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The synthesis of multilayer graphene materialsby the fluorination of carbon nanodiscs/nanocones

Yasser Ahmad a,b, Elodie Disa a,b, Marc Dubois a,b,*, Katia Guerin a,b, Vincent Dubois a,b,Wei Zhang a,b,c, Pierre Bonnet a,b, Francis Masin d, Loıc Vidal e, Dimitri A. Ivanov e,Andre Hamwi a,b

a Clermont Universite, Universite Blaise Pascal, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000 Clermont-Ferrand, Franceb CNRS, UMR 6296, ICCF, BP 80026, F-63171 Aubiere, Francec Ecole Centrale de Pekin, Beijing University of Aeronautics and Astronautics (BUAA), Road 37, HaiDian District, Beijing 100191, Chinad Matiere Condensee et Resonance Magnetique, Universite Libre de Bruxelles (U.L.B.), CP 223, Boulevard du Triomphe,

B-1050 Bruxelles, Belgiume Institut de Sciences des Materiaux de Mulhouse, 15, rue Jean Starcky, BP 2488, 68057 Mulhouse Cedex, France

A R T I C L E I N F O

Article history:

Received 9 February 2012

Accepted 9 April 2012

Available online 13 April 2012

0008-6223/$ - see front matter � 2012 Elsevihttp://dx.doi.org/10.1016/j.carbon.2012.04.034

* Corresponding author at: Clermont UniverClermont-Ferrand, France. Fax: +33 4 73 40 7

E-mail address: marc.dubois@univ-bpcler

A B S T R A C T

Multilayer carbonaceous nanomaterial has been synthesized using a two-step process: car-

bon nanodiscs/nanocones were fluorinated using either the direct reaction with pure F2 gas

or the thermal decomposition of solid fluorinating agent (TbF4). Then the fluorinated parts

were removed by treatment at 600 �C in air. When the fluorine atoms are homogenously

dispersed, using fluorination by TbF4, thinning due to thermal defluorination results in

multilayer materials with 7–10 nm of thickness and 400–500 nm of width. Such resulting

materials and the fluorinated precursors have been characterized by solid state NMR,

TGA, XRD, SEM, TEM, AFM and Raman spectroscopy.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction

Graphene, an infinite two-dimensional layer consisting of sp2

hybridized carbon atoms, has been attracting considerable

interest in recent years because of its unique band structure

and physical properties. Among them the extremely high car-

rier mobility makes it a promising material for applications in

the semiconductor industry, for composite materials, molec-

ular gas sensors, energy storage and conversion [1–3]. Never-

theless, graphene sheets are still difficult to fabricate. Several

methods are reported: (i) the micromechanical cleavage tech-

nique, graphene of high quality can be obtained but only in

minute quantities. Novoselov et al. [4] were able to isolate

graphene sheets using a simple micromechanical exfoliation

method placing a scotch-tape over commercial highly ordered

er Ltd. All rights reserved

site, Universite Blaise Pa1 08.mont.fr (M. Dubois).

pyrolytic graphite (HOPG). The latter technique allowed scien-

tists all over the world to isolate single and double-layered

graphene flakes (SLG and DLG). (ii) Intercalation–exfoliation

of graphite via thermal shock, acid treatment and intercala-

tion of graphite results in a few monolayer sheets (1%) and

mainly three-layer sheets. Exfoliation of natural graphite

using a liquid phase method has also been reported [5]. (iii)

‘Oxidation–exfoliation’ processes involve the synthesis of

graphite oxide (GO) and its subsequent reduction by exfolia-

tion at high temperature (thermal exfoliation), under vacuum

(vacuum exfoliation), or via chemical treatments (chemical

exfoliation) [6–8]. In particular, hydrogen induced reduction–

exfoliation of GO at a low temperature of 200 �C is reported

[9]. (iv) Heat treatments were also used, e.g. at 1600 �C of

diamond nanoparticles deposited on HOPG [10]. (v) Other

.

scal, Institut de Chimie de Clermont-Ferrand, BP 10448, F-63000

3898 C A R B O N 5 0 ( 2 0 1 2 ) 3 8 9 7 – 3 9 0 8

methods to obtain graphene flakes were reported including

its epitaxial growth over SiC substrates [11], and chemical va-

pour deposition (CVD) growth over thin metal layers [12,13].

Moreover, there is a wide spectrum of methods available to

produce carbon nanoribbons (CNRs), from CVD, through

chemical treatments of graphite to the unzipping of carbon

nanotubes: (i) intercalation-exfoliation of MWCNT, involving

treatments in liquid NH3 and lithium, and subsequent exfoli-

ation using HCl and heat treatments [14], (ii) chemical routes,

involving acid reactions that start to break carbon–carbon

bonds (e.g. H2SO4 and KMnO4 as oxidizing agents) [15], (iii)

catalytic approach, in which metal nanoparticles ‘‘cut’’ the

nanotube longitudinally like a pair of scissors [16], (iv) the

electrical method, by passing electric current through a nano-

tube [17], (v) physicochemical method by embedding the

tubes in a polymer matrix followed by Ar plasma treatment

[18], and (vi) reducing the fluorinated parts of graphene using

electron beam irradiation [19]. The resulting structures are

either CNRs or graphene sheets.

