In Situ TEM Reveals Direct One-Step Reduction of van der Waals Crystal MoO₃ to Mo

Molybdenum oxides (MOs) exhibit rich polymorphism and tunable properties, yet their phase transformation pathways are poorly understood. Here, we employ in situ environmental transmission electron microscopy (TEM) to reveal a direct reduction of MoO3 to metallic Mo, bypassing known intermediate phases such as MoO2 and Mo4O11. Surface nucleation begins at approximately 800 °C and is completed at 900 °C. Molecular dynamics (MD) and density functional theory (DFT) calculations attribute this unexpected transformation to the van der Waals (vdW) layered structure of MoO3, which lowers both the oxygen binding energy and the Gibbs free energy (ΔG) for oxygen desorption under high-vacuum and high-temperature conditions. Preferential oxygen removal from the weakly bonded vdW layers facilitates a rapid reduction to the metallic phase. These findings uncover a nonclassical reduction mechanism and provide a pathway for the rational design of MOs with controllable phases and properties.