Investigation of per- and polyfluoroalkyl substances (PFAS) in soils and sewage sludges by fluorine K-edge XANES spectroscopy and combustion ion chromatography

For the first time, fluorine K-edge X-ray absorption near-edge structure (XANES) spectroscopy was applied to detect per- and polyfluoroalkyl substances (PFAS) in various soil and sewage sludge samples. The method can be used to determine the speciation of inorganic and organic fluorides, without pre-treatment of solid samples. Therefore, XANES spectra of several inorganic fluorides as well as selected fluorinated organic compounds were recorded. While inorganic fluorides partially exhibit a variety of sharp spectral features in the XANES spectrum, almost all inspected organofluorine compounds show two distinct broad features at 688.5 and 692.0 eV. Moreover, the peak intensity ratio 688.5 eV/692.0 eV in the PFAS XANES spectrum can be inversely correlated to the chain length of the perfluoro sulfonic acid group. The detection of targeted PFAS by bulk-XANES spectroscopy in combination with linear combination fitting in soils and sewage sludges was not applicable due to the low organic fluorine to total fluorine ratio of the samples (0.01–1.84%). Nonetheless, direct analysis of pure PFAS revealed that analysis of organofluorine species might be achieved in higher concentrated samples. Furthermore, quantitative measurements by combustion ion chromatography (CIC) evaluated as sum parameters extractable organically bound fluorine (EOF) and total fluorine (TF) emphasize that besides soils, sewage sludges are a significant source of organic fluorine in agriculture (154–7209 µg/kg).

Spinel Iron Oxide by the Co-Precipitation Method: Effect of the Reaction Atmosphere

Synthesis atmosphere (i.e., air and nitrogen) effects on the physical properties and formation mechanism of spinel iron oxide nanoparticles prepared via the co-precipitation method have been investigated using a multi-technique approach. The obtained magnetic nanoparticles (MNPs) were characterized using the X-ray diffraction, transmission electron microscopy (TEM), SQUID magnetometry, Mössbauer spectroscopy and X-ray absorption near-edge Structure spectroscopy techniques. The synthesis procedure leads to the formation of a spinel structure with an average crystallite size of 9.0(9) nm. The morphology of the particles synthetized under an inert atmosphere was quasi-spherical, while the nanoparticles prepared in air present a faceted shape. The small differences observed in morphological properties are explained by the influence of the reaction atmosphere on the formation mechanism of the MNPs. The magnetic characterization indicates that both samples exhibit superparamagnetic behavior at 300 K. The investigation by means of the Langevin approach at 300 K also leads to equal values for the mean size of the magnetic cores (Dm). Additionally, the analysis of the Mössbauer spectra revealed the lack of spin disorder for both samples, resulting in a high saturation magnetization. The fit of XANES spectrum suggests that about 2/3 of the iron ions reside in a local environment close to that of γ-Fe2O3 and about 1/3 close to that of Fe3O4 for the sample synthetized in inert atmosphere.

Site-Specific Electronic Properties of [Ag25(SR)18]- Nanoclusters by X-Ray Spectroscopy

We present the site-specific electronic properties of Ag₂₅(SR)₁₈and Au₂₅(SR)₁₈ using X-ray spectroscopy experiments and quantum simulations. To overcome the final state effect observed in X-ray photoelectron spectroscopy (XPS), a unique method was developed to reliably analyze the charge transfer behavior of the NCs. Density functional theory calculations were combined with XPS to provide more insight into the electronic properties of the NCs. The differences in the XPS valence bands of these two NCs were further compared and interpreted using the relativistic effect. The first derivative of the X-ray absorption near-edge structure (XANES) spectrum was further used as a tool to sensitively probe the bonding properties of Ag₂₅(SR)₁₈. By combining the experimental XANES data and their site-specific quantum simulations, the large impact of the staple motif on the bonding properties of the NC was demonstrated. These findings highlight the unique electronic properties of each atomic site in Ag₂₅(SR)₁₈; the effective X-ray analysis techniques developed here can offer new opportunities for the site-specific study of other NCs.

Site-Specific Electronic Properties of [Ag25(SR)18]- Nanoclusters by X-Ray Spectroscopy

We present the site-specific electronic properties of Ag₂₅(SR)₁₈and Au₂₅(SR)₁₈ using X-ray spectroscopy experiments and quantum simulations. To overcome the final state effect observed in X-ray photoelectron spectroscopy (XPS), a unique method was developed to reliably analyze the charge transfer behavior of the NCs. Density functional theory calculations were combined with XPS to provide more insight into the electronic properties of the NCs. The differences in the XPS valence bands of these two NCs were further compared and interpreted using the relativistic effect. The first derivative of the X-ray absorption near-edge structure (XANES) spectrum was further used as a tool to sensitively probe the bonding properties of Ag₂₅(SR)₁₈. By combining the experimental XANES data and their site-specific quantum simulations, the large impact of the staple motif on the bonding properties of the NC was demonstrated. These findings highlight the unique electronic properties of each atomic site in Ag₂₅(SR)₁₈; the effective X-ray analysis techniques developed here can offer new opportunities for the site-specific study of other NCs.