Our two-step strategy for producing multilayer graphene

materials consists first in the synthesis of sub-fluorinated

graphitized nanocarbons and then the thermal defluorination

in the air atmosphere. Sub-fluorination means that some car-

bon atoms are not fluorinated and the value x in the global

chemical composition CFx is lower than unity. A recent study

on the thermal stability of fluorinated carbon nanofibres

(CNF) [20] underlines that the fluorine atoms are thermally re-

moved from the outer tubes by a mechanism similar to a peel-

ing which preserves the non-fluorinated core. On the

contrary, if the fluorine atoms are homogenously dispersed

in the whole volume of the fibres, the gas evolution during

the thermal defluorination leads to the destructuration of

the carbonaceous matrix. As a matter of fact, the decomposi-

tion reaction of (CF)n graphite fluoride is described by the fol-

lowing reaction [21]:

2ðCFÞn ! nCþ XCF2 þ YC2F4 ð1Þ

where X + 2Y = n. The evolution of gaseous fluorocarbon spe-

cies such as CF2 and/or C2F4 during the decomposition of CxF

[22,23] and (CF)n [21–24] must be accompanied by a C–C bond

cleavage on the carbon sheets in the compounds. The decom-

position temperatures of the (CF)n samples prepared from var-

ious carbon materials is ranged from 320 to 610 �C; the higher

the graphitization degree of the carbonaceous precursor, the

higher the thermal stability, [25,26]. Moreover, in spite of sim-

ilar CFx chemical composition and C–F bonding, carbon nano-

fibres fluorinated using different ways, direct fluorination

with F2 or controlled process using solid fluorinating agent

TbF4, exhibit different thermal stabilities and different prod-

ucts after heating. Indeed, during the controlled fluorination,

the reactive species is mainly atomic fluorine FÆ and its diffu-

sion rate and reactivity allow a homogenous dispersion of the

fluorine atoms. The method using the thermal decomposition

of solid fluorinating agent (TbF4) is called controlled fluorina-

tion. Using a flux of F2 gas, first the outer tubes react and the

fluorination progresses then towards the core when the reac-

tion temperature increases. In the present study, both pro-

cesses were carried out and evaluated for the synthesis of

graphene materials. With this aim, our choice for the carbo-

naceous precursor was the graphitized nanocones/discs.

The commercially available product for nanocones/discs, pro-

vided by N-Tec Corporation, consists of a mixture of 70 weight

percent (wt.%) of nanodiscs (CND), 20 wt.% of nanocones and

10 wt.% of amorphous carbons. Due to the content of nano-

discs, the mixture will be called CND. In the present study,

fluorinated carbon nanocones/discs have been prepared from

CND post treated at 2700 �C under argon in order to increase

the graphitization degree because the order can be partly

maintained upon fluorination, although this process obliga-

tory results in a progressive disorganization of the carbona-

ceous matrix due to the accommodation of the fluorine

atoms and the change of carbon hybridization from sp2 to

sp3. Moreover, the more remarkable benefits of the use of

graphitized sample as starting material are, on one hand, that

cracks and irregularities of both cones and discs are avoided

contrary to as-prepared CND after fluorination and, in the

other hand, structural defects such as CF2 and CF3 and dan-

gling bonds, are significantly reduced [27]. Those defects act

on the thermal stability of the fluorinated CND.

Our wet chemistry method avoids the use of liquid-phase

extraction contrary to a recent work [26], for which the exfo-

liation of commercial CF�0.5 graphite fluoride in dimethyl-

formamide has been first carried out; the liquid-phase

extraction of graphene fluoride was then necessary. Finally,

a subsequent reduction of graphene fluoride with triethylsi-

lane or zinc particles results in graphene.

Carbon nanodisc/nanocones treated by direct fluorination

and by controlled fluorination using TbF4 as fluorinating

agent will be firstly compared using complementary tech-

niques including X-ray diffraction (XRD), scanning and trans-

mission electron microscopies (SEM and TEM), atomic force

microscopy (AFM), Raman spectroscopy, and 19F and 13C high

resolution nuclear magnetic resonance (NMR). These tech-

niques allow to investigate at various scales both the fluori-

nated parts (XRD, 19F and 13C NMR) and the non-fluorinated

carbons (13C NMR, Raman and XRD). The differences will be

discussed and used to explain the thermal defluorination

and the different final products, among which multilayer car-

bonaceous nanomaterials have been obtained.

