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

Synthesis of Iron Nanoclusters by Pulsed Current

Method

Hassan Karami Ozra Rostami-Ostadkalayeh

Received: 3 January 2009 / Published online: 28 July 2009 Springer Science+Business Media, LLC 2009

Abstract The main objective of this research is to prepare iron nanoclusters with

more porous structure by the pulsed current electrochemical method. In this method,

there are some effective parameters including; pulse amplitude (current amount),

pulse frequency, iron salt concentration, sulfuric acid concentration and synthesis

temperature, which were optimized by the one at a time method. An iron optimized

nanopowder was synthesized by using iron sulfate (0.005 M) as precursor and silver

nitrate as a nucleation agent (at 0.5% mole of iron sulfate in the starting solution) bypulsed current of 20 mA cm-2 with a frequency of 14 Hz. The relative gravimetrical

density was used as an optimizing parameter for iron nanoparticles synthesis. The

morphology and particle size of each synthesized sample was studied by SEM, TEM

and XRD. The iron nanopowder synthesized in the optimum conditions has excellent

uniform and a more porous structure including nanoclusters with a particle size of

approximately 2040 nm. The obtained results indicate that the pulsed current elec-

trochemical method can be used as a confident and controllable method for the

preparation of iron nanoparticles. XRD, EDX and ICP-AES results showed that the

iron nanoclusters can be synthesized with high purity by the proposed method.

Keywords Pulsed current Nanoclusters Iron Electrochemical

Nanoparticle Nanopowder

Introduction

Small metal particles in the nanometer range display different chemical and

physical properties from bulk materials, and play an important role as a

H. Karami (&) O. Rostami-Ostadkalayeh

Electrochemistry Laboratory, Department of chemistry, Payame Noor University (PNU),

P.O. Box 97, Abhar, Iran

e-mail: [email protected]

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DOI 10.1007/s10876-009-0262-5

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heterogeneous catalyst. The reactivity of the nanosized catalyst particle is associated

with their high ratio of surface to bulk atoms [1]. The metal nanoparticles display

unique properties that differ from the bulk substances, i.e., a different heat capacity,

vapor pressure, and melting point. Moreover, when decreasing the metal particle

size sufficiently enough, there occurs a transition of the electronic state from ametallic to non-metallic one. Additionally, metal nanoparticles exhibit a large

surface-to-volume ratio and an increased number of edges, corners and faces leading

to altered catalytic activity and selectivity [2]. Nanometre-sized particles of

magnetic metals have been attracting substantial interest in basic and applied

research for many years [36]. This activity is mainly driven by the envisaged

application of such nanoparticles in high-density data storage [7], in magnetic

resonance imaging [8,9], catalysts [10,11] and especially for soil remediation [12].

In many cases they show improved or even new properties due to their size in the

nanometre range [13]. State-of-the-art production methods of metal nanoparticlesinclude arc discharge [4], colloidal chemistry [14], sputtering or laser ablation [15],

and ripening from gel-like films [10]. Each method has its specific advantages and

disadvantages. Critical issues are the stability of the particles with time, their

protection against oxidation [16], and the control of their size and shape [17].

Magnetic nanoparticles have received much attention because of their applica-

tions in various fields, and thus, a lot of methods for synthesis of magnetic

nanoparticles have been developed and reported. As for the metal nanoparticles,

methods through decomposition or chemical reduction of organometallic com-

pounds at high temperature are investigated extensively. In the present study, wefocus on a two-phase liquidliquid system, which has been established for the

synthesis of gold nanoparticles [18], as a novel approach to the synthesis of

magnetic nanoparticles containing iron. Because the two-phase liquidliquid system

has been applied to the synthesis of gold-alloys including goldplatinum and gold

palladium [19], it is expected to be useful for its application to the magnetic iron-

alloy nanoparticles if the synthesis of iron nanoparticles is conveniently achieved. In

addition, the oxidation of iron nanoparticles has been reported to bring about the

formation of monodisperse iron-oxide nanoparticles [2022]. Magnetic nanoparti-

cles can be used for many applications, including magnetic recording media,

catalysis, MRI contrast enhancement, drug delivery and hyperthermia [23, 24].

Hyperthermia is a promising form of cancer therapy that locally heats tissue to

greater than 42 C for to destroy tissue, particularly tumors. The difficulty of this

therapy lies in selectively heating cancer tumors [23]. By localizing magnetic

nanoparticles to the desired area and applying an alternating magnetic field (AMF),

the particles can generate heat by hysteresis loss, thus killing cancer cells with

minimal injury to normal tissues. Highly hysteretic particles are desirable because

they will give off the most heat with the lowest dose to a patient. Iron, which has a

very high magnetic saturation value (220 emu/g), is ideal for hyperthermia

application [23,24].

