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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 25960−25966

Fabrication of MOF Thin Films at Miscible Liquid−Liquid Interface by Spray Method Xiao-jue Bai,† Dan Chen,† Lin-lin Li,† Lei Shao,† Wen-xiu He,† Huan Chen,† Yu-nong Li,† Xue-min Zhang,† Li-ying Zhang,† Tie-qiang Wang,† Yu Fu,*,† and Wei Qi*,‡ †

Department of Chemistry, College of Sciences, Northeastern University, Shenyang 110819, P. R. China Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, P. R. China

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‡

S Supporting Information *

ABSTRACT: Metal−organic frameworks (MOFs) are an intriguing class of porous crystalline inorganic−organic hybrid materials. The fabrication of oriented, crystalline thin film of MOFs is expected to open novel avenues to traditional applications and to serve myriad advanced technologies. Here, a facile spray-assisted miscible liquid−liquid interface (MLLI) synthetic strategy is carried out and reported under mild condition that utilizes miscible interface for the rapid and controllable fabrication of large-area free-standing MOF thin films. The methodology can employ various metal nodes and organic ligands to yield various high quality lamellar/ granulous MOF thin films at MLLI, which indicates the universality of the MLLI strategy. KEYWORDS: metal−organic frameworks, thin films, spray technology, miscible liquid−liquid interface synthesis, catalysis membrane reactor

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and so on.16−18 However, the intricate chemical per-treatments of substrates are inevitable to introduce additional functional groups for MOFs growth/deposition, which complicates the fabrication process and limits the scope of applications to certain types of MOFs or substrates. Alternatively, the indirect strategy commonly employs the liquid interface to fabricate MOF thin films, which is typically divided into two steps. First, the synthesis of free-standing MOF thin films takes place on the liquid interface between two phases via self-coordination of metal ions and organic linkers. Second, a free-standing MOF thin film is transferred to the target substrate. The liquid−liquid/liquid−air interfacial synthesis of MOF thin films has drawn great attention due to the fact that it is not limited to the surface properties of substrate. Since Ameloot et al. prepared HKUST-1 film at the water/1octanol interface for the first time,19 the interface synthetic approach has been applied to many different MOF structures, including ZIF-8,20 CoTCPP-py-Cu,21 and so on. 22−24 However, the currently reported liquid−liquid interfaces are mostly formed between immiscible organic/aqueous phase or organic/organic phase, which is difficult to control the orientation and thickness of the prepared MOF thin films

etal−organic frameworks (MOFs) have emerged as intriguing microporous crystalline coordination polymers that consist of metal ions or clusters connected by multifarious organic ligand groups.1 MOFs have been applied to a variety of fields because of their unique physicochemical properties such as ultrahigh specific surface areas originated from their porosity and highly ordered molecular structures.2−5 However, their inherent fragile and rigid characteristics seriously limit their processability, device fabrication and related practical applications.6 Indeed, it would be highly desirable to make MOFs into robust thin films in order that they could be widely used in various fields including chemical sensors,7 membrane reactors8 and other advanced nanodevices.9 Generally speaking, there are two strategies to prepare MOF thin films on supporting substrates: the direct and indirect approaches. The direct approach mainly utilizes solid surface as the starting point for the growth/deposition of MOF crystals. As early as 2005, Fischer and co-workers employed the solvothermal method to successfully synthesize MOF-5 thin film on COOH- modified gold substrate.10 Subsequently, Shekhah et al. demonstrated the controllable growth of HKUST-1 film via stepwise layer-by-layer method on COOH-/OH- modified substrate.11 Besides that, a series of methods have been applied to fabricate the substratessupported homogeneous MOF thin films, such as epitaxial growth,12,13 electrochemical deposition,14 gel-layer approach,15 © 2018 American Chemical Society

Received: June 12, 2018 Accepted: July 27, 2018 Published: July 27, 2018 25960

DOI: 10.1021/acsami.8b09812 ACS Appl. Mater. Interfaces 2018, 10, 25960−25966

Letter

ACS Applied Materials & Interfaces

Figure 1. (a) Schematic illustration of the fabrication process of CuBDC thin films by spray method. The photos: (b) CuBDC free-standing film, (c, d) CuBDC/hydrophobic filter membrane composite membrane.

