Luminescence of Eu³⁺ Activated CaF₂ and SrF₂ Nanoparticles: Effect of the Particle Size and Codoping with Alkaline Ions

Eu³⁺ doped CaF₂ and SrF₂ nanoparticles were synthesized through a facile hydrothermal technique, using citrate ions as capping agents and Na⁺ or K⁺ as charge compensator ions. A proper tuning of the reaction time can modulate the nanoparticle size, from few to several tens of nanometers. Analysis of EXAFS spectra indicate that the Eu³⁺ ions enter into the fluorite CaF₂ and SrF₂ structure as substitutional defects on the metal site. Laser site selective spectroscopy demonstrates that the Eu³⁺ ions are mainly accommodated in two sites with different symmetries. The relative site distribution for lanthanide ions depends on the nanoparticle size, and higher symmetry Eu³⁺ sites are prevalent for bigger nanoparticles. Eu³⁺ ions in high symmetry sites present lifetimes of the ⁵D₀ level around 27 ms, among the longest lifetimes found in the literature for Eu³⁺ activated materials. As a proof of concept of possible use of the Eu³⁺ activated alkaline-earth fluoride nanoparticles in nanomedicine, the red luminescence generated by two-photon absorption using pulsed laser excitation at 790 nm (in the first biological window) has been detected. The long Eu³⁺ lifetimes suggest that the present nanomaterials can be interesting as luminescent probes in time-resolved fluorescence techniques in biomedical imaging (e.g., FLIM) where fast autofluorescence is a drawback to avoid

Fixed Energy X‑ray Absorption Voltammetry

In this paper, the fixed energy X-ray absorption voltammetry (FEXRAV) is introduced. FEXRAV represents a novel in situ X-ray absorption technique for fast and easy preliminary characterization of electrode materials and consists of recording the absorption coefficient at a fixed energy while varying at will the electrode potential. The energy is chosen close to an X-ray absorption edge, in order to give the maximum contrast between different oxidation states of an element. It follows that any shift from the original oxidation state determines a variation of the absorption coefficient. Although the information given by FEXRAV obviously does not supply the detailed information of X-ray absorption near edge structure (XANES) or extended X-ray absorption fine structure (EXAFS), it allows to quickly map the oxidation states of the element under consideration within the selected potential windows. This leads to the rapid screening of several systems under different experimental conditions (e.g., nature of the electrolyte, potential window) and is preliminary to more deep X-ray absorption spectroscopy (XAS) characterizations, like XANES or EXAFS. In addition, the time-length of the experiment is much shorter than a series of XAS spectra and opens the door to kinetic analysis

α- and γ‑FeOOH: Stability, Reversibility, and Nature of the Active Phase under Hydrogen Evolution

α-FeOOH (goethite) and γ-FeOOH (lepidocrocite) were found to be the main corrosion products of the steel cathode in the sodium chlorate process; the identification of the phases formed under reducing potentials, along with the study of the electrodes during the reoxidation, is fundamental to understanding their role in this process. In this work, FeOOH-based electrodes were investigated through in situ and in operando X-ray absorption spectroscopy (XAS), combined to electrochemical measurements (e.g., voltammetry and chronoamperometry). At sufficiently negative potentials (below −0.4 V vs RHE ca.) and under hydrogen evolution conditions an unknown iron­(II)-containing phase is formed. A comprehensive analysis of the whole XAS spectrum allowed proposing a structure bearing a relation with that of green rust (space group <i>P</i>3̅1<i>m</i>). This phase occurs independently of the nature of the starting electrode (α- or γ-FeOOH). During electrochemical reoxidation, however, the original phase is restored, meaning that the reduced phase brings some memory of the structure of the starting material. Spontaneous reoxidation in air suppresses the memory effect, producing a mixture of α and γ phases

Structural and Thermodynamic Properties of Nanoparticle–Protein Complexes: A Combined SAXS and SANS Study

We propose a novel method for determining the structural and thermodynamic properties of nanoparticle–protein complexes under physiological conditions. The method consists of collecting a full set of small-angle X-ray and neutron-scattering measurements in solutions with different concentrations of nanoparticles and protein. The nanoparticle–protein dissociation process is described in the framework of the Hill cooperative model, based on which the whole set of X-ray and neutron-scattering data is fitted simultaneously. This method is applied to water solutions of gold nanoparticles in the presence of human serum albumin without any previous manipulation and can be, in principle, extended to all systems. We demonstrate that the protein dissociation constant, the Hill coefficient, and the stoichiometry of the nanoparticle–protein complex are obtained with a high degree of confidence

Structure and Stability of a Copper(II) Lactate Complex in Alkaline Solution: a Case Study by Energy-Dispersive X‑ray Absorption Spectroscopy

Energy-dispersive X-ray absorption spectroscopy was applied, aimed at solving the problem of the structure and stability of a copper­(II) lactate complex in alkaline solution, used as a precursor for the electrodeposition of Cu₂O. The application of multiple scattering calculations to the simulation of the X-ray absorption near-edge structure part of the spectra allowed an accurate resolution of the structure: the copper­(II) cation is surrounded by four lactate ions in a distorted tetrahedral environment, with the lactate anions acting as monodentate ligands. This results in an atomic arrangement where copper is surrounded by four oxygen atoms located at quite a short distance (ca. 1.87 Å) and four oxygen atoms located quite far apart (ca. 3.1–3.2 Å). The complex was finally found to be stable in a wide range of applied potentials

