Controllable preparation of porous ZnO microspheres with a niosome soft template and their photocatalytic properties
Introduction
ZnO is a kind of direct wide band-gap semiconductor material with unique photoelectric characteristic and gas sensing performance. A variety of morphologies and structures of ZnO nanomaterials lead to their outstanding physical properties and potential applications. In recent years, there has been a great deal of interest in the arrangement of “manipulated” nano-scale building units into nano-or microspheres with well-defined structures due to their potential wide-ranging applications, such as catalysts, adsorbents, chromatography fillers, delivery carriers, sensing devices and biomedical uses [1], [2], [3]. The self-assembly of nano-scale building units into the microscale hierarchical structures, inspiring and simulating the natural phenomena, has been a hotspot in the field of materials. Specially, hierarchically intricate self-assembled ZnO super-structures with the controlled size, morphology based on nanopieces and nanospheres units have been extensively investigated due to their outstanding electronic and optical properties, and the potential wide-ranging applications for nanolasers, solar cells, and other functional devices. Recently, self-assembly of ZnO micro- and nano-spheres has been documented by different methods. Among these approaches, the hydrothermal method has been widely employed, because of its advantages of simple operation and mild reaction condition. Now the solution method combined with the template technique is proved to be an extremely useful approach and easily implemented for the synthesis of spherical structures, in which the template is either removed by post-treatment or “sacrificed” during the reaction process. To date, a variety of porous ZnO nanostructures have been prepared successfully by means of many methods, including porous pyramids [4], porous nanotubes [5], mesoporous polyhedral cages [6], porous nanowires [7], porous nanobelts [8] and multilayered nanosheets [9]. Many techniques have been developed to fabricate the micro/nanomaterial, in which template-directed synthesis is the most versatile method with either soft or hard templates [10]. The hard template need to be selectively removed by solvent extractions or high temperature calcination. The separation of the nanostructures from the hard template is time-consuming and expensive processes. Although the hard-template method has advantages in providing a well controllable size distribution and hollow structures dimensions, the removal of template cores usually requires some toxic solvents. Moreover, the preparation and removal procedures of the sacrificial templates are generally complicated, uneconomic, and time consuming [11]. Compared with the hard-template method, consequently, the soft template method is more popular owing to its high efficiency, facile, inexpensive and environment friendly. Up to now, various self-aggregation structure of micelles [12], vesicles [13], [14], and oil-in-water emulsion [15] have been adopted for the preparation of hollow cavity in combination with surfactants as the soft-template. However, to best of our knowledge, the report on synthesis of pure porous ZnO microspheres has rarely been described by using Span80/polyethyleneglycol400 (PEG400)/H2O niosome as a soft template. Niosome template is usually used as a soft template directing the microsphere assembly of the ZnO nanostructures. Our groups have reported that Span80/polyethyleneglycol400 (PEG400)/H2O niosome was prepared [16].
In this paper, we report a successful synthesis of hierarchical ZnO porous microspheres via a facile hydrothermal process and their enhanced photocatalytic degradation activity for methylene blue under the UV light. The crystallinity, morphology and microstructure of the as-synthesized samples were characterized by X-ray diffraction(XRD)analysis and transmission electron microscopy (TEM). UV–vis spectroscopy was taken to explore their optical properties. Moreover, a new multistep mechanism was proposed to illustrate how to form such a structure by this simple soft template method.
Section snippets
Materials
All chemical reagents were of analytical grade and used with no further purification. Span80/polyethyleneglycol 400 (PEG400)/H2O niosome was prepared according to the protocol as our previous report [16].
Synthesis of sphere-like ZCHH
In a typical experiment, The zinc nitrate [Zn(NO3)2·6H2O] (0.2 mM), urea (1 mM) and cinnamate (0.2 mM) were first dissolved into distilled water (40 mL), followed by strong stirring for 1 h using magnetic stirrer. The solution was then transferred into autoclaves which were heated to 120 °C and
XRD curves of as-synthesized samples
XRD spectra of the samples before and after thermal treatment are presented in Fig. 1. Curve (a) shows the typical XRD pattern of the hydrothermally as-synthesized precursor, All the peaks in Fig. 1(a) can be indexed as monoclinic zinc hydroxide carbonate (JCPDS Card No. 14-0256). After heat treatment, the phase has changed dramatically, as shown in curve (b). Peaks at 31.75°, 34.44°, 36.25°, 47.54°, 56.55°, 62.87°, 66.39°, 67.92° and 69.06° were observed in all patterns, which are representing
Conclusions
In summary, we have successfully prepared hierarchical ZnO porous microspheres via the simple efficient hydrothermal approach followed by calcination of the ZCHH precursor. The role of the additives on the morphology of the samples is investigated. Photocatalytic tests indicated that the as-synthesized ZnO porous microspheres exhibited excellent photocatalytic activities after UV illumination for a short time (60 min), which show that the as synthesized ZnO porous microspheres may be a promising
Acknowledgments
This research was financially supported by Jiangsu Key Laboratory of Environmental Material and Environmental Engineering China (No: K13065), Priority Academic Program Development of Jiangsu Higher Education Institutions, Senior visiting scholar program of Jiangsu Higher Vocational College (No: 2015FX089) and Qing Lan project of Jiangsu Province.
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