**2.2. Results**

The TiO2 films obtained by sol-gel process using the dip-coating technique are transparent, homogeneous, adherent, durable, and free of micro-cracks. **Figure 2a** shows thin films removed

withdrawal speed, and solution viscosity. After the deposition, the gel film is formed by a solid structure impregnated with the solvent, and a drying process can be used to convert the wet gel in a dry porous film. Denser film can be tailored by different temperatures of thermal

The advantage of the dip-coating process is the ease of deposition in substrates of any size and

 thin films were prepared by sol-gel process [2, 5] using titanium isopropoxide (Aldrich, 98%) as the precursor of titania mixed with isopropyl alcohol and hydrochloric acid in stoichiometric amounts. The precursor solution was kept under agitation at room temperature for 1 h and rested until the viscosity reaches the best value condition, between 2 and 5 cP. The films were prepared using solutions with 2 < pH < 4 and atmosphere relative humidity <40%, since they are opaque and not adherent for other pH and relative humidity values. The films were deposited onto clean substrates (borosilicate glass, steel, silicon, and magnets) at room conditions (25°C, relative air humidity lower than 30%), using a dip-coating equipment with withdrawal speed between 0.2 and 1.5 mm/s. The substrates were washed with standard cleaning method before dipping. After each dip-coating process, the wet films were dried in air for 30 min and thermally treated at temperatures between 100 and 500°C for a range of time (between 10 and 60 min) to convert them into porous or densified oxide films. Depending on the thermal treatment temperature, the films can be amorphous or nanocrystalline. Some samples were submitted at UV-C light (lamp Girardi RSE20B, 254 nm—15 W) to crystallize without increasing the temperature. Crystalline structures were investigated by an X-ray diffraction (incidence angle of 5°) using a diffractometer Rigaku (Geigerflex model 3034). The samples were analyzed by atomic force microscopy (AFM) in an Asylum Research, model MFP-3D-SA, to observe the topography and possible coating defects, such as cracks and peeling. Morphological characterization was evaluated by transmission electron microscopy (FEI TECNAI G2 20 at acceleration tension of 200 kV). Electron diffraction was also used to determine the structure of the crystalline phases. The films were pulled from glass substrates and mounted onto 200 mesh copper grids coated with holey carbon films for examination. The morphology and composition were evaluated by a scanning electron microscope (SEM) FEI Quanta 200 FEG with an energy-dispersive spectrometer (EDS). The transparency and thickness of the films deposited on glasses were verified by the optical transmission spectra mea-

films obtained by sol-gel process using the dip-coating technique are transparent,

homogeneous, adherent, durable, and free of micro-cracks. **Figure 2a** shows thin films removed

treatment, leading to films with different specific surface areas and porosities.

sured with an ultraviolet and visible spectrometer (U3010, Hitachi).

shape, facilitating the industrial process.

84 Titanium Dioxide - Material for a Sustainable Environment

 **thin films**

**2. TiO2**

TiO2

**2.2. Results**

The TiO2

**2.1. Experimental**

**Figure 2.** (a) TiO2 thin films removed from a glass substrate and (b) thickness and refractive index of TiO<sup>2</sup> thin films in function of thermal treatment temperature.

from a glass substrate. The thickness of the films deposited in glass and dried in air can range from 40 to 800 nm for each coating, depending on the withdrawal speed and viscosity. After heating, the film thickness decreases due to the densification process, reaching values between 20 and 300 nm each coating. When the number of coating increased, the thickness can reach 800 nm after calcination without cracks.

After drying, the films are porous when treated at low temperatures, and the density increases as a function of heating temperature and time. The porosity of the films leads to a variation in the refraction index that can change from 1.9 to 2.3 (λ = 550 nm) for porosities between 20 and 5%, respectively. **Figure 2b** shows an example of the variation of thickness and refractive index in the function of thermal treatment temperature of TiO<sup>2</sup> film. When the TiO<sup>2</sup> films are deposited in substrates that cannot be thermally treated, such as polymers and cotton, the densification and crystallization can be made by UV light treatment. **Figure 3** shows images of TiO<sup>2</sup> films heated at 100 and 400°C for 10 min.

The film formed after drying at room temperature is amorphous and contains organic contaminants in the network. With increasing in temperature of thermal treatment, the film structure changes to anatase phase around ~300°C and to rutile phase above ~600°C.

According to the literature, the values of the phase transition of TiO<sup>2</sup> can change in some degrees also depending on the type and time of drying, used dopant, and particle size, among others factors. **Figure 4** shows typical diffractograms of TiO<sup>2</sup> films deposited in glass substrate in two temperatures, generating an amorphous material at 100°C and a nanocrystalline material at 400°C. **Figure 5** shows SEM and TEM images of the film and the respective electron diffraction that confirm its anatase phase.

the bandgap of the TiO<sup>2</sup>

TiO2

for example.

in the surface.

**Figure 5.** TiO2

(d) electron diffraction of the film.

