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Carbon Nanomaterials for Energy and Biorelated Catalysis: Recent Advances and Looking Forward Chuangang Hu,† Jia Qu,*,§ Ying Xiao,# Shenlong Zhao,*,‡ Hao Chen,§ and Liming Dai*,†,§,#,‡ †

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Center of Advanced Science and Engineering for Carbon (Case4Carbon), Department of Macromolecular Science and Engineering, Case Western Reserve University (CWRU), 10900 Euclid Avenue, Cleveland, Ohio 44106, United States § Institute of Advanced Materials for Nano-Bio Applications, School of Ophthalmology & Optometry, Wenzhou Medical University, 270 Xueyuan Xi Road, Wenzhou, Zhejiang 325027, China # College of Energy, Beijing University of Chemical Technology, Beijing, China ‡ UNSW-BUCT-CWRU International Joint Laboratory, School of Chemical Engineering, The University of New South Wales, Sydney, New South Wales 2052, Australia ABSTRACT: Along with the wide investigation activities in developing carbon-based, metal-free catalysts to replace precious metal (e.g., Pt) catalysts for various green energy devices, carbon nanomaterials have also shown great potential for biorelated applications. This article provides a focused, critical review on the recent advances in these emerging research areas. The structure−property relationship and mechanistic understanding of recently developed carbon-based, metal-free catalysts for chemical/biocatalytic reactions will be discussed along with the challenges and perspectives in this exciting field, providing a look forward for the rational design and fabrication of new carbon-based, metal-free catalysts with high activities, remarkable selectivity, and outstanding durability for various energy-related/biocatalytic processes.

carrier and green oxidizer),17 I3− to I− reduction in dyesensitized solar cells,18 CO2 reduction reaction (CO2RR) for direct conversion of CO2 into fuels,19,20 N2 reduction reaction (N2RR) for synthesis of NH3 under ambient environment,21 sustainable generation of green energy from sunlight and water,22 biosensing, environmental monitoring,23 and even commodity chemical production.24,25

1. INTRODUCTION Catalysis is the core for many sustainable energy conversion devices and advanced biorelated technologies.1−3 Precious metals (e.g., Pt, Pd, Rh, Ru, Ir) and their oxides are commonly used catalysts. However, their scarcity and high price have limited certain commercial applications.4 Most carbon materials are earth-abundant, ecofriendly, and biocompatible, and, some of them are even catalytically active and stable. Therefore, carbon-based, metal-free catalysts (C-MFCs) have attracted worldwide interest as alternatives to precious metalbased catalysts, particularly for energy/biorelated applications.4−7 Compared with conventional metal-based catalysts, C-MFCs also display a high and broad tunability because of rich surface chemistries and lack of metal dissolution and poisoning. In this context, the introduction of heteroatom(s) into the carbon skeleton (i.e., heteroatom-doping), by either in situ doping during the nanocarbon synthesis or through posttreatment (i.e., postdoping) of preformed carbon nanostructures,4,5,8 has been demonstrated to cause electron modulation of carbon atoms for facilitating catalytic reactions4,6 and the surface property changes for biorelated applications.7,9 Since the discovery of nitrogen-doped vertically aligned CNTs (VANCNTs) for oxygen reduction reaction (ORR) in 2009,10 worldwide efforts have been dedicated to the development of various C-MFCs for the ORR,4,11−13 oxygen evolution reaction (OER),14 hydrogen evolution reaction (HER),15,16 twoelectron (2e−) transfer ORR to produce H2O2 (an energy © XXXX American Chemical Society

By creating a variety of coexisting active sites, C-MFCs can possess multiple catalytic functionalities, which is otherwise difficult, if not impossible, using metal-based catalysts. By creating a variety of coexisting active sites, C-MFCs can possess multiple catalytic functionalities, which is otherwise difficult, if not impossible, using metal-based catalysts. This significant advantage allows for the design of new C-MFCs capable of catalyzing different chemical reactions and/or bioprocesses simultaneously. Of particular interest, certain CMFCs have been demonstrated to be effective bifunctional electrocatalysts for OER and ORR in rechargeable metal-air Received: October 4, 2018

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DOI: 10.1021/acscentsci.8b00714 ACS Cent. Sci. XXXX, XXX, XXX−XXX

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batteries for efficient energy storage26 as well as OER and HER in photocatalytic/photoelectrochemical water splitting systems to produce H2 and O2 gases from water and sunlight.27 In conjugation with photocatalysis, these bifunctional electrocatalysts could be employed to harvest, convert, and then store the solar energy, offering the possibility for developing lightdriven energy systems. Apart from the fabrication of C-MFCs for energy conversion and storage, nanocarbons have also been recently used for various biomedical applications.28 Particularly, certain carbon nanomaterials were demonstrated as stable and effective CMFCs for detecting H2O2 released from living cells, while a novel solid-state fluorescent sensor was fabricated by simply dipping a piece of filter paper into carbon dots with polyhedral oligomeric silsesquioxane (CDs@POSS) solutions for efficient detection of ions (e.g., Fe3+) of biological importance.29 More recently, certain rationally designed biocompatible carbon nanomaterials have shown great potential in photodynamic therapy, sonodynamic therapy, and catalytic nanomedicine.30−33 In this focused and critical review, we summarize recent advances in developing C-MFCs for energy and biorelated applications. The challenges and opportunities in this exciting field are presented as well, along with elucidation of the structure−property relationship and mechanistic understanding of recently developed C-MFCs in energy and biorelated processes, providing a look forward for rational design and fabrication of various C-MFCs with high activities, remarkable selectivity, and outstanding durability for various energy/ biocatalytic processes.

blocks, three-dimensional (3D) carbon architectures (Figure 1) have been devised as efficient porous C-MFCs, exhibiting a large specific surface area (SSA) with numerous accessible active centers, high electrical conductivity and ion diffusibility, and even good mechanical strength.4,34−36 In addition to the graphitic carbon materials mentioned above, nanodiamond (ND) derived materials consisting of sp3 carbon atoms have also been developed as superior catalysts for special chemical reactions,37−40 nanocarriers/probes for drug delivery,41,42 and biomedical imaging.43,44 Because of the interplay between B- and N- doping,40,45 B,N codoped nanodiamond (BND) exhibited a high efficiency and stability toward selective CO2 reduction to ethanol as it passivated the competitive HER reaction.39 On the other hand, as ND materials are both scalable and biocompatible, functionalized ND particles have been used for targeted delivery of drugs,41,42 and in vitro or in vivo imaging.46−48 For instance, Ho et al. demonstrated nanodiamonds ensured great enhancements toward high efficacy and safe drug (doxorubicin) delivery in both murine liver tumor and mammary carcinoma models.49 Specifically, fluorescent NDs (ND-phosphorylcholine composites) exhibited good dispersibility in water, strong fluorescence, and low toxicity, making the ND-based materials ideal for cell imaging.44 In particular, Zhang et al. have developed a versatile ND construct that incorporates a targeting, imaging, and chemo-therapeutic agent attractive for drug delivery with capabilities of targeting, imaging, and controlled drug releasing, simultaneously.50 Therefore, the structure/charge modulation and surface functionalization of carbon-based materials offer possibilities for advancing C-MFCs for energy and biorelated applications.

