Mehrdad Mahkama , Javad Aboudia and Milad Nouralieib,*
a Chemistry Department, Faculty of Science, Azarbaijan Shahid Madani University, Tabriz, Iran
b Young Researchers and Elite Club, Central Tehran Branch, Islamic Azad University Tehran,Iran
Corresponding author: Milad Nouraliei
ABSTRACT: We performed a computational study (density functional theory, DFT) on the adsorption of TNT,RDX and Nitroguanidine molecules on single-walled carbon nanotubes in gas phase. The TNT,RDX and Nitroguanidine molecules adsorption is energetically favorable with adsorption equal to -0.13 ,-0.16 and -0,57 eV respectively. The electronic properties analysis reveals that physisorbed TNT and RDX and chemisorbed Nitroguanidine significantly change the highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) energy gap of the nanotubes by about 1.21eV and after interaction with TNT,RDX and Nitroguanidine is 1.19 , 1.01 and 1.08eV respectively. We conclude that the three molecules outside carbon nanotube make changes to the set of energy molecule and the nanotube is also conductive structure. This phenomenon suggesting that the Carbon nanotube is potential sensor for TNT , RDX and Nitroguanidine molecules detection.
Keywords: DFT ,Sensor,Single-walled carbon nanotubes , TNT, RDX, NQ.
A carbon nanotube (CNT) is a tubular structure made of carbon atoms, having diameter of nanometer order but length in micrometers. Although this kind of structure was synthesized, studied and reported by several researchers during 1952–1989, Iijima’s detailed analysis of helical arrangement of the carbon atoms on seamless coaxial cylinders in 1991 proved to be a discovery report. Since then, CNT has remained an exciting material ever. Some of Its so-called extraordinary properties are: many-fold stronger than steel, harder than diamond, electrical conductivity higher than copper, thermal conductivity higher than diamond, etc. Set off a gold rush in academic and industrial laboratories all over the world to find practical uses of the CNTs. This sprouted thousands of publications and patents on innumerous potential applications of the CNTs in almost all the walks of life such as media, entertainment, communication, transport, health and environment. The gold rush turned into a stampede when NASA scientists and many others predicted the possibility of making space elevators, lighter and stronger aircrafts, collapsible and reshapable cars, incredible new fabric, portable X-ray machines etc. by using the CNTs. Consequently, the CNT has become a material of common interest today, and society is eagerly waiting for seeing the charisma of the CNT in household products. We, the CNT researchers, know how to make single-electron transistors from the individual CNTs, but we do not know how to make a CNT of the required structure. Hence it is necessary to retrospect. The skytower of the ambitious nanotechnology (in particular, CNT-based technology) cannot be erected without a firm foundation of the growth-mechanism understanding. Environmental applications of the CNTs capitalize on the changes induced in the material properties of the CNT by the presence of an externally adsorbed species. Adsorption of chemicals on the CNTs is an important topic for fundamental researches and applications of the nanotubes. The adsorptive characteristics of nanotubes allow their use as sensors and in the storage of fuels. Knowledge of the interactions between molecules and CNTs and of their response to molecules is essential for fabricating novel CNT-based molecular adsorbents for toxic compounds removal or detection in different matrixes. So far, many different aspects of nanotube/adsorbates have been experimentally and theoretically explored. We have selected one explosives for which experimental data are available. These compounds are 2,4,6-trinitrotoluene (TNT).The chemical structures of these molecules are shown in Figure 1. Results of thermodynamic property predictions of these compounds were discussed. The TNT was first prepared in 1863 by German chemist Julius Wilbrand and originally used as a yellow dye. Its potential as an explosive was not appreciated for several years mainly because it was so difficult to detonate and because it was less powerful than alternatives. The TNT can be safely poured when liquided into shell cases, and is so insensitive that, it was exempted from the UK's Explosives Act in 1875 and was not considered an explosive for the purposes of manufacturing and storage. Ingale et al. demonstrated that the silica xerogels incorporated with nanocrystalline PETN or PETN/ TNT can be synthesized, in which the content of energetic materials ranges from 50% to 90% (w/w) . TNT is still widely used by the United States military, as well as construction companies around the world. The majority of TNT currently used by the US military is manufactured by Radford Army Ammunition Plant near