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 μm 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 body-centered tetragonal structure (space group I 4/ m ) with unit cell parameters a = 9.8241(5) Å and c = 2.8523(1) Å 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 −l, which represents specific fingerprints of the vibrational features of MnO 2 materials containing (2 × 2) + (1 × 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.