**4.1. Degradation experiment**

**3.3. Photoluminescence study**

106 Titanium Dioxide - Material for a Sustainable Environment

recombination of electrons and holes.

Na doping in TiO<sup>2</sup>

increases for Ti0.9Na0.1O<sup>2</sup>

**4. Photocatalytic activity**

with increase in doping concentration of Na in TiO<sup>2</sup>

vacancies might act as recombination centers [70].

Photoluminescence (PL) spectroscopy is a versatile and powerful optical method to investigate the energy levels in materials. The material is irradiated with light photons of energy greater than or equal to band gap energy of material. This results in the excitation of electrons from valence band to the excited states (**Figure 4(A)**) of material. These electrons relax to conduction band by losing excess energy via nonradiative process. When these electrons return to their valence band, the energy is released in the form of photons and the process is known as photoluminescence. The energy of emitted photons is determined by the difference in atomic energy levels, while the intensity of emitted light gives us information regarding

**Figure 4.** (A) Principle of photoluminescence spectroscopy. (B) PL spectra of pristine and Na doped TiO<sup>2</sup>

The energy and intensity of emitted light in PL spectra of particular material is highly affected

excitation wavelength 390 nm. The shape of PL signal remains same while intensity reduces

lying between valence band and conduction band that trap the electrons form conduction band and thereby reduce electron-hole recombination [68]. However, the PL signal intensity

The photocatalytic activity of prepared photo catalyst can be measured by many different test methods [71], which have been accepted at national and international standards. We have used German standard DIN 52980<sup>11</sup> for the determination of photocatalytic activity. This standard

matrix results in formation of oxygen vacancies resulting in energy states

sample. This is attributed to fact that excessive formation of oxygen

nanopowder at

nanopowder.

matrix. This is attributed to the fact that

by doping. **Figure 4(B)** shows the PL spectra of pristine and Na doped TiO<sup>2</sup>

The photocatalytic activity of pristine and metal doped TiO<sup>2</sup> is evaluated by degrading aqueous solution of MB dye under ultraviolet (UV) irradiation. In order to compare photocatalytic activity of photo catalyst prepared by nonaqueous solvent controlled sol-gel route with commercial photo catalyst, the photocatalytic activity of well-known commercially available Degussa P25 TiO<sup>2</sup> is also evaluated. The initial concentration of MB aqueous solution is 5 mg/L. About 100 ml of MB dye aqueous solution (pH 5) is taken in 100 ml borosil glass beaker and 60 mg of photo catalyst is added in the solution. The beaker is kept on magnetic stirrer for uniform suspension of photo catalyst in the solution throughout the experiment. The reaction mixture is irradiated by UV light of peak wavelength at 365 nm and the intensity of UV light at the surface of reaction mixture is 10 mW/cm<sup>2</sup> . The distance between reaction mixture and UV light source is 20 cm resulting in light intensity of 8 × 104 lux over reaction mixture. In order to complete adsorption-desorption equilibrium between photo catalyst and dye, the reaction mixture is stirred for 20 minutes in completely dark chamber. After achieving adsorption-desorption equilibrium, the reaction mixture is irradiated with UV light and a small amount of solution is withdrawn at regular time intervals. The withdrawn sample is centrifuged to separate out nanoparticles from the solution and absorbance of supernatant measured using UV-Visible spectrometer. The photocatalytic degradation percentage of dye for different time intervals is plotted for several photo catalysts and without catalyst (WC).

The degradation percentage of dye is calculated using Eq. 1. The rate constant (k) for the photocatalytic degradation of dye is determined from pseudo first order law using Eq. 2.

$$\text{Degradation } \%= \left( \langle \mathbf{C}\_0 - \mathbf{C}\_0 \rangle / \mathbf{C}\_0 \right) \times 100 \tag{1}$$

$$\ln\left(\mathbb{C}\_{\boldsymbol{\vartheta}}/\mathbb{C}\_{\boldsymbol{\vartheta}}\right) = \text{kt} \tag{2}$$

where C0 is dye concentration before UV irradiation and C<sup>t</sup> is dye concentration after t time of UV irradiation.

In order to further confirm the degradation results, TOC of irradiated and nonirradiated dye aqueous solution is measured and TOC removal rate percentage is calculated using Eq. 3.

$$\text{TOC} \,\%= \left( \left( \text{TOC}\_o - \text{TOC}\_i \right) / \text{TOC}\_o \right) \times 100 \tag{3}$$

where TOC<sup>0</sup> and TOC<sup>t</sup> are the TOC values of dye solution before and after time 't' of UV irradiation respectively.

