Homogeneous decoration of zeolitic imidazolate framework-8 (ZIF-8) with core–shell structures on carbon nanotubes

Abstract

Considerable attention has focused on the combination of carbon nanotubes (CNTs) and metal–organic frameworks (MOFs) since both nanomaterials have outstanding properties. We describe a method for the homogeneous decoration of a MOF (ZIF-8 was chosen) onto the surfaces of CNTs dispersed by polyvinylpyrrolidones (PVPs) in methanol, which was revealed by a scanning electron microscopic study. The homogeneous coating of the MOF on the CNTs, and nanostructures of the CNT-MOF were controlled by simply changing the concentrations of the MOFs. Furthermore, this method was also applicable to graphene and graphene oxide (GO). CO₂ uptakes of the CNT-MOF and graphene-MOF were significantly improved as compared to the nonhomogeneous composites synthesized without the PVP functionalization, and a good reproducibility of the CO₂ adsorption was confirmed by the cycling test.

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1 Introduction

Carbon nanotubes (CNTs) are nanomaterials with remarkable electrical, thermal, optical and mechanical properties.^1–4 They are also an attractive material for gas storage due to their high surface areas (50–1315 m² g⁻¹).⁵ Especially, specific nano-spaces in the CNT bundles, such as interstitial channels and external grooves, have significant interactions with aromatic⁶ and gas molecules, such as Ar,⁷ Ne,⁸ He,^8,9 CF₄,¹⁰ H₂,¹¹ N₂,^12,13 O₂,¹³ and C_nH_2n+2,^14,15 and also exhibit selectivity for molecules.^6,16,17 However, the rather low gas selectivity and storage capacity on the π-conjugated surfaces of the CNTs has limited their applications to gas sensors and separations. In order to improve such a drawback, covalent¹⁸ and non-covalent functionalization^19,20 and coating by inorganic materials²¹ have been carried out.

Metal–organic frameworks (MOFs) with extremely-high surface areas caused by the three-dimensional porous structures including metal centers and organic linkers have emerged as one of the most promising gas absorbent material because the pore sizes and chemical affinities are easily controlled by modifying the metals and linkers.^22–28 MOFs have been investigated in many fields including gas absorbents and separations, catalysts²⁹ and drug delivery system (DDS) and fluorescence imaging of cancer cells,³⁰ therefore, the homogeneous decorations of the MOFs are very important for many applications.

Zeolitic imidazolate frameworks (ZIFs), which is one of the MOFs, are known to have the highest thermal and chemical stabilities among the MOF families,³¹ and their facile synthetic method³² enables complexations with various polymers,^33,34 graphene quantum dots (GQDs),³⁵ graphene oxide (GO),³⁶ Cu₃(BTC)₂-modified CNTs,³⁷ etc. Especially, the ZIF-8 synthesized by 2-methylimidazole (denoted as 2-MeIm) and zinc ions shows good separation properties for CO₂/CH₄ (ref. 38) and CO₂/N₂ (ref. 39) compared to other ZIFs, such as ZIF-7, ZIF-22 and ZIF-90.⁴⁰ Flexible structures opened by rotations of the 2-MeIm would be advantageous to control its morphology, such as core–shell structures. ZIF-8 has been used as a material for the encapsulation of GQDs³⁵ and preparation of composites with polymers,^41–46 CNTs⁴⁷ and graphene oxide.³⁶ However, the formation of core–shell structures using the ZIF-8 as the shell has not yet been reported. We here focus on the homogeneous decorations of the MOFs since such a coating is very important for many applications.

In this study, as shown in Fig. 1, polyvinylpyrrolidone (PVP) was chosen as the structure-directing and stabilizing agent for the preparation of the CNT@ZIF-8 core–shell structure due to its shape-selective properties for inorganic nanowires^48–52 and core–shell structures,^53–55 and it was expected that its strong interaction and high dispersing property for the CNTs would help to form a homogeneous structure without aggregations of the CNTs.⁵⁶ Furthermore, graphene and graphene oxide were also used as the core materials for the ZIF-8 shell. This is the first report about the synthesis of extremely well-controlled shell structures of the ZIF-8 coating the surface of CNTs with the high aspect ratio.

