Kinetics and Thermodynamics of Zinc Phosphate Hydrate Synthesized

Mar 1, 2010 - Science, and College of KMITL Nanotechnology, King Mongkut's Institute of Technology Ladkrabang,. Bangkok 10520, Thailand...
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Ind. Eng. Chem. Res. 2010, 49, 3571–3576

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Kinetics and Thermodynamics of Zinc Phosphate Hydrate Synthesized by a Simple Route in Aqueous and Acetone Media Banjong Boonchom,*,†,‡ Rattanai Baitahe,‡,§,| Samart Kongtaweelert,§ and Naratip Vittayakorn‡,§,| King Mongkut’s Institute of Technology Ladkrabang, Chumphon Campus, 17/1 M. 6 Pha Thiew District, Chumphon 86160, Thailand, AdVanced Materials Science Research Unit, Department of Chemistry, Faculty of Science, and College of KMITL Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang, Bangkok 10520, Thailand

Zinc phosphate (Zn3(PO4)2 · 4H2O) was prepared by a simple route using ZnO and H3PO4 in aqueous and acetone media at ambient temperature. Zn3(PO4)2 · 4H2O decomposes to Zn3(PO4)2 by three dehydration reactions, as revealed by thermal analytical (TG/DTG/DTA) and differential scanning calorimetry (DSC) techniques. The synthesized Zn3(PO4)2 · 4H2O and its dehydration product Zn3(PO4)2 were characterized by powder X-ray diffraction, Fourier transfer infrared, and scanning electron microscopy. Kinetic triplet parameters (activation energy, E; preexponential, A; Avrami exponent, n) and thermodynamic functions (∆H*, ∆G*, and ∆S*) of three dehydration reactions are calculated by DSC experiments. Evaluation and control of the specific characteristics of three dehydration processes of Zn3(PO4)2 · 4H2O are essentially important in the variously oriented studies, which were discussed from the viewpoints of thermal stability based on kinetics and thermodynamics. 1. Introduction Zinc phosphates have found wide applications such as ionexchange materials, chelating agents, corrosion-resistant coatings, glass ceramics, biomedical cements, and high-quality fertilizers.1–4 The most frequent applications of zinc phosphates were as pigments for coating products (iron and steel alloys), which possess very good anticorrosive properties and are nontoxic substances (the environmentally friendly pigments).5,6 Especially, nowadays these materials have been used in electric motors and transformers and in the automotive industry.7,8 Thus, several methods of zinc phosphate synthesis on the basis of different raw materials and reaction conditions in their respective preparation methods have been developed because obtaining anticorrosive properties depends on the polycrystalline homogeneous particles.9–11 Zn3(PO4)2 · 4H2O, one of the important zinc phosphates, has found widespread application as mentioned above. Thermal treatment of zinc phosphate hydrate has great synthetic potential because it may turn simple compounds into advanced materials, which are related to the hydrate in the conventional crystal form.11–13 This compound is transformed to Zn3(PO4)2 by dehydration reactions at high temperature. In this respect, studies on the thermodynamics, mechanisms, and kinetics of the dehydration reaction of this compound are challenging and difficult tasks, with complexity resulting from a great variety of factors. The presence of water molecules of Zn3(PO4)2 · 4H2O influences the intermolecular interactions (affecting the internal energy and enthalpy) as well as the crystalline disorder (entropy) * To whom correspondence should be addressed. Tel: +66-77506422, ext. 4546. Fax: +66-7750-6410. E-mail: [email protected]. † King Mongkut’s Institute of Technology Ladkrabang, Chumphon Campus. ‡ Advanced Materials Science Research Unit, Department of Chemistry, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang. § Department of Chemistry, Faculty of Science, King Mongkut’s Institute of Technology Ladkrabang. | College of KMITL Nanotechnology, King Mongkut’s Institute of Technology Ladkrabang.

