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Hybridization with Metal Oxide 2D Nanosheet: An Effective Way to Optimize Functionality and Stability of Metal-Organic-Framework Yun Kyung Jo, Minho Kim, Xiaoyan Jin, In Young Kim, Joohyun Lim, Nam-Suk Lee, Young Kyu Hwang, Jong-San Chang, Hyungjun Kim, and Seong-Ju Hwang Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.6b03788 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 20, 2016
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Chemistry of Materials
Hybridization with Metal Oxide 2D Nanosheet: An Effective Way to Optimize Functionality and Stability of Metal-Organic-Framework Yun Kyung Jo†, Minho Kim‡, Xiaoyan Jin†, In Young Kim†, Joohyun Lim†, Nam-Suk Lee∥, Young Kyu Hwang§, Jong-San Chang§, Hyungjun Kim‡*, and Seong-Ju Hwang†* †Department of Chemistry and Nanoscience, Ewha Womans University, Seoul 03760, Korea ‡Graduate School of EEWS, Korea Advanced Institute of Science and Technology (KAIST) Daejeon 34141, Korea ∥National Institute for Nanomaterials Technology (NINT), Pohang University of Science and Technology (POSTECH), Pohang 37673, Korea §Research Center for Nanocatalysts, Korea Research Institute of Chemical Technology (KRICT), Daejeon 34114, Korea KEYWORDS metal-organic-framework, nanosheet, stability, gas adsorptivity, hybridization
ABSTRACT: An effective way to improve the functionalities and stabilities of metal-organic-framework (MOF) is developed by employing exfoliated metal oxide 2D nanosheet as an immobilization matrix. A crystal growth of ZIF-8 nanocrystals on the surface of layered titanate nanosheet yields intimately-coupled nanohybrids of zeolitic imidazolate frameworks-8 (ZIF-8)−layered titanate. The resulting nanohybrids show much greater surface areas and larger pore volumes than do the pristine ZIF-8, leading to the remarkable improvement of the CO2 adsorption ability of MOF upon hybridization. Of prime importance is that the thermal and hydro-stabilities of ZIF-8 are significantly enhanced by a strong chemical interaction with the robust titanate nanosheet. A strong interfacial interaction between ZIF-8 and layered titanate is verified by molecular mechanics simulation and spectroscopies. The universal applicability of the present strategy for other couple of MOF and metal oxide nanosheet is confirmed by the stabilization of Ti-MOF-NH2 via the immobilization on exfoliated V2O5 nanosheet. The present study underscores that the hybridization with metal oxide 2D nanosheet provides an efficient and universal synthetic route to novel MOF-based hybrid material with enhanced gas adsorptivity and stability.
1. INTRODUCTION Metal-organic-framework (MOF) is an inorganic−organic hybrid compound consisting of metal ions or clusters coordinated by organic linkers.1−5 The MOF compounds attract intense research interest because of their useful functionalities as gas storage, gas separation, catalysis, energy storage, and chemical sensors.1−5 However, the MOF suffers from several drawbacks including poor thermal and hydro-stability, and the limited controllability of pore size, which frustrate its commercial use. The hybridization with robust nanostructured materials might provide an efficient way to circumvent such drawbacks of MOF. In one instance, the homogeneous dispersion of MOF crystal on the surface of chemically-inert nanostructure would be effective in optimizing its pore structure and functionalities and also in improving its poor stability. To dates, several attempts have been made to hybrid MOF with nanostructured materials like carbon
nanotube and graphene.6−10 As inorganic analogues to graphene, exfoliated 2D nanosheets of layered metal oxides evoke intense research interest because of their unique characteristics such as subnanometer-level thickness, wide surface area, and diverse physicochemical properties.11−13 The exfoliated metal oxide 2D nanosheet can act as an effective substrate and also as a structuredirecting agent for the growth and immobilization of MOF nanocrystals, since the coordinative binding of MOF can occur through surface-exposed oxygen species. The protective hybridization of MOF by robust metal oxide nanosheet would significantly improve the stability of MOF. Additionally the stacking of MOF crystals with metal oxide nanosheets leads to the creation of mesopores via the house-of-cards-type stacking of nanosheets,14−16 yielding hierarchical micro-/meso-porous structure with enhanced gas adsorptivity. Taking into account the diverse functionalities of metal oxide nanosheet as catalyst, electrode, and sensor,17−20 the syn-
