Bottom-Up Fabrication of Ultrathin 2D Zr MetalâOrganic Framework

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Bottom-up Fabrication of Ultrathin 2D Zr Metal-Organic Framework Nanosheets through a Facile Continuous Microdroplet Flow Reaction Ying Wang, Liangjun Li, Liting Yan, Xin Gu, Pengcheng Dai, Dandan Liu, Jon G. Bell, Guoming Zhao, Xuebo Zhao, and K. Mark Thomas Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.8b00765 • Publication Date (Web): 09 Apr 2018 Downloaded from http://pubs.acs.org on April 9, 2018

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Chemistry of Materials

Bottom-up Fabrication of Ultrathin 2D Zr Metal-Organic Framework Nanosheets through a Facile Continuous Microdroplet Flow Reaction Ying Wang1, Liangjun Li1, Liting Yan1, Xin Gu1, Pengcheng Dai1, Dandan Liu1, Jon G. Bell2, Guoming Zhao3, Xuebo Zhao1* and K. Mark Thomas2* 1

Research Centre of New Energy Science and Technology, Research Institute of Unconventional Oil & Gas and Renewable Energy, China University of Petroleum (East China), Qingdao, 266580, P. R. China. E-mail: [email protected]

2

Wolfson Northern Carbon Reduction Laboratories, School of Engineering, Newcastle University, Newcastle upon Tyne, NE1 7RU, UK. Email: [email protected]

3

College of Chemical and Environmental Engineering, Shandong University of Science and Technology, Qingdao 266590, China ABSTRACT Direct synthesis of the ultrathin and discrete 2D MOF nanosheets is extremely challenging. Herein, we present the first facile, continuous bottom-up strategy for preparing ultrathin (~3 nm) 2D MOF nanosheets with high crystallinity comprising of assemblies of a few layers. Unlike conventional solvothermal synthetic methods for 2D MOFs, the weak interlayer interaction in the vertical direction of the 2D materials is restricted under microdroplet flow reaction conditions. 2D MOF nanosheets with a large lateral area and a few layers thick were directly synthesized by suppressing the lamellar stacking of the nanosheets under the dynamic growth conditions. The 2D MOF nanosheets were characterized by scanning and transmission electron microscopy, atomic force microscopy, X-ray photoelectron spectroscopy, powder Xray diffraction, infrared spectroscopy, thermogravimetric analysis, gas adsorption and light scattering techniques, which were supported by Density Functional Theory (DFT) calculations and molecular simulations. The properties of the 'as-prepared' 2D MOF nanosheets were compared with the corresponding pristine solvothermal MOF with extended structure perpendicular to the laminar assembly. The ultrathin 2D MOF nanosheets have greater external surface area, resulting in a far higher gas adsorption and colloidal suspensions

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that exhibit the Tyndall effect. This synthetic methodology for 2D MOF nanosheets has potential for scale-up of materials production.

1.

INTODUCTION Two-dimensional (2D) nanomaterials, such as graphene,1-6 silicone,7-9 hexagonal boron

nitride,10-12 and transition metal dichalcogenides,13-15 have attracted tremendous research attention because of their exotic dimension-related properties and physicochemical properties.16, 17 These nanomaterials have high aspect ratios, because of the micrometer lateral dimensions and nanometer thickness, making them ideal for widespread applications in gas storage/separation,18-21

electrochemistry,22-24

batteries,25,

26

supercapacitors27-29

and

catalysis.30-32 As a new member of the class of 2D nanomaterials, 2D metal-organic frameworks (MOFs) have recently attracted increasing research interest.33-35 MOFs are an emerging class of crystalline porous materials fabricated from metal centers and organic linkers.36-39 Diverse metal coordination and design of organic ligands allows MOFs to be synthesized with regular porous structures, large surface area and adjustable functionality.40-44 These properties of MOF materials have many potential applications,45-48 particularly the use of 2D MOFs to fulfill the specific requirements of their unique 2D morphology.49-51 However, like other types of 2D materials, 2D MOFs always tend to stack closely along the vertical direction because of the existence of π-π interactions, van der Waals forces, and/or hydrogen bonding.52 Furthermore, the lamellar stacking structures limit the performance of the bulk 2D MOFs, which can be attributed to the slow diffusion into bulk 2D MOFs and the limited accessibility of active sites inside the 1D channels.53 As compared to the bulk 2D MOFs, isolated 2D MOF nanosheets with an atomic or molecular thickness of sub-10 nm, which expose more accessible active sites on the surface and have larger surface areas, are promising candidates for separation,54-56 catalysis,57 and sensing applications.58, 59


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These unique properties and promising applications indicate that the synthesis of ultrathin 2D MOF nanosheets with higher production and crystallinity are urgently required. Conventionally, ultrathin 2D MOF nanosheets are based on the top-down exfoliation of 2D bulk-layered MOFs, including physical, chemical, or liquid phase exfoliation.60,

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Unfortunately, this method often leads to mixed MOF nanosheets with a wide distribution of thickness and lateral size. Moreover, these exfoliated MOF nanosheets are prone to restacking because of strong interlayer interactions.54 To address these challenges, the bottom-up synthesis method reported here is used for directly preparing the ultrathin 2D MOF nanosheets, in a similar manner to interface-mediated synthesis62-64 and surfactants/small molecules-assisted synthesis,53,

