Self-cleaning window glazes offer the opportunity to have low-maintenance self-cleaning windows that absorb UV-light as part of their process for breaking down organic matter and removing it from the glass surface. The glazes work via a process called photocatalysis. The coating is applied to the exterior face of the glass and acts as a catalyst, breaking down organic compounds metal catalyzed redox reactions. Unlike simple catalysis reactions thaat are activated by heat, photocatalysis reactions are activated by UV radiation.
Pilkington Activ was the first commercially available glass to exploit photocatalysis to reduce the cleaning needs of windows and structures. The glass will naturally clean itself during cycles of sun and rain via titanium dioxide (TiO2) applied as a nanothin layer to the glass' surface. Titanium dioxide was used as the metal catalyst in this case for its long-term stability, allowing the glass to maintain its properties for a few years. Once installed, the windows must be activated in direct sunlight for five to seven days before becoming active. Once active, the organic compounds in dirt will react with the titanium dioxide layeras the dirt settles on the surface. When it rains, or when clean water is sprayed on the surface during dry spells, the hydrophilic nature of the titanium dioxide coating causes the water to sheet and the dissolved compounds to run off the surface, leaving a clean and spot free surface.
Pilkington Activ uses a 15-30 nm thick coating of titanium dixide. This coating is applied through Chemical Vapor Deposition (CVD). CVD is a nanofabrication process that allows for continuous and industrial scale applications of nanomaterials as thin films through vaporization. Saint Pancras Station in the United Kingdom has used this nanoparticle enhanced glass to reduce maintenance costs on their glass roof and building structure.
Titanium dioxide coated glass has the potential to reduce water and solvent use in building and window maintenance. It also reduces exposure to harmful organic compounds created by pollutants and fertilizers, breaking these compounds down into other compounds that may be less harmful. These attributes have the potential to lower the maintenance and operation costs of buildings and structures.
Though the material itself is considered safe for human use, some organics are partially converted into carbon dioxide and mineral acids, which may have ecological or health risks. Additionally, titanium dioxide may be toxic to organisms that are particularly sensitive to oxidative stress. More research will have to be conducted on the cost/benefit ratio as well as the risk characterizations of the widespread use of titanium dioxide nanoparticles.