Research Highlights
Probing Isolated Water Molecules in Aqueous Acetonitrile Solutions Using Soft 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. The electronic structures of the isolated water molecules can be analyzed using O K-edge XAS spectra, separating the contributions of small water clusters.
Mechanism of poly(N-isopropylacrylamide) cononsolvency in aqueous methanol solutions explored via oxygen K-edge X-ray absorption spectroscopy
The cononsolvency mechanism of poly(N-isopropylacrylamide) (PNIPAM), dissolving in pure methanol (MeOH) and water but being insoluble in aqueous MeOH solutions, were investigated by oxygen K-edge X-ray absorption spectroscopy with theoretical calculations executed in molecular dynamics simulations and inner-shell calculations. It was found that the cononsolvency emerges from the aggregation of PNIPAM with MeOH clusters owing to the hydrophobic interactions between PNIPAM and MeOH clusters leading to the collapse of the hydrophobic hydration of PNIPAM.
[1] M. Nagasaka et al., Phys. Chem. Chem. Phys. 26, 13634 (2024).
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 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. The decay process of the high spin state to the low spin state in the iron complex has been obtained from the ligand side by N K-edge XAS, and the time constant is close to that obtained from the central metal side by time-resolved Fe K-edge XAS in the previous studies.
[1] F. Kumaki, M. Nagasaka et al., J. Chem. Phys. 158, 104201 (2023).
Carbon K-edge X-ray absorption spectra of liquid alcohols from quantum chemical calculations of liquid structures obtained by molecular dynamics simulations
For reproducing carbon K-edge X-ray absorption spectra of liquid alcohols, the 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 CH2-CH2 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 CH3-CH3 distance of 6 Å. The calculated C K-edge inner-shell spectra of liquid alcohols well reproduced the spectral shapes of the experimentally obtained X-ray absorption spectra and the spectral changes from gas to liquid phases.
Site selective analysis of water in hydrogen bond network of aqueous dimethyl sulfoxide solutions by oxygen K-edge X-ray absorption spectroscopy
Hydrogen bond (HB) network in aqueous dimethyl sulfoxide (DMSO) solutions at different concentrations has been observed by oxygen K-edge X-ray absorption spectroscopy with a site selective analysis that separates donor and acceptor sites of water (H2O), where the S=O π* peak in DMSO reflects the donor site of H2O and the 4a1 peak in H2O reflects the acceptor site of H2O, respectively. From the comparison of the XAS experiments with molecular dynamics simulations and inner-shell quantum chemical calculations, we have revealed that the HB network in aqueous DMSO solutions is influenced with not only the HB interaction of the S=O group with the donor site of H2O but also the dipole interaction of the S atom with the acceptor site of H2O, which breaks the HB network between H2O. Four concentration regions were found in the HB network of aqueous DMSO solutions, which would be related to the anomalies of physical properties and solvent effects in chemical and biological reactions.
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. On the other hand, 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. From the comparison of C K-edge XAS with the inner-shell calculations, we have found that ethanol clusters are easily formed in the middle concentration region due to the hydrophobic interaction of the ethyl group in ethanol, resulting in the enhancement of the hydrogen bond structures among water molecules. This behavior is different from aqueous methanol solutions, where the methanol ‒ water mixed clusters are more predominant in the middle concentration region due to the relatively weak hydrophobic interactions of the methyl group in methanol.
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. In this study, we have developed a photoelectron based soft x-ray (PBSX) detector, where the Au 4f photoelectrons emitted by the first order x rays are separated from those by the high order x rays using a difference in kinetic energies of photoelectrons. By using the PBSX detector, we have successfully obtained Si L-edge XAS spectra of the SiC and polymer/SiC films that mainly include the first order x rays by removing the major contributions of the second order x rays at the C K-edge and the fifth order x rays at the O K-edge.
[1] M. Nagasaka and H. Iwayama, Rev. Sci. Instrum. 91, 083103 (2020).
