ZIF-8 Hybrid Aerogel: Preparation

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Hierarchically Porous Graphene/ZIF-8 Hybrid Aerogel: Preparation, CO2 Uptake Capacity and Mechanical Property Min Jiang, Houzhi Li, Lijuan Zhou, Ruofei Xing, and Jianming Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b17728 • Publication Date (Web): 20 Dec 2017 Downloaded from http://pubs.acs.org on December 21, 2017

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ACS Applied Materials & Interfaces

Hierarchically Porous Graphene/ZIF-8 Hybrid Aerogel: Preparation, CO2 Uptake Capacity and Mechanical Property Min Jiang*, Houzhi Li, Lijuan Zhou, Ruofei Xing, Jianming Zhang* Key laboratory of of Rubber-Plastics, Ministry of Education/Shandong Provincial Key Laboratory of Rubber-Plastics, Qingdao University of Science & Technology KEYWORDS: zeolitic imidazole framework, graphene, hierarchical pore, aerogel, carbon dioxide storage

ABSTRACT: Hierarchical zeolitic imidazole framework (ZIFs) combining micropore with mesoporous structure is desirable to enhance mass transport and give rise to novel applications. Here, hierarchically porous graphene/ZIF-8 hybrid aerogel (GZAn) materials were successfully prepared by a two-step reduction strategy and layer-by-layer assembly method. To avoid a tedious dry step and a use of energy-consuming freeze-drying technology, a reduced graphene oxide hydrogel with different reduction degree was chosen as template to in-situ grow ZIF-8 crystals. The parameter of density and elemental analysis were adopted to calculate the amount of ZIF-8 in GZAn materials for different assembly cycles. The distribution of micropore and mesopore of GZAn materials was controlled by changing the loading of ZIFs in GZAn materials. Furthermore, GZA8 materials performed enhanced CO2 uptake capacity (0.99 mmol g-1, 298K,

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1bar) than pure ZIF-s crystal and pure graphene aerogels, showing excellent synergistic effect of hierarchical pore structures. Meantime, with the increase of ZIF-8 loading, the mechanical robustness of GZAn was uplifted obviously. This work provides an efficient method to prepare hierarchically porous ZIFs-based materials with good CO2 uptake capacity and tunable mechanical robustness.

INTRODUCTION During the past decade, a new sub-class of metal organic frameworks, zeolitic imidazole framework (ZIFs),1 has received remarkable attention from many research fields. ZIFs own a well-defined porous structure with ultrahigh surface area and exceptional thermal stability, which render ZIFs promising and potential in CO2 capture,2 gas separation,3 heterogeneous catalysis,4 and chemical sensing.5 In order to improve machinability of ZIFs in practical application and give rise to novel applications, many ZIFs-based composite materials have been invented, including metal oxide/ZIFs,6 surfactant/ZIFs,7 and polymer/ZIFs.8 Within all known ZIFs materials, ZIF-8 (2-methylimidazole zinc salt) is acknowledged as an excellent member and has been studied widely owing to its low cost, easy preparation, exceptional hydrothermal stability. Recently, there have been increasing research efforts to develop ZIF-8/graphene oxide (GO) composites9-18 and ZIF-8/graphene composites.19-21 It is well-known that GO has a basal plane with anchoring groups like hydroxy, epoxy or carboxyl groups. These functional groups are critical for the assembly of graphene-based materials and hybrid composites. Kumar et al. firstly described that the growth and stabilization of nanoscale ZIF-8 on a graphene oxide surface through coordination modulation.10 Wang et al. reported that ZIF-8 crystals were selectively seeded and grown at the large pores of ultrathin GO membranes and improved the hydrogen selectivity of GO membranes.11 Hu et al. developed a 2D nano-hybrid seeding strategy for the


