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Supported by a National Research Council Senior Research Associateship (G.J.C.), the FP7 People program under the project Marie Curie IOF-275148 (R.V.M.), National Security Science and Engineering Faculty Fellowship N00014-15-1-0030 (J.L.); NSF grants CMMI-0547636, CMMI-1436305, CMMI-1120577, and CNS-1126688; NIH grant 1R01RR026273-01; Defense Threat Reduction Agency grant HDTRA1-10-1-0106; and Office of Naval Research grant N00014-11-1-0678. We thank S. Suslov for assistance with TEM and X. Xu for help with NSOM. SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/346/6215/1352/suppl/DC1 Materials and Methods Supplementary Text Figs. S1 to S16 References (31–55) 18 August 2014; accepted 14 November 2014 10.1126/science.1260139
MEMBRANES
Metal-organic framework nanosheets as building blocks for molecular sieving membranes Yuan Peng,1,2 Yanshuo Li,1* Yujie Ban,1,2 Hua Jin,1,2 Wenmei Jiao,1,2 Xinlei Liu,1 Weishen Yang1* Layered metal-organic frameworks would be a diverse source of crystalline sheets with nanometer thickness for molecular sieving if they could be exfoliated, but there is a challenge in retaining the morphological and structural integrity. We report the preparation of 1-nanometer-thick sheets with large lateral area and high crystallinity from layered MOFs. They are used as building blocks for ultrathin molecular sieve membranes, which achieve hydrogen gas (H2) permeance of up to several thousand gas permeation units (GPUs) with H2/CO2 selectivity greater than 200. We found an unusual proportional relationship between H2 permeance and H2 selectivity for the membranes, and achieved a simultaneous increase in both permeance and selectivity by suppressing lamellar stacking of the nanosheets.
G
as separation with membranes is an energyefficient and environmentally friendly alternative to cryogenic and adsorptive or absorptive gas separation processes. Polymer membranes are subject to a trade-off between productivity (permeability) and efficiency (selectivity), known as Robeson’s upper bound (1, 2). Membranes based on molecular sieve materials are expected to overcome this limitation by relying on their ability to distinguish molecules based on size and shape (3–8). Molecular sieve nanosheets (MSNs) with large lateral area and small thickness are the most appropriate building 1
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China. 2University of Chinese Academy of Sciences, Beijing 100049, China.
*Corresponding author. E-mail: [email protected] (Y.L.); [email protected] (W.Y.)
blocks for ultrathin, and thus ultrapermeable, molecular sieve membranes (9). The permeance of such membranes is measured in gas permeation units (1 GPU = 10−6 cm3 cm–2 s–1 cmHg–1 at STP). Tsapatsis et al. demonstrated the fabrication of molecular sieve membranes based on exfoliated zeolite nanosheets with thickness at the unit cell level (~3 nm) (10), whereas the types of zeolites that can be easily exfoliated are rather limited (11). Recently, graphene oxide (GO) nanosheets with selective defects were used to produce ultrathin membranes with thickness of as low as 1.8 nm (12). However, the measured H2 permeance of these extremely thin GO membranes, ~300 GPUs, was still at the same level as conventional microporous membranes (13). This can be attributed to the low density of selective defects and their random distribution in the GO nanosheets. sciencemag.org SCIENCE
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intensity is larger than 0.6 GW/cm2, graphene will be cut into nanogaps (Fig. 4H). The Raman spectrum shows that D peak (related to defect level) is high, while G peak is not shifted relative to that in the unstrained graphene, since strain is released after cut. When the laser intensity is less than 0.5 GW/cm2, the graphene is continuous and conformal on nanoshaped Cu. This is indicated in the Raman spectrum (Fig. 4J) where the D peak is low and the G peak is shifted to the left because of straining. In order to study the SERS of nanopyramids, graphene is transferred onto the Au nanopyramids array (Fig. 4K) to measure the localized plasmonenhanced Raman spectra and verify the enhancement of the Raman signatures of graphene. As shown in Fig. 4M, the Raman signature of a monolayer of graphene (both G and 2D peak) is highly increased when it is placed on top of the nanopyramids, demonstrating the capability of local field enhancement of these structures. The ratio of 2D/G intensities has decreased from around 3 to 1, which could be a result from charge doping of graphene from the imprinted plasmonic nanostructures under laser excitation. FDTD simulations confirmed the field enhancement around the tip at the resonant frequency (Fig. 4L and fig. S16, C and D). LSI is a high-throughput 3D nanoimprinting technique capable of producing nanoscale crystalline metallic nanostructures over a 6-inch wafer within 30 s, using laser pulses (pulse energies of 150 to 250 mJ, a beam size of 3 mm, and pulse frequency of 10 Hz). The crystalline nanostructures fabricated by LSI have several characteristics that make them especially suited for electronic, plasmonic, or sensing applications: (i) precise and ultrasmooth nanoshaping due to better formability and low-temperature processing; (ii) complex 3D nanoshaping, sub–20-nm lateral sizes, and aspect ratios up to 5 when fabricated using silicon nanomolds; (iii) beneficial crystalline metallic nanostructure for device performance, because it has the potential to improve electronic stability and to reduce optical loses, noise, and energy consumption. Our results show that LSI has the potential to provide new insights into the role of crystallinity in the electronic, optical, and mechanical behavior of metallic nanostructures. Further, LSI is an attractive fabrication method for the development of future electronics, optics, and sensing devices.
