**2. Self-cleaning action of TiO2 as photocatalyst**

Once a metal oxide semiconductor is irradiated with an energy source that is higher than its band gap (3.2 eV) [2] leads to the absorption of photons and excitation of an electron (e−) in which way generate an electron–hole pair, with the hole (h+) left behind in the valence band. These electron–hole pair in turn undergoes recombination and non-radiatively emits the excess energy in the form of heat or light [3, 4]. This recombination process or charge obliteration reduces the efficiency of TiO2 photocatalytic activity. Due to crystal defects and impurities present on the surface of TiO2 this reaction would take place on the surface or within the bulk of TiO2. Over the surface of TiO2 excitons (electron–hole pair) were undergoes recombination, this electron–hole pair can combine with the adsorbed molecules and leads to the decomposition of organic volatile compounds. Evidently, electrons in the CB reacts with the oxygen molecules present on the surface and reduces it into superoxide radicals which can react with water to give hydroxyl radicals (% OH); on the other hand holes (h+) in the valence band reacts with water to form %OH radicals. These oxygen-generated species decompose the volatile organic compounds into CO2 and H2O on the catalytic surface by the process of free radical mechanism [5–8].

Thus the mechanism of superhydrophilicity on the catalytic surface can be explained by the combination of two processes. The first mechanism of superhydrophilicity includes the surface hydroxylation upon photoexcitation. This step consists of two steps (a) oxygen vacancy generation and (b) reconstruction of photoinduced Ti-OH bonds. The mechanism of superhydrophilicity propagates by the formation of photoinduced electrons in the system by the reduction of metal centers. For example, TiO2 (IV) gets reduced into TiO2 (III) by the movement of conduction band electrons, however, the holes formed in the valence band oxidized into O2 − anions, and thus vacancies are formed by the ejection of oxygen atoms. The hydroxyl anions are formed at these vacancies during the photocatalytic mechanism and water molecules get absorbed, leading to the hydrophilic nature of the surface [9]. Sakai et al. proposed that for the superhydrophilic characteristics of a surface photoinduced holes are more important than electrons, these holes can diffuse on the surface of catalyst

and get trapped at the oxygen lattice because these photo-induced holes can diffuse on the surface and get trapped at the lattice oxygen sites, this leads to the formation of new hydroxyl bonds with the adsorbed water molecules [10].
