Synthesis and characterization of magnetic porous clayheterostructure

Natthaphon Bunnak • Sarute Ummartyotin •

Pitak Laoratanakul • Amar S. Bhalla •

Hathaikarn Manuspiya

� Springer Science+Business Media New York 2013

Abstract Magnetic porous clay heterostructure (magnetic

PCH) was successfully synthesized using a simple precipita-

tion method of applying magnetite onto a PCH surface. X-ray

techniques were used to confirm the presence of magnetite in

the composite. The magnetite particles, as investigated by the

transmission electron microscopy, were spherical nanoparti-

cles (*12.07 nm). The magnetic PCH exhibited characteris-

tics of mesoporous material type IV, similar to PCH.

Significant enhancement of the magnetic and dielectric prop-

erties in the high frequency range was also observed.

Keywords Magnetic materials � Dielectric

properties � Magnetic properties � Porous materials

1 Introduction

The development of mineral clay for commercial use has

attracted significant interest in the past several decades.

The use of advanced composite materials has progressed

from a laboratory curiosity to a production reality. From

a fundamental point of view, the dispersion of clay into

a polymer is generally considered the first step of clay

composite preparation. Depending on the physical state

of the materials, various processing methods have been

used such as in situ polymerization [1], high shear

mixing [2], three-roll milling [3], as well as twin screw

extrusion [4]. Therefore, the use of clay-based compos-

ites has been investigated for applications in aerospace

and aircraft technology, sports equipment, and military

hardware [5].

Bentonite (BTN), absorbent aluminium phyllosilicate, is

an impure clay mostly consisting of montmorillonite. It is

used in a wide range of applications including drilling mud

[6, 7], bleaching earth [8], and water- and solvent-based

rheological additives [9, 10]. Due to its high viscosity [11],

low filtration loss [12], and stable swelling [13], BTN is in

high demand for commercial applications. The function of

BTN can be altered by alkali activation or by introducing

cations on its porous structure [14]. These activations

typically employ various types of cations in order to tailor

the engineering properties.

We have previously reported the successful modification

of clay-based materials by the introduction of cations. In

2008, Mattayan et al. [15] studied the enhancement of

magnetic properties of poly (lactic acid) (PLA) film for

food packaging by introducing iron oxide into the PLA

matrix. They reported that the magnetic properties of

the composite films were enhanced, and there was an

improvement in anti-bacterial properties. Then, in 2010,

Jindapech et al. [16] reported the successful modification of

magnetic porous clay heterostructure by the addition of

manganese ions. The purpose of their study was to

induce the magnetic properties of clay for anti-corrosion

enhancement in active packaging.

Similarly, these novel materials have been developed for

telecommunication systems to obtain superior performance

N. Bunnak � S. Ummartyotin � H. Manuspiya (&)

The Petroleum and Petrochemical College, Chulalongkorn

University, Bangkok 10330, Thailand

e-mail: hathaikarn.m@chula.ac.th

P. Laoratanakul

National Metal and Materials Technology Center,

Pathumthani 12120, Thailand

A. S. Bhalla

Department of Electrical and Computer Engineering, College of

Engineering, The University of Texas at San Antonio,

San Antonio, TX 78249, USA

123

J Porous Mater

DOI 10.1007/s10934-013-9739-6

in radio frequency identification (RFID) products. For use

in electromagnetic systems, the relationship between

structural, dielectric, and magnetic properties has to be

carefully considered. Thus, the development of clay-based

materials is highly desirable because properties of clay can

be modified by the placement of metal particles onto the

surface. This has led to further studies of clay-based

material for applications in electronic devices.

In 2013, Bunnak et al. [17] found that the introduction of

cations on the surface of clay can provide significant

enhancement of dielectric properties. In general, it is

known that clay has a layer-like structure, but in their study

they synthesized porous clay and suggested that cations

trapped in the porous structure lead to the enhancement of

dielectric properties due to the inclusion of the dipole-

dipole mechanism. Therefore, it is important to note that

adsorption of metal ions on clay surfaces is the most

desirable approach to produce significant changes in both

physical and chemical properties of clay-based materials.

This approach is easily exploitable and also useful in

electronic device applications.

It is time for the next generation of radio frequency

identification devices—prepared with clay—to be devel-

oped. We aimed to enhance both the magnetic and dielectric

properties of clay, which is suitable for RFID applications.

