A facile synthesis method of hierarchically porous NiO nanosheets
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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