(530b) Nanocrystalline TiO2/CdSe Heterostructures for the Photocatalytic Reduction of CO2 Using Visible Light

Titanium oxide is an abundant material capable of photocatalyzing a variety of chemical reactions. Literature reports have shown that TiO2 can be used for the photocatalytic splitting of H2O into H2 and O2, as well as the photodecomposition of organic molecules. Titania-based photocatalysts have also shown activity for the photoreduction of CO2 in the presence of H2O leading to the production of hydrocarbons. One potential technical barrier preventing the widespread use of TiO2 for the photoreduction of CO2 is its large band gap which requires ultraviolet photons to initiate catalytic activity. This aspect of the material limits its efficiency for harnessing solar energy for this process since only 2-3% of the total solar spectrum reaching the earth's surface is in the ultraviolet. One way to enhance the visible light activity of TiO2 is to use small band gap nanocrystalline semiconductors as photosensitizers. These materials have excellent photochemical and thermal stability, have been shown to sensitize dye molecule photooxidation into the red region of the light spectrum, and have size tunable bandgaps that can be utilized to enhance their photoefficiency in the solar spectrum. Nanoparticles, such as CdSe, also form a type II band alignment with TiO2 which makes this material suitable for photosensitizing TiO2. A series of nanocrystalline TiO2/CdSe heterstructures have been synthesized and characterized. The visible light photoactivity of these materials has been tested using methanol oxidation as well as the reduction of CO2 in the presence of H2O. Methanol was shown to decompose in the presence of O2 under visible light (>420 nm) irradiation when TiO2 was sensitized with CdSe quantum dots. Doping of the TiO2/CdSe heterostructures with catalytic metals resulted in a system cable of photoreducing CO2 with visible light (>420 nm) in the presence of H2O. Gas chromatographic analysis shows that the reaction products are CH4, H2, and other light hydrocarbons. Control experiments show that TiO2 is not photoactive in the visible light spectrum without the CdSe sensitizer.