International Journal of Bio-Inorganic Hybrid Nanomaterials by ijbihn

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

International Journal of Bio-Inorganic Hybrid Nanomaterials

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 2

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

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

Assistant Professor, Department of Chemistry, Science and Research Branch, Islamic Azad University, Tehran Iran Received: 20 November 2012; Accepted: 28 Jannuary 2013

ABSTRACT 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 characterized 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.

1. INTRODUCTION 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 (*) Corresponding Author - e-mail: Rostam_Somayyeh@yahoo.com

of their broad applications in several technological fields including permanent magnets, magnetic fluids, magnetic drug delivery, and high density

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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),

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Ammonia solution (25%), cetyltrimethylammonium 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 70째C 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 50째C 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 40째C for 24 h [20].

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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

(d)

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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 2Î¸= 22-25Â° (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

(b)

(c)

(b)

(a)

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.

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(b)

(a)

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

(b)

(a)

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

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

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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

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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

(a)

(b)

(c)

(d)

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.

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

CoFe2O4

CoFe2O4/SiO2

Element

wt%

At%

Fe2O3

64.89

46.45

CoO

35.11

53.55

Fe2O3

46.58

30.68

CoO

28.61

17.86

SiO2

5.62

7.36

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

Mn0.02Co0.98Fe2O4

Element

wt%

At%

46.83

35.32

28.49

20.30

13.44

35.28

9.12

6.98

100

Total

Mn0.02Co0.98Fe2O4/SiO2

48.88

41.84

28.41

23.o4

8.31

7.23

4.07

12.17

8.42

14.32

100

Total

silica coated Mn0.02Co0.98Fe2O4/SiO2 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, that confirms the formation of manganese ferrite.

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

nanostructures. These images clearly show also well distributed particles of Mn doped nanostructures 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.

3.3. FESEM and EDAX Figure 4 (a, b) represents FESEM images of CoFe2O4 and CoFe2O4/SiO2 core-shell nanoparticles. 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

3.4. Magnetic properties of CoFe2O4 nanoparticles Figure 5 (a, b and c) and Table 3 represent magnetic field dependent magnetization parameters, M(H) for CoFe2O4 in the size of 13.26, 21.06

Table 3: Magnetic parameters of CoFe2O4 in different sizes and CoFe2O4 /SiO2 nanoparticles. Remanent Saturation magnetization magnetization Ms(emu/g) Mr(emu/g)

CoFe2O4 nanoparticles

Average particle size XRD (nm)

CoFe2O4

13.26

32.1

CoFe2O4

21.06

CoFe2O4 CoFe2O0.84 /SiO2

276

Coercivity Hc (Oe)

Remanence ratio (Mr/Ms)

Ms emu/g (Bulk)

Mr/Ms (Bulk)

240

80.8

0.84

12.74

2000

0.47

80.8

0.84

10.58

4.9

2300

0.46

80.8

0.84

15.14

8.5

80.8

0.84

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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.

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= 20002500 (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 ferromagnetic 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 superparamagnetic 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 magnetization, Ms of 35.0 emu/g for a size of 35.30 nm and

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

CoFe2O4 nanoparticles

Average Saturation Remanent particle size XRD magnetization magnetization (nm) Ms(emu/g Mr(emu/g)

Coercivity Hc (Oe)

Remanence ratio (Mr/Ms)

CoFe2O4

10.85

4.9

2500

0.46

Mn0.02Co0.98Fe2O4

35.30

500

0.37

Mn0.2Co0.98Fe2O4/SiO2

3.5

0.75

250

0.21

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.

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 magnetization, 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

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used to tune the magnetic properties of nanoparticles, 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.

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4. CONCLUSIONS

ACKNOWLEDGMENTS

3 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. 3 The synthesis processes explored in this study are simple and easy to achieve the desired particle size distribution. 3 Characterization of the prepared cobalt ferrite and Manganese doped cobalt ferrite nanoparticles were performed using XRD, FTIR and FESM-EDAX technigues. 3 The magnetic properties of the cobalt ferrite and Manganese doped cobalt ferrite nanoparticles evaluated by VSM and the decreases of saturation magnetization with increasing SiO2 coatings were reported earlier separately by authors [24]. 3 Magnetic properties of small ferromagnetic particles such as coercivity, are dominated by two key features: the single domain size and superparamagnetic size. 3 The magnetic measurements on these particles showed strong dependence of the magnetic properties with the particle size. 3 A size limit exist for CoFe2O4 nanoparticles and cobalt ferrite with size greater than 12 nm showed ferromagnetic behavior at room temperature. 3 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. 3 Room temperature magnetic properties strongly depend on (1) doping content (x) (2) particles size and ions distributions. 3 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.

