Progress in Research into 2D Graphdiyne-Based Materials

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Cite This: Chem. Rev. 2018, 118, 7744−7803

Progress in Research into 2D Graphdiyne-Based Materials Changshui Huang,†,‡ Yongjun Li,† Ning Wang,‡ Yurui Xue,† Zicheng Zuo,† Huibiao Liu,† and Yuliang Li*,† †

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Beijing National Laboratory for Molecular Sciences (BNLMS), CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R. China ‡ Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, P.R. China ABSTRACT: Graphynes (GYs) are carbon allotropes with single-atom thickness that feature layered 2D structure assembled by carbon atoms with sp- and sp2- hybridization form. Various functional theories have predicted GYs to have natural band gap with Dirac cones structure, presumably originating from inhomogeneous π-bonding between those carbon atoms with different hybridization and overlap of the carbon 2pz orbitals. Among all the GYs, graphdiyne (GDY) was the first reported to be prepared practically and, hence, attracted the attention of many researchers toward this new planar, layered material, as well as other GYs. Several approaches have been reported to be able to modify the band gap of GDY, containing invoking strain, boron/nitrogen doping, nanoribbon architectures, hydrogenation, and so on. GDY has been well-prepared in many different morphologies, like nanowires, nanotube arrays, nanowalls, nanosheets, ordered stripe arrays, and 3D framwork. The fascinating structure and electronic properties of GDY make it a potential candidate carbon material with many applications. It has recently revealed the practicality of GDY as catalyst; in rechargeable batteries, solar cells, electronic devices, magnetism, detector, biomedicine, and therapy; and for gas separation as well as water purification.

CONTENTS 1. Introduction 2. Basic Structure and Band Gap Engineering: Theoretical Study of GYs 2.1. Molecular Structure 2.2. Electronic Structure 2.3. Mechanical Properties 2.4. Layer Structure of Bulk GDY 2.5. Band Gap Engineering of GDY 3. Preparation and Characterization 3.1. Preparation 3.1.1. Films 3.1.2. Nanotube Arrays and Nanowires 3.1.3. Nanowalls 3.1.4. Nanosheets 3.1.5. Ordered Stripe Arrays 3.1.6. GDY 3D Framework 3.2. Characterization Methods 3.3. Functionalization of GYs 3.3.1. Hydrogenation 3.3.2. Fluorination 3.3.3. Metal Decoration 3.3.4. Absorption of Guest Molecules 4. Properties and Potential Application of GDY 4.1. Catalysts 4.1.1. Photocatalysts 4.1.2. Electrocatalyst 4.1.3. Photoelectrochemical Water Splitting 4.1.4. Other Catalysis 4.2. Rechargeable Batteries

© 2018 American Chemical Society

4.3. Solar Cells 4.3.1. Perovskite Solar Cells 4.3.2. Dye-Sensitized Solar Cells 4.3.3. Quantum Dots Solar Cells 4.4. Electronics, Thermoelectrics, and Magnetism Devices 4.4.1. Electronic Devices 4.4.2. Thermoelectric Materials 4.4.3. Electrochemical Actuators 4.4.4. Magnetism 4.5. Detector 4.6. Biomedicine and Therapy 4.7. Gas Separation and Capture 4.8. Water Purification 5. Conclusion Associated Content Special Issue Paper Author Information Corresponding Author ORCID Notes Biographies Acknowledgments References

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Received: May 4, 2018 Published: July 26, 2018 7744

DOI: 10.1021/acs.chemrev.8b00288 Chem. Rev. 2018, 118, 7744−7803

Chemical Reviews

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Figure 1. Structure of GYs. Reproduced with permission from ref 33. Copyright 2012 American Physical Society.

1. INTRODUCTION The development of new materials allows continued progress on breaking bottlenecks of both science and technology. Accordingly, much effort has been exerted to discover new materials having unconventional architectures.1−5 Among them, carbonbased nanomaterials (e.g., fullerenes, carbon nanotubes (CNTs), and graphene) have drawn particular attention for their special structures and chemical-physical properties.6−10 Because most of those carbon nanostructures have been formed from sp2- or sp3-hybridized carbon atoms, there is tremendous interest in fabricating carbon allotropes with high degrees of sp hybridization, because the presence of ethynyl units can considerably affect the properties of carbon-based materials.11,12 In 1987, Baughman et al. first predicted that graphynes (GYs) would be a series of stable crystalline carbon allotropes featuring a high degree of sp hybridization.13 Comprising ethynyl units (sp-hybridized carbon) and aromatic moiety rings (sp2hybridized carbon), GYs were named for their similar structure to graphite and ethyne. On the basis of the number of ethyne units between two neighboring aromatic rings, GYs can be distinguished as graphyne (GY), graphdiyne (GDY), graphtriyne (GTY), and so on. The extended existence of sp-like and sp2-like carbon atoms endow GYs with high π-conjunction, tunable electronic properties, two-dimensional plane framework, and uniformly distributed pores. Many efforts including theoretical predictions and practical experiments have been made to study the GYs during the last few decades, exploring the potential mechanical, optical, and electronic properties of GYs. Like graphene, the existence of Dirac cones was also found in GYs.14 Besides, GYs were predicted to have natural semiconductor band gap.15 The GYs exhibit superior electrical properties, for example, high carrier mobility and small carrier effective masses.16 Both the intrinsic holes and electrons mobility of GYs at room temperature could reach up to 105 cm2 V−1 s−1.17 Mechanical properties of GYs were thought of as a function of number and different arrangements of acetylenic linkages.18−20 The Poisson’s ratio and the in-plane Young’s modulus of GYs were calculated as 162 N m−1 and 0.429, respectively.21,22 Accordingly, the GYs are drawing much attention from chemists, physicists, material scientists, and so on, who want to exploit the outstanding properties of GYs. Although many synthetic and theoretical chemists have attempted to prepare GYs,23−31 it was not until 2010 that Li et al. synthesized GDY through in situ cross-coupling method.32 As a

member of the GYs family, GDY units contain two acetylenic groups between two adjacent benzene rings. This first synthesis of GDY greatly prompted related research into its properties and practical applications, helping us to take a glance into the advantages of GYs. This Review underscores the important properties of GYs, especially GDY, including molecular structure, electronic structure, and mechanical properties, and offers an overview of current progress of the research in this field through examinations of the most recent and important results from different research groups. The band gap of GDY has been reported to be varied through several ways. In addition to films, GDY has been prepared in the forms of nanowires, nanotube arrays, nanowalls, three-dimensional (3D) foams, nanosheets, ordered stripe arrays, etc. The successful preparation of GDY allowed its study to extend beyond theoretical predictions and into the realm of practical experiments, where it has displayed many applications, for example, as catalyst; applied in rechargeable batteries, solar cells, electronic devices, detectors, biomedicine, and therapy; and for water purification and gas separation. The theoretical predictions and practical advances in GYs and GDY research discussed herein suggest that these new two-dimensional (2D) carbon allotropes have a bright future for use as a new generation of materials of the carbon family.

2. BASIC STRUCTURE AND BAND GAP ENGINEERING: THEORETICAL STUDY OF GYS 2.1. Molecular Structure

Normally GYs were composed of benzene rings and acetylenic linkers (Figure 1). As we mentioned, by accounting for the numbers of acetylenic linkers between two benzene rings, GYs can be distinguished as GY, GDY, GTY, and so on. By varying the linkages of acetylene bonds, the GYs can display the flat layer of carbon atoms in various structures, such as 6,6,12-, α-, and βGY, which have also been theoretically discussed and reviewed.33−36 In contrast to the other carbon materials, four types of carbon−carbon bonds can be observed in GYs: (1) the Csp2− Csp2 bonds of the benzene rings; (2) the Csp2−Csp bonds between neighboring CC double and CC triple bonds; (3) the Csp−Csp triple carbon bonds; and (4) the Csp−Csp single carbon bonds linking adjacent CC bonds. Generally for GYs, the predicted bond lengths are 1.48−1.50, 1.18−1.19, and 1.46− 1.48 Å for aromatic bonds, triple bonds, and single bonds, 7745


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Figure 2. (a) Band structure of α-GY. (b) Density of states (DOS) of α-GY. (c) First Brillouin zone of α-GY. (d) Dirac cone of α-GY. (e) Band structure and (f) DOS of 6,6,12-GY. (g) First Brillouin zone of 6,6,12-GY. (h) Dirac cone I and (i) Dirac cone II of 6,6,12-GY. The Fermi level is indicated by gray planes in panels. Reproduced with permission from ref 33. Copyright 2012 American Physical Society.

linkages in GYs. While the content of −CC− linkages is increased, the stability of GYs will be decreased.43 It was wellknown that the system with smaller Gibbs free energy is more stable. So Gibbs free energy was reported to be applied as one of the reference points to compare GYs with other different materials, especially the carbon materials.45 In contrast to diamond, carbine, graphite, fullerene, CNTs (6,6), and graphene, of which the Gibbs free energies are about −0.022, 1.037, −0.008, 0.364, 0.114, and 0 eV per atom, respectively, the corresponding calculated value of GDY is 0.803 eV. As regarding GDY nanoribbons (NRs), the Gibbs free energy becomes smaller than the value of GDY, which is ∼0.520−0.775 eV. For the computational results of chemical bonds, besides closed-shell species of ground-state, open-shell singlet might be an alternative concept. Sheka and co-workers believed that the radical character of a molecule is commonly perceived as a oneelectron property.46−52 Sheka performed a further study on the triple carbon bonds and structure of GDY.49 On the basis of this concept, she thought GDY as a whole could be highly radicalized, which means chemically reactive. But the status of its radicalization depends on the surrounding structure. The GDY main radicalization is concentrated on the ring while each ligament is about half less reactive, so it will be meaningful to consider the defects, which will considerably enhance the body reactivity, while discussing a possible controlling of electronic properties of GDY devices.

respectively.37 The carbon atom hybrid effect of the sp2- and sphybridization can be observed as the carbon−carbon single bonds are shrunk and the related aromatic bonds are stretched to the characteristic values, which are attributed to the weak conjugation between the aromatic ring moieties and the alkynyl units. The carbon−carbon single bond, aromatic, and triple bond lengths of the GY monolayer can be calculated as 1.407, 1.426, and 1.223 Å, respectively, through first-principles calculations.22 Although the bond length of single bond is shorter than that of the aromatic bond, the single bond is easier to be broken than the aromatic and triple bonds. In GDY, the bond lengths are 1.41 Å for the Csp2−Csp2 bonds in benzene rings, 1.40 Å for the Csp2−Csp bonds in-between neighboring CC double and CC triple bonds, 1.24 Å for the triple Csp−Csp bonds, and 1.33 Å for the single Csp−Csp bonds linking two adjacent CC bonds.21,38,39 The latter has some character of double bonds for the effect of conjugation. The GDY layers structurally exhibit the similar hexagonal symmetry (p6m) as graphene. The optimized crystal parameters a, b, c, and θ of the energy-minimized GDY structure are 9.38 Å, 9.38 Å, 3.63 Å, and 120°, respectively.40 The interlayer distance of GDY layers is 3.7 Å.41,42 Qualitatively, due to the weak conjugation, the bond length between the benzene ring and the alkynyl units is extended, which also resulted in the aromatic properties of benzene units being reduced. Through full atomistic molecular dynamics (MD) calculations,43 the lattice spacing can be increased by 2.66 Å, in terms of the addition of a single ethyne linkage. On the basis of quantum-level analysis, the single ethyne linkage can lead to a lattice spacing increase of ∼2.58 Å.16 Due to the sp and sp2 carbon atoms and their related bonds, GYs display greater structural flexibility than graphene. This structural flexibility offered the opportunity to GYs forming curved structures, but accompanied by the disadvantage that mechanical stiffnesses of GYs were weakened. In contrast to graphite, the formation energy of GYs is high, but actually it was predicted that GYs possess low formation energy while their thermal stability is high.44 It was first predicted by Baughman et al. that a high-temperature stability was displayed by GY.13 In contrast to graphene and other sp2-like graphene allotropes, the stability of GYs was decreased for the existence of −CC−

2.2. Electronic Structure

Electronic structure is the fundamental property of materials. To well understand the instinct of GYs, it is necessary to analyze and describe their electronic structures accurately.53−60 Through the theoretical calculations it can be confirmed that GYs display a natural band gap that is different from graphene, of which the band gap value is zero.33,61,62 For the special presence of the C C triple bonds, the Dirac cones of GYs are competent. The effective hopping matrix elements of GYs are modulated and their signs are reversed for the carbon−carbon triple bonds, which result in the reversed chirality properties and momentum shift of GYs’ Dirac cones, as well as tunability of their energy gap.63 7746


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Figure 3. (a) Structure of GDY and its first Brillouin zone. (b) Band structures and DOS of GDY. (c) Electron densities of the lowest doubly degenerate bright excitons (the upper figure is top view and the lower one is side view). Reproduced with permission from ref 64. Copyright 2011 American Physical Society.

Existing in the band structure and leading to the exceptional electronic properties, the Dirac cones were considered to be graphene’s special feature previously among all the 2D materials. However, Görling and co-workers demonstrated with firstprinciples calculations that the electronic structures of GYs also possess Dirac cones (Figure 2).33 For example, the valence and conduction band of α-GY contact in one spot at the Fermi level and locate at the same positions in the Brillouin zone, which are the same as the Dirac points of graphene (Figure 2a−d). Consistently, at the Fermi level, the DOS of α-GY is zero. Besides, different from graphene’s hexagonal symmetry, the 6,6,12-GY displays a rectangular symmetry, leading to four Dirac points in its Brillouin zone and two pairs of distorted Dirac points as I and II as shown in Figure 2h and i. It is interesting that 6,6,12-GY is self-doped in a manner because its Dirac cone I, at which electrons are worked as charge carriers, slightly lies below the Fermi level, while Dirac cone II, at which holes are charge carriers, slightly lies above the Fermi level. Using density functional theory (DFT), the band gap value of GDY was calculated as 0.44 eV at the local density approximation (LDA) level, while at the theory of GW manybody level this value is increased to 1.10 eV (Figure 3a and b).15,64 The quasiparticle correction was explained to be attributed to the strengthened Coulomb interaction that occurred in decreased dimensionality. The quasiparticle band gap of GDY is 1.17 eV for experimental value and 1.29 eV for theoretical value, which is quite close to that of Si.15 Three optical absorption peaks in the spectrum were supposed to be investigated. The one peak is centered on 0.66 eV and originates from transitions around the band gap, while the other two peaks, located around 4.02 and 1.77 eV, derive from transitions around the Van Hove singularities at the K and M points, respectively. It was deduced that the excitons of GDY possess both the exciton characteristics of Wannier−Mott and Frenkel. While fixing the hole (yellow spot) at the density maximum of the πz orbitals, the electron density of the lowest doubly bright degenerated excitons A1 and excitons A2 (Figure 3c). The CNTs can be formed by rolling up graphene sheet. Similarly, GYs, especially GDY with zigzag, armchair, or chiral configuration, can also form GY NTs (Figure 4).65−95 According

Figure 4. (a) Schematic imaging of the GDY nanosheet. (b) GDY NTs rolled by GDY sheet. Reproduced with permission from ref 65. Copyright 2015 Springer Nature.

to the previously calculated results, the intrinsic hole and electron mobility of GDY NTs could reach the orders of 102 and 104 cm2 V−1 s−1, respectively.65 Moreover, it also has been revealed theoretically that the deformation potential (DP) constants of GDY or GDY NTs with monolayer stucture are much larger than that of graphene or CNTs, which owes to the stronger bonding of triple bonds than that of double bonds. Notably, compared to GDY with single-layer structure, GDY NTs present a weak electron−phonon coupling for its small elastic and DP constant. 2.3. Mechanical Properties

