Adsorption studies on air pollutants using blue phosphorene nanosheet as a chemical sensor – DFT approach

Highlights

• Using DFT, the structural stability of BPNS is verified using formation energy.

• Electronic property of BPNS is explored by DOS and bandstructure spectrum.

• Using Bader charge transfer, the adsorption of air pollutants on BPNS is studied.

• The BPNS is highly sensitive towards NO and NO₂ molecules.

• BPNS can be efficiently utilized for probing CO, CO₂, NO, NO₂ and H₂O molecules.

Abstract

The proposed research article investigates on the adsorption behaviour of the prime substance blue phosphorene nanosheet (BPNS) with the pollutant vapours CO, CO₂, NO and NO₂ by utilizing ab-initio method. Initially the BPNS stability is guaranteed by calculating its formation energy and it is calculated as −4.149 eV/atom. Before performing the adsorption studies, the structural and electronic characteristics of BPNS are investigated. Then the interaction between the prime substance BPNS and the pollutant vapours are analysed with the support of adsorption energy, energy band gap, Bader charge transfer, and a change in band gap together with DOS and energy band structure spectrum. The humidity effects on BPNS have also been studied by sensing the H₂O molecule. The energy band gap of BPNS without pollutant adsorption is observed as 1.999 eV. Though, the adsorption of CO, CO₂, NO and NO₂ vary the energy gap of BPNS. Thus, the outcome of this research suggests that BPNS can be used as an efficient sensing material for pollutant vapours especially NO and NO₂ even in the humidity environment.

Introduction

In the modern world, for designing chemical and biosensors, much more interests are shown on nanomaterials because of their advantages on greater surface area, superior-conducting abilities, and possible different methods to synthesis nanomaterials in the form of tubes, sheets and ribbons [1]. Recent studies states that for identifying the presence of gas and vapors, the two-dimensional (2D) substances such as graphene, graphdiyne, germanene and analogous to semiconducting basic materials may be used as promising sensors [2], [3], [4], [5]. Rad and groups [6], [7], [8], [9], [10], [11], [12] have demonstrated the adsorption behavior of different small gas molecules namely CO, CO₂, H₂O, NH₃, SO₃, H₂S, HCN, CH₄ and CO on 2D graphene including polymer (Polythiophene) and Ca₁₂O₁₂ material using density functional theory technique. Further, the authors have enhanced the sensing behaviour of target molecules by the substitution of proper dopant atom. It is studied that phosphorus is a pnictogen which is abundantly present in the earth crust. Also phosphorous has dissimilar allotropic forms. The different allotropic forms of phosphorous are red phosphorous, blue, white phosphorous and black phosphorous, [13], [14]. White phosphorous gets ignited around 30 °C. To achieve stability at high temperatures white phosphorous is transformed in to red phosphorous. This red phosphorous does not ignite up to the temperature of 240 °C [15]. Additionally, it is possible to synthesize violet and black phosphorus using the thermal processing of white or red phosphorous [16]. Zhang et al. in the year 2018 described that by using molecular beam epitaxy, it is possible to achieve epitaxial synthesis of blue phosphorene on Au (1 1 1) [17]. Also, the phosphorene possesses different electronic properties such as high carrier mobility, which is layer dependent. In phosphorene, the phosphorus atom is made by sp³ hybridisation. It shares the single pair and the balance three valance electrons with the adjacent electrons of phosphorus [18]. Thus, the electronic arrangement of phosphorene shows an improved sensitivity in the neighbouring environment and identifying it as an acceptable candidate for chemical sensor [19], [4]. Blue phosporene is one among the allotropes of phosphorus. In addition, blue phosphorene also finds its importance in field-effect transistors and lithium-ion batteries [20]. Both black phosphorene and blue phosphorene are majorly employed as biosensors and chemical sensors [21]. Chandiramouli et al. [22], [23], [24] have recently proposed various allotropes of phosphorene material and to be utilized for chemical sensor in connection with various toxic vapour molecules. Recently, Soltani and co-workers [25], [26], [27], [28], [29] have reported the various form of 2D material including phosphorene nanosheet, Al₁₂N₁₂ nano-cage, boron nitride nanotube, B₁₁XN₁₂ (X = Mg, Ge, Ga) nano-clusters, Al₁₂P₁₂ fullerene and MgO nanotube to be utilized as a sensor to detect various toxic molecules (NO, NO₂, CO₂, NH₃, SO₂, etc.). The harmful effects of fossil fuel burning are becoming progressively noticeable. When coal, natural gas and petroleum, are kindled for energy, carbon from these fossil fuels are mainly emitted to the atmosphere as carbon dioxide (CO₂), which is the main greenhouse gas that is accountable for the majority of anthropogenic climate change [30]. Fossil fuel kindling simultaneously releases other air pollutants, such as nitrogen oxides (NO_x), mercury, volatile organic compounds (VOCs) and particulate matter (PM) [31]. The partial burning of carbon-carrying fuels, such as oil, coal, wood, gasoline and natural gas produce the odourless, tasteless and colorless toxic air pollutant carbon monoxide (CO). Inhaling the excessive concentrations of CO from a polluted environment cause the decreased oxygen (O₂) transport in blood by haemoglobin. This less oxygen transport in blood leads to ill health effects as headaches, greater possibility of chest pain of persons having heart disease, and so on. Nitrogen monoxide (NO) and Nitrogen dioxide (NO₂) are also produced by the burning of fossil fuels (coal, gas and oil) and gets spread in to the air. The major sources of NO and NO₂ are the exhaust emissions from cars, buses and trucks, power plants, and off-road equipments. More exposure of NO₂ for a longer duration may leads to the development of asthma and increase the chance of getting respiratory infections. Hence the children’s and elders are at increased risk for the ill effects of NO₂. Volcanoes and bacteria are also the natural source of nitrogen oxides. The main constituent of earth's hydrosphere is water, which is a transparent, odorless, tasteless, nearly colorless and inorganic chemical substance. Also it is the fluids of all living organisms. Its chemical formula is H₂O that means each of its molecules hold two hydrogen atoms and one oxygen atom fastened by covalent bonds. At standard ambient temperature and pressure, the liquid state of H₂O is called as water. The concentration of the gaseous state of water known as water vapour and invisible to human eye, present in the air is known as humidity. It is one of the fundamental abiotic factors that determine which animals and plants can prosper in a given environment [32]. The human body dissipates heat through evaporation and perspiration. The human body maintains its temperature by transporting the heat from the body to the surrounding air by heat convection, and thermal radiation. When the humidity is high in air due to increase in atmospheric temperature, the rate of evaporation of sweat from the skin decreases. Thus because of this humidity effect the heat brought by the blood to the surface of the body cannot be dissipated in to atmospheric air. Since much more blood is used for heat dissipation through the surface of the body, only less amount of blood flows through the brain, the active muscles and other internal organs of the body. Thus the physical strength decreases and fatigue appears. In addition the mental capacity and physical alertness will be affected, which may result in heat stroke or hyperthermia. Importantly, the sensing performance of the chief blue phosphorene nanosheet may get deteriorated in the humidity environment. Therefore, in the present work we have also considered the humidity effect by studying the adsorbing behaviour of H₂O molecule on base material. This research work concentrates on the use of blue phosphorene nanosheets as a recognizing element to sense the existence of toxic CO, CO₂, NO and NO₂ molecule even in the humidity atmosphere. By considering the adsorption studies of CO, CO₂, NO and NO₂ on BPNS, the feasibility of employing BPNS as recognizing element for chemical nanosensors are explored.