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preparation of ultrathin ZIF-8/GO membranes showing excellent molecular sieving gas separation properties.3 These pioneering studies proved adequately that the electrostatic interaction and coordination interaction between ZIF-8 and GO played a key role in the formation of ZIFs/GO composites. Recently, Kim et al. adopted a thermal annealing of GO at 250 ºC to prepare reduced graphene oxide with retaining some -C=O groups for further assembly of ZIF-8 on its surface.19 Although ZIF-8 also grew on the reduced GO nanosheets, it was not clear for us to know the relationship between the amount of oxygen functional group and the loading of ZIF-8. With the rapid development of graphene materials, three-dimensional graphene-based framework materials are emerging in many forms, such as aerogels, hydrogel, foams and sponges.22-26 These materials exhibit continuously interconnected meso-/macro- porous structure, low mass density, and large surface area, showing tremendous potential in energy dissipation, conductive sensors, catalyst support and recyclable absorbents.27-29 Therefore, some researchers aimed to make use of 3D graphene network as template to upload ZIF-8. For example, Li et al. combined a hydrothermal process with solvothermal process to prepare 3D graphene supported ZIFs materials for studying their electrochemical capacitance.30 Cao et al. used the chemical vapor deposition to prepare 3D graphene network on nickle foam substrates and treated them by strong acid for the following growth of ZIF-8.31 Wan et al. prepared graphene aerogel (GA) by a freeze-drying process and then chose GA as a template for the further growth of ZIF-8 to form all-solid-state supercapacitor.32 It is well-known that high microporosity and enormous internal surface areas are two crucial characters for ZIF-8 materials to perform strong bonding and high guest selectivity with gas molecule. Therefore, another merit of ZIF-8 loaded by 3D graphene network is to eliminate a


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reduction of the intrinsic surface area and appearance of pore-blocking in ZIF-8 crystal caused by conventional composite technologies. Simultaneously, the incorporation of ZIFs into meso/macro- porous graphene materials can produce the hybrid materials with hierarchically porous structures.33 In view of dynamic gas sorption process, hierarchical pore structure can avoid hysteresis behavior and low kinetic efficiency resulting from small-sized micropores of ZIF-8. In this regard, it is highly desirable to investigate the gas adsorption behavior of ZIF-8/3D graphene hybrid materials to confirm the synergistic effect between different sized pores. Herein, we reported a facile approach to prepare graphene/ZIF-8 hybrid aerogels (GZAn) by combining a two-step reduction strategy with a layer-by-layer assembly method. In this preparation method, a universal third-party template and an energy-consuming freeze-drying technology are refused. The substitute is the use of graphene hydrogel as template and ambient pressure drying method. The amount of oxygen functionalities of graphene hydrogel could be adjusted by changing the reduction time of GO. The loading of ZIF-8 crystals strongly depended on the amount of oxygen functional group of the reduced GO in graphene hydrogel. The asprepared GZAn material owned a hierarchical micro-/mesoporous structure and it performed enhanced CO2 uptake capacity than single component and tunable mechanical robustness. EXPERIMENTAL SECTION Synthesis of pure ZIF-8 crystals. Typical ZIF-8 with rhombic dodecahedron morphology was prepared in methanol solution at the room temperature. 250 mL methanol solution containing 3.66 g Zn(NO3)26H2O was slowly added into a 250 mL methanol solution containing 8.11 g 2-methylimidazole and stirred for 24h. A white suspension was formed and subsequently treated by centrifugation, washing with methanol for three times and dried under ambient conditions. Finally, a white powder was obtained.


