Emerging group-11 monochalcogenide MX (M = Cu, Ag, Au; X = S, Se, Te) monolayers: Two-dimensional polarized metals and visible-light-driven photocatalysts

Abstract

Two-dimensional (2D) transition metal chalcogenides have garnered significant research interest owing to their novel properties and potential applications. In this paper, we report a novel class of group-11 chalcogenide MX (M = Cu, Ag, Au; X = S, Se, Te) monolayers based on first-principles calculations, which demonstrate significant application potential as polarized metals and visible-light-driven photo-catalysts. ι-CuS, ι-AgS ι-AgTe, η-CuSe, and η-CuTe were found to be metals, while β-AgSe, α-AuS, α-AuSe, and α-AuTe were observed to be semiconductors with a bandgap of 1.46, 2.14, 1.77, and 1.37 eV respectively. Interestingly, the work functions of the bottom and top sides of ι-CuS, ι-AgS, and ι-AgTe differed owing to the out-of-plane inversion asymmetry of the ι phase. Moreover, these metallic MX monolayers were used to form graphene-based metal-metal heterostructures; accordingly, the work function of graphene could be modulated from 4.35 eV to the range of 3.87–5.04 eV. For semiconducting MX monolayers, a-phase and β-phase monolayers just satisfied the band edge requirement for the H⁺/H₂ and H₂O/O₂ reaction at pH 0/7, respectively. Further investigations on the α-AuSe/α-AuTe heterostructure demonstrated its great application potential as a Z-scheme photocatalyst in visible-light-driven overall water splitting with appropriate band alignment, enhanced optical absorbance and high solar-to-hydrogen energy conversion efficiency (20.47%). Benefitting from these excellent properties, 2D group-11 chalcogenide MX monolayers can be potentially used as polarized metals and visible-light-driven water splitting photocatalysts.