Direct Synthesis of Crystalline Graphtetrayne—A New Graphyne Allotrope
At a Glance
Section titled “At a Glance”| Metadata | Details |
|---|---|
| Publication Date | 2020-07-09 |
| Journal | CCS Chemistry |
| Authors | Qingyan Pan, Siqi Chen, Chenyu Wu, Feng Shao, Jing Sun |
| Institutions | UNSW Sydney, Qingdao University of Science and Technology |
| Citations | 43 |
Abstract
Section titled “Abstract”Open AccessCCS ChemistryCOMMUNICATION1 Apr 2021Direct Synthesis of Crystalline Graphtetrayne—A New Graphyne Allotrope Qingyan Pan, Siqi Chen, Chenyu Wu, Feng Shao, Jing Sun, Lishui Sun, Zhaohui Zhang, Yixiao Man, Zhibo Li, Lixia He and Yingjie Zhao Qingyan Pan College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Siqi Chen College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Chenyu Wu College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 The University of New South Wales, Sydney, NSW 2052 , Feng Shao Department of Chemistry, Faculty of Science, National University of Singapore, Singapore 117543 , Jing Sun College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Lishui Sun College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Zhaohui Zhang College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Yixiao Man College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Zhibo Li *Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 , Lixia He Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 and Yingjie Zhao Corresponding authors: E-mail Address: [email protected] E-mail Address: [email protected] E-mail Address: [email protected] College of Polymer Science and Engineering, Qingdao University of Science and Technology, Qingdao 266042 https://doi.org/10.31635/ccschem.020.202000377 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesTrack Citations ShareFacebookTwitterLinked InEmail We report a bottom-up synthesis of a new graphyne allotrope-graphtetrayne (GTTY), which features four acetylene linkages between the adjacent benzene on the surface of copper under low temperature. The as-prepared GTTY exhibited nanosheet morphology with an average thickness of 4.5 nm. Structural analysis based on high-resolution transmission electron microscopy (HR-TEM)/selected area electron diffraction (SAED) characterizations revealed high crystallinity formed with a ninefold stacking structure. The GTTY displayed excellent semiconductor properties bearing a band gap of 1.3 eV and a conductivity of 0.285 S m−1 at room temperature. Moreover, by the space-charge-limited current (SCLC) method, the measured hole and electron mobilities of the GTTY films reached up to 1.47 × 103 cm2 V−1 S−1 and 2.98 × 103 cm2 V−1 S−1, respectively. Download figure Download PowerPoint Introduction Over the past 30 years, a great variety of carbon-based nanomaterials with unique structures has emerged. They have shown excellent chemical, physical, electrochemical, and photophysical properties in vast applications across different areas. Most commonly, these materials are composed of sp2- or sp3-hybridized carbon atoms, such as diamond, fullerenes, carbon nanotubes, and graphene.1-3 Nevertheless, many efforts have been made to discover new carbon allotropes with rich sp-hybridized carbon atoms.4-10 Indeed, the presence of sp-hybridized state was found to affect the structures and properties of carbon-based materials significantly. As a typical example, graphyne (GY) emerged as a series of carbon allotropes composed of sp- and sp2-hybridization carbon whose existence was first theoretically predicted by Baughman et al. in 1987.4 GYs could be designated further as GY, graphdiyne (GDY), graphtriyne (GTRY), graphtetrayne (GTTY), and so on, according to the number of linking ethyne units between two neighboring benzene rings. The GY family is of great versatility in terms of both structures and properties. On the one hand, the length of the acetylenic linkage could be adjusted, which leads to the rational design of topological pore sizes. On the other hand, tailorable composition ratios between the sp- and sp2-hybridized carbon in the GY family have enabled largely tunable electronic properties and also make them novel two-dimensional (2D) Dirac materials. Many synthetic and theoretical chemists have made great