C A R B O N 4 7 ( 2 0 0 9 ) 5 7 8 – 5 8 8

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Multifunctional hybrid materials composed of [60]fullerene-based functionalized-single-walled carbon nanotubes

Silvia Giordania,b, Jean-Francois Colomerc, Fabrizio Cattaruzzaa, Jessica Alfonsid,Moreno Meneghettid, Maurizio Pratoa, Davide Bonifazia,c,*

aCenter of Excellence for Nanostructured Materials, CENMAT, Dipartimento di Scienze Farmaceutiche, INSTM UdR di Trieste,

Universita degli Studi di Trieste, Piazzale Europa 1, 34127 Trieste, ItalybSchool of Chemistry/Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN),

University of Dublin Trinity College, Dublin 2, IrelandcChemistry Department, University of Namur, Rue de Bruxelles 61, 5000 Namur, BelgiumdDipartimento di Scienze Chimiche, Universita degli Studi di Padova, 35131 Padova, Italy

A R T I C L E I N F O

Article history:

Received 6 September 2008

Accepted 23 October 2008

Available online 7 November 2008

0008-6223/$ - see front matter � 2008 Elsevidoi:10.1016/j.carbon.2008.10.036

* Corresponding author: Address: Chemistry725433.

E-mail addresses: dbonifazi@units.it, dav

A B S T R A C T

We report the synthesis and characterization of several hybrid [60]fullerene–SWCNT mate-

rials that combine [60]fullerenes with appended photoactive ferrocenyl or porphyrinyl

functionalities and SWCNTs into a single multifunctional structure, where the dyads are

covalently attached to the exo-surface of SWCNTs. The structural properties of all hybrids

have been characterized using a large variety of spectroscopic and HR-TEM techniques.

Raman spectra showed how all SWCNTs were functionalized and the presence of func-

tional groups in the nanotube derivatives. Furthermore, these spectra reveal a new elec-

tronic activity of the compounds due to the interaction of the functional groups with the

SWCNT frameworks. XPS investigations have documented the presence of [60]fullerene

derivatives around the exo-surface of the oxidized SWCNTwalls, exhibiting a characteristic

photoelectron N 1s emission peak at 400.3 eV. Very importantly, by means of HR-TEM inves-

tigations we have also observed the presence of the [60]fullerene functions on the SWCNT

outer surface by imaging spherical structures. The presence of the porphyrinyl and ferroce-

nyl fragments, which can act as effective chromophores and electroactive species, makes

this class of materials very interesting for applications in optoelectronics and photovolta-

ics, and bio-applications, for example in the field of diagnosis and treatment.

� 2008 Elsevier Ltd. All rights reserved.

1. Introduction

The discoveries of C60 in 1985 [1] and the milestone paper by

Iijima about carbon nanotubes in 1991 [2] have arguably

inspired a new interdisciplinary era in material science and

technology. Both [60]fullerene and carbon nanotubes (CNTs)

[3] display unique structures that bring with them remarkable

mechanical, thermal, and optical properties which, in turn,

er Ltd. All rights reserved

Department, University o

ide.bonifazi@fundp.ac.be

may be harnessed into devices that are suitable for applica-

tions in electronics, advanced materials, and nanomedicine

[4,5]. The ability to functionalize [60]fullerenes [6] and CNTs

[7–11] through controlled structural modifications [12,13] is

an essential prerequisite for manufacturing purposes as it

may lead to increased solubility and processability, as well

as to opportunities for fine-tuning the physical properties

thereby enabling the construction of more responsive, selec-

.

f Namur, Rue de Bruxelles 61, 5000 Namur, Belgium. Fax: +32 (0) 81

(D. Bonifazi).

C A R B O N 4 7 ( 2 0 0 9 ) 5 7 8 – 5 8 8 579

tive, or faster practical devices [14–18]. The major chemical

methodologies employed to modify CNTs are divided into

two categories: [7] (i) non-covalent (or supramolecular) and

(ii) covalent functionalization. While the non-covalent meth-

ods do not lead to a structural modifications of the nanotube

framework, the covalent modifications often cause great

structural (and accordingly electronic) alterations as a conse-

quence of the change in the carbon hybridization.

