Journal of Microscopy Society of Thailand 2009, 23(1): 75-78 75 Preparation of ZnO Nanostructures by Solvothermal Method Doungporn Yiamsawas, Kanittha Boonpavanitchakul, and Wiyong Kangwansupamonkon* National Nanotechnology Center, 111 Thailand Science Park, Paholyothin Road, Klong 1, Klong Luang, Phathumthani, 12120 Thailand *Corresponding author, e-mail: [email protected]Abstract The solvothermal method was employed to synthesize ZnO nanostructure. This technique is based on thermal decomposition of organometallic compound in organic solvent and has been successfully applied for the synthesis of various types of nanosized metal oxide with large surface area, high crystallinity and high thermal stability. The influences of type of on shape of the synthesized nanostructures as well as the mechanism were investigated. An interesting correlation between aspect ratio of the ZnO products and physical properties of the solvent was observed and presented in this work. The size and morphology from different environmental were investigated. The different environments during ZnO preparation led to the different morphology with wide and length in the range of 8.9-69 nm and 68-108 nm, respectively and 0.17-0.93 in the term of the aspect-ratio. The TEM image clearly reveals the lattice spacing is about 0.52 corresponding to the (0 0 1) plane of hexagonal- structured ZnO. Background Zinc oxide is an important basic material due to its low cost, large band gap (3.37 eV), large exciton binding energy (60 MeV), and luminescent proper-ties [1]. It is widely used in many applications, such as catalyst, gas sensor, filtering materials for ultraviolet light, and also antimicrobial material. Currently many interesting ZnO nanostructures, including nanorods, nanobridges and nanonails have been fabricated. These structures are expected to have potential applications in building functional nanoelectronic devices. Control of the particle shape is of interest for nanostructured material synthesis because electrical and optical properties of nanomaterials depend sensitively on both size and shape of the particles. Different methods have been reported for ZnO nanostructure, therein, a vapor-phase transport process with the assistance of metal catalysts. However, expensive raw materials, complex process control and sophisticated equipment are often needed and this is unfavorable for large-scale synthesis. As a better alternative route, soft chemical methods have proven to be appealing because of their low growth temperature and good potential for scale- up [2,3]. Various chemical method are applied for the prepa-ration of metal oxide nanoparticles, such as the gas phase methods, sol-gel methods, evaporative decomposition of solution and wet chemical synthesis [4]. To promote the formation of ZnO nanostructures, poly(vinyl pyrrolidone) (PVP) is frequently used as a templating molecule [5]. In this research, we report a solvothermal- assisted heat treatment method to synthesize ZnO nano-structure. This synthesis involved precursors of Zn(CH 3 COO) 2 .2H 2 O and carried out at low temperature (80°C). Different solvents were used and investigated to have an effect on crystals morphologies. Materials and Methods In typical, synthesis of ZnO was carried out by solvothermal process at 80°C. Poly(vinyl pyrrolidone) PVP 30K was dissolved in absolute ethanol under stirring at room temperature, then zinc acetate dihydrate was slowly added to the solution. Consequently, the solid NaOH was put into the reaction mixture. The resulting solution was stirred for several minutes. The solution was then transferred to polypropylene vessel, then sealed and heated in temperature-controlled autoclave at 80°C for 24 h. After cooling to room temperature, the white powder was precipitated and then washed with absolute ethanol several times to dissolve other impurities. Finally, the powder was dried under vacuum at 60°C overnight and determined in terms of their structural, morphology and optical properties. The mixed solvent of ethylene glycol and absolute ethanol without polymer template was also investigated. The crystalline structures of the products were characterized by X-ray diffraction (XRD) analysis using a JEOL JDX-3530 diffractometer. Dry powder of samples which were re-dispersed in ethanol and consequently deposited on a glass slide were used for the structural measurement. The X-ray diffraction (XRD) patterns were recorded from 20°C to 80°C in 2θ with a scanning rate of 0.2°/s. The absorbance spectra were recorded on a PerkinElmer Lambda 650 UV-vis spectrophotometer. Size and morphology of the products were observed by transmission
Transcript
Journal of Microscopy Society of Thailand 2009, 23(1): 75-78
75
Preparation of ZnO Nanostructures by Solvothermal Method
Doungporn Yiamsawas, Kanittha Boonpavanitchakul, and Wiyong Kangwansupamonkon*
National Nanotechnology Center, 111 Thailand Science Park, Paholyothin Road, Klong 1, Klong Luang, Phathumthani, 12120 Thailand *Corresponding author, e-mail: [email protected] Abstract
The solvothermal method was employed to synthesize ZnO nanostructure. This technique is based on thermal decomposition of organometallic compound in organic solvent and has been successfully applied for the synthesis of various types of nanosized metal oxide with large surface area, high crystallinity and high thermal stability. The influences of type of on shape of the synthesized nanostructures as well as the mechanism were investigated. An interesting correlation between aspect ratio of the ZnO products and physical properties of the solvent was observed and presented in this work. The size and morphology from different environmental were investigated. The different environments during ZnO preparation led to the different morphology with wide and length in the range of 8.9-69 nm and 68-108 nm, respectively and 0.17-0.93 in the term of the aspect-ratio. The TEM image clearly reveals the lattice spacing is about 0.52 corresponding to the (0 0 1) plane of hexagonal-structured ZnO.
