## 4 Discussion
\[\text{TiO}_{2}+\text{h}\
u\rightarrow\text{e}\text{ }_{\text{CB}}{}^{-}+\text{h}_{\text{VB}}{}^{+} \tag{8}\]  
\[\text{TiO}_{2}(\text{h}_{\text{VB}}{}^{+})+\text{H}_{2}\text{O}\rightarrow \text{TiO}_{2}+\text{H}{}^{+}+\text{OH}^{\bullet} \tag{9}\]  
\[\text{TiO}_{2}(\text{h}_{\text{VB}}{}^{+})+\text{OH}^{-}\rightarrow\text{TiO }_{2}+\text{OH}^{\bullet} \tag{10}\]  
\[\text{TiO}_{2}(\text{e}_{\text{CB}}{}^{-})+\text{O}_{2}\rightarrow\text{TiO }_{2}+\text{O}_{2}{}^{-\bullet} \tag{11}\]  
\[\text{O}_{2}{}^{-\bullet}+\text{H}{}^{+}\rightarrow\text{HO}_{2}{}^{\bullet} \tag{12}\]  
\[2\text{HO}_{2}{}^{\bullet}\rightarrow\text{H}_{2}\text{O}_{2}+\text{O}_{2} \tag{13}\]  
\[\text{H}_{2}\text{O}_{2}+\text{e}{}^{-}\rightarrow\text{OH}^{\bullet}+\text{OH }^{-} \tag{14}\]  
\[\text{Tannic acid}+\text{OH}^{\bullet}\rightarrow\text{degradation products} \tag{15}\]  
\[\text{Tannic acid}+\text{h}_{\text{VB}}{}^{+}\rightarrow\text{oxidation products} \tag{16}\]  
\[\text{TiO}_{2}+\text{h}\
u\rightarrow\text{e}\text{ }_{\text{CB}}{}^{-}+\text{h}_{\text{VB}}{}^{+} \tag{8}\]0Figure 7 shows the rate constants obtained for the different TiO\({}_{2}\)NT surfaces under different pHs. The results indicate that, in general, as the pH values decrease, the rate of tannic acid decomposition decreases; however, this does not vary in a linear fashion. To understand the influence of pH on photocatalytic degradation, several factors need to be taken into consideration, including the charge of the surface and the tannic acid at each pH and also the concentration of radical precursors in solution. The surface charge is greatly affected by the pH of the solution. An isoelectric point at approximately pH 6 is widely reported for titanium dioxide surfaces in contact with aqueous solution [38; 39; 40]. Thus, it is expected that, below the isoelectric point, the TiO\({}_{2}\) surfaces would be positively charged, while, above the isoelectric point, the TiO\({}_{2}\) surfaces would be negatively charged due to the following ionization reactions. \[\text{TiO}_{2}+\text{h}\
u\rightarrow\text{e}\text{ }_{\text{CB}}{}^{-}+\text{h}_{\text{VB}}{}^{+} \tag{8}\]1  
\[\text{TiO}_{2}+\text{h}\
u\rightarrow\text{e}\text{ }_{\text{CB}}{}^{-}+\text{h}_{\text{VB}}{}^{+} \tag{8}\]2  
Similarly, tannic acid being a weak organic acid (pKa = 6), at acidic pH, the molecule exists at its protonated or undissociated form, while, in alkaline conditions, above pH 6, it exists as a deprotonated dissociated form of anions. Thus, it is expected that, at low pH conditions (pH 2 and 3), there would be an electrostatic repulsion between the positive charges carried onto the surface and the tannic acid molecule; in turn, the rate of degradation would be much slower. The rate of reaction increases drastically at pH 6, which correspond to the isoelectric point of both the tannic acid and surface, thus resulting in an equal amount of attraction and repulsive forces between the two. Increasing the pH beyond 7 results in a slight drop in the reaction constant. This is attributed to the repulsive forces between the negative surface and the deprotonated forms of tannic acid. A further increase in pH should surely cause the Coulombic repulsion [41; 42] to be accentuated; however, similar to what was observed by a number of researchers [43; 44], this was not the case, as the rate of reaction increased drastically. It has been proposed [45] that a higher concentration of OH\({}^{\bullet}\) radicals is generated by photogenerated holes due to the high concentration OH\({}^{\text{-}}\) ions. Table 4 summarizes the electrostatic forces and the charges that occur with an aqueous solution of tannic acid exposed to TiO\({}_{2}\) surfaces. Figure 7 further demonstrates that TiO\({}_{2}\)NT and TiO\({}_{2}\)NT-Ag surfaces had a larger rate constant irrelevant of the pH of the medium. This further indicates that the layer thickness had an effect where a lower generation of photoholes decreases the production of hydroxyl radicals and active sites for the oxidation of tannic acid.