[60]Fullerene is one of the most widely used electron

acceptor component in molecular dyads and much effort

has been focused on the development of hybrid materials

[19] containing C60 and its derivatives with the aim to study

their electronic properties [20–28]. Covalent and supramolec-

ular modification of the [60]fullerene modules with electron

donating moieties, such as porphyrins or ferrocenes, is cur-

rently of high importance, owing to their efficiency in under-

going photoinduced energy/electron transfer [27,29–31] from

the electron donor moiety to the [60]fullerene acceptor and

to their enhancement of the non-linear [32] optical properties.

In this context, we have recently prepared some porphyrin–

and ferrocene–fullerene conjugates, which displayed efficient

photoinduced electron transfer and NLO properties [33].

Therefore, new hybrid materials comprising both [60]ful-

lerene and CNTs would be extremely attractive since the

inherent mechanical/structural strength imparted by the car-

bon skeleton of CNTs could be complemented by a range of

desirable tunable optical properties provided by [60]fullerene

derivatives that decorate the carbon exo-surface [34]. It is

thus possible to envision how the versatility and usefulness

of such macromolecular species can be further expanded

and refined by coupling them to the optoelectronic properties

that can be offered by C60 and its derivatives [34]. A major

challenge in this field, therefore, is to develop a range of reli-

able and effective synthetic methods for the construction of

[60]fullerene/CNT based hybrid materials, and only a small

number of approaches have been reported to date. First,

[60]fullerene–pyrene derivatives have been immobilized onto

the surface of single-walled carbon nanotubes (SWCNTs) via

non-covalent interactions between the pyrene and the

SWCNTs [35]. Second, [60]fullerene has been covalently at-

tached to the surface of SWCNTs by combining pre-made iron

catalyst particles and SWCNTs during CO disproportionation

[36], and by allowing amine-functionalized [60]fullerenes to

react with acid [37] or acyl-chloride functionalized SWCNTs

[38]. The goal of our current efforts is thus to produce, devel-

op, and comprehensively characterize novel functional hybrid

materials by combining [60]fullerenes with appended electro-

active and chromophoric species, e.g., ferrocene or porphyrin

functionalities, and SWCNTs into single multifunctional

structures, where the [60]fullerenes are covalently attached

to the exo-surface of the SWCNTs. Both porphyrins and ferro-

cene are suitable partners for carbon nanotubes in nanocom-

posites due to their (electro) luminescence and light

absorption properties [39–44], which can be exploited in the

engineering of new materials for applications in optoelec-

tronics and photovoltaics. In this article, we report the syn-

thesis and characterization of several [60]fullerene–SWCNT

hybrid materials prepared by amidation reactions between

acid-functionalized SWCNTs and amine-functionalized

[60]fullerene derivatives bearing an appended ferrocenyl (1a-

f-SWCNT), a Zn(II)–tetraphenylporphyrinyl (1b-f-SWCNT), or

a N,N-dimethylanilinyl (1c-f-SWCNT) unit (Fig. 1).

2. Experimental

2.1. Materials

Purified single-walled nanotubes were produced by the HiPCO

method, purchased from Carbon Nanotechnologies Inc. (lot

R0510C). [60]Fullerene derivatives 1a–c were synthesized

according to literature procedures [45]. Reagents and solvents

were purchased reagent-grade from Fluka, Aldrich, or J.T. Baker

and used without further purification unless otherwise stated.

Moisture-sensitive reactions were performed under argon (N2)

in oven-dried (180 �C) glassware. CH2Cl2 was freshly distilled

from CaH2, THF from Na/benzophenone ketyl, and DMF and

CCl4 dried over 4 A molecular sieves. Molecular sieves (4 A) were

activated immediately before use by heating in a vacuum oven

to 300 �C at 0.1 Torr for 2 d. Evaporation and concentration un-

der reduced pressure were performed below 50 �C at water aspi-

rator pressure, and compounds were dried at 10�2 Torr.

2.2. Preparation of 1a-f-SWCNT

(A) A suspension of 25.0 mg of SWCNT–COOH in 5.0 mL of

SOCl2 was refluxed for 24 h under a nitrogen atmosphere (a

few drops of DMF were added). SOCl2 was removed under vac-

uum, and then 10 mg of 1a in 20 mL of dry pyridine were

added. After stirring at 120 �C for 6 days, the reaction mixture

was cooled down to room temperature, then filtered using a

0.2 lm pore-sized polycarbonate membrane filter. After being

washed with DMF, CH2Cl2, and Et2O to remove the excess of

1a, the black solid was dried under vacuum to afford 36 mg

of 1a-f-SWCNT. (B) Alternatively, a suspension of 7.0 mg of

SWCNT–COOH in 10.0 mL of DMF was sonicated in sonic bath

for 10 min. DMF solutions of NHS (10 mg) and EDC (7 mg) were

added consecutively and the suspension sonicated for 10 min

after each addition, then stirred at room temperature for 1 h.

