Hydrothermal synthesis nanomaterials

Hydrothermal synthesis is a powerful method used for the fabrication of nanophase materials due to the relatively low temperature during synthesis, facile separation of nanopartides in the product, and ready availability of apparatus for such syntheses. Versatile physical and chemical properties of nanomaterials can be obtained with the use of this method that involves various techniques (e.g., control of reaction time, temperature and choice of oxidant and its concentration). Several extensive reviews are available that discuss the fundamental properties and applications of this method [2, 3]. These reviews cover the synthesis of nanomaterials with different pore textures, different types of composition [2, 4—6], and different dimensionalities in terms of morphology [6-8]. 

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. 

Schaf, O., Ghobarkar, H., Knauth, R Hydrothermal synthesis of nanomaterials. Nanostruct. Mater Electron. Mater Sci. Technol. 8 23-41 (2004)... 

Hydrothermal synthesis of structure- and shape-controlled manganese oxide octahedral molecular sieve nanomaterials. Advanced Functional Materials, 16,... 

The formation of vanadium oxide nanotubes (VOx-NTs) is sensitive to agitation during hydrothermal synthesis. The ordering of the VOx layers is lower and the interlayer distance increases. The pH range for VOx-NT synthesis could be extended to pH values below 5. At low pH, the VOx layers ate better ordered and there is a trend towards shorter tubes. Below a critical pH aroimd 4.5, other morphologies next to VOx-NTs appear. Only amine templates were foimd to be suitable while platelets and star-Uke VOx nanomaterials were obtained using alcohols and thiols. 

Studies on the use of hydrothermal, microwave-assisted, and reflux synthesis methods for the development and application of nanomaterials have been reviewed. An important aspect of the green synthesis of metallic nanopartides involves techniques that make use of biological materials such as plant extracts and microorganisms. The design of nanomaterials and control of their desired properties have been reviewed. The unique properties of manufactured nanomaterials offer many potential benefits. 

A third possibility for the synthesis of nanomaterials in constrained volumes is the use of molds (Figure 3.1c). Advantages of this method include its simplicity, versatility, and precise control over the shape of the solid, even with intricate forms. An elegant example of this strategy is the preparation of zeolites which precisely replicate the complex microstructure of wood. To do this, Dong et al. [43] infiltrated a zeolite synthesis solution into a wood sample. After the necessary hydrothermal treatment, and subsequent calcination to remove the template as well as the wood, a zeolitic structure was obtained that reproduced with full detail and fidelity the wooden sample used as a mold. 

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. 

The preparation of nanomaterials is one of the most active fields in material science. Number of techniques have been used for the production of nanoparticles gas-evaporation [11], sputtering [12], sol-gel method [13], hydrothermal [14], microemulsion [15, 16], polyols [17], laser pyrolysis [18], sonochemical synthesis [19], chemical coprecipitation [20-22], and so on. Among them, the surfactant assembly mediated synthesis is attracting more attention because it allows for a good... 

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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