PHYSICAL REVIEW B 85, 085426 (2012)

Chlorination of carbon nanotubes

Dogan Erbahar* and Savas BerberPhysics Department, Gebze Institute of Technology, Gebze, Kocaeli 41400, Turkey

(Received 3 October 2011; revised manuscript received 25 January 2012; published 21 February 2012)

We report ab initio density functional theory calculations for chlorinated single-wall carbon nanotubes andinvestigate the atomic structure, energetics, and electronic structure of the chlorinated nanotubes, as well as theenergetics of the desorption reaction. We find that the Cl atoms should be adsorbed in pairs and thus focus ondoubly chlorinated nanotubes. Using the terminology of arene substitution patterns, ortho and para configurationsare the most stable. The physisorption is preferable to the chemisorption in large-diameter nanotubes. Theimpurity states appear near the Fermi level EF in the electronic structure and may alter the electronic propertiesconsiderably. The bonding character for adsorption outside the nanotube is mainly covalent, but inside it consistsof physical bonding. The adsorption of several Cl atoms inside a carbon nanotube leads to the formation of acharged Cl chain. Our calculated desorption barrier of �1.4 eV per Cl atom pair indicates that the cleansing bychlorination is a less damaging alternative with removable residue.

DOI: 10.1103/PhysRevB.85.085426 PACS number(s): 61.48.De, 81.07.De, 61.46.−w, 68.65.−k

I. INTRODUCTION

Nanotubes1 are generally purified by a postprocessingsince pristine nanotube samples contain catalyst particles andamorphous overcoating.2 Such processes usually involve harshchemicals and elevated temperatures and are responsible fromthe elimination of small-diameter nanotubes.2,3 In contrast, thetreatment of nanotubes by halogen gases,4 such as Cl2, standsout as a method that should not damage the carbon nanotubesas much. The correctness of this naive expectation stronglydepends on the nature of the interaction between the chlorineatoms and the carbon nanotube.

The physisorption of chlorine on a carbon nanotube isnot expected to make big changes in its physical properties.However, if the chlorine binds to the nanotube throughchemisorption, there should be significant changes in the phys-ical properties, especially in the electronic structure, whichis modified extensively by other halogens.5 We suggest thatcontrolling the physical properties of the nanotubes by chlori-nation, similar to fluorination experiments,6,7 may be possible.

Carbon nanotubes can be chlorinated by alkyl halides orCl2 gas,8 and they can decompose halogenized compounds.9

The functionalization of carbon nanotubes by alkanes wasfound to improve the solubility of carbon nanotubes invarious solvents.10 The binding and electronic structure ofchlorinated nanotubes were studied by x-ray absorption andscanning photoelectron microscopy.11 In theoretical works,the adsorption of a single Cl atom was considered.12–14 It isknown that covalently binding adsorbates should prefer to beadsorbed in pairs on carbon nanotubes,15 but the adsorptionof additional chlorine atoms were not studied before. Inthis work, we focus on the adsorption of a second Cl atomnear the first adsorption site and check whether there isa clustering tendency of the adsorbates. The linear defectdensity dependence of the adsorption energetics has also beeninvestigated. The adsorptions of Cl inside and outside thenanotube are compared. In addition, the energy landscapeduring desorption of 2 neighboring Cl atoms, which form aCl2 molecule after desorption, has been calculated.

The organization of this paper is as follows: A briefdescription of our calculation method, and computational

details are presented in Sec. II, followed by our resultsin Sec. III. The bonding character of the chlorine atomsinteracting with the nanotube is discussed in Sec. III A.Modification of atomic and electronic structures in chlorinated(5,5), (9,0), and (10,0) nanotubes are presented in Secs. III B 1,III B 2, and III B 3, respectively. After presenting the energeticsof chlorine desorption in Sec. III D, we summarize our findingsin Sec. IV.

