Advances in the Organometallic Chemistry of Carbon Nanomaterials

Review pubs.acs.org/Organometallics

Advances in the Organometallic Chemistry of Carbon Nanomaterials Lorcan J. Brennan† and Yurii K. Gun’ko*,†,‡ †

School of Chemistry and CRANN Institute, Trinity College Dublin, Dublin 2, Ireland ITMO University, 197101 St. Petersburg, Russia

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ABSTRACT: Carbon nanomaterials demonstrate remarkable electrical, thermal, and mechanical properties that allow for a number of very important and exciting potential applications. In this review, we present an overview of the major advances in the organometallic chemistry of carbon nanotubes and graphene over the past decade. The review is focused only on carbon nanocomposites with metal−carbon bonds. We review and discuss various organometallic functionalization approaches and their role in the modification of carbon nanotubes and graphene nanomaterials. Particular attention is paid to reactions of carbon nanotubes with organolithium compounds and to organometallic η6 transition metal complexes with carbon nanotubes and graphene. We also consider the main applications of organometallic reactions of carbon nanomaterials for the preparation of polymer-grafted carbon nanotubes and the corresponding reinforced polymer composites and review the potential applications of transition metal derivatives of carbon nanomaterials in nanoelectronics, fabrication of high-mobility organometallic transistor devices, memory components, spintronics, and catalysis. Finally, we give a future outlook for the further development and applications of carbon nanomaterials containing metal−carbon bonds.

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INTRODUCTION Carbon nanomaterials play a key role in modern science and technology because of their unique electrical, thermal, chemical, and mechanical properties. The nanomaterials have found a range of important applications in energy storage devices, supercapacitors, fuel cells, sensors, nanoscale electronic components, ultrastrong composite materials, and many others. Conjugated sp2-hybridized carbon nanomaterials include fullerenes, carbon nanotubes (CNTs), and graphene. There have been a number of reviews of the reactivity of fullerenes (including their organometallic chemistry).1−13 In addition, a themed issue on “Organometallic and Coordination Chemistry of Carbon Nanomaterials” covering some recent advances in organometallic chemistry of fullerenes was recently published in Dalton Transactions.14 However, the organometallic reactivity of CNTs and graphene has received substantially less attention and has not been systematically reviewed. Therefore, in this review we will focus our attention on the main advances in organometallic chemistry of CNTs and graphene. We consider only CNT and graphene derivatives that contain metal−carbon bonds. Graphene is a flat monolayer of sp2-hybridized carbon atoms tightly packed into a two-dimensional (2D) honeycomb crystal lattice. Graphene is the basic building block for graphitic materials of all other dimensionalities, such as 0D fullerenes, 1D nanotubes, and 3D graphite (Figure 1).15 Considering a topdown approach, individual graphene layers are found in graphite, where the layers are stacked on top of each other to form the bulk graphite crystal. The interest in graphene has grown significantly in recent years. This is largely due to the unique physical attributes associated with pristine graphene © XXXX American Chemical Society

Figure 1. Graphene is the 2D building block for carbon materials of all other dimensions. Graphene can be wrapped into 0D fullerenes, rolled into 1D nanotubes, or stacked into 3D graphite. Reproduced with permission from ref 15. Copyright 2007 Nature Publishing Group.

Special Issue: Mike Lappert Memorial Issue Received: December 9, 2014

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DOI: 10.1021/om501258j Organometallics XXXX, XXX, XXX−XXX

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to such an extent. Here we focus only on organometallic aspects of carbon nanotube chemistry, which involve carbon− metal bond formation. 2.1. Reactions of Carbon Nanotubes with Lithium Alkyls and Organolithium Derivatives of CNTs. One of the first organometallic reactions of carbon nanotubes was reported by Ajayan and co-workers.51 These researchers treated single-walled carbon nanotubes (SWNTs) with sec-butyllithium and then used the corresponding derivatives as initiators of anionic polymerization for the in situ preparation of polystyrene-grafted nanotubes (Figure 2) .

