**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 reproduced.

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 treatment of the oxide films [36].

Less attention was given to study the reactions that take place in the sol–gel solutions during MW irradiation [42–45].

#### **4.1 Pure and doped oxide nanostructures**

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

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 also observed.

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 V-doped TiO2 nanostructures.

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 Huang et al. [55].

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 HNO3 as catalyst were used.

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

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 the formation of different molecular species.

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

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

morphology of the resulted nanoparticles [9, 35].

because water absorption is maximum at this value.

and metallic nanomaterials [38].

between the solvent and reactant [40].

nanostructures are improved [9, 34].

to being cheap and environmentally friendly heating methods, offer the advantage of using shorter synthesis time, and allow the control of crystallinity, size and

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

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,

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

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

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 nano-

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

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

atoms and molecules from the solution in the form of heat [38, 39].

**102**

crystals [31, 41].

reaction [41].

The solutions were used for obtaining thin films and the resulted gels were investigated for their structural and morphological properties.

In our studies for **TiO2 samples** synthesized by sol–gel and microwave-assisted sol–gel methods, the TG/DTG/DTA curves corresponding to the decomposition of the obtained gels are presented in **Figure 1**.

It could be noticed that the thermal decomposition of the gels is not essentially influenced by the method of preparation. Only a small increase of the thermal effect at 195o C is observed for the TiO2 sample obtained by MW assisted sol–gel method. The fact could be explained by the positive influence of microwaves on the formation of the molecular species that decompose at the mentioned temperature.

Based on the TG/DTG/DTA results, the samples prepared by both methods were thermally treated at 450°C for 1 h. By X-ray diffraction of the samples thermally treated at this temperature only anatase phase was detected (according to

**Figure 1.**

*TG/DTG/DTA curves of the TiO2 samples obtained by SG and MW methods [42] (Reproduced with the permission of Springer Nature).*

**Figure 2.**

*The XRD patterns of the TiO2 samples obtained by SG and MW-assisted SG methods thermally treated at 450°C.*

**105**

**Figure 4.**

*permission from Springer Nature).*

**Figure 3.**

*(Reproduced with the permission of Springer Nature).*

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

JCPDS card no. 21–1272), but a higher crystallinity is noticed in the case of sample

In the case of **the V-doped TiO2** the TG/DTG/DTA measurements in the air are presented in **Figure 3** for the gel containing 2 mol% V. In this case, increased thermal stability and a more complex decomposition of the gels obtained by the

Confirmations of the TG/DTG/DTA results on the gels with 2 mol% V were obtained by Differential Scanning Calorimetry (DSC). The obtained DSC curves

According to the DSC results, the thermal stability of the gel obtained from the solution prepared in the presence of microwaves, is significantly higher (with about 100°C), as compared with the gel with similar compositions, but obtained by the

At the same time, the number and temperatures of the thermal effects are different in the two discussed cases underlying the different compositions of the gels

*TGA/DTG/DTA curves of the V-doped TiO2 samples obtained by SG and MW-assisted SG methods [42]* 

*DCS curves of the V-doped TiO2 obtained by SG and MW-assisted SG methods [43] (Reproduced with the* 

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

obtained by MW-assisted procedure (**Figure 2**).

microwave-assisted sol–gel method is observed.

obtained in the presence or the absence of the microwaves.

are presented in **Figure 4**.

classical sol–gel method.

*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*

JCPDS card no. 21–1272), but a higher crystallinity is noticed in the case of sample obtained by MW-assisted procedure (**Figure 2**).

In the case of **the V-doped TiO2** the TG/DTG/DTA measurements in the air are presented in **Figure 3** for the gel containing 2 mol% V. In this case, increased thermal stability and a more complex decomposition of the gels obtained by the microwave-assisted sol–gel method is observed.

Confirmations of the TG/DTG/DTA results on the gels with 2 mol% V were obtained by Differential Scanning Calorimetry (DSC). The obtained DSC curves are presented in **Figure 4**.

According to the DSC results, the thermal stability of the gel obtained from the solution prepared in the presence of microwaves, is significantly higher (with about 100°C), as compared with the gel with similar compositions, but obtained by the classical sol–gel method.

At the same time, the number and temperatures of the thermal effects are different in the two discussed cases underlying the different compositions of the gels obtained in the presence or the absence of the microwaves.

