Simultaneous phase- and morphology-controlled synthesis of MnO 2 crystals through controlled release of cuprous ions in
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Ka Wai Wongb) Institute of Precision Engineering and Department of Physics, The Chinese University of Hong Kong, Shatin, Hong Kong, China
Fengling Yang Institute of Surface Micro and Nano Materials, Xuchang University, Xuchang, Henan 461000, China
Zude Zhang Department of Chemistry, University of Science and Technology of China, Hefei, Anhui 230026, China (Received 16 June 2008; accepted 28 August 2008)
The a-, b-, and d-MnO2 with various morphologies have been synthesized by a novel redox system of KMnO4 and CuCl with HCl added under a hydrothermal condition. The resultant MnO2 products have been characterized by x-ray diffraction, scanning electron microscopy, and transmission electron microscopy. Upon control of reaction temperature and duration, it was observed that MnO2 polymorphs of different morphology (e.g., flowery d-MnO2, b-MnO2 nanowires and octahedrons, a-MnO2 nanowires) can be prepared in an adjustable manner. The phenomenon is mainly attributed to the effect of cuprous ions controllably released from CuCl by the action of HCl at different experimental conditions. The corresponding formation mechanism for the MnO2 crystals will also be proposed and discussed.
I. INTRODUCTION
Manganese dioxide (MnO2) is known to be an important material used as catalyst,1 ion-sieve,2 and especially as electrode material in Li/MnO2 batteries.3–5 There are different polymorphs of MnO2, such as a-, b-, g-, e- and d-type.6 Over the past few years, great efforts have been made to prepare micro- or nanostructured MnO2 crystals with different phase and morphology, such as a-MnO2 nanorods and nanofibers,7–11 b-MnO2 microrods and nanorods,7,11–16 g-MnO2 nanowires17 and threedimensional (3D) urchinlike structures,18 and d-MnO2 nanofibers.19 Among these reports, a-MnO2 has been mostly investigated. Song et al.20 obtained a-MnO2 nanostructures with sea urchin shapes by a surfactant-assisted redox hydrothermal treatment of MnSO4 and KClO3. Kumar et al.21 obtained highly dispersed a-MnO2 nanoneedles by sonochemical hydrolysis of manganese(III) acetate. The b-MnO2 nanorods were synthesized by the post-treatment of precursor MnOOH nanorods.22 Wang et al.7 have reported the phase transformation of b- and a-MnO2 through the addition of (NH4)2SO4 by a redox method. They also suggested that formation of a specific Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2009.0022 58
J. Mater. Res., Vol. 24, No. 1, Jan 2009
http://journals.cambridge.org
Downloaded: 27 Mar 2015
phase of MnO2 is influenced by the anions and pH value of the reaction system. However, the two crystalline phases of MnO2 obtained were both one-dimensional (1D) rodlike structures. Despite the fact that the morphology, size, and crystal orientation are influential factors affecting the performance of MnO2 in various applications, there are still few reports of a single system or method that allows synthesis of different phases of MnO2 nanocrystals with different morphology by simply adj
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