- Email: [email protected]
A simple method for preparation of micro-sized ZnO flakes
Materials Letters 91 (2013) 255–257
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A simple method for preparation of micro-sized ZnO flakes Mehdi Zareie a,n, Abed Gholami b, Mohammad Bahrami b, Amir Hossein Rezaei a, Mohammad Hossein Keshavarz a a b
Department of Chemistry, Malek-ashtar University of Technology, Shahin-shahr P.O. Box 83145/115, Islamic Republic of Iran Department of Chemistry, Isfahan University, Isfahan, Islamic Republic of Iran
a r t i c l e i n f o
abstract
Article history: Received 24 July 2012 Accepted 4 October 2012 Available online 13 October 2012
In this paper, ZnO flakes were synthesized by calcinations of the precursor of Zn(OH)2. The reactant Zn(OH)2 was also obtained by the precipitation method via the reaction between zinc sulfate (ZnSO4) and sodium hydroxide (NaOH) in aqueous solutions with appropriate ratio. The precursor precipitates of ZnO were subjected to thermal calcinations, which yielded the ZnO flakes. The synthesized ZnO flakes were characterized by X-ray diffraction measurements (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and ultraviolet–visible (UV–Vis) spectroscopy. XRD analysis revealed the broad pattern for crystal structure of ZnO flakes. The results show that asprepared ZnO flakes have a uniform structure with high purity. & 2012 Published by Elsevier B.V.
Keywords: Microstructure Crystal structure Zinc oxide Micro-sized ZnO flakes Inorganic materials
1. Introduction
2. Experimental
Zinc oxide (ZnO) is a material of particular interest because it has unique optical and electronic properties. ZnO has some characteristics: (1) it is a wide-band gap semiconductor (3.37 ev); (2) it has a large exciton binding energy of 60 mev, which is also luminescent; (3) it has emerged as a good candidate for many applications such as solar cells, photo catalysts, gas sensor, photo detectors and luminescent oxides [1–5]. Various physical or chemical synthetic approaches have been developed to produce ZnO particles, including vapor phase oxidation [6], precipitation [7–9], chemical vapor deposition (CVD) [10], sol–gel [11,12], microemulsion [13], hydrothermal [14], solvothermal [15], and sonochemical methods [16]. Among these synthetic routes, precipitation approach provides a facile way for low caste and large-scale production as compared with other traditional methods because it does not need expensive raw materials and complication equipments. Recently, Li et al. [17] have obtained nano-ZnO flakes by calcinations of the precursor of Zn(OH)2 via the reactive ion-exchange method between an ion-exchange resin and zinc sulfate solution at room temperature. Al-Heniti et al. [18] have synthesized nanocrystalline ZnO flakes via solution process using zinc acetate and diethyl amine under refluxing at 85 1C for 4 h. In the present work, a direct precipitation method was employed to synthesize micro-size ZnO flakes using suitable alternative raw materials, which are attractive for chemical industries.
All the chemicals used in this study were of analytical grade and were used without further purification. Zinc sulfate pentahydrate (ZnSO4(H2O)5) and sodium hydroxide (NaOH) are the two starting materials for the synthesis of ZnO flakes. The synthetic procedure for ZnO flakes by direct precipitations method are briefly summarized in Scheme 1. In this synthesis, ZnO flakes were prepared by thermal calcinations of zinc hydroxide as precursor. Zinc sulfate pentahydrate (20 g) were dissolved in 40 ml deionized water. Sodium hydroxide solution (4 M) was slowly dropped into zinc sulfate solution with vigorous stirring until the pH of reaction mixture become basic (pH¼7.5–8). The precipitates derived from the reaction between the zinc sulfate and the sodium hydroxide solutions were collected by filtration and rinsed three times with distilled water. The washed precipitates were dried at 100 1C for one day to form the precursors of ZnO. Finally, the precursors were calcined at a temperature of 600 1C for 4 h in the furnace to obtain the ZnO flakes. The morphology of the powder was determined by scanning electron microscopy (SEM), equipped with an energy dispersive Xray spectroscope. The compositional analysis was done by energy dispersive X-ray (EDX). X-ray diffraction (XRD) analysis was used for phase composition and line broadening on the synthesized powder with Cu Ka radiation. The electronic spectra of the ZnO flakes were taken on a Shimadzu UV–Vis scanning spectrometer.