ascribing to the heterogeneous nucleation.25 Actually, the liquid−liquid interface does not only exist between the immiscible solvents, but also widely appears between the miscible solvents. Just like layered cocktails constituted by different kinds of wine, when two miscible liquids with proper polarity and viscosity are contacted at an extremely low initial velocity, a relatively clear interface will appear and maintain for a period of time. The foremost reason is that there is no obvious irregular pulsation in the trajectory of liquid molecules but only the momentum exchange between adjacent liquid layers caused by the molecular thermal motion.26 That is, the laminar flow dominates the movement of the liquid contact surface, which is represented as a macroscopic stratification. The miscible interface has the advantages of both the interfacial reaction and the homogeneous nucleation, because its characterization falls between the homogeneous system and the two completely immiscible solutions system. Therefore, miscible liquid−liquid interface (MLLI) has excellent potential to controllably fabricate homogeneous thin film. In this work, a facile spray-assisted MLLI synthetic strategy is reported for the rapid and controllable fabrication of largearea free-standing MOF thin films at room temperature. Furthermore, the present work demonstrates that the synthesized MOF thin films were deposited onto hydrophobic filter membrane to form the MOF/filter composite membrane, which acts as a flowing membrane reactor and shows an outstanding continuous catalytic performance in the reduction of 4-nitrophenol (4-NP). Copper 1,4-benzenedicarboxylate (CuBDC)27 was selected as the model to illustrate the constructing approach. As demonstrated in Figure 1a, the atomized N, N-dimethylformamide/acetonitrile (DMF/CH3CN) (V/V = 1:2) solution of metal ions (Cu2+) was uniformly sprayed onto the surface of DMF/CH3CN (V/V = 2:1) solution containing ligand (BDC) with the flow rate of 1 μL/s, and then the blue substance

appeared at the surface of ligand solution after 30 s. A complete free-standing blue film floating on the liquid interface was observed after 50 μL of spray, as shown in Figure 1b. For a clearer observation, a crack was artificially cut on the film. In addition, the free-standing thin film can be deposited onto an arbitrary substrate by removing the residual reaction solution. Figure 1c, d indicates that the thin film was completely transferred onto the filter membrane. The membrane is macroscopically uniform and dense with good flexibility, which provides more possibilities for the applications of MOFs in various devices or reactors. The proposed fabrication method provides possibility for practical applications of MOF materials, because there is almost no limitation on the dimensions of MOF thin films,28,29 and spray procedures are easily industrialized. Scanning electron microscope (SEM) images (Figure 2a, b) show that the thin film consists of horizontally oriented nanosheets with square shapes (∼200 nm) and has a typical lamellar structure (∼610 nm), which leads to the orientation of the thin film. Moreover, the thickness of the thin film could be adjusted by controlling the amount of sprayed metal ions to the ligand solution. As shown in Figure S1, the thickness of MOF thin films is adjusted from 0.25 to 5.55 μm, which exhibits approximately positive linear dependence on the amount of sprayed metal ions within a certain range. Energydispersive X-ray (EDX) elemental map images of CuBDC thin films in Figure S2 indicate that Cu, O, and C dispersed homogeneously across the entire membrane. In addition, the utilization percentage of Cu ions could reach 90% suggesting the relatively high efficiency (yield of CuBDC thin film) of the proposed spray fabrication method. The chemical coordination interaction between Cu and COO− in CuBDC thin film is verified by Fourier transform-Infrared (FT-IR) spectroscopy. As shown in Figure 2c, the prepared sample lacks the signal belonging to free protonated acid groups (at around 1700 and 25961

DOI: 10.1021/acsami.8b09812 ACS Appl. Mater. Interfaces 2018, 10, 25960−25966

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ACS Applied Materials & Interfaces

Figure 2. (a) SEM top view image and (b) cross-section SEM image of CuBDC thin film.(c) FT-IR spectra of CuBDC thin film and H2BDC. (d) XRD patterns of CuBDC thin film and CuBDC simulation. (e) N2 adsorption/desorption isotherm and the pore-size distribution (inset) of CuBDC thin film. (f) Crystalline structure of CuBDC. Copper, oxygen, hydrogen, and carbon atoms are shown in brown, red, white and gray, respectively.

