**5. Importance and applications of TiO2 thin films**

Due to its interesting intrinsic properties, TiO2 thin films have great importance and significance for a large variety of industrial applications. Titanium oxide, which

**49**

*T i* 4+ [

−*O* − *CH* (*CH*)3

*Titanium Dioxide Versatile Solid Crystalline: An Overview*

belongs to the metal transition oxide family, was the most studied during the last two decades and demanded material in many fields of applications such as transparent electrodes, gas sensors, solar cells (PV), photocatalytic process, etc. To improve the performance of this oxide, doping TiO2 with suitable metal ion dopants offers an effective method to adjust some of its physical properties. Generally, the doping of semiconductors with appropriate metallic element (Al, Nb, Sn, Ge, Fe, Ni and Cr) is one of the most effective ways in research for developing sensitivity applications such as photovoltaic solar cells, photocatalysis and pollution sensors. However, the interaction between the doping metals and the semiconductor is complicated because the interaction relates to the carrier concentration, defect level and surface

The sol-gel technique is a suitable method for deriving nano-TiO2 having unique

metastable structure at low temperatures and excellent chemical homogeneity [33]. The novelty of the work is to synthesize nanosized (<25 nm) TiO2 particles by optimized preparatory parameters associated with sol-gel method and characterize

for their structural and optical behavior useful to photovoltaic application. The synthesis process includes that the precursor TTIP, 3.5 mL, was slowly added to the mixture of isopropanol (25 mL), concentric hydrochloric acid (0.1 mL, catalyst) and distilled water (0.2 mL) with constant stirring for 30 min. The mixture undergoes hydrolysis reaction resulting in transparent pale yellowish solution. Then, the solution was allowed 24 h for gelation period. The gel was dried at 373 K for an hour and finely ground with mortar. Finally, the TiO2 powder was calcinated at 673 K for an hour to obtain a nanosized particle with desired phase [34]. The different steps are involved in synthesis of nano-TiO2 powder by sol-gel method and

it was represented by the chemical reaction [35] in the system is

]4 *H*2*O*<sup>→</sup>

*H*+ *Isopropanal*

*Ti O*<sup>2</sup> + 2 *H*2*O*

Characterization of TiO2 powder was carried out by using X-ray diffraction analysis (**Figure 5**) with XPERT-PRO X-ray diffractometer in the range of 2θ values from 20° to 80° (λ = 0.1540 nm). The 7.7-nm-sized particles were determined from Debye-Scherrer's formula. The surface morphology of TiO2 pellet (**Figure6**) obtained using the VEGA3 TESCAN scanning electron microscope confirms nanosize spherical-shaped particles uniformly distributed without any aggregation and atomic force microscope AFM XE-100 topography images (**Figure 7**) exhibit the distribution of uniform spherical-shaped particles. The chemical compositions such as Ti (26.36 at. wt.%), O (68.5 at. wt.%) and C (5.07 at. wt.%) in the prepared TiO2 powder were confirmed by the energy dispersive X-ray (EDX) spectroscopy (**Figure 8**). The absorption and transmission spectra were obtained using Perkin Elmer Lambda 35 UV/ Vis Spectrophotometer and its transmittance is about 60% (**Figure 9**). The Fourier transform infrared (FTIR) spectra (**Figure 10**) were taken using SPECTRUM-RX 2 and represents the characteristic peaks in the range of wave numbers 4000–400 cm<sup>−</sup><sup>1</sup>

The TiO2 nanoparticles of size 7.7 nm have been prepared by optimized sol-gel technique. The XRD analysis reveals that the TiO2 powder was highly crystalline (anatase phase) and nanostructured with tetragonal system. The SEM images exhibit the nanosized TiO2 particles with less densification nature. The AFM

*Ti* (*OH*)<sup>4</sup> <sup>+</sup> 4[(*<sup>C</sup> <sup>H</sup>*3)2*CH* <sup>−</sup> *OH*]

↓*Δ*(450*<sup>o</sup>*

*C*)

(1)

.

states of the semiconductor, electronic, optical properties, and so on.

