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One-step asymmetric growth of continueous metal-organic framework thin films on 2-D colloidal crystal arrays: a facile approach towards multifunctional superstructures Limei Li, Xiuling Jiao, Dairong Chen, and Cheng Li Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.5b01817 • Publication Date (Web): 04 Apr 2016 Downloaded from http://pubs.acs.org on April 10, 2016

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Crystal Growth & Design

December 26, 2015 Dear Editor,

Enclosed please find the pdf version of our article entitled "One-step asymmetric growth of continuous metal-organic framework thin films on 2-D colloidal crystal arrays: a facile approach towards multifunctional supersturctures" with the kind request to consider it for publication in the journal Crystal Growth & Design. In this work, we represent a facile approach towards multifunctional MOF superstructures by asymmetric growth of continuous MOFs thin films on 2-D colloidal crystal arrays (CCAs) anchored at the air-solution interfaces, with which the control over spatial configuration, structural hierarchy, and overall dimensionality of MOF superstructures can be realized all at once. This interfacial growth method also endows MOF superstructures with an unprecedented transferability, which greatly facilitates the interfacing of MOF materials with other functional surfaces. We have demonstrated this by the construction of layered structures (including hybrid ones) that are promising for device applications. Taking advantage of the resulting periodic and hierarchical porous structures, the as-grown MOF superstructures lend themselves for efficient vapor sensing, size-screening of nanoparticles, and removal of dye molecules from aqueous solutions, and have exhibited a superior performance as compared to its unstructured counterpart. Therefore, this work does not only present an efficient route in well-organizing MOF nanocrystals at the meso/macroscopic scale but also provides an inspiring example of enriching the material performance of MOFs by shaping their physical forms. We believe this work falls within the scope of your distinguished journal and interesting for the journal readers. RECOMMENDED REVEIWERS Thomas Bein, [email protected] Bettina V. Lotch, [email protected] Paolo Falcaro, [email protected] Dirk E. De Vos, [email protected] Andreas Terfort, [email protected] GRAPHICS FOR WHICH COLOR IS REQUESTED Figure 3 and Figure 4

ACS Paragon Plus Environment


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The authors claim that none of the material in the paper has been published or is under consideration for publication elsewhere. Sincerely Yours, Cheng Li


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One-step asymmetric growth of continuous metal-organic frameworks thin films on 2-D colloidal crystal arrays: a facile approach towards multifunctional superstructures Limei Li, Xiuling Jiao, Dairong Chen,* and Cheng Li* National Engineering Research Center for Colloidal Materials, and School of Chemistry and Chemical Engineering, Shandong University, Shanda’nan Road 27, 250100 Ji’nan, Shandong Province, China.

ABSTRACT: In this work, we represent a facile approach towards multifunctional MOF superstructures by asymmetric growth of continuous MOFs thin films on 2-D colloidal crystal arrays (CCAs) anchored at the air-solution interfaces, with which the control over spatial configuration, structural hierarchy, and overall dimensionality of MOF superstructures can be realized all at once. This interfacial growth method also endows MOF superstructures with an unprecedented transferability, which greatly facilitates the interfacing of MOF materials with other functional surfaces. We have demonstrated this by the construction of layered structures (including hybrid ones) that are promising for device applications. Taking advantage of the resultant periodic and hierarchical porous structures, the as-grown MOF superstructures lend themselves for efficient vapor sensing, size-screening of nanoparticles, and removal of dye molecules from aqueous solutions, and have exhibited a superior performance as compared to its unstructured counterpart. Therefore, this work does not only present an efficient route in well-organizing MOF nanocrystals at the meso/macroscopic scale but also provides an inspiring example of enriching the material performance of MOFs by shaping their physical forms. Prof. C. Li, Shanda’nan Road 27, 250100 Ji’nan, China Phone: +86-053188362285 E-mail: [email protected]



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One-step asymmetric growth of continuous metal-organic frameworks thin films on 2-D colloidal crystal arrays: a facile approach towards multifunctional superstructures Limei Li, Xiuling Jiao, Dairong Chen,* and Cheng Li*

National Engineering Research Center for Colloidal Materials, and School of Chemistry and Chemical Engineering, Shandong University, Shanda’nan Road 27, 250100 Ji’nan, Shandong Province, China.

