Enhanced Visible-Light TiO2-Photocatalysis for a Sustainable Future

Despite the pollution created by the use of them and their limited availability, fossil fuels are still the most commonly used energy source. Photocatalysis can play a major role in addressing that pollution and in a further transition away from their use. Because of a good score in a variety of properties that are important for photocatalyst materials, titanium dioxide (TiO2) is commonly used as photoactive material in photocatalysts. TiO2 exhibits, however, some major drawbacks limiting its efficiency. First, charge carrier recombination is a big issue. Second, because of the wide bandgap, the photoresponse of TiO2 is limited to the UV region of the solar spectrum. This thesis deals with both issues by exploring cost-efficient, sustainable methods to raise the catalyst's efficiency. First, deposition of a Au bowtie nanostructure on a TiO2 surface using nanosphere lithography and subsequent photoactivity analysis by methylene blue degradation as well as photocurrent generation, provided quantifiable information about the role of the metal. Activity enhancement factors up to 2.1 were found for the Au/TiO2 system. Using the same system, more insight was also obtained about the underlying mechanism of noble metal-enhanced photocatalysis. By increasing the Au particle size, a correlation between the extinction spectra of the Au structures and the photoresponse was found. Activities were observed up to a band of 630-670 nm for Au bowtie structures derived with 625 nm sized mask particles. The correlation between extinction and activity enhancement strongly indicates that 'hot'-electron transfer is the most predominant mechanism in metal-enhanced photocatalysis. Second, direct femtosecond laser writing is presented for direct and precise formation of Au nanostructures on TiO2 surfaces. It provides advantages over conventional top-down approaches in terms of fabrication costs and contamination, while still confining nanostructures within spots well below the size of the diffraction limited focal spot. A detailed study on the growth of the Au nanostructures distinguished a nucleation stage in solution, induced by the femtosecond laser, and a further growth stage and deposition on the surface, assisted by the TiO2. Ultimately, applicability of the deposition method was demonstrated in visible-light photocurrent generation experiments, where an activity increase up to a factor 1.5 was found when focused on the Au structures compared to activities on the bare TiO2-film. Last, adsorption of small polyols onto the TiO2 surface is shown to engineer the bandgap in such a way that visible excitation is possible. The existence of bandgap engineering was found using density functional theory (DFT) calculations on the density of states (DOS) of TiO2 films with adsorbed polyol molecules. The theoretical results are supported by the fact that polyols for which an extra mid-gap energy state was found (ethylene glycol (EG), glycerol (GC) and propylene glycol (PG)) also showcased the highest enhancement factors amongst the studied polyols, with enhancement factors up to 2.8 for EG. All of the results provide useful information to enable a further transition from the use of fossil fuels as energy source to the use of solar light-driven photocatalysis.status: publishe