Stable photocatalysts with excellent optical adsorption and low reaction barrier are the key for the water splitting. Here, we find that a two-dimensional Janus WSSe monolayer possesses the compelling photocatalytic properties from density functional theory simulations, which can be well modulated with strain deformation. Comprehensive investigations indicate that the Janus material not only exhibits strong optical absorbance in the visible spectrum, suitable band edge potentials, high carrier separation, and transfer efficiency but also has adequate driving forces of photoexcited carrier for water redox reaction and good resistance against photoinduced corrosion. Janus WSSe is therefore predicted to be a promising photocatalyst for water splitting. Moreover, we also find that tensile strains could further improve the photocatalytic performance for water splitting by effectively increasing the energy conversion efficiency and reducing the exciton binding energy. Our results not only predict a photocatalyst, which can utilize the visible light for overall water splitting, but also propose an effective path to extend the absorption spectra and raise the photocatalytic efficiency.

Janus two-dimensional (2D) materials, referring to the layers with different surfaces, have attracted intensive research interest due to the unique properties induced by symmetry breaking, and promising applications in energy conversion. Based on the successful experimental synthesis of Janus transition metal dichalcogenides (TMDC), here we present a review on their potential application in photocatalytic overall water splitting, from the perspectives of the latest theoretical and experimental progress. Four aspects which are related to photocatalytic reaction, including the adsorption of water molecules, utilization of sunlight, charge separation and transport, and surface chemical reactions have been discussed, and it is concluded that the Janus structures have better performances than symmetric TMDCs. At the end of this review, we raise further challenges and possible future research directions for Janus 2D materials as water-splitting photocatalysts.

Two-dimensional Janus van der Waals (vdW) heterojunctions, referring to the junction containing at least one Janus material, are found to exhibit tuneable electronic structures, wide light adsorption spectra, controllable contact resistance, and sufficient redox potential due to the intrinsic polarization and unique interlayer coupling. These novel structures and properties are promising for the potential applications in electronics and energy conversion devices. To provide a comprehensive picture about the research progress and guide the following investigations, here we summarize their fundamental properties of different types of two-dimensional Janus vdW heterostructures including electronic structure, interface contact and optical properties, and discuss the potential applications in electronics and energy conversion devices. The further challenges and possible research directions of the novel heterojunctions are discussed at the end of this review.

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