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The synthesis of most chemicals relevant to industrial processes requires at least one catalytic step. The goods and products derived from these processes have played a fundamental role in shaping our social structure and continue to be essential for sustaining it. However, a major challenge in many of the catalytic methods currently used in the chemical industry is their reliance on high temperatures and pressures, making them energy-intensive. Recently, new catalytic strategies capable of operating under milder and more efficient conditions have been developed. 

One such strategy is photocatalysis, which has emerged as a promising alternative to conventional catalytic methods. In photocatalysis, the catalytic reactions are driven by light, reducing the need for energy demanding conditions. Among the various photocatalysts under investigation, those based on plasmonic particles have garnered considerable attention. These materials benefit from strong light absorption and high spectral tunability, making them particularly interesting for catalytic applications.

After a brief review of the basic principles of plasmonic phenomena and their role in plasmonic photocatalytic systems, the results of this thesis are presented in three chapters. Each chapter addresses specific aspects of plasmonic photocatalysis, some of which are still under investigation. The first chapter examines the effect of temperature on hole transfer from a plasmonic particle to the p-type semiconductor copper iodide, and delves into the unusual response of this semiconductor to photoexcitation. The second chapter explores an approach to integrate the advantages of plasmonic-based and molecular-based photocatalytic systems through the functionalization of plasmonic particles with thiolate ligands. Finally, the third chapter reviews a plasmonic photocatalytic system with the potential to mitigate the photothermal effects inherent in plasmonic excitation.

The generation and dynamics of plasmon-induced hot carriers in gold nanoparticles offer crucial insights into nonequilibrium states for energy applications, yet the underlying mechanisms remain experimentally elusive. Here, we leverage ultrafast X-ray absorption spectroscopy (XAS) to directly capture hot carrier dynamics with sub-50 fs temporal resolution, providing clear evidence of plasmon decay mechanisms. We observe the sequential processes of Landau damping (similar to 25 fs) and hot carrier thermalization (similar to 1.5 ps), identifying hot carrier formation as a significant decay pathway. Energy distribution measurements reveal carriers in non-Fermi-Dirac states persisting beyond 500 fs and observe electron populations exceeding single-photon excitation energy, indicating the role of an Auger heating mechanism alongside traditional impact excitation. These findings deepen the understanding of hot carrier behavior under localized surface plasmon resonance, offering valuable implications for applications in photocatalysis, photovoltaics, and phototherapy. This work establishes a methodological framework for studying hot carrier dynamics, opening avenues for optimizing energy transfer processes in nanoscale plasmonic systems.

Increased attention has been directed toward generating nonequilibrium hot carriers resulting from the decay of collective electronic oscillations on metal known as surface plasmons. Despite numerous experimental endeavors, demonstrating hot carrier-mediated photocatalysis without a heating contribution has proven challenging, particularly for single electron transfer reactions where the thermal contribution is generally detrimental. An innovative engineering solution is proposed to enable single electron transfer reactions with plasmonics. It consists of a photoelectrode designed as an energy filter and photocatalysis performed with light function modulation instead of continuously. The photoelectrode, consisting of FTO/TiO₂ amorphous (10 nm)/Au nanoparticles, with TiO₂ acting as a step-shape energy filter to enhance hot electron extraction and charge-separated state lifetime. The extracted hot electrons were directed toward the counter electrode, while the hot holes performed a single electron transfer oxidation reaction. Light modulation prevented local heat accumulation, effectively decoupling hot carrier catalysis from the thermal contribution.

Plasmonic materials convert light into hot carriers and heat to mediate catalytic transformation. The participation of hot carriers (photocatalysis) remains a subject of vigorous debate, often argued on the basis that carriers have ultrashort lifetime incompatible with drive photochemical processes. This study utilises plasmon hot electrons directly in the photoelectrocatalytic reduction of CO₂ to CO via a Ppasmonic nanohybrid. Through the deliberate construction of a plasmonic nanohybrid system comprising NiO/Au/Re^I(phen-NH₂)(CO)₃Cl (phen-NH₂ = 1,10-Phenanthrolin-5-amine) that is unstable above 580 K; it was possible to demonstrate hot electrons are the main culprit in CO₂ reduction. The engagement of hot electrons in the catalytic process is derived from many approaches that cover the processes in real-time, from ultrafast charge generation and separation to catalysis occurring on the minute scale. Unbiased in situ FTIR spectroscopy confirmed the stepwise reduction of the catalytic system. This, coupled with the low thermal stability of the Re^I(phen-NH₂)(CO)₃Cl complex, explicitly establishes plasmonic hot carriers as the primary contributors to the process. Therefore, mediating catalytic reactions by plasmon hot carriers is feasible and holds promise for further exploration. Plasmonic nanohybrid systems can leverage plasmon’s unique photophysics and capabilities because they expedite the carrier’s lifetime.

Plasmonic systems convert light into electrical charges and heat, mediating catalytic transformations. However, there is ongoing controversy regarding the involvement of hot carriers in the catalytic process. In this study, we demonstrate the direct utilisation of plasmon hot electrons in the hydrogen evolution reaction with visible light. We intentionally assemble a plasmonic nanohybrid system comprising NiO/Au/[Co(1,10-Phenanthrolin-5-amine)2(H2O)2], which is unstable at water thermolysis temperatures. This assembly limits the plasmon thermal contribution while ensuring that hot carriers are the primary contributors to the catalytic process. By combining photoelectrocatalysis with advanced in situ spectroscopies, we can substantiate a reaction mechanism in which plasmon-induced hot electrons play a crucial role. These plasmonic hot electrons are directed into phenanthroline ligands, facilitating the rapid, concerted proton-electron transfer steps essential for hydrogen generation. The catalytic response to light modulation aligns with the distinctive profile of a hot carrier-mediated process, featuring a positive, though non-essential, heat contribution. Direct participation of plasmon-induced hot electrons in the photoelectrocatalytic synthesis of hydrogen. This report solves a long-lasting contentious issue surrounding plasmonic materials on catalytic applications.

