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Broad Family of Carbon Nanoallotropes: Classification, Chemistry, and Applications of Fullerenes, Carbon Dots, Nanotubes, Graphene, Nanodiamonds, and Combined Superstructures Vasilios Georgakilas,† Jason A. Perman,‡ Jiri Tucek,‡ and Radek Zboril*,‡ †
Material Science Department, University of Patras, 26504 Rio Patras, Greece Regional Centre of Advanced Technologies and Materials, Department of Physical Chemistry, Faculty of Science, Palacky University in Olomouc, 17 listopadu 1192/12, 771 46 Olomouc, Czech Republic
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‡
4.2. Photoluminescence of Carbon Dots and Graphene Quantum Dots 4.3. Fundamental Properties of Nanodiamonds 4.4. Mechanical Properties of Graphene, Carbon Nanotubes, and Carbon Nanofibers 4.5. Electronic and Related Properties of Graphene, Graphene Nanoribbons, Carbon Nanotubes, and Carbon Nanohorns 4.6. Magnetic Properties of Carbon Nanoallotropes 4.7. Chemical Reactivity 4.7.1. Addition to sp2 Carbon Atoms 4.7.2. Reactions at Edges and Defect Sites 4.7.3. Noncovalent Surface Interactions 4.7.4. Internal Spaces as Nanoreactors 5. Interconversions of Carbon Nanoallotropes 6. Combining Nanoarchitectures To Produce Advanced Allotropic Hybrids 6.1. Fullerene Aggregates 6.2. Carbon Clusters 6.3. Assembled Nanostructures Containing Graphene Quantum Dots 6.4. Carbon Nanotubes and 2D Graphene Nanostructures 6.4.1. Transparent Thin Films 6.4.2. Conductive Membranes and Papers 6.5. Graphenic Hybrid Nanocomposites 6.5.1. Graphene−CNT Thin Films 6.5.2. Graphene−CNT Membranes 6.5.3. Graphene−CNT 3D Hybrids and Pillared Structures 6.6. 3D Graphenic Hybrid Superstructures 6.6.1. Aerogels, Nanofoams, and Spongelike Nanoarchitectures 6.6.2. Hollow 3D Microspheres 6.7. Nanoreactors Based on Carbon Nanotubes 6.8. Nanodiamonds and C60-Functionalized Graphene and Carbon Nanotubes 7. Predicted, Rare, and High-Pressure Carbon Nanoallotrope Structures 8. Summary, Outlook, and Selected Challenging Applications Author Information
CONTENTS 1. Introduction 2. Classification of Carbon Nanoallotropes: Structural Description and Characterization 2.1. 0D Carbon Nanoallotropes 2.1.1. Fullerenes and Onion-like Carbon 2.1.2. Carbon Dots 2.1.3. Graphene Quantum Dots 2.1.4. Nanodiamonds 2.2. 1D Carbon Nanoallotropes 2.2.1. Carbon Nanotubes and Nanofibers 2.2.2. Carbon Nanohorns 2.3. 2D Carbon Nanoallotropes 2.3.1. Graphene 2.3.2. Multilayer Graphitic Nanosheets 2.3.3. Graphene Nanoribbons 3. Methods for Preparing Carbon Nanostructures 3.1. 0D Carbon Nanoallotropes 3.1.1. Fullerenes and Onion-like Carbon 3.1.2. Carbon Dots 3.1.3. Graphene Quantum Dots 3.1.4. Nanodiamonds 3.2. 1D Carbon Nanoallotropes 3.2.1. Carbon Nanotubes and Nanofibers 3.2.2. Carbon Nanohorns 3.3. 2D Carbon Nanoallotropes 3.3.1. Graphene 3.3.2. Multilayer Graphitic Nanosheets 3.3.3. Graphene Nanoribbons 4. Fundamental Physicochemical Properties of Carbon Nanoallotropes 4.1. Fundamental Properties of C60
© 2015 American Chemical Society
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nanostructure is graphene, the building block of graphite. While its existence had been predicted decades ago6 and experimentally identified in 1962 by Boehm et al.,7 it was first isolated and characterized in 2004 by Andre Geim and Konstantin Novoselov.8 The graphene family includes several similar nanostructures that consist of a single graphene monolayer or a few graphene monolayers. There are various methods for manufacturing graphene, each of which yields products with different sizes and contents of impurities such as oxygen. In recent years, there has been growing interest in graphene nanosheets (also known as graphene quantum dots, or GQDs), which consist of a monolayer or a few monolayers and have interesting optoelectronic properties. All of these carbon nanoallotropes can be regarded as members of the same group because they consist primarily of sp2 carbon atoms arranged in a hexagonal network. This common structure means that they all have some common properties, although they also have significant differences due to their different sizes and shapes. They all have similar levels of electrical conductivity, mechanical strength, chemical reactivity, and optical properties. Their biggest differences relate to their dispersibility in organic solvents: C60 is the only readily soluble nanostructure, although graphene is dispersible in specific organic solvents. Many of the other materials are only slightly dispersible in organic solvents, forming unstable suspensions. Another group of carbon nanostructures that have been developed in parallel with the graphitic (sp2) species discussed above are carbon dots (C-dots) and nanodiamonds. Nanodiamonds consist primarily of tetrahedral sp3 carbon atoms, while carbon dots contain mixtures of sp3 and sp2 carbon atoms in various ratios. The preparation, properties, and potential applications of carbon nanoallotropes (particularly fullerenes, CNTs, graphene, and C-dots) have been studied extensively. Consequently, several review papers covering these topics have been published. However, many recent reports have described carbon superstructures or all-carbon composite structures generated by combining different carbon nanostructures. Such superstructures can be formed by carefully mixing components such as C60, CNTs, graphene, and so on. Alternatively, they can be prepared directly from appropriate precursors. By combining 0D, 1D, and 2D carbon nanostructures, it is possible to form 1D nanowires, 2D thin films, 3D microspheres, carbon aerogels, and potentially many more interesting materials. Therefore, carbon nanostructures can be regarded as bridges between nanoscience and the microarchitectures that are used in the development of electronic, photonic, and optoelectronic microdevices. The aim of this review is to classify all of the currently known carbon nanoallotropes and to describe their properties in relation to their structural characteristics. In addition to describing individual carbon nanoallotropes, this review also covers superstructures consisting of combinations of carbon nanoallotropes such as fullerene aggregates, CNTs or graphene thin films, fibers, membranes, aerogels, and so on. In brief, the section 2 focuses on the structures and basic characterization of the known carbon nanoallotropes, which are classified as being 0D, 1D, or 2D on the basis of their dimensionality. Section 3 describes the established methods for preparing the most important carbon nanoallotropes as well as their relative merits and drawbacks. Section 4 is devoted to the major and most extensively studied physicochemical properties of carbon nanoallotropes, which are discussed in the context of their diverse potential applications in nanotechnology. It also covers
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1. INTRODUCTION The unique ability of carbon atoms to participate in robust covalent bonds with other carbon atoms in diverse hybridization states (sp, sp2, sp3) or with nonmetallic elements enables them to form a wide range of structures, from small molecules to long chains. This property underpins the immense importance of organic chemistry and biochemistry in life. It was two centuries ago that carbon was first shown to be present in organic molecules and biomolecules as well as natural carbon materials such as the various types of amorphous carbon, diamond, and graphite. Although diamond and graphite both consist exclusively of carbon atoms, their properties are very different. Diamond is a transparent electrical insulator and the hardest known material. Conversely, graphite is a black opaque soft material with remarkable electrical conductivity. These differences derive from the way that the carbon atoms are connected in each case. Diamond consists of tetrahedral sp3 carbon atoms that form unique large crystals. In contrast, graphite is made up of stacked graphene monolayers that are held together by van der Waals interactions. These graphene monolayers consist of sp2 carbon atoms that are packed densely in a two-dimensional hexagonal lattice. Over the past few years, the previously empty space between organic molecules and natural carbon materials has been partially filled by the identification of a range of new materials with surprising properties and diverse potential applications in technology. The first of these carbon nanostructures to be discovered was the C60 molecule, which is known as fullerene and was initially reported in 1985.1,2 Several other fullerenes were subsequently discovered, including C20, C70, and even larger species, but C60 is by far the most widely studied to date. Each C60 molecule consists of 60 sp2 carbon atoms arranged in a series of hexagons and pentagons to form a spherical (truncated icosahedral) structure. Fullerenes are the smallest known stable carbon nanostructures and lie on the boundary between molecules and nanomaterials. As an example, C60 could reasonably be seen as a large spherical organic molecule given its solubility in organic solvents (toluene in particular). Another major step in the development of carbon nanomaterials was taken six years later, with the discovery of carbon nanotubes (CNTs) by Iijima.3−5 Due to their size and shape, the properties of CNTs are completely different from those of C 60 . Consequently, their potential applications are different: among other things, they have been studied as components of composite materials with polymers and as substrates for the deposition of catalytic nanoparticles. The discoveries of these two key materials, fullerenes and CNTs, were followed by the development of other carbon nanostructures with unique shapes such as single-walled carbon nanohorns (SWNHs), onion-like carbon (OLC) spheres, and bamboo-like nanotubes. While the discoveries of these materials were less revelatory than that of CNTs, their existence is equally important because it further demonstrates the ability of carbon to form unique nanostructures that would not have been imagined only a few years previously. The most recently isolated carbon 4745
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can accommodate guest molecules, metals, atoms, or other nanostructures. In certain cases, they can even provide nanoenvironments that facilitate specific reactions. Robust nanostructures with no internal spaces such as nanodiamonds, C-dots, and OLC spheres would belong to a separate second category under this scheme. This category could also accommodate graphene because it has no internal spaces too. A final approach to carbon nanostructure classification that is used in this work is based on the structures’ dimensionality. This scheme distinguishes between (i) 0D carbon nanostructures such as fullerenes, OLC structures, C-dots, and nanodiamonds, (ii) 1D nanoallotropes such as CNTs, carbon nanofibers, and SWNHs (although the latter are organized into 3D aggregates), and (iii) 2D nanoallotropes such as graphene, graphene nanoribbons, and few-layer graphenes.
the issue of chemical modification of carbon nanoallotropes with division into several sections based on the origins of the chemical reactivity in each case. Because many review papers focusing on the functionalization of the most reactive carbon nanostructures have already been published, the chemical reactivity section only provides general descriptions of the known functionalization procedures and their characteristic paradigms. Section 5 discusses transformation among various carbon nanoallotropes with graphene viewed as the building block. Section 6 is devoted to superstructures and hybrids that are formed by combining different carbon nanoallotropes. Next, section 7 discusses hypothetical/predicted carbon nanoallotropes along with structures that have been predicted to exist and prepared at very high pressures. The properties of these model systems can be predicted on the basis of computational studies prior to their synthesis, facilitating their discovery and providing motivation for their preparation. Finally, in section 8, we highlight the application potential of individual carbon nanoallotropes and their hybrids and propose new carbon-based nanoarchitectures with properties imparted due to synergetic effects.