#### **Figure 3.**

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

investigated for their structural and morphological properties.

the obtained gels are presented in **Figure 1**.

effect at 195o

The solutions were used for obtaining thin films and the resulted gels were

In our studies for **TiO2 samples** synthesized by sol–gel and microwave-assisted sol–gel methods, the TG/DTG/DTA curves corresponding to the decomposition of

It could be noticed that the thermal decomposition of the gels is not essentially influenced by the method of preparation. Only a small increase of the thermal

method. The fact could be explained by the positive influence of microwaves on the formation of the molecular species that decompose at the mentioned temperature. Based on the TG/DTG/DTA results, the samples prepared by both methods were thermally treated at 450°C for 1 h. By X-ray diffraction of the samples thermally treated at this temperature only anatase phase was detected (according to

*TG/DTG/DTA curves of the TiO2 samples obtained by SG and MW methods [42] (Reproduced with the* 

*The XRD patterns of the TiO2 samples obtained by SG and MW-assisted SG methods thermally treated at* 

C is observed for the TiO2 sample obtained by MW assisted sol–gel

**104**

**Figure 2.**

*450°C.*

**Figure 1.**

*permission of Springer Nature).*

*TGA/DTG/DTA curves of the V-doped TiO2 samples obtained by SG and MW-assisted SG methods [42] (Reproduced with the permission of Springer Nature).*

#### **Figure 4.**

*DCS curves of the V-doped TiO2 obtained by SG and MW-assisted SG methods [43] (Reproduced with the permission from Springer Nature).*

**Figure 5.**

*TG/DTG/DTA/EGA curves of V-doped TiO2 obtained by (a) SG and (b) MW-assisted SG methods [42] (Reproduced with the permission of Springer Nature).*

The TG/DTG/DTA/EGA measurements, presented in **Figure 5**, have confirmed, once more, the results discussed above, regarding the different thermal behavior of the gels obtained by the microwave-assisted sol–gel method.

In the case of the microwave-assisted sol–gel method the same gasses are evolved, namely H2O and CO2, but a more complex thermal decomposition is observed, with different ratios among the two mentioned gases at the different temperatures. This result is assigned to the higher number of molecular species present in the gel, having different chemical composition and different thermal stability.

By X-ray diffraction of the V-doped TiO2 with 2 mol% V samples thermally treated at 450°C (**Figure 6**) only anatase phase was detected (according to JCPDS card no. 21–1272). As in the case of un-doped TiO2, a higher crystallinity is noticed in the case of samples obtained by MW assisted procedure.

Before gelation, the solutions prepared in the presence and in the absence of MWs were used for thin film deposition by dip-coating on glass substrates [43].

In our studies for **the TiO2 films** obtained by the sol–gel method, the SEM micrographs show surface cavities that were not observed in the case of microwaves-assisted sol–gel films (**Figure 7a** and **c**).

*The XRD patterns of the V-doped TiO2 samples obtained by SG and MW-assisted SG methods, thermally treated at 450°C.*

**107**

**Figure 8.**

*methods.*

solutions.

**Figure 7.**

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

**The sol–gel TiO2 based films** present also a similar variation of the morphology according to the method of preparation. A more dense and homogeneous aspect is observed in the film obtained in the presence of microwaves (**Figure 7b** and **d**). Thickness values are around 200 nm both for TiO2 and V-doped TiO2 films, but slightly higher in the case of the films obtained from microwave-assisted sol–gel

The transmission spectra of obtained films are presented in **Figure 8** show

*Optical transmission of the TiO2 and V-doped TiO2 films obtained by (a) SG and (b) MW-assisted SG* 

To explain the differences induced by the microwave-assisted sol–gel process on the properties of the resulted films, their influence on the starting solution, and the evolution of the sol–gel process, should be taken into consideration. Based on the results obtained up to now, it could be assumed that in the presence of microwaves, different and more stable molecular species are formed as compared to the classical sol–gel method and this a fact influences the properties of the resulted films.

optical transmittance values mainly over 80% in the visible range.

