TiO2, (Titanium Dioxide) is a chemical containing one titanium atom and two oxygen atoms and used normally as a white pigment in many products like paint, paper, inks and cosmetics. But when TiO2 is synthesised into nano-sized particles it exhibits photocatalytic properties. Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate and the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron–hole pairs, which generates free radicals (hydroxyl radicals: •OH) capable of undergoing secondary reactions.. The principle of photocatalytic activity is that when TiO2 is illuminated with light of sufficient energy, electron-hole pairs are excited so that additional electrons go across the band gap to conduction band, while holes stay in the valence band. Natural sunlight comprises more than sufficient UV energy to initiate such photocatalysis.

Using a thin film titanium dioxide coating the first self-cleaning glass was made. Self-cleaning glass is a specific type of glass with a surface which keeps itself free of dirt and grime through photocatalytic decomposition. The glass cleans itself in two stages. The "photocatalytic" stage of the process breaks down the organic dirt on the glass using UV light and makes the glass hydrophilic (normally glass is hydrophobic). During the following "hydrophilic" stage rain washes away the dirt, leaving almost no streaks, because hydrophilic glass spreads the water evenly over its surface.

Self cleaning of coated surface is achieved by the breaking down the bond between the surface and the dirt by which dirt gets easily washed away by water. The coatings come in colours, including transparent one, which can be applied to things such as monuments - to reduce stone cleaning costs. Self Cleaning property can be made use of in street furniture, highway structures, buildings, bridges, monuments, road signs etc.The self cleaning and pollution reducing properties of a TiO2 coating arise from its ability to break down unwanted chemicals such as algae and fungi (self-cleaning) or noxious gases such as nitrogen dioxide and benzene and sulphur present in vehicle exhaust are converted into harmless molecules. Toxic and unpleasant odours such as VOC's are also neutralised.

TiO2 Nano-coatings made on a surface is found to reduce pollutants such as fine dust (Particulate matter), benzene, nitrogen dioxide, sulphur dioxide and ozone from air and also from water running over surfaces. Examples of pollution reduction in public utilities can be in pavements, roads, tunnels, subways, urban playgrounds, car parking places etc.

As a catalyst TiO2 just catalyses the reaction and is not consumed in the reactions and is characterized by high photocatalytic properties, chemical stability and low price.