2. Materials and methods

2.1. Synthesis and chemical composition of the fluorinatedCND

The carbon nanocones and nanodiscs were produced by pyro-

lysis of heavy oil using the Kvaerner carbon black and hydro-

gen process (CBH) [28]. The CBH is an emission-free industrial

process that decomposes hydrocarbons directly into carbon

and H2, based on a specially designed plasma torch, with a

plasma temperature above 2000 �C. The solid output was

found to consist of a significant amount of opened carbon

nanocones (20 wt.%), as well as a large number of flat carbon

discs (70 wt.%), and the rest being carbon black. Some impu-

rities are also present such as Fe (257 ppm), Si (85 ppm), Ca

(65 ppm) and Al (34 ppm) [29]. The size of the crystalline

domains, i.e. the coherence length along the c-axis (Lc) deter-

mined by XRD, is around 2 nm which means that the as-

product exhibits low degree of order. An annealing treatment

C A R B O N 5 0 ( 2 0 1 2 ) 3 8 9 7 – 3 9 0 8 3899

under argon at 2700 �C was then performed (the resulting

sample is denoted CND). It improves the crystallinity since

Lc reaches a much higher value of 39 nm.

Scanning electronic microscopy (SEM) images (Fig. 1a

and b) underline, firstly, the size dispersion of nanodiscs

and nanocones for the raw material. In order to ‘‘quantify’’

this dispersion, the frequency of occurrence was estimated

as a function of the diameter for discs and cones. Compared

to the data provided by N-Tec, no difference was observed be-

tween the as-prepared and graphitized samples indicating

that the post-treatment at 2700 �C did not significantly

change the geometry. Whatever the considered parameters,

the dispersion is rather large with a maximum giving the

average diameter centred at 1.5 lm for nanodiscs and at

1.0 lm for nanocones. Moreover, the same average thickness

of 35 nm is found for both cones and discs.

Fluorination was carried out with pure fluorine gas flow in

a Monel reactor. The reaction temperatures TF were ranged

between 480 and 520 �C. The duration of each experiment

was of 3 h. The resulting samples are denoted D�TF.

Terbium tetrafluoride was used as fluorinating agent. It

was obtained from TbF3 (Aldrich, 99.9%) in pure F2 gas at

500 �C. Its purity (i.e. the absence of residual TbF3) was sys-

tematically checked by X-ray diffraction. The thermogravi-

metric analysis (TGA) of TbF4, realized until 350 �C,

indicated that exactly one mole of FÆ was released per mole

of TbF4 between 100 and 300 �C. This experiment was carried

out under argon flow in order to avoid the formation of

oxyfluorides.

For the CND fluorination by TbF4, a nickel closed reactor

was used in order to preserve the defined fluorine amount

(atomic and/or molecular) released by the thermal decompo-

sition of TbF4. A two-temperature oven was used: the part

containing TbF4 was heated at 450 �C whatever the

Fig. 1 – SEM images of as-prepared CND (a) and treated at

2700 �C (b).

experiment whereas CND were heated at temperatures TF of

500 and 550 �C.

A reaction time of 16 h was used. Prior to the heating, a

primary vacuum (�10�2 atm) was applied into the reactor.

The resulting samples are denoted C�TF, C means controlled

fluorination. The reactions involved during the fluorination

are the following ones:

TbF4!D

TbF3 þ F� or12

F2 ð2Þ

Cþ xF� orx2

F2 ! CFx ð3Þ

The total conversion of TbF4 into TbF3 was systematically

checked by both the weight loss and XRD.

The fluorination level ‘x’ (i.e. F:C molar ratio) of nanocar-

bons fluorinated by F2 (denoted D�TF) was determined by

weight uptake as for C�TF samples.

2.2. Physicochemical characterizations

SEM micrographs were recorded using a Cambridge Scan

360 SEM operating at 1 kV.

The different samples were characterized by Transmission

Electron Microscopy (TEM, FEI CM200 operating at 200 kV).

The nanomaterials were dispersed in chloroform using ultra-

sonic treatment and a few drops of suspension were depos-

ited onto copper observation grids covered with ultrathin

carbon/formvar films. The grids were subsequently dried at

ambient conditions.

The quantitative analysis of TEM images was performed in

reciprocal space. The details of the method can be found else-

where [30]. The two-dimensional power spectral density

function (P2(s)) was computed from TEM images (u(r)) up to

the critical, or Nyquist, frequency depending upon the exper-

imental sampling interval as:

P2 �1A

ZA

uðrÞwðrÞexpð2pisrÞd2r

�������� ð4Þ

where A denotes the image area, W(r) window function [31]

and s the 2D reciprocal space vector. The P2(s) function was

then transformed into the one-dimensional PSD (P1(s)), where

s stands for the norm of s, according to:

p1ðsÞ ¼ ð2psÞ�1Z

P2ðS0djS0j � sÞdS0 ð5Þ

The samples have been also investigated by AFM (Dimen-

sion V from Veeco) in tapping mode. CND have been dis-

persed in acetonitrile and sonicated for few minutes. Then,

they have been drop cast on freshly clived mica surfaces.

X-ray diffractogramms were obtained using a PHILIPS dif-

fractometer with a Cu(Ka) radiation (k = 1.5406 A).

NMR experiments were carried out with Bruker Avance

spectrometer, with working frequencies for 13C and 19F of

73.4 and 282.2 MHz, respectively. A Magic Angle Spinning

probe (Bruker) operating with 2.5 mm rotors was used. For

MAS spectra, a simple sequence was performed with a single

p/2 pulse length of 4.0 and 3.5 ls for 19F and 13C, respectively.13C chemical shifts were externally referenced to tetramethyl-

silane (TMS). 19F chemical shifts were referenced with respect

to CFCl3.

