Characterization and photocatalytic activity of different molar ratios of TiO 2 thin films prepared by Sol-Gel process

Abstract

Photocatalytic application has been of large interest due to its new technology for organic pollution. Among the photocatalyst, titanium dioxide thin film is known as a photocatalyst for the elimination of organic pollutants. In this research, the structural and optical properties of TiO₂ thin films prepared on glass substrates were studied, using a sol-gel (spin coating) technique. TiO₂ thin films were deposited with molar ratio y between [TTIP] and [AcAc] concentration, which varied from 0.5 to 2. Photocatalytic properties of TiO₂ thin films are studied by using sunlight and photodegradation of methylene blue as a water pollutant evaluated until the photocatalytic process enhances with the molar ratio. Absorbance spectra were measured using a spectrophotometer. Test results showed that the photocatalytic sunlight /TiO₂ thin film is a promising method for treating wastewater. The quantity of acetylacetone in the precursor solution had no significant effect on the optical and structural properties of the TiO₂ thin films. Films consisted of the anatase phase, with a band gap in the range of 3.38–3.51 eV and thickness in the range of 534–618 nm. However, the molar ratio effect on photocatalytic preperties, the best photodegradation rate reaches to 94% in y = 0.66 at t = 180 min.

Keywords

Thin films Sol Gel (spin coating)photocatalytic activity sunlight environmental pollution

1 Introduction

Titanium dioxide (TiO₂) has attracted considerable attention for various applications such as pigments, photocatalysis, dye-sensitized solar cells, sensor devices, cosmetics and protective coatings. In particular, TiO₂ is the most studied photocatalyst because of its high efficiency, non-toxicity, chemical and biological stability, and low cost [1].

The organic pollutants serious problem on human health and environment, which are discharged in the environment by industries and houses. Before discharging these harmful organic pollutants must be Environmental, pollution is the area of concern and there is need to pay more attention on it. Due to industrial growth, the environmental pollution has been increased and it has attained a level high in developing countries. Photo-oxidation by using TiO2 photo-catalyst is being widely studied as a relatively new technique of pollution abatement [1 –3]. TiO2 is a commonly used photo-catalyst because of its stability UV light and water. However, the need of an ultraviolet (UV) excitation source restricts its technological utility for limited applications. For widespread applicationns, TiO2 photo catalyst effective in visible radiation or solar light needs to be developed as future generation photo-catalytic material degraded or destroyed. The design of highly efficient and selective photocatalytic systems that work with no loss of energy for applications in reducing global atmospheric pollution and/or degradation of various toxic compounds in the most desirable and vital goals in environmental friendly- catalyst research [4].

For commercial use, it is very important to find a cost-effective and efficient technology for thin film deposition. TiO₂ films have been prepared using a variety of deposition methods such as chemical vapor deposition [5], sputtering [6], hydrothermal [7], electrophoretic [8], sol-gel [9, 10], and spray pyrolysis [11, 12]. Among the above-mentioned methods, deposition of thin films with sol gel (spin coating) has the advantage of being a simple, inexpensive method that allows us to obtain transparent films with high stability and quality [13].

Through this paper, TiO₂ thin films were prepared by Sol Gel (spin coating) with changing the molar ratio between the concentration of titanium isopropoxide (IV) and the acetone acetone. The objective of this study was to determine the optimal molar ratio $y = [\frac{TTIP}{AcAc}]$ in the deposition of photocatalytic activity TiO₂ thin films by sol gel (spin coating) technique. The structural, optical properties and photocatalytic activity of TiO₂ thin films with different molar ratios $y = [\frac{TTIP}{AcAc}]$ were investigated (y varied from 0.5 to 2). The photocatalytic activity in this study was tested by methylene blue dye test under sunlight.