Background Zinc oxide is an important basic material due
to its low cost, large band gap (3.37 eV), large exciton binding energy (60 MeV), and luminescent proper-ties [1]. It is widely used in many applications, such as catalyst, gas sensor, filtering materials for ultraviolet light, and also antimicrobial material. Currently many interesting ZnO nanostructures, including nanorods, nanobridges and nanonails have been fabricated. These structures are expected to have potential applications in building functional nanoelectronic devices. Control of the particle shape is of interest for nanostructured material synthesis because electrical and optical properties of nanomaterials depend sensitively on both size and shape of the particles. Different methods have been reported for ZnO nanostructure, therein, a vapor-phase transport process with the assistance of metal catalysts. However, expensive raw materials, complex process control and sophisticated equipment are often needed and this is unfavorable for large-scale synthesis. As a better alternative route, soft chemical methods have proven to be appealing because of their low growth temperature and good potential for scale-up [2,3]. Various chemical method are applied for the prepa-ration of metal oxide nanoparticles, such as the gas phase methods, sol-gel methods, evaporative decomposition of solution and wet chemical synthesis [4]. To promote the formation of ZnO nanostructures, poly(vinyl pyrrolidone) (PVP) is frequently used as a templating molecule [5]. In this research, we report a solvothermal-assisted heat treatment method to synthesize ZnO nano-structure. This synthesis involved precursors of Zn(CH3COO)2.2H2O and carried out at low
temperature (80°C). Different solvents were used and investigated to have an effect on crystals morphologies.
Materials and Methods In typical, synthesis of ZnO was carried out by
solvothermal process at 80°C. Poly(vinyl pyrrolidone) PVP 30K was dissolved in absolute ethanol under stirring at room temperature, then zinc acetate dihydrate was slowly added to the solution. Consequently, the solid NaOH was put into the reaction mixture. The resulting solution was stirred for several minutes. The solution was then transferred to polypropylene vessel, then sealed and heated in temperature-controlled autoclave at 80°C for 24 h. After cooling to room temperature, the white powder was precipitated and then washed with absolute ethanol several times to dissolve other impurities. Finally, the powder was dried under vacuum at 60°C overnight and determined in terms of their structural, morphology and optical properties. The mixed solvent of ethylene glycol and absolute ethanol without polymer template was also investigated.
The crystalline structures of the products were characterized by X-ray diffraction (XRD) analysis using a JEOL JDX-3530 diffractometer. Dry powder of samples which were re-dispersed in ethanol and consequently deposited on a glass slide were used for the structural measurement. The X-ray diffraction (XRD) patterns were recorded from 20°C to 80°C in 2θ with a scanning rate of 0.2°/s. The absorbance spectra were recorded on a PerkinElmer Lambda 650 UV-vis spectrophotometer. Size and morphology of the products were observed by transmission
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electron microscopy (TEM) which was taken on a JEOL JEM-2010 electron microscopy using an accelerating voltage of 200 kV in bright field and electron diffraction (ED) modes. A small drop of the ZnO powder re-dispersed by ethanol was dropped on a carbon film-coated copper grid. The sizes of ZnO were measured and averaged by several TEM images (n = 20).