Mechanical property is one of the attractive features of GYs that was studied extensively after the prediction of GYs structure.20,96−108 Peng et al. have shown that the in-plane Young’s modulus of GY (162 N m−1) was calculated to be 53% percent lower than that of graphene.22 This could be understood from the lower average coordination number of carbon atom in GY than those in graphene, which leads to the smaller in-plane atomic mass density and electronic charge density of GY. Furthermore, the ultimate strain of GY is lower than that of graphene under the zigzag and biaxial strains; however, the result is opposite under the armchair strain. The Poisson ratio (0.429) of GYs is 2 times larger than that of graphene and quite close to that of perfectly incompressible material (0.5), indicating that the volume of GY can be well-conserved under uniaxial strain. According to the related calculated results, GY with single-layer structure was predicted to be unstable under large tension, 7747


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Figure 5. Illustration of uniaxial tension tests and the corresponding stress−strain results along the direction of (a) reclined chair and (b) zigzag. Reproduced with permission from ref 18. Copyright 2011 Elsevier.

especially while the strength of the external stress is higher than the ultimate strain. Under uniaxial tension by using MD simulations, Zhang et al. have theoretically studied the mechanical property of GY.20 Moreover, the failure mechanism also has been studied. The calculated results showed that the mechanical properties of GYs depended highly on the existence of the acetylenic groups. It was observed that the Young’s modulus and fracture stress are decreased with the percentage of the acetylenic linkage being increased. However, the changing trend is opposite for the fracture strain, which could be ascribe to the fewer bond connections and low atom density in the molecular plane of GYs. Among the different GYs including 6,6,12-, α-, β-, and γGY, the directional anisotropy of 6,6,12-GY is most obvious; hence, it also has versatile application potential for its excellent anisotropic electrical conductivity. Displayed in Figure 5, Buehler and co-workers theoretically studied the mechanical properties of GY in both zigzag (y-axis) and reclined-chair (x-axis) directions.18,19,43 The ultimate fracture point of stress and strain applied on GY is highly related to the orientation of linear carbon triple bonds; meanwhile, the direction of the extern stress is also a key factor. Without considering any sheet thickness and boundary effects, the stress−strain results indicate that the values of Young’s modulus as 532.5 and 700.0 GPa were calculated in armchair and zigzag modulus, respectively. Particularly, the ultimate stress values of 48.2 and 107.5 GPa were determined in the above two directions, respectively. Thus, it can be seen that mechanical properties of GYs including the out-of-plane bending stiffnesses and intersheet adhesion are comparative to those of graphene, which could be explained from the fact that the carbon atoms in GY linked with directional acetylenic groups can efficiently cause the stiffening of GY molecular plane and lead to the outstanding nonlinear stress−strain behavior. The elastic modulus (i.e., stiffness) can be an effective penalty by altering the length of introduced acetylene links. That means elastic modulus can be theoretically approximated as a function of acetylene groups in the GY molecular plane.18,53,109−112

Ahangari and co-workers have investigated Young’s modulus and in-plane stiffness of GDY sheet with different size.53 It should be noted that, in their wok, the structure of GDY is optimized through a tight-binding method based on selfconsistent charge density function. Their calculated results revealed that the Young’s modulus and in-plane stiffness increase with the size of the selected GDY models increasing. On the other hand, these two physical parameters are also highly dependent on the temperature. With respect to the increase of temperature, GYs’ mechanical properties have a weaker resistance than that of graphene.113 Vacancy defects were reported to affect the mechanical properties of GYs.53,109 To study the influence of vacancy defects, two different defects as random and mapped defects on GYs were designed by Ansari and co-workers.109 Their calculated results indicate that the random vacancy defects show stronger effect on the mechanical properties of GYs than those of mapped ones, which could be understood from the point that the GY sheet is weakened randomly in different areas while the defects are added irregularly. The boundary effects will be noticeable because some defects can be added near the edge of the GY sheet. However, the result is quite different for mapped defect, in which the boundary effect will be dramatically reduced, for the defect is regularly located in the center, rather than in the edge of the GY sheet. Moreover, an increased Poisson’s ratio can be observed for the formation of plenty of empty spaces for the inclusion of defects. As a result, a higher lateral strain can be observed because the atoms can move into these empty spaces, which will compensate the reduction of ultimate stress and strain of the GY sheet caused by the existence of defects. The elastic properties of GY NRs, which have various edges and widths, are also theoretically analyzed through density functional theory.45,114−116 Taking GDY as example, the GDYNRs can be prepared by cutting the GDY sheet. The calculated results indicate that GDY NRs with armchair edges show Young’s moduli in the range of 584−834 GPa. Comparably, GDY NRs with zigzag edges have Young’s moduli in the range of 7748


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2.5. Band Gap Engineering of GDY

523−575 GPa. All these showed that the elastic properties of the GDY NRs are not highly related to their edges and widths. The Young’s moduli of the GDY NRs are much smaller than those of graphene NRs or single-walled carbon nanotubes (SWNTs) because of the low density of the carbon atom in the GDY NRs induced by the existence of acetylenic groups.

Unlike graphene, which has zero band gaps, GYs, especially GDY, are characterized by a direct natural band gap. This band gap presumably originates from the overlap of carbon 2pz orbitals and inhomogeneous π-bonding between the carbon atoms with sp2 and sp hybridization.120−122 The electrondistribution conditions and conducting behavior are quite different in the various parts of GDY such as those around the sp2 carbon hexagons or in the sp carbon linkage.123−127 Lu and co-workers compared the density of states and energy-band distribution of GDY, which were calculated at the GW level and the LDA level, respectively.64 The band gap at the LDA level is 0.44 eV, while at the GW level it is 1.10 eV. Notably, the theoretical quasiparticle band gap of GDY is comparable to that of Si.15,128 Nevertheless, unlike Si, the direct band gap of GDY results in higher luminescence efficiency. As a result, GDY has a great potential for practical application in both optoelectronic and semiconductor devices. Until now, many functional theories, including DFT,50,120,129−131 the self-consistent field crystal orbital (SCF-CO) method,49 and the generalized gradient approximation (GGA),65 have been utilized for calculating the GDY energy band gap,132 with each calculated value falling between 1.10 and 0.44 eV. On the basis of those studies, several approaches, including invoking strain, boron (B)/nitrogen (N) doping, nanoribbon architectures, hydrogenation, and so on, have also been shown to be able to tune the band gap of GDY.133 The semiconductor−semimetal transition in GDY under external strain stimulation was studied by Su and co-workers through DFT method with tight-binding approximation (TBA) (Figure 7).134,135 Previous reports have shown the high carrier mobility of graphene with ballistic charge-transport characteristic. The electrons and holes of graphene exhibited comparable transportability. The special band structure of graphene featuring Dirac points and cones directly led to its amazing electronic properties. Similarly, GDYs also possess an electronic structure of Dirac cones (Figure 8). The band gap of GDY could be efficiently tuned under tensile strain. GDY’s band gap can be increased from 0.47 to ∼1.39 eV or decreased from 0.47 to ∼0 eV while the biaxial or uniaxial tensile strain is increased, respectively. The electronic structure changes of GDY under various strains are quite different from that of graphene.136 Hybrid carbon with boron and nitrogen (BN) doped into carbon networks should exhibit interesting band gap properties for the density of states and electrical conductivity could be affected by BN doping.137 Not only global but also local reactivity parameters of the carbon materials could be changed after the BN was introduced. By investigating the absorption properties and electronic structures of GY with BN and its relative derivatives,138−141 Zhang, Sun, Sarkar, and co-workers suggested a potential approach of modulating GY’s band gap. By using the method of first-principles calculations to investigate the stable configuration of nBN-doped GDY,38 Zhao and coworkers have found that the BN atoms prefer to replace the carbon atoms of sp hybridization and form a linear BN bond between benzene rings at low doping level (n ≤ 4) (Figure 8). On the contrary, the BN bond was first generated in the hexagon structure of benzene and then the sp-hybridized carbon chain at high doping level (n ≥ 5). As a control experiment, that substitution was not easy to happen on graphene with only sp2hybridized carbon. Because the π-bindings and electron state localization are inhomogeneous, GDY band gap increased gradually initially and then became abrupt with the increase of

2.4. Layer Structure of Bulk GDY

To understand how the structure of GDY in the bulk state differs from that in the monolayer, it is better to first understand the ways of the GDY layer stack. The directed investigation of the GDY layers is discussed in the Characterization Methods part of this Review. The carbon rings in bilayer and trilayer structures of GDY are stably stacked in AB and ABA mode of Bernal manner, respectively.118,119 In the case of GDY model with bilayer structure, the direct band gaps were calculated to be 0.35 for the most-stable packing configuration and 0.14 eV for the secondmost-stable packing configuration. As for the trilayer GDYs, the band gaps were estimated to range from 0.18 to 0.33 eV. Moreover, the band gaps of GDY with trilayer or bilayer structures usually decrease, while the strength of the external vertical electric field increases. The conducting characteristic of bulk GDY is highly related to its packing mode of the molecular plane, which can even convert from a semiconductor to a metal.117 For example, the calculated results have shown that GDY with AA packing mode, which is the least stable configuration, has a metal characteristic. However, the GDY with stable AB-3, AB-2, and AB-1 configurations can be regarded as a semiconductor (Figure 6). Furthermore, a systematic study

Figure 6. Schematic illustration of bulk GDY with (a) AB-1, (b) AB-2, (c) AB-3, and (d) AA configuration. (e) Total energy surface of the bulk GDY with AB configuration as a function of the relative in-plane shift while the interlayer distance in the selected cell was set as 3.20 Å. The AA configuration corresponds to the original conditions of the graph. The AB-1 (triangle), AB-2 (dot), and AB-3 (star) packing modes are colored with black. Reproduced with permission from ref 117. Copyright 2013 American Chemical Society.

with van der Waals (vdW) corrections has revealed that the binding energy of AB-3 and AB-2 stacking structures is ∼56 meV atom−1, which is distinguished by a potential barrier of ca. 1.3 meV atom−1. The differences below 1 eV in the approximated absorption spectra could be applied to identify each of the four stacking structures. 7749


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Figure 7. Illustration at (a) armchair and (b) zigzag directions of GDY: the optimized configuration, Brillouin zones, partial density of states, and energy bands at the Γ point. Reproduced with permission from ref 134. Copyright 2013 Royal Society of Chemistry.

Figure 8. (a) Kohn− Sham state isosurfaces of the nBN-GDYs (n = 0, 2, 4, and 7). (b) Formation energy and (c) band gap of nBN-GDYs and BNdoped graphene structure varied with different BN unit numbers. Reproduced with permission from ref 38. Copyright 2012 American Chemical Society.

the BN content, but at the γ point it kept the intact feature of the direct band gap. Systematic research with variations on the nanoribbon edge configurations, edge functionalizations, and widths of GDY nanoribbon (GDY-NR) has been performed, providing us a fundamental understanding of the GDY-NR on its chemical, electronic, mechanical, and magnetic properties.45,120,130,142−153 GDY sheet can be theoretically cut along the nearest-neighbor benzene rings to obtain GDY-NR with divan-like edges (Figure 9a-I), or along the next-nearest-neighbor benzene rings to achieve GDY-NR with zigzag-like edges (Figure 9a-II and -III).120 Regarding the GDY-NRs with zigzag-like edges, there are also two kinds of structures, which are the one with uniform width (Figure 9a-II) and the one with nonuniform width (Figure 9a-III). After studying the electronic states carefully by firstprinciples calculations,120,145 an approximate value of 0.8 eV minimum band gap of the GDY NRs with D2 configuration (Figure 9b) at the Γ-point was predicted, which is a potential valuable feature of semiconductor channel when employed in devices like field-effect transistors (FETs) (Figure 9c).

The value of GDY NR band gaps could be adjusted and controlled through changing the ribbon widths or employing the transverse electric fields.145,146 For example, the band gap of GDY NRs is decreased with the increase of ribbon width. When a transverse electric field is employed, the edge states of valence band (VB) are localized strongly at the ribbon’s low-potential edges, and the conduction band is localized at high-potential edges. Because of wave-function localization, the band gap decreased upon increasing the field intensity, and a transition of semiconductor−metal could occur under a certain value of the threshold field. In principle, the sp-hybridized carbon of GDY could be extended to 2D hydrocarbons consisting of benzene rings joined with either sp3- or sp2-hybridized carbon chains by hydrogenation.149,156,157 For the hydrogenation of GDY (Figure 10), free energy calculations suggested that such thoroughly hydrogenated carbon material is thermodynamically executable at high temperatures. The direct band gap varied from 0.47 to 2.65 eV at the Γ point, relying on adjusting the coverage of 7750


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Moreover, the theoretical band gap of GDY could be variable while the structure of GDY is modified. For example, while functional groups such as oxygen, hydrogen, or nitrogen are introduced to the GDY framework, defects with different types are generated in GDY. So, the band gap values in a range of 0.6− 0.9 eV of GDY could be obtained, indicating an efficient method to tune the band gaps of GDY.159 Another example is that the nanostructure comprising N-doped GDY and fullerene shows a tunable band gaps for the different distributions of electron in the hybrid system.160 Besides, the band gap and dielectric constant of GDY and its family could be systematically engineered, while external dielectric field or rotating strain are applied to the investigated system.161,162

3. PREPARATION AND CHARACTERIZATION

Figure 9. (a) Schematic diagram of GDY-NR with three different building structures: divan- (I) and zigzag-like (II) edged with same width, as well as zigzag-like (III) with different widths. Various (b) building structures and (c) band structures of GDY-NRs. Reproduced with permission from ref 120. Copyright 2011 American Chemical Society.

3.1. Preparation

Since the first successful preparation of GDY by Li and coworkers through a chemical method of template coupling in organic solvent, great efforts have been carried out to prepare GDY with different morphologies and with different reaction conditions. As shown in Table 1, various synthesis methods like two-phase reaction and thermal coupling have been developed. In addition, different experimental conditions such as catalyst system, reaction category, selected template, and structure of precursor have also been tested. In this section, the preparation of GDY is systematically discussed. 3.1.1. Films. GDY has been predicted theoretically before it was ever prepared artificially (Figure 11). In 1987, the structural models of GYs have been discussed by Baughman, Eckhardt, and Kertesz.13 They also proposed some possible subunits like macrocyclic groups that are suitable for creating the GYs network. Ten years later, Haley et al. attempted to synthesize GDY but obtained only substructures (small organic diyne segments containing hydrogen atoms).23,182−186 Many efforts

hydrogen.154,155 Thus, the electronic property or the band gap of GDY could possibly be modified by hydrogenation. Fluorination is another way to adjust the electronic property of GDY, which has been reported.158 By calculating the corresponding changes including the electronic and band structures of fluorination at different sites, fluoro-GY has been shown by Sarkar and co-workers as a semiconductor with a widely tunable band gap. The details of the bonding nature can be studied using the analysis method of projected density of state, while the participation of the orbit in antibonding and bonding can be revealed by Hamilton population analysis of crystal orbital, demonstrating the band gap adaptability of GYs for nanoelectronic devices.

