Microstructures and Spectroscopic Properties of Cryptomelane-type Manganese Dioxide Nanofibers

Cryptomelane-type manganese dioxide (K-MnO₂) nanottbers with typical diameters ot 20-60 nm and lengths of 1 -6 μ 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 14/m) with unit cell parameters a = 9.8241(5) Åand c = 2.8523(1) Å and elongate along the 10.1021/jp804924flt;00110.1021/jp804924fgt; direction. The K-MnO 2 nanofibers had a mean chemical composition of K 0.11(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-1, 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.