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Fabrication of tubular ZnO by vesicle–template fusion
Materials Letters 61 (2007) 2195 – 2199 www.elsevier.com/locate/matlet
Fabrication of tubular ZnO by vesicle–template fusion Yanyan Ding a , Zhou Gui a,⁎, Jixin Zhu a , Shanshan Yan b , Jian Liu b , Yuan Hu a , Zhengzhou Wang a a
State Key Lab of Fire Science, University of Science and Technology of China, Hefei, 230027, P. R. China b Department of Chemistry, University of Science and Technology of China, Hefei, 230027, P. R. China Received 10 July 2006; accepted 22 August 2006 Available online 8 September 2006
Abstract Zinc oxide microtubes were synthesized by hydrothermal method. The cooperation activities of surfactant cetyltrimethylammonium bromide (CTAB) and bidentate ethylenediamine ligand were found to be appropriate for the formation of final tubes. The morphologies and structures of the samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). A possible vesicle-template elongating fusion mechanism was proposed and discussed. © 2006 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a well-known semiconductor for its wide bandgap (3.37 eV) and high exciton binding energy of 60 meV at room temperature. As a consequence, it possesses unique optical, acoustical, and electronic properties that stimulate wide research interest in its potential applications [1]. Recently, a particularly striking observation is that of room temperature lasing action in ZnO nanorod arrays [2], highlighting the prospects of corresponding research interests in a morphologically controllable synthesis of ZnO to meet the demand for the development of novel devices. Up to now, there have been two basic templating methods (either hard or soft templates) to fabricate hollow materials. For example, carbon nanotubes, alumina porous membranes, gelator fibers, and sacrificial crystals have been used as hard templates to synthesize tubular inorganic materials [3]. In the soft-template method, the vesicle-template method has been considered as an effective interpretation for the formation of the inorganic and polymer materials with hollow structure [4]. The vesicle-template synthesis method based on the hydrolysis and crosslinking of inorganic precursors at the vesicle bilayer co-organized by supramolecular surfactants has been extended to prepare a variety of inorganic mesostructures. The prerequisite of co⁎ Corresponding author. E-mail address: [email protected] (Z. Gui). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.08.045
organization is being fulfilled due to electrostatic charge-density matching or hydrogen bonding between template and inorganic precursor [5]. During the synthesis, the vesicles are also used as directing and structuring agents and the transcription of the surface of the vesicles automatically results in an imprinting of the template morphology. In the vesicle-template method, ionic organic surfactants or non-ionic polymer surfactants have been widely used to form inorganic hollow structures [4h,e,5a]. But to our knowledge, there is little literature concerning the soft vesicle–template fusion in the synthesizing of the inorganic tubes and hollow spheres. In the present communication, we report a hydrothermal preparation of single crystal tubular ZnO by the cooperative effect of Zn(CH3COO)2–ethylenediamine–cetyltrimethylammonium bromide complex vesicles as soft templates. A possible “vesicle elongating fusion” mechanism for the tubular structure formation is proposed based on the intermediate products obtained. It may provide a helpful clue to further understand the formation of large tube with closed tips and to provide an effective way for artificial fabrication of hollow structures. 2. Experimental All reactants were analytically pure, and no further purification was needed. 0.005 mol Zn(CH 3 COO) 2 ·2H 2 O, 0.0125 mol ethylenediamine (10% wt) and 50 mL distilled water were added to a Teflon-lined stainless autoclave with
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Fig. 1. Powder X-ray diffraction patterns for hexagonal ZnO microtubes.
