**2. General consideration on the sol-gel chemistry**

Among the chemical methods in the liquid phase, the sol–gel technique is a versatile and efficient method for pure or doped metal oxide films or powders, as well as for oxide compounds preparation [21–24].

A comprehensive definition of sol–gel method assumes that the process represents the formation of an inorganic polymeric network by reactions in the solution at low temperatures. In the second step, by adequate thermal treatments, the conversion of the inorganic amorphous polymers takes place either into glasses or into crystalline materials [1, 22].

Based on the type of the precursors and the reaction medium used, two types of sol–gel processes were developed: on the bases of the alcoholic (organic) or aqueous medium.

According to Pierre [25] in both polymeric and aqueous sol–gel routes, the precursors undertake the succession of the following transformations in the presence of water:

**101**

coordination sphere.

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

In the case of the **polymeric route**, using alkoxides (non-ionized precursors),

M OR OH M OR OH M OR OH MOM OR OH H O ( )nx x <sup>−</sup> ( ) + → ( )nx x <sup>−</sup> ( ) ( )nx x1 −− − ( ) ( )n x ( ) + <sup>2</sup> (3)

**The aqueous sol–gel route** has also two pathways: the colloidal route [26] and

In the case of the **aqueous route,** which starts from colloidal solutions in

( ) <sup>z</sup> <sup>z</sup> M nH O M OH 2 2 <sup>n</sup>

( ) ( ) ( ) <sup>z</sup> (z 1) M H O M H O OH H 2 2 <sup>n</sup> n 1

( ) ( ) ( ) ( ) ( ) <sup>z</sup> xz y a xM H O yOH aA M O OH H O A xn u n H O <sup>2</sup> <sup>n</sup> x u y 2u n 2 a <sup>2</sup> + − −− +

In the case of transition metals, it is more difficult to obtain gels, the metals having very high reactivity due to their higher electronegativity and their not satisfied

To favor the gelling process, in case of the transition metals, chelating agents, as

( ) ( ) ( ) ( ) ( ) <sup>z</sup> <sup>z</sup> M H O OH ] mAc M H O OH Ac mOH x 2 p y x 2 p ym m

It is important to underline that in all mentioned cases the reactions take place simultaneously, not consequently, and they are also reversible, fact that determines

Prior to gelation, the sol–gel solution can be used to obtain thin films by using

Besides the fact that it offers the possibility of obtaining both films and powders of metal oxides at nanometric dimensions, the sol–gel method has also some advantages over other preparation techniques. Such advantages are purity, homogeneity, the possibility to introducing dopants in large quantities, ease of manufacturing, low processing temperature, control over the stoichiometry, composition, viscosity [13, 27, 29] and, in the case of thin films, easy control of thickness, as well as the

Lately, ultrasonic [32, 33] or microwave irradiation [9, 17, 18, 34–36] in sol–gel oxide nanomaterials synthesis have become methods of interest because, in addition

carboxylic acids or polyols, are used. A typical reaction is the following

a complex composition of the sol–gel solutions.

simple techniques such as dip or spin coating [23, 28].

ability to cover large and different type of surfaces [30, 31].

<sup>−</sup> + +→ + +− (8)

<sup>−</sup> − − <sup>−</sup> + → <sup>+</sup> (9)

the aqueous route using different chelating agents [23, 26, 27].

aqueous medium, the following reactions take place:

( ) <sup>2</sup> ( ) ( ) M OR xH O M OR OH xROH <sup>n</sup> nx x <sup>−</sup> +→ + (2)

− − + − −→− − − −+ M OH HO M M O M H O2 (4)

<sup>z</sup> *MX*<sup>z</sup> M X → ++ − (5)

<sup>+</sup> <sup>+</sup> + → (6)

+ − <sup>+</sup> → + <sup>−</sup> (7)

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

the reactions that occur are the following:

*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*

In the case of the **polymeric route**, using alkoxides (non-ionized precursors), the reactions that occur are the following:

$$\text{M(OR)}\_{\text{n}} + \text{xH}\_{2}\text{O} \rightarrow \text{M(OR)}\_{\text{n-x}}\text{(OH)}\_{\text{x}} + \text{xROH} \tag{2}$$

$$\mathrm{M(OR)\_{n-x}(OH)\_x} + \mathrm{M(OR)\_{n-x}(OH)\_x} \rightarrow \mathrm{M(OR)\_{n-x}(OH)\_{x-1}\bullet\\\mathrm{MOM(OR)\_{n-x}(OH)} + \mathrm{H\_2O}} \tag{3}$$

$$\text{--M--OH} + \text{HO} - \text{M--M--} \rightarrow \text{-M--O} - \text{M--H}\_2\text{O} \tag{4}$$

**The aqueous sol–gel route** has also two pathways: the colloidal route [26] and the aqueous route using different chelating agents [23, 26, 27].

