Synthesis of Iron Oxide Nanoparticles Using Isobutanol

3 (hematite).Magnetite andmaghemite exhibit superparamagnetic properties and high saturation magnetization, leading to their biomedical applications [1–4] such as tumor targeting [5–12] and magnetic resonance imaging [13, 14]. Magnetite and maghemite have been synthesized by coprecipitation of ferrous/ferric salts [15–17], thermal decomposition of hydrazinated iron(II) oxalate [18], microemulsion [19], and sol-gel syntheses [20], hydrolysis, and pyrosol [21]. In the coprecipitation, the size andmorphology of nanoparticles vary with synthesis conditions such as iron salts and their concentration, surfactants, pH, and temperature [1, 3, 17, 22]. Magnetite is transformed to maghemite by calcination in air as 4Fe

In the present work magnetic iron nanoparticles were prepared by precipitation using one iron salt and isobutanol, which was expected to play a role of a surfactant.Ferrous chloride and ferric nitrate were individually used in the synthesis process where NaOH and NH 4 OH were used as a base.The resulting nanoparticles were calcined for 100 min to 5 h at various temperatures.Their size and phase composition were examined by X-ray powder diffraction (XRD), while their size and morphology were studied by scanning electron microscope (SEM).Their absorption infrared spectra were measured by Fourier-transform infrared spectroscopy (FTIR) and magnetic properties were examined by a vibrating sample magnetometer (VSM).
The phase composition and size of iron oxide nanoparticles were deduced from measurements by X-ray diffraction (XRD, PANalytical X'pert PRO MPD) with wavelength of Cu K  , 1.5405 Å at room temperature.The size and morphology were measured by an ultrahigh resolution scanning electron microscope (UHR-SEM, Hitachi S-4800).The absorption spectra were obtained by Fourier-transform infrared spectroscopy (FTIR, PerkinElmer Frontier) with a MIR source and an optical KBR beam splitter covering the wavelength range from 2.5 to 25 m (wavenumber range from 4000 to 400 cm −1 ).
The magnetic properties of the iron oxide nanoparticles were examined by a vibrating sample magnetometer (VSM, LakeShore).The magnetization of the nanoparticles was measured as a function of magnetic field strength at room temperature to yield their saturation and remanent magnetization and coercivity.
Figure 2 shows the X-ray diffraction (XRD) patterns of samples S12, S13, and S10.The first two samples were synthesized with ferric nitrate and NH 4 OH in isobutanol and sample S10 with ferrous chloride and NH 4 OH in isobutanol.Sample S13 was calcined at 400 ∘ C for 100 min and sample S10 at 530 ∘ C for 2 h.Figures 2(a The average particle size was estimated from XRD data using Scherrer equation [15,17]: where  is the average particle size,  the dimensionless shape factor whose typical value is about 0.9,  the Xray wavelength used in XRD (Cu K  = 1.5405Å),  the broadening of the observed diffraction line at half the maximum intensity in radians, and  the Bragg angle.Using  the diffraction peaks at 2 = 30.2∘ , 35.5 ∘ , 43.2 ∘ , 53.6 ∘ , 57.1 ∘ , and 62.7 ∘ , the average crystallite size of samples S4 to S8 was deduced to be 28, 31, 31, 23, and 56 nm, respectively.The average particle size of S8 synthesized without isobutanol turned out to be the largest, implying that isobutanol played a role of a surfactant to some extent and contributed to the reduction of the nanoparticle size.Using the diffraction peaks at 2 = 24.1 ∘ , 33.1 ∘ , 35.6 ∘ , 40.8 ∘ , 49.4 ∘ , 54.0 ∘ , 57.5 ∘ , 62.4 ∘ , and 64.0 ∘ , the average crystallite size of samples S10, S11, S13, and S14 was deduced to be 27, 30, 21, and 28 nm, respectively.Figure 3 shows FTIR spectra of iron oxide samples S1, S3, S5, and S14.Their characteristic wavenumbers are listed in Table 2 along with ten other samples.IR bands of 575 and 420 cm −1 in Figure 3 2 are characteristic peaks of maghemite [17], implying that calcination at 300 ∘ C for 2 h contributed to its phase transformation.The similar IR bands in Table 2 are present in sample S7 at 638 and 561 cm −1 .Samples S2, S5, and S7 contain maghemite mixed with magnetite.The IR band in the vicinity of 3400 cm −1 arises from -OH [15] or water molecules on the sample surface [17].The absorption peaks around 1629 and 1384 cm −1 in Figure 3(a) are due to C=O vibrations [15].The IR bands at 539 and 448 cm −1 in sample S14 are characteristic of hematite and related to the band near 635 cm −1 in maghemite.The synthesis of magnetite and maghemite can be accomplished using isobutanol with ferrous salt and NaOH.Similarly, hematite was synthesized using isobutanol with ferric nitrate and ammonium hydroxide, which was confirmed by XRD and FTIR results.
SEM micrographs of samples S1 to S14 are shown in Figures 4-7.The average crystalline sizes of all the samples were deduced from SEM micrographs and are listed in Table 3 with those deduced from the XRD data using (1).Samples S1  to S11 were synthesized with ferrous chloride, while samples S12 to S14 were synthesized with ferric nitrate.The average particles of sample S1 in Figures 4(a) and 4(b) are smaller than those of S2 in Figures 4(c) and 4(d) and S3 in Figures 4(e) and 4(f).Calcination at 300 and 500 ∘ C for 2 h contributed to increase of the average particle sizes from 11 to 22 nm.
The average particle size of sample S4 in Figures 5(a) and 5(b) and Table 3 is larger than that of S1, indicating that the average nanoparticle size increased from 11 to 30 nm as the concentration of ferrous chlorides increased from 0.09 to 0.4 M. The average particle size of sample S5 slightly increased due to calcination as seen in Figure 4.The much larger sizes of sample S8 in Figures 5(e) and 5(f) synthesized without isobutanol demonstrated that the role of isobutanol was a crucial size-determining factor in the process.The particle sizes of sample S6 in Figures 6(a Figures 7(d), 7(e), and 7(f) show SEM micrographs of samples S12 to S14, respectively.They were synthesized with ferric nitrate and ammonium hydroxide in isobutanol.The average particle size of sample S12 in Figure 7(d) turned out to be 8 nm, which was the smallest among them.The average particle size of sample S13 in Figure 7(e) agrees reasonably well with that of XRD, while the corresponding size of sample S14 in Figure 7(f) is slightly off from that of XRD as seen in sample S7.As shown in Table 3, the values of the average particle sizes deduced from SEM micrographs and their corresponding XRD agree reasonably well in samples S4, S5,  S6, S8, S10, S11, and S13, while those in samples S7 and S14 disagree to some extent.The magnetic hysteresis curves were measured at room temperature for samples S1 to S8 and their coercivity, the remanent magnetization and the saturation magnetization are listed in Table 4, and the magnetic hystereses of samples S1, S2, S4, and S5 are shown in Figures 8(a  0.09 M FeCl 2 ⋅4H 2 O, while S4 and S5 were prepared with 0.4 M FeCl 2 ⋅4H 2 O.Samples S2 and S5 were calcined at 300 ∘ C for 2 h.The saturation magnetization of sample S4 is nearly 3 times higher than the corresponding value of sample S1.The coercivity value tends to decrease from 32.0 to 16.1 Oe as calcination temperature increases to 500 ∘ C, while the remanent magnetization decreases from 1.27 to 0.32 emu/g.The  exhibited superparamagnetic properties at room temperature [15,17].Their corresponding sizes were 11, 22, and 22 nm, respectively.The coercivities and remanent magnetizations of samples S4 to S8 in Table 4 range from 39.8 to 129 Oe and from 4.39 to 12.1 emu/g, respectively, whose values were rather high so that they failed to exhibit superparamagnetic behavior.The values of the coercivity and remanent magnetization are the largest in sample S8 as 129 Oe and 12.1 emu/g, respectively.Sample S8 was synthesized without isobutanol, whose particle sizes were widely distributed around its average of 52 nm.Superparamagnetic properties result from nanoparticles that are smaller than their critical size of 25 nm [4].

