www.elsevier.com/locate/carbon

Carbon 45 (2007) 1669–1678

Degree of functionalization of carbon nanofiberswith benzenesulfonic groups in an acid medium

F. Barroso-Bujans a, J.L.G. Fierro b, S. Rojas b, S. Sanchez-Cortes c,M. Arroyo a, M.A. Lopez-Manchado a,*

a Instituto de Ciencia y Tecnologıa de Polımeros, CSIC, Juan de la Cierva, 3, 28006 Madrid, Spainb Instituto de Catalisis y Petroleoquımica, CSIC, Marie Curie, 2, Cantoblanco, 28049 Madrid, Spain

c Instituto de Estructura de la Materia, CSIC, Serrano, 121, 28006 Madrid, Spain

Received 11 January 2007; accepted 22 March 2007Available online 30 March 2007

Abstract

Benzene sulfonic groups have been successfully attached to a carbon nanofiber surface by reaction of diazonium benzenesulfonic saltin sulfuric acid. The extent of the functionalization reaction was determined by X-ray photoelectron spectroscopy, energy dispersiveX-ray analysis, elemental analysis, and thermogravimetric analysis complemented with temperature-programmed desorption experi-ments. Good agreement between the degrees of functionalization provided by these techniques was observed. The results pointed to ahigher extent of anchorage of –SO3H groups when the nanofibers were treated in fuming sulfuric acid, for which a surface S/C (%)atomic ratio of 2.4 was obtained. Raman spectroscopy revealed that the D-band does not fully disappear after CNF treatment, indicatingthat a certain degree of structural disorder is maintained. However, a decrease in the D-band was observed after the diazotization reac-tion and this was attributed to the chemical change occurring at the edges. No significant changes to the morphological and texturalcharacteristics of the CNFs by surface treatment were observed. This study may offer an important guideline in the application of CNFsmodified with benzenesulfonic groups in polymeric membranes for fuel cells.� 2007 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon nanofibers (CNFs) and carbon nanotubes(CNTs) have attracted huge interest over the past decadedue to their potential applications arising from the advan-tageous mechanical and chemical properties of these kindsof materials. CNFs are graphitic materials that can be pre-pared by catalytic decomposition of hydrocarbons oversmall metal particles, usually Ni catalysts [1]. They are pro-duced at larger scale and at lower cost than CNTs. In addi-tion to the low cost, the CNFs display high electrical andthermal conductivities, good mechanical strength, high sur-face areas, and chemical stability. These properties makethem excellent candidates for advanced materials, e.g. as

0008-6223/$ - see front matter � 2007 Elsevier Ltd. All rights reserved.doi:10.1016/j.carbon.2007.03.039

* Corresponding author. Fax: +34 91 5644853.E-mail address: lmanchado@ictp.csic.es (M.A. Lopez-Manchado).

a nanoreinforcement of polymer composites [2], catalystsupports [3–5], chemical/biochemical sensing [6], neuraland orthopedic implants [7], hydrogen-storage material[8] and as anode materials in lithium batteries [9]. Theapplications of CNFs can be substantially broadened bythe chemical bonding of different groups to the outer layerof the carbon fibers. For instance, surface functionalizationof CNFs is an attractive route for increasing their compat-ibility with polymers in composites [10,11], the dispersabil-ity in raw materials [12] and wettability [11]. Someprocedures carried out to generate hydroxyl, carbonyland carboxyl groups on the fiber surfaces consist in apply-ing oxygen plasma treatment [13], HNO3 [14] or electro-chemical oxidations [15]. It has been demonstrated thatsuch surface groups imprinted by plasma treatmentimprove the adhesion of CNFs to nitrile butadienerubber (NBR) [16]. The surface oxidation of CNFs and

1670 F. Barroso-Bujans et al. / Carbon 45 (2007) 1669–1678

derivatization by a linker molecule derived from diaminesor triamines followed by step-wise process involving ami-dation have also been reported [17]. Thus, aliphatic andaromatic amines have been attached to the CNF surfaces.An interesting route to link organic groups to the fiber sur-face is through chemical reaction with diazonium salts.This procedure allows the anchorage of benzene-substi-tuted groups to the surface, thus allowing one to select adesired functionality R:

Table 1CNF concentrations of acid suspensions (commercial sulfuric acid oroleum) in diazotization reactions (from 1 to 3) and acid treatment ofCNFs

Sample c (mg of CNFs/mL of sulfuric acid)

Commercial Oleum

CNF-BzSO3H (1) 1 –CNF-BzSO3H (2) 2.5 –CNF-BzSO3H (3) – 2.5

CNF-Ac 2.5 –CNF-oleum – 2.5

R

n (R=Cl, Br, t-butyl, CO2CH3, NO2, SO3H, CH2CH2OH)

The diazonium reaction route was used to graft 4-nitro-benzene groups onto CNF surfaces, which were covalentlylinked to thio-terminated DNA oligonucleotides in subse-quent chemical steps [18]. Different methodologies forgrafting benzene-substituted groups to CNTs by meansof the diazonium route have been developed, includingelectrochemical reduction of the salt [19] as well as thetreatment of surfactant-wrapped nanotubes [20]. Arene-diazonium species can be generated in situ in media suchas organic solvents [21], oleum (H2SO4 with dissolvedSO3) [22], or without any solvent, achieving high degreesof functionalization of up to 1 in 9 carbons along theirbackbones bearing an organic moiety.

