Morphological Control of Hydrothermal Ni(OH)2 in the Presence of Polymers and Surfactants: Nanocrystals, Mesocrystals, and Superstructures

Polymers with different hydrophilic groups [polyvinylpyrrolidone (PVP), ammonium polyacrylate (APA), and hydroxypropylmethyl cellulose (HPMC)] and surfactants [cetyltrimethylammonium bromide (CTAB) and sodium dodecylbenzensulfonate (SDBS)] were used as additives to modify the crystallization of β-Ni(OH)2 in hydrothermal conditions. Marked morphological changes in the β-Ni(OH)2 particles were observed depending on the additive concentration and on the duration of the hydrothermal treatment. The final morphology is the result of a complex, time-dependent self-assembly and growth process. Well-defined particles with sizes from submicrometer range to a few micrometers corresponding to hexagonal lamellae, hexagonal tabular mesocrystals, rosette- and flowerlike aggregates of lamellae, hexagonal prismatic mesocrystals, and acicular nanocrystals were easily obtained after a short time (2−24 h) aging at 150−200 °C. With PVP and CTAB, there is evidence of a growth process dominated by self-assembly of nanocrystals to produce mesocrystals. The formation of spherical superstructures (with SDBS, up to 70 μm in diameter) and hollow spheres (with PVP) is observed at long times (>24 h) as a result of solvent-mediated recrystallization processes, like Ostwald ripening. The overall results show that hydrothermal synthesis of β-Ni(OH)2 in the presence of polymers with hydrophilic groups and surfactants is a versatile tool for crystal morphogenesis.