**4. Conclusion**

228 Heat Treatment – Conventional and Novel Applications

following equation known as the Tauc plot [83]:

different dipping.

become [84, 85]:

eV at 350°C and 450°C respectively.

that of undoped TiO2 (3.50 eV).

and different thickness.

*3.3.2.3. Optical band gap:* 

*Thickness d (nm)* 

*T (°C) 4 Dipping 6 Dipping 8 Dipping 350°C* 127 194 268 *400°C* 139 216 274 *450°C* 158 233 289 **Table 6.** Variation of calculated lm thicknesses d (nm) for different annealing temperatures and

The band gap is then found as the intercept of the linear portion of the plot. For a direct band gap semi-conductor, the absorption near the band edge can be estimated from the

> ( ) ( )*<sup>n</sup> <sup>g</sup> ah C h E*

Where C is a constant, Eg the optical band, α is the optical absorption coefcient, h*ν* is the photon energy gap, h the Plank's constant and the exponent n characterizes the nature of band transition; the values of n = 1/2 and 3/2 correspond to direct allowed and direct forbidden transitions, n = 2 and 3 are related to indirect allowed and indirect forbidden transitions [83] and in the cases for a direct band gap semi-conductor like TiO2 the relation

( ) <sup>2</sup> () *<sup>g</sup> ah C h E*

The energy band gap (Eg) of the lms can be estimated by plotting (αh*ν*)2 versus h*ν* (Figure 17), then extrapolating the straight-line part of the plot to the photon energy axis. The energy band gap of 5% ZrO2-doped TiO2 lms, given in table 7, decrease owing to an increase in annealing temperatures and the number of dipping. The values are 3.65 and 3.54

This decrease was correlated with grains size increases with temperature, when the latter increases the defects and impurities tend to disappear causing a reorganization of the structure. We find that doping with ZrO2 causes an increase in the band gap by contrast to

*T (°C) 3 Dipping 4 Dipping 6 Dipping 8 Dipping 350°C* 3,79 3,74 3,71 3,65 *400°C* 3,73 3,70 3,68 3,59 *450°C* 3,67 3,63 3,61 3,54 **Table 7.** Variation of band gap of 5% ZrO2-doped TiO2 thin films for different annealing temperatures

*Band gap (eV)* 

 ν= −

 ν= −

ν

ν

In this study, we investigated the transformation behaviors and the effect; of a smaller ratio range of ZrO2; doping on the surface area of TiO2 thin films, band gap energy, variations of crystal granularity, phase composition and especially on the evolution of the crystallite size and defects concentration with annealing treatments and layers thickness of the samples produced. So that in this chapter, we report the study of structural, thermal and optical properties of ZrO2-doped TiO2 thin films deposited by the sol–gel process

Analyses of doped TiO2 xerogel show that addition of 5% ZrO2 would be largely sufficient to form nanoparticles of anatase (size of grain of 14.78 nm) by contrast to that of undoped TiO2. X-ray diffraction and Raman spectroscopy analyses exhibit that doped thin films obtained starting from annealing at 350°C crystallize in both anatase and brookite phases. Calculation of grain sizes by Scherrer's formula, gives sizes ranging from 8.58 to 20.56 nm and we note an increase in grain sizes by increasing the annealing temperature for all structures. Raman spectroscopy studies confirms the results found by XRD and reveal that

#### 230 Heat Treatment – Conventional and Novel Applications

the films annealed from 350 to 450◦C crystallizes in anatase and brookite structure. From the DSC analysis, we have demonstrated that an annealing temperature equal or higher than 340 °C for undoped and 260 ◦C for 5% ZrO2-doped would be sufficient to form titanium oxide. The addition of 5% of zirconium oxide led to a shift of exothermic peak phase towards lower temperatures, due to the speeding up of the crystallization of titanium oxide compared to the undoped one.

Synthesis, Characterization and Properties of

Zirconium Oxide (ZrO2)-Doped Titanium Oxide (TiO2) Thin Films Obtained via Sol-Gel Process 231

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Analysis of UV-VIS transmission spectra shows that the 5% ZrO2-doped TiO2 thin films are transparent in the visible range and opaque in the UV region, whatever the annealing temperature and the number of dipping. Refractive index of the thin films of titanium oxide increases with increasing annealing temperature and number of dipping, but the porosity decreases, due to phase transition (anatase, anatase–brookite), which increases grain sizes and/or density of layers. Energy band gap of 5% ZrO2-doped TiO2 lms decrease owing to an increase in annealing temperatures, also we find that doping with ZrO2 causes an increase in the band gap by contrast to that of undoped TiO2.

The optical properties of the films are found to be closely related to the microstructure and crystallographic structure which depend on the annealing temperature. In summary, In this study, we successfully fabricated ZrO2-doped TiO2 thin films, with desired structural and optical properties by sol–gel dip coating using the titanium alkoxide (tetrabutylorthotitanate) as a starting material.
