DFT, QTAIM, and NBO Study of Adsorption of Rare Gases into and on

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DFT, QTAIM and NBO Study of Adsorption of Rare Gases into and on the Surface of Sulfur Doped, Single-Wall Carbon Nanotubes Hossein Tavakol, and Dana Shahabi J. Phys. Chem. C, Just Accepted Manuscript • Publication Date (Web): 05 Mar 2015 Downloaded from http://pubs.acs.org on March 7, 2015

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The Journal of Physical Chemistry

DFT, QTAIM and NBO Study of Adsorption of Rare Gases into and on the Surface of Sulfur Doped, Single-Wall Carbon Nanotubes

Hossein Tavakol* and Dana Shahabi

Department of Chemistry, Isfahan University of Technology, Isfahan 84156-83111, Iran Phone: +98-31-33913241 Fax: +98-31-33912350 Email: [email protected] , [email protected]

Keywords: Sensors; S-doped carbon nanotubes; Rare Gas; Adsorption; DFT

ACS Paragon Plus Environment

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ABSTRACT

In this work, the interactions of S-doped single-wall carbon nanotube (SCNT) with rare gases (RGs: He, Ne, Ar and Kr) were fully considered using density functional theory (DFT), QTAIM and NBO calculations. Different number of doped atoms and four RGs (He, Ne, Ar and Kr) were considered for their adsorption on the surface and into the SCNTs. The adsorption energies indicated that RGs could be adsorbed on the surface of the S-doped carbon nanotubes in endothermic process. Partial charges showed the small charge transfer from RGs to SCNTs. The calculated Ead values showed that the He and then Ar are the best RGs for adsorption by SCNTs. QTAIM calculations confirmed the close-shell (non-covalent) interactions between SCNTs and RGs. In accordance with the results of QTAIM, both NBO charges and E2 interaction energies showed the interaction of SCNTs with RGs. Moreover, population analyses were performed to obtain electronic properties, reactivity parameters and density of states (DOS) plots of all structures.

Keywords: Sensors; S-doped carbon nanotubes; Rare Gas; Adsorption; DFT


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1. INTRODUCTION Carbon nanotube (CNT) is an allotrope of carbon with a cylindrical nanostructure1 and unusual properties, which make it valuable for many fields in materials science and technology.2,3 During recent decades, there was a great focus on the structure and properties of single walled carbon nanotubes (SWCNTs) which present unique tubular structure, high specific surface and thermal and chemical stability.4,5 Therefore, numerous works were carried out to investigate their significance and the wide range applications of CNTs in nanoelectronics, nanoscaling, biotechnology, biosensors, optics and other fields of materials science.6-12 The functionalization or surface modification of CNTs is one of the most effective routes to modify their chemical, structural and electronic properties.13,14 These methods can induce localized changes within the nanotube framework. In this line, some effective methods such as substitutional doping of heteroatoms like boron, nitrogen, sulfur and silicon have been reported.15-20 Doping heteroatoms into the honeycomb-like lattice of CNTs forms one of the fundamentally adopted and effective ways for functionalization21,22 of CNTs. This doping leads to the activation of the surface of CNTs, possibility of introducing new functionalization and introducing additional electronic states in the Fermi levels.23–25 CNTs are doped to achieve some especial applications. One of the most important applications of doped nanotubes (DNTs) is adsorption of small molecules into and on the surface of nanotubes. Reported studies showed that doping CNTs could enhance their adsorption potencies.26-28 This ability could be used for many purposes such as drug delivery, removing pollutions, elimination of side products and designing new sensors.29-32 Sensors are important devices with the ability to detect or measure physical and chemical quantities to provide an immediate feedback from the environment. Recognizing and sensing molecules is crucial to


