9 research outputs found
Luminescence of Eu<sup>3+</sup> Activated CaF<sub>2</sub> and SrF<sub>2</sub> Nanoparticles: Effect of the Particle Size and Codoping with Alkaline Ions
Eu<sup>3+</sup> doped CaF<sub>2</sub> and SrF<sub>2</sub> nanoparticles
were synthesized through a facile hydrothermal technique, using citrate
ions as capping agents and Na<sup>+</sup> or K<sup>+</sup> 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 Eu<sup>3+</sup> ions enter into
the fluorite CaF<sub>2</sub> and SrF<sub>2</sub> structure as substitutional
defects on the metal site. Laser site selective spectroscopy demonstrates
that the Eu<sup>3+</sup> 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 Eu<sup>3+</sup> sites are prevalent for bigger nanoparticles. Eu<sup>3+</sup> ions in high symmetry sites present lifetimes of the <sup>5</sup>D<sub>0</sub> level around 27 ms, among the longest lifetimes found
in the literature for Eu<sup>3+</sup> activated materials. As a proof
of concept of possible use of the Eu<sup>3+</sup> 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 Eu<sup>3+</sup> 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 Xâray 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 ÎłâFeOOH: Stability, Reversibility, and Nature of the Active Phase under Hydrogen Evolution
α-FeOOH
(goethite) and Îł-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 â0.4 V vs RHE ca.) and under hydrogen evolution
conditions an unknown ironÂ(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>3Ì
1<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 NanoparticleâProtein Complexes: A Combined SAXS and SANS Study
We
propose a novel method for determining the structural and thermodynamic
properties of nanoparticleâprotein 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 nanoparticleâprotein
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 nanoparticleâprotein
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 Xâray Absorption Spectroscopy
Energy-dispersive
X-ray absorption spectroscopy was applied, aimed at solving the problem
of the structure and stability of a copperÂ(II) lactate complex in
alkaline solution, used as a precursor for the electrodeposition of
Cu<sub>2</sub>O. 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
copperÂ(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 Ă
)
and four oxygen atoms located quite far apart (ca. 3.1â3.2
Ă
). 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 + Al<sub>2</sub>O<sub>3</sub> â ZnAl<sub>2</sub>O<sub>4</sub> Solid-State Reaction: I. Reactivity of Films Deposited onto the Sapphire (110) and (012) Faces
The
initial steps of the reaction between ZnO and Al<sub>2</sub>O<sub>3</sub> have been investigated with X-ray diffraction, atomic
force microscopy, and X-ray absorption spectroscopy at the ZnâK
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)<sub>zincite</sub> â„ (110)<sub>sapphire</sub> 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)
Ă 10<sup>â9</sup> cms<sup>â1</sup> at 1000 °C.
At the (110)<sub>zincite</sub> â„ (012)<sub>sapphire</sub> 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/IrO<sub>2</sub> electrodes, made of an Ir core and an IrO<sub>2</sub> shell,
that are designed and synthesized with an innovative, green approach,
in order to have a higher surface/bulk ratio of IrâO 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<sub><i>x</i></sub>. The main conclusion of this study is that the Ir core is noninteracting
with the IrO<sub><i>x</i></sub> 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 IrÂ(III) and (V), as previously established for the
highly hydrated IrO<sub><i>x</i></sub> material
An Efficient Cu<sub><i>x</i></sub>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
copperÂ(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<sup>â2</sup> 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 MottâSchottky 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 â 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 âelectron spongeâ 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 CâH bonds and the formation of methoxy- and/or methyl-tin species at the surface