Successive Free-Radical C(sp ² )–C(sp ² ) Coupling Reactions to Form Graphene

Abstract

Open AccessCCS ChemistryRESEARCH ARTICLE1 Feb 2022Successive Free-Radical C(sp2)–C(sp2) Coupling Reactions to Form Graphene Huaqiang Cao†, Cheng Wang, Baojun Li, Tianyu Chen, Peng Han, Yan Zhang, Haijun Yang, Qunyang Li and Anthony K. Cheetham† Huaqiang Cao† *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Chemistry, Tsinghua University, Beijing 100084 †H. Cao and A. K. Cheetham contributed equally to this work.Google Scholar More articles by this author , Cheng Wang Department of Chemistry, Tsinghua University, Beijing 100084 Google Scholar More articles by this author , Baojun Li Department of Chemistry, Tsinghua University, Beijing 100084 Google Scholar More articles by this author , Tianyu Chen Beijing Advanced Innovation Center for Imaging Technology and Key Laboratory of Terahertz Optoelectronics (MoE), Department of Physics, Capital Normal University, Beijing 100048 Google Scholar More articles by this author , Peng Han Beijing Advanced Innovation Center for Imaging Technology and Key Laboratory of Terahertz Optoelectronics (MoE), Department of Physics, Capital Normal University, Beijing 100048 Google Scholar More articles by this author , Yan Zhang Beijing Advanced Innovation Center for Imaging Technology and Key Laboratory of Terahertz Optoelectronics (MoE), Department of Physics, Capital Normal University, Beijing 100048 Google Scholar More articles by this author , Haijun Yang Department of Chemistry, Tsinghua University, Beijing 100084 Google Scholar More articles by this author , Qunyang Li Applied Mechanics Laboratory, Department of Engineering Mechanics, School of Aerospace Engineering, Tsinghua University, Beijing 100084 Google Scholar More articles by this author and Anthony K. Cheetham† *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] Department of Materials Science and Metallurgy, University of Cambridge, Cambridge CB3 0FS Materials Research Laboratory, University of California, Santa Barbara, Santa Barbara, CA 93106 †H. Cao and A. K. Cheetham contributed equally to this work.Google Scholar More articles by this author https://doi.org/10.31635/ccschem.021.202100919 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Graphene is of great interest because of its exciting properties and potential applications, but its production on a large-scale still presents considerable challenges. Herein, we report the synthesis of predominately few-layer graphene, due to π–π stacking, and single-layer graphene from reaction between hexabromobenzene and Na metal, followed by annealing to improve crystallinity. The reaction proceeds via a free-radical C(sp2)–C(sp2) coupling mechanism, which is supported by theoretical calculations. The graphene can host unpaired spin electrons, leading to a short acquisition time for a solid-state nuclear magnetic resonance 13C spectrum from unlabeled graphene, which is ascribed to the very short spin-lattice relaxation time. High catalytic activity for transforming amine to imine with a conversion of >99% and a yield of ∼97% is demonstrated, and high electronic conductivity of ∼105 S·m−1 is found by terahertz spectroscopy. The reaction delivers a method for synthesizing graphene with a high spin concentration from perbrominated benzene molecules by using an active metallic agent, such as Na, Li, or Mg. Download figure Download PowerPoint Introduction Graphene, in the strictest term, is a single-layer of carbon atoms tightly packed into a two-dimensional (2D) honeycomb lattice; in a broader sense, graphene can be distinguished as three different types of 2D graphene materials, that is, single-, double-, and few-layer (3–10 layers) samples.1 Many methods have been reported for the synthesis of graphene,2 including bottom-up approaches such as solvothermal synthesis and sonication based on reactions between ethanol and sodium in solution,3 surface-assisted coupling and cyclodehydrogenation gas-phase reactions on metal gold or silver surfaces through a radical addition process (albeit with technological limitations substrate extension),4 and on Cu surfaces through radical-coupling reactions.5 Graphene films have also been synthesized by chemical vapor deposition techniques using a mixture of methane and hydrogen at 1000 °C on Cu foils.6 Several top-down approaches have also been explored, including the exfoliation–reintercalation–expansion of graphite to generate single-layer graphene sheets ∼250 nm in size7; nonchemical, solution-phase exfoliation of graphite in organic solvents to form monolayer and few-layer graphene with a lateral size of a few micrometers8; and exfoliation of graphite by brief (60 s) heating to 1000 °C in forming gas (3% hydrogen in argon) to generate sub-10-nanometer graphene nanoribbons.9 