efforts to discover new variants of the GY family from both theoretical predictions and practical experiments in the last few decades.11-16 GDY, as one example of the GY family, was first synthesized in the lab by Li et al. in 2010 (Scheme 1).15 The successful synthesis of GDY greatly inspired related researches in uncovering its properties and a vast range of practical applications,17-23 including energy storage,24-26 photoelectric device,27,28 catalysis,29-35 biological science,36,37 and so on.38-40 Despite the relative prosperity in GDY studies, other members of the GY family have been least studied.14,16 In fact, most of the existing reports are still limited within the stage of theoretical predictions.11-13,41,42 Notwithstanding, based on these theoretical studies, other members of the GY family with various acetylenic linkage lengths turn out to be ideal candidates for gas separation and water purification.11,41 Scheme 1 | (a) Chemical structure representation of typical variants of the GY family with different contents of sp-carbon atoms. (b) The synthesis route and topological structure of GTTY. GY, graphyne; GTTY, graphtetrayne. Download figure Download PowerPoint More importantly, recently, it was discovered that GDY possesses excellent single-atom catalyst support.32-34 The GY family with tunable (especially larger) pore sizes shows vast potential for supporting larger sized metal atoms and allowing more possibilities in topologically anchoring these chelated single atoms. From a mechanical point of view, the rotatable -C≡C- groups along the acetylenic linkage of the GY family have their π/π orbitals readily accessible by the single metal atoms sitting in pores. Therefore, the GY family could chelate readily with a single metal atom in each equivalent site within the pore.43 Moreover, we confirmed further the strong chemical interaction and electronic coupling between the single transition metal (Mn, Fe, Co, Ni, Cu, etc.) atom and GDY, which has mostly allowed for effective charge transport between the active catalytic site and the support.32-34,44 Acknowledging these facts, the GY family, especially allotropes, with higher sp-carbon content, is believed to be readily ideal candidates as the atomic catalyst supports without any pretreatment. Additionally, the sp carbon has shown the ability to reduce the aromatic character of the benzene ring, which leads to elongated bonds between the acetylenic group and the benzene ring.7,45 Thus, the increasing number of sp-carbon atoms within acetylene bonds could yield varied electronic structures with respect to many properties, such as tuned band gaps, electrochemical electrode performance, photocatalytic properties, and so on, as revealed by both experiments and their subsequent theoretical analysis.4 However, the synthesis of other members (other than GDY) with prolonged acetylenic linkages is still considered very challenging.14,16 The major difficulty lies in achieving a controllable synthesis of GTTY, whose precursor bears repeated alkyne groups exhibiting much higher reactivity than the single alkyne group counterpart. This has caused a lot of trouble in the control of the reaction. Indeed, if performed through usual GDY synthetic procedures, the growth process of the 2D films is disordered by fast coupling reactions in solution. Consequently, there is scarcely any ordered GTTY film formed on the Cu surface. To date, no report has addressed the direct and successful synthesis of GTTY. Here, we put forward a well-planned strategy to break through this synthetic bottleneck. We achieved the fabrication of highly ordered crystalline GTTY with precisely controlled and well-defined chemical structure from hexa(buta-1,3-diyn-1-yl)benzene precursors via Glaser-Hay coupling reaction on Cu surface. A low-temperature of -20 °C was adopted here, to decrease the monomer reactivity substantially and to enable formation of good crystalline GTTY. The as-prepared GTTY film was well characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), atom force microscopy (AFM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). The internal structure of the GTTY was evaluated by selected area electron diffraction (SAED) from TEM in conjunction with