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Review
Nitrogen Dopants in Carbon Nanomaterials: Defects
or a New Opportunity?
Won Jun Lee, Joonwon Lim, and Sang Ouk Kim*
recent years, intriguing novel N-doping
effects have been discovered, such as
increased control of charge-carrier density, surface energy, and surface reactivity,
based on tailored chemical pathways.[11] As
a consequence of the remarkable growth
of research interest, it has been appealing
to develop this concept with state-of-the-art
applications. including chemical energy
conversion,[12] chemical scissors,[13] and
site-selective molecular self-assembly.[14]
A major emerging direction in
N-doping research is aimed at utilizing
N defects as distinctive reactive sites. A
well-known example is the oxygen-reduction-reaction (ORR) catalysis of N-doped
CNMs, firstly suggested by Dai et al.;[15]
while several issues such as the exact catalytic sites and reaction
mechanism are still controversial, general consideration of the
possible ORR mechanism of N-doped CNMs is based on the
peculiar modification of the atomic and electronic structures
induced by N-doping.[5,16] Interestingly, electron-rich substitution via N-doping only leads to a minor distortion of the lattice,
which prevents significant disruption of the graphitic plane.[17]
A fascinating recent example for the application of N-dopants is
their ability to initiate reduction/oxidation with the adsorption
of molecules at the interface by charge transfer.[18] Electron
transfer from the N-dopants to the adsorbed molecules enables the formation of interfacial bonding, which results in
self-assembled hybrid or composite structures.[19] The role
of N-dopants has to be carefully understood, as they not only
reorganize chemical bonds, but also frequently accompany the
rupture of chemical bonds.[13] More recently, N-doping control
has been introduced as a strategy to tailor the size of graphene
having sub-10 nm width, which may transform the graphene
into semiconductors with quantum confinement and edge
effects.[20] Strikingly, N-dopants have enabled the longitudinal
cutting of graphene at the atomic level, controlling the edge
configuration without compromising the intact crystallinity.[13]
Although several different elements have been doped into
CNMs, including boron (B), sulphur (S), and phosphorus (P),
N is known to be the most popular dopant to induce catalytic
activity. In contrast, still little is known about other functionalities including: i) energy capture, ii) direct redox reactions,
and iii) dopant/defect-induced bond engineering. Here, we
highlight recent pioneering research work associated with
N-dopants in CNMs (Figure 1). Based on the fundamental
features of N-doping, we particularly add new aspects for the
utilization of N-dopants, shedding much light not only on the
novel defect chemistry, but also on defect engineering.
Substitutional N-doping of carbon nanomaterials refers to the chemical functionalization method that replaces a part of the carbon atoms in fullerene,
carbon nanotubes, or graphene by nitrogen. N-doping has attracted a tremendous amount of research attention for their unique possibilities, spanning from its ability to engineer various physiochemical properties of carbon
nanomaterials in a stable manner with different dopant configurations. Many
viable configurations of N-dopants are accompanied by typical structural
defects, while still preserving the structural symmetry in the basal graphitic
plane. Here, the physicochemical features are highlighted and the exciting
challenges of N-dopants in carbon nanomaterials identified, with particular
emphasis on the broad tunability of the material properties and relevant
emerging applications.
1. Introduction
Nitrogen (N)-doped carbon nanomaterials, such as carbon
nanotubes (CNTs) and graphene, have a myriad of potential scientific possibilities, due to their atomic structure at the
graphitic edge and basal planes, and related controlled physicochemical properties.[1] A large variety of reaction routes for
N-doping involving in situ synthetic or post-synthetic methods[2]
leads to the transformation of chemical bonds, which can also
be referred to as defects with topological imperfections.[3]
However, these defects can play a pivotal role for novel multifunctional properties, such as elevated charge-carrier density,[4] superior catalytic activity,[5] and high chemical affinity to
other materials principally involved with modulated electronic
structures.[6] Since the first introduction of N-doping into an
amorphous carbon film by IBM in the late 1980s,[7] N-doping
of carbon allotropes, including fullerene,[8] CNTs,[2] graphene,[9]
and graphite nanoplatelets,[10] has been reported. Nonetheless,
research interests in the heteroelement doping of carbon nanomaterials (CNMs) has been relatively limited due to concern for
the deterioration of the charge-carrier mobility. Meanwhile, in
Dr. W. J. Lee, Dr. J. Lim, Prof. S. O. Kim
National Creative Research Initiative (CRI) Center
for Multi-Dimensional Directed Nanoscale Assembly
Department of Material Science and Engineering
KAIST, Daejeon 34141, Republic of Korea
E-mail: [email protected]
Dr. W. J. Lee
Department of Chemistry
Imperial College London
London SW7 2AZ, UK
DOI: 10.1002/smtd.201600014
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Figure 1. Pioneering utilization of N-dopants in the graphitic carbon
plane of graphene and carbon nanotubes
2. Nitrogen Dopants as Structural Defects?
N-doping is known to give rise to remarkable chemical properties, such as enhanced catalytic activity and surface reactivity. To
delineate the effect of N-dopants, it is essential to understand
the atomic structure and electron delocalization of N-dopants
with neighboring C atoms. The incorporation of N into the
graphitic plane is energetically favored as native point-defects
and N-dopants attract each other. Moreover, initial N-dopants
prompt the creation of point defects at a lowered formation
energy,[21] which drives perceptible bond disorders and lattice distortion.[22] The two distinct electronic states of C and N
generate permanent dipoles, which can be modulated through
their defect types with different configurations.[2a] Recent precise design of doping methods has attained a noticeably high
doping level (16.4 at%),[23] which approaches the theoretical
limit (19.1 at%).[24] Obviously, it is anticipated that the doping
level is strongly dependent upon the doping method.
