Advanced Photonic Crystals for Efficient Light-Trapping in Photocatalytic Applications

Abstract

Despite advances in solar technologies, there is still a growing and urgent demand for light-harnessing materials that enable efficient utilisation of solar energy for solar-to-fuel conversion and environmental remediation applications. Existing photocatalytic technologies present inherent limitations to achieve these goals due to wide energy bandgap and poor electrochemical properties of conventional materials. A combination of fundamental and applied materials science, nanotechnology, chemistry, photonics and applied physics offers a way forward for developing new light-confining photocatalyst platforms with improved capabilities, versatilities, cost-effectiveness and sustainability to address global energy and environmental issues. This thesis presents the development of rationally engineered composite photocatalyst platforms based on nanoporous anodic alumina photonic crystals (NAA-PCs) and photoactive materials. The fabrication of these photocatalytic systems with enhanced performances is achieved through structural engineering and chemical modification of NAA-PCs. Various forms of NAA-PCs were produced by pulse-like anodisation strategies with a view to optimising optical properties to harness light–matter interactions at the nanoscale efficiently, within high-irradiance spectral regions. The essential photocatalytic properties of these PC structures, well-defined energy bandgap capable of photogeneration of charge carriers, were provided by chemical functionalisation, using photoactive layers of titanium dioxide (TiO₂) deposited onto the inner surface of NAA-PCs through sol-gel method. Photocatalytic performances of photo-active NAA-PCs as well as photocatalytic enhancements associated with distinct forms of light–matter interactions were demonstrated through photodegradation of model organics such as methylene blue, methyl orange, rhodamine B and 4-chlorophenol, under simulated solar light irradiation conditions. Photocatalytic enhancements associated with slow photons, light confinement, and plasmonic effects in noble metal nanostructures with and without NAA-PCs were also analysed. This thesis demonstrated that: (i) high-quality nanoporous anodic alumina gradient-index filters (NAA-GIFs) and hybrid NAA-PCs can be developed with tunable optical properties across the UV-visible-NIR spectrum, (ii) various forms of photo-active NAA-PCs with and without noble metal nanostructures are found to have superior performances to benchmark photocatalyst materials in many cases due to “slow photon” effect and light confinement, and (iii) 2D gold nanodot plasmonic single-lattices show outstanding performances due to efficient utilisation of solar energy at high-irradiance spectral regions and harnessing plasmonic light-matter interactions. The studies completed in this thesis advance both fundamental understanding and applied knowledge on the photocatalytic performance of chemically-modified NAA-PCs with optimised structural, optical, chemical and photocatalytic properties. These advanced materials could potentially be integrated into fully functional and marketable real-life photocatalytic devices for addressing global energy challenges and environmental pollution remediation.