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Preparation of graphene oxide (GO). GO was synthesized from natural graphite (325 mesh) by a modified Hummers method. Briefly, 2 g graphite powder, 6 g KMnO4 and 120 mL concentrated H2SO4 solution were stored in a refrigerator (-18 °C) for 30 min and taken out to mix together with stirring for 2 h in an ice bath. Next, the mixture was heated in an oil bath (50 °C) and stirred for 6 hours. The result mixture was slowly poured into 240 mL of distilled water. 30 % H2O2 solution was dropwise added into the above mixture solution until no bubble appeared. Finally, the crude product was washed repeatedly with deionized water by centrifuge until the filtrate became closely neutral. The obtained brown-yellow graphene oxide was saved for use. Preparation of graphene hydrogel (GH). L-ascorbic acid was added into 4 mg L-1 GO aqueous dispersion and the mixture was stirred at 95 °C for different times (25 min, 35min, 50min and 110min, respectively) to form a partially reduced graphene hydrogel (GH). Then GH was completely frozen in an ordinary refrigerator (-18°C) followed by thawing at room temperature. Another 10 min reduction period at 95 °C was carried out to produce different GH sample (GH35, GH45, GH60 and GH120, respectively). GH was solvent-exchanged with methanol three times for the further assembly of ZIF-8. Fabrication of graphene/ZIF-8 aerogel (GZAn). GZAn was synthesized by layer-by-layer (LbL) assembly method. First, GH (after solvent-exchange with methanol) was soaked in 5 mL methanol solution containing Zn(NO3)26H2O (73.2 mg) for 2 h. Second, the GH was immersed in 5 mL methanol solution containing 2-methylimidazole (162.2 mg) for 2 h followed by washing with methanol to remove the excess 2-methylimidazole. ZIF-8 crystals were in-situ formed in graphene wet gel. Different LbL assembly cycles could be adopted to produce ZIF-8


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loaded graphene wet gel samples, which were further dried in a 60 °C oven for 24 hours to prepare GZAn materials (n represents the number of immersion cycle). Characterization. Elemental analysis were carried out with a German Elementary Vario EL cube instrument. X-ray diffraction (XRD) measurements were carried out on a Rigaku Smart Lab diffractometer equipped with Cu-Kα radiation (λ=0.154 nm). Scanning electron microscopy (SEM) images were obtained using a field emission SEM (JSM-7500F) at an accelerating voltage of 10 kV. The N2 ( at 77 K) and CO2 (at 298 K) sorption experiments were performed on a static volumetric sorption analyzer (ASAP2020, Micrometrics, and USA). The studied experimental pressure ranged from 0.02 to 1 atm. The compressive tests were performed on a dynamic mechanical analysis (DMA) (TA, Q800) at room temperature. The cylindrical GZAn samples (10 mm thickness and 11 mm diameter) were compressed to 50 % of the pristine thickness with a strain rate of 10 % per minute and then recovered with a rate of 5 % per minute. The maximum stress was gained when the sample was compressed to 50%. Young’s modulus was calculated by the ratio of the stress and the strain when the strain is not over 5%. XPS measurements were recorded on Thermo ESCALAB 250 spectroscopy with a monochromic Xray source (Al-Ka, 1486.6 eV). Fourier transform infrared (FT-IR) measurements were performed on a Nicolet 6700 FTIR spectrometer in the range of 400–4000 cm-1. RESULTS AND DISCUSSION The preparation procedure for graphene hybrid aerogels is illustrated in Scheme 1. Firstly, GO dispersion was transformed into graphene hydrogel (GH) at 95 °C with the addition of Lascorbic acid as reducing agent by two-step reduction strategy. The two-step reduction strategy involved a partial reduction, a frozen/thawed process, and a further reduction, which was proved to be a crucial technique for the formation of large cellular structure in our previous results.25 In


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order to investigate the influence of oxygen functional group of graphene on the assembly of ZIF-8, different reduction times were used to provide several GH samples. Secondly, GH was alternately immersed in Zn(NO3)2 and 2-methylimidazole methanol solution for different cycles (n=2, 4, 6, 8 and 10 cycles for GZAn) and ZIF-8 nanocrystals could in-situ grow on both the internal surface and external surface of GA materials. The whole procedure is facile, moreover, the property of the resulted GZAn can be tailored by controlling reduction time and cycle numbers of ZIF-8 growth.

Scheme 1. Schematic of the preparation of a graphene/ZIF-8 hybrid aerogel by a two-step reduction strategy and a layer-by-layer assembly method. GA is a well-known elastic material with meso- and macropores, while ZIFs crystal own a high modulus and microporous structure. As for the GA/ZIF-8 hybrid materials, their properties are closely interrelated with the composite ratios. It is necessary to learn the composite content of ZIF-8 in GZAn, which is neglected in many reports on ZIF-8 based composites. The parameter of density was adopted to calculate the amount of ZIF-8 in GZAn materials. Pure GA and ZIF-8 nanocrystal have a density of ca. 3 mg cm-3 and 0.95 g cm-3,34 respectively. Based on the


