TiO2 as a photocatalyst
As a nanomaterial,photocatalytic TiO2 notably affords some advantages such as complete degradation of most organic compounds,applicable to both water and air treatment,working under ambient temperature and pressure,cheap and mass-produced materials,safe and simple process. TiO2 has also some disadvantages like low photo-conversion efficiency,inactive to visible light,not suitable to large scale applications,need of separation and recycle of photocatalysts in slurry process,need of catalyst immobilization,high maintenance cost of lamps and difficulty in uniform irradiation of all catalyst surfaces.
In principle,any semiconductor with an appropriate magnitude of the band gap and the position of band edges is able to initiate photo induced redox reactions on its surface. Although wide band gap semiconductors with high positive values of EVB such as ZnO2,WO3,and SnO2 often show some oxidative photocatalytic reactivities but in most cases their photoactivities are lower than those of TiO2.
Titanium dioxide generates a pair of a conduction band (CB) electron and a valence band (VB) hole in the solid oxide lattice upon absorbing a photon with energy greater than 3.2 eV (or λ<388 nm) and the subsequent charge transfers at the interface initiate various kind of redox reactions under the ambient condition (in both air and water).

Pollutant degradation
Photocatalytic oxidation of organic and biological molecules is of great interest for environmental applications,especially in the destruction of hazardous wastes. The ideal outcome is complete mineralization of the organic or biological compounds,including aliphatic and aromatic chlorinated hydrocarbons,into small inorganic,non- or less- hazardous molecules,such as HCl,HBr ,H2O,CO2 ,SO4-2,NO3 etc. Compounds that have been degraded by semiconductor photocatalysis include alkanes,haloalkanes,aliphatic alcohols,carboxylic acids,alkenes,aromatics,haloaromatics,polymers,surfactants,herbicides,pesticides and dyes.The photocatalytic reactions initiating on the TiO2 surface have the multi-phasic character. The photocatalytic degradation reactions of organic substances take place not only at the TiO2/water and TiO2/air interfaces but also at the TiO2/solid interfaces.

In addition to the photo-oxidative effect observed on TiO2 surfaces and in suspensions of TiO2 colloids,TiO2 surfaces have been more recently studied with the aim to understand the UV-induced hydrophilicity effect. Both polycrystalline films and single-crystal (rutile and anatase) samples show hydrophilicity,thus it can be concluded that the observed UV-induced hydrophilicity is an inherent property of TiO2 and does not change on the basis of the character of the specific type of TiO2 material. Therefore it is proposed that the UV light generates oxygen vacancies which are postulated to be active sites for water dissociation. A large number of commercial products have been developed based on this technology,including windows and mirrors that are self-cleaning and anti-fogging.
From the survey of the massive amount of literature dealing with the UV-induced hydrophilicity effect on TiO2 surfaces,three possible mechanisms have been proposed as discussed below. The first model involves the production of Ti3+ ions at the surface as a result of oxygen atom ejection from the lattice. The defects,known as oxygen vacancies or Ti3+ sites,when produced on the surface by thermal activation are known to cause water dissociation and thus,the production of adsorbed -OH species,which are known to be hydrophilic in nature. As a result,this was a first logical explanation for the UV-induced hydrophilicity effect.
Second model suggests that the UV-induced hydrophilicity effect is due to surface modifications,which,in the presence of H2O,lead to an increased surface coverage of Ti-OH groups. This model was proposed on the basis of XPS,IR,and electrochemical measurements. The effect is explained in the following way:-OH groups that are bound in 2-fold coordination to Ti atoms are converted by H2O adsorption into two -OH groups that are singly coordinated each to their own Ti atom. Further XPS investigations suggest a similar mechanism,where Ti-OH groups are converted into Ti-OH groups that are associated with dangling bonds. Experiments have now been done which suggest that the mechanisms suggested are invalid. These experiments involve the use of a powdered TiO2 sample that has been exposed to UV light under high-vacuum conditions to study how UV light affects chemisorbed H2O or Ti-OH bonds found on the TiO2 surface.
A third model for the UV-induced hydrophilicity effect on TiO2 simply involves the removal of a hydrophobic hydrocarbon monolayer by photo-oxidation. The aromically clean TiO2 surface produced is hydrophilic.