layers. Theoretical and experimental studies of graphene predict that graphene and graphene-based materials may possess superior mechanical and electrical properties.16 Graphene is known to possess extremely high mobility, on the order of 4000−20000 cm2 V−1 s−1, when deposited onto Si/SiO2 substrates.17 Electron transport in graphene can be described by the Dirac equation, which allows access to quantum electrodynamics and novel graphene-based nanodevices. Graphene is also mechanically and chemically stable and has remarkable properties when extended to bilayer and multilayer form.18 The intrinsic strength of defect-free sheets has established graphene as the strongest material ever measured, with a breaking strength of 42 N m−1,19 while it also possesses excellent thermal stability, which will benefit its use in electronic applications.20,21 Carbon nanotubes are another very well known and extensively explored representative of carbon nanomaterials. The field of carbon nanotubes and their composites has been progressing extremely rapidly over last two decades. CNTs can be conducting or semiconducting, depending on their diameter and chirality. Furthermore, CNTs can carry 1000 times more electric current than copper wire.22 These properties make CNTs very promising nanomaterials for applications in nanoelectronics. For example, CNTs have been envisaged as very promising candidates for nano-interconnects, high-speed field-effect transistors, and few- or single-electron memories for future nanoelectronics.23−27 The small diameter and long length of nanotubes lead to very large aspect ratios, making them almost ideal 1D quantum wires. This in turn implies drastically reduced carrier scattering and the possibility of ballistic devices. In addition, carbon nanotubes have exceptional mechanical properties, such as elastic moduli and strengths 10− 100 times higher than those of the strongest steel at only at a fraction of the weight.28 This area is very topical, and there are a huge number of publications and good reviews on the mechanical properties of carbon nanotubes and their polymer composites.29−44 With carbon nanotubes becoming easier to produce and much cheaper to buy, CNT-based manufacturing could potentially overtake that of the carbon fiber industry and result in a number of new commercial polymer composite products. Furthermore, CNTs demonstrate extremely high mechanical, thermal, and chemical stability. The lack of interface states as in the silicon−silicon dioxide interface provides a greater flexibility to the fabrication process. It is also expected that CNTs could solve thermal dissipation problems because of their high thermal conductivity. Therefore, there is an enormous interest in the development of new carbon nanotube−metal or nonmetal composite materials with controlled electronic properties.

Figure 2. Schematic presentation of carbanion formation and subsequent initiation of polymerization. (a) Section of SWNT sidewall showing the addition of sec-butyllithium to a double bond (the green arrow indicates the bond to which it adds) and the formation of an anion via transfer of charge. (b) The carbanion attacks the double bond in styrene and in turn transfers the negative charge to the monomer. Reproduced from ref 51. Copyright 2003 American Chemical Society.

An ab initio study of the SWNT−sec-butyllithium system showed that both free radical and anionic functionalization processes are energetically favorable. It was also demonstrated that the resulting CNT radical and CNT anion can react with ethylene and epoxide, respectively, and that the resulting products have free electrons and negative charges on the carbon and oxygen atoms at the free ends of ethylene and epoxide, respectively.52 After these publications, several groups used this strategy to graft various polymers to carbon nanotube surfaces. For example, sec-butyllithium was also used to prepare soluble carbon nanotubes and generate carbanions that were used as initiators of anionic polymerization of tert-butyl acrylate (t-BA) and methyl methacrylate (MMA) and the block copolymerization of t-BA and MMA.53 The same approach was also used to graft polyisoprene (PIp) to SWNTs. The formation of carbanions on the SWNT surface facilitated the separation of the nanotubes in solution and enabled the production of homogeneous PIp rubber composites with well-dispersed nanotubes. The new PIp−SWNT nanocomposites showed a significant improvement in thermal properties.54 Similarly, secbutyllithium was employed to introduce negative charges on carbon nanocapsules, and these carbanions were used as initiators for anionic polymerization of styrene. The process resulted in polystyrene-grafted carbon nanocapsules.55 Another group used sec-butyllithium-treated SWNTs to produce nanotubes functionalized with alkyl and carboxyl groups by a direct reaction with carbon dioxide.56 Finally, it was shown that the treatment of SWNTs with sec-butyllithium and then with epoxyethane results in SWNTs functionalized with both butyl and hydroxyethyl groups.57

2. ORGANOMETALLIC CHEMISTRY OF CARBON NANOTUBES As mentioned above, CNTs have remarkable electronic, thermal, and mechanical properties, making them attractive building blocks for creating next-generation electronic devices, networks, and components of various composite materials. It is known that solubility, dispersion, and solution stability of carbon nanotubes depend on their surface functionalization. Therefore, functionalization of nanotubes is extremely important for their processing and potential applications. The chemical functionalization of carbon nanotubes has been the subject of several good reviews.45−50 However, the organometallic chemistry of carbon nanotubes has not been explored B


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Organometallics In 2004, Gun’ko and co-workers reported the use of nbutyllithium-functionalized multiwalled carbon nanotubes (MWNTs) to prepare new reinforced polymer composites by a coupling reaction with halogenated polymers.58 The process involved the binding of butyl groups to the MWNT surface with the formation of very air- and moisture-sensitive (MWNTn−)(Li+)n adducts (Figure 3). It was found that the

