international journal of bio-inorganic hybrid nanomaterials

A survey on the scientific literature indicates lots of

the researches have been devoted to the synthesis of

magnetic NPs, with spinel ferrite structures because

of their broad applications in several technological

fields including permanent magnets, magnetic

fluids, magnetic drug delivery, and high density

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 271-280

An Investigation on Synthesis and Magnetic Properties of

Manganese Doped Cobalt Ferrite Silica Core-Shell

Nanoparticles for Possible Biological Application

Somayyeh Rostamzadehmansour1*, Mirabdullah Seyedsadjadi2, Kheyrollah Mehrani3

1 Ph.D., Department of Chemistry, Ardabil Branch, Islamic Azad University, Ardabil, Iran

2 Associated Professor, Department of Chemistry, Science and Research Branch, Islamic Azad University,

Tehran Iran

3 Assistant Professor, Department of Chemistry, Science and Research Branch, Islamic Azad University,

Tehran Iran

Received: 20 November 2012; Accepted: 28 Jannuary 2013

In this work, we investigated synthesis, magnetic properties of silica coated metal ferrite,

(CoFe2O4)/SiO2 and Manganese doped cobalt ferrite nanoparticles (MnxCo1-xFe2O4 with

x= 0.02, 0.04 and 0.06)/SiO2 for possible biomedical application. All the ferrites nanoparticles

were prepared by co-precipitation method using FeCl3.6H2O, CoCl2.6H2O and MnCl2.2H2O as

precursors, and were silica coated by Stober process in directly ethanol. The composition, phase

structure and morphology of the prepared core-shell cobalt ferrites nanostructures were charac-

terized by powder X-ray diffraction (XRD), Fourier Transform Infra-red spectra (FT-IR), Field

Emission Scanning Electron Microscopy and energy dispersive X-ray analysis (FESEM-EDAX).

The results revealed that all the samples maintain ferrite spinel structure. While, the cell

parameters decrease monotonously by increase of Mn content indicating that the Mn ions are

substituted into the lattice of CoFe2O4. The magnetic properties of the prepared samples were

investigated at room temperature using Vibrating Sample Magnetometer (VSM). The results

revealed strongly dependence of room temperature magnetic properties on (1) doping content, x;

(2) particles size and ions distributions.

Keyword: Magnetic properties; Silica coated magnetic nanoparticles; Manganese doped ferrite

nanoparticles; Core-Shell.

ABSTRACT

1. INTRODUCTION

International Journal of Bio-Inorganic Hybrid Nanomaterials

(*) Corresponding Author - e-mail: [email protected]

recording media [1-4]. Structure of these magnetic

ferrite NPs, MFe2O4 (M= Fe, Co), is cubic inverse

spinel formed by oxygen atoms in a closed packing

structure where, M2+ and Fe3+ occupy either

tetrahedral or octahedral sites. Interesting point is

that, the magnetic configuration in these kinds of

materials can be engineered by changing or

adjusting the chemical identity of M2+ to provide a

wide range of magnetic properties [5, 6]. There are

many reports about this area in literature.

According to one of the recent research studies,

substitution of Co2+ in CoFe2O4 nanoparticle

structure with Zn2+ (ZnxCo1-xFe2O4) exhibited

improvement in properties such as excellent

chemical stability, high corrosion resistivity,

magneto crystalline anisotropy, magneto striation,

and magneto optical properties [7-13]. A very

interesting point in all of these reports and in many

of applications is that, the synthesis of uniform size

nanoparticles is of key importance, because the

nanoparticles magnetic properties depend strongly

on their dimensions. Therefore recently great

efforts have been made by various groups to

achieve a fine tuning of the size of ferrite and

substituted nanoparticles employing different

synthesis techniques. In other studies, magnetic

properties of cobalt ferrite and silica coated cobalt

ferrite were studied [14, 15], but the novelty of this

work was that we attempted to study magnetic

properties of manganese doped derivatives

(MnxCo1-x Fe2O4 with x= 0.02, 0.04 and 0.06)/

SiO2 for possible biomedical application. Silica and

its derivatives coated onto the surfaces of magnetic

nanoparticles may change their particles surface

properties and provide a chemically inert layer for

the nanoparticles, which is particularly useful in

biological systems [16-17].

2. EXPERIMENTAL

2.1. Materials

All chemicals were of analytical grade and were

used without further purification. Cobalt chloride

hexa hydrate (CoCl2.6H2O), ferric chloride hexa

hydrate (FeCl3.6H2O), sodium hydroxide (NaOH),

Ammonia solution (25%), cetyltrimethylammoni-

um bromide (CTAB) and tetraethyl orthosilicate

(TEOS), anhydrous ethanol (C2H5OH), were

purchased from MERCK company. Deionized

water was used throughout the experiments.

2.2. Synthesis of CoFe2O4

Cobalt ferrite nanoparticles, CoFe2O4 was

synthesized via a coprecipitation method by adding

a mixture of 2.5 mL of CoCl2.6H2O (0.5 M) and 5

mL of FeCl3.6H2O (0.5 M) into a solution mixture

of 1 g CTAB in 20 mL distilled water and 5 mL of

sodium hydroxide solution (3 M) and stirred under

nitrogen protection for 10 min. The resulting black

solution was then maintained at 70°C for 1 h and

cooled at room temperature. Stable colloidal

solution was then separated by centrifugation and

the black products obtained were washed by

distilled water for several times and dried at room

temperature [18].

2.3. Synthesis of MnxCo1-xFe2O4 nanoparticles

The above described experiments were repeated for

preparation of MnxCo1-xFe2O4/SiO2 nanoparticles

by adding a mixture of 2.5 mL of CoCl2.6H2O

(1-x) M, 5 mL of FeCl3.6H2O (0.5M) and 2.5 mL

of MnCl2.2H2O (xM) with (x= 0.02, 0.04, 0.06)

into a solution mixture of 1 g CTAB in 20 mL

distilled water and 5 mL of sodium hydroxide

solution (3 M) and stirred under nitrogen protection

for 10 min [19].

2.4. Synthesis of Core-Shell CoFe2O4/SiO2 and

MnxCo1-xFe2O4/SiO2 nanoparticles

Silica coated magnetic nanoparticles were prepared

using a modified Stober method by dispersing of

the prepared nanoparticles in 200 mL of ethanol and

adding then 2 mL of 25% ammonia, 20 mL of

deionized water and 2 mL of TEOS respectively.

The mixture was degassed and stirred vigorously at

50°C for 3 h under nitrogen gas protection to obtain

core-shell nanoparticles of CoFe2O4/SiO2 and

MnxCo1-xFe2O4/SiO2. The products obtained were

separated and washed with ethanol and water for

several times and dried at 40°C for 24 h [20].

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 271-280 Rostamzadehmansour S et al

272

2.5. Characterization

X-ray diffraction patterns (PW 1800 PHILIPS),

Energy Dispersion Spectrum (Hitachi F4160,

Oxford), and FT-IR spectra (A NICOLET 5700)

were used to determine the crystal structure of the

silica coated Fe3O4 nanoparticles and the chemical

bonds of Fe-O-Si, respectively. The magnetic

properties were analyzed with a Vibration Sample

Magnetometer (VSM, Quantum Design PPMS-9).

3. RESULT AND DISCUSSIONS

3.1. XRD characterization

Figure 1 left (a, b, c and d) show X-ray diffraction

patterns of CoFe2O4 and MnxCo1-xFe2O4 (with

x= 0.02, 0.04 and 0.06) nanoparticles. The peaks

observed in these patterns assigned to scattering

from the planes of (220), (311), (400), (422), (511)

and (440), all were consistent with those of standard

XRD pattern of spinel, CoFe2O4 (JCPDS card No.

86-2267) and confirm that all the prepared samples

maintain ferrite spinel structure. While, the cell

parameters decrease monotonously with the

increase of Mn content indicating that Mn ions are

substituted into the lattice of CoFe2O4. The

average of crystalline size of CoFe2O4, MnxCo1-x

Fe2O4 nanoparticle at the characteristic peak (311)

were calculated by using Scherrer formula. The

results of D values, using 311 planes of the spinel

structures were 13.36, 35.4 and 30.26 nm

respectively. Figure 1 right (a, b) shows X-ray

diffraction patterns of CoFe2O4 and CoFe2O4/SiO2

nanoparticles. All the peaks observed in these

patterns were consistent with those of standard

XRD pattern reported data (JCPDS card No.

86-2267) and confirm crystallinity of CoFe2O4 and

CoFe2O4/SiO2 nanoparticles. In addition to the

characteristic diffraction peaks of spinel phase, a

wide peak appeared at about 2θ= 22-25°

(Figure 2b) can be related to the formation of a

SiO2 phase due to the addition of TEOS in basic

condition.

3.2. FT-IR spectra

Figure 2 (a, b) represents FT-IR spectra of the

CoFe2O4 and CoFe2O4/SiO2 nanoparticles. The

strong peaks at about 592 cm-1 and 516 cm-1

(in Figure 2a) are due to the stretching vibrations of

Fe-O and Co-O bonds and the peaks around

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 271-280Rostamzadehmansour S et al

273

Figure 1 A: XRD patterns of: a) CoFe2O4, b, c, d) MnxCo1-xFe2O4 (with x= 0.02, 0.04 and 0.06); B: a) CoFe2O4; b)

CoFe2O4 /SiO2. Peak broadening observed in SiO2 coated nanostructures can be related to the decrease in crystallinity.

A B

(a)

(b)

(c)

(d)

(a)

(b)

3000-3500 cm-1 and 1624 cm-1 have been assigned

to the stretching and bending vibrations of the

H-O-H bond, respectively, showing the physical

absorption of H2O molecules on the surfaces. In

Figure 2b shows IR spectrum of silica coated

CoFe2O4 nanoparticles confirms the presence of

the finger print bands below 1100 cm-1 which are

characteristic of asymmetric (1085 cm-1) and

symmetric (810 cm-1) stretching vibrations of

framework Si-O-Si.

Figure 3 (a, b) represents FT-IR spectra of the

Mn0.02Co0.98Fe2O4 and Mn0.02Co0.98Fe2O4/SiO2


274

Figure 2: FT-IR spectrum of (a) CoFe2O4 and (b) CoFe2O4 /SiO2 nanoparticles

Figure 3: FT-IR spectrum of (a) Mn0.02Co0.98Fe2O4 and (b) Mn0.02Co0.98Fe2O4 /SiO2 nanoparticles

(a)

(b)

(a)

(b)

nanoparticles. The strong broad peaks at about

592 cm-1 and 516 cm-1 (in Figure 3a) are due to the

stretching vibrations of Fe-O and Co-O bonds and

the peaks around 3000-3500 cm-1 and 1624 cm-1

have been assigned to the stretching and bending

vibrations of the H-O-H bond, respectively,

showing the physical absorption of H2O molecules

on the surfaces. In Figure 3b shows IR spectrum of


275

Figure 4: FESEM images of: a) CoFe2O4; b) CoFe2O4 /SiO2 core-shell nanoparticles; c) Mn0.02Co0.98Fe2O4; and d)

Mn0.02Co0.98Fe2O4 /SiO2 core-shell nanostructures.

Sample Element wt% At%

CoFe2O4

Fe2O3

CoO

64.89

35.11

46.45

53.55

CoFe2O4/SiO2

Fe2O3

CoO

SiO2

46.58

28.61

5.62

30.68

17.86

7.36

Table 1: EDAX ZAF quantification (standardless) element normalized for CoFe2O4 and CoFe2O4 /SiO2 nanoparticles.

(a) (b)

(c) (d)

silica coated Mn0.02Co0.98Fe2O4/SiO2 nano-

particles confirms the presence of the finger print

bands below 1100 cm-1 which are characteristic of

asymmetric (1085 cm-1) and symmetric (810 cm-1)

stretching vibrations of framework Si-O-Si, that

confirms the formation of manganese ferrite.

3.3. FESEM and EDAX

Figure 4 (a, b) represents FESEM images of

CoFe2O4 and CoFe2O4/SiO2 core-shell nano-

particles. These images show well homogeneous

distribution of spheric nanoparticles in the prepared

samples. Elemental analysis data (EDAX) for these

two composite materials is given in Table 1.

Figure 4 (c, d) represents the FESEM images of

Mn0.02Co0.98Fe2O4 and (b) Mn0.02Co0.98Fe2O4/SiO2

nanostructures. These images clearly show also

well distributed particles of Mn doped nano-

structures in Mn0.02Co0.98Fe2O4, and in its

silica coated nanoparticles, Mn0.02Co0.98Fe2O4

/SiO2. X-ray dispersive analysis data for these two

nanostructure is represented in Table 2. This results

confirm presence of Co, Fe, Mn and silica in the

related nanoparticles.

3.4. Magnetic properties of CoFe2O4 nano-

particles

Figure 5 (a, b and c) and Table 3 represent

magnetic field dependent magnetization parame-

ters, M(H) for CoFe2O4 in the size of 13.26, 21.06


276

Table 2: EDAX ZAF quantification (standardless) element normalized for Mn0.02Co0.98Fe2O4 and Mn0.02Co0.98Fe2O4

nanoparticles.

Sample Element wt% At%

Mn0.02Co0.98Fe2O4

Fe

Co

O

Mn

46.83

28.49

13.44

9.12

35.32

20.30

35.28

6.98

Total 100 100

Mn0.02Co0.98Fe2O4/SiO2

Fe

Co

Mn

O

Si

48.88

28.41

8.31

4.07

8.42

41.84

23.o4

7.23

12.17

14.32

Total 100 100

CoFe2O4

nanoparticles

Average

particle size

XRD (nm)

Saturation

magnetization

Ms(emu/g)

Remanent

magnetization

Mr(emu/g)

Coercivity

Hc (Oe)

Remanence

ratio (Mr/Ms)

Ms emu/g

(Bulk)

Mr/Ms

(Bulk)

CoFe2O4 13.26 32.1 0 240 0 80.8 0.84

CoFe2O4 21.06 12.74 5 2000 0.47 80.8 0.84

CoFe2O4 34 10.58 4.9 2300 0.46 80.8 0.84

CoFe2O0.84 /SiO2 15.14 8.5 0 50 0 80.8 0.84

Table 3: Magnetic parameters of CoFe2O4 in different sizes and CoFe2O4 /SiO2 nanoparticles.

and 34 nm at room temperature, using vibrating

sample magnetometer with a peak field of 15 kOe.

The hystersis loops for CoFe2O4 in the size of

13.26 nm, with a finite low value of coercivity

(Hc= 240 Oe) and remanence (Mr= 0) indicate a

ferromagnetism at 300 K. Shi-Yong Zhao et al.,

have reported the same results for their prepared

CoFe2O4 nanoparticles in the size of 14.8 nm and

proposed a superparamagnetism properties at the

same condition [21]. These results seems

contraversal since superparamagnetic particles

shohld exhibit no remanence or coercivity, or there

are no hysteresis in the magnetization curve. A very

important parameter that has to be considered for

this kind of materials is coercivity. Coercivity is the

key to distinguish between hard and soft phase

magnetic materials. Materials with a typically low

intrinsic coercivity less than 100 Oe, with a high

saturation ted magnetization Ms and low Mr are

magnetically called soft and materials that have an

intrinsic coercivity of greater than 1000 Oe,

typically high remanance, Mr are hard magnetic

materials.

So, CoFe2O4 in the size of 13.26 nm with a low

finite value of coercivity and remanance (Mr);

(Hc= 240 Oe; Mr= 0.0 emu/g can be considered as

a good example for weak ferromagnetism.

Whereas, CoFe2O4 nanoparticles, in the sizes of

21.06 and 34 nm with a high coercivity (Hc= 2000-

2500 (Oe) and remanance (Mr) (Hc= 5000 Oe;

Mr= 5.0 emu/g, are magnetically hard magnetic

materials. A new interesting change on the hystersis

loop is observed for silicad coated. CoFe2O4/SiO2

(Figure 3d) with a saturation magnetization

decreased to 8.5 emu/g and a finite zero value for

coercivity (Hc) and remanence (Mr); (Hc= 50 Oe;

Mr= 0.0 emu/g) indicating a soft ferromagnetism at

RT. Decrease of magnetic saturation in this case can

be related to the separation of neighbors

nanoparticle by a layer silica leading to the decrease

of magnetostatic coupling between the particles.

A closer look at the above mentioned results

confirm that magnetic properties of small ferromag-

netic particles such as coercivity, as reported by

other autors [22, 23], are dominated by two key

features: (1) a size limit that below which the

specimen cannot be broken into domains, hence it

remains with single domain; (2) the thermal energy

in the small particles which give rise to the

phenomenon of superparamagnetism. These two

key features are represented by two key sizes (on

length scale): the single domain size and superpara-

magnetic size (Figure 5) [22, 23].