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control and monitor the technical and industrial processes.33-35 In all of the sensor applications, there is strong demand to improve the sensitivity, selectivity and stability of sensors. To comply this demand, nanocompounds such as CNTs and DNTs are employed to provide new materials, devices and systems that can exhibit novel and significant improvement in physical and chemical properties. Since the first use of CNT as sensor, demonstrated by Kong et al,36 there have been a growing interests to introduce new nanostructures for sensing and adsorbing various molecules.37 Among various DNTs, we have focused on S-doped CNTs (SCNTs) in our researches because only a few reports on their synthesis and applications like as sensing and adsorption properties have reported while many studies about the sensing and adsorption properties of other doped systems could be observed.38,39 In addition, it is well known that sulfur atom acts as strong electron donor when it doped into nanostructures such as carbon nanotubes and this doping enhance the conductivity and sensor properties of nanotubes.40 In recent years, the possibility of preparation and stability of SCNTs was investigated and proved theoretically by the study of Denis et al.41 According to this report, SCNTs with sulfur concentrations of 1.7–4 atom% could be prepared and the calculated formation energies are between 0.9 and 3.8 eV for various ((3,3), (5.5) and (10,0)) SCNTs. These values are comparable with that of 0.7 eV obtained for a nitrogen-doped (5,5) CNT. However, only two reports have observed on the synthesis of SCNTs using chemical vapor deposition (CVD) method starting from liquid sulfur-containing materials such as carbon disulfide and dimethyl sulfide.42,43 Insertion of sulfur make some changes on its electronic and chemical behaviors and despite the novelty of these materials, SCNTs have shown some potencies in sensors and energy applications such as fuel cells, supercapacitors or batteries.26,44 In addition, sulfur atom could enhance the encapsulation of ferromagnetic materials and thus


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improve the soft magnetic properties of CNTs, which is favorable to the applications of SCNTs as electromagnetic wave-absorbing and magnetic data storage areas. Therefore in the course of our studies about the adsorption processes and chemical sensing, especially on the surface of sulfur-doped nanostructures,26,29,37,45 we have decided to study the adsorption of rare gases (RGs) into and on the surface of SCNTs. The RGs were selected for this study because RGs have many applications in science, life and they are used in many cases when a stable element is needed to maintain a safe and constant environment. Therefore, study of their adsorption, storage and chemical sensing is important topic and should be worked out clearly. In addition, by reviewing the literature, a research about the adsorption of xenon on the surface of simple graphene have reported.46 To expand this interesting area, it should be useful to study of their adsorption on (or into) the doped nanostructures. In this work, the structures of some SCNTs (with one or two doped sulfur atoms) with adsorbed RGs (He, Ne, Ar, Kr) into and on their surface were optimized and then, their adsorption energies, reactivity parameters and molecular orbital properties (including DOS plots) were calculated using density functional theory (DFT) calculations. Moreover, interaction parameters were studied clearly using AIM and NBO calculations. Details of computations and the results obtained in this work are presented in the next sections. 2. COMPUTATIONAL METHOD Gaussian 09 program package47 have been used to perform calculations such as geometry optimizations and solvent effects. All the calculations were carried out using density functional theory (DFT).48,49 The DFT method was validated to give results similar to those of the more computationally expensive MP2 theory in calculation of energy, molecular geometry and frequency.50,51 Optimizations of all structures, single point calculations and other calculations


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were carried out using CAM-B3LYP method in combination with 6-31G, 6-31G(d) and 6311+G(d) basis sets.52,53 This method showed good abilities in study of long-range interactions as one of the best DFT methods for this purpose.54,55 The absence of imaginary frequency verified that structure was true minima at its respective levels of theory. The results of frequency calculations were used after applying an appropriate scaling factor.56 The effect of solvent was calculated using SCRF keyword using Tomasi's polarized continuum (PCM) model.57 Three solvents, two non-polar solvents (benzene with ε=2.271 and cycloHexane with ε=2.017) and one polar solvent (acetone with ε=20.493) were selected to study of the solvent effect on adsorption energies. Adsorption energies (Ead) in both gas and solvent were calculated according to the equation 1. Ead=ESCNT-RG-(ESCNT+ERG)