3900 C A R B O N 5 0 ( 2 0 1 2 ) 3 8 9 7 – 3 9 0 8

Raman spectra were recorded at room temperature using a

Jobin Yvon T64000 with a charge coupled device multichannel

detector. The radiation source was a 514.5 nm Argon laser

line. The laser power was tuned to 10 mW.

2.3. Thermal treatment

In order to select the most efficient thermal treatment for the

synthesis of multilayer graphene, thermogravimetric analysis

(TGA) experiments were carried out on Shimadzu TGA-50.

Measurements were made in air at a heating rate of

2 �C min�1 from room temperature to 600 �C on about 10 mg

of sample (accuracy of 0.001 mg). The same conditions were

used for each sample. Different parameters are extracted

from the TGA curve: (i) T10 is the temperature determined

from the TGA curve for 10% weight loss, (ii) TC–F is determined

from the derivative TGA curve and corresponds to the decom-

position temperature of the C–F bond (defluorination of the

fluorinated CND) and (iii) TC represents the temperature of

the oxidation of the non-fluorinated carbon parts, i.e. burning

in air atmosphere.

Fig. 2 – SEM images of C-500 (a), C-550 (b) and D-500 (c). The

fluorine contents are 0.70, 0.95 and 0.96, respectively.

3. Results and discussion

3.1. Fluorination

The fluorine contents x in CFx, after fluorination, obtained by

weight uptake, are 0.70, 0.95, 0.78 and 0.96 for C-500, C-550, D-

480 and D-500, respectively. The fluorination level increases

with TF. Among the different samples, the highly fluorinated

CND were obtained at 550 �C using TbF4 and 500 �C using F2

with a corresponding fluorination level of 0.95 and 0.96,

respectively. Increase of the reaction temperature to 520 �Cduring the direct fluorination results in partial exfoliation of

the sample, which is already visible by the macroscopic vol-

ume expansion and confirmed at the nanoscale by SEM.

The volume expansion due to hyperfluorination and gas evo-

lution of volatile species such as CF4, and C2F6 results in

cracks and swelling on the sheet edges. Any further investiga-

tion was carried out with this damaged sample. Nevertheless,

this observation underlines the efficient reaction temperature

range for the direct fluorination, up to 500 �C. Similar phe-

nomenon has been found for fluorination of nanofibres at

temperature higher than 480 �C.

Scanning electron microscopy (SEM) images (Fig. 2) under-

line the maintaining of the geometry for both direct and con-

trol fluorinations, whatever the reaction temperature.

Nevertheless, the disc and cone edges are more irregular in

the case of D-500. Moreover, a nanocarbon swelling occurred

due to the accommodation of the fluorine atoms. This swell-

ing differs after the direct and controlled processes as re-

vealed by atomic force microscopy.

AFM was performed in tapping mode. Topography and er-

ror images of the raw nanodiscs (D-500) and fluorinated nano-

discs thermally treated at 2700 �C (C-550) show a clear

difference in the surface morphology (Fig. 3). The accommo-

dation of the fluorine atoms occurs by a swelling, which is

more pronounced on the edges for D-500 (Fig. 3a); the maxi-

mum extent of swelling is observed in the overall perimeter.

The edge seems then to be the preferential area for the direct

fluorination. On the contrary, the swelling is homogenous for

C-550 and the accommodation of fluorine atoms results in

small cracks homogenously dispersed on the disc surface

(Fig. 3c).

The fluorination progressively changes the structure as a

function of the fluorination temperature (TF). The XRD pat-

terns of the fluorinated CND are compared in Fig. 4. The pris-

tine CND pattern is similar to graphite. The main peaks

correspond to the graphite structure (002), (100), (101) and

(004) diffraction lines for 2h values of 26.4� (interlayer dis-

tance d = 0.337 nm), 42.9� (0.211 nm), 44.8� (0.202 nm) and

54.4� (0.169 nm). CND exhibit a high structural organization

thanks to the post treatment at 2700 �C (coherence length

Fig. 3 – AFM images of D-500 (a), and C-500 (b and c).

Fig. 4 – XRD diagrams of C-500, C-550, D-480 and D-500.

C A R B O N 5 0 ( 2 0 1 2 ) 3 8 9 7 – 3 9 0 8 3901

along the c axis, LC = 39 and 2 nm for post-treated and raw

CND, respectively). Considering that the value of 39 nm is

close to the average thickness of the graphitized sample, the

rebaked discs and cones can be considered to be nearly sin-

gle-crystal-like in the c-direction. For C–TF a new phase ap-

pears with corresponding peaks at 2h values centred at 14.8�and 40.9� attributed to the (001) and (100) peaks of a fluoro-

graphite matrix in an hexagonal system. The corresponding

interlayer spacing of 0.60 nm is characteristic of (CF)n-type

structure. On the contrary, an intermediate phase is deduced

for D-480 and D-500. Because of the coexistence of (C2F)n and

(CF)n phases, the value d equal to 0.68 nm is measured instead

of 0.81–0.82 nm and 0.6 nm for (C2F)n and (CF)n structural

types, respectively. Moreover, because of this coexistence of

phases, the full width at half maximum is larger for D-480

and D-500 (Dh = 3.7�) than for C-500 and C-550 (Dh = 2.7�).In order to complete the investigation on the non-fluori-

