Comprehensive Mechanism for CO Electroreduction on Dual-Atom-Catalyst-Anchored N-Doped Graphene.

Carbon neutrality has drawn increasing attention for realizing the carbon cyclization and reducing the greenhouse effect. Although the C1 products, such as CO, can be achieved with a high Faraday efficiency, the targeted production of C2 fuels as well as the mechanism have not been systematically investigated. In this work, we carry out a first-principles study to screen dual-atom catalysts (DACs) for producing C2 fuels through the electrocatalytic carbon monoxide reduction reaction (e-CORR). We find that methanol, ethanol and ethylene can be produced on both DAC-Co and DAC-Cu, while acetate can be achieved on DAC-Cu only. Importantly, methanol and ethylene are preferred on DAC-Co, while acetate and ethylene on DAC-Cu. Furthermore, we show that the explicit solvent can enhance the adsorption and influence the protonation steps, which subsequently affects the protonation and dimerization behavior as well as the performance and selectivity of e-CORR on DACs. We further demonstrate that the C-C coupling is easy to be formed and stabilized if the Integrated Crystal Orbital Hamilton Population (ICOHP) is low because of the low energy barrier. Our findings provide not only guidance on the design of novel catalysts for e-CORR, but an insightful understanding on the reduction mechanism.

Two-Dimensional Dirac Nodal Line Carbon Nitride to Anchor Single-Atom Catalyst for Oxygen Reduction Reaction.

Two-dimensional carbon nitride (2DCN) materials have emerged as an important class of 2D materials beyond graphene. However, 2DCN materials with nodal-line semimetal characteristic are rarely reported. In this work, a new nodal-line semimetal 2DCN with the stoichiometry C4 N4 is designed by using density functional theory (DFT) calculations and its application to anchor single-atom catalysts (SACs) for the oxygen reduction reaction (ORR) is investigated. C4 N4 is a planar covalent network (sp2 hybridization) with regular holes formed by the four N atoms, which is dynamically, thermodynamically, and mechanically stable. The nodal line is contributed by the pz orbitals of C and px/y orbitals of N atoms. C4 N4 shows an anisotropic Fermi velocity and high electron mobility. Because of its porous structure, C4 N4 can anchor heteroatoms as SACs for electrocatalysis. C4 N4 anchored with Fe or Co is shown to be highly active for the ORR with a rather high half-wave potential of around 0.90 V, which is higher than those of SACs on other carbon nitrides. These findings may provide a new strategy to design novel substrates for SACs.

Highly Efficient Photocatalytic CO2 Reduction in Two-Dimensional Ferroelectric CuInP2S6 Bilayers.

Photocatalytic CO2 conversion into reproducible chemical fuels (e.g., CO, CH3OH, or CH4) provides a promising scheme to solve the increasing environmental problems and energy demands simultaneously. However, the efficiency is severely restricted by the high overpotential of the CO2 reduction reaction (CO2RR) and rapid recombination of photoexcited carriers. Here, we propose that a novel type-II photocatalytic mechanism based on two-dimensional (2D) ferroelectric multilayers would be ideal for addressing these issues. Using density-functional theory and nonadiabatic molecular dynamics calculations, we find that the ferroelectric CuInP2S6 bilayers exhibit a staggered band structure induced by the vertical intrinsic electric fields. Different from the traditional type-II band alignment, the unique structure of the CuInP2S6 bilayer not only effectively suppresses the recombination of photogenerated electron-hole (e-h) pairs but also produces a sufficient photovoltage to drive the CO2RR. The predicted recombination time of photogenerated e-h pairs, 1.03 ns, is much longer than the transferring times of photoinduced electrons and holes, 5.45 and 0.27 ps, respectively. Moreover, the overpotential of the CO2RR will decrease by substituting an S atom with a Cu atom, making the redox reaction proceed spontaneously under solar radiation. The solar-to-fuel efficiency with an upper limit of 8.40% is achieved in the CuInP2S6 bilayer and can be further improved to 32.57% for the CuInP2S6 five-layer. Our results indicate that this novel type-II photocatalytic mechanism would be a promising way to achieve highly efficient photocatalytic CO2 conversion based on the 2D ferroelectric multilayers.

Development of Perovskite Oxide-Based Electrocatalysts for Oxygen Evolution Reaction.

Perovskite oxides are studied as electrocatalysts for oxygen evolution reactions (OER) because of their low cost, tunable structure, high stability, and good catalytic activity. However, there are two main challenges for most perovskite oxides to be efficient in OER, namely less active sites and low electrical conductivity, leading to limited catalytic performance. To overcome these intrinsic obstacles, various strategies are developed to enhance their catalytic activities in OER. In this review, the recent developments of these strategies is comprehensively summarized and systematically discussed, including composition engineering, crystal facet control, morphology modulation, defect engineering, and hybridization. Finally, perspectives on the design of perovskite oxide-based electrocatalysts for practical applications in OER are given.

CNSi/MXene/CNSi: Unique Structure with Specific Electronic Properties for Nanodevices.