2 Experimental details

2.1 Preparation of TiO₂ thin films

Titanium tetra (IV) isopropoxide (Ti [OC–H (CH₃)₂]₄) was used as precursor, acetyl acetone (CH₃CH) was used as a stabilizer and ethanol (C₂H₅OH) was used as a solvent. The solution for Sol-Gel (spin-coating) deposition by mixing Titanium tetra (IV) isopropoxide and acetyl acetone in ethanol, the titanium tetra (IV) isopropoxide acetyl acetone. The ratio between titanium tetra (IV) isopropoxide concentration [TTIP] and acetyl acetone concentration [AcAc] was expressed by the following relationship: $y = [\frac{TTIP}{AcAc}]$ this ratio varied from 0.5 to 2. The solution was let under agitation to plug closed during 3 h to 50°C. Titanium dioxide (TiO₂) thin films were deposited on glass substrate. The substrate were chemically cleaned before the deposition. The films were dried at 250 °C for 10 min followed by annealing in a temperature 500°C for 2 hours.

2.2 Photocatalytic experiments

The photocatalytic performance of TiO₂ thin films prepared at different molar ratios using methylene blue (MB) dye in aqueous solution of concentration 15 mg/L as a model pollutant under sun light irradiation. The prepared thin films TiO₂ were settled in methylene blue solution in a closed glass bottle. The solutions were irradiated in sun light for 90 min and 180 min.

2.3 Thin films characterization

To identify the crystal structure of TiO₂, the elaborated films was characterized by X-ray diffraction (XRD) using a diffractometer with monochromatic Cu Ka radiation of λ_kα= 1.54056 Å. The optical transmission of the films was obtained by using an UV-Visible Perkin Elmer lambda 950 spectrophotometer.

3 Results and discussion

3.1 Structural characteristics

Figure 1 shows the XRD spectra of TiO₂ thin films with different molar ratio and annealing 500°C. The peak of XRD is about 2θ= 25.41, 48.05° which corresponds to (101), (200) reflections of the anatase phase of TiO₂ according to (JCPDS 21-1271) [14]. The growth along the (101) plane is due to the low free surface energy found in this direction for TiO₂ tetragonale structure.

Fig. 1

XRD spectra of TiO₂ thin films with different molar ratio.

The grain size D of the TiO₂ films was estimated using the classical Shearer [15]:

$D = \frac{0.9 λ}{β cos θ}$ (1)

The mean crystallite size of the films varies in the range of 20–23 nm. Slight changes in the values of crystallite size are not in correlation with the molar ratio. The results of literature demonstrates that an increase in the amount of the acetylacetonein the precursor solution has no significant effect on the structural properties of TiO₂ thin films, since all the obtained films were annealed at the same temperature 500 C [16, 17]. As seen, the higher value of grain size was found at the molar ratio y = 0.5. This indicates an improvement in the crystalline state of the thin films of titanium oxide. In addition, the lower value of the grain size was found in the ratio y = 0.66.

The dislocation density (δ) is defined as the length of dislocation lines per unit volume of the crystal has been calculated by using the Williamson and Smallman [15]: $δ = \frac{1}{D^{2}}$ (2)

Table 1

Dislocation density and grain size as a function of molar ratio y

Molar ratio (y)	D(nm)	δ×10¹⁵(line/m³)	Thickness d (nm)
0.5	23	1.8	618
0.6	20	2.7	555
1	22	2	547
1.5	23	1.8	540
2	23	1.8	534

3.2 Optical properties

The transmittance spectra of TiO₂ thin films versus molar ratio are displayed in Fig. 2. The optical transmittance spectra of TiO₂ thin films were measured in the spectral region of 300–800 nm for all obtained films are transparent in the visible light region. The transmittance is about 90% in the spectral region of 400–800 nm.It can be observed that an optical transmittance increases with increasing the molar ratio y. According to the interferences fringe, the elaborated films have a smooth surface. It can also be seen that as the molar ratio y increases, the interferences fringe increase, indicating that the surface roughness is decreasing.

Fig. 2

Transmittance spectra of TiO₂ thin films deposited at different molar ratio y.

TiO₂ has an indirect gap, a plot of (αhυ) ^1/2 =f(hυ) was used (Fig. 3). Table 2 shows the indirect band gaps of the obtained TiO₂ thin films as a function of molar ratio. A minimum value of band gap was found at y = 0.66 (Eg = 3.38 eV).

Fig. 3

(αhυ)^0.5 versus hυ plotted as a function of molar ratio.

Table 2

Indirect band gap values as a function of molar ratio y

Molar ratio y	Eg (indirect) (eV)
0.5	3.40
0.66	3.38
1	3.49
1.5	3.47
2	3.51

Overall, it was found that the direct and indirect band gap of the TiO₂ films are not significantly affected by the ratio molar y [16].

