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Controllable Synthesis of ZnO Nanostructures with Various Morphologies
Available online at www.sciencedirect.com
ScienceDirect Physics Procedia 48 (2013) 235 – 240
The XIII International Conference on the Physics of Non-Crystalline Solids
Controllable synthesis of ZnO nanostructures with various morphologies Duofa Wanga,b, Fangjie Wangb, Haizheng Taoa*,, Xuecai Hana, Xiujian Zhaoa, Tianjin Zhangb, Hui Huangc, Hualiang Fuc, Wu Qiuc a
State key Laboratory of Silicate Materials for architechture (Wuhan University of Technology), Wuhan, 430070, PR China, b Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, P.R.China c Hubei Yingtong Telecommunication Cable Holding Co., Ltd., Tongcheng, 437400, P.R.China
Abstract Utilizing the silicon as substrate, ZnO nanostructures with various morphologies, including nanorod, nanotube, nanoinjector and nanoplate, were synthesized by thermal chemical vapor deposition without catalysts. The control on the morphology of nanostructures was achieved by forming a temperature gradient in the tube furnace and controlling the oxygen partial pressure in the reaction system. The as-synthesized nanostructures were characterized by x-ray diffraction, scanning electron microscopy and transmission electron microscopy. Moreover, merged ZnO nanoplates were prepared at proper conditions, which are expected to be merged further and eventually grown into a ZnO singlecrystalline film with high quality.
2013 The Authors. Published © 2013 The©Authors. Published by Elsevier B.V. by Elsevier Ltd. Selection and/or of Prof. XiujianofZhao, Wuhan University of Selection and peer-review underpeer-review responsibilityunder of Prof.responsibility Xiujian Zhao, Wuhan University Technology Technology
Keywords: ZnO; controllable synthesis; nanostructure
1. Introduction Since the first discovery of carbon nanotubes, one-dimensional nanometer-sized semiconductor materials
* Corresponding author. Tel/Fax.: +86-27-87669729 E-mail address: [email protected](HZ Tao)
1875-3892 © 2013 The Authors. Published by Elsevier B.V. Selection and peer-review under responsibility of Prof. Xiujian Zhao, Wuhan University of Technology doi:10.1016/j.phpro.2013.07.037
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Duofa Wang et al. / Physics Procedia 48 (2013) 235 – 240
such as nanowires have attracted extensive interest due to their great potential for the fundamental studies about the roles of dimensionality and size on their physical properties as well as for their application to optoelectronic nanodevices [1-6]. Among various semiconductor materials, zinc oxide (ZnO) with a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV provides potential applications in nanoscaled optoelectronic device, such as laser diodes and ultraviolet sensors [7]. Various synthesis methods and nanostructure morphologies have been reported, including Nanobelts, nanowires, tetrapod nanostructures [8-10]. However, the controllable synthesis and the deep understanding of growth mechanism of these nanostructures has not been achieved yet. Moreover, the preparation of ZnO nanostructures with complicated morphology are also needed to meet the requirements for advanced optical and electronic devices. In the present work, ZnO nanostructures with complex morphology, including nanorod, nanotube, nanoinjector and nanoplate, were synthesized by providing a temperature gradient between Zn powders and substrate in a tube furnace together with the change of oxygen partial pressure in the reaction system. And the growth mechanism of all these nanostructures was discussed in detail.
