Confined Synthesis of Two-Dimensional Covalent Organic Framework

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Confined Synthesis of Two-Dimensional Covalent Organic Framework Thin Films within Superspreading Water Layer Qing Hao, Chuangqi Zhao, Bing Sun, Cheng Lu, Jian Liu, Mingjie Liu, Li-Jun Wan, and Dong Wang J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b07120 • Publication Date (Web): 04 Sep 2018 Downloaded from http://pubs.acs.org on September 4, 2018

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Journal of the American Chemical Society

Confined Synthesis of Two-Dimensional Covalent Organic Framework Thin Films within Superspreading Water Layer Qing Hao, †,§ Chuangqi Zhao, ‡ Bing Sun,‖ Cheng Lu, †,§ Jian Liu, †,§ MingJie Liu,*‡ Li-Jun Wan,† Dong Wang*†,§ †Key

Laboratory of Molecular Nanostructure and Nanotechnology, Institute of Chemistry, Chinese Academy of Sciences and Beijing National Laboratory for Molecular Sciences, Beijing 100190, P.R. China ‡Key Laboratory of Bio-Inspired Smart Interfacial Science and Technology of Ministry of Education, School of Chemistry, Beijing Advanced Innovation Center for Biomedical Engineering, International Research Institute for Multidisciplinary Science, Beihang University, Beijing 100191, P.R. China §University of the Chinese Academy of Sciences, Beijing 100049, P.R. China ‖

School of Science, China University of Geosciences (Beijing), Beijing 100083, P.R. China

ABSTRACT: The confined synthesis of two-dimensional covalent organic frameworks (2D COF) thin films was developed by using thin superspreading water on the hydrogel immersed under oil as reactor. Through loading two monomers into oil and hydrogel respectively, COF thin films are synthesized at the oil/water/hydrogel interface. This strategy provides a new way for synthesis of freestanding 2D COF thin films. Detailed characterizations of the COF thin films reveal homogeneous topography, large area, controllable thickness from 4 nm to 150 nm and crystallinity with certain orientation. Young’s modulus of COF film is measured by AFM indentation as 25.9 ± 0.6 GPa, showing good mechanical properties. Based on the freestanding COF films, nanofilter membrane and photoelectrochemical sensors for Ru3+ were developed successfully. Moreover, the strategy was extended to the synthesis of crystalline zeolitic imidazolate framework-8 thin film, exhibited high application potential.

INTRODUCTION Covalent organic frameworks (COFs) are a class of crystalline porous materials and have attracted the growing research enthusiasm in recent years.1 Due to their tunable molecular structures, micro-or meso-porosity, high surface area,2 COFs are widely applied in gas storage and separation,3 selective membranes,4 catalytic science,5 organic electronic device,6 electrochemical energy storage,7 superhydrophobic interface8 and photoelectrochemistry.9 However, due to the poor solubility, COF powders are typically hard to be processed, which makes it difficult for both intrinsic properties investigation and their application. Therefore, many efforts have been made to obtain freestanding COF or covalent organic polymer films.10 In particular, interfacial synthesis has been the promising way to obtain large area COF films with nanometer thickness. For example, Langmuir-Blodgett technique was introduced to prepare covalent organic monolayer on air/water interface based on specifically designed monomers.10a COFs or 2D polymer films can be synthesized at the interface of two immiscible solvents loaded with two monomers. 4b However, due to the entangled issue of solubility, diffusion, reaction, and crystallization, it is still challenging to obtain freestanding COF films with tunable thickness and crystallinity. Meanwhile, solid/liquid interface based method could generate crystalline and oriented COF films on specific substrate due to the templating effect of the solid sup-

port.10d However, it is not straightforward to achieve freestanding film by this strategy. The essential principle to obtain thin film by interface synthesis is to use interface as a confined reactor.11 Superspreading technique could provide a thin liquid layer as a confined reactor for controllable and stable environment for preparation of thin films. Superspreading means complete liquid spreading and homogeneous liquid layer formation at interface.12 Recently, a liquid/liquid/gel interface system was established to achieve superspreading liquid layer on the immersed gel surfaces.13 Compared to air/liquid or liquid/liquid interface, this tri-phase system could form an adjustable thin confined superspreading liquid layer between liquid and gel rapidly, and provide a robust interface for chemical reaction. In this liquid/liquid/gel tri-phase system, gels could be regarded as a liquid phase with capability to release monomer slowly, while owning solid-like behavior to provide robust support. Based these advantages of superspreading on immersed gel, we envision that this system could bring together the advantages of other interfaces for the preparation of high-quality free-standing COF thin films. In this work, we presented an ingenious strategy for confined synthesis of two-dimensional (2D) COF thin films within superspreading water layers. When dropping water droplets on hydrogel immersed in oil phase,