4 Photocatalytic activity studies

4.1 UV–Vis test of methylene blue by TiO₂/sunlight

The photocatalytic activities of TiO₂ thin films prepared at various molar ratio y varying from 0.5 to 2 was tested by using methylene blue solution as organic pollution. Figure 4 shows UV-Vis absorption spectra of MB solution after reacting with TiO₂ thin films was prepared at different molar ratio in different time (0 min, 90 min, and 180 min). It is clear from this figure that the intensity of the absorption peaks decreases with the time of sun light radiation exposure, which implies that the MB is degraded. As seen, in all samples a minimum value of peak intensity was found at 180 min. In addition that the absorption peak intensity changed with varying the molar ratio. The light absorption properties of TiO₂ thin films, including absorption region, which decide the maximum amount of photons that can be converted into electrons and holes from incident light. It can also be seen that as the molar ratio y increases, the interferences fringe increase, indicating that the surface roughness is decreasing (Fig. 2). It was also observed that the surface roughness increases with molar ration leading to the increase of the degradation efficiency of dyes (Fig. 4). The TiO₂ films with a high rough surface present a higher efficiency of degradation than those with a smoother surface [17, 18].

Fig. 4

UV-Vis absorption spectra of MB solution after reacting with TiO₂ thin films was prepared at different molar ratio in different time (0 min, 90 min, and 180 min).

Figure 5 represents the absorption spectra of an aqueous solution of MB in the presence of the TiO₂ thin Films at different molar ratio. It was observed that the lowest value of absorption intensity of methylene blue using TiO₂ thin films with molar ratio y = 0.66. While the maximum value of the absorption intensity is at y = 1. This indicates that the decomposition of methylene blue pigment is significant when using thin films at molar ratio of 0.66.

Fig. 5

The absorption spectra of an aqueous solution of MB in the presence of the TiO₂ thin films at different molar ratio.

4.2 The photocatalytic reaction rate constant (k_app)

The photocatalytic reaction rate constant (k_app) (Kinetics) was determined from the plots represented in Fig. 6. It was calculated from Langmuir–Hinshelwood kinetics model that express the first order reaction kinetics for the samples. With this approach, it can be inferred that the higher the slope of the linear plot, the higher the degradation reaction rate [19]. The k_app parameter indicates the reaction speed. As the k_app values increase, the reaction velocity on the surface of the thin layer increases, lead to increased degradation of methylene blue dye. It was also observed that the surface roughness increases with molar ration leading to the increase of the degradation efficiency of dyes. The TiO₂ films with a high rough surface present a higher efficiency of degradation than those with a smoother surface. The porous surface increases the active surface sites, thus enhancing the catalytic activity of the films, as has been established by several authors [20, 21].

Fig. 6

Corresponding plots for reaction rate constant (k_app) for methylene blue (MB) dye under UV–Vis light irradiation.

4.3 Photocatalytic degradation rate

The photocatalytic degradation at 90 min and 180 min values as a function of molar ratio y are shown in Table 3.

Table 3
The photocatalytic degradation at 90 min and 180 min values as a function of molar ratio y

Molar ratio y Photocatalytic degradation at 90 min Photocatalytic degradation at 180 min

0.5 40 % 90%

0.66 57% 94%

1 36% 84%

1.5 49% 89%

2 41% 89%

Molar ratio y	Photocatalytic degradation at 90 min	Photocatalytic degradation at 180 min
0.5	40 %	90%
0.66	57%	94%
1	36%	84%
1.5	49%	89%
2	41%	89%

The Table 3 shows that the photocatalytic degradation rate of methylene blue was high, when the molar ratio was not equal to 1. While we achieved the lowest value at molar ratio 1. This means that photocatalytic degradation rate is more effective when the molar concentration of the TTIP is not equal to the molar concentration of the AcAc. TiO₂ thin films were deposited at a molar ratio y = 0.66 exhibited a better photocatalytic degradation rate than those TiO₂ thin films were deposited further, it was found of value photocatalytic degradation rate 94%. In this case the catalytic reaction velocity on the surface is high which is indicated by the value of K_app . The change in photocatalytic degradation rate can be explained by existence Ti^3 + and O^- defect site because charge carriers can be trapped as Ti^3 + and O^- defect sites in the TiO₂ lattice, or they can recombine, dissipating energy [22, 23].

