Nitrogen-doped carbon nanotubes and graphene composite structures for energy and catalytic applications.

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Nitrogen-doped carbon nanotubes and graphene composite structures for energy and catalytic applications Won Jun Lee,†a Uday Narayan Maiti,†a Ju Min Lee,a Joonwon Lim,a Tae Hee Han*b and Sang Ouk Kim*a Substitutional heteroatom doping is a promising route to modulate the outstanding material properties of carbon nanotubes and graphene for customized applications. Recently, (nitrogen-) N-doping has been introduced to ensure tunable work-function, enhanced n-type carrier concentration, diminished surface energy, and manageable polarization. Along with the promising assessment of N-doping effects, research on the N-doped carbon based composite structures is emerging for the synergistic integration with various functional materials. This invited feature article reviews the current research progress, emerging trends, and opening opportunities in N-doped carbon based composite structures. Underlying basic principles are introduced for the effective modulation of material properties of graphitic carbons by N-doping. Composite

Received 7th January 2014, Accepted 27th March 2014

structures of N-doped graphitic carbons with various functional materials, including (i) polymers, (ii) transition

DOI: 10.1039/c4cc00146j

benefits of the synergistic composite structures are investigated in energy and catalytic applications, such as organic photovoltaics, photo/electro-catalysts, lithium ion batteries and supercapacitors, with a particular

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emphasis on the optimized interfacial structures and properties.

metals, (iii) metal oxides, nitrides, sulphides, and (iv) semiconducting quantum dots are highlighted. Practical

1. Introduction Carbon based composites have been a principal application field since the birth of carbon nanotubes (CNTs) and graphene a

Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Materials Science & Engineering, KAIST, Daejeon 305-701, Korea. E-mail: [email protected] b Department of Organic and Nano Engineering, Hanyang University, Seoul 133-791, Korea. E-mail: [email protected] † Contributed equally to this feature article.

to exploit the ideal nanostructures and outstanding material properties of the graphitic carbons.1,2 Carbon composite structures synergistically integrated with other functional components, such as metal–metal oxide nanoparticles, polymers and biomolecules, may offer novel material properties potentially useful for many different purposes.3–5 Meanwhile, carbon composite fabrication generally requires the effective functionalization of the graphitic surface for the robust interfacial interaction with other components.6 Currently, oxygen-containing functional groups, such as carboxylic acids, carbonyl, and hydroxyl

Dr Won Jun Lee has been working as a research associate under Prof. Sang Ouk Kim since 2013. He received his PhD degree from Department of Materials Science & Engineering, KAIST, Daejeon, Korea, in 2013 under the supervision of Prof. Sang Ouk Kim. His research focuses on the development of graphitic carbonbased hybrid materials for energy applications. Won Jun Lee

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Dr Uday Narayan Maiti has been working as a postdoc fellow under Prof. Sang Ouk Kim since 2011. He received his PhD degree from Department of Physics, Jadavpur University, Kolkata, India, in 2010 under the supervision of Prof. Kalyan Kumar Chattopadhyay. His research interest includes synthesis and applications of graphene and inorganic nanostructures. Uday Narayan Maiti

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groups that are usually obtained by random chemical oxidation, are widely employed to enhance the surface energy and facilitate the binding sites.6–9 Unfortunately, such oxygen-functionalization severely deteriorates the electrical properties, thermal/mechanical stability, and carrier mobility of CNTs and graphene. Oxygen functional groups inevitably accompany defect formation, thereby reducing the crystallinity and electrical conductivity.10 Moreover, electronegative oxygen traps carriers.11,12 Alternative functionalization methods are also developed exploiting the noncovalent interaction of biomolecules or aromatic compounds with the graphitic surface. However, the functionalized layers interfere with the tight mechanical and electrical contact of composite components with the graphitic surface such that the synergistic functionalities are not fully realized.13 Substitutional heteroatom doping has become a central research focus in the recent CNT and graphene research, given the delicate and robust controllability of electrical, chemical and other properties.14,171 Among various possible dopants (B,15,16 N,17 S,18 F,19 and P20), N-doping possesses advantages in several aspects: a facile doping process and effective modulation of graphitic structure and properties while maintaining high electrical conductivity.21–23 More specifically, N with excessive valence electrons may provide additional p-electrons in a graphitic plane.21,24,25 This feature along with the considerable electronegativity difference between N and C gives rise to the reduced work-function,26 increased n-type carrier concentration,21 high surface energy,27 and tunable polarization28 of graphitic carbons. Accordingly, relevant applications of graphitic carbons, such as flexible electronics,26,29 energy conversion/storage devices,30–32 and catalysts,33 may benefit from the modified electrical properties34 and surface reactivity of N-doped sites.35 Owing to these apparent advantages of N-doping, N-doped carbon composite structures are attracting enormous scientific and technological interest.36 As mentioned above, substitutional N-doping may effectively enhance the surface energy

Prof. Sang Ouk Kim is the KAIST Chair Professor in the Department of Materials Science and Engineering at KAIST and the group leader in the Center for Nanomaterials and Chemical Reactions, Institute for Basic Science (IBS), Daejeon, Korea. He received his PhD from the Department of Chemical Engineering, KAIST in 2000. His main research interest focuses on the Directed Molecular Assembly of Soft Nanomaterials, Sang Ouk Kim which includes: (i) block copolymer self-assembly, (ii) carbon nanotubes and graphene assembly and chemical modification, and (iii) flexible optoelectronics and energy devices. Detailed information can be found at http://snml.kaist.ac.kr.