2. ADVANCED CARBON NANOMATERIALS Depending on the arrangement of carbon atoms, carbon has been traditionally divided into three categories: amorphous carbon, graphite, and diamond.4 The recent discoveries of C60, CNTs, and graphene (graphene nanosheets, graphene quantum dots, graphene nanoribbon) opened up an important field in carbon material science and technology (Figure 1).4 Using these individual carbon nanomaterials as building

3. CREATION OF ACTIVE SITES IN C-MFCs In 2009, the first heteroatom-doped C-MFC was reported when a VA-NCNT array was discovered to be an effective electrode toward catalyzing the ORR in an alkaline medium.10 The high catalytic performance of the metal-free VA-NCNTs for ORR was ascribed to the charge redistribution among C atoms and the adjacent N heteroatoms induced by N-doping, which changed the oxygen molecule adsorption mode to weaken the “O−O” bond, facilitating the ORR (Figure 2a,b).10 This groundbreaking work was followed by enormous research worldwide.4,6,11,51−54 Subsequently, various efficient C-MFCs, including nonmetal atom (e.g., N, O, S, F, B, P, Br, Cl, or As)doped graphene, CNTs, carbon quantum dots, carbon nitride, and graphite nanoplates, have been developed for advanced energy conversion/storage involving ORR,11−13 HER,16,53 OER,54,55 CO2RR,56 and/or N2RR,21 and biorelated applications (e.g., reactive oxygen species, ROS, generation/ detection).29,57 The heteroatom-doping induced catalytic performance of CMFCs is strongly dependent on the dopant type, dopant location, and co-/multidoping effect(s).58 It was found that the ball-milling generated edge-selenated graphene nanoplatelets (SeGnPs) were applied as the counter electrode for dyesensitized solar cells (DSSCs) with I−/I3− or Co(II)/Co(III) electrolytes to display an even higher photovoltaic performance than those of DSSCs with the Pt counter electrode in SM315 or N719 sensitizers (Figure 2c).59 The calculations based on density functional theory (DFT) and nonequilibrium Green’s function clarified the I3− reduction mechanism by this special edge doping (Figure 2d).

Figure 1. Structure of carbon nanomaterials: diamond, graphite, C60, CNTs, graphene, and 3D graphene-CNT hybrid materials.34 Images reprinted with permission from ref 34. Copyright 2012, Elsevier. B


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Figure 2. (a) Charge density distribution based on calculations for nitrogen-doped CNTs (N-CNTs). (b) Possible adsorption modes of an O2 molecule on CNTs (top) and N-CNTs (bottom).10 (c) Atomic-resolution transmission electron microscopy (AR-TEM) image of the SeGnP edge. (d) IRR mimetic diagram on the SeGnP surface.59 (e) Schematic representation of charge transfer and of ORR process on PDDA-CNT.60 (f) Freeenergy values for ORR of different kinds of defects.67 (g) AR-TEM image of DG. Hexagons: orange, pentagons: green, heptagons: blue, and octagons: red.68 Images reprinted with permission from refs 10, 59, 60, 67, and 68. Copyright 2009 AAAS, 2016 AAAS, 2011 American Chemical Society, 2015 American Chemical Society, and 2016 Wiley−VCH.

number/density of accessible active sites, improve the number of active sites to be exposed, and facilitate electron transport, mass transfer of reactants, and electrolyte diffusion.4,6,16,58,70−77 Thus, the first reported N-doped CNTs with aligned structure exhibited an ORR performance that greatly surpassed the nonaligned counterpart due to the enhanced electrolyte/reactant diffusion and accessibility to the active sites on the vertically aligned N-CNT surface.10

Furthermore, intermolecular charge transfer can be realized by physically doping nanocarbon materials with electron acceptor(s) or donor(s) (Figure 2e), as exemplified by good ORR activities from the polyelectrolyte (e.g., PDDA) adsorbed graphitic carbon materials (e.g., CNTs, graphene).60−62 In this particular case, amine groups in the PDDA possess a strong electron acceptability, which can extract electrons from C atoms in the carbon skeleton, forming partially positively charged carbon atoms as active centers for efficient electrocatalysis of ORR,60−62 OER,63,64 and CO2RR.65 Recent studies indicate that certain pure carbon nanocages and graphene quantum dots/graphene nanoribbon hybrids without any dopants and free from physically adsorbed polyelectrolyte can also have high ORR activities.66,67 DFT calculations disclose the charge transfer between the defects of pentagon/zigzag edge and the basal plane (Figure 2f), indicating ORR activities arising from the charge transfer induced by defects.67 Compared to catalytic activities induced by the heteroatom-doping-induced intramolecular charge transfer and polyelectrolyte-adsorption-induced intermolecular charge transfer discussed above, the investigation of defective induced catalytic actives in C-MFCs is still in its fancy. However, many carbon nanomaterials with well-controlled defects (e.g., porous graphene, Figure 2g) have been reported to show good catalytic performance, even superior to some CMFCs from heteroatom-doped graphene, for various electrochemical reactions (e.g., ORR, OER, HER).68 This is obviously a valuable field for future work.69

Unusual surface, mechanical, and electrical properties can be obtained by tuning micro-/meso-/ macroporous 3D carbon architectures. Such properties, along with remarkable electrolyte transport capability for achieving a large triphase (gas−liquid− solid) interface with abundant accessible active sites, could significantly enhance the sluggish kinetics of the catalytic reactions. Unusual surface, mechanical, and electrical properties can be obtained by tuning micro-/meso-/macroporous 3D carbon architectures. Such properties, along with remarkable electrolyte transport capability for achieving a large triphase (gas− liquid−solid) interface with abundant accessible active sites, could significantly enhance the sluggish kinetics of the catalytic reactions.34 Functional reduced graphene oxide (RGO) with a 3D porous structure can be obtained by high-temperature thermal annealing GO with other precursor containing heteroatom(s). In this context, Sheng et al., developed a facile approach to N-doped graphene with a 3D porous structure by pyrolysis of a mixture containing GO and melamine.78 The N content in the resulting N-doped graphene was up to 10.1 atom %, and the resultant C-MFC catalyzed a high-efficient four electron (4e−) ORR process in 0.1 M alkaline medium.78 Additionally, a powerful template-free approach, involving

4. RECENT ADVANCES IN THE DEVELOPMENT OF C-MFCs An exciting feature of C-MFCs is that different dopant types and chemical complexities of catalytic active centers can introduce different catalytic functions (e.g., ORR, OER, HER), and the catalytic performance could be enhanced by improving the density and distribution of individual catalytic active sites. Thus, the structural control and surface modification of carbon-based materials can serve as a promising approach to further enhance the catalytic activity of C-MFCs. Of particular interest, the 3D assembly structures (morphologies) of C-MFCs can be used to increase the C


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Figure 3. (a, b) Schematic and scanning electron microscope (SEM) image of the 3D SWCNT-bridged graphene block on the cross-section.87 (c) Schematic for the synthesis and microstructures of a 3D graphene-RACNT fiber. (d) SEM image (top view) of the 3D graphene-RACNT fiber. (e, f) TEM images of graphene sheet connecting to the open tips of RACNTs under different magnifications. (g) SEM image of the wire-shaped DSSC from the cross-section view.36 Images reprinted with permission from refs 87 and 36. Copyright 2015 American Chemical Society, 2015 AAAS.