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Figure 1. (A)Schematic for the fabrication of thin COF film at hydrogel surfaces based on the confined superspreading water layers under oil. (B) Chemical structures of DHTA, TTA and COFTTA-DHTA. superspreading water layers emerge with the help of water-like hydrogel surface.13 By introducing amine monomers and aldehyde monomers into hydrogel and oil phase respectively, these reactants diffuse into the thin superspreading water layers to afford homogeneous COFs thin films. The imine-based COFTTA-DHTA system with pore size around 3.4 nm 14 (TTA: 4,4',4''-(1,3,5-triazine-2,4,6triyl)trianiline and DHTA: 2,5-dihydroxyterethaldehyde) was chosen as a model structure to grow thin films (Figure 1). COFTTA-DHTA films obtained on the surface of immersed hydrogel showed tunable thickness, homogeneous morphology, crystallinity, and large area only limited by the size of the hydrogel. Based on the freestanding state of COF films, mechanical properties of ultrathin COF films were evaluated, and nano-filter membranes were also successfully prepared. Moreover, a photoelectrochemical (PEC) sensor of ruthenium ions (Ru3+) was developed based on the COFTTA-DHTA films transferred onto indium tin oxide (ITO) slices. Furthermore, this strategy can be extended to the synthesis of other COFs and crystalline zeolitic imidazolate framework-8 (ZIF-8) thin film systems, showing generally used application potential.

EXPERIMENTAL PROCEDURES Preparation of PAAm hydrogel. Typically, an aqueous solution mixture of Acrylamide (Aam) (15 g), N,N’methylenebis(acrylamide) (MBAAm) (0.3 g), and ammonium persulphate (APS) (0.3 g) in 100 mL distilled water was prepared. The PAAm hydrogel was synthesized by radical polymerization for 5 min after the addition of N,N,N',N'-Tetramethylethylenediamine (TEMED) (300 μL) at room temperature. Then the obtained hydrogels were rinsed in abundant water to wash off the unreacted components and make sure the hydrogels were fully swollen.

Synthesis and transfer of COFTTA-DHTA thin films. 2,5dihydroxyterethaldehyde (DHTA) and 4,4',4''-(1,3,5triazine-2,4,6-triyl)trianiline (TTA) were synthesized from 1,4-dimethoxy-benzen and 4-nitrobenzonitrile, respectively, according to previous reported procedures.15 DHTA was dissolved in oil phase (tridecane). TTA was dissolved in 4 M acetic acid solution, and PAAm hydrogel was put into the solutions for 3 hours to realize the TTA-swollen hydrogels. The TTA-swollen hydrogel was firstly put into tridecane solution of DHTA at a depth of 1 cm. Then, 10 µL water was superspreading on the hydrogel surface and formed a confined water layer. After reaction 12 h, free-standing thin COFTTA-DHTA films were successfully fabricated at the oil/water/hydrogel interface. By controlling the concentration of TTA loaded in hydrogel and DHTA dissolved in tridecane from 20 µM to 0.5 mM, the thickness of the COFTTA-DHTA films ranging from 4 nm to 150 nm could be regulated. The synthesized thin COFTTA-DHTA film was stand on the surface of hydrogel. To get a clean film and transfer to other substrates, the hydrogel was pulled out from tridecane solution of DHTA, and put into class tridecane for 1 hour to remove free DHTA. Then the hydrogel was put into clean water. As prepared COFTTA-DHTA films were then floating on the water surface to get a freestanding COFTTA-DHTA film, and at the same time removing free TTA. Then, using different substrates such as Si wafer, SiO2 wafer, quazert and filter membranes to dredge up films by approaching the sample perpendicularly. After drying at heat stage, COFTTADHTA films on different substrates were finally put into tetrahydrofuran 12 h to further remove monomers. Synthesis of COFTpPA thin films. 1,3,5triformylphloroglucinol (TFP) was dissolved in oil phase (tridecane), and p-Phenylene diamine dihydrochlorie was dissolved in water and PAAm hydrogel was put into the solutions for 3 hours to realize the PA-swollen hydrogels. Like the process of COFTTA-DHTA film synthesis, the PAswollen hydrogels was firstly put into tridecane solution of TFP at a depth of 1 cm. Then, 10 µL water was superspreading on the hydrogel surface and formed a confined water layer. After reaction 12 h, free-standing thin COFTpPA films were successfully fabricated at the oil/water/hydrogel interface. The thickness of the PA films ranging from 1.8 nm to 200 nm could be also regulated by controlling the concentration of TFP and PA from 20 µM to 0.5 mM. Synthesis of thin ZIF-8 films. Zn(NO3)2•6H2O 1.17 mg and 2-methylimidazole 22.7 mg were dissolved in 8 mL and 80 mL water respectively. The water-swollen hydrogels was firstly put into tridecane at a depth of 1 cm. Then, the Zn(NO3)2•6H2O and 2-methylimidazole solutions were premixed, and 10 µL achieved mixture was superspreading on the hydrogel surface and formed a confined water layer. After a reaction 12 h, a free-standing thin ZIF-8 film was successfully fabricated at the oil/water/hydrogel interface. AFM indentation experiments of COFTTA-DHTA film. Young’s modulus of COFTTA-DHTA film was calculated from AFM indentation experiments. Firstly, a Si wafer was patterned with circular hole (230 nm in diameter, 200 nm in depth) array by reactive ion etching. Then, a 4.7 nm COFTTA-DHTA film obtained from hydrogel surface was trans-