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Scheme 1 Schematic illustration of N-doped CNTs and graphene based composites with various functional materials.

and surface reactivity37 of graphitic carbons with the minimum loss of material properties.38 The improved surface energy and chemical activity can be utilized for facile synthesis of robust composite structures using various functional materials.39–45 In this feature article, we review the recent research progress in exploiting the N-doped graphitic carbons as promising components of novel composite structures (Scheme 1). In the front part, the basic principles are overviewed associated with how N-doping can effectively modulate the electrical and surface properties of graphitic carbons. Afterwards, many different N-doped carbon based composite structures are highlighted focusing on their energy and catalytic applications. Table 1 briefly summarizes various previously reported N-doped carbon composite structures and their relevant versatile applications.

2. Modulated electrical and surface properties of N-doped CNTs and graphene Substitutional N-doping can effectively modify the electronic and surface properties of CNTs and graphene (Table 2). Nitrogen with excess valence electrons introduces excessive p-electrons in the graphitic plane, which may lead to the reduction of workfunction and enhanced n-type carrier density. It is noteworthy that the additional electron carriers may at least partially compensate the carrier mobility reduction caused by defect formation. Since N has a stronger electronegativity than C, C–N bonding generates permanent dipoles in the graphitic carbon plane to enhance surface energy and wettability. Moreover, the lone pair electrons and permanent dipoles associated with N-dopants offer surface reactivity and catalytic activity. In this section, the basic principles of the modulation of electronic and surface properties of CNTs and graphene are overviewed. 2.1.

Electronic properties of N-doped CNTs and graphene

A hexagonal two-dimensional graphitic lattice consists of the robust s-bonding between planar sp2-hybrid C orbitals. The electrical conductivity is given by the out-of-plane p-bonding electrons in the p-orbitals. The overlap of p-electron states splits into the conduction and the valence bands in CNTs and graphene. Due to the typical equivalence of two sub-lattices, graphene

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Various N-doped graphitic carbon composite structures with their versatile applications

Materials (applications) Category

N-doped CNTs 47

Polymers

DNA (dispersing agent), polystyrene (mechanical reinforcement), polyaniline (supercapacitor),48 polythiophene (photovoltaics)49

Metals

Au (sensor),50,51,172 Ag (reinforcement),52 Pt (ORR catalyst),53,54 Mn (field emission electrode),55 Fe (ORR catalyst),56 Co (ORR catalyst),57 Ni (magnetization),58 Ru (NH3 decomposition),59 Pd (H2O2 sensor)60 SiOx (mineralization),61 CaCO3 (mineralization),61 TiO2 (photocatalyst),13 VOx (supercapacitor),62 LiFePO4 (LIB cathode),63 Fe3PO4 (LIB anode),64,65 CoOx (ORR catalyst),66 ZnO (OLED),67 RuO2 (supercapacitor),68 SnO2 (NO sensor),69 CeO2 (NO sensor),69 TiSiOx (ORR catalyst)70

Metal oxides


N-doped graphene 46

DNA (patterning),72 peptides (supercapacitor),73 polystyrene-co-butadiene-co-styrene (electroconductive matrix)74 Au (sensor),75,76 Ag (ORR catalyst),77 Pt (ORR catalyst),78 Li (hydrogen storage),79 Fe (ORR catalyst),80 Pd (hydrogen storage)40 SiOx (porous reinforcement),81 TiO2 (photocatalyst),82 VOx (LIB anode),83 MnO2 (Li-O2 battery catalyst, supercapacitor),43,84,85 FeOx (LIB anode, ORR catalyst),86 ZnO (photoluminescence, field emission electrode),87,88 SnOx (LIB anode),39,89 NiCo2O4 (electrocatalyst),90 Zn2GeO4 (LIB anode)91 TiN (catalyst),92,93 FeN (ORR catalyst),94 VN (ORR catalyst)92,93 NiSx (LIB anode),95 MoSx (photocatalyst),96 WS2 (LIB anode)97

Metal nitrides Metal sulfides Quantum dots

Table 2

CdSe (electroluminscence),41 InP (OPVs)71

Electrical and surface properties of graphene and CNTs before and after N-doping

Properties

Pristine graphene

Mobility (cm2 V1 s1)

B15 000 for electron and hole98 5–45023,99

Electrical conductivity (S cm1) Work-functiona (eV)

106 103

8, 333 [rGO film]

4.5105,106

Band gapa (eV)

098

3.98 [1% NQ]107 4.83 [1% NP]107 4.92 [1% NN]107 0–0.2 [0–2.9%]110

Surface energy (water contact angle) Chemical and catalytic activity

Equals to the underlying substrate (single layer)116 Pseudo-capacitive inactive,118 ORR inactive119

75.41 (few layer NrGO) — [advancing mode]117 120 Pseudo-capacitive active, Pseudo-capacitive Pseudo-capacitive active,123 ORR active121 inactive,118 ORR inactive122 ORR active33

a

N-doped graphene

Pristine CNTs

N-doped CNTs

B1000 for electron, 4000 for hole100 0.17–2 105 104

18–895101,102

4.47–4.8108,109

3.9–4.52 [0–7%]102,108

0 [Metallic CNTs]111 0.767/diameter [semiconducting SWNT]112,113 —

0 [NQ-CNTs]114,115 Open [NP-CNTs]114

219 [CNT mat]102

NQ – quaternary; NP – pyridinic; NN – nitrile.

