X-ray Absorption Spectroscopy Measurements Research Articles

Achieving High OER Performance by Tuning the Co/Mn Content in Prussian Blue Analogues.

The need for efficient, economical, and clean energy systems is increasing, and as a result, interest in water-splitting techniques to produce green hydrogen is also increasing. However, the sluggish kinetics of the oxygen evolution reaction (OER) hinders the practical application and widespread use of water-splitting technologies; therefore, to address this challenge, it is essential to develop cost-effective and efficient OER catalysts. In this work, we have synthesized an inexpensive and tunable FeCoMn Prussian blue analogue (PBAs) as an efficient OER catalyst via a straightforward process. The ratio of the Co and Mn to optimize the electrochemical performance, and as a result, the FeCo0.41Mn0.42 PBA catalyst demonstrated the best electrochemical performance (260/304 mV overpotential at 10/50 mA cm-2, a low Tafel slope of 48 mV dec-1 and a good stability of 72 h at 10 mA cm-2). Additionally, X-ray absorption spectroscopy (XAS) measurements and density functional theory (DFT) calculations suggest that the FeCo0.41Mn0.42 PBA possesses the optimized electronic density distribution at the active site (Co), and the doping of Mn and Fe can not only increase the electricity conductivity but also activate the critical H2O deprotonation step.

Revealing the Role of Ruthenium on the Performance of P2-Type Na0.67Mn1-xRuxO2 Cathodes for Na-Ion Full-Cells.

Herein, P2-type layered manganese and ruthenium oxide is synthesized as an outstanding intercalation cathode material for high-energy density Na-ion batteries (NIBs). P2-type sodium deficient transition metal oxide structure, Na0.67Mn1-xRuxO2 cathodes where x varied between 0.05 and 0.5 are fabricated. The partially substituted main phase where x = 0.4 exhibits the best electrochemical performance with a discharge capacity of ≈170mAhg-1. The in situ X-ray Absorption Spectroscopy (XAS) and time-resolved X-ray Diffraction (TR-XRD) measurements are performed to elucidate the neighborhood of the local structure and lattice parameters during cycling. X-ray photoelectron spectroscopy (XPS) revealed the oxygen-rich structure when Ru is introduced. Density of States (DOS) calculations revealed the Fermi-Level bandgap increases when Ru is doped, which enhances the electronic conductivity of the cathode. Furthermore, magnetization calculations revealed the presence of stronger Ru─O bonds and the stabilizing effect of Ru-doping on MnO6 octahedra. The results of Time-of-flight secondary-ion mass spectroscopy (TOF-SIMS) revealed that the Ru-doped sample has more sodium and oxygenated-based species on the surface, while the inner layers mainly contain Ru-O and Mn-O species. The full cell study demonstrated the outstanding capacity retention where the cell maintained 70% of its initial capacity at 1C-rate after 500 cycles.

Mechanism Behind the Enhanced Ferromagnetic Contributions Upon Ru Substitution in Ca3Mn2O7 from X-Ray Absorption Spectroscopies.

We have studied the local structure and electronic and magnetic properties of hybrid improper ferroelectric Ca3Mn2O7 upon Ru substitution at the Mn site by a combination of atomic-selective X-ray absorption spectroscopies in the soft and hard X-ray energy regimes. Ru substitution enhances the macroscopic ferromagnetic contributions, whose origin is here elucidated. In particular, soft X-ray magnetic circular dichroism (XMCD) data indicate that the spin moments of Mn and Ru are aligned in opposite directions, with the effective magnetic moments of Ru being about 1 order of magnitude smaller than for Mn. The XMCD results also demonstrate the presence of an intrinsic ferromagnetic component in the Mn sublattice for Ru contents x ≥ 0.3, although the values of the net Mn magnetic moment are much smaller than expected for a fully saturated Mn4+ magnetic sublattice. Soft and hard X-ray absorption spectroscopy measurements show that Mn keeps a +4 valence state independently of the Ru content, whereas Ru is in an intermediate valence state, ranging between +4.7 (x ≤ 0.1) and +4.4 (x ≥ 0.7). Analysis of the extended X-ray absorption fine structure (EXAFS) signals at the Mn and Ru K-edges suggests the lack of a complete solid solution in the local structure across the entire range of compositions. These findings disclose the existence of charge transfer between Mn and Ru atoms, so the weak ferromagnetic component is attributed to a canting of the antiferromagnetically ordered Mn4+ spins caused by the oxygen-mediated hybridization between localized Mn4+ 3d and itinerant intermediate valence Ru 4d bands, in analogy to the mechanism proposed for ferromagnetism in ordered doubled perovskites with 3d and 4d/5d transition-metal ions. In the present case, disorder prevents the formation of long-range ferromagnetism.