65, 66

and this can help to selectively control the growth

direction of the MOF crystals. The facile and low-cost bottom-up synthesis for ultrathin MOF nanosheets using a gluconate assisted method was reported recently.67 Although the bottomup method allows direct growth for ensuring the intact morphology and the controlled thickness of the MOF nanosheets, the applied auxiliary substances will inevitably lead to an insufficient utilization of the active sites on the surface,68, 69 thus lowering the performance of the 2D MOF nanosheets. Therefore, the direct synthesis of ultrathin 2D MOF nanosheets, which does not affect the growth along the lateral direction, but breaks the interlayer interactions within the layers, is limited. ZrBTB is a typical 2D layered MOF of the kgd topology, which is composed of 6connected Zr6O4(OH)4 as secondary building units (SBU) clusters and 3-connected H3BTB as linkers.70, 71 The 2D layered network of ZrBTB material prepared by solvothermal methods (ST-ZrBTB where “ST” stands for “solvothermal”) exhibits excellent stability and high crystallinity. These 2D kgd monolayers tend to stack on each other and form a 3-dimensional structure because of the existence of van der Waals interactions and hydrogen bond between the layers. Recently, several isostructural 2D MOFs with the structure of monolayer or multilayers have been reported,57, 72 and 2D ZrBTB nanosheets with a thickness of 10-20 nm 3 ACS Paragon Plus Environment


have been obtained via a surfactant-free modulated hydrothermal approach.73 However, the bottom-up fabrication of well-dispersed ultrathin 2D ZrBTB nanosheets under a microdroplet flow reaction method has not been achieved thus far. Therefore, in this work, we choose this 2D layered MOF structure as a candidate for the synthesis of ultrathin nanosheets with a thickness of sub-10 nm via a microdroplet flow reaction. Dynamic mixing reactions dominated by diffusion and forced convection in the microdroplet flow system,74, 75 lead to suppression of longitudinal growth for the stacking of the 2D ZrBTB. Thus, ultrathin and separated 2D ZrBTB nanosheets (MF-ZrBTB wherein “microdroplet flow reaction” was abbreviated to “MF”) with a lateral dimension of 1~5 µm and a thickness of ~3 nm were prepared directly, and the space-time yield (STY) was as high as 385.03 kg m-3 d-1. Furthermore, compared with the bulk 2D ZrBTB featured multiple-stacking structures, ultrathin 2D ZrBTB nanosheets exhibit higher surface areas and a considerably better gas adsorption performance. These observations not only disclose a new approach for directly fabricating ultrathin 2D ZrBTB nanosheets, which is capable of being scaled up for bulk samples, but also provide a reference value for the future preparation of other ultrathin layered materials. 2.

EXPERIMENTAL

2.1 Synthesis of MF-ZrBTB under the microdroplet flow reaction system The ZrCl4, (0.0873 g, 0.37 mol L-1) was dissolved in a solvent mixture of formic acid (FA, 5 mL) and N, N`- dimethylformamide (DMF, 10 mL). The 1,3,5-tris(4-carboxyphenyl) benzene acid, (H3BTB, 0.110 g, 0.25 mol L-1) was dissolved in a solvent mixture of formic acid (5 mL) and N, N`- dimethylformamide (DMF, 10 mL). In the microdroplet flow reaction system, these solutions were pumped continuously into a static micro-mixer equipped with magnetic stirring to form a homogeneous solution. At a constant velocity ratio (υsilicone oil: υreactant = 9:4), the solution of reactants was dispersed into uniform microdroplets by shear forces between the two co-current fluid flows, which were immiscible with each other. These 4 ACS Paragon Plus Environment

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microdroplets pass along the channel (2 mm i.d. and 3 mm o.d.) and enter into the reaction coil that was immersed in an oil heating bath (T = 130︒C). After cooling to room temperature, the mixed solution was filtered and washed with DMF and petroleum ether solution. The products were activated by Soxhlet extraction using petroleum ether and acetone as the eluent. After drying at 120 oC for 12 h under vacuum, 252 mg of sample was obtained with the calculated STY of 385.03 kg m-3 d-1 and a 0.72 g h-1 production rate. 2.2 Synthesis of solvothermal ST-ZrBTB ST-ZrBTB was synthesized by dissolving ZrCl4, (0.0873 g, 0.37 mol L-1) and the 1,3,5tris(4-carboxyphenyl)benzene acid, (H3BTB, 0.110 g, 0.25 mol L-1), formic acid (10 mL) and N,N`- dimethylformamide (DMF, 20 mL) at room temperature.71, 73 The mixture was stirred and placed in a vessel at 130 ˚C for 96 hours. After cooling to room temperature, the solution was filtered and washed with DMF solution, a white powder was collected. The product was activated by Soxhlet extraction using acetone as the eluent. The samples were soaked in acetone before they were subjected to further characterization. 2.3 Structural Characterization Powder X-ray diffraction (PXRD). Powder X-ray Diffraction (PXRD) analysis of the MFZrBTB and ST-ZrBTB were used to confirm the crystallinity as well as the phase purity of the materials by comparison with the simulated pattern of ZrBTB. Powder X-ray diffraction data were measured using a XRD-7000 diffractometer (Shimadzu, Japan) with Cu Kα radiation from 5 to 60 o (λ = 1.5406 Å). Thermogravimetric analysis (TGA). Thermogravimetric analysis (TGA) experiments were conducted on a ZRT-A thermogravimetric analyzer (Gaoke Instrument Co. Ltd., China.), with a heating rate of 10 °C min-1.



Fourier

Transform

Infrared

Spectroscopy

(FT-IR).

