Molybdenum sulfides are very attractive noble-metal-free electrocatalysts for the hydrogen evolution reaction (HER) from water. The atomic structure and identity of the catalytically active sites ...have been well established for crystalline molybdenum disulfide (c-MoS2) but not for amorphous molybdenum sulfide (a-MoSx), which exhibits significantly higher HER activity compared to its crystalline counterpart. Here we show that HER-active a-MoSx, prepared either as nanoparticles or as films, is a molecular-based coordination polymer consisting of discrete Mo3S13(2-) building blocks. Of the three terminal disulfide (S2(2-)) ligands within these clusters, two are shared to form the polymer chain. The third one remains free and generates molybdenum hydride moieties as the active site under H2 evolution conditions. Such a molecular structure therefore provides a basis for revisiting the mechanism of a-MoSx catalytic activity, as well as explaining some of its special properties such as reductive activation and corrosion. Our findings open up new avenues for the rational optimization of this HER electrocatalyst as an alternative to platinum.
Direct observation of demagnesiated structure of MgMn2O4 spinel oxides at atomic scale has been achieved using spherical aberration-corrected scanning transmission electron microscopy (STEM) with ...high-angle annular-dark-field (HAADF) and annular-bright-field (ABF) techniques. Upon the Mg ions extraction from MgMn2O4 spinel oxides, structural transition from cubic to tetragonal phases was observed. The phase transition is a result of Mn octahedral distortion due to cooperative Jahn–Teller distortion of six-coordinate MnIII (t2g3–eg1). On the basis of HAADF/ABF micrographs and electron energy-loss spectrometer (EELS), it is suggested that Mn3+/Mn4+ and/or Mn3+/Mn5+ redox couples play a crucial role in the first electrochemical Mg deintercalation. This is the first report on observation of cubic-tetragonal phase transition in charged spinel MgMn2O4 which shed new insight into the magnesium storage mechanism in this important cathode material for Mg-ion batteries.
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•Direct observation of demagnesiated structure of MgMn2O4 spinel oxides•Structural transition from cubic to tetragonal phases was observed.•The phase transition is due to cooperative Jahn–Teller distortion.
Arbuscular mycorrhizal (AM) fungi function as extended roots and take an active part in plant acquisition of nutrients and also soil pollutants, such as heavy metals. The objective of this study was ...to establish a method to observe the localization of cadmium (Cd) Kα at subcellular levels using X-ray fluorescence (XRF) imaging with a synchrotron irradiation microbeam in resin-embedded sections of mycorrhizas. To evaluate the methodology, distributions of Cd in high-pressure-frozen Lotus japonicus—Rhizophagus irregularis mycorrhizal roots were compared between two treatments; Cd was exposed either to the roots or to the extraradical hyphae. Results showed that, in the latter treatment, Cd was restricted to fungal structures, whereas in the former, Cd was detected in cell walls of the two organisms. Plunge-frozen extraradical mycelium of Gigaspora margarita exposed to Cd showed high signals of Cd in the cell walls and vacuoles, and low in the cytoplasm. With selective staining and elemental mapping by electron-dispersive X-ray spectrometry (EDS), a positive correlation between distributions of Cd and P was revealed in the vacuole, which suggested polyP as a counter ion of Cd. These results indicated that there was no Cd relocation in rapidly frozen resin-embedded materials, therefore supporting the usefulness of this methodology.
Arbuscular mycorrhizal (AM) fungi can improve plant tolerance to heavy metal contamination. This detoxification ability may largely depend on how AM fungi influence the uptake and distribution of ...metals in host plants. Two experiments were performed in order to gain insights into the mechanisms underlying cadmium (Cd) tolerance in mycorrhizal plants. Stable isotope Cd106 and compartmented pots were adopted to quantify the contribution of the AM fungus, Rhizophagus irregularis, to the uptake of Cd by Lotus japonicus. Moreover, synchrotron radiation μX-ray fluorescence (SR-μXRF) was applied to localize Cd in the mycorrhizal roots at the sub-cellular level. The results obtained indicated that mycorrhizal colonization markedly enhanced Cd immobilization in plant roots. Less Cd was partitioned to plant shoots when only hyphae had access to Cd in the hyphal compartment than when roots also had direct access to the Cd pool. SR-μXRF imaging indicated that Cd absorbed by extraradical hyphae was translocated into intraradical fungal structures, in which arbuscules accumulated large amounts of Cd; however, plant cells without fungal structures and plant cell walls contained negligible amounts of Cd. The present results provide direct evidence for the intraradical immobilization of Cd absorbed by AM fungi, which may largely contribute to the enhanced tolerance of plants to Cd. Therefore, AM fungi may play a role in the phytostabilization of Cd-contaminated soil.