capacity of 80 mL and stirred to a solution. Then 0.0025 mol cetyltrimethylammonium bromide (CTAB) was added into the solution. After the mixture was stirred to a clear aqueous solution, the autoclave was sealed and heated at 140 °C for 4–6 h and cooled to room temperature. A white precipitate was collected after filtration, washed with water and ethanol, and later dried at 80 °C for 5 h. X-ray powder diffraction (XRD) analysis was conducted out on a Rigaku D/Max X-ray diffractometer with graphite monochromated CuKα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) images, and selected area electron diffraction (SAED) patterns of the samples were collected on Hitachi H-800 electron microscope operated at 200 keV. Scanning electron microscopy (SEM) images were recorded on a JSM-6700F scanning electron microscope, working at 100 keV acceleration voltages. 3. Results and discussion Fig. 1 shows a typical X-ray powder diffraction (XRD) pattern of the hydrothermal samples at 140 °C. All diffraction peaks can be
Fig. 3. TEM image of a single ZnO microtube. The inset is the selected area electron diffraction pattern of the ZnO microtube.
indexed to a hexagonal structure of the bulk ZnO. No characteristic peaks from other crystalline forms are detected in the XRD pattern. The morphology of the as-prepared ZnO was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 2 shows SEM images of the as-prepared hexagonal ZnO. The SEM images reveal that the product ZnO has a 1-dimensional structure. The open-ended tips (Fig. 2b and inset) and some broken shells (Fig. 2a) clearly show that they are hollow inside, although most
Fig. 2. SEM images of ZnO microtubes. a) Low-magnification SEM image and b) high-magnification image of ZnO microtubes with open tips. The areas as the arrows indicated in (a) clearly show some broken shells, which indicate that the products are hollow inside. The inset of (b) is a single tube with an open end.
Y. Ding et al. / Materials Letters 61 (2007) 2195–2199
of the samples have the closed tips. A small quantity of microtubes with open ends is due to mechanical fracture. The SEM results further show the ZnO microtubes with diameter ranging from 1.0 to 2.0 μm and lengths ranging from 10 to 20 μm. The average thickness of wall is about 400–600 nm. The images of Fig. 3 represent a single ZnO microtube and its selected area electron diffraction (SAED) pattern. It indicates that the obtained ZnO samples are single crystal. Elongated diffraction spots in the SAED pattern are due to the cylindrical structure of the sample. The {0001} plane is perpendicular to the growth direction, showing that the preferred orientation of the ZnO microtubes is along the c-axis. We tried to investigate high-resolution TEM (HRTEM) image of the ZnO microtubes, but we cannot succeed in getting a lattice image because the tube wall was too thick to obtain HRTEM images.
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It is well-known that the formation mechanism for the tube structures is very important in exploring synthetic methods. Li et al. [6] synthesized WS2 and vanadium oxide nanotubes, respectively, and provided strong evidence for their rolling model of layered structures. Mallouk et al. [7] also provided an obvious proof for the transformation of lamellar oxides into tubular structures. In the present system, we propose a different scheme for the formation of ZnO microtubes, which is named “vesicle elongating fusion mechanism”, which could be mainly divided into three steps: (1) With the efficacy of CTAB surfactant-based template, the Zn(CH3COO)2–ethylenediamine–CTAB system self-organized into a lamella-like phase, which is no longer planar but a vesicle shape with hollow cavity. The self-organized process takes place continuously and leads to more vesicle assemblies. (2) The vesicle aggregation, coalescence and fusion and the sequential vesicle
Fig. 4. TEM images of a) many small vesicles with hollow cavity, b) an individual elongating vesicle, c) the fusing of small vesicles with elongated vesicle, and d) the fusing of the tubelike vesicles.
