21 research outputs found
Carbon Core Electron Spectra of Polycyclic Aromatic Hydrocarbons
Aromaticity
profoundly affects molecular orbitals in polycyclic
aromatic hydrocarbons. X-ray core electron spectroscopy has observed
that carbon 1sâĎ* transitions can be broadened or even
split in some polycyclic systems, although the origin of the effect
has remained obscure. The Ď electrons in polycyclic systems
are typically classified in the Clar model as belonging to either
true aromatic sextets (similar to benzene) or isolated double bonds
(similar to olefins). Here, bulk-sensitive carbon core excitation
spectra are presented for a series of polycyclic systems and show
that the magnitude of the 1sâĎ* splitting is determined
primarily by the ratio of true aromatic sextets to isolated double
bonds. The observed splitting can be rationalized in terms of ground
state energetics as described by HuĚckel, driven by the Ď
electron structure described by Clar. This simple model including
only ground state energetics is shown to explain the basics physics
behind the spectral evolution for a broad set of polycyclic aromatic
hydrocarbons, although some residual deviations between this model
and experiment can likely be improved by including a more detailed
electronic structure and the core hole effect
Dioxygen Oxidation Cu(II) â Cu(III) in the Copper Complex of <i>cyclo</i>(LysâdHis-βAla-His): A Case Study by EXAFS and XANES Approach
A former spectroscopic study of CuÂ(II) coordination by
the 13-membered
ring cyclic tetrapeptide <i>c</i>(Lys-dHis-βAla-His)
(DK13), revealed the presence, at alkaline pH, of a stable peptide/CuÂ(III)
complex formed in solution by atmospheric dioxygen oxidation. To understand
the nature of this coordination compound and to investigate the role
of the His residues in the CuÂ(III) species formation, Cu K-edge XANES,
and EXAFS spectra have been collected for DK13 and two other 13-membered
cyclo-peptides: the diastereoisomer <i>c</i>(Lys-His-βAla-His)
(LK13), and <i>c</i>(Gly-βAla-Gly-Lys) (GK13), devoid
of His residues. Comparison of pre-edge peak features with those of
Cu model compounds, allowed us to get information on copper oxidation
state in two of the three peptides, DK13 and GK13: DK13 contains only
CuÂ(III) ions in the experimental conditions, while GK13 binds only
with CuÂ(II). For LK13/Cu complex, EXAFS spectrum suggested and UVâvis
analysis confirmed the presence of a mixture of CuÂ(II) and CuÂ(III)
coordinated species. Theoretical XANES spectra have been calculated
by means of the MXAN code. The good agreement between theoretical
and experimental XANES data collected for DK13, suggests that the
refined structure, at least in the first coordination shell around
Cu, is a good approximation of the DK13/CuÂ(III) coordination species
present at strongly alkaline pH. All the data are consistent with
a slightly distorted pyramidal CuN<sub>4</sub> unit, coming from the
peptide bonds. Surprisingly, the His side-chains seemed not involved
in the final, stable, CuÂ(III) scaffold
Charge and Spin-State Characterization of Cobalt Bis(<i>o</i>âdioxolene) Valence Tautomers Using Co Kβ Xâray Emission and LâEdge Xâray Absorption Spectroscopies
The valence tautomeric
states of CoÂ(phen)Â(3,5-DBQ)<sub>2</sub> and
CoÂ(tmeda)Â(3,5-DBQ)<sub>2</sub>, where 3,5-DBQ is either the semiquinone
(SQ<sup>â</sup>) or catecholate (Cat<sup>2â</sup>) form
of 3,5-di-<i>tert</i>-butyl-1,2-benzoquinone, have been
examined by a series of cobalt-specific X-ray spectroscopies. In this
work, we have utilized the sensitivity of 1s3p X-ray emission spectroscopy
(Kβ XES) to the oxidation and spin states of 3d transition-metal
ions to determine the cobalt-specific electronic structure of valence
tautomers. A comparison of their Kβ XES spectra with the spectra
of cobalt coordination complexes with known oxidation and spin states
demonstrates that the low-temperature valence tautomer can be described
as a low-spin Co<sup>III</sup> configuration and the high-temperature
valence tautomer as a high-spin Co<sup>II</sup> configuration. This
conclusion is further supported by Co L-edge X-ray absorption spectroscopy
(L-edge XAS) of the high-temperature valence tautomers and ligand-field
atomic-multiplet calculations of the Kβ XES and L-edge XAS spectra.
