Enhanced electrocatalytic hydrogen evolution from nitrogen plasma-tailored MoS2 nanostructures.

Two-dimensional (2D) layered transition metal dichalcogenides such as MoS2 have been viewed as the most favorable candidates for replacing noble metals in catalyzing the hydrogen evolution reaction in water splitting owing to their earth abundance, superb chemical stability, and appropriate Gibbs free energy. However, due to its low number of catalytic sites and basal catalytic inertia, the pristine MoS2 displayed intrinsically unsatisfactory HER catalytic activity. Here, the hydrogen evolution catalytic activities of nanostructured MoS2 powder before and after plasma modification with nitrogen doping were experimentally compared, and the influence of treatment parameters on the hydrogen evolution catalytic performance of MoS2 has been studied. The feasibility of regulating hydrogen evolution catalytic activity by nitrogen doping of MoS2 was verified based on density functional theory calculations. Our work demonstrates a more convenient and faster way to develop cheap and efficient MoS2-based catalysts for electrochemical hydrogen evolution reactions. Additionally, theoretical studies reveal that N-doped MoS2 exhibits strong hybridization between Mo-d and N-p states, causing magnetism to evolve, as confirmed by experiments.

Tailoring the Microstructure of Porous Carbon Spheres as High Rate Performance Anodes for Lithium-Ion Batteries.

Benefiting from their high surface areas, excellent conductivity, and environmental-friendliness, porous carbon nanospheres (PCSs) are of particular attraction for the anodes of lithium-ion batteries (LIBs). However, the regulation of carbon nanospheres with controlled pore distribution and graphitization for delivering high Li+ storage behavior is still under investigation. Here, we provide a facile approach to obtain PCSs with different microstructures via modulating the carbonization temperatures. With the processing temperature of 850 °C, the optimized PCSs exhibit an increased surface area, electrical conductivity, and enhanced specific capacity (202 mA h g-1 at 2 A g-1) compared to the PCSs carbonized at lower temperatures. Additionally, PCSs 850 provide excellent cyclability with a capacity retention of 83% for 500 cycles. Such work can pave a new pathway to achieve carbon nanospheres with excellent performances in LIBs.

Second-order Jahn-Teller effect induced high-temperature ferroelectricity in two-dimensional NbO2X (X = I, Br).

Based on the first-principles calculations, we investigated the ferroelectric properties of two-dimensional (2D) materials NbO2X (X = I, Br). Our cleavage energy analysis shows that exfoliating one NbO2I monolayer from its existing bulk counterpart is feasible. The phonon spectrum and molecular dynamics simulations confirm the dynamic and thermal stability of the monolayer structures for both NbO2I and NbO2Br. Total energy calculations show that the ferroelectric phase is the ground state for both materials, with the calculated in-plane ferroelectric polarizations being 384.5 pC m-1 and 375.2 pC m-1 for monolayers NbO2I and NbO2Br, respectively. Moreover, the intrinsic Curie temperature TC of monolayer NbO2I (NbO2Br) is as high as 1700 K (1500 K) from Monte Carlo simulation. Furthermore, with the orbital selective external potential method, the origin of ferroelectricity in NbO2X is revealed as the second-order Jahn-Teller effect. Our findings suggest that monolayers NbO2I and NbO2Br are promising candidate materials for practical ferroelectric applications.

Exploring the catalytic activity of graphene-based TM-NxC4-x single atom catalysts for the oxygen reduction reaction via density functional theory calculation.

