Engineering Interfacial Built-in Electric Field in Polymetallic Phosphide Heterostructures for Superior Supercapacitors and Electrocatalytic Hydrogen Evolution.

Herein, a patterned rod-like CoP@NiCoP core-shell heterostructure is designed to consist of CoP nanowires cross-linked with NiCoP nanosheets in tight strings. The interfacial interaction within the heterojunction between the two components generates a built-in electric field that adjusts the interfacial charge state and create more active sites, accelerating the charge transfer and improving supercapacitor and electrocatalytic performance. The unique core-shell structure suppresses the volume expansion during charging and discharging, achieving excellent stability. As a result, CoP@NiCoP exhibits a high specific capacitance of 2.9 F cm-2 at a current density of 3 mA cm-2 and a high ion diffusion rate (Dion is 2.95 × 10-14  cm2  s-1 ) during charging/discharging. The assembled asymmetric supercapacitor CoP@NiCoP//AC exhibits a high energy density of 42.2 Wh kg-1 at a power density of 126.5 W kg-1 and excellent stability with a capacitance retention rate of 83.8% after 10 000 cycles. Furthermore, the modulated effect induced by the interfacial interaction also endows the self-supported electrode with excellent electrocatalytic HER performance with an overpotential of 71 mV at 10 mA cm-2 . This research may provide a new perspective on the generation of built-in electric field through the rational design of heterogeneous structures for improving the electrochemical and electrocatalytical performance.

Mechanical properties of hydrogenated ψ-graphene.

CONTEXT: Hydrogenation is an effective way to open a band gap of the metallic ψ-graphene, expanding its application in electronics. Evaluating the mechanical properties of hydrogenated ψ-graphene, especially the effect of hydrogen coverage, is also crucial to the application of ψ-graphene. Here, we demonstrate the mechanical properties of ψ-graphene depend closely on the hydrogen coverage and arrangement. Upon hydrogenation, Young's modulus and intrinsic strength of ψ-graphene decrease due to breaking of sp2 carbon networks. Both the ψ-graphene and hydrogenated ψ-graphene exhibit mechanical anisotropy. During changing the hydrogen coverage, the variation of mechanical strength of the hydrogenated ψ-graphene relies on the tensile direction. In addition, the arrangement of hydrogen also contributes to the mechanical strength and fracture behavior of hydrogenated ψ-graphene. Our results not only present a comprehensive understanding of the mechanical properties of hydrogenated ψ-graphene, but also provide a reference to tailor the mechanical properties of other graphene allotropes, which are of potential interest in materials science. METHODS: Vienna ab initio simulation package based on the plane-wave pseudopotential technique was employed for the calculations. The exchange-correlation interaction was described by the Perdew-Burke-Ernzerhof functional within the general gradient approximation and the ion-electron interaction was treated with the projected augmented wave pseudopotential.

Boosting Electrocatalytic Ammonia Synthesis of Bio-Inspired Porous Mo-Doped Hematite via Nitrogen Activation.

Electrochemical N2 reduction reaction (NRR) emerges as a highly attractive alternative to the Haber-Bosch process for producing ammonia (NH3) under ambient circumstances. Currently, this technology still faces tremendous challenges due to the low ammonia production rate and low Faradaic efficiency, urgently prompting researchers to explore highly efficient electrocatalysts. Inspired by the Fe-Mo cofactor in nitrogenase, we report Mo-doped hematite (Fe2O3) porous nanospheres containing Fe-O-Mo subunits for enhanced activity and selectivity in the electrochemical reduction from N2 to NH3. Mo-doping induces the morphology change from a solid sphere to a porous sphere and enriches lattice defects, creating more active sites. It also regulates the electronic structures of Fe2O3 to accelerate charge transfer and enhance the intrinsic activity. As a consequence, Mo-doped Fe2O3 achieves effective N2 fixation with a high ammonia production rate of 21.3 ± 1.1 µg h-1 mgcat.-1 as well as a prominent Faradaic efficiency (FE) of 11.2 ± 0.6%, superior to the undoped Fe2O3 and other iron oxide catalysts. Density functional theory (DFT) calculations further unravel that the Mo-doping in Fe2O3 (110) narrows the band gap, promotes the N2 activation on the Mo site with an elongated N≡N bond length of 1.132 Å in the end-on configuration, and optimizes an associative distal pathway with a decreased energy barrier. Our results may pave the way toward enhancing the electrocatalytic NRR performance of iron-based materials by atomic-scale heteroatom doping.

Bending modulus of the rippled graphene: the role of thickness.

Bending modulus is a key parameter to characterize the stiffness of materials. Commonly, it is believed that the bending modulus is closely related to the thickness as described by the thin plate theory. However, the thin plate theory fails in multilayer van der Waals materials like multilayer graphene, suggesting a more complex relationship between the bending modulus and thickness. Here, rippled graphene structures containing non-hexagonal carbon rings with different thicknesses are constructed to study the thickness-dependent bending modulus by the first-principles calculations. It is found that the bending modulus of rippled graphene depends on several factors, such as geometry, bending curvature, and thickness. Particularly, for the egg-tray graphene structures with similar structural pattern and bending curvature, i.e., eliminating the effects of structural pattern and bending curvature, the bending modulus could show a linear relationship to the thickness. Moreover, this linear relationship is very robust even in the case of changing the thickness through heteroatom doping.

