**4.2 Oxide compounds**

Let us deal with the results regarding the synthesis by microwave-assisted sol– gel methods of the precursor powders for SrCu2O2 preparation.

The interest for the SrCu2O2 compound are connected to its possible applications as thermoelectric or full oxide electronic devices, solar cells, liquid-crystal displays, touchscreen, and so on [45].

Among the CuO-based p-type TCOs, Cu-Sr-O has received attention due to its wide direct band gap, and its potential use in transparent optoelectronic devices; such as light-emitting diodes, laser diodes, solar cells, display technology, and other technologies [69]. In most of the published reports, Cu based p-type TCO thin films are deposited by high vacuum processes which are costly. Some of the processes include pulsed laser deposition (PLD), reactive evaporation, magnetron sputtering, thermal co-evaporation and radio frequency [70–76]. To date, few studies have reported on the preparation of a Cu-based p-type TCO by a non-vacuum solution chemical route.

Roy et al. [77] used sol–gel and annealing methods to prepare Cu2SrO2 thin films. They used different oxygen pressure, annealing time, and temperature combinations to attempt to obtain phase pure Cu2SrO2 thin films. Copper (II) methoxide and triethanolamine were mixed in the ration 1:1. Pure Sr-metal was dissolved separately in distilled anhydrous isopropanol under argon. The Cu-solution was then mixed drop-wise into the Sr-solution while stirring. The mixture was stirred continuously for 2 hrs at room temperature. The sol was spin-coated on clean substrates with 3000 rpm for 30 s. The coated films were heated at 225°C for 2 min in the air for partial pyrolysis. This coating/heating cycle was repeated ten times to obtain films of the desired thickness of 500 nm. After deposition, the film was annealed further under controlled oxygen pressure. Different annealing procedures were used to avoid the presence of excess Cu2O phase.

XRD analysis (**Figure 11b**) showed the films had a mixed-phase of excess Cu2O and Cu2SrO2 after final reduced-oxygen pressure annealing. Films annealed at lower oxygen pressure (1.3 × 10−2 and 1.3× 10−3 Pa) had similar phase composition and in all the three films Cu2O formed as a secondary phase with Cu2SrO2. For the film annealed at the highest oxygen pressure (1.3 × 10−1 Pa), CuSrO2 was observed as the amount of Cu2SrO2 decreased and the intensity of the Cu2O peaks did not change.

Both SEM and TEM images (**Figure 11a** and **c**) show that two phases are present. The light-gray particles (differing sizes) in the SEM and large particles in TEM images are the Cu2SrO2 phases. The dark gray phase in the SEM image is a mixture of small Cu2SrO2 and Cu2O particles, as confirmed by the TEM images. The SEM and TEM images reveal that the Cu2O and Cu2SrO2 phases are intermingled with each other.

Ginley et al. [78] used sol–gel and annealing to prepare pure phased Cu2SrO2 films. Stoichiometric amounts of aqueous solutions copper formate and strontium acetate were mixed in methanol and stirred. Triethanolamine was added, the

**111**

**Figure 12.**

**Figure 11.**

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

mixture stirred and evaporated at 80°C to form sol which was diluted by isopropyl alcohol and spin-coated on MgO (100) substrates for 20 s, at 3000 revs per min. The resulting films were annealed at 200°C temperature for 2 min and then pyrolyzed at 500°C for 2 min. The spin-coating and pyrolysis cycles were repeated 8–10 times. After the cycles, the films were first annealed at 750°C for 30 min in air and then at 775°C under 2.7 × 10−6 Torr oxygen. The films were characterized by XRD

*(a) TEM image, (b) XRD spectra (c) SEM image of the films after final annealing at 750°C under* 

Predoana et al., are the first to report the synthesis of Sr-Cu-O gels by microwave (MW) assisted sol–gel methods [45]. Pure strontium acetyl acetonate (Sr(C5H7O2)2 and copper (II) acetyl acetonate (Cu(C5H7O2)2) were used as precursors for strontium and copper, respectively. The 0.25 M aqueous solutions of Sr.(C5H7O2)2 and Cu(C5H7O2)2 solution in absolute ethanol were mixed with triethanolamine, in the ratio 1:1. In the case of the sol–gel method, the starting solution was homogenized under vigorous stirring for 2 h at 80 C. For MW assisted sol–gel method, the same starting solution was homogenized by stirring and exposing to microwaves having power ~ 300 W and 2.45 GHz frequency for 5 minutes. The sol–gel and the microwaveassisted sol–gel prepared Sr-Cu-O were characterized by SEM, FTIR, XRD, and their thermal properties investigated by TG/DTA-MS in air, inert and reducing atmospheres.

(**Figure 12**) and FTIR and showed to be phase pure.

*XRD patterns of Cu2SrO2 films as a function of processing time [78].*

*1.3 × 10−2 Pa oxygen pressure [77] (Reproduced by the permission of Elsevier).*

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

*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 11.**

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

synthesized, but the characteristics of the non-coated samples were done.

of the samples.

