Fine-structure Spectra Research Articles

Manipulation of the Electronic Structure of Ruthenium Nanoclusters by Ni‐N4 Sites Enhances the Alkaline Hydrogen Evolution Reaction

AbstractDesigning electrocatalysts that are both highly efficient and durable is crucial for the industrial implementation of alkaline electrocatalytic hydrogen production technologies. A limitation of the current Ru‐based catalysts is that the water dissociation energy barrier tends to be too high. Here, the electronic structure of ruthenium nanoclusters (Ru NCs) is modulated by single atom Ni‐N4 sites leading to leading to lowering of the water dissociation barrier. X‐ray absorption fine structure spectrum confirms that Ru NCs are stably anchored on the carbon support through the formation of Ru‐N bonds, significantly enhancing catalytic stability. The resulting Ru/Ni‐N4C‐300 catalyst shows excellent catalytic activity toward alkaline hydrogen evolution reaction with a low overpotential of 15.0 mV at 10 mA cm−2 together with robust durability. An anion exchange membrane water electrolyzer employing Ru/Ni‐N4C‐300 can be stably operated under 500 mA cm−2 for over 1370 h, surpassing the parameters required for industrialization. Theoretical calculation indicates the single atom Ni‐N4 sites in Ru/Ni‐N4C‐300 optimize the electron distribution of Ru NCs, thereby reducing the Gibbs free energy of intermediates species in water dissociation process.

NEXAFS spectroscopy of alkylated benzothienobenzothiophene thin films at the carbon and sulfur K-edges.

Alkylated benzothienobenzothiophenes are an important class of organic semiconductors that exhibit high performance in solution-processed organic field-effect transistors. In this work, we study the near-edge x-ray absorption fine-structure (NEXAFS) spectra of 2,7-didecyl[1]benzothieno[3,2-b][1]benzothiophene (C10-BTBT) at both the carbon and sulfur K-edges. Angle-resolved experiments of thin films are performed to characterize the dichroism associated with molecular orientation. First-principles calculations using the density functional theory-based many-body x-ray absorption spectroscopy (MBXAS) method are also performed to correlate the peaks observed and their dichroism with transitions to specific antibonding molecular orbitals. Interestingly, the dichroism of the dominant, lowest energy peak is opposite at the carbon and sulfur K-edges. While the low-energy peak at the carbon K-edge is assigned to carbon 1s → π* transitions with transition dipole moment (TDM) perpendicular to the planar BTBT core, the dominant low energy peak at the sulfur K-edge is assigned to sulfur 1s → σ* transitions with TDM oriented along the long axis of the BTBT core. These differences at the sulfur and carbon K-edges are understood through the MBAXS simulations that find a reordering of the energy of the lowest energy π* and σ* transitions at the sulfur K-edge due to the strong localization of the σ* orbital over the sulfur atom. This work highlights differences in the NEXAFS spectra of organic semiconductors at carbon and sulfur K-edges and provides new insights into peak assignment and x-ray dichroism relevant for studying the molecular orientation of organic semiconductor films.

Visualizing the Structure and Dynamics of Transition Metal-Based Electrocatalysts Using Synchrotron X-Ray Absorption Spectroscopy.

As the global energy structure evolves and clean energy technologies advance, electrocatalysis has become a focal point as a critical conversion pathway in the new energy sector. Transitional metal electrocatalysts (TMEs) with their distinctive electronic structures and redox properties show great potential in electrocatalytic reactions. However, complex reaction mechanisms and kinetic limitations hinder the improvement of energy conversion efficiency, highlighting the necessity for comprehensive studies on structure and performance of electrocatalysts. X-ray Absorption Fine Structure (XAFS) spectra stand out as a robust tool for examining the electrocatalyst's structures and performance due to its atomic selectivity and sensitivity to local environments. This review delves into the application of XAFS technology in characterizing TMEs, providing in-depth analyses of X-ray Absorption Near-Edge Structure (XANES) spectra, and Extended XAFS (EXAFS) spectra in both R-space and k-space. These analyses reveal intrinsic structural information, electronic interactions, catalyst stability, and aggregation morphology. Furthermore, the paper examines advancements in in-situ XAFS techniques for real-time monitoring of active site changes, capturing critical intermediate and transitional states, and elucidating the evolution of active species during electrocatalytic reactions. These insights deepen our understanding on structure-activity relationship of electrocatalysts and offer valuable guidance for designing and developing highly active and stable electrocatalysts.

