Preparation of Magnetic Nanoparticles via a Chemically Induced ...

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Dec 13, 2015 - The dependence of magnetic transition on the treatment solution used in the preparation of magnetic nanoparticles was investigated.
Hindawi Publishing Corporation Journal of Chemistry Volume 2016, Article ID 7604748, 9 pages http://dx.doi.org/10.1155/2016/7604748

Research Article Preparation of Magnetic Nanoparticles via a Chemically Induced Transition: Presence/Absence of Magnetic Transition on the Treatment Solution Used Yanshuang Chen, Qin Chen, Hong Mao, Yueqiang Lin, and Jian Li School of Physical Science and Technology, Southwest University, Chongqing 400715, China Correspondence should be addressed to Jian Li; [email protected] Received 26 November 2015; Accepted 13 December 2015 Academic Editor: JosΒ΄e L. A. Mediano Copyright Β© 2016 Yanshuang Chen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. The dependence of magnetic transition on the treatment solution used in the preparation of magnetic nanoparticles was investigated using as-prepared products from paramagnetic FeOOH/Mg(OH)2 via a chemically induced transition. Treatment using FeCl3 and CuCl solutions led to a product that showed no magnetic transition, whereas the product after treatment with FeSO4 or FeCl2 solutions showed ferromagnetism. Experiments revealed that the magnetism was caused by the ferrimagnetic 𝛾-Fe2 O3 phase in the nanoparticles, which had a coating of ferric compound. This observation suggests that Fe2+ in the treatment solution underwent oxidation to Fe3+ , thereby inducing the magnetic transition. The magnetic nanoparticles prepared via treatment with an FeSO4 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 FeCl2 solution. The magnetic transition of the precursor (FeOOH/Mg(OH)2 ) was dependent upon treatment solutions and was essentially induced by the oxidation of Fe2+ 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.

1. Introduction Nanotechnology involves the understanding and control of matter with dimensions of roughly 1–100 nm [1]. Magnetic nanoparticles have attracted increasing interest because their size range enables the investigation of the fundamental aspects of magnetic-ordering phenomena in materials with reduced dimensions, and this has the potential to lead to new technological applications [2, 3]. Studies on magnetic nanoparticles have focused on the development of simple and effective methods for the fabrication of nanoparticles with controlled size, morphology, and properties [4, 5]. Iron oxide nanoparticles including magnetite (Fe3 O4 ), hematite (𝛼-Fe2 O3 ), maghemite (𝛾-Fe2 O3 ), goethite (𝛼-FeOOH), and akaganeite (𝛽-FeOOH) nanoparticles have attracted enormous attention due to their interesting properties [6–10]. Many methods have been developed for the preparation of 𝛾-Fe2 O3 magnetic nanoparticles, including coprecipitation

[11], a gas-phase reaction technique [12], direct thermal decomposition [13], thermal decomposition-oxidation [14], sonochemical synthesis [15], a microemulsion technique [16], hydrothermal synthesis [17], a vaporization-condensation method [18], and a sol-gel approach [19]. 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 𝛾-Fe2 O3 particles involves three or more steps [22], but we recently found a new route for the production of 𝛾-Fe2 O3 magnetic nanoparticles that requires

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Journal of Chemistry

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 (FeCl2 ) solution [23]. During this treatment, Mg(OH)2 dissolves, and the paramagnetic FeOOH precursor transforms into ferrimagnetic 𝛾-Fe2 O3 nanoparticles via dehydration: Mg (OH)2 󳨀→ Mg2+ + 2OHβˆ’ 2FeOOH (paramagnetic) 󳨀→ 𝛾-Fe2 O3 (ferrimagnetic) + H2 O

(1) (2)

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 (FeCl3 ), cuprous chloride (CuCl), ferrous sulfate (FeSO4 ), and ferrous chloride (FeCl2 ) 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.

