Hydrothermal growth of ZnO nanostructures in supercritical domain: Effect of the metal salt concentration (Zn(NO3)2) in alkali medium (KOH)

• A continuous process of hydrothermal synthesis is used to prepare ZnO powders.
• The effect of [Zn(II)] on the ZnO crystallite characteristics is studied.
• The ZnO crystal is considered as a cylindrical crystallite (XRD, Loüer method).
• Growth units and oriented attachment concepts are taken into account.
• A schematic synoptic of ZnO crystal growth (size and morphology) is suggested.

The metal salt concentration effect on the size and morphology of ZnO NPs was highlighted through its synthesis thanks to a continuous one-step method at 401 ± 15 °C and 306 ± 8 bar. Experiments were performed from Zn(NO3)2 and KOH as reactants in concentration ranges of 10–480 mM and 40–1920 mM, respectively. A constant [KOH]/[Zn(NO3)2] ratio of 4 was fixed in order to maintain a constant pH value between 12.5 and 13.0. The as-prepared NPs were characterized by X-ray diffraction and (high-resolution) transmission electron microscopy. Based on Loüer's method, ZnO crystal exhibiting a hexagonal structure was considered as cylinder with a diameter D and a height H. The D/H parameter has already been used to observe the change of ZnO crystallite shape and correlated with (HR)TEM observations. The evolutions of crystal size and morphology according to [Zn(NO3)2] in alkali medium are investigated and the particle growth mechanisms in supercritical water are discussed.

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