The RFID applications, generally used in frequency ranges

of the industrial-scientific-medical (ISM) arena, are

categorized into four ranges; (1) low frequency (LF;

\150 kHz), (2) high frequency (HF; 13.56 MHz), (3) ultra-

high frequency (UHF; 433/868/915 MHz), and (4) micro-

wave frequency (MF; 2.4/58 GHz). We aimed to make

composites for high-frequency applications, therefore, the

electrical properties were studied in the high frequency

range [18–20].

2 Material preparation and measurement

2.1 Material

Bentonite clay was obtained from a commercial source,

Thai Nippon Chemical Industry Co., Ltd. The cation

exchange capacity (CEC) of the BTN was 44.5 mmol/

100 g of clay. Cetyltrimethylammonium bromide (CTAB)

and tetraethyl orthosilicate (TEOS) came from Fluka and

were used as the cationic surfactant and silica source,

respectively. Methanol (CH3OH) and ammonium hydrox-

ide (NH4OH) were purchased from Lab Scan. Dodecyl-

amine and barium chloride (BaCl2) solution were obtained

from Sigma Aldrich. Ferric chloride (FeCl3�6H2O) and

ferrous chloride (FeCl2�4H20) were purchased from Merck.

All of the chemical reagents were used as received without

any further purification.

2.2 Sample preparation

Porous clay heterostructure (PCH) was successfully syn-

thesized as follows: approximately 50 g of BTN was added

into 0.1 M CTAB solution at 323 K and continuously stir-

red for 3 h. The synthesized product was filtered and

washed with 1,000 mL of 1:1 vol% mixture of water and

methanol. Organoclay (OGN) was obtained after the prod-

uct was dried at 334 K. The OGN was stirred in dodecyl-

amine at 323 K, followed by the addition of TEOS, and

stirred at room temperature for a further 4 h (OGN:Dode-

cylamine:TEOS = 1:20:150 mmol). The resulting product

was calcined at 873 K for 5 h and PCH was obtained.

Magnetic PCH was prepared as follows. Complex salts

(ferric and ferrous chloride), 0–50 wt%, were added to the

PCH suspension, and dispersed in distilled water under N2

atmosphere. Then, the solution was stirred at room temper-

ature for 1 h. Next, ammonium hydroxide was slowly drop-

ped into the solution to allow precipitation of the magnetite

[21]. The reactants were stirred for a further 30 min to allow

complete precipitation of the magnetite. The obtained solu-

tion was sonicated for 15 min, then filtered and washed with

1,000 mL of distilled water. The obtained powder was dried

at 343 K for 24 h. Finally, magnetic PCH powder was

obtained. In addition, the magnetite was synthesized and can

be represented by the reaction shown [22]

Fe2þ + 2Fe3þ + 8OH� ! Fe3O4 + 4H2O ð1Þ

2.3 Measurements

2.3.1 Fourier transform infrared spectroscopy (FTIR)

Fourier transform infrared spectra of PCH and magnetic

PCH were obtained with a Nicolet Nexus 670 FTIR

spectrometer in the frequency range of 4,000–400 cm-1 at

64 scans with a resolution of 4 cm-1. The samples were

prepared using a mixture with KBr in pellet form.

2.3.2 X-ray diffraction (XRD)

X-ray diffraction patterns were collected by using a Rigaku

Model Dmax 2002 diffractrometer, with Ni-filtered CuKa

radiation, operated at 40 kV and 30 mA. The powder

samples were observed in the 2h range of 10�–70� with a

scan speed of 2�/min and a scan step of 0.02�.

2.3.3 Field emission scanning electron microscopy

(FE-SEM)

The morphological properties of PCH and magnetic PCH

were investigated by FE-SEM (a Hitachi S-4800 model) at

an acceleration voltage of 2.0 kV. Prior to investigation,

J Porous Mater

123

the samples were sputter coated with platinum under vac-

uum for 3 min to reduce particle charging.

2.3.4 Energy dispersive x-ray analysis (EDX)

The elemental composition of PCH and magnetic PCH was

determined by using a Hitachi S-4800 field emission

scanning electron microscope (FE-SEM). The samples

were coated with platinum nanoparticles before measure-

ment. The tests were operated at an acceleration voltage of

10.0 kV.

2.3.5 X-ray fluorescence spectroscopy (XRF)

The chemical composition was determined with an AXIOS

PW440 XRF with a silver x-ray tube, operated at 60 kV.

