X-ray Spectroscopy at the SuperXAS and Debye Beamlines: from in situ to Operando.

Understanding structure-performance relationships are essential for the rational design of new functional materials or in the further optimization of (catalytic) processes. Due to the high penetration depth of the radiation used, synchrotron-based hard X-ray techniques (with energy > 4.5 keV) allow the study of materials under realistic conditions (in situ and operando) and thus play an important role in uncovering structure-performance relationships. X-ray absorption and emission spectroscopies (XAS and XES) give insight into the electronic structure (oxidation state, spin state) and local geometric structure (type and number of nearest neighbor atoms, bond distances, disorder) up to ~5 Å around the element of interest. In this mini review, we will give an overview of the in situ and operando capabilities of the SuperXAS beamline, a facility for hard X-ray spectroscopy, through recent examples from studies of heterogeneous catalysts, electrochemical systems, and photoinduced processes. The possibilities for time-resolved experiments in the time range from ns to seconds and longer are illustrated. The extension of X-ray spectroscopy at the new Debye beamline combined with operando X-ray scattering and diffraction and further developments of time-resolved XES at SuperXAS will open new possibilities after the Swiss Light Source upgrade mid 2025.

Quantifying Photoinduced Polaronic Distortions in Inorganic Lead Halide Perovskite Nanocrystals.

The development of next-generation perovskite-based optoelectronic devices relies critically on the understanding of the interaction between charge carriers and the polar lattice in out-of-equilibrium conditions. While it has become increasingly evident for CsPbBr3 perovskites that the Pb-Br framework flexibility plays a key role in their light-activated functionality, the corresponding local structural rearrangement has not yet been unambiguously identified. In this work, we demonstrate that the photoinduced lattice changes in the system are due to a specific polaronic distortion, associated with the activation of a longitudinal optical phonon mode at 18 meV by electron-phonon coupling, and we quantify the associated structural changes with atomic-level precision. Key to this achievement is the combination of time-resolved and temperature-dependent studies at Br K and Pb L3 X-ray absorption edges with refined ab initio simulations, which fully account for the screened core-hole final state effects on the X-ray absorption spectra. From the temporal kinetics, we show that carrier recombination reversibly unlocks the structural deformation at both Br and Pb sites. The comparison with the temperature-dependent XAS results rules out thermal effects as the primary source of distortion of the Pb-Br bonding motif during photoexcitation. Our work provides a comprehensive description of the CsPbBr3 perovskites' photophysics, offering novel insights on the light-induced response of the system and its exceptional optoelectronic properties.

Excited-state structure of copper phenanthroline-based photosensitizers.

Cu diimine complexes present a noble metal free alternative to classical Ru, Re, Ir and Pt based photosensitizers in solution photochemistry, photoelectrochemical or dye-sensitized solar cells. Optimization of these dyes requires understanding of factors governing the key photochemical properties: excited state lifetime and emission quantum yield. The involvement of exciplex formation in the deactivation of the photoexcited state is a key question. We investigate the excited-state structure of [Cu(dmp)2]+ and [Cu(dsbtmp)2]+ (dmp = 2,9-dimethyl-1,10-phenanthroline, dsbtmp = 2,9-di-sec-butyl-3,4,7,8-tetramethyl-1,10-phenanthroline) using pump-probe X-ray absorption spectroscopy (XAS) and DFT. Features of XAS that distinguish flattened tetrahedral site and 5-coordinated geometry with an additional solvent near Cu(II) center are identified. Pump-probe XAS demonstrates that for both complexes the excited state is 4-coordinated. For [Cu(dmp)2]+ the exciplex is 0.24 eV higher in energy than the flattened triplet state, therefore it can be involved in deactivation pathways as a non-observable state that forms slower than it decays. For [Cu(dsbtmp)2]+ the excited-state structure is characterized by Cu-N distances of 1.98 and 2.07 Å and minor distortions, leading to a 3 orders of magnitude longer excited-state lifetime.

