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

Abstract

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.

First-Principles Calculations on Semiconducting ε-GeS and ε-SnS Monolayer Nanosheets with Photocatalytic Activity for Sunlight-Driven Water Splitting

“…Accordingly, both ε-GeS and ε-SnS monolayers possess a better adsorption capacity for water molecules, suggesting their outstanding photocatalytic potential. Similar to various other hexagonal structures, 27,29 the band structures also exhibit anisotropy of both ε-GeS and ε-SnS monolayers under investigation. The band dispersion of CBM along Γ−X is very weak, whereas that along Γ−Y is quite notable, suggesting the easy drift of electrons along Γ−Y yet difficult drift of electrons along Γ−X.…”

“…Similar to various other hexagonal structures, , the band structures also exhibit anisotropy of both ε-GeS and ε-SnS monolayers under investigation. The band dispersion of CBM along Γ– X is very weak, whereas that along Γ– Y is quite notable, suggesting the easy drift of electrons along Γ– Y yet difficult drift of electrons along Γ– X .…”

“…Thermodynamic stability was examined by ab initio molecular dynamics (AIMD) of 6 ps with a time step of 1 fs . Moreover, for electronic structures and optical properties, compared to the inaccurate description with the PBE functional, the Heyd–Scuseria–Ernzerhof (HSE06) hybrid functional was selected to ensure more accurate calculations . In terms of the exciton effect, for optical properties, the G 0 W 0 approximation combined with the Bethe–Salpeter equation (G 0 W 0 + BSE) calculations was also performed. , The quantitative description of charge transfer could be gained via Bader charge analysis.…”

“…Moreover, when γ-SnS (blue-phosphorenelike SnS) is stacked on γ-GeSe (blue-phosphorene-like GeSe), this stacked Janus structure induces an out-of-plane polarization electric field, enabling photocatalytic HER and OER to proceed spontaneously under sunlight irradiation. 27 Our recent work predicted that the SnS monolayer with a δ-phosphorenelike skeleton (δ-SnS) 29 is also a promising photocatalyst for water splitting due to its well-aligned band edge, pronounced visible-light absorption, and small overpotentials of photocatalytic HER and OER. These studies fully demonstrated that the phosphorene-like structure can make these group IV−VI monochalcogenides novel platforms for application in photocatalysis.…”

A novel two-dimensional transition metal dichalcogenide as water splitting photocatalyst with excellent performances

“…In addition, the carrier effective masses of MoWS 4 monolayer have a small difference along the x and y directions. The electron effective masses of MoWS 4 monolayer are much lower than its hole effective masses ( m e = 0.377 m 0 and 0.374 m 0 along the x and y directions), which are smaller than that of many 2D photocatalytic materials, such as Penta-PdSSe ( m e = 2.16 m 0 ), Penta-PdSe 2 ( m e = 1.88 m 0 ), δ-SnS ( m e = 1.01 m 0 ), SnP 3 ( m e = 0.9 m 0 ) ( Long et al, 2018 ; Sun et al, 2018 ; Zhang et al, 2021b ; Xiao et al, 2021 ). The smaller carrier effective masses benefit the transfer rate of photogenerated carriers in the photocatalytic process.…”