Hydrothermal and Solvothermal Methods

An alternative approach to the use of high-temperature solvents, which can be both toxic and expensive, is to use more usual solvents conventionally limited by their rather low boiling points. However, solvents can be used well-above their boiling point at atmospheric pressure if heated in a sealed vessel (an autoclave or bomb ) the autogenous pressure then far exceeds the ambient pressure raising the boiling point of the solvent. Such solvothermal reaction conditions are extensively used in the preparation of inorganic solids, especially zeolites [64]. Eor a comprehensive review of this method for nanopartides see [65]. 

Cu2xSe particles vere obtained by Qian and coworkers [69] starting from Cut, 

and ethylenediamine, with T = 90 °C, t = 4 h. The particles are spherical and quite monodisperse. Qian and coworkers have also reported [70] a solvothermal preparation of CuInSe2, obtaining 15 nm particles from CuCh, InCh and Se in either ethylenediamine or diethylamine, at 180 °C for 15 h in ethylenediamine, and 36 h for diethylamine. CuInSe2 S [71] also has been prepared by Qian and coworkers using InCl3, CuCl2, S and Se with ethylenediamine as the solvent. 

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Another feature of hydro(solvo)thermal synthesis is the operability and tunability of hydrothermal and solvothermal chemistry, which bridges the synthetic chemistry and physical properties of synthesized materials. With deepening studies on hydrothermal and solvothermal synthesis chemistry, more and more reaction types have been discovered. Compared with other synthesis and preparation techniques, hydro(solvo)thermal synthesis methodology and techniques have irreplaceable advantages. So far, a variety of materials and crystals used in many fields could be hydrothermally or solvothermally synthesized, and the quality and properties of the resulting products are often much better than those prepared by other methods. 

In this book, we briefly examine the different types of reactions and methods employed in the synthesis of inorganic solid materials. Besides the traditional ceramic procedures, we discuss precursor methods, combustion method, topochemical reactions, intercalation reactions, ion-exchange reactions, alkali-flux method, sol-gel method, mechanochemical synthesis, microwave synthesis, electrochemical methods, pyrosol process, arc and skull methods and high-pressure methods. Hydrothermal and solvothermal syntheses are discussed separately and also in sections dealing with specific materials. Superconducting cuprates and intergrowth structures are discussed in separate sections. Synthesis of nanomaterials is dealt with in some detail. Synthetic methods for metal borides, carbides, nitrides, fluorides, sili-cides, phosphides and chalcogenides are also outlined. 

Different methods have been used for the preparation of TiOa, such as hydrothermal methods [134], sol-gel processes [135], emulsion precipitation [136], and solvothermal methods [137,138],... 

Synthetic methods of preparing materials are broadly divided into two categories physical and chemical methods, and include coprecipitation, hydrothermal and solvothermal, sol-gel, microemulsion, microwave, sonochemical, impregnation, combustion, ball milling and so on. There are several parameters, which influence the size and morphology of the finished material.By controlling these parameters it is possible to make well-defined ceria-based oxides as shown in Fig. 8.1. 

It should also be briefly recalled that semiconductors can be added to nanocarbons in different ways, such as using sol-gel, hydrothermal, solvothermal and other methods (see Chapter 5). These procedures lead to different sizes and shapes in semiconductor particles resulting in different types of nanocarbon-semiconductor interactions which may significantly influence the electron-transfer charge carrier mobility, and interface states. The latter play a relevant role in introducing radiative paths (carrier-trapped-centers and electron-hole recombination centers), but also in strain-induced band gap modification [72]. These are aspects scarcely studied, particularly in relation to nanocarbon-semiconductor (Ti02) hybrids, but which are a critical element for their rational design. 

An important point is that the precursor gels thus prepared contain significant amounts of water, even if the gels are dried by some suitable method. One of the weak points of solvothermal crystallization may be difficulty in controlling the water content in the precursor gel. Water facilitates hydrothermal crystallization of the precursor gel, and therefore the essential chemistry here may be hydrothermal. Actually, solvothermal crystallization usually requires higher temperatures and more prolonged reaction times than hydrothermal crystallization. Moreover, in the solvothermal crystallization of stabilized zirconia, the presence of an adequate amount of water was reported to be so critical in dissolving the oxide powder that no crystallization occurred in absolute alcohol. " ... 

Recently, Rajamathi and Seshadri [65] have reviewed the uses of solvothermal methods for the preparation of oxide and chalcogenide nanoparticles. For oxide nanoparticles, these methods can involve hydrolysis, oxidation and thermolysis, all performed under hydrothermal or solvothermal conditions. Some of the more striking examples are provided here. 

Fig. 7.5. TEM images of InP nanocrystals prepared by hydrothermal (a) and solvothermal (b) methods.

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