Structural- and optical-property characterization of three-dimensional branched ZnO nanospikes

Highlights

• Pyrolysis of zinc stearate in synthesizing 3-D ZnO nanospikes

• ZnO nanospikes possess bundle of nanorods that sprout out from the hexagonal stump

• Growth mechanism is deduced to elucidate the morphological evolution from nanobullet to nanospike with branching topology

• PL spectrum indicate that the nanospike exhibit prominent visible-light emission that ranged from green- to red-region

Abstract

Current study reports the synthesis of three-dimensional (3-D) ZnO nanospikes with anomalous optical property, where zinc stearate is adopted as a safe, common and low-cost precursor that undergoes thermal pyrolysis under non-hydrolytic approach. High resolution transmission electron microscope (HRTEM) and scanning electron microscope (SEM) result show that the as-synthesized 3-D ZnO nanospikes are constructed by bundle of nanorods that sprout radially outwards in random orientation. The possible growth mechanism is discussed by referring to the microscopy results. X-ray diffraction (XRD) pattern confirms that the nanospikes are highly crystalline, which existed in hexagonal wurtzite crystal structure. Optical absorption characterization shows that the onset absorption for the nanospikes is slightly red-shifted if compared to commercial ZnO and the corresponding bandgap energy is estimated to be 3.1 eV. The photoluminescene (PL) result of ZnO nanospikes indicate that its optical emission exhibits weak UV emission but very intense visible-light emission that ranged from green- up to red-region. The factors that contributed to the intriguing PL characteristic are discussed. Current finding would offer a versatile synthesis scheme in engineering advanced nanostructures with new design that exhibit congruent optical property.

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.