Surface Modification and Optical Behavior of Tio 2 Nanostructures.
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SURFACE MODIFICATION AND OPTICAL BEHAVIOR OF TIO2 NANOSTRUCTURES. S.M. Prokes*, W.E. Carlos*, James L. Gole**, Chunxing She*** and T. Lian*** * Naval Research Laboratory, Washington DC ** Department of Physics, Georgia Institute of Technology, Atlanta, GA *** Department of Chemistry, Emory University, Atlanta, GA. ABSTRACT TiO2 is a very important material from the perspective of photocatalytic applications, but due to its 3.2 eV bandgap, it is not possible to make this material efficient under visible illumination. Anatase TiO2 nanoclusters in the 3 – 30 nm range have been formed by a wet chemical technique and surface modified in order to enhance the absorption of visible light. Nitridation of the highly reactive TiO2 nanosphere surface has been achieved by a quick and simple treatment in alkyl ammonium compounds and the metallization of this surface has been achieved by electroless plating. Although the structure of the resultant material remains anatase, some of the treated material exhibits strong emission between 550-560 nm, which red shifts and drops in intensity with aging in the atmosphere. Electron Spin Resonance performed on these samples identify a resonance at g=2.0035, which increases significantly with the nitridation step. This resonance is attributed to an oxygen hole center created near the surface of the nanoclusters, which correlates well the noted optical activity. INTRODUCTION Recently, a significant amount of attention has been focused on the photocatalytic properties of TiO2, which has good stability in the outdoor environment. One specific area of interest is the photodegradation of organic molecules, such as water and air purification, as well as antibacterial and self-cleaning coatings [1- 7]. However, in order that such a TiO2 photocatalyst be used outdoors, it must respond to visible light with good efficiency. Untreated TiO2, however, exhibits a high reactivity and stability only under ultraviolet light (wavelength < 387 nm), due to the fact that the bandgap of anatase TiO2 is 3.2 eV, and thus absorption into TiO2 can only occur with light energies which exceed the bandgap value. Several different approaches have been attempted in order to enhance the absorption of visible light. One approach has been to dope transition metals into TiO2 [8,9], but these doped materials suffer from problems such as thermal instability or an increase in the number of recombination centers [9]. Another route has been to reduce the TiO2 using H plasma, which has been reported in both the rutile [10,11] and anatase [12] crystalline structures of TiO2. However, it has been found that the reduction of TiO2 in this way results in the formation of deep localized oxygen vacancy states. This means that the energy levels of the optically excited electrons would be 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 would result in low efficiency of the photodegradation process using visible light, since the photo
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