A Simple Way to Synthesize Superparamagnetic Iron Oxide ...

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IEEE TRANSACTIONS ON MAGNETICS, VOL. 46, NO. 12, DECEMBER 2010

A Simple Way to Synthesize Superparamagnetic Iron Oxide Nanoparticles in Air Atmosphere: Iron Ion Concentration Effect Oznur Karaagac1 , Hakan Kockar1 , Seda Beyaz2 , and Taner Tanrisever2 Physics Department, Science and Literature Faculty, Balikesir University, Balikesir 10145, Turkey Chemistry Department, Science and Literature Faculty, Balikesir University, Balikesir 10145, Turkey We have synthesized iron oxide nanoparticles in air atmosphere by coprecipitation. The ratio of [Fe+2 ] [Fe+3 ] was fixed at 2/3. The total amount of iron ions in the solution was varied from 250 mmol to 12.5 mmol. X-ray diffraction (XRD) patterns showed the characteristic peaks of iron oxide phases. We observed that the crystallinity reduced and particle sizes calculated from XRD patterns decreased as the iron ion concentration in the solution decreased. Fourier transform infrared spectroscopy analysis exhibited the Fe-O vibration band at 560–580 cm 1 confirming the iron oxide formation. The mean physical sizes obtained from a transmission electron microscope are around 8 nm. According to the magnetic measurements, all samples are superparamagnetic at room temperature showing an increase in saturation magnetization up to 63.71 emu/g. Magnetic sizes of the particles vary from 7.45 nm to 4.88 nm with the change of iron ion concentration. Index Terms—Magnetic analysis, magnetic materials, nanotechnology.

I. INTRODUCTION

M

AGNETIC nanoparticles have attracted a lot of interest because of their distinguished properties that are characteristics of neither the atom nor the bulk counterparts [1]. Quantum size effects, large surface area, and superparamagnetic behavior of magnetic nanoparticles offer a high potential of applications in different areas such as magnetic storage media [2], [3], ferrofluids [4]–[6], and biomedicine [7]–[17]. In superparamagnetic nanoparticles, there is no remanent magnetization when the magnetic field is removed [18], . Due to this special property, magnetic nanoparticles have been studied extensively in biomedical areas such as contrast agents in magnetic resonance imaging (MRI) [8]–[10], targeted drug delivery [11], [12], magnetic separation [13], and hyperthermia treatment [6], [14], [15], [19]. In particular, iron oxide nanoparticles are interesting because of their biocompatibility and low toxicity in the human body [20], [21]. One of the most widespread techniques to synthesize iron oxide is coprecipitation. In coprecipitation, nanoparticles are ) obtained by addition of a base (such as NaOH, KOH, to an aqueous solution of iron salts under vigorous stirring in the presence of inert gas (especially nitrogen) atmosphere [22]–[26]. The synthesis procedure is easier and the yield is more than other techniques for example thermal decomposition, hydrothermal reaction, microemulsion [27]. The size and saturation magnetization of magnetic nanoparticles show dependence on the reaction conditions such as iron ion concentration. The size of the nanoparticles can be estimated due to the growth mechanisms of the nanoparticles. According to the

Manuscript received March 15, 2010; revised July 21, 2010; accepted July 28, 2010. Date of publication September 23, 2010; date of current version November 30, 2010. Corresponding author: O. Karaagac (e-mail: [email protected]). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TMAG.2010.2076824

classical model of LaMer [28], which is the first explanation for formation mechanism of sulfur colloids, a critical concentration called supersaturation is defined as the nucleation starts to grow by diffusion. Furthermore, the particles can be obtained as a result of aggregation of smaller subunits rather than continuous growth by diffusion [29] or multiple nucleation and growth process (Ostwald ripening) [7], [30]. It is shown that iron ion concentration is an important parameter in the synthesis of iron oxide nanoparticles of equivalent size but different properties [31]. In this study, superparamagnetic iron oxide nanoparticles were synthesized using coprecipitation technique at room temperature in air atmosphere, which is a simpler way to nitrogen ones. The effect of iron ion concentration on the structural properties, particle sizes and magnetic properties of these nanoparticles were studied. The highest saturation magnetizawas found to be 63.71 emu/g that is compatible with tion, the superparamagnetic iron oxide nanoparticles synthesized in nitrogen atmosphere [23]–[25]. II. EXPERIMENTAL SETUP Merck , Ferrous chloride tetrahydrate Merck salts ferric chloride hexahydrate Merck, 25% of ammonia) and ammonium hydroxide ( were used for the synthesis of iron oxide nanoparticles. All chemicals were of reagent grade and used without further purification. In nitrogen medium the ratio of is used to be 1/2 since nitrogen prevents the oxidation of to . Howconverts to and iniever, in oxidizing environment . Therefore, in air tial ratio changes with the increase of medium the ratio of 2/3 is preferred [32]. In this study, and were dissolved in 50 ml deionized water with molar ratio of 2/3. Total amount of iron ions in the solution was varied from 250 mmol to 12.5 mmol. 50 ml of ammonium hydroxide (25%) was added to 50 ml mixture of iron salts under vigorous mechanical stirring at 1500 rpm. The reaction was performed for 30 minutes at 20 C in air medium. After the reaction, the

