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.
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.
Results and discussion
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).
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