Among all the possibilities offered by diazonium treat-ment, we chose the incorporation of benzenesulfonicgroups onto the CNFs in an acid medium. It is our futureobjective to prepare proton-conducting fuel cell mem-branes, based on sulfonated ethylene–propylene diene ter-polymer rubber, as an alternative material to Nafion.Polymeric membranes for fuel cells are widely describedin the literature but their poor mechanical behaviour limitstheir applicability [23]. It is expected that incorporation ofsmall amounts of carbon nanofibers will improve the ther-mal and dimensional stability of the membrane, and alsothe presence of sulfonic groups will increase its protonicconductivity. Within this framework, we believe it to beof crucial interest the optimization of the carbon nanofiberfunctionalization reaction. For this reason, the presentstudy reports the complete characterization of such sulfo-nated CNFs and a comparison between the techniquesused to analyze the sulfonic group content. The degree offunctionalization of modified CNTs is generally calculatedby means of thermogravimetric analysis (TGA) [19,24].However, in this work it is shown to which extent theresults obtained by bulk techniques (TGA and elementalanalysis), and surface techniques (X-ray photoelectronspectroscopy (XPS) and energy dispersive X-ray analysis(EDX)), are comparable. TGA was complemented withtemperature-programmed desorption analysis in order toidentify the gases given off at different temperature stages.The textural characteristics of CNFs were evaluated bynitrogen adsorption isotherms at low-temperature, and

the morphology was assessed using transmission electronmicroscopy (TEM). X-ray diffraction (XRD) and Ramanspectroscopy were also used to reveal possible changes inthe graphitic structure induced by the chemical treatmentsemployed during the functionalization step.

2. Experimental

2.1. Materials

CNFs with diameters of 20–80 nm and lengths of 30 lm were kindlysupplied by Grupo Antolın Ingenierıa, S.A., Spain, and were obtainedby a submicron vapor-grown carbon fiber (s-VGCF) process by usingNi catalyst and natural gas as the carbon source. Sodium nitrite (Panreac),sulfanilic acid (Fluka), a,a 0-azoisobutyronitrile (AIBN) (Fluka), fumingsulfuric acid (oleum) (Riedel-de Haen) and commercial sulfuric acid (Pan-reac) were used as received.

2.2. Functionalization reaction

CNFs were functionalized following the procedure described by Hud-son et al. [22]. They were dispersed in commercial-grade sulfuric acid andoleum (20% of SO3) for 3 h at the concentrations indicated in Table 1.Sodium nitrite (4 mol/mol C), sulfanilic acid (4 mol/mol C), and AIBN(0.2 mol/mol C) were slowly added to the acid dispersion in the sameorder. The reaction mixture was heated to 80 �C for 2 h. The productformed was cooled to room temperature and carefully diluted with water.The modified CNFs were filtered using polycarbonate filter membranes(0.2 lm pore-size), washed thoroughly with distilled water, and driedunder a vacuum at room temperature. As reference samples, the CNFswere submitted to the same treatment conditions as samples of CNF-BzSO3H (2) and CNF-BzSO3H (3), using only sulfuric acid as the reactant(CNF-Ac and CNF-oleum, Table 1).

2.3. Characterization of modified-CNFs

The nature and relative concentrations of sulfonic groups were evalu-ated by XPS. XP spectra were recorded using an Escalab 200R spectrom-eter provided with a hemispherical analyser, operated in a constant passenergy mode and non-monochromatized Mg Ka X-ray radiation(hm = 1253.6 eV) operated at 10 mA and 12 kV. The binding energies(BE) were referenced to the C1s peak at 284.9 eV. Data processing wasperformed with the XPS peak program. The spectra were decomposedwith the least squares fitting routine provided with the software, with aGauss/Lorentz product function and after subtracting a Shirley back-ground. Surface S/C atomic ratios were estimated from the integratedintensities of S2p and C1s lines after background subtraction and cor-rected for atomic sensitivity factors [25].

The atomic composition of carbon nanofibers was measured by energydispersive X-ray (EDX) in a Philips XL30 environmental scanning elec-tron microscope (ESEM).

F. Barroso-Bujans et al. / Carbon 45 (2007) 1669–1678 1671

Thermal gravimetric analyses were performed in a Mettler ToledoTGA/STDA 851e device. Samples placed in 70 lL alumina pans wereheated from 40 to 1000 �C at a heating rate of 10 �C/min under a constantN2flow of 20 mL/min.

For temperature-programmed desorption experiments (TPD), ca.25 mg of the sample was loaded in a U-shaped quartz reactor. Sampleswere pretreated in Ar at 150 �C (10 �C/min) for 30 min in order to removephysisorbed water. The sample was cooled to room temperature under anAr flow. The reactivity of the species remaining at the surface of the solidswas tested by passing Ar (EGA-MS) (evolved gas analysis-mass spec-trum). Temperature programs were run from 25 to 1100 �C at a rate of10 �C/min, with a gas flow rate of 50 mL/min. The evolution of differentcompounds was monitored by selected m/z fragments that were followedwith a quadrupolar mass spectrometer connected on-line to the reactor.Downstream lines were heated to 120 �C to prevent the condensation ofproducts.

Titration of CNFs was carried out as follows [26]. CNFs (30 mg) werestirred in 25 mL of 8 · 10�4 or 6 · 10�3 N NaOH aqueous solution for24 h. The mixture was then filtered using a polycarbonate filter membrane(0.2 lm pore-size). 20 mL of the filtrate were titrated with 8 · 10�4 or6 · 10�3 N HCl aqueous solution. The amount of acid groups in the CNFswas estimated by the NaOH consumed. The titration procedure is shownbelow.

280 284 288 292BE (eV)

CNF

CNF-oleum

CNF-Ac

cou

nts

per s

econ

d (a

u)

CNF-BzSO3H(1)

CNF-BzSO3H(2)

CNF-BzSO3H(3)

C 1sC-C

C-OH

C-OOH

Fig. 1. C 1s core-level spectra of pristine CNFs, diazotized CNFs, andsulfuric acid-treated CNFs.

