Probing the Crystal and Electronic Structures of Molybdenum Oxide in Redox Process: Implications for Energy Applications

Knowledge regarding the phase and valence state evolution of molybdenum (Mo) and its oxides during the redox reaction is essential for advancing their energy applications (e.g., electrocatalysis), which unfortunately remains largely unexplored. Herein, the effects of atomic and electronic structures on the electrocatalytic performance of Mo/oxides core–shell structures are investigated on the basis of the combination of ex situ and in situ experiments. First, a two-step reaction pathway is revealed during the oxidation of nanoscale Mo: the formation of amorphous MoO3 (A-MoO3) shells followed by the nucleation of crystalline α-MoO3. It is shown that the electrocatalytic performance of A-MoO3 is superior to that of α-MoO3, mainly due to more catalytically active sites in the former material. Furthermore, in situ transmission electron microscopy observations show that the A-MoO3 shell can be rapidly reduced into metallic MoO2 under reductive environment, which is likely to occur during the hydrogen evolution reaction measurement. Our in-depth characterization may contribute to the thorough and comprehensive understanding of the structural transition in Mo and its oxides during oxidative and reductive environments and thus serves as a reference for understanding the structure–property interplay for real energy applications.