Potential-Induced Spin Changes in Fe/N/C Electrocatalysts Assessed by In Situ X-ray Emission Spectroscopy.

The commercial success of the electrochemical energy conversion technologies required for the decarbonization of the energy sector requires the replacement of the noble metal-based electrocatalysts currently used in (co-)electrolyzers and fuel cells with inexpensive, platinum-group metal-free analogs. Among these, Fe/N/C-type catalysts display promising performances for the reduction of O2 or CO2 , but their insufficient activity and stability jeopardize their implementation in such devices. To circumvent these issues, a better understanding of the local geometric and electronic structure of their catalytic active sites under reaction conditions is needed. Herein we shed light on the electronic structure of the molecular sites in two Fe/N/C catalysts by probing their average spin state with X-ray emission spectroscopy (XES). Chiefly, our in situ XES measurements reveal for the first time the existence of reversible, potential-induced spin state changes in these materials.

Fluorescence-detected XAS with sub-second time resolution reveals new details about the redox activity of Pt/CeO2 catalyst.

A setup for fluorescence-detected X-ray absorption spectroscopy (XAS) with sub-second time resolution has been developed. This technique allows chemical speciation of low-concentrated materials embedded in highly absorbing matrices, which cannot be studied using transmission XAS. Using this setup, the reactivity of 1.5 wt% Pt/CeO2 catalyst was studied with 100 ms resolution during periodic cycling in CO- and oxygen-containing atmospheres in a plug-flow reactor. Measurements were performed at the Pt L3- and Ce L3-edges. The reactivity of platinum and cerium demonstrated a strong correlation. The oxidation of the catalyst starts on the ceria support helping the oxidation of platinum nanoparticles. The new time-resolved XAS setup can be applied to various systems, capable of reproducible cycling between different states triggered by gas atmosphere, light, temperature, etc. It opens up new perspectives for mechanistic studies on automotive catalysts, selective oxidation catalysts and photocatalysts.

Pump-probe XAS investigation of the triplet state of an Ir photosensitizer with chromenopyridinone ligands.

The triplet excited state of a new Ir-based photosensitizer with two chromenopyridinone and one bipyridine-based ligands has been studied by pump-probe X-ray absorption near edge structure (XANES) spectroscopy coupled with DFT calculations. The excited state has a lifetime of 0.5 µs in acetonitrile and is characterized by very small changes of the local atomic structure with an average metal-ligand bond length change of less than 0.01 Å. DFT-based calculations allow the interpretation of the XANES in the energy range of ∼50 eV around the absorption edge. The observed transient XANES signal arises from an additional metal-centered Ir 5d vacancy in the excited state which appears as a result of electron transfer from the metal to the ligand. The overall energy shift of the excited state spectrum originates from the shift of 2p and unoccupied states induced by screening effects. The approach for the analysis of time-resolved spectra of 5d metal complexes is quite general and can also be used if excited and ground state structures are significantly different.

Shedding Light on the Nature of Photoinduced States Formed in a Hydrogen-Generating Supramolecular RuPt Photocatalyst by Ultrafast Spectroscopy.

Photoinduced electronic and structural changes of a hydrogen-generating supramolecular RuPt photocatalyst are studied by a combination of time-resolved photoluminescence, optical transient absorption, and X-ray absorption spectroscopy. This work uses the element specificity of X-ray techniques to focus on the interplay between the photophysical and -chemical processes and the associated time scales at the catalytic Pt moiety. We observe very fast (<30 ps) photoreduction of the Pt catalytic site, followed by an ∼600 ps step into a strongly oxidized Pt center. The latter process is likely induced by oxidative addition of reactive iodine species. The oxidized Pt species is long-lived and fully recovers to the original ground state complex on a >10 µs time scale. However, the photosensitizing Ru moiety is fully restored on a much shorter ∼300 ns time scale. This reaction scheme implies that we may withdraw two electrons from a catalyst that is activated by a single photon.