Conclusions
Iron oxide nanoparticles were synthesized as magnetite, maghemite, and hematite using isobutanol.The synthesis process involved precipitation using a single ferrous or ferric salt in isobutanol with sodium hydroxide or ammonium  hydroxide.Isobutanol played a role of a surfactant in the process.The average size of Fe 3 O 4 nanoparticles prepared at the lower concentration of ferrous chloride was in the range of 11 nm.The corresponding size increased to 30 and 34 nm in samples S4 and S5 prepared with 0.4 M FeCl 2 ⋅4H 2 O at 75 ∘ C, while it increased to 39 and 48 nm in samples S6 and S7 prepared with 0.09 M FeCl 2 ⋅4H 2 O at 90 ∘ C. Phase transformation of magnetite to maghemite occurred in calcining magnetite at 300 ∘ C for 2 h.As-synthesized and calcined nanoparticles ranging from 11 to 22 nm prepared at the lower concentration of ferrous chloride at 75 ∘ C exhibited superparamagnetic properties.Nanoparticles synthesized with ferrous chloride and ammonium hydroxide at 75 ∘ C and calcined at 530 ∘ C for 2 h were -Fe 2 O 3 (hematite).Superparamagnetic iron oxide nanoparticles with size of interest using isobutanol are to be stabilized for biomedical applications.
(a), 557 and 443 cm −1 in Figure 3(b), 637, 559, and 441 cm −1 in Figure 3(c), and 539 and 448 cm −1 in Figure 3(d) are due to the Fe-O bonds in tetrahedral and octahedral sites, which are shifted from the corresponding bands of bulk Fe 3 O 4 around 570 and 375 cm −1 [17].IR bands of 635 and 559 cm −1 of sample S2 and 637 and 559 cm −1 of sample S5 in Table
) and 6(b) and Table3are larger than those of samples S1 to S5.This suggests that the particle sizes increased as synthesis temperature changed from 75 to 90 ∘ C. The average particle size of sample S7 deduced from Figures6(a) and 6(b) disagrees with the value estimated with (1) from the corresponding XRD data.Figures 7(a), 7(b), and 7(c) show SEM micrographs of samples S9 to S11, respectively.They were synthesized with ferrous chloride and ammonium hydroxide in isobutanol.The particle sizes of sample S9 in Figure 7(a) are much widely distributed and larger than those in samples S10 in Figure 7(b) and S11 in Figure 7(c) calcined at 530 and 600 ∘ C, respectively.
S5 calcined at 300 ∘ C for 2 h

Table 1 :
Synthesis conditions of the iron oxide samples at 75 ∘ C.
*The sample was synthesized at 90 ∘ C.

Table 2 :
Major wavenumbers of FTIR spectra for the iron oxide samples in Table1.

Table 3 :
Average nanoparticle size for the iron oxide samples from SEM and XRD.± 9.8 * Synthesis temperature was 90 ∘ C. + The size was deduced by the Scherrer equation (1).

Table 4 :
Magnetic characteristics of the iron nanoparticle samples synthesized at 75 ∘ C.
* Synthesis temperature was 90 ∘ C. smaller values of the coercivity and remanent magnetization are related to superparamagnetic properties.As shown in Figures 8(a) and 8(b) andTable 4, samples S1, S2, and S3