formation of species containing Sn−C and Si−C bonds. Then the butyllithium-functionalized MWNTs were further reacted with chlorinated polypropylene (CPP) to give nanotubes covalently bonded to chlorinated polypropylene (CPP− MWNT) (Figure 3). The polymer-functionalized CNTs were found to be easily dispersed in organic solvents and used to produce blends of CNTs with the CPP polymer. Organolithium-treated carbon nanotubes were also used as nucleophilic reactive species that were reacted with various halogenated electrophiles. This synthetic strategy has been used with both single-walled and multiwalled nanotubes and a series of electrophiles.59 Hirsch and co-workers demonstrated that a reaction sequence of SWNTs with t-BuLi followed by an oxidation, can be repeated several times.60 This approach has been used to obtain SWNTs with a high degree of tert-butyl functionalization (Figure 4) and enabled a carbon-to-addend ratio of up to 31. The use of scanning transmission microscopy (STM) enabled the visualization of the covalently bonded tert-butyl groups attached to the sidewalls of nanotubes. It was also found that addition of the nucleophile to metallic nanotubes was preferential over the addition to semiconducting tubes. The same group continued their research in this area and later published a very comprehensive study of covalent sidewall addition of a series of organolithium and organomagnesium compounds (n-BuLi, t-BuLi, EtLi, n-HexLi, n-BuMgCl, and tBuMgCl) to SWNTs followed by reoxidation. It was confirmed that in the reaction of nanotubes with organolithium and -magnesium, metallic tubes are more reactive than semiconducting tubes. This can be explained by the finite density of states above the Fermi level in metallic tubes. Interestingly, it was also found that the reactivity of SWNTs in the addition of organometallic compounds is inversely proportional to the diameter of the nanotubes. In this case, nanotubes with a smaller diameter have more highly pyramidalized sp2 C atoms, resulting in higher strain energy. The strain energy can be reduced after sidewall addition, and therefore, nanotubes of smaller diameter are more reactive. The reactivity of SWNTs also depends on the size and steric demands of the alkyl groups. For example, binding of bulky t-Bu to SWNTs is less favorable than the addition of primary alkyl groups such as n-Bu. However, n-BuLi was found to be less selective than t-BuLi in the preferred reaction with metallic tubes. This was explained by the competitive fast electron transfer to the metallic SWNTs, which have low-lying electronic states close to the

Figure 3. Schematic presentation of the functinalization of MWNTs with BuLi and chlorinated polypropylene (CPP). Reproduced from ref 58. Copyright 2004 American Chemical Society.

butyllithium-functionalized nanotubes are susceptible to metathetic exchange reactions with the halogenated species via elimination of lithium halides. For example, reactions of lithiated carbon nanotubes with SnCl4 and SiCl4 resulted in the

Figure 4. Schematic presentation of t-BuLi functionalization followed by oxidation. Reproduced from ref 60. Copyright 2006 American Chemical Society. C


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Figure 5. (a) Electrophilic trapping of negatively charged (Nu)n(SWCNTn−) intermediates obtained by primary addition of a nucleophile. (b) Reductive SWCNT functionalization by electrophiles via Birch-reduced SWCNTn− species. Reproduced from ref 62. Copyright 2011 American Chemical Society.

Fermi level. As t-BuLi has the highest HOMO energy and lowest oxidation potential in this series of reagents, it is more selective in binding to sidewalls of metallic SWNTs.61 More recently, the same group reported a modification of the reductive alkylation/arylation sequence (the so-called Billups reaction) (Figure 5).62 This approach enabled the use of a broad variety of carbonyl-based electrophiles (ketones, esters, carboxylic acid chlorides) for the covalent functionalization of SWCNT sidewalls under modified Birch conditions. The researchers found that the degree of functionalization accessible by this reaction sequence is directly correlated with the stability/reactivity of the intermediately generated ketyl radical species. In addition, the carbonyl-based electrophiles can also be used as secondary functionalization or electrophilic trapping reagents for anionic SWCNT intermediates derived from an initial nucleophilic addition stage. As a result, this method allows the production of very interesting mixed (e.g., hydroxyl and carbonyl anchor groups) functional SWCNT derivatives that can be used for further functionalization for the development of various nanotube-based materials. 2.2. Transition Metal Derivatives of CNTs. There are only a very limited number of reports on transition metal derivatives of CNTs containing a metal−carbon bond. Haddon and co-workers developed a range of carbon nanomaterials based on η6 complexation of group 6 transition metals (Cr, Mo, and W) with graphene (see below), graphite, and carbon nanotubes.63−65 In this case, the carbon nanostructures with extended periodic π-electron systems can react for example with reagents such as Cr(CO)6 and (η6-benzene)Cr(CO)3 and serve as large η6 ligands (Figure 6). This group of researchers reported several new types of organometallic sidewall complexes of SWNTs, including (η6-SWNT)Cr(CO)3, (η6SWNT)Cr(η6-C6H6), and (η6-SWNT)2Cr (Figure 7). It was suggested that endohedral binding results in a very stable and kinetically inert mode of organometallic bonding. In addition, it was found that the electrical conductivity of SWNT thin films substantially increases in the case of sidewall bonding to the metal (M = Cr, Mo, W). This can be explained by the formation of SWNT interconnects of the (η6-SWNT)2M type that results in the significant reduction of the inter-CNT electrical junction resistance.64 The same group has identified the main processes that occur during this functionalization, including (a) weak physisorption; (b) ionic chemisorption, in which there is charge transfer to the graphitic structure and

