Solvothermal processing

Fig. 1.4 SEM images of caved cuboctahedral hexagonal copper sulfide (CuS covellite) crystals, synthesized by a solvothermal process in ethylene glycol, at 140 °C. (Reprinted in gray scale with permission from [52], Copyright 2009, American Chemical Society)...

Lu Q, Hu J, Tang K, Qian Y, Zhou G, Liu X (2000) Synthesis of nanocrystaUine CuMS2 (M = In or Ga) through a solvothermal process. Inorg Chem 39 1606-1607 Wu C, Yu S-H, Antoniette M (2006) Complex concaved cuboctahedrons of copper sulfide crystals with highly geometrical symmetry created by a solution process. Chem Mater 18 3599-3601... 

Li, F. Zhang, Z. 2005. Synthesis of high quality CdS nanorods by solvothermal process, and their photoluminescence./. Nanoparticle Research 7 685-689. 

Aita Y, Komatsu M, Yin S, Sato T (2004) Phase-composihonal control and visible light photocatalyhc achvity of nitrogen-doped htania via solvothermal process. J Solid state Chem 177 3235-3238. 

Jiang, L. and Gao, L., Carbon nanotubes-magnetite nanocomposites from solvothermal processes formation, characterization, and enhanced electrical properties , Chemistry of Materials, 2003, 15, 2848-2853. 

In this chapter we discuss preparative routes for inorganic materials in three basic types of systems involving the presence of a distinct solid-liquid interface those in which the liquid and solid phases are of the same chemical identity (solidification and vitrification processes), those in which the liquid and solid phases are not of the same chemical identity (crystallization, precipitation), and the special case in which the liquid phase is a pure ionic liquid or molten salt. Ionic liquids can serve as the solvent as well as a templating agent, and the liquid components may or may not become incorporated into the final solid product. We also discuss two areas where the distinct solid-liquid interface becomes somewhat blurred namely, sol-gel and solvothermal processes. 

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. 

Solvothermal process is now becoming a powerful technique for preparing nanomaterials. It is analogous to hydrothermal synthesis, except that non-aqueous solvents replace water as reaction medium. From the chemical reaction point of view, solvents in supercritical conditions play a significant role in reaction and crystallization. New materials, especially those having metastable phases and special nanostructures, can be obtained under mild conditions. By sealing the reaction system in an autoclave, the reactants and products prevent effectively from oxidation, hydrolysis and volatilization, and the reaction and crystallization can be realized synchronously. 

We have developed solvothermal synthesis as an important method in research of metastable structures. In the benzene-thermal synthesis of nanocrystalline GaN at 280°C through the metathesis reaction of GaClj and U3N, the ultrahigh pressure rocksalt type GaN metastable phase, which was previously prepared at 37 GPa, was obtained at ambient condition [5]. Diamond crystallites were prepared from catalytic reduction of CCI4 by metallic sodium in an autoclave at 700°C (Fig.l) [6]. In our recent studies, diamond was also prepared via the solvothermal process. In the solvothermal catalytic metathesis reaction of carbides of transition metals and CX4 (X = F, Cl, Br) at 600-700°C, Raman spectrum of the prepared sample shows a sharp peak at 1330 cm" (Fig. 1), indicating existence of diamond. In another process, multiwalled carbon nanotubes were synthesized at 350°C by the solvothermal catalytic reaction of CgCle with metallic potassium (Fig. 2) [7]. 

Figure 1. Raman spectra of the sample prepared by solvothermal process through the catalytic reduction [6] (left) and the catalytic metathesis reaction (right).

Hydrothermal synthesis is one of the important methods for producing fine powders of oxides. A hydrothermal system is usually maintained at a temperature beyond 100 °C and the autogenous pressure of water exceeds the ambient pressure, which is favorable for the crystallization of products. Recent research indicates that the hydrothermal method is also a practical means for preparing chal-cogenide and phosphide nanomaterials, and hydrothermal treatment is an effective method for passivating porous silicons. Similar to hydrothermal synthesis, in a solvothermal process, a non-aqueous solvent, which is sealed in an autoclave and maintained in its superheated state, is the reaction medium, where the reactants and products are prevented effectively from oxidation and volatilization and the reaction and crystallization can be realized simultaneously. Furthermore, organic solvents may be favorable for the dispersion of non-oxide nanocrystallites and may stabilize some metastable phases. 

We tried to obtain fully non-agglomerated YAG powder and succeeded to do so together with colleagues from the Institute of Experimental Mineralogy of RAS, Chemogolovka, by modified solvothermal process (Fig. 2) but the quantity of YAG powder obtained till now was not sufficient... 

Yu SH (2001) Hydrothermal/solvothermal processing of advanced ceramic materials. J Ceram Soc Jpn 109 S65-S75... 

Gnoe C, Jinmin W, Xiangwen L, Kaixnn H. Self-assembly synthesis of singlecrystalline tin oxide nanostructures by a poly (acrylic acid)-assisted solvothermal process. J Phys Chem B 2006 110 16208-11. 

Solvothermal processes have been also used to synthesize metal nanoparticles other than metal oxide nanoparticles. Palladium nanoparticles of about 1 pm have been synthesized by a solvothermal route, as shown by following reaction ... 

Li2Mri2(Mo04)3 has been S3mthesized both in micro and nanoscale by employing two different S3mthesis protocols involving aqueous solutions of the respective nitrates with a fuel for mild combustion (we have named it as soft-combustion) and low temperature solvothermal process as described below. 

Solvothermal Process Precipitation fi"om a nonaqueous solution of precursor salts under high temperature. Application of high pressure is optional Particle size control By reaction temperature and duration, precursor concentration, pH, surfactant additive, precursor materials, and solvent General attributes High crystallinity spherical shapes as well as unique morphologies not only oxides but also sulfides and other chalcogenide compounds... 

Devaraju MK, Honma I (2012) Hydrothermal and solvothermal process towards development of LiMP04 (M=Fe, Mn) nanomaterials for lithium-ion batteries. Adv Ener Mater 2 284-297... 

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