Preparing Fe 3 O 4 Nanoparticles from Fe 2+ Ions

Preparing Fe 3 O 4 Nanoparticles from Fe 2+ Ions Source by Coprecipitation Process in Various pH Darminto, Machida N. Cholishoh, Feby A. Perdana, Malik A. Baqiya, Mashuri, Yoyok Cahyono, and Triwikantoro Citation: AIP Conference Proceedings 1415, 234 (2011); doi: 10.1063/1.3667264 View online: http://dx.doi.org/10.1063/1.3667264 View Table of Contents: http://scitation.aip.org/content/aip/proceeding/aipcp/1415?ver=pdfcov Published by the AIP Publishing

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Preparing Fe3O4 Nanoparticles from Fe2+ Ions Source by Co-precipitation Process in Various pH Darminto*, Machida N. Cholishoh, Feby A. Perdana, Malik A. Baqiya, Mashuri, Yoyok Cahyono, and Triwikantoro Department of Physics, Faculty of Mathematics and Natural Sciences, Sepuluh Nopember Institute of Technology, Kampus ITS Keputih Sukolilo Surabaya,60111, Indonesia * Email: [email protected] Abstract. Magnetite (Fe3O4) nanoparticles were synthesized from the Fe2+ ions in the form of ferrous chloride tetrahydrate (FeCl2.4H2O) by coprecipitation method at low temperature (≤ 70 °C). During the precipitation process pH was kept constant at 7.37, 8.07, 9.12, 10.37 and 10.55 respectively. It was found that the magnetite nanoparticles was formed as a result of the dehydration reaction of ferrous hydroxide and ferric oxyhydroxide, in which the latter compound was produced by the partial oxidation of ferrous hydroxide by O 2 in dissolved air during the synthesis. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) have been used to characterize these resulted particles. The average crystalline size of Fe3O4 was obtained in the range between 18 – 55 nm for the corresponding range of pH used in the process. These results show that the pH of precipitation and the valence of the metal salt used in the synthesis have played an important role in influencing the particle size. Keywords: Fe3O4 nanoparticle, co-precipitation, ferrous chloride tetrahydrate, pH PACS: 73.63.Bd, 75.50.Tt, 81.16.Be

INTRODUCTION Magnetic nanoparticle is an interesting field of investigation not only in the field of magnetic recording but also in various field such as biomedical, optical, electronics, chemical and mechanical applications [1-8]. For these applications, size and size distribution of particles, physical properties of magnetic nanoparticles such as magnetic, optical and electronic, also surface reactivity are critical parameters. Especially, magnetite (Fe3O4) nanoparticles are reported to be applicable as a material for cancer therapy, pigments, catalysts, ferrofluid, magnetic drug delivery systems (MDDS), resonance imaging (MRI) [5, 9-13]. Numerous and diverse methodologies exist to synthesize magnetic nanoparticles of magnetite in order to obtain different characteristic and size, including ball milling, chemical precipitation, thermal decomposition, and hydrogels [9, 10, 14-16]. Chemical precipitation is the most common approach, which yields relatively good control over the size and morphology of nanoparticles. Synthesis of magnetite through chemical precipitation includes: (1) co-precipitation of Fe2+ and Fe3+, (2) partial reduction of Fe3+, and (3) partial oxidation of Fe2+, followed by co-precipitation [17]. The coprecipitation of Fe2+ and Fe3+ ions at the ratio of 1 to 2 in an alkaline medium is one of the most popular, easy and also cheap route of synthesizing method, because Fe3O4 contains ferrous and ferric iron at this ratio [9]. The most commonly noticed impurity compounds in magnetite synthesized from co-

precipitation technique, are goethite and maghemite [18]. In this paper, Fe3O4 nanoparticles were synthesized from the Fe2+ ions in the form of ferrous chloride tetrahydrate (FeCl2.4H2O) by coprecipitation method at low temperature (≤ 70 °C). We report especially on the effect of precipitation process pH on the size of Fe3O4 nanoparticles. The X-ray diffraction (XRD) and transmission electron microscopy (TEM) have been used to characterize these resulted particles.

EXPERIMENTAL The chemicals used for the synthesis, ferrous chloride tetrahydrate (FeCl2.4H2O), 20% aqueous ammonia, 37% hydrochloric acid, were procured from E-Merck. All these chemicals were used without any further purification. Fe3O4 nanoparticles were prepared by co-precipitation method from FeCl2.4H2O at alkaline medium on with low temperature. For all the experiments, FeCl 2.4H2O was dissolved in aqua DM. Then hydrochloric acid was added and stirred at room temperature. The pH of precipitation process was systematically varied to be 7.37 (±0.01), 8.07(±0.01), 9.12(±0.01), 10.37(±0.01) and 10.55(±0.01) by adding 20% aqueous ammonia and digested for 30 min at 70 °C. Then, the precipitate was washed with aqua DM to remove the excess of ammonia molecules presenting in the precipitate. The Fe3O4 nanoparticles were finally collected as a black powder after drying at 70 °C.