2D materials have been interesting for applications into nanodevices due to their intriguing physical properties. In this work, four types of unique structures are designed that are composed of MXenes and C/N-Si layers (CNSi), where MXene is sandwiched by the CNSi layers with different thicknesses, for their practical applications into integrated devices. The systematic calculations on their elastic constants, phonon dispersions, and thermodynamic properties show that these structures are stable, depending on the composition of MXene. It is found: 1) different from MXene or N-functionalized MXene (M2 CN2 ), SiN2 /M2 X/SiN2 possess new electronic properties with free carriers only in the middle, leading to 2D free electron gas; 2) CNSi/MXene/CNSi shows an intrinsic Ohmic semiconductor-metal-semiconductor (S-M-S) contact, which is potential for applications into nanodevices; and 3) O/M2 C/SiN2 and N/M2 C/OSiN are also stable and show different electronic properties, which can be semiconductor or metal as a whole depending on the interface. A method is further proposed to fabricate the 2D structures based on the industrial availability. The findings may provide a novel strategy to design and fabricate the 2D structures for their application into nanodevices and integrated circuits.

Design of 2D materials - MSi2CxN4-x (M = Cr, Mo, and W; x = 1 and 2) - with tunable electronic and magnetic properties.

Two-dimensional (2D) materials have attracted increasing interest in the past decades due to their unique physical and chemical properties for diverse applications. In this work, we present a first-principles design on a novel 2D family, MSi2CxN4-x (M = Cr, Mo, and W; x = 1 and 2), based on density-functional theory (DFT). We find that all MSi2CxN4-x monolayers are stable by investigating their mechanic, dynamic, and thermodynamic properties. Interestingly, we see that the alignment of magnetic moments can be tuned to achieve non-magnetism (NM), ferromagnetism (FM), anti-ferromagnetism (AFM) or paramagnetism (PM) by arranging the positions of carbon atoms in the 2D systems. Accordingly, their electronic properties can be controlled to obtain semiconductor, half-metal, or metal. The FM states in half-metallic 2D systems are contributed to the hole-mediated double exchange, while the AFM states are induced by super-exchange. Our findings show that the physical properties of 2D systems can be tuned by compositional and structural engineering, especially the layer of C atoms, which may provide guidance on the design and fabrication of novel 2D materials with projected properties for multi-functional applications.

Multi-Phase Heterostructure of CoNiP/Cox P for Enhanced Hydrogen Evolution Under Alkaline and Seawater Conditions by Promoting H2 O Dissociation.

Hydrogen evolution reaction (HER) is a key step for electrochemical energy conversion and storage. Developing well defined nanostructures as noble-metal-free electrocatalysts for HER is promising for the application of hydrogen technology. Herein, it is reported that 3D porous hierarchical CoNiP/Cox P multi-phase heterostructure on Ni foam via an electrodeposition method followed by phosphorization exhibits ultra-highly catalytic activity for HER. The optimized CoNiP/Cox P multi-phase heterostructure achieves an excellent HER performance with an ultralow overpotential of 36 mV at 10 mA cm-2 , superior to commercial Pt/C. Importantly, the multi-phase heterostructure shows exceptional stability as confirmed by the long-term potential cycles (30,000 cycles) and extended electrocatalysis (up to 500 h) in alkaline solution and natural seawater. Experimental characterizations and DFT calculations demonstrate that the strong electronic interaction at the heterointerface of CoNiP/CoP is achieved via the electron transfer from CoNiP to the heterointerface, which directly promotes the dissociation of water at heterointerface and desorption of hydrogen on CoNiP. These findings may provide deep understanding on the HER mechanism of heterostructure electrocatalysts and guidance on the design of earth-abundant, cost-effective electrocatalysts with superior HER activity for practical applications.

Theoretical evidence of the spin-valley coupling and valley polarization in two-dimensional MoSi2X4 (X = N, P, and As).

Very recently, the centimeter-scale MoSi2N4 monolayer was synthesized experimentally and exhibited a semiconducting nature with high mobility (Hong et al., Science, 2020, 369, 670-674). Here, we show that MoSi2N4 and its analogues, MoSi2P4 and MoSi2As4, are potential two-dimensional (2D) materials for valleytronics based on first-principles calculations. We demonstrate that the intrinsic inversion symmetry breaking and strong spin-orbital coupling lead to the remarkable spin-valley coupling in the inequivalent valleys at K and K' points, which result in not only the valley-contrasting transport properties, but also the spin and valley coupled optical selection rules. Moreover, the in-plane strain can tune the bandgaps and spin splitting or even induce an indirect-to-direct bandgap transition for promising application in the strain-tunable valleytronics. We find that the valley polarization can be generated by doping magnetic element. Our findings offer theoretical insight into the exotic physical properties of novel MoSi2N4-family materials beyond transition metal dichalcogenides.

Magnetic and electronic properties of 2D TiX3 (X = F, Cl, Br and I).