Figure 6. Reactions of SWNT (1) and SWNT−CONH(CH2)17CH3 (2) with chromium hexacarbonyl and (η6-benzene)chromium tricarbonyl. Reproduced with permission from ref 64. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

preservation of the conjugation and band structure; (c) covalent chemisorption, in which there is strong rehybridization of the graphitic band structure; and (d) covalent chemisorption with the formation of an η6 organometallic−metal bond that largely preserves the graphitic band structure (constructive rehybridization).66 Moreover, since first-row transition metals (M = Ti, V, Cr, Mn, Fe) can spontaneously form (η6-C6H6)2M complexes by low-temperature metal vapor synthesis (MVS), it was envisaged that the MVS technique can be used in the construction of various (η6-SWNT)M(η6-SWNT) oligomeric architectures.65,66 There are also several publications on the interactions of carbon nanotubes with transition metal carbonyl derivatives. D


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As a means of increasing the solubility of graphene in a wider range of solvents, functionalization of the graphene layers is generally required. Oxidation of graphite to graphene oxide (GO) is the most widely employed method, as this allows for polar groups such as hydroxyl, epoxide, and carboxylic acids to be present on the surface of the 2D sheets.73 This method greatly increases the solubility of the graphene layers and hence the processability of the material.74 The functionality enables further chemistry to be performed on the surface of the GO sheet, and such approaches are outlined in an extremely thorough review by Bielawski and Ruoff.75 Metallic and semiconducting functionalization is also possible, as GO can be used as a support material for metallic nanoparticles76−78 and optically active semiconducting quantum dots.79,80 The added functionality offered by such particles allows the material to find applications in a range of electrochemical devices such as solar cells,81 batteries,82 and supercapacitors,83 primarily as a result of the large surface area offered by the 2D support material and the capability offered by the nanoparticles. Chemical functionalization of graphene can also be used to tailor the electronic properties of the material. The ability to control the electronic properties of graphene will be of utmost importance if graphene is to emerge as a material for future electronic applications. Lau and co-workers demonstrated that functionalization of graphene sheets with aryl groups can be employed as a method for engineering the material’s band gap.84 Their transport measurements showed that upon functionalization the electronic properties change from those of a granular metal to those of a semiconductor, whereby transport occurs via thermal activation of the carriers over the band gap. The introduction of the band gap upon functionalization is attributed to the modification of the inplane π conjugation in the graphene sheet. For example, Wang and co-workers theoretically demonstrated band-gap opening in graphene upon the [2 +1] cycloaddition of nitrenes to the graphene surface.85 Cycloaddition of the arene perturbs the π conjugation, and the corresponding electronic properties change from metallic to semiconducting. The formation of the cyclopropane three-membered ring upon [2 + 1] cycloaddition changes the hybridization across a graphene C−C bond from sp2 to sp3. In general, this is a drawback of most functionalization methodologies employed for graphenebased materials. The creation of sp3 centers at the expense of sp2 centers is useful for creating a band gap in the material but comes at the cost of reducing the mobility of the materials, as the in-plane hybridization is lost. These functionalization routes also have the major drawback that controlled restoration of the sp2 lattice is very difficult to achieve. For example, it is known that GO can be restored to a more “graphene-like” material upon treatment with reducing agents such as hydrazine86 and sodium borohydride.87 However, such materials are generally termed reduced graphene oxide (rGO), as the sp2 lattice is never fully restored and the material is subject to doping from the reducing agents.88 By means of an organometallic approach it is possible to circumnavigate some of the issues usually associated with traditional graphene functionalization methods. Organometallic functionalization of graphene layers is a promising method for introducing chemical functionality onto graphene without causing disruption of the π−π network. Such a method provides functionality of graphene sheets without the creation of sp3 centers, which are known to the reduce the charge mobility and strength and to increase the material’s optical

Figure 7. Schematic presentation of organometallic chromium sidewall complexes of SWNT with different modes of bonding. Reproduced with permission from ref 64. Copyright 2012 Wiley-VCH Verlag GmbH & Co. KGaA.

For example, it was reported that photochemical decomposition of the dimer [CpMo(CO)3]2 resulted in metalcentered free radicals [CpMo(CO)3]•, which interacted with fullerenes (C60, C70) or SWNTs in toluene to give the radical adducts.67 Similarly, homolytic dissociation of (pentamethylcyclopentadienyl)chromium tricarbonyl dimer in toluene resulted in organometallic chromium-centered free radicals that reacted with SWNTs.68 In another work,69 it was demonstrated that selected acyclic disubstituted η4-(1E,3E)-dienyl-Fe(CO)3 iron complexes can bind non-covalently and reversibly to MWCNT sidewalls via hydrophobic and/or π−π stacking interactions (Figure 8). It was found that these iron-complexed MWCNTs may be readily dissociated in CH3CN as a result of the weak non-covalent binding to the nanotubes.

