Improved Water Electrolysis Over Cobalt Oxyhydroxide by Carbonate Anion Adsorption: Direct Observation of Active Species Using Operando Hard/Soft X-Ray Absorption Spectroscopy.

Cobalt oxide electrocatalysts electrodeposited from carbonate-based electrolyte solutions (Co-C) and exhibiting high oxygen evolution reaction (OER) activity at neutral pH are investigated using operando hard/soft X-ray absorption fine structure (XAFS) analyses. Operando Co K-edge XAFS data indicate that the Co-C comprises a collection of CoOOH nanoclusters and that the Co in this material is oxidized to higher valence states upon applying an electrode potential. Operando C K-edge XAFS results show that carbonate anions are adsorbed on the CoOOH nanoclusters and that this phenomenon is promoted at the active potential for the OER.

Solvatochromism Observed in the X-ray Absorption Spectrum of Indole Dissolved in Water.

Current developments in X-ray absorption spectroscopy (XAS) for liquid samples in the water window demand a rigorous understanding of the interactions between molecules or solute-solvent interactions observed in the spectra. Meanwhile, a theoretical description of such effects, in addition to inner-shell excitations, remains controversial. The controversy is mainly over whether the orbitals should be optimized in the final states or whether the orbital optimizations can be expressed by dynamic electron correlation.

Oxygen K-edge inner-shell calculations of polymers in solutions realized by the extraction of local structures from molecular dynamics simulations.

Inner-shell quantum chemical calculations of large molecular systems, such as polymers and soft matter in solution, were performed to understand the phase transition dynamics of these systems using soft x-ray absorption spectroscopy (XAS). The molecular structures of 40-mer poly(N-isopropylacrylamide) (PNIPAM) chains in solutions were obtained using molecular dynamics simulations. The 5-mer PNIPAM chains with terminated H atoms, including the second coordination shells of the solvent methanol and water molecules, were extracted from the 40-mer PNIPAM chains in solutions.

Emission enhancement in a luminescent polychlorinated diphenylpyridylmethyl radical through coordination to silver(I).

A luminescent silver(I) complex containing a luminescent radical ligand was prepared for the first time. Coordination to Ag enhanced and red-shifted the radical-centered emission. This study demonstrates similar effects in the luminescence of the radical by complexation with group 11 d-metal ions.

Metal-ligand delocalization of iron and cobalt porphyrin complexes in aqueous solutions probed by soft X-ray absorption spectroscopy.

Metal-ligand delocalization of metal porphyrin complexes in aqueous solutions was investigated by analyzing the electronic structure of both the metal and ligand sides using soft X-ray absorption spectroscopy (XAS) at the metal L-edges and nitrogen K-edge, respectively. In the N K-edge XAS spectra of the ligands, the energies of the CN π* peaks of cobalt protoporphyrin IX (CoPPIX) are higher than iron protoporphyrin IX (FePPIX). The energy difference between the two lowest peaks in the XAS spectrum of CoPPIX is also larger than that of FePPIX.

Probing Isolated Water Molecules in Aqueous Acetonitrile Solutions Using Oxygen K-Edge X-ray Absorption Spectroscopy.

Oxygen K-edge X-ray absorption spectroscopy (XAS) of an aqueous acetonitrile solution exhibited a sharp peak at approximately 537 eV, which was similar to that of water vapor and was not observed in liquid water. The inner-shell spectra of isolated water molecules and water clusters of different sizes surrounded by acetonitrile molecules were obtained by extracting these water structures from the liquid structures of aqueous acetonitrile solutions, as calculated using molecular dynamics simulations. The sharp peak profiles of the O K-edge XAS spectra were derived not from water clusters but from isolated water molecules surrounded by acetonitrile molecules.

Mechanism of poly(-isopropylacrylamide) cononsolvency in aqueous methanol solutions explored oxygen K-edge X-ray absorption spectroscopy.

The cononsolvency mechanism of poly(-isopropylacrylamide) (PNIPAM), dissolving in pure methanol (MeOH) and water (HO) but being insoluble in MeOH-HO mixtures, was investigated by O K-edge X-ray absorption spectroscopy (XAS). The cononsolvency emerges from the aggregation of PNIPAM with MeOH clusters, leading to the collapse of the hydrophobic hydration of PNIPAM.

Experimental and Theoretical Study for Core Excitation of Firefly Luciferin in Carbon K-Edge Spectra.

Carbon (C) K-edge X-ray absorption spectra for firefly luciferin were measured and assigned using time-dependent density functional theoretical calculations for luciferin anion and dianion to elucidate the effect of hydroxy-group deprotonation. It was found that the C K-edge spectra for luciferin had four characteristic peaks. The effect of deprotonation of the hydroxy group appears in the energy difference of the first and second peaks of these spectra.

Stacking of a Cofacially Stacked Iron Phthalocyanine Dimer on Graphite Achieved High Catalytic CH Oxidation Activity Comparable to That of pMMO.