Electrocatalysts for the oxygen reduction reaction (ORR) are extremely crucial for advanced energy conversion technologies, such as fuel cell batteries. A promising ORR catalyst usually should have low overpotentials, rich catalytic sites and low cost. In the past decade, single-atom catalyst (SAC) TM-N4 (TM = Fe, Co, etc.) embedded graphene matrixes have been widely studied for their promising performance and low cost for ORR catalysis, but the effect of coordination on the ORR activity is not fully understood. In this work, we will employ density functional theory (DFT) calculations to systematically investigate the ORR activity of 40 different 3d transition metal single-atom catalysts (SACs) supported on nitrogen-doped graphene supports, ranging from vanadium to zinc. Five different nitrogen coordination configurations (TM-NxC4-x with x = 0, 1, 2, 3, and 4) were studied to reveal how C/N substitution affects the ORR activity. By looking at the stability, free energy diagram, overpotential, and scaling relationship, our calculation showed that partial C substitution can effectively improve the ORR performance of Mn, Co, Ni, and Zn-based SACs. The volcano plot obtained from the scaling relationship indicated that the substitution of N by C could distinctively affect the potential-limiting step in the ORR, which leads to the enhanced or weakened ORR performance. Density of states and d-band center analysis suggested that this coordination-tuned ORR activity can be explained by the shift of the d-band center due to the coordination effect. Finally, four candidates with optimal ORR activity and dynamic stability were proposed from the pool: NiC4, CoNC3, CrN4, and ZnN3C. Our work provides a feasible designing strategy to improve the ORR activity of graphene-based TM-N4 SACs by tuning the coordination environment, which may have potential implication in the high-performance fuel cell development.

Optimized Pinecone-Squama-Structure MoS2-Coated CNT and Graphene Framework as Binder-Free Anode for Li-Ion Battery with High Capacity and Cycling Stability.

Extensive research has been conducted on the development of high-rate and cyclic stability anodes for lithium batteries (LIBs) due to their high energy density. Molybdenum disulfide (MoS2) with layered structure has garnered significant interest due to its exceptional theoretic Li+ storage behavior as anodes (670 mA h g-1). However, achieving a high rate and long cyclic life of anode materials remains a challenge. Herein, we designed and synthesized a free-standing carbon nanotubes-graphene (CGF) foam, then presented a facile strategy to fabricate the MoS2-coated CGF self-assembly anodes with different MoS2 distributions. Such binder-free electrode possesses the advantages of both MoS2 and graphene-based materials. Through rational regulation of the ratio of MoS2, the MoS2-coated CGF with uniformly distributed MoS2 exhibits a nano pinecone-squama-like structure that can accommodate the large volume change during the cycle process, thereby significantly enhancing the cycling stability (417 mA h g-1 after 1000 cycles), ideal rate performance, and high pseudocapacitive behavior (with a 76.6% contribution at 1 mV s-1). Such a neat nano-pinecone structure can effectively coordinate MoS2 and carbon framework, providing valuable insights for the construction of advanced anode materials.

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.

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.

Ultra-High-Temperature Ferromagnetism in Intrinsic Tetrahedral Semiconductors.

Ferromagnetic semiconductors exhibit novel spin-dependent optical, electrical, and transport properties, which are promising for next-generation highly functional spintronic devices. However, the possibility of practical applications is hindered by their low Curie temperature. Currently, whether semiconducting ferromagnetism can exist at room temperature is still unclear because of the absence of a solid physical mechanism. Here, on the basis of tight-binding model analysis and first-principles calculations, we report that ferromagnetism in a tetrahedral semiconductor originating from superexchange interactions can be strong enough to survive at room temperature because of the weakening of antiferromagnetic direct-exchange interactions. On the basis of the explored mechanism, a zinc-blende binary transition metal compound, chromium carbide, is predicted to be an intrinsic ferromagnetic tetrahedral semiconductor with a Curie temperature that is as high as ∼1900 K. These findings not only expand the understandings of magnetism in semiconductors but also are of great interest for room-temperature spintronic applications.

Toward Intrinsic Room-Temperature Ferromagnetism in Two-Dimensional Semiconductors.

Two-dimensional (2D) ferromagnetic semiconductors have been recognized as the cornerstone for next-generation electric devices, but the development is highly limited by the weak ferromagnetic coupling and low Curie temperature ( TC). Here, we reported a general mechanism which can significantly enhance the ferromagnetic coupling in 2D semiconductors without introducing carriers. On the basis of a double-orbital model, we revealed that the superexchange-driven ferromagnetism is closely related to the virtual exchange gap, and lowering this gap by isovalent alloying can significantly enhance the ferromagnetic (FM) coupling. On the basis of the experimentally available two-dimensional CrI3 and CrGeTe3, the FM coupling in two semiconducting alloy compounds CrWI6 and CrWGe2Te6 monolayers are calculated to be enhanced by 3∼5 times without introducing any carriers. Furthermore, a room-temperature ferromagnetic semiconductor is achieved under a small in-plane strain (4%). Thus, our findings not only deepen the understanding of FM semiconductors but also open a new door for realistic spintronics.