The TiO2

anatase phase to the visible region to make it a competitive energy

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source with application in photocatalysis, solar cells, and artificial photosynthesis.

 films are also used in the preparation of hydrophobic and self-cleaning surfaces in several substrates. **Figure 7** shows water drops over the film surface and over the glass substrate surface. The contact angle can be change varying the film porosity and the number of layers,

self-cleaning surfaces have the ability to remove greasy dirt and bacteria from their

thin film images: (a) SEM image of the film over a glass substrate, (b) and (c) TEM images of the film, and

surfaces due to their photocatalytic property, which promotes the breakdown of fat molecules or destroys the membranes of bacteria. The self-cleaning property is frequently connected to hydrophobic surfaces, because the dusts can be removed by the rolling of the water droplets

**Figure 3.** Atomic force microscopy of TiO<sup>2</sup> films heated at (a) 100°C and (b) 400°C.

TiO2 thin films are used in the confection of optical devices (linear and nonlinear) due to the transparency throughout the visible spectrum, high linear and nonlinear refractive index that change in function of the wavelength, and dielectric properties. Their nonlinearity can make possible operations such as logic, all-optical switching, and wavelength conversion. Their high linear index of refraction can improve optical confinement as waveguide. The optical and electric properties of the thin films made by sol-gel process can be modulated according to the desired application. **Figure 6** shows transmittance curves of TiO<sup>2</sup> thin films deposited on glass substrates as a function of the number of layers. Each layer measures approximately 60 nm.

By these spectra it is possible to calculate the bandgap of the films using, for example, the Tauc method. The value measured in this case was 3.4 eV, meaning that the photocatalytic activity occurs at a wavelength in the UV region. Several studies are made aiming to reduce

**Figure 4.** XRD patterns of TiO<sup>2</sup> thin films heated at (a) 100°C and (b) 400°C.

the bandgap of the TiO<sup>2</sup> anatase phase to the visible region to make it a competitive energy source with application in photocatalysis, solar cells, and artificial photosynthesis.

TiO2 films are also used in the preparation of hydrophobic and self-cleaning surfaces in several substrates. **Figure 7** shows water drops over the film surface and over the glass substrate surface. The contact angle can be change varying the film porosity and the number of layers, for example.

The TiO2 self-cleaning surfaces have the ability to remove greasy dirt and bacteria from their surfaces due to their photocatalytic property, which promotes the breakdown of fat molecules or destroys the membranes of bacteria. The self-cleaning property is frequently connected to hydrophobic surfaces, because the dusts can be removed by the rolling of the water droplets in the surface.

**Figure 5.** TiO2 thin film images: (a) SEM image of the film over a glass substrate, (b) and (c) TEM images of the film, and (d) electron diffraction of the film.

**Figure 4.** XRD patterns of TiO<sup>2</sup>

approximately 60 nm.

**Figure 3.** Atomic force microscopy of TiO<sup>2</sup>

86 Titanium Dioxide - Material for a Sustainable Environment

TiO2

thin films heated at (a) 100°C and (b) 400°C.

 thin films are used in the confection of optical devices (linear and nonlinear) due to the transparency throughout the visible spectrum, high linear and nonlinear refractive index that change in function of the wavelength, and dielectric properties. Their nonlinearity can make possible operations such as logic, all-optical switching, and wavelength conversion. Their high linear index of refraction can improve optical confinement as waveguide. The optical and electric properties of the thin films made by sol-gel process can be modulated

films heated at (a) 100°C and (b) 400°C.

thin films

according to the desired application. **Figure 6** shows transmittance curves of TiO<sup>2</sup>

deposited on glass substrates as a function of the number of layers. Each layer measures

By these spectra it is possible to calculate the bandgap of the films using, for example, the Tauc method. The value measured in this case was 3.4 eV, meaning that the photocatalytic activity occurs at a wavelength in the UV region. Several studies are made aiming to reduce

hydrochloric acid, and distilled water to prepare the SiO<sup>2</sup>

prepared solutions were aged for 24 h before the mixture. Then, TiO<sup>2</sup>

solutions was maintained in 3. SiO<sup>2</sup>

**Figure 8.** 3D topographies of the five-layer xTiO<sup>2</sup>

and (f) SiO<sup>2</sup>

.

/(100-x)SiO<sup>2</sup>

films: (a) TiO<sup>2</sup>

, (b) Si20Ti80, (c) Si40Ti60, (d) Si60Ti40, (e) Si80Ti20,

precursor solution. The pH of both

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precursor solutions

89

/SiO<sup>2</sup>

precursor solution was refluxed for 24 h at 60°C. Both the

Pure and Nanocomposite Thin Films Based on TiO2 Prepared by Sol-Gel Process…

**Figure 6.** UV–Vis spectra of the films deposited on (a) both sides of a glass substrate and heated at 100°C: (a) glass, (b) one layer, (c) two layers, (d) three layers, (e) four layers, (f) five layers; (b) one side of the glass substrate and heated at 400°C: (a) one layer, (b) two layers, (c) three layers, and (d) five layers.

**Figure 7.** TiO2 thin film deposited on (a) the film surface and on (b) the glass surface.

#### **3. SiO2 /TiO2 thin films**

When Si alkoxide is mixed with Ti alkoxide to the preparation of precursors of TiO<sup>2</sup> /SiO<sup>2</sup> thin films for utilization of the sol-gel process, the nanocomposites produced can combine or enhance the properties of the well-known pure oxides: TiO<sup>2</sup> and SiO<sup>2</sup> [6]. These nanomaterials can offer enhanced photocatalytic activities, persistent superhydrophilicity, modulated refractive index, enhanced resistance to corrosion, and superior mechanical properties such as larger mechanical resistance and hardness. The deposition of TiO<sup>2</sup> /SiO<sup>2</sup> thin films in different substrates such as glasses, metals, ceramics, and polymers enables the application of these films in many purposes such as self-cleaning surfaces, antireflection surfaces, anticorrosion protection, wear resistance protection, fungicide and bactericide surfaces, water and air treatment devices, planar waveguides, nonlinear optical devices, etc. The most important fact is that two or more of these applications can be combined in TiO<sup>2</sup> /SiO<sup>2</sup> multifunctional surfaces [7, 8].

