Self-Cleaning: Photocatalysis
Total Page：16
File Type：pdf, Size：1020Kb
Self-cleaning: Photocatalysis PHOTOCATALYSIS Hydrophilic surfaces. Deposited dirt is broken down and lies loose on the surface. A water film washes dirt away. UV light and water are required. Reduces maintenance requirement. Photocatalytic self-cleaning is probably the most wide- Generally speaking, photocatalytic self-cleaning is a ly used nano-function in building construction, with low-maintenace and trouble-free solution. Japan leading the field. There are numerous buildings A further advantage is that light transmission for glaz- of all sizes around the world that make use of this func- ing and translucent membranes is improved as day- tion. Its primary effect is that it greatly reduces the ex- light is obscured less by surface dirt and grime. Energy tent of dirt adhesion on surfaces. It is important to note costs for lighting can be reduced accordingly. that the term "self-cleaning" in this context is mislead- For the function to work, UV light, oxygen and air hu- ing and does not mean, as commonly assumed, that a midity are required. The level of UV light present in surface need not be cleaned at all. The interval between normal daylight is sufficient to activate the photocata- cleaning cycles can, however, be extended significantly, lytic reaction. Organic dirt on the surface of a material a fact that is particularly relevant in the context of fa- is decomposed with the help of a catalyst - usually ti- cility management. Fewer detergents are required, re- tanium dioxide (Ti02, and the particularly reactive de- sulting in less environmental pollution and less wear rivative Anatase). The nanoscalar dimension of Ti02 and tear of materials. Likewise reduced cleaning cycles makes it a highly reactive catalyst, speeding up the de- lead to savings in personnel costs and the fact that the composition process rapidly without being used up so dirt adheres less means that it is also easier to remove. that the effect is lasting. Since its discovery in 1908 Ti02, Oieophobic surfaces are resistant against oils and fats. Before and after: On conventional tiles, water forms droplets that dry leaving behind dirt deposits. On the hydrophlllc surfaces of photocataiytic tiles, water forms a film that runs off taking any loose dirt deposits with it. Self-cleaning: Photocatalysis • 72 | 73 The diagrams show the bask process: Organic dirt and grime is broicen down and "decomposed". Until now UV light, such as present in sunlight, is necessary to initiate photocatalysis. When water impacts on the surface, it spreads to form a film washing away the loose dirt. The result: clean surfaces! also known as titanium white, has been used in all In addition to the catalyst, the UV component of light, kinds of products from washing detergents to salami with a wavelength of less than 390 nm is considered skin - the prosperity of a culture can be measured by- essential for the reaction to occur, and its intensity its consumption of Ti02. At a nanoscalar dimension, ti- plays an important role. As such, photocatalytic self- tanium appears no longer white but transparent. cleaning surfaces are generally speaking more effective The production of self-cleaning surfaces using photo- outdoors than indoors. The method is predestined, for catalysis originates from Japan. In 1967, the photocata- example, for use on building fagades. lytic property of titanium dioxide was discovered by The hydrophilic properties of Ti02 were discovered Akira Fujishima at the University of Tokyo. Together much later. Due to its increased surface energy such with his professor at the time a report was published surfaces are hydrophilic (water-attracting), which on the phenomenon entitled the "Honda-Fujishima means that water runs off from any inclined surface in Effect". The first self-cleaning house was of course a film rather than in droplets. In comparison to Lotus- Fujishima's own house. Effect surfaces, this coating is transparent and can be Together with the University of Tokyo, the Japanese tile applied to glass invisibly. Photocatalytic surface coat- producer Toto Ltd. developed photocatalytic tiles that ings are often applied to fagade panels made of glass or were launched on the market in 1994 and continue to ceramics or to membranes. As the self-cleaning effect be successfully marketed today, in Japan as well as un- does not function without water, eaves should be de- der licence in various other countries. At the RCAST signed so that they do not prevent rainwater or dew University in Tokyo, Kazuhito Hashimoto and Toshiyo from reaching the fagade. It is also necessary in glazing Watanabe have researched photocatalysis in more de- to abstain from the use of silicon-based seals and seal- tail, including its process in visible light. ants - the oils they contain transfer to the glass and are incompatible with the surface coating, rendering it par- TiOj and PVC coated white membranes in weathering tests. The difference is readily apparent: after five months the former is still white, the latter grey and unsightly. tially hydrophobic and resulting in unsightly streaking. road building the transparent coating can also be used, In combination with photocatalytic coatings silicon- for example for noise barriers. Tiles with baked-on du- free sealants have to be used. Similarly, contact with rable coatings are available for use both indoors and any kind of film-forming detergents (e.g. high-gloss outdoors. Likewise, concrete, another common building coatings) must also be abandoned. material for fagades, can also be equipped with a self- In production, it is only economical for mass-produced cleaning surface. glass as the coating is usually applied in the factory Photocatalytic glass can be combined with other typ- using chemical vapour deposition (CVD), a vacuum ical functions such as solar-protection glass. The mar- coating technique in which an ultra-thin coating is ap- ket for self-cleaning coatings is expanding most rapidly plied in vapour form. Such coatings cannot be retrofit- in Japan, where it has become common practice in ted. However, this does not limit its application exclu- many cases for new glazed fagades. The variety of pho- sively to large buildings; it can be equally appropriate tocatalytic applications already available on the mar- for example for conservatories and winter gardens. In ket is quite varied and ranges from windows to vacuum Self-cleaning: Photocatalysis • 74 | 75 Water washes off the dirt. cleaners to fridge-freezer deodorising units. In addition the hydrophilic property of the surface, the water im- to the self-cleaning properties, photocatalysis can also mediately formed a thin film, which evaporates quickly, be used to achieve air-purifying, water-purifying as well absorbing in the process ambient warmth and thereby as antimicrobial properties. reducing the indoor temperature noticeably. Initial esti- An interesting application, which has already been mates suggest a potential energy reduction of between implemented in experimental prototype form, is the 10 and 20% in comparison to conventional air condi- exploitation of the cooling effect of evaporating water. tioning. In Japan, this effect is traditionally achieved The canvas and steel roofing as well as the windows of a by sprinkling water on the ground - in this case nano- trade fair pavilion in Japan (Expo 2005, Aichi, Nagakute technology represents a perfect symbiosis of tradition area) were equipped with a photocatalytic TiOj coating and high-tech. and subjected to a constant stream of water. Due to These roof tiles, which have been on the market for some time, have self-cleaning properties thanks to photocatalysis. .