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. In this study, the argon gas window is proposed as a new soft X-ray transmission window in the low-energy region. High-order X-rays are removed by the absorption of the Ar L-edge (240 eV), and first-order X-rays become the major contribution of transmitted soft X-rays in the low-energy region. From the soft X-ray transmission spectra, we confirmed that the argon gas window is effective for soft X-ray transmission in the low-energy region from 60 eV to 240 eV.
Microheterogeneity in Aqueous Acetonitrile Solution Probed by Soft X-ray Absorption Spectroscopy
Chemical processes in solution are influenced with 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, by using soft X-ray absorption spectroscopy (XAS). Molecular interactions of acetonitrile were revealed by the C and N K-edge XAS at different concentrations, and those of solvent water were separately revealed by the O K-edge XAS. The peak energy shift at the C K-edge shows three characteristic concentration regions and a phase transition-like behavior between them. By comparing the energy shifts in XAS spectra with ab initio quantum chemical inner-shell calculations, we have determined local structures of acetonitrile-water mixtures at three concentration regions and found that the dipole interaction (DP) between acetonitrile and water is the key structure to emerge the MH state in the middle concentration region. The MH state continues when the DP interaction is predominant over the hydrogen bond (HB) interactions.
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 penetrate deeply a microfluidic cell. In this study, we have newly developed a microfluidic cell for spatially-resolved XAS, where a 100 nm thick Si3N4 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 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. The distributions of the molar fractions of pyridine and water near the liquid-liquid interface have been determined from the energy shifts probed at different geometric positions, where pyridine is mixed in the water part of the laminar flow and vice versa. The spatial distribution of both species has also been studied by infrared microscopy, using the same microfluidic setup. The present work clearly shows that these spectroscopic techniques are easily applicable to chemical and biological reactions prepared by microfluidics.
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 as a sensitive local probe. The π* peak in liquid benzene unexpectedly shows an opposite temperature behavior, approaching the solid peak apart from the gaseous benzene with increasing temperature. In the radial distribution function of parallel structures in molecuar dynamics simulation of liquid benzene, the components of small intermolecular distance become abundant by increasing temperature. This is rationalized by inner-shell calculations providing insights that structural changes from parallel displaced structures to sandwich (parallel nondisplaced) structures cause the unexpected temperature-dependent spectral shift of the π* peak. These results are confirmed by infrared spectroscopy with additional support of vibrational mode calculations. From these consistent results we anticipate that there are temperature-dependent changes of ordered structures of benzene in the liquid state that may affect the mechanisms of chemical and biological phenomena.
[1] M. Nagasaka et al., J. Phys. Chem. Lett. 9, 5827 (2018).
Molecular Interactions of Pyridine in Liquid Phase and Aqueous Solution Studied by Soft X-ray Absorption Spectroscopy
Molecular interactions of pyridine in liquid and in aqueous solution are studied by soft X-ray absorption spectroscopy (XAS) at the C, N, and O K-edges. XAS of liquid pyridine shows that the N 1s → π* peak is blue shifted and the C 1s → π* peak of the meta and para sites is red shifted, respectively, as compared with XAS of pyridine gas. These C and N 1s → π* shifts in liquid are smaller than those in clusters, indicating that the interaction of liquid pyridine is weaker than that of pyridine cluster, as supported by the combination of quantum chemical calculations and molecular dynamics simulations. On the other hand, XAS spectra of aqueous pyridine solutions (C5H5N)x(H2O)1−x measured at different molar fractions show that in the pyridine rich region, x > 0.7, the π* peak energies are not so different from pure liquid pyridine (x = 1.0). In this region, antiparallel displaced structures of pyridine molecules are dominant as in pure pyridine liquid. In the O K-edge XAS, the pre-edge peaks sensitive to the hydrogen bond (HB) network of water molecules show the red shift of −0.15 eV from that of bulk water, indicating that small water clusters with no large-scale HB network are formed in the gap space of structured pyridine molecules. In the water rich region, 0.7 > x, the N peaks and the O 1s pre-edge peaks are blue shifted, and the C peaks of the meta and para sites are red-shifted by increasing molar fraction of water. The HB network of bulk water is dominant, but quantum chemical calculations indicate that small pyridine clusters with the HB interaction between the H atom in water and the N atom in pyridine are still existent even in very dilute pyridine solutions.