*DOI: http://dx.doi.org/10.5772/intechopen.92056*

**6. Synthesis of titania**

*Titanium Dioxide Versatile Solid Crystalline: An Overview DOI: http://dx.doi.org/10.5772/intechopen.92056*

belongs to the metal transition oxide family, was the most studied during the last two decades and demanded material in many fields of applications such as transparent electrodes, gas sensors, solar cells (PV), photocatalytic process, etc. To improve the performance of this oxide, doping TiO2 with suitable metal ion dopants offers an effective method to adjust some of its physical properties. Generally, the doping of semiconductors with appropriate metallic element (Al, Nb, Sn, Ge, Fe, Ni and Cr) is one of the most effective ways in research for developing sensitivity applications such as photovoltaic solar cells, photocatalysis and pollution sensors. However, the interaction between the doping metals and the semiconductor is complicated because the interaction relates to the carrier concentration, defect level and surface states of the semiconductor, electronic, optical properties, and so on.

#### **6. Synthesis of titania**

*Assorted Dimensional Reconfigurable Materials*

conversion to rutile involves a collapse of the anatase structure, which is irreversible [21, 22]. Although rutile and anatase are both of tetragonal crystallographic structure,

The mechanism for the sintering and transformation of anatase into rutile involves several steps. Initially, the smallest particles coalesce, forming bigger particles. The fractions of particles that are already large have been shown not to undergo sintering. The heat evolved from the exothermic sintering process causes the local nucleation of the rutile phase. Finally, as the conversion to rutile is also an exothermic process, this results in the transformation of the whole particle to rutile.

In many research works, researchers have observed that the inclusion of a certain amount of impurities into TiO2 can drastically alter the physical properties of the film. It has been shown that silicon and phosphorus inhibit the transformation from anatase to rutile, with 100% anatase phase being retained at temperatures as high as 870*°*C for up to 3 h for thin films 80 K and 1500 K for bulk samples [29]. The retardation of the anatase-rutile transformation can be achieved with impurities [29]. Most researchers agree that oxygen vacancies are responsible for the overall transformation mechanism [30]. Thus, the oxides and fluorides that assist the transformation can substitute for Ti+4 in the anatase lattice, resulting in the creation of oxygen vacancies. On the other hand, the inhibiting effect of other impurities involves the

*4.2.2 The effect of impurities on the anatase-rutile phase transformation*

reduction of oxygen vacancies due its substitution into the anatase lattice.

precursor, and this results in chlorine contamination of the deposited film.

**5. Importance and applications of TiO2 thin films**

Titanium alkoxides are common TiO2 precursors, with the most frequently used being titanium tetra isopropoxide (TTIP) (also called tetra isopropyl titanate). The residue of the organic binders results in carbon contamination of typically a few atomic weight percentage (at. wt.%), but as high as 13 at. wt.% being observed [31]. It is likely that carbon incorporation could be higher at low growth temperatures, as when higher temperatures were used the carbonate species decomposed, resulting in the removal of hydrocarbon fragments [32]. Titanium tetrachloride (TiCl4) is another common TiO2

Due to its interesting intrinsic properties, TiO2 thin films have great importance and significance for a large variety of industrial applications. Titanium oxide, which

The deposition of TiO2 thin films is formed by chemical reaction, using chemical vapor deposition (CVD), and spray pyrolysis and hydrolysis systems. In this scenario, the substrate temperature is the primary means of controlling the deposited phase of the material. In contrast, physical vapor deposition (PVD) systems, such as evaporation, sputtering, and ion-beam deposition, are used to determine the structure with its phase primarily by the kinetic energy of the impinging atoms. Therefore, the progression through the amorphous, anatase and rutile phases may not necessarily be expected. This is confirmed by the occurrence of rutile films at low deposition temperatures (*<* 450*°*C) by carefully optimized deposition methods, [23, 24] ion-assisted deposition [25] and reactive evaporation [26]. The TiO2 films are formed by a chemical reaction, where the substrate temperature dominates film growth characteristics. Several researchers observed that the processing temperatures required to convert an anatase film into a rutile one are much higher than temperature required depositing a rutile film directly [26–28]. The variation in physical and chemical properties of the films is determined solely by the maximum processing temperature, whether the deposition temperature or a subsequent

rutile is more densely packed and thus possesses a greater density.

annealing temperature was observed by researchers [20].