3. Results and discussion n
Corresponding author. Tel.: þ98 312 522 5071; fax: þ 98 312 522 5068. E-mail address: [email protected] (M. Zareie).
0167-577X/$ - see front matter & 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2012.10.013
Fig. 1 shows SEM image of ZnO flakes powder resulted from calcinations of zinc hydroxide at 600 1C for 4 h. It demonstrates
256
M. Zareie et al. / Materials Letters 91 (2013) 255–257
Scheme 1. Preparation of ZnO flakes.
Fig. 1. SEM photograph of the ZnO flakes: (a) the low-resolution image and (b) the high-resolution image.
Fig. 2. Typical (a) EDX pattern and (b) X-ray diffraction pattern of the as-synthesized ZnO flake.
the typical SEM images of micro-sized ZnO flakes, which has a large quantity of flake shape with a narrow size distribution. It is confirmed from the low-resolution image that the flakes are synthesized in very large quantity (Fig. 1a). The high-resolution image of the synthesized flakes reveals that the products have uniform flake shape (Fig. 1b). Chemical purity of the samples was tested by EDX. The corresponding EDX spectrum (Fig. 2a) suggests that the as-synthesized ZnO flakes are composed only of zinc and oxygen elements, which indicates that the product is high-pure ZnO. To check the crystallinity and crystal phase of the as-synthesized ZnO flakes, XRD was performed and results are shown in Fig. 2b. All the peaks in the pattern can be indexed to hexagonal phase of bulk ZnO. The sharp diffraction peaks show that the obtained ZnO flakes have high crystallinity. No characteristic peaks from impurities such as Zn(OH)2 are detected. The room-temperature optical properties of the ZnO flakes were investigated by UV–Vis spectra at room temperature (Fig. 3). For the UV–Vis measurement, ZnO flakes were dispersed in water and measured from these dispersions to detect absorption over the range of 300–600 nm. The obtained UV–Vis absorbance spectrum exhibits a well defined excitonic absorption peak at
376 nm, a corresponding peak for wurtzite hexagonal phase bulk ZnO [19]. As seen in Fig. 3, UV–Vis absorption spectrum of the sample has a red-shift (376 nm) compared to that of bulk ZnO ( 370 nm). This red-shift can be explained by the formation of shallow levels inside the band gap due to impurity atoms present in the lattice [20]. In particular, ZnO is also a promising material for spintronics since it can possess the ferromagnetic properties. The micrographs (Fig. 1) witness that the obtained ZnO flakes contain the very developed free surface and UV–Vis-spectrum (Fig. 3) demonstrates the presence of optically active defects. In turn, the ferromagnetic behavior depends on the presence of defects like grain boundaries [21] and on the presence of (invisible for XRD) amorphous intergranular layers [22].
4. Conclusion In summary, the micro-sized ZnO flakes were prepared via thermal calcinations of zinc hydroxide. This method employed an inexpensive, reproducible process for the large-scale synthesis of
M. Zareie et al. / Materials Letters 91 (2013) 255–257
Fig. 3. Typical UV–Vis spectrum for the as-synthesized ZnO flakes.
micro-sized ZnO flakes. SEM images, EDX pattern and XRD pattern indicate that the as-prepared ZnO flakes have uniform structure and high purity. References [1] Ohshima E, Ogino H, Kiikura I, Macda K, Sato M, Ito M, et al. Growth of the 2-in-size bulk ZnO single crystals by the hydrothermal method. J Cryst Growth 2004;260:166–70. [2] Arii T, Kishi A. Humidity controlled thermal analysis. J Therm Anal Calorimetry 2006;83:253–60.
