Microscale sphere assembly of ZnO nanotubes
Introduction
Over the past several years, large-scale self-organization of micro-, meso-, and nano-structured building units into desired superstructures has attracted significant interest in materials synthesis and device fabrication [1]. Self-organization driven by various interactions is an effective strategy for forming versatile soft nanocrystal-organization motifs [2]. Understanding factors and mechanisms governing the formation of nanocrystal assemblies would allow the design of desired nanostructures for optical, microelectronic, chemical, and biological applications [3]. As a result of rapid advancements in synthetic strategies, highly organized building blocks of metals [4], semiconductors [5], copolymers [6], organic–inorganic hybrid materials [7], and biominerals [8] have been synthesized by using various methods. However, controlled organization into complex 2D and 3D superstructures from rod-like building blocks remains a challenge, such a capability is attractive to scientists not only because of its importance in understanding the concept of self-organization with artificial building blocks but also for its great application potential [9]. Regarding generation of curved architectures from prefabricated components, Mirkin and his coworkers had successfully examined control factors to organize artificial building blocks, i.e., one-dimensional organic–inorganic hybrid rods, into various curved single-layer superstructures consisting of bundles, tubes, or sheets [10].
Zinc oxide, a wide band gap (3.37 eV) semiconductor with large excitation binding energy (60 m eV) at room temperature, has been widely investigated due to their potential wide-ranging applications [11]. It is a versatile smart material that has unique applications in catalysts, sensors, piezoelectric transducers and actuators, photovoltaic and surface acoustic wave devices. ZnO is also a biosafe and biocompatible material, which can be directly used for biomedical applications without coating [12]. These properties have stimulated the search for new synthetic methodologies for well-controlled ZnO nanostructures. Synthesis of ZnO with different forms of controlled nanostructures have been widely explored in recent years and many kinds of interesting and delicate ZnO nanostructures have been obtained by using different methods based on vapor phase processes and solution processes [11], [12], [13], [14], [15], [16], [17], [18], [19], [20]. Well-defined ZnO nanostructures with various morphologies such as nanoparticles [13], nanowires and nanorods [11], [14], nanobelts [12], [15], helical nanorods and columns [16], nano-tetrapods [17], nanocables and nanotubes [18], nanosheets [19], nanocoils [20], and complicated hierarchical ZnO nanostructures [21] have been fabricated. To our knowledge, only a few have obtained ZnO nanotubes, Vayssieres et al. [18] had reported the synthesis of only ZnO microtubes at low temperatures in aqueous solution, Hu et al. and Sun et al. obtained ZnO nanotubes with diameters of 30 and 400 nm, respectively [22], [23]. Although these one-dimensional (1D) and 2D ZnO nanoscale building units can be routinely obtained in large quantities and in a massively parallel manner via various physical and chemical routes, their self-organization into complex 2D and 3D ordered superstructures is still considerably difficult, Liu Fabricated ZnO “Dandelions” via a Modified Kirkendall Process by using microsphere zinc powder as hard template [14], Korgel had assembled ZnO nanoribbons into nanocoils through electrostatic interaction [20], and Wang and coworkers had also converted ZnO nanobelts into superlattice-structured nanohelices by using electrostatic interaction [5]. To our best knowledge, hollow-structured ZnO nanorods, i.e., a special ZnO nanotubes with closed −c-ends and their microsphere assemblies with () crystallographic plane exposed outwards have not been reported until now, though such a structure has long been a focus of the surface chemistry since ZnO () crystallographic plane are largely motivated by using ZnO as a model catalyst support in methanol synthesis [24].
Herein, we report an ultrasonic-assisted approach for the large-scale growth of microsphere organization of ZnO nanotubes by using PEG as directing agents. All ZnO O-terminated () crystallographic planes were closed and concisely arranged outwards on the surface of the microsphere, these assemblies show remarkable PL response in yellow region. The formation mechanism of ZnO nanotubes and their microsphere organization were also proposed in details. It is more interesting that one of the structural features of this microsphere organization is that one- or two-dimensional building blocks (anisotropic) can be selectively made and aligned into highly symmetrical three-dimensional conformations (isotropic), which may promise us new types of ZnO applications. For example, this class of metal-oxides and metal-semiconductor microsphere-organization composites should be potentially useful for applications such as storage of light-generated electrons, three-dimensional laser [11], and new ways of photo catalyst [14], [21].
Section snippets
Experimental
All chemicals are analytical-grade reagents and were purchased from Shanghai Chemical Reagent Corp. Poly ethylene glycol (PEG, average molecular weight 2000), ethanol (EtOH), Zn(NO3)2·6H2O, NaOH were used as reactants without further purification, doubly deionized distilled water was used throughout. 0.003 mole of Zn(NO3)2·6H2O and 0.06 mole of NaOH and 2 g of poly ethylene glycol (PEG 2, 0 0 0) were dissolved in 5 mL of deionized distilled water. Then 50 mL anhydrous ethanol and 10 mL of deionized
Morphologies and structures of the microsphere organization of ZnO nanotubes
As shown in Fig. 1A and B, ZnO nanotubes (in 100% microsphere organization of hexagonal nanotube morphological yield) were arranged in the form of sphere organization with a diameter of ca. 3–4.5 μm. Furthermore, ZnO nanotubes projecting out with a flat O-termination crystallographic () plane, which is −c-end, i.e., the slowest growth direction [14], can also be observed in this type of sphere aggregation. We conclude that the inner end pointed to the interior of the microsphere is
Conclusions
In summary, we have reported here a facile method to perform microsphere organization of ZnO nanotubes, the constituent ZnO nanotube is hollow along c-axis, all the −c-ends closed with O-termination () crystallographic plane were arranged outwards on the surface of the microsphere, these microsphere assemblies of ZnO nanotubes have a visible yellow emission at 600 cm−1. Study shows that addition of metallic zinc species into linear PEG led to aggregation of the polymer coils to
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