Hydrothermal or solvothermal methods

Another method somewhat related to crystal growth is hydrothermal or solvothermal (if another solvent than water is used) reactions [1]. It involves heating the reactants and a suitable solvent in an sealed autoclave ( bomb ) for a long time (hours to days). The pressure will built up and the solvent thus can be heated well above its normal boiling point. The advantages are several  

We might also add that for some systems in situ generated ligands are possible, [41] and may give products different from normal synthesis. 

Currently, it may be hard to rationalise all differences between normal synthesis and solvothermal methods, but it has for example been argued that when there is competition between ligand bridge formation and water metal binding the T-AS term favours free water compared to coordinated water at higher temperatures [40], 

Solvothermal methods have successfully been applied to the synthesis of molecule-based 3D nets for some 10 years, one of the first examples being the diamond nets of K2[M(2,3-pyridinedicarboxyolate)2] (M=Mn, Zn) [42], and the number of reports are rising and we here cite some recent work [27,43-53], 

This method has some resemblance to the microwave methods currently of much interest in organic synthesis where sealed vessels are heated by microwaves [54,55]. It would probably be interesting to use this technique also for hydrogen bonded or coordination polymer 3D-nets. 

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TABLE 7.2 Nanorods Synthesized by Hydrothermal or Solvothermal Methods... 

Haase et al. obtained the monodisperse fluorescent NPs of doped LaP04 through hydrothermal reactions, solvothermal methods, or precipitation in high-boiling solvents (Meyssamy et al., 1999 Riwotzki et al., 2000,2001). Caruso et al. demonstrated biofunctionalization of nano-sized... 

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. 

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. 

Hydrothermal or solvothermal reactions are also an important method for the synthesis of unconventional zeolite-like compounds. The reagents are usually dissolved or dispersed in water and then heat-treated at elevated temperatures in an autoclave. For reactions at moderate temperatures up to about 250 °C the autoclaves are normally lined with Teflon. If higher reaction temperatures are desired, the reactions are, for instance, carried out in stainless steel or gold tubes. However, in favorable cases, the use of higher temperatures might not even be necessary. The products sometimes just precipitate from solution at room temperature. 

Inorganic nanotubes, particularly oxide nanotube structures, have been synthesized via chemical routes without the use of a sacrificial template or an additional deposition process. Hydrothermal and/or solvothermal synthetic methods are very simple means of producing nanotube structures. Hydrothermal synthesis is usually accomplished at a high pressure and high temperature using an autoclave containing aqueous solutions that consist of an inorganic precursor... 

The wide applicability of the hydrothermal method has already been stressed. The method was extended to other solvents in recent years and is now frequently characterized more generally as the solvothermal approach. Since it has been observed that even such inert materials like C02 or even Xe act as quite good solvents under supercritical conditions, an obvious extension was to evaluate fluorides such as NF3, SF6, AsF5, and, especially, HF under comparable conditions [322], NF3 was regarded as particularly interesting from a synthetic point of view, as it is chemically inert at ambient conditions, but above about 200°C it becomes a rather strong... 

In many cases, anhydrous metal oxides have been prepared by solvothermal treatments of sol-gel or micro-emulsion-based precursors. Wu and coworkers prepared anatase and rutile Ti02 by a micro-emulsion-mediated method, in which the micro-emulsion medium was further treated by hydrothermal reaction [171]. This micro-emulsion-mediated hydrothermal (MMH) method could lead to the formation of crystalline titania powders under much milder reaction conditions. 

In preparing fine particles of inorganic metal oxides, the hydrothermal method consists of three types of processes hydrothermal synthesis, hydrothermal oxidation, and hydrothermal crystallization. Hydrothermal synthesis is used to synthesize mixed oxides from their component oxides or hydroxides. The particles obtained are small, uniform crystallites of 0.3-200 jim in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fme oxide particles can be prepared from metals, alloys, and intermciallic compounds by oxidation with high temperature and pressure solvent, that is, the starting metals are changed into fine oxide powders directly. For example, the solvothermal oxidation of cerium metal in 2-mcthoxycthanol at 473-523 K yields ultrafine ceria particles (ca 2 nm). 

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