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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International Journal of Bio-Inorganic Hybrid Nanomaterials

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 3

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

M.Sc. Student, Department of Chemistry Engineering, Faculty of Sciences, Arak Azad University, Arak, Iran Received: 28 November 2012; Accepted: 30 Jannuary 2013

ABSTRACT In this work, ZnCaO2 (zinc oxide-calcium oxide) nanocomposite were synthesized at different temperatures (500-700째C) 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 synthesized nanoparticles (at 600째C) 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 characterized 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 chromatographymass 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.

(*) Corresponding Author - e-mail: meysamsadeghi45@yahoo.com

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1. INTRODUCTION One of the first successful applications of nanotechnology was the use of oxides as catalysts for adsorption of toxic sulfurous and organophosphate. 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 nanocomposites via sol-gel pyrolysis method at temperatures 500, 600 and 700°C is reported. Then, we have focused our attention on the nanocomposite 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]. S Cl

(a)

(b)

OCH3

OCH3 CH3

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

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.

DMMP.

2. EXPERIMENTAL 2.1. Materials Zn(NO3)2.6H2O, Ca(NO3)2.4H2O, ethanol, poly

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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

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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 nanocomposite/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).

two tips by heat. Then, 37 µL DMMP, 10 mL n-pentane as solvent and 0.48 g ZnCaO2 nanocomposite 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

Figure 2: The temperature program for GC set.

2.5. Preparation of ZnCaO2 nanocomposite/ DMMP sample For investigation of the reaction ZnCaO2 nanocomposite 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

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 nanocomposites 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

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)

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 3: SEM images of ZnCaO2 nanocomposites, (a) and (b) 500째C, (c) and (d) 600째C, (e) and (f) 700째C with different magnification (15000X and 30000X).

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,

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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.

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(a)

(b)

(a)

(b) (c)

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

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)

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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 nanocomposite (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).

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(a)

(b)

3.5. 31PNMR and FT-IR studies 31PNMR 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, respectively. The intensity of the DMMP area under curve

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(g) (f) (e) (d) (c) (b) (a)

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. Sample a

Time(h) Blank(0)

Adsorbed and Destructed % by ZnCaO2 nanocomposite 100.00

82.25

64.43

56.67

25.15

8.60

00.00

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

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(b)

(a)

Figure 9: FT-IR spectra of 2-CEES/ZnCaO2 nanocomposite (600째C), 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.

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(a)

(c)

(d)

(e)

(f)

(g)

Figure 11:

31PNMR

(b)

(h)

spectra of the adsorption DMMP on the ZnCaO2 nanocomposite at differernt times.

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Table 3: The results of

Sample

31PNMR

Time(h)

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spectra in the presence of different times. Concentration (DMMP) after

DMMP AUC / phosphoric acid blank

reaction(Molar)

AUC

% Adsorption(DMMP) by ZnCaO2 nanocomposite

0(blank)

0.03

199.14

0.0246

163.34

17.98

0.0229

151.93

23.71

0.0188

124.73

37.37

0.0161

106.58

46.48

0.0121

80.25

59.70

0.0114

63.14

67.34

100

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

(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% organophosphosphate molecule was adsorbed. After the reaction, the adsorption of DMMP on the nanocomposite 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.

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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

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

+ S

Cl S 2-CEES

Cl S

Cl-

O -HCl Zn

Zn O

i elim

t duc pro s i ys rol H 2O hyd

on nati

-H2O

uct prod

OH S HEES S EVS

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

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,

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CH3 CH3O

OCH3 P O CH3 OH3C

O Zn O

O O

Mechanism A -CH3OH

Zn O

-H2O

CH3 CH3O

CH3

OCH3

OH3C

CH3

P O Zn O

O O

Mechanism B

Zn O

O O

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

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 (500700째C) 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 (600째C) showed that CaO wt% and the average particles size was 2.05 wt% and 33 nm, respectively. In the other,

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synthesized nanocomposite (500 and 700째C), average particles size is higher. Thus, in this research, the best of temperature for the synthesized nanocomposite ZnCaO2 is 600째C. 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.

