Synthesis and microstructure of single‐crystalline cobalt oxyhydroxide and topotactic transformation to cobalt oxide

ABSTRACT Plate‐like cobalt oxyhydroxide (β‐CoOOH, heterogenite) single crystals, with a width of ~5 mm and a thickness of ~100 μm, were successfully synthesized using a simple method of heating a mixture of Co 3 O 4 and NaOH in air at 650°C, and then soaking the resultant crystals in water at room temperature. The crystals were topotactically formed from sodium cobalt oxyhydrate (Na 0.2 (H 3 O) 0.54 CoO 2 ·0.48H 2 O) by ion exchange between Na + and H 3 O + , and by intercalation of water molecules in the as‐grown NaCoO 2 crystals. The sodium cobalt oxyhydroxide crystals had spherical nanopores with diameters of ~5 nm.