Shape-Tunable Co₃O₄ Nanocrystals Possessing Surface-Dependent Optical and Magnetic Properties

Small Co 3 O 4 cubes and cuboctahedra were synthesized at 110 and 150 °C in just 2 h, respectively. Size-tunable nanocubes were also prepared at 150 °C for 2 h. Despite their high crystalline quality as seen from X-ray and electron diffraction patterns, cubes and cuboctahedra formed at 150 °C present clear lattice deviations, while cubes synthesized at 110 °C show crystal perfection. Large amounts of Co 3 O 4 cubes, octahedra, cuboctahedra, and truncated octahedra can also be obtained. These Co 3 O 4 crystals exhibit both size- and facet-dependent absorption positions and band gaps. Remarkably, while smaller magnetization differences can be identified at 300 K, octahedra become much more magnetized than cubes and cuboctahedra at 2 K. Their Néel temperatures also vary. Octahedra have notably larger coercivity and retentivity values than cubes and cuboctahedra at 2 K. Thus, in addition to various facet-dependent properties of semiconductor materials, magnetic behaviors also reveal surface effects. This means that there is the potential to prepare more powerful magnets through surface control.