B18-Core EXAFS
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Diamond Proposal Number(s):
[14239]
Abstract: The rutiles (M,Ru)O2 (M = Mg, Zn, Co, Ni, Cu) are formed directly under hydrothermal conditions at 240 °C from potassium perruthenate and either peroxides of zinc or magnesium, or poorly crystalline oxides of cobalt, nickel or copper. The polycrystalline powders consist of lath-shaped crystallites, tens of nanometres in maximum dimension. Powder neutron diffraction shows that the materials have expanded a axis and contracted c axis compared to the parent RuO2, but there is no evidence of lowering of symmetry to other AO2-type structures, supported by Raman spectroscopy. Rietveld refinement shows no evidence for oxide non-stoichiometry and provides a formula (MxRu1-x)O2 with 0.14 < x < 0.2, depending on the substituent metal. This is supported by energy-dispersive X-ray analysis on the transmission electron microscope, while Ru K-edge XANES spectroscopy shows that upon inclusion of the substituent the average Ru oxidation state is increased to balance charge. Variable temperature magnetic measurements provide evidence for atomic homogeneity of the mixed metal materials, with suppression of the high temperature antiferromagnetism of RuO2 and increased magnetic moment. The new rutiles all show enhanced electrocatalysis compared to reference RuO2 materials for oxygen evolution in 1 M H2SO4 electrolyte at 60 °C, with higher specific and mass activity (per Ru) than a low surface area crystalline RuO2, and with less Ru dissolution over 1000 cycles compared to an RuO2 with a similar surface area. Magnesium substitution provides the optimum balance between stability and activity, despite leaching of the Mg2+ into solution, and this was proved in membrane electrode assemblies.
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Jun 2020
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E01-JEM ARM 200CF
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Diamond Proposal Number(s):
[22572, 21641]
Abstract: Porous polymer catalysts possess the potential to combine the advantages of heterogeneous and homogeneous cataly-sis, namely easy post-reaction recycling and high dispersion of active sites. Here we designed a -SO3H functionalized polyphenylene (PPhen) framework with purely sp2-hybridized carbons, which exhibited high activity in the hydration of alkynes including challenging aliphatic substrates such as 1-octyne. The superiority of the structure lies in its cova-lent crosslink in the xy-plane with π-π stacking interaction between the planes, enabling simultaneously high swella-bility and porosity (653 m2·g-1). High acidic sites density (2.12 mmol/g) was achieved at mild sulfonation condition. Similar turnover frequencies (0.015 ± 0.001 min-1) were obtained regardless of acidic density and crosslink content, suggesting high accessibility for all active sites over PPhen. In addition, the substituted benzene groups can activate alkynes through a T-shape CH/π interaction, as indicated by the 8 cm-1 and 16 cm-1 red shift of the alkyne C-H stretch-ing peak for phenylacetylene and 1-octyne repectively in the IR spectra. These advantages render PPhen-SO3H a prom-ising candidate as a solid catalyst replacing the highly toxic liquid phase acids such as the mercury salt.
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May 2020
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B18-Core EXAFS
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Diamond Proposal Number(s):
[16478]
Abstract: The methodology for experimentally verifying the point-defect chemistry (site of substitution, valence, and charge compensation mechanisms) in manganese-doped SrTiO3 ceramics is presented for dilute and non-dilute dopant concentrations. Experimental and theoretical techniques have strengths and weaknesses depending upon defect types and concentrations, so a combinatorial characterization approach is required. Using electron-paramagnetic-resonance and X-ray-absorption-fine-structure measurements combined with density functional theory calculations, the charge compensation mechanisms and local structural relaxations for five unique manganese defect centers are identified. Mn4+, as an isovalent dopant, occupies the octahedrally coordinated Ti sites without the need for charge compensation; its smaller ionic radius relative to Ti is accommodated by isotropic contraction of the [MnO6] octahedra. Mn3+ is an aliovalent dopant that also favors the Ti sites with the [MnO6] octahedra exhibiting Jahn-Teller distortions. The charge difference associated with the Mn3+Ti4+ substitution is compensated by formation of oxygen vacancies. A more complex behavior is observed for the Mn2+ species which can occupy either the Sr or Ti sites depending on the Sr/Ti ratio. The effects of Mn concentration or high-temperature annealing on the site preference (i.e., Sr vs Ti) are negligible. The Mn2+ species on the Sr sites are strongly off-centered in the relatively large cuboctahedral cages within the oxygen framework. The dynamic nature of the Mn displacements in these configurations is confirmed using ab initio molecular dynamics simulations.
