Carbon-carbon allotropic hybrids and composites: synthesis

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Carbon-carbon allotropic hybrids and composites: synthesis, properties, and applications. Oxana Vasilievna Kharissova, Boris Ildusovich Kharisov, and Cesar Maximo Oliva Gonzalez Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b05857 • Publication Date (Web): 14 Feb 2019 Downloaded from http://pubs.acs.org on February 14, 2019

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Industrial & Engineering Chemistry Research

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Carbon-carbon allotropic hybrids and composites: synthesis, properties, and applications. Oxana V. Kharissova, Boris I. Kharisov,* Cesar M. Oliva González. Universidad Autónoma de Nuevo León, Monterrey, Mexico. *Corresponding author. E-mail [email protected]. Abstract Carbon-carbon allotropic hybrids and their composites are reviewed. Carbon-carbon hybrids are mainly composed of graphene, graphite, graphene oxide, reduced graphene oxide, carbon nanotubes, carbyne chains, fullerenes and more complex spherical and ring-like carbon forms, graphene quantum dots and carbon nanodots. Aerogels and xerogels on their basis are also discussed. Most of these hybrides consist of 3D architectures with either covalent or van der Waals interactions between carbon atoms, possessing unusual properties as compared to their counterparts, due to the simultaneous presence of carbon structures of distinct dimensionality and reactivity. These composites can be prepared by a variety of methods starting from already existing allotropes, or by their synthesis in situ, such as chemical vapor deposition, solvothermal techniques, pyrolysis, ultrasonication in solution, and other liquidphase methods, frequently including redox steps. Current and potential applications of hybrids and their metal-doped composites as supercapacitors, sensors, catalysts, and environmental remediation reactants are described. Keywords: Carbon allotropes; hybrid nanostructures; graphene; carbon nanotubes; nanodiamonds; carbon nanodots. Index Introduction Graphite hybrids - Composites of graphite (graphite oxide) with carbon nanotubes - Other graphite-carbon composites Graphene and graphene oxide composites Graphene – carbon nanochain composites Graphene (graphene oxide) – carbon nanofiber composites Graphene-fullerene composites Graphene hybrids with carbon nanocages Graphene (graphene oxide)-nanodiamond composites Other graphene-carbon composites Carbon nanotube composites Graphene (G, GO, rGO) – carbon nanotube composites CNT hybrids with other nanocarbons Nanoballs and nanospheres Nanorings (nanotori) composites

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Composites of carbon nanodots (graphene quantum dots) Aerogels and xerogels on the basis of carbon-carbon composites Amorphous and glassy carbon composites Conclusions and further outlook Abbreviations AC = amorphous carbon ACNT = amorphous carbon nanotube CA = carbon aerogel CB = carbon black CNCs = carbon nanocages CCVD = catalytic chemical vapor deposition CDs = carbon nanodots CNFls = carbon nanoflowers CNFs = carbon nanofibers CNR = carbon nanoring CNSs = carbon nanoshperes CNTs = carbon nanotubes CNW = carbon nanochain webs CVD = chemical vapor deposition CX = carbon xerogel FLG = few-layer-graphene DFT = density functional theory G = graphene GNFs = graphitic nanofibers GNs = graphene nanosheets GO = graphene oxide Gr = graphite GrO = graphite oxide HOPG = pyrolytic graphite HSAC = high surface area activated carbon MLG = multilayer graphene MPC = microporous carbon MWCNTs = multi-wall carbon nanotubes NBs = nanobuds NCD = nanocrystalline diamond NDs = nanodiamonds NS-CD = n-type nitrogen, sulfur co-doped carbon dot RF = resorcinol–formaldehyde rGO = reduced graphene oxide rGrO = reduced graphite oxide SGFC = sandwiched graphene-fullerene composites SWCNTs = single-wall carbon nanotubes UGF = ultra-thin graphite film UHMWPE = ultrahigh molecular weight polyethylene


