Crystal formation and growth during the hydrothermal synthesis of β-Ni(OH)₂ in one-dimensional nano space

Hydrothermal synthesis of β-Ni(OH)₂ was performed inside uniform carbon-coated nanochannels of an anodic aluminium oxide film. The time course of crystal formation and growth of Ni(OH)₂ in such one-dimensional nano space was observed using transmission electron microscopy (TEM), and the changes in the number and size of crystals with the hydrothermal reaction period were quantitatively analyzed using the TEM images. Moreover, the effect of the channel size (25, 100 and 300 nm in diameter) on the crystal growth was examined. In the early stage of the reaction, the crystal formation and growth of β-Ni(OH)₂ in the one-dimensional channels took place in the same manner as in conventional hydrothermal synthesis. However, except for the 300 nm-channels, further crystal growth was hampered by the spatial restriction, and it allowed only the growth toward the channel axis. In the case of the 25 nm-channels, many Ni(OH)₂ crystals of less than 40 nm formed initially, but slowly disappeared except for a few that grew larger at the expense of the small crystals. This finding clearly indicates that the crystal growth of Ni(OH)₂ during the whole hydrothermal process was governed by the Ostwald ripening. With this mechanism and the spatial restriction, single crystals of β-Ni(OH)₂ nanorods with a length of over 150 nm were finally formed.