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Abstract: Using standard X-ray diffraction (XRD) techniques for crystal structure refinement often leaves the precise location of protons ambiguous. Core level spectroscopies can be used as a tool for proton location refinement due to the direct sensitivity of the core electron binding energy to the acceptor-proton distance. We have demonstrated how both Near Edge X-ray Absorption Fine Structure (NEXAFS) and X-ray Photoelectron Spectroscopy (XPS) can be used as a diagnostic tool for proton transfer in hydrogen bonds through a consistent core level shift of +2 eV in the nitrogen acceptor 1s emission where proton transfer is observed.1,2 We have studied a range of organic crystals consisting of both salts and cocrystals, where the core level shift appears to be a universal effect.1,2 This core level shift can be linked directly to the electrostatic effect of a shorter N-H distance.
Using a model based entirely on classical electrostatics, we have modelled the effect of the proton position on the 1s -> π* transition of N 1s electrons, to understand the contribution of the electrostatic interaction to the core level shift observed with changing proton position. From this classical electrostatic model, we observe a continuous change in core electron binding energy with change in proton position. A shift of +2 eV is observed between N-H distances of 1 Å - 2 Å, as observed experimentally 1,2, with structures containing centred protons resulting in an intermediate shift, fully in keeping with a continuum.1 This indicates that the primary interaction involved is electrostatic, further suggesting that the effect observed in XPS and NEXAFS is direct electrostatic sensitivity to the proton position. Future work will investigate the use of NEXAFS spectroscopy with additional information from Density Functional Theory calculations to more accurately refine proton positions in organic crystals than the current standard methods used in XRD.
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Jul 2021
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B07-B-Versatile Soft X-ray beamline: High Throughput
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Diamond Proposal Number(s):
[29340]
Abstract: The development of low-cost, robust and efficient non-noble metal electrocatalysts is still a pursuit for the hydrogen evolution reaction (HER). Herein, a self-standing electrocatalyst, Ni2P/CoP nanosheet, was fabricated directly on three-dimensional Ni foams by two facile steps, which illustrated both high activity and stability for HER in different electrolytes. Benefiting from the porous structures of nanosheets with large specific surface area and the hybrid Ni2P/CoP, the as-prepared electrocatalyst presented remarkable HER with overpotentials of 65.2 mV and 87.8 mV to reach a current density of -10 mA cm-2 in neutral and alkaline media, respectively. Density function theory calculations revealed a lower activation energy of water dissociation and efficient HER steps of hybrid Ni2P/CoP nanosheets compared with mono CoP. The self-standing electrocatalyst maintained excellent chemical stability. Additionally, the HER process in domestic wastewater was realized with more impressive performance by using Ni2P/CoP nanosheets compared with commercial Pt/C. Hydrogen was continuously generated for 20 h in mildly alkaline dishwashing wastewater. This work provides a feasible way to fabricate non-noble metal and self-standing hybrid bimetallic phosphides for HER in neutral and alkaline media, showing great potential for efficient hydrogen production by re-utilizing wastewater resources.
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Apr 2022
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B07-B-Versatile Soft X-ray beamline: High Throughput
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Isabel
Rodríguez-García
,
Dmitry
Galyamin
,
Laura
Pascual
,
Pilar
Ferrer
,
Miguel A.
Peña
,
David
Grinter
,
Georg
Held
,
Mohamed
Abdel Salam
,
Mohamed
Mokhtar
,
Katabathini
Narasimharao
,
Maria
Retuerto
,
Sergio
Rojas
Diamond Proposal Number(s):
[28150]
Open Access
Abstract: Ru mixed oxides are the most active catalysts for the oxygen evolution reaction (OER) in acid electrolyte. However, their stability is seriously compromised during the reaction. In this work we show that it is possible to enhance both OER activity and durability of SrRuO3 mixed oxide by the partial doping with K+ in Sr2+ sites. Sr1-xKxRuO3 perovskites (x = 0.00, 0.05, 0.10 and 0.20) have been synthesized by wet chemistry. The partial doping with K+ cations led to oxides with Ru atoms in a higher oxidation state. In addition, K-doping resulted in perovskites with slightly higher symmetry. The performance of the K-doped perovskites for the OER was assessed in acid electrolyte. Clearly, the K-doped materials, especially Sr0.80K0.20RuO3, display higher activity (lower E10) and significantly higher durability than the undoped sample SrRuO3. The results indicate that chemical modifications on Ru perovskites can be a suitable strategy to improve the stability of Ru phases during the OER.