II. COMPUTATIONAL METHOD

Our geometry optimization and total energy calculationsare based on the density functional theory16,17 within thelocal density approximation (LDA), as implemented in theSIESTA18,19 code. We used Perdew-Zunger20 parametrizationfor the exchange-correlation functional, and a double-ζ basisset augmented by polarization orbitals. The interaction be-tween the core and valence electrons is handled by Troullier-Martins norm-conserving pseudopotentials21 in their fullyseparable form.22

In our calculations, we use (5,5), (9,0), and (10,0) nan-otubes, which are metallic, small-gap semiconducting, andsemiconducting systems, respectively. To describe isolatednanotubes while using periodic boundary conditions, wearrange nanotubes on a tetragonal lattice with a large intertubeseparation of 20 A. In the supercell, we include 3 primitivecells for zigzag nanotubes and 6 primitive cells for armchairnanotubes in order to minimize the interaction between theadsorbates. We use a k-point sampling that is the equivalentof 36 k points in the 1D Brillouin zone of a primitive unitcell of an armchair nanotube. Charge density and potentialsare determined on a real-space mesh that corresponds to theplane wave cutoff energy of 200 Ry. Optimized geometries areobtained in a conjugate-gradient algorithm without symmetryconstraints until all force components on each atom are lessthan 0.01 eV/A. The possibility of a magnetic orderingis investigated using the local spin density approximation(LSDA). The reaction energetics during desorption of 2 nearbyCl atoms, which form a Cl2 molecule after desorption, issearched by constrained structure optimizations.

085426-11098-0121/2012/85(8)/085426(7) ©2012 American Physical Society

DOGAN ERBAHAR AND SAVAS BERBER PHYSICAL REVIEW B 85, 085426 (2012)

bridge

on-tophexagon-

center

(a) (b)

FIG. 1. (Color online) (a) Adsorption sites of a Cl atom on ahoneycomb lattice. (b) Optimized atomic configuration of the Clatom adsorbed on a (5,5) nanotube is shown in top and side view.The light color denotes the C atoms, and a darker color is used forthe Cl atoms.

III. RESULTS AND DISCUSSION

We first investigate the adsorption of a single Cl atom inorder to determine initial geometries for successive chlorineabsorptions. There are 3 different sites for a Cl atom to beadsorbed on a honeycomb lattice: on-top, bridge, and onhexagon center positions, which are depicted in Fig. 1(a). Ourstructure optimizations are started with these 3 different atomicconfigurations of a Cl atom on a (5,5) nanotube to obtain allstable and metastable adsorption configurations. The relaxedatomic structures show only on-top adsorption geometries,indicating that the adsorptions on other sites are unstable. Wetherefore consider only the on-top adsorption position in ourcalculations.

When a Cl atom is adsorbed on the (5,5) nanotube, theclosest C atom to the adsorbed Cl atom moves outward thetube wall as shown in Fig. 1(b). This C atom acquires ansp3 hybridization that results in pyramidalization around this4-coordinated C atom. The C-C bond lengths increase from≈1.42 A to ≈1.5 A around this C atom, and the C-Cl bondlength is found to be ≈1.932 A. This increased C-C bondlength is another indicator of the sp3 hybridization.

The rehybridization disrupts the sp2 bonding near the Cl ad-sorbate, and thus the carbon atoms near the first adsorption siteare expected to be more reactive. Therefore, we concentrate onthe adsorption of additional Cl atoms with the anticipation ofbetter adsorption energetics. Although the increased activityenhances the probability of populating adjacent adsorptionsites, the Cl atoms may become too close to each otherand form physisorbed diatomic molecules. Therefore, in thefollowing we compare the total energies of adsorbed chlorineatom pairs to this competing phase.

A. The bonding character between the Cl adsorbates andthe carbon nanotube

In Fig. 2, the valence charge density contour plots inplanes perpendicular to the tube axis are shown. The chargedensity contours allow us to determine the character of bondingin chlorinated carbon nanotubes. Atomic structure for eachcontour plot is shown on the left, and the plane on whichthe contours are drawn is indicated by a dashed line. Thevalence charge density of a single chlorine atom adsorbed ona (5,5) nanotube, shown in Fig. 2(a), demonstrates a covalentcharacter.

(a)

y)(r

30/ ae

0.1

0.2

0.3

(b)

y

0.1

0.2

0.3

(c)

y

0.1

0.2

0.3

)(r

)(r

0.0

0.0

0.0

Cl

C

Cl

C

Cl

C

-5

0

5

y

-5

0

5

-5

0

5

0

0 5-5

5-5x

yy

FIG. 2. (Color online) The valence charge density ρ(r) in a planeperpendicular to the nanotube axis. The ρ(r) (a) for a single adsorbedCl atom, (b) for a pair of Cl atoms adsorbed on top of 2 adjacentC atoms, and (c) for an adsorbed Cl2 molecule. The scales for thecontours are displayed in right panels. The planes, on which thecontours are drawn, are indicated by dashed lines on the ball-and-stickmodels, which are shown in the left panels. The C atoms and Cl atomsare distinguished by the light and the dark colors, respectively.