SO3H NaOH

SO3Na NaOHH2O

n

+

n

+

(in excess)

the concentration is determinedby titration with HCl

+

X-ray powder diffraction patterns were collected using a XPert High-score Philips Analytical Diffractometer at a Cu Ka wavelength of1.54 A, a tube voltage of 45 kV, and a tube current of 40 mA. XRD dif-fractograms were collected in the 2–60� 2h range, in steps of 1� min�1.

Raman spectra were recorded in a micro-Raman Renishaw RM2000instrument, using the 514.5 nm radiation line of a Spectra Physics Model163-C4210 Ar+ laser. The instrument was coupled to a Leica microscope,an electrically refrigerated CCD camera, and a notch-filter to eliminateelastic scattering. The laser power at the sample was 0.1 mW. Resolutionwas set at 4 cm�1, and the geometry of the micro-Raman measurementswas 180�. Raman spectra are the result of the sum of three spectrarecorded on different points of the sample using a 50· lens.

Morphological changes after treatment were evaluated using the trans-mission electron microscopy (TEM) images obtained in a JEOL JEM-4000 EX microscope operated with an accelerating voltage of 400 kV.Many micrographs were taken, but only representative ones are shownhere.

Specific surface areas were calculated using the BET method fromnitrogen adsorption isotherms, recorded at the temperature of liquid nitro-gen on a Micromeritics ASAP 2000 apparatus. Prior to the adsorptionmeasurements, samples were degassed at 80 �C for 12 h.

3. Results and discussion

3.1. Quantification of functional groups on the fiber surface

Photoelectron spectroscopy (XPS) is a technique partic-ularly suited to monitor the evolution of functional groupsin the surface region of carbon-based materials [27,28].Thus, the XPS technique was used in this work to deter-mine the nature and relative abundance of functional

groups present on the CNF surface. The C1s, O1s andS2p spectra of the samples are shown in Figs. 1–3. Allpeaks were decomposed into several symmetrical compo-nents: three for C1s; two (or three) for O1s, and one (ortwo) for S2p. It should be stressed in this point that inthe case of the S2p peaks the spin–orbit splitting is ratherlarge (ca. 1.1 eV) and hence the two components (S2p3/2

and S2p1/2) for a given S-containing species were resolved.The C1s peaks were satisfactorily fitted to three compo-nents (Fig. 1) according to the peak assignment usedby Hiura et al. [29]. The most intense peak at284.7–284.8 eV can be unambiguously assigned to sp2 C–C bonds of graphitic carbon. The broad shoulder of themain component was fitted to either one peak at 286.3 eV(CNF and CNF-BzSO3H (2)) or to two peaks at 286.3and 288.0–288.5 eV in the other samples. The componentat 286.3 eV has been often assigned to C–OH and that at288.0–288.5 eV to carboxyl carbon –COOH species

528 532 536 540BE (eV)

CNF

CNF-oleum

CNF-Ac

coun

ts p

er s

econ

d (a

u)

CNF-BzSO3H(1)

CNF-BzSO3H(2)

C-OC=O

CNF-BzSO3H(3)

O 1s

H-O-H

Fig. 2. O 1s core-level spectra of pristine CNFs, diazotized CNFs, andsulfuric acid-treated CNFs.

160 164 168 172 176CNF-oleum

BE (eV)

R-SH

CNF-Ac

CNF-BzSO3H(1)

CNF-BzSO3H(2)

coun

ts p

er s

econ

d (a

u)

-SO3H

CNF-BzSO3H(3)

S 2p

Fig. 3. S 2p core-level spectra of diazotized CNFs and sulfuric acid-treated CNFs.

1672 F. Barroso-Bujans et al. / Carbon 45 (2007) 1669–1678

[27,30]. The presence of some carbonyl (C@O) groups can-not be precluded, since the binding energy of these speciesis about 287.7 eV. It should also be noted that the C1s pro-files did not show a broad, weak component at around291.0 eV, which comes from the p! p* transition of car-bon atoms in graphene structures [27,30,31]. This featuremay be indicative of a disordered and/or defective graph-ene surface of the fibers.

Similarly, the O1s spectra were fitted to three compo-nents (Fig. 2). A first component at 531.1–531.5 eVcorresponded to O@C surface groups, a second one at532.7–533.2 eV was associated with O–C bonds, and athird one above 534.2 eV came from strongly adsorbedmolecular water (H–O–H) [31]. The only exception wasthe unmodified CNF sample, which displayed only the firsttwo components. The absence of chemisorbed molecularwater in the CNF sample could be explained on the basis

of the absence of strong polar sulfonic groups in this sam-ple. Unfortunately, the BE of the O1s core-level for O–Cand S–O bonds fell in the same energy region which madeit extremely difficult, if not impossible, to distinguishbetween the contribution of both species. The S2p spectra(Fig. 3) of the S-functionalized samples revealed the princi-pal S2p3/2 peak at a binding energy of 168.1–168.3 eV,characteristic of –SO3H groups [32]. In addition, theCNF, CNF-Ac and CNF-oleum samples displayed a sec-ond S2p peak at 163.9 eV. This peak does not appear tobe related to any oxidized –SOx species, although a similarbinding energy has been associated in the literature withS-containing organic structures [33]. The observation ofC–SH/C–S–C bonds at the surface of CNFs is not surpris-ing since the C-source employed to synthesise these fiberswas natural gas, in which ppb levels of S-containingorganic compounds persisted even after the S-removal stepemployed just before the pyrolysis reactor. In addition, thepristine CNF sample also showed a small component at168.1 eV (spectrum not shown) which may be due to ametallic sulfate, probably produced by air oxidation ofthe metal catalyst employed in the synthesis of the CNFs.No N1s photoelectron peaks were observed in the func-tionalized samples, suggesting that the diazonium groups(�Nþ2 ) were released during the diazotization process.