Figure 10. DFT-optimized configurations of (a) GDY, (b) graphdiene, and (c) graphbutane with 3 × 3 supercells, respectively. Reproduced with permission from ref 154. Copyright 2012 American Chemical Society. (d) Width of the band gap of GY and GDY with different hydrogen and carbon atomic proportions. Reproduced with permission from ref 155. Copyright 2012 Elsevier. 7751


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Table 1. Some Synthesis Experiments for GDY precursor

catalyst

reaction

substrate

temperaturea

ref

HEB HEB HEB HEB HEB HEB tetraethynylethene HEB 2,4,6-tristriethynylpyridine 2,4,6-triethynylpyrimidine HEB HEB HEB 1,3,6,8-tetraethylenepyrene HEB HEB 2,4,6-triethynyl-1,3,5-triazine 2,4,6-triethynyl-1,3,5-triazine 2,3,5,6-tetraethynylpyrazine HEB 2,4,6-triethynyl-1,3,5-triazine 2,3,4,5,6-pentaethynylpyridine HEB 1,3,5-trichloro-2,4,6-triethynylbenzene 1,3,5-triethynylbenzene

Cu Cu Zn Cu/TMEDAf N/A CuCl Cu(OAc)2 Cu(OAc)2 Cu Cu Cu(OH)2 Cu/TMEDA Cu Cu/TMEDA Cu(OAc)2 Cu envelope Cu Cu(OAc)2 Cu(OAc)2 N/A N/A N/A Cu/TMEDA Cu Cu

Glaser−Hay Glaser−Hay thermal coupling Glaser−Hay thermal coupling Glaser−Hay Eglinton Glaser−Hay Glaser−Hay Glaser−Hay Glaser−Hay Glaser−Hay Glaser−Hay Glaser−Hay Glaser−Hay Glaser−Hay Glaser−Hay Glaser−Hay Glaser−Hay thermal coupling thermal coupling thermal coupling Glaser−Hay Glaser−Hay Glaser−Hay

Cu foil AAOd ZnO nano arrays Cu foil Ag foil graphene graphene Cu(OAc)2/PVP Cu foil Cu foil Cu(OH)2nanowires Cu foam Cu nanowires Cu foil N/A substratei Cu foil N/A N/A N/A N/A N/A diatomite Cu foil Cu foil

60 60 540 50 150 65−70 25 25 70 70 50 50 25 50 25 50 60 25 25 120 120 120 60 110 60

32 163 164 165 166 167 168 169 170 170 171 172 173 174 175 47 176 177 177 178 179 179 41 180 181

methods b

solution solution VLSe solution CVDg solution solution solution solution solution solution solution solution solution two-phaseh solution solution two-phase two-phase explosionj explosion explosion solution solution solution

c

a The unit is °C. bRepresents the template coupling method in organic solvent. cHEB represents hexaethynylbenzene. dAAO represents the anodic aluminum oxide. eVapor−liquid−solid growth process. fN,N,N′,N′-Tetramethylethylenediamine. gChemical vapor deposition method. hRepresents the two-phase coupling method. iThe selected substrate included Ni, Au, W foils, Si nanowires, quartz, stainless steel mesh, and graphene foam. j Represents the explosive coupling method.

uniform multilayers was finally generated in situ on the Cu surface. GDY samples with different numbers of layers can also be prepared through the vapor−liquid−solid (VLS) process. This VLS process was performed by strictly controlling the serving weight of GDY powder and moving the position of the quartz boat accordingly in the heating tube (Figure 13).164 During the heating process, a spot of zinc oxide (ZnO) was reduced to Zn droplets, which worked as both catalysts and growth anchor of GDY film. The layer distances of GDY with various thicknesses were all 0.365 nm, investigated by high-resolution transmission electron microscopy (HRTEM). The VLS method provides a new approach for preparing high-quality GDY films with high conductivity of 2800 S cm−1. Through a chemical vapor deposition (CVD) process with HEB, GDY monolayer and multilayer were also prepared on Ag substrate later (Figure 14). This CVD method provides another simple and possible way for preparing GDY with few layers.166 The as-prepared film exhibited conductivity of 6.72 S cm−1, which is the feature of a semiconductor. The GDY film can be used as a substrate to repress fluorescence and enhance Raman signals of attached molecules. Later, Liu, Zhang, and co-workers successfully synthesized very thin GDY film (∼2.9 nm) on graphene (Figure 15).167 The atomic molecular plane of graphene has an interaction with the repeat units of GDY, which is considered as the pivotal factor for the successful preparation of ultrathin GDY film. In addition, the ultrathin films of GDY analogues were also prepared though the above-mentioned methods. For example, the β-GDY-like films with smooth morphology and excellent crystallinity were

Figure 11. Schematic representation of the GDY structure.

since then were continually made but produced GDY substructures such as oligomeric or even monomeric.24,187−201 Until 2010, Li et al. proposed a new synthetic strategy, which benefited from the development of alkyne metathesis, metalcatalyzed cross-coupling, and template-assisted synthesis, to successfully prepare GDY on copper substrate through in situ cross-coupling reaction.32 To prepare GDY film, hexaethynylbenzene (HEB) was first synthesized and used as precursor. The Cu foil acted as both the reaction substrate and the catalyst of the directional polymerization (Figure 12).32 Immersed in pyridine, Cu foil gently provided the copper ions (Cu2+) to catalyze the cross-coupling reaction. Cu acetylide species with C−Cu2+ bond were the intermediate of the coupling reaction. GDY film composed of 7752


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Figure 12. (a) Structures of precursor HEB and GDY, (b) scanning electron microscopy (SEM) and photographic images, and (c) atomic force microscopy (AFM) images of GDY films with its I−V curve. Reproduced with permission from ref 32. Copyright 2010 Royal Society of Chemistry.

Figure 13. (a, b) Illustration of the growth process of GDY film on ZnO nanorods through VLS model. (c) SEM images of the related growth process. (d) TEM and (e) HRTEM images of GDY film samples with thicknesses of 540 nm. (f) TEM and (g) HRTEM images of GDY film samples with thicknesses of 42.6 nm. Reproduced with permission from ref 164. Copyright 2015 Springer Nature.

supporting characteristic, which can form a transparent film with super large area over 80 cm−2 (Figure 16c), and are bendable from different directions (Figure 16d−f). The chemical structures and the two nitrogen-substituted GDYs are characterized through solid-state 13C NMR spectra, which clearly displayed the chemical shifts of carbon assigned to butadiyne linkage and central nitrogen heteroaromatic rings (Figure 16g and h). 3.1.2. Nanotube Arrays and Nanowires. On the basis of their earlier cross-coupling method, Li et al. also prepared GDY NT arrays using a template like anodic aluminum oxide.163 Showing a smooth surface, the thickness of the GDY NTs wall is ∼40 nm. After the annealing process, this wall thickness was

fabricated though Eglinton coupling reaction with graphene as reaction template.168 Most recently, a versatile method that can further control the release speed of copper ions was developed in which the composite film containing the polyvinylpyrrolidone and copper acetate is spin-coated on the selected substrate and used as catalyst for the coupling reaction. As a result, the morphology, thickness, and conductivity of the as-prepared GDY nanostructure can be systematically tuned and optimized.169 Two nitrogen-substituted GDY films named as PM-GDY and PY-GDY were prepared through directly coupling the corresponding nitrogen-containing precursors (Figure 16a and b).170 Notably, the as-prepared GDY films have an excellent self7753


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Figure 14. (a) Illustration of the CVD system for the GDY growth on silver surface. (b) Illustration of the growth process. (c) Optical microscope image of the as-prepared GDY film on the surface of SiO2/Si substrate. (d) Selected-area electron diffraction (SAED) pattern and TEM image of the film. (e) Height profile and AFM image of the film on SiO2/Si substrate. (f) Raman spectrum of the as-grown single-layer and 10-layer films. Reproduced with permission from ref 166. Copyright 2017 John Wiley and Sons.

the copper foam-supported Cu(OH)2 nanowires worked as the catalyst and substrate for the growth of GDY on Cu(OH)2 nanowires. After annealing the as-prepared GDY/Cu(OH)2 nanowires at 180 °C, Cu(OH)2 nanowires were dehydrated and converted to CuO nanowires. The composite structure of GDY/CuO is confirmed by SEM, TEM, and X-ray diffraction (XRD) patterns. 3.1.3. Nanowalls. In collaboration with Li, Liu and coworkers modified the Glaser−Hay coupling reaction to prepare GDY nanowalls (Figure 18).165 Similarly, a copper plate and HEB were used as the reaction substrate and precursor, respectively. However, with pyridine and a tiny amount of N,N,N′,N′-tetramethylethylenediamine (TMEDA) as catalyst, the copper was alternated to copper ions, which functioned as catalytic reaction sites for the production of GDY (Figure 18a). The volume radio of solvent (acetone), pyridine, and TMEDA is set to be 100:5:1. First, GDY grew directly at those catalytic reaction sites through Glaser−Hay coupling reaction at a rapid rate. Then, as the amount of dissolved copper ions increased, new GDY grew along the as-prepared GDY, formed uniform GDY nanowalls on the copper plate (Figure 18b and c). AFM images (Figure 18d and e) of an exfoliated GDY sample, which was transferred onto a substrate of Si/SiO2, indicated the layered structure of the as-prepared nanowalls with thicknesses in the range from several to scores of nanometers. The as-obtained GDY nanowalls were also observed using TEM (Figure 18f). The GDY nanowalls have a high crystallinity, which was verified by the results of HRTEM. Notably, the space between the lattice fringes was estimated to be 0.466 nm, which is well-consistent with the theoretical result (Figure 18g). Moreover, the curved streaks (space ∼0.365 nm) with certain lattice fringe can be originated from the layer spacing of GDY (Figure 18h). In their subsequent work, a copper foam was used as the reaction template instead of copper plate, and a super hydrophobic foam based on GDY was obtained.172 In addition, this preparation method can also be applied to prepare GDY analogues such as pyrediyne nanowalls.174 On the basis of the preparation process of GDY nanowalls, a smart strategy with copper foil parcel as catalysis supplier for the selected substrate was developed to fabricate GDY nanowalls on arbitrary substrates.47 As shown in Figure 19, the function of the copper foil can be regarded as an “envelope”, which can ensure the concentration of the copper ions catalyst for the coupling of

Figure 15. (a) Illustration of the preparation strategy of GDY film with graphene as substrate. (b) Optical, (c) AFM, and (d) SEM images of the as-prepared GDY film. Note that the region of bared SiO2/Si is marked by arrows. Reproduced with permission from ref 167. Copyright 2018 American Chemical Society.

decreased to ∼15 nm. When it was applied for devices such as field-emission device, quite low threshold and turn-on field of 8.83 and 4.20 V μm−1, respectively, were observed. Besides, with better stability, the GDY NTs displayed lower helmholtz function than that of CNTs.93 The electronic and structural properties of GDY NTs with zigzag and armchair configurations have been calculated through the DFT.67,93 Notably, all the investigated nanotubes showed semiconducting behavior. GDY nanowires could also be prepared by VLS growth process using the synthesized GDY powder.202 With lengths of ∼0.6−1.8 mm and diameters ranging from 20 to 50 nm, the asprepared GDY nanowires showed mobility with a value of 7.1 × 102 cm2 V−1 s−1. Moreover, the conductivity was measured to be 1.9 × 103 S m−1, indicating that GDY nanowires can be applied as potential materials in photoelectronics and electronics.73,145 Besides Cu foil, GDY can also grow on the copper oxide (CuO) nanowires, forming a hierarchical structure like coaxial nanowires with GDY and CuO in the outer and inner sides, respectively (Figure 17).171 First, copper hydroxide (Cu(OH)2) nanowires could be prepared with oxidized copper foam. Then 7754


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Figure 16. (a) Illustration for preparation equipment and (b) reaction mechanism for the pyridinic nitrogen-substituted GDY. (c) Photograph of selfsupporting PM-GDY film with super large area over 80 cm2. (d) Photographs of the self-supported PY-GDY film, which is bendable in the ways of (e) both opposite side and (f) the diagonal. The results of solid-state 13C NMR spectra for (g) PM- and (h) PY-GDY. Reproduced with permission from ref 170. Copyright 2018 American Chemical Society.

Figure 17. (a) Preparation process, (b, c, d) SEM images, and (e) TEM image, as well as (f) HRTEM images of GDY/CuO coaxial nanowires. Reproduced with permission from ref 171. Copyright 2017 American Chemical Society.

the precursors on the arbitrary substrates. GDY can grow on different types of substrates according to this method. 3.1.4. Nanosheets. Sakamoto, Nishihara, and co-workers reported the bottom-up synthesis of GDY crystalline nanosheets at the interface of liquid/liquid or gas/liquid with HEB as the precursor (Figure 20).175 The copper(II) acetate and pyridine were used to catalyze the coupling reaction and were dispersed in the upper aqueous layer. The precursor was dissolved in the lower layer of dichloromethane. The GDY nanosheets grew gradually in between the interface of those two liquid phases.

Especially the gas/liquid interfacial synthesis generated multilayer GDY nanosheets with single-crystalline characteristic, which has regular hexagonal domains. In addition, the asprepared GDY nanosheets also have a side size of 1.5 μm and a uniform thickness of ca. 3.0 nm. In their subsequent work, hexaethynyltriphenylene precursor was also coupled to form free-standing films through a similar procedure.203 This liquid− liquid interfacial synthetic strategy also could be used to prepare ultrathin films of nitrogen-substituted GDY.177 The thickness of this nitrogen-substituted GDY film was measured to be ∼4 nm. 7755


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Figure 18. (a) Experimental schematic of preparing GDY nanowalls. SEM images of (b) the top and (c) the cross section for GDY nanowalls. (d) SEM and optical microscope image of the GDY sample exfoliated from GDY nanowalls. (e) AFM image of the exfoliated GDY sample with a thickness of ∼15.5 nm. (f) TEM and (g, h) HRTEM images of GDY. The inset graphs show the SAED patterns. Reproduced with permission from ref 165. Copyright 2015 American Chemical Society.

Figure 19. (a) Illustration of preparing GDY nanowalls following a method of the envelope copper catalysis. SEM images of the substrates: (b) silicon nanowires, (c) Au foil, and (d) graphene foam (GF) grown on Ni foam. SEM images of the GDY nanowalls grown on (e−g) the corresponding substrates. Reproduced with permission from ref 47. Copyright 2017 John Wiley and Sons.

Figure 21, the CuNWs first served as templates.173 The crystal boundaries on CuNW are supposed to be highly reactive, which

GDY NTs and ultrathin nanosheets have been reported to be prepared on free-standing Cu nanowire (CuNW). As shown in 7756


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Figure 20. (a) Illustration and (b) an experiment setup of the synthetic procedure for GDY in the interface of liquid−liquid. Few-layer GDY (c) SEM micrograph. (d) TEM micrograph on an elastic carbon grid. (e) SAED pattern. (f) AFM image and the height analysis of the blue line region. Reproduced with permission from ref 175. Copyright 2017 American Chemical Society.

Figure 21. (a) Schematic illustration of preparing GDY on CuNW. (b) TEM and (c, d) energy-dispersive spectrometry (EDS) of GDY nanosheets on CuNW. (e) GDY nanotube TEM image. GDY nanosheets HRTEM images: (f) without Cu and (g) with Cu. Reproduced with permission from ref 173. Copyright 2018 John Wiley and Sons.

Following this method, GDY patterns with controllable microshapes can be achieved, which can be further used in the electronic devices. 3.1.6. GDY 3D Framework. Although the synthesis of GDY was reported in 2010 by Li et al., it is still interesting and a challenge to explore new ways to prepare GDY on a large scale. Recently, an explosion method was proposed to prepare GDY in a large amount with different nanostructures.178 As shown in Figure 23, three kinds of thermal treatments were carried out to control the cross-coupling reaction of HEB. This explosion method was performed by treating HEB at 120 °C without any metal catalyst. GDY nanoribbons, 3D framework, and nanochains can be obtained by control the heating process. Especially when adding HEB directly to a preheated (∼120 °C) environment in air, a violent explosion was immediately initiated to get GDY samples with a high yield of 98%. Such a method

can offer more reactive sites for growing GDY. The interlayer distance of the GDY nanosheet with average wrinkle thickness of 3.75 nm is consistent with the previous reports as 0.365 nm. Cu nanoparticles can be clearly observed in the nanosheets, indicating their strong interaction with GDY nanosheets. GDY nanotube can be obtained after removing the CuNWs using the HCl and FeCl3 mixed solution. 3.1.5. Ordered Stripe Arrays. GDY can be used as a flexible and even stretchable electronic sensor because it has both high elasticity and excellent electronic characteristics. In addition, precise patterning of GDY could be achieved while predefined silicon template with different shapes in microlevel is used in the process of in situ growth of GDY on copper foil (Figure 22).52 Notably, it is crucial to select a template with superhydrophilicity, so that the reactants could continuously diffuse to the surface of the substrate with microscale spacing. 7757


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Figure 22. Illustration of silicon templates and corresponding SEM images of GDY patterns on silicon templates: (a−c) predefined groove with different size, (d) circle, and (e) spindle. Reproduced with permission from ref 52. Copyright 2017 John Wiley and Sons.

