Polymer–metal–organic framework core–shell framework nanofibers via electrospinning and their gas adsorption activities

Abstract

In this study, we have fabricated of Polyacrylonitrile/Zeoliticimidazolate frameworks (PAN@ZIF-8) core–shell nanofibers by combining electrospinning techniques and the MOF synthesis method. In the first step, 2MI ligand was dispersed on PAN by using electrospinning, and then 2MI was made to coordinate Zn²⁺ ions derived from a zinc acetate solution. In the second step, the nanofiber mats were immersed in a ZIF-8 seed solution, and continuous and compact ZIF-8 was formed on the PAN surface by a second round of crystal growth. Analyses of XPS results and of SEM and TEM images revealed the core–shell structure of PAN@ZIF-8 nanofibers, and showed them to have a uniform nanoshell but a variety of crystal diameters. In addition, the core–shell PAN@ZIF-8 nanofibers were found to display unique properties such as a stable and flexible structure and an excellent gas adsorption capability. Our findings suggest that the core–shell PAN@ZIF-8 nanofiber mats may form a good filter material because of their gas absorption properties and because of the structural flexibility and stability of ZIF-8.

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Introduction

Zeolitic imidazolate frameworks (ZIFs) are an emerging subclass of metal–organic framework (MOF) materials that could serve as an effective platform for gas adsorption,¹ carbon dioxide separation,² catalysis,³ and hydrogen storage⁴ due to their ordered structures, large internal surface areas, thermal stabilities and adjustable chemical functionalities.⁵ ZIF-8 (Zn(2MI)₂, 2MI = 2-methylimidazolate) in particular is one of the most studied ZIFs; it features a cubic crystal lattice with sodalite (SOD) topology, can be prepared in situ, and shows excellent chemical and thermal stabilities, which are advantageous for membrane applications. Therefore, various substrates and techniques have been used to prepare ZIF-8 films/membranes, such as α-Al₂O₃ (ref. 6) and nanofibers.^7,8 The electrospinning technique has become a versatile method for producing multifunctional nanofibers from various materials such as polymers, polymer blends, and composites.^9–11 More importantly, electrospun nanofibers have unique properties including a very high specific surface area, pore sizes in the nano range, high porosity, and a very light weight since these fibers have diameters ranging from one micrometer down to a few tens of nanometers.¹² The core–shell structure nanofiber is one of the most well-known and interesting nanostructures, and has attracted considerable attention since it has been successfully applied in various fields, such as gas purification, catalysts support, pervaporation, liquid chromatography, and information storage.^13–17

In this work, we report a facile and scalable strategy to obtain integrated, binder-free, flexible PAN@ZIF-8 core–shell nanofiber mats, which showed remarkable gas absorption properties due to their unique morphological features including high surface area and nanoporosity. First, we applied electrospinning. This technique has attracted growing interest in the past decade because it is quite versatile and cost-effective for producing multifunctional nanofibers with a variety of materials such as polymers, polymer blends, metal oxides, composite structures and nonpolymeric systems.^18–25 Four different types of inorganic materials with different functional MOFs have been employed for the formation of composites and applied as functional materials.^26–28 In particular, in 2011, Rainer Ostermann and his co-workers published the synthesis and characterization of ZIF-8/organic polymer nanofibers for the first time.²⁹ However, few scientists have tried to generate hierarchical ZIF-8 nanostructures by using electrospinning techniques.^30,31 However, we have successfully used this technique to achieve integrated nanofibers by growing ZIF-8 nanocrystals along the fiber and forming a continuous and compact ZIF-8 sheath. Moreover, we were able to quite feasibly control the surface morphology of the fiber and have the ZIF-8 nanocrystals grow along the nanofiber direction, and hence obtain multifunctional electrospun nanofibers. Herein, by using ZIF-8 as an example, 2MI/PAN nanofibers were obtained by electrospinning, and then 2MI on the PAN surface underwent mutual coordination chelation with Zn²⁺ ion provided by zinc acetate. After that, through the second crystal growth, the continuous and compact ZIF-8 was formed on the surface of PAN. Using this strategy, a uniform distribution of the MOF seeds was achieved. Free-standing flexible HKUST-1 and ZIF-8 membranes were obtained by Wu et al.^32,33 after the secondary growth step.

The core–shell PAN@ZIF-8 structure offers obvious advantages. First, polymer–MOF composite nanofiber structures have intriguing properties that combine the advantages of polymers and MOFs such as structural flexibility, light weight, high thermal stability, excellent adsorption, hydrogen storage, and catalysis. Secondly, the aggregation of 2MI on the surface during electrospinning of the 2MI/PAN blending nanofiber can replace traditional pretreatment of the PAN nanofibers to grow ZIF-8 crystals. Thirdly, the ZIF-8 nanocrystals grown along the fibers formed the continuous and compact ZIF-8 shell. Also, the as-synthesized core–shell PAN@ZIF-8 displays a high gas adsorption capacity.