Radford, Virginia. It was reported that the TNT is contain 2.8 mega joules per kilogram explosive energy. Structurally diagnostic ion/molecule reactions of ethyl vinyl ether with specific fragment ions generated from explosives such as trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) were also used for selective detection and these molecules are shown in Figure 1. Non-aromatic explosive compound, 1,3,5-trinitro-1,3,5-triazacyclohexane (RDX), have also been investigated by 1,2-dichloroethane chemical ionization. Unlike nitrobenzene, the analogous arylnitrenium ions were not formed, probably due to the lack of resonance stabilization of the aromatic ring. However, the ring contraction of the adduct ion [RDX + CH2ClCH2]+ (m/z285) occurs to give rise to the characteristic 1-chloroethyl-1,3-dinitro-1,3-diazacyclobutane cation [RDX+CH2ClCH2-CH2NNO2]+ (m/z 211) by loss of CH2NNO2. In addition, the abundance of the ion of m/z211 is much higher than that of the deprotonated or protonated RDX ions which is potentially useful for the detection of RDX with increased selectivity and sensitivity . Nitroguanidine (NQ) , molecules shown in Figure 1,was first prepared by Jousseline in 1877 and its properties investigated and described by Vielle in 1901. During the First World War, the mixture of nitroguanidine and nitrocellulose was used as a flashless propellant. However, it was difficult to maintain the stability of such composition because nitroguanidine was reacting with nitrocellulose. The company Dynamit AG overcame that problem in 1937 by developing “Gudol Pulver” composition. At that time, it was used as a high explosive mixed with ammonium nitrate(V) to fill mortar ammunition. After the Second World War, nitroguanidine was used to produce triple-base propellants. Nitroguanidine can be also used for solid rocket propellants, particularly propellants designed for the so called rocket engines, in which the steady and stable combustion takes place. Nowadays, the studies on developing the installation for the production of bulk density, spherical and cubical nitroguanidine are conducted in the USA. The aim of these works is to use nitroguanidine as a component of low sensitive explosive mixtures characterised by high detonation parameters. The main purpose of this study is to gain fundamental insights of the TNT,RDX and NQ molecules on the electronic properties of the side-wall CNT, and how these effects could be used to design more sensitive molecules sensing devices.
Geometry optimizations, molecular electrostatic potential (MEP) and density of states (DOS)
analyses were performed on a 8 × 0 CNT and different configurations of the CNT molecules at the spinunrestricted M06-2X/6-311G(d,p) level of theory as implemented in GAMESS suite of program. The M06-2X/6-311G(d,p) is a reliable and common used level of theory for nanotube structures[30-32]. We define the adsorption energy (Ead) as follows:
Ead = Etot(molecule/CNT) – Etot(CNT) – Etot(molecule)
Where E(Molecule/CNT) is the total energy of a molecule TNT,RDX and NQ adsorbed on the external pristine CNT,and E(CNT) and E(Molecule) are the total energies of the pristine CNT and a TNT,RDX and NQ molecules,respectively. By the definition , a negative value of Ead corresponds to exothermic adsorption.
RESULTS AND DISCUSSION
A)Adsorption of TNT on the CNT:
In this study, we investigated the adsorption of the TNT on the exterior surface of the tube. Confinement the TNT outside (8,0) carbon nanotube is treated as a triclinic cell size6.3×6.3×13.9 Å. All confined TNT molecules outside CNT (Figure 2), were optimized with DFT method. Interestingly, after full relaxation with no constraints, exothermic adsorption energy (Eads) was 0.13 eV.
Figure 1: Chemical structures of studied molecule (TNT,RDX and NQ) and nanotube(CNT)
Figure 2:The structural geometric of TNT@CNT(8,0).(A) , RDX@CNT(8,0).(B) and NQ@CNT(8,0).(C)
TNT adsorbed on the external of the carbon nanotube (Figure 2A). The calculations show that the adsorption of TNT molecule from its OH head on the environs to C atoms is an exothermic process with negative Eads of -0.13 eV and interaction distance of 2.57 Å and this interaction indicating that the TNT is physisorbed on the CNT. The interaction leads to charge transfer of 0.016 |e| from the TNT to the nanotube (Table 1), indicating that the TNT acts as an electron acceptor and the CNT as an electron donor. The calculated MEP (Figure 3A) obviously shows this phenomenon where the red color on the adsorbed TNT represents the negative charge . Performance of a FMO analysis on the TNT molecule, showing that its HOMO is slightly more localized , therefore, the interaction between the OH head of TNT and C (LUMO of nanotube is localized on C atom) is somewhat stronge.
Table 1: The adsorption energies Eads, net partial charge (QT) on TNT molecules and the HOMO–LUMO gap (Eg) of mentioned systems.