2.1. Structural Configuration of Nitrogen Atoms
Incomplete-bonding defects such as monovacancy (MV), divacancy (DV), and Stone–Thrower–Wales (STW) defects can
be observed in many different crystalline materials.[25] Whilst
several configurations of N-dopants have been reported thus
far, the most common types of N-dopants are substituted
or accompanied by these incomplete-bonding defects.[3,22,26]
Where a C atom is substituted with a N atom in the graphitic
plane, it is called “graphitic N” or “quaternary N” (labelled
NQ) (Figure 2a).[27] In particular, the position of NQ is related
to the chirality of SWCNTs,[28] which is also found at the edge
chirality of the graphene.[29] The quaternary N possesses an
n-type band structure, since the N has one extra electron
compared with C, which can be delocalized around the N.[22]
Figure 2. Different configurations of representative N-dopants: a) quaternary N (NQ) with substitution, b) pyridinic N on the edge (Pyr-N1),
c) pyridinic N with monovacancy (Pyr-N3), d) Porphyrinic N with
di-vacancy (Por-N4), and e) Pyrrolic N (NPY) with Stone–Thrower–Wales
defect.
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Won Jun Lee is currently a
research associate at the
Department of Chemistry,
Imperial College London, UK.
He received his Ph.D degree
from the Department
of Materials Science &
Engineering at KAIST in 2013
under the supervision of
Prof. Sang Ouk Kim. His
research focuses on the
molecular assembly of functional nanomaterials for energy applications.
Sang Ouk Kim is the Chair
Professor in the Department
of Materials Science and
Engineering at KAIST, and
the director of National
Creative Research Initiative
Center for Multi-Dimensional
Directed Nanoscale
Assembly, Daejeon, Korea.
He obtained his Ph.D from
the Department of Chemical
Engineering, KAIST in 2000
and carried out postdoctoral research at the Department
of Chemical and Biological Engineering, University of
Wisconsin-Madison. He has a broad research interest
in the “directed molecular assembly of soft nanomaterials”, which includes: i) block-copolymer self-assembly,
ii) graphene-based materials assembly and chemical
modification, and iii) flexible and wearable energy devices.
A pyridinic N (labelled Pyr-N3) usually refers to N atoms that
are bonded to two C atoms. In the vicinity of MV, the most
stable configuration for the N-dopants is the Pyr-N3, which
has two-fold coordination with a C–N bond length of 1.33 Å
(Figure 2b,c).[26] While two carbon atoms around the Pyr-N1
can form a pentagon-like structure (from D3h to Cs), Pyr-N3
preserves the graphitic-plane structural symmetry (D3h).[22]
As Pyr-N3 has fewer electrons compared to pristine graphene
(considering MV defects), it exhibits a p-type band structure
due to the electron deficiency.[30] For N arrangement around
a DV defect, where two C atoms are removed, theoretical and
experimental studies have shown that four N atoms energetically prefer to substitute the C atom with unpaired electrons
and form Pyr-N3 (Figure 2d).[22,30] In this case, porphyrinic N
(labelled Por-N4) refers to a configuration with four pyridinic N
atoms in a porphyrinic planar architecture, which has a similar formation energy (2.55 eV) to that of Pyr-N3 (2.51 eV).[22]
As in the case of Pyr-N3, the electron deficiency of Por-N4 gives
rise to an acceptor level that shows a p-type band structure as
well. It is interesting to note that several N-related impurity
states are spatially localized around the DV and form p-like
orbital shapes localized at N atoms, which results in different
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2.2. Modification of Physical and Chemical Properties
Different N-dopant configurations influence various physicochemical properties of CNMs. Table 1 gives a summary of the
electronic properties (work function (eV) and binding energy
(eV)), structural properties (bond length (Å) and formation
energy (eV)), and surface-active properties (energy barrier
of oxygen dissociation (eV) and oxygen reduction pathway).
Notably, due to the limited number of methods for chiral selection, only a few detailed studies are available for the different
chiralities of SWCNTs.[33] As briefly described in Section 2.1,
the band structure and density of states are determined by
the quantity of lone pair states with their configurations.
