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J Nanopart Res (2012) 14:807 DOI 10.1007/s11051-012-0807-7

RESEARCH PAPER

One-step synthesis of silica-coated magnetite nanoparticles by electrooxidation of iron in sodium silicate solution Heru Setyawan • Fauziatul Fajaroh • W. Widiyastuti • Sugeng Winardi • I. Wuled Lenggoro • Nandang Mufti

Received: 18 July 2011 / Accepted: 27 February 2012 Ó Springer Science+Business Media B.V. 2012

Abstract Silica-coated magnetite nanoparticles have been synthesized successfully using a one-step electrochemical method. In this method, pure iron in a dilute aqueous sodium silicate solution that served as a silica precursor was electrooxidized. We show that the presence of silicate can significantly enhance the purity of the magnetite formed. Impurities in the form of FeOOH (found in the magnetite prepared in water) are not found. The magnetite nanoparticles produced by this method are nearly spherical with a mean size ranging from 6 to 10 nm, which is lower than the size of particles prepared in water, and this size range

H. Setyawan (&) W. Widiyastuti S. Winardi Department of Chemical Engineering, Faculty of Industrial Technology, Sepuluh Nopember Institute of Technology, Kampus ITS Sukolilo, Surabaya 60111, Indonesia e-mail: [email protected] F. Fajaroh Department of Chemistry, Faculty of Mathematics and Science, Malang State University, Jl. Semarang 5, Malang 65119, Indonesia I. W. Lenggoro Institute of Symbiotic Science and Technology, Tokyo University of Agriculture and Technology, Nakacho 2-24-16, Koganei, Tokyo 184-8588, Japan N. Mufti Department of Physics, Faculty of Mathematics and Science, Malang State University, Jl. Semarang 5, Malang 65119, Indonesia

depends on the applied voltage and the sodium silicate concentration. The magnetite nanoparticles exhibit superparamagnetic properties with saturation magnetization ranging from 15 to 22 emu g-1, which is lower than the saturation magnetization of the Fe3O4 bulk materials (Ms = 92 emu g-1). This facile method appears to be promising as a synthetic route for producing silica-coated magnetite nanoparticles. Keywords Magnetite nanoparticles Electrochemical synthesis Sodium silicate Magnetic properties Aqueous system

Introduction Nanosized materials have been the subject of extensive investigations in a variety of research areas due to their unique properties, i.e., very large surface area and reactivity, that differ from the properties of the corresponding bulk materials. Magnetic nanoparticles, especially magnetite (Fe3O4), have attracted increasing interest because of their good biocompatibility, strong superparamagnetic properties, low toxicity, and easy preparation processes (Teja and Koh 2009). Such materials can be manipulated using an external magnetic field because of their superparamagnetic properties. Magnetic nanoparticles therefore have many applications in the biomedical industry, such as targeted drug delivery, hyperthermia treatment, cell

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J Nanopart Res (2012) 14:807

separation, magnetic resonance imaging, immunoassays, and the separation of biochemical products (Nishio et al. 2007; Berry and Curtis 2003). Magnetic nanoparticles are also useful for environmental processes, such as the treatment of water and wastewater for the magnetic separation of metals or anions (Mayo et al. 2007; Pang et al. 2007). For most of the applications mentioned above, magnetite nanoparticles must have a narrow size distribution, a uniform morphology, high-magnetization values, and a size smaller than 100 nm (Gupta and Gupta 2005). Recently, we have successfully prepared nearly monodispersed magnetite nanoparticles using a surfactant-free electrochemical method with a sacrificial iron anode and water as the electrolyte (Fajaroh et al. 2011). The magnetite nanoparticles produced by our method are nearly spherical with a mean size ranging from 10 to 30 nm, depending on the experimental conditions. The magnetite nanoparticles exhibit ferromagnetic properties with a relatively high-saturation magnetization (*76 % relative to the corresponding bulk of Fe3O4), although some impurities in the form of FeOOH, a nonmagnetic material, were retained. FeOOH is an intermediate product created during the formation of Fe3O4 in the electrooxidation of iron in water. For some biological and biomedical applications, high-purity magnetite nanoparticles are required. The presence of impurities in the nanoparticles may hinder their applications in some biological and biomedical areas. We must therefore eliminate FeOOH impurities in the magnetite nanoparticles to obtain a high-purity product. The formation of Fe3O4 particles taking place during the electrooxidation of iron in our system can be enhanced by the reaction of ferrous ions with a base according to the reactions scheme below (Fajaroh et al. 2011): FeðsÞ ! Fe2þ ðaqÞ þ 2e

