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Aug 27, 2011 - of Superparamagnetic Iron Oxide Nanoparticles. Oznur Karaagac · Hakan Kockar. Received: 6 July 2011 / Accepted: 14 July 2011 / Published ...
J Supercond Nov Magn (2012) 25:2777–2781 DOI 10.1007/s10948-011-1264-8

O R I G I N A L PA P E R

Effect of Synthesis Parameters on the Properties of Superparamagnetic Iron Oxide Nanoparticles Oznur Karaagac · Hakan Kockar

Received: 6 July 2011 / Accepted: 14 July 2011 / Published online: 27 August 2011 © Springer Science+Business Media, LLC 2011

Abstract Iron oxide nanoparticles were coprecipitated in air medium using different sodium hydroxide (NaOH) concentrations, and their structural and magnetic properties were studied. It was observed that the precipitation of superparamagnetic iron oxide nanoparticles could be achieved above a critical NaOH concentration. This was followed by the investigation of the effect of the stirring rate on the structural and magnetic properties of the nanoparticles precipitated at 8.5 M NaOH and over. Morphological observation made by a transmission electron microscope (TEM) showed that the particle size of iron oxide nanoparticles was around 7.5 nm. Magnetization curves measured by a vibrating sample magnetometer showed zero coercivity indicating that the samples are superparamagnetic and the highest saturation magnetization (70.4 emu/g) was obtained at the stirring rate of 1100 rpm. The mean particle sizes of iron oxide nanoparticles calculated from the magnetization data are found to be consistent with the particle sizes obtained from the TEM images.

particles is below a critical value they show superparamagnetism which permits the use of magnetic nanoparticles in specific applications [4, 5]. Coprecipitation is an economic and a simple way to obtain superparamagnetic nanoparticles. In this technique, iron oxide nanoparticles can be obtained from ferric and ferrous salts solutions under vigorous stirring [6]. The precipitate can be obtained in a basic medium. The structural and magnetic properties of these nanoparticles depend on the synthesis conditions such as base concentration, stirring rate, etc. [7]. In this study, iron oxide nanoparticles were synthesized in air medium by coprecipitation and the properties of the nanoparticles were investigated in terms of the base (sodium hydroxide, NaOH) concentration and the stirring rate. It was seen that the superparamagnetic nanoparticles can be synthesized over a critical NaOH concentration, and also the effect of the stirring rate on the properties of the nanoparticles was observed.

2 Experimental Keywords Coprecipitation · Iron oxide nanoparticles · Superparamagnetism · Stirring rate

1 Introduction Study of iron oxide nanoparticles were substantially increased because of their technological applications especially in biomedical science [1–3]. When the size of the O. Karaagac () · H. Kockar Physics Department, Science & Literature Faculty, Balikesir University, 10145, Cagis, Balikesir, Turkey e-mail: [email protected]

For coprecipitation, ferrous chloride tetrahydrate (FeCl2 ·4H2 O, Merck > 99%), ferric chloride hexahydrate (FeCl3 ·6H2 O Merck > 99%) and sodium hydroxide (NaOH, Merck) were used. The desired NaOH solutions (5.5, 8.5 and 14.5 M) were prepared and stored before the synthesis. Twelve grams of FeCl2 and 24 g of FeCl3 were dissolved in 50 ml de-ionized water and then 50 ml NaOH solution was added to the mixture of iron salts under a vigorous mechanical stirring. The reaction period was 30 minutes at room temperature in air medium. After the reaction, the precipitate was washed three times with distilled water and was dried in an oven. The structural characterizations of the nanoparticles were investigated with X-ray diffractometer system (XRD,

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Fig. 1 XRD patterns of the samples synthesized at different NaOH concentrations at 1500 rpm. (S1:14.5 M, S2: 8.5 M, S3: 5.5 M) (*corresponds to the peaks of oxyhydroxide phases)

Fig. 2 FT-IR spectrum of iron oxide nanoparticles synthesized at different NaOH concentrations at 1500 rpm (S1:14.5 M, S3: 5.5 M)

Rigaku-rint 2200) and Fourier transform infrared spectroscopy (FT-IR, Perkin Elmer-1600 Series). XRD patterns were obtained using Cu-Kα radiation (λ = 1.54056 Å) between 20◦ and 80◦ . FT-IR spectrum was recorded at 400– 1400 cm−1 wavelengths using KBr pellets. High-resolution transmission electron microscope (HRTEM, FEI TECNAI G2 F30 model) with an accelerating voltage of 200 kV was used to estimate the particle size and also observe the morphology of the particles. Vibrating sample magnetometer (VSM, ADE Technologies EV9) was used for the magnetic measurements of the nanoparticles at ±20 kOe.

