Hydrothermal Methods in Synthesis of Inorganic Materials

Hydrothermal synthesis is a bottom-up, liquid-phase synthesis method and a heterogeneous reaction, utilizing a water solvent at a temperature of >25 °C and a pressure of ≥1 atm to dissolve and to precipitate crystalline or amorphous materials or to get solutions by using a reflux, autoclave, or flow reactor. Microwave-assisted hydrothermal and supercritical flow reactors successfully reduced the synthesis times from hours or days to minutes and seconds. Substitutions for precursors, reductors, or stabilizer chemicals with plant extracts successfully created greener hydrothermal methods, but they still need relatively long times (hours) and high temperatures (>100 °C). The stronger critical perseptives include the inhibited standarization and reproducibility due to plant species variant, plant growth conditions, and plant extraction methods. The plant extract can’t substitute surfactant as mesoporous template or the organic solvents for water-organic sol-vent mixture, and it is possibly photodegraded by microwave. Strategies to reduce time and temperature by mechanical hydrothermal synthesis using plant extracts, with safety prioritized, utilizing non-toxic products, degradable products, and non-harmful reactants, are suggested for future research. One mechanistic question is still not resolved: how distiguish crystalization mechanism by using the temperature reduction method and by using temperature different method.