Annemarie Huijser

We have investigated how Al3+ doping affects the facet-specific performance of SrTiO3 in gas-phase photocatalytic oxidation of acetone, comparing cubic and truncated structures. In situ Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS) indicates that Al-doping suppresses facet-specific adsorption of acetone and results in very similar relatively weakly adsorbed states of acetone on both, cubic and truncated structures. Oxidation of adsorbed acetone on both Al3+ doped SrTiO3 structures proceeds via α-cleavage, yielding acetaldehyde and formaldehyde, followed by the formation of surface-bound acetate and formate. The absence of CO2 suggests these species deactivate the surface leading to rapid termination of the oxidation process. Interestingly, the rate at which surface bound products are formed appears higher for the Al3+-doped truncated SrTiO3 than for the cubic morphology─contrary to the undoped analogues showing higher rates for the cubic structure. Time-resolved Photoluminescence (TRPL) analysis shows that Al3+ doping enhances charge carrier lifetimes in both morphologies, compared to undoped SrTiO3 having the same morphologies. Importantly, the Al-doped cubic STO shows shorter carrier lifetimes (τ1 = 86 ± 0.1 ps; τ2 = 709 ± 1 ps) compared to the Al-doped truncated material (τ1 = 116 ± 0.1 ps; τ2 = 1070 ± 1 ps)─which is consistent with the observed faster acetone transformations for the latter. This study shows Al3+ doping of SrTiO3 induces a transition from crystallographically controlled, to defect-dominated surface chemistry upon illumination, prohibiting complete mineralization of acetone, despite a doping induced lengthening of charge carrier lifetimes of cubic and truncated SrTiO3 structures.

Quasi-two-dimensional (quasi-2D) lead halide perovskites with alternating cations in the interlayer (ACI) space represent a promising type of material for optoelectronics. Similar to the Ruddlesden–Popper and Dion–Jacobson types of perovskites, domains with different thicknesses (n) and bandgaps are formed within a single film. This work focuses on ⟨n⟩ = 5 ACI perovskites based on guanidinium (GA⁺) and methylammonium (MA⁺) cations and investigates the influence of the GA:MA ratio in the interlayer space on the photophysical processes after photoexcitation. Using a combination of time-resolved photoluminescence (TRPL) and femtosecond transient absorption (TA) spectroscopy, hot carrier cooling, the occurrence and directionality of energy or charge transfer between the different domains, and the exciton and charge carrier dynamics are studied and modeled using target analysis. After the thermalization of hot carriers and excitons, exciton transfer from low-n to high-low-n domains occurs within 10 ps, after which they dissociate into free charges. From there, charge transfer into the intermediate-n domains occurs in about 22–54 ps. In the layers with excess GA, this process possibly occurs in an undesirable competition with self-trapped exciton (STE) formation. From the intermediate-n domains, charges are transferred into the high-n domains in 95–159 ps, which process occurs the fastest in the GA-MA layer. Finally, charge carriers decay intrinsically on the nanosecond scale with the longest lifetimes for the GA-MA and GA-2MA systems, which is beneficial for PV applications.

Ru/TiO₂ catalysts are well known for their high activity in the hydrogenation of CO₂ to CH₄ (the Sabatier reaction). This activity is commonly attributed to strong metal–support interactions (SMSIs), associated with reducible oxide layers partly covering the Ru-metal particles. Moreover, isothermal rates of formation of CH₄ can be significantly enhanced by the exposure of Ru/TiO₂ to light of UV/visible wavelengths, even at relatively low intensities. In this study, we confirm the significant enhancement in the rate of formation of methane in the conversion of CO₂, e.g., at 200 °C from ~1.2 mol g_Ru⁻¹·h⁻¹ to ~1.8 mol g_Ru⁻¹·h⁻¹ by UV/Vis illumination of a hydrogen-treated Ru/TiO_x catalyst. The activation energy does not change upon illumination—the rate enhancement coincides with a temperature increase of approximately 10 °C in steady state (flow) conditions. In-situ DRIFT experiments, performed in batch mode, demonstrate that the Ru–CO absorption frequency is shifted and the intensity reduced by combined UV/Vis illumination in the temperature range of 200–350 °C, which is more significant than can be explained by temperature enhancement alone. Moreover, exposing the catalyst to either UV (predominantly exciting TiO₂) or visible illumination (exclusively exciting Ru) at small intensities leads to very similar effects on Ru–CO IR intensities, formed in situ by exposure to CO₂. This further confirms that the temperature increase is likely not the only explanation for the enhancement in the reaction rates. Rather, as corroborated by photophysical studies reported in the literature, we propose that illumination induces changes in the electron density of Ru partly covered by a thin layer of TiO_x, lowering the CO coverage, and thus enhancing the methane formation rate upon illumination.

