The nickel hydroxides (β-Ni(OH)2) with different shapes such as hexagonal nanosheets, irregular nanosheets, and nanoparticles were synthesized in the absence of SO4 2− ions and in the presence of 0.00−3.00 mmol of the added NaOH using a hydrothermal method. The β-Ni(OH)2 phase with brucite-type structure was confirmed by an energy-dispersive spectrum (EDS), Fourier transform infrared spectra (FTIR), and powder X-ray diffraction (XRD). The effect of the free OH− ions and the SO4 2− ions in the hydrothermal system on morphologies of the Ni(OH)2 products was investigated in detail. In the absence of SO4 2− ions system, when the added NaOH amount is less than 0.02 mmol, the irregular thin nickel hydroxide nanosheets with thickness of about 20−50 nm were obtained; when the added NaOH amount is between 0.35 and 0.55 mmol, the products have the regular hexagonal morphology with a width of 150−500 nm and thickness of 40−80 nm; while when the added NaOH amount is 1.50−3.00 mmol, the nickel hydroxide products became polyhedral nanoparticles with an average diameter of ca. 50−90 nm. The Ni(SO4)0.3(OH)1.4 nanowires or α-Ni(OH)2 nanowires containing the intercalated SO4 2− anions can only be obtained in the presence of SO4 2− ions. They have lengths of several micrometers and widths of 20−30 nm. A possible growth mechanism of the nanosheets, nanoparticles, and nanowires is suggested.

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