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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 adsorbents for biological and chemical contamination", 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).

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International Journal of Bio-Inorganic Hybrid Nanomaterials

The Study of Pure and Mn Doped ZnO Nanocrystals for Gas-sensing Applications Meysam Mazhdi1*, Jabar Saydi1, 2, Faezeh Mazhdi3 1 2

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

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

ABSTRACT 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 nanocrystals 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 360째C and 347째C for ethanol vapor and about 287째C and 335째C 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.

1. INTRODUCTION 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, (*) Corresponding Author - e-mail: meisam.physics@gmail.com

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

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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)

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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 250째C for 3 hours [3, 4]. The nanocrystals were compressed as tablets and were annealed at 500째C 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.

(a)

(b)

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 500째C 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 temperaturecontrolled micro-heater, and was mounted on a refractory stand, so that the temperature of the sample could be adjusted in the 160-430째C 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

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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.

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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].

(a) Figure 2: Gas sensor Electric circuit.

2.3. Characterization Obtained nanocrystals were analyzed by X-ray diffractometer (Scifert, 3003 TT) with Cu-kα radiation, 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 (b)

3.1. XRD analysis Figure 3 shows the XRD patterns of ZnO and

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

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The broadening of the XRD lines is attributed to the nanocrystalline characteristics of the samples, which indicates that the particle size is in nanometer range. Inter planar spacing (d) is evaluated using the relation (1):

1 4  h 2 + hk + k 2  l 2  + 2 =  d 2 3  a2  c

(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]:

Kλ β cos θ

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β cos θ =

kλ + 4ε sin θ D

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.

(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]:

3.3. 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

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

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(3)

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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.

V  Rs =  c − 1 R0  V0 

(4) (a)

Sensor sensitivity for reducer gases defines as relative of sensor electric resistance in air to sensor electric resistance in presence of reducer gas:

Ra Rg

(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:

(b) Figure 5: Conductivity logarithmic graphs (Arrhenius graph) in the presence of air for ZnO (a), ZnO: Mn (b) nanocrystals.

O2→O2 (physical adsorption) O2 (physical adsorption) + 2e-→2OO- + e-→O2-

(6) (7) (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

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band. This leads to an increase in carrier concentration 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).

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.

(a)

(b) (b) Figure 7: Sensitivity versus temperature for 1000 ppm of Figure 6: Sensitivity versus temperature for 1000 ppm of

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

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

Optimum working temperature to ethanol vapor for pure and doped zinc oxide nanocrystals sample is 362째C and 345째C (Figure 6 a, b) and also for gasoline vapor is 285째C and 333째C (Figure 7 a, b)

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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.

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(a)

(b)

Figure 8: Sensor response (Sensitivity) to different

Figure 9: Sensor response (Sensitivity) to different

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

concentration for Gasoline 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

of gasoline vapor achieves to saturation state whereas 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

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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 362째C and 345째C and also for gasoline vapor was 285째C and 333째C 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.

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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.

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International Journal of Bio-Inorganic Hybrid Nanomaterials

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 Received: 19 December 2012; Accepted: 21 February 2013

ABSTRACT 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 throughout 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.

1. INTRODUCTION Shortage of wood resources and natural regeneration 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 composite 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

(*) Corresponding Author - e-mail: htaghiyari@srttu.edu & htaghiyari@yahoo.com

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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 nanoparticles [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-

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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 nanoparticles 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 concentration 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

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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.

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 0.1째C 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

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Figure 3: Temperature at the core section of the medium-density fiberboard mat with five-second intervals (NS= nanosilver content mL/kg).

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.

3. RESULTS AND DISCUSSION 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

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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

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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).

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

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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.

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Wood Wood Prod., DOI 10.1007/s00107-0120644-5, 71(1) (2013), 69.