E-mail: [email protected], [email protected]

ABSTRACT: In this work, we represent a facile approach towards multifunctional MOF superstructures by asymmetric growth of continuous MOFs thin films on 2-D colloidal crystal arrays (CCAs) anchored at the air-solution interfaces, with which the control over spatial configuration, structural hierarchy, and overall dimensionality of MOF superstructures can be realized all at once. This interfacial growth method also endows MOF superstructures with an unprecedented transferability, which greatly facilitates the interfacing of MOF materials with other functional surfaces. We have demonstrated this by the construction of layered structures (including hybrid ones) that are promising for device applications. Taking advantage of the resultant periodic


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and hierarchical porous structures, the as-grown MOF superstructures lend themselves for efficient vapor sensing, size-screening of nanoparticles, and removal of dye molecules from aqueous solutions, and have exhibited a superior performance as compared to its unstructured counterpart. Therefore, this work does not only present an efficient route in well-organizing MOF nanocrystals at the meso/macroscopic scale but also provides an inspiring example of enriching the material performance of MOFs by shaping their physical forms.

1. INTRODUCTION Metal-organic frameworks (MOFs), also known as polymer coordination polymers (PCPs), are novel crystalline inorganic-organic hybrid materials with a well-defined porous structure.1,2 Compared with conventional porous materials, MOF features ultrahigh porosity, large internal surface areas, and flexible frameworks that enable rational synthesis. Consequently, MOFs have been shown to be useful in a wide range of areas, including but not limited to gas storage and separation,3,4 catalysis,5 sensing,6 opto- and micro-electronics,7 biomedicines,8 light-harvesting and energy transfer,9 and pollutants degradation.10 The progressive broadening of this list relies fundamentally on the development of MOF materials with novel properties and functionalities. In parallel with the effort to tailor frameworks at the molecular level,11,12 the construction of complex architectures with higher-order superstructures



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at the mesoscopic or macroscopic scale using MOF nanocrystals as building units has also drawn much attention of materials scientists because it opens a new opportunity to improve performance and generate novel functionality via design of the physical form rather than altering the chemical component.13-19 To date, a variety of MOF superstructures with different dimensionality have been synthesized toward various applications. By employing “soft” or “hard” templates including emulsions,20,21 interfaces,22,23 and polymer spheres,24,25 or self-organizing strategy,26 MOF superstructures with hollow interiors were prepared, and have demonstrated potential as reactors and capsules with improved catalytic performance and selective permeability in comparison with loose particulates.22,25 In two dimensions, the processing of MOFs into films and membranes is of paramount importance for many applications, such as sensing and separation.27-29 Various techniques for fabricating MOF thin films and membranes have been developed, including direct synthesis,30-34 seeded growth,35-37 layer-by-layer deposition, 38-40 and conversion from oxides.41-43 The ultimate goal is to realize MOF films with continuity (defect-free) and precisely tunable thickness, which can suit specific applications. For instance, an ideal separation membrane of MOF prefers homogenous structures with a relative small thickness so that high selectivity and low mass transport barriers can be achieved at the same time. Besides growth techniques, site-specific deposition of


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MOF thin film on a given substrate is also important because it is a prerequisite for fabricating miniature systems and devices.44,45 Not limiting to 2-D substrate, sitespecific deposition of MOF nanocrystals has been very recently extended to 3-D objects to form MOF-patched asymmetric microparticles.46 In addition to positioning and patterning, the introduction of hierarchy to MOF supersturctures is also intriguing to obtain novel functionalities. For example, Li et al. have incorporated 3-D ordered macropore arrays into a thick MOF film, which could act as dynamic photonic materials.47 However, a simultaneous control over the spatial configuration, structural hierarchy, and overall dimensionality of MOF superstructures is still challenging with existing synthetic strategies. Moreover, the realization of multifunction within a single MOF superstructure is desirable but has seldom been reported. Herein, we report on a facile yet versatile approach towards multifunctional MOF superstructures with highly ordered hierarchical structures by asymmetric growth of MOF thin films on 2-D colloidal crystal arrays (CCAs) anchored at the air-solution interfaces. 2-D CCAs usually consist of close-packed arrays of colloidal spheres and have been used as masks or templates to fabricate 2D ordered nanostructure arrays and macroporous films in processes termed “nanosphere lithography”.48 Recently, they have also been applied at the air-solution interface to prepare macroporous films of noble metal,49,50 metal sulfides,51-53 and calcium carbonate.54 Unlike previous



