One-Step Synthesis of Ultrathin Carbon Nanoribbons from Metal–Organic Framework Nanorods for Oxygen Reduction and Zinc–Air Batteries

Open AccessCCS ChemistryRESEARCH ARTICLE1 Jan 2022One-Step Synthesis of Ultrathin Carbon Nanoribbons from Metal–Organic Framework Nanorods for Oxygen Reduction and Zinc–Air Batteries Lianli Zou, Yong-Sheng Wei, Chun-Chao Hou, Miao Wang, Yu Wang, Hao-Fan Wang, Zheng Liu and Qiang Xu Lianli Zou AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), National Institute of Advanced Industrial Science and Technology (AIST), Sakyo-ku, Kyoto 606-8501 , Yong-Sheng Wei AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), National Institute of Advanced Industrial Science and Technology (AIST), Sakyo-ku, Kyoto 606-8501 , Chun-Chao Hou AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), National Institute of Advanced Industrial Science and Technology (AIST), Sakyo-ku, Kyoto 606-8501 , Miao Wang AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), National Institute of Advanced Industrial Science and Technology (AIST), Sakyo-ku, Kyoto 606-8501 , Yu Wang AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), National Institute of Advanced Industrial Science and Technology (AIST), Sakyo-ku, Kyoto 606-8501 , Hao-Fan Wang AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), National Institute of Advanced Industrial Science and Technology (AIST), Sakyo-ku, Kyoto 606-8501 , Zheng Liu Innovative Functional Materials Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Shimoshidami, Moriyamaku, Nagoya, Aichi 463-8560 and Qiang Xu *Corresponding authors: E-mail Address: email protected E-mail Address: email protected AIST-Kyoto University Chemical Energy Materials Open Innovation Laboratory (ChEM-OIL), National Institute of Advanced Industrial Science and Technology (AIST), Sakyo-ku, Kyoto 606-8501 Institute for Integrated Cell-Material Sciences (iCeMS), Kyoto University, Sakyo-ku, Kyoto 606-8501 Department of Materials Science and Engineering, SUSTech Academy for Advanced Interdisciplinary Studies and Guangdong-Hong Kong-Macao Joint Laboratory for Photonic-Thermal-Electrical Energy Materials and Devices, Southern University of Science and Technology (SUSTech), Nanshan, Shenzhen, Guangdong 518055 https://doi.org/10.31635/ccschem.021.202101160 SectionsSupplemental MaterialAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Two-dimensional (2D) carbon nanostructures play a critical role in energy-related applications, but developing facile and efficient strategies to synthesize these kinds of nanostructures is extremely rare. Herein, ultrathin carbon nanoribbons (CNRibs), with a thickness of 2–6 nm and length over 100 nm, have been strategically fabricated via a one-step pyrolysis of one-dimensional (1D) metal–organic framework nanorods (MOF NRods). Manipulating the diameters of MOF NRods will result in the formation of porous carbon nanostructures in 1D or 2D morphologies. Functional CNRibs with N doping or metal active site immobilization have also been studied. The CNRibs decorated with iron nanoclusters and single atoms have been used as excellent catalysts for the oxygen reduction reaction under both alkaline and acidic conditions, as well as zinc–air batteries. This work gives deep insights into the structural evolution from 1D to 2D morphology, providing an efficient approach to fabricate low-dimensional nanomaterials with controllable morphologies and functionalities for electrochemical applications. Download figure Download PowerPoint Introduction The upsurge on the study of low-dimensional nanomaterials can be attributed to their great potential in various applications.1–4 Two-dimensional (2D) carbon nanostructures, including graphene, carbon nanosheets, and carbon nanoribbons (CNRibs), have emerged as a group of new-generation functional materials, and have been extensively studied in the fields of optics, catalysis, energy storage, and so on.5–7 Among these fascinating carbon materials, ultrathin CNRibs constructed by a few layers of carbon atoms not only acquire the exceptional advantages of graphene but also present the anisotropic