The performance of dye-sensitized photoelectrochemical cells is presently limited by the photocathode component. Here, we investigate the impact of nuclear dynamics on the photoinduced charge separation of the benchmark NiO/P1 system (P1 = 4-(bis-4-(5-(2,2-dicyano-vinyl)-thiophene-2-yl)-phenyl-amino)-benzoic acid). Transient absorption (TA) studies in aqueous environments with different viscosities show that photoinduced hole injection either proceeds ultrafast (<100 fs) or in a sub-ps time window. We assign the fastest component to a surface species strongly coupled to the NiO. Interestingly, the slower injection component and charge recombination are slowed down considerably in more viscous media. Quantum-classical dynamics simulations of a system with the dye standing perpendicular to the surface yield an injection lifetime remarkably close to the slow component from kinetic modeling of the TA results. Simulations including nuclear thermal motion yield a 2-fold increase in hole transfer rate compared to simulations on fixed nuclei, highlighting the role of nuclear motion and providing new design principles for dye-sensitized photocathodes.

Quasi‐2D lead‐halide perovskites consist of conducting inorganic layers with tunable thickness (n) separated by large organic spacer cations. Typically, domains with different n and bandgaps are formed within a single film. Here, the crystallization of the films is tuned by mixing Dion‐Jacobson (DJ) with Ruddlesden‐Popper (RP) spacer cations. Compared to the quasi‐2D perovskite film based on solely the DJ type spacer 1,4‐phenylenedimethylammonium (PDMA), a film with less defects and more vertically aligned crystallization is achieved by addition of the RP type spacer propylammonium (PA). As the film structure plays an important role in the photophysics, time‐resolved photoluminescence (TRPL) and femtosecond transient absorption (TA) are used to investigate the impact of mixing these spacer cations on the dynamics of hot carrier cooling, the occurrence and directionality of energy or electron transfer between the different domains, and the exciton and charge carrier dynamics. Exciton transfer from low‐n to high‐n domains occurs at a favorable faster rate for the PDMA‐based film (0.0640 ps⁻¹) compared to the PA‐based film (0.0365 ps⁻¹), while the mixed spacer film demonstrates intermediate behavior (0.0473 ps⁻¹). This study facilitates the design of advanced materials with optimized photophysical characteristics for a next generation of optoelectronic devices.

The donor–acceptor (D–A) dye 4‐(bis‐4‐(5‐(2,2‐dicyano‐vinyl)‐thiophene‐2‐yl)‐phenyl‐amino)‐benzoic acid (P1) has been frequently used to functionalize NiO photocathodes and induce photoelectrochemical reduction of protons when coupled to a suitable catalyst. Photoinduced twisting of the P1 dye is steered on NiO by co‐adsorption of tetradecanoic acid (C₁₄, myristic acid (MA)). Density Functional Theory and time‐resolved photoluminescence studies confirm that twisting lowers the energy levels of the photoexcited D–A dye. The apolar environment provided by the MA suppresses photoinduced D–A twisting, retards charge recombination following photoinduced charge separation between P1 and NiO, and provides a larger electrochemical potential increasing the photocurrent. Very interestingly, co‐adsorption of MA induces H₂ evolution upon photoexcitation without the presence of an H₂ evolution catalyst. Based on prior art, the formation of H₂ is assigned to the dissolution of Ni²⁺, followed by reduction and re‐deposition of Ni nanoparticles acting as the catalytically active site. It propose that only excited P1 with suppressed twisting provides the sufficient electrochemical potential to induce deposition of Ni nanoparticles. The work illustrates the importance of understanding the effects of photoinduced intramolecular twisting and highlights the promise of designing twisting‐limited D–A dyes to create efficient solar fuel devices.