2. Materials and Methods 2.1. Chemicals. FeCl3 , Mg(NO3 )2 , NaOH, CuCl, FeSO4 , and FeCl2 of analytical grade and all other chemicals were used as received without further purification. Distilled water was used throughout the experiments. 2.2. Preparation. The precursor was synthesized via coprecipitation. An aqueous mixture of FeCl3 (1 M, 40 mL) and Mg(NO3 )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 HNO3 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 FeCl3 , CuCl, and FeSO4 solutions produced samples (1), (2), and (3), respectively. For comparison, treatment was also performed in an FeCl2 solution to produce sample (0). 2.3. 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.

Table 1: Atomic percentages (π‘Žπ‘– ) determined from EDS measurements performed on samples (0) and (3).

Sample (0) Sample (3)

π‘ŽO 60.48 62.35

π‘ŽFe 37.93 32.35

π‘ŽCl 1.59

π‘ŽS

π‘ŽMg

3.93

1.37

3. Results and Discussion 3.1. Results. XRD spectra are shown in Figure 1 for all of the samples. The crystal structure of samples (1) and (2) was different from that of sample (0), whereas the main structure of sample (3) was identical to that of sample (0), which corresponded to ferrite-like 𝛾-Fe2 O3 . The results showed that sample (3) contained MgSO4 β‹…5H2 O and Fe2 (SO4 )3 β‹…11H2 O. The crystal phase of samples (1) and (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 (πœŽπ‘  ) for samples (0) and (3) were determined from a plot of 𝜎 versus 1/𝐻 (where 𝐻 is the strength of magnetic field) in the high-field region, which was linear for strong magnetic fields [26]. 𝜎 versus 1/𝐻 (𝐻 β‰₯ 7.5 kOe) plots were represented as inserts of Figure 2, and the πœŽπ‘  values were determined by extrapolating plots to 𝜎-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 𝛾Fe2 O3 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 FeCl3 β‹…6H2 O [25], but that the content of FeCl3 β‹…6H2 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 MgSO4 β‹…5H2 O and Fe2 (SO4 )3 β‹…11H2 O. Accordingly, the Fe2p3/2 lines and O1s lines observed for the two samples and the S2p3/2 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 MgSO4 β‹…5H2 O was approximately 1304.9 eV. In addition, it is noticed that the ferrite-like spinel structure, 𝛾-Fe2 O3 and Fe3 O4 , is difficult to discriminate by XRD

3

20

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40 2πœƒ (deg.)

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60

70

Intensity (a.u.)

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40 2πœƒ (deg.)

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FeCl3 Β· 6H2 O

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40 2πœƒ (deg.)

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40 2πœƒ (deg.)

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MgSO4 Β· 5H2 O (PDF#25-0532) 20

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(PDF#33-0645)

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Intensity (a.u.)

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𝛾-Fe2 O3

10 20

(440)

(422) (511)

(400)

(202) Cl

(220)

(311)Cl

(311)

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𝛾-Fe2 O3

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40 2πœƒ (deg.)

Intensity (a.u.)

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(PDF#39-1346)

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(PDF#39-1346)

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Intensity (a.u.)

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Intensity (a.u.)

(2)

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40 2πœƒ (deg.)

(440)

(021) Fe (130)Fe (111)

Intensity (a.u.)

Intensity (a.u.) 10

30

(151)Mg (422) (511)

(3)

(1)

20

(400)

10

70

(212)Mg (311)

60

(111)

Intensity (a.u.)

(440)

(511) 50

(220)

40 2πœƒ (deg.)

(422)

(400)

(220) 30

(111)Mg

20

(0)

(131)Fe (021) Mg

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(311)Cl

(111)

Intensity (a.u.)

(0)

(202) Cl

(311)

Journal of Chemistry

10

Fe2 (SO4 )3 Β· 11H2 O (PDF#28-0496)

20

30

40 2πœƒ (deg.)

50

60

70

Figure 1: XRD patterns of all samples, with (hkl), (hkl)Cl , (hkl)Mg , and (hkl)Fe corresponding to 𝛾-Fe2 O3 , FeCl3 β‹…6H2 O, MgSO4 β‹…5H2 O, and Fe2 (SO4 )3 β‹…11H2 O, respectively.

due to peak broadening [27] and by XPS because the data are very close (see Table 2). However, Fe3 O4 is not very stable and is sensitive to oxidation [28]. It was found that Fe3 O4 nanocrystallites transformed into 𝛾-Fe2 O3 nanocrystallites using ferric nitrate treatment [29]. Therefore, it is judged that the magnetic phase for samples (0) and (3) is 𝛾-Fe2 O3 , rather than Fe3 O4 or mixed phase of both 𝛾-Fe2 O3 and Fe3 O4 .

