Carbon Core Electron Spectra of Polycyclic Aromatic Hydrocarbons

Aromaticity profoundly affects molecular orbitals in polycyclic aromatic hydrocarbons. X-ray core electron spectroscopy has observed that carbon 1s−π* transitions can be broadened or even split in some polycyclic systems, although the origin of the effect has remained obscure. The π electrons in polycyclic systems are typically classified in the Clar model as belonging to either true aromatic sextets (similar to benzene) or isolated double bonds (similar to olefins). Here, bulk-sensitive carbon core excitation spectra are presented for a series of polycyclic systems and show that the magnitude of the 1s−π* splitting is determined primarily by the ratio of true aromatic sextets to isolated double bonds. The observed splitting can be rationalized in terms of ground state energetics as described by Hückel, driven by the π electron structure described by Clar. This simple model including only ground state energetics is shown to explain the basics physics behind the spectral evolution for a broad set of polycyclic aromatic hydrocarbons, although some residual deviations between this model and experiment can likely be improved by including a more detailed electronic structure and the core hole effect

Dioxygen Oxidation Cu(II) → Cu(III) in the Copper Complex of <i>cyclo</i>(Lys‑dHis-βAla-His): A Case Study by EXAFS and XANES Approach

A former spectroscopic study of Cu­(II) coordination by the 13-membered ring cyclic tetrapeptide <i>c</i>(Lys-dHis-βAla-His) (DK13), revealed the presence, at alkaline pH, of a stable peptide/Cu­(III) complex formed in solution by atmospheric dioxygen oxidation. To understand the nature of this coordination compound and to investigate the role of the His residues in the Cu­(III) species formation, Cu K-edge XANES, and EXAFS spectra have been collected for DK13 and two other 13-membered cyclo-peptides: the diastereoisomer <i>c</i>(Lys-His-βAla-His) (LK13), and <i>c</i>(Gly-βAla-Gly-Lys) (GK13), devoid of His residues. Comparison of pre-edge peak features with those of Cu model compounds, allowed us to get information on copper oxidation state in two of the three peptides, DK13 and GK13: DK13 contains only Cu­(III) ions in the experimental conditions, while GK13 binds only with Cu­(II). For LK13/Cu complex, EXAFS spectrum suggested and UV–vis analysis confirmed the presence of a mixture of Cu­(II) and Cu­(III) coordinated species. Theoretical XANES spectra have been calculated by means of the MXAN code. The good agreement between theoretical and experimental XANES data collected for DK13, suggests that the refined structure, at least in the first coordination shell around Cu, is a good approximation of the DK13/Cu­(III) coordination species present at strongly alkaline pH. All the data are consistent with a slightly distorted pyramidal CuN₄ unit, coming from the peptide bonds. Surprisingly, the His side-chains seemed not involved in the final, stable, Cu­(III) scaffold

Charge and Spin-State Characterization of Cobalt Bis(<i>o</i>‑dioxolene) Valence Tautomers Using Co Kβ X‑ray Emission and L‑Edge X‑ray Absorption Spectroscopies

The valence tautomeric states of Co­(phen)­(3,5-DBQ)₂ and Co­(tmeda)­(3,5-DBQ)₂, where 3,5-DBQ is either the semiquinone (SQ^–) or catecholate (Cat^2–) form of 3,5-di-<i>tert</i>-butyl-1,2-benzoquinone, have been examined by a series of cobalt-specific X-ray spectroscopies. In this work, we have utilized the sensitivity of 1s3p X-ray emission spectroscopy (Kβ XES) to the oxidation and spin states of 3d transition-metal ions to determine the cobalt-specific electronic structure of valence tautomers. A comparison of their Kβ XES spectra with the spectra of cobalt coordination complexes with known oxidation and spin states demonstrates that the low-temperature valence tautomer can be described as a low-spin Co^III configuration and the high-temperature valence tautomer as a high-spin Co^II configuration. This conclusion is further supported by Co L-edge X-ray absorption spectroscopy (L-edge XAS) of the high-temperature valence tautomers and ligand-field atomic-multiplet calculations of the Kβ XES and L-edge XAS spectra. The nature and strength of the magnetic exchange interaction between the cobalt center and SQ^– in cobalt valence tautomers is discussed in view of the effective spin at the Co site from Kβ XES and the molecular spin moment from magnetic susceptibility measurements

