**4.2. Dye degradation**

Comparing between the bacterial inactivation under UV and visible light with and without photocatalyst, it is similar to the UV photolysis system [63, 64]. Visible light photolysis alone is observed to play a role in microorganism inactivation. The bacterial inactivation in UV-A or visible light is observed due to the synergetic effects of radiating energy and mild heat pro-

UV region due to its large band gap energy. On the other hand, doping with metal is mainly

bial property of silver, the germicidal property of UV and photocatalytic activity of the TiO2 photocatalyst [63, 64]. It should be noted that the antibacterial inactivation is dependent on

Photocatalytic inactivation reactions are highly dependent on the irradiation intensity of the light source [34, 63]. The increase in the visible light intensity from 2 to 8 W.m<sup>2</sup>

ment in the inactivation is due to the increase in the number of photons produced as light

The addition of catalyst to the solution resulted in a better bacterial inactivation than the control experiments. At low catalyst dose (0.1 g/L), the observed inactivation was not significant because of less availability of OH radicals to target a large number of bacteria. The increase in the photocatalyst dose to 0.25 g/L shows the possibility of an increase in the number of OH radicals sufficient enough to target the microorganism number, which improves bacterial inactivation. At 0.25 g/L, the maximum inactivation shows the maximum availability of OH radicals in the solution. When the photocatalyst loading is increased above 0.25 g/L, the inactivation process becomes slow where more bacterial colonies are detected for photocatalyst loading of 0.5 and 1.0 g/L. A high amount of catalyst in the solution results in turbidity increase that blocks the radiation to reach to microorganisms and other catalyst particles (shadowing or screening effect), which leads to a low rate of inactivation. The similar effect is

It is reported that the inactivation of bacteria was not influenced by changing the pH of the solution. The rate constant also remains constant for all pH range [64]. The zero point of


(*E. coli*) = 2.5, respectively. At all the pH values studied, both *E. coli* and the catalyst had a negative surface charge. Therefore, the electrostatic repulsion between bacterial cells and

rates of inactivation were higher in the presence of UV than that of visible light.

several operating conditions such as visible light intensity and catalyst amount.


can


itself possesses higher photocatalytic activity in

: Ag catalyst is a synergetic effect of antimicro-


to the visible region. Therefore, the photocatalytic inac-

duced during the irradiation [63]. In the presence of TiO2

UV light has germicidal property, and TiO2

340 Titanium Dioxide - Material for a Sustainable Environment

tivation in the presence of UV light and TiO2

enhance the bacterial inactivation over TiO2

observed for the UV photocatalytic inactivation [63, 64].

catalyst particles could result in the similar inactivation effect.

done to extend the absorbance of TiO2

*4.1.1. Effect of visible light intensity*

*4.1.2. Effect of the photocatalyst dose*

intensity rise.

*4.1.3. Effect of pH*

charge (ZPC) for TiO2

The performance of TiO2 -AgNP photocatalyst is investigated for dye photodegradation under visible light. Among dyes studied as examples in photodegradation test include methylene blue [31, 66], rhodamine-B (RD-B) [33], and acid read 8S [67].

#### *4.2.1. The activity of TiO<sup>2</sup> -AgNP photocatalyst under visible light*

TiO2 -AgNP photocatalyst shows higher activity under visible light in the photodegradation of dyes than that of bare TiO2 [33, 31]. It means that TiO2 -AgNP is also photoactive in the visible region. The energy of the visible light (2.2–3.0 eV) is near the band gap energy of TiO2 - AgNP, which is about 2.7–2.9 eV [72, 75]. The visible light is required to activate TiO2 -AgNP for the dye photodegradation. The energy of the visible light is slightly lower than the band gap of TiO2 anatase (3.2 eV) and TiO2 rutile (3.0 eV). The visible light is unable to excite an electron in TiO2 . Therefore, the photocatalytic performance of TiO2 is weak.

Increasing Ag content in TiO2 -AgNP promotes photodegradation as shown in many works. But further increase in Ag content could have a detrimental effect on the photodegradation result [31, 33, 66, 68]. The optimum Ag content in TiO2 -AgNP is found be at about 2.5 w% [33], 0.80% mol [66], and 0.25 mol% [68].

The effect of Ag content in TiO<sup>2</sup> -AgNP on photocatalytic activity can be explained as follows. The appropriate amount of Ag-doped TiO2 allows effective capture of the photoinduced electrons [31, 33, 66, 67]. The photoinduced electrons during light irradiation results in negatively charged Ag. The photoinduced electrons can be immediately transferred to oxygen atoms of TiO2 . The electron transfer from the TiO2 conduction band to metallic silver particles at the interface is thermodynamically favorable because the Fermi level of TiO2 is higher than that of silver metals [31, 33]. It results in the formation of the Schottky barrier at metal-semiconductor contact region, which improves the charge separation. Accordingly, the recombination of the electron and the OH radicals can be inhibited more [33]. This condition explains the significant enhancement of the photocatalytic activity of TiO2 -AgNP. The increase in Ag content will keep the photodegradation improve until it reaches its optimum.

