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Green synthesis of flower-like ZnO decorated reduced graphene oxide composites

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Abstract

Flower-like ZnO decorated reduced graphene oxide composites have been controllably synthesized, using sodium citrate as the green reducing agent by a hydrothermal method, which combines the formation of the flower-like ZnO particles and the reduction of graphite oxide in one single step. The as-synthesized composites were characterized by scanning electron microscope and X-ray powder diffraction, which indicate that the ZnO particles are decorated on the surfaces and on the interlayers of the reduced graphene oxide sheets, and act as spacers to prevent the aggregation of the sheets. The electrochemical tests show the composites to have good capacitive behavior, which predicts their potential application in energy storage.

Introduction

Graphene (GN), a one-atom-thick and two-dimensional (2D) honeycomb lattice structure, has initiated enormous scientific activities due to its extraordinary electrical, thermal, and mechanical properties [1], [2], [3]. Currently, about the synthesis of GN, the solution-based chemical reduction method is the primary route because of the low cost and large-scale production [4], however, the GN sheets are not stable in solution and tend to aggregate due to the van der Waals interactions between them. Recently, graphene-based metal oxide composites consisting of highly conductive carbon film, serving as an anchor for the metal oxide particles, with potential application in optoelectronics and energy conversion devices have attracted increasing attention [5], [6]. ZnO, an important semiconductor material has been viewed as one of the most promising nanomaterials in the applications of gas sensor, photocatalysis and solar cell [7]. Consequently, it prompts researchers to synthesize the GN–ZnO composites and explore their potential applications. Some GN–ZnO composites have been synthesized and exhibited potential applications in the fields such as supercapacitor and photocatalysis [8], [9], [10]. Wang et al. have reported that the synthesis of GN nanosheets/ZnO composites via a two-step process and explored the electrochemical properties [8]. Pan et al. have reported the synthesis of ZnO-reduced graphene oxide (ZnO-RGO) composites via microwave-assisted reaction and investigated the photocatalytic properties [9].

In this work, we report a facile one-step hydrothermal method for the synthesis of ZnO-RGO composites, using sodium citrate as the green reducing agent, in which flower-like ZnO particles were decorated on the surfaces and on the interlayers of GN sheets and inhibited the restack of the GN sheets. Electrochemical tests indicated that the composites had better capacitive behavior than the GN sheets.

Section snippets

Synthesis of ZnO-RGO composites

Graphite oxide (GO) was prepared by a modified Hummers method [11]. Zn(NO3)2·6H2O aqueous solution (0.005 M) was first added into the aqueous solution of GO (10 mg), and the mixture was sonicated for 1 h, then NaOH (0.025 M), ethylene glycol (0.05 M) and sodium citrate (0.2 g) were added into the mixture. After stirring for 1 h, the mixture was transferred into a Teflon-lined stainless steel autoclave, and reacted at 100 °C for 12 h. The obtained samples were washed and dried in a vacuum oven at 60 °C.

Results and discussion

The XRD patterns of the GO, GN and GN–ZnO composites are shown in Fig. 1. As shown in Fig. 1a, the feature diffraction sharp peak at 12.0°, corresponds to the (001) crystalline plane of GO, and the interlayer spacing (d-spacing) of GO is 0.73 nm, which is larger than the d-spacing (0.34 nm) of natural graphite as a result of the introduction of oxygen-containing groups on carbon nanosheets [12]. For the GN (Fig. 1b), a weak and broad diffraction peak appears around 24.3°, corresponding to the

Conclusions

In summary, ZnO-reduced graphene oxide composites were controllably synthesized in a one-step green hydrothermal process, using sodium citrate as the green reducing agent. It suggests that flower-like ZnO particles are decorated evenly on the surfaces and on the interlayers of the GN sheets and inhibit the restack of the GN sheets. Electrochemical tests indicate that the composites enhance the capacitive behavior by comparison to graphene. The simple method is universal and can be easily

Acknowledgment

This work was supported by National Natural Scientific Foundation of China (No. 51102180).

References (16)

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