nated carbon atoms and their content in the samples, 19F

MAS NMR operating at high spinning rates of 15.0 and

30.0 kHz was carried out; the spectra are shown in Fig. 5. This

technique can highlight the nature of the interaction between

carbon and fluorine atoms, i.e. the C–F bonding. The compar-

ison of samples with similar fluorination level obtained by di-

rect and controlled fluorination using TbF4 indicates different

fluorination mechanisms. The spectra of covalent graphite

fluorides with (C2F)n and (CF)n structural types are added to

this figure for comparison. Whatever the sample, fluorinated

C-500, C-550, D-480 and D-500 or graphite fluorides, the main

line at �190 ppm (Fig. 5a and b) is assigned to covalent C–F

bonds [32–36]. Thanks to high spinning rate, a second line

at �175 ppm can be observed, present only for D-500 and

(C2F)n type graphite fluoride (Fig. 5b). This line can be unam-

biguously assigned to fluorine nuclei present only in the

(C2F)n structure: 19F in weak interaction with non-fluorinated

sp3 carbon atoms. The structure of (C2F)n can be described

using a F/C/C/F stacking sequence for fluorocarbon sheets

whereas for (CF) the sequence is F/C/F/F/C/F. All carbon atoms

exhibit a sp3 hybridization. Half of the carbon atoms are fluo-

rinated in (C2F)n while the others being linked to carbon

atoms. By analogy with the hyperconjugation effect in fluo-

rine-graphite intercalation compounds (F-GICs), that involves

C–C in non-fluorinated parts and C–F bonds [37], the cova-

lence of the C–F bonds located close to these diamond-like

atoms could be weakened in the same manner. The 19F chem-

ical shift is then of �175 ppm instead of �190 ppm for pure

covalent C–F. In the case of F-GICs, �170 ppm is recorded.

The line at �175 ppm, which is observed thanks to a high

Table 1 – Key parameters deduced from TGA curves.

Sample F/C T10 (�C)±5 �C

TC–F (�C)±5 �C

C-500 0.70 432 521C-550 0.95 517 569D-480 0.78 460 520D-500 0.96 430 520

0 -50 -100 -150 -200 -250 -300 -350

D-500

D-480

C-550

C-500

C-F

CF2

*

*

*

*

*

*

*

*

*

*

* *

δ19F/ CFCl3 (ppm)

CF3

(a)

150 100 50 0

C-C-F sp3 C

δ13C / TMS (ppm)

(c)C-F

CF2

C-C-F

sp2 C D-500

D-480

C-500

C-550

(b)

Fig. 5 – 19F MAS NMR spectra of fluorinated CND with 14 kHz

(a) and 30 kHz (b) spinning speed ( * are the spinning

sidebands). 13C MAS spectra (10 kHz) (c).

3902 C A R B O N 5 0 ( 2 0 1 2 ) 3 8 9 7 – 3 9 0 8

spinning rate, can be considered as an indicator of the pres-

ence of (C2F)n structural type. (C2F)n type is maintained in a

wide temperature range for the direct fluorination whereas

(CF)n type is formed during the controlled fluorination using

TbF4 whatever the temperature. This is due both to progres-

sive generation of atomic fluorine by thermal decomposition

of TbF4 and to its high reactivity and diffusion.

The 19F lines close to �80 and �120 ppm are assigned

respectively to CF3 and CF2 groups, their intensities are higher

for D-480, D-500, (C2F)n and (CF)n prepared with F2 gas at high

temperature (380 and 600 �C for (C2F)n and (CF)n, respectively).

Indeed, the control of fluorination kinetic avoids the hyperflu-

orination at high temperature. The generated atomic fluorine

penetrates towards the core without formation of important

structural defects. Moreover, excess of fluorine is avoided

contrary to the case of a F2 gas flux. This results in a decrease

of structural defects such as CF2, CF3 and dangling bonds and

could change the thermal stability. These defects are always

well correlated with structural disorder [38–41].13C NMR confirms the structural phase formed during fluo-

rination. As a matter of fact, in the F/C/C/F stacking sequence

of (C2F)n, half of the carbon atoms are fluorinated. The other

atoms, also with sp3 hybridization, are linked only to other

carbon atoms, as in a diamond phase. The typical 13C NMR

line of those 13C nuclei is observed at 42 ppm and appears

as a good indicator of the presence of (C2F)n phase. This line

is observed only for D-480 and, with lower intensity, for D-

500, in accordance with both 19F NMR and XRD. A common

line whatever the synthesis way appears at 88 ppm and is as-

signed to covalent C–F bonds.

The high relative intensity of the line at 110 ppm related to

CF2 groups also confirms the large amount of those groups in

D-500 and D-480. Finally, for the lower fluorination tempera-

ture using both F2 and TbF4, an additional line is measured

at 141 ppm. Such a chemical shift, which differs from the

one of pure graphite at 120 ppm, is explained by the interac-

tion of non-fluorinated carbon atoms with neighbouring C–F

bonds (the corresponding atoms are denoted C–C–F on

Fig. 5c). The C–F bonding is weakened due to this interaction.