ð1Þ

Fe2þ ðaqÞ þ 2OH ðaqÞ ! FeðOHÞ2 ðsÞ

ð2Þ

3FeðOHÞ2 ðsÞ þ 1=2O2 ðgÞ ! FeðOHÞ2 ðsÞ þ 2FeOOHðsÞ þ H2 OðlÞ

ð3Þ

FeðOHÞ2 ðsÞ þ FeOOHðsÞ ! Fe3 O4 ðsÞ þ 2H2 OðlÞ ð4Þ -

The OH ions required to create the basic medium necessary for the formation of ferrous hydroxide [Fe(OH)2] [reaction (2)] and to produce oxygen to

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partially oxidize Fe(OH)2 to form ferric oxyhydroxide (FeOOH) [reaction (3)] arise from the reduction of water at the cathode. The mechanism controlling the conversion of ferrous hydroxide to magnetite is the partial oxidation of ferrous hydroxide by the dissolved oxygen [reaction (3)] (Ozkaya et al. 2009). The purity of magnetite nanoparticles could be improved by blocking the further reaction of partial oxidation of Fe(OH)2 after the formation of Fe3O4. To block this further reaction, magnetite nanoparticles must be isolated from their reaction environment once they are formed by coating the particles with a substance that can improve the stability of the nanoparticles in this environment. Silica is a nontoxic and biocompatible material and has been widely used to improve the stability of nanoparticles in a basic environment (Nishio et al. 2007; Yang et al. 2010). Silica is surface terminated with silanols (–SiOH), allowing for silica-coated magnetite nanoparticles can be easily modified with many functional groups, such as amines, thiols, and carboxyl groups. This covalent modification of the particle surfaces with biological molecules, such as drugs, proteins, enzymes, antibodies, or nucleotides, creates magnetite nanoparticles that can be used for biomedical applications. These modified nanoparticles can be directed to an organ, tissue, or tumor using an external magnetic field or can be heated in the alternating magnetic fields for use in hyperthermia therapy. Magnetite nanoparticles without a coating often suffered from aggregation in water or tissue fluid; this aggregation may limit in vitro magneticbased isolation and detection strategies (Cheng et al. 2005). The most common method to prepare silica-coated magnetite nanoparticles involves a two-step process: (i) the synthesis of magnetite nanoparticles and (ii) silica coating (Deng et al. 2005; Girginova et al. 2010; Hong et al. 2009; Li et al. 2010). The magnetite nanoparticles were first synthesized by either hydrolysis of FeSO4 or coprecipitation of Fe2? and Fe3? with ammonium hydroxide. The particles produced were then coated with silica through a sol–gel process using tetraethyl orthosilicate (TEOS) as the silica source. The most common method for the silica coating of magnetite nanoparticles appears to be a sol–gel method involving the hydrolysis and polycondensation of silicon alkoxides in an alcoholic environment. Alkoxide compounds are expensive and


hazardous, and thus, the process that uses such compounds is environmentally unfriendly. Silicate precursors may be used as a substitute for the alkoxide compounds because silicates are inexpensive and nontoxic, and thus preferable to alkoxide compounds as environmentally friendly reagents (Setyawan and Balgis 2011). In addition, silicates may be compatible with an aqueous electrochemical system, such as the system used in our method, which allows the use of silicates for a one-step synthetic process. In the present work, we report a one-step synthesis of silica-coated magnetite nanoparticles by the electrooxidation of iron in an aqueous system using sodium silicate as the silica precursor. The role of sodium silicate in this system is not only as a silica source but also as a supporting electrolyte and dispersing agent. Sodium silicate will influence the conductivity of the electrolyte and the dispersion of the particles. We therefore investigated the effect of sodium silicate on magnetite formation and on the properties of the magnetite nanoparticles produced.

