410_Term_Paper - Neil Mucha Chem410 Term Paper 14 November...

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Neil Mucha Chem410 Term Paper 14 November 2007 Self-cleaning Glass Self-cleaning glass is a recent innovation brought about by extensive research in the area of semiconductor photochemistry. Self-cleaning glass is essentially normal glass coated with a thin film of titanium dioxide (TiO 2 ). Through the properties of photocatalysis and superhydrophilicity that TiO 2 exhibits, photochemical reactions break down organic compounds and the glass appears to clean itself. This paper will explore the properties, chemistry, and proposed mechanisms of this exciting new technology. Of all semiconductors tested, TiO 2 is the most favored due to its chemical and biological inertness, high photocatalytic activity, high mechanical durability, and cheap price. Titanium dioxide exists in three crystalline forms, namely anatase, rutile, and brookite 1 . The form used in semiconductor photochemistry is anatase as it appears to be the most active and easiest to produce of the three. Most of the early work with semiconductor photocatalysis used TiO 2 in the form of powder dispersions. However, for more practical applications and the added requirement of a filtration step in powder dispersions a new method needed to be developed. Because the TiO 2 powder films are mechanically frail, research has been done into the production and testing of nanocrystalline semiconductor films. Such films have been produced almost exclusively by the sol–gel process, although they can also be produced by the process of chemical vapor deposition, (CVD) which is favored in commercial situations 2 . 1
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The TiO 2 coatings came about as a result of semiconductor photocatalysis being used for air and water purification. The basic photocatalytic reaction for the process can be summarized as: (1) Self-cleaning films have their scientific basis in semiconductor photocatalysis. Although many companies currently promote products that work via semiconductor photochemistry, the semiconductor materials function largely due to one or more of the following properties: photomineralisation (PCO), photo-sterilization, and photoinduced superhydrophilicity (PSH). In the case of self-cleaning glass, photoinduced super hydrophilicity is the mechanism of interest. The superhydrophilic property of TiO 2 allows water to spread completely across the surface rather than remain in droplets 3 . The effect in TiO 2 coated glass is antifogging and self-cleaning properties. To understand PSH and other mechanisms relating to semiconductor photochemistry, it is necessary to examine some features of electron energy levels in semiconductor materials. A semiconducting material possesses many electron energy levels filled with electrons called the valence band (VB), and, at a higher energy, many largely vacant electron energy levels called the conduction band (CB) 1 . The energy difference between these two bands is called the bandgap (EBG). Promoting an electron from the valence band to the conduction band can be realized by the absorption of a photon of ultra-
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This note was uploaded on 03/30/2008 for the course CHEM 410 taught by Professor Mallouk,thomaselarrabee,thomasj during the Fall '08 term at Pennsylvania State University, University Park.

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410_Term_Paper - Neil Mucha Chem410 Term Paper 14 November...

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