In this work, the preparation of TiO<sup>2</sup> /SiO<sup>2</sup> nanocomposite thin films was made using titanium isopropoxide (Aldrich, purity >98%), isopropyl alcohol and hydrochloric acid, to prepare the TiO2 precursor solution and tetraethyl orthosilicate (Aldrich, purity >98%), isopropyl alcohol, hydrochloric acid, and distilled water to prepare the SiO<sup>2</sup> precursor solution. The pH of both solutions was maintained in 3. SiO<sup>2</sup> precursor solution was refluxed for 24 h at 60°C. Both the prepared solutions were aged for 24 h before the mixture. Then, TiO<sup>2</sup> /SiO<sup>2</sup> precursor solutions

**3. SiO2**

**Figure 7.** TiO2

surfaces [7, 8].

TiO2

In this work, the preparation of TiO<sup>2</sup>

**/TiO2**

 **thin films**

400°C: (a) one layer, (b) two layers, (c) three layers, and (d) five layers.

88 Titanium Dioxide - Material for a Sustainable Environment

enhance the properties of the well-known pure oxides: TiO<sup>2</sup>

as larger mechanical resistance and hardness. The deposition of TiO<sup>2</sup>

fact is that two or more of these applications can be combined in TiO<sup>2</sup>

/SiO<sup>2</sup>

When Si alkoxide is mixed with Ti alkoxide to the preparation of precursors of TiO<sup>2</sup>

thin film deposited on (a) the film surface and on (b) the glass surface.

thin films for utilization of the sol-gel process, the nanocomposites produced can combine or

**Figure 6.** UV–Vis spectra of the films deposited on (a) both sides of a glass substrate and heated at 100°C: (a) glass, (b) one layer, (c) two layers, (d) three layers, (e) four layers, (f) five layers; (b) one side of the glass substrate and heated at

als can offer enhanced photocatalytic activities, persistent superhydrophilicity, modulated refractive index, enhanced resistance to corrosion, and superior mechanical properties such

ferent substrates such as glasses, metals, ceramics, and polymers enables the application of these films in many purposes such as self-cleaning surfaces, antireflection surfaces, anticorrosion protection, wear resistance protection, fungicide and bactericide surfaces, water and air treatment devices, planar waveguides, nonlinear optical devices, etc. The most important

isopropoxide (Aldrich, purity >98%), isopropyl alcohol and hydrochloric acid, to prepare the

precursor solution and tetraethyl orthosilicate (Aldrich, purity >98%), isopropyl alcohol,

and SiO<sup>2</sup>

/SiO<sup>2</sup>

/SiO<sup>2</sup>

nanocomposite thin films was made using titanium

/SiO<sup>2</sup>

[6]. These nanomateri-

thin films in dif-

multifunctional

**Figure 8.** 3D topographies of the five-layer xTiO<sup>2</sup> /(100-x)SiO<sup>2</sup> films: (a) TiO<sup>2</sup> , (b) Si20Ti80, (c) Si40Ti60, (d) Si60Ti40, (e) Si80Ti20, and (f) SiO<sup>2</sup> .

construction of different structural models of devices. The variation in the refraction index in function of incident light wavelength of 2.0–2.8 (**Figure 9b**) is also very important to the

 exhibits a high energy bandgap (3.2–3.8 eV) which corresponds to UV irradiation with a wavelength smaller than 388 nm. To overcome this limitation, several studies have been per-

coatings were prepared from alcoholic solution containing titanium isopropoxide

and silver nitrate dissolved in a mixture of isopropyl alcohol in several atomic ratios. Acid conditions (pH = 4) were reached after acetic acid addition. This precursor solution was stirred at room temperature during 1 h and submitted to UV-C irradiation (254 nm) treatment

films were deposited onto clean substrates as borosilicate, silicon, 316 L stainless steel, and magnets (NdFeB) with withdrawal speed of 8 mm s−1. After deposition, the coatings with one to five layers were dried in air for 20 min and were thermally treated for 1 h between 100 and

According to XRD patterns, the coatings heated at 400°C show indexed peak characteristic of crystalline metallic Ag and anatase phase (PDF #1-562). The diffractogram of the film heated at 100°C was characteristic of a noncrystalline material, as expected. The substrates of 316 L

The structure of the used substrates has induced the formation of nano- and microstructures

faces. This formation occurs due to thermal treatment that induces the diffusion of the metal nanoparticles to the film surface. In the borosilicate substrate (**Figure 11a**), the formation of spherical Ag nanoparticles with a bimodal particle size distribution is observed. When substrates of 316 L stainless steel and magnets (NdFeB) were used, Ag dendrite micro- and nanostructures were formed (**Figure 11b** and **c**). A trimodal size distribution is observed for

in air for 100 min. This procedure has been used to produce metallic Ag from Ag<sup>+</sup>

stainless steel and magnets showed similar XRD patterns. SEM images of Ag/TiO<sup>2</sup>

of metallic silver with different sizes and morphologies supported on the TiO<sup>2</sup>

Particularly in this film, the Ag particles show dimensions of 5–150 nm.

