MX Anti-MXenes from Non-van der Waals Bulks for Electrochemical Applications: The Merit of Metallicity and Active Basal Plane

Exploring the potential of MB2 MBene family as promising anodes for Li-ion batteries

In recent years, finding high-performance energy storage materials has become a major challenge for Li-ion batteries. B-based two-dimensional materials have become the focus of attention because of their abundant reserves and non-toxic characteristics. A series of two-dimensional transition metal borides (MBenes) are reported and their electrochemical properties as anode materials for Li-ion batteries are investigated by density functional theory (DFT) calculations. The surface of MB2 possesses medium adsorption strength and diffusion energy barrier for Li atoms, which are conducive to the insertion and extraction of Li-ions during the charge/discharge process of Li-ion batteries. Herein, we explore the potential of MB2 (M = Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Mn, Fe, Co, Ni, Cu and Zn) as the anode material for LIBs. Excitingly, the Li atom can be stably adsorbed on the surface of MB2 (M = Sc, Ti, V, Nb, Mo, W) monolayers, and the theoretical capacity of the MB2 monolayer is high (521.77-1610.20 mA h g-1). The average open circuit voltage range is within 0.10-1.00 V (vs. Li/Li+). The relationship between the p-band center of the B atom and the adsorption energy of Li on the surface of MB2 is also investigated. Furthermore, it is found that the charge transfer of Li atom and metallic center in the most stable position is strongly related to the corresponding value of diffusion energy barrier. These results confirm that MB2 monolayers are promising 2D anode materials for Li-ion batteries, demonstrating the application prospects of B-based 2D materials.

Enriching 2D transition metal borides via MB XMenes (M = Fe, Co, Ir): Strong correlation and magnetism

Recently, two-dimensional (2D) FeSe-like anti-MXenes (or XMenes), composed of late d-block transition metal M and p-block nonmetal X elements, have been both experimentally and theoretically investigated. Here, we select three 2D borides FeB, CoB and IrB for a deeper investigation by including strong correlation effects, as a fertile ground for understanding and applications. Using a combination of Hubbard corrected first-principles calculations and Monte Carlo simulations, FeB and CoB are found to be ferro- and anti-ferro magnetic, contrasting with the non-magnetic nature of IrB. The metallic FeB XMene monolayer, superior to most of the MXenes or MBenes, exhibits robust ferromagnetism, driven by intertwined direct-exchange and super-exchange interactions between adjacent Fe atoms. The predicted Curie temperature (TC) of the FeB monolayer via the Heisenberg model reaches an impressive 425 K, with the easy-axis oriented out-of-plane and high magnetic anisotropic energy (MAE). The asymmetry in the spin-resolved transmission spectrum induces a thermal spin current, providing an opportunity for spin filtration. This novel 2D FeB material is expected to hold great promise as an information storage medium and find applications in emerging spintronic devices.

Band alignment type I, II transformations in Hf2CO2/MoS2 heterostructures using biaxial strain, external electric field, and interlayer coupling: a first principal investigation

The transition to neuromorphic devices is relevant to the development of materials capable of providing electronic switching in response to external stimuli. In the present work, the Hf2CO2/MoS2 heterostructure under biaxial strain, interlayer coupling, and an electric field was investigated by first-principles calculations based on density functional theory. We have shown that the influence of lateral deformation as well as the perpendicular external electric field is more significant compared to the influence of external vertical pressure on changes in the heterojunction type of heterostructure. The lateral stretching leads to a type-I and lateral compression results in a type-II heterojunction, and an external electric field also has an effect on heterojunction type. The combination of these impacts can tune the Hf2CO2/MoS2 heterostructure. The current work suggests a compelling way to make type-I and type-II heterostructure types consisting of Hf2CO2 and MoS2 monolayers for new nanodevices in fields like photonics, electronics, optoelectronic and neuromorphic applications.

