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Preparation of Mg-doped ZnO nanoparticles by mechanical milling and their optical properties
Available online at www.sciencedirect.com Available online at www.sciencedirect.com
Procedia Engineering
ProcediaProcedia Engineering 00 (2012) 000–000 Engineering 32 (2012) 821 – 826 www.elsevier.com/locate/procedia
I-SEEC2011
Preparation of Mg-doped ZnO nanoparticles by mechanical milling and their optical properties S. Suwanboona∗ and P. Amornpitoksukb a
Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Songkhla, 90112, Thailand Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Songkhla, 90112, Thailand
b
Elsevier use only: Received 30 September 2011; Revised 10 November 2011; Accepted 25 November 2011.
Abstract Mg-doped ZnO nanoparticles were synthesized by planetary ball milling at a speed of 400 rpm and milled for 20 h. The samples were characterized by XRD, SEM and UV-Vis spectrophotometer. The crystallite size of the samples increased and the lattice strain decreased with an increase of MgO loading. The increase in crystallite size of the samples as a function of MgO loading can be explained by the effect of Ostwald ripening. The absorption edge of the samples shifted to a lower wavelength when MgO loading was increased. The energy band gap of the samples varied in a range of 2.96-3.13 eV depending on the loading content.
© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of I-SEEC2011 Keywords: Mg-doped ZnO; nanoparticles; mechanical milling; X-ray method; optical properties
1. Introduction Nanostructured materials have been receiving considerable attention for a decade due to their excellent properties compared to the same materials, consisting of a large grain. Recently, nanostructured ZnO is one of semiconducting metal oxide materials that has been investigated widely for fundamental and advanced research. ZnO is an n-type II-VI semiconducting compound and possesses a wide band gap (3.2 eV) and a large exciton binding energy (60 meV) [1]. ZnO can be extensively used for varistors [2], sensors [3] and photocatalysts [4]. Nanostructured ZnO material can be synthesized via a bottom-up technique including sol-gel, precipitation and hydrothermal
* Corresponding author. Tel.: +66-74-288-250; fax: +66-74-288-395. E-mail address: [email protected].
1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.02.018
822 2
S. Suwanboon andAmornpitoksuk P. Amornpitoksuk / Procedia Engineering 32 (2012) 821 – 826 S. Suwanboon and P. / Procedia Engineering 00 (2012) 000–000
method and top-down technique such as grinding, conventional milling and high-energy milling [5]. As far as the above methods are concerned, only a few researchers [6,7] reported a preparation of Mg-doped ZnO nanoparticles through the mechanical milling method. Therefore, this study focused on preparation of Mg-doped ZnO nanoparticle by mechanical milling and investigated the dependence of optical band gap on structural properties of milled samples. 2. Experimental ZnO (99%, Fluka Chimie, Switzerland) and MgO (Riedel-deHaën) powders were used as starting materials. Zn1-xMgxO nanoparticles were prepared by a Pulverisette 7 FRITSCH planetary ball mill. The stoichiometric quantities were weighted to obtain the mixtures of ZnO with 1, 3 and 5 mol% of Mg or x = 0.01, 0.03 and 0.05. The milling was performed in a silicon nitride vial with silicon nitride balls. The ballto-powder ratio (BPR) was 10:1, the speed was 400 rpm and the milling time was 20 h. The mixtures were milled for 10 min, alternating with a stop for 5 min so as to prevent over heating. Structural properties of all samples were studied by X-ray diffraction using CuKα radiation (XRD, X’Pert MPD, PHILIPS). The morphology was carried out by a scanning electron microscope (SEM, JEOL-5800 LV) and diffuse reflection spectra were measured by a UV-Vis spectrophotometer (UV-Vis 2450, Shimadzu). 3. Results and discussion 3.1 Structural study
(112)
(103)
(110)
(102)
intensity (a.u.)
(100) (002) (101)
The effect of Mg contents on Mg-doped ZnO powders formation was studied by milling at 400 rpm for 20 h. The crystalline phase of as-milled samples was determined by XRD technique and all results are presented in Fig. 1.