Recently, atomic-scale structural defects in graphene,10–20 which can host unpaired spins,10–12 has aroused great interest. Such defect engineering21–26 can be used to produce materials that have applications in quantum information processing,27 spins,28 photons,29 and so on. Defects in graphene are usually generated during synthesis, device preparation, or transfer procedures, and include point defects,11 line defects containing pentagon–heptagon pairs,13 holes and edge structures,14 tilt grain boundaries,15 dislocations,16 topological defects,17 atomic-scale lattice defects,18 extended defects,19 and so on; these can greatly alter graphene’s physical and chemical properties. Herein, we report a simple method, and the associated free-radical C(sp2)–C(sp2) coupling reaction mechanism, for the synthesis of graphene from the reaction of the hexabromobenzene (HBB), with metallic Na, Li, or Mg. Although the synthesis of graphene quantum dots from HBB and Na has been reported,30 the detailed mechanism of this C(sp2)–C(sp2) coupling reaction has not been described previously.30,31 Our free-radical C–C coupling reaction to form graphene is neither a Wurtz reaction [coupling between alkyl halide-C(sp3) and alkyl halide-C(sp3)], nor a Wurtz–Fittig reaction [coupling between alkyl halide-C(sp3) and aryl halide-C(sp2)].32 Neither is it an Ullmann reaction (the coupling of aryl halides with copper undergoing an oxidative addition step),32 a free-radical polymerization reaction without the chair propagation step,33 nor a general inorganic nanoparticle growth mechanism.34,35 It is a free-radical aryl halide C(sp2)–C(sp2) coupling reaction, but not a free-radical chain reaction.36 It has been stated that “the coupling of two aryl halides with sodium is impractical,”30,32 but biaryl compounds can be formed via the Ullmann reaction.32 Our mechanism has similarities to the classical mechanisms, but there are also differences which deserve to be understood and elaborated upon. Our synthetic method is also different from a free-radical polymerization involving successive additions of free-radical building blocks33 and from the formation of inorganic nanocrystals via nucleation and subsequent growth (the Ostwald ripening mechanism34 or oriented attachment mechanism35). However, as described later, it progresses from an organic molecular HBB to a “macromolecule” with fused six-membered rings—“graphene molecules”—to graphene via successive C–C coupling steps in a radical step-growth manner. The as-synthesized graphene has been characterized by many physical and chemical techniques, which clearly demonstrate the formation of the 2D nanostructured carbon product and shed light on the underlying reaction mechanism. In addition to establishing the reaction mechanism, we show that our graphene contains stable free radicals and can function as a metal-free catalyst for an amine to imine oxidation reaction with a conversion of >99% and a yield of ∼97%. Experimental Methods Materials All chemical reagents, such as hexabromobenzene [HBB, C6Br6, Analytical Reagent (AR), 99.0%] purchased from J&K Scientific Ltd. (Beijing, China) and Na (AR, >99.8%) from Acros Organics (France), were used without purification unless otherwise noted. Toluene (AR, >99.5%; Beijing Chemical Works) was treated with metal Na to remove traces of water. Synthesis of graphene In a typical synthesis, 1000 mg of HBB powder was dissolved in 400 mL of dehydrated toluene with stirring to form a solution in a Teflon-lined stainless steel autoclave (500 mL in capacity), followed by rapid addition of 600 mg of metal Na in small pieces into the above autoclave with stirring. Then the mixture was sealed, heated to 180 °C, and maintained at this temperature for 50 h. After the autoclave was allowed to cool to room temperature, the products were collected and treated with absolute alcohol to deplete unreacted Na, followed by washing with deionized water. After drying under vacuum, the as-obtained solid product was placed into a porcelain crucible, which was placed inside a tube furnace for annealing. Argon gas was first fed into the tube furnace at room temperature for 1 h to ensure that the furnace was full of inert gas. Then, the tube furnace was heated from room temperature to 450 °C [we also selected temperatures of 500 and 550 °C for Brunauer–Emmett–Teller (BET) measurements] with a heating rate of 10 °C·min−1, followed by cooling to room temperature and then holding at room temperature for 3 h. After the above annealing procedure, the final product was collected. Also, the same reaction system was heated at 180 °C under atmospheric pressure and reflux conditions for 15 h, while all the other reaction parameters remained unchanged. In addition, metallic Li, Mg, Fe, Co, Ni, Cu, and Zn were independently