(Eq. 1)

NBO analyses of all structures were done using NBO program58 as implemented in the Gaussian program package. QTAIM calculations were performed using AIMAll 10.05.04 program.59 Density of states (DOS) plot was obtained from GaussSumm 2.2.1 program.60 To examine the reactivities, the chemical potential (µ), chemical hardness (η), global softness (S) and electrophilicity index (ω) were calculated as defined in equations 2-5, according to Koopman’s theorem.61 µ= (ELUMO+EHOMO)/2

(Eq. 2)

η= (ELUMO-EHOMO)/2

(Eq. 3)

S=1/ η

(Eq. 4)

ω= µ2/2η

(Eq. 5)

3. RESULTS AND DISCUSSIONS 3.1. Optimized structures


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In this study, first, the structures of S-doped carbon nanotubes (SCNTs), consisted of one or two doped sulfur atoms in the zigzag (5,5) CNT with 60 carbon atoms, were optimized. To simplify the calculations, both ends of SCNTs were saturated with hydrogen atoms. Subsequently, one or two (for 2S-doped) rare gases (He, Ne, Ar and Kr) was located on the surface or into the optimized SCNTs and the new structures were optimized again. The xenon has not been considered in this work because of the impossibility of applying accurate basis sets on this atom without serious problems. For SCNTs with one doped sulfur atom (briefly named as 1S), one RG was placed on the surface and into the SCNTs (named as In-1S) to study of their adsorptions and interactions. Moreover, for SCNTs with two doped sulfur atoms (briefly named as 2S), two RGs were considered only on the surfaces of SCNT and the resulting structures have been studied. The graphical pictures of the optimized structures were shown in Fig. 1. In addition, the important molecular parameters were extracted from the optimized structures and the results were listed in Table 1 in attendant with the partial charges of sulfur and RGs, obtained from the NBO calculations. In the 2S-RG structures, the distance between two S atoms was 6.09 Å in the optimized structure. The average C-S bond lengths in 1S-RG structures (between 1.775-1.933 Å) are smaller than those values in 2S structures (between 1.810-1.812 Å) and all these values are not related to the nature of connected RG. However, in In-1S structures, the average C-S bond length (between 1.777-1.878 Å) increases by the increasing the size of RG. The S-RG distance is certainly a function of RG and surprisingly, the highest value is related to S-Ar (not S-Kr) while the atomic radii of krypton is larger than argon. Moreover, in inner insertion of RGs, these distances are shorter than outer cases. The S1-RG bond lengths (between 2.977- 4.011 Å) are comparable with the S2-RG bond lengths (between 3.019-4.410 Å) and any meaningful relation between the number of doped sulfur and the S-RG distance could not be observed.


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The partial charges of S atoms in 2S complexes are between 0.864-0.866 Å, larger than those values for 1S complexes (0.852-0.854 Å). Although, in In-1S complexes, by the increase of RG size, its charge decreases extensively because of the small size of the SCNT cavity. In addition, in 1S and 2S complexes, the RG charges are very small and their values lie in the range of 0.001 -0.013 au. These values show only small electron transfer could be observed in these complexes while in In-1S complexes with large RGs (Ar and Kr), these charges increase to higher value (0.209 for Kr). Moreover, because of the positive charges of RGs and unlike our previous studies,26,37 SCNTs act as acceptor in these interactions.

1S-He

2S-He

In-1S-He

1S-Ne

2S-Ne

In-1S-Ne

1S-Ar

2S-Ar

In-1S-Ar


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1S-Kr

2S-Kr

In-1S-Kr

Figure 1. The optimized structures of SCNTs in interaction with RGs Table 1. Important bond lengths (in angstrom) and NBO atomic charges for all structures Molecule C-Sa S-Rg S charge Rg charge 1S-He 1.785 3.362 0.854 0.001 1S-Ne 1.786 2.977 0.852 0.013 1S-Ar 1.785 4.011 0.854 0.001 1S-Kr 1.785 3.572 0.854 0.004 b 2S-He 1.810 3.361 0.866 0.001 b 2S-Ne 1.812 3.019 0.864 0.011 b 2S-Ar 1.812 4.410 0.866 0.002 2S-Krb 1.811 3.562 0.866 0.005 In-1S-He 1.777 2.796 0.839 0.005 In-1S-Ne 1.777 2.678 0.810 0.036 In-1S-Ar 1.826 2.828 0.396 0.080 In-1S-Kr 1.878 2.936 0.336 0.209 a

