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1.
Rev Sci Instrum ; 90(11): 113101, 2019 Nov 01.
Artigo em Inglês | MEDLINE | ID: mdl-31779391

RESUMO

We present results obtained with a new soft X-ray spectrometer based on transition-edge sensors (TESs) composed of Mo/Cu bilayers coupled to bismuth absorbers. This spectrometer simultaneously provides excellent energy resolution, high detection efficiency, and broadband spectral coverage. The new spectrometer is optimized for incident X-ray energies below 2 keV. Each pixel serves as both a highly sensitive calorimeter and an X-ray absorber with near unity quantum efficiency. We have commissioned this 240-pixel TES spectrometer at the Stanford Synchrotron Radiation Lightsource beamline 10-1 (BL 10-1) and used it to probe the local electronic structure of sample materials with unprecedented sensitivity in the soft X-ray regime. As mounted, the TES spectrometer has a maximum detection solid angle of 2 × 10-3 sr. The energy resolution of all pixels combined is 1.5 eV full width at half maximum at 500 eV. We describe the performance of the TES spectrometer in terms of its energy resolution and count-rate capability and demonstrate its utility as a high throughput detector for synchrotron-based X-ray spectroscopy. Results from initial X-ray emission spectroscopy and resonant inelastic X-ray scattering experiments obtained with the spectrometer are presented.

2.
Nanomaterials (Basel) ; 9(7)2019 Jul 05.
Artigo em Inglês | MEDLINE | ID: mdl-31284375

RESUMO

Deposition of atoms or molecules on a solid surface is a flexible way to prepare various novel two-dimensional materials if the growth conditions, such as suitable surface and optimum temperature, could be predicted theoretically. However, prediction challenges modern theory of material design because the free energy criteria can hardly be applied to this issue due to the long-standing problem in statistical physics of the calculations of the free energy. Herein, we present an approach to the problem by the demonstrations of graphene and γ-graphyne on the surface of copper crystal, as well as silicene on a silver substrate. Compared with previous state-of-the-art algorithms for calculations of the free energy, our approach is capable of achieving computational precisions at least 10-times higher, which was confirmed by molecular dynamics simulations, and working at least four orders of magnitude faster, which enables us to obtain free energy based on ab initio calculations of the interaction potential instead of the empirical one. The approach was applied to predict the optimum conditions for silicene growth on different surfaces of solid silver based on density functional theory, and the results are in good agreement with previous experimental observations.

3.
J Synchrotron Radiat ; 26(Pt 3): 629-634, 2019 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-31074425

RESUMO

An X-ray emission spectrometer that can detect the sulfur Kα emission lines with large throughput and a high energy resolution is presented. The instrument is based on a large d-spacing perfect Bragg analyzer that diffracts the sulfur Kα emission at close to backscattering angles. This facilitates the application of efficient concepts routinely employed in hard X-ray spectrometers towards the tender X-ray regime. The instrument described in this work is based on an energy-dispersive von Hamos geometry that is well suited for photon-in photon-out spectroscopy at X-ray free-electron laser and synchrotron sources. Comparison of its performance with previously used instrumentation is presented through measurements using sulfur-containing species performed at the LCLS. It is shown that the overall signal intensity is increased by a factor of ∼15. Implementation of this approach in the design of a tender X-ray spectroscopy endstation for LCLS-II is also discussed.

4.
J Synchrotron Radiat ; 26(Pt 3): 811-818, 2019 May 01.
Artigo em Inglês | MEDLINE | ID: mdl-31074446

RESUMO

The evolution in local structure and electronic properties of cobalt was investigated during in situ sulfurization. Using a combination of 1s X-ray absorption (XAS) and 1s3p resonant inelastic X-ray scattering (RIXS), the valence, coordination and symmetry of cobalt ions were tracked in two cobalt-promoted molybdenum oxide precursors of the hydrodesulfurization catalyst system, namely Co-Mo/Al2O3 and Co-Ni-Mo/Al2O3. Extended X-ray absorption fine structure shows that the Co-O bonds were replaced with Co-S bonds as a function of reaction temperature. The cobalt K pre-edge intensity shows that the symmetry of cobalt was modified from Co3+ Oh and Co2+ Oh to a Co2+ ion where the inversion symmetry is broken, in agreement with a square-pyramidal site. The 1s3p RIXS data revealed the presence of an intermediate cobalt oxy-sulfide species. This species was not detected from XAS and was determined from the increased information obtained from the 1s3p RIXS data. The cobalt XAS and RIXS data show that nickel has a significant influence on the formation of the cobalt oxy-sulfide intermediate species prior to achieving the fully sulfided state at T > 400°C.

