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Carbon Nanotubes
Direct Growth of Polyaniline Chains from N-Doped Sites of Carbon Nanotubes
Atta Ul Haq , Joonwon Lim , Je Moon Yun , Won Jun Lee , Tae Hee Han , * and Sang Ouk Kim *
Polymer grafting from graphitic carbon materials has been pursued for several decades. Unfortunately, currently available methods mostly rely on the harsh chemical treatment of graphitic carbons which causes severe degradation of chemical structure and material properties. A straightforward growth of polyaniline chain from the nitrogen (N)-doped sites of carbon nanotubes (CNTs) is presented. N-doping sites along the CNT wall nucleate the polymerization of aniline, which generates seamless hybrids consisting of polyaniline directly grafted onto the CNT walls. The resultant materials exhibit excellent synergistic electrochemical performance, and can be employed for charge collectors of supercapacitors. This approach introduces an effi cient route to hybrid systems consisting of conducting polymers directly grafted from graphitic dopant sites.
1. Introduction
Substitutional doping of graphitic carbon compounds with
heteroelements such as nitrogen, boron, and sulfur has been
investigated in an effort to control and optimize the struc-
ture and properties of graphitic carbon species. [ 1–4 ] Previous
research efforts have proven that such heterodopants can
induce dramatic modifi cation of material structures and prop-
erties of graphitic carbon species. [ 5–9 ] In addition, it has been
reported that heterodopants can be exploited for nanocom-
posite production. [ 10–12 ] The enhancement of surface energy,
as well as the introduction of local charge via N- or B-doping,
© 2013 Wiley-VCH Verlag Gm
DOI: 10.1002/smll.201300625
A. U. Haq, J. Lim, Dr. J. M. Yun, W. J. Lee, Prof. S. O. KimCenter for Nanomaterials and Chemical Reactions Institute for Basic Science (IBS) Department of Materials Science and Engineering KAIST, Daejeon 305-701, S. Korea E-mail: [email protected]
Prof. T. H. HanDepartment of Organic and Nano Engineering Hanyang University Seoul 133-791, S. Korea E-mail: [email protected]
small 2013, 9, No. 22, 3829–3833
facilitates the deposition of various minerals without degra-
dative surface modifi cation or an adhesive interlayer. [ 11–13 ]
Polyaniline (PANI) is a widely used electroconducting
polymer with strong electrochemical properties. The high
redox activity of PANI originates from the presence of dif-
ferent possible oxidation states and their doping/dedoping
behaviors. [ 14 , 15 ] PANI’s excellent properties, [ 16–18 ] low cost,
easy synthesis, and environmental stability can be further
exploited by producing PANI–carbon nanotube (CNT) com-
posites. [ 19–22 ] Even though the synergistic effects of such a
composite have been recognized in terms of chemical and
electrical properties, the rational mechanism underlying the
property enhancement has not been clearly understood yet.
Meanwhile, one main criterion for improved electrochemical
performance of CNT–PANI electrodes is the strong inter-
facial interactions between the CNTs and PANI, which can
facilitate mutual charge transfer. [ 23 ] In this regard, an effi -
cient approach to graft polyaniline chains directly onto CNTs
becomes a crucial issue. However, the majority of previous
approaches utilizes harsh chemical treatments (usually acidic
oxidation) to generate surface reactive sites at the inert gra-
phitic carbon plane, which can signifi cantly degrade the mate-
rial properties of CNTs. [ 24 , 25 ]
Herein, we report a straightforward approach in which
polyaniline chains are directly grown from the N-doping sites
of CNTs. In contrast to the widely used harsh acid treatment,
N-doping of CNTs, which can occur during CNT synthesis,
3829bH & Co. KGaA, Weinheim wileyonlinelibrary.com
A. U. Haq et al.
38
full papers
does not degrade the material properties of CNTs. N-dopingprovides additional electrons to the graphitic plane while
enhancing the surface energy such that the electrical prop-
erties and interfacial adhesion can be improved coopera-
tively. [ 11 , 26 , 27 ] We also demonstrate supercapacitor electrode
performance of these idealized hybrid materials.