3.5. Magnetic properties of (Mn0.02Co0.98Fe2O4)

nanoparticles

Figure 6 and Table 4 Compare field dependent

magnetization parameters for CoFe2O4,

Mn0.02Co0.98 Fe2O4 and Mn0.02Co0.98Fe2O4/SiO2

nanoparticles in a similar sizes. The hystersis loop

for Mn0.02Co0.98Fe2O4 show a saturation magneti-

zation, Ms of 35.0 emu/g for a size of 35.30 nm and


277

Figure 5: Magnetization curves versus applied field for synthsized CoFe2O4 in a size of: a) 13.26 nm; b) 21.06 nm; c)

34 nm; d) 15.4 nm for silica caoted CoFe2O4 at 300 K in a magnetic field of 15 kOe.

a finite low value of coercivity (Hc) with increased

remanence (Mr); (Hc= 500 Oe; Mr= 12.0 emu/g).

The large value of HC for CoFe2O4 is known to be

originated from the anisotropy of the octahedral

Co2+ ion [24]. So, decrease of HC for Mn doped

material can be attributed to a decrease in the

octahedral Co2+ ions due to their migration to

tetrahedral sites. The increase in MS in doped

material can also be explained in terms of the Co2+

migration [25]. Decrease of the saturation magneti-

zation, remanence (Mr) and coercivity to 250 Oe

(Hc= 250 Oe; Mr= 0.75 emu/g), for Mn0.02Co0.98

Fe2O4/SiO2 due to the surface coating effects has

been explained via different mechanisms, such as

existence of a magnetically dead layer on the

particles surface, the existence of canted spins, or

the existence of a spin-glass-like behavior of the

surface spins [26]. However, a silica coating can be

used to tune the magnetic properties of nanoparti-

cles, since the extent of dipolar coupling is related

to the distance between particles and this in turn

depends on the thickness of the inert silica shell

[27]. A thin silica layer will separate the particles,

thereby preventing a cooperative switching which

is desirable in magnetic storage data. The MS of the

MnxCo1-xFe2O4 films is seen to increase with

increasing x. Also, the coercive field (HC) is found

to decrease with increasing Mn doping as shown in

Table 6, while the MS increases gradually with x.

The large HC of CoFe2O4 is known to originate

from the anisotropy of the octahedral Co2+ ion.

Thus, the decrease in HC with increasing x is

attributed to a decrease in the octahedral Co2+ ions

due to their migration to tetrahedral sites. The

increase in MS can also be explained in terms of the

Co2+ migration.


278

CoFe2O4 nanoparticlesAverage

particle size XRD

(nm)

Saturation

magnetization

Ms(emu/g

Remanent

magnetization

Mr(emu/g)

Coercivity

Hc (Oe)

Remanence

ratio (Mr/Ms)

CoFe2O4 34 10.85 4.9 2500 0.46

Mn0.02Co0.98Fe2O4 35.30 35 12 500 0.37

Mn0.2Co0.98Fe2O4/SiO2 37 3.5 0.75 250 0.21

Table 4: Magnetic parameters of Mn0.2Co0.98Fe2O4 and Mn0.2Co0.98Fe2O4 /SiO2 nanoparticles.

Figure 6: Magnetization curves versus applied field for synthsized: a) CoFe2O4; b) Mn0.02Co0.98Fe2O4 and

c)Mn0.02Co0.98Fe2O4 /SiO2 in the similar size at 300 K in a magnetic field of 15 kOe.

4. CONCLUSIONS

In this study, the process of preparing hard and

soft magnetic nanoparticles having potential for

many technological applications such as ultra

high density recording media, biotechnology

ferrofluids, and fabrication of exchange coupled

nanocomposite has been explained.

The synthesis processes explored in this study

are simple and easy to achieve the desired

particle size distribution.

Characterization of the prepared cobalt ferrite

and Manganese doped cobalt ferrite nano-

particles were performed using XRD, FTIR and

FESM-EDAX technigues.

The magnetic properties of the cobalt ferrite and

Manganese doped cobalt ferrite nanoparticles

evaluated by VSM and the decreases of satura-

tion magnetization with increasing SiO2

coatings were reported earlier separately by

authors [24].

Magnetic properties of small ferromagnetic

particles such as coercivity, are dominated by

two key features: the single domain size and

superparamagnetic size.

The magnetic measurements on these particles

showed strong dependence of the magnetic

properties with the particle size.

A size limit exist for CoFe2O4 nanoparticles

and cobalt ferrite with size greater than 12 nm

showed ferromagnetic behavior at room

temperature.

High coercivity values higher than 1 kOe and

2-2.5 kOe were obtained for 21.06 and 34 nm

CoFe2O4 nanoparticles at room temperature.

Room temperature magnetic properties strongly

depend on (1) doping content (x) (2) particles

size and ions distributions.

The saturation magnetization is strongly

dependent on the Mn doping content, x and

increased average magnetic moment improved

for Mn1-xCoxFe2O4 (x= 0.02), have been

decreased by Mn content due to the antiferro

magnetic super exchange interaction with in the

neighbor Mn2+ ions through O2- ions for the

samples with higher Mn doping.

ACKNOWLEDGMENTS

The authors express their thanks to the vice

presidency of Islamic Azad University, Science and

Research Branch and Iran Nanotechnology

Initiative for their encouragement, and financial

supports.

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280


Synthesis and Characterization of ZnCaO2 Nanocomposite

Catalyst and the Evaluation of its Adsorption/Destruction

Reactions with 2-CEES and DMMP

Meysam Sadeghi1*, Mirhasan Hosseini2, Hadi Tafi3

1,2 M.Sc., Department of Chemistry, Faculty of Sciences, I.H.U, Tehran, Iran

3 M.Sc. Student, Department of Chemistry Engineering, Faculty of Sciences, Arak Azad University, Arak,

Iran

Received: 28 November 2012; Accepted: 30 Jannuary 2013

In this work, ZnCaO2 (zinc oxide-calcium oxide) nanocomposite were synthesized at different

temperatures (500-700°C) by sol-gel method based on polymeric network of polyvinyl alcohol

(PVA). The synthesized samples were characterized by SEM/EDAX, FT-IR and XRD techniques.

It was found that synthesized nanocomposites have 1.62, 2.05 and 13.91%wt of CaO,

respectively. The obtained results show that each particle of nanocomposite has been made of a

CaO core which is completely covered by ZnO layers. The smaller average diameter of synthe-

sized nanoparticles (at 600°C) calculated by XRD technique found to be 33 nm for prepared

ZnCaO2 nanocomposite. This compound has been used as adsorbing removal for agricultural

pesticide. The 2-chloroethyl ethyl sulfide (2-CEES) and dimethyl methyl phosphonate (DMMP)

are for the class of compounds containing phosphonate esters and sulfurous with the highly toxic

that used such as pesticides, respectively. The adsorption/destruction reactions of 2-CEES and

DMMP have been investigated by using ZnCaO2 nanocomposite. Reactions were monitored by

GC-FID (gas chromatography) and FT-IR techniques and the reaction products were character-

ized by GC-MS. The results of GC analysis for the weight ratio of 1:40 (2-CEES/DMMP: ZnCaO2

nanocomposite) at room temperature showed that 2-CEES molecule is destructed about

perfectly in the n-pentane solvent by nanocomposite after 12 hours and it changed to less toxic

chemical hydrolysis and elimination products and identified via GC-MS (gas chromatography-

mass spectrometry) instrument, were hydroxyl ethyl ethyl sulfide (HEES) and ethyl vinyl sulfide

(EVS), respectively. On the other hand, the 31PNMR analysis emphasized that 100% of DMMP

molecule after 14 hours in the n-pentane solvent was adsorbed.

Keyword: ZnCaO2 Nanocomposite; Sol-Gel; 2-CEES and DMMP; Adsorption/Destruction;31PNMR.

ABSTRACT



One of the first successful applications of

nanotechnology was the use of oxides as catalysts

for adsorption of toxic sulfurous and organo-

phosphate. Recently, wurtzitic group-II oxides such

as ZnO have attracted attention due to their

potential applications in adsorption and destruction

of toxic pollutants [1]. Heterostructures or alloys of

ZnO with CaO [2] are important for band-gap

tailoring, since it is possible to open up the energy

band-gap from 3.4 eV [wurtzite (wz) ZnO] to

almost more of 4 eV in CaxZn1-xO alloys [3, 4]. A

suitable process for control of nanoparticles is using

of sol-gel pyrolysis method [5]. In this research, the

synthesis and characterization of ZnCaO2 nano-

composites via sol-gel pyrolysis method at

temperatures 500, 600 and 700°C is reported. Then,

we have focused our attention on the nano-

composite due to good catalytic properties and high

performance for the adsorption/destruction of the

2-CEES and DMMP molecules (Figure 1a and b).

The 2-CEES and DMMP molecules are used for the

class of compounds such as agricultural pesticides

which containing and sulfurous phosphonate esters,

respectively [6-13].

Figure 1: Molecular structures of (a) 2-CEES and (b)

DMMP.

2. EXPERIMENTAL

2.1. Materials

Zn(NO3)2.6H2O, Ca(NO3)2.4H2O, ethanol, poly

vinyl alcohol (PVA) and phosphoric acid 85%

(d= 1.5 g/mL), are purchased from Merck Co.

(Germany). N-pentane, toluene, CDCl3, 2-CEES

(2-chloroethyl ethyl sulfide) and DMMP (dimethyl

methyl phosphonate) form Sigma-Aldrich Co.

(USA) were used as received.

2.2. Physical characterization

The morphology of the products was evaluated by

using Emission Scanning Electron Microscope and

Energy Dispersive X-ray Spectroscopy (SEM/

EDAX, LEO-1530VP). The IR spectrum was

scanned using a Perkin-Elmer FT-IR (Model 2000)

in the wavelength range of 400 to 4000 cm-1 with

KBr pellets method. X-ray diffraction (XRD)

analysis was carried out on a Philips X-ray

diffractometer using CuKα radiation (40 kV, 40 mA

and λ= 0.15418 nm). Sample were scanned at

2°/min in the range of 2θ = 10-90°.

2.3. Synthesis of ZnCaO2 nanocomposite catalyst

by sol-gel pyrolysis method

ZnCaO2 nanocomposite was synthesized according

to the following procedure: First, 100 ml

ethanol/water solution with ratio of 50:50 for

solvent was added to a 200 mL Erlenmeyer flask.

Then, 87 g solvent that was prepared at previous

stage was transferred to a cruicible. In the next step,

2 g Zn(NO3)2.6H2O and 2 g Ca(NO3)2.4H2O were

added to solution and stirred until the solution

became clear. 9 g poly vinyl alcohol (PVA) was

added to clear solution until 100 g sample to be

produced. The sample was stirred vigorously for

1 h and the temperature slowly increased until at

80°C to form a homogeneous sol solution. The final

gel was cooled and then calcined at 500, 600 and

700°C for 16 h.

2.4. Preparation of ZnCaO2 nanocomposite/

2-CEES sample

For each sample, 10 µL of 2-CEES, 5 mL n-pentane

solvent and 10 µL toluene (internal standard) and

150 mg ZnCaO2 nanocomposite were added to the

50 mL Erlenmeyer flask. To do a complete reaction

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 281-293 Sadeghi M et al

282

1. INTRODUCTION

S

Cl

P

O

OCH3

CH3

OCH3

(a)

(b)

between catalyst and sulfurous compound, all

samples were attached to a shaker and were shaked

for about 0, 2, 4, 6, 8, 10 and 12 h. Then, by

micropipette extracted 10 µL of ZnCaO2 nano-

composite/2-CEES sample solutions and injected to

GC and GC-MS (Varian Star 3400 CX, OV-101

CW HP 80/100 2m×1.8 in and DB 5 MS, 101 mic,

30 m×0.25 mm) instruments. Temperature program

for GC: The initial and final temperature of the

oven was programmed to 60°C (held for 4 min) and

220°C, to reach the final temperature(after for 4

min); the temperature was increased at rate of

20°C/ min for 13 min. Also, detector temperature

was 230°C (Figure 2).

Figure 2: The temperature program for GC set.

2.5. Preparation of ZnCaO2 nanocomposite/

DMMP sample

For investigation of the reaction ZnCaO2 nano-

composite and DMMP, ZnCaO2/DMMP sample

were prepared according to the following method:

For the preparation of the phosphoric acid solution

blank (0.03 M), first, 0.05 mL phosphoric acid 85%

(d= 1.5 g/mL) was diluted with 25 mL deionized

water and injected to a capillary column and closed

two tips by heat. Then, 37 µL DMMP, 10 mL

n-pentane as solvent and 0.48 g ZnCaO2 nano-

composite were added to the 50 mL Erlenmeyer

flask and the mixture was stirred for 0, 1, 2, 3, 4, 5,

6 and 14 h at ambient temperature. In the next step,

1 mL solution was placed in centrifuge instrument

(CAT.NO.1004, Universal) by 500 rpm for 5 min

for doing the extraction operation. Now, 0.3 mL of

the ZnCaO2 nanocomposite/DMMP sample

solution and 0.1 ml CDCl3 were added to a NMR

tube and capillary column was added to the tube for

the blank. After that, the presence of the DMMP in

the sample was investigated by the 31PNMR) 250

MHz Bruker) instrument.

3. RESULT AND DISCUSSION

3.1. SEM/EDAX analysis

The SEM images with different magnification and

EDAX analysis of the ZnCaO2 nanocomposites at

500, 600 and 700°C are shown in Figures 3 and 4.

This micrographs show that with increasing of

calcination temperature, the particles size and the

morphology of nanoparticles are changed. The

results of EDAX analysis were emphasized that,

percent of CaO (wt%) in the synthesized nano-

composites has increased (Table 1). On the other

hand, the smaller of the particle size is

corresponded to the synthesized nanocomposite at

600°C and forming as spherical.

3.2. FT-IR studies

FT-IR spectra of ZnCaO2 nanocomposites at

different temperature (500, 600 and 700°C) are

shown in Figure 5. The peaks at 1630 and 1710

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 281-293Sadeghi M et al

283

Table 1: The results of EDAX analysis for the synthesized ZnCaO2 nanocomposites.

Temperature(°C) ZnO(wt%) CaO(wt%) Average particle size

500 98.38 1.62 70 - 100

600 97.95 2.05 30 - 40

700 86.09 13.91 200 nm (diameter)

cm-1 are assigned to CO2 absorbed on the surface of

nanoparticles. The peaks at 1350 and 847 cm-1 are

assigned to C-H and C-C bonding vibrations of

organic impures in the synthesized sample,

respectively. The peak around 3450 cm-1 is

corresponded to (OH) stretching vibration. The

strong absorbed peak around 450 cm-1 is

corresponded to ZnO and CaO bonds.


284

Figure 3: SEM images of ZnCaO2 nanocomposites, (a) and (b) 500°C, (c) and (d) 600°C, (e) and (f) 700°C with different

magnification (15000X and 30000X).

(a) (b)

(c) (d)

(e) (f)

3.3. X-ray diffraction (XRD) study

The structure of prepared ZnCaO2 nanocomposite

at 500-700°C was investigated via X-ray diffraction

(XRD) measurement (Figure 6). The average

particle size of nanocomposite was investigated

from line broadening of the peak at 2θ= 10-90° via

using Debye-Scherrer formula (1):

d= 0.94λ/βcosθ (1)


285

Figure 4: EDAX analysis of ZnCaO2 nanocomposites, (a) 500, (b) 600 and (c) 700°C.

Figure 5: FTIR spectra of ZnCaO2 nanocomposite, (a) 500, (b) 600 and (c) 700°C.

(a) (b)

(c)

(a)

(b)

(c)

Where d is the crystal size, λ is wavelength of

X-ray source, β is the full width at half maximum

(FWHM), and θ is the Bragg diffraction angle. The

smaller average particles size by Debye-Scherrer

formula was estimated to be 33 nm for 600°C.

2θ= 33.001°, 38.285°, 55.258°, 65.910°, 69.288

(black points) corresponded CaO nanoparticles

(FCC phase) and 2θ= 31.72°, 34.4°, 36.24°, 47.52°,

56.6°, 62.8°, 66.3°, 67.9°, 69.1° corresponded ZnO

nanoparticles. All diffraction peaks are indicating to

the hexagonal phase with wurtzite structure for

ZnO. After the characterization, obtained ZnCaO2

nanocomposite (600°C) sample was used to study

the interaction with 2-chloroethyl ethyl sulfide

(2-CEES) and dimethyl methyl phosphonate

(DMMP) at room temperature (25±1°C).