Brightening dark excitons in inorganic halide perovskites by local symmetry breaking

Thermally activated delayed fluorescence (TADF) phenomenon, defined as thermal activation of triplet dark-state excitons into singlet state excitons through reverse intersystem crossing (RISC), has become a prevalent principle for designing organic-based electro-/photo-luminescent molecules. However, activating dark excitons in inorganic metal halide perovskites (MHPs) still remains challenging. Herein, we report an efficient strategy of doping-induced local symmetry breaking to brighten dark excitons in inorganic halide perovskite Cs2ZrCl6, which result in a 500 % enhancement of low-temperature luminescence emission. Lattice symmetry breaking confirmed by the X-ray absorption spectroscopy measurements could disturb the local coordination environment at the Zr site. The singlet-triplet energy gap (ΔEST) fitted from temperature-dependent photoluminescence decay curves decreases from 0.087 to 0.071 eV, thus reducing up-conversion barrier of dark excitons. Theoretical calculation indicates large spatial separation of charge density favorable for enhanced TADF emission. Furthermore, a flexible X-ray scintillator screen with poly (dimethylsiloxane) as the matrix could obtain high-quality X-ray imaging with a 14 lp mm−1 resolution. This work gives insights into improving photoemission of inorganic MHPs via brightening dark excitons.

Towards an EPR on a Chip Spectrometer for Monitoring Radiation Damage During X-ray Absorption Spectroscopy

AbstractElectron paramagnetic resonance (EPR) spectroscopy is an essential tool to investigate the effects of ionizing radiation, which is routinely administered for reducing contaminations and waste in food products and cosmetics as well as for sterilization in industry and medicine. In materials research, EPR methods are not only employed as a spectroscopic method of structural investigations, but also have been employed for detection of changes in electronic structure due to radiation damage from high energy X-rays, for example, to monitor radical formation inside biomolecules caused by X-ray irradiation at carbon, nitrogen, and oxygen K-edges at synchrotron facilities. Here a compact EPR spectrometer, based on EPR-on-a-chip (EPRoC) sensor and a portable electromagnet, has been developed as a solution for monitoring radiation damage of samples during their investigation by X-ray absorption spectroscopy (XAS) at synchrotron facilities. A portable electromagnet with a soft iron core and forced air temperature stabilization was constructed as the source of the external magnetic field. The sweep range of magnetic field inside the most homogeneous region of the portable electromagnet is 12–290 mT. The compact spectrometer performance was evaluated by placing the EPRoC sensor inside either a commercial electromagnet or the portable electromagnet to record the EPR spectrum of tempol, irradiated alanine, and dilithium phthalocyanine (Li2Pc). The potential performance of the portable spectrometer for the detection of radiation damage in organic compounds and transition metal-containing catalysts during XAS measurements in both fluorescence and transmission modes was calculated with promising implications for measurements after implementation in a synchrotron-based XAS spectrometer.

(Invited) Nitrogen-Doped High-Entropy Alloy as Efficient and Stable Electrocatalyst