Numerous biomimetic molecular catalysts inspired by methane monooxygenases (MMOs) that utilize iron or copper-oxo species as key intermediates have been developed. However, the catalytic methane oxidation activities of biomimetic molecule-based catalysts are still much lower than those of MMOs. Herein, we report that the close stacking of a μ-nitrido-bridged iron phthalocyanine dimer onto a graphite surface is effective in achieving high catalytic methane oxidation activity.

Operando time-resolved soft x-ray absorption spectroscopy for photoexcitation processes of metal complexes in solutions.

Operando time-resolved soft x-ray absorption spectroscopy (TR-SXAS) is an effective method to reveal the photochemical processes of metal complexes in solutions. In this study, we have developed the TR-SXAS measurement system for observing various photochemical reactions in solutions by the combination of laser pump pulses with soft x-ray probe pulses from the synchrotron radiation. For the evaluation of the developed TR-SXAS system, we have measured nitrogen K-edge x-ray absorption spectroscopy (XAS) spectra of aqueous iron phenanthroline solutions during a photoinduced spin transition process.

Carbon K-edge x-ray absorption spectra of liquid alcohols from quantum chemical calculations of liquid structures obtained by molecular dynamics simulations.

For reproducing the carbon K-edge x-ray absorption spectra of liquid alcohols, inner-shell quantum chemical calculations based on the Hartree-Fock method were performed with the snapshots of the liquid structures obtained by molecular dynamics simulations. The C K-edge inner-shell spectrum of liquid ethanol (EtOH) was obtained by the summation of one thousand calculated spectra of EtOH molecules including neighbor EtOH molecules within the CH-CH distance of 6 Å. For the C K-edge inner-shell spectrum of liquid methanol (MeOH), we have calculated one thousand spectra of MeOH molecules including neighbor MeOH molecules within the CH-CH distance of 6 Å.

Hydrophobic Cluster Formation in Aqueous Ethanol Solutions Probed by Soft X-ray Absorption Spectroscopy.

Hydrophobic cluster structures in aqueous ethanol solutions at different concentrations have been investigated by soft X-ray absorption spectroscopy (XAS). In the O K-edge XAS, we have found that hydrogen bond structures among water molecules are enhanced in the middle-concentration region by the hydrophobic interaction of the ethyl groups in ethanol. In the C K-edge XAS, the lower energy features arise from a transition from the terminal methyl C 1s electron to an unoccupied orbital of 3s Rydberg character, which is sensitive to the nearest-neighbor intermolecular interactions.

X-ray absorption spectra of aqueous cellobiose: Experiment and theory.

The hydration structure of cellulose is very important for understanding the hydrolysis of cellulose at the molecular level. In this paper, we report a joint experimental and theoretical study on x-ray absorption spectroscopy (XAS) of aqueous cellobiose, a disaccharide unit of cellulose. In the experimental part, high resolution measurements of the carbon K-edge XAS spectra were taken.

Solvent Effects in the Ultraviolet and X-ray Absorption Spectra of Pyridazine in Aqueous Solution.

Electrostatic interaction of the solvent with the solute and fluctuations of the solvent configurations may make excitation energies of the solute different from those in the gas phase. These effects may dominate photoinduced or chemical reaction dynamics in solution systems and can be observed as shifts or broadening of peaks in absorption spectra. In this work, the nitrogen K-edge X-ray absorption spectra were measured for pyridazine in the gas phase and in aqueous solution.

Electrochemical Oxidation Enables Regioselective and Scalable α-C(sp)-H Acyloxylation of Sulfides.

A highly selective, environmentally friendly, and scalable electrochemical protocol for the construction of α-acyloxy sulfides, through the synergistic effect of self-assembly-induced C(sp)-H/O-H cross-coupling, is reported. It features exceptionally broad substrate scope, high regioselectivity, gram-scale synthesis, construction of complex molecules, and applicability to a variety of nucleophiles. Moreover, the soft X-ray absorption technique and a series of control experiments have been utilized to demonstrate the pivotal role of the self-assembly of the substrates, which indeed is responsible for the excellent compatibility and precise control of high regioselectivity in our electrochemical protocol.

Soft X-ray absorption spectroscopy in the low-energy region explored using an argon gas window.

The soft X-ray region below 200 eV is important for investigating chemical and biological phenomena since it covers K-edges of Li and B and L-edges of Si, P, S and Cl. Helium gas is generally used as the soft X-ray transmission window for soft X-ray absorption spectroscopy (XAS) under atmospheric conditions. However, the helium gas window cannot be applied to XAS in the low-energy region since transmitted soft X-rays mostly consist of high-order X-rays due to the low transmission of first-order X-rays.

Quantum Materials Exploration by Sequential Screening Technique of Heteroatomicity.