257
[3] Zeng H, Cui J, Cao B, Gibson U, Bando Y, Golberg D. Electrochemical deposition of ZnO nanowire arrays: organization, doping, and properties. Sci Adv Mater 2010;2:336–58. [4] Wu W, Bai S, Cui N, Ma F, Wei Z, Qin Y, et al. Increasing UV photon response of ZnO sensor with nanowires array. Sci Adv Mater 2010;2:402–6. [5] Sharma SP, Ting JM, Chang ZK. Growth of ZnO nanowires on carbon fibers by RF magnetron sputtering technique. Adv Sci Lett 2010;3:74–9. [6] Hu JQ, Li Q, Wong NB, Lee CS, Lee ST. Synthesis of uniform hexagonal prismatic ZnO whiskers. Chem Mater 2002;14:1216–9. [7] Pesika NS, Hu Z, Stebe KJ, Searson PC. The quenching of growth of ZnO nanoparticles by adsorption of octanethiol. J Phys Chem B 2002;106: 6985–90. [8] Radovanovic PV, Norberg NS, McNally KE, Gamelin DR. Colloidal transitionmetal-doped ZnO quantum dots. J Am Chem Soc 2002;124:15193. [9] Seelig EW, Tang B, Yamilov A, Cao H, Chang RPH. Self-assembled 3D photonic crystals from ZnO colloidal spheres. Mater Chem Phys 2003;20:257–63. [10] Wu JJ, Liu SC. Catalyst-free growth and characterization of ZnO nanorods. J Phys Chem B 2002;106:9546–51. [11] Hoyer P, Weller H. Potential-dependent electron injection in nanoporous colloidal ZnO films. J Phys Chem 1995;99(38):14096–100. [12] Paul GK, Bandyopadhyay S, Sen SK, Sen S. Structural, optical and electrical studies on sol–gel deposited Zr doped ZnO films. Mater Chem Phys 2003;79:71–5. [13] Gao L, Ji YL, Xu H, Simon P, Wu Z. Regularly shaped, single-crystalline ZnO nanorods with wurtzite structure. J Am Chem Soc 2002;124:14864–5. [14] Cheng B, Samulski ET. Hydrothermal synthesis of one-dimensional ZnO nanostructures with different aspect ratios. Chem Commun 2004;8:986–7. [15] Jia Z, Yue L, Zheng Y, Xu Z. Rod-like zinc oxide constructed by nanoparticles: synthesis, characterization and optical properties. Mater Chem Phys 2008;107(1):137–41. [16] Qian D, Jiang JZ, Hansen PL. Preparation of ZnO nanocrystals via ultrasonic irradiation. Chem Commun 2003:1078–9. [17] Li YS, Li G, Wang SX, Gao H, Ta ZC. Preparation and characterization of nanoZnO flakes prepared by reactive ion exchange method. Therm Anal Calorimetry 2009;95(2):671–4. [18] Al-Heniti S, Umar A, Singh P, Sayari A, Al-Hajry A, Al-Ghamdi AA. Nanocrystalline ZnO flakes for photovoltaic device applications. Adv Sci Lett 2010;3:543–7. [19] Al-Hajry A, Umar A, Hahn YB, Kim DH. Growth, properties and dye-sensitized solar cells–applications of ZnO nanorods grown by low-temperature solution process. Superlattices Microstruct 2009;45:529–34. [20] Ischenko V, Polarz S, Grote D, Stavarache V, Fink K, Driess M. Zinc oxide nanoparticles with defects. Adv Funct Mater 2005;15:1945–54. ¨ G, [21] Straumal BB, Protasova SG, Mazilkin AA, Myatiev AA, Straumal PB, Schutz et al. Ferromagnetic properties of the Mn-doped nanograined ZnO films. J Appl Phys 2010;108:073923–6. [22] Straumal BB, Myatiev AA, Straumal PB, Mazilkin AA, Protasova SG, Goering E, et al. Grain boundary layers in nanocrystalline ferromagnetic zinc oxide. JETP Lett 2010;92:396–400.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
A simple method for preparation of micro-sized ZnO flakes Mehdi Zareie a,n, Abed Gholami b, Mohammad Bahrami b, Amir Hossein Rezaei a, Mohammad Hossein Keshavarz a a b
Department of Chemistry, Malek-ashtar University of Technology, Shahin-shahr P.O. Box 83145/115, Islamic Republic of Iran Department of Chemistry, Isfahan University, Isfahan, Islamic Republic of Iran
a r t i c l e i n f o
abstract
Article history: Received 24 July 2012 Accepted 4 October 2012 Available online 13 October 2012
