Shape-controllable Synthesis of Ultrafine ZnO Powders of Different Morphologies Xiaoyi Shen, Yuan Liang, Yuchun Zhai * , Zhiqiang Ning School of Materials and Metallurgy, Northeastern University, Shenyang 110819, China [Manuscript received January 17, 2012, in revised form May 24, 2012, Available online 24 December 2012] By employing zinc acetate and sodium hydroxide as raw materials, ultrafine ZnO powders with different morphologies were successfully synthesized through hydrothermal method. The influences of the reaction temperature, the OH À /Zn 2þ mol ratio and the reaction time on the morphologies of the ZnO powders were discussed. The reaction conditions were obtained, under which the ZnO of flower-like particles, micro-rods and flake particles was synthesized, respectively. The crystal structures and morphologies of those ZnO particles were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The ZnO with flower-like structures was composed of lots of micro-rods with hexagon morphology. The XRD patterns indicated that the ZnO powders were hexagonal wurtzite structures with high purity. Finally, the growth mechanism of the ZnO particles was discussed. KEY WORDS: ZnO particles; Hexagonal wurtzite structure; Hydrothermal method; Growth mechanism 1. Introduction Micro/nano-materials have stimulated great interest due to their special characteristic in optical, catalyst, thermal properties that differ from those of bulk materials and their importance in basic scientific research and potential technology application [1,2] . The ultrafine crystalline of ZnO is a good example, which is an attractive material due to its wide variety of application including pigments, catalysts and phosphors [3,4] . There are lots of ways for synthesis of micro/nano ZnO powder, among which the wet chemical processes are of special interest due to their simplicity and low temperature [5,6] . As it is known, the so-called hydro- thermal method is a relatively simple wet chemical method to prepare micro/nano ZnO particles with different morphology, such as prism-like, flower-like, rods and spherical particles. And the controllable hydrothermal method has its unique advantages of simplicity, low temperature and high yield [7,8] . In addition to these, the hydrothermal method can produce micro/nano parti- cles with different morphology and better crystal quality, without using metal catalyst or template, and it is an easy and economical process [9,10] . The ZnO with flower-like structures was obtained in previous work [11e13] , but detailed discussion was not finished. In this study, the influencing factors were investigated in more detail and the ultrafine ZnO powders of flower-like particles, rods and flake particles were successfully synthesized, respec- tively. The influences of the reaction temperature, the OH À /Zn 2þ mol ratio and the reaction time on the morphologies of ZnO powders were discussed in detail. The crystal structures and morphologies of the ZnO powders were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Finally, the growth mechanism was discussed. 2. Experimental 2.1. Materials Zinc acetate and sodium hydroxide with analytic grade were used as raw materials. Polyethylene glycol (PEG20000) with analytic grade was used as dispersant agent. Distilled water was obtained in laboratory. 2.2. Preparation The preparation of the ultrafine ZnO powder involved several steps. First, 0.5 mol L À1 zinc acetate solution and 1, 5, 10 mol L À1 sodium hydroxide solutions were formulated, respectively. Second, according to the mol ratio of OH À /Zn 2þ , zinc acetate and sodium hydroxide solutions were added into a reaction kettle with lining Teflon, and then the PEG20000 was added. Afterward the solution was stirred with a magnetic agitator to homogeneous. Third, the reaction kettle was placed in * Corresponding author. Prof., Ph.D.; Tel./Fax: þ86 24 83687731; E-mail address: zhaiyc@smm.neu.edu.cn (Y. Zhai). 1005-0302/$ e see front matter Copyright Ó 2013, The editorial office of Journal of Materials Science & Technology. Published by Elsevier Limited. All rights reserved. http://dx.doi.org/10.1016/j.jmst.2012.11.004 Available online at SciVerse ScienceDirect J. Mater. Sci. Technol., 2013, 29(1), 44e48 an oven at a desired temperature for several hours. When the reaction ended, the reaction kettle was cooled down to room temperature naturally. The product was centrifugated and washed by using distilled water repeatedly and using absolute ethanol twice. Finally, the ultrafine ZnO powder was obtained by drying in an oven. 2.3. Characterization The structures of the ZnO powders were measured with a Japan Rigaku X-ray diffractometer. The morphologies and sizes of the ZnO powders were observed by using a Japan shi- madzu SSX-550 scanning electron microscope. 