**1. Introduction**

The availability of water is of great importance for the development of economic activities and mainly for human health. However, the rapid increase in industrial production resulted in serious consequences to the environment by generating waste and contaminating the water reserves [1–3]. The pharmaceutical products, pesticides, azo dyes, herbicides, and hormones are the main contaminants in water [4–6]. The textile activities are responsible for 15% of the industrial consumption of water [7]. It is estimated that approximately 15–20% of the chemical species, including dyes, are disposed of as effluent after processing [8].

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2018 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Among the more than 10,000 types of dyes available [9], the azo compounds stand out and are characterized by the presence of one or more chromophore groups (─N═N─) in their chemical structure [10–12]. The intensity of absorption and also the shades of color vary according to the other electrons *π* and *n* which are in conjunction [13]. An example of azo dye is the methyl orange (MO).

peroxide (H2

observed that TiO2

(TiO2

O2

molecules and ionized species [5, 6].

*Photocatalyst*(*ecb*

*Photocatalyst*(*ecb*

*Photocatalyst*(*hvb*

*Photocatalyst*(*hvb*

**Figure 2.** Principal advanced oxidation processes.

*Photocatalyst* + *hv* → *Photocatalyst*(*ecb*

)—oxidants with high degrading power—semiconductors such as titanium dioxide

exposed to the sunlight could produce the photocatalytic dissociation of

Titanium Dioxide Films for Photocatalytic Degradation of Methyl Orange Dye

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213

<sup>−</sup> + *hvb*

<sup>+</sup> ) (1)

<sup>+</sup> → *H*<sup>2</sup> (2)

*O*(*ads*) → *H*<sup>+</sup> + *OH* (3)

*O*(*ads*) → *H*<sup>+</sup> + *OH*• (4)

<sup>−</sup> → *OH*• (5)

) and ultraviolet irradiation (UV) [25, 26]. **Figure 2** presents the main techniques of AOP [22].

The first studies in photocatalysis were developed by Fukushima and Honda [27], when they

water, producing hydrogen. Recent researches deal with heterogeneous photocatalysis for applications in water treatment [2, 6, 23, 26]. **Figure 3** shows the schematic diagram of photocatalytic process, showing the photoactivation of a catalyst semiconductor and the production of oxidizing radicals. This process is based on the electronic excitation of certain semiconducting oxides (catalyst) by means of radiant energy—visible or UV light [28, 29]. The reaction is activated by absorption of a photon with energy equal or higher than the bandgap energy (Ebg) of the catalyst [5]. When an electron is promoted from the valence band (VB) to the conduction band (CB), a hole (h+) is generated in the VB (Eq. (1)). The electrons transferred to CB are responsible for reduction reactions, producing gaseous hydrogen and other oxidizing species (Eqs. (2) and (3)). The holes react with the adsorbed water molecules on the surface of the photocatalyst to generate OH• radicals (Eqs. (4) and (5)), allowing the oxidation of organic

−

− ) + *H*<sup>2</sup>

<sup>+</sup> ) + *H*<sup>2</sup>

<sup>+</sup> ) + *OH*(*ads*)

) + 2*H*(*ads*)

Methyl orange dye (C14H14N3 NaO<sup>3</sup> S− Na+ ) is a compound generally used as acid-base indicator [14]. Sha et al. [14] shows that a decrease in pH causes a shift in the absorption band of MO, and a change in its coloration occur, being orange in basic pH and red in acidic condition [15]. The MO structure is characterized by the azo group among the aromatic rings [15, 16] and is shown schematically in **Figure 1**.

Several researches have been done to develop new technologies to remove dyes and others pollutants from wastewater effluent. Hassan et al. [17] studied the use of heterogeneous photocatalysis for the treatment of landfill leachate, comparing the efficiency of this technique to other methods, besides the parameters that influence the process results. The study revealed to be possible to remove pollutants found in landfill leachate using TiO2 as catalyst. The best photocatalytic results were obtained for anatase and anatase-rutile mixture. Jorfi et al. [18] developed TiO2 catalyst supported on magnetic activated carbon for the oxidative degradation of benzotriazole (BTA) by UV-Fenton process. According to the authors, the catalyst showed good reusability, since after five cycles of reuse, the efficiency in the degradation of BTA dropped from 92.2 to 71.6%. Konstantinou and Albanis [19] and Sleiman et al. [20] evaluated the photocatalytic degradation of azo dyes by photocatalytic oxidation using TiO2 under UV-vis irradiation. Both works state the efficiency of the heterogeneous photocatalysis method in water treatment. Akrout and Bousselmi [21] carried out the electrochemical degradation of synthetic wastewater containing biazo dye on boron-doped diamond anode (BDD) at current densities from 8 to 48 Am−2. The authors verified the influence of pH and of the applied current density on degradation. The decrease of pH and the increase of current density showed a positive effect on the oxidation. Vallejo et al. [22] presented a review about the capacity for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) remediation by means of Advanced Oxidation Processes.

Advanced Oxidation Processes (AOPs) are the most attractive methods used to degrade polluting compounds based on the use of highly oxidizing species to promote greater efficiency in the treatment process [23].