5.
J Phys Chem A ; 122(26): 5730-5734, 2018 Jul 05.
Artigo em Inglês | MEDLINE | ID: mdl-29897245

RESUMO

Aromaticity profoundly affects molecular orbitals in polycyclic aromatic hydrocarbons. X-ray core electron spectroscopy has observed that carbon 1s-π* transitions can be broadened or even split in some polycyclic systems, although the origin of the effect has remained obscure. The π electrons in polycyclic systems are typically classified in the Clar model as belonging to either true aromatic sextets (similar to benzene) or isolated double bonds (similar to olefins). Here, bulk-sensitive carbon core excitation spectra are presented for a series of polycyclic systems and show that the magnitude of the 1s-π* splitting is determined primarily by the ratio of true aromatic sextets to isolated double bonds. The observed splitting can be rationalized in terms of ground state energetics as described by Hückel, driven by the π electron structure described by Clar. This simple model including only ground state energetics is shown to explain the basics physics behind the spectral evolution for a broad set of polycyclic aromatic hydrocarbons, although some residual deviations between this model and experiment can likely be improved by including a more detailed electronic structure and the core hole effect.

6.
Nano Lett ; 18(5): 3241-3249, 2018 05 09.
Artigo em Inglês | MEDLINE | ID: mdl-29667835

RESUMO

Chemical and mechanical properties interplay on the nanometric scale and collectively govern the functionalities of battery materials. Understanding the relationship between the two can inform the design of battery materials with optimal chemomechanical properties for long-life lithium batteries. Herein, we report a mechanism of nanoscale mechanical breakdown in layered oxide cathode materials, originating from oxygen release at high states of charge under thermal abuse conditions. We observe that the mechanical breakdown of charged Li1- xNi0.4Mn0.4Co0.2O2 materials proceeds via a two-step pathway involving intergranular and intragranular crack formation. Owing to the oxygen release, sporadic phase transformations from the layered structure to the spinel and/or rocksalt structures introduce local stress, which initiates microcracks along grain boundaries and ultimately leads to the detachment of primary particles, i.e., intergranular crack formation. Furthermore, intragranular cracks (pores and exfoliations) form, likely due to the accumulation of oxygen vacancies and continuous phase transformations at the surfaces of primary particles. Finally, finite element modeling confirms our experimental observation that the crack formation is attributable to the formation of oxygen vacancies, oxygen release, and phase transformations. This study is designed to directly observe the chemomechanical behavior of layered oxide cathode materials and provides a chemical basis for strengthening primary and secondary particles by stabilizing the oxygen anions in the lattice.

7.
J Chem Phys ; 147(21): 214201, 2017 Dec 07.
Artigo em Inglês | MEDLINE | ID: mdl-29221417

RESUMO

We present X-ray absorption spectroscopy and resonant inelastic X-ray scattering (RIXS) measurements on the iron L-edge of 0.5 mM aqueous ferricyanide. These measurements demonstrate the ability of high-throughput transition-edge-sensor (TES) spectrometers to access the rich soft X-ray (100-2000 eV) spectroscopy regime for dilute and radiation-sensitive samples. Our low-concentration data are in agreement with high-concentration measurements recorded by grating spectrometers. These results show that soft-X-ray RIXS spectroscopy acquired by high-throughput TES spectrometers can be used to study the local electronic structure of dilute metal-centered complexes relevant to biology, chemistry, and catalysis. In particular, TES spectrometers have a unique ability to characterize frozen solutions of radiation- and temperature-sensitive samples.