To the activated nanotube dispersion, 100 mg of 1a in 20 mL of

dry pyridine were added. After stirring at room temperature

for 10 days, the reaction mixture was then filtered on a

0.2 lm pore-sized polycarbonate membrane filter. The black

solid collected on the filter was dissolved in toluene, soni-

cated in the sonic bath for 10 min and filtered again on a 0.2

lm pore-sized polycarbonate membrane filter. This procedure

was repeated many times for each of the following solvents:

CH2Cl2, DMF, pyridine, and Et2O to remove the excess of 1a.

After being washed with MeOH, the black solid was dried un-

der vacuum to afford 20 mg of 1a-f-SWCNT.

2.3. Preparation of 1b-f-SWCNT

A suspension of 20 mg of SWCNT–COOH in 25 mL of DMF was

sonicated in sonic bath for 60 min. DMF solutions of NHS

(8 mg) and EDC (12 mg) were added consecutively and the sus-

pension sonicated for 10 min after each addition, then stirred

at room temperature for 1 h. To the activated nanotube disper-

sion, 100 mg of 1b in 20 mL of DMF were added. After stirring

at room temperature for 4 days, the reaction mixture was then

filtered on a 0.2 lm pore-sized polycarbonate membrane filter.

580 C A R B O N 4 7 ( 2 0 0 9 ) 5 7 8 – 5 8 8

The black solid collected on the filter was dissolved in pyridine,

sonicated in the sonic bath for 10 min and filtered again on a

0.2 lm pore-sized polycarbonate membrane filter. This proce-

dure was repeated many times for each of the following solvents:

Na2CO3 in H2O, H2O, DMF, Et2O, DMF, and toluene (4 times) to re-

move the excess of 1b. After being washed with MeOH, the black

solid was dried under vacuum to afford 20 mg of 1b-f-SWCNT.

2.4. Preparation of 1c-f-SWCNT

A suspension of 20 mg of SWCNT–COOH in 30 mL of DMF was

sonicated in sonic bath for 20 min. DMF solutions of NHS

(8 mg) and EDC (12 mg) were added consecutively and the sus-

pension sonicated for 10 min after each addition, then stirred

at room temperature for 1 h. To the activated nanotube dis-

persion, 50 mg of 1c in 5 mL of DMF and 1.5 mL of pyridine

were added. After stirring at room temperature for 4 days,

the reaction mixture was then filtered on a 0.2 lm pore-sized

polycarbonate membrane filter. The black solid collected on

the filter was dissolved in pyridine, sonicated in the sonic

bath for 10 min and filtered again on a 0.2 lm pore-sized poly-

carbonate membrane filter. This procedure was repeated

many times for each of the following solvents: DMF, toluene,

CH2Cl2, and Et2O to remove the excess of 1c completely. After

being washed with MeOH, the black solid was dried under

vacuum to afford 32 mg of 1c-f-SWCNT.

2.5. Characterization

Thermogravimetric analysis were performed with a TGA Q500

instrument from TA instruments. The micro-Raman spectra

were collected with an inVia Renishaw system equipped with

633 and 488 nm laser sources. Absorption spectra were re-

corded with a Varian Cary 5000i. X-ray photoelectron spectros-

copy analyses were performed with a SSX-100 system (Surface

Science Instrument). The photon source was a monochroma-

tised Al Ka line (hm = 1486.6 eV) applied with a takeoff angle

of 35�. The nominal resolution of the system (source + ana-

lyser) was 0.9 eV. In the spectrum analysis, the background sig-

nal was subtracted by Shirley’s method. The C 1s core level

peak position of carbon atoms was taken as the reference at

284.7 eV. The spectrum analysis was carried out by fitting the

peak shape obtained in the same analysing conditions and

other components with mixed (Gaussian + Lorentzian) line

shapes. XPS atomic ratios have been estimated from the

experimentally determined area ratios of the relevant core

lines, corrected for the corresponding theoretical atomic

cross-sections and for a square root dependence of the photo-

electrons kinetics energies. Transmission electron microscopy

images were obtained with a JEOL 4000 EX microscope operat-

ing at 400 kV. A few milligrams of the sample were dispersed in

ethanol and one droplet was deposited onto a holey-carbon

TEM grid and allowed to dry. The images are typical and repre-

sentative of the samples under observation.