and, hence, influences the free energy, thermodynamic activity, solubility, stability, and electrochemical and catalytic activity.11–15 These results obtained from such studies can be directly applied in materials science for the preparation of various metals and alloys, cements, ceramics, glasses, enamels, glazes, and polymer and composite materials. The objective of this study was to elaborate a new method of preparation of zinc phosphate with the crystallite size as small as possible with utilization of aqueous solution and acetone as the media agents. The presence of acetone reduced the hot reaction and prevented the evolved H2(g) in the precipitation process. This method is a simple, rapid, cost-effective, and environmentally friendly route for the synthesis of Zn3(PO4)2 · 4H2O. Furthermore, this work seeks to characterize the thermal dehydration processes of Zn3(PO4)2 · 4H2O, in relation to its thermodynamic (∆H*, ∆S*, and ∆G*) and kinetic (E, A, mechanism, and model) properties, which are discussed for the first time. 2. Experimental Section 2.1. Synthesis. In this study, ZnO (99.99%, Merck), H3PO4 [86.4% (w/w), Merck], and acetone (99.99%, Merck) were used as starting materials. In a typical procedure of the synthesized Zn3(PO4)2 · 4H2O (1), 10 mL of acetone was added to 2.45 g of ZnO, and this mixture was referred to as suspension A. Next, 81.02 mL of 86.4% (w/w) H3PO4 was diluted in 18.98 mL of deionized water, and this solution was referred to as solution B. Then, 5 mL of solution B was added slowly to suspension A with continuous stirring at ambient temperature for 15 min. Finally, a white precipitation of Zn3(PO4)2 · 4H2O was obtained, filtered by a suction pump, washed with acetone until free from phosphate ions, and dried in air. 3ZnO(s) + 2H3PO4 + C3H6O, acetone

H2O 98 Zn3(PO4)2·4H2O(s) (1) ambient temperature

2.2. Characterization. Thermal analysis of Zn3(PO4)2 · 4H2O was carried out in a flow rate of air (100 mL min-1) over the

10.1021/ie901626z  2010 American Chemical Society Published on Web 03/01/2010

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temperature range from 303 to 1173 K with a heating rate of 10 K min-1 using a Perkin-Elmer Instruments Pyris Diamond thermogravimetric/differential thermal analyzer. The structures and crystallite sizes of the prepared sample and its dehydration product were identified by powder X-ray diffraction (XRD) using a D8 Advanced powder diffractometer (Bruker AXS, Karlsruhe, Germany) with Cu KR radiation (λ ) 0.1546 nm). The Scherrer method was used to evaluate the crystallite size [i.e., D ) Kλ/β cos θ, where λ is the wavelength of X-ray radiation, K is a constant taken as 0.89, θ is the diffraction angle, and β is the full width at half-maximum (fwhm)].16 The room temperature Fourier transform infrared (FTIR) spectra were recorded in the range of 4000-370 cm-1 with eight scans on a Perkin-Elmer Spectrum GX FTIR/FT-Raman spectrometer with a resolution of 4 cm-1 using KBr pellets (KBr, spectroscopy grade, Merck). The kinetic and thermodynamic properties of zinc phosphates were carried out using a Perkin-Elmer Diamond differential scanning calorimetry (DSC) apparatus with R-Al2O3 powder as the reference material. The experiments were performed at heating rates of 2, 4, 6, and 8 K min-1 over a temperature range from 303.15 to 723.15 K under a N2 atmosphere at a flow rate of 20 mL min-1. The particle sizes and external morphologies of the prepared sample and its dehydration powder were characterized by scanning electron microscopy (SEM) using LEO SEM VP1450 after gold coating. 2.3. Kinetic and Thermodynamic Studies. Kinetic parameters of the dehydration step of zinc phosphate hydrate Zn3(PO4)2 · 4H2O, the activation energy (Ea/kJ mol-1), and the frequency factor (A/min-1) were calculated from DSC and obtained at four different heating rates (2, 4, 6, and 8 K min-1), using Kissinger’s equation17 ln

( )

( )

Ea Ea β )+ ln 2 RTp RA Tp

(2)

where β is the heating rate (K min-1), Tp is the DSC peak temperature, and R is the gas constant (8.314 J mol-1 K-1). The thermal dehydration mechanism could be determined from the shape factor (n) of the endothermic peak represented by the equation18–20 2 2.5 Tp n) ∆D Ea /R