Each sample was placed in a sample holder with a diameter

of 37 mm. The chemical composition of PCH and mag-

netic PCH is shown in Table 1.

2.3.6 Surface area analysis (SAA)

Nitrogen adsorption desorption isotherms were obtained at

77 K by using a Quantachrome Autosorb-1. Powder sam-

ples were degased at 422 K for 16 h under vacuum prior to

analysis. Specific surface area and pore size were calcu-

lated by using the BET equation [23]. The pore size dis-

tributions were investigated based on the Barrett, Joyner,

and Halenda (BJH) method [24].

2.3.7 Vibration sample magnetometer (VSM)

Magnetic properties can be measured in terms of magne-

tization. The vibration sample magnetometer (LakeShore-

7404), in continuous mode, was used to perform the

analysis. The maximum field and ramp rate were set at

10 kOe and 50.63 Oe/s, respectively.

2.3.8 Transmission electron microscopy (TEM)

Transmission electron microscopy images of the sample

were taken by using a Hitachi H-7650 TEM. The powder

was prepared as a dilute solution. The test was operated

with an accelerating voltage of 100 kV.

2.3.9 Dielectric properties measurement

All samples had an electrode placed on both sides prior

to taking electrical property measurements. The sample

pellets, with a diameter of 12 mm and thickness of

*0.50 mm, were coated with platinum nanoparticles on

both sides (diameter *10 mm) for 180 s to ensure good

electrode contact. The dielectric data were collected by

using a network analyzer (Agilent E4991A) interfaced with

a 16453A test fixture. The capacitance of the samples was

measured as a function of frequency (1 MHz–1 GHz) at

constant temperature (293 K). The dielectric constant (k) of

the composite was calculated by using Eq. 2:

C ¼ e0kA

dð2Þ

where C is the capacitance of the composite (F), e0 is the

dielectric constant of the free space (8.85 9 10-12 F/m),

A is the electrode area (m2), and d is the distance between

the electrodes, i.e. thickness of the pellet (m).

3 Results and discussion

Magnetic PCH was synthesized by applying magnetite

(Fe2? and Fe3?) onto the PCH surface via the precipita-

tion method. To verify the magnetic properties, a 0.3T

magnet was used. The entire magnetic PCH powder

sample was attracted to the magnet, confirming that the

powder was magnetic. Figure 1 displays typical FTIR

spectra of the materials. It was found that the spectrum of

magnetic PCH is similar to the spectrum of PCH, indi-

cating Si–O–Si bending vibrations (476 and 475 cm-1),

Si–O–Si symmetric stretching (785 and 796 cm-1), Si–

O–Si asymmetric stretching (1,085 and 1,097 cm-1), H–

O–H bending vibrations of physically adsorbed water

(1,618 and 1,640 cm-1), and O–H stretching vibrations of

hydrogen-bonded surface silanol groups and physically

adsorbed water (3,398 and 3,440 cm-1) [25]. The FTIR

spectra are in good agreement with previously reported

results [15–17, 25]. There was no observed peak indi-

cating the presence of free iron oxide after synthesis;

consequently, x-ray techniques were used to confirm the

presence of magnetite.

Table 1 Chemical composition of PCH and magnetic PCH

Sample Concentration (%)

