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.