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 (𝑑𝑔 ) for samples (0) and (3) were 10.55 and 13.16 nm, respectively, and the associated standard

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Journal of Chemistry

(0)

(1)

0.0 βˆ’0.5 βˆ’0.05

6 0.00 0.05 H (kOe)

0 𝜎 (emu/g)

60

βˆ’30

βˆ’60 βˆ’10

βˆ’5

58 56 54 0.00

0

0

(2)

βˆ’6 0.07 0.14 1/H (kOe) 5

βˆ’12 βˆ’10

10

βˆ’5

0 H (kOe)

H (kOe)

5

10

0.5 (3)

0.0 βˆ’0.5 βˆ’0.05

0.00 0.05 H (kOe)

0

44 𝜎 (emu/g)

𝜎 (emu/g)

25

𝜎 (emu/g)

50

βˆ’25

42 0.000

βˆ’50 βˆ’10

βˆ’5

0 H (kOe)

0.125 0.250 1/H (kOe) 5

10

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

Fe

(0)

(3)

O

Counts (a.u.)

Fe Counts (a.u.)

𝜎 (emu/g)

30

12

0.5

𝜎 (emu/g)

𝜎 (emu/g)

60

S

O Mg

Cl 2

4

6

8

2

Energy (keV)

Figure 3: EDS spectra for samples (0) and (3).

4 Energy (keV)

6

8

Journal of Chemistry

5

(0) Count (a.u.)

Count (a.u.)

Fe2p3/2

(3)

O1s

Mg1s

Fe2p3/2

O1s

Cl2p3/2 1200 1000 800 600 400 Binding energy (eV) (0)

720

1200 1000 800 600 400 Binding energy (eV)

0

(0)

716 712 Binding energy (eV)

536

708

534

532 530 528 Binding energy (eV)

716 712 Binding energy (eV)

708

536

(0)

Cl

526

204

202

534

P1

532 530 528 Binding energy (eV) (0)

526

204

202

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(0)

Mg

170 168 166 Binding energy (eV)

164

1308

1306 1304 1302 Binding energy (eV)

1300 (3)

Counts (a.u.)

Counts (a.u.)

172

170 168 166 Binding energy (eV)

164

1308

1306 1304 1302 Binding energy (eV)

Figure 4: XPS spectra for samples (0) and (3).

198

Binding energy (eV)

Counts (a.u.)

Counts (a.u.)

172

194

(3)

(3)

174

200 198 196 Binding energy (eV)

Counts (a.u.)

P2

S

174

0

(3) Counts (a.u.)

Counts (a.u.)

(3)

720

200

Counts (a.u.)

O Counts (a.u.)

Counts (a.u.)

Fe

200

S2p3/2

1300

196

194

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Journal of Chemistry

(3)

(0)

100 nm

100 nm

Figure 5: Typical TEM images for samples (0) and (3). Table 2: Binding energies (eV) of elements, determined from XPS measurements performed on samples (0) and (3).

Sample (0) Fe2 O3 Fe3 O4 FeCl3 Sample (3) Fe2 O3 Fe3 O4 Fe2 (SO4 )3 MgSO4

Fe2p3/2 710.81 710.9 710.8 711.3 Fe2p3/2 711.41 710.9 710.8 711.05

O1s 530.19 530.2 530.1

Cl2p3/2 198.84

199 O1s 530.40 (P1); 530.2 530.1

531.65 (P2)

531.6 ?

S2p3/2 168.99

Mg1s 1304.92

169.1 168.6

?