Figure 23. Illustrations of the preparation processes. (a) Photo of HEB before the reaction; (b) reaction under three treatments; (c) GDY morphologies; and (d) sample photos showing the volume change after the reaction. Reproduced with permission from ref 178. Copyright 2017 Royal Society of Chemistry.

3.2. Characterization Methods

could also be utilized to tune the porous structures of GDYs and N-content as well as N-configurations of GDYs, exhibiting good controllability.179 Another way to prepare GDY 3D framework was reported by Zhang and co-workers utilizing diatomite as template (Figure 24).41 Diatomite is naturally abundant and possesses strong ability as an adsorbent. Thus, Cu nanoparticles were absorbed inside the pores of diatomite as catalyst. GDY was prepared with porous diatomite as substrate. The freestanding 3D GDY could be realized by etching the diatomite and Cu. The collapse of 3D GDY was prevented by the support provided from the hollow GDY columns that connected the GDY flakes.

It has always been difficult to directly image and determine the crystal structures of 2D plane materials with few layers. However, recently Lu and Luo et al. successfully employed low-voltage and low-current-density TEM to directly image an as-synthesized crystalline GDY of ABC stacking with 6-layer thickness.204 The TEM and HRTEM images of GDY nanosheet are shown in Figure 25 with its experimental selected-area electron diffraction (SAED) pattern. Three stacking modes, AA, AB, and ABC of GDY, with corresponding simulated SAED patterns are also presented (Figure 25b and c). Comparing the SAED patterns in experiment and simulation, the experimental 7758


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Figure 24. (a) Preparation processes of 3D GDY. SEM images of GDY grown on diatomite with Cu nanoparticles being absorbed: (b) zoomed-out and (c) zoomed-in images. SEM images of freestanding 3D GDY without diatomite template: (d) zoomed-out and (e) zoomed-in images. Reproduced with permission from ref 41. Copyright 2018 John Wiley and Sons.

Figure 25. (a) TEM, (b) HRTEM, and (c) related SAED pattern of a crystal GDY nanosheet; the inset shows the indexed [001] zone axis. (d) Simulated images of HRTEM. The Δf represents the defocus values along the axis of [001] zone with units of nm. (e) Three stacking modes named as AA, AB, and ABC of GDY and their simulated SAED patterns. Yellow, green, and purple represent A, B, and C layers, respectively. Reproduced with permission from ref 204. Copyright 2018 Springer Nature.

by the XPS spectra. This result was the same as that observed by energy-dispersive spectrometry (EDS).32,165,205,206 The fitted C 1s peak was able to be divided into four subpeaks, including the sp2 orbital carbons of benzene rings around 284.5 eV, the sp2 orbital carbons of CC bonds at 285.2 eV, CO bonds close to 287.5 eV, and C−O bonds near the binding energy of 286.4 eV, respectively.32,165,207 The presence of oxygen was presumed to be the air adsorbed in the GDY sample, as well as some terminal alkyne groups’ oxidation in GDY. Raman spectroscopy can be conveniently applied to characterize GDY, especially the typical D band, G band, and CC bonds of GDY. Liu, Zhang, and co-workers predicted that

one matches with that of the ABC rather than those of the AA and the AB. Thus, the nanosheet of GDY is confirmed to have the ABC stacking. To further examine the GDY crystal structure by using Wiener filtering, the noise existing in the HRTEM images was also filtered. The HRTEM image by simulation is in accordance with the HRTEM images in experiment or by filter, as shown in Figure 25d. Finally, the GDY nanosheet composed of six layers with a thickness value of 2.19 nm, as well as the mode of ABC stacking, has been further confirmed (Figure 25e). The bonding structure and elemental composition of GDY could be studied by X-ray photoelectron spectroscopy (XPS). The carbon was the main element in GDY, having been verified 7759


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that it is softened, compared to those on the other sp2-hybridized carbons such as graphene. The structure variation and strain effect on the Raman shifts of GDY sample were also considered. The inherent relationship between the applied mode of strain and the shift of Raman spectra could also provide useful information for the characterization of GDY-based sample. Experimentally, the Raman spectrum of GDY showed four characteristic peaks at 2189.8, 1926.2, 1569.5, and 1382.2 cm−1, consisting of the reported values for carbon bonds.32,40,165,172,202,207,208 The peak located around 1382.2 cm−1 comes from the carbon atoms vibration in benzene rings band. The strong peak appearing at 1569.5 cm−1 derived from the stretching of aromatic carbon bonds, which could correspond to the G band. Two weak peaks of 1926.2 and 2189.8 cm−1 can be assigned to the diynyl link (−CC−C C−) vibrations. It can be observed that the theoretical and experimental results compare well with each other. As an explicit elemental absorption spectroscopic technique, X-ray absorption spectroscopy (XAS) can be measured by exciting the core electrons to quasi bound states and is widely applied for determining the electronic structure and local geometry of complexes.209 Using XAS, the photon energy peak for GDY has been observed around 285.5 eV, which is able to be ascribed to π* excitation of the carbon−carbon bonds in aromatic rings, while a new peak at 285.8 eV can be ascribed to π* excitation that comes from triple bonds of carbon.

there were six peaks of GDY that can be intensively observed through the Raman spectrum (Figure 26).40 The benzene and

Figure 26. Predicted Raman spectra of GDY. Reproduced with permission from ref 40. Copyright 2016 American Chemical Society.

alkyne-corresponding ring’s breathing vibration generated the B peak. The atom’s scissoring vibration in phenyl group led to the G″ peak. Between doubly coordinated atoms and their neighboring triply coordinated atoms, the carbon−carbon single-bond vibrations resulted in the G′ peak. The aromatic bonds stretching mostly brought about the G peak. The carbon−carbon triple bonds’ synchronous stretching and contracting caused the symmetric mode Y peak. The out-ofphase vibrations of carbon−carbon triple bonds initiated the Y′ peak. On the basis of the theoretical analysis of the above vibrational modes, the G bands were supposed to be useful to detect and characterize the GDY sample, which is due to the fact

Figure 27. Top-view structure of (a) GY, (b) GY with a coverage of 0.5 hydrogen atoms, and (c) graphine. The band structure and corresponding total DOS of (d) GY, (e) GY with 0.5 hydrogen atoms coverage, and (f) graphine, with their Fermi energy marked by magenta line. The wave functions of valence band maximum of (g) GY, (h) GY with 0.5 hydrogen atoms coverage, and (i) graphine, and conduction band minimum of (j) GY, (k) GY with 0.5 hydrogen atoms coverage, and (l) graphine. The negative and positive signs are displayed, respectively, by yellow (light gray) and blue (dark gray) colors. Reproduced with permission from ref 155. Copyright 2012 Elsevier. 7760


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Figure 28. (a−c) Atomic structure of three kinds of GYs and (d−f) their related electronic bands. (g−i) Optimized structures of the fluorinated GYs and (j−l) their related electronic bands. Reproduced with permission from ref 228. Copyright 2013 Elsevier.

displayed two hybridization features like sp3 and sp2, with adsorption energy of hydrogen atoms as −1.80 and −2.27 eV, suggesting the absorption preference of hydrogen atoms on sphybridized carbon. To complete the hydrogenation, it needs to break three π-bonds of carbon atom for the conversion from sp3hybridization to sp2-hybridization, while it would break one πbond of the carbon atom for changing sp-hybridization to sp2hybridization. With the increase in the number of hydrogen atoms, it is expected naturally that one new whole conjugated hydrocarbon material with 2D structure can be obtained when all these carbon atoms of sp-hybridization are hydrogenated to sp2hybridization. Such hydrogenated GYs exhibit the following features: (1) The planar structure of GY is twisted, which induces the noncoplanarity of all the carbon atoms. (2) The hybridization forms of the carbon atoms are all sp2 type. (3) There are two orientations of C−H bonds, two-thirds of which tend to carbon plane, while the others are perpendicular to the carbon plane. On the basis of the calculated results, the band gap of the hydrogenated GYs is highly related to the hydrogenation extent. While only the carbon atoms in the linkage are hydrogenated, the band gap of the as-prepared material is calculated to be 1.01 eV. This value can be increased to 4.43 eV for the fullhydrogenated GY materials. Besides, the hydrogenated GY has great potential in the field of gas separation. For example, hydrogenated α-GY without defects can be used to efficiently separate the mixture of methane (CH4) and hydrogen (H2).227 Moreover, the separation selectivity of this membrane can be ∼700 for the mixture gas of nitrogen (N2) and H2 at room temperature. Increasing the temperature can reduce this value.

3.3. Functionalization of GYs

Functionalization of GYs will result in many interesting carbonbased materials, varying the band gap and the mechanical property of GYs, and so on.210 Although the way of functionalizing GYs is still being investigated, there are some theoretical studies of modifying GYs, such as hydrogenation, fluorination, and metal decorated on GYs.46,157,158,211−223 Besides, there are also some other studies about absorption method, like absorption of polycyclic aromatic hydrocarbons, or small molecules, like boron, halogenated on GYs.224,225 3.3.1. Hydrogenation. To functionalize the carbon materials like CNTs and graphene, hydrogenation was reported as an efficient method. It has been demonstrated by theoretical and experimental studies that the electronic properties, like spinpolarization and electron conductivity of carbon nanomaterials, can be modified by adsorption of hydrogen. The hydrogen adsorption of GY and GY derivatives can be very easy for the hybridized networks of sp and sp2 carbon.226 The optimized configurations of hydrogenated GYs and their related electronic structures have been studied by Zhao and co-workers through first-principles calculations (Figure 27).155 Characterized with different orbital hybridizations, two kinds of carbon atoms were displayed by GY to anchor hydrogen, which was different from the CNTs and graphene. The hydrogen atom can be located above either the sp2-hybridized carbon atom or the sphybridized carbon atom. Chemical bonds with lengths of 1.11 and 1.12 Å were formed between hydrogen atoms and carbon atoms in both cases, which can be confirmed by structural optimization. The π-bond nearby was broken by the adsorbed hydrogen atoms, while the carbon atoms under the hydrogen atoms were pulled out of the plane. Those carbon atoms 7761


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However, the existence of the defects can dramatically degrade the separation selectivity of the hydrogenated α-GY film. Interestingly, the separation selectivity of the defected hydrogenated α-GY membrane can be enhanced while the temperature increases. It should be noted that hydrogenated γ-GY cannot be used as the material for gas separation, because no permeability capability is observed in the membrane, even for this kind of hydrogenated GY without defects. 3.3.2. Fluorination. While GYs are fluorinated, both sp and sp2 carbon atoms can be converted into the sp3 carbon for the addition of fluorine atoms to the double and triple carbon− carbon bonds, which will result in the variable ratio of fluorine atoms to carbon atoms.228 All C−C bonds of GYs are elongated for the fluorination (Figure 28). This elongation could be regarded as obvious evidence for the transition of sp2 or sp carbon to sp3-hybridized carbon. The lengths of the three types of carbon−carbon bonds in GYs, which can be expressed as the bonds of alkynyl bridge ring, the bonds of aromatic benzene ring, and the alkynyl bond between the aromatic ring and the alkynyl, are estimated to be 1.265, 1.454, 1.444 Å, respectively. After being fluorinated, the lengths of the three types of bonds are separately increased to 1.729, 1.651, and 1.700 Å, respectively. Moreover, plenty of conformers for the fluorinated sphybridized carbon atoms in GY exist, which can be assigned to the various possible configurations of the difluoroethylene moieties with numerous relative orientations of carbon−fluorine bond. Nevertheless, the steric effect can efficiently limit the number of possible configurations of fluorinated GYs. The basis for the major role of steric effects on the stability of fluorinated GYs can be obtained through the comparison of its formation energy. On the basis of its unique structure, the fluorinated GYs show excellent properties like good mechanical strength and chemical and thermal stability, which make fluorinated GYs attractive materials for optoelectronics or a candidate for protective coatings.158,213,220,228 3.3.3. Metal Decoration. The adsorption of single metal atoms such as K, Na, Li, Ti, Sc, Ca, etc. on GY sheets provided another possible way to functionalize GYs theoretically (Figure 29).46,86,87,212,215,217−219,221,222,229−252 It is regarded as a novel way to hold the metal atoms in the molecular pores evenly distributed in the molecular plane of GYs, which is thanks to strong binding energy between the metal atoms and the acetylenic groups for the existence of in-plane π/π* states and proper pore size. According to the calculation results, the binding energies for K, Na, Li, Ti, Sc, and Ca are estimated to be 1.92, 1.82, 2.67, 5.11, 4.85, and 2.41 eV, respectively.215,229 Metal-decorated GY can be used as novel hydrogen-storage material. Especially, GYs decorated with Li show a storage capacity as large as 18.6% for hydrogen and proper adsorption energy (ca. 0.27 eV per hydrogen molecule), which is ascribed to the polarization of the hybrids under the electric field derived from the adsorbed Li on GY.249 The discussed study reveals that Li-decorated GY is an ideal candidate for the hydrogen storage. As another example, the Ca-decorated GY system also shows a strong adsorption situation for hydrogen similar to the Cadecorated fullerene with a binding energy of 0.2 eV per hydrogen molecule. This can be ascribed to the calculation result that single Ca metal atoms can be strongly combined in the molecular pores surrounded by the sp and sp2 carbon in GY,217,221 which leads to the presence of the additional in-plane π states. Different from the above-discussed metal-decorated GY system, the stability of Na-decorated GY system is decreased,

Figure 29. Charge density of GYs decorated with (a) Li, (b) Ca, (c) Sc, and (d) Ti. Note that the charge accumulation and depletion are represented by the warm and cold colors, respectively. Reprinted with permission from ref 229. Copyright 2013 Elsevier.

but metallic properties are shown after decorating Na in GYs, which was demonstrated through first-principles calculations.238 The Löwdin charge analysis results also revealed that a certain amount of charge transfers from sodium to the carbon pz orbital, which is assigned as the reason for the metallic properties. As for the energy-level distribution, the conduction band was contributed mainly by the decorated Na atom. As a result, the Fermi level of the Na-decorated system rises while the number of sodium atoms decorated to GY increases. Moreover, the contribution of particular orbitals of Na to the Fermi level corresponds to the number of Na atoms decorated in the molecular pores of GY. While increasing the number of decorated Na, the p orbitals contribute more, while the s orbitals contribute less. The metallic nature can endow Nadecorated GY with many applications, for example, being applied in a Na-ion battery as the electrodes. 3.3.4. Absorption of Guest Molecules. Because of the porous character of GYs, the adsorption strength of compounds onto GYs’ surfaces was anticipated to be much lower than that onto graphene (a perfect π-extended system).224,225,253−256 The adsorption of polycyclic aromatic hydrocarbons (PAHs) onto low-dimensional carbon allotropes has been of interest, because PAHs can be used as nonaggressive dopant molecules to tune the electronic character of carbon materials. DFT calculations were implemented by Cortés-Arriagada to expand the knowledge about GY and its interaction with PAHs (Figure 30a−c).224 PAHs were calculated to be strongly adsorbed onto GY with high adsorption energy, which is a decrease of only 12−23% in the adsorption strength compared to graphene due to an abatement in the dispersive interactions of the same order. The interaction energy of GY with graphene was predicted to be ∼31 meV/atom. Moreover, nonsubstituted PAHs behave as ndopants for GY, also inducing significant decreases in its band gap of up to 0.5 eV (such as ovalene). With the GY substituted by strong donor and adsorbed with semiconductor PAHs (such 7762


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Figure 30. (a) Energy of one carbon atom (Eads/C atom) of PAHs adsorbed onto graphene (G) and γ-GY (GY) versus the H/C ratio of the PAH molecules. (b) Configurations of benzene and pyrene adsorbed on G. (c) Configurations of pyrene adsorbed on GY. Reproduced with permission from ref 224. Copyright 2017 John Wiley and Sons. (d) Diagram of valence band maximum (VBM). (e) Optimized structure of the GY adsorbed with boron triiodide (purple), boron trichloride (green), and boron trifluoride (cyan). Reproduced with permission from ref 225. Copyright 2016 Springer Nature.

