Ultrathin, Polycrystalline, Two-Dimensional Co3O4 for Low-Temperature CO Oxidation

Abstract

Free-standing, hierarchical, ultrathin, and two-dimensional (2D) polycrystalline Co3O4 flowers were synthesized by a hydrothermal and topotactic transformation process. Aberration-corrected electron microscopy study of both the CoOx precursor structure and the subsequent topotactic transformation processes revealed the nucleation and growth mechanisms of the 2D polycrystalline Co3O4 nanosheets. The free-standing flower-shaped CoOx 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 CoOx nanocrystallites structurally transformed into ultrathin 2D Co3O4 nanoplates with thicknesses ranging from 1 to 5 nm (average thickness ∼2.4 nm). The final, free-standing, ultrathin 2D polycrystalline Co3O4 flowers possess a BET surface area of 138 m2/g. Statistical structural analyses revealed that the exposed surfaces of the Co3O4 flowers are d...