(3)

where n is the Avrami constant, Tp is the endothermic peak temperature, and ∆D is the fwhm of the endothermic peak at four different heating rates.21 Thermodynamic parameters, i.e., enthalpy change (∆H*/kJ mol-1), heat capacity (Cp/kJ mol-1 K-1), entropy change (∆S*/ kJ mol-1 K-1), and Gibbs’ free energy (∆G*/kJ mol-1), were calculated from DSC experiments carried out in a N2 atmosphere at different heating rates as follows. The enthalpy change was calculated directly from the amount of heat change involved to calculate the specific heat capacity (Cp) using the equation21–23 Cp )

∆H* ∆T

(4)

where ∆T ) T2 - T1, in which T1 is the temperature at which the DSC peak begins to depart the baseline and T2 is the temperature at which the peak lands.22,23 The entropy change (∆S*) was calculated using the relationship ∆S* ) 2.303Cp log

T2 T1

(5)

Figure 1. TG/DTG/DTA curves of the synthesized Zn3(PO4)2 · 4H2O at a heating rate of 10 K min-1 in static air.

and the Gibbs’ free energy (∆G*) from activated complex formation from a reagent can be calculated using the well-known thermodynamic equation ∆G* ) ∆H* - Tp∆S*

(6)

3. Results and Discussion 3.1. Characterization. The thermogravimetry/differential thermogravimetry/differential thermal analysis (TG/DTG/DTA) curves of Zn3(PO4)2 · 4H2O are shown in Figure 1. The TG curve relating to the elimination of water molecules in the crystallohydrate shows three stages of mass loss between 303 and 1173 K. Three mass losses were observed at 350-412, 412-505, and 505-644 K and were accompanied by mass losses of 7.75, 3.81, and 4.33 mol, which correspond to 1.97, 0.97, and 1.10 mol of water for Zn3(PO4)2 · 4H2O, respectively. These three mass loss stages appear in the respective DTA (three endothermic effects) and DTG curves as three peaks (380, 437, and 574 K). The thermal dehydration of Zn3(PO4)2 · 4H2O in the range of 350-773 K involves dehydration of the coordinated water molecules (4 mol of H2O) as shown in eqs 7-9. Zn3(PO4)2·4H2O f Zn3(PO4)2·2H2O + ∼2H2O

(7)

Zn3(PO4)2·2H2O f Zn3(PO4)2·2H2O + ∼1H2O

(8)

Zn3(PO4)2·H2O f Zn3(PO4)2 + ∼1H2O

(9)

The total mass loss of 15.89% (4.04 mol of H2O) is close to the theoretical value (15.72%, 4 mol of H2O). Zn3(PO4)2 was found to be the product of thermal dehydration at T > 600 K, as revealed by the TG curve. This result may not be in agreement with the higher temperature (>823 K) of thermal transformation of Zn3(PO4)2 · 4H2O to Zn3(PO4)2 reported in the literature.3 In order to gain complete dehydration of Zn3(PO4)2 · 4H2O, a sample of Zn3(PO4)2 · 4H2O was heated in a furnace at 673 K for 3 h. The thermal stability, mechanism, and phase transition temperature of the synthesized Zn3(PO4)2 · 4H2O in aqueous and acetone media observed in this work are significantly different from those of Zn3(PO4)2 · 4H2O reported in the literature.3 This result indicates that the medium reagents for precipitation have strong effects on the thermal transformation of Zn3(PO4)2 · 4H2O. The XRD patterns of the synthesized Zn3(PO4)2 · 4H2O and its dehydration product Zn3(PO4)2 are shown in Figure 2. All detectable peaks of the obtained Zn3(PO4)2 · 4H2O and Zn3(PO4)2 samples are indexed as the standard data of PDF no. 37-0316 for Zn3(PO4)2 · 4H2O and PDF no. 29-1390 for Zn3(PO4)2, respectively. These results indicated that the two crystal structures are in a monoclinic system with space group P21/c (Z ) 2) for Zn3(PO4)2 · 4H2O and C2/c (Z ) 4) for Zn3(PO4)2.

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Figure 2. XRD patterns of the synthesized Zn3(PO4)2 · 4H2O (a) and its dehydration product Zn3(PO4)2 (b).

Figure 3. FTIR spectra of the synthesized Zn3(PO4)2 · 4H2O (a) and its dehydration product Zn3(PO4)2 (b).