8.
Chem Rev ; 117(21): 13123-13186, 2017 Nov 08.
Artigo em Inglês | MEDLINE | ID: mdl-28960962

RESUMO

Rechargeable battery technologies have ignited major breakthroughs in contemporary society, including but not limited to revolutions in transportation, electronics, and grid energy storage. The remarkable development of rechargeable batteries is largely attributed to in-depth efforts to improve battery electrode and electrolyte materials. There are, however, still intimidating challenges of lower cost, longer cycle and calendar life, higher energy density, and better safety for large scale energy storage and vehicular applications. Further progress with rechargeable batteries may require new chemistries (lithium ion batteries and beyond) and better understanding of materials electrochemistry in the various battery technologies. In the past decade, advancement of battery materials has been complemented by new analytical techniques that are capable of probing battery chemistries at various length and time scales. Synchrotron X-ray techniques stand out as one of the most effective methods that allow for nearly nondestructive probing of materials characteristics such as electronic and geometric structures with various depth sensitivities through spectroscopy, scattering, and imaging capabilities. This article begins with the discussion of various rechargeable batteries and associated important scientific questions in the field, followed by a review of synchrotron X-ray based analytical tools (scattering, spectroscopy, and imaging) and their successful applications (ex situ, in situ, and in operando) in gaining fundamental insights into these scientific questions. Furthermore, electron microscopy and spectroscopy complement the detection length scales of synchrotron X-ray tools and are also discussed toward the end. We highlight the importance of studying battery materials by combining analytical techniques with complementary length sensitivities, such as the combination of X-ray absorption spectroscopy and electron spectroscopy with spatial resolution, because a sole technique may lead to biased and inaccurate conclusions. We then discuss the current progress of experimental design for synchrotron experiments and methods to mitigate beam effects. Finally, a perspective is provided to elaborate how synchrotron techniques can impact the development of next-generation battery chemistries.

9.
Struct Dyn ; 4(4): 044030, 2017 07.
Artigo em Inglês | MEDLINE | ID: mdl-28653021

RESUMO

We have used femtosecond resolution UV-visible and Kß x-ray emission spectroscopy to characterize the electronic excited state dynamics of [Fe(bpy)2(CN)2], where bpy=2,2'-bipyridine, initiated by metal-to-ligand charge transfer (MLCT) excitation. The excited-state absorption in the transient UV-visible spectra, associated with the 2,2'-bipyridine radical anion, provides a robust marker for the MLCT excited state, while the transient Kß x-ray emission spectra provide a clear measure of intermediate and high spin metal-centered excited states. From these measurements, we conclude that the MLCT state of [Fe(bpy)2(CN)2] undergoes ultrafast spin crossover to a metal-centered quintet excited state through a short lived metal-centered triplet transient species. These measurements of [Fe(bpy)2(CN)2] complement prior measurement performed on [Fe(bpy)3]2+ and [Fe(bpy)(CN)4]2- in dimethylsulfoxide solution and help complete the chemical series [Fe(bpy)N(CN)6-2N]2N-4, where N = 1-3. The measurements confirm that simple ligand modifications can significantly change the relaxation pathways and excited state lifetimes and support the further investigation of light harvesting and photocatalytic applications of 3d transition metal complexes.

10.
Chem Sci ; 8(1): 515-523, 2017 Jan 01.
Artigo em Inglês | MEDLINE | ID: mdl-28451198

RESUMO

Developing light-harvesting and photocatalytic molecules made with iron could provide a cost effective, scalable, and environmentally benign path for solar energy conversion. To date these developments have been limited by the sub-picosecond metal-to-ligand charge transfer (MLCT) electronic excited state lifetime of iron based complexes due to spin crossover - the extremely fast intersystem crossing and internal conversion to high spin metal-centered excited states. We revitalize a 30 year old synthetic strategy for extending the MLCT excited state lifetimes of iron complexes by making mixed ligand iron complexes with four cyanide (CN-) ligands and one 2,2'-bipyridine (bpy) ligand. This enables MLCT excited state and metal-centered excited state energies to be manipulated with partial independence and provides a path to suppressing spin crossover. We have combined X-ray Free-Electron Laser (XFEL) Kß hard X-ray fluorescence spectroscopy with femtosecond time-resolved UV-visible absorption spectroscopy to characterize the electronic excited state dynamics initiated by MLCT excitation of [Fe(CN)4(bpy)]2-. The two experimental techniques are highly complementary; the time-resolved UV-visible measurement probes allowed electronic transitions between valence states making it sensitive to ligand-centered electronic states such as MLCT states, whereas the Kß fluorescence spectroscopy provides a sensitive measure of changes in the Fe spin state characteristic of metal-centered excited states. We conclude that the MLCT excited state of [Fe(CN)4(bpy)]2- decays with roughly a 20 ps lifetime without undergoing spin crossover, exceeding the MLCT excited state lifetime of [Fe(2,2'-bipyridine)3]2+ by more than two orders of magnitude.

