Controllable synthesis of α- and β-MnO2: cationic effect on hydrothermal crystallization

α- and β-MnO2 were controllably synthesized by hydrothermally treating amorphous MnO2 obtained via a reaction between Mn2+ and MnO4−, and cationic effects on the hydrothermal crystallization of MnO2 were investigated systematically. The crystallization is believed to proceed by a dissolution–recrystallization mechanism; i.e. amorphous MnO2 dissolves first under hydrothermal conditions, then condenses to recrystallize, and the polymorphs formed are significantly affected by added cations such as K+, NH4+ and H+ in the hydrothermal systems. The experimental results showed that K+/NH4+ were in competition with H+ to form polymorphs of α- and β-MnO2, i.e., higher relative K+/NH4+ concentration favoured α-MnO2, while higher relative H+ concentration favoured β-MnO2.