The average crystallite size of 34 ( 8 nm for the Zn3(PO4)2 · 4H2O sample was calculated from X-ray line broadening of the reflections of (0, 1, 3), (1, 1, 3), (1, 1, -4), (3, 0, -2), and (1, 0, -6), using the Scherrer equation (i.e., D ) 0.89λ/β cos θ), where λ is the wavelength of X-ray radiation, D is a constant taken as 0.89, θ is the diffraction angle, and β is the fwhm.16 Similarly, the average crystallite size of 26 ( 12 nm for calcined sample (Zn3(PO4)2) was calculated from X-ray line broadening of the reflections of (0, 1, 1), (0, 0, 2), (-3, 1, 1), (-4, 1, 1), and (2, 2, 0). The crystallite sizes and lattice parameters are also tabulated in Table 1. As shown Table 1, the lattice parameters of Zn3(PO4)2 · 4H2O and Zn3(PO4)2 are close to those of the standard data of PDF nos. 37-0316 and 29-1390, respectively. The FTIR spectra of the obtained Zn3(PO4)2 · 4H2O and Zn3(PO4)2 samples shown in Figure 3 are very similar to those observed by Pawlig and Trettin.3 Vibrational bands are assigned to the fundamental vibrating units, H2O and PO43- for Zn3(PO4)2 · 4H2O and PO43- for Zn3(PO4)2. The observed bands in the 1600-1700 and 3000-3500 cm-1 regions are attributed to the water bending band (ν2) and stretching vibration bands (ν1 and ν3), respectively. These water bands disappeared or are very weak (due to moisture) in the FTIR spectrum of Zn3(PO4)2, while the free PO43- anion has the four normal modes of vibration of a tetrahedral ion. These are symmetric stretching [ν1(A1); singly degenerate], antisymmetric stretching [ν3(F2); triply degenerate], symmetric bending [ν2(E); doubly degener-

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ate], and antisymmetric bending [ν4(F2); triply degenerate] vibrations. Vibrational bands of the PO43- anion are observed in the regions of 370-400, 450-600, 900-1000, and 1000-1200 cm-1. These bands are assigned to the ν2(PO43-), ν4(PO43-), ν1(PO43-), and ν3(PO43-) vibrations, respectively, while the FTIR spectra in the 1200-400 cm-1 region of the Zn3(PO4)2 · 4H2O and Zn3(PO4)2 samples are different because of the separation of the vibrations in the crystalline state into internal and external modes of the phosphate Td point group. The number of bands in this spectral region confirms the existence of distinct nonequivalent phosphate units in each structure and the loss of degeneracy of the vibrational modes, which were affected by factor group analysis.3,24 Additionally, the observation of a strong νs(POP) band (748 cm-1) for the FTIR spectrum of Zn3(PO4)2 is known to be the most striking feature of polyphosphate spectra. This result is consistent in XRD data. The SEM images of Zn3(PO4)2 · 4H2O (Figure 4a) and Zn3(PO4)2 (Figure 4b) show nonuniform morphology and were aggregates of small and large particles. The SEM micrographs of Zn3(PO4)2 · 4H2O and Zn3(PO4)2 powders appear as aggregates of particles with homogeneous and heterogeneous composition, which are caused primarily by the process of dissolution and a rapid precipitation and dehydration process, respectively. 3.2. Kinetic and Thermodynamic Results. Figure 5 shows the DSC curves of thermal dehydration of Zn3(PO4)2 · 4H2O at four heating rates (2, 4, 6, and 8 K min-1). DSC curves of Zn3(PO4)2 · 4H2O show three clearly distinguishable endothermic peaks in the range of 373-673 K. All DSC curves are approximately the same shape, which indicates that the transformative phase is independent of the heating rate. However, three dehydration stages were shifted toward higher temperatures when the heating rates were increases. The DSC peaks are in good agreement with the DTG and DTA peaks, as shown in Figure 1. According to the Kissinger method, the basic data of T are collected from the DSC curves of dehydration of Zn3(PO4)2 · 4H2O at various heating rates (2, 4, 6, and 8 K min-1; Figure 5). Figure 6 shows the Kissinger plots of the dehydration reaction of the synthesized Zn3(PO4)2 · 4H2O. From the slopes and y intercept of the curves (Figure 6), the activation energy values and preexponential factors in three dehydration steps of the synthesized Zn3(PO4)2 · 4H2O were determined and are also tabulated in Table 2. From Table 2, these activation energies are consistent with the former hypothesis that the intermediate nucleates and crystallizes as a metastable phase with adequate growth kinetics before the stable phase Zn3(PO4)2. The second step exhibits a higher activation energy in comparison with the other ones. The reason is relevant to the strengths of binding of water molecules in the crystal lattice, which indicates the metastable intermediate as Zn3(PO4)2 · 2H2O. The lowest activation energy in the first step indicates the ease of elimination of two water molecules in Zn3(PO4)2 · 4H2O before transformation to Zn3(PO4)2 · 2H2O. Whereas the big difference between the values of the preexponential factor A for the three processes of dehydration of the studied compound is interesting, the values of the