2. Results and Discussion
Highly aligned vertical N-doped carbon nanotubes (NCNTs)
were grown from Fe catalyst particles by using plasma-
enhanced chemical vapor deposition (PECVD) in an NH 3
environment, which is an effective source for the N-doping of
CNTs. [ 28–30 ] The fully grown NCNTs were then immersed in
a 0.01 m m aniline solution at a low temperature (2 ° C). After
a few hours, 0.01 m m ammonium peroxydisulfate (APS) was
added to the solution for the oxidation and polymerization of
aniline monomers. As illustrated in Figure 1 a, aniline mono-
mers were adsorbed on the NCNT wall and polymerized
from the N-doped sites. Figures 1 b and d compare the
30 www.small-journal.com © 2013 Wiley-VCH V
Figure 1 . a) Proposed reaction mechanism for in situ PANI grafting processwere adsorbed on the NCNT wall and polymerized from the N-doped siteNCNTs at b) low and c) high magnifi cations. FESEM images of synthesizedmaterials at d) low and e) high magnifi cations.
fi eld-emission scanning electron microscopy (FESEM)
images of vertical NCNTs and NCNT–PANI hybrids arrays.
It is obvious that the diameter of nanotubes signifi cantly
increases with polymerization time (Figures 1 c,e). The dia-
meter increased from 15 to 115 nm approximately after 2
h of polymerization (Figure S1). The vertical NCNTs were
uniformly coated with polyaniline, which indicates a dense
and uniform nucleation of polymerization reaction at the
NCNT surface. After this in situ polymer-grafting process,
the vertical alignment of individual NCNTs could be well
maintained, preserving the large surface area of this ideal
structure.
The formation mechanism of NCNT–PANI hybrids is
described in Figure 1 a (see also Figure S2). For the oxidative
polymerization of polyaniline, N-doping sites on the CNT
backbone provide active nucleation sites for the initiation
stage of the polymerization process. The lone-pair electrons
of the pyridinic nitrogens are protonated, while aniline
monomers are oxidized by adding APS (Figure S2). After-
wards, aniline radical cations are generated due to the res-
onance of the benzene ring and interact with the pyridinic
erlag GmbH & Co. KGaA,
. Aniline monomers s. FESEM images of NCNT–PANI hybrid
nitrogen atom at the CNT wall via rad-
ical-cation coupling. [ 11 , 31 ] Further aniline
growth through this radical coupling is
repeated in the presence of excess aniline
radicals in the solution.
The role of N-doping sites as the
covalent grafting sites for polyaniline
chain was verifi ed by the control experi-
ment employing acid-functionalized oxy-
genated CNTs (oCNTs). PANI could be
grown on the surface of oCNTs in similar
experimental conditions. In the Raman
spectra of as grown PANI–NCNT and
PANI–oCNT hybrids ( Figures 2 a,b),
the characteristic peaks of PANI are
observed at 1164, 1217, 1493 cm − 1 . [ 32 ]
However, after thorough washing using
sonication for 2 hours in N -methyl-
2-pyrrolidone (NMP) and sequential
fi ltration to remove physisorbed PANI
chains, oCNT-PANI exhibits signifi cantly
weakened peak intensities, and the peaks
of CNT (D and G) became dominant.
This result suggests that unbound PANI
was washed away from the oCNT–PANI
(Figure 2 b). By contrast, the NCNT–
PANI shows a consistent spectrum even
after NMP washing (Figure 2 a), which
confi rms that the PANI chains grown
from NCNTs are strongly covalently
bonded to the NCNTs.
Figure 2 c compares the nitrogen
X-ray photelectron (XPS) spectra of bare
NCNTs and NCNT–PANI hybrids. The
deconvoluted spectra of bare NCNTs
clearly show the presence of pyridinic
nitrogen (N1, at ca. 398 eV), quaternary
nitrogen (N3, at ca. 401 eV) and nitrogen
Weinheim small 2013, 9, No. 22, 3829–3833
Direct Growth of Polyaniline Chains from N-Doped Sites of Carbon Nanotubes
structure to form in a facile single reac-
Figure 2 . Raman spectra for a) NCNT and b) oCNT. There is a signifi cant difference in the spectra of oCNT–PANI before and after washing with NMP. c) N1s XPS spectra of NCNT and NCNT–PANI with different polymerization times (1, 5, and 30 min).
oxide (N5, at ca. 406 eV). The XPS spectra of NCNT–PANI
confi rm the presence of PANI with free amines (N2, at ca.