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Fourier

transform

infrared

measurements were performed on a Nicolet 6700 spectrometer (Thermo Fisher Co. Ltd., US). The spectra were collected over the range 500-4000 cm-1 with a resolution of 4 cm-1. X-ray photoelectron spectroscopy (XPS). X-ray photoelectron spectroscopy (XPS) spectra was conducted on an EscaLab 250Xi photoelectron spectrometer. The shift of the binding energy due to the surface electrostatic charging is corrected using the C 1s as an internal standard at 284.6 eV. Scanning electron microscopy (SEM). The scanning electron microscopy (SEM) images were acquired using a JSM-6700F microscope (JEOL Co. Ltd., Japan). Transmission electron microscopy (TEM). The TEM images were obtained using a JEM2100F transmission electron microscope (JEOL Co. Ltd., Japan). X-ray spectroscopy (EDS). EDS analysis were carried out by using IE250X-Max50 spectrometer (Oxford Co. Ltd., UK). High-resolution TEM (HR-TEM). The HR-TEM images were obtained using the JEM2100F high resolution transmission electron microscopy (JEOL Co. Ltd., Japan). Atom force microscopy (AFM). The AFM images were carried out using Bruker Multimode 8 scanning probe microscope (Bruker Co. Ltd., Germany) Gas Adsorption Studies. These studies were carried out to investigate the porosity of the MFZrBTB and ST-ZrBTB. Nitrogen sorption isotherms were measured at 77 K using a Autosorb volumetric gas sorption analyzer (Quantachrome, USA). All of samples were degassed in vacuum for 12 h at 135 °C to fully remove guest molecules prior to analysis. The pore size distribution is obtained from non-local density function theory (NLDFT) based on N2 isotherms. High pressure CO2 and CH4 adsorption isotherms were measured using a XEMIS magnetic suspension balance sorption analyser (Hiden Isochema, UK) equipped with a 6 ACS Paragon Plus Environment

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circulating water bath. Before sorption measurements, samples were activated at 130 °C for 8 h under ultrahigh vacuum (10-6 mbar). The pressure ranges for CO2 and CH4 were 0-20 bar and 0-100 bar, respectively. The absolute isotherms were calculated from the surface excess using the density measured in the XEMIS instrument.

3.

RESULTS AND DISCUSSION

3.1

Synthesis Methodology

The synthesis of ultrathin 2D ZrBTB nanosheets is schematically shown in Figure 1a. During the microdroplet flow synthesis system reported here (Figures 1b and S1),76, 77 the solution of ZrCl4 and 1,3,5-tris(4-carboxyphenyl) benzene acid (H3BTB) in DMF/formic acid were injected into the oil phase and well-distributed microdroplets were generated by shear stresses on the interface of the two immiscible phases (see Figure S1b). When these microdroplets rotated and flowed through the cylindrical microchannel into the heating zone, the effect of diffusion and forced convection combined together to accelerate mixing.75 Thus, the crystalline reaction occurred quickly because of the excellent efficiency of heat/mass transfer.78,

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The entire process of crystallization growth was affected by the flow

characteristics induced by diffusion and forced convection in the microdroplets (see Figure S1b),80-84 which also increased the probability for breaking the stacked growth of the layers. After passing the resulting product through the cooling zone and washing off the oil phase, the white powder was collected. The residence time was determined by the length of the heating zone and the total flow velocity of the fluid. In this work, the sample was prepared in a 2 m microchannel at 130 oC and at total flow rate of 200 µL min-1, which resulted in the residence time of 32 min and was denoted as MF-ZrBTB. The space-time yield (STY)85, 86 of MF-ZrBTB was estimated to be 385.03 kg m-3 d-1, which was sufficiently high for mass production. ST-ZrBTB with an extensive stacking structure along the c crystallographic axis



perpendicular to the lamellar structure, was synthesized using a conventional solvothermal method71 and was used as a reference sample for comparison with MF-ZrBTB.

Figure 1. Schematic illustrations of (a) the overall process developed to produce 2D MF-ZrBTB and STZrBTB via microdroplet flow reaction and conventional solvothermal methods and (b) the experimental arrangement for microdroplet flow synthesis of MF-ZrBTB.

3.2 Structural Characterization 8 ACS Paragon Plus Environment

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Powder X-ray diffraction (PXRD) measurements confirmed the crystallinity and purity of MF-ZrBTB and ST-ZrBTB (see Figure 2a). Both MF-ZrBTB and ST-ZrBTB exhibited two intense peaks at 5.1o and 8.8o, corresponding to the (100) and (110) crystal planes of 2D ZrBTB, respectively.71 The slightly broader PXRD peaks of MF-ZrBTB compared with STZrBTB are attributed to the smaller number of layers.87,

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Diffraction peaks of the other

crystal planes were not detected, indicating the absence of any inter-penetration structures. These PXRD profiles confirm that both materials are isostructural with UMCM-309a or NUS8 (Zr), which has a typical 2D layered net with the kgd topology.70, 71, 73 MF-ZrBTB and ST-ZrBTB have essentially the same crystal structure, but different structural morphologies and these were compared using scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM) and atomic force microscopy (AFM). The plate-like structure of MF-ZrBTB with the lateral sizes ranging from 1 to 5 µm are shown in Figures 2b and S2. These 2D layers are divided into isolated or separated ultrathin nanosheets (< 10 nm) (Figure 2b, inset).89 The morphology of MF-ZrBTB nanosheets was different from ST-ZrBTB, which has a rose-shape morphology with a wider size distribution from 3 to 15 µm (Figure S3). ST-ZrBTB had ordered and hexagonal monolithic structures with thicknesses of ~ 45 nm, which were closely stacked together and rolled up to form the flower-shape structures (Figures 2c, 2c inset and S4). TEM images of MF-ZrBTB showed slightly wrinkled and curled planes (Figures 2d and S5a), resulting from the dimensional characteristics of the nanosheets and indicate successful synthesis of the ultrathin and flexible nanosheets. MF-ZrBTB ultrathin nanosheets viewed along the c axis also showed the features of the transparent layers (Figure S5b). In contrast, ST-ZrBTB exhibited a superposed and opaque morphology (Figure 2g), and its hexagonal monolithic and staggered lamellar structures were as expected (Figure S6). The nanometer particle size and dispersion of MF-ZrBTB nanosheets were confirmed by light scattering. The Tyndall effect was clearly observed in the colloid suspension of ultrathin MF-ZrBTB 9 ACS Paragon Plus Environment


