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Synthesis, and characterization of Iron-doped Zinc Oxide nanoparticles; Influence of drying
Available online at www.sciencedirect.com
ScienceDirect Materials Today: Proceedings 17 (2019) 1479–1486
www.materialstoday.com/proceedings
MRS-Thailand 2017
Synthesis, and characterization of Iron-doped Zinc Oxide nanoparticles; Influence of drying Suradchadaporn Wiengnona, Piched Anuragudomb* a
Department of Chemistry, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaeng Saen Campus, Nakhon Pathom 73140, Thailand bDepartment of Chemistry, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaeng Saen Campus, Nakhon Pathom 73140, Thailand
Abstract Iron-doped ZnO nanoparticles have been prepared by solution chemical method from zinc acetate dihydrate and ammonium hydroxide as precursors. Fe-doped ZnO nanoparticles containing different concentration of Fe3+ (0.1 wt.% to 2.0 wt.%) were prepared. The obtained mixed solution was separated into two parts for drying processes. One part (P1) was washed with methanol and dried in an oven at room temperature to 300 oC and another part (P2) was submitted to freeze dryer and dried at room temperature to 300 oC. Structural characterizations by X-ray Diffraction (XRD) confirmed the phase purity of the samples. The XRD patterns of pure ZnO nanoparticles as well as Fe-doped ZnO nanoparticles prepared by solution chemical method. Its clear form the XRD patterns are quite sharp and intense owing to the high crystallinity and corresponding to the pure hexagonal wurtzite structure. No secondary phases or cluster are observed in the samples within the detection limits of XRD system, thus providing evidence for the incorporation of Fe at the Zn sites. The crystallite size of the samples calculated using Scherrer’s equation was found to be 33.88-52.61 for P1 and 29.26-78.01 nm for P2. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference. Keywords: Fe-doped ZnO nanoparticles; Drying processes; Freeze dryer;
* Corresponding author. Tel.: +66 3430 0481; fax: +66 3435 1402. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference.
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1. Introduction Zinc oxide nanoparticles (ZnO) are one of the well-known semiconductor metal oxides with a wide direct band gap (3.37 eV) and large exciton binding energy (60 meV), providing various applications such as varistors, catalysts, pigments, cosmetics, gas sensors, ceramics, sunscreen lotion (cream), medicated creams and photocatalytic [1-3]. ZnO nanoparticles doped with different elements such as Al, In, Ga, Fe exhibit improved electrical, optical, and catalytic properties [4]. Iron (Fe) is chemically stable and exists in two possible oxidation states, Fe2+ and Fe3+ having ionic radii (0.78 Å and 0.64 Å) close to the ionic radius of Zn2+ (0.74 Å). Thus, it can easily enter into Zn lattice sites either substitutionally or interstitially without disturbing the crystal structure of ZnO and can contribute more charge carriers in order to improve its conductivity. Nowadays various methods have been presented for preparing ZnO nanoparticles such as sol-gel [5], spray pyrolysis [6], precipitation [7], hydrothermal processes [8] and solution method [9]. Among these methods, the solution method had attracted considerable attention because of its simplicity, acceptable costs and the good crystalline quality of the obtained ZnO [10]. Advanced oxidation processes constitute an effective technology for the treatment of wastewaters containing non-easily removable organic compounds. In recent times, advanced oxidation processes are considered very promising alternatives to conventional effluents treatments due to their efficiency to oxidize a wide variety of organic contaminants by the generation of highly oxidative hydroxyl radicals. The semiconductor photocatalysis of ZnO, the most common used photocatalyst [11], is presented below. The photocatalytic process is initiated by the illumination of a semiconductor catalyst with radiation of energy higher than the band gap energy of the semiconductor. This irradiation generates electrons (e–) and holes (h+) in the conduction band (CB) and valence band (VB) respectively shown in Scheme 1 and as given by Eq. (1).
