One-step synthesis of silica-coated magnetite nanoparticles by electrooxidation of iron in sodium...

RESEARCH PAPER

One-step synthesis of silica-coated magnetite nanoparticlesby 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

� Springer Science+Business Media B.V. 2012

Abstract Silica-coated magnetite nanoparticles have

been synthesized successfully using a one-step elec-

trochemical 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

depends on the applied voltage and the sodium silicate

concentration. The magnetite nanoparticles exhibit

superparamagnetic properties with saturation magne-

tization 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 exten-

sive 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 increas-

ing 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

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

123

J Nanopart Res (2012) 14:807

DOI 10.1007/s11051-012-0807-7

separation, magnetic resonance imaging, immunoas-

says, and the separation of biochemical products

(Nishio et al. 2007; Berry and Curtis 2003). Magnetic

nanoparticles are also useful for environmental pro-

cesses, 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-magnetiza-

tion 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 sacri-

ficial 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 exper-

imental 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 magne-

tite 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):

Fe sð Þ ! Fe2þ aqð Þ þ 2e� ð1Þ

Fe2þ aqð Þ þ 2OH� aqð Þ ! Fe OHð Þ2 sð Þ ð2Þ

3Fe OHð Þ2 sð Þ þ 1=2O2 gð Þ ! Fe OHð Þ2 sð Þþ 2FeOOH sð Þ þ H2O lð Þ

ð3Þ

Fe OHð Þ2 sð Þ þ FeOOH sð Þ ! Fe3O4 sð Þ þ 2H2O lð Þð4Þ

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

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 nanoparti-

cles 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 magnetic-

based 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 hydro-

lysis 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 polycon-

densation of silicon alkoxides in an alcoholic envi-

ronment. Alkoxide compounds are expensive and

of 9 J Nanopart Res (2012) 14:807

123

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 elec-

trooxidation 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.

Experimental work

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 electro-

plating 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

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 mea-

suring 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 spher-

ical and dense such that the particle diameter could be

calculated using the equation

dp ¼6

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.

Results and discussion

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 parame-

ters, 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

J Nanopart Res (2012) 14:807 of 9

123

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 mag-

netite 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 nanopar-

ticles 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 spec-

trum of the particles was obtained using an FTIR

spectrophotometer. Figure 3 shows the infrared

spectrum of the samples prepared with various con-

centrations 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

FeOOHFe3O4

0 ppm.)si

ty (

a.u.

150 ppm

Inte

ns

300 ppm

200 ppm

25 30 35 40 45 50 55 60 65

2θ

Fig. 1 XRD patterns of particles prepared at various concen-

trations of sodium silicate at a constant applied voltage of 20 V

20 Vu.)

sity

(a.

u

15 V

Inte

n

25 30 35 40 45 50 55 60 65

2θ

Fig. 2 XRD patterns of particles prepared at two different

applied voltages at a sodium silicate concentration of 200 ppm

ceSi-O-Si

Si-O

200 ppmsm

itta

nc

Fe-O-Si150 ppm

%Tr

ans

0 ppm

H2 OOHOH

Fe-OH2 O

4000 3500 3000 2500 2000 1500 1000 500

Wavenumber (cm-1 )

Fig. 3 FTIR spectra of particles prepared at various concen-

trations of sodium silicate at a constant applied voltage of 20 V


123

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 corrob-

orate 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 per-

turbation 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 electrooxi-

dation 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

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 absor-

bance for the two samples. Higher absorbance indi-

cates 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 dissolu-

tion rate. These observations confirm that silica in the

samples prepared in sodium silicate solution as

0.12

0.10

006

0.08

Water

0.04 Sodium silicate

Ab

sorb

ance

0.00

0.02

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


123

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

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 concentra-

tion is increased (Fig. 1). As discussed above, in very

dilute solution, sodium silicate dissociates completely

to form Na? and SiO32-. The SiO3

2- 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 hydro-

xyl ions are generated by water reduction at the

Fig. 5 Illustration of a

simple mechanism for the

formation of a silica layer on

the surface of magnetite

particles


123

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 base-

catalyzed reaction of ferrous ions produced by the

electrooxidation of iron (Fajaroh et al. 2011). The size

of the particles increases (Table 1), and the crystal-

linity also increases.

The magnetic properties of the silica-coated mag-

netite 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

1.0

1.2 davg

= 12.5 nm (dBET

= 10 nm)Std. dev. = 5.9 nm

1.0

1.2

davg

= 19.4 nm (dBET

= 22.56 nm)Std. dev. = 9.3 nm

0.6

0.8

actio

n/nm

0.6

0.8

ract

ion/

nm

0.0

0.2

0.4Fra

0 5 10 15 20 25 30 35 400.0

0.2

0.4Fr

(b)(a)

0 5 10 15 20 25 30 35 40

Particle size (nm)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


123

thermal vibrations. The saturation magnetization

decreases with an increase in sodium silicate concen-

tration. The saturation magnetization assumes a value

of *22 and 16 emu g-1 for sodium silicate concen-

trations 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

200 ppm) at an applied voltage of 20 V. For both the

cases, the nanoparticles exhibit a cusp that corre-

sponds 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.

Conclusion

This study demonstrated that silica-coated magnetite

nanoparticles can be prepared using a one-step elec-

trochemical method in which pure iron is electrooxi-

dized 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 concentra-

tion 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 superparamag-

netic 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.

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.

20 150 ppm

10

(em

u/g) 200 ppm

0

e tiz

atio

n (

-20

-10

Mag

ne

-4000 -2000 0 2000 4000

Magnetic Field (Oe)

Fig. 7 Magnetization curve at room temperature for silica-

coated magnetite nanoparticles prepared at two different sodium

silicate concentrations at an applied voltage of 20 V

15

16

150 ppm

12

13

14

10

11

Mag

neti

zati

on (

emu/

g)

200 ppm

50 100 150 200 250 300

9

Temperature (K)

Fig. 8 The temperature dependence of magnetization for

silica-coated magnetite nanoparticles


123

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