The valence charge density for 2 chemically bonded Clatoms on a (5,5) carbon nanotube is shown in Fig. 2(b). Theplane on which the contour is drawn encloses the centers ofthe Cl atoms. The charge density contour plot in Fig. 2(b)suggests a covalent character. The C-Cl bond lengths are in therange of 1.9–2.0 A, indicating that the adsorption is throughchemisorption since it is similar to the values for a single Clatom adsorbed on a (5,5) nanotube. The existence of nonzeroelectron densities in the region connecting the 2 Cl atoms inFig. 2(b) hints at an interaction between the adsorbates, whichmay become important at higher Cl coverages.

The chemical bonding that is illustrated in Fig. 2(b) cannotbe achieved at high Cl coverages. In our calculations, weachieve higher coverages through increasing either the axialadsorbate density or the circumferential adsorbate density.

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CHLORINATION OF CARBON NANOTUBES PHYSICAL REVIEW B 85, 085426 (2012)

In both cases, the residual Cl-Cl interaction between theadsorbate atoms leads to energetically degenerate adsorp-tion geometries that consist of both chemisorption andphysisorption.

The charge density contours for a Cl2 molecule physisorbedon a (5,5) nanotube at the distance of 2.9 A is shown in Fig. 2(c)for comparison, where the plane on which the contours aredrawn contains the centers of chlorine atoms.

We also investigate the chlorine adsorption inside carbonnanotubes, and find that the formation of a chemical bond withthe inner wall is inhibited by curvature effects and that thecharge density plot, not shown here for simplicity, is similar tothat of the physisorbed Cl2 molecule. Our Mulliken populationanalysis indicates that there is a charge transfer from chlorineadsorbates to the carbon nanotube. Thus, the bonding shouldalways have some ionic contribution.

B. Atomic and electronic structure of chlorinated nanotubes

We first determine the required axial size of the supercellfor negligible interadsorbate interaction by calculating thelinear defect density dependence of the adsorption geometries.We find that the atomic structure and the adsorption energyresults converge at the axial length value of ≈7 A. In thefollowing, we present the results obtained for the supercellsthat are longer than ≈14 A for the sake of a better convergence.We concentrate on the adsorption of Cl pairs since our initialcalculations showed that the adsorption of the second Cl atomnear a previously adsorbed Cl atom is more favorable by≈0.5 eV than the adsorption of a single Cl atom. We believethat the adsorption of Cl pairs is more relevant to experimentsfor carbon nanotube chlorination.

We define the adsorption energy per chlorine atom as Eads =1n{E(nCl-CNT)−E(NT)−nE(Clatom)}, where E(nCl-CNT) is

the total energy of n Cl atoms adsorbed on the carbon nanotube,E(NT) the total energy of the isolated carbon nanotube, andE(Clatom) the total energy of an isolated Cl atom. Total energyof an isolated Cl atom E(Clatom) is taken from a spin-polarizedcalculation.

Since a competing processes to the adsorption of Clatom pairs is the physisorption of a Cl2 molecule aftertheir desorption, we often compare Eads values for differentchemisorption configurations of Cl pairs to the Eads valuefor physisorption. In the case of physisorption, the adsorptionenergies per chlorine atom Eads show almost the same valueindependent of nanotube diameter and adsorption orientation.We use dispersion correction23 and basis set superpositionerror correction for the physisorption of a Cl2 molecule on thenanotube. Our corrected adsorption energy Eads has the valueof ≈ − 2.08 eV/atom.

