Dental caries is the most prevalent oral disease that causes structural and compositional changes of the dental hard tissues due to a chronic demineralisation (combined with possible phases of remineralisation) process. Though considerable efforts have been directed at studying natural and artificial carious lesions, most characterisations remain either constrained to 2D analyses or have been unable to achieve fine resolution in 3D due to limited field of view. To overcome this challenge, the present study combined X-ray diffraction (XRD) and scanning transmission X-ray microscopy (STXM) tomography techniques to analyse the mineral density, scattering intensity, and crystallite size in normal, carious, 30 % artificially demineralised, and 50 % artificially demineralised dentine. Combined XRD and STXM tomography was performed on the I18 beamline at Diamond Light Source, using a 15 keV monochromatic beam with 2 × 2 µm spotsize and scanning with translation steps of 2 µm, providing a reconstructed voxel size of 2 × 2 × 2 µm. Natural carious dentine showed a reduction in hydroxyapatite (HAp) crystallite size due to chronic demineralisation. This was unlike artificially demineralised dentine samples that underwent short, continuous demineralisation, which created a zone of fully demineralised dentine, near the sample surface, and a zone of partially demineralised dentine that had a reduced mineral density but an increased average crystallite size.

The application of high-temperature proton exchange membrane fuel cells (HT-PEMFCs) addresses challenges in water management, fuel purity, and overheating under high current density. However, phosphoric acid (PA) migration hinders their development. This study uses synchrotron-based X-ray fluorescence spectroscopy to investigate PA and catalyst migration. Interventions with single-layer graphene and electrochemically exfoliated graphene oxide improve performance and durability. X-ray absorption spectroscopy provides insights into relevant mechanisms, advancing understanding of membrane electrode assembly preparation and the intricate influences of PA and catalyst migration on performance and durability in HT-PEMFCs.

PEO-LiX solid polymer electrolyte (SPE) with the addition of Li6.4La3Zr1.4Ta0.6O12 (LLZTO) fillers is considered as a promising solid-state electrolyte for solid-state Li-ion batteries. However, the developments of the SPE have caused additional challenges, such as poor contact interface and SPE/Li interface stability during cycling, which always lead to potentially catastrophic battery failure. The main problem is that the real impact of LLZTO fillers on the interfacial properties between SPE and Li metal is still unclear. Herein, we combined the electrochemical measurement and in situ synchrotron-based X-ray absorption near-edge structure (XANES) imaging technology to study the role of LLZTO fillers in directing SPE/Li interface electrochemical performance. In situ XRF-XANES mapping during cycling showed that addition of an appropriate amount of LLZTO fillers (50 wt %) can improve the interfacial contact and stability between SPE and Li metal without reacting with the PEO and Li salts. Additionally, it also demonstrated the beneficial effect of LLZTO particles for suppressing the interface reactions between the Li metal and PEO-LiTFSI SPE and further inhibiting Li-metal dendrite growth. The Li|LiFePO4 batteries deliver long cycling for over 700 cycles with a low-capacity fade rate of 0.08% per cycle at a rate of 0.3C, revealing tremendous potential in promoting the large-scale application of future solid-state Li-ion batteries.

Producing green hydrogen in a cost-competitive manner via water electrolysis will make the long-held dream of hydrogen economy a reality. Although platinum (Pt)-based catalysts show good performance toward hydrogen evolution reaction (HER), the high cost and scarce abundance challenge their economic viability and sustainability. Here, a non-Pt, high-performance electrocatalyst for HER achieved by engineering high fractions of stacking fault (SF) defects for MoNi4/MoO2 nanosheets (d-MoNi) through a combined chemical and thermal reduction strategy is shown. The d-MoNi catalyst offers ultralow overpotentials of 78 and 121 mV for HER at current densities of 500 and 1000 mA cm-2 in 1 M KOH, respectively. The defect-rich d-MoNi exhibits four times higher turnover frequency than the benchmark 20% Pt/C, together with its excellent durability (> 100 h), making it one of the best-performing non-Pt catalysts for HER. The experimental and theoretical results reveal that the abundant SFs in d-MoNi induce a compressive strain, decreasing the proton adsorption energy and promoting the associated combination of *H into hydrogen and molecular hydrogen desorption, enhancing the HER performance. This work provides a new synthetic route to engineer defective metal and metal alloy electrocatalysts for emerging electrochemical energy conversion and storage applications.