The preparation methods influence the properties of nanoscale iron particles.

Mostly chemical and physical methods are used to generate iron nanoparticles in the

form of colloids as well as powders. From the preparation point of view, one of the

important subjects in the study of magnetic nanoparticles is the preparation of stable

588 H. Karami, O. Rostami-Ostadkalayeh

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nanoparticles with novel properties. In the present study we report a simple way to

synthesis iron nanoclusters. Iron nanoclusters were electrodeposited by applying a

pulsed current technique from an iron salt acidic solution containing sulfuric acid.

The synthesized samples were characterized by relative gravimetrical density

(RGD), scanning electron microscopy (SEM), X-ray diffraction (XRD) anddetermination of magnetic momentum.

Experiment

Materials

All materials and reagents used in these experiments were of reagent grade and were

produced in the Loba Chemie Co. (India). Double-distilled water was used in all theexperiments.

Instrumentals

A power source of the MPS-3010L model, made by the Taiwan Matrix Company

was used for producing the constant current. A home-made electrical pulse

apparatus was applied to make the reproducible current pulses. Figure 1shows the

used laboratory systems, including the power supply, pulse apparatus, and

electrochemical cell. The solution temperature was kept constant with usage of awater bath (Optima, Tokyo, Japan).

A scanning electron microscope with EDX instrument from Philips Co. (XL30)

was used for studying of morphology, particle size and surface analysis of the

prepared iron nanopowders. X-ray diffraction (XRD) studies were performed by a

Fig. 1 Laboratory system used to synthesis the lead dioxide nanoparticles, including power supply, pulse

maker apparatus, and electrochemical cell

Synthesis of Iron Nanoclusters by Pulsed Current Method 589

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Decker D8 instrument. Bulk analysis of the sample was performed by inductively

coupled plasma atomic emission spectroscopy (ICP-AES) from Varian Co.

Procedure

Reagent grade iron sulfate (FeSO4) was used as the precursor and, silver nitrate

(AgNO3) was chosen as the source of Ag as a nucleation agent for iron synthesis. The

appropriate amounts of iron sulfate (0.005 M) and AgNO3 (at 0.5% mole of iron in

the starting solution) were first dissolved in low acidic water (pH = 5), which was

then heated to the desired temperature for the reaction. The obtained solution was

used in the electrochemical synthesis cell with two stainless steel (316L) electrodes as

anodes and one stainless steel (316L) as a cathode. In theses cases, the precipitate was

separated from the solution, washed with water and ethanol, and dried at room

temperature in inert atmosphere. After milling, RGD of the obtained powder wasdetermined by a simple gravimetrical method. For this concept; a glass vessel was

made with constant volume (0.5 cm3); the powder sample was pressed into this vessel

under constant pressure (1.0 bar). RGD was calculated from powder weight and

vessel volume. The morphology, dimension size, and structure of iron nanoparticles

were studied by scanning electron microscopy and XRD. The magnetic saturation and

coercivity of the nanoparticles were determined by examining the hysteresis loop

produced by the vibrating sample magnetometer (VSM).

At this method, there are effective parameters which include pulse amplitude

(current amount or pulse height), pulse frequency, iron salt concentration, solutionpH, and temperature of synthesis solution which were optimized by the one at a time

method.

Results and Discussion

In this procedure, a pulse current is exerted on the electrochemical solution

containing a stainless steel (316L) cathode and two stainless steel (316L) anodes by

the electrical pulse instrumentation. Experimental observations showed that the

synthesized iron nanoclusters do not stick to the electrode surface in this method. The

synthesized particles immediately leave the electrode surface. Therefore, electrode

surface is not completely covered by the synthesized particles. In this method, there

are some effective parameters which include, pulse amplitude (current amount or

pulse height), pulse frequency, iron salt concentration, solution pH, and temperature

of synthesis solution which were optimized by the one at a time method.

Before optimizing of pulse parameters, the effect of pH was studied. Initial

studies showed that the pH range of 2.83.8 is suitable for iron synthesis by this

method.

Determination of RGD

Our initial studies showed that RGD can be used as an easy and cheep optimizing

parameter when phase composition is constant. For this propose, many iron powder


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samples with different porosity were synthesized and then, their porosity were

determined by mercury adsorption method and then, RGD of all samples were

determined by the proposed method. Figure2 shows the experimental correlation

between RGD and porosity of iron powder. The presented results show good

correlation between RGD and porosity so that, RGD can be used as an optimizing

parameter to obtain a more porous powder.