{Co4O4} and {CoxNi4-xO4} Cubane Water Oxidation Catalysts as Surface Cut-Outs of Cobalt Oxides.

The future of artificial photosynthesis depends on economic and robust water oxidation catalysts (WOCs). Cobalt-based WOCs are especially promising for knowledge transfer between homogeneous and heterogeneous catalyst design. We introduce the active and stable {CoII4O4} cubane [CoII4(dpy{OH}O)4(OAc)2(H2O)2](ClO4)2 (Co4O4-dpk) as the first molecular WOC with the characteristic {H2O-Co2(OR)2-OH2} edge-site motif representing the sine qua non moiety of the most efficient heterogeneous Co-oxide WOCs. DFT-MD modelings as well as in situ EXAFS measurements indicate the stability of the cubane cage in solution. The stability of Co4O4-dpk under photocatalytic conditions ([Ru(bpy)3]2+/S2O82-) was underscored with a wide range of further analytical methods and recycling tests. FT-IR monitoring and HR-ESI-MS spectra point to a stable coordination of the acetate ligands, and DFT-MD simulations along with 1H/2H exchange experiments highlight a favorable intramolecular base functionality of the dpy{OH}O ligands. All three ligand types enhance proton mobility at the edge site through a unique bioinspired environment with multiple hydrogen-bonding interactions. In situ XANES experiments under photocatalytic conditions show that the {CoII4O4} core undergoes oxidation to Co(III) or higher valent states, which recover rather slowly to Co(II). Complementary ex situ chemical oxidation experiments with [Ru(bpy)3]3+ furthermore indicate that the oxidation of all Co(II) centers of Co4O4-dpk to Co(III) is not a mandatory prerequisite for oxygen evolution. Moreover, we present the [CoIIxNi4-x(dpy{OH}O)4(OAc)2(H2O)2](ClO4)2 (CoxNi4-xO4-dpk) series as the first mixed Co/Ni-cubane WOCs. They newly bridge homogeneous and heterogeneous catalyst design through fine-tuned edge-site environments of the Co centers.

Kinetic Studies of the Pt Carbonate-Mediated, Room-Temperature Oxidation of Carbon Monoxide by Oxygen over Pt/Al2O3 Using Combined, Time-Resolved XAFS, DRIFTS, and Mass Spectrometry.

The kinetics involved in a recently revealed ambient-temperature mechanism for the catalytic oxidation of carbon monoxide by oxygen over a 5 wt % Pt/Al2O3 catalyst are evaluated within a periodic, plug flow, redox operation paradigm using combined mass spectrometry (MS), diffuse reflectance infrared spectroscopy (DRIFTS), and time-resolved Pt L3-edge XAFS. The species that are the most active at room temperature are shown to be a high-wavenumber (ca. 1690 cm-1) carbonate that we associate directly with a room-temperature redox process occurring in a fraction of the Pt atoms present in the catalyst. Our results, however, do not exclude the participation of carbonate species native to the Al2O3 support, though these species tend to store CO at ambient temperature and become significant participants in CO oxidation catalysis only at slightly higher temperatures (323-333 K). Pt carbonate formation (1690 cm-1) under CO and the reaction to yield CO2 is shown to be extremely rapid and subject to an average apparent activation energy (Eapp), across the techniques applied, of 8.7 kJ mol-1, within the temperature range investigated (276-343 K). Reoxidation of Pt (XANES) and subsequent CO2 production mediated by Pt carbonates under O2 (MS/IR) displays a first-order dependence upon O2 partial pressure and a negative dependence upon the coverage of CO adsorbed on the Pt nanoparticles also present in this catalyst. This oxidative regeneration/CO2 production step is subject to an apparent activation energy (Eapp) of 56.5 (±5) kJ mol-1, is kinetically limited by the desorption of molecular CO from Pt nanoparticles, and also is shown to be dependent upon the partial pressure of O2 present in the oxidizing half of the cycle that we associate with the direct interaction of O2 with molecular CO adsorbed on the nanoparticles that promotes their desorption. Finally, a minority reactive state producing CO2 in the oxidizing cycle that displays no dependence upon the CO coverage of the nanoparticles can be induced through simple thermal treatment of the catalyst. These results are discussed in terms of the number and types of species present within the reactive system and in terms of the wider possibilities for the development of effective low-temperature CO oxidation using Pt/Al2O3 catalysts.

