**4. Structural modifications of TiO2**

Various structural modifications in the crystal structure of titanium dioxide have been proposed for the improvement of its photocatalytic performance. All these modifications enhance the photocatalytic performance by decreasing the rate of recombination of positive holes and electrons. These modifications allow the absorption of visible light as well. Two types of structural modifications are commonly used for enhancement in the photocatalytic performance of titanium dioxide. These modifications are doping of titanium dioxide with metals or nonmetals and formation of heterojunctions with other semiconductors.

#### **4.1 Doping**

Doping is the addition of elements (impurities) into the inner structure of titanium dioxide. The doping of elements causes a red shift in the absorption of light. It also causes the prevention of recombination of photo-induced positive holes and electrons. Hence, doping improves the photocatalytic performance by absorbing of wide range of radiations and separation of charge carriers. Silver (Ag) is one of the elements used for doping of titanium dioxide. The silver (Ag) doping is used for improvement of photocatalytic performance of titanium dioxide towards photo degradation of organic pollutants [34–36]. Saeed et al., [37] have reported the improvement in photocatalytic performance of titanium dioxide for photodegradation of methylene blue and rhodamine B dyes by incorporation of silver (Ag) in the structure of titanium dioxide. They investigated the effect of silver (Ag) on the photocatalytic performance of titanium dioxide using 2, 4, 6, and 8% loading of Ag on titanium dioxide. They reported 65, 84, 97, and 78% photodegradation of methylene blue over 2, 4, 6, and 8% Ag-TiO2 as a catalyst, respectively. It was found that doping of Ag enhanced the photocatalytic performance of titanium dioxide significantly. **Figure 4** shows the comparison of photocatalytic activity of TiO2 and 6% Ag-TiO2 for photodegradation of methylene blue and rhodamine B. The data given in **Figure 4** was obtained by performing degradation experiments with 0.1 g of Ag-TiO2. A 50 mL solution of methylene blue and/or rhodamine B having concentration of 100 mg/L was used asmodel solution separately for degradation study.

It was found that higher loading of Ag decreased the photocatalytic performance of titanium dioxide. Higher loading of Ag blocks the active sites of the catalyst, therefore the photocatalytic performance decreased [38].

Similarly, Ag-TiO2 with 1, 3, 5, 7, and 10% Ag have been also reported for photodegradation of methylene blue and methyl orange dyes [39]. In this study, the Ag-TiO2 loaded with 5% Ag showed the highest photocatalytic performance for

#### **Figure 4.**

*Comparison of photocatalytic performance of TiO2 and Ag-TiO2 towards photodegradation of methylene blue dye (a) and rhodamine B dye (b) [24].*

photodegradation of dyes. it was reported that doping of silver generates the surface defects by the creation of oxygen vacancies and Ti (III) sites in the structure of TiO2. The formation of Ti (III) has been proposed by the flow of electrons from Ag to Ti (IV). The generation of oxygen vacancies leads to the formation of defect energy levels below the conduction band of TiO2. Also, the incorporation of Ag narrows the band gap of titanium dioxide due to the formation of Ag 4 d states. These modifications favor the excitation of electrons from the valence band to the conduction band under visible light irradiations. The photogenerated positive holes and electrons flow to the surface of titanium dioxide. The positive holes initiate oxidation by reaction with H2O or OH ions and generate reactive hydroxyl radicals (OH• ). Similarly, the photogenerated electrons initiate reduction by reaction with adsorbed oxygen and give rise to super oxide anion radicals. The photo-generated positive holes are trapped by Ag as well and produce Ag (II). The Ag (II) also

#### *Photocatalytic Applications of Titanium Dioxide (TiO2) DOI: http://dx.doi.org/10.5772/intechopen.99598*

initiate oxidation reactions by reaction with H2O or OH ions and ultimately produce hydroxyl radicals (OH• ). Similarly, the Ag (I) traps the photo-generated electrons and produces Ag (0) and then these trapped electrons are transferred to oxygen or Ti (IV). Hence, the recombination of positive holes and electrons is decreased and ultimately the photocatalytic performance is increased [40–44]. Chemical reactions 11 to 18 explain the whole mechanism.