behaves as a semi-metal where the conduction and valence bands are in contact.124,125 By contrast, according to the chirality of graphene sheet wrapping, single-walled carbon nanotubes (SWCNTs) can be either metallic or semiconducting. Multiwalled carbon nanotubes (MWCNTs) are composed of several densely stacked graphene sheets such that the electrical conduction is usually dominated by a metallic shell. Nitrogen can be incorporated into the graphitic lattice in several different configurations,126 including pyridinic (NP, 1), pyrrolic (NPY, 2), quaternary (NQ, 3), nitrile (NN, 4), primary amine (NA, 5), vacancy pyridinic doping complexes (VNP, 6, 7), and ‘interstitial’ divalent nitrogen atom (8), as schematically described in Fig. 1a. Among those configurations, the NQ and NP are the most common types. N-dopants in the graphitic plane can significantly alter the electronic structures, which strongly depend on the bonding configurations.127,128 In the case of NQ, four out of five valence electrons of N form s- and p-bonding, similar to C atoms. The remaining one forms a shallow donor state in the bandgap of semiconducting SWCNTs129 or is completely delocalized in the p-electron system of graphene or metallic SWCNTs.127 By contrast, in the case

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of NP, two valence electrons of N take part in s-bonding with neighboring C atoms, two others form a lone pair, and the rest stays in the p-state.127 Due to the vacancy site, the equivalent p-electron system is missing, which leads to the p-type doping effect for graphene or metallic CNTs,114,127 and the acceptor state in the bandgap of semiconducting SWCNTs.114 It is noteworthy that N-doping may generate vacancy or other defects in the graphitic lattice plane, yet the defect types are relatively well-defined. Moreover, the carrier mobility reduction caused by the defect formation can be partially compensated by the excessive p-electrons from the substituted N, while electrical conductivity is generally given by the charge carrier density multiplied by charge carrier mobility. 2.2.

Catalytic activities of N-doped CNTs and graphene

Pristine graphitic surfaces of CNTs and graphene are composed of neutral C–C bonds coupled with low wettability and chemical inertness. By contrast, due to the significant difference in the electronegativity of N (Pauling scale: 3.04) and C (Pauling scale: 2.55), C–N bonds are permanently polarized with weak negative charge over the N atom and weak positive charge over the


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Fig. 1 (a) Different possible configurations of N-doping. Reproduced with permission from ref. 126. (b) Change in the adsorption mode of formaldehyde over a Pt catalyst with a N-doped CNT support. Reproduced with permission from ref. 134. (c) Nucleation and growth of polyaniline from Np-doped CNTs. Reproduced with permission from ref. 48.

nearby C atoms. This charge distribution of the N-doped graphitic plane plays a vital role in the catalytic activity. For instance, N-doped graphene acts as a highly stable catalyst for oxygen reduction reaction (ORR). In an ORR process, a positive charge over the C atom adjacent to N facilitates the O2 absorption, an initial step for the effective ORR process. Subsequently, an increase of the local density of states around the Fermi level of N-doped graphitic carbons facilitates a charge transfer to the adsorbed O2 molecule, which reduces the dissociation energy barrier. The catalytic activity of N-doped graphitic carbon is strongly dependent on the doping configuration. NQ is excellent for the ORR process, but NP and NPY are less active due to the presence of lone-pair electrons, which hinder the absorption of O2 molecules at the graphitic surface. By contrast, owing to the lone pair electrons, NP serves as a Bronsted–Lowry base which is readily protonated in an aqueous solution. Such protonation renders the doping site positively charged, thereby introducing a mild basic behaviour, and rendering base catalyst functionality.35,130 Lone pair electrons are donated to an acceptor molecule in a chemical reaction to stabilize an intermediate.131 The electron transfer from the NP site to the acceptor reagent


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increases its nucleophilic or electrophobic characteristic. The enhanced nucleophilic or electrophobic characteristic of the intermediate state triggers the subsequent chemical reactions, during which the graphitic base catalyst returns to its initial state. Dommele et al. firstly demonstrated the basic catalytic behaviour of N-doped CNTs for Knoevenagel condensation reactions.132 N-doped CNTs and graphene can also be used as excellent catalytic supports. Experimental and theoretical research suggests that, along with the extremely large surface area of CNTs and graphene, the principal role of the N-dopants in the catalytic support is twofold. (i) They increase the support– catalyst interaction for improved catalyst durability.133 (ii) They modify the electronic structure of the catalyst particle to enhance the intrinsic catalytic activity.54,134 Simulation studies showed that transition-metal nanoparticles have higher binding energies in the N-doped graphitic plane than their undoped counterpart due to the strong interaction of their d-orbitals with the p-orbitals of N.135,136 Interestingly, the interaction depends on the configuration of N-dopants. NP shows much stronger binding than NQ due to the larger overlap of the dorbital of the transition metal with the nonbonding state of lone pair electrons.137 Catalytic activity enhancement of noble-metal nanoparticles at the oxide base substrate is relatively well-known. Recent experimental research suggests that the N-doped carbon catalytic support can also promote the intrinsic activity of catalyst particles. While the theoretical basis to support this finding has been lacking, recently, Feng et al. gave an account of this interesting behaviour.134 They performed simulation and demonstrated that the electronic structure modification of transition metal nanoparticles by the neighboring 2p-orbital of N gives rise to a higher adsorption energy of the substances to be catalyzed (Fig. 1b). 2.3.