3. Results and discussion

Purified and oxidized SWCNTs, SWCNT–COOH, were ob-

tained by reaction of pristine HiPCO SWCNTs with HNO3

(2.6 M) and by oxidation with H2SO4–H2O2 at 318 K for 1 h, fol-

lowing an oxidative protocol as that recently reported by us

(Fig. 1) [46–48]. Both purification and oxidation processes pro-

duce defects on the sidewall and the formation of open ends

that are both terminated by carboxylic acid groups. The

resulting oxidized material was then allowed to react with

NHS and EDC to convert the carboxylic acid groups into acti-

vated NHS-esters, via O-acyl-isourea intermediates. The acti-

vated ester was then condensed with the corresponding

[60]fullerene-bearing amine derivatives 1i (1a-c) [45] in the

presence of pyridine to afford [60]fullerene-functionalized

carbon nanotubes 1i-f-SWCNT (Fig. 1). Alternatively, nanohy-

brid 1a-f-SWCNT could be synthesized following the acyl-

chloride route, allowing the SWCNT–COOH material to react

with SOCl2 under N2 in the presence of a catalytic amount

of DMF. The resulting acid chloride derivative, SWCNT–COCl,

was then heated under reflux with [60]fullerene derivative 1a

to afford the [60]fullerene-bearing SWCNTs, 1a-f-SWCNT. All

SWCNT intermediates as well as the target nanohybrid com-

pounds 1i-f-SWCNTwere fully characterized via thermogravi-

metric analysis, Raman spectroscopy, UV–vis–NIR absorption

spectroscopy, FTIR spectroscopy (see Supplementary mate-

rial), X-ray photoelectron spectroscopy, and high-resolution

transmission electron to obtain a complete picture of the

structural, electronic and chemical properties of the function-

alized carbon nanotubes.

3.1. Thermogravimetric analysis

Thermogravimetric analysis has been used to evaluate the

oxidized (SWCNT–COOH) and functionalized materials (1a-f-

SWCNT, 1b-f-SWCNT, and 1c-f-SWCNT) (Fig. 2). The TGA of

compounds 1a-f-SWCNT, 1b-f-SWCNT, and 1c-f-SWCNT

show a weight loss of about 22%, 20% and 24% at 600 �C,

respectively, as compared to about only 4% of the pristine

SWCNTs and 15% of the oxidized material, SWCNT–COOH.

We assume that this weight loss occurring during fragmenta-

tion is due to pyrolysis of the hydrogenated carbon residues.

On the basis of this assumption, we estimated that the degree

of functionalization is of one [60]fullerene derivative each 290,

600 and 240 carbon atoms, respectively.

3.2. Steady-state UV–vis–NIR absorption measurements

The modification of the sidewalls in the oxidized and [60]ful-

lerene-functionalized materials was also evident in the stea-

dy state absorption spectra in the UV–vis–NIR regions

because of the partial loss of the sharp van Hove singularities

(vHS) typical of pristine SWCNTs. The UV–vis–NIR absorption

spectra of SWCNT–COOH, 1a-f-SWCNT, and 1b-f-SWCNT are

shown in Fig. 3. In addition, the typical porphyrin-centered

bands at 429 (Soret band), 561 (Q band), and 602 (Q band)

nm are easily identified in the spectra for 1b-f-SWCNT, con-

firming the presence of the [60]fullerene–porphyrin fragment

attached to the nanotube framework. Notably, no energy

shifts have been observed for the main porphyrin-centered

absorption bands with respect to the absorption spectra of

1b taken in DMF. This accounts for a lack of strong perturba-

tions of the porphyrin-centered absorption properties as a

consequence of strong ground-state interactions (e.g., p–p or

Fig. 1 – Purification and oxidation (SWCNT–COOH) and synthesis of nanohybrids 1i-f-SWCNT. 1i = 1a, 1b, or 1c [45]. DMF: N,N 0-

dimethylformamide; EDC: 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide; NHS: N-hydroxysuccinimide.

