Preparation of Magnetic Nanoparticles via a Chemically Induced Transition : Presence / Absence of Magnetic Transition on the Treatment Solution Used

Thedependence ofmagnetic transition on the treatment solution used in the preparation ofmagnetic nanoparticles was investigated using as-prepared products from paramagnetic FeOOH/Mg(OH) 2 via a chemically induced transition. Treatment using FeCl 3 and CuCl solutions led to a product that showed no magnetic transition, whereas the product after treatment with FeSO 4 or FeCl 2 solutions showed ferromagnetism. Experiments revealed that the magnetism was caused by the ferrimagnetic γ-Fe 2 O 3 phase in the nanoparticles, which had a coating of ferric compound. This observation suggests that Fe in the treatment solution underwent oxidation to Fe, thereby inducing the magnetic transition. The magnetic nanoparticles prepared via treatment with an FeSO 4 solution contained a larger amount of the nonmagnetic phase. This resulted in weaker magnetization even though these nanoparticles were larger than those prepared by treatment with an FeCl 2 solution. The magnetic transition of the precursor (FeOOH/Mg(OH) 2 ) was dependent upon treatment solutions and was essentially induced by the oxidation of Fe and simultaneous dehydration of FeOOH phase. The transition was independent of the acid radical ions in the treatment solution, but the coating on the magnetic crystallites varied with changes in the acid radical ion.

Reactions for the synthesis of oxide nanoparticles using the coprecipitation method can be grouped into two categories.In the first category, the oxide is produced directly; in the second category, a precursor is produced initially and then subjected to further processing (drying, calcination, and subsequent steps) [20].During the chemical reaction, which is followed by calcination or annealing, a new phase is formed.In addition to the transition from amorphous to crystalline, the particle size increases, and the crystallites aggregate as the calcination temperature increases [21].The conventional aqueous synthesis of -Fe 2 O 3 particles involves three or more steps [22], but we recently found a new route for the production of -Fe 2 O 3 magnetic nanoparticles that requires only two steps.This method involves the preparation of the paramagnetic FeOOH/Mg(OH) 2 precursor via a coprecipitation method, followed by treatment of the hydroxide precursor in the liquid phase using a ferrous chloride (FeCl 2 ) solution [23].During this treatment, Mg(OH) 2 dissolves, and the paramagnetic FeOOH precursor transforms into ferrimagnetic -Fe 2 O 3 nanoparticles via dehydration: This method is referred to as a chemically induced transition [24,25].However, it remains unclear whether such a magnetic transition can be produced using other solutions.In the present work, we used aqueous solutions of ferric chloride (FeCl 3 ), cuprous chloride (CuCl), ferrous sulfate (FeSO 4 ), and ferrous chloride (FeCl 2 ) as treatment solutions to investigate the dependence of the magnetic transition on the treatment solution used and to attempt to explain the mechanism of the transition in the liquid phase.

Materials and Methods
2.1.Chemicals.FeCl 3 , Mg(NO 3 ) 2 , NaOH, CuCl, FeSO 4 , and FeCl 2 of analytical grade and all other chemicals were used as received without further purification.Distilled water was used throughout the experiments.

Preparation.
The precursor was synthesized via coprecipitation.An aqueous mixture of FeCl 3 (1 M, 40 mL) and Mg(NO 3 ) 2 (2 M, with 0.05 mol HCl; 10 mL) was added to an aqueous NaOH solution (0.7 M, 500 mL).The resulting solution was then heated to boiling for 5 min under stirring.The red-brown precursor precipitated gradually during this process.The precursor was collected and washed with dilute HNO 3 solution (0.01 M) until the pH of the supernatant liquid was 7-8.
The as-prepared precursor was mixed with the treatment solution (0.25 M, 400 mL), and the resulting mixture was allowed to boil for 30 min.The products were then dehydrated with acetone and allowed to air-dry.Treatment with FeCl 3 , CuCl, and FeSO 4 solutions produced samples (1), (2), and (3), respectively.For comparison, treatment was also performed in an FeCl 2 solution to produce sample (0).