Surface reactivity of N-doped CNTs and graphene

Taking advantage of N-doped CNTs and graphene, a wide range of composite structures can be produced either by in situ preparation or by post-synthetic adsorption through electrostatic interaction. In general, the in situ growth of the N-doped graphitic surface can be proceeded by two successive steps of nucleation and growth. The positively charged Np site electrostatically attracts any negatively charged precursor for facile nucleation. Subsequently, preferential growth of nanomaterials from the nuclei gives rise to a various functional composite structures with directly grown N-doped graphitic carbon surfaces. Based on this straightforward scheme, our research group successfully demonstrated distinct composite structures of polymers and metal-oxides with N-doped CNTs (Fig. 1c).13,48 The nucleation at N-doped sites can be supplementary facilitated by enhanced surface energy of N-doped graphitic plane. The increment in the surface energy may reduce the energy barrier for heterogeneous nucleation. Graphitic carbon composites with metal nanoparticles can be readily formed via spontaneous electroless reduction at the graphitic surface as long as the reduction potential of the metal

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precursor is lower than the Fermi level of the CNTs or graphene.138 This difference in the energy levels facilitates a spontaneous electron transfer from CNTs or graphene to the metal precursor causing reduction. In general, NQ-doping increases the Fermi level of graphitic carbons, which is favorable for the spontaneous reduction. By contrast, NP-dopants can be readily protonated while the electron transfers to a metal precursor. Taken together, N-doped CNTs or graphene with either NQ or NP configuration allow the spontaneous reduction process thus benefiting the construction of composite material. Besides, ex situ attachment of negatively charged molecules, polymers or nanoparticles by electrostatic attraction at positively charged Np is also a useful route for facile composite structure formation.

3. Various composite materials with N-doped CNTs and graphene In the previous section, how substitutional N-doping can effectively influence the electronic and surface properties of CNTs and graphene is studied. In this section, current research progress in the N-doped graphitic carbon based composite structures is reviewed. Various functional composite structures with (1) polymers, biomolecules, (2) transition metals, (3) metal oxides, nitrides and sulfides, and (4) semiconducting quantum dots are highlighted. 3.1.

Polymers and biomolecules

While pristine CNTs and graphene generally show hydrophobic characteristics with chemical inertness, N-doped counterparts can provide highly specific adsorption sites for hydrophilic biomolecules, (e.g. DNAs),44 and initiation sites for polymerization reaction.47,49 Significantly, such specific surface reactivity at N-doped sites is potentially useful for the spatial control of organic molecular absorption for desired architectures. Our research group demonstrates that negatively charged DNA origami can be stabilized at the N-doped graphene surface for unique DNA nanopatterning.72 Fig. 2a illustrates the procedure of DNA origami stabilization on graphene surfaces. The exact stabilization mechanism was investigated through Mg2p XPS measurements, which verified that the significance of electrostatic binding between Mg2+ and lone-pair electrons at NP sites is crucial. Mg2+ decorates negatively charged DNA and facilitates the formation of a stable origami structure. The Mg2p binding energy at oxygenfunctionalized graphene and N-doped graphene was measured to be 50.1 and 50.7 eV, respectively (Fig. 2b), indicating that the lone-pair electrons of N can strongly interact with Mg2+. When coupled with the polymer matrix, graphitic carbons have been anticipated to provide ideal composite structures for diverse applications. Effective reinforcement of stiffness,139 strength,140 toughness,141 and electrical/thermal conductivity142 have been reported at a low loading level of high aspect ratio graphitic fillers. Nonetheless, the longstanding technological challenge still remains for the effective exfoliation of

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Fig. 2 (a) Schematic presentation of DNA origami patterning over N-doped graphene. Spin-cast GO films are lithographically patterned and chemically modified by reduction or N-doping. DNA origami assembly on the patterned graphene surface from buffer solution (RIE = reactive ion etching). Reproduced with permission from ref. 72. (b) AFM images and XPS Mg2p spectra of DNA origami assembled on N-doped graphene. Reproduced with permission. (c) and (d) SEM images of N-doped CNT– polyaniline core–shell composite materials. Reproduced with permission from ref. 48.

individual CNT strands and graphene monolayers in the polymer matrix. Non-uniform dispersion of fillers leads to a high filler loading for percolation, non-uniform material properties and local stress concentrations,143 In this regard, polymer grafting on the graphitic carbon surface has been considered as a straightforward route for the effective exfoliation of graphitic carbons. Recently, we reported that conducting polymer chains (polyaniline) are directly grown from the N-doped sites at CNTs to form a core–shell heterogeneous composite structure.48 As depicted in Fig. 2c, N-doped sites provide active initiation sites for polymerization reaction. When a sufficient amount of acidic initiators (ammonium peroxydisulfate (APS)) is added, the lone-pair electrons of Np are protonated, while aniline monomers are oxidized. This reaction generates radical cations of the aniline monomer that strongly interact with the Np at the graphitic surface via radical–cation coupling. The resulting polyaniline shell thickness can be systematically controlled in a wide range from 10 to 100 nm (Fig. 2d). 3.2.

Transition metals

Transition metals retain partially filled d- or f-orbitals in various oxidation states, and have been extensively explored in various fields of materials chemistry due to their excellent electrical conductivity, catalytic activity and other intriguing properties.144 Nonetheless, their high cost and inevitable corrosion issues triggered research interest shift to their composite structures. Transition metal–graphitic carbon composites have been widely investigated taking advantage of the characteristics of transition metals in graphitic carbon composite form, including their (1) low cost, (2) high surface area, and (3) good electrical/thermal conductivity. Moreover, graphitic carbon may strengthen the catalytic activity of the transition metal catalyst


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as a co-catalyst and provide a strong backbone to sustain under harsh conditions.145 A recent study confirmed that metal– graphitic carbon composite structures can effectively improve the durability of transition metal based electrodes or catalysts under harsh environmental conditions.54,70,78,86,94,172 Recently, our research group reported the facile fabrication of vertical N-doped CNT arrays decorated with transition metal nanoparticles.55 Electrostatic attraction between carboxylterminated metal particles (Au) and N-doped CNTs could readily achieve the composite structures under weak acidic conditions. Ru and Mn nanoparticles were decorated over N-doped CNTs via in situ deposition of their corresponding metal oxides and post thermal reduction (Fig. 3b and c). We also presented another interesting approach of spontaneous Au reduction on an N-doped graphene surface.146,147 It is widely recognized that NP, NPY or NQ dopants at the graphitic surface may serve as reduction and coordination sites for metal salts. Thus, simple immersion of N-doped graphene film in gold (HAuCl4) precursor solution generates reduced Au particles at the graphitic surface. As depicted in Fig. 3d, Au nanoparticles densely decorate the surfaces of porous graphene networks. It is noteworthy that this spontaneous reduction behaviour can be strengthened by the reduced work-function by N-doping and the resultant enlarged redox potential difference.