2.1. 0D Carbon Nanoallotropes
2.1.1. Fullerenes and Onion-like Carbon. The study of carbon nanostructures began with the discovery of the fullerenes. In general, fullerenes are closed hollow cages made of sp2hybridized carbon atoms arranged into 12 pentagons and a calculable number of hexagons that depends on the total number of carbon atoms. A fullerene with 20 + 2n carbon atoms will have n hexagons. The number of pentagons is predetermined by the closed shapes of the fullerenes and will always be 12 in those with perfect structures. C60 is a simple spherical molecule with an external diameter of 0.71 nm, and its chemical properties are very similar to those of an organic molecule. However, it can also be regarded as the smallest carbon nanostructure and a representative 0D carbon nanoallotrope. The C60 molecule is frequently said to be not superaromatic due to its tendency to avoid formation of double bonds in the pentagonal rings. Although experimentally produced fullerene soot consists of several similar structures with higher or lower numbers of C atoms (e.g., C70, C76, C82, and C84), C60 is the most abundant, the most widely studied, and the one that has been used most heavily to date. It takes the form of a truncated icosahedron containing 12 pentagons and 20 hexagons. All of its carbon atoms are sp2hybridized. However, as the arrangement of carbon atoms is not planar but rather pyramidalized, a “pseudo”-sp3-bonding component must be present in the essentially sp2 carbons. Thus, C60 and other larger fullerenes can be viewed as a carbon nanoallotrope with hybridization between sp2 and sp3. The presence of pentagons is essential, introducing curvature and, hence, allowing closing of the cage. Two different types of bonds are identified from the refinement of the C60 X-ray diffraction pattern, i.e., a bond termed a 66 bond with a length of 1.38 Å connecting C atoms common to the two adjacent hexagons and a bond termed a 56 bond with a length of 1.45 Å connecting C atoms common to the pentagon−hexagon pair. In the solid phase, C60 either exists as aggregates or forms a crystalline structure with a face-centered cubic (fcc) lattice. Its aggregates and crystals are both soluble in many organic solvents, especially toluene (2.8 mg/mL) and carbon disulfide (8.0 mg/ mL).10 C60 has quite a stable structure; cage destruction occurs at temperatures above 1000 °C. Fullerenes have been characterized using various spectroscopic and spectrometric techniques that are used widely in organic chemistry, such as UV−vis, NMR, FTIR, and Raman spectroscopy and mass spectrometry.11 In some cases, especially when C60 is combined with other carbon nanostructures, the C60 molecules can be visualized using microscopic techniques such as high-resolution transmission electron microscopy (HRTEM). The 13C NMR spectrum of C60
2. CLASSIFICATION OF CARBON NANOALLOTROPES: STRUCTURAL DESCRIPTION AND CHARACTERIZATION Carbon nanostructures can be separated into two general groups on the basis of the predominant types of covalent bonds linking their C atoms. The first group involves the graphenic nanostructures, which are primarily made up of sp2 carbon atoms that are densely packed in a hexagonal honeycomb crystal lattice, although they may also contain some sp3 carbon atoms at defect sites or edges. This group includes graphene, graphenic nanosheets, CNTs, nanohorns, onion-like carbon nanospheres, and C-dots. Graphenic nanoallotropes are all based on the ability of carbon to form three identical covalent bonds with other carbon atoms using sp2 orbitals, generating a 2D lattice of densely packed hexagons. The simplest and most basic representative of this group is graphenea two-dimensional, one atom thick sheet of sp2-hybridized carbon arranged in a hexagonal lattice. The dimensions of the layer are not predetermined but are usually in excess of 500 nm. From a theoretical perspective, graphene can be regarded as the “building block” of the other graphenic/ graphitic nanoallotropes. For example, a properly cut piece of a graphene sheet could in principle be wrapped up to form the 0D fullerene, rolled up to form a 1D CNT, or stacked with additional sheets of graphene to form multilayered 2D carbon nanosheets or graphite.9 More complex graphenic nanostructures such as nanohorns can conceptually be obtained by elaborating on the structure of graphene in more sophisticated ways. The second group of carbon nanostructures contain both sp3 and sp2 carbon atoms in various ratios and have mixtures of amorphous and graphitic regions, or consist predominantly of sp3 carbon atoms. At present, nanodiamond is the only known member of this group. However, some types of C-dots with nongraphitic structures could also be regarded as members. The main characteristic feature of these nanoforms is that they are not built from graphene parts or monolayers such as CNTs or SWNHs. It is also possible to classify carbon nanoallotropes according to their morphological characteristics. The first category in such a classification scheme would contain nanostructures with empty internal spaces such as fullerene, carbon nanotubes, and nanohorns. The internal voids of these hollow nanostructures 4746
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is very simple because all of its carbon nuclei are equivalent: it features only a single resonance at 143.2 ppm (see Figure 1a).11
Figure 1. (a) 13C NMR spectrum of pure C60 in deuterated benzene, C6D6. (b) Mass spectrum of the toluene extract of fullerene soot, which consists primarily of C60 and C70. (c) UV−vis spectra of C60 and C70 in hexane. Reprinted with permission from ref 11. Copyright 1994 Elsevier B.V.
Fullerenes have also been characterized by mass spectrometry. This requires careful control over several key experimental parameters, particularly those relating to electron impact desorption and ionization, to avoid the detection of spurious impurities corresponding to carbon clusters generated by the destruction of the fullerenes following their ionization. If an appropriate ionization technique is used (e.g., fast atom bombardment, FAB), the mass spectrum of C60 contains a peak at m/z = 720 corresponding to C60+ (see Figure 1b).11−13 The optical properties of fullerenes are determined by several factors such as the size and morphology of the molecules, the size of the clusters they form, the nature of the solution, and so on. C60 in toluene has a deep purple color, while C70 is red, and solutions of larger fullerenes shift from yellow to green as their size increases. In keeping with these observations, the UV−vis absorption spectrum of C60 contains intense bands at 213, 257, and 329 nm and a weak band between 500 and 700 nm, while that of C70 has several intense peaks at 214, 236, 360, 378, and 468 nm (see Figure 1c).11 The latter of these absorption bands represents the characteristic difference between the spectra of the two fullerenes. The Fourier transform infrared (FTIR) spectrum of C60 contains characteristic bands at 1428, 1183, 577, and 527 cm−1.11 Several research papers have discussed a fullerene-type graphitic nanoallotropea multishell spherical carbon nanostructure that is often called onion-like carbon. OLC structures consist of concentric graphenic shells such as giant fullerenes that enclose a series of progressively smaller fullerenes (see Figure 2).14−18 They were first identified by Ugarte19 in a mixture of carbon nanotubes after strong electron beam irradiation. 2.1.2. Carbon Dots. C-dots are quasi-spherical carbon nanoparticles with diameters of 2−10 nm that have high oxygen contents and consist of combinations of graphitic and turbostratic carbon in various volumetric ratios (see Figure 3).20−50 C-dots contain mostly sp3-hybridized carbon and are usually of amorphous nature. The most characteristic and
Figure 2. (a) TEM images of onion-like carbon structures produced by annealing nanodiamonds. Reprinted with permission from ref 16. Copyright 2005 American Physical Society. (b) Carbon onion generated by electron irradiation of a polyhedral particle at high temperature. Reprinted with permission from ref 18. Copyright 1991 Wiley-VCH.
Figure 3. TEM and HRTEM (inset) images of C-dots prepared from histidine. The left-hand side inset shows a size distribution histogram for the C-dots. Reprinted with permission from ref 20. Copyright 2012 Royal Society of Chemistry.
significant property of C-dots is relatively strong photoluminescence, which depends on their size, the excitation wavelength, and the surface functionalization.48 The double character of C-dots is indicated by the characteristic peaks of 4747
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that chemical functionalization of GQDs affects their band gaps and PL.58 The details of the relationship between functionalization and PL are not currently well understood, but it has been reported that PL shifts due to chemical functionalization may be due to the suppression of defect state emissions, which would make intrinsic state emissions from the band gap dominant.59 GQDs can be visualized using TEM and atomic force microscopy (AFM) as well-defined dots with quasi-cyclic structures having clear graphitic elements (see Figure 5).60,61 They have
their X-ray diffraction (XRD) patterns at 18.26° and 23.82°, which are attributed to their amorphous and graphitic (d002 = 3.4 Å) components, respectively (see Figure 4a). The graphitic structure of C-dots is also apparent in HRTEM images such as that of C-dots prepared from histidine shown in the inset of Figure 3.20
Figure 4. (a) XRD patterns, (b) Raman spectra (λex = 633 nm), (c) C 1s XPS spectra, and (d) FT-IR spectra of graphite and C-dots. Reprinted with permission from ref 21. Copyright 2012 Royal Society of Chemistry.
The Raman spectra of C-dots contain characteristic D and G bands at 1350 and 1600 cm−1, respectively. Their ID/IG ratios demonstrate that they have high contents of sp3 carbon atoms. The functional groups present on the surfaces of C-dots can be identified by the presence of specific peaks in their FTIR spectra: hydroxyl groups generate peaks at 3442 cm−1, carboxyl groups give peaks at 1710 cm−1, and epoxide peaks occur at 1244 cm−1. Peaks at 1444, 865, and 606 cm−1 are attributed to aromatic C C and C−H bonds (see Figure 4).21 2.1.3. Graphene Quantum Dots. GQDs are defined as the products obtained by cutting a graphene monolayer into small pieces (disks) with dimensions of a few nanometers (2−20 nm). GQDs are composed mainly of sp2-hybridized carbon, and they are crystalline.51 In practice, however, they usually consist of a few stacked graphene monolayers based on the structures of their graphenic nanosheet precursors. They owe their evolution to the observation that quantum confinement and edge effects in graphene nanosheets with sizes of less than 100 nm become more pronounced as the sheets get smaller, particularly once their dimensions fall below 10 nm.51 Whereas large graphene nanosheets have a band gap of zero width, which limits their usefulness in electronic and optoelectronic applications, GQDs have nonzero band gaps due to quantum confinement and edge effects. This gives them very interesting electronic and optical properties.52−54 Multiple studies have shown that the band gaps of GQDs (and hence their optoelectronic properties) can be tuned by changing their size, shape, and geometry, as well as the nature of their edges. The correlation between size and band gap has been demonstrated by scanning tunneling spectroscopy experiments; in general, smaller GQDs have larger band gaps.55,56 GQDs also exhibit strong photoluminescence (PL), which shifts in parallel with changes in the band gap size.57 It has been demonstrated
Figure 5. (a) AFM images of GQDs. Reprinted with permission from ref 61. Copyright 2012 Royal Society of Chemistry. (b) TEM image of GQDs. Reprinted with permission from ref 60. Copyright 2010 WileyVCH.
characteristic UV−vis absorption spectra similar to that of graphene oxide, and their PL spectra feature an intense emission band between 400 and 600 nm (see Figure 6).62
Figure 6. (a) UV−vis and (b) photoluminescence emission spectra of GQDs. Reprinted with permission from ref 62. Copyright 2013 Elsevier B.V. 4748
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The Raman spectra of GQDs are usually similar to that of graphene, with the main D and G bands occurring at 1329 and 1600 cm−1, respectively. Their ID/IG ratios are relatively high because they have a large number of sp3 carbons at the edges relative to the number of sp2 carbons in the core of their structure. GQDs have potential applications in photovoltaic devices,52,58,63,64 bioimaging,59,65,66 phototransistors,67 and photocatalysis.68 2.1.4. Nanodiamonds. Nanodiamonds69 are sp3 carbon nanoparticles that consist of crystal domains with a diamondoidlike topology and diameters that are greater than 1−2 nm but less than 20 nm. They are not dispersible and are usually prepared by top-down methods such as jet milling or abrasion of microdiamonds.70,71 Nanostructures of this sort that have diameters above 20 nm behave like bulk diamonds. Conversely, sp3 carbon nanostructures with diameters of less than 1 nm are usually called diamondoids and occur naturally in petroleum deposits.72 The sp3-hybridized surface-bound carbon atoms of these small diamondoids are normally bonded to hydrogen or other noncarbon atoms. Consequently, their properties resemble those of organic molecules rather than bulk diamonds. As the diameter of the sp3 carbon cluster increases, the percentage of carbon atoms located at the surface decreases and the diamond character of the nanoparticles becomes important. However, the lower limit of nanodiamond diameters is not clear. The archetypal nanodiamonds are those produced by the detonation method (described in section 3), which have a mean diameter around 4 nm. Fifteen percent of the carbon atoms in these nanodiamonds are located at the surface and contribute to the stabilization of the structure as a whole by forming bonds with hydrogen or other atoms. The surfaces of nanodiamonds are usually decorated with several organic functional groups, the identity of which depends on the chemical conditions applied during purification. Their presence can be demonstrated by FTIR spectroscopy: OH species adsorbed on or covalently bonded to the surface generate characteristic bands at 3200−3600 and 1630−1640 cm−1, CO groups give bands at 1700−1800 cm−1, and CH2 or CH3 groups produce bands at 2850−3000 cm−1. The Raman spectra of nanodiamonds can also provide important information about their structures.73 However, they are often dominated by intense signals arising from impurities containing sp2-hybridized carbon atoms. It is therefore necessary to extensively purify nanodiamond samples before recording their high-resolution Raman spectra. The Raman spectra of nanodiamonds usually contain a broad peak near 1320−1330 cm−1 representing the first-order diamond line. This line is down-shifted relative to its position in the spectrum of bulk diamond. There is also usually a second broad peak between 1500 and 1800 cm−1 that is attributed to the surface environment of the nanodiamonds.74 Figure 7 shows the Raman spectrum of some nanodiamonds formed by irradiating carbon black.75 In this case, the first Raman line occurs at 1331 cm−1. The broad band around 1600 cm−1 was identified as a superposition of the G bands of graphitic impurities, while that at 1611 cm−1 was attributed to paired 3-fold-coordinated defects.
