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 (MoO 3 ). The metastable phase in the layered framework, i.e., MoO 3 -II, was a result of the topotactic phase transformation of hexagonal-MoO 3 (h-MoO 3 ) into MoO 3 -II. This transformation was realized through the removal of the water and ammonium molecules from the framework of h-MoO 3 . Next, a facile exfoliation strategy was adopted to prepare MoO 3 -II nanosheets, consisting of double-layers of MoO 6 octahedra. The as-exfoliated MoO 3 -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 MoO 6 octahedra in MoO 3 -II, respectively, after taking into account their respective van der Waals gaps (thickness of a MoO 3 -II double-layer ∼0.709 nm). This work is the first attempt to synthesize MoO 3 -II nanosheets and shed light on their formation mechanism.