5 mol% Mg 3 mol% Mg 1 mol% Mg 0 mol% Mg
10
20
30
40
50
60
70
80
90
o
2θ ( ) Fig. 1. XRD patterns of Mg-doped ZnO nanoparticles at different Mg contents
100
S.S.Suwanboon Engineering 3200 (2012) 821 – 826 Suwanboonand andP.P.Amornpitoksuk Amornpitoksuk/ Procedia / Procedia Engineering (2012) 000–000
3
From Fig. 1, it was evident that no diffraction peaks of MgO were detected under the limitation of XRD used. The diffraction peaks of all samples exhibited a ZnO hexagonal wurtzite structure with highquality crystallinity in good agreement with the JCPDS standard card number of 36-1451. In this study, the crystallite size and lattice strain were evaluated by the Williamson-Hall equation [8] and the results are given in Table 1.
β cos θ 0.9 η sin θ = + D λ λ
(1)
where β is the full-width at the half-maximum, θ is the Bragg angle, λ is the wavelength of X-ray used, D is the crystallite size and η is the lattice strain. Besides, the lattice parameters were estimated from the following relation [1] and the calculated results are also displayed in Table 1. 1 2 d hkl
=
4 ⎡ h 2 + hk + k 2 ⎤ l 2 ⎥+ 2 ⎢ 3 ⎣⎢ a2 ⎦⎥ c
(2)
where dhkl is the lattice spacing of the (hkl) plane and a and c are the lattice parameters. As mentioned in [1], the change in lattice parameters of metal-doped ZnO powders depends upon the ionic radius of doping ion. In this study, the ionic radius of Mg2+ ion (72 pm) is smaller than that of the ionic radius of Zn2+ ion (74 pm). Therefore, the lattice parameters contracted when the Mg ion substituted Zn ion in the lattice and the contraction of lattice parameters is a function of Mg content. This is in good agreement with the reduction of lattice volume. For this reason, it could be concluded that the Zn1-xMgxO can form in the whole range of Mg content used. The crystallite size increased as Mg content was increased, this might be due to the coarsening of the crystal and the growth occurred through the Ostwald ripening mechanism [9]. Generally, the heat was generated during milling process because of the collision between the vial wall and media. This evidence promoted the Ostwald ripening and brought about the reduction of lattice strain. The morphology of Mg-doped ZnO nanoparticles at different Mg contents is presented in Fig. 2. The spherical nanoparticles are agglomerated so as to reduce the total surface free energy. This evidence always occurs when the particles are small [1, 8]. In this study, the particle size increased as Mg content was increased in accordance with the crystallite size obtained by XRD analysis. Table 1. XRD analysis and optical properties of Mg-doped ZnO nanoparticles Mg content
D
(mol%)
(nm)
Eg
E0
lattice parameter (Å)
V
(eV)
(eV)
a
c
(Å)3
0
22.12
1
22.99
1.36
2.96
0.39
3.2593
5.2220
48.04
1.31
3.06
0.34
3.2571
5.2110
47.87
3
28.30
1.05
3.11
0.30
3.2552
5.2123
47.83
5
30.77
1.01
3.13
0.30
3.2524
5.2076
47.70
η
823
824 4
S. Suwanboon andAmornpitoksuk P. Amornpitoksuk / Procedia Engineering 32 (2012) 821 – 826 S. Suwanboon and P. / Procedia Engineering 00 (2012) 000–000 0 mol%
1 mol%
3 mol%
5 mol%
Fig. 2. SEM images of Mg-doped ZnO nanoparticle at different Mg contents
3.2 Optical study In this part, the absorbance of Mg-doped ZnO nanoparticles was measured in the wavelength of 200800 nm as shown in Fig. 3. 1.60
0 mol%
1.40
1 mol% 3 mol%
absorbance
1.20
5 mol%
1.00 0.80 0.60 0.40 0.20 0.00 200
300
400
500
600
700
wavelength (nm) Fig. 3. Absorbance of Mg-doped ZnO nanoparticles at different Mg contents
800
S.S.Suwanboon Engineering 3200 (2012) 821 – 826 Suwanboonand andP.P.Amornpitoksuk Amornpitoksuk/ Procedia / Procedia Engineering (2012) 000–000
5