1280 cm−1) due to the complete coordination of H2BDC, and the sample shows the typical C−O−Cu stretching vibration band at 1110 cm−1. In addition, the characterization signals belonging to the asymmetric and symmetric stretching vibrations of COO− groups (1620 and 1395 cm−1) and the ring vibration of benzene (1507 cm−1) could also be observed in the IR spectrum of the film, showing the coordination interactions in CuBDC. As shown in Figure 2d, the XRD patterns of the proposed CuBDC thin film only exhibit two sharp peaks at 16.9° and 34.1° corresponding to [201] and [402] crystallographic planes of CuBDC, which are all perpendicular to the stacking direction of the nanosheets and to the pore openings (Figure 2f).30 None of other diffraction signals of its bulk counterpart are detected, which also confirms the 2D lamellar morphology of CuBDC. The N2 adsorption/ desorption isotherms of the CuBDC thin film (Figure 2e) are identified as type I at low and middle relative pressures (P/P0 < 0.7), which exhibits the typical feature of microporous materials. According to the IUPAC classification, the isotherm at high pressures (P/P0 > 0.7) could be assigned as H3-type

hysteresis loop, indicating the presence of interlaminar pores between the stacked nanosheets. The specific surface area of the CuBDC film is found to be at 145 m2/g, and the average pore size of the material is calculated at 6.5 Å with a relatively narrow pore-size distribution (Figure 2e). The nitrogen adsorption/desorption isotherm of bulk CuBDC is also provided in Figure S3, and the BET surface area of the bulk CuBDC reaches 483 m2/g. By comparison, bulk CuBDC has a higher adsorption capacity at low relative pressure due to its rich and well-developed internal micropores. However, CuBDC thin film exhibits stronger adsorption performance in the high relative pressure region, indicating that it has a larger external surface area. Two-dimensional lamellar CuBDC nanofilm has more active sites exposed on the outer surface rather than inside the topological structure. In our approach, the choice of solvents is critical for the formation of MOF thin films. A control experiment was carried out using pure DMF instead of the mixed solvent of DMF/ CH3CN, and only blue CuBDC floccule could be observed in this process (Figure S4). The control experiment results 25962

DOI: 10.1021/acsami.8b09812 ACS Appl. Mater. Interfaces 2018, 10, 25960−25966

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ACS Applied Materials & Interfaces

Figure 3. Versatility of the MLLI strategy to produce MOF thin films. SEM images of MOF thin films: (a) Cu(1,4-NDC), (b) Cu(2,6-NDC), (c) copper(BDC-NH2), (d) Cu2(DOBDC), (e) Co−Ni(BDC), and (f) ZIF-8 fabricated by MLLI synthesis strategy. Insets in a-f are corresponding XRD patterns.

indispensable conditions to fabricate MOF thin films at MLLI. To quantify this metastable MLLI, DMF/CH3CN (V/V = 1:2) as upper solution containing the copper acetate was sprayed onto pure DMF/CH3CN (V/V = 2:1), and the downward diffusion rate of Cu ions was detected by UV−vis absorption spectra at 0.1 cm below the miscible liquid−liquid interface, The corresponding results showed that Cu ions could slowly cross the interface into the lower phase after 3 h using the proposed spray method (more information see Figure S5b). Besides DMF/CH3CN, several other solvent systems, such as DMF/methanol, ethanediol/methanol, DMSO/DMF, and acetonitrile/ethanol etc., could also be applied to create such interface and to fabricate MOF film as shown in Figure S5c. In the present CuBDC thin film fabrication process, copper acetate as metal precursor could help to the deprotonation of H2BDC, and spray method could benefit the dispersion of the reactant molecules into very small particles, which both enhance the CuBDC crystallization rate. In addition, DMF is used as a reaction solvent and CH3CN is used as a cosolvent (1/2−2/1) to form a steady MLLI where the crystal tends to grow in the lateral direction without extra longitudinal growth to produce CuBDC nanosheets. Consequently, with the aid of surface tension and weak interaction between the construction units, the synthesis and assembly of MOF nanosheets occur simultaneously, leading to the formation of free-standing

suggest that the formation of CuBDC thin film may occur at the interface between the ligand and the metal ion solution. Unfortunately, it is rather difficult to observe directly this liquid−liquid interface, and another series of control experiments using Coomassie Brilliant Blue dyed solvent (without MOF precursors) were carried out to confirm the existence of such interface between the two miscible liquids. As shown in Figure S5a, it is feasible to use dyed DMF/CH3CN (V/V from 1:2 to2:1) as two-phase solvents to construct the liquid−liquid interface by spray method. Interestingly, the upper phase and the lower phase can be reversed from each other rather than being limited by the density within a certain range. However, the sprayed layer would directly penetrate into the solution, and the solvents mixed immediately, if pure DMF is used as the solvent. Besides the proper solvents, another important factor in the proposed MOF film fabrication process is the advantage of the spray method, which avoids the gradient diffusion phenomenon caused by directly mixing of the two precursor solutions (DMF/CH3CN) even via slowly dropwise addition. Thanks to the unique feature of the spray technology which could disperse the solution into extremely small droplets and reduce the interference to the interface upon the contact of the two phases, so that the liquid−liquid interface could maintain for a relatively long time. Therefore, the choice of proper solvents and the utilization of spray technology are two 25963

