (ϕFe) measured by EDS is greater for the former than for the latter. Based on these results, we infer that the epitaxial Fe2O3 layer, which forms on the initial seed crystallites, may have a disordered surface layer due to the breaking of crystal symmetry. This expanse of the disordered layer is similar to the amorphous component and it does not influence XRD measurement because only the crystalline phase is detected with XRD [29]. The thickness of the disordered layer increases as the temperature of the treating solution is increased. Such a disordered layer seems to be magnetically silent, and it does not stimulate the apparent magnetization in any way [38]. The contents of both FeCl3·6H2O and epitaxial Fe2O3 in sample (5) are more than those in sample (3); however, the content of γ-Fe2O3 crystal, that is, the effective magnetic component is almost the same in samples (5) and (3), so the magnetization of sample (5) is weaker than that of sample (3). Accordingly, it can summarized that as the treating solution’s temperature was increased from 70 to 90 °C, the content of both FeCl3∙6H2O and the disordered Fe2O3 increased so that the magnetization behavior of as-prepared samples became weak with a steady increase in temperature.
The zero-field cooled (ZFC) and field-cooled (FC) measurements for magnetic behaviors can reveal the super paramagnetic behavior of a sample. This could be interesting to clarify possible interactions between the different magnetic phases in the sample [39], and will be performed in further work. Mӧssbauer spectroscopy may be used to distinguish γ-Fe2O3 from Fe3O4, since γ-Fe2O3 and Fe3O4 give quite a different spectrum, both above and below the Verwey transition [40]. It will be considered in further works that using Mӧssbauer spectroscopy can determine the maghemite phase in the nanoparticles.

5. Conclusions

Using FeOOH/Mg(OH)2 as a precursor, we prepared γ-Fe2O3 based magnetic nanoparticles in FeCl2 solution. In this chemical reaction, we found that the magnetization of as-prepared products had a non-monotonical variation with an increase in the temperature of treated solution (FeCl2). Experimental results indicate that the magnetization behavior of the as-prepared samples is not only related to the chemical compounds present in the particles, but it also governs the formation of nanoparticles and their effectively magnetic compounds. When the treating solution’s temperature was below 70 °C, for example 40–60 °C, the hydration reaction involving FeOOH species from the precursor does not reach completion. However, the γ-Fe2O3 crystallite core is formed initially in this reaction. As a result, the as-prepared samples contained FeOOH nanoparticles along with γ-Fe2O3/FeCl3·6H2O nanoparticles, and their magnetization levels were weaker. When the temperature of the treated FeCl2 solution was increased from 70 to 90 °C, we could obtain as-prepared samples containing only γ-Fe2O3/FeCl3·6H2O nanoparticles. Both Fe2O3 and FeCl3·6H2O contents increased completely with an increase in temperature. Furthermore, we infer that the FeCl2 treating solution, which has a catalytic effect on the dehydration of FeOOH and its subsequent transformation into γ-Fe2O3 seed crystals, could appear as a two-step reaction involving dismutation and oxygenation; the reaction led to the formation of FeCl3 and Fe2O3 as products of dismutation and oxygenation, respectively. Moreover, the contents of both FeCl3 and Fe2O3 would increase with an increase in temperature. In this synthesis reaction, Fe2O3 grows epitaxially on the initial seed crystals of γ-Fe2O3, whereas FeCl3 is absorbed to form FeCl3·6H2O on the outermost layer of the particles. This epitaxial Fe2O3 could have a γ-Fe2O3 phase layer and disordered surface layer. The disordered surface has the breaking of crystal symmetry, so it seems to be a magnetically silent layer. As a result, it does not have any role in the apparent magnetization of nanoparticles. As the treating solution’s temperature was increased tom 70 to 90 °C, the content of both the products, namely, FeCl3·6H2O and the disordered Fe2O3 increased sharply. Consequently, the magnetization behavior of as-prepared samples became weak with a steady increase in temperature.

Acknowledgments

The financial support for this work was provided by the following institutions: the Fundament Research Fund for the Central University of China (grant number XDJK2017D139), and National Natural Science Foundation of China (grant number 11274257), the Natural Science Foundation of Chongqing (grant number CSTC 2011jeyjA40029).

