- Nano Express
- Open Access
Transformation of ZnO polycrystalline sheets into hexagon-like mesocrystalline ZnO rods (tubes) under ultrasonic vibration
© Ding et al.; licensee Springer. 2014
- Received: 11 February 2014
- Accepted: 24 April 2014
- Published: 7 May 2014
The mesoscale assembly process is sensitive to additives that can modify the interactions of the crystal nucleus and the developing crystals with solid surfaces and soluble molecules. However, the presence of additives is not a prerequisite for the mesoscale transformation process. In this study, ZnO sheet networks were synthesized on Al foils by a hydrothermal process. Scanning electron microscopy and transmission electron microscopy images confirmed that under ultrasonic vibration, monolithic polycrystalline ZnO sheets transformed into hexagon-like mesocrystalline tubes or rods. The formation mechanism was discussed.
- Polycrystalline sheet
- Hexagonal-like rod
- Hexagonal-like tube
Zinc oxide (ZnO), a wide-band gap II-VI semiconductor, has a wurtzite structure, belongs to the space group C 6mc, and has lattice parameters of a = 0.3249 nm and c = 0.5207 nm . The wurtzite structure of ZnO can be described as a number of alternating planes composed of tetrahedrally coordinated O2− and Zn2+ ions stacked along the c-axis. The oppositely charged ions produce positively charged Zn (0001) and negatively charged O polar surfaces . Together with the polar surfaces, three fast growth directions along , , and facilitated anisotropic growth of the one-dimensional (1D) ZnO structures, including c-axis-oriented nanowires and a-axis-oriented nanobelts [2–5].
Recently, a new class of nanostructured solid materials, mesocrystals, consisting of self-assembled crystallographically oriented nanoparticles [6–8] has attracted much attention. A large variety of ZnO mesocrystals grown using different additives has been obtained [9–14]. During the crystal growth of mesocrystals, the primary particles involved are usually scattered in the solution and are formed through the spontaneous organization to produce crystallographically continuous particles and ordered structures. For example, hexagonal, nanoplatelet-based, mesocrystalline ZnO microspheres were grown using a facile solution-based route . Several mechanisms of mesocrystal formation have been proposed: biomineralization, roles of organic additives, alignment by capillary forces, hydrophobic forces, a mechanical stress field, magnetic fields, dipole and polarization forces, external electric fields, minimization of the interfacial energy, and so on [16–23]. However, the mechanisms are, however, still under debate.
In this work, ZnO polycrystalline sheets were synthesized on Al foils by a hydrothermal process. It is very interesting to find that the monolithic polycrystalline sheets could be transformed into hexagon-like mesocrystalline tubes or rods under ultrasonic vibration. To the best of our knowledge, this is the first report of such a transformation.
ZnO sheet networks were synthesized on Al foils by a hydrothermal process. Previous to growing, the Al foil surface was processed with ultrasonic cleaning in acetone, alcohol, and deionized water for 20 min, respectively. The hydrothermal growth was carried out by immersing the Al foils in an aqueous solution containing zinc nitrate hexahydrate (Zn(NO3)2 · 6H2O, 10 mM) and methenamine ((CH2)6 N4, also called hexamethylenetetramine or HMT, 10 mM) at 90°C in a stainless steel autoclave for 2 h. After cooling to room temperature naturally, the ZnO-coated Al foils were first washed with water and then ethanol to remove the organic residues. The foils were then baked at 70°C for 1 h to obtain dried ZnO-coated Al foils. An X-ray diffractometer with Cu K α radiation (D/max 2500 PC, Rigaku Corporation, Shibuya-ku, Japan, 2θ/θ, = 0.1542 nm) at 40 kV was used to analyze the crystalline structures of the as-grown ZnO on Al foils.
The dried ZnO-coated Al foils were placed in ethanol for exposure to ultrasonic vibration at 0°C for 20 to 50 min to observe the morphological transformation of the ZnO on the Al foils. Besides, the ZnO nanosheets on Al substrate were scraped off from the substrate and were added into ethanol to be dispersed by ultrasonication for 0.5 h. The dispersed ZnO samples are also investigated. Field-emission scanning electron microscope (FESEM, SUPRA55, German) images were obtained and recorded on a LEO 1530 VP, with the voltage of 5 kV and spot size of 20 mm. Transmission electron microscope (TEM, JEOL JEM-2100,200 kV, Akishima-shi, Japan) images were observed on a JEM 200CX to further investigate the morphological and structural transformation of ZnO.
