Micro-FTIR and micro-raman studies of a carbon film prepared from furfuryl alcohol polymerization

Article abstract of DOI:10.1021/jp9050534

The synthesis of a carbon film by the acid-catalyzed polymerization and resinification of furfuryl alcohol with a diluted solution of HCl is studied by combining micro-FTIR and micro-Raman spectroscopies. The detailed study of the evolution of spectra as

Full text of DOI:10.1021/jp9050534

10571
2009, 113, 10571–10574
Published on Web 07/13/2009
Micro-FTIR and Micro-Raman Studies of a Carbon Film Prepared from Furfuryl Alcohol
Polymerization
S. Bertarione,* F. Bonino,† F. Cesano,‡ S. Jain,§ M. Zanetti,⊥ D. Scarano,# and A. Zecchina¶
Department of Inorganic, Physical, and Materials Chemistry, NIS Centre of Excellence, and INSTM Centre of
Reference, UniVersity of Turin, Via P. Giuria 7, I-10125 Torino, Italy
ReceiVed: May 29, 2009; ReVised Manuscript ReceiVed: July 2, 2009
The synthesis of a carbon film by the acid-catalyzed polymerization and resinification of furfuryl alcohol
with a diluted solution of HCl is studied by combining micro-FTIR and micro-Raman spectroscopies. The
detailed study of the evolution of spectra as a function of dosage of furfuryl alcohol and temperature shows
that neutral and protonated species are formed at 80 °C, while upon gradually increasing the temperature up
to 600 °C, the viscous polyfurfuryl alcohol resin is transformed into a carbon phase, containing a heterogeneous
distribution of pores, with a size in the 100-2000 nm range, as shown by SEM and AFM analyses.
It is well-known that carbonaceous materials derived from
the pyrolysis of furfuryl alcohol (FA) have a wide range of
applications in the production of gas separation membranes,
electrode materials, acid-resistant materials, corrosion inhibitors,
and the binding phase in cement.1-5 The acid-catalyzed oligo-
merization and resinification processes have been studied
intensively in the past, and in general terms, the features of the
processes are understood.6-9 The most widely accepted mech-
anism is the formation of conjugated sequences in the polymer
chains, associated with the formation of colored species.6 The
acid-catalyzed reaction of two furfuryl alcohol molecules
initially produces a colorless bis(furyl)methyl group, as reported
in structure I in Scheme 1. Following Scheme 1, these oligomers
undergo hydride ion exchanges from the carbon atom joining
two furan rings with the carbenium ion, formed by water
elimination from a protonated methylol termination of another
polymer. It must be emphasized that the first, second, third, and
so forth formed methyl groups are accompanied by the formation
of carbenium ions containing two, three, four, and so forth
conjugated rings in positively charged oligomeric species.
the insolubility of the products.9,10 The structure of the resin
depends on the experimental conditions, such as temperature,
solvent, and concentration of the acid mainly. When the viscous
(and colored) product is subjected to thermal treatment in a flow
of inert gas, by gradually increasing the temperature up to 1000
°C, the material becomes rigid and black due to the process of
carbonization, leading finally to the formation of amorphous
carbon. The acid-catalyzed FA polymerization by hydrochloric
acid (HCl) has not been yet studied extensively. It has been
reported that when the monomer is exposed to the vapors of a
highly concentrated HCl solution, a highly exothermic reaction
occurs, which gives rise to the formation of oligomer clusters,
which bind each other to generate a porous and electrically
conductive pellicle.11,12 In more detail, there are three critical
temperatures for the pyrolytic process of carbonaceous materi-
als,13 the carbonization temperature at about 400 °C, when the
molecular structure of polyfurfuryl alcohol (PFA) is degraded
and carbonization begins; the nucleation temperature at 550 °C,
when a large number of small graphitic crystalline nuclei are
formed; and the crystallite growth temperature at 700 °C, when
the nuclei grow and large crystalline grains are observed. The
size of the microcrystallites increases quickly by increasing the
temperature as well as by increasing heating treatment time.
The reaction leading to colored species is not the only one
occurring during the process; in fact, the formation of γ-diketone
structures, via the furan ring opening, has been suggested too8
(Scheme 2).
In the present work, we report a combined study of vibrational
properties and surface morphology of a carbon film, obtained
from polymerization of FA with HCl and pyrolyzed from 100
to 600 °C. PFA resin was prepared by stirring a diluted solution
of HCl [HCl/H2O, 1/3] in FA at 80 °C for 10 min. The resulting
viscous PFA resin was then activated at 100, 200, 300, 400,
500, and 600 °C under Ar gas flow (100 mL/min). In order to
control the polymerization mechanism and the structure of the
carbon phase, the as-prepared samples were characterized after
each thermal treatment by means of Fourier transform infrared
microspectroscopy (AutoImage micro-FTIR microscope system,
Perkin-Elmer, with a 4 cm-1 resolution and an area of analysis
of 100 × 100 µm2), micro-Raman (Renishaw in Via Raman
Microscope with a 50× objective and a He-Cd laser emitting
at 442 nm), scanning electron microscopy (Leica Cambridge
When the monomer is mixed with an acid catalyst, a highly
exothermic reaction occurs, which gives rise to the formation
of oligomers with an intense brown color, which turn to black
resins as the reaction reaches its end. Furthermore, at a later
stage of the polymerization process, the polymer growth is
characterized by the formation of branched species of the
insulating resin, whose study is made particularly difficult by
* To whom correspondence should be addressed. Tel: +39 011 6707858.
Fax: +39 011 6707855. E-mail: serena.bertarione@unito.it.
†
E-mail: francesca.bonino@unito.it.
E-mail: federico.cesano@unito.it.
E-mail: sagarmjain@gmail.com.
E-mail: marco.zanetti@unito.it.
E-mail: domenica.scarano@unito.it.
E-mail: adriano.zecchina@unito.it.
‡
§
⊥
#
¶
10.1021/jp9050534 CCC: $40.75  2009 American Chemical Society