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Journal of the American Chemical Society ferred onto the patterned Si wafer surface. To get forceseparation curves at each hole, an AFM topographical image of the suspended area was firstly obtained to move the AFM tip to center position of the film. Then a typical forceseparation curve was recorded by indentation experiment. Considering the hexagonal COFTTA-DHTA films owns threefold rotation symmetry and the suspended area has circular symmetry, respectively, we fit the force-separation data using the Schwering-type solution as16 F =σ 02Dπδ +E 2D

q 3δ 3 r2

2D

where δ is the separation, σ0 is the pretension, E2D is the 2D elastic modulus, r is the radius of the hole, and q is a dimensionless constant determined by Poisson’s ratio v (q=1/(1.05-0.15v-0.16v2)). Here, we don’t have the exactly value of v, but we take 0.3 as v from literature of similar COF system.17 Fitting force-separation curve could provide 2D the value of σ0 and E2Dq3/r2, and then the value of E2D and Young’s modulus (combining thickness value) could be calculated. PEC measurements. PEC measurements were performed with a home-built PEC sensing system. A 500 W Xe lamp was used as the light source. Photocurrent was measured on a CHI 630D electrochemical workstation (CH Instruments, Austin, TX). A conventional three-electrode system was using as sensing system: working electrode is a modified ITO glass electrode, a platinum wire as the auxiliary electrode, and a reference electrode is saturated calomel electrode. All the PEC experiments were carried out at room temperature. General characterization methods. Powder X-ray diffraction (PXRD) patterns were recorded on a PANalytical Empyrean Diffractometer operated at 40 kV and 40 mA with Cu Kα radiation (λ = 1.5416 Å) ranging from 2.5 to 35° with a speed of 2 °/min at ambient temperature. Films morphology, thickness and Young’s modulus experiments were studied by atomic force microscopy (AFM, Veeco Multimode). Morphology observation was performed using FEM-6700F scanning electron microscopy (SEM) at an accelerating voltage of 10 kV and JEM-2011F transmission electron microscopy (TEM) at an accelerating voltage of 120 kV. The SEM and TEM samples were prepared by transferring the films from air/water interface to copper grids without supporting film or Si wafers. Raman spectra were recorded with a Thermo Scientific DXR instrument, with 532 nm laser source, in the range of 1200 to 1800 cm1. The surface elemental composition was analyzed by the X-ray photoelectron spectroscopy (XPS) on the Thermo Scientific ESCALab 250Xi with 200W Al Kα radiation. Twodimensional synchrotron radiation grazing incidence wide-angle X-ray scattering (GIWAXS) were performed at BL14B beamline, Shanghai synchrotron Radiation Facility with a wavelength of 1.2387 Å to analyze the crystallinity and orientation within the COFs and MOFs films. 2D GIWAXS data were acquired by using a MarCCD with a distance c.a. 400 mm from the samples. UV−vis−NIR absorption spectra were recorded on a Shimadzu UV-2600 UV−vis spectrophotometer. Thermogravimetric analysis of the COF materials was carried out on a Mettler Toledo TGA/DSC 1.