Deciphering Photochemical Reaction Pathways of Aqueous Tetrachloroauric Acid by X‑ray Transient Absorption Spectroscopy

Photolysis reaction pathways of [Au(III)Cl4]− in aqueous solution have been investigated by time-resolved X-ray absorption spectroscopy. Ultraviolet excitation directly breaks the Au–Cl bond in [Au(III)Cl4]− to form [Au(II)Cl3]− that becomes highly reactive within 79 ps. Disproportionation of [Au(II)Cl3]− generates [Au(I)Cl2]−, which is stable for ≤10 μs. In contrast, intense near-infrared lasers photolyze water to generate hydrated electrons, which then reduce [Au(III)Cl4]− to [Au(II)Cl3]− at 5 ns. Hydrated electrons further induce a chain reaction from [Au(II)Cl3]− to [Au(0)Cl]− by successively removing one Cl–. The zero-valency Au anions quickly polymerize and condense to form Au nanoparticles, which become the dominating product after 400 s. Our results reveal that the condensation of zero-valency Au starts with dimerization of gold clusters coordinated with chloride ions rather than direct condensation of pristine Au atoms

Structural and Chemical Evolution of Amorphous Nickel Iron Complex Hydroxide upon Lithiation/Delithiation

Development of novel electrode materials is essential to achieve high-performance lithium ion batteries. Here, we demonstrate that amorphous nickel iron complex hydroxides (Ni–Fe–OH) synthesized by a laser–chemical method can be used as a potential conversion anode material for lithium storage. Complementary characterizations, including ensemble-averaged X-ray absorption spectroscopy, spatially resolved electron energy-loss spectroscopy, and energy dispersive X-ray spectroscopy in a scanning transmission electron microscope, were performed to reveal the chemical and structural evolutions of the active hydroxide particles undergoing electrochemical cycling. The solid–electrolyte interphase (SEI) layer with a primary component of lithium fluoride (LiF) was found and remained robust on the particle surface during the charge/discharge processes, which suggests that the LiF-containing SEI layer plays a critical role in maintaining the stable capacity retention and good reversibility of the Ni–Fe–OH anode

Kβ Valence to Core X‑ray Emission Studies of Cu(I) Binding Proteins with Mixed Methionine – Histidine Coordination. Relevance to the Reactivity of the M- and H‑sites of Peptidylglycine Monooxygenase

Biological systems use copper as a redox center in many metalloproteins, where the role of the metal is to cycle between its +1 and +2 oxidation states. This chemistry requires the redox potential to be in a range that can stabilize both Cu­(I) and Cu­(II) states and often involves protein-derived ligand sets involving mixed histidine–methionine coordination that balance the preferences of both oxidation states. Transport proteins, on the other hand, utilize copper in the Cu­(I) state and often contain sites comprised predominately of the cuprophilic residue methionine. The electronic factors that allow enzymes and transporters to balance their redox requirements are complex and are often elusive due to the dearth of spectroscopic probes of the Cu­(I) state. Here we present the novel application of X-ray emission spectroscopy to copper proteins via a study of a series of mixed His-Met copper sites where the ligand set varies in a systematic way between the His₃ and Met₃ limits. The sites are derived from the wild-type peptidylglycine monooxygenase (PHM), two single-site variants which replicate each of its two copper sites (Cu_M-site and Cu_H-site), and the transporters CusF and CusB. Clear differences are observed in the Kβ_2,5 region at the Met₃ and His₃ limits. CusB (Met₃) has a distinct peak at 8978.4 eV with a broad shoulder at 8975.6 eV, whereas Cu_H (His₃) has two well-resolved features: a more intense feature at 8974.8 eV and a second at 8977.2 eV. The mixed coordination sphere CusF (Met₂His) and the PHM Cu_M variant (Met₁His₂) have very similar spectra consisting of two features at 8975.2 and 8977.8 eV. An analysis of DFT calculated spectra indicate that the intensity of the higher energy peak near 8978 eV is mediated by mixing of ligand-based orbitals into the Cu d¹⁰ manifold, with S from Met providing more intensity by facilitating increased Cu p–d mixing. Furthermore, reaction of WT PHM with CO (an oxygen analogue) produced the M site CO complex, which showed a unique XES spectrum that could be computationally reproduced by including interactions between Cu­(I) and the CO ligand. The study suggests that the valence-to-core (VtC) region can not only serve as a probe of ligand speciation but also offer insight into the coordination geometry, in a fashion similar to XAS pre-edges, and may be sufficiently sensitive to the coordination of exogenous ligands to be useful in the study of reaction mechanisms