$$\text{Ag}-\text{TiO}\_2 + \text{Irradiation} \rightarrow \text{Ag}-\text{TiO}\_2\\\text{(h}^+) + \text{Ag}-\text{TiO}\_2 \text{(e}^-) \tag{11}$$

$$\mathrm{H\_2O} \text{ or } \mathrm{OH^-} + \mathrm{Ag} - \mathrm{TiO\_2} \text{(h}^+) \rightarrow \mathrm{OH^\bullet} \tag{12}$$

$$\rm O\_2(ads) + Ag \rightarrow TiO\_2(e^-) \rightarrow O\_2^- \tag{13}$$

$$\text{Ag } (I) + \text{Ag} - \text{TiO}\_2 (h^+) \to \text{Ag} (II) \tag{14}$$

$$\mathrm{H\_2O} \text{ or } \mathrm{OH^-} + \mathrm{Ag} \text{ (II)} \rightarrow \mathrm{OH^{\bullet}} + \mathrm{Ag} \text{ (I)}\tag{15}$$

$$\text{Ag } (I) + \text{Ag} - \text{TiO}\_2(e^-) \to \text{Ag } (\mathbf{0}) \tag{16}$$

$$\text{Ag} \ (0) + \text{O}\_2 \text{(ads)} \to \text{O}\_2^- + \text{Ag} \ (I) \tag{17}$$

$$\text{Ag } (\mathbf{0}) + \text{Ti } (IV) \to \text{Ag } (I) + \text{Ti } (III) \tag{18}$$

$$\text{Ti} \ (\text{III}) + \text{O}\_2 (\text{ads}) \to \text{Ti} \ (IV) + \text{O}\_2^- \tag{19}$$

$$\text{Ag} - \text{Ti}\_2\text{(}h^+\text{)} + \text{O}\_2^- \rightarrow \text{O}^- \tag{20}$$

$$O^{-} + H\_{2}O \rightarrow OH^{-} + OH^{\bullet} \tag{21}$$

$$\text{Polultants} + \text{OH}^{\bullet} \rightarrow \text{Degradation products} \tag{22}$$

Similarly, other elements can also be used for doping TiO2. Doping of nitrogen is also one of the most studied approaches for the enhancement of photocatalytic performance of titanium dioxide. The doping of nitrogen has been used to extend the absorption of light towards the visible wavelength side. Some of the others have reported that doping of nitrogen results in narrowing of the band gap of titanium dioxide. Some researchers have argued the interaction between valance band, conduction band, and energy states of doping element cause the narrowing of band gap [13, 45, 46]. Di Valentin and co-workers [47] have reported the density functional theory (DFT) study for the evaluation of the photocatalytic performance of Ndoped-TiO2. They predicted that nitrogen atoms occupy either interstitial or substitutional sites in the lattice of titanium dioxide. As a result, localized energy states are generated. In the case of the interstitial position of nitrogen, discrete energy states are formed above the valence band. In the case of the substitutional position of nitrogen atoms, energy levels are formed in extension to the valence band. In the same way, the doping of other elements like carbon can also improve the catalytic performance of titanium dioxide by narrowing its band gap [48]. It has also been reported that modifications in the (101) plane of titanium dioxide take place with doping of elements. The modifications resulted from the doping of elements enhance the movement of photo-generated electrons to other places within the structure. This flow of electrons to other places increases the lifetime of photoinduced charge carriers and ultimately causes an improvement in photocatalytic performance [49].

Although the doping of elements in the lattice of titanium dioxide has been used for enhancement in the photocatalytic performance of titanium dioxide, however, these dopants may also decrease the photocatalytic performance as these dopants can promote the recombination of photogenerated positive holes and electron. Therefore, the doping of elements in high concentrations must be avoided [50].

The photo catalytic performance of TiO2 can be enhanced by doping of iron as well. The doping ot TiO2 with iron produces mixed oxides as well as mixture composed of mixed oxides and simple oxides. The Fe (III) and Ti (IV) have almost similar radii, therefore, Fe(III) occupies the substitutional positions. The presence of Fe (III) decrease the rate of recombination of positive holes and electrons by separating them and hence ultimately increases the catalytic performance. The Fe (III) traps the positive hole and produces Fe(IV). Then, the Fe (IV) reacts with hydroxyl ions and produce the hydroxyl radicals and O2 [51, 52]. The doping of TiO2 with Fe shifts the light absorption ability towards visible light region. Under irradiation by visible light, excitation takes place (Fe(III)/Fe(IV) to conduction band of TiO2. By irradiation, the Fe(III) changes to Fe(IV) by absorption of visible radiation because the t2g level of d orbital of Fe(III) is above the valence band of TiO2. The electron released from Fe(III) is shifted to conduction band of TiO2. The shifted electron produces hydroxyl radicals by further reactions.

