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.
Ferrite nanoparticles from Fe(NO3)3 and Me(NO3)2 (Me = Ni, Cu, Zn) aqueous solutions could be synthesized
continuously with a flow reactor at 673 K, 30 MPa, and 4 s residence time. The particles were characterized
by TEM, XRD, and ICP to obtain sizes, crystal structures, lattice parameters, and Me/Fe molar ratios. Solid-solution nanoparticles of MeFe2O4 and γ-Fe2O3 with a cubic spinel structure and an average particle size
under 10 nm were obtained. Conversion of Fe3+ was more than 0.97 at the given residence time, and conversions
of Me2+ increased from 0 to 0.62 with increasing residence time. The particle size increased with increasing
residence time, and no significant difference in divalent cations was observed. The Me/Fe molar ratio in the
obtained solid-solution nanoparticles of MeFe2O4 and γ-Fe2O3 increased with increasing residence time and
KOH molality. The Me/Fe molar ratio also increased in the order Zn < Cu < Ni at a given residence time,
and this trend could be explained on the basis of ZnO, CuO, and NiO solubilities. The increase of the size
and the Me/Fe molar ratio in the obtained particles with increasing residence time shows that, in the formation
mechanism, primary solid-solution particles with low Me/Fe molar ratios probably nucleated and, after surface
dissolution of the particles, dissolved Fe3+ recrystallized on the surface with incorporation of Me2+.
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