10572 J. Phys. Chem. B, Vol. 113, No. 31, 2009
Letters
SCHEME 1: Formation Reaction of Conjugated Sequences in the Polymer Chains
SCHEME 2: Ring-Opening Reaction with the Formation of γ-Diketone Structures
Stereoscan 420 instrument equipped with an energy dispersive
X-ray spectroscopy), and atomic force microscopy (Park
Scientific Instrument Auto Probe LS).
In a previous work, a similar sequence of FTIR spectra for
FA adsorbed on H-Y zeolite at temperatures ranging from 25
Figure 1 shows micro-FTIR spectra of FA polymerization
with HCl after thermal treatment, under Ar gas flow at 100,
200, 300, 400, 500, and 600 °C (curves b-g). Curve (a) reports
the spectrum obtained after stirring FA with a diluted solution
of HCl at 80 °C, while the gray spectrum corresponds to liquid
FA at 25 °C.
Figure 1. Micro-FTIR spectra of the viscous PFA resin as prepared
after stirring FA with a diluted solution of HCl at 80 °C (a) and after
activation at 100 (b), 200 (c), 300 (d), 400 (e), 500 (f), and 600 °C (g)
under Ar gas flow. Gray line reports the spectrum of liquid FA at 25
°C.
Figure 2. Micro-Raman spectra of the FA and diluted HCl solution at
25 °C (a) and of the viscous PFA resin after activation at 600 °C under Ar
gas flow (b). The gray line reports the spectrum of liquid FA at 25 °C.

Letters
J. Phys. Chem. B, Vol. 113, No. 31, 2009 10573
Figure 3. (a) SEM image of the carbon material treated at 600 °C; in the inset, a SEM enlarged view of a selected area in (a). (b) CM-AFM images
of the (1400 × 1400) nm carbon surface and (c) CM-AFM enlarged view, (500 × 500) nm in size.
up to 400 °C has been obtained in order to understand whether
longer oligomers can be formed under different conditions by
imposing a constraint on the growth of the oligomeric chains.14
This is in line with previous works concerning the polymeri-
zation of unsatured hydrocarbons within zeolitic frameworks.15
From Figure 1, it is clearly evident that the as-prepared sample
(curve a) still shows the spectral features of liquid FA (gray
line, astericks on curve a). The band at 1715 cm-1 can be
assigned to the stretching vibration of carbonyl species, thus
supporting the hypothesis of the formation of diketonic struc-
tures.8 The absorption observed at 1565 cm-1 could be ascribed
to ν(CdC) modes of neutral or cationic polyunsatured species.14
This last assignment could support the coexistence of both
neutral conjugated species, formed by FA polycondensation,
and the cationic species derived from protonation with HCl.
We shall return to this point in the following when we illustrate
the Raman results. After heat treatments at 100-200 °C, the
absorptions at 1675 and 1422 cm-1, ascribed to the ring
stretching modes of the 2-substituted and 2,3-disubstituted furan
rings of PFA, and the components at 1215-1180 and ∼1015
cm-1, assigned to the asymmetric and symmetric stretching
vibration of )C-O-C groups belonging to 2-substituted furan
rings,16 significantly grow. These last assignments support the
formation of branched polymeric species. From this sequence
of spectra, it comes out that the formation of polymerized PFA
occurs already at 100 °C (curve b), and it seems to be complete
at 200 °C (curve c). On the sample treated at 300-500 °C
(curves d-f), all of the absorptions in the 1675-1015 cm-1
range disappear, and the component at 1715 cm-1 significantly
decreases in intensity. At the same time, two main broader bands
grow at ∼1600 and ∼1300 cm-1, which become predominant
after thermal treatment at 600 °C (curve g). On the basis of the
literature data concerning amorphous carbon and carbon
nanotubes,17,18 absorptions at ∼1600 and ∼1300 cm-1 indicate
the formation of the carbonaceous material.
In order to investigate the nature of the formed carbonaceous
species, two types of experiments have been performed by
means of the micro-Raman technique. First, we obtained the