11.
Angew Chem Int Ed Engl ; 56(21): 5780-5784, 2017 05 15.
Artigo em Inglês | MEDLINE | ID: mdl-28407339

RESUMO

For overall water-splitting systems, it is essential to establish O2 -insensitive cathodes that allow cogeneration of H2 and O2 . An acid-tolerant electrocatalyst is described, which employs a Mo-coating on a metal surface to achieve selective H2 evolution in the presence of O2 . In operando X-ray absorption spectroscopy identified reduced Pt covered with an amorphous molybdenum oxyhydroxide hydrate with a local structural order composed of polyanionic trimeric units of molybdenum(IV). The Mo layer likely hinders O2 gas permeation, impeding contact with active Pt. Photocatalytic overall water splitting proceeded using MoOx /Pt/SrTiO3 with inhibited water formation from H2 and O2 , which is the prevailing back reaction on the bare Pt/SrTiO3 photocatalyst. The Mo coating was stable in acidic media for multiple hours of overall water splitting by membraneless electrolysis and photocatalysis.

12.
Inorg Chem ; 56(2): 737-747, 2017 Jan 17.
Artigo em Inglês | MEDLINE | ID: mdl-28035824

RESUMO

The valence tautomeric states of Co(phen)(3,5-DBQ)2 and Co(tmeda)(3,5-DBQ)2, where 3,5-DBQ is either the semiquinone (SQ-) or catecholate (Cat2-) form of 3,5-di-tert-butyl-1,2-benzoquinone, have been examined by a series of cobalt-specific X-ray spectroscopies. In this work, we have utilized the sensitivity of 1s3p X-ray emission spectroscopy (Kß XES) to the oxidation and spin states of 3d transition-metal ions to determine the cobalt-specific electronic structure of valence tautomers. A comparison of their Kß XES spectra with the spectra of cobalt coordination complexes with known oxidation and spin states demonstrates that the low-temperature valence tautomer can be described as a low-spin CoIII configuration and the high-temperature valence tautomer as a high-spin CoII configuration. This conclusion is further supported by Co L-edge X-ray absorption spectroscopy (L-edge XAS) of the high-temperature valence tautomers and ligand-field atomic-multiplet calculations of the Kß XES and L-edge XAS spectra. The nature and strength of the magnetic exchange interaction between the cobalt center and SQ- in cobalt valence tautomers is discussed in view of the effective spin at the Co site from Kß XES and the molecular spin moment from magnetic susceptibility measurements.

13.
Nature ; 540(7633): 453-457, 2016 12 15.
Artigo em Inglês | MEDLINE | ID: mdl-27871088

RESUMO

Light-induced oxidation of water by photosystem II (PS II) in plants, algae and cyanobacteria has generated most of the dioxygen in the atmosphere. PS II, a membrane-bound multi-subunit pigment protein complex, couples the one-electron photochemistry at the reaction centre with the four-electron redox chemistry of water oxidation at the Mn4CaO5 cluster in the oxygen-evolving complex (OEC). Under illumination, the OEC cycles through five intermediate S-states (S0 to S4), in which S1 is the dark-stable state and S3 is the last semi-stable state before O-O bond formation and O2 evolution. A detailed understanding of the O-O bond formation mechanism remains a challenge, and will require elucidation of both the structures of the OEC in the different S-states and the binding of the two substrate waters to the catalytic site. Here we report the use of femtosecond pulses from an X-ray free electron laser (XFEL) to obtain damage-free, room temperature structures of dark-adapted (S1), two-flash illuminated (2F; S3-enriched), and ammonia-bound two-flash illuminated (2F-NH3; S3-enriched) PS II. Although the recent 1.95 Å resolution structure of PS II at cryogenic temperature using an XFEL provided a damage-free view of the S1 state, measurements at room temperature are required to study the structural landscape of proteins under functional conditions, and also for in situ advancement of the S-states. To investigate the water-binding site(s), ammonia, a water analogue, has been used as a marker, as it binds to the Mn4CaO5 cluster in the S2 and S3 states. Since the ammonia-bound OEC is active, the ammonia-binding Mn site is not a substrate water site. This approach, together with a comparison of the native dark and 2F states, is used to discriminate between proposed O-O bond formation mechanisms.


