Two dimensional B₂CX (X=O, S, Se) monolayer semiconductors for water splitting application

Highlights

• A new stable B₂CX (X = O, S, Se) monolayers are predicted.

• The thermodynamic, kinetic, and thermal stabilities of the predicted monolayers are proved by simulation.

• 2D B₂CO, B₂CS, B₂CSe monolayers are semiconductors with bandgaps of about 2.81, 2.63, 1.57 eV respectively.

• 2D B₂CX (X = O, S, Se) monolayers are promising photocatalysts materials for water splitting applications.

Abstract

The structural and electronic stabilities as well as photocatalytic properties of two-dimensional (2D) Janus B₂CX (X = O, S, Se) monolayers are theoretically assessed. First, with the help of cohesive energy computations and phonon dispersion simulations, the energetically and kinetically stabilities of the proposed 2D B₂CX (X = O, S, Se) monolayers are confirmed. Next, electronic properties computations using hybrid density functional (HSE06) indicates that 2D B₂CO, B₂CS, B₂CSe monolayers show semiconducting properties with moderate bandgaps of about 2.81, 2.63, 1.57 eV respectively. Finally, comparing the positions of band edge of the proposed monolayers with the redox potentials of water shows that 2D B₂CX (X = O, S, Se) monolayers are promising photocatalysts materials for water splitting applications.

Introduction

Increasing global energy demand coupled with climate change and environmental degradation regarding the growing use of fossil fuels can be considered as the greatest challenges facing the human societies for the time being, therefore, to find alternative clean and sustainable energy sources is an important task related to the issue. Solar and hydrogen energy are most common clean energy sources, have been gradually increasingly interested many efforts to be used as replacement of traditional resources [[1], [2], [3], [4], [5]].

Hydrogen in its pure form, is a good energy carrier, has been employed as one of the most useful substitute energy sources can be used in several different ways. In recent decades, different forms of hydrogen are employed in many practical applications, as examples, liquid hydrogen is used as rocket fuel, and hydrogen fuel cells are widely used in power electric cars. Unfortunately, hydrogen rarely exists on its pure form and it must be obtained from hydrogen containing compounds such as water [6]. Regarding this task, to find an efficient photocatalyst material which can utilize solar energy to produce pure hydrogen by water splitting procedure is the main challenge [7,8]. In principle, in a photocatalytic water splitting process based on a semiconductor, by absorbing a photon from sunlight by the surface of the semiconducting photocatalyst an electron–hole pairs is generated via an electron excitation from the valence band to the conduction band, and at last, the redox reaction splits water molecules into hydrogen and oxygen. For the aim of semiconducting photocatalyst based water splitting technology, to find an efficient photocatalyst is the root problem. To be used in practical applications, for overall water splitting, a promising efficient photocatalyst must have three characteristics: (a) a good photocatalyst semiconductor needs to have a bandgap of 1.2–3.0 eV to absorb a significant portion of solar energy; (b) the photo generated charge carriers need to promptly transfer as well as they need to separate to prevent their recombination; (c) The valence band maximum (VBM) must be lower than −5.67 eV, and the conduction band minimum (CBM) should be higher than (−4.44 eV) to further produce hydrogen. These two values are the oxidation potential of O₂/H₂O and the reduction potential of H/H₂. A schematic representation of photocatalyst water splitting is shown in Fig. 1.

In recent years many efforts have been done to introduce new photocatalyst semiconductor materials to be used in water splitting and a large number of bulk and nanostructured photocatalyst semiconductors have been proposed [[9], [10], [11], [12], [13], [14], [15]], however, that most of the proposed bulk photocatalyst semiconductors exhibits low efficiencies. It is well known that due to their special properties, generally two dimensional (2D) materials may have promising applications in water splitting technology. Therefore, a huge number of research activities have been conducted to design promising two dimensional (2D) photo-catalyst semiconductors. Since now, the water splitting photocatalytic properties of phosphorene, 2D group III-VI monochalcogenides [16,17], 2D IV−VI compounds [18,19], 2D IV-IV compounds [20]^, 2D transition-metal dichalcogenides [21,22] have been investigated. Very recently, the water splitting application of 2D Ga₂XY and In₂XY (X = S, Se, Te; Y = S, Se, Te) Janus monolayers (MLs) have been investigated by Da Silva et al. [23], it has been shown that the proposed Janus monolayers Janus monolayers exhibit required conditions to be used in water splitting applications.

In this work, First Principles Calculations based on Density Functional Theory (DFT) were preformed to investigate, the structural stability, electronic and photocatalytic properties of two-dimensional (2D) Janus B₂CX (X = O, S, Se) monolayers. In these calculations, once the stability of the proposed monolayers was confirmed, the electronic properties of the designed 2D monolayers was evaluated and it was found that all of the proposed monolayers shows semiconductor nature with strain tunable moderate bandgaps. Moreover, by analyzing the band edge positions of the proposed monolayers with the redox potentials of water shows that 2D B₂CX (X = O, S, Se) monolayers are promising photocatalysts for using in water splitting technologies.