3.4. GC, FT-IR and GC-MS studies

The evaluation of the reaction ZnCaO2 nano-

composite (600°C) with 2-CEES at ambient

temperature (25±1°C) via GC analysis shows that a

high potential exists for degradation of

2-chloroethyl ethyl sulfide. Generally, with

increasing the time, higher values sulfurous

molecules have destructed. Thus, after 12 hours,

100% of 2-CEES molecules in contact with the

ZnCaO2 nanocomposite catalyst (in the n-pentane

solvent) were destructed. The GC chromatograms

and data's curve for the different times are shown in

Figures 7, 8 and Table 2. After the reaction, the

structure of nanocomposite was investigated via

FTIR spectrum (Figure 9). The any new peaks in

corresponded to adsorbed 2-CEES. Hence, 2-CEES

molecules were destructed about perfectly.

Thereafter, the reaction mixtures were analyzed by

GC-MS (gas chromatography coupled with mass

spectrometry) for the characterization of reaction

products. Data illustrates the formation of two

products by detector. One of the spectra has m/z

values at 88, 71, 61, 47 and 27 thus indicating the

formation of EVS and another one has the m/z

values at 106, 89, 75, 61, 48 and 28, and indicates

the formation of HEES thus emphasizing the role of

elimination and hydrolysis reaction in the removal

of 2-CEES (m/z values at 123, 109, 91, 75, 61, 47

and 28) thereby rendering it non-toxic (Figure 10).

Figure 6: XRD patterns of synthesized ZnCaO2 nano-

composites at a) 500, b) 600 and c) 700°C.

3.5. 31PNMR and FT-IR studies31PNMR spectra and data's curve for interaction

between ZnCaO2 nanocomposite (600°C) and

DMMP (dimethyl methyl phosphonate) in the

presence of different times are shown in Figure 11,

Tables 3 and 12, respectively. The chemical shift for

DMMP H3PO4 were δ= 33 and 0 ppm, respective-

ly. The intensity of the DMMP area under curve


286

(a)

(b)

(c)


287

Figure 7: GC chromatograms for 2-CEES on ZnCaO2 nanocomposite.

Table 2: The results of GC analysis in the presence of different times and pentane solvent.

Figure 8: The curve of destructed 2-CEES% versus time.

Sample Time(h) Adsorbed and Destructed % by ZnCaO2 nanocomposite

a Blank(0) 100.00

b 2 82.25

c 4 64.43

d 6 56.67

e 8 25.15

f 10 8.60

g 12 00.00

(a)

(b)

(c)

(d)

(e)

(f)

(g)


288

Figure 9: FT-IR spectra of 2-CEES/ZnCaO2 nanocomposite (600°C), a) before and b) after the reaction.

Figure 10: GC-MS analysis results for reaction of 2-CEES/ZnCaO2 nanocomposite and it's the destruction products.

(a)

(b)


289

Figure 11: 31PNMR spectra of the adsorption DMMP on the ZnCaO2 nanocomposite at differernt times.

(b)(a)

(e) (f)

(c) (d)

(g) (h)

(AUC) in comparison to AUC phosphoric acid

blank and also concentration (DMMP) after

reaction with increasing the time was decreasedbut,

any new peak appears for the destruction product.

Hence, we can say that after 14 h, 100% organo-

phosphosphate molecule was adsorbed. After the

reaction, the adsorption of DMMP on the nano-

composite was investigated via FT-IR spectrum

(Figure 13). The new peaks in 1186.05, 1066.04 and

3637.68 cm-1 are corresponded to adsorb DMMP.

Hence, the structure of catalyst after the interaction

was remained. After investigation of reactions

2-CEES and DMMP with ZnCaO2 nanocomposite

catalyst, that's proposed mechanisms in the

presence of nanocomposite which are shown in

Schemes 1 and 2. For the interaction between

sulfurous compound and nanocomposite two

sections were investigated. Section I) Adsorption

reaction with nucleophillic attack the H atoms of

composite to the chlorine and sulfur atoms of


290

Figure 12: The curve of adsorbed DMMP% versus time.

Table 3: The results of 31PNMR spectra in the presence of different times.

Concentration DMMP AUC / % Adsorption(DMMP)

Sample Time(h) (DMMP) after phosphoric acid blank by ZnCaO2

reaction(Molar) AUC nanocomposite

a 0(blank) 0.03 199.14 0

b 1 0.0246 163.34 17.98

c 2 0.0229 151.93 23.71

d 3 0.0188 124.73 37.37

e 4 0.0161 106.58 46.48

f 5 0.0121 80.25 59.70

g 6 0.0114 63.14 67.34

h 14 0 0 100

2-CEES molecule. In this interaction, the chlorine

atom in 2-chloroethyl ethyl sulfide will be removed

(the dehalogenation reaction). Section II) in the

present and absence of H2O molecule, the

hydrolysis and elimination products were revealed,

respectively. Also, in the interaction DMMP

/ZnCaO2 nanocomposite, two mechanisms were

shown. In mechanism A: the bonding between

oxygen atom of organo phosphorous compound and

active sites of nanocomposite are produced. Then,


291

SCl

S+

Cl-

OZn

OCa

OZn

O

HS

Cl

OZn

OCa

OZn

OS

-HCl

hydrolysisproduct

elimin

ationp

rodu

ct

SOH

S

2-CEES

HEES

EVS

-H2O

H2O

Scheme 1: Proposed mechanism for the adsorption/destruction of 2-CEES on ZnCaO2 nanocomposite catalyst.

Figure 13: FT-IR spectrum of the adsorption DMMP on the ZnCaO2 nanocomposite.

by elimination of CH3OH, DMMP on the catalyst

was adsorbed. In mechanism B: the bonding

between oxygen atom of organo phosphorous

compound and Zn or Ca atoms of nanocomposite

are produced that by elimination of CH3OH and its

adsorption on the catalyst any product was seen.

4. CONCLUSIONS

In summary, sulfurous and organo phosphorous

compounds such as 2-CEES and DMMP are the

ideal conditions for the adsorption/destruction of

SCH2CH2Cl and P=O groups containing

pollutants. The sol-gel pyrolysis method has been

successfully used for synthesis of ZnCaO2

nanocomposites at different temperatures (500-

700°C) with to the hexagonal phase with wurtzite

structure of zinc oxide and calcium oxide with FCC

phase. This method is simple, environmentally

friendly and low cost for production nanocomposite

catalyst. The structure and the morphology of

nanoparticles were investigated by XRD, SEM

/EDAX and FT-IR techniques. The EDAX analysis

for the synthesized nanocomposite (600°C) showed

that CaO wt% and the average particles size was

2.05 wt% and 33 nm, respectively. In the other,

synthesized nanocomposite (500 and 700°C),

average particles size is higher. Thus, in this

research, the best of temperature for the synthesized

nanocomposite ZnCaO2 is 600°C. The results

obtained in this study demonstrate that ZnCaO2

nanocomposite has a high catalyst potential for

adsorption/destruction of 2-CEES and DMMP

molecules that wereinvestigated via GC, GC-MS

and 31PNMR analyses, respectively. The 2-CEES

and DMMP are absorbed about perfectly after 12

and 14 hours respectively and the destruction

products of 2-CEES with nanocomposite; i.e.

hydroxyl ethyl ethyl sulfide (HEES) and ethyl vinyl

sulfide (EVS) were identified.

ACKNOELDGMENTS

The authors acknowledge the department of

chemistry, Imam Hossein University for his

constructive advice in this research.

REFERENCES

1. Joseph M., Tabata H., and Kawai T., Jpn. J.

Appl. Phys., 38(1999), 1205.


292

OZn

OCa

OZn

OZn

OCa

OZn

OO

H

O

P

OCH3

CH3

CH3O

O

P

OH3CCH3

-H2O

-CH3OH

OZn

OCa

OZn

O

P

OCH3

CH3

CH3O

OZn

OCa

OZn

O

CH3

O O

P

OH3CCH3

Mechanism A

Mechanism B

Scheme 2: Proposed mechanisms for the adsorption of DMMP on ZnCaO2 nanocomposite catalyst.

2. Ozgr U., Alivov Y.I., Liu C., Teke A.,

Reshchikov M.A., Doan S., Avrutin V., Cho S.J.,

and Mork H., J. Appl. Phys., 98(2004), 1301.

3. Ohtomo A., and Tsukazaki A., Semicond. Sci.

Technol., 20(2005), S1.

4. Schmidt R., Rheinlnder B., Schubert M.,

Spemann D., Butz T., Lenzner J., Kaidashev

E.M., Lorenz M., Rahm A., Semmelhack H.C.,

and Grundmann M., Appl. Phys. Lett., 82(2003),

2260.

5. Wang F., Liu B., Zhang Z., Yuan S., Physica E,

41(2009), 879.

6. Bhattacharyya P., Basu P.K., Saha H., Basu S.,

Sens. Act. B., 124(2007), 62.

7. Chao L.C., Huang J.W., Chang C.W., Physica

B., 404(2009), 1301.

8. Osterlund L., Stengl V., Mattsson A.,

Bakardjieva S., Andersson P.O., Oplustil F.,

Appl. Catal. B: Environ, 88(2009), 194.

9. Koper O., Klabunde K.J., Martin L.S.,

Knappenberger K.B., Hladky L.L., Decker S.P.,

"Reactive nanoparticles as destructive adsor-

bents for biological and chemical contamina-

tion", US Pat. No. 20 (2002).

10. Prasad G.K., J. Sci. Indust. Research, 68(2009),

379.

11. Saxena A., Srivastava A. K., Sharma A., Singh

B., J. Hazard. Mater., 03(2009), 112.

12. Prasad G.K., Mahato T.H., Pandey P., Singh B.,

Suryanarayana M.V.S., Saxena A., Shekhar K.,

Micropor. Mesopor. Mater, 106(2007), 256.

13. Klabunde K.J., "Nanometer sized metal oxide

particles for ambient temperature adsorption of

toxic chemicals", US Pat. No.05 (1999).

293


Semiconductor nanocrystals have attracted great

application during the past two decades. Compared

with the corresponding bulk materials, new devices

from semiconductor nanocrystals may possess

novel optical and electronic properties, which are

potentially useful for technological applications,

[1-3]. Extremely high surface area to volume ratio

can be obtained with the decrease of particle size,

which leads to an increase in surface specific active

sites for chemical reactions and photon absorptions.

The enhanced surface area also affects chemical

reaction dynamics. The size quantization increases

the energy band gap between the conduction band

electrons and valence band holes which leads to

change in their optical properties [3]. Zinc oxide, a

typical II-VI compounding semiconductor, with a


The Study of Pure and Mn Doped ZnO Nanocrystals for

Gas-sensing Applications

Meysam Mazhdi1*, Jabar Saydi1, 2, Faezeh Mazhdi3

1 M.Sc., Department of Physics, Faculty of Sciences, I.H.U, Tehran, Iran

2 Ph.D. Student, Department of Physics, Faculty of Sciences, Young Research Club, Islamic Azad

University of East Branch, Tehran, Iran

3 B.Sc., Department of Electrical and Robotics Engineering, University of Shahrood Technology,

Shahrood, Iran

Received: 10 December 2012; Accepted: 16 February 2013

ZnO and ZnO: Mn nanocrystals were synthesized via reverse micelle method. The structural

properties of nanocrystals were investigated by XRD. The XRD results indicated that the

synthesized nanocrystals had a pure wurtzite (hexagonal phase) structure. Resistive gas sensors

were fabricated by providing ohmic contacts on the tablet obtained from compressed nano-

crystals powder and the installation of a custom made micro heater beneath the substrate.

Sensitivity (S= Ra/Rg) of ZnO and ZnO: Mn nanocrystals were investigated as a function of

temperature and concentration of ethanol and gasoline vapor. The obtained data indicated that

optimum working temperatures of the ZnO and ZnO: Mn nanocrystals sensors are about 360°Cand 347°C for ethanol vapor and about 287°C and 335°C for gasoline vapor. Based on gas

sensing results, although Mn impurity reduces the Sensitivity but the sensor got saturated at

much higher gas concentration.

Keyword: ZnO Nanocrystals; Gas sensor; Response time; Recovery time; Micelle method.

ABSTRACT

1. INTRODUCTION



direct band gap of 3.2 eV at room temperature and

60 meV as excitonic binding energy, is a very good

luminescent material used in displays, ultraviolet

and visible lasers, solar cells components, gas

sensors and varistors [1, 2]. Recently, a number of

techniques such as reverse micelle, hydrothermal,

sol-gel, and wet chemical have been employed in

the synthesis of zinc oxide nanocrystals [1-4].

However, the reverse micelle technique is one of

the more widely recognized methods due to its

advantages, for instance, soft chemistry, demanding

no extreme pressure or temperature control, easy to

handle, and requiring no special or expensive

equipment [4].

In current material science research, the use of

nano sized materials for gas sensors is rapidly

arousing interest in the scientific community. One

reason is that the surface to bulk ratio for the

nano sized materials is much greater than those for

coarse materials. As another reason, the conduction

type of the material is determined by the grain size

of the material. When the grain size is small enough

(the actual grain size D is less than twice the

space-charge depth L), the material resistivity is

determined by grain control, and the material

conduction type becomes surface conduction type

[5]. Hence, the grainsize reduction is one of the

main factors in enhancing the gas sensing

properties of semiconducting oxides.

In this scientific work, ZnO and ZnO: Mn

nanocrystals were synthesized through the reverse

micelle method. The structural characteristics of

these nanocrystals were analyzed. The sensitivity of

the fabricated nanocrystals to gasoline and ethanol

vapor contamination was measured. The results

indicated a profound increase in the gas sensitivity

due to nanocrystalline nature of the sensors.

2. EXPERIMENTAL

2.1. Materials

ZnO and ZnO: Mn nanocrystals were fabricated

through the mixture of two equal microemulsion

systems. In micro emulsion I, butanol, PVP and

aqueous solution of zinc acetate (0.1 molar ratios)

with the molar ratio of 1:1:0.4 was used as oil,

surfactant and aqueous phase. Microemulsion II

had similar ingredients but instead of aqueous

solution, a solution of potassium hydroxide and

water was used as aqueous media. Both micro

emulsion solutions I and II were mixed vigorously

with a magnetic stirrer. After centrifugation, ZnO

and Mn doped ZnO precipitates were collected and

were dried at 250°C for 3 hours [3, 4]. The

nanocrystals were compressed as tablets and were

annealed at 500°C for 2 hours by using a

temperature controlled heating element. The tablets

were used as gas sensor. The system employed is

schematically shown in Figure 1.

Figure 1: Schematic illustrations of the fabricated gas

sensor (a) and the sensor probe (b).

2.2. Sensor fabrication and measurements

The produced tablet annealing temperature was

kept at 500°C for 2 hours by using a temperature

controlled heating element. Therefore we use the

tablet as a gas sensor. Ohmic connections are used

in order to create relation between sensor and

electric circuit that has been shown in Figure 2.

These connections contact through platinum wire

and paste. The used paste kind is similar to tablet.

The sample was then attached to a temperature-

controlled micro-heater, and was mounted on a

refractory stand, so that the temperature of the

sample could be adjusted in the 160-430°C

temperature range. The structure of the device is

schematically presented in Figure 1 (a). A sensor

probe was formed by mounting the sample on an

insulated layer through which two insulated

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 295-302 Mazhdi M et al

296

(a) (b)

connection cables were guided to the temperature

control unit and the impedance measurement device

respectively. For each sensitivity measurement, the

sensor probe was set at the desired operating

temperature and a 10 min time was allowed for the

probe temperature to stabilize. Then, a constant AC

voltage (4 v, 80 Hz) was applied to the sensor, while

the current passing through the device was

recorded. DC fields could cause ionic migration and

electrode instability which were of much lesser

concern in the case of AC voltages applied. The

sensitivity measurement was then achieved by the

insertion of the probe into a 1.5 Lit glass tank

containing air with a predetermined contamination

(in this work, gasoline and ethanol) level. To avoid

errors caused by condensation of the contaminating

gas on the walls of the tank, it was externally

heated.

Figure 2: Gas sensor Electric circuit.


Obtained nanocrystals were analyzed by X-ray

diffractometer (Scifert, 3003 TT) with Cu-kαradiation, Atomic absorption spectrometer

(PERKIN ELMER, 1100 B) and sensitivity of these

nanocrystals investigated by resistive gas sensors

for ethanol and gasoline vapor.