Further information on how the dopant concentration and
defect type can affect the band structure is detailed in previous literature.[2a,32] Work functions are commonly quoted in
terms of equivalent electrical potential, which is related to the
Fermi level.[34] For this reason, NQ doping moves the Fermi
level closer to the vacuum level and thus reduces the work
function (4.22 eV).[35] Conversely, Pyr-N3 (4.69 eV)[35] and PorN4 (4.7 eV)[16a] move the Fermi level to the valence band, thus
increasing the work function. In the case of NPY, the resulting
work function change is negligible.[35]
The emitted electron’s kinetic energy (Ek) can be measured by X-ray photoelectron spectroscopy (XPS), and the
atomic core-level binding energy (Eb), which is related to the
Fermi level, can be calculated.[36] Spectroscopy measurements
of N1s have indicated that NQ, Pyr-N3, Por-N4, and NPY have
core-level binding energies of 401.4 ± 0.3 eV, 398.7 ± 0.2 eV,
398.9 ± 1.0 eV, and 400.3 ± 0.2 eV, respectively.[5,37] Significantly,
the structural properties directly provide useful information for
the most stable configuration, controlled by the different types
of defects and the different chemical potentials of N (directly
proportional to free molar energy). While the average C–C bond
length is approximately 1.42 Å in sp2 carbon, the bond length of
C–N is shorter, due to the difference of atom size and electronegativity.[22] Considering the bond strength and atomic arrangement, the C–N bond length of the NQ (1.39 Å)[27] is longer than
Pyr-N3(1.33 Å),[26] Por-N4(1.32-1.33 Å),[22] and NPY (1.372 Å).[38]
The structural stability has been analyzed based on the calculated formation energy (Ef). It is noteworthy that curvature and
chirality in CNTs affect the electronic structure and formation
energy, together with the configuration of N-dopants. The Ef of
the NQ in graphene was found to be the lowest, 0.32 eV.[22] It
can be attributed to the energetic preference for N placement in
a perfect graphene sheet. For N-dopants with vacancies, the Ef
of Pyr-N3 and Por-N4 in graphene sheets are 2.51 and 2.55 eV,
respectively.[22] Although this small difference implies the existence of both configurations, when the N chemical potential is
high, the Por-N4 is found to be stabilized further with an Fe
atom, which resembles the Fe-Porphyrin structure.[16a] In addition to its structural and electronic properties, N-dopants have
exceptionally low energy barriers for guest molecules, making
them superior for molecular assembly and catalysts. The
outstanding reactivity of N-dopants relies on its lowered energy
Table 1. Physical properties of various N-dopants.
Quaternary N
(NQ)
Work function
[Φ]
Binding energy
[eV]
C–N bond length
[Å]
Formation energy
[eV]
Energy barrier of oxygen
dissociation [eV]
ORR pathway (4e−/2e−)
4.22 (SWCNT)[35]
401.4 ± 0.3[5]
1.39[27]
0.32 (Graphene)[22]
0.86 ((8,0) SWCNT)[35]
High 4e− pathway[48]
0.93 ((10,0) SWCNT)[53]
1.87 (Graphene)[35]
0.97 ((5,5)
Pyridinic N
(Pyr-Nx) (x = 1,3)
4.69 (MV with N3
in SWCNT)[35]
398.7 ± 0.2[5]
1.33 (MV with N1)[26]
SWCNT)[53]
3.16 (MV with N3,
(10,0) SWCNT)[53]
1.28 (MV with N3 in
(8,0) SWCNT)[35]
2.96 (MV with N3,
(5,5) SWCNT)[53]
2.34 (MV with N3 in
graphene)[35]
High 4e− pathway[17]
5.61 (N1, graphene)
2.51 (MV with N3, graphene)
Porphyrinic N
(Por-N4)
Pyrrolic N (NPY)
Graphitic carbon
(undoped)
4.7 ((18,0) SWCNT
with Fe)[16a]
4.55 (SWCNT)[35]
4.5 (graphene)
398.9 ± 1.0[37]
400.3 ± 0.2[37]
–
1.372[38]
1.372[38]
1.42 (C–C in
graphene)
2.55 (graphene)[22]
0.009 (graphene)[54]
3.2 ((10,0)
SWCNT)[53]
0.602 ((10,10) SWCNT)
0.12[37]
0.49 ((8,0) SWCNT)[35]
–
[41]
1.61 ((8,0)
SWCNT)[35]
High 4e− pathway[55]
Low 2e− pathway
2.71 (Graphene)[35]
4.47–4.84 (SWCNT)
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catalytic activities.[16a] STW defects, which result from a simple
90° rotation of two carbon atoms, produces two pentagons and
two heptagons in the hexagonal graphitic lattice.[25b] In the presence of STW defects, the N atom can be placed into a pentagon
ring, which is denoted as a pyrrolic N (labelled NPY, Figure 2e).
It is noteworthy that NPY can be placed into the edge of the graphitic plane as well. Compared with other N-dopants, the NPY
configuration does not induce electron/hole doping in the electronic structure, as the defects are stabilized by hydrogen (H)
atoms.[31] Detailed information on band-structure modification
through N-doping can be found elsewhere.[2a,32]
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barrier for other molecules such as oxygen for catalytic reactions. As briefly discussed above, the curvature of a CNT
influences the electronic structure and eventually lowers the
energy barrier of oxygen dissociation compared to flat graphene. Generally, the energy barrier for oxygen dissociation
is related to the number of electrons that occupy the π* antibonding orbitals around N, parallel to the pz axis.[35] Electrons
in the π* anti-bonding orbitals can decrease the stability of
oxygen molecules and facilitate dissociation.[39] As NQ (0.86 eV)
has two pz electrons, π* anti-bonding orbitals around N can be
partially occupied.[35] However, for Pyr-N3 (1.28 eV), a lone pair
on N has only one electron for the pz orbital, which means the
occupation of the π* anti-bonding orbital is negligible.[35] It is
believed that NQ has the lowest energy barrier for oxygen dissociation and is the dominant site for catalytic reactions.[40]
Notably, very recent investigations have shown that Por-N4
(0.602 eV) also has a low energy barrier,[41] which can be further
improved by complexation with transition metals.[16a] The ORR
proceeds through either: i) a one-step, four-electron process, O2
+ 4H+ + 4e− → H2O, or ii) a two-step, two-electron process, O2 +
2H+ + 2e− → H2O2 and H2O2 + 2H+ + 2e− → 2H2O.[5] The fourelectron process is generally very fast, whereas the two-electron
process is rather slow because of the relatively stable hydrogen
peroxide intermediates.[5] Recently, Wang et al. reviewed the
four-electron ORR pathway for N-dopants, particularly guided
by a modulated electronic structure, spin density and charge
density.[5]
N-dopants with different degrees of chemical potential (µN).[16a]
Surprisingly, there have already been pre-existing observations
that in situ doping easily forms Pyr-N, while NQ is dominant
after a high-temperature post-treatment, observed in a precise
XPS study.[40] The increase in µN could be attributed to N-rich
conditions, e.g., under elevated gas pressure or increased
external energy density, multi-N defects (Pyr-N3 and Por-N4)
could be dominant with lowered Ef.[32a] However, the precise
control of selective N-doping still remains a profound challenge
in CNMs.
