**2.1. Characterization of the films**

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

[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

for rutile; Eg = 3.1 eV for brookite) [6, 33, 34], the material can only be activated by UV irradia-

gel 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

tion (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].

) is the most crystalline semiconductor used in photocatalytic process

bandgap energy being relatively wide (Eg = 3.2 eV for anatase; Eg = 3.0 eV

nanoparticles in anatase, rutile, and brookite phases by sol-

semiconductor.

[38–41], the chemical vapor deposi-

in the irradiation intensity intensified the dye degradation.

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

214 Titanium Dioxide - Material for a Sustainable Environment

the material active in the region of the visible spectrum of light.

Among several techniques used on the synthesis of TiO2

Titanium dioxide (TiO2

[17]. Due to the TiO2

tion with λ < 380 nm [35].

Absalan et al. [36] developed TiO2

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

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

**Figure 5.** Schematic diagram of the photocatalytic reactor.

the species near the solid surface. A *Thermo Scientific, K-Alpha model* equipment was used. After the acquisition of the high-resolution spectra, the deconvolution was done using the algorithm Smart in the software Avantage®. The binding energies were corrected considering the C1s reference peak at 214.8 eV.

#### **2.2. Photocatalytic tests**

The photocatalytic activities of the TiO2 films were estimated by measuring the degradation of methyl orange dye (MO, 5 mg L−1) in an aqueous solution (pH = 2), under UV light irradiation (Sankyo Denki Co., Ltd., 15 W, λ = 352 nm). The changes in the MO concentration were monitored using a UV-vis spectrophotometer (*Global Trade Technology*). For this purpose, the films were placed in a reactor (**Figure 5**) containing 40 mL of the dye solution and were illuminated by two tubular UV lamps for 300 min. Synthetic air (N2 /20 wt.% O2 ) was bubbled into the solution during the tests.

The photocatalytic reactor consists of a glass chamber containing the MO dye solution to be degraded, the TiO2 catalyst supported in borosilicate, and the source of radiation. The components of the photoreactor were arranged in a box to prevent loss of photons and to protect users against the emitted UV radiation. The distance between the photocatalyst and the UV lamps was set at 25 cm.

The RMS (*Root Mean Square*) roughness expresses the values of a roughness profile that moves away from the midline, in other words, it is the standard deviation of the mean height *Z* [47],

film with thickness of 280 nm: (a) topography and (b) 3D image.

∑ *N*=1 *N*

where *N* is the number of peaks; *ZN* is the height of each peak; and *Z* is the mean height of *N* peaks.

presents a mean grain size of 214 nm and roughness values of the order of 16 nm (**Table 1**). It can be observed as a dense film with columnar structure (**Figure 6b**). Results presented by Krumdieck et al. [48] showed that the increase of the thickness of the films caused a decreased of the roughness, similar trend with the values exhibited in the present work. The results clearly show that the increase of the growth time (**Table 1**) caused an increase in the film

\_\_\_\_\_\_\_\_\_\_

(*ZN* − *Z*)<sup>2</sup> \_\_\_\_\_\_\_\_\_

films: (a) thickness of 280 nm and (b) thickness of 468 nm.

Titanium Dioxide Films for Photocatalytic Degradation of Methyl Orange Dye

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

217

*<sup>N</sup>* <sup>−</sup> <sup>1</sup> (6)

film grown with thickness of 468 nm (**Figure 8a**)

being mathematically expressed as:

**Figure 7.** AFM images of TiO2

**Figure 6.** FE-SEM fracture images for TiO2

*RMS* = √

The morphology of the surface of the TiO2

thickness, as evidenced by Antunes et al. [49].