In recent years, metal-organic frameworks (MOFs) have emerged as a family of nanoporous molecular sieves. A large number of layered MOFs have been reported (14), and they could serve as a diverse source of MSNs if they could be exfoliated to nanometer-scale thickness (15–17). However, structural deterioration and morphological damage have hampered success in obtaining highquality MOF nanosheets, which has hindered the application of the nanosheets as building blocks for molecular sieve membranes. We report the preparation of 1-nm-thick MSNs with large lateral area and high crystallinity from layered MOF precursors and demonstrate their use in fabricating ultrapermeable membranes that have excellent molecular sieving properties for H2/CO2 separation. H2/CO2 membrane separation is considered one of the key technologies for zero-emission fossil fuel power generation and hydrogen production (18). The layered MOF precursor exemplified here is poly[Zn2(benzimidazole)4], denoted Zn2(bim)4 (19). The corresponding scanning electron microscopy (SEM) images are shown in Fig. 1A. In this structure, two-dimensional (2D) layers are oriented normal to the c axis, connected by weak van der Waals interactions (Fig. 1A, inset). In the layers, each Zn ion is coordinated by four benzimidazole (bim) ligands in a distorted tetrahedral geometry, and each bim ligand bridges two Zn atoms via a bis-monodentate linkage (Fig. 1B). Powder x-ray diffraction (XRD) measurements confirm that our product is isostructural to a previously
determined MOF (Fig. 1C) [ref. no. 675375, Cambridge Crystallographic Data Centre (CCDC)] (20). Zn2(bim)4 can also be obtained in high yield via hydrothermal transformation of the wellknown three-dimensional zeolitic imidazolate framework ZIF-7 (19, 21) (fig. S1), which implies that Zn2(bim)4 possesses excellent thermodynamic stability (fig. S2). Conventional physical exfoliation may damage the in-plane structure of MOF nanosheets. Softchemical exfoliation is a potential alternative (22), but requires chemicals that may negatively affect application of the nanosheets. To address these problems, we developed a soft-physical process: Pristine Zn2(bim)4 crystals were first wet ball-milled at very low speed (60 rpm), followed by exfoliation in volatile solvent with the aid of ultrasonication. We found that a mixture of methanol and propanol is the most appropriate for this process (fig. S3), based on our analysis of a number of solvents. We hypothesize that wet ball-milling facilitates the penetration of small methanol molecules into the galleries of the layered Zn2(bim)4, and propanol helps to stabilize the exfoliated nanosheets by adsorbing on the sheets with its hydrophobic alkane tails. The colloid suspension of Zn2(bim)4 MSNs is clear but exhibits a Tyndall effect due to light scattering by the nanosheets in the colloid (Fig. 1D, inset). Figure 1E illustrates the network structure of a single-layered Zn2(bim)4 MSN. The square Zn4(bim)4 unit, as shown in the red circle, can be considered to be the subunit of the 2D layer.
The aperture size of the Zn4(bim)4 unit, a fourmembered ring, is ~0.21 nm as estimated from crystallographic data (Fig. 1F). The effective pore size should be slightly larger, considering the structural flexibility of the sheet. Thus, we can expect that membranes based on Zn2(bim)4 MSNs will achieve a high selectivity of H2 (0.29 nm) over CO2 (0.33 nm) through molecular sieving. Moreover, unlike zeolite nanosheets, which consist of cages (or half cages) or zigzag channels (10), the pores of Zn2(bim)4 MSNs are constructed with four flat bim molecules, ensuring the rapid transport of H2 molecules. A colloidal dispersion containing ~15 mg/liter Zn2(bim)4 MSNs was obtained by removing the larger unexfoliated particles after being left to stand for 2 weeks. The colloid remained stable at room temperature for several months. Zn2(bim)4 MSNs with wrinkles were frequently observed under transmission electron microscopy (TEM), indicating the flexibility of the nanosheets (fig. S4), which is beneficial for achieving a conformal layer on macroporous supports with rough surfaces (10). An isolated Zn2(bim)4 MSN with side length of ~600 nm is shown in Fig. 2A, where folds and curled edges are observed. The nanosheet is very thin, as determined from the contrast in the TEM image. As is common for MOF materials characterized using high-resolution TEM (HRTEM), the Zn2(bim)4 nanosheets visibly and rapidly degraded during imaging, as a result of electron beam irradiation. Thus, it is difficult to visualize the lattice fringes of the (hk0) planes
Fig. 1. Top-down fabrication of molecular sieve nanosheets. (A) Scanning electron microscopy (SEM) image of as-synthesized Zn2(bim)4 crystals. The inset image shows the typical flake-like morphology of Zn2(bim)4 crystals. (B) Architecture of the layered MOF precursor. The ab planes are highlighted in purple to better illustrate the layered structure. (C) Powder XRD patterns of Zn2 (bim)4. The top trace is the experimental pattern, whereas the bottom trace is the pattern simulated based on the single-crystal data (CCDC-675375). The asymmetric unit of Zn2 (bim)4 is also presented to illustrate the coordination environment of Zn atoms. (D) Transmission electron microscopy (TEM) image of Zn2(bim)4 MSNs. The inset shows the Tyndall effect of a colloidal suspension. (E) Illustration of the grid-like structure of the Zn2(bim)4 MSN. The Zn coordination polyhedra are depicted in blue, whereas the bim links are represented by sticks. H atoms are omitted for clarity. Symmetry code: A 2–x, y, 1.5–z. (F) Space-filling representation of a four-membered ring of the Zn2(bim)4 MSN. SCIENCE sciencemag.org