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International Journal of Bio-Inorganic Hybrid Nanomaterials

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

Assistant Professor, Department of Chemistry, Varamin (Pishva) Branch Islamic Azad University, Varamin, Iran Received: 22 December 2012; Accepted: 25 February 2013

ABSTRACT In this work, we report synthesis of superparamagnetic iron oxide nanoparticles at room temperature using microemulsion template phase consisting of cyclohexane, water, cetyltrimethylammonium 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.

1. INTRODUCTION 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, (*) Corresponding Author - e-mail: af.sharafi@yahoo.com

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

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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.

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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).

2. EXPERIMENTAL 3. RESULTS AND DISCUSSION 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

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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)

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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.

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.

(a)

(b)

Figure 2: Energy-dispersive X-ray spectroscopy Edx result of silica coated Fe3O4 nanoparticles. 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

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

(a)

(b)

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

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(b)

(a)

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

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.

a homogenous particle size and distribution. This result has been confirmed by Dynamic Light Scattering analysis data.

Table 1: EDAX quantification element normalized. Figure 5: Magnetization vs. applied magnetic field for Elements

Wt.%

At.%

95.39

11.29

4.08

68.15

0.53

16.8

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

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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

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properties at 300 K with a saturation magnetization of 65 emu/g for Fe3O4 and 34 emu/g for Fe3O4@SiO2.

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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.

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., 85(1990), 3. 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.

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International Journal of Bio-Inorganic Hybrid Nanomaterials

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

Professer, Department of Chemistry Science and Research Branch Islamic Azad University, Tehran, Iran Received: 1 January 2013; Accepted: 3 March 2013

ABSTRACT 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.

1. INTRODUCTION 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 (*) Corresponding Author - e-mail: faranak.asgari@gmail.com

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

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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. 2.2. Characterization 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 photoluminscence spectrophotometer Ls-50B Perkin Elmer equipped with Xe lamp (位= 320 nm) as an excitation light source. A JEON 360 Transmission Electron Microscope (TEM) operated at 100 W was

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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掳C under helium-gas conditions.

3. RESULTS AND DISCUSSION 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

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

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Figure 2: Fluorescence spectra and absorbtions of CdZnTe (10%) quantum dots prepared at different reaction times.

Figure 3: Fluorescence spectra and absorbtions of CdZnTe (15%) 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 24.0째, 39.2째, 46.3째 and 56.8째 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.

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4. CONCLUSIONS

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

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

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

Figure 6: Band Gap and Size of CdZnTe quantum dots compared at different reaction times.

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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.

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International Journal of Bio-Inorganic Hybrid Nanomaterials

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,

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

Varamin, Iran

Received: 10 January 2013; Accepted: 8 March 2013

ABSTRACT 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 15.2Âą0.8 mg of ions. The limit of detection (3Ď&#x192;) 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.

1. INTRODUCTION 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

(*) Corresponding Author - e-mail: alimoghimi@iauvaramin.ac.ir; kamran9537@yahoo.com

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phase extraction techniques [3, 4]. The application of solid phase extraction technique for preconcentration 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 preconcentrators 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-

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centration of various cations [21, 28]and 2-mercaptobenzothiazol modified silica gel for online preconcentration and separation of silver for atomic absorption spectrometric determinations [22]. Ammonium hexa-hydroazepin-1-dithiocarboxylate (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-dithiocaboxylic acid (ACDA) for the extraction of silver(I), Co(II) and palladium(II) [25], 2-[2-triethoxysilyl-ethylthio]aniline for the selective extraction and separation of palladium from other interfering metal ions [26] as well as thiosemicarbazide 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

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particle size 30 mL) by the Hummers [2]. Graphite oxide dried for a week over phosphorus pentoxide in a vacuum desiccators before use. 4-Isocyanatobenzenesulfonyl 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 Âľm 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.

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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 performed by a Varian Spect AA-10. Raman spectrophotometer 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 spectrometer equipped with a high intensity hallow cathode lamp (HI-HCl) according to the recommendations 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.

2.2.2. Column preparation IGOs (40 mg) were packed into an SPE minicolumn (6.0 cm Ă&#x2014; 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

Slit width

0.7 nm

Operation current of HI-HCL

10 mA

Resonance fine

283.3

Type of background correction Type of flame

Deuterium lamp Air/acetylene

Air flow

7.0 mL.min-1

Acetylene flow

1.7 mL.min-1

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

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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 Âľm 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 (Chelex100) 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 Č?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

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Cobalt content of each eluted solution was determined by FAAS.

