Supersaturation of aqueous species and hydrothermal crystal growth of ZnO

• ZnO was hydrothermally prepared at 200 °C with EDTA for pH 8–12.
• Supersaturation was calculated and examined for zinc complex ions.
• Optical microscopy and digital size analysis were performed.
• pH 10–11 needle to prismatic habit correlate with supersaturation changes.
• [001] growth rates correlate with supersaturation changes of specific zinc ions.

Synthesis of ZnO crystals prepared with zinc acetate or chloride, disodium dihydrogen ethylenediaminetetraacetate (EDTA), potassium hydroxide and sodium triflate at 200 °C and variable pH 8–12 is reported. Crystals were imaged and size-analyzed with optical microscopy. Using aqueous speciation modeling software, supersaturation dependence on pH was calculated for five zinc species—Zn2+, Zn(OH)+, Zn(OH)2, Zn(OH)3–and Zn(OH)42–—to investigate connections between predominate crystal habits at different pH and dominant aqueous species. For zinc acetate and chloride systems, the zinc species with highest supersaturation was Zn(OH)42– throughout the pH 8–12 range, and the second highest was Zn2+ or Zn(OH)3–, with a crossover pH of 10.2–10.4 depending on counterion. The prominence of the tetrahydroxyl zinc species in ZnO crystal growth is supported by these calculations, and total supersaturation is inversely proportional to average crystal sizes, as expected. Optical microscopy and size analysis on products revealed crystals with a needle or prismatic habit throughout the studied pH range, and the change in aspect ratio correlates with supersaturation changes for the Zn2+ in this pH range, thus suggesting that growth rates along the [001] crystallographic direction are affected by small concentration changes of this ion.