**4.1. Bacterial inactivation**

particles (>100 nm) is observed in the TEM images [33, 67]. The particle size of Ag-doped TiO2

tion method, Ag is detected to have a larger size than that of sol-gel. In the impregnation

The surface area is determined by surface area analyzer based on the BET method. In gen-

By impregnation method, the metallic silver is formed as large agglomerate that may block

contrast, by a sol-gel method, the surface area increases with the enlargement of Ag content in

By the photo-deposition method, the addition of Ag at only 0.25 mol%, an appreciable increase

large surface area. The Ag loading higher than 0.25 mol% leads to the decrease in surface area

The other reasons for the surface area improvement are proposed as follows [31, 68, 76]. The Ag doping with a suitable amount (ca. 2–6 mol%) promotes the phase transformation of TiO2 from anatase to rutile since the surface area of rutile is larger than that of the anatase. The

also has a depressing effect on the anatase grain growth. The average crystallite

/Ag-0.25%, and (c) TiO2


by impregnation method is observed to decrease with increase in the Ag-doped TiO2

method, and the calcination temperature [33, 68, 72]. The surface area of TiO2

in the specific surface area is observed [33, 68]. The surface area of TiO2

/Ag-0.05%, (b) TiO2

as 0.25 mol% seems to be well dispersed on the surface of the TiO2

tent smaller than 0.25 mol% is not significantly different from that of the pure TiO<sup>2</sup>


particle. Such surface blocking leads to the surface area to decline. In

structure. Ag in TiO2

/Ag-0.50% [71].

are suspended in water in the TiO2

solution that allows them to have mutual interaction and inhibit their particle


structure that enables them to form





grain that contributes to a

due to the



[72].

is also directed by the preparation methods [33]. In TiO2

Consequently, the Ag particle is not limited by the TiO2

a large agglomerate. Meanwhile, in the sol-gel method, TiO2

growth. As a result, it forms the small grain size (**Figure 3**).

**-AgNP**

vast and thin dispersion of the Ag particles on the TiO2

that is resulted from by large silver aggregate.

salt and TiO2

338 Titanium Dioxide - Material for a Sustainable Environment

process, AgNO3

**3.4. Surface area of TiO2**

eral, the surface area of TiO2

the surface of the TiO2

Ag-doped TiO2

**Figure 3.** The TEM images of (a) TiO2

TiO2

sol and Ag+

The doping TiO2 with metallic silver to produce TiO2 -AgNP has been investigated as a potential antibacterial agent in inactivating *Escherichia coli* [63–65, 78, 79] under visible light exposure. The TiO2 -AgNP exposure to UV light for bacterial inactivation is also essential.

TiO2 -AgNP demonstrates a significant activity in the bacterial inactivation under visible light. The antibacterial performance of TiO2 -AgNP is found to be higher than that of unmodified TiO2 . Both TiO<sup>2</sup> and TiO2 -AgNP can kill bacteria because TiO2 provides OH radicals during UV or visible irradiation at a suitable wavelength. The OH radicals attack and destroy the bacterial wall [10, 11]. Under visible light, TiO2 -AgNP can be activated since TiO2 -AgNP has a low band gap energy (Eg) that matches with the visible region wavelength. Meanwhile, due to its high band gap energy (Eg), which is in the same order as UV light, TiO2 is less active under visible light. Addition of Ag to TiO2 also gives the excellent antibacterial agent, that is by penetrating the metallic Ag nanoparticles into the cell membrane of the bacteria [84]. There is a synergic effect of TiO<sup>2</sup> and Ag in inhibiting bacteria [63–65, 78, 79]. The activity of Ag in the bacteria inactivation process is examined by applying TiO2 -AgNP in dark condition [81]. It has been postulated that silver disrupts the cell wall and affects the rapid penetration of the metallic ions into the cell where irreversible precipitation of the bacteria's DNA occurs [84].

The role of Ag in TiO2 -AgNP in the bacterial inhibition under visible light is affected by its ability as a center of the separation of photoinduced electron and OH radicals that delay the recombination of electron and hole [34, 63–65, 78, 79]. The other role of Ag in the improving the bacterial inactivation corresponds to the electron capture that can prevent the recombination. The inhibition of the recombination creates more OH radicals, which improves the bacterial inactivation.

The Ag content in TiO2 -AgNP is subjected to test further. Increase in Ag content leads to decline in bacterial inactivation [34, 63–65, 78, 79]. Increasing Ag in TiO2 -AgNP can enhance the electron capture, forming anion Ag, which allows more OH radical available. The more OH radicals available, the better bacterial inactivation. However, a further increase in Ag content can block the TiO2 surface and prevent the light absorption, producing a lower amount of OH radicals. The other possible reason is the attachment of OH radical with excess Ag anion. The depletion of OH radicals leads to the inactivation declined [64].

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 produced during the irradiation [63]. In the presence of TiO2 -AgNP photocatalyst, it is found the rates of inactivation were higher in the presence of UV than that of visible light.

**4.2. Dye degradation**

The performance of TiO2

*4.2.1. The activity of TiO<sup>2</sup>*

of dyes than that of bare TiO2

Increasing Ag content in TiO2

0.80% mol [66], and 0.25 mol% [68].

The appropriate amount of Ag-doped TiO2

. The electron transfer from the TiO2

cant enhancement of the photocatalytic activity of TiO2

keep the photodegradation improve until it reaches its optimum.

The effect of Ag content in TiO<sup>2</sup>

*4.2.2. The performance of TiO2*

TiO2

TiO2

gap of TiO2

electron in TiO2


Silver Nanoparticle Incorporated Titanium Oxide for Bacterial Inactivation and Dye Degradation


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

rutile (3.0 eV). The visible light is unable to excite an



allows effective capture of the photoinduced elec-

conduction band to metallic silver particles at the

is weak.



is higher than that of

crystal. The metal



341


visible light. Among dyes studied as examples in photodegradation test include methylene


[33, 31]. It means that TiO2

AgNP, which is about 2.7–2.9 eV [72, 75]. The visible light is required to activate TiO2

. Therefore, the photocatalytic performance of TiO2

interface is thermodynamically favorable because the Fermi level of TiO2

possible reason is the formation of silver metallic clusters inside the TiO2

recombination [44]. As a result, they reduce photodegradation reaction.

clusters give small contact surface area of the photocatalyst. The atomic Ag in TiO2

visible region. The energy of the visible light (2.2–3.0 eV) is near the band gap energy of TiO2

for the dye photodegradation. The energy of the visible light is slightly lower than the band

But further increase in Ag content could have a detrimental effect on the photodegradation

trons [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

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 signifi-

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

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

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

*-AgNP under UV light in the dye photodegradation*

*-AgNP photocatalyst under visible light*

blue [31, 66], rhodamine-B (RD-B) [33], and acid read 8S [67].

anatase (3.2 eV) and TiO2

result [31, 33, 66, 68]. The optimum Ag content in TiO2

UV light has germicidal property, and TiO2 itself possesses higher photocatalytic activity in UV region due to its large band gap energy. On the other hand, doping with metal is mainly done to extend the absorbance of TiO2 to the visible region. Therefore, the photocatalytic inactivation in the presence of UV light and TiO2 : Ag catalyst is a synergetic effect of antimicrobial 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 several operating conditions such as visible light intensity and catalyst amount.