2. Experimental The growth was performed in two horizontal tube furnaces with and without temperature gradient, which was made by controlling the density of heaters as shown in Figs. 1(a) and 1(b). Quartz boats loaded with Zn powders (purity: 99.99%) and p-type Si substrates were introduced into the tube furnaces. The distance between Si substrate and Zn powders was around 5 mm. In furnace (a), another Si substrate was also put on the quartz boat, 5 cm away from Zn powders where the temperature is lower to investigate the effect of temperature on the morphology of ZnO nanostructures. Then, the temperatures near Zn powders were raised to 480°C at a rate of 15°C/min in furnaces (a) and (b). At this time, the temperature of substrate A and substrate B in furnace (a) was 430°C and 480°C, respectively. Afterwards, oxygen and Ar gases were introduced into the chamber as reactant and carrier gases, respectively. The reaction was maintained for 1 h. During the reaction, the pressure in the tube furnace was keep at 20 Pa. In furnace (a) the flow of oxygen was kept to be constant at 10 sccm during the reaction, while it was varied in furnace (b) to synthesize nanostructures with complex morphology aimed at the application to advanced optoelectronic devices. The crystal structure of the as-prepared samples was analyzed by x-ray diffraction (XRD). The morphologies of samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Fig. 1. Schematic diagrams of thermal chemical vapor deposition system (a) with and (b) without temperature gradient
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3. Results and discussion Analysis on the XRD patterns (not shown) of the synthesized samples reveals that all the deposited nanostructures on Si substrate are wurtzite ZnO. The morphologies of these nanostructures were measured by SEM, shown in Figure 2. Fig. 2(a) and (c) are the SEM images of nanostructures synthesized on substrates A (low temperature) and B (high temperature) in furnace (a), respectively. And Fig. 2(b) are the high magnification of (a). It can be seen that curved nanostructures was grown on substrate A densely and uniformly, with a diameter of 180 - 350 nm. High-magnification SEM image of it [Fig. 2(b)] shows that some of the nanostructures (marked by arrows) are brokenˈfrom which it is noticed that the nanostructures deposited on substrate A have a tubular structure. TEM was additionally employed to investigate the unbroken part for further investigation of the microstructure. From the TEM image [Fig. 2(d)], we can see clearly that the whole nanostructure is tubular, not only the broken port. The selected area electron diffraction pattern, shown as the inset of Fig. 2(d), reveals that the synthesized nanotube is single-crystalline ZnO and that no metallic Zn phase is found, which is consistent with XRD results. Due to the tubular structure, ZnO nanotube has great potential for fundamental studies and for applications in catalyst, intramolecular absorption, storage and release system, etc.[11-14]. The SEM image in Fig. 2(c) indicates that the sample prepared at higher temperature has the morphology of nanorod.
Fig.2. (a) low- and (b) high-magnification SEM images of nanotube, (c) SEM image of nanorod, and (d) TEM image of nanotube; inset is the corresponding selected-area electron diffraction pattern.
Since no catalyst was used in this study, the growth was different from traditional vapor-liquid-solid mechanism where noble metal is needed as catalyst [15-17]. The growth of nanotube in our study has been proposed to be a process composed of the solidification of liquid droplets, surface oxidation and sublimation as below. Zn powders are evaporated and carried downstream to the substrate by the Ar carrier gas, where liquid Zn droplets are formed and then solidified. From surface-energy point of view, the lowest energy facets for Zn are (0001) and accordingly the [0001] direction has the fastest growth rate [18]. Thus Zn rod is formed as schematically shown in Fig. 3(a). On the other hand, the facets with higher energy grow slower, but have a higher oxidation rate. Therefore, the surface of synthesized hexagonal Zn nanorod is oxidized,
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forming a Zn/ZnO core-shell structure [Fig. 3(b)]. At the same time, Zn inside will sublimate owing to the low melting temperature (420°C). Because of ZnO shell layer, the Zn vapor inside cannot move out which leads to the increase of pressure, and finally breaches the ZnO shell layer and sublimate it out. Then nanotubes with some broken wall are formed as shown in Fig. 3(c). In the case of nanorod synthesized on substrate B, Zn can be fully oxidized owing to a lower deposition rate of Zn and a faster oxidation rate at a higher temperature. Since oxygen partial pressure in the reaction system has strong effect on the morphology of ZnO nanostructure [19], this was applied in synthesizing ZnO nanostructures with complex morphology by tuning the oxygen partial pressure during reaction in our study. The flow of oxygen was set at 17 and 5 sccm for the first and the second 30 min of the reaction in furnace (b). Under this circumstances, ZnO nanorod with several nanowires on the head, namely nanoinjector, were prepared, as shown in Fig. 4(a). The diameter of nanorod at the bottom and nanowire at the top is about 200 and 15 nm, respectively. It is suggested that they were grown in the first and the second 30 min of the reaction, respectively. Dramatically decrease of oxygen partial pressure reduces the oxidation rate, leading to the formation of nanowire with a small diameter. Moreover, nanorod formed at the first stage reaction favors the nucleation of nanowire [20]. To verify the two-stage growth mechanism proposed for nanoinjector, we prepared another sample at the same experimental conditions as nanoinjector except without the second 30 min reaction process, and found that ZnO nanorod without nanowires on the head was formed, as in Fig. 4(b). Slight difference in the morphology between two kinds of nanorods is suggested to be ascribed to the slightly different experimental conditions which are out of our controlling ability. The as-synthesized nanoinjector can be used as efficient electron emitter due to its high aspect ratio of the nanowire on the top [21,22]. It may also have potential applications in multifunctional devices capable of communication with multiple receiver, such as multichannel optoelectronic device, since several nanowires are grown from one nanorod at the bottom.