Figure 7. (a) TEM image of nanodiamonds prepared by the detonation method. (b) Raman spectra of carbon black and nanodiamonds (labeled “sample”) formed by its irradiation. Reprinted with permission from ref 75. Copyright 2009 Elsevier B.V.
cylindrical tube with a high aspect ratio. The simplest CNT has a single graphenic wall and is capped at both ends. Single-walled carbon nanotubes (SWNTs) have diameters of around 0.4−2 nm and are several micrometers long, with an empty internal space. CNTs can also be double-walled (DWNTs)76,77 or multiwalled (MWNTs) carbon nanotubes depending on the number of graphenic layers in the walls of the cylindrical structure (see Figure 8).76 The aspect ratio (i.e., length-to-diameter ratio) of carbon nanotubes frequently exceeds 10 000, and thus, carbon nanotubes are regarded as the most anisotropic materials ever produced. Apart from diameter and length, chirality (the angle between the hexagons and the nanotube axis) is another key parameter of the carbon nanotubes. Depending on the chirality, carbon atoms around the nanotube circumference can be arranged in several ways; armchair, zigzag, and chiral patterns are the most common examples (see section 4.5). CNTs are not dispersible in organic solvents or water and are usually held strongly together in bundles by significant van der Waals interactions.78−80 A similar group of cylindrical carbon nanostructures that have recently appeared in the literature are referred to as bamboo-like CNTs due to the division of their internal spaces into small cavities similar to those found in bamboo (see Figure 9).81 The Raman spectra of carbon nanotubes have a characteristic G band between 1500 and 1600 cm−1 that appears in the spectra of all graphitic structures, a D band at around 1300−1400 cm−1, and a G′ band at 2600 cm−1 (see Figure 10).82 The ID/IG ratio depends on the number of sp3 carbon atoms present at the defect sites and the open ends. The most characteristic features in the Raman spectra of CNTs are the low-energy modes known as “radial breathing modes” (RBMs) that occur between 150 and
2.2. 1D Carbon Nanoallotropes
2.2.1. Carbon Nanotubes and Nanofibers. Carbon nanotubes were discovered and characterized several years before the isolation of graphene.3−5 “Carbon nanotubes” is a general term that refers to a wide range of tubular nanostructures with similar structures and shapes. Ideally, they are based on a hexagonal lattice of sp2 carbon atoms such as graphene. However, in nanotubes, the edges of the graphene sheet are fused to form a 4749
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Figure 10. Raman spectra of SWNTs, DWNTs, and MWNTs. Reprinted with permission from ref 82. Copyright 2008 Oficyna Wydawnicza Politechniki Wroclawskiej.
Depending on the internal structure of the carbon nanofiber (i.e., on the way the graphene sheets are arranged), several types of carbon nanofibers have been recognized so far (see Figure 11a− c):85,86 (i) platelet carbon nanofibers with graphene layers lying in a perpendicular manner with respect to the fiber axis; (ii) herringbone (or fishbone) carbon nanofibers consisting of graphene sheets tilted to the fiber axis; (iii) ribbon (or tubular) carbon nanofibers with graphene layers forming a stacked organization parallel to the fiber axis; (iv) stacked-up and conestacked carbon nanofibers with similar structure composed of truncated cones arranged to leave a hollow core; (v) cone-helix carbon nanofibers with a graphite layer forming a continuous helix-spiral architecture and with an internal hollow core. On the contrary, carbon nanofibers lack the hollow cavity, which is viewed as a main difference from carbon nanotubes. To distinguish carbon nanotubes and carbon nanofibers with cones and tilted graphene sheets, an angle α is defined (see Figure 11d,e)83 reflecting the positioning of the graphene sheet near the sidewall surface with respect to the axis of the carbon nanofiber. If α = 0, a continuous structure of the carbon nanotube is restored. Carbon nanofibers with cone and tilted-graphenesheet motifs are characterized with α > 0; the angle determines the physicochemical properties carbon nanofibers show. 2.2.2. Carbon Nanohorns. SWNHs were discovered by Iijima in 1999 while studying CNT formation.87 Nanohorns are tubule-like/conelike structures made from a single graphenic layer. They usually exist in large spherical aggregates that are around 80−100 nm in diameter and resemble dahlia flowers. Individual nanohorns have diameters of 1−2 nm at the tips and 4−5 nm at the base of the cone (see Figure 12).88 The wall-towall distance between adjacent SWNHs is about 0.4 nm. Other types of SWNH aggregations were observed resembling features of buds and seeds.89 Cones are formed by cutting a wedge from the single graphenic layer; the exposed edges are then connected in a seamless fashion. The opening angle of the wedge is called the declination angle and is defined as n(π/3), where 0 ≤ n ≤ 6. The relation between the declination angle and the opening angle of the cone, θ, is then given as θ = 2 arcsin(1 − n/6).90 The limiting cases with n = 0 and n = 6 correspond to two-dimensional planar structures (e.g., graphene) and one-dimensional cylindrical structures (e.g., carbon nanotubes), respectively. Following
Figure 8. TEM images of (a) an SWNT and (b) a DWNT. (c) Crosssectional HRTEM image of a bundle of DWNTs. Reprinted with permission from ref 76. Copyright 2006 Wiley-VCH.
Figure 9. TEM images of bamboo-like CNTs in different magnifications. Reprinted with permission from ref 81. Copyright 2010 Elsevier B.V.
350 cm−1. Their frequencies depend on the tubes’ diameters, and they only occur in SWNTs. Carbon nanofibers are described as a noncontinuous 1D carbon nanoallotrope of cylindrical or conical shape, consisting of stacked and curved graphene sheets arranged in various ways.83−86 They were identified and extensively studied long back before the discovery of carbon nanotubes. They are frequently described as sp2-based linear filaments with a diameter ranging from 50 to 200 nm and a high aspect ratio exceeding 100. 4750
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Figure 11. Examples of carbon nanofibers: (a) platelet carbon nanofibers with graphene layers lying in a perpendicular manner with respect to the fiber axis, (b) ribbon carbon nanofibers with graphene layers forming a stacked organization parallel to the fiber axis, and (c) herringbone carbon nanofibers consisting of graphene sheets tilted to the fiber axis. Structural difference between the (d) carbon nanofiber and (e) carbon nanotube. Panels d and e reprinted with permission from ref 83. Copyright 2005 American Institute of Physics. Figure 12. (a) A TEM micrograph of SWNH aggregates. (b) A magnified TEM micrograph of graphitic carbon particles showing aggregations of tubule-like structures protruding from the particle’s surface. (c) A highly magnified TEM micrograph of the edge regions of the nanotubes showing the SWNH structure. Reprinted with permission from ref 88. Copyright 2008 Springer.
Euler’s rule, the terminating cap of the cone with a declination angle of n(π/3) consists of n pentagons, substituting for the hexagonal planar graphite. For nanohorns, θ ≈ 20°, which corresponds to a declination angle of 5π/3.90 This implies that nanohorns contain exactly five pentagons near the tip. The nanohorns are then classified with respect to the relative positions of carbon pentagons at the apex, determining the morphology of the terminating cap (see Figure 13).90 The Raman spectra of SWNHs exhibit a broad graphitic band around 1550−1600 cm−1 (the G band) that is assigned to tangential vibrations in the sp2-bonded carbon network. There is also a broad peak of similar height at around 1320−1345 cm−1 (the D band), which is attributed to single-bonded sp3 carbon atoms that are present within the SWNH aggregates (see Figure 14).91
carbon atoms by covalent σ bonds, creating a robust honeycomb lattice (hcb). Graphene is currently the strongest known material. The unhybridized p orbitals of the carbon atoms are oriented perpendicularly to the planar structure of the graphene sheet and interact with one another to form the half-full π band that gives graphene its aromatic character. Typical TEM images of graphene monolayers show them as transparent films on the supporting substrate, whereas HRTEM images clearly show the hexagonal lattice (see Figure 15a,b).92,93 The optical transmittance of a graphene monolayer has been measured at nearly 97.5%,94but the transmittance decreases rapidly as the number of stacked monolayers increases. TEM is not powerful enough to discriminate between graphene monolayers and stacked nano-
2.3. 2D Carbon Nanoallotropes
2.3.1. Graphene. Graphene is a very abundant material because it is the building block of natural graphite. Every carbon atom in a graphene sheet is connected to three neighboring 4751
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Figure 13. Examples of the end structure of an SWNH when all pentagons are located (a−c) at the conical shoulder or (d−f) at the apex. Reprinted with permission from ref 90. Copyright 2000 American Physical Society.
Figure 14. Raman spectra of pristine SWNHs (black line) and watersoluble f-SWNHs (red line). Reprinted from ref 91. Copyright 2010 American Chemical Society. Figure 15. (a) TEM image of a single graphene monolayer. Reprinted with permission from ref 92. Copyright 2008 Nature Publishing Group. (b) HRTEM image of a single graphene monolayer. Reprinted from ref 93. Copyright 2008 American Chemical Society. (c) AFM image of a single graphene monolayer. Reprinted with permission from ref 9. Copyright 2007 Nature Publishing Group.
sheets with two or more layers. However, the thickness of the nanosheets can be measured using AFM techniques (see Figure 15c).9 The quality of a batch of graphene depends on the method by which it was produced; methods that yield higher quality material are generally more expensive. Low-quality graphene normally exhibits poor electrical conductivity or mechanical strength due to defects or oxygen sites that are dispersed in the graphenic lattice. In addition, its nanoplatelets are usually limited in size, and it is often mixed with multilayered graphenic nanosheets. Like other graphitic materials, the Raman spectra of graphene nanoplatelets contain a prominent G band at 1580 cm−1, a D band at 1350 cm−1, and a 2D band (also known as the G′ band) at 2700 cm−1.82 The G band is usually assigned to the E2g phonon of the sp2 carbon atoms. The D band is due to the A1g mode breathing vibrations of the aromatic rings and is activated only for the rings that are bound directly to sp3 carbon atoms at defect sites and near edges. The 2D band is the overtone of the D band and is a second-order two-phonon mode (see Figure 16).95−98 The ratio of the first two bands, ID/IG, reflects the percentage of sp3 carbon in the graphitic nanostructure and thus the number of defect sites and edge atoms. These distinctive spectroscopic features make Raman spectroscopy a powerful tool for assessing the quality of graphene samples. For example, graphene produced by liquid exfoliation of graphite is of high quality, so its Raman spectra exhibit either no D band or a very weak D band in the case of small flakes, which have a comparatively high proportion of edge atoms.92 Conversely, graphene nanoplatelets produced by reducing graphene oxide usually have a greater
number of defects and oxygen sites and therefore have a higher ID/IG ratio than liquid-exfoliated graphene (see Figure 16).97 The third characteristic feature in the Raman spectrum of graphene nanosheets is the 2D or G′ band, which appears near 2700 cm−1. Its shape and shift are determined by the number of layers in the graphene nanosheet. In bulk graphite, this band appears as a broad double peak above 2700 cm−1, with the two peak components having an intensity ratio of 1:2. Its height is usually half that of the G band. In graphene sheets of intermediate thickness, the G′ peak shifts to progressively lower wavenumbers and its shape profile becomes sharper as the number of graphene monolayers decreases. In the spectra of pure single graphene monolayers, the G′ band occurs below 2700 cm−1 as a single sharp peak.96 2.3.2. Multilayer Graphitic Nanosheets. Multilayer graphitic nanosheets (MGNs) consist of between 2 and 10 graphene monolayers (see Figure 17).99 Their properties are similar to those of graphene. MGNs can be dispersed in organic solvents, forming stable transparent suspensions. The Raman spectra of MGNs clearly indicate that they have defective graphitic structures. This is demonstrated by their G′ bands (as discussed in the preceding subsection) and the presence of both G (1600 cm−1) and D (1385 cm−1) bands. The 4752
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Figure 17. TEM images of multilayer graphitic nanosheets. Reprinted with permission from ref 99. Copyright 2009 Springer.
Figure 16. (a) Raman spectra of graphite and graphene monolayers. (b) Evolution of the G′ band as the number of graphene layers decreases. Reprinted with permission from ref 96. Copyright 2006 American Physical Society. (c) Raman spectrum of pristine graphene nanoplatelets. Reprinted with permission from ref 98. Copyright 2009 Elsevier B.V. (d) Raman spectrum of chemically reduced graphene oxide compared to that of pristine graphite. Reprinted from ref 97. Copyright 2008 American Chemical Society.
Figure 18. (a, b) TEM images of graphene nanoribbons formed upon unzipping the carbon nanotubes. Reprinted with permission from ref 101. Copyright 2009 Nature Publishing Group. (c) Schemes of the structures of armchair and zigzag graphene nanoribbons. (d) Model of the graphene nanoribbon edge showing a junction between the armchair and zigzag patterns. Reprinted with permission from ref 102. Copyright 2010 Elsevier Ltd. (e) High-resolution transmission electron microscopy image of graphene edges showing overlapping armchair edges together with zigzag edges (the structure was obtained after application of Joule heating to the graphitic ribbon). Reprinted with permission from ref 106. Copyright 2009 American Association for the Advancement of Science.