ZnO
hv
e(−CB ) + h(+VB)
(1)
The electron-hole pair formed may recombine in the bulk lattice (or) migrate to the surface where they can react with the adsorbents [12]. Holes are trapping reaction proceeds with the formation of hydroxyl radicals as given by equation.
h + + OH − → •OH +
•
h + H 2O → OH + H
(2) +
(3)
The electrons are trapped by dissolved oxygen resulting in the formation of superoxide ion.
e− + O2 → O2•−
(4)
The photocatalytic degradability by ZnO involved the hydroxy radical and holes for oxidation of organic molecules (OM). The reactions are given below.
h + + OM → OM + +
OM + O2 → Products
(5) (6)
This work studied the synthesis of 0.1, 0.2, 0.5, 1.0 and 2.0% for Fe-doped ZnO and was prepared with solution method. Freeze-drying and filtration technique were applied to purify the synthesized samples. The photocatalytic activities of iron-doped ZnO nanoparticles were tested in the photocatalytic oxidation of methylene blue dyes.
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2. Experimental 2.1 Synthesis 0.1, 0.2, 0.5, 1.0 and 0.2 wt% iron-doped ZnO nanoparticle were prepared with solution method by using zinc acetate dihydrate (Zn(CH3COO)2.2H2O) (99%), iron (III) nitrate and concentrated ammonium hydroxide (NaOH)
(98%) as precursors. The solution was adjusted to make the pH 9.8 with a concentration of NH4OH. The solution was refluxed at 90 °C for 1 hr. Freeze-drying and filtration techniques were applied to dry the synthesized samples. Calcination of Fe-doped ZnO nanoparticles was done at a temperatures ranging from 200-300 ๐C. 2.2 Characterization The structure, phase, and size of zinc oxide nanoparticles were characterized by X-ray diffraction (XRD) technique using a DMAX/2200/PC X-ray RIKAGU diffractometer. 2.3 Photocatalysis Photocatalytic activity of Fe-ZnO was tested by photocatalytic oxidation of methylene blue dyes. The preparation process is shown in a diagram in Fig. 1. UV (λ = 365 nm) irradiation and absorption measured by a UVVis Spectrophotometer (PG Instrument T80) every 10 min for 3 hrs.The process
Fe-ZnO nanoparticles 0.1, 02, 0.5, 1.0 and 2.0% into beakers
3 ppm of Methylene blue solution
Sonicated for 15 min and Stirrer in the black room for 2-4 hrs.
Fig. 1. Diagram showing the preparation for UV irradiation and absorption measurements.
3. Results and Discussion Based on the experiment, the complete hydrolysis of zinc acetate with the aid of NH4OH resulted in the formation of ZnO. The final product was obtained as a result of the equilibrium between the hydrolysis and condensation reaction. Due to the heating, zinc acetate within the solution undergoes hydrolysis forming acetate ions and zinc ions. The abundance of electrons in the oxygen atoms makes the hydroxyl groups (-OH) of alcohol molecules bond with the zinc ions [13]. The overall chemical reaction to form ZnO nano-powder when ammonium hydroxide was used as solvent stated as follow in Eq. 7 and 8: Zn(OAc)2 (aq) + NH4OH (NH4)2ZnO2 + H2O
∆
(NH4)2ZnO2 + H2O ZnO(s) + NH4OH
(7) (8)
With prolonged refluxing, ZnO can easily be formed at higher temperatures. Ammonium hydroxide is water soluble and could be removed from the end product. High purity ZnO nanoparticles could be obtained successfully by solution method. Filtration is another ancient and widely used technique that removes particles but the freeze drying process minimalizes chemical decomposition and the resulting product has a very high surface area.
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3.1 X-ray Diffraction analysis The XRD patterns of the prepared samples are shown in Fig. 2. All the detectable peaks could be indexed as the hexagonal ZnO phase, with the wurtzite structure found in the standard reference data (JCPDs: 36-1451). It can be concluded that the prepared samples have high crystallinity. The highest peak is at the intensity of 0.2% Fe-ZnO.
Fig. 2. The XRD patterns of Fe-ZnO nanoparticles via a freeze-drying techniques (a), (c) and (e) and a filtration techniques (b), (d) and (f) calcinated at 200, 250 and 300 oC, respectively.
