Light-driven control of electronic structure in correlated metal oxides offers new opportunities for optimizing materials used in photovoltaic and photoelectrochemical technologies. We show that photoexcitation of NiO, a prototypical transparent semiconductor and hole-transport material, across its charge-transfer gap produces a long-lived metastable state with enhanced Ni 3d-O 2p orbital hybridization. We characterize this state using Ni K-edge X-ray absorption spectroscopy, which probes how structural and electronic changes affect the unoccupied p density of states during continuous and pulsed ultraviolet excitation. Under pulsed excitation, high carrier densities of approximately 10^20 per cubic centimeter generate a state with a lifetime of approximately 600 picoseconds, in which enhanced hybridization coexists with lattice heating. By contrast, continuous ultraviolet irradiation at much lower carrier densities of approximately 10^13 per cubic centimeter stabilizes a similar electronic state with negligible lattice heating, demonstrating that its formation is not solely thermally driven. First-principles DFT+U+V calculations attribute the spectral changes to stronger Ni 3d-O 2p hybridization, which alters the unoccupied Ni 4p states probed by dipole-allowed K-edge transitions. We attribute this change to the dynamic screening of on-site electronic correlations following photoexcitation, which redistributes the charge density. Because orbital hybridization governs carrier transport and charge-transfer energetics, our results identify photoinduced screening as a mechanism for dynamically tuning correlated oxides and suggest new design principles for optoelectronic materials.

Hot carrier generation in plasmonic nanostructures underpins a wide range of energy-conversion technologies, yet its fundamental mechanisms remain elusive due to ultrafast dynamics and detection challenges. This work demonstrates that femtosecond time-resolved X-ray absorption spectroscopy (TR-XAS) at X-ray free-electron lasers (XFELs) enables direct, quantitative probing of hot carrier generation and relaxation in low-concentration gold nanoparticle suspensions. The measured Au L3-edge spectra exhibit good agreement with synchrotron references (Pearson correlation coefficient = 0.995), validating the method's reliability in capturing subtle electronic changes. XFEL-based TR-XAS resolved statistically significant transient signals within 100 fs of localized surface plasmon resonance (LSPR) excitation, allowing for the quantification of hot hole populations down to 0.06 per atom, setting a new sensitivity benchmark for plasmonic systems. By correlating TR-XAS signal intensities with calibrated electronic structure shifts, a precise framework for tracking charge dynamics in metallic nanostructures was established. Furthermore, conducted ab initio calculations, reproduced the obtained experimental data, and provided insight into changes in occupied states expected upon optical illumination in Au NPs. This high-fidelity, element-specific methodology opens new avenues for optimizing hot carrier generation and transfer in plasmon-enhanced applications spanning catalysis, photovoltaics, optoelectronics, and phototherapy.

Abstract Electrochemical CO2 reduction (eCO2R) is a sustainable strategy for converting CO2 into value-added chemicals. Here, we report the Cu-azolate metal−organic framework (MOF) CuBBTA (Cu2Cl2-bbta, MAF-X29) as a stable electrocatalyst for eCO2R in a zero-gap electrolyzer. CuBBTA was spray-coated onto carbon paper using either PTFE or Nafion binders to form gas diffusion electrodes that achieved Faradaic efficiencies of up to 50% for carbon-containing products (CH4, C2H4, and CO) at current densities as high as 100 mA cm−2 under a flow of CO2. Post-electrolysis studies confirmed retention of the MOF structure. DFT calculations reveal that CO2 reduction proceeds via COOH* and CO* intermediates, with a thermodynamically favourable and kinetically accessible CO*−CO* dimerization pathway for ethylene formation. Importantly, polymer binder selection significantly influences product selectivity. While PTFE favours methane formation, Nafion shifts selectivity toward CO, highlighting the critical yet underexplored role of electrode environment in MOF-based eCO2R catalysis.