1. Chlorinated (5,5) armchair nanotube

In armchair nanotubes, we identify 4 unique adsorptionpositions for the second Cl atom in the vicinity of the firstadsorbed Cl atom and define them in Fig. 3(a), where the red(dark) 3D ball is indicating the position of the first adsorbed Clatom. We use the terminology of the arene substitution patternsin order to refer the position of the second Cl atom with respectto the first adsorption site. The two ortho positions, named as

(a) (b)

(c)

(d)

(e)

(f)

ortho1

ortho2

para

meta

-5

-3

-1

1

3

5

0 10 20

E-E

F (

eV)

DOS (arb. units)

ortho2

-5

-3

-1

1

3

5

0 10 20

E-E

F (

eV)

DOS (arb. units)

para

-3

-2

-1

EF

1

2

3

Γ X

Ene

rgy

(eV

)

k

-3

-2

-1

EF

1

2

3

Γ X

Ene

rgy

(eV

)

k

-3

-2

-1

EF

1

2

3

Γ X

Ene

rgy

(eV

)

k

FIG. 3. (Color online) The modification of the physical propertiesof a (5,5) nanotube by the adsorption of a pair of Cl atoms. (a) Ournomenclature for the adsorption position of the second Cl relativeto the first Cl atom for an armchair nanotube. The first Cl atomis the red (dark) ball. (b) The electronic structure of a pristine(5,5) nanotube is shown for comparison. The electronic structureof the chlorinated (5,5) nanotube for (c) ortho2 configuration and for(d) para configuration. The charge density that corresponds to theelectronic states of the peak in DOS is depicted as an isosurface for(e) ortho2 and (f) para positions. The energies are given with respectto the Fermi level EF , which is indicated by the dashed line.

ortho1 and ortho2, differ by the angle between the tube axisand the line connecting the Cl atoms.

Our calculated adsorption energies per atom Eads of −2.30eV/atom for both ortho2 and para positions indicate thatthese are the most favorable adsorption configurations. SinceEads for a single Cl atom is −2.07 eV/atom, the Cl shouldprefer to be adsorbed in pairs. The adsorption energy Eads forchemisorption is better by −0.22 eV per Cl atom than thatfor the physisorption. Therefore, the chemisorption is slightlymore preferable to the physisorption on a (5,5) nanotube. Theadsorption energy per atom Eads for the ortho1 configurationis −2.02 eV/atom, and Eads for meta configuration is −1.93eV/atom. Since the energetics of the meta configuration is not

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DOGAN ERBAHAR AND SAVAS BERBER PHYSICAL REVIEW B 85, 085426 (2012)

better than that of the physisorption it is less likely to occur inthermodynamic equilibrium.

When a single Cl atom is adsorbed on a (5,5) nanotube,the C-Cl bond length is 1.93 A. The C-Cl bond lengthsfor the most favorable adsorption geometries of ortho2 andpara configurations are ≈1.82 A and ≈1.87 A, respectively,indicating a stronger bond formation for the adsorption of a Clpair. As expected from the sp3 rehybridization, the C-C bondlengths around the four-coordinated carbon atoms increasefrom the value of ≈1.42 A to the value of ≈1.49–1.56 A.

The band structure of a pristine (5,5) nanotube is shownin Fig. 3(b) for comparison. The band structure of ortho2configuration, presented in the left panel of Fig. 3(c), preservesthe metallic character of a pristine (5,5) nanotube. However,the details of the electronic structure are far from being similarto that of the pristine tube. A close inspection of the electronicdensity of states (DOS), shown in the right panel of Fig. 3(c),reveals an occurrence of a defect state above the Fermi levelEF . The band lines above the Fermi level EF seem to be theresult of a mixing between the pristine nanotube states and thisdefect state. In order to visualize the spatial distribution of thedefect state, we calculated the charged density that correspondsto the electronic states of the peak in DOS by summing up thesquares of wave functions within a narrow energy interval. Theisosurface of this charge density, shown in Fig. 3(e), indicatesthat the localized defect state of the C-Cl bond is completelymixed with the extended nanotube states.

The double chlorination of a (5,5) nanotube in the paraposition opens a small gap in the electronic structure as shownin Fig. 3(d). An impurity state above the Fermi level EF resultsin a peak in DOS. Unlike in the ortho2 configuration, the bandstructure in the left panel of Fig. 3(d) suggests that the defectstate does not mix with the extended π states of the pristinenanotube. The charge density that corresponds to the electronicstates of the peak in DOS for para position is depicted as anisosurface in Fig. 3(f), where the localized character of thedefect state is apparent. The calculated spatial distribution ofthe impurity state indicates contribution from p orbitals aroundthe adsorbed Cl atoms.