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 X-ray absorption spectroscopy (XAS). 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. On the other hand, the pre-edge component arising from water molecules in the first hydration shell of Li+ ions is not evidently dependent on the temperature, indicating that the hydration water molecules are more strongly bound with Li+ ions than the other water molecules. These experimental results are supported by the results of radial distribution functions of the first hydration shell evaluated by molecular dynamics simulations.
Reliable Absorbance Measurement of Liquid Samples by Soft X-ray Absorption Spectroscopy in Transmission Mode
In order to measure reliable absorbance of liquid samples by soft X-ray absorption spectroscopy (XAS), it is necessary to optimize the thickness of thin liquid layers and keep the sample thickness flat within a photon beam. In this study, we have developed a liquid flow cell for XAS of liquid samples in transmission mode. A thin liquid layer is sandwiched between two 100 nm thick Si3N4 membranes, and the liquid thickness is optimized by using the elasticity of the membranes under controlling the helium pressure. The flatness of the liquid sample within a photon beam is investigated with measuring position-dependent O K-edge XAS spectra of liquid water. The pre-edge vs. main edge ratio is 0.38 for the center of the sample area and becomes considerably larger than 0.38 for the off-center position. The deviation from 0.38 is caused by the inhomogeneous thickness and is evaluated in comparison with a model simulation. Positioning of as small a beam as possible on the center of the liquid sample area is essential to obtain reliable XAS spectra.
[1] M. Nagasaka et al., J. Electron Spectrosc. Relat. Phenom. 224, 93 (2018).
Operando Observation System for Electrochemical Reaction by Soft X-ray Absorption Spectroscopy with Potential Modulation Method
In order to investigate local structures of electrolytes in electrochemical reactions under the same scan rate as a typical value 100 mV/s in cyclic voltammetry (CV), we have developed an operando observation system for electrochemical reactions by soft X-ray absorption spectroscopy (XAS) with a potential modulation method. The electrode potential is swept with a scan rate of 100 mV/s at a fixed photon energy, and soft X-ray absorption coefficients at different potentials are measured at the same time. By repeating the potential modulation at each fixed photon energy, it is possible to measure XAS of electrochemical reaction at the same scan rate as in CV. We have demonstrated successful measurement of the Fe L-edge XAS spectra of aqueous iron sulfate solutions and of the change in valence of Fe ions at different potentials in the Fe redox reaction. The mechanism of these Fe redox processes is discussed by correlating the XAS results with those at different scan rates (50 mV/s).
[1] M. Nagasaka et al., Rev. Sci. Instrum. 85, 104105 (2014).
Local Structures of Methanol-Water Binary Solutions Studied by Soft X-ray Absorption Spectroscopy
The local structure of methanol-water binary solutions was studied by the O and C K-edge soft X-ray absorption spectroscopy (XAS). Liquid methanol shows one- and two-dimensional (1D/2D) hydrogen bond (HB) networks and liquid water shows three-dimensional (3D) HB networks. The pre-edge peak in the O K-edge XAS reflects the HB interaction of oxygen atoms and shows almost linear concentration dependence. It indicates that the HB interaction of methanol with surrounding water molecules is nearly the same as in pure liquid methanol and the HB interaction of water with surrounding methanol molecules is nearly the same as in pure liquid water. The C K-edge XAS enables us exclusively to investigate local structures around the methyl group of methanol molecules in the binary solution. We have clearly found three different local structures around the methyl group of methanol molecule in the methanol-water binary solutions (CH3OH)X(H2O)1-X. With the help of molecular dynamics simulations, we have discussed the concentration dependence of the hydrophobic interaction at the methyl group of methanol molecule. In the methanol-rich region I (1.0>X>0.7), a small amount of water molecules exists separately around dominant 1D/2D HB networks of methanol clusters. In the region II (0.7>X>0.3), the hydrophobic interaction of the methyl group is dominant due to the increase of mixed methanol-water 3D network structures. In the water-rich region III (0.3>X>0.05), methanol molecules are separately embedded in dominant 3D HB networks of water.
