I11-High Resolution Powder Diffraction
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Ashok S.
Menon
,
Seda
Ulusoy
,
Dickson O.
Ojwang
,
Lars
Riekehr
,
Christophe
Didier
,
Vanessa K.
Peterson
,
German
Salazar-alvarez
,
Peter
Svedlindh
,
Kristina
Edström
,
Cesar Pay
Gomez
,
William R.
Brant
Diamond Proposal Number(s):
[21804]
Open Access
Abstract: Li- and Mn-rich layered oxides show significant promise as electrode materials for future Li-ion batteries. However, an accurate description of its crystallography remains elusive, with both single-phase solid solution and multiphase structures being proposed for high performing materials such as Li1.2Mn0.54Ni0.13Co0.13O2. Herein, we report the synthesis of single- and multiphase variants of this material through sol–gel and solid-state methods, respectively, and demonstrate that its crystallography is a direct consequence of the synthetic route and not necessarily an inherent property of the composition, as previously argued. This was accomplished via complementary techniques that probe the bulk and local structure followed by in situ methods to map the synthetic progression. As the electrochemical performance and anionic redox behavior are often rationalized on the basis of the presumed crystal structure, clarifying the structural ambiguities is an important step toward harnessing its potential as an electrode material.
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Feb 2021
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I22-Small angle scattering & Diffraction
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Diamond Proposal Number(s):
[20757]
Open Access
Abstract: Understanding the kinetics of the crystallization process for organometal halide perovskite formation is critical in determining the crystalline, nanoscale morphology and therefore the electronic properties of the films produced during thin film formation from solution. In this work, in situ grazing incidence small-angle X-ray scattering (GISAXS) and optical microscopy measurements are used to investigate the processes of nucleation and growth of pristine mixed halide perovskite (MAPbI3–xClx) crystalline films deposited by bar coating at 60 °C, with and without additives in the solution. A small amount of 1,8-diiodooctane (DIO) and hydriodic acid (HI) added to MAPbI3–xClx is shown to increase the numbers of nucleation centers promoting heterogeneous nucleation and accelerate and modify the size of nuclei during nucleation and growth. A generalized formation mechanism is derived from the overlapping parameters obtained from real-time GISAXS and optical microscopy, which revealed that during nucleation, perovskite precursors cluster before becoming the nuclei that function as elemental units for subsequent formation of perovskite crystals. Additive-free MAPbI3–xClx follows reaction-controlled growth, in contrast with when DIO and HI are present, and it is highly possible that the growth then follows a hindered diffusion-controlled mechanism. These results provide important details of the crystallization mechanisms occurring and will help to develop greater control over perovskite films produced.
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Feb 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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I09-Surface and Interface Structural Analysis
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Diamond Proposal Number(s):
[19067]
Open Access
Abstract: High-end organic–inorganic lead halide perovskite semitransparent p–i–n solar cells for tandem applications use a phenyl-C61-butyric acid methyl ester (PCBM)/atomic layer deposition (ALD)-SnOx electron transport layer stack. Omitting the PCBM would be preferred for manufacturing, but has in previous studies on (FA,MA)Pb(Br,I)3 and (Cs,FA)Pb(Br,I)3 and in this study on Cs0.05FA0.79MA0.16PbBr0.51I2.49 (perovskite) led to poor solar cell performance because of a bias-dependent light-generated current. A direct ALD-SnOx exposure was therefore suggested to form a nonideal perovskite/SnOx interface that acts as a transport barrier for the light-generated current. To further investigate the interface formation during the initial ALD SnOx growth on the perovskite, the mass dynamics of monitor crystals coated by partial p–i–n solar cell stacks were recorded in situ prior to and during the ALD using a quartz crystal microbalance. Two major finds were made. A mass loss was observed prior to ALD for growth temperatures above 60 °C, suggesting the decomposition of the perovskite. In addition, a mostly irreversible mass gain was observed during the first exposure to the Sn precursor tetrakis(dimethylamino)tin(IV) that is independent of growth temperature and that disrupts the mass gain of the following 20–50 ALD cycles. The chemical environments of the buried interface were analyzed by soft and hard X-ray photoelectron spectroscopy for a sample with 50 ALD cycles of SnOx on the perovskite. Although measurements on the perovskite bulk below and the SnOx film above did not show chemical changes, additional chemical states for Pb, Br, and N as well as a decrease in the amount of I were observed in the interfacial region. From the analysis, these states and not the heating of the perovskite were concluded to be the cause of the barrier. This strongly suggests that the detrimental effects can be avoided by controlling the interfacial design.