The development of Pt-based catalysts for use in fuel cells that meet performance targets of high activity, maximized stability, and low cost remains a huge challenge. The high-entropy alloy (HEA) is one of the most promising candidates for electrocatalysts, owing to their unique properties involving the high entropy effect, lattice distortion effect, sluggish diffusion effect, and “cocktail” effect.1 Alternatively, nitrogen (N)-doping effects have been well documented and demonstrated in our previous studies, which further fine-tune the adsorption energetics of intermediates and improve the corrosion resistance by the pinning effect of metal-nitrogen bonds.2-3 Combining the HEA and N-doping strategies, the tailored lattice and interstice atomic sites lead to the formation of chemically stable multiple metal-nitrogen (M-N) bonds and the generation of the possible synergistic effect on promoting ORR performance. Herein, we report a N-doped high-entropy alloy (HEA) electrocatalyst that consists of a Pt-rich shell and a N-doped PtCoFeNiCu core on a carbon support (denoted as N-Pt/HEA/C). The N-Pt/HEA/C catalyst showed high mass activity of 1.34 A mgPt -1 at 0.9 V for the oxygen reduction reaction (ORR), which substantially outperformed commercial Pt/C and most of the other binary/ternary Pt-based catalysts. The N-Pt/HEA/C catalyst also demonstrated excellent stability in both rotating disk electrode (RDE) and membrane electrode assembly (MEA) testing. Using operando X-ray absorption spectroscopy (XAS) measurements and theoretical calculations, we revealed that the enhanced ORR activity of N-Pt/HEA/C originated from the optimized adsorption energy of intermediates, consequent of the tailored electronic structure formed upon the N-doping. Furthermore, we showed that the multiple metal-nitrogen bonds formed synergistically improved the corrosion resistance of the 3d transition metals and enhanced the ORR durability.References Yao, Y.; Dong, Q.; Brozena, A.; Luo, J.; Miao, J.; Chi, M.; Wang, C.; Kevrekidis, I. G.; Ren, Z. J.; Greeley, J.; Wang, G.; Anapolsky, A.; Hu, L., High-entropy nanoparticles: Synthesis-structure-property relationships and data-driven discovery. Science 2022, 376 (6589), eabn3103.Zhao, X.; Xi, C.; Zhang, R.; Song, L.; Wang, C.; Spendelow, J. S.; Frenkel, A. I.; Yang, J.; Xin, H. L.; Sasaki, K., High-Performance Nitrogen-Doped Intermetallic PtNi Catalyst for the Oxygen Reduction Reaction. ACS Catalysis 2020, 10 (18), 10637-10645.Zhao, X.; Cheng, H.; Song, L.; Han, L.; Zhang, R.; Kwon, G.; Ma, L.; Ehrlich, S. N.; Frenkel, A. I.; Yang, J.; Sasaki, K.; Xin, H. L., Rhombohedral Ordered Intermetallic Nanocatalyst Boosts the Oxygen Reduction Reaction. ACS Catalysis 2020, 11 (1), 184-192.

Advanced Nanostructured Textile Catalysts for Enhanced Hydrogen Production in PEM Water Electrolysis: An Overview and Recent Developments

As the global focus shifts towards a zero-emission society, hydrogen has emerged as a critical element in various industries, particularly in green hydrogen production. A promising method for this is Polymer Electrolyte Membrane Water Electrolysis (PEMWE), which uses pure water and electricity to produce high-purity hydrogen. However, the efficiency of PEMWE is highly dependent on the effectiveness of its catalysts, particularly in the context of the catalyst-coated membranes (CCMs) employed in these systems. Conventional preparation of these CCMs involves multiple steps, including synthesis and application of a catalyst ink, which often leads to catalyst agglomeration and requires high Ir (Iridium) loadings to maintain uniformity and performance.In our latest study, we have developed a novel approach using ultralight conductive IrO2 nanostructured textiles (NST) as a catalyst layer for PEMWE. This innovative method successfully circumvents the issue of electrically isolated inactive catalysts, a common problem with lower Ir amounts in traditional catalyst layers. By employing a process of sputtering IrO2 onto water-soluble polymer nanofibers, we have achieved facile CCM preparation and demonstrated unprecedented long-term hydrogen evolution with ultra-low Ir mass loadings at high rates. Our study revealed that the high performance of these CCMs is derived from their low resistivity and unique porous three-dimensional structure.The preparation of NST involves dissolving polyvinylpyrrolidone (PVP) in methanol, followed by electrospinning this solution onto a metal-foil collector. The resulting PVP nanofibers then undergo a sputtering process to deposit IrO2, forming the nanostructured textiles. This method eliminates the need for wet chemical routes and labor-intensive processes typically associated with conventional catalyst layer preparations, thus offering a more environmentally friendly and scalable alternative.Our performance evaluations of these NST CCMs, with varying Ir mass loadings, have shown significant advancements. The operational cell voltages of CCMs containing as little as 0.05 mg-Ir cm−2 were suppressed across all investigated current densities compared to conventional CCMs. Notably, operational voltages remained below 2 V even at the highest current density of 7 A cm−2. This level of performance is unprecedented for CCMs with such low catalyst amounts. Additionally, the long-term stability of these CCMs was remarkable, with operational times exceeding 1500 hours without significant increases in voltage, showcasing their potential for practical applications.An essential aspect of our study was the analysis of the electrical properties and the surface structure of the IrO2 NSTs. We observed that the electrical conductivity of these textiles was significantly higher than that of conventional catalyst layers. This high conductivity, coupled with the unique interconnected structure of the textiles, ensures efficient electrical contact between nano catalysts and the current collector, thus avoiding the formation of isolated catalysts. Our analyses revealed that the IrO2 NSTs have unique surface terminations and a high number of undercoordinated surface Ir atoms, contributing to their high intrinsic catalytic activity.Further insights were gained through operando X-ray absorption spectroscopy (XAS) measurements, which showed that the IrO2-nanostructured textiles possess a large number of D-band holes and are intrinsically highly active. This finding is crucial as it links the unique chemical state of the textiles to their high activity in oxygen evolution reactions (OER), a key process in PEMWE.Our recent developments in nanostructured textile catalysts for PEMWE mark a significant leap in hydrogen production technology. By demonstrating high-rate hydrogen production at low operational voltages and with significantly lower Ir mass loadings compared to conventional CCMs, we have addressed major challenges in the field. These advancements not only enhance the efficiency and sustainability of hydrogen production but also open avenues for applying this technology to other electrochemical conversion cells, paving the way for broader applications and further research. In this talk, we discuss the latest activity on NST, and highlight the challenges and opportunities for hydrogen production.