Role of Interfacial Energy and Crystallographic Orientation on the Mechanism of the ZnO + Al₂O₃ → ZnAl₂O₄ Solid-State Reaction: I. Reactivity of Films Deposited onto the Sapphire (110) and (012) Faces

The initial steps of the reaction between ZnO and Al₂O₃ have been investigated with X-ray diffraction, atomic force microscopy, and X-ray absorption spectroscopy at the Zn–K edge starting from 45 nm thick zincite films deposited onto (110)- and (102)-oriented sapphire single crystals. The formation of nonequilibrium phase(s) has been detected for both orientations. For the (001)_zincite ∥ (110)_sapphire interface, the rate-determining step is the motion of the interface(s); the growth of the spinel layer is linear with time, with a rate constant <i>k</i> = 1.1(2) × 10^–9 cms^–1 at 1000 °C. At the (110)_zincite ∥ (012)_sapphire interface, the reaction shows dumped oscillations. The results are discussed along with a comparison with previous results on thinner films to clarify the role of interfacial free energy and crystallographic orientation

Easy Accommodation of Different Oxidation States in Iridium Oxide Nanoparticles with Different Hydration Degree as Water Oxidation Electrocatalysts

In this paper, we present a comprehensive study on low hydration Ir/IrO₂ electrodes, made of an Ir core and an IrO₂ shell, that are designed and synthesized with an innovative, green approach, in order to have a higher surface/bulk ratio of Ir–O active centers. Three materials with different hydration degrees have been deeply investigated in terms of structure and microstructure by means of transmission electron microscopy (TEM) and synchrotron radiation techniques such as high-resolution (HR) and pair distribution function (PDF) quality X-ray powder diffraction (XRPD), X-ray absorption spectroscopy (XAS), and for what concerns their electrochemical properties by means of cyclic voltammetry and steady-state <i>I</i>/<i>E</i> curves. The activity of these materials is compared and discussed in the light of our most recent results on hydrous IrO_<i>x</i>. The main conclusion of this study is that the Ir core is noninteracting with the IrO_<i>x</i> shell, the latter being able to easily accommodate Ir in different oxidation states, as previously suggested for the hydrated form, thus explaining the activity as electrocatalysts. In addition, in operando XAS experiments assessed that the catalytic cycle involves Ir­(III) and (V), as previously established for the highly hydrated IrO_<i>x</i> material

An Efficient Cu_<i>x</i>O Photocathode for Hydrogen Production at Neutral pH: New Insights from Combined Spectroscopy and Electrochemistry

Light-driven water splitting is one of the most promising approaches for using solar energy in light of more sustainable development. In this paper, a highly efficient p-type copper­(II) oxide photocathode is studied. The material, prepared by thermal treatment of CuI nanoparticles, is initially partially reduced upon working conditions and soon reaches a stable form. Upon visible-light illumination, the material yields a photocurrent of 1.3 mA cm^–2 at a potential of 0.2 V vs a reversible hydrogen electrode at mild pH under illumination by AM 1.5 G and retains 30% of its photoactivity after 6 h. This represents an unprecedented result for a nonprotected Cu oxide photocathode at neutral pH. The photocurrent efficiency as a function of the applied potential was determined using scanning electrochemical microscopy. The material was characterized in terms of photoelectrochemical features; X-ray photoelectron spectroscopy, X-ray absorption near-edge structure, fixed-energy X-ray absorption voltammetry, and extended X-ray absorption fine structure analyses were carried out on pristine and used samples, which were used to explain the photoelectrochemical behavior. The optical features of the oxide are evidenced by direct reflectance spectroscopy and fluorescence spectroscopy, and Mott–Schottky analysis at different pH values explains the exceptional activity at neutral pH

Understanding solid-gas reaction mechanisms by operando soft X-ray absorption spectroscopy at ambient pressure

Ambient-pressure operando soft X-ray absorption spectroscopy (soft-XAS) was applied to study the reactivity of hydroxylated SnO2 nanoparticles toward reducing gases. H2 was first used as a test case, showing that the gas phase and surface states can be simultaneously probed: Soft-XAS at the O K-edge gains sensitivity toward the gas phase, while at the Sn M4,5-edges, tin surface states are explicitly probed. Results obtained by flowing hydrocarbons (CH4 and CH3CHCH2) unequivocally show that these gases react with surface hydroxyl groups to produce water without producing carbon oxides and release electrons that localize on Sn to eventually form SnO. The partially reduced SnO2 – x layer at the surface of SnO2 is readily reoxidized to SnO2 by treating the sample with O2 at mild temperatures (>200 °C), revealing the nature of “electron sponge” of tin oxide. The experiments, combined with DFT calculations, allowed devising of a mechanism for dissociative hydrocarbon adsorption on SnO2, involving direct reduction of Sn sites at the surface via cleavage of C–H bonds and the formation of methoxy- and/or methyl-tin species at the surface