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methods,44-46 our strategy does not rely on surface functionalization to attain spatioselective deposition of MOFs on an object. The asymmetric growth of MOF thin films on 2-D CCAs is achieved with the assistance of the natural confinement of air-solution interfaces, since MOF growth only takes place in the solution phase (Scheme 1). In this scenario, 2-D CCAs act as “hard” templates that direct the growth of MOF thin films, leading to highly ordered structures. By modifying the growth condition, MOF films with great continuity and ultrathin thickness can be fabricated in only one simple step. Benefiting from this interfacial method, the as-prepared MOF thin films are highly transferable. Sequential transfer of monolayer films to a solid substrate can lead to layered architectures with controllable porosities and component, which efficiently expands this strategy to generate more complex 3-D superstructures from 2-D elements. Although MOF thin films have been successfully grown on various substrates (solid or porous ones), as far as we know, none of the previous works has demonstrated transferable MOF thin films in large areas, which is highly desirable for fabricating devices with layered architectures, especially hybrid ones. Our approach of asymmetric growth of thin-film MOFs on 2-D CCA directly leads to useful platforms for sensing and separation. Showing distinct and tunable photonic properties, the MOF-grown 2-D CCA films are capable of discriminating vapors based on the guest-uptake-induced refractive index (RI) change of MOFs. By


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removing 2-D CCAs, 2-D ordered macroporous (2DOM) MOF thin film with a highly uniform dual porosity was obtained, which can be applied for size-screening of nanoparticles and removal of dye molecules from aqueous solutions, respectively, representing their possible utility in separation at two distinctively different length scales. Therefore, this work should not only present an efficient route in wellorganizing MOF materials at the meso/macroscopic scale but also provides an inspiring example of enriching the material performance of MOFs by shaping their physical forms. As a prototypical MOF with permanent porosity and high thermal and chemical stability, ZIF-8 ([Zn(MeIM)2]n) (MeIM = 2-methylimidazole) characterized by the sodalite (SOD) zeolite-type structures with large cavities (ca. 11.6 Å) and small, flexible apertures (ca. 3.4 Å),55 was chosen in this work. Polystyrene (PS) spheres were chosen for 2-D CCA assembly at the air-solution interface as their negatively charged surfaces can facilitate MOF nucleation by anchoring metal ions. 2. EXPERIMENTAL SECTION

Chemicals and materials. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O, ≥ 99.0%), 2methylimidazole (MeIM, 99%) and poly(2-vinyl pyridine) (P2VP) (Mw = 159 kg/mol)

were

purchased

from

Sigma-Aldrich

and

used

as

received.

Sodiumdodecylsulfate (SDS, 99.0%) and organic solvents of analytical grade were



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purchased from Sinopharm Chemical Reagent and used without further purification. Ultrapure water (≥ 18.2 MΩ) was obtained from a Milli-Q Reference system. Monodisperse polystyrene (PS) spheres of 130, 380, 520 and 590 nm in diameter (standard deviation < 10%) were synthesized using an emulsion-free polymerization method.56 Thermal release tape (single coated adhesive type) was received from Nitto Denko Co. Polyvinylidene fluoride (PVDF) membrane filters ( pore size: 1 µm, diameter: 13 mm) were purchased from Beijing Haicheng Shijie Co.

Characterizations. The crystalline structure was measured by a Bruker D8 Advance X-ray diffractometer (XRD) in theta-theta geometry (Cu Kα radiation, λ = 1.5406 Å). The morphology was examined by a Zeiss Supra55 field emission scanning electron microscope (FE-SEM) equipped with a Bruker XFlash 6 ︱ 60 energy dispersive spectrometer (EDS) system. A thin layer of gold was sputtered on the samples for clearer images. Zeta potential was measured with a Beckman Coulter DelsaNano C particle size and zeta potential analyzer. Nitrogen physisorption isotherms were obtained from a Micromeritics ASAP2020HD88 adsorption system at 77 K. Static contact angle of water were measured on a Krüss DSA 10 drop shape analysis system at ambient temperature. Transmission spectra of the thin films in the visible region were acquired with an Ocean Optics USB2000 fiber optic


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spectrophotometer coupled to a Leica DM2700 M optical microscope. Absorbance of dye solutions was measured by a PerkinElmer Lambda 35 UV/Vis spectrometer.

