**3. Microwaves and their influence on the chemical reactions**

Microwave radiation is a source of energy of great interest for chemical synthesis because, among other benefits, it has been observed that the use of microwaves improves the properties of obtained nanomaterials. The first reporting on the use of microwaves in a chemical synthesis dates back to 1986 [37]. Although initially microwaves have been applied in organic synthesis, lately their use has become quite widespread in obtaining inorganic products like metal oxides nanomaterials and metallic nanomaterials [38].

Microwaves are electromagnetic radiations located between infrared radiation and radio waves with frequencies between 300 MHz (100 cm) and 300 GHz (0.1 cm). For the nanomaterials synthesis in which aqueous solutions are used, 2.45 GHz frequency is commonly applied for microwave heating of the solutions, because water absorption is maximum at this value.

Subjected to a microwave field, the substances behave differently: absorb, transmit, reflect received radiation, or any combination of these three interactions. Polar substances absorb microwaves radiation, non-polar substances are transparent environments for this type of radiation, and electrical conductors reflect microwaves radiation. Therefore, microwave heating process is used for heating the materials which can absorb the microwave energy and convert it into heat especially by dipolar polarization or conduction mechanism [1, 39]. The interactions of polar molecules and ions with the electromagnetic field have already been described by many researchers. Shortly, the collisions resulting from the rotation of the dipoles during polarization and the load carriers during conduction give energy to the atoms and molecules from the solution in the form of heat [38, 39].

While conventional heating methods are slow enough and the heat transfer from the surface to the inner material or solution, producing non-homogeneous heating, microwave heating is done quickly because microwaves can penetrate the materials to a depth that depends on the dielectric properties of the material, heating them homogeneously [38]. Consequently, microwave heating can have certain benefits over conventional heating, like faster reaction, higher reproducibility, enhancement of product quality. It is instantaneous, with no heat dissipation effects, and advantageous for selective dielectric heating, as a result of the dielectric constant difference between the solvent and reactant [40].

In sol–gel synthesis, due to rapid and direct heating of the sample with microwave radiation, the instantaneous decomposition of the precursors and the obtaining of a supersaturated solution occur. In this way, the conditions for obtaining monodispersed nanoparticles (rapid and short nucleation in a supersaturated solution) can be obtained experimentally. At the same time, the *in-situ* approach of conversion of energy results in a minimized thermal gradient due to the fast heating rate consequently is providing perfect conditions for the uniform growth of nanocrystals [31, 41].

More, in the case of sol–gel synthesis using organic solvents, characterized by slow kinetics, microwave heating is an optimal method of increasing the rate of reaction [41].

From the research carried out so far, it has been observed that, by combining the sol–gel method with the microwave heating, the properties of the obtained oxide nanostructures are improved [9, 34].

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*Influence of the Microwaves on the Sol-Gel Syntheses and on the Properties of the Resulting...*

**4. Oxide nanostructures obtained by MW assisted sol–gel method**

Up to now, there have been several reports regarding the synthesis of metal oxide nanomaterials by microwave-assisted sol–gel method. However, many of them have been performed using domestic microwave ovens, in which the reaction conditions cannot be accurately measured, making the experiments difficult to be

According to the literature data, the MW irradiation in the sol–gel synthesis was used, most frequently, for precipitation of nanocrystalline metal oxides, for thermal treatment of amorphous oxide nanopowders as well as for drying and thermally

Less attention was given to study the reactions that take place in the sol–gel

A large number of oxides were prepared by sol–gel and microwave assisted sol– gel methods. Using MW irradiations of the solutions, preparation of several oxides were mentioned in the literature data, as MgO [46], RuO2 [47], ZnO [16], ZrO2 [48], WO3 [49], SiO2 [50], TiO2 [35, 51]. The power of the used microwaves ranged

Among them, considerable interest is given to pure and doped TiO2. The doping of TiO2 was realized with a high number of elements, such as Cr [13], Ag [52], Au, Pt [14, 53], Sn-Cu-Ni [54], Fe, Pt, Pd [51] and V [55]. Doping TiO2 with different elements the properties of the resulted nanostructures are improved, while using microwave assisted preparation, supplementary improvement was

Our studies regarding the influence of the microwaves on the reactions in the sol–gel solutions were published by Predoana et al. [42] in the case of TiO2 and

The use of vanadium as a doping agent has a beneficial influence on the TiO2 properties: it can reduce the band gap energy, enhance the absorption of visible light and increase the specific surface area of the powder. The mentioned properties are reflected mainly in its photocatalytic activity, previously presented by

In our studies, the reagents used in the synthesis were titanium(IV) ethoxide Ti(OC2H5)4 in the case of TiO2, as well as, titanium(IV) ethoxide Ti(OC2H5)4 and vanadylacetylacetonate VO(AcAc), for V-doped TiO2. In both cases, ethanol C2H5OH as a solvent, 2,4 pentanedione (AcAc), as a chelating agent, and nitric acid

By the classical sol–gel method the reagents were mixed for 2 hours at room temperature. By the microwave-assisted sol–gel method, the same mixture was

The first important result of using the microwave-assisted sol–gel method is the significantly increasing of the stability of the prepared solutions against gelation, having a great advantage for multilayer film deposition. This effect was assigned to

exposed for 5 min at 300 W and a frequency of 2.45 GHz.

the formation of different molecular species.

Because the presence of MW, the interaction of the electromagnetic field with each molecule in the solution differs during the hydrolysis-condensation process, we can expect the formation of different molecular species as compared to the clas-

*DOI: http://dx.doi.org/10.5772/intechopen.94931*

sical sol–gel synthesis.

treatment of the oxide films [36].

from 140 W [51] to 850 W [47].

V-doped TiO2 nanostructures.

HNO3 as catalyst were used.

also observed.

Huang et al. [55].

solutions during MW irradiation [42–45].

**4.1 Pure and doped oxide nanostructures**

reproduced.

*Influence of the Microwaves on the Sol-Gel Syntheses and on the Properties of the Resulting... DOI: http://dx.doi.org/10.5772/intechopen.94931*

Because the presence of MW, the interaction of the electromagnetic field with each molecule in the solution differs during the hydrolysis-condensation process, we can expect the formation of different molecular species as compared to the classical sol–gel synthesis.
