Chemical Physics Letters 534 (2012) 43–47

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Chemical Physics Letters

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Fluorination and defluorination of carbon nanotubes: A nanoscale perspective

JungHo Kang a, Dharmpal Takhar a, Oleksandr V. Kuznetsov b, Valery N. Khabashesku c,⇑, Kevin F. Kelly a,⇑a Department of Electrical and Computer Engineering, Rice University, 6100 Main Street, Houston, TX 77005, USAb Department of Physics and Astronomy, Rice University, 6100 Main Street, Houston, TX 77005, USAc Department of Chemical and Biomolecular Engineering, University of Houston, 4800 Calhoun Avenue, Houston, TX 77204, USA

a r t i c l e i n f o a b s t r a c t

Article history:Received 30 December 2011In final form 6 March 2012Available online 14 March 2012

0009-2614/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cplett.2012.03.026

⇑ Corresponding authors. Fax: +1 713 743 4323 (V.5686 (K.F. Kelly).

E-mail addresses: valery@uh.edu (V.N. KhabashKelly).

In this Letter, scanning tunneling microscopy (STM) was used to study the sidewall fluorination and thereverse process of defluorination of single wall carbon nanotubes (SWNTs) and double wall carbon nano-tubes (DWNTs). The same single SWNT was imaged in situ before and after defluorination to show thediameter changes and tube cuttings, and these confirm previously reported statistical shortening ofSWNTs. In comparison, the STM image of annealed fluorinated DWNT reveals the inner tube to remainintact while the outer tube is cut. Finally, the ex situ Raman spectroscopy was used to confirm the fluo-rination and defluorination processes.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

Single-wall carbon nanotubes (SWNTs) have shown potentialfor novel electronic devices [1], and double-wall carbon nanotubes(DWNTs) have aroused interest among scientists due to their phys-ical structure resembling bilayer graphene, which possesses inter-esting quantum behavior [2,3]. Theoretical [4–8] and experimental[9–17] studies with various functional groups have reported themodified chemical and electronic properties of carbon nanotubes(CNTs) with functionalization. Due to the high reactivity, fluorinecan readily functionalize SWNTs [18–20], and it has been reportedthat sidewall fluorination can enhance the low solubility of bareSWNTs [21,22]. Fluorine atoms can form covalent bonds with thecarbon atoms without interrupting the tube-like structure, andthus can significantly increase the resistivity of CNTs. In addition,the fluorination can be reversed by heat treatment to remove theattached fluorine [19].

Most studies on fluoronanotubes have been performed withSWNTs, but recently successful fluorination of DWNTs as well asMWNTs have been reported [23,24]. Fluorination of DWNTs canbe particularly interesting because under controlled reaction con-ditions only the carbon atoms in the outer tubes participate inthe fluorination while keeping inner tubes intact [23]. PristineSWNTs and DWNTs are thermally stable up to 1200 �C [25] and2000 �C [26] respectively, while the fluorinated CNTs (F-CNTs) startto lose their thermal stability already at �200 �C [19]. These fea-tures indicate that F-CNTs perhaps can be used for novel electronic

ll rights reserved.

N. Khabashesku), +1 723 348

esku), kkelly@rice.edu (K.F.

devices that require temperature sensitive materials. Understand-ing the detailed physical structure changes of F-CNTs should openup a range of applications.

In this work we have observed with an in situ STM imaging thatdesorption of fluorine through annealing can recover the originalSWNT structure to a certain extent and will cut the SWNTs inthe radial direction thus obtaining a direct evidence for the SWNTcutting proposed earlier on basis of ex situ AFM studies of annealedfluoronanotubes [27]. Similarly, we have observed that fluorineatoms on the outer-wall of a DWNT were removed and created de-fects in the same manner, while leaving the inner nanotube intact.

