The method is based on the ability of water and aqueous solutions to dissolve at high temperatures (up to 500 ° C) and pressure (10-80 MPa, sometimes up to 300 MPa) substances that are practically insoluble under normal conditions — some oxides, silicates, sulfides. The main parameters of hydrothermal synthesis, which determine both the kinetics of the processes taking place and the properties of the products formed, are the initial pH value of the medium, the duration and temperature of synthesis, and the pressure in the system. Synthesis is carried out in autoclaves, which are sealed steel cylinders capable of withstanding high temperatures and pressure for a long time.
To obtain nanopowders, either reactions of high—temperature hydrolysis of various compounds directly in an autoclave or hydrothermal treatment of reaction products at room temperature are usually used - this uses a sharp increase in the crystallization rate of many amorphous phases under hydrothermal conditions. In the first case, an aqueous solution of precursor salts is loaded into the autoclave, in the second case, a suspension of reaction products in a solution carried out under normal conditions. There is usually no need to use special equipment and the presence of a temperature gradient.
The advantages of the hydrothermal synthesis method are the possibility of synthesizing crystals of substances unstable near the melting point, the possibility of synthesizing large crystals of high quality. As disadvantages, it is worth noting the high cost of equipment and the inability to observe crystals during growth.
Hydrothermal synthesis is possible both at temperatures and pressures below the critical point for a given solvent, above which the differences between liquid and vapor disappear, and under supercritical conditions. The solubility of many oxides in hydrothermal salt solutions is significantly higher than in pure water; the corresponding salts are called mineralizers. There is also a group of solvothermal synthesis methods related to hydrothermal, based on the use of organic solvents and supercritical CO2.
The application of additional external influences on the reaction medium during the synthesis process contributes to a significant expansion of the capabilities of the hydrothermal method. Currently, a similar approach is implemented in hydrothermal-microwave, hydrothermal-ultrasonic, hydrothermal-electrochemical and hydrothermal-mechanochemical synthesis methods.
One of the most well-known nanomaterials produced by the hydrothermal method is synthetic zeolites. A necessary condition for their production is the presence in solution of some surfactants (surfactants) that actively influence the morphological evolution of oxide compounds in hydrothermal solutions. The choice of synthesis conditions and the type of surfactant makes it possible to purposefully obtain porous nanomaterials with a given pore size, regulated within a fairly wide range.
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