X-ray-absorption spectroscopic studies of sodium polyphosphate glasses

Energy Level Alignment at the Cobalt Phosphate/Electrolyte Interface: Intrinsic Stability vs Interfacial Chemical Reactions in 5 V Lithium Ion Batteries

The intrinsic stability of the 5 V LiCoPO₄-LiCo₂P₃O₁₀ thin-film (carbon-free) cathode material coated with MoO₃ thin layer is studied using a comprehensive synchrotron electron spectroscopy in situ approach combined with first-principle calculations. The atomic-molecular level study demonstrates fully reversible electronic properties of the cathode after the first electrochemical cycle. The polyanionic oxide is not involved in chemical reactions with the fluoroethylene-containing liquid electrolyte even when charged to 5.1 V vs Li⁺/Li. The high stability of the cathode is explained on the basis of the developed energy level model. In contrast, the chemical composition of the cathode-electrolyte interface evolves continuously by involving MoO₃ in the decomposition reaction with consequent leaching of oxide from the surface. The proposed mechanisms of chemical reactions are attributed to external electrolyte oxidation via charge transfer from the relevant electron level to the MoO₃ valence band state and internal electrolyte oxidation via proton transfer to the solvents. This study provides a deeper insight into the development of both a doping strategy to enhance the electronic conductivity of high-voltage cathode materials and an efficient surface coating against unfavorable interfacial chemical reactions.

Raman and XANES Spectroscopic Study of the Influence of Coordination Atomic and Molecular Environments in Biomimetic Composite Materials Integrated with Dental Tissue

In this work, for the first time, the influence of the coordination environment as well as Ca and P atomic states on biomimetic composites integrated with dental tissue was investigated. Bioinspired dental composites were synthesised based on nanocrystalline calcium carbonate-substituted hydroxyapatite Ca4ICa6IIPO46−xCO3x+yOH2−y (nano-cHAp) obtained from a biogenic source and a set of polar amino acids that modelled the organic matrix. Biomimetic composites, as well as natural dental tissue samples, were investigated using Raman spectromicroscopy and synchrotron X-ray absorption near edge structure (XANES) spectroscopy. Molecular structure and energy structure studies revealed several important features related to the different calcium atomic environments. It was shown that biomimetic composites created in order to reproduce the physicochemical properties of dental tissue provide good imitation of molecular and electron energetic properties, including the carbonate anion CO₃²⁻ and the atomic Ca/P ratio in nanocrystals. The features of the molecular structure of biomimetic composites are inherited from the nano-cHAp (to a greater extent) and the amino acid cocktail used for their creation, and are caused by the ratio between the mineral and organic components, which is similar to the composition of natural enamel and dentine. In this case, violation of the nano-cHAp stoichiometry, which is the mineral basis of the natural and bioinspired composites, as well as the inclusion of different molecular groups in the nano-cHAp lattice, do not affect the coordination environment of phosphorus atoms. The differences observed in the molecular and electron energetic structures of the natural enamel and dentine and the imitation of their properties by biomimetic materials are caused by rearrangement in the local environment of the calcium atoms in the HAp crystal lattice. The surface of the nano-cHAp crystals in the natural enamel and dentine involved in the formation of bonds with the organic matrix is characterised by the coordination environment of the calcium atom, corresponding to its location in the Ca^I position—that is, bound through common oxygen atoms with PO₄ tetrahedrons. At the same time, on the surface of nano-cHAp crystals in bioinspired dental materials, the calcium atom is characteristically located in the Ca^II position, bound to the hydroxyl OH group. The features detected in the atomic and molecular coordination environment in nano-cHAp play a fundamental role in recreating a biomimetic dental composite of the natural organomineral interaction in mineralised tissue and will help to find an optimal way to integrate the dental biocomposite with natural tissue.