Figure 8. Model of nonbonding interactions that develop during complex adsorption onto the MWCNT sidewall. Reproduced with permission from ref 69. Copyright 2008 Royal Society of Chemistry.

3. ORGANOMETALLIC CHEMISTRY OF GRAPHENE The functionalization of graphene using traditional organic and inorganic chemistry approaches is widely known and has been well-documented. Generally, functionalization of graphene is employed as a means of increasing the processing capability of the material. Pristine graphene is insoluble in most solvent regimes. Solubility can be forced through periods of ultrasonication; however this generally results in destabilization of the suspension and agglomeration of the layers after a short period of time. The factors affecting the solubility of graphene in solution have been well-documented elsewhere.70−72 It can generally be considered that the main factors affecting the solubility of graphene layers are the interplay of forces between individual graphene layers and the corresponding forces that may exist between graphene sheets and the solvent in question. Successful solvents for exfoliation are generally characterized as having surface tension values close to 40 mJ m−2 and a Hildebrand solubility parameter close to 23 MPa1/2.71 E


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Organometallics absorption profile. Organometallic functionalization also allows the band energy of the material to be controlled. Crucially, the organometallic approach can also facilitate reversible functionalization reactions, enabling the restoration of the pristine graphene sheets. Hence, there are many advantages in considering organometallic functionalization of graphene and related arene-based structures to control the intrinsic properties of the material. Organometallic functionalization of graphene is facilitated through the overlap of vacant metal dπ orbitals with graphene’s occupied π orbitals, which exist perpendicular to the graphene sheet. This results in bonding of the metal atom to the aromatic graphene rings. This type of covalent chemisorption is generally termed constructive rehybridization, as such a bonding mechanism causes no disruption of the in-plane hybridization of the graphene and hence the sp2 character is not disrupted. Therefore, this approach allows for a much milder form of chemical modification of the graphene surface without the need for σ-bond formation. Such approaches have been studied in depth by Haddon’s group, who examined the η6 complexation of graphene with chromium to form the η6 arene−metal bond. Their initial approaches focused on functionalization of various arene systems (exfoliated graphene (XG), SWNTs, epitaxial graphene (EG), highly oriented pyrolytic graphite (HOPG)) with chromium hexacarbonyl (Cr(CO)6). It was found that all of the functionalized arene systems could be isolated, and clear stability trends based on the surface curvature of the arene systems emerged. It was observed that graphitic arenes (HOPG and XG) provided greater reactivity and stability than SWCNTs. This is attributed to the intrinsic curvature of CNTs, which causes strain in the formation of the hexahapto bond. Samples of XG were found to give multiple products, such as (η6-arene)Cr(CO)3 and (η6-arene)2Cr. The formation of the latter complex opens up routes to introduce conductivity in the z plane, offering three-dimensional interconnection between graphene sheets. The synthesis of organometallic-functionalized graphene has followed two routes. The first method uses solvent-exfoliated graphene for the reaction with the metal complex. Such a process is useful for reacting large quantities of graphene with the metal complex, but it suffers from the random nature of solvent-exfoliated graphene samples. Typically, solvent-exfoliated graphene contains a mixture of flake sizes and a large variation in graphene flake thickness, ranging from monolayer graphene to multilayered graphene structures. The resulting organometallic-functionalized graphene products thus have a random distribution of flake dimensions. In a typical reaction, the metallic complex is added to a solution of exfoliated graphene in an appropriate solvent. The reaction mixture is refluxed (48 h), and the resulting product is filtered onto a PTFE membrane, washed, and dried. The removal of three carbon monoxide molecules from the metal complex is promoted by the polar solvents used, which also act to stabilize the intermediates.65 The overlap of the vacant metal d orbitals with the occupied π orbitals of graphene facilitates bond formation. Considering the relatively flat nature of the graphene surface (compared with CNTs or fullerenes), the hexahapto bond can form with relative stability on the 2D surface. Haddon and co-workers prepared such organometallicfunctionalized graphene using both chromium hexacarbonyl and (η6-benzene)chromium tricarbonyl as coordination agents to graphene according to the procedures outlined in Figure 9.

Figure 9. Reactions of highly oriented pyrolytic graphite (HOPG), exfoliated graphene (XG), and epitaxial graphene (EG) with (a) (η6benzene)chromium tricarbonyl and (b) chromium hexacarbonyl for the formation of organometallic-functionalized exfoliated graphene. Reproduced with permission from ref 63. Copyright 2011 Royal Society of Chemistry.