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Introduction Currently, carbon in its zero-valent state is one of the most investigated elements of the Periodic Table due to its unique ability to form C-C covalent bonds in different hybridizations (sp, sp2, sp3). This well-known fact causes a grand variety of carbon allotropes, both classic as diamond or graphite, and the relatively recent discovered nanostructures as graphene or carbon nanotubes. Carbon allotropes can be classified in different manners, in particular according to the hybridization of carbon atoms (containing mainly sp2 or sp2+sp3 with predominant sp3 hybridized carbons), or by morphological characteristics (with or without empty volumen, such as, for example, in nanocages or fullerenes), or by their dimensionality. Carbon allotropes have very distinct properties, ranging from 3D superhard diamond crystals containing tetrahedral sp3 carbon atoms, to soft graphite containing sp2 carbon atoms costructed from 2D graphene sheets united by van der Waals forces, including the 1D carbon nanotubes1 that possess outstanding mechanical, electrical and other properties, and the 0D fullerenes, nano-onions, nanodots or nanodiamonds, among other lesser-common carbon arquitectures which are already in use, or are predicted to exist. The synthesis and properties of carbon nanoallotropes have been intensively studied, resulting in numerous current and potential applications. At the same time, it is known that carbon allotropes can form their composites or hybrids due to the spontaneous formation of covalent or van der Waals bonds between carbon atoms, initially belonging to the precursors.2 3 4 5 6 7 8 A certain number of reports are dedicated to combinations of carbon nanostructurized allotropes, as well as graphite/carbon nanoallotrope hybrids, which can be prepared from already formed nanocarbons or in situ from their hydrocarbon precursors as a result of pyrolysis or solvothermal reactions. Thus, produced hybrids can possess unusual properties due to the simultaneous presence of carbon structures of distinct dimensionality and reactivity. For instance, the carbon nanobuds (hybrids of carbon nanotubes and fullerenes), where the counterparts can be connected by a variety of modes through different number of covalent bonds, contain less-reactive carbon atoms of CNTs and higher-reactive carbon atoms of a fullerene moiety, so final properties of nanobuds are not a sum of properties of CNTs and fullerenes. Such carbon-based composite nanostructures can have certain advantages derived from their low-dimensional precursor materials; their development is currently in progress in the context of creating highperfomance all carbon-based devices of low cost and non-toxic. In this review we discuss available carbon-carbon composites made of different carbon units, their synthesis methods, structures, properties, and applications, paying certain attention to less-common carbon nanostructures, such as nanoflowers or nanocages. The hybrids, constructed by the same carbon units, for instance fullerene aggregates on templates (HOPG, alumina, Au/Ni(110), Cu(110) or Au(111) surfaces), carbon clusters Cn, which are well described in an excellent recent review,9 are omitted here. Graphite hybrids Composites of graphite (or graphite oxide) with carbon nanotubes



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Graphite-CNT composites are common, known for both main types of CNTs and can be prepared by a plenty of methods, most frequently by chemical vapor deposition (CVD), among others. Thus, synthesis of SWCNTs/multi-graphene and SWCNTs/graphite composites on nickel foam was carried out by three different routes10 by CVD from acetylene gas, a carbon precursor (equation 1). Other synthesis methods are also available. For instance, graphite fiber composites with SWCNTs (this process could be scaled up after additional improving efforts and adaptation to the existing manufacturing process) were obtained by air-spraying SWCNTs onto the surface of epoxy/graphite.11 The resulting out-of-plane electrical conductivity was improved by 144% for 2 wt.% SWCNT samples, being compared to samples non-containing SWCNTs. The composite of treading CNTs and coated graphite was obtained as a result of the pyrolysis of CNT/polyaniline composites at 1500oC.12 A specific orientation relationship between graphene layers and the CNTs axis, having an angle of 110o between them, was observed. This synthesis method can be used for fabrication high performance carbon materials via the alignment of graphene layers. 3H-CC-H  6C + 3H2 (1) Several intriguing differences between GrO and rGrO behavior in carbon nanotubes were established.13 In particular, electrically insulating GrO, being present in a SWCNT network (Figure 1), considerably increases electrical conductivity; at the same time, rGrO, being even though electrically conductive, decreases electrical conductivity, testifying the “non-direct” role of the oxide groups. These groups, being present in GrO inside the SWCNT-GrO composite, work through electronic doping of metallic SWCNTs. The following main factors, limiting and controling electronic transport, were proposed: a) in SWCNT networks: poor coupling between SWCNTs vs. their high intrinsic conductivity; b) in rGrO networks: poor intrinsic conductivity within the sheet vs. good coupling between the sheets.

Figure 1. TEM images of the composite SWCNT-GrO: (a) Top view of the composite; the inset shows electron diffraction, (b) Side view of a bundle of SWCNTs; the inset shows electron diffraction. Reprinted from Skákalová, V.; Vretenár, V.; Kopera, L.; et al. Electronic Transport in Composites of Graphite Oxide with Carbon Nanotubes. Carbon, 2014, 72, 224. Copyright 2014. Elsevier Science.


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Graphite-CNT composites alone, as well as their more complex derivatives with metal oxides, are mainly applied for Li storage and as supercapacitors. In one of them, the graphiteCNT composite (Figure 2), the CNTs, being mixed with graphite, are dispersed evenly on the surface of graphite sheets.14 For a three-phase-containing hybrid (Figure 3), consisting of commercial graphite particles, CNTs, and graphene oxide sheets, prepared with aid of strong oxidants (KMnO4 and H2O2) and ultrasonication, it was revealed15 that, after formation of a CNTs composite with graphite, the GO conserves its typical wrinkled structure. The graphite particle and CNTs are covered by GO sheets, and the CNTs are randomly aligned, resulting the formation of a conductive bridge. This composite was found to have a reversible lithium storage capacity of 1172.5 mA h g-1 at 0.5C (1C = 372 mA g-1); this is a very high value, exceeding the capacity of the theoretical sum of all the three components. This hybrid can be used as material for anodes, possessing excellent electrochemical properties and characteristics.

