δ-SnS: An Emerging Bidirectional Auxetic Direct Semiconductor with Desirable Carrier Mobility and High-Performance Catalytic Behavior toward the Water-Splitting Reaction.

We propose a novel two-dimensional SnS allotrope (monolayer δ-SnS) based on an auxetic δ-phosphorene configuration using first-principles calculations. This monolayer appears to have outstanding stability as revealed by its energetic, kinetic, thermodynamic, and mechanic calculations, and it can withstand temperatures as high as 900 K. Monolayer δ-SnS is a wide direct-bandgap (2.354 eV) semiconductor, and its electron mobility is as high as ∼1.25 × 103 cm2 V-1 s -1, higher than that of monolayer KTlO (∼450 cm2 V-1 s-1) and MoS2 (∼200 cm2 V-1 s-1). Optical absorption spectra, reaching up to the order of ∼105 cm-1, are obviously excellent in the visible-light region, suggesting efficient harvesting of solar radiation. Because of its unique atomic motif, monolayer δ-SnS presents an unusual bidirectional auxetic effect: a high negative in-plane Poisson's ratio (-0.048 and -0.068), which is larger than those for many recently reported two-dimensional auxetic materials, e.g., black phosphorene (-0.027), borophene (-0.04), and monolayer penta-B2N4 (-0.02). The bandgap and band edge can be substantially manipulated under strain to meet the requirement of the water-splitting reaction. Particularly, when pH = 7, suitable band-edge alignments and small overpotentials of the photocatalytic OER (oxygen evolution reaction) and HER (hydrogen evolution reaction) appear, endowing monolayer δ-SnS with great potential as an efficient visible-light-driven bifunctional photocatalyst for water splitting.