A Novel Approach for the Quantification of Different Inorganic and Organic Phosphorus Compounds in Environmental Samples by P L2,3-Edge X-ray Absorption Near-Edge Structure (XANES) Spectroscopy

Phosphorus (P) is an essential element for life on Earth, with an important and oftentimes unaccounted organic biogeochemical component. Current methods for the quantification of different organic P compounds in environmental samples (e.g., soils, sediments) are based on extraction techniques and often associated with incomplete P recovery or sample changes. In this study, we present a protocol for the quantification of different organic and inorganic P species in soils using synchrotron-based X-ray absorption near-edge structure (XANES) spectroscopy at the P L_2,3-edge. Its accuracy and precision was evaluated by analyzing 40 standard mixtures composed of seven different inorganic and organic P compounds (with a mean of R² = 0.85). In addition, we quantified the P species of two soils and two agro-industrial byproducts using P L_2,3-edge XANES spectroscopy and the results were compared with those obtained by P K-edge XANES or ³¹P NMR spectroscopy. Using the P L_2,3-edge, we identified different organic P species, including those not identified by the common P K-edge XANES. However, there is a consistent underestimation of organic polyphosphates. Overall, the application of P L_2,3-edge XANES provides a higher level of information than by P K-edge XANES, although the ubiquitous use of this novel methodology is still limited to samples with a phosphorus content above 3 mg g^-1.

Unveiling the Interfacial Instability of the Phosphorus/Carbon Anode for Sodium-Ion Batteries

As a competitive anode material for sodium-ion batteries (SIBs), a commercially available red phosphorus, featured with a high theoretical capacity (2596 mA h g^-1) and a suitable operating voltage plateau (0.1-0.6 V), has been confronted with a severe structural instability and a rapid capacity degradation upon large volumetric change. In particular, the fundamental determining factors for phosphorus anode materials are yet poorly understood, and their interfacial stability against ambient air has not been explored and clarified. Herein, a high-performance phosphorus/carbon anode material has been fabricated simply through ball-milling the carbon black and red phosphorus, delivering a high reversible capacity of 1070 mA h g^-1 at 400 mA g^-1 after 200 cycles and a superior rate capability of 479 mA h g^-1 at 3200 mA g^-1. More importantly, we first reveal the significance of inhibiting the exposure of phosphorus/carbon electrode materials to air, even for a short period, for achieving a good electrochemical performance, which would sharply decrease the reversible capacities. With the assistance of synchrotron-based X-ray techniques, the formation and accumulation of insulating phosphate compounds can be spectroscopically identified, leading to the decay of electrochemical performance. At the same time, these passivation layers on the surface of electrode were found to occur via a self-oxidation process in ambient air. To maintain the electrochemical advantages of phosphorus anodes, it is necessary to inhibit their contact with air through a rational coating or an optimal storage condition. Additionally, the employment of a fluoroethylene carbonate (FEC) additive facilitates the decomposition of the electrolyte and favors the formation of a robust solid electrolyte interphase layer, which may suppress the side reactions between the active Na-P compounds and the electrolyte. These findings could help improve the surface protection and interfacial stability of phosphorus anodes for high-performance SIBs.

Tracking Internal Electron Shuttle Using X-ray Spectroscopies in La/Zr Hydroxide for Reconciliation of Charge-Transfer Interaction and Coordination toward Phosphate

Metallic hydroxides have been applied as the adsorbents of oxyanion contaminants, with the oxygen-bonded metal (M-O) considered as the core site for adsorption. For enhanced adsorption toward oxyanions, multimetallic modification of M-O is a promising approach for high removal performance. Here, bimetallic La/Zr hydroxides were prepared via a solvothermal route with varying La/Zr dosages. Bimetallic La/Zr hydroxides exhibited higher oxyanion adsorption capacity than La or Zr hydroxide. A maximum phosphate adsorption capacity of ∼160 mg g^-1 was achieved under the La/Zr atomic ratio of 1:1, representing a new record among comparable adsorbents. X-ray photoelectron spectroscopy and X-ray absorption near-edge structure (XANES) spectroscopy showed that the incorporation of [LaO₆] and [ZrO₆] can induce an internal charge shuttle owing to the electronegativity difference of La and Zr. The charge transfers from La to Zr through the surrounding O 2p ligand, where the electrons in the highest occupied molecular orbitals of the [LaO₆] octahedron filled unoccupied π orbitals of [ZrO₆]. The as-induced internal electron shuttle in the bimetallic hydroxides primarily strengthened the formation of [MO₆]···PO₄ species (backdonation interaction). Further, the σ_P-O donation interaction between [LaO₆] and [PO₄] was clearly increased via intensification of the covalent coordination with O, as confirmed by the P K-edge XANES spectra for the as-used samples.

