*4.2.3.2. Effect of initial pH*

In the acidic pH, the effectiveness of the dye photocatalytic degradation over TiO<sup>2</sup> -AgNP is found to be low. The photodegradation improves as pH increasing, but when the pH is increased further the photodegradation declines. For methyl orange photodegradation, the optimum pH is reached at 3 [68]. The dye degradation over heterogeneous photocatalyst of TiO2 is initiated by adsorption on photocatalyst surface, leading to sequentially or simultaneously dye degradation. The effectiveness of the adsorption and degradation of dye depends on the surface charge of the catalyst and solution pH. The pH is an effective parameter to 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 characteristics of TiO2 , the following equilibriums take place:

$$\text{Ti}-\text{OH} + \text{H}^\* \rightleftharpoons \text{Ti}-\text{OH}\_2^\* \tag{4}$$

$$\text{Ti}-\text{OH} + \text{OH}^- \rightleftharpoons \text{Ti}-\text{O}^- + \text{H}\_2\text{O} \tag{5}$$

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 adsorption.

At pH lower than 3, the H+ ions cause the dye to become positively charged. Note that 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 efficiency [68].

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