Table 2S/C ratio of pristine and modified CNF samples determined by elemental analysis, XPS and EDX, BzSO3H/C ratio determined by TGA; graphitizationdegree (ID/IG) measured by means of Raman spectroscopy, and surface area measured by nitrogen physisorption

Sample XPS, S/C (at%) EDX, S/C (at%) Elemental analysis, S/C (at%) TGA BzSO3H/C (mol%) Raman ID/IG BET, S (m2/g)

CNF 0.10 0.31 0.48 0 1.19 159CNF-oleum 0.40 0.51 0.32 – 1.04 89CNF-Ac 0.40 0.38 0.38 – 1.01 98CNF-BzSO3H (1) 0.70 0.58 0.38 0.31 0.71 163CNF-BzSO3H (2) 1.90 1.00 0.72 0.66 0.75 126CNF-BzSO3H (3) 2.40 1.53 1.09 0.71 0.79 122

90

95

100 CNF

CNF-Ac

CNF-OleumCNF-BzSO3H (2)

t (%

)

CNF-BzSO3H (1)

F. Barroso-Bujans et al. / Carbon 45 (2007) 1669–1678 1673

Quantitative S/C atomic ratios (Table 2) and atomicpercentages (Table 3) indicate that the surface density of–SO3H species strongly depends on the functionalizationmethodology applied. Indeed, CNF functionalization by–BzSO3H seemed to be the most effective one, whereasAc and oleum pretreatments afforded a very small densityof SO3H groups. It is worth noting that the sample treatedwith fuming sulfuric acid (CNF-BzSO3H (3)) showed thehighest % S/C surface atomic ratio (2.4%), indicating thatan acid source plays an important role in the sulfonationof nanofibers (Table 2). Another crucial parameter to beconsidered is the concentration of sulfuric acid used duringthe reaction. From the data in Tables 2 and 3 it is clear thatthe S/C atomic ratio and sulfur percentage increased by afactor of ca. 2.7 in sample CNF-BzSO3H (2) with respectto CNF-BzSO3H (1). From the surface oxygen percentagesin Table 3 it appears that the increase in surface oxygenruns in parallel with the sulfur content.

Quantification of S/C superficial atomic ratios is alsopossible by means of EDX analysis. This technique isknown to have limited application in the meaning of suchquantifications due to heterogeneities in the powder sam-ples and the low sensitivity of detectors of carbon atoms[34]. Nevertheless, comparative values were obtained withXPS (Table 2). Some nickel was identified in both pristineand treated samples, originating from the catalyst used togrow the nanofibers. Moreover, sulfur was also detectedin pristine CNFs, coming from the S impurities still presentin the natural gas feed stream employed in CNF synthesis.

The S/C bulk content of both pristine and functional-ized CNFs was determined by means of elemental analysis(Table 2). It should be noted that the analysis of these sam-ples has serious limitations, since to burn the sample fully itis necessary to use amounts of sample lower than thoseemployed in typical analyses. As a consequence, the sulfurpercentage determined, which was in the order of 50-fold

Table 3Surface composition determined by XPS

Sample C (at%) S (at%) O (at%)

CNF 95.1 0.1 4.8CNF-oleum 93.5 0.4 6.1CNF-Ac 93.6 0.4 6.0CNF-BzSO3H (1) 91.3 0.6 8.1CNF-BzSO3H (2) 89.3 1.7 9.0CNF-BzSO3H (3) 88.0 2.1 9.9

lower with respect to carbon, had a significant error. How-ever, very good agreement with the results obtained byother techniques was found. It may be seen, in concordancewith the EDX analysis, that the original pristine CNFscontained sulfur.

Thermogravimetry is usually used to determine thedegree of functionalization of CNTs [19,24]. Since acidtreatment incorporates –COOH and –SO3H groups at theCNF surface, as demonstrated by the XPS spectra, someaspects should be clarified before calculations are made.The TGA profiles, obtained in a nitrogen atmosphere, ofboth pristine and treated CNF are shown in Fig. 4. PristineCNFs remained thermally stable until 550 �C. Then, anabrupt weight loss was observed, probably due to therelease of oxidized groups from the surface [35]. Acid-trea-ted fibers (with oleum or commercial sulfuric acid) showeddifferent TGA profiles than benzenesulfonated samples,losing some mass at 100 �C before the functionalized ones.

In order to complement this analysis, temperature-pro-grammed desorption experiments were carried out. TheEGA-MS profiles of the different samples are compiled inFig. 5. The fragments H2O, CO, CO2, SO2 and SO3 weremonitored. In general, a CO-desorption band was observedat low-temperatures: ca. 300 �C. The process occurredsimultaneous to the evolution of CO2 and H2O, and hencecan be ascribed to the decomposition of carboxylic acids[36]. From the desorption profiles of the CNF samples,

0 100 200 300 400 500 600 700 800 900 100075

80

85

CNF-BzSO3H (3)

Wei

gh

T(ºC)

Fig. 4. TGA curves of pristine CNFs, diazotized CNFs, and sulfuric acid-treated CNFs obtained in N2 atmosphere.

0 200 400 600 800 1000

MS

sign

al (I

/I Ar)

Temperature (ºC)

CNF

0.0005

SO3

SO2

CO

CO2

H2O

0 200 400 600 800 1000

MS

sign

al (I

/I Ar)

Temperature (ºC)

CNF-Ac

0.0001

H2O

CO

CO2

SO2

SO3

0 200 400 600 800 1000

CNF-Oleum

MS

sign

al (I

/I Ar)

Temperature (ºC)

0.0002

H2O

CO

CO2

SO2

SO3

0 200 400 600 800 1000

MS

sign

al (I

/I Ar)

MS

sign

al (I

/I Ar)

Temperature (ºC)

CNF-BzSO3H(2)

0.0005

H2O

CO

CO2

SO2

SO3

0 200 400 600 800 1000

Temperature (ºC)

CNF-BzSO3H (3)

0.005

H2O

CO

CO2

SO2

SO3

Fig. 5. EGA-MS profiles of pristine CNFs, diazotized CNFs, and sulfuric acid-treated CNFs.

