Controlling Low Temperature Aqueous Synthesis of ZnO. 1. Thermodynamic Analysis

A thermodynamic model based on published data is presented for calculating the solubility and speciation of ZnO in aqueous solutions as a function of pH, ammonia concentration, and temperature. These calculations are used to explain how solubility and speciation control nucleation and growth behavior from an aqueous solution. The nucleation and growth behavior, in turn, can lead to the different morphologies of epitaxial and particulate ZnO obtained experimentally using low temperature aqueous methods. The calculations indicate that the presence of the zinc amine complex, Zn(NH3)4 2+, causes ZnO to display a substantial decrease in solubility when the temperature is raised from room temperature to 90 °C. This retrograde solubility is the basis for a novel continuous circulation reactor for synthesizing ZnO, which is presented in part 2 of this series.