Insights into the Structure and Activity of Bimetallic Au/Cu2O Catalysts during CO2 Electroreduction to C2 products

Despite its promising performance for C2+ product formation in electrochemical reduction of CO2 (CO2RR), Cu-based catalysts exhibit a relatively poor selectivity towards important chemical intermediates such as ethylene and ethanol. Up to 16 different products can typically be formed during CO2RR.1 Cu is the only element that can form C-C bonds at an appreciable rate.2 Therefore, it is critical to better understand the active sites of Cu-based electrocatalysts, as it can lead to better catalyst design with improved selectivity towards desired products. A common approach is to investigate the reactivity of well-defined surface terminations of Cu, which can be achieved by employing shape-controlled (nano)particles.3 Here, we expanded on such studies by evaluating the impact of Au on well-defined Cu2O surfaces for the electrochemical reduction of CO2. This promoter metal was chosen, because it can reduce CO2 to CO, which is widely accepted as the key intermediate for C-C coupling.2,4 In this study, 40 nm Cu2O nanocubes that predominantly expose the (100) facet were synthesized with a ligand-free method. The (100) facet is more active in the production of ethylene at the expense of unwanted methane formation, which takes place predominantly at the (111) facet.3 The surface of the nanocubes was decorated with Au nanoparticles by a galvanic replacement reaction. HAADF-STEM images of the catalysts with various Au loadings are shown in Figure 1a-d. After addition of Au nanoparticles, the shape of the Cu2O nanocubes was preserved. However, the Au nanoparticles were highly dispersed at low loadings (1 mol % Au) whereas Au clusters were observed at higher loadings (10 mol % Au). This result was in line with synchrotron X-ray diffraction (XRD) results, whereby the Au(111) reflection appears for the 10Au/Cu2O sample. Besides, the formation of a Au-Cu alloy and CuO phase was confirmed as well. The highly dispersed nature of the Au nanoparticles at low loadings was further established by Au 4f X-ray photoelectron spectroscopy (XPS), whereby a shift towards higher binding energies was observed as compared to the samples with higher Au loadings. The electrocatalytic performance of the catalysts was assessed over a range of potentials in neutral electrolyte (0.1 M KHCO3) in H-cell configuration. With highly dispersed Au as promotor, we found that the formation of ethylene and ethanol was significantly enhanced (Figure 1e-f). More specifically, the Faradaic effiency towards ethanol increased by roughly 1.5 times by introducing 1 mol % Au to the Cu2O nanocubes. Surprisingly, the 10Au/Cu2O sample displayed the lowest Faradaic efficiency for C2 products. We believe that the dispersion of Au on the Cu2O surface plays an important role in the product formation, whereby CO can be formed at Au sites and diffuse to Cu sites in proximity for further reduction to C2 products (CO spillover). To further study structure-activity relationships, we performed in-situ X-ray absorption spectroscopy (XAS) measurements. Figure 1g shows the normalized Cu K-edge X-ray absorption near-edge structure (XANES) of the Cu2O and Au/Cu2O samples in the final state. The evolution of the Cu(0), Cu(I) and Cu(II) fractions was followed by Multivariate Curve Resolution-Alternating Least Squares (MCR-ALS) analysis. This analysis revealed that the final oxidation state of the catalysts is different. The Cu(I)/Cu(0) ratio shows the following trend 1Au/Cu2O > 5Au/Cu2O > Cu2O > 10Au/Cu2O. Based on these observations, we hypothesize that the oxidation state of Cu affects the product distribution and in particular, the formation of oxygenates. The presence of Cu(I) species and the formation of Cu(0) species during CO2 electroreduction was further confirmed with in-situ XRD measurements. Finally, we performed quasi in-situ XPS measurements to study the reduction of the surface of the catalysts. Acknowledgements This publication is part of the research programme 'Reversible Large-scale Energy Storage' (RELEASE) with project number 17621 which is financed by the Dutch Research Council (NWO).

