Hydrothermal Synthesis of ZnO Crystals from Zn(OH)2 Metastable Phases at Room to Supercritical Conditions

The originality of this work is to highlight the effect of temperature and pressure on the size and morphology of hydrothermal ZnO particles from ambient to supercritical conditions (T > 374 °C and P > 221 bar) using a unique continuous one-step process. Experiments were carried out from zinc nitrate (Zn(NO3)2) and potassium hydroxide (KOH) solutions in the ranges of 1–300 bar and 30–400 °C. The as-prepared particles of ZnO (flower, ellipsoid, and sphere) and ε-Zn(OH)2 (polyhedral) sized from nano to micrometers were characterized by X-ray diffraction and electronic microscopy. The wulfingite phase (ε-Zn(OH)2) was detected inside some powders especially at room temperature for higher pressures. The proportion of each phase was determined by the quantitative Rietveld analysis. On the basis of Loüer's method, ZnO crystals exhibiting a hexagonal structure were considered as cylinders with a diameter D and a height H. The D/H parameter, also known as the aspect ratio, was correlated with electronic microscopy observations to investigate the ZnO nanoparticle morphology evolution according to the temperature and pressure. Thus, a schematic synoptic summarizing these results is presented.