Mg Al Si K Ca Fe Cu Cl Ti

PCH 0.72 4.91 92.55 0.18 1.04 0.30 0.05 0.06 0.10

Magnetic PCH 0.52 4.71 66.45 0.08 0.47 26.23 0.23 1.24 0.07

J Porous Mater

123

The XRF results indicate that 26.23 % of magnetic PCH

is compose of iron ions (Table 1). The XRD technique was

also used to study the crystal structure of the materials and

to confirm the presence of magnetite. The XRD pattern of

magnetic PCH (Fig. 2) indicates strong characteristic peaks

at (210) and (311) identifying PCH [26, 27] and magnetite

[28, 29], respectively. Additionally, the XRD pattern of

magnetite (insert Fig. 2) exhibits six peaks at 30.16� (220),

35.58� (311), 43.04� (400), 53.62� (422), 57.22� (511), and

62.78� (440) from which the lattice parameter was calcu-

lated as 0.837 ± 0.002 nm, thus identifying the charac-

teristic of magnetite (0.839 nm) [30–36]. Generally,

magnetite (Fe3O4) is easily oxidized and transformed to

maghemite (c-Fe2O3) and hematite (a-Fe2O3) with

increased temperature [28, 37]. The characteristic peaks at

(113), (210), and (213), (210) are attributed to the ma-

ghemite and hematite, respectively; however, these peaks

are not found in the XRD patterns (magnetic PCH and

magnetite) implying the successful inducement of magne-

tite in PCH. The composite provides excellent magnetic

properties (Fig. 6). The presence of the magnetite does not

change the structure of the magnetic PCH, confirmed by

the XRD results and surface area analysis. The XRD pat-

tern of magnetic PCH exhibits the characteristic peaks of

both PCH and magnetite, while the surface area analysis

indicates that the magnetic PCH also exhibits characteristic

of porous materials type IV (Fig. 3), which is similar to

that of the PCH starting material. Therefore, it can be

concluded that the magnetite can only be attached on the

surface of PCH, suggesting that PCH acts as a host material

while magnetite acts as a guest material. Magnetic PCH

still retains the pore characteristics.

Magnetic PCH exhibits the nitrogen adsorption isotherm

type IV (Fig. 3), similar to that of PCH, indicating meso-

porosity of the material [38, 39] with a very narrow pore

size of *10.07 nm. The BET surface area and BJH pore

volume of magnetic PCH were carefully calculated as

267 m2/g and 0.67 cc/g, respectively. Additionally, the

BET surface area and BJH pore volume of PCH were

calculated as 222 m2/g and 0.47 cc/g, respectively. It is

interesting to note that the magnetic PCH showed higher

surface area and pore volume than PCH. Significant

changes in surface area and pore volume can be explained

as an increase in the adsorption of the magnetic nanopar-

ticles either in the pores or on the surface of PCH. We

believe that magnetite was formed in the medium and then

the particles adhered to each other on the clay surface

resulting in increased surface area. Oliveria et al. [28] has

reported that this probably relates to the porous texture of

the formed newly magnetite contributing to the increase of

surface area and porosity.

It can be suggested that the magnetic PCH exhibits a

rough surface, similar to the surface of PCH (Fig. 4a, b).

Fig. 1 FTIR spectra of PCH and magnetic PCHFig. 2 XRD patterns of PCH, magnetic PCH, and magnetite (insert)

Fig. 3 Nitrogen adsorption isotherm of magnetic PCH and PCH

(insert)

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After the synthesis of magnetic PCH, spherical nanoparti-

cles filled in the pores of PCH. The morphology of mag-

netite is observed by SEM, as shown in Fig. 4c, implying

that the magnetite is spherical with a median value of

25.43 ± 5.96 nm. However, when looking at magnetite via

TEM, the magnetite spheres are relatively uniform, with a

diameter of 12.07 ± 2.78 nm (Fig. 4d). Interestingly, the

average particle size of magnetite as observed by using the

SEM technique is larger than that as observed by using the

TEM technique because the magnetite particles attached to

each other, which is a characteristic of the magnetite. In

fact, during sample preparation—in regards to the TEM

technique—the magnetite was dispersed in a solvent to

observe the real size without agglomeration; while for the

SEM technique, the magnetite was observed without dis-

persion in solvent, consequently, an agglomeration of

particles was found. The EDX technique was used to

identify the elements and their distribution in the PCH

matrix. The results indicate that the magnetic PCH surface

was covered with iron atoms (43.65 %), clear evidence of

magnetite in the PCH matrix. Consequently, the iron

mapping of magnetic PCH, with 30 wt% magnetite, clearly

indicates good distribution of magnetite (green dot) in PCH

(Fig. 4e). All magnetic PCH samples, with various mag-

netite contents, also exhibited good distribution of mag-

netite in the PCH; therefore, they are not shown.

Ideally, the addition of guest atoms helps to increase the

dielectric constant of the host material. The random

attachment of guest atoms (magnetite) causes increased

dipole polarization of the host material (PCH); thus, the

total dipole effect of the composite (magnetic PCH) may

increase; and as a result, the dielectric constant may also

increase [40, 41]. Bunnak et al. [17, 42] in 2013, reported

that the addition of metal atoms to PCH resulted in an

Fig. 4 Morphologies of (a) PCH (b) magnetic PCH and (c) magnetite as observed by using SEM technique (d) magnetite as observed by using

TEM technique, and iron mapping of (e) magnetic PCH

J Porous Mater

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improved dielectric constant. Industry requirements for

electronic devices were also met. In the current work, the

dielectric constant of magnetic PCH was investigated in the

high frequency region. The dielectric constant of magnetic

PCH was higher than that of PCH in all frequency ranges

and it affected the loss factor (Fig. 5). The dielectric

characteristic of magnetic PCH was similar to that of PCH,

which strongly decreased with increased frequency. It is

important to note that in the high frequency ranges,

polarization is less responsive to frequency. The addition of

magnetite to PCH caused the creation of a dipole moment,

resulting in an enhancement of the dielectric constant of

magnetic PCH.