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 MgSO4 .

deviation values (ln πœŽπ‘” , where πœŽπ‘” is the geometry deviation) were 0.37 and 0.31, respectively. 3.2. Discussion. Experimental XRD and magnetization curve measurements indicated that the products after treatment with an FeCl3 solution were not ferromagnetic. The product after treatment with an FeSO4 solution exhibited a magnetic transition, similar to that observed for the products generated after treatment with an FeCl2 solution. These products mainly consisted of a ferrimagnetic 𝛾-Fe2 O3 phase. These results show that the magnetic transition was induced by Fe2+ , rather than by the acid radical ions. Additionally, samples (0) and (3) contained FeCl3 β‹…6H2 O and Fe2 (SO4 )3 β‹…11H2 O, which, respectively, correspond to the ferrous salts FeCl2 and FeSO4 treatment solution. This observation suggests that the ferrous salts induced the transformation of FeOOH in the precursor into 𝛾-Fe2 O3 via dehydration, and this resulted in the simultaneous oxidation of Fe2+ to Fe3+ . 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 (Cu2+ ) 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 FeSO4 solution contained MgSO4 , but the products after treatment with an FeCl2 solution had no corresponding Mg-containing constituent. This result suggests that FeSO4 in the treatment solution not only induced the transformation of FeOOH into 𝛾-Fe2 O3 via dehydration and then underwent oxidation into Fe2 (SO4 )3 , but also reacted with dissolved Mg(OH)2 , producing MgSO4 : Mg2+ + 4 (Fe3+ + SO4 2βˆ’ ) 󳨀→ MgSO4 + Fe2 (SO4 )3 + 2Fe3+

(3)

According to (3), MgSO4 first adsorbed onto the 𝛾-Fe2 O3 crystallites, forming MgSO4 β‹…5H2 O. Some Fe3+ and SO4 2βˆ’ ions also adsorbed, forming Fe2 (SO4 )3 β‹…11H2 O on the outermost layer. Thus, the solutions of ferrous salts with different acid radicals (e.g., FeCl2 and FeSO4 ) could induce the transformation of the FeOOH/Mg(OH)2 precursor into a 𝛾-Fe2 O3

Journal of Chemistry

7 Table 3: Molar and mass percentages of phases in samples (0) and (3).

Molar percentage (𝑦) 𝛾-Fe2 O3 /FeCl3 𝛾-Fe2 O3 /MgSO4 /Fe2 (SO4 )3 Sample (0) Sample (3)

Mass percentage (𝑧) 𝛾-Fe2 O3 /FeCl3 β‹…6H2 O 𝛾-Fe2 O3 /MgSO4 β‹…5H2 O/Fe2 (SO4 )3 β‹…11H2 O

97.24/2.76

95.42/4.58 87.34/7.81/4.85

75.44/8.87/15.69

percentages of 𝛾-Fe2 O3 , MgSO4 β‹…5H2 O, and Fe2 (SO4 )3 β‹…11H2 O (𝑦𝛾 , 𝑦Mg , and 𝑦S , resp.) could be described as follows:

FeCl3 Β· 6H2 O

eCl 2 In F Mg(OH)2

𝛾-Fe2 O3 + Mg2+ + OHβˆ’

𝑦Mg =

FeOOH

In F eSO

4

(π‘ŽFe βˆ’ 2 (π‘ŽS βˆ’ π‘ŽMg ) /3) /2 π‘ŽFe /2 + π‘ŽMg π‘ŽMg π‘ŽFe /2 + π‘ŽMg

Γ— 100, (5)

Γ— 100,

𝑦S = 100 βˆ’ 𝑦𝛾 βˆ’ 𝑦Mg , 𝛾-Fe2 O3

Fe2 (SO4 )3 Β· 11H2 O

+ Fe3+ + OHβˆ’

MgSO4 Β· 5H2 O

Figure 6: Schematic diagram of the formation of magnetic nanoparticles using FeCl2 and FeSO4 solutions.

magnetic phase via dehydration. However, the coating on the 𝛾-Fe2 O3 crystallites varied when the treatment solution was changed. The results revealed that sample (0) contained 𝛾Fe2 O3 and FeCl3 β‹…6H2 O, and sample (3) contained 𝛾-Fe2 O3 , MgSO4 β‹…5H2 O, and Fe2 (SO4 )3 β‹…11H2 O. A schematic diagram illustrating the formation of the nanoparticles using FeCl2 and FeSO4 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 𝛾-Fe2 O3 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 𝛾-Fe2 O3 content. For sample (0), the molar percentages of 𝛾-Fe2 O3 and FeCl3 β‹…6H2 O (𝑦𝛾 and 𝑦Cl , resp.) could be described as follows: 𝑦𝛾 =

