In this study, hafnium oxide nanoparticles with different sizes and morphologies were synthesized under sub- and supercritical hydrothermal conditions and with and without organic surface modifiers. Transmission electron microscopy images showed that various shapes such as trapezohedron, oval-like and irregular shaped particles with sizes from 8 to 20 nm were obtained depending on the reaction conditions. The synthesized particles had monoclinic crystal structures as evidenced by their X-ray diffraction patterns. Chemical bonding of organic surface modifiers and their density on the particle surfaces were evaluated by Fourier transform infrared spectroscopy and thermogravimetric analysis. Furthermore, the effects of sub- and supercritical water state as the reaction medium and the presence of organic surface modifiers with different functional groups on the size and morphology of the particles are discussed.
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