Figure 2. Morphological images of graphite, CNT and their composite, (a) CNT as received, (b) CNT treated at 200°C in air, (c) CNT treated at 200°C in vacuum, (d) graphite, and (e and f) graphite–CNT composite with 5 wt.% CNT treated at 200°C in vacuum. Reprinted from Zhu, H.Q.; Zhang,

Figure 3. SEM images of (a) graphite, (b) CNT, (c) graphene oxide, and (e) GGCC composite. TEM images of (d) graphene oxide and (f) GGCC composite. Reproduced with permission of the Elsevier Science. Reprinted from Zhu, H.Q.; Zhang, Y.M.; Y.M.; Yue, L.; et al. Graphite–carbon Yue, L.; et al. Graphite–carbon nanotube nanotube composite electrodes for all composite electrodes for all vanadium redox vanadium redox flow battery. J. Power flow battery. J. Power Sources, 2008, 184, 637. Copyright 2008. Elsevier Science.

Sources, 2008, 184, 637. Copyright 2008. Elsevier Science.



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As an example of complex composites “graphite-CNTs-metal oxide”, the CVD and subsequent electrodeposition (Figure 4) were used to obtain 3D ultra-thin graphite film (UGF)/carbon nanotubes, which is covered by NiO nanosheets in an uniform manner (Figure 5).16 The formed intermediate product (UGF/CNTs) acted as spacers to stabilize the whole composite structure, and also served as substrates for NiO nanosheet. The composite material showed increased Li-storage properties as anode materials for batteries. A similar composite with multiple layers of graphite, CNT/GT/ZnO composite (carbon nanotube/graphite/zinc oxide), was employed as supercapacitor.17 Another application field is in the sensor area: the β-cyclodextrin−graphite oxide−carbon nanotube (βCD−GrO−CNT) composite was used18 to recognize three types of biomolecules (thioridazine, L-tyrosine, and dopamine) and presented an good sensor properties for supramolecular recognition, better than those for individual CNT and βCD−CNT hybrids.

Figure 4. Schematics of fabrication process for 3D nickel foam/UGF/CNTs/NiO composite: CVD growth of UGF on nickel foam (step 1), CVD growth of CNTs on nickel foam/UGF (step 2), and deposition and annealing treatment of nickel foam/UGF/CNTs/Ni(OH)2 precursor (step 3). Reprinted from W. Liu, W.; Lu, C.; Wang, X.; Lianga, K.; Tay, B. K. In situ fabrication of three-dimensional, ultrathin graphite/carbon nanotube/NiO composite as binder-free electrode for high-performance energy storage. J. Mater. Chem. A, 2015, 3, 624 Copyright 2015. Royal Society of Chemistry.

Figure 5. SEM images of (a) 3D nickel foam/UGF; (b and c) 3D nickel foam/UGF/CNTs network; and (e, f and g) 3D nickel foam/UGF/CNTs/NiO core–shell structure. The inset in


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(a) shows high-magnification SEM image of (a). The inset in (b) shows a low-magnification SEM image of (b). The inset in (d) shows the element (Ni, O, and C) mapping of marked area in (d). Reprinted from W. Liu, W.; Lu, C.; Wang, X.; Lianga, K.; Tay, B. K. In situ fabrication of three-dimensional, ultrathin graphite/carbon nanotube/NiO composite as binder-free electrode for high-performance energy storage. J. Mater. Chem. A, 2015, 3, 624 Copyright 2015. Royal Society of Chemistry. This type of hybrids can also be applied for further loading of polymers. For instance, the graphite−carbon nanotube (Gr-CNT) hybrid formed a composite (Figure 6) with polyethylene of an ultrahigh molecular weight (UHMWPE).19 In this composite possessing a segregated structure, the Gr-CNT hybrid form interconnected networks, being selectively distributed at the UHMWPE interfaces. This Gr-CNT/UHMWPE composite exhibited highly conductive networks and served to develop EMI shielding materials. Another example of a triple system is the Si/Gr/CNTs composite material (silicon/graphite/carbon nanotubes), fabricated by annealing after ball milling.20 This material exhibited an initial specific discharge capacity of 2326 mAh.g−1. These enhanced electrochemical properties were explained as being due to the uniform dispersion of carbon nanotubes in the internal surface of silicon and graphite phases.

