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 Co3O4 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 (Na0.2(H3O)0.54CoO2·0.48H2O) by ion exchange between Na+ and H3O+, and by intercalation of water molecules in the as‐grown NaCoO2 crystals. The sodium cobalt oxyhydroxide crystals had spherical nanopores with diameters of ~5 nm. Single‐crystalline sodium cobalt oxyhydroxide, β‐NaxCo4+yCo3+1−yOOH1−x−y (x = 0.08, y = 0.07), with a width of ~5 mm and a thickness of ~100 μm, was successfully synthesized using a novel and simple method. These crystals were formed from sodium cobalt oxyhydrate. The sodium cobalt oxyhydroxide crystals had spherical nanopores with diameters of ~5 nm.