NEXAFS Chemical State and Bond Lengths of p-Aminobenzoic Acid in Solution and Solid State

Solid-state and solution pH-dependent NEXAFS studies allow direct observation of the electronic state of para-aminobenzoic acid (PABA) as a function of its chemical environment, revealing the chemical state and bonding of the chemical species. Variations in the ionization potential (IP) and 1s→π* resonances unequivocally identify the chemical species (neutral, cationic, or anionic) present and the varying local environment. Shifts in σ* shape resonances relative to the IP in the NEXAFS spectra vary with C-N bond length, and the important effect of minor alterations in bond length is confirmed with nitrogen FEFF calculations, leading to the possibility of bond length determination in solution

X ray absorption spectroscopy of TiO2 nanoparticles in water using a holey membrane based flow cell

Many applications of TiO2 nanoparticles, such as photocatalytic water splitting or water remediation, occur in aqueous environment. However, the impact of solvation on TiO2 electronic structure remains unclear because only few experimental methods are currently available to probe nanoparticle water interface. Soft X ray absorption spectroscopy has been extensively used to characterize the electronic structure of TiO2 materials, but so far only in vacuum conditions. Here, oxygen K edge and titanium L edge X ray absorption spectroscopy characterization of TiO2 nanoparticles measured directly in aqueous dispersion is presented. For this purpose, a new method to probe nanomaterials in liquid using a holey membrane based flow cell is introduced. With this approach, the X ray transmission of the membrane is increased, especially in the water window, compared to solid membrane

Introducing Ionic Current Detection for X Ray Absorption Spectroscopy in Liquid Cells

Photons and electrons are two common relaxation products upon X ray absorption, enabling fluorescence yield and electron yield detections for X ray absorption spectroscopy XAS . The ions that are created during the electron yield process are relaxation products too, which are exploited in this study to produce ion yield for XA detection. The ionic currents measured in a liquid cell filled with water or iron III nitrate aqueous solutions exhibit characteristic O K edge and Fe L edge absorption profiles as a function of excitation energy. Application of two electrodes installed in the cell is crucial for obtaining the XA spectra of the liquids behind membranes. Using a single electrode can only probe the species adsorbed on the membrane surface. The ionic current detection, termed as total ion yield TIY in this study, also produces an undistorted Fe L edge XA spectrum, indicating its promising role as a novel detection method for XAS studies in liquid cell

Accessing the solid electrolyte interphase on silicon anodes for lithium ion batteries in situ through transmission soft X ray absorption spectroscopy

Silicon offers nine times higher theoretical storage capacity than commercial graphite anodes for Li ion batteries. For cycling stability, the electrolyte needs to be kinetically stabilized by the so called Solid Electrolyte Interphase SEI , a layer which ideally forms once from decomposition products of the electrolyte. While it works for graphite, the SEI on silicon fails to stabilize the electrolyte sufficiently, partly due to the large volume changes upon de lithiation. To investigate the SEI on silicon anodes, we developed a novel approach for X ray Absorption Spectroscopy XAS , that puts a twist to conventional SiNx window based liquid cells by utilizing a deliberately induced gas bubble to form a soft X ray transparent electrolyte layer. We demonstrate our approach to allow transmission XAS in the soft X ray regime on liquids and electrode thin film materials under in situ conditions. In our case XAS study of the SEI on silicon anodes, we reveal the main SEI constituents as Li acetate, Li ethylene di carbonate or Li ethylene mono carbonate, Li acetylacetonate, LiOH, and LiF. Additionally, we see evidence for aldehyde species which we attribute to possible liquid inclusions within a porous SEI morphology.We consider our method an appropriate tool for the successful engineering of a stable, efficient SEI in the futur

Studies Combining UV-Vis, NEXAFS and RIXS Spectroscopies

NEXAFS-RIXS and home laboratory-based UV-Vis absorption spectroscopy are combined to examine the speciation and electronic structure of para- aminobenzoic acid (PABA) in aqueous solution as a function of pH. DFT and TD- DFT electronic structure calculations reproduce the experimental trends and provide a correlation between the experimental HOMO↔LUMO gap as well as the electronic transitions between molecular orbitals in the non-ionic, anionic and cationic forms of PABA

The Structure of p-Aminobenzoic Acid in Water: Studies Combining UV-Vis, NEXAFS and RIXS Spectroscopies