**48**

The sol-gel technique is a suitable method for deriving nano-TiO2 having unique metastable structure at low temperatures and excellent chemical homogeneity [33]. The novelty of the work is to synthesize nanosized (<25 nm) TiO2 particles by optimized preparatory parameters associated with sol-gel method and characterize for their structural and optical behavior useful to photovoltaic application.

The synthesis process includes that the precursor TTIP, 3.5 mL, was slowly added to the mixture of isopropanol (25 mL), concentric hydrochloric acid (0.1 mL, catalyst) and distilled water (0.2 mL) with constant stirring for 30 min. The mixture undergoes hydrolysis reaction resulting in transparent pale yellowish solution. Then, the solution was allowed 24 h for gelation period. The gel was dried at 373 K for an hour and finely ground with mortar. Finally, the TiO2 powder was calcinated at 673 K for an hour to obtain a nanosized particle with desired phase [34]. The different steps are involved in synthesis of nano-TiO2 powder by sol-gel method and it was represented by the chemical reaction [35] in the system is

$$\begin{aligned} \text{Mass represented by une cennica reaction } \{\text{\textperig\\_in un une système} \\\\ Ti^4 \left[ -O - CH \{CH\}\_3 \right] \downarrow H\_2O \stackrel{\text{Jomponal}}{\underset{H}{\rightarrow}} Ti \left( OH \right)\_4 + 4 \left[ \{CH\_3\}\_2CH - OH \right] \\\\ \downarrow \Delta \{ 450^\circ C\} \\ TiO\_2 + 2H\_2O \end{aligned} \tag{1}$$

Characterization of TiO2 powder was carried out by using X-ray diffraction analysis (**Figure 5**) with XPERT-PRO X-ray diffractometer in the range of 2θ values from 20° to 80° (λ = 0.1540 nm). The 7.7-nm-sized particles were determined from Debye-Scherrer's formula. The surface morphology of TiO2 pellet (**Figure6**) obtained using the VEGA3 TESCAN scanning electron microscope confirms nanosize spherical-shaped particles uniformly distributed without any aggregation and atomic force microscope AFM XE-100 topography images (**Figure 7**) exhibit the distribution of uniform spherical-shaped particles. The chemical compositions such as Ti (26.36 at. wt.%), O (68.5 at. wt.%) and C (5.07 at. wt.%) in the prepared TiO2 powder were confirmed by the energy dispersive X-ray (EDX) spectroscopy (**Figure 8**). The absorption and transmission spectra were obtained using Perkin Elmer Lambda 35 UV/ Vis Spectrophotometer and its transmittance is about 60% (**Figure 9**). The Fourier transform infrared (FTIR) spectra (**Figure 10**) were taken using SPECTRUM-RX 2 and represents the characteristic peaks in the range of wave numbers 4000–400 cm<sup>−</sup><sup>1</sup> .

The TiO2 nanoparticles of size 7.7 nm have been prepared by optimized sol-gel technique. The XRD analysis reveals that the TiO2 powder was highly crystalline (anatase phase) and nanostructured with tetragonal system. The SEM images exhibit the nanosized TiO2 particles with less densification nature. The AFM

study confirms the uniform distribution of spherical-shaped particles. The optical band gap of the TiO2 is found to be 3.45 eV making it suitable for solar cell applications.

