Continuous synthesis of surface-modified zinc oxide nanoparticles in supercritical methanol

Continuous synthesis of surface-modified zinc oxide (ZnO) nanoparticles was examined using surface modifiers (oleic acid and decanoic acid) in supercritical methanol at 400 °C, 30 MPa and a residence time of ∼40 s. Wide angle X-ray diffraction (WAXD) analysis revealed that the surface-modified nanoparticles retained ZnO crystalline structure. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) showed that the surface modifiers changed drastically the size and morphology of the ZnO nanoparticles. When the molar ratio of oleic acid to Zn precursor ratio was 30, 10 nm size particles with low degree of aggregation were produced. The surface-modified ZnO nanoparticles had higher BET surface areas (29–36 m2/g) compared to unmodified ZnO particles synthesized in supercritical water (0.7 m2/g). Fourier transform infrared (FT-IR) and thermogravimetric analysis (TGA) indicated that aliphatic, carboxylate and hydroxyl groups were chemically attached on the surface of ZnO nanoparticles. Long-term (80 days) dispersion test using ultraviolet transmittance showed that the surface-modified ZnO particles had enhanced dispersion stability in ethylene glycol.