Nanostructured materials for solar energy conversion
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The energy requirements of our planet will continue to grow with increasing world population and the modernization of currently underdeveloped countries. This will force us to search for environmental friendly alternative energy resources. Solar energy by far provides the largest of all renewable energy resources with an average power of 120 000 TW irradiated from the sun which can be exploited through solar electricity, solar fuel, and biomass. Nanostructured materials have been the subject of extensive research as the building block for construction of solar energy conversion devices for the past decades. The nanostructured materials sometimes have peculiar electrical and optical properties that are often shape and size dependent and are not expected in the bulk phase. Recent research has focused on new strategies to control nanostructured morphologies and compositions of semiconductor materials to optimize their solar conversion efficiency. In this dissertation, we discuss the synthesis and characterizations of one dimensional nanostructured TiO₂ based materials and their solar energy conversion applications. We have developed a solvothermal synthesis method for growing densely packed, vertical, single crystalline TiO₂ rutile nanowire arrays with unprecedented small feature sizes of 5 nm and lengths up to 4.4 [mu]m. Because of TiO₂'s large band gap, the working spectrum of TiO₂ is limited to the ultra violet region with photons shorter than 420 nm. We demonstrate that the active spectrum of TiO₂ can be shifted to ~ 520 nm with incorporation of N via nitridation of TiO₂ nanowires in NH₃ flow. In addition, we demonstrate a synergistic effect involving hydrogenation and nitridation cotreatment of TiO₂ nanowires that further redshift the active spectrum of TiO₂ to 570 nm. The Ta and N co-incorporated TiO₂ nanowires were also prepared and showed significant enhancement in photoelectrochemical performance compared to mono-incorporation of Ta or N. This enhancement is due to fewer recombination centers from charge compensation effects and suppression of the formation of an amorphous layer on the nanowires during the nitridation process. Finally, we have developed hydrothermal synthesis of single crystalline TiO₂ nanoplatelet arrays on virtually all substrates and demonstrated their applications in water photo-oxidation and dye sensitized solar cells.