Pulse Frequency Optimization

In the pulsed current method, the current vs. time was applied as shown in Fig.3. As

it can be seen in Fig. 3, the used current pulse has four variable parameters: pulse

0.7

0.9

1.1

1.3

1.5

1.7

1.9

2.1

30 40 50 60 70 80 90 100 110 120 130 140 150

Porosity (m2.g

-1)

RGD(g.c

m2-)

Fig. 2 Correlation between RGD and porosity of iron powder

Fig. 3 Used pulse diagram, including pulse time, relaxation time, and pulse height


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height (current amplitude), pulse time, relaxation time, and pulse frequency. Our

initial studies show that the relaxation time/pulse time ratio of three is suitable and

optimum to obtain a suitable synthesis, thus the ratio of three was selected for later

studies. At constant ratio of relaxation time to pulse time, a pulse system has three

variable parameters: pulse height, pulse time, and pulse frequency. The effect of alleffective parameters was optimized by the one at a time method.

Our initial studies showed that the pulse current method is more effective than

the simple constant voltage and simple constant current methods. The pulsed current

electrochemical method can be used as a confident and controllable method for the

production of nanoparticles. Each current pulse contains an on time (pulse time)

and an off time (relaxation time), so that the pulse frequency can be related to the

sum of current pulses which can be applied in to the system at one unit of time. At

constant ratio of relaxation time/pulse time (which is equal to 3), any variation in

pulse frequency makes a new pulse and relaxation time.In order to investigate the effect of pulse frequency on particle size, the pulse

frequency was varied from 0 to 18 Hz. The SEM results showed that at a frequency

of 14 Hz, the synthesized iron nanopowder is more uniform, it has smaller particles

and more porous, exceptional nanostructures. Figure4 shows the effect of pulse

frequency on the relative gravimetrical density (RGD) of yield powder. As it was

shown in Fig.4, RGD is decreased when the pulse frequency increased from 0 to

14 Hz. Applying the synthesis current in pulse form decreases the nuclear growth,

thus the uniform and more porous powder with low RGD is synthesized. At higher

pulse frequencies (more than 14 Hz), the relaxation time between two successivepulses is very short so that the new synthesis cycle (according to the new pulse)

starts before the finishing of the previous synthesis cycle (according to the previous

pulse). It should be mentioned that the nuclear growth has little hindrance time with

respect to pulse time (reduction of iron ion). At lower frequencies, the nuclear

growth kinetic is faster than the nuclear production. For the pulse frequency of

14 Hz, the on and off durations were 18 and 54 ms, respectively. All samples

of frequency optimizing experiments also studied with SEM. The obtained results

by SEM were confirmed those of RGD experiments. Figure5shows the SEM image

Fig. 4 Effect of the pulse frequency on relative gravimetrical density (RGD) of iron nanoparticles


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of the sample which synthesized at pulse frequency of 14 Hz. The sample has a

more porous structure in nanometric scale.

Optimization of Pulsed Current Amplitude

In order to investigate the effect of pulse current amplitude (pulse height) on the

RGD, morphology and particle size, the pulse height varied from 10 to 70 mA/cm2

was exerted on the electrochemical solution while the other parameters were kept

constant. Figure6shows the effect of pulse height (current density) on the relative

Fig. 5 SEM image of the sample which synthesized at pulse frequency of 14 Hz

Fig. 6 Effect of pulse height on the relative gravimetrical density (RGD) of yield powder


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gravimetrical density (RGD) of yield powder. As it was shown in Fig. 6, RGD is

increased when the pulse height increased from 20 to 70 mA cm-2

. This result can

be related to this fact that in the electrochemical pulse system, particle growth andagglomeration rates increases making the pulse height increase, which therefore

causes a decrease in that porosity and consequently an increase in RGD. The

obtained results showed that the current density (pulse height) of 20 mA cm-2 and

or lower cause to more porous structure and smaller and more uniform structures.

Use of lower current density not only makes to increase the experiment time but

also, makes no considerably improvement in the structure so that the amount of

20 mA cm-2

was select as optimum pulse height. Figure7shows the SEM image

of the sample with porous and uniform structure which synthesized under optimum

current density.

Temperature Optimization

For investigation of the synthesis temperature effect on the morphology and

particle size of iron nanopowder, five synthesizes were carried out at temperatures

of 0, 30, 50, 75 and 95 C. RGD and SEM studies showed that samples

synthesized at higher temperature have uniform nanoparticles with smaller

diameters. Therefore, temperature of 95 C was chosen as an optimum temperature

for further studies. All samples which synthesized at different temperatures were

studied by XRD. XRD results showed that the temperature has not any effect on

the composition of samples. All analyzed sample at these series of experiments had

only a-Fe form.