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May 2020
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I11-High Resolution Powder Diffraction
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Diamond Proposal Number(s):
[13284]
Abstract: There has been much interest recently in hybrid improper ferroelectrics – materials which adopt polar, ferroelectric structures due to a complex tilting and twisting of the MO6 octahedra which constitute perovskite and related struc-tures. Using a combination of synchrotron X-ray powder diffraction and high-resolution neutron powder diffraction data the temperature-dependent phase transitions of a series of n = 2 Dion-Jacobson oxides have been investigated. RbNdM2O7 undergoes a transition from a polar, aˉaˉc+/-(aˉaˉc+) distorted I2cm phase to an antipolar, aˉb0cˉ/-(aˉb0)cˉ distorted Cmca phase at T = 790 K and 500 K for M = Nb and Ta respectively. There is a subsequent transition to an a0a0cˉ/a0a0-cˉ distorted I4/mcm structure at 865 K and 950 K for M = Nb and Ta respectively, before a transition to the undistorted P4/mmm aristotype structure. In contrast CsNdM2O7 undergoes a transition from a polar, aˉaˉc+ distorted P21am structure to an antipolar, aˉb0cˉ distorted C2/m phase at T = 625 K and 330 K for M = Nb and Ta respectively, with a subsequent phase transition to the undistorted P4/mmm aristotype structure at 800 K and 820 K for M = Nb and Ta respectively. A plot of Tc against the relative stability of the 4 polar Dion-Jacobson phases compared to the corre-sponding aristotype P4/mmm structures (calculated from first-principles DFT), yields a strong linear relation suggest-ing Tc is not proportional to the enthalpy change at the ferroelectric phase transition.
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Apr 2020
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I09-Surface and Interface Structural Analysis
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Theodore D. C.
Hobson
,
Laurie J
Phillips
,
Oliver S
Hutter
,
Huw
Shiel
,
Jack E. N.
Swallow
,
Christopher N.
Savory
,
Pabitra K
Nayak
,
Silvia
Mariotti
,
Bhaskar
Das
,
Leon
Bowen
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Matthew J.
Smiles
,
Mark A
Farnworth
,
Guillaume
Zoppi
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Henry J.
Snaith
,
Chris
Leighton
,
David O.
Scanlon
,
Vinod R.
Dhanak
,
Ken
Durose
,
Tim D.
Veal
,
Jonathan D
Major
Diamond Proposal Number(s):
[21431]
Open Access
Abstract: The carrier type of Sb2Se3 was evaluated for both thin films and bulk crystals via a range of complementary techniques. X-ray photoelectron spectroscopy (XPS), hot-probe, hall effect and surface photo-voltage spectroscopy showed material synthesized from Sb2Se3 granulate mate-rial to be n-type with chlorine identified as an unintentional n-type dopant via secondary ion mass spectrometry analysis. The validity of chlorine as a dopant was con-firmed by synthesis of intrinsic crystals from metallic precursors and subsequent n-type doping by the addition of MgCl2. Chlorine was also shown to be a substitutional n-type shallow dopant by density functional theory calculations. TiO2/Sb2Se3 n-n isotype heterojunction solar cells of 7.3% efficiency based are demonstrated with band alignment analyzed via XPS.