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of the single-layered nanosheet by HRTEM. The inset in Fig. 2A shows the cross-sectional HRTEM image of a five-layered Zn2(bim)4 nanosheet. The distance between the adjacent layers is ~1.18 nm, which is close to the d002 of the pristine crystals. The selected-area electron diffraction (SAED) pattern of a few-layered nanosheet collected along the c axis is shown in Fig. 2B, which corresponds to the diffraction pattern of the (hk0) planes of the nanosheet. The simulated SAED pattern is in good agreement with the experimental result, confirming that the exfoliated nanosheets are highly crystalline and retain the same in-plane crystal structure of pristine bulk materials. In Fig. 2C, a tapping-mode atomic force microscopy (AFM) image shows a Zn2(bim)4 MSN with large lateral dimension up to 1.5 mm. The height profile reveals that the nanosheet has a fairly flat, smooth terrace with a uniform thickness of 1.12 nm. Noting that 2D materials are often raised by a few angstroms above the supporting surface (23), the nanosheets in Fig. 2C should be monolayers of Zn2(bim)4. Additionally, it should be noted that Zn2(bim)4 MSNs with lateral size up to several microns were occasionally observed by TEM (fig. S4) and AFM (fig. S5). The exfoliation of layered Zn2(bim)4 precursors into nanosheets leads to a significant increase in the BrunauerEmmett-Teller (BET) surface area, from 19.9 to 112.4 m2/g (Fig. 2D). According to the classification by IUPAC (24), the isotherm of Zn2(bim)4 nanosheets can be identified as type II with an identifiable H4-type hysteresis loop, which is associated with the slit-like pores formed by aggregation of nanosheets during freeze drying. The Fourier transform infrared (FTIR) spectrum of Zn2(bim)4 nanosheets is identical to that of pristine Zn2(bim)4, indicating that the soft-physical exfoliation process has little effect on the in-plane structure of the Zn2(bim)4 nanosheets (fig. S6). Thermal analysis revealed that the Zn2(bim)4 nanosheets remain stable up to 200°C despite their nanometer-scale thickness (fig. S7). Filtration of nanosheet suspensions through porous supports has been successfully applied to fabricate separating membranes in previous studies (10, 12). This method, however, could not be applied in the current case. We attribute this to the fast restacking of nanosheets back to ordered pristine structures at elevated concentration when the solvent is filtered out (fig. S8). Ordered restacking of the MSNs will result in partial or total blockage of the molecular sieve pores. A hot-drop coating process addressed this problem, with the aim of achieving a disordered stacking of the nanosheets in the membrane layer. In a typical preparation, 3 ml of an MSN suspension was diluted five times with methanol and then deposited dropwise onto the surface of an a-Al2O3 disk (Inocermic GmbH, pore size 70 nm, Fig. 3A), which was heated at 120°C on a heating plate (19) (fig. S9). A uniform surface coverage of Zn2(bim)4 MSNs on a-Al2O3 support was achieved (fig. S10). Figure 3B presents a top-view SEM image of a Zn2(bim)4 MSN membrane at high magnification. Figure 3C shows the cross-sectional view of a membrane, where an unsupported por1358
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tion of the nanosheet layer folds around the corner of the fractured support, clearly illustrating the high flexibility of Zn2(bim)4 nanosheets. In Fig. 3, B and C, the texture of the underlying a-Al2O3 support is distinguishable, indicating a very thin layer of Zn2(bim)4 MSNs on the support. In addition, a substantial aluminum signal from the underlying a-Al2O3 support was detected by x-ray photoelectron spectroscopy (XPS) for a Zn2(bim)4 MSN membrane, indicating that the MSN layer is only several nanometers thick (fig. S11). The MSN membranes were sealed into WickeKallenbach cells for measuring the permeance and selectivity of both single and binary feed of H2 and CO2. There was no absolute pressure differential across the membrane to avoid deforming the thin MSN layers (19) (fig. S12). We first conducted a control experiment using the a-Al2O3 support. The support showed an H2 permeance of >50,000 GPUs with a low H2/CO2 selectivity (~1.2). Figure 3D shows the H2/CO2 selectivities measured on 15 membranes prepared via hot-drop coating at different support surface temperatures (80° to 200°C; table S1). In our investiga-
tion, 120°C optimizes the average selectivity and reproducibility. We hypothesize that this temperature is just high enough to maintain a solvent evaporation rate that avoids ordered restacking of the nanosheets during the coating process, which occurs at lower temperature (fig. S13). At higher temperature, pinholes or gaps might form between nanosheets when the solvent evaporates too quickly. The three membranes prepared at 120°C exhibited H2/CO2 selectivity of 261 T 39. Nine membranes were prepared at 120°C with different drop volumes (1 to 15 ml), which proved to be a less significant factor than the support surface temperature (fig. S14 and table S2). A membrane prepared at 120°C was tested for single gas permeation of H2 (0.29 nm), CO2 (0.33 nm), N2 (0.36 nm), CH4 (0.38 nm), and C2H6 (0.44 nm). There was a clear cutoff between H2 and CO2 (fig. S15). H2/CO2 separation measurements at different temperatures indicate activated diffusion for both H2 and CO2 (fig. S16). These results suggest a size exclusion mechanism for separation of H2 from CO2. H2/CO2 separation with different feed compositions indicates that
Fig. 2. Characterizations of molecular sieve nanosheets. (A) Low-magnification TEM image of a piece of Zn2(bim)4 nanosheets. The inset image shows a high-resolution TEM image of a five-layered Zn2(bim)4 nanosheet. (B) SAED pattern (white circle) shows the diffraction from (hk0) planes within a few-layered nanosheet. A simulated SAED pattern of Zn2(bim)4 nanosheet down the c axis is also shown. (C) Tappingmode AFM topographical image of Zn2(bim)4 nanosheets on silicon wafer. The height profile of the nanosheets along the black lines was marked in the image. (D) N2 adsorption-desorption isotherms (77 K) on pristine Zn2(bim)4 and Zn2(bim)4 nanosheets. The inset presents photographs comparing the pristine Zn2(bim)4 and Zn2(bim)4 nanosheets obtained after exfoliation. sciencemag.org SCIENCE