Figure 2. (A) Pictures of COFTTA-DHTA films on SiO2 wafer. (B) Optical microscopy image of the COFTTA-DHTA film. (C) SEM images of COFTTA-DHTA film on TEM grids. (D) AFM height image of COFTTA-DHTA film on SiO2 wafer and the line profiles.

RESULTS AND DISCUSSION Synthesis process of COFTTA-DHTA thin films. The iminelinked COFTTA-DHTA thin films were synthesized from two monomers TTA and DHTA, as shown in Figure 1. Tridecane was chosen as an oil phase to form oil/water/hydrogel triphase system, due to its stability and safety. Polyacrylamide (PAAm) hydrogel was used as the immersed hydrogel phase to realize superspreading. Certain concentration of DHTA was dissolved in tridecane as oil phase, and TTA swollen hydrogel was placed in this oil phase, followed by forming superspreading water layers (Figure S1). Then, DHTA and TTA diffused to the thin water layer from tridecane and hydrogel respectively, thus confined synthesis of COFTTA-DHTA film was carried out by the Schiff-base reaction of these two monomers (Figure 1B). After reaction, a brick red, shiny thin film emerged at the oil/water/hydrogel triphase interface (Figure S3). The freestanding COFTTA-DHTA films were obtained onto various substrates for structural and property characterization following the transferring method (Figure S2). Figure 2A shows a typical COF films on silicon wafer with size up to centimeters. Characterizations of COFTTA-DHTA thin films. Characterizations of the obtained COFTTA-DHTA films by optical microscopy, scanning electron microscopy (SEM), and atomic force microscopy (AFM) were carried out after transferred on silicon wafers with oxide layer and transmission electron microscopy (TEM) grids (Figure 2). In optical microscope (Figure 2B), the COF film shows homogeneous light blue contrast on silicon oxide surface (dark purple area), indicating thin thickness of the film. A homogenous film suspended on TEM grid was observed in the SEM images (Figure 2C). The thickness of prepared film was measured to be 4.5 nm by AFM (Figure 2D). COFTTA-DHTA films were also characterized by Raman spectroscopy (Figure 3A). Comparing with monomers TTA



Figure 3. (A) Raman spectra of COFTTA-DHTA film, powder, and the corresponding monomers (TTA and DHTA) (B) TEM images of COFTTA-DHTA film on TEM grids. Inset: SAED pattern of the films (C) GIWAXS data of COFTTA-DHTA thin film on Si wafer. (D) Projections of GIWAXS data of COFTTADHTA films sets near qz = 0 (Black) and XRD data of COFTTADHTA powder (Red). and DHTA (curves blue and green), COFTTA-DHTA powders and films exhibit the similar Raman spectra (curves black and red), and emerged Raman shift at around 1597 cm-1, which corresponds to the stretching vibration mode of C=N- bond. Meanwhile, the shift at 1360 cm-1 in the spectrum of TTA corresponding to the stretching vibration of the β-ring and wagging of -NH2 groups, and the aldehyde stretching vibration of DHTA at 1673 cm-1,10a almost disappeared in the COFTTA-DHTA. These results indicate the Schiff base reaction of -NH2 groups and -CHO groups, and the covalent formation of imine-linkers in both COFTTA-DHTA film and powders. TGA curves of Peaks at 398.3 eV of N1s spectrum and 286.3 eV of C1s spectrum in XPS spectra of the COFTTA-DHTA films correspond to -C=N bond (Figure S5), which also confirms the formation of imine bonds in COFTTA-DHTA films. TEM images showed that the COFTTA-DHTA film exhibits locally ordered pore channels, which are consistent with the expected 2D structure of COFTTA-DHTA crystalline (Figure 3B). At the same time, hexagonal pattern emerged when selected area electron diffraction (SAED) experiment was carried out on the suspended film, (Fig. 3B, inset) which is consistent with the lattice geometry for the COFTTA-DHTA structure.14 Furthermore, two-dimensional synchrotron