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ergistic hybridization with metal oxide nanosheet provides useful opportunity to explore novel multifunctional nanohybrids. Yet, at the time of the submission of this study, we are unaware of other report about the application of exfoliated metal oxide nanosheet as an immobilization substrate for MOF crystals to improve their functionalities and stabilities. Here we report the synthesis of novel nanohybrids of MOF−layered metal oxide nanosheet with improved stability and CO2 adsorptivity. The zeolitic imidazolate framework-8 (ZIF-8) nanocrystals are directly grown on the surface of exfoliated titanate nanosheets, see Figure 1A. The evolutions of the crystal structure, morphology, pore structure, and stability of ZIF-8 upon hybridization with layered titanate nanosheets are systematically investigated together with the applicability of the obtained nanohybrids as CO2 adsorbents. To probe the effect of chemical compositions on the physicochemical properties and functionalities of the resulting nanohybrids, several compositions are applied for the synthesis of the ZIF8−layered titanate (ZT) nanohybrids; the resulting materials with the titanate content of 2, 5, 10, 20, and 30 wt% are denoted as ZT-1, ZT-2, ZT-3, ZT-4, and ZT-5, respectively. The universal applicability of the present synthetic strategy for other MOF compound is verified from the enhancement of the stability of Ti-MOF-NH2 via a selfassembly with exfoliated V2O5 nanosheet.
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methyl formamide (DMF), and methanol at 150 °C for 16 h.24 The hybridization between V2O5 nanosheet and TiMOF-NH2 nanocrystal was achieved by reaction between the suspension of V2O5 nanosheet in methanol/DMF mixture and Ti-MOF-NH2 nanocrystal in hydrothermal reactor at 150 °C for 5 h. The intimate hybridization between MOF and metal oxide nanosheet were confirmed by various optical and spectroscopic tools. Interfacial structures between ZIF-8 and lepidocrocite type titanate layer were calculated with MM simulations using Cerius2 software. The evolutions of the porosity, gas adsorptivity, and stability of MOF nanocrystal upon the immobilization on the layered metal oxide nanosheet were systematically investigated. Experimental and computational details for characterization of the present materials are provided in Supporting Information.
3. RESULTS AND DISCUSSION
2. EXPERIMENTAL SECTION The pristine layered cesium titanate with lepidocrocitetype structure was prepared by conventional solid state reaction and its protonated derivative was obtained by the following acid treatment of the pristine material.21 The exfoliated nanosheet of layered titanate was obtained by the reaction of protonated titanate with tetrabutylammonium (TBA) ions, leading to the formation of stable colloidal suspension of titanate nanosheet.21,22 The ZT nanohybrids were synthesized by adding the colloidal suspension of exfoliated titanate nanosheet into the methanolic solution of zinc nitrate hexahydrate (Zn(NO3)2⋅6H2O) and 2-methylimidazole under stirring. To probe the effect of chemical composition on the physicochemical properties of the resulting nanohybrids, several weight percent of titanate suspensions (2, 5, 10, 20, and 30 wt%) were adopted for the synthesis of the ZT nanohybrids. To verify the universal role of metal oxide nanosheet as an efficient substrate for MOF, the hybridization between Ti-MOF-NH2 (MIL-125(Ti)-NH2) nanocrystal and V2O5 nanosheet was also carried out via a simple solvothermal treatment. The precursor of exfoliated V2O5 nanosheet was prepared by the previously-reported hydrothermal method,23 first, V2O5 powder was dispersed into a mixture of H2O2 and deionized water, and then the mixture was sonicated for 2−3 min. The obtained suspension was reacted in hydrothermal vessel at 150 °C for 10 h, leading to the formation of the exfoliated V2O5 nanosheets. As another precursor, Ti-MOF-NH2 was synthesized by the hydrothermal reaction of the mixture of tetrapropyl orthotitanate, 2-aminoterephthalic acid, di-
Figure 1. (A) Schematic diagram for synthesis route to ZT nanohybrids. (B) Powder XRD, (C) Ti K-edge XANES, and (D) FT-IR data of ZT nanohybrids and their references.
