Materials harbouring exotic quasiparticles, such as massless Dirac and Weyl fermions, have garnered much attention from physics and material science communities due to their exceptional physical properties such as ultra-high mobility and extremely large magnetoresistances. Here, we show that the highly stable, non-toxic and earth-abundant material, ZrSiS, has an electronic band structure that hosts several Dirac cones that form a Fermi surface with a diamond-shaped line of Dirac nodes. We also show that the square Si lattice in ZrSiS is an excellent template for realizing new types of two-dimensional Dirac cones recently predicted by Young and Kane. Finally, we find that the energy range of the linearly dispersed bands is as high as 2 eV above and below the Fermi level; much larger than of other known Dirac materials. This makes ZrSiS a very promising candidate to study Dirac electrons, as well as the properties of lines of Dirac nodes.
The solid lithium-ion electrolyte ''Li 7 La 3 Zr 2 O 12 '' (LLZO) with a garnet-type structure has been prepared in the cubic and tetragonal modification following conventional ceramic syntheses routes. Without aluminium doping tetragonal LLZO was obtained, which shows a two orders of magnitude lower room temperature conductivity than the cubic modification. Small concentrations of Al in the order of 1 wt% were sufficient to stabilize the cubic phase, which is known as a fast lithium-ion conductor. The structure and ion dynamics of Al-doped cubic LLZO were studied by impedance spectroscopy, dc conductivity measurements, 6 Li and 7 Li NMR, XRD, neutron powder diffraction, and TEM precession electron diffraction. From the results we conclude that aluminium is incorporated in the garnet lattice on the tetrahedral 24d Li site, thus stabilizing the cubic LLZO modification. Simulations based on diffraction data show that even at the low temperature of 4 K the Li ions are blurred over various crystallographic sites. This strong Li ion disorder in cubic Al-stabilized LLZO contributes to the high conductivity observed. The Li jump rates and the activation energy probed by NMR are in very good agreement with the transport parameters obtained from electrical conductivity measurements. The activation energy E a characterizing longrange ion transport in the Al-stabilized cubic LLZO amounts to 0.34 eV. Total electric conductivities determined by ac impedance and a four point dc technique also agree very well and range from 1 Â 10 À4 Scm À1 to 4 Â 10 À4 Scm À1 depending on the Al content of the samples. The room temperature conductivity of Al-free tetragonal LLZO is about two orders of magnitude lower (2 Â 10 À6 Scm À1 , E a = 0.49 eV activation energy). The electronic partial conductivity of cubic LLZO was measured using the Hebb-Wagner polarization technique. The electronic transference number t eÀ is of the order of 10 À7. Thus, cubic LLZO is an almost exclusive lithium ion conductor at ambient temperature.
The heptazine-based polymer melon (also known as graphitic carbon nitride, g-C3N4) is a promising photocatalyst for hydrogen evolution. Nonetheless, attempts to improve its inherently low activity are rarely based on rational approaches because of a lack of fundamental understanding of its mechanistic operation. Here we employ molecular heptazine-based model catalysts to identify the cyanamide moiety as a photocatalytically relevant ‘defect'. We exploit this knowledge for the rational design of a carbon nitride polymer populated with cyanamide groups, yielding a material with 12 and 16 times the hydrogen evolution rate and apparent quantum efficiency (400 nm), respectively, compared with the unmodified melon. Computational modelling and material characterization suggest that this moiety improves coordination (and, in turn, charge transfer kinetics) to the platinum co-catalyst and enhances the separation of the photogenerated charge carriers. The demonstrated knowledge transfer for rational catalyst design presented here provides the conceptual framework for engineering high-performance heptazine-based photocatalysts.
Nanosheets of a crystalline 2D carbon nitride were obtained by ionothermal synthesis of the layered bulk material poly(triazine imide), PTI, followed by one-step liquid exfoliation in water. Triazine-based nanosheets are 1-2 nm in height and afford chemically and colloidally stable suspensions under both basic and acidic conditions. We use solid-state NMR spectroscopy of isotopically enriched, restacked nanosheets as a tool to indirectly monitor the exfoliation process and carve out the chemical changes occurring upon exfoliation, as well as to determine the nanosheet thickness. PTI nanosheets show significantly enhanced visible-light driven photocatalytic activity toward hydrogen evolution compared to their bulk counterpart, which highlights the crucial role of morphology and surface area on the photocatalytic performance of carbon nitride materials.
Solving the structure of carbon nitrides has been a long-standing challenge due to the low crystallinity and complex structures observed within this class of earth-abundant photocatalysts. Herein, we report on two-dimensional layered potassium poly(heptazine imide) (K-PHI) and its proton-exchanged counterpart (H-PHI), obtained by ionothermal synthesis using a molecular precursor route. We present a comprehensive analysis of the in-plane and three-dimensional structure of PHI. Transmission electron microscopy and solid-state NMR spectroscopy, supported by quantum-chemical calculations, suggest a planar, imide-bridged heptazine backbone with trigonal symmetry in both K-PHI and H-PHI, whereas pair distribution function analyses and X-ray powder diffraction using recursive-like simulations of planar defects point to a structure-directing function of the pore content. While the out-of-plane structure of K-PHI exhibits a unidirectional layer offset, mediated by hydrated potassium ions, H-PHI is characterized by a high degree of stacking faults due to the weaker structure directing influence of pore water. Structure–property relationships in PHI reveal that a loss of in-plane coherence, materializing in smaller lateral platelet dimensions and increased terminal cyanamide groups, correlates with improved photocatalytic performance. Size-optimized H-PHI is highly active toward photocatalytic hydrogen evolution, with a rate of 3363 μmol/gh H2 placing it on par with the most active carbon nitrides. K- and H-PHI adopt a uniquely long-lived photoreduced polaronic state in which light-induced electrons are stored for more than 6 h in the dark and released upon addition of a Pt cocatalyst. This work highlights the importance of structure–property relationships in carbon nitrides for the rational design of highly active hydrogen evolution photocatalysts.
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