Electrochemical Reaction of Aqueous Iron Sulfate Solutions Studied by Fe L-Edge Soft X-ray Absorption Spectroscopy
Change in valence of Fe ions in aqueous iron sulfate solutions at different potentials has been studied by Fe L-edge soft X-ray absorption spectroscopy (XAS) in transmission mode. Each XAS spectrum is measured at a constant potential by using a liquid cell with built-in electrodes, such as working, counter, and reference electrodes. The XAS spectra have signals from Fe(II) and Fe(III) ions and show an isosbestic point, indicating only two species are involved. The fractions of Fe(II) and Fe(III) ions at different potentials are determined from the curve fitting of the XAS spectra by the superposition of the reference spectra of Fe(II) and Fe(III) ions. A nonlinear oxidation of Fe(II) to Fe(III) ions is observed when the potential is increased from 0.0 to 0.9 V. Two processes are found in the oxidation: One is a simple oxidation process and the other is a process involving the sulfate ions. The potential peak in the latter process is changed with different scanning rates because the sulfate ions affect electrode kinetic parameters and diffusion coefficients. The reduction of Fe(III) to Fe(II) ions shows a linear profile when the potential is decreased from 0.9 to -0.4 V. The mechanism of these Fe redox processes is discussed by correlating the XAS results with cyclic voltammetry results at different scanning rates.
Structures of Small Mixed Kr-Xe Clusters Studied by Soft X-ray Photoelectron Spectroscopy
Structures of small mixed Kr-Xe clusters of different compositions with an average size of 30-37 atoms are investigated. The Kr 3d5/2 and Xe 4d5/2 surface core level shifts and photoelectron intensities originating from corner, edge, and face/bulk sites are analyzed by using soft X-ray photoelectron spectroscopy. Structural models are derived from these experiments, which are confirmed by theoretical simulation taking induced dipole interactions into account. It is found that one or two small Xe cores are partly embedded in the surface of the Kr clusters. These may grow and merge leading to a phase separation between the two rare gas moieties in mixed clusters with increasing the Xe content.
Exchange Interaction of Rydberg Electrons in Small Kr Clusters Studied by Inner-shell Spectroscopy
The exchange interaction of the Rydberg electron bound by a singly charged ion EX(+1) in small van der Waals Kr clusters is obtained from surface-site resolved Kr 3d5/2-15p and 3d5/2-16p Rydberg excited states by X-ray absorption spectroscopy. EX(+1) of the 5p electron is increased due to the large overlap with the nearest neighbor atoms compared to that of the 6p electron. Furthermore, the exchange interaction of the Rydberg electron bound by a doubly charged ion EX(+2) in small Kr clusters is obtained from surface-site resolved Kr 4s-25p and 4s-26p shakeup-like Rydberg states by resonant Auger electron spectroscopy. We have found that EX(+2) is larger than EX(+1) in Rydberg states with the same principal quantum number. This is reasonable considering that overlap between the Rydberg electron and the nearest neighbor atoms is increased by contraction of the Rydberg orbital in the doubly-ionized atom. The site- and state-dependent EX is a key to understand the surrounding structure of different surface sites in variable size clusters. This is because EX corresponds to the short-range interaction with the nearest neighbor atoms of the ionized atom.
[1] M. Nagasaka et al., J. Electron Spectrosc. Relat. Phenom. 183, 29 (2011).
[2] M. Nagasaka et al., J. Electron Spectrosc. Relat. Phenom. 166-167, 16 (2008).