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X-ray diffraction (XRD) patterns of the particles were determined using an X-ray diffractometer (X-Pert, Philips). The infrared (IR) spectra were recorded using a Fourier transform infrared (FTIR) spectrophotometer (FTIR 8400s, Shimadzu). The mean size of the prepared particles was determined indirectly by measuring their Brunauer–Emmett–Teller (BET) specific surface area using the multi-point nitrogen adsorption at its boiling point (Nova 1200, Quantachrome). In this measurement, the particles were assumed to be spherical and dense such that the particle diameter could be calculated using the equation dp ¼

6 qAs

ð5Þ

where q is the particle density and As is the specific surface area. The mean particle size estimated from the BET specific surface area is in good agreement with that derived from SEM measurements (Fajaroh et al. 2011). Magnetic characterization was performed using a SQUID magnetometer (Quantum Design MPMS-5T) at 300 K.

Experimental work Results and discussion Experiments were carried out in an electrochemical cell consisting of a sacrificial iron anode and an iron-based cathode of the same dimensions (23 mm 9 13 mm and 0.25 mm thick). The highly pure iron layer that weakly adhered to the anode surface was prepared by electroplating a steel plate in FeSO4 solution under a constant current density. Details of the electroplating experiment can be found elsewhere (Fajaroh et al. 2011). The electrochemical cell was filled with a dilute sodium silicate solution (molar ratio SiO2/Na2O = 3.3) as the supporting electrolyte and as the silica source for particle coating. The concentration of sodium silicate was varied from 0 to 300 ppm. The distance between electrodes was set at 2 cm, and the applied voltage was varied from 15 to 20 V, corresponding to current densities of *5.30–7.75 mA/cm2 from a DC power supply (GW Instek GPC-M Series). All experiments were conducted at room temperature and had a reaction time of 12 h unless otherwise stated. The product obtained was washed with deionized water and dried at 60 °C for subsequent analysis. The morphologies of the particles prepared in this reaction were observed using scanning electron microscopy (SEM; FE-SEM JSM-6335F, JEOL). The

The effect of sodium silicate on magnetite formation The effect of sodium silicate on the formation of magnetite nanoparticles was studied by varying the concentration of sodium silicate while other parameters, such as potential and distance between electrodes, were kept constant. The influence of sodium silicate on the purity of the magnetite nanoparticles, as shown by their XRD patterns, is illustrated in Fig. 1. Although for both conditions (with and without the presence of sodium silicate), a black precipitate was obtained, the purity of the powders was very different. As shown in Fig. 1, for the sample obtained without sodium silicate, the characteristic peaks of the XRD pattern correspond to the mixture of magnetite (JCPDS 19-0629) and FeOOH (JCPDS 44-1415). FeOOH is an intermediate product in the formation of Fe3O4 during the synthesis of iron in an aqueous system by electrooxidation (Fajaroh et al. 2011). In the samples obtained by adding a small amount of sodium silicate, the characteristic peaks of FeOOH in the XRD patterns completely disappear. Figure 1

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FeOOH

Intensity (a.u u.)

Intens sity (a.u..)

Fe3O4

0 ppm

150 ppm

20 V

15 V 200 ppm 300 ppm

25

30

35

40

45

50

55

60

65

25

30

35

40

2θ

shows that the patterns for the products of all reactions that occurred in dilute sodium silicate solution with concentrations varying from 100 to 300 ppm have only seven characteristic peaks at 30.5° (220), 35.9° (311), 37° (222), 43.5° (400), 53.6° (422), 57.3° (511), and 63.1° (440). These seven characteristic peaks match the standard pattern of Fe3O4 (JCPDS 19-0629) well. There are no diffraction peaks other than those of Fe3O4. Sodium silicate appears to stabilize the magnetite nanoparticles once they are formed, and all FeOOH as an intermediate product is completely converted into Fe3O4. Impurities in the form of FeOOH can be eliminated from the product by adding a small amount of sodium silicate to the water electrolyte during the synthesis of magnetite nanoparticles using the electrooxidation of iron. The effect of the applied voltage on the formation of magnetic particles was also investigated. Figure 2 shows the XRD patterns of samples prepared in 200 ppm sodium silicate solution at two different voltages, 15 and 20 V. The patterns for the products formed at the two voltages have only the seven characteristic peaks corresponding to Fe3O4 (JCPDS 19-0629). The purity of magnetite particles does not appear to be influenced by the applied voltage but instead by the presence of sodium silicate. However, the crystallinity of the magnetite that is formed is influenced by the voltage. To understand the role of sodium silicate in stabilizing the magnetite particles, the infrared spectrum of the particles was obtained using an FTIR spectrophotometer. Figure 3 shows the infrared