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CO2 adsorption has influence on the H2 permeation to some extent (fig. S17). Normally, researchers prepare thicker membranes to minimize nonselective defects. However, there is an inverse relationship between membrane permeance and selectivity. In contrast, we found an anomalous proportional relationship between permeance and selectivity for the Zn2(bim)4 MSN membranes, as shown in Fig. 3E. The H2 permeance varies from 760 to 3760 GPUs along with the variation of H2/CO2 selectivity from 53 to 291. The linear regression of these data accounts for 90.6% of the variance (table S1). Meanwhile, a small fluctuation in CO2 permeance (12.3 T 4.5 GPUs) was measured on all 15 membranes (table S1). Theoretically, only H2 molecules can pass through the four-membered rings of the MSNs. The non-
zero CO2 permeance could be attributed to the non–size-selective mass transport through imperfect sealing or through boundaries of the nanosheet layer. To gain insight into the structure-performance relationship of the MSN membranes, four membranes with different performance were characterized by powder XRD (Fig. 3F). The proportional relationship between permeance and selectivity can be observed in the table in Fig. 3F. Membranes with higher permeance are more selective, and vice versa. Membrane performance can be correlated with the order of nanosheet stacking, as indicated by XRD patterns. The low-angle hump suggests regions with expanded stacking of nanosheets. The appearance of (002) reflection coincides with the formation of bulk Zn2(bim)4
via ordered restacking. Lamellar ordering of nanosheets would block the permeation pathway for H2, but have only a slight effect on CO2 leakage, resulting in a reduction in membrane performance. Noting that a tiny hump is still identifiable for the high-performance membrane, we speculate that an ultrapermeable and superselective molecular sieve membrane could be obtained if the membrane consisted of fully disordered MSNs. The performance of our membranes exceeds the latest Robeson’s upper-bound for H2/CO2 gas pair and is higher than that of the molecular sieve membranes reported to date, including polycrystalline membranes composed of three dimensional MOFs (fig. S18). For practical use, thermal and hydrothermal stability is an important issue for H2 selective membranes (25). A Zn2(bim)4 MSN membrane was tested under different conditions for more than 400 hours in total, including two temperature cycles, showing no degradation in membrane performances. When exposed to an equimolar H2/CO2 feed containing ~4 mol % steam at 150°C, the membrane showed good stability after a 120-hour test (fig. S19). REFERENCES AND NOTES
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AC KNOWLED GME NTS
Fig. 3. Morphology and performance of membranes derived from molecular sieve nanosheets. (A) SEM image of a bare porous a-Al2O3 support. (B) SEM top view and (C) cross-sectional view of a Zn2(bim)4 nanosheet layer on a-Al2O3 support. (D) Scatterplot of H2/CO2 selectivities measured from 15 membranes. The red line with symbols shows the average selectivity and dispersion of selectivity of the membranes prepared at different coating temperatures. (E) Anomalous relationship between selectivity and permeance measured from 15 membranes. (F) Powder XRD patterns of four membranes with different separation properties.The cartoons at the left schematically illustrate the microstructural features of the nanosheet layers. The yellow and green portions correspond to the low-angle humps and the (002) peaks in the XRD patterns, respectively. All the membranes were measured for equimolar mixtures at room temperature and 1 atm. SCIENCE sciencemag.org
We thank the National Natural Science Foundation of China and the Key Research Program of the Chinese Academy of Sciences for funding, L. Liu for experimental assistance in wet ball milling, S. Miao for experimental assistance in TEM and SAED, and H. Duan for experimental assistance in TG analysis. Y.P., Y.L., and W.Y. are inventors on a Chinese patent filed through the Dalian Institute of Chemical Physics (CN102974229A, “Exfoliation and application of two-dimensional layered metal-organic frameworks”). SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/346/6215/1356/suppl/DC1 Materials and Methods Figs. S1 to S19 Tables S1 and S2 References (26–40) 1 April 2014; accepted 12 November 2014 10.1126/science.1254227
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Editor's Summary Metal-organic framework material membranes There continues to be a lot of interest in developing membranes for gas separations that go beyond the current polymer membranes used commercially for this purpose. Peng et al. took a porous metal-organic framework material with a layered structure and exfoliated it to give nanometer-thick molecular sieves. The membranes were exceptionally good at separating hydrogen gas from carbon dioxide both in terms of permeance and selectivity. Science, this issue p. 1356
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Metal-organic framework nanosheets as building blocks for molecular sieving membranes Yuan Peng, Yanshuo Li, Yujie Ban, Hua Jin, Wenmei Jiao, Xinlei Liu and Weishen Yang (December 11, 2014) Science 346 (6215), 1356-1359. [doi: 10.1126/science.1254227]
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12. W. Kurnia, M. Yoshino, J. Micromech. Microeng. 19, 125028 (2009). 13. S. Y. Chou, C. Keimel, J. Gu, Nature 417, 835–837 (2002). 14. K. A. Lister et al., J. Vac. Sci. Technol. B 22, 3257–3259 (2004). 15. B. Cui, C. Keimel, S. Y. Chou, Nanotechnology 21, 045303 (2010). 16. R. F. Oulton, Nat. Photonics 6, 219–221 (2012). 17. C. C. Liang et al., Opt. Express 19, 4768–4776 (2011). 18. N. Yu et al., Science 334, 333–337 (2011). 19. J. Lin et al., Science 340, 331–334 (2013). 20. W. Zhou, T. W. Odom, Nat. Nanotechnol. 6, 423–427 (2011). 21. P. Nagpal, N. C. Lindquist, S. H. Oh, D. J. Norris, Science 325, 594–597 (2009). 22. R. S. Mishra, T. R. Bieler, A. K. Mukherjee, Acta Metall. Mater. 43, 877–891 (1995). 23. T. G. Nieh, J. Wadsworth, T. Imai, Scr. Metall. 26, 703–708 (1992). 24. R. Fabbro, J. Fournier, P. Ballard, D. Devaux, J. Virmont, J. Appl. Phys. 68, 775–784 (1990). 25. A. Poddubny, I. Iorsh, P. Belov, Y. Kivshar, Nat. Photonics 7, 948–957 (2013). 26. S. I. Bozhevolnyi, V. S. Volkov, E. Devaux, T. W. Ebbesen, Phys. Rev. Lett. 95, 046802 (2005). 27. D. K. Gramotnev, S. I. Bozhevolnyi, Nat. Photonics 4, 83–91 (2010). 28. Q. Li, Y. Xu, Z. Lai, L. Shen, Y. Bai, J. Mater. Sci. Technol. 15, 435 (1999).