Table 1. Average Crystallite Sizes and Lattice Parameters of Zn3(PO4)2 · 4H2O and Zn3(PO4)2 Calculated from XRD Data compound

system

a (nm)

b (nm)

c (nm)

β (deg)

Zn3(PO4)2 · 4H2O

PDF no. 37-0316 this work

0.8695 0.8648(3) -0.0047 1.5000 1.5323(0) +0.0323

0.4891 0.4834 (2) -0.0057 0.5635 0.5597(0) -0.0038

1.6695 1.6531(4) -0.0164 0.8183 0.8179(1) -0.0004

94.94 94.08(0) -0.86 105.00 105.06(3) +0.06

DIF. this work, PDF Zn3(PO4)2 DIF. this work, PDF

PDF no. 29-1390 this work

average crystallite size (nm) 24 ( 8 26 ( 12

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Figure 6. Dehydration kinetic analysis of Zn3(PO4)2 · 4H2O by the Kissinger model. Table 2. Kinetics and Thermodynamics for Thermal Events Encountered during the Dehydration Course of Zinc Phosphate in a N2 Atmosphere dehydration parameter -1

Ea/kJ mol A/min-1 corrn determn n ∆H*/kJ mol-1 Cp/kJ mol-1 K-1 ∆S*/kJ mol-1 K-1 ∆G*/kJ mol-1

Figure 4. SEM micrographs of the synthesized Zn3(PO4)2 · 4H2O (a) and its dehydration product Zn3(PO4)2 (b).

Figure 5. DSC curves of Zn3(PO4)2 · 4H2O at the four heating rates (2, 4, 6, and 8 K min-1) in a N2 atmosphere.

preexponential factor A in the Arrhenius equation for solidphase reactions are expected to be quite widely ranged (6 or 7 orders of magnitude), even after correction of the surface area effect.25–28 The empirical first-order preexponential factors may vary from 105 to 1018 s-1. The low factors will often indicate a surface reaction, but if the reactions are not dependent on the surface area, the low factor may indicate a “tight” complex. The high factors will usually indicate a “loose” complex. Even higher factors (after correction of the surface area) can be obtained if the complexes have a free transition on the surface. Because the concentrations in the solids are not controllable in