3.3.

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applications. Unfortunately, many transition metal complexes suffer from poor electrical conductivity151 and low chemical stability.152 Thus, an appropriate support material is required to compensate the chemical stability and electrical conductivity of metal complexes.153 N-doped graphitic carbons are attractive candidates for such support material owing to their high electrical conductivity and exceptional stability. Lee et al. reported the efficient and environmentally benign biomimetic mineralization of various metal oxides on a graphitic carbon surface. A stable carbon–metal oxide core–shell composite structure could be created without harsh surface treatment and an interfacial adhesive layer. The N-doped sites on the graphitic plane trigger the spontaneous deposition of uniformly thick metal oxides (SiO2, CaCO3, and TiO2) through coordinative electron transfer mechanisms (Fig. 4a–e).13,61 Such a deposition process can be conducted in neutral aqueous solution at ambient pressures and temperatures. Moreover, the direct contact between the metal oxide nanoshell and the N-doped graphitic core enables the modulation of the resulting work-function and electrical properties.

Metal oxides, nitrides, and sulfides

The family of transition metal complexes, including oxides,148 nitrides,149 and sulphides,150 has been intensively explored discovering their various crystalline phases and diverse tailored

Fig. 3 TEM images of (a) Au particle/N-doped CNTs (NCNTs). Reproduced with permission from ref. 172. (b) Ru particle/NCNTs, (c) Mn particle/NCNTs. Reproduced with permission from ref. 55. (d) Nucleation and growth of Au particles on the N-doped graphene network. Reproduced with permission from ref. 147.


Fig. 4 (a) Schematic illustration of NCNT–metal oxide core–shell composite fabrication. N-doping sites act as nucleation sites. (b) TEM and (c) SEM images of NCNT–TiO2 core–shell composites. The inset shows the lattice distance of the anatase phase. (d) NCNT–TiO2 core–shell (top) composites and bare NCNTs (bottom). Reproduced with permission from ref. 13. (e) SEM and TEM (inset) images of NCNT–SiO2 (top) and NCNT– CaCO3 (bottom) core–shell composites. Reproduced with permission from ref. 61.

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For nitride complexes, their abundant grain boundaries and defects may readily degrade their electron transport efficiency and the diffusion of reactive species in catalytic activities. To date, many studies have been devoted to facilitate electron transport and provide multiple diffusion pathways via composite preparation of different materials. For instance, Zhang et al. reported that various binary nitrides (MoN, TiN, and VN) could be spontaneously anchored on N-doped graphene by physisorption and effectively improve the electrical conductivity and electrocatalytic activity.93 Fechler et al. recently reported the one-pot synthesis of ternary nitrides on N-doped graphitic carbons for enhanced stability and electrical conductivity.92 Similar to metal nitrides, sulfide complexes frequently suffer from less active sites and poor charge transport. Very recently, Meng et al. reported a nanoscale p–n junction of p-type MoS2 and n-type N-doped graphene with a highly efficient photocatalytic property for hydrogen evolution reactions.96 The heterojunction between the N-doped graphene and MoS2 greatly reduced the charge recombination and enhanced the photogeneration of electron–hole pairs.

3.4.

Semiconducting quantum dots

Nanocrystalline semiconducting quantum dots (QDs) of transition metal chalcogenide (MX2, M = Zn, Cd, In, Pb and Bi/X = Se, Te) exhibit unique optical and electrical properties through their size dependent quantum confinement effect.154 To date, a great deal of research attention has been paid to these materials and relevant applications such as photocatalysts and photovoltaics have emerged. Unfortunately, rapid recombination of excitons, sluggish charge transfer via inter-dot hopping, and strong tendency for particle agglomeration have hindered the realization of ultimate application.155 In this regard, novel metal chalcogenide based carbon composites has been employed to overcome these shortcomings. Recently, Jia et al. demonstrated that CdS nanoparticles deposited on N-doped graphene greatly enhance the photocatalytic activity, taking advantage of fast charge transport and robust support.156 Due to a similar lattice spacing of the h100i plane of hexagonal CdS and the basal plane of N-doped graphene, defects functioned as the essential nucleation sites of CdS nanoparticle growth. Recently, Lee et al. successfully synthesized colloidal InP QDs stabilized on the surfaces of N-doped CNTs71 without the assistance of an intermediate adhesive layer. The weakly etched In3+-rich surface of bare QDs shows positive charge and strongly interacts with the lone-pair electrons of the N-doped sites of CNTs. The attractive interaction between the InP QDs and the N-doped CNTs was confirmed by X-ray photoelectron spectroscopy (XPS). The In peaks shifted 0.8–1.0 eV higher due to the strongly bound In3+ at the N-doped CNTs. Their strong interaction was also confirmed by extended X-ray absorption fine structure (EXAFS) analysis. Because In3+ ions at the QDs undergo strong coordinative interactions with N-doped sites, N atoms at the graphitic plane pose considerable steric hindrance, subsequently resulting in an increased bonding distance with In.

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4. Energy and catalytic applications of N-doped CNTs and graphene composites 4.1.