Boosting Hydrogen Evolution Electrocatalysis via Regulating the Electronic Structure in a Crystalline-Amorphous CoP/CeOx p-n Heterojunction.

The modulation of the electronic structure is the effective access to achieve highly active electrocatalysts for the hydrogen evolution reaction (HER). Transition-metal phosphide-based heterostructures are very promising in enhancing HER performance but the facile fabrication and an in-depth study of the catalytic mechanisms still remain a challenge. In this work, the catalytically inactive n-type CeOx is successfully combined with p-type CoP to form the CoP/CeOx heterojunction. The crystalline-amorphous CoP/CeOx heterojunction is fabricated by the phosphorization of predesigned Co(OH)2/CeOx via the as-developed reduction-hydrolysis strategy. The p-n CoP/CeOx heterojunction with a strong built-in potential of 1.38 V enables the regulation of the electronic structure of active CoP within the space-charge region to enhance its intrinsic activity and facilitate the electron transfer. The functional CeOx entity and the negatively charged CoP can promote the water dissociation and optimize H adsorption, synergistically boosting the electrocatalytic HER output. As expected, the heterostructured CoP/CeOx-20:1 with the optimal ratio of Co/Ce shows significantly improved HER activity and favorable kinetics (overpotential of 118 mV at a current density of 10 mA cm-2 and Tafel slope of 77.26 mV dec-1). The present study may provide new insight into the integration of crystalline and amorphous entities into the p-n heterojunction as a highly efficient electrocatalyst for energy storage and conversion.

Synergically engineering defect and interlayer in SnS2 for enhanced room-temperature NO2 sensing.

Defect and interlayer engineering are considered as two promising strategies to alter the electronic structures of sensing materials for improved gas sensing properties. Herein, ethylene glycol intercalated Al-doped SnS2 (EG-Al-SnS2) featuring Al doping, sulfur (S) vacancies, and an expanded interlayer spacing was prepared and developed as an active NO2 sensing material. Compared to the pristine SnS2 with failure in detecting NO2 at room temperature, the developed EG-Al-SnS2 exhibited a better conductivity, which was beneficial for realizing the room-temperature NO2 sensing. As a result, a high sensing response of 410% toward 2 ppm NO2 was achieved at room temperature by using the 3% EG-Al-SnS2 as the sensing material. Such outstanding sensing performance was attributed to the enhanced electronic interaction of NO2 on the surface of SnS2 induced by the synergistic effect of Al doping, S vacancies, and the expanded interlayer spacing, which is directly revealed by the in-suit measurement based on near-ambient pressure X-ray photoelectronic spectroscopy (NAP-XPS). Furthermore, to identify the role of Al doping, S vacancies, and the expanded interlayer spacing in enhancing the NO2 sensing properties, a series of comparative experiments and theoretical calculations were performed.

Remote Passivation in Two-Dimensional Materials: The Case of the Monolayer-Bilayer Lateral Junction of MoSe2.

Two-dimensional (2D) monolayer-bilayer (ML-BL) lateral junctions (LJs) have recently attracted attention due to their straightforward synthesis and resulting clean interface. Such systems consist of an extended ML with a secondary layer present only over half of the system, leading to an interface that is associated with the terminating edge of the secondary half layer. Our first-principles calculations reveal that the edges of the half layer completely lack reconstruction in the presence of unintentional dopants, in this case, Re. This observation is in startling contrast to the known physics of three-dimensional (3D) semiconductor surfaces where reconstruction has been widely observed. Herein, the electrostatics of the reduced dimensionality allows for greater separation between compensating defects, enabling dopants to remotely passivate edge states without needing to directly participate in the chemistry.

Theoretical study of the influence of doped niobium on the electronic properties of CsPbBr3.

In the family of inorganic perovskite solar cells (PSCs), CsPbBr3 has attracted widespread attention due to its excellent stability under high humidity and high temperature conditions. However, power conversion efficiency (PCE) improvement of CsPbBr3-based PSCs is markedly limited by the large optical absorption loss coming from the wide band gap and serious charge recombination at interfaces and/or within the perovskite film. In this work, using density functional theory calculations, we systemically studied the electronic properties of niobium (Nb)-doped CsPbBr3 with different concentration ratios. As a result, it is found that doped CsPbBr3 compounds are metallic at high Nb doping concentration but semiconducting at low Nb doping concentration. The calculated electronic density of states shows that the conduction band is predominantly constructed of doped Nb. These characteristics make them very suitable for solar cell and energy storage applications.

Tunable bending modulus and bending limit of oxidized graphene.

