Hydrothermal and Solvothermal Synthesis

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 0,m in size and dispersed each other. Pressures, temperatures, and mineralizer concentrations control the size and morphology of the particles. In the hydrothermal oxidation method, fine oxide particles can be prepared from metals, alloys, and intermetallic 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-methoxyethanol at 473-523 K yields ultrafine ceria particles (ca. 2 nm). 

The hydrothermal crystallization is the most popular technique in preparing ceria-based nanoparticles. Precipitation from aqueous solutions under elevated temperature and high pressure are involved in the process. Usually the hydrothermal crystallization is carried out as follows. An excess amount of precipitates is added to the cerium salt solutions. The precipitated gels are sealed in Teflon-lined autoclaves and hydrothermally treated at 423-573 K for several hours. The autoclaves are quenched and the crystalline powder products are washed and dried. Using the hydrothermal crystallization method, a number of ceria-based nanoparticles have been prepared as summarized in Table 3.2. The particle size clearly depends on the reaction temperature and the starting materials used. It is shown that by heating at low temperature and by using tetravalent cerium salt solutions smaller particles can be obtained. 

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With the wide use of hydrothermal and solvothermal synthesis, many polyoxogermanates with unique stmctures have been prepared. PolyoxoaUcoxovanadium germanate, (NH4)2[H2V9Ge8O26(L)6]-0.65H2O (H2T = HOCH2CH2-... 

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. 

Hydro(solvo)thermal synthesis chemistry focuses on the chemistry in preparation, synthesis, and assembly of special compounds or materials under hydro(solvo)thermal conditions. More importantly, hydrothermal or solvothermal synthesis routes can be used to prepare materials with special structures and properties, or phases, types, and morphologies which cannot be obtained by using solid-state reactions. In some cases, the materials can be obtained under mild conditions by using hydrothermal and solvothermal synthesis instead of under critical conditions by using a solid-phase reaction synthesis route. 

According to the reaction temperature, hydrothermal and solvothermal synthesis can be classified into subcritical and supercritical synthesis reactions. In subcritical synthesis, the temperature is in the range of 100 to 240 °C, while in supercritical synthesis, the temperature could reach 1000 °C and the pressure could reach 0.3 GPa. By using the special properties of solvent water and other reactants under supercritical high temperature and pressure, various syntheses with specific features could be conducted, resulting in the formation of numerous crystal materials with simple to very complex structures. In addition, it should be pointed out that some crystal materials cannot be obtained by using other preparation approaches except for using hydrothermal or solvothermal synthesis routes. 

So far, hydrothermal and solvothermal synthesis have been widely used in the 1) modification, 2) crystal growth and morphology control, 3) phase-transition study, and 4) discovery of new species of zeolites and porous materials. 

SYNTHESIS AND X-RAY CRYSTAL STRUCTURES OF LOWDIMENSIONAL BORATES FROM HYDROTHERMAL AND SOLVOTHERMAL SYSTEMS... 

To increase the crystallization rate and to alter the product phase, an alkaline mineralizer is sometimes added to the solvothermal reaction. Some researchers believe that, compared with the hydrothermal process, solvothermal synthesis allows the product to be free from foreign ions because the organic solution, having a low relative permittivity, is free from ionic species. When precursor gels are prepared from alkoxide, one can prepare products free of foreign ions. However, when the precursor gel is prepared by precipitation from salt solutions, or when alkali/acid mineralizer or ionic surfactant is added to the solvothermal crystallization system, the above statement is a myth. In fact, ions are easily adsorbed or occluded in the product particles because of the low dielectric constant of the organic solvent. 

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. 

A solvothermal process is one in which a material is either recrystallized or chemically synthesized from solution in a sealed container above ambient temperature and pressure. The recrystallization process was discussed in Section 1.5.1. In the present chapter we consider synthesis. The first solvothermal syntheses were carried out by Robert Wilhelm Bunsen (1811-1899) in 1839 at the University of Marburg. Bunsen grew barium carbonate and strontium carbonate at temperatures above 200°C and pressures above 100 bar (Laudise, 1987). In 1845, C. E. Shafhautl observed tiny quartz crystals upon transformation of freshly precipitated silicic acid in a Papin s digester or pressure cooker (Rabenau, 1985). Often, the name solvothermal is replaced with a term to more closely refer to the solvent used. For example, solvothermal becomes hydrothermal if an aqueous solution is used as the solvent, or ammothermal if ammonia is used. In extreme cases, solvothermal synthesis takes place at or over the supercritical point of the solvent. But in most cases, the pressures and temperatures are in the subcritical realm, where the physical properties of the solvent (e.g., density, viscosity, dielectric constant) can be controlled as a function of temperature and pressure. By far, most syntheses have taken place in the subcritical realm of water. Therefore, we focus our discussion of the materials synthesis on the hydrothermal process. 

Hydrothermal/solvethermal s)mthesis of RMO3 is also extensively adopted, by virtue of the low reaction temperature and well-crystallized products. Vazquez-Vazquez and Lopez-Quintela (2006) reported the solvothermal synthesis of Lai j-A MnOs (A = Ca, Sr, Ba) NPs in benzyl alcohol and acetophenone. The obtained precipitate was annealed to form crystalline products and acetophenone was found to be more suited to obtain clean perovskite phase. Zhu et al. (2008a) prepared single-crystalline YbMnOs and LuMnOa nanoplates via hydrothermal method. The products were found to be hexagonal phases. A possible formation mechanism was proposed, which involves the formation of ROOH phase as intermediate. 

Besides water for hydrothermal reactions, liquid ammonia (bp, 78°C Tc, 132°C Pc, 113 atm) is also used for the solvothermal synthesis of nitrides. Metastable or otherwise unobtainable nitride materials were reported to be formed by this rnethod. " Ammonium and amide (NH2) ions are the strongest acid and base, respectively, for the liquid ammonia system, and therefore ammonium salt acts as the acid mineralizer, while amide ion can be prepared by addition of alkali metals to the solvent. Since ammonia has a low boiling point, the reaction pressure is usually quite high. 

Solvothermal reactions in alcohols are sometimes called alcohothermal reactions this word is derived from alcoholysis based on the similarity between hydrothermal and hydrolysis. Alcohols are the most common solvents for sol-gel synthesis. Primary alcohols are fairly stable at higher temperatures (up to 360°C) and therefore are widely used for the solvothermal reactions." For example, amorphous gel derived by hydrolysis of metal alkoxides can be crystallized by solvothermal treatment in alcohols. Since lower alcohols (methanol, ethanol, and 1-propanol) are completely miscible with water, water molecules present in the precursor gel may be replaced with the solvent alcohols. Therefore the precursor gel is easily dispersed in the solvent, where crystallization takes place. Detailed mechanisms for the formation of crystals are not yet fully elucidated. Crystallization of metal oxides is usually reported to take place by dissolution-recrystallization mechanisms, but the mechanism seems to depend on the gel structure. Moreover, water molecules dissolved from the gel in the reaction medium may facilitate crystallization of the product. More discussion is given in Section III.D of this chapter. 

So far, a large number of nondense aluminophosphates have been successfully synthesized via the solvothermal synthesis system. Most of them have anionic frameworks except for those crystallized from hydrothermal synthesis systems, such as AlP04-5, A1P04-11, and A1P04-21. The stoichiometries of these aluminophosphates include... 

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