Figure 31. (a) Schematic of the TiO2/GDY composite for photodegradation of methylene blue (MB). (b) Alteration tendency of the Kubelka−Munk function with the energy of light irradiating over samples. Photocatalytic degradation extent of MB versus the irradiation time under (c) UV and (d) visible light. Reproduced with permission from ref 270. Copyright 2012 John Wiley and Sons.

system adsorbing on it, as reported by Sarkar and co-workers.225 The GY interacting with boron triiodide and boron trichloride molecules acts as an n-type semiconductor. While boron trifluoride molecule is absorbed, the GY system behaves as a p-type semiconductor. According to the Mulliken and Hirshfeld charge analysis, that can be a result of the associated bottom movement of the conduction band together with top movement of the valence band for the interaction. The dipole moment of pristine GY is zero, but it can suddenly increase after absorbing the halogenated boron. The detectable increase of the dipole

as bis[1,2,5]-thiadiazolo-p-quinobis(1,3-dithiole)), the band gap decreased ∼0.8 eV. These results show that GY should be considered as a good adsorbent material for sorption and analyses of PAH pollutants in environmental applications. In addition, PAHs can be applied for tailoring the electronic properties of GY by means of nonaggressive molecular doping, which allows for enhancement of its metallic character. Small molecules adsorbing on pristine GY could also influence its electronic properties. Parts d and e of Figure 30 showed the means of increasing the band gap of GY by boron-halogenated 7763


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Figure 32. (a) Illustration of the photodegradation by TiO2−graphene and TiO2−GDY catalysis. (b) Conduction band (CB) and valence band (VB) positions of different TiO2−GDY, TiO2−GR, and TiO2−graphene composites. Difference of electron density at the interfaces of the composites: (c) TiO2(001)−GDY and (d) TiO2(001)−GR. The electrons accumulation is displayed by blue, and depletion is shown by yellow. (e) Mulliken population of TiO2(001)−GR and TiO2(001) − GDY. Reproduced with permission from ref 272. Copyright 2013 American Chemical Society.

reaction, the part of diacetylenic units in GDY can be converted into an electronic transmission preferred 2D π-conjugated structure, which can avail GDY to be applied in photodegradation. GDY has been used to promote the catalytic activity of TiO2 (Figure 31).270 Titania and GDY nanocomposites with different mixed ratios have been fabricated as photocatalysts for degrading methylene blue. The band gap of TiO2 has been efficaciously decreased, and the light range it absorbed has been obviously extended after the generation of chemical bonds between GDY and TiO2. When the TiO2/GDY ratio was 0.6 wt %, the photocatalytic activity of TiO2−GDY was prominently higher than those of TiO2, TiO2−graphene, and TiO2−CNTs, especially exposured under visible light. The electronic properties and chemical structures of TiO2− graphene and TiO2−GDY composites featuring different TiO2 facets have also been studied using first-principles DFT.258,272 The TiO2−GDY composite exhibited better charge separation and longer lifetimes for photoexcited carriers than all the other TiO2-containing 2D composites. The lower valence band (VB) positions are observed in TiO2−GDY composites in comparison with TiO2−graphene composites. Generally, the more negative the value of VB, the higher is the oxidation ability, thus bringing about better photodegradation ability (Figure 32). When applied as photocatalyst for methylene blue degradation, TiO2−GDY shows a rate constant that is 1.27 and 1.63 times larger than that of TiO2 (001)−graphene and TiO2, respectively, indicating the great potential of applying TiO2−GDY as a very efficient photocatalyst. The electron mobility of ZnO is very high, almost several hundred times that of TiO2. Thus, the photocatalytic efficiency of ZnO is high, because of the increased lifetime of the photogenerated charge carriers. GDY−ZnO nanohybrids have also displayed photocatalytic properties superior to those of bare ZnO nanoparticles.274 It should be noted that GDY has excellent conductivity, which guarantees it as a promising candidate to

moment could be suggested as an application of GY in gas sensor.

4. PROPERTIES AND POTENTIAL APPLICATION OF GDY GDY shows excellent π-conjugated structure, perfect pores distribution, and adjustable electronic properties for its sp2- and sp-hybridized flat framework.206,257−269 The fascinating structures and electronic properties suggest that GDY-based materials will have possible applications as catalyst; in rechargeable batteries, solar cells, electronic devices, magnetism, detector, biomedicine, and therapy; and for gas separation as well as water purification. Lots of reports have already exhibited experimentally the practical application of GDY. 4.1. Catalysts

Usually, excellent carbon-based catalysts should display large surface area, good pore structure, and stability. As discussed above, GDY has a two-dimensional conjugated and highly porous plane, which can not only increase the reactive sites for the specific reaction but also provide an atomic-level dispersion framework for other efficient catalysts. Over the past decade, plenty of GDY-based catalysts have been developed, which showed bright application potential in many fields such as photocatalyst, electrochemical and photoelectrochemical water splitting reaction, etc. 4.1.1. Photocatalysts. The application of titanium oxide (TiO2) on the elimination of organic contaminants has been researched widely for its extremely low toxicity, cheap cost, and outstanding physical and chemical stability. The sunlight exploitation of TiO2 nanoparticles is confined in only the ultraviolet range for the large intrinsic band gap of titanium oxide. The band gaps of rutile and anatase are 3.0 and 3.2 eV, respectively. As a 2D planar semiconductor material, GDY can facilitate the photocatalytic improvement of TiO2 and its nitrogen-doped derivatives.270−273 Through a hydrothermal 7764


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nearby N atoms being highly positive charged to easily draw electrons from the anode and enhance the ORR activity. Although N-doped GDY (NGDY) shows enhanced electrocatalytic performance in the ORR, the activity including fourelectron selectivity and onset potential of NGDY remains inferior to that of well-known Pt/C catalyst.285 Further improvements like dual-heteroatom (N, F) codoping were developed for NGDY (Figure 34).284,286 Enhanced carbon oxide and methanol tolerance, considerable long-term durability, and excellent selectivity of four-electron ORR method in alkaline solution were obtained for N and F codoped GDY (NFGDY) catalysis. Moreover, the NFGDY also exhibited comparable performance with that of the Pt/C in a manual Zn-air battery. In our opinion, this experimental evidence is good evidence for confirming the predicted beneficial “doping effect” in GDYbased ORR electrocatalysts. The configuration of N doping is the key to affect the catalytic performance of N-doped carbon materials.287−291 Especially Nakamura, Kondo, and co-workers applied graphite as a model catalyst to demonstrate that the active sites of ORR were contributed mainly by pyridinic N.288 Herein recently, pyridine and ammonia were used as the nitrogen source in succession under heat treatment, to prepare N-doped GDY with more valid N-doping configurations (Figure 35).287 Through annealing treatment, the nitrogen heteroatom can be doped into GDY; in addition, small pieces of GDY and some groups containing oxygen in GDY can also be removed. These will enhance the conductivity of the N-doped GDY. The new N-doped GDY exhibits comparable catalytic activity to the Pt/C that is commercially available for ORR and superior activity to the other metal-free catalysts shown in that referred work. Hydrogen evolution reaction (HER), which is currently considered as an important half-reaction in the process of water splitting, can efficiently produce hydrogen in large quantities and high purity. In the aspect of materials, core−shell nanowires array Cu@GDY NA/CF, which was composed of GDY as the outer layer with copper nanowire as the inside, was fabricated on the copper foams through an in situ method (Figure 36). Notably, this catalyst composite displayed not only selfsupporting characteristic but also high HER catalytic activity after cycling pretreatment in acid environment containing sulfuric acid with a concentration of 0.5 M. The onset overpotential and Tafel slope of the Cu@GDY NA/CF catalyst were measured to be 52 mV and 69 mV dec−1, respectively. To get 10 (or 100) mA cm−2 catalytic current density, the corresponding overpotential only needed to be 79 (or 162) mV. The outstanding catalytic performance of Cu@GDY NA/ CF can be explained by the synergetic interaction between GDY shell and Cu core. Moreover, this catalyst also showed an excellent stability for up to 20 h.292 The composite catalyst system containing GDY cobalt nanoparticles, which is covered by carbon layers with nitrogen dopant (CoNC/GDY), also showed excellent HER electrochemical activity. The onset potential of CoNC/GDY, which was measured to be ca. 170 mV, was a much lower value than that of GDY, NC/GDY, or Co/GDY (Figure 37).293 Compared with Pt/C (10 wt %), the current density of the composite CoNC/GDY catalyst system was large while a >406 mV applied potential was employed, indicating the higher activity of the asprepared catalyst system. Markedly, even after a continuous scan of 36 000 cycles in the cyclic voltammetry measurement, the obtained polarization plot of the as-prepared CoNC/GDY catalyst maintained no change compared to that of the initial

efficiently transfer the photoinduced electrons from ZnO nanoparticles to GDY. Besides, the recombination of carriers will be suppressed. The electron that is trapped on GDY is able to have a reaction with the dissolved oxygen molecules to generate superoxide radicals. Meanwhile, the generated holes are able to have a reaction with H2O to generate hydroxyl radicals that can make the azo dyes be decomposed. Just like graphene oxide (GO) being used as the agent of cross-linking, GDY can also be hybridized with Ag/AgBr in a facile manner. The photocatalytic ability of Ag/AgBr/GO/GDY for methyl orange pollutant degradation irradiated under visible light was higher than that of Ag/AgBr/GDY, Ag/AgBr/GO, and Ag/AgBr.275 Thus, GDY can be regarded as an excellent candidate for application in photovoltaics and photocatalysis. Another example is using GDY together with Ag3PO4 and graphitic carbon nitride to fabricate Z-scheme nanocomposite as a photocatalyst for oxygen production.404 Not only as substrate, GDY can also accelerate the kinetics of O2 evolution via collecting the photogenerated electrons.276 In addition, the composite material containing GDY and CdS, which is prepared through in situ route, also shows improved activity in the photocatalytic reaction of hydrogen production.277 4.1.2. Electrocatalyst. GDY and its derivatives with nonequivalent distorted rather than hexagonal symmetry Dirac cones are theoretically predicted to show superior electronic properties than those of graphene. The maximum pore size of ∼2.5 Å would benefit the air diffused into the abandoned pores of GDY while in the atmosphere. For the prediction by theoretical simulations that the butadiyne units could result in positive charging of some carbon atoms in GDY, GDY can directly act as the metal-free catalysts for the application in oxygen reduction reaction (ORR).278−283 After doping with nitrogen, with the competitive electrocatalytic activity to Pt/C, the N-doped GDY showed superior stability and tolerance (Figure 33).278 It was elucidated by quantum mechanical calculations that the N doping would lead to the carbon atoms

Figure 33. (a) Possible structure of N-doped GDY, with gray, red, yellow, and purple spheres standing for C, pyridyl N, imino 1 N, and imino 2 N atoms, respectively. Cyclic voltammetry (CV) curves of (b) N-doped GDY and GDY in O2-saturated aqueous solution containing KOH in a concentration of 0.1 M, (c) N 550-GDY, and (d) the commercial Pt/C with Ar- (black) or O2-saturated (red) or with 3 M methanol and O2-saturated (blue) in 0.1 M KOH. The scan rate is 10 mV s−1. Reproduced with permission from ref 278. Copyright 2014 Royal Society of Chemistry. 7765


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Figure 34. (a) Illustration of preparing GDY-based materials doped with multielement. (b) Linear sweep voltammetry (LSV) curves of GDY, NFGDY, NSGDY, and NBGDY in aqueous O2-saturated solution containing KOH at a concentration of 0.1 M. Note that the scan rate is set to 10 mV s−1. (c) Polarization plots and the related power density curves for NFGDY and commercial Pt/C as the cathode catalyst in Zn-air batteries. Reproduced with permission from ref 284. Copyright 2016 Royal Society of Chemistry.

Figure 35. (a) Scheme of the N-doping GDY fabrication process. (b) LSV curves of N′N-GDY, N-GDY-900 °C, and Pt/C in the O2-saturated aqueous solution containing KOH; the inset shows the enlarged LSVs. The comparison of the durability of the (c) as-prepared N′N-GDY and (d) Pt/C (commercial) during the 5000 cycles of scan in CVs test. Note that the reaction was carried out in the O2-saturated aqueous solution containing 0.1 M KOH as electrolyte. Reproduced with permission from ref 287. Copyright 2017 American Chemical Society.

38c). These results exhibited the advancement of strategy in that work to prepare single-atom transition-metal catalyst. The welldispersed Ni and Fe atoms in the GDY were fully verified by the high-angle annular dark field scanning tunneling electron microscopy (HAADF-STEM) images (Figure 38d−g). Most recently, Lu and co-workers also successfully developed a method to fabricate Pt single-atom catalysts with GDY as supporter, which showed excellent catalytic activity for HER reaction. The highest mass activity of as-prepared composite catalyst system is 26.9 times larger than that of commercial Pt/C catalyst.295 As electrocatalyst support materials, the metal substrate always suffers severe corrosion/dissolution problems during the whole water splitting process, which can significantly affect the stability of the electrocatalysts. GDYs were reported as efficient support materials for bifunctional electrocatalysts with enhanced catalytic activity and stability for overall water splitting.296 Using 3D GDY foam as scaffolds and NiCo2S4 nanowires as building blocks (shown in Figure 39), the fabricated bifunctional electrode (NiCo2S4 NW/3D GDF) exhibited extraordinary durability and excellent catalytic activity

one. In contrast, regarding the well-known catalyst system of Pt/ C with ratio of 10 wt %, a significant decrease of catalyst activity was observed after only 8 000 cycles at low-current-density range. Herein, the stability of the CoNC/GDY catalyst system is apparently much better for HER in the environment of alkaline electrolytes than that of commercial Pt/C. Li and co-workers recently developed a two-step method to prepare atomic catalysts comprising GDY and isolated metal atoms (Figure 38a).294 The 3D GDY foam was prepared on the surface of carbon cloth. Then the single metal atoms including Ni or Fe were adsorbed on GDY foam through electrochemical reduction method. The possible absorption site and the optimized geometry of Ni/GDY and Fe/GDY were investigated through DFT method, which indicated that the metal atoms were anchored on site 1 (S1) instead of site 2 (S2) (Figure 38b). The as-prepared GDY anchored with isolated Fe or Ni showed improved catalytic activity compared to that of commercially available Pt/C in HER. Particularly, the mass activities of Fe/ GDY and Ni/GDY were measured to be 80.0 and 16.6 A mg−1, while the overpotential was set to be 0.2 V, which were 7.19 and 34.6 times larger than that of Pt/C catalyst, respectively (Figure 7766


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Figure 36. (a) Schematic illustration of synthetic process of the Cu@GDY NA/CF (upper row) and the corresponding optical images (bottom row). HER polarization curves of (b) the as-prepared and reference electrodes, (c) initial electrode and CV-treated after 2000 cycles from 0 to −0.7 V. Reproduced with permission from ref 292.Copyright 2016 Elsevier.