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Jan 2021
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I09-Surface and Interface Structural Analysis
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Huw
Shiel
,
Oliver S.
Hutter
,
Laurie J.
Phillips
,
Jack E. N.
Swallow
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Matthew J.
Smiles
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Vinod R.
Dhanak
,
Jonathan D.
Major
,
Tim D.
Veal
Diamond Proposal Number(s):
[23160]
Abstract: Sb2Se3 is a promising material for use in photovoltaics, but the optimum device structure has not yet been identified. This study provides band alignment measurements between Sb2Se3, identical to that used in high-efficiency photovoltaic devices, and its two most commonly used window layers, namely, CdS and TiO2. Band alignments are measured via two different approaches: Anderson’s rule was used to predict an interface band alignment from measured natural band alignments, and the Kraut method was used in conjunction with hard X-ray photoemission spectroscopy to directly measure the band offsets at the interface. This allows examination of the effect of interface formation on the band alignments. The conduction band minimum (CBM) of TiO2 is found by the Kraut method to lie 0.82 eV below that of Sb2Se3, whereas the CdS CBM is only 0.01 eV below that of Sb2Se3. Furthermore, a significant difference is observed between the natural alignment- and Kraut method-determined offsets for TiO2/Sb2Se3, whereas there is little difference for CdS/Sb2Se3. Finally, these results are related to device performance, taking into consideration how these results may guide the future development of Sb2Se3 solar cells and providing a methodology that can be used to assess band alignments in device-relevant systems.
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Dec 2020
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B18-Core EXAFS
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Diamond Proposal Number(s):
[14239]
Abstract: Lithium-rich layered oxides and sodium layered oxides represent attractive positive electrode materials exhibiting excess capacity delivered by additional oxygen redox activity. However, structural degradation in the bulk and detrimental reactions with the electrolyte on the surface often occur, leading to limited reversibility of oxygen redox processes. Here we present the properties of P3-type Na0.67Mg0.2Mn0.8O2 synthesized under both air and oxygen. Both materials exhibit stable cycling performance in the voltage range 1.8-3.8 V where the Mn3+/Mn4+ redox couple entirely dominates the electrochemical reaction. Oxygen redox activity is triggered for both compounds in the wider voltage window 1.8-4.3 V with typical large voltage hysteresis from non-bonding O 2p states generated by substituted Mg. Interestingly, for the compound prepared under oxygen, an additional reversible oxygen redox activity is shown with exceptionally small voltage hysteresis (20 mV). The presence of vacancies in the transition metal layers is shown to play a critical role not only in forming unpaired O 2p states independent of substituted elements but also in stabilising the P3 structure during charge with reduced structural transformation to the O’3 phase at the end of discharge. This study reveals the important role of vacancies in P3-type sodium layered oxides to increase energy density using both cationic and anionic redox processes.
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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):
[21526]
Open Access
Abstract: Herein, the influence of iron on nanocasting of cobalt oxide nanowires and the performance of these materials for the oxygen evolution reaction (OER) are investigated. Pristine Co3O4 and mixed cobalt iron oxide nanowires with a diameter of 7 nm have been synthesized via a nanocasting route by using SBA 15 silica as a template. A small amount of iron added during the synthesis results in a decrease in the nanowires’ array length and induces the formation of a bimodal pore size distribution. Raman Spectroscopy, X-ray emission and high-energy resolution X-ray absorption spectroscopies further show that Fe incorporation alters the electronic structure by increasing the average distortion around the cobalt centers and the amount of Co2+ in tetrahedral sites. These affect the OER activity significantly; the overpotential of pristine Co3O4 at 10 mA/cm2 decreases from 398 to 378 mV, and the current density at 1.7 V increases from 107 to 150 mA/cm2 with the addition of iron at Co/Fe atomic ratio of 32. Furthermore, post-reaction characterization confirmed that both the morphology and electronic structure of nanowires remain intact after a long-term stability test.