3. Discussion 3.1. Influence of reaction temperature on the morphologies of ZnO powders The influence of the reaction temperature on the morphologies of ZnO powders was studied, and the results were shown in Fig. 1. Fig. 1(a)e(c) was corresponding to 120, 150 and 180  C for 10 h, respectively as the mol ratio of OH À /Zn 2þ was 5. The ZnO with flower-like structures was observed both in Fig. 1(a) and (b), but the flower-like structures were more regular in Fig. 1(b) than those in Fig. 1(a). The flower-like structures were composed of micro-rods with 4 m m in length, which aligned in a radical way from a center and grew longer and bigger with increasing temperature. However, when the reaction temperature was up to 180  C, the flower-like structures of ZnO were destroyed and regular ZnO micro-rods with hexagon figure and flat top were obtained, as shown in Fig. 1(c). The ZnO micro- rods were hexagon with perfect morphology and smooth surface, of which the diameter and the length were about 1 m m and more than 20 m m, respectively. That is to say, with increasing temperature, the flower-like structures of ZnO grew more regular and uniform. However, when the temperature was too high, the structures were broken down and yielded out micro-rods. 3.2. Influence of the OH À /Zn 2þ mol ratio on the morphologies of ZnO powders The influence of the OH À /Zn 2þ mol ratio on the morphologies of the ZnO particles was also investigated, as shown in Fig. 2. Fig. 2(a)e(c) was corresponding to the OH À /Zn 2þ mol ratio 2, 5 and 10 at 150  C for 10 h, respectively. In Fig. 2(a), the ZnO powder looked like flake. In Fig. 2(b), the ZnO with flower-like structures were observed with regular figure and uniform micro-rods, and the micro-rods aligned in a radical way from a center. The micro-rods of the ZnO flower-like structures grew longer and bigger with increasing the OH À /Zn 2þ mol ratio. When the OH À /Zn 2þ mol ratio was 10, the micro-rods grew so big that the flower-like structures were damaged and the micro- rods were yielded out, as shown in Fig. 2(c). This could be confirmed by the residual flower-like structure in Fig. 2(c), from the center of which the residual of the stripping of those micro-rods could be found. The micro-rods were hexagon and were more than 20 m m in length with smooth surface and regular tip. From the above, the influence of the OH À /Zn 2þ mol ratio on the morphologies of ZnO particles was significant. The morphologies of ZnO particles transited from flake to flower-like to micro-rods with increasing the OH À /Zn 2þ mol ratio. This is because the NaOH solution was alkaline with strong polarity, and the polarity of the solution was enhanced when the OH À / Zn 2þ mol ratio increased. It was evident that the ZnO particles grew with self-organizing. 3.3. Influence of the reaction time on the morphologies of ZnO powders The influence of the reaction time on the morphologies of ZnO powders was also discussed, as shown in Fig. 3. Fig. 3(a) and (b) Fig. 1 SEM images of ZnO powder prepared at: (a) 120  C; (b) 150  C; (c) 180  C. Fig. 2 SEM images of ZnO particles synthesized at: (a) OH À /Zn 2þ ¼ 2; (b) OH À /Zn 2þ ¼ 5; (c) OH À /Zn 2þ ¼ 10. X. Shen et al.: J. Mater. Sci. Technol., 2013, 29(1), 44e48 45 was corresponding to reaction time for 4 and 10 h at 150  Cas the OH À /Zn 2þ mol ratio was 5, respectively. The ZnO with flower-like structures were observed in both Fig. 3(a) and (b). However, the flower-like structures in Fig. 3(a) were still in growth stage with irregular figures. The flower-like structures, shown in Fig. 3(b), had enough time to grow by extending time to 10 h, and the micro-rods of the flower-like structures grew more regular. In summary, the influences of the reaction temperature, the OH À /Zn 2þ mol ratio and the reaction time on the morphologies of the ultrafine ZnO powders were notable, among which the OH À /Zn 2þ mol ratio was the most remarkable. It could be concluded that the appropriate reaction conditions for obtaining ZnO with flower-like structures were reaction temperature 150  C, OH À /Zn 2þ mol ratio 5 and reaction time 10 h. 3.4. XRD patterns of ZnO powders and EDS pattern of flower-like structure The XRD patterns of ZnO of flower-like particles, rods and flake, and the EDS pattern of the ZnO with flower-like structures are shown in Fig. 4. The XRD patterns were obtained by employing CuK a radiation with a voltage of 40 kV, at a scanning rate of 6 deg/min with 2 q ranging from 20 to 80 deg. The crystal planes of (100), (002), (101) etc. belonged to the hexagonal system, and the crystal parameters were a ¼ 0.3249 nm and c ¼ 0.5206 nm. All diffraction data of Fig. 4(a)e(c) were in good agreement with JCPDS files No. 361451 (a ¼ 0.324982 nm and c ¼ 0.520661 nm). No other phases were detected, and the reflections were sharp, which indicated that the ZnO powders with flower-like, rods and Fig. 3 SEM images of ZnO with flower-like structures reaction for: (a) 4 h, (b) 10 h. Fig. 4 XRD patterns of ZnO powders of (a) flower-like, (b) rods and (c) flake shape and (d) EDS pattern of ZnO powder with flower-like structures. 