Quasi-2D perovskites have attracted attention as potential solar energy absorber materials due to their balanced efficiency and stability and their unique quantum-well structures. In order to facilitate directional excitons and charge carrier transport and preferential energy transfer landscape in photovoltaic thin films, the phase distribution formed by different types of microstructural domains should be regulated. In this work, the Dion–Jacobson-type spacer 1,4-phenylenedimethanammonium (PDMA) was used, and different strategies were pursued to control the phase distribution in formamidinium-based (FA) quasi-2D perovskites based on the composition of (PDMA)FA₄Pb₅I₁₆. In general, doping with FACl modulated the crystallization kinetics, forming 2D low-n crystals on the top surface or a reversed-gradient phase distribution, depending on whether excess or substitutional doping was employed. Alternatively, mixing with a Ruddlesden–Popper spacer helped bridging to adjacent octahedra in pure PDMA-based perovskites and improved crystallization, while regulating the quantum-well structures to give a normal-gradient phase distribution, where 2D domains resided on the bottom side. By combining FACl doping and spacer mixing, the film showed both a reversed-gradient phase distribution and larger vertically aligned grains. This work contributes to the knowledge of how to manipulate and regulate the phase distribution in FA-based quasi-2D perovskites and further paves the way for fabricating corresponding devices with high efficiency and stability.

Dye-sensitized photocathodes attract wide interest because of their ease of fabrication and tunability in properties for light-driven proton or CO2 reduction. They are often based on nanocrystalline NiO, functionalized with a photosensitizer and catalyst, immersed into an aqueous electrolyte optionally saturated with CO2. Developing fundamental insight into their photo- and structural dynamics, especially as a function of the nanostructure and process conditions to which the photocathode is exposed, is essential to realize efficient design approaches. The present work reviews research progress over the last two decades, including (i) the design of an impressive variety in NiO nanostructures, photosensitizers, catalysts, and combined assemblies and (ii) the present mechanistic understanding. We highlight the importance of time-resolved spectroscopy studies under operating conditions to associate the early-time photodynamical processes with the overall photocatalytic performance and to understand the effects of surface-adsorbed species on the photodynamics. The nature and concentration of the ions in the electrolyte can play a major role in steering light-induced charge separation and recombination dynamics. We also discuss the importance of developing a better understanding of the interplay between the structural dynamics and the photoinduced charge and proton transfer dynamics to improve the photocathode performance and present several research perspectives.

Observations of low-lying dark states in several photosynthetic complexes challenge our understanding of the mechanisms behind their efficient energy transfer processes. Computational models are necessary for providing novel insights into the nature and function of dark states, especially since these are not directly accessible in spectroscopy experiments. Here, we will focus on signatures of dark-type states in chlorosomes, a light-harvesting complex from green sulfur bacteria well-known for uniting a broad absorption band with very efficient energy transfer. In agreement with experiments, our simulations of two-dimensional electronic spectra capture the ultrafast exciton transfer occurring in 100s of femtoseconds within a single chlorosome cylinder. The sub-100 fs process corresponds to relaxation within the single-excitation manifold in a single chlorosome tube, where all initially created populations in the bright exciton states are quickly transferred to dark-type exciton states. Structural inhomogeneities on the local scale cause a redistribution of the oscillator strength, leading to the emergence of these dark-type exciton states, which dominate ultrafast energy transfer. The presence of the dark-type exciton states suppresses energy loss from an isolated chlorosome via fluorescence quenching, as observed experimentally. Our results further question whether relaxation to dark-exciton states is a leading process or merely competes with transfer to the baseplate within the photosynthetic apparatus of green sulfur bacteria.

The structure of the bchQ mutant of Cba. tepidum, with predominantly [8-Et, 12-Et]BChl c, is resolved using solid-state NMR, cryo-EM, and optical spectroscopy. Syn–anti parallel stacks run vertically with a subunit axial displacement of 1.49 nm.