Because S is connected to three carbon atoms, the average value of theses distances was reported In all 2S structures, the reported value is the average value of parameters related to both sulfur atoms

b

3.2. Adsorption energies and solvation effects One of the most important parameters for each interaction is adsorption energy. It is advantageous to estimate the probability and strengths of the interaction using adsorption energy. The adsorption energies (shown in Table 2) for all complexes in the gas phase were calculated using 6-31G and 6-31G(d) basis sets of optimizations and 6-311+G(d) basis set for single point calculations. Moreover, considering solvation effects in the adsorption energies is useful to relate our systems to the real cases. Therefore, adsorption energies in three solvent (benzene, acetone and cyclohexane) were calculated by PCM model using the highest basis set (6-311+G(d)) and


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the results were shown in Table 2. It should be noticed that in the real sensor experiments, based on the nature and solubility of investigated molecule, various solvents could be employed. Therefore, since rare gases are non-polar species, we selected two non-polar solvents (benzene with ε=2.271 and cyclohexane with ε=2.017) to study of the solvent effects. In addition, to consider the effect of polar solvent for possible case, acetone (with ε=20.493) was selected as a solvent with moderate polarity because high polar solvents (such as water, methanol, acetonitrile) are not suitable for both CNT and rare gases. Table 2. Adsorption energies (kcal/mol) of RGs on and into the SCNTs using different basis sets in the gas phase and three solvent using PCM model in comparison with the gas phase adsorption energies of simple CNT Gas phase

Solvent using 6-311+g(d) basis set

Molecule

6-31G

6-31G(d)

6-311+G(d)

1S-He

-0.19

-0.21

-0.30

a

Cyclohexane

Acetone

Benzene

-0.29

-0.29

-0.29

1S-Ne

-3.48

-3.26

0.31

0.33

0.45

0.34

1S-Ar

-0.16

-0.18

-0.11

-0.11

-0.11

-0.11

1S-Kr

-4.05

-1.02

0.01

0.02

0.02

0.02

2S-He

-0.39

-0.41

-0.64

-0.62

-0.63

-2.07

2S-Ne

-6.51

-6.17

0.42

0.52

0.61

0.53

2S-Ar

-0.80

-0.83

-0.25

-0.11

0.12

-0.08

2S-Kr

-8.27

-2.62

0.19

0.42

0.82

0.46

In-1S-He

32.52

33.55

34.02

33.79

33.28

33.75

In-1S-Ne

68.68

68.46

80.69

80.12

79.01

80.03

In-1S-Ar

230.45

232.48

232.25

233.15

235.26

233.30

In-1S-Kr

271.30

282.26

304.50

305.40

307.53

305.55

CNT-He

-0.28

-0.30

-0.32

CNT-Ne

-2.64

-2.50

0.28

CNT-Ar

-0.45

-0.48

-0.29

CNT-Kr 2.45 2.90 3.59 a These energies obtained from the 6-311+G(d) single point calculations after 6-31G(d) geometry optimizations

The results obtained from the calculations using 6-31G and 6-31G(d) basis sets are nearly the same but they are different from the results of 6-311+G(d) calculations. Therefore, we discuss about the results of 6-311+G(d) calculations in this section because this basis set is more precise than the other employed basis sets and its data is more valuable. In the gas phase data, all