3. RESULTS AND DISCUSSION 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

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

group (1275-1263 cm-1), indicating that the treatment of GO with phenyl isocyanate results in chemical reactions and not mere absorption/intercalation 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 experimental 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 investigations 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.

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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

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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

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

Amounts taken (mg) added to 50 mL

Found % Determination of Co2+

Recovery % 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)

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)

5.0

2.2(2.6)

94.5(2.2)

5.0

2.9(3.0)

98.7(2.1)

Ni2+

CH3

COO-

SO422-

5.6

1.8(2.5)

96.3(2.5)

PO43-

2.5

2.1(2.0)

98.9(2.0)

CO3

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

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.

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

Found (µg)

%Recovery

0.005 0.100 -

0.043(2.40)a 0.094(2.90) 0.015(3.54)

96 97 -

0.050

0.066(2.42)

0.048(2.25)

0.100

0.157(2.65)

98.0

0.042(2.25)

0.100

0.106(2.45)

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)

added (µg) Sample distilled water (100 mL) Tap water (100 mL)

Snow water (50 mL)

Rain water (100 mL)

Synthetic sample 1 Na+, Ca2+, Fe3+, Co2+, Cr3+, Hg2+, 1 mg L-1

Synthetic sample 2 K+ Ba2+,

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.

Tap water (1000 mL)

Column

water sample (1000 mL)a

River Water (50 mL)

Co(II) (ȝg)

Dowex 1X8

Silica C-18

0.104 (2.3)

0.103 (2.2)

Chelex-100

0.012

(4.4)b

a: This was a solution containing 0.1 ȝg 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.

REFERENCES 1. Leyden D.E., Luttrell G.H., Nonidez W.K., Werho D.B., Anal. Chem., 48(1976), 67. 2. Jones J.S., Harrington D.E., Leone B.A., Bramdstedt W.R., Atom. Spectrosc., 4(1983), 49.

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3. Nambiar D.C., Patil N.N., Shinde V.M., Fresenius J., Anal. Chem., 360(1998), 205. 4. Caroli C., Alimanti A., Petrucci F., Horvath Z., Anal. Chim. Acta, 248(1991), 241. 5. Alexandrova A., Arpadjan S., Analyst, 118(1993), 1309.

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6. Arpadjan S., Vuchkova L., Kostadinova E., Analyst, 122(1997), 243. 7. Leyden D.E., Luttrell G.H., Anal. Chim., 47(1975), 1612. 8. Gennaro M.C., Baiocchi C., Campi E., Mentasti E., Aruga R., Anal. Chim. Acta, 151(1983), 339. 9. Grote M., Kettrup A., Anal. Chim. Acta, 175(1985), 239. 10. K. Unger, 1979. Porous Silica, Elsevier, Amsterdam. 11. Boudreau S.P., Cooper W.T., Anal. Chem., 61(1989), 41. 12. Kvitek R.J., Evans J.F., Carr P.W., Anal. Chim. Acta, 144(1982), 93. 13. Bruening M.L., Mitchell D.M., Bradshaw J.S., Izatt R.M., Bruening R.L., Anal. Chem., 63(1991), 21. 14. Mahmoud M.E., Talanta, 45(1997), 309. 15. Mahmoud M.E., Soliman E.M., Talanta, 44(1997), 15. 16. Mahmoud M.E., Soliman E.M., Talanta, 44(1997), 1063. 17. Tong A., Akama Y., Tanaka S., Anal. Chim. Acta, 230(1990), 179. 18. Dadler V., Lindoy L.F., Sallin D., Schlaepfer C.W., Aust. J. Chem., 40(1987), 1557. 19. M.E. Mahmoud, 1998. In: Proceeding of the 25th FACSS Conference, Austin, TX, USA. 20. Hashemi O.R., Razi Kargar M., Raoufi F., Moghimi A., Aghabozorg H., Ganjali M.R., Microchem. J., 62(2001), 1. 21. M.B. Smith, J. March, 2001. March's advanced organic chemistry: reactions, mechanisms, and structure. New York: John Wiley & Sons Inc. 22. Mermoux M., Chabre Y., Rousseau A., Carbon, 29(3) (1991), 469. 23. Cataldo F., Fuller Nanotube Car N, 11(1) (2003), 1. 24. H. Gunzler, H.U. Gremlich, 2002. IR spectroscopy, Winheim: Wiley-VSH; p. 223. 25. H. Effery, 1991. Chemical Analysis, 5th ed., John Wiley & Sons, Inc., New York. 26. Abollino O., Aceto M., Sarzanini C., Mentasti E., Anal. Chim. Acta, 411(2000), 233. 27. Bingye D., Meirong C., Guozhen F., Bing L., X.D., Mingfei P., Shuo W., J. Hazard. Mater.,

