I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[20589, 23523]
Abstract: The design and performance of an electrochemical cell and solution flow system optimized for the collection of X-ray absorption spectra from solutions of species sensitive to photodamage is described. A combination of 3D CAD and 3D printing techniques facilitates highly optimized design with low unit cost and short production time. Precise control of the solution flow is critical to both minimizing the volume of solution needed and minimizing the photodamage that occurs during data acquisition. The details of an integrated four-syringe stepper-motor-driven pump and associated software are described. It is shown that combined electrochemical and flow control can allow repeated measurement of a defined volume of solution, 100 µl, of samples sensitive to photoreduction without significant change to the X-ray absorption near-edge structure and is demonstrated by measurements of copper(II) complexes. The flow in situ electrochemical cell allows the collection of high-quality X-ray spectral measurements both in the near-edge region and over an extended energy region as is needed for structural analysis from solution samples. This approach provides control over photodamage at a level at least comparable with that achieved using cryogenic techniques and at the same time eliminates problems associated with interference due to Bragg peaks.
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Mar 2021
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B18-Core EXAFS
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[16558]
Abstract: Copper aluminate spinel (CuO.CuAl2O4) is the favoured Cr-free substitute for the copper chromite catalyst (CuO.CuCr2O4) in the industrial hydrogenation of aldehydes. New insights in the catalytic mechanism were obtained by systematically studying the structure and activity of these catalysts including effects of manganese as a catalyst component. The hydrogenation of butyraldehyde to butanol was studied as a model reaction and the active structure was characterised using X-ray diffraction, temperature programmed reduction, N2O chemisorption, EXAFS and XANES, including in-situ investigations. The active catalyst is a reduced spinel lattice that is stabilised by protons, with copper metal nanoparticles grown upon its surface. Incorporation of Mn into the spinel lattice has a profound effect on the spinel structure. Mn stabilises the spinel towards reduction of CuII to Cu0 by occupation of tetrahedral sites with Mn cations, but also causes decreased catalytic activity. Structural data, combined with the effect on catalysis, indicate a predominantly interface-based reaction mechanism, involving both the spinel and copper nanoparticle surface in protonation and reduction of the aldehyde. The electron reservoir of the metallic copper particles is regenerated by the dissociative adsorption and oxidation of H2 on the metal surface. The generated protons are stored in the spinel phase, acting as proton reservoir. Cu(I) species located within the spinel and identified by XANES are probably not involved in the catalytic cycle.
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Mar 2021
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B18-Core EXAFS
I20-Scanning-X-ray spectroscopy (XAS/XES)
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V.
Celorrio
,
D. J.
Fermin
,
L.
Calvillo
,
A.
Leach
,
H.
Huang
,
G.
Granozzi
,
J. A.
Alonso
,
A.
Aguadero
,
R. M.
Pinacca
,
A. E.
Russell
,
D.
Tiwari
Diamond Proposal Number(s):
[10306, 15151, 16479]
Abstract: Oxygen electrocatalysis at transition metal oxides is one of the key challenges underpinning electrochemical energy conversion systems, involving a delicate interplay of the bulk electronic structure and surface coordination of the active sites. In this work, we investigate for the first time the structure–activity relationship of A2RuMnO7 (A = Dy3+, Ho3+, and Er3+) nanoparticles, demonstrating how orbital mixing of Ru, Mn, and O promotes high density of states at the appropriate energy range for oxygen electrocatalysis. The bulk structure and surface composition of these multicomponent pyrochlores are investigated by high-resolution transmission electron microscopy, X-ray diffraction, X-ray absorption spectroscopy, X-ray emission spectroscopy (XES), and X-ray photoemission spectroscopy (XPS). The materials exhibit high phase purity (cubic fcc with a space group Fd3̅m) in which variations in M–O bonds length are less than 1% upon replacing the A-site lanthanide. XES and XPS show that the mean oxidation state at the Mn-site as well as the nanoparticle surface composition was slightly affected by the lanthanide. The pyrochlore nanoparticles are significantly more active than the binary RuO2 and MnO2 toward the 4-electron oxygen reduction reaction in alkaline solutions. Interestingly, normalization of kinetic parameters by the number density of electroactive sites concludes that Dy2RuMnO7 shows twice higher activity than benchmark materials such as LaMnO3. Analysis of the electrochemical profiles supported by density functional theory calculations reveals that the origin of the enhanced catalytic activity is linked to the mixing of Ru and Mn d-orbitals and O p-orbitals at the conduction band which strongly overlap with the formal redox energy of O2 in solution. The activity enhancement strongly manifests in the case of Dy2RuMnO7 where the Ru/Mn ratio is closer to 1 in comparison with the Ho3+ and Er3+ analogs. These electronic effects are discussed in the context of the Gerischer formalism for electron transfer at the semiconductor/electrolyte junctions.
