Encapsulation and Adsorption of Halogens into Single-Walled Carbon Nanotubes

Functionalisation of single-walled carbon nanotubes (SWNTs) with atoms and molecules has the potential to prepare charge–transfer complexes for numerous applications. Here, we used density functional theory with dispersion correction (DFT + D) to examine the encapsulation and adsorption efficacy of single-walled carbon nanotubes to trap halogens. Our calculations show that encapsulation is exoergic with respect to gas-phase atoms. The stability of atoms inside SWNTs is revealed by the charge transfer between nanotubes and halogens. Encapsulation of halogens in the form of diatomic molecules is favourable with respect to both atoms and diatomic molecules as reference states. The adsorption of halogens on the outer surfaces of SWNTs is also exothermic. In all cases, the degree of encapsulation, adsorption, and charge transfer is reflected by the electronegativity of halogens.


Introduction
Carbon nanotubes have been studied for the last three decades for their applications in nanoelectronic devices [1][2][3][4], energy storage devices [5][6][7][8], biology, and medicine [9][10][11][12]. As they exhibit good chemical, thermal, and mechanical properties, they are used as building blocks for designing supramolecular arrays. This type of design is expected to modify the structural and electronic properties of nanotubes.
Carbon nanotubes and graphene have been effectively considered for the removal of surfactants [33][34][35]. Surfactants should be removed before they enter the environment as they can cause skin irritation [36]. Many experimental studies are available in the literature addressing the functionalisation of carbon surfaces to adsorb surfactants [37][38][39]. Zelikman et al. [40] used molecular simulations to investigate the interactions between SWNTs and surfactants. Enhancement in the binding energy was noted with the adsorption of a benzoic ring with the graphitic surface.
Halogens are also candidate species that should be considered for the encapsulation or adsorption with nanotubes as they are toxic to humans at their exceeded level. Molecular bromine and iodine are important radioactive fission products that are released from nuclear plants. In a previous simulation study [41], the interaction of single F and Cl atoms has been studied. In a combined experimental and theoretical study [42], adsorption of ICl, In this study, we used spin-polarized density functional theory together with dispersion correction to study the encapsulation and adsorption of halogens (of Cl, Br, and I) in the form of atoms and molecules the three different armchair SWNTs [(8,8), (9,9) and (10,10)]. Dispersion correction is important for atoms or molecules to interact noncovalently with nanotubes. The current simulation method enabled us to calculate encapsulation or adsorption energies, the charge transfer between halogens and nanotubes, and electronic structures of encapsulated or adsorbed composites.

Computational Methods
We used plane wave-based DFT simulations, as implemented in the Vienna Ab initio simulation package (VASP) code [43]. The exchange-correlation term was included in the form of generalised gradient approximation (GGA) described by Perdew, Burke, and Ernzerhof (PBE) [44]. The valence electronic configurations for C, Cl, Br, and I were 2s 2 2p 2 , 3s 2 3p 5 , 4s 2 4p 5 , and 6s 2 6p 5 , respectively. A plane-wave basis set with a cut-off of 500 eV and the projected augmented wave (PAW) potentials [45] were used. For the pristine tubes and atom-encapsulated or -adsorbed tubes, a 1 × 1 × 4 Monkhorst-Pack [46] k-point mesh was used. Structure optimisations were performed using a conjugate gradient algorithm [47], together with Hellman-Feynman theorem including Pulay corrections. Forces on the atoms were smaller than 0.01 eV/Å in all relaxed configurations. Dispersive attractive interactions were modelled using a semi-empirical pair-wise force field, as implemented in the VASP code [48].
Periodic boundary conditions were applied to all three nanotubes along the c axis, and a minimum lateral separation of 30 Å between adjacent structures in the other two directions was maintained. Encapsulation energy was calculated using the following Equation (1): where E (X@SWNT) is the total energy of a halogen atom (X = Cl, Br and I) encapsulated within an SWNT; E (SWNT) and E (X or 1 2 X 2 ) are the total energies of an SWNT and an isolated gas-phase halogen atom in the form of atomic or molecular state. A similar equation was also used for the calculation of adsorption energy. Bader charge analysis [49] was carried out to determine the charge transfer between tubes and halogen atoms. The density-of-states (DOS) plots were used to calculate the electronic structures of pristine SWNTs and halogen-encapsulated or -adsorbed SWNT composites.

