A Janus MoSSe monolayer, synthesized recently though the chemical vapor deposition method [A. Y. Lu, H. Zhu, J. Xiao, C. P. Chuu, Y. Han, M. H. Chiu, C. C. Cheng, C. W. Yang, K. H. Wei Y. Yang, Y. Wang, D. Sokaras, D. Nordlund, P. Yang, D. A. Muller, M. Y. Chou, X. Zhang and L. J. Li, Nat. Nanotechnol., 2017, 12, 744–749], has drawn considerable attention as a new two-dimensional (2D) material owing to its fascinating electronic and optical properties. In this study, based on first-principles calculations, we systematically explore for the first time the performance of Janus MoSSe monolayers as a channel material contacting with graphene to form van der Waals (vdW) heterostructures. Our calculations show that the intrinsic electronic properties of both the graphene and MoSSe monolayer are preserved well in our proposed two graphene/MoSSe heterostructures (i.e. G/SMoSe and G/SeMoS heterostructures), and n-type Schottky contacts with a small Schottky barrier height (SBH) are formed at their respective interfaces. An analytical model is presented for the barrier heights. Moreover, the n-type Schottky barrier at the G/SMoSe heterostructure interface can be reduced by increasing the interlayer distance and can even be changed to an Ohmic contact by applying a negative electric field. More interestingly, varying the interlayer distance or applying an external electric field can effectively modulate the Schottky barrier and the Schottky contact (n-type and p-type) of the G/SeMoS heterostructure interface. These theoretical findings not only provide insights into the fundamental properties of the graphene/MoSSe interfaces but also open the possibility of designing high-performance field-effect transistors (FETs) based on the graphene/MoSSe heterostructures.

With crystal structure prediction using a genetic algorithm method, we propose a novel two-dimensional (2D) carbon allotrope with relatively low energy, which is composed of tetragonal and heptagonal carbon rings, named thgraphene. The robust thermodynamic, dynamic, and mechanic stability of thgraphene signify the high possibility for experimental synthesis. As a metallic 2D carbon material, thgraphene is proved to be an efficient bifunctional electrocatalyst for hydrogen and oxygen evolution reactions (HER and OER), with low overpotentials of 0.14 V and 0.46 V, respectively. Moreover, the HER/OER catalytic activities can be further tuned close to the ideal values by applying a certain amount of strain. Also, we find that thgraphene is a promising anode material for potassium-ion batteries due to its excellent ductility (Poisson’s ratio ∼ 0.39), ultra-low diffusion barrier (0.06 eV), high theoretical specific capacity (744 mA h g⁻¹), and moderate average open-circuit voltage (0.30 V). Our findings highlight a new 2D carbon allotrope as a bifunctional HER/OER electrocatalyst and potassium-ion anode material.

Computational studies of the GaSe/β-SnS heterostructure indicate that it is a promising type-II photocatalyst for solar-driven water splitting.

Mimicking the natural photosynthesis in green plants, artificial Z-scheme photocatalysis enables more efficient utilization of solar energy for photocatalytic water splitting. Most currently designed g-C ₃ N ₄ -based Z-scheme heterojunctions are usually based on metal-containing semiconductor photocatalysts, thus exploiting metal-free photocatalysts for Z-scheme water splitting is of huge interest. Herein, we propose two metal-free C ₃ N/g-C ₃ N ₄ heterojunctions with the C ₃ N monolayer covering g-C ₃ N ₄ sheet (monolayer or bilayer) and systematically explore their electronic structures, charge distributions and photocatalytic properties by performing extensive hybrid density functional calculations. We clearly reveal that the relative strong built-in electric fields around their respective interface regions, caused by the charge transfer from C ₃ N monolayer to g-C ₃ N ₄ monolayer or bilayer, result in the bands bending, renders the transfer of photogenerated carriers in these two heterojunctions following the Z-scheme instead of the type-II pathway. Moreover, the photogenerated electrons and holes in these two C ₃ N/g-C ₃ N ₄ heterojunctions can not only be efficiently separated, but also have strong redox abilities for water oxidation and reduction. Compared with the isolated g-C ₃ N ₄ sheets, the light absorption in visible to near-infrared region are significantly enhanced in these proposed heterojunctions. These theoretical findings suggest that these proposed metal-free C ₃ N/g-C ₃ N ₄ heterojunctions are promising direct Z-scheme photocatalysts for solar water splitting.