I22-Small angle scattering & Diffraction
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Giuseppe
Sforazzini
,
Axel
Kahnt
,
Michael
Wykes
,
Johannes
Sprafke
,
Sergio
Brovelli
,
Damien
Montarnal
,
Francesco
Meinardi
,
Franco
Cacialli
,
David
Beljonne
,
Bo
Albinsson
,
Harry
Anderson
Abstract: Conjugated polyrotaxanes jacketed with hole-transport groups have been synthesized from water-soluble polyrotaxanes consisting of a polyfluorene-alt-biphenylene (PFBP) conjugated polymer threaded through beta-cyclodextrin macrocycles. The hydroxyl groups of the oligosaccharides were efficiently functionalized with triphenylamine (TPA) so that every polyrotaxane molecule carries a coat of about 200 TPA units, forming a supramolecular coaxial structure. This architecture was characterized using a range of techniques, including small-angle X-ray scattering. Absorption of light by the TPA units results in excitation energy transfer (EET) and photoinduced electron transfer (ET) to the inner conjugated polymer core. These energy- and charge-transfer processes were explored by steady-state and time-resolved fluorescence spectroscopy, femtosecond transient absorption spectroscopy, and molecular modeling. The time-resolved measurements yielded insights into the heterogeneity of the TPA coat: those TPA units which are close to the central polymer core tend to undergo ET, whereas those on the outer surface of the polyrotaxane, far from the core, undergo EET. Sections of the backbone that are excited indirectly via EET tend to be more remote from the TPA units and thus are less susceptible to electron-transfer quenching. The rate of EET from the TPA units to the PFBP core was effectively modeled by taking account of the heterogeneity in the TPA-PFBP distance, using a distributed monopole approach. This work represents a new strategy for building and studying well-defined arrays of >100 covalently linked chromophores.
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Feb 2014
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I07-Surface & interface diffraction
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Diamond Proposal Number(s):
[6799, 7698]
Open Access
Abstract: The phase changes that occur during discharge of an electrode comprised of LiFePO4, carbon, and PTFE binder have been studied in lithium half cells by using X-ray diffraction measurements in reflection geometry. Differences in the state of charge between the front and the back of LiFePO4 electrodes have been visualized. By modifying the X-ray incident angle the depth of penetration of the X-ray beam into the electrode was altered, allowing for the examination of any concentration gradients that were present within the electrode. At high rates of discharge the electrode side facing the current collector underwent limited lithium insertion while the electrode as a whole underwent greater than 50% of discharge. This behavior is consistent with depletion at high rate of the lithium content of the electrolyte contained in the electrode pores. Increases in the diffraction peak widths indicated a breakdown of crystallinity within the active material during cycling even during the relatively short duration of these experiments, which can also be linked to cycling at high rate.
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Feb 2014
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I07-Surface & interface diffraction
I11-High Resolution Powder Diffraction
I22-Small angle scattering & Diffraction
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Diamond Proposal Number(s):
[8370, 4857]
Abstract: We report the structures of submonolayer films of the homologous series of normal-alkanes hexane (C6H14) to hexadecane (C16H34) adsorbed on the surface of hexagonal-boron nitride (h-BN), as determined by X-ray powder diffraction. These n-alkanes are demonstrated to form solid monolayers and exhibit structures that are similar to those seen on graphite but with some important differences. Each n-alkane studied shows a fully commensurate structure, even at submonolayer coverage. Hexane and octane adopt structures in which the molecules are arranged in a “herringbone” pattern (2 × m√3abn for CnHn+2,
WHERE abn is the unit cell parameter of the underlying h-BN surface, n = 6 or 8 and m = (2n – 2)/2). Heptane, nonane, and undecane–hexadecane adopt structures in which the molecules within adjacent lamellae are arranged with their principal axis parallel to each other (√3 × mabn for CnHn+2,
WHERE n = 7, 9, 11–16 and m = n + 2). Decane is a transitional case with evidence of both structure types, depending on coverage and temperature. This transitional case occurs for shorter molecules on h-BN than on graphite, indicating a difference in the relative balance of intermolecular and molecule–surface interactions that has significant implications for the phase behavior of adsorbates on these two surfaces.
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Jan 2014
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I11-High Resolution Powder Diffraction
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Shyam
Biswas
,
Danny E. P.
