Controllable self-catalytic fabrication of carbon nanomaterials mediated by a nickel metal organic framework

Abstract

Carbon nanomaterials and metal–organic frameworks are all research hotspots in the field of energy which have important applications in catalysis and electrochemistry. We present a self-catalytic and controllable strategy for the fabrication of different morphologies and species of carbon nanomaterials under different temperatures via a nickel MOF (Ni₂(bdc)₂dabco) as the precursor.

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Environmental significance

Metal–organic frameworks and carbon nanomaterials are all hotspots in the field of energy which have important applications in the environment and catalysis. It has been shown that the direct carbonization of a MOF precursor without an additional carbon source is another way to prepare highly porous carbon materials. They showed similar morphology to the original MOFs and the metal flowed out at its boiling point, leaving pure porous carbons. Besides, the carbons obtained from small crystals are mainly micropores while the carbons derived from big crystals show more mesopores. Gas sorption showed that the carbons exhibit high CO₂ adsorption, implying their potential in clean energy storage and carbon capture.

Carbon nanomaterials have attracted much attention^1,2 in a variety of energy and environmental applications including gas storage and separation,³ catalysis,⁴ sensing,⁵ and energy harvesting.⁶ Recently, the fabrication of porous carbons with hierarchical pore structures has provided new insights into the advanced utilization of carbon materials.^7,8 Among the various types of porous and crystalline materials, metal–organic frameworks have attracted a great deal of attention because of their fascinating architectures as well as their useful properties.⁹

The metal–organic framework scaffolds are composed of metal ions and organic units, which can be transformed into carbons, nanostructured metals/metal oxides or their hybrid materials under thermolysis conditions. It has been reported for the first time that MOFs can be used as templates or precursors for porous carbon synthesis.¹⁰ However, in general, the thermal transformation of MOFs is accompanied by partial or complete collapse of their original morphology.⁹ In recent research, there are some reports about carbon nanotubes as templates combined with metal–organic frameworks.^11,12 Thus, the transformation of MOFs into well-defined one-dimensional and two-dimensional carbon materials is still a significant challenge.^13–17

Multi-walled carbon nanotubes were applied as highly conductive skeletons in the final electrocatalyst with N-decoration leading to abundant catalytic sites on the CNT skeleton without sacrificing structural integrity and electrical conductivity, noting that the N-containing MOFs were chosen as they formed continuous layers on the CNTs.¹¹ Over the past few years, although great progress has been achieved by using MOFs as templates or precursors to synthesize porous carbon, it is still quite difficult to effectively control the pore structure and its morphology.¹⁸ A double-template approach has been reported for the first time to control the pore structures of MOF-derived carbon. Metal ions in the MOF play a significant role during the carbonization process.¹²

It has been utilized for MOF preparation because MW synthesis of porous materials has the advantages of fast crystallization, improved yield and purity, phase selectivity, good reproducibility, diverse morphology/size, reduced environmental impact, and facile evaluation of reaction parameters. Microwave energy can be directly and uniformly absorbed throughout the entire volume of an object, causing it to heat evenly and rapidly. For the synthesis of microporous materials, this advantage leads to homogeneous nucleation and substantial reduction in crystallization time compared to conventional oven heating. Whilst fundamentally the same findings were confirmed for MOFs prepared by microwave heating, the synthesis scales attempted were very small (1–10 mL) and, accordingly, the reaction time scale reported in these studies can be misleading for a larger batch size. Furthermore, relatively little information has been provided regarding the synthesis details. Compound Ni₂(bdc)₂dabco was selected as a typical example of a layer-based MOF, exhibiting a two-dimensional open framework of interconnected Ni₂(bdc)₄ paddle-wheel units. These 2D nickel carboxylate layers are linked together by dabco via the vacant coordination sites at the Ni²⁺ centers of the nickel carboxylate layers. And the network is rigid in two dimensions but flexible in the third one. Therefore, this compound is able to change reversibly its structure and properties through external stimuli.