Fig. 3. Schematic diagrams of growth process of nanotube.
Fig. 4. SEM images of (a) nanoinjector and (b) nanorod.
More interestingly, ZnO nanoplate was formed and merged [Figs. 5(a) and 5(b)] when the flow of oxygen is increased to 25 sccm. Enlarged images of nanoplate in Fig. 5(b) show that each nanoplate before merging has a regular hexagonal-faceted shape. It is not strange since ZnO has hexagonal structures. An interesting thing is that these hexagonal faceted nanoplates were merged by self-assembly. Since regular hexagons can
Duofa Wang et al. / Physics Procedia 48 (2013) 235 – 240
build up a plane as shown in Fig. 5(c), it is reasonable to expect that these hexagonal nanoplates can be merged more and grown into a film at improved experimental conditions. Provided that this method is feasible, ZnO single-crystalline thin film with high quality, which is of considerable interest [23,24], can be achieved, because it was reported that each ZnO nanostructure is single-crystalline with perfect crystalline quality [19,20]. Moreover, the smooth surface of each hexagonal nanoplate [see Fig. 5(b)] promises a rather smooth surface of ZnO single crystal fabricated by this method.
Fig. 5. (a) low- and (b) high-magnification SEM images of merged nanoplates, (c) Schematic diagram of growth process of ZnO singlecrystalline film from nanoplate.
4. Conclusion ZnO nanostructures, including nanorod, nanotube, nanoinjector and nanoplate were synthesized by thermal chemical vapor deposition. The control on the morphology of nanostructures was realized by employing a temperature gradient together with the tuning of oxygen partial pressure. Moreover, merged ZnO nanoplates were prepared at the proper conditions, which is expected to be merged further and grown into the ZnO single crystalline film with high quality.
Acknowledgement This work was partially supported by NSFC (No. 51172169, 51032005), NCET (NCET-11-0687), and the Fundamental Research Funds for the Central Universities (Wuhan University of Technology). References 1. K. Xuan, X. H. Yan, S. L. Ding, Y. R . Yang, X. Xiao, Z. H. Guo, 2006. Field Emission Performance of Randomly Oriented ZnO Hexagonal Nanoprisms: Catalyst-Free Chemical Vapor Deposition, J. Phys. Soc. Jpn. 75, p. 014711. 2. T. Makino, A. Tsukazaki, A. Ohtomo, M. Kawasaki and H. Koinuma, 2006. Shifting Donor–Acceptor Photoluminescence in N-doped ZnO, J. Phys. Soc. Jpn. 75, p.075703. 3. Takayuki Makino and Yusaburo Segawa, 2002. Magnesium Concentration Dependence of Room-Temperature Absorption-Edge Singularity in Alloyed MgZnO Epilayers, J. Phys. Soc. Jpn. 71, p. 2855.