The crystal structures of the obtained ZT nanohybrids are examined with powder X-ray diffraction (XRD) analysis. As presented in Figure 1B, all the ZT nanohybrids exhibit well-developed Bragg reflections of ZIF-8 phase, indicating the efficient growth of well-crystalline MOF material. On the basis of Scherrer equation,25 the particle size of ZIF-8 nanocrystal is estimated as 23.4, 24.1, 22.9, 21.7, and 21.3 nm for ZT-1, ZT-2, ZT-3, ZT-4, and ZT-5, respectively, which are smaller than that of ZIF-8 nanocrystal (28.7 nm), underscoring the limited growth of MOF crystal on the surface of nanosheet. Conversely no XRD peaks of layered titanate phase are discernible for the ZT nanohybrids. Since the (0k0) reflections of titanate nanosheet become observable only for case of wellinterstratified stacking,13,26 no observation of the (0k0) reflections strongly suggests the homogeneous dispersion of titanate nanosheets without any phase segregation. Although no titanate-related reflections are discernible in the present XRD patterns of the ZT nanohybrids, the presence of layered titanate nanosheet in these materials is obviously evidenced by Ti K-edge X-ray absorption near-edge structure (XANES) spectroscopy. As depicted in Figure 1C, all the present ZT nanohybrids show similar spectral features for the pre-edge peaks P1, P2, and P3 corresponding to the dipole-forbidden 1s → 3d transitions, which are typical of the lepidocrocite-type layered
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Chemistry of Materials
Figure 2. FE-SEM images, EDS−elemental maps, and HR-TEM images of ZT nanohybrids and EF-TEM images of the pristine ZIF-8 nanocrystal and ZT-4. titanate phase.27,28 Such a spectral similarity between the ZT nanohybrids and lepidocrocite-type layered titanate is also distinct for the main-edge spectral features corresponding to the dipole-allowed 1s → 4p transitions.27,28 The present XANES results clearly demonstrate the incorporation of layered titanate nanosheet into the ZT nanohybrids without significant structural modification. The hybridization between titanate nanosheet and MOF is further confirmed by Fourier transformed-infrared (FTIR) spectroscopy, see Figure 1D. The distinct IR bands related to (Ti−O) stretching modes are discernible at 500 cm–1 for the ZT nanohybids,29 together with those of the imidazole unit of ZIF-8, confirming the co-existence of both the components.30 The crystal morphologies and hybrid structures of the present ZT nanohybrids are probed with field emissionscanning electron microscopy (FE-SEM) and high resolution-transmission microscopy (HR-TEM). As depicted in Figure 2, rhombic dodecahedral-shaped ZIF-8 nanocrystals with the average size of ~20−30 nm are immobilized on the surface of flat titanate nanosheet without agglomeration,25 indicating the effective dispersion of ZIF-8 nanocrystal on the titanate nanosheet with the maintenance of their original morphologies (Figure S1). The observation of clear lattice fringes corresponding to the (044) plane of cubic ZIF-8 in energy filtering-TEM (EFTEM) images provides additional evidence for intimate hybridization between titanate nanosheet and ZIF-8 nanocrystal. A homogeneous hybridization between ZIF8 and titanate is further evidenced by energy dispersive spectrometry (EDS)−elemental maps showing a uniform distribution of Ti and Zn elements in the entire region of the nanohybrids. The chemical compositions of the present ZT nanohybrids are determined by inductively coupled plasma (ICP)-atomic emission spectrometry. As listed in Table S1, there occurs a gradual increase of titanate content in the ZT nanohybrids with increasing the concentration of titanate nanosheet in the precursors, underscoring the
controllability of the chemical compositions of the ZT nanohybrids. The chemical interaction between layered titanate nanosheet and ZIF-8 nanocrystal is investigated with micro-Raman and X-ray photoelectron spectroscopy (XPS). As presented in Figure 3A, the micro-Raman spectra of the ZT nanohybrids clearly demonstrate marked blueshifts of the phonon lines of (Ti−O) bonds upon the hybridization with ZIF-8, underscoring significant reinforcement of these bonds caused by chemical interaction between layered titanate and ZIF-8.31 In the high wavenumber region, typical Raman peaks of ZIF-8 phase commonly appear for all the ZT nanohybrids, confirming the existence of ZIF-8 species.