molecular-mechanics-based structural simulation. The GTTY possessed a high ratio of sp-hybridized carbon atoms (as high as 80% sp-carbon content) and 2D lamellar networks structure with 0.9 nm pore size.46 In addition, the GTTY exhibited excellent semiconductor properties, including a band gap of 1.3 eV, and a conductivity of 0.285 S m−1 at room temperature, as well as high carrier mobility. Results and Discussion The synthesis of the GTTY monomer was presented in the Supporting Information. Briefly, compound 3 was prepared as an essential intermediate following the typical procedures. Compound 3 was unstable in the air and had to be used immediately after purification. Compound 4 was synthesized through Sonogashira cross-coupling reaction between hexabromobenzene and 3. Then the monomer was obtained through the deprotection of 4 by tetrabutylammonium fluoride (TBAF) to remove the triisopropylsilyl (TIPS) groups under the argon atmosphere at 0 °C. The GTTY was then grown through Glaser-Hay coupling reaction on the surface of a copper foil in the presence of pyridine for approximately 72 h at −20 °C. Here, the copper foil not only played the role of a catalyst for the Glaser-Hay coupling reaction but also served as the template for growing the films by facilitating the orientation of the monomers to lead to the final formation of the illustrated nanopores structure. This method has been evaluated to be effective in the preparation of 2D GDY and its analogs.15-18,20,21,24,32,47 As the control molecules, the linear structures of 1,3,5,7-tetrayne and even longer polyynes have confirmed to have very high degrees of linearity for conjugated polyynes.48-52 Under −20 °C, the 2D films’ growth could still happen due to the high reactivity of the diacetylene and the side reaction can be hindered to some extent.53-56 As a result, the good crystallinity of GTTY could be obtained. Nothing was observed on the copper foil surface except some bulk materials in the solution at 0 °C, room temperature or even higher temperature. The low-temperature condition not only reduced monomer activity but also slowed down the diffusion of the catalyst. In a diluted monomer solution, the heterogeneous nucleation and crystalline film growth preferred to occur on Cu substrate to form initial nucleation sites to minimize the energy barrier at the interface while the GTTY was forming on the Cu surface.57 After the reaction, transparent brown GTTY films were observed on the copper foil surface. Finally, the copper foils were washed by heated acetone and dimethylformamide (DMF) to get rid of the unreacted monomers and oligomers, followed by drying under nitrogen. The as-prepared GTTY films exhibited a large sheet morphology as revealed by SEM, TEM, and AFM images shown in Figure 1. Continuous thin films were apparent, with a lateral size of 4 μm (Figures 1a and 1b). Based on the statistical analysis of 50 flakes, the average thickness of the GTTY film was determined to be 4.5 nm by AFM (Figure 1d and Supporting Information Figure S13). In addition, the curved streaks with a distance of 0.4 nm between each other were observed, which were designated as the spacing between carbon layers (Figure 1c). Both SAED (Figure 1f) and high-resolution TEM (HR-TEM; Figure 1e) demonstrated that a high crystallinity of the 2D GTTY film. Figure 1 | Morphology characterizations of the GTTY film: (a) SEM image, (b) TEM image, (d) AFM image, (c and e) HR-TEM image, and (f) SAED pattern. GTTY, graphtetrayne; SEM, scanning electron microscopy; TEM, transmission electron microscopy; AFM, atomic force microscopy; HR-TEM, high-resolution transmission electron microscopy; SAED, selected area electron diffraction. Download figure Download PowerPoint Subsequently, XPS analysis indicated that the GTTY consisted mainly of carbon and oxygen (Figure 2a). The high-resolution C 1s spectra could confirm further the presence of C≡C and C=C among the Gaussian curves (Figure 2b). The C 1s peak could be mainly deconvoluted into four subpeaks, C=C (sp2) of 284.4 eV, C≡C (sp) of 285.0 eV, C-O of 287.1 eV, and -C=O of 288.6 eV, respectively. The existence of C=O and C-O groups might be due to the oxygenation of the acetylene groups in the margin of the crystalline GTTY domains. The