𝑦𝛾 =

(π‘ŽFe βˆ’ π‘ŽCl /3) /2 Γ— 100, (π‘ŽFe βˆ’ π‘ŽCl /3) /2 + π‘ŽCl /3

(4)

𝑦Cl = 100 βˆ’ 𝑦𝛾 , where π‘ŽFe and π‘ŽCl are the atomic percentages of Fe and Cl in sample (0), respectively. For sample (3), the molar

where π‘ŽFe , π‘ŽMg , and π‘ŽS are the atomic percentages of Fe, Mg, and S in sample (3), respectively. Thus, the molar percentages of each phase (𝑦𝑖 ) in samples (0) and (3) could be calculated from the π‘Žπ‘– values, which were measured using EDS, and are listed in Table 3. The mass percentages of each phase (𝑧𝑖 ) in a sample were deduced using the following expression: 𝑧𝑖 =

𝑦𝑖 𝐴 𝑖 Γ— 100, βˆ‘ 𝑦𝑖 𝐴 𝑖

(6)

where 𝐴 𝑖 is the molar weight of the 𝑖th phase. Accordingly, the mass percentage of each phase was calculated for samples (0) and (3) from the molar percentages (𝑦𝑖 ) and molar weights of 𝛾-Fe2 O3 , FeCl3 β‹…6H2 O, MgSO4 β‹…5H2 O, and Fe2 (SO4 )3 β‹…11H2 O. The mass percentage values are listed in Table 3. Table 3 indicates that the main species in samples (0) and (3) was 𝛾Fe2 O3 . The πœŽπ‘  values for samples (0) and (3) could therefore be expressed as πœŽπ‘  β‰ˆ 𝑧𝛾 β‹…

πœŽπ›Ύ,𝑠 100

,

(7)

where 𝑧𝛾 is the mass percentage of the 𝛾-Fe2 O3 phase and πœŽπ›Ύ,𝑠 is the specific saturation magnetization of the 𝛾-Fe2 O3 phase. For samples (3) and (0), their πœŽπ›Ύ,𝑠 can be regarded as same since the 𝛾-Fe2 O3 phase resulted from the same FeOOH phase. Thus, the ratio of the πœŽπ‘  values for samples (3) and (0) was proportional to the mass fraction 𝑧𝛾 of 𝛾-Fe2 O3 . From the experimental results for the magnetization, we found that the ratio of the πœŽπ‘  values for samples (3) and (0) was 0.73, which agreed with the ratio of the mass percentages 𝑧𝛾 of the 𝛾-Fe2 O3 phase (0.79). As a consequence, the mass percentage was available. The volume percentage of each phase of the samples (Φ𝑖 ) could be obtained from the 𝑧𝑖 value: Φ𝑖 =

𝑧𝑖 /πœŒπ‘– Γ— 100, βˆ‘ 𝑧𝑖 /πœŒπ‘–

(8)

where πœŒπ‘– is the density of the 𝑖 phase. The πœŒπ‘– values for 𝛾-Fe2 O3 , FeCl3 β‹…6H2 O, MgSO4 β‹…5H2 O, and Fe2 (SO4 )3 β‹…11H2 O

8

Journal of Chemistry Table 4: Volume percentages of phases (Φ𝑖 ) in samples (0) and (3). 𝛾-Fe2 O3 /FeCl3 β‹…6H2 O 88.69/11.31

Sample (0) Sample (3)

𝛾-Fe2 O3 /MgSO4 β‹…5H2 O/Fe2 (SO4 )3 β‹…11H2 O 56.18/17.07/26.75

were 4.90, 1.844, 1.896, and 2.14 g/cm3 , respectively. The values calculated for Φ𝑖 are listed in Table 4. Because the particles in samples (0) and (3) were nearly spherical, the ratio of the average particle volume in sample (0) (⟨𝜐⟩(0) ) to the average particle volume in sample (3) (⟨𝜐⟩(3) ) could be described as follows: 3 ⟨𝜐⟩(0) βŸ¨π‘‘βŸ©πœ,(0) = , ⟨𝜐⟩(3) βŸ¨π‘‘βŸ©3𝜐,(3)

