Chemical treatment of photocatalytic degradation (semiconductor based materials like ZnO and TiO2 ), has been used extensively due to its excellent catalytic activity, eco-friendly as well as low secondary pollution.

Unfortunately metal oxide nanomaterials have no efficient absorption in the visible light region and require activation/irradiation in the UV region.

Metal nanoparticles, on the other hand exhibit strong absorption and excellent catalytic activity in the visible range and gained much attention due to its potential application in degradation and removal of organic pollutants under irradiation of visible light.

As you know there are various pollutants in waste water which are either organic or inorganic ones. There are also many methods for the removal or degradation of organic based pollutants.One of the attracting method for the decrease of organic pollutants is the use of photocatalytic materials. In this method the photocatalyst can convert harmful pollutants to much safer molecules under light radiation. The most attracting photocatalysts are those which are active under Sun light.

The Catalysts are utilized for speeding up the chemical reaction and similarly, a photocatalyst employ the catalyst for speeding up chemical reactions in the presence of UV light. In this way, the absorption of light produces the electron–hole pairs that enable chemical transformations of the reaction participants and regenerate its chemical composition after each cycle of such interactions. There are two types of photocatalytic reactions, i.e., homogeneous photocatalysis and heterogeneous photocatalysis. Significant features of the photocatalytic systems are: proper band gap, morphology of the material, more exposed surface area, stability and its reusability. Photocatalysis is widely being practiced for the degradation and mineralization of hazardous organic compounds to CO2 and H2O and thus leads to the reduction of toxic metal ions into non-toxic states, deactivate and destruct all the water borne microorganisms, decomposes the air pollutants such as NO2, CO and NH3, degraded the waste plastics and green synthesis of industrially important chemicals. Thus, Photocatalysis refers to the oxidation and reduction reactions on the surfaces of photocatalyst material, mediated by the valence band (VB) such as holes(h+) and conduction band (CB) such as electrons (e-) generated by the absorption of UV-VIS light radiation. Such photo-generated pairs of h+ and e-induces the formation of aggressive species such as hydroxyl (OH-) or superoxide radicals from the moisture and atmospheric oxygen. These species are strong enough to oxidize and decompose organic materials or smelling gas and kill bacteria. Photocatalysis has been established as an efficient process for the mineralization of toxic organic compounds, hazardous inorganic materials and microbial disinfection as a result of the formation of the OH- ions, which acts as a strong oxidizing agent.

Photocatalyst as its name imply will bring about the hastening of degradation of pollutants or contaminants either by oxidation of organic matter or reduction of inorganic matter in presence of light. The product profile or the intermediates formed however have to be analyzed to check for its toxic potential. If photocatalysis is coupled with reagents which have oxidation or reduction potential the process of remediation can be further hastened.

Chemical treatment of photocatalytic degradation (semiconductor based materials like ZnO and TiO2 ), has been used extensively due to its excellent catalytic activity, eco-friendly as well as low secondary pollution.

Unfortunately metal oxide nanomaterials have no efficient absorption in the visible light region and require activation/irradiation in the UV region.

Metal nanoparticles, on the other hand exhibit strong absorption and excellent catalytic activity in the visible range and gained much attention due to its potential application in degradation and removal of organic pollutants under irradiation of visible light.

Dear Bhaskar Bhatt! To see a bunch of works at this area you may want to look for examle through the google search for the question "TiO2 water purification". Or look for example at

https://yandex.ru/search/?lr=213&clid=2233626&text=tio2%20water%20purification

Sir, how do we improve the efficiency of catalyst?

and what are limitations of photo-catalysis

Efficiency of catalyst could be measure using stability and reusability of catalyst