The fluorination using TbF4 decomposition generates the

formation of a unique highly fluorinated phase, i.e. (CF)n type,

whatever the fluorination temperature, the C–F bonding being

covalent. This fluorinated phase is always mixed with some

residual non-fluorinated CND and a high fluorination level

can be obtained (x = 0.95) without hyperfluorination contrary

to the F2 process. The higher homogeneity of the materials

obtained using this controlled process by comparison with

the direct fluorination using fluorine gas can be explained

by the low kinetic of decomposition of TbF4 that allows a con-

tinuous addition of fluorine to carbon matrix and a more pro-

gressive fluorination.

3.2. Thermal treatment

Fig. 6 displays the TGA curves for samples fluorinated by F2

and TbF4. Table 1 summarizes the key values to evaluate the

thermal stability, namely T10 and TC–F. Whatever the fluorina-

tion way, the TGA curves may exhibit two weight losses as-

signed to the defluorination at lower temperature and to

oxidation of non-fluorinated carbon atoms with dioxygen

from air (burning) at higher temperature. The derivative

curves clearly underline one main process by a well-defined

peak (insert on Fig. 6 for C-550 as a representative example).

The temperature of burning in air (TC) is higher than 600 �C

Fig. 6 – Evolution of the residual weight of CND fluorinated

by TbF4 decomposition (straight lines for C-500 and C-550)

and by F2 ( d for D-480 and s for D-500) versus the

temperature (air atmosphere, 2 �C min�1); insert: derivative

curve of TGA curve of C-550 sample showing TC–F.

C A R B O N 5 0 ( 2 0 1 2 ) 3 8 9 7 – 3 9 0 8 3903

for all the samples. The derivative TGA curve does not exhibit

a second peak related to TC because the experiment is con-

ducted up to 600 �C (Fig. 6). TC–F does not depend on the fluo-

rination temperature using F2, being equal to 520 ± 5 �C. The

sample fluorinated by controlled fluorination exhibits an en-

hanced thermal stability when compared to the ones ob-

Fig. 7 – SEM images after thermal treatment at 600 �C of C-550 (a–

Image of exfoliated D-520 is added for comparison (h).

tained by molecular fluorine F2. The physico-chemical

characterization underlines that the controlled process using

TbF4 results in more homogeneous fluorine atom distribution

than the direct fluorination and leads to lower structural

defects such as CF3, CF2 groups and dangling bonds (always

related to disorder). Such disorder favours the thermal

decomposition. The higher defluorination temperature TC–F

was then measured for C-550 with a value of 569 ± 5 �C(Table 1).

The thermal defluorination occurs at lower temperatures

for C-500 (TC–F = 521 ± 5 �C) than for C-550 (TC–F = 569 ± 5 �C).

An additional effect must be considered. The presence of

weakened C–F bonds has been underlined by 13C NMR and

their thermal defluorination must take place at lower temper-

ature because of the weakening of the C–F bonding. Such pro-

cess could destabilize the fluorocarbon matrix and decrease

both TC–F and T10. As a consequence of the presence of weak-

ened covalent C–F, the temperatures T10 for 10% weight loss

are 432 ± 5 �C for C-500, lower than 517 ± 5 �C for C-550. Similar

phenomenon has been found for fluorinated nanofibres [20].

With the aim to produce multilayer carbonaceous nanom-

aterials, the treatment temperature has been selected taking

into account both the TGA derivative curves and TC–F and TC

values. Both C-550, and D-500 were thermally defluorinated

at 600 �C. It is to note that the residual weight% is calculated

equal to 12n/[2n(12 + 19)] � 20% taking into account reaction

(1); such value is in good accordance with the TGA data for

these samples. For comparison, similar treatment was per-

formed with non-fluorinated nanodiscs/nanocones. The

c), D-500 (d–f) and non-fluorinated nanodiscs/nanocones (g).

Fig. 8 – TEM images of C-550 (left) and D-500 (right) heated at 600 �C in air. The magnifications are 5000, 50 000, 115 000 and

310 000.

3904 C A R B O N 5 0 ( 2 0 1 2 ) 3 8 9 7 – 3 9 0 8

resulting samples have been characterized by Raman spec-

troscopy, SEM and TEM.

Discs and cones are still observed by SEM (Fig. 7 d–f) after

the thermal treatment of sample fluorinated by F2. Neverthe-

less, their thickness is irregular because of the thermal deflu-

orination. The sheet edges appear thinner on the discs

(Fig. 7e) and the cones (Fig. 7f). On the contrary, most of the

cones and discs disappear during the defluorination of

C-550 (Fig. 7a–c). The resulting particles of about 400–500 nm

of width and 7–10 nm of thickness seem like flakes folded

up on themselves.