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KARAAGAC et al.: A SIMPLE WAY TO SYNTHESIZE SUPERPARAMAGNETIC IRON OXIDE NANOPARTICLES IN AIR ATMOSPHERE

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precipitate was washed three times with distilled water. To obtain the powder, the precipitate was dried in an oven. The crystalline structure of nanoparticles were investigated with a Rigaku—rint 2200 X-ray diffractometer system (XRD) radiation between 20 and 80 . using The samples were characterized by Fourier transform infrared spectroscopy (FT-IR, Perkin Elmer-1600 Series). High-resolution transmission electron microscope (HRTEM, FEI TECNAI G2 F30 model) with an accelerating voltage of 200 kV was used to obtain information about the morphology and size of the nanoparticles. For TEM measurements, samples were prepared by placing a drop of very dilute magnetic dispersion on a copper grid covered by formvar foil. The particle sizes were obtained from TEM images by using the software ImageJ. The magnetic properties of nanoparticles were studied by vibrating sample magnetometer (VSM-ADE EV9 Model) in field range up to 20 kOe.

III. RESULTS AND DISCUSSION Experiments were carried out by adjusting the amount of iron ion in the solution while keeping the other parameters constant. It was observed that the color of the samples changed from black to reddish-brown as the amount of iron ion in the medium decreased from 250 mmol to 12.5 mmol. This change may indicate the phase transform of magnetite to another iron oxide phase (maghemite, hematite) and/or iron oxyhydroxides since the color of magnetite is black while it is reddish-brown for others. The XRD patterns of samples (S1–S8) in Fig. 1(a) are corresponding to nanoparticles synthesized with the total amount of iron ion at 250, 200, 150, 100, 75, 50, 25, and 12.5 mmol, respectively, see also Table I. S1–S5 samples have the characteristic (220), (311), (400), (422), (511), (440), and (533) peaks of a face-centered cubic spinel structure at around 2 30 , 35 , 43 , 53 , 57 , 63 , and 74 , respectively. Whereas in patterns of S6–S8, the intensity of (311) and (440) peaks were reduced and the other peaks observed in previous patterns disappeared as seen from Fig. 1(a). Thus, it can be concluded that the decrease of total iron ion concentration results in a remarkable broadening and weakening of the peaks. Fig. 1(b) shows the experimentally observed peak profile fit of sample S1 which was fitted by using pseudo-Voigt function that is a linear combination of Gaussian and Cauchy functions [33]. For good fit correlation, the peaks of powder samples were selected but the fit correlation of samples S5–S8 was found very low probably due to amorphous products. From the half-maximum width of the fitted peaks, average crystallite sizes and size distributions were estimated using the Scherrer equation [34]–[36]. It should be noted that the calculated crystal sizes for S5–S8 includes error because of low fit correlation. The average crystal size (dXRD) decreased from 8.34 nm to 1.55 nm when the amount of iron ion decreased from 250 mmol to 12.5 mmol as seen in Table I. The broadening of the peaks may be a sign of a poor crystallization of the structure and/or small particle size. A nucleation stage could occur when the concentration of consistent species reached the supersaturation and the nuclei might

Fig. 1. (a) XRD patterns of nanoparticles. (b) Theoretical profile fit of XRD pattern for S1.