Defect states in ortho2 and para configurations must havedifferent origins. There is a substantial Cl-Cl interactionin the ortho2 position. However, in the para configuration,the rehybridization allows a further separation of Cl atoms.Therefore, the defect state in the ortho2 configuration is relatedto weak Cl-Cl interaction and can be mixed with the states onboth sublattices of the nanotube. In the para configuration, thisstate remains as an unpaired electron state.

2. Chlorinated (9,0) zigzag nanotube

We use the (9,0) carbon nanotube to explore the effect ofdouble chlorination on the electronic structure of small-gapsemiconducting carbon nanotubes. We identify six relativepositions for a Cl adsorbate pair, and define our nomenclaturefor the adsorption positions on zigzag nanotubes in Fig. 4(a).The first adsorbed Cl atom is denoted by the red 3D ball andthe possible adsorption sites for the second Cl atom are labeledin Fig. 4(a).

Our calculated adsorption energies Eads of −2.25 eV/atomfor the ortho1 configuration and −2.21 eV/atom for the

-3

-2

-1

EF

1

2

3

Γ Xk

-3

-2

-1

EF

1

2

3

Γ Xk

-3

-2

-1

EF

1

2

3

Γ X

Ene

rgy

(eV

)

k

(a)

(b) (c)

ortho1

ortho2

para2meta1

(d)

para1

meta2

ortho1 para2(9,0)

FIG. 4. (Color online) The adsorption of Cl atom pairs on a (9,0)nanotube. (a) The relative adsorption positions of 2 Cl atoms ona zigzag carbon nanotube. The first adsorbed atom is denoted bya red 3D ball. (b) The calculated electronic structure of a pristine(9,0) nanotube. The electronic structure for (c) the ortho1 adsorptiongeometry and (d) para2 configuration. The energies are given withrespect to the Fermi level EF , which is denoted by the dashed line.

para2 configuration indicate that there are 2 almost degenerateadsorption geometries, which are more favorable than thephysisorption by 0.13 and 0.17 eV/atom. Since Eads for asingle adsorbed Cl atom is −2.00 eV/atom Cl atoms shouldbe adsorbed in pairs also on a small-gap semiconducting nan-otube. The para1 adsorption geometry with −2.13 eV/atomadsorption energy is less favorable than the most stableadsorption configurations and slightly more favorable thanthe physisorption. Because both meta1 and meta2 geometrieshave the adsorption energy Eads of ≈ − 1.91 eV/atom, theyare only metastable and are not likely to occur. It is notpossible to obtain a stable chemisorption geometry in theortho2 configuration because the strain energy that occursduring the transformation of some C atoms from sp2 to sp3

hybridization is not compensated by the new bonds.The band structure of the pristine (9,0) nanotube, shown

in Fig. 4(b), has a narrow energy gap and the bands near theFermi level EF are doubly degenerate. The band structuresfor the most stable adsorption geometries of the ortho1and para2 configurations are displayed in Figs. 4(c) and4(d), and demonstrate that this degeneracy is lifted after thechlorination. Similar to the electronic structure of chlorinated(5,5) nanotubes, a defect state appears above the Fermi levelEF for both ortho1 and para2 geometries. The low dispersionof the defect-related state suggests that this state should resultin small charge mobility in electron-doped systems.

3. Chlorinated (10,0) zigzag nanotube

In order to investigate double-chlorinated semiconductingnanotubes, we use the (10,0) zigzag nanotube, which is

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CHLORINATION OF CARBON NANOTUBES PHYSICAL REVIEW B 85, 085426 (2012)

-3

-2

-1

EF

1

2

3

Γ Xk

-3

-2

-1

EF

1

2

3

Γ Xk

-3

-2

-1

EF

1

2

3

Γ X

Ene

rgy

(eV

)

k

(a) (b) (c)

ortho1 para2(10,0)

FIG. 5. (Color online) The modification of the electronic structureof a (10,0) zigzag nanotube by double chlorination. (a) The bandstructure of the pristine (10,0) nanotube. A gap state appears in theelectronic structure of the chlorinated (10,0) nanotube for both (b)ortho1 and (c) para2 adsorption geometries. The energies are givenwith respect to the Fermi level EF , which is denoted by the dashedline.

semiconducting as shown in Fig. 5(a), where the Fermilevel EF is chosen to be the midpoint of the band gap. Weobtained the optimized structures of doubly chlorinated (10,0)nanotubes using the adsorption configurations that are definedin Fig. 4(a).