step I

step II

step III

201.78 6.90 × 1011 0.984 1.42 242.90 2.64 1.86 -28.61

359.82 2.75 × 1022 0.993 4.97 42.22 1.27 0.16 -1.45

283.84 5.96 × 1013 0.997 1.57 19.62 0.64 0.07 -0.20

many cases, it would be convenient if the magnitude of the preexponential gives an indication of the reaction molecularity. On the basis of these reasons, three steps of the dehydration reactions of Zn3(PO4)2 · 4H2O may be interpreted as tight, loose, and tight complexes, respectively. This may, most likely, occur on a surface where the activated complexes have free conditions, unlike the reactants, and rotate parallel to the surface. For this bimolecular case, the complex is expected to expand in size into a bulk solid phase and hence interact more intensely with its neighbors. The second step exhibits a higher preexponential factor (A) in comparison with the other steps, and this is consistent with the results of the activation energy value. The value of the Avrami exponent provides information regarding the morphology of the growing crystal.18–20,29 The value of n reflects a volume change upon crystallization, different crystal geometries, and different crystal branching. Here, smaller n values indicate an explanation of large nuclei numbers followed by a rather limited growth, slowed down by the production of a rigid amorphous portion between the microphase-separated crystals. On the other hand, larger n values are expected only in the case of increasing nucleation rates followed by a nonlimited random nucleation and growth of nuclei. For Zn3(PO4)2 · 4H2O, the larger n values estimated for all dehydration steps correspond to a random nucleation and growth of nuclei. Thermodynamic parameters, i.e., enthalpy change (∆H*/J mol-1), heat capacity (Cp/J mol-1 K-1), entropy change (∆S*/J mol-1 K-1), and Gibbs’ free-energy change (∆G*/J mol-1), were calculated from DSC experiments according to eqs 4-6 and are presented in Table 2. As can be seen from Table 2, the entropy of activation (∆S*) values for three dehydration steps are positive. This means that the corresponding activated complexes had a lower degree of arrangement (higher entropy) than the initial state. Because dehydration of Zn3(PO4)2 · 4H2O proceeds as three consecutive reactions, the formation of the second and third activated complexes passed in situ. In terms of the activated complex theory (transition theory),30–32 a

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positive value of ∆S* indicates a malleable activated complex, which leads to a large number of degrees of freedom of rotation and vibration. This result may be interpreted as a “fast” stage. On the other hand, a negative value of ∆S* indicates a highly ordered activated complex, and the degrees of freedom of rotation as well as of vibration are less than those in the nonactivated complex. This result may indicate a “slow” stage.30 On the basis of these results, all stages of thermal dehydration of Zn3(PO4)2 · 4H2O may be interpreted as “fast” stages. The change of the activation enthalpy ∆H* shows the energy differences between the activated complex and the reagents. If this difference is small, the formation of the activated complex is favored, because the potential energy barrier is low. The change of the Gibbs’ free energy ∆G* reflects the total energy increase of the system at the approach of the reagents and the formation of the activated complex. Near-zero Gibbs free energy ∆G* means that the material has just passed through some kind of physical or chemical aging process, bringing it to a state near its own thermodynamic equilibrium. In this situation, the material shows little reactivity, increasing the time taken to form the activated complex. On the other hand, when high positive or negative Gibbs free-energy values are observed, the material is far from its own thermodynamic equilibrium. In this case, the reactivity is high and the system can react faster to produce the activated complex, which resulted in the short reaction times observed.25 The high negative ∆G* value indicates that the first step needs a higher-energy pathway than the other ones. That means that the rate of the first step is higher than that of other steps of dehydration and indicates that the first dehydration step occurs softer than the other steps. 4. Conclusion The Zn3(PO4)2 · 4H2O sample was successfully synthesized by a rapid and simple solid-state method from ZnO and H3PO4 in aqueous and acetone media at ambient temperature with short time consumption (15 min). The presence of acetone prevented H2 gas from evolving, strongly reduced the hot reaction (ZnO and H3PO4), and gave a rapid drying powder. Thermal dehydration of Zn3(PO4)2 · 4H2O to Zn3(PO4)2 involves three stages of dehydration reactions, which are different from those of other works because of the media agents and conditions of preparation. The parameters characterizing the kinetics and thermodynamics of the three dehydration reactions of the synthesized Zn3(PO4)2 · 4H2O were different because the four water molecules are not in equivalent positions, which causes the ordered or disordered structures of the reactant, activated complex, and product. On the basis of correctly established values of kinetic triplet parameters (E, A, and n) and thermodynamic functions (∆H*, ∆S*, and ∆G*), certain conclusions can be made concerning the mechanisms and characteristics of the processes. Thus, various scientific and practical problems involving the thermal transformation of Zn3(PO4)2 · 4H2O to Zn3(PO4)2 can be solved. The results can be applied to the production of Zn3(PO4)2, which plays a large role in industrial applications. Acknowledgment This work was financially supported by the Thailand Research Fund, the Commission on Higher Education (Research Grant for New Scholar), and the National Nanotechnology Center (NANOTEC) NSTDA, Ministry of Science and Technology, Thailand.

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ReceiVed for reView October 20, 2009 ReVised manuscript receiVed February 11, 2010 Accepted February 14, 2010 IE901626Z