**Figure 10** shows the characteristic diffractogram of Ag/TiO<sup>2</sup>

400°C deposited on different substrates are shown in **Figure 11**.

the particles present on the surface of the Ag/TiO<sup>2</sup>

the light absorption to the visible range and simultaneously increasing the recombination time of the electron-hole pairs formed. In particular, nanocomposite thin films of silver and titania have been of considerable interest since silver nanoparticles can act as electron traps, contributing to electron-hole separation and creating a local electric field capable of facilitating the electron excitation and consequently their photocatalytic properties. The improvement in the photocatalytic properties leads to surfaces with better bactericide, hydrophobicity, and

with metal and nonmetal species aiming to extend

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91

ions. The

heated at

thin film sur-

thin films with five layers depos-

film deposited on silicon (**Figure 11d**).

construction of nonlinear optic devices [9].

 **thin films**

formed showing the modification of TiO<sup>2</sup>

self-cleaning characteristics [10].

ited on glass and heated at 400°C.

**4. Ag/TiO2**

TiO2

Ag/TiO<sup>2</sup>

400°C [5, 11].

**Figure 9.** Optical properties modulation of the xTiO<sup>2</sup> /(100-x)SiO<sup>2</sup> films: (a) transmittance and (b) refractive index in the function of wavelength.

with different xTiO<sup>2</sup> /(100-x)SiO<sup>2</sup> molar ratios (x = 0, 20, 40, 60, 80, and 100%) were prepared and stirred for 1 h. The final viscosity of the solutions was maintained in approximately 2.2 cP. The films were deposited on properly clean glass substrates with a constant withdraw speed of 1.0 mm/s at 25°C and relative air humidity about 30%. The drying process occurred at 80°C in air for 10 min. This stage (deposition and drying) was repeated five times for thickness control. Finally, the samples were thermally treated at 500°C for 1 h.

The TiO2 thin films were formed by anatase phase, and the SiO<sup>2</sup> thin films were amorphous according to XRD patterns and Raman spectroscopy results. The TiO<sup>2</sup> /SiO<sup>2</sup> thin films are formed by the anatase phase dispersed in a vitreous matrix. The anatase phase is fundamental for the desired applications due to their optical and photocatalytic property. The microstructure, morphology, and texture of the xTiO<sup>2</sup> /(100-x)SiO<sup>2</sup> thin films change substantially due to the mixture of the titanium and silicon oxide, as seen in AFM images of **Figure 8**.

With the addition of SiO<sup>2</sup> , the titania nanoparticles remain dispersed in the vitreous matrix, and because of that, TiO<sup>2</sup> and SiO<sup>2</sup> pure films have higher root-mean-square (RMS) roughness (2.2 and 6.0 nm, respectively) than TiO<sup>2</sup> /SiO<sup>2</sup> films (between 0.2 and 1.2 nm). The surface smoothing, after the mixture of TiO<sup>2</sup> and SiO<sup>2</sup> , resulted in an enhanced hardness that changes to 4.5 GPa for both pure films and to approximately 7.4 GPa for all nanocomposite thin films. These properties are essential for outdoor applications, special windows, glasses of cars, and other vehicles, among others, since the film surfaces can be subjected to intense mechanical wear of air particles. Moreover, TiO<sup>2</sup> /SiO<sup>2</sup> nanocomposite thin films present a persistent superhydrophilicity, which is required for application on self-cleaning surfaces and water/air treatment, promoting a better washing of the contaminants in the surface, which can be obtained with the rain precipitation. These nanocomposites increase the adsorption of pollutants by the surface.

The optical properties of the xTiO<sup>2</sup> /(100-x)SiO<sup>2</sup> films are modulated by Ti/Si rate variation, as seen in **Figure 9**.

The possibility to modulate the transmittance and refractive index (n) of the xTiO<sup>2</sup> /(100-x) SiO<sup>2</sup> thin films is essential in applications as antireflection surfaces, filters, and planar waveguides, since this wide variation of n (from 1.45 to 2.18 in visible light) permits the construction of different structural models of devices. The variation in the refraction index in function of incident light wavelength of 2.0–2.8 (**Figure 9b**) is also very important to the construction of nonlinear optic devices [9].

#### **4. Ag/TiO2 thin films**

**Figure 9.** Optical properties modulation of the xTiO<sup>2</sup>

90 Titanium Dioxide - Material for a Sustainable Environment

ture, morphology, and texture of the xTiO<sup>2</sup>

and SiO<sup>2</sup>

/SiO<sup>2</sup>

and SiO<sup>2</sup>

/(100-x)SiO<sup>2</sup>

Finally, the samples were thermally treated at 500°C for 1 h.

thin films were formed by anatase phase, and the SiO<sup>2</sup>

according to XRD patterns and Raman spectroscopy results. The TiO<sup>2</sup>

/SiO<sup>2</sup>

the mixture of the titanium and silicon oxide, as seen in AFM images of **Figure 8**.

tation. These nanocomposites increase the adsorption of pollutants by the surface.

/(100-x)SiO<sup>2</sup>

The possibility to modulate the transmittance and refractive index (n) of the xTiO<sup>2</sup>

function of wavelength.

with different xTiO<sup>2</sup>

With the addition of SiO<sup>2</sup>

after the mixture of TiO<sup>2</sup>

ticles. Moreover, TiO<sup>2</sup>

seen in **Figure 9**.

