Identifying suitable photocatalysts for photocatalytic water splitting to produce hydrogen fuel via sunlight is an arduous task by the traditional try-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 on 48 two-dimensional (2D) transition metal carbides also referred to as MXenes to understand their photocatalytic property. Our results highlight 2D Zr 2 CO 2 and Hf 2 CO 2 as the candidate single photocatalysts for possible high efficient photocatalytic water splitting. The significantly property of 2D Zr 2 CO 2 and Hf 2 CO 2 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 Zr 2 CO 2 and Hf 2 CO 2 in liquid water is expected to be good based on ab initio molecular dynamics simulations. Finally, the absorption and decomposition of water molecule on the 2D Zr 2 CO 2 surface and the subsequently followed formation process of hydrogen were studied, which contributes to the unraveling of the micro-mechanism of photocatalytic hydrogen production on MXene.Our findings will open a new way to facilitate the discovery and application of MXenes for photocatalytic water splitting. Fig. 3. The electronic band edge positions with respect to the water reduction and oxidation potential levels.Fig. 4. Imaginary part of the dielectric function of 2D Zr 2 CO 2 and Hf 2 CO 2 .

This review highlights recent advances of MXenes and their composites in the environment-related applications including catalysis, water purification and sensors.

Phosphorene has been attracted intense interest due to its unexpected high carrier mobility and distinguished anisotropic optoelectronic and electronic properties. In this work, we unraveled strain engineered phosphorene as a photocatalyst in the application of water splitting hydrogen production based on density functional theory calculations. Lattice dynamic calculations demonstrated the stability for such kind of artificial materials under different strains. The phosphorene lattice is unstable under compression strains and could be crashed. Whereas, phosphorene lattice shows very good stability under tensile strains. Further guarantee of the stability of phosphorene in liquid water is studied by ab initio molecular dynamics simulations. Tunable band gap from 1.54 eV at ambient condition to 1.82 eV under tensile strains for phosphorene is evaluated using parameter-free hybrid functional calculations.Appropriate band gaps and band edge alignments at certain pH demonstrate the potential application of phosphorene as a sufficiently efficient photocatalyst for visible light water splitting. We found that the strained phosphorene exhibits significantly improved photocatalytic properties under visible-light irradiation by 2 calculating optical absorption spectra. Negative splitting energy of absorbed H 2 O indicates the water splitting on phosphorene is energy favorable both without and with strains.

Graphene has intrigued the science community by many unique properties not found in conventional materials. In particular, it is the strongest two-dimensional material ever measured, being able to sustain reversible tensile elastic strain larger than 20%, which yields an interesting possibility to tune the properties of graphene by strain and thus opens a new field called "straintronics". In this article, the current progress in the strain engineering of graphene is reviewed. We first summarize the strain effects on the electronic structure and Raman spectra of graphene. We then highlight the electron-phonon coupling greatly enhanced by the biaxial strain and the strong pseudomagnetic field induced by the non-uniform strain with specific distribution. Finally, the potential application of strain-engineering in the self-assembly of foreign atoms on the graphene surface is also discussed. Given the short history of graphene straintronics research, the current progress has been notable, and many further advances in this field are expected.

Graphene-like two-dimensional materials have garnered tremendous interest as emerging device materials for nanoelectronics due to their remarkable properties. However, their applications in spintronics have been limited by the lack of intrinsic magnetism. Here, using hybrid density functional theory, we predict ferromagnetic behavior in a graphene-like two-dimensional Cr2C crystal that belongs to the MXenes family. The ferromagnetism, arising from the itinerant Cr d electrons, introduces intrinsic half-metallicity in Cr2C MXene, with the half-metallic gap as large as 2.85 eV. We also demonstrate a ferromagnetic-antiferromagnetic transition accompanied by a metal to insulator transition in Cr2C, caused by surface functionalization with F, OH, H, or Cl groups. Moreover, the energy gap of the antiferromagnetic insulating state is controllable by changing the type of functional groups. We further point out that the localization of Cr d electrons induced by the surface functionalization is responsible for the ferromagnetic-antiferromagnetic and metal to insulator transitions. Our results highlight a new promising material with tunable magnetic and electronic properties toward nanoscale spintronics and electronics applications.