Fig. 2 – Thermogravimetric traces for pristine SWCNTs (––), SWCNT–COOH (–––) and 1a-f-SWCNT (–Æ–Æ–Æ), 1b-f-SWCNT (� � �� � �),and 1c-f-SWCNT (short dash line). The temperature interval (200–600 �C) represents the steepest weight loss due to organic

decomposition.

C A R B O N 4 7 ( 2 0 0 9 ) 5 7 8 – 5 8 8 581

charge-transfer) between the porphyrin moiety and the nano-

tube surface, supporting our hypothesis that only a covalent

linkage is responsible for the interaction between the [60]ful-

lerene derivative and the SWCNT framework.

3.3. Raman spectroscopy

Raman spectra of the SWCNTs were also recorded to investi-

gate the effect of the derivatization. The strong increase of

the D-band at 1311 cm�1 observed in these spectra (Fig. 4,

exciting line: 632.8 nm) shows that a large number of defects

(sp3 carbon atoms) are introduced during the functionaliza-

tion of the nanotubes. Furthermore, the RBM spectral region

below 350 cm�1 shows that the oxidation is operated on all

type of nanotubes (both metallic and semiconducting, nano-

tubes, observed below and above 230 cm�1, respectively) and

those smaller in diameters (below about 0.9 nm) are de-

stroyed, when a strong oxidation is used [49]. Relative varia-

tions of the intensity of the RBM bands can be interpreted

recalling that metallic SWCNTs are functionalized more eas-

ily those semiconducting, because their electronic states are

more available to reactions [50]. As a consequence, the reso-

nance condition of the Raman spectrum is more influenced

for the metallic nanotubes. Therefore, the variation of the

intensity of the band at about 195 cm�1, which is related to

metallic nanotubes (in particular (13,4) and (12,6)) [51], derives

400 600 800 1000 1200 1400

600 800 1000 1200 1400

Abs

orba

nce

(a.u

.)

Wavelenght (nm)

Fig. 3 – UV–vis–NIR absorption spectra of SWCNT–COOH (–––),

1a-f-SWCNTs (� � �� � �) and 1b-f-SWCNT (––) in DMF at 298 K.

Insert: SWCNT-centered vHS region.

Fig. 4 – Normalized Raman spectra (kexc = 632.8 nm) of

pristine SWCNTs, SWCNT–COOH, and 1i-f-SWCNT. Insert:

magnified RBM region.

582 C A R B O N 4 7 ( 2 0 0 9 ) 5 7 8 – 5 8 8

from a change of the resonance conditions whereas that of

the band at about 255 cm�1, related to semiconducting tubes

((10,3), (9,4)) [51], is also due to some selection operated by the

oxidation reaction during the first step of the functionaliza-

tion. The Raman signal of [60]fullerene was clearly found in

the spectrum of 1a-f-SWCNT (see Fig. 5). The main [60]fuller-

ene band, related to the vibrational mode Ag(2) of the non-

functionalized molecule, is observed in the spectrum of

[60]fullerene–ferrocene dyad 1a at 1462 cm�1 (see Fig. 5a),

and at the same frequency it is found in the spectrum of

1a-f-SWCNT when excitation light at 488 nm is used (see

Fig. 5b). In Fig. 5b, one also observes the band related to the

[60]fullerene moiety, Hg(7) at 1424 cm�1. A luminescence sig-

nal can be observed in the spectrum of 1a excited at

633 nm, with a maximum at about 2500 cm�1 (13,300 cm�1

in absolute values which corresponds to about 750 nm). This

is not observed in the corresponding spectrum of 1a-f-

SWCNT probably due to the interaction of the molecule with

the nanotube, which opens different deactivation paths like,

for example, charge transfer processes. Raman spectra (see

Fig. 6) of the dyad 1b containing the [60]fullerene cage coupled

to the porphyrin fragment show spectral features which can

be also recognized in the spectrum of the nanotube derivative

1b-f-SWCNT like the band at 1344 cm�1 attributed to the por-

phyrin fragment (see Supplementary material for the porphy-

rin-centered Raman reference spectra). In this case the low

functionalization degree (ca. one functional group per 600 C)

does not allow to detect lower intensity bands like those of

the [60]fullerene fragment that are relatively weak in the

spectrum of 1b. However, one can observe that the strong

luminescence signal at 1950 cm�1 in the spectrum excited at

632.8 nm (13850 cm�1 in absolute values which corresponds

to about 720 nm) is not found in the spectrum of the nanotube

derivative 1b-f-SWCNT indicating that, also in this case, some

new deactivation paths must now be present as a conse-

quence of the interaction of the [60]fullerene dyad with the

nanotube framework.