3. RESULTS AND DISCUSSION

3.1. XRD analysis

Figure 3 shows the XRD patterns of ZnO and

ZnO: Mn nanocrystals. The spectrums show three

broad peaks for ZnO and ZnO: Mn at the

2θ= 31.744, 34.398, 36.223 and 2θ= 31.647,

34.313, and 36.131 positions. The three diffraction

peaks correspond to the (100), (002), and (101)

crystalline planes of hexagonal ZnO. All the peaks

in the XRD patterns of ZnO and ZnO: Mn samples

could be fitted with the hexagonal wurtzite

structure having slightly increased lattice parameter

values for Mn doped sample (For ZnO

Nanocrystals a= 3.250 A°, c= 5.207 A° and

ZnO: Mn nanocrystals, a= 3.256 A°, c= 5.212 A°)

in comparison to that of pristine ZnO sample

(a= 3.249A°, c= 5.205A°, JCPDS no. 36-1451).

The increased lattice parameter values of Mn

doped ZnO indicates the incorporation of

manganese at zinc sites [4].

Figure 3: XRD patterns of ZnO (a) and ZnO: Mn (b)

nanocrystals.

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 295-302Mazhdi M et al

297

(a)

(b)

The broadening of the XRD lines is attributed to the

nanocrystalline characteristics of the samples,

which indicates that the particle size is in nano-

meter range.

Inter planar spacing (d) is evaluated using the

relation (1):

(1)

D-spacing for (100), (002), and (101) planes are

2.8146, 2.6035, 2.4760 A° and 2.8204, 2.6062,

2.4806 A° for ZnO and ZnO: Mn nanocrystals,

respectively. But, due to the size effect, the XRD

peaks are broad. From the width of the XRD peak

broadening, the mean crystalline size has been

calculated using Scherer's equation [6]:

(2)

Where D is the diameter of the particle, K is a

geometric factor taken to be 0.9, λ is the X-ray

wavelength, θ is the diffraction angle and β is the

full width at half maximum of the diffraction main

peak at 2θ. The mean crystal size of ZnO and ZnO:

Mn nanocrystals resulted to be 21 and 18 nm.

Also, we used the Williamson-Hall equation to

calculate the strain and particle size of the samples.

The Williamson-Hall equation is expressed as

follows [6]:

(3)

In this equation, βcosθ is plotted against sinθ.

Using a linear extrapolation to this plot, the

intercept gives the particle size kλ/D and the slope

represents the strain (ε) for ZnO and ZnO: Mn

nanoparticles. The size value and internal lattice

strain value were found to be 23 and 21nm and

1.46×10-3 and 1.41×10-3 for ZnO and ZnO: Mn

nanoparticles, respectively [3].

3.2. Atomic absorption study

The atomic absorption studies confirmed

attendance of manganese at Zinc sites in ZnO: Mn

nanocrystals. It supported the result obtained by

XRD analysis. The amount of Mn doping is about

1% by weight.

3.3. TEM studies

TEM high magnification imaging allows the

determination of size and individual crystallite

morphology. TEM micrographs of the ZnO powder

and size distribution histogram of nanocrystals

obtained by TEM micrograph is presented in

Figure 4. The main products are the spherical or

quasi spherical nanocrystals and the average crystal

size is related to 18-23 nm.

3.4. Sensitivity study

Figure 2 shows schematic of resistance gas sensor


298

Figure 4: TEM micrograph at high magnification and size distribution histogram of ZnO nanocrystals.

2

2

2

22

2 3

41

c

l

a

khkh

d+

++=

θβλ

cos

KD =

θελ

θβ sin4cos +=D

k

that used in this experimental work. Sensor

resistance Rs and constant resistance R0 are

connected continuously via power supply (Vc= 4

volts) and signal generator (80 Hz). Voltage loss V0

of R0, determine sensor conductivity changes in the

presence of purpose gas and without it. Sensor

resistance (Rs) obtains by the relation (4). R0, V0

and Vc are circuit resistance, voltage loss of R0 and

power supply voltage respectively.

(4)

Sensor sensitivity for reducer gases defines as

relative of sensor electric resistance in air to sensor

electric resistance in presence of reducer gas:

(5)

The duration between the states that external

voltage last until from the lowest value (i.e. in the

presence of air) arrives to 90 % of highest value

(i.e. in the presence of reducer gas) is called

response time and restore time define as duration

between the states that external voltage (by

elimination of reducer gas) last until from the

highest value arrives to 10 % of the lowest value

[7, 8]. Semiconductor materials in ordinary

temperature and pressure conditions almost are of

electric insulator but when they are under the heat,

their conductivity increase gradually. Figure 5

shows conductivity logarithmic graph (Arrhenius

graph) of pure and doped zinc oxide and it indicates

that increase in temperature lead to increase in

conductivity.

In surface conduction sensors change in

conduction resulted from reactions that occur in the

semiconductor surface. In the presence of air and

low temperatures, oxygen molecules adsorb on the

surface of sensor physically. At the temperature

between 240- 420°C, there is O2- or O- species on

the surface of sensor that O- is more stability and in

high temperature dominant species is O2- [9]. The

reactions on surface of sensor are as:

Figure 5: Conductivity logarithmic graphs (Arrhenius

graph) in the presence of air for ZnO (a), ZnO: Mn (b)

nanocrystals.

O2→O2 (physical adsorption) (6)

O2 (physical adsorption) + 2e-→2O- (7)

O- + e-→O2- (8)

Oxygen vacancies in ZnO, acts as electron donors

and also makes it an ntype semiconductor. Oxygen

molecules in the ambient are adsorbed at the grain

boundaries, which capture electrons from the

conduction band and forming adsorbed oxygen ion.

This causes a decrease in carrier concentration and

increase in resistance of the sample. When the

sensor is exposed to a reducing gas (in this work,

ethanol and gasoline vapor), it reacts with the

adsorbed oxygen and resulting in the release of the

trapped electrons, come back into the conduction


299

0

0

1 RV

VR c

s

−=

g

a

R

RS =

(a)

(b)

band. This leads to an increase in carrier concentra-

tion and decrease in the resistance of the sensor.

The properties of the sensors such as sensitivity,

response and recovery times are known that to be

temperature dependent.

Sensitivity versus temperature for 1000 ppm of

ethanol and gasoline vapor as a function of

temperature is shown in Figures 6, 7. The

temperature that sensor sensitivity arrives to

maximum value defines as optimum working

temperature (i.e. peak of Figures 6, 7).

Figure 6: Sensitivity versus temperature for 1000 ppm of

ethanol vapor; ZnO (a), ZnO: Mn (b) nanocrystals.

Optimum working temperature to ethanol vapor for

pure and doped zinc oxide nanocrystals sample is

362°C and 345°C (Figure 6 a, b) and also for

gasoline vapor is 285°C and 333°C (Figure 7 a, b)

respectively. Response and recovery times to

ethanol vapor in working temperature of sensor for

pure zinc oxide nanocrystals sensor are 5 and 80

seconds and also for zinc oxide nanocrystals doped

by manganese are 19 and 13 seconds respectively.

Response and recovery times to gasoline vapor in

working temperature of sensor for pure zinc oxide

nanocrystals sensor, 20 and 225 seconds and alsofor

zinc oxide nanocrystals doped by manganese,

5 and 3 seconds have obtained respectively.

Figure 7: Sensitivity versus temperature for 1000 ppm of

gasoline vapor; ZnO (a), ZnO: Mn (b) nanocrystals.

In this case study, we also investigated response of

sensors to different concentration that has shown in

Figures 8, 9. Increase of ethanol and gasoline vapor

concentration lead to increase in nanocrystals

sensing response.


300

(b)

(a) (a)

(b)

Figure 8: Sensor response (Sensitivity) to different

concentration for ethanol vapor; ZnO (a), ZnO: Mn (b)

nanocrystals.

Pure zinc oxide nanocrystals sensor will saturate in

2000 ppm of ethanol vapor whereas zinc oxide

nanocrystals sensor doped by manganese achieves

to saturation state in exceed of 70000 ppm. The

doped sensor is able to sense the ethanol vapor by

high concentration than pure one. By increase of

ethanol vapor concentration, response and recovery

times for pure sensor at first increase and then

decrease and also are in order of 1 and 10 second

respectively but for doped sensor response time

decrease gradually and recovery time do not change

tangible as for 1000 and 70000 ppm response and

recovery times are 2, 12 and 2, 8 second

respectively. Pure zinc oxide sensor in 30000 ppm

Figure 9: Sensor response (Sensitivity) to different

concentration for Gasoline vapor; ZnO (a), ZnO: Mn (b)

nanocrystals.

of gasoline vapor achieves to saturation statewhereas doped zinc oxide sensor in 70000 ppm of

gasoline vapor achieves to saturation state therefore

doped zinc oxide sensor is a good choice for

measurement of sample gas in high concentration.

The results for sensor response to concentration

show that response time of pure zinc oxide sensor

to gasoline vapor decrease and recovery time

increase regularly that are in order of 10 and 1000

second respectively. For 1000 and 30000 ppm of

gasoline vapor, response and recovery times are

240, 6630 and 5, 20000 second respectively. By

increase of gasoline vapor concentration, doped

sensor response time decrease and its recovery time


301

(a) (a)

(b) (b)

increase respectively, as for 1000 and 70000 ppm of

gasoline vapor, response and recovery times are 19,

70 and 5, 171 second respectively.

4. CONCLUSIONS

ZnO and ZnO: Mn nanocrystals with hexagonal

structure were synthesized by the reverse micelle

method using PVP as surfactant. The XRD studies

of these nanocrystals revealed that the average

particle size is about 21 and 18 nm for ZnO and

ZnO: Mn nanocrystals, respectively. The atomic

absorption studies confirmed attendance of

manganese at zinc sites in ZnO: Mn nanocrystals.

Sensor devices were fabricated and their gas

sensing properties with respect to gasoline and

ethanol vapor at different concentrations were

measured. Gas sensing properties of these sensors

showed that those are sensitive to both gasoline and

ethanol vapors. Optimum working temperature to

ethanol vapor for pure and doped zinc oxide

nanocrystals sample was 362°C and 345°C and also

for gasoline vapor was 285°C and 333°C

respectively. Pure zinc oxide sensor achieve to

saturation state at 2000 ppm and 30000 ppm for

ethanol and gasoline vapor, respectively. Doped

zinc oxide sensor for both gas sample (i.e. ethanol

and gasoline) in 70000 ppm achieve to saturation

state. Further studies showed that Mn doped ZnO

nanoparticles based sensors have faster response

and recovery time and the sensor will be saturated

at higher concentrations.

ACKNOWLEDGMENTS

The financial support of the Laboratory at the

Department of Physics in Imam Hossein University

is gratefully acknowledged.

REFERENCES

1. Maensiri S., Masingboon C., Promarak V.,

Seraphin S., Opt. Mater., 29(2007), 1700.

2. Mazhdi M., Hossein Khani P., Chitsazan

Moghadam M., IJND, 2(2011), 117.

3. Mazhdi M., Hossein Khani P., IJND, 2(2012),

233.

4. Jayakumar O.D., Gopalakrishnan I.K., Kadam

R.M., Vinu A., Asthana A., Tyagi A.K., J. Cryst.

Growth, 300(2007), 358.

5. Xu C., Tamaki J., Miura N., Yamazor N., Sens.

Actuators B, 3(1991), 147.

6. Khorsand Z., Abd. Majid W.H., Abrishami M.E.,

Yousefi R., Solid-State Sci., 13(2011), 251.

7. Mohamadrezaei A., Afzalzadeh R., Sensor Lett.,

8(2010), 777.

8. Tan O.K., Cao W., Hu Y., Zhu W., Ceram. Int.,

30(2004), 1127.

9. Takata M., Tsubone D., Yanagida H., J. Am.

Ceram. Soc., 59(1976), 4.


302

Shortage of wood resources and natural regenera-

tion of forests necessitates the use of fast growing

trees as well as harvesting them at short rotations

[1]. The harvested wood of these trees usually are

not suitable for furniture industry; however, they

provide a sustainable source for paper and

composite manufacturing industries. Wood com-

posite panels offer the advantage of a homogeneous

structure which may be important for many design

purposes [2]. Due to the low thermal conductivity

coefficient of wood [3], many studies have so far

been carried out to increase the rate of heat transfer


Effect of Nanosilver on the Rate of Heat Transfer to the

Core of the Medium Density Fiberboard Mat

Hamid Reza Taghiyari1*, Asaad Moradiyan2, Amir Farazi3

1 Assisstant Professor, Wood Science & Technology Department, the Faculty of Civil Engineering, Shahid

Rajaee Teacher Training University, Tehran, Iran

2, 3 B.Sc. Student, Wood Science & Technology Department, the Faculty of Civil Engineering, Shahid

Rajaee Teacher Training University, Tehran, Iran


Effect of nanosilver (NS) on the heat-transferring rate to the core section of medium

density fiberboard (MDF) mat was studied here. A 400 ppm aqueous nanosilver suspension was

used at three consumption levels of 100, 150, and 200 mL/kg, based on the weight of dry wood

fibers; the results were then compared with the control MDF panels. The size range of nanosilver

was 30-80 nm. Results showed that the uniform and even dispersion of nanoparticles through-

out the MDFmatrix significantly contributed to the faster transfer of heat to the core section. As to

the loss of mat water content after the first 3-4 minutes under the hot press, the core temperature

slightly decreased in the control panels. However, heat transferring prope rty of nanosilver

contributed to keeping the core temperature rather constant in the nanosilver-150 and 200

treatments. The surface layers of the mat rapidly absorbed the heat, resulting in the

depolymerization of part of the resin. It can therefore be concluded that the optimum nano

suspension content should not necessarily be the highest one.

Keyword: Composite board; Heat transferring property; Metal nanoparticles; Nanosilver;

Thermal conductivity coefficient; Wood fiber.

ABSTRACT

1. INTRODUCTION


(*) Corresponding Author - e-mail: [email protected] & [email protected]

to the core of the wood composite mat. Hot press

time is dependant on the thickness of the composite

mat, press temperature, closing rate, and most

importantly, moisture distribution throughout the

mat [4]. Moisture of the mat can not always be

increased as it in turn increases the hot press time,

or causes blows in wood composite panels.

Furthermore, for urea-formaldehyde (UF) resin,

there is a limitation of moisture content (MC) level

[5]. Finding new ways to increase the heat

transferring rate to the core section of the

composite mat is always a challenge before the

wood composite manufacturing industry.

Heat transferring property of metal nano-

particles [6-8] was reported to improve some

properties in solid woods as well as wood

composite materials. However, little or no direct

measurements at the core section of the mat was

carried out to practically investigate if the

temperature was really increased, or the

improvement in physical and mechanical properties

was merely due to the formation of bonds between

wood fibers or particles and the nano materials used

in the composite matrix. Therefore, the present

study was therefore carried out to directly measure

the temperature at the core section of the composite

mat and find out the probable increasing trend.

2. MATERIALS AND METHODS

2.1. Specimen procurement

Wood fibers were procured from Sanaye Choobe

Khazar Company in Iran (MDF Caspian Khazar).

The fibers comprised a mixture of five species of

beech, alder, maple, hornbeam, and poplar from

forests of Gillan province. Boards were 16 mm in

thickness and 0.68 g/cm3 in density. A laboratory

hot press produced by Mehrabadi Machinery Mfg.

Co. was used; the size of the hot plates was 50×50

cm. The total nominal pressure of the hot plates was

160 bars. The total nominal pressure of the plates

was 160 bars. The temperature of the plates was

fixed at 150°C. Hot pressing continued for 10

minutes. Urea Formaldehyde resin (UF), as a

popular thermosetting resin in composite manufac-

turing factories of Iran, was procured from Pars

Chemical Industries Company, Iran. 10% of UF

with 200-400 cP in viscosity, 47 seconds of gel

time, and 1.277 g/cm3 in density was used in the

composite based on the dry weight of wood fibers.

Specimens were kept in conditioning chamber

(30±2°C, and 45±2% relative humidity) for three

weeks before the tests were carried out on them.

The moisture content of the specimens at the time

of testing was 7%. Five boards were made for each

treatment group.

2.2. Nanosilver application

A 400 ppm aqueous suspension of silver nano-

particles nanosilver (NS) was produced and applied

to the specimens using electrochemical technique.

The nano suspension was prepared by transferring

the silver metal ion from the aqueous phase to the

organic phase, where it reacted with a monomer.