29. C. M. Watts, X. Liu, W. J. Padilla, Adv. Mater. 24, OP98–OP120 (2012). 30. J. Li, T. F. Chung, Y. P. Chen, G. J. Cheng, Nano Lett. 12, 4577–4583 (2012). AC KNOWLED GME NTS
Supported by a National Research Council Senior Research Associateship (G.J.C.), the FP7 People program under the project Marie Curie IOF-275148 (R.V.M.), National Security Science and Engineering Faculty Fellowship N00014-15-1-0030 (J.L.); NSF grants CMMI-0547636, CMMI-1436305, CMMI-1120577, and CNS-1126688; NIH grant 1R01RR026273-01; Defense Threat Reduction Agency grant HDTRA1-10-1-0106; and Office of Naval Research grant N00014-11-1-0678. We thank S. Suslov for assistance with TEM and X. Xu for help with NSOM. SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/346/6215/1352/suppl/DC1 Materials and Methods Supplementary Text Figs. S1 to S16 References (31–55) 18 August 2014; accepted 14 November 2014 10.1126/science.1260139
MEMBRANES
Metal-organic framework nanosheets as building blocks for molecular sieving membranes Yuan Peng,1,2 Yanshuo Li,1* Yujie Ban,1,2 Hua Jin,1,2 Wenmei Jiao,1,2 Xinlei Liu,1 Weishen Yang1* Layered metal-organic frameworks would be a diverse source of crystalline sheets with nanometer thickness for molecular sieving if they could be exfoliated, but there is a challenge in retaining the morphological and structural integrity. We report the preparation of 1-nanometer-thick sheets with large lateral area and high crystallinity from layered MOFs. They are used as building blocks for ultrathin molecular sieve membranes, which achieve hydrogen gas (H2) permeance of up to several thousand gas permeation units (GPUs) with H2/CO2 selectivity greater than 200. We found an unusual proportional relationship between H2 permeance and H2 selectivity for the membranes, and achieved a simultaneous increase in both permeance and selectivity by suppressing lamellar stacking of the nanosheets.
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as separation with membranes is an energyefficient and environmentally friendly alternative to cryogenic and adsorptive or absorptive gas separation processes. Polymer membranes are subject to a trade-off between productivity (permeability) and efficiency (selectivity), known as Robeson’s upper bound (1, 2). Membranes based on molecular sieve materials are expected to overcome this limitation by relying on their ability to distinguish molecules based on size and shape (3–8). Molecular sieve nanosheets (MSNs) with large lateral area and small thickness are the most appropriate building 1
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China. 2University of Chinese Academy of Sciences, Beijing 100049, China.
*Corresponding author. E-mail: [email protected] (Y.L.); [email protected] (W.Y.)
blocks for ultrathin, and thus ultrapermeable, molecular sieve membranes (9). The permeance of such membranes is measured in gas permeation units (1 GPU = 10−6 cm3 cm–2 s–1 cmHg–1 at STP). Tsapatsis et al. demonstrated the fabrication of molecular sieve membranes based on exfoliated zeolite nanosheets with thickness at the unit cell level (~3 nm) (10), whereas the types of zeolites that can be easily exfoliated are rather limited (11). Recently, graphene oxide (GO) nanosheets with selective defects were used to produce ultrathin membranes with thickness of as low as 1.8 nm (12). However, the measured H2 permeance of these extremely thin GO membranes, ~300 GPUs, was still at the same level as conventional microporous membranes (13). This can be attributed to the low density of selective defects and their random distribution in the GO nanosheets. sciencemag.org SCIENCE
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intensity is larger than 0.6 GW/cm2, graphene will be cut into nanogaps (Fig. 4H). The Raman spectrum shows that D peak (related to defect level) is high, while G peak is not shifted relative to that in the unstrained graphene, since strain is released after cut. When the laser intensity is less than 0.5 GW/cm2, the graphene is continuous and conformal on nanoshaped Cu. This is indicated in the Raman spectrum (Fig. 4J) where the D peak is low and the G peak is shifted to the left because of straining. In order to study the SERS of nanopyramids, graphene is transferred onto the Au nanopyramids array (Fig. 4K) to measure the localized plasmonenhanced Raman spectra and verify the enhancement of the Raman signatures of graphene. As shown in Fig. 4M, the Raman signature of a monolayer of graphene (both G and 2D peak) is highly increased when it is placed on top of the nanopyramids, demonstrating the capability of local field enhancement of these structures. The ratio of 2D/G intensities has decreased from around 3 to 1, which could be a result from charge doping of graphene from the imprinted plasmonic nanostructures under laser excitation. FDTD simulations confirmed the field enhancement around the tip at the resonant frequency (Fig. 4L and fig. S16, C and D). LSI is a high-throughput 3D nanoimprinting technique capable of producing nanoscale crystalline metallic nanostructures over a 6-inch wafer within 30 s, using laser pulses (pulse energies of 150 to 250 mJ, a beam size of 3 mm, and pulse frequency of 10 Hz). The crystalline nanostructures fabricated by LSI have several characteristics that make them especially suited for electronic, plasmonic, or sensing applications: (i) precise and ultrasmooth nanoshaping due to better formability and low-temperature processing; (ii) complex 3D nanoshaping, sub–20-nm lateral sizes, and aspect ratios up to 5 when fabricated using silicon nanomolds; (iii) beneficial crystalline metallic nanostructure for device performance, because it has the potential to improve electronic stability and to reduce optical loses, noise, and energy consumption. Our results show that LSI has the potential to provide new insights into the role of crystallinity in the electronic, optical, and mechanical behavior of metallic nanostructures. Further, LSI is an attractive fabrication method for the development of future electronics, optics, and sensing devices.
