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Hydrothermal synthesis of metal oxide (AlOOH/Al 2 O 3 , CuO, Fe 2 O 3 , NiO, ZrO 2 ) nanoparticles from metal nitrate aqueous solution was carried out at 673 K and pressures ranging from 25 MPa to 37.5 MPa with a flow-through supercritical water method. Size, phase and crystallinity of the obtained particles were characterized by TEM, XRD and TG, respectively. Effect of the difference of the metals in starting materials, pressures and concentrations on particle size and crystallinity was analyzed on the basis of supersaturation, which was evaluated by estimated metal oxide solubility. The result suggests that supersaturation should be set to higher than around 10 4 in this method to obtain particles under 10 nm in diameter. Further, crystallinity of the obtained particles was evaluated as weight loss through TG analysis. It was found that higher supersaturation decreased the crystallinity. This result can be explained that high supersaturation led to the inclusion of water molecules during the formation of particles.

The solubilities of lead oxide (PbO) and copper oxide (CuO) in subcritical and supercritical water were measured at temperatures from 250 °C to 500 °C and pressures from 26 MPa to 34 MPa, in a flow-type apparatus. The solubility of lead oxide varied from 351 μmol/kg H2O at 424.9 °C and 25.9 MPa to 4406 μmol/kg H2O at 350.3 °C and 30.2 MPa. The solubility of copper oxide varied from 1.3 μmol/kg H2O at 449.8 °C and 28.0 MPa to 8.0 μmol/kg H2O at 324.9 °C and 28.1 MPa. A hydration reaction model was applied to correlate the data.

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