nanosheets in ethanol (Figures 2d inset, 2g inset and S7). Furthermore, the HRTEM images showed that the ultrathin MF-ZrBTB nanosheets still maintained the same crystal structure as ST-ZrBTB, even when the thickness was reduced to nanometers. The HRTEM image of the MF-ZrBTB nanosheets shows clear lattice fringes, in which the Zr6 clusters appeared as the dark spots (see Figure 2e). The arrangement of these dark spots matched the atomic model of the kgd monolayer, according to the crystallographic data of ST-ZrBTB. Moreover, the interplanar distance of these lattice fringes was approximately 1.73 nm, which was consistent with the X-ray crystallographic value of 1.70 nm for the (100) crystal plane of ST-ZrBTB. The corresponding Fast Fourier transform (FFT) analysis of the MF-ZrBTB nanosheets revealed a six-fold symmetry (Figure 2e, inset), indicating the hexagonal crystal structure for MF-ZrBTB, which was also in good agreement with the crystal structure of ST-ZrBTB. Although the crystal structure of the ultrathin MF-ZrBTB nanosheets was maintained, it was different from that of ST-ZrBTB with the highly ordered lattice fringes (Figures 2h and S8). A number of random lattice fringes, complex lines or even ripples were observed for MFZrBTB, which may be caused by the wrinkles and curls or tilted sample orientations of these ultrathin nanosheets resulting from the micrometer lateral size and nanometer thickness.57, 72 MF-ZrBTB nanosheets were also characterized by AFM to further confirm the successful preparation of ultrathin nanosheets and gain insight into their thickness. As shown in the Figures 2f and S9, the tapping-mode AFM topographical images revealed that MF-ZrBTB was fairly flat with a lateral dimension of up to ~3 µm. The height profiles showed that the thickness of most of the MF-ZrBTB nanosheets was approximately 3-4 nm (Figures 2f and S10). However, the thickness of ST-ZrBTB was an order of magnitude larger with an average value of ~45 nm (Figures 2i and S11-12). The crystallographic interlayer distance of ZrBTB was ~1.00 nm,73 indicating that the number of layers in the ultrathin MF-ZrBTB nanosheets was approximately 3-4. The crystallographic structures of MF-ZrBTB and ST-ZrBTB are essentially the same and microscopic techniques have shown that discrete ultrathin 2D MF10 ACS Paragon Plus Environment

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ZrBTB nanosheets have been successfully prepared by the facile continuous microdroplet flow reaction method. However, microscopic methods only characterize very small amounts of material. Therefore, further characterization of MF-ZrBTB was undertaken in order to confirm that the microscopic characteristics described above are representative of larger scale properties of MF-ZrBTB and different from ST-ZrBTB. Thermogravimetric analysis (TGA) revealed that the ultrathin MF-ZrBTB nanosheets remained stable up to 494 °C in air atmosphere despite their nanometer-scale thickness, which was similar to ST-ZrBTB (Figure S13). The Fourier transform infrared (FTIR) spectrum of MF-ZrBTB nanosheets was identical to that of ST-ZrBTB, indicating that the chemical composition as well as the bonding mode remains intact even when the thickness was reduced to several nanometers (Figure S14). X-ray photoelectron spectroscopy (XPS) and energydispersive X-ray spectroscopy (EDS) were carried out on the activated samples of the MFZrBTB and ST-ZrBTB to examine their compositions. The identical XPS survey spectra, combined with similar high-resolution spectra for C 1s, O 1s and Zr 2d, revealed that ultrathin MF-ZrBTB nanosheets have an identical composition and share the same coordination spheres as pristine ST-ZrBTB (Figure S15). Elemental mapping results showed a similar composition of MF-ZrBTB and ST-ZrBTB (Figures S16 and S17). Therefore, all the aforementioned results proved that the stability, functional groups, chemical composition and coordination spheres remained unchanged for the ultrathin 2D MF-ZrBTB nanosheets obtained from the microdroplet flow reaction. The apparent low density of MF-ZrBTB compared with ST-ZrBTB is shown in Figure S18. These special characteristics also support broad-scale potential applications of the ultrathin 2D MF-ZrBTB nanosheets.



Figure 2. (a) PXRD profiles for MF-ZrBTB and ST-ZrBTB. The top image is the architecture of the layered ZrBTB MOF that is based on single-crystal data. (b) SEM image of MF-ZrBTB nanosheets. Inset: single-layered MF-ZrBTB nanosheets; (c) SEM image of ST-ZrBTB. Inset: single-layered ST-ZrBTB; (d) Transmission electron microscopy (TEM) image of MF-ZrBTB. The inset figure shows the Tyndall light scattering effect of MF-ZrBTB colloidal suspension; (e) HR-TEM and FFT images of MF-ZrBTB nanosheets; (f) AFM image and corresponding height profiles of MF-ZrBTB nanosheets along the red and blue lines, respectively; (g) Transmission electron microscopy (TEM) image of ST-ZrBTB. The inset figure shows the Tyndall effect for ST-ZrBTB colloidal suspension; (h) HR-TEM and FFT images of ST-ZrBTB; (i) AFM image and corresponding height profiles of ST-ZrBTB along the red and blue lines, respectively

Gas adsorption characteristics were investigated in detail in order to establish that the bulk properties of MF-ZrBTB were consistent with the microscopic structural characteristics and different from ST-ZrBTB. The efficient microdroplet flow synthesis for ultrathin discrete 2D MOF nanosheets was further proven by the enhanced Brunauer-Emmett-Teller (BET) surface area. The BET surface area of previously reported for ZrBTB, which has elliptical windows with a calculated size of 1.26 nm × 0.89 nm (Figure S19), was 810 m2 g-1.71 N2 sorption isotherms at 77 K were performed on the activated samples of ST-ZrBTB and MFZrBTB nanosheets. As shown in Figure 3a, typical Type-I/II isotherms were observed for both ST-ZrBTB and MF-ZrBTB. The steep increase in the low pressure range and a plateau in 12 ACS Paragon Plus Environment