FTIR spectra of these materials contain a clear band at 1725 cm−1, which is assigned to carbonyl groups, and a broad band
between 1500 and 900 cm−1, which is assigned to the O−H deformations of C−OH groups and to C−O stretching 4753
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produced from carbon sources such as graphite, organic gases, or volatile organic compounds by using instrumental techniques to reorganize carbon atoms. The most common such techniques are carbon vapor deposition (CVD), laser ablation, and arc discharge.
vibrations. The presence of these functional groups is attributed to the method used to form the MGNs.99 2.3.3. Graphene Nanoribbons. Thin ribbons of graphene monolayers are an alternative form of graphene that are currently receiving significant research attention. Most studies on these materials have focused on the thin elongated graphene monolayer strips that can be produced by “unzipping” CNTs (see Figure 18a,b).100,101 Graphene nanoribbons are frequently described as a onedimensional sp2-hybridized carbon strip of finite dimension with defined edges at which carbon atoms are of non-3-coordinated nature. Three types of graphene nanoribbons are currently recognized depending on the edge termination:102−105 (i) armchair, (ii) zigzag, (iii) and chiral nanoribbons. In particular, the width of the armchair graphene nanoribbon is expressed as the number of dimer C−C lines (Na) across the nanoribbon, while in the case of the graphene nanoribbon with zigzag edges, the width is given by the number of zigzag chains (Nz) across the nanoribbon (see Figure 18c). As the edge carbon atoms are not bound saturated, edge reconstruction may occur. While the edge pattern of the armchair graphene nanoribbon is stable due to the presence of strong dangling bonds, edge reconstructions are expected for zigzag graphene nanoribbons at elevated temperatures. To stabilize the edge structure, hydrogen saturation is commonly applied. Other edge profiles have been observed involving pentagonal and heptagonal carbon rings; however, such edge reconstructions are very rare.102 Sometimes, graphene nanoribbons can feature a combination of different edges (see Figure 18d,e);102,106 such structures are termed hybrid graphene nanoribbons, most frequently with heterojuctions of armchair and zigzag patterns. Analogously to graphene, graphene nanoribbons can show bilayered or few-layered arrangements; if more layers of finite graphene strips are stacked together, the architecture is termed a graphitic nanoribbon.102 The Raman spectra of graphene nanoribbons derived from MWNTs contain a stronger D band than those of the starting materials due to the greater numbers of defect sites and higher edge-to-surface ratios of the nanoribbons (see Figure 19).107
3.1. 0D Carbon Nanoallotropes
3.1.1. Fullerenes and Onion-like Carbon. C60 and other fullerenes are produced from graphite by vaporization using arc and plasma discharges2,13,108,109 or laser irradiation.1,110 Alternative methods include naphthalene pyrolysis111 and hydrocarbon combustion.112,113 The latter technique was introduced by Howard et al.112 and is useful for the large-scale commercial production of fullerenes.114 The crude products obtained using all of these methods contain a small proportion of fullerenes, with C60 being the most abundant. The fullerene mixture is isolated from the initially formed soot by simple extraction with benzene or toluene, after which the desired products are isolated by column or liquid chromatography.11 The main disadvantages of most fullerene preparation methods are their low yields and the difficulty of isolating and purifying the desired products, which greatly increases the cost and environmental impact of fullerene production even though it uses cheap and abundant raw materials such as graphite.115,116 To explain the formation of fullerenes, several models have been proposed; they have been reviewed at length by Mojica et al.116 The first mechanism, known as icospiral particle nucleation,110,116,117 assumes a corannulene-like C20 molecule as a starting structure with one pentagon surrounded by five hexagons. Owing to its high reactivity, it forms nautilus-like open spiral shells by accretion of small carbon fragments that are adsorbed on the surface of these shells. Further growth is accompanied by edge bypass; the final closure of the structure is believed to occur on a statistical basis when a proper number of pentagons are incorporated into the structure. Another mechanism describing formation of fullerenes involves four steps.116,118 First, vaporized carbon atoms from graphite tend to form carbon chains with a length of up to 10 atoms (up to C10). These carbon chains then progressively grow to monocyclic rings (C10C20). In the third step, further growth leads to threedimensional carbon networks resembling features of curved shells. The last step then involves growth of small fullerene cages due to the shell-closing mechanism. The formation of C60 and C70 can be alternatively explained by the so-called ring-stacking model.116,119 In this model, C10 ring is considered as the starting structure that undergoes deformation to form a molecule with two hexagons and eight dangling bonds. This molecule is then stacked with C18 so that initial dangling bonds are eliminated. At the same time, new dangling bonds emerge, the number of which decreases in sequence upon stacking with C18, C12, and C2 molecules, ending with formation of the C60 cage. For C70, C10 is stacked with C18, C20, C16, and 3C2 molecules. Annealing of carbon clusters was suggested as another model to understand fullerene formation.116,120 Carbon clusters include chains and rings of carbon atoms, bicycles or tricycles involving 34−60 carbon atoms, larger clusters, or other fullerenes.116 The fullerenes are then formed upon the process of either sequential isomeric transformations of carbon clusters or crystallization of liquid-like clusters. Annealing of carbon atoms is accompanied by emission of atoms or small clusters. The theory (quantum chemical molecular dynamics) predicts formation of fullerenes adopting the so-called shrinking hot giant road.116,121 The proposed mechanism involves two steps: (i) the size-up process
Figure 19. Raman spectra of MWNTs (black line), carbon nanoribbons prepared from them (blue line), and the nanoribbons after reduction (red line). Reprinted from ref 107. Copyright 2010 American Chemical Society.
3. METHODS FOR PREPARING CARBON NANOSTRUCTURES Most carbon nanostructures are constructed from 2D hexagonal carbon lattices. However, in practice, it is not generally possible to fabricate carbon nanostructures using a carbon lattice as the starting material. In fact, aside from graphene nanoplatelets and multilayer carbon nanosheets that can be isolated from naturally occurring graphite, graphitic nanostructures are generally 4754
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when giant fullerenes are formed from hot carbon vapor and (ii) the size-down process when giant fullerenes are shrunken to C60 and/or C70 by irreversible elimination of C2 molecules as a result of the vibrationally excited, highly irregular, and defective nature of cages of giant fullerenes.121 Besides graphite, other precursors have been successfully used to produce fullerenes by the bottom-up approach. Corannulene, a polycyclic aromatic hydrocarbon (C20H10) and the smallest subunit of C60, is viewed as the most suitable C60 precursor due to a curved π surface.116,122 Other promising hydrocarbon molecules include decacyclene, tribenzodecacyclene, and trinaphtodecacyclene.116 However, syntheses employing such precursors are very complex with many steps involving, at first, preparation of precursors themselves and, then, subsequent treatment to fullerenes, frequently using flash vacuum pyrolysis to promote the ring-closure process.116,122 It was theoretically suggested to use simple precursors such as methane and ethylene together with supported metallic nanoparticles acting as a template and catalyst.123 However, three conditions must be secured during fullerene synthesis, including (i) complete dehydrogenation of precursor molecules by the metallic nanoparticles, (ii) persisting attachment of carbon atoms and carbon intermediate phases at the surface of metallic nanoparticles, and (iii) energy preference for formation of fullerenes over other carbon nanoallotropes such as graphene.123 Other routes to fullerenes include conversion (via intramolecular multimember ring formation reactions) of tetrahedral precursors such as benzene rings and acetylene rods or inverse conversion of CO2 upon elevated temperature and pressure.124 OLC structures were first prepared by intense irradiation of carbon nanotubes with electron beams.19 Macroscopic quantities of OLC structures are prepared by heating carbon soot under vacuum at 2100−2250 °C.125 The nanostructures produced in this way have 2−8 graphenic shells with diameters of 3−10 nm and various nonspherical shapes. Sano et al.126,127 and Alexandrou et al.128 produced high-quality spherical OLC structures with mean diameters of 25−30 nm on a milligram scale by means of an arc discharge between graphite electrodes submerged in water. In addition, Cabioc’h et al.129,130 have produced significant quantities of spherical OLC structures with diameters between 10 and 20 nm by implanting carbon ions into matrixes of metals such as silver (see Figure 20). Kuznetsov et al.17 developed a gram-scale method for preparing OLC structures by annealing an ultradispersed diamond powder with an electron beam at 1000−1500 °C under high vacuum. Different types of OLC structures can be produced by varying the temperature and duration of the annealing process.131−133 3.1.2. Carbon Dots. C-dots were serendipitously discovered by Xu et al.22 during the purification by gel electrophoresis of SWNTs prepared using the arc-discharge technique. In keeping with the great interest in carbon nanostructures and their fascinating properties, a great variety of techniques and methods for preparing C-dots have been developed in recent years. The synthesis, properties, and applications of C-dots have been described at length by Li et al.45 They are typically prepared using a top-down approach based on laser ablation of appropriately treated mixtures of graphite powder and cement. The laserablation step forms the basic structure of the C-dots and is followed by treatment with an oxidative acid that enriches the surfaces of the newly formed dots with reactive oxygen groups. This in turn is followed by a surface-passivation step in which organic molecules and oligomers are entrapped on the dots’
Figure 20. HRTEM micrographs of carbon onions synthesized by carbon ion implantation into a silver substrate. Reprinted with permission from ref 130. Copyright 1998 American Institute of Physics.
surfaces. The resulting nanoparticles have a mean diameter of about 5 nm.23 An interesting alternative method for C-dot preparation uses a simple electrochemical apparatus featuring two graphite electrodes in pure water.21 This procedure is schematically outlined in Figure 21. HRTEM images of material prepared in this way show C-dots that have a mean diameter of 4.5 nm with a narrow size distribution and which contain both graphitic and amorphous components. Similar electrochemical processes have been exploited to prepare C-dots using graphite electrodes24,25 or MWNTs26 as the carbon source.
Figure 21. (a) Schematic illustration of C-dot preparation. Digital photos of C-dots (b) before and (c) after treatment. (d) DLS histogram of a C-dot sample. (e) TEM and (f) HRTEM images of C-dots. Reprinted with permission from ref 21. Copyright 2012 Royal Society of Chemistry. 4755
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Zhang et al.39 have reported that N-doped C-dots with tunable luminescence can be formed by heating toluene solutions of carbon tetrachloride and NaNH2 (as carbon and nitrogen sources, respectively) in an autoclave at 200 °C. Bourlinos et al.31 presented a single-step method based on the thermal decomposition of substituted ammonium citrate salts that yields C-dots with predetermined dispersibility. The citrate serves as the carbon source, while the ammonium cations determine the dispersibility of the formed C-dots. When octadecylammonium citrate is used as the starting material, this method furnishes very organophilic C-dots. Conversely, the decomposition of diethylene glycol ammonium citrate yields strongly hydrophilic Cdots. Citric acid is one the most widely used carbon sources in C-dot preparation due to its low carbonization temperature. For example, when dissolved in octadecene, citric acid can be carbonized by heating to 300 °C under argon.41 Solutions of branched poly(ethylenimine) (BPEI) and citric acid in water are carbonized at even lower temperatures (1500 °C) and pressures (>9 GPa).577−579 If nanotubes are used as the starting materials in this process, their diameter determines the size of the resulting nanodiamonds.579 It was recently shown that exposing C60 to high pressures (5−8 GPa) causes it to collapse into 7−12 graphene clusters with lateral dimensions of 2−4 nm. These clusters consist of disordered sp2 carbon atoms along with some sp3-hybridized carbon.580 The transformations between carbon nanoallotropes observed so far are schematically depicted in Figure 82, highlighting the change in dimensionalities.
Figure 80. Coalescence of two fullerenes (C60−C60 and C70−C70) into carbon nanotubes with (6,5), (7,6), and (9,4) chirality. Reprinted with permission from ref 566. Copyright 2013 Royal Society of Chemistry.
were previously unique to fullerenes among carbon nanostructures, such as superconductivity.569,570 The coalescence of carbon allotropes such as nanotube− nanotube, fullerene−nanotube, and fullerene−fullerene has been theoretically well addressed in several works by Zhao et al.571−573 Lower energy reorganization of the carbon−carbon bonds through the Stone−Wales bond switching is discussed as the most likely route during these types of coalescence.574 It was recently suggested that graphene nanoribbons could be ideal platforms for the synthesis of carbon nanotubes with predetermined chiralities.575 The necessary transformations can be achieved inside well-defined carbon nanotubes, which serve as templates for the formation of the new nanotubes. In contrast to the well-characterized fullerene pathway (see Figure 81), the nanoribbon pathway uses precursors such as perylene-3,4,9,10-
6. COMBINING NANOARCHITECTURES TO PRODUCE ADVANCED ALLOTROPIC HYBRIDS 6.1. Fullerene Aggregates
An interesting property of C60 and its organically functionalized derivatives is their ability to form nanostructured crystals with
Figure 81. Proposed pathways for the nanotube-templated synthesis of new nanotubes: (1) conventional deformed fullerene path and (2) twisted graphene nanoribbon path. Reprinted with permission from ref 575. Copyright 2013 Nature Publishing Group.
Figure 82. Scheme showing interconversions among various carbon nanoallotropes highlighting the change in dimensionality. The onesided arrow represents one-way transformation, while the two-sided arrow denotes both-way transformation. 4784
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different shapes. These shapes are usually determined by the methods used in their preparation and the organic functional groups that they bear.581−583 Nanosized needle-like crystals of C60 form at the interface between the solvents when 2-propanol is diffused into a toluene/C60 solution or at the solid−liquid interface when an m-xylene/C60 solution is allowed to evaporate on silica or glass substrates (see Figure 83).584−586 The crystals
Figure 84. (a) Structures of fullerene derivatives 1−5. (b) AFM image of derivative 1 on HOPG. (c) High-resolution STM image of 1 on HOPG. (d) Schematic illustration showing the molecular organization of 1. Reprinted from ref 588. Copyright 2006 American Chemical Society.
