micro Article Encapsulation and Adsorption of Halogens into Single-Walled Carbon Nanotubes Navaratnarajah Kuganathan 1,* and Sashikesh Ganeshalingam 2 1 Department of Materials, Imperial College London, London SW7 2AZ, UK 2 Department of Chemistry, University of Jaffna, Sir. Pon Ramanathan Road, Thirunelvely, Jaffna 40000, Sri Lanka; [email protected] * Correspondence: [email protected] Abstract: 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. Keywords: halogens; carbon nanotubes; DFT; encapsulation; charge transfer 1. Introduction Citation: Kuganathan, N.; Carbon nanotubes have been studied for the last three decades for their applications in Ganeshalingam, S. Encapsulation and nanoelectronic devices [1–4], energy storage devices [5–8], biology, and medicine [9–12]. As Adsorption of Halogens into they exhibit good chemical, thermal, and mechanical properties, they are used as building Single-Walled Carbon Nanotubes. blocks for designing supramolecular arrays. This type of design is expected to modify the Micro 2021, 1, 140–150. https:// structural and electronic properties of nanotubes. doi.org/10.3390/micro1010011 A variety of atoms [13,14], molecules [15,16], one-dimensional nanocrystals [17–21], nanowires [22,23], and nanoribbons [24,25] have been encapsulated experimentally inside Received: 16 June 2021 the SWNTs. The filling of bulk material has led to the formation of the one-dimensional Accepted: 18 August 2021 Published: 21 August 2021 nanocrystal, which is different from the bulk structure [26,27]. For example, bulk zinc blende HgTe formed a three-coordinated tubular structure inside SWNT [28]. The outer surface of the nanotubes has been extensively studied theoretically for the adsorption Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in of a variety of molecules [29–32]. Such adsorption studies have been recognised for published maps and institutional affil- sensor applications. iations. Carbon nanotubes and graphene have been effectively considered for the removal of surfactants [33–35]. 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 [37–39]. Zelikman et al. [40] used molecular simulations to investigate the interactions between Copyright: © 2021 by the authors. SWNTs and surfactants. Enhancement in the binding energy was noted with the adsorption Licensee MDPI, Basel, Switzerland. This article is an open access article of a benzoic ring with the graphitic surface. distributed under the terms and Halogens are also candidate species that should be considered for the encapsulation or conditions of the Creative Commons adsorption with nanotubes as they are toxic to humans at their exceeded level. Molecular Attribution (CC BY) license (https:// bromine and iodine are important radioactive fission products that are released from creativecommons.org/licenses/by/ nuclear plants. In a previous simulation study [41], the interaction of single F and Cl atoms 4.0/). has been studied. In a combined experimental and theoretical study [42], adsorption of ICl, Micro 2021, 1, 140–150. https://doi.org/10.3390/micro1010011 https://www.mdpi.com/journal/micro Micro 2021, 1 141 Br2, IBr, and I2 molecules was considered. However, there are no studies available in the literature that focus on halogens interacting with carbon nanotubes. In this study, we used spin-polarized density functional theory together with disper- sion 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. 2. 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 2s22p2, 3s23p5, 4s24p5, and 6s26p5, 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): 1 E = E (X@SWNT) − E (SWNT) - E (X or X ) (1) enc 2 2 where E (X@SWNT) is the total energy of a halogen atom (X = Cl, Br and I) encapsulated 1 within an SWNT; E (SWNT) and E (X or 2 X2) 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. Three different nanotubes [(8,8), (9,9), and (10,10)] were selected. Seven unit cells were extended along the tube axis to make a supercell. Table1 lists the number of carbon atoms in each nanotube supercell and their diameters. Table 1. Diameters and number of carbon atoms in the supercells of (8,8), (9,9), and (10,10) SWNTs. Type Diameter (Å) Number of Carbon Atoms in the Super Cell (8,8) 10.86 224 (9,9) 12.21 252 (10,10) 13.57 280 The quality of the PAW potentials for C, Cl, Br, and I used in this study was reported in our previous simulation studies [50,51]. Micro 2021, 1 142 Micro 2021, 1, FOR PEER REVIEW 3 3. Results and Discussion 3.1. EncapsulationFirst, we ofconsidered Halogen Atoms the within encapsulation SWNTs of halogen atoms within SWNTs. Relaxed structuresFirst, we of considered Cl encapsulated the encapsulation within of (8,8), halogen (9,9), atoms and within (10,10) SWNTs. SWNTs Relaxed and corresponding structurescharge density of Cl encapsulated plots are shown within (8,8), in Figure (9,9), and 1. Relaxed (10,10) SWNTs structures and corresponding of Br and I atoms encapsu- chargelated densitywithin plots SWNTs are shown are similar in Figure to1. those Relaxed pres structuresented ofin Br Figure and I atoms 1 and encap- are provided in the sulated within SWNTs are similar to those presented in Figure1 and are provided in the supplementarysupplementary information information (ESI, Figures (ESI, Figures S1 and S2). S1 In and all cases, S2). In halogen all cases, atoms halogen are closer atoms are closer toto the the centre centre of the of nanotubes.the nanotubes. FigureFigure 1. Relaxed1. Relaxed structures structures of (a) Cl@(8,8), of (a) Cl@(8,8), (b) Cl@(9,9), (b) and Cl@(9,9), (c) Cl@(10,10). and (c) Corresponding Cl@(10,10). chargeCorresponding charge densitydensity plots plots (d– f()d are–f) also are shown. also shown. The stability of single halogen atoms was examined by calculating encapsulation energies.The In stability Table2, we of provide single encapsulation halogen atoms energies was calculated examined with by respect calculating to gaseous encapsulation en- halogenergies. atoms In Table and diatomic2, we provide molecules encapsulation and Bader charges energies on encapsulated calculated halogen with atoms. respect to gaseous Encapsulationhalogen atoms energies and calculateddiatomic with molecules respect to and gaseous Bader atoms charges are negative, on encapsulated meaning halogen at- thatoms. they Encapsulation are stable inside energies the SWNTs. calculated However, with encapsulation respect energiesto gaseous are positive atoms inare all negative, mean- cases with respect to dimers as references. This is because of the additional endothermic energying that required