In recent years, metal-organic frameworks (MOFs) have emerged as a family of nanoporous molecular sieves. A large number of layered MOFs have been reported (14), and they could serve as a diverse source of MSNs if they could be exfoliated to nanometer-scale thickness (15–17). However, structural deterioration and morphological damage have hampered success in obtaining highquality MOF nanosheets, which has hindered the application of the nanosheets as building blocks for molecular sieve membranes. We report the preparation of 1-nm-thick MSNs with large lateral area and high crystallinity from layered MOF precursors and demonstrate their use in fabricating ultrapermeable membranes that have excellent molecular sieving properties for H2/CO2 separation. H2/CO2 membrane separation is considered one of the key technologies for zero-emission fossil fuel power generation and hydrogen production (18). The layered MOF precursor exemplified here is poly[Zn2(benzimidazole)4], denoted Zn2(bim)4 (19). The corresponding scanning electron microscopy (SEM) images are shown in Fig. 1A. In this structure, two-dimensional (2D) layers are oriented normal to the c axis, connected by weak van der Waals interactions (Fig. 1A, inset). In the layers, each Zn ion is coordinated by four benzimidazole (bim) ligands in a distorted tetrahedral geometry, and each bim ligand bridges two Zn atoms via a bis-monodentate linkage (Fig. 1B). Powder x-ray diffraction (XRD) measurements confirm that our product is isostructural to a previously
determined MOF (Fig. 1C) [ref. no. 675375, Cambridge Crystallographic Data Centre (CCDC)] (20). Zn2(bim)4 can also be obtained in high yield via hydrothermal transformation of the wellknown three-dimensional zeolitic imidazolate framework ZIF-7 (19, 21) (fig. S1), which implies that Zn2(bim)4 possesses excellent thermodynamic stability (fig. S2). Conventional physical exfoliation may damage the in-plane structure of MOF nanosheets. Softchemical exfoliation is a potential alternative (22), but requires chemicals that may negatively affect application of the nanosheets. To address these problems, we developed a soft-physical process: Pristine Zn2(bim)4 crystals were first wet ball-milled at very low speed (60 rpm), followed by exfoliation in volatile solvent with the aid of ultrasonication. We found that a mixture of methanol and propanol is the most appropriate for this process (fig. S3), based on our analysis of a number of solvents. We hypothesize that wet ball-milling facilitates the penetration of small methanol molecules into the galleries of the layered Zn2(bim)4, and propanol helps to stabilize the exfoliated nanosheets by adsorbing on the sheets with its hydrophobic alkane tails. The colloid suspension of Zn2(bim)4 MSNs is clear but exhibits a Tyndall effect due to light scattering by the nanosheets in the colloid (Fig. 1D, inset). Figure 1E illustrates the network structure of a single-layered Zn2(bim)4 MSN. The square Zn4(bim)4 unit, as shown in the red circle, can be considered to be the subunit of the 2D layer.
The aperture size of the Zn4(bim)4 unit, a fourmembered ring, is ~0.21 nm as estimated from crystallographic data (Fig. 1F). The effective pore size should be slightly larger, considering the structural flexibility of the sheet. Thus, we can expect that membranes based on Zn2(bim)4 MSNs will achieve a high selectivity of H2 (0.29 nm) over CO2 (0.33 nm) through molecular sieving. Moreover, unlike zeolite nanosheets, which consist of cages (or half cages) or zigzag channels (10), the pores of Zn2(bim)4 MSNs are constructed with four flat bim molecules, ensuring the rapid transport of H2 molecules. A colloidal dispersion containing ~15 mg/liter Zn2(bim)4 MSNs was obtained by removing the larger unexfoliated particles after being left to stand for 2 weeks. The colloid remained stable at room temperature for several months. Zn2(bim)4 MSNs with wrinkles were frequently observed under transmission electron microscopy (TEM), indicating the flexibility of the nanosheets (fig. S4), which is beneficial for achieving a conformal layer on macroporous supports with rough surfaces (10). An isolated Zn2(bim)4 MSN with side length of ~600 nm is shown in Fig. 2A, where folds and curled edges are observed. The nanosheet is very thin, as determined from the contrast in the TEM image. As is common for MOF materials characterized using high-resolution TEM (HRTEM), the Zn2(bim)4 nanosheets visibly and rapidly degraded during imaging, as a result of electron beam irradiation. Thus, it is difficult to visualize the lattice fringes of the (hk0) planes
Fig. 1. Top-down fabrication of molecular sieve nanosheets. (A) Scanning electron microscopy (SEM) image of as-synthesized Zn2(bim)4 crystals. The inset image shows the typical flake-like morphology of Zn2(bim)4 crystals. (B) Architecture of the layered MOF precursor. The ab planes are highlighted in purple to better illustrate the layered structure. (C) Powder XRD patterns of Zn2 (bim)4. The top trace is the experimental pattern, whereas the bottom trace is the pattern simulated based on the single-crystal data (CCDC-675375). The asymmetric unit of Zn2 (bim)4 is also presented to illustrate the coordination environment of Zn atoms. (D) Transmission electron microscopy (TEM) image of Zn2(bim)4 MSNs. The inset shows the Tyndall effect of a colloidal suspension. (E) Illustration of the grid-like structure of the Zn2(bim)4 MSN. The Zn coordination polyhedra are depicted in blue, whereas the bim links are represented by sticks. H atoms are omitted for clarity. Symmetry code: A 2–x, y, 1.5–z. (F) Space-filling representation of a four-membered ring of the Zn2(bim)4 MSN. SCIENCE sciencemag.org
