Nanopowders are normally produced by means of either high temperature hydrolysis reactions of various compounds directly in the autoclave or hydrothermal treatment of reaction products at room temperature; the latter case is based on the sharp increase in the rate of crystallisation of many amorphous phases in hydrothermal conditions. In the first case the autoclave is loaded with aqueous solution of precursor salts, in the second case – with suspension of products derived from solution reactions flowing under normal conditions. There is normally no need to use special equipment and maintain a temperature gradient.
Advantages of the hydrothermal synthesis method include the ability to synthesise crystals of substances which are unstable near the melting point, and the ability to synthesise large crystals of high quality. Disadvantages are the high cost of equipment and the inability to monitor crystals in the process of their growth. Hydrothermal synthesis can be effected both under temperatures and pressures below the critical point for a specific solvent above which differences between liquid and vapour disappear, and under supercritical conditions. The solubility of many oxides in hydrothermal solutions of salts is much higher than in pure water; such salts are called mineralizers. There is also a group of solvothermal synthesis methods, relational to hydrothermal methods; this group of methods is based on the use of organic solvents and supercritical CO2.
Substantial enhancement of the hydrothermal method facilitates the use of additional external factors to control the reaction medium during the synthesis process. As of now, this approach is implemented in the hydrothermal-microwave, hydrothermal-ultrasonic, hydrothermal-electrochemical and hydrothermal-mechanochemical synthesis methods.
One of the most widely known nanomaterials produced by the hydrothermal method are synthetic zeolites. A necessary condition for their production is the presence in the solution of some surface active agents (SAA) that actively influence morphological evolution of oxide compounds in hydrothermal solutions. The choice of synthesis conditions and type of surfactants can ensure the production of targeted porous nanomaterials with given pore size controlled in a fairly wide range of values.
Nanocrystalline TiO2 produced by the hydrothermal technique. Author: Bulat R. Churagulov, Lomonosov Moscow State University.
- Shlyakhtin Oleg A.
- Zaitsev Dmitry D.
- Churagulov Bulat R.
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