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the middle pressure range suggested the overall microporous nature of ST-ZrBTB and MFZrBTB. Both materials show convex curvature to the P/P0 axis at values > 0.8, which can be attributed to multilayer adsorption in interstices between nanosheets in MF-ZrBTB or defects/cracks in ST-ZrBTB particles. Very small hysteresis loops were observed in the highrelative pressure range of N2 sorption, which was characteristic for the narrow slit pores formed by the aggregation of layered materials. The ultrathin MF-ZrBTB nanosheets had a N2 BET surface area of 1516 ± 10 m2 g-1, which was almost twice the literature value for ZrBTB (810 m2 g-1)

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and ST-ZrBTB (855 ± 4 m2 g-1) (Figures S20 and S21, and Table S1).

Meanwhile, the micropore surface area of MF-ZrBTB was relatively high, this could be attributed to the large number of adsorption sites exposed on the external surface of these isolated or separated nanosheets. The porous structures of ultrathin MF-ZrBTB nanosheets and ST-ZrBTB were confirmed by pore size distribution (PSD) derived from the N2 isotherm. As shown in the PSD plots (Figure 3b), calculated using the non-local density functional theory (NLDFT), MF-ZrBTB revealed a major peak at ~1.0 nm, which was identical to the pore size of ST-ZrBTB and agreed with the pore size calculated using the crystallographic data of ZrBTB. In addition, a very weak peak was observed at ~1.8 nm for MF-ZrBTB (Figure 3b, inset), possibly representing the silt-like pores formed by the stacking of ultrathin nanosheets as seen from the SEM images. MF-ZrBTB possesses total pore and micropore volumes of 0.81 and 0.52 cm3 g-1, respectively, which are larger than those of the ST-ZrBTB (v total volume: 0.50 cm3 g-1 and v micropore: 0.30 cm3 g-1). Comparison of these results confirmed that the thickness of the layered 2D MOFs had a significant effect on the surface area and the pore volume. This phenomenon has also been verified previously for other delaminated 2D materials.56, 89, 90 To further understand the increase in the surface areas of the ultrathin MF-ZrBTB nanosheets, N2 isotherms at 77 K were calculated using the Sorption module within the Materials Studio software. Figure 3c shows a visualization of the N2 sorption in the 2D 13 ACS Paragon Plus Environment


ZrBTB MOF, where the green points represent the density distribution of the N2 molecules. As the critical dimensions of N2 (3.054 × 2.991 × 4.046 Å)91 was larger than the minimum distance between two identical layers of ZrBTB (~ 0.29 nm, see detail in Figure S19), the pristine ZrBTB exhibited the major adsorption of N2 in the elliptical pores formed by the stacking of monolayers along the c axis, but showed an efficient blockage of N2 molecules within interlayers. The simulated N2 sorption of ZrBTB was calculated as 838 m2 g-1,72 which is close to the experimental BET surface area of 855 ± 4 m2 g-1 (ST-ZrBTB). Reduction of the thickness of a stacked structure to three-layered nanosheets gave a larger external surface resulting in a higher surface area. The surface areas of three-layered ZrBTB was calculated to be 1522 m2 g-1, and this agrees with the experimental BET surface area of MF-ZrBTB (1516 ± 10 m2 g-1). Agreement between experimental and molecular simulation surface area values confirm the successful preparation of the separated or isolated ultrathin 2D ZrBTB nanosheets with three-layered structures.

Figure 3. (a) N2 adsorption-desorption isotherms at 77 K on ST-ZrBTB and ultrathin MF-ZrBTB nanosheets; (b) Pore size distribution of ST-ZrBTB and MF-ZrBTB calculated using non-local density function theory (NLDFT); (c) Molecular simulation of N2 adsorption in the pristine bulky ZrBTB, the three-layered ZrBTB and the single-layered ZrBTB. (the green points shows the density distribution of N2 molecules)


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To synthesize the single-layered ZrBTB nanosheets and understand the growth mechanism of the ultrathin nanosheets under the MF condition, a series of MF-ZrBTB-t (in which t stands for the residence time in minutes) were performed under the same microdroplets flow reaction conditions. As shown in Figure 4a, the experimental PXRD of MF-ZrBTB-8 and MF-ZrBTB64 coincided with the simulated pattern of ZrBTB. However, the diffraction peak of MFZrBTB-8 was fairly broad and some impurity peaks were observed, indicating lower crystallinity that may be caused by the unfinished crystallization process at a short residence time under the MF condition. The SEM image of MF-ZrBTB-8 showed that a number of amorphous substances co-exist with flake-like crystal structures (Figure S22), which also agreed with the XRD results. The corresponding magnified SEM image of MF-ZrBTB-8 is shown in Figure 4b and in this structure, nanometer-scale monolayers were stacked along the longitudinal axis. MF-ZrBTB-64 grew to a regular flower-like morphology accompanied with an increase in both the thickness and the lateral size as a result of increased residence time (Figures 4c and S23). AFM analysis confirmed that irregular nanosheets of MF-ZrBTB-8, as thin as 2.8 nm, were obtained at the shortest residence time of 8 min (Figure 4d). After 64 min, more regular and fairly flat nanosheets with a thickness of ~ 25 nm were observed. The height profile along the edge of the MF-ZrBTB-64 nanosheets revealed several smooth terraces with a uniform thickness of ~ 6 nm, indicating the continuous growth in the thickness of the monolayers and the stacking of these monolayers back to the ordered bulk structures with a prolonged residence time (Figure 4e). Similar Type II isotherms and pore size distributions were observed on MF-ZrBTB-8 and MF-ZrBTB-64. However, their BET surface areas decreased to 1150 ± 5 m2 g-1 and 946 ± 5 m2 g-1, respectively (Figures S24-S27 and Table S1). This phenonmen may be caused by the lower crystalline of the MF-ZrBTB-8 nanosheets and the increased thickness of the MF-ZrBTB-64 nanosheets. Experimental results showed that because of the strong adhesion and the role of stacking between these monolayers, it was 15 ACS Paragon Plus Environment


difficult to achieve well-dispersed single-layer ZrBTB nanosheets with a simulated BET surface area of 1977 m2 g-1.