DOI: 10.1021/acsami.8b09812 ACS Appl. Mater. Interfaces 2018, 10, 25960−25966

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ACS Applied Materials & Interfaces

Figure 4. (a) Schematic illustration of membrane reactor for continuous reduction of 4-NP with CuBDC/hydrophobic filter membrane. (b) UV− vis absorption spectra of the outlet solution. (c) Conversion of 4-NP at different time.

CuBDC thin film at MLLI. The same MLLI synthesis methodology could be successfully extended to produce a variety of different MOF thin films via exploring different MOF building blocks and solvents. As shown in Figure 3a−f, similar nanosheet and particle MOFs could also be constructed into thin films, indicating that the proposed MLLI method is a universal strategy to fabricate MOF films with adjustable porosity and functionality. The free-standing CuBDC thin film could be transferred onto a porous filter membrane, and the composite could act as a high performance flowing membrane reactor for the continuous catalytic operation of 4-NP reduction. As shown in Figure 4a, the aqueous solution of 4-NP and NaBH4 was pumped through CuBDC/filter composite membrane using a peristaltic pump (the details can be found in the Supporting Inforation). Cu-MOF was reduced easily in the presence of NaBH4, and CuBDC thin film was converted into the porous film of Cu/Cu2O nanoparticles with the size at about 40 nm (Figure S6). Thanks to the porous structure and sufficient active site of the film, the 4-NP solution could be almost completely degraded after passing through the composite membrane, as shown in Figure 4b and Figure S7. The conversion of 4-NP remained above 80% without obvious deactivation even after 24 h of continuous operation (Figure 4c). Blank control experiment results have shown that the catalytic activity originates from Cu species (Figure S8). In conclusion, a facile spray-assisted MLLI synthetic method is utilized to prepare a series of free-standing MOF thin films under mild conditions. The MOF-film/filter composite membrane is designed and fabricated serving as an efficient flowing membrane reactor for the continuous 4-NP reduction to corresponding 4-AP. The present research provides a new concept for the fabrication of MOF thin films at MLLI, which

sheds light on the large-scale industrial production of large-area MOF films and their potential practical applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b09812. Experimental procedure, figures of miscible liquid− liquid interface, cross-section SEM images and EDX mapping images of CuBDC thin film, nitrogen adsorption/desorption isotherm of bulk CuBDC, 1H NMR spectrum of reactant (4-NP) and product (4-AP), reference experiment about influence of solvent on constructing MOF thin film, reference experiment about catalyst membrane reactor (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.F.). *E-mail: [email protected] (W.Q.). ORCID

Yu-nong Li: 0000-0002-0872-5976 Tie-qiang Wang: 0000-0003-3104-8421 Yu Fu: 0000-0003-0962-7152 Wei Qi: 0000-0003-1553-7508 Author Contributions

Y.F. and W.Q. designed and supervised this work. X.-j.B., D.C., H.C., and L.-l.L. carried out experiments. X.-j.B., W.-x.H., and T.-q.W. analyzed experimental results. X.-j.B. and L.S. drew schematics. X.-j.B., Y.-n.L., L.-y.Z. and X.-m.Z. wrote and modified the manuscript. 25964

DOI: 10.1021/acsami.8b09812 ACS Appl. Mater. Interfaces 2018, 10, 25960−25966

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ACS Applied Materials & Interfaces Funding

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W.Q. received funding from The National Natural Science Foundation of China (21761132010, 91645114, and 21573256). X.-m.Z. received funding from The National Natural Science Foundation of China (21503037). Y.-n.L. received funding from Fundamental Research Funds for the Central Universities (N160504002). L.-y.Z. received funding from Fundamental Research Funds for the Central Universities (N170503010). Y.F. received funding from Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm201804). T.-q.W. received funding from Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm201822). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support from the National Natural Science Foundation of China (21503037, 21761132010, 91645114, and 21573256), Fundamental Research Funds for the Central Universities (N160504002, N170503010) and Open Project of State Key Laboratory of Supramolecular Structure and Materials (sklssm201804, sklssm201822).

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ABBREVIATIONS MLLI, miscible liquid−liquid interface REFERENCES

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