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 similar to 0.709 nm). This work is the first attempt to synthesize MoO3-II nanosheets and shed light on their formation mechanism.