Figure 3. (A) Micro-Raman and (B) Ti 2p, (C) Zn 2p, and (D) N 1s XPS spectra of ZT nanohybrids and their references.
The significant charge transfer between ZIF-8 and titanate is further evidenced by XPS results in Figures 3B−3D. While the ZIF-8 exhibits marked red-shifts of N 1s and Zn 2p XPS peaks upon the hybridization with titanate nanosheet, the Ti 2p XPS peaks of ZT-4 show higher binding energies than those of the precursor titanate nanosheet, which is in good agreement with the blue shift of the IR bands corresponding to (Ti−O) vibrations. This observation can be interpreted as strong evidence for significant charge transfer from layered titanate nanosheet to ZIF-8. Atomistic details on the interfacial structure of the ZT nanohybrids are examined with molecular mechanics (MM) simulations. As illustrated in Figure 4, three different facets of sodalite-like ZIF-8 framework are simulated;
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(100) facet exposing 4-membered Zn ring (4MR), (111) facet exposing 6-membered Zn ring (6MR), and (100) facet exposing 8-membered Zn ring (8MR). It is of noted that low index facets, where the surface Zn atom loses only one coordination (out of four) with an imidazole linker, are considered. Adhesion energy (Ead) between each facet of ZIF-8 and basal plane of 2D layered titanate nanosheet is then calculated. Simulation details are provided in Supporting Information. The strongest adhesion is found from the interface between 4MR at ZIF-8 (100) facet and titanate surface (Ead = −0.52 kcal/mol/Å2), which is 3~4 times larger than both 6-membered Zn ring (6MR) at (111) surface (−0.17 kcal/mol/Å2) and 8-membered Zn ring (8MR) at (100) surface (−0.12 kcal/mol/Å2).
Figure 4. Molecular mechanics simulation for the interfacial interactions in the ZT nanohybrid. H atoms are not displayed for simplicity. (A) Schematic representation of three different facets: 4-membered ring (4MR) and 8-membered ring (8MR) at the (100) surface and 6-membered ring (6MR) at the (111) surface of ZIF-8 structure. (B)−(D) The local geometry and the calculated adhesion energy (Ead) between each facet of ZIF-8 and a lepidocrocite-type titanate single layer (Zn: blue, N: light blue, C: grey, O: red for anchoring sites and light red in otherwise). The numbers are the distances (in Å) between a Zn atom and an O atom (black dashed lines). The distances within 3.65 Å are highlighted in yellow; otherwise, highlighted in cyan.
In Figures 4B-4D, interfacial interaction is manipulated by negatively-charged surface O atoms of titanate and positively-charged undercoordinated surface Zn atoms of ZIF. Due to the geometrical atomic arrangement and lattice mismatches, however, all surface Zn atoms cannot develop close interaction with O atoms of titanate. Our simulation reveals that 50% Zn surface atoms of (100) 4MR ZIF facet develop strong coordinating interaction with O atoms of titanate within van der Waals contact distance of 3.65 Å,32 which is a much higher ratio than the other two cases; 33% and 37.5% of surface Zn atoms of (111) 6MR facet and (111) 6MR facet respectively develop close interaction with titanate surface. This supports the calculated strong adhesion between the ZIF-8 (100) 4MR facet and titanate surface. In accordance with the microRaman and XPS results (Figure 3), the theoretical simulation presented here highlights a significant chemical interaction between titanate nanosheet and ZIF-8 nanocrystal, which is beneficial in enhancing the stability of hybridized MOF species.
