title: Photophysics and Electronic Structure of Molecular Catalysts and Chromophores
Creators: name: Hammon, Sebastian
Publisher: name: University of Bayreuth
Year: 2022
Subjects: subject: renewable energies
subject: hydrogen production
subject: solar technologies
subject: water splitting
subject: catalyst development
subject: visible photocatalysis
subject: earth-abundant catalysts
subject: noble metal-free hydrogen evolution catalysts
subject: molecules
subject: chromophore
subject: metal nanoparticle
subject: photosensitizer
subject: photocatalyst
subject: Ir(bpy)(ppy)2
subject: Ir(dmOHbpy)(ppy)2
subject: CdS
subject: CdS/TiO2
subject: MOF
subject: MIL-101
subject: Ni
subject: Ni13
subject: Ni38
subject: nickel
subject: Pd
subject: Pd13
subject: palladium
subject: Pt
subject: Pt13, platinum
subject: quantum chemistry
subject: quantum mechanics
subject: theoretical physics
subject: density functional theory
subject: time-dependent DFT
subject: real-time TDDFT
subject: real-space grid
subject: simulation
subject: computation
subject: angle-resolved photoemission spectroscopy
subject: ARPES
subject: time-resolved spectroscopy
subject: two-photon photoemission spectroscopy
subject: pump-probe photoemission spectroscopy
subject: electronic structure
subject: electron dynamics
subject: charge-transfer excitation
subject: 530
subject: 540

description: Solar hydrogen production via water splitting promises to sustainably produce clean fuel for various applications by primarily relying on two of the most abundant resources on Earth, sunlight and water. Photocatalysis is one of the technologies that has attracted increasing research interest for water splitting, along with the degradation of organic pollutants and the synthesis of value-added organic products. Experimental approaches rely on suitable light-active compounds that catalyze the respective chemical reactions which mainly occur in a solution of a substrate mixture. In modern catalysis, metal nanoparticles (MNPs) excel as (co)catalysts in various organic reactions due to their synergistic effects on catalytic performance. By the same token, MNPs are being explored in photocatalysis. However, obtaining detailed insights into the photocatalytic mechanism typically requires state-of-the-art experimental techniques, such as angle-resolved photoemission spectroscopy (ARPES). While these techniques provide a wealth of data, their interpretation can be challenging: On the one hand, the underlying photophysical and electronic phenomena are of complicated quantum nature. On the other hand, viable system designs must meet additional requirements, such as preventing the aggregation of MNPs, which adds to the multi-faceted nature of the systems. Therefore, it has proven expedient to use experimental and theoretical methods to characterize photocatalytic systems jointly. Due to its favorable ratio of accuracy to computational cost, ab-initio density-functional theory (DFT) in its standard Kohn-Sham formulation is currently the most popular electron-structure method in photocatalysis and, moreover, in most interdisciplinary fields of physics with chemistry, biology, and materials science. This thesis's first of two project lines concerns predicting structural, electronic, and photophysical properties of molecular building blocks of (photo)catalytic systems containing MNPs using ground-state DFT and time-dependent DFT (TDDFT). A primary focus lies on understanding performance-related differences between certain cocatalytic MNP species (Ni, Pd, Pt) found in photocatalytic experiments for hydrogen production. Regarding the conceptual basis, I first present a DFT-based procedure to obtain low-energy molecular structures of systems containing MNPs, since these generally exhibit many geometries that are stable and similar in energy. This procedure is first applied to investigate whether MNPs and solvents interact (significantly). Studying small Pd nanoparticles (clusters) in solution with ketones shows that the interaction can affect the electronic and molecular structure of the metal particles. The interaction manifests itself, \textit{inter alia}: (i) In changes in the electronic density of states of the metal-solvent systems near the Fermi level (compared to their components). (ii) In the quenching of the magnetic moment that Pd clusters otherwise exhibit in the gas phase. The results suggest that the electronic interaction is more pronounced with aromatic than non-aromatic solvents. In the course of a collaboration of physics and chemistry in a joint research center (SFB840, "From Particulate Nanosystems to esotechnology"), we explore new design strategies in photocatalytic hydrogen production to replace cocatalytic noble MNPs with earth-abundant Ni: These novel approaches utilize the metal-organic framework MIL-101 to combine MNPs with either an Ir-based molecular photosensitizer or solid-state photocatalysts CdS, CdS/TiO2 without surface blocking ligands. The former enables hydrogen production via proton reduction in water under visible light. Encapsulating the MNPs, and photosensitizer into the nanopores of MIL-101 prevents metal aggregation. CdS/TiO2 and CdS decorated with MNPs accomplish the visible light-driven acceptorless dehydrogenation of alcohols and benzylamine under liberation of hydrogen, respectively. The MNPs reduce charge recombination and stabilize the CdS component against photooxidation. In most cases, the combination with MNPs promotes hydrogen production compared to the pure photosensitizer and photocatalyst, respectively. Encouragingly, Ni promotes hydrogen evolution in all cases, consistently outperforming the paradigmatic noble metals Pd and Pt. My studies contribute to a first understanding of the general role of MNPs and the synergistic effects of Ni in these systems: (i) DFT calculations reveal that the Ir photosensitizer and the substrates (benzyl alcohol, benzylamine) bind stronger to Ni than to the noble metal clusters. (ii) TDDFT calculations with optimally tuned range-separated hybrid functionals show that all three metals directly impact the photophysical properties of the photosensitizer via electronic interaction. The respective optical excitations feature a pronounced charge transfer from the metal cluster to the photosensitizer. The second project line focuses on pump-probe ARPES. This technique is a powerful tool for characterizing the photoactivated state of materials, as it allows direct insights into the excited electronic structure. Here, I develop a method for predicting pump-probe ARPES from molecular systems using TDDFT in real space and real time. To this end, I present a method that unites the key elements of this technique - excitation, ionization, and detection - in a single TDDFT simulation. I first provide a proof of concept. Finally, studying the organic semiconductor molecule perylene-3,4,9,10-tetracarboxylic dianhydride shows that this approach accomplishes the challenging task of capturing many-body signatures of excitations. In other words, this method goes beyond the popular DFT-based single-particle interpretation of ARPES (experiments), and this study provides an example of when many-particle effects are so prominent that they cannot be disregarded. Overall, this method constitutes a viable extension to existing methods that can now be utilized to interpret many-particle effects in pump-probe experiments.