3.4. X-ray photoelectron spectroscopy

XPS was also employed to investigate the composition of the

functionalized SWCNTs. This technique is a very important

tool to confirm the functionalization of carbon nanotubes

[52–54], since it gives information about the atomic composi-

tions and the type of bond that occurs between the atoms. In

Fig. 7, the low-resolution survey XPS spectrum for samples

containing nanohybrid 1c-f-SWCNT revealing peaks from C

1s (284.7 eV), N 1s (399.5 eV) and O 1s (533.1 eV) core levels,

is reported [45].

The presence of N atoms in the sample (2.7%) shows that

the carbon nanotubes have been modified by the derivatiza-

tion treatment. The high-resolution C 1s core level spectrum

is displayed in Fig. 8a. This spectrum is made of four compo-

nents that can be identified as follows: (1) a main peak at

284.6 eV (generated by photoelectrons emitted from carbon

atoms in graphite configuration, the shake-up structure of

which is out of the considered window); (2) a second peak

centered at 285.4 eV (from the photoelectron contribution of

the sp3 carbon atoms); (3) and (4) two others components

localized at 287 and 288.6 eV attributed to the photoelectrons

emitted from –CH2– carbon atoms bonded to N atoms (C–N)

and CO carbon atoms belonging to the amidic functional

group –C(O)NH–, respectively [52,53,55,56]. The profiles of

the C 1s core level are similar for the two other compounds,

1a-f-SWCNT and 1b-f-SWCNT, and will be not further dis-

cussed here. Considering now the high-resolution N 1s core

level spectrum (Fig. 8b), two nitrogen species must contribute

as components to the spectrum: two tertiary amines (the N 1s

peak for the pyrrolidine ring and that for the N,N 0-dimethyl-

anilinyl group have almost the same energy and can not be

resolved in two different components) and an amide func-

tion. Indeed, we could assign to photoelectrons emitted from

the N atom of tertiary amines and of the amidic function the

peaks centered at 399.5 and 400.2 eV, respectively. Unexpect-

edly, we could also observe a third, very-well visible compo-

nent at 401.8 eV, which we have unambiguously assigned to

photoelectrons emitted from a quaternary N atom. This result

has already been observed and attributed to a protonation

reaction [57]. In this [60]fullerene fragment (i.e., that deriving

Fig. 5 – Raman spectra of [60]fullerene–ferrocene conjugate 1a and nanohybrid 1a-f-SWCNT. Inset: zoomed region.

(a) kexc = 633 nm and (b) kexc = 488 nm.

Fig. 6 – Raman spectra at of [60]fullerene–porphyrin conjugate 1b and nanohybrid 1b-f-SWCNT. Inset: zoomed region.

(a) kexc = 633 nm and (b) kexc = 488 nm.

C A R B O N 4 7 ( 2 0 0 9 ) 5 7 8 – 5 8 8 583

1000 800 600 400 200 0

0

2000

4000

6000

8000

10000

Fe 2p3

C 1s

N 1s

O 1s

O a

Cou

nts

Bending Energy (eV)

Fig. 9 – XPS survey spectrum for nanohybrid 1a-f-SWCNT.

1000 800 600 400 200 0

0

2000

4000

6000

8000

10000

O a

C 1s

N 1s

O 1s

Cou

nts

Bending Energy (eV)

Fig. 7 – XPS survey spectrum for nanohybrid 1c-f-SWCNT.

584 C A R B O N 4 7 ( 2 0 0 9 ) 5 7 8 – 5 8 8

from 1c), the protonation can occur on both tertiary nitrogen

atoms. If now we look at the acid–base properties of the N

atom of the fulleropyrrolidine ring, its pKa is six orders of

magnitude higher (5.6 in 85% [D8]Dioxane/D2O) than that of

isolated pyrrolidine rings (11.1 in 80% Dioxane/Water and

11.6 in 85% [D8]Dioxane/D2O) [58]. Therefore, we can tenta-

tively suggest that the protonation occurs on the N atom of

the N,N 0-dimethylanilinyl substituent. This is consistent with

the observations made with the materials containing 1a-f-

SWCNT, in which no protonation has been detected (see the

high-resolution N 1s core level spectrum, Fig. 10b) since no

additional peaks localized at values superior to 401 eV in term

of bending energy have been observed.