First principles insights into stability of defected MXenes in water

Two-dimensional transition metal carbides and nitrides, known as MXenes, have demonstrated remarkable performance in electrochemical energy storage and various other applications. Despite their potential, MXenes exhibit instability in aqueous solutions, and the role of defects in their aqueous stability is unclear. In this study, we report on the interfacial chemistry between water and defected Ti3C2O2 MXene at room temperature using first principles molecular dynamics simulations. We investigate how defects such as O vacancy, Ti vacancy, F terminal groups, and Ti-O vacancy pair influence the chemical interaction of water molecules with the basal plane of Ti3C2O2. Our results show that water molecules can repair the surface O vacancies, by dissociating to hydroxide and hydronium. On the other hand, F terminal groups cannot effectively block water chemisorption on the surface Ti, while Ti vacancies behave as a spectator species on the surface with respect to interaction with water. Ti3C2O2 with a Ti-O vacancy pair is found to behave like Ti3C2O2 with an O vacancy where a water molecule dissociates and refills the vacancy. These findings enrich our understanding of water interaction with defects on the MXene surfaces.

Versatile transition metal monolayers with catalytic and superconducting properties: a computational study

Due to their abundant valence electrons and unique electronic properties, transition metals have garnered great interest in the search for novel materials displaying various properties, including superconductivity, catalysis and so on. XRu₂ (X: V, Mn, Fe, etc.) compounds, which are isostructural to AlB₂, were used as examples for which we performed extensive simulations to screen for superconductivity and possible potential catalytic activity. On this basis, we found that VRu₂ could achieve a superconducting critical temperature (T_c) of about 13 K. Meanwhile, our simulations showed the lowest adsorption free energy of atomic hydrogen (ΔG_H) on the (0 0 1) surface of VRu₂ to be about 2 meV, indicating its almost zero free energy of hydrogen adsorption with excellent catalytic performance. In addition, the results suggested potential superconducting and catalytic properties of VXRu (X = Os, Fe). Our current results have shed light on possible applications of Ru-based AlB₂-type intermetallic compounds and have presented a new strategy for further designing superconductors and catalysts based on transition metals.

Halogen-Doped Chevrel Phase Janus Monolayers for Photocatalytic Water Splitting

Chevrel non-van der Waals crystals are promising candidates for the fabrication of novel 2D materials due to their versatile crystal structure formed by covalently bonded (Mo₆X₈) clusters (X-chalcogen atom). Here, we present a comprehensive theoretical study of the stability and properties of Mo-based Janus 2D structures with Chevrel structures consisting of chalcogen and halogen atoms via density functional theory calculations. Based on the analysis performed, we determined that the S₂Mo₃I₂ monolayer is the most promising structure for overall photocatalytic water-splitting application due to its appropriate band alignment and its ability to absorb visible light. The modulated Raman spectra for the representative structures can serve as a blueprint for future experimental verification of the proposed structures.

Element-Doped Mxenes: Mechanism, Synthesis, and Applications

Heteroatom doping can endow MXenes with various new or improved electromagnetic, physicochemical, optical, and structural properties. This greatly extends the arsenal of MXenes materials and their potential for a spectrum of applications. This article comprehensively and critically discusses the syntheses, properties, and emerging applications of the growing family of heteroatom-doped MXenes materials. First, the doping strategies, synthesis methods, and theoretical simulations of high-performance MXenes materials are summarized. In order to achieve high-performance MXenes materials, the mechanism of atomic element doping from three aspects of lattice optimization, functional substitution, and interface modification is analyzed and summarized, aiming to provide clues for developing new and controllable synthetic routes. The mechanisms underlying their advantageous uses for energy storage, catalysis, sensors, environmental purification and biomedicine are highlighted. Finally, future opportunities and challenges for the study and application of multifunctional high-performance MXenes are presented. This work could open up new prospects for the development of high-performance MXenes.

Janus MoPC Monolayer with Superior Electrocatalytic Performance for the Hydrogen Evolution Reaction

Designing the earth's abundant and high-performance electrocatalysts, which possess high stability, excellent electrical conductivity, inherent active sites, and catalytic activity identical with Pt, is challenging but crucial for the hydrogen evolution reaction (HER). By first-principles structure search simulations, we identify a new two-dimensional (2D) MoPC material with the Janus structure as a promising catalyst. This novel 2D monolayer has superior stability and metallic conductivity. Especially, it exhibits a remarkable HER catalytic activity, where all of the constituent atoms, including Mo, P, and C, can uniformly act as active sites in view of the near-zero ΔG_H* value. Its active site density counts up to 1.46 × 10¹⁵ site/cm², larger than that of many reported materials and even comparable to Pt. The excellent HER catalytic activity can also be maintained at a very high H coverage with or without external strain. The MoPC monolayer can produce H₂ spontaneously through the favorable Volmer-Heyrovsky pathway. The detailed catalytic mechanism behind the high HER activity has been also analyzed. Our work provides a feasible action for the experimental synthesis of excellent HER catalysts.