Subnanoparticles (SNPs) exhibit unique properties and functions due to their extremely small particle sizes which extend into the quantum scale. Although the synthesis of SNPs requiring precise control of atomicity and composition has not been accomplished, we recently developed an atom-hybridization method (AHM) that realizes such atomic-level control using a macromolecular template. As a next step in the quest for innovative quantum materials, the practical creation of functional subnanomaterials will become a central subject.

Photoelectron based soft x-ray detector for removing high order x rays.

Soft x-ray absorption spectroscopy (XAS) in the low energy region below 200 eV is important to investigate chemical and biological phenomena under an atmospheric condition since it covers K-edges of Li and B and L-edges of Si, P, S, and Cl. Monochromatic soft x rays usually include not only first order x rays but also high order x rays due to the high order diffraction of a plane grating monochromator. It is difficult to measure XAS in the low energy region under an atmospheric helium condition since the transmitted soft x rays mostly consist of the high order x rays due to the low transmission of the first order x rays.

Microheterogeneity in Aqueous Acetonitrile Solution Probed by Soft X-ray Absorption Spectroscopy.

Chemical processes in solution are influenced by microheterogeneity (MH), where two liquids seem to be mixed in a macroscopic scale but are microscopically inhomogeneous. We have investigated one of the simplest MH systems, aqueous acetonitrile solution, using soft X-ray absorption spectroscopy (XAS). Molecular interactions of acetonitrile were revealed by C and N K-edge XAS at different concentrations, and those of solvent water were separately revealed by O K-edge XAS.

Soft X-ray Absorption Spectroscopy of Liquids for Understanding Chemical Processes in Solution.

Soft X-ray absorption spectroscopy (XAS) involving excitation processes of a core electron to unoccupied states is an effective method to study local structures around excited C, N, and O atoms in liquid samples. Since soft X-rays are strongly absorbed by air and liquid itself, we have developed transmission-type liquid flow cells, where the absorbance of liquid samples can be easily reduced and optimized by controlling the liquid thickness. By using the transmission-mode XAS techniques, we have investigated local structures of several liquid samples such as concentration dependence of aqueous pyridine solutions and unexpected temperature-dependent structural changes in liquid benzene from the precise energy shift measurements in XAS spectra with the help of molecular dynamics simulation and inner-shell calculations.

Laminar flow in microfluidics investigated by spatially-resolved soft X-ray absorption and infrared spectroscopy.

The application of soft X-ray absorption spectroscopy (XAS) to liquid cells based on microfluidics for chemical state analysis of light elements is much more difficult than hard X-ray absorption since soft X-rays cannot deeply penetrate a microfluidic cell. In this study, we have newly developed a microfluidic cell for spatially resolved XAS, where a 100 nm thick SiN membrane is used for the measurement window to transmit soft X-rays for keeping the microfluidic flow at a width and depth of 50 µm. The π peak of pyridine near the N K-edge XAS shows characteristic energy shifts near the liquid-liquid interface in a laminar flow of pyridine and water.

Uncovering the Charge Transfer between Carbon Dots and Water by In Situ Soft X-ray Absorption Spectroscopy.

Carbon dots (CDs) exhibit outstanding physicochemical properties that render them excellent materials for various applications, often occurring in an aqueous environment, such as light harvesting and fluorescence bioimaging. Here we characterize the electronic structures of CDs and water molecules in aqueous dispersions using in situ X-ray absorption spectroscopy. Three types of CDs with different core structures (amorphous vs graphitic) and compositions (undoped vs nitrogen-doped) were investigated.

Temperature-Dependent Structural Changes in Liquid Benzene.

Benzene is the simplest aromatic molecule with intermolecular π-π interactions. Because ordered liquids are key structures used to study chemical and biological phenomena in the liquid state, ordered structures of benzene confined in nanopores have been extensively studied, whereas those in the liquid state are still unknown. In this study, we address fundamental questions regarding whether ordered structures of benzene are formed in the liquid state by using carbon K-edge X-ray absorption spectroscopy (XAS) as a sensitive local probe.

Interaction between Water and Alkali Metal Ions and Its Temperature Dependence Revealed by Oxygen K-Edge X-ray Absorption Spectroscopy.

Interaction between water molecules and alkali metal ions in aqueous salt solutions has been studied by the oxygen K-edge soft X-ray absorption spectroscopy (XAS) in transmission mode. In the measurement of several alkali halide aqueous solutions with different alkali chlorides (Li, Na, and K) and different sodium halides (Cl, Br, and I), the pre-edge component arising from the hydration water molecules shows a blue shift in peak energy as strongly depending on cations but not on anions. In the temperature dependent measurement, the pre-edge component arising from water molecules beyond the first hydration shell shows the same behavior as that of pure liquid water.

Impacts of Conformational Geometries in Fluorinated Alkanes.

Research of blood substitute formulations and their base materials is of high scientific interest. Especially fluorinated microemulsions based on perfluorocarbons, with their interesting chemical properties, offer opportunities for applications in biomedicine and physical chemistry. In this work, carbon K-edge absorption spectra of liquid perfluoroalkanes and their parent hydrocarbons are presented and compared.