Metal-organic frameworks (MOFs) containing photoactive organic linkers are promising platforms for artificial photosynthesis, yet the nature of the photoactive state in Zr(IV)-based MOFs remains controversial, particularly the proposed involvement of transient Zr(III) centers formed through linker-to-node charge transfer. Here, we elucidate the mechanism of CO 2 -to-formate conversion in MOFs by comparing redox-active PCN-223 with redox-inactive UiO-66-NH 2 . Although both MOFs contain the same Zr 6 -oxo cluster, their linkers differ in redox activity and interchromophore coupling, which allowed us to decouple linker-driven excited-state interactions from cluster-centered contributions to photocatalysis. Using in situ X-ray absorption spectroscopy, electron paramagnetic resonance, infrared spectroscopy, and transient optical spectroscopy, we show that the Zr 6 -oxo cluster remains redox-inactive during photocatalysis, ruling out linker-to-node electron transfer as the dominant pathway. Instead, framework assembly in PCN-223 induces strong interporphyrin coupling that promotes exciton delocalization and symmetry-breaking charge separation, generating long-lived linker-centered radical states absent in molecular porphyrins. Triethanolamine selectively scavenges the oxidized porphyrin species, stabilizing the reduced radical and enabling accumulation of photogenerated reducing equivalents within the framework. Remarkably, CO 2 reduction proceeds even after illumination ceases, revealing photocharging behavior in which stored linker-centered charges sustain catalysis in the dark. We further provide direct evidence for transient CO 2 •− species, which is consistent with a proton-coupled electron-transfer pathway inside the MOF pores. Together, these findings show that collective chromophore interactions, rather than metal-node redox chemistry, govern charge separation, charge storage, and catalytic reactivity in photoactive Zr-MOFs.

Electrocatalytic transformations of oxygen, i.e., the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), are key processes in renewable energy conversion, defining to a large extent, the efficiency of numerous energy conversion technologies, such as fuel cells, metal-air batteries or water electrolyzers. However, the development of highly effective, stable and inexpensive materials for such conversion processes is a bottleneck. Hence, establishing generic catalyst design principles by identifying structural features of catalysts that influence their performance would constitute a major step towards the rational engineering of advanced electrocatalysts. In this study, by investigating a series of metal-substituted manganese oxide (spinel), Mn3O4:M (M = Sr, Ca, Mg, Zn, Cu), nanoparticles as a model system, we demonstrate experimentally and rationalized the dependence of the activity of Mn3O4:M for ORR and the oxygen binding strength in Mn3O4:M oxides on the properties of the M substituent, viz. the enthalpy of formation of the binary MO oxide and the Lewis acidity of the M2+ substituent. Incorporation of elements M that have a low enthalpy of formation of MO (i.e., highly exothermic oxides featuring relatively strong M‒O bonding) enhances the oxygen binding strength in Mn3O4:M, which increases its activity in ORR due to the established correlation between ORR activity and the binding energy of *O/*OH/*OOH species on the catalyst surface. Our work provides a new perspective on the design of new compositions for oxygen electrocatalysis relying on the substitution by redox-inactive elements affecting the binding energy of oxygen to the surface of the complex metal oxide catalysts (ORR/OER activity descriptor). We speculate that this concept is general and transferrable to a broad selection of materials and processes involving oxygen adsorption and redox, beyond electrocatalysis.

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 (~25 fs) and hot carrier thermalization (~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.

The application of in situ and operando spectroscopic techniques has significantly advanced the understanding of reticular materials, particularly metal-organic frameworks (MOFs) and covalent organic frameworks (COFs). These techniques offer real-time insights into the dynamic structural, electronic, and chemical changes that occur within these materials during various processes, such as catalysis, sorption, and material synthesis. This review offers a comprehensive overview of key in situ and operando techniques used to investigate the formation, functionalization, and catalytic behavior of reticular materials. How these techniques have elucidated the roles of active sites, reaction intermediates, and structural transformations under reaction conditions, especially in single-site catalysis, electrocatalysis, and photocatalysis, is highlighted. The review also discusses the challenges and opportunities that lie ahead in integrating advanced spectroscopic methods with reticular materials, aiming to foster further innovation in the design and application of these versatile materials.

The reactivity towards C-H bond activation of alkanes with transition metals is determined by the ability of the metal to donate and withdraw electron density in due proportion. 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 in a C-H activating Rh complex and reveal changes in oxidation state as well as valence-orbital energies and character from femtosecond Rh-alkane bond formation to nanosecond C-H bond cleavage. Our X-ray spectroscopic signatures reflect how alkane-to- metal donation determines metal-alkane bond 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 new opportunities for manipulating reactivity for C-H activation with transition metals.