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Mar 2020
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E02-JEM ARM 300CF
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Diamond Proposal Number(s):
[21980]
Abstract: Advances in the production of two-dimensional (2D) materials such as graphene and MoS2 during the past two decades have spurred the search for other van der Waals materials with distinct functional properties. However, reducing the dimensionality of bulk van der Waals materials can lead to structural rearrangement and chemical degradation, especially in the presence of air. These challenges have slowed the progress of the discovery and analysis of chemically diverse 2D materials. Here, we provide a case study on the shear exfoliation of a class of wide band gap van der Waals materials termed II–VI layered hybrids (II–VI LHs) and show how reducing their dimension influences their structural and chemical stabilities. ZnSe(butylamine) and ZnSe(octylamine) are exfoliated, yielding shear-thinned material whose resistance toward degradation via oxidation is studied in depth by a variety of macro- and microscopic characterization techniques. Mechanical energy input, solvent–ligand interaction, and exposure to ambient conditions all play important roles in the stability of these materials. Our findings suggest that moderately coordinating alkylamine layers stabilize 2D materials that would otherwise degrade during exfoliation and exposure to air.
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Mar 2020
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Philip A. E.
Murgatroyd
,
Matthew J.
Smiles
,
Christopher N.
Savory
,
Thomas P.
Shalvey
,
Jack E. N.
Swallow
,
Nicole
Fleck
,
Craig M.
Robertson
,
Frank
Jaeckel
,
Jonathan
Alaria
,
Jonathan D.
Major
,
David O.
Scanlon
,
Tim D.
Veal
Abstract: The van der Waals material GeSe is a potential solar absorber, but its optoelectronic properties are not yet fully understood. Here, through a combined theoretical and experimental approach, the optoelectronic and structural properties of GeSe are determined. A fundamental absorption onset of 1.30 eV is found at room temperature, close to the optimum value according to the Shockley-Queisser detailed balance limit, in contrast to previous reports of an indirect fundamental transition of 1.10 eV. The measured absorption spectra and first-principles joint density of states are mutually consistent, both exhibiting an additional distinct onset $\sim$0.3~eV above the fundamental absorption edge. The band gap values obtained from first-principles calculations converge, as the level of theory and corresponding computational cost increases, to 1.33 eV from the quasiparticle self-consistent GW method, including the solution to the Bethe-Salpeter equation. This agrees with the 0~K value determined from temperature-dependent optical absorption measurements. Relaxed structures based on hybrid functionals reveal a direct fundamental transition in contrast to previous reports. The optoelectronic properties of GeSe are resolved with the system described as a direct semiconductor with a 1.30 eV room temperature band gap. The high level of agreement between experiment and theory encourages the application of this computational methodology to other van der Waals materials.
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Mar 2020
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I15-1-X-ray Pair Distribution Function (XPDF)
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Patrick
Hirschle
,
Christian
Hirschle
,
Konstantin
Böll
,
Markus
Döblinger
,
Miriam
Höhn
,
Joshua M.
Tuffnell
,
Christopher W.
Ashling
,
David
Keen
,
Thomas D.
Bennett
,
Joachim O.
Rädler
,
Ernst
Wagner
,
Michael
Peller
,
Ulrich
Lächelt
,
Stefan
Wuttke
Diamond Proposal Number(s):
[21260]
Abstract: Iron(III) fumarate materials are well suited for biomedical applications as they feature biocompatible building blocks, porosity, chemical functionalizability, and magnetic resonance imaging (MRI) activity. The synthesis of these materials however is difficult to control, and it has been challenging to produce monodisperse particle sizes and morphologies that are required in medical use. Here, we report the optimization of iron(III) fumarate nano- and microparticle synthesis by surfactant-free methods, including room temperature, solvothermal, microwave, and microfluidic conditions. Four variants of iron(III) fumarate with distinct morphologies were isolated and are characterized in detail. Structural characterization shows that all iron(III) fumarate variants exhibit the metal–organic framework (MOF) structure of MIL-88A. Nanoparticles with a diameter of 50 nm were produced, which contain crystalline areas not exceeding 5 nm. Solvent-dependent swelling of the crystalline particles was monitored using in situ X-ray diffraction. Cytotoxicity experiments showed that all iron(III) fumarate variants feature adequate biotolerability and no distinct interference with cellular metabolism at low concentrations. Magnetic resonance relaxivity studies using clinical MRI equipment, on the other hand, proved that the MRI contrast characteristics depend on particle size and morphology. All in all, this study demonstrates the possibility of tuning the morphological appearance of iron(III) fumarate particles and illustrates the importance of optimizing synthesis conditions for the development of new biomedical materials.