The formation and size of the nanosilver was

monitored by transmission electron microscopy

(TEM). Samples for TEM were prepared by drop

coating the Ag nanoparticle suspensions on to

carbon coated copper grids. Micrographs were

obtained using an EM-900 ZEISS transmission

electron microscope. The size range of nanosilver

was 30-80 nm. The pH of the suspension was 6-7;

two kinds of surfactants (anionic and cationic) were

used in the suspension as stabilizer; the concentra-

tion of the surfactants was two times the nanosilver

particles. The nano suspension was applied at three

consumption levels, including nanosilver 100

(NS-100; 100 mL/kg), nanosilver 150 (NS-150;

150 mL/kg), and nanosilver 200 (NS-200; 200

mL/kg). After impregnating the wood specimens

with the silver nano suspension, SEM micrographs

showed uniform dispersion of nanoparticles on

wood fibers (Figure 1).

2.3. Temperature measurement at the core section

of the mat

A digital thermometer with temperature sensor

probe was used to measure the temperature at the

core section of the mat at 5 second intervals

(Figure 2). The probe of the thermometer was

directly inserted for about 50 mm into the core of

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 303-308 Taghiyari HR et al

304

the mat (from the edge boarder of the mat), in

the horizontal direction. Temperature measurement

was started immediately after the two hot plates

reached the stop bars. Temperature was measured

with 0.1°C precision.

2.4. SEM imaging

SEM imaging was done at thin film laboratory,

FE-SEM lab (Field Emission), School of Electrical

& Computer Engineering, The University of

Tehran; a field emission cathode in the electron gun

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 303-308Taghiyari HR et al

305

Figure 1: SEM micrograph showing nanosilver scattered all over the fibers.

Figure 2: Temperature measurement using a digital thermometer with its sensor probe inserted into

the core section of the composite board mat.

of a scanning electron microscope provided

narrower probing beams at low as well as high

electron energy, resulting in both improved spatial

resolution and minimized sample charging and

damage.


Measurement of temperature at the core section of

the mat (immediately after the upper plate of the hot

press reached the stopbars) indicated significant

difference between the temperatures of the four

treatments of control, NS-100, NS-150, and

NS-200 (Figure 3). During the first minute of hot

pressing, the increasing rate of heat in the core

section of the mat showed significant higher rate in

the nanosilver treated mats in comparison to the

control panels. As the times passed (during the

second minute), NS-100 came closer to the control

mat, although it was still significantly different.

NS-150 and NS-200 were both higher than both

NS-100 and control treatments; however, no

significant difference was observed between them

in the first two minutes of hot pressing.

During the third to the seventh minutes of

hot pressing, control panels showed significantly

lower temperatures in comparison to all three

NS treated mats (Figure 4). Although NS-100 was a

bit lower in temperature, but no significant

difference was observed in the temperatures of the

NS treated panels. During this time (the third to the

seventh minutes of hot pressing), a decreasing trend

in temperature of all treatments was observed; it

was due to the decrease in moisture content of the

mat. In fact, the evaporation of water content

resulted in decreasing of the heat transferred to the

core; consequently, it decreased the core

temperature. In the final two minutes of hot

pressing though (the 8th to 10th minutes), an

increasing trend in temperature was seen in NS-150

and NS-200 treatments. This slight increasing trend

was because much of the moisture content of the

mat was evaporated by this time; therefore, the heat


306

Figure 3: Temperature at the core section of the medium-density fiberboard mat with

five-second intervals (NS= nanosilver content mL/kg).

transferred to the core resulted in the increase in the

temperature. The control and NS-100 treatments

showed no increasing trend in the last two minutes

of hot pressing; in fact they tended to be rather flat.

The depolymerization of the surface resin bonds

in the surface layers of panels with high metal

nanoparticle content can be related to the increasing

trend in the final minutes of the hot pressing; that

is, in the final minutes when all moisture content

was nearly evaporated in the surface layers, the heat

resulted in the depolymerization and breaking down

of resin bonds. The depolymerization increased the

fluid flow in the composite matrix. Similarly, the

increase in the internal bond in nanocopper treated

panels was due to the higher heat transfer rate to the

core section of the composite mat, resulting in

better polymerization of UF resin [9].

As to the fact that rapid transfer of heat to the

surface layers of the mat would eventually result in

the depolymerization of resin, ending up in

decrease in some of the physical and mechanical

properties, authors are working on possible spread

of metal nanoparticles or mineral nanofibers in only

the core section of composite mats to facilitate the

heat transfer to this part; this would also prevent

over heating of the surface layers and the

consequent resin break down.

4. CONCLUSIONS

Effects of a 400 ppm aqueous suspension of

nanosilver on the heat transferring rate from the hot

press plates to the core section of medium density

fiberboards (MDF) was studied here. Nanosilver

suspension was applied to the mat at three

consumption levels of 100, 150, and 200 mL/kg

based on the dry weight of wood fibers. The

obtained results proved significant higher

heat transferring rate to the core of the mat in the

NS treated panels. The high heat transferring rate

was also the reason for the depolymerization of

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 303-308Taghiyari HR et al

307

Figure 4: Temperature at the core section of the medium density fiberboard mat after the third minute

of hot pressing with five second intervals (NS= nanosilver content mL/kg).

resin bonds in the surface layers of composite

boards. It may therefore be concluded that addition

of metal nanoparticles to increase the heat

transferring rate to the core section of composite

mats should not necessarily improve all physical

and mechanical properties. Furthermore, the

optimum consumption level for metal nanoparticles

is dependent on many factors, including the hot

press temperature, hot press duration, thermal

conductivity coefficient of metal nanoparticles, and

the type and density of composite panels.

ACKNOWLEDGMENTS

The authors are grateful to Mr. Majid Ghazizadeh,

the internal sales manager of Pars Chemical

Industries Company, for the procurement of the

resin for the present study. We also appreciate Mr.

Mossyyeb Abbasi for schematic diagram design of

the apparatus.

REFERENCES

1. Ruprecht H., Vacik H., Steiner H., & Frank G.,

Aust. J. For. Sci., 129(2) (2012), 67.

2. Valenzuela J., Von Leyser E., Pizzi A.,

Westermeyer C., Gorrini B., Eur. J. Wood Wood

Prod., DOI 10.1007/ s00107-012-0610-2,

(2012).

3. K. Doosthoseini, 2001. Wood Composite

Materials Technology, Manufacture, and

Applications, The University of Tehran Press.

4. Taghiyari HR., Rangavar H., Farajpour Bibalan

O., Bioresources, 6(4) (2011), 4067.

5. Papadopoulos A.N., Bioresources, 1(12) (2006),

201.

6. Sadeghi B. & Rastgo S., IJBIHN, 1(1) (2012),

33.

7. Yu Y., Jiang Z., Wang G., Tian G., Wang H. &

Song Y., Wood Sci. Technol., DOI

10.1007/s00226-011-0446-7, 46(2012), 781.

8. Khojier K., Zolghadr S., Zare N., IJBIHN, 1(3)

(2012), 199.

9. Taghiyari HR. & Farajpour Bibalan O., Eur. J.

Wood Wood Prod., DOI 10.1007/s00107-012-

0644-5, 71(1) (2013), 69.


308

Magnetic drug delivery systems are a promising

technology for cancer cells treatment. In such a

system, some smart particles have to be associated

with the magnetic core to direct magnetic

nanostructures to the vicinity of the target for

hyperthermia or for temperature enhanced release

of the drug. The best magnetic particle size, for

these kind applications has to be below a critical

value, which is dependent on the material species,

but is typically around 10-20 nm [1-4]. In this

condition, each nanostructure will be able to pass

through the cell membrane and can acts as a single

domain paramagnetic substance with a fast

response to applied magnetic fields and zero

remanence (residual magnetism) and coercivity (the

field required to bring the magnetization to zero)

[5, 6]. Multifunctional silica nanoparticles (NPs)

have tremendous potential applications as magnetic


Preparation of Fe3O4@SiO2 Nanostructures via Inverse

Micelle Method and Study of Their Magnetic Properties for

Biological Applications

Afsaneh Sharafi1*, Nazanin Farhadyar2

1 Ph.D., Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran

2 Assistant Professor, Department of Chemistry, Varamin (Pishva) Branch Islamic Azad University,

Varamin, Iran


In this work, we report synthesis of superparamagnetic iron oxide nanoparticles at room

temperature using microemulsion template phase consisting of cyclohexane, water, cetyltrimethy-

lammonium bromide CTAB as cationic surfactant and butanol as a cosurfactant. Silica surface

modification of the as prepared nanoparticles was performed by adding tetraethoxysilane TEOS

to alkaline medium. The structure, morphology, and magnetic properties of the products were

characterized by X-ray powder diffraction (XRD), energy dispersive X-ray spectroscopy (EDX),,

Scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), and

vibrating sample magnetometer (VSM) at room temperature. The results revealed formation of

iron oxide nanoparticles, with an average size of 8.8-12 nm, a superparamagnetism behavior with

fast response to applied magnetic fields and Zero remanence and coercivity.

Keyword: Inverse micelle; Surface modification; Superparamagnetism; Magnetic nanoparticles;

Fe3O4@SiO2 Nanostructures.

ABSTRACT

1. INTRODUCTION



indicators and/or photon sources for a number of

biotechnological and information technologies.

Indeed, the chemistry of silica gained recently in

interest in the design of new nano sized particles

with functional architecture for applications in

biotechnology and photonics [7, 8]. Silica NPs are

actually very promising candidates in the fields of

biomedical, imaging, separation, diagnosis and

therapy [9-11] and band-gap photonic materials

when assembled in colloidal crystals [12, 13].

These applications all require size controlled,

monodispersed, bright and/or magnetic NPs that

can be specifically conjugated to biological

macromolecules or arranged in higher ordered

structures. Also the preparation of such functional

NPs involves a very good understanding of the

influence of the synthesis parameters in order to

control the properties of the final product such as

size, morphology, effects of the shell on the core

particle, etc.

In the paper, a room temperature microemulsion

method has been employed to synthesize Fe3O4

nanoparticles, and the sol-gel processes were

selected for coating magnetic nanoparticles with

silica, and magnetic resonance property were

investigated.

2. EXPERIMENTAL

2.1. Chemicals and reagents

Iron (III) chloride hexahydrate (Fe3Cl3.6H2O), Iron

(II) chloride tetrahydrate (FeCl2.4H2O), aqueous

ammonia (16%), cetyltrimetylammonium bromide

(CTAB), n-butanol (C4H4OH), tetraethoxysilane

(TEOS), in analytical grade were purchased from

Merck Company (Darmstadt, Germany).

2.2. Preparation of magnetite iron oxide

nanoparticles

The magnetic nanoparticles were prepared by the

reverse microemulsion method. First 3 gr of

cetyltrimetyl ammonium bromide (CTAB) and

10 mL n-butanol were added in 60 mL of n-hexane.

The mixture was stirred at 100 rpm for 20 min and

was added dropping aqueous solution of

FeCl2/FeCl3 (0.14 g /0.06 g, 2.7 mL water) under

nitrogen (N2) atmosphere and purging with N2 for

20 min. An ammonium hydroxide solution (16%

NH4OH in water, 0.7 mL) was finally dropped in

the solution under N2 protection. By adding 1.5 mL

TEOS dropwisely to the mixture and stirred for 16

h. The reaction was finally stopped by addition of

ethanol and the surfactant was removed through

centrifugting the solution.

2.3. Preparation of Fe3O4@SiO2 nanoparticles

Fe3O4@SiO2 nanoparticles were prepared by the

stober method. The magnetic nanoparticals Fe3O4

(0.01 g) was dissolved in mixed solution of water

(10 mL) and ethanol (50 mL). Ammonia solution

(1.2 mL) and TEOS (1.8 mL) were added to the

mixed solution with stirring and reactant for 1.5 h.

The nanoparticles were isolated by centrifugating

and washed with ethanol.

X-ray diffraction patterns (PW 1800 PHILIPS),

Energy Dispersion Spectrum (Hitachi F4160,

Oxford), and FT-IR spectra (A NICOLET 5700)

were used to determine the crystal structure of the

silica coated Fe3O4 nanoparticles and the chemical

bonds of Fe-O-Si, respectively. The magnetic

properties were analyzed with a Vibration Sample

Magnetometer (VSM, Quantum Design PPMS-9).


3.1. X-Ray study

Figure 1(a, b) represents X-ray diffraction pattern

of Fe3O4 and silica coated Fe3O4 nanoparticles. All

the diffraction peaks observed at (220), (311),

(400), (422), (511), (440) in this Figure 1 (a, b)

were consistent with those of standard XRD pattern

of Fe3O4 crystal with spinal structure (JCPDS card

No. 65-3107). Whereas, no peaks were detected for

silica coated Fe3O4 nanoparticles which could be

assigned to impurities as shown in Figure 1 (b). The

average crystalline size of Fe3O4 and Fe3O4@SiO2

nanostructures at the characteristic peak (311) were

calculated by using Scherer formula:

D = kλ/βcosθ (1)

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 309-313 Sharafi A et al

310

Where, D is the mean grain size, k is a

geometric factor, λ is the X-ray wavelength, β is the

FWHM of diffraction peak and θ is the diffraction

angle. The results of D values, using the peak (311)

planes of the spinel structures was 11 nm for

uncoated and 17 nm for silica coated magnetic iron

oxide.

Figure 1: X-ray powder diffraction patterns of: a) Fe3O4

nanoparticles and (b) Fe3O4@SiO2 composite particles.

3.2. Edx study

The surface composition of silica coated sample

was qualitatively determined by energy dispersion

spectrum (EDS) as shown in Figure 2. It shows that

Fe and Si peak are obtained and atomic (%) ratio of

Fe/Si= 11.29/16.8. It is therefore assumed that

silica particles are coated onto the surface of Fe3O4

nanoparticles.

Figure 2: Energy-dispersive X-ray spectroscopy Edx

result of silica coated Fe3O4 nanoparticles.

Figure 3 represents FT-IR spectra for Fe3O4 and

Fe3O4@SiO2, The strong broad peaks at about 630

cm-1 and 568 cm-1 (in Figure 3a) are due to the

stretching vibrations of Fe-O and Fe-O bonds and

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 309-313Sharafi A et al

311

Figure 3: Fourier transforms infrared (FT-IR) spectra of: a) Fe3O4; b) Fe3O4@SiO2.

(b)

(a)

(a)

(b)

the peaks around 3000-3500 cm-1 and 1625 cm-1

have been assigned to the stretching and bending

vibrations of the H-O-H bond, respectively,

showing the physical absorption of H2O molecules

on the surfaces. In Figure 3b shows IR spectrum of

silica coated Fe3O4@SiO2 nanoparticles confirms

the presence of the finger print bands at around

1090 cm-1 which are characteristic of stretching

vibrations of framework Si-O-Si. Peak 634 cm-1

(in Figure 3b) not absorbed indicates the formation

of Si-O-Fe, Si-O-Si bond was assumed that

absorption bands in (1090 cm-1), (989 cm-1), (801

cm-1), respectively, assigned to stretching vibration

of Si-O-Si bond, Si-OH bond, Si-O-Fe bond.

Table 1: EDAX quantification element normalized.

3.3. Morphological study

Figure 4 (a, b) represents SEM images of Fe3O4 and

Fe3O4@SiO2 nanoparticles. These images clearly

show spheric particle shapes and morphology with

a homogenous particle size and distribution. This

result has been confirmed by Dynamic Light

Scattering analysis data.

Figure 5: Magnetization vs. applied magnetic field for

a) Fe3O4; b) Fe3O4@SiO2 at room temperature.

3.4. Magnetic study

Figure 5 (a, b) represents magnetic-field-dependent

magnetization parameters, M(H) for Fe3O4; and

Fe3O4/SiO2 in the size of 13.26, 21.06 and 34 nm at

room temperature, using vibrating sample

magnetometer with a peak field of 15 kOe. The

hystersis loops for Fe3O4; and Fe3O4@SiO2 in the

size of 11 nm, with coercivity (Hc= 0.0 Oe) and

remanence (Mr= 0) indicate a superparagnetism

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 309-313 Sharafi A et al

312

Elements Wt.% At.%

Fe 95.39 11.29

O 4.08 68.15

Si 0.53 16.8

(a) (b)

Figure 4 (a, b): a) SEM image of Fe3O4; b) SEM image of Fe3O4@SiO2 Core-Shell nanostructures.

properties at 300 K with a saturation magnetization

of 65 emu/g for Fe3O4 and 34 emu/g for

Fe3O4@SiO2.

