Solvothermal synthesis of shape-controlled manganese oxide materials and their electrochemical capacitive performances

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We present a simple and quick procedure for the one-pot synthesis of manganese oxides under a basic solvothermal condition in the presence of cationic surfactants acting as the template in a 2-butanol/water solution. Three-dimensional spinel-type MnO2 microspheres composed of small nanoparticles have been fabricated for the first time using our method. Their corresponding electrochemical performances in the applications of supercapacitor electrodes exhibit a good specific capacitance (SC) value of ;190 F/g at 0.5 A/g and excellent SC retention and Coulombic efficiency of ;100% and ;95% after 1000 charge/discharge cycles at 1 A/g, respectively. This suggests its potential applications in energy storage devices. Further, we demonstrate that this solvothermal technique enables the morphological tuning of manganese oxides in various forms such as schists, rods, fibers, and nanoparticles. This work describes a rapid and low-cost technique to fabricate novel architectures of manganese oxides having the desired crystal phase, which will highly benefit various supercapacitor applications.

I. INTRODUCTION

Over the past few decades, the synthesis of inorganic nanomaterials with desired sizes and shapes has been intensively pursued due to their unique electronic, magnetic, optical, and catalytic properties with respect to multiple fundamental and practical technological applications.1–7 By means of further investigations on crystal growth kinetics and shape evolution, nanomaterials with different architectures have been successfully fabricated in the solution phase.8,9 By means of the cooperative self-assembly of inorganic species and surfactants, where the surfactant surface aggregates as templates, the surfactant-assisted strategy is known to be a general and efficient method for the preparation of uniform inorganic nanomaterials with controllable sizes and shapes. Recently, various nano- and microstructures of metal oxides (particularly those of MnO2, SiO2, and V2O5) in the form of spheres, porous shapes, layers, tubes, wires, and rods have been successfully fabricated by using this approach.10–14 Manganese oxide (MnO2)—an important functional metal oxide—and its derivative compounds offer the following advantages: possibility of numerous attainable oxidation states of Mn, cost-effectiveness, variety of electrochemical behaviors, and environmental compatibility; because of these advantages, they have been the subject of intense investigations for a wide range of applications such as catalysts, water purification, water splitting, molecular Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2013.238 J. Mater. Res., Vol. 29, No. 1, Jan 14, 2014

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absorption, and battery materials.15–17 In addition, they also provide reversible Faradaic (redox) reaction, appropriate electrode potential window, and high theoretical capacitance; therefore, they have been considered for use as a promising electrode