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4. Y. Huang, X. Duan, Y. Cui, L. J. Lauhon, K. H. Kim, C. M. Lieber, 2001. Logic Gates and Computation from Assembled Nanowire Building Blocks, Science 294, p. 1313. 5. J. R. Kim, H. M. So, J. W. Park, J. J. Kim, J. Kim, C. J. Lee, S. C. Lyu, 2002. Electrical transport properties of individual gallium nitride nanowires synthesized by chemical-vapor-deposition, Appl. Phys. Lett. 80, p. 3548. 6. C. J. Lee, T. J. Lee, S. C. Lyu, Y. Zhang, H. Ruh, H. J. Lee, 2002. Field emission from well-aligned zinc oxide nanowires grown at low temperature, Appl. Phys. Lett. 81, p. 3648. 7. M. Zamfirescu, A. Kavokin, B. Gil, G. Malpuech, M. Kaliteevski, 2002. ZnO as a material mostly adapted for the realization of room-temperature polariton lasers, Phys. Rev. B 65, p. 161205. 8. J. Park, H. H. Choi, K. Siebein, R. K. Singh, 2003. Two-step evaporation process for formation of aligned zinc oxide nanowires, J. Cryst. Growth 258, p. 342. 9. Z. W. Pan, Z. R. Dai, Z. L. Wang, 2001. Nanobelts of semiconducting oxides, Science 291, p. 1947 10. H. Y. Dang, J. Wang, S. S. Fan, 2003. The synthesis of metal oxide nanowires by directly heating metal samples in appropriate oxygen atmospheres, Nanotechnology 14, p. 738. 11. Z. Yao, H. W. C. Postma, L. Balents, C. Dekker, 1999. Carbon nanotube intramolecular junctions, Nature. 402, p. 273. 12. W. Tremel, 1999. Inorganic Nanotubes, Angew. Chem., Int. Ed., 38, p. 2175. 13. Y. Lin, A. M. Rao, B. Sadanadan, E. A. Kenik, Y. P. Sun, 2002. Functionalizing Multiple-Walled Carbon Nanotubes with Aminopolymers, J. Phys. Chem. B 106, p. 1294. 14. J. E. Riggs, D. B. Walker, D. L. Carroll, Y. P. Sun, 2000. Soluble Dendron-Functionalized Carbon Nanotubes: Preparation, Characterization, and Properties , J. Phys. Chem. B 104, p. 7071. 15. J. T. Hua, T. W. Odom, C. M. Lieber, 1999. Chemistry and Physics in One Dimension: Synthesis and Properties of Nanowires and Nanotubes, Acc. Chem. Res. 32, p. 435. 16. P. Yang, Y. Wu, R. Fan, 2002. Ab Initio Modeling of Diamond Nanawire Structures, Int. J. Nanosci. 1, p. 1. 17. Y. Wu, P. Yang, 2001. Direct observation of Vapor-Liquid-Solid Nanowire Growth, J. Am. Chem Soc. 123, p. 3165. 18. P. X. Gao, C. S. Lao, Y. Ding, Z. L. Wang, 2006. Metal/Semiconductor Core/Shell Nanodisks and Nanotubes, Adv. Funct. Mater. 16, p. 53. 19. H. Yan, R. He, J. Pham, P. Yang, 2006. In situ observation of ZnO nanowire growth on Zinc film in environmental scanning electron microscope, Adv. Mater. 15, 402. 20. H. B. Lu, L. Liao, J. C. Li, D. F. Wang, H. He, Q. Fu, L. Xu, Y. Tian, 2006. One-Step Hydrothermal Formation of Bi2O3 Nanourchins with Radially Ultrathin Nanotubes, J. Phys. Chem. B 110, p. 23211. 21. Q. H. Li, Q. Wan, Y. J. Chen, T. H. Wang, Q. H. Li, Q. Wan, Y. J. Chen, T. H. Wang, 2004. Stable field emission from tetrapodlike ZnO nanostructures, Appl. Phys. Lett. 85, p. 636. 22. C. X. Xu, X. W. Sun, B. J. Chen, 2004. Field emission from gallium doped zinc oxide nanofiber array,Appl. Phys. Lett. 84, p. 1540. 23. Y. Chen, D. M. Bagnall, Z. Zhu, T. Sekiguchi, K. Park, K. Hiraga, T. Yao, S. Koyama, M. Y. Shen, T. Goto, 1997. Growth of ZnO single crystal thin films on c-plane (0 0 0 1) sapphire by plasma enhanced molecular beam epitaxy, J. Crystal Growth 181, p. 165. 24. M. Kawasaki, A. Ohtomo, I. Ohkubo, H. Koinuma1, Z. K. Tang, P. Yu, G. K. L. Wong, B. P. Zhang, Y. Segawa, 1998. Excitonic ultraviolet laser emission at room temperature from naturally made cavity in ZnO nanocrytal thin films, Mater. Sci. Eng. B 56, p. 239
