Amplified Photochemistry with Slow Photons
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Amplified Photochemistry with Slow Photons Jennifer I. L. Chen1, Georg von Freymann2, Sung Yeun Choi1, Vladimir Kitaev3, and Geoffrey A. Ozin1 1 Chemistry, University of Toronto, 80 St. George Street, Toronto, M5S 3H6, Canada 2 Institut für Nanotechnologie, Forschungszentrum Karlsruhe in der Helmholtz-Gemeinschaft, Karlsruhe, 76021, Germany 3 Chemistry, Wilfrid Laurier University, Waterloo, N2L 3C5, Canada
ABSTRACT We demonstrate monochromatic and white light optical amplification of the photooxidation of adsorbed methylene blue using an inverse colloidal photonic crystal fashioned from anatase nanocrystals, denoted i-nc-TiO2-o. Enhanced photo-activity that drives the oxidation of the dye is attributed to slow photons in i-nc-TiO2-o. When the slow photon wavelength is optimized with respect to the electronic excitation energy of i-nc-TiO2-o, the photo-oxidation rate of the dye is doubled compared to conventional nc-TiO2. By increasing the probability of absorbing photons in i-nc-TiO2-o relative to nc-TiO2, a larger population of electron-hole pairs is generated enabling more efficient photo-oxidation. Slow photons in photonic crystals portend a myriad of opportunities for amplified photo-processes in chemistry and biology. INTRODUCTION Titania in the anatase phase is an important candidate material for environmental remediation and water purification owing to its ability to degrade a variety of organic compounds and microbes [1,2]. Anatase TiO2 is an efficient photocatalyst as a result of the highly oxidizing holes and reducing electrons it generates upon excitation with ultra-violet light. Intense research has been devoted to increase its photocatalytic efficiency, most commonly by increasing the lifetime of electron-hole pairs or by extending its absorption to the visible range [3,4]. Light absorption can also be enhanced when the interaction of light with the material is increased [5]. A phenomenon found in photonic crystals that can significantly increase the path length of light is slow photon, which is observed at energies just above and below the photonic stop band [6]. If the energy of the slow photons overlaps with the absorbance of the material, then an enhancement of the absorption can be expected as a result of the increased matter-radiation interaction. Herein we demonstrate the influence of the photonic structure on the photo-activity of TiO2 via a solid-state photo-degradation experiment with adsorbed methylene blue as the probe molecule [7].
EXPERIMENTAL METHODS The films of i-nc-TiO2-o with stop-band maxima centered at 280, 300, 325, 345, 370, 430 and 500 nm were prepared by infiltrating polystyrene sphere opal templates of diameters 130, 150, 180, 210, 240, 300 and 380 nm with titanium butoxide. The templates for the inverse opals had thicknesses varying from 15 to 20 layers of polystyrene spheres self-assembled via the solvent evaporation method. Infiltration was done by placing the opal templates in ethanol solutions containing 0.5 - 1 vol% of titanium butoxide under low pressure
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