Confinement boosts CO oxidation on an Ni atom embedded inside boron nitride nanotubes.

To date, most studies of heterogeneous catalysis have focused on metal particles supported on the surface of substrates. However, studies of the catalytic properties of metallic nanoparticles supported on the interior surface of nanotubes are rare. Using first-principles calculations based on density functional theory, we have studied the CO oxidation on a single nickel atom confined in a nitrogen vacancy on the inside surface of boron nitride nanotubes (BNNT). By exploring the Eley-Rideal mechanism, we find that an Ni atom embedded on the interior surface of BNNTs exhibits a much higher catalytic activity for CO oxidation when compared with Ni doped on their outside surface. In addition, the energy barriers of the rate-determining step for CO oxidation on Ni embedded on the inside wall of BNNT(5,5), BNNT(6,6) and BNNT(7,7) are 0.39, 0.29 and 0.33 eV, respectively. The results illustrate the merit of confinement for CO oxidation.

Effect of Coulomb Correlation on the Magnetic Properties of Mn Clusters.

In spite of decades of research, a fundamental understanding of the unusual magnetic behavior of small Mn clusters remains a challenge. Experiments show that Mn2 is antiferromagnetic while small clusters containing up to five Mn atoms are ferromagnetic with magnetic moments of 5 µB/atom and become ferrimagnetic as they grow further. Theoretical studies based on density functional theory (DFT), however, find Mn2 to be ferromagnetic, with ferrimagnetic order setting in at different sizes that depend upon the computational methods used. While quantum chemical techniques correctly account for the antiferromagnetic ground state of Mn2, they are computationally too demanding to treat larger clusters, making it difficult to understand the evolution of magnetism. These studies clearly point to the importance of correlation and the need to find ways to treat it effectively for larger clusters and nanostructures. Here, we show that the DFT+ U method can be used to account for strong correlation. We determine the on-site Coulomb correlation, Hubbard U self-consistently by using the linear response theory and study its effect on the magnetic coupling of Mn clusters containing up to five atoms. With a calculated U value of 4.8 eV, we show that the ground state of Mn2 is antiferromagnetic with a Mn-Mn distance of 3.34 Å, which agrees well with the electron spin resonance experiment. Equally important, we show that on-site Coulomb correlation also plays an important role in the evolution of magnetic coupling in larger clusters, as the results differ significantly from standard DFT calculations. We conclude that for a proper understanding of magnetism of Mn nanostructures (clusters, chains, and layers) one must take into account the effect of strong correlation.

Prediction of Intrinsic Ferromagnetic Ferroelectricity in a Transition-Metal Halide Monolayer.

The realization of multiferroics in nanostructures, combined with a large electric dipole and ferromagnetic ordering, could lead to new applications, such as high-density multistate data storage. Although multiferroics have been broadly studied for decades, ferromagnetic ferroelectricity is rarely explored, especially in two-dimensional (2D) systems. Here we report the discovery of 2D ferromagnetic ferroelectricity in layered transition-metal halide systems. On the basis of first-principles calculations, we reveal that a charged CrBr_{3} monolayer exhibits in-plane multiferroicity, which is ensured by the combination of orbital and charge ordering as realized by the asymmetric Jahn-Teller distortions of octahedral Cr─Br_{6} units. As an example, we further show that (CrBr_{3})_{2}Li is a ferromagnetic ferroelectric multiferroic. The explored phenomena and mechanism of multiferroics in this 2D system not only are useful for fundamental research in multiferroics but also enable a wide range of applications in nanodevices.

Diverse carrier mobility of monolayer BNC x : a combined density functional theory and Boltzmann transport theory study.