Figure 83. (a) SEM and (b) HRTEM images of C60 nanorods formed by the slow evaporation of the solvent from a C60/xylene solution. (c) C60 nanosheets formed by interfacial precipitation from an alcohol/CCl4 system. Reprinted with permission from ref 585. Copyright 2006 WileyVCH.
that grow during the evaporation of m-xylene have mean widths and lengths of around 200 nm and 100 μm, respectively. Thin, transparent, and flexible nanosheets with a characteristic hexagonal shape are formed by the interfacial precipitation of a biphasic alcohol/CCl4 system (see Figure 83). The diameters of these hexagonal nanosheets depend on the alcohol used, ranging from 500 to 700 nm for methanol, from 2 to 3 μm for ethanol, and from 7 to 9 μm for 2-propanol.587 C60 molecules are known to accumulate in rows when they are deposited on templating surfaces such as highly oriented pyrolytic graphite (HOPG) (see Figure 84),588 pure metallic surfaces such as Au/Ni(110),589 oxygen-treated Cu(110),590 or a Au(111) surface covered with a self-assembled alkanethiol layer.591 Yang et al.592 reported that hybrid rGO−C60 nanocomposites can be formed at liquid−liquid interfaces by slowly diffusing a suspension of rGO in 2-propanol into a saturated solution of C60 in m-xylene (see Figure 85). C60 and rGO nanoplatelets interact favorably via π−π interactions to form long nanowires. These nanowires are micrometers in length with diameters of 200−800 nm. Whereas their individual components, rGO and C60, are ambipolar and n-type semiconductors, respectively, the hybrid
Figure 85. SEM images of (a) C60 nanowires, (b) rGO−C60 wires, and (c) rGO after it was mixed with a solution of C60 in m-xylene. (d) TEM images of an rGO−C60 nanowire. The magnified image shows the rGO layers. The inset shows the Raman spectrum of an rGO−C60 wire in which bands corresponding to both rGO and C60 can be seen. Reprinted from ref 592. Copyright 2011 American Chemical Society.
nanowires exhibit p-type semiconducting behavior. This is probably due to electron transfer from the rGO to C60.592 Various interesting C60-based nanoarchitectures can be created using superstructural substrates within which the molecules can order themselves. For example, the pores of anodic aluminum oxide (AAO) membranes can serve as host sites for C60 molecules. C60 can then be electrocoated and polymerized within the pores, after which the template can be removed, leaving intact C60 nanowires behind (see Figure 86).593 4785
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Figure 87. Dodecahedral structure of the C20 carbon cluster. Figure 86. (a) Schematic illustration of the immobilization of C60 in the channels of AAO. (b) TEM image of polycrystalline C60 nanowires. Reprinted with permission from ref 593. Copyright 2003 Wiley-VCH.
producing gram quantities of C20H20 that will greatly facilitate further studies in this area. Belau et al.604 used vacuum ultraviolet (VUV) photoionization mass spectrometry in conjunction with ab initio calculations to study small carbon clusters (Cn; n = 2−15) that were formed using an advanced light source. The VUV source provided the necessary energy to study the small carbon clusters, whose ionization potentials lie in the 9−13 eV range. It was observed that when using relatively high ionization energies (>10 eV), the even-numbered carbon cluster ions with between 9 and 15 carbon atoms were more prominent than their odd-numbered counterparts in the acquired spectra. Increasing the ionization energy to more than 12.5 eV caused the C3 ion to become dominant, but no C2 ions were observed under these conditions, suggesting that larger odd- and even-numbered clusters may grow by different mechanisms. Ionization potentials were determined experimentally for the C3 to C15 clusters. Computational studies were then performed to determine the relative energies of the linear and cyclic structures of the C4 to C10 clusters as both neutral species and cations. It was found that the odd-numbered neutral clusters prefer linear motifs whereas their even-numbered counterparts favor cyclic structures. Similar results were obtained for the cations, but the C7+ and C9+ clusters preferred a cyclic configuration. The CCSD(T)-optimized structures of the linear and cyclic neutral carbon clusters from C4 to C10 are shown in Figure 88.
6.2. Carbon Clusters
Research on pure carbon molecules (Cn) has its origins in extraterrestrial observations as noted by Hinkle et al.,594 Kirkwood et al.,595 and Hartquist and Williams.596 Early terrestrial studies by Rohlfing et al.597 focused on carbon clusters generated by irradiating graphite using laser vaporization techniques, which produced a bimodal distribution of Cn carbon clusters (n = 1−190). The first peak of the bimodal distribution corresponded to smaller Cn clusters (1 ≤ n ≤ 30) with both odd and even numbers of carbon atoms. The second peak corresponded to larger carbon clusters with even numbers of atoms, C2n, where 20 ≤ n ≤ 100. This pattern was attributed in part to the clusters’ ionization potentials. A comprehensive description of the studies that have since been conducted in this area would be beyond the scope of this review. However, the early reviews by Weltner and Van Zee in 1989598 began to pull together the available characterization data and theoretical information concerning these clusters. Then, in 1998, van Orden and Saykally599 published a new review that augmented the older ones with a more comprehensive set of theoretical data and experimental observations. More recently, Lifshitz600 published a personal perspective on the detection and analysis of carbon clusters using mass spectrometry. The first two of these reviews initially discuss smaller carbon clusters (C2 to C10), but their main focus is on the Cn molecules and Cnm ions (m ≠ 0). These species can exist as either linear or cyclic molecules with cumulene- or acetylene-like bonding arrangements between carbon atoms that may be in their either ground or excited states based on the results of computational studies. Computational results have been particularly useful in analyzing the spectra of these carbon clusters in cases where they have been detected optically. Prinzbach et al.601 have described their attempts to construct the smallest possible fullerene: the dodecahedral C20, which has only 12 pentagonal faces (see Figure 87). Their brute force approach was to replace the hydrogen atoms of C20H20, which was first synthesized by Paquette et al.,602 with bromine atoms and to then debrominate this material by electron impact ionization. This resulted in the formation of C20 ions that could be detected by mass spectrometry and differentiated from brominated corannulene molecules by photoelectron spectroscopy. Prinzbach et al.603 subsequently demonstrated that the spectra of C20 ions generated from C20H0−3Br14−12 tri/tetraene, corannulene, perylene, and graphite are all different from one another. In addition, they have developed a method for
Figure 88. Calculated minimum energy structures for the linear or cyclic clusters of C4 (top) to C10 (bottom).
Dunk et al.605 have prepared and studied stabilized endohedral complexes of the very small fullerene C28. The incorporation of a stabilizing guest metal atom into the fullerene’s cavity is essential for its synthesis, and the stabilization of C28 by Ti, other group IV metals, or U is unique. Two isomers of C28 can exist, one with Td symmetry and one with D2 symmetry. Computational studies 4786
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Fernandez-Lima et al.609 generated cationic C2+ to C16+ clusters by electronic sputtering from 252Cf fission fragments onto frozen CO and by UV laser vaporization of amorphous carbon and graphite. They characterized the resulting clusters experimentally and proposed new bicyclic, tricyclic, and rhombic structures for them along with the established linear and monocyclic structures. The polycyclic, linear, and cyclic structures were optimized at the DFT/B3LYP/6-311G** level of theory, and the computed results were compared to the experimental measurements. In keeping with the results of previous studies, the linear and monocyclic structures were found to be the most stable, regardless of the ionization source used. Baranovski610 optimized the geometries of the cyclic carbon clusters C8, C10, and C12 at the CCSD(T)/cc-pVDZ level of theory. The clusters were found to have D4d, D5h, and D6h symmetries, respectively. The calculated atomization energies for cyclic C8, C10, and C12 were 989, 1336, and 1595 kcal M−1, respectively, which are in good agreement with previous experimental results and quantum chemical calculations.611−613
indicate that the Td-C28 isomer, which is based on four fused triple pentagons, is less strained than its D2-C28 counterpart, which is based on four joined pentagons (see Figure 89). Likewise, Ti@Td-C28 is calculated to be more stable than the Ti@ D2-C28 isomer.
Figure 89. Two isomers of the C28 carbon cluster: (a) Td-C28 and (b) D2-C28.
6.3. Assembled Nanostructures Containing Graphene Quantum Dots
Research by Koyasu et al.606 on the smaller C7+ to C10+ cationic carbon clusters showed that they can be formed in relatively large quantities by laser ablation and that their mechanisms of dissociation depend on whether they exist as linear or cyclic clusters. The linear and cyclic isomers were separated using ion mobility spectrometry and fragmented by either photodissociation or collision-induced dissociation (CID). The CID fragmentation of the linear isomers C8+ to C10+ preferentially occurred via the loss of a C3 fragment, while the cyclic isomers tended to lose a C2 fragment along with the C3 fragment. The C7+ isomers were observed to fragment by losing either C2 or C3 clusters, with C4 or C5 clusters being generated simultaneously. Kong et al.607 generated large carbon cluster anions Cn− (60 ≤ n ≤ 500) by performing laser ablation on graphene samples prepared from graphene oxide, and by ablating graphene oxide itself. The ablation of graphene produced carbon cluster anions with even numbers of carbon atoms, generating a series of mass spectral peaks separated by 24 m/z units (i.e., two carbon atoms). The ablation of graphene oxide generated carbon cluster anions ranging in size from C90− to C500−, albeit in smaller quantities than the ions produced by ablating graphene. Both the accumulation period and the energy of the laser affected the distribution of carbon cluster anions, with longer accumulation periods enabling the formation of larger cluster anions. The anion size distribution centered around C216− when using comparatively low laser energies, while higher laser energies yielded cluster distributions centered around C180−. Individual anions fragmented in specific ways, by losing C2, C4, C6, or C8 clusters. Kassaee et al.608 studied nanoannular carbon clusters from C4 to C20 using atoms in molecules (AIM) analysis at the B3LYP/631+G(d) level of theory. The optimized structures they obtained were similar to those predicted in previous studies. In addition, they found that the C6, C10, C14, and C18 carbon clusters are of quasi-aromatic character with uniform C−C bond lengths of 1.327, 1.298, 1.289, and 1.285 Å, respectively. In contrast, C4, C8, C12, C16, and C20 had two distinct types of carbon−carbon bonds with different lengths, suggesting that these clusters have a more acetylenic character. The lowest energy optimized structures of the C5 and C9 clusters were nonplanar. The C7, C11, C13, C15, C17, and C19 clusters were predicted to have bond lengths that were intermediate between those of the double bond of ethene and the triple bond of ethyne.
The small size of GQDs combined with the presence of diverse functional groups on their surfaces facilitates their self-assembly into well-defined nanostructures such as nanotubes, honeycomb nanostructures, or hollow microspheres. GQDs prepared by the electrochemical oxidation of graphene contain many carboxylate groups, making them highly dispersible in water. This in turn facilitates the direct electrophoretic deposition of GQDs into the nanochannels of an anodic aluminum membrane (AAO; see Figure 90).614 Once the AAO is removed, this results in the
Figure 90. (a) TEM image of GQDs. (b−d) SEM images of GDQ nanotubes at various magnifications (the scale bar represents 1 μm). (e, f) TEM images of a part of an individual nanotube and part of its wall. Reprinted from ref 614. Copyright 2012 American Chemical Society. 4787
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formation of hollow nanotubes with diameters of 200−300 nm. These tubes have exhibited promising results as substrates for efficient surface-enhanced Raman scattering (SERS). Using a similar templating technique, GQDs have been selforganized to form a microporous honeycomb structure.615 In this case, GQDs were deposited by electrophoresis on a densely packed assembly of SiO2 nanospheres (see Figures 91 and 92).615 The GQDs were then linked to one another in an annealing step, forming a stable honeycomb nanostructure that remained intact after the SiO2 template was removed.
Figure 91. Formation of a GQD honeycomb assembled on SiO2 microspheres. Reprinted with permission from ref 615. Copyright 2012 Royal Society of Chemistry.
Finally, GQDs have been organized into microspheres using a water-in-oil emulsion technique (see Figure 93).616 Due to the presence of surface carboxylates, GQDs disperse readily in solutions of ammonia, forming the corresponding GQD ammonium salts (GQDs−NH4). When an emulsion was formed by mixing such a solution with olive oil under mechanical agitation, GQDs−NH4 accumulated in water microdroplets at the liquid−liquid interface. After removal of water from the emulsion by evaporation, spherical nanoassemblies of the GQDs−NH4 remained in the oil phase and were isolated by removing the oil. The resulting GQD microspheres had diameters of 0.5−3 μm and symmetrical shapes with smooth surfaces. The same method can be used to prepare GQD microspheres with other speciesmolecules or nanostructuresencapsulated in their internal spaces. For example, Fe3O4 nanoparticles have been entrapped within GQDs−NH4 microspheres simply by adding them to the mixture during the formation of the water/oil emulsion, thus forming magnetically modified microspheres. By reducing these Fe3O4 nanoparticles to Fe, the microspheres can be used as substrates for the formation of CNTs by CVD. This procedure results in the formation of new GQD−CNT nanocomposite superstructures. As shown in Figure 94,616 the high-quality CNTs that are developed around the microspheres are multiwalled and firmly bound.