a high HER activity for this unique C-MFCeven higher than that of some non-noble metallic catalysts. Furthermore, Ndoped holey graphene (N-HG) and B,N codoped HG (B,NHG) with hierarchically porous structures were successfully prepared as highly efficient air electrodes in Li-O2 batteries and Li-CO2 batteries, respectively.8,86 The efficient utilization of active sites in HG with abundant holes, coupled with the enhanced density of active sites and electrolyte/gases/electron transports, imparts superb cell performance. This provides new opportunities for designing reversible and stable metal-air batteries with a high energy efficiency. Recent carbon architectures have been developed as CMFCs from zero-dimensional (0D)-carbon quantum dots, one-dimensional (1D)-CNTs, two-dimensional (2D)-graphene, and 3D-porous carbon materials.70 Of particular interest, Pham et al. prepared a 3D carbon architecture of CNT-bridged graphene via Coulombic interactions between cationic surfactants-grafted CNTs and graphene oxide sheets with negatively charge (Figure 3a,b).87 Within the pillared 3D structures, graphene layers were intercalated by CNTs to enhance the accessible SSA and allow for fast ion diffusion, leading to a high-performance electrochemical catalyst for metal-free catalysis or supercapacitor for energy storage. Since the covalent bonding between carbon atoms in the carbon skeleton of CNTs and graphene is stronger than the van der Waals interaction in the transverse direction between the carbon layers, thermal, electrical, and mechanical properties of 1D CNTs and 2D layered graphene are strongly direction-dependent. However, high through-thickness thermal and/or electrical conductivities are usually required for many

polymerization followed by pyrolysis, has also been developed to produce various C-MFCs with 3D porous structure.26,78−81 The pyrolysis of a polyaniline aerogel derived from phytic acid and aniline monomers generated a mesoporous C-MFCs with N,P codoping, which exhibited an SSA over 1600 m2 g−1 and high electrocatalytic activities for both OER and ORR26 attributable to the codoping in conjunction with the smooth mass and charge transfer associated with the 3D mesoporous carbon. In addition, silica colloids were employed as a hard template for polymerization of N-contained monomers, followed by pyrolysis, to produce 3D N-doped mesoporous carbons.82 Subsequent heat treatment under NH3 further modified the mesoporous carbons into meso-/microstructures with a high SSA, leading to a high improvement in the ORR activity.82 Also, 3D carbon nanocages with N doping were successfully synthesized by annealing the pyridine in the presence of MgO or AAO template,83 which is attractive for metal-free electrocatalysis in various applications. These results suggest the importance of the hierarchical structures to the electrocatalytic performance of C-MFCs. Hierarchical porous structures were also constructed to further enlarge the SSA and improve the electrolyte diffusion. It was found that abundant edges in the pores could enhance interfacial catalytic performances.84 In this context, a highly efficient HER electrocatalyst was developed by in situ growing mesh-on-mesh networks of mesoporous g-C3N4 sheets on a mesoporous graphene support.85 The in-plane mesoporous structure in this system affords abundant active sites at the exposed edges and defects for hydrogen adsorption, leading to D


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Figure 4. (a) N 1s XPS spectra of HOPG model catalysts. (b) ORR activities for these model catalysts in (a). The inset: nitrogen contents of the catalyst models. (c) Schematic pathway for ORR on N-doped carbon nanomaterials.90 (d) Geometries of N atoms (sp-N, pyri-N, amino-N, and grap-N atoms) in graphdiyne. (e) N K-edge XANES of N doped graphdiyne (NFLGDY) prepared under temperatures of 700, 800, and 900 °C. A broaden and red-shifted peak ca. 397.4 eV appeared when the temperature increased, indicating the presence of the sp-N doping type. (f) ORR current densities @0.65 V for samples with different amounts of sp-N atoms.92 (g) Schemes of proposed active sites for the H2O2 generation on mild reduction of GO (F-mrGO) and F-mrGO(600) (F-mrGO annealed at 600 °C).98 Images reprinted with permission from refs 90, 92, and 98. Copyright 2016 AAAS, 2018 Nature Publishing Group, and 2018 Nature Publishing Group.

standing of catalytic reactions and discovery of fundamentally new catalysis concepts and active site structures,89 which, ultimately, will lead to great advances in catalysis applicable to critical reactions in energy and biorelated systems. Therefore, it is important to identify specific active centers for C-MFCs or their model catalysts. Among many of the recent breakthroughs, the clarification of which type of “N” is responsible for ORR from experiment and theory is another milestone for the development of CMFCs.90 The intrinsic active sites toward ORR in N-doped CMFCs were determined by using a suite of well-defined Ndoping species within various highly oriented pyrolytic graphites (HOPGs), including pyridinic N-/graphitic N-/ edges-/clean-HOPG (Figure 4a).90 On the basis of studies of these structurally well-controlled HOPG model catalysts, Nakamura and co-workers have demonstrated that the positively charged carbon atoms (C+) around pyridinic nitrogen in N-doped C-MFCs are the active sites for ORR (Figure 4b).90 An oxygen molecule initially adsorbs on the C+ around the pyridinic N, which is then followed by protoncoupled electron transfer to the adsorbed oxygen via either a 2e− or 4e− pathway (Figure 4c). Therefore, the improved electrocatalytic performance of C-MFCs results from the

practical applications. Theoretical studies indicated CNTs and graphene with a seamless nodal junction could have both satisfied out-of-plane and in-plane properties.88 In this context, Dai and co-workers have developed a one-step approach (template-assisted chemical vapor deposition) to a special 3D carbon architecturegraphene-CNT hollow fibers with radially aligned CNTs (RACNTs) seamlessly sheathed by a cylindrical graphene layer (Figure 3c−f).36 The resultant fibers, with meso-and micropores, a large SSA, and extraordinary electrical properties, are superior electrodes, as exemplified by their use as counter electrodes in all-solid-state wire-shaped DSSCs with high-performance (Figure 3g). The concept of “seamless nodal junction” can be extended to other carbon architectures, such as CNTs and C60, graphene and C60, even graphene and C3N4, for the fabrication of new kinds of CMFCs with unexpected properties. These are just the first few examples for the special 3D carbon architectures, and many more will be sure to follow.

5. RECENT ADVANCES IN MECHANISTIC UNDERSTANDING OF C-MFCs A robust atomic-scale understanding of the active sites and related catalytic activities will enable the mechanistic underE


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Figure 5. (a) Schematic of bifunctional SiNC with catalytic activity for both CO2RR and OER for the CO2 overall splitting. (b) HAADF image of porous SiCN. (c) Current density, cathode, and electrode potentials of CO2 overall splitting with two SiNC electrodes.107 (d) A generic mechanism for N2RR to NH3 on heterogeneous catalysts.111 (e) Schematic illustration of the preparation of NPCs. (f) Production rates of NH3 and current efficiency of NPCs sample during 10 cycles at a given potential of −0.9 V.21 (g) The catalyst model with N-doped highly disordered carbon for N2RR. (h) TEM image of the corresponding catalyst of (g).114 Images reprinted with permission from refs 107, 111, 21, and 114. Copyright 2018 Wiley−VCH, 2016 Elsevier, 2018 American Chemical Society, and 2018 Elsevier.

nology currently used industrially to synthesize H2O2 is based on the anthraquinone process, which is very energy intensive,95 and cannot compete financially with other reagents as it involves the costly and energy-intensive separation and concentration steps. Direct synthesis of H2O2 that combines H2 and O2, through 2e− ORR has been achieved using noble metal catalysts through an electrochemical reaction. 95 Recently, it turns out to be possible that carbon catalysts with heteroatom-doping could be employed as effective catalysts for electricity generation and H2O2 production in fuel cells.17,96−99 Of particular interest, single-wall graphitic carbon nanohorns doped with N were obtained by annealing of polydopamine as 2e− transfer ORR C-MFCs for high-yield H2O2 production.96 The as-prepared C-MFCs showed a high performance over a wide pH range (1−13) with an optimized Faradaic efficiency as high as 98%. Furthermore, both the activity and selectivity for H2O2 generation were correlated with the oxygen content of the surface oxidized carbon nanomaterials,97,98 and the carbon atoms adjacent to oxygen functional groups (−COOH and C−O−C) were confirmed as the active sites for ORR via the 2e− pathway (Figure 4g).98 Thus, the O-CNTs exhibited greatly improved activity and selectivity (∼90%) for H2O2 production when compared with pure CNTs. Electrochemical Reduction of CO2 (CO2RR). Following the discovery of N-doped CNTs for ORR in 2009,10 numerous

intrinsic activities of heteroatom-doped carbon coupled with doping-induced charge transfer,58 rather than metal residuals.91 This fundamental understanding of the ORR mechanism in Ndoped carbon materials is critical as this principle can also provide the guideline to design C-MFCs beyond ORR for fuel cells. More recently, Zhao et al. introduced sp-hybridized N (sp-N) atoms into a few-layer graphdiyne host by replacing the carbon atoms in acetylene groups (Figure 4d,e),92 and found that sp-N-doped graphdiyne exhibited overall enhanced ORR activities in the presence of sp-N atoms (Figure 4f), which facilitated O2 adsorption and electron transfer on the carbon surface. Thus, the introduction of sp-N atoms in carbon skeleton provides a powerful approach for the preparation of highly active C-MFCs.