At high Ag loading above its optimum level, an excess amount of negatively charged silver species are available. A significant amount of the negatively charged silver particles allows silver atoms to attract more OH radicals. However, it reduces charge separation efficiency [31, 35, 66] or raises electron-hole recombination and decrease dye photodegradation. Another possible reason is the formation of silver metallic clusters inside the TiO2 crystal. The metal clusters give small contact surface area of the photocatalyst. The atomic Ag in TiO2 -AgNP may act as a barrier to obstruct light absorption by titania. It also prevents organic substrates from contacting the photocatalyst surface. Silver atoms may become media for electron-hole recombination [44]. As a result, they reduce photodegradation reaction.

#### *4.2.2. The performance of TiO2 -AgNP under UV light in the dye photodegradation*

The photocatalyst working under the UV light is frequently assessed for dye photodegradation such as methyl orange [68], diazo type dye of DR 23 and DB 53 [69, 70] and methylene blue [74, 82]. It is evident that in the presence of Ag, the photocatalytic performance of the TiO2 -AgNP under UV light improves. The photocatalytic activity of TiO2 -AgNP under the UV light is higher than that of unmodified one. The role of the Ag in the improvement of the dye photodegradation under UV light can explain similarly to the effect of Ag under visible light.

affect the surface state [68, 74]. The amphoteric characteristics of synthesized oxides influence the surface charge of the photocatalyst. The pH of dye solution varies with the surface charge of the photocatalyst and shifts the position of redox reaction [68, 74]. Based on the amphoteric

Silver Nanoparticle Incorporated Titanium Oxide for Bacterial Inactivation and Dye Degradation

Ti − OH + OH<sup>−</sup> ⇄ Ti − O<sup>−</sup> + H2 O (5)

Concerning the reactions (4) and (5), it is evident that the surface of the photocatalyst can become positively charged in acidic medium and negatively charged in alkaline medium. On the other side, methyl orange in the aqueous medium is in the anionic state that can also affect

the surface of the catalyst is also positive. Since both dye and photocatalyst are positively charged, it will inhibit adsorption and photodegradation. At pH 3, the dye becomes anionic, while the photocatalyst surface is still positively charged. It facilitates better electrostatic attraction between dye molecules and positively charged photocatalyst surface, which speeds up the photodegradation. At pH greater than 7, the surface of the photocatalyst has become negatively charged, which leads to electrostatic repulsion between methyl orange and photocatalyst. Therefore, it results in a decrease in the dye photodegradation

Different from the anionic dye, a cationic dye such as methylene blue shows the maximum adsorption and photodegradation at neutral to basic pH. At low pH, the photodegradation may occur less efficient due to electrostatic repulsion between methylene blue molecules and photocatalyst, since both dye molecules and photocatalyst have positive charges. The electrostatic repulsion can inhibit adsorption that results in a decline in the dye degradation. In neutral pH, the dye species is positively charged, whereas photocatalyst is neutral so that they create electrostatic interaction. At higher pH, the dye is neutral, whereas the photocatalyst is in the anionic state, which facilitates efficient adsorption and photodegradation [68]. The

It is apparent that the dye photodegradation reduces gradually when dye concentration improves [66, 75]. At low dye concentration, a few dye molecules in solution can move freely into the active surface of the photocatalyst. When the abundant active sites of the photocatalyst are available to absorb the dye, the dye photodegradation becomes efficient. High dye concentration gives more dye molecules that hinder their movement close to the photocatalyst. Therefore, the adsorption and the photodegradation decrease. For the photocatalyst, the surface has been occupied by much dye that diminishes the active sites at the surface. It leads

maximum photodegradation for this dye takes place at pH 9 [74].

to less dye adsorption and declines photodegradation [66, 74, 75].

*4.2.3.3. Effect of the initial dye concentration*

ions cause the dye to become positively charged. Note that

<sup>+</sup> (4)

http://dx.doi.org/10.5772/intechopen.75918

343

, the following equilibriums take place:

Ti − OH + H+ ⇄ Ti − OH2

characteristics of TiO2

the adsorption.

efficiency [68].

At pH lower than 3, the H+

The photodegradation performance of TiO2 -AgNP under visible light is better than under UV light. In the case of rhodamine-B degradation, the dye can be adsorbed by Ag particle in TiO2 -AgNP. The dye adsorbed on the Ag surface can be activated by visible light because the dye absorbs the electromagnetic radiation in the range of visible light. The activated dye molecules are unstable and start to degrade. On the one hand, the lower UV light photocatalytic activity of TiO2 -AgNP may be due to surface plasmon resonance of metallic Ag that reduces UV light excitation [66]. This unexcitability of the photocatalyst leads to the low dye photodegradation.