Figure 6. (a) Schematic for the fabrication of segregated Gr-CNT/UH. SEM images of (b) pure UHMWPE granules, (c) 2.0 wt % Gr1-CNT3 hybrid coated UHMWPE complex granules. (d) Digital images of the Gr-CNT/UH with wafer shape. Reprinted from Jia, L. C.; Yan, D.X.; Jiang, X.; et al. Synergistic Effect of Graphite and Carbon Nanotubes on Improved Electromagnetic Interference Shielding Performance in Segregated CompositesInd. Eng. Chem. Res. 2018, 57, 11929. Copyright 2018. American Chemical Society. Other graphite-carbon composites



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Insertion of other carbon allotropes as dopants into the graphite structure can lead to considerable changes of the original graphite support. Thus, for the sandwiched fullerenegraphite hybrids, synthesized via a solution mixing/evaporation technique, the dopation of fullerene into graphite was found to have minimal effect on the electrical properties of the graphite phase, but considerably increased its electromagnetic absorption capacity.21 As well as for other combinations of the “carbon/carbon” type, the main application of graphite hybrids is as supercapacitors. For instance, a composite of carbon black, graphene oxide, and commercial graphite was synthesized by mixing these components and were used to prepare the anode slurries that were applied for a good material directly on Li-ion batteries.22 GO was responsible for the binder and lithium storage functions, the graphite was for conductivity and capacity, and the carbon black, being dispersed between the sheets of graphene oxide in an uniform manner, had the role of enhancing conductivity. This composite showed excellent rate capability and cycle performance, among other characteristics. Carbon nanofiber/graphite-felt composites (Figure 7) were synthesized by the catalytic CVD (CCVD) through the CNFs growth on the graphite microfibers.23 Both the apparent density and the compressive strength varied with the CNF yield (w/w %), reaching maximum values of 6 w/w %. It was noted that an excessive CNFs growth could affect and destroy the felt framework. A high mechanical strength, distinct mesoporous character, and large external surface area make them attractive as catalysts for the oxidative dehydrogenation of ethylbenzene to styrene, possessing much greater catalytic stability than activated carbon. As an example of another catalytic use, the carbon nanofiber/graphite-felt composite was offered to be applied as a support for iridium catalyst (30 wt.%) in reactions of hydrazine catalytic decomposition (equations 2-4),24 exhibiting high thermal conductivity and strong mechanical resistance. This catalyst is resistant at high pressure values and it can be compared to an industrial version based on Ir supported on alumina possessing a high surface area. In addition, carbon nanofibers (CNFs) and reduced graphite oxide (rGrO) nanocomposites with distinct composition (10-40 wt.% CNFs), were offered for removal of different ions by electrosorption.25 These composites have different surface areas and capacitance, indicating that both the electrode conductivity and porosity are important for the electrochemical performance. N2H4 = 4 NH3 + N2 (2) 3 N2H4 = 3 N2 + 6 H2 (3) 4 NH3 + N2H4 = 3 N2 + 8 H2 (4)


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Figure 7. Typical SEM images of graphite fibers and carbon nanofibers illustrating the evolution of CNF/graphite-felt composite synthesis: (a) graphite fibers of felt; (b) graphite fibers deposited by nickel precursor; (c) graphite fibers covered by carbon filaments having been grown for 3 h; (d) graphite fibers covered by filaments with carbon agglomerates within felt after reaction for 8 h; (e) fleecy filaments grown on the surface of graphite fibers; (f) high-magnification image of carbon filaments showing the nanofiber structure; (g) highmagnification image of carbon agglomerates showing the nanofiber structure; and (h) amorphous carbon in agglomerates. Reprinted from Li, P.; Li, T.; Zhou, J. H. et al. Synthesis of carbon nanofiber/graphite-felt composite as a catalyst. Microporous Mesoporous Mater., 2006, 95, 1. Copyright 2006. Elsevier Science. Graphene and graphene oxide composites Graphene – carbon nanochain composites



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Graphene composites with other nanocarbons are obviously the most studied subjects, as well as those with carbon nanotubes below. Among hybrids with 1D nanoobjects, the simplest case corresponds to the graphene-carbon nanochains. This way, the composite of sandwiched graphene-carbon nanochain webs, prepared (Figure 8) by carbonization after in situ polymerization,26 possesses high conductivity and is useful for Li-ion batteries, showing, after 50 cycles, a considerable charge capacity of 1103.2 mA h g-1 at 0.05 A g-1. In its micronano structure, webs of carbon nanochains were found to be inserted between graphene layers resulting sandwiched plates. The webs of carbon nanochains derived from PPy as precursor (PPy carbonization is necessary for prevention of volume expansion/) prevent the overlap of graphene layers, acting as the isolator. The graphene layers provide a large surface area for the deposition of carbon nanochain webs, and for the attainment of high conductivity. This composite consists of uniform granules (10 mm in diameter from the macroscopic perspective), made of a lot of grids containing thin plates. 1D carbon nanochains (7-8 nm of thickness) are dispersed on the graphene film surfaces, being coated by graphene and resulting sandwich-like plates (G/CNW/G) (Figure 9). The N-atomic content for CNW was found to be 12.08%; this value is much higher being compared with other N-doped carbon materials, and GCNW still has 5.11% of nitrogen. Lithium ions can reversibly enter and exit into/from such a structure, demonstrating excellent properties as an anode and also high reversible Li-storage capacities.