4. CONCLUSIONS

Fe3O4 nanoparticles were prepared by the

microemulsion technique using Fe3+ and Fe2+ and

silica coated uniformly by hydrolysis and

condensation of TEOS in a sol-gel process. Using

this method nanosized Fe3O4 in 11 nm size and

17 nm size silica coated Fe3O4 were prepared

successfully. FT-IR spectra showed formation

chemical bonds of Fe-O-Si on to the surface of

Fe3O4 nanoparticles and FT-IR spectra, edx

analysis data showed the presence of silica in our

prepared sample. Microemulsion (inverse micelle)

is a suitable way for obtaining the uniform and size

controllable nanoparticles.

REFERENCES

1. Yamguchi K., Matsumoto K., Fiji T., J. Appl.

Phys., 67(1990), 4493.

2. Odenbach S., Adv. Colloid Interface Sci.,

46(1993), 263.

3. Atarshi T., Imai T., J. Magn Magn Mater.,

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4. Caceres P.G., Behbehani M.H., Appl. Catal A,

109(1994), 211.

5. Chikov V., Kuznetsow A., J. Magn Magn Mater.,

122(1993), 367.

6. Fan R., Chen X.H., Gui Z., Mater Res Bull.,

36(2001), 497.

7. Burns A., Hooisweng O., Weisner U., Chem. Soc.

Rev., 35(2006), 1028.

8. Wang L., Zhao W., Tan W., Nano Res., 1(2)

(2008), 99.

9. Yan J., Estevez M.C., Smith J.E., Wang K., He

X., Wang L., Tan W., Nano Today, 2(3) (2007),

44.

10. Trewyn B.G., Slowing I.I., Giri S., Chen H.T.,

Lin V.S.Y., Acc. Chem. Res., 40(2007) 846.

11. Slowing I.I., Trewyn B.G., Lin V.S.L., J. Am.

Chem. Soc., 129(2007), 8845.

12. Masse P., Pouclet G., Ravaine S., Adv. Mater.,

20(2008), 584.

13. Ge J., Yin J., Adv. Mater., 20(18) (2008), 3485.

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 309-313Sharafi A et al

313

Research on semiconductor quantum dots has

increased rapidly in the past few decades.

Luminescent semiconductor quantum dots have

been intensely studied due to their unique optical

properties [1]. In particular, semiconductor

Quantum dots are very attractive as biological

labels because of their small size, emission

tunability, superior photostability and longer

photoluminescence decay times in comparison with

conventional organic dyes [2-6]. These highly

luminescent quantum dots have photophysical

properties superior to organic dyes but the high

temperature required to synthesize them can be

problematic for some applications [4, 5, and 7]. One

of the major challenges is to obtain water soluble

Quantum dots with a high PL quantum efficiency.

Arrested precipitation in water in the presence of

stabilizers (e.g., thiols) is a faster and simpler

method to synthesize water soluble Quantum dots

and has been applied to several semiconductors

potentially relevant to biolabeling (e.g., CdS, CdSe,

CdTe). For CdS and CdSe, this yielded Quantum

dots with defect related emission and a low

quantum efficiency. For CdTe Quantum dots, both

excitonic and defect related emission bands were

observed. Although samples with no observable


Synthesis and Optical Study of CdZnTe Quantum Dots

Faranak Asgari1*, Shankar Lal Gargh2, Karim Zare3

1 Ph.D. Student, Department of Chemistry Science and Research Branch Islamic Azad University, Tehran,

Iran

2 Professer, Research Journal of BioTechnology, Sector AG/80, Scheme no.54, A.B.Road, Indore, India

3 Professer, Department of Chemistry Science and Research Branch Islamic Azad University, Tehran, Iran

Received: 1 January 2013; Accepted: 3 March 2013

The comparison of growth processes and fluorescent properties of CdZnTe semiconductor

quantum dots that are synthesized in different concentrations of Zn2+ in water are discussed in

this paper. The samples are characterized through absorbtion (UV) and photoluminescence

spectra (PL). The results show that when the reaction time is prolonged, the absorption peak and

fluorescent emission peak present obvious red shifts and the diameters of the Quantum dots

continuously increase. Under the best reaction conditions, the highest quantum yield can be

attained by using thioglycollic acid (TGA) as modifier when the reaction time is 300 min.

Keyword: Quantum dots; Modifier; Fluorescence; Photoluminescence; Thioglycollic acid;

Emission.

ABSTRACT

1. INTRODUCTION



trap luminescence were also obtained. In this study,

we report a novel method that yields highly

luminescent water soluble CdZnTe Quantum dots.

The results show that when the reaction time is

prolonged, the absorption peak and fluorescent

emission peak present obvious red shifts and the

diameters of the Quantum dots continuously

increase [8].

2. EXPERIMENTAL

2.1. Materials

Cadmium chloride (CdCl2, 2.5H2O), zinc chloride

(ZnCl2), tellurium (Te reagent powder), and sodium

borohydride (NaBH4), Thioglycolic acid (TGA).

All chemicals were received from Merck chemical

company and used without any further purification.

Deionized and distilled water were used in this

work.


A Varian Cary 100 spectrophotometer in the range

of 200-800 nm was used to record the UV-Vis

absorption spectra. The PL emission measurments

were performed at room temperature on a photolu-

minscence spectrophotometer Ls-50B Perkin Elmer

equipped with Xe lamp (λ= 320 nm) as an

excitation light source. A JEON 360 Transmission

Electron Microscope (TEM) operated at 100 W was

used to observe morphology and size of the

synthesized Quantum dots.

2.3. Synthesis CdZnTe quantum dots

In a typical synthesis 2.5 mmol of CdCl2, 2.5H2O

and 5, 10.15 weight percent of Zn2+ is dissolved in

110 mL of water, and 12 mmol of the thiol

stabilizer (TGA) is added under stirring, followed

by adjusting the pH to appropriate values by

dropwise addition of 1M solution of NaOH. The

solution may be slightly turbid at this stage. The

reaction mixture is placed in a three necked flask

fitted. Under stirring, NaHTe (a purple clear liquid

generated by the reaction of 2.4 mmol of Te powder

with 5 mmol NaBH4 in 8 mL water and stirring then

cooling in an icebath for 10 min) is passed through

the solution together for 20 min. CdZnTe precursors

are formed at this stage. The precursors are

converted to CdZnTe quantum dots by refluxing the

reaction mixture at 95°C under helium-gas

conditions.


3.1. Optical properties of CdZnTe quantum dots

Figures 1, 2, 3 show photoluminescence (PL)

spectra and absorbtions (UV) of a size series of

CdZnTe quantum dots. The spectra were measured

on as prepared CdZnTe colloidal solutions which

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No.1 (2013), 315-318 Asgari F et al

316

Figure 1: Fluorescence spectra and absorbtions of CdZnTe (5%) quantum dots prepared at different reaction times.

were taken from the refluxing reaction mixture at

different intervals of time. A clearly resolved

absorption maximum of the first electronic

transition of CdZnTe Quantum dots appears which

shifts to longer wavelengths as the particles grow in

the reaction process. The size of the growing

CdZnTe Quantum dots is further controlled by the

duration of reflux and can be easily monitored by

absorption and PL spectra. The PL excitation

spectra also display electronic transitions at higher

energies when the heating time is extended from 30

min to 300 min in the presence of thioglycolic acid

is used as the stabilizer. PL technique allows

detection of the luminescence emitted by particles

with selected size.

3.2. Morpholgy study and structure analysis of

CdZnTe quantum dots

Figure 4 shows typical XRD patterns obtained from

powdered precipitated fractions of CdZnTe

quantum dots synthesized when the stabilizer is

TGA. Five distinct diffraction peaks were observed

values of 24.0°, 39.2°, 46.3° and 56.8° respectively,

corresponding to the (111), (220), (311) and (400)

crystalline planes. Figure 5 shows TEM obtained

from powdered precipitated fractions of CdZnTe

quantum dots. This distribution of spherical image

shows a well homogenized quantum dots. Figure 6

shows the band gap of the Quantum dots decrease

and the diameters of the Quantum dots

continuously increase.

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No.1 (2013), 315-318Asgari F et al

317



Figure 4: XRD pattern of the CdZnTe (5%) quantum

dots.

Figure 5: TEM of the CdZnTe (5%) quantum dots.

Figure 6: Band Gap and Size of CdZnTe quantum dots

compared at different reaction times.

4. CONCLUSIONS

Water soluble CdZnTe Quantum dots have been

reported in this paper with 0.8 nm in diameters. The

Fluorescence spectra of CdZnTe quantum dots

prepared at different reaction times functional

groups of the thiol capping molecules of the

quantum dots provide their water solubility. The

method reported here is also very attractive for its

simplicity compared to other methods for

producing water soluble semiconductor Quantum

dots. It also yields water soluble Quantum dots with

photophysical properties superior to those

presented by Quantum dots prepared directly in

water.

REFERENCES

1. Chan W., Nie S., Science, 281(5385) (1998),

2016.

2. Bruchez M., Moronne M., Gin P., Weiss S.,

Alivisatos A.P., Science, 281(5385) (1998),

2013.

3. Alivisatos A.P., J. Phys. Chem., 100(31) (1996),

13226.

4. Colvin V.L., Schlamp M.C., Alivisatos A.P.,

Nature, 370(1994), 354.

5. Klimov V.I., Mikhailovsky A.A., Xu S., Malko

A., Hollingsworth J.A., Leatherdale C.A., Eisler,

H.J., Bawendi M.G., Science, 290(5490) (2000),

314.

6. Brus L., J. Phys. Chem., 90(12) (1986), 2555.

7. Meng L., Song Z.X., Biochem. Biophys. Dev.,

31(2) (2004), 185.

8. Santra S., Yang H., Holloway P.H., Stanley J.T.,

Mericle R.A., J. Am. Chem. Soc., 127(6) (2005),

1656.

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No.1 (2013), 315-318 Asgari F et al

318

The direct determination of trace metals especially

toxic metal ions such as Co, Sn, As, Pb, Sb and Se

from various samples require mostly an initial and

efficient preconcentration step [1]. This preconcen-

tration is required to meet the detection limits as

well as to determine the lower concentration levels

of the analyte of interest [2]. This can be performed

simply in many ways including liquid and solid


Extraction of Co(II) by Isocyanate Treated Graphite Oxides

(iGOs) Adsorbed on Surfactant Coated C18 Before

Determination by FAAS

Ali Moghimi1*, Majid Abdouss2

1 Associated Professer, Department of Chemistry, Varamin (Pishva) Branch Islamic Azad University,

Varamin, Iran

2 Associated Professer, Department of Chemistry, Amir Kabir University of Technology, Tehran, Iran


A simple, highly sensitive, accurate and selective method for determination of trace amounts of

Co(II) in water samples is presented. Isocyanate treated graphite oxides (iGOs) solid phase

extraction adsorbent was synthesized by covalently isocyanate onto the surfaces of graphite

oxides. The stability of a chemically (iGOs) especially in concentrated hydrochloric acid which

was then used as a recycling and pre concentration reagent for further uses of (iGOs). The

method is based on (iGOs) of Co(II) on surfactant coated C18, modified with a isocyanate

treated graphite oxides (iGOs). The retained ions were then eluted with 4 mL of 4 M nitric acid

and determined by flame atomic absorption spectrometry (FAAS) at 283.3 nm for Co. The

influence of flow rates of sample and eluent solutions, pH, breakthrough volume, effect of foreign

ions on chelation and recovery were investigated. 1.5 g of surfactant coated C18 adsorbs 40 mg

of the iGOs which in turn can retain 15.2±0.8 mg of ions. The limit of detection (3σ) for Co(II) was

found to be 3.20 ng L-1. The enrichment factor for both ions is 100. The mentioned method was

successfully applied on determination of Cobalt in different water samples. The ions were also

speciated by means of three columns system.

Keyword: Extraction of cobalt; Preconcentration; Isocyanate treated graphite oxides (iGOs);

Flame atomic absorption spectrometry.

ABSTRACT

1. INTRODUCTION


(*) Corresponding Author - e-mail: [email protected]; [email protected]

phase extraction techniques [3, 4]. The application

of solid phase extraction technique for preconcen-

tration of trace metals from different samples

results in several advantages such as the minimal

waste generation, reduction of sample matrix

effects as well as sorption of the target species on

the solid surface in a more stable chemical form [5].

The normal and selective solid phase extractors

are those derived from the immobilization of the

organic compounds on the surface of solid supports

which are mainly polyurethane foams [6], filter

paper [7], cellulose [8] and ion exchange resins [9].

Silica gel, alumina, magnesia and zirconia are the

major inorganic solid matrices used to immobilize

the target organic modifiers on their surfaces [10] of

which silica gel is the most widely used solid

support due to the well documented thermal,

chemical and mechanical stability properties

compared to other organic and inorganic solid

supports [11]. The surface of silica gel is

characterized by the presence of silanol groups,

which are known as weak ion exchangers, causing

low interaction, binding and extraction of the target

analysts [12]. For this reason, modification of the

silica gel surface with certain functional groups has

successfully been employed to produce the solid

phase with certain selectivity characters [13]. Two

approaches are known for loading the surface of

solid phases with certain organic compounds and

these are defined as the chemical immobilization

which is based on chemical bond formation

between the silica gel surface groups and those of

the organic modifier, and the other approach is

known as the physical adsorption in which direct

adsorption of the organic modifier with the active

silanol groups takes place [10].

Selective solid phase extractors and preconcen-

trators are mainly based on impregnation of the

solid surface with certain donor atoms such as

oxygen, nitrogen and sulfur containing compounds

[14-18]. The most successful selective solid phases

for soft metal ions are sulfur containing

compounds, which are widely used in different

analytical fields. Amongst these sulfur containing

compounds are dithiocarbamate derivatives for

selective extraction of Co(II) [19, 20] and precon-

centration of various cations [21, 28]and 2-mercap-

tobenzothiazol modified silica gel for online

preconcentration and separation of silver for

atomic absorption spectrometric determinations

[22]. Ammonium hexa-hydroazepin-1-dithiocar-

boxylate (HMDC) loaded on silica gel as solid

phase pre-concentration column for atomic

absorption spectrometry (AAS) and inductively

coupled plasma atomic emission spectrometry

(ICP-AES) was reported [5]. Mercapto modified

silica gel phase was used in preconcentration of

some trace metals from seawater [23]. Sorption of

Co(II) by some sulfur containing complexing

agents loaded on various solid supports [24] was

also reported. 2-Amino-1-cyclopentene-1-dithio-

caboxylic acid (ACDA) for the extraction of

silver(I), Co(II) and palladium(II) [25], 2-[2-tri-

ethoxysilyl-ethylthio]aniline for the selective

extraction and separation of palladium from other

interfering metal ions [26] as well as thiosemicar-

bazide for sorption of different metal ions [27] and

thioanilide loaded on silica gel for preconcentration

of palladium(II) from water [28-33] are also sulfur

contaning silica gel phases. The main goal of the

present work is the development of a fast, sensitive

and efficient way for enrichment and extraction of

trace amounts of Co(II) from aqueous media by

means of a surfactant coated C18 modified with

isocyanate-treated graphite oxides (iGOs). Such a

determination has not been reported in the

literature. The structure of isocyanate treated

graphite oxides (iGOs) is shown in Figure 1. The

chelated ions were desorbed and determined by

FAAS. The modified solid phase could be used at

least 50 times with acceptable reproducibility

without any change in the composition of the

sorbent, iGOs or SDS. On the other hand, in terms

of economy it is much cheaper than those in the

market, like C18 SPE mini-column.

2. EXPERIMENTAL

2.1. Reagents and apparatus

Graphite oxide was prepared from purified natural

graphite (SP-1, Bay Carbon, Michigan, average

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 319-327 Moghimi A et al

320

particle size 30 mL) by the Hummers [2]. Graphite

oxide dried for a week over phosphorus pentoxide

in a vacuum desiccators before use. 4-Isocyanato-

benzenesulfonyl azide was prepared from

4-carboxybenzenesulfonyl azide via a published

procedure [17]. All solutions were prepared with

doubly distilled deionized water. C18 powder for

chromatography with diameter of about 50 µm

obtained from Katayama Chemicals. It was

conditioned before use by suspending in 4 M nitric

acid for 20 min, and then washed two times with

water. Sodium dodecyl solfate (SDS) obtained from

Merck and used without any further purification.