*SEM micrographs showing the film cross-section for samples (a) (TiO2)SG; (b) (TiO2)MW; (c)* 

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

*(V-dopedTiO2)SG; (d) (V-dopedTiO2)MW [43].*

*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*

#### **Figure 7.**

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

The TG/DTG/DTA/EGA measurements, presented in **Figure 5**, have confirmed, once more, the results discussed above, regarding the different thermal behavior of

In the case of the microwave-assisted sol–gel method the same gasses are evolved, namely H2O and CO2, but a more complex thermal decomposition is observed, with different ratios among the two mentioned gases at the different temperatures. This result is assigned to the higher number of molecular species present in the gel, having different chemical composition and different thermal

*TG/DTG/DTA/EGA curves of V-doped TiO2 obtained by (a) SG and (b) MW-assisted SG methods [42]* 

By X-ray diffraction of the V-doped TiO2 with 2 mol% V samples thermally treated at 450°C (**Figure 6**) only anatase phase was detected (according to JCPDS card no. 21–1272). As in the case of un-doped TiO2, a higher crystallinity is noticed

Before gelation, the solutions prepared in the presence and in the absence of MWs were used for thin film deposition by dip-coating on glass substrates [43]. In our studies for **the TiO2 films** obtained by the sol–gel method, the SEM micrographs show surface cavities that were not observed in the case of micro-

*The XRD patterns of the V-doped TiO2 samples obtained by SG and MW-assisted SG methods, thermally* 

the gels obtained by the microwave-assisted sol–gel method.

*(Reproduced with the permission of Springer Nature).*

in the case of samples obtained by MW assisted procedure.

waves-assisted sol–gel films (**Figure 7a** and **c**).

**106**

**Figure 6.**

*treated at 450°C.*

stability.

**Figure 5.**

*SEM micrographs showing the film cross-section for samples (a) (TiO2)SG; (b) (TiO2)MW; (c) (V-dopedTiO2)SG; (d) (V-dopedTiO2)MW [43].*

**The sol–gel TiO2 based films** present also a similar variation of the morphology according to the method of preparation. A more dense and homogeneous aspect is observed in the film obtained in the presence of microwaves (**Figure 7b** and **d**).

Thickness values are around 200 nm both for TiO2 and V-doped TiO2 films, but slightly higher in the case of the films obtained from microwave-assisted sol–gel solutions.

The transmission spectra of obtained films are presented in **Figure 8** show optical transmittance values mainly over 80% in the visible range.

To explain the differences induced by the microwave-assisted sol–gel process on the properties of the resulted films, their influence on the starting solution, and the evolution of the sol–gel process, should be taken into consideration. Based on the results obtained up to now, it could be assumed that in the presence of microwaves, different and more stable molecular species are formed as compared to the classical sol–gel method and this a fact influences the properties of the resulted films.

**Figure 8.** *Optical transmission of the TiO2 and V-doped TiO2 films obtained by (a) SG and (b) MW-assisted SG methods.*

It was also observed that the effect of microwaves on the properties of the resulted materials is higher in the case of V-doped TiO2 samples, fact that could be correlated to an enhancement of the reactions between Ti and V reagents during the sol–gel process in the presence of the microwaves.

As presented in the several references, **WO3 based nanomaterials** are widely investigated in the field of electrochromic devices [56], gas sensing [57], and photocatalysis [58] in different morphologies and structures. Even though the sol– gel process has a long past and is an intensely researched method [59] the literature of sol–gel preparation of WO3 using microwave-assistance is scarce. The following articles are all from the 2010s so further researches are to be expected.

Different nanostructures were prepared by microwave assisted sol–gel method with sodium tungstate as a precursor material by Kharade et al. [60–62]. The research group synthesizes various nanoparticles and nanofilms for electrochromic purposes. In 2012 WO3 nanofilms were deposited on the FTO substrate, which was the first time used MW-assisted two-step process. In the first step, the preparation of the gel was conducted with microwave assistance, then in the second step, the deposition of the thin film occurred by a chemical growth set up. Scanning electron microscope (SEM) showed that the surface is coated with petal-like WO3 nanodisks with dimensions of 450–600 nm length, 350–400 nm width, and 20–35 nm thickness. The X-ray diffraction (XRD) analysis (**Figure 9**) points out that WO3 is in the hexagonal crystal form. Narrow and intense XRD peaks indicate that the material has good crystallinity and calculations determined that the crystal size is 71 nm, which is comparable to samples made by the regular sol–gel method [63]. X-ray photoelectron spectroscopy (XPS) revealed that the W:O ratio is nonstoichiometric(2.89). Electrochromic capabilities were determined with different electroanalytical methods [60]. Comparing this to a regular sol–gel method shows that the morphology of the surface, namely the platelet like nanodisks is nearly the same with a small difference in size (regular sol–gel platelets: 10–30 nm thick and few hundred nm lengths and width). However, to achieve the same crystallinity a 500°C annealing process is required for the regular sol–gel method, in contrast to the 150°C drying of the MW-assisted sol–gel method [64].