When the non-fluorinated nanodiscs/nanocones were

treated at 600 �C in air, strong damages were generated

(Fig. 7g): the edges are irregular on both discs and cones (see

arrows on Fig. 7g) and pseudo-hexagonal holes are observed

(insert). Another route to produce carbon multilayers could

have been exfoliation during a fast increase of the tempera-

ture up to 520 �C in F2 atmosphere. But the resulting sample

(D-520 in the same notation than before) exhibits inhomoge-

neity in sizes, both in thickness and width (Fig. 7h). The flakes

have also a curvature. This method has not been retained.

Contrary to C-550 heated at 600 �C, numerous discs are still

observed by TEM for CND fluorinated by F2 and thermally

defluorinated. In addition, smaller particles are present. Their

shapes are similar to those of the defluorinated C-550. A high-

er magnification, the particles reveal their substructure

Fig. 9 – TEM images of heat-treated C-550 (a) and D-500 (b) showing the presence of graphitic layers and corresponding power

spectral density (PSD) functions (c). The curves are offset vertically for clarity. The dotted line indicates the periodicity of

graphitic layers. One-dimensional correlation functions computed from the TEM images (d).

C A R B O N 5 0 ( 2 0 1 2 ) 3 8 9 7 – 3 9 0 8 3905

consisting of multilayers folded up on themselves. As the ini-

tial shape is the disc; the thinning occurring during the ther-

mal defluorination results in the discs folding up. This

process systematically occurs for C-550 during the defluorina-

tion and punctually for D-500 resulting in the smaller parti-

cles. The thickness of the edges is ranged in between 7 and

10 nm corresponding to about 20–30 layers (Fig. 9).

Magnification of TEM bright-field images of the heat-trea-

ted fluorinated CND underlines the presence of graphitic lay-

ers (Fig. 9a and b). The periodicity of the layers is reflected by a

Bragg peak in the corresponding PSD curves (Fig. 9c), which is

positioned at about 0.35 nm for both samples. This value is in

accordance with non-fluorinated graphene layers. For fluori-

nated nanofibres, in addition to the usual graphene layer peri-

odicity the PSD function also displays a broad peak with a

maximum at about 1.5–2.0 nm�1. This additional feature in

the PSD curve was assigned to the presence of layers, which

were less ordered and are more spatially separated due to

the accommodation of fluorine atoms [42]. Importantly, the

additional line is absent for treated C-550 and D-500 in accor-

dance with a quasi-total defluorination.

The one-dimensional correlation function (CF) denoted

here as c (r) was computed as the real part of the Fourier

transform of the one-dimensional PSD function, P1(s), cor-

rected for the so-called sigmoidal-gradient transition zones

with thickness r:

cð1Þ ffi Ref2pZ 1

0

P1ðsÞs expð2pislÞexpð4p2r2s2Þds ð6Þ

For the sake of simplicity the CFs were normalized to unity

at the origin. The method is described in full detail in Ref. [30].

The CFs show that treated D-500 generates the first subsidiary

maximum corresponding to the nearest-neighbor distance,

which is located at a slightly smaller distance than the one

of the C-550 sample. Also, the subsidiary maximum itself is

somewhat broader for this sample (Fig. 9d). Therefore the

structure of treated D-500 reveals less order than the one of

heated C-550. Raman spectroscopy in the following section

will confirm this fact.

The Raman spectra of the treated samples are displayed

on Fig. 10. First, it is to note that the spectra of the fluorinated

CND cannot be recorded because of the strong fluorescence

1000 1200 1400 1600 1800

CNDs

Heated C-550

Raman Shift (cm-1)

Heated D-500

Fig. 10 – Raman spectra of thermally treated C-550 and D-

500 and raw non-fluorinated CND.

3906 C A R B O N 5 0 ( 2 0 1 2 ) 3 8 9 7 – 3 9 0 8

typical of highly fluorinated carbons. The microRaman spec-

tra were recorded in numerous regions of the samples, heated

C-550 and D-500. Any difference was found whatever the

location. For heated C-550 and D-500, three narrow vibration

bands with full width at half maximum of about 20–40 cm�1

are visible: two of them around 1356 and 1620 cm�1 are as-

signed to the D and D 0 modes. For their activation D and D 0

resonances require a defect, such as bond dislocations,

Fig. 11 – Effect of thermal treatment of nanodisc; the graphitic c

after defluorination. Contrary to C-550, the thinning is incompl

fluorine atoms.

missing atoms at the edges of the sample and sp3-hybridized

carbon atoms; their presence is associated with an increased

degree of disorder [43–45]. The third band at 1589 cm�1 is as-

signed to the conventional G mode, which is related to the

graphitization degree of carbon material and is also called

tangential modes (where the carbons in sp2 hybridization vi-

brate in parallel to the axis of the nanotube) [46]. The value

measured for heated fluorinated samples differ from the ones

of the raw CND, for which only D and G are recorded at 1345

and 1565 cm�1. On the contrary, some similitudes appear with

fully fluorinated graphene; in those materials the D and D 0

peak appear at 1350 and 1620 cm�1 and their intensity is high-

er than that of the G band at 1580 cm�1. Contribution to the D

peak of fully fluorinated graphene comes from sp3-bonded

carbon atoms rather than the edges of the fully fluorinated

flakes. The fluorination/defluorination induces defect into

the resulting graphene sheets.