To investigate the magnetic properties of PCH and

magnetic PCH, a magnetic field was applied by using VSM

at a constant temperature (room temperature) with results

in Fig. 6. PCH exhibits paramagnetic behavior, which is

completely reversible in nature [43]. The magnetic moment

can be induced by applying a magnetic field. However, the

paramagnetic material does not retain any magnetization in

the absence of an externally applied magnetic field because

the thermal motion causes electron spinning to become

Fig. 5 Dielectric properties of

PCH and magnetic PCH as

measured in the high frequency

region at constant temperature

Fig. 6 Magnetization of PCH and magnetic PCH in relation to

applied magnetic field Fig. 7 Effect of magnetite content on magnetization of magnetic

PCH

J Porous Mater

123

randomly oriented. Subsequently, total magnetization

drops to zero when the applied field is removed [44].

Moreover, the hysteresis loop of PCH is a tight loop

indicating a single magnetic phase in PCH with a saturated

magnetization of 0.1405 emu/g. The coercivity and per-

meability were calculated as 43.1 G and 0.0002, respec-

tively. The magnetization of PCH can be adjusted by

introducing magnetite. The saturated magnetization of

magnetic PCH increases with increased magnetite content,

as shown in Fig. 7. Typically, the saturated magnetization

of magnetite is very high due to its electron spin. Chen

et al. [31] has reported the saturated magnetization of

magnetite as 65 emu/g while Petcharoen et al. [30] has

reported the saturated magnetization of magnetite as

57 emu/g. In this work, the saturated magnetization of

magnetic PCH was found to be 1.2820 emu/g. The coer-

civity and permeability were also calculated as 26.6 G and

0.0009, respectively. Magnetization of magnetic PCH

jumps with an applied magnetic field; therefore, it is clear

that magnetization could be easily adjusted by applying a

magnetic field. In addition, the total magnetization of

magnetic PCH falls down to zero without an applied

magnetic field, known as superparamagnetism [37, 45].

Similarly, the completely reversible hysteresis loop with a

tight loop was also found, indicating a single magnetic

phase [43, 44].

4 Conclusion

Porous clay heterostructure is environmental friendly. Its

properties can be adjusted to make magnetic PCH, by

introducing magnetite. Magnetic porous clay heterostruc-

ture (magnetic PCH) was successfully synthesized using

the simple precipitation method by applying magnetite

onto the surface of PCH to enhance the magnetic and

dielectric properties. The magnetic PCH exhibited the

characteristic of mesoporous material type IV similar to

PCH. X-ray results indicated that the precipitated complex

salts were magnetite nanoparticles, with a median size of

12.07 nm (as investigated by TEM). Furthermore, it was

found that the magnetic PCH offered significantly

enhanced magnetic and dielectric properties in the high

frequency range. This magnetic PCH meets industry

requirements for electronic devices in radio frequency

identification labels. It can be used as an alternative

material for antenna wire in RFID labels, replacing the

conductive metal wires. In the high frequency range it can

be useful for smart cards, smart shelves, library books, as

well as airline baggage handling tags.

Acknowledgments The authors would like to acknowledge the

receipt of a research grant from the National Research Council of

Thailand (NRCT), and partial support from the 90th Anniversary of

Chulalongkorn University Fund (Ratchadaphiseksomphot Endow-

ment Fund). Additionally, the authors are thankful for the use of the

experimental facilities at the Polymer Processing and Polymer

Nanomaterials Research Unit of the Petroleum and Petrochemical

College, Chulalongkorn University. We sincerely appreciate to Asst.

Prof. Dr. Pongsakorn Jantaratana, Department of Physics, Kasetsart

University, for his kind assistance with the Vibration Sample Mag-

netometer measurement. Finally, NB would like to acknowledge his

PhD scholarship from the Thailand Graduate Institute of Science and

Technology (TGIST).

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