radiation grazing incidence wide-angle X-ray scattering (GIWAXS) experiments verified the crystallinity of the COFTTA-DHTA films. Figure 3C showed 2D X-ray diffraction patterns obtained from a 17 nm COFTTA-DHTA film on Si wafer. It can be clearly found that the in-plane Bragg peaks intensities were significantly stronger than that at outplane. And at the same time, a weak diffraction (001) peak at around 1.85 Å-1 (Figure S6) emerge at around qxy = 0, and is not observed at inplane direction. The distribution of the Bragg peaks reveals a certain orientation of the COFTTA-DHTA film, as their c-axis orientations are centered on the surface of COFTTA-DHTA film. Projections of these data sets near qz = 0 (Figure 3D) give the peaks at 0.20, 0.35, 0.40, 0.53 and 0.70 Å-1 corresponding to the (100), (110), (200), (210) and (220) reflection planes,14 respectively. The projection data (Figure 3D) agree well with the COFTTADHTA powders XRD spectrum and the simulated pattern in eclipsed conformation from literature.14 The cell parameters calculated from the projection of GIWAXS data are a = b = 36.27 Å. Based on Debye−Scherrer analysis, the grain size of COFTTA-DHTA film on average is calculated as 27.3 ± 0.4 nm across two-dimensional planes. These results indicated the well-ordered structure and crystallinity of COFTTA-DHTA films obtained by the present method. Based on unique properties of hydrogel, COF films with tunable thickness and low roughness could be easily prepared within superspreading water layer. By increasing the volume of superspreading water, the thickness of water layer increase, thus the thickness of films changes in the same trend (Figure S7A). Meanwhile, by changing the concentration of monomers, films with thickness from 4 nm to 150 nm were obtained (Figure S7B). Using super-spreading water layer as reactor shows distinct features compared to liquid/liquid interface synthesis method. It is well-studied that solute diffusion can be retarded within and out of the hydrogel.18 The experimental results shows that TTA diffusion from hydrogel to water reaches equilibrium at time scale of an hour. (Figure S8). The slow release of monomers to the superspreading water layer is beneficial for the growth of crystalline COF. In fact, GIWAXS maps of COFTTA-DHTA films obtained from liquid/liquid interface at the same concentration didn’t show obvious diffraction rings. Secondly, As shown in Figure S9, the defects decrease as the reaction time increases, and uniform films typically form after more than 6 h. The quasi-solid hydrogel surface provides a robust interface to sustain long time film preparation process. Homogeneous films with large area could be obtained on hydrogel surface even under continuous stirring, while no COF films can be collected at liquid/liquid interface under stirring. Direct comparison of the COF films obtained at gel interface and liquid/liquid interface indicates that COF films synthesized at gel interface shows lower thickness and lower roughness (Figure S10). When comparing with the freestanding COF films prepared by other interfacial synthesis methods, this superspreading method can fabricate nanometer thickness COF thin films with crystallinity and orientation (Table S1). Mechanical properties of 2D COF thin films. Mechanical properties of 2D COF thin films is very important parameters for practical application. AFM nano-indentation


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Journal of the American Chemical Society cant inter-group deviation, due to the significant nonuniformity of COF powders suspension.

Figure 4. (A) Schematic of the COFTTA-DHTA film on patterned Si wafer. (B) Schematic of determination of Young’s modulus by AFM. (C) AFM images of a 4.7 nm COFTTA-DHTA film on the patterned Si wafer with 230nm holes. (D) Force-separation curve obtained at the center of the suspended COFTTA-DHTA film. has been an established method to measure the nanomechanical property of 2D inorganic materials such as graphene,19a MoS219b and WSe219c. The synthesis of freestanding ultrathin COF films makes it possible and facile to directly measure the mechanical property of the 2D organic COF thin film. We transfer the films onto the patterned Si wafer with periodic 230 nm diameter holes to form a freely suspended film sample (Figure 4).19 By carrying out AFM indentation experiments (details in supporting information), force-separation (deformation of film) curves were obtained at the center of suspended films. By fitting force-separation curve (solid red line) based Schweringtype solution, 2D elastic modulus E2D of a 4.7 nm COFTTADHTA film is calculate as 119.1 ± 2.9 N m-1, which corresponding Young’s modulus value is 25.9 ± 0.6 GPa. These data reveal a better mechanical property of COF film comparing with some polymer materials like polyimide and polydimethylsiloxane.19a PEC sensor of COFTTA-DHTA thin film. COF materials, especially those with 2D conjugated frameworks, have attracted great attention of optoelectronic applications. However, due to the poor solubility and processability of COF powders, it’s desirable to obtain uniform COF thin film for device integration. UV-visible spectra reveal obvious absorbance of COFTTA-DHTA film on ITO, providing a potential for the application as PEC sensor (Figure S11). We found the homogenous freestanding COFTTA-DHTA films transferred on ITO electrode exhibited obvious photoelectrochemical activity. The schematic experiment setup is shown in Figure 5A. As shown in Figure 5B, COFTTA-DHTA generate around 1.2 µA cm-2 photocurrent in the 0.1 M Na2SO4 solutions with 0.5 mM ascorbic acid as electron donors under white light irradiation, while blank ITO didn’t show obvious response. Meanwhile, the photocurrent of COFTTA-DHTA thin films exhibit good reproducibility, which benefits from the uniform topography. In contrast, electrodes made from COFTTA-DHTA powders show signifi-