SiO<sup>2</sup>

and 6.0 nm, respectively) than TiO<sup>2</sup>

The optical properties of the xTiO<sup>2</sup>

because of that, TiO<sup>2</sup>

The TiO2

/(100-x)SiO<sup>2</sup>

stirred for 1 h. The final viscosity of the solutions was maintained in approximately 2.2 cP. The films were deposited on properly clean glass substrates with a constant withdraw speed of 1.0 mm/s at 25°C and relative air humidity about 30%. The drying process occurred at 80°C in air for 10 min. This stage (deposition and drying) was repeated five times for thickness control.

formed by the anatase phase dispersed in a vitreous matrix. The anatase phase is fundamental for the desired applications due to their optical and photocatalytic property. The microstruc-

both pure films and to approximately 7.4 GPa for all nanocomposite thin films. These properties are essential for outdoor applications, special windows, glasses of cars, and other vehicles, among others, since the film surfaces can be subjected to intense mechanical wear of air par-

which is required for application on self-cleaning surfaces and water/air treatment, promoting a better washing of the contaminants in the surface, which can be obtained with the rain precipi-

 thin films is essential in applications as antireflection surfaces, filters, and planar waveguides, since this wide variation of n (from 1.45 to 2.18 in visible light) permits the

/(100-x)SiO<sup>2</sup>

films: (a) transmittance and (b) refractive index in the

thin films were amorphous

thin films are

/(100-x)

/SiO<sup>2</sup>

thin films change substantially due to

molar ratios (x = 0, 20, 40, 60, 80, and 100%) were prepared and

, the titania nanoparticles remain dispersed in the vitreous matrix, and

nanocomposite thin films present a persistent superhydrophilicity,

pure films have higher root-mean-square (RMS) roughness (2.2

, resulted in an enhanced hardness that changes to 4.5 GPa for

films (between 0.2 and 1.2 nm). The surface smoothing,

films are modulated by Ti/Si rate variation, as

TiO2 exhibits a high energy bandgap (3.2–3.8 eV) which corresponds to UV irradiation with a wavelength smaller than 388 nm. To overcome this limitation, several studies have been performed showing the modification of TiO<sup>2</sup> with metal and nonmetal species aiming to extend the light absorption to the visible range and simultaneously increasing the recombination time of the electron-hole pairs formed. In particular, nanocomposite thin films of silver and titania have been of considerable interest since silver nanoparticles can act as electron traps, contributing to electron-hole separation and creating a local electric field capable of facilitating the electron excitation and consequently their photocatalytic properties. The improvement in the photocatalytic properties leads to surfaces with better bactericide, hydrophobicity, and self-cleaning characteristics [10].

Ag/TiO<sup>2</sup> coatings were prepared from alcoholic solution containing titanium isopropoxide and silver nitrate dissolved in a mixture of isopropyl alcohol in several atomic ratios. Acid conditions (pH = 4) were reached after acetic acid addition. This precursor solution was stirred at room temperature during 1 h and submitted to UV-C irradiation (254 nm) treatment in air for 100 min. This procedure has been used to produce metallic Ag from Ag<sup>+</sup> ions. The films were deposited onto clean substrates as borosilicate, silicon, 316 L stainless steel, and magnets (NdFeB) with withdrawal speed of 8 mm s−1. After deposition, the coatings with one to five layers were dried in air for 20 min and were thermally treated for 1 h between 100 and 400°C [5, 11].

**Figure 10** shows the characteristic diffractogram of Ag/TiO<sup>2</sup> thin films with five layers deposited on glass and heated at 400°C.

According to XRD patterns, the coatings heated at 400°C show indexed peak characteristic of crystalline metallic Ag and anatase phase (PDF #1-562). The diffractogram of the film heated at 100°C was characteristic of a noncrystalline material, as expected. The substrates of 316 L stainless steel and magnets showed similar XRD patterns. SEM images of Ag/TiO<sup>2</sup> heated at 400°C deposited on different substrates are shown in **Figure 11**.

The structure of the used substrates has induced the formation of nano- and microstructures of metallic silver with different sizes and morphologies supported on the TiO<sup>2</sup> thin film surfaces. This formation occurs due to thermal treatment that induces the diffusion of the metal nanoparticles to the film surface. In the borosilicate substrate (**Figure 11a**), the formation of spherical Ag nanoparticles with a bimodal particle size distribution is observed. When substrates of 316 L stainless steel and magnets (NdFeB) were used, Ag dendrite micro- and nanostructures were formed (**Figure 11b** and **c**). A trimodal size distribution is observed for the particles present on the surface of the Ag/TiO<sup>2</sup> film deposited on silicon (**Figure 11d**). Particularly in this film, the Ag particles show dimensions of 5–150 nm.

**Figure 10.** XRD patterns of Ag/TiO<sup>2</sup> thin films with five layers heated at 100°C and 400°C and deposited on (a) borosilicate and (b) silicon substrates (400°C).

**Figure 12.** SEM-EDS of Ag/TiO<sup>2</sup>

different regions in the surface.

and heated at 400° C (one layer).

