Facile synthesis and electrochemical characterization of hierarchical α-MnO₂ spheres

Abstract

Hierarchical α-MnO₂ spheres have been synthesized by a facile wet chemical method involving no templates. The products were characterized by XRD, SEM, EDX, and TEM. It was found that the products are composed of a large quantity of submicron-spheres with diameters of 300–500 nm and a few microspheres with diameters of several micrometers. The MnO₂ submicron-spheres were constructed of many small nanorods with diameters of ca. 10 nm and lengths up to 50 nm. The influences of experimental parameters on the morphology and phase of the products were investigated. The possible formation mechanism for the hierarchical α-MnO₂ spheres was discussed based on the experimental results. Electrochemical properties of the as-synthesized MnO₂ spheres as cathode materials in Li-ion cells were also studied and compared.

Introduction

Manganese dioxide (MnO₂) has long been investigated as an important transitional metal oxide because it forms porous structures. Generally, MnO₂ can exist in many kinds of crystal structures such as α, β, γ and δ-type on the basis of different linking manners of its basic MnO₆ octahedral structure. For example, α-MnO₂ is constructed of double chains of MnO₆ octahedra forming 2 × 2 tunnels. β-MnO₂ is constructed of single chains of edge-sharing octahedra to form 1 × 1 tunnels. γ-MnO₂ is a random intergrowth of ramsdellite (1 × 2 tunnels) and pyrolusite (1 × 1 tunnels). The variety of morphologies and polymorphs of MnO₂ has endowed it with wide applications such as molecular/ion sieves [1], redox catalysts [2] and especially electrode materials in supercapacitors and batteries [3], [4], [5], [6], [7]. Based on various redox reactions between Mn²⁺ and MnO₄⁻, MnO₂ materials have been successfully prepared by several methods, such as sol–gel [4], thermal decomposition [8], solid state reaction [2], refluxing [9] and hydrothermal [10]. In addition, it has been stressed that [11] the electrochemical properties of MnO₂ materials strongly rely on the parameters of their structural characteristics, such as particle morphology, phase and bulk density. So far, much effort has been devoted to the fabrication and characterization of one-dimensional (1D) MnO₂ nanomaterials. Various 1D MnO₂ nanostructures, including wires, rods and tubes have been appropriately synthesized by many approaches [9], [12], [13], [14], [15], [16], [17], [18], [19]. Further, it is found that the 1D MnO₂ nanomaterials displayed favorable electrochemical properties as compared to their bulk counterparts [7]. Recently, it is highly desirable to assemble metal oxides nanoparticles into three-dimensional (3D) spherical nanostructures, which are expected to bring novel properties. For instance, hierarchical TiO₂ spheres with improved photocatalytic performance and Li storage capacity have been reported [20], [21], [22]. Very recently, 3D MnO₂ spherical nanostructures have also been synthesized via a catalytic or hydrothermal route [23], [24], [25], [26], [27], [28], [29], [30], [31].

In a recent paper [32], we proposed the synthesis of hierarchical α-MnO₂ spheres by a mild redox reaction between MnSO₄ and K₂S₂O₈ with the addition of CuSO₄ in an acidic solution. In this work, the effects of experimental parameters, such as the existence of CuSO₄, reaction time and temperature, on the phase and morphology of the final products were further investigated in detail. Meantime, electrochemical properties of the MnO₂ spheres synthesized at different reaction conditions as cathode materials in Li-ion cells were also studied and compared.