10574 J. Phys. Chem. B, Vol. 113, No. 31, 2009
Letters
spectra under the local heating effect of the laser beam, in
particular, the spectra of liquid FA (Figure 2, gray line) and
that of the FA/diluted HCl solution (Figure 2, curve a). Then,
the spectrum of the viscous PFA resin after thermal treatment
at 600 °C, under Ar gas flow, was recorded (Figure. 2, curve
Figure 3c reports an AFM image obtained in a region free
from pores, which clearly shows the structure of the carbon
phase, characterized by a surface average roughness of about 2
nm with a mean height of about 9 nm. The values reported
were obtained from the region analysis within the selected area,
which allows estimation of the distribution of heights (histogram
plots, not reported for the sake of brevity).
In conclusion, the synthesis of a carbon film by the resini-
fication/pyrolysis processes of HCl-catalyzed FA has been
investigated by means of micro-FTIR/Raman techniques. The
studies successfully show that neutral and protonated species
are formed at low temperature (∼80 °C), leading to the
formation of solidified PFA already at 100 °C, whereas the
amorphous carbon phase becomes dominating at high temper-
ature of treatment (∼500-600 °C). Furthermore, from SEM
and AFM images on samples treated at 600 °C, a turbostratic
carbon phase characterized by a heterogeneous distribution of
pores is highlighted.
b). Furthermore the exciting line at λ ) 442 nm (22624 cm-1
)
is expected to cause Raman resonant effects on species
adsorbing in the visible region, where both neutral and cationic
conjugated species absorb;6,14 consequently, we expect that the
Raman spectra are dominated by the vibrational manifestations
of them.
From Figure 2, it is clearly observable that the spectrum of
the FA/diluted HCl solution (curve a) shows still the spectral
features of liquid FA (gray line, astericks on curve a); they can
be assigned to the vibrational modes of PFA chains. On the
contrary, the peaks at ∼1650, ∼1560, and ∼1522 cm-1 can be
attributed to cationic unsatured moieties, which derive from
protonation by HCl and are inserted into the polymeric chains.14
From this, it can be argued that the infrared band at 1565 cm-1
is due to carbocationic oligomeric species (Figure 1). The
contemporary presence of all of these features reveals the
coexistence of both cationic and neutral (carbon-type) species.
This implies that the micro-Raman spectrum of samples,
obtained under the laser beam by adding a diluted HCl solution
to liquid FA at 25 °C, can be compared to the micro-FTIR one,
even if recorded on samples prepared after stirring FA with a
diluted solution of HCl at 80 °C (Figure 1, curve a). In fact, it
can be considered that there are both thermal and photoactivation
effects of the laser beam on the samples.
When the so-obtained sample is treated at 600 °C under Ar
gas flow (curve b), all of the features ascribed to oligomeric
and cationic species disappear, while two broader Raman
absorptions centered at ∼1600 and ∼1350 cm-1 (G and D peaks,
respectively) grow. From the position and the shape of these
components, the complete transformation of the FA polymer
into a turbostratic carbon phase can be further highlighted.9,13,19,20
Figure 3 shows SEM and AFM images of the morphology
of the carbon material treated at 600 °C. In particular, from the
SEM analysis (Figure 3a), the presence of a heterogeneous
distribution of holes with sizes in the 500-2000 nm range is
clearly evidenced. Furthermore, from AFM images, holes with