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The peak broadening from XRD results can be utilized to evaluate the crystallite size and lattice strain, due to dislocation [14]. The particle size of the Fe-ZnO nanoparticles were determined by the X-ray line broadening method using the Scherrer equation: D= (kʎ/βcosθ), where D is the particle size in nanometer, ʎ is the wavelength of the radiation (1.54056 Ao) for CuKα radiation), k is a constant equal to 0.94, β is the peak width at half-maximum intensity and θ is the peak position [15]. The average crystallite sizes of 0.2% Fe-ZnO (freeze-dried) and 0.2% Fe-ZnO (filtrated) samples are given in tables 1 and 2, respectively. It was clearly seen that the reflection peaks became sharper with increasing calcination temperature, indicating the enhancement of crystallinity [16]. Table1. Average crystallite size of 0.2% Fe-ZnO (freeze-dried samples) calcinated at different temperatures. Calcination Temp. (OC)
D (nm)
0.2%Fe-ZnO (Freeze)
49.79
o
49.25
o
49.78
o
50.34
0.2%Fe-ZnO 200 C (Freeze) 0.2%Fe-ZnO 250 C (Freeze) 0.2%Fe-ZnO 300 C (Freeze)
Table 2. Average crystallite size of 0.2% Fe-ZnO (filtrated sample) calcinated at different temperatures Calcination Temp. (OC)
D (nm)
0.2%Fe-ZnO
60.71 o
47.76
o
51.07
o
56.12
0.2%Fe-ZnO 200 C 0.2%Fe-ZnO 250 C 0.2%Fe-ZnO 300 C 3.2 Photocatalysis
A good catalyst should be stable under operation conditions [17]. Therefore, the chemical stability of the FeZnO photocatalyst was assessed. Photocatalytic activity of 0.1, 0.2, 0.5, 1.0 and 2.0% Fe-ZnO (borh freeze-dried and filtrated nanoparticles) was studied with prolonged exposure to UV light in aqueous solution. Figures 3 and 4 show the optical absorption spectra of the methylene blue solution at different time intervals of the photodegradation reactions over Fe- ZnO. For both samples, the degradation behaviours were found to be similar but with different time responses.
Fig. 3. Photocatalysis of (a) 0.2% Fe-ZnO (Freeze) (a) 200 oC, (b) 250 oC and (c) 300 oC.
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The results gave us an impression about the effect of the morphology on the degradation efficiency for the two different morphologies of Fe-ZnO catalysts. The effect of morphology on the photodegradation efficiency can be ascribed to the following reason; when the size of ZnO crystals decreases, the number of the dispersion particles per volume in the solution will increase, resulting in the enhancement of the photon absorbance [18]. Table 3 shows the percentage of degradation of the 0.2% Fe-ZnO (freeze-dried) that was calcinated at different temperatures. The sample calcinated at 200 oC exhibited the best catalytic efficiency and 80.73 % degradation. Table 3. The percentage of degradation of 0.2% Fe-ZnO (both freeze-dried and filtrated samples) calcinated at different temperatures under UV irradiation. Filtration Freeze Dry η (%) η (%) K K 0.2%Fe-ZnO 36.02 0.0032 60.80 0.0071 0.2%Fe-ZnO, 200 oC 65.48 0.0076 80.73 0.0012 0.2%Fe-ZnO, 250 oC 61.82 0.0069 76.83 0.0105 0.2%Fe-ZnO, 300 oC 53.38 0.0059 63.44 0.0073 The photocatalytic activity of 0.1, 0.2, 0.5, 1.0 and 2.0% Fe-ZnO 200 oC of freeze and filtration after prolonged exposure to UV light in aqueous solution were shown in Fig. 4. The optical absorption spectra of the methylene blue solution at different time intervals of the photodegradation reactions over Fe-ZnO.
