**2.1. Sol-gel**

The sol-gel method involves the interaction of titania sol with silver ion the solution, which forms a gel. The gel is heated at a low temperature to evaporate the solvent followed by calcination at high temperature [31, 35, 72, 73, 78, 80, 82, 83]. The main precursor of titania is commonly liquid titanium(IV) isopropoxide (TTiP) with a chemical formula of Ti(OC3 H7 ) 4 [31, 35, 72, 75], but titanium n-butoxide (Ti(OC4 H9 ) 4 [82, 83] is also potential. For silver starting material, AgNO3 is frequently employed [31, 72, 75, 77, 80, 82, 83]; however, silver acetylacetonate is also chosen sometime [35].

Titania sol is prepared by mixing TTip solution in ethanol and water accompanied by stirring vigorously at room temperature for about 15 min. During the dissolution of TTip in the ethanol medium, an exchange of propyl group from TTip with ethyl group from the ethanol to yield titania tetra ethoxides takes places by the release of propanol. Titanium tetra ethoxide is hydrolyzed in acidic medium to form sol titanol of Ti(OH)4 .

The AgNO3 solution is mixed with ethanol and stirred [31, 72, 75, 77, 80, 82, 83], giving the sol of silver. Finally, the silver sol is added dropwise to the titania sol with stirring, where transparent titania sol changes its viscous yellow solution. The reaction between titania sol and silver sol is commonly written as follows:

$$\text{Ti(OH)}\_{4} + \text{C}\_{2}\text{H}\_{5}\text{O}-\text{Ag} \xrightarrow{\Delta} \text{Ti(OH)}\_{3}-\text{Ag} + \text{C}\_{2}\text{H}\_{5}\text{OH} \tag{1}$$

The viscous sol was heated at about 60°C to evaporate organic solvents to yield dry gel [72]. The dry gel is calcined at about 350°C to produce TiO2 -AgNP powders, according to the reaction (2):

$$\text{4Ti(OH)}\_{3}-\text{Ag} + \text{O}\_{2}\text{(350}^{\circ}\text{C)}\\\Delta \text{TiO}\_{2}-\text{AgNP} + 6\text{ H}\_{2}\text{O}\tag{2}$$

In the preparation of TiO2 -AgNP photocatalyst by a sol-gel method, some modifications have been done including the use of acetic acid [83], micellar medium [75], spin coating technique [82], with the purpose to improve TiO2 -AgNP photocatalyst characteristics and performance.