Asymmetric growth of thin-film ZIF-8 on 2-D CCAs. In a typical experiment, 6.568 g MeIM and 1.859 g Zn(NO3)2·6H2O were dissolved in 100 mL methanol and 250 mL water, respectively, to form stock solution A and B. 2-D CCAs in an area of 1 × 1 cm2 were assembled on a glass substrate adopting a previous method,57 and rapidly transferred onto the surface of a freshly mixed solution of 4 mL A and 8 mL B by inserting the substrate into the solution with an inclining angle. The system was kept undisturbed at room temperature for 10 minutes to complete the growth. Then another clean substrate (either silicon wafer or glass slide) was inserted into the solution, placed below the as-grown floating thin film. The film was deposited on the substrate by lifting the substrate out of the solution vertically. The as-grown film was transferred onto the surface of a mixed solvent (water: methanol = 4:1) by inserting the substrate into the solvent with an inclining angle so as to wash off redundant ligands and particles. Before this 2 uL 10 wt% SDS was added carefully onto the solution surface with a pipette to lower the surface tension. Finally, the as-washed film was picked up by a clean substrate using the same procedure above and dried under a nitrogen flow.



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Fabrication of 2-D ordered macroporous (2DOM) ZIF-8 thin films and their layered architectures. To obtain the 2DOM ZIF-8 thin film, the ZIF-8 grown 2-D CCAs on a substrate were immersed in copious toluene to dissolve PS spheres. By repeating the process, i.e. ZIF-8 growth, film transfer and PS removal, a multilayer of desired layer numbers was obtained. The layered hybrid with P2VP was fabricated by depositing the P2VP layer and the 2DOM ZIF-8 thin film alternately on a substrate. The P2VP layer was prepared by spin-coating according to a literature method.58

Preparation of ZIF-8 nanoparticles and their thin films. ZIF-8 nanoparticles were synthesized according to a previously reported method.59 Stable colloidal suspensions of ZIF-8 were obtained by re-dispersing the product in 4 mL methanol using ultrasound for 30 min. To fabricate ZIF-8 thin films, such colloidal suspension was spin-coated onto a substrate (i.e. silicon wafer, glass slide or PVDF membrane) at 4000 rpm for 1 min.

Vapor sensing. Prior to sensing experiments, samples were evacuated under a dynamic vacuum at room temperature to remove entrapped solvent molecules. Then they were fixed inside a homemade PS flow cell (14 mm × 14 mm × 48 mm), through which a vapor saturated-nitrogen flow were passed. Different saturated vapors were obtained by bubbling ultrahigh purity nitrogen at a flow rate of 100 mL/min through


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vessels containing corresponding solvents at 293 K. Normal-incidence transmission spectra of the samples were recorded before and after vapor exposure.

Nanoparticle separation. An aqueous mixture containing 130 nm (0.0025 wt%) and 590 nm (0.0075 wt%) PS spheres was prepared and treated with ultrasonication for hours. Then a drop of this suspension was cast onto the as-prepared 2DOM ZIF-8 thin film supported by a silicon wafer. After drying at ambient conditions, the particle-loaded 2DOM thin film was brought into contact with an adhesive tape. By peeling off the tape, particles out of the pores were successfully separated from the ones in the pores.

Dye removal and separation. The 2DOM ZIF-8 thin film was first deposited on a PVDF filter to form an asymmetric membrane. For separation experiments, two pieces of such membranes were put face to face so as to prevent the ZIF-8 layer from destroying by solution flux. As a control membrane, unstructured ZIF-8 thin films composed of nanoparticles (ca. 80 nm in size) with an equivalent mass (6 µg) to the 2DOM thin films were also prepared in between two PVDF filters by spin-coating. For filtration experiments, the as-fabricated membrane was immobilized in a Swinnex syringe filter housing (0.50 cm2 filter area), through which a dye solution (0.010 mM, 0.30 mL) was driven using a 1.0 mL syringe pump at a flux rate of 1.1 L/h. The dye



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retention rate was determined by analyzing the absorbance at λ = 601 nm for MB and 471 nm for MO of the filtrates. 3. RESULTS AND DISCUSSION