Development of a Liquid Cell for Soft X-ray Absorption Spectroscopy of Liquids in Transmission Mode
X-ray absorption spectroscopy (XAS) is an element specific method to study local electronic structures of liquids and aqueous solutions. Especially, soft X-ray region has many chemically important absorption edges such as C, N, and O K-edges. However, XAS of liquid sample needs a thin liquid layer below a few micrometers because soft X-rays are strongly absorbed by solvent water molecules. In this study, we have developed a liquid cell for XAS in transmission mode. The liquid layer is sandwiched between two 100 nm-thick Si3N4 membranes under atmospheric condition. The thickness of the liquid layer is controllable between 20 and 2000 nm by changing the He pressure outside the liquid layer. The O K-edge XAS spectra of liquid water at different thickess are obtained by using the transmitted-type liquid cell.
[1] M. Nagasaka et al., J. Electron Spectrosc. Relat. Phenom. 177, 130 (2010).
Proton Transfer in a Two-Dimensional Hydrogen-Bonding Network Revealed by Spatially-Resolved X-ray Photoelectron Spectroscopy
The time scale of proton transfer between H2O and OH adsorbed on a Pt(111) surface was determined by a combination of laser-induced thermal desorption (LITD) and spatially-resolved X-ray photoelectron spectroscopy (micro-XPS). The patterned distribution OH+H2O/H2O/OH+H2O was initially prepared on the Pt(111) surface by the LITD method and the time evolution of the spatial distribution of H2O and OH was observed by the micro-XPS. From quantitative analyses based on a diffusion equation, we found two proton-transfer pathway, which were attributed to direct proton transfer to the neighbor site and H3O+-mediated transfer to the next-nearest site, respectively.
[1] M. Nagasaka et al., Phys. Rev. Lett. 100, 106101 (2008).
CO Oxidation Reaction on Pt(111) Studied by Dynamic Monte Carlo Simulations Combined with Density Functional Theory
The dynamics of surface adsorbate during CO oxidation reaction on Pt(111) surfaces were studied by dynamic Monte Carlo (DMC) simulations combined with density functional theory (DFT). The lateral interaction energies between adsorbed species were calculated by DFT. DMC simulations were performed for the oxidation reaction over a mesoscopic scale, where the experimentally determined activation energies of elementary paths were altered by the lateral interaction energies calculated by DFT. The simulated results reproduced the characteristics of the microscopic and mesoscopic scale adsorbate structures formed during the reaction, which were observed by the reported results of scanning tunnelling microscopy. The simulated reaction kinetics of surface adsorbates also reproduced the reported results of time-resolved near-edge X-ray absorption fine structure spectroscopy. These results clearly revealed that the complicated reaction kinetics is comprehensively explained by a single reaction path affected by the surrounding adsorbates. We also proposed from the simulations that weakly adsorbed CO molecules at domain boundaries promote the island-periphery specific reaction.
Water Formation Reaction on Pt(111) Studied by Time-Resolved NEXAFS Spectroscopy and Kinetic Monte Carlo Simulations
Catalytic water formation reaction on a Pt(111) surface was investigated by time-resolved near-edge X-ray absorption fine structure (time-resolved NEXAFS) spectroscopy and kinetic Monte Carlo (KMC) simulations. An oxygen covered Pt(111) surface was exposed to hydrogen gas at 130 K. O K-edge NEXAFS spectra were measured during the reaction with a time interval of 35 sec. Quantitative analyses of the spectra provided the coverage changes of the reaction species (O, OH, and H2O). KMC simulation has well reproduced the reaction kinetics obtained by NEXAFS. The surface configurations also clearly reproduced the reported results of scanning tunneling microscopy, which show that the OH reaction fronts proceed on the O-covered Pt(111) surface by producing H2O islands backwards. The KMC simulation also revealed that proton transfer between H2O and OH plays a significant role for the propagation of the reaction fronts.
