Inserting auxeticity into graphene oxide via bottom-up strategy.

Carbon-based materials that process a wide bandgap, high mechanical performance, thermal stability and adjustable characteristics are in high demand. Auxeticity is one of the factors that helps enhances the mechanical performance. Based on this concept, two stable layered carbon-based materials, namely α-C2O and ß-C2O, are proposed. A new mechanism (multi-directional negative Poisson's ratio (NPR) effect) is induced, which is attributed to the interaction of modified pz orbitals between interfacial layers. This effect introduces high mechanical properties into materials. Besides, all layered materials are ultrawide bandgap semiconductors, which endows them comparable dielectric properties to those of diamond. Furthermore, α-BK-C2O would maintain its configuration over 2000 K, thereby guaranteeing extremely high thermodynamic stability. So far, these advantages suggested that these carbon-based layer materials could be used in nanoelectronics, especially in electromechanical devices.

First-principles calculations of monolayered Al2Te5: a promising 2D donor semiconductor with ultrahigh visible light harvesting.

Atomically thin two-dimensional (2D) crystals have piqued the curiosity of researchers due to their unique features and potential applications, such as catalysis and ion batteries. One essential and desirable aspect of 2D materials is that they have a large photoreactive contact surface for optical absorption. Here, a 2D crystal is proposed that possesses a moderate adjustable indirect band gap of 1.95 eV (HSE06) and exhibits ultrahigh visible light harvesting with a absorption coefficient of up to 108 cm-1 in the ∼380 to 800 nm range of the visible light spectrum. Besides that, the indirect band gap can be converted to a direct one under biaxial strain. By means of density functional theory, the 2D Al2Te5 monolayer displays great stability and promise of experimental fabrication. These advantages will provide considerable application potential for future photovoltaics (PV) devices.

Two-dimensional metal-free boron chalcogenides B2X3 (X = Se and Te) as photocatalysts for water splitting under visible light.

Finding photocatalysts that fully utilize the visible solar light to split water into hydrogen and oxygen has been a challenging problem for a long time. In this regard, compared to traditional three-dimensional materials, graphene-like two-dimensional materials offer many advantages such as ultra-high surface area for photochemical reactions and minimal migration distance for carriers. Herein, using density functional theory (DFT), we examine the potential of a new series of two-dimensional boron chalcogenides, B2X3 (X = S, Se, Te) as candidates for such photocatalysts. We show that B2Se3 and B2Te3 possess the ideal energy levels for photon excitation for both oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Furthermore, a bilayer van der Waals heterostructure consisting of B2Te3/B2Se3 is found to have the greatest potential for two-step photo-excitation for water splitting reaction. Our results can stimulate the synthesis of new two dimensional materials for photocatalysis.

Designing New Metal Chalcogenide Nanoclusters through Atom-by-Atom Substitution.

Atom-by-atom substitution is a promising strategy for designing new cluster-based materials, which has been used to generate new gold- and silver-containing clusters. Here, the first study focused on atom-by-atom substitution of Fe and Ni to the core of a well-defined cobalt sulfide superatom [Co6 S8 L6 ]+ ligated with triethylphosphine (L = PEt3 ) to produce [Co5 MS8 L6 ]+ (M = Fe, Ni) is reported. Electrospray ionization mass spectrometry confirms the substitution of 1-6 Fe atoms with the single Fe-substituted cluster being the dominant species. The Fe-substituted clusters oxidize in solution to generate dicationic species. In contrast, only a single Ni-substituted cluster is observed, which remains stable as a singly charged species. Collision-induced dissociation experiments indicate the reduced stability of the [Co5 FeS8 L6 ]+ toward ligand loss in comparison with the unsubstituted and Ni-substituted counterparts. Density functional theory calculations provide insights into the effect of metal atom substitution on the stability and electronic structures of the clusters. The results indicate that Fe and Ni have a different impact on the electronic structure, optical, and magnetic properties, as well as ligand-core interaction of [Co6 S8 L6 ]. This study extends the atom-by-atom substitution strategy to the metal chalcogenide superatoms providing a direct path toward designing novel atomically precise core-tailored superatoms.

Polymeric heptazine imide by O doping and constructing van der Waals heterostructures for photocatalytic water splitting: a theoretical perspective from transition dipole moment analyses.

Semiconductor-based photocatalysts have received extensive attention for their promising capacity in confronting global energy and environmental issues. In photocatalysis, a large band gap with suitable edge-position is necessary to warrant enough driving force for reaction, whereas a much smaller band gap is needed for visible-light response and high solar energy conversion efficiency. This paradox hinders the development of photocatalysts. Via state-of-the-art first-principles calculations, we find that the transition dipole moments (TDMs) are changed significantly in O-doped partly polymerized g-C3N4, i.e., OH-terminated polymeric heptazine imide (PHI-OH), and concomitantly, an enhancement of visible-light absorption is achieved; meanwhile a large enough band gap can provide a powerful driving force in the photocatalytic watersplitting reaction. Furthermore, by using TDM analysis of the PHI-OH/BC3N heterostructure, direct light excited transition between two building layers can be confirmed, suggesting it as a candidate catalyst for hydrogen evolution. From TDM analysis of the PHI-OH/BCN heterostructure, we also verify a Z-scheme process, which involves simultaneous photoexcitations with strong reducibility and oxidizability. Thus, TDM could be a good referential descriptor for revealing photocatalytic mechanisms in semiconductor photocatalysts and interlayer photoexcitation behavior in layered heterostructures. Hopefully, more strategies via modification of TDMs would be proposed to enhance the visible-light response of a semiconductor without sacrificing its photocatalytic driving force.

Nanocrystalline Cr-Ni Alloying Layer Induced by High-Current Pulsed Electron Beam.

In this investigation, chromium (Cr) was adopted as an alloying element on a nickel substrate, and the alloying process was materialized via high-current pulsed electron beam (HCPEB) irradiation. X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were also conducted for microstructure characterization. The results showed that after HCPEB irradiation a nanocrystalline Cr-Ni alloying layer was formed and numerous dislocations were generated, resulting in a great deal of diffusion paths for Cr elements. Moreover, properties including hardness, wear and electrochemical performance were significantly improved after HCPEB irradiation, which was mainly due to the formation of the nanocrystalline Cr⁻Ni alloying layer. In addition, each strengthening mechanism that contributed to the hardness of the HCPEB-irradiated sample was mathematically analyzed, and solid solution strengthening was found to be of great importance.