Microstructures and spectroscopic properties of cryptomelane-type manganese dioxide nanofibers

Cryptomelane-type manganese dioxide (K-MnO(2)) nanofibers with typical diameters of 20-60 nm and lengths of 1-6 mu m were prepared by reacting KMnO(4) with MnSO(4) under hydrothermal conditions. Rietveld refinement from synchrotron X-ray powder diffraction data showed that the K-MnO(2) nanofibers crystallize in a bodycentered tetragonal structure (space group 14/m) with unit cell parameters a = 9.8241(5) angstrom and c = 2.8523(1) and elongate along the < 001 > direction. The K-MnO(2) nanofibers had a mean chemical composition of K(0.11)(H(3)O)(0.05)MnO(2). The optical band gap of the K-MnO(2) nanofibers was estimated to be 1.32 eV based on the UV-visible absorption. The K-MnO(2) nanofibers had four diagnostic infrared absorptions at 722, 593, 524, and 466 cm(-1), which represents specific fingerprints of the vibrational features of MnO(2) materials containing (2 x 2) + (1 x 1) tunnel structures. The Raman scattering spectrum of the K-MnO(2) nanofibers had nine Raman bands with four main contributions at 183, 386, 574, and 634 cm(-1) along with five weak ones at 286, 330, 470, 512, and 753 cm(-1), which are attributed to the Mn-O lattice vibrations within the MnO(6) octahedral frameworks. These intrinsic vibrational features can be conveniently used for online and/or in situ analyses of the K-MnO(2) nanofibers during electrochemical and/or ion-exchange reactions.