smaller diameters in the 100-300 nm range are very clearly
imaged (Figure 3b).
Acknowledgment. This work was supported by MIUR
(Ministero dell’Istruzione, dell’Universita` e della Ricerca),
INSTM Consorzio, Centre of Reference INSTM (Torino), and
NIS (Nanostructured Interfaces and Surface) Centre of Excel-
lence. The authors acknowledge financial support by Regione
Piemonte (Progetto NANOMAT, Docup 2000-2006, Linea
2.4a).
References and Notes
(1) Dong, Y. R.; Nishiyama, N.; Egashira, Y.; Ueyama, K. Ind. Eng.
Chem. Res. 2007, 46, 4040–4044.
(2) Kawashima, D.; Aihara, T.; Kobayashi, Y.; Kyotani, T.; Tomita,
A. Chem. Mater. 2000, 12, 3397–3401.
(3) Muthukumar, M.; Mohan, D. J. Polym. Res. 2005, 12, 231–241.
(4) Yi, B.; Rajagopalan, R.; Foley, H. C.; Kim, U. J.; Liu, X.; Eklund,
P. C. J. Am. Chem. Soc. 2006, 128, 11307–11313.
(5) Vishwanatham, S.; Haldar, N. Corros. Sci. 2008, 50, 2999–3004.
(6) Choura, M.; Belgacem, N. M.; Gandini, A. Macromolecules 1996,
29, 3839–3850.
(7) Muller, H.; Rehak, P.; Ja¨ger, C.; Hartmann, J.; Meyer, N.; Spange,
S. AdV. Mater. 2000, 12, 1671–1675.
(8) Conley, R. T.; Metil, I. J. Appl. Polym. Sci. 1963, 7, 37–52.
(9) Wang, Z.; Lu, Z.; Huang, X.; Xue, R.; Chen, L. Carbon 1998, 36,
51–59.
(10) Gonzalez, R.; Figueroa, J. M.; Gonzalez, H. Eur. Polym. J. 2002,
38, 287–297.
(11) Batista, P. S.; De Souza, M. F. Synth. Met. 1999, 101, 635–636.
(12) Batista, P. S.; De Souza, M. F. Polymer 2000, 41, 8263–8269.
(13) Li, G.; Lu, Z.; Huang, B.; Wang, Z.; Huang, H.; Xue, R.; Chen, L.
Solid State Ionics 1996, 89, 327–331.
(14) Bertarione, S.; Bonino, F.; Cesano, F.; Damin, A.; Scarano, D.;
Zecchina, A. J. Phys. Chem. B 2008, 112, 2580–2589.
(15) Bordiga, S.; Ricchiardi, G.; Spoto, G.; Scarano, D.; Carnelli, L.;
Zecchina, A.; Otero Area`n, C. J. Chem. Soc., Faraday Trans. 1993, 89,
1843–1855.
(16) Sugama, T.; Kukacka, L. E. J. Mater. Sci. 1982, 17, 2067–2076.
(17) Branca, C.; Frusteri, F.; Magaz, V.; Mangione, A. J. Phys. Chem.
B 2004, 108, 3469–3473.
(18) Verdejo, R.; Lamoriniere, S.; Cottam, B.; Bismarck, A.; Shaffer,
M. Chem. Commun. 2007, 5, 513–515.
In a previous work from us,21 a porous carbon surface was
produced after thermal treatment at 800 °C of ZnO-carbon
composites, which was obtained from a homogeneous mixtures
of FA with ZnCl2. In that case, the ZnCl2 catalyst was acting
as a templating agent for the production of pores, whose
distribution and diameters depend on the Zn concentration. In
fact, the carbon phase gradually changes from a homogeneous
distribution of holes (less than 1 µm in size) (low Zn concentra-
tion, 4.8% w/w) to a more disordered situation (high Zn
concentration, 17% w/w), with wide pockets and small cavities
simultaneously present (200-500 nm in size).21 On this basis,
we can highlight the role of the catalyst in affecting the
distribution and size of holes inside of the porous carbon phase.
In previous studies, advanced analytical techniques have been
used in order to characterize carbonaceous materials.22 In
particular, structural studies using X-ray have demonstrated the
relationships of the starting structure to the final graphitizability.
(19) Ferrari, A. C.; Robertson, J. Philos. Trans. R. Soc. London, Ser. A
2004, 362, 2477–2512.
(20) Ferrari, A. C.; Robertson, J. Phys. ReV. B 2000, 61, 14095–14107.
(21) Cesano, F.; Scarano, D.; Bertarione, S.; Bonino, F.; Damin, A.;
Bordiga, S.; Prestipino, C.; Lamberti, C.; Zecchina, A. J. Photochem.
Photobiol. A: Chem. 2008, 196, 143–153.
(22) Lewis, I. C. Carbon 1982, 20, 519–529.
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