Redox conditions influence the chemical composition of iron-associated organic carbon in boreal lake sediments: A synchrotron-based NEXAFS study

The global carbon and iron cycles are intimately linked as redox-sensitive iron oxides readily bind organic carbon in a variety of environmental settings, including marine and lacustrine sediments. While these iron-organic carbon complexes sequester vast quantities of organic carbon, the composition of the organic matter within them remains unknown for lacustrine environments. Here we present C K1s and Fe L3,2 edge Near Edge X-ray Absorption Fine Structure (NEXAFS) spectra of surface sediments and authigenic iron complexes from adjacent basins of a pristine boreal lake located in Québec, Canada, with contrasting oxygen exposure regimes. We demonstrate differences in organic carbon speciation in sediments from both basins, as well as co-localization of organic carbon and iron on a sub-micron scale in 100 nm thick samples. Differences in redox cycling across these two basins allow for a direct comparison of the effect of oscillating redox conditions on the composition of organic carbon sequestered by iron. Our results suggest that reactive organic molecules, which may be polysaccharides, were found preferentially associated with iron in the perennially oxic sediments compared to more phenol rich organics in the seasonally anoxic sediments, highlighting the importance of iron oxides in the protection and preservation of labile organic compounds. Traces of aliphatic carbon were observed in sediments from the anoxic basin, alongside carboxyl and aromatic functionalities. This carboxyl-rich aliphatic material could possibly interact with the sediment mineral matrix either through a ligand exchange mechanism between the mineral phases and the carboxyl functionalities, or via non-specific hydrophobic interactions involving the aliphatic moieties. Finally, our work also shows that OC:Fe ratios should be used with caution when inferring a binding mechanism between OC and iron oxides.

Highly Stable Solid Contact Calcium Ion-Selective Electrodes: Rapid Ion-Electron Transduction Triggered by Lipophilic Anions Participating in Redox Reactions of CunS Nanoflowers.

Solid contact (SC) calcium ion-selective electrodes (Ca2+-ISEs) have been widely applied in the analysis of water quality and body fluids by virtue of the unique advantages of easy operation and rapid response. However, the potential drift during the long-term stability test hinders their further practical applications. Designing novel redox SC layers with large capacitance and high hydrophobicity is a promising approach to stabilize the potential stability, meanwhile, exploring the transduction mechanism is also of great guiding significance for the precise design of SC layer materials. Herein, flower-like copper sulfide (CunS-50) composed of nanosheets is meticulously designed as the redox SC layer by modification with the surfactant (CTAB). The CunS-50-based Ca2+-ISE (CunS-50/Ca2+-ISE) demonstrates a near-Nernstian slope of 28.23 mV/dec for Ca2+ in a wide activity linear range of 10-7 to 10-1 M, with a low detection limit of 3.16 × 10-8 M. CunS-50/Ca2+-ISE possesses an extremely low potential drift of only 1.23 ± 0.13 μV/h in the long-term potential stability test. Notably, X-ray absorption fine-structure (XAFS) spectra and electrochemical experiments are adopted to elucidate the transduction mechanism that the lipophilic anion (TFPB-) participates in the redox reaction of CunS-50 at the solid-solid interface of ion-selective membrane (ISM) and redox inorganic SC layer (CunS-50), thereby promoting the generation of free electrons to accelerate ion-electron transduction. This work provides an in-depth comprehension of the transduction mechanism of the potentiometric response and an effective strategy for designing redox materials of ion-electron transduction triggered by lipophilic anions.

Reduction of Cofed Carbon Dioxide Modifies the Local Coordination Environment of Zeolite-Supported, Atomically Dispersed Chromium to Promote Ethane Dehydrogenation.