#### **4.2. Degradation results and mechanism**

The photocatalytic activity of prepared Na doped TiO<sup>2</sup> photo catalyst can be compared from **Figure 5**. Clearly, the degradation percentage of MB dye is highest for Ti0.92Na0.08O<sup>2</sup> , even superior to commercially available Degussa P25 catalyst. The k values for degradation of MB dye by WC, P25, pristine TiO<sup>2</sup> and Ti0.92Na0.08O<sup>2</sup> are 0.86 × 10−3, 30.52 × 10−3, 3.02 × 10−3 and 43.24 × 10−3 min−1*,* respectively. TOC results are in accordance as well. This enhancement in photocatalytic activity of TiO<sup>2</sup> with Na doping is attributed to its smaller crystallite size and reduced rate of electron-hole recombination. Similarly, Ti0.9Zr0.1O<sup>2</sup> shows enhanced photocatalytic activity [10] for which, in addition to reduction in crystallite size and electron-hole recombination, band gap was also reduced due to Zr doping.

The photocatalytic degradation of dye at the surface of TiO<sup>2</sup> is well explained in the existing literature [56, 72]. The reactions occurring at the surface of semiconductor TiO<sup>2</sup> under UV irradiation are depicted in the Eqs. 4–8. The electrons are excited to conduction band while holes are formed in conduction band after absorption of photons having energy greater than or equal to energy gap of semiconductor. In case of pristine TiO<sup>2</sup> , most of these electrons recombine with holes; few of them react with adsorbed oxygen forming reactive oxygen active specie (ROS) O<sup>2</sup> ¯. Similarly holes in the valence band reacts with water molecules and form ROS OH\* radicals. These ROS actually reacts with dye molecules and degrade them into simple hydrocarbons H2 O and CO<sup>2</sup> .

$$\text{TiO}\_2 + \text{h}\upsilon = \text{e}^- + \text{h}^+\tag{4}$$

$$\mathbf{e}^- \star \mathbf{O}\_2 = \mathbf{O}\_2^- \tag{5}$$

In pristine TiO<sup>2</sup>

depicted in **Figure 6**.

**5. Conclusion**

large metal ions in TiO<sup>2</sup>

than pristine TiO<sup>2</sup>

the higher recombination rate of electron-hole results in fewer number of ROS

.

Novel TiO2 Photocatalyst Using Nonaqueous Solvent-Controlled Sol-Gel Route

http://dx.doi.org/10.5772/intechopen.74568

109

results in large surface area,

with improved photocatalytic properties dis-

nanoparticles have been prepared by this

nanopowder is found to be much higher

is

photo cata-

radicals and hence less photocatalytic activity. In addition, large energy gap and small surface

band gap tuning toward visible range and reduced electron-hole recombination rate. This most general proposed mechanism for degradation of dye at surface of metal doped TiO<sup>2</sup>

In this chapter the importance of recently reported nonaqueous solvent controlled sol-gel

modified sol-gel route and their photocatalytic activity evaluated. Successful doping of these

, and even superior to commercially available Degussa P25 TiO<sup>2</sup>

lyst. This is attributed to large surface area due to small grain size and reduced electron hole

lattice using this synthesis route was confirmed by shifts in XRD peak

area also limit the degradation efficiency. Metal doping in TiO<sup>2</sup>

**Figure 6.** Effect of metal doping on degradation mechanism of pristine TiO<sup>2</sup>

positions and increase in d spacing observed from HRTEM images.

route for the synthesis of metal doped TiO<sup>2</sup>

cussed. Pristine as well as Zr and Na doped TiO<sup>2</sup>

The photocatalytic activity of metal doped TiO<sup>2</sup>

$$\rm H\_2O + h^+ = OH^\cdot + H^\cdot \tag{6}$$

$$\stackrel{\circ}{\text{O}}\_2^{\text{-}} + 2\text{H}^{+} + \text{e}^{-} = 2\text{ }\text{OH}^{\*}\tag{7}$$

$$\text{MB} + \text{OH}^\* = \text{Degradiation products} \tag{8}$$

**Figure 5.** Photocatalytic degradation (A) and mineralization (B) of MB dye under 60 minute of UV irradiation.

**Figure 6.** Effect of metal doping on degradation mechanism of pristine TiO<sup>2</sup> .

In pristine TiO<sup>2</sup> the higher recombination rate of electron-hole results in fewer number of ROS radicals and hence less photocatalytic activity. In addition, large energy gap and small surface area also limit the degradation efficiency. Metal doping in TiO<sup>2</sup> results in large surface area, band gap tuning toward visible range and reduced electron-hole recombination rate. This most general proposed mechanism for degradation of dye at surface of metal doped TiO<sup>2</sup> is depicted in **Figure 6**.