2.2. Synthetic procedures

2.2.1. Preparation of isocyanate-treated graphite

oxides (iGOs)

In a typical procedure, graphite oxide (50 mg) was

loaded into a 10 mL round bottom flask equipped

with a magnetic stir bar and anhydrous DMF

(5 mL) was then added under nitrogen to create an

inhomogeneous suspension. The organic isocyanate

(2 mmol) was next added and the mixture was

allowed to stir under nitrogen for 24 h [17]. (In the

case of solid isocyanates, both the isocyanate and

graphite oxide were loaded into the flask prior to

adding DMF.) After 24 h the slurry reaction mixture

was poured into methylene chloride (50 mL) to

coagulate the product. The product was filtered,

washed with additional methylene chloride

(50 mL), and dried under vacuum.

2.2.2. Column preparation

IGOs (40 mg) were packed into an SPE mini-

column (6.0 cm × 9 mm i.d., polypropylene). A

polypropylene frit was placed at each end of the

column to prevent loss of the adsorbent. Before use,

0.5 mol L-1 HNO3 and (double) distilled water were

passed through the column to clean it.

2.3. Apparatus

The pH measurements were conducted by an ATC

pH meter (EDT instruments, GP 353) calibrated

against two standard buffer solutions of pH 4.0 and

9.2. Infrared spectra of iGOs were carried out from

KBr pellet by a Perkin-Elmer 1430 ratio recording

spectrophotometer. Atomic absorption analysis of

all the metal ions except Zn(II) were performed

with a Perkin-Elmer 2380 flame atomic absorption

spectrometer. Zn(II) determinations were per-

formed by a Varian Spect AA-10. Raman spec-

trophotometer analysis was performed with a

Perkin-Elmer.

2.3.1. Preparation of admicell column

To 40 mL of water containing 1.5 g of C18, 150 mg

of the above iGOs was loaded after washing

acetone, 1 mol L-1 HNO3 solution and water,

respectively, solution was added. The pH of the

suspension was adjusted to 2.0 by addition of 4 M

HNO3 and stirred by mechanical stirrer for 20 min.

Then the top liquid was decanted (and discarded)

and the remained C18 was washed three times with

water, then with 5 mL of 4 M HNO3 and again three

times with water. The prepared sorbent was

transferred to a polypropylene tube (i.d 5 mm,

length 10 mm). Determination of Co2+ contents in

working samples were carried out by a Varian

spectra A.200 model atomic absorption spectro-

meter equipped with a high intensity hallow

cathode lamp (HI-HCl) according to the recommen-

dations of the manufacturers. These characteristics

are tabulated in (Table 1). A metrohm 691 pH meter

equipped with a combined glass calomel electrode

was used for pH measurements.

Table 1: The operational conditions of flame for

determination of Cobalt.

2.3.2. Procedure

The pH of a solution containing 100 ng of Co(II)

was adjusted to 2.0. This solution was passed

through the admicell column with a flow rate of

5 mL min-1. The column was washed with 10 mL of

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 319-327Moghimi A et al

321

Slit width 0.7 nm

Operation current of HI-HCL 10 mA

Resonance fine 283.3

Type of background correction Deuterium lamp

Type of flame Air/acetylene

Air flow 7.0 mL.min-1

Acetylene flow 1.7 mL.min-1

water and the retained ions were desorbed with

1 mL of 4 M HNO3 with a flow rate of 2 mL

min-1. Desorption procedure was repeated 3 more

times. All the acid solutions (4 mL all together)

were collected in a 10 mL volumetric flask and

diluted to the mark with water. The concentrations

of Cobalt in the solution were determined by FAAS

at 283.3.

2.3.3. Determination of cobalt in water samples

Polyethylene bottles, soaked in 1 M HNO3

overnight, and washed two times with water were

used for sampling. The water sample was filtered

through a 0.45 µm pores filter. The pH of a 1000

mL portion of each sample was adjusted to 2.0 (4 M

HNO3) and passed through the column under a flow

rate of 5 mL min-1. The column was washed with

water and the ions were desorbed and determined as

the above mentioned procedure.

2.3.4. Speciation of cobalt in water samples

This procedure is reported in several articles. The

method has been evaluated and optimized for

speciation and its application on complex mixtures

[26-29]. The chelating cation exchanger (Chelex-

100) and anion exchanger, Dowex 1X-8 resins were

washed with 1 M HCl, water, 1 M NaOH and water

respectively. 1.2 g of each resin was transfered to

separate polyethylene columns. Each column was

washed with 10 mL of 2 M HNO3 and then 30 mL

of water. The C18 bounded silica adsorber in a

separate column was conditioned with 5 ml of

methanol, then 5 mL of 2 M HNO3 and at the end

with 20 mL of water. 5 mL of methanol was added

on top of the adsorber, and passed through it until

the level of methanol reached just the surface of the

adsorber. Then water was added on it and

connected to the other two columns. A certain

volume of water sample was filtered through a

0.45 m filter and then passed through the three

columns system, Dowex 1X-8, RP-C18 silica

adsorber and Chelex-100 respectively. The columns

were then separated. The anion and cation

exchanger columns were washed with 10 mL of 2

M HNO3 and the C18 column with 10 mL of 1 M

HCl. The flow rate of eluents was 1 mL min-1. The

Cobalt content of each eluted solution was

determined by FAAS.


The treatment of GO with organic isocyanates can

Cobalt to the derivatization of both the edge

carboxyl and surface hydroxyl functional groups

via formation of amides [20] or carbamate esters

[21], respectively (Figure 1a). The chemical

changes occurring upon treatment of GO with

isocyanates can be observed by FT-IR spectroscopy

as both GO and its isocyanate-treated derivatives

display characteristic IR spectra. Figure 1b

illustrates the changes occurring in the FT-IR

spectrum of GO upon treatment with phenyl

isocyanate (for FT-IR spectra of all iGO derivatives

see Electronic Supporting Information (ESI)). The

most characteristic features in the FT-IR spectrum

of GO are the adsorption bands corresponding to

the C=O carbonyl stretching at 1733 cm-1, the O-H

deformation vibration at 1412 cm-1, the C-OH

stretching at 1226 cm-1, and the C-O stretching at

1053 cm-1 [5,8,12]. Besides the ubiquitous O-H

stretches which appear at 3400 cm-1 as a broad and

intense signal (not shown), the resonance at

1621 cm-1 can be assigned to the vibrations of the

adsorbed water molecules, but may also contain

components from the skeletal vibrations of

un-oxidized graphitic domains [5, 22, 23].

Upon treatment with phenyl isocyanate, the

C=O stretching vibration at 1733 cm-1 in GO

becomes obscured by the appearance of a stronger

absorption at 1703 cm-1 that can be attributed to the

carbonyl stretching vibration of the carbamate

esters of the surface hydroxyls in iGO. The new

stretch at 1646 cm-1 can be assigned to an amide

carbonyl-stretching mode (the so-called Amide I

vibrational stretch). The new band at 1543 cm-1 can

originate from either amides or carbamate esters

and corresponds to the coupling of the C-N

stretching vibration with the CHN deformation

vibration (the so-called Amide II vibration) [24].

Significantly, the FT-IR spectra of iGOs do not

contain signals associated with the isocyanate


322

group (1275-1263 cm-1), indicating that the

treatment of GO with phenyl isocyanate results in

chemical reactions and not mere absorption/interca-

lation of the organic isocyanate [30].

3.1. Stability studies

The stability of the newly synthesized iGO phases

was performed in different buffer solutions (pH 1,

2, 3, 4, 5, 6 and 0.1 M sodium acetate) in order to

assess the possible leaching or hydrolysis

processes. Because the metal capacity values

determined in Section 3.2 revealed that the highest

one corresponds to Co(II)s, this ion was used to

evaluate the stability measurements for the iGO

phase [14].

The results of this study proved that the iGO is

more resistant than the chemically adsorbed analog

especially in 1.0, 5.0 and 10.0 M hydrochloric acid

with hydrolysis percentage of 2.25, 6.10 and 10.50

for phase, respectively. Thus, these stability studies

indicated the suitability of phase for application in

various acid solutions especially concentrated

hydrochloric acid and extension of the experimen-

tal range to very strong acidic media which is not

suitable for other normal and selective chelating ion

exchangers based on a nano poly-meric matrix [9].

Finally, the iGO phases were also found to be stable

over a range of 1 year during the course of this work

the iGO is insoluble in water. Primary investiga-

tions revealed that surfactant coated C18 could not

retain Co(II) cations, but when modified with the

iGO retains these cations selectively. It was then

decided to investigate the capability of the iGO as a

ligand for simultaneous preconcentration and

determination of Cobalt on admicell. The C18

surface in acidic media (1<pH<6) attracts protons

and becomes positively charged. The hydrophyl

part of SDS (-SO3-) is attached strongly to these

protons. On the other hand, the iGO are attached to

hydrophobe part of SDS and retain small quantities

of metallic cations [22].

3.2. Effect of pH in extraction

The effect of pH of the aqueous solution on the

extraction of 100 ng of each of the cations Co(II)

was studied in the pH rang of 1-10. The pH of the

solution was adjusted by means of either 0.01 M

HNO3 or 0.01 M NaOH. The results indicate that

complete chelation and recovery of Co(II) occurs in

pH range of 2-4 and that of in 2-8 and are shown in

Figure 2. It is probable that at higher pH values, the

cations might be hydrolysed and complete

desorbeption does not occur. Hence, in order to

prevent hydrolysis of the cations and also keeping

SDS on the C18, pH=2.0 was chosen for further

studies.


323

Figure 1: (a) Proposed reactions during the isocyanate treatment of GO where organic isocyanates react with the hydr-

oxyl (left oval) and carboxyl groups (right oval) of graphene oxide sheets to form carbamate and amide functionalities,

respectively. (b) FT-IR spectra of GO and phenyl isocyanate treated GO.

Figure 2: Extraction percentage of Co(II) against pH.

3.3. Effect of flow rates of solutions in extraction

Effect of flow rate of the solutions of the cations on

chelation of them on the substrate was also studied.

It was indicated that flow rates of 1-5 mL min-1

would not affect the retention efficiency of the

substrate. Higher flow rates cause incomplete

chelation of the cations on the sorbent. The similar

range of flow rate for chelation of cations on

modified C18 with SDS and an iGO has been

reported in literature [21, 22]. Flow rate of

1-2 mL min-1 for desorption of the cations with 4

mL of 4 M HNO3 has been found suitable. Higher

flow rates need larger volume of acid. Hence, flow

rates of 5 mL min-1 and 2 mL min-1 were used for

sample solution and eluting solvent throughout

respectively.

3.4. Effect of the iGO quantity in extraction

To study optimum quantity of the iGO on

quanti-tative extraction of Cobalt, 50 mL portions

of solutions containing 100 ng of each cation were

passed through different columns the sorbent of

which were modified with various amounts,

between 10-50 mg of the iGO. The best result was

obtained on the sorbent which was modified with

40 mg of the iGO.

3.5. Figures of merit

The breakthrough volume is of prime importance

for solid phase extractions. Hence, the effect of

sample volume on the recovery of the Co (II) was

studied. 100 ng of each cation was dissolved in 50,

100, 500 and 1000 mL of water. It was indicated

that in all the cases, chelation and desorption of the

cations were quantitative. It was then concluded

that the breakthrough volume could be even more

than 1000 mL. Because the sample volume was

1000 mL and the cations were eluted into 10 mL

solution, the enrichment factor for both cations is

100, which is easily achievable. In other experiment

for maximum capacity of 1.5 g of the substrate was

determined as follow; 500 mL of a solution

containing 50 mg of each cation was passed

through the column. The chelated ions were eluted

and determined by FAAS. The maximum capacity

of the sorbent for three individual replecates was

found to be 15.2±0.8 µg of each cation. The limit of

detection (3σ) for the catoins [30] was found to be

3.20 ng.L-1 for Cobalt ions. Reproducibility of the

method for extraction and determination of 100 ng

of each cation in a 50 mL solution was examined.

Results of seven individual replicate measurements

indicated 2.85%.

3.6. Effect of foreign ions

Effect of foreign ions was also investigated on the

measurements of Cobalt. Here a certain amount of

foreign ion was added to 50 ml of sample solution

containing 100 ng of each Co(II) with a pH of 2.5.

The amounts of the foreign ions and the

percentages of the recovery of Cobalt are listed in

Table 2. As it is seen, it is possible to determine

Cobalt without being affected by the mentioned

ions. According to the Table 2 and comparison

between the amount of cobalt (ng) and foreign ions

(mg), recover trace of cobalt ions.

3.7. Analysis of the water samples

The prepared sorbent was used for analysis of real

samples. To do this, the amounts of Cobalt were

determined in different water samples namely:

distilled water, tap water of Tehran (Tehran, taken

after 10 min operation of the tap), rain water

(Tehran, 25 January, 2013), Snow water (Tehran, 7

February, 2013), and two synthetic samples

containing different cations. The results are


324

tabulated in Table 3. As it is seen, the amounts of

Cobalt added to the water samples are extracted and

determined quantitatively which indicates accuracy

and precision of the present method.

Separation and speciation of cations by three

columns system is possible to preconcentrate and at

the same time separate the neutral metal

complexes of iGO, anionic complexes and free ions

from each other by this method [27]. Water samples

were passed through the three connected columns:

anoin exchanger, C18 silica adsorber and chelating

cation exchanger. Each species of Cobalt is retained

in one of the columns; anionic complexes in the

first column, neutral complexes of iGO in the

second, and the free ions in the third. The results of

passing certain volumes of different water samples

through the columns are listed in Table 4.

According to the results, it is indicated that Cobalt

present only as cations. On the other hand the t-test

comparing the obtained mean values of the present

work with those published indicate no significant

difference between them. We have proposed a

method for determination and preconcentration of

Co in water samples using surfactant coated C18

impregnated with a Sciff's base. The proposed

method offers simple, highly sensitive, accurate and

selective method for determination of trace

amounts of Co(II) in water samples.

ACKNOWLEDGMENTS

The author wish to thank the Chemistry Department

of Varamin (Pishva) branch, Islamic Azad

University for financial support.


325

Table 2: Effect of foreign ions on the recovery of 100 ng of Co.

a: Values in parenthesis are CVs based on three individual replicate measurements.

Diverse ion Amounts taken Found % Recovery %

(mg) added to 50 mL Determination of Co2+ of Co2+ ion

Na+ 92.4 1.19(2.9)a 98.3(1.9)

K+ 92.5 1.38(2.1) 98.9(2.2)

Mg2+ 14.5 0.8(1.8) 98.6(2.7)

Ca2+ 28.3 1.29(2.0) 95.4(1.9)

Sr2+ 3.42 2.81(2.2) 98.2(2.1)

Ba2+ 2.66 3.16(2.4) 98.3(2.0)

Mn2+ 2.64 1.75(2.3) 98.5(1.8)

Ni2+ 2.65 2.0(2.14) 98.4(2.4)

Zn2+ 2.74 1.97(2.1) 98.7(2.2)

Cd2+ 2.53 1.9(2.0) 98.8(2.8)

Bi3+ 2.55 2.7(1.4) 98.4(2.7)

Cu2+ 2.46 2.81(2.3) 97.7(2.5)

Fe3+ 2.60 3.45(2.4) 96.6(2.8)

Cr3+ 1.70 2.92(2.2) 97.3(2.4)

UO2+ 2.89 1.3(2.2) 98.3(2.2)

NO3- 5.8 2.3 (2.3) 98.4(2.6)

CH3COO- 5.0 2.2(2.6) 94.5(2.2)

SO42- 5.0 2.9(3.0) 98.7(2.1)

CO32- 5.6 1.8(2.5) 96.3(2.5)

PO43- 2.5 2.1(2.0) 98.9(2.0)

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Werho D.B., Anal. Chem., 48(1976), 67.

2. Jones J.S., Harrington D.E., Leone B.A.,

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Anal. Chim. Acta, 248(1991), 241.

5. Alexandrova A., Arpadjan S., Analyst,

118(1993), 1309.


326

Table 3: Recovery of Co (II) contents in different water samples.

a: Values in parenthesis are CVs based on three individual replicate measurements.

Table 4: Results of speciation of Co2+ in different samples by three columns system.

a: This was a solution containing 0.1 g of Co(II) in 1000 mL of distilled water.

b: Values in parenthesis are CVs based on three replicate analysis. The samples are the same as those mentioned in Table 4.

