Insight into the photoelectric characteristics and photocatalytic water splitting performances of van der Waals heterostructures Cs ₂ PbI ₄ /MX ₂ (M = Mo, W; X = Se, S)

The photoelectric characteristics and photocatalytic water splitting performances of van der Waals heterostructures Cs₂PbI₄/MX₂ (M = Mo, W; X = Se, S) were investigated using first principles calculations. Four stable heterostructures were obtained with indirect narrow gaps. Among them, Cs₂PbI₄/MoSe₂, Cs₂PbI₄/MoS₂ and Cs₂PbI₄/WSe₂ belong to type-II heterostructures, and Cs₂PbI₄/WS₂ is a type-I heterostructure. High optical absorption efficiency and high carrier mobility imply that type-I heterostructure Cs₂PbI₄/WS₂ has broad potential application prospects in light-emitting devices. Moreover, the optical absorption efficiency up to 10^-5 cm^-1, the spatial separation interval of photogenerated electron-hole (∼3.30), and type-II band edge alignment mean that heterostructures can readily realize electron-hole separation and transfer, reduce the recombination probability of electrons and holes, and thus enable solar energy conversion and highly efficient photocatalytic water splitting. In particular, the Cs₂PbI₄/MoSe₂ heterostructure can trigger the HER and OER spontaneously at pH = 0 under an equilibrium potential of 1.23 V with a limiting reaction barrier of 1.16 eV for the OER, which is close to 1.128 eV in the WSe₂/MoSe₂ heterojunction reported recently, implying that it may be an excellent photocatalyst. In addition, the limiting reaction barrier of the Cs₂PbI₄/WSe₂ heterostructure is 1.27 V, the low overpotential for the OER is 0.31 V, and the high power conversion efficiency indicates that it may have potential applications in both solar cells and photocatalytic water splitting.