2. Experimental section

2.1. Material preparation

As starting CNT materials, we used commercially availableSWNTs and purified DWNTs. SWNTs were purchased from MERcompany. DWNTs, synthesized by a catalytic CVD method and sub-sequently purified by HCl treatment and air oxidation at 500 �C,were provided by Endo and Kim [28]. The CNTs were fluorinatedby 20% F2/80% He gas mixture in a Monel reactor for 2 h at150 �C (SWNTs) and 200 �C (DWNTs). The average C/F ratios inthe obtained F-SWNT and F-DWNT products estimated from theXPS analysis data were approximately 2/1 and 3/1, respectively.XPS data were collected with PHI Quantera X-ray photoelectronspectrometer using a monochromatic Al Ka radiation source(hm = 1486.6 eV) with a power setting of 95.4 W and analyzer passenergy of 26 eV. STM experiments were performed with anOmicron variable temperature ultra-high vacuum (UHV) STMcontrolled by RHK electronics. F-CNTs were dissolved in ethanol

Figure 1. (a) Deconvolution of high resolution C 1s XPS spectrum of F-DWNTs, curve-fitted to reveal different bonding states of carbon: (1) sp2 carbon in C@C units, (2) semi-ionic and covalent CAF bonding, (3) C(@O)AF bonding (since O@CF2 is the first volatile product to be formed during annealing of F-CNTs) [27] and (4) CF2 bonding. (b)Deconvolution of high resolution F 1s XPS spectrum of F-DWNTs: (1) semi-ionic CAF bonding, (2) covalent CAF bonding, (3) C(@O)AF bonding (c) ATR-FTIR spectrum of F-DWNTs. The peak at 1758.4 cm�1 is due to C(@O)F moieties produced by fluorination of COOH groups introduced into DWNTs during purification, while peaks at 2362.4,2335.0, and 1623.8 cm1 are due to CO2 and moisture impurities.

44 J. Kang et al. / Chemical Physics Letters 534 (2012) 43–47

by bath-sonicating for 10 min which produced homogeneoussolution. Usually, about 5 lL of this solution was spun cast ontoAu(111) substrates. UHV STM was also used for thermal treatmentof the sample at 400 �C and in situ imaging. Confocal Raman spec-troscopy was performed with the WiTec alpha300 S with 785 nmexcitation on the same samples as used in STM.

2.2. Characterization

To confirm the existence of CAF bonding in fluorinated DWNTs,X-ray photoelectron spectroscopy (XPS) was performed. Accordingto XPS analysis, the C 1s and F 1s peak positions at 289.5 and688.5 eV, respectively, confirmed the presence of CAF covalentbonding in fluorinated DWNTs. Deconvolution of C 1s peak inFigure 1a reveals four main peaks, that represent sp2 carbon (peak1), semi-ionic CAF (peak 2), covalent CAF (peak 3) and covalent CF2

(peak 4). This analysis indicates that fluorine is semi-ionically andcovalently bonded to the sidewall. Since F 1s peak is better indica-tion of the bonding type, deconvolution of F 1s peak was performedas well (Figure 1b). F 1s peak of DWNTs consists of three mainpeaks that can be attributed to semi-ionic bonding (peaks 1 and2) and covalent attachment of fluorine atoms (peak 3). This result

is in agreement with the deconvolution of C 1s peak. On average,one fluorine atom is attached to every three carbon atoms forDWNTs (C3F) while the same reaction gives C2F for SWNTs. C2Fstoichiometry corresponds to the highest possible coverage ofnanotube sidewall by fluorine as reported by Mickelson et al[18]. The broad peak at 1150 cm�1 in the attenuated total reflec-tance Fourier transform infrared (ATR-FTIR) spectrum of F-DWNTin Figure 1c confirms XPS data by showing all bond types of fluo-rine to carbon atoms on the sidewall.