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219(2012), 103. 28. Groschner M., Appriou P., Anal. Chim. Acta, 297(1994), 369. 29. Lewis B.L., Landing W.M., Mar. Chem., 40(1992), 105. 30. Stankovich S., Piner R.D., Nguyen S.T., Ruoff R.S., Carbon, 44(2006), 3342. 31. Nayebi P., Moghimi A., Orient. J. Chem., 22(3) (2006), 507. 32. Moghimi A., Orient. J. Chem., 22(3) (2006), 527. 33. Dilovic I., Rubcic M., Vrdoljak V., Pavelic S.K., Kralj M., Piantanidab I., Cindrica M, Bioorg Med Chem., 16(2008), 518.

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International Journal of Bio-Inorganic Hybrid Nanomaterials

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

Assistant Professor, Electrical and Electronic Department, University of Sistan and Baluchestan, Zahedan, Iran

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

Professor,Chemical Engineering Department, University of Sistan and Baluchestan, Zahedan, Iran Received: 12 January 2013; Accepted: 18 March 2013

ABSTRACT 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.

1. INTRODUCTION 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 (*) Corresponding Author - e-mail: tahere.fanaei@ece.usb.ac.ir

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,

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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] established 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.

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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:

& ∂ρ = −∇ ⋅ ρV ∂t

( )

(1)

Conservation of Momentum:

& && & ∂ρV = −∇ ⋅ ρVV + ∇ ⋅τ − ∇P + ρg ∂t

(

)

(2)

For Newtonian fluids such as existent gases in CVD reactors, viscous stress tensor is as follows:

& & τ = µ  ∇V + ∇V 

( )  T

 2  + k − µ    3 

( ∇ ⋅V ) I

(3)

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These equations are coupled with energy equation. Energy Equation:

& ∂ρT = −C p ∇.( ρVT ) + ∇.(λ∇T )+ Cp ∂t

i ik

g k

− R−gk

i =1 k =1

)

(4)

and Species Transport Equation:

∂ (ρω i ) = −∇. ∂t

( ρV&ω ) − ∇.J& + i

(

mi ∑ vik Rkg − R−gk k =1

) = m ∑σ i

Rls

(8)

i =1

∑∑ H v (R N

(

4. NUMERICAL ANALYSIS

  H & D ∇. RT ∑ ∇(ln f i ) + ∑ i ∇.J i − i =1 mi   i =1 mi T i

& & & & n. ρωiV + J ic + J iT

)

(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: L

Pi = mi ∑ σ il Rls

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.

(6)

l =1

Thus the normal velocity on the surface of furnace can be express by:

& & 1 n.V = ρ

i =1

l =1

∑ mi ∑ σ il Rls

(7)

Total net mass flux of ith specie, normal to the surface of furnace must be equal to Pi. Thus:

Figure 1: CVD Reactor Scheme.

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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 deactivation assumption.

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5%). Thus the numerical procedure is reliable and can well predict the process throughout the reactor.

Table 1: Gas phase reaction [13]. Gas Phase Reactions C8H10+H2→ C7H8+CH4

PEF

2.512e+8 1.674e+8

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

TE 0 0

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

Table 2: Surface reactions [13]. Gas Phase Reactions

PEF

C8H10 → 8C + 5H2

0.00034

C7H8 → 7C + 4H2

0.00034

C6H6 → 6C + 3H2

0.00034

CH4 → C + 2H2

0.008

PEF= Pre-Exponential Factor, AE= Activation Energy, TE= Temperature Exponent

Figure 3:

Local growth rate in furnace region with

6. VALIDATION OF NUMERICAL RESULTS

different furnace temperature.