start to grow by flocculation or aggregation of nuclei into larger particles until the final size is obtained as stated in [7]. The decrease of iron ion concentration might impede to reach the sufficient number of particles for the nucleation and growth stages, thus, the number of iron oxide nanoparticles formed in the medium could be limited. Furthermore, the end product might show amorphous structures depending on the iron ion concentration [37]. Consequently, the amorphous particles were observed in the product (to be mentioned below; see TEM images in Fig. 3) with the decrease of iron ion concentration. FT-IR analysis was performed to confirm the formation of iron oxide nanoparticles. Iron oxide shows bands indicating the vibrations of - , where and correspond to the metal occupying tetrahedral and octahedral positions, respectively [38]. FT-IR spectra of the samples region are shown in Fig. 2. A broad band in the 1200–400 was detected at about 560–580 which was related to the vibrations of Fe-O bond. A shoulder peak was observed in the indicating the spectrum of samples S1–S4 at around 620 maghemite phase [39]. In all samples, Fe-O vibration band was widened and weakened with the decrease of iron ion concentration. In the spectrum of S4, two weak peaks around 790 and 890 were observed. Since these two peaks indicate iron oxyhydroxide phase [40], it can be said that the samples from S4 to S8 have an additional FeOOH phase with main iron

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TABLE I PARTICLE SIZES AND MAGNETIC PROPERTIES OF IRON OXIDE NANOPARTICLES

Total solvent amount is 100 ml.

Fig. 2. FT-IR spectra of nanoparticles.

oxide phase. Fe-O vibrations deduced and weak Fe-O(OH) vibrations appeared with the decrease of iron ion concentration which is consistent with the XRD results. TEM pictures of the samples were taken to determine the morphology and size of the nanoparticles and compare them with the particle sizes obtained using other techniques. The TEM images of samples S2 and S6 are given in Fig. 3(a) and (b), respectively. In the image of S6 rod-like particles some of

Fig. 3. TEM images of nanoparticles: (a) sample S2; (b) sample S6. (Arrows show the amorphous by-products of coprecipitation.)

which are pointed out with arrows in the figure are observed. This shows that the sample includes by-products of coprecipitation besides small amount of iron oxide nanoparticles. These


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also been carried out from the magnetization data using the approach of Chantrell et al. [43] which assumes a log-normal distribution of particles. A model called core-shell model in which each particle has an inner single-domain core with the spontaneous magnetization of the bulk material and an outer paramagnetic shell was used for the fitting procedure [44]. The magnetic and standard deviation were departicle size rived from the following equations [43], [45]:

(1)

Fig. 4. Hysteresis curves of the superparamagnetic iron oxide nanoparticles. (Inset shows the magnetization curves at 150 Oe.)

6

Fig. 5. Change of the saturation magnetization of the nanoparticles versus the total iron ion amount in the solution.

formations may be in amorphous phase as no peaks of another phase were observed in the XRD pattern of the sample. The are found to be around 8 nm. and particle sizes of the samples S1–S5 are in good agreement as shown in Table I. Magnetization curves of the nanoparticles measured at room temperature are shown in Fig. 4. Magnetic measurements of all iron oxide nanoparticles indicate that the particles are superparamagnetic at room temperature. As can be seen in the inset of Fig. 4, the coercivity of all synthesized samples is zero implying the superparamagnetic behavior as in other studies [41], [42]. It is harder to magnetize the samples in the order of S1–S8 due to from 9342 Oe to 18808 Oe the increase of saturation field, caused by the decrease of the iron ion concentration. Fig. 5 indicates the change of the saturation magnetization of the particles versus the total iron ion amount in the solution. Saturation magnetization, of the samples decreased from 63.71 emu/g to 32.86 emu/g with the decrease of the iron ion from 200 mmol to 75 mmol, see also Table I. When the amount of iron ion reduces below 75 mmol, of samples S6–S8 becomes almost conreflects the structural stant (14–16 emu/g). The change in the changes of nanoparticles. Slight drop in the of the sample S1 (from 63.71 emu/g to 60.05 emu/g) may have been the result of the longer time to prepare the solution of 250 mmol iron ion. values of iron It is known that the particle size affects the oxide nanoparticles. Estimation of the size of nanoparticles has

