Bonding-restricted structure search for novel 2D materials with dispersed C2 dimers

First-principles Density Functional Theory Elucidation of the Hydrogen Evolution Reaction on TM-promoted TiC2 (TM=Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au)

Single-atom-catalyst-based systems have been attractive by virtue of their desirable catalytic performance. Herein, the possibility of the 15 transition-metal (TM)-promoted (TM=Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, Os, Ir, Pt, Au, and Hg) and their hydrogen evolution reaction (HER) performance were investigated on two-dimensional titanium carbides (TiC₂ ). It is found that the adsorption strength of TMs on TiC₂ is stronger than that of TMs on γ-graphyne and weaker than that of TMs on Ti₃ C₂ . Among the fifteen investigated catalysts, Ru-TiC₂ , Ag-TiC₂ , Ir-TiC₂ , Au-TiC₂ , and Fe-TiC₂ exhibits overpotential of -0.18, -0.15, -0.18, -0.17, and -0.04 V, respectively. In addition, the Volmer-Tafel step was preferred to the Volmer-Heyrovsky step on Fe-TiC₂ . This work suggests that Fe-TiC₂ is possibly a superior HER electrocatalyst.

A DFT prediction of two-dimensional MB3 (M = V, Nb, and Ta) monolayers as excellent anode materials for lithium-ion batteries

Transition metal borides (MBenes) have recently drawn great attention due to their excellent electrochemical performance as anode materials for lithium-ion batteries (LIBs). Using the structural search code and first-principles calculations, we identify a group of the MB3 monolayers (M = V, Nb and Ta) consisting of multiple MB4 units interpenetrating with each other. The MB3 monolayers with non-chemically active surfaces are stable and have metal-like conduction. As the anode materials for Li-ion storage, the low diffusion barrier, high theoretical capacity, and suitable average open circuit voltage indicate that the MB3 monolayers have excellent electrochemical performance, due to the B3 chain exposed on the surface improving the Li atoms' direct adsorption. In addition, the adsorbed Li-ions are in an ordered hierarchical arrangement and the substrate structure remains intact at room temperature, which ensures excellent cycling performance. This work provides a novel idea for designing high-performance anode materials for LIBs.

Anisotropic and High-Mobility C3S Monolayer as a Photocatalyst for Water Splitting

Taking into account the high conductivity and stability of carbon materials, such as graphene, and the strong polar covalent bonding character of main-group compounds, we explore potential 2D materials in the C-S binary system through first-principles structure search calculations. Herein, a hitherto unknown semiconducting C₃S monolayer is identified, consisting of well-known n-biphenyl and S atom linked benzenes, exhibiting an obvious direction-dependent atomic arrangement. Thus, it exhibits anisotropic mechanical properties and carrier mobility. Its electron mobility reaches 2.14 × 10⁴ cm² V^-1 s^-1 in the b direction, along which n-biphenyl units are arranged, and is much higher than that in the well-used MoS₂ monolayer and black phosphorus. Meanwhile, the C₃S monolayer has high optical absorption coefficients (10⁵ cm^-1), high thermal and dynamical stabilities, and a moderate ability to split water. All these desirable properties make the C₃S monolayer a promising candidate for applications in novel optoelectronic devices.

First-Principles Observation of Bonded 2D B4C3 Bilayers

Two-dimensional (2D) B-C compounds possess rich allotropic structures with many applications. Obtaining new 2D B₄C₃ structures is highly desirable due to the novel applications of three-dimensional (3D) B₄C₃ in protections. In this work, we proposed a new family of 2D B₄C₃ from the first-principles calculations. Distinct from previous observations, this family of 2D B₄C₃ consists of bonded 2D B₄C₃ bilayers. Six different types of bilayers with distinct bonded structures are found. The phonon spectrum calculations and ab initio molecular dynamics simulations at room temperature demonstrate their dynamic and thermal stabilities. Low formation energies suggest the high possibility of realizing such structures in experiments. Rich electronic structures are found, and the predicted Young's moduli are even higher than those of the previous ones. It is revealed that the unique electronic and mechanical properties are rooted in the bonding structures, indicating the prompting applications of this family of 2D B₄C₃ materials in photovoltaics, nanoelectronics, and nanomechanics.