(9)

where βŸ¨π‘‘βŸ©πœ,(0) and βŸ¨π‘‘βŸ©πœ,(3) are the diameters corresponding to the average volumes of the particles in samples (0) and (3), respectively. βŸ¨π‘‘βŸ©πœ could be calculated directly from the size distribution parameters obtained using TEM by using the equation βŸ¨π‘‘βŸ©πœ = exp(ln 𝑑𝑔 + 1.5 ln2 πœŽπ‘” ) [30]. Accordingly, a βŸ¨π‘‘βŸ©3𝜐,(0) /βŸ¨π‘‘βŸ©3𝜐,(3) value of 0.62 was obtained. Additionally, the average volume of the 𝛾-Fe2 O3 based nanoparticles, which contained many phases, (⟨𝜐⟩), can be expressed as ⟨𝜐⟩ = βŸ¨πœπ›Ύ ⟩ βˆ‘ Φ𝑖 /Φ𝛾 , where βŸ¨πœπ›Ύ ⟩ is the average volume of the 𝛾Fe2 O3 phase and Φ𝛾 is the volume percentage of the 𝛾Fe2 O3 phase. Because the 𝛾-Fe2 O3 phase was produced from FeOOH, the βŸ¨πœπ›Ύ ⟩ values for samples (0) and (3) were the same. Therefore, the ratio of ⟨𝜐⟩(0) to ⟨𝜐⟩(3) could be described by Φ𝛾,(3) /Φ𝛾,(0) ; that is, ⟨𝜐⟩(0) Φ𝛾,(3) = , ⟨𝜐⟩(3) Φ𝛾,(0)

(10)

where Φ𝛾,(0) and Φ𝛾,(3) are the volume percentages of the 𝛾Fe2 O3 phase in samples (0) and (3), respectively. Thus, the ⟨𝜐⟩(0) /⟨𝜐⟩(3) value obtained from the volume percentages was 0.63, which agreed with the value of βŸ¨π‘‘βŸ©3𝜐,(0) /βŸ¨π‘‘βŸ©3𝜐,(3) (0.62) and confirmed the validity of both the coating model of the nanoparticle structure and the volume percentage.

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 FeCl3 and CuCl were used as treatment solutions; in contrast, the product obtained using treatment with an FeSO4 solution, which is similar to that obtained using an FeCl2 solution, showed ferromagnetic behavior. The magnetic phase of the two products was 𝛾-Fe2 O3 . Experimental results revealed FeCl3 and Fe2 (SO4 )3 in the products after treatment with FeCl2 and FeSO4 solutions, respectively. Thus, Fe2+ , which likely has a stronger dehydrating ability than the cuprous ion Cu+ , might have undergone oxidation to Fe3+ , thereby inducing a magnetic transition via dehydration. Treatment

with an FeSO4 solution resulted in the formation of MgSO4 on the 𝛾-Fe2 O3 crystallites (the binding energy of Mg1s is ∼1304.9 eV), whereas no Mg-containing compound formed in the product of the treatment using an FeCl2 solution. This shows that the nonmagnetic coating on the magnetic 𝛾-Fe2 O3 crystallites varied with changes in the acid radical ions in the treatment solution. Treatment with an FeSO4 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 FeCl2 solution. Due to the surface pinned effect, such 𝛾Fe2 O3 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 FeCl2 solution or FeSO4 solution) as a simple and effective method for the preparation of 𝛾-Fe2 O3 -based magnetic nanoparticles. Obviously, the alkali-oxide FeOOH dehydrating into 𝛾-Fe2 O3 was induced with Fe2+ in ferrous salt solution transforming into Fe3+ . 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.

Conflict of Interests The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments Financial support for this work was provided by the Innovation Foundation Project of Southwest University, China (no. 1318001), and the National Science Foundation of China (no. 11074205).

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