Development of XANES nanoscopy on BL7C at PLS-II

X-ray absorption near-edge structure (XANES) imaging is a powerful tool to visualize the chemical state distribution of transition-metal-based materials at synchrotron radiation facilities. In recent years, the electrochemical working rechargeable battery has been the most studied material in XANES imaging owing to the large increase of portable electronics and electric vehicles. This work acknowledges the importance of battery analysis and has developed the XANES imaging system on BL7C at Pohang Light Source-II (PLS-II). BL7C employs an undulator taper configuration to obtain an energy band >130 eV near the K-absorption edge of the target element with a minimum energy interval >0.2 eV. While measuring energy-dependent images, the zone plate translation maintains the best focus, and then various data processes such as background correction, image registration and clustering allow single XANES spectrum extraction and chemical distribution mapping. Here, the XANES imaging process is described, the XANES spectrum quality is identified and the chemical states of the partially charged cathode material used in lithium-ion batteries as an application example are examined.

PyXAS – an open-source package for 2D X-ray near-edge spectroscopy analysis

In the synchrotron X-ray community, X-ray absorption near-edge spectroscopy (XANES) is a widely used technique to probe the local coordination environment and the oxidation states of specific elements within a sample. Although this technique is usually applied to bulk samples, the advent of new synchrotron sources has enabled spatially resolved versions of this technique (2D XANES). This development has been extremely powerful for the study of heterogeneous systems, which is the case for nearly all real applications. However, associated with the development of 2D XANES comes the challenge of analyzing very large volumes of data. As an example, a single 2D XANES measurement at a synchrotron can easily produce ∼106 spatially resolved XANES spectra. Conventional manual analysis of an individual XANES spectrum is no longer feasible. Here, a software package is described that has been developed for high-throughput 2D XANES analysis. A detailed description of the software as well as example applications are provided.

Machine learning approaches for ELNES/XANES

Materials characterization is indispensable for materials development. In particular, spectroscopy provides atomic configuration, chemical bonding and vibrational information, which are crucial for understanding the mechanism underlying the functions of a material. Despite its importance, the interpretation of spectra using human-driven methods, such as manual comparison of experimental spectra with reference/simulated spectra, is becoming difficult owing to the rapid increase in experimental spectral data. To overcome the limitations of such methods, we develop new data-driven approaches based on machine learning. Specifically, we use hierarchical clustering, a decision tree and a feedforward neural network to investigate the electron energy loss near edge structures (ELNES) spectrum, which is identical to the X-ray absorption near edge structure (XANES) spectrum. Hierarchical clustering and the decision tree are used to interpret and predict ELNES/XANES, while the feedforward neural network is used to obtain hidden information about the material structure and properties from the spectra. Further, we construct a prediction model that is robust against noise by data augmentation. Finally, we apply our method to noisy spectra and predict six properties accurately. In summary, the proposed approaches can pave the way for fast and accurate spectrum interpretation/prediction as well as local measurement of material functions.

Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-edge XANES

<p>Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a ⁵T_2g state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)₃²⁺ and conclusively identify a ³T intermediate that forms in 170 fs and decays to a vibrationally hot ⁵T_2g state in 40 fs. A coherent vibrational wavepacket with a period of 250 fs and damping time of 0.66 ps is observed on the ⁵T_2g surface, and the spectrum of this oscillation serves as a fingerprint for the Fe-N symmetric stretch. The results show that the shape of the M_2,3-edge X-ray absorption near edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high spin sensitivity, fast time resolution, and tabletop convenience of XUV transient absorption make it a powerful tool for studying the complex photophysics of transition metal complexes.<br></p>

Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-edge XANES

<p>Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a ⁵T_2g state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)₃²⁺ and conclusively identify a ³T intermediate that forms in 170 fs and decays to a vibrationally hot ⁵T_2g state in 40 fs. A coherent vibrational wavepacket with a period of 250 fs and damping time of 0.66 ps is observed on the ⁵T_2g surface, and the spectrum of this oscillation serves as a fingerprint for the Fe-N symmetric stretch. The results show that the shape of the M_2,3-edge X-ray absorption near edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high spin sensitivity, fast time resolution, and tabletop convenience of XUV transient absorption make it a powerful tool for studying the complex photophysics of transition metal complexes.<br></p>

Tracking the Metal-Centered Triplet in Photoinduced Spin Crossover of Fe(phen)32+ with Tabletop Femtosecond M-edge XANES

<p>Fe(II) coordination complexes are promising alternatives to Ru(II) and Ir(III) chromophores for photoredox chemistry and solar energy conversion, but rapid deactivation of the initial metal-to-ligand charge transfer (MLCT) state to low-lying (d,d) states limits their performance. Relaxation to a ⁵T_2g state is postulated to occur via a metal-centered triplet state, but this mechanism remains controversial. We use femtosecond extreme ultraviolet (XUV) transient absorption spectroscopy to measure the excited-state relaxation of Fe(phen)₃²⁺ and conclusively identify a ³T intermediate that forms in 170 fs and decays to a vibrationally hot ⁵T_2g state in 40 fs. The shape of this M_2,3-edge X-ray absorption near edge structure (XANES) spectrum is sensitive to the electronic structure of the metal center, and the high spin sensitivity, fast time resolution, and tabletop convenience of XUV transient absorption make it a powerful new tool for measuring the complex photophysics of transition metal complexes.</p>