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adsorption processes are between -0.30 and 0.31 kcal/mol for 1S complexes, between -0.64 and 0.42 kcal/mol for 2S complexes and between 34.02 and 304.50 kcal/mol for IN-1S complexes. These values show partially favorable adsorption of RGs in the gas phase because even we have positive values, the adsorption is possible with less equilibrium constants and only adsorptions with highly positive Ead are impossible. In addition, the values of Ead for different RGs are in this order: Ne>Kr >Ar>He. This means that the He and then Ar are the best RGs for adsorption by SCNTs though the differences between them are negligible (the difference between the Ead of the best and the worst case is 0.61 kcal/mol). This is noticeable that adsorption energies have not related to the size of RGs. In 2S complexes, the order of adsorption energies for 2S structures are the same with the order of 1S complexes and these values are nearly twice of adsorption values for 1S structures because these values are related to the adsorption of two RGs. In In-1S complexes, because of the small size of SCNT cavity, the adsorption of RGs are so endothermic and unfavorable (34.02-304.50 kcal/mol) and this endothermicity increases by the increase of RG size, especially for Ar and Kr. However, if we increase the size of SCNT to avoid spatial hindering, these quantities might be tended to negative values. By considering solvent effect, the adsorption energies (their values and orders) in different solvents are nearly the same with each other and with gas phase values. Therefore, it could be said that solvent has not important effect on the adsorption of RGs by SCNTs. To complete this sections, it could be useful to compare this results with the results of adsorption energies by simple CNT. It should be mentioned that there were only two studies related to the adsorption of RGs on the surface of CNT by molecular dynamic simulations and by reviewing the literature, any research about the quantum mechanical study of adsorption of RGs on the simple or doped CNTs have not been observed.62,63 Therefore, the interaction of RGs with simple CNT (with


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similar structure with the studied SCNT) have examined and the results were added to the end of Table 2. Moreover, the study of adsorption of RGS on the other doped CNTs (by N, B, …) is time-consuming, need to independent studies to be fully investigated and could be followed in the future projects. The obtained adsorption energies of RGs on CNT show that for He, Ne and Ar, CNT and SCNTs have nearly the same Ead values and both of them are the same in these processes. However, SCNT is more appropriate than CNT in adsorption of Kr because SCNT has smaller (less positive) adsorption energies than CNT (Ead for adsorption of Kr by CNT and SCNT are respectively 3.59 and 0.01 kcal mol from the 6-311+G(d) calculations). 3.3. QTAIM analyses Quantum Atom in Molecule (AIM) analyses are useful tools to study the intermolecular interactions. In Table 3, the electron density (ρ) and Laplacian of electron densities (▼2ρ) were shown for the bond critical points placed between S-doped carbon nanotubes and the RGs in all complexes. The positive values for the Laplacian of electron density confirmed that there are close-shell (non-covalent) interactions between SCNTs and RGs. The Laplacian of ρ (▼2ρ) values are in the range of 8.18×10-3-4.57×10-2 e/a05 for 1S and 2S structures and between 1.27×10-1-1.88×10-1 e/a05 for In-1S structures. Table 3. Electron densities (ρ) and laplacian of electron densities (▼2ρ) for interaction of SCNTs with RGs ▼2ρ (e/a05) Molecules ρ (e/a03) 1S-He 2.70E-03 1.06E-02 1S-Ne 8.04E-03 4.57E-02 1S-Ar 2.55E-03 8.18E-03 1S-Kr 7.77E-03 2.54E-02 2S-He 2.59E-03 1.04E-02 2S-Ne 7.96E-03 4.48E-02 2S-Ar 3.31E-03 1.06E-02 2S-Kr 7.80E-03 2.58E-02 In-1S-He 2.37E-02 1.27E-01 In-1S-Ne 3.54E-02 1.72E-01