3 Results and Discussion Iron oxide nanoparticles were synthesized in the order of 5.5, 8.5 and 14.5 M of NaOH. Initial structural analysis of the samples were done by the XRD. Figure 1 shows the X-ray patterns of the samples synthesized at different NaOH

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concentrations. The iron oxide peaks of cubic spinel structure at 2θ ≈ 31◦ , 35◦ , 43◦ , 57◦ and 62◦ were detected for the samples S1 (14.5 M NaOH) and S2 (8.5 M NaOH). The sample S3 obtained by using 5.5 M NaOH has peaks of 2θ ≈ 27◦ , 31◦ , 36◦ , 44◦ , 57◦ and 75◦ , indicating that the sample may be a mixture of α and β phases of iron oxyhydroxide corresponding to the JCPDS 81-0464 and JCPDS 34-1266 cards. To gain further information about the structure of the samples, FT-IR analysis was done, and the spectra of S1 and S3 are shown in Fig. 2. In the figure, the transmittance peak of magnetite at about 580 cm−1 and maghemite at 620 cm−1 [8, 9] was observed for the sample S1. However, in the spectrum of S3, these peaks disappeared and new transmittance peaks appeared at 790 and 880 cm−1 which seem to correspond to α-iron oxyhydroxide as stated in [10]. Magnetization curves of the nanoparticles were measured at ±20 kOe. Hysteresis curves of the samples S1–S3 are given in Fig. 3. Samples S1 and S2 are superparamagnetic with zero coercivities whereas sample S3 shows paramagnetic behaviour. From the hysteresis curves, it is obtained that the saturation magnetization, Ms , of the samples S1 and S2 is 51.8 and 53.1 emu/g, respectively, and saturation field, Hs , is 12348 and 12089 Oe for S1 and S2, respectively. They are listed in Table 1. The mean particle sizes, DXRD , were calculated from the most intense peak (311) using Scherrer equation [11] and the results are summarized in Table 1. The DXRD values are 8.4 and 9.3 nm for S1 and S2, respectively. The mean magnetic particle size, DMAG , was derived from the Langevin function with a log-normal particle size distribution [12, 13]. Samples S1 and S2 are superparamagnetic, therefore the DMAG values can only be calculated for the samples S1 and S2 and are 6.3 ± 0.6 and 6.1 ± 0.6 nm, respectively. The physical particles sizes, DTEM , of iron oxide nanoparticles were examined by TEM analysis. TEM images of S1 and S2 are demonstrated in Fig. 4, (a) and (b), respectively. The DTEM were 7.4 ± 2.4 and 7.5 ± 2.5 nm for samples S1 and S2, respectively. The mean DMAG is slightly smaller than the DXRD and DTEM that may originate from the magnetically dead surface layer as reported [14]. The final pH values of the solutions are 13.6, 11.8 and 4.2 for the samples S1, S2 and S3, respectively. The pH values of S1 and S2 are suitable for the precipitation process for the production of superparamagnetic iron oxide nanoparticles, whereas the pH value of S3 prevents the precipitation to take place. At low pH (4.2) paramagnetic particles were produced since a complete precipitation of iron oxide occurs at the pHs between 8 and 14 [15]. It was observed that the NaOH concentration with certain pH values is an important parameter to precipitate the superparamagnetic iron oxide nanoparticles. Therefore, in this study, it might be concluded that the superparamagnetic iron oxide nanoparticles can be obtained at 5.5 M and over NaOH concentration.

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Fig. 3 Magnetization curves of the iron oxide nanoparticles synthesized at different base concentrations at 1500 rpm (S1: 14.5 M, S2: 8.5 M, S3: 5.5 M)

