Hydrothermal/solvothermal synthesis nanomaterials

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. 

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. 

In this chapter, we briefly review our recent progress in the solvothermal preparation of non-oxide nanomaterials, y-ray irradiation and room temperature synthesis of chalcogenide nanocrystallites are also briefly described but first we present some progress in hydrothermal synthesis. 

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. 

Besides the co-precipitation method, other common methods for the synthesis of nanomaterials, such as the hydrothermal method, the solvothermal method, the sonochemical method, pyrolysis, the sol-gel process, and the reverse micelles method have also been applied for the synthesis of ceria nanomaterials. The typical shape of as-prepared nanocrystals is always polyhedral because of the fluorite crystal structure of ceria (Fig. 6.1). 

Another kind of ID ceria nanostructure is the spindle. Zheng and co-workers described a hydrothermal method to obtain spindles. Urea was used as a precipitant and the reaction time was relatively shorter than for nanotubes. Zhang, Shi and co-workers also described a method to prepare ceria nanospindles. Ce(III) was precipitated by urea in the presence of glycerin. Ce(0H)G03 was realized as intermediate and the mechanism involved the assembly of small particles assisted by glycerin molecules adsorbed on the surface of the parti-cles. ° Sun et al used a similar solvothermal method to assemble shutde-like ceria nanomaterials from nanorods. Zhang, Tong and co-workers described the synthesis of bowknot-like ceria bundles assembled from nanorods obtained via a hydrothermal reaction. ... 

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