Photochemically prepared transition-metal complexes are known to be effective at cleaving the strong C-H bonds of organic molecules in room temperature solutions. There is also ample theoretical evidence that the bidirectional charge-transfer between an incoming alkane C-H group and the transition metal is the decisive interaction in the C-H activation reaction. What is missing, however, are experimental methods to directly probe these interactions in order to reveal what determines reactivity of intermediates and the rate of the reaction. Here, we propose metal specific and time-resolved valence-to-core resonant inelastic X-ray scattering (VtC-RIXS) at the transition metal L-edge as a method to provide a full account of the evolution of metal-alkane interactions during transition-metal mediated C-H activation reactions. For the model system cyclopentadienyl rhodium dicarbonyl (CpRh(CO)2), we demonstrate with a combination of experiment and quantum chemical simulation how the Rh-centered valence-excited final states probed with VtC-RIXS directly reflect changes in donation and back-donation between the alkane C-H group and the transition metal as the reaction proceeds via its intermediates. Following the initial photo-triggered CO dissociation, we find substantial reduction in charge donation onto the metal and the resulting stabilization of metal-centered states as the alkane coordinates to the Rh center in a σ-complex intermediate. C-H bond cleavage in the final oxidative addition step is instead characterized by a substantial increase in back-donation as the new Rh-hydrogen and Rh-carbon bonds are formed. We benchmark and validate our simulations against experimental steady-state measurements. With our study, we predict the key spectral fingerprints for future time-resolved experiments of C-H activation reactions with CpRh(CO)2 and related compounds.

We propose, using simulated RIXS signatures, a way of spectroscopically accessing the “two-way” charge transfer interactions between the C–H bond and the metal during C–H activation, thereby verifying and motivating orbital correlation diagrams.

Monitoring the spontaneous reconstruction of the surface of metal oxides under electrocatalytic reaction conditions is critical to identifying the active sites and establishing structure-activity relationships. Here, we report on a self-terminated surface reconstruction of Ruddlesden-Popper lanthanum nickel oxide (La2NiO4+delta) that occurs spontaneously during reaction with alkaline electrolyte species. Using a combination of high-resolution scanning transmission electron microscopy (HR-STEM), surface-sensitive X-ray photoelectron spectroscopy (XPS), and soft X-ray absorption spectroscopy (sXAS), as well as electrochemical techniques, we identify the structure of the reconstructed surface layer as an amorphous (oxy)hydroxide phase that features abundant under-coordinated nickel sites. No further amorphization of the crystalline oxide lattice (beyond the similar to 2 nm thick layer formed) was observed during oxygen evolution reaction (OER) cycling experiments. Notably, the formation of the reconstructed surface layer increases the material's oxygen evolution reaction (OER) activity by a factor of 45 when compared to that of the pristine crystalline surface. In contrast, a related perovskite phase, i.e., LaNiO3, did not show noticeable surface reconstruction, and also no increase in its OER activity was observed. This work provides detailed insight into a surface reconstruction behavior dictated by the crystal structure of the parent oxide and highlights the importance of surface dynamics under reaction conditions.

Research on gold nanoparticles (Au NPs) remains a field of intense activity due to their broad range of applications in diverse fields like catalysis, renewable energy, environmental science, and medicine. Herein, the morphological and electronic structures investigation of Au NPs prepared at different pH values is reported. The dependence of the localized surface plasmon resonance wavelength and electronic structure with size was determined by combining transmission electron microscopy, and various spectroscopic methods led by X-ray absorption spectroscopy. The X-ray absorption experiments evidenced that the citrate-stabilized Au NPs bulk electronic structure remains intact over a broad range of pHs, and changes were detected resulting from differences in NPs surface terminations.

ERI and SSZ-13 were subjected to post-synthetic treatments (depending on the zeolite topology) to create micro-/mesoporous materials. The results in terms of NH3-SCR-DeNOx show that the applied treatments improved the catalytic activity of the Cu-containing ERI-based materials; however, the NO conversion did not vary for the different materials treated with NaOH or NaOH/HNO3. For the micro-/mesoporous Cu-containing SSZ-13, a lower NO conversion in NH3-SCR-DeNOx was observed. Thus, our findings challenge the current paradigm of enhanced activity of micro-/mesoporous catalysts in NH3-SCR-DeNOx. The modification of the supports results in the presence of different amounts and kinds of copper species (especially isolated Cu2+ and aggregated Cu species) in the case of ERI- and SSZ-13-based samples. The present copper species further differentiate the formation of reactive reaction intermediates. Our studies show that besides the μ-η2,η2-peroxo dicopper(II) complexes (verified by in situ DR UV-Vis spectroscopy), copper nitrates (evidenced by in situ FT-IR spectroscopy) also act as reactive intermediates in these catalytic systems.