Fig. 1 shows the process used to fabricate the integrated core–shell PAN@ZIF-8 grown on the PAN nanofiber. Initially, a spot of ZIF-8 was grown on the PAN substrate, which involved the chelation of the zinc ions by 2MI. After a second round of growth, the ZIF-8 crystals grown along the fibers formed the continuous and compact ZIF-8 sheath.

Fig. 1 Schematic illustrating the procedures used to synthesize the PAN@ZIF-8 core–shell.

Experimental section

Chemicals

All of the chemicals were of analytical grade and used as received without further purification. These chemicals included polyacrylonitrile (PAN, MW = 90 000, Beijing Yili Fine Chemical), zinc acetate dihydrate (Zn(OAc)₂·2H₂O, 99% purity, Sigma), 2-methylimidazole, N,N-dimethylformamide (DMF, 99.5%, Tianjin Chemicals), and 2-methylimidazole (2MI, 99%, Aladdin).

Preparation of the flexible 2MI/PAN substrates

2MI/PAN substrates were prepared by blending imidazolate into electrospinning solutions. A vertical-axis electrospinning installation was used to electrospin a solution including PAN (1 g), 2MI (1 mmol) and N,N-dimethylformamide (11 g). The electrospun fibers were collected on a grounded aluminum foil.

Synthesis of the core–shell PAN@ZIF-8

The nanocrystals on the PAN fiber surface were prepared by adding the 2MI/PAN substrates into a solution of Zn(OAc)₂·2H₂O (0.5 mmol) dissolved in 40 ml methanol at 60 °C for 2 h. Then the products were washed with methanol several times. For the synthesis of the core–shell PAN@ZIF-8, the growth solution was prepared by adding MI, zinc acetate and methanol solution at 60 °C. Mole ratios of 1 : 4 : 625 and 1 : 2 : 625 for 2MI, zinc acetate and methanol were examined. Finally, the synthesis of the core–shell PAN@ZIF-8 was allowed to proceed by removing the solvent under vacuum for 12 h at room temperature.

Characterization

The samples were characterized by using various techniques. X-ray diffraction (XRD; Rigaku D/MAX-YA) was applied with Cu Kalpha radiation, λ = 0.154 nm, and scans performed from (2θ) 3–50° at a rate of 5 min⁻¹. Scanning electron microscopy (SEM; JSM-6360LA, Japan) was carried out using an acceleration voltage of 15 kV, and transmission electron microscopy (TEM; JEM-2100, Japan) was carried out using a 200 kV accelerating voltage. The electrospun fibers were dispersed in DMF and then dip-coated onto copper grids. The specimens were analyzed using an XPS system (XPS, Thermo Electron Corporation, Escalab 250, Germany). The photoelectron take-off angle used was 45 degrees with respect to the sample plane. Survey spectra were acquired in the binding energy range of 0–1300 eV and high-resolution spectra of Zn 1s, O 1s, and N 1s were acquired. The N₂ adsorption–desorption isotherm measurements were carried out on a Micromeritics ASAP2010 analyzer at 77 K. Prior to the measurement, the sample was degassed at 120 °C for 6 h in the vacuum line. The gas adsorption was tested by using this same device (ASAP2020) with the same pretreatment method, but at 20 °C.

Results and discussion

The XRD patterns of pristine ZIF-8 nanocrystals and core–shell PAN@ZIF-8 nanofibers are shown in Fig. 2

The presence of strong peaks implied a high crystallinity of the prepared ZIF-8, which is in good agreement with previous experimental work.³⁴ The peaks at 2θ = 7.30, 10.35, 12.70, 14.80, 16.40 and 18.00 degrees derived from the (110), (200), (211), (220), (310), and (222) planes, respectively. Fig. 2a shows the XRD pattern of the as-synthesized product. The diffraction peaks matched well with the simulated XRD pattern of ZIF-8 according to the published crystal structure data,³⁵ indicative of the phase purity of the as-synthesized ZIF-8. The considerably high patterns of PAN@ZIF-8 nanofiber mats Fig. 2b with the simulated sample also shows that the ZIF-8 nanocrystals grown along the fibers formed a continuous and compact ZIF-8 sheath. Fig. 2 (right) shows an SEM image of the core–shell PAN@ZIF-8 nanofiber. The prepared ZIF-8 nanocrystals on the surface of PAN nanofiber were observed to have a rhombic dodecahedron morphology, which is in good agreement with the literature.³⁴ The particle sizes of the prepared samples were measured by using a Nanomeasurer 1.2 as shown in Fig. S2.† Also, FTIR analysis (see Fig. S1†) revealed that the ZIF-8 nanocrystal was successfully grown on the PAN fiber.

