Green solvent for green materials: a supercritical hydrothermal method and shape-controlled synthesis of Cr-doped CeO ₂ nanoparticles

Abstract

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 CeO(2) 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 CeO(2) nanoparticles with sizes of 15-30 nm and cubic Cr-doped CeO(2) nanoparticles with sizes of 5-8 nm. Octahedral Cr-doped CeO(2) nanoparticles exposing (111) facets and cubic Cr-doped CeO(2) 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.

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

“…The rapid reaction owing to high supersaturation causes a metastable or non-equilibrium state in supercritical water. The formation of unique structures such as metastable, highly doped, and deficient structures has been reported (Hakuta Y. et al, 1998;Yamamoto K. et al, 2014Yamamoto K. et al, , 2015Zhu Y. et al, 2015;Yoko A. et al, 2014Yoko A. et al, , 2016aYoko A. et al, , 2016bYoko A. et al, , 2017. Barium hexaferrite could be synthesized in approximately 1 s in a continuous flow reactor with excessive barium (Hakuta Y. et al, 1998).…”

“…The synthesized barium hexaferrite was transformed to barium di-ferrite and barium mono-ferrite after it was treated in batch reactors under the same temperature and pressure as the synthesis conditions, suggesting that the synthesized barium hexaferrite phase was a non-equilibrium state. Recently, Gd-doped ceria nanocubes (Yamamoto K. et al, 2014(Yamamoto K. et al, , 2015 and Cr-doped ceria (Zhu Y. et al, 2015) were synthesized by the supercritical hydrothermal method. High doping concentration, including nonequilibrium composition, can be achieved through rapid reactions in a flow reactor.…”

“…It was found that OSC was significant at temperatures as low as 150 °C, whereas oxygen mobility was observed for ordinary, non-facet-controlled CeO 2 at approximately 400 °C (Zhang J. et al, 2011a). In addition, 5 mol% Cr-doped CeO 2 nanocubes were synthesized, exhibiting higher OSC than undoped CeO 2 nanocubes (Zhu Y. et al, 2015). As explained in Section 4, the supercritical hydrothermal method can be applied for high-concentration doping of metal oxide nanoparticles to further improve these nanocatalysts.…”

A techno-economic assessment of the potential for combining supercritical water oxidation with ‘in-situ’ hydrothermal synthesis of nanocatalysts using a counter current mixing reactor

“…A series of experiments was carried out to investigate whether the addition of various metal salt solutions would impact on COD reduction. In principle these metal salts form nanoparticles, via the SCWHS process [52]. These newly formed metal oxides nanoparticles would then act as catalysts during the decomposition of acrylic acid.…”

Green Synthesis of Carbon Quantum Dots Doped on Nickel Oxide Nanoparticles as Recyclable Visible Light Photocatalysts for Enhanced Degradation of Malachite Green

“…As you know, the most environmentally benign solvent is water. Hydrothermal synthesis is a method that uses water as a solvent to synthesize nanomaterials [7] . Also, in this method because of using renewable carbon sources at low reaction temperatures, carbonization and functionalization processes can occur in a single step [8] .…”