Assuntos
Cianobactérias/química , Elétrons , Lasers , Complexo de Proteína do Fotossistema II/química , Complexo de Proteína do Fotossistema II/metabolismo , Temperatura Ambiente , Amônia/química , Amônia/metabolismo , Proteínas de Bactérias/química , Proteínas de Bactérias/metabolismo , Sítios de Ligação , Cristalização , Manganês/metabolismo , Modelos Moleculares , Oxigênio/metabolismo , Especificidade por Substrato , Água/metabolismo
14.
Inorg Chem ; 55(7): 3431-9, 2016 Apr 04.
Artigo em Inglês | MEDLINE | ID: mdl-26965786

RESUMO

Biological systems use copper as a redox center in many metalloproteins, where the role of the metal is to cycle between its +1 and +2 oxidation states. This chemistry requires the redox potential to be in a range that can stabilize both Cu(I) and Cu(II) states and often involves protein-derived ligand sets involving mixed histidine-methionine coordination that balance the preferences of both oxidation states. Transport proteins, on the other hand, utilize copper in the Cu(I) state and often contain sites comprised predominately of the cuprophilic residue methionine. The electronic factors that allow enzymes and transporters to balance their redox requirements are complex and are often elusive due to the dearth of spectroscopic probes of the Cu(I) state. Here we present the novel application of X-ray emission spectroscopy to copper proteins via a study of a series of mixed His-Met copper sites where the ligand set varies in a systematic way between the His3 and Met3 limits. The sites are derived from the wild-type peptidylglycine monooxygenase (PHM), two single-site variants which replicate each of its two copper sites (CuM-site and CuH-site), and the transporters CusF and CusB. Clear differences are observed in the Kß2,5 region at the Met3 and His3 limits. CusB (Met3) has a distinct peak at 8978.4 eV with a broad shoulder at 8975.6 eV, whereas CuH (His3) has two well-resolved features: a more intense feature at 8974.8 eV and a second at 8977.2 eV. The mixed coordination sphere CusF (Met2His) and the PHM CuM variant (Met1His2) have very similar spectra consisting of two features at 8975.2 and 8977.8 eV. An analysis of DFT calculated spectra indicate that the intensity of the higher energy peak near 8978 eV is mediated by mixing of ligand-based orbitals into the Cu d(10) manifold, with S from Met providing more intensity by facilitating increased Cu p-d mixing. Furthermore, reaction of WT PHM with CO (an oxygen analogue) produced the M site CO complex, which showed a unique XES spectrum that could be computationally reproduced by including interactions between Cu(I) and the CO ligand. The study suggests that the valence-to-core (VtC) region can not only serve as a probe of ligand speciation but also offer insight into the coordination geometry, in a fashion similar to XAS pre-edges, and may be sufficiently sensitive to the coordination of exogenous ligands to be useful in the study of reaction mechanisms.


Assuntos
Proteínas de Transporte de Cátions/química , Cobre/metabolismo , Proteínas de Escherichia coli/química , Escherichia coli/química , Proteínas de Membrana Transportadoras/química , Oxigenases de Função Mista/química , Complexos Multienzimáticos/química , Animais , Sítios de Ligação , Proteínas de Transporte de Cátions/metabolismo , Escherichia coli/metabolismo , Proteínas de Escherichia coli/metabolismo , Histidina/química , Histidina/metabolismo , Proteínas de Membrana Transportadoras/metabolismo , Metionina/química , Metionina/metabolismo , Oxigenases de Função Mista/metabolismo , Modelos Moleculares , Complexos Multienzimáticos/metabolismo , Ligação Proteica , Ratos , Espectrometria por Raios X
15.
Chem Sci ; 7(8): 5236-5248, 2016 Aug 01.
Artigo em Inglês | MEDLINE | ID: mdl-28044099

