**2.1 Photoinduced hydrophilicity**

Furthermore, it is evident that the hydrophilic conversion rate of TiO2 can be increased by applying high positive electrode potential, and can be decreased by the usage of hole-scavengers in the photocatalytic reaction [11]. This interpretation suggests that for the hydrophilic conversion of the photocatalytic material, the diffusion of holes to the catalytic surface is very essential. The formation of Ti-OH bands on the TiO2 surface is attributed to photoinduced hydrophilicity [12]. The diffusion of holes to the catalytic surface can weaken the Ti-O bonds, and water molecules adsorption to this surface can then break the weakened bond. This will lead to the charge separation and the formation of a new OH bond on the surface. Therefore one OH group which is doubly coordinated to Ti atoms converted into two OH groups. It is revealed that the highly amphiphilic catalytic surface of TiO2 can leads to the reconversion of the surface wettability, which is due to the deposition of a hydroxyl group from the surface. The second mechanism of photocatalytic activity on the surface initiates the decomposition of the adsorbed pollutants on the surface which initiates by the absorption of UV light and thus reduced the organic pollutants on the surface. It has been recommended that in addition to the photocatalytic degradation of organic pollutants on a TiO2 surface, desorption of water molecules on the surface is also initiated by UV irradiation. The subsequent breakdown in the H-bonded network is supposed to be essential for the hydrophilic conversion [13]. The results reveal that the photocatalytic activity on the surface is not an exclusive requirement for any surface to be hydrophilic or superhydrophjilic. It has been reported that many metal oxides such as vanadium oxide and tungsten oxide have superhydrophilic nature under UV light illumination without and photocatalytic activity mechanism on their surface. In consequence, it has been suggested that a combination of various mechanisms is required to explain the photoinduced hydrophilicity or photocatalytic activity on the surface. The combination of photoinduced hydrophilicity and photocatalytic activity are the major requirements to degrade the organic pollutants, and light-induced superhydrophilicity to sheet water for cleaning the surface by washing off the degraded pollutants, good optical transparency, photostability, and durability.