Figure 37. (a) Schematic preparation process of the CoNC/GDY. (b) HER polarization plot of the CoNC/GDY. (c) Comparison of related curves of Pt/C catalyst with a weight percent of 10% before and after 8000 cycles, and the as-prepared CoNC/GDY catalyst before and after 36 000 cycles in aqueous KOH solution. Note that the concentration of KOH is 1 M and the scan rate is set to 100 mV s−1. Reproduced with permission from ref 293. Copyright 2016 American Chemical Society.

It is a great challenge to prepare catalyst with good durability and high activity for HER in a solution with pH ranging from alkaline to acid. The electron-rich GDY (e-rich GDY), which is GDY grown on carbon cloths (CCs), was reported to achieve that challenge when molybdenum disulfide (MoS2) was grown on its surface (Figure 40).297 Besides strong electronic

for HER, OER, and water splitting in potassium hydroxide solution (1.0 M). The retated two-electrode alkaline water electrolyzer needed low cell voltages of merely 1.53 and 1.56 V to achieve 10 and 20 mA cm−2, respectively, with remarkable stability of electrolysis operation over 140 h. 7767


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Figure 38. (a) Preparation strategy of GDY anchored with single metal atoms including Fe and Ni. (b) Calculated possible adsorbed sites (left) and optimized configuration of metal atoms Ni and Fe on GDY. (c) Comparison of mass activity of GDY with single metal atoms and Pt/C. The inset table shows the mass activities achieved at overpotentials of 0.05 and 0.2 V. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) results of Ni/GDY (d, e) and Fe/GDY (f, g). Reproduced with permission from ref 294. Copyright 2018 Springer Nature.

Figure 39. (a) Schematic of the preparation process of the NiCo2S4 nanowire (NiCo2S4 NW) on 3D GDY foam (GDF). Corresponding (b) polarization curves, (c) Tafel slopes, and (d) stability test for oxygen evolution reaction (OER). Corresponding (e) polarization curves, (f) Tafel slopes, and (g) stability test for HER. Reproduced with permission from ref 296. Copyright 2017 John Wiley and Sons.

interaction between e-rich GDY and MoS2, theoretical calculation revealed that the hydrogen adsorption-free energy of e-rich GDY and MoS2 heterostructure is smaller (−0.58 eV) than that of e-rich GDY (1.18 eV) or MoS2 (1.94 eV), indicating good catalytic activity of e-rich GDY and MoS2 heterostructure for HER. On the basis of that fundamental analysis, over-

potential values of 128 mV in sulfuric acid solution and 99 mV in hydroxide potassium solution have been obtained by Li and coworkers with e-rich GDY and MoS2 heterostructure as catalyst for HER. A hybrid electrocatalyst system containing GDY and nickel− iron-layered double hydroxide (GDY/NiFe-LDH) showed 7768


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Figure 40. (a) Difference maps of charge density for e-rich GDY (eGDY) and MoS2 (MDS) heterostructure. (b) Adsorption-free energy of hydrogen. (c) LSV plots and (d) Tafel slopes in sulfuric acid solution. (e) LSV plots and (f) Tafel slopes in hydroxide potassium solution. Reproduced with permission from ref 297. Copyright 2018 John Wiley and Sons.

Figure 41. (a) TEM image and (b) HRTEM image of NGDY/MoS2 hybrids. HRTEM images of (c) MoS2 part and (d) NGDY part in NGDY/MoS2 hybrids. (e) LSV plots in H2SO4 with a concentration of 0.5 M. (d) Contrast of overpotentials. Reproduced with permission from ref 302. Copyright 2018 John Wiley and Sons.

structures were unstable and poorly conductive. GDY can be used to make hybrid layered composite with WS2 for good conductive ability and stability. The lower Fermi level of GDY than that of WS2 facilitates effective electron transfer. The activity of GDY and WS2 hybrid layered composite is apparently competitive with other catalysts. This method opened a new way to prepare hybrid layered composite with interesting properties. Although large amounts of catalysts have been prepared for OER, the limited abundance and expensive price limited their application. GDY was also reported to be able to present as a catalyst support material of those OER catalysts.301 Take cobalt nanoparticles, for example; the interaction between the πconjugated networks and Co2+/Co of aryl and alkyne would benefit prevention of the aggregation of nanoparticles. The cobalt nanoparticles can be stored in the unique porous

excellent catalytic performance toward water oxidation reaction, which can be attributed to the special 2D structure of GDY and its superior electron-capture and -transfer abilities.298,299 The unique distributed holes in the GDY plane can also benefit the accommodation of selected ions to effectively transport through the GDY sheet. Thus, GDY can be used as an effective stabilizer and as a reducing agent with other layered materials like WS2 to form a 2D nanohybrid of layered structure that is applied as HER catalyst, which can be a candidate to replace the noble metal catalyst such as Pt.300 As a layered structure material, WS2 has been found to show efficient electrocatalysis activity in HER. Compared to the basal plane, the WS2 edge sites were reported as the main contribution part of electrocatalytic activity, so WS2 with monolayer configuration or even quantum dot type was thought to be the ideal HER catalyst. However, both of these 7769


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Figure 42. (a) SEM and (b) TEM results of the GDY/BiVO4 nanocomposites. (c) Illustration of experimental setup and operating principle of photoelectrochemical (PEC) devices with GDY/BiVO4 as photoanodes. (d) Current−voltage and (e) J−t curves of photoanodes based on BiVO4 and GDY/BiVO4, respectively. (f) Photoluminescence spectra of BiVO4 and GDY/BiVO4 films. (g) Hole injection yield testament of photoanodes based on BiVO4 and GDY/BiVO4. Reproduced with permission from ref 47. Copyright 2017 John Wiley and Sons.

structure of GDY for its strong adsorption energy. Besides, the outstanding electrical conductivity and chemical stability properties of GDY could also benefit it to serve as support materials for OER. NGDY can also be applied as efficient electrocatalysts for HER in acidic media while working together with MoS2.302,303 NGDY/MoS2 hybrids have been fabricated by growing MoS2 on NGDY nanosheets (Figure 41). The direct growth of MoS2 on NGDY nanosheets resulted in enhanced conductivity and convenient charge transfer between NGDY and MoS2, benefiting the HER efficiency. With instinct pores on the NGDY plane, the 3D porous NGDY/MoS2 hybrids exhibited many exposed active sites and large catalytically active surface area, which are good for the mass diffusion. Thus, the electrocatalyst based on NGDY/MoS2 hybrids showed outstanding electrocatalytic performance in acidic conditions for HER, such as large exchange current density, low overpotential, and great competitive stability. 4.1.3. Photoelectrochemical Water Splitting. The composite GDY/BiVO4 photoanode was fabricated by direct growth of GDY nanowalls on the surface of a BiVO4 electrode, showing good performance of photoelectrochemical water splitting (Figure 42).47 Although BiVO4 was thought to be one of the most prospective photoanode materials, it still has the disadvantages of high recombination rate of hole and electron, as well as poor oxidation kinetics for water. Many attempts have been made to establish practical ways to solve these issues, but they remain a challenge. In constract to the BiVO4, GDY/ BiVO4-based photoanode showed not only improved activity but also enhanced stability, for the reason that the GDY with controlled nanostructure can facilitate the extraction of the photogenerated holes, which can subsequently improve the efficiency of water oxidation. As metal-free catalyst for photocatalytic water splitting, the graphitic carbon nitride (g-C3N4) has been studied extensively. Nevertheless, the catalytic performance of g-C3N4 was restricted because of its poor mobility of photoinduced hole. Lu and coworkers recently reported a method to enhance the hole mobility of g-C3N4 by preparing GDY and g-C3N4 heterojunction (Figure 43).304 The GDY and g-C3N4 heterojunction exhibited superior photocatalytic performance. The photocurrent generated by the GDY and g-C3N4 heterojunction after deposition with Pt nanoparticles is the best among all the known g-C3N4-based photocatalysts. Even the photocurrent obtained

Figure 43. (a) Preparation process of the GDY and g-C3 N 4 heterojunction. (b) HRTEM images of the heterojunction of GDY and g-C3N4. (c) Photocurrent curve generated by the g-C3N4 and the heterojunction. Reproduced with permission from ref 304. Copyright 2018 John Wiley and Sons.

by the GDY and g-C3N4 heterojunction without Pt nanoparticles is three times as large as that of g-C3N4. Those improvements were ascribed to the photoinduced holes injected from g-C3N4 into GDY. With high hole mobility, GDY can facilitate the transfer of the photoinduced holes. A new GDY-based electrocatalyst system named FeCH@ GDY/NF was obtained by selecting iron carbonate hydroxide (FeCH) nanosheets prepared on nickel foam (NF) as the substrate to grow GDY (Figure 44a).305 As shown in parts b and c of Figure 44, the FeCH@GDY showed a crystalline lattice with a smaller spacing of 0.242 nm than that of FeCH. This was ascribed to the strong interaction between FeCH and GDY. The electrochemical experiment results (Figure 44d−f) indicated that the FeCH@GDY/NF catalyst could be applied efficiently for water splitting reaction, which is for the high electrochemical surface area, facilitated kinetics, and good stability of the asprepared catalyst system.306 4.1.4. Other Catalysis. Owing to the highly conjugated structure with low work function, GDY can be utilized as the stabilizer and reductant for facially preparing surfactant-free Pd 7770


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Figure 44. (a) Photographs of NF, FeCH/NF, and the as-prepared FeCH@GDY/NF (from top to bottom). (b, c) HRTEM images of as-prepared FeCH@GDY nanosheets. (d) CV curves of different catalyst electrodes toward OER in aqueous solution of 1.0 M KOH. (e) CV curves for different catalyst samples in the system of two electrodes. (f) Current density of as-prepared FeCH@GDY/NF versus reaction time. Reproduced with permission from ref 305. Copyright 2018 American Chemical Society.

nanoparticles of well-dispersed state.307,308 Furthermore, GDY oxide (GDYO) is an excellent substrate on which the Pd clusters can be deposited to form Pd/GDYO nanocomposites exhibiting high catalytic performance for 4-nitrophenol (4-NP) reduction, presumably arising from synergetic effects of the components of the nanocomposite (Figure 45).307 Benefiting from the unique

electronic structure and excellent atomic arrangement, GDY can be used to load Pd nanoparticles by a redox reaction with GDY and PdCl42− as reductants and oxidants, respectively. The asprepared composite catalyst system, which was named Pd/ GDYO, can be used as an efficient catalyst to reduce 4-nitrogen phenol in the presence of mediated sodium borohydride (NaBH4). A rate constant of 0.322 min−1 was obtained for the reaction of reduction, indicating that the catalyst activity of Pd/ GDYO was much higher than that of other well-known catalyst systems such as Pd/C, Pd/graphene oxide (GO), etc. This can be ascribed to the good distribution of small Pd clusters in the pores of GDYO. As shown in Figure 46, the adsorption behavior of noble metals on GDY was systematically investigated by Yang and coworkers through the method of density functional calculations corrected by dispersion (DFT-D).309 It was observed that the noble metals such as Pd, Pt, Rh, and Ir interacted strongly with the GDY substrate. The noble metals are inclined to be adsorbed in the 18C-hexagons distributed in the molecular plane of GDY. Notably, the number concentration of embedded noble metals on the GDY grew with the mobility barrier energy increasing upon increasing the embedded adsorption energy.310 This result implies the outstanding property of GDY to support noble metals for efficient catalyst. In recent years, for the important role of solving the CO emissions, which resulted in serious environmental issues, and improving the endurance of catalyst in the fuel cells, there has been tremendous interest in the oxidation of CO. However, as the most commonly used nanocatalysts, noble metals suffer from the drawbacks of high cost and critical reaction conditions such as high temperatures. The previous studies have indicated that the reaction barrier of CO oxidation process has to be controlled at the level of 1000 mAh g−1 while the current density was 0.1 A/ g. Notably, all the substituted nitrogen in triazine-GDY, PMGDY, and PY-GDY is in pyridinic nitrogen form, which proved to be beneficial for improving the capacity of the electrode though both theoretical analysis and experimental measurement in that work. Chlorine atoms improve the performance of GDY in LIBs. With the chlorine atoms that are homogeneously distributed in the two-dimensional molecular plane, the chlorine atoms would interact with the Li atoms anchored in the carbon network and provide more sites for storage. Recently, a well-defined 2D carbon-rich material named chlorine-substituted GDY (ClGDY) was prepared by the strategy of bottom-up (Figure 53). 7776


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Figure 56. (a) Schematic illustration of the preparation process of graphtetrayne (GTY) with expanded molecular pores. (b) Rate performance and (c) 200 cycling of lithium-ion batteries tested under a current density of 187 mA/g with as-prepared GTY film as electrode material. Reproduced with permission from ref 339. Copyright 2018 Elsevier.

Figure 57. (a) J−V curves and (b) incident photon conversion efficiency (IPCE) spectra of P3HT/GDY- and P3HT-based PSCs. (c) Schematic interaction of P3HT and GDY. (d) Energy levels of PSCs with P3HT/GDY. Reproduced with permission from ref 340. Copyright 2015 John Wiley and Sons.

Moreover, GDY materials with boron heteroatoms even distributed on the molecular plane were also prepared and used as electrode in SIBs. As displayed in Figure 55a, boronsubstituted GDYs (BGDYs) were prepared through coupling the corresponding alkynyl-containing precursors following the bottom-up strategy.338 The DFT calculated results indicated that the sodium atoms can be stabilized by a synergistic effect of alkynyl linkage and boron heteroatoms. Particularly, position 1, which is located in the corner surrounded by the alkynyl groups and boron atoms, showed stable binding energy (Figure 55b.) As a result, an excellent electrochemical performance of electrode based on BGDY in sodium-ion batteries (Figure 55c and d) was obtained, which could be ascribed to the improved storage sites and enlarged transfer channels in BGDY-based electrode.