Characterization.
Crystal structures and specific magnetization curves were obtained for samples (0)-(3) using X-ray diffraction (XRD; XRD-7000, Shimadzu, Japan) and vibrating-sample magnetometry (VSM; HH-15, Nju-yq, China) at room temperature, respectively.The bulk chemical species, surface chemical composition, and morphology of samples (0) and (3) were determined using energy-dispersive X-ray spectroscopy (EDS; Quanta-200, Genesis, USA), X-ray photoelectron spectroscopy (XPS; XSAM800, Krator, UK), and transmission electron microscopy (TEM; Tecnai G20 ST, FEI, USA), respectively.2) is difficult to be distinguished from the XRD spectra, but it can be determined that both did not contain ferrite-like phase.Specific magnetization curves measured at room temperature are shown for all samples in Figure 2. The curves for samples (1) and ( 2) exhibited similar, apparently paramagnetic, behavior.Similar to sample (0), sample (3) appeared to be ferromagnetic having coercivity (see the windows in Figure 2), but its magnetization was weaker than that of sample (0).According to the XRD and VSM results, samples ( 1) and (2) showed no magnetic transition; in contrast, sample (3) exhibited a transition similar to that shown by sample (0).The specific saturation magnetization values (  ) for samples (0) and (3) were determined from a plot of  versus 1/ (where  is the strength of magnetic field) in the high-field region, which was linear for strong magnetic fields [26]. versus 1/ ( ≥ 7.5 kOe) plots were represented as inserts of Figure 2, and the   values were determined by extrapolating plots to -axis to be 59.20 and 43.36 emu/g for samples (0) and (3), respectively.
Figure 3 shows EDS spectra for samples (0) and (3).Sample (0) contained Fe, O, and Cl, and sample (3) contained S and Mg, in addition to Fe and O.The quantitative results are listed in Table 1.XPS measurements revealed the same chemical species that were identified by the EDS analysis in samples (0) and (3).The XPS spectra are shown in Figure 4.The XRD and EDS results suggested that the -Fe 2 O 3 phase was dominant in samples (0) and (3).There was also a Cl-containing phase in sample (0), and two Scontaining phases in sample (3).It is possible that the Clcontaining phase in sample (0) was FeCl 3 ⋅6H 2 O [25], but that the content of FeCl 3 ⋅6H 2 O may be so low that it did not produce clear diffraction peaks in the XRD spectrum (see Figure 1).The XRD results also implied that the Scontaining phases in sample (3) were MgSO 4 ⋅5H 2 O and Fe 2 (SO 4 ) 3 ⋅11H 2 O. Accordingly, the Fe2p 3/2 lines and O1s lines observed for the two samples and the S2p 3/2 line observed for sample (3) may be associated with two or more species.Detailed data on binding energies are listed in Table 2.We deduced from the experimental results that the binding energy of Mg1s in MgSO 4 ⋅5H 2 O was approximately 1304.9 eV.In addition, it is noticed that the ferrite-like spinel structure, -Fe 2 O 3 and Fe 3 O 4 , is difficult to discriminate by XRD due to peak broadening [27] and by XPS because the data are very close (see Table 2).However, Fe 3 O 4 is not very stable and is sensitive to oxidation [28].It was found that Fe 3 O 4 nanocrystallites transformed into -Fe 2 O 3 nanocrystallites using ferric nitrate treatment [29].Therefore, it is judged that the magnetic phase for samples (0) and ( 3 TEM observations revealed that sample (3) was identical to sample (0) and that it consisted of nearly spherical nanoparticles.Typical TEM images for both samples are shown in Figure 5. Statistical analysis [30] indicated that the diameter of the nanoparticles fitted a lognormal distribution.The median diameters (  ) for samples (0) and (3) were 10.55 and 13.16 nm, respectively, and the associated standard (    deviation values (ln   , where   is the geometry deviation) were 0.37 and 0.31, respectively.