features of one-dimensional (1D) nanostructures, rendering them unconventional physical and chemical properties.8–10 To date, several methods, including mechanical or chemical exfoliation process,11,12 bottoms-up organic synthesis,13 and electro deposition,14 have been developed to synthesize 2D CNRibs, while the morphologies and compositions of the resulting materials are difficult to be modulated, especially the engineering pores or grafted metal active species on the surface. Additionally, the complex, time-consuming, and low-yield features of these approaches make them unsuitable for industrial production. Thus, establishing cheap and easy-to-achieve methods with controllable morphologies and functionalities is both highly desired and challenging. In recent years, metal–organic frameworks (MOFs)15,16 have been widely used as precursors or templates for fabricating porous carbons,17–21 metal oxides,22,23 and their composites.24–26 General methods to fabricate 2D carbon nanomaterials from MOF precursors are based on a morphology-preserving strategy, which, to a large extent, requires designing MOFs into desired shapes, and subsequently transforms them into 2D carbons via pyrolysis process.27–29 The sonochemical treatment of MOF-derived carbon nanorods (CNRods) with subsequent chemical activation is a promising method to fabricate CNRibs, while multiple steps are still needed.30 Seeking a reliable approach to directly synthesize 2D CNRibs from bulk or 1D MOFs is of immense value, which opens new avenues in developing low-dimensional nanostructures but poses significant challenges. Owing to the decomposition of ligands and aggregation of metals during pyrolysis, thermal transformation of MOFs is accompanied by the partial or complete collapse of their original morphologies,31,32 where the falling fragments might reorganize into a new profile under high pyrolysis temperatures and provide a rare opportunity to tailor the structure of the resultant carbons, such as from 1D MOF nanorods to porous 2D carbon nanostructures. In this work, we have successfully converted 1D MOF NRods into ultrathin CNRibs through a one-step pyrolysis process, based on a facile size-mediated strategy (Scheme 1). The 1D porous CNRods or 2D ultrathin CNRibs can be easily achieved by the carbonization of MOF NRods with controlled diameters. As a result, large-sized zinc-MOF NRods (MOF-NRod-L, 100–300 nm in diameter) would just retain the 1D morphology after pyrolysis, while small-sized MOF NRods (MOF-NRod-S, 20–40 nm in diameter) could be directly transformed into 2D CNRibs with a thickness of only 2–6 nm. To improve the electrochemical application of this kind of nanostructures, N-doped CNRibs decorated with ultrafine metal nanoclusters (<2 nm) and single metal atoms have been developed that show excellent catalytic performance for the oxygen reduction reaction (ORR)33 in both alkaline and acidic conditions, as well as outstanding capabilities for Zn–air batteries.34,35 Scheme 1 | Synthetic strategies for fabricating functional 2D CNRibs from small-sized 1D MOF NRods. Download figure Download PowerPoint Experimental Methods Syntheses MOF-NRod-S Fifty milligrams of 2,5-dihydroxyterephthalic acid and 50 mg zinc acetate dihydrate were dissolved in 15 mL ethanol and 2 mL methanol to form homogeneous solutions, respectively. Then, the zinc-containing solution was poured into the ligand solution and ultrasonicated for about 5 min. Next, the mixture was transferred into a Teflon-lined autoclave, heated at 150 °C for 18 h and cooled down naturally to room temperature (RT). The resulting yellow precipitate (MOF-NRod-S) was collected and washed with ethanol several times and dried in a vacuum oven for further use. Fe-doped MOF-NRod-S Similar to the synthesis of MOF-NRod-S, 1.5 mL methanol containing 40 mg zinc acetate dihydrate was poured into 12 mL ethanol solution with 40 mg 2,5-dihydroxyterephthalic acid. After ultrasonication for about 5 min, 0.5 mg FeCl3•6H2O dissolved in 0.1 mL ethanol was added and mixed homogeneously under stirring. The mixture was transferred into a Teflon-lined autoclave, heated at 150 °C