Fig. 4. Photocatalysis of (a) 0.1% Fe-ZnO Filtration Temp. 200 oC, (b) 0.5% Fe-ZnO Filtration Temp. 200 oC, 1.0% Fe-ZnO Filtration Temp. 200 oC and 0.1% Fe-ZnO Filtration Temp. 200 oC
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Table 4. The percentage of degradation under UV irradiation for Fe-ZnO (freeze-dried samples) calcinated at 200 oC. Freeze Dry η (%) K 0.1%Fe-ZnO 48.18 0.0051 0.2%Fe-ZnO 80.73 0.0012 0.5%Fe-ZnO 42.90 0.0042 1.0%Fe-ZnO 29.88 0.0029 2.0%Fe-ZnO 22.12 0.0019 ZnO 32.84 0.0031 The results of Fig. 4, and Table 4 gave us an impression about the degradation efficiency for the five different morphologies of iron dope ZnO catalysts. The 0.2% Fe-ZnO freeze-dried samples that were calcinated at 200 o C provided the best catalytic efficiency. 4. Conclusion ZnO nanoparticles doped with 0.1, 0.2, 0.5, 1.0 and 2.0 wt% Fe were successfully synthesized by using solution method. The XRD patterns of Fe-ZnO exhibited the hexagonal phase (Wurtzite structure) when comparing the results with the data from JCPDs file no. 36-1451. It can be concluded that the prepared sample has high crystallinity. The photocatalytic degradation of methylene blue was also conducted. The degradation of the 0.2% FeZnO (filtrated and freeze-dried samples) calcinated at 200 oC were 65.48% and 80.73% respectively. The sample with 0.2% Fe that was prepared by a freeze-drying techniques and calcinated at 200 oC provided the best catalytic efficiency. Acknowledgements The presented research is financed from Chemistry Division, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaengsaen Campus. We would like to acknowledge the Department of Primary Industries and Mines, Region 3 Chiang Mai for providing access to the XRD equipment. References [1] C.R. Bhattacharjee, D.D. Purkayastha, S. Bhattacharjee and A. Nath. Homogeneous chemical precipitation route to ZnO nanosphericals, Assam University Journal of Sci. &Tech.: Phys. Sci. and Tech., 7(2): 122-127. [2]C.C. Hwang, T.-Y. Wu, Synthesis and characterization of nanocrystalline ZnO powders by a novel combustion synthesis method Mater, Sci. Eng, 111(2-3):197-206. [3] F. Xu, X. Liu, S.D. Tse, F. Cosandey, B.H. Kear, Flame synthesis of zinc oxide nanocrystals, Materials Sci. and Eng. B, 178: 127–134. [4] W. Khan, A. A. Saad, S. Shervani, A. Saleem, A. H. Naqvi, Synthesis and characterization of al doped ZnO nanoparticles, Int. Conference on Ceramics, 22(2013): 630-636. [5] A. Z. Khorsand, W.H. Abd. Majid, M.E Abrishami, R. Yousefi, X-ray analysis of ZnO nanoparticles by Williamson-Hall and size-strain plot methods, Solid State Sci., 13(2011):251-256. [6]N. Lehraki, M.s. Aida, S. Aida, N. Attaf, A. Attaf, M. Poulaine, ZnO thin films deposition by spray pyrolysis: Influence of precursor solution properties, Current Appl. Phys., 12(2012): 1283-1287. [7] H.R. Ghorbani, F.P. Mehr., H. Pazoki, B.M. Rahmani, Synthesis of ZnO Nanoparticles by Precipitation Method. Oriental journal of chem., 31(2): 1219-1221. [8]S. Baruah, J. Dutta, Hydrothermal growth of ZnO nanostructures, Sci. and Tech. of Advanced Materials, 10(2009). [9] S.Y. Park, B.J. kim, K. Kim, Low-Temperature, Solution-Processed and Alkali Metal Doped ZnO for High-Performance Thin-Film Transistors, Advanced Materals, 24(2012): 834-938. [10] S. Liu, E. Lei, Y. Jing, Y. Xin, Growth of ZnO nanorods by aqueous solution method with electrodeposited ZnO seed layers, Appl. Surface Sci.,12(2009): 6415-6420.
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[11] V. C. Sriivastava, Photocatalytic Oxidation of Dry Bearing Wastewater by Iron Doped Zinc Oxide, Ind. Eng. Chem. Res.,52(2012): 17790-17799. [12] C.S. Turichi, D.F. Ollis, Photocatalytic degradation of an organic pollutant by zinc oxide – solar process, J. Catal., 122 (1990): 178-192. [13] T. Shokuhfa., M.R. Vaezi, S.K. Sadrnezhad, Synthesis of zinc oxide nanopowder and nanolayer via chemical processing, Int. J. Nanomanufacturing, 44(2009) :149. [14] H. Xu, H. Wang, Y. Zhang, W. He, M. Zhu, B. Wang, H. Yan, Hydrothermal synthesis of zinc oxide powders with controllable morphology. Ceramics Int., 30(2004): 93-97. [15] R. Hong, T. Pan., J. Qian, H. Li, Synthesis and surface modification of ZnO nanoparticles. Chem. Eng. Journal 119: 71-81. [16] K.H. Harbbi, A.A. thsan, Restriction of Particle Size and Lattice strain through X-Ray Diffraction Peak Broadening Analysis of ZnO Nanoparticles, Advances in Phys Theorier and Appl., 49(2015). [17] Y.H. Katayama, Material design for transparent ferromagnets with ZnO-based magnetic semiconductors, Jpn. J. Appl. Phys, 39(6): 555-558. [18] N. Elamin., A. Elsanousi, Synthesis of ZnO Nanostructures and their Photocatalytic Activity. Journal of Appl. and Industrial Sci., 1(1): 32-35.