3. Results and discussion

3.1. Single-wall carbon nanotubes

STM images of the same fluorinated SWNTs (F-SWNTs) in Fig-ure 2 show different diameter values depending on the sidewall re-gions which indicate that the fluorine chemisorption is nothomogeneous, as also shown by the banding structures. The pro-posed origin of this banded structure is the boundary of two differ-ent domains of F-SWNT isomers [29]. F-SWNTs show a much largerdiameter than bare SWNTs based on increasing coverage offluorine.

Figure 2. STM images of fluorinated SWNTs: C2F, and C16F for comparison. Morefluorination results in greater increases in height measured in STM images.

21nm

(a)

3

2

1

0

Hei

ght (

nm)

(c)

Figure 3. STM images of a fluorinated SWNT (a) before defluorination and (b) after defluoscale image highlighting with a blue box the high resolution areas. (c) Cross-section of (arecovered atomic resolution from a pristine region. (For interpretation of the references to

J. Kang et al. / Chemical Physics Letters 534 (2012) 43–47 45

Previous studies on the thermal treatment of fluoronanotubesreported a decrease in the length of SWNT bundles [27], but a directobservation of the same individual SWNT undergoing structuralchanges through the treatment has not been reported hitherto.Here, we report a direct observation of the same single F-SWNT be-fore and after the in situ thermal treatment for 1 h at 400 �C. Figure3a and b show images of the same single F-SWNT taken both be-fore and after defluorination, with the corresponding insets show-ing that this is indeed the same region of the nanotube. It should benoted that F-SWNTs in Figure 3 look very wide due to the tip arti-fact, therefore in order to find the correct diameter we have to usethe height in the cross-section. On average, the diameter of a givenbare SWNTs ranges between 1.2 and 1.5 nm, but the diameter ofthe F-SWNT in Figure 3a is measured as 3 nm. This indicates thatthe fluorination alters the electronic structure of SWNTs consider-ably, as STM measures the density of states of the sample near theFermi energy level. It was calculated that a carbon–carbon bondlength at the chemisorption site can increase to 1.56 from 1.42 Åwith fluorination [7], and this fact as well as the modified elec-tronic states of F-SWNTs contribute to the diameter increasearound the fluorine bonding sites. After the defluorination, thediameter decreases to the value of bare SWNTs. The decrease indiameter of the defluorinated SWNT confirms the recovering ofelectronic states of the scanned SWNT. Atomic resolution on fluo-rinated SWNTs was not readily obtained, but after defluorination,which causes restoration of electric conductivity of the nanotube,atomic resolution was recovered on the defluorinated region. Fig-ure 3d shows bare regions on either side of a defect after defluori-nation at 400 �C for an hour. The defect is further proof ofpermanent nanoscale damage done to the nanotubes from thefluorination/defluorination process. Atomic resolution of the pris-

24nm

(b)

(d)

rination with thermal treatment at 400 �C for an hour. Inset in each image is a larger) and (b). (d) STM image of defluorinated SWNT showing a remnant defect as well as

colour in this figure legend, the reader is referred to the web version of this article.)

Figure 4. Raman spectra, obtained with the 785 nm laser excitation, of bareSWNTs, fluorinated SWNTs, and defluorinated SWNTs.

Figure 6. Raman spectra (785 nm laser excitation) from pristine DWNTs, fluori-nated DWNTs, and defluorinated DWNTs.

46 J. Kang et al. / Chemical Physics Letters 534 (2012) 43–47

tine nanotubes lattice is recovered away from the defect in thebare regions.

The occurrence of fluorination and defluorination has also beenconfirmed by Raman spectroscopy. Raman spectra for the differenttreatment stages of SWNT bundles are shown in Figure 4. Afterfluorination, the radial breathing mode (RBM) peak disappears be-cause the added fluorine atoms transform the sp2 aromatic carbonson the tube into an sp3-state carbons carrying ‘heavy’ fluorine sub-stituents attached to a stretched CAC bonds which yield a less flex-ible breathing of the tube. Once the fluorine atoms are thermallydesorbed, the breathing oscillation of the tube becomes relaxed,so the RBM peak around 200 cm�1 is seen to recover. StartingSWNT samples used in this study contain sidewall structural de-fects such as pentagon–heptagon pairs and also interstitials andmetallic impurities which explain why a weak D-band in Ramanspectrum of bare SWNT sample is observed.