Figure 8. Schematic illustration of the three-step formation process of GCNW. Reprinted from Zou, Y.; Zhou, X.; Yang, J. In situ fabrication of graphene–carbon nanochain webs as anodes for Li-ion batteries. Phys. Chem. Chem. Phys., 2014, 16, 10429. Copyright 2014. Royal Society of Chemistry.


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Figure 9. (a) SEM, (b) TEM and (c) HETEM images of CNW (carbon nanochain webs), (d) HETEM image of GCNW (graphene–carbon nanochain web). Reprinted from Zou, Y.; Zhou, X.; Yang, J. In situ fabrication of graphene–carbon nanochain webs as anodes for Li-ion batteries. Phys. Chem. Chem. Phys., 2014, 16, 10429. Copyright 2014. Royal Society of Chemistry. Graphene(or graphene oxide) – carbon nanofiber composites Carbon nanofibers (CNFs) are very small cylindrical nanometer-scale nanostructures consisting of layers of graphene.27 Their synthesis, properties, functionalization, perfomance and applications are currently generalized in a book,28 in a chapter,29 and in a review.30 We note that, despite a series of similarities of CNTs and CNFs, the second type of so similar nanoobjects attracted a considerably lesser attention of investigators. However, some of their composites with graphene, graphene oxide (GO), and graphite oxide (GrO) are known; almost all of them are described by authors as 3D freestanding materials and have mainly electrochemical device applications (including more complex composites, as, for example, graphene-coated carbon nanofiber/sulphur composite material for applications in batteries31). Thus, the freestanding material based on highly conductive rGO sheets (1–3 nm thickness) with tightly intertwined mechanically stable CNFs (50–500 nm diameter) scaffolds, was prepared from cellulose/graphene oxide mats by one-step carbonization at 800oC.32 In the interpenetrated CNF/rGO network, the CNFs have a role of nanospacers, thus increasing excellent volumetric electrochemical performance of this supercapacitor. This mesoporous material, having well-interconnected graphene layers, can be used as supercapacitor composite electrode material (Figure 10), showing promising volumetric data of power density, energy, and capacitance.



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Figure 10. Schematic illustration of the fabrication process of CNF/rGO composite electrodes for a compact supercapacitor. Reprinted from Kuzmenko, V.; Wang, N.; Haque, M.; et al. Cellulose-derived carbon nanofibers/graphene composite electrodes for powerful compact supercapacitors. RSC Adv., 2017, 7, 45968. Copyright 2017. Royal Society of Chemistry. Similar 3D freestanding graphene hydrogel/carbon nanofibers (GH/CNFs) composites, where CNFs are uniformly embedded into graphene nanosheets, were obtained33 by chemical reduction of GO/CNFs mixtures at low temperature. The product can be used directly as an electrode; no necessity to add any conductive substances, binders or additives. One more related example corresponds to carbon nanofiber/graphene composites (CNF/GN) which were fabricated (Figure 11) using a membrane–liquid interface culture technique with further carbonization.34 The CNFs and GNs are uniformly dispersed in a 3D conductive architecture, providing sufficient stability, fine flexibility, and enhanced surface area, becoming very promising materials for superior supercapacitor electrodes, effective devices for energy storage, and particularly for use in flexible devices.


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Figure 11. The schematic illustration of the synthesis of the CNF/GN nanocomposite paper using a membrane–liquid interface culture and carbonization approach (each GN is supported and separated by CNF nanofibers). Reprinted from Luo, H.; Xiong, P.; Xie, J.; et al. Uniformly Dispersed Freestanding Carbon Nanofiber/graphene Electrodes Made by a Scalable Biological Method for High‐Performance Flexible Supercapacitors. Adv. Funct. Mater. 2018, 1803075. Copyright 2018. Wiley. Carbon nanofiber composites with graphene have also other applications, which are directly related to the outstanding properties of graphene. Thus, flexible paper on the basis of graphene–carbon fiber composite was prepared by depositing GO into the precursor of carbon fiber with subsequent carbonization.35 In this structure, the hierarchically arranged 2D graphene in the framework contributes a reasonable route for acoustic phonon transmission (responsible for the high thermal conductivity), while 1D carbon fiber is used for reinforcement of the mechanical strength. This material seems to be highly useful for commercial portable electronics of next generation, acting as lateral heat spreader. Also, GO