6. RECENT ADVANCES IN METAL-FREE ELECTROCATALYSIS BY C-MFCs Electrochemical Production of H2O2. ORR can, in principle, go through either a 4e process to produce electricity and H2O in a fuel cell or a 2e− process with H2O2 as an intermediate.4 H2O2 has been commonly used not only as a green oxidizer for cleaning,93,94 but also as fuel to power a rocket and a clean energy carrier to run automobiles with water and O2 as byproducts (http://www. americanenergyindependence.com/peroxide.aspx). The techF


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Figure 6. (a) The redox potentials of various reactions with respect to the theoretical position of the g-C3N4 band edges at pH 7.116 (b) TEM image of the CDots-C3N4 composite. The inset: A magnified TEM image from region marked in red. (c) Band structure diagram for CDots-C3N4. VB, valence band; CB, conduction band. (d) H2 and O2 production from water splitting with CDots-C3N4 as photochemical catalyst.145 (e) AFM image of the mesoporous g-C3N4 nanomesh with monolayer structure. The inset: enlarged view of the rectangle area. (f) Height profiles along the lines in (e).147 (g) UV−vis diffuse reflectance spectra of OCN-tubes.153 (h) Schematic illustration of valence and conduction bands of Hterminated diamond, respectively. (i) Comparison of NH3 yield from H-terminated and O-terminated diamond samples.124 Images reprinted with permission from refs 116, 145, 147, 153, and 124. Copyright 2017 Elsevier, 2015 AAAS, 2016 American Chemistry Society, 2017 Wiley-VCH, and 2013 Nature Publishing Group.

the center of the carbon skeleton that often makes their lone pairs difficult to access by CO2 molecule(s). Recently, a bifunctional C-MFC, porous Si and N codoped carbon (SiNC), with catalytic activity for both CO2RR and OER, was first realized (Figure 5a,b), and allowed for costeffective electrochemical splitting of CO2 into CO and O2 to supplement the natural carbon cycle (Figure 5c).107 Partial current densities of CO2-to-CO and OER on SiNC in neutral medium were enhanced by 2 orders of magnitude in the Tafel regime relative to single-doped comparisons (SiC and NC). Mechanism studies suggested the elastic electron structure of -Si(O)−C-N- unit in SiNC as the highly active site for CO2RR and OER simultaneously by lowering the free energy of CO2RR and OER intermediates adsorption.107 Some detailed studies on CO2RR by C-MFCs, including their synthesis, structure characterization, and performance, are summarized in one of our previous review articles.58 Electrochemical Reduction of N2 (N2RR). Converting earth-abundant N2 to NH3 is an important approach for acquiring the widely used agricultural fertilizer108 and hydrogen intermediate for energy storage.109 As an alternative green fuel, NH3 has a volumetric energy density nearly double that of liquid hydrogen, and is easier for storage, shipment, and distribution than hydrogen.109 Industrial synthesis of NH3 involves the energy- and capital-intensive Haber-Bosch process at high temperature of 350−550 °C and high pressure between 150 and 350 atm. Under ambient conditions, the photo-/

C-MFCs have been developed for various reactions, including graphene foams doped with N as counter electrodes for DSSCs in 2012,18 N-doped graphite nanomaterials for OER,14 and gC3N4/N-doped graphene catalyst for HER in 2014,16 N-doped carbon nanofibers (N-CNFs), and N-doped porous carbon for CO2RR19 in 2013 and N2RR21 in 2018, respectively. Since the research and development of C-MFCs for ORR, OER, HER, including their multifunctional systems, and counter electrodes for DSSCs have been reviewed recently,4,57,69,100,101 a focus of this article is given to the recent advances in the development of C-MFCs for emerging multielectron transfer electrocatalysis, such as CO2RR, N2RR, and photo/electro-/biorelated reactions. Electrochemical reduction of CO2 to the value-added organic molecules (e.g., HCOOH, CH3OH, C2H4, CH4, CO) is a potential way for the preparation of carbon compounds to store chemical energy.58 In this regard, Kumar et al., developed N-CNFs by electrospinning polyacrylonitrile, followed by pyrolysis, as the first C-MFC for CO2RR in 2013.19 Recently, a series of carbon materials doped with heteroatom (e.g., N, B, F, N/B, N/S) were developed as highly active, selective, and stable C-MFCs for CO2RR.19,36−39,102−106 In practical, it was demonstrated that pyridinic N, among those doping N species in carbon skeletons, is responsible for CO2RR.102 Electrons of graphitic N are limited in the π antibonding orbital, which are less easily for binding CO2, while pyrrolic N atoms are usually located in G


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Figure 6a.116 As can be seen, g-C3N4 owns a moderate bandgap of 2.7 eV, which corresponds to an optical wavelength around 460 nm for visible light irradiation. More importantly, the energy level of the top conductive band (CB) of g-C3N4 is much more negative when compared with those of conventional inorganic semiconductors, the H2-evolution, and CO2/ O2 theoretical reduction potentials (Figure 6a,c), which suggests that photoinduced electrons in g-C3N4 have a strong thermodynamic driving force to reduce different kinds of molecules. As such, g-C3N4 based materials are useful for many photocatalytic applications, including photocatalytic H2 evolution and overall water splitting,117,118 environmental remediation,119,120 CO2RR,121,122 N2RR,123,124 and selective organic transformations and disinfection125,126 under light irradiation with suitable wavelength(s). However, the bulk gC3N4 materials often exhibit a low photocatalytic efficiency resulting from multiple drawbacks: a high recombination rate of electron−hole, insufficient visible light absorption, limited SSA (∼10 m2 g−1) and insufficient active centers for interfacial photocatalytic reactions, slow kinetics for reaction on the photocatalyst surface, and low charge mobility.116,127 Various bandgap engineering strategies, such as heteroatom-doping, defective site control, materials dimensionality tuning, pore texture tailoring, heterojunction constructing, cocatalyst coupling, and nanocarbon hybridization, have been introduced to address the above issues, and thus improve the performance of g-C3N4 photocatalysts.116,127