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

7. RESULTS AND DISCUSSION

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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

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(a)

(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).

(a)

(b) (b)

(c)

Figure 5: Temperature distribution in reactor with different furnace temperatures (a) 1000 K, (b) 1100 K and (c) 1150 K.

(c) Figure 4: Reaction product concentration throughout the reactor at different furnace temperature (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)

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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.

(a)

(b)

(c)

Figure 6: Non dimensional inlet hydrocarbon (xylene) concentration

reactor

with

various

furnace

(b)

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

different inlet hydrocarbon concentrations (a) 1000 ppm,

different inlet hydrocarbon concentrations.

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

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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 concentrations. It is seen that lower hydrocarbon concentrations 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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REFERENCES 1. Iijima S., Nature, 354(1991), 56. 2. Liang Y.X., Wang T.H., Physica E, 23(2004), 232. 3. Klinke C., Afzali A., Chem. Phys. Lett., 430(2006), 75. 4. Odom T.W., Huang J.L., Kim P., Lieber C.M., Nature, 39(1998), 62. 5. J. Michael, O. Connel, 2006. Carbon nanotubes properties and applications, New York: Taylor & Francis. 6. Lee C.J., Lyu S.C., Kim H., Park C., Yang C., Chem. Phys. Lett., 359(2002), 109. 7. Cho Y.S., Seok Choi G., Hong G.S., Kim D., Cryst. Growth., 24(2002), 224. 8. Liu W.W., Aziz A., Chai S.P., Mohamed A.R, Physica E, 43(2011),1535. 9. Zahed B., Fanaei S.T., Ateshi H., Transport Phenomena in Nano and Micro Scales, 1(2013), 38. 10. Andrew C.L., Chui W.K.S., Nanotechnology, 19(16) (2008), 165607. 11. Pan L., Nakayama Y., Ma H., Carbon, 49(2011), 854. 12. Grujicic M., Cao G., Gersten B., J. Mater Sci., 38(8) (2003), 1819. 13. Endo H., Kuwana K., Saito K., Qian D., Andrews R., Grulke E.A., Chem. Phys. Lett., 387(2004), 307. 14. Kuwana K., Saito K., Carbon, 43(10) (2005), 2088. 15. Ma H., Pan L., Nakayama Y., Carbon, 49(2011), 845. 16. Zhou Z.P., Ci L.J., Chen X.H., Tang D.S., Yan X.Q., Liu D.F., Carbon, 41(2) (2003), 337. 17. Porro S., Musso S., Giorcelli M., Chiodoni A., Tagliaferro A., Physica E, 37(1-2), (2007), 16. 18. Lin C.H., Chang H.L., Hsu C.M., Lo A.Y., Kuo C.T., Diam Relate Mater, 12(10-11) (2003), 1851. 19. Bower C., Zhou O., Zhu W., Werder D.J., Jin S., Appl. Phys. Lett., 77(17) (2000), 2767. 20. Flahaut E., Govindaraj A., Peigney A., Laurent C., Rousset A., Rao C.N.R., Chem. Phys. Lett., 300(1-2) (1999), 236.

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

Multicomponent

Rls

thermal

diffusion

Reaction rate for the lth surface reaction (mole.m-2.s-1)

Time (s)

Temperature (K)

& V

Velocity vector (m.s-1)

Greek Symbols κ

Volume viscosity (kg.m-1.s-1)

Thermal conductivity of the gas mixture (W.m-1.K-1)

µ νik ρ σil τ ω

NOMENCLATURE Cp

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Dynamic viscosity of the gas mixture (kg.m-1.K-1) Stoichiometric coefficient for the ith gaseous species in the kth gas phase reaction Density (kg.m-3) Stoichiometric coefficient for the ith gaseous species in the lth surface reaction Viscous stress tensor (N.m-2) Species mass fraction

Subscripts i,j

With respect to the ith/jth species

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

Species mole fraction

& g

Gravity vector

Molar enthalpy (J.mole-1)

Unity tensor

* j

Diffusive mass flux vector (kg.m-2.s-1) Mole mass of the ith species (kg.mole-1)

& n

Unity vector normal to the inflow/outflow opening or wall

Pressure (pa)

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

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)

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Superscripts c T

Due to ordinary diffusion Due to thermal diffusion