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and oxidized carbon nanofibers (O-CNF) were incorporated36 into the hydroxyapatite (mineralized Ca10(PO4)6(OH)2, M-HAP). The resulting product (M-HAP/O-CNF/GO composite) was found to be similar to natural bone (M-HAP); its mechanical strength is enhanced by GO presence (corrosion protection and high mechanical strength). This composite can be potential candidate for orthopedic and dental applications, as opposed to hydroxyapatite which is unsuitable for engineering applications in bone tissue. Graphene-fullerene composites A series of hybrid graphene/fullerene architectures, called graphene nanobuds, were constructed using molecular dynamics simulations, by attaching or fusing C60 molecules on a defect graphene sheet.37 The tensile strengths were found to be above 50 GPa and the elastic moduli were observed to degrade by a certain amount, but remaining still high. The Young’s muduli were calculated to be in the range from 0.43 to 0.77 TPa, depending largely on the structure configuration. In such C60/graphene composites, the C60 molecules are attached onto the graphene surface.38 Due to its predicted good characteristics in specific capacitance, it could be a suitable material for use as supercapacitor. Such graphene-based supercapacitor electrodes are indeed very attractive due to their chemical inertness, featured high electrical conductivity and surface area.39 Due to these and other useful properties, some theoretical researches have been carried out on the elucidation of possible structural features and the stability of distinct graphene/fullerene composites of distinct sizes of C60. Thus, sandwiched graphene-fullerene composites (SGFC) were considered in numerical simulations using Monte Carlo calculations, considering covalent junctions between randomly dispersed fullerene units and graphene layers.40 Distinct types of fullerenes (i.e. C180, C320 and C540, whose approximate radii a 0.6, 0.8 and 1.0 nm, respectively) were considered as sandwich core in these calculations (Figure 12); the purpose was to examine the behavior of hydrogen. The main result is that, for of SGFCs, the performance of hydrogen storage (maximum 5 wt.% at 77 K and 1 bar) can be enhanced by selection of fullerene compositions; in addition, their capacity of hydrogen adsorption can also be increased considerably after Li doping. The key features of this structure are as follows a) H2 molecules can easily enter the nanostructure and pass between the graphene layers and fullerenes, 2) to increase the accessible surface area, the pore size and interlayer spacing could be easily adjusted.


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Figure 12. The proposed atomistic models: (a) SGFC180, (b) SGFC320 and (c) SGFC540. Reprinted from Ozturk, Z.; Baykasoglu, C.; Kirca, M. Sandwiched graphene-fullerene composite: A novel 3-D nanostructured material for hydrogen storage. Int. J. Hydrogen Energy, 2016, 41(15), 6403. Copyright 2016. Elsevier Science. DFT study was made also to elucidate the electronic structure of porous graphene (PG) composite with fullerene (PG/F).41 It was revealed that, upon photoexcitation, smaller sized composites contribute to make more efficient the charge separation, and so, these hybrids can be used in the design of solar cells, as opposed to the larger ones. These systems could be useful in a series of devices, such as electronics, photovoltaics, and optoelectronic. In addition, two different hybridized fullerene-graphene composites (Figure 13), a) C60 fullerenes physisorbed on a monolayer of graphene and b) graphene nanoribbons functionalized using different-size fullerene molecules such as C20, C60, C70 and C60 derivative PCMB ([6,6]-phenyl-C61-butyric acid methyl ester), were DFT-studied.42 It was revealed that the C60, being adsorbed on a graphene layer, is not subjected to change its low energy states. However, this adsorption strongly affects its optical spectrum, revealing new absorption peaks. In addition, the longitudinally polarized electro-absorption spectrum was found to be enriched with a host of new transitions.

Figure 13. Schematic view of different complexes of fullerenes physisorbed onto a graphene monolayer and armchair graphene nanoribbons. On top SLG–C60 and SLG–2C60 (SLG = single layer graphene) and at the middle from left to right: nGR–C20, nGR–C60, nGR–PCMB and nGR–C70. At the bottom the coordination modes for each system are shown. Reprinted from Correa, J. D.; Orellana, P. A.; Pacheco, M. Optoelectronic Properties of Van Der Waals Hybrid Structures: Fullerenes on Graphene Nanoribbons. Nanomaterials, 2017, 7, 69. Copyright 2017. MDPI. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5388171/



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Such hybrid rGO−C60 nanocomposites were obtained practically (Figure 14) via slow diffusion of a GO suspension in 2-PrOH through C60 solution in m-xylene.43 In this case, long nanowires (200-800 nm in lenght) with p-type semiconducting behavior are formed due to - interactions between rGO and fullerene molecules. An electron transfer between counterparts and hole transport through rGO were proposed. In addition, various functionalized graphene nanobuds are known, for instance those containing pyrene units, attached to C60,44 water-soluble fullerol–graphene nanobuds (Figure 15),45 and organicfunctionalized pG-C60 (pG = pristine graphene) hybrids.46 The curved sp2 carbon atoms in C60 provide high chemical reactivity, resulting that organic groups are selectively covalently attached onto the fullerene molecule without affecting the aromatic system characteristics of graphene. This method works also for nanobuds of carbon nanotubes, where the properties of nanobuds are not a sum of properties of individual components. Such graphene nanobuds, possessing conductivity, could be used as sensors, thin films, fillers, and conductive inks, as well as effective Li absorption material in batteries.