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of the single-layered nanosheet by HRTEM. The inset in Fig. 2A shows the cross-sectional HRTEM image of a five-layered Zn2(bim)4 nanosheet. The distance between the adjacent layers is ~1.18 nm, which is close to the d002 of the pristine crystals. The selected-area electron diffraction (SAED) pattern of a few-layered nanosheet collected along the c axis is shown in Fig. 2B, which corresponds to the diffraction pattern of the (hk0) planes of the nanosheet. The simulated SAED pattern is in good agreement with the experimental result, confirming that the exfoliated nanosheets are highly crystalline and retain the same in-plane crystal structure of pristine bulk materials. In Fig. 2C, a tapping-mode atomic force microscopy (AFM) image shows a Zn2(bim)4 MSN with large lateral dimension up to 1.5 mm. The height profile reveals that the nanosheet has a fairly flat, smooth terrace with a uniform thickness of 1.12 nm. Noting that 2D materials are often raised by a few angstroms above the supporting surface (23), the nanosheets in Fig. 2C should be monolayers of Zn2(bim)4. Additionally, it should be noted that Zn2(bim)4 MSNs with lateral size up to several microns were occasionally observed by TEM (fig. S4) and AFM (fig. S5). The exfoliation of layered Zn2(bim)4 precursors into nanosheets leads to a significant increase in the BrunauerEmmett-Teller (BET) surface area, from 19.9 to 112.4 m2/g (Fig. 2D). According to the classification by IUPAC (24), the isotherm of Zn2(bim)4 nanosheets can be identified as type II with an identifiable H4-type hysteresis loop, which is associated with the slit-like pores formed by aggregation of nanosheets during freeze drying. The Fourier transform infrared (FTIR) spectrum of Zn2(bim)4 nanosheets is identical to that of pristine Zn2(bim)4, indicating that the soft-physical exfoliation process has little effect on the in-plane structure of the Zn2(bim)4 nanosheets (fig. S6). Thermal analysis revealed that the Zn2(bim)4 nanosheets remain stable up to 200°C despite their nanometer-scale thickness (fig. S7). Filtration of nanosheet suspensions through porous supports has been successfully applied to fabricate separating membranes in previous studies (10, 12). This method, however, could not be applied in the current case. We attribute this to the fast restacking of nanosheets back to ordered pristine structures at elevated concentration when the solvent is filtered out (fig. S8). Ordered restacking of the MSNs will result in partial or total blockage of the molecular sieve pores. A hot-drop coating process addressed this problem, with the aim of achieving a disordered stacking of the nanosheets in the membrane layer. In a typical preparation, 3 ml of an MSN suspension was diluted five times with methanol and then deposited dropwise onto the surface of an a-Al2O3 disk (Inocermic GmbH, pore size 70 nm, Fig. 3A), which was heated at 120°C on a heating plate (19) (fig. S9). A uniform surface coverage of Zn2(bim)4 MSNs on a-Al2O3 support was achieved (fig. S10). Figure 3B presents a top-view SEM image of a Zn2(bim)4 MSN membrane at high magnification. Figure 3C shows the cross-sectional view of a membrane, where an unsupported por1358
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tion of the nanosheet layer folds around the corner of the fractured support, clearly illustrating the high flexibility of Zn2(bim)4 nanosheets. In Fig. 3, B and C, the texture of the underlying a-Al2O3 support is distinguishable, indicating a very thin layer of Zn2(bim)4 MSNs on the support. In addition, a substantial aluminum signal from the underlying a-Al2O3 support was detected by x-ray photoelectron spectroscopy (XPS) for a Zn2(bim)4 MSN membrane, indicating that the MSN layer is only several nanometers thick (fig. S11). The MSN membranes were sealed into WickeKallenbach cells for measuring the permeance and selectivity of both single and binary feed of H2 and CO2. There was no absolute pressure differential across the membrane to avoid deforming the thin MSN layers (19) (fig. S12). We first conducted a control experiment using the a-Al2O3 support. The support showed an H2 permeance of >50,000 GPUs with a low H2/CO2 selectivity (~1.2). Figure 3D shows the H2/CO2 selectivities measured on 15 membranes prepared via hot-drop coating at different support surface temperatures (80° to 200°C; table S1). In our investiga-
tion, 120°C optimizes the average selectivity and reproducibility. We hypothesize that this temperature is just high enough to maintain a solvent evaporation rate that avoids ordered restacking of the nanosheets during the coating process, which occurs at lower temperature (fig. S13). At higher temperature, pinholes or gaps might form between nanosheets when the solvent evaporates too quickly. The three membranes prepared at 120°C exhibited H2/CO2 selectivity of 261 T 39. Nine membranes were prepared at 120°C with different drop volumes (1 to 15 ml), which proved to be a less significant factor than the support surface temperature (fig. S14 and table S2). A membrane prepared at 120°C was tested for single gas permeation of H2 (0.29 nm), CO2 (0.33 nm), N2 (0.36 nm), CH4 (0.38 nm), and C2H6 (0.44 nm). There was a clear cutoff between H2 and CO2 (fig. S15). H2/CO2 separation measurements at different temperatures indicate activated diffusion for both H2 and CO2 (fig. S16). These results suggest a size exclusion mechanism for separation of H2 from CO2. H2/CO2 separation with different feed compositions indicates that