The nature and strength of the magnetic exchange interaction between
the cobalt center and SQ<sup>â</sup> in cobalt valence tautomers
is discussed in view of the effective spin at the Co site from Kβ
XES and the molecular spin moment from magnetic susceptibility measurements
Deciphering Photochemical Reaction Pathways of Aqueous Tetrachloroauric Acid by Xâray Transient Absorption Spectroscopy
Photolysis reaction pathways of [Au(III)Cl4]â in aqueous solution have been investigated by
time-resolved X-ray
absorption spectroscopy. Ultraviolet excitation directly breaks the
AuâCl bond in [Au(III)Cl4]â to
form [Au(II)Cl3]â that becomes highly
reactive within 79 ps. Disproportionation of [Au(II)Cl3]â generates [Au(I)Cl2]â, which is stable for â¤10 Îźs. In contrast, intense near-infrared
lasers photolyze water to generate hydrated electrons, which then
reduce [Au(III)Cl4]â to [Au(II)Cl3]â at 5 ns. Hydrated electrons further induce
a chain reaction from [Au(II)Cl3]â to
[Au(0)Cl]â by successively removing one Clâ. The zero-valency Au anions quickly polymerize and condense to form
Au nanoparticles, which become the dominating product after 400 s.
Our results reveal that the condensation of zero-valency Au starts
with dimerization of gold clusters coordinated with chloride ions
rather than direct condensation of pristine Au atoms
Structural and Chemical Evolution of Amorphous Nickel Iron Complex Hydroxide upon Lithiation/Delithiation
Development of novel electrode materials
is essential to achieve
high-performance lithium ion batteries. Here, we demonstrate that
amorphous nickel iron complex hydroxides (NiâFeâOH)
synthesized by a laserâchemical method can be used as a potential
conversion anode material for lithium storage. Complementary characterizations,
including ensemble-averaged X-ray absorption spectroscopy, spatially
resolved electron energy-loss spectroscopy, and energy dispersive
X-ray spectroscopy in a scanning transmission electron microscope,
were performed to reveal the chemical and structural evolutions of
the active hydroxide particles undergoing electrochemical cycling.
The solidâelectrolyte interphase (SEI) layer with a primary
component of lithium fluoride (LiF) was found and remained robust
on the particle surface during the charge/discharge processes, which
suggests that the LiF-containing SEI layer plays a critical role in
maintaining the stable capacity retention and good reversibility of
the NiâFeâOH anode
Kβ Valence to Core Xâray Emission Studies of Cu(I) Binding Proteins with Mixed Methionine â Histidine Coordination. Relevance to the Reactivity of the M- and Hâsites of Peptidylglycine Monooxygenase
Biological systems use copper as
a redox center in many metalloproteins,
where the role of the metal is to cycle between its +1 and +2 oxidation
states. This chemistry requires the redox potential to be in a range
that can stabilize both CuÂ(I) and CuÂ(II) states and often involves
protein-derived ligand sets involving mixed histidineâmethionine
coordination that balance the preferences of both oxidation states.
Transport proteins, on the other hand, utilize copper in the CuÂ(I)
state and often contain sites comprised predominately of the cuprophilic
residue methionine. The electronic factors that allow enzymes and
transporters to balance their redox requirements are complex and are
often elusive due to the dearth of spectroscopic probes of the CuÂ(I)
state. Here we present the novel application of X-ray emission spectroscopy
to copper proteins via a study of a series of mixed His-Met copper
sites where the ligand set varies in a systematic way between the
His<sub>3</sub> and Met<sub>3</sub> limits. The sites are derived
from the wild-type peptidylglycine monooxygenase (PHM), two single-site
variants which replicate each of its two copper sites (Cu<sub>M</sub>-site and Cu<sub>H</sub>-site), and the transporters CusF and CusB.
Clear differences are observed in the Kβ<sub>2,5</sub> region
at the Met<sub>3</sub> and His<sub>3</sub> limits. CusB (Met<sub>3</sub>) has a distinct peak at 8978.4 eV with a broad shoulder at 8975.6
eV, whereas Cu<sub>H</sub> (His<sub>3</sub>) has two well-resolved
features: a more intense feature at 8974.8 eV and a second at 8977.2
eV. The mixed coordination sphere CusF (Met<sub>2</sub>His) and the
PHM Cu<sub>M</sub> variant (Met<sub>1</sub>His<sub>2</sub>) have very
similar spectra consisting of two features at 8975.2 and 8977.8 eV.
An analysis of DFT calculated spectra indicate that the intensity
of the higher energy peak near 8978 eV is mediated by mixing of ligand-based
orbitals into the Cu d<sup>10</sup> manifold, with S from Met providing
more intensity by facilitating increased Cu pâd mixing. Furthermore,
reaction of WT PHM with CO (an oxygen analogue) produced the M site
CO complex, which showed a unique XES spectrum that could be computationally
reproduced by including interactions between CuÂ(I) and the CO ligand.