Fig. 5 XRD pattern of the sample synthesized at 1100 rpm, S6

Fig. 4 TEM images of iron oxide nanoparticles (a) S1: 14.5 M and (b) S2: 8.5 M

In order to investigate the effect of the stirring rate on the properties of iron oxide nanoparticles, the superparamagnetic nanoparticles were synthesized at different stirring rates from 300 to 1800 rpm. In this study, the 8.5 M NaOH concentration was selected since the superparamagnetic iron oxide nanoparticles can be obtained at over 5.5 M NaOH concentration; see also Table 1. XRD measurements showed that all samples have a face-centred cubic

spinel structure with the characteristic (220), (311), (400), (422), (511), (440) and (622) peaks of iron oxide at around 2θ ≈ 30◦ , 35◦ , 43◦ , 53◦ , 57◦ , 63◦ and 74◦ , respectively. As an example, XRD pattern of the sample synthesized at 1100 rpm is given in Fig. 5. The DXRD increased from 12.0 to 13.5 nm and then decreased from 13.5 to 6.9 nm as the stirring rate increased from 300 to 1800 rpm. FT-IR spectra of the samples are also shown in the 400–1400 cm−1 region in Fig. 6. The transmittance peak observed at 570 ± 10 cm−1 confirms the magnetite formation and ∼620 cm−1 peak declares the maghemite phase [8, 9]. The 570 ± 10 cm−1 peak has only appeared at the spectrum of sample S6 which can be interpreted that the main phase of the sample was magnetite. Hysteresis curves of the samples synthesized at different stirring rates were measured and presented in Fig. 7. As seen in Table 1, the Ms obtained from the hysteresis curves increases from 65.3 to 70.4 emu/g and then decreases from 70.4 to 51.6 emu/g as the stirring rate increases from 300 to 1800 rpm. In order to explain the change of Ms , considerable ratio of maghemite phase accompanying the magnetite phase should also be taken into consideration as indicated by the FT-IR analysis. The highest Ms (70.4 emu/g) was obtained at the stirring rate of 1100 rpm which was confirmed from the major phase of magnetite; see the FT-IR spectrum in Fig. 6. The Hs changes from 10128 to 12089 Oe with

2780 Table 1 Particle sizes and magnetic properties of iron oxide nanoparticles

J Supercond Nov Magn (2012) 25:2777–2781 Sample

NaOH Concentration

Stirring Rate

Particle Size

(M)

(rpm)

DXRD

DVSM

DTEM

Ms

Hs

(nm)

(nm±nm)

(nm±nm)

(emu/g)

(Oe)

8.4

6.3±0.6

7.4±2.3

53.1

12348

9.3

6.1±0.6

7.5±2.5

51.8

12089











S1

14.5

S2

8.5

S3

5.5

1500

Magnetic Properties

S4

300

12.0

7.0±0.6



65.3

10324

S5

700

12.7

6.7±0.6



67.5

11430

1100

13.5

6.9±0.6



70.4

10128

S2

1500

9.3

6.1±0.6

7.5±2.5

51.8

12089

S7

1800

6.9

6.5±0.6



51.6

11846

S6

8.5

Fig. 7 Magnetization curves of the iron oxide nanoparticles synthesized at stirring rates of 300 rpm (S4), 700 rpm (S5), 1100 rpm (S6), 1500 rpm (S2), 1800 rpm (S7)

4 Conclusions

Fig. 6 FT-IR spectra of the samples synthesized at stirring rates of 300 rpm (S4), 700 rpm (S5), 1100 rpm (S6), 1500 rpm (S2), 1800 rpm (S7)

the change of the stirring rate. Slight variations (6.1 ± 0.6 to 7.0 ± 0.6 nm) in the DMAG were observed with the change of the stirring rate as can be seen in Table 1. Although the trend of Ms closely resembles the values of the DXRD , it is known that the DXRD does not necessarily reflect the real size of the particles. Thus it is suggested that smaller particles are coated by an amorphous spin dead layer, which strongly decreases the total magnetic moment [16].

Superparamagnetic iron oxide nanoparticles were synthesized in air medium at different NaOH concentrations. It was observed that the superparamagnetic iron oxide nanoparticles can be obtained at 5.5 M and over NaOH concentration. The influence of the different stirring rates on the structural and magnetic characteristics of the particles was also studied. The XRD patterns and FT-IR spectroscopy showed that the samples have both magnetite and maghemite phases of iron oxide. It was seen that the saturation magnetization was altered by the change of the stirring rate and the highest Ms value was 70.4 emu/g at 1100 rpm. The particle sizes are around 7.5 nm according to the TEM analysis. Acknowledgements The authors would like to thank the State Planning Organization, Turkey, under Grant No. 2005K120170 for VSM system. The authors also thank the Department of Materials Science and Engineering, Anadolu University, Turkey for XRD measurements; the Chemistry Department, Balikesir University, Turkey for FTIR measurements and the National Nanotechnology Research Centre (UNAM), Bilkent University, Turkey for TEM analysis.

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