The adsorption energy Eads for a single adsorbed Cl atom ona (10,0) nanotube is calculated to be −1.73 eV/atom, whichis higher by ≈0.30 eV/atom than those for (5,5) and (9,0)nanotubes. The reactivity of the (10,0) nanotube is lower sinceits diameter is bigger and it is a semiconducting nanotube. Thesame trend in the adsorption energetics is expected also forthe adsorption of Cl atom pairs. However, the energetics ofphysisorption is not significantly affected by diameter increaseor variations in the electronic structure.

We find that the adsorption energies Eads for the ortho1,para1, and para2 configurations are −2.06 eV/atom, −2.03eV/atom, and −2.10 eV/atom, respectively. Since these en-ergy values are comparable to the Eads value of −2.08 eV/atomfor the physisorption, the chlorination is lightly favorable tothe physisorption only in the best adsorption configuration ofpara2. Moreover, the 2 meta configurations have Eads valuesof −1.7 eV/atom, making the meta configurations highlyunlikely.

The energetics of the chlorination is severely affectedby the diameter increase and the introduction of an energygap in the electronic structure. In contrary, the energetics ofphysisorption changes very little by diameter increase. Wetherefore suspect that the physisorption of Cl2 molecules ispreferred to the chemisorption of Cl atoms for larger nanotubediameter values.

The electronic structures of pristine and double-chlorinated(10,0) nanotubes are presented in Fig. 5, where only themost stable configurations of ortho1 and para2 are chosen.The most striking feature in the electronic structures of thechlorinated (10,0) nanotubes, shown in Figs. 5(b) and 5(c), isthe appearance of a defect state inside the band gap, which wasreported also in hydrogenated nanotubes.24 After the doublechlorination, the band degeneracies are removed because ofbroken translational symmetry. The midgap defect states mayact as acceptor levels. In all our calculations, the possibility of

(a)

y

)(r30/ ae

0.1

0.2

0.3(b)

y

(c))(r

Cl

C

Cl

CCl

C

y

)(r

-5 0 5-5 0 5-5 0 5

-5

0

5

x x x

y

FIG. 6. (Color online) The adsorption of Cl atoms inside thecarbon nanotubes. The bonding character is demonstrated by thevalence charge density ρ(r) contour plots in the planes perpendicularto the tube axis for (a) the (5,5) nanotube, (b) the (9,0) nanotube, and(c) the (10,0) nanotube. Common contour scale is given in the rightpanel of (c). Corresponding atomic structures are shown as insets,where the planes in which the contour plots are drawn are denoted bydashed lines.

a spin-polarized solution is searched for, but the most stableconfigurations of double-chlorinated nanotubes do not havespin-polarized self-consistent solutions.

In the most stable adsorption configurations, the Cl atomsadsorb on different sublattices of the graphitic surface. Whena single atom is adsorbed on a nanotube, a vacancy is formedin one of the sublattices of the sp2 network, and an unpairedπ level on the other sublattice appears near the Fermi level. Ifthe next atom is adsorbed on the other sublattice this unpairedπ level is shifted down and the total energy is lowered.

C. Chlorine adsorption inside carbon nanotubes

We next investigate the successive adsorption of Cl atomsinside the carbon nanotubes, starting from the adsorption ofa single atom. We find that even a single Cl atom, which issupposed to have a dangling bond and to be reactive, cannotform a chemical bond with the nanotube from inside. In orderto present the state of the encapsulated Cl atom in the carbonnanotube, the valence charge densities are shown as contourplots for (5,5), (9,0), and (10,0) carbon nanotubes in Fig. 6,where ball-and-stick models of the atomic structures are shownas insets and the planes on which the contours are drawnare indicated by dashed lines. The lack of covalent bondingis apparent from the near-zero charge densities in the spacebetween the Cl and the nanotube wall. Thus, the energy gainby the encapsulation of an atom is originated from physicalbonding, which is partly because of the charge transfer fromthe nanotube to the Cl atom.

As shown in Fig. 6, the Cl atom stays on the tube axisinside the (5,5) and (9,0) nanotubes, but it shows an off-axisdisplacement in the (10,0) nanotube since the diameter of the(10,0) nanotube is larger. A close inspection of the electronicstructures for single atoms encapsulated inside nanotubesreveals that a charge transfer from the nanotube to the Cl atomoccurs. Thus, the bonding must be ionic for the adsorptioninside the nanotube.