The same hexagonal WO3 thin film was synthesized and its electrochromic properties were enhanced with different amounts of Ag nanoparticles [61]. The microwave-assisted sol–gel method was also used to produce WO3/MoO3 mixed

**109**

**Figure 10.**

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

oxide thin films. First, the WO3 layer were produced with the two-step method explained earlier, then MoO3 was deposited with vacuum evaporation [62].

Hilaire et al. [49] prepared WO3 nanoparticles using a nonaqueous microwaveassisted sol–gel method for photoanodes. The synthesized nanoparticles were analyzed with FT-IR, which showed that no organic contaminant remained on the surface of the particles, but a weight indicates that there are a 4.4% water and organic residue after 800°C heating. XRD studies confirm the monoclinic crystal-

Transmission electron microscopy (TEM) showed that the platelets like WO3 nanoparticles size is 20–40 nm and thickness of 3 nm. Moreover, TEM measurements indicate that the WO3 platelets face having the crystalline orientation of [0 0 2]. The WO3 nanoparticles were used for the production of photoanodes, which was proven to be an efficient method for water splitting. The comparison of this result with another nonaqueous regular sol–gel method shows that the morphology of the particles differs, but this can be caused by the usage of a different solvent

The regular method resulted in larger (58 nm) rod-like nanoparticles. The case of the WO3 particle's crystallinity is similar to the thin layer's: without after anneal-

It was also established [66] that microwave heating is more convenient than resistive heating to fabricate WO3 nanoparticles with high specific surfaces and very small particle sizes also in the case of hydrothermal method of preparation. In our studies [67, 68] hexagonal structured WO3 nanoparticles and wires were prepared using MW

ing process, the MW assisted method provides better crystallinity [65].

assisted hydrothermal process. SEM images are presented in the **Figure 10**.

WO3 took the form of nanorods with 10 μm length and 10 nm diameter. Nevertheless, the Au decorated nanowires showed great photocatalytic activities. Nanowires and nanoparticles coated with TiO2 using ALD were also

The Au decorated h-WO3 nanowires were prepared for photocatalysis. The pre-decorated WO3 nanowires showed crystallinity and were composed of W and O only. The morphology also differs from nanodisks, the hydrothermally produced

*SEM images of the (a,b) hexagonal WO3 nanowire coated with TiO2 and (c, d) monoclinic WO3 nanoparticle* 

*coated with TiO2 [67] (Reprinted with permission from [67] copyright from RSC Advances).*

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

line structure of the WO3 nanoparticles.

(dicarboxylic acid) and modifier (polyethylene glycol).

**Figure 9.** *X-ray diffractogram of the WO3 thin film [60] (Copyright (2012), with permission from Elsevier).*

#### *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*

oxide thin films. First, the WO3 layer were produced with the two-step method explained earlier, then MoO3 was deposited with vacuum evaporation [62].

Hilaire et al. [49] prepared WO3 nanoparticles using a nonaqueous microwaveassisted sol–gel method for photoanodes. The synthesized nanoparticles were analyzed with FT-IR, which showed that no organic contaminant remained on the surface of the particles, but a weight indicates that there are a 4.4% water and organic residue after 800°C heating. XRD studies confirm the monoclinic crystalline structure of the WO3 nanoparticles.

Transmission electron microscopy (TEM) showed that the platelets like WO3 nanoparticles size is 20–40 nm and thickness of 3 nm. Moreover, TEM measurements indicate that the WO3 platelets face having the crystalline orientation of [0 0 2]. The WO3 nanoparticles were used for the production of photoanodes, which was proven to be an efficient method for water splitting. The comparison of this result with another nonaqueous regular sol–gel method shows that the morphology of the particles differs, but this can be caused by the usage of a different solvent (dicarboxylic acid) and modifier (polyethylene glycol).