The reduction of CO2 is known to promote increased alkene yields from alkane dehydrogenations when the reactions are cocatalyzed. The mechanism of this promotion is not understood in the context of catalyst active-site environments because CO2 is amphoteric, and even general aspects of the chemistry, including the significance of competing side reactions, differ significantly across catalysts. Atomically dispersed chromium cations stabilized in highly siliceous MFI zeolite are shown here to enable the study of the role of parallel CO2 reduction during ethylene-selective ethane dehydrogenation. Based on infrared spectroscopy and X-ray absorption spectroscopy data interpreted through calculations using density functional theory (DFT), the synthesized catalyst contains atomically dispersed Cr cations stabilized by silanol nests in micropores. Reactor studies show that cofeeding CO2 increases stable ethylene-selective ethane dehydrogenation rates over a wide range of partial pressures. Operando X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine-structure (EXAFS) spectra indicate that during reaction at 650 °C the Cr cations maintain a nominal 2+ charge and a total Cr-O coordination number of approximately 2. However, CO2 reduction induces a change, correlated with the CO2 partial pressure, in the population of two distinct Cr-O scattering paths. This indicates that the promotional effect of parallel CO2 reduction can be attributed to a subtle change in Cr-O bond lengths in the local coordination environment of the active site. These insights are made possible by simultaneously fitting multiple EXAFS spectra recorded in different reaction conditions; this novel procedure is expected to be generally applicable for interpreting operando catalysis EXAFS data.

Local laser heating effects in diamond probed by photoluminescence of SiV− centers at low temperatures

The accurate measurement of thermal conductivity of diamond below 10 K has always been a challenge, mainly due to significant error in temperature sensing using the thermocouple method. Diamond is generally considered to have high thermal conductivity, so little attention has been paid to the laser heating effects. Here, we observed the dynamic redshift and broadening of zero phonon line of silicon-vacancy (SiV−) centers at 4 K. Utilizing the intrinsic temperature response of the fine structure spectra of SiV− as a probe, we confirmed that laser heating effect appears and the temperature rising results from high defect concentration. By simulating the thermal diffusion process, we have estimated the thermal conductivity of around 1 W/(m K), which is a two-order magnitude lower than that of single-crystal diamond. Our results provide a feasible scheme for all-optical non-contact temperature sensing and help to solve the problem of accurate measurement of thermal conductivity at cryogenic temperatures.

Unraveling the chemical and structural evolution of novel Li-rich layered/rocksalt intergrown cathode for Li-ion batteries

The prototype material, Li1.23Ru0.41Ni0.36O2, is proposed to gain the deep and comprehensive understanding of chemical and structural changes of the novel layered/rocksalt intergrown cathodes. Synchrotron-based X-ray absorption spectra and resonant inelastic X-ray scattering reveal that both cationic and anionic redox evolves in the charge compensation process of the intergrown material, while synchrotron-based extended X-ray fine structure spectra and in situ X-ray diffraction measurements demonstrates that the intergrown material undergoes minimal local- and long-range structural variations at deep de/lithiation. This work highlights the great potential of the intergrown structure to inspire the design of advanced cathode materials for lithium-ion batteries.

Structural identification of single boron-doped graphdiynes by computational XPS and NEXAFS spectroscopy.

Boron-doped graphdiyne (B-GDY) material exhibits an excellent performance in electrocatalysis, ion transport, and energy storage. However, accurately identifying the structures of B-GDY in experiments remains a challenge, hindering further selection of suitable structures with the most ideal performance for various practical applications. In the present work, we employed density functional theory (DFT) to simulate the X-ray photoelectron spectra (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectra of pristine graphdiyne (GDY) and six representative single boron-doped graphdiynes at the B and C K-edges to establish the structure-spectroscopy relationship. A notable disparity in the C 1s ionization potentials (IPs) between substituted and adsorbed structures is observed upon doping with a boron atom. By analyzing the C and B 1s NEXAFS spectra on energy positions, spectral widths, spectral intensities, and different spectral profiles, we found that the six single boron-doped graphdiyne configurations can be sensitively identified. Moreover, this study provides a reliable theoretical reference for distinguishing different single boron-doped graphdiyne structures, enabling accurate selection of B-GDY structures for diverse practical applications.