electrochemical N2RR to NH3 without additional reducing agent (H2) and greenhouse emission has attracted a great deal of attention.110 As the strong NN bond with no dipole moment in N2 molecules, it is a challenge to find an efficient way for activating N2.110 Generally speaking, electrocatalytic N2RR on the surface of a heterogeneous catalyst involves two reaction processes: associative and dissociative.111 In an associative process, the two nitrogen centers bind together as a N2 molecule is hydrogenated, and the product (NH3) is released along with the cleavage of the N−N bond. The alternating approach requires each of the two nitrogen centers to be adsorbed on the active sites in the catalyst, and the second NH3 molecule will be generated just after the removal of the first NH3 molecule (Figure 5d). The dissociative mechanism involves the cleavage of the NN bond before hydrogenation occurs, forming adsorbed N atoms on the catalytic surface, followed by the final step to produce NH3. Ru, Au, and Fe/CNTs have been used for N2RR,112,113 though their large overpotential and poor N2 adsorption seriously impede the efficiencies and yields of NH3. Also, various N-doped porous carbon materials (NPCs) obtained by the pyrolysis of ZIFs (Figure 5e) have recently been used for electrocatalytic N2RR.21 The high SSA of the NPCs (896 cm3 g−1) can provide a large amount of active sites, and benefit the contact and aggregation of N2 on the catalytic surface as well, and hence the resultant high ammonia production rate of ∼1.4 mmol g−1 h−1 and energy conversion efficiency of ∼1.42% at −0.9 V (Figure 5f).21 As the maximum reaction free energies for N2RR on pyridinic N and pyrrolic N sites are 0.45 and 0.56 eV (form DFT calculations), respectively. The values are quite similar; therefore, pyridinic and pyrrolic N are considered the active sites for ammonia generation on NPCs. The possible catalytic mechanism of N2RR on NPC, followed the alternating approach, was also presented (*NN →*NHNH → *NH2−NH2 → 2NH3), as we discussed above. To date, the investigation of C-MFCs for electrochemical N2RR is still a very early development, and it is desirable to also study N2RR induced by the defective and intermolecular charge transfer, and the mechanistic understanding of relevant reactions. It was found that the performance of N2RR on N-doped carbon can be enhanced by coupling with defective effects (Figure 5g,h).114 The moiety, consisting of three adjacent pyridinic N atoms around one carbon vacancy in the carbon skeleton, can effectively adsorb a N2 molecule to dissociate “NN” for the following protonation process. Thus, N-doped porous carbon showed a remarkable NH3 production rate of as high as 3.4 × 10−6 mol cm−2 h−1 with a Faradaic efficiency (FE) of 10.2% at a given potential of 0.3 V vs RHE in 0.1 M KOH medium. Photoelectrochemical Catalysis. Heterogeneous photocatalysis could be employed to harvest, convert, and store solar energy for generating green fuels and environmental protections. In this context, carbon nanomaterials with tunable energy bands are a class of promising C-MFCs for photoelectrochemical catalysis.115 Due to their distinctive physicochemical/optical/electrical properties, graphitic carbon nitride (g-C3N4)-based materials have been widely studied as polymeric semiconducting photocatalysts to promote various photoelectrochemical reactions. Like most photocatalysts, the photocatalytic activity of g-C3N4-based materials is strongly determined by its band gap, which, along with some theoretical potentials of various redox reactions at pH 7, are shown in

Specifically, doping with heteroatoms could enhance the πconjugated electron delocalization, which is significant for the enhancement of the conductivity, charge mobility, and separation of photoinduced electrons to improve the photocatalytic performance of g-C3N4. Of particular interest, heteroatom (e.g., S,128,129 O,130 C,131 I,132,133 and P134,135 ) doping and C, P-codoping136 of g-C3N4 have been confirmed as effective approaches to narrow its bandgap and improve the light-harvesting capability. Specifically, doping with heteroatoms could enhance the πconjugated electron delocalization, which is significant for the enhancement of the conductivity, charge mobility, and separation of photoinduced electrons to improve the photocatalytic performance of g-C3N4. Copolymerization with appropriate aromatic derivatives or organic small molecules could also enhance π-electron delocalization to modulate the band gap, electronic structures, physical/chemical properties for enhancing the photochemical activities of g-C3N4.115,137 Apart from the heteroatom doping, the enhancement in crystallinity of g-C3N4 could also lead to significantly improved photocatalytic activity by improving the charge-carrier mobility and separation.118,140 Because of the defect-induced catalytic activities,66−69,139−142 however, the generation of nitrogen vacancies143 or amorphous structures144 in g-C3N4 has been recently developed as an effective strategy to enhance the visible-light performance. Such disorder structure in amorphous g-C3N4 can narrow the bandgap to 1.9 from 2.7 eV, H


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results demonstrated that the C dots-C3N4 can be used as a promising photocatalyst for water splitting driven by visible light. To further improve the photocatalytic activity of g-C3N4, Qu and co-workers produced atomically thin mesoporous nanomesh (Figure 6e,f)147 by exfoliating the g-C3N4 bulk with mesoporous to tune the energy levels and enhance the electron transfer, light harvesting, and accessible active sites. The resultant g-C3N4 nanomesh exhibited a superb photocatalytic HER rate (8510 μmol h−1 g−1) under a wavelength >420 nm and a quantum efficiency of 5.1% at a wavelength of 420 nm, which outperformed all the metal-free g-C3N4 nanosheet photocatalysts. Delamination of layered bulk into single atom sheets could induce extraordinary physical properties, such as a large SSA, ultrafast carrier mobility, and fine-tuned energy band structure, leading to highly efficient photocatalytic water splitting. Production of H2 and O2 gases from water splitting driven by visible light is a promising approach for green energy conversion. Apart from the electrochemical reduction of CO2 and N2 discussed above, the use of solar energy and CO2 for artificial photosynthesis of hydrocarbon fuels is also considered as one of the ideal routes for solving the greenhouse effect.148−152 In this context, Nam and co-workers coupled the N-doped graphene quantum sheets (N-GQSs) with a silicon nanowire as the photocatalyst for photocatalyzing CO2 into CO.149 Odoped g-C3N4 nanotubes (OCN-tube) with a porous structure possess a CB potential of −0.88 V, which was more positive than that of the bulk g-C3N4 (−1.02 V) and attractive for photocatalytic CO2 into liquid fuels (Figure 6g).153 OCN-tube was a promising photocatalyst for CH3OH generation with a photocatalytic production rate up to 0.88 μmol/(g·h) without obvious decay after three cycles. The high performance of OCN-tube was assigned to a good CO2 uptake capacity, excellent light harvesting, and high separation efficiency of

which corresponds to a red-shift of the absorption wavelength edge from 460 to 682 nm.144 The above results provide a new approach to develop visible light-driven photocatalysts from amorphous/defective g-C3N4 samples.

Apart from the heteroatom-doping, the enhancement in crystallinity of g-C3N4 could also lead to significantly improved photocatalytic activity by improving the charge-carrier mobility and separation. In 2015, Liu et al.145 reported that a C-MFC of carbon dots and C3N4 (C dots-C3N4, Figure 6b) can photocatalytic water into H2 and O2 with a high quantum efficiency (QE) of 16% and 6.3% at λ = 420 ± 20 and 580 ± 15 nm, respectively. In this particular case, C dots-C3N4 catalyzed water splitting into H2 and O2 molecules via a two-electron, two-step approach under visible light; C3N4 and C dots are responsible for the first step (photocatalysis) and second step (chemical catalysis), respectively. C3N4 usually suffers from poisoning by H2O2 produced by a 2e− process, which is usually difficult to be removed from the photocatalyst (C3N4) surface.146 Nevertheless, the synergism between C3N4 (H2O2 generation sites) and C dots (H2O2 decomposition sites) in the composite offered a smooth approach for high efficient O2 generation. Also, the incorporation of C dots into the C3N4 results in an expanded ultraviolet visible (UV−vis) absorption of the (photo)catalyst to the entire wavelength range, and thus enhanced values of the QE and “solar-to-hydrogen” conversion efficiency (Figure 6c). More importantly, C dots-C 3N 4 maintains a high durability for the generation rate of H2 and O2 for 200 cycles (more than 200 days, Figure 6d). These

