Structural- and optical-property characterization of three-dimensional branched ZnO nanospikes
Graphical abstract
Introduction
ZnO is an ideal semiconductor material that shows high potential to be used in various optical devices such as LEDs [1], sensors [2], [3], and solar cells [4]. Its wide bandgap (3.36 eV at RT) as well as large exciton binding energy (60 meV) has continued motivate researchers worldwide to unravel its attractive optical characteristic [5] particularly the origin of the visible-light photo-emission (from green up to red region). Various mechanisms have been hypothesized to explain the reasons behind this anomalous optical feature, with particularly on the aspect of defect induced deep level emission. It has been identified that numerous reasons can be accounted for this optical phenomenon, which consisted of geometrical effects [6], crystal imperfection [7], impurities [8], deviation stoichiometry [9] or dopant effects [10]. Therefore, all these possible reasons had been taken into account during the formulation of ZnO as an efficient route in engineering ZnO with sophisticated optical characteristics.
With respect to the aforementioned reasons, geometrical factor (such as size and shape) is one of the crucial factors that can be used in tailoring the physical and chemical properties, which will strictly-govern the optical characteristics. For few decades, researchers have been working actively in fine-tuning the optical characteristic of ZnO nanostructures by exploring various types of efficient synthetic schemes in producing ZnO nanostructures with a controlled size and shape. Among these include physical method synthesis in the controlled growth of advanced nanostructures ranged from 0-D [11], 1-D [12], [13], 2-D [5], [14] and even up to 3-D [15], [16]. In conjunction with this, wet chemistry synthesis through nonhydrolytic colloidal approach also provides a facile yet reproducible route in synthesizing nanostructures that are highly monodispersed in size as well as possesses well-defined shape. This offers a merit for the as-produced nanostructures to be easily dispersed in either organic or aqueous media for variety applications such as organic-based LED applications [17] or biological labelling [18]. Generally, the synthesis via nonhydrolytic colloidal approach is particularly suitable for oxide-based nanomaterial preparation, where it is realized by pyrolysis of molecular precursor (in the form of organometallic compounds) and it offers an effective route for the precisely-controlled nanostructures with advanced nano-architecture. Under finite induction period with sufficient supplying of the heat, the as-obtained nanostructures could own excellent crystallinity and uniformity in terms of both size and shapes. Moreover, both isotropic and anisotropic products can easily be obtained by manipulating the growth parameters such as choice of the molecular precursor and capping ligands, the reaction duration and growth temperature [19].
Hereby, for the first time, we successfully use zinc-stearate (Zn(C18H35O2)2), an environmentally benign metal-soap precursor for the controlled-synthesis of 3-D ZnO nanospikes. Current study demonstrates a single step nonhydrolytic reaction instead of multiple step synthesis in the producing of truly 3-D ZnO nanostructures as compared to earlier reports [20], [21]. Systematic characterization on the as-synthesized ZnO nanospikes has been carried out to probe its structural- and optical-property. The nanospikes exhibit branched topology with good crystallinity, in which bundle of nanorods that sprout radially outwards in three-dimensional space to form spiky nanostructure. The possible growth mechanism is deduced based on the microscopy results. Moreover, the optical measurement indicates the absorption onset of nanospikes is red shifted; while its PL spectra reveal a weak UV-light emission but strong visible-light emission (ranged from green up to red region). The possible reasons accounted for these emissions are also discussed.
Section snippets
Experimental details
All chemicals were used as received without further purification. The reaction was performed in an inert atmosphere under continuous flow of argon (Ar) gas. In a typical synthesis, 3.3 mmol of zinc stearate (Zn(C18H35O2)2, Meryer Chemicals), 1.5 mmol of oleylamine (C18H37N, Sigma-Aldrich, technical grade 70%) and 50 mL of n-octadecene (C18H36, Sigma-Aldrich, technical grade 90%) were loaded into a 250 mL four neck flask equipped with an Allihn condenser. The mixture was heated to 180 °C for 1 h by
Result and discussion
Fig. 1(a) shows the low-magnification TEM micrograph for the as-synthesized ZnO nanospikes with average dimension of 200 nm. It is perceived that the nanospikes are constructed by bundle of nanorods which are bound together at a neck that located at one end of the rectangular basement (or the so-called stump afterwards). Occasionally, it is observed that some of the nanospikes appear to have smaller dimension due to incomplete grow, where the densities of the nanorods are found to be lower as
Conclusion
In summary, 3-D ZnO nanospikes have been successfully synthesized by pyrolysis of zinc stearate through organometallic approach for the first time. The as-synthesized ZnO nanospikes are constructed by the bundle of nanorods that are stacked randomly in three-dimensional space and exhibit good crystallinity. The X-ray diffraction and Raman spectra confirm the formation of ZnO with hexagonal wurtzite phase. The optical bandgap energy of the as-synthesized nanospikes is calculated to be 3.1 eV and
Acknowledgements
The financial support by Chancellory of University Malaya under High Impact Research Grant (HIR-UM) (J-21002-73810) and HIR-MOHE (UM.C/625/1/HIR/MOHE/SC/06) are acknowledged. W. S. Chiu is also grateful for the research grants that funded by the Ministry of Science, Technology and Innovation (MOSTI) (ScienceFund: 03-01-03-SF0658), Ministry of Higher Education (MOHE) (FRGS: FP-038-2014B; ERGS: ER002-2013A) and University Malaya Research Grant (UMRG: RP007B-13AFR). S. N. H. Daud would like to
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