Symmetry-protected metallic and topological phases in penta-materials

We analyze the symmetry and topological features of a family of materials closely related to penta-graphene, derived from it by adsorption or substitution of different atoms. Our description is based on a novel approach, called topological quantum chemistry, that allows to characterize the topology of the electronic bands, based on the mapping between real and reciprocal space. In particular, by adsorption of alkaline (Li or Na) atoms we obtain a nodal line metal at room temperature, with a continuum of Dirac points around the perimeter of the Brillouin zone. This behavior is also observed in some substitutional derivatives of penta-graphene, such as penta-PC₂. Breaking of time-reversal symmetry can be achieved by the use of magnetic atoms; we study penta-MnC₂, which also presents spin-orbit coupling and reveals a Chern insulator phase. We find that for this family of materials, symmetry is the source of protection for metallic and nontrivial topological phases that can be associated to the presence of fractional band filling, spin-orbit coupling and time-reversal symmetry breaking.

Origins of Dirac cone formation in AB3 and A3B (A, B = C, Si, and Ge) binary monolayers

Compared to the pure two-dimensional (2D) graphene and silicene, the binary 2D system silagraphenes, consisting of both C and Si atoms, possess more diverse electronic structures depending on their various chemical stoichiometry and arrangement pattern of binary components. By performing calculations with both density functional theory and a Tight-binding model, we elucidated the formation of Dirac cone (DC) band structures in SiC₃ and Si₃C as well as their analogous binary monolayers including SiGe₃, Si₃Ge, GeC₃, and Ge₃C. A “ring coupling” mechanism, referring to the couplings among the six ring atoms, was proposed to explain the origin of DCs in AB₃ and A₃B binary systems, based on which we discussed the methods tuning the SiC₃ systems into self-doped systems. The first-principles quantum transport calculations by non-equilibrium Green’s function method combined with density functional theory showed that the electron conductance of SiC₃ and Si₃C lie between those of graphene and silicene, proportional to the carbon concentrations. Understanding the DC formation mechanism and electronic properties sheds light onto the design principles for novel Fermi Dirac systems used in nanoelectronic devices.

Penta-graphene: A Promising Anode Material as the Li/Na-Ion Battery with Both Extremely High Theoretical Capacity and Fast Charge/Discharge Rate

Recently, a new two-dimensional (2D) carbon allotrope named penta-graphene was theoretically proposed ( Zhang , S. ; et al. Proc. Natl. Acad. Sci. U.S.A. 2015 , 112 , 2372 ) and has been predicted to be the promising candidate for broad applications due to its intriguing properties. In this work, by using first-principles simulation, we have further extended the potential application of penta-graphene as the anode material for a Li/Na-ion battery. Our results show that the theoretical capacity of Li/Na ions on penta-graphene reaches up to 1489 mAh·g^-1, which is much higher than that of most of the previously reported 2D anode materials. Meanwhile, the calculated low open-circuit voltages (from 0.24 to 0.60 V), in combination with the low diffusion barriers (≤0.33 eV) and the high electronic conductivity during the whole Li/Na ions intercalation processes, further show the advantages of penta-graphene as the anode material. Particularly, molecular dynamics simulation (300 K) reveals that Li ion could freely diffuse on the surface of penta-graphene, and thus the ultrafast Li ion diffusivity is expected. Superior performance of penta-graphene is further confirmed by comparing with the other 2D anode materials. The light weight and unique atomic arrangement (with isotropic furrow paths on the surface) of penta-graphene are found to be mainly responsible for the high Li/Na ions storage capacity and fast diffusivity. In this regard, except penta-graphene, many other recently proposed 2D metal-free materials with pentagonal Cairo-tiled structures may be the potential candidates as the Li/Na-ion battery anodes.