Figure 7. (a) Schematic illustrations for the assembly of a MEA with the VA-NCNTs. C.E., R.E., W.E., stand for counter electrode, reference electrode, and working electrode, respectively. (b) SEM image of the VA-NCNTs. (c) Digital photo and SEM images of the MEA after a durability test. (d) Cross-section SEM image of the N-G-CNT/KB/Nafion catalyst layer. The purple arrows indicate the parallelly separated N-G-CNT sheets with interdispersed KB particles. (e, f) Schematic illustrations of O2 diffusion through KB separated N-G-CNT sheets and the densely packed N-G-CNT sheets, respectively. Images reprinted with permission from ref 157. Copyright 2016, AAAS. I


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Figure 8. (a) SEM image of NPMC-1000. (b) Scheme of the structure of NPMC foam and its building block. (b, c) Linear sweep voltammetry (LSV) curves of NPMC-1000, NPMC-1100, the RuO2, and the Pt/C on an RDE electrode in 0.1 M KOH medium. (d) ORR and (e) OER volcano plots for N-doped graphene, P-doped graphene, and N,P-doped graphene, respectively. (f) Schematic of a primary Zn-air battery. A carbon paper coated with NPMC was employed as the air cathode (enlarge part), a Zn foil was used as anode, and the separator a glass fiber membrane filled KOH solution.26 (g) Schematic of graphene with N, P, and F tridoping as a multifunctional C-MFC for simultaneous ORR, OER, and OER. (h) Polarization and power density curves of Zn-air batteries using N, P, and F tridoped graphene as ORR/OER catalyst in the air electrode. (i) O2 and H2 generation volumes with respected to the water-splitting time, the N, P, and F tridoped graphene was used as the HER/OER catalyst.170 (j) Schematic of the integrated green energy devices based on a superior multifunctional C-MFC (N,S-3DPG).22 Images reprinted with permission from refs 26, 170, and 22. Copyright 2015 Nature Publishing Group, 2016 Wiley-VCH, and 2017 Elsevier.

photogenerated charge carriers resulting from its hierarchical porous structure coupled with oxygen doping. In recent years, C-MFCs have also been developed for photocatalytic nitrogen fixation.123 In particular, it was found that the boron-doped diamond (BDD) yielded electron emissions into liquids to form solvated electrons, which reacted quickly with protons to form neutral atomic hydrogen to reduce N2 into NH3.124 In this case, diamond is a semiconductor (bandgap: 5.5 eV) and lies ∼0.8−1.3 eV above the vacuum level when its surface is terminated with hydrogen (Figure 6h). Thus, the photocatalytic activity of BDD depends strongly on the surface property and correlates well with the solvated electron generation (Figure 6i). The optimized activity of the C-MFC of BDD is even superior to that of Ru/TiO2 under same conditions. In a separate study,

heterostructure junctions between g-C3N4 and rGO were constructed,154 leading to a superior charge transfer to overwhelm the electron−hole recombination to significantly improve the NH4+ generation rate and catalytic stability.154 Therefore, the rational design and development of hierarchically structured carbon hybrids provide a promising approach for high-performance C-MFCs for efficient photocatalytic nitrogen fixation and beyond.

7. RECENT ADVANCES IN ENERGY HARVESTING ON C-MFCs Although many C-MFCs have shown comparable, or even better, performance to commercial Pt/C catalysts toward ORR in alkaline electrolytes,10,13 their performance in acidic electrolytes needs to be further enhanced to commercialize J


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the polymer electrolyte membrane fuel cells (PEMFCs) that have a more significant economic impact than alkaline fuel cells.155,156 Therefore, it is important, but still challenging, to develop effective C-MFCs for ORR in acidic electrolytes.58,157,158 By properly tuning the species and types of the dopants, together with the hierarchical carbon structures, considerable improvement has been realized in the development of C-MFCs toward ORR in acidic electrolytes. In particular, VA-NCNTs as ORR catalysts produced strong ORR signals in a PEMFC analogous acidic medium due to its superior vertical array structure.158 Subsequently, CNTs with N,S- codoping (NSCNTs),159 graphite nanofibers with N,F-codoping,160 RGO with F,N,S-tridoping (F,N,S-rGO),161 and zigzag-edged graphene nanoribbons162 were also demonstrated to show superior ORR activities in both acidic and alkaline medium with respect to their single doping counters. A confined polymerization/carbonization within layered montmorillonite approach was introduced to prepare pyridinic N-doping dominant graphene with relatively good ORR performance under acidic conditions.79 Graphene/CNT layer-by-layer composites (Figure 7a−c) also displayed higher ORR catalytic activities in acidic medium than that of their individual components of graphene or CNTs.157 Shui et al.157 reported a well-defined 3D N-doped carbon architecture constructed with graphene, CNTs, and Ketjen black (KB) particles with N doping (N-G-CNT-KB). In this rationally designed carbon system, N-doped graphene (N-G) ensures a large SSA to provide enormous active sites for ORR reaction, CNTs contribute conductivity, and KB particles separate the graphene sheets for efficient electrolyte and O2 diffusion, which showed a catalytic activity for ORR as high as the state-of-the-art nonprecious metal catalyst (i.e., Fe−N−C) but a better long-term stability in acidic PEMFCs.157 Specifically, the C-MFC of N-G-CNT-KB catalyst, delivered a current density of 30 A g−1 at a given potential of 0.8 V and a peak power density of 300 W g−1. Moreover, this N-G-CNTKB catalyst exhibited a much higher stability in acidic PEMFCs than that of the Fe/N/C nonprecious metal catalyst running at 0.5 V,157 whether at a high or low loading amount of 2.0 mg cm−2 or 0.5 mg cm−2, respectively. This study opens avenues for green energy production from PEMFCs based on welldefined C-MFCs. On another front, N,P-doped porous nanocarbon (NPMC) was developed as the first bifunctional C-MFC for both ORR and OER in 2015 (Figure 8a−f),26 and then was employed as air electrodes for highly efficient primary and secondary Zn-air batteries. Since then, research achievements have further illustrated that many well-defined carbon-based materials could be used as bifunctional C-MFCs for OER and HER in water splitting devices to generate H2 and O2 gases,163−165 for ORR/OER in metal-air batteries,8,166−169 and even trifunctional catalysis for ORR/OER/HER in integrated energy systems (Figure 8j).22,170 For high-performance Zn-air batteries, one of the major challenges is to increase ORR and OER efficiency of the air electrodes. Thus, it is important to prepare inexpensive ORR/ OER bifunctional electrocatalysts to facilitate the discharge and charge processes and to reduce the cost for a Zn-air battery. In this context, ORR and OER bifunctional metal-free 3D NPMC foams (Figure 8a, b) were prepared through polymerization of aniline in the presence of phytic acid and subsequent pyrolysis.26