Figure 92. SEM images of a GQD honeycomb. The scale bar corresponds to 100 nm. Reprinted with permission from ref 615. Copyright 2012 Royal Society of Chemistry.
6.4. Carbon Nanotubes and 2D Graphene Nanostructures
The optoelectronic and mechanical properties of graphene and carbon nanotubes make these carbon nanomaterials particularly interesting for the production of transparent conductive films or electrodes and membranes that could enhance the performance
Figure 93. (a−d) Synthesis of GQD microspheres. (e−g) SEM images of GQD−NH4 microspheres at several magnifications. Reprinted with permission from ref 616. Copyright 2012 Institute of Physics. 4788
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Figure 95. (a) An 80 nm thick CNT film on a sapphire substrate. (b) An AFM image of a 150 nm thick CNT film (color scale: brown to bright yellow, 30 nm). Reprinted with permission from ref 618. Copyright 2004 American Association for the Advancement of Science.
necessary to remove oxygenated groups and restore the aromatic system of graphene. For example, a graphene film prepared by spin coating on a quartz substrate displayed a sheet resistance of 100−1000 Ω/square with 80% transmittance after reductive treatment.628 Other attempts to produce graphene films have yielded similar results.621,629 An interesting method for the deposition of CNTs and graphenes on nonspecific substrates to form transparent conductive films has been developed by D’Arcy et al.630 In this method, a liquid−liquid interface is used to organize and align CNTs or graphene nanoplatelets in a thin film, which can then be transferred onto various substrates. CVD techniques have been used to produce transparent thin graphene films on a nickel substrate. The resulting films consist of a few graphene layers and exhibit a sheet resistance of 230 Ω/square at 72% transparency.627 Thin films with similar properties have also been prepared by CVD on Ni or Cu.631,632 6.4.2. Conductive Membranes and Papers. 2D carbon microstructures with thicknesses of up to several micrometers can be prepared using methods similar to those for the production of thin films. So-called CNT or graphene membranes or papers are more conductive than thin films but sacrifice their transparency (see Figure 96).245,633−638 Graphene paper has
Figure 94. (a, b) SEM images (the scale bar corresponds to 1 μm) and (c, d) TEM images of GQDs−CNTs. Reprinted with permission from ref 616. Copyright 2012 Institute of Physics.
and capabilities of electronic devices. An interesting review that compares the performance of these two widely used carbon nanomaterials has been presented by Lee and Biswas.617 6.4.1. Transparent Thin Films. Thin films of graphene or CNTs are transparent and have remarkable flexibility, mechanical stability, and electrical conductivity.618 Given these advantages and their relatively low cost, thin films of carbon nanostructures could potentially replace indium tin oxide (ITO) electrodes in a great variety of applications. The remarkable properties of CNT and graphene thin films are due to the extended aromatic system and atomic thickness of graphene, the minute diameters of CNTs relative to their lengths, and the strong van der Waals interactions they form. High electrical conductivity is a fundamental property of graphitic nanostructures. However, the conductivity of thin films is primarily determined by the tube−tube or sheet−sheet junctions between individual CNTs or graphene sheets. Consequently, films often have much lower conductivities than might be expected on the basis of theoretical calculations. The electrical conductivity and optical transparency of CNTs are also influenced by the ratio of metallic to semiconducting CNTs, which cannot yet be reliably controlled or specified during CNT synthesis.619 CNTs and graphene thin films are easily prepared by vacuum filtration from dilute surfactant-assisted suspensions,618,620 dip coating,621,622 Langmuir−Blodgett film or layer-by-layer (LBL) assembly, 246,623−625 drawing from CNT arrays,626 and CVD processes.627 A 50 nm thin SWNT film prepared by vacuum filtration from an SWNT suspension exhibited an optical transmittance in excess of 90% and a sheet resistance of around 30 Ω/square (see Figure 95).618 These values are comparable to those achieved for ITO, which typically exhibits a sheet resistance of 5−10 Ω/square and >85% optical transparency. In principle, graphene nanoplatelets have an advantage over SWNTs in terms of their optical transparency and electrical conductivity because a monolayer of graphene is only one atom thick whereas an ideal SWNT monolayer would be at least two atoms thick. However, thin films prepared from GO suspensions are less electrically conductive than equivalent CNT films, even after being reduced by high-temperature annealing, which is
Figure 96. (a) Photograph of free-standing graphene paper. (b) Sideview SEM images of a ∼6 μm thick graphene paper sample. Reprinted with permission from ref 633. Copyright 2008 Wiley-VCH.
been prepared by filtering a graphene suspension through an anodisc membrane to yield a material with an electrical conductivity of around 7200 S/m.241 Other techniques for preparing such structures are based on the electrophoretic deposition of GO,635 self-assembly of oxidized short-cut CNTs636 or GO637 on hydrophilic glass surfaces, and LBL deposition.638 4789
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6.5. Graphenic Hybrid Nanocomposites
Many graphene−CNT hybrids have recently been prepared in an effort to identify a material that combines the properties of its individual components.639 Such hybrids have been prepared by liquid-phase interaction, self-assembly,640−654 and CVD,655−667 among other methods. Recently, Yan et al.668 reported on a 2D SWNT reinforced graphene hybrid (rebar graphene) prepared employing annealing treatment of an alkyl-functionalized SWNT by hydrogen. It is believed that the functional groups from the SWNT surface are the carbon source for the formation of graphene monolayers that are directly connected with the SWNT by π−π stacking or covalent bonding. The graphene− CNT hybrid in the form of fibers showed high electrical conductivity, electrocatalytic activity, and mechanical properties.669 6.5.1. Graphene−CNT Thin Films. Several attempts to prepare graphene−CNT hybrids have focused on thin films due to the interesting electrical properties of such materials.640,641,670 A self-assembled GO and CNT hybrid film with high conductivity and transparency has been prepared by using sonication to disperse chemically reduced graphene nanoplatelets and CNTs in dry hydrazine. The self-assembled graphene−CNT composite is then isolated after removal of the hydrazine at an elevated temperature or by using it to spincoat a substrate. Thin films prepared in this way exhibit an optical transparency of 92% and a promising sheet resistance of 636 Ω/ square. The film’s resistance can be further reduced to 240 Ω/ square by anion doping, which is achieved by exposing the film to SOCl2 vapors. However, this slightly reduces its optical transmittance (to 88%).640 King et al.671 demonstrated that adding graphene to nanotube films affects their transparency and conductivity. Their results indicate that the optimal graphene content of a graphene−CNT thin film is about 3% by weight. In practice, an acid-treated CNT film with a thickness of 35 nm containing 3% graphene exhibited a sheet resistance of 100 Ω/ square with an optical transparency of 80%. 6.5.2. Graphene−CNT Membranes. In addition to thin films, self-assembly techniques have been used to prepare nanostructured membranes and papers. Well-dispersed GO nanoplatelets can act as surfactants that facilitate the dispersion of CNTs in several solvents, leading to the formation of GO− CNT composite nanostructures that are held together by van der Waals interactions. If such nanostructures are formed in an aqueous emulsion, the slow evaporation of the organic solvent leads to the formation of very flexible composite membranes suspended in water (see Figure 97).642 Cai et al.643 showed that MWNT−GO membranes prepared by self-assembly from DMF solutions have remarkably low sheet resistances that decrease as the ratio of MWNTs to GO increases. The highest resistance achieved for such membranes was around 1.3 × 10−4 Ω/square at an MWNT:GO ratio of 5:1 by weight. Of course, these resistances are impractically low, and much lower than those achieved with thin films. However, these materials have not been extensively studied, so there is probably scope for improvement, and it is important to note that these resistances were achieved for membranes that are around 8 μm thick whereas typical thin films have thicknesses of only a few nanometers. 6.5.3. Graphene−CNT 3D Hybrids and Pillared Structures. Three-dimensional nanostructures with extremely high active surface areas can be generated by intercalating 1D wirelike nanomaterials such as SWNTs or MWNTs between stacked 2D sheets of materials such as graphene nanoplatelets. The active surface areas of such composite materials are particularly large if
Figure 97. Top: formation of a GO−CNT composite membrane by self-assembly in an aqueous suspension. Bottom: (a) typical digital photo and (b−d) cross-sectional SEM images of the GO−CNT membrane. Reprinted with permission from ref 642. Copyright 2012 Royal Society of Chemistry.
Figure 98. (a) Schematic representation of CNT development on graphene surfaces. (b, c) TEM images of graphene−CNT composites. Reprinted from ref 655. Copyright 2012 American Chemical Society.
the wirelike components are oriented perpendicularly to the 2D sheets, as is preferable for applications in catalysis and storage. The catalytic growth of CNTs on graphene surfaces has been identified as a suitable strategy for the preparation of such 3D carbon nanostructures because it favors the perpendicular connection of CNTs and graphene nanoplatelets.655−657,659−667 667 An easy way to prepare graphene−CNT composite 4790
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nanostructures involves the direct growth of CNTs on graphene surfaces by means of CVD or some similar technique. For example, CNTs can be developed on graphene surfaces by catalytic growth over Fe nanoparticles at high temperature. The catalytic nanoparticles are formed during the procedure by the reduction of Fe cations from FeMgAl layered double hydroxide flakes that are laid over the graphene layer. CNTs are then formed by the decomposition of hydrocarbons over the catalytic centers at high temperature (see Figure 98).655 In addition, plasma-enhanced CVD has been used to deposit vertically aligned CNTs on a graphene substrate in distinct dots. The development of these CNT “pillars” was mediated by catalytic Fe/Al nanoparticles that were deposited symmetrically by photolithography (see Figure 99).656 The final hybrid graphene−CNT composite that was deposited on the quartz film substrate was conductive, flexible, and highly transparent because the CNT pillars were uniformly dispersed and covered only a small part of the graphene surface.656 Low-pressure CVD techniques have also made it possible to deposit carbon nanofibers on few-layer graphene nanosheets.665 An alternative approach has been presented by Rout et al.666 in which the growth of aligned CNTs on a Si substrate by microwave plasma CVD is followed by the growth of graphene nanoplatelets on top of the CNT forest using the same technique. The growth of CNTs between graphene nanoplatelets by the thermal decomposition of iron phthalocyanine leads to the formation of pillared graphene hybrid systems as described by Du et al. (see Figure 100).659 In this method, thermally expanded HOPG sheets whose surfaces have been coated with SiO2 serve as the skeleton over which CNTs are grown. The length of the CNT pillars can by tuned by adjusting their growth conditions. Computational studies indicate that the resulting 3D pillared graphene−CNT architectures could be useful for H2 storage if doped with Li ions.672 Alternative pillared graphene systems have been constructed by using commercially available carbon nanospheres (CNs) as the pillars that separate GO nanosheets. The CNs are immobilized on the graphene surface by simply mixing them with exfoliated GO nanosheets in water. The CN-pillared graphenic structure is then obtained by lyophilization to remove water. The graphenic structure of the GO sheets can be partially restored by heating the product at 300 °C under H2. The CN pillars help to stabilize and increase the active graphenic surface area of the composite material, which is crucial in catalytic applications, by preventing the restacking of dispersed graphene after the removal of the solvent. The 3D graphitic network generated in this way had a moderately high Brunauer− Emmett−Teller (BET) surface area (∼219 m2/g) and was used as a substrate for the deposition of catalytic Pt nanoparticles. The resulting Pt-decorated pillared graphene system was used to create Pt nanocatalysts for use in PEM fuel cells.653 A similar pillared graphene has been prepared by inserting functionalized carbon nanoparticles between GO nanosheets. The nanoparticles used in this case were derived from commercially available EC300 carbon black that had been functionalized with SO3H moieties. After the composite was formed, the GO sheets were reduced with hydrazine. The final nanocomposite has a layered structure with high porosity and a remarkably high measured surface area of 1256 m2/g, indicating that the graphene pillaring in this case was much more effective than in the previously discussed example. This was probably because the functionalized carbon nanoparticles were more effectively dispersed across the GO surface than the grown CNTs
Figure 99. (a) Preparation of vertically aligned CNTs (VACNTs) on graphene. (b) SEM images of graphene−VACNTs deposited on a quartz substrate. The inset shows a digital photographic image of the optical element. Reprinted with permission from ref 656. Copyright 2011 Wiley-VCH.