Fig. 2 Left: XRD patterns of ZIF-8 (a), PAN@ZIF-8 (b), and a simulated ZIF-8 (c). Right: SEM image showing the morphology of the prepared PAN@ZIF-8.

Morphology of the core–shell PAN@ZIF-8 nanofibers

SEM images showed the ZIF-8 nanocrystals grown along the fibers, which formed the continuous and compact ZIF-8 shell. 2MI/PAN was fabricated via blending and spinning 2MI and PAN with N,N-dimethylformamide. In this strategy, the 2MI/PAN surface served as a template for the ZIF-8 nanocrystal growth and provided the ligand; the Zn²⁺ ions were provided by dissolving zinc acetate with methanol. As clearly seen in the scanning electron microscopy (SEM) images (Fig. 3a), ZIF-8 adhered uniformly on the PAN substrate, resulting in a stronger adhesive force, which solved the problem of fiber surface pretreatment.³⁶ Fig. 3b and c show the SEM images of core–shell PAN@ZIF-8 nanofibers obtained, respectively, at 1 : 4 : 625 and 1 : 2 : 625 mole ratios of MI, zinc acetate and methanol. The average grain diameter of the ZIF-8 nanocrystals obtained using the 1 : 2 : 625 ratio was about 614 nm, which was larger than the 206 nm average grain diameter of the ZIF-8 nanocrystals obtained using the 1 : 4: : 625 ratio. The increased size resulted from the different concentrations of MI and zinc acetate. As shown in Fig. 3b and c, the PAN surface was still completely covered by the ZIF-8 nanocrystals. These nanocrystals grew along the fibers, were well aligned, and formed the continuous and compact ZIF-8 sheath after the two-step reaction.³²

Fig. 3 SEM images of ZIF-8 grown on PAN. (a) ZIF-8 on PAN. (b, c) The core–shell PAN@ZIF-8 nanofibers obtained using 1 : 4 : 625 and 1 : 2 : 625 mole ratios of MI, zinc acetate and methanol.

It is worth mentioning that the synthesized PAN@ZIF-8 remained intact after twenty minutes of ultrasonic vibration, indicating good bonding strength between the crystal layer and substrate.³⁷ More importantly, as a result of further examining the morphologies of the core–shell PAN@ZIF-8 nanofibers using TEM (see Fig. S3†), the ZIF-8 shell layer was observed to be of uniform thickness on the PAN fiber surface.

Surface analysis of the core–shell PAN@ZIF-8 nanofibers

The surface chemical composition and bonding states of the pristine ZIF-8 nanofibers and core–shell PAN@ZIF-8 nanofibers were investigated by using XPS.

Table 1 summarizes the compositional data, in atomic concentrations, of the pristine ZIF-8 nanocrystals and core–shell PAN@ZIF-8 nanofibers. As anticipated, all of the expected ZIF-8 features were observed, including the presence of zinc (the coordinating metal), nitrogen, and carbon (the imidazole linker)³⁸ (see Fig. S4†). The probe depth for XPS is usually approximately the top atomic layers (∼10 nm),³⁹ depending on the take-off angle between the photoelectrons and the sample surface. Since the average diameter of the ZIF-8 nanoparticles was 34.0 ± 2.9 nm, the core of PAN was not observed in an SEM image of the core–shell PAN@ZIF-8 nanofiber (Fig. 4). An oxygen signal was observed (Fig. 4a) due to ambient exposure to air during sample preparation, as reactions are possible between under-coordinated Zn sites, CO₂, and water in air.⁴⁰

Table 1 Atomic concentrations generated from XPS wide-energy survey scans

Samples	C (%)	O (%)	Zn (%)	N (%)
ZIF-8	62.5	11.89	7.84	17.77
PAN@ZIF-8	69.26	8.55	5.73	16.46

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Fig. 4 Top: Ball-and-stick model of the ZIF-8 structure and an image of the core–shell PAN@ZIF-8 nanofiber. Bottom: High-resolution XPS spectra of core–shell PAN@ZIF-8 nanofiber mats: (a) N 1s, (b) C 1s, (c) O 1s, and (d) Zn 2p regions.