Results and Discussion XRD pattern of synthesized ZnO nanostructures
by solvothermal reaction at 80°C is shown in Fig. 1. All peaks of the obtained product are corresponding to the hexagonal wurtzite structure of ZnO reported in JCDDS card (NO. 36-1451, a = 3.249 Å, c = 5.206 Å). No characteristic peaks were observed for the other impurities. According to the intensity and half width of the characteristic peaks in the XRD pattern, the ZnO nanostructures still crystallized well even though the preparation was carried out at low temperatures. This result confirmed that ZnO was successfully synthesized by the solvothermal reaction. However, it was found that the signal intensity due to the 0 0 2 plane in Fig. 1 (a) and (b) is quite low. This may be resulted from the inadequate ZnO film coating on the glass slide, causing some of the crystal plane hard to be detected.
The two common reaction media (ethanol and ethylene glycol) were used for studying the morphology control of the ZnO nanostructure. It was found that the difference of solvent leads to the different morphology of ZnO nano-structures as shown in TEM images (Fig. 2).
The nanoparticles shown in Fig. 2 (a.1, a.2) which were prepared in ethylene glycol have the
diameters of 68.1±7 nm. Moreover, the prepared-ZnO consists of a large quantity of well dispersed and uniform ZnO nanostructure with spherical shape. Fig 2 (b.1, b.2) shows rod shape synthesized in absolute ethanol. The diameter and length of these nanorods are 8.2±2 nm and 54.3±11 nm, respectively. Note that the short ZnO nanorods could be observed when the ultrasonication was used in the sample preparation for TEM analysis. The high resolution TEM images of nanostructures show that the lattice fringes between two adjacent planes about 0.52 nm which is equal to the lattice constant of the ZnO. This further indicates that the obtained structures have a wurtzite hexagonal phase and preferentially grew along the c-axis [0 0 1] direction. The selected area electron diffractions (SAED) in Fig. 2 (a.3, b.3, c.3) indicate that all the ZnO nanostructures are crystalline.
In addition, the semi-sphere ZnO nano-crystals, which were obtained in the mixed alcohol solution without using PVP at the same condition in the presence of the polymer template molecules were shown in Fig. 2 (c1, c2). TEM images show approximately uniform morphology with diameter and length of 68.9±9 nm and 108.4±9 nm, respectively.
The results demonstrate that the PVP plays an important role in controlling the ZnO size and shape. PVP act as a template, which can form chain structures. With the polymer template, ZnO can grow up along these chains to form nanostructure. On the other hand, PVP can form a shell surrounding the particles to prevent them from aggregating to larger particles and grain growth as a result of its steric effect. It has been reported that the presence of a capping molecule (such as
Fig. 1 XRD patterns of synthesized ZnO sample from solvothermal process 24 h at 80°C with (a) PVP K30 and ethylene glycol, (b) PVP K 30 and ethanol and (c) no polymer template and 50:50 of ethylene glycol:ethanol.
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poly(vinyl pyrrolidone)) can alter the surface energy of crystallographic surfaces, in order to promote the anisotropic growth of the nanocrystals [6,7]. The PVP adsorbs on crystal nuclei to serve as growth director. With this condition, the structure of the solvent led to the morphology control depending on its interaction with the growing crystal. Ethylene glycol with its structure and bulkier functional groups (OH) comparing to ethanol limits the growth of all crystal planes, thus producing a more uniform dimensional shape like sphere. In the other hand, ethanol which has less complicated structure allows the growth of some crystal planes, thus giving the nanorod structure. In deed, when the PVP template was not used the growth of the crystal planes in the mixed solvent become no orientation. This led to the semi-spherical structure of the synthesized ZnO.