Fig. 2. Characterizations of molecular sieve nanosheets. (A) Low-magnification TEM image of a piece of Zn2(bim)4 nanosheets. The inset image shows a high-resolution TEM image of a five-layered Zn2(bim)4 nanosheet. (B) SAED pattern (white circle) shows the diffraction from (hk0) planes within a few-layered nanosheet. A simulated SAED pattern of Zn2(bim)4 nanosheet down the c axis is also shown. (C) Tappingmode AFM topographical image of Zn2(bim)4 nanosheets on silicon wafer. The height profile of the nanosheets along the black lines was marked in the image. (D) N2 adsorption-desorption isotherms (77 K) on pristine Zn2(bim)4 and Zn2(bim)4 nanosheets. The inset presents photographs comparing the pristine Zn2(bim)4 and Zn2(bim)4 nanosheets obtained after exfoliation. sciencemag.org SCIENCE
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CO2 adsorption has influence on the H2 permeation to some extent (fig. S17). Normally, researchers prepare thicker membranes to minimize nonselective defects. However, there is an inverse relationship between membrane permeance and selectivity. In contrast, we found an anomalous proportional relationship between permeance and selectivity for the Zn2(bim)4 MSN membranes, as shown in Fig. 3E. The H2 permeance varies from 760 to 3760 GPUs along with the variation of H2/CO2 selectivity from 53 to 291. The linear regression of these data accounts for 90.6% of the variance (table S1). Meanwhile, a small fluctuation in CO2 permeance (12.3 T 4.5 GPUs) was measured on all 15 membranes (table S1). Theoretically, only H2 molecules can pass through the four-membered rings of the MSNs. The non-
zero CO2 permeance could be attributed to the non–size-selective mass transport through imperfect sealing or through boundaries of the nanosheet layer. To gain insight into the structure-performance relationship of the MSN membranes, four membranes with different performance were characterized by powder XRD (Fig. 3F). The proportional relationship between permeance and selectivity can be observed in the table in Fig. 3F. Membranes with higher permeance are more selective, and vice versa. Membrane performance can be correlated with the order of nanosheet stacking, as indicated by XRD patterns. The low-angle hump suggests regions with expanded stacking of nanosheets. The appearance of (002) reflection coincides with the formation of bulk Zn2(bim)4
via ordered restacking. Lamellar ordering of nanosheets would block the permeation pathway for H2, but have only a slight effect on CO2 leakage, resulting in a reduction in membrane performance. Noting that a tiny hump is still identifiable for the high-performance membrane, we speculate that an ultrapermeable and superselective molecular sieve membrane could be obtained if the membrane consisted of fully disordered MSNs. The performance of our membranes exceeds the latest Robeson’s upper-bound for H2/CO2 gas pair and is higher than that of the molecular sieve membranes reported to date, including polycrystalline membranes composed of three dimensional MOFs (fig. S18). For practical use, thermal and hydrothermal stability is an important issue for H2 selective membranes (25). A Zn2(bim)4 MSN membrane was tested under different conditions for more than 400 hours in total, including two temperature cycles, showing no degradation in membrane performances. When exposed to an equimolar H2/CO2 feed containing ~4 mol % steam at 150°C, the membrane showed good stability after a 120-hour test (fig. S19). REFERENCES AND NOTES
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AC KNOWLED GME NTS
Fig. 3. Morphology and performance of membranes derived from molecular sieve nanosheets. (A) SEM image of a bare porous a-Al2O3 support. (B) SEM top view and (C) cross-sectional view of a Zn2(bim)4 nanosheet layer on a-Al2O3 support. (D) Scatterplot of H2/CO2 selectivities measured from 15 membranes. The red line with symbols shows the average selectivity and dispersion of selectivity of the membranes prepared at different coating temperatures. (E) Anomalous relationship between selectivity and permeance measured from 15 membranes. (F) Powder XRD patterns of four membranes with different separation properties.The cartoons at the left schematically illustrate the microstructural features of the nanosheet layers. The yellow and green portions correspond to the low-angle humps and the (002) peaks in the XRD patterns, respectively. All the membranes were measured for equimolar mixtures at room temperature and 1 atm. SCIENCE sciencemag.org
We thank the National Natural Science Foundation of China and the Key Research Program of the Chinese Academy of Sciences for funding, L. Liu for experimental assistance in wet ball milling, S. Miao for experimental assistance in TEM and SAED, and H. Duan for experimental assistance in TG analysis. Y.P., Y.L., and W.Y. are inventors on a Chinese patent filed through the Dalian Institute of Chemical Physics (CN102974229A, “Exfoliation and application of two-dimensional layered metal-organic frameworks”). SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/346/6215/1356/suppl/DC1 Materials and Methods Figs. S1 to S19 Tables S1 and S2 References (26–40) 1 April 2014; accepted 12 November 2014 10.1126/science.1254227
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Editor's Summary Metal-organic framework material membranes There continues to be a lot of interest in developing membranes for gas separations that go beyond the current polymer membranes used commercially for this purpose. Peng et al. took a porous metal-organic framework material with a layered structure and exfoliated it to give nanometer-thick molecular sieves. The membranes were exceptionally good at separating hydrogen gas from carbon dioxide both in terms of permeance and selectivity. Science, this issue p. 1356
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Metal-organic framework nanosheets as building blocks for molecular sieving membranes Yuan Peng, Yanshuo Li, Yujie Ban, Hua Jin, Wenmei Jiao, Xinlei Liu and Weishen Yang (December 11, 2014) Science 346 (6215), 1356-1359. [doi: 10.1126/science.1254227]