Organic photovoltaics

Current organic photovoltaic (OPV) devices are primarily based on the bulk heterojunction (BHJ) of semiconducting polymers and fullerene derivatives, which may overcome the short diffusion length of excitons (10–20 nm) and enhance the exciton dissociation with the organic active layer.157 Nonetheless, the BHJ structure has a limited number of dedicated pathways for each type of carrier, leading to insufficient charge transport.158 To resolve this obstacle, manifold nanomaterials have been introduced to facilitate the additional charge transport pathways, none of which was successful thus far (Fig. 5a).159 In fact, many research groups have introduced CNTs in the active layers of OPVs, but CNTs act as quenching sites rather than the transport pathway, resulting in spontaneous injection of both electrons and holes. During recent years, our research group successfully overcame this longstanding technological issue and demonstrated remarkable OPV performances with B- and N-doped CNTs in the BHJ active layers.160 Commercially available CNTs are doped by thermal treatment with NH3 and B2O3 powder for gas for N-doping and B-doping, respectively. The N- and B-doping can control the work-function of CNTs (4.6 eV for undoped CNTs, 4.4 eV for N-doped CNTs and 5.2 eV for B-doped CNTs) and thereby enable the charge selective transport of photo-excited carriers (N-doped CNTs for electron only transport and B-doped CNTs for hole only transport) with minimum charge recombination. These dopings also facilitate the uniform dispersion of CNTs within BHJ layers. The power conversion efficiency of devices based on poly(3-hexylthiophene) (P3HT)/[6,6]-phenyl C61 butyric acid methyl ester (PC61BM) with doped CNTs has been enhanced from 3.0% to 4.1%, marking a

Fig. 5 (a) Schematic OPV architecture with B- and N-doped CNTs (BCNTs and NCNTs) and QDs:NCNTs. J–V characteristics with (b) NCNTs and (c) QDs:NCNTs. Reproduced with permission from ref. 71 and 160.


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37% improvement over conventional OPVs (Fig. 5b). Outstanding performance of N-doped CNTs for OPVs was also confirmed with low-band-gap semiconducting polymers.42 Initially, the incorporation of N-doped CNTs within a polythieno[3,4-b]thiophene/benzodithiophene (PTB7)/[6,6]-phenyl C71 butyric acid methyl ester (PC71BM) BHJ layer promoted exciton dissociation and enhanced the degree of light absorption. Additionally, the cohesive nanoscale ordering of PBT7 within N-doped CNTs gives rise to remarkable enhancement in OPV efficiency from 7.3% to 8.6%. Recently, we reported OPVs with composite nanomaterials consisting of indium phosphite (InP) QDs/N-doped CNTs. N-doped sites of CNTs have considerable local polarities, and mediate direct interaction with positively charged bare InP QDs.71 The conduction band of InP QDs matched well with the lowest unoccupied molecular orbital of semiconducting polymers. Thus, InP QDs and NCNTs are connected via ohmic contact. This composite structure promotes the efficient exciton dissociation and spontaneous charge transfer to N-doped CNTs through InP QDs. The efficiency of P3HT/indene-C60 bisadduct (ICBA) OPVs with InP QD–N-CNT composites was improved from 4.68% to 6.11%, which correspond to a 31% enhancement. The substitutional doping of CNTs not only encourages the selective charge transport but also mediates tight electric binding with other nanomaterials (Fig. 5c). 4.2.

Composite catalysts

4.2.1. Photocatalysts. Recent research interest in photocatalysts is particularly motivated by mimicking natural photosynthesis to produce chemical fuels from solar energy.161 In principle, photocatalysis is based on the sensitized photoreaction consisting of two steps: (i) photo-induced excitation and (ii) electron transfer to the ground-state molecules. Among numerous photocatalysts, titanium dioxide (TiO2) has been of central research interest due to its feasible band-gap (anatase: B3.2 eV) and extraordinary resistance to harsh environments. The photoexcitation of electron–hole pairs in TiO2 can effectively sensitize and catalyze a light-induced redox process. Nevertheless, there has been significant demand and effort to replace TiO2 based photocatalysts with other alternatives due to the limited absorbance range of the ultraviolet (UV) spectrum. In fact, heterogeneous photocatalysts with a narrow band-gap are desirable to achieve effective photocatalyst systems utilizing a wide spectrum of visible light.155 Lee et al. reported N-doped CNT–TiO2 core–shell composites mimicking biomineralization.13 In this approach, N-dopant plays a pivotal role in the nucleation of uniform thick TiO2 nanoshells at the surface of N-doped CNTs. Through this strategy, high-yield TiO2 deposition was attained in a costeffective, energy-efficient, and environmentally benign manner. Significantly, the large surface area of CNTs provides an extremely large TiO2/carbon interface for effective electron–hole separation. Such interfacial hot spots can introduce a new carbon energy level in the middle of the TiO2 band-gap thereby lowering the effective band-gap energy (Fig. 6a). Consequently, the photocatalytic efficiency of N-doped CNT–TiO2 core–shell


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Fig. 6 (a) Schematic illustration of band-gap reduction in N-doped carbon–TiO2 heterogeneous photocatalysts. (b) UV-Vis diffuse reflectance absorption spectra of N-doped CNT–TiO2 core–shell nanotubes. Variation of absorption spectra of MB under visible light irradiation in the presence of (c) TiO2, and (d) N-doped CNT–TiO2 nanotubes. Reproduced with permission ref. 13.

