Topotactic phase transformation of hexagonal MoO₃ to layered MoO₃-II and its two-dimensional (2D) nanosheets

This study reports a novel strategy to obtain a layered metastable phase of molybdenum trioxide (MoO₃). The metastable phase in the layered framework, i.e., MoO₃-II, was a result of the topotactic phase transformation of hexagonal-MoO₃ (h-MoO₃) into MoO₃-II. This transformation was realized through the removal of the water and ammonium molecules from the framework of h-MoO₃. Next, a facile exfoliation strategy was adopted to prepare MoO₃-II nanosheets, consisting of double-layers of MoO₆ octahedra. The as-exfoliated MoO₃-II nanosheets had measured thicknesses of 1.4, 2.4, and 11.2 nm, as quantified using atomic force microscopy (AFM), equivalent to 2, 3, and 16 double-layers of the MoO₆ octahedra in MoO₃-II, respectively, after taking into account their respective van der Waals gaps (thickness of a MoO₃-II double-layer ∼0.709 nm). This work is the first attempt to synthesize MoO₃-II nanosheets and shed light on their formation mechanism. (Figure Presented).