Fig. 9 shows the low-resolution survey spectrum for nano-

hybrids 1a-f-SWCNT containing an appended ferrocenyl

group. The XPS analysis of this compound reveals peaks from

C 1s, N 1s, O 1s and Fe 2p core levels. As shown for the previ-

ous compound, the presence of N (3.7%) and of Fe (1.5%) sup-

ports our assumption that 1a-f-SWCNT has been successfully

attached through the functionalization treatment. Notably,

the experimental atomic ratio N/Fe of 2.4 is very similar to

the theoretical stoichiometric value of 2.

The Fe 2p spectrum (Fig. 10a) is composed of two peaks at

707.8 and 720.6 eV corresponding to the Fe 2p3/2 and Fe 2p1/2

208258209

C 1s

4

3

2

1

Inte

nsity

(a. u

.)

Bending Energy (eV)

a b

Fig. 8 – XPS high-resolution spectra (a) the C 1s and

core levels, respectively. These values are perfectly consistent

with those previously reported for ferrocenyl derivatives

[59,60] revealing the Fe atoms in its doubly charged oxidation

state. The absolute atomic percentage of measured Fe(II) is

about 1.5%. The high-resolution N 1s core level spectrum,

shown in Fig. 10b, agrees with the presence of two different

N atoms in the structure: one belonging to the tertiary amine

group at 399.4 eV and one to the amidic group at 400.3 eV with

a 1:1 peak-area ratio.

Fig. 11 shows the low-resolution survey spectrum for 1b-f-

SWCNT containing an appended Zn(II)–tetraphenylporphyri-

nyl group. The XPS analysis reveals peaks from C 1s, N 1s, O

1s and Zn core levels. The others peaks (Na, Cl, etc.) come

from byproducts remained during the workup and the purifi-

cation processes. The detection of N atoms (3.1%) and of Zn(II)

ions (0.43%) further confirm the presence of the [60]fullerene–

porphyrin dyad 1b in the analysed SWCNT material.

The high-resolution Zn 2p core level spectrum (Fig. 12a) is

characterized by two peaks at 1021.5 and 1044.7 eV corre-

sponding to the Zn 2p3/2 and Zn 2p1/2 core levels, respectively

[61]. These values are perfectly consistent with those previ-

ously reported for Zn(II)–porphyrins. In Fig. 12b, the high-res-

olution spectrum of the N 1s region is shown. A free-base

porphyrin has two symmetry-distinct and electronically

408 406 404 402 400 398 396 394 392 390

N 1s

Inte

nsity

(a. u

.)

Bending Energy (eV)(b) the N 1s regions for nanohybrid 1c-f-SWCNT.

408 406 404 402 400 398 396 394 392 390

N 1s

Inte

nsity

(a. u

.)

Bending Energy (eV)740 735 730 725 720 715 710 705 700

Fe 2p3/2

Fe 2p1/2

Inte

nsity

(a. u

.)

Bending Energy (eV)

a b

Fig. 10 – XPS high-resolution spectrum (a) the Fe 2p region and (b) the N 1s regions for nanohybrid 1a-f-SWCNT.

C A R B O N 4 7 ( 2 0 0 9 ) 5 7 8 – 5 8 8 585

non-equivalent N atoms. As widely described in the literature,

these non-equivalent atoms are related to the unprotonated

and protonated porphyrin N atoms [62]. When a porphyrin

is complexed to a Zn(II) ion, as in our study, four equivalent

metal-ligated N atoms replace the two different N atoms

1000 800 600 400 200 0

0

2000

4000

6000

8000

10000

Cl 2pCl 2sK 2s

Na a N 1s

O 1s

C 1s

F 1s

Zn 2p3

O aNa 1s

Cou

nts

Bending Energy (eV)

Fig. 11 – XPS survey spectrum for nanohybrid 1b-f-SWCNT.

Inte

nsity

(a. u

.)

