Continuous Flow Synthesis of Nanoparticles Using Supercritical Water: Process Design, Surface Control, and Nanohybrid Materials

Abstract

A continuous flow reaction process in which a metal salt solution is rapidly mixed with high-temperature water was employed to achieve rapid heating up to supercritical conditions. A quarter of a century has passed since the supercritical hydrothermal method was first proposed. This paper introduces recent advances in science and technology related to the supercritical process. Process design, kinetics, reaction atmosphere (redox) control, morphology control, organic modification of particles, nanocatalysts, and organic-inorganic hybrid materials are reviewed for promising applications of the supercritical process.

Highly Cr-Substituted CeO2Nanoparticles Synthesized Using a Non-equilibrium Supercritical Hydrothermal Process: High Oxygen Storage Capacity Materials Designed for a Low-Temperature Bitumen Upgrading Process

“…According to the reaction temperature, the crystal size tends to decrease as the temperature increases from 300 to 400 °C because crystal growth occurs predominantly under subcritical conditions, where solubility of metal ions is higher than that in the supercritical state. By contrast, a drastic decrease in the solubility of metal ions and numerous nucleation under supercritical conditions results in smaller particles. − From the viewpoint of Cr substitution, because the Cr 3+ ions (61.5 Å) are much smaller than Ce 4+ (97 Å), the crystal lattice decreases and the XRD peak shifts in the high-angle direction. , …”

“…As described in this paper, the synthesis of highly Cr-substituted CeO 2 nanoparticles (Cr-CeO 2 NPs) is demonstrated using a supercritical hydrothermal method, − including the subcritical region. A continuous flow reactor that can provide a non-equilibrium reaction was used to improve the Cr-substitution concentration significantly in CeO 2 .…”

Continuous-flow syntheses of alloy nanoparticles

“…In total, oxide, metallic, and semiconductor NPs have widely been studied and have occupied about 45%, 32% and 7% of the reports, respectively. In the oxide NPs, flow syntheses using supercritical water are well studied 23 because supercritical water (>374 °C and >22.1 MPa) has unique features such as a very low dielectric constant leading to the formation of metal oxides. In particular, the syntheses of FeO x , TiO x , ZnO x , and CeO x for applications including catalysis, magnetics, and ion conductors have been investigated.…”

Fabrication of Liquid Scintillators Loaded with 6-Phenylhexanoic Acid-Modified ZrO2 Nanoparticles for Observation of Neutrinoless Double Beta Decay

“…Organic molecules are miscible with water at high temperatures under sub/supercritical conditions, which allows the surface modification of nanoparticles [ 18 , 19 , 20 ]. To date, many studies have been conducted toward the sub/supercritical hydrothermal synthesis of surface-modified metal oxide nanoparticles [ 20 , 21 , 22 , 23 ].…”