Hydrothermal synthesis of nanostructured Co3O4 materials under pulsed magnetic field and with an aging technique, and their electrochemical performance as anode for lithium-ion battery
Introduction
In recent years, great efforts have been put into the reinvestigation of materials that were thought of as being electrochemically inactive in bulk form, but that could present improved electrochemical performance at the nanoscale. One good example is the demonstration that nanoparticles of some simple transition-metal oxides, sulfides, fluorides, and nitrides can provide innovative anode materials for lithium-ion batteries [1]. Transition-metal oxides in the nanometer size regime display many interesting size-dependent optical, electronic, magnetic, and chemical properties [2], [3], [4]. Such nanoscale materials have potential applications in chemical sensors, spintronics, magnetic data storage systems, and shape memory alloys [5], [6], [7]. Among these oxides, Co3O4 is universally known as a widely applied material used in electrodes [8], catalysts [9], and gas sensors [10]. With continuing progress on electrode materials, Co3O4 has attracted considerable attention due to its high theoretical reversible capacity of 1100 mAh g−1 when discharged to 0 V vs. Li+/Li [11], as well as its electrochemical stability [12], [13], [10]. As a result, it is believed that the Co3O4 nanomaterials can exhibit superior Li-battery performance.
It is well known that the morphology and microstructure of Co3O4, including the crystal size and orientation, have a great influence on its properties. So, various morphologies of nanosized Co3O4 have been synthesized by different methods, such as the thermal decomposition of a solid cobalt nitrate (380 °C) [4], chemical spray pyrolysis (350–400 °C) [14], [15], chemical vapor deposition (CVD, 550 °C) [16], pulsed laser deposition [17], and the traditional sol–gel method (above 260 °C) [18]. It has been reported that porous nanotubes of Co3O4 were synthesized by the micro-emulsion method [19], and Co3O4 nanorods were prepared by improving the traditional molten salt synthesis [20] and by the solvothermal method [21]. Nevertheless, most of the above-mentioned methods need some special instruments, harsh conditions, or a relatively high processing temperature, so that the production of nanocrystalline Co3O4 is difficult and inconvenient [22]. In addition to these considerations, another limitation of the traditional methods is the necessity of post-reaction thermal treatment of the materials to increase the crystallinity, which leads to particle aggregation and uncontrolled crystal growth [23].
As is well known, Co3O4 particles have magnetic properties [24], [25], so there may be advantages to their fabrication under external field. As an example of using such properties, single-crystalline Fe3O4 (an allomer of Co3O4) nanowires [26], [27] have been successfully synthesized under low magnetic field. However, most of the above methods may be not compatible with magnetic field, or the magnetic field involved can only be very small. Recently, high magnetic field has been recognized not only for study of the physical properties of a material, but also as a tool to control the microstructure and functions of the material [28]. Yet until now, there have been scarcely any reports on the synthesis of Co3O4 nanoparticles under high magnetic field. So, it is very interesting to explore the synthesis of Co3O4 nanoparticles by the hydrothermal method under pulsed magnetic field and with an aging technique, and to examine their electrochemical performance as anode for the lithium-ion battery.
Section snippets
Materials synthesis
The chemicals used in this work were cobalt nitrate, ammonia, ammonium chloride, hydrogen peroxide, polyethylene glycol (PEG), n-butanol, and absolute ethanol. All chemicals were analytical-grade reagents, and Co3O4 materials were synthesized according to the procedure described in a previous report by some of us [29]. Cobalt nitrate (0.0378 mole) and PEG (0.125 g) as surfactant were dissolved in an appropriate amount of deionized water, and then an excess amount of NH3–NH4Cl buffer solution (pH
Results and discussion
Fig. 1 shows the XRD patterns for the samples produced at 180 °C and 0 T magnetic field (Co3O4-0T), at 180 °C and 4 T magnetic field (Co3O4-4T), and with 12 h aging time followed by 0 T processing at 180 °C (Co3O4-Aging).
All the diffraction peaks are readily indexed to cubic structure [space group: Fd3m (2 2 7)] Co3O4, which is consistent with the literature results (Joint Committee on Powder Diffraction Standards (JCPDS) File No. 43-1003). The lattice constants and the peak positions (observed and
Conclusions
In this study, high pulsed magnetic field and an aging technique have successfully been used in the synthesis of nanocrystalline Co3O4 via the hydrothermal method. The pulsed magnetic field processing produces a more compact and smooth surface composed of Co3O4 microspheres that consist of numerous nanograins. The aging technique introduced into the Co3O4 synthesis process makes large Co3O4 hollow spheres consisting of a large quantity of nanospheres. So, both processes have been proved to be
Acknowledgements
The authors are grateful for funding from the Australian Research Council under an ARC Centre of Excellence Program (CE0561616). The authors also thank Dr. T. Silver for critical reading of the manuscript.
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