Zn 2p3/2

Zn 2p1/2

Bending Energy (eV)1060 1050 1040 1030 1020 1010

a

Fig. 12 – XPS high-resolution spectra (a) the Zn 2p and

and a single peak appears at around 400 eV with a energy sep-

aration from the C 1s peak of 114 ± 0.3 eV [62]. In our spec-

trum, this peak appears at 399.5 eV. Nevertheless, the peak

also contains the contribution of the other two N atoms (that

from the tertiary amine group and that from the amide func-

tion), similar to the others compounds. The energy separation

of the N 1s from C 1s possesses a value of 114.8 eV, just

slightly larger than those already reported. This can be ex-

plained by the fact that in our system other two N-centered

components contribute to the shape and formation of the

peak. As to the N/Zn atomic ratio, the experimental value is

again similar (7.2) that the theoretical one (6). The atomic per-

centage of Zn(II) ions measured in the samples of 1b-f-

SWCNT amount to around 0.43%, suggesting that lower load-

ing of fullerene derivative has been achieved for such samples

as compared to that obtained with the ferrocenyl derivative

(1.4%) for the synthesis of 1a-f-SWCNT, probably caused by

the larger steric hindrance of the porphyrinyl macrocycle [62].

3.5. Microscopic characterization

The results of the HR-TEM investigations of the SWCNT mate-

rials containing the [60]fullerene derivatives (in particular

that of 1b-f-SWCNT) are shown in Fig. 13. In the HR-TEM im-

410 405 400 395 390

N 1s

Inte

nsity

(a. u

.)

Bending Energy (eV)

b

(b) the N 1s regions for nanohybrid 1b-f-SWCNT.

Fig. 13 – HR-TEM micrographs images for nanohybrid 1b-f-SWCNT showing dark spherical features (which we have

attributed to the [60]fullerene cage) almost in front of each other separated by two dark lines representing the SWCNT’s walls.

Inset: zoomed region.

586 C A R B O N 4 7 ( 2 0 0 9 ) 5 7 8 – 5 8 8

age of an isolated SWCNT (appearing as two dark linear

fringes in the central part of the picture), two spherical struc-

tures (pointed by two arrows) imaged as dark circles display-

ing a central white spot in close proximity to the outer surface

of the tube wall are visible (see also the magnification as dis-

played in the inset). We can tentatively attribute these spher-

ical features to the structure of the covalently-linked

[60]fullerene derivatives. This observation further confirm

the presence of functionalized [60]fullerene appends on car-

bon nanotube substrate, in this case containing a Zn(II)– tet-

raphenylporphyrin function.

4. Conclusions

In conclusion, we have synthesized new hybrid nanomateri-

als by amidation reaction between oxidized SWCNTs and

amine-functionalized [60]fullerene derivatives bearing elec-

tron-active moieties. The structural properties of all hybrids

have been comprehensively characterized via a large variety

of spectroscopic and HR-TEM techniques. Raman spectra

showed how all SWCNTs were functionalized and the pres-

ence of the functional groups in the nanotube derivatives.

Furthermore, these spectra reveal a new electronic activity

of the compounds due to the interaction of the functional

groups with the SWCNT frameworks. High-resolution XPS

investigations have documented the presence of [60]fullerene

derivatives around the exo-surface of the oxidized SWCNT

walls, exhibiting a characteristic photoelectron N 1s emission

peak at 400.3 eV. By means of HR-TEM investigations we have

also confirmed the presence of the [60]fullerene fragments on

the SWCNT outer surface by imaging spherical structures.

Although more work is needed in order to determine the po-

tential properties of this novel hybrid materials, this new

materials could open new possibilities of designing function-

alized SWCNTs, not only for photovoltaic or optoelectronic

applications, but also for medical and bio-applications, for

example in the field of diagnosis and treatment by simply

linking molecular modules that contain Gd(III) ions.

Acknowledgements

This work was supported by the European Union through the

Marie-Curie Research Training Network ‘‘PRAIRIES’’, Contract

MRTN-CT-2006-035810, Marie-Curie Initial Training Network

‘‘FINELUMEN’’, grant agreement PITN-GA-2008-215399, the

University of Trieste, INSTM, MUR (PRIN 2006, prot.

2006034372 and FIRB, prot. RBIN04HC3S), University of Na-

mur, the Belgian National Research Foundation (FRS-FNRS,

through the Contract No. 2.4.625.08 F), and the Science Foun-

dation Ireland for a PIYRA award to S.G. J.-F.C. is supported by

the Belgian FNRS as Postdoctoral Researcher and thanks Dr. O.

C A R B O N 4 7 ( 2 0 0 9 ) 5 7 8 – 5 8 8 587

Lebedev for assistance in HRTEM measurements, as well as

Prof. Van Tendeloo for his agreement to access to EMAT labo-

ratory facilities.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at doi:10.1016/j.carbon.2008.10.036.

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