Hydrothermal synthesis of cobalt sulfide nanotubes: The size control and its application in supercapacitors
Introduction
Supercapacitors have recently attracted main interests because they can provide higher power density than batteries and higher energy density than conventional dielectric capacitors, which probably make them the most important next generation energy storage device [1], [2], [3]. Transition metal sulfides, such as CoS2 [4], NiS [5], MoS2 [6], CuS [7] have been considered as one of the most promising pseudocapacitor electrode materials with respect to both their specific capacitance and cost effectiveness.
As an important class of transition metal chalcogenides, cobalt sulfides are versatile materials with potential applications in supercapacitors, lithium-ion batteries, solar cells and catalysts [8], [9]. Up to now, nanostructured cobalt sulfides have been investigated as a new type of energy storage materials and have achieved good performance [10]. For instance, Co9S8 nanotubes act as an electrode material in lithium batteries [11]. The three-dimensional cobalt sulfide hierarchitectures exhibit superior specific capacitance of 555 F g−1 at 5 mA cm−2 and excellent cycle life [12]. However, the study of the cobalt sulfide system has not been worked out well. The literature survey reveals that there are no reports on the applicative properties of cobalt sulfides nanotubes for supercapacitors except as a lithium-ion battery [11].
Herein, we investigate the electrochemical capacitor performance of cobalt sulfide nanotubes synthesized by a hydrothermal method, and the sizes of nanotubes are controlled by adjusting reaction temperature. The mechanism can be explained by Kirkendall effect based on different diffusion velocity of materials [13], [14]. The as-prepared cobalt sulfide nanotubes electrode exhibits excellent electrochemical performance as electrode materials for supercapacitors.
Section snippets
Preparation of precursors
In a typical synthesis, the nanorods of precursors were prepared by a hydrothermal route. For example, 5 mmol CoCl2·6H2O and 5 mmol urea were dissolved into 30 mL of deionized water under magnetic stirring at room temperature. The resultant mixture was continually stirred for 15 min and then transferred to a 50 mL Teflon lined stainless-steel autoclave. The autoclave was sealed and maintained at different temperatures (80–160 °C) for 10 h. Then, the system was then cooled to room temperature
Characterizations of the precursor
Fig. 1a displays a typical XRD pattern of the precursor prepared at 90 °C for 10 h. All the reflection peaks in this pattern could be readily indexed to crystalline Co(CO3)0.35Cl0.20(OH)0.11 (JCPDS Card file No. 38-547), without any obvious impurity peaks [11], [15]. Fig. 1b shows the FTIR spectrum of the precursor. A broad band at 3606 cm−1 corresponds to the O–H vibration of a hydrogen-bonded water molecule. With the exception of the δH2O vibration at 1620 cm−1, the bands located below 1500 cm
Conclusions
In summary, cobalt sulfide nanotubes were synthesized by a hydrothermal method, whose sizes were controlled by adjusting reaction temperature. The Co-90 precursor was characterized by XRD and FTIR. We have studied the influence of temperature on the evolution of this special coarse shape nanostructure. The sizes of the precursors increased when the deposition temperature was increased from 80 to 160 °C. We have analyzed the sizes of cobalt sulfide nanotubes related with the capacitive
Acknowledgments
This work was supported by the National Natural Science Foundation of China (Grant 61172003). All electron microscopy was carried out in Analytical and Testing Center, Huazhong University of Science and Technology.
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