The 4th Nanoscience and Nanotechnology Symposium (NNS2011) AIP Conf. Proc. 1415, 234-237 (2011); doi: 10.1063/1.3667264 © 2011 American Institute of Physics 978-0-7354-0992-7/$30.00

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The samples were characterized by X-ray diffraction (XRD) for the crystal structure, average particle size and the concentration of impurity compounds present. A JEOL-3530 Shimadzu Philips X-Pert Multi Purpose Diffractometer System was used for X-ray diffraction patterns of these samples. The 2θ values were taken from 5° to 90° with a step size of 0.02° and 30° to 90° with a step size of 0.04° using Cu Kα radiation (λ value of 1.54056 Å). Further, the image examination using a transmission electron microscope (TEM) of Tecnai G2 (Philips, 200-kV) will give more evidence of the particles size which are in nanoscale.

RESULTS AND DISCUSSION Fig. 1 shows the XRD patterns of magnetite nanoparticles obtained at different initiation pH of the precipitating reaction. The formation of Fe3O4 was indicated for all samples based on the comparison of their XRD patterns with the standard pattern of Fe3O4 (JCPDS# 19-0629). No other diffraction peaks corresponding to ferrite nitrite or other iron oxide, such as goethite and maghemite,

FIGURE 2. Average crystalline size of samples prepared with precipitation pH 7.37, 8.07, 9.12, 10.37 and 10.55.

Variation of magnetite crystalline size obtained at different precipitation pH (Fig. 2) was estimated from Scherrer’s formula using RIETICA with ICSD card file No. 84098 (a = 8.398 Å). Crystalline size of samples have tendency to decrease with increasing precipitation pH until pH 9.12 which indicated no significant modification of particle size up to pH

FIGURE 1. XRD patterns of samples synthesized with precipitation pH 7.37, 8.07, 9.12, 10.37 and 10.55. 8.381 ± 1,5 × 10-3 Å; 8.364 ± 2 × 10-3 Å; 8.377 ± 2 × 10-3 Å and 8.368 ± 5 × 10-4 Å, respectively. In the present case, it was concluded that the product with increasing pH contained little maghemite, based on the typical unit cell parameter reported for magnetite is ~8.39 Å and for maghemite ~8.34 Å [18].

can be observed, but those of Fe3O4. In general, it is difficult to distinguish Fe3O4 from maghemite because maghemite has a crystal structure and a lattice spacing similar to those of Fe3O4. The exact identification of the nanoparticles structure (maghemite or magnetite) is possible from Mössbauer spectroscopy although it is still only semi-quantitative [18]. The unit cell parameter of the magnetite samples prepared with various precipitation pH is presented in the caption of Fig. 1.

9.12. Fe3O4 synthesized with precipitation pH 9.12, 10.37 and 10.55 actually have crystalline size in same range. Vayssières reported that the mean particles size decreases wiyh increasing precipitation pH and ionic strength and it is consistence with this our results. Furthermore, size distributions of Fe3O4 nanoparticles synthesized with precipitation pH 7.37, 8.07, 9.12, 10.37 and 10.55 were evaluated using MAUD sofware. The decrease in size with increasing pH correlatively reduces the size distribution width is presented in Fig. 3, while the TEM image of one of the samples is exhibited by Fig. 4.


FIGURE 3. Size distributions analyzed by MAUD software of Fe3O4 nanoparticles synthesized with precipitation pH 7.37, 8.07, 9.12, 10.37 and 10.55.

Large Fe3O4 particles at low pH was produced as a result of Ostwald ripening. At high pH, the size is negligible, indicating that no variation phenomenon occurs. The pH range where ripening does not occur and particles appear dimensionally stable broadens is above a critical pH value, pH*, which depends on the ionic strength and temperature of the salt solution. The sample prepared with an precipitation pH of 10.55 shows lower particle size than others. On addition of ammonia to the salt solution, it also neutralizes HCl in the system and subsequently the ionic strength of the solution goes up. The driving force for dissolution–recrystallization process during growth is interfacial tension, which leads to an increase in particle growth or decrease in specific surface area [19].

FIGURE 4. The TEM image of Fe3O4 particles synthesized with pH of 10.37, showing the average size as that estimated from the XRD spectra.

