## 4 Discussion
In fact, with respect to DPPH, ABTS and Cu\({}^{2+}\) ions, the reaction rate was found to decrease by almost half when the TiO\({}_{2}\)NT-S and TiO\({}_{2}\)NT-P surfaces were compared to TiO\({}_{2}\)NT and TiO\({}_{2}\)NT-Ag surfaces. Of the different morphological features presented in Table 1, photocatalytic activity is largely reliant on the thickness of the nanomaterial layer, and hence the exposed area. This is inferred when the thickness of the samples prepared in ethylene glycol, namely the TiO\({}_{2}\)NT and TiO\({}_{2}\)NT-Ag surfaces are compared to the TiO\({}_{2}\)NT-S and TiO\({}_{2}\)NT-P surfaces, prepared in the aqueous media. The thickness of the former is several times larger than that of the other two materials. The difference in activity between TiO\({}_{2}\)NT-S and TiO\({}_{2}\)NT-P towards the redox probes is not a clear cut and indicates different affinity of the molecules to the surface. This may be due to a different number of hydroxyl groups on the surface. The addition of silver to the bare TiO\({}_{2}\)NT did not provide any appreciable benefits. This could be due to the low surface coverage with the spherical nanoparticles. The high content of impregnated silver can be misleading, in that while it is present it is not contributing to the formation of the Schottky junction which will promote a higher activity. This is likely to be due to the silver being incorporated along the tube length, resulting in little to no contact with the solution or the UV source. Whilst ABTS, DPPH and CUPRAC assays are based on the reduction-based reactions, the removal of tannic acid and microorganisms are attributed to the oxidation reactions that occur at the surface of the titanium dioxide nanotubes. This photodegradation mechanism is based on the conversion of organic compounds into harmless gaseous CO\({}_{2}\) and water. This can occur through the direct interaction between tannic acid and the generated holes to produce an oxidized product (Equation (17)) or else through the photolysis of water, which generates free oxidizing radicals (Equation (18)) that, in turn, oxidize tannic acid. The generation of free radicals generally follows a ten-step process. The initiation step involves the absorbance of UV light by TiO\({}_{2}\) (Equation (8)), which causes an electron propagation to the conduction band and a positive hole generation in the valence band [34; 35]. The photohole generated reacts with water causing photolysis of water to H\({}^{+}\) and OH\({}^{\bullet}\) (Equation (9)). Furthermore, the photoholes neutralize OH\({}^{-}\) groups to produce more OH\({}^{\bullet}\) radicals (Equation (10)). The promoted electron in the conduction band reacts with dissolved oxygen found within the solvent to produce superoxide anions (Equation (11)), which are neutralized by protons in solutions to form perhydroxyl radicals (Equation (12)) that combine and form hydrogen peroxide (Equation (13)) [36]. The hydrogen peroxide formed under UV light decomposes to release more hydroxyl radicals and anions (Equation (14)). The oxidation of tannic acid occurs through the reaction with hydroxyl radicals generated through the photolysis of water [37].