#### **Figure 5.**

*X-ray diffraction pattern of TiO2 powder calcined at 450°C.*

**Figure 6.**

*SEM micrographs of TiO2 powder (a) before (b) and (c) calcined at 450°C with different magnifications.*

**51**

**Figure 10.**

*Titanium Dioxide Versatile Solid Crystalline: An Overview*

*DOI: http://dx.doi.org/10.5772/intechopen.92056*

*AFM topographic images of TiO2 powder calcined at 450°C.*

*UV (a) transmittance and (b) absorption spectra of TiO2 powder calcined at 450°C.*

*The Fourier transform infrared (FTIR) spectra of TiO2 powder calcined at 450°C.*

**Figure 8.**

**Figure 9.**

**Figure 7.** *EDAX spectra of TiO2 powder calcined at 450°C.*

*Titanium Dioxide Versatile Solid Crystalline: An Overview DOI: http://dx.doi.org/10.5772/intechopen.92056*

#### **Figure 8.**

*Assorted Dimensional Reconfigurable Materials*

*X-ray diffraction pattern of TiO2 powder calcined at 450°C.*

applications.

study confirms the uniform distribution of spherical-shaped particles. The optical band gap of the TiO2 is found to be 3.45 eV making it suitable for solar cell

*SEM micrographs of TiO2 powder (a) before (b) and (c) calcined at 450°C with different magnifications.*

**50**

**Figure 7.**

**Figure 6.**

**Figure 5.**

*EDAX spectra of TiO2 powder calcined at 450°C.*

**Figure 10.** *The Fourier transform infrared (FTIR) spectra of TiO2 powder calcined at 450°C.*

## **7. Preparation of sol-gel routed nano-TiO2 thin films and their effect of molarity**

Nanocrystalline titanium dioxide (TiO2) films are extensively studied because of their interesting chemical, electrical and optical properties. TiO2 is one of the most important transition metal oxide semiconductors with wide band gap. A wide variety of techniques have been used to prepare titania films. Among these, the sol-gel routed spin coating technique has emerged as one of the most promising methods as it produces films by simple synthetic route with good homogeneity, low cost, excellent compositional control and feasibility of producing thin films on large complex shapes with low crystallization temperature.

This work is keeping the optimization of the processing parameters such as pH value (~8), amount of catalyst (HCl), spin speed (3000 rpm) and calcination temperature (450°C) constant to prepare nano-TiO2 thin films with molar concentrations 0.05 M, 0.1 M, 0.15 M and 0.2 M by sol-gel routed spin coated technique. And also the study of the effect of molarity on structural, optical and electrical behaviors is useful to photovoltaic applications.

The titanium tetra isopropoxide (TTIP) was used as a precursor, hydrochloric acid as a chelating agent, isopropanol and deionized water as a solvent. Triton X-100 was used as a stabilizer to avoid precipitation in solution and at the same time used to increase the conductivity of films. TTIP (3.5 ml) was slowly added to the mixture of isopropanol (25 ml), concentric Hydrochloric acid (0.1 ml, catalyst) and distilled water (0.2 ml) with constant stirring for 30 min. Introduction of isopropanol prior to TTIP induces immediate precipitation due to highly reactive alkoxide, therefore Triton X-100 was added as a stabilizing agent for the hydrolysis reaction. The resultant alkoxide solution was kept at room temperature for hydrolysis reaction for 2 hours, resulting in a transparent pale yellowish TiO2 sol. The hydrolysis and the poly condensation of titanium alkoxides proceeds according to the mechanism Eq. (1)). TiO2 sol was deposited on to a glass substrate by a spin coating unit with spin rate at 3000 rpm for 60s

**Figure 11.** *X-ray diffraction pattern of TiO2 thin films at different molar concentrations calcined at 450°C.*

**53**

**Figure 12.**

**Figure 13.**

*Titanium Dioxide Versatile Solid Crystalline: An Overview*

in air and dried on a hot plate at 100°C for 60 seconds. The prepared samples

*Optical band gap energy of TiO2 thin films at different molar concentrations calcined at 450°C.*

*SEM micrographs of doped and undoped TiO2 thin films at different molar concentrations calcined at 450°C.*

The nanostructured titanium dioxide (TiO2) thin films were prepared using the sol–gel routed spin coating technique. The X-ray diffraction pattern of TiO2 thin films (**Figure 11**) exhibits that TiO2 particles are crystallized as anatase phase and nanostructured with the tetragonal system. The SEM images (**Figure 12**) exhibit that the particles are spherical in nature. TiO2 thin film prepared at 0.2 M concentration has a smooth surface. The roughness of the TiO2 thin film increases with the increase of molarity. The optical transmittance is found to depend on the molarity and the higher value of molarity leads to lower optical band gap energy (**Figure 13**). Hence, the nano-TiO2 thin films with higher molar concentration will be useful for photovoltaic applications due to their structural, optical and electrical behaviors.

were calcinated at 450°C for 1 hour [36].