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The gas adsorption properties of the ZT nanohybrids are investigated for N2 and CO2. As shown in Figure 5A, all the ZT nanohybrids show strong N2 adsorption in very low pressure region with a distinct hysteresis in high pressure region, indicating the co-existence of micropores and meso/macropores.33 The hysteresis of ZIF-8 nanocrystal becomes more prominent upon the hybridization with titanate nanosheet, clearly demonstrating the enhancement of mesoporosity originating from the stacking structure of titanate nanosheet. On the basis of Langmuir equation (Table S2), the surface areas of the ZT-1, ZT-2, ZT-3, ZT-4, and ZT-5 nanohybrids are estimated as 1995, 1915, 1829, 1803, and 1800 m2g−1, respectively, which are greater than that of ZIF-8 (1797 m2g−1). This is mainly attributable to the homogeneous dispersion of MOF nanocrystals on the surface of titanate nanosheet without significant aggregation. This interpretation is supported by the smallest ratio of the Vmeso+macro/Vtotal for ZT-1 with the largest surface area. Beyond the composition of ZT-1, a further increase of titanate content gives rise to the depression of surface area and micropore volume, which is ascribed to the decrease of ZIF-8 content. This is supported by t-plot analysis showing the decrease of the micropore volume of ZT with increasing titanate content (Table S3). The increase of the Vmeso+macro/Vtotal ratio with the increase of titanate content provides clear evidence for the formation of mesoporous stacking structure of nanosheets. The present findings clearly demonstrate that the hybridization with titanate nanosheet provides an effective tool to tailor the hierarchical pore structure of MOF nanocrystal with the increase of surface area.
Figure 5. (A) N2 adsorption−desorption isotherms and (B) CO2 adsorption isotherms of ZT nanohybrids. (C) Schematic model of the formation of chemical bonds between CO2 and titanate nanosheet.
As plotted in Figure 5B, all the ZT nanohybrids exhibit excellent CO2 storage capabilities, which are much superior to that of the ZIF-8 nanocrystal. Despite the accompanying decrease of surface area and micropore volume, a gradual increase of titanate content results in the enhancement of CO2 adsorptivity. This finding can be explained by the formation of more open pore structure via the stacking of titanate nanosheets, which enhances the
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Chemistry of Materials
accessibility of CO2 molecules to the micropores of MOF crystals. The reactivity of surface hydroxyl group of layered titanate nanosheet for CO2 would make additional contribution to the observed improvement of CO2 adsorptivity, see Figure 5C.34,35 Like hydroxyl group, the coordinatively-unsaturated metal ions exposed on the surface of layered titanate can also act as efficient adsorption sites for CO2, which is partly responsible for the enhanced CO2 adsorptivity of the ZT nanohybrids.36,37 However, beyond the optimal content of ZT-4, a further increase of titanate content gives rise to the decrease of the specific surface area, micropore volume, and CO2 adsorptivity, since excess titanate nanosheet blocks micropores of MOF and severely lowers the content of microporous MOF.
Figure 6. (A) TGA/DTG curves of ZT nanohybrids. (B) Schematic model for evolution of the thermal stability of MOF upon hybridization.
The evolution of the thermal stability of MOF upon hybridization is examined with thermogravimetric analysis (TGA) and differential thermogravimetry (DTG). As plotted in Figure 6A, all the present materials show weak mass loss upto 100 °C corresponding to the evaporation of trapped solvent. In case of unhybridized ZIF-8 nanocrystal, there occurs a gradual mass loss from 410 °C due to the degradation of the organic moiety. All the ZT nanohybrids show the same mass loss at much higher temperature of >545 °C, underscoring remarkable enhancement of the thermal stability of MOF upon hybridization. As depicted in Figure 6B, the additional coordination of Zn2+ ion by surface-exposed oxygen of robust titanate nanosheet is mainly responsible for the improved thermal stability of ZT nanohybrids, as verified by theoretical MM simulation (Figure 4). The partial replacement from imodazolate organic moiety to oxygen ion with imidazolate organic ligands coordinated Zn2+ ion can make the intimate chemical interaction between ZIF-8 and surface of layered titanate. The effect of hybridization on the hydrostabilty of ZIF-8 is also studied by monitoring structural change of MOF after exposure in controlled humidity (RH = 80%) for 48 h (see Figure S2). While the pristine ZIF-8 shows a significant depression of XRD peak by 35%, a much weaker depression of