ScienceDirect Materials Today: Proceedings 17 (2019) 1479–1486
www.materialstoday.com/proceedings
MRS-Thailand 2017
Synthesis, and characterization of Iron-doped Zinc Oxide nanoparticles; Influence of drying Suradchadaporn Wiengnona, Piched Anuragudomb* a
Department of Chemistry, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaeng Saen Campus, Nakhon Pathom 73140, Thailand bDepartment of Chemistry, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaeng Saen Campus, Nakhon Pathom 73140, Thailand
Abstract Iron-doped ZnO nanoparticles have been prepared by solution chemical method from zinc acetate dihydrate and ammonium hydroxide as precursors. Fe-doped ZnO nanoparticles containing different concentration of Fe3+ (0.1 wt.% to 2.0 wt.%) were prepared. The obtained mixed solution was separated into two parts for drying processes. One part (P1) was washed with methanol and dried in an oven at room temperature to 300 oC and another part (P2) was submitted to freeze dryer and dried at room temperature to 300 oC. Structural characterizations by X-ray Diffraction (XRD) confirmed the phase purity of the samples. The XRD patterns of pure ZnO nanoparticles as well as Fe-doped ZnO nanoparticles prepared by solution chemical method. Its clear form the XRD patterns are quite sharp and intense owing to the high crystallinity and corresponding to the pure hexagonal wurtzite structure. No secondary phases or cluster are observed in the samples within the detection limits of XRD system, thus providing evidence for the incorporation of Fe at the Zn sites. The crystallite size of the samples calculated using Scherrer’s equation was found to be 33.88-52.61 for P1 and 29.26-78.01 nm for P2. © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference. Keywords: Fe-doped ZnO nanoparticles; Drying processes; Freeze dryer;
* Corresponding author. Tel.: +66 3430 0481; fax: +66 3435 1402. E-mail address: [email protected] 2214-7853 © 2019 Elsevier Ltd. All rights reserved. Selection and/or Peer-review under responsibility of The First Materials Research Society of Thailand International Conference.
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1. Introduction Zinc oxide nanoparticles (ZnO) are one of the well-known semiconductor metal oxides with a wide direct band gap (3.37 eV) and large exciton binding energy (60 meV), providing various applications such as varistors, catalysts, pigments, cosmetics, gas sensors, ceramics, sunscreen lotion (cream), medicated creams and photocatalytic [1-3]. ZnO nanoparticles doped with different elements such as Al, In, Ga, Fe exhibit improved electrical, optical, and catalytic properties [4]. Iron (Fe) is chemically stable and exists in two possible oxidation states, Fe2+ and Fe3+ having ionic radii (0.78 Å and 0.64 Å) close to the ionic radius of Zn2+ (0.74 Å). Thus, it can easily enter into Zn lattice sites either substitutionally or interstitially without disturbing the crystal structure of ZnO and can contribute more charge carriers in order to improve its conductivity. Nowadays various methods have been presented for preparing ZnO nanoparticles such as sol-gel [5], spray pyrolysis [6], precipitation [7], hydrothermal processes [8] and solution method [9]. Among these methods, the solution method had attracted considerable attention because of its simplicity, acceptable costs and the good crystalline quality of the obtained ZnO [10]. Advanced oxidation processes constitute an effective technology for the treatment of wastewaters containing non-easily removable organic compounds. In recent times, advanced oxidation processes are considered very promising alternatives to conventional effluents treatments due to their efficiency to oxidize a wide variety of organic contaminants by the generation of highly oxidative hydroxyl radicals. The semiconductor photocatalysis of ZnO, the most common used photocatalyst [11], is presented below. The photocatalytic process is initiated by the illumination of a semiconductor catalyst with radiation of energy higher than the band gap energy of the semiconductor. This irradiation generates electrons (e–) and holes (h+) in the conduction band (CB) and valence band (VB) respectively shown in Scheme 1 and as given by Eq. (1).