Coupled Biphase (1T-2H)-MoSe2 on Mold Spore Carbon for Advanced Hydrogen Evolution Reaction

Performance breakthrough of MoSe₂ -based hydrogen evolution reaction (HER) electrocatalysts largely relies on sophisticated phase modulation and judicious innovation on conductive matrix/support. In this work the controllable synthesis of phosphate ion (PO₄^3- ) intercalation induced-MoSe₂ (P-MoSe₂ ) nanosheets on N-doped mold spore carbon (N-MSC) forming P-MoSe₂ /N-MSC composite electrocatalysts is realized. Impressively, a novel conductive N-MSC matrix is constructed by a facile mold fermentation method. Furthermore, the phase of MoSe₂ can be modulated by a simple phosphorization strategy to realize the conversion from 2H-MoSe₂ to 1T-MoSe₂ to produce biphase-coexisted (1T-2H)-MoSe₂ by PO₄^3- intercalation (namely, P-MoSe₂ ), confirmed by synchrotron radiation technology and spherical aberration-corrected TEM (SACTEM). Notably, higher conductivity, lower bandgap and adsorption energy of H⁺ are verified for the P-MoSe₂ /N-MSC with the help of density functional theory (DFT) calculation. Benefiting from these unique advantages, the P-MoSe₂ /N-MSC composites show superior HER performance with a low Tafel slope (≈51 mV dec^-1 ) and overpotential (≈126 mV at 10 mA cm^-1 ) and excellent electrochemical stability, better than 2H-MoSe₂ /N-MSC and MoSe₂ /carbon nanosphere (MoSe₂ /CNS) counterparts. This work demonstrates a new kind of carbon material via biological cultivation, and simultaneously unravels the phase transformation mechanism of MoSe₂ by PO₄^3- intercalation.

Catalysis by design: development of a bifunctional water splitting catalyst through an operando measurement directed optimization cycle

A critical challenge in energy research is the development of earth abundant and cost-effective materials that catalyze the electrochemical splitting of water into hydrogen and oxygen at high rates and low overpotentials. Key to addressing this issue lies not only in the synthesis of new materials, but also in the elucidation of their active sites, their structure under operating conditions and ultimately, extraction of the structure-function relationships used to spearhead the next generation of catalyst development. In this work, we present a complete cycle of synthesis, operando characterization, and redesign of an amorphous cobalt phosphide (CoP x ) bifunctional catalyst. The research was driven by integrated electrochemical analysis, Raman spectroscopy and gravimetric measurements utilizing a novel quartz crystal microbalance spectroelectrochemical cell to uncover the catalytically active species of amorphous CoP x and subsequently modify the material to enhance the activity of the elucidated catalytic phases. Illustrating the power of our approach, the second generation cobalt-iron phosphide (CoFePx) catalyst, developed through an iteration of the operando measurement directed optimization cycle, is superior in both hydrogen and oxygen evolution reactivity over the previous material and is capable of overall water electrolysis at a current density of 10 mA cm-2 with 1.5 V applied bias in 1 M KOH electrolyte solution.

Beam-induced redox transformation of arsenic during As K-edge XAS measurements: availability of reducing or oxidizing agents and As speciation

During X-ray absorption spectroscopy (XAS) measurements of arsenic (As), beam-induced redox transformation is often observed. In this study, the As species immobilized by poorly crystallized mackinawite (FeS) was assessed for the susceptibility to beam-induced redox reactions as a function of sample properties including the redox state of FeS and the solid-phase As speciation. The beam-induced oxidation of reduced As species was found to be mediated by the atmospheric O₂ and the oxidation products of FeS [e.g. Fe(III) (oxyhydr)oxides and intermediate sulfurs]. Regardless of the redox state of FeS, both arsenic sulfide and surface-complexed As(III) readily underwent the photo-oxidation upon exposure to the atmospheric O₂ during XAS measurements. With strict O₂ exclusion, however, both As(0) and arsenic sulfide were less prone to the photo-oxidation by Fe(III) (oxyhydr)oxides than NaAsO₂ and/or surface-complexed As(III). In case of unaerated As(V)-reacted FeS samples, surface-complexed As(V) was photocatalytically reduced during XAS measurements, but arsenic sulfide did not undergo the photo-reduction.

Spectroscopic characterization of lithium thiophosphates by XPS and XAS – a model to help monitor interfacial reactions in all-solid-state batteries

Shift of binding energies upon depolymerization of superionic lithium thiophosphates.

