We propose a new method to synthesize organic-inorganic hybrid nanoparticles at supercritical hydrothermal conditions. By introducing organic legands (aminoacids, carboxylic acids, amines, alcohols, aldehydes etc.) into supercritical hydrothermal synthesis atmosphere, organic - inorganic hybrid nanocrystals were synthesized. Particle size was in the range from 2.5 nm to 10 nm, and particle size dispersion was extremely narrow. Crystal shape can be controlled to be sphere, nano-cube etc. by changing concentration of organic modeifiers. By selecting a proper modifier (affinity with the solvent), particles could be dispersed perfectly in organic solvents or in aqueous solutions. This implies a variety of applications of the nanoparticels including nanohybrid polymers, nano-ink, nano-paints, and bio-imaging probe. In this paper, mechanism of the reaction was discussed. Important is homogeneous phase formation for the organic substance and metal salt aqueous solutions at supercritical conditions. Through the experiments of changing pH, it was found that the matching of surface charge of the nanoparticles and the dissociation of organic molecules under supercritical conditions was essential for the surface modification.
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