Reversible MnO to Mn3 O4 Oxidation in Manganese Oxide Nanoparticles.

Manganese is an attractive element for sustainable solutions. It is largely available in the earth's crust, making it ideal for cost-effective and large-scale applications. Especially MnO nanoparticles have recently received attention for applications in battery technology. However, manganese has many oxidation states that are energetically very similar, indicating that they may easily transform from one to the other. Herein, the reversible oxidation of MnO nanoparticles to Mn3 O4 studied with in situ transmission electron microscopy is shown. The oxygen sublattices of MnO and Mn3 O4 are found to be perfectly aligned, and an atomic mechanism where the transformation is facilitated by the migration of Mn cations on the shared O sublattice is proposed. Even when protected with an amorphous carbon layer, MnO particles are highly unstable and oxidize to Mn3 O4 in ethanol. The poor stability of MnO lacks discussion in many battery-related works, and strategies aimed at avoiding this should be developed.

Thermal Reduction of MoO3 Particles and Formation of MoO2 Nanosheets Monitored by In Situ Transmission Electron Microscopy.

Nanoscale forms of molybdenum trioxide have found widespread use in optoelectronic, sensing, and battery applications. Here, we investigate the thermal evolution of micrometer-sized molybdenum trioxide particles during in situ heating in vacuum using transmission electron microscopy and observed drastic structural and chemical changes that are strongly dependent on the heating rate. Rapid heating (flash heating) of MoO3 particles to a temperature of 600 °C resulted in large-scale formation of MoO2(001) nanosheets that were formed in a wide area around the reducing MoO3 particles, within a few minutes of time frame. In contrast, when heated more gently, the initially single-crystal MoO3 particles were reduced into hollow nanostructures with polycrystalline MoO2 shells. Using density functional theory calculations employing the DFT-D3 functional, the surface energy of MoO3(010) was calculated to be 0.187 J m-2, and the activation energy for exfoliation of the van der Waals bonded MoO3 (010) layers was calculated to be 0.478 J m-2. Ab initio molecular dynamics simulations show strong fluctuations in the distance between the (010) layers, where thermal vibrations lead to additional separations of up to 1.8 Å at 600 °C. This study shows efficient pathways for the generation of either MoO2 nanosheets or hollow MoO2 nanostructures with very high effective surface areas beneficial for applications.