Fig. 1 A schematic drawing of a CNT@ZIF-8 core–shell structure.

2 Materials and methods

2.1 Materials

Multi-walled carbon nanotubes (MWNTs) (C_tube120, metal oxide <3 wt%, average diameter: ∼20 nm, length: 1–25 μm, CNT Co., Ltd), graphene (3 nm graphene nanopowder, grade AO-1, Graphene Supermarket), graphene oxide (GO) (dry platelets, Graphene Supermarket) and methanol (>99.8%, J. T. Baker®) were used as received. Polyvinylpyrrolidone (PVP) (Mw: ∼360 000), 2-methylimidazole (99%) and zinc nitrate hexahydrate (98%) were purchased from Sigma-Aldrich.

2.2 Measurements

The field emission scanning electron microscopy (FE-SEM) and scanning transmission electron microscopy (STEM) were conducted using an SU-8020 (Hitachi, Tokyo, Japan) at 1 kV and 30 kV. Fourier transform infrared (FT-IR) and Raman spectroscopy measurements using a 532 nm laser were carried out by a Varian 660-IR (Varian Medical Systems, Inc., California, USA) and a SENTERRA Raman microscope spectrometer (Bruker Corporation, Billerica, MA, USA), respectively. X-ray diffraction (XRD) measurements in the range of 1° < 2θ < 30° were performed by a SmartLab (Rigaku) at 40 kV and 30 mA (CuK_α radiation, λ = 0.154 nm).

2.3 Preparations of CNT, graphene, and GO composites with the ZIF-8

Composites of carbon materials with the ZIF-8 were fabricated by the following in situ ZIF-8 synthesis. The CNTs (30 mg) were added to a PVP methanol solution (2 mg mL⁻¹, 60 mL), then sonicated in a bath-type sonicator (JAC-3010, KODO) for 1 h. After centrifugation (20 000g, 1 h), the supernatant was removed, the precipitates were redispersed in methanol (15 mL) to which 2-methylimidazole (2-MeIm) in methanol (22 mg mL⁻¹, 60 mL) was added, then zinc nitrate hexahydrate in methanol (11 mg mL⁻¹, 12 mL) was carefully added to the dispersion while stirring. The generated precipitates were collected after centrifugation (20 000g, 0.5 h), then rinsed with methanol, and dried in a vacuum oven at 40 °C.

Graphene and GO composites with the ZIF-8 were also fabricated by the same procedure.

2.4 TGA analysis

CO₂ adsorption experiments were carried out by a TGA Q500 (TA Instruments, New Castle, USA) using a previously reported method.^57–59 Nitrogen (N₂) and carbon dioxide (CO₂) gases were used as the purge and furnace gases at the flow rates of 40 and 60 mL min⁻¹, respectively, and all experiments were performed after verification of no weight reduction after N₂ flowing for 4 h at 100 °C in order to dehydrate and degas the samples. The temperature of the furnace was raised to 70 °C at the rate of 20 °C min⁻¹, then a furnace gas was changed to the CO₂. After the isothermal process for 1 h at each temperature (70, 55, 40, and 25 °C), the furnace gas was changed from the CO₂ to N₂ during heating from 25 to 70 °C. Recycle test was performed by a similar method at 25 °C, and thermogravimetric analysis (TGA) was carried out in the N₂ atmosphere by heating to 900 °C at the rate of 5 °C min⁻¹.

3 Results and discussion

3.1 Preparations of CNT, graphene, and GO composites with the ZIF-8

PVP has been reported not only as a good structure-directing agent for nanowires^48–52 and core–shell structures,^53–55 but also as a good dispersant for CNTs.⁵⁶ Recently, the encapsulation of metal nanoparticles into the ZIF-8 has been investigated using the PVP as the capping agent due to its structure-selective property for the nucleation and growth of the ZIF-8 crystals.⁶⁰ The core–shell structure formation of the CNT@ZIF-8 is shown in Scheme 1, in which the PVP-functionalized CNTs (PVP-CNTs) are redispersed in methanol after the removal of the excess PVPs, and the ZIF-8 was synthesized using the 2-MeIm and zinc ions in the presence of the PVP-CNTs. Graphene and GO were also used as supporting materials in place of the CNTs.