for 18 h, and cooled naturally to RT. The obtained precipitate (Fe/MOF-NRod-S) was washed with ethanol several times and dried in a vacuum oven for further use. For comparison, samples with different amounts of Fe were prepared by adding 0.2, 2.0, and 4.0 mg FeCl3•6H2O. CNRib With a heating rate of 5 °C/min, 0.6 g MOF-NRod-S was placed in a tube furnace and heated at different temperatures for 2 h in argon, and resulting products were denoted as CNRib-T, where T is 400, 500, 600, 700, 800, 900, and 1000 °C. The weights of samples obtained at 1000 °C were about 84 mg, corresponding to a yield of 14 wt %. Fe–N/CNRib To adjust the composition of the carbon nanostructures, N-doped CNRibs were prepared by the pyrolysis of MOF NRods with additional nitrogen sources. The as-prepared Fe/MOF-NRod-S (0.3 g) and melamine (0.9 g) were placed in a tube furnace with melamine upstream of Ar flow. With a ramp rate of 5 °C/min, the temperature was increased from RT to 1000 °C and kept at this temperature for 2 h. The resulting samples (55 mg, corresponding to the yields of 18 wt %) were denoted as Fe–N/CNRib. The inductively coupled plasma (ICP) results confirmed that the Fe content of the samples containing 0.5 mg FeCl3•6H2O was about 1 wt %. Characterizations Powder X-ray diffraction (PXRD) measurements were performed on a Rigaku Ultima IV X-ray diffractometer (Rigaku Corp., Tokyo, Japan) with a Cu Kα source (40 kV, 40 mA). Raman scattering spectra were recorded on a laser Raman microscope system (Nanophoton RAMANtouch, Nanophoton Corp., Osaka, Japan) with an excitation wavelength of 532 nm. Fourier-transform infrared (FTIR) spectroscopy analyses were carried out on a Shimadzu IRTracer-100 (Shimadzu Corp., Kyoto, Japan) in air mode. The specific surface areas and pore structures were analyzed by N2 adsorption/desorption isotherms at 77 K after dehydration under vacuum at 120 °C for 12 h using automatic volumetric adsorption equipment (Belsorp-max, MicrotracBEL Corp., Osaka, Japan). Pore volumes were calculated using the nonlocalized density functional theory (NLDFT) method. Thermogravimetric analysis (TGA) was recorded on a Rigaku Thermo plus EVO2/TG-DTA equipment (Rigaku Corp., Tokyo, Japan). X-ray photoelectron spectroscopy (XPS) analyses were conducted on a KRATOS ULTRA2 instrument (Shimadzu Corp., Kyoto, Japan). Scanning electron microscopy (SEM) analyses were carried out with a scanning electron microscope (JEOL JSMIT100, JEOL Ltd., Tokyo, Japan; Hitachi S-5000, Hitachi Corp., Tokyo, Japan). All transmission electron microscopy (TEM) and two of the high-angle annular dark-field scanning TEM (HAADF-STEM) images were taken using an FEI Tecnai G2 F20 at 200 kV (Thermo Fisher Scientific, OR, USA); other HAADF-STEM images and annular bright-field (ABF)-STEM images were recorded on a JEM-ARM200FC equipped with Corrected Electron Optical Systems (CEOS) Cs correctors at 120 kV. Energy dispersive X-ray spectrometry (EDS) mapping was collected from the JED-2300 attached onto a JEM-ARM200FC (JEOL Ltd., Tokyo, Japan). Atomic force microscopy (AFM) images were recorded on the FastScan Dimension XR (Bruker Corp., MA, USA). Metal contents in samples were detected by ICP-optical emission spectroscopy (ICP-OES) on a Thermo Scientific iCAP6300 (Thermo Fisher Scientific, OR, USA). Electrochemical measurements Electrocatalytic measurements were carried out in a three-electrode cell using a rotating ring-disk electrode (RRDE) constant rotation system (RRDE-3A) with a CHI7088E electrochemical workstation at ambient conditions. A platinum wire and an Ag/AgCl electrode in saturated aqueous KCl solution were used as the counter and reference electrodes, respectively. A catalyst-loaded glassy carbon (GC) rotating disk electrode (RDE, 5 mm in diameter, 0.196 cm2 geometric surface areas) or RRDE (GC disk: 4 mm in diameter; Pt ring: 5 mm ID/7 mm OD) was used as the working electrode. All potentials in this study refer to the reversible hydrogen electrode (RHE; ERHE = EAg/AgCl + 0.059 pH + 0.198 V). Catalyst inks were prepared by dispersing catalysts (2.5 mg) in a 1.0 mL mixture of ethanol–water solution (0.49 mL