Amounts Found (µg) %Recovery

added (µg)

Sample distilled water (100 mL) - - -

0.005 0.043(2.40)a 96

Tap water (100 mL) 0.100 0.094(2.90) 97

- 0.015(3.54) -

Snow water (50 mL) 0.050 0.066(2.42) 96

- 0.048(2.25) -

Rain water (100 mL) 0.100 0.157(2.65) 98.0

- 0.042(2.25) -

Synthetic sample 1 Na+, Ca2+,

Fe3+, Co2+, Cr3+, Hg2+, 1 mg L-1 0.100 0.106(2.45) 98

- - -

Synthetic sample 2 K+ Ba2+,

Mn2+, Cd2+, Ni2+, Zn2+ 0.100 0.104(2.46) 0.100

1 mg L-1 of each cation - - -

0.100 0.103(2.73) 99

Tap water (1000 mL) water sample (1000 mL)a River Water (50 mL)

Column Co(II) ( g) Co(II) ( g) Co(II) ( g)

Dowex 1X8 - - -

Silica C-18 - - -

Chelex-100 0.012 (4.4)b 0.104 (2.3) 0.103 (2.2)

6. Arpadjan S., Vuchkova L., Kostadinova E.,

Analyst, 122(1997), 243.

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47(1975), 1612.

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327

Carbon nanotubes (CNTs) are tubular structures,

formed by carbon atoms with the diameter in range

of one to tens nanometer. After Iijima's discovery in

1991 [1] an extensive academic and industrial

researches have been conducted on CNT, because

of its interesting electrical, thermal and mechanical

properties [2-5].

However, one of the most important issues in

this field that must be well addressed is mass

production and high cost production of CNT. Thus,


Numerical Study of Furnace Temperature and Inlet

Hydrocarbon Concentration Effect on Carbon Nanotube

Growth Rate

Babak Zahed1, Tahereh Fanaei Sheikholeslami2*, Amin Behzadmehr3, Hossein Atashi4

1 M.Sc., Mechanical Engineering Department, University of Sistan and Baluchestan, Zahedan, Iran

2 Assistant Professor, Electrical and Electronic Department, University of Sistan and Baluchestan,

Zahedan, Iran

3 Professor, Mechanical Engineering Department, University of Sistan and Baluchestan, Zahedan, Iran

4 Professor,Chemical Engineering Department, University of Sistan and Baluchestan, Zahedan, Iran


Chemical Vapor Deposition (CVD) is one of the most important methods for producing Carbon

Nanotubes (CNTs). In this research, a numerical model, based on finite volume method, is

investigated. The applied method solves the conservation of mass, momentum, energy and

species transport equations with aid of ideal gas law. Using this model, the growth rate and

thickness uniformity of produced CNTs, in a horizontal CVD reactor, at atmospheric pressure, are

calculated. The furnace temperature and inlet hydrocarbon concentration variations are studied

as the effective parameters on CNT growth rate and thickness uniformity. It is indicated that by

increasing the furnace temperature, the CNT growth rate increases, while the thickness

uniformity shows decreasing. The results show that the growth rate of produced CNTs could be

improved by increasing the inlet hydrocarbon concentration, but the latter causes more non

uniformity on the CNTs height.

Keyword: CNT growth rate; CVD; Furnace temperature; Hydrocarbon concentration; Numerical

analysis.

ABSTRACT

1. INTRODUCTION



the researchers have examined different methods to

produce high quality CNTs, easier and with lower

cost. Among the various methods for CNT

production, Chemical Vapour Deposition (CVD)

due to its handling procedure, simplicity and

possibility of high production rate is vastly used

[6-8]. CNT growth rate in an atmospheric pressure

CVD (APCVD) reactor depends on various

parameters such as inlet flow rate, deposition

temperature, inlet hydrocarbon concentration and

catalyst types [9-11]. To obtain desired CNT growth

rate and acceptable thickness uniformity, numerical

analyses are used prior to experimental studies.

Numerical studies can be used to investigate the

effect of various conditions and also to help

understanding the details of processes for well

interpretation of different parameters such as

growth rate, transport rates and reaction

mechanisms. In this regard, Grujicic et al. [12]

proposed a model, where the detailed gas-phase

reactions of CH4, surface reactions for CNT growth

were contained. The growth rates of CNTs and also

distribution of velocity, temperature and

concentration under different growth conditions,

were investigated. Also, Endo et al. [13] estab-

lished a CFD model that predicted the production

rate of nanotubes via catalytic decomposition of

xylene in a CVD reactor. They predicted

velocity and temperature distributions and

concentration distributions in the reactor. Using this

model, they calculated and measured the total

production rates with various inlet xylene

concentrations. Similar works were done with the

other researchers to model the CNT growth rate

with different deposition conditions [14-16].

The horizontal quartz tube reactor is a simple

system which is vastly used in catalytic CVD. Two

carrier gases that can be used are argon and nitrogen

[16-18] with certain amount of hydrogen that added

into carrier gas to prevent the oxidation of catalyst

particles and the formation of other carbon

impurities during the CNT production. The carbon

sources can be in gas state (methane, acetylene,

ethylene, etc) [19-25] or in liquid state (alcohol,

benzene, toluene, xylene, etc.) [26-28].

In this research, a catalytic APCVD technique

for production of CNT is modelled, numerically.

The inlet gas mixture includes xylene as carbon

source and a mixture of argon with 10% hydrogen,

as carrier gas. The effects of furnace temperature

and inlet hydrocarbon concentration on growth rate

and thickness uniformity has been studied and

discussed.

2. PROBLEM DESCRIPTION

Ferrocene vapor with carrier gas (argon) is entered

into a horizontal reactor that works at atmosphere

pressure. A uniform layer of iron atoms on the

furnace wall is considered as a catalyst for surface

reaction. Therefore, inlet gas mixture including

xylene (C8H10) and carrier gas (argon with 10%

hydrogen), enters into reactor continuously. Reactor

processes is modeled with two gas phase reactions

and four surface reactions. These reactions release

carbon atoms to produce CNTs on the catalyst

particle that layout on the reactor hot walls.

3. GOVERNING EQUATIONS

Considering two dimensional axisymmetric model

and steady state process, the governing equations

are as follow:

Conservation of Mass:

(1)

Conservation of Momentum:

(2)

For Newtonian fluids such as existent gases in

CVD reactors, viscous stress tensor is as follows:

(3)

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1(2013), 329-336 Zahed B et al

330

( )Vt

ρρ

⋅−∇=∂∂

( ) gPVVt

Vρτρ

ρ+∇−⋅∇+⋅−∇=

∂∂

( ) ( ) IVkVVT

⋅∇

−+

∇+∇= µµτ

3

2

These equations are coupled with energy

equation. Energy Equation:

(4)

and Species Transport Equation:

(5)

These equations solve subject to the following

boundary conditions:

• Reactor walls are impermeable and no slip

condition is considered for the velocity at walls.

• Constant temperature at the heated walls and

zero heat flux at the adiabatic walls.

• At the nonreactant walls, mass flux vector for

each species must be zero.

• Due to surface reactions, net mass production

rate Pi for ith gas specie at the surface of

furnace is:

(6)

Thus the normal velocity on the surface of

furnace can be express by:

(7)

Total net mass flux of ith specie, normal to the

surface of furnace must be equal to Pi. Thus:

(8)

4. NUMERICAL ANALYSIS

The governing equations are discretized using finite

volume approach. SIMPLE algorithm is adopted for

the pressure-velocity coupling.

Physical properties (viscosity, thermal

conductivity and specific heat capacity) for each

species are assumed to be thermal dependent [29].

These properties for the gas mixture were obtained

using the mixing law.

Non-uniform structured grid distribution that is

refined near walls is considered. Convergence

criterion for energy equation is 10-10 and for other

equations (continuity, momentum and species

transport) is 10-6.

5. MODELING

Tubular hot wall reactor that is worked at

atmosphere pressure is modelled. It has 34 mm

diameter and 1.5 meter length with 17 mm

inlet/outlet diameter (Figure 1). Inlet gas mixture

including xylene and argon with 10% hydrogen as

carrier gas enters into the reactor. Its temperature

and inlet mass flow rate are 300 K and 685 sccm

(standard cubic centimeters per minute)

respectively. Then inlet gas mixture is heated in

preheater up to 513 K and then enters to the furnace

region.

Figure 1: CVD Reactor Scheme.

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 329-336Zahed B et al

331

( )g

k

g

kik

N

i

K

k

i RRvH −= =

−∑∑1 1

( ) ( ) +∇−−∇=∂

∂ii

i JVt

.. ωρρω

s

l

L

l

ilii RmP ∑=

=1

σ

s

l

L

l

il

N

i

i RmVn ∑∑==

=11

1. σ

ρ

( ) −∇+

∇∇ ∑∑

==i

N

i i

iN

i

i

i

T

i Jm

Hf

m

DRT .ln.

11

( )g

k

g

k

k

k

iki RRvm −=

−∑1

( ) s

l

L

i

ili

T

i

c

ii RmJJVn ∑=

=++1

. σρω

( )+∇∇+∇−=∂

∂TTVC

t

TC pp λρ

ρ.).(

Preheater zone is considered from 20 to 50 cm and

furnace zone from 60 to 125 cm from the inlet

section. Except of preheater and furnace walls that

are isothermal walls, other walls are considered to

be adiabatic. A schematic of the considered

modelled is shown in Figure 1.

Reactions that is used in this model, is shown in

Tables 1 and 2. There are two gas phase reactions

and four surface reactions apply for this model. All

reactions are irreversible. Also kinetic rate

coefficients determined with no catalyst deactiva-

tion assumption.

Table 1: Gas phase reaction [13].

Table 2: Surface reactions [13].

PEF= Pre-Exponential Factor, AE= Activation Energy,

TE= Temperature Exponent

6. VALIDATION OF NUMERICAL RESULTS

Non-uniform structured grid, that is refined at the

near walls where the gradient of the parameters are

important, is selected. Several different grid

distributions have been tested to ensure the results

are grid independence. The selected grid number is

10998. In addition to show the accuracy of the

results, comparisons are made between the obtained

numerical results and numerical results of Endo

et al. [13] for two different xylene concentrations. It

is shown in Figure 2, as seen good concordance

between the results is obtained (Maximum of error

5%). Thus the numerical procedure is reliable and

can well predict the process throughout the reactor.

Figure 2: Validation with Endo et al. work [13].

Figure 3: Local growth rate in furnace region with

different furnace temperature.


In this research the effects of furnace temperature

and inlet hydrocarbon concentration on CNT

growth rate and thickness uniformity of produced

CNT has been studied. For a given inlet

hydrocarbon concentration (3750 ppm) the effects

of different furnace temperature on the growth rate

of CNTs is shown in Figure 3. As seen, in general,

CNT growth rate increases with increasing the

furnace temperature. At low furnace temperature

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 329-336 Zahed B et al

332

Gas Phase Reactions PEF AE TE

C8H10+H2→ C7H8+CH4 2.512e+8 1.674e+8 0

C7H8 + H2 →C6H6+CH4 1.259e+11 2.2243e+8 0

Gas Phase Reactions PEF AE TE

C8H10 → 8C + 5H2 0.00034 0 0

C7H8 → 7C + 4H2 0.00034 0 0

C6H6 → 6C + 3H2 0.00034 0 0

CH4 → C + 2H2 0.008 0 0

Figure 4: Reaction product concentration throughout the

reactor at different furnace temperature (a) 1000 K, (b)

1100 K and (c) 1150 K.

(1000 K), the local growth rate monotonically

decreases along the reactor length. However, for

higher furnace temperature (1100 K and 1150 K) it

is increased up to 0.2 m along the furnace length

(between 0.6 to 0.8 meter from inlet as seen in

Figure 3) then the CNT growth rate decrease with

different gradient. To understand the reasons for

such variations Figure 4 is presented. Figure 4

shows the effect of furnace temperature on the

reactions products throughout the furnace. The

balance of different products materials based on the

surface reactions (Table 2) along the furnace length

clearly explains the variations of the CNTs growth

rate at different furnace temperature. It is known

that the Arrhenius equation is used to quantify the

temperature dependence of a reaction rate. Thus to

better see the reason for such variations on the

reactant inside of the reactor is presented (see

Figure 5).

Figure 5: Temperature distribution in reactor with

different furnace temperatures (a) 1000 K, (b) 1100 K and

(c) 1150 K.

As seen in the unheated region (0 to 0.6 m) the

temperature profiles are similar for different

furnace temperatures. However, increasing the

furnace temperature augments the reactor crosswise

temperature more rapidly. The latter consumes

more C8H10 through a gas reaction (see Table 1)

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 1, No. 3 (2012), 329-336Zahed B et al

333

(a)

(b)

(c)

(a)

(b)

(c)

and produces more C7H8, C6H6 and CH4. These

productions, through the surface reactions causes to

growth of CNTs on the catalyst layer (on the

furnace wall). The contour of variations of the

normalized C8H10 (with inlet C8H10) is shown in

Figure 6. This clearly shows the rate of C8H10

consumption entire the reactor. Increasing the

furnace temperature augments the rate of C8H10

consumption more rapidly.

Figure 6: Non dimensional inlet hydrocarbon (xylene)

concentration in reactor with various furnace

temperatures (a) 1000, (b) 1100 and (c) 1150 K.

Figure 7: Local growth rate in furnace region with

different inlet hydrocarbon concentrations.

Figure 8: Material concentrations on the reactor axis for

different inlet hydrocarbon concentrations (a) 1000 ppm,

(b) 3000 ppm and (c) 5000 ppm.

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 329-336 Zahed Bet al

334

(a)

(b)

(c)

(a)

(b)

(c)

To see the effects of inlet hydrocarbon

concentrations on the CNTs growth rate Figure 7 is

presented for a given furnace temperature (975 K).

As expected by increasing the inlet hydrocarbon

concentrations the rate of CNTs growth rate

augments along the reactor length. However the

variation of CNT growth rate along the furnace

increases with the inlet hydrocarbon concentra-

tions. It is seen that lower hydrocarbon concentra-

tions results more uniform CNTs growth rate. This

could point out that using higher concentration than

the necessary amount of hydrocarbon may increase

non-uniformity of the CNTs.

As seen in Figure 8 the rate of consumption and

production of different materials in the reactor are

similar. However, in the case of 1000 ppm of inlet

hydrocarbon the amount of C8H10 and C7H8 that

exit the reactor are 400 ppm and 75 ppm

respectively (Figure 8a). Increasing the inlet

hydrocarbon concentration to 3000 ppm, 1300 ppm

and 200 ppm of C8H10 and C7H8 exit the reactor

(Figure 8b). While for higher inlet hydrocarbon

concentration (5000 ppm) 2100 ppm of C8H10 and

300 ppm of C7H8 is remained at the reactor outlet.

8. CONCLUSIONS

CNT deposition process in an APCVD reactor was

modeled and discussed. Results indicated that

increasing furnace temperature has a positive effect

on CNTs growth rate but decreases their

uniformity. This occurrence was related to the

different temperature distributions in three

mentioned cases and so different material

concentrations.

The effect of inlet hydrocarbon concentration on

growth rate and uniformity of produced CNTs was

considered also. Results showed that increasing the

inlet hydrocarbon concentration leads to more

growth rate due to more availability of carbon

source in reactor and near reactant surfaces,

particularly. In addition, increasing the inlet

hydrocarbon concentration causes decreasing the

CNTs uniformity due to the various carbon source

consumption and productions.

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NOMENCLATURE

Cp Specific heat of the gas mixture (J.kg-1.K-1)

DT Multicomponent thermal diffusion

coefficient (kg.m-1.s-1)

f Species mole fraction

Gravity vector

H Molar enthalpy (J.mole-1)

I Unity tensor

Diffusive mass flux vector (kg.m-2.s-1)

mi Mole mass of the ith species (kg.mole-1)

Unity vector normal to the inflow/outflow

opening or wall

P Pressure (pa)

R Universal gas constant= 8.314 (J.mole.K-1)

Rk Forward reaction rate of the kth gas phase

reaction (mole.m-3.s-1)

R-k Reverse reaction rate of the kth gas phase

reaction (mole.m-3.s-1)

Rls Reaction rate for the lth surface reaction

(mole.m-2.s-1)

t Time (s)

T Temperature (K)

Velocity vector (m.s-1)

Greek Symbols

κ Volume viscosity (kg.m-1.s-1)

λ Thermal conductivity of the gas mixture

(W.m-1.K-1)

µ Dynamic viscosity of the gas mixture

(kg.m-1.K-1)

νik Stoichiometric coefficient for the ith

gaseous species in the kth gas phase

reaction

ρ Density (kg.m-3)

σil Stoichiometric coefficient for the ith

gaseous species in the lth surface reaction

τ Viscous stress tensor (N.m-2)

ω Species mass fraction

Subscripts

i,j With respect to the ith/jth species

Superscripts

c Due to ordinary diffusion

T Due to thermal diffusion

Int. J. Bio-Inorg. Hybd. Nanomat., Vol. 2, No. 1 (2013), 329-336 Zahed B et al

336

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