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fusion result in the formation of the elongated tubular vesicles. (3) The elongating process stopped after the mineralization of the inorganic precursors on the shell of the elongated tubular vesicles, resulting in the obtained ZnO microtubes with tips closed. To further comprehend the vesicle elongating mechanism we proposed, we performed a series of parallel experiments through intercepting the intermediates at different conditions. Under a lower temperature of 100 °C, a lot of hollow spheres (vesicles) with diameter about 100 nm have been observed from the products (Fig. 4a). It is known that CTAB has the tendency to form vesicles under proper experimental conditions [8]. The as-formed vesicle is metastable and characterized by active ligands on the surface [8d]. Elongated vesicles and tubelike samples are also found in the same products (Fig. 4b, c, d). Large amount of articles have been published on the membrane fusion that plays an important role in the formation of many complex organs, and it is even believed to be one of the key events in the origin of life [9]. Most interesting is the recent report on polymer vesicle fusion [10], although the mechanism of the fusion is controversial [9b,11]. Fig. 4b is a typical example of the resultant fusion vesicle. The existence of the neck indicated by arrow 1 and the center wall indicated by arrow 2 is the trace for the fusion [10d]. Compared with the vesicles shown in Fig. 4a, the thickness of the vesicle shell shown in Fig. 4b is much larger than 100 nm, and the vesicles are elongating to the flattening shape. It is probably due to the repetitious fusion. In other words, the vesicles seem to be more elongated and the thickness of the shell is increased after every fusion process. In fact, the repetitious fusion has been found in polymer vesicles [10d]. To our knowledge, this is the first report about elongating fusion of vesicles. The mechanism for such elongating fusion is still unclear, and the elongating fusion should be a complicated process. More detailed investigation is needed. From our opinions, the component and structure of the vesicle shell may play a very important role in developing an anisotropic orientation growth. As shown in Fig. 4b, the spots on the shell of vesicle indicate that the vesicles are unstable under irradiation by a strong electron beam, which is probably due to the presence of organic molecules on the shell. More evidence for the elongating fusion mechanism is found in the fusing of small vesicles with elongated vesicle (Fig. 4c) and the tubelike vesicles (Fig. 4d). The observation shown in Fig. 4 suggests that the tubelike vesicles are obtained with several fusions. Recently, Shi et al. reported a transition of block copolymer PS80-b-P4VP110 from vesicles to giant tubular vesicles in a heating process. It is the only work to report the transition of vesicles to tubular vesicles [12]. With the fusion reaction proceeding, especially after the mineralization [8d,13], the elongating fusing process stops, resulting in the formation of ZnO microtubes with closed tips and a large thickness of the wall that is consistent with the proposed formation mechanism. We replaced the ethylenediamine by hydrazine or 1,6-hexamethylendiamine with a shorter or longer hydrocarbon chain, respectively, keeping other experimental variables unchanged. In the hydrazine, CTAB and zinc acetate solution system, the product is large rod-like particles. But in 1,6-hexamethylendiamine system, the obtained products are ZnO nanoparticles, confirmed by XRD and TEM analysis. Similarly, the obtained precipitate is the large cube-like particles when CTAB was not added to the reactants. Recently, many literatures reported the synthesis of different inorganic materials with hollow structure by using soft vesicle-template. But the soft vesicle–template fusion is little reported. According to the literatures [10d,14], the whole fusion process of vesicles is very quick. It lasted about 1.5 min for the polymer vesicles, in the timescale of seconds for the liposomes and of milliseconds for the biomembranes. Therefore, it is difficult for the scientists to “watch” the fusing sequences by recording the vesicle fusion intermediates in real-time.