composites surpassed that of commercial TiO2 catalysts particularly under the visible light spectrum (Fig. 6b–d). Recently, Jia et al. reported N-doped graphene–CdS composites for enhanced photocatalytic hydrogen evolution reaction (HER) under the visible light spectrum.156 Essentially, N-doped graphene plays a significant role as a charge collector to promote the electron–hole separation and the transfer of photoinduced carriers. As such, N-doped graphene assists facile HER compared to single CdS materials. It is evident that N-doped carbon composites not only enhance the photocatalytic activity but also modify the intrinsic band-gap by the modulation of electronic structures. 4.2.2. Electrocatalysts. N-doped carbon composites are also attractive for energy conversion catalysts owing to their excellent long-term stability, low cost, and excellent electrontransfer kinetics.162–164 Among various energy conversion catalysts, efficient electrocatalysts for oxygen reduction reactions (ORRs) are essential for the practical application of fuel cells. It is widely recognized that the principal bottleneck for fuel cell technology is the electrocatalysts, mostly based on Pt or other noble metals. Their high cost and poor stability retard the practical applications of fuel cells. In this regard, development of electrocatalysts with novel nanocomposite structures is a promising route to address this longstanding technological challenge.165,166 Since Gong et al. reported that vertical N-doped CNTs have catalytic activity for ORR, there has been enormous research interest in N-doped carbon based ORR catalysts.33 In principle, a change in the adsorption energy resulting from N-doping is known to effectively weaken the O–O bonds. Thus, N-dopant derives charge delocalization and provides host sites for O2 adsorption. Although the N-doped graphitic carbon catalyst for ORR has attracted research attention given its scalability, low cost, and durability, its inferior

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performance, including high onset potential and low current density particularly under acidic operating conditions, hamper further development of this type of catalyst. Alternatively, interesting studies were carried out to combine N-doped graphitic carbon materials with catalytic transition metals (i.e., Pt,78,173,174 Fe,80 Co,57 Ni, Cu, and Pd60) and metal oxides (i.e., Fe3O4,86 MnO2,85 and TiSi2Ox70). N-doped carbon materials could also be used as excellent supports for catalytic particles. Vijayaraghavan et al. reported that Pt particles supported by N-doped CNTs show the enhancement of catalytic activity and durability along with N dopant contents.53 Recently a unique study has been reported based on the controllable assembly of nonprecious Fe3O4 metal-oxide nanoparticles supported on three-dimensional N-doped graphene aerogels.86 The composite catalyst exhibits a more positive onset potential, higher current density, lower peroxide yield, and better durability than the commercial Pt/C catalyst. It is noteworthy that the synergistic integration of inexpensive metal-oxide particles with N-doped carbon materials could show such a remarkable ORR activity. Recently, Lee et al. demonstrated in situ synthesis of Fe-porphyrin-like CNTs, where Fe atoms can be directly incorporated into the di-vacant sites of the NP-doped graphitic plane to form ‘Fe–N4’ complex structures.56 Apart from the novel synthetic method, the seamless and well-defined incorporation of ‘Fe–N4’ complexes into the graphitic plane offers dense catalytic sites with the robust electrical contact along CNT strands (Fig. 7a and b).

Fig. 7 (a) Fe–N4 complex hybridized CNTs, (b) half-wave RDE voltammograms for ORR at the GC (black), Pt/C/GC (purple), CNT/GC (blue), NQ-doped CNT (NQ-CNT)/GC (green), and Fe–N4-CNT/GC (red) electrodes in O2-saturated 0.1 M KOH. The scan rate was 5 mV s1, and the electrode rotation rate was 1600 rpm. (c) Koutecky–Levich plots for the Pt/C/GC (purple), NQ-doped CNT/GC (green), and Fe–N4-CNT/GC (red) electrodes with a rotating limiting disk current of 0.5 V vs. Ag/AgCl in O2-saturated 0.1 M KOH. Reproduced with permission from ref. 56.

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4.3.

Lithium ion batteries

Li-ion battery (LIB) is a rapidly growing field where N-doped graphitic carbons can be utilized.31 To date, anode materials for LIBs have relied upon graphite materials (soft or hard carbon). Unfortunately, graphite based anode materials cannot meet the current increasing demand for large storage capacity due to the inherent limit in the theoretical capacity (B372 mA h g1). In this regard, N-doped graphitic carbons have been introduced with the expectation of increment in the Li-ion storage capacity. On the basis of previous theoretical studies, the modified electronic configuration by N-doping may facilitate Li-ion absorption and enhance ion diffusion through the structural defects. Moreover, the structural defects may provide additional binding sites with Li-ions. Reddy et al. demonstrated that the reversible discharge capacity of N-doped graphene is nearly doubled compared to pristine graphene due to the dense surface defects.167 Limitations of N-doping are also presented, primarily related to the intrinsic capacity limit relative to other competing anode materials, such as Sn,168 Si,169 SnOx,39 SiOx,81 and FeOx.65 Among various large capacity LIB anode candidates, Si is receiving great research interest due to its large theoretical capacity close to 4000 mA h g1, which is more than ten times higher than that of graphite (B372 mA h g1). Nonetheless, lithiation of Si causes very extreme expansion (up to approximately 400%) during the charging–discharging process, which commonly results in a fracture of the silicon with significant capacity reduction. Semiconducting Si also reveals relatively low electrical conductivity that can significantly lower the performance of LIB particularly in the rapid charging–discharging conditions. These intrinsic limitations can be addressed by the integration of Si anode particles with graphitic carbon materials. To supplement the inherent weakness of Si, graphitic carbon should complement the following criteria: (i) void space generation to accommodate the pulverization of Si, (ii) compensation for the low electrical conductivity of Si, and (iii) stable SEI layer formation on the carbon surface. Recently, our research group reported Si–C composite LIB anodes created by the spontaneous encapsulation of N-doped CNTs and graphene over commercial Si particle surfaces.169 The N-doped sites trigger the spontaneous and rapid encapsulation of commercial Si particles via simple surface charge tuning by pH control. In addition, the overall procedure was under ambient pressures, low temperatures, and aqueous media, which contrast to conventional carbonization process where high temperature treatments are required. The resultant N-doped CNT encapsulated Si particles demonstrate remarkable anode performance, especially in terms of the cycle life and rate performance (Fig. 8a and b). A superior capacity retention rate of 79.4% is obtained after 100 cycles at a rate of 2C, and an excellent rate capability of 914 mA h g1 is observed at a rate of 10C, the highest values ever reported for commercial Si-based anodes (Fig. 8c–f). While N-doped graphitic carbons have been employed as anode materials for LIBs, the adaptation into cathodes is less explored due to the unmatched electrode potentials.