Rapid Synthesis of Ruthenium-Copper Nanocomposites as High-Performance Bifunctional Electrocatalysts for Electrochemical Water Splitting.

Development of high-performance, low-cost catalysts for electrochemical water splitting is key to sustainable hydrogen production. Herein, ultrafast synthesis of carbon-supported ruthenium-copper (RuCu/C) nanocomposites is reported by magnetic induction heating, where the rapid Joule's heating of RuCl3 and CuCl2 at 200A for 10s produces Ru-Cl residues-decorated Ru nanocrystals dispersed on a CuClx scaffold, featuring effective Ru to Cu charge transfer. Among the series, the RuCu/C-3 sample exhibits the best activity in 1m KOH toward both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), with an overpotential of only -23 and +270mV to reach 10mAcm-2, respectively. When RuCu/C-3 is used as bifunctional catalysts for electrochemical water splitting, a low cell voltage of 1.53V is needed to produce 10mAcm-2, markedly better than that with a mixture of commercial Pt/C+RuO2 (1.59V). In situ X-ray absorption spectroscopy measurements show that the bifunctional activity is due to reduction of the Ru-Cl residues at low electrode potentials that enriches metallic Ru and oxidation at high electrode potentials that facilitates the formation of amorphous RuOx. These findings highlight the unique potential of MIH in the ultrafast synthesis of high-performance catalysts for electrochemical water splitting.

Extended X-ray absorption spectroscopy using an ultrashort pulse laboratory-scale laser-plasma accelerator

Laser-driven compact particle accelerators can provide ultrashort pulses of broadband X-rays, well suited for undertaking X-ray absorption spectroscopy measurements on a femtosecond timescale. Here the Extended X-ray Absorption Fine Structure (EXAFS) features of the K-edge of a copper sample have been observed over a 250 eV window in a single shot using a laser wakefield accelerator, providing information on both the electronic and ionic structure simultaneously. This capability will allow the investigation of ultrafast processes, and in particular, probing high-energy-density matter and physics far-from-equilibrium where the sample refresh rate is slow and shot number is limited. For example, states that replicate the tremendous pressures and temperatures of planetary bodies or the conditions inside nuclear fusion reactions. Using high-power lasers to pump these samples also has the advantage of being inherently synchronised to the laser-driven X-ray probe. A perspective on the additional strengths of a laboratory-based ultrafast X-ray absorption source is presented.

Catalytic Water Electrolysis by Co-Cu-W Mixed Metal Oxides: Insights from X-ray Absorption Spectroelectrochemistry.

Mixed metal oxides (MMOs) are a promising class of electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Despite their importance for sustainable energy schemes, our understanding of relevant reaction pathways, catalytically active sites, and synergistic effects is rather limited. Here, we applied synchrotron-based X-ray absorption spectroscopy (XAS) to explore the evolution of the amorphous Co-Cu-W MMO electrocatalyst, shown previously to be an efficient bifunctional OER and HER catalyst for water splitting. Ex situ XAS measurements provided structural environments and the oxidation state of the metals involved, revealing Co2+ (octahedral), Cu+/2+ (tetrahedral/square-planar), and W6+ (octahedral) centers. Operando XAS investigations, including X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), elucidated the dynamic structural transformations of Co, Cu, and W metal centers during the OER and HER. The experimental results indicate that Co3+ and Cu0 are the active catalytic sites involved in the OER and HER, respectively, while Cu2+ and W6+ play crucial roles as structure stabilizers, suggesting strong synergistic interactions within the Co-Cu-W MMO system. These results, combined with the Tafel slope analysis, revealed that the bottleneck intermediate during the OER is Co3+ hydroperoxide, whose formation is accompanied by changes in the Cu-O bond lengths, pointing to a possible synergistic effect between Co and Cu ions. Our study reveals important structural effects taking place during MMO-driven OER/HER electrocatalysis and provides essential experimental insights into the complex catalytic mechanism of emerging noble-metal-free MMO electrocatalysts for full water splitting.