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Feb 2022
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B07-B-Versatile Soft X-ray beamline: High Throughput
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Diamond Proposal Number(s):
[29334]
Open Access
Abstract: It is important to be able to identify the precise position of H-atoms in hydrogen bonding interactions to fully understand the effects on the structure and properties of organic crystals. Using a combination of near-edge X-ray absorption fine structure (NEXAFS) spectroscopy and density functional theory (DFT) quantum chemistry calculations, we demonstrate the sensitivity of core-level X-ray spectroscopy to the precise H-atom position within a donor-proton-acceptor system. Exploiting this sensitivity, we then combine the predictive power of DFT with the experimental NEXAFS, confirming the H-atom position identified using single-crystal X-ray diffraction (XRD) techniques more easily than using other H-atom sensitive techniques, such as neutron diffraction. This proof of principle experiment confirms the H-atom positions in structures obtained from XRD, providing evidence for the potential use of NEXAFS as a more accurate and easier method of locating H-atoms within organic crystals.
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May 2022
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B07-B-Versatile Soft X-ray beamline: High Throughput
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Diamond Proposal Number(s):
[31119]
Open Access
Abstract: Tin-containing layers with different degrees of oxidation are uniformly distributed along the length of silicon nanowires formed by a top-down method by applying metalorganic chemical vapor deposition. The electronic and atomic structure of the obtained layers is investigated by applying nondestructive surface-sensitive X-ray absorption near edge spectroscopy using synchrotron radiation. The results demonstrated, for the first time, a distribution effect of the tin-containing phases in the nanostructured silicon matrix compared to the results obtained for planar structures at the same deposition temperatures. The amount and distribution of tin-containing phases can be effectively varied and controlled by adjusting the geometric parameters (pore diameter and length) of the initial matrix of nanostructured silicon. Due to the occurrence of intense interactions between precursor molecules and decomposition by-products in the nanocapillary, as a consequence of random thermal motion of molecules in the nanocapillary, which leads to additional kinetic energy and formation of reducing agents, resulting in effective reduction of tin-based compounds to a metallic tin state for molecules with the highest penetration depth in the nanostructured silicon matrix. This effect will enable clear control of the phase distributions of functional materials in 3D matrices for a wide range of applications.
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Jan 2023
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B07-B-Versatile Soft X-ray beamline: High Throughput
B18-Core EXAFS
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Jichao
Zhang
,
Xuedan
Song
,
Liqun
Kang
,
Jiexin
Zhu
,
Longxiang
Liu
,
Qing
Zhang
,
Dan J. I.
Brett
,
Paul R.
Shearing
,
Liqiang
Mai
,
Ivan P.
Parkin
,
Guanjie
He
Diamond Proposal Number(s):
[29340, 29271]
Open Access
Abstract: Layered hydroxides have shown superior catalytic activity for the electrocatalytic organic compound oxidation reaction. However, metal leaching can lead to uncontrollable structural phase transformation. Here, we report a Cr-Ni(OH)2 electrocatalyst as a model of a pre-catalyst for the identification of the structure-performance relationship. The optimized electrocatalyst delivered superb performances, i.e., a low potential of 1.38 V (versus reversible hydrogen electrode [RHE]) to reach 100 mA cm−2 and stable activity over 200 h at 10 mA cm−2. In situ analyses and theoretical calculations demonstrate that well-tuned electronic structures and the superhydrophilic-superaerophobic surface can enable rapid urea oxidation reaction (UOR) kinetics, which reduces the specific adsorption OH− and significantly depresses Cr dopants leaching, and this helps to maintain high UOR performance. Furthermore, the crucial role of mass transfer improvement to alleviate the structural decay under high potentials is disclosed.
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Oct 2022
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B07-B-Versatile Soft X-ray beamline: High Throughput
E02-JEM ARM 300CF
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Longxiang
Liu
,
Liqun
Kang
,
Arunabhiram
Chutia
,
Jianrui
Feng
,
Martyna
Michalska
,
Pilar
Ferrer
,
David
Grinter
,
Georg
Held
,
Yeshu
Tan
,
Fangjia
Zhao
,
Fei
Guo
,
David
Hopkinson
,
Christopher
Allen
,
Yanbei
Hou
,
Junwen
Gu
,
Ioannis
Papakonstantinou
,
Paul
Shearing
,
Dan
Brett
,
Ivan P.