3. Nitrogen Dopants for Pioneering Applications
Understanding the relationships between electronic structure
and structural features with different dopant configurations
is at the heart of many different forms of materials design. It
may offer a mechanism for energy storage and conversion,
while also commonly dictating the chemical reactivity via electron transfer. As the fundamental understanding of N-dopants
in CNMs matures, a number of pioneering applications have
emerged, where energy conversion and storage are the most
prevalent, yet N-dopants have also been exploited for the
assembly of molecules, as well as in the engineering of new
carbon allotropes.
3.1. Energy Storage/Conversion via N-Dopants
2.3. Preparation Methods for N-Doped Carbon Nanomaterials
Although graphitic carbon layers consisting of sp2 carbons
are relatively inert, various approaches to introducing substitutional N-dopants into the hexagonal C plane have been
investigated.[11b] Those synthesis methods can be classified into
two categories: in situ doping and post-synthetic treatment.[42]
As briefly discussed in Section 2.2., the energetic preference
for N placement relies on the number of defect sites and the
chemical potential of N. Intrinsically, at a typical surface density
of a few SWCNTs per μm2, the areal defect density is a mere
109 sites per cm2.[43] This is significantly low, particularly compared with conventional Si, which implies that engineering of
sp2 carbon should be harder than the simultaneous incorporation of N into sp2 carbon structures. Therefore, the doping
level attained via the post-treatment method is relatively low
(10.1 at%) compared with that via in situ doping (16.4 at%).[23,44]
For post-treatment, thermal annealing at high temperatures
(800–1200 °C), plasma and irradiation treatments with high
energy have been reported.[42] For in situ doping, high-temperature arc discharge,[45] chemical vapor deposition (CVD),[2]
solvothermal synthesis,[23] and laser ablation methods[46] are
available. Interestingly, many of the most promising synthesis
methods for N-doped carbon rely on CVD, by using different C
sources, including methane, acetylene, ethylene, and benzene,
and a N source, including ammonia, ethylene diamine, and
benzylamine.[6,47] Moreover, controlling the overall portion and
flow rate of the N source in CVD, which is directly related to
the chemical potential, enables selective N doping with various
defects.[16a] Recent research has targeted selective formation of
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The majority of research studies performed up to now have
focused on the ORR application of N-doped CNMs. N-doped
CNMs have several advantages for energy materials, including:
i) relatively low costs, ii) long-term durability, and iii) chemical
resistance.[2a] Recent work has shown that selective doping of
N is the major issue for both CNTs and graphene, which determines the limiting current density and onset potential for catalytic reactions.[48] Our previous work suggests that the use of
the Por-N4 structure of CNTs for the ORR increases the current density with an onset potential shift by selective doping of
Por-N4 with an Fe atom (Figure 3a).[16a] Particularly, when Fe is
supplied, a single Fe atom with the oxidation state of (2+) stabilizes the Por-N4 structure, which lowers the formation energy
of the Por-N4 dopant (Figure 3b). The Fe-Por-N4 complex facilitates the breakage of the O–O bond by lowering the energy
barrier (Figure 3c), which leads to a lower onset potential and
an increase in the current density (Figure 3d). Very recently,
self-size selection of graphene by liquid-crystal assembly was
experimentally demonstrated.[48] Interestingly, when the same
N-doping conditions are applied to small and large graphene
flakes, small graphene flakes predominantly produce Pyr-N3,
whereas NQ is dominant for large graphene flakes (Figure 3e),
which also effectively enhances the ORR catalytic activity with
a low energy barrier for oxygen (Figure 3f). Selective N-doping
also plays an important role in energy devices. Generally,
Pyr-N3 in the basal plane is believed to enhance the storage
capacity of CNMs with its tunable hydrophobicity and vacancy
sites. Furthermore, when the Ef or work function is modified by selective N-doping, it can help the charge transport of
photo­excited carriers, which contributes to the improvement
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water), which do not require harsh reaction conditions. To date, most hybrids/composites based on N-doped CNMs have been
fabricated based on relatively weak intermolecular interactions: i) electrostatic attractions
(in water), ii) redox potential difference, and
iii) van der Waals interactions. Considerable efforts have been invested in developing and exploring molecular assembly
of various functional materials, including:
i) polymers and biomolecules,[49] ii) transition
metals,[50] iii) metal oxides,[19b] nitrides, and
sulfides, and iv) semiconducting quantum
dots, at the surface of N-doped CNMs. A
particularly versatile and fascinating utilization of molecular hybrids is in energy
applications, which includes: i) catalysts
for the hydrogen-evolution reaction (HER)
(2H+ + 2e− → H2),[12b] the oxygen-evolution
reaction (OER) (2H2O → 4e− + 4H+ + O2),[12a]
and photoconversion;[19b] ii) energy storage,
such as pseudocapacitors[13] and lithium-ion
batteries;[50] and iii) organic photovoltaics and
organic light-emitting diodes.[51]
The recent ability to control atomicscale features in N-doped CNTs is increasingly exploited for the development of novel
bifunctional catalysts or high-performance
monofunctional
catalysts.
