B18-Core EXAFS
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Diamond Proposal Number(s):
[11646, 8986]
Abstract: Iridate pyrochlore oxides (Na,Ca)2-x(Ir2-yMy)O6·nH2O (M = Sb, Zr, Ru, Rh) are studied as resilient electrocatalysts for the oxygen evolution reaction under acid conditions. The materials crystallise from aqueous solution under alkali hydrothermal conditions with 10-40 nm crystallite size. Refinement of their crystal structures using both powder neutron and X-ray diffraction determined the composition of the materials, and Ir LIII-edge XANES spectroscopy shows the average Ir oxidation state to be close to 4.5 in all materials, consistent with bond valence sums. All materials show high electrocatalytic activity for the oxygen evolution reaction and the electrocatalyst which best maintains activity on cycling is the sodium-free Ca2-xIr2O6·nH2O, while the (Na,Ca)2-xIr2O6·nH2O material shows highest activity when normalised for surface area. In membrane electrode assemblies, carbon corrosion is minimised, making the materials suitable for use in catalyst layers in proton exchange membrane devices, such as electrolysers and fuel cells. Under strongly acidic conditions it is proved that while A-site Ca and Na are readily leached, the average pyrochlore structure is maintained, as is electrocatalytic activity, with charge balance achieved by inclusion of protons in the pyrochlore structure in the form of bridging hydroxyls, as seen using inelastic neutron scattering spectroscopy.
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Nov 2021
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B18-Core EXAFS
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Diamond Proposal Number(s):
[22410]
Abstract: Bismuth-based coordination polymers (CPs) have recently attracted attention as catalyst precursors for the electrocatalytic CO2 reduction reaction (eCO2RR). We present a comparative study by investigating six bismuth-based compounds in-depth to elucidate the correlation between their structures and their catalytic CO2-to-formate conversion. Thereby, we identify structural indicators of the pristine CPs resulting in optimized catalytic performance, paving the way for future design of CP derived catalysts. The structural properties of the six pristine materials vary in terms of porosity (from non-porous to 495 m2 g−1), linker type (carboxylate- or phenolate-based), thermal- and chemical stability, and metal content. Herein, electrochemical studies are combined with comprehensive structural investigations using electron microscopy, powder X-ray diffraction, and X-ray absorption spectroscopy. Our study reveals that low chemical stability of the pristine CPs is crucial for the conversion of the precursors into active Bi2O2CO3 and of paramount importance for the eCO2RR activity, while the nature of the pristine material mostly influence the catalyst morphology and transport properties. Of the six investigated CPs, the best performing compounds selectively convert CO2 to formate with faradaic efficiencies in the range 80(3)–95(3)% and current densities of 5(1)–8(1) mA cm−2 at −0.97 VRHE.
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Nov 2021
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B18-Core EXAFS
E01-JEM ARM 200CF
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Emerson Cristofer
Kohlrausch
,
Higor A.
Centurion
,
Rhys
Lodge
,
Xuanli
Luo
,
Thomas
Slater
,
Marcos J. L.
Santos
,
Sanliang
Ling
,
Valmor R.
Mastelaro
,
Matthew J.
Cliffe
,
Renato
Vitalino Goncalves
,
Jesum
Alves Fernandes
Diamond Proposal Number(s):
[25120, 17198, 24914]
Open Access
Abstract: Atomically-dispersed metal catalysts (ADMCs) on surfaces have demonstrated
high activity and selectivity in many catalytic reactions. However, dispersing and stabilising
individual atoms in support materials in an atom/energy-efficient scalable way still presents a
significant challenge. Currently, the synthesis of ADMCs involves many steps and further
filtration procedures, creating a substantial hurdle to their production at industrial scale. In this
work, we develop a new pathway for producing ADMCs in which Pt atoms are stabilised in the
nitrogen-interstices of a graphitic carbon nitride (g-C3N4) framework using scalable, solvent-free,
one-pot magnetron sputtering deposition. Our approach has the highest reported rate of ADMC
production of 4.8 mg h-1 and generates no chemical waste. Deposition of only 0.5 weight percent
of Pt onto g-C3N4 led to improved hydrogen production by factor of ca. 3333 ± 450 when compared to bare g-C3N4. PL analysis showed that the deposition of Pt atoms onto g-C3N4 suppressed the
charge carrier recombination from the photogenerated electron-hole pairs of Pt/g-C3N4 thereby
enhance hydrogen evolution. Scanning transmission electron microscope imaging before and after
the hydrogen evolution reaction revealed that the Pt atoms stabilised in g-C3N4 have a high
stability, with no agglomeration observed. Herein, it is shown that this scalable and clean approach
can produce effective ADMCs with no further synthetic steps required, and that they can be readily
used for catalytic reactions.