It has been report that the interaction between zinc acetate and alcohol under the solvothermal conditions involved in esterification reaction, which proceeded to form ZnO, ester and water, according to the following reaction [8]:
Zn(CH3COO)2 + 2R-OH �
ZnO + 2CH3-CO-R + H2O The room temperature UV-visible spectra were
also measured. The UV-visible spectra of the prepared ZnO colloidal suspensions are shown in Fig 3. The spectra exhibit a strong absorption with an on set around 355 nm except the semi-spherical shape which shows absorption peak around 366 nm. It is known that the bulk ZnO has absorption at 375 nm in the UV-visible spectrum [9] which is obviously larger than the prepared ZnO nanostructures, corresponding to the fact that the excitonic peak shifts to blue results from decreasing particle size.
(a.1)
(b.1)
(c.1)
(a.2) (a.3)
(b.2) (b.3)
(c.2) (c.3)
0. 52
0.52
Fig. 2 TEM images and the corresponding selected area diffraction pattern of the synthesized ZnO sample from solvothermal process 24 h at 80°C with PVP K30 as polymer template and different solvent; (a) PVP K30 and ethylene glycol, (b) PVP K 30 and ethanol and (c) no polymer template and 50:50 of ethylene glycol:ethanol.
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Conclusion In summary, ZnO nanocrystals were success-
fully obtained by using solvothermal process at 80°C. Zinc acetate dihydrate was used as the zinc source. The synthesized ZnO nanoparticles prepared in ethylene glycol have the diameters of 68.1±7 nm. The ZnO nanorod was obtained by using absolute ethanol as solvent, giving the diameter and length of 8.2±2 nm and 54.3±11 nm, respectively. The absence of PVP with the use of mixed solvent gave the ZnO semi-sphere shape with its diameter and length of 68.1±9 nm and 108.4±9 nm, respectively.
Acknowledgements
The authors are grateful for the research funding from National Nanotechnology Center and Institute of Solar Energy Technology Development, Thailand (Grant number: B22 AR010110RDAR01).
References 1. Zhong J, Kitai AH, Mascher P and Puff W.
The influence of processing conditions on point defects and luminescence centers in ZnO. J. Electrochem. Soc. 1993, 140: 3644-3649.
2. Zhang H, Yang D, Ji Y, Ma X, Xu J and Que D. Low temperature synthesis of flowerlike ZnO nanostructures by cetyltrimethyl-ammonium bromide-assisted hydrothermal process. J. Phys. Chem. B. 2004, 108: 3955-3958.
3. Liu B, Yu S-H, Zhang F, Li L, Zhang Q, Ren L and Jiang K. Ring-like nanosheets standing on spindle-like rods: Unusual ZnO superstructures synthesized from a flakelike precursor Zn5(OH)8Cl2·H2O. J. Phys. Chem.
B. 2004, 108: 4338-4341. 4. Moballegh A, Shahverdi HR,
Aghababazadehn R and Mirhabibi AR. ZnO nanoparticles obtained by mechanochemical technique and the optical properties. Surf. Sci. 2007, 601: 2850-2854.
5. Lepot N, Van Bael MK, Van den Rul H, D’Haen J, Peeters R, Franco D and Mullens J. Synthesis of ZnO nanorods from aqueous solution. Mater. Lett. 2007, 61: 2624-2627.
6. Park WI, Kim DH, Jung SW and Yi G-C. Metalorganic vapor-phase epitaxial growth of vertically well-aligned ZnO nano-rods. Appl.
H, Wenjing L and Guanying Y. Control of ZnO morphologies via surfactants assisted route in the subcritical water. J. Cryst Growth.
2005, 280: 126-134. 8. Tonto P, Mekasuwandumrong O, Phatanasri
S, Pavarajarn V and Piyasan P. Preparation of ZnO nanorod by solvothermal reaction of zinc acetate in various alcohols. Ceram. Int. 2008, 34: 57-62.
9. Srikant V and Clarke DR. On the optical band gap of zinc oxide. J. Appl. Phys. 1998, 83: 5447-5451.
Fig. 3 UV-vis spectra of the prepared ZnO (a) PVP K30 and ethylene glycol (nanoparticles), (b) PVP K 30 and ethanol (nanorods) and (c) no polymer template and 50:50 of ethylene glycol:ethanol (semi-nanoparticles).