Figure 4. (a) XRD patterns for MF-ZrBTB-8 and MF-ZrBTB-64; (b) SEM image of MF-ZrBTB-8 and (c) MF-ZrBTB-64 nanosheets; (d) AFM image and corresponding height profiles of MF-ZrBTB-8 nanosheets along the red and blue lines, respectively; (e) AFM image and corresponding height profiles of MF-ZrBTB64 nanosheets along the red and blue lines, respectively.

These observations also demonstrate the growth mechanism of 2D ZrBTB under the MF conditions. At the early stage of the crystallization process under MF, the nucleation and crystal growth process of monolayers occurred rapidly because of the high-efficiency solution diffusion in the MF system.92, 93 However, these monolayers tended to stack due to weak van der Waals and hydrogen bonding interactions. As a result of the existence of the diffusion and forced convection in flowing and rotating microdroplets, the weak interlayer interaction was difficult to form and the growth along the vertical direction was suppressed effectively at this stage. With an increase in the residence time, these ultrathin nanosheets grew gradually, the lateral and vertical dimensions increased and the morphology became more regular. Then a certain extent of aggregation will occur for these nanosheets to reduce the surface free energy in this system.94 Based on this, the crystal growth process can be terminated at the desired 16 ACS Paragon Plus Environment

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stage to ensure 2D MOF nanosheets with controlled thickness and shapes under microdroplet flow system are produced. Thus, the MF method allowed for an efficient fabrication of ultrathin 2D MOF nanosheets with the thickness of sub-10 nm by precise control of the residence time To further investigate the indispensable role of the dynamic reaction conditions, the conventional solvothermal synthesis of ST-ZrBTB-t at different reaction times was performed for comparison. However, no products were formed when the reaction time was 8 min or 32 min under solvothermal conditions. The experiments indicated that 60 minutes was the critical time required to produce ST-ZrBTB-t by using the conventional ST method. The morphology and structure of ST-ZrBTB-t were examined using SEM and XRD. As shown in Figures 5a-5d, with the passage of time, the layer structure of ST-ZrBTB-t tended to grow larger and more uniform, and evolved from the scattered and inhomogeneous nanosheets of ST-ZrBTB-60 to the uniform rose-shape morphology of ST-ZrBTB-1440. Meanwhile, the control of the crystal growth, via an adjustment of the reaction time, enabled the variation of the thickness of the ST-ZrBTB-t (Figures 5e-5h). Obviously, the ST-ZrBTB-60 nanosheets with a high crystallinity exhibited the largest aspect ratios, as a result of the smallest thickness along the stacking direction (Figures 5e and 5i). AFM analysis of ST-ZrBTB-60 showed regular hexagon lamellae with a thickness of approximately 40 nm (Figure 5j), it was far larger than the samples obtained under the MF condition. N2 sorption at 77 K further verified that the stacked bulk structure of ST-ZrBTB-60 with a BET surface areas of 834 ± 4 m2 g-1 (Figures 5k, S28 and Table S1) as simlar to STZrBTB obtained with 4 days reaction under conventional ST conditions. These contrasting results between the two synthetic methods highlighted the advantages of MF with respect to the synthesis of ultrathin 2D MOF nanosheets. Firstly, the mixing rate and the heat/mass transfer effciency can be significantly improved due to the small volumes, individual microdroplets and the complicated diffusion and forced convection process in the MF 17 ACS Paragon Plus Environment


system,95-98 ensuring the 2D MOF materials can be produced rapidly with good crystallinity. Furthermore, the dynamic synthesis conditions were considered an efficient method to suppress the growth of the 2D MOF along the vertical direction. Also, the thickness distribution of these nanosheets can be decreased using small volumes of individual microreactors,80, 84 resulting in separate and homogeneous MOFs nanosheets. Additionally, this method continuously produced ultrathin 2D MOF nanosheets with a high STY. Therefore, the bottom-up synthesis of ultrathin 2D MOF nanosheets with high crystallinity and good dispersion could be controlled by adjusting the residence time in the microdroplet flow reaction system without any template assistance, and was compatible with the continuity and scale-up production.

Figure 5. SEM images of (a) ST-ZrBTB-60, (b) ST-ZrBTB-360, (c) ST-ZrBTB-720 (d) ST-ZrBTB-1440; Magnified SEM images of (e) ST-ZrBTB-60, (f) ST-ZrBTB-360, (g) ST-ZrBTB-720 and (h) ST-ZrBTB1440. (i) XRD patterns for ST-ZrBTB-60, -360, -720, and -1440; (j) AFM image and corresponding height profiles of ST-ZrBTB-60 along the red and blue lines, respectively; (k) N2 adsorption and desorption isotherms for ST-ZrBTB-60 at 77 K.