Herein, we report a facile and effective method to synthesize microporous core–shell-like nano-nickel, carbon nanotubes and multi-layer graphene only via a metal–organic framework as the precursor. The process was controllable and adjustable. Microporous core–shell-like nano-nickel could be applied in gas phase and electrochemical catalysis. Recently, much research has been focused on core/shell nanostructures absorbing materials.^19–21 The nanoshell not only protects the nanostructures against oxidation or corrosion^22,23 but also improves the impedance match between the metal and carbon components in the core/shell structure which can enhance the adsorption and catalytic performance.²⁴ The metallic material of the cores is important for their application in catalysis.²⁵ By only elevating the calcination temperature, carbon nanotubes can be obtained. At higher temperature, hollow carbon nanospheres can be formed. The metal nickel as the core escaped from the nanosphere. Through washing with dilute hydrochloric acid, metal nickel can be removed and absolute hollow carbon nanospheres can be prepared. A more important point was that graphene sheets were obtained when the reaction achieved certain conditions. This synthetic method was simple and effective, which made carbon nanomaterials with different morphologies available and obtainable. The experimental results and procedure are displayed in Fig. 1A. All analysis about the MOF is presented in the ESI.† Powder X-ray diffraction analysis of the porous materials was performed to ensure the high purity of crystalline phases (ESI† Fig. S1). The X-ray diffractograms are identical to those calculated from single crystal analysis data reported in the literature,²⁶ and the amount of side products is below the limit of detection as determined by means of powder X-ray diffraction. The specific surface area of the MOF Ni₂(bdc)₂dabco is 2004 m² g⁻¹ with a pore volume of 0.996 cm³ g⁻¹. As seen from Fig. 1B, the SEM (B) image of the metal–organic framework shows that its morphology is cubic. The SEM (C) image shows that the carbon material is in the form of nanotubes prepared from the MOF Ni₂(bdc)₂dabco. The agglomerated particles were metal nickel clusters attached to the carbon nanotubes after high temperature calcination. The size of the MOFs was about 10 μm but not all had the same size. The MOF precursor was synthesized under microwave in 1 min. The characteristic of this kind of microwave synthesis action was inhomogeneous. The size of the MOF materials was smaller generally than that of MOFs synthesized by conventional heating. More specific characterization of the metal–organic framework is demonstrated in the ESI.†

Fig. 1 (A) Procedure for the synthesis under microwave of metal–organic frameworks as precursors to prepare carbon materials; (B) SEM image of the metal–organic framework Ni₂(bdc)₂dabco; (C) SEM image of carbon nanotubes (CNT-900) prepared from MOF Ni₂(bdc)₂dabco.

Because the solvent of the synthesis was N,N-dimethyl formamide without deionized water, the stability of the MOF was not good in water solution. Its framework easily collapsed. With this in mind, the application of this MOF was limited to water environments. But it could be applied in gas phase catalysis, gas adsorption,²⁷ storage²⁸ and separation,^29,30 electrochemical catalysis,^11,31 colorimetric sensing,³² lithium-ion batteries,^33,34 and so on.

This nickel MOF has a peculiar framework structure which can be used as a precursor to create nanoporous carbon materials owing to its large surface area and pore volume. Usually, there are two methods to fabricate carbon materials from metal–organic frameworks. One is using MOFs as sacrificial templates and precursors with incorporation of an additional carbon source. The thermolysis of the MOFs with loading of the additional carbon source leads to the generation of porous carbon materials. Secondly, direct carbonization of MOFs is another way to prepare carbon nanomaterials since their scaffolds contain abundant organic species that could be used as carbon precursors. In this research, some kinds of carbon materials were prepared through the same nickel metal–organic framework Ni₂(bdc)₂dabco.

TEM images of some carbon nanomaterials are shown in Fig. 2. When the temperature of calcination was 800 °C, as shown in Fig. 2a and b, core–shell-like nano-nickel was obtained. Metal nickel was the core and multi-walled carbon was the shell. The nanoparticles (NPs) coated with carbon are normally synthesized by a chemical vapor deposition (CVD) method using the vaporization of the metal and carbon, which are deposited onto a substrate. After several attempts, the carrier gas chosen was nitrogen. The Kratschmer carbon arc process (standard arc method) has been used in the synthesis of the carbon shells combined with metal NPs. Briefly, the standard arc method is used to bury the selected metals or metal oxide powders inside the drilled hole of the graphite anode. And at high temperatures, the metal core particles obtained from metal vapor are coated with carbon to form the carbon-encapsulated metal NPs. The obtained carbon shell-coated metallic NPs resist etching by acids. The disadvantage of this method is that the yield of the carbon-encapsulated metal particles is much lower than that of other carbon products including carbon nanotubes (CNTs), nanopowders, and graphite. For the conventional chemical vapor deposition (CVD) method, a catalyst is needed to synthesize carbon materials. In our research, there was no catalyst in the whole preparation process. In the study of the carbon precursor, three kinds of metal–organic frameworks M₂(bdc)₂dabco (M = Zn, Ni, Co) were prepared by the same method. Characterization of specific data is demonstrated in the ESI.† In the subsequent calcination process, it was found that only Ni₂(bdc)₂dabco can be the precursor to synthesize core–shell-like microporous nano-nickel and carbon nanotubes. Thus, it can be seen that the metal nickel played an important role in the self-catalytic reaction. Especially in the synthesis of carbon nanotubes, metal nickel has not only the effect of traction, but also the effect of catalyst. In addition, this was an in situ growth process.

