Synthesis of Nickel Hydroxide by Homogeneous Alkalinization. Precipitation Mechanism

Uniform microcrystalline nickel(II) hydroxide particles have been prepared via the homogeneous alkalinization of nickel(II) nitrate solutions by urea hydrolysis. The nature of the precipitated solids depends on the temperature of synthesis. At ca. 423 K, i.e., under the hydrothermal conditions attained by microwave heating, β-Ni(OH)2 forms upon the fast ripening of α-Ni(OH)2. In the range 363−343 K, α-Ni(OH)2 is always the final product. The evolution of the systems during the precipitation of α-Ni(OH)2 is analyzed in terms of the kinetic factors that control nucleation and growth. A precipitation mechanism, based on the principle of minimal structural change, is proposed; it is suggested that the edge-on condensation of Ni4(OH)44+ tetramers is a key step in the sequence of events that conduct from hexaaquo Ni2+ to α-Ni(OH)2.