In this paper, ZnO flakes were synthesized by calcinations of the precursor of Zn(OH)2. The reactant Zn(OH)2 was also obtained by the precipitation method via the reaction between zinc sulfate (ZnSO4) and sodium hydroxide (NaOH) in aqueous solutions with appropriate ratio. The precursor precipitates of ZnO were subjected to thermal calcinations, which yielded the ZnO flakes. The synthesized ZnO flakes were characterized by X-ray diffraction measurements (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and ultraviolet–visible (UV–Vis) spectroscopy. XRD analysis revealed the broad pattern for crystal structure of ZnO flakes. The results show that asprepared ZnO flakes have a uniform structure with high purity. & 2012 Published by Elsevier B.V.
Keywords: Microstructure Crystal structure Zinc oxide Micro-sized ZnO flakes Inorganic materials
1. Introduction
2. Experimental
Zinc oxide (ZnO) is a material of particular interest because it has unique optical and electronic properties. ZnO has some characteristics: (1) it is a wide-band gap semiconductor (3.37 ev); (2) it has a large exciton binding energy of 60 mev, which is also luminescent; (3) it has emerged as a good candidate for many applications such as solar cells, photo catalysts, gas sensor, photo detectors and luminescent oxides [1–5]. Various physical or chemical synthetic approaches have been developed to produce ZnO particles, including vapor phase oxidation [6], precipitation [7–9], chemical vapor deposition (CVD) [10], sol–gel [11,12], microemulsion [13], hydrothermal [14], solvothermal [15], and sonochemical methods [16]. Among these synthetic routes, precipitation approach provides a facile way for low caste and large-scale production as compared with other traditional methods because it does not need expensive raw materials and complication equipments. Recently, Li et al. [17] have obtained nano-ZnO flakes by calcinations of the precursor of Zn(OH)2 via the reactive ion-exchange method between an ion-exchange resin and zinc sulfate solution at room temperature. Al-Heniti et al. [18] have synthesized nanocrystalline ZnO flakes via solution process using zinc acetate and diethyl amine under refluxing at 85 1C for 4 h. In the present work, a direct precipitation method was employed to synthesize micro-size ZnO flakes using suitable alternative raw materials, which are attractive for chemical industries.
All the chemicals used in this study were of analytical grade and were used without further purification. Zinc sulfate pentahydrate (ZnSO4(H2O)5) and sodium hydroxide (NaOH) are the two starting materials for the synthesis of ZnO flakes. The synthetic procedure for ZnO flakes by direct precipitations method are briefly summarized in Scheme 1. In this synthesis, ZnO flakes were prepared by thermal calcinations of zinc hydroxide as precursor. Zinc sulfate pentahydrate (20 g) were dissolved in 40 ml deionized water. Sodium hydroxide solution (4 M) was slowly dropped into zinc sulfate solution with vigorous stirring until the pH of reaction mixture become basic (pH¼7.5–8). The precipitates derived from the reaction between the zinc sulfate and the sodium hydroxide solutions were collected by filtration and rinsed three times with distilled water. The washed precipitates were dried at 100 1C for one day to form the precursors of ZnO. Finally, the precursors were calcined at a temperature of 600 1C for 4 h in the furnace to obtain the ZnO flakes. The morphology of the powder was determined by scanning electron microscopy (SEM), equipped with an energy dispersive Xray spectroscope. The compositional analysis was done by energy dispersive X-ray (EDX). X-ray diffraction (XRD) analysis was used for phase composition and line broadening on the synthesized powder with Cu Ka radiation. The electronic spectra of the ZnO flakes were taken on a Shimadzu UV–Vis scanning spectrometer.