ScienceDirect Physics Procedia 48 (2013) 235 – 240
The XIII International Conference on the Physics of Non-Crystalline Solids
Controllable synthesis of ZnO nanostructures with various morphologies Duofa Wanga,b, Fangjie Wangb, Haizheng Taoa*,, Xuecai Hana, Xiujian Zhaoa, Tianjin Zhangb, Hui Huangc, Hualiang Fuc, Wu Qiuc a
State key Laboratory of Silicate Materials for architechture (Wuhan University of Technology), Wuhan, 430070, PR China, b Faculty of Materials Science and Engineering, Hubei University, Wuhan 430062, P.R.China c Hubei Yingtong Telecommunication Cable Holding Co., Ltd., Tongcheng, 437400, P.R.China
Abstract Utilizing the silicon as substrate, ZnO nanostructures with various morphologies, including nanorod, nanotube, nanoinjector and nanoplate, were synthesized by thermal chemical vapor deposition without catalysts. The control on the morphology of nanostructures was achieved by forming a temperature gradient in the tube furnace and controlling the oxygen partial pressure in the reaction system. The as-synthesized nanostructures were characterized by x-ray diffraction, scanning electron microscopy and transmission electron microscopy. Moreover, merged ZnO nanoplates were prepared at proper conditions, which are expected to be merged further and eventually grown into a ZnO singlecrystalline film with high quality.
2013 The Authors. Published © 2013 The©Authors. Published by Elsevier B.V. by Elsevier Ltd. Selection and/or of Prof. XiujianofZhao, Wuhan University of Selection and peer-review underpeer-review responsibilityunder of Prof.responsibility Xiujian Zhao, Wuhan University Technology Technology
Keywords: ZnO; controllable synthesis; nanostructure
1. Introduction Since the first discovery of carbon nanotubes, one-dimensional nanometer-sized semiconductor materials
* Corresponding author. Tel/Fax.: +86-27-87669729 E-mail address: [email protected](HZ Tao)
1875-3892 © 2013 The Authors. Published by Elsevier B.V. Selection and peer-review under responsibility of Prof. Xiujian Zhao, Wuhan University of Technology doi:10.1016/j.phpro.2013.07.037
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Duofa Wang et al. / Physics Procedia 48 (2013) 235 – 240
such as nanowires have attracted extensive interest due to their great potential for the fundamental studies about the roles of dimensionality and size on their physical properties as well as for their application to optoelectronic nanodevices [1-6]. Among various semiconductor materials, zinc oxide (ZnO) with a direct band gap of 3.37 eV and a large exciton binding energy of 60 meV provides potential applications in nanoscaled optoelectronic device, such as laser diodes and ultraviolet sensors [7]. Various synthesis methods and nanostructure morphologies have been reported, including Nanobelts, nanowires, tetrapod nanostructures [8-10]. However, the controllable synthesis and the deep understanding of growth mechanism of these nanostructures has not been achieved yet. Moreover, the preparation of ZnO nanostructures with complicated morphology are also needed to meet the requirements for advanced optical and electronic devices. In the present work, ZnO nanostructures with complex morphology, including nanorod, nanotube, nanoinjector and nanoplate, were synthesized by providing a temperature gradient between Zn powders and substrate in a tube furnace together with the change of oxygen partial pressure in the reaction system. And the growth mechanism of all these nanostructures was discussed in detail.