**Figure 13.** AFM images of (a and b) 316 L stainless steel and (c and d) Ag/TiO<sup>2</sup>

thin film heated at 400°C and deposited on 316 L stainless steel and EDS spectra from

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thin film deposited on 316 L stainless steel

Energy-dispersive spectra (EDS) shown in **Figure 12** has confirmed the elemental composition of the Ag/TiO<sup>2</sup> films treated at 400°C deposited on 316 L stainless steel. In this film's circular, micrometric and submicrometric structures also are observed besides the dendrites mentioned above. Brightness regions on the micrograph are constituted only by Ag, while the other regions are formed by TiO<sup>2</sup> matrix in the anatase phase, according to the XRD results. The analyses for the other substrates were similar.

**Figure 11.** SEM images of Ag/TiO<sup>2</sup> heated at 400°C and deposited on (a) borosilicate, (b) magnet, (c) 316 L stainless steel, and (d) silicon substrates.

**Figure 12.** SEM-EDS of Ag/TiO<sup>2</sup> thin film heated at 400°C and deposited on 316 L stainless steel and EDS spectra from different regions in the surface.

Energy-dispersive spectra (EDS) shown in **Figure 12** has confirmed the elemental composi-

circular, micrometric and submicrometric structures also are observed besides the dendrites mentioned above. Brightness regions on the micrograph are constituted only by Ag, while the

films treated at 400°C deposited on 316 L stainless steel. In this film's

thin films with five layers heated at 100°C and 400°C and deposited on (a) borosilicate

matrix in the anatase phase, according to the XRD results.

heated at 400°C and deposited on (a) borosilicate, (b) magnet, (c) 316 L stainless steel,

tion of the Ag/TiO<sup>2</sup>

**Figure 10.** XRD patterns of Ag/TiO<sup>2</sup>

92 Titanium Dioxide - Material for a Sustainable Environment

and (b) silicon substrates (400°C).

other regions are formed by TiO<sup>2</sup>

**Figure 11.** SEM images of Ag/TiO<sup>2</sup>

and (d) silicon substrates.

The analyses for the other substrates were similar.

**Figure 13.** AFM images of (a and b) 316 L stainless steel and (c and d) Ag/TiO<sup>2</sup> thin film deposited on 316 L stainless steel and heated at 400° C (one layer).

substitutional defects, has been studied to improve several material properties, such as TiO<sup>2</sup>

cells, gas sensors, magnetic properties, and transparent conductive oxide (TCO) for several

types of substrates. However, the most used deposition methods are chemical vapor deposition

coatings were prepared from alcoholic solution containing titanium isopropoxide and ammonium-(bisaquo oxobisoxalato) niobate-trihydrate (produced by CBMM, Brazil) dissolved in a mixture of isopropyl alcohol. Acid conditions (pH = 4) were reached after acetic acid addition. The precursor solution was stirred at room temperature during 1 hour and deposited by dipcoating process in clean glass substrates with withdrawal speed between 0.8 and 3.7 mm s−1. After deposition, the coatings with one to five layers were dried in air for 20 min and were ther-

visual appearance more homogeneous than the other deposited thin films. The niobium

A theoretical study using density functional theory (DFT) showed that the insertion of niobium in the titanium dioxide matrix, causing the substitution of Ti4+ cations for Nb5+ cations, changes its lattice parameters, cell volume, and bandgap [14]. Therefore, the structures of the materials calcined at 500°C were found to be crystalline in the anatase phase (PDF #1-562). The thin films doped with 0.5, 1, and 3% molar ratio Nb:Ti showed a displacement of the 101 and 200 peaks to lower angles, evidencing the substitution of the niobium inside the crystal

) that are very expensive is reported in the literature [13]. In this work, Nb/TiO<sup>2</sup>

thin films obtained are transparent, adherent, free of micro-cracks, and with

thin films with different niobium concentrations, calcined at 500°C.

Several methods are being used to synthesize and deposit Nb-doped TiO<sup>2</sup>

mainly two niobium precursors, niobium ethoxide [Nb(OCH<sup>2</sup>

(CVD), sputtering, and sol-gel process. In the sol-gel synthesis of Nb-doped TiO<sup>2</sup>

are its use as photocatalyst, dye-sensitized solar

http://dx.doi.org/10.5772/intechopen.74335

CH3 ) 5

Pure and Nanocomposite Thin Films Based on TiO2 Prepared by Sol-Gel Process…

Examples of applications of Nb-doped TiO<sup>2</sup>

mally treated for 1 h between 100 and 500°C.

structure, as shown in **Figure 15**.

**Figure 15.** XRD patterns of TiO<sup>2</sup>

increases the mechanical resistance of the surface.

electronic devices.

ride (NbCl5

The Nb-TiO<sup>2</sup>

.

95

thin films in different

], and niobium pentachlo-

, the use of

**Figure 14.** (a) Ag/TiO<sup>2</sup> thin film transmittance in the function of wavelength: (a) glass, (b) one layer, (c) two layers, (d) three layers, and (e) four layers. (b) Absorption spectra of (a) TiO<sup>2</sup> ; (b) Ag/TiO<sup>2</sup> , Ag:Ti = 1:6; and (c) Ag/TiO<sup>2</sup> , Ag:Ti = 1:100.