The formation of the (η6-graphene)Cr(η6-graphene) product was also observed. Atomic interconnects between graphene sheets are expected to increase the conductivity of the material and allow for electron transport in three dimensions. The formation of organometallic-functionalized graphene was confirmed using a range of characterization procedures, including Raman, IR, and X-ray photoelectron spectroscopy, and the electrical properties of the functionalized product were also analyzed. The group extended this work further and examined multiple routes for the formation of this system through the reaction of Cr(CO)6 with single-layer graphene deposited on a Si/SiO2 substrate. This method allows the formation of functionalized nanostructured-graphene-based devices on the surface of the Si substrate. The effect of organometallic functionalization on individual monolayers can also be observed using this approach, as individual flakes can be examined with microscopic and electrical characterization. The organometallic-functionalized graphene nanodevices were fabricated following the procedure outlined in Figure 10. Graphene was isolated from bulk graphite using micromechanical exfoliation and placed onto an oxidized Si substrate. Metal contacts were deposited by electron-beam lithography. The organometallic functionalization of the graphene followed three routes (A, B, and C) as outlined in Figure 10.89 It was found that the reactivity of the graphene was dependent on the number of layers present in the graphene flakes, with single-layer graphene being the most reactive, followed by few-layer graphene and finally HOPG. X-ray photoelectron spectroscopy was used to analyze the coverage of organometallic functionalization on the graphene surface. The analysis gave a C:Cr ratio of 18:1. It has been successfully demonstrated for both synthetic methods that the organometallic functionalization is reversible via decomplexation F


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Figure 10. Organometallic functionalization of single-layer graphene devices on oxidized Si substrates. Reproduced with permission from ref 89. Copyright 2013 Wiley-VCH Verlag GmbH & Co. KGaA.

Figure 11. Example of a decomplexation reaction using mesitylene as an arene-exchange ligand. Adapted with permission from ref 63. Copyright 2011 Royal Society of Chemistry.

demonstrated that organometallic-functionalized n-butyllithium MWNTs can be covalently bonded to chlorinated polypropylene via a coupling reaction with elimination of LiCl. Subsequent addition of the CPP-grafted nanotubes (Figure 12) to the CPP polymer matrix resulted in a significant increase in the mechanical properties. Mechanical tensile testing was performed on the corresponding CNT polymer composites, and the stress−strain curves are shown in Figure 12. It was found that as the nanotube content was increased to 0.6 vol %, the Young’s modulus increased by a factor of 3 compared with the pure polymer. In addition, both the tensile strength and the toughness (energy required to break) increased by factors of 3.8 and 4, respectively. Thus, this work demonstrated that butyllithium-treated nanotubes can be covalently functionalized with a halogenated polymer, providing efficient dispersion and excellent interfacial stress transfer between the nanotubes and the polymer matrix.58,90 MWNTs covalently functionalized with CPP were also used as a filler material for reinforcement of other polymers such as polystyrene and poly(vinyl chloride) polymer matrices. Close to a 2-fold increase in both Young’s

reactions. Raman spectroscopy can be employed effectively as a tool to follow the decomplexation reactions, as the intensity of the D band decreases significantly as disorder in the material is reduced upon decomplexation. The decomplexation of (η6graphene)Cr(η6-benzene) and (η6-graphene)Cr(CO)3 can be achieved by exposure of the complex to sunlight and air in the presence of diethyl ether or acetonitrile. Arene exchange reactions (Figure 11) can also be employed to cleave the metal−graphene bond using suitable electron-rich ligands such as anisole and mesitylene, allowing for restoration of the graphene surface and a return of metallic-like conductivity to the material.63

4. APPLICATIONS OF ORGANOMETALLIC-FUNCTIONALIZED CARBON NANOMATERIALS 4.1. Applications of Organometallic-Functionalized Carbon Nanotubes. One of the major applications of organometallic-functionalized carbon nanotubes was found in polymer composite reinforcement. Gun’ko and co-workers have G


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Figure 13. (left) SEM image of a metal−SWNT film. (right) Bis(hexahapto) bond formation at the inter-nanotube junction via a chromium atom. Reproduced from ref 94. Copyright 2014 American Chemical Society.

cycloaddition of electron-rich dienes (Figure 14).95 Thus, there is a broad range of potential applications for carbon nanotubes that have been functionalized using organometallic chemistry approaches.

