Synthesis of MnO2 nanoparticles from sonochemical reduction of MnO4(-) in water under different pH conditions.

MnO2 was synthesized by sonochemical reduction of MnO4(-) in water under Ar atmosphere at 20°C, where the effects of solution pH on the reduction of MnO4(-) were investigated. The obtained XRD results showed that poor crystallinity δ-MnO2 was formed at pH 2.2, 6.0 and 9.3. When solution pH was increased from 2.2 to 9.3, the morphologies of δ-MnO2 changed from aggregated sheet-like or needle-like structures to spherical nanoparticles and finally to cubic or polyhedron nanoparticles. After further irradiation, MnO2 was readily reduced to Mn(2+). It was confirmed that H2O2 and H atoms formed in the sonolysis of water acted as reductants for both reduction for MnO4(-) to MnO2 and MnO2 to Mn(2+). The optimum irradiation time for the effective synthesis of MnO2 was 13 min at pH 2.2, 9 min at pH 6.0, 8 min at pH 9.3, respectively.