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Mar 2020
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B18-Core EXAFS
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Ronghuan
Zhang
,
Paul E.
Pearce
,
Vanessa
Pimenta
,
Jordi
Cabana
,
Haifeng
Li
,
Daniel
Alves Dalla Corte
,
Artem M.
Abakumov
,
Gwenaëlle
Rousse
,
Domitille
Giaume
,
Michael
Deschamps
,
Alexis
Grimaud
Diamond Proposal Number(s):
[12559]
Abstract: Water electrolysis is considered as a promising way to store and convert excess renewable energies into hydrogen, which is of high value for many chemical transformation processes such as the Haber-Bosch process. However, to allow for the wide-spread of the polymer electrolyte membrane water electrolysis (PEMWE) technology, the main challenge lies in the design of robust catalysts for oxygen evolution reaction (OER) under acidic conditions since most of transition metal-based oxides un-dergo structural degradation under these harsh acidic conditions. To broaden the variety of candidate materials as OER cata-lysts, a cation-exchange synthetic route was recently explored to reach crystalline pronated iridates with unique structural properties and stability. In this work, a new protonated phase H3.6IrO4∙3.7H2O, prepared via Sr2+/H+ cation exchange at room temperature starting from the parent Ruddlesden-Popper Sr2IrO4 phase, is described. This is the first discovery of crystalline protonated iridate forming from a perovskite-like phase, adopting a layered structure with apex-linked IrO6 octahedra. Fur-thermore, H3.6IrO4∙3.7H2O is found to possess not only an enhanced specific catalytic activity, superior to that of other perov-skite-based iridates, but also a mass activity comparable to that of nanosized IrOx particles, while showing an improved cata-lytic stability owing to its ability to reversibly exchange protons in acid.
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Mar 2020
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I09-Surface and Interface Structural Analysis
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Benjamin A. D.
Williamson
,
Thomas J.
Featherstone
,
Sanjayan S.
Sathasivam
,
Jack E. N.
Swallow
,
Huw
Shiel
,
Leanne A. H.
Jones
,
Matthew J
Smiles
,
Anna
Regoutz
,
Tien-lin
Lee
,
Xueming
Xia
,
Christopher
Blackman
,
Pardeep K.
Thakur
,
Claire J.
Carmalt
,
Ivan P.
Parkin
,
Tim D.
Veal
,
David O.
Scanlon
Diamond Proposal Number(s):
[18195, 21431]
Abstract: Transparent conducting oxides (TCOs) are ubiquitous in modern consumer electronics. SnO2 is an earth abundant, cheaper alternative to In2O3 as a TCO however, its performance in terms of electrical properties lags behind that of In2O3. Based on the recent discovery of mobility and conductivity enhancements in In2O3 from resonant dopants, we use a combination of state-of-the-art hybrid density functional theory calculations, high resolution photoelectron spectroscopy and semiconductor statistics modelling to understand what the optimal dopant is to maximise performance of SnO2-based TCOs. We demonstrate that Ta is the optimal dopant for high performance SnO2, as it is a resonant dopant which is readily incorporated into SnO2 with the Ta 5d states sitting ca. 1.4 eV above the conduction band minimum. Experimentally, the electron effective mass of Ta doped SnO2 was shown to be 0.23m0, compared to 0.29m0 seen with conventional Sb doping, explaining its ability to yield higher mobilities and conductivities.
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Feb 2020
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