399.6 eV) and the charged nitrogen atoms (overlapped with
N3 at ca. 401 eV and N4 at ca. 402.6 eV), [ 33 ] which were
contributed by amines and imines. As polymerization time
increases, three distinct PANI peaks of N2, N3, and N4 grow.
The C/N ratio also decreases with polymerization reaction
time (Figure S3).
Owing to the synergistic effect from the strong electro-
chemical character of PANI shell and highly electroconduc-
tive NCNT core, PANI–NCNT hybrids are expected to be
an excellent candidate for supercapacitor electrodes. The
performance of supercapacitor electrodes was analyzed
using cyclic voltammetry (CV) at room temperature. A
typical three-electrode confi guration was employed in 0.1 m
Na 2 SO 4 electrolyte. Figure 3 a shows the CVs of the NCNT
and NCNT–PANI supercapacitors, scanned at 50 mV s − 1 in
a potential range of –0.2 to 0.8 V. The current values were
normalized with the mass of the active material in the elec-
trodes. The voltammograms of the NCNTs and NCNT–PANI
exhibit a nearly rectangular shape, representing effi cient
proton diffusion-migration. [ 34 ] The NCNT–PANI super-
capacitors showed their Faradaic redox behavior near 0 V.
As the thickness of the PANI layer on NCNTs increases,
the cathodic peak also shifts to approximately 0.2 V. [ 35 ] The
redox peaks at 0 to 0.2 V are attributed to the redox transi-
tion of PANI between the leucoemeraldine and emeraldine
forms. [ 36 ] In addition, the second redox transition of PANI
observed at 0.3 to 0.5 V appears in response to the emeral-
dine–pernigraniline transition of PANI. [ 37 ] Taken together,
© 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheismall 2013, 9, No. 22, 3829–3833
the capacitance of NCNT–PANI hybrids
mainly stems from the synergistic effect
of the pseudocapacitive contribution of
PANI at the electrode/electrolyte surface
and the electric-double-layer capacitance
of CNTs. The specifi c capacitance esti-
mated from the CVs is represented as a
function of the scan rate in Figure 3 b. As
shown in Figure 3 a,b, the specifi c capaci-
tances gradually increase with polyaniline
grafting time, up to as high as ca. 250 F g − 1
(30 min reaction time).
Our direct growth of PANI chains
on NCNTs can also incorporate man-
ganese (MnOx) or ruthenium oxide
(RuOx) nanoparticles into the hybrid
structures. All reaction precursors (typi-
cally 10 m m of metal oxide precursors
and 0.01 m m of aniline solution) were
mixed in the reactor from the beginning.
When small amounts of an oxidizing
agent (APS) are added, in situ poly-
merization and metal oxide precipita-
tion occurs simultaneously. The synthetic
method allows a metal oxide/PANI/
NCNT three-component hybrid nano-
tion step. Figures 4 a and b show the SEM images of the
NCNT–PANI–MnOx and NCNT–PANI–RuOx structures,
respectively. These SEM images confi rm that both PANI
and metal oxides are uniformly coated on the surface of
the NCNTs. Owing to the redox-active metal oxides [ 38 ] and
PANI, [ 39 ] these three-component hybrids are expected to be
ideal capacitor materials for current collectors. As shown
in Figure 4 c, NCNT–PANI–MnOx and NCNT–PANI–
RuOx exhibit remarkably enhanced specifi c capacitances
compared to NCNTs–PANI. The specifi c capacitances of
NCNTs, NCNT–PANI–MnOx, and NCNT–PANI–RuOx
were 111, 261, and 304 F g − 1 , respectively. This enhancement
in specifi c capacitance is attributed to the Faradaic pseudo-
capacitance of the RuOx and MnOx.
3. Conclusion
In summary, we have demonstrated idealized NCNT–PANI
core-shell hybrid materials through a straightforward direct
‘grafting from’ method. Nitrogen atoms, doped into CNTs,
played a key role in ensuring the initiation and growth of
PANI. Furthermore, three-component hybrids of NCNT,
PANI, and metal oxides could be produced via a facile single-
step reaction. Owing to the synergistic effects from carbon,
polymer, and metal oxide components, those hybrids demon-
strate remarkable performances as supercapacitor electrodes.