Fig. 2 TEM images of microporous nano-nickel and carbon nanotubes. (a and b) Microporous core–shell-like nano-nickel MNN-800 calcined under 800 °C; (c and d) carbon nanotubes (CNT-900) calcined under 900 °C; (e and f) hollow carbon nanospheres (HCNS-1000) calcined under 1000 °C.

For the particles of these carbon materials, Fig. 2 can give the distinct characteristics of the core/shell structure and a close view of the core. In the inset of Fig. 2b, the thickness of every graphite shell of the nanocapsules was 0.35 nm (5d = 1.75 nm). A distance of 0.185 nm was observed for the (111) planes of face-centered-cubic (FCC) Ni in the core. For the carbon nanotubes in Fig. 2c and d, the outside diameter of the nanotubes was 10.12 nm and the hollow cavity diameter was about 1.82 nm. The thickness of the carbon nanotubes was 8.30 nm. The average size of the hollow carbon nanospheres was 11.41 nm. In addition, it is also difficult to obtain pure and uncontaminated ferromagnetic transition metal cores. The yield of the carbon-coated metal NPs is dependent on the metal species and also critically on the operating conditions such as the geometry of the graphite crucible–metal anode assembly and the inner diameter of the crucible. Interface motion and the formation of pores have been studied because of their impact on the reproducibility and reliability of solders, passivation layers, diffusion barriers, etc., but not generally as a method for preparing porous materials.³⁵Fig. 2 shows a comparison of the different calcination temperatures.

In this research, we only used the metal–organic framework Ni₂(bdc)₂dabco as a precursor to prepare core–shell-like nanostructured nickel and carbon nanotubes. From this particular result, it is clear that the preparation of carbon nanomaterials can be achieved by a self-templated catalyst-free synthesis procedure, avoiding the use of secondary removable templates or transition-metal catalysts.⁹ In terms of the synthesis mechanism, for carbon nanotubes, chemical vapor deposition (CVD) is one of the most widely used synthesis methods. But in this preparation, the nanotube diameter was irregular, and a catalyst must be used in the process. The main research objective of this method is to control the arrangement of the catalyst on the template to control the generation of the structure of carbon nanotubes. In our research, there was no catalyst participating in the reaction. Based on the experimental results, this was related to the special structure of the nickel metal–organic framework. This may be a self-catalytic process. The metal nickel played an important role in the formation of the carbon nanotubes. In an inert gas atmosphere, the scavenging rate was kept at 50 mL min⁻¹. The growth of the carbon nanotubes was drived by the metal nickel, which was located in the top of the carbon nanotubes. The longer the carbon nanotube was, the thinner the wall it had. As seen from Fig. 2a and b, microporous core–shell-like nano-nickel MNN-800 was obtained by calcination under 800 °C, and the average grain diameter was 15 nm. The core of this nanoparticle was nickel, while the shell of which was filmy multilayer carbon. With the increase of calcination temperature, nano-nickel began to move slowly under a nitrogen atmosphere to form carbon nanotubes with nano-nickel particles (CNT-900 mentioned in Fig. 2c) located in the top of the nanotubes. This procedure was similar to CVD method, which was used commonly in the preparation of nanotubes. Further, when the temperature was increased to 1000 °C, the top of the nanotubes was broken, and the metal nano-nickel particles fell off. Then, metal nickel assembled to form larger nanoparticles which can be removed by washing with dilute hydrochloric acid. The sample that remained was hollow carbon nanospheres (HCNS-1000). XRD analysis of microporous nano-nickel MNN-800 is demonstrated in Fig. 3b. We used atomic force microscopy (AFM) to characterize the carbon nanomaterial. As seen from the results (Fig. 3c and d), the height of the sample was about 0.5 nm. Fig. 3e shows a TEM image of the multi-layer graphene. Electron diffraction of multi-layer graphene prepared at 1100 °C can also prove its structure. Single-layer graphene can be obtained in the next series of experiments.

Fig. 3 TEM (a), XRD (b) and AFM (c and d) analyses of microporous nano-nickel MNN-800; (e) TEM of multi-layer graphene-1100; (f) electron diffraction of multi-layer graphene-1100.