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In-1S-Ar In-1S-Kr

4.15E-02 4.54E-02

1.88E-01 1.87E-01

*X is Rare-Gas such as: He, Ne, Ar, Kr

The values of electron density were in the range of 2.55×10-3-8.04×10-3 e/a03 for 1S and 2S structures and between 2.37×10-2-4.57×10-3 e/a05 for In-1S structures. For both ρ and ▼2ρ, 1S and 2S structures have nearly the same values. Therefore, it could be said that doping more sulfur atoms has not negative effect on the adsorption processes. In addition, the highest and the least values in both ρ and ▼2ρ have observed respectively in Ne and Ar complexes. For In-1S structures, the values of ρ and ▼2ρ increase by the increase of RG size and all of these values are larger than those in 1S complexes. However, despite the small size of cavity and maximum force employed on RGs, all ▼2ρ values of In-1S complexes are positive and covalent bond between RG and SCNT was not observed. 3.4. NBO analyses NBO calculations are useful tools to calculate molecular properties such as hybridizations, atomic charges (listed in Table 1) and the interactions between different parts of molecule (second-order perturbation energies, E2) with high precision. The most important acceptor– donor second order perturbation energies (in kcal/mol) for interaction of S-doped carbon nanotubes with RGs extracted from NBO calculations are listed in Table 4. Table 4. The most important acceptor–donor second order perturbation energies (in kcal/mol) for interaction of SCNTs with RGs (LP is lone pair and RY is Rydberg orbitals). Hybridizationb

Interactions

Molecules

C

S

LPRG→σ*c-s

LPRG→ RY*S

1S-He

3.43

4.15

0.05

0.24

1S-Ne

3.42

4.15

0.72

0.46

1S-Ar

3.43

4.15

0.06

0.11

1S-Kr

3.41

4.16

0.36

0.75

2S-He

3.66

4.57

0.09

0.23

2S-Ne

3.67

4.57

0.23

0.25

2S-Ar

3.66

4.57

--

0.12a

2S-Kr

3.63

4.59

0.20

0.46


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a

In-1S-He

3.42

4.10

--

0.16a

In-1S-Ne

3.43

3.97

0.13

0.11

In-1S-Ar

3.01

5.46

0.16

--

In-1S-Kr

3.01

6.47

0.34

0.18

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LPNG→σ*c-c

b

the reported values refer to n in spn hybridization

Noticeably, NBO charges and E2 energies show the RG is donor and SCNT is acceptor in these interactions. The quantities of these electron transfers are not so promising and all E2 values are smaller than 1 kcal/mol. The electrons were transferred from the LP of the RGs (LPRG) to the Rydberg orbital of the S (RY*S) atom with the energy in the range of 0.11-0.75 kcal/mol and from the LPRG to the σ* antibonding orbitals of C-S bonds (σ*c-s) with the E2 values in the range of 0.05-0.72 kcal/mol. Although, there is not important difference between the E2 values of 1S and 2S complexes. More importantly, the E2 values in In-1S complexes are in the range of other complexes. These values show us that the adsorption on the surface and into the SCNTs is energetically the same. This will be useful when we know that because of the small size of the SCNT cavity we could not obtain any relation between the calculated values (other than E2 values) and the adsorption type (into or on the surface). The results of hybridizations for RG and S atoms were also shown in Table 4. The values reported in the table refer to the n in spn hybridized orbital. In both sulfur and carbon atoms, these values are larger than 3 because of participating the pure p orbital in these interactions in addition to hybridized orbitals. The share of p orbital in sulfur atoms is larger than that in carbon atoms. Moreover, these values in 1S structures (4.15-4.16 for S and 3.41-3.43 for c) are smaller than 2S structures (4.57-4.59 for S and 3.42-3.67 for C) while these differences have not important changes on the strength of adsorption (see QTAIM and Ead data). The share of p orbital in In-1S structures are decreased for carbon atoms and increased extensively for sulfur atom.


