Preparation and photoluminescence of ZnO complex structures with controlled morphology
Introduction
It is well known that nanostructured materials are expected to have improved optical properties compared with bulk materials, and the optical response depends on their particular size, shape and local dielectric environment [1]. In recent years, synthesis of inorganic materials with specific size and morphology has attracted a lot of interest because of their significant mechanical, electrical, optical and magnetic properties for the potential applications in various fields [2]. ZnO is an II–VI compound semiconductor with a wide and direct band gap of 3.3 eV and large exciton binding energy (60 meV), exhibiting important application in optoelectronics, sensors, transducers, and biomedical applications. Recently, ZnO has been investigated as a short-wavelength light-emitting and laser diodes, transparent conducting and piezoelectric material [3], [4], [5], [6], [7], [8].
Until now, one-dimensional ZnO nanostructures such as nanorods, nanowires and nanotubes; and two-dimensional structures including nanosheets, nanoribbons, have been successfully synthesized [9], [10], [11], [12], [13] by vapor transport, physical vapor deposition and wet chemical approaches. In addition, the three-dimensional ZnO complex structures with special morphologies, which are another important category and could be built up through the low-dimensional components, have received increasing attention recently. For example, ZnO nanorings, nanosprings, nanosaws, nanopropellers, nanojunction arrays, tetrapodlike and dendritic patterns were obtained through a high temperature process in Wang's research group [14], [15], [16]. ZnO cage-like patterns [17] and nanoaeroplanes [18] were fabricated by a thermal evaporation technique. ZnO flower-like structures were synthesized by chemical vapor deposition (CVD) with [19a,b] or without [20] catalyst, and also by the hydrothermal process in the presence or absence of surfactants [11], [21], [22], [23],etc.
ZnO exhibits numerous morphological configurations due to its unique crystal structure and polar surfaces [14]. From this point of view, different ZnO structures can be systematically obtained just by designing appropriate experimental parameters. Since various types of morphologies of ZnO have been separately synthesized by different research groups and different methods, it is our goal to develop synthetic routes for simply modulating the morphology of ZnO in one system. On the other hand, it is rationally expected that some specific morphologies of ZnO would be promising candidates for practical applications such as UV light source and lasers [11], [14]. Therefore, studying the relationship between ZnO morphologies and reaction conditions for exploring a simple route to control the morphology of ZnO is essentially necessary. Furthermore, in order to meet the industrial needs of low-cost, large-scale production and to systematically study the formation theory of ZnO materials, controlled synthesis under mild conditions is very crucial and still not well exploited. Herein, encouraged by the previous work, we demonstrate a simple wet chemical method to grow ZnO complex structures with different shapes controllably at 60 °C. The novel self-assembled ZnO spheres, ellipsoids, flowers and propellers are all with single-crystal nature and exhibit strong UV emission properties. To the best of our knowledge, the controlled synthesis of these four structures simply by varying the pH value or the solution concentration at low temperature has never been reported.
Section snippets
Experimental
The experimental process is similar to that reported in our previous work [23d,e]. For ZnO spheres, flowers and propellers, 2.0 g of Zn(NO3)2·6H2O was put into 270 mL of distilled water under stirring. After several minutes, 50 mL of 0.85, 1.15 and 1.45 M NH3·H2O aqueous solutions containing 0.15 g of polyethylene glycol (PEG, molecular weight: 10,000 g mol− 1) were introduced into above solutions (the final pH values are ∼7.5, 8.5 and 10), respectively. For ZnO ellipsoids, 2.0 g of Zn(NO3)2·6H2
Results and discussion
The XRD patterns of the as-grown spheres and flowers are shown in Fig. 1, respectively. All the peaks of both spheres [Fig. 1b] and flowers [Fig. 1a] can be indexed to the wurtzite ZnO (JCPDS card No. 36-1451, a = 0.3249 nm, c = 0.5206 nm). No characteristic peaks were observed for the other impurities such as Zn(OH)2. The XRD pattern of ZnO ellipsoids is similar to that of ZnO spheres and the XRD pattern of ZnO propellers is also similar to that of ZnO flowers. It is noticed the (002) reflection
Conclusions
In summary, a simple and facile wet chemical method has been successfully and controllably employed to grow ZnO complex structures including spheres, ellipsoids, flowers and propellers at 60 °C. All the self-assembled ZnO structures are single crystalline in nature and exhibit sharp UV emissions, indicating that they are with high crystal and optical quality. The pH value or the solution concentration could be easily changed for controlling the morphology of ZnO crystals. During the period of
Acknowledgment
The authors appreciate the financial support of National Science Foundation of China (No. 50202007).
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