First-principle DFT calculations (Figure 8d,e) revealed that the 3D NPMC foam required lower minimum ORR and OER overpotentials than those on the N-doped or P-doped counterparts, due to the synergistic effect between the N,P codoping and abundant edges in the 3D carbon structure. When used as the C-MFC for air electrodes in primary or rechargeable Zn-air batteries (Figure 8f), the NPMC-based batteries displayed a cell performance comparable to that of batteries with commercial Pt/RuO2 electrodes. This work has initiated worldwide studies on the development of highly efficient bifunctional/multifunctional C-MFCs for metal (Zn/ Li/Mg/Al)-air batteries and beyond.166−170 Besides, a porous carbon codoped with N and O, fabricated by the pyrolysis of a zeolitic imidazolate framework-8 (ZIF-8) and subsequent H2SO4 treatment, was demonstrated as a highperformance OER/HER bifunctional catalyst for overall water splitting. The catalyst delivered a current density of 10 mA cm−2 for more than 8 h at a cell voltage of ca. 1.82 V under alkaline medium.164 Similarly, porous carbon with N, P, and O tridopants has also been developed for water splitting with a current density of 10 mA cm−2 at a potential of 1.66 V and a long-term electrochemical durability in alkaline.165 This catalyst also showed the high catalytic performance and good long-term durability in acidic or neutral environments. Furthermore, N, F, and P tridoped graphene materials were used as a trifunctional C-MFCs (Figure 8g) for electrochemical water splitting (OER and HER) for the generation of O2 and H2 gases, driven by a Zn-air battery with the same electrocatalyst as an air electrode (ORR and OER).170 The achieved high gas generation rates for H2 (0.496 μL s−1) and O2 (0.254 μL s−1) showed great potential for practical applications (Figure 8h,i). Subsequently, trifunctional CMFCs for ORR, OER, and HER simultaneously have also been realized by the fabrication of N, S codoped graphitic sheets with stereoscopic holes.171 More recently, Yao and coworkers reported highly efficient defect-abundant C-MFCs for simultaneous ORR, OER, and HER by removal of N from a Ndoped graphene.68 Furthermore, trifunctional C-MFCs with a low-cost have great potential for widely applications, especially in lightderived integrated clean energy systems.22 By rationally designing the cross-linked 3D graphitic networks with N, S codoping (N,S-3DPG), Dai and co-workers reported a highly efficient trifunctional catalyst toward HER, OER, and ORR in alkaline medium.22 The extraordinary trifunctional electrocatalytic activity of N,S-3DPG was assigned to the combined effects induced from the N,S-codoping and the unique 3D porous architecture. On the basis of the emerging trifunctional C-MFC toward ORR/OER/HER, an integrated energy system was further developed, in which photoelectrochemical water splitting was driven by perovskite solar cells to generate H2 and O2 gases for a PEMFC for the renewable generation of green electricity, representing a fresh concept for generation of clean electricity just from sunlight and water without any pollutant emission (Figure 8j).22

8. RECENT ADVANCES IN BIORELATED APPLICATIONS OF CARBON NANOMATERIALS Owing to their unique chemical, physical, optical, and even magnetic properties, carbon nanomaterials have been widely studied for various biorelated applications. Carbon nanomaterials have successfully attracted tremendous interest for the development of novel biomedical tools, including biosensors, K


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Figure 9. (a) Schematic drawing for the fabrication route of CDs@POSS. (b) TEM image of CD@POSS. Inset is the size distributions of the CDs.29 (c) Illustration of colorimetric detection of glucose by using glucose oxidase (GOx) and g-C3N4 peroxidase-like catalytic reactions. (d) Schematic of H2O2 reduction with TMB catalyzed by g-C3N4.177 Images reprinted with permission from refs 29 and 177. Copyright 2016 WileyVCH and 2014 Elsevier.

Figure 10. (a) SEM image of N-HMCS, the inset is the corresponding TEM image. (b) Current−time response on N-HMCS electrode with successive injection of O2•− into 0.1 M deoxidized PBS with pH 7.4 at a given potential of −0.15 V.181 (c) LSV curves of the N,P-CMP-1000 catalyst in common PBS solution (top) and artificial tear (bottom), respectively. Inset: dissolved oxygen (DO) concentrations detected by a commercial DO sensor. In vitro cytotoxicity of N,P-CMP-1000 extracts against (d) human corneal epithelial cells determined by CCK-8 assay and (e) live/dead cell double staining assay. (f) Schematic of the potential using N,P-CMP-1000 for sensing DO in the wearable glasses to monitor eye health.182 Images reprinted with permission from refs181 and 182. Copyright 2018 Elsevier, 2018 Wiley-VCH.

imaging agents, and drug/gene carriers for medical diagnostics and clinical treatments. Great progress has been made, and many excellent reviews and books have appeared.28 Therefore, this section will focus on the recent advances in biorelated applications of carbon nanomaterials, particularly C-MFCs. Among various biorelated applications of carbon nanomaterials, one of the fascinating recent developments is related to their photoluminescence (PL) features. In this regard,

researchers have paid much attention to graphene quantum dots (GQDs) for biological imaging.172 For formaldehyde detection and bioimaging, optical and fluorescent properties of GQDs were found to be tunable by heteroatom doping and/or surface functionalization.173 The quantum yield of N-doped GQDs (NGQDs) can be reached as high as 22.9%, which is around 23 times the pristine GQDs (1%). Of particular interest, the fluorescence emission of the functionalized L


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Figure 11. (a) Processes of PTT and PDT using CNTs.31 (b) Schematic drawing of the sonosensitization process of PMCS for cancer therapy.32 (c) Synthetic procedures for GODFe3O4@DMSNs nanocatalysts. (d) The schematic diagram of the catalytic-therapeutic mechanism of GFD NCs for generating hydroxyl radicals toward cancer therapy.33 Images reprinted with permission from refs 31−33. Copyright 2016 Dove Medical Press, 2018 Wiley-VCH, and 2017 Nature Publishing Group.

NGQDs can be reversibly switched “on−off” via redox reactions. Furthermore, the PL emission can be switched from excitation-independent to excitation-dependent by modifying the surface states of NGQDs. For instance, Dai and co-workers have successfully designed carbon dots (CDs) and amine functionalized polyhedral oligomeric silsesquioxane (CDs@ POSS) nanocomposites for multifunctional applications (Figure 9a,b).29 The emission intensity and fluorescence quantum yield of the CDs@POSS nanocomposites were superior to those of the pure CDs as the surface passivation of CDs in CDs@POSS. Furthermore, the fabricated CDs@ POSS-based liquid marbles showed obvious fluorescence under UV excitation, demonstrating its potential for sensing and drug delivery. More interestingly, the formation of a complex between Fe3+ ions and CDs@POSS can effectively quench the fluorescence of CDs@POSS. On this basis, a novel solid-state fluorescent sensor was fabricated by simply dipping a piece of filter paper into a CDs@POSS solution, and the resultant solid-state fluorescent sensor exhibited a good sensitivity for the detection of Fe3+.29 Having good catalytic properties and biocompatibility, CMFCs have been widely used in many biorelated applications.2,174,175 Certain carbon nanomaterials have been demonstrated as stable and effective C-MFCs for detecting H2O2 released from living cells.176 It was demonstrated that the g-C3N4 owned an intrinsic catalytic activity for the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) to generate a colored solution (easily to be detected by the naked eye) in the presence of H2 O 2 (Figure 9c,d). 177 Using g-C3 N 4 peroxidase-like C-MFC and glucose oxidase (GOx), therefore, a colorimetric approach for detecting glucose in serum samples was developed, exhibiting a high sensitivity and selectivity with a minimum detection density of 1.0 μM. In a separated study, aminated 3,4,9,10-perylenetetracarboxylic acid (PTCA) coupled with GQDs (GQDs/PTCA-NH2) realized solid-state GQD sensing.178 Furthermore, the fabricated electrochemilu-

minescence signal tag was decorated on the surface of the electrode, displaying good film-forming property, excellent electronic conductivity, and high stability. Carbon fibers sheathed with VA-CNTs have also been developed for monitoring of ascorbate in rat brains,179 which displayed a good selectivity, reproducibility, and durability. Superoxide anion (O2•−) is one of the important ROS species, and closely related to many diseases, including atherosclerosis, diabetes, neurodegeneration, and cancer.180 Just as carbon nanomaterials could be employed as superior CMFCs for 4e− and 2e− process ORR, hollow mesoporous carbon spheres with N-doping (N-HMCS, Figure 10a) were used as a sensitive enzyme-free C-MFC sensor for electrochemical quantification of O2•− directly (Figure 10b). As unique features, for example, good conductivity, effective N doping, apposite pore size/volume, and large SSA, N-HMCS showed a higher sensitivity for detection of O2•− than solid carbon spheres and hollow mesoporous carbon spheres without N doping, even some of metal-/enzyme-based sensors.181 More recently, microporous graphitic nanosheets with N,P-codoping (N,P-CMP-1000) were fabricated by the pyrolysis of as-prepared conjugated polymers and phytic acid, which was then employed as highly active C-MFC toward overall ORR in a broad pH range. Specifically, having a high electrocatalytic activity for ORR in neutral solution (Figure 10c), N,P-CMP-1000 was successfully used for DO electrochemical quantification with a low detection limit of 1.89 μAmg−1 L and wide detection range from 2.56 to 16.65 mg L−1.182 In vitro cytotoxicity tests (CCK-8 and live/dead cell double staining assay) further demonstrated a high biocompatibility of N,P-CMP-1000 to human corneal epithelial cells (Figure 10d, e), indicating a good promise as an eyewearable biosensor for detecting DO in tears to monitor eye health (Figure 10f).182 Additionally, because of their unique structure and property advantages, including large SSA, rich surface chemical functionalities, nanoscale size, and biocompatibility, carbon M