The excess ions in the reaction mixture lower sufficiently the interfacial tension to allow the formation of dimensionally stable particles. Therefore, due to higher ionic strength in the solutions where the precipitation pH was 10.55, the particle growth is less compared to those prepared with low precipitation pH. This explains smaller particles obtained at 10.55 pH. Compared to 10.55

pH, the ionic strength of the mixture at 8.07 pH is less and hence a higher interfacial tension, which is the contributing parameter for the increasing particle size. If the pH of precipitation is higher than pH* at a given ionic strength, the surfaces of the particles are always completely charged during the formation and the particle size does not vary with time. On lowering the pH of the solution (pH < pH*) after precipitation, the particles spontaneously grow by an Ostwald ripening process in order to reach a new equilibrium state. The valence of the iron salt used in the synthesis appears to play an important role in determining the size of Fe3O4 nanoparticles. In the most common co-precipitation for producing synthetic magnetite nanoparticles with solution of Fe2+ and Fe3+ ions (molar ratio 1:2), a black precipitate of Fe3O4 nanoparticles was formed immediately after mixing the iron salt solution with the base solution, reacting as indicated in the equation (1) below: 2Fe3+ + Fe2+ + 8OHˉ → FeO.Fe2O3 + 4H2O (1) where FeO·Fe2O3 signifies that the divalent, trivalent iron atoms in magnetite. On rapid raise in the pH, Fe3+ ions are immediately precipitated at pH of 3 as highly soluble form ferrihydrite which then reacts with the existing Fe2+ ions in the solution to form magnetite. Many papers report on the synthesis of Fe3O4 nanoparticles with the diameter in the range of 2 to 51 nm are prepared by the co-precipitation of Fe2+ and Fe3+ ions at the ratio of 1 to 2, which is carried out by controlling the base concentration or the solution temperature, and, sometimes by the addition of surfactants [20]. In this experiment, however, a solution containing Fe2+ ions alone produced a dark green precipitate just after being mixed with the base solution, and the precipitate gradually turned black in color, which indicated that intermediates were formed prior to the formation of Fe3O4. This observation shows that the absence of Fe3+ ions calls for a different reaction mechanism for the formation of large Fe3O4 nanoparticles in the range between 18 – 55 nm for the corresponding range of pH used in the process. The formation of an intermediate after mixing the two aqueous solutions is likely to be responsible for the formation of large Fe3O4 nanoparticles. Refait and Olowe [21] studied the alkalization reaction of ferrous ions as regards the formation of iron hydroxide and iron oxide, and they proposed the following reactions as the mechanism of formation of Fe3O4: Fe2+ + 2OHˉ → Fe(OH)2

(2)

3Fe(OH)2+1/2O2 → Fe(OH)2+2FeOOH+H2O

(3)

Fe(OH)2 + 2FeOOH → Fe3O4 + 2H2O

(4)

Thus, it is concluded that in the synthesis with ferrous ions alone, Fe3O4 is formed as a result of the


dehydration reaction of ferrous hydroxide and ferric oxyhydroxide represented by Eq. (4), in which the latter compound is produced by the partial oxidation of ferrous hydroxide by O2 in dissolved air according to Eq. (3). A large number of ferric oxyhydroxide (FeOOH), which were formed from Fe(OH)2 during formation process of Fe3O4 is reported in the literature [22]. In addition, the intermediates formed during the synthesis are suggested to be ferrous and ferric hydroxides which are expected to grow in size via hydroxylation. It was well known that Fe2+ ions require a minimum pH of 7 to precipitate as dark green of ferrous hydroxide, Fe(OH)2[18]. Furthermore, ferric hydroxides is formed as a result of partial oxidation of ferrous hydroxide by O2 in dissolved air during the synthesis following reaction: 4Fe(OH)2 + 2H2O + O2 → 4Fe(OH)3

(5)

Consequently, it is likely that large Fe3O4 particles were produced as a result of dehydration of ferrous and ferric hydroxides according to Eq. (6), which had grown in size via the hydration process.

(6) If the sample obtained crystallizes completely, the products would not include -OH, just like the reactions (4) and (6). This observation was consistent as described above that the formation of Fe3O4 involved the dissolution-recrystallization process. Fe2+ adsorbed onto ferric hydroxide and followed by the structural rearrangement due to the electron transfer of Fe2+ and Fe3+ through overlapping d-orbital.

CONCLUSION The method of synthesis of Fe3O4 nanoparticles by co-precipitation was investigated with a focus on the control of the size of Fe3O4 nanoparticles. It was clarified that the valence of the metal salt used in the synthesis and the pH of precipitation have played an important role in influencing the particle size. In this work, Fe3O4 nanoparticles was formed as a result of the dehydration reaction of ferrous hydroxide and ferric oxyhydroxide, in which the latter compound was produced by the partial oxidation of ferrous hydroxide by O2 in dissolved air during the synthesis. Furthermore, the average crystalline size of Fe3O4

was shown to be influenced and controlled in the range of 18 to 55 nm for the corresponding range of pH used in the process. These finding are very useful for the synthesis of pure magnetite nanoparticles through co-precipitation method for various applications, and it is found that the increasing pH will surpress the average size of the resulting nanoparticles.

ACKNOWLEDGEMENT This research was partialy suported by Hibah Pascasarjana and Penelitian Kompetensi PUM, 2011, provided respectively by DP2M Dikti 2010 and LPPM ITS 2011.

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