RESUMO

The Mn4CaO5 cluster in Photosystem II catalyzes the four-electron redox reaction of water oxidation in natural photosynthesis. This catalytic reaction cycles through four intermediate states (Si, i = 0 to 4), involving changes in the redox state of the four Mn atoms in the cluster. Recent studies suggest the presence and importance of isomorphous structures within the same redox/intermediate S-state. It is highly likely that geometric and electronic structural flexibility play a role in the catalytic mechanism. Among the catalytic intermediates that have been identified experimentally thus far, there is clear evidence of such isomorphism in the S2 state, with a high-spin (5/2) (HS) and a low spin (1/2) (LS) form, identified and characterized by their distinct electron paramagnetic resonance (EPR spectroscopy) signals. We studied these two S2 isomers with Mn extended X-ray absorption fine structure (EXAFS) and absorption and emission spectroscopy (XANES/XES) to characterize the structural and electronic structural properties. The geometric and electronic structure of the HS and LS S2 states are different as determined using Mn EXAFS and XANES/XES, respectively. The Mn K-edge XANES and XES for the HS form are different from the LS and indicate a slightly lower positive charge on the Mn atoms compared to the LS form. Based on the EXAFS results which are clearly different, we propose possible structural differences between the two spin states. Such structural and magnetic redox-isomers if present at room temperature, will likely play a role in the mechanism for water-exchange/oxidation in photosynthesis.

16.
Chem Sci ; 7(6): 3640-3644, 2016 Jun 01.
Artigo em Inglês | MEDLINE | ID: mdl-30008996

RESUMO

Carbon dioxide is expected to be employed as an inexpensive and potential feedstock of C1 sources for the mass production of valuable chemicals and fuel. Versatile chemical transformations of CO2, i.e. insertion of CO2 producing bicarbonate/acetate/formate, cleavage of CO2 yielding µ-CO/µ-oxo transition-metal complexes, and electrocatalytic reduction of CO2 affording CO/HCOOH/CH3OH/CH4/C2H4/oxalate were well documented. Herein, we report a novel pathway for the reductive activation of CO2 by the [NiIII(OMe)(P(C6H3-3-SiMe3-2-S)3)]- complex, yielding the [NiIII(κ1-OCO˙-)(P(C6H3-3-SiMe3-2-S)3)]- complex. The formation of this unusual NiIII(κ1-OCO˙-) complex was characterized by single-crystal X-ray diffraction, EPR, IR, SQUID, Ni/S K-edge X-ray absorption spectroscopy, and Ni valence-to-core X-ray emission spectroscopy. The inertness of the analogous complexes [NiIII(SPh)], [NiII(CO)], and [NiII(N2H4)] toward CO2, in contrast, demonstrates that the ionic [NiIII(OMe)] core attracts the binding of weak σ-donor CO2 and triggers the subsequent reduction of CO2 by the nucleophilic [OMe]- in the immediate vicinity. This metal-ligand cooperative activation of CO2 may open a novel pathway promoting the subsequent incorporation of CO2 in the buildup of functionalized products.

17.
Proc Natl Acad Sci U S A ; 112(45): 13845-9, 2015 Nov 10.
Artigo em Inglês | MEDLINE | ID: mdl-26515097

RESUMO

Biocatalysis by nitrogenase, particularly the reduction of N2 and CO by this enzyme, has tremendous significance in environment- and energy-related areas. Elucidation of the detailed mechanism of nitrogenase has been hampered by the inability to trap substrates or intermediates in a well-defined state. Here, we report the capture of substrate CO on the resting-state vanadium-nitrogenase in a catalytically competent conformation. The close resemblance of this active CO-bound conformation to the recently described structure of CO-inhibited molybdenum-nitrogenase points to the mechanistic relevance of sulfur displacement to the activation of iron sites in the cofactor for CO binding. Moreover, the ability of vanadium-nitrogenase to bind substrate in the resting-state uncouples substrate binding from subsequent turnover, providing a platform for generation of defined intermediate(s) of both CO and N2 reduction.