Graphene is highly flexible and widely used in flexible devices. However, is the oxidized graphene more flexible than graphene? This is still under debate between simulations and experiments. By employing density functional theory calculations, we show that the bending modulus of oxidized graphene is quite tunable by changing the type and coverage of the functional groups, as well as the bending direction. The hydroxyl increases the bending modulus of graphene, but epoxide can degrade the bending modulus in the armchair bending direction, making the oxidized graphene more flexible than graphene. On the other hand, there exists a curvature limit during bending the oxidized graphene, where OH hydrogen bonds start to transform into O-H covalent bonds. Generally, our results demonstrate the effects of the functional groups and bending direction on the flexibility of oxidized graphene, which should be helpful to design graphene-based flexible devices.

Hole defects and nitrogen doping in graphene: implication for supercapacitor applications.

One great challenge for supercapacitor is to achieve high energy capacity and fast charge/discharge rates simultaneously. Porous graphene with large surface area is a promising candidate for electrode materials of supercapacitor. Using first-principles calculations and non-equilibrium Green's function technique, we have explored the formation energies, mechanical properties, diffusion behaviors and electrical conductance of graphene sheets with various hole defects and/or nitrogen doping. Interestingly, graphene sheets with pyridinic-like holes (especially hexagonal holes) can be more easily doped with nitrogen and still retain the excellent mechanical properties of pristine graphene that is beneficial for the long cycle life. Porous graphene electrode with moderate hole diameter of 4.2-10 Å facilitates efficient access of electrolyte and exhibit excellent rate capability. In addition, doping with nitrogen as electron donors or proton attractors leads to charge accumulation and generates higher pseudocapacitance. Transmission coefficients of N-doped graphene sheets with pyridinic-like holes are only moderately reduced with regard to that of pristine graphene and are insensitive to the detailed geometry parameters. Overall, N-doped graphene with pyridinic-like holes exhibits exciting potentials for high performance energy storage in supercapacitor devices.

Electromechanical properties of zigzag-shaped carbon nanotubes.

Atomic structural models of zigzag-shaped carbon nanotubes (Z-CNTs) were constructed by periodically introducing pentagons and heptagons into pristine CNTs. In terms of formation energies, the Z-CNTs present comparable energetic stabilities to those of the pristine CNTs and are more stable than C60 fullerene. The mechanical properties of these Z-CNTs, including the Young's modulus, intrinsic strength and failure behaviour, were systematically investigated by first-principles computations. Compared with the pristine CNTs with an average Young's modulus of about 1.0 TPa, incorporation of pentagons and heptagons in the Z-CNTs will reduce the average Young's modulus to several hundreds of GPa. Moreover, the computational results also showed that under uniaxial tensile strain, the intrinsic strength and failure strain of the Z-CNTs are also lower than those of the pristine CNTs. Generally, the Young's modulus and intrinsic strength of the Z-CNTs are exponentially inverse to curvature, which can be expressed by simple formulae. In particular, the electronic properties of the armchair Z-CNTs can be tailored by uniaxial tensile strain. It was also found that through applying tensile strain, a semiconductor-metal or metal-semiconductor transition can be triggered. The localized-delocalized partial charge distribution near the Fermi energy for the strained Z-CNTs can explain the semiconductor-metal or metal-semiconductor transition. This significant electromechanical coupling effect suggests the Z-CNTs have potential applications in nanoscale electromechanical sensors and switches.

Solid-solution semiconductor nanowires in pseudobinary systems.

Pseudobinary solid-solution semiconductor nanowires made of (GaP)(1-x)(ZnS)(x), (ZnS)(1-x)(GaP)(x) and (GaN)(1-x)(ZnO)(x) were synthesized based on an elaborative compositional, structural, and synthetic designs. Using analytical high-resolution transmission electron microscopy (HRTEM) and energy dispersive X-ray spectroscopy (EDS), we confirmed that the structure uniformity and a lattice match between the two constituting binary components play the key roles in the formation of quaternary solid-solution nanostructures. Electrical transport measurements on individual GaP and (GaP)(1-x)(ZnS)(x) nanowires indicated that a slight invasion of ZnS in the GaP host could lead to the abrupt resistance increase, resulting in the semiconductor-to-insulator transition. The method proposed here may be extended to the rational synthesis of many other multicomponent nanosystems with tunable and intriguing optoelectronic properties for specific applications.

Mechanical properties of graphene oxides.

The mechanical properties, including the Young's modulus and intrinsic strength, of graphene oxides are investigated by first-principles computations. Structural models of both ordered and amorphous graphene oxides are considered and compared. For the ordered graphene oxides, the Young's modulus is found to vary from 380 to 470 GPa as the coverage of oxygen groups changes, respectively. The corresponding variations in the Young's modulus of the amorphous graphene oxides with comparable coverage are smaller at 290-430 GPa. Similarly, the ordered graphene oxides also possess higher intrinsic strength compared with the amorphous ones. As coverage increases, both the Young's modulus and intrinsic strength decrease monotonically due to the breaking of the sp(2) carbon network and lowering of the energetic stability for the ordered and amorphous graphene oxides. In addition, the band gap of the graphene oxide becomes narrower under uniaxial tensile strain, providing an efficient way to tune the electronic properties of graphene oxide-based materials.