Titanium dioxide (TiO2) due to its low cost and high chemical stability is widely used as a photocatalyst in a variety of applications ranging from air purification and deodorization to self cleaning coatings on buildings (1,2). However, TiO2 shows photocatalytic activity only under UV light (3.2 eV band gap) which constitutes approximately 5% of the total incident solar energy. As a result, many strategies have been explored to reduce the energy band gap of TiO2 in order to enhance its photoactivity in visible light (approximately 45% of the total incident solar energy). In particular, transition metal (V, Cr, Mn, Fe) doping has been shown to effectively reduce the energy band gap. However, this approach has only had limited success in extending the photocatalytic activity of TiO2 in the visible region due to the thermal instability of the doped materials and the fact that the dopant sites can act as electron-hole recombination centers (3). In the case of oxygen deficient titanium dioxide, represented as TiOx with x<2, oxygen vacancy states are formed at 0.75 to 1.18 eV below the conduction band minimum (4). Therefore, introduction of the oxygen vacancy states reduce the band gap (2.02-2.45 eV) with respect to the stoichiometric TiO2 so that photoexcitation of electrons from the valance band can take place with visible light. In this study, we have developed a high throughput flame synthesis technique for production of visible-light-active oxygen deficient titanium dioxide (TiOx with x<2) photocatalysts. Specifically, TiOx nanoparticles (size range of 10-50 nm, 1.85< x<1.95 determined with Energy Loss Spectroscopy) are synthesized by controlling the oxygen concentration in the particle formation zone in a diffusion flame aerosol reactor equipped with a flame quenching system utilizing titanium tetraisopropoxide (TTIP) as a precursor. The absorption spectra of the particles, measured via diffuse reflectance spectrometer, clearly demonstrated light absorption in the visible region. Furthermore, the visible light photocatlytic activity of TiOx particles is demonstrated by degradation of Methyl Orange solutions under visible light illumination. Flame aerosol reactors provide a single step process for synthesis of nanostructured titanium dioxide for visible light activated applications.

References: 1) Thimsen E. and Biswas P. to appear, AIChE J (2007) 2) Almquist, C. and Biswas, P. J. Catalysis 212, 145 (2002) 3) W. Choi, A. Termin, and M. R. Hoffmann, J. Phys. Chem. 98, 13669 (1994) 4) Cronemeyer, D. C., Physical Review 113, 1222 (1959)