area ratio of sp- and sp2-hybridized carbon atoms was close to 4∶1, which matched well with the chemical composition of the GTTY structure (one benzene ring with six sp2 carbons linked to 12 -C≡C- triple bonds containing 24 sp carbons). Moreover, the Raman spectra displayed two prominent peaks in the range of 1300-1600 cm−1, ascribed to D and G bands, respectively (Figure 2c). As we ascertained previously, the D band is strongly related to structural defects, whereas the G band represents the first-order scattering of the E2g stretching vibration mode observed for sp2-hybridized carbon atom domains in aromatic rings. The intensity ratio of the D and G bands was approximately 0.14, which indicated that the GTTY had relatively high structural regularity. Furthermore, two peaks at 2055 and 2220 cm−1 were ascribed to the stretching modes of acetylenic bond (Figure 2c and Supporting Information Figure S12c and Table S1, density functional theory [DFT] calculation), where their attenuated relative intensities were attributable to the surface selection rules on the metallic Cu substrates.58,59 By comparison of the Fourier transform infrared (FT-IR) spectra for GTTY film and compound 4 in Figure 2d, a substantial decrease of the CH3 (from TIPS group in 4) vibration over 2800-3000 cm−1 was observed. Meanwhile, the presence of the stretching modes of the C≡C bond (2100 cm−1) and the aromatic ring breathing modes combined C-C chain stretching modes (1390 and 1030-1050 cm−1) indicated the intactness of tetrayne groups in the GTTY film ( Supporting Information Figure S12d, DFT calculation). Besides, as shown in Figure 2e the UV-Vis absorption spectrum of GTTY indicated that the optical band gap between the conduction band minimum (CBM) and the valence band maximum (VBM) was 1.3 eV (Ebg = 1.3 eV). Moreover, the cyclic voltammetry of GTTY film in dichloromethane showed an obvious reduction peak (E = −1.44 V vs Ag/AgCl) in Figure 2f. Thereby, the determined CBM energy level of GTTY was approximately −2.85 eV, and the determined VBM energy level of GTTY was approximately −4.15 eV based on the band gap of 1.3 eV. Figure 2 | Spectroscopic characterizations and cyclic voltammogram of GTTY films. (a) XPS survey spectrum and (b) high-resolution C 1s spectra of GTTY film on the copper foil substrate, (c) Raman spectra of GTTY film, (d) Infrared spectra of 4 (red) and GTTY film (black), (e) the UV-Vis absorption spectrum of GTTY film, and (f) cyclic voltammetry curve of GTTY film collected at 100 mV/s in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF6)/CH2Cl2 electrolyte. GTTY, graphtetrayne; XPS, X-ray photoelectron spectroscopy. Download figure Download PowerPoint By SAED (Figure 1f) analysis of the synthesized GTTY, we found a typical diffraction pattern with hexagonal symmetry, which confirmed the 2D periodic hexagonal lattice of GTTY with high crystallinity. As could be seen in the Supporting Information Figure S20, the {300} plane distance was obtained as 0.473 nm by averaging measurements of 10 diffraction points, then listed all possible stacking modes of GTTY in Supporting Information Figure S21 and performed plane-wave DFT calculations for models with each stacking mode. As shown in Supporting Information Table S3, the ninefold stacking GTTY (dmax,calc = 0.421 nm) was consistent with the experimental SAED pattern (dmax,SAED = 0.473 nm; Supporting Information Figure S20) with a relative error, δ, as low as 12.4%, whereas all other stacking modes deviated hugely from SAED. A ninefold stacking structure was, thus, confirmed for the synthesized GTTY (Figures 3a and 3b). Correspondingly, systematic extinction of diffraction points resulted from the intrinsic 2D symmetry of this ninefold stacking GTTY structure yielded {300} diffraction (Figures 3a and 3b) as the nearest diffraction points to the central transmission spot (Figure 1f). The DFT-optimized structure with unit-cell parameters (Figures 3a and 3b; d{300} = 0.421 nm, a = b = 1.46 nm, c = 3.21 nm, α = β = 90°, and γ = 120°) matched well with those derived from the experimental SAED pattern (d{300} = 0.473 nm, a = b = 1.42 nm, γ = 118°, and relative error Er = 12.4%). More detailed structural analysis is presented in the Supporting Information Section “1.6 Structural Simulation by SAED Analysis.” HR-TEM revealed 0.258 nm interval lattice fringes corresponding to {330} plane (Figure 1e). Furthermore, a = b = 1.48 nm characterized by HR-TEM was generally consistent with the simulated value. Figure 3 | Crystal modeling and simulation of GTTY (a) top view, (b) side view, and (c) calculated energy-band structure with GGA/Perdew-Burke-Ernzerhof approximation and PDOS for GTTY. The Fermi energy was set to zero. (d) Top views of for the visualized LUMO and HOMO at G point (0 0 0). GTTY, graphtetrayne; GGA, generalized gradient approximation; PDOS, partial density of states; LUMO; lowest unoccupied molecular orbitals; HOMO; highest occupied molecular orbitals. Download figure Download PowerPoint DFT calculation was performed to evaluate the energy-band structure and the corresponding partial density of states (PDOS) in the Brillouin zone. Figure 3c indicates that GTTY is a typical direct band gap semiconductor, with a calculated band gap of 1.0 eV, which almost matched the measured optical band gap of 1.3 eV. The slight underestimation in the DFT calculation might be attributable to the self-interaction error and the missing discontinuity associated with the generalized gradient approximation (GGA) and local-density approximation (LDA) to the exchange-correlation potential. Moreover, according to the visualization of frontier molecular orbitals (Figure 3d), the lowest unoccupied molecular orbitals (LUMO) of GTTY were located at the σ bond connecting the attached benzene rings and that connecting the C≡C groups. However, the highest occupied molecular orbitals (HOMO) were located mainly at the απ orbital of benzene rings and C≡C bonds (side views are shown in Supporting Information Figures S17 and S18). Furthermore, the conductivity of the GTTY film was determined to be 0.285 S m−1 derived from the current-voltage (I-V) curve in Supporting Information Figure S15, indicating superior electron transport property of GTTY, compared with that of GDY (2.5 × 10−4 S m−1).15 We explored its electrical characteristics further, by performing the space-charge-limited current (SCLC) method to test the electron mobility (μe) and hole mobility (μh) of GTTY. The μe and μh values were measured to be 2.98 × 103 cm2 V−1 S−1 and 1.47 × 103 cm2 V−1 S−1 according to the curves in Supporting Information Figure S16. These results were consistent with the predicted intrinsic charge carrier mobility values ( Supporting Information Table S2); those of in-plane (G-M and G-K) directions were higher than that of the interlayer (G-A) direction, as shown in Supporting Information Figure S19 and Table S2. Above all, both experimental data and calculated results revealed that GTTY is a potential intrinsic semiconductor with remarkable carrier transport capabilities, which originated from its extended π-conjugated system and narrow band gap. Conclusion We described a bottom-up synthesis of a novel 2D carbon allotrope-GTTY, featuring four acetylene linkages between the pairs of benzene rings to form extra-large topological trigonal pores each composed of 30 carbon atoms per layer. The GTTY films’ growth was well controlled on the surface of copper, yielding large surface areas and an average thickness of 4.5 nm under low temperature. This pure carbon-based material bears a record of as high as 80% sp-carbon content, super large topological cavities, and high crystallinity. Accordingly, the GTTY displays excellent semiconductor properties exhibiting a band gap of 1.3 eV and a conductivity of 0.285 S m−1 at room temperature. The hole and electron mobilities of GTTY films are as high as 1.47 × 103 cm2 V−1 S−1 and 2.98 × 103 cm2 V−1 S−1, respectively, by the SCLC method. Supporting Information Supporting Information is available. Conflict of Interest There is no conflict of interest to report. Acknowledgment This study was supported by the National Natural Science Foundation of China (nos. 51722302, 21674054, and 51972185). References 1. Franlin A. D.Electronics: The Road to Carbon Nanotube Transistors.Nature2013, 498, 443-444. Google Scholar 2. Geim A. K.; Novoselov K. S.The Rise of Graphene.Nat. Mater.2007, 6, 183-191. Google Scholar 3. 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