Tungsten speciation in hydrothermal fluids

Understanding the speciation and thermodynamic properties of aqueous tungsten (W) complexes under various conditions is essential for predicting W transport in hydrothermal fluids relevant to ore formation and geothermal systems. Although previous experimental and geochemical modelling studies have provided insights into W solubility in hydrothermal systems, a comprehensive molecular-level understanding of W in hydrothermal fluids remains elusive.In this study, we employed ab initio molecular dynamics (MD) simulations to determine the speciation and coordination geometries of W(VI) complexes in NaCl, NaHS, and NaF-bearing brines at temperatures up to 600 °C and pressures up to 2 kbar. These theoretical calculations were complemented by synchrotron in-situ X-ray Absorption Spectroscopy measurements of W(VI) in chloride-, sulfide-, and fluoride-rich solutions under pressures of 600 bar and temperatures ranging from 25 to 429 °C. The speciation and geometrical properties obtained from ab initio MD simulations are in reasonably good agreement with the in-situ X-ray Absorption Spectroscopy data. Our study reveals that W-Cl complexes are not stable, and W is transported as tungstates (H2WO4(aq), HWO4− and WO42−)in NaCl-rich fluids. In sulfur-rich fluids under near-neutral pH and reduced conditions (sulfide predominant), S2− ions gradually replace O2− in tungstates to form thiotungstate complexes (WO4-xSx2−, where x = 1, 2, 3, 4). The MD results suggest that fluoride (F−) plays a significant role in W transport by forming WO3F− and WO3F22− complexes, or their hydrated ions. We employed thermodynamic integration to determine the formation constants of the WO3F− and WO3F22− complexes at temperatures up to 600 °C and 2 kbar, and extrapolated these properties across a broader range of temperatures and pressures. This study underscores the significance of W-F complexes in W transportation in fluoride-bearing, acidic to neutral (pH < 8) hydrothermal fluids. In contrast, W is most likely transported as thiotungstate complexes in sulfur-bearing hydrothermal fluids within a neutral to alkaline pH range (e.g., pH 5–8.5 at 300 °C) under reduced (sulfide-stable) conditions in the Earth’s crust. Existing models for W transport in hydrothermal ore fluids need to consider the influence of W-F and thiotungstate species.

Epitaxial superlattices as a framework for stabilizing different LaNiO3 structures

Targeted material design is at the center of research efforts on epitaxial, complex oxide heterostructures. Due to the strong electron-lattice coupling, small structural modifications are often decisive for the resulting macroscopic, physical properties of artificially layered materials. Here we report on a detailed structural analysis of a set of differently stacked LaNiO3−LaGaO3 superlattices by transmission electron microscopy. We find that the relative thickness ratio of the two superlattice sublayers affects the structural network, resulting in different Ni–O bond lengths and Ni–O–Ni angles under the same epitaxial strain conditions. Whereas the bond length values depend mainly on the LaNiO3 layer thickness, bond angles are mainly influenced by the LaGaO3 layer thickness. The orbital polarization determined from x-ray absorption spectroscopy measurements shows that the superlattice with the smallest deviation of both values compared to bulk shows the highest orbital polarization. Therefore, the thickness ratio of the two superlattice components can be regarded as an additional effective tool to tune the functional properties of nickelates. Published by the American Physical Society 2024

Real-time and operando observation of intermediates on TiO2 photoelectrocatalysis by soft X-ray absorption spectroscopy

The oxygen evolution reaction (OER) on the TiO2 surface at the solid–liquid interface was observed in real-time and operando using fluorescence-yield X-ray absorption spectroscopy (XAS) in the soft X-ray region. The OER was observed during a potential sweep with UV light on or off, focusing on the oxygen K-edge XAS. Although conventional XAS measurements require long measurement times, the spectra during the reaction were obtained every 3 s in the present experiment via wavelength-dispersive XAS technique. An increase in the X-ray absorption intensity at approximately 533.8 eV was observed during the potential sweep only with UV light on. The observed spectral change is discussed in relation to the reaction mechanism of the OER. The present technique can be applied to a wide range of analyses of (photo-) electrocatalysis and electrochemical reactions at solid–liquid interfaces to observe their products and intermediates during the reaction.

Absorption Correction for 3D Elemental Distributions of Dental Composite Materials Using Laboratory Confocal Micro-X-ray Fluorescence Spectroscopy.