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traditional method of mass divided by volume, the density of GZAn materials was determined. As shown in Figure 1a, it is clear that the density of GZAn increases with the growth cycle of ZIF-8 nonlinearly. The GH (all data below refer to GH35, except for special instructions) was floated on the top of methanol before the beginning of growth cycles, while it gradually sank onto the bottom of vessel after four immersion cycles (insert photos in Figure 1a). This phenomenon visually demonstrated that the increasing ZIF-8 nanocrystals were formed and adsorbed stably on the surface of reduced graphene oxide with continuous growth cycles. The calculated amounts of ZIF-8 in GZAn are 71.8 wt %, 73.5 wt %, 85.0 wt %, 86.7 wt %, and 88.6 wt % after 2, 4, 6, 8 and 10 cycles, respectively. The nucleation rate and growth of ZIF-8 nanocrystal were initially fast and gradually tended to be saturated adhesion on graphene surface in the eighth cycle, as shown in Figure 1a. The other method based on elemental analysis was also used to evaluate the amount of ZIF-8 in GZAn. No nitrogen species exist in pure GA materials so that all N atoms in GZAn origin from ZIF-8 crystals. According to the measured carbon-to-nitrogen ratio (C/N) of GZAn and the theoretical C/N of pure ZIF-8 nanocrystal, the calculated amount of ZIF-8 in GZAn are 69.2 wt %, 70.6 wt %, 76.1 wt %, 85.2 wt %, and 86.2 wt % after 2, 4, 6, 8 and 10 cycles, respectively. The results from elemental analysis were almost in accordance with density (Figure 1b).


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Figure 1. (a) Apparent density of GZAn and ZIF-8 content as a function of immersion cycle. Inset is photographs of as-prepared GH in methanol and GH with ZIF-8 loading (GZH) in methanol after six immersion cycles. (b) The C to N ratio of GZAn from elemental analysis and the corresponding calculated content of ZIF-8 in GZAn. Besides the immersion cycle, the reduction time of GO is the main factor to affect the loading of ZIF-8 on GZAn. According to our previous results, the C/O value of the reduce GO increased with the extension of reduction time.25 In other words, the amount of oxygen functional group gradually decreased. However, the formation and growth of ZIF-8 on graphene sheets directly depend on the electrostatic interaction and coordination interaction between Zn2+ and oxygencontaining groups. As shown in Figure 2, the loading of ZIF-8 in GZA8 is different when


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different GH sample acts as template to grow ZIF-8 crystal. It is obvious that the GH template with highly reduced degree is not favor for the formation and growth of ZIF-8 crystals.

Figure 2. The loading content of ZIF-8 in GZA8 as a function of GH template with different reduction time.

The morphology of GZAn sample was evaluated by SEM technology. The appearance of asprepared GA depended on the shape of experimental vessel. GZAn materials (Figure 3b) still kept the same shapes and volume with GA samples (Figure 3a, obtained from drying GH precursors) after the deposition of ZIF-8 crystals. From the cross-sectional SEM image of GA sample in Figure 3c, we can observe a honeycomb-like and oriented cellular structure. Such large cell dimensions can afford GA materials with excellent elasticity. The dimension of large cell in GZAn (Figure 3e and 3h) is similar to the data of pure GA material (Figure 3c). If GH120 (120min reduction) was used as template for the deposition of ZIF-8 crystals, the coverage of ZIF-8 grown on graphene surface was low and the size of crystal was small (Figure 3e). Long reduction time decreased the oxygen-containing groups, which did not benefit for the nucleation


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and growth of ZIF-8 crystal. When GH35 (35min reduction) was chosen as template to grow ZIF-8, the situation was different. After four immersion cycles, ZIF-8 crystals with heterogeneous sizes (Figure 3f) occupied the surface of GH35 with high coverage. After eight cycles, it was visible that the size of ZIF-8 nanocrystals became larger and uniform. Perfect ZIF8 nanocrystals with an average size of 200 nm could homogeneously nucleate and grow on the surface of graphene wall (Figure 3i and 3j). ZIF-8 nanocrystal had a clear rhombic dodecahedron structure, which was in accordance with its crystal space-filling model (inset in Figure 3j). The phenomenon just supports the result about the saturated adsorption of ZIF-8 from Figure 1a.

