Shape-regulated synthesis of cobalt oxide and its gas-sensing property
Introduction
As an important functional material, cobalt oxide (Co3O4) has been applied in many fields with promising properties, such as catalysis [1], [2], energy storage [3], [4], [5], [6], [7], [8], [9], [10], gas sensors [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], and magnetism [24], [25], [26]. Recent development of nanotechnology makes nano-scale Co3O4 structure to be a very outstanding candidate for technological application. The performances of Co3O4 nanostructure are greatly dependent on their assembly and morphologies. Investigations have demonstrated that the properties of nano-scale Co3O4 are greatly size- or shape-dependent, giving the nanomaterials excellent performances. A lot of literatures have been published on the controllable fabrication of Co3O4 nanostructures with various morphologies and excellent properties [27], [28], [29], [30], [31], [32], [33], [34].
Co3O4 is the most stable phase in the Co–O system and it is a mixed valence compound with a normal spinel structure. It is generally known that the nanostructured Co3O4 can be easily obtained via a two-step method, where cobalt-based intermediate compounds, like cobalt carbonate, cobalt hydroxide, and cobalt carbonate-hydroxide, are first synthesized followed by a thermal annealing in air. As the control over the morphology of these intermediates is relatively less challenging, the synthesis of the intermediate compounds with abundant morphologies is of great importance for the fabrication of Co3O4 with a variety of morphologies and much effort has been undertaken for this topic. Thus, the synthesis of Co(OH)2 precursor with variously controllable shapes via a facile route still remains interesting.
These metal oxides having the spinel-type structure attracted a great deal of attention from scientists because of their semi-conducting properties. As an important p-type semiconductor, Co3O4 has been reported as a potential gas sensing material due to its less-expensive sources and abundant morphologies. The importance of Co3O4 is its tendency to form hierarchical structures through simple processing routes. Because of its advanced geometric structure and atom arrangement on specific facets, it usually exhibits novel properties. Thus, Co3O4 with different morphologies such as nanospheres, nanocubes, nanowires, and mesoporous structure has been prepared and characterized as sensor materials [15], [35], [36], [37], [38], [39], [40], [41]. Meanwhile, it is found that the gas-sensing characteristics are also greatly influenced by the morphology, dimension and porosity of Co3O4 nanostructures [38]. Therefore, it is worthwhile to investigate the gas-sensing properties of p-type Co3O4 with various morphologies and nanostructures.
Hence in this work, we adopt a simple precipitation route followed by calcination to prepare porous nanostructured Co3O4 with various shapes. This issue offers an opportunity to control the structures and morphologies of the precipitated precursor particles, i.e. cobalt hydroxide, by simply changing the used chemical agents. Then the shape of the cobalt hydroxide has a decisive effect on the shape of the final oxide particles after calcination, well retaining the shape of the hydroxide precursors. The second aim of this work is to develop an efficient gas sensitive material of Co3O4, investigate its gas sensing properties and compare them with each other.
Section snippets
Experimental
All chemicals used in this work were of analytical grade reagents and used as-received without further purification. The approach to synthesize Co(OH)2 was described as follows. Cobalt salt (nitrate or acetate) was dissolved into a solution of 40 mL water with a concentration of 0.25 M and it was heat to 70 °C. Then 10 mL ammonia solution (or 0.02 Mole NaOH) was added into above solution under stirring and the mixture was kept at 70 °C for 1 more hour to yield a suspension containing pink solid. The
Results and discussion
Fig. 1 shows the XRD patterns of Co(OH)2 prepared using different chemicals. All the diffraction peaks in the patterns could be well indexed as the hexagonal cell of brucite-like β-Co(OH)2 (PDF No. 74-1057). These sharp peaks are a sign of good crystallinity of the intermediate compounds. Except for β-Co(OH)2, no diffraction peaks from other phases could be detected, indicating a high purity of the cobalt hydroxide product. The broadening (0 0 1) peak in pattern c for the sample prepared by
Conclusions
In this work, we have reported a precipitation method to synthesize Co(OH)2 with various shapes, whose morphology was readily tuned by changing the chemical agents. After annealing Co(OH)2 under moderate temperature in air, Co3O4 products were obtained and they reserved the shape of Co(OH)2 with a porous feature. High annealing temperature leaded to a large crystal size and macropore volume. These Co3O4 products exhibited a high response and stability as ethanol and acetone sensor under gas
Acknowledgements
This work was financially supported by Experimental technology research project of Zhejiang University.
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