Chemical and topological disordering in M23C6 due to irradiation: An atomic-scale observation

To clarify the stability of M23C6 precipitate in steels under irradiation, the chemical and topological disordering behaviors in M23C6 subjected to ion irradiation at elevated temperatures were investigated with the combined application of high-resolution electron microscopy (HREM) and electron energy-loss spectroscopy (EELS). Results show that the duplex structured M23C6 which is the core-crystalline and shell-amorphous was observed in the specimen irradiated at 623 K, nevertheless, no amorphous phase was observed in the specimen irradiated at 673 K. HREM observation confirmed the occurrence of the chemical disordering in irradiated M23C6, and the disordering of W atoms in the M23C6 unit cell was clarified by the peak intensity profile of the atomic column from the HREM image. EELS analyses indicate that no obvious chemical shift was observed by the core-loss spectrum, however, the change in the electron mean free path (λ) due to irradiation was confirmed by the low-loss spectrum, which is related to the formation of both irradiation defects and amorphous phase. It is proposed that the decreased material density upon irradiation is ascribed to the formation of W-vacancy-dominated defect clusters. An obvious change in interatomic distance was not observed in the irradiated M23C6 based on the radical distribution function analysis by extended energy-loss fine structure spectrum. However, the peak intensity decreased by irradiation, confirming the occurrence of chemical and topological disordering, regardless of the carbide being crystalline or amorphous.

Structural characterization of nitrogen-doped [formula omitted]-graphynes by computational X-ray spectroscopy

Heterogeneous atomic doping, especially with nitrogens, can improve the interfacial properties and electrical conductivity of carbon-based materials like γ-graphyne (γ-GY), thereby improving the energy storage capacity and expanding various electrical applications. Properties of doped structures are affected by the local structures around the impurity atoms, which can be sensitively determined by the X-ray photoelectron (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectra. In the present work, we have simulated the XPS and NEXAFS spectra of pristine γ-GY as well as five nitrogen-doped γ-GYs in both the C and N K-edges with the density functional theory. The theoretical C and N 1s ionic potentials of the N-doped systems can be well fitted to the experimentally XPS spectra, which can accurately characterize the experimental peaks. These five N-doped γ-GYs can be well distinguished by NEXAFS. Our results provide clear local structure-spectroscopy relation and a theoretical reference for identifying different N-doped γ-GYs, which are also insightful for general two-dimensional carbon-based materials.

A new satellite of manganese revealed by extended-range high-energy-resolution fluorescence detection.

The discovery of a new physical process in manganese metal is reported. This process will also be present for all manganese-containing materials in condensed matter. The process was discovered by applying our new technique of XR-HERFD (extended-range high-energy-resolution fluorescence detection), which was developed from the popular high-resolution RIXS (resonant inelastic X-ray scattering) and HERFD approaches. The acquired data are accurate to many hundreds of standard deviations beyond what is regarded as the criterion for `discovery'. Identification and characterization of many-body processes can shed light on the X-ray absorption fine-structure spectra and inform the scientist on how to interpret them, hence leading to the ability to measure the dynamical nanostructures which are observable using the XR-HERFD method. Although the many-body reduction factor has been used universally in X-ray absorption spectroscopy in analysis over the past 30 years (thousands of papers per year), this experimental result proves that many-body effects are not representable by any constant reduction factor parameter. This paradigm change will provide the foundation for many future studies and X-ray spectroscopy.

Potency of Extended X-ray Absorption Fine Structure Spectroscopy toward the Determination of Individual Bond Grüneisen Parameter for High Thermoelectric Performance

