Search

The development of transition metal intermetallic compounds, in which active sites are incorporated in lattice frameworks, has great potential for modulating the local structure and the electronic properties of active sites, and enhancing the catalytic activity and stability. Here we report that a new copper-based intermetallic electride catalyst, LaCu0.67Si1.33, in which Cu sites activated by anionic electrons with low work function are atomically dispersed in the lattice framework and affords selective hydrogenation of nitroarenes with above 40-times higher turnover frequencies (TOFs up to 5084 h-1) than well-studied metal-loaded catalysts. Kinetic analysis utilizing isotope effect reveals that the cleavage of the H-H bond is the rate-determining step. Surprisingly, the high carrier density and low work function (LWF) properties of LaCu0.67Si1.33 enable the activation of hydrogen molecules with extreme low activation energy (Ea = 14.8 kJ•mol-1). Furthermore, preferential adsorption of nitroarenes via a nitro group is achieved by high oxygen affinity of LaCu0.67Si1.33 surface, resulting in high chemoselectivity. The present efficient catalyst can further trigger the hydrogenation of other oxygen-containing functional groups such as aldehydes and ketones with high activities. These findings demonstrate that the transition metals incorporated in the specific lattice site function as catalytically active centers and surpass the conventional metal-loaded catalysts in activity and stability. [ABSTRACT FROM AUTHOR]

We report an air and water stable electride Y5Si3 and its catalytic activity for direct ammonia synthesis. It crystallizes in the Mn5Si3-type structure and confines 0.79/f.u. anionic electrons in the quasi-one-dimensional holes. These anionic electrons strongly hybridize with yttrium 4d electrons, giving rise to improved chemical stability. The ammonia synthesis rate using Ru(7.8 wt %)-loaded Y5Si3 was as high as 1.9 mmol/g/h under 0.1 MPa and at 400 °C with activation energy of ~50 kJ/mol. Its strong electron-donating ability to Ru metal of Y5Si3 is considered to enhance nitrogen dissociation and reduce the activation energy of ammonia synthesis reaction. Catalytic activity was not suppressed even after Y5Si3, once dipped into water, was used as the catalyst promoter. These findings provide novel insights into the design of simple catalysts for ammonia synthesis. [ABSTRACT FROM AUTHOR]

We report that the interlayer states common to the compounds AeAlSi (Ae=Ca, Sr, Ba) arise from F-center-like electrons arrayed in periodic cavities. The SrPtSb-type intermetallic phases exhibit electrons localized to columns of the trigonal bipyramidal Ae3Al2 cages running perpendicular to the honeycomb layers. Ab initio calculations in combination with hard/soft x-ray photoemission spectroscopic measurements reveal that these features correspond to the anionic electrons that hybridize with apical Al3pz orbitals from the honeycomb layers above and below. Extra bands with a significant dispersion along the kz direction therefore contribute to the Fermi level in contrast to the apparent two-dimensional connectivity of the bonding in the compounds, and completely account for the presence of interlayer states. Our study demonstrates how the cage centers may serve as electronically important crystallographic sites, and extend the anionic electron concept into honeycomb lattice compounds. [ABSTRACT FROM AUTHOR]

The excitonic insulator is an electronically driven phase of matter that emerges upon the spontaneous formation and Bose condensation of excitons. Detecting this exotic order in candidate materials is a subject of paramount importance, as the size of the excitonic gap in the band structure establishes the potential of this collective state for superfluid energy transport. However, the identification of this phase in real solids is hindered by the coexistence of a structural order parameter with the same symmetry as the excitonic order. Only a few materials are currently believed to host a dominant excitonic phase, Ta2NiSe5 being the most promising. Here, we test this scenario by using an ultrashort laser pulse to quench the broken-symmetry phase of this transition metal chalcogenide. Tracking the dynamics of the material's electronic and crystal structure after light excitation reveals spectroscopic fingerprints that are compatible only with a primary order parameter of phononic nature. We rationalize our findings through state-of-the-art calculations, confirming that the structural order accounts for most of the gap opening. Our results suggest that the spontaneous symmetry breaking in Ta2NiSe5 is mostly of structural character, hampering the possibility to realize quasi-dissipationless energy transport. [ABSTRACT FROM AUTHOR]

The article presents a study of the collective excitations in Ta2NiSe5 to prove the coherent order parameter oscillations in the ground state of the bandgap semiconductor. It demonstrates the existence of a coherent amplitude response in the excitonic insulator (EI) Ta2NiSe5, identifying a phonon-coupled state of the condensate characterizing the transient order parameter of the EI as a function of excitation and temperature density.

In this paper, two types of hybrid semiconductors made up of silicon nanowires and ZnO nanostructures, namely Si/ZnO core–shell nanowire arrays and ZnO quantum dots (QDs)-decorated Si nanowire arrays, have been prepared by combining metal-assisted wet-chemical etching and metal–organic chemical vapor deposition (MOCVD). We demonstrate that ZnO QDs and thin ZnO layers can be grown on Si nanowires in a controlled manner by varying growth parameters including working pressure and growth time. Meanwhile, porous silicon and porous Si/ZnO nanowire arrays have also been fabricated. The morphology and optical properties of both hybrid nanostructures have been carefully investigated for their potential applications in nanowire optoelectronics. A quantum confinement effect in ZnO QDs was confirmed by the blue-shifted photoluminescence. Porous Si/ZnO core–shell nanowires display a very broad emission band throughout the entire visible light range. [ABSTRACT FROM AUTHOR]