Encapsulation of Halogen Atoms within SWNTs
First, we considered the encapsulation of halogen atoms within SWNTs. Relaxed structures of Cl encapsulated within (8,8), (9,9), and (10,10) SWNTs and corresponding charge density plots are shown in Figure 1. Relaxed structures of Br and I atoms encapsulated within SWNTs are similar to those presented in Figure 1 and are provided in the supplementary information (ESI, Figures S1 and S2). In all cases, halogen atoms are closer to the centre of the nanotubes.
Micro 2021, 1, FOR PEER REVIEW First, we considered the encapsulation of halogen atoms within SWNTs. structures of Cl encapsulated within (8,8), (9,9), and (10,10) SWNTs and corres charge density plots are shown in Figure 1. Relaxed structures of Br and I atoms lated within SWNTs are similar to those presented in Figure 1 and are provide supplementary information (ESI, Figures S1 and S2). In all cases, halogen atoms a to the centre of the nanotubes. The stability of single halogen atoms was examined by calculating encapsul ergies. In Table 2, we provide encapsulation energies calculated with respect to halogen atoms and diatomic molecules and Bader charges on encapsulated ha oms. Encapsulation energies calculated with respect to gaseous atoms are negativ ing that they are stable inside the SWNTs. However, encapsulation energies are in all cases with respect to dimers as references. This is because of the addition thermic energy required to break dimers to form gaseous atoms. The degree of lation decreases from Cl to I with any of the three SWNTs. This is clearly due to that the electronegativity order of halogen is Cl > Br > I. Electronegativity values and I are 3.16, 2.96, and 2.66, respectively [52]. When the size of the nanotube i the encapsulation energy of a particular halogen decreases as expected. For exam capsulation energies of Cl in (8,8), (9,9), and (10,10) tubes are −1.15 eV, −1.00 eV, a eV, respectively. The importance of inclusion of dispersion is indicated by the hi ues of encapsulation energies than those calculated without dispersion. In all small amount of charge is transferred from SWNTs to halogen. The amount of observed to be dependent on the electronegativity of halogen. The charge tran creases when the diameter of SWNTs increases as the distance between the halo the tube wall increases. The stability of single halogen atoms was examined by calculating encapsulation energies. In Table 2, we provide encapsulation energies calculated with respect to gaseous halogen atoms and diatomic molecules and Bader charges on encapsulated halogen atoms. Encapsulation energies calculated with respect to gaseous atoms are negative, meaning that they are stable inside the SWNTs. However, encapsulation energies are positive in all cases with respect to dimers as references. This is because of the additional endothermic energy required to break dimers to form gaseous atoms. The degree of encapsulation decreases from Cl to I with any of the three SWNTs. This is clearly due to the fact that the electronegativity order of halogen is Cl > Br > I. Electronegativity values of Cl, Br, and I are 3.16, 2.96, and 2.66, respectively [52]. When the size of the nanotube increases, the encapsulation energy of a particular halogen decreases as expected. For example, encapsulation energies of Cl in (8,8), (9,9), and (10,10) tubes are −1.15 eV, −1.00 eV, and −0.92 eV, respectively. The importance of inclusion of dispersion is indicated by the higher values of encapsulation energies than those calculated without dispersion. In all cases, a small amount of charge is transferred from SWNTs to halogen. The amount of charge is observed to be dependent on the electronegativity of halogen. The charge transfer decreases when the diameter of SWNTs increases as the distance between the halogen and the tube wall increases.  The calculated density of plots for (8,8) and (9,9) tubes are shown in Figure 2. Both tubes exhibit metallic character in agreement with the fact that armchair tubes (n,n) are metallic.   The calculated density of plots for (8,8) and (9,9) tubes are shown in Figur tubes exhibit metallic character in agreement with the fact that armchair tubes metallic.  (9,9) tubes. Figure 3 shows the calculated total DOS plots of Cl, Br, and I encapsulate (9,9) tube and atomic DOS plot of Cl, Br, and I. SWNTs still exhibit metallic char  (9,9) tubes. Figure 3 shows the calculated total DOS plots of Cl, Br, and I encapsulated within (9,9) tube and atomic DOS plot of Cl, Br, and I. SWNTs still exhibit metallic character.