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Aug 2020
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[14937]
Open Access
Abstract: In this study in situ wide angle X-ray scattering (WAXS) has been measured during the spin coating process used to make the precursor films required for the formation of thin films of perovskite. A customized hollow axis spin coater was developed to permit the scattered X-rays to be collected in transmission geometry during the deposition process. Spin coating is the technique most commonly used in laboratories to make thin perovskite films. The dynamics of spin casting MAPbI3-xClx and FAPbI3-xClx films have been investigated and compared to investigate the differences between the dynamics of MAPbI3-xClx and FAPbI3-xClx film formation. In particular we focus on the crystallization dynamics of the precursor film formation. When casting MAPbI3-xClx we observed relatively fast 1D crystallization of the intermediate product MA2PbI3Cl. There was an absence of the desired perovskite phase formed directly; it only appeared after an annealing step which converted the MA2PbI3Cl to MAPbI3. In contrast, slower crystallization via a 3D precursor was observed for FAPbI3-xClx film formation compared to MAPbI3-xClx. Another important finding was that some FAPbI3-xClx perovskite was generated directly during spin casting before annealing. These findings indicate that there are significant differences between the crystallization pathways for these two perovskite materials. These are likely to explain the differences in the lifetime of the resulting perovskite solar cell devices produced using FA and MA cations.
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Jun 2020
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I09-Surface and Interface Structural Analysis
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Andrew J.
Naylor
,
Ida
Kallquist
,
David
Peralta
,
Jean-frederic
Martin
,
Adrien
Boulineau
,
Jean-francois
Colin
,
Christian
Baur
,
Johann
Chable
,
Maximilian
Fichtner
,
Kristina
Edstrom
,
Maria
Hahlin
,
Daniel
Brandell
Diamond Proposal Number(s):
[20870]
Abstract: Promising theoretical capacities and high voltages are offered by Li-rich disordered rocksalt oxyfluoride materials as cathodes in lithium ion batteries. However, as has been discovered for many other Li-rich materials, the oxyfluorides suffer from extensive surface degradation, leading to severe capacity fading. In the case of Li2VO2F, we have previously determined this to be a result of detrimental reactions between an unstable surface layer and the organic electrolyte. Herein, we present the protection of Li2VO2F particles with AlF3 surface modification, resulting in a much enhanced capacity retention over 50 cycles. While the specific capacity for the untreated material drops below 100 mA h g-1 after only 50 cycles, the treated materials retain almost 200 mA h g-1. Photoelectron spectroscopy depth profiling confirms the stabilisation of the active material surface by the surface modification and reveals its suppression of electrolyte decomposition.
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May 2020
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B18-Core EXAFS
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Diamond Proposal Number(s):
[14239]
Abstract: Increasing dependence on rechargeable batteries for energy storage calls for the improvement of energy density of batteries. Toward this goal, introduction of positive electrode materials with high voltage and/or high capacity is in high demand. The use of oxygen chemistry in lithium and sodium layered oxides has been of interest to achieve high capacity. Nevertheless, a complete understanding of oxygen-based redox processes remains elusive especially in sodium ion batteries. Herein, a novel P3-type Na0.67Ni0.2Mn0.8O2, synthesized at low temperature, exhibits oxygen redox activity in high potentials. Characterization using a range of spectroscopic techniques reveals the anionic redox activity is stabilized by the reduction of Ni, because of the strong Ni 3d–O 2p hybridization states created during charge. This observation suggests that different route of oxygen redox processes occur in P3 structure materials, which can lead to the exploration of oxygen redox chemistry for further development in rechargeable batteries.
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Jan 2020
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