Chemical-state distributions in charged LiCoO2 cathode particles visualized by soft X-ray spectromicroscopy.

Lithium-ion deintercalation/intercalation during charge/discharge processes is one of the essential reactions that occur in the layered cathodes of lithium-ion batteries, and the performance of the cathode can be expressed as the sum of the reactions that occur in the local area of the individual cathode particles. In this study, the spatial distributions of the chemical states present in prototypical layered LiCoO2 cathode particles were determined at different charging conditions using scanning transmission X-ray microscopy (STXM) with a spatial resolution of approximately 100 nm. The Co L3- and O K-edge X-ray absorption spectroscopy (XAS) spectra, extracted from the same area of the corresponding STXM images, at the initial state as well as after charging to 4.5 V demonstrate the spatial distribution of the chemical state changes depending on individual particles. In addition to the Co L3-edge XAS spectra, the O K-edge XAS spectra of the initial and charged LiCoO2 particles are different, indicating that both the Co and O sites participate in charge compensation during the charging process possibly through the hybridization between the Co 3d and O 2p orbitals. Furthermore, the element maps of both the Co and O sites, derived from the STXM stack images, reveal the spatial distribution of the chemical states inside individual particles after charging to 4.5 V. The element mapping analysis suggests that inhomogeneous reactions occur on the active particles and confirm the existence of non-active particles. The results of this study demonstrate that an STXM-based spatially resolved electronic structural analysis method is useful for understanding the charging and discharging of battery materials.

Redox Reaction in Ti-Mn Redox Flow Battery Studied by X-ray Absorption Spectroscopy.

We performed X-ray absorption studies for the electrolytes of a Ti-Mn redox flow battery (RFB) to understand the redox reaction of the Ti/Mn ions and formation of precipitates in charged catholyte, because suppression of the disproportionation reaction is a key to improve the cyclability of Ti-Mn RFB and enhance the energy density. Hard X-ray absorption spectroscopy with a high transmittance and soft X-ray absorption spectroscopy to directly observe the 3d orbitals were complementarily employed. Moreover, the Ti/Mn 3d electronic structure for each precipitate and solution in the charged catholyte was investigated by using scanning transmission X-ray microscopy: the valence of Mn in the precipitate is mostly attributed to 4+, and the solution includes only Mn2+ . This charge disproportionation reaction should occur after the Mn ions in the catholyte should be oxidized from Mn2+ to Mn3+ by charge.

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. 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.

Improved Skin Permeability after Topical Treatment with Serine Protease: Probing the Penetration of Rapamycin by Scanning Transmission X-ray Microscopy.

Drug penetration in human skin ex vivo following a modification of skin barrier permeability is systematically investigated by scanning transmission X-ray microscopy. Element-selective excitation is used in the O 1s regime for probing quantitatively the penetration of topically applied rapamycin in different formulations with a spatial resolution reaching <75 nm. The data were analyzed by a comparison of two methods: (i) two-photon energies employing the Beer-Lambert law and (ii) a singular value decomposition approach making use of the full spectral information in each pixel of the X-ray micrographs. The latter approach yields local drug concentrations more reliably and sensitively probed than the former. The present results from both approaches indicate that rapamycin is not observed within the stratum corneum of nontreated skin ex vivo, providing evidence for the observation that this high-molecular-weight drug inefficiently penetrates intact skin. However, rapamycin is observed to penetrate more efficiently the stratum corneum when modifications of the skin barrier are induced by the topical pretreatment with the serine protease trypsin for variable time periods ranging from 2 to 16 h. After the longest exposure time to serine protease, the drug is even found in the viable epidermis. High-resolution micrographs indicate that the lipophilic drug preferably associates with corneocytes, while signals found in the intercellular lipid compartment were less pronounced. This result is discussed in comparison to previous work obtained from low-molecular-weight lipophilic drugs as well as polymer nanocarriers, which were found to penetrate the intact stratum corneum exclusively via the lipid layers between the corneocytes. Also, the role of the tight junction barrier in the stratum granulosum is briefly discussed with respect to modifications of the skin barrier induced by enhanced serine protease activity, a phenomenon of clinical relevance in a range of inflammatory skin disorders.

