Formation Mechanisms of Nanocrystalline MnO₂ Polymorphs under Hydrothermal Conditions

Abstract

Understanding and controlling the polymorphism of manganese dioxide (MnO 2 ) is of vital importance in many nanoscale applications. Here in situ powder X-ray diffraction (PXRD) in combination with in situ total X-ray scattering are used to reveal the formation mechanism as well as polymorph evolution of MnO 2 under hydrothermal synthesis conditions. A "PXRD invisible" amorphous phase with a local structure resembling α-MnO 2 (denoted α-MnO 2 (A)) is observed at all reaction stages, and it never fully disappears from the reaction solution. The MnO 2 phase evolution involves initial formation of δ-MnO 2 , which transforms to α-MnO 2 , and then subsequently to β-MnO 2 . The phase transformations between different polymorphs do not involve dissolution−recrystallization, but they occur via solid-state mechanisms. However, the amorphous α-MnO 2 (A) phase plays a key role since it is consumed in growing both the αand β-MnO 2 polymorphs. Overall, the polymorphism of the crystalline product can be controlled through reaction time and temperature to form either nanocrystalline and disordered δ-MnO 2 , nanocrystalline α-MnO 2 , or nanocrystalline β-MnO 2 . At the lowest temperature (200 °C) the very early growth of α-MnO 2 appears to be by oriented attachment along (110) crystal planes of primary nanorods, but this is quickly followed by rapid growth along the c-direction supported by consumption of α-MnO 2 (A).

Non-Equilibrium Crystallization Pathways of Manganese Oxides in Aqueous Solution

“…A more quantitative understanding of the thermochemistry of the electrochemical double layer, especially as a function of surface chemistry and electronic structure, will provide an additional handle to engineer crystallization pathways in solution. Additionally, solid-solid transformations, for example, by H-ion or Mn-ion migration within an oxygen sublattice 53,54 , could be a competing structure transformation mechanism to dissolution-reprecipitation 55 . The energy barriers of ion-diffusion (units of meV/atomic hop) and crystal nucleation (units of meV/ nucleus) have different units, and therefore the relative kinetics between these two competing mechanisms cannot be directly compared.…”

There's no place like real-space: elucidating size-dependent atomic structure of nanomaterials using pair distribution function analysis

“…[203][204][205][206] In situ PDF studies, on the other hand, allow for identication and characterization of disordered or amorphous intermediate structures and species in solution, which are important in nanomaterials. For example, in studies of nanomaterial synthesis, in situ PDF analysis gives the possibility for studying the atomic structure of the species present before, during and aer crystallization of nanoparticles, 183,[207][208][209][210][211][212][213][214][215][216][217][218][219] which is essential for gaining a deeper understanding of material nucleation processes, where atomic scale mechanistic information generally is scarce. Knowledge of the relation between chemical synthesis and the resulting atomic structure of the material is crucial in the advancement of nanoscience.…”

Understanding crystallization pathways leading to manganese oxide polymorph formation

“…While phase diagrams offer guiding insights on the relative stability of reaction end-products, little is known about the energy landscape that a material traverses as it nucleates and grows. There is growing evidence of the complexity of this energy landscape, as well-crystallized transient metastable phases are often observed to form before the equilibrium phase, during both solid-state 5 – 8 and aqueous synthesis 9 , 10 . Furthermore, minor variations in synthesis conditions can affect the lifetimes of these metastable phases 11 , or even redirect a crystallization pathway through very different metastable phases 12 , 13 .…”