Letter
Fabrication of ZnO ring-like nanostructures at a moderate temperature via a thermal evaporation process

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Abstract

ZnO ring-like nanostructures have been successfully synthesized by thermal evaporation of a mixture of ZnS and active carbon powders at 900 °C. Studies found that as-grown ZnO nanorings grew along the [011¯0] direction with the top/bottom surfaces as the (21¯1¯0) planes, and the (0 0 0 1) polar surface pointing to the center of the rings. The ring-like structures were believed to be formed by bending the (0 0 0 1) polar surface dominated ZnO nanobelts, which was induced by the spontaneous polarization. The synthesis method may be extended to synthesize other semiconducting nanomaterials with similar structures.

Introduction

As an important semiconductor material with a direct wide band gap, one-dimensional (1D) ZnO nanostructures have potential applications in electronics and optoelectronics [1], [2]. Therefore, rational design and synthesis of ZnO nanostructures with unique morphologies and different sizes are of significant importance from the fundamental research to the fabrication of novel devices. Many kinds of 1D nanostructures with interesting morphologies have been discovered for ZnO, which make ZnO being a material with the richest morphologies so far [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13]. It is well known that ZnO has three outstanding properties. First, it is a semiconductor material with a band gap of 3.37 eV and an exciton binding energy of 60 meV, which make it showing unique optical, mechanical, electronic, and optoelectronic properties [14], [15]. Second, ZnO is a piezoelectric semiconducting material. Utilizing the coupling of piezoelectric and semiconducting properties, Wang et al. made nanogenerator with well-aligned ZnO nanowire arrays [16], [17]. Third, ZnO is a biosafe and biocompatible material and it can be directly applied to cosmetics and bioengineering field without coating.

Additionally, ZnO has a stable noncentral symmetric wurtzite crystal structure with two typical polar surfaces: ±(0 0 0 1) and (011¯1). Once a nanobelt is dominated by a polar surface, electrostatic energy will be generated due to the spontaneous polarization, induced by the dipole moment. The nanobelt then tends to be bended to decrease the generated electrostatic energy. However, bending a nanobelt will induce elastic energy. Thus, the competition between electrostatic energy and elastic energy finally results in the formation of nanosprings or nanorings. Recently, a variety of polar surface dominated nanosprings, nanobows, and nanohelices have been synthesized for several materials, such as ZnO, SnO2, Ag2V4O11, InP, etc. [18], [19], [20], [21], [22], [23]. Though ZnO nanorings have been reported by Wang et al., a growth temperature higher than 1300 °C is usually required and the yield of nanorings is still relatively low [24], [25], [26], [27]. Herein, we report the synthesis of ring-like ZnO nanostructure on a large scale by thermal evaporating the mixture of ZnS and active carbon powders. The growth temperature can be efficiently decreased to as low as 900 °C and the yield of nanorings can be increased to about 50%.

Section snippets

Experimental details

In a typical procedure, an alumina crucible with a mixture of equivalent amount of ZnS powder and active carbon was loaded into the center of a quartz tube furnace. Silicon substrate coated with 10 nm gold film was then put downstream near the crucible. Pumped with a mechanical pump for about 3 h, the furnace was heated to 900 °C at a heating rate of 30 °C per minute and held at this temperature for 2 h. The chamber pressure was maintained at 300 Torr throughout the whole experiment. After cooled to

Results and discussion

The morphology of as-synthesized products was first checked using SEM and the results are shown in Fig. 1. Fig. 1a is a general image of the as-synthesized product. Novel ring-like products were clearly seen from this image and the yield of nanorings is about 50%. Fig. 1b depicts a SEM image of several nanorings connected with nanowires. In some cases, a nanoring was found to connect with more than one nanowires, for instance, two nanowires, as shown in Fig. 1c. In other cases, a single perfect

Conclusions

In conclusion, ring-like ZnO nanostructures with a yield of ∼50% have been successfully synthesized by thermal evaporation of the mixture of ZnS and active carbon powders at 900 °C. As-grown ZnO rings have mean diameters of 300 nm, and thickness of several to tens of nanometers. The formation of ring-like structures was attributed to the spontaneously polar surface-induced polarization. The synthesized rings-like ZnO nanostructure presented here may find potential applications in sensor, actuator

Acknowledgements

This work was supported by “973” National Key Basic Research Project (2003CB716900), Doctor Start-up Fund of Harbin Normal University (KGB200802), the Natural Science Foundation of China (20871037), the Natural Science Foundation of Heilongjiang Province (B2007-2) and the Science Technology and Research Project of Education Bureau, Heilongjiang Province (11531229).

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