Microwave Assisted Hydrothermal Method

Cao, C., Cui, Z., Chen, C., Song, W. and Cai, W. (2010) Ceria hollow nanospheres produced by a template-free microwave-assisted hydrothermal method for heavymetal ion removal and catalysis. Journal of Physical Chemistry C, 114, 9865-9870. 

Xu, L., Ding, Y., Chen, C., Zhao, L., Rimkus, C., Joesten, R. and Suib, S.L. (2008) 3D flowerlike a-nickel hydroxide with enhanced electrochemical activity synthesized by microwave-assisted hydrothermal method. Chemistry of Materials, 20, 308-316. 

Moreira, M.L., Andres, J., Varela, J.A. and Longo, E. (2009) Synthesis of fine microsized BaZr03 powders based on a decaoctahedron shape by the microwave-assisted hydrothermal method. Crystal Growth and Design, 9, 833-839. 

Microwave-assisted hydrothermal methods can also be used for preparing ceramics. This involves placing a sealed vessel containing the reagents into the micro-wave cavity. One example of the use of microwave-assisted hydrothermal synthesis involves the preparation of cubic-shaped In(OH)3 particles from a mixed aqueous solution of InClj and urea. These particles were then used to probe the kinetics of the thermal decomposition of ln(OH)3 to cubic-In203. 

Even more, for layered second-row transition metal oxides it was possible to obtain binder-free, nanobelt-Mo03/G film electrodes by using microwave-assisted hydrothermal methods. Irrespective of the large molecular weight of the oxide, the gravimetric capacity could reach 291 mA h at 100 mA g with retention of ca. 60% after 100 cycles [142]. An exciting achievement was also the flexible properties of the electrode that could finally lead to new applications in bendable batteries or the origami concept [143]. 

The control of shape and particle size is not easy with this method. Therefore, various modifications have been introduced to improve the material properties and include the surfactant-assisted hydrothermal method, the microwave-assisted hydrothermal method, the supercritical hydrothermal method, etc. 

The application of microwaves in both drying and calcination processes of coprecipitated powders seems to be an efficient method of improving powder quality, leading to shorter processing schedules and enhanced colour development. Bondioli et aV have synthesized praseodymium-doped ceria powders by a microwave-assisted hydrothermal method. The powders obtained had nanosized faceted polyhedral morphologies and the particle size was 30 nm as shown in Fig. 8.16. 

Glucose derived water-soluble crystalline graphene quantum dots (GQDs) were prepared by a facile microwave-assisted hydrothermal method with an average diameter as small as 1.65 run. These GQDs exhibit typical excitation wavelength dependent properties as expected in carbon-based quantum dots (Tang et al., 2012). 

Iron oxide nanoparticles supported on almninosilicate catalysts were found to be efficient and easily recoverable materials in the aqueous selective oxidation of alcohols to their corresponding carbonyl compoimds using hydrogen peroxide under both conventional as well as microwave heating (Rajabi et al., 2013). A new and versatile approach to synthesize SnO nanociystals (ratile-type stmcture) using microwave-assisted hydrothermal method was reported by Mendes et al. (2012). They observed that there are substantial changes in optical absorbance of tin (IV) oxide nanoparticles. 

In order to accelerate the synthesis of both zeolite seeds and membranes, microwave-assisted hydrothermal synthesis (MW-HT) revealed an attractive method. Colloidal zeolite seeds can be successfully prepared by MW-HT synthesis [100,112,113]. Starting from a seeded support,... 

Supported zeolite membranes have been prepared using numerous procedures [4] such as alignment of crystals in electrical fields, electroplating, self-assembly, growth on organic molecular layers, covalent linkages, hydrothermal synthesis (in situ and ex situ), microwave-heating-assisted hydrothermal method, dry gel method (vapor-phase transport method and steam-assisted crystallization), synthesis at the interface between two fluid phases, and direct filtration of pre-exfoliated zeolite nanosheets. 

In this part of the chapter, we will focus on microwave application in the synthesis of perovskite materials (ABOs-type oxides). There are many examples in the literature of chemical routes that can be applied in the synthesis of perovskite oxides, such as hydrothermal, sol-gel, combustion, or citrate processing, among others (see Figure 5.1) [11]. Table 5.1 compiles some detailed information about microwave-assisted synthesis methods of perovskite materials made by selected research groups. 

Over the years, several conventional methods such as hydrothermal, fusion prior to hydrothermal, microwave assisted hydrothermal and molten salt techniques have been developed by researchers for producing FAZ. However, one to two step reactions between the fly ash and alkali have been mostly reported and the reaction liquid by-product has been disposed of in the open environment without utilizing its chemical potentials. On the contrary, such alkaline wastes may cause environmental contaminations. 

Various methods have been used to synthesize metal oxide nanopartides. The methods discussed here indude hydrothermal alteration, reflux, and microwave-assisted methods. 

MW-HT techniques can shorten the reaction time. For example, Ivanov et al. [38] synthesized ZnO nanocrystals in 10 min with the assistance of microwave hydro-thermal methods. Huang et al. [39] also demonstrated the use of microwave radiation in the hydrothermal synthesis of ZnO complex nanostructures. These nanocrystals showed high photocatalytic activity in a model reaction of Methyl Orange photodegradation. Li et al. [40] synthesized W03 nanorods in 20 min and showed high... 

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. 

ABSTRACT Synthesis of nanostructured LiCoOj and TiO via microwave assisted and conventional hydrothermal methods, respectively, is presented. The physical properties of the resulting material are characterised via X-ray powder diffraction and scanning electron microscopy. Cyclic voltammetry and Galvanostatic Intermittent Titration Technique are used to study differences of the electrochemical properties compared to bulk material. 

In summary, self-organized transition metal oxide nanostructures were successfully synthesised by means of hydrothermal methods. In particular, the microwave-assisted method provides a fast and reliable technique to produce reasonable amounts of material. The nanosized morphology of the materials can influence the reaction behaviour so that higher capacities can be reached (TiOj) than in the bulk or metastable phases can be stabilised (LiCoOj). 

Reactions are usually conducted in solution and often under hydrothermal conditions. This being stated, solvent-free cogrinding of the reactants (mechanochemistry), microwave-assisted, and sonochemical methods are receiving increasing attention as alternative methods for synthesis. Beyond synthesis, there is increasing interest in fabricating thin fllms, membranes, and nanofibers comprising coordination polymers. 

---