Elsevier

Materials Letters

Volume 69, 15 February 2012, Pages 69-71
Materials Letters

A facile synthesis method of hierarchically porous NiO nanosheets

https://doi.org/10.1016/j.matlet.2011.11.063Get rights and content

Abstract

In this study, highly hierarchically porous NiO nanosheet arrays were synthesized on glass by a hydrothermal growth method. The effects of temperature and the reactant concentrations on the morphology of the nanosheets were examined. SEM revealed uniform and hierarchical porous NiO nanosheets at 50 mM of a nickel nitrate hexahydrate solution at 90 °C in 2 h. After heat treatment in oxygen at 450 °C for 1 h, the resistivity of the annealed NiO nanosheet samples was five orders of magnitude lower than the as-prepared samples. In addition, the resistivity of the nanosheet arrays decreased significantly with increasing reactant concentration. The hierarchically porous nanosheets synthesized under optimal conditions were 200–400 nm in thickness and 6–8 μm in height. These nanosheets might have potential electrochromic applications.

Graphical abstract

Highlights

► A facile, simple and fast hydrothermal method to synthesize NiO nanosheets. ► Highly hierarchically porous NiO nanosheets were obtained. ► All NiO nanosheets showed a polycrystalline Ni2O3 phase. ► Optimal NiO nanosheets had high structural uniformity and porosity. ► Hierarchically porous nanosheets were 200–400 nm in thickness and 6–8 μm in height.

Introduction

Nickel oxide (NiO) is used extensively in many areas, such as catalysis, battery electrodes, electrochromic film, gas sensors and magnetic materials [1], [2], [3]. Several approaches have been used to prepare NiO nanostructures, including sputtering [4], spray pyrolysis [2], chemical vapor deposition [5] and sol–gel method [6]. Among these, the chemical solution method [6] is the most cost-effective method that provides excellent control of the composition and homogeneity. Materials with hierarchically multimodal pore-size have superior electrical properties compared to their counterpart thin films due to the higher surface area to volume ratio and quantum size effect. Therefore, the synthesis of a two dimensional (2-D) NiO nanostructure, such as nanofilms and nanosheets, is technically promising. On the other hand, the development of simple, fast and versatile methods for the synthesis of hierarchically structured metal oxides, which can greatly facilitate future applications, is still a challenge. Thus far, many hydrothermal growth methods of NiO nanomaterial were synthesized in an autoclave using many complicated precursor solutions and for a reaction time of up to 15 h [7]. Therefore, it is important to develop an alternative simple and fast method to synthesize NiO nanomaterials.

This study investigated the structural and electrical properties of the NiO nanosheets as well as their dependence on the growth temperature and reactant concentration. Compared to existing studies [7], [8], [9], [10], [11], this proposed method uses a smaller amount of precursor chemicals. NiO nanosheets could be synthesized at low temperatures (90 °C) over a 2 h period.

Section snippets

Hydrothermal growth of NiO nanosheets

Nickel acetate tetrahydrate, nickel nitrate hexahydrate and hexamethylenetetramine were purchased from Sigma Aldrich. All chemicals were of analytic grade and used as received. To grow the NiO nanosheets, the substrate (Corning glass E2000) was first spin-coated with an ethanolic solution of 8 mM nickel acetate tetrahydrate at 1000 rpm for 10 s, and then 2000 rpm for 20 s, followed by heating on a hotplate at 200 °C for 1 min to dry the films. After this step, the substrates were placed into a 350 °C

Results and discussion

Fig. 1 shows the impact of the reaction temperature on the morphology of the NiO nanostructures in a 100 mM nickel nitrate hexahydrate solution. At 70 °C (Fig. 1a), nanosheets were observed only on some regions of the substrate, indicating less uniform growth. Moreover, the nanosheets exhibited a rough surface area. Increasing the growth temperature to 90 °C (Fig. 1b) resulted in more uniform and smoother nanosheets than those grown at 70 °C. Further increases in growth temperature to 100 °C

Conclusions

Well-defined hierarchically porous NiO nanosheets were prepared using a facile, simple and fast hydrothermal method. The effect of the preparation conditions, i.e., reaction temperature and reactant concentration, was systematically studied. These conditions are important in the formed morphology and porosity of the synthesized NiO nanosheets. Uniform and well-defined hierarchically porous NiO nanosheets were obtained under the following conditions: reaction temperature of 90 °C, nickel nitrate

Acknowledgements

This work is partly supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (2010-0023418), and partly supported by the Institute for Research & Industry Cooperation, Pusan National University (PNUIRIC, Research Development Promotion Fund) (PNUIRIC-2010-606 and PNUIRIC-2011-711). We acknowledge financial support from the Centre for Functional Photonics (CFP), City University of Hong Kong.

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