Synthesis of colloidal MnO2 with a sheet-like structure by one-pot plasma discharge in permanganate aqueous solution

Abstract

Stable colloidal MnO₂—consisting of MnO₂ with a sheet-like structure—was synthesized by solution plasma process (SPP). The synthesis was completed in one-step by discharging the plasma in potassium permanganate (KMnO₄) aqueous solution without using any dispersants or stabilizers. An alteration of the manganese oxide oxidation state, from MnO₄⁻ to MnO₂ as a function of the discharge time, was investigated by UV-vis spectroscopy. Morphology and elemental constituents were observed by TEM and EDS mapping. Results indicated that the discharge time was an important feature for the formation of MnO₂ in the SPP system. Specifically at neutral pH (pH = 7), MnO₄⁻ was completely reduced to MnO₂ within a discharge time of 18 min. After that the obtained MnO₂ was converted rapidly to Mn²⁺. To better understand the possible pathways of MnO₂ formation by the SPP, we compared aspects of the reaction under different pH conditions. Formation of MnO₂ under additional, controlled pH conditions, i.e. 2 and 12, was studied. Results suggested that hydrogen species played a key role for the reduction of MnO₄⁻ in the water-based SPP system. In comparison to the existing routes for the synthesis of MnO₂ nanosheets with a single or low number of layers, the SPP holds excellent promise as an effective alternative means regarding its simplicity, time-energy preserving, and scalability.