Microstructures and Spectroscopic Properties of Cryptomelane-type Manganese Dioxide Nanofibers

Cryptomelane-type manganese dioxide (K-MnO2) nanofibers with typical diameters of 20−60 nm and lengths of 1−6 μm were prepared by reacting KMnO4 with MnSO4 under hydrothermal conditions. Rietveld refinement from synchrotron X-ray powder diffraction data showed that the K-MnO2 nanofibers crystallize in a body-centered tetragonal structure (space group I4/m) with unit cell parameters a = 9.8241(5) Å and c = 2.8523(1) Å and elongate along the <001> direction. The K-MnO2 nanofibers had a mean chemical composition of K0.11(H3O)0.05MnO2. The optical band gap of the K-MnO2 nanofibers was estimated to be 1.32 eV based on the UV−visible absorption. The K-MnO2 nanofibers had four diagnostic infrared absorptions at 722, 593, 524, and 466 cm−l, which represents specific fingerprints of the vibrational features of MnO2 materials containing (2 × 2) + (1 × 1) tunnel structures. The Raman scattering spectrum of the K-MnO2 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 MnO6 octahedral frameworks. These intrinsic vibrational features can be conveniently used for online and/or in situ analyses of the K-MnO2 nanofibers during electrochemical and/or ion-exchange reactions.