ZnO
hv
e(−CB ) + h(+VB)
(1)
The electron-hole pair formed may recombine in the bulk lattice (or) migrate to the surface where they can react with the adsorbents [12]. Holes are trapping reaction proceeds with the formation of hydroxyl radicals as given by equation.
h + + OH − → •OH +
•
h + H 2O → OH + H
(2) +
(3)
The electrons are trapped by dissolved oxygen resulting in the formation of superoxide ion.
e− + O2 → O2•−
(4)
The photocatalytic degradability by ZnO involved the hydroxy radical and holes for oxidation of organic molecules (OM). The reactions are given below.
h + + OM → OM + +
OM + O2 → Products
(5) (6)
This work studied the synthesis of 0.1, 0.2, 0.5, 1.0 and 2.0% for Fe-doped ZnO and was prepared with solution method. Freeze-drying and filtration technique were applied to purify the synthesized samples. The photocatalytic activities of iron-doped ZnO nanoparticles were tested in the photocatalytic oxidation of methylene blue dyes.
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2. Experimental 2.1 Synthesis 0.1, 0.2, 0.5, 1.0 and 0.2 wt% iron-doped ZnO nanoparticle were prepared with solution method by using zinc acetate dihydrate (Zn(CH3COO)2.2H2O) (99%), iron (III) nitrate and concentrated ammonium hydroxide (NaOH)
(98%) as precursors. The solution was adjusted to make the pH 9.8 with a concentration of NH4OH. The solution was refluxed at 90 °C for 1 hr. Freeze-drying and filtration techniques were applied to dry the synthesized samples. Calcination of Fe-doped ZnO nanoparticles was done at a temperatures ranging from 200-300 ๐C. 2.2 Characterization The structure, phase, and size of zinc oxide nanoparticles were characterized by X-ray diffraction (XRD) technique using a DMAX/2200/PC X-ray RIKAGU diffractometer. 2.3 Photocatalysis Photocatalytic activity of Fe-ZnO was tested by photocatalytic oxidation of methylene blue dyes. The preparation process is shown in a diagram in Fig. 1. UV (λ = 365 nm) irradiation and absorption measured by a UVVis Spectrophotometer (PG Instrument T80) every 10 min for 3 hrs.The process
Fe-ZnO nanoparticles 0.1, 02, 0.5, 1.0 and 2.0% into beakers
3 ppm of Methylene blue solution
Sonicated for 15 min and Stirrer in the black room for 2-4 hrs.
Fig. 1. Diagram showing the preparation for UV irradiation and absorption measurements.
3. Results and Discussion Based on the experiment, the complete hydrolysis of zinc acetate with the aid of NH4OH resulted in the formation of ZnO. The final product was obtained as a result of the equilibrium between the hydrolysis and condensation reaction. Due to the heating, zinc acetate within the solution undergoes hydrolysis forming acetate ions and zinc ions. The abundance of electrons in the oxygen atoms makes the hydroxyl groups (-OH) of alcohol molecules bond with the zinc ions [13]. The overall chemical reaction to form ZnO nano-powder when ammonium hydroxide was used as solvent stated as follow in Eq. 7 and 8: Zn(OAc)2 (aq) + NH4OH (NH4)2ZnO2 + H2O
∆
(NH4)2ZnO2 + H2O ZnO(s) + NH4OH
(7) (8)
With prolonged refluxing, ZnO can easily be formed at higher temperatures. Ammonium hydroxide is water soluble and could be removed from the end product. High purity ZnO nanoparticles could be obtained successfully by solution method. Filtration is another ancient and widely used technique that removes particles but the freeze drying process minimalizes chemical decomposition and the resulting product has a very high surface area.
