A precursor route to synthesize mesoporous γ-MnO₂ microcrystals and their applications in lithium battery and water treatment

Abstract

MnCO₃ microstructures, including 2.3 μm microplates with the thickness of 200 nm and 3.1 μm microspheres stacked with 50 nm-thick sheets, were hydrothermally prepared in the assistance of sodium dodecyl benzene sulphonate (SDBS) and dodecyl sulfonic acid sodium (SDS), respectively. With the as-synthesized MnCO₃ as precursors followed by annealing at 400 °C for 4 h, mesoporous γ-MnO₂ microplates and microspheres with the pore size of 4–50 nm, which basically preserved the initial shapes, were obtained. The Brunauer–Emmett–Teller surface areas of the as-prepared γ-MnO₂ microplates and microspheres were 52.1 m² g⁻¹ and 50.2 m² g⁻¹, respectively. The electrochemical property tests over Li⁺ batteries showed that the initial discharge capacity of γ-MnO₂ microplates and microspheres were 1997 mAh g⁻¹ and 1533 mAh g⁻¹. Noticeably, even after 100 cycles, the discharge capacity of γ-MnO₂ microplates was still as high as 626 mAh g⁻¹, indicating the decent cycle behavior. In addition, mesoporous γ-MnO₂ was also applied as adsorbents in water treatment, and γ-MnO₂ microplates and microspheres could remove about 55% and 80% of Congo red.

Introduction

It is well known that manganese dioxide can form several kinds of polymorphs such as α-, β-, γ- and δ-type according to the different linkage patterns of basic unit [MnO₆] octahedral [1]. Among all the structures, γ-MnO₂, which is considered to be an intergrowth of ramsdellite (1 × 2 tunnels) and pyrolusite (1 × 1 tunnels) [2], has wide applications in lithium ion battery [3], [4], [5], electrochemical supercapacitors [6], [7], dry-cell batteries [8], [9], catalysts [10], [11], [12], [13], [14], and water-purifying agents [15]. Therefore, study of synthesis and application over γ-MnO₂ has attracted much more interest.

Recently, a variety of strategies have been developed and employed to fabricate γ-MnO₂ with well-defined morphologies because of the strong size- or structure-dependent properties. In general, these methods can be divided into two routes. One is direct growth of γ-MnO₂. For instance, electrolytic method [16], [17], [18], [19], solid-state reaction [6], room temperature reduction [7], and hydrothermal method [20], [21] have been adopted to synthesize γ-MnO₂ with various shapes. Recently, precursor-based routes followed by certain post-treatments have already been designed to obtain inorganic nanomaterials efficiently owing to the definite transformation to the aimed materials through the removal of organic ingredients [22], [23], [24], [25]. It generally involves procedures of shape-controlled synthesis and subsequent process of the precursors treated at certain conditions to get γ-MnO₂ structure. For example, MnCO₃ solid microspheres and microcubes synthesized in hydrothermal condition were oxidized by KMnO₄ solution at room temperature, and then hollow microspheres and microcubes composed of γ-MnO₂ nanosheets were finally obtained [26]. Porous γ-MnO₂ hollow microspheres and hollow microcubes composed of particles have also been produced by calcining the MnCO₃ synthesized with ammonium hydrogen carbonate as the precipitant during hydrothermal courses [3].

Herein, we report a facile route to fabricate manganese carbonate (MnCO₃) structures with the shapes of microplates and microspheres just by changing the surfactants applied in the synthesis system. After annealing at 400 °C for 4 h in air, mesoporous γ-MnO₂ microcrystal with the pore size of 4–50 nm was obtained, which still maintained the initial morphologies. The electrochemical measurement results over Li⁺ batteries indicated that the initial discharge and charge capacities of the as-prepared γ-MnO₂ microplates were 1997 mAh g⁻¹ and 1271 mAh g⁻¹, respectively. And the discharge capacity of γ-MnO₂ microplates was still 626 mAh g⁻¹ in the current density of 100 mA·g⁻¹ even after 100 cycles, indicating its potential application in Li-ion batteries. The adsorptive experiments for Congo red also indicated the potential application as the water-purifying agents.