BNC x monolayer as a kind of two-dimensional material has numerous chemical atomic ratios and arrangements with different electronic structures. Via calculations on the basis of density functional theory and Boltzmann transport theory under deformation potential approximation, the band structures and carrier mobilities of BNC x (x = 1,2,3,4) nanosheets are systematically investigated. The calculated results show that BNC2-1 is a material with very small band gap (0.02 eV) among all the structures while other BNC x monolayers are semiconductors with band gap ranging from 0.51 eV to 1.32 eV. The carrier mobility of BNC x varies considerably from tens to millions of cm2 V-1 s-1. For BNC2-1, the hole mobility and electron mobility along both x and y directions can reach 105 orders of magnitude, which is similar to the carrier mobility of graphene. Besides, all studied BNC x monolayers obviously have anisotropic hole mobility and electron mobility. In particular, for semiconductor BNC4, its hole mobility along the y direction and electron mobility along the x direction unexpectedly reach 106 orders of magnitude, even higher than that of graphene. Our findings suggest that BNC x layered materials with the proper ratio and arrangement of carbon atoms will possess desirable charge transport properties, exhibiting potential applications in nanoelectronic devices.

Ca-Embedded C2N: an efficient adsorbent for CO2 capture.

Carbon dioxide as a greenhouse gas causes severe impacts on the environment, whereas it is also a necessary chemical feedstock that can be converted into carbon-based fuels via electrochemical reduction. To efficiently and reversibly capture CO2, it is important to find novel materials for a good balance between adsorption and desorption. In this study, we performed first-principles calculations and grand canonical Monte Carlo (GCMC) simulations, to systematically study metal-embedded carbon nitride (C2N) nanosheets for CO2 capture. Our first-principles results indicated that Ca atoms can be uniformly trapped in the cavity center of C2N structure, while the transition metals (Sc, Ti, V, Cr, Mn, Fe, Co) are favorably embedded in the sites off the center of the cavity. The determined maximum number of CO2 molecules with strong physisorption showed that Ca-embedded C2N monolayer is the most promising CO2 adsorbent among all considered metal-embedded materials. Moreover, GCMC simulations revealed that at room temperature the gravimetric density for CO2 adsorbed on Ca-embedded C2N reached 50 wt% at 30 bar and 23 wt% at 1 bar, higher than other layered materials, thus providing a satisfactory system for the CO2 capture and utilization.

Nanoporous MoS2 monolayer as a promising membrane for purifying hydrogen and enriching methane.

We present a theoretical prediction of a highly efficient membrane for hydrogen purification and natural gas upgrading, i.e. laminar MoS2 material with triangular sulfur-edged nanopores. We calculated from first principles the diffusion barriers of H2 and CO2 across monolayer MoS2 to be, respectively, 0.07 eV and 0.17 eV, which are low enough to warrant their great permeability. The permeance values for H2 and CO2 far exceed the industrially accepted standard. Meanwhile, such a porous MoS2 membrane shows excellent selectivity in terms of H2/CO, H2/N2, H2/CH4, and CO2/CH4 separation (>103, > 103, > 106, and > 104, respectively) at room temperature. We expect that the findings in this work will expedite theoretical or experimental exploration on gas separation membranes based on transition metal dichalcogenides.

Stabilization of the Metastable Lead Iodide Perovskite Phase via Surface Functionalization.

Metastable structural polymorphs can have superior properties and applications to their thermodynamically stable phases, but the rational synthesis of metastable phases is a challenge. Here, a new strategy for stabilizing metastable phases using surface functionalization is demonstrated using the example of formamidinium lead iodide (FAPbI3) perovskite, which is metastable at room temperature (RT) but holds great promises in solar and light-emitting applications. We show that, through surface ligand functionalization during direct solution growth at RT, pure FAPbI3 in the cubic perovskite phase can be stabilized in nanostructures and thin films at RT without cation or anion alloying. Surface characterizations reveal that long-chain alkyl or aromatic ammonium (LA) cations bind to the surface of perovskite structure. Calculations show that such functionalization reduces the surface energy and plays a dominant role in stabilizing the metastable perovskite phase. Excellent photophysics and optically pumped lasing from the stabilized single-crystal FAPbI3 nanoplates with low thresholds were demonstrated. High-performance solar cells can be fabricated with such directly synthesized stabilized phase-pure FAPbI3 with a lower bandgap. Our results offer new insights on the surface chemistry of perovskite materials and provide a new strategy for stabilizing metastable perovskites and metastable polymorphs of solid materials in general.

