Introduction
The adsorption of gaseous molecules on the surface of carbon nanomaterials, especially carbon nanotubes (CNTs), has awakened an enormous interest in industrial, environmental, and medical monitoring.1-5 Various small molecules such as NO2, NH3, O2, CH4, H2O and N2 were determined to be adsorbed on the surface of various nanotubes and their related nanostructures.6-10 In particular, the adsorption of carbon monoxide (CO) on the surface of nanomaterials has attracted a great deal of attention due to its effect on air pollution and human’s health. CO detection technology has been developed over the common approaches contain electrochemical, catalytic combustion and semiconductor devices.11-13 In recent years, different kinds of nanotubes have been designed to detect small concentration of CO such as boron nitride
nanotubes (BNNTs),14-15 silicon carbide nanotubes,16 Gallium phosphide nanotubes,17 TiO2nanotubes18-19 and MgO nanotubes.20Among different sensor materials, carbon nanotubes have been widely used for the adsorption of CO molecules due to their extraordinary electronic, mechanical and chemical properties.21-22Also, the sensitivity of graphene to CO has been indicated by experimental and theoretical studies.23
Nowadays, doped carbon nanostructures have attracted widespread attention due to their excellent physical, chemical and mechanical properties as well as their potential applications in electronics, sensors, supercapacitors and batteries.24 It is demonstrated that surface-modifications via doping of heteroatoms (such as boron and nitrogen) can tailor the gas sensitivity of nanostructures. Substitutional doping of carbon nanostructures creates charged sites and consequences in redistribution of spin and charge densities in nanostructures.25 For example, B and N-doped carbon nanostructures are p-type and n-type conductors, respectively. Therefore, N and B impurities could enhance the conductivity and chemical reactivity of carbon nanostructures.26-30 An early study showed that nitrogen-doped carbon nanotubes (NCNTs) have significantly modified the physical and chemical properties of simple CNTs and NCNTs are more sensitive to CO molecules.31It is well known that S-doped carbon nanostructures could act as n-type conductors. This is mainly believed that due to the S lone-pair electrons, it can provide additional negative charges for the π system of carbon lattices.32 The design of new nanostructures plays a key role in the development of sensor device technologies and applications. In this way, calculating the structural properties and interactive energies (in the gas or solvent) is a key to tailoring the technologically important physical properties of sensing materials. Therefore, density functional theory (DFT) has been widely used to calculate adsorption energies of gas molecules on the surface of doped nanotubes because these properties cannot be studied completely by experiments.33 For instance, our previous studies indicated that doped carbon nanostructures were dramatically sensitive to the adsorption of small molecules.34-40 However, to date there have been no systematic comparative studies of doped nanostructures with different doping atoms. The current investigations suggest that DNTs with different dopant atoms (N, S and B) will show various chemical activities to adsorption of CO molecule.
In spite of several theoretical studies on the adsorption of CO molecule by several nanomaterials, only a few works have been reported on the sulfur doped CNTs and any works related to the comparison between various dopant atoms have not been observed in the adsorption of CO molecule by DNTs. In this study, ONIOM calculations were employed to investigate the comparison between the CO-sensor abilities of simple and N, B, S-doped carbon nanotubes. Moreover, adsorption energies, optimized parameters and the molecular orbital properties like HOMO and LUMO energies, energy gap (Eg) and density of states (DOS) plots have been calculated using ωB97X-D/6-31G* calculations. The results of these computations and the related details will be presented in the next sections.