Ultrathin, Polycrystalline, Two-Dimensional Co ₃ O ₄ for Low-Temperature CO Oxidation

Free-standing, hierarchical, ultrathin, and two-dimensional (2D) polycrystalline Co ₃ O ₄ flowers were synthesized by a hydrothermal and topotactic transformation process. Aberration-corrected electron microscopy study of both the CoO _x precursor structure and the subsequent topotactic transformation processes revealed the nucleation and growth mechanisms of the 2D polycrystalline Co ₃ O ₄ nanosheets. The free-standing flower-shaped CoO _x powders (1-5 μm) consist of numerous self-assembled nanocrystallites (average size ∼1.8 nm). After the topotactic transformation, via a rapid calcination process, the powders maintained their hierarchical flower-like shape, but the CoO _x nanocrystallites structurally transformed into ultrathin 2D Co ₃ O ₄ nanoplates with thicknesses ranging from 1 to 5 nm (average thickness ∼2.4 nm). The final, free-standing, ultrathin 2D polycrystalline Co ₃ O ₄ flowers possess a BET surface area of 138 m ² /g. Statistical structural analyses revealed that the exposed surfaces of the Co ₃ O ₄ flowers are dominated by the Co ₃ O ₄ {112} (∼70%). The hierarchical Co ₃ O ₄ flowers contain many grain boundaries, pockets, surface steps, and other types of surface defects. CO oxidation on the as-synthesized hierarchical Co ₃ O ₄ flowers showed a specific activity (normalized to the surface area) of 0.377 μmol·m ^-2 ·s ^-1 , about 5 times that of the most active Co ₃ O ₄ at 70 °C reported in literature. Furthermore, even under moisture-saturated condition (∼3% H ₂ O), the ultrathin 2D Co ₃ O ₄ catalyst demonstrated a high specific rate and is stable for at least 40 h at 90 and 150 °C. The abundance of accessible coordinatively unsaturated Co ³⁺ , active oxygen species, and surface defects on the polycrystalline Co ₃ O ₄ {112} nanosheets are responsible for the experimentally observed high catalytic activity.