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3.1 X-ray Diffraction analysis The XRD patterns of the prepared samples are shown in Fig. 2. All the detectable peaks could be indexed as the hexagonal ZnO phase, with the wurtzite structure found in the standard reference data (JCPDs: 36-1451). It can be concluded that the prepared samples have high crystallinity. The highest peak is at the intensity of 0.2% Fe-ZnO.
Fig. 2. The XRD patterns of Fe-ZnO nanoparticles via a freeze-drying techniques (a), (c) and (e) and a filtration techniques (b), (d) and (f) calcinated at 200, 250 and 300 oC, respectively.
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The peak broadening from XRD results can be utilized to evaluate the crystallite size and lattice strain, due to dislocation [14]. The particle size of the Fe-ZnO nanoparticles were determined by the X-ray line broadening method using the Scherrer equation: D= (kʎ/βcosθ), where D is the particle size in nanometer, ʎ is the wavelength of the radiation (1.54056 Ao) for CuKα radiation), k is a constant equal to 0.94, β is the peak width at half-maximum intensity and θ is the peak position [15]. The average crystallite sizes of 0.2% Fe-ZnO (freeze-dried) and 0.2% Fe-ZnO (filtrated) samples are given in tables 1 and 2, respectively. It was clearly seen that the reflection peaks became sharper with increasing calcination temperature, indicating the enhancement of crystallinity [16]. Table1. Average crystallite size of 0.2% Fe-ZnO (freeze-dried samples) calcinated at different temperatures. Calcination Temp. (OC)
D (nm)
0.2%Fe-ZnO (Freeze)
49.79
o
49.25
o
49.78
o
50.34
0.2%Fe-ZnO 200 C (Freeze) 0.2%Fe-ZnO 250 C (Freeze) 0.2%Fe-ZnO 300 C (Freeze)
Table 2. Average crystallite size of 0.2% Fe-ZnO (filtrated sample) calcinated at different temperatures Calcination Temp. (OC)
D (nm)
0.2%Fe-ZnO
60.71 o
47.76
o
51.07
o
56.12
0.2%Fe-ZnO 200 C 0.2%Fe-ZnO 250 C 0.2%Fe-ZnO 300 C 3.2 Photocatalysis
A good catalyst should be stable under operation conditions [17]. Therefore, the chemical stability of the FeZnO photocatalyst was assessed. Photocatalytic activity of 0.1, 0.2, 0.5, 1.0 and 2.0% Fe-ZnO (borh freeze-dried and filtrated nanoparticles) was studied with prolonged exposure to UV light in aqueous solution. Figures 3 and 4 show the optical absorption spectra of the methylene blue solution at different time intervals of the photodegradation reactions over Fe- ZnO. For both samples, the degradation behaviours were found to be similar but with different time responses.
Fig. 3. Photocatalysis of (a) 0.2% Fe-ZnO (Freeze) (a) 200 oC, (b) 250 oC and (c) 300 oC.
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The results gave us an impression about the effect of the morphology on the degradation efficiency for the two different morphologies of Fe-ZnO catalysts. The effect of morphology on the photodegradation efficiency can be ascribed to the following reason; when the size of ZnO crystals decreases, the number of the dispersion particles per volume in the solution will increase, resulting in the enhancement of the photon absorbance [18]. Table 3 shows the percentage of degradation of the 0.2% Fe-ZnO (freeze-dried) that was calcinated at different temperatures. The sample calcinated at 200 oC exhibited the best catalytic efficiency and 80.73 % degradation. Table 3. The percentage of degradation of 0.2% Fe-ZnO (both freeze-dried and filtrated samples) calcinated at different temperatures under UV irradiation. Filtration Freeze Dry η (%) η (%) K K 0.2%Fe-ZnO 36.02 0.0032 60.80 0.0071 0.2%Fe-ZnO, 200 oC 65.48 0.0076 80.73 0.0012 0.2%Fe-ZnO, 250 oC 61.82 0.0069 76.83 0.0105 0.2%Fe-ZnO, 300 oC 53.38 0.0059 63.44 0.0073 The photocatalytic activity of 0.1, 0.2, 0.5, 1.0 and 2.0% Fe-ZnO 200 oC of freeze and filtration after prolonged exposure to UV light in aqueous solution were shown in Fig. 4. The optical absorption spectra of the methylene blue solution at different time intervals of the photodegradation reactions over Fe-ZnO.
