Solvothermal Synthesis

Solvothermal processes can be defined as chemical reactions or transformations in a solvent under supercritical conditions [36, 376]. The specific physico-chemical properties of solvents near such a pressure-temperature (100 MPa, 300 °C) domain can markedly improve the diffusion of chemical species [377]. These processes are used to synthesize new materials (meta-stable phases) as well as to develop new processes for obtaining functional materials and the shaping of materials, for example, monodispersed nano-sized particles [378-380]. Although water is largely used as the reaction medium (hydrothermal synthesis) in most solvothermal reactions, several nonaqueous solvents (e.g., ammonia, hydrazine, alcohols) have been investigated for the preparation of different materials [381]. The first step of a hydrothermal reaction is either a sol-gel precipitation or a coprecipitation process, the precursors mostly being alkoxides, nitrates, chlorides, or sulfates, which are first dissolved in a suitable medium or water to form a homogeneous solution. Typically, sol-gel or coprecipitation-derived precipitates are amorphous in nature, which are hydrother-mally treated in Teflon-lined steel autoclaves at constant temperature to induce crystallization (Scheme 12). Finally, the crystallized powders are repeatedly washed and dried at room temperature. Hydrothermal synthesis does not require expensive precursors and apparatus. Moreover, it is possible to predict the optimum synthesis conditions by theoretical methods based on electrolyte thermodynamics. One of the peculiar advantages of the hydrothermal synthesis is that besides the synthesis of inorganic materials, it is also possible to synthesize new crystalline precursors that can be structurally characterized. For example, a new hydroxo-stannate, Sr2Sn(OH)8, has been obtained from hydrothermal studies of the SrO-SnO2 system. The compound acts as a single source for SrSnO3 (<700 °C) and Sr2SnO4 (>900 °C) [382], subject to the thermal treatment.

Figure 10. A schematic of various steps involved in the preparation of BaTiO3 by a coprecipitation technique.

Scheme 12.

Solvothermal methods offer a simple, direct, and low-temperature route for obtaining nanometric particles with narrow size dispersions and represent an alternative to calcinations for promoting crystallization under milder temperatures. Another feature of the hydrothermal treatment performed under near-supercritical conditions concerns the morphology of the materials that can be governed by monitoring temperature, pressure, reaction time, and the nature of the solvent. The easy access to the shaping of materials (crystal growth, preparation of fine microcrystallites) and low crystallization temperatures make hydrothermal synthesis an attractive method for producing submicrometer particles with a narrow size distribution, avoiding the firing steps required in sol-gel processing; however, interest in this method is limited compared with sol-gel methods. The drawbacks include the trial-and-error nature of the method to understand the chemical reactivity of supercritical fluids, which governs the course of the hydrothermal synthesis and consequently the composition, structure, and morphology of the nanomaterial. There are many experimental variables, like composition of the starting mixture (stoichiometric or not), choice of reagents, temperature, reaction time, and percentage fill of the reaction vessel. In addition, it remains difficult to predict the outcome of new reactions. Furthermore, observation of the reaction course is difficult because the reaction takes place in a sealed vessel. The future of hydrothermal processing for nanocrystal growth and materials synthesis requires lower pressure and temperature, which will facilitate in-situ observation of the reaction. Finally, the use of molecular compounds as stoichiometric synthons can substantially augment the advantages of solvothermal routes to produce nanopowders with high crystallinity and regular morphology.

0 0

Post a comment