Vanpoucke
,
Toon
Verstraelen
,
Matthias
Vandichel
,
Sarah
Couck
,
Karen
Leus
,
Ying-ya
Liu
,
Michel
Waroquier
,
Veronique
Van Speybroeck
,
Joeri F. M.
Denayer
,
Pascal
Van Der Voort
Abstract: Six new functionalized vanadium hydroxo terephthalates [VIII(OH)(BDC-X)]·n(guests) (MIL-47(VIII)-X-AS) (BDC = 1,4-benzenedicarboxylate; X = −Cl, −Br, −CH3, −CF3, −OH, −OCH3; AS = as-synthesized) along with the parent MIL-47 were synthesized under rapid microwave-assisted hydrothermal conditions (170 °C, 30 min, 150 W). The unreacted H2BDC-X and/or occluded solvent molecules can be removed by thermal activation under vacuum, leading to the empty-pore forms of the title compounds (MIL-47(VIV)-X). Except pristine MIL-47 (+III oxidation state), the vanadium atoms in all the evacuated functionalized solids stayed in the +IV oxidation state. The phase purity of the compounds was ascertained by X-ray powder diffraction (XRPD), diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, Raman, thermogravimetric (TG), and elemental analysis. The structural similarity of the filled and empty-pore forms of the functionalized compounds with the respective forms of parent MIL-47 was verified by cell parameter determination from XRPD data. TGA and temperature-dependent XRPD (TDXRPD) experiments in an air atmosphere indicate high thermal stability in the 330–385 °C range. All the thermally activated compounds exhibit significant microporosity (SBET in the 305–897 m2 g–1 range), as verified by the N2 and CO2 sorption analysis. Among the six functionalized compounds, MIL-47(VIV)-OCH3 shows the highest CO2 uptake, demonstrating the determining role of functional groups on the CO2 sorption behavior. For this compound and pristine MIL-47(VIV), Widom particle insertion simulations were performed based on ab initio calculated crystal structures. The theoretical Henry coefficients show a good agreement with the experimental values, and calculated isosurfaces for the local excess chemical potential indicate the enhanced CO2 affinity is due to two effects: (i) the interaction between the methoxy group and CO2 and (ii) the collapse of the MIL-47(VIV)-OCH3 framework.
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Nov 2013
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B18-Core EXAFS
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Diamond Proposal Number(s):
[7221]
Abstract: Carbon supported Au@Pd core@shell nanoparticles were prepared using two methods based on displacement of a Cu-under potential deposited (upd) layer; the standard method, in which a single Cu-upd layer is formed and then displaced, and a Cu-upd-mediated deposition method, wherein the Pd displaces the Cu-upd layer during the Cu deposition. The resulting materials were characterized using in situ extended X-ray absorption fine structure as a function of the applied potential in an electrochemical cell at both the Au L3 and Pd K absorption edges. This detailed structural analysis shows that the standard, single Cu-upd layer method results in the formation of Pd clusters or islands on the Au core rather than a complete monolayer shell, while the Cu-upd-mediated method produces a mixed/alloyed PdAu shell.
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Oct 2013
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I15-Extreme Conditions
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Abstract: We have performed an experimental study of the
crystal structure, lattice dynamics, and optical properties of silver
chromate (Ag2CrO4) at ambient temperature and high pressures.
In particular, the crystal structure, Raman-active phonons, and
electronic band gap have been accurately determined. When the
initial orthorhombic Pnma Ag2CrO4 structure (phase I) is
compressed up to 4.5 GPa, a previously undetected phase
(phase II) has been observed with a 0.95% volume collapse.
The structure of phase II can be indexed to a similar orthorhombic cell as phase I, and the transition can be considered to be an
isostructural transition. This collapse is mainly due to the drastic contraction of the a axis (1.3%). A second phase transition to
phase III occurs at 13 GPa to a structure not yet determined. First-principles calculations have been unable to reproduce the
isostructural phase transition, but they propose the stabilization of a spinel-type structure at 11 GPa. This phase is not detected in
experiments probably because of the presence of kinetic barriers. Experiments and calculations therefore seem to indicate that a
new structural and electronic description is required to model the properties of silver chromate.