On the basis of the research mentioned in this manuscript, besides the nickel MOF, three-dimensional hollow sphere ultrathin graphene with a hierarchical structure was synthesized with a copper metal–organic framework, which has excellent applications in electrochemistry. This kind of nano and porous carbon material and its applications in catalysis are the research emphasis in the next study.

Conclusions

Core–shell-like microporous nano-nickel, carbon nanotubes, hollow carbon nanospheres and multi-layer graphene can be obtained only by changing the temperature via a nickel metal–organic framework (Ni₂(bdc)₂dabco) as the precursor. The highlight was that this was a self-catalytic and controllable process. This was a simple and effective path from metal–organic frameworks to carbon materials. By this method, we can obtain relatively large surface areas of microporous carbon materials. Meanwhile, the pore characteristics can be well preserved. To some extent, it can expand the application domain of metal–organic frameworks. The morphology diversity of the carbon nanomaterials, which was determined by different calcination temperatures, can influence the material properties.

Experimental section

Synthesis of metal–organic framework Ni₂(bdc)₂dabco

Ni(NO₃)₂ and 1,4-benzenedicarboxylic acid were purchased from Sigma-Aldrich. 1,4-Diazabicyclo[2.2.2]octane and N,N-dimethylformamide were purchased from SCRC.

A novel and highly porous metal–organic framework with rigidity and flexibility, Ni₂(bdc)₂dabco (H₂bdc = 1,4-benzenedicarboxylic acid, dabco = 1,4-diazabicyclo[2.2.2]octane), was prepared at 120 °C over 2 days by Kim.³⁶ In the MOF synthesis of Ni₂(bdc)₂dabco, 0.5000 g (1.68 mmol) Ni(NO₃)₂·6H₂O, 0.2800 g (1.68 mmol) H₂bdc and 0.0935 g dabco (0.84 mmol) were mixed in 20 mL N,N-dimethylformamide (DMF, >99%). To obtain better reaction results, the mixture was stirred for 2 h at room temperature. After that, the reactant mixture was loaded into a vessel (35 mL), sealed and placed in a microwave oven (Discover, CEM, maximum power of 300 W, 50 W used in this work). The vessel was heated to 120 °C in 10 min and kept at this temperature for 1 min. After the reaction time, the reactor was cooled to room temperature. A fine powder was obtained as the major product. To obtain higher porosity, the as-synthesized Ni₂(bdc)₂dabco was purified. The contents were completely transferred into a conical flask, and DMF was added incrementally with continuous stirring to dissolve the residual ligands H₂bdc and dabco. Upon repeated washing with DMF and centrifugation, most of the excess H₂bdc was removed from the MOFs and the resulting product was centrifuged and washed with ethanol. Finally, the product was dried at 120 °C overnight. The color of the as-synthesized MOF Ni₂(bdc)₂dabco was green.

Synthesis of nano-nickel, nanotubes, hollow carbon nanospheres and graphene sheets

The MOF Ni₂(bdc)₂dabco was calcined at 800 °C, 900 °C, 1000 °C and 1100 °C, respectively, under a high-purity nitrogen atmosphere in the whole calcination process. The nitrogen gas flow rate was 45 mL min⁻¹. The temperature was increased from room temperature (20 °C) to 800 °C, 900 °C, 1000 °C and 1100 °C, respectively. The heating rate was 5 °C min⁻¹. Every calcination was held for 5 h in the temperature process. After calcination, the black samples were washed two times with dilute hydrochloric acid (the concentration was 4 mol L⁻¹). Then the samples were dried in an oven at 120 °C for 5 h.

Material characterization

The phase and crystallinity of all the samples were determined from 2° to 80° 2θ using an X-ray diffractometer (CuK_α, Bruker D8) in θ–2θ geometry and capillary mode. The crystal size and morphology were examined using a scanning electron microscope (SEM, Vega 3 Tescan). The TEM and EDX were acquired using a field emission TEM (JEM-2100F, JEOL, Japan). The three-dimensional map of the nanoparticles was examined by atomic force microscopy (AFM, Bruker Dimension Icon). The surface area and the micropore volume were determined using a BET experiment (BELSORP-mini, Ankersmid) and the BET equation.

Acknowledgements

This work was financially supported by the NSFC (21376123, 21603107, U1403293), MOE (IRT-13R30 and 113016A), and the Research Fund for 111 Project (B12015).

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6en00441e

‡ These authors contributed equally.

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