Microsecond X-ray Absorption Spectroscopy Identification of Co(I) Intermediates in Cobaloxime-Catalyzed Hydrogen Evolution.

Rational development of efficient photocatalytic systems for hydrogen production requires understanding the catalytic mechanism and detailed information about the structure of intermediates in the catalytic cycle. We demonstrate how time-resolved X-ray absorption spectroscopy in the microsecond time range can be used to identify such intermediates and to determine their local geometric structure. This method was used to obtain the solution structure of the Co(I) intermediate of cobaloxime, which is a non-noble metal catalyst for solar hydrogen production from water. Distances between cobalt and the nearest ligands including two solvent molecules and displacement of the cobalt atom out of plane formed by the planar ligands have been determined. Combining in situ X-ray absorption and UV/Vis data, we demonstrate how slight modification of the catalyst structure can lead to the formation of a catalytically inactive Co(I) state under similar conditions. Possible deactivation mechanisms are discussed.

Palladium versus platinum: the metal in the catalytic center of a molecular photocatalyst determines the mechanism of the hydrogen production with visible light.

To develop highly efficient molecular photocatalysts for visible light-driven hydrogen production, a thorough understanding of the photophysical and chemical processes in the photocatalyst is of vital importance. In this context, in situ X-ray absorption spectroscopic (XAS) investigations show that the nature of the catalytically active metal center in a (N^N)MCl2 (M=Pd or Pt) coordination sphere has a significant impact on the mechanism of the hydrogen formation. Pd as the catalytic center showed a substantially altered chemical environment and a formation of metal colloids during catalysis, whereas no changes of the coordination sphere were observed for Pt as catalytic center. The high stability of the Pt center was confirmed by chloride addition and mercury poisoning experiments. Thus, for Pt a fundamentally different catalytic mechanism without the involvement of colloids is confirmed.

Highly accurate excited-state structure of [Os(bpy)2dcbpy](2+) determined by X-ray transient absorption spectroscopy.

Determining the electronic and geometric structures of photoexcited transient species with high accuracy is crucial for understanding their fundamental photochemistry and controlling their photoreactivity. We have applied X-ray transient absorption spectroscopy to measure the XANES and EXAFS spectra of a dilute (submillimolar) solution of the osmium(II) polypyridyl complex [Os(bpy)2dcbpy](PF6)2 (dcbpy = 4,4'-dicarboxy-2,2'-bipyridine) (OsL2L') in methanol at the Os LIII edge. We have obtained spectra of superb quality for both the ground state and the photoinduced (3)MLCT excited state that have allowed us not only to extract detailed information about the Os 5d orbitals but also to resolve very small differences of 0.010 ± 0.008 Å in the average Os-N bond lengths of the ground and excited states. Theoretical calculations using a recently developed DFT-based approach support the measured electronic structures and further identify the nature of the molecular orbitals that contribute to the main absorption bands in the XANES spectra.

Structural basis of the Meinwald rearrangement catalysed by styrene oxide isomerase.

Membrane-bound styrene oxide isomerase (SOI) catalyses the Meinwald rearrangement-a Lewis-acid-catalysed isomerization of an epoxide to a carbonyl compound-and has been used in single and cascade reactions. However, the structural information that explains its reaction mechanism has remained elusive. Here we determine cryo-electron microscopy (cryo-EM) structures of SOI bound to a single-domain antibody with and without the competitive inhibitor benzylamine, and elucidate the catalytic mechanism using electron paramagnetic resonance spectroscopy, functional assays, biophysical methods and docking experiments. We find ferric haem b bound at the subunit interface of the trimeric enzyme through H58, where Fe(III) acts as the Lewis acid by binding to the epoxide oxygen. Y103 and N64 and a hydrophobic pocket binding the oxygen of the epoxide and the aryl group, respectively, position substrates in a manner that explains the high regio-selectivity and stereo-specificity of SOI. Our findings can support extending the range of epoxide substrates and be used to potentially repurpose SOI for the catalysis of new-to-nature Fe-based chemical reactions.