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Fig. 8 HR-SEM images of (a) N-doped CNTs and (b) N-doped graphene encapsulated Si particles (Si@N-CNTs). Electrochemical characterization of bare and graphitic encapsulated Si anodes. (c) Potential profiles of bare Si and Si/N-doped CNT composites during the first cycles at the C/10 rate. (d) Reversible discharge capacities and (e) cycling performance of Si/N-doped CNT composites measured at 2C. (f) Rate capability tests for Si/N-doped CNT composites at various C-rates. The gravimetric capacities are normalized with the total mass of Si and N-doped CNT encapsulants. Reproduced with permission from ref. 169.

Nonetheless, charge transport in LIB cathodes can be enhanced by the incorporation of graphitic carbons from the excellent electrical transport through the graphitic carbon network. Yang et al. reported porous composite cathodes consisting of LiFePO4/N-doped CNTs hierarchical structures.63 In this structure, highly conductive and dispersive N-doped CNTs could facilitate the electrical conductivity and Li-ion diffusion. Eventually, this LiFePO4/N-doped CNT composites show a reversible discharge capacity of 138 mA h g1 at a current density of 17 mA g1. In addition to LIBs, N-doped carbon based composites can be utilized in the newly emerging Li–air (Li–O2) or Li–sulfur (Li–S) batteries. Only a few studies have been reported on these emerging batteries, but N-doped carbon materials are anticipated to show promising results owing to the aforementioned advantages. 4.4.

Supercapacitors

Rapidly growing portable electronics and hybrid electric vehicles inevitably require smart energy storage with high power density.170 Among various energy storage devices, supercapacitors are the classical high power storage devices with rapid charging–discharging capability. Intensive research efforts have been devoted to realize graphitic carbon based electrical double-layer supercapacitors. The extremely large surface area


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Fig. 9 SEM image (left) and the corresponding supercapacitor performance (right) of (a) macroporous N-doped graphene film (b) N-doped CNT–polyaniline (PANI) composites. (c) N-doped CNT/PANI/RuOx ternary composites, (right). Reproduced with permission from ref. 48 and 147.

and the high electrical conductivity of CNTs and graphene may offer the key to success in this area. Significantly, the supercapacitor performance of graphitic carbons can further be enhanced by means of N-doping. Our research group have shown that N-doping can enhance the storage capacity of three-dimensional graphene networks while it enhances the surface wettability with electrolytes (Fig. 9a).147 In addition, N-doping induces surface functionalities that can accommodate various pseudocapacitive materials. The charge storage of these materials relies on a reversible redox reaction among the different oxidation states. For instance, polyaniline is an excellent capacitive polymeric material with three different oxidation states, which can be mutually exchanged by redox reactions. Therefore, creating N-doped CNT–polyaniline composite structures should have a synergistic effect on the charge storage capacity (Fig. 9b). We recently succeeded in the direct growth of polyaniline chains from the N-doped sites on CNTs and demonstrated greatly improved charge storage capacity with acceptable rate capability.48 Its high performance results from the synergistic interplay between the high pseudo-capacitive storage at polyaniline overcoat and the fluent electron transport along with the CNT backbone. The capacity can be further increased by forming ternary composite structures via sequential anchoring of polyaniline and transition metal oxides at N-doped CNT surfaces (Fig. 9c).

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5. Conclusions N-doping of CNTs and graphene has revealed the unique roles of controlling the work-function, n-type carrier concentration, surface energy and surface polarization. With these customized features, the N-doped graphitic carbons can be synergistically integrated with distinct functional materials, such as polymers/ biomolecules, transition metals, metal oxides/nitrides/sulfides and semiconducting QDs. In these composite structures, N-doping provides several key functions as follows: (i) it provides versatile reactive sites for a robust interface. (ii) It lowers the work-function for facile charge injection and redox reaction. (iii) It modifies the charge transfer characteristics with excessive electrons. (iv) It lowers surface energy to improve the dispersibility in solvents and other media. (v) It modifies atomic level structures to interlock guests, such as transition metal atoms. (vi) It offers permanent dipoles that can activate the catalytic activity and surface reactivity. (vii) Strong N-doped graphitic carbons tightly support the thermal, chemical and mechanical stability of other composite components for durable applications. We note that it is hard to include all aspects of the explosively rising field of N-doped carbon composites in this feature article. The selected aspects of this article focus on the potential use of N-doped carbon composites for energy conversion and storage with the customized features based on N-doping. Noteworthy in this context is the recent development of N-doped carbon composites concerning their straightforward synthetic protocols and robust control of structures and properties. Overall, the rise of N-doped graphitic carbons has renewed our interest in carbon based composites, particularly in their grand prospects for energy and catalytic applications.

Acknowledgements This work was supported by Institute for Basic Science (IBS) [CA1301-02].

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