Identifying suitable photocatalysts for photocatalytic water splitting to produce hydrogen fuel via sunlight is an arduous task by the traditional trial-and-error method. Thanks to the progress of density functional theory, one can nowadays accelerate the process of finding candidate photocatalysts. In this work, by ab initio calculations, we investigated 48 two-dimensional (2D) transition metal carbides also referred to as MXenes to understand their photocatalytic properties. Our results highlight 2D Zr₂CO₂ and Hf₂CO₂ as the candidate single photocatalysts for possible high efficiency photocatalytic water splitting. A significant property of 2D Zr₂CO₂ and Hf₂CO₂ is that they exhibit unexpectedly high and directionally anisotropic carrier mobility, which may effectively facilitate the migration and separation of photogenerated electron–hole pairs. Meanwhile, these two MXenes also exhibit very good optical absorption performance in the wavelength ranging approximately from 300 to 500 nm. The stability of 2D Zr₂CO₂ and Hf₂CO₂ in liquid water is expected to be good based on ab initio molecular dynamics simulations. Finally, the adsorption and decomposition of water molecules on the 2D Zr₂CO₂ surface and the subsequent formation process of hydrogen were studied, which contributes to the unravelling of the micro-mechanism of photocatalytic hydrogen production on MXenes. Our findings will open a new way to facilitate the discovery and application of MXenes for photocatalytic water splitting.

Exploring new two-dimensional (2D) crystals attracts great interest in the materials community due to their potential intriguing properties. Here, we report a new family of 2D transition metal borides (labeled as MBenes) that can be produced by selectively etching the A layer from a family of layered transition metal borides (MAB phases). The emerged MBenes are demonstrated to possess great stability with isotropic and ultrahigh Young's modulus. Meanwhile, our results show that 2D Mo₂B₂ and Fe₂B₂ MBenes are metallic with excellent electronic conductivity, which are highly desirable for applications in Li-ion batteries (LIB) and electrocatalysis. Furthermore, 2D Mo₂B₂ and Fe₂B₂ are confirmed to have an omnidirectional small diffusion energy barrier and high storage capacity for Li atoms, which highlight MBenes as appealing electrode materials for LIBs. Moreover, 2D Fe₂B₂ MBene also exhibits superior catalytic activity for the hydrogen evolution reaction (HER) with hydrogen adsorption Gibbs free energy close to the optimal value (0 eV), indicating its promising application as an electrocatalyst for hydrogen evolution. Considering the large number of possible MAB phases, more MBenes with attractive applications are anticipated theoretically and/or experimentally in the near future.

Two-dimensional (2D) transition metal carbides/nitrides M _n+1 X _n labeled as MXenes are attracting increasing interest due to promising applications as Li-ion battery anodes and hybrid electro-chemical capacitors.

Development of novel van der Waals (vdW) heterostructures from various two-dimensional (2D) materials shows unprecedented possibilities by combining the advantageous properties of their building layers. In particular, transforming the vdW heterostructures from type-I to type-II is of great interest and importance to achieve efficient charge separation in photocatalytic, photovoltaic, and optoelectronic devices. In this work, by means of ab initio calculations, we have systematically investigated the electronic structures, optical properties, and mechanical properties of MXene/Blue Phosphorene (BlueP) vdW heterostructures under various deformations. We highlight that, under strain, the type-I heterostructures can be transformed to type-II with their conduction band minimum (CBM) and valence band maximum (VBM) separated in different layers. Interestingly, the locations of the CBM or VBM in MXene/BlueP vdW heterostructures can also be reversed by compressive or tensile strain between the building layers, which indicates that either layer can be utilized as an electron donor or acceptor by varying its deformation conditions. Meanwhile, this compressive (tensile) strain can also induce a red (blue) shift in the optical absorption spectra of MXene/BlueP vdW heterostructures. Finally, our results on the mechanical flexibility and deformation mechanism of MXene/BlueP vdW heterostructures suggest their great long-term stability as well as promising applications in flexible devices. We believe that our findings will open a new way for the modulation and development of vdW heterostructures in flexible optical/electronic devices.

Mo _x C _y monolayers and their potential application as anodes for LIBs/SIBs.