Tuning Complex Transition Metal Hydroxide Nanostructures as Active Catalysts for Water Oxidation by a Laser–Chemical Route

Diverse transition metal hydroxide nanostructures were synthesized by laser-induced hydrolysis in a liquid precursor solution for alkaline oxygen evolution reaction (OER). Several active OER catalysts with fine control of composition, structure, and valence state were obtained including (Li_<i>x</i>)­[Ni_0.66Mn_0.34(OH)₂]­(NO₃)­(CO₃) · mH₂O, Li_<i>x</i>[Ni_0.67Co_0.33(OH)₂]­(NO₃)_0.25(ORO)_0.35 · mH₂O, etc. An operate overpotential less than 0.34 V at current density of 10 mA cm^–2 was achieved. Such a controllable laser–chemical route for assessing complex nanostructures in liquids opens many opportunities to design novel functional materials for advanced applications

Highly Active Surface Structure in Nanosized Spinel Cobalt-Based Oxides for Electrocatalytic Water Splitting

Spinel cobalt-based oxides are a promising family of materials for water splitting to replace currently used noble-metal catalysts. Identifying the highly active facet and the corresponding coordinated structure of surface redox centers is pivotal for the rational design of low-cost and efficient nanosized catalysts. Using high-resolution transmission electron microscopy and advanced X-ray techniques, as well as ab initio modeling, we found that the activity of Co³⁺ ions exhibits the surface dependence owing to the variability of its electronic configurations. Our calculation shows that the Co³⁺ site in {100} facet of nanosized Li₂Co₂O₄ exhibits an impressive intrinsic activity with low overpotential, far lower than that of the {110} and {111} facets. The unique, well-defined CoO₅ square-pyramidal structure in this nonpolar surface stabilizes the unusual intermediate-spin states of the Co³⁺ ion. Specially, we unraveled that oxygen ion anticipates the redox process via the strong hybridization Co 3d–O 2p state, which produces a 3d_<i>z</i>^₂1.1 filling orbit. Finally, a spin-correlated energy diagram as a function of Co–O distance was devised, showing that the covalency of Co–O significantly affects the spin state of Co³⁺ ions. We suggest that the nonpolar surface that contains CoO₅ units in the edge-sharing systems with the short Co–O bond distance is a potential candidate for alkaline water electrolysis

Electrochemical Oxidation of Size-Selected Pt Nanoparticles Studied Using in Situ High-Energy-Resolution X‑ray Absorption Spectroscopy

High-energy-resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) has been applied to study the chemical state of ∼1.2 nm size-selected Pt nanoparticles (NPs) in an electrochemical environment under potential control. Spectral features due to chemisorbed hydrogen, chemisorbed O/OH, and platinum oxides can be distinguished with increasing potential. Pt electro-oxidation follows two competitive pathways involving both oxide formation and Pt dissolution

Oxygen Release Induced Chemomechanical Breakdown of Layered Cathode Materials

Chemical and mechanical properties interplay on the nanometric scale and collectively govern the functionalities of battery materials. Understanding the relationship between the two can inform the design of battery materials with optimal chemomechanical properties for long-life lithium batteries. Herein, we report a mechanism of nanoscale mechanical breakdown in layered oxide cathode materials, originating from oxygen release at high states of charge under thermal abuse conditions. We observe that the mechanical breakdown of charged Li_1–<i>x</i>Ni_0.4Mn_0.4Co_0.2O₂ materials proceeds via a two-step pathway involving intergranular and intragranular crack formation. Owing to the oxygen release, sporadic phase transformations from the layered structure to the spinel and/or rocksalt structures introduce local stress, which initiates microcracks along grain boundaries and ultimately leads to the detachment of primary particles, <i>i.e.</i>, intergranular crack formation. Furthermore, intragranular cracks (pores and exfoliations) form, likely due to the accumulation of oxygen vacancies and continuous phase transformations at the surfaces of primary particles. Finally, finite element modeling confirms our experimental observation that the crack formation is attributable to the formation of oxygen vacancies, oxygen release, and phase transformations. This study is designed to directly observe the chemomechanical behavior of layered oxide cathode materials and provides a chemical basis for strengthening primary and secondary particles by stabilizing the oxygen anions in the lattice