ethanol, 0.49 mL water, and 20 μL 5%-Nafion solution). After sonication for about 30 min, a certain volume of catalyst ink was dropped onto the GC surface to give a loading of 0.4 mg cm−2 for all samples, except for commercial 20% Pt/C (0.2 mg cm−2). The working electrode was dried at RT naturally and then tested in the Ar- or O2-saturated electrolyte (0.1 M KOH or 0.5 M H2SO4). The linear sweep voltammogram (LSV) data presented in this work deducted the value obtained in Ar-saturated conditions to eliminate the effects of double-layer capacitance (Cdl). Zn–air batteries Zinc–air batteries were assembled with a Zn anode, an aqueous solution with 6 M KOH and 0.2 M zinc acetate, and an air cathode comprised of a gas-diffusion layer, catalyst layer, and a separator to prevent electrolyte leakage. The catalyst layer was prepared by dropping a certain volume of catalyst ink on a nickel foam, forming a circular plane (0.7 cm in diameter) with a loading of 0.5 mg cm−2. The battery was placed at ambient conditions overnight to ensure the electrodes were well-immersed in the electrolyte, and then tested at RT. Results and Discussion Morphology control of MOF NRods and their derivatives For the synthesis of 1D MOF NRods in different diameters, the mass ratio of metals to ligands (RM/L) has been adjusted. Generally, when RM/L was 1∶1, small-sized Zn-MOF NRods with a diameter less than 40 nm could be obtained, while the Zn-MOF NRods with larger sizes (100–300 nm in diameters and lengths of several micrometers) were prepared by adjusting RM/L up to 2∶1 ( Supporting Information Figure S1). PXRD characterization was used to analyze the crystal structure of these MOF NRods, and their diffraction profiles match well with the diffraction curve of Zn-MOF-74 (Figures 1a and 1b).36 After carbonization in argon flow at 1000 °C, PXRD curves of products only show two broad peaks at about 25 and 44°, corresponding to the characteristic peaks of graphitic carbon (002) and (101), respectively.17 Figure 1 | (a) Crystal structure of MOF-74 with Zn (green), O (blue), and C (grey) atomic distribution and the photos (right) of MOF-NRod-L (up) and MOF-NRod-S (below); (b) PXRD, (c) N2 sorption curves of MOF NRods and their corresponding carbon products; (d) SEM images of MOF-NRod-L; (e) SEM images of CNRods; (f) SEM and (g) ABF-STEM images of MOF-NRod-S; (h) SEM and (i) TEM images of CNRibs; (j and k) AFM image of CNRibs with the corresponding height profile along the line scan. Download figure Download PowerPoint Compared with MOF NRods and CNRods, CNRibs exhibit a sharp increase of N2 adsorption at the high relative pressure section (0.9–1.0) in the isotherms (Figure 1c). The Brunauer–Emmett–Teller (BET) surface area of CNRibs is about 1201 m2 g−1, comparable to CNRods with a value of 1480 m2 g−1 ( Supporting Information Table S1). Attributable to the presence of large pores or abundant slits caused by the stacked CNRibs, CNRibs (2.94 cm3 g−1) show a much higher pore volume than that of CNRods (1.33 cm3 g−1).32 It has been demonstrated that the large porosity of electrode materials facilitates mass transport and electrolyte diffusion during catalytic reactions.37,38 The SEM images display that the MOF-NRod-L can retain the original 1D morphology after carbonization, giving porous CNRods with diameters of about 100–200 nm and lengths of 2–3 μm (Figures 1d and 1e and Supporting Information Figure S2). Unexpectedly, the MOF-NRod-S (20–40 nm in diameter) will transform into 2D ultrathin CNRibs with a width distribution between 20 and 50 nm after pyrolysis at the same conditions (Figures 1f–1i and Supporting Information Figure S3). HAADF-STEM and TEM images clearly show the ribbon-like structure of samples with folds along their lengths (200–500 nm), suggesting the excellent flexibility as well as few-layer thickness of CNRibs. The high purity of CNRibs demonstrates the usefulness of this synthesis methodology, which shows potential in scalable production on industrial levels. As shown in AFM images and height profiles (Figures 1j and 1k), CNRibs display an average height between 2 and 6 nm, further demonstrating