Exploring novel electrode materials is critical for the development of a next-generation rechargeable magnesium battery with high volumetric capacity. Here, we showed that a distinct amorphous ...molybdenum sulfide, being a coordination polymer of disulfide-bridged (Mo3S11) clusters, has great potential as a rechargeable magnesium battery cathode. This material provided good reversible capacity, attributed to its unique structure with high flexibility and capability of deformation upon Mg insertion. Free-terminal disulfide moiety may act as the active site for reversible insertion and extraction of magnesium.
Mono- and few-layer exfoliated MoSe2 with great variety of vacancies and edge defects which act favorably as catalytic centers for the H2 evolution, showed interesting catalytic activity for the HER ...in water.
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•Single- to few-layers MoSe2 nanosheets were obtained by exfoliation.•Great variety of vacancies and edge defects in MoSe2 nanosheets.•MoSe2 nanosheets showed attractive catalytic performance.
Single- to few-layers MoSe2 nanosheets were obtained from bulk MoSe2 by a supercritical fluid exfoliation. High angle annular dark field (HAADF) imaging showed the presence of a great variety of vacancies and edge defects within the exfoliated MoSe2 nanosheets. Thanks to these defects, the exfoliated MoSe2 nanosheets showed attractive catalytic performance and robustness for the hydrogen evolution reaction in water. The best electrode made of these nanosheets required 300 mV overpotential to generate a catalytic current of 10 mA/cm2 and showed a Tafel slop of 90 mV/s. The catalytic performance was further enhanced by twice when the MoSe2 nanosheets catalyst was conditioned at a mild oxidative potential, e.g.+0.73 V vs. RHE. The mild oxidation treatment was proposed to create novel Se-vacancy and Se2-vacancy while an oxidation treatment at higher potential, e.g. +1.23 V vs. RHE, completely oxidized MoSe2 into inactive MoO3. As a consequence, the later caused a rapid degradation of catalytic performance.
Lithium-ion battery that consists of a cathode made of (Mo3S11)n polymer and an anode of Li metal exhibits a high gravimetric-capacity, 673.3 mAh g−1. A flexible structure of the (Mo3S11) n polymer ...enables consecutive redox reactions of the S22− dimer and the Mo atoms. According to X-ray absorption near-edge spectroscopy, X-ray photoelectron spectroscopy and Raman spectroscopy, the chemical bonds of Mo–S and S–S in the polymer elongate by accepting electrons up to 16, while the Mo–Mo bond does not change much during the redox reactions. Although the polymer cathode is put in a solid-state electrolyte, the S22− dimer that is redoxed by the reaction of S22− + 2e−→ 2S2− forms Li–S–Li bonds, which is an origin of the high capacity of the battery. The redox reactions in the (LixMo3S11)n polymer cathode is theoretically confirmed by first principles calculation.
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Structural defects, including point defects, dislocation and planar defects, significantly affect the physical and chemical properties of low-dimensional materials, such as layered compounds. In ...particular, inversion domain boundary is an intrinsic defect surrounded by a 60° grain boundary, which significantly influences electronic transport properties. We study atomic structures of the inversion domain grain boundaries (IDBs) in layered transition metal dichalcogenides (MoSe2 and MoS2) obtained by an exfoliation method, based on the aberration-corrected scanning transmission electron microscopy observation and density functional theory (DFT) calculation. The atomic-scale observation shows that the grain boundaries consist of two different types of 4-fold ring point shared and 8-fold ring edge shared chains. The results of DFT calculations indicate that the inversion domain grain boundary behaves as a metallic one-dimensional chain embedded in the semiconducting MoSe2 matrix with the occurrence of a new state within the band gap.
This study presents a simple one-pot synthesis method to achieve few-layered and defective Mo(S,Se)
2
and (Mo,W)S
2
by using supercritical water with organic reducing agents from simple and ...less-toxic precursors. This synthesis process is expected to be suitable for preparing other various kinds of TMD solid solutions.
Defective Mo(S,Se)
2
and (Mo,W)S
2
nanosheets are synthesized by a supercritical hydrothermal process without highly toxic sources in a short time.
In arbuscular mycorrhizas (AM), the supply of phosphorus from the fungi is one of the most important benefits to the host plant. Here we describe for the first time the ultrastructure and ...polyphosphate (poly P) distribution in rapidly frozen and freeze-substituted germ tubes of the AM fungus Gigaspora margarita. At the ultrastructural level, phosphorus distribution was analysed using energy-filtering transmission electron microscopy, and poly P was detected using an enzyme-affinity method. Semithin sections and live cells were also stained with 4',6-diamidino-2-phenylindole, which is not specific but fluoresces yellow when viewed under UV irradiation by binding with poly P. The cryotechnique method showed that extensive elongate ellipsoid vacuoles containing a uniform electron-opaque material occupied most of the cell volume. Combining the results of multiple methods revealed that poly P was localized in a dispersed form in vacuoles and in the outer fungal cell wall. These results show the significant potential of AM fungi for phosphorus storage based on its localization in the extensive complement of vacuoles in thick hyphae. The mechanism of translocation of poly P in tubular vacuoles, and the role of poly P in the cell wall, need to be elucidated.
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