In the case of the **aqueous route,** which starts from colloidal solutions in aqueous medium, the following reactions take place:

$$\mathbf{M}\mathbf{X}\_{\mathbf{z}} \to \mathbf{M}^{\mathbf{z}\*} + \mathbf{X}^{-} \tag{5}$$

$$\text{M}^{x\*} + \text{nH}\_2\text{O} \rightarrow \left[\text{M}(\text{OH}\_2)\_n\right]^{x\*} \tag{6}$$

$$\left(\mathrm{M}(\mathrm{H}\_{2}\mathrm{O})\right)\_{\mathrm{n}}^{\mathrm{x+}} \rightarrow \mathrm{M}(\mathrm{H}\_{2}\mathrm{O})\_{\mathrm{n-1}}\left(\mathrm{OH}\right)^{\mathrm{(z-1)}} + \mathrm{H}^{\*}\tag{7}$$

$$\text{xM(H}\_{2}\text{O)}\_{\text{n}}^{\text{+}} + \text{yOH}^{-} + \text{a}\\ \text{A} \rightarrow \text{M}\_{\text{x}}\text{O}\_{\text{u}}\text{(OH)}\_{\text{y-2u}}\text{(H}\_{2}\text{O)}\_{\text{n}}\\ \text{A}\_{\text{a}}^{(\text{x-y-a})^{+}} + (\text{xn} + \text{u} - \text{n})\text{H}\_{2}\text{O} \quad \text{(8)}$$

In the case of transition metals, it is more difficult to obtain gels, the metals having very high reactivity due to their higher electronegativity and their not satisfied coordination sphere.

To favor the gelling process, in case of the transition metals, chelating agents, as carboxylic acids or polyols, are used. A typical reaction is the following

$$\mathrm{M\_x\left(H\_2O\right)\_p\left(OH\right)\_y}\mathrm{I^{x-}} + \mathrm{mAc} \rightarrow \left[\mathrm{M\_x\left(H\_2O\right)\_p\left(OH\right)\_{y-m}\left(Ac\right)\_m}\right]^{x-} + \mathrm{mOH^-} \tag{9}$$

It is important to underline that in all mentioned cases the reactions take place simultaneously, not consequently, and they are also reversible, fact that determines a complex composition of the sol–gel solutions.

Prior to gelation, the sol–gel solution can be used to obtain thin films by using simple techniques such as dip or spin coating [23, 28].

Besides the fact that it offers the possibility of obtaining both films and powders of metal oxides at nanometric dimensions, the sol–gel method has also some advantages over other preparation techniques. Such advantages are purity, homogeneity, the possibility to introducing dopants in large quantities, ease of manufacturing, low processing temperature, control over the stoichiometry, composition, viscosity [13, 27, 29] and, in the case of thin films, easy control of thickness, as well as the ability to cover large and different type of surfaces [30, 31].

Lately, ultrasonic [32, 33] or microwave irradiation [9, 17, 18, 34–36] in sol–gel oxide nanomaterials synthesis have become methods of interest because, in addition

*Microwave Heating - Electromagnetic Fields Causing Thermal and Non-Thermal Effects*

significantly on the obtaining method. A large number of available synthesis methods underlies the continuous interest in obtaining oxide nanostructures that can be used successfully in specific areas [1, 7]. However, most of these methods are limited due to the use of toxic reagents, high processing temperatures, high

vacuum, expensive equipment, or long reaction times [8, 9].

**2. General consideration on the sol-gel chemistry**

well as for oxide compounds preparation [21–24].

into crystalline materials [1, 22].

conditions used [10, 11].

various substrates [8, 19, 20].

present chapter.

[12–14].

The structure, morphology, and properties of the oxide nanostructures depend

Although physical methods have the advantage of high reproducibility, chemical methods in the liquid phase are more often used to obtain oxide nanostructures due to their advantages, such as low production temperature, homogeneous mixing of precursors at the molecular scale, design and control of the physico-chemical properties of final products, depending on the precursors, and the experimental

Among the various chemical procedures, the sol–gel method gained increasing importance in the field of materials science because it is cheap, simple, allows the introduction of dopants in large quantities, ensures high purity, and homogeneity, allows control of size, shape, and size distribution of the obtained nanomaterials

Lately, for the preparation of functional nanomaterials, more and more attention is being paid to the use of microwave as the energy source for carrying out a chemical reaction [1, 15]. The microwave (MW) assisted sol–gel method is reported to be a simple, cheap, faster, more energy-saving, and efficient process as compared to conventional heating methods [16–18]. The use of microwaves has received increased attention in the technological field because, among other things, it reduces the reaction time from days to minutes or hours, improves the properties of synthesized nanostructures, and allows obtaining oxide nanocrystalline films on

The improved properties of the oxide nanostructures obtained by microwaves assisted sol–gel method could be correlated to the influence of the microwaves on the chemical reactions that take place during the sol–gel synthesis, leading to the formation of different molecular species. Results on the influence of the microwaves on the chemical reactions during the sol–gel synthesis will be discussed in the

Among the chemical methods in the liquid phase, the sol–gel technique is a versatile and efficient method for pure or doped metal oxide films or powders, as

A comprehensive definition of sol–gel method assumes that the process represents the formation of an inorganic polymeric network by reactions in the solution at low temperatures. In the second step, by adequate thermal treatments, the conversion of the inorganic amorphous polymers takes place either into glasses or

Based on the type of the precursors and the reaction medium used, two types of sol–gel processes were developed: on the bases of the alcoholic (organic) or aque-

Hydrolysis polymerization nucleation growth → →→ (1)

According to Pierre [25] in both polymeric and aqueous sol–gel routes, the precursors undertake the succession of the following transformations in the pres-

**100**

ous medium.

ence of water:

to being cheap and environmentally friendly heating methods, offer the advantage of using shorter synthesis time, and allow the control of crystallinity, size and morphology of the resulted nanoparticles [9, 35].