Different from the halogen atoms, hydrogen atoms have the smallest atomic radius and the most modest electronegativity among all the dopant atoms to the carbon materials. On the basis of this, the inclusion of the hydrogen might increase the storage sites in some carbon material systems for the affinity of alkali metals such as lithium and sodium to the hydrogen. Recently, Li, Huang, and co-workers introduced an aromatic hydrogen unit into the benzene of GDY (Figure 54a) to stabilize the chemical structure, enlarge the diffusion path of ions, and generate binding sites that are much more active for both lithium and sodium storage (Figure 54b).181 Notably, this new free-standing anode material can work as an outstanding transparent bendable electrode (Figure 54c) and exhibited good electrochemical performance in both sodium- (Figure 54d) and lithium-ion batteries (Figure 54e). 7777


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Figure 58. (a) Device structure of PSC with GDY as the interfacial layer. (b) J−V characteristic results with phenyl-C61-butyric acid methyl ester (PCBM) and the composite of PCBM and GDY. The inset graph shows the external quantum efficiency (EQE) spectra. (c) Photocurrent density (black) and steady-state efficiency (blue) of the PCBM:GDY-based PSC device. Reproduced with permission from ref 345. Copyright 2015 American Chemical Society.

cell performance. Indeed, hole extraction was accelerated in the P3HT/GDY hole-transport material (HTM) compared with that in pristine P3HT. Furthermore, the aggregation of GDY in the films of P3HT resulted in good scattering. The lower transmittance contributed by the scattering distinctly increased the harvesting of long-wavelength light in the corresponding perovskite solar cells. Compared to the PSCs based on pristine P3HT, a higher photoelectric conversion efficiency as 14.58% has been realized by P3HT/GDY HTM. Besides P3HT, another hole-transport polyelectrolyte with similar structure named P3CT-K is also used as a dopant for GDY in perovskite devices with MAPbI3 as the active layer. An enhanced conversion efficiency of 19.5% is achieved, which is mainly ascribed to the improved Jsc and fill factor (FF).344 As a dopant, GDY can not only increase the charge-extraction ability, electron conductivity, and electron mobility in the electron-transfer layer (ETL) but also improve the contact situation between the ETL layer and the perovskite layer, a vital aspect for data repeatability.345,346 The PCBM layer doped with GDY in a PSC with an inverted device configuration could improve the electron transport. The GDY might also be helpful for the passivation of the grain boundary, then efficiently avoiding the recombination through reducing the interface trap states. An optimized power conversion of 14.8% with small hysteresis has been observed for PCBM:GDY ETL-based PSCs with planar heterojunction structures (Figure 58).345 On the basis of the previous report, the interfacial layer is a key issue to affect the stability and performance of the PSCs. Recently, Liao, Li, Wang, and co-workers fabricated a new electron-transport layer (ETL) containing GDY and fullerene derivative (PCBSD), which could stack well in the face-on orientation according to the 2D grazing incidence X-ray diffraction (2D GIXRD) results.347 This was because of the strong intermolecular interaction between the cross-linkable PCBSD and the conjugated GDY (Figure 59a). The thickness of

Another rational strategy to optimize the electrochemical performance of GDY-based electrode is to modify the skeleton of the GDY framework. For example, Li and co-workers successively synthesized carbon ene-yne and graphtetrayne (GTY).34,339 Particularly, it should be noted that the enlarged molecular pores on GTY (Figure 56) could efficiently facilitate the transfer of lithium ions and provide more storage sites for lithium with much more sp-hybridized carbon atoms and bigger pore size than GDY. As a result, the electrode based on GTY films showed excellent cycling stability and rate performance in LIBs. 4.3. Solar Cells

In the last few decades, the rapid development of solar cells provides an important solution for the worldwide energy shortage. Researchers all over the world are trying to develop an efficient, low-cost photovoltaic system. The highly conjugated two-dimensional structure, wide interlayer distance, and tunable electronic band gaps endow GDY with extraordinary physical properties. GDY-based photovoltaic devices including perovskite, dye-sensitized, and quantum dots have exhibited excellent performance. 4.3.1. Perovskite Solar Cells. The incorporation of GDY into polymer solar cells could effectively improve the power conversion efficiency (PCE) and short-circuit current (Jsc). This improvement can be ascribed to the face that GDY exhibits high charge-transport capability and can form an efficient percolation pathway in the active layer.145,205,340−343 Compared with the solar cells device without GDY, the Jsc and PCE values of the device containing 2.5 wt % GDY were increased by 2.4 mA cm−2 and 56%, respectively. Subsequently, GDY can be doped into the hole-transporting material (e.g., poly(3-hexylthiophene) (P3HT)) layer of perovskite solar cells (PSCs) (Figure 57).340 The strong π-stacking interaction of P3HT and the GDY nanoparticles layer favored hole transport and thus improved the 7778


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Figure 59. (a) Chemical structure of GDY and fullerene derivative (PCBSD) and the schematic illustration for the packing mode of interfacial layer containing GDY and PSBSD. (b) Device structure of the photovoltaic device. (c) SEM image of photovoltaic devices in the cross-sectional direction. (d) Device performance of PSCs with the complex of PCBSD and GDY as electron-transfer layer. (e) Stability of the devices. Reproduced with permission from ref 347. Copyright 2018 Elsevier.

Figure 60. (a) Device structure of the photovoltaic device with dual doping GDY as interfacial layer. (b) Illustration for the process of electron transfer in the devices with different ETLs. (c) Device performance of the photovoltaic device with dual doping GDY as the interfacial layer. Comparison of (d) photoluminescence (PL) quenching and (e) impedance value of perovskite active layer coved with different ETLs. (f) Stability of the PSCs. Reproduced with permission from ref 348. Copyright 2018 Elsevier.

accelerated the electron transfer from the active layer to the contact electrode. Li, Jiu, and co-workers reported an inspiring dual doping method to fabricate a GDY-containing electron-transport layer with a cascade structure (Figure 60a).348 The good dispersion of GDY in organic solvent such as chlorobenzene fully guarantees the subsequent doping of PCBM or ZnO to GDY, which is beneficial to the electron transfer though each layer of devices (Figure 60b). As a result, the as-prepared photovoltaic devices with MAPbI3 as active layer and GDY/ZnO and GDY/PCBM as cascade dual ETL showed a high PCE of 20.0% with reduced J− V hysteresis phenomenon (Figure 60c). The inclusion of GDY

the ETL containing the composites of GDY and PCBSD in typical perovskite devices (Figure 59b) was estimated to be 15 nm according to the cross-sectional SEM images (Figure 59c). The best photovoltaic device showed a PCE of 20.19%, together with a Voc of 1.11 V, a FF of 78%, and a Jsc of 23.30 mA/cm2 (Figure 59d). Moreover, compared to the reference devices with TiO2 as ETL, an enhanced stability of GDY-containing device was observed, which can be assigned to the solvent resistance function of the planar structure of GDY and cross-linked fullerene derivatives. The excellent device performance was ascribed to the inclusion of GDY in the ETL, which efficiently 7779


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Figure 61. (a) Schematic illustration of GDY quantum dots (QDs). (b) Device structure of PSCs. (c) Comparison of champion and (d) average performance of optimized GDY QDs compressive doped devices and those of reference devices. Reproduced with permission from ref 349. Copyright 2018 John Wiley and Sons.

in the dual ETL can efficiently avoid the electron accumulation and reduce the transport resistance, which can be well-verified by the photoluminescence quenching (Figure 60d) and smaller impedance (Figure 60e) of GDY-containing devices. Moreover, the stability of the GDY-containing devices was also significantly improved (Figure 60f). This work indicates that the dual GDYdoped electron-transport layer could build a cascade layered device configuration for the PSCs, which can efficiently improve the device performance. On the basis of the above discussion, the performance of the perovskite photovoltaic devices can be improved through doping GDY in either hole-transport layer (HTL) or ETL. In a recent report, GDY quantum dots (QDs) (Figure 61a) were used as a comprehensive dopant for both the interfacial layer and the active layer, which showed stacked-up improvement for the performance of PSCs with CH3NH3PbI3 as active layer and TiO2 and Spiro-OMeTAD as interfacial layer (Figure 61b).349 Shown as Figure 61c and d, the measured distribution of PCEs of GDY-doped devices are in a range of 17.17−19.89%, which are much higher values than those of reference devices, indicating that doping GDY QDs is an efficient method to optimize the device performance of perovskite photovoltaic devices. 4.3.2. Dye-Sensitized Solar Cells. The chief hindrance to the application of dye-sensitized solar cells (DSSCs) is the limited natural reserves of the precious metal Pt. As a transition metal, Pt has a 3d unoccupied orbital; GDY displays n-type semiconductor behavior. Thereby, a unique structure as “p−n” junction may be formed by Pt and GDY (Figure 62).350 For the reaction of redox pairs I3−/I−, the catalytic activity is enhanced due to the improved charge transfer from GDY to Pt nanoparticles, which is caused by the interactions via the dz2 orbital of Pt and the pz orbital of C. Through both DFT calculations and experiments, as counter electrodes in DSSCs, it

Figure 62. (a) Mayer bond-order analysis and maps of localized orbital locator (LOL); (b) electrostatic potential (ESP) surfaces for the Pt2− GDY. (c) Cyclic voltammetry characteristics and (d) DSSCs’ J−V curves with various counter electrodes. Reproduced with permission from ref 350. Copyright 2015 John Wiley and Sons.

was confirmed by Yu and co-workers that the performance of GDY/Pt nanoparticle composites was not only the same as that of Pt foil but also superior to that of rGO/Pt nanoparticle composites as well as Pt nanoparticles. Furthermore, in addition to Pt, GDY can also interact with other noble metals (e.g., Pd, Rh, and Ir) to form composites for use as high-activity catalysts or sensors for gas molecules.146 7780


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Figure 63. (a) Diagrammatic sketch of the photocathode comprising CdSe QDs/GDY. (b) Open-circuit voltage and (c) controlled potential electrolysis of the composite photocathode comprising CdSe QD and GDY. Reproduced with permission from ref 351. Copyright 2016 American Chemical Society.

Figure 64. (a) Graphical representation of the PbS colloidal QDs solar cells using GDY as buffer layer. (b) SEM micrograph of the solar cell device cross section. (c) J−V characteristics and (d) corresponding EQE plots of the solar cells. Reproduced with permission from ref 352. Copyright 2016 John Wiley and Sons.

4.3.3. Quantum Dots Solar Cells. In addition, because the theoretical calculated hole mobility value of GDY is estimated to be ∼104 cm2 V−1 s−1, GDY can be applied directly as a holetransfer material. As illustrated with the schematic diagram in Figure 63a, the CdSe QDs with 4-mercaptopyridine surface functionalization were adhered to the GDY film surface by the π−π interaction.351 Because the potential value of GDY’s valence band is more negative than that of the of CdSe QDs, it can be demonstrated by the test of open-circuit potential (Figure 63b) that the photoinduced holes could be smoothly injected to GDY from CdSe QDs. Exhibiting utilities such as reducing the charge recombination, improving the conductivity, and decreasing the photocathode’s resistance, GDY can be applied as a promising hole-transport material for the photocathode. For hydrogen production, the GDY-based photocathodes showed

high faradaic efficiency and moderate photoactivity within 12 h.351 The colloidal quantum dot-sensitized solar cells (CQDSCs) using GDY as buffer layer of anode were reported to display long-term and good stability (Figure 64).352 Compared with the CQDSC device without GDY, the PCE was remarkably improved to 10.64% from the initial 9.49%. It was possible that, because the optimized interface between the Au anode and the PbS/EDT active layer was formed, the carrier lifetime was enhanced and the carrier recombination was suppressed inside the devices by GDY. 4.4. Electronics, Thermoelectrics, and Magnetism Devices

Different from the well-known carbon materials consisting of sp2- or sp3-hybridized carbon, the existence of sp-hybridized carbon and highly conjugated two-dimensional plane structure 7781


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Figure 65. (a) Illustration and (b) photograph of the GDY field-effect transistor. (c) Ids−Vds curves and (d) transfer characteristic curve of the GDY field-effect transistor device. The source−drain voltage Vds = 20 V. Reproduced with permission from ref 164. Copyright 2015 Springer Nature.

Figure 66. (a) Structure of GDY and conjugated polymer o-fluoro-p-alkoxy-phenyl-substituted benzo[1,2-b:4,5-b′]dithiophene (PFC). (b) Photograph of GDY and the GDY/PFC mixture in chlorobenzene solution. (c) Schematic illustration of FET device and the energy levels. (d) Comparison of the measured ISD−Vg curves for GDY/PFC and PFC films. The inset show the photograph of photoetching devices. Reproduced with permission from ref 358. Copyright 2017 American Chemical Society.

of GDY exhibits unique and excellent performance in the field of electronics, thermoelectrics, and magnetism devices. 4.4.1. Electronic Devices. It was demonstrated by both experimental investigation and theoretical studies that, as a novel carbon allotrope, GDY is a semiconductor. Using the Boltzmann transport equation based relaxation time approximation, as well as DFT calculations, the carrier mobility of GDY-NRs and GDY sheets have been studied through longitudinal acoustic phonon scattering.17,56,120,353−356 The hole and electron mobilities of GDYs with a single layer were theoretically predicted to be around 2 × 104 and 2 × 105 cm2 V−1 s−1 at 300 K, respectively.17 The electron mobility of GDY-NRs at room temperature is apparently larger than its hole mobility and can reach the order of magnitudes in 105 cm2 V−1 s−1. It was

also shown that the increase of the GDY-NR width can result in its charge mobility increase. Besides, the mobility of GDY-NRs with armchair edge is larger than that of GDY-NRs with zigzag edge. Similar results regarding the mobility of GDY-NRs also have been obtained with deformation potential theory.45 The charge carrier mobility of GDY has been evaluated from fabricating transistor devices based on GDY-NWs and GDY films (Figure 65). Measured with the GDY thin-film transistors of bottom gate, the average mobilities of GDY-NWs and GDY films are about 7.1 × 102 and 1 × 102 cm2 V−1 s−1, respectively. The difference between these two mobility values might be attributed to their morphology differences.164,202 So far these practical values of the mobility are much smaller than the theoretic results of single layer GDY, some optimization 7782


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Figure 67. (a) Illustration of GDY nanowalls preparation. (b) SEM images displaying the growth process of GDY nanowalls. (c) Electron emission properties of GDY nanowalls and random structure. (d) Fowler−Nordheim (F−N) plots and their fitting line. Reproduced with permission from ref 165. Copyright 2015 American Chemical Society.

methods were proposed to increase those, e.g., improve the quality of GDY by reducing its defects, or screen the better contact metal and channel length for the devices.357 Huang, Long, Zhang, and co-workers fabricated a FET device based on GDY and conjugated polymers (Figure 66a), which can be well-dispersed or soluble in organic solvent such as chlorobenzene (Figure 66b).358 The device performance of asprepared FET devices (Figure 66c and d) indicated that doping GDY to the conjugated polymers could efficiently improve both the on/off ratio and the threshold voltage of the fabricated devices, indicating the application potential of GDY in the field of organic electronics. GDY nanowalls are expected to show outstanding fieldemission performance (Figure 67).165 The threshold field (Ethr) is 10.7 V μm−1, while the turn-on field (Eto) is ∼6.6 V μm−1. The curve is in accordance with the Fowler−Nordheim mechanism, which indicates that the electron emission of the as-prepared samples is originated from electron tunneling. Sheets of GDY nanowalls having a regular structure can endure high currents because of the conjugated structure. These results indicate that GDY should be a suitable candidate for use in all-carbon electronic devices. 4.4.2. Thermoelectric Materials. GDY can be utilized as a thermoelectric material to convert electricity into heat or heat into electricity directly for its high conductivity and thermal resistance.359,360 The ideal model for thermoelectric material was proposed to work like an electron crystal and phonon glass with a ZT value > 3.0. Combining MD and first-principles simulations together with Boltzmann theory, Liu and co-workers demonstrated that the improved ZT value of 3.0 for p-type holes and 4.8 for the n-type electrons can be obtained for GDY at room temperature (Figure 68),132 and these are higher than most of the current experimental results, indicating GDY is an achievable candidate material for high-powered thermoelectric devices. 4.4.3. Electrochemical Actuators. Electrochemical actuators, which are important components of artificial intelligence,

Figure 68. (a) Conductivity σ, (b) Seebeck coefficient S, (c) ZT (dimensionless figure of merit) value, and (d) power factor S2σ of GDY plotted versus the carrier concentration n at room temperature. Reproduced with permission from ref 132. Copyright 2015 Elsevier.

are used to convert electrical energy to mechanical energy. So far, the energy transduction efficiency of the existing actuators is 50 °C within 300 s of photo irradiation, a much higher value than that of the control group. Moreover, the fluorescence confocal images also revealed that the doxorubicin drug can be released from the GDY delivery system in the acid environment (Figure 77c). For unique conjugated structure comprising not only the benzene rings but also acetylenic moieties, GDY is also expected to have strong free radioprotection ability. Most recently, a composite system containing GDY nanoparticles and bovine serum albumin (BSA) was used for the radiation protection in in vivo models including cell and animal (Figure 78a).382 As shown in Figure 78b and c, the GDY nanoparticles modified by BSA

Figure 72. (a) Fluorescence spectra curves of H1N1 ssDNA labeled with FAM GDY nanosheets. (b) Fluorescence intensity of ssDNA and dsDNA probes with and without GDY. (c) Schematic illustration for the detection principle of DNA through GDY-based multiplex. Reproduced with permission from ref 370. Copyright 2017 John Wiley and Sons.

