Electrochemical and surface properties of nanocrystalline β-MnO2 in aqueous electrolyte

Surface and electrochemical properties of micrometric and nanometric tetragonal β-MnO2 of, respectively, 4 and 61 m2 g−1 surface area were studied. The monodispersed nanocrystalline phase prepared by spray pyrolysis and characterized by transmission electron microscopy (TEM), is made of facetted crystals with 70 nm × 40 nm edges. Three types of energy domains were identified by surface acid–base titration. Slow step linear voltammetry reduction (10 mV/2 h scan rate) showed one current peak at −0.6 V (versus Hg–HgO 1 M KOH) for the micrometric phase, but three current peaks at −0.18, −0.36 and −0.55 V for the nanometric phase. They were assigned to the reduction of: (1) surface Mn atoms related to 〈1 1 1〉 and 〈3 1 1〉 faces; (2) more stable surface Mn atoms related to 〈1 0 0〉 and 〈1 1 0〉 faces; (3) bulk Mn atoms, respectively. The presence of new reduction peaks at higher potential is ascribed to the increase in Gibbs free energy caused by the surface energy of the nanometric particle.