Windows, self-cleaning

The oxidative degradation of organic pollutants in water and air streams is considered as one of the so-called advanced oxidation processes. Photocatalytic decomposition of organics found widespread industrial interest for air purification (e.g., decomposition of aldehydes, removal of NO , ), deodorization, sterilization, and disinfection. Domestic applications based on Ti02 photocatalysts such as window self-cleaning, bathroom paints that work under illumination with room light, or filters for air conditioners operating under UV lamp illumination have already been commercialized. Literature-based information on the multidisciplinary field of photocatalytic anti-pollutant systems can be found in a number of publications, such as Bahnemann s [237, 238] (and references therein). 

Quite recently (2001) titanium dioxide has been used to coat "self-cleaning" windows with a very thin film about 5 x 10-6 cm thick. The titanium dioxide acts as a catalyst for the decomposition of dust and grime by solar ultraviolet light. Probably more important, it lowers surface tension so that rain water "sheets down" the window, washing away dirt. 

A new kind of self-cleaning window was launched by Henkel in 2004 and it made use of silica nanoparticles which align themselves on the glass to form an invisible film with a negative charge which also makes the glass attractive to water, in this case to the hydrogen atoms of water molecules, but the result is the same the surface is covered with a film of water and not droplets. 

Keep windows clean Clean with squeegee Self-cleaning glass... 

Window-glass has evolved through the centuries and is still changing today with the development of newer types of glasses, the latest of which is "self-cleaning" and "smart" glasses. All of these new developments are useful for their intended purposes, which may be room darkening, UV protection or thermal mediation or protection. However, from a scientific standpoint they are all different and will bias the obtained results. 

The Lotus Effect simulates the properties of the lotus flower in nature, by microstructured hydrophobic protrusions which enable surfaces to clean themselves by water in motion. By making use of innovative technologies, the Lotus Effect can be used on many different products. For example, self-cleaning surfaces could be used for self-adhesive films, injection moulded parts or painted components in the construction industry, for facade elements or window frames, for traffic facilities such as road signs, and not to forget cars themselves. 

Photocatalytic titanium dioxide coatings can be selfcleaning, converting organic materials, such as bacteria and dirt, to carbon dioxide at practical rates at room temperature.219 This offers the possibility of self-cleaning buildings and windows. 

The lotus effect has inspired scientists to design superhydrophobic surfaces for applications such as self-cleaning windows and water-repellent clothing. To understand the lotus effect and other phenomena involving liquids and solids, we must understand intermoiecuiar forces, the forces that exist between molecules. Only by understanding the nature and strength of these forces can we understand how the composition and structure of a substance are related to its physical properties in the liquid or solid state. 

As well as water purification, there are many other areas where photo-catalytic contaminant oxidations can he applied such as self-cleaning windows and surfaces and air purification. 

Self-cleaning windows have a coating of nano titanium dioxide. The coating becomes activated when exposed to sunlight and catalytically bums off the oils and grime on the surface. 

From a commercial point of view, cleaning of windows is expensive and cumbersome, especially if the windows are on a skyscraper. The hydrophilic route to self-cleaning windows has meanwhile been reaUzed by several companies and has been released to the market. How far these windows will be a commercial success remains to be seen. The Germany-based website Lotus-EflFekt.de, dedicated to the commercial application of patented self-cleaning superhydrophobic micro-to-nano structured surfaces, states the following ... 

Thin solid films are layers that are present on a surface and have their top interface exposed to the environment Their extreme thinness in comparison with their lateral dimensions makes them systems that are infinite in two dimensions and are confined between an infinite gaseous phase and an infinite solid phase in the third one. As a result the global property of a layer is a combination of bulk and interface properties thus, one has to take into accoimt the thickness of the layer and the nature of the substrate when designing a coating for a specific application. Thin layers can be used for various purposes, depending on their surfece and/or bulk intrinsic properties. They can be dense, porous, patterned, multilayered, composite, and so on. Sol-gel films can be found in many different application domains such as optics (e.g., antireflection, self-cleaning, smart windows, and conductive transparent layers), electronics (e.g., microfabrication, low-/ , and self-assembled monolayers (SAMs)), protection (e.g., anticorrosion, anti-abrasion, and antistatic), and analysis (e.g., selective sensors). In most of these applications, the function must be identical on the whole surface of the substrate, and thus the thickness has to be controlled as much as possible and must be as uniform as possible. 

Figure 4.5 Photograph of windows taken from indoor looking out with one ordinary window glass (left) and self-cleaning glass (right). (Copyright Saint Gobain.)...

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