Si–Ge is a well-known disordered alloy exhibiting high anharmonicity. These two features, disorder and anharmonicity, arise due to the unique nature of bonding instead of structural complexity and are beneficial as they limit the thermal conduction and are widely used in high-temperature thermoelectric applications. In this regard, a comprehensive understanding of the nature of interatomic bonds and the calculation of the Grüneisen parameter, which is a characteristic metric of anharmonicity, of the individual bond are essential to design a crystalline solid along with the development of a high-throughput thermoelectric alloy exhibiting ultralow lattice thermal conductivity. Here, we demonstrate the origin of the low lattice thermal conductivity, κl, of ∼0.8 and 0.4 Wm–1 K–1 for the transition-metal (Ni and Cr)-doped Si–Ge alloy, respectively, due to the difference in the anharmonic pair potential achieved using synchrotron-based X-ray absorption fine structure spectroscopy. The technique enables the determination of the Grüneisen parameter of the individual bond in the alloy and thus reveals the unpaired electron-induced bond anharmonicities and bonding heterogeneity. The technique also determines the bond Grüneisen parameter more precisely in comparison to the values obtained from the speed of sound, as the latter neglects the existence of the soft TO mode, which also contributes to lattice dynamics. Thus, the synergistic presence of (i) heteroatoms as point scatters (mass contrast), (ii) bonding anharmonicities, and (iii) electron–phonon scattering suppresses the lattice thermal conductivity beyond the theoretical alloy limit. Furthermore, the analysis imparts a conception for the estimation of the bond Grüneisen parameter through the acquisition and analysis of extended X-ray analysis of fine structure (EXAFS) spectra.

First-principles simulation of X-ray spectra of graphdiyne and graphdiyne oxides at the carbon K-edge.

The experimental C 1s near-edge X-ray absorption fine-structure (NEXAFS) spectra of graphdiyne (GDY) show an evident change at different exposure periods, which is explained by oxidation. Herein, to better understand the structure-spectra relationship and the influence of oxidization, we performed a first-principles simulation of the NEXAFS spectra and X-ray photoelectron spectra (XPS) of both pure GDY and its four typical graphdiyne oxides (GDO) at the carbon K-edge. Pure GDY contains one sp2-hybridized (C1) and two sp-hybridized (C2, C3) carbons, while oxidation introduces more nonequivalent carbons. The experimental NEXAFS spectrum exhibits the lowest peak at ca. 285.8 eV. It was found that the C 1s → π* excitation from the sp2-hybridized carbon atoms (C1) in pure GDY and the sp-hybridized atoms (C2, C3) in GDOs contributes to this peak. The two weak resonances at around 289.0 and 290.6 eV in the experiment are contributed by the carbon atoms bonded to the oxygen atoms. Meanwhile, we found that oxidization leads to an increase in the C 1s ionization potentials (IPs) by 0.3-2.7 eV, which is consistent with the XPS experiments. Our calculations provide a clear explanation of the structure-spectra relationships of GDY and GDOs, and the signatures are useful for estimating the degree of oxidation.

Fine structure of spectrum of secondary electron, 12

Fine structure of spectrum of secondary electron, 12

Experimental and theoretical near-edge x-ray-absorption fine-structure studies of NO+

Experimental near-edge x-ray-absorption fine-structure (NEXAFS) spectra of the nitrosonium ${\mathrm{NO}}^{+}$ ion are presented and theoretically analyzed. While neutral NO has an open shell, the cation is a closed-shell species, which for NEXAFS leads to the simplicity of a closed-shell spectrum. Compared to neutral NO, the electrons in the cation experience a stronger Coulomb potential, which introduces a shift of the ionization potential towards higher energies, a depletion of intensity in a large interval above the ${\ensuremath{\pi}}^{*}$ resonance, and a shift of the ${\ensuremath{\sigma}}^{*}$ resonance from the continuum to below the ionization threshold. NEXAFS features at the nitrogen and oxygen $K$ edges of ${\mathrm{NO}}^{+}$ are compared, as well as NEXAFS features at the nitrogen edges of the isoelectronic closed-shell species ${\mathrm{NO}}^{+}, {\mathrm{N}}_{2}$, and ${\mathrm{N}}_{2}{\mathrm{H}}^{+}$.