A low-pass filtering Fresnel zone plate for soft x-ray microscopic analysis down to the lithium K-edge region.

We have designed a new low-pass Fresnel zone plate (LPFZP) to extend soft x-ray absorption spectroscopy (XAS) to the lithium K absorption edge in a scanning transmission x-ray microscope (STXM). The performance of the LPFZP was evaluated in the STXM beamline at the UVSOR-III Synchrotron (Okazaki, Japan); the contribution of the higher-order harmonics is successfully suppressed to 0.1% of the fundamental energy, and a spatial resolution of 72 nm and an energy resolution (E/∆E) above 1000 are achieved as expected. XAS spectra of lithium are measured successfully in an electrode of a lithium-ion battery.

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. The energy shift of the C≡N π* peak 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 in three concentration regions and found that the dipole interaction between acetonitrile and water is the key structure to emerge the MH state in the middle concentration region.

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. These XAS techniques are also applied to in situ/operando observation of chemical processes in solutions such as catalytic and electrochemical reactions.

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 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 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. The distributions of the molar fractions of pyridine and water near the liquid-liquid interface have been determined from the energy shifts of the π* peak 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.

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. Depending on the CD core structure, different ionic currents generated upon X-ray irradiation of the CD dispersions at the carbon K-edge were detected, which are interpreted in terms of different charge transfer to the surrounding solvent molecules. The hydrogen bonding networks of water molecules upon interaction with the different CDs were also probed at the oxygen K-edge. Both core graphitization and nitrogen doping were found to endow the CDs with enhanced electron transfer and hydrogen bonding capabilities with the surrounding water molecules.

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. By comparing unexpected temperature behaviors of the π* peak in XAS spectra with model calculations, we have investigated temperature-dependent changes of ordered structures in liquid benzene caused by the increase in abundance of the parallel sandwich orientation relative to parallel displaced structures for the higher temperature. These results are confirmed by infrared spectroscopy with additional support of vibrational mode calculations.

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. 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 and their temperature dependence, evaluated by molecular dynamics simulations.

Probing Interfacial Water on Nanodiamonds in Colloidal Dispersion.

The structure of interfacial water layers around nanoparticles dispersed in an aqueous environment may have a significant impact on their reactivity and on their interaction with biological species. Using transmission soft X-ray absorption spectroscopy in liquid, we demonstrate that the unoccupied electronic states of oxygen atoms from water molecules in aqueous colloidal dispersions of nanodiamonds have a different signature than bulk water. X-ray absorption spectroscopy can thus probe interfacial water molecules in colloidal dispersions. The impacts of nanodiamond surface chemistry and concentration on interfacial water electronic signature are discussed.

In 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 in operando observation system for electrochemical reactions by soft X-ray absorption spectroscopy (XAS) with a potential modulation method. XAS spectra of electrolytes are measured by using a transmission-type liquid flow cell with built-in electrodes. 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.

A heterogeneous palladium catalyst hybridised with a titanium dioxide photocatalyst for direct C-C bond formation between an aromatic ring and acetonitrile.

A palladium catalyst hybridised with a titanium dioxide photocatalyst can promote cyanomethylation of an aromatic ring by using acetonitrile, where the photocatalyst activates acetonitrile to form a cyanomethyl radical before the C-C bond formation using the palladium catalyst.

Direct aromatic-ring amination by aqueous ammonia with a platinum loaded titanium oxide photocatalyst.

Active species generated from aqueous ammonia with a platinum-loaded titanium oxide photocatalyst can selectively aminate the aromatic ring of benzene and some substituted benzenes.

Photocatalytic hydroxylation of aromatic ring by using water as an oxidant.

Electrophilic oxygen species photocatalytically derived from water molecules can selectively react with the aromatic ring of both benzene and its derivatives to produce the corresponding phenols and hydrogen over platinum-loaded titanium oxide when illuminated with light of appropriate wavelength in the absence of oxygen.