selected as a reagent in place of Na for this reaction, while all the other reaction parameters remained unchanged. Also, 4,4′-dibromobiphenyl and 1,3,5-trichlorobenzene, in place of HBB, were treated with Na, respectively, while all the other reaction parameters remained unchanged. The resultant product was washed by absolute alcohol and deionized water alternately, using centrifugation at 10,000 rpm (revolution per minute) for 5 min several times until the suspension was colorless. The first deionized-water-washing supernatant was retained for consecutive 72 h-dialysis process, which was used for transmission electron microscopy (TEM) observation, while the solid product was dried in a vacuum oven at 60 °C for 24 h for other characterization. The yield is affected by the size of the Na pieces in the air, which in turn is affected by the humidity in the air. Results and Discussion Characterization of graphene Representative TEM images and the corresponding electron diffraction (ED) patterns, atomic force microscopy (AFM), and optical microscopy images of the graphene samples are shown in Figures 1a and 1b and Supporting Information Figures S1–S5. The maximum observed size of the graphene nanosheet is ∼80 μm × 80 μm ( Supporting Information Figure S2b). Annealing the sample generates single-crystalline areas with lattice fringe spacing of ∼0.38 nm, corresponding to the (002) ( Supporting Information Figure S1). This interlayer spacing is larger than the distance between adjacent graphene layers in AB stacked graphite, 0.335 nm, which is due to the oxygen functional groups, generated by O2 or CO2 in the termination step of the C–C coupling (detailed discussion in Supporting Information), on the edge or plane of graphene. Also, the lattice fringes of ∼0.38 nm agrees with the X-ray diffraction (XRD) result (d(002) = 0.381 nm). When different regions of the as-synthesized sample are inspected, well-defined ED spots instead of ring patterns, are always observed ( Supporting Information Figure S1b), indicating the crystallinity of all regions examined.37 Figure 1c clearly shows the short-range ordering of nanosheets, demonstrated by both XRD and Raman results. Figure 1 | Physical characterization of the as-synthesized graphene. (a) TEM image. (b) AFM images of the samples of the first four largest number of layers deposited on an oxidized silicon wafer substrate. (c) Raman spectrum (532 nm) of the samples deposited on an oxidized silicon wafer substrate. (d) XPS image of the sample. Download figure Download PowerPoint The step height between the Si wafer (SiO2/Si) substrate surface and the thinnest graphene surface is ∼0.53 nm, as measured by AFM. This is important evidence that we can synthesize graphene only a single-atom-layer thick.2 According to the AFM thickness statistics, the thickness distribution of graphene is 15% for single-layer, 22.5% for bi-layer, and 62.5% for few-layers (3–10 layers), respectively ( Supporting Information Figures S4 and S5 and Table S1). The XRD pattern of the powder sample is shown in Supporting Information Figure S6. For the peak related to any stacking, we calculated the interlayer distance as 0.381 nm (corresponding to 2θ = 23.3°) based on Bragg’s law, 2dsinθ = λ (λ =1.5406 Å), which corresponds to the (002) crystallographic plane of stacked graphene sheets.38 The carbon (002) diffraction peak is extremely broad, which may stem from any short-range ordering of layers or the existence of defects in the sample39; note the successful removal of NaBr by washing with water ( Supporting Information Figure S6a). Micro-Raman spectroscopy characterization presents two intense peaks, the G-peak at ∼1578 cm−1 and the D-peak at ∼1361 cm−1, as well as three weak peaks at ∼2867 cm−1 (2D-peak) (historically called G′, related to the second order of zone-boundary phonons, but having nothing to do with the G-peak), ∼3002 cm−1 (D +D′-peak, which is caused by a two-phonon defect-assisted process; this peak is often labeled as D + G in the literature), and ∼3171 cm−1 (2D′-peak, an overtone of D′), respectively (Figure 1c and Supporting Information Figures S7 and S8). The large overlap between the D- and the G-peaks indicates that the material has a partially disordered structure. The Raman mapping IG/ID ratios at the edges and in the inner regions are approximately 1.27 and 1.67 ( Supporting Information Figure S8), respectively, indicating that the peripheral edges have more defects than the interior edges (detailed discussion in Supporting Information). X-ray photoelectron spectroscopy (XPS) analysis (Figure 1d and Supporting Information Figure S9 and Tables S2–S5) reveals the predominant C1s peak (∼97.2 atomic percentage (at. %)) and an O1s peak (∼2. 