Discussion
. Experimental XRD and magnetization curve measurements indicated that the products after treatment with an FeCl 3 solution were not ferromagnetic.The product after treatment with an FeSO 4 solution exhibited a magnetic transition, similar to that observed for the products generated after treatment with an FeCl 2 solution.These products mainly consisted of a ferrimagnetic -Fe 2 O 3 phase.These results show that the magnetic transition was induced by Fe 2+ , rather than by the acid radical ions.Additionally, samples (0) and (3) contained FeCl 3 ⋅6H 2 O and Fe 2 (SO 4 ) 3 ⋅11H 2 O, which, respectively, correspond to the ferrous salts FeCl 2 and FeSO 4 treatment solution.This observation suggests that the ferrous salts induced the transformation of FeOOH in the precursor into -Fe 2 O 3 via dehydration, and this resulted in the simultaneous oxidation of Fe 2+ to Fe 3+ .The results for sample (1) show that the ferric salt did not induce a transition, suggesting that the oxidization of the ferrous ions in the liquid phase was necessary for the dehydration of amorphous FeOOH to form magnetic phase.The experimental results for sample (2) show that the cuprous ions (Cu + ), which may not have had the tendency to oxidize to cupric ions (Cu 2+ ) in the liquid phase [31], did not cause FeOOH to form magnetic phase.This implies that the dehydrating action of the cuprous ions was weaker than that of the ferrous ions in the chemically induced transition in the liquid phase.The experimental results also indicated that the products after treatment with an FeSO 4 solution contained MgSO 4 , but the products after treatment with an FeCl 2 solution had no corresponding Mg-containing constituent.This result suggests that FeSO 4 in the treatment solution not only induced the transformation of FeOOH into -Fe 2 O 3 via dehydration and then underwent oxidation into Fe 2 (SO 4 ) 3 , but also reacted with dissolved Mg(OH) 2 , producing MgSO 4 : According to (3)   and FeSO 4 solutions is shown in Figure 6.As the magnetic nanoparticles have surface heterolayers that are different from the magnetic core, the magnetic core spins close to surface can be pinned by the layer, and the pinning would cause an unusually large coercivity [32,33].So, such -Fe 2 O 3 based magnetic nanoparticles appeared to be apparently ferromagnetic due to surface pinning rather than shape effects [34].
Because the metal salts were paramagnetic, the difference in the apparent magnetization of samples (0) and (3) depended on the -Fe 2 O 3 content.For sample (0), the molar percentages of -Fe 2 O 3 and FeCl 3 ⋅6H 2 O (  and  Cl , resp.) could be described as follows: where  Fe and  Cl are the atomic percentages of Fe and Cl in sample (0), respectively.For sample (3), the molar percentages of -Fe 2 O 3 , MgSO 4 ⋅5H 2 O, and Fe 2 (SO 4 ) 3 ⋅11H 2 O (  ,  Mg , and  S , resp.) could be described as follows: where  Fe ,  Mg , and  S are the atomic percentages of Fe, Mg, and S in sample (3), respectively.Thus, the molar percentages of each phase (  ) in samples (0) and ( 3) could be calculated from the   values, which were measured using EDS, and are listed in Table 3.The mass percentages of each phase (  ) in a sample were deduced using the following expression: where   is the molar weight of the th phase.Accordingly, the mass percentage of each phase was calculated for samples (0) and (3) from the molar percentages (  ) and molar weights of -Fe The mass percentage values are listed in Table 3. Table 3 indicates that the main species in samples (0) and (3) was -Fe 2 O 3 .The   values for samples (0) and (3) could therefore be expressed as where   is the mass percentage of the -Fe 2 O 3 phase and  , is the specific saturation magnetization of the -Fe 2 O 3 phase.For samples (3) and (0), their  , can be regarded as same since the -Fe 2 O 3 phase resulted from the same FeOOH phase.Thus, the ratio of the   values for samples (3) and (0) was proportional to the mass fraction   of -Fe 2 O 3 .From the experimental results for the magnetization, we found that the ratio of the   values for samples (3) and (0) was 0.73, which agreed with the ratio of the mass percentages   of the -Fe 2 O 3 phase (0.79).As a consequence, the mass percentage was available.The volume percentage of each phase of the samples (Φ  ) could be obtained from the   value: where   is the density of the  phase.4.

Conclusions
Metal ions in the treatment solution play a key role in the magnetic transition during the preparation of magnetic nanoparticles via the chemically induced transition of FeOOH/Mg(OH) 2 .No magnetic transition occurred when FeCl 3 and CuCl were used as treatment solutions; in contrast, the product obtained using treatment with an FeSO 4 solution, which is similar to that obtained using an FeCl 2 solution, showed ferromagnetic behavior.The magnetic phase of the two products was -Fe 2 O 3 .Experimental results revealed FeCl 3 and Fe 2 (SO 4 ) 3 in the products after treatment with FeCl 2 and FeSO 4 solutions, respectively.Thus, Fe 2+ , which likely has a stronger dehydrating ability than the cuprous ion Cu + , might have undergone oxidation to Fe 3+ , thereby inducing a magnetic transition via dehydration.Treatment with an FeSO 4 solution resulted in the formation of MgSO 4 on the -Fe 2 O 3 crystallites (the binding energy of Mg1s is ∼1304.9eV), whereas no Mg-containing compound formed in the product of the treatment using an FeCl 2 solution.This shows that the nonmagnetic coating on the magnetic -Fe 2 O 3 crystallites varied with changes in the acid radical ions in the treatment solution.Treatment with an FeSO 4 solution led to a higher content of the nonmagnetic phase in the product, and thus weaker magnetization, despite the fact that these particles were larger than those formed after treatment with an FeCl 2 solution.Due to the surface pinned effect, such -Fe 2 O 3 based magnetic nanoparticles, which were produced via the chemically induced transition, exhibited apparently ferromagnetism.
Here, we demonstrated the use of an FeOOH/Mg(OH) 2 precursor and a ferrous salt treatment solution (as FeCl 2 solution or FeSO 4 solution) as a simple and effective method for the preparation of -Fe 2 O 3 -based magnetic nanoparticles.Obviously, the alkali-oxide FeOOH dehydrating into -Fe 2 O 3 was induced with Fe 2+ in ferrous salt solution transforming into Fe 3+ .For the preparation of magnetic nanoparticles via chemically induced transition (CIT) method, the relation between acting energy of the alkali-oxide dehydrating and oxidation of the ferrous ions is interesting, and this mechanism will be further investigated in future work.
) is -Fe 2 O 3 , rather than Fe 3 O 4 or mixed phase of both -Fe 2 O 3 and Fe 3 O 4 .

Figure 2 :Figure 3 :
Figure 2: Specific magnetization curves measured at room temperature for all samples.

Figure 6 :
Figure 6: Schematic diagram of the formation of magnetic nanoparticles using FeCl 2 and FeSO 4 solutions.

Table 2 :
Binding energies (eV) of elements, determined from XPS measurements performed on samples (0) and (3).Note: the standard data are from the NIST online database for XPS at http://www.nist.gov;there are no O1s and Mg1s binding energies for MgSO 4 .