Figure 5. (A) Schematic for photocurrent generation process of COFTTA-DHTA film on ITO electrode. (B) Photocurrent responses of blank ITO (red curve) and COFTTA-DHTA film electrode (blue curve), in the presence of 0.5 mM ascorbic acid with on–off illumination under Xe lamp irradiation. (C) Photocurrent responses of 50 nm COFTTA-DHTA film ITO electrode towards Ru3+ at increasing concentrations (from a to h, 0.01, 0.03, 0.10, 0.30, 1.0, 3.0, 10, 30 µM) in the supporting electrolyte of 0.1 M Na2SO4. (D) Relative photocurrents of 50 nm COFTTA-DHTA film electrodes in the presence of 10 µM of Ru3+ and 100 µM of each cations. Based on the photoelectrochemical activity, the COFTTAfilm could be used to fabricate PEC sensor. When immersed in Ru3+ solutions for 10 min, the photocurrent of COFTTA-DHTA film increased significantly. As shown in Figure 5C, the photocurrent increased with the increasing of Ru3+ concentration. The calibration plots show a linear relationship between the photocurrent increasing (Ip / I0, Ip and I0 is the photocurrent of electrode with and without Ru3+ incubation, respectively) and the concentration of Ru3+ in a range of 0.3 µM to 30 µM (Figure S12). According to previous literature,20 Schiff bases derivatives with nitrogen and oxygen as donor atoms could absorb Ru3+ ions to form Ru complexes. It is proposed that the imine-linkage based COFTTA-DHTA could form Ru complexes when immersed in Ru3+ solution. It is well-known that Ru complexes can promote the photoelectrochemical activity.21 Therefore, we suggest that the increasing of photocurrent exposed to Ru3+ solution could be attributed to the generation of Ru complexes. Furthermore, the selectivity of COFTTA-DHTA film ITO electrode was investigated. No significant response was obtained at 0.1 mM of interferences cations such as Al3+, Fe3+, Cu2+, Zn2+, Mg2+, Co2+, Mn2+, Cd2+, and K+. In contrast, the COF thin film incubation with 10 µM Ru3+ exhibited around 6 times increasing photocurrent response, showing excellent selectivity of the detection of Ru3+ (Figure 5D). Nanofilter membrane of COFTTA-DHTA thin film. Based on the porosity with nanometer size, good chemical stability and mechanical properties of the COFTTA-DHTA films, they could be applied for size-selective nanofiltration application when transferred onto the filter membranes (Figure DHTA



S13). We chose macro-porous cellulose acetate filters as support membrane to load large area 40 nm COFTTA-DHTA