Figure 14. Schematic illustration and SEM images (a–e) showing steps of formation of rGOwrapped C60 wires. Reprinted from Yang, J.; Heo, M.; Lee, H. J.; Park, S. M.; Kim, J. Y.; Shin, H. S. Reduced Graphene Oxide (rGO)-Wrapped Fullerene (C60) Wires. ACS Nano, 2011, 5, 8365. Copyright 2011. American Chemical Society.


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Figure 15. Synthesis and tentative structure of the water-dispersible graphene–fullerol (GFL) hybrid. Reprinted from Bourlinos, A.B.; Georgakilas, V.; Mouselimis, V.; et al. Fullerol– graphene nanobuds: Novel water dispersible and highly conductive nanocarbon for electrochemical sensing. Appl. Mater. Today, 2017, 9, 71. Copyright 2017. Elsevier Science. Graphene hybrids with carbon nanocages Carbon nanocages (CNCs), representing nanosize cage-like mesoporous carbon with nanometric graphene shell, have attained high recent interest, due to applications in such topics, as rechargeable batteries, sensing, hydrogen storage and production, catalysis, gene and drug delivery, among others. However, composites “carbon cage – another carbon form” are practically unknown (except fullerene hybrids). As an example, stable nanocomposites (Figure 16) of reduced graphene oxide (rGO) with carbon nanocages (CNCs), in situ prepared by solvothermal reaction at 180oC for 5 h from carbon nanocages (obtained by Hammer’s method) and GO,47 can be applied for the electrochemical detection of hydroquinone (HQ) and catechol (CC). The detection limits (S/N = 3) for CC and HQ were found to be 0.40 and 0.87 µM respectively.



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Figure 16. TEM (a) and HRTEM (b) images of CNCs; (c) SEM image of GO; (d) TEM image of rGO-CNCs. Reprinted from Huang, Y.H.; Chen, J. H.; Sun, X.; et al. One-pot hydrothermal synthesis carbon nanocages-reduced grapheneoxide composites for simultaneous electrochemical detection ofcatechol and hydroquinone. Sens. Actuators, B, 2015, 212, 165. Copyright 2015. Elsevier Science. Graphene(or graphene oxide)-nanodiamond composites Graphene-nanodiamond heterostructures are not common composites, but they possess certain applications in some types of transistors and other high current devices,48 as well as in catalysis. Thus, the composites of rGO modified with ND particles were obtained by heating aqueous GO suspensions, after 3 h ultrasonication for better exfoliation, with NDs at different ratios (1/1, 4/1, 10/1 and 20/1) at 100oC for 48 h (Figure 17).49 The molar ratio of OH active groups on NDs is considered to be sufficient for GO reduction. The resulting rGO/NDs composites (Figure 18) were found to be stable for 1 week in EtOH, acetone, CH3CN, DMF, and N-methyl-2-pyrrolidone, while their low dispersibility in THF, DMSO, water, and toluene was revealed. It was established that the electrochemical behavior (best for a GO:NDs ratio 10:1) depends on the starting ratio of GO/NDs precursors, which were used for the synthesis of the nanocomposites, being useful in the field of supercapacitors.


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Graphene oxide (GO)

rGO/ND

Figure 17. Schematic illustration of the preparation of rGO/NDs nanocomposites. Reprinted from Wang, Q.; Plylahan, N.; Shelke, M.V.; et al. Nanodiamond particles/reduced graphene oxide composites as efficient supercapacitor electrodes. Carbon, 2014, 68, 175. Copyrigt 2014. Elsevier Science.

Figure 18. (A) SEM images and (B) typical HR-TEM images of rGO/NDs matrices. The HR-TEM image of ND-OH is also included. Reprinted from Wang, Q.; Plylahan, N.; Shelke, M.V.; et al. Nanodiamond particles/reduced graphene oxide composites as efficient supercapacitor electrodes. Carbon, 2014, 68, 175. Copyrigt 2014. Elsevier Science. Not only the individual properties of ND and graphene can be potentially displayed in their hybrids, but also those properties appearing because of synergism upon their close interaction. Thus, ND/FLG (FLG: 2D support, few-layer-graphene, from 5 to 20, ND: nanospacer for prevention of the FLG sheets re-stacking) composite, synthesized by mixing NDs with a FLG suspension in ethanol, was further used for catalytic purposes. This stable metalfree catalyst can serve for the steam-free direct ethylbenzene dehydrogenation leading to styrene.50 An oxidative treatment of a deactivated catalyst in mild conditions allows easily regenerate it. The same authors reported more complex systems for the same catalytic purpose: the exfoliation (17% yield) of few-layer-graphene (FLG) under sonication for 5 h of expanded graphite in water, carried out using GO, which acts a surfactant, was used51 as a template medium for coating with NDs, resulting in 3D-laminated sandwich-like nanostructures FLG–GO@NDs containing spherical NDs (4–8 nm in diameter), which are homogeneously distributed on their surface. The GO double role was established, being ACS Paragon Plus Environment