3.2. Double-wall carbon nanotubes

Based on the XPS analysis, the chemical composition of the F-DWNT sample that is used in this study is C3F. Since only the outer

(a) (

(b)

Figure 5. STM images of the same (a) fluorinated DWNT. Image size: (26.9 � 141.6 nm2)DWNT due to the double-tip structure. (45 � 151 nm2) (c) Cross-sections of the reveale

tubes are functionalized, the chemical composition of the outerwall is approximately C2F which is the densest arrangement offluorine atoms possible. In STM images, fluorinated DWNTs (F-DWNTs) also show banding structures much like SWNTs do, andsome appear to be only partially fluorinated. The direct observationof the same DWNT before and after the defluoration process wasnot done yet, but the previous experiment with F-SWNTs indicatesthat after defluorination, the diameter observed by STM decreases.After thermal treatment at 400 �C, F-DWNTs show the same diam-eter as bare DWNTs, which is approximately 2 nm as measured bySTM. Defluorination also results in tube cutting for DWNTs asshown in Figure 5. The in situ STM shows that sometimes onlythe outer tube is cut revealing the pristine inner tube, or boththe inner and outer tubes break at the same time. In Figure 5b,the revealed inner tube is about 0.9 nm in diameter and the deflu-orinated outer wall is about 1.7 nm, which gives an intertube spac-ing of about 4 Å, in good agreement with the reported [30]measurement of �4 Å or more. The defluorinated DWNT alsoshows some sidewall defects or partially fluorinated regions asshown by the remaining banding structures.

1.6

1.2

0.8

0.4

0

0.

9 nm

1.7

nm

Hei

gh

t (n

m)

c)

, and (b) Defluorinated DWNT with revealed inner tube. Double imaging of the samed pristine inner tube and the defluorinated outer tube.

J. Kang et al. / Chemical Physics Letters 534 (2012) 43–47 47

The fluorination and defluorination of DWNTs was also ana-lyzed by Raman spectroscopy. The Raman spectrum (Figure 6)produced from F-DWNT bundles shows only RBM peak corre-sponding to the inner tubes (between 200 and 300 cm�1), butafter defluorination the outer tube RBM (between 100 cm�1

and 200 cm�1) is recovered. The relative height of D-bandaround 1300 cm�1 compared to G-band at 1590 cm�1 alsodecreases with the defluorination, which indicates that thereare less sp3 CAC(F) bonds on the outer tube left after thedefluorination.

4. Conclusion

Sidewall fluorination of SWNTs and DWNTs and defluorinationhave been studied with STM and Raman spectroscopy. The modi-fied electronic structure due to the fluorination gives rise to an in-crease in diameter and the banding structure along the tube asshown in STM images. Annealing of the fluorinated CNTs at400 �C can remove the fluorine adatoms. During this process theF-SWNTs experience cutting at the domain boundaries betweendifferent F-SWNT isomers, and this fact makes the defluorinationnot entirely recovering the original state. For F-DWNTs, we can ob-serve that when only outer tubes are cut the pristine inner tubesare revealed. Further studies of defluorination processes and con-trolled peeling off the outer shells of DWNTs can open up theopportunity for synthesis of new interesting nanostructures ofDWNTs with partially revealed inner shells.

Acknowledgements

This work was supported by Office of Naval Research (STTR con-tract N00014-06-M-0316) and the Welch Foundation (C-1605). Wethank Profs. Y.A. Kim and M. Endo at Shinshu University, Japan, forproviding double-wall nanotubes.

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