Tailoring p-Type Behavior in ZnO Quantum Dots through Enhanced Sol-Gel Synthesis: Mechanistic Insights into Zinc Vacancies.

The synthesis and control of properties of p-type ZnO is crucial for a variety of optoelectronic and spintronic applications; however, it remains challenging due to the control of intrinsic midgap (defect) states. In this study, we demonstrate a synthetic route to yield colloidal ZnO quantum dots (QD) via an enhanced sol-gel process that effectively eliminates the residual intermediate reaction molecules, which would otherwise weaken the excitonic emission. This process supports the creation of ZnO with p-type properties or compensation of inherited n-type defects, primarily due to zinc vacancies under oxygen-rich conditions. The in-depth analysis of carrier recombination in the midgap across several time scales reveals microsecond carrier lifetimes at room temperature which are expected to occur via zinc vacancy defects, supporting the promoted p-type character of the synthesized ZnO QDs.

A setup for hard x-ray time-resolved resonant inelastic x-ray scattering at SwissFEL.

We present a new setup for resonant inelastic hard x-ray scattering at the Bernina beamline of SwissFEL with energy, momentum, and temporal resolution. The compact R = 0.5 m Johann-type spectrometer can be equipped with up to three crystal analyzers and allows efficient collection of RIXS spectra. Optical pumping for time-resolved studies can be realized with a broad span of optical wavelengths. We demonstrate the performance of the setup at an overall ∼180 meV resolution in a study of ground-state and photoexcited (at 400 nm) honeycomb 5d iridate α-Li2IrO3. Steady-state RIXS spectra at the iridium L3-edge (11.214 keV) have been collected and are in very good agreement with data collected at synchrotrons. The time-resolved RIXS transients exhibit changes in the energy loss region <2 eV, whose features mostly result from the hopping nature of 5d electrons in the honeycomb lattice. These changes are ascribed to modulations of the Ir-to-Ir inter-site transition scattering efficiency, which we associate to a transient screening of the on-site Coulomb interaction.

High energy resolution core-level X-ray spectroscopy for electronic and structural characterization of osmium compounds.

A comprehensive study of the bulk solid OsCl3 and the molecular ion [Os(bpy)2(CO)Cl](+) is presented illustrating the application of RIXS and HERFD XANES spectroscopies to the investigation of both bulk materials and molecular complexes. In order to analyze the experimental results, DFT simulations were performed taking into account spin-orbit interaction. Calculations for both compounds resulted in good agreement with the experimental RIXS and HERFD XANES data, shedding light on the details of their local atomic and electronic structure. In particular, the spatial distribution of molecular orbitals was obtained, which allowed the determination of the origin of the absorption peaks. It was shown that for materials containing heavy atoms, only the application of advanced RIXS and HERFD XANES spectroscopies makes it possible to extract the information on local atomic and electronic structure details from XANES data.

Promoting Photocatalytic Activity of NH2-MIL-125(Ti) for H2 Evolution Reaction through Creation of TiIII- and CoI-Based Proton Reduction Sites.

Titanium-based metal-organic framework, NH2-MIL-125(Ti), has been widely investigated for photocatalytic applications but has low activity in the hydrogen evolution reaction (HER). In this work, we show a one-step low-cost postmodification of NH2-MIL-125(Ti) via impregnation of Co(NO3)2. The resulting Co@NH2-MIL-125(Ti) with embedded single-site CoII species, confirmed by XPS and XAS measurements, shows enhanced activity under visible light exposure. The increased H2 production is likely triggered by the presence of active CoI transient sites detected upon collection of pump-flow-probe XANES spectra. Furthermore, both photocatalysts demonstrated a drastic increase in HER performance after consecutive reuse while maintaining their structural integrity and consistent H2 production. Via thorough characterization, we revealed two mechanisms for the formation of highly active proton reduction sites: nondestructive linker elimination resulting in coordinatively unsaturated Ti sites and restructuring of single CoII sites. Overall, this straightforward manner of confinement of CoII cocatalysts within NH2-MIL-125(Ti) offers a highly stable visible-light-responsive photocatalyst.