The absorption edge of Mg-doped ZnO nanoparticles was shifted to a lower wavelength (blue-shift) when the Mg content was altered from 0 to 5 mol%. This might be due to a decrease in lattice strain of sample doping with Mg ion. This blue-shift behavior indicated that a wider band gap (Eg) was achieved when higher Mg content was doped in ZnO lattice. The Eg value of Mg-doped ZnO nanoparticles, as shown in Fig. 4(a), was estimated from a linear portion of the (αE)2 versus E curves using the relation as follow [8]: (αE)2 =ED(E-Eg)
(3)
where α is the absorption coefficient, E is the photon energy, ED is a constant, Eg is the optical band gap. The Eg values of all samples are given in Table 1. It was clearly observed that the Eg value increased when the Mg content was increased or when the crystallite size increased. As we know, the Eg value depended upon many key parameters such as particle size, particle shape and defect concentration [10]. In this study, the particle shape is similar for the whole range of Mg doping content. So, only particle size and defect concentration were considered. The wider Eg value was normally obtained from the sample with a smaller particle size, but the narrower Eg value was obtained from Mg-doped ZnO nanoparticles with a smaller size. Therefore, it could be surmised that the defect concentration played an important role in a reduction of Eg value. So, the curve of ln(α) versus E was plotted as shown in Fig. 4 (b). The reciprocal value of the slope identified the defect concentration and the results are given in Table 1. From the results obtained, it is reasonable to conclude that the defect concentration influenced a reduction of Eg value. (a)
0 mol% 1 mol%
1 mol% 3 mol% 5 mol%
5 mol%
ln(α )
(αE)2 (eV/cm)2
3 mol%
2.90
(b)
0 mol%
3.00
3.10
3.20
3.30 E (eV)
3.40
3.50
3.60
2.90
3.00
3.10
3.20
3.30
3.40
3.50
3.60
E (eV)
Fig. 4. (a) Evaluation of (αE)2 versus E for estimating the Eg value and (b) plots of ln(α) versus E for evaluating the defect concentration of Mg-doped ZnO nanoparticles at different Mg contents
4. Conclusions The hexagonal wurtzite Mg-doped ZnO nanoparticles that are in the form of Zn1-xMgxO (x = 0-0.05) were successfully synthesized by high-energy ball milling method at a speed of 400 rpm for 20 h. As increase in Mg contents, the crystallite size increased and the lattice strain decreased because the crystals grow via the Ostwald ripening. The spherical nanoparticles were agglomerated into a cluster in order to reduce the surface free energy. The absorption edge of Mg-doped ZnO nanoparticles performed a blueshift behavior compared to pure ZnO nanoparticles and the Eg values of Mg-doped ZnO nanoparticles
825
826 6
S. Suwanboon andAmornpitoksuk P. Amornpitoksuk / Procedia Engineering 32 (2012) 821 – 826 S. Suwanboon and P. / Procedia Engineering 00 (2012) 000–000
were wider than the Eg value of pure ZnO nanoparticles. In this study, the defect concentration affected significantly the reduction of Eg value. Acknowledgements This work has been supported by Science Research Fund, Faculty of Science, Prince of Songkla University (Fiscal Year 2011) under the Contact Number 154001. The authors also would like to acknowledge the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education and the authors would like to acknowledge Michael Benjamin Lane for his English manuscript proofreading service. References [1] Suwanboon S, Amornpitoksuk P, Sukolrat A. Dependence of optical properties on doping metal, crystallite size and defect concentration of M-doped ZnO nanopowders (M = Al, Mg, Ti). Ceram Int 2011; 37: 1359-65. [2] Houabes M, Metz R. Rare earth oxides effects on both the threshold voltage and energy absorption capability of ZnO varistors. Ceram Int 2007; 33: 1191-7. [3] Hongsith N, Wongrat E, Kerdcharoen