Defect Formation in Surface Modified TiO 2 Nanostructures.
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Defect Formation in Surface Modified TiO2 Nanostructures. S.M. Prokes1 and James L. Gole2, 1 Naval Research Laboratory, Washington DC 2 Department of Physics, Georgia Institute of Technology, Atlanta, GA ABSTRACT The defect formation in surface modified TiO2 has been studied. Anatase TiO2 structures in the 3-20 nm range formed by a wet chemical technique were surface modified and nitridation of the highly reactive TiO2 nanocolloid surface was achieved by a quick and simple treatment in alkyl ammonium compounds. The nitriding process was also accompanied by a metal surface coating process using electroless plating techniques, resulting in a thin metal surface layer on the modified TiO2 nanostructures. The crystal structure of the resultant TiO2-xNx nano-colloids remained anatase and the freshly prepared samples exhibited a strong light emission near 560nm (2.21 eV), which red shifted to 660 nm (1.88 eV) and dropped in intensity with aging in the atmosphere. This behavior was also evident in some of the combined nitridized and metal coated TiO2 nano-colloids. Electron Spin Resonance performed on these samples identified a resonance at g = 2.0035, which increased significantly with nitridation. This resonance is attributed to an oxygen hole center created near the surface of the nanocolloid, which correlates well with the observed optical activity. INTRODUCTION There has been significant attention focused on the photocatalytic properties of TiO2, due to its excellent stability in a diversity of environments. It’s photocatalytic properties can provide an important route for the photodegradation of organic molecules, it is also useful for water and air purification, and it can be used to produce antibacterial and self-cleaning coatings [1- 7]. Although untreated TiO2 exhibits a high reactivity and stability, it does so only under ultraviolet light excitation (wavelength < 387 nm), since the band gap of anatase TiO2 is 3.2 eV. For this reason, the use of untreated TiO2 as a photocatalyst can only occur in the ultraviolet spectral region. However, because of the availability and cost of UV light, before such a TiO2 – based photocatalyst can be effectively used, it must be efficient under visible light excitation. A number of different approaches have been pursued to enhance the absorption of visible light of TiO2. One route has been to dope transition metals into TiO2 [8,9], but these doped materials suffer from problems such as thermal instability and/or an increase in the number of recombination centers [9]. Another route has been to reduce the TiO2 using hydrogen plasma, which has been reported for both the rutile [10,11] and anatase [12] crystalline structures of TiO2. However, the reduction of TiO2 by this method results in the formation of deep localized oxygen vacancy states. This means that the energy levels of the optically excited electrons are lower than the redox potential of the hydrogen evolution in H2O, which is located close to the conduction band minimum of TiO2 [13]. This, of course, results in l
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