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tunable intrinsic structure and surface structure of the carbon nanomaterials, the well-defined carbon could further act as a C-MFC to reduce/replace Fe3O4 NPs. In this context, CMFCs show great potential as advanced materials for cancer therapy for various biomedical applications.

nanomaterials have shown great potential as drug carriers and mediators for target cancer therapy to minimize the side effects to surrounding normal tissues caused by conventional cancer therapies, such as radiation,183 ultrasound,30 and microwave therapies.184 Through well-controlled functionalization, for example, CNTs could be employed as nanocarriers for the targeted delivery of anticancer drugs, genes, and immunotherapy components for chemotherapy.31 They have been used as mediators to damage cancer cells without severe damage to normal tissue in photothermal therapy (PTT) and photodynamic therapy (PDT) (Figure 11a). Because of the therapy safety, relatively long tissue penetration depth, and cost-effectiveness, sonodynamic therapy (SDT) has also led to widespread interest recently.185 However, the discovery of sonosensitizers with high sonosensitization efficacy and good stability is still a great challenge. Very recently, a metal−organic framework (MOF)derived carbon nanostructure (PMCS) was developed as a superior sonosensitizer (Figure 11b).32 Compared to amorphous carbon nanospheres, the sonosensitization effect of PMCS was shown to be closely linked to the porphyrin-like macrocycle in MOF derived carbon nanomaterials. Their intrinsic large bandgap allowed for a high yield ROS production (Figure 11b), as validated by electron spin resonance and dye measurement, for a high tumor inhibition efficiency (85%). This study offers a good perspective for understanding of structure-dependent SDT enhancement, which should lead to new carbon-based materials with advanced structures for high-performance SDT. To achieve a high tumor specificity, specific tumor microenvironment (TME) has been deliberately developed,186 in which the metabolism, biosynthetic process, and physical− chemical environment are different from those in normal tissues.187,188 Nevertheless, the used toxic anticancer medicine could cause some destruction of normal tissues.189,190 If the substances in the TME are toxicity-free and biocompatible, and could be converted into effective chemicals just against tumors, the stimuli of intratumor-delivered nontoxic agent/ agents would trigger the therapeutic process in tumorous tissues without any damage to normal tissue. Very recently, Hou et al. developed a smart biocompatible catalyst for catalytic nanomedicine via TME-responsive reactions with a highly efficient, tumor-specific therapy.33 During the therapy process, glucose oxidase (GOD) as the starting enzyme catalyst was introduced to catalyze the glucose into a certain amount of H2O2 in the tumor region (as the intracellular H2O2 level is low). The generated H2O2 was then reacted with Fe3O4 NPs supported by dendritic mesoporous silica nanoparticles via Fenton-like reactions for hydroxyl radicals generation,191 further leading to apoptosis of the tumor (Figure 11c,d), resulting in a tumor-specific therapy without damage to normal organisms and tissues. As can be seen, mesoporous silica-based versatile nanomaterials were usually chosen as catalyst supports for Fe3O4 loading (Figure 11c).33,192 In this respect, nontoxic carbon-based nanomaterials (biodegradable mesoporous carbon)70,193 with a large specific area and good biocompatibility could be alternatives to silica-based versatile nanosystems.33 Indeed, various simple and feasible methods have been designed for developing multidimensional and multifunctional carbon materials with controlled architectures and pore sizes4,26,18,58,70,171,194 to support various C-MFC active sites and/or other catalyst NPs for biomedical sensing, imaging, diagnosis, and therapy. Having the unique advantages of the

9. LOOKING FORWARD: FUTURE DIRECTIONS AND PERSPECTIVES The introduction of heteroatoms into the carbon skeleton causes electron modulation that could generate desirable electronic structures of C-MFCs for various catalysis. Although significant progress has been achieved in developing C-MFCs for many important reactions associated with energy conversion/storage and biomedical applications, fundamental understanding of the reaction mechanisms associated with CMFCs is still largely lacking with respect to metal-based catalysts. Further research work on the active sites engineering (e.g., dopant type, location, content) and characterization are needed to reveal the structure−performance relationship for the development of efficient heteroatom-doped C-MFCs. Compared with the strategy of heteroatom doping, the investigation on the defective effect is an emerging direction. Some research work has indicated that the catalytic performance induced by defects for ORR, OER, and HER could be even superior to that of N-doping.68 This finding prompted more and more studies on the effects of defect types (e.g., topological defects, edge defects) on the catalytic performance, along with the defect control and identification. The atomic level insight into the relationship between active sites and catalytic activities is still largely lacking. In-depth theoretical and experimental investigations should be devoted to explaining the nature of the active sites and the electrochemical catalytic mechanisms, providing more guidelines and opportunities for discovery of C-MFCs with breakthrough activities for specific reactions. At the current stage, the theoretical calculations have been employed to explain the experimental phenomena in most of the cases. However, more attention should be paid to predict structures of promising catalytic active sites and then provide guidance for designing highly efficient C-MFCs for various reactions. On one hand, the in situ real-time characterization techniques (e.g., X-ray absorption spectra, scanning tunneling microscope, Raman, and/or atomic force microscopy) should be developed and employed to detect chemical reactions and disclose the electrochemical active sites. On the other hand, considerations on the optimization of synthetic methods should be taken to enhance the density and exposure of the corresponding catalytic active sites. Multifunctional C-MFCs are currently highly desirable for advanced energy-related technologies, including Zn-air batteries, water-splitting systems self-powered by Zn-air batteries, and integrated devices of water-splitting with fuel cells for the continuous generation of clean electricity. In another frontier, intracellular catalysis is very promising for biomedical applications (e.g., cancer therapy) while carbon nanomaterials have been used for biosensing, bioimaging, and drug/gene delivery. Because of the large difference between in vivo and in vitro environments, however, much more work is required before clinic implications to be within insight. Continued work in these emerging fields is of great value. N


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AUTHOR INFORMATION

Corresponding Authors

*(J.Q.) E-mail: [email protected]. *(S.Z.) E-mail: [email protected]. *(L.D.) E-mail: [email protected]. ORCID

Liming Dai: 0000-0001-7536-160X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank our colleagues for their contributions to the work cited. We are also grateful for financial support from The National Key Research and Development Program of China (2017YFA0206500), ARC DP190103881, China Postdoctoral Science Foundation (2017M610036), and Key Program of National Natural Science Foundation of China (51732002 and 21620102007), NSF (CMMI-1400274), NASA (NNX16AD48A), Distinguished Scientist Program at BUCT (buctylkxj02), WMU (SPN2330), and CWRU.

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