In-situ X-ray spectroscopy of the electric double layer around TiO2 nanoparticles dispersed in aqueous solution: Implications for H2 generation

We report an experimental observation of a significant amount of hydroxide (OH–) created upon water dissociation and subsequently trapped around TiO2 nanoparticles dispersed in NH4OH aqueous solution. The hydroxide species is identified and quantified by a combination of photoemission and photon emission X-ray spectroscopies conducted on liquid samples using a liquid microjet. Unlike previous X-ray studies that observed only a few monolayers of water coverage on TiO2 surfaces and found maximally submonolayer of OH–, the true aqueous environment adopted in this study enables ion mobility and the separation of the water dissociation products H+/OH–. This facilitates the formation of OH– diffused multilayer in which the trapped OH– ions are discovered to coordinate with three water molecules to form a tetrahedral hydration configuration. The negatively charged diffuse layers, together with the positive NH4+ Stern layers, constitute >0.8 nm thick electric double layers around the TiO2 nanoparticles. The large observed amount of hydroxide indicates a high efficiency of water dissociation for the TiO2 catalyst, a promising result for H2 generation in true aqueous environments

LiXEdrom: High Energy Resolution RIXS Station dedicated to Liquid Investigation at BESSY II

LiXEdrom is an experimental station dedicated to high resolution RIXS measurements on liquid samples. It is equipped with two VLS gratings and advanced photon detector (MCP/phosphorous screen/CCD), covering soft X-ray range of 200 – 1200 eV. The efficient differential pumping and cooling systems ensure successful executions of X-ray spectroscopy on liquid samples in vacuum. Liquid samples are introduced into the vacuum chamber by micro-jet or flow-cell techniques

an iron L-edge X-ray absorption study of the active centre

Iron L-edge X-ray absorption spectra of the active centre of myoglobin in the met-form, in the reduced form and upon ligation to O2, CO, NO and CN are presented. The strength of ligation with the iron centre is finger-printed through the variation of the L3 : L2 intensity ratio. Charge Transfer Multiplet calculations are performed and give qualitative information about oxidation states as well as charge transfer

spin-state and metal coordination revealed from resonant inelastic X-ray scattering and electronic structure calculations

The local electronic structure of the cobalt centre-ion of Co(III) protoporphyrin IX chloride dissolved in dimethyl sulfoxide (DMSO) liquid solution is studied by resonant inelastic X-ray scattering (RIXS) spectroscopy at the cobalt L-edge. The resulting cobalt 2p partial-fluorescence-yield (PFY) X-ray absorption (XA) spectrum, integrated from RIXS spectra, is simulated for various possible spin-states and coordination of the cobalt centre by using the newly developed density functional theory/restricted open shell single excitation configuration interaction (DFT/ROCIS) method. Comparison between experiment and calculation shows that the cobalt ion (3d6 electronic configuration) adopts a low-spin state with all six 3d electrons paired, and the cobalt centre is either 5-coordinated by its natural ligands (one chloride ion and four nitrogen atoms), or 6-coordinated, when binding to an oxygen atom of a DMSO solvent molecule. Analysis of the measured RIXS spectra reveals weak 3d–3d electron correlation, and in addition a value of the local HOMO–LUMO gap at the Co sites is obtained

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.Volkswagen foundation (Freigeist Fellowship No. 89592), Christian Doppler Research Association (Austrian Federal Ministry for Digital and Economic Affairs National Foundation for Research, Technology and Development) OMV

pH and Solvent Influence on p-Aminobenzoic Acid

Through X-ray absorption and emission spectroscopies, the chemical, electronic and structural properties of organic species in solution can be observed. Near-edge X-ray absorption fine structure (NEXAFS) and resonant inelastic X-ray scattering (RIXS) measurements at the nitrogen K-edge of para- aminobenzoic acid reveal both pH- and solvent-dependent variations in the ionisation potential (IP), 1s→π* resonances and HOMO–LUMO gap. These changes unequivocally identify the chemical species (neutral, cationic or anionic) present in solution. It is shown how this incisive chemical state sensitivity is further enhanced by the possibility of quantitative bond length determination, based on the analysis of chemical shifts in IPs and σ* shape resonances in the NEXAFS spectra. This provides experimental access to detecting even minor variations in the molecular structure of solutes in solution, thereby providing an avenue to examining computational predictions of solute properties and solute–solvent interactions

hydrogen bonds and nuclear dynamics

Knowledge about the hydrogen bond network of water is essential for understanding its anomalies as well as its special role for biochemical systems. Different types of x-ray spectroscopy allow probing of the molecular orbitals of water, revealing the electronic structure which reflects the hydrogen bond conformations. In this work a recently developed high-resolution x-ray emission spectrometer was used in combination with the microjet technique for recording spectra of liquid H2O and D2O and their mixtures with acetonitrile. Variation of the nuclear dynamics via isotope substitution and variation of the hydrogen bond conformation via dissolution in acetonitrile was investigated. These two effects have two clearly distinguishable spectral fingerprints

oxygen K-edge X-ray absorption and emission spectroscopy on micro-jets

Oxygen K-edge X-ray absorption, emission, and resonant inelastic X-ray scattering spectra were measured to site selectively gain insights into the electronic structure of aqueous zinc acetate solution. The character of the acetate ion and the influence of zinc and water on its local electronic structure are discussed

Intermolecular Bonding of Hemin in Solution and in Solid State Probed by N K-edge X-ray Spectroscopies

X-ray absorption/emission spectroscopy (XAS/XES) at the N K-edge of iron protoporphyrin IX chloride (FePPIX-Cl, or hemin) has been carried out for dissolved monomers in DMSO, dimers in water and for the solid state. This sequence of samples permits identification of characteristic spectral features associated with the hemin intermolecular bonding. These characteristic features are further analyzed and understood at the molecular orbital (MO) level based on the DFT calculations

Time-of-flight electron spectrometer for a broad range of kinetic energies

A newly constructed time-of-flight electron spectrometer of the magnetic bottle type is characterized for electron detection in a broad range of kinetic energies. The instrument is designed to measure the energy spectra of electrons generated from liquids excited by strong laser fields and photons in the range of extreme ultra violet and soft X-rays. Argon inner shell electrons were recorded to calibrate the spectrometer and investigate its characteristics, such as energy resolution and collection efficiency. Its energy resolution ΔE/E of 1.6% allows resolving the Ar 2p spin orbit structure at kinetic energies higher than 100 eV. The collection efficiency is determined and compared to that of the spectrometer in its field-free configuration