The synthesis of MnO2 smooth balls was performed similarly
under hydrothermal conditions. At first, 1.0245 g MnSO4$H2O and
1.397 g (NH4)2S2O8 were dissolved in 15 mL of deionized water.
Meanwhile, 0.5 g PVP (polyvinyl pyrrolidone K30) was added
slowly with vigorous stirring. When the solution was clarified, this
mixed solution was transferred into 20 mL Teflon-lined stainless
steel autoclaves and allowed to react at 120 C. Then, MnO2 smooth
balls were obtained from the reaction