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Nov 2021
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I15-Extreme Conditions
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Diamond Proposal Number(s):
[21610]
Abstract: The high pressure stability of α-Cu3V2O8 has been investigated via complementary high pressure synchrotron X-ray diffraction experiments and theoretical density functional theory calculations. The results of both experiment and theory are in close agreement. The main result of this work is that α-Cu3V2O8 undergoes a pressure-induced chemical decomposition into CuO and V2O5 at a modest pressure of ∼1.35 GPa according to the experimental observations, and at ∼2.45 GPa according to the calculations. The decomposition is investigated with enthalpy calculations and one of the main driving factors is the stability of the octhedral oxygen-coordination of the metal atoms in the decompositon products. Additionally, we determine the bulk modulus of α-Cu3V2O8 to be the lowest of all known vanadates, at 52(2) GPa, due to its crystal structure, and determine the corresponding isothermal compressibility tensor.
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Sep 2021
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Open Access
Abstract: The environmental burden of fossil fuels and the rising impact of global warming have created an urgent need for sustainable clean energy sources. This has led to widespread interest in thermoelectric (TE) materials to recover part of the ∼60% of global energy currently wasted as heat as usable electricity. Oxides are particularly attractive as they are thermally stable, chemically inert, and formed of earth-abundant elements, but despite intensive efforts there have been no reports of oxide TEs matching the performance of flagship chalcogenide materials such as PbTe, Bi2Te3 and SnSe. A number of ternary X4Y2Z mixed-anion systems, including oxides, have predicted band gaps in the useful range for several renewable-energy applications, including as TEs, and some also show the complex crystal structures indicative of low lattice thermal conductivity. In this study, we use ab initio calculations to investigate the TE performance of two structurally-similar mixed-anion oxypnictides, Ca4Sb2O and Ca4Bi2O. Electronic-structure and band-alignment calculations using hybrid density-functional theory (DFT), including spin–orbit coupling, suggest that both materials are likely to be p-type dopable with large charge-carrier mobilities. Lattice-dynamics calculations using third-order perturbation theory predict ultra-low lattice thermal conductivities of ∼0.8 and ∼0.5 W m−1 K−1 above 750 K. Nanostructuring to a crystal grain size of 20 nm is predicted to further reduce the room temperature thermal conductivity by around 40%. Finally, we use the electronic- and thermal-transport calculations to estimate the thermoelectric figure of merit ZT, and show that with p-type doping both oxides could potentially serve as promising earth-abundant oxide TEs for high-temperature applications.
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Sep 2021
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I09-Surface and Interface Structural Analysis
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Matthew J.
Smiles
,
Jonathan M.
Skelton
,
Huw
Shiel
,
Leanne A. H.
Jones
,
Jack E. N.
Swallow
,
Holly J.
Edwards
,
Thomas
Featherstone
,
Philip A. E.
Murgatroyd
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Vinod R.
Dhanak
,
Tim D.
Veal
Diamond Proposal Number(s):
[21431, 23160]
Open Access
Abstract: Germanium sulfide and germanium selenide bulk crystals were prepared using a melt growth technique. X-ray photoemission spectroscopy (XPS) was used to determine ionisation potentials of 5.74 and 5.48 eV for GeS and GeSe respectively. These values were used with the previously-measured band gaps to establish the natural band alignments with potential window layers for solar cells and to identify CdS and TiO2 as sensible choices. The ionisation potential of GeS is found to be smaller than in comparable materials. Using XPS and hard x-ray photoemission (HAXPES) measurements in conjunction with density-functional theory calculations, we demonstrate that stereochemically active Ge 4s lone pairs are present at the valence-band maxima. Our work thus provides direct evidence for active lone pairs in GeS and GeSe, with important implications for the applications of these and related materials, such as Ge-based perovskites.
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Sep 2021
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Open Access
Abstract: Transparent conducting materials (TCMs) are crucial in the operation of modern opto-electronic devices, combining the lucrative properties of optical transparency and electronic conductivity. More than ever we rely on display and touch screens, energy efficient windows and solar cells in our day to day lives. The market for transparent electronics is projected to surpass $3.8 billion by 2026 as the automotive industry seek to incorporate pop-up displays into driver windshields, and the prospect of touch-enabled transparent displays challenges the traditional mouse and keyboard mode of computer operation. However, these new technologies rely heavily on the development of high performance p-type TCMs, a task that has posed a significant challenge to researchers for decades. This review will cover the basic theory and design principles of transparent conductors, followed by an overview of early p-type TCMs and their shortcomings. We discuss the impact of high-throughput screening studies on materials discovery and critically assess the family of p-type halide perovskites that emerged from these, ruling them as unsuitable candidates for high-performance applications. We find that phosphides, selenides, tellurides and halides are the most promising emerging materials, capable of achieving greater valence band dispersion than traditional oxides, and we discuss the challenges facing these more exotic systems. The smorgasbord of materials presented in this review should guide experimental and computational scientists alike in the next phase of p-type transparent conductor research.
