Controlled Preparation of MnO2 Hierarchical Hollow Nanostructures and Their Application in Water Treatment

Abstract

Manganese oxides are of considerable importance in technological applications, including ion-exchange, molecular adsorption, catalysis, and electrochemical supercapacitors owing to their structural flexibility combined with novel chemical and physical properties. Up to now, various nanostructures of MnO2, such as nanoparticles, [6] nanorods/-belts/-wires/-tubes/fibers, nanosheets, mesoporous/molecular sieves and branched structures, urchins/orchids, and other hierarchical structures have been synthesized by different methods. Over the past years, fabrication of hierarchical hollow nanostructures has attracted significant interest because of their widespread potential applications in catalysis, drug delivery, acoustic insulation, photonic crystals, and other areas. Until now, the general approach for preparation of hollow structures has involved the use of various removable or sacrificial templates, referred to as “hard”, such as monodispersed silica, polymer latex spheres and reducing metal nanoparticles, as well as “soft” ones, for example, emulsion droplets/ micelles and gas bubbles. Furthermore, lots of one-pot template-free methods for generating hollow inorganic microand nanostructures have been developed employing novel mechanisms, including the nanoscale corrosion-based insideout evacuation and Kirkendall effect. Recently, rhombododecahedral silver cages have been prepared by self-assembly coupled with the precursor crystal-templating approach. By treating the external morphologies of hollow structures, unique properties can be obtained. Thus, it is desirable to develop easy methods to control the morphologies of assembled systems with well-defined hierarchical structures. Herein, we report a simple controlled preparation of hierarchical hollow microspheres and microcubes of MnO2 nanosheets through self-assembly with an intermediate crystaltemplating process. As shown in Figure 1, the synthesis is performed by a three-step process. Particularly, discrete spherical and cubic hollow MnO2 nanostructures with controlled morphologies can be prepared by changing the morphologies of MnCO3 precursors, which can be simply obtained by adding the (NH4)2SO4 solution in the reaction system, and the