1674 F. Barroso-Bujans et al. / Carbon 45 (2007) 1669–1678

CNF-oleum and CNF-Ac, CO, CO2 and H2O were theonly fragments observed, at least at low-temperatures. Infact, the three desorption processes occurred simulta-neously, probably indicating that such fragments wouldbe due to the decomposition of carboxylic acids species.At higher temperatures, starting from ca. 460 �C a strongCO-desorption can be observed. This process was notaccompanied by H2O desorption and, for the CNF-oleumsample, only a moderate degree of CO2 desorption tookplace. For these samples, the maximum of the CO-desorp-tion band is located beyond 900 �C. At such high-tempera-tures, CO-desorption has been ascribed to the presence ofphenol, ether, hydroquinone structures and, at the highesttemperatures, quinone [36]. It is also worth remarking thatfor the CNF-Ac sample at ca. 230 �C some SO2 + H2Ofragments were detected.

The picture is slightly different for the CNF-BzSO3H (2)and CNF-BzSO3H samples (3). The low-temperature CO-desorption process was scarcely observed. However, even

at low-temperatures H2O desorption was quite strong. Thisprocess coincides with a strong SO2 desorption band.HSO3 groups are known to decompose into SO2 andH2O. Thus, it is reasonable to assume that even if the evo-lution of SO3 species had been detected, they would havebeen present within the solid, evolving as SO2 + H2O.Nonetheless, some CO2 and CO-desorption processes,especially for sample CNF-BzSO3H (3) at ca. 290 �C (peakmaximum), were also observed. Additionally, a strong CO-desorption process starting at ca. 350 �C and displayingmaximum intensity at ca. 700 �C was seen. The processwas accompanied by desorption of CO2 and H2O, althoughfrom the figures it may be deduced that the amount of des-orbed CO was much larger than that of CO2 and H2O. Thisfeature can probably be accounted for by the presence ofphenol-, hydroquinone- or ether-type species. However,no high-temperature CO-desorption band can be seen inthe EGA profile of these samples. For the CNF, CNF-oleum and CNF-Ac samples a rather intense CO-desorp-

0.000 0.001 0.002 0.0030

1

2

3

NFC

-BzS

O3H

(3)

NFC

-BzS

O3H

(2)

NFC

-BzS

O3H

(1)

NFC

-Ac

S/C

(%)

[H+ ](mol/g)

XPS EDX TGA Elemental Analysis

NFC

Fig. 6. Comparison of S/C ratio obtained by XPS, EDX, TGA andElemental Analysis, with the acid content of the nanofibers determined bytitration.

F. Barroso-Bujans et al. / Carbon 45 (2007) 1669–1678 1675

tion band displaying a maximum at ca. 900 �C and ascribedto the presence of quinone and phenol species is observed.Such a maximum in the CO-desorption profile is notobserved for the CNF-BzSO3 samples, although someCO-desorption could be still observed at such high-temper-atures. Apparently, treatment with benzenesulfonic diazo-nium salt either inhibits (partially) or modifies (probablyby interacting with them during the diazotization reaction)the species responsible for CO release at high-temperature.As discussed above, such species are formed during acidtreatment of the support. When the nanofibers were treatedsimultaneously with both acid and benzenesulfonic diazo-nium salt the nature of the species formed on the surfaceof the nanofibers was controlled by the latter species, i.e.,by the diazotization reaction.

From the EGA-MS and XPS results, it may be deducedthat acid treatment of the CNFs produces carboxy speciesat the surface that compete during the diazotization reac-tion with the addition of benzenesulfonic groups, sulfonatespecies preferentially appearing over oxidized ones. Thisfeature gives rise to two different TGA profiles for the acidand benzenesulfonated CNFs, which permits the degree offunctionalization of the latter to be calculated.

The calculation of the degree of functionalization ofmodified CNFs was performed using the percentage valuesobtained at 500 �C, considering that the released groupswere –BzSO3H. The highest value obtained was 0.71% offunctional group per carbon atom in sample CNF-BzSO3H(3) (Table 2). These results showed a good correlation withthose obtained with elemental analyses, since both tech-niques analyze the bulk mass of the CNFs. Evidently, theS/C ratios obtained from these techniques are lower thanthose determined with XPS and EDX, which detect theatoms from the surface.

An attempt has been made to rationalize both concen-tration and location of sulfur-containing groups on theCNFs. This can be understood by taking into account thatthe depth analyzed by XPS and EDX differs markedly fromthe bulk material determined by TGA and chemical analy-sis. It must also be considered that the sulfonic acid groupsare essentially exposed on the CNF surface and thereforethe proton should be quantitatively titrated. In Fig. 6,experimental S/C ratios determined by XPS, EDX , TGAand chemical analysis are plotted as a function of H+ sur-face concentration. As sulfonic acid functionalization isessentially a surface process, the largest S/C ratios havedetected by XPS since the analysis depth of this techniqueis only confined to a few atomic layers of the CNF sub-strate. It can be noted that the (S/C) XPS ratio –[H+]dependence is almost linear in the region of low and med-ium [H+] concentration although it deviates in sample 3,displaying the largest [H+]. This may be due to the fact thatin sample CNF-BzSO3H (3) there is a distribution gradientof –SO3H groups from the external surface and pore mouthtoward the inner walls, that is the inner pore surfaceappears to be carpeted by a higher surface density of –SO3H groups than the outer CNF surface. Almost linear