Fig. 7 SEM image of iron nanopowder which were synthesized at current density of 20 mA cm-2


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Optimization of the Initial Iron Sulfate Concentration

The effect of the initial iron sulfate concentration on the RGD, particle size and

morphology of iron nanopowder was investigated by varying the iron salt

concentration from 0.005 to 0.1 M. RGD and SEM studies showed that the lowerconcentrations of iron salts create a uniform structure and the smallest particle size

for the final iron nanopowder. As the initial concentration of iron sulfate in the

synthesis solution decreases from 0.1 to 0.005 M, the average particle size of iron

nanopowder also decreases from 150 nm to 40 nm. The obtained results can

probably be related to the reduction of particle growth and the agglomeration rate at

lower concentrations. At initial iron sulfate concentrations which are lower than

0.005 M, the amount of iron nanoparticles precipitation is very low, thus it can not

be filtered and collected. Therefore, the initial concentration of 0.005 M was

selected for the optimization of other parameters.

Optimized Iron Nanoclusters

The excellent uniform nanoclusters of iron in nanopowder form can be synthesized

at the optimum conditions as followings:

(1) Pulse frequency of 14 Hz

(2) Pulse height (current amplitude) of 20 mA cm-2

(3) Synthesis solution temperature of 95

C(4) Initial iron ion concentration of 0.005 M in sulfate salt form

When a synthesis is carried out under the above-mentioned conditions, iron

nanoclusters can be obtained in powder form. Figure8 shows the final SEM

images of the obtained iron powder samples for the optimization of the effective

parameters (See Determination of RGD, Pulse Frequency Optimization, Optimi-

zation of Pulsed Current Amplitude and Temperature Optimization sections). In

Fig.8, for each effective parameter (pulse frequency, pulse height, temperature,

and initial concentration of iron ion) the final SEM image of the sample which

synthesized in the optimum value of the proposed parameter has been presented.As observed in Fig. 8, in the duration of the optimization steps, the morphology

and particle size of the synthesized iron change from agglomerated particles to

perfect uniform nanoclusters. For more clarification, TEM images of the

optimized sample were shown in Fig.9. As it can be seen in Fig.8e and

Fig.9, the sample synthesized at optimum conditions has excellent uniform

nanoclusters. Figure10 shows the particle size distribution in the optimum

condition. Based the obtained results, major of the synthesized iron nanoparticles

was 20 to 40 nm in diameter.

For the determination of sample purity, the optimum sample was analyzed byEDX, XRD and ICP-AES. EDX peaks and XRD patterns for the optimum iron

nanoclusters were shown in Fig. 11. As it is obvious in Fig.11, the sample is very

pure and the EDX and XRD could not detect any impurity in the sample. It should

be mentioned that the EDX and XRD can be used only for surface analysis, but


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ICP-AES can reveals bulk analysis with more precision. EDX and ICP-AES

analysis showed that the Silver content of each sample is very small and negligible(lower than 0.01 %wt). Silver is a starting agent for iron crystallization so that, iron

synthesis in the absence of silver ion is very slow. The silver impurity was not

detected by XRD and EDX, because there is this element only in the center of each

iron nanoparticle.

Fig. 8 Final SEM images (with magnification of 150009) of the obtained iron nanopowder at the

different optimization steps: a optimized pulse frequency, b optimized current amplitude, c optimized

solution temperature, d optimized initial concentration of iron salt, and e sample synthesized in the

optimum conditions with magnification of 300009


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Magnetization Studies

The present study focuses on the magnetic properties of these novel iron

nanoclusters. Figure12shows the magnetization measurements of iron nanoclusters

and bulk iron. The measurements were performed on powder samples. It is observed

that the saturation magnetization is about 110 and 100 emu/g for iron nanoclustersand bulk iron powders, respectively. The obtained result showed that the

synthesized iron nanoclusters exhibit saturation magnetization 10% more than bulk

iron.

Fig. 9 TEM images of the iron nanoclusters synthesized in the optimum conditions

Fig. 10 Size distribution histogram for the iron nanoparticles


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Conclusion

The experimental results indicate which pulse current electrochemical method can

be used as a confident and controllable method for the preparation of iron

nanoclusters. When a synthesis is carried out under a pulse frequency of 14 Hz, a

pulse height (current amplitude) of 20 mA cm-2, a synthesis solution temperature

of 95 C, and an initial iron ion concentration of 0.005 M in sulfate salt form, ironnanopowder can be obtained in the perfect uniform structures in nanocluster form

with an average particle size of 20 to 40 nm.

Fig. 11 EDX and XRD patterns for the iron nanoclusters; all indexed and non-indexed peaks are related

to different atomic lines of Iron


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Acknowledgments We gratefully acknowledge the support of Abhar Payame Noor University

Research Council, throughout these research experiments.

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