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3.5. Population analysis and DOS plots Molecular orbital populations for all structures were calculated using population analyses to examine their reactivates and conductivities (based on the HOMO-LUMO energy gap, Eg). The shapes of HOMO and LUMO for complexes with He and Kr were depicted in Fig. 2 and for all structures were shown in Fig S1 (to save space). In All complexes, the orbital densities of both FMOs are placed on SCNT part. In 1s structures, the orbital density in HOMOs are kept out from the RGS and in LUMOs are directed toward RGs. To obtain more information from these calculations, the energies of HOMO and LUMO (in eV), the energy gap between them (Eg) and the reactivity parameters such as chemical potential (µ), chemical hardness (η), global softness (S) and electrophilicity index (ω) were calculated for all structures and listed in Table 5. Moreover, density of states (DOS) plots were depicted in Fig. 3 to show more details about the electronic configurations of all structures. Comparing alone SCNTs with their complexes shows an increasing in energy gap when the SCNT interacts with RGs. In both 1S and 2S structures ELUMO vales are identical in SCNTs and SCNT-RGs while the EHOMO decreased. The values of EHOMO and ELUMO in 1S and 2S complexes are nearly the same, which shows the quantity of doping has not important effect on these properties. However, 1S (without RG) has smaller EHOMO and higher Eg than 2S that shows the effect of number of doped S atoms on the electronic properties. Comparing various RGs, only 1S-He complex has small difference with the other RGs (less EHOMO and higher Eg values). Therefore, these results show that the interaction of SCNTs with RGs decrease their reactivities. These changes are more intensive in In-1S structures versus adsorption of RGs on the surface of SCNTs.


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1S-He-LUMO

In-1S-He-LUMO

2S-He-LUMO

1S-He-HOMO

In-1S-He-HOMO

2S-He-HOMO

1S- Kr -LUMO

In-1S- Kr -LUMO

2S- Kr -LUMO

1S-Kr-HOMO

In-1S- Kr -HOMO

2S- Kr -HOMO

Figure 2. The shapes of HOMO and LUMO of SCNTs in interaction with He and Kr (the shapes for all rare gases could be found in supplementary documents) Table 5. The energies of HOMO and LUMO, energy gap (Eg), energy of Fermi level (EFL) and reactivity parameters for SCNTs and their complexes with RGs. All energy values are reported in eV. Molecules EHOMO EFL (Φ) ELUMO Eg ω µ η S 1S -0.181 -0.134 -0.088 0.093 -0.158 0.047 21.28 0.266 1S-He -0.199 -0.144 -0.088 0.111 -0.144 0.055 18.025 0.187 1S-Ne -0.191 -0.139 -0.088 0.102 -0.139 0.051 19.514 0.189 1S-Ar -0.191 -0.140 -0.088 0.102 -0.140 0.051 19.518 0.190 1S-Kr -0.191 -0.139 -0.088 0.102 -0.139 0.051 19.531 0.190 2S -0.169 -0.126 -0.083 0.086 -0.148 0.043 23.262 0.255 2S-He -0.192 -0.139 -0.085 0.107 -0.139 0.053 18.728 0.180


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2S-Ne 2S-Ar 2S-Kr In-1S-He In-1S-Ne In-1S-Ar In-1S-Kr

-0.191 -0.192 -0.192 -0.190 -0.192 -0.217 -0.215

-0.138 -0.139 -0.138 -0.139 -0.139 -0.156 -0.155

-0.084 -0.085 -0.085 -0.088 -0.086 -0.095 -0.095

0.107 0.107 0.107 0.102 0.106 0.122 0.120

-0.138 -0.139 -0.138 -0.139 -0.139 -0.156 -0.155

0.053 0.053 0.053 0.051 0.053 0.061 0.060

18.704 18.721 18.713 19.643 18.847 16.420 16.711

0.177 0.180 0.178 0.189 0.183 0.199 0.200

The order of chemical potential (µ) values are 2S-RGs>1S-RGs>2S>1S, which the interaction of SCNTs with RGs and increasing the number of doped sulfur atoms decrease their reactivities and probably increase their stabilities. The chemical hardness of structures decrease by the number of doped atoms while interestingly increase when they interact with RGs and the reverse order could be found in softness values. Therefore, maximum softness could be observed for 2S and the minimum is belonging to 2S-RGs. Moreover, the electrophilicity indexes (ω) are mainly decrease in complexes versus simple SCNTs and decreases by the increasing number of doped atoms. Therefore, 2S-RGs have minimum ω values and 1S has the maximum value. This is noticeable that in all complexes, reactivity indexes are not so different when the RG is changed.