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Jun 2013
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I11-High Resolution Powder Diffraction
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Abstract: We have performed simulations utilizing the dispersion-corrected density functional theory method (DFT-D) as parametrized by Grimme on selected polymorphs of RDX (cyclotrimethylenetrinitramine). Additionally, we present the first experimental determination of the enthalpy of fusion (?Hfus) of the highly metastable ?-form of RDX. The characteristics of fusion for ?-RDX were determined to be 186.7 ± 0.8 °C, 188.5 ± 0.4 °C, and 12.63 ± 0.28 kJ mol–1 for the onset temperature, peak temperature (or melting point), and ?Hfus, respectively. The difference in experimental ?Hfus for the ?- and ?-forms of RDX is 20.46 ± 0.92 kJ mol–1. Ambient-pressure lattice energies (EL) of the ?- and ?-forms of RDX have been calculated and are in excellent agreement with experiment. In addition the computationally predicted difference in EL (20.35 kJ mol–1) between the ?- and ?-forms is in excellent agreement with the experimental difference in ?Hfus. The response of the lattice parameters and unit-cell volumes to pressure for the ?- and ?-forms have been investigated, in addition to the first high-pressure computational study of the ?-form of RDX—these results are in very good agreement with experimental data. Phonon calculations provide good agreement for vibrational frequencies obtained from Raman spectroscopy, and a predicted inelastic neutron scattering (INS) spectrum of ?-RDX shows excellent agreement with experimental INS data determined in this study. The transition energies and intensities are reproduced, confirming that both the eigenvalues and the eigenvectors of the vibrations are correctly described by the DFT-D method. The results of the high-pressure phonon calculations have been used to show that the heat capacities of the ?-, ?-, and ?-forms of RDX are only weakly affected by pressure.
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Mar 2013
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I18-Microfocus Spectroscopy
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Diamond Proposal Number(s):
[7405]
Abstract: A windowless electrochemical cell for the spectroscopic investigation of the liquid–liquid interface, using a dual droplet configuration, has been designed. The setup permits in situ probing of the bulk solutions and the interfacial region by fiber-optic UV–vis spectroscopy, microfocus X-ray fluorescence (XRF) elemental mapping, and microfocus X-ray absorption near-edge structure (?XANES) spectroscopy. The electrodeposition of Au, induced by ion transfer of the tetrachloroaurate complex from a halogenated solvent (containing a weak reducing agent) to the aqueous phase, has been monitored by a combination of the three techniques. The reaction can be followed in situ by UV–vis spectroscopy by detecting the oxidized form of the reducing agent. Voltammetric evidence suggests the formation of interfacial Au(I) species, whereas ?XANES detect the presence of metallic Au(0).
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Feb 2013
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Abstract: We report a careful characterization of the interaction of NH3 with the Cu(II) sites of the [Cu2C4O8] paddle-wheel cornerstone of the HKUST-1 metallorganic framework, also known as Cu3(BTC)2. The general picture emerging from combining XRPD, EXAFS, XANES, mid- and far-IR, DRUV–vis, and EPR techniques is that the presence of traces of water has relevant consequences on the effect of ammonia on the MOF framework. NH3 adsorption on the dry system results in a strong chemisorption on Cu(II) sites that distorts the framework, keeping the crystallinity of the material. Perturbation observed upon NH3 adsorption is analogous to that observed for H2O, but noticeably enhanced. When the adsorption of ammonia occurs in humid conditions, a time-dependent, much deeper modification of the system is observed by all of the considered techniques. On a methodological ground, it is worth noticing that we used the optimization of XANES spectra to validate the bond distance obtained by EXAFS.
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Sep 2012
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Abstract: We report X-ray diffraction measurements in CdIn2S4, MgIn2S4, and MnIn2S4 thiospinels at room temperature and high pressures. The pressure dependences of the structural parameters have been determined and compared to those from theoretical calculations. It is found that the three thiospinels have similar bulk moduli (B0) between 75 and 80 GPa (B0′ ∼ 3). The degree of inversion of these thiospinels has also been determined. The three thiospinels undergo a phase transition toward a defect LiTiO2-type structure above 9.5, 8.3, and 6.8 GPa in CdIn2S4, MgIn2S4, and MnIn2S4, respectively. Interestingly, the low- and high-pressure phases belong to the same symmetry group (Fd-3m), the transition mechanism being associated to the migration of the tetrahedrally coordinated cations to a Wyckoff position of higher multiplicity, where these cations present an octahedral environment. The new postspinel phase exhibits a larger compressibility than the spinel phase for the three compounds, likely due to the presence of stoichiometric vacancies in the unit cell as it occurs for defect chalcopyrites and stannites. The relation between the bulk and the polyhedral compressibilities is discussed as well.
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Jul 2012
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