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Jan 2021
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I20-EDE-Energy Dispersive EXAFS (EDE)
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[17574]
Open Access
Abstract: Deep eutectic solvents (DES) and their hydrated mixtures are used for solvothermal routes towards greener functional nanomaterials. Here we present the first static structural and in situ studies of the formation of iron oxide (hematite) nanoparticles in a DES of choline chloride[thin space (1/6-em)]:[thin space (1/6-em)]urea where xurea = 0.67 (aka. reline) as an exemplar solvothermal reaction, and observe the effects of water on the reaction. The initial speciation of Fe3+ in DES solutions was measured using extended X-ray absorption fine structure (EXAFS), while the atomistic structure of the mixture was resolved from neutron and X-ray diffraction and empirical potential structure refinement (EPSR) modelling. The reaction was monitored using in situ small-angle neutron scattering (SANS), to determine mesoscale changes, and EXAFS, to determine local rearrangements of order around iron ions. It is shown that iron salts form an octahedral [Fe(L)3(Cl)3] complex where (L) represents various O-containing ligands. Solubilised Fe3+ induced subtle structural rearrangements in the DES due to abstraction of chloride into complexes and distortion of H-bonding around complexes. EXAFS suggests the complex forms [–O–Fe–O–] oligomers by reaction with the products of thermal hydrolysis of urea, and is thus pseudo-zero-order in iron. In the hydrated DES, the reaction, nucleation and growth proceeds rapidly, whereas in the pure DES, the reaction initially proceeds quickly, but suddenly slows after 5000 s. In situ SANS and static small-angle X-ray scattering (SAXS) experiments reveal that nanoparticles spontaneously nucleate after 5000 s of reaction time in the pure DES before slow growth. Contrast effects observed in SANS measurements suggest that hydrated DES preferentially form 1D particle morphologies because of choline selectively capping surface crystal facets to direct growth along certain axes, whereas capping is restricted by the solvent structure in the pure DES.
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Jan 2021
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Open Access
Abstract: The heterogeneously catalyzed oxidation of bioethanol offers a promising route to bio-based acetic acid. Here, we assess an alternative method to support gold nanoparticles, which aims to improve selectivity to acetic acid through minimizing over-oxidation to carbon dioxide. The most promising support system is 5 wt % titanium on silica, which combines the high surface area of silica with the stabilizing effect of titania on the gold particles. Compared to gold–silica systems, which require a complex synthesis method, small quantities of titanium promoted the formation of gold nanoparticles during a simple deposition–precipitation. Characterization of the catalyst with X-ray absorption spectroscopy shows that titanium is highly dispersed in the form of small, possibly dimeric, titanium(IV) structures, which are isolated and stabilize gold nanoparticles, possibly minimizing sintering effects during synthesis. The size of the gold particles depends on the pre-treatment of the titanium–silica support before gold deposition, with larger titanium structures hosting larger gold particles. Acetic acid yield over the titanium–silica-supported gold systems improved by about 1.6 times, compared to pure titania-supported gold. The high activity of those catalysts suggests that bulk, crystalline titania is not required for the reaction, encouraging the use of mixed supports to combine their benefits. Those support systems, besides improving selectivity, offer high surface area and a low-cost filler material, which brings ethanol oxidation one step further to the industry. Additionally, the low loading of titanium permits studying the reaction mechanisms on the gold–titanium interface with bulk characterization techniques.
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Dec 2020
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I10-Beamline for Advanced Dichroism
I20-Scanning-X-ray spectroscopy (XAS/XES)
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Myron S.