8 at. %). Peaks for Br and Na were not found, indicating that NaBr had been removed and that no C–Br appears in the final product, which is further supported by inductively coupled plasma mass spectrometry (ICP-MS) results ( Supporting Information Figure S10 and Table S6). The high-resolution XPS data ( Supporting Information Figure S9 and Tables S2 and S3) reveal the existence of sp2 aromatic carbon, carbon-containing functional groups (C–OH, C=O, –COOH), sp3-carbon, and π–π*-carbon in the final product, among which ∼91.8 at. % is for sp2-carbon, ∼2.7 at. % is for sp3-carbon, and ∼2.7 at. % is for other carbon covalently bound to oxygen.40,41 The nitrogen adsorption–desorption isotherms and Barret–Joyner–Halender (BJH) pore size distribution curves of the powder sample ( Supporting Information Figure S11 and Table S7) indicate aggregates of plate-like particles impart slit-shaped pores ( Supporting Information Figures S11a, S11c, S11e, and S11g).42 The BET specific surface area (SSA) value for our sample is 443.6 m2·g−1 ( Supporting Information Figures S11b, S11d, S11f, and S11h), and the average pore diameters are ∼1.43 (for DBJH) and ∼2.65 nm (for DBET) from the corresponding pore diameter distributions ( Supporting Information Figures S11a and S11b; detailed discussion in Supporting Information). The electron spin resonance (ESR) spin trapping technique is a general approach that can provide direct evidence for the presence of free radicals.43 For the detection of free radicals in this work, 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and phenyl-N-tert-butylnitrone (PBN) spin traps were employed. Both can confirm the existence of free-radical intermediates and differentiate between carbon- and oxygen-centered radicals (Figure 2a and Supporting Information Figures S12–S15; detailed discussion in Supporting Information). In addition, we have measured the ESR spectrum for a powder sample ( Supporting Information Figures S16 and S17 and Tables S8 and S9), which shows a strong peak with a g-factor close to 2.003 and a linewidth <1 mT ( Supporting Information Table S8). This indicates that it is incompatible with metal ions ( Supporting Information Figure S16 and Table S8).21 Figure 2 | Chemical characterization of the as-synthesized graphene. (a) First-derivative ESR of the reaction solution after 30 min. The ESR spectra for the HBB radical produced by metal Na in toluene trapped by DMPO (upper left) and PBN (upper right). (b) 1D MAS ssNMR spectrum of the unlabeled graphene. (c) Matrix-free LDI-TOF-MS spectrum in the negative-ion mode. (d) FT-IR spectroscopy spectra of the toluene solvent and the as-synthesized graphene. Download figure Download PowerPoint Solid-state nuclear magnetic resonance (ssNMR) can provide important structural insights into carbon-based materials, and high-resolution ssNMR using magic-angle spinning (MAS) is a primary technique for the characterization of graphene-related structures such as graphite oxide,44 graphene oxide (GO),45 and reduced GO (rGO) electrodes.46 The 13C MAS ssNMR spectrum reveals only one peak at a chemical shift of δ ∼128.6 ppm with full width at half maxima of approximately 17 ppm, which is characteristic of sp2 aromatic carbon atoms (Figure 2b); no peaks for other carbon-containing functional groups, such epoxide C (δ 59.7 ppm) or alcohol C–OH (δ 69.6 ppm),44 or the methyl carbon of toluene (δ ∼20 ppm)47,48 are observed. The ssNMR peak is wider than that obtained with liquid-state NMR due to anisotropic effects that are caused by restricted molecular motion in the solid state (detailed discussion in Supporting Information). The slightly asymmetric character of our peak reflects the presence of other unresolved components, but our one-dimensional (1D) ssNMR approach cannot distinguish these sp2-carbons. 2D ssNMR for 13C-unlabeled samples would require 108-fold more acquisition time due to the low natural abundance of 13C, whereas the use of 13C-labeling is prohibitively expensive. Thus, it is currently difficult to resolve the components of the peak at 128.6 ppm. Our ssNMR observations further demonstrate that the as-synthesized sample is overwhelmingly composed of an sp2-carbon network (i.e., graphene). To confirm that the chemical shift at δ ∼128.6 ppm comes from graphene and not from other organic aromatic molecules,49 the spin-lattice relaxation time (T1 = 4.29 s) and solid-state ESR (ssESR) spectra were measured. We find that the short measurement time of 16.5 min for an ssNMR experiment can be ascribed to the very short T1 caused by paramagnetic relaxation enhancement effects ( Supporting Information Figures S18 and S19) because of the paramagnetic centers that are observed in the ESR spectra ( Supporting Information Figures S16 and S17 and Tables S8 and S9, detailed discussion in Supporting Information). Thus, these data show that the ∼128.6 ppm peak in the