Figure 6. (A) Optical microscopy image of the ZIF-8 film. Dark blue area is the SiO2/Si wafer, and the light blue area is the film. (B) AFM images of ZIF-8 film on Si wafer. (C) GIWAXS data of COFTTA-DHTA thin film on Si wafer. (D) Projections of GIWAXS data of ZIF-8 films sets near qz = 0. films to form hybrid filter membrane. (Figure S13A). Au nanoparticles (AuNPs) solutions were chosen as template solute. Considering the holes size of COF films (3.4 nm), the 4 nm AuNPs could be rejected while small dye molecule rhodamine B (RhB) could cross over the hybrid filter membrane. For 4 nm AuNPs, COFTTA-DHTA films exhibits rejection values as high as 97%, while the rejection value for RhB, was calculated smaller than 3%. These data show the size-selective capabilities for nanoscale selective filtration application. Preparation of other COF films. The confined synthesis of 2D COF thin films strategy could be extended to other system. TpPA (1,3,5-triformylphloroglucinol (TFP) and ρPhenylene diamine dihydrochlorie (PA) system22 is a typical enamine linking COF. (Figure S14) Using the same strategy, the large area, freestanding COFTpPA thin films was obtained successfully at the interface of oil/water/hydrogel, and also could be easily transferred to other substrates using the same method. Topography of COFTpPA films were investigated by microscopes (Figure S14). While transferred onto SiO2 wafer, the films can be clearly found with naked eye due to the good contrast. Under optical microscopy, these films showed homogenous topography, large area and clean surface. Suspended COFTpPA films could easily observed on TEM grids in the SEM images, and is composed by small COFTpPA particles. The thickness of COFTpPA film was measured to be as low as 1.8 nm by AFM. Raman and XPS characterization confirms the formation of the β-ketoenamine bond. XRD of the COFTpPA powders synthesized in water at room temperature showed crystallinity. HRTEM of the thin film gives the clear ordered lines with 0.34 nm distance referring to the stacking of the COFTpPA layers, which could provide an indirect evidence to confirm the orderliness of the COFTpPA film (Figure S15). Besides, several other Schiff-base based COF

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system films were also successfully prepared using same method (Figure S16). Preparation of ZIF-8 films. To further increase the scope of this synthesis strategy, we also applied it to the synthesis of crystalline metal organic frameworks (MOF) films. The classical Zeolitic Imidazolate Framework 8 (ZIF8),23 which could get crystalline products in water at room temperature easily, was chosen as the model system. Similarly, white shinny thin films of ZIF-8 appeared on the top of hydrogel. Optical microscope shows the homogeneous ZIF-8 films on the SiO2 wafer with large area (Figure 6A). SEM images revealed the ZIF-8 film was formed by small ZIF-8 microcrystalline (Figure S17A). And TEM images show similar topography with SEM image, while polycrystalline pattern emerged in the SAED pattern (Figure S17B). The thickness of ZIF-8 film was determined as around 100 nm by AFM (Figure 6B). Clear diffraction rings emerge in the GIWAXS map (Figure 6C), and projections of these data indicate peaks at 0.52, 0.73, 0.88, 1.02, 1.14, and 1.24 Å-1, corresponding to (011), (002), (112), (022), (013) and (222) reflection planes (Figure 6D), respectively, which is consistent with simulated ZIF-8 spectrum.23 The cell parameters calculated are a = b = c = 17.01 Å from the GIWAXS data. Meanwhile, the grain size of ZIF-8 film on average is calculated as 63.0 ± 0.2 nm according to Debye−Scherrer equation. The GIWAXS data provide direct evidence for the successful synthesis of ZIF-8. These data demonstrate that, in addition to COF, this new strategy could also be widely utilized in the preparation of thin film of other framework materials.

CONCLUSION In summary, we have demonstrated an ingenious strategy for confined synthesis of 2D COF thin films within superspreading water layer on immersed hydrogel. By the polymerization reaction of two monomers from oil and hydrogel in thin water layer, large area, thickness controlled, freestanding and crystalline COF thin films were successfully prepared. The topography and chemical structure of the imine based COF thin films were characterized by different techniques. Crystallinity and certain orientation of COF thin film was confirmed by GIWAXS measurement. Using AFM indentation, Young’s modulus of ultrathin COF films was determined as around 25 GPa. We demonstrated the thin films can be utilized as selective nanofilter membranes. Moreover, a COF film based PEC sensor of Ru3+ was successfully fabricated and showed good selectivity. This strategy could be extended to the synthesis freestanding homogeneous crystalline films of MOFs, showing general-used potential in preparation of thin film materials. Due to availability of the wide library of gel materials of different composition and properties, it is facile to further tune the diffusion of monomers, catalysts, and other parameters to improve the quality of COF materials, which is currently undergoing in our group.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:


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Journal of the American Chemical Society Experimental details and characterization supplementary Figures S1-S17 (PDF)

data,

and

AUTHOR INFORMATION Corresponding Author *Email: [email protected]; [email protected]

Notes The authors declare no competing financial interests.

ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Grant Nos. 21725306, 21433011 and 91527303), and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDB12020100).

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Confined Synthesis of Two-Dimensional Covalent Organic Framework

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