In this study, we investigated the adsorption of the RDX on the exterior surface of the tube. All confined RDX molecules outside CNT (Figure 2B), were optimized using M06-2X method. The RDX adsorbed on the external of the carbon nanotube (Figure 2B). The calculations show that the adsorption of RDX molecule from its OH head on the environs to C atoms is an exothermic process with negative Eads of -0.16 eV and interaction distance of 2.48 Å and this interaction indicating that the RDX is physisorbed on the CNT. The interaction leads to charge transfer of -0.007|e| from the RDX to the nanotube (Table 1), indicating that the RDX acts as an electron donor and the CNT as an electron acceptor. The calculated MEP (Figure 3B) obviously shows this phenomenon where the red color on the adsorbed RDX represents the negative charge . Performence a FMO analysis on the RDX molecule, showing that its HOMO is slightly more localized , therefore, the interaction between the OH head of RDX and C (LUMO of nanotube is localized on C atom) is somewhat stronge.
C)Adsorption of NQ on the CNT:
NQ adsorbed on the external of the carbon nanotube (Figure 2C). The calculations show that the adsorption of the NQ molecule from its NH2 head on the environs to C atoms is an exothermic process with negative Eads of -0.57 eV and interaction distance of 2.71 Å and this interaction indicating that the NQ is chemisorbed on the CNT. The interaction leads to charge transfer of -0.009|e| from the NQ to the nanotube (Table 1), indicating that the NQ acts as an electron donor and the CNT as an electron acceptor. The calculated MEP (Figure 3B) obviously shows this phenomenon where the red color on the adsorbed NQ represents the negative charge. Performance a FMO analysis on the NQ molecule, showing that its HOMO is slightly more localized , therefore, the interaction between the NH2 head of NQ and C (LUMO of nanotube is localized on C atom) is somewhat stronge.
Figure3: Calculated molecular electrostatic potential surfaces for TNT@CNT(A) ,RDX@CNT(B) and NQ@CNT(C). The surfaces are defined by the 0.0004 electrons/b3 contour of the electronic density. Color ranges, in a.u.: blue, more positive than 0.050; red, more negative than-0.050.
Figure 4: Calculated density of states(DOS) for pristine Carbon Nanotube (A),TNT@Nanotube (B), RDX@Nanotube (C) and NQ@Nanotube (D).
D)The DOS of the molecule/nanotube systems:
To verify effects of the adsorption of the TNT,RDX and NQ molecules on the nanotube, electronic properties electronic density of states (DOS) of the nanotube and adsorbate/nanotube complexe were calculated (Figure 4). For all three configuration, it can be found that the DOS near the Fermi level are not affected by the TNT,RDX and NQ adsorption (Figure 4). So the Eg of the nanotube has no significant change external the TNT adsorptions (Eg = 1.19 for the B and RDX adsorptions) (Eg = 1.01 for the C and NQ adsorptions) (Eg = 1.08 for the D). As depicted the DOS in Figure 4, in the three configuration external the adsorption some energy states appear above the Fermi level ,so that the Fermi level shifts to lower energy levels. This occurrence is expected to bring about obvious change in the corresponding electrical conductivity because it is well known that the Eg (or band gap in bulk materials) is a major factor determining the electrical conductivity of a material and a classic relation between them is as follows:
where r is the electrical conductivity and k is the Boltzmann’s constant.
According to Eq. above, the smaller Eg at a given temperature leads to the higher electrical conductivity. However, the Eg of TNT,RDX and NQ/nanotube complex is quantitatively reduced compared to that of the nanotube. Since the conductivity is exponentially correlated with negative value of Eg, it is expected that it become larger with reducing the Eg. It demonstrates the high sensitivity of the electronic properties of the Carbon nanotube towards the adsorption of the TNT,RDX and NQ on molecules. We believe that the carbon nanotube can transform the presence of the TNT,RDX and NQ molecules directly into an electrical signal.
DFT calculations were performed to study the equilibrium geometries, stabilities, and electronic properties of TNT,RDX and NQ molecules adsorbed in the exterior surface of Carbon nanotube. It was found that the three molecule can be strongly adsorbed in the nanotube exterior surface with adsorption energies -0.13, -0.16 and -0.57eV respectively, and in the case of changes GAP(Eg), quantitative decreases,which leading increase the electrical conductivity. Thus, it was deduced that the carbon nanotube can acts as a TNT,RDX and NQ molecule sensor.
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