Remarkable
effects have been observed from the hybridization of electrocatalysts for the OER and
the ORR into one, which could be used for
both regenerative fuel cells and rechargeable metal–air batteries.[12a] Sub-nano­
meter Co(OH)x-anchored N-doped CNTs
(Figure 4a) have shown a great potential for
bifunctional catalysis, exhibiting moderate
Figure 3. a) Schematic illustration of N-doped graphene plane for the ORR. The blue, red,
grey, yellow, and green indicate C, NQ, Pyr-N3, Por-N4, and Fe atoms, respectively. b) Calculated ORR activity and excellent OER activity with
formation energies as a function of N chemical potential µN. c) Effective oxygen dissociation on
superior cycling stability.[12a] Phenomenal
Por-N4 sites based on density functional theory (DFT) calculation. d) Half-wave RDE voltammoperformance for the HER is also achieved
grams for the ORR of various N-doped CNTs. b–d) Reproduced with permission.[16a] Copyright by a MoS /N-doped CNT-forest hybrid catax
2011, American Physical Society. e) Schematic illustration of N-dopant selective graphene with
lyst (Figure 4b), which determines the pertheir size. f) Linear sweep voltammograms of various N-doped graphene. e,f) Reproduced with
formance of electrochemical water splitting
permission.[48] Copyright 2014, American Chemical Society.
and direct photoelectrolysis.[12b] Our recent
work on Li-ion batteries has revealed that
N-dopants trigger spontaneous and rapid encapsulation of
in power conversion efficiency of organic photovoltaics, for
electrode materials via electrostatic attractions.[50] Graphitic
instance. Indeed, experiments have even shown that selective
N-doping can promote exciton dissociation in bulk-heterojuncencapsulation of the boundary layer exposed to the electrolyte
tion (BHJ) organic solar cells. Most importantly, such selective
supports the mechanical robustness of the electrode, which can
behavior, both in energy conversion and energy storage, has
be easily deteriorated by migration of Li ions (Figure 4c). As
been exploited to engineer various functional energy materials
mentioned above, N-doping with a controlled work function
for diverse applications, which will be described in Section 3.2.
demonstrates a great potential for charge transport in organic
semiconductors, while avoiding the recombination of charge
carriers.[51a] Moreover, nanoparticles self-assembled at N-doped
3.2. Molecular Hybrids/Composites
CNT surfaces can supplement additional effects, such as plasmonic properties, which can boost the device performance of
From a practical viewpoint, research interest in N-doped
organic photovoltaics.[51b]
CNMs for hybrid materials has been principally motivated by
For graphene, the molecular assembly for a broad spectrum
facile assembly reactions (mostly simple solution mixing in
of other materials, including biomolecular, semiconductors,
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and metals, has been tailored by various types of N-doping.
Pyr-N3 predominantly tweaks DNA origami through the interaction between lone pair electrons of Pyr-N3 and Mg ions in
DNA (Figure 5a).[49b] The p-type band structure of Pyr-N3
is appropriate for the electrostatic attraction, which enables
spontaneous Au nanoparticle decoration on the basal planes
of N-CNTs under weak acidic conditions (Figure 5b).[52] Interestingly, Si particles with native oxide layers a few nanometers
thick can also be wrapped with flexible flakes of N-doped graphene, which confirms the ease of molecular assembly with
N-dopants.[50] Conversely, where NQ is dominant, extra electrons from N raise the Ef and lower the work function, which
drives a redox potential difference. Consequently, it allows
spontaneous bimetal electroless deposition of Pt and Pd at
the surface of graphene (Figure 5d), which is applicable for
electrocatalysts.[18a]
3.3. Structural Transformation of Carbon Allotropes
Figure 4. Multifunctional composites based on N-doped CNTs with
various applications. a) Cobalt-species-anchored NCNT hybrid catalyst
for the ORR and OER. Reproduced with permission.[12a] Copyright 2016,
American Chemical Society. b) MoSx/NCNT hybrid catalyst for the HER.
Reproduced with permission.[12b] Copyright 2014, American Chemical
Society. c) Si/NCNT hybrid anode for lithium-ion batteries. Reproduced
with permission.[50] Copyright 2014, Royal Society of Chemistry. d) Au/
NCNT active layer for organic solar cells. Reproduced with permission.[51b]
Copyright 2015, Wiley.
Figure 5. Multifunctional composites based on N-doped graphene with
various applications. a) DNA-origami assembly on N-doped graphene.
Reproduced with permission.[49b] Copyright 2012, Wiley-VCH. b) Au/Ngraphene film for electrically conducting layer. Reproduced with permission.[52] Copyright 2010, Wiley-VCH. c) Si/N-graphene hybrid anode for
lithium-ion batteries. Reproduced with permission.[50] Copyright 2014,
Royal Society of Chemistry. d) PtPd/NCNT catalyst for the ORR. Reproduced with permission.[18a] Copyright 2014, Wiley-VCH.
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Atomic-scale structural engineering of CNMs has been recognized as one of the essential routes to tailor desired physicochemical properties, exploiting strong structure-dependent
material properties at the nanoscale. Fullerene, carbon nanotubes, graphene, and graphene nanoribbons are the types of
carbon allotropes being constructed with sp2 hybridized carbon
networks, and they possess physicochemical properties that
are distinguishable from each other, including their energy
bandgap and chemical affinity, according to their geometries.