Searching for two-dimensional (2D) materials with a high phase-transition temperature and magnetic anisotropy is critical to the development of spintronics. Herein, we investigate the electronic and magnetic properties of 2D TiX3 (X = F, Cl, Br and I) monolayers based on density-functional theory (DFT). We show that the 2D TiX3 monolayers are stable dynamically and thermodynamically as evidenced by phonon and molecular dynamics calculations, respectively, and show their semiconducting nature. We find that the TiBr3 and TiI3 monolayers are ferromagnetic with magnetic anisotropy out of plane, which are intrinsic without the need for external intervention. The magnetic anisotropy energies of the TiBr3 and TiI3 monolayers are 0.8 and 2.5 meV per s.f., respectively. The Curie temperatures of TiBr3 and TiI3 are 75 K and 90 K, respectively. We further show that the interlayer magnetic coupling and magnetic anisotropy energies (MAE) of the bilayer TiI3 can be tuned by the interlayer distance. Additionally, a two-step transition in the magnetic state is observed in the bilayer TiI3 with AB' stacking under applied strain in a vertical direction. It is expected that our design may enrich two-dimensional functional materials, which may find versatile applications.

Valley polarization and ferroelectricity in a two-dimensional GaAsC6 monolayer.

Valley polarization and ferroelectricity are the two basic concepts in electronic device applications. However, the coexistence of these two scenarios in one material has not been reported. Here, using first-principles calculations, we demonstrated that the two-dimensional GaAsC6 monolayer which is a hybrid structure of GaAs and graphene has a pair of inequivalent valleys with opposite Berry curvatures and an intrinsic out-of-plane spontaneous electric polarization. It also has a direct band gap of about 1.937 eV and a high carrier mobility of about 1.80 × 105 cm2 V-1 s-1, which are promising for electronic device applications. The integration of valley polarization and ferroelectricity in a single material offers a promising platform for the design of electronic devices.

Intrinsic multiferroicity in two-dimensional VOCl2 monolayers.

The coexistence of ferroelectricity and magnetism in two-dimensional (2D) multiferroic materials with the thickness of few atomic layers offers a tantalizing potential for high-density multistate data storage but has been rarely verified in experiments. Herein, we propose a realistic 2D multiferroic material, VOCl2 monolayer, which is mechanically strippable from the bulk material. It has a large intrinsic in-plane spontaneous electric polarization of 312 pC m-1 and stable antiferromagnetism with the Néel temperature of 177 K. The off-center displacement of V ions that contributes to the ferroelectricity can be ascribed to the pseudo Jahn-Teller distortion. The energy barrier (0.18 eV) between two ferroelectric states with opposite electronic polarization renders the thermodynamic stability of the ferroelectricity and the switchability of the electric polarizations. The interplay between electric polarization and magnetism would lead to tunable ferroelectricity. Our findings are expected to offer a realistic platform for the study of 2D multiferroic materials as well as their applications in miniaturized memory devices.

Valley-selective circular dichroism and high carrier mobility of graphene-like BC6N.

The two-dimensional (2D) hybrid structures of boron nitride (BN) and graphene with properties superior to the individuals are long desired. In this work, we demonstrate theoretically that this goal can be reached in a new graphene-like borocarbonitride (g-BC6N) whose domain has been synthesized in recent experiments. It has a direct band gap of 1.833 eV and a high carrier mobility comparable to that of black phosphorene. The inversion symmetry breaking in g-BC6N leads to a pair of inequivalent valleys with opposite Berry curvatures in the vicinities of the vertices (K and K') of the Brillouin zone. The coexistence of valley-selective circular dichroism and high carrier mobility in g-BNC6 is beneficial to realize the valley Hall effect. We also propose a tight-binding (TB) model to describe the intrinsic features of this type of lattice, revealing a new class of 2D valleytronic materials.

Silicene and germanene on InSe substrates: structures and tunable electronic properties.

Using first-principles calculations, we show that the recently synthesized two-dimensional (2D) van der Waals layered material indium selenide (InSe) nanosheets can serve as a suitable substrate for silicene and germanene, which form commensurate and stable silicene/InSe (Si/InSe) and germanene/InSe (Ge/InSe) heterolayers (HLs). The buckled honeycomb geometries and Dirac-cone-like band structures of silicene and germanene are well preserved in these HLs. The interaction between silicene (or germanene) and the InSe substrate opens up a band gap of 141 meV (or 149 meV) at the Dirac points, while electron effective masses (EEM) remain as small as 0.059 and 0.067 times the free-electron mass (m0). The band gap and the EEM of the HLs can be further modulated effectively by applying an external electric field or strain. These features are attributed to the built-in electric field due to the interlayer charge transfer of the HLs which breaks the equivalence of the two sublattices of silicene and germanene. Multilayer (ML) InSe substrates have also been considered. We also proposed a parallel plate capacitor model to describe the interaction between silicene (or germanene) and the InSe substrate as well as the electronic band structure modification in response to an external field. This work is expected to offer an ideal substrate material for the growth of silicene and germanene and a promising van der Waals (vdW) layered heterostructure for electronic devices.