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responsable for the exfoliation of expanded graphite and incorporation of NDs for the formation of composites, concentrating ND nanoparticles on the FLG–GO surface. This metal-free catalyst showed excellent perfomance in the dehydrogenation of ethylbenzene (98.6% styrene selectivity and 35.1% of ethylbenzene conversion). Air calcination at 400oC leads to efficient regeneration of the deactivated catalyst. Also, defective nanodiamondgraphene (ND@G) composite was applied to synthesize metal catalyst (Pd, Figure 19) in the form of an atomically dispersed species, which was used for selective acetylene hydrogenation upon presence of abundant ethylene.52 This catalyst was shown to be stable, having high ethylene selectivity (90%) and high conversion (100%). Its structure, containing atomically dispersed Pd atoms on graphene surface due to formation of Pd-C bonds, allowed avoiding the formation of subsurface hydrogen species. Ethylene is easily desorbed and not hydrogenized to undesired ethane. This acetylene hydrogenation reaction, characterized by high selectivity, is related with the competition of ethylene desorption at the active sites of the catalyst Pd1/ND@G.

Figure 19. HAADF-STEM images of Pd1/ND@G at low (a, b) and high (c, d) magnifications; STEM (e) and HAADF-STEM (f) images of Pdn/ND@G. (The inset in image b is a diamonds diffraction rings image; atomically dispersed Pd atoms in image d are highlighted by the white circles; and Pd nanoclusters in image f are highlighted by the yellow circles.). Huang, F.; Deng, Y.; Chen, Y.; et al. Atomically Dispersed Pd on


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Nanodiamond/Graphene Hybrid for Selective Hydrogenation of Acetylene. J. Am. Chem. Soc. 2018, 140, 13142. Copyright 2018. American Chemical Society. Other graphene-carbon composites Other graphene-carbon composites are rare and have similar applications in the fields of energy saving and catalysis. Thus, microporous carbon/graphene composites (MPC/Gs) were prepared from GO and coal tar pitch upon KOH activation.53 These composites possess two advantages: abundant micropores for ion storage and incorporated graphene, responsible for high electron conduction, exhibiting an excellent cycle stability and high specific capacitance. In core‐shell composite (GNs@AC), consisting of graphene nanosheets (GNs) and amorphous carbon (AC), synthesized by a chlorination method, the GNs acted as a shell, possessing high surface area, corrosion resistance, and excellent conductivity, and as a protecting layer to prevent the degradation of the amorphous carbon core.54 The amorphous carbon nanoparticles are coated with a few GNs layers, yielding in a core‐shell GNs@AC structure (Figure 20). Upon further homogeneous deposit of platinum nanoparticles, the resulting Pt/GNs@AC catalyst showed considerably higher stability and activity in comparison with the industrially prepared Pt/C catalyst. Good conductivity of graphene contributes to this activity, being also effective in inhibition of migration and aggregation of platinum nanoparticles by covering the core of amorphous carbon. This composite also has promising applications in fuel cells. Also, studying the effect of reaction temperature in the range of 400 to 1000°C on the synthesis of thin film of graphite on nickel support (Figure 21), it was established that temperature is a critical factor in the synthesis of multilayer graphene (MLG) and AC films: the hybrid films of MLG and amorphous carbon can be prepared at 600°C, while MLG and AC film can be CVD-obtained at 800°C and 400°C, respectively.55 These films are promising materials for Simple and low-cost carbon-based solar cells on carbon basis can be fabricated fusing these films.

Figure 20. (a, b) HRTEM images of GNs@AC. CV plots of GNs@AC NPs. Reprinted from Wu, H.; Peng, T.; Kou, Z. et al. Core‐shell graphene@amorphous carbon composites supported platinum catalysts for oxygen reduction reaction. Chin. J. Catal., 2015, 36, 490. Copyright 2015. Elsevier Science.



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Figure 21. Illustration and Raman spectra of as-grown films at different temperatures. (a) Illustration of films produced at different reaction temperatures. (b) Raman spectra of asgrown films obtained at different temperatures. Cui, T.; Lv, R.; Huang, Z. H.; et al. Lowtemperature synthesis of multilayer graphene/amorphous carbon hybrid films and their potential application in solar cells. Nanoscale Res. Lett., 2012, 7, 453. Copyright 2012. Springer. In addition, the thin (

Carbon-carbon allotropic hybrids and composites: synthesis

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