3.1 Growth mechanism and characterization of MOF superstructures. The asymmetric growth of ZIF-8 thin film on the PS 2-D CCA template was performed using an in-situ crystallization method. Its success requires heterogeneous nucleation and rapid crystallization of ZIF-8 nanocrystals at the PS-solution interface. The ζ potential of PS spheres was measured to be -12.4 mV in a pure solvent and -3.40 mV in the Zn2+ solution, indicating Zn2+ could adsorb on the PS surface through electrostatic interactions. To quantitatively determine the amount of adsorbed Zn 2+ on PS surfaces, 2-D CCAs with a known weight were kept floating on the surface of the Zn2+ solution for different times and then taken out for energy dispersive spectrometry (EDS) measurement. The amount of adsorbed Zn 2+ on PS spheres were calculated to be 62.3, 80.5, 82.6 mg g-1 for a floating time of 1, 5, 10 min, respectively. It can be seen that the amount of adsorbed Zn ions increased with time, and the adsorption was very fast in the beginning of one minute and slowed sharply down with time, indicating a saturation of adsorption. While the adsorption of metal ions on PS spheres facilitates heterogeneous nucleation, rapid crystallization of ZIF-8 can be achieved by increasing the ligand content. It was reported that a large excess of MeIM


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could result in rapid crystallization processes, which was used for the synthesis of size-controllable ZIF-8 nanocrystals.59-61 It has been explained that MeIM can act both as a linker unit in its deprotonated form and as a stabilizing unit in its neutral form because enough neutral MeIM should terminate growth and stabilizing positively charged nanocrystals.59 In our experiment, we found that by using a 1:16 molecular ratio of Zn2+ and MeIM, ZIF-8 thin film with excellent uniformity was grown on the 2-D CCA (Figure 1), whereas only dispersed particles were obtained at lower molecular ratios of 1:2, 1:4, and 1:8 (Figure S1). This result indicates that a properly high rapid crystallization rate is required for the successful coating of PS spheres with ZIF-8 thin films consisting of nanocrystals. The morphology of both sides (sides facing the air and the solution) as well as the cross section of the as-prepared film was carefully examined using scanning electronic microscopy (SEM). For a large area, the triangular arrays of the template were well retained after ZIF-8 growth (Figure 1a, c). The PS spheres (590 nm in diameter) were found to be embedded in a continuous film with grain sizes ranging from 50 to 150 nm, and seemed to form a non-close-packed array (Figure 1b). This implies that when the 2-D CCA floats on the solution surface the meniscus lies above the tangent-point plane under the action of capillary force.49 Due to the suitably high rate of nucleation, crystal nuclei were uniformly formed on the PS surfaces that



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submerged in the solution, and grew subsequently upon absorbing the corresponding species. In the end, quasi-hemispherical shell formed on the PS spheres (Figure 1d, e). Although the growth period was controlled to be as short as 10 minutes, the as-grown nanocrystals were already well-faceted, indicating their inherent high crystallinity. Xray diffraction (XRD) patterns of the film as compared to the simulated data confirmed that the crystalline phase was ZIF-8 (Figure 1f). In the meantime, large quantities of nanocrystallites were formed in the bulk solution, which turned milky white rapidly. After removal of PS spheres by dissolving in toluene, ZIF-8 superstructures with highly ordered macropores were obtained in a large area (Figure 2). The crystallinity of ZIF-8 thin film is still present according to the corresponding XRD pattern (Figure 1f). Nitrogen adsorption-desorption isotherms of the as-obtained 2-D ordered macroporous (2DOM) ZIF-8 thin films indicated a BET surface area of 1240 m2 g−1 (Figure S2). Due to the intergrowth of ZIF-8 nanocrystals, the as-formed thin film showed great integrity. As viewed from the side originally facing the air, the quasihemispherical macropores were hexagonally arranged with a uniform aperture size of around 400 nm, which was smaller than the diameter of the original PS spheres because adjacent pores were not tangential (Figure 2a, b). The bottom of the pores, which did not reveal itself in the top-view SEM images due to the depth of focus, was