**Figure 13** shows AFM images of Ag/TiO<sup>2</sup> thin films with one layer deposited on 316 L stainless steel substrate. The surface roughness of the 316 L stainless steel, whose texture is shown in **Figure 13a** and **b**, is ~40 nm, a much higher value compared to the roughness value of the borosilicate substrate, which is about 0.20 nm. It is observed that the Ag/TiO<sup>2</sup> films deposited on the steel substrates reduce their roughness as a function of the number of layers deposited. With four layers, the roughness value decreases to 7 nm. In addition, the Ag/TiO<sup>2</sup> films are formed by silver nanoparticles dispersed on the surface of the TiO<sup>2</sup> matrix with sizes between 20 and 50 nm.

The introduction of silver in the TiO<sup>2</sup> structure changes their optic properties as can be seen in **Figure 14a**, represented by the variation of transmittance in the function of wavelength. The bandgap decreases, depending on the amount of silver in the crystalline structure (Tauc method), until values of 1.75 eV depend on the concentration of silver, according to the literature [9].

**Figure 14b** shows the absorption spectra of pure and doped TiO<sup>2</sup> , emphasizing the photonic property of the Ag/TiO<sup>2</sup> thin films, with absorption peaks between 490 and 520 nm that changed with the variation of the molar ratio (Ag:Ti). These photonic surfaces provide new possibilities to increase the efficiency of solar energy conversion by confinement of the light, improve bandgap effects, and enhance optical transmission as well as nonlinear optical switching in surface polaritonic structures.

Other utilizations of Ag/TiO<sup>2</sup> thin films are in hydrophilic/hydrophobic surfaces and in bactericide and fungicide devices [5], since the silver increases the TiO<sup>2</sup> efficiency.

#### **5. Nb/TiO2 thin films**

Traditionally, the niobium is used mainly in the confection of metallic alloys for several industrial applications [12]. However, the use of niobium to produce ceramic materials is increasing in the last few years with several applications into catalysis, supercapacitor, and battery components, among others. The incorporation of the niobium in other material structures, causing substitutional defects, has been studied to improve several material properties, such as TiO<sup>2</sup> . Examples of applications of Nb-doped TiO<sup>2</sup> are its use as photocatalyst, dye-sensitized solar cells, gas sensors, magnetic properties, and transparent conductive oxide (TCO) for several electronic devices.

Several methods are being used to synthesize and deposit Nb-doped TiO<sup>2</sup> thin films in different types of substrates. However, the most used deposition methods are chemical vapor deposition (CVD), sputtering, and sol-gel process. In the sol-gel synthesis of Nb-doped TiO<sup>2</sup> , the use of mainly two niobium precursors, niobium ethoxide [Nb(OCH<sup>2</sup> CH3 ) 5 ], and niobium pentachloride (NbCl5 ) that are very expensive is reported in the literature [13]. In this work, Nb/TiO<sup>2</sup> coatings were prepared from alcoholic solution containing titanium isopropoxide and ammonium-(bisaquo oxobisoxalato) niobate-trihydrate (produced by CBMM, Brazil) dissolved in a mixture of isopropyl alcohol. Acid conditions (pH = 4) were reached after acetic acid addition. The precursor solution was stirred at room temperature during 1 hour and deposited by dipcoating process in clean glass substrates with withdrawal speed between 0.8 and 3.7 mm s−1. After deposition, the coatings with one to five layers were dried in air for 20 min and were thermally treated for 1 h between 100 and 500°C.

The Nb-TiO<sup>2</sup> thin films obtained are transparent, adherent, free of micro-cracks, and with visual appearance more homogeneous than the other deposited thin films. The niobium increases the mechanical resistance of the surface.

**Figure 14.** (a) Ag/TiO<sup>2</sup>

three layers, and (e) four layers. (b) Absorption spectra of (a) TiO<sup>2</sup>

silver nanoparticles dispersed on the surface of the TiO<sup>2</sup>

**Figure 13** shows AFM images of Ag/TiO<sup>2</sup>

94 Titanium Dioxide - Material for a Sustainable Environment

The introduction of silver in the TiO<sup>2</sup>

tonic property of the Ag/TiO<sup>2</sup>

Other utilizations of Ag/TiO<sup>2</sup>

**5. Nb/TiO2**

switching in surface polaritonic structures.

 **thin films**

thin film transmittance in the function of wavelength: (a) glass, (b) one layer, (c) two layers, (d)

; (b) Ag/TiO<sup>2</sup>

less steel substrate. The surface roughness of the 316 L stainless steel, whose texture is shown in **Figure 13a** and **b**, is ~40 nm, a much higher value compared to the roughness value of the

the steel substrates reduce their roughness as a function of the number of layers deposited. With

**Figure 14a**, represented by the variation of transmittance in the function of wavelength. The bandgap decreases, depending on the amount of silver in the crystalline structure (Tauc method), until values of 1.75 eV depend on the concentration of silver, according to the literature [9].

that changed with the variation of the molar ratio (Ag:Ti). These photonic surfaces provide new possibilities to increase the efficiency of solar energy conversion by confinement of the light, improve bandgap effects, and enhance optical transmission as well as nonlinear optical

Traditionally, the niobium is used mainly in the confection of metallic alloys for several industrial applications [12]. However, the use of niobium to produce ceramic materials is increasing in the last few years with several applications into catalysis, supercapacitor, and battery components, among others. The incorporation of the niobium in other material structures, causing

borosilicate substrate, which is about 0.20 nm. It is observed that the Ag/TiO<sup>2</sup>

four layers, the roughness value decreases to 7 nm. In addition, the Ag/TiO<sup>2</sup>