4. Conclusion In present paper, single crystal ZnO microtube was successfully synthesized, and we propose a vesicle–template fusion mechanism that provides a helpful clue to further understand the tube or hollow sphere formation. If the mechanism could be confirmed by future investigations, it may have implications for the fabrication of large hollow inorganic materials by soft vesicle–template fusion in general. Acknowledgments This work was supported by the National Natural Foundation of China (No. 20571070) and the special foundation of State Key Lab of Fire Science. References [1] a) Y. Xia, P. Yang, Y. Sun, Y. Wu, B. Mayers, B. Gates, Y. Yin, F. Kim, H. Yan, Adv. Mater. 15 (2003) 353; b) M. Hiramatsu, K. Imaeda, N. Horio, M. Nawata, J. Vac. Sci. Technol., A, Vac. Surf. Films 16 (1998) 669. [2] M.H. Huang, S. Mao, H. Feick, H. Yan, Y. Wu, H. Kind, E. Weber, R. Russo, P. Yang, Science 292 (2001) 1897. [3] a) C.N.R. Rao, B.C. Satishkumar, A. Govindaraj, Chem. Commun. (1997) 1581; b) W.Q. Han, S.S. Fan, Q.Q. Li, Y.D. Hu, Science 277 (1997) 1287; c) B.B. Lakshmi, P.K. Dorhout, C.R. Martin, Chem. Mater. 9 (1997) 857; d) J.H. Jung, S. Shinkai, T. Shimizu, Nano Lett. 2 (2002) 17; e) S. Kobayashi, K. Hanbasa, N. Nakasaki, M. Kimura, H. Shirai, Chem. Mater. 12 (2000) 1523. [4] a) D. Hubert, M. Jung, A. German, Adv. Mater. 12 (2000) 1291; b) D. Discher, A. Eisenberg, Science 297 (2002) 967; c) T. Zemb, M. Dubois, B. Deme, T. Krzywicki, Science 283 (1999) 816; d) B. Tan, H. Lehmler, S. Vyas, B. Knutson, S. Rankin, Adv. Mater. 17 (2005) 2368; e) B. Tan, S. Vyas, H. Lehmler, B. Knutson, S. Rankin, J. Phys. Chem., B 109 (2005) 23226; f) D. Zhang, L. Qi, J. Ma, H. Cheng, Adv. Mater. 14 (2002) 1499; g) C. Aguirre, T. Kaspar, C. Radloff, N. Halas, Nano Lett. 3 (2002) 1707; h) T. Peng, A. Hasegawa, J. Qiu, K. Hirao, Chem. Mater. 15 (2003) 2011. [5] a) D. Hubert, M. Jung, P. Frederik, P. Bomans, J. Meuldijk, A. German, Adv. Mater. 12 (2000) 1286; b) J. Blin, A. Leonard, Z. Yuan, L. Gigot, A. Vantomme, A. Cheetham, B. Su, Angew. Chem., Int. Ed. 42 (2003) 2872; c) Z. Yuan, T. Ren, B. Su, Adv. Mater. 15 (2003) 1462. [6] a) Y. Li, X. Li, R. He, J. Zhu, Z. Deng, J. Am. Chem. Soc. 124 (2002) 1411; b) X. Chen, X. Sun, Y. Li, Inorg. Chem. 41 (2002) 4524. [7] a) G. Saupe, C. Waraksa, H. Kin, Y. Han, D. Kaschak, D. Shinner, T. Mallouk, Chem. Mater. 12 (2000) 1556; b) R. Schaak, T. Mallouk, Chem. Mater. 12 (2000) 3427. [8] a) H.P. Hentze, S.R. Raghavan, C.A. MaKelvey, E.W. Kaler, Langmuir 19 (2003) 1069; b) M.P. Pileni, Nat. Mater. 2 (2003) 145; c) D. Discher, A. Eisenberg, Science 297 (2002) 967; d) S. Wang, F. Gu, M. Lu, Langmuir 22 (2006) 398. [9] a) B.R. Lentz, V. Malinin, M.E. Haque, K. Evans, Curr. Opin. Struck. Biol. 10 (2000) 607; b) A. Mayer, Trends Biochem. Sci. 26 (2001) 717; c) V. Norris, D. Raine, J. Orig. Life Evol. Biosph. 28 (1998) 523. [10] a) L. Luo, A. Eisenberg, Langmuir 17 (2001) 6804; b) A.A. Choucair, A.H. Kycia, A. Eisenberg, Langmuir 19 (2003) 1001; c) F. Liu, A. Eisenberg, J. Am. Chem. Soc. 125 (2003) 15059; d) Y. Zhou, D. Yan, J. Am. Chem. Soc. 127 (2005) 10468.