Through the results of structural, it was found that thin films were elaborated with ratio y = 0.66 have the smallest grain size.The smaller the grain size the greater is the surface area for the degradation. Thus, the observed percent photodegradation of MB clearly exhibits that photocatalytic efficiency of films strongly depends on thickness and crystallite size of the titanium dioxide thin films.

M. Danish and all found that as thickness of film decreases the effect of calcination on the films increases leading to an increase in the crystallinity and crystallite size of TiO₂ crystallites and therefore the photocatalytic efficiency decreases [24].

4.4 Mechanism of photocatalytic reactions

The first step involves the adsorption of MB dye molecules on to the surface of nanostructured TiO₂ thin films. When TiO₂ thin film samples with MB dye molecules adsorbed on their surfaces are exposed to sunlight, TiO₂ nanostructures absorb photons leading to the generation of photoexcited electrons in the conduction band and holes in the valance band of TiO₂ (Equation 3) [25]. In addition, sun light resulting in the transfer of electrons into the conduction band of TiO₂ excites the MB dye molecules. The electrons in the conduction band of TiO₂ react with oxygen molecules present at the surface and form super oxide radicals ( $O_{2}^{\cdot -}$ ) (Equation 4). The holes generated in the valance band interact with water molecules to form hydroxyl radicals (OH•) (Equation 5). Some of the super oxide radicals react with H⁺ to form ${HO}_{2}^{\cdot}$ , which get converted to hydrogen peroxide (H₂O₂) (Equation 7), which leads to an enhancement in the photocatalytic activity.

The highly reactive super oxide and hydroxyl radicals interact with the MB dye molecules adsorbed on the surface of nanostructured TiO₂ thin films and lead to their photocatalytic degradation involving the following reactions [25 –27]. $Ti O_{2} + hv \to e_{CB}^{-} + h_{VB}^{+}$ (3) $O_{2} + \dot{e} \to O_{2}^{\cdot -}$ (4) $H_{2} O + h_{VB}^{+} \to H O^{\cdot} + H^{+}$ (5) $O_{2}^{\cdot -} + H^{+} \to {HO}_{2}^{\cdot -}$ (6) $2 {HO}_{2}^{\cdot} \to H_{2} O_{2} + O_{2}$ (7) $H O^{\cdot} + MB \to degradation product$ (8)

$O_{2}^{\cdot -} + MB \to degradation product$ (9)

5 Conclusion

TiO₂ thin films were prepared by sol-gel (spin coating). The effect of the molar ratio on the properties of titanium dioxide was studied. The prepared films are crystallized in the anatase phase. The preferred growth plane is (101). The average grain size around 23 nm. Good transparency in the visible area. The obtained results shows that no significant impact of molar ratio y on structural and optical properties of TiO₂ thin films. However, the molar ratio effect on photocatalytic preperties, the best photodegradation rate reaches to 94% in y = 0.66 at t = 180 min.

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Characterization and photocatalytic activity of different molar ratios of TiO 2 thin films prepared by Sol-Gel process

Abstract

Keywords

1 Introduction

2 Experimental details

2.1 Preparation of TiO2 thin films

2.2 Photocatalytic experiments

2.3 Thin films characterization

3 Results and discussion

3.1 Structural characteristics

4.1 UV–Vis test of methylene blue by TiO2/sunlight

Table 3 The photocatalytic degradation at 90 min and 180 min values as a function of molar ratio y Molar ratio y Photocatalytic degradation at 90 min Photocatalytic degradation at 180 min 0.5 40 % 90% 0.66 57% 94% 1 36% 84% 1.5 49% 89% 2 41% 89%

References

2.1 Preparation of TiO₂ thin films

4.1 UV–Vis test of methylene blue by TiO₂/sunlight

Table 3
The photocatalytic degradation at 90 min and 180 min values as a function of molar ratio y

Molar ratio y Photocatalytic degradation at 90 min Photocatalytic degradation at 180 min

0.5 40 % 90%

0.66 57% 94%

1 36% 84%

1.5 49% 89%

2 41% 89%