(2) where and are saturation magnetization of the bulk phase ) and magnetic nanoparticles, respectively. The (471 was calculated by considering the density of iron oxide 5.24 [44]. is the initial susceptibility and is calculated by extrapolating M to zero at high fields where the relationship between M and 1/H is linear [46]. k is Boltzman constant and T is 298.15 K. The magnetic particle sizes were calculated by assuming the bulk saturation magnetization of the magnetite or maghemite and they both showed the same trend of the decrease with the decrease of the iron ion concentration. Therefore, only the results of the calculations using the magnetite phase were listed in Table I. Magnetic particle sizes decreased from 7.45 0.45 nm to 4.88 0.63 nm as the amount of iron ion in the solution decreased. The size estimations made from magnetic data are slightly lower than the sizes obtained from TEM; this may have come from the magnetically dead surface layer as reported in [47]. The difference between and for samples S6–S8 may be related to the mismatch of the sizes calculated from XRD as mentioned before. Various studies have revealed that the size of iron oxide nanoparticles has a linear relationship with the saturation magnetization value [48]–[50] when the size of the particles is below the critical ferromagnetic exchange length of 27 nm [51]. The change values are correlated with the change of magnetic particle of sizes as can be seen in Table I. However, it may be inadequate to only related to the change of particle explain the change of size. The oxidized phase should also be taken into consideration may be related to the oxidized since a substantial change in may come from the change phase [52]. The decrease of the in the participation of maghemite to magnetite phase as indicated in structural analysis. Therefore, in our study, values may be explained not only with a change in particle size but with the existence of the oxidized phase within the nanoparticles. This is most probably the case occurred in our study. With the decrease of iron ion concentration the crystal structure of nanoparticles was slightly composed of magnetite. However, of the superparamagnetic nanoparticles is comparable with the ones synthesized in nitrogen medium. In our study, 63.71 emu/g is the highest value obtained at 200 mmol while the superparamagnetic counterparts synthesized in nitrogen atmosphere are 42.10 emu/g, 60 emu/g, and 60.57 emu/g in some studies [25], [24], and [23], respectively.

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In order to achieve a complete precipitation of iron oxide, the pH should be between 8 and 14 [53], [54], according to the following reaction:

(3) The final pH’s are above 9 for all reactions indicating that the reaction medium of all samples at the beginning was convenient to produce iron oxide nanoparticles but the structural and hence the magnetic properties of the product are different from each other. Thus, the iron ion concentration has a significant effect on the synthesis of iron oxide nanoparticles by coprecipitation. IV. CONCLUSION Superparamagnetic iron oxide nanoparticles with a high of 63.71 emu/g were synthesized at 200 mmol iron ion in air medium using coprecipitation technique. Their size was calculated to be 7.02 nm from the hysteresis curve, which is in good agreement with the 8.39 nm and 8.77 nm derived from XRD and pattern and TEM image, respectively. Due to their high superparamagnetic behavior, they can be used in the relevant physical and especially biomedical applications. ACKNOWLEDGMENT This work was supported by Balikesir University Research Grant BAP 2010/35. The authors would like to thank the State Planning Organization, Turkey, under Grant 2005K120170 for VSM system. O. Karaagac would like to thank TUBITAK for the BIDEB 2211 Scholarship. The authors also thank the Department of Materials Science and Engineering, Anadolu University, Turkey for XRD measurements; Chemistry Department, Balikesir University, Turkey for FT-IR measurements and National Nanotechnology Research Center (UNAM), Bilkent University, Turkey for TEM analysis. REFERENCES [1] B. H. Sohn and R. E. Cohen, “Processible optically transparent block copolymer films containing superparamagnetic iron oxide nanoclusters,” Chem. Mater., vol. 9, pp. 264–269, 1997. [2] S. H. Sun, C. B. Murray, D. Weller, L. Folks, and A. Moser, “Monodisperse FePt nanoparticles and ferromagnetic FePt nanocrystal superlattices,” Science, vol. 287, pp. 1989–1992, 2000. [3] Y. Shiroishi, K. Fukuda, I. Tagawa, H. Iwasaki, S. Takenoiri, H. Tanaka, H. Mutoh, and N. Yoshikawa, “Future options for HDD storage,” IEEE Trans. Magn., vol. 45, no. 10, pp. 3816–3822, Oct. 2009. [4] H. Xu, T. Song, X. Bao, and L. Hu, “Site-directed research of magnetic nanoparticles in magnetic drug targeting,” J. Magn. Magn. Mater., vol. 293, pp. 514–519, 2005. [5] D. Jamon, S. Robert, F. Donatini, J. J. Rousseau, C. Bovier, H. Roux, J. Serrughetti, V. Cabuil, and D. Zins, “Optical investigation of -Fe O nanoparticle-doped silica gel matrix for birefringent components,” IEEE Trans. Magn., vol. 37, no. 5, pp. 3803–3806, Sep. 2001. [6] R. Hiergeist, W. Andrä, N. Buske, R. Hergt, I. Hilger, U. Richter, and W. Kaiser, “Application of magnetite ferrofluids for hyperthermia,” J. Magn. Magn. Mater., vol. 201, pp. 420–422, 1999. [7] P. Tartaj, M. del Puerto Morales, S. Veintemillas-Verdaguer, T. González-Carreño, and C. J. Serna, “The preparation of magnetic nanoparticles for applications in biomedicine,” J. Phys. D: Appl. Phys., vol. 36, pp. 182–197, 2003.

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