Producing green hydrogen in a cost-competitive manner via water electrolysis will make the long-held dream of meeting energy needs with hydrogen instead of fossil fuel a reality. Although platinum-based catalysts show good performance towards hydrogen evolution reaction (HER), the high cost and scarce abundance challenge their economic viability for large-scale implementation. Here, we engineer high fractions of stacking fault defects in MoNi nanosheets to form a high-performance electrocatalyst (d-MoNi) through a combined chemical and thermal reduction strategy. Using d-MoNi as an electrode for HER afforded ultralow overpotential of 63 and 120 mV at current densities of -500 and -1000 mA cm-2 in 1 M KOH, respectively. Besides, the stacking fault defect-rich d-MoNi exhibits 4 orders of magnitude higher turnover frequency than benchmark 20% Pt/C, together with excellent durability (>100 h) at high current density of 1000 mA cm-2, making it one of the best-performing non-platinum catalysts for HER. The experimental and theoretical results reveal that the abundant stacking faults in d-MoNi induce a compressive strain, decreasing the proton adsorption energy and promoting the concomitant combination of adsorbed hydrogen into molecular hydrogen and the desorption of the molecular hydrogen, therefore enhancing the HER performance. This work provides a new synthetic route to engineer defective metal and metal alloy electrocatalysts for emerging applications in electrochemical energy conversion and storage.

Using operando X-ray absorption spectroscopy in a continuous-flow microfluidic cell, we have investigated the nucleation of platinum nanoparticles from aqueous hexachlor-oplatinate solution in the presence of the reducing agent ethylene glycol. By adjusting flow rates in the microfluidic channel, we resolved the temporal evolution of the reaction system in the first few seconds, generating the time profiles for speciation, ligand exchange, and reduction of Pt. Detailed analysis of the X-ray absorption near-edge structure and extended X-ray absorption fine structure spectra with multivariate data analysis shows that at least two reaction intermediates are involved in the transformation of the precursor H2PtCl6 to metallic platinum nanoparticles, including the formation of clusters with Pt-Pt bonding before complete reduction to Pt nanoparticles.

X-ray absorption spectroscopy (XAS) has become a very important analytical method for studying reactions under operating conditions and the method relies heavily on the use of tunable, high intensity X-ray radiation generated at synchrotron and X-ray Free Electron Laser (XFEL) facilities. Advances in methodology and instrumentation mean that X-ray absorption spectra can be measured in short time periods and can be used to probe catalytic reactions on a microsecond timescale. In this chapter the focus is TRansient X-ray Absorption Spectroscopy (TR-XAS) which operates effectively on submicrosecond timescales and is targeted at elucidating structural changes in molecular systems under photoactivation. The experimental set ups combine the use of laser pulses (to pump the reactions) with the X-ray pulses from the synchrotron (to probe the activated species) are described as well as the best methods to prepare and deliver the chemical samples into the reaction chamber. Recent results showing the benefits of using XAS in establishing the nature of catalytic processes are highlighted for a range of transition metal-based catalytic systems.

The study aims to understand the role of the transient bonding in the interplay between the structural and electronic changes in heteroleptic Cu(i) diimine diphosphine complexes. This is an emerging class of photosensitisers which absorb in the red region of the spectrum, whilst retaining a sufficiently long excited state lifetime. Here, the dynamics of these complexes are explored by transient absorption (TA) and time-resolved infrared (TRIR) spectroscopy, which reveal ultrafast intersystem crossing and structural distortion occurring. Two potential mechanisms affecting excited state decay in these complexes involve a transient formation of a solvent adduct, made possible by the opening up of the Cu coordination centre in the excited state due to structural distortion, and by a transient coordination of the O-atom of the phosphine ligand to the copper center. X-ray absorption studies of the ground electronic state have been conducted as a prerequisite for the upcoming X-ray spectroscopy studies which will directly determine structural dynamics. The potential for these complexes to be used in bimolecular applications is confirmed by a significant yield of singlet oxygen production.

Nickel‐catalyzed cross‐coupling reactions have become a powerful methodology to construct C‐heteroatom bonds. However, many protocols suffer from competitive off‐cycle reaction pathways and require non‐equimolar amounts of coupling partners to suppress them. Here, we report on mechanistic examination of carboxylate O ‐arylation under thermal conditions, in both the presence and absence of an exogeneous bipyridine‐ligand. Furthermore, spectroscopic studies of the novel ligand‐free carboxylate O ‐arylation reaction unveiled the resting state of the nickel catalyst, the crucial role of the alkylamine base and the formation of an off‐cycle Ni I −Ni II dimer upon reduction. This study provides insights into the competition between productive catalysis and deleterious pathways (comproportionation and protodehalogenation) in the commonly proposed self‐sustained Ni I /Ni III catalytic cycle. Thereby we show that for productive nickel‐catalyzed carboxylate O ‐arylation a choice must be made between either mild conditions or equimolar ratios of substrates.