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Aug 2021
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B07-C-Versatile Soft X-ray beamline: Ambient Pressure XPS and NEXAFS
I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[21754, 22365]
Abstract: SnS and SnS2 are earth abundant layered semiconductors that owing to their optoelectronic properties have been proposed as materials for different photovoltaic, photosensing and photocatalytic applications. The intrinsic efficiency of these materials for such applications is driven by their charge transfer dynamics, which in turn depend on their electronic structure and the interaction of the molecular orbitals involved in the charge transfer process. In this publication, we provide a step-by-step description of the use of the core hole clock method to obtain orbital dependent charge transfer times for SnS2 and SnS down to the attosecond time scale. We use both S 1s and Sn 3d core holes, with natural core hole lifetimes of 1.3 fs and 300 as, as time references to obtain a complete picture of electron delocalisation times across the conduction band of both materials. Ultrafast electron delocalisation times, lower than 5 femtoseconds and as low as tens of attoseconds, are measured for electrons excited to the unoccupied molecular orbitals of both materials. SnS delocalisation times are found to be shorter than those for SnS2 for all probed molecular orbitals, with delocalisation times being more than an order of magnitude shorter for higher lying molecular orbitals. DFT calculations show that charge transfer dynamics are strongly influenced by the interlayer interaction within these materials. The slower delocalisation of electrons in SnS2 can be linked to the restricted out of plane spatial dispersion of the molecular orbitals in the conduction band and consequent limitation of the electron delocalisation pathways. The results presented in this paper highlight the high potential of combining the core hole clock method with high-level DFT calculations to study orbital dependent charge transfer in semiconductor materials for optoelectronic applications down to the attosecond scale.
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Jul 2021
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B18-Core EXAFS
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Diamond Proposal Number(s):
[25651]
Open Access
Abstract: In this paper, we demonstrate a novel synthetic route to assemble reduced graphene oxide (rGO) uniformly coated on BiSI composite and investigate its potential as the active electrode material for supercapacitors. In this strategy, graphene oxide (GO) was not a simple physical mixture with the BiSI material but bismuth cations were uniformly anchored on the surface of GO by chemical bonding during material growth and the size of GO can determine the final size of rGO coated BiSI composite. The galvanostatic charge–discharge measurement results show that the BiSI–rGO electrode has a maximum specific capacity of 234 C g−1 at the current density of 1 A g−1 and excellent capacity retention of 92.4% after 2000 cycles. In situ XANES and EXAFS were employed to study the electrochemical oxidation and reduction processes of the bismuth-based material with rGO coating and investigate the origins of the structural stabilities. The results show that our novel rGO coating route can not only significantly increase the capacity but also improve cycling stability.
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Jun 2021
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B18-Core EXAFS
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Jamie W.
Gittins
,
Chloe J.
Balhatchet
,
Yuan
Chen
,
Cheng
Liu
,
David G.
Madden
,
Sylvia
Britto
,
Matthias J.
Golomb
,
Aron
Walsh
,
David
Fairen-Jimenez
,
Sian E.
Dutton
,
Alexander C.
Forse
Diamond Proposal Number(s):
[14239]
Open Access
Abstract: Two-dimensional electrically conductive metal–organic frameworks (MOFs) have emerged as promising model electrodes for use in electric double-layer capacitors (EDLCs). However, a number of fundamental questions about the behaviour of this class of materials in EDLCs remain unanswered, including the effect of the identity of the metal node and organic linker molecule on capacitive performance, and the limitations of current conductive MOFs in these devices relative to traditional activated carbon electrode materials. Herein, we address both these questions via a detailed study of the capacitive performance of the framework Cu3(HHTP)2 (HHTP = 2,3,6,7,10,11-hexahydroxytriphenylene) with an acetonitrile-based electrolyte, finding a specific capacitance of 110–114 F g−1 at current densities of 0.04–0.05 A g−1 and a modest rate capability. By directly comparing its performance with the previously reported analogue, Ni3(HITP)2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene), we illustrate that capacitive performance is largely independent of the identity of the metal node and organic linker molecule in these nearly isostructural MOFs. Importantly, this result suggests that EDLC performance in general is uniquely defined by the 3D structure of the electrodes and the electrolyte, a significant finding not demonstrated using traditional electrode materials. Finally, we probe the limitations of Cu3(HHTP)2 in EDLCs, finding a limited stable double-layer voltage window of 1 V and only a modest capacitance retention of 81% over 30 000 cycles, both significantly lower than state-of-the-art porous carbons. These important insights will aid the design of future conductive MOFs with greater EDLC performances.
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Jun 2021
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