The regular method resulted in larger (58 nm) rod-like nanoparticles. The case of the WO3 particle's crystallinity is similar to the thin layer's: without after annealing process, the MW assisted method provides better crystallinity [65].

It was also established [66] that microwave heating is more convenient than resistive heating to fabricate WO3 nanoparticles with high specific surfaces and very small particle sizes also in the case of hydrothermal method of preparation. In our studies [67, 68] hexagonal structured WO3 nanoparticles and wires were prepared using MW assisted hydrothermal process. SEM images are presented in the **Figure 10**.

The Au decorated h-WO3 nanowires were prepared for photocatalysis. The pre-decorated WO3 nanowires showed crystallinity and were composed of W and O only. The morphology also differs from nanodisks, the hydrothermally produced WO3 took the form of nanorods with 10 μm length and 10 nm diameter.

Nevertheless, the Au decorated nanowires showed great photocatalytic activities. Nanowires and nanoparticles coated with TiO2 using ALD were also

#### **Figure 10.**

*SEM images of the (a,b) hexagonal WO3 nanowire coated with TiO2 and (c, d) monoclinic WO3 nanoparticle coated with TiO2 [67] (Reprinted with permission from [67] copyright from RSC Advances).*

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

articles are all from the 2010s so further researches are to be expected.

the 150°C drying of the MW-assisted sol–gel method [64].

The same hexagonal WO3 thin film was synthesized and its electrochromic properties were enhanced with different amounts of Ag nanoparticles [61]. The microwave-assisted sol–gel method was also used to produce WO3/MoO3 mixed

*X-ray diffractogram of the WO3 thin film [60] (Copyright (2012), with permission from Elsevier).*

with sodium tungstate as a precursor material by Kharade et al. [60–62]. The research group synthesizes various nanoparticles and nanofilms for electrochromic purposes. In 2012 WO3 nanofilms were deposited on the FTO substrate, which was the first time used MW-assisted two-step process. In the first step, the preparation of the gel was conducted with microwave assistance, then in the second step, the deposition of the thin film occurred by a chemical growth set up. Scanning electron microscope (SEM) showed that the surface is coated with petal-like WO3 nanodisks with dimensions of 450–600 nm length, 350–400 nm width, and 20–35 nm thickness. The X-ray diffraction (XRD) analysis (**Figure 9**) points out that WO3 is in the hexagonal crystal form. Narrow and intense XRD peaks indicate that the material has good crystallinity and calculations determined that the crystal size is 71 nm, which is comparable to samples made by the regular sol–gel method [63]. X-ray photoelectron spectroscopy (XPS) revealed that the W:O ratio is nonstoichiometric(2.89). Electrochromic capabilities were determined with different electroanalytical methods [60]. Comparing this to a regular sol–gel method shows that the morphology of the surface, namely the platelet like nanodisks is nearly the same with a small difference in size (regular sol–gel platelets: 10–30 nm thick and few hundred nm lengths and width). However, to achieve the same crystallinity a 500°C annealing process is required for the regular sol–gel method, in contrast to

sol–gel process in the presence of the microwaves.

It was also observed that the effect of microwaves on the properties of the resulted materials is higher in the case of V-doped TiO2 samples, fact that could be correlated to an enhancement of the reactions between Ti and V reagents during the

As presented in the several references, **WO3 based nanomaterials** are widely investigated in the field of electrochromic devices [56], gas sensing [57], and photocatalysis [58] in different morphologies and structures. Even though the sol– gel process has a long past and is an intensely researched method [59] the literature of sol–gel preparation of WO3 using microwave-assistance is scarce. The following

Different nanostructures were prepared by microwave assisted sol–gel method

**108**

**Figure 9.**

synthesized, but the characteristics of the non-coated samples were done. Hexagonal and monoclinic nanoparticles were prepared using controlled annealing of the samples.

Similarly, further annealing is needed to reach a comparable crystallinity, but for the monoclinic structure it's obligatory. The size of the crystals was 50–70 nm and 60–90 nm for hexagonal, and for the irregular shaped monoclinic WO3 nanoparticles respectively. The hexagonal WO3 nanowires were analogous to the earlier nanowire, several μm long and 5–10 nm diameter. The TiO2 coated nanostructures proved to be efficient photocatalysts [67, 68].