T, Choopun S. Sensor response formula for sensor based on ZnO nanostructures. Sens Actuators B 2010; 144: 67-72. [4] Sun JH, Dong SY, Feng JL, Yin XJ, Zhao XC. Enhanced sunlight photocatalytic performance of Sn-doped ZnO for Methylene Blue degradation. J Mol Catal A: Chem 2011; 335: 145-50. [5] Suwanboon S, Amornpitoksuk P. Preparation and characterization of nanocrystalline La-doped ZnO powders through a mechanical milling and their optical properties. Ceram Int 2011; doi:10.1016/j.ceramint.2011.06.007. [6] Damonte LC, Hernández-Fenollosa MA, Marŕ B. Cation substitution in ZnO obtained by mechanical milling. J Alloys Compd 2007; 434-435: 813-5. [7] Nursyahadah MZ, Nurul SS, Azlan Z, Kumar MT. Effect of magnesium doping on structural and optical properties of ZnO nanoparticles synthesized by mechanochemical processing. AIP Conference Proceeding 2011; 1328: 211-3. [8] Suwanboon S, Amornpitoksuk P, Bangrak P. Synthesis, characterization and optical properties of Zn1-xTixO nanoparticles prepared via a high-energy ball milling technique. Ceram Int 2011; 37: 333-40. [9] Singh RG, Singh F, Kumar V, Mehra RM. Growth kinetics of ZnO nanocrystallites: Structural, optical and photoluminescence properties tuned by thermal annealing. Curr Appl Phys 2011; 11: 624-30. [10] Suwanboon S, Amornpitoksuk P, Bangrak P, Sukolrat A, Muensit N. The dependence of optical properties on the morphology and defects of nanocrystalline ZnO powders and their antibacterial activity. J Ceram Process Res 2010; 11: 547-51.
Procedia Engineering
ProcediaProcedia Engineering 00 (2012) 000–000 Engineering 32 (2012) 821 – 826 www.elsevier.com/locate/procedia
I-SEEC2011
Preparation of Mg-doped ZnO nanoparticles by mechanical milling and their optical properties S. Suwanboona∗ and P. Amornpitoksukb a
Department of Materials Science and Technology, Faculty of Science, Prince of Songkla University, Songkhla, 90112, Thailand Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Prince of Songkla University, Songkhla, 90112, Thailand
b
Elsevier use only: Received 30 September 2011; Revised 10 November 2011; Accepted 25 November 2011.
Abstract Mg-doped ZnO nanoparticles were synthesized by planetary ball milling at a speed of 400 rpm and milled for 20 h. The samples were characterized by XRD, SEM and UV-Vis spectrophotometer. The crystallite size of the samples increased and the lattice strain decreased with an increase of MgO loading. The increase in crystallite size of the samples as a function of MgO loading can be explained by the effect of Ostwald ripening. The absorption edge of the samples shifted to a lower wavelength when MgO loading was increased. The energy band gap of the samples varied in a range of 2.96-3.13 eV depending on the loading content.
© 2010 Published by Elsevier Ltd. Selection and/or peer-review under responsibility of I-SEEC2011 Keywords: Mg-doped ZnO; nanoparticles; mechanical milling; X-ray method; optical properties
1. Introduction Nanostructured materials have been receiving considerable attention for a decade due to their excellent properties compared to the same materials, consisting of a large grain. Recently, nanostructured ZnO is one of semiconducting metal oxide materials that has been investigated widely for fundamental and advanced research. ZnO is an n-type II-VI semiconducting compound and possesses a wide band gap (3.2 eV) and a large exciton binding energy (60 meV) [1]. ZnO can be extensively used for varistors [2], sensors [3] and photocatalysts [4]. Nanostructured ZnO material can be synthesized via a bottom-up technique including sol-gel, precipitation and hydrothermal
* Corresponding author. Tel.: +66-74-288-250; fax: +66-74-288-395. E-mail address: [email protected].
