Synthesize of hierarchical sisal-like cobalt hydroxide and its electrochemical applications
Graphical abstract
Sisal-shaped hierarchical cobalt hydroxide was obtained by a simple hydrothermal method, and cobalt oxide was also easily obtained by heating the cobalt hydroxide in air. The obtained cobalt oxide possessed a similar hierarchical structure to the cobalt hydroxide. The prepared cobalt hydroxide samples were made of building blocks which took the shape of tower of Hanoi. The prepared samples exhibited promising electrochemical applications.
Introduction
Due to the ability of complex stereoscopic morphologies to provide novel properties which are quite different from those of bulk materials [1], the synthesis of inorganic materials with three dimensional micro/nanostructures is of great interest, and is currently being widely applied in materials science and industry [2], [3]. One-dimensional nanostructures possess attractive electronic, mechanical and optical properties [4], and many of them exhibit potential applications in many fields, such as alkaline rechargeable batteries, supercapacitors and catalysts [5], [6], [7]. Using one-dimensional nanostructures as building blocks to construct hierarchical inorganic micro/nanostructures with simple configurations may improve the physical and chemical properties of the nanometer materials [8], [9], [10]. When one-dimensional nanostructures have been prepared, the next step is to organize them into complex three-dimensional structures [11]. So many efforts have been made to synthesize nanostructures with well-controlled shapes with complex three-dimensional structures, such as sisal-like, dandelion-like, rose-like [12], butterfly-like [13] and flower-like [14] structures.
Cobalt hydroxide and cobalt oxide exhibit exceptional physical and chemical properties. They have been chosen as promising materials widely applied in many fields, such as rechargeable Li-ion batteries [15], [16], catalysis [17], [18], ionic exchanges [19], magnetic materials [20] and super capacitor [21]. And nanoscale materials with well-controlled morphologies possess novel properties, which make them capable of potential application in the fields of alkaline rechargeable batteries and electrode materials [22].
Generally, there are two main methods of achieving inorganic materials with specific morphologies: vapor phase growth and solution phase techniques [23]. Vapor phase growth needs great power to prepare complex structure; high reaction temperature and expensive equipment are necessary for this method. Therefore, as one of the solution phase techniques, hydrothermal process has often been introduced to synthesize samples. This is a relatively easy way to synthesize cobalt hydroxide with micro/nanostructures: low reaction temperatures, inexpensive reaction equipment and ease in procedural control. In this paper, a new method is introduced to synthesize cobalt hydroxide. Cobalt hydroxide was directly precipitated from the Co(II) solution when the hydroxide ions were added. At the same time, the samples were fabricated in a short period of time. In order to gain interesting morphologies and structures, triethylenetetramine (TETA) was employed as the morphology directing agent and chelating agent. This helped to simplify the preparation procedure and save time and cost. Cobalt oxide possessing similar sisal-like hierarchical structure was easily obtained by heat treatment of the synthesized cobalt hydroxide in air.
Section snippets
Materials
Cobalt nitrate hexahydrate (Co(NO3)2 ⋅ 6H2O), triethylenetetramine (TETA, trien), sodium hydroxide (NaOH), anhydrous ethanol (EtOH) were purchased from Beijing Chemical Factory. All solutions were prepared in deionized water, and all reagents were used without further purification.
Synthesis
First, 5 mL cobalt nitrate (0.4 M) and 5 mL TETA (0.4 M) was mixed, chelating for about 30 min at room temperature; then 8 mL sodium hydroxide (7 M) was added. The solution was heated in a beaker to 80 °C, then kept at a
Characterization of the morphologies and structures
Fig. 1a shows the FESEM image of the synthesized sample, which indicates a uniform three-dimensional microsphere array with a diameter of 6–8 μm. And the sample is composed of nanorod, whose diameter and length are approximately 1 μm and 10 μm respectively. X-ray diffraction (XRD) analysis was used to determine the crystalline structure and chemical composition of the synthesized material. Fig. 1b provides the X-ray powder diffraction pattern of the sisal-shaped ordered arrays of cobalt hydroxide
Formation mechanism
In order to understand the formation mechanism of the sisal-shaped hierarchical cobalt hydroxide, FESEM images of cobalt hydroxide samples were examined at various hydrothermal intervals, as shown in Fig. 5. To guarantee the accuracy of the hydrothermal intervals, the reaction system was dropped into a large mass of cooled deionized water at the specific hydrothermal interval. Then the morphologies of the precipitations were examined. With the exception of the hydrothermal interval, all of the
Electrochemical evaluation
Fig. 6a indicates that during the potential sweep of the electrode, two possible reactions occur in this electrochemical system [24]:
The redox peaks P1 and P3 contribute to the redox reaction between Co(OH)2 and CoO(OH), which correspond to Eq. (1); and the redox peaks P2 and P4 are due to the redox reaction between CoO(OH) and CoO2, which correspond to Eq. (2). This means that the capacitance characteristics are
Conclusions
Three-dimensional sisal-shaped hierarchical β-cobalt hydroxide was synthesized by a two-step formation mechanism by hydrothermal method. Cobalt oxide was easily obtained by heating cobalt hydroxide in air, and it also possessed a similar hierarchical structure to cobalt hydroxide. The morphologies, structures and sizes of as-received samples could be easily controlled by changing the synthesis conditions. The specific capacitance of the synthesized samples was about 176 F/g at the scan rate of 5
Acknowledgments
We are grateful to the National Natural Science Foundation of China (Grant No. 51272028) and Key Project of Chinese National Programs for Fundamental Research and Development (973 program) (Grant No. 2012CB720404) for financial support.
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