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clearly observed from the bird’s eye side-view of an edge of the film (Figure 2c, d). The macropores were interconnected via small windows on the walls as a result of the close packing of PS spheres. The thickness of the hemispherical shell was measured to be about 80 nm from the fracture SEM image (Figure 2d), which is roughly equivalent to a monolayer of constituent nanoparticles. ZIF-8 thin films with larger thickness can be readily obtained by repeating the growth process. It is worth noting that the inner walls of the macropores appear much smoother than the outer ones, indicating ZIF-8 nucleation was initiated closely at the PS surface, and therefore, resulting in a faithful inverse replica of the 2-D CCA template. Although various macroporous films have been fabricated with 2-D CCA as templates, this is the first MOF inverse opal monolayer that has been reported so far. Besides, this facile onestep growth method is much more convenient compared with previous methods to prepare ZIF-8 thin films on a substrate, and can be readily extended to other polymer substrates. For example, ZIF-8 thin film was also successfully grown on 2-D CCAs consisting of poly (styrene-methyl methacrylate-acrylic acid) (PS-MMA-AA) monodisperse spheres using the same method (Figure S3).

3.2 Versatility in fabricating layered architectures. Due to the flexibility of airsolution interface, the ZIF-8 grown 2-D CCAs can be readily transferred to arbitrary substrates, which enables the interfacing of ZIF-8 thin films with different surfaces.



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For instance, by sequential deposition on a solid substrate, vertically layered structures of 2DOM ZIF-8 thin films with either homogeneous or heterogeneous porosities can be easily obtained (Figure 3a, b). The macropores in individual layers remained intact and the as-formed multilayer exhibited a regular cross section, suggesting the utility of this method in structuring complex 3-D MOF superstructures. Thus we have successfully expanded our strategy to construct 3-D MOF superstructures from 2-D elements. Compared with conventional routes to prepare 3D ordered macroporus materials,62 our method allows for a precise control over microstructures, i.e. porosity, film thickness, and surface area, as well as the composition of individual layers. More importantly, it enables MOF thin films to be easily integrated with other functional surfaces. As a proof-of-concept, we prepared a poly (2-vinyl pyridine) (P2VP) gel/2DOM ZIF-8/ P2VP gel tri-layer structure with a sub-micrometer thickness (Figure 3c). Such vertically layered structures of MOFs and polymeric gels may function as stimuli-responsive films. These results demonstrate the exceptional robustness of the transferable MOF thin films in device fabrication, which circumvents the many challenges in direct growth of MOF thin films on surfaces carrying various chemical environments.


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3.3 Photonic sensing performance. Photonic structures are capable of modulating light at a specific wavelength that depends in part on their refractive index (RI), making them useful as RI-based sensors. Recently, MOF-incorporated photonic structures have been demonstrated as promising sensors for gases and vapors.6,47, 63-66 Sorption of analytes within the initially vacant pores will lead to a large increase in RI, which can be monitored optically from the wavelength shift of the photonic peak or dip. To this end, we characterized the possible photonic properties of our films by transmission spectra measurement. For the ZIF-8 grown 2-D CCAs, three major dips existed in the visible region at 444, 529, 627 nm and 504, 607, 721 nm for films prepared with 520 and 590 nm PS spheres, respectively (Figure 4a). If these dips are caused by the scattering of light, their wavelength position should obey the scaling rule of photonic crystals,67 and only depend on the PS diameter (D) when other parameters are fixed. To verify this, we plot the as-recorded transmission spectra vs. the dimensionless ratio of light wavelength to particle diameter λ/D, which could eliminate the particle size effect (Figure 4b). The dips for D = 520 nm and 590 nm almost overlapped at 0.85, 1.0 and 1.2, supporting their same physical origination of scattering. The slight discrepancy is caused by the coating of ZIF-8 layer. For the 2DOM ZIF-8 thin films, only one broad shoulder appeared in the spectra, which also



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results from their long-range periodic structure as compared to the unstructured ZIF-8 film (Figure S4). The sharp dips of the ZIF-8 grown 2-D CCA films make them very suitable for sensing as it is convenient to determine the spectra shift upon guest sorption-induced RI changes. The adsorption of vapor molecules in the pores of ZIF-8 will increase the effective RI and lead to a red shift of the transmission spectra, the extent of which depends on the amount of molecules adsorbed as well as its RI. The sorption behavior of ZIF-8 towards various vapors largely depends on their kinetic diameters, steric effects and chemical affinities. The exclusion of large-sized and low-affinity molecules from the pores provides the sensor with selectivity. Figure 4c and d shows the optical response of the ZIF-8 grown 2-D CCA film to various chemical vapors. Due to the hydrophobicity of ZIF-8, water can hardly enter the frameworks even though it is smaller than the size of the pore aperture (3.4 Å). As a result, minute shift was observed upon exposure to a saturated water vapor. For alcohols, it is known that both the kinetic diameter and RI increase as the carbon number increases. The extent of dip shift of the ZIF-8 grown 2-D CCA film in response to various alcohols was given in the order of 1-propanol > ethanol > 1-butanol > 1-pentanol > tert-pentanol. Owing to the flipping of imidazolate groups, alcohols of larger sizes can still enter the pores and cause distinct red shifts. Although butanol and pentanol have larger RI than