Figure 12. Stress−strain curves for the CPP−MWNT composites with various nanotube contents. Reproduced from ref 58. Copyright 2004 American Chemical Society.

modulus and strength were measured at nanotube loading levels of less than 1 vol %. Reinforcement values of dY/dVf = 115 ± 34 GPa and dY/dVf = 304 ± 90 GPa were observed for polystyrene- and poly(vinyl chloride)-based composites, respectively.91 The same group also performed functionalization of MWNTs with n-butyllithium followed by hydrolysis and oxidation of the derivatives. This procedure was repeated two more times to achieve a higher degree of MWNT functionalization. Then the n-butyl-functionalized nanotubes were used to make nanotube polymer composites. The mechanical properties of these new composites were tested and found to show increases of up to 25% in Young’s modulus and up to 50% in tensile strength over pure polystyrene.92 The use of butyllithium-functionalized nanotubes for polymer grafting and as additives for the preparation of polymer composites and comparison of the mechanical properties with other various CNT-reinforced composites have also been considered in detail in some previous reviews.36,37,93 The recent development of η6 functionalization of carbon nanotubes with transition metals opened up new so-called atomtronic approaches in nanotechnology of these nanomaterials, where for example one individual metal atom can be used to bridge two carbon nanotubes.63−65 It was recently shown that the formation of covalent η6 bonds of chromium atoms with the benzenoid rings of single-walled carbon nanotube composite films results in a significant increase in the film conductivity due to the reduced inter-nanotube junction resistance (Figure 13). Spectroscopic studies demonstrated that the interaction between the transition metal and the SWNTs does not induce significant charge transfer, indicating the covalent nature of the bond.94 This approach can open up new horizons in the development of carbon nanotube interconnects, high-mobility organometallic transistor devices, new high-frequency nanoelectronic devices, 3D electronics, spintronics, memory devices, and potentially the next generation of energy harvesting devices.65,66 In addition, the new transition metal derivatives of carbon nanotubes can find a range of applications in organometallic catalysis. For example, Cr(CO)6-activated pristine SWNTs were used as an activated substrate in the Diels−Alder reaction to enhance the reactivity of the nanotube toward the

Figure 14. Schematic presentation of the Diels−Alder reaction on Cr(CO)6-activated pristine SWNTs. Reproduced from ref 95. Copyright 2006 American Chemical Society.

4.2. Applications of Organometallic-Functionalized Graphene. The most significant application of organometallicfunctionalized graphene has been the production of the graphene field-effect transistor (FET). The lack of σ-bond formation during organometallic functionalization ensures that the FETs retain the high mobility of graphene, as the in-plane hybridization is not disturbed and the functionalized carbon atoms remain part of the electronic band structure of the material. This has enabled the design of FETs with a field-effect mobility (μ) in the range of ∼200−2000 cm 2 V−1 s−1 and an on/off ratio of 5 to 13. Transport in nanostructured carbon electrodes is known to be hampered by the large junction resistance between individual carbon entities. For example, it has been demonstrated that the junction resistance between individual SWCNTs is high and leads to poor conductivity in nanotube networks.96,97 Atomic interconnects between graphene sheets may provide scope for decreasing the resistance of graphenebased electrodes for applications in solar cells,98,99 batteries,100 and supercapacitors101 and as replacement materials for transparent conducting oxides such as indium- and fluorinedoped tin oxide.102 In particular, the development of η6 functionalization of graphene with transition metals has opened up new opportunities in the design of high-mobility organometallic graphene transistor devices (Figure 15).65,66 The effect of transition metal (Cr, W, Mo) intercalation via the hexahapto η6 functionalization approach has been shown to increase the conductivity of free-standing films of organometallic-functionalized graphene nanoplatelets, with the higher group 6 metal atoms showing the highest conductivities.65,66,103 It has also H


Review

Organometallics

development of polymer−nanotube composite materials, which would have a broad range of important applications including new ultrastrong polymer−nanotube materials for bulletproof vests and protective clothing and high-performance composites for the aircraft and automotive industries (e.g., seat belts, cables, reinforcement of tires, break linings, bumpers, etc.) These large sectors will require huge quantities of carbon nanotubes. Therefore, it will be important to optimize and further develop organometallic approaches for large and cost-effective production of functionalized CNTs and the fabrication of corresponding CNT−polymer composites. Organometallic transition metal complexes of carbon nanotubes and graphene (such as η6-coordinated transition metal derivatives)65,66 have opened up a range of new unique opportunities in nanoelectronics, fabrication of high-mobility organometallic transistor devices, memory components, spintronics, advanced energy devices, and catalysis. This area of research will be of very high priority in the nearest future. It will be important to investigate in detail and understand the nature of the interactions between different transition metal atoms and π-conjugated graphitic surfaces. This can lead to discoveries of new physicochemical phenomena (e.g., mobility of metal atoms on graphene surfaces) related to self-assembled metal nanoclusters with unique morphology and atomic interconnects for 3D electronics based on 1D SWNTs or 2D graphene structures.65 The above-discussed atomtronic approach is particularly interesting as it enables not only the interconnection of carbon nanotubes and graphene layers using individual transition metal atoms but also control (e.g., enhancement) of the conductivity at the nanotube junctions. The possibility of using MVS and electron-beam evaporation techniques should facilitate further developments in this area, enabling extension in the choice of metals and scale-up of the fabrication.65,66This should open up a range of new potential applications for carbon-based nanomaterials. Another interesting research area that should be explored in more detail is the direct incorporation of transition metal atoms into the carbon network. For example, using density functional theory it was shown that selective substitution of a single atom within a large carbon framework with a Pt atom results in species demonstrating a strong preference for interactions with small organic molecules, particularly polar substances, via a Lewis acid/Lewis base complexation.106 This can open up completely new approaches in organic catalysis. In addition, carbon nanostructures with transition metal atoms incorporated into the carbon network should have unique physical properties that might open new avenues in electronics and fabrication of memory and energy storage devices. However, the direct incorporation of metal atoms into carbon networks still presents a big technical challenge that must be resolved in the future. Overall, carbon nanomaterials containing metal−carbon bonds should be of great interest for both future science and technology and is still a relatively young area of research that is expected to result in very important discoveries and many exciting applications.