Figure 3. Photographs of (a) GA and (b) GZA8. SEM images of cross-section of (c) GA and (d) GZA8 using GH120 as template, the cell wall of (c-g) GZA4 and (h-j) GZA8 using GH35 as template. Inset in (g) is ZIF-8 crystal structure with a space-filling model.


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Figure 4. XRD pattern of (a) GA, ZIF-8 and GZA8, (b) outer and inner sheet of GZA8 sample. In order to further verify the formation of ZIF-8 crystal in GZAn, XRD pattern revealed the related information. As shown in Figure 4a, GZA8 sample shows the same crystallographic planes with pure ZIF-8 crystal, which is the strong evidence of ZIF-8 formation in GZAn. In addition, the uniform distribution of ZIF-8 in GZA8 materials was also checked by XRD. Both of the outer surface and inner surface of GZA8 aerogel materials display the crystallographic planes of ZIF-8 crystals (Figure 4b). This result illustrates that the homogeneous GZAn materials can be prepared by choosing GH as template. In order to reveal the formation and adhesion of ZIF-8 crystal on the surface of reduced graphene oxide, their interaction must be clearly investigated. Figure 5 displays the IR spectra of pure ZIF-8 crystal and GZA8 aerogel material. There is an obvious peak in 1735 cm-1 ascribed to C=O stretching vibration in the IR spectrum of GZA8 sample. This appearance of this peak also


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proved that the precursor GH was not highly reduced graphene oxide, while it surface still kept some oxygen-containing group. Hence, the zinc ions could adhere strongly to the GH surface by electrostatic interaction and further make use of coordination interaction with imidazole group as driving forces to form ZIF-8 crystal.

Figure 5. IR spectra of ZIF-8 and GZA8. XPS analysis is another convictive means to make clear the interaction of Zn ions and oxygen-containing groups. As shown in Figure 6a, GZA8 exhibits N1s and Zn2p spectrum while GA does not show such spectrum in XPS measurement. The peak centered at 1022.0 eV in the Zn2p3/2 spectrum (Figure 6b) corresponds to Zn-O bonds. A higher binding energy at 1022.8 eV may be attributed to the interaction of Zn and carboxyl group, while the peak at 1021.4 eV is assigned to the interaction of Zn and hydroxyl group. The O1s peaks of GA (the dried GH35 sample) in Figure 6c can be deconvoluted into three peaks corresponding to the low binding energy (LP), middle binding energy (MP), and high binding energy (HP) components centered at 531.2, 532.1, and 533.0eV. It reveals that the surface of GH template is covered by a number of oxygen atoms. The LP at 531.2 eV was attributed to –COO- ions, the MP centered at 532.1 eV is associated with –C-O-C, and the MP centered at 533.0 eV is typically attributed to −OH groups


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on the surface.35 When ZIF-8 nanocrystals grow on the surface of GH, the O1s peaks display a little difference in Figure 6d. The area ratio calculated for the three LP, MP, and HP peaks is obviously changed. This phenomenon also suggests the presence of Zn ions interacting with oxygen-containing groups. These above results from XPS demonstrates that GH with more oxygen-containing reactive sites is a good precursor for the formation and growth of ZIF-8. Hence, the homogeneous GZAn materials were successfully prepared by our method.

Figure 6. (a) XPS spectra of GA and GZA8. (b) Zn2p3/2 spectrum of GZA8. O1s spectrum of (c) GA and (d) GZA8. Here GA is the dried sample of GH35. The porosity of GA and GZAn samples was investigated through nitrogen adsorptiondesorption isotherms analysis. The Brunauer-Emmett-Teller (BET) surface areas and pore volumes of the samples are summarized in Table 1. As shown in Figure 7a, GA exhibited typeIII adsorption-desorption isotherms with negligible uptake at low relative pressures (P/P0

ZIF-8 Hybrid Aerogel: Preparation

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