ssNMR cannot be derived from sp2-carbons in aromatic molecules, such as the HBB reagent or the toluene solvent ( Supporting Information Table S10), but must stem from sp2-carbon in graphene. The 1H ssNMR measurement reveals a strong peak at δ ∼6.8 ppm accompanied by a weak peak at δ ∼2.5 ppm ( Supporting Information Figure S20). The peak at ∼6.8 ppm is attributed to hydroxyl hydrogen attached to the edge of graphene (formation of functional groups in Supporting Information of the defects of the graphene by high electron spin concentration from the ESR data ( Supporting Information Figure S16 and Table and Raman mapping ( Supporting Information Figure S8), the weak peak at ppm may be attributed to a hydroxyl by the adjacent defect electron the defect edge of To the reaction and we mass spectrometry on the reaction solution after 30 min at 180 °C (Figure and Supporting Information Figures and Tables we do not to be to peak in the mass we can the ( at of and to for with of and to for with of and and the to for of and to be the four structures generated by C–C coupling from the HBB molecules, and this result to the reaction mechanism, which is shown in Figure The peaks from aromatic containing oxygen can also be (Figure and Supporting Information Figure and Table that these structures are also formed by the O2 or CO2 ( Supporting the radical concentration is intermediates are dissolved in water due to can be removed from the final products after washing the oxygen concentration in the final products as observed in the XPS data (Figure 1d and Supporting Information Table More peaks can also be observed because is a method than We on molecular 600 because the intermediates obtained by coupling two or three benzene from the first two steps of the radical reaction are 600 such as the largest in the first two reaction, with molecular mass of the peaks above 600 in LDI-TOF-MS which are graphene of different are and weak to Figure 3 | synthetic mechanism of graphene from HBB under solvothermal conditions with three to generate the HBB free-radical formed via electron by Na, and of HBB free-radical to radicals Coupling C–C coupling of to form of Coupling between 5 and followed by by Na to form Coupling to generate due to the same as 1 in Coupling the same as 2 in of from Coupling between and to form the same as 1 in Coupling the same as 2 in of from Coupling by three radicals to form of above C(sp2)–C(sp2) coupling steps to produce graphene. radicals with O2 or CO2 to generate structures Download figure Download PowerPoint To further the reaction mechanism, we used spectroscopy to the products after a reaction time of 30 min (Figure In with the solvent we a formed peak at cm−1 in the reaction which is attributed to from groups in or It not to water because the for water at cm−1 (detailed discussion in Supporting Information). mechanism ESR analysis (Figure 2a and Supporting Information Table direct evidence for the presence of radicals in the reaction which is incompatible with metal ions ( Supporting Information Figure S10 and Table S8).21 The reaction of the synthesis of graphene is shown in and proceeds by a step-growth radical coupling mechanism (Figure with structures and an average yield of The reaction mechanism can be into radical and termination step a radical be generated after an electron to a and then the radical to form a free radical and an The reaction of the aromatic aryl halides C6Br6, by metallic Na in toluene is an electron transfer reaction, which a and generates the radical ( The potential of sodium = indicates that it is a strong The the associated with an aromatic and after the electron is the product is radical The resonance structures of approximately different structures in a of and an unpaired electron which are on two carbon atoms of the and the three can be on any two or atoms of the the electron in the radical a this the structure. Thus, it the C–Br to the stable radical ( and coupling and termination steps two radicals can with one in one of two or reactions are usually very and are but radical concentration an important in such high radical the radicals to rapid reactions to yield more than a radical The C–C coupling of radical 2 can yield which can be by the C–C between two benzene radicals are the above mechanism, the reaction in to generate the from radical 2 other of radicals can be obtained by of atoms from different in the In addition, ( and ( can also be generated from respectively, and other radicals generated by Na can also be formed in a manner. coupling of 5 with followed by Na to generate ( can form ( The 8 is the for formation of graphene. free radicals (the number of atoms on three benzene and = respectively, due to the can also be After the above fused

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