Anisotropic Crystal Growth of Layered Nickel Hydroxide along the Stacking Direction Using Amine Ligands

Edge surfaces of two-dimensional crystals play crucial roles in their properties, such as intercalation behavior and catalytic activities; however, reports on the preparation of crystals with a high aspect ratio of thickness to lateral size, typically a prism-like crystal morphology composed of stacked layers, are scarce. We report the anisotropic crystal growth of β-Ni(OH)₂ along the stacking direction using bidentate amine ligands, which act as both the base and the reservoir of Ni²⁺ through the formation of Ni-diamine complexes. Various characterization results of the crystal structure, composition, and crystal orientation indicate the formation of hexagonal prisms of β-Ni(OH)₂ with an unusually high aspect ratio of the thickness to the lateral size higher than 1. A systematic investigation focusing on the molar ratio of amine ligands to Ni²⁺, the concentration of Ni-diamine complexes, and stability constants of the complexes revealed that anisotropic growth was promoted when the supersaturation was relatively high and was maintained constant for a long time. We clarified the role of amine ligands in controlling supersaturation through the controlled release of metal ions from stable complexes. β-Co(OH)₂ with a hexagonal prism shape was prepared using this protocol. This study provides valuable indications for developing synthetic chemistry for various layered compounds to achieve a controlled aspect ratio.