dependence between S/C ratios, determined by TGA andchemical analysis, and [H+] is obtained, although thevalues of this ratio are substantially lower than those deter-mined by XPS, i.e. ca. one third for sample CNF-BzSO3H(2). This is expected since both TGA and elemental analysismeasure the bulk material. Obviously, there are many Catoms in sub-surface regions, e.g. inside the pore structure,inside a non-accessible C–C network, and hence impossibleto functionalize. The relatively large S/C ratio of the pris-tine CNFs is consistent with the formation of some car-bon–sulfur bonds, and also oxidized –SO3H groups,coming respectively from the carbon source and the cata-lyst employed in the CNF synthesis. Finally, EDX providea linear dependence between the S/C ratio and [H+]. As theanalysis depth of EDX is confined to ca. 1 lm3, thisvolume of solid includes not only the S/C ratio of theexternal surface, but also the S/C ratio of many pores(micro, meso and very likely some macropores) presentin 1 lm3. Thus S/C ratios derived from EDX are inbetween that determined by XPS and, TGA and elementalanalysis.

3.2. Graphitic structure of the nanofibers

The graphitic structure of the carbon nanofibers aftertreatment in sulfuric medium was evaluated by X-ray dif-fraction and Raman spectroscopy.

X-ray diffractograms of both pristine and treated CNFsamples are shown in Fig. 7. As expected, the untreatedfibers exhibited several graphite peaks, the most visiblebeing the d(002) reflection located at 26�. Further peakswere discernible in the 42� and 45� regions correspondingto (100) and (101) reflections and that close to 50� was

0 10 20 30 40 50 60

004

2θ

002

100

CNF

101

CNF-BzSO3H (1)CNF-BzSO3H (2)

CNF-BzSO3H (3)CNF-AcCNF-Oleum

Fig. 7. XRD patterns of pristine CNFs, diazotized CNFs, and sulfuricacid-treated CNFs. Assignation of reflection lines.

1676 F. Barroso-Bujans et al. / Carbon 45 (2007) 1669–1678

assigned to the (004) line [37]. Treatment of the CNFshardly changed the position or width of these reflections.Accordingly, it may be concluded that the graphitic struc-ture of the bulk nanofibers does not change upon treatmentwith sulfuric acid.

The Raman spectra of both pristine and sulfuric acid-treated carbon nanofibers in the 400–1800 cm�1 range areshown in Fig. 8. Two typical bands may be clearlyobserved for all the materials studied: the D-band at1347 cm�1 can be attributed to the presence of ‘‘disorder’’

400 600 800 1000 1200 1400 1600 1800

CNF-BzSO3H (3)

CNF-BzSO3H (2)

CNF-BzSO3H (1)

CNF-Oleum

CNF-Ac

CNF

Wavenumber (cm-1)

Fig. 8. Raman spectra of pristine CNFs, diazotized CNFs, and sulfuricacid-treated CNFs. Decrease in D-bands with acid and diazoniumtreatment.

or defects in the sp2 graphitic structure while the G-band at1583 cm�1 would correspond to ordered graphite [38]. Sev-eral defects can be described as ‘‘disorder’’: namely thepresence of edges in small crystals, deviations from planar-ity, the presence of a certain number of C atoms in the sp3

hybridization state, etc.It is usually accepted that the ratio between the inte-

grated intensities of both bands, ID/IG, provides usefulinformation about the crystalline order of the graphitic sys-tem [37,38]. Thus, a high value indicates a low graphitizedsystem, with superficial defects and the presence of amor-phous carbon, as revealed by TEM micrographs (Fig. 9).It may be seen that this ratio drops moderately upon treat-ment of the CNFs with sulfuric acid. This treatmentinduces a decrease in the intensity of the D-band. Accord-ing to the molecular model of Negri et al. [39,40], the chem-ical structure and the size of the graphitic layer can affectthe intensity of the D-band. Hence, we attribute thedecrease in ID/IG to different factors: a decrease in amor-phous carbon and/or the oxidation of C atoms located atthe edges, leading to a decrease in the intensity of the D-band. The oxidation of nanofibers has been observed withthe above mentioned techniques.

The anchorage of benzenesulfonic groups on the edge ofthe graphene sheets induces a further decrease in the D-band intensity. This can be also attributed to the chemicalchange occurring at the edges, leading to a variation in theD-band Raman cross-section. Thus, the changes observedin ID and IG can be attributed to a modification in thechemical structure of the edges rather than to a change inthe order/disorder relationship. This effect seems to bemore important than a possible decrease in amorphous car-bon because this material is still observed in the TEMmicrographs of treated CNFs. This is consistent with theXPS results, which did not show the appearance of anyp! p* transition of carbon atoms in graphene structuresafter any treatment with sulfuric acid.

3.3. Morphological and textural characteristics

The nanoscopic morphology of the carbon nanofiberswas observed by TEM (Fig. 9). The graphite layers of suchfibers seem to have a fishbone structure in pristine CNFs.After acid and diazonium treatment of CNFs, a clear phys-ical change is observed due to the appearance of roughnessin the graphite part of CNFs. A small fraction of amor-phous carbon can also be discerned; this is usually formedwhen the synthesis temperature of CNFs is somewhatbelow 700 �C [41]. The amorphous region in the CNF-oleum and CNF-BzSO3 (3) seems to be unaltered afterthe acid and diazonium treatments.

The textural characteristics of the samples are shown inTable 2 (BET surface area (S)). All samples displayed typeII isotherms characteristics of either non-porous or macro-porous materials according to the BDDT’s classification[42]. Nevertheless the isotherms do display a certain hyster-esis loop (H3 type), characteristic of solids consisting of

Fig. 9. Representative TEM images of pristine CNFs, CNF-oleum and CNF-BzSO3H (3).

