$TiO_2$ photocatalysts

Abstract

Recently, titanium dioxide (TiO2) has been studied extensively due to its high photocatalytic activity for handling of several types of environmental problems. Major areas of activity in TiO2 photocatalysis are; water purification, photochemical cancer treatment, air purification, self-sterilizing, fog-proof and self-cleaning surfaces.Photocatalysis can be defined as “acceleration of a photoreaction by the presence of a photocatalyst”. Photocatalytic reactions necessitate a photocatalyst that absorbs the phonons and drives the redox reactions.TiO2 is a semiconductor and it can be chemically activated by UV light. TiO2 has three different crystal structures which are anatase, brookite and rutile. TiO2 in the anatase form is the most efficient of photocatalysts for many applications. The band gap energy of anatase TiO2 is 3.2 eV and it can be only activated by UV light. Although UV light is present in the solar spectrum it is only a very limited part. For practical applications the photocatalytic activity of TiO2 needs further improvement. Doping TiO2 with transition metals or noble metals is an effective way to improve photocatalytic activity. When TiO2 is exposed to UV light, electron-hole pairs are created. The photogenerated holes in the valence band, which has strong oxidizing power, diffuse to the surface and react with adsorbed water in order to produce hydroxyl radicals (•OH). These hydroxyl radicals participate in oxidizing organic molecules. On the other hand, electrons in the conduction band react with molecular oxygen in the air to produce the superoxide radical anion (O2-•), which also participates in further oxidation processes. The photocatalytic efficiency of TiO2 strongly depends on surface area and electron-hole recombination rate. The surface area of the photocatalyst increases with a smaller particle size and the active surface sites increase. For improving photocatalytic efficiency, electron-hole recombination rate should be reduced. An effective way to seperate electron-hole pairs is to introduce foreign materials into TiO2 matrix. As mentioned above, TiO2 can be used in different application areas. One of the remarkable property of TiO2 is its self-cleaning effect. The surfaces of glasses, ceramic tiles can be contaminated by organic particles such as smoke residue, oil and dirt. TiO2 thin films can be applied to these surfaces in order to decompose those organic species. Another excellent property of TiO2 photocatalysts is their anti-bacterial effect. TiO2 can decompose bacteria and virus when it is exposed to UV light. TiO2 has advantages over conventional self-sterilizing surfaces. For instance, in the case of E.coli, TiO2 decompose both the living cells as well as the endotoxin released from these cells during their death. TiO2 photocatalysts can also be used for cancer treatment. TiO2 particles which are injected to the tumor clearly inhibit the tumor growth. In literature, there are several studies which are related to the doping effect of silver. Studies which are performed on the effect of silver dopant are focused on the change of optical and electronical properties of TiO2. Moreover, since silver itself is known as strong anti-bacterial agent it is used as dopant for improving anti-bacterial properties of TiO2. Doping silver can give rise to the separation of electron-hole pairs and can accelerate the forma-tions of oxidative species. In addition to this, silver can reduce particle size which is needed for increas-ing surface area of TiO2.In order to obtain anti-bacterial effect in the dark, energy storage photocatalyst can be produced by doping TiO2 with tungsten. TiO2-WO3 photocatalyst can be photo-charged by irradiating their surfaces with UV light. Photo-charged tungsten doped TiO2 films are able to show anti-bacterial effect when the light is turned off. Molybdenum also is an energy storage material and it can be used as an alternative to those of tungsten. In this study, the mechanism of photocatalytic activity is discussed and the application fields of TiO2 photocatalysts were summarized by focussing on the bacterial activity of TiO2. Moreover, the effect of silver, tungsten and molybdenum dopants on the bacterial activity of TiO2 were discussed.

Özet

Son yıllarda, titanyum dioksit (TiO2) üzerinde, fotokatalitik aktivite özelliğinden dolayı yoğun olarak çalışılmaktadır. TiO2, UV ışığı ile uyarıldığı zaman fotoaktif özellik gösteren ve organik grupları parçalayabilen yarıiletken bir malzemedir. TiO2, ışığa maruz bırakıldığında, suyun arıtılmasında, kendi kendini temizleyebilen, buğulanmayan yüzeylerin elde edilmesinde, fotokimyasal olarak kanser tedavisi uygulamalarında, havanın arındırılmasında kullanılabilir. TiO2 filmler, kimyasal buhar biriktirme, sıçratma, elektron demeti ile buharlaştırma, iyon ışını destekli biriktirme ve sol-jel gibi yöntemlerle değişik yüzeyler üzerine kaplanabilirler. TiO2, anataz, rutil ve brukit olmak üzere üç farklı kristal yapıya sahiptir. Birçok uygulamada TiO2’in anataz formu en iyi fotoaktivite özelliği göstermektedir. TiO2’in, solar spektrumun çok az bir bölümünü oluşturan UV ışığı ile aktive edilebiliyor olması bu malzemenin pratik uygulamalardaki kullanımını sınırlandırmaktadır. Bundan dolayı, pratik uygulamalar için, TiO2’in fotoaktivitesinin geliştirilmesi gerekmektedir. Fotoaktiviteyi arttırmanın bir yolu, TiO2’in geçiş metalleri veya soy metallerle katkılandırma işlemi yaparak soğurma (absorption) bandının UV bölgesinden, görünür bölgeye kaydırılmasıdır. Literatürde, titanyum oksit filmlere gümüş, tungsten ve molibden katkılandırılmasına yönelik çalışmalar yapılmış ve üç katkılandırmanın da, titanyum oksit filmlerin fonksiyonalitesine farklı mekanizmalar üzerinden ciddi katkılar yapacak nitelikte olduğu belirtilmiştir. Bu çalışmada, TiO2’in fotokatalitik aktivite mekanizması tartışılmış ve TiO2’in kullanım alanları anti-bakteriyel özelliklerine odaklanarak özetlenmiştir. Buna ek olarak, gümüş, tungsten ve molibden katkısının TiO2’in, anti-bakteriyel aktivitesine olan etkileri tartışılmıştır.