Large scale hydrothermal synthesis of β-Co(OH)2 hexagonal nanoplates and their conversion into porous Co3O4 nanoplates

β-Co(OH)2 hexagonal nanoplates with a diameter of 200–400 nm and a thickness of 30–50 nm have been synthesized successfully by a template and surfactant-free hydrothermal route in a very simple system composed only of water, CoCl2 and NaOH, and many of them are regular hexagons with the angles of adjacent edges of 120°. This method is high yield, simple and environmentally benign. According to the systematical investigation of the reaction parameters (temperature, time and the concentration of NaOH), the optimal conditions to β-Co(OH)2 hexagonal nanoplates were concluded, including the temperature of 120 °C, the hydrothermal time of 3 h and the NaOH dosage of 5 mmol. A dissolution–recrystallization mechanism was put forward. Furthermore, the as-obtained Co(OH)2 hexagonal nanoplates can be easily converted into porous Co3O4 nanoplates by calcining Co(OH)2 nanoplates in air at 500 °C for 2 h.