*DOI: http://dx.doi.org/10.5772/intechopen.92056*

*Titanium Dioxide Versatile Solid Crystalline: An Overview DOI: http://dx.doi.org/10.5772/intechopen.92056*

*Assorted Dimensional Reconfigurable Materials*

complex shapes with low crystallization temperature.

behaviors is useful to photovoltaic applications.

**molarity**

**7. Preparation of sol-gel routed nano-TiO2 thin films and their effect of** 

Nanocrystalline titanium dioxide (TiO2) films are extensively studied because of their interesting chemical, electrical and optical properties. TiO2 is one of the most important transition metal oxide semiconductors with wide band gap. A wide variety of techniques have been used to prepare titania films. Among these, the sol-gel routed spin coating technique has emerged as one of the most promising methods as it produces films by simple synthetic route with good homogeneity, low cost, excellent compositional control and feasibility of producing thin films on large

This work is keeping the optimization of the processing parameters such as pH value (~8), amount of catalyst (HCl), spin speed (3000 rpm) and calcination temperature (450°C) constant to prepare nano-TiO2 thin films with molar concentrations 0.05 M, 0.1 M, 0.15 M and 0.2 M by sol-gel routed spin coated technique. And also the study of the effect of molarity on structural, optical and electrical

The titanium tetra isopropoxide (TTIP) was used as a precursor, hydrochloric acid as a chelating agent, isopropanol and deionized water as a solvent. Triton X-100 was used as a stabilizer to avoid precipitation in solution and at the same time used to increase the conductivity of films. TTIP (3.5 ml) was slowly added to the mixture of isopropanol (25 ml), concentric Hydrochloric acid (0.1 ml, catalyst) and distilled water (0.2 ml) with constant stirring for 30 min. Introduction of isopropanol prior to TTIP induces immediate precipitation due to highly reactive alkoxide, therefore Triton X-100 was added as a stabilizing agent for the hydrolysis reaction. The resultant alkoxide solution was kept at room temperature for hydrolysis reaction for 2 hours, resulting in a transparent pale yellowish TiO2 sol. The hydrolysis and the poly condensation of titanium alkoxides proceeds according to the mechanism Eq. (1)). TiO2 sol was deposited on to a glass substrate by a spin coating unit with spin rate at 3000 rpm for 60s

*X-ray diffraction pattern of TiO2 thin films at different molar concentrations calcined at 450°C.*

**52**

**Figure 11.**

**Figure 12.** *SEM micrographs of doped and undoped TiO2 thin films at different molar concentrations calcined at 450°C.*

**Figure 13.** *Optical band gap energy of TiO2 thin films at different molar concentrations calcined at 450°C.*

in air and dried on a hot plate at 100°C for 60 seconds. The prepared samples were calcinated at 450°C for 1 hour [36].

The nanostructured titanium dioxide (TiO2) thin films were prepared using the sol–gel routed spin coating technique. The X-ray diffraction pattern of TiO2 thin films (**Figure 11**) exhibits that TiO2 particles are crystallized as anatase phase and nanostructured with the tetragonal system. The SEM images (**Figure 12**) exhibit that the particles are spherical in nature. TiO2 thin film prepared at 0.2 M concentration has a smooth surface. The roughness of the TiO2 thin film increases with the increase of molarity. The optical transmittance is found to depend on the molarity and the higher value of molarity leads to lower optical band gap energy (**Figure 13**). Hence, the nano-TiO2 thin films with higher molar concentration will be useful for photovoltaic applications due to their structural, optical and electrical behaviors.
