Effect of KOH on the continuous synthesis of cobalt oxide and manganese oxide nanoparticles in supercritical water

•Cobalt oxide and manganese oxide synthesized continuously in supercritical water.•KOH concentrations have effect on oxidation state of metal oxides.•OH− of KOH may act as a reducing agent as well as a supersaturation enhancing agent.

The effects of KOH on the supercritical hydrothermal synthesis of cobalt oxide and manganese oxide particles are investigated using a continuous-flow reactor. Significant changes in morphology, particle size, and oxidation state are observed by adding KOH. The spinel Co3O4 phase is transformed to a rocksalt CoO phase and the pyrolusite MnO2 phase is transformed to a hausmannite Mn3O4 phase in the presence of 0.5 M KOH. The average particle size of the metal oxides decreased with an addition of KOH. The OH− ions of KOH may act as a reducing agent as well as a supersaturation enhancing agent under supercritical water conditions.