Confocal micro-X-ray fluorescence (micro-XRF) spectroscopy facilitates three-dimensional (3D) elemental imaging of heterogeneous samples in the micrometer range. Laboratory setups using X-ray tube excitation render the method accessible for diverse research fields but interpretation of results and quantification remain challenging. The attenuation of X-rays in composites depends on the photon energy as well as on the composition and density of the material. For confocal micro-XRF, attenuation severely impacts elemental distribution information, as the signal from deeper layers is distorted by superficial layers. Absorption correction and quantification of fluorescence measurements in heterogeneous composite samples have so far not been reported. Here, an absorption correction approach for confocal micro-XRF combining density information from microcomputed tomography (micro-CT) data with laboratory X-ray absorption spectroscopy (XAS) and synchrotron transmission measurements is presented. The energy dependency of the probing volume is considered during the correction. The methodology is demonstrated on a model composite sample consisting of a bovine tooth with a clinically used restoration material.

Co-Ni co-sputter deposited bimetallic thin film catalysts for alkaline hydrogen and oxygen evolution reactions

Co-Ni bi-metallic thin films with different stoichiometries have been deposited by magnetron co-sputtering technique on Ni foam (NF), Si and carbon paper substrates. The structural and morphological characterisations of these films have been done by X-ray diffraction, X-ray absorption spectroscopy and Field Emission Scanning Electron Microscope measurements. The performances of these films for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) have been evaluated through electrochemical measurements like Linear Sweep Voltammetry, Cyclic Voltammetry and Electrochemical Impedance Spectroscopy measurements. Operando X ray absorption near edge spectroscopy measurements have also been carried out during HER and OER to decipher the redox reaction mechanisms. It has been observed that the HER and OER performance of all the Co Ni bi-metallic thin films on NF are found to be better than that of bare NF. The CoNi sample with composition of 58.7 % Co and 41.3 % Ni shows the best catalytic activity and stability towards HER and OER reactions. Though the HER activity is not the best but is comparable to many results reported in the literature, however the catalyst shows excellent OER activity with overpotential of 214 mV for 10 mA/cm2 current density. This is much better than that of the OER benchmark catalyst RuO2 and most of those reported in the literature. The OER is kinetically slower than HER and requires more overpotential, hence a significant improvement in the catalytic OER activity will definitely enhance the overall water splitting activity and hydrogen production efficiency. Thus the Co-Ni bi-metallic catalyst prepared by an easily scalable and highly reproducible magnetron co-sputtering technique has significant potential to emerge out as a technologically viable water splitting catalyst.

Nanostructure and dynamics of N-truncated copper amyloid-β peptides from advanced X-ray absorption fine structure.

An X-ray absorption spectroscopy (XAS) electrochemical cell was used to collect high-quality XAS measurements of N-truncated Cu:amyloid-β (Cu:Aβ) samples under near-physiological conditions. N-truncated Cu:Aβ peptide complexes contribute to oxidative stress and neurotoxicity in Alzheimer's patients' brains. However, the redox properties of copper in different Aβ peptide sequences are inconsistent. Therefore, the geometry of binding sites for the copper binding in Aβ4-8/12/16 was determined using novel advanced extended X-ray absorption fine structure (EXAFS) analysis. This enables these peptides to perform redox cycles in a manner that might produce toxicity in human brains. Fluorescence XAS measurements were corrected for systematic errors including defective-pixel data, monochromator glitches and dispersion of pixel spectra. Experimental uncertainties at each data point were measured explicitly from the point-wise variance of corrected pixel measurements. The copper-binding environments of Aβ4-8/12/16 were precisely determined by fitting XAS measurements with propagated experimental uncertainties, advanced analysis and hypothesis testing, providing a mechanism to pursue many similarly complex questions in bioscience. The low-temperature XAS measurements here determine that CuII is bound to the first amino acids in the high-affinity amino-terminal copper and nickel (ATCUN) binding motif with an oxygen in a tetragonal pyramid geometry in the Aβ4-8/12/16 peptides. Room-temperature XAS electrochemical-cell measurements observe metal reduction in the Aβ4-16 peptide. Robust investigations of XAS provide structural details of CuII binding with a very different bis-His motif and a water oxygen in a quasi-tetrahedral geometry. Oxidized XAS measurements of Aβ4-12/16 imply that both CuII and CuIII are accommodated in an ATCUN-like binding site. Hypotheses for these CuI, CuII and CuIII geometries were proven and disproven using the novel data and statistical analysis including F tests. Structural parameters were determined with an accuracy some tenfold better than literature claims of past work. A new protocol was also developed using EXAFS data analysis for monitoring radiation damage. This gives a template for advanced analysis of complex biosystems.