Local electronic structure and nanolevel hierarchical organization of bone tissue: theory and NEXAFS study

Theoretical and experimental investigations of native bone are carried out to understand relationships between its hierarchical organization and local electronic and atomic structure of the mineralized phase. The 3D superlattice model of a coplanar assembly of the hydroxyapatite (HAP) nanocrystallites separated by the hydrated nanolayers is introduced to account the interplay of short-, long- and super-range order parameters in bone tissue. The model is applied to (i) predict and rationalize the HAP-to-bone spectral changes in the electronic structure and (ii) describe the mechanisms ensuring the link of the hierarchical organization with the electronic structure of the mineralized phase in bone. To check the predictions the near-edge x-ray absorption fine structure (NEXAFS) at the Ca 2p, P 2p and O 1s thresholds is measured for native bone and compared with NEXAFS for reference compounds. The NEXAFS analysis has demonstrated the essential hierarchy induced HAP-to-bone red shifts of the Ca and P 2p-to-valence transitions. The lowest O 1s excitation line at 532.2 eV in bone is assigned with superposition of core transitions in the hydroxide OH^-(H₂O) _m anions, Ca²⁺(H₂O) _n cations, the carboxyl groups inside the collagen and [PO₄]^2- and [PO₄]^- anions with unsaturated P-O bonds.

Chemical nature of alkaline polyphosphate boundary film at heated rubbing surfaces

Alkaline polyphosphate has been demonstrated to be able to reduce significant wear and friction of sliding interfaces under heavy loads (>1 GPa) and elevated temperature (800 °C and above) conditions, e.g. hot metal manufacturing. The chemical composition and fine structure of polyphosphate lubricating film is not well understood as well as the role of alkaline elements within the reaction film at hot rubbing surface. This work makes use of the coupling surface analytical techniques on the alkaline polyphosphate tribofilm, XANES, TOF-SIMS and FIB/TEM. The data show the composition in gradient distribution and trilaminar structure of tribofilm: a shorter chain phosphate overlying a long chain polyphosphate that adheres onto oxide steel base through a short chain phosphate. The chemical hardness model well explains the anti-abrasive mechanism of alkaline polyphosphate at elevated temperatures and also predicts a depolymerisation and simultaneous cross-linking of the polyphosphate glass. The role of alkaline elements in the lubrication mechanism is especially explained. This work firstly serves as a basis for a detailed study of alkaline polyphosphate tribofilm at temperature over 600 °C.

Chemical–mechanical properties of tribofilms and their relationship to ionic liquid chemistry

Hardness as a function of indentation depth for tribofilms formed from ZDDP and N_DEHP ionic liquid. Superimposed on the data is the corresponding phosphate chain length information acquired from XANES.

Interaction of phosphonium ionic liquids with borate esters at tribological interfaces

Chemical make-up of the tribofilms formed using phosphonium-phosphate IL and borate ester.

Polyphosphates as inorganic polyelectrolytes interacting with oppositely charged ions, polymers and deposited on surfaces: fundamentals and applications

Polyphosphates are important but neglected polyelectrolytes that play a major role in biology and in surface science for the stabilization of colloids against flocculation and for the preservation of food. They are also known as "Calgon" ® and intensively used as additives in washing powders. This review aims to review recent developments in which linear polyphosphates are used for the design of new functional coatings using sol-gel processes and layer-by-layer deposition methods. All these methods rely on the high charge density of polyphosphates as inorganic polyelectrolytes, therefore the structure and properties of these molecules are also reviewed. New perspectives will also been given for the design of stimuli responsive coatings at the tiny frontier between biology and materials science.

The contribution of XANES spectroscopy to tribology

X-ray absorption near edge structure spectroscopy (XANES) analysis has been routinely used to study the complex chemical interactions between additives in engine oil and metallic surfaces during high-temperature and pressure reciprocating wear conditions. XANES analysis provides detailed chemical and structural information on the resultant antiwear and tribo films formed on metallic surfaces. The following review will illustrate how XANES analysis on the macro scale can provide the information required to elucidate complex film formation mechanisms, then describes the use of emerging XANES spectromicroscopy to such systems, and concludes by showing the complementary nature of the macro and micro scale spatially resolved XANES analysis; Professor Bancroft has utilized the combination of these to stay at the forefront of XANES research in the field of tribology and in spectroscopy science in general.Key words: tribology, XANES, X-ray absorption near edge structure spectroscopy, metals, thiophosphates, spectromicroscopy, tribochemistry.