NEXAFS-RIXS and home laboratory-based UV-Vis absorption spectroscopy are combined to examine the speciation and electronic structure of para-aminobenzoic acid (PABA) in aqueous solution as a function of pH. DFT and TD-DFT electronic structure calculations reproduce the experimental trends and provide a correlation between the experimental HOMO↔LUMO gap as well as the electronic transitions between molecular orbitals in the non-ionic, anionic and cationic forms of PABA

Reversible and irreversible processes during cyclic voltammetry of an electrodeposited manganese oxide as catalyst for the oxygen evolution reaction

Manganese oxides have received much attention over the years among the wide range of electrocatalysts for the oxygen evolution reaction OER due to their low toxicity, high abundance and rich redox chemistry. While many previous studies focused on the activity of these materials, a better understanding of the material transformations relating to activation or degradation is highly desirable, both from a scientific perspective and for applications. We electrodeposited Na containing MnOx without long range order from an alkaline solution to investigate these aspects by cyclic voltammetry CV , scanning electron microscopy SEM and X ray absorption spectroscopy XAS at the Mn K and Mn L edges. The pristine film was assigned to a layered edge sharing Mn3 4 oxide with Mn O bond lengths of mainly 1.87 and some at 2.30 as well as Mn Mn bond lengths of 2.87 based on fits to the extended X ray fine structure EXAFS . The decrease of the currents at voltages before the onset of the OER followed power laws with three different exponents depending on the number of cycles and the Tafel slope decreases from 186 48 to 114 18 mV dec 1 after 100 cycles, which we interpret in the context of surface coverage with unreacted intermediates. Post mortem microscopy and bulk spectroscopy at the Mn K edge showed no change of the microstructure, bulk local structure or bulk Mn valence. Yet, the surface region of MnOx oxidized toward Mn4 , which explains the reduction of the currents in agreement with literature. Surprisingly, we find that MnOx reactivates after 30 minutes at open circuit OC , where the currents and also the Tafel slope increase. Reactivation processes during OC are crucial because OC is unavoidable when coupling the electrocatalysts to intermittent power sources such as solar energy for sustainable energy productio

Vacuum compatible flow-cell for high-quality in situ and operando soft X-ray photon-in–photon-out spectroelectrochemical studies of energy materials

Abstract: Soft X‐ray spectroscopy is a powerful method to investigate materials on an element selective level with respect to their atomic and electronic structure. However, its application is technically challenging for in situ or operando investigations of materials for electrochemical applications. Herein, we present a spectroelectrochemical flow‐cell designed to enable state‐of‐the‐art electrochemical characterization while being installed in a vacuum chamber for the direct accessibility of the electroactive sample to soft X‐rays. An overview of the application of soft X‐ray photon‐in–photon‐out spectroscopic studies to electromaterials is provided, along with discussions of experimental and technical considerations specific to this highly sensitive mode of analysis. Application of the cell for the in situ spectroelectrochemical characterization of an electrodeposited nickel oxide water electrooxidation catalyst is demonstrated

Impact of Cation Intercalation on the Electronic Structure of Ti3C2Tx MXenes in Sulfuric Acid

Intercalation in Ti3C2Tx MXene is essential for a diverse set of applications such as water purification, desalination, electrochemical energy storage, and sensing. The interlayer spacing between the Ti3C2Tx nanosheets can be controlled by cation intercalation; however, the impact of intercalation on the Ti3C2Tx MXene chemical and electronic structures is not well understood. Herein, we characterized the electronic structure of pristine, Li , Na , K , and Mg intercalated Ti3C2Tx MXenes dispersed initially in water and 10 mM sulfuric acid H2SO4 using X ray absorption spectroscopy XAS . The cation intercalation is found to dramatically influence the chemical environment of Ti atoms. The Ti oxidation of the MXene increases progressively upon intercalation of cations of larger sizes after drying in air, while interestingly a low Ti oxidation is observed for all intercalated MXenes after dispersion in diluted H2SO4. In situ XAS at the Ti L edge was conducted during electrochemical oxidation to probe the changes in the Ti oxidation state in the presence of different cations in H2SO4 aqueous electrolyte. By applying the sensitivity of the Ti L edge to probe the oxidation state of Ti atoms, we demonstrate that cation intercalation and H2SO4 environment significantly alter the Ti3C2Tx surface chemistr