3. Results and discussion n
Corresponding author. Tel.: þ98 312 522 5071; fax: þ 98 312 522 5068. E-mail address: [email protected] (M. Zareie).
0167-577X/$ - see front matter & 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.matlet.2012.10.013
Fig. 1 shows SEM image of ZnO flakes powder resulted from calcinations of zinc hydroxide at 600 1C for 4 h. It demonstrates
256
M. Zareie et al. / Materials Letters 91 (2013) 255–257
Scheme 1. Preparation of ZnO flakes.
Fig. 1. SEM photograph of the ZnO flakes: (a) the low-resolution image and (b) the high-resolution image.
Fig. 2. Typical (a) EDX pattern and (b) X-ray diffraction pattern of the as-synthesized ZnO flake.
the typical SEM images of micro-sized ZnO flakes, which has a large quantity of flake shape with a narrow size distribution. It is confirmed from the low-resolution image that the flakes are synthesized in very large quantity (Fig. 1a). The high-resolution image of the synthesized flakes reveals that the products have uniform flake shape (Fig. 1b). Chemical purity of the samples was tested by EDX. The corresponding EDX spectrum (Fig. 2a) suggests that the as-synthesized ZnO flakes are composed only of zinc and oxygen elements, which indicates that the product is high-pure ZnO. To check the crystallinity and crystal phase of the as-synthesized ZnO flakes, XRD was performed and results are shown in Fig. 2b. All the peaks in the pattern can be indexed to hexagonal phase of bulk ZnO. The sharp diffraction peaks show that the obtained ZnO flakes have high crystallinity. No characteristic peaks from impurities such as Zn(OH)2 are detected. The room-temperature optical properties of the ZnO flakes were investigated by UV–Vis spectra at room temperature (Fig. 3). For the UV–Vis measurement, ZnO flakes were dispersed in water and measured from these dispersions to detect absorption over the range of 300–600 nm. The obtained UV–Vis absorbance spectrum exhibits a well defined excitonic absorption peak at
376 nm, a corresponding peak for wurtzite hexagonal phase bulk ZnO [19]. As seen in Fig. 3, UV–Vis absorption spectrum of the sample has a red-shift (376 nm) compared to that of bulk ZnO ( 370 nm). This red-shift can be explained by the formation of shallow levels inside the band gap due to impurity atoms present in the lattice [20]. In particular, ZnO is also a promising material for spintronics since it can possess the ferromagnetic properties. The micrographs (Fig. 1) witness that the obtained ZnO flakes contain the very developed free surface and UV–Vis-spectrum (Fig. 3) demonstrates the presence of optically active defects. In turn, the ferromagnetic behavior depends on the presence of defects like grain boundaries [21] and on the presence of (invisible for XRD) amorphous intergranular layers [22].
4. Conclusion In summary, the micro-sized ZnO flakes were prepared via thermal calcinations of zinc hydroxide. This method employed an inexpensive, reproducible process for the large-scale synthesis of
M. Zareie et al. / Materials Letters 91 (2013) 255–257
Fig. 3. Typical UV–Vis spectrum for the as-synthesized ZnO flakes.
micro-sized ZnO flakes. SEM images, EDX pattern and XRD pattern indicate that the as-prepared ZnO flakes have uniform structure and high purity. References [1] Ohshima E, Ogino H, Kiikura I, Macda K, Sato M, Ito M, et al. Growth of the 2-in-size bulk ZnO single crystals by the hydrothermal method. J Cryst Growth 2004;260:166–70. [2] Arii T, Kishi A. Humidity controlled thermal analysis. J Therm Anal Calorimetry 2006;83:253–60.