Huzan
,
Manuel
Fix
,
Matteo
Aramini
,
Peter
Bencok
,
J. Frederick W.
Mosselmans
,
Shusaku
Hayama
,
Franziska A.
Breitner
,
Leland B.
Gee
,
Charles J.
Titus
,
Marie-anne
Arrio
,
Anton
Jesche
,
Michael L.
Baker
Diamond Proposal Number(s):
[21117, 23982]
Open Access
Abstract: Large single-ion magnetic anisotropy is observed in lithium nitride doped with iron. The iron sites are two-coordinate, putting iron doped lithium nitride amongst a growing number of two coordinate transition metal single-ion magnets (SIMs). Uniquely, the relaxation times to magnetisation reversal are over two orders of magnitude longer in iron doped lithium nitride than other 3d-metal SIMs, and comparable with high-performance lanthanide-based SIMs. To understand the origin of these enhanced magnetic properties a detailed characterisation of electronic structure is presented. Access to dopant electronic structure calls for atomic specific techniques, hence a combination of detailed single-crystal X-ray absorption and emission spectroscopies are applied. Together K-edge, L2,3-edge and Kβ X-ray spectroscopies probe local geometry and electronic structure, identifying iron doped lithium nitride to be a prototype, solid-state SIM, clean of stoichiometric vacancies where Fe lattice sites are geometrically equivalent. Extended X-ray absorption fine structure and angular dependent single-crystal X-ray absorption near edge spectroscopy measurements determine FeI dopant ions to be linearly coordinated, occupying a D6h symmetry pocket. The dopant engages in strong 3dπ-bonding, resulting in an exceptionally short Fe–N bond length (1.873(7) Å) and rigorous linearity. It is proposed that this structure protects dopant sites from Renner–Teller vibronic coupling and pseudo Jahn–Teller distortions, enhancing magnetic properties with respect to molecular-based linear complexes. The Fe ligand field is quantified by L2,3-edge XAS from which the energy reduction of 3dz2 due to strong 4s mixing is deduced. Quantification of magnetic anisotropy barriers in low concentration dopant sites is inhibited by many established methods, including far-infrared and neutron scattering. We deduce variable temperature L3-edge XAS can be applied to quantify the J = 7/2 magnetic anisotropy barrier, 34.80 meV (∼280 cm−1), that corresponds with Orbach relaxation via the first excited, MJ = ±5/2 doublet. The results demonstrate that dopant sites within solid-state host lattices could offer a viable alternative to rare-earth bulk magnets and high-performance SIMs, where the host matrix can be tailored to impose high symmetry and control lattice induced relaxation effects.
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Oct 2020
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[23538]
Abstract: Phase-pure magnesium ferrite (MgFe2O4) spinel nanocrystals are synthesized by a fast microwave-assisted route. The elemental composition is optimized via the ratio of the precursor mixture and controlled by energy-dispersive X-ray spectroscopy. Fine-tuning of the magnetic properties without changing the overall elemental composition is demonstrated by superconducting quantum interference device (SQUID) magnetometry and Mössbauer spectroscopy. Together with X-ray absorption spectroscopy and X-ray emission spectroscopy, we confirm that the degree of cation inversion is altered by thermal annealing. We can correlate the magnetic properties with both the nanosize influence and the degree of inversion. The resulting nonlinear course of saturation magnetization (Ms) in correlation with the particle diameter allows to decouple crystallite size and saturation magnetization, by this providing a parameter for the production of very small nanoparticles with high Ms with great potential for magnetic applications like ferrofluids or targeted drug delivery. Our results also suggest that the optical band gap of MgFe2O4 is considerably larger than the fundamental electronic band gap because of the d5 electronic configuration of the iron centers. The presented different electronic transitions contributing to the absorption of visible light are the explanation for the large dissent among the band gaps and band potentials found in the literature.
