Highly enhanced NH₃-sensing performance of BC₆N monolayer with single vacancy and Stone-Wales defects: A DFT study

Highlights

• Pure BC₆N monolayer cannot be used as high-performance gas sensors.

• BC₆N monolayer with single vacancy and Stone-Wales defects are dynamically stable.

• Introduction of monovacancies in BC₆N monolayer enhances strongly NH₃-sensing performance.

• BC₆N monolayer with MV_C, MV_N, and rippled-SW defects is promising candidates for NH₃ sensors.

Abstract

Searching for suitable NH₃-sensing materials has important scientific significance and application value. Using DFT calculations, the adsorption behaviors, electronic, gas-sensing, and optical properties of NH₃ and other common molecules on the BC₆N monolayer without and with single vacancy (MV) and Stone-Wales (SW) defects were investigated to fully exploit the possibilities of the BC₆N monolayer as a NH₃ gas sensor. Our results showed that the pure BC₆N monolayer has bad gas-sensing performance for NH₃ detection. The BC₆N monolayer with MV_C, MV_N, MV_B, and rippled-SW defects are all dynamically stable. The studies on adsorption behaviors (adsorption energy, geometric structures, and adsorption mode), charge transfer, electron density difference, electronic and optical properties indicate that the BC₆N monolayer with MV_C, MV_N, and rippled-SW defects is promising candidates for NH₃ sensors, indicating the introduction of monovacancies (MV_C, MV_N, and rippled-SW) in BC₆N monolayer can enhance strongly the NH₃-sensing performance.

Introduction

Toxic gases, deriving from a very wide range of fields such as combustion/chemical reactions and leaks of harmful industrial gases and vapors, is a serious threat to the environment as well as to humans. It is well known that ammonia (NH₃) is frequently used for industrial cleaning, food and chemical manufacturing as a refrigerant and so on [1]. In addition, as a metabolite in human life, ammonia has been used as an important indicator in the diagnosis of diseases such as diabetes, kidney disease, malignant tumors, and lung cancer. Despite its usefulness, NH₃ is a colorless toxic gas with a pungent odor, which is harmful to human health. For example, people exposure to 25 ppm of NH₃ can cause skin, eye, and lung irritation [2], and work with NH₃ at high concentrations are often poisoned [3]. Thus, it is of great importance to detect NH₃ gas and to monitor their concentrations, and the exploration and design of new suitable materials for NH₃ detection is a challenging and urgent task.

Two-dimensional (2D) nanomaterials has taken the front row in innovative applications as gas sensors in the past decade after the successful experimental exfoliations of graphene due to their outstanding mechanical performance, large surface-to-volume ratio (i.e. large sensing area per unit volume), high mobility, low electronic temperature noise, and high chemical stability [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30]. It is well known that the utility of pure graphene as gas sensors is limited by the lack of a band gap, and substitutional doping with B and N atoms can enhance effectively the gas-sensing properties of graphene [4], [6], [7], [8], [9]. The graphene with B or N substitutional doping can be referred to B_xC_y and N_xC_y monolayers, and previous studies have demonstrated that these monolayers have good gas-sensing performance for toxic gases detection [31], [32], [33], [34], [35], [36], [37], [38], [39]. Recently, a BN-co-doped graphene with a similar hexagonal atomic structure was synthesized [40], and this graphene-like borocarbonitride (namely BC₆N) monolayer shows a semiconducting behavior and offers high carrier mobility, thermal conductivity, and thermal stability [41], [42], [43], [44], [45], [46], and has potential applications in hydrogen storage [47], electronic and photovoltaic devices [48], Z-scheme photocatalysts [49], and sensors [50], [51].

Based on the reported studies in the literature [42], [46], there are two configurations for the most stable BC₆N structure with a unit cell consisting of 8 atoms in hexagonal form: one configuration is demonstrated as BN-doped nanographene in which one B atom and one N atom in a six-membered ring are farthest from each other, and the other is found that B and N atom are also located in a six-membered ring but formed a B-N bond, which are named as BC₆N-1 and BC₆N-2, respectively [46]. The electronic properties and potential applications of the first structure (BC₆N-1) were widely investigated [40], [41], [42], [43], [44], [45], [46], [48], [49], [50], [51], while only a few work have focused on the second configuration (BC₆N-2), although the second configuration is more stable than the first one [42], [46], [47]. On the other hand, the electronic properties (especially gas-sensing properties) of monolayers toward gases can be modified effectively through the introduction of defects [50], [51], [52], [53]. For example, using the BC6N-1 structure as modes, Aghaei et al. studied the adsorption behaviors of several volatile organic compound (VOCs) and interfering gases in exhaled breath on the BC₆N-1 monolayer, and found that defective BC₆N-1 monolayer can be viewed as chemiresistive sensors for applications in room temperature breath analysis of VOCs [50]. Babar et al. investigated the application potential of pristine and monovacant BC₆N-1 monolayer for sensing gaseous pollutants (CO, CO₂, NO, NO₂, NH₃, H₂S, and SO₂), and their results showed that the monovacancies in BC6N-1 monolayer can enhance remarkably the sensitivity of the material for molecule detection [51]. These works indicate that the defects in monolayer can enhance obviously the sensitivity of the materials.

In this work, we investigated the adsorption behaviors, electronic and optical properties of gaseous molecules on the most stable BC₆N-2 monolayer as depicted in Fig. 1, (we just called it as BC₆N monolayer in the study for clarity), and understanding the effect of various defects, such as Stone-Wales (SW) defect and single vacancy (MV) defects by removing one atom (including removal of B, N, and C, respectively), on the gas-sensing properties of BC₆N monolayer as a NH₃ sensor, by using first-principles calculations based on density functional theory (DFT). Our results demonstrated that the pure BC₆N monolayer has bad gas-sensing performance for NH₃ detection, but the introduction of monovacancies (MV_C, MV_N, and rippled-SW) in BC₆N monolayer can enhance strongly the NH₃-sensing performance.