The Cover Feature shows the novel exogeneous ligand-free nickel-catalyzed coupling of aryl halides and carboxylic acids. In their Research Article, J. N. H. Reek, M. Tromp, T. J. Korstanje and co-workers provide insight into the competition between productive catalysis and deleterious pathways in the commonly proposed self-sustained NiI/NiIII catalytic cycle. Spectroscopic studies of the exogeneous ligand-free carboxylate O-arylation reaction unveiled the resting state of the nickel catalyst, the crucial role of the alkylamine base and the formation of an off-cycle NiINiII dimer upon reduction. More information can be found in the Research Article by J. N. H. Reek, M. Tromp, T. J. Korstanje and co-workers.

Nickel-catalyzed cross-coupling reactions have become a powerful methodology to construct C–heteroatom bonds. How-ever, many protocols suffer from competitive off-cycle reaction pathways and require non-equimolar amounts of cou-pling partners to suppress them. Here, we report on mechanistic examination of carboxylate O-arylation under thermal conditions, in both the presence and absence of an exogeneous bipyridine-ligand. Furthermore, spectroscopic studies of the novel ligand-free carboxylate O-arylation reaction unveiled the resting state of the nickel catalyst, the crucial role of the alkylamine base and the formation of a catalytically relevant NiI–NiII dimer upon reduction. This study provides in-sights into the competition between productive catalysis and deleterious pathways (comproportionation and pro-todehalogenation) that exist for all elementary steps in the commonly proposed self-sustained NiI/NiIII catalytic cycle. Thereby we show that for productive nickel-catalyzed carboxylate O-arylation a choice must be made between either mild conditions or equimolar ratios of substrates.

CO2 utilisation as a chemical feedstock could transform fuels production and help mitigate climate change. Direct CO2 reduction for energy production requires the development of active, stable, and low-cost catalysts selective for methane. A bimetallic Ni@Rh core-shell catalyst prepared by galvanic replacement (GR) exhibits a 3.5-fold rate enhancement for CO2 methanation relative to an analogue prepared by chemical reduction (CR) and is twice as active as monometallic Rh/Al2O3. Superior performance of RhNi/Al2O3 (GR) is attributed to Rh dispersion as an atomically thin RhOx shell encapsulating Ni nanoparticles, stabilised by a strong Rh-Ni interaction. Operando IR spectroscopy identifies reactively-formed CO from the dissociative chemisorption of CO2 over Rh as the key intermediate for methane production. Surface formate from the dissociative chemisorption of CO2 and subsequent hydrogenation (via spillover from Rh sites) over alumina is a catalytic spectator. This mechanistic insight paves the way to high activity nanostructured catalysts for CO2 methanation.

Core/shell nanocrystals with a graded interface between core and shell exhibit improved optoelectronic properties compared with particles with an abrupt, sharp interface. Material gradients mitigate interfacial defects and define the shape of the confinement potential. So far, few works exist that allow to quantify the width of the gradient. In this study, ZnSe/CdS nanocrystals with graded shells made at different temperatures are characterized using extended X-ray absorption fine structure (EXAFS) and Raman spectroscopies. The average coordination number of the probed element with respect to the two possible counterions is fit to a simple, geometric model. It is shown that at the lower temperature limit for shell growth (260 °C), substantial interfacial alloying can be attributed mainly to cation migration. At higher temperature (290 °C), strain minimization leads to atomic ordering of the metal ions and an anomalously low degree of phase mixing.

The structural form and elemental distribution of material originating from different Fukushima Daiichi Nuclear Power Plant reactors (Units 1 and 3) is hereby examined to elucidate their contrasting release dynamics and the current in-reactor conditions to influence future decommissioning challenges. Complimentary computed X-ray absorption tomography and X-ray fluorescence data show that the two suites of Si-based material sourced from the different reactor Units have contrasting internal structure and compositional distribution. The known event and condition chronology correlate with the observed internal and external structures of the particulates examined, which suggest that Unit 1 ejecta material sustained a greater degree of melting than that likely derived from reactor Unit 3. In particular, we attribute the near-spherical shape of Unit 1 ejecta and their internal voids to there being sufficient time for surface tension to round these objects before the hot (and so relatively low viscosity) silicate melt cooled to form glass. In contrast, a more complex internal form associated with the sub-mm particulates invoked to originate from Unit 3 suggest a lower peak temperature, over a longer duration. Using volcanic analogues, we consider the structural form of this material and how it relates to its environmental particulate stability and the bulk removal of residual materials from the damaged reactors. We conclude that the brittle and angular Unit 3 particulate are more susceptible to further fragmentation and particulate generation hazard than the round, higher-strength, more homogenous Unit 1 material.