**4.2 Oxide compounds**

touchscreen, and so on [45].

proved to be efficient photocatalysts [67, 68].

Hexagonal and monoclinic nanoparticles were prepared using controlled annealing

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

Let us deal with the results regarding the synthesis by microwave-assisted sol–

Among the CuO-based p-type TCOs, Cu-Sr-O has received attention due to its wide direct band gap, and its potential use in transparent optoelectronic devices; such as light-emitting diodes, laser diodes, solar cells, display technology, and other technologies [69]. In most of the published reports, Cu based p-type TCO thin films are deposited by high vacuum processes which are costly. Some of the processes include pulsed laser deposition (PLD), reactive evaporation, magnetron sputtering, thermal co-evaporation and radio frequency [70–76]. To date, few studies have reported on the preparation of a Cu-based p-type TCO by a non-vacuum solution chemical route. Roy et al. [77] used sol–gel and annealing methods to prepare Cu2SrO2 thin films. They used different oxygen pressure, annealing time, and temperature combinations to attempt to obtain phase pure Cu2SrO2 thin films. Copper (II) methoxide and triethanolamine were mixed in the ration 1:1. Pure Sr-metal was dissolved separately in distilled anhydrous isopropanol under argon. The Cu-solution was then mixed drop-wise into the Sr-solution while stirring. The mixture was stirred continuously for 2 hrs at room temperature. The sol was spin-coated on clean substrates with 3000 rpm for 30 s. The coated films were heated at 225°C for 2 min in the air for partial pyrolysis. This coating/heating cycle was repeated ten times to obtain films of the desired thickness of 500 nm. After deposition, the film was annealed further under controlled oxygen pressure. Different annealing procedures

XRD analysis (**Figure 11b**) showed the films had a mixed-phase of excess Cu2O

Both SEM and TEM images (**Figure 11a** and **c**) show that two phases are present. The light-gray particles (differing sizes) in the SEM and large particles in TEM images are the Cu2SrO2 phases. The dark gray phase in the SEM image is a mixture of small Cu2SrO2 and Cu2O particles, as confirmed by the TEM images. The SEM and TEM images reveal that the Cu2O and Cu2SrO2 phases are intermingled with each other. Ginley et al. [78] used sol–gel and annealing to prepare pure phased Cu2SrO2 films. Stoichiometric amounts of aqueous solutions copper formate and strontium acetate were mixed in methanol and stirred. Triethanolamine was added, the

and Cu2SrO2 after final reduced-oxygen pressure annealing. Films annealed at lower oxygen pressure (1.3 × 10−2 and 1.3× 10−3 Pa) had similar phase composition and in all the three films Cu2O formed as a secondary phase with Cu2SrO2. For the film annealed at the highest oxygen pressure (1.3 × 10−1 Pa), CuSrO2 was observed as the amount of Cu2SrO2 decreased and the intensity of the Cu2O peaks did

The interest for the SrCu2O2 compound are connected to its possible applications as thermoelectric or full oxide electronic devices, solar cells, liquid-crystal displays,

gel methods of the precursor powders for SrCu2O2 preparation.

were used to avoid the presence of excess Cu2O phase.

**110**

not change.

*(a) TEM image, (b) XRD spectra (c) SEM image of the films after final annealing at 750°C under 1.3 × 10−2 Pa oxygen pressure [77] (Reproduced by the permission of Elsevier).*

mixture stirred and evaporated at 80°C to form sol which was diluted by isopropyl alcohol and spin-coated on MgO (100) substrates for 20 s, at 3000 revs per min. The resulting films were annealed at 200°C temperature for 2 min and then pyrolyzed at 500°C for 2 min. The spin-coating and pyrolysis cycles were repeated 8–10 times. After the cycles, the films were first annealed at 750°C for 30 min in air and then at 775°C under 2.7 × 10−6 Torr oxygen. The films were characterized by XRD (**Figure 12**) and FTIR and showed to be phase pure.

Predoana et al., are the first to report the synthesis of Sr-Cu-O gels by microwave (MW) assisted sol–gel methods [45]. Pure strontium acetyl acetonate (Sr(C5H7O2)2 and copper (II) acetyl acetonate (Cu(C5H7O2)2) were used as precursors for strontium and copper, respectively. The 0.25 M aqueous solutions of Sr.(C5H7O2)2 and Cu(C5H7O2)2 solution in absolute ethanol were mixed with triethanolamine, in the ratio 1:1. In the case of the sol–gel method, the starting solution was homogenized under vigorous stirring for 2 h at 80 C. For MW assisted sol–gel method, the same starting solution was homogenized by stirring and exposing to microwaves having power ~ 300 W and 2.45 GHz frequency for 5 minutes. The sol–gel and the microwaveassisted sol–gel prepared Sr-Cu-O were characterized by SEM, FTIR, XRD, and their thermal properties investigated by TG/DTA-MS in air, inert and reducing atmospheres.