AOPs are characterized by generating free radicals, especially the hydroxyl radical (OH•), transforming the organic contaminants in simpler species, such as carbon dioxide, water, and inorganic anions [24]. Hydroxyl radicals can be generated from reactions involving ozone (O<sup>3</sup> ) and hydrogen

**Figure 1.** Structure of the methyl orange dye molecule.

peroxide (H2 O2 )—oxidants with high degrading power—semiconductors such as titanium dioxide (TiO2 ) and ultraviolet irradiation (UV) [25, 26]. **Figure 2** presents the main techniques of AOP [22].

The first studies in photocatalysis were developed by Fukushima and Honda [27], when they observed that TiO2 exposed to the sunlight could produce the photocatalytic dissociation of water, producing hydrogen. Recent researches deal with heterogeneous photocatalysis for applications in water treatment [2, 6, 23, 26]. **Figure 3** shows the schematic diagram of photocatalytic process, showing the photoactivation of a catalyst semiconductor and the production of oxidizing radicals. This process is based on the electronic excitation of certain semiconducting oxides (catalyst) by means of radiant energy—visible or UV light [28, 29]. The reaction is activated by absorption of a photon with energy equal or higher than the bandgap energy (Ebg) of the catalyst [5]. When an electron is promoted from the valence band (VB) to the conduction band (CB), a hole (h+) is generated in the VB (Eq. (1)). The electrons transferred to CB are responsible for reduction reactions, producing gaseous hydrogen and other oxidizing species (Eqs. (2) and (3)). The holes react with the adsorbed water molecules on the surface of the photocatalyst to generate OH• radicals (Eqs. (4) and (5)), allowing the oxidation of organic molecules and ionized species [5, 6].

$$\text{Photocatalyst} \star h\nu \rightarrow \text{Photocatality} \text{(e}\_{ab}^{-} + h\_{ib}^{\*}\text{)}\tag{1}$$

$$\text{Photocatality}(e^{-}\_{ab}) \text{ + } 2H^{+}\_{(ab)} \rightarrow H\_{2} \tag{2}$$

$$\text{Photocatalyst} \{ e\_{ab}^{-}\} + H\_2O\_{\text{(ads)}} \rightarrow H^\* + OH \tag{3}$$

$$\text{Photocatalyst} \text{(h}^{\star}\_{\text{ub}}\text{)} + \text{H}\_{2}\text{O}\_{\text{(ads)}} \rightarrow \text{H}^{\star} + \text{OH}^{\star} \tag{4}$$

$$\text{Photocatality} \text{(h}^{\star}\_{\text{vb}}\text{)} \text{+ OH}^{-}\_{\text{(ads)}} \rightarrow \text{OH}^{\star} \tag{5}$$

**Figure 2.** Principal advanced oxidation processes.

Among the more than 10,000 types of dyes available [9], the azo compounds stand out and are characterized by the presence of one or more chromophore groups (─N═N─) in their chemical structure [10–12]. The intensity of absorption and also the shades of color vary according to the other electrons *π* and *n* which are in conjunction [13]. An example of azo dye is the

[14]. Sha et al. [14] shows that a decrease in pH causes a shift in the absorption band of MO, and a change in its coloration occur, being orange in basic pH and red in acidic condition [15]. The MO structure is characterized by the azo group among the aromatic rings [15, 16] and is

Several researches have been done to develop new technologies to remove dyes and others pollutants from wastewater effluent. Hassan et al. [17] studied the use of heterogeneous photocatalysis for the treatment of landfill leachate, comparing the efficiency of this technique to other methods, besides the parameters that influence the process results. The study

The best photocatalytic results were obtained for anatase and anatase-rutile mixture. Jorfi

degradation of benzotriazole (BTA) by UV-Fenton process. According to the authors, the catalyst showed good reusability, since after five cycles of reuse, the efficiency in the degradation of BTA dropped from 92.2 to 71.6%. Konstantinou and Albanis [19] and Sleiman et al. [20] evaluated the photocatalytic degradation of azo dyes by photocatalytic oxidation using TiO2 under UV-vis irradiation. Both works state the efficiency of the heterogeneous photocatalysis method in water treatment. Akrout and Bousselmi [21] carried out the electrochemical degradation of synthetic wastewater containing biazo dye on boron-doped diamond anode (BDD) at current densities from 8 to 48 Am−2. The authors verified the influence of pH and of the applied current density on degradation. The decrease of pH and the increase of current density showed a positive effect on the oxidation. Vallejo et al. [22] presented a review about the capacity for polychlorinated dibenzo-p-dioxins and dibenzofurans (PCDD/Fs) remedia-

Advanced Oxidation Processes (AOPs) are the most attractive methods used to degrade polluting compounds based on the use of highly oxidizing species to promote greater efficiency

AOPs are characterized by generating free radicals, especially the hydroxyl radical (OH•), transforming the organic contaminants in simpler species, such as carbon dioxide, water, and inorganic

anions [24]. Hydroxyl radicals can be generated from reactions involving ozone (O<sup>3</sup>

catalyst supported on magnetic activated carbon for the oxidative

revealed to be possible to remove pollutants found in landfill leachate using TiO2

) is a compound generally used as acid-base indicator

as catalyst.