Y. Ding et al. / Materials Letters 61 (2007) 2195–2199 [11] a) S.J. Marrink, A.E. Mark, J. Am. Chem. Soc. 125 (2003) 11144. [12] L. Gao, L. Shi, W. Zhang, Y. An, Z. Liu, G. Li, Q. Meng, Macromolecules 38 (2005) 4548. [13] a) H.P. Hentze, S.R. Raghavan, C.A. MaKelvey, E.W. Kaler, Langmuir 19 (2003) 1069;
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b) D. Lootens, C. Vautrin, H.V. Damme, T. Zemb, J. Mater. Chem. 13 (2003) 2072. [14] a) F. Nomura, T. Inaba, S. Ishikawa, M. Nagata, S. Takahashi, H. Hotani, K. Takiguchi, Proc. Natl. Acad. Sci. U. S. A. 101 (2004) 3420; b) J.C. Shillcock, R. Lipowsky, Nat. Mater. 4 (2005) 225.
Fabrication of tubular ZnO by vesicle–template fusion Yanyan Ding a , Zhou Gui a,⁎, Jixin Zhu a , Shanshan Yan b , Jian Liu b , Yuan Hu a , Zhengzhou Wang a a
State Key Lab of Fire Science, University of Science and Technology of China, Hefei, 230027, P. R. China b Department of Chemistry, University of Science and Technology of China, Hefei, 230027, P. R. China Received 10 July 2006; accepted 22 August 2006 Available online 8 September 2006
Abstract Zinc oxide microtubes were synthesized by hydrothermal method. The cooperation activities of surfactant cetyltrimethylammonium bromide (CTAB) and bidentate ethylenediamine ligand were found to be appropriate for the formation of final tubes. The morphologies and structures of the samples were characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). A possible vesicle-template elongating fusion mechanism was proposed and discussed. © 2006 Elsevier B.V. All rights reserved.
1. Introduction Zinc oxide (ZnO) is a well-known semiconductor for its wide bandgap (3.37 eV) and high exciton binding energy of 60 meV at room temperature. As a consequence, it possesses unique optical, acoustical, and electronic properties that stimulate wide research interest in its potential applications [1]. Recently, a particularly striking observation is that of room temperature lasing action in ZnO nanorod arrays [2], highlighting the prospects of corresponding research interests in a morphologically controllable synthesis of ZnO to meet the demand for the development of novel devices. Up to now, there have been two basic templating methods (either hard or soft templates) to fabricate hollow materials. For example, carbon nanotubes, alumina porous membranes, gelator fibers, and sacrificial crystals have been used as hard templates to synthesize tubular inorganic materials [3]. In the soft-template method, the vesicle-template method has been considered as an effective interpretation for the formation of the inorganic and polymer materials with hollow structure [4]. The vesicle-template synthesis method based on the hydrolysis and crosslinking of inorganic precursors at the vesicle bilayer co-organized by supramolecular surfactants has been extended to prepare a variety of inorganic mesostructures. The prerequisite of co⁎ Corresponding author. E-mail address: [email protected] (Z. Gui). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.08.045
organization is being fulfilled due to electrostatic charge-density matching or hydrogen bonding between template and inorganic precursor [5]. During the synthesis, the vesicles are also used as directing and structuring agents and the transcription of the surface of the vesicles automatically results in an imprinting of the template morphology. In the vesicle-template method, ionic organic surfactants or non-ionic polymer surfactants have been widely used to form inorganic hollow structures [4h,e,5a]. But to our knowledge, there is little literature concerning the soft vesicle–template fusion in the synthesizing of the inorganic tubes and hollow spheres. In the present communication, we report a hydrothermal preparation of single crystal tubular ZnO by the cooperative effect of Zn(CH3COO)2–ethylenediamine–cetyltrimethylammonium bromide complex vesicles as soft templates. A possible “vesicle elongating fusion” mechanism for the tubular structure formation is proposed based on the intermediate products obtained. It may provide a helpful clue to further understand the formation of large tube with closed tips and to provide an effective way for artificial fabrication of hollow structures. 2. Experimental All reactants were analytically pure, and no further purification was needed. 0.005 mol Zn(CH 3 COO) 2 ·2H 2 O, 0.0125 mol ethylenediamine (10% wt) and 50 mL distilled water were added to a Teflon-lined stainless autoclave with
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Fig. 1. Powder X-ray diffraction patterns for hexagonal ZnO microtubes.