Structural transformation from tubular CNT to planar graphene, called “unzipping”, has been considered a promising
way to tailor the structures of graphene-based nanomaterials.
Recently, we reported a controlled unzipping principle based
on electrochemical oxidation using N-dopants in CNTs.[13]
Pyr-N3 incorporated in hexagonal carbon networks modifies
the electronic structure of neighboring carbon atoms of PyrN3, and makes them more favorable for the unzipping reaction, compared to other carbon atoms in the pristine graphene
plane (Figure 6a). The enhanced reactivity for unzipping by
Pyr-N3 dopants is clearly verified by the lowered critical applied
potential (0.6 V) of N-doped CNTs, compared to that of pristine
undoped CNTs (0.8 V) (Figure 6b,c). This reactivity difference
enables the initiation of the unzipping reaction exclusively
from Pyr-N3 sites, rather than random reaction sites. The
initiated unzipping reaction sequentially propagates the longitudinal axis of CNTs due to the generated strain at opened
sites during unzipping, originating from its tubular structure. Consequently, CNTs are transformed to planar graphene
nanostructures or graphene–nanotube complexes as shown in
Figure 6d–f. It is noteworthy that the unzipped nanostructures
possess intact crystallinity, resulting from the highly specific
unzipping principle when utilizing Pyr-N3 (Figure 6g). Moreover, the degree of unzipping is precisely controlled by the
density of N-dopants, applied electrical potential, reaction time,
and so on. This highly controllable graphene-scissoring mechanism based on N-dopant sites offers unprecedented opportunities for tailored intact crystalline CNMs and relevant novel
properties and applications.
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 6. Structural transformation of carbon nanotubes via N-dopant specific unzipping. a) Schematic illustration of N-dopant specific unzipping
process. b,c) Chemical-reactivity difference between CNTs (b) and NCNTs 8c). d) Diverse structural transformation of CNTs by N-dopant specific unzipping. e) Graphene nanoribbons, f) CNT–graphene nanoribbon complexes, and g) intact crystallinity of the unzipped carbon structures. Reproduced
with permission.[13] Copyright 2016, Nature Publishing Group.
4. Conclusion and Perspective
We have highlighted that N-doping enables precise control over
electronic structures and molecular configurations of CNMs at
the atomic scale and facilitates distinct physicochemical features
and applications of these interesting materials. Over the past
years, while a variety of novel methods have been proposed for
the effective N-doping of CNMs, research activities for novel
applications have been limited to several specific fields, such as
ORR catalysts. Further exploitation of their exciting properties
is an open challenge for a broad spectrum of emerging fields,
including: i) mapping and control of the density of states for
individual N-dopants, ii) further reduction of the energy barrier for chemisorption, iii) controlled distortion of the electron cloud around atomic orbitals, and iv) tailored engineering
of chemical bonding states and related defects. Recently, we
Small Methods 2017, 1600014
have already witnessed momentous progress in several fields:
i) energy storage/conversion, ii) molecular hybrids/composites, and iii) structural transformation of carbon allotropes, as
summarized here. While further development is anticipated,
this scope is still sufficient to demonstrate the great potential
of N-doped CNMs, particularly when it is strengthened by new
insights into underlying physics and chemistry. Hopefully, this
brief summary of novel concepts along with an intuitive outlook paves the way for further innovation in materials science
and nanotechnology.
Acknowledgements
This work was financially supported by the National Creative Research
Initiative (CRI) Center for Multi-Dimensional Directed Nanoscale
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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Assembly (Grant 2015R1A3A2033061) and the Nano Material
Technology Development Program (Grant 2016M3A7B4905613) through
the National Research Foundation of Korea (NRF) funded by the
Ministry of Science, ICT, & Future Planning.
Received: October 3, 2016
Revised: October 26, 2016
Published online:
[1] a) O. Stephan, P. M. Ajayan, C. Colliex, P. Redlich, J. M. Lambert,
P. Bernier, P. Lefin, Science 1994, 266, 1683; b) J. R. Pels, F. Kapteijn,
J. A. Moulijn, Q. Zhu, K. M. Thomas, Carbon 1995, 33, 1641;
c) A. Lherbier, X. Blase, Y.-M. Niquet, F. Triozon, S. Roche, Phys.
Rev. Lett. 2008, 101, 036808.
[2] a) U. N. Maiti, W. J. Lee, J. M. Lee, Y. Oh, J. Y. Kim, J. E. Kim,
J. Shim, T. H. Han, S. O. Kim, Adv. Mater. 2014, 26, 40; b) D. H. Lee,
W. J. Lee, S. O. Kim, Nano Lett. 2009, 9, 1427.
[3] D. W. Boukhvalov, M. I. Katsnelson, Nano Lett. 2008, 8, 4373.
[4] R. Czerw, M. Terrones, J. C. Charlier, X. Blase, B. Foley,
R. Kamalakaran, N. Grobert, H. Terrones, D. Tekleab, P. M. Ajayan,
W. Blau, M. Rühle, D. L. Carroll, Nano Lett. 2001, 1, 457.
[5] H. Wang, T. Maiyalagan, X. Wang, ACS Catal. 2012, 2, 781.
[6] S. Maldonado, S. Morin, K. J. Stevenson, Carbon 2006, 44,
1429.
[7] J. H. Kaufman, S. Metin, D. D. Saperstein, Phys. Rev. B 1989, 39,
13053.
[8] R. Yu, M. Zhan, D. Cheng, S. Yang, Z. Liu, L. Zheng, J. Phys. Chem.