2. Experimental The growth was performed in two horizontal tube furnaces with and without temperature gradient, which was made by controlling the density of heaters as shown in Figs. 1(a) and 1(b). Quartz boats loaded with Zn powders (purity: 99.99%) and p-type Si substrates were introduced into the tube furnaces. The distance between Si substrate and Zn powders was around 5 mm. In furnace (a), another Si substrate was also put on the quartz boat, 5 cm away from Zn powders where the temperature is lower to investigate the effect of temperature on the morphology of ZnO nanostructures. Then, the temperatures near Zn powders were raised to 480°C at a rate of 15°C/min in furnaces (a) and (b). At this time, the temperature of substrate A and substrate B in furnace (a) was 430°C and 480°C, respectively. Afterwards, oxygen and Ar gases were introduced into the chamber as reactant and carrier gases, respectively. The reaction was maintained for 1 h. During the reaction, the pressure in the tube furnace was keep at 20 Pa. In furnace (a) the flow of oxygen was kept to be constant at 10 sccm during the reaction, while it was varied in furnace (b) to synthesize nanostructures with complex morphology aimed at the application to advanced optoelectronic devices. The crystal structure of the as-prepared samples was analyzed by x-ray diffraction (XRD). The morphologies of samples were characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM).
Fig. 1. Schematic diagrams of thermal chemical vapor deposition system (a) with and (b) without temperature gradient
Duofa Wang et al. / Physics Procedia 48 (2013) 235 – 240
3. Results and discussion Analysis on the XRD patterns (not shown) of the synthesized samples reveals that all the deposited nanostructures on Si substrate are wurtzite ZnO. The morphologies of these nanostructures were measured by SEM, shown in Figure 2. Fig. 2(a) and (c) are the SEM images of nanostructures synthesized on substrates A (low temperature) and B (high temperature) in furnace (a), respectively. And Fig. 2(b) are the high magnification of (a). It can be seen that curved nanostructures was grown on substrate A densely and uniformly, with a diameter of 180 - 350 nm. High-magnification SEM image of it [Fig. 2(b)] shows that some of the nanostructures (marked by arrows) are brokenˈfrom which it is noticed that the nanostructures deposited on substrate A have a tubular structure. TEM was additionally employed to investigate the unbroken part for further investigation of the microstructure. From the TEM image [Fig. 2(d)], we can see clearly that the whole nanostructure is tubular, not only the broken port. The selected area electron diffraction pattern, shown as the inset of Fig. 2(d), reveals that the synthesized nanotube is single-crystalline ZnO and that no metallic Zn phase is found, which is consistent with XRD results. Due to the tubular structure, ZnO nanotube has great potential for fundamental studies and for applications in catalyst, intramolecular absorption, storage and release system, etc.[11-14]. The SEM image in Fig. 2(c) indicates that the sample prepared at higher temperature has the morphology of nanorod.
Fig.2. (a) low- and (b) high-magnification SEM images of nanotube, (c) SEM image of nanorod, and (d) TEM image of nanotube; inset is the corresponding selected-area electron diffraction pattern.
Since no catalyst was used in this study, the growth was different from traditional vapor-liquid-solid mechanism where noble metal is needed as catalyst [15-17]. The growth of nanotube in our study has been proposed to be a process composed of the solidification of liquid droplets, surface oxidation and sublimation as below. Zn powders are evaporated and carried downstream to the substrate by the Ar carrier gas, where liquid Zn droplets are formed and then solidified. From surface-energy point of view, the lowest energy facets for Zn are (0001) and accordingly the [0001] direction has the fastest growth rate [18]. Thus Zn rod is formed as schematically shown in Fig. 3(a). On the other hand, the facets with higher energy grow slower, but have a higher oxidation rate. Therefore, the surface of synthesized hexagonal Zn nanorod is oxidized,