It is well known that the fastest growth rate of ZnO is along the  direction owing to the lowest surface energy of the (0002) facet under thermodynamic equilibrium conditions, resulting in the growth of ZnO nanorods on most occasions. However, when Al was used as a substrate in our study, it absorbed OH− ions to form Al(OH)4− on the surface, which adhered to the Zn2+-terminated (0001) surface and suppressed growth along the  direction, resulting in lateral growth of ZnO [25, 26]. Meanwhile, the precipitation of aluminum hydroxide (Al(OH)3) also reduced OH− concentration, supersaturating the growth solution. Owing to the influence of Al foils, 1D nanorods with the c-axis along the  direction were not formed. In contrast, two-dimensional (2D) ZnO sheets were formed, which exhibited crooked nanoplate morphology instead of a freely stretched shape, suggesting that there was stress in the ZnO sheets.
It was suggested that the nanosheet rolled up along the  direction primarily as a result of the minimization of the surface energy. As shown in Figure 1b,c, the interlinked ZnO nanosheets were in crooked rather than freely stretched shapes, which indicated that there existed stress in ZnO nanosheets. When the ZnO nanosheets were separated from the substrates under ultrasound vibration, the stress would be released. And the nanosheets would begin to wind around each other layer by layer, and the short-range chemical bonds among these layers resulted in nanorods or nanotubes. The reduced surface area and the formation of chemical bonds (short-range forces) between the layers should be responsible for stabilizing the coiled structure. As for the formation of mesocrystalline ZnO rods (tubes) rather than polycrystalline ones, the dipole-dipole interaction was considered the driving force [27–30]. For the polycrystalline ZnO sheets, the measured interplanar distances of most single-crystalline nanosize grains are 0.265 nm, corresponding (0001) axis of ZnO. Along (0001) axis, the oppositely charged ions produce positively charged Zn (0001) and negatively charged O , which forms a dipole. Under ultrasonic vibration, these dipoles were aligned by the dipole-dipole interaction, and the mesocrystalline ZnO rods were formed. The dipole-dipole interaction has been suggested as the mechanism of mesocrystal formation [31–33]. Differently, in our work, the nanocrystals were not dispersed in the organic solvent. The hexagon-like external morphology of mesocrystal ZnO rods or tubes were thought to be determined by hexagonal wurtzite structure of ZnO.
ZnO nanosheets with a large area and a small thickness were prepared on Al substrates. Under ultrasonic vibration, these monolithic polycrystal ZnO nanosheets rolled up and transformed into mesocrystalline nanorods or nanotubes. It was suggested that the transformation of nanorods or nanotubes from nanosheet primarily as a result of the minimization of the surface energy. The mesocrystal formation was thought ascribed to the dipole-dipole interaction.
This work was supported by the National High Technology Research and Development Program 863 (2011AA050511), National Natural Science Foundation of China (NSFC) (51272033), Jiangsu ‘333’ Project, the Priority Academic Program Development of Jiangsu Higher Education Institutions, and the Jiangsu Education Department Project (EEKJA48000).