1995, 99, 1818.
[9] L. S. Panchakarla, K. S. Subrahmanyam, S. K. Saha, A. Govindaraj,
H. R. Krishnamurthy, U. V. Waghmare, C. N. R. Rao, Adv. Mater.
2009, 21, 4726.
[10] J. O. Hwang, J. S. Park, D. S. Choi, J. Y. Kim, S. H. Lee, K. E. Lee,
Y.-H. Kim, M. H. Song, S. Yoo, S. O. Kim, ACS Nano 2012, 6,
159.
[11] W. J. Lee, U. N. Maiti, J. M. Lee, J. Lim, T. H. Han, S. O. Kim, Chem.
Commun. 2014, 50, 6818.
[12] a) J. E. Kim, J. Lim, G. Y. Lee, S. H. Choi, U. N. Maiti, W. J. Lee,
H. J. Lee, S. O. Kim, ACS Appl. Mater. Interfaces 2016, 8, 1571;
b) D. J. Li, U. N. Maiti, J. Lim, D. S. Choi, W. J. Lee, Y. Oh, G. Y. Lee,
S. O. Kim, Nano Lett. 2014, 14, 1228.
[13] J. Lim, U. N. Maiti, N.-Y. Kim, R. Narayan, W. J. Lee, D. S. Choi,
Y. Oh, J. M. Lee, G. Y. Lee, S. H. Kang, H. Kim, Y.-H. Kim, S. O. Kim,
Nat. Commun. 2016, 7, 10364.
[14] S. H. Lee, D. H. Lee, W. J. Lee, S. O. Kim, Adv. Funct. Mater. 2011,
21, 1338.
[15] K. Gong, F. Du, Z. Xia, M. Durstock, L. Dai, Science 2009, 323,
760.
[16] a) D. H. Lee, W. J. Lee, W. J. Lee, S. O. Kim, Y. H. Kim, Phys.
Rev. Lett. 2011, 106, 175502; b) A. Zitolo, V. Goellner, V. Armel,
M.-T. Sougrati, T. Mineva, L. Stievano, E. Fonda, F. Jaouen, Nat.
Mater. 2015, 14, 937; c) D. Guo, R. Shibuya, C. Akiba, S. Saji,
T. Kondo, J. Nakamura, Science 2016, 351, 361.
[17] D. Geng, Y. Chen, Y. Chen, Y. Li, R. Li, X. Sun, S. Ye, S. Knights,
Energy Environ. Sci. 2011, 4, 760.
[18] a) W. J. Lee, D. S. Choi, S. H. Lee, J. Lim, J. E. Kim, D. J. Li,
G. Y. Lee, S. O. Kim, Part. Part. Syst. Char. 2014, 31, 965;
b) M. Lefèvre, E. Proietti, F. Jaouen, J.-P. Dodelet, Science 2009, 324,
71; c) G. Wu, K. L. More, C. M. Johnston, P. Zelenay, Science 2011,
332, 443.
[19] a) W. J. Lee, D. H. Lee, T. H. Han, S. H. Lee, H.-S. Moon, J. A. Lee,
S. O. Kim, Chem. Commun. 2011, 47, 535; b) W. J. Lee, J. M. Lee,
1600014 (8 of 9)
wileyonlinelibrary.com
S. T. Kochuveedu, T. H. Han, H. Y. Jeong, M. Park, J. M. Yun,
J. Kwon, K. No, D. H. Kim, S. O. Kim, ACS Nano 2012, 6,
935.
[20] a) D. V. Kosynkin, A. L. Higginbotham, A. Sinitskii, J. R. Lomeda,
A. Dimiev, B. K. Price, J. M. Tour, Nature 2009, 458, 872;
b) M. Terrones, ACS Nano 2010, 4, 1775.
[21] Z. Hou, X. Wang, T. Ikeda, K. Terakura, M. Oshima, M.-a. Kakimoto,
S. Miyata, Phys. Rev. B 2012, 85, 165439.
[22] Y. Fujimoto, S. Saito, Phys. Rev. B 2011, 84, 245446.
[23] D. Deng, X. Pan, L. Yu, Y. Cui, Y. Jiang, J. Qi, W.-X. Li, Q. Fu, X. Ma,
Q. Xue, G. Sun, X. Bao, Chem. Mater. 2011, 23, 1188.
[24] S. Zhang, S. Tsuzuki, K. Ueno, K. Dokko, M. Watanabe, Angew.
Chem., Int. Ed. 2015, 54, 1302.
[25] a) J. C. Charlier, Acc. Chem. Res. 2002, 35, 1063; b) F. Banhart,
J. Kotakoski, A. V. Krasheninnikov, ACS Nano 2011, 5, 26.
[26] Y.-C. Lin, P.-Y. Teng, C.-H. Yeh, M. Koshino, P.-W. Chiu, K. Suenaga,
Nano Lett. 2015, 15, 7408.
[27] T. Kondo, S. Casolo, T. Suzuki, T. Shikano, M. Sakurai, Y. Harada,
M. Saito, M. Oshima, M. I. Trioni, G. F. Tantardini, J. Nakamura,
Phys. Rev. B 2012, 86, 035436.
[28] L. G. Bulusheva, O. V. Sedelnikova, A. V. Okotrub, Int. J. Quantum
Chem 2011, 111, 2696.
[29] T. G. Pedersen, Phys. Rev. B 2015, 91, 085428.
[30] H. S. Kim, H. S. Kim, S. S. Kim, Y.-H. Kim, Nanoscale 2014, 6,
14911.