**Figure 12.** *XRD patterns of Cu2SrO2 films as a function of processing time [78].*

#### **Figure 13.**

*Thermal decomposition in air (a) sol–gel synthesized sample, (b) MW assisted sol–gel synthesized sample, (c) FTIR spectra of sol–gel synthesized sample, (d) FTIR spectra of MW assisted sol–gel synthesized sample [45] (Reproduced by the permission of Elsevier).*

In the experimental conditions presented above pieces of gels of different size, and blueish-green color were obtained for both preparation methods. The results obtained by TG/DTA-MS analysis (**Figure 13a** and **b**) of the obtained gels demonstrated the influence of MW on the sol–gel synthesis. MW treated samples had one more mass loss step when heated in air attributed to complex compositions of the resulted gels that contain a higher number of molecular species with higher thermal stability. The results were confirmed with the FTIR spectra (**Figure 13c** and **d**) showing more vibration bands for the samples prepared by the MW sol-gel method, assigned according to [79–81].

Based on the XRD patterns of the residues (**Figure 14**), the final product is composed of a mixture of phases that depend on the synthesis route and the annealing conditions.

For samples annealed in air, Sr–Cu–O phase was also present for the sol–gel synthesized sample, while the MW sample had CuO as the main component. In different atmosphere (N2 and H2/Ar) several compounds (Sr2CuO3, SrO and CuO) are present in varying amounts. Only traces of SrCO3 can be detected. In all annealing atmospheres, in the case of the samples synthesized by MW-assisted sol–gel method, powders with a lower degree of crystallization is formed. This result could be attributed to the formation of a higher number of molecular species with higher thermal stability.

**113**

**5. Conclusions**

thermally treatment of the oxide films.

place during the sol–gel synthesis is less investigated.

ence on the properties of the resulted nanostructure.

synthesis and obtaining nanostructures with improved properties.

properties.

**Figure 14.**

*and 5%H2/95%Ar [45].*

presented.

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

The powders prepared in the mentioned conditions are intended to be investi-

*(a) XRD patterns of sol–gel synthesized samples (b) MW-assisted sol–gel samples annealed at 900°C in air, N2*

The presented results are important revealing the effect of MW on the reactions that take place during the sol–gel synthesis but should be considered preliminary. Direct methods of the solutions investigations, as High-Pressure Liquid Cromatogaphy (HPLC), are underway in order to bring more information on the

The interest of using microwaves in obtaining oxide nanostructures by reactions

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 to crystallize the amorphous oxide nanopowders as well as for drying and

However, the influence of the microwaves on the chemical reactions that take

Results regarding the formation of pure or doped nanostructures, as well as oxide compound, by sol–gel method in the presence or absence of microwave are

The main results of the studies have shown that in all cases in the presence of microwave formation different molecular species is observed with a positive influ-

The advantage of using the MW-assited sol–gel method is a more shorter time of

in solutions is rather high, leading to obtaining powders or films with enhanced

gated as precursors for SrCu2O2 compound preparation.

sol–gel chemistry in the presence and the absence of microwaves.

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

*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 14.**

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

In the experimental conditions presented above pieces of gels of different size, and blueish-green color were obtained for both preparation methods. The results obtained by TG/DTA-MS analysis (**Figure 13a** and **b**) of the obtained gels demonstrated the influence of MW on the sol–gel synthesis. MW treated samples had one more mass loss step when heated in air attributed to complex compositions of the resulted gels that contain a higher number of molecular species with higher thermal stability. The results were confirmed with the FTIR spectra (**Figure 13c** and **d**) showing more vibration bands for the samples pre-

*Thermal decomposition in air (a) sol–gel synthesized sample, (b) MW assisted sol–gel synthesized sample, (c) FTIR spectra of sol–gel synthesized sample, (d) FTIR spectra of MW assisted sol–gel synthesized sample [45]* 

Based on the XRD patterns of the residues (**Figure 14**), the final product is composed of a mixture of phases that depend on the synthesis route and the annealing

For samples annealed in air, Sr–Cu–O phase was also present for the sol–gel synthesized sample, while the MW sample had CuO as the main component. In different atmosphere (N2 and H2/Ar) several compounds (Sr2CuO3, SrO and CuO) are present in varying amounts. Only traces of SrCO3 can be detected. In all annealing atmospheres, in the case of the samples synthesized by MW-assisted sol–gel method, powders with a lower degree of crystallization is formed. This result could be attributed to the formation of a higher number of molecular species with higher

pared by the MW sol-gel method, assigned according to [79–81].

**112**

conditions.

**Figure 13.**

*(Reproduced by the permission of Elsevier).*

thermal stability.

*(a) XRD patterns of sol–gel synthesized samples (b) MW-assisted sol–gel samples annealed at 900°C in air, N2 and 5%H2/95%Ar [45].*

The powders prepared in the mentioned conditions are intended to be investigated as precursors for SrCu2O2 compound preparation.

The presented results are important revealing the effect of MW on the reactions that take place during the sol–gel synthesis but should be considered preliminary. Direct methods of the solutions investigations, as High-Pressure Liquid Cromatogaphy (HPLC), are underway in order to bring more information on the sol–gel chemistry in the presence and the absence of microwaves.