in the previous case, thereby minimizing sheet aggregation (see Figure 101).654 4791
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Figure 101. (a) Formation of a graphene−carbon nanoparticle nanocomposite. Cryo-TEM images of the solution of functionalized nanocarbon particles and graphene oxide at (b) 0 and (c) 8 h after reduction. Green arrows indicate the functionalized carbon nanoparticles and their aggregates, the yellow arrow indicates the graphene edge, and blue and red arrows indicate the carbon nanoparticles sandwiched between graphene layers. Reprinted with permission from ref 654. Copyright 2012 Elsevier B.V.
generate a robust aerogel. Carbon aerogels are generally lowdensity materials with high porosity, large surface areas, and high electrical conductivity. A simple technique for constructing CNT aerogels involves dispersing carbon nanotubes in a solution and then forming a stable hydrogel using ferrocene-grafted poly(p-phenyleneethynylene) as the cross-linker. The aerogel is then obtained by drying the product using supercritical CO2 to remove unwanted solvent. Subsequent thermal annealing greatly improves the electrical conductivity of the initially formed aerogel to yield a product with a conductivity of 1−2 S/cm, favorable mechanical properties, a high active surface area (590−680 m2/g), and high porosity (99%).675 In a similar process, MWNTs were dispersed and stabilized in a solvent by cross-linking functionalization using poly(3-(trimethoxysilyl)propyl methacrylate) after hydrolysis. Subsequent removal of the solvent yielded an ultralight aerogel with a density of 4 mg/cm3, a surface area of 580 m2/g, and an electrical conductivity of 3.2 × 10−2 S/cm, which was increased to 0.67 S/cm using a high current pulse method.676 Aerogels have also been constructed from MWNTs and chitosan (see Figure 102),677 DWNTs,678 and graphene nanoplatelets,680,681 and by using resorcinol−formaldehyde sol−gel chemistry followed by drying with supercritical CO2 and pyrolysis at 1050 °C under N2. Graphene aerogels prepared in this way exhibited very high surface areas (584−1200 m2/g) that depended on the initial ratio of graphene to resorcinol−formaldehyde, and had densities of 0.016−0.025 g/cm3.680
Figure 100. (a−d) Development of vertically aligned CNTs between the surfaces of thermally expanded HOPG. (e−g) SEM images of the pillared structure at various points during pyrolysis. (h) Height of the pillars as a function of the deposition time. Reprinted from ref 659. Copyright 2011 American Chemical Society.
6.6. 3D Graphenic Hybrid Superstructures
6.6.1. Aerogels, Nanofoams, and Spongelike Nanoarchitectures. A range of more complex 3D architectures based on CNTs and graphene have been described as hydrogels, aerogels or organogels, nanofoams, or spongelike superstructures depending on their properties and basic characteristics.673−689 In these 3D superstructures, growth is equally developed in all three dimensions. Conversely, in graphene−CNT CVD hybrids, growth is limited by the length of the CNTs in one dimension and by the extent of the graphene surface in the other two. However, both types of composite material have threedimensional structures. Carbon aerogels containing CNTs or graphene units can be formed by connections based on stable chemical bonds or via noncovalent interactions. Both approaches can generate stable skeletons. They are usually prepared by either the sol−gel or the aerogel approach. In the former, a dispersion of a carbon nanostructure is mixed with an organic compound that will facilitate the cross-linking of the separate carbon units in a solvent that will form a hydrogel. In the latter approach, the solvent is gently removed from the nanostructure dispersion to 4792
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aqueous suspension, GO is partially reduced (to rGO) and captured in a monolithic construction that is transformed into an aerogel after freeze-drying to remove water (see Figure 104).687
Figure 102. SEM micrographs showing a cross section of MWNT/ chitosan 3D structures formed using various initial ratios of the two components. The inset shows the walls of the structure, which are made up of interconnected MWNTs. Reprinted from ref 677. Copyright 2007 American Chemical Society. Figure 104. Digital photos of (a) a GO solution, (b) the black suspension obtained after the GO solution was reduced with mercaptoacetic acid, (c) a dried rGO aerogel, and (d) the dried aerogel before and after being pressed with a 1000 g load (shown in the inset). Digital photos of the dried aerogel supporting loads of (e) 500 g and (f) 1000 g. (g, h) SEM images of the rGO aerogels with 1% GO reduced by 0.05 mL of mercaptoacetic acid. Reprinted with permission from ref 687. Copyright 2013 Royal Society of Chemistry.
Graphene aerogels can also be prepared from graphene hydrogels (which are in turn prepared by heating aqueous mixtures of graphene oxide with L-ascorbic acid) by drying with supercritical CO2 or by freeze-drying (see Figure 103).681 The resulting aerogels have low densities (0.012−0.096 g/cm3), high electrical conductivity (102 S/m), large surface areas (512 m2/g), and porous structures. A method for the simultaneous reduction of graphene oxide and formation of reduced GO aerogels has been developed by Chen et al.687 By heating mercaptoacetic acid and GO in an
Dried rGO aerogels with rGO concentrations in excess of 0.5% have relatively high mechanical strengths and porosities in excess of 90%.687 Although the exact mechanism by which these structures are formed remains unclear, it seems that mercaptoacetic acid or mercaptoethanol reacts with epoxy and hydroxyl groups on the graphene surface. The functionalized graphene is enriched in carboxylic acid and hydroxyl moieties that facilitate the formation of graphene monolayer assemblies via hydrogen bonding. The thicker and more stable graphene sheets formed in this way probably resemble the final 3D material. Finally, carbon aerogels with skeletons containing CNTs and graphene nanoplatelets were constructed using vitamin C as a sol gelator. The final product obtained in this way has a surface area of 435 m2/g, a pore volume of 2.58 cm3/g, and a high conductivity of 7.5 S/m.685 In addition to the sol−gel method, 3D CNT spongelike architectures679 and graphene−CNT foams686 have been prepared by CVD. CNTs formed by CVD using ferrocene and dichlorobenzene as precursors self-assemble into an interconnected skeleton that forms a flexible but robust ultralight aerogel with a porosity of >99%, a density of 5−10 mg/cm3, and a surface area of 400 m2/g. These spongelike CNT nanostructures can adsorb 80−180 times their own weight of various solvents and oils.679 In addition, a graphene foamlike 3D architecture with remarkable properties has been prepared by CVD following the decomposition of CH4 over a foamlike Ni template at 1000 °C. Before the Ni template is removed by treatment with HCl, the
Figure 103. Photos of (a) a graphene oxide dispersion in water, (b) a hydrogel formed by heating a GO suspension with ascorbic acid, (c) the aerogels formed after the hydrogel was dried with supercritical CO2 (left) or after freeze-drying (right), and (d) a 7.1 mg aerogel pillar supporting a 100 g counterpoise. Reprinted with permission from ref 681. Copyright 2011 Royal Society of Chemistry. 4793
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newly formed graphene skeleton is coated with a protective PMMA layer. The Ni is then removed, and the resulting foamlike graphene superstructure is isolated by removing the PMMA coating with hot acetone. The porous 3D graphene architecture generated in this way is very light and flexible, with a density of 5 mg/cm3, a porosity of 99.7%, a high surface area of 850 m2/g, and good electrical conductivity of about 10 S/cm.688 6.6.2. Hollow 3D Microspheres. Graphene oxide and CNTs can be organized into hollow 3D microspheres in a water/ oil emulsion.690−692 Graphene oxide has numerous carboxylic acid moieties at its edges and defect sites that facilitate its dispersion in water. Therefore, due to their amphiphilic character, GO nanoplatelets can be used to coat the oil-facing surface of water microdroplets in water/oil emulsions, stabilizing them. These GO nanoplatelets are held together by van der Waals interactions and hydrogen bonding. Hollow GO microspheres are subsequently obtained by separating them from the oil and removing water from their internal spaces. A similar approach has been used to prepare oxidized CNT-based microspheres (see Figure 105).691
Figure 106. (a, b) HRTEM images of isolated SWNTs containing Gd@ C82 fullerenes. (c) Schematic representation of the hybrid product. Reprinted with permission from ref 529. Copyright 2000 American Physical Society.
Figure 105. Formation of GO microspheres from a water/oil emulsion. Reprinted with permission from ref 691. Copyright 2010 Royal Society of Chemistry.
6.7. Nanoreactors Based on Carbon Nanotubes
these encapsulation techniques are partially determined by the diameter of the template nanotubes. Like the success of nanoribbon formation inside an SWNT, which produces a series of nanoribbons depending on the tube radius, similar uses for peapod structures were discovered and reviewed earlier by Monthioux.698 Published less than 10 years after the discovery of peapods, this early review highlights the best way of preparing fullerenes encased in CNTs: the two components are placed in a vacuum-sealed quartz ampule, which is then heated to the sublimation temperature of the fullerene. It is well-known that a square peg cannot fit into a round hole whose diameter is smaller than the length of the square’s edges. Similarly, one might reasonably expect that steric interactions would make it impossible to place a fullerene inside a CNT whose radius is smaller than that of the fullerene guest. However, it is not immediately clear how large a guest fullerene can be while still fitting inside a given SWNT. Early theoretical studies on fullerene encapsulation in nanotubes by Okada et al.699 focused on SWNT peapods of the C60@(n,n) (n = 8, 9, 10) type (see Figure 108). These authors found that the encapsulation of C60 in (10,10) SWNTs is exothermic. However, it was found to be endothermic for both (8,8) and (9,9) SWNTs. On the basis of these findings, they calculated that a nanotube must have a radius
Novel hybrid carbon nanoallotropes have been formed by encapsulating carbon nanostructures or other guests in the internal spaces of CNTs.693 The most well-known of these structures are based on C60 and were first described by Smith et al.528 Another notable early example was the encapsulation of metallofullerenes529 inside SWNTs to produce so-called nanotube peapods (see Figure 106), as partly touched on in section 5. It was subsequently shown that C60 molecules encapsulated in this way coalesce inside the host nanotube, generating nested pairs of graphene cylinders.694 After the potential usefulness of the internal spaces of carbon nanostructures was first recognized, they were rapidly used to promote the synthesis of graphene nanoribbons by the condensation of encapsulated polyaromatic molecules. Coronene or perylene molecules were inserted into SWNTs by sublimation and aligned in rows. After heating, one-dimensional nanoribbons were obtained by the successful fusion of the encapsulated polyaromatic molecules.695 Similarly, Chamberlain et al.696 and Chuvilin et al.697 recently prepared well-defined nanoribbons by condensing sulfur-terminated tetrathiafulvalene molecules that were aligned and encapsulated in SWNTs (see Figure 107). The properties of the nanoribbons prepared using 4794
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which C60 enters (10,10) SWNTs. They found that the velocity and entry angle of the C60 molecule are both important for encapsulation, and that it is easier for the fullerene to enter via large defects in the tube’s walls rather than through its open ends. Their calculations suggest that these processes should occur at around 400 °C, which is consistent with the experimental findings. Fullerenes encapsulated in SWNTs are known to exhibit interesting properties. Briefly, Chiu et al.701 have observed that nanotubes containing encapsulated endohedral fullerenes such as (Dy@C82)@SWNT exhibit p-type to n-type conductive behavior between room temperature and 215 K. Upon further cooling, this peapod exhibited metallic behavior, which was attributed to constriction of the nanotube, which would force the fullerenes to distort along the tube’s axial direction. This distortion would change the fullerene’s symmetry, facilitating charge transfer between Dy@C82 and the antibonding π* orbitals of the SWNT. The optical properties of SWNTs with high loadings of C60 or C70 (up to 85% and 72%, respectively) were investigated by Kataura et al.702 using high-quality carbon nanotubes with diameters of 1.29−1.36 nm. Fullerenes were sublimed into these tubes at 605 °C in a sealed ampule. In separate studies, Lee et al.703 found that the band gap of the chiral Gd@C82@(11,9) complex (see Figure 109) shifted from 0.43 to
Figure 109. A model of the chiral Gd@C82@(11,9) complex.
0.17 eV in the vicinity of the encapsulated endohedral metallofullerene. However, Shiozawa et al.704 found that both the valence-band and inverse photoemission spectra of C60@ SWNT were similar to those of the isolated components. Once fullerenes are inside CNTs, they are not necessarily stationary. Instead, they appear to act as dynamic entities. Under electron irradiation, the fullerenes can coalesce to form larger carbon clusters. These may eventually become a second CNT inside the original SWNT, converting it into a DWNT. Hernandez et al.562 performed both experimental studies and theoretical calculations on the formation of such internal nanotubes from encapsulated pairs of fullerenes under the influence of both thermal annealing and electron beam irradiation. In the computational studies, thermal annealing at temperatures ranging from 1300 to 3600 K (see Figure 110) was investigated using an approach whereby the reacting fullerenes were given appropriate amounts of energy for the chosen temperature but not permitted to lose any atoms. Under these constraints, the fullerenes merged and then underwent various rearrangements to minimize their surface energy. In the simulations of electron beam irradiation, carbon atoms were removed at random from the fullerenes to mimic the creation of vacancies by knock-on effects caused by electron irradiation. This creates dangling bonds, which drive the coupling of the fullerenes and their subsequent transformation into a new nanotube. The collaborative studies of Nasibulin et al.705 provided the first direct evidence and visualizations of the coalescence of fullerenes on the surfaces of SWNTs to form so-called nanobuds. These nanobuds resemble arteries with saccular aneurysms: the SWNT (which is the “artery” in this simile) is distorted and covered with ball-like protrusions (see Figure 111). These
Figure 107. (a−c) Schematic representation of nanoribbon formation by the condensation of tetrathiafulvalene (TTF) inside a nanotube’s internal space. (d−g) HRTEM images of nanoribbons formed in this way. Reprinted from ref 696. Copyright 2012 American Chemical Society.