46 X. Shen et al.: J. Mater. Sci. Technol., 2013, 29(1), 44e48 flake morphologies were hexagonal wurtzite structure with regular crystal form and high purity. The EDS pattern was performed on one of the ZnO flower- like structures, and the composition analysis of the powder showed the existence of Zn and O (from the sample), as well as Au (from the coating sprayed before testing). This provided an additional evidence that the ZnO with flower-like structures was pure. 3.5. Growth mechanism As it is well known, the crystal formation in solution can be divided into two steps: nucleation and growth. The o b- tained particles morphology depends on both the nucleation and crystal grow th rate [7,14,15] . Based on the experimental results, the growth mechanism of ZnO particles could be represented by the followin g chemical reactions, and Fig. 5 shows the fundamental schematic growth diagram of ZnO particles. The similar results were also reported in literature [16e18] . ZnðCH 3 COOÞ 2 / Zn 2þ þ 2CH 3 COO À (1) Zn 2þ þ 2OH À / ZnðOHÞ 2 (2) ZnðOHÞ 2 þ 2OH À / ZnðOHÞ 2À 4 (3) ZnðOHÞ 2À 4 / ZnO þ H 2 O þ 2OH À (4) In the process of preparing ZnO with flower-like structures, both nucleation and crystal growth were fast. At the initial stage, large amounts of ZnO nuclei were formed, and some ZnO nuclei agglomerated together and then became small crystals by driving force of the surface energy and electrostatic effect. This was a spontaneous process that occurred, since small crystals were more active than larger ones. Therefore, small particles nucleated in supersaturated solution kinetically favored to grow larger particles. The growth of ZnO with flower-like structures was controlled by nucleation and growth process in aqueous solution. In hexagonal system, the crystal plane of (001) is close-over plane [19] . The energy is minimum when the crystal grows along with (001) plane. Based on surface energy minimization, ZnO crystals developed along the c-axis from ZnðOHÞ 2À 4 supersaturation solution [7,16] , and then micro-rod crystals were formed, which were branches of ZnO with the flower-like structures. ZnO is a polar crystal, whose positive polar plane is rich in Zn 2þ and the negative plane is rich in O 2À[15,16,20] . The Zn and O atoms were arranged alternatively along the c-axis, and the top surfaces were Zn terminated (001). Therefore, the top surfaces were energetically active, which made the micro-rods growing in radical way once the nuclei were formed [15,16] . The reason was that the Zn (001) surface with positive charge was chemically active and the O (0 0 À1) surface with negative charge was inert. However, when the reaction equilibrium was broken down, the flower-like structures were destroyed to form micro-rods. 4. Conclusions (1) The reaction temperature, OH À /Zn 2+ mol ratio and reaction time all had significant influences on the morphologies of the ZnO powders, among which the OH À /Zn 2+ mol ratio was the most notable one. The micro-rods of the flower- like structures grew larger as the reaction temperature or the OH À /Zn 2+ mol ratio increased. When the reaction temperature or the mol ratio was increased to a certain degree, the flower-like structures would be destroyed to get micro-rods. The appropriate reaction conditions for preparing ZnO with flower-like structures were reactio n temperature 150  C, OH À /Zn 2+ molratio5andreaction time 10 h. (2) The synthesized ZnO powders of different morphologies were hexagon wurtzite structure with high purity and regular crystal form. The ZnO with flower-like structures was regular with uniform micro-rods that were hexagon with smooth surface. (3) The growth mechanism of ZnO with flower-like structures, rods and flake morphologies was proposed, which was a complex process including: the formation of ZnðOHÞ 2À 4 from Zn 2+ , the dehydration of ZnðOHÞ 2À 4 , the appearance of ZnO nuclei and the growth of ZnO crystals with different morphologies. Fig. 5 Schematic growth diagram of ZnO particles. X. Shen et al.: J. Mater. Sci. Technol., 2013, 29(1), 44e48 47 Acknowledgments This work was supported by the National Natural Science Foundation of China (No. 51204054) and the Fundamental Research Funds for the Central Universities, China (No. N110402012). REFERENCES [1] Y.F. Chen, M.T. Tang, S.H. Yang, B.P. Zhang, J.G. Yang, J. Cent. South. Univ. Technol. 11 (2004) 51e54. [2] J.W. Zhao, L.R. Qin, Z.D. Xiao, L.D. Zhang, Mater. Chem. Phys. 105 (2007) 194e198. [3] L.Q. Jing, Z.L. Xu, J. Shang, X.J. Sun, W.M. Cai, Mater. Sci. Eng. A 232 (2002) 356e361. [4] X.Y. Chen, X.F. Zhang, Z.J. Zhang, J.W. Liu, Y.T. Qian, J. Cryst. 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XRD patterns of ZnO powders and EDS pattern of flower-like structure The XRD patterns of ZnO of flower-like particles, rods and flake, and the EDS pattern of the ZnO with flower-like structures are. formation of ZnðOHÞ 2À 4 from Zn 2+ , the dehydration of ZnðOHÞ 2À 4 , the appearance of ZnO nuclei and the growth of ZnO crystals with different morphologies. Fig. 5 Schematic growth diagram of ZnO