Parkin
,
Guanjie
He
Diamond Proposal Number(s):
[29340, 32501, 30614, 29809, 32058]
Open Access
Abstract: The electrochemical synthesis of hydrogen peroxide (H2O2) via a two-electron (2e-) oxygen reduction reaction (ORR) process provides a promising alternative to replace the energy-intensive anthraquinone process. However, the development of efficient electrocatalysts is still facing lots of challenges like insufficient understanding of active sites. Herein, we develop a facile template-protected strategy to synthesize a highly active quinone-rich porous carbon catalyst (PCC) for H2O2 electrochemical production. The optimized PCC900 exhibits unprecedented activity and selectivity, of which the onset potential reaches 0.83 V vs. reversible hydrogen electrode in 0.1 M KOH and the H2O2 selectivity is over 95 % in a wide potential range. Comprehensive synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy combined with electrocatalytic characterizations reveals the positive correlation between quinone content and 2e- ORR performance. The effectiveness of chair-form quinone groups as the most efficient active sites is highlighted by the molecule-mimic strategy and theoretical analysis.
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Mar 2023
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B07-B-Versatile Soft X-ray beamline: High Throughput
I10-Beamline for Advanced Dichroism
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Jack E. N.
Swallow
,
Michael W.
Fraser
,
Nis-Julian H.
Kneusels
,
Jodie F.
Charlton
,
Christopher G.
Sole
,
Conor M. E.
Phelan
,
Erik
Bjorklund
,
Peter
Bencok
,
Carlos
Escudero
,
Virginia
Perez-Dieste
,
Clare P.
Grey
,
Rebecca J.
Nicholls
,
Robert S
Weatherup
Diamond Proposal Number(s):
[25647, 29213, 30816]
Open Access
Abstract: The solid electrolyte interphase (SEI) that forms on Li-ion battery anodes is critical to their long-term performance, however observing SEI formation processes at the buried electrode-electrolyte interface is a significant challenge. Here we show that operando soft X-ray absorption spectroscopy in total electron yield mode can resolve the chemical evolution of the SEI during electrochemical formation in a Li-ion cell, with nm-scale interface sensitivity. O, F, and Si K-edge spectra, acquired as a function of potential, reveal when key reactions occur on high-capacity amorphous Si anodes cycled with and without fluoroethylene carbonate (FEC). The sequential formation of inorganic (LiF) and organic (-(C=O)O-) components is thereby revealed, and results in layering of the SEI. The addition of FEC leads to SEI formation at higher potentials which is implicated in the rapid healing of SEI defects and the improved cycling performance observed. Operando TEY-XAS offers new insights into the formation mechanisms of electrode-electrolyte interphases and their stability for a wide variety of electrode materials and electrolyte formulations.
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Oct 2022
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Abstract: Solar fuels production is a cornerstone in the development of emerging sustainable energy conversion and storage technologies. Light-induced H2 production from water represents one of the most crucial challenges to produce renewable fuel. Metal-organic frameworks (MOFs) are being investigated, due to the ability to assemble new structures with the use of suitable photoactive building blocks. However, the identification of the reaction intermediates remains elusive, which negatively impacts in the design of more efficient materials. Here, we report the synthesis and characterization of a new MOF prepared with the use of bismuth and dithieno[3,2-b:2',3'-d]thiophene-2,6-dicarboxylic acid (DTTDC), an electron-rich linker with hole transport ability. By combining theoretical studies and time-resolved spectroscopies, such as core hole clock and laser flash photolysis measurements, we have completed a comprehensive study at different time scales (fs to-ms) to determine the effect of competitive reactions on the overall H2 production. We found that an intermediate radical anion is formed upon reaction of photogenerated holes with an electron donor, which play a key role in the photoelectrocatalytic processes. These results shed new light on the use of MOFs for solar fuel production.
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Dec 2019
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
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Diamond Proposal Number(s):
[19299]
Abstract: Methanol is a promising chemical for the safe and efficient storage of hydrogen, where methanol conversion reactions can generate a hydrogen-containing gas mixture. Understanding the chemical state of the catalyst over which these reactions occur and the interplay with the adsorbed species present is key to the design of improved catalysts and process conditions. Here we study polycrystalline Cu foils using ambient pressure X-ray spectroscopies to reveal the Cu oxidation state and identify the adsorbed species during partial oxidation (CH3OH + O2), steam reforming (CH3OH + H2O), and autothermal reforming (CH3OH + O2 + H2O) of methanol at 200 °C surface temperature and in the mbar pressure range. We find that the Cu surface remains highly metallic throughout partial oxidation and steam reforming reactions, even for oxygen-rich conditions. However, for autothermal reforming the Cu surface shows significant oxidation towards Cu2O. We rationalise this behaviour on the basis of the shift in equilibrium of the CH3OH* + O* ⇌ CH3O* + OH* caused by the addition of H2O.
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Mar 2020
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