Figure 15. Schematic presentation of a high-mobility organometallic graphene transistor device. Reproduced from ref 65. Copyright 2013 American Chemical Society.

been proposed that hybrids of η6-graphene−transition metal− semiconductor composites may find useful applications in water splitting reactions for H2 production.65 It is expected that η6graphene−transition metal systems will act as efficient acceptors of photoexcited electrons from the semiconductor and allow for efficient exciton separation. Popov and co-workers have also demonstrated the potential of graphene|metal|ligand systems in atomic spintronics.104 They showed that it is possible to build a chemical analogue of an optical lattice by trapping metal atoms in the 2D potential lattice of the graphene−ligand interface. The electronic properties of the material can be controlled by varying the substituent number of the graphene|metal|ligand system, and it was possible to achieve an open-gap state or a zero-gap material with linear dispersion at the Fermi level provided by the interplay of graphene π and metallic 3d states. It is expected that such control over the system will bring the realization of quantum computing one step closer. The immobilization of group-4-based metallocenes, Cp2ZrCl2 and Cp2TiCl2, on N-doped graphene nanoplatelets enabled the production of new catalytically active nanohybrids.105 The use of these organometallic graphene composites in polymerization of ethylene resulted in the production of ultrahigh-molecular-weight polyethylene (UHMWPE) with excellent mechanical properties. This research demonstrated that hybrids of graphene and organometallic compounds are very promising candidates for organometallic catalysis. In addition, these hybrids can serve as nanofillers that can be incorporated into the polymer matrix by in situ polymerization. This approach might open up unique opportunities for tuning organometallic catalysts using carbon nanomaterials and at the same time enable the production of new graphene-based polymer composite materials with a range of potential applications.105

5. CONCLUSIONS AND FUTURE OUTLOOK There has been significant progress in the development of organometallic chemistry of carbon nanotubes and graphene over the past decade. A range of new unique organometallic composites of carbon nanomaterials have been developed and studied. The treatment of carbon nanotubes with organolithium and organomagnesium compounds has deserved particular attention and has not only resulted in very interesting and unique methods for nanotube functionalization but also enabled the production of new CNT-based additives for very efficient polymer reinforcement. There is great interest in the further

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. I


Organometallics Biographies

Review

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DEDICATION

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REFERENCES

This review is dedicated to the memory of Professor Michael Lappert.

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Lorcan Brennan received his B.Sc. in Chemistry from Trinity College Dublin in 2010. Following graduation he joined the materials research group of Prof. Yurii Gun’ko, where his Ph.D. research interests focused on carbon nanomaterials and plasmonic nanoparticles for solar energy harvesting. Lorcan currently works as a postdoctoral research associate under the supervision of Dr. Rachel C. Evans at Trinity College Dublin and also lectures in physical chemistry. His present research focuses on the development of novel hybrid organic−inorganic waveguides for use as luminescent solar concentrators.

Yurii Gun’ko graduated from the Chemistry Department of Moscow State University in 1987 and later received his Ph.D. degree in Inorganic Chemistry from Moscow State University in 1990. Then he worked as a lecturer in chemistry at the Belorussian Institute of Technology (Belarus). In 1994 he received a postdoctoral position in the group of Prof. M. F. Lappert at the University of Sussex (U.K.). Then in 1995 he was awarded an Alexander von Humboldt Fellowship and worked at the University of Magdeburg (Germany) with Prof. F. T. Edelmann. After that he returned to the University of Sussex and worked as a postdoctoral researcher with Prof. Lappert. In 1999 Dr. Gun’ko moved to the Chemistry Department of Trinity College Dublin (Ireland) to take up the position of Lecturer in Inorganic Chemistry. Currently, he works as a Professor of Inorganic Chemistry in the School of Chemistry at Trinity College. His main research interests and activities are carbon nanomaterials, magnetic nanostructures, and plasmonic and quantum-dot-based materials.

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ACKNOWLEDGMENTS We acknowledge the financial support from Science Foundation Ireland (Grant SFI 12/IA/1300) and the Ministry of Education and Science of the Russian Federation (Grant 14.B25.31.0002). J


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