3.3 High Pressure Gas Adsorption Studies 18 ACS Paragon Plus Environment

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Considering the high surface areas and excellent stability of the ultrathin 2D MF-ZrBTB nanosheets, CO2 and CH4 adsorption were investigated for gas storage capacity at high pressures and compared to ST-ZrBTB (Figures 6 and S29-S34). The CO2 adsorption and desorption isotherms for MF-ZrBTB and ST-ZrBTB were measured at 278, 288, 298 and 313 K over the pressure range of 0-20 bar and no isotherm adsorption/desorption hysteresis was observed (see Figures 6b and S30-S32). However, the CO2 adsorption isotherm for MFZrBTB at 278 K had an unusual curvature change, which gradually becomes less pronounced with increasing temperature (see Figure 6a and S30). The adsorption characteristics and shapes of the CO2 isotherms for MF-ZrBTB and STZrBTB were compared by fitting the experimental data to the Langmuir and DubininRadushkevich (D-R) equations. The Langmuir model is derived for monolayer adsorption on an open surface and therefore should be a suitable model for MF-ZrBTB. The D-R equation is derived for pore filling in a material with a Gaussian pore size distribution and this should be more suitable for ST-ZrBTB. The experimental CO2 isotherms for MF-ZrBTB at 278 and 298 K deviate from the simulated Langmuir equation to a relatively small extent in the pressure range 1-10 bar where the curvature change is most prominent (see Figure 6a) and are shown in Figures S35-40 for 278 and 298 K, respectively. The deviation is most prominent at lower temperature and is attributed to a gradual transition from monolayer to multilayer adsorption on nanosheets leading to adsorption in interstices between nanosheets at higher pressure (Figures S37 and S40). There is also the possibility of the presence of a small amount of adsorption in porosity in the nanosheets layers. The results are consistent with adsorption and desorption isotherms for N2 at 77 K, where there is convex curvature relative to the P/P0 axis at high values (see Figure 3a). The D-R graphs for CO2 adsorption isotherm on MF-ZrBTB at 278 and 298 K only exhibited a linear relationship at low pressures range (0.1-1 bar) and deviated at high



pressure (Figures S41-S46). It is evident that the D-R equation does not provide a good model for the CO2 isotherms of MF-ZrBTB at 278 and 298 K (see Figures S43 and S46). The D-R graphs show a very good linear relationship for CO2 adsorption on ST-ZrBTB at 278 and 298 K with regression coefficients of 0.99986 and 0.99981(Figures S47 and S49). The simulated CO2 isotherms at 278 and 298 K based on the D-R equation are in good agreement with the experimental data (see Figures S48 and S50). However, the description of the experimental CO2 isotherms for ST-ZrBTB at 278 and 298 K based on the Langmuir equation deviates considerably from the experimental data (see Figures S51-S56 for the CO2 isotherms and Figures S53 and S56 for the comparison of simulated and experimental data at 278 and 298 K, respectively). Therefore, the D-R equation gives an accurate model for the pore filling mechanism of CO2 on microporous ST-ZrBTB. CH4 is a supercritical gas and therefore, the methane uptake cannot be modelled using the D-R equation.99 Similar trends were observed for Langmuir analysis CH4 adsorption on MF-ZrBTB and ST-ZrBTB (see Figures S57-S62). Both ST-ZrBTB and MF-ZrBTB fit the Langmuir equation well at high pressure (60-100 bar). The Langmuir equation provides higher accuracy for the description of experimental methane isotherms at 298 K for MFZrBTB rather than ST-ZrBTB (see Figures S59 and S62). This is consistent with the CO2 isotherms and is attributed to the larger open external surface area for the ultrathin 2D MOF nanosheets compared to the microporous ST-ZrBTB. MF-ZrBTB had almost the same CO2 working capacity as that of ST-ZrBTB at a low pressure (< 1 bar), but there is a large divergence in capacities at high pressure (Figures 6b and S30-32). At 298 K and 1 bar, the CO2 uptake on MF-ZrBTB was 1.26 mmol g-1, which was slightly higher than that of ST-ZrBTB (1.20 mmol g-1). An obvious demarcation point between the two CO2 isotherms was observed at 2.0 bar, where the CO2 uptake in MF-ZrBTB increased sharply. While the amount of CO2 adsorbed on the ST-ZrBTB was relatively low, it almost reached the saturated sorption capacity at high pressure (Figure 6b). When the pressure 20 ACS Paragon Plus Environment

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reached 20 bar, MF-ZrBTB absorbed 12.01 mmol g-1 of CO2, which was approximately three times higher than that adsorbed by ST-ZrBTB (4.09 mmol g-1) under the same conditions (see details in Table S2). The remarkable higher CO2 adsorption for MF-ZrBTB compared with ST-ZrBTB is consistent with N2 adsorption at 77 K, which reveals that MF-ZrBTB has a much larger surface area and total pore volume than ST-ZrBTB (see Figure 3a). This is ascribed to the larger external surface of the separated nanosheets and multilayer fomation leading to the filling of interticies between nanosheets in MF-ZrBTB. ST-ZrBTB is a microporous material and the total pore volume limits adsorption capacity. To determine whether there were stronger interactions between MF-ZrBTB and CO2, the adsorption isotherms on MF-ZrBTB and ST-ZrBTB were analyzed using the virial method and the Clausius-Clapeyron equation,100 respectively. The virial graphs for the CO2 on MF-ZrBTB and ST-ZrBTB indicated that both of these samples have very good linearity in the low pressure region (Figures S63-S66 for MF-ZrBTB and Figures S67-S70 for STZrBTB). The isosteric adsorption enthalpies of MF-ZrBTB and ST-ZrBTB at zero surface coverage (Qst,n=0) for CO2 were calculated from the graphs of A0 versus 1/T. The enthalpy of adsorption at zero-coverage (Qst,n=0) for CO2 adsorption on MF-ZrBTB (29.3 ± 1.8 kJ mol-1) was higher than ST-ZrBTB (21.7 ± 1.8 kJ mol-1), reflecting the stronger interaction with the adsorbent surface of MF-ZrBTB (Figures S71 and S72).101,