#### **4.2 Heterojunction**

A heterojunction is an interface that occurs between two layers or regions of dissimilar crystalline semiconductors. As stated in an earlier section that titanium dioxide is very important in photocatalysis. However, two factors limit the photocatalytic activity of titanium dioxide. These factors include the wide band gap and fast recombination of positive holes and electrons. The formation of heterojunction of titanium dioxide with other semiconductor metals oxide is also an attempt to improve the photocatalytic performance by separation of positive holes and electrons and narrowing the band gap. The synthesis of heterojunction or composite of titanium dioxide with other semiconductors has gained much attention [53]. The formation of heterojunction shifts the absorption capacity of titanium dioxide towards the visible wavelength side and thus improves the catalytic performance. Different semiconductor metal oxides can be used for the formation of heterojunction. Zinc oxide (ZnO) is one of the semiconductors that can be used for the formation of heterojunction of titanium dioxide. The zinc oxide has band gap similar to titanium dioxide and it possesses good catalytic activity. Therefore, the TiO2-ZnO heterojunction is expected to show good photocatalytic performance under visible light irradiation [54–56]. Saeed and his coworkers [57] have reported the synthesis of ZnO-TiO2 heterojunction as an efficient photocatalyst for the photodegradation of methyl orange. They found that photodegradation of methyl orange was 98% with ZnO-TiO2 catalyst. The photocatalytic performance was much higher than the photocatalytic performance of ZnO and TiO2 alone having 75 and 60% activity, respectively. **Figure 5** shows the comparison of photocatalytic performance of ZnO-TiO2 heterojunction with pure semiconductor ZnO and TiO2. The data given in **Figure 5** was obtained by performing degradation experiments with 50 mg of ZnO or TiO2 or ZnO-TiO2. A 50 mL solution of methyl orange having concentration of 100 mg/L was used asmodel solution for degradation study.

The ZnO-TiO2 exhibited higher photocatalytic performance due to the synergistic effect between zinc oxide and titanium dioxide. This synergetic effect arises due to the formation of heterojunction. When ZnO-TiO2 heterojunction is irradiated with light, positive holes and electrons are formed in valence band and conduction band respectively. The positive hole flows from the titanium dioxide valence band to the zinc oxide valence band. At the same time, the electrons flow from the zinc oxide conduction band to the titanium dioxide conduction band. This flow of positive holes and electrons has been explained in **Figure 6**. The flow of positive holes and electrons separates the positive holes and electrons from one another. As a result, the recombination of positive holes and electrons is suppressed. Therefore,

*Photocatalytic Applications of Titanium Dioxide (TiO2) DOI: http://dx.doi.org/10.5772/intechopen.99598*

#### **Figure 5.**

*Comparison of photo catalytic performance of ZnO-TiO2, ZnO and TiO2 towards photodegradation of methyl orange [43] (this figure has reproduced with the permission of DeGruyter).*

#### **Figure 6.**

*Process of separation of positive holes and electrons for the improvement of photocatalytic performance towards photodegradation of methyl orange [43] (this figure has reproduced with the permission of DeGruyter).*

these positive holes and electrons take proceed the redox reactions. Hence, the photocatalytic performance is increased.

Similarly, heterojunctions of titanium dioxide with other semiconductors have also been reported. For example, Abd-Rabboh and his co-workers [53] have reported the synthesis of BiVO4-TiO2 heterojunction as an effective photocatalyst for photodegradation of rhodamine B dye. They reported the heterojunction between BiVO4 and TiO2 for the production of hydrogen gas and photo degradation of rhodamine B dye. it was found that formation of heterojunction shifted that absorption of radiation by TiO2 towards visible light region. The prepared heterojunction was tested as catalysts for degradation of rhodamine B dye. It was


#### **Table 2.**

*A summary of photocatalytic degradation of pollutant in the presence of TiO2 based photocatalysts.*

found that BiVO4-TiO2 heterojunction showed a photo catalytic performance of ten times greater than bare TiO2. The rate constant for photodegradation of rhodamine B were 0.021 and 0.0023 per minute with BiVO4-TiO2 and TiO2 as catalyst respectively. Mousavi and Ghasemi [58] have reported TiO2-CoTiO3 heterojunction as photocatalyst for photodegradation of different dyes. They reported 99% photodegradation of methyl orange, methylene blue, and rhodamine b dye over TiO2-CoTiO3 heterojunction as photocatalyst under visible light irradiation. Another research group [59] has used TiO2-Ti3C2 heterojunction as a catalyst for photodegradation of methyl orange with 99% performance under sunlight irradiation. CuO-TiO2 heterojunction has also been reported for photodegradation of phenol with excellent photocatalytic performance [60]. Hence, it is concluded that the formation of heterojunction for titanium dioxide with other semiconductor metal oxide enhanced the photocatalytic performance of titanium dioxide.

Although a lot of literature is available on photocatalytic degradation of dyes and other organic pollutants in the presence of TiO2 based photocatalysts, a summary is given in **Table 2**.
