|Title:||Broadband plasmonic absorbers for sunlight photocatalysis|
|Subject:||Hong Kong Polytechnic University -- Dissertations|
|Department:||Department of Applied Physics|
|Pages:||xxii, 153 pages : color illustrations|
|Abstract:||Recently, plasmonic photocatalysis has been regarded as a very promising technology since the localized surface plasmon resonance (LSPR) effect of noble metal nanoparticles (NPs) can boost up the utilization of visible light significantly. As the visible light takes up about 43% of solar energy, plasmonic photocatalysis increases the prospect of sunlight for environmental and energy applications such as water treatment, water splitting and photosynthesis. Plasmonic resonance of the noble metal NPs can enhance the visible response of wide bandgap photocatalysts like TiO₂ drastically, but the current technology has two fundamental problems: narrow absorption band and low absorption, which limit the efficiency of photocatalysis using sunlight energy. In this study, different types of hybrids based on TiO₂ are designed, fabricated and characterized with the aims to enhance the absorption, to improve the charge carrier separation and transfer, and subsequently to enhance the photoreactivity under the irradiation of visible light. In this thesis, original work focuses on a couple of broadband plasmonic absorbers. In the first work, a TiO₂-Au bilayer that consists of a rough Au film under a TiO₂ film is investigated using the atomic laser deposition (ALD) method. This TiO₂ -Au bilayer aims to enhance the photocurrent of TiO₂ over the whole visible region and is a novel attempt to use rough Au films to sensitize TiO₂. Experiments show that the bilayer structure gives the optimal optical and photoelectrochemical performance when the TiO₂ layer is 30 nm thick, measuring the absorption 80% - 90% over 400 - 800 nm and the photocurrent intensity of 15 μA.cm-₂ , much better than those of the TiO₂-AuNP hybrid (i.e., Au NPs covered by the TiO₂ film) and the bare TiO2 film. The superior properties of the TiO₂-Au bilayer can be attributed to the plasmonic resonance of the rough Au film as the hot electron generator and the photoactive TiO₂ film as the electron accepter. Although this TiO₂ Au bilayer exhibits excellent photocatalytic activity, the fabrication complexity and the relative low electron transfer from the device to the electrolyte limit its large-scale applications. The second work further proposes and investigates another broadband plasmonic absorber with a sandwich structure that makes use of the combined plasmonic effects of the rough Au surface (100 nm thick) and the random Au NPs that are separated by a 150-nm TiO₂ layer made by the sol-gel method. The absorber measures a strong absorption (72% - 91%) over 400 - 900 nm and significantly enhances the photocurrent (by 20 folds) as compared to the bare TiO₂ film. The FDTD simulations have proved the broadband strong absorption is the result of interaction of rough Au film and TiO₂ film. In summary, novel broadband plasmonic absorbers have been introduced to help improve the performance of the photocatalysis under visible light, such as the visible light absorption and the generation and transfer of electrons and holes. These broadband plasmonic absorbers, as excellent photoelectrodes, may find niche applications in solar photocatalysis and may also be promising for large-scale industrial applications with a long-term stability.|
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