102

The Qst values for CO2

adsorption as a function of uptake were in the ranges 19.0-29.5 kJ mol-1 and 19.0-26.0 kJ mol1

for MF-ZrBTB and ST-ZrBTB, respectively (Figure S75-S77). At low uptakes, the Qst of

CO2 on MF-ZrBTB was slightly higher than ST-ZrBTB, which was accordingly attributed to the stronger binding strength of CO2 molecules with the MF-ZrBTB framework (Figure S77), and this is in agreement with the result from the virial method. In addition, the second virial coefficients A1 for CO2 on MF-ZrBTB increased significantly with the increasing temperature, and the values of A1 were approximately linear with temperature (Figure S73). In contrast, the values for the virial parameter A1 for CO2 on ST-ZrBTB were not strongly temperature 21 ACS Paragon Plus Environment


dependent (Figure S74). The contrasting trends and ranges of values for the A1 virial coefficients for CO2 adsorption demonstate the different adsorbate-adsorbate interactions in MF-ZrBTB and ST-ZrBTB structures. Similar behavior was observed for CH4 adsorption over the temperature range 278-308 K, and pressure range of 0-100 bar (Figures 6c, 6d and S33-34). As shown in Figure 6d, MFZrBTB exhibited a considerably higher CH4 uptake with respect to ST-ZrBTB at 298 K and 100 bar. The adsorption amount of CH4 in MF-ZrBTB was high (up to 14.8 mmol g-1), which was 2.6 times higher than that of ST- ZrBTB (5.6 mmol g-1, see detail in Table S2). This also showed significantly higher CH4 uptakes on MF-ZrBTB due to the high surface area of the ultrathin nanosheets. The isosteric adsorption enthalpies (Qst) of CH4 on MF-ZrBTB decreased with increasing uptake and were in the range 5.6-14.5 kJ mol-1, which was within the normal range of Qst of the CH4 adsorption on porous materials (Figure S78-S79).103-105 The considerable improvements in the CO2 capture and CH4 storage capacity on a weight basis are exhibited by ultrathin MF-ZrBTB nanosheets compared with microporous STZrBTB in gas adsorption applications. PXRD shows that the crystallographic structures are essentially very similar and the marked differences in the adsorption characteristics of MFZrBTB and ST-ZrBTB are attributed to the nanosheet and microporous structural morphologies of these materials.


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Figure 6. (a) CO2 adsorption isotherms of MF-ZrBTB at 278, 288, 298 and 313 K, respectively; (b) CO2 adsorption and desorption isotherms of ST-ZrBTB and ultrathin MF-ZrBTB nanosheets at 298 K and 0-20 bar; (c) CH4 adsorption isotherms of MF-ZrBTB at 278, 288, 298 and 308 K, respectively; (d) CH4 adsorption and desorption isotherms of ST-ZrBTB and ultrathin MF-ZrBTB nanosheets at 298 K and 0-100 bar.

4.

CONCLUSIONS

A new continuous and facile bottom-up approach was used to synthesize ultrathin 2D MFZrBTB nanosheets directly via the microdroplet flow reaction. This continuous flow process proceeded efficiently to produce ultrathin 2D MF-ZrBTB nanosheets with a high STY of up to ~385 kg m-3 d-1. The dynamic reaction process dominated by mass transfer processes, facilitates restricting growth of the 2D MF-ZrBTB along the vertical direction. SEM and TEM demonstrates that well-dispersed 2D MF-ZrBTB sheets were successfully synthesized with consistent nanometer thickness and lateral dimensions of micrometers. Wrinkled and curled sheets were also observed in HRTEM, suggesting the ultrathin nature of MF-ZrBTB. MF-ZrBTB was characterized by AFM, and ~ 3 nm high ultrathin multilayered nanosheets were observed. N2 sorption studies on larger samples showed that MF-ZrBTB had much 23 ACS Paragon Plus Environment


higher BET surface area than that of ST-ZrBTB and that of the previously reported ZrBTB. The significant increase in N2 BET surface areas was attributed to the larger external surface area of the ultrathin and isolated or separated nanosheets, and was validated with molecular simulations studies. Owing to the enhanced external surface areas, the ultrathin MF-ZrBTB nanosheets exhibited a far superior gas adsorption performance (~3 times for CO2 and 2.6 times for CH4) to that of the corresponding ST-ZrBTB. The continuous microdroplet flow reaction for the bottom-up synthesis of the ultrathin 2D MOF materials reported in this work has opened a new avenue and versatile approach for developing other promising ultrathin 2D MOF materials. Furthermore, the continuous microdroplet flow reaction technique can be extended easily to realize large-scale production for materials. Therefore, this method represents a breakthrough for the synthesis of ultrathin 2D MOF nanosheets.

ASSOCIATED CONTENT Supporting Information. Section S1 contains details of materials used. Section 2 contains further information on the microdroplet flow reaction system. Scanning electron microscopy, transmission electron microscopy data are given in Section S3 and for the effect of residence time in Section S12. Atomic force microscopy is given in Section S4. Thermogravimetric analysis, infrared spectra, X-ray photoelectron and Energy dispersive spectroscopy are presented in Sections S5, S6, S7 and S8, respectively. The comparison of the density of the materials is shown in Figure S9. Gas adsorption isotherms, virial, BET, DubininRadushkevich and Langmuir graphs, isosteric adsorption enthalpy calculations are presented in sections S10, S11 and S13-S21. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors: 24 ACS Paragon Plus Environment

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E-mail: [email protected] E-mail: [email protected] Author contributions: This manuscript was written through the contributions of all authors. ACKNOWLEDEMENTS This work was supported by grants from the National Natural Science Foundation of China (Grant No. 21473254 and 21401215), the Special Project Fund of “Taishan Scholars” of Shandong Province (ts201511017) and the Fundamental Research Funds for the Central Universities (18CX02068A). The work was also received support from EPSRC award EP/K005499/1.

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