Figure 73. (a) Schematic illustration of the preparation route of nanocomposites composing GDY and ZnO. (b) Device structure of photodetectors (PDs) based on GDY:ZnO. (c) Rise and decay times of PD with different active layers. (d) Illustration of energy levels of the as-prepared ZnO NPs and GDY NPs. The transfer direction of hole from ZnO NP to GDY NP is also presented by the red arrows. Reproduced with permission from ref 374. Copyright 2016 John Wiley and Sons. 7786


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Figure 74. (a) Schematic image, (b) SEM image, and (c) IR mapping of the GDY and SWNTs-based IR detector. (d) I−V response plots under light or without light. (e) Frequency-dependent detectivity and responsivity. (f) Photocurrent of the detector. Reproduced with permission from ref 376. Copyright 2017 John Wiley and Sons.

Figure 75. (a) Illustration of the chemical structures of oxides of GDY (GDO) and graphene (GO). The inset graphs show the Tyndall effect of GDO and GO dispersed in aqueous solution. (b) I−V curves recorded at modified Au interdigital electrodes. Note that the blue, red, black, and cyan curves represent the GO-modified and GDO-modified electrodes under 44% relative humidity (RH) and dry N2, respectively. (c) Change of current versus time for the sensors based on GDO and GO under dry N2 and various RH levels in a range of 23%−95%. (d) Normalized response of the sensors based on GDO and GO (the thickness of the active material is 100 nm) toward the humid air flow. (e) Response signal of the sensor base on GDO toward different human respiratory rhythms. (f) Respiratory measurements for an anesthetized rat. (g) Current and RH distribution versus the distance of the fingertip from the GDO-based sensor. Reproduced with permission from ref 377. Copyright 2018 John Wiley and Sons.

showed good O2−· and ·OH scavenging activity in both enzymatic and nonenzymatic systems. Moreover, both the in vitro and in vivo studies indicated that the existence of GDYBSA nanoparticles could efficiently decrease the damage of cells under radiation and improve its viability, indicating that the GDY is a new material candidate to protect normal tissues in the radiation therapy.

Consequently, it can be expected that the membrane with only one-atom thick could be a prospective candidate to be applied for gas separation. Particularly, GDY and its derivatives have not only high surface areas but also unique pores distributed uniformly in their 2D plane, showing great promise for the fabrication of molecular sieve membranes, which would meet various separation requirements and objectives.1 Compared to the other materials, the great superiorities of GDYs for gas separation lie in the unique, mechanically stable, and chemically inert structures, which provide the platforms for separating gas selectively through homogeneous material systems under ambient pressures. In other words, using GDYs for gas separation can avoid the cumbersome process of

4.7. Gas Separation and Capture

Until now, lots of membranes such as polymer, silica, metallic, zeolite, and carbon-based membranes have been widely used in hydrogen separation. Usually, the permeance of the used membranes is inversely proportional to their thickness. 7787


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Figure 76. (a) Schematic illustration of the preparation route for the GDY nanosheets with pegylation (GDY-PEG). (b) Photos of mice at different stages of treatment. (c) Changing trend of relative tumor volume with treatment time. (d) Survival percentage of mice after being treated under different conditions. Reproduced with permission from ref 380. Copyright 2017 American Chemical Society.

Figure 77. (a) Schematic illustration of application of GDY nanosheets GDY for drug delivery in the combined cancer treatment of photothermal and chemotherapy. (b) Photothermal treatment effect comparison of GDY/doxorubicin system irradiated under the laser. Note that the wavelength is 808 nm. (c) Images of fluorescence confocal investigation for the changing trend of MDA-MB-231 cells with time after being treated with GDY/ doxorubicin. Reproduced with permission from ref 381. Copyright 2018 American Chemical Society.

notably affect the separation of H2 over other gases such as N2, CO, CO2, and CH4.384−386 There are two other approaches that can efficiently improve the purification of H2 by GDY. One is to dope nitrogen on GDY framework, and the other is to introduce positive charges to the purification system. It was observed that the configuration of transition state would be changed after inclusion of the nitrogen heteroatom or positive charge, which would further alter the energy barrier for the passing of H2 and other gases (Figure 80).387,388 For example, for H2 to traverse the pores of nitrogendoped GDY, the energy barrier is 0.08 eV, lower than that of GDY. The associated diffusion energy barrier for CO is 0.73 eV, and that for CH4 is 0.38 eV. The opposite changing trend for the diffusion barrier for H2 and CO/CH4 in nitrogen-doped system results in the enhancement of purification capability of the GDY system. Moreover, a similar phenomenon is also found in the GDY injected with positive charges. Helium (He) is an irreplaceable natural resource because it is in growing need in many scientific and industrial applications. It was found that the VdW’s diameter of He atom is 2.6 Å, which is

chemical modifications and functionalization, or of introducing extensive molecular pores. According to its van der Waals (VdW) surface, which is smaller than the VdW’s dimension of CH4/CO but larger than that of H2, the side length of GDY’s triangle pore is ∼3.8 Å.128 As a result, the energy barrier for H2 to traverse the molecular pores of GDY was calculated to be 0.10 eV, while that for CH4 was 0.72 eV and that for CO was 0.33 eV. As a result, the selectivity of H2 to CH4 is ∼1010 while that of H2 to CO is ∼103 at room temperature. Thus, it is possible to use GDY as a separation membrane to extract H2 from a synthesis mixture gas containing CH4, H2, and CO (Figure 79). Cranford and Buehler predicted that,19 through MD simulations at temperatures in a range of 300−500 K, to prevent the passing of CH4 and CO molecules in the synthesis gas, the mass flux when molecules of H2 pass through a membrane of GDY would be ∼7−10 g cm−2 s−1. Moreover, the additional marginal applied force can effectively improve the separation efficiency of H2.383 Besides that, it can be observed that the size and shape of the pores in GDY can also 7788


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Figure 78. (a) Schematic illustration of application of GDY nanoparticles for the radiation protection during the radiotherapy for cancer. Superoxide radical scavenging by GDY-bovine serum albumin nanoparticle through nonenzymatic (b) and enzymatic (c) methods. Reproduced with permission from ref 382. Copyright 2018 American Chemical Society.

Figure 79. (a) Illustration of the required energy barrier for hydrogen gas to pass through GDY molecular planes according to the Arrhenius relation. (b) Snapshot of the simulation on 30 ps at a temperature of 500 K with 30 H2 molecules. (c) Decrease of the required energy for passing through the GDY membrane by extra force. (d) Snapshot of the simulation that CH4, CO, and H2 pass through the GDY membrane. Reproduced with permission from ref 19. Copyright 2012 Royal Society of Chemistry. When diffusing through the pores of GDY, the gas molecules related (e) diffusion rate coefficient and the (f) selectivity. Reproduced with permission from ref 128. Copyright 2011 Royal Society of Chemistry.

the gases of chlorine (Cl2), hydrogen chloride (HCl), hydrocyanic acid (HCN), cyan chloride (CNCl), sulfur dioxide (SO2), hydrogen sulfide (H2S), ammonia (NH3), and formaldehyde (CH2O), with high permeance and selectivity (Figure 81).391 Especially for O2 to pass through the molecular pores of GDY, the diffusion barrier was calculated to be 0.21 eV, which is the smallest value among all the investigated gases. This result intuitively revealed that O2 molecule can be separated efficiently though GDY membrane from the above-mentioned mixed gas. Gas capture and sequestration are very interesting technologies for wide applications such as gas storage and environmental gas issues. GDY has been regarded as one of the most promising candidates that can be applied for carbon dioxide separation and absorption (Figure 82).392 The uniformly distributed pores composed by the alkyne ligands and benzene ring on the GDY plane facilitate the transportation

smaller than the inner triangle pores side length (3.8 Å) distributed in GDY molecular plane. The quantum dynamical simulations results have also revealed that the penetration barrier for He atom to pass through the molecular pores of GDY is ∼0.033 eV, which is quite a lower value than that for CH4.389 On the basis of this, GDY membrane exhibited the highest theoretical selectivity among the so far known materials for the separation of He and CH4 in a very wide temperature range and showed great potential for the extraction of He from natural gas. Moreover, GDY is also a promising candidate for the separation of isotopic He at ambient temperature in spite of the tiny differences between 3He and 4He in the tunneling probabilities.389,390 Via detailed first-principles calculations, GDY was also predicted to be an ideal membrane for the efficient separation of oxygen gas (O2) from other mixed gas molecules, including 7789


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Figure 80. (a) Top views of electron density differences and framework distortions of H2, CH4, and CO in their corresponding transition states in NGDY. Reproduced with permission from ref 387. Copyright 2015 Royal Society of Chemistry. (b) Total charge density distribution of neutral and oneelectron positively charged GDY. (c) Interaction energies between gas molecules and neutral (dash) and one-electron positively charged (solid) GDY, plotted concerning the adsorption height. Reproduced with permission from ref 388. Copyright 2016 Taylor & Francis.

Figure 81. (a) Schematic illustration for the O2 passing through the molecular plane of GDY. Note that the energy of the initial state is set to be 0 eV; the carbon and oxygen atoms are represented by green balls and red balls, respectively. (b) Electron densities structure for a series of gas molecules penetrating through the GDY membrane. (c) Selectivity and (d) permeance changes with the rising of temperature for different gas molecules passing through GDY. Reproduced with permission from ref 391. Copyright 2016 American Chemical Society.

of gas molecules. After being functionalized by various groups like −COOH, −OH, −F, etc., and combined with the doping of lithium, GDY was revealed to exhibit an unexpected synergistic effect for extremely high CO2 uptake, efficient CO2 capture, and superior selectivity for CO2 over CH4.

growing interest. GDY has also showed great capacity for water purification, which is beneficial from its porous structure, tunable surface energy, and super hydrophobic characteristic. Lin and Buehler,395 Guo and co-workers,396 and Fan and coworkers397 have demonstrated that the monovalent salt ions, hydrophobic organic chemicals, and divalent heavy metal ions from water can be effectively separated by GYs, suggesting the promising application potential of GYs as water-purification

4.8. Water Purification

With the rapid exhaustion of many water resources, the desalination of brackish water and seawater has been paid 7790


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There are two key attractions for using GDY as a molecular filter for water purification. One is the abundant nanopores in the GDY structure, which can be regarded as not only the welldefined infiltration for the water but also the ideal blockage for the salt ions infiltration and ideal salt ion blockage. Notably, these nanopores are naturally occurring in the process of preparation and need no additional process such as postcreating or chemical functionalization. The other advantage of GDY is its great potential for future practical use, because it is possible to prepare the GDY membranes with a large area.393−401 Moreover, according to the theoretical simulation results, GYs-4, which is endowed with four acetylene bonds as linkage for the phenyl rings, has the most suitable pore size for the separation of the water molecule and salts and should provide the best performance. This prediction was verified by the calculated results that the salt rejection rate of GYs-4 membrane could be predicted to be 100%. In addition, a very high water permeability of 13 L cm−2 day−1 MPa−1 was determined. It should be noted that this value is ∼3 orders of magnitude higher than that of current commercial reverse osmosis membranes and ∼10 times that of the most advanced nanoporous graphene. A smart design for preparing ordered superior superhydrophobicity GDY nanostructures by utilizing copper foam as catalyst and 3D substrate has been reported (Figure 84).172 Those GDY samples were also obtained via Glaser−Hay crosscoupling reaction. A mixture of CH2Cl2/H2O can be wellseparated by the superhydrophobic GDY foam specially coated with a poly(dimethylsiloxane) (PDMS). With a large contact angle in air of 160.1° and in oil of 171.0°, such GDY foam

Figure 82. Schematic diagram of the GDY being applied for gas capture and separation. Reproduced with permission from ref 392. Copyright 2017 American Chemical Society.

systems in the future. The molecular size of water (ca. 3 Å) is smaller than the pore size of GDY. Thus, GDY semipermeable membranes containing continuous channels should pass a greater volume of water (than GY membranes) at a given pressure while blocking the passage of ions (Figure 83).393,394 A low water penetration barrier (ca. 70 times the known weight of GDYMS (Figure 85e). More importantly, the fabrication procedure of GDYMS is simple, low-cost, and scalable, which shows the great application potential of this material in practical environmental settings.

5. CONCLUSION GYs, which feature both sp- and sp2-hybridized carbon atoms as a layered structure, containing members like GY, GDY, etc., are a series of very interesting carbon allotropes. Since the first successful preparation of GDY, increasing numbers of researchers are focusing their attention on these new planar, layered materials. A direct natural band gap for GYs and a Dirac cones structure, which is considered to originate from overlap of 7792


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might be competitive with conventional sp2-hybridized carbon in various potential applications to meet the increasing demand for new nanomaterials.

on the design and synthesis of photo- and electro-active organic− inorganic hybrid materials, and nanoscale and nanostructural materials.

ACKNOWLEDGMENTS This study was supported by the National Natural Science Foundation of China (21790050, 21790051, and 21771187), the Hundred Talents Program and Frontier Science Research Project (QYZDB-SSW-JSC052 and QYZDY-SSW-SLH015) of the Chinese Academy of Sciences, the Natural Science Foundation of Shandong Province (China) for Distinguished Young Scholars (JQ201610), and the National Key Research and Development Project of China (2016YFA0200104).

ASSOCIATED CONTENT Special Issue Paper

This paper is an additional review for Chem. Rev. 2018, volume 118, issue 3, “2D Materials Chemistry”.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

REFERENCES

Changshui Huang: 0000-0001-5169-0855 Yongjun Li: 0000-0003-1359-1260 Zicheng Zuo: 0000-0001-7002-9886 Huibiao Liu: 0000-0002-9017-6872 Yuliang Li: 0000-0001-5279-0399

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Notes

The authors declare no competing financial interest. Biographies Changshui Huang earned his doctorate in 2008 at Institute of Chemistry, the Chinese Academy of Sciences (ICCAS). Then he worked as a postdoc during the period of 2010−2014 in University of WisconsinMadison. Now he is a professor in Prof. Yuliang Li’s group, working at Qingdao Institute of Bioenergy and Bioprocess Technology (QIBEBT) and ICCAS. His current research includes the application of carbon-based nanomaterials for energy and catalysis. Yongjun Li earned his Master’s degree in 2001 at Sichuan University; then he received his doctorate in the major of organic chemistry from ICCAS in 2006. During 2006−2008 he worked as a postdoc researcher in Indiana University. Currently, he is a professor in Prof. Yuliang Li’s group, ICCAS. His current work covers the design, synthesis, and characterization of functional molecules. Ning Wang earned his doctorate from ICCAS in July 2007. From August 2007 to 2009, he worked as a postdoc in Carleton University at Ottawa, Canada. Currently, he is an associate professor in Prof. Yuliang Li’s group, working at QIBEBT. His research includes the synthesis and application of GDY-based carbon materials. Yurui Xue earned his doctorate in the major of Polymer Chemistry and Physics in 2014 from Jilin University. Currently, he is a postdoc under the supervision of Prof. Yuliang Li and Prof. Wensheng Yang, working in Prof. Yuliang Li’s group, ICCAS. His current research focuses on the preparation of carbon nanomaterials and its application as an electrochemical catalyst. Zicheng Zuo earned his doctorate from ICCAS in 2011. Currently, he works as an Associate Professor in Professor Yuliang Li’s group at ICCAS. His research includes the preparation and application of nanomaterials based on carbon and the corresponding application in energy-storage devices. Huibiao Liu earned his doctorate from the Nanjing University in 2001. Currently, he is a professor in Prof. Yuliang Li’s group, ICCAS. His research covers inorganic−organic hybrid nanomaterials. Yuliang Li, professor of ICCAS, has carried out research in the fields of design and synthesis of functional molecules, self-assembly methodologies of low-dimension and large-size molecular aggregation structures, chemistry of carbon and rich carbon, with particular focus 7793


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Chemical Reviews

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Progress in Research into 2D Graphdiyne-Based Materials

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