Adsorption of Halogen Atoms on the Surfaces of SWNTs
Next, halogen atoms were allowed to adsorb on the surfaces of SWNTs. Three possible sites (H, 66, and C) were considered (see Figure 4). In configuration H, the atom is positioned above the centre of the hexagonal ring. Atoms are positioned above the centre of the bond connecting adjacent six-membered rings and the three coordinated carbon in 66 and C configurations.  (9,9) tubes. Figure 3 shows the calculated total DOS plots of Cl, Br, and I encapsulat (9,9) tube and atomic DOS plot of Cl, Br, and I. SWNTs still exhibit metallic char Figure 3. Calculated total DOS plots for (a) Cl@ (9,9), (b) Br@ (9,9), and (c) I@ (9,9). Corresponding atomic DOS plots for Cl, Br, and I (d-f) are also shown.

Adsorption of Halogen Atoms on the Surfaces of SWNTs
Next, halogen atoms were allowed to adsorb on the surfaces of SWNTs. Three poss ble sites (H, 66, and C) were considered (see Figure 4). In configuration H, the atom i positioned above the centre of the hexagonal ring. Atoms are positioned above the centr of the bond connecting adjacent six-membered rings and the three coordinated carbon i 66 and C configurations. All three configurations were fully relaxed for the (8,8) tube. Relative energies ar shown in Table 3. The lowest energy structure is calculated to be the C configuration.

Configuration
Relative Energy (eV) C 0.00 66 0.02 H 0.05 Next, we considered the adsorption of halogen atoms on the surfaces of all thre SWNTs. The relaxed structures of halogens adsorbed on the (9,9) tube are shown in Figur 5. Adsorption is exoergic with respect to gas-phase atoms (refer to Table 4), meaning tha atoms are stable on the surface of SWNTs. Stronger adsorption is calculated with disper sion correction in all cases. The diameter of the nanotube does not significantly change th adsorption energies. Negative Bader charges on the halogen atoms indicate that there is charge transfer from tubes to halogen atoms. The electronegativity of halogen reflects i the amount of charge transferred. This is further confirmed by the C-X distances, as liste in Table 4. All three configurations were fully relaxed for the (8,8) tube. Relative energies are shown in Table 3. The lowest energy structure is calculated to be the C configuration. Next, we considered the adsorption of halogen atoms on the surfaces of all three SWNTs. The relaxed structures of halogens adsorbed on the (9,9) tube are shown in Figure 5. Adsorption is exoergic with respect to gas-phase atoms (refer to Table 4), meaning that atoms are stable on the surface of SWNTs. Stronger adsorption is calculated with dispersion correction in all cases. The diameter of the nanotube does not significantly change the adsorption energies. Negative Bader charges on the halogen atoms indicate that there is a charge transfer from tubes to halogen atoms. The electronegativity of halogen reflects in the amount of charge transferred. This is further confirmed by the C-X distances, as listed in Table 4.   Figure 6 shows the total and atomic DOS plots calculated for Cl, Br, and I a sorbed on the surface of (9,9) tube. All three resultant complexes exhibit metallic c as calculated for pristine nanotubes. The Fermi energy levels are partly occupie p-state halogen atoms (see Figure 6).

Structures Adsorption Energy (eV)
Bader Charge |e| C-X (Å) X (Cl or Br or I)  Figure 6 shows the total and atomic DOS plots calculated for Cl, Br, and I atoms adsorbed on the surface of (9,9) tube. All three resultant complexes exhibit metallic character, as calculated for pristine nanotubes. The Fermi energy levels are partly occupied by the p-state halogen atoms (see Figure 6).  Figure 6. Calculated total DOS plots for (a) Cl@ (9,9), (b) Br@ (9,9), and (c) I@ (9,9). Correspond atomic DOS plots for Cl, Br, and I (d-f) are also shown.