the ultrathin feature of these CNRibs.30 It is obvious that the ultrathin CNRibs can be directly achieved by using small-sized MOF NRods as precursors, avoiding complex exfoliation or template-removing processes. Thermal evolution of CNRibs from MOF NRods To trace the thermal transformation process of MOF-NRod-S to CNRibs, PXRD analyses were initially performed to monitor the decomposition process of MOF NRods (Figure 2a). Diffraction curves of samples calcined at 300 and 400 °C are different from that of MOF NRods, indicating the change of crystal structure at these temperatures ( Supporting Information Figures S4–S6). By increasing the carbonization temperature, diffraction peaks corresponding to ZnO were observed in samples obtained at 500–800 °C, suggesting the complete decomposition of MOFs to carbons and metal oxides. At 900 °C, the Zn-related diffraction peaks disappeared, demonstrating the reduction and removal of Zn species at high pyrolysis temperatures. Two peaks located at about 25 and 44° illustrate the existence of graphitic carbons, which can be further confirmed from Raman spectra with two bands centered at about 1347 cm−1 (D band) and 1610 cm−1 (G band) ( Supporting Information Figure S7).39 Then, SEM and TEM analyses were used to observe the morphology of intermediates during the carbonization process (Figure 2 and Supporting Information Figures S8–S12). Samples carbonized at 300 °C show many ZnO nuclei, which are evenly distributed in the whole MOF NRods (Figure 2b and Supporting Information Figure S9). With increasing pyrolysis temperature, these nuclei begin to grow and aggregate into large-sized nanoparticles (NPs), which finally move to the surface of MOF-NRods and cause the collapse of 1D MOF NRods, inducing the transformation of rod-like structures into a ribbon-like morphology surrounded by many ZnO NPs (Figure 2c and Supporting Information Figures S8 and S10). Increasing the temperature to 700 °C will further accelerate the growth of ZnO NPs and repair the carbon framework. Note that some ZnO NPs are reduced to metallic Zn species, which are directly evaporated due to the relatively low evaporation temperature. Therefore, most NPs disappear at this temperature and the well-defined CNRibs with a few immobilized NPs are observed (Figure 2d and Supporting Information Figure S11). Uniform CNRibs with nearly no NPs are obtained at 900 °C, demonstrating the successful formation of pure CNRibs (Figure 2e and Supporting Information Figure S12). The that the in Zn-related NPs show great to the transformation of MOF NRods to CNRibs. The aggregation and of Zn or ZnO NPs are for morphology which or the rod-like morphology into ribbon-like structures along the resulting in the formation of carbon intermediates with many ZnO the Zn-related species will be reduced and evaporated with increasing temperature, which will a great of carbon and the thickness of carbon Thus, small-sized MOF NRods are finally transformed into ultrathin CNRibs. With the removal of zinc species at high temperature, pure CNRibs with well-defined morphologies are obtained (Figure In with large-sized MOF was that small-sized MOF the formation of carbon show a structure which can the aggregation and transport of NPs during Therefore, only 1D porous CNRods are obtained when using large-sized MOF NRods as Figure 2 | (a) PXRD of MOF-NRod-S carbonized at different TEM images the morphology of samples obtained at 500, 700, and 900 °C, (f) of the transformation process from MOF NRods to CNRibs. Download figure Download PowerPoint Metal nanoclusters immobilized on CNRibs from the and thermal 2D ultrathin carbon nanostructures are excellent to metal which have been widely in various To the of metal nanoclusters on ultrathin CNRibs, metal and are initially in the pores or on the surface of MOF precursors through a synthesis After a carbonization process, catalysts with metal nanoclusters immobilized on ultrathin CNRibs were obtained ( Supporting Information Figures It is of that the chemical composition of the resulting catalysts can be further by the pyrolysis conditions. The of additional precursors