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50

55

60

65

Fig. 2 XRD patterns of particles prepared at two different applied voltages at a sodium silicate concentration of 200 ppm

Si-O-Si

200 ppm

Si-O

%Trans smittanc ce

Fig. 1 XRD patterns of particles prepared at various concentrations of sodium silicate at a constant applied voltage of 20 V

45

2θ

0 ppm

H2 O

4000

Fe-O-Si

150 ppm

3500

H2 O

OH

3000

2500

2000

OH

1500

1000

Fe-O

500

-1

Wavenumber (cm ) Fig. 3 FTIR spectra of particles prepared at various concentrations of sodium silicate at a constant applied voltage of 20 V

spectrum of the samples prepared with various concentrations of sodium silicate ranging from 0 to 200 ppm. For all cases, two main bands corresponding to Fe3O4 can be observed. The bands at 580 and 442 cm-1 are metal–oxygen bands that correspond to the intrinsic stretching frequencies of the tetrahedral and octahedral sites, respectively, of the inverse spinel cubic of Fe3O4 (Sen and Bruce 2009; Socrates 1994). The main difference between the spectrum of samples prepared in a dilute solution of sodium silicate and the spectrum of samples prepared in water is the bands appearing at 1,022 and at 3,200 cm-1. These two bands do not appear in the spectrum of samples prepared in water. The bands can be attributed to the


periodically sampling 3 mL aliquots from each of the solutions to examine the dissociation of magnetite into Fe3? and Fe2? ions. The aliquot was then added to an excess of KSCN solution to form a deep-red complex of nondissociated iron(III) thiocyanate. The absorbance of this complex was measured using a UV–vis spectrophotometer (Model Genesys 10uv, Thermo Scientific) at a wavelength of 463 nm. The solids remaining after acid dissolution were dried, and the dried samples were weighed. Figure 4 shows the change in the solution absorbance for the two samples. Higher absorbance indicates that there are more Fe3? ions in the aliquot, i.e., more magnetite dissolves into the HCl solution. The absorbance increases more rapidly for the sample prepared in water, indicating a faster dissolution rate. The absorbance is nearly constant at a very low value for the sample prepared in sodium silicate solution. Magnetite is soluble under acid conditions, while amorphous silica is barely soluble. After digestion in acid solution for 24 h, the remaining solids from the sample prepared in water and from the sample prepared in sodium silicate solution weighed *2.34 and 13.66 mg, respectively. The sample prepared in water dissolves, while the sample prepared in sodium silicate solution does not dissolve. Magnetite prepared in the sodium silicate solution appears to be protected by the silica on the surface, which slows the dissolution rate. These observations confirm that silica in the samples prepared in sodium silicate solution as

0.12 0.10

Absorbance

O–H stretching that probably comes from FeOOH (Fajaroh et al. 2011), as also shown in the XRD patterns discussed earlier. These bands disappear from the spectrum of all particles prepared in a dilute sodium silicate solution. These FTIR results corroborate the results of the XRD analysis, indicating that the presence of a small amount of sodium silicate in the electrolyte solution during electrosynthesis can eliminate impurities in the form of FeOOH, resulting in the formation of high-purity magnetite. Moreover, other bands from 1,000 to 1,200 as well as at 970, 561, and 954 cm-1 that appear in the spectrum of the samples prepared in dilute sodium silicate solution cannot be observed in the spectrum of the samples prepared in water. The bands appearing from 1,000 to 1,200 and at 970 cm-1 can be attributed to Si–O–Si and Si–O stretching, respectively, that probably arise from SiO2. The bands at 561 and 954 cm-1 are possibly due to the Fe–O–Si and Si–O– Si stretching vibrations, respectively, caused by perturbation of the metallic ion in SiO4. Other bands that do not appear in the spectrum of samples prepared in water occur at 1,630 and 3,430 cm-1 and at 3,443 and 1,629 cm-1. The bands at 1,630 and 3,430 cm-1 can be attributed to O–H bending and stretching of the associated water molecules. The silica surface is terminated with silanols that have very strong polar interactions. The silanol groups are hydrogen bonded to water and constitute at least part of the strongly held water, which may explain why the intensity of the bands increases with the increase in sodium silicate concentration. The peaks at 3,443 and 1,629 cm-1 correspond to –OH on the silica surface (Girginova et al. 2010; Chang et al. 2009; Souza et al. 2009). These results clearly demonstrate that silica exists in the magnetite formed during synthesis by electrooxidation of iron in dilute sodium silicate solution, although it is not detected in the XRD analysis, i.e., there is no peak corresponding to SiO2 in the diffraction pattern. To understand whether the silica is embedded within the magnetite particles or only forms a layer on the particle surface, we have performed dissolution experiments with the powder samples. Two powder samples of 20 mg each, one prepared in water and the other in sodium silicate solution, were digested in 100 mL of 10-5 M HCl solution under constant shaking in an orbital shaker for 24 h. The progress of the magnetite dissolution was followed by