F. Barroso-Bujans et al. / Carbon 45 (2007) 1669–1678 1677

lamellar aggregates displaying a broad pore distribution[43]. The pore-size distribution was calculated by applyingthe BJH method [44] to the desorption branch of the iso-therms. It reveals a broad distribution of pores, showinga maximum centered between 30 and 40 nm, characteristicof mesoporous materials (CNF: 45 nm; CNF-oleum: 35 nmand CNF-BzSO3H (3): a bimodal distribution with max-ima at 20 and 30 nm). Surface treatment of the pristinesample gave rise to a decrease in the specific area in thenanofibers. No significant changes between the differenttreatments were observed.

4. Conclusions

Surface treatment of CNFs with sulfanilic acid in sulfu-ric media by means of the diazotization reaction leads tothe incorporation of benzenesulfonic groups. XPS providesclear evidence of surface sulfur in these modified CNFs andits evaluation resulted in 2.4% of the S/C surface atomicratio as the highest obtained value within several samples.Fuming sulfuric acid used as solvent in the diazotizationreaction favours the incorporation of –BzSO3H and –HSO3 groups giving the highest S/C ratio. Moreover, ahigher sulfuric acid concentration increases the S/C con-tent. Surface and bulk analysis of CNFs by means ofXPS, EDX, elemental analysis and thermogravimetryrevealed the increase in –BzSO3H due to changes in theaforementioned reaction conditions. Sulfuric acid is effec-tive for producing some oxidation of the carbon layers tosome –COOH groups and graft –SO3H entities. Ramanspectra are also sensitive to the functionalization of CNF,which mainly affects the D-band. The changes occurringin the latter band can be attributed to the chemical changeoccurring at the edges, leading to a variation in the D-bandRaman cross-section rather than a decrease in amorphouscarbon.

Acknowledgements

F. Barroso-Bujans thanks the Ministerio de Educacion yCiencia (Spain) for the mobility program and S. Rojasthanks the Ramon y Cajal program from this ministry.

References

[1] Rodriguez NM. A review of catalytically grown carbon nanofibers. JMater Res 1993;8(12):3233–50.

[2] Kelarakis A, Yoon K, Somani R, Sics I, Chen XM, Hsiao BS, et al.Relationship between structure and dynamic mechanical properties ofa carbon nanofiber reinforced elastomeric nanocomposite. Polymer2006;47(19):6797–807.

[3] Radovic LR, Rodrıguez-Reinoso F. Carbon materials in catalysis. In:Thrower PA, editor. Chemistry and physics of carbon, vol. 25. NewYork: Dekker; 1997. p. 243–358.

[4] Rodrıguez Reinoso F. The role of carbon materials in heterogeneouscatalysis. Carbon 1998;36:159–75.

[5] Serp P, Corrias M, Kalck P. Carbon nanotubes and nanofibers incatalysis. Appl Catal A 2003;253:337–58.

[6] Baker SE, Tse KY, Lee CS, Hamers RJ. Fabrication and character-ization of vertically aligned carbon nanofiber electrodes for biosen-sing applications. Diam Relat Mater 2006;15:433–9.

[7] Webster TJ, Waid MC, McKenzie JL, Price RL, Ejiofor JU. Nano-biotechnology: carbon nanofibers as improved neural and orthopae-dic implants. Nanotechnology 2004;15:48–54.

[8] Rzepka M, Bauer E, Reichenauer G, Schliermann T, Bernhardt B,Bohmhammel K, et al. Hydrogen storage capacity of catalyticallygrown carbon nanofibers. J Phys Chem B 2005;109:14979–89.

[9] Subramanian V, Zhu H, Wei B. High rate reversibility anodematerials of lithium batteries from vapor-grown carbon nanofibers. JPhys Chem B 2006;110:7178–83.

[10] Ran S, Burger C, Sics I, Yoon K, Fang D, Kim K, et al. In situsynchrotron SAXS/WAXD studies during melt spinning of modifiedcarbon nanofiber and isotactic polypropylene nanocomposite. Col-loid Polym Sci 2004;282:802–9.

[11] Bubert H, Ai X, Haiber S, Heintze M, Bruser V, Pasch E, et al. Basicanalytical investigation of plasma-chemically modified carbon fibers.Spectrochim Acta B 2002;57:1601–10.

[12] Brandl W, Marginean G. Functionalisation of the carbon nanofibersby plasma treatment. Thin Solid Films 2004;447–448:181–6.

[13] Kubota S, Nishikiori H, Tanaka N, Endo M, Fujii T. Dispersion ofacid-treated carbon nanofibers into gel matrices prepared by the sol–gel method. J Phys Chem B 2005;109:23170–4.

[14] Gardner S, Singamsetty CSK, Booth GL, He GR, Pittman Jr CU.Surface characterization of carbon fibers using angle-resolved XPSand ISS. Carbon 1995;33(5):587–95.

[15] Pittman Jr CU, Jiang W, Yue ZR, Gardner S, Wang L, Toghiani H,et al. Surface properties of electrochemically oxidized carbon fibers.Carbon 1999;37:1797–807.

[16] Valentın JL, Lopez-Manchado MA, Posadas P, Ibarra L. Nanocom-posites de caucho butadieno reforzado con nanofibras de carbono.Efecto del tratamiento superficial. Actas del VI congreso nacional demateriales compuestos. Valencia: Asociacion Espanola de Materi-ales Compuestos (AEMAC); 2005. p. 979–86.

1678 F. Barroso-Bujans et al. / Carbon 45 (2007) 1669–1678

[17] Li J, Vergne MJ, Mowles ED, Zhong WH, Hercules DM, LukehartCM. Surface functionalization and characterization of graphiticcarbon nanofibers (GCNFs). Carbon 2005;43:2883–93.