1877-7058 © 2012 Published by Elsevier Ltd. doi:10.1016/j.proeng.2012.02.018
822 2
S. Suwanboon andAmornpitoksuk P. Amornpitoksuk / Procedia Engineering 32 (2012) 821 – 826 S. Suwanboon and P. / Procedia Engineering 00 (2012) 000–000
method and top-down technique such as grinding, conventional milling and high-energy milling [5]. As far as the above methods are concerned, only a few researchers [6,7] reported a preparation of Mg-doped ZnO nanoparticles through the mechanical milling method. Therefore, this study focused on preparation of Mg-doped ZnO nanoparticle by mechanical milling and investigated the dependence of optical band gap on structural properties of milled samples. 2. Experimental ZnO (99%, Fluka Chimie, Switzerland) and MgO (Riedel-deHaën) powders were used as starting materials. Zn1-xMgxO nanoparticles were prepared by a Pulverisette 7 FRITSCH planetary ball mill. The stoichiometric quantities were weighted to obtain the mixtures of ZnO with 1, 3 and 5 mol% of Mg or x = 0.01, 0.03 and 0.05. The milling was performed in a silicon nitride vial with silicon nitride balls. The ballto-powder ratio (BPR) was 10:1, the speed was 400 rpm and the milling time was 20 h. The mixtures were milled for 10 min, alternating with a stop for 5 min so as to prevent over heating. Structural properties of all samples were studied by X-ray diffraction using CuKα radiation (XRD, X’Pert MPD, PHILIPS). The morphology was carried out by a scanning electron microscope (SEM, JEOL-5800 LV) and diffuse reflection spectra were measured by a UV-Vis spectrophotometer (UV-Vis 2450, Shimadzu). 3. Results and discussion 3.1 Structural study
(112)
(103)
(110)
(102)
intensity (a.u.)
(100) (002) (101)
The effect of Mg contents on Mg-doped ZnO powders formation was studied by milling at 400 rpm for 20 h. The crystalline phase of as-milled samples was determined by XRD technique and all results are presented in Fig. 1.
5 mol% Mg 3 mol% Mg 1 mol% Mg 0 mol% Mg
10
20
30
40
50
60
70
80
90
o
2θ ( ) Fig. 1. XRD patterns of Mg-doped ZnO nanoparticles at different Mg contents
100
S.S.Suwanboon Engineering 3200 (2012) 821 – 826 Suwanboonand andP.P.Amornpitoksuk Amornpitoksuk/ Procedia / Procedia Engineering (2012) 000–000
3
From Fig. 1, it was evident that no diffraction peaks of MgO were detected under the limitation of XRD used. The diffraction peaks of all samples exhibited a ZnO hexagonal wurtzite structure with highquality crystallinity in good agreement with the JCPDS standard card number of 36-1451. In this study, the crystallite size and lattice strain were evaluated by the Williamson-Hall equation [8] and the results are given in Table 1.
