Formation Mechanisms of Nanocrystalline MnO2 Polymorphs under Hydrothermal Conditions

Understanding and:controlling the. olymorphism of manganese dioxide (MnO2) is of vital importance in many nanoscale applications. Here in situ powder Xray diffraction (PXRD) in 'combination with in situ total X-ray scattering are used to reveal the formation mechariisin as well as polymorph-evolution of MnO2 under hydrothermal synthesis conditions. A "PXRD invisible" amorphous phase with a local structure :resembling alpha-MnO2 (denoted alpha-MnO2:(A)) is observed at all reaction stages, and it never fully disappears from the reaction solution. The MnO2 phase evolution involves initial formation:of delta-MnO2, which transforms to alpha-MnO2, and then subsequently to beta-MnO2. The phase transformations between-different polymorphs, do not involve dissolution-recrystallization, but they occur via solid-state mechanisms. However, the amorphous alpha-MnO2(A) phase, plays a key role since it is consumed in growing both the alpha-and beta-MnO2 polymorphs. Overall, the polymorphism of the crystalline product can be controlled through reaction time and temperature to forth either nanocrystalline and-disordered delta-MnO2, nanocrystalline alpha-MnO2, or nanocrystalline beta-MnO2. At the lowest temperature (200 degrees C) the very early growth of alpha-MnO2 appears to be by oriented attachment along (110) crystal planes of primary nanorbds, but this is quickly followed by rapid growth along the c-direction supported by consumption of alpha-MnO2(A).