ZnO nanotriangles: Synthesis, characterization and optical properties

https://doi.org/10.1016/j.jallcom.2008.09.196Get rights and content

Abstract

Zinc oxalate, [Zn(C2O4)·2H2O] was used as a precursor to prepare zinc oxide nanotriangles by thermal decomposition. The different combinations of triphenylphosphine, and oleylamine were added as surfactants to control the particle size. The products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), photoluminescence spectroscopy (PL), Fourier transform infrared (FT-IR) spectroscopy, Ultraviolet–visible (UV–vis) spectroscopy and X-ray photoelectron spectroscopy (XPS). The synthesized ZnO nanoparticles have a hexagonal zincite structure.

Introduction

In recent years, there has been an increasing interest in developing materials with low-dimensional nanostructure such as nanotubes and nanocrystals due to their potential technological applications in nanoscale devices [1], [2], [3], [4]. It has been obvious that their properties depend sensitively on their size and shape. Therefore, the challenges in nanocrystals synthesis are to control not only the crystal size but also the shape and morphology [5], [6].

Zinc oxide is a material of particular interest because of its unique optical and electronic properties. ZnO is a wide bandgap semiconductor (3.37 eV) and has a large excition binding energy of 60 meV, which is also luminescent, and has emerged as a good candidate for many applications. Nanosized ZnO has great potentiality for being used in preparing solar cell [7], [8], gas sensors [9], chemical absorbent [10] varistors [11], electrical and optical devices [12], electrostatic dissipative coating [13], catalysts for liquid phase hydrogenation [14], and catalysts for photo-catalytic degradation [15] instead of titania nanoparticles [16].

In the past decade, various different physical or chemical synthetic approaches have been developed to produce ZnO, including vapor phase oxidation [17], thermal vapor transport and condensation (TVTC) [18], [19], chemical vapor deposition (CVD) [20], precipitation [21], [22], [23], sol–gel [24], [25], microemulsion [26], hydrothermal [27], solvothermal [28] and sonochemical methods [29].

Among various techniques for synthesis of ZnO nanoparticles, thermal decomposition is one of the most common to produce stable monodisperse suspensions with the ability of self-assembly. Nucleation occurs when the metal precursor is added into a heated solution in the presence of surfactant, while the growth state take place at a higher reaction temperature [30]. ZnO nanostructures can also be synthesized from different precursors [31], [32]. Choi et al. [33] reported a method for the large-scale synthesis of uniform-sized hexagonal pyramid-shaped ZnO nanocrystals by the thermolysis of Zn–oleate complex. Zinc acetylaceton has been reported to be a good precursor for ZnO nanoparticles from thermal decomposition. Salavati-Niasari et al. [34] have synthesized uniform ZnO nanoparticles with size 12 nm by thermal decomposition of zinc acetylacetonate in oleylamine. And so, monodisperse ZnO nanoparticles were successfully prepared through the decomposition of zinc acetylacetonate precursor [35]. Recently Ahmad et al. [36] have synthesized zinc oxide nanoparticles with average size 55 nm by thermal decomposition of zinc oxalate, which were prepared with emulsion method involving cetyltrimethyl ammonium bromide (CTAB) as the surfactant. Kanade et al. [37] have investigated the effect of particle size of ZnO synthesized via thermal decomposition of zinc oxalate at 450 °C. Muruganandham and Wu [38] have reported synthesis of ZnO nanobundles from zinc oxalate by decomposition at 400 °C for 12 h. We decided to prepare ZnO nanocrystals from zinc oxalate at the less temperature and shorter time than the last works done by others. However, an improvement in the thermal decomposition process should be made in preparing ZnO nanotriangles with uniform size in order to extend the application areas and satisfy the needs of fundamental research. To the best of our knowledge the used of zinc oxalate as precursor in thermal decomposition at 240 °C is the first time. Here, we report a simple, green, low-cost, and reproducible process for the synthesis of ZnO nanotriangles from zinc oxalate, [Zn(C2O4)·2H2O], as precursor. In this process, oleylamine was used as both the medium and the stabilizing reagent.

Section snippets

Materials

All the chemicals reagents used in our experiments were of analytical grade and were used as received without further purification. Zinc oxalate, [Zn(C2O4)·2H2O], precursor was synthesized according to this procedure: the 10 mmol of zinc acetate dihydrate Zn(CH3COO)2·2H2O was dissolved into 10 mL of distilled water to form a homogeneous solution. A stoichiometric amount of potassium oxalate dissolved in an equal volume of distilled water was dropwise added into the above solution under magnetic

Results and discussion

X-ray powder diffraction pattern of ZnO nanotriangles at room temperature is shown in Fig. 1a. The XRD pattern is consistent with the spectrum of ZnO, and no peak attributable to possible impurities is observed. All of the reflection peaks of the XRD pattern can be indexed well to hexagonal phase ZnO (space group: P63mc; JCPDS No. 79–0205, with calculated cell parameters a = 3.24 Å and c = 5.18 Å). The sharp diffraction peaks manifested show that the obtained ZnO nanocrystals have high crystallinity.

Conclusion

In summary, ZnO nanotriangles were synthesized via thermal decomposition of [Zn(C2O4)]–oleylamine complex. This method employed an inexpensive, reproducible process for the large-scale synthesis of ZnO nanocrystals.

Acknowledgement

Authors are grateful to council of University of Kashan for providing financial support to undertake this work.

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