β cos θ 0.9 η sin θ = + D λ λ
(1)
where β is the full-width at the half-maximum, θ is the Bragg angle, λ is the wavelength of X-ray used, D is the crystallite size and η is the lattice strain. Besides, the lattice parameters were estimated from the following relation [1] and the calculated results are also displayed in Table 1. 1 2 d hkl
=
4 ⎡ h 2 + hk + k 2 ⎤ l 2 ⎥+ 2 ⎢ 3 ⎣⎢ a2 ⎦⎥ c
(2)
where dhkl is the lattice spacing of the (hkl) plane and a and c are the lattice parameters. As mentioned in [1], the change in lattice parameters of metal-doped ZnO powders depends upon the ionic radius of doping ion. In this study, the ionic radius of Mg2+ ion (72 pm) is smaller than that of the ionic radius of Zn2+ ion (74 pm). Therefore, the lattice parameters contracted when the Mg ion substituted Zn ion in the lattice and the contraction of lattice parameters is a function of Mg content. This is in good agreement with the reduction of lattice volume. For this reason, it could be concluded that the Zn1-xMgxO can form in the whole range of Mg content used. The crystallite size increased as Mg content was increased, this might be due to the coarsening of the crystal and the growth occurred through the Ostwald ripening mechanism [9]. Generally, the heat was generated during milling process because of the collision between the vial wall and media. This evidence promoted the Ostwald ripening and brought about the reduction of lattice strain. The morphology of Mg-doped ZnO nanoparticles at different Mg contents is presented in Fig. 2. The spherical nanoparticles are agglomerated so as to reduce the total surface free energy. This evidence always occurs when the particles are small [1, 8]. In this study, the particle size increased as Mg content was increased in accordance with the crystallite size obtained by XRD analysis. Table 1. XRD analysis and optical properties of Mg-doped ZnO nanoparticles Mg content
D
(mol%)
(nm)
Eg
E0
lattice parameter (Å)
V
(eV)
(eV)
a
c
(Å)3
0
22.12
1
22.99
1.36
2.96
0.39
3.2593
5.2220
48.04
1.31
3.06
0.34
3.2571
5.2110
47.87
3
28.30
1.05
3.11
0.30
3.2552
5.2123
47.83
5
30.77
1.01
3.13
0.30
3.2524
5.2076
47.70
η
823
824 4
S. Suwanboon andAmornpitoksuk P. Amornpitoksuk / Procedia Engineering 32 (2012) 821 – 826 S. Suwanboon and P. / Procedia Engineering 00 (2012) 000–000 0 mol%
1 mol%
3 mol%
5 mol%
Fig. 2. SEM images of Mg-doped ZnO nanoparticle at different Mg contents
3.2 Optical study In this part, the absorbance of Mg-doped ZnO nanoparticles was measured in the wavelength of 200800 nm as shown in Fig. 3. 1.60
0 mol%
1.40
1 mol% 3 mol%
absorbance
1.20
5 mol%
1.00 0.80 0.60 0.40 0.20 0.00 200
300
400
500
600
700
wavelength (nm) Fig. 3. Absorbance of Mg-doped ZnO nanoparticles at different Mg contents
800
S.S.Suwanboon Engineering 3200 (2012) 821 – 826 Suwanboonand andP.P.Amornpitoksuk Amornpitoksuk/ Procedia / Procedia Engineering (2012) 000–000
5
The absorption edge of Mg-doped ZnO nanoparticles was shifted to a lower wavelength (blue-shift) when the Mg content was altered from 0 to 5 mol%. This might be due to a decrease in lattice strain of sample doping with Mg ion. This blue-shift behavior indicated that a wider band gap (Eg) was achieved when higher Mg content was doped in ZnO lattice. The Eg value of Mg-doped ZnO nanoparticles, as shown in Fig. 4(a), was estimated from a linear portion of the (αE)2 versus E curves using the relation as follow [8]: (αE)2 =ED(E-Eg)
(3)
where α is the absorption coefficient, E is the photon energy, ED is a constant, Eg is the optical band gap. The Eg values of all samples are given in Table 1. It was clearly observed that the Eg value increased when the Mg content was increased or when the crystallite size increased. As we know, the Eg value depended upon many key parameters such as particle size, particle shape and defect concentration [10]. In this study, the particle shape is similar for the whole range of Mg doping content. So, only particle size and defect concentration were considered. The wider Eg value was normally obtained from the sample with a smaller particle size, but the narrower Eg value was obtained from Mg-doped ZnO nanoparticles with a smaller size. Therefore, it could be surmised that the defect concentration played an important role in a reduction of Eg value. So, the curve of ln(α) versus E was plotted as shown in Fig. 4 (b). The reciprocal value of the slope identified the defect concentration and the results are given in Table 1. From the results obtained, it is reasonable to conclude that the defect concentration influenced a reduction of Eg value. (a)