Tracking C-H activation with orbital resolution.

Transition metal reactivity toward carbon-hydrogen (C-H) bonds hinges on the interplay of electron donation and withdrawal at the metal center. Manipulating this reactivity in a controlled way is difficult because the hypothesized metal-alkane charge-transfer interactions are challenging to access experimentally. Using time-resolved x-ray spectroscopy, we track the charge-transfer interactions during C-H activation of octane by a cyclopentadienyl rhodium carbonyl complex. Changes in oxidation state as well as valence-orbital energies and character emerge in the data on a femtosecond to nanosecond timescale. The x-ray spectroscopic signatures reflect how alkane-to-metal donation determines metal-alkane complex stability and how metal-to-alkane back-donation facilitates C-H bond cleavage by oxidative addition. The ability to dissect charge-transfer interactions on an orbital level provides opportunities for manipulating C-H reactivity at transition metals.

Atomic-Level Description of Thermal Fluctuations in Inorganic Lead Halide Perovskites.

A comprehensive microscopic description of thermally induced distortions in lead halide perovskites is crucial for their realistic applications, yet still unclear. Here, we quantify the effects of thermal activation in CsPbBr3 nanocrystals across length scales with atomic-level precision, and we provide a framework for the description of phase transitions therein, beyond the simplistic picture of unit-cell symmetry increase upon heating. The temperature increase significantly enhances the short-range structural distortions of the lead halide framework as a consequence of the phonon anharmonicity, which causes the excess free energy surface to change as a function of temperature. As a result, phase transitions can be rationalized via the soft-mode model, which also describes displacive thermal phase transitions in oxide perovskites. Our findings allow to reconcile temperature-dependent modifications of physical properties, such as changes in the optical band gap, that are incompatible with the perovskite time- and space-average structures.

Understanding the electronic structure of 4d metal complexes: from molecular spinors to L-edge spectra of a di-Ru catalyst.

L(2,3)-edge X-ray absorption spectroscopy (XAS) has demonstrated unique capabilities for the analysis of the electronic structure of di-Ru complexes such as the blue dimer cis,cis-[Ru(III)(2)O(H(2)O)(2)(bpy)(4)](4+) water oxidation catalyst. Spectra of the blue dimer and the monomeric [Ru(NH(3))(6)](3+) model complex show considerably different splitting of the Ru L(2,3) absorption edge, which reflects changes in the relative energies of the Ru 4d orbitals caused by hybridization with a bridging ligand and spin-orbit coupling effects. To aid the interpretation of spectroscopic data, we developed a new approach, which computes L(2,3)-edges XAS spectra as dipole transitions between molecular spinors of 4d transition metal complexes. This allows for careful inclusion of the spin-orbit coupling effects and the hybridization of the Ru 4d and ligand orbitals. The obtained theoretical Ru L(2,3)-edge spectra are in close agreement with experiment. Critically, existing single-electron methods (FEFF, FDMNES) broadly used to simulate XAS could not reproduce the experimental Ru L-edge spectra for the [Ru(NH(3))(6)](3+) model complex nor for the blue dimer, while charge transfer multiplet (CTM) calculations were not applicable due to the complexity and low symmetry of the blue dimer water oxidation catalyst. We demonstrated that L-edge spectroscopy is informative for analysis of bridging metal complexes. The developed computational approach enhances L-edge spectroscopy as a tool for analysis of the electronic structures of complexes, materials, catalysts, and reactive intermediates with 4d transition metals.