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forming a Zn/ZnO core-shell structure [Fig. 3(b)]. At the same time, Zn inside will sublimate owing to the low melting temperature (420°C). Because of ZnO shell layer, the Zn vapor inside cannot move out which leads to the increase of pressure, and finally breaches the ZnO shell layer and sublimate it out. Then nanotubes with some broken wall are formed as shown in Fig. 3(c). In the case of nanorod synthesized on substrate B, Zn can be fully oxidized owing to a lower deposition rate of Zn and a faster oxidation rate at a higher temperature. Since oxygen partial pressure in the reaction system has strong effect on the morphology of ZnO nanostructure [19], this was applied in synthesizing ZnO nanostructures with complex morphology by tuning the oxygen partial pressure during reaction in our study. The flow of oxygen was set at 17 and 5 sccm for the first and the second 30 min of the reaction in furnace (b). Under this circumstances, ZnO nanorod with several nanowires on the head, namely nanoinjector, were prepared, as shown in Fig. 4(a). The diameter of nanorod at the bottom and nanowire at the top is about 200 and 15 nm, respectively. It is suggested that they were grown in the first and the second 30 min of the reaction, respectively. Dramatically decrease of oxygen partial pressure reduces the oxidation rate, leading to the formation of nanowire with a small diameter. Moreover, nanorod formed at the first stage reaction favors the nucleation of nanowire [20]. To verify the two-stage growth mechanism proposed for nanoinjector, we prepared another sample at the same experimental conditions as nanoinjector except without the second 30 min reaction process, and found that ZnO nanorod without nanowires on the head was formed, as in Fig. 4(b). Slight difference in the morphology between two kinds of nanorods is suggested to be ascribed to the slightly different experimental conditions which are out of our controlling ability. The as-synthesized nanoinjector can be used as efficient electron emitter due to its high aspect ratio of the nanowire on the top [21,22]. It may also have potential applications in multifunctional devices capable of communication with multiple receiver, such as multichannel optoelectronic device, since several nanowires are grown from one nanorod at the bottom.
Fig. 3. Schematic diagrams of growth process of nanotube.
Fig. 4. SEM images of (a) nanoinjector and (b) nanorod.
More interestingly, ZnO nanoplate was formed and merged [Figs. 5(a) and 5(b)] when the flow of oxygen is increased to 25 sccm. Enlarged images of nanoplate in Fig. 5(b) show that each nanoplate before merging has a regular hexagonal-faceted shape. It is not strange since ZnO has hexagonal structures. An interesting thing is that these hexagonal faceted nanoplates were merged by self-assembly. Since regular hexagons can
Duofa Wang et al. / Physics Procedia 48 (2013) 235 – 240
build up a plane as shown in Fig. 5(c), it is reasonable to expect that these hexagonal nanoplates can be merged more and grown into a film at improved experimental conditions. Provided that this method is feasible, ZnO single-crystalline thin film with high quality, which is of considerable interest [23,24], can be achieved, because it was reported that each ZnO nanostructure is single-crystalline with perfect crystalline quality [19,20]. Moreover, the smooth surface of each hexagonal nanoplate [see Fig. 5(b)] promises a rather smooth surface of ZnO single crystal fabricated by this method.
Fig. 5. (a) low- and (b) high-magnification SEM images of merged nanoplates, (c) Schematic diagram of growth process of ZnO singlecrystalline film from nanoplate.
4. Conclusion ZnO nanostructures, including nanorod, nanotube, nanoinjector and nanoplate were synthesized by thermal chemical vapor deposition. The control on the morphology of nanostructures was realized by employing a temperature gradient together with the tuning of oxygen partial pressure. Moreover, merged ZnO nanoplates were prepared at the proper conditions, which is expected to be merged further and grown into the ZnO single crystalline film with high quality.
Acknowledgement This work was partially supported by NSFC (No. 51172169, 51032005), NCET (NCET-11-0687), and the Fundamental Research Funds for the Central Universities (Wuhan University of Technology). References 1. K. Xuan, X. H. Yan, S. L. Ding, Y. R . Yang, X. Xiao, Z. H. Guo, 2006. Field Emission Performance of Randomly Oriented ZnO Hexagonal Nanoprisms: Catalyst-Free Chemical Vapor Deposition, J. Phys. Soc. Jpn. 75, p. 014711. 2. T. Makino, A. Tsukazaki, A. Ohtomo, M. Kawasaki and H. Koinuma, 2006. Shifting Donor–Acceptor Photoluminescence in N-doped ZnO, J. Phys. Soc. Jpn. 75, p.075703. 3. Takayuki Makino and Yusaburo Segawa, 2002. Magnesium Concentration Dependence of Room-Temperature Absorption-Edge Singularity in Alloyed MgZnO Epilayers, J. Phys. Soc. Jpn. 71, p. 2855.
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