The study suggests that the valence-to-core (VtC) region can not only
serve as a probe of ligand speciation but also offer insight into
the coordination geometry, in a fashion similar to XAS pre-edges,
and may be sufficiently sensitive to the coordination of exogenous
ligands to be useful in the study of reaction mechanisms
Tuning Complex Transition Metal Hydroxide Nanostructures as Active Catalysts for Water Oxidation by a LaserâChemical Route
Diverse transition metal hydroxide
nanostructures were synthesized by laser-induced hydrolysis in a liquid
precursor solution for alkaline oxygen evolution reaction (OER). Several
active OER catalysts with fine control of composition, structure,
and valence state were obtained including (Li<sub><i>x</i></sub>)Â[Ni<sub>0.66</sub>Mn<sub>0.34</sub>(OH)<sub>2</sub>]Â(NO<sub>3</sub>)Â(CO<sub>3</sub>) ¡ mH<sub>2</sub>O, Li<sub><i>x</i></sub>[Ni<sub>0.67</sub>Co<sub>0.33</sub>(OH)<sub>2</sub>]Â(NO<sub>3</sub>)<sub>0.25</sub>(ORO)<sub>0.35</sub> ¡ mH<sub>2</sub>O, etc. An operate overpotential less than 0.34 V at current density
of 10 mA cm<sup>â2</sup> was achieved. Such a controllable
laserâchemical route for assessing complex nanostructures in
liquids opens many opportunities to design novel functional materials
for advanced applications
Highly Active Surface Structure in Nanosized Spinel Cobalt-Based Oxides for Electrocatalytic Water Splitting
Spinel cobalt-based
oxides are a promising family of materials
for water splitting to replace currently used noble-metal catalysts.
Identifying the highly active facet and the corresponding coordinated
structure of surface redox centers is pivotal for the rational design
of low-cost and efficient nanosized catalysts. Using high-resolution
transmission electron microscopy and advanced X-ray techniques, as
well as ab initio modeling, we found that the activity of Co<sup>3+</sup> ions exhibits the surface dependence owing to the variability of
its electronic configurations. Our calculation shows that the Co<sup>3+</sup> site in {100} facet of nanosized Li<sub>2</sub>Co<sub>2</sub>O<sub>4</sub> exhibits an impressive intrinsic activity with low
overpotential, far lower than that of the {110} and {111} facets.
The unique, well-defined CoO<sub>5</sub> square-pyramidal structure
in this nonpolar surface stabilizes the unusual intermediate-spin
states of the Co<sup>3+</sup> ion. Specially, we unraveled that oxygen
ion anticipates the redox process via the strong hybridization Co
3dâO 2p state, which produces a 3d<sub><i>z</i></sub><sup><sub>2</sub>1.1</sup> filling orbit. Finally, a spin-correlated
energy diagram as a function of CoâO distance was devised,
showing that the covalency of CoâO significantly affects the
spin state of Co<sup>3+</sup> ions. We suggest that the nonpolar surface
that contains CoO<sub>5</sub> units in the edge-sharing systems with
the short CoâO bond distance is a potential candidate for alkaline
water electrolysis
Electrochemical Oxidation of Size-Selected Pt Nanoparticles Studied Using in Situ High-Energy-Resolution Xâray Absorption Spectroscopy
High-energy-resolution fluorescence-detected X-ray absorption
spectroscopy
(HERFD-XAS) has been applied to study the chemical state of âź1.2
nm size-selected Pt nanoparticles (NPs) in an electrochemical environment
under potential control. Spectral features due to chemisorbed hydrogen,
chemisorbed O/OH, and platinum oxides can be distinguished with increasing
potential. Pt electro-oxidation follows two competitive pathways involving
both oxide formation and Pt dissolution
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Oxygen Release Induced Chemomechanical Breakdown of Layered Cathode Materials
Chemical and mechanical
properties interplay on the nanometric
scale and collectively govern the functionalities of battery materials.
Understanding the relationship between the two can inform the design
of battery materials with optimal chemomechanical properties for long-life
lithium batteries. Herein, we report a mechanism of nanoscale mechanical
breakdown in layered oxide cathode materials, originating from oxygen
release at high states of charge under thermal abuse conditions. We
observe that the mechanical breakdown of charged Li<sub>1â<i>x</i></sub>Ni<sub>0.4</sub>Mn<sub>0.4</sub>Co<sub>0.2</sub>O<sub>2</sub> materials proceeds via a two-step pathway involving intergranular
and intragranular crack formation. Owing to the oxygen release, sporadic
phase transformations from the layered structure to the spinel and/or
rocksalt structures introduce local stress, which initiates microcracks
along grain boundaries and ultimately leads to the detachment of primary
particles, <i>i.e.</i>, intergranular crack formation. Furthermore,
intragranular cracks (pores and exfoliations) form, likely due to
the accumulation of oxygen vacancies and continuous phase transformations
at the surfaces of primary particles. Finally, finite element modeling
confirms our experimental observation that the crack formation is
attributable to the formation of oxygen vacancies, oxygen release,
and phase transformations. This study is designed to directly observe
the chemomechanical behavior of layered oxide cathode materials and
provides a chemical basis for strengthening primary and secondary
particles by stabilizing the oxygen anions in the lattice