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Oct 2020
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[11356]
Abstract: We have investigated the role of oxygen stoichiometry and structural properties in the modulation of Co valence and spin state in single-layer La2–xAxCoO4±δ (A = Sr, Ca; 0 ≤ x ≤ 1) perovskites as well as the interplay between their local structural properties and the magnetic and charge-ordering phenomena. We show the results of high angular resolution powder X-ray diffraction and Co K-edge X-ray absorption and emission spectroscopy experiments on polycrystalline and single-crystal samples. The different doping-induced changes in the Co valence and spin state by Ca (or Sr) substitution can be understood in terms of the evolving oxygen stoichiometry. For Ca doping, the interstitial oxygen excess around the La/Ca atoms in underdoped samples is rapidly lost upon increasing the Ca content. The creation of oxygen vacancies leads to the stabilization of a mixed-valence Co2.5+ independently of the Ca content. In contrast, Sr substitution leads to almost stoichiometric samples and a lower oxygen vacancy concentration, which allows higher mixed-valence states for Co up to Co2.9+. The Co mixed-valence state along the two series is fluctuating between two valence states, Co2.4+ as in La2CoO4.2 and Co2.9+ as in LaSrCoO3.91, that become periodically ordered for the charge-ordered phases around the half-doping. The X-ray emission derived spin states agree well with the Co fluctuating mixed-valence state derived from X-ray absorption spectroscopy on consideration of a distribution of high-spin Co2+ and low-spin Co3+. Furthermore, there is no quenching of the orbital contribution for the high-spin Co2+, as concluded from a comparison with macroscopic magnetization measurements. Doping holes are mainly located in the ab plane and have a strong oxygen 2p character. The major lattice distortions, which are different for Sr and Ca doping, occur along the c axis, where changes in the oxygen stoichiometry take place. Moreover, charge-order transitions are clearly shown from the anomalous increase of the c lattice parameter with an increase in the temperature above 500 K but there is no signature for a temperature-dependent spin-state transition.
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Oct 2020
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[19013]
Open Access
Abstract: Surfactant-mediated chemical routes allow one to synthesize highly engineered shape- and size-controlled nanocrystals. However, the occurrence of capping agents on the surface of the nanocrystals is undesirable for selected applications. Here, a novel approach to the production of shape-controlled nanocrystals which exhibit high thermal stability is demonstrated. Ceria nanocubes obtained by surfactant-mediated synthesis are embedded inside a highly porous silica aerogel and thermally treated to remove the capping agent. Powder X-ray Diffraction and Scanning Transmission Electron Microscopy show the homogeneous dispersion of the nanocubes within the aerogel matrix. Remarkably, both the size and the shape of the ceria nanocubes are retained not only throughout the aerogel syntheses but also upon thermal treatments up to 900 °C, while avoiding their agglomeration. The reactivity of ceria is measured by in situ High-Energy Resolution Fluorescence Detected - X-ray Absorption Near Edge Spectroscopy at the Ce L3 edge, and shows the reversibility of redox cycles of ceria nanocubes when they are embedded in the aerogel. This demonstrates that the enhanced reactivity due to their prominent {100} crystal facets is preserved. In contrast, unsupported ceria nanocubes begin to agglomerate as soon as the capping agent decomposes, leading to a degradation of their reactivity already at 275 °C.
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Sep 2020
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I20-Scanning-X-ray spectroscopy (XAS/XES)
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Diamond Proposal Number(s):
[18039]
Abstract: Cu2O is an attractive photocathode for important renewable energy reactions such as water splitting and CO2 reduction. Electrodeposition is commonly used to deposit Cu2O films on conductive substrates due to its simplicity and consistency. However, structural descriptors, linking electrodeposition parameters, film structure and the catalytic properties are elusive. A variety of Cu2O films reported by many research groups would often display vastly different electronic properties and catalytic activity, while appear indistinguishable under common characterisation tools. In this work, we take a systematic look into electrochemically deposited Cu2O and investigate the impact of deposition parameters towards the bulk and surface chemistry of the deposited film. Specifically, we employ high resolution XANES for thorough quantitative analysis of the Cu2O films, alongside more common characterisation methods like XRD, SEM and Raman spectroscopy. Photoelectrochemical (PEC) studies reveal an unexpected trend, where the highest PEC activity appears to correlate with the amount of Cu2+ content. Other factors which also affect the PEC activity and stability are film thickness and crystallite grain size. Our study shows that the use of high resolution XANES, though not perfect due to possible self-absorption issue, is apt for extracting compositional descriptor in concentrated thin film samples from the pre-edge energy position analysis. This descriptor can serve as a guide for future development of more active Cu2O based films for wide range of PEC processes as well as for solar cell applications.
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Sep 2020
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