Our approach not only provides a novel synthetic route for
nanomaterials but also suggests how to optimize the struc-
ture, interface, and corresponding properties of graphitic
carbon-based hybrid materials.
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A. U. Haq et al.
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full papers
Figure 4 . SEM images of a) NCNT–PANI–MnOx and b) NCNT–PANI–RuOx. c) Cyclic voltammetry curves of NCNTs, NCNT–PANI–MnOx, and NCNT–PANI–RuOx.
Figure 3 . a) Cyclic voltammetry curves and b) specifi c capacitance vs. scan rates for of NCNT and NCNT–PANI with different polymerization times.
4. Experimental Section
Vertical N-Doped Carbon Nanotube Synthesis : Firstly, iron thin fi lm was deposited on a Si/SiO 2 wafer (1 cm × 1 cm) via e-beam evaporation. Then, the wafer was heated to 750 ° C along with hydrogen and ammonia gas mixture (H 2 : 60 ccm, NH 3 : 40 ccm) using plasma-enhanced chemical vapor deposition (PECVD). At this temperature the Fe thin fi lm became agglomerated within 2 min. Afterwards, the chamber pressure and DC voltage were increased to 4.6 Torr and 470 V, respectively, which resulted in the production of plasma inside the chamber between the electrodes. Acetylene gas was injected into the chamber and dissociated into carbon atoms on the Fe particle surfaces, which promoted the growth of carbon in the form of N-doped CNTs.
Polyaniline Grafting from NCNT Surfaces : Aniline (C 6 H 5 NH 2 ) and ammonium peroxydisulfate ((NH 4 ) 2 S 2 O 8 , APS) were purchased from Sigma Aldrich. The NCNT samples were put in a 0.01 m M (in 1 M HCl) solution of aniline and the container was kept at 2 ° C for 2 to 4 h. Then 0.01 m M of APS (in 1 M HCl) was injected into the aniline solution dropwise and the polymerization reaction was controlled by varying the reaction time from 0 to 60 min. For the control experiments, aniline polymerization was also performed with pristine and acid-functionalized CNTs. The acid functionaliza-tion of CNTs was done by means of an acid refl ux using a 1:3 ratio of concentrated HNO 3 :H 2 SO 4 at 120 ° C.
Synthesis of NCNT–PANI/Metal Oxide Hybrid Nanostructures : Metal oxide precursor solutions (10 m M RuCl 3 and KMnO 4 ) were prepared for the respective MnOx and RuOx precipitations. The aniline solution (0.01 m M ) and NCNT samples were mixed into the precursor solutions. After 30 to 60 min, APS was added to initiate polymerization. The metal oxides were precipitated at 50 ° C.
Characterization : The microstructural characterization was done using a Hitachi S-4800 FESEM. Raman spectroscopy (Horiba Jobin Yvon, ARAMIS) with 514 nm laser excitation and XPS (Thermo VG Scientifi c, Sigma Probe) were performed to characterize the chemical structures of CNTs and their hybrids. For Raman char-acterization, PANI–CNT hybrid was sonicated in NMP for 2 h and washed with excess NMP in a vacuum fi ltration kit (membrane: Whatman, Anodisc 47, pore 0.1 μ m). All the electrochemical exper-iments were performed using a three-electrode cell confi guration. Cyclic voltammetry tests were used to evaluate the electrochemical performance of the electrodes for supercapacitor applications with Bio-Logic (SP-200) in Na 2 SO 4 electrolyte. Supercapacitor elec-trodes were prepared by the transfer of CNT hybrids from silicon oxide wafer to titanium electrodes as described previously. [ 11 , 12 ]
Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.
Acknowledgements
This work was supported by Institute for Basic Science (IBS) in Korea, the Converging Research Center Program through the
erlag GmbH & Co. KGaA, Weinheim small 2013, 9, No. 22, 3829–3833
Direct Growth of Polyaniline Chains from N-Doped Sites of Carbon Nanotubes
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Received: February 27, 2013 Revised: March 19, 2013 Published online: May 2, 2013
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