**Figure 14b** shows the absorption spectra of pure and doped TiO<sup>2</sup>

ricide and fungicide devices [5], since the silver increases the TiO<sup>2</sup>

, Ag:Ti = 1:6; and (c) Ag/TiO<sup>2</sup>

matrix with sizes between 20 and 50 nm.

efficiency.

thin films with one layer deposited on 316 L stain-

structure changes their optic properties as can be seen in

thin films, with absorption peaks between 490 and 520 nm

thin films are in hydrophilic/hydrophobic surfaces and in bacte-

, Ag:Ti = 1:100.

films deposited on

films are formed by

, emphasizing the pho-

A theoretical study using density functional theory (DFT) showed that the insertion of niobium in the titanium dioxide matrix, causing the substitution of Ti4+ cations for Nb5+ cations, changes its lattice parameters, cell volume, and bandgap [14]. Therefore, the structures of the materials calcined at 500°C were found to be crystalline in the anatase phase (PDF #1-562). The thin films doped with 0.5, 1, and 3% molar ratio Nb:Ti showed a displacement of the 101 and 200 peaks to lower angles, evidencing the substitution of the niobium inside the crystal structure, as shown in **Figure 15**.

**Figure 15.** XRD patterns of TiO<sup>2</sup> thin films with different niobium concentrations, calcined at 500°C.

AFM 3D micrographs (**Figure 16a** and **b**) show that the TiO<sup>2</sup>

thin films, with smaller nanoparticles and RMS roughness and, therefore, more

structure led to a denser film with higher refractive index and high

Pure and Nanocomposite Thin Films Based on TiO2 Prepared by Sol-Gel Process…

UV-Vis spectra seen in **Figure 17** show that also it is possible to modulate the transmittance of the thin films as a function of the wavelength to obtain optical filters. All studied films showed similar bandgap values obtained by the Tauc method, between 3.6 and 3.4 eV. The insertion

The sol-gel deposition parameters such as the density of the precursor solution, concentration of oxides, viscosity, withdrawal velocity, number of dips, and drying temperature influence the characteristics of the films such as thickness, porosity, refractive index, particle size, particle shape, and oxidation degree. Someway, all dopants used improved the quality and

their mechanical, optical, and surface properties. The addition of Ag increases its photocatalytic activity, improving fungicide and bactericide properties of the films. The hydrophobicity/hydrophilicity change capacity was improved too. The doping with Nb improves the mechanical resistance of the films. All these properties can be applied in the confection of best photocatalytic surfaces to be used in the production of solar energy, self-cleaning surface, and

The authors would like to thank FAPEMIG, CNPq, and CAPES for their financial support and

and Ellen Denise Lopes Alves<sup>2</sup>

1 Laboratory of Nanostructured Materials, Universidade Federal de Minas Gerais, Belo

,

, Gustavo Henrique de Magalhães Gomes1

films. The addition of SiO<sup>2</sup>

RMS roughness of 2.2 ± 0.1 nm, while the 2% Nb/TiO<sup>2</sup>

0.6 ± 0.2 nm and smaller nanoparticles. All Nb/TiO<sup>2</sup>

homogeneity, adherence, and visual quality.

the range of application of the TiO<sup>2</sup>

optical and nonlinear optical devices.

UFMG's Microscopy Center for the images.

Magnum Augusto Moraes Lopes de Jesus1

\*Address all correspondence to: nelcy@ufmg.br

Luiz Fernando de Sousa Lima1

Horizonte, Minas Gerais, Brazil

2 REDEMAT, MG, Brazil

Nelcy Della Santina Mohallem1,2\*, Marcelo Machado Viana1

**Acknowledgements**

**Author details**

than TiO2

of niobium on the TiO<sup>2</sup>

mechanical resistance.

**6. Conclusion**

has larger particle size and

97

film presents a RMS roughness of

http://dx.doi.org/10.5772/intechopen.74335

thin films presented different profiles

in the TiO2

films changes

,

**Figure 16.** 3D micrographs of (a) 2% Nb-doped TiO<sup>2</sup> and (b) pure TiO<sup>2</sup> , calcined at 500°C.

**Figure 17.** UV-Vis spectra of several TiO<sup>2</sup> and Nb/TiO<sup>2</sup> thin films with different calcination temperatures and numbers of layers.

The increase of niobium content in the thin film promoted a considerable variation in the lattice parameters, whose d101 changed to 3.49 for pure TiO<sup>2</sup> and to 3.55 for 3% Nb/TiO<sup>2</sup> . The crystallite size decreased from 11 to 7 nm, which agreed with the DFT results previously reported. AFM 3D micrographs (**Figure 16a** and **b**) show that the TiO<sup>2</sup> has larger particle size and RMS roughness of 2.2 ± 0.1 nm, while the 2% Nb/TiO<sup>2</sup> film presents a RMS roughness of 0.6 ± 0.2 nm and smaller nanoparticles. All Nb/TiO<sup>2</sup> thin films presented different profiles than TiO2 thin films, with smaller nanoparticles and RMS roughness and, therefore, more homogeneity, adherence, and visual quality.

UV-Vis spectra seen in **Figure 17** show that also it is possible to modulate the transmittance of the thin films as a function of the wavelength to obtain optical filters. All studied films showed similar bandgap values obtained by the Tauc method, between 3.6 and 3.4 eV. The insertion of niobium on the TiO<sup>2</sup> structure led to a denser film with higher refractive index and high mechanical resistance.
