Continuous hydrothermal synthesis of Fe2O3, NiO, and CuO nanoparticles by superrapid heating using a T-type micro mixer at 673 K and 30 MPa

Continuous hydrothermal synthesis of Fe2O3, NiO, and CuO nanoparticles from starting solutions of metal nitrates was carried out at 673 K and 30 MPa with a flow-through apparatus. A T-type micro mixer was used for superrapid heating of the starting solutions. Residence time was varied from very short time of 0.002–2 s. Conversion of metal nitrates to products, average particle size (APS), crystal phase and crystallinity were characterized by ICP, TEM, XRD and TG, respectively. In the Fe2O3 synthesis, the conversion became 97.9% at shortest residence time of 0.002 s and APS slightly increased from 4.0 to 6.7 nm with increasing residence time. In the NiO synthesis, the conversion was 11.4% at 0.002 s and increased to 70.1% at 2 s. In parallel, APS increased from 15.4 to 18.7 nm. In the CuO synthesis, Cu(NO3)2 was not converted to CuO at 0.002 s. The conversion and APS sharply increased with increasing residence time to 0.257 s. After that, the conversion, and APS gradually increased from 25.6 to 42.5% and from 28.7 to 34.3 nm, respectively. Mechanism of particle nucleation and growth at different metal species was discussed on the basis of the experimental results and the metal oxide solubility.