A bright future for engineering piezoelectric 2D crystals

The piezoelectric effect, mechanical-to-electrical and electrical-to-mechanical energy conversion, is highly beneficial for functional and responsive electronic devices. To fully exploit this property, miniaturization of piezoelectric materials is the subject of intense research. Indeed, select atomically thin 2D materials strongly exhibit the piezoelectric effect. The family of 2D crystals consists of over 7000 chemically distinct members that can be further manipulated in terms of strain, functionalization, elemental substitution (i.e. Janus 2D crystals), and defect engineering to induce a piezoelectric response. Additionally, most 2D crystals can stack with other similar or dissimilar 2D crystals to form a much greater number of complex 2D heterostructures whose properties are quite different to those of the individual constituents. The unprecedented flexibility in tailoring 2D crystal properties, coupled with their minimal thickness, make these emerging highly attractive for advanced piezoelectric applications that include pressure sensing, piezocatalysis, piezotronics, and energy harvesting. This review summarizes literature on piezoelectricity, particularly out-of-plane piezoelectricity, in the vast family of 2D materials as well as their heterostructures. It also describes methods to induce, enhance, and control the piezoelectric properties. The volume of data and role of machine learning in predicting piezoelectricity is discussed in detail, and a prospective outlook on the 2D piezoelectric field is provided.

Janus Mo2P3 Monolayer as an Electrocatalyst for Hydrogen Evolution

The rational design of low-cost electrocatalysts with the desired performance is the core of the large-scale hydrogen production from water. Two-dimensional materials with high specific surface area and excellent electron properties are ideal candidates for electrocatalytic water splitting. Herein, we identify a hitherto unknown Mo₂P₃ monolayer with a Janus structure (i.e., out-of-plane asymmetry) through first-principle structure search calculations. Its inherent metallicity ensures good electrical conductivity. Notably, its catalytic activity is comparable to that of Pt and the density of active sites is up to 2.65 × 10¹⁵ site/cm² owing to the Mo → P charge transfer enhancing the catalytic activity of P atoms and asymmetric structure exposing more active sites to the surface. The Mo₂P₃ monolayer can spontaneously produce hydrogen through the Volmer-Heyrovsky pathway. These excellent performances can be well maintained under strain. The coexistence of covalent and ionic bonds results in Mo₂P₃ having high stability. All these excellent properties make the Mo₂P₃ monolayer a promising candidate for electrocatalytic water splitting.

Multilevel Theoretical Screening of Novel Two-Dimensional MA2Z4 Family for Hydrogen Evolution

The synthesis of 2D MoSi₂N₄ marked the birth of a new family of 2D MA₂Z₄ materials, whose potential applications are expected to sweep the field of nanoscale devices and catalysis in the future. In this work, we propose a multilevel screening workflow to systematically explore the mechanic stabilities, electronic properties, and hydrogen evolution performances of the 2D MA₂Z₄ family, among which seven stable, metallic, and highly active 2D MA₂Z₄ monolayers (2H-α-VGe₂N₄, 2H-α-NbGe₂N₄, 2H-α-TaGe₂N₄, 2H-α-NbSi₂N₄, 2H-β-VGe₂N₄, 2H-β-NbGe₂N₄, and 2H-β-TiGe₂P₄) are predicted as promising hydrogen evolution reaction (HER) catalysts with near-zero hydrogen adsorption free energy (ΔG_H). The lowest unoccupied state energy (E_LUS) of the MA₂Z₄ basal plane is identified as a concise descriptor to influence the electron filling and eventually determine its ΔG_H. The criteria of -6.0 < E_LUS < -5.6 eV is confirmed as the HER active window to explore novel HER catalysts. In particular, group-VI-terminated MA₂Z₄ basal planes have a higher E_LUS (> -5.6 eV), which leads to weak H adsorption and poor HER activities accordingly.