Nobuaki Aoki

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.

Supercritical Hydrothermal Synthesis

This chapter describes specific features of a supercritical hydrothermal synthesis method. First, the some characteristic properties of supercritical water are summarized, and then the mechanism of supercritical hydrothermal synthesis is explained. Higher reaction rate and lower solubility are the key factors to synthesize nanosize crystals in a short reaction time. The flow reaction system to achieve rapid heating is explained. Some commercialized process is introduced. This method is useful to synthesize organic molecule modified nanoparticles (NPs). Since the organic molecules and metal–salt aqueous solutions are miscible under the supercritical state, and water molecule works as an acid/base catalyst for the reactions, organic–inorganic conjugate NPs can be synthesized under the condition. The mechanism of the conjugate forming reaction is explained. NPs’ superlattice structure and bioconjugate materials are also formed under the condition. The hybrid NPs show high affinity for the organic solvent or the polymer matrix, which leads to fabricate the organic–inorganic hybrid nanomaterials with the trade-off function (superhybrid nanomaterials). One of the applications is to fabricate a heat-conductive flexible hybrid-polymer sheet. By the surface modification of BN particles by supercritical method, affinity of BN and polymers could be improved so that high BN content of hybrid materials, thus high thermal conductivity materials, could be synthesized.

Environmentally Benign Route for Nanomaterial Synthesis by Using SCW

Nanomaterials are known for their unique mechanical, chemical, physical, thermal, electrical, optical, electronic, magnetic, and specific surface area properties. Supercritical fluid technology is also expected to contribute to new materials synthesis with green sustainable chemistry, especially tonanomaterials. In this chapter, we first introduce the principles of supercritical hydrothermal synthesis. Second, designing the reactor, especially for rapid mixing and precise temperature control, is explained. Then, we summarize some specific features of supercritical hydrothermal synthesis method. This method is extended to organice inorganic hybrid NP synthesis. This provides the fabrication of “superhybrid nanomaterials”.