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0.08 006

Water Sodium silicate

0.04 0.02 0.00

0

200

400

600

800

1000

1200

1400

Time (min)

Fig. 4 The change in solution absorbance after complexing with KSCN for particles prepared in water and in sodium silicate solution

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identified by the FTIR spectrum forms a layer on the surface of the magnetite particles. Simple mechanisms for the formation of a silica layer on the surface of the magnetite particles during the electrooxidation of iron in a sodium silicate solution are illustrated schematically in Fig. 5. Because the concentration of sodium silicate is very low, the sodium silicate may dissociate completely to form Na? and SiO32- ions. Silicate ions may exist in a conjugated form with the Si atom as the center of positive charge. In a conjugated form, these ions would tend to bond to oxygen as a part of the magnetite. The other end of the silica surface that contains the silanol groups may react further by a condensation reaction to form a silica layer to cover the magnetite particles. This condensation reaction may explain why FeOOH impurities disappear under these conditions such that nearly pure magnetite is obtained. Properties of silica-coated magnetite particles Figure 6 shows the SEM images of magnetite particles prepared in water (a) and in sodium silicate solution (b). The corresponding size distributions derived from the images are also shown. The size of the particles for both cases is of the order of several nanometers. Although the particles are nearly spherical in shape in both cases, the particles prepared in the sodium silicate solution appear to be more uniform in size as indicated by the smaller value of the standard deviation of the Fig. 5 Illustration of a simple mechanism for the formation of a silica layer on the surface of magnetite particles

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size distribution. The mean size of the particles, as measured by BET surface area at various conditions, is shown in Table 1. The mean particle sizes estimated from the specific surface area are in good agreement with those measured by SEM. The size of the particles prepared in water at a voltage of 20 V is *22.56 nm. The particle size decreases significantly to 9.98 and 8.67 nm by adding sodium silicate at concentrations of 150 and 200 ppm, respectively. Decreasing the applied voltage has the same effect as increasing the sodium silicate concentration. The same effect was observed for the cases without sodium silicate (Fajaroh et al. 2011). The crystallinity of the powders produced by the electrosynthesis depends on both sodium silicate concentration and applied voltage. The crystallinity tends to decrease when the sodium silicate concentration is increased (Fig. 1). As discussed above, in very dilute solution, sodium silicate dissociates completely to form Na? and SiO32-. The SiO32- ions tend to bond oxygen as a part of magnetite, and the other end reacts further by a condensation reaction to form a silica layer to cover magnetite particles. Therefore, the opportunity to form silica film on the magnetite surface is greater when the silicate concentration is larger that causes the particle growth is hindered and the particle size decreases (Table 1). As a result, the crystallinity of the particles decreases. The crystallinity of the particles increases with an increase in the applied voltage (Fig. 2). More hydroxyl ions are generated by water reduction at the


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(b)

(a)

davg = 19.4 nm (dBET = 22.56 nm) Std. dev. = 9.3 nm

1.0

= 12.5 nm (dBET = 10 nm) = 5.9 nm

1.0 0.8

0.8

Fra action/nm

Frraction/nm

davg Std. dev.