The intimate mechanism of N2O decomposition on bare and redox-tuned Co3O4 nanocubes (achieved by single (Li or K) and double (Li and K) doping) was elucidated. The catalysts synthesized by the hydrothermal method were characterized by X-ray electron absorption fine structure measurements, X-ray diffraction, Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and Kelvin Probe techniques. TPSR and steady-state isothermal catalytic tests reveal that the N2O turnover frequencies are critically sensitive to the work function of the catalysts, adjusted purposely by doping. For the catalysts obtained by one-pot hydrothermal synthesis, lithiation of the Co3O4 nanocubes leads to the formation of {Li'(8a), Co(16d)} species, decreasing steadily the work function and the activity, while for the catalysts prepared by postsynthesis impregnation, formation of {Li'(8a), Co'(16d), Co(16c)} species leads to a volcano-type dependence of the catalytic activity and the work function in parallel. The beneficial effect of potassium was discussed in terms of mitigation of surface potential buildup due to the accumulation of ionosorbed oxygen intermediates (surface electrostatics), which hinders the interfacial electron transfer. Analysis of the catalytic activity response to the redox tuning of Co3O4, substantiated by DFT calculations, allowed for a straightforward conceptualization of the redox nature of the N2O decomposition in terms of the lineup of frontier orbitals of the N2O/N2O- and O-2(-)/O-2 reactants with the surface DOS structure and the resultant molecular orbital interactions. The positions of the virtual bonding 3 pi g 0(N2O)-alpha 3d(z2) and the occupied 2 pi(1)(g)(O-2(-))-alpha-3d(z2) states relative to the Fermi energy level play a crucial role in the regulation of the forward and backward interfacial electron transfer events, which drive the redox process.

A new method for time-resolved X-ray absorption near edge structure (XANES) spectroscopy that enables faster data acquisition and requires smaller sample quantities for high-quality data, thus allowing the analysis of more samples in a shorter time is introduced. The method uses large bandwidth free electron laser pulses to measure laser-excited XANES spectra in transmission mode. A beam-splitting grating configuration allows simultaneous measurements of the spectra of the incoming X-ray Free Electron Laser (XFEL) pulses and transmission XANES, which is crucial for compensating the pulse-dependent intensity and spectrum fluctuations due to the self-amplified spontaneous emission operation. The implementation of this new methodology is applied on a liquid solution of ammonium iron(III) oxalate jet and is compared to previous results, showing great improvements in the speed of acquisition and spectral resolution, and the ability to measure a large 2-D spectral-time map quickly.

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 (H 2 O) 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.

Precise control and characterization of nanomaterials at working conditions are essential for further rational applications in many areas important for modern society. Penetrating properties of X-ray radiation in combination with advanced spectroscopy schemes are an ideal tool to investigate modifications of nanomaterials with extraordinary precision. Here, we present preliminary results on the controlled oxidation of copper nanoparticles and exploration of X-ray absorption spectroscopy to follow electronic and structural changes. The described pilot experiment raises questions on the applicability of high energy resolution X-ray detection schemes in potential future investigations aimed at following reversible reduction/oxidation processes at nanoparticle surfaces.

Abstract Plasmonic systems convert light into electrical charges and heat that mediate catalytic transformations. However, the debate about the involvement of hot carriers in the catalytic process remains shredded in controversy. Here, we demonstrate the direct use of plasmon hot electrons in the hydrogen evolution with visible light. A plasmonic nanohybrid system consisting of NiO/Au/[Co II (phen-NH 2 ) 2 (H 2 O) 2 ] (phen-NH 2 = 1,10-Phenanthrolin-5-amine) that is unstable at water thermolysis temperatures was consciously assembled, ensuring that the plasmon contribution to the catalytic process is solely from hot carriers. With the combination of photoelectrocatalysis and advanced in situ spectroscopies, one could establish the reaction mechanism, which consisted of electron injection into the phenanthroline-ligands followed by two quick, concerted proton-coupled electron transfer steps resulting in the evolution of hydrogen. Light-driven hydrogen evolution with plasmons provides a sustainable route for producing green hydrogen, which modern society strives to achieve.