0 mol% 1 mol%
1 mol% 3 mol% 5 mol%
5 mol%
ln(α )
(αE)2 (eV/cm)2
3 mol%
2.90
(b)
0 mol%
3.00
3.10
3.20
3.30 E (eV)
3.40
3.50
3.60
2.90
3.00
3.10
3.20
3.30
3.40
3.50
3.60
E (eV)
Fig. 4. (a) Evaluation of (αE)2 versus E for estimating the Eg value and (b) plots of ln(α) versus E for evaluating the defect concentration of Mg-doped ZnO nanoparticles at different Mg contents
4. Conclusions The hexagonal wurtzite Mg-doped ZnO nanoparticles that are in the form of Zn1-xMgxO (x = 0-0.05) were successfully synthesized by high-energy ball milling method at a speed of 400 rpm for 20 h. As increase in Mg contents, the crystallite size increased and the lattice strain decreased because the crystals grow via the Ostwald ripening. The spherical nanoparticles were agglomerated into a cluster in order to reduce the surface free energy. The absorption edge of Mg-doped ZnO nanoparticles performed a blueshift behavior compared to pure ZnO nanoparticles and the Eg values of Mg-doped ZnO nanoparticles
825
826 6
S. Suwanboon andAmornpitoksuk P. Amornpitoksuk / Procedia Engineering 32 (2012) 821 – 826 S. Suwanboon and P. / Procedia Engineering 00 (2012) 000–000
were wider than the Eg value of pure ZnO nanoparticles. In this study, the defect concentration affected significantly the reduction of Eg value. Acknowledgements This work has been supported by Science Research Fund, Faculty of Science, Prince of Songkla University (Fiscal Year 2011) under the Contact Number 154001. The authors also would like to acknowledge the Center for Innovation in Chemistry (PERCH-CIC), Commission on Higher Education, Ministry of Education and the authors would like to acknowledge Michael Benjamin Lane for his English manuscript proofreading service. References [1] Suwanboon S, Amornpitoksuk P, Sukolrat A. Dependence of optical properties on doping metal, crystallite size and defect concentration of M-doped ZnO nanopowders (M = Al, Mg, Ti). Ceram Int 2011; 37: 1359-65. [2] Houabes M, Metz R. Rare earth oxides effects on both the threshold voltage and energy absorption capability of ZnO varistors. Ceram Int 2007; 33: 1191-7. [3] Hongsith N, Wongrat E, Kerdcharoen T, Choopun S. Sensor response formula for sensor based on ZnO nanostructures. Sens Actuators B 2010; 144: 67-72. [4] Sun JH, Dong SY, Feng JL, Yin XJ, Zhao XC. Enhanced sunlight photocatalytic performance of Sn-doped ZnO for Methylene Blue degradation. J Mol Catal A: Chem 2011; 335: 145-50. [5] Suwanboon S, Amornpitoksuk P. Preparation and characterization of nanocrystalline La-doped ZnO powders through a mechanical milling and their optical properties. Ceram Int 2011; doi:10.1016/j.ceramint.2011.06.007. [6] Damonte LC, Hernández-Fenollosa MA, Marŕ B. Cation substitution in ZnO obtained by mechanical milling. J Alloys Compd 2007; 434-435: 813-5. [7] Nursyahadah MZ, Nurul SS, Azlan Z, Kumar MT. Effect of magnesium doping on structural and optical properties of ZnO nanoparticles synthesized by mechanochemical processing. AIP Conference Proceeding 2011; 1328: 211-3. [8] Suwanboon S, Amornpitoksuk P, Bangrak P. Synthesis, characterization and optical properties of Zn1-xTixO nanoparticles prepared via a high-energy ball milling technique. Ceram Int 2011; 37: 333-40. [9] Singh RG, Singh F, Kumar V, Mehra RM. Growth kinetics of ZnO nanocrystallites: Structural, optical and photoluminescence properties tuned by thermal annealing. Curr Appl Phys 2011; 11: 624-30. [10] Suwanboon S, Amornpitoksuk P, Bangrak P, Sukolrat A, Muensit N. The dependence of optical properties on the morphology and defects of nanocrystalline ZnO powders and their antibacterial activity. J Ceram Process Res 2010; 11: 547-51.