Author Contributions

T.Z. carried out preparation of samples and analysis of the characterization results, and drafted the manuscript. X.M. participated in the sequence alignment and help to draft the manuscript. Z.H. help to carry out characteristical studies. Y.L. and X.L. carried out the measurements of both VSM and XRD. D.L. participated in the design of the study. J.L. and X.Q. conceived of the study, and participated in the design and coordination. All authors read and approved the final manuscript.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

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Figure 1. Specific magnetization curves of the precursor and as-prepared samples (1)–(3) (a), and as-prepared samples (3)–(5) (b).
Figure 1. Specific magnetization curves of the precursor and as-prepared samples (1)–(3) (a), and as-prepared samples (3)–(5) (b).
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Figure 2. Typical TEM images for samples (1), (3), and (5). The insets are the histograms of the particle sizes for samples (3) and (5), and a High resolution TEM (HRTEM) image for sample (3).
Figure 2. Typical TEM images for samples (1), (3), and (5). The insets are the histograms of the particle sizes for samples (3) and (5), and a High resolution TEM (HRTEM) image for sample (3).
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Figure 3. XRD spectra of samples (1), (3), and (5) with (hkl), (hkl)Cl and (hkl)Mg corresponding to γ-Fe2O3, FeCl3·6H2O and FeOOH phases, respectively.
Figure 3. XRD spectra of samples (1), (3), and (5) with (hkl), (hkl)Cl and (hkl)Mg corresponding to γ-Fe2O3, FeCl3·6H2O and FeOOH phases, respectively.
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Figure 4. EDS spectra of samples (1), (3), and (5).
Figure 4. EDS spectra of samples (1), (3), and (5).
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Figure 5. XPS spectra of samples (1), (3), and (5), representing O ls (a) Fe 2p3/2 (b) and Cl 2p (c) line regions.
Figure 5. XPS spectra of samples (1), (3), and (5), representing O ls (a) Fe 2p3/2 (b) and Cl 2p (c) line regions.
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Figure 6. Schematic diagram of the XPS measurement’s region, dx, which is the depth detected by XPS (a). Schematic model of nanoparticle structure in samples (3) and (5) (b).
Figure 6. Schematic diagram of the XPS measurement’s region, dx, which is the depth detected by XPS (a). Schematic model of nanoparticle structure in samples (3) and (5) (b).
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Table 1. Median size (dg), standard deviation (lnσg) based on the TEM results, and grain size (dc) based on the XRD results for samples (3) and (5).
Table 1. Median size (dg), standard deviation (lnσg) based on the TEM results, and grain size (dc) based on the XRD results for samples (3) and (5).
Samplesdg (nm)lnσgdc (nm)
(3)11.20.39.0
(5)12.00.39.0
Table 2. Atomic percentages (ai) of O, Fe, and Cl obtained by EDS and XPS measurements in samples (1), (3), and (5).
Table 2. Atomic percentages (ai) of O, Fe, and Cl obtained by EDS and XPS measurements in samples (1), (3), and (5).
SamplesEDSXPS
OFeClOFeCl
(1)40.4351.581.9964.0631.993.96
(3)45.1652.562.2864.1535.983.87
(5)45.3152.921.7758.0232.895.17
Table 3. Binding energies data from XPS (eV) for elements present in samples (1), (3), and (5).
Table 3. Binding energies data from XPS (eV) for elements present in samples (1), (3), and (5).
SamplesO lsFe 2p3/2Cl 2p3/2
(1)529.32(P1)530.46(P2)709.66(P1)711.19(P2)198.7
(3)529.38 709.67(P1)711.44(P2)198.5
(5)529.36 709.63(P1)711.40(P2)198.5
Fe2O3529.5 709.9
FeCl3 711.3199.0
FeOOH 530.1 711.5
Note: Standard data from the NIST online database for XPS at http://www.nist.gov.
Table 4. Molar fraction percentages (yi), mass fraction percentages (zi), and volume fraction percentages (ϕi) determined by (a) EDS and (b) XPS.
Table 4. Molar fraction percentages (yi), mass fraction percentages (zi), and volume fraction percentages (ϕi) determined by (a) EDS and (b) XPS.
SamplesyFeyClzFezClϕFeϕClϕCl/ϕFe
a
(3)97.142.8595.274.7388.4111.590.13
(5)97.802.2096.333.6790.889.120.10
b
(3)93.086.9288.8211.7874.2925.910.35
(5)90.039.9784.2215.7866.9433.060.49
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