capacity of 80 mL and stirred to a solution. Then 0.0025 mol cetyltrimethylammonium bromide (CTAB) was added into the solution. After the mixture was stirred to a clear aqueous solution, the autoclave was sealed and heated at 140 °C for 4–6 h and cooled to room temperature. A white precipitate was collected after filtration, washed with water and ethanol, and later dried at 80 °C for 5 h. X-ray powder diffraction (XRD) analysis was conducted out on a Rigaku D/Max X-ray diffractometer with graphite monochromated CuKα radiation (λ = 1.5418 Å). Transmission electron microscopy (TEM) images, and selected area electron diffraction (SAED) patterns of the samples were collected on Hitachi H-800 electron microscope operated at 200 keV. Scanning electron microscopy (SEM) images were recorded on a JSM-6700F scanning electron microscope, working at 100 keV acceleration voltages. 3. Results and discussion Fig. 1 shows a typical X-ray powder diffraction (XRD) pattern of the hydrothermal samples at 140 °C. All diffraction peaks can be
Fig. 3. TEM image of a single ZnO microtube. The inset is the selected area electron diffraction pattern of the ZnO microtube.
indexed to a hexagonal structure of the bulk ZnO. No characteristic peaks from other crystalline forms are detected in the XRD pattern. The morphology of the as-prepared ZnO was characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 2 shows SEM images of the as-prepared hexagonal ZnO. The SEM images reveal that the product ZnO has a 1-dimensional structure. The open-ended tips (Fig. 2b and inset) and some broken shells (Fig. 2a) clearly show that they are hollow inside, although most
Fig. 2. SEM images of ZnO microtubes. a) Low-magnification SEM image and b) high-magnification image of ZnO microtubes with open tips. The areas as the arrows indicated in (a) clearly show some broken shells, which indicate that the products are hollow inside. The inset of (b) is a single tube with an open end.
Y. Ding et al. / Materials Letters 61 (2007) 2195–2199
of the samples have the closed tips. A small quantity of microtubes with open ends is due to mechanical fracture. The SEM results further show the ZnO microtubes with diameter ranging from 1.0 to 2.0 μm and lengths ranging from 10 to 20 μm. The average thickness of wall is about 400–600 nm. The images of Fig. 3 represent a single ZnO microtube and its selected area electron diffraction (SAED) pattern. It indicates that the obtained ZnO samples are single crystal. Elongated diffraction spots in the SAED pattern are due to the cylindrical structure of the sample. The {0001} plane is perpendicular to the growth direction, showing that the preferred orientation of the ZnO microtubes is along the c-axis. We tried to investigate high-resolution TEM (HRTEM) image of the ZnO microtubes, but we cannot succeed in getting a lattice image because the tube wall was too thick to obtain HRTEM images.
2197
It is well-known that the formation mechanism for the tube structures is very important in exploring synthetic methods. Li et al. [6] synthesized WS2 and vanadium oxide nanotubes, respectively, and provided strong evidence for their rolling model of layered structures. Mallouk et al. [7] also provided an obvious proof for the transformation of lamellar oxides into tubular structures. In the present system, we propose a different scheme for the formation of ZnO microtubes, which is named “vesicle elongating fusion mechanism”, which could be mainly divided into three steps: (1) With the efficacy of CTAB surfactant-based template, the Zn(CH3COO)2–ethylenediamine–CTAB system self-organized into a lamella-like phase, which is no longer planar but a vesicle shape with hollow cavity. The self-organized process takes place continuously and leads to more vesicle assemblies. (2) The vesicle aggregation, coalescence and fusion and the sequential vesicle
Fig. 4. TEM images of a) many small vesicles with hollow cavity, b) an individual elongating vesicle, c) the fusing of small vesicles with elongated vesicle, and d) the fusing of the tubelike vesicles.