We define the encapsulation energy per chlorine atomas Eenc = 1

n{E(nCl@CNT)−E(NT)−nE(Clatom)}, where

E(nCl@CNT) is the total energy of n Cl atoms encapsulatedinside the carbon nanotube, E(NT) the total energy of theisolated carbon nanotube, and E(Clatom) the total energy of anisolated Cl atom.

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DOGAN ERBAHAR AND SAVAS BERBER PHYSICAL REVIEW B 85, 085426 (2012)

The encapsulation energy Eenc of a single Cl atom inside(5,5), (9,0), and (10,0) nanotubes are −2.53 eV/atom, −2.40eV/atom, and −1.79 eV/atom respectively. The encapsulationof the Cl atom in snug-fit nanotubes (5,5) and (9,0) isenergetically better since the interaction area is maximizedin these nanotubes. Our calculated encapsulation energiesindicate that adsorption inside the nanotube is favorable onlyin (5,5) and (9,0) nanotubes.

Although the adsorption of individual Cl atoms is notfavorable in large-diameter nanotubes, larger Cl species, suchas small clusters, may be adsorbed inside the nanotube.Therefore, we continue successive encapsulation of Cl atoms.When a second atom is encapsulated inside the nanotube, 2Cl atoms form a Cl2 molecule, and it forms a physical bondwith the nanotube. When this Cl2 molecule is encapsulated,we observe that the Cl-Cl bond length increases from ≈2 A to≈2.3–2.4 A in (9,0) and (5,5) nanotubes while the bond lengthincrease is only ≈0.06 A in the (10,0) nanotube. We attributethis bond length increase to charge transfer from the nanotubeto the encapsulated species. Since there are no levels in theband gap of semiconducting nanotubes, the charge transferin the semiconducting nanotube is quenched, and the bondalteration in this case is not significant.

When 3 or more number of Cl atoms are encapsulatedinside the carbon nanotube, we find that the Cl-Cl bonds areweakened by the charge transfer also in this case. Since thedefect levels appear in the valence band for the encapsulationof more than 2 Cl atoms, the charge transfer exists even insemiconducting nanotubes when 3 or more Cl atoms are insidethe nanotube.

The atomic structure of 3 Cl atoms in our carbon nanotubesis a homogenous linear chain with the Cl-Cl bond lengthsof 2.32 A. Starting with the encapsulation of 4 Cl atomsinside the nanotube, an expanded Cl chain interacting with thenanotube through physical bonding is formed. The structureof this chain is not homogenous because of charging effects.For example, the Cl-Cl bond length in the center differsfrom the bond lengths at the ends. In this 4-atom Cl chainencapsulated in (5,5) and (9,0) nanotubes, the Cl-Cl distanceat the ends is ≈2.5–2.6 A while it is ≈2.3 A in the center. Inthe semiconducting (10,0) nanotube, which shows a smallercharge transfer, the Cl-Cl distance at the ends is ≈2.4 A.The adsorption of several Cl atoms inside a carbon nanotubethus leads to the formation of a charged Cl chain that isinteracting with the nanotube through physical instead ofchemical bonding. Since (5,5) and (9,0) nanotubes are narrowthis chain is linear in (5,5) and (9,0) nanotubes. However, itbecomes a zigzag chain in larger diameter nanotubes, such asin the (10,0) nanotube.

D. The energetics of desorption

Last, we investigate the reaction energetics for desorp-tion of a Cl atom pair. We consider that the 2 Cl atomsform a Cl2 molecule after desorption. In this reaction,the C-Cl bond breaking and Cl-Cl bond formation occurssimultaneously.

Because of the symmetry in the system, the distancebetween the Cl atom and its closest C neighbor d is takenas the same for both Cl atoms during the reaction, and we use

(a) (b)

physisorbed

chemisorbed−0.6

−0.4

−0.2

0.0

0.2

0.4

0.6

0.8

1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6

ΔE (

eV)

reaction coordinate (Å)

(5,5) nanotube(9,0) nanotube

FIG. 7. The energetics of desorption. (a) Schematic of 2 Cl atomsadsorbed on a nanotube, where the reaction coordinate d is shown.(b) The change in the total energy �E versus the reaction coordinated while 2 Cl atoms are desorbed and form a Cl2 molecule. Thereference energy is chosen as the total energy of the system when theCl2 molecule is physisorbed.

this distance d as the reaction coordinate, shown schematicallyin Fig. 7(a). The reaction coordinate is varied in steps of0.1 A from the value of the equilibrium distance for thechemisorption to the value of 3.5 A, and constrained structureoptimizations are performed.

