A promising direct Z-scheme GaAs/HfS2 van der Waals heterostructure photocatalyst for overall water-splitting using infrared, visible and ultraviolet solar spectrum

High solar energy utilization is a key issue for increasing solar-to‑hydrogen (STH) energy conversion efficiency in photocatalytic overall water-splitting. However, the infrared light of the solar light spectrum, which constitutes almost half of the solar energy, has not been used in non-polar material. Here, the GaAs/HfS₂ van der Waals heterostructure has been constructed along the vertical direction and its structural stability, electronic property, photocatalysis water-splitting mechanism, optical absorption, STH energy conversion efficiency, and Gibbs free energy changes in redox reactions have been investigated through first-principles calculation. The thermodynamic, thermal and dynamical stabilities of the GaAs/HfS₂ heterostructure with H1 configuration are verified from binding energy, ab-initio molecular dynamics simulation and phonon spectra, respectively. The smaller work function 5.573 eV of the monolayer GaAs than 6.689 eV of the monolayer HfS₂ makes 0.149 |e| electron transfer and thus a built-in electric field from GaAs side to HfS₂ side. The smaller direct bandgap of 0.44 eV of the GaAs/HfS₂ heterostructure than 1.96 eV and 1.94 eV of the GaAs and HfS₂ monolayers at central Γ point of the Brillouin zone, resulting in a high optical absorption in infrared, visible and ultraviolet light regions. The type-II band arrangement, especially the direct Z-scheme photocatalysis mechanism with infrared optical absorption and high STH efficiency 38.2 %, conform the GaAs/HfS₂ heterostructure is a promising photocatalyst for overall water-splitting.