) and hydrogen

methyl orange (MO).

Methyl orange dye (C14H14N3

212 Titanium Dioxide - Material for a Sustainable Environment

shown schematically in **Figure 1**.

et al. [18] developed TiO2

in the treatment process [23].

tion by means of Advanced Oxidation Processes.

**Figure 1.** Structure of the methyl orange dye molecule.

NaO<sup>3</sup> S− Na+

The crystalline structure, grain size, and, mainly, the chemical composition and the thickness of the films are essential parameters to define its properties and applications [44]. The present

400°C by MOCVD process and to study the influence of the thickness on the photocatalytic

(MOCVD) in a conventional horizontal reactor at 400°C under a pressure of 50 mbar. Titanium(IV) isopropoxide (*Sigma-Aldrich*, 99.999%) was used as precursor of titanium and oxygen. Nitrogen (flow rate of 0.5 mL min−1) was used as carrier and purge gas. The boro-

tion, rinsed in deionized water, dried in nitrogen, and immediately inserted into the reactor. **Figure 4** shows schematically the MOCVD equipment [45]. The main components are the reaction chamber, which consists of a quartz tube heated by an infrared oven containing the sample holder, and a vacuum pump that keeps the reaction chamber under a pressure below the atmospheric. The TTiP is maintained in a bubbler heated to 39°C. The gas conduction lines are made of stainless steel and are kept heated to prevent condensation and premature

X-ray diffraction (XRD) diagrams, obtained in a *Rigaku Multiflex* equipment using CuKα radiation (λ = 1.54148 Å), incidence angle of 2.5°, and scan rate of 0.02°, were used to identify the phases formed. Measurements of surface roughness and mean grain size were performed by atomic force microscopy (AFM) operating in the Tapping mode (*SPM Bruker NanoScope IIIA*), employing a silicon tip with a curvature radius of 15 nm. The thickness of the films was measured in the cross section of the samples by using a field emission scanning electron microscope (FE-SEM) *JSM*6701*F* X-ray photoelectron spectroscopy (XPS) measurements with spot size beam of 400 μm were conducted in order to determine the chemical state of

thin films was realized by metalorganic chemical vapor deposition

Titanium Dioxide Films for Photocatalytic Degradation of Methyl Orange Dye

thin films grown at

215

http://dx.doi.org/10.5772/intechopen.75528

aqueous solu-

SO4

research aims the structural and morphological characterization of TiO2

silicate substrates (25 × 76 × 1 mm) were previously cleaned in a 5% H2

**Figure 4.** MOCVD equipment shown schematically (adapted from reference 45).

efficiency to degrade methyl orange dye under UV light.

**2. Experimental procedure**

The growth of TiO2

pyrolysis of the precursor.

**2.1. Characterization of the films**

**Figure 3.** Schematic diagram of photocatalytic process and bandgap of TiO2 semiconductor.

The main factors that influence the photocatalytic degradation are pH, initial concentration of dyes, reaction temperature, catalyst concentration, oxidizing agents, light intensity, and irradiation time [6, 17, 20]. Acid pH is more favorable for photocatalytic applications than neutral or alkaline pH [30]. Chanathaworn et al. [31] studied the effects of irradiation intensity of black light lamp on the degradation of the Rhodamine B. According to the results, an increase in the irradiation intensity intensified the dye degradation.

Titanium dioxide (TiO2 ) is the most crystalline semiconductor used in photocatalytic process [26, 32]. It presents three polymorphic phases: anatase and rutile, with tetragonal structure; and brookite, orthorhombic [33, 34], being anatase the phase of greater degradative efficiency [17]. Due to the TiO2 bandgap energy being relatively wide (Eg = 3.2 eV for anatase; Eg = 3.0 eV for rutile; Eg = 3.1 eV for brookite) [6, 33, 34], the material can only be activated by UV irradiation with λ < 380 nm [35].

Absalan et al. [36] developed TiO2 nanoparticles in anatase, rutile, and brookite phases by solgel method using different calcination temperature and time and doped by transition metals (cadmium, chromium, nickel, manganese, iron, and cobalt). According to the authors, the proportion of anatase phase increased after doping process, besides improving the photocatalytic efficiency. According to Carp et al. [37], the doping process reduces the bandgap, making the material active in the region of the visible spectrum of light.

Among several techniques used on the synthesis of TiO2 [38–41], the chemical vapor deposition (CVD) is widely employed [42]. Pierson [43] defines CVD as the deposition of a solid on a heated surface from a chemical reaction in the vapor phase. In this process, the vapor of a volatile compound reacts near or over the surface to be coated (substrate), forming a solid deposit by nucleation of the chemical element that composes the material to be deposited, from a movement governed by processes of diffusion and convection of matter [43].

The crystalline structure, grain size, and, mainly, the chemical composition and the thickness of the films are essential parameters to define its properties and applications [44]. The present research aims the structural and morphological characterization of TiO2 thin films grown at 400°C by MOCVD process and to study the influence of the thickness on the photocatalytic efficiency to degrade methyl orange dye under UV light.