[18] Baker SE, Tse KY, Hindin E, Nichols BM, Clare TL, Hamers RJ.Covalent functionalization for biomolecular recognition on verticallyaligned carbon nanofibers. Chem Mater 2005;17:4971–8.

[19] Bahr JL, Yang J, Kosynkin DV, Bronikowski MJ, Smalley RE, TourJM. Functionalization of carbon nanotubes by electrochemicalreduction of aryl diazonium salts: a bucky paper electrode. J AmChem Soc 2001;123:6536–42.

[20] Dyke CA, Tour JM. Unbundled and highly functionalized carbonnanotubes from aqueous reactions. Nanoletter 2003;3:1215–8.

[21] Bahr JL, Tour JM. Highly functionalized carbon nanotubes usingin situ generated diazonium compounds. Chem Mater 2001;13:3823–4.

[22] Hudson JL, Casavant MJ, Tour JM. Water-soluble, exfoliated,nonroping single-wall carbon nanotubes. J Am Chem Soc2004;126:11158–9.

[23] Hickner MA, Ghassemi H, Kim YS, McGrath JE. Alternativepolymer systems for proton exchange membranes (PEMs). Chem Rev2004;104(10):4587–611.

[24] Ying Y, Saini RK, Liang F, Sadana A, Billups WE. Functionalizationof carbon nanotubes by free radicals. Organ Lett 2003;5(9):1471–3.

[25] Wagner CD, Davis LE, Zeller MV, Taylor JA, Raymond RH, GaleLH. Empirical atomic sensitivity factors for quantitative analysis byelectron spectroscopy for chemical analysis. Surf Interface Anal1981;3:211–25.

[26] Hu H, Bhowmik P, Zhao B, Hamon MA, Itkis ME, Haddon RC.Determination of the acidic sites of purified single-walled carbonnanotubes by acid–base titration. Chem Phys Lett 2001;345:25–8.

[27] Boehm HP. Surface oxides on carbon and their analysis: a criticalassessment. Carbon 2002;40:145–9.

[28] Lennon D, Lundie DT, Jackson SD, Kelly JG, Parker SF. Injectionand stabilization of mesophase pitch in the fabrication of carbon–carbon composites. Part III: Mesophase stabilization at lowtemperatures and elevated oxidation pressures. Langmuir 2002;18:4667–73.

[29] Hiura H, Ebbesen TW, Tanigaki K. Opening and purification ofcarbon nanotubes in high yields. Adv Mater 1995;7:275–6.

[30] Martınez MT, Callejas MA, Benito AM, Cochet M, Seeger T, AnsonA, et al. Sensitivity of single wall carbon nanotubes to oxidativeprocessing: structural modification, intercalation and functionalisa-tion. Carbon 2003;41:2247–56.

[31] Okpalugo TIT, Papakonstantinou P, Murphy H, McLaughlin J,Brown NMD. High resolution XPS characterization of chemicalfunctionalised MWCNTs and SWCNTs. Carbon 2005;43(1):153–61.

[32] Duesberg GS, Graupner R, Downes P, Minett A, Ley L, Roth S,et al. Hydrothermal functionalisation of single-walled carbon nano-tubes. Synth Metal 2004;142:263–6.

[33] Jimenez JM, Fierro JLG. X-ray photoelectron spectroscopic study ofpetroleum cokes. Surf Interface Anal 1996;24:223–36.

[34] Zackrisson J, Andren H-O. Effect of carbon content on themicrostructure and mechanical properties of (Ti, W, Ta, Mo)(C,N)–(Co, Ni) cermets. Int J Refract Metal Hard Mater1999;17(4):265–73.

[35] Ros TG, van Dillen AJ, Geus JW, Koningsberger DC. Surfaceoxidation of carbon nanofibers. Chem Eur J 2002;8(5):1151–62.

[36] Figueiredo JL, Pereira MFR, Freitas MMA, Orfao JMM. Modifi-cation of the surface chemistry of activated carbons. Carbon1999;37:1379–89.

[37] Paredes JI, Burghard M, Martınez-Alonso A, Tascon JMD. Graph-itization of carbon nanofibers: visualizing the structural evolution onthe nanometer and atomic scales by scanning tunneling microscopy.Appl Phys A 2005;80:675–82.

[38] Lim S, Yoon SH, Mochida I, Chi JH. Surface modification of carbonnanofiber with high degree of graphitization. J Phys Chem B2004;108(5):1533–6.

[39] Negri F, Castiglioni C, Tommasini M, Zerbi GA. Computationalstudy of the Raman spectra of large polycyclic aromatic hydrocar-bons: toward molecularly defined subunits of graphite. J Phys ChemA 2002;106:3306–17.

[40] Negri F, Donato E, Tommasini M, Castiglioni C, Zerbi G, Mullen K.Resonance Raman contribution to the D band of carbon materials:modeling defects with quantum chemistry. J Chem Phys2004;120(24):11889–900.

[41] Lee S-Y, Yamada M, Miyake M. Synthesis of carbon nanotubes overgold nanoparticle supported catalysts. Carbon 2005;43(13):2654–63.

[42] Brunauer S, Deming LS, Deming WS, Teller E. On a theory of thevan der Waals adsorption of gases. J Am Chem Soc 1940;62:1723–32.

[43] Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA,Rouquerol J, et al. Reporting physisorption data for gas/solidsystems with special reference to the determination of surface areaand porosity. Pure Appl Chem 1985;57:603–19.

[44] Barret EP, Joyner LS, Halenda PP. The determination of porevolume and area distributions in porous substances. I. Computationsfrom nitrogen isotherms. J Am Chem Soc 1951;73:373–80.