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fusion result in the formation of the elongated tubular vesicles. (3) The elongating process stopped after the mineralization of the inorganic precursors on the shell of the elongated tubular vesicles, resulting in the obtained ZnO microtubes with tips closed. To further comprehend the vesicle elongating mechanism we proposed, we performed a series of parallel experiments through intercepting the intermediates at different conditions. Under a lower temperature of 100 °C, a lot of hollow spheres (vesicles) with diameter about 100 nm have been observed from the products (Fig. 4a). It is known that CTAB has the tendency to form vesicles under proper experimental conditions [8]. The as-formed vesicle is metastable and characterized by active ligands on the surface [8d]. Elongated vesicles and tubelike samples are also found in the same products (Fig. 4b, c, d). Large amount of articles have been published on the membrane fusion that plays an important role in the formation of many complex organs, and it is even believed to be one of the key events in the origin of life [9]. Most interesting is the recent report on polymer vesicle fusion [10], although the mechanism of the fusion is controversial [9b,11]. Fig. 4b is a typical example of the resultant fusion vesicle. The existence of the neck indicated by arrow 1 and the center wall indicated by arrow 2 is the trace for the fusion [10d]. Compared with the vesicles shown in Fig. 4a, the thickness of the vesicle shell shown in Fig. 4b is much larger than 100 nm, and the vesicles are elongating to the flattening shape. It is probably due to the repetitious fusion. In other words, the vesicles seem to be more elongated and the thickness of the shell is increased after every fusion process. In fact, the repetitious fusion has been found in polymer vesicles [10d]. To our knowledge, this is the first report about elongating fusion of vesicles. The mechanism for such elongating fusion is still unclear, and the elongating fusion should be a complicated process. More detailed investigation is needed. From our opinions, the component and structure of the vesicle shell may play a very important role in developing an anisotropic orientation growth. As shown in Fig. 4b, the spots on the shell of vesicle indicate that the vesicles are unstable under irradiation by a strong electron beam, which is probably due to the presence of organic molecules on the shell. More evidence for the elongating fusion mechanism is found in the fusing of small vesicles with elongated vesicle (Fig. 4c) and the tubelike vesicles (Fig. 4d). The observation shown in Fig. 4 suggests that the tubelike vesicles are obtained with several fusions. Recently, Shi et al. reported a transition of block copolymer PS80-b-P4VP110 from vesicles to giant tubular vesicles in a heating process. It is the only work to report the transition of vesicles to tubular vesicles [12]. With the fusion reaction proceeding, especially after the mineralization [8d,13], the elongating fusing process stops, resulting in the formation of ZnO microtubes with closed tips and a large thickness of the wall that is consistent with the proposed formation mechanism. We replaced the ethylenediamine by hydrazine or 1,6-hexamethylendiamine with a shorter or longer hydrocarbon chain, respectively, keeping other experimental variables unchanged. In the hydrazine, CTAB and zinc acetate solution system, the product is large rod-like particles. But in 1,6-hexamethylendiamine system, the obtained products are ZnO nanoparticles, confirmed by XRD and TEM analysis. Similarly, the obtained precipitate is the large cube-like particles when CTAB was not added to the reactants. Recently, many literatures reported the synthesis of different inorganic materials with hollow structure by using soft vesicle-template. But the soft vesicle–template fusion is little reported. According to the literatures [10d,14], the whole fusion process of vesicles is very quick. It lasted about 1.5 min for the polymer vesicles, in the timescale of seconds for the liposomes and of milliseconds for the biomembranes. Therefore, it is difficult for the scientists to “watch” the fusing sequences by recording the vesicle fusion intermediates in real-time.
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