Ultrathin, Polycrystalline, Two-Dimensional Co₃O₄ 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.