The physical and photo electrochemical characterization of the crednerite CuMnO2

Abstract

CuMnO2 is prepared via Cu+ → Li+ exchange in molten copper (I) chloride. It crystallizes in a monoclinic structure (SG C2/m) where the MnO6 octahedra elongation is ascribed to the Yahn–Teller (Y–T) effect of Mn3+ ions. From chemical analysis, the oxide is more accurately formulated as CuMnO2.01. Above 250 °C, it undergoes a reversible transition to spinel CuxMn3−xO4 and beyond 940 °C it converts back to Cu1.1Mn0.9O2. Extrapolation of high-temperature magnetic data indicates T-intercept θ p of −450 K and an effective moment of 5.22 μB, consistent with strong antiferromagnetism in the basal plans and high spin (HS) configuration Mn3+. This value is slightly larger than that of the spin only moment, a behavior ascribed to Cu2+ originating from oxygen insertion. As prepared, CuMnO2 displays p-type conductivity with an activation energy of 0.16 eV. Most holes generated upon band gap excitation are trapped on Cu+ ions and the conduction occurs by small polarons hopping between neighboring sites. The linear increase of thermopower for Cu1.05Mn0.95O2 with temperature indicates a hole mobility μ300 K (3.5 × 10-6 cm2 V−1 s−1) thermally activated. CuMnO2 is made p- and n-type and the difference in the carriers mobilities is attributed to different oxygen polyhedra. The title oxide, characterized photo electrochemically, exhibits a pH-insensitive flat band potential (+0.13 VSCE). The valence band, located at 5.3 eV below vacuum, is made up of Cu 3d orbital. As application, the powder showed a good performance for the H2-photo evolution.