The rise in nitrogen-containing compounds in water sources due to modern agricultural practices has intensified the need to effectively convert nitrate to ammonia, a valuable fertiliser and fuel. We developed a photo-assisted electrocatalytic system using a NiO/Au plasmon/TiO2 composite to selectively reduce nitrate to ammonia under visible light, at neutral pH, and at room temperature. TiO2 was found to be the active catalyst, but the precise active structure responsible for each catalytic step remains unclear, as the reaction involves a complex, multistep process. By analyzing the catalytic activity of different TiO2 phases, we found that amorphous TiO2 significantly enhances the nitrate-to-nitrite reduction step, increasing nitrite concentration in solution by nearly 50 % and resulting in a 10 % increase in Faradaic efficiency for this product. Conversely, the rutile phase plays a crucial role in the subsequent conversion of nitrite to ammonia. When the rutile phase was present, the ammonia yield more than doubled, leading to a 30 % increase in Faradaic efficiency. This phase-dependent behaviour provides critical insights into improving nitrate reduction efficiency, enabling sustainable agricultural practices that recycle nutrients, reduce fertiliser costs, and promote economic sustainability.

Changes in farming techniques have facilitated the movement of nitrogen-containing species, making converting nitrate into ammonia (fertilizer) highly desirable. Recently, we introduced a photosystem comprising NiO/Au plasmon/TiO2 that can selectively convert nitrate to ammonia at neutral pH and room temperature using visible light in a photo-electrochemical approach. The study evaluated the role of adding alcohol to the overall process activity and selectivity. Adding small quantities of alcohol to the electrolyte leads to changes in the catalytic behaviour, which cannot be attributed exclusively to improvement in counter-electrode reaction kinetics. Analysis of product Faradaic efficiency and photo-current measurements revealed that alcohols act as proton donors in nitrate/nitrite reduction, possibly through a concerted proton-couple electron transfer mechanism. These initial findings offer new handles for nitrate reduction to ammonia efficacy at neutral pH. Ultimately, this opens up avenues for agricultural practices that recycle nutrients, improve process circularity, and reduce fertilizer costs, thus contributing to economic sustainability.

Plasmonic nanomaterials have garnered considerable attention in the scientific community due to their applicability in light-mediated technologies, owing to tunability, large optical cross-sections and scalability. Plasmonic nanoparticles with uniform morphology exhibit substantial optical cross-sections but limited energy absorption windows, reducing effectiveness for applications using polychromatic illumination like sunlight. Integrating plasmonics electrodes with a Fabry–Pérot nanocavity is a promising approach to broaden the absorption energy range of single morphology particles. Traditionally, the fabrication of these nanocavities involves clean room processes, posing scalability challenges and limiting the materials' scope. This study presents a successful approach for enhancing light absorption in a plasmonic photoelectrode system through a Fabry–Pérot nanocavity created using bottom-up solution methods. This approach technique overcomes some of the existing scalability issues while enabling the fabrication of a photocathode that can be rendered semitransparent or opaque. Such versatility opens up many application possibilities for these photosystems, from photocatalysis to optical devices.

There is an urgent need for efficient solutionprocessable p-type semiconductors. Copper(I) iodide (CuI) has attracted attention as a potential candidate due to its good electrical properties and ease of preparation. However, its carrier dynamics still need to be better understood. Carrier dynamics after bandgap excitation yielded a convoluted signal of free carriers (positive signal) and a negative feature, which was also present when the material was excited with sub-bandgap excitation energies. This previously unseen feature was found to be dependent on measurement temperature and attributed to negative photoconductivity. The unexpected signal relates to the formation of polarons or strongly bound excitons. The possibility of coupling CuI to plasmonic sensitizers is also tested, yielding positive results. The outcomes mentioned above could have profound implications regarding the applicability of CuI in photocatalytic and photovoltaic systems and could also open a whole new range of possible applications.

The existing literature survey reports rare and conflicting studies on the effect of the preparation method of metal-based semiconductor photocatalysts on structural/morphological features, electronic properties, and kinetics regulating the photocatalytic H-2 generation reaction. In this investigation, we compare the different copper/titania-based photocatalysts for H-2 generation synthesized via distinct methods (i.e., photodeposition and impregnation). Our study aims to establish a stringent correlation between physicochemical/electronic properties and photocatalytic performances for H-2 generation based on material characterization and kinetic modeling of the experimental outcomes. Estimating unknown kinetic parameters, such as charge recombination rate and quantum yield, suggests a mechanism regulating charge carrier lifetime depending on copper distribution on the TiO2 surface. We demonstrate that H-2 generation photoefficiency recorded over impregnated CuxOy/TiO2 is related to an even distribution of Cu(0)/Cu(I) on TiO2, and the formation of an Ohmic junction concertedly extended charge carrier lifetime and separation. The outcomes of the kinetic analysis and the related modeling investigation underpin photocatalyst physicochemical and electronic properties. Overall, the present study lays the groundwork for the future design of metal-based semiconductor photocatalysts with high photoefficiencies for H-2 evolution.