257
[3] Zeng H, Cui J, Cao B, Gibson U, Bando Y, Golberg D. Electrochemical deposition of ZnO nanowire arrays: organization, doping, and properties. Sci Adv Mater 2010;2:336–58. [4] Wu W, Bai S, Cui N, Ma F, Wei Z, Qin Y, et al. Increasing UV photon response of ZnO sensor with nanowires array. Sci Adv Mater 2010;2:402–6. [5] Sharma SP, Ting JM, Chang ZK. Growth of ZnO nanowires on carbon fibers by RF magnetron sputtering technique. Adv Sci Lett 2010;3:74–9. [6] Hu JQ, Li Q, Wong NB, Lee CS, Lee ST. Synthesis of uniform hexagonal prismatic ZnO whiskers. Chem Mater 2002;14:1216–9. [7] Pesika NS, Hu Z, Stebe KJ, Searson PC. The quenching of growth of ZnO nanoparticles by adsorption of octanethiol. J Phys Chem B 2002;106: 6985–90. [8] Radovanovic PV, Norberg NS, McNally KE, Gamelin DR. Colloidal transitionmetal-doped ZnO quantum dots. J Am Chem Soc 2002;124:15193. [9] Seelig EW, Tang B, Yamilov A, Cao H, Chang RPH. Self-assembled 3D photonic crystals from ZnO colloidal spheres. Mater Chem Phys 2003;20:257–63. [10] Wu JJ, Liu SC. Catalyst-free growth and characterization of ZnO nanorods. J Phys Chem B 2002;106:9546–51. [11] Hoyer P, Weller H. Potential-dependent electron injection in nanoporous colloidal ZnO films. J Phys Chem 1995;99(38):14096–100. [12] Paul GK, Bandyopadhyay S, Sen SK, Sen S. Structural, optical and electrical studies on sol–gel deposited Zr doped ZnO films. Mater Chem Phys 2003;79:71–5. [13] Gao L, Ji YL, Xu H, Simon P, Wu Z. Regularly shaped, single-crystalline ZnO nanorods with wurtzite structure. J Am Chem Soc 2002;124:14864–5. [14] Cheng B, Samulski ET. Hydrothermal synthesis of one-dimensional ZnO nanostructures with different aspect ratios. Chem Commun 2004;8:986–7. [15] Jia Z, Yue L, Zheng Y, Xu Z. Rod-like zinc oxide constructed by nanoparticles: synthesis, characterization and optical properties. Mater Chem Phys 2008;107(1):137–41. [16] Qian D, Jiang JZ, Hansen PL. Preparation of ZnO nanocrystals via ultrasonic irradiation. Chem Commun 2003:1078–9. [17] Li YS, Li G, Wang SX, Gao H, Ta ZC. Preparation and characterization of nanoZnO flakes prepared by reactive ion exchange method. Therm Anal Calorimetry 2009;95(2):671–4. [18] Al-Heniti S, Umar A, Singh P, Sayari A, Al-Hajry A, Al-Ghamdi AA. Nanocrystalline ZnO flakes for photovoltaic device applications. Adv Sci Lett 2010;3:543–7. [19] Al-Hajry A, Umar A, Hahn YB, Kim DH. Growth, properties and dye-sensitized solar cells–applications of ZnO nanorods grown by low-temperature solution process. Superlattices Microstruct 2009;45:529–34. [20] Ischenko V, Polarz S, Grote D, Stavarache V, Fink K, Driess M. Zinc oxide nanoparticles with defects. Adv Funct Mater 2005;15:1945–54. ¨ G, [21] Straumal BB, Protasova SG, Mazilkin AA, Myatiev AA, Straumal PB, Schutz et al. Ferromagnetic properties of the Mn-doped nanograined ZnO films. J Appl Phys 2010;108:073923–6. [22] Straumal BB, Myatiev AA, Straumal PB, Mazilkin AA, Protasova SG, Goering E, et al. Grain boundary layers in nanocrystalline ferromagnetic zinc oxide. JETP Lett 2010;92:396–400.