Photoactive covalent organic frameworks incorporating Au and RuO2 nanoparticles as a hybrid photocatalyst for artificial photosynthesis

The following dissertation focuses on the design of advanced photocatalytic materials to be applied in solar-driven CO2 conversion into fuels and other chemicals. The work is presented over several sections, beginning with an introduction into the main topics involved within the investigational work. Following this, the experimental research is divided into five chapters, moving from the construction of a photosensitising framework, to the incorporation of catalytically active metal sites into this stable matrix to create a novel, hybrid photocatalyst. These chapters are summarised below: Chapter 1: Rising atmospheric CO₂ levels, along with the global ambition to meet the Paris Agreement targets, have intensified the need to transition away from fossil fuels towards sustainable energy sources. Artificial photosynthesis offers a promising solution by using sunlight to convert CO₂ into solar fuels. This process requires efficient photocatalysts, which often struggle with low activity, instability, and rapid charge recombination. This chapter outlines the current state-of-the-art in advanced photocatalyst design for solar fuel production. Hybrid systems involving covalent organic frameworks (COFs) are a burgeoning area of research due to their strong light absorption, stability, conductivity, and charge separation ability. Their high surface areas can incorporate catalytically active sites, such as metal nanoparticles, to form nanocomposite photocatalysts. Both components work synergistically to enhance the overall properties of the material– metal nanoparticles can extend the lifetime of photoseparated charges within the COF to facilitate redox reactions, while COFs provide a stable support to prevent nanoparticle aggregation, thereby extending their photocatalytic lifetime. Chapter 4: A polyimide-based COF was chosen as the photo-active, robust support that would later form the porous network onto which the nanoparticles would be incorporated. Porphyrin and perylene building blocks were chosen for their photoactivity and favourable bandgap alignment, which should create a powerful photosensitiser when combined in a donor acceptor COF architecture. The desired material should exhibit visible light-harvesting ability, long-range order for effective charge transport, a homogeneous morphology to ensure reproducible properties, and be thermally stable to endure high temperature conditions during photocatalysis. Three synthetic routes were investigated for synthesising polyimide COFs from porphyrin and perylene starting monomers, along with a thorough assessment and comparison of their structures, thermal stabilities, morphologies, and optical properties. Using this information, the best synthetic strategy was chosen for making the final nanocomposite material. Chapter 5: Morphological modulation of COFs is a promising technique for tuning the properties of COFs, which tailors their functionality towards specific applications. COF nucleation and growth during synthesis are highly dependent on the chosen parameters, which can be altered to produce different morphologies and as a consequence, endow the COF with wholly unique properties. One of the synthetic strategies tested in Chapter 4 produced a mixed morphology, composed of nanoclusters embedded within a fibrous matrix. This chapter explores modifying reaction temperature during synthesis to favour a specific morphology, and testing the morphology-activity relationship in real-world applications. To test this, the COFs were subjected to dye adsorption experiments to assess suitability for water remediation, as well as advanced spectroscopic analysis to evaluate their efficiency as light-harvesters in future photocatalytic applications. Chapter 6: The self-assembly of planar, 2D COFs into multi-layered, columnar systems creates a highly conductive material that provides efficient charge transport pathways. Since little is known about the COF self-assembly process itself, a theoretical study was performed first on the Zn-porphyrin-perylene COF monolayer, then on the stacked system to investigate changes in structural and opto-electronic properties as the number of layers increased. To validate predicted properties from the theoretical model, an experimental investigation was performed in tandem with Zn-porphyrin-perylene COF, which was synthesised solvothermally and characterised using IR spectroscopy, electron microscopy, PXRD, UV-Vis spectroscopy, and fluorescence spectroscopy. Chapter 7: The highly ordered and porous nature of COFs can effectively confine nanoparticles to prevent their aggregation and improve photocatalytic performance. Because of their tunable structures, COFs can be modified with specific functional groups that strongly interact with nanoparticle surfaces, thereby enhancing their dispersibility and stability within the COF matrix. Herein, a thiol-functionalised Zn-porphyrin-perylene COF was synthesised to provide a robust support for growing Au nanoparticles, which serve as the CO2 reduction catalyst, by leveraging the Au–S interaction. Furthermore, by altering Au/S ratio, the size of Au NPs can be modulated to alter their properties, such as the LSPR effect, surface area, and product selectivity. Chapter 8: Artificial photosynthesis involves using water as a sacrificial agent to produce the necessary electrons to drive CO2 reduction. RuO2 nanoparticles, a robust and well-studied OER catalyst, was incorporated as a co-catalyst into the Au@COF system created in chapter 7, thus creating the final advanced photocatalytic material. By spatially separating Au and RuO2 sites within the COF scaffold, RuO2 preserves holes to increase charge separation lifetime and boost photocatalytic performance. The combined Au/RuO2@COF photocatalyst was evaluated for visible-light-driven CO2-to-syngas conversion using water as the sacrificial donor, initially under batch conditions, and then in-flow, where key reaction parameters were optimised, such as reactor type, temperature, and CO2 flow rate.

Publisher

University of Galway

Rights

CC BY-NC-ND

cbn

Collections

University of Galway Theses (PhD Theses)