Two-Dimensional Defective MoO_3–<i>x</i> Layers: Formation of a Magnéli-Type Nanophase

The reduction of MoO₃ profoundly influences its physical and chemical properties, making it a material of central importance across a wide range of applications. While bulk reduction processes and the resulting substoichiometric Magnéli phases─composed of extended shear planes─have been thoroughly investigated, the reduction chemistry of MoO₃ nanoscale objects remains largely unexplored and nanoscale-specific mechanisms of oxygen deficiency accommodation are poorly understood. In this study, we employ a combination of atomic-resolution scanning tunneling microscopy (STM), low energy electron diffraction (LEED), X-ray photoelectron spectroscopy (XPS), and density functional theory (DFT) modeling to systematically investigate the reduction of an ultrathin MoO₃ bilayer supported on a Pd(100) surface. Our results reveal that the bilayer decomposes upon reduction into reduced monolayer and trilayer phases, with the trilayers consistently exhibiting ordered surface defect structures characterized by (2 × 3) and (2 × 4) periodicities. Through DFT modeling, we assign these defects to a peculiar type of surface oxygen vacancy, formed by a transformation of MoO₃ octahedra configurations from corner-sharing to edge-sharing. Ordered assemblies of such defects form lattices of substoichiometric shear lines, which are the two-dimensional analogs of the bulk Magnéli crystallographic shear planes. Their genuinely nanoscale-specific character is proved by the instability of alike phases on surfaces of bulk MoO₃. Their detection on MoO₃ films suggests that the formation of similar two-dimensional (2D) Magnéli-type phases may be a more general phenomenon and occur also in other reducible oxide nanolayers, where enhanced structural flexibility enables the formation of phases not accessible in bulk crystals.