- Lieber CM: The incredible shrinking circuit. Sci Am 2001, 285: 50. 10.1038/scientificamerican1101-50View ArticleGoogle Scholar
- Li WJ, Shi EW, Zhong WZ, Yin ZW: Growth mechanism and habit of oxide crystals. J Cryst Growth 1999, 203: 186. 10.1016/S0022-0248(99)00076-7View ArticleGoogle Scholar
- Wander A, Schedin F, Steadman P, Norris A, McGrath R, Turner TS, Thornton G, Harrison NM: Stability of polar oxide surfaces. Phys Rev Lett 2001, 86: 3811. 10.1103/PhysRevLett.86.3811View ArticleGoogle Scholar
- Ding Y, Gao PX, Wang ZL: Formation of piezoelectric single-crystal nanorings and nanobows. J Am Chem Soc 2004, 126: 6703. 10.1021/ja049266mView ArticleGoogle Scholar
- Fan HJ, Fuhrmann B, Scholz R, Himcinschi C, Berger A, Leipner H, Dadgar A, Krost A, Christiansen S, Gösele U, Zacjarias M: Vapour-transport-deposition growth of ZnO nanostructures: switch between c-axial wires and a-axial belts by indium doping. Nanotechnology 2006, 17: S231. 10.1088/0957-4484/17/11/S02View ArticleGoogle Scholar
- Cölfen H, Antonietti M: Mesocrystals: inorganic superstructures made by highly parallel crystallization and controlled alignment. Angew Chem Int Ed 2005, 44: 5576. 10.1002/anie.200500496View ArticleGoogle Scholar
- Niederberger M, Cölfen H: Oriented attachment and mesocrystals: non-classical crystallization mechanisms based on nanoparticle assembly. Phys Chem Chem Phys 2006, 8: 3271. 10.1039/b604589hView ArticleGoogle Scholar
- Song RQ, Cölfen H: Mesocrystals-ordered nanoparticle superstructures. Adv Mater 2010, 22: 1301. 10.1002/adma.200901365View ArticleGoogle Scholar
- Zhang T, Dong W, Keeter-Brewer M, Konor S, Njabon RN, Tian ZR: Site-specific nucleation and growth kinetics in hierarchical nanosyntheses of branched ZnO crystallites. J Am Chem Soc 2006, 128: 10960. 10.1021/ja0631596View ArticleGoogle Scholar
- Cong H-P, Yu S-H: Hybrid ZnO-dye hollow spheres with new optical properties by a self-assembly process based on evans blue dye and cetyltrimethylammonium bromide. Adv Funct Mater 2007, 17: 1814. 10.1002/adfm.200601082View ArticleGoogle Scholar
- Cho S, Jung S-H, Lee KH: Morphology-controlled growth of ZnO nanostructures using microwave irradiation: from basic to complex structures. J Phys Chem C 2008, 112: 12769.View ArticleGoogle Scholar
- Liu Z, Wen D, Wu XL, Gao YJ, Chen HT, Zhu J, Chu PK: Intrinsic dipole-field-driven mesoscale crystallization of core-shell ZnO mesocrystal microspheres. J Am Chem Soc 2009, 131: 9405. 10.1021/ja9039136View ArticleGoogle Scholar
- Liu X, Afzaal M, Ramasamy K, Ò Brien P, Akhtar J: Synthesis of ZnO hexagonal single-crystal slices with predominant (0001) and (0001) facets by poly (ethylene glycol)-assisted chemical bath deposition. J Am Chem Soc 2009, 131: 15106. 10.1021/ja906992sView ArticleGoogle Scholar
- Raula M, Rashid MH, Paira TK, Dinda E, Mandal TK: Ascorbate-assisted growth of hierarchical ZnO nanostructures: sphere, spindle, and flower and their catalytic properties. Langmuir 2010, 26: 8769. 10.1021/la904507qView ArticleGoogle Scholar
- Wang SS, Xu AW: Template-free facile solution synthesis and optical properties of ZnO mesocrystals. CrystEngComm 2013, 15: 376. 10.1039/c2ce26638eView ArticleGoogle Scholar
- Simon P, Zahn D, Lichte H, Kniep R: Intrinsic electric dipole fields and the induction of hierarchical form developments in fluorapatite-gelatine nanocomposites: A general principle for morphogenesis of biominerals. Angew Chem Int Ed 2006, 45: 1911. 10.1002/anie.200504465View ArticleGoogle Scholar
