New honeycomb-like M-based (M = C, Si, Ge and Sn) monochalcogenides polymorphs: An extended family as isoelectronic photocatalysts of Group-VA for water splitting

Abstract

Employing density functional theory (DFT), designing isoelectronic counterparts of phosphorene, the lattice-structure-dependent geometric, energetic, stable, electronic and photocatalytic performances of phosphorene-like group-IV monochalcogenides are investigated systematically. For each monolayer, three possible atomic motifs were considered, namely, the δ-phosphorene-like, ε-phosphorene-like and γ-phosphorene-like structures, in which we choose the binary monolayers constituted by same numbers of group IV and group VI atoms. The binding energy, phonon and molecular dynamics simulations confirm that most of those two-dimensional (2D) group IV-VI systems are energetically, dynamically and thermodynamically stable synchronously. In electronics, most of them present semiconducting behaviors, among which ten monolayers are direct bandgap semiconductors. Interestingly, we note that many of those indirect semiconducting systems possess rather small difference between their direct and indirect bandgaps. Hence, there is great feasibility of making those indirect monolayers experience an indirect-to-direct bandgap transition by suitable modulations. In fact, under fairly small uniaxial load (⩽0.05), our results predict that as many as seven indirect semiconductors experience an indirect-to-direct bandgap transition, which greatly enrich the members of the 2D direct bandgap family. Moreover, the band-edge alignments of all 2D semiconductors reveal several sheets can be served as an intrinsic photocatalyst for application in the photocatalytic water splitting. Particularly, for δ-GeO, δ-SnO and δ-GeS monolayers, high visible light absorption, desirable carriers mobility, strong stability in aqueous and acidic environments make them efficient photocatalysts.