A sub-100 nm thickness flat jet for extreme ultraviolet to soft X-ray absorption spectroscopy.

Experimental characterization of the structural, electronic and dynamic properties of dilute systems in aqueous solvents, such as nanoparticles, molecules and proteins, are nowadays an open challenge. X-ray absorption spectroscopy (XAS) is probably one of the most established approaches to this aim as it is element-specific. However, typical dilute systems of interest are often composed of light elements that require extreme-ultraviolet to soft X-ray photons. In this spectral regime, water and other solvents are rather opaque, thus demanding radical reduction of the solvent volume and removal of the liquid to minimize background absorption. Here, we present an experimental endstation designed to operate a liquid flat jet of sub-micrometre thickness in a vacuum environment compatible with extreme ultraviolet/soft XAS measurements in transmission geometry. The apparatus developed can be easily connected to synchrotron and free-electron-laser user-facility beamlines dedicated to XAS experiments. The conditions for stable generation and control of the liquid flat jet are analyzed and discussed. Preliminary soft XAS measurements on some test solutions are shown.

Photocatalytic Activity of BaAl2O4 for Water Purification.

Degradation of dyes under natural light sources is one of the most active research areas in basic science for greener technology. In this context, the photocatalytic activity of semiconductors has received massive attention in solving water treatment-related issues as these possess enormous potential for degrading organic impurities. Here, we report that barium aluminate (BaAl2O4, BAO), which has been extensively studied for photoluminescence applications, is found to be a highly potent candidate for photocatalytic activities. We have explored the degradation of dyes (meant for water purification) by using the photocatalytic properties of pure and Dy- and Yb-codoped BAO. Crystal structure, electron microscopy, and Raman analysis of the autocombustion-synthesized pure and codoped BAO samples revealed significant morphological changes such as increased particle size and stabilization of rod-like structures. UV-vis absorbance measurements confirm the presence of multiple bandgaps in the BAO samples, which is substantiated by X-ray absorption spectroscopy measurements. Photocatalytic degradation studies of methylene blue (MB) dye (with different catalyst concentrations, dopings, and MB dye concentrations) have been carried out by using BAO. The kinetics of the photocatalytic degradation measurements has been explained by the Boltzmann distribution function, and the fastest (in less than 40 min), with more than 99% degradation of MB impurity, is reported here for the first time in BAO compounds. Synthesized BAO samples show excellent cyclic stability, which is essential for their potential applications in environmental remediation. The trade-off between the enhancement of surface area and increased particle size is considered the key parameter for controlling the photocatalytic performance of the BAO catalyst after Dy and Yb codopings.

Unveiling the non-innocence of vanadium dopant in TiO2 nanocrystals for advanced energy storage and smart windows

Vanadium doped TiO2 NCs stand out as a promising candidate for energy storage applications due to its high electrical conductivity and redox properties. However, the thermodynamical behavior of the material under working conditions has not been explored and the reasons for its superior performance remain unlocked. This study explores the use of a combination of advanced in situ spectroscopy techniques, including x-ray absorption spectroscopy (XAS), spectro-electrochemistry (SEC), and electrochemical impedance spectroscopy (EIS) to provide unprecedented insights into the intricate electrochemical reaction mechanisms within these nanocrystals. Density functional theory calculations and EIS reveal the active role of substitutional V ions in the TiO2 anatase network as electron donors, enhancing surface charge and carrier density and improving pseudocapacitive properties. Cyclic voltammetry and in situ SEC reveal that V-doped TiO2 NCs exhibit significantly improved charge storage capacities, particularly in the pseudo-capacitance storage mechanism. In situ SEC and XAS analyses indicate that a more effective reduction of Ti4+ ions occurs during the electrochemical process in doped NCs, leading to higher charge capacitance and faster processes. Furthermore, in situ XAS measurements of the V K-edge revealed that the vanadium ions, beyond improving the redox behavior of the host, also actively participate in the reduction process. The significant changes in the V K-edge XANES and extended x-ray absorption fine structure spectra observed under reduction conditions can be ascribed to a change in the structure and oxidation state of the vanadium ions during the electrochemical reaction.