Carrier mobility in double-helix DNA and RNA: A quantum chemistry study with Marcus-Hush theory.

Charge mobilities of six DNAs and RNAs have been computed using quantum chemistry calculation combined with the Marcus-Hush theory. Based on this simulation model, we obtained quite reasonable results when compared with the experiment, and the obtained charge mobility strongly depends on the molecular reorganization and electronic coupling. Besides, we find that hole mobilities are larger than electron mobilities no matter in DNAs or in RNAs, and the hole mobility of 2L8I can reach 1.09 × 10-1 cm2 V-1 s-1 which can be applied in the molecular wire. The findings also show that our theoretical model can be regarded as a promising candidate for screening DNA- and RNA-based molecular electronic devices.

New Ferroelectric Phase in Atomic-Thick Phosphorene Nanoribbons: Existence of in-Plane Electric Polarization.

Ferroelectrics have many significant applications in electric devices, such as capacitor or random-access memory, tuning the efficiency of solar cell. Although atomic-thick ferroelectrics are the necessary components for high-density electric devices or nanoscale devices, the development of such materials still faces a big challenge because of the limitation of intrinsic mechanism. Here, we reported that in-plane atomic-thick ferroelectricity can be induced by vertical electric field in phosphorene nanoribbons (PNRs). Through symmetry arguments, we predicted that ferroelectric direction is perpendicular to the direction of external electric field and lies in the plane. Further confirmed by the comprehensive first-principles calculations, we showed that such ferroelectricity is induced by the electron-polarization, which is different from the structural distortion in traditional ferroelectrics and the recent experimental discovery of in-plane atomic-thick ferroelectrics (Science 2016, 353, 274). Moreover, we found that the value of electronic polarization in bilayer is much larger than that in monolayer. Our results show that electron-polarization ferroelectricity maybe the most promising candidate for atomic-thick ferroelectrics.

Superhalogens as building blocks of two-dimensional organic-inorganic hybrid perovskites for optoelectronics applications.

Organic-inorganic hybrid perovskites, well known for their potential as the next generation solar cells, have found another niche application in optoelectronics. This was demonstrated in a recent experiment (L. Dou, et al., Science, 2015, 349, 1518) on atomically thin (C4H9NH3)2PbBr4, where, due to quantum confinement, the bandgap and the exciton binding energy are enhanced over their corresponding values in the three-dimensional bulk phase. Using density functional theory we show that when halogen atoms (e.g. I) are sequentially replaced with superhalogen molecules (e.g. BH4) the bandgap and exciton binding energy increase monotonically with the superhalogen content with the exciton binding energy of (C4H9NH3)2Pb(BH4)4 approaching the value in monolayer black phosphorus. Lead-free admixtures (C4H9NH3)2MI4-x(BH4)x (M = Sn and Ge; x = 0-4) also show a similar trend. Thus, a combination of quantum confinement and compositional change can be used as an effective strategy to tailor the bandgap and the exciton binding energy of two-dimensional hybrid perovskites, making them promising candidates for optoelectronic applications.

Graphdiyne as a High-Efficiency Membrane for Separating Oxygen from Harmful Gases: A First-Principles Study.

We theoretically explored the adsorption and diffusion properties of oxygen and several harmful gases penetrating the graphdiyne monolayer. According to our first-principles calculations, the oxidation of the acetylenic bond in graphdiyne needs to surmount an energy barrier of ca. 1.97 eV, implying that graphdiyne remains unaffected under oxygen-rich conditions. In a broad temperature range, graphdiyne with well-defined nanosized pores exhibits a perfect performance for oxygen separation from typical noxious gases, which should be of great potential in medical treatment and industry.