The structure of a highly active pyridine-alkoxide iridium water oxidation catalyst (WOC) is examined by X-ray absorption spectroscopy (XAS). A detailed comparison with IrO2 points to a rigid molecular unit of low nuclearity, with the best analysis suggesting a novel tetrameric iridium-oxo cubane as the resting state.

The suitability of N-ethylpyridinium bromide as a potential electrolyte additive for zinc electrodeposition in aqueous solution has been investigated by a combination of cyclic voltammetry, bulk electrolysis, scanning electron microscopy, synthesis and NMR spectroscopy. The current magnitudes of the Zn/Zn(II) redox reaction observed at the electrode decreased dramatically after a single deposition/stripping cycle in the presence of N-ethylpyridinium bromide at switching potentials more negative than -1.30 V vs Ag/AgCl. This is attributed to the reduction of the N-ethylpyridinium cation to form a pyridyl radical that subsequently and rapidly dimerizes. The proposed dimer was synthesized and shown to have a low solubility in aqueous solutions, consistent with a passivation of the electrode surface by precipitation and inhibition of redox behavior. Based on this understanding for the pyridinium cation reduction, alternative pyridinium-based additives have been examined in order to determine their influence on zinc electrodeposition. (C) 2019 The Electrochemical Society.

Carbon nitride polymeric semiconductors with nanorod morphology are explored as photocatalysts for efficient hydrogen evolution from water.

Complexes with weakly coordinating ligands are often formed in chemical reactions and can play key roles in determining the reactivity, particularly in catalytic reactions. Using time-resolved X-ray absorption fine structure (XAFS) spectroscopy in combination with time-resolved IR (TRIR) spectroscopy and tungsten hexacarbonyl, W(CO)6, we are able to structurally characterize the formation of an organometallic alkane complex, determine the W-C distances, and monitor the reactivity with silane to form an organometallic silane complex. Experiments in perfluorosolvents doped with xenon afford initially the corresponding solvated complex, which is sufficiently reactive in the presence of Xe that we can then observe the coordination of Xe to the metal center, providing a unique insight into the metal-xenon bonding. These results offer a step toward elucidating the structure, bonding, and chemical reactivity of transient species by X-ray absorption spectroscopy, which has sensitivity to small structural changes. The XAFS results indicate that the bond lengths of metal-alkane (W-H-C) bond in W(CO)5(heptane) as 3.07 (±0.06) Å, which is longer than the calculated W-C (2.86 Å) for binding of the primary C-H, but shorter than the calculated W-C (3.12 Å) for the secondary C-H. A statistical average of the calculated W-C alkane bond lengths is 3.02 Å, and comparison of this value indicates that the value derived from the XAFS measurements is averaged over coordination of all C-H bonds consistent with alkane chain walking. Photolysis of W(CO)6 in the presence of HSiBu3 allows the conversion of W(CO)5(heptane) to W(CO)5(HSiBu3) with an estimated W-Si distance of 3.20 (±0.03) Å. Time-resolved TRIR and XAFS experiments following photolysis of W(CO)6 in perfluoromethylcyclohexane (PFMCH) allows the characterization of W(CO)5(PFMCH) with a W-F distance of 2.65 (±0.06) Å, and doping PFMCH with Xe allows the characterization of W(CO)5Xe with a W-Xe bond length of 3.10 (±0.02) Å.

The rational design of low-cost transition metal catalysts that exhibit high activity and selectivity may be the most significantly investigated in heterogeneous catalysis. In this study, Co3-xMnxO4 (x = 0.75, 1.0, and 1.5) mixed metal oxides were successfully synthesized by a controlled template-free autoclave strategy and studied for toluene oxidation. It is found that the Co-rich sample showed markedly enhanced activity and the 3D dandelion-like Co2.25Mn0.75O4 catalyst exhibited in the highest toluene oxidation rate (8.9 mu mol/(g(cat)s)) and a 100% toluene conversion at 239 degrees C. In situ DRIFTS study indicates that toluene was sequentially oxidized to benzyl radical, benzaldehyde, benzene, oxalic acid, and finally to CO2 and H2O. The interaction between Co and Mn, in conjunction with the high concentration of surface oxygen species and rich surface oxygen vacancies, reasonably explains the elevated catalytic activity and thermal stability for toluene oxidation over 3D flower-like Co3-xMnxO4 spinel catalysts.