- Cölfen H, Antonietti M: Mesocrystals and Nonclassical Crystallization. Chichester, U.K.: John Wiley & Sons; 2008.View ArticleGoogle Scholar
- Li ZH, Gessner A, Richters JP, Kalden J, Voss T, Kübel C, Taubert A: Hollow zinc oxide mesocrystals from an ionic liquid precursor (ILP). Adv Mater 2008, 20: 1279. 10.1002/adma.200700935View ArticleGoogle Scholar
- Liu XH, Afzaal M, Badcock T, Dawson P, Ò Brien P: Conducting ZnO thin films with an unusual morphology: Large flat microcrystals with (0001) facets perpendicular to the plane by chemical bath deposition. Mater Chem Phys 2011, 127: 174. 10.1016/j.matchemphys.2011.01.054View ArticleGoogle Scholar
- Zhu YC, Liu YY, Ruan QC, Zeng Y, Xiao JW, Liu ZW, Cheng LF, Xu FF, Zhang LL: Superstructures and mineralization of laminated vaterite mesocrystals via mesoscale transformation and self-assembly. J Phys Chem C 2009, 113: 6584. 10.1021/jp900475rView ArticleGoogle Scholar
- Song RQ, Cölfen H, Xu AW, Hartmann J, Antonietti M: Polyelectrolyte-directed nanoparticle aggregation: systematic morphogenesis of calcium carbonate by nonclassical crystallization. ACS Nano 2009, 3: 1996.Google Scholar
- Peng Y, Xu AW, Deng B, Antonietti M, Cölfen H: Polymer-controlled crystallization of zinc oxide hexagonal nanorings and disks. J Phys Chem B 2006, 110: 2988. 10.1021/jp056246dView ArticleGoogle Scholar
- Song RQ, Cölfen H: Additive controlled crystallization. Cryst Eng Comm 2011, 13: 1249. 10.1039/c0ce00419gView ArticleGoogle Scholar
- Cheng JP, Liao ZM, Shi D, Liu F, Zhang XB: Oriented ZnO nanoplates on Al substrate by solution growth technique. J Alloys Compd 2009, 480: 741. 10.1016/j.jallcom.2009.02.041View ArticleGoogle Scholar
- Ye CH, Bando Y, Shen GZ, Golberg D: Thickness-dependent photocatalytic performance of ZnO nanoplatelets. J Phys Chem B 2006, 110: 15146. 10.1021/jp061874wView ArticleGoogle Scholar
- Cheng JP, Zhang XB, Luo ZQ: Oriented growth of ZnO nanostructures on Si and Al substrates. Surf Coat Tech 2008, 202: 4681. 10.1016/j.surfcoat.2008.03.032View ArticleGoogle Scholar
- Tang Z, Kotov NA, Giersig M: Spontaneous organization of single CdTe nanoparticles into luminescent nanowires. Science 2002, 297: 237. 10.1126/science.1072086View ArticleGoogle Scholar
- Tang Z, Zhang Z, Wang Y, Glotzer SC, Kotov NA: Self-assembly of CdTe nanocrystals into free-floating sheets. Science 2006, 314: 274. 10.1126/science.1128045View ArticleGoogle Scholar
- Talapin DV, Shevchenko EV, Murray CB, Titov A, Kral VP: Dipole-dipole interactions in nanoparticle superlattices. Nano Lett 2007, 7: 1213. 10.1021/nl070058cView ArticleGoogle Scholar
- Gunning RD, O’Sullivan C, Ryan KM: A multi-rate kinetic model for spontaneous oriented attachment of CdS nanorods. Phys Chem Chem Phys 2010, 12: 12430. 10.1039/c0cp00196aView ArticleGoogle Scholar
- Li JM, Dai LG, Wang XP, Zeng XL: An “edge to edge” jigsaw-puzzle two-dimensional vapor-phase transport growth of high-quality large-area wurtzite-type ZnO (0001) nanohexagons. Appl Phys Lett 2012, 101: 173105. 10.1063/1.4761942View ArticleGoogle Scholar
- Li JM, Wang XP, Dai LG, Xu ZA: Non-layered wurtzite-type extralarge-area flexible ZnO (0110) paper-like nanostructures grown by electrostatically induced vapor-phase transport. Cryst Eng Comm 2013, 15: 1179. 10.1039/c2ce26381eView ArticleGoogle Scholar
- Tian ZR, Voigt JA, Liu J, Mchenzie B, Mcdermott MJ, Rodriguez MA, Konishi H, Xu HF: Complex and oriented ZnO nanostructures. Nat Mater 2003, 2: 821. 10.1038/nmat1014View ArticleGoogle Scholar
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