Scheme 1 Schematic drawing for the preparation of CNT@ZIF-8 core–shell structures.

As shown in Fig. 2, the ZIF-8 composites with the PVP-CNTs (CNT@ZIF-8) and CNTs (CNT/ZIF-8) clearly exhibited a thicker diameter than that of the pristine CNTs, suggesting the ZIF-8 decoration. In the figure, we recognized that ZIF-8 particles were hardly seen in the CNT@ZIF-8 composite; however in sharp contrast, many ZIF-8 aggregates were observed in the CNT/ZIF-8 composite. It is expected that the PVP worked as a structure-directing agent on the CNT surfaces (see ESI, Fig. S1†), and a similar behavior also appeared when using the PVP-functionalized graphene (PVP-G) and GO (PVP-GO). In the ZIF-8 composites with the PVP-G (PVP-G/ZIF-8) and PVP-GO (PVP-GO/ZIF-8), the sizes of the decorated ZIF-8 nanoparticles were smaller and significantly more homogeneous than those of the pristine graphene (G/ZIF-8) and GO (GO/ZIF-8) as can be seen in Fig. 3. As shown in Fig. 4, the scanning transmission electron microscope (STEM) image of the CNT@ZIF-8 clearly shows shell structures on the individually-dispersed CNT surfaces, and the diameter of the decorated ZIF-8 nanoparticles on the PVP-G/ZIF-8 and PVP-GO/ZIF-8 was found to be less than 35 nm.

Fig. 2 Scanning electron microscope (SEM) images of the (A) CNT, (B) CNT/ZIF-8, and (C) CNT@ZIF-8. The yellow bars in (B) indicate aggregations of the ZIF-8 nanoparticles. Scale bars: 200 nm.

Fig. 3 SEM images of the (A) graphene, (B) G/ZIF-8, (C) PVP-G/ZIF-8, (D) GO, (E) GO/ZIF-8, and (F) PVP-GO/ZIF-8. Scale bars: 200 nm.

Fig. 4 Scanning transmission electron microscope (STEM) images of the (A) CNT@ZIF-8, (B) PVP-G/ZIF-8, and (C) PVP-GO/ZIF-8. Scale bars: 100 nm.

All composites showed characteristic peaks in the Raman spectra, which were attributed to the G-band (∼1590 cm⁻¹) and D-band (∼1350 cm⁻¹) of the CNTs, graphene, and GO (see ESI, Fig. S2†). We also observed Fourier transform infrared spectroscopy (FT-IR) peaks, which appeared at almost the same wavenumbers as that of the ZIF-8 synthesized without the supported carbon materials (see ESI, Fig. S3 and S4†). Furthermore, X-ray diffraction (XRD) patterns of the composites showed patterns similar to the ZIF-8 (see ESI, Fig. S5†). Since the formation of the ZIF-8 crystals is evident from the FT-IR and XRD studies together with the results from previous reports,^36,61 all the obtained results strongly suggested the formation of ZIF-8-decorated nanostructures.

It was found that during the synthesis of the CNT@ZIF-8 core–shell structures, the concentrations of the 2-MeIm and zinc nitrate hexahydrate played a key role in the shell formation, and the suitable concentrations were ∼22 mg mL⁻¹ of 2-MeIm and ∼11 mg mL⁻¹ of zinc nitrate hexahydrate. However, when using four and eight times higher concentrations, no such CNT@ZIF-8 core–shell structures were formed due to the expected rapid nucleation and growth of the ZIF-8 as shown in Fig. 5. It was found that when using two times higher concentrations, kebab-like structures (see Fig. 5B) were observed in the CNT@ZIF-8. The obtained results indicated that the ZIF-8 formations on the surfaces of the PVP-CNTs can be simply controlled by changing the concentrations of the 2-MeIm and zinc nitrate hexahydrate. In contrast, reaction times did not influence on the nanostructure of the CNT@ZIF-8 even when the experiment was carried out at the suitable concentrations of ∼22 mg mL⁻¹ of 2-MeIm and ∼11 mg mL⁻¹ of zinc nitrate hexahydrate with stirring for over 240 min (see ESI, Fig. S6†).