This procedure is repeated for all the adsorption positionsof the (5,5) and (9,0) nanotubes. However, the atomicstructure falls to one of the more stable configurations ifthe initial structure is metastable and the transition-statesearch fails for metastable configurations. Therefore, thegeometries that are found to be metastable in our staticadsorption energy calculations are not likely to occur be-cause of dynamic reasons in addition to the pure energeticsreasons.

The reaction energetics for the (5,5) and (9,0) nanotubesare shown in Fig. 7(b). The zero energy level is chosen as thetotal energy of the system when a Cl2 molecule is physisorbedon the nanotube since it is the same for both nanotubes. Thesolid circles are used for the (5,5) nanotube, and the opencircles for the (9,0) nanotube in Fig. 7(b), where the linesare the spline fits to the data. The �E values for the (5,5)and (9,0) nanotubes differ in the attractive region, and thesaddle point is reached at the reaction coordinate of 2.4 A.The energy barrier for desorption is found to be ≈1.2–1.4 eVper Cl atom pair. We also calculate the reaction energeticsusing the generalized gradient approximation (GGA)25 andconstrained structure optimizations. The energy landscape onthe physisorption side changes by changing the exchange-correlation functional. However, we find the same desorptionenergy barrier in GGA perhaps due to cancellation of errors inthe chemisorption side of the energy landscape. Constrainedenergy minimizations may overestimate energy barriers. Thus,our energy barrier value of ≈1.4 eV is the upper limit for thedesorption energy barrier. It is expected to be lowered in largerdiameter nanotubes.

Using the Arrhenius formula with a typical attempt fre-quency of 1013 Hz, which is of the order of phonon frequencies,it is concluded that the desorption may be achieved atmoderate temperatures. Therefore, the removal of the chlorineimpurities may be achieved with minimal heating damage tothe nanotubes. Our results indicate that the cleaning of as-grown nanotubes by chlorination is a less damaging alternativeto current harsh methods.

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IV. SUMMARY AND CONCLUSIONS

In summary, we investigate the atomic structure, energetics,and the electronic structure of double-chlorinated carbonnanotubes as well as the energetics of the desorption re-action. We find that the Cl atoms prefer to be adsorbedin pairs and thus concentrate on the adsorption of 2 Clatoms on the carbon nanotube. We use the terminology ofarene substitution patterns and identify ortho, meta, and paraadsorption configurations. Ortho and para are most stableconfigurations while meta configurations are only metastable.The chemisorption energies are reduced by the increasingdiameter while the values for the physisorption do not changesignificantly. Therefore, the physisorption of Cl2 is preferableto the chemisorption of Cl atoms in large-diameter nanotubes.

The impurity states in the electronic structure of chlorinatednanotubes appear near the Fermi level for metallic and small-gap semiconducting nanotubes, and thus the electronic densityof states show peaks near the Fermi level. The chlorinationopens small energy gaps in the electronic structure of metallicnanotubes. In semiconducting nanotubes, the impurity levels

appear as midgap levels, which should make noticeablechanges in the electrical properties.

The Cl is adsorbed inside the nanotube by physical bonding,and there is a charge transfer from the nanotube to theencapsulated Cl atoms. The adsorption of several Cl atomsinside a carbon nanotube leads to the formation of a chargedCl chain, which shows the modulation of Cl-Cl distances asexpected in a charged chain system.

Our calculated desorption energy barriers are less thanthe value of 1.4 eV for a pair of Cl atoms. The removal ofchlorine impurities may be achieved with minimal heatingdamage to the nanotubes. The cleaning of as-grown nanotubesby chlorination is a less damaging alternative to current harshmethods.

ACKNOWLEDGMENTS

This work was funded by the TUBITAK under Grant No.108T740. Computational resources have been provided bythe Michigan State University High Performance ComputingCenter.

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