title: Engineering Co-Catalytically Active Sites for Enhancing the Photocatalytic Hydrogen Production on Titanium Dioxide
Creators: name: Qin, Shanshan
Publisher: name: Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU)
Year: 2025

description: The photocatalytic generation of hydrogen (H2) using titanium dioxide (TiO2) has garnered significant attention due to its potential for sustainable energy production. This thesis presents a comprehensive investigation into the enhancement of photocatalytic H2 production through defect engineering and single atom (SA) co-catalysis. Initially, we explore the hydrogenation of anatase titania nanosheets (NSs) enriched with (001) facets, revealing that prevention of deformation and sintering during thermal treatments is key to activating titania-oxygen vacancy (Ti3+-Ov) defects for co-catalyst-free H2 production. Comprehensive characterization techniques, including scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, electron paramagnetic resonance, and X-ray photoelectron spectroscopy, elucidate the structural and electronic changes resulting in enhanced photocatalytic activity. Further investigations into pristine and hydrogenated anatase NSs reveal that increased photocatalytic activity in grey titania is attributed to accelerated hole transfer rates rather than improved conductivity or visible light absorption. We identify that the hole transfer bottleneck significantly impacts photocatalytic performance, particularly in pure water, where reaction product accumulation can hinder H2 evolution. The research also introduces a novel self-assembling photocatalytic system using grey titania nanoparticles in a plain aqueous Ni2+ solution, where UV illumination induces H2 production via a unique Ti3+-OV defect structure and Ni(I) electron transfer mediator, eliminating the need for noble metals or sacrificial agents. Additionally, we examine the effect of reactive facets in anatase crystallites, comparing (001) facet-enriched nanosheets and (101) facet-rich nanocubes. Our findings show that (101) facets significantly enhance H2 production in the absence of a co-catalyst, a benefit that vanishes when a Pt co-catalyst is present on all facets. Explorations into SA Pt co-catalysts on different TiO2 polymorphs (anatase, rutile, P25) reveal that SA Pt on anatase and P25 dramatically improves H2 generation efficiency, particularly at low Pt concentrations. We demonstrate that SA Pt provides a superior co-catalytic effect compared to nanoparticles and that optimal co-catalytic activity is achieved at specific Pt loading densities. We also investigate into Pt SA deposition on titania nanosheets, using cyanide leaching to identify the active Pt species. The results indicate that a minority of Pt SAs drive the photocatalytic activity, with a remarkable efficiency for H2 evolution. Finally, we address the stability of Pt SAs in photocatalytic systems by incorporating them into hierarchical structures of metal-organic frameworks (MOFs) and aligned TiO2 nanotubes. This hybrid approach leverages the light-harvesting and charge-transport properties of TiO2 nanotubes and the stable anchoring sites of MOFs, yielding high efficiency and long-term stability in H2 evolution. Additionally, by employing tetrakis-(4-carboxyphenyl)-porphyrin (TCPP) and its Pt-coordinated analogue (Pt-TCPP) on anatase TiO2 surfaces, we achieve a controlled spatial distribution of Pt SAs. The Pt SA density is finely tuned via co-adsorption of Pt-TCPP and TCPP, achieving an optimal SA density. This distribution is preserved in post-thermal treatments, achieving optimal photocatalytic activity and SA stability. Overall, this work provides critical insights into the defect engineering of TiO2 and the strategic design of Pt SAs, advancing the understanding and practical application of TiO2-based photocatalysts for efficient and stable hydrogen production.