such as and to excellent catalysts with various The carbonization of Fe-doped MOF-NRods with melamine as a nitrogen source to the formation of N-doped CNRibs immobilized with ultrafine Fe nanoclusters and single metal atoms that much Fe doping might result in the formation of large Fe NPs or other ( Supporting Information Figures and samples with Fe content of about 1.0 wt have been diffraction peaks on PXRD curves the of Fe nanoclusters (Figure The obtained not only the exceptional features of CNRibs including high surface area cm2 g−1) and large pore volume cm3 g−1) (Figure but also the ultrathin 2D morphology and ultrafine metal nanoclusters into a which the of metals and the mass during the catalytic analyses show the chemical of the in which the and Fe have been The C the presence of and species (Figure The N can be into peaks located at and corresponding to the and graphitic N species, (Figure As shown in the Fe the large area of from the existence of Fe while the and might be attributed to the formation of or in the (Figure As Fe–N/CNRib a 2D which is different from the with 1D morphology, suggesting the of this approach (Figure and Supporting Information Figure the ultrafine of metal nanoclusters that are observed from SEM and TEM ( Supporting Information Figure HAADF-STEM and were performed for further As shown in Figure HAADF-STEM images clearly show the distribution of Fe nanoclusters on N-doped CNRibs, with an average diameter of 1.5 nm. The the existence of distributed Fe single atoms (Figures and and Supporting Information and which also play a role in catalytic It is that the highly distributed N and Fe atoms and 2D ultrathin structure the with active for the reaction to Figure | (a) PXRD and (b) N2 sorption and pore distribution curves of Fe–N/CNRib and results of Fe–N/CNRib the C N and Fe (f) and and (i) mapping images of Fe–N/CNRib. Download figure Download PowerPoint and Zn–air battery a critical reaction in has been widely studied in recent years, but still in the of catalysts with high and low of the ultrathin morphology the of electron and mass diffusion and the distribution of ultrafine metal nanoclusters providing abundant active 2D exhibit excellent performance in both alkaline and acidic conditions. In 0.1 M KOH aqueous the Fe–N/CNRib shows outstanding with an potential of and potential as high as much than of commercial Pt/C and and comparable with the catalysts (Figure and Supporting Information Figures and Table The density of Fe–N/CNRib at is about which is up to cm−2 at curves from curves are shown in Figure The Fe–N/CNRib shows the with the to that of Pt/C and CNRib This result that the Fe and N species are to the of as the of Fe–N/CNRib is to and much than 2D CNRibs with and high the diffusion length of and accelerate the mass and improve their electrochemical The RRDE was used for the yield of intermediates and the of Fe–N/CNRib (Figure The yield is when the potential is corresponding to the electron larger than ( Supporting Information Figure Figure 4 | (a) of curves at (b) the corresponding curves in and show the RRDE measurements providing the electron and yield of Fe–N/CNRib. (d) of Fe–N/CNRib and Pt/C at 0.5 shows the corresponding methanol (e) curves of Fe–N/CNRib tested in 0.5 M aqueous (f) (g) corresponding density and (h) curves at a density of 25 of Zn–air batteries using Fe–N/CNRib and Pt/C + as the Download figure Download PowerPoint The electrochemical surface areas of Fe–N/CNRib and were by the The of Fe–N/CNRib catalyst is larger than that of the suggesting that the Fe–N/CNRib can provide active for ( Supporting Information Figure It is that the of highly on the Fe and N contents in CNRibs. The N atoms in carbon frameworks will the and distribution of Fe functional species, such as the active in the much Fe doping on CNRibs will their of the aggregation of Fe NPs ( Supporting Information Figures and of the ultrathin 2D morphology along with highly Fe single the Fe–N/CNRib excellent with a over 20 h at 0.5 in alkaline conditions (Figure In the commercial Pt/C catalyst from a at the same with only of after 20 h. Additionally, the