Encapsulation of Molecular Halogens inside SWNTs
Here, we examine the formation of halogen dimers inside SWNTs. Two different p sible configurations (along the tube axis and perpendicular to the tube axis) were cons ered. Relaxed structures of Cl2, Br2, and I2 molecules encapsulated within SWNTs shown in Figure 7. Table 5 reports the encapsulation energies, Bader charges on halog molecules, and intermolecular distances of halogens.   (9,9), (b) Br@ (9,9), and (c) I@ (9,9). Corresponding atomic DOS plots for Cl, Br, and I (d-f) are also shown.

Encapsulation of Molecular Halogens inside SWNTs
Here, we examine the formation of halogen dimers inside SWNTs. Two different possible configurations (along the tube axis and perpendicular to the tube axis) were considered. Relaxed structures of Cl 2 , Br 2, and I 2 molecules encapsulated within SWNTs are shown in Figure 7. Table 5 reports the encapsulation energies, Bader charges on halogen molecules, and intermolecular distances of halogens.

Encapsulation of Molecular Halogens inside SWNTs
Here, we examine the formation of halogen dimers inside SWNTs. Two different possible configurations (along the tube axis and perpendicular to the tube axis) were considered. Relaxed structures of Cl2, Br2, and I2 molecules encapsulated within SWNTs are shown in Figure 7. Table 5 reports the encapsulation energies, Bader charges on halogen molecules, and intermolecular distances of halogens.

Structures
Encapsulation Energy (eV)/X Atom Bader Charge |e| X-X (Å) X (Cl or Br) X 2 (Cl or Br) Encapsulation energies are negative with respect to both atom and molecule as reference states indicate that the formation of dimers is possible inside SWNTs. The inclusion of dispersion strengthens the encapsulation. The electronegativity of halogens also influences encapsulation. Stronger encapsulation is reflected by higher electronegative halogen. Bader charge analysis shows that dimers are polarised in most cases, and the total charge on the molecules is very small. There is only a small energy difference between the two configurations (along the axis and perpendicular to the axis). Isolated dimer distances of Cl 2 , Br 2 , and I 2 in this study are 1.99 Å, 2.32 Å, and 2.69 Å, respectively. Encapsulation has a small effect on dimer distances. Elongation in the dimer distances increases with the size of the molecules.

Adsorption of Molecular Halogens on the Surfaces of SWNTs
Finally, we considered the stability of dimers adsorbed on the surfaces of SWNTs. Figure 8 shows the relaxed structures of Cl 2 , Br 2 , and I 2 adsorbed on the (9,9) tube. Table 6 lists the adsorption energies, Bader charges on the molecules, and dimer distances.

Adsorption of Molecular Halogens on the Surfaces of SWNTs
Finally, we considered the stability of dimers adsorbed on the surfaces of SWNTs. Figure 8 shows the relaxed structures of Cl2, Br2, and I2 adsorbed on the (9,9) tube. Table 6 lists the adsorption energies, Bader charges on the molecules, and dimer distances.  In all cases, adsorption is exothermic with respect to gas-phase atoms and dimers. The highest adsorption energy is calculated for the Cl due to its highest electronegativity. Bader charge analysis shows that dimers are polarised, and there is a negligible charge transfer between tubes and molecules. Dimer distances in the relaxed configurations are closer to the distances calculated in the isolated dimers.

Conclusions
In this study, we used DFT simulation, together with dispersion, to examine the encapsulation and adsorption of gas-phase halogen atoms and dimers. Strong encapsulation energies were calculated for all three halogen atoms with respect to their gas-phase atoms. A significant charge transfer from nanotubes to halogens was noted in all cases. Diatomic molecules were also stable inside SWNTs with respect to both atoms and diatomic molecules as reference states. Exothermic adsorption energies were calculated for all three halogens, meaning that they can be trapped via the outer surfaces of SWNTs. Electronegativity of halogens determines the nature of encapsulation or adsorption and the amount of charge transferred between SWNTs and halogens.