Figure 108. (a) C60@(8,8), (b) C60@(9,9), and (c) C60@(10,10). The van der Waals space-filling model of C60 placed at the open end of each nanotube is used to show the relative sizes of the fullerene and the SWNT cavities.
of at least 6.4 Å to comfortably accommodate C60 in its internal cavity without distorting its walls. The observation of fullerene@SWNT structures by HRTEM convincingly demonstrates that fullerene encapsulation in nanotubes is possible. However, there is some debate as to whether the fullerenes enter the tubes’ cavities via their open ends or via defects in the SWNTs. Therefore, Berber et al.700 used computational methods to determine the mechanism by 4795
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One way of preparing a particularly interesting class of new nanomaterials would be to use endohedral metallofullerenes as transfer agents that could deliver metal atoms to the interior of SWNTs. The coalescence of the encapsulated fullerenes would then convert the host SWNT into a DWNT and simultaneously deposit metal ions into the inner cavity of the new DWNT, creating a metal-cored nanowire. Kitaura and co-workers706 showed that such metallic nanowires inside DWNTs can be formed from (Gd@C82)@CNT. Various sizes of nanowire can be formed depending on the radial size of the initial SWNT: nanotubes with larger radii produce wires whose crystal packing resembles the normal fcc or body-centered cubic (bcc) crystalline states more closely (see Figure 112).
Figure 112. A scheme describing formation of a metal-cored nanowire inside a DWNT.
6.8. Nanodiamonds and C60-Functionalized Graphene and Carbon Nanotubes
The use of C60 derivatives in organic photovoltaic cells is well established in the literature due to their strong ability to accept electrons. However, their optoelectronic properties could be greatly enhanced by their incorporation into composite nanomaterials with highly conductive graphene or CNTs.707,708 C60 can be covalently linked to graphene or CNTs via organic linkers.709 A more “pure” all-carbon hybrid was prepared by using n-BuLi to lithiate graphene, which then reacted with C60 by nucleophilic addition.710 C60 and higher fullerenes have also been attached to well-dispersed organically modified SWNTs in an odichlorobenzene−acetonitrile solvent system (see Figure 113).711,712
Figure 110. A model showing the coalescence of three fullerenes in the absence of a templating CNT.
Figure 113. Attachment of C60 and clusters to organically modified SWNTs. Reprinted from ref 711. Copyright 2010 American Chemical Society. Figure 111. A model of a nanobud.
A nearly all-carbon composite was formed by coupling C60 with SWNTs under microwave irradiation. In this way, individual C60 molecules or small clusters of them were dispersed over the SWNT surface and weakly bound to it. The amphiphilic character of GO provided the driving force for the formation of these ternary all-carbon composites based on C60, SWNTs, and GO.713
authors’ findings demonstrated that nanobuds are essentially covalently bound detritus left behind by various types of fullerenes since they remain intact when the SWNTs are subjected to various cleaning procedures that remove noncovalently bound fullerenes. 4796
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suggesting potentially fruitful new lines of inquiry. However, computational tools can also be used to tentatively answer more far-reaching questions such as that Feynman posed in his famous lecture of 1959 at an American Physical Society meeting at Caltech (California Institute of Technology, Pasadena, CA): “What could we achieve if we had total control over atomic placement?” Such complete control would enable the construction of carbon materials with unprecedented architectures that could be built from the bottom up. Carbon atoms normally exist in one of the sp, sp2, or sp3 hybridization states, which are associated with linear, trigonal, and tetrahedral geometries, respectively. These geometries could be combined in a great variety of ways to create new allotropes with potentially interesting physical and chemical properties. This is illustrated in Figure 115, where the vertices of the ternary phase diagram
GO has been used as a surfactant for the effective dispersion of a few-layer graphene (FLG) in water. The resulting graphenic structure (FLG−GO) has been further decorated with nanodiamonds (NDs) in a self-organized 3D superstructure (see Figure 114) showing high catalytic properties (FLG−GO@ NDs).714
Figure 114. TEM images of the FLG−GO@ND hybrid superstructure: (a) general view evidencing the homogeneous dispersions of NDs over the surface of the composite and (b) image showing the high affinity of NDs to adsorb on the GO surface (FLG is few-layer graphene, GO is graphene oxide, and ND is nanodiamond). Reprinted with permission from ref 714. Copyright 2014 Royal Society of Chemistry.
Figure 115. A ternary phase diagram showing materials consisting of carbon in a single hybridization state at the vertices, materials containing mixtures of two hybridization states along the edges, and materials with all three hybridization states within the triangle. Adapted with permission from ref 716 and complemented with new carbon nanoallotropes reviewed here. Copyright 1997 Elsevier B.V.
Finally, the remarkable electrical properties of MWNTs are combined with the hardness and inertness of nanocrystalline diamond (NCD) in a characteristic hybrid nanomaterial. A paper was produced by filtration of a suspension of MWNTs used as a substrate and coated with NCD by microwave plasma chemical vapor deposition. The hybrid presented an impressive increase of the Young’s modulus from 0.3 to 300 GPa after NCD deposition. Regarding the electrical conductivity of the hybrid, the presence of NCD in the MWNT surface resulted in an anisotropic behavior, strongly increasing the transverse resistivity of the hybrid in comparison with MWNT paper free of NCD.715
represent the three potential hybridization states of carbon.716 Materials located on the lines connecting two vertices would contain atoms in the two connected hybridization states (in various ratios), while those within the triangle would contain all three hybridization states in some ratio. At the sp3 vertex, one would find cubic and hexagonal (lonsdaleite) diamond, graphene and graphite would be located at the sp2 vertex, and the pure sp vertex would be home to polyyne/carbyne. A potential structure that would lie on the sp−sp3 edge would be a form of expanded diamond in which the diamond topology is retained but −C C− “spacers” are inserted between adjacent tetrahedral sp3 carbons such that the ratio of sp to sp3 carbons is 4:1. Along the sp−sp2 edge, you might find extended graphene (graphyne) with an sp:sp2 ratio of 1:1, while, along the sp2−sp3 edge, one might encounter nanotubes that were joined along the tube. Depending on the ratio of the different hybridization states, the
7. PREDICTED, RARE, AND HIGH-PRESSURE CARBON NANOALLOTROPE STRUCTURES The preceding sections discussed carbon nanoallotropes in terms of their synthesis, characterization, and properties. However, in many cases, before these structures were prepared and characterized experimentally, their properties had been predicted in computational studies. Such studies facilitated the identification of the synthesized materials and also provided guidance, 4797
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bond lengths in crystallized polyynes and found that the triple bond lengths increase and the single bond lengths decrease asymptotically as the number of carbon atoms in the linear chain increases. By extrapolating from these results, bond lengths of 1.25 and 1.33 Å were predicted for the triple and single bonds of the hypothetical infinitely long polyyne, respectively.730 This result appears to rule out a linear >CCCC< cumulenelike structure because if this resonance structure were representative of the bonding in polyynes, all their bond lengths would be equal. Certain properties of the end-capped polyynes depend on the number of sp carbon atoms in the linear chain. Notably, as the linear chain gets longer, the highest occupied molecular orbital (HOMO)−lowest unoccupied molecular orbital (LUMO) gap decreases and the second hyperpolarizability tensor for third-order nonlinear optical measurements increases.731 Theoretical studies on carbyne/polyyne molecules using different levels of theory have established their band gap to be approximately 2.2 eV.732 Nair et al.733 have reported on the calculated physical properties of carbynes/polyynes with 4, 6, 8, 12, 24, and 46 carbon atoms. Under uniaxial tension (i.e., tension imposed by a “tug-of-war”), these polyynes display a Young’s modulus (stress/strain) of approximately 288 GPa. This result is lower than the value for graphene, which is unsurprising because sp carbon atoms have 33% fewer connections to other atoms than sp2 carbon atoms. Additional properties from the Yakobson group have revealed carbyne to possess a high tensile stiffness, ∼109 (N m)/kg, with a Young’s modulus of 32.7 TPa.734 Under different amounts of induced strain, single chains of carbon can undergo transitions of electrical properties from being dielectric to metallic.735 Crystallographic studies indicate that polyynes are somewhat curved in the solid state.736 The synthesis of longer-end-capped polyynes, up to 22 yne units, was used to determine the gap between HOMO and LUMO for carbyne, revealing a gap of ∼2.56 eV or ∼485 nm.737 The formation of single atom chains from few-layer graphene upon in situ TEM irradiation was experimentally reported by Casillas et al.738 These free-standing carbon chains were observed also in carbon nanotubes which subsequently broke upon further irradiation. Longer polyynes should be able to form “belts” or loops. Another set of conjugated systems described in the work by Estrada and Simon-Manso739 consisted of discrete molecules with two loops in either the Möbius (twisted) or Hückel (untwisted) conformations. Both were calculated to be stable (see Figure 116). Kaing and Goddard740 have proposed that carbon nanotubes may be grown from nucleation sites on polyyne rings. Our discussion of polyynes is regrettably but necessarily limited. However, acetylene units can also be used to link carbon atoms in other hybridization states; such linkers are quite well understood. Further exploration of this “old but new material” is expected to yield diverse new carbon allotropes with mixed hybridization states in the future.741 The pioneering work of Baughman et al.742 on the design and physical properties of new forms of planar carbon has recently been revisited at length. Graphyne, an analogue of graphene in which half of the sp2 carbon atoms are replaced with acetylenic (−CC−) moieties, forms a hexagonal (hxl) network, or a honeycomb network if benzene is retained. Its stability is predicted to be comparable to that of diamond and graphite. The idealized picture in Figure 117 shows the familiar benzene rings of graphene transformed into 6-connected nodes that are linked to one another via acetylene units. Kang et al.743 calculate a single graphyne layer to be weaker than graphene in terms of in-plane
materials might exhibit more characteristics of one hybridization state than the other. In light of all these synthesized and predicted allotropes, it is clear that we will need a systematic way of comparing new allotropes to those that have already been reported in the literature. It is simple to look at a diamond or graphene structure and realize that they have significantly different topologies. Several databases have established naming systems based on the connectivity within the networks of these allotropes, which have been extremely useful when comparing coordination polymers (also known as metal−organic frameworks). For example, diamond consists of 4-coordinate sp3 carbon atoms (nodes) that are connected to one another to form a 3-periodic diamond (dia) network. Conversely, in graphene, 3-coordinate sp2 carbon atoms (nodes) are connected in a honeycomb (hcb) network. The cumulative number of atoms within the first 10 coordination shells of topological neighbors is referred to as the TD10. This parameter is particularly useful for characterizing topologies that are similar but not identical to known ones. For example, the coordination sequence of the hexagonal lonsdaleite (lon) is slightly different from that of dia, and they have slightly different TD10 values. The three-letter terms shown in parentheses in this section are assigned by the RCSR (Reticular Chemistry Structure Resource) to new topologies as they are confirmed.717 The Database of Zeolite Structures contains, as the name implies, zeolites. The topologies and structures of zeolites resemble those of the sp3 carbon allotropes except that their 4-coordinate zeolite nodes (Al, Si) are bridged/linked by oxygen atoms.718 These oxygen atoms are not considered in topological analyses because they function as connectors/linkers/rods rather than nodes. EPINET also explores 2D hyperbolic tilings for networks.719 All of the materials discussed in this section are given RCSR threeletter codes unless their topology is novel, in which case they are given a point symbol and TD10 value. Further details concerning networks can be obtained from the work of Wells and, more recently, that of O’Keeffe, Delgado-Friedrichs, Blatov, and Proserpio.720−723 Programs such as TOPOS are useful for determining the connectivity of new networks.724 Along with this, a recent analysis of sp3 carbon allotropes with zeolite topologies has been presented by Baburin et al.725 Nodes for the theoretical allotropes that contain benzene or an ethylene unit can be considered in various ways. The sp2 carbons in benzene can individually be regarded as 3-coordinate nodes, or if benzene is taken as a whole, it can be anywhere between 3- and 6-coordinate depending on its substituents. For instance, a 1,3,5substituted benzene would be treated as a 3-coordinate node, whereas a 1,2,4,5-substituted benzene would be considered a 4coordinate node, and so on. Ethylene moieties can be regarded as two joined 3-coordinate nodes (>CC
