Optoelectronic and photocatalytic properties of stable pentagonal B₂S and B₂Se monolayers

Highlights

• Pentagonal B2S and B2Se monolayers exhibit a moderate bandgap, high holes’ mobility and optical response in the visible region.

• Application of strains results in indirect-to-direct bandgap transitions.

• These monolayers can be useful for photocatalytic water splitting under suitable solution conditions and strain.

Abstract

Boron-based 2D monolayers have attracted tremendous interest due to their unique physical and chemical properties. In this paper, we report novel pentagonal monolayers, B₂S and B₂Se, which are predicted to be energetically, dynamically, and thermally stable based on density functional theory. At the HSE06 level of theory, they exhibit a moderate indirect bandgap of (e.g., 1.82 eV for Penta-B₂S and 1.94 eV for Penta-B₂Se). Strain-induced indirect-to-direct bandgap transition, high hole mobility ($~{10}^3$ ${\mathit{Cm}}^2{V}^{-1}{S}^{-1}$) and strong optical absorption (α $~{10}^5$ ${\mathit{Cm}}^{-1}$) in the visible region are observed for these monolayers. Moreover, the electronic band structures and optical spectra are tunable by mechanical strains suggesting their visible light-harvesting capabilities for optoelectronic applications. In this way, the pentagonal family of 2D materials is now expanded to include boron-containing photocatalytic materials for water splitting applications.

Introduction

Two-dimensional (2D) materials with pentagonal structures have generated great interest after the theoretical prediction of Penta-graphene [1]. Many efforts have been made to predict and synthesize pentagonal 2D materials theoretically and experimentally with interesting properties with a growing range of applications. Many 2D materials having pentagonal network such as Si-based Pentagonal monolayers [2], Penta-B₂C [3], Penta-CN₂ [4], Penta-AlN₂ [5], Penta-BN₂ [6], Penta-BP₅ [7], Penta-NiS₂ [8], Penta-XS₂ (X = Ni, Pd, Pt) [9], Penta-BCN [10], Penta-NiX₂ (X = S, Se and Te) [11], and Penta-PtM₂ [12] etc. are theoretically predicted. Interestingly, in recent studies, Penta-PdSe2 has been experimentally synthesized with high air stability (by mechanical exfoliation and chemical synthesis) [13], [14], [15]. These experimental studies mark a new hope for exploring pentagonal structures as a new family of 2D materials.

It is known that the properties of any material are correlated closely to the structures of the materials. Therefore, properties can be significantly modulated from unitary (Penta-graphene) to binary (Penta-PdSe₂). For instance, unitary Penta-graphene has an intrinsic quasi–direct bandgap of 3.25 eV [1], superior ultrahigh strength, and is found to be suitable anode material for Li/Na-Ion batteries [16]. On the other hand, binary Penta structures such as Penta-PdSe₂ can be used as a phototransistor [17], thermoelectric material [18], field-effect transistor [19] and photocatalyst for water splitting [20].

The wide and tunable bandgap of Penta-BP₅ has shown superior high-frequency–response to transition metal dichalcogenides (TMDs) and black phosphorus [7]. Other pentagonal structures such as Penta-BCN show high spontaneous polarization and prominent piezoelectricity, larger than the values associated with an h-BN sheet and functionalized Penta-graphene [10], [21]. Combining Penta-BN₂ with Penta-graphene vertically, the Schottky barrier becomes tunable for electronic applications [22]. Further, Penta-BN₂ has also been used as an anode material for Li-Ion batteries [23]. Recently, Penta-PC₂ being metallic, has shown an ultrahigh theoretical storage capacity for Li/Na absorption, greater than graphite [24]. On the other hand, Pentagonal PtTe₂ has a promising potential due to the significant thermoelectric response with a zT value of 5.03 along x-direction at 600 K [12]. Strain tunable solar cell power conversion efficiency of Penta-PdQ₂ (Q = S, Se) monolayers is predicted to be as high as 33.9% at the compressive strain of 5%.[25]. These attractive properties exhibited by pentagonal structures make them the potential candidates for future nanodevices.

The boron-based monolayer structures have received much attention in recent years [26], [27], [28]. The honeycomb monolayer B₂S is a new anisotropic Dirac cone material [29]. Recently, two-dimensional boron pnictides have shown photocatalytic behavior and suitable electronic and optical properties [30]. Our recent study has demonstrated excellent electronic and optical properties of boron-based hybrid monolayers, BX (X = As, Sb, Bi) [31].

This study reports novel 2D Penta-B₂S and Penta-B₂Se monolayers using density functional theory (DFT). The stability of these monolayers is confirmed with phonon spectra and ab-initio molecular dynamics (AIMD) simulations. The influence of the uniaxial strain on the electronic and optical properties is also investigated. We have also explored the suitability of these monolayers under applied strain and varied $\mathit{pH}$ of medium to be used as photocatalyst for water splitting.