To obtain quantitative chemical information on the PAN@ZIF-8 nanofiber, multiple regions including Zn 2p, O 1s, N 1s, and C 1s were scanned at higher resolution as shown in Fig. 4 (bottom). To determine the nature of the surface terminations after by fitting the N 1s region. Also after using the main peak at 400.5 eV shown in Fig. 3a, which we assigned to the imidazole groups based on comparisons to analogous species,^41,42 which were found to be almost stoichiometric as expected. On the other hand, the absence nitrogen peak at 397.9 ev of nitrile groups in the XPS survey scans of the core–shell PAN@ZIF-8 nanofibers indicated that the surface of the PAN nanofibers was successfully coated by a ZIF-8 layer. Formation of ZIF-8 nanocrystals on the surfaces of PAN nanofibers was also confirmed by a high-resolution Zn 2p XPS scan (Fig. 4d). Zn 2p_3/2 and Zn 2p_1/2 subpeaks of the Zn 2p doublet were located at 1021.73 and 1044.76 eV, which is compatible with Zn 2p of the ZIF-8 structure seen in crystals of this MOF.⁴³

Specific surface area analysis of the core–shell PAN@ZIF-8 nanofibers

The adsorption properties of the PAN@ZIF-8 nanofibers were measured by acquiring N₂ adsorption curves and carrying out a micropore analysis. The results for pure nanocrystalline ZIF-8, the 2MI/PAN nanofiber, and core–shell ZIF-8@PAN are shown in Fig. 5 and Table 2, which indicated a type I isotherm. The high nitrogen adsorption at relatively very low pressures was due to the presence of micropores.⁴⁴ The micropore volumes of the 2MI/PAN nanofiber, ZIF-8 nanocrystal and core–shell PAN@ZIF-8 nanofiber were determined to be about 0.0005, 0.62 and 0.51 cm³ g⁻¹, respectively. The specific surface areas of 2MI/PAN, the pure ZIF-8 nanocrystal and the core–shell PAN@ZIF-8 nanofiber were determined to be 5, 1219 and 983 m² g⁻¹, respectively. The greater specific surface area of the pure ZIF-8 nanocrystal than of the core–shell PAN@ZIF-8 nanofiber was due to the inclusion of PAN, and its low specific surface area, in the ZIF-8 nanocrystal layers of the PAN@ZIF-8 nanofiber. Compared with the previous results of Zhou lian,³⁶ our strategy, which was based on the use of a 2MI/PAN substrate, yielded a much higher gas adsorption capacity. In a word, the 1D electrospun PAN nanofibers contributed to the orderly growth of ZIF-8 nanocrystals along the nanofiber direction, and contributed to the formation of the core–shell PAN@ZIF-8 nanofiber, while at the same time aggregation of nanocrystals over a large area was avoided.

Fig. 5 N₂ adsorption–desorption analysis of ZIF-8, PAN@ZIF-8 and 2MI/PAN.

Table 2 N₂ adsorption–desorption data

Sample	Surface area^a (m^2 g^−1)	Micropore volume^b (cm^3 g^−1)
a According to the BET model.b According to the t-plot model.
ZIF-8	1219	0.62
PAN@ZIF-8	983	0.51
2MI/PAN	5	0.0005

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Gas adsorption of the core–shell PAN@ZIF-8 nanofibers

Here, we demonstrate that the core–shell PAN@ZIF-8 nanofiber mat can serve as a very good filtering material candidate because of the flexible polymeric core and the gas adsorption of the ZIF-8 shell layer. These nanofiber mats can be easily handled and folded as a free-standing material. We tested the gas adsorption of the core–shell PAN@ZIF-8 nanofiber mats at 20 °C by using the ASAP2020, which has been shown to have good CO₂ and H₂ adsorption performances. From Fig. 6, at 800 mm Hg, the volumes for the adsorption of H₂ and CO₂ were determined to be 8.1 and 13.3 cm³ g⁻¹, respectively. The clearly higher adsorption capacity of PAN@ZIF-8 for CO₂ than for H₂ reveals the potential application of PAN@ZIF-8 in membranes that preferentially adsorb CO₂. These materials, which combine the excellent toughness of PAN nanofibers with the outstanding adsorption ability of ZIF-8, have potential applications in the air purification of clothes.

Fig. 6 Gas adsorption isotherms of PAN@ZIF-8 at room temperature (20 °C).

Conclusions

In conclusion, we have successfully fabricated core–shell PAN@ZIF-8 nanofibers based on 2MI/PAN for the first time. Analyses of XPS results and of EM and TEM images revealed a continuous and compact core–shell structure for the PAN@ZIF-8 nanofibers. The core–shell PAN@ZIF-8 was found, based on N₂ adsorption–desorption experiments, to have a high surface area of 983 cm² g⁻¹; it was also found to have volumes of 13.3 and 8.1 cm³ g⁻¹ for the adsorption of CO₂ and H₂, respectively. Our results indicate that core–shell PAN@ZIF-8 nanofiber mats can serve as a filtering/membrane material for gas adsorption. Due to the flexible 2MI/PAN substrates, core–shell PAN@ZIF-8 nanofibers can be fabricated for many applications in addition to filters/membranes, including catalyst supports, air purification, gas sensors and special gas adsorption applications.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra23147g

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