Fig. 5 SEM images of the CNT@ZIF-8s prepared from 2-MeIm and zinc nitrate solutions at concentrations of (A) 22 and 11 mg mL⁻¹, respectively, and the (B) 2, (C) 4, and (D) 8 times-concentrated solutions. Scale bars: 300 nm.

3.2 CO₂ adsorption properties

The CO₂ adsorption capability was examined using a previously reported method.^57–59 As shown in Fig. 6, the CO₂ uptake in the CNT@ZIF-8 was significantly improved compared to the pristine CNTs and CNT/ZIF-8s. Since it has been reported that homogeneous decorations of nanoparticles with smaller sizes improved the adsorption capacity for hydrogen gases,⁶² it is expected that the homogeneous decorating of the ZIF-8 on the CNT@ZIF-8, in which the particle size of the ZIF-8 was small, significantly improved the adsorption properties even though the ZIF-8 content in the CNT@ZIF-8 was lower than that in the CNT/ZIF-8 (ESI, Fig. S7†). The PVP-G/ZIF-8 also showed a greater CO₂ uptake than those of the pristine graphene and G/ZIF-8; however, such an improvement was not observed from the PVP-GO/ZIF-8 although the adsorption rate of the CO₂ was improved as compared to the pristine GO (ESI, Fig. S8†). The difference of CO₂ uptakes between the CNT@ZIF-8 and PVP-G/ZIF-8 was due to the difference of the contents of the ZIF-8 in the composites (ESI, Fig. S7†); namely the higher ZIF-8 content in the PVP-G/ZIF-8 provided a higher CO₂ uptake than that of the CNT@ZIF-8. Meanwhile, stable reproducibility of the CO₂ adsorption on the CNT@ZIF-8 was confirmed from cycling tests without any decrease in the adsorption capacities as shown in Fig. 7. Although the presented CO₂ uptake of the ZIF-8 on the CNTs and graphenes is not very sufficient, the uptake amount is expected to be enhanced by optimizing the experimental conditions.

Fig. 6 TGA showing mass changes due to the adsorption and desorption of CO₂ gases obtained from the (A, blue line) CNT@ZIF-8, (A, red line) CNT/ZIF-8, (A, black line) CNT, (B, blue line) PVP-G/ZIF-8, (B, red line) G/ZIF-8, and (B, black line) graphene.

Fig. 7 Cycling of the CO₂ adsorption on the CNT@ZIF-8.

4 Conclusions

CNT@ZIF-8 core–shell structures have been successfully fabricated by the in situ ZIF-8 synthesis in the presence of the PVP-functionalized CNTs, and the ZIF-8 nanostructures in the composites were simply controlled by changing the concentrations of the starting chemicals for the synthesis of the ZIF-8. Similar homogeneous decorations were possible when using PVP-functionalized graphene and GO in place of the PVP-functionalized CNTs. Such a homogeneous ZIF-8 decoration was important for the improved CO₂ gas adsorption compared to the composites prepared using none-PVP modified CNTs and graphenes. The presented method for fabricating homogeneous ZIF-8 decorations is very simple and the coated ZIF-8 shell structures were easily controlled, suggesting that the method is highly useful for the design and synthesis of many nanomaterials including catalysts, materials for DDS and imaging bionanotechnology, etc.

Acknowledgements

This work was supported by the R&D program of MOTIE/KEIT (no. 100039993, Development of product safety information service solution using nano hydrid smart packaging), Korea.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra06792d

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