Mechanistic studies on TiO2-SiO2 core-shell nanoparticles for photocatalytic hydrogen production and degradation of organics

Abstract

Oil, coal and gas together account for the majority of global primary energy consumption and they are projected to remain the dominant energy source until at least 2050. These fuels, however, impose serious pollution and emissions that impose a growing strain on both our economy and environment. Therefore, we need alternative solutions that provide sustainable fuels from renewable energy sources. We also need better strategies to protect our environment, such as converting organic waste in air and water to harmless chemicals. Photocatalysis can provide a solution to both of these challenges; it promises an efficient purification of air and water as well as the production of hydrogen and solar fuels via light-assisted water splitting and conversion of organic compounds.The aim of this thesis was to modify TiO2-based photocatalysts and to investigatethe mechanisms and deactivation kinetics during photocatalytic degradation of organic molecules and sacrificial hydrogen evolution. For this, I converted TiO2 nanoparticles into core-shell nanostructures through the deposition of ultrathin layers of an insulating metal oxide (SiO2) with tuneable thickness in the range of 0 to 1 nm. The film thickness and uniformity, interfacial and surface properties as well as charge separation dynamics and charge transfer properties and were studied with a range of state-of-theart techniques.In the first part, the deposition of SiO2 has enabled the direct tuning of the kinetics and selectivity towards degradation of differently charged organic dyes, by modifying the adsorption characteristics and accessibility of active Lewis sites. It was also possible to extract the respective rate limiting processes for different thickness regimes. In the second part, I investigated the early-stage deactivation phenomenon during hydrogen evolution reaction (HER) that was recently discovered for TiO2 photocatalysts modified with Pt nanoparticles. I investigated the mechanism for different process conditions (type of sacrificial agent, Pt content, deposition method) and used the coreiii shell strategy to uncover the origins for deactivation. The results show that the choice of deposition method plays a key role as it affects the dispersion and amount of the adsorbed Pt precursor, the Pt reduction kinetics and the initial size of the Pt nanoparticles,and – consequently – both the kinetics and the extent of deactivation. Moreover,I demonstrated that it is possible to minimize the deactivation with one monolayer of SiO2 shell while retaining a high charge transfer efficiency to the adsorbed reactant molecules.In the third part, I introduced nanocavities (< 1nm) into the SiO2 shells on TiO2.Preliminary studies showed that these cavities allowed for highly size-selective degradation of primary and secondary alcohols. Moreover, this strategy can be used to distinguish between a direct and indirect charge transfer-based oxidation mechanism of sacrificial agents during HER. Future studies are required to increase the activity for better results, to correlate reaction kinetics and selectivities with the number and topology of these nanocavities and to analyze intermediates for understanding the overall reaction mechanism.