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.