[31] J. Wei, H. Hu, H. Zeng, Z. Wang, L. Wang, P. Peng, Appl. Phys. Lett.
2007, 91, 092121.
[32] a) F. Joucken, Y. Tison, P. Le Fèvre, A. Tejeda, A. Taleb-Ibrahimi,
E. Conrad, V. Repain, C. Chacon, A. Bellec, Y. Girard, S. Rousset,
J. Ghijsen, R. Sporken, H. Amara, F. Ducastelle, J. Lagoute, Sci.
Rep. 2015, 5, 14564; b) Y. Tison, J. Lagoute, V. Repain, C. Chacon,
Y. Girard, S. Rousset, F. Joucken, D. Sharma, L. Henrard, H. Amara,
A. Ghedjatti, F. Ducastelle, ACS Nano 2015, 9, 670.
[33] H. Wang, Y. Yuan, L. Wei, K. Goh, D. Yu, Y. Chen, Carbon 2015, 81, 1.
[34] Y.-J. Yu, Y. Zhao, S. Ryu, L. E. Brus, K. S. Kim, P. Kim, Nano Lett.
2009, 9, 3430.
[35] S. Ni, Z. Li, J. Yang, Nanoscale 2012, 4, 1184.
[36] W. F. Egelhoff, Phys. Rev. B 1984, 30, 1052.
[37] Z. Tian, S. Dai, D.-e. Jiang, Chem. Mater. 2015, 27, 5775.
[38] Y. Nam, D. Cho, J. Y. Lee, J. Phys. Chem. C 2016, 120,
11237.
[39] J. Sun, Y.-H. Fang, Z.-P. Liu, Phys. Chem. Chem. Phys. 2014, 16,
13733.
[40] T. Sharifi, G. Hu, X. Jia, T. Wågberg, ACS Nano 2012, 6, 8904.
[41] D. Srivastava, T. Susi, M. Borghei, L. Kari, RSC Adv. 2014, 4,
15225.
[42] Q. Wei, X. Tong, G. Zhang, J. Qiao, Q. Gong, S. Sun, Catalysts 2015,
5, 1574.
[43] Y. Fan, B. R. Goldsmith, P. G. Collins, Nat Mater 2005, 4,
906.
[44] Z.-H. Sheng, L. Shao, J.-J. Chen, W.-J. Bao, F.-B. Wang, X.-H. Xia,
ACS Nano 2011, 5, 4350.
[45] M. Glerup, J. Steinmetz, D. Samaille, O. Stéphan, S. Enouz,
A. Loiseau, S. Roth, P. Bernier, Chem. Phys. Lett. 2004, 387,
193.
[46] Y. Zhang, H. Gu, S. Iijima, Appl. Phys. Lett. 1998, 73, 3827.
[47] Y. Ito, C. Christodoulou, M. V. Nardi, N. Koch, H. Sachdev,
K. Müllen, ACS Nano 2014, 8, 3337.
[48] K. E. Lee, J. E. Kim, U. N. Maiti, J. Lim, J. O. Hwang, J. Shim,
J. J. Oh, T. Yun, S. O. Kim, ACS Nano 2014, 8, 9073.
[49] a) A. Ul Haq, J. Lim, J. M. Yun, W. J. Lee, T. H. Han, S. O. Kim,
Small 2013, 9, 3829; b) J. M. Yun, K. N. Kim, J. Y. Kim, D. O. Shin,
W. J. Lee, S. H. Lee, M. Lieberman, S. O. Kim, Angew. Chem., Int.
Ed. 2012, 51, 912.
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Small Methods 2017, 1600014
www.advancedsciencenews.com
www.small-methods.com
Small Methods 2017, 1600014
[52] S. H. Lee, H. W. Kim, J. O. Hwang, W. J. Lee, J. Kwon,
C. W. Bielawski, R. S. Ruoff, S. O. Kim, Angew. Chem., Int. Ed. 2010,
49, 10084.
[53] M. Mananghaya, E. Rodulfo, G. N. Santos, A. R. Villagracia,
A. N. Ladines, J. Nanomater. 2012, 2012, 14.
[54] Y. Li, S. Zhang, J. Yu, Q. Wang, Q. Sun, P. Jena, Nano Res. 2015, 8,
2901.
[55] W. Ding, Z. Wei, S. Chen, X. Qi, T. Yang, J. Hu, D. Wang, L.-J. Wan,
S. F. Alvi, L. Li, Angew. Chem., Int. Ed. 2013, 52, 11755.
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
wileyonlinelibrary.com
(9 of 9) 1600014
Review
[50] W. J. Lee, T. H. Hwang, J. O. Hwang, H. W. Kim, J. Lim, H. Y. Jeong,
J. Shim, T. H. Han, J. Y. Kim, J. W. Choi, S. O. Kim, Energy Environ.
Sci. 2014, 7, 621.
[51] a) J. M. Lee, J. S. Park, S. H. Lee, H. Kim, S. Yoo, S. O. Kim,
Adv. Mater. 2011, 23, 629; b) J. M. Lee, J. Lim, N. Lee, H. I. Park,
K. E. Lee, T. Jeon, S. A. Nam, J. Kim, J. Shin, S. O. Kim, Adv. Mater.
2015, 27, 1519; c) J. M. Lee, B.-H. Kwon, H. I. Park, H. Kim,
M. G. Kim, J. S. Park, E. S. Kim, S. Yoo, D. Y. Jeon, S. O. Kim, Adv.
Mater. 2013, 25, 2011.