Fig. 4. Photocatalysis of (a) 0.1% Fe-ZnO Filtration Temp. 200 oC, (b) 0.5% Fe-ZnO Filtration Temp. 200 oC, 1.0% Fe-ZnO Filtration Temp. 200 oC and 0.1% Fe-ZnO Filtration Temp. 200 oC
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Table 4. The percentage of degradation under UV irradiation for Fe-ZnO (freeze-dried samples) calcinated at 200 oC. Freeze Dry η (%) K 0.1%Fe-ZnO 48.18 0.0051 0.2%Fe-ZnO 80.73 0.0012 0.5%Fe-ZnO 42.90 0.0042 1.0%Fe-ZnO 29.88 0.0029 2.0%Fe-ZnO 22.12 0.0019 ZnO 32.84 0.0031 The results of Fig. 4, and Table 4 gave us an impression about the degradation efficiency for the five different morphologies of iron dope ZnO catalysts. The 0.2% Fe-ZnO freeze-dried samples that were calcinated at 200 o C provided the best catalytic efficiency. 4. Conclusion ZnO nanoparticles doped with 0.1, 0.2, 0.5, 1.0 and 2.0 wt% Fe were successfully synthesized by using solution method. The XRD patterns of Fe-ZnO exhibited the hexagonal phase (Wurtzite structure) when comparing the results with the data from JCPDs file no. 36-1451. It can be concluded that the prepared sample has high crystallinity. The photocatalytic degradation of methylene blue was also conducted. The degradation of the 0.2% FeZnO (filtrated and freeze-dried samples) calcinated at 200 oC were 65.48% and 80.73% respectively. The sample with 0.2% Fe that was prepared by a freeze-drying techniques and calcinated at 200 oC provided the best catalytic efficiency. Acknowledgements The presented research is financed from Chemistry Division, Faculty of Liberal Arts and Science, Kasetsart University, Kamphaengsaen Campus. We would like to acknowledge the Department of Primary Industries and Mines, Region 3 Chiang Mai for providing access to the XRD equipment. References [1] C.R. Bhattacharjee, D.D. Purkayastha, S. Bhattacharjee and A. Nath. Homogeneous chemical precipitation route to ZnO nanosphericals, Assam University Journal of Sci. &Tech.: Phys. Sci. and Tech., 7(2): 122-127. [2]C.C. Hwang, T.-Y. Wu, Synthesis and characterization of nanocrystalline ZnO powders by a novel combustion synthesis method Mater, Sci. Eng, 111(2-3):197-206. [3] F. Xu, X. Liu, S.D. Tse, F. Cosandey, B.H. Kear, Flame synthesis of zinc oxide nanocrystals, Materials Sci. and Eng. B, 178: 127–134. [4] W. Khan, A. A. Saad, S. Shervani, A. Saleem, A. H. Naqvi, Synthesis and characterization of al doped ZnO nanoparticles, Int. Conference on Ceramics, 22(2013): 630-636. [5] A. Z. Khorsand, W.H. Abd. Majid, M.E Abrishami, R. Yousefi, X-ray analysis of ZnO nanoparticles by Williamson-Hall and size-strain plot methods, Solid State Sci., 13(2011):251-256. [6]N. Lehraki, M.s. Aida, S. Aida, N. Attaf, A. Attaf, M. Poulaine, ZnO thin films deposition by spray pyrolysis: Influence of precursor solution properties, Current Appl. Phys., 12(2012): 1283-1287. [7] H.R. Ghorbani, F.P. Mehr., H. Pazoki, B.M. Rahmani, Synthesis of ZnO Nanoparticles by Precipitation Method. Oriental journal of chem., 31(2): 1219-1221. [8]S. Baruah, J. Dutta, Hydrothermal growth of ZnO nanostructures, Sci. and Tech. of Advanced Materials, 10(2009). [9] S.Y. Park, B.J. kim, K. Kim, Low-Temperature, Solution-Processed and Alkali Metal Doped ZnO for High-Performance Thin-Film Transistors, Advanced Materals, 24(2012): 834-938. [10] S. Liu, E. Lei, Y. Jing, Y. Xin, Growth of ZnO nanorods by aqueous solution method with electrodeposited ZnO seed layers, Appl. Surface Sci.,12(2009): 6415-6420.
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