1S-He

2S-He

In-1S-He

1S-Ne

2S- Ne

In-1S- Ne


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1S-Ar

2S-Ar

In-1S-Ar

1S-Kr

2S-Kr

In-1S-Kr

Figure 3. Density of states (DOS) plots of SCNTs with RGs

In the final part of this report, the density of states (DOS) plots (Fig. 3) were depicted for all complexes. These plots show that except for In-1S-Ar and In-2S-Kr, the energy gaps for all structures are between 0.102-0.111 eV and all values are so closed together. In addition, 2S complexes have more Eg vales versus 1S complexes and both have larger values than simple SCNTs. Therefore, it could be said that interaction of SCNTs with RGs change their energy gaps and also their electronic properties, these changes might be useful to design new sensors for RGs based on SCNTs, in addition to employ the ability of SCNTs in adsorption of RGS.

4. CONCLUSION In this work, density functional theory (DFT), QTAIM and NBO calculations were employed to study the adsorption of RGs by SCNTs. Different number of doped atoms and four RGs (He, Ne, Ar and Kr) were considered for their adsorption on the surface and into the SCNTs. The adsorption energies indicated that RGs could be adsorbed on the surface of the S-doped carbon nanotubes. Partial charges showed the small charge transfer from RGs to SCNTs. The Ead values in both gas and solvent for different RGs are in this order: Ne>Kr>Ar>He for adsorption on the


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surface of SCNTs. This means that the He and then Ar are the best RGs for adsorption by SCNTs. QTAIM calculations showed that the positive values for the Laplacian of electron density confirmed that there are close-shell (non-covalent) interactions between SCNTs and RGs. The NBO calculations showed that the electrons were transferred from the LP of the RGs (LPRG) to the Rydberg orbital of the S (SRY) atom and from the LPRG to the σ* antibonding orbitals of C-S bonds (σ*C-S). Finally, population analyses were performed to obtain electronic properties of all structures. From these calculations, it was found that the interaction of SCNTs with RGs decrease their reactivities. These changes are more intensive in In-1S structures versus adsorption of RGs on the surface of SCNTs. Moreover, reactivity parameters and density of states plots (DOS) were calculated for all systems to obtain more insight about these interactions and examine their sensor properties. Since the interaction of SCNTs with RGs change their energy gaps and their electronic properties, these changes might be useful to design new sensors for RGs. Corresponding Author * Hossein Tavakol: Department of Chemistry, Isfahan University of Technology, Isfahan 8415683111, Iran. Phone: +98-31-33913241. Fax: +98-31-33912350; Email: [email protected]. ACKNOWLEDGMENT We are grateful from National High-Performance Computing Center (NHPCC) at Isfahan University of Technology (http://nhpcc.iut.ac.ir) for providing computational facilities (Rakhsh supercomputer) for this work. This work also has been supported by the research affair of Isfahan University of Technology (IUT).

ABBREVIATIONS


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BCP, bond critical point; CNTs, carbon nanotubes; CVD, chemical vapor deposition; DFT, Density Functional Theory; DOS, density of states; HOMO, highest occupied molecular orbital; LUMO, lowest unoccupied molecular orbital; NBO, natural bond orbital; QTAIM, quantum theory of atom in molecule; SCNTs, sulfur doped carbon nanotubes.

Supporting information are available for HOMO and LUMO shapes of all structures. This information is available free of charge via the internet at http://pubs.acs.org


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Table of Contents graphic image


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DFT, QTAIM, and NBO Study of Adsorption of Rare Gases into and on

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