The relative integrated intensities of D and G bands ID/IGare 2.0 and 2.3 for heated C-550 and D-500, respectively, the

value for the raw CND being 0.06. Those ratio are in accor-

dance with TEM study that underlines the slightly lower dis-

order for heated C-550 than treated D-500. For comparison,

fully highly fluorinated graphene exhibits ID/IG of 3.8 [47].

3.3. Discussion

Using a freeze fracturing setup, Naess et al. [48] observed for

the same nanodiscs from N-Tec an in-plane cleavage that

was usually parallel to the disc surface. The particles were

ore (in white), which acts as a reinforcement, is maintained

ete for D-500 because of inhomogeneous distribution of the

C A R B O N 5 0 ( 2 0 1 2 ) 3 8 9 7 – 3 9 0 8 3907

fractured in a plane close to the geometrical centre of the

disc. The splitting occurs between the graphitic layers, which

are only bonded by weak van der Waals forces, and not in the

amorphous carbon surrounding the disc. The authors con-

cluded then that the as-synthesized discs can be described

as a graphitic core within an envelope of non-crystalline car-

bon. The graphitic core is first grown to its full size, where-

upon layers of amorphous carbon are deposited, the

thickness of the graphitic core being estimated to only

2–5 nm. Moreover, if the material is heat-treated at 2700 �C,

the graphitization takes place simultaneously in different

parts of the amorphous carbon. Thus the temperature-

induced conversion from amorphous carbon to layers of

graphite is not fully coupled geometrically to the original

graphite core. This history of the nanodiscs has important

consequence during the fluorination/defluorination. The level

of fluorination may differ between the core and the outer

parts and the difference is maintained after fluorination

(Fig. 11). Moreover, the core could act in a reinforcement ef-

fect. Regarding the thermal defluorination, a peeling of the

fluorinated layer occurs in a similar way than fluorinated

nanofibres. A thinning of the discs progressively occurs by

gaseous removal of the fluorinated parts. The final thickness

is close to 7–10 nm, not so far from the initial core size. Two

cases must be distinguished: When the fluorine atoms are ini-

tially homogenously dispersed in the whole outer volume

thanks to the controlled fluorination (TbF4), the thinning is

equal whatever the location along the diameter. Moreover,

the discs fold up itself when the thickness is about ten nm

and appear as ruffled paper (Figs. 7 and 8).

On the contrary, as irregular swelling takes place upon di-

rect fluorination (see AFM section), the thermal defluorina-

tion results in different thickness and the planar

configuration is partly maintained. The planarity is main-

tained because of the thicker parts.

The homogenous thinning of the discs cannot be per-

formed by a simple burning at 600 �C in air atmosphere be-

cause the resulting discs are damaged with holes and edge

cut down (Fig. 7g). Fluorination allows the decomposition

temperature to be decreased and the gap between defluorina-

tion and burning in air is sufficient to separate the two pro-

cesses (for C-550, TC–F is equal to 569 ± 5 �C whereas TC is

higher than 600 �C). The chemical thinning is then possible.

This discussion focuses on 70 wt.% of the sample, corre-

sponding to the disc percentage. 10% amorphous parts are

probably burned under fluorine atmosphere [49]. 20 wt.% of

the sample are nanocones. They are structurally similar to

flat carbon discs, which can be regarded as carbon cones with

apex angle equal to 180, or alternatively, with zero pentagon

at the tip. The change of the geometry of the cones is more

difficult to be observed than for discs using SEM and TEM

but we believe that the mechanisms are nearly similar due

to the structural similarities.

4. Conclusions

The fluorination of opened structures such as nanodiscs (or

nanocones) may be the first step for the production of multi-

layer carbonaceous nanomaterials. The fluorinated parts

must be homogenously dispersed in the carbon lattice with

a non-fluorinated core (sub-fluorination). A controlled

fluorination using solid fluorinating agent must be then pre-

ferred rather than the direct process using molecular fluorine

F2. Whatever the fluorination method, the removal of the

fluorinated parts during a process similar to a peeling with

CF4 and C2F6 gas evolution does not result in damages of

the non-fluorinated region when the structure is opened con-

trary to fluorinated nanofibres. The structure of those latter

consists of either concentric cylinders of graphite sheets

(Russian doll model) or of a single graphite sheet rolled in

around itself (parchment model). Gas evolution during the

thermal defluorination into this confined structure may cause

damages in the resulting material such as exfoliation and loss

of the tubular shape. Multilayer carbonaceous nanomaterials

were synthesized with 7–10 nm thickness in a form of ruffled

paper. The discs fold up on themselves during thinning. Fur-

ther studies are necessary to decrease the thickness by using

the controlled fluorination and by increasing the fluorine con-

tent closer to the limit of CF1. A few non-fluorinated layers

must be conserved in order to form thinner multilayers. The

present work is a promising first stage towards the synthesis

of multilayer graphene materials via fluorination.

Acknowledgements

The authors are thankful to Elodie Petit for her helpful contri-

bution for Raman spectra.

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