In situ gas cell for the analysis of adsorption behaviour on surfaces using X-ray spectroscopy

A gas cell for in-situ measurements of Volatile Organic Compounds (VOCs) and their adsorption behaviour on different surfaces by means of X-ray Fluorescence (XRF) and X-ray Absorption Fine-Structure (XAFS) spectroscopy has been developed. The cell is especially designed to allow for the efficient excitation and detection of low-Z elements such as carbon, oxygen or nitrogen as main components of VOCs. Two measurement modes are available. In the surface mode, adsorption on a surface can be studied using XAFS by fluorescence detection under shallow angles of incidence. The transmission mode enables the simultaneous investigation of gaseous samples via XAFS in transmittance and fluorescence detection modes. Proof-of-principle experiments were performed at the PTB plane grating monochromator beamline for soft X-ray radiation at the synchrotron radiation facility BESSY II. The flexible design and high versatility of the cell are demonstrated with the investigation of ethanol (EtOH) as one of the most abundant VOCs. The comparison of Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectra under transmission and fluorescence detection in the gas phase with measurements of adsorbed molecules on a Si-wafer surface both at the C and O-K absorption edges proves the applicability of the cell for the monitoring of adsorption processes.

Identification of oxidation states in [formula omitted]-graphyne by computational XPS and NEXAFS spectra

Oxidation is an inevitable process for γ-graphyne (γ-GY), a two-dimensional (2D) material of carbon, both in its natural state and during large-scale processing. Understanding the oxidation states within γ-GY is of fundamental importance for further applications. In this work, with density functional theory (DFT) simulations of seven representative oxygen-doped γ-GY structures, we demonstrate that the X-ray photoelectron (XPS) and near-edge X-ray absorption fine-structure (NEXAFS) spectra at the oxygen and carbon K-edges are sensitive for different local structures of oxygen dopants. The theoretical calculation results of γ-GY and seven oxygen-doped γ-GY configurations fit well with the experiment, and all characteristic experimental XPS peaks are assigned. We found that the C 1s ionization potential (IP) of sp2-hybridized carbons is higher than that of sp-hybridized carbons for the pristine γ-GY. By complete and elaborate analysis of the NEXAFS spectrum of each structure at the carbon and oxygen K edges, the seven oxygen-doped γ-GY configurations have been successfully identified. The results can be considered as a benchmark for the determination of oxygen-doped γ-GY configurations, and provide a deep insight into the structure–spectroscopy relationships.

Laser heating induced spatial homogenization of phase separated Na2O-B2O3-SiO2 glass plate with bearing NiO for heat center and structural probe

Na2O-B2O3-SiO2 glass plate phase separated in spinodal was partially remelted and homogenized by laser heating. As heat generation source, 0.8 mol% NiO was added to absorb continuous-wave laser with λ=1064 nm. After acid leaching, borate phase with NiO was remaining partially in laser irradiated area. Smooth, non-porous surface was found at cross section of laser irradiated area below surface, and nano-scale porous structure was found at non-irradiated area observed by scanning electron microscopy (SEM). X-ray Absorption Fine Structure (XAFS) spectra revealed that Ni2+ in laser irradiated area showed coordination change from 6 coordinated octahedral to 5 coordinated in square pyramid along with apparent color change from green to brown. Similar change was observed in water-quenched specimen from 1000°C. Ni2+ transition gave additional structural information in phase separate / homogeneous glass, which was difficult to be distinguished by spectroscopy which focuses on covalent bonding of glass network.

The Valence Band Structure of the [Ni(Salen)] Complex: An Ultraviolet, Soft X-ray and Resonant Photoemission Spectroscopy Study

The valence band photoemission (VB PE) spectra of the [Ni(Salen)] molecular complex were measured by ultraviolet, soft X-ray and resonant photoemission (ResPE) using photons with energies ranging from 21.2 eV to 860 eV. It was found that the Ni 3d atomic orbitals’ (AOs) contributions are most significant for molecular orbitals (MOs), which are responsible for the low-energy PE band at a binding energy of 3.8 eV in the VB PE spectra. In turn, the PE bands in the binding energies range of 8–16 eV are due to the photoionization of the MOs of the [Ni(Salen)] complex with dominant contributions from C 2p AOs. A detailed consideration was made for the ResPE spectra obtained using photons with absorption resonance energies in the Ni 2p3/2, N 1s, and O 1s Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectra. A strong increase in the intensity of the PE band ab was found when using photons with an energy 854.4 eV in the Ni 2p3/2 NEXAFS spectrum. This finding is due to the high probability of the participator-Auger decay of the Ni 2p3/2−13d9 excitation and confirms the relationship between the PE band ab with the Ni 3d-derived MOs.