Assuntos
Monóxido de Carbono/metabolismo , Nitrogenase/metabolismo , Monóxido de Carbono/química , Espectroscopia de Ressonância de Spin Eletrônica , Modelos Moleculares , Nitrogenase/química , Especificidade por Substrato
18.
Chemistry ; 21(49): 17570-3, 2015 Dec 01.
Artigo em Inglês | MEDLINE | ID: mdl-26437878

RESUMO

A positive myocardial inotropic effect achieved using HNO/NO(-) , compared with NO⋅, triggered attempts to explore novel nitroxyl donors for use in clinical applications in vascular and myocardial pharmacology. To develop M-NO complexes for nitroxyl chemistry and biology, modulation of direct nitroxyl-transfer reactivity of dinitrosyl iron complexes (DNICs) is investigated in this study using a Fe(III) -porphyrin complex and proteins as a specific probe. Stable dinuclear {Fe(NO)2 }(9) DNIC [Fe(µ-(Me) Pyr)(NO)2 ]2 was discovered as a potent nitroxyl donor for nitroxylation of Fe(III) -heme centers through an associative mechanism. Beyond the efficient nitroxyl transfer, transformation of DNICs into a chemical biology probe for nitroxyl and for pharmaceutical applications demands further efforts using in vitro/in vivo studies.

19.
Phys Chem Chem Phys ; 17(39): 26369-77, 2015 Oct 21.
Artigo em Inglês | MEDLINE | ID: mdl-26388021

RESUMO

LiFePO4 is a battery cathode material with high safety standards due to its unique electronic structure. We performed systematic experimental and theoretical studies based on soft X-ray emission, absorption, and hard X-ray Raman spectroscopy of LixFePO4 nanoparticles and single crystals. The results clearly show a non-rigid electron-state reconfiguration of both the occupied and unoccupied Fe-3d and O-2p states during the (de)lithiation process. We focus on the energy configurations of the occupied states of LiFePO4 and the unoccupied states of FePO4, which are the critical states where electrons are removed and injected during the charge and discharge process, respectively. In LiFePO4, the soft X-ray emission spectroscopy shows that, due to the Coulomb repulsion effect, the occupied Fe-3d states with the minority spin sit close to the Fermi level. In FePO4, the soft X-ray absorption and hard X-ray Raman spectroscopy show that the unoccupied Fe-3d states again sit close to the Fermi level. These critical 3d electron state configurations are consistent with the calculations based on modified Becke and Johnson potentials GGA+U (MBJGGA+U) framework, which improves the overall lineshape prediction compared with the conventionally used GGA+U method. The combined experimental and theoretical studies show that the non-rigid electron state reshuffling guarantees the stability of oxygen during the redox reaction throughout the charge and discharge process of LiFePO4 electrodes, leading to the intrinsic safe performance of the electrodes.

20.
Nano Lett ; 15(9): 5755-63, 2015 Sep 09.
Artigo em Inglês | MEDLINE | ID: mdl-26288360

RESUMO

The development of sodium ion batteries (NIBs) can provide an alternative to lithium ion batteries (LIBs) for sustainable, low-cost energy storage. However, due to the larger size and higher m/e ratio of the sodium ion compared to lithium, sodiation reactions of candidate electrodes are expected to differ in significant ways from the corresponding lithium ones. In this work, we investigated the sodiation mechanism of a typical transition metal-oxide, NiO, through a set of correlated techniques, including electrochemical and synchrotron studies, real-time electron microscopy observation, and ab initio molecular dynamics (MD) simulations. We found that a crystalline Na2O reaction layer that was formed at the beginning of sodiation plays an important role in blocking the further transport of sodium ions. In addition, sodiation in NiO exhibits a "shrinking-core" mode that results from a layer-by-layer reaction, as identified by ab initio MD simulations. For lithiation, however, the formation of Li antisite defects significantly distorts the local NiO lattice that facilitates Li insertion, thus enhancing the overall reaction rate. These observations delineate the mechanistic difference between sodiation and lithiation in metal-oxide conversion materials. More importantly, our findings identify the importance of understanding the role of reaction layers on the functioning of electrodes and thus provide critical insights into further optimizing NIB materials through surface engineering.

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