Green solvent for green materials: a supercritical hydrothermal method and shape-controlled synthesis of Cr-doped CeO2 nanoparticles

This paper describes a supercritical hydrothermal synthesis method as a green solvent process, along with products based on this method that can be used as green materials that contribute to solving environmental problems. The first part of this paper summarizes the basics of this method, including the mechanism of the reactions, specific features of the supercritical state for nanoparticle synthesis, the continuous flow-type reactor and applications; this provides a better understanding of the suitability of this method to synthesize green materials. The second part of the paper describes the method used to synthesize Cr-doped CeO2 nanoparticles, which show an extremely high oxygen storage capacity, suggesting their high potential as an environmental catalyst. Transmission electron microscopy and scanning electron microscope images showed octahedral Cr-doped CeO2 nanoparticles with sizes of 15-30 nm and cubic Cr-doped CeO2 nanoparticles with sizes of 5-8 nm. Octahedral Cr-doped CeO2 nanoparticles exposing (111) facets and cubic Cr-doped CeO2 nanoparticles exposing (100) facets were determined by high-resolution transmission electron microscopy and selected area electron diffraction. The X-ray diffraction peaks shifted to a high angle because the radius of the Cr ion is smaller than that of the Ce ion.