1.2

1.2

0.6 0.4

0.6 0.4 0.2

0.2

0.0

0.0 0

5

10

15

20

25

30

35

40

Particle size (nm)

0

5

10

15

20

25

30

35

40

Particle size (nm)

Fig. 6 SEM images of magnetite particles prepared in a sodium silicate solution and b water Table 1 Mean diameter of magnetite particles prepared under various conditions SS concentration (ppm)

Potential (V)

Particle diameter (nm)

0

20

22.56

150

20

9.98

200

20

8.67

200

15

5.75

cathode at higher applied voltages. These hydroxyl ions can also move to the anode faster by diffusion and migration at a higher applied voltage. Hydroxyl ions play a prominent role in the formation of magnetite nanoparticles around the anode through the basecatalyzed reaction of ferrous ions produced by the electrooxidation of iron (Fajaroh et al. 2011). The size

of the particles increases (Table 1), and the crystallinity also increases. The magnetic properties of the silica-coated magnetite nanoparticles were examined with the SQUID magnetometer. Figure 7 shows the magnetization curve at room temperature for silica-coated magnetite nanoparticles prepared at an applied voltage of 20 V at two sodium silicate concentrations, 150 and 200 ppm. The result suggests that the silica-coated nanoparticles probably possess superparamagnetic properties, as indicated by a nearly zero remanence and a negligible coercivity in the absence of an external magnetic field. The superparamagnetism behavior is typically observed for materials composed of very small crystals (1–10 nm). In this size range, each particle becomes a single magnetic domain, and the energy barrier for its spin reversal is easily overcome by

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Magneetization ((emu/g)

20

150 ppm 200 ppm

10

0

-10

-20 -4000

-2000

0

2000

4000

Magnetic Field (Oe)

Fig. 7 Magnetization curve at room temperature for silicacoated magnetite nanoparticles prepared at two different sodium silicate concentrations at an applied voltage of 20 V

thermal vibrations. The saturation magnetization decreases with an increase in sodium silicate concentration. The saturation magnetization assumes a value of *22 and 16 emu g-1 for sodium silicate concentrations of 150 and 200 ppm, respectively. The values are less than the values for uncoated magnetite nanoparticles prepared in water (Fajaroh et al. 2011) and for the Fe3O4 bulk materials (Ms = 92 emu g-1). The temperature dependence of magnetization of the prepared nanoparticles was determined under an external field of 200 Oe at temperatures ranging from 10 to 350 K. Figure 8 shows the effect of temperature on magnetization for nanoparticles prepared at two different sodium silicate concentrations (150 and

Magnetization (emu/g)

16 15

150 ppm

14 13 12

Conclusion This study demonstrated that silica-coated magnetite nanoparticles can be prepared using a one-step electrochemical method in which pure iron is electrooxidized in a dilute aqueous sodium silicate solution that serves as the silica precursor. The presence of silicate significantly enhances the purity of the magnetite formed. The formation of magnetite nanoparticles is influenced by the applied voltage and the sodium silicate concentration. An increase in the applied voltage tends to promote the formation of magnetite nanoparticles, and the particle size becomes larger. In contrast, an increase in the sodium silicate concentration tends to decrease the particle size. The magnetite nanoparticles produced by the electrooxidation of iron in a sodium silicate solution have a mean size ranging from 6 to 10 nm, depending upon the experimental conditions. The nanoparticles exhibit superparamagnetic properties with a saturation magnetization lower than the saturation magnetization of the Fe3O4 bulk materials. These results demonstrate the possibility of using this synthetic technique as an environmentally friendly method for the production of silica-coated magnetite nanoparticles.

200 ppm

11 10 9 50

100

150

200

250

300

Temperature (K)

Fig. 8 The temperature dependence of magnetization for silica-coated magnetite nanoparticles

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200 ppm) at an applied voltage of 20 V. For both the cases, the nanoparticles exhibit a cusp that corresponds to the blocking temperature, TB. The blocking temperatures assume the values of *300 and 25 K, respectively, for nanoparticles prepared in 150 and 200 ppm sodium silicate solutions. Various TB values are reported for magnetite nanoparticles prepared using different techniques (Ozkaya et al. 2009). The blocking temperature is another parameter that depends strongly on particle size (Buschow 2006). This maximum peak indicates a transition from a ferromagnetic property at low temperatures to a superparamagnetic behavior at high temperatures.

Acknowledgments We would like to thank the Directorate General of Higher Education (DGHE), the Ministry of National Education, Indonesia, for funding through a Fundamental Research Grant and for the financial support for one of the authors (FF) to obtain a doctorate through BPPS and to visit the Tokyo University of Agriculture and Technology as a research fellow via a Sandwich-Like Program. We also thank Ms. Ratih Y. Utomo and Ms. Kartikasari Sutrisno for their assistance with the experiments.


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