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methanol and pentanol, they caused less shifts of the transmission dip, indicating their amounts of sorption were very low owing to the size-exclusion effect of the pores. In comparison with 1-pentanol, tert-pentanol is sterically more demanding and more difficult to enter the pores, and hence resulted in a smaller shift. Despite of a similar molecular size and RI with methanol, acetonitrile gave rise to a much larger red shift than alcohols, suggesting a stronger affinity of the -CN group with the framework than the -OH group. As vapors are physisorbed on ZIF-8, such sensor could be rapidly regenerated by exposing it in a dynamic air or nitrogen flow, and therefore can be reused for many times without degrading. In comparison with existing photonic MOF sensors, our sensor features structural openness and a sub-micrometer thickness, which were reported to be helpful for the improvement of sensing performance.66 Moreover, it is among the few examples of photonic MOF sensors, whose fabrication process does not resort to the tedious layer-by-layer or cycled growth method,47 making it more attractive for further applications.

3.4 Separation membrane performance. The wettability of MOF membranes can directly relate to the permeability and flux of solvents and therefore is of great significance for their application in separation. It is known that the wettability of MOFs can be effectively altered by postsynthetic modifications of the frameworks,68 however, little attention has been paid on modulating their wettability via control over



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the morphology of MOF superstructures. Recently, Chin et al. have reported the tuning of ominiphobicity via morphological control of MOF functionalized surfaces.69 In the present case, the uniform structure of the 2DOM ZIF-8 film provides us with a unique chance to examine the pure effect of morphology on the wettability of MOF superstructures. For comparison, an unstructured ZIF-8 film of 300 nm thick was prepared by spin-coating a colloid of ZIF-8 (ca. 80 nm in size) on a silicon wafer (Figure S5). The contact angles of water drops on both kinds of ZIF-8 films were measured at three different spots. Surprisingly, without any chemical modification, the 2DOM ZIF-8 film showed a much smaller contact angle of 94.8° than that of the unstructured one, which is 111.3° (Figure S6). Such a decrease in the contact angle should be ascribed to the surface structure of the 2DOM film. The ordered arrays of uniform macropores are capable of generating capillary forces, under which the water droplet possibly turns to Wenzel’s state. The resultant less hydrophobic surface may be more favorable for separation in aqueous systems since it can enhance water permeability and flux.70 As far as we know, this is the first report on modulation of the wettability of MOF thin films by purely fashioning their physical forms, which expand the available methods of modifying the properties of MOFs. Different from existing MOF membranes, the 2DOM ZIF-8 thin film possesses a unique dual porosity that combines the macropores inherited from the PS template


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and the micropores of ZIF-8, both of which are uniform in size and regular in order. Such an attribute intrigues us to explore their possible utility for separation at two distinctively different length scales. On the micrometer scale, the macropores with uniform openings can be possibly used to separate particles by holding small ones in the pores while keeping large ones out. The closely packed “bowl” arrays can provide capillary forces for particles to be filtered and trapped. As a proof-of-concept, we placed a drop of an aqueous mixture of 130 and 590 nm PS particles on top of the film with a pore opening of ca. 400 nm. After water evaporation, large particles were totally excluded from the pores, and sat on top of the “bowls”, resembling marbles on the pitted-wood game board of Chinese checkers (Figure S7a, b). This result suggests that particles were driven by the capillary force to positions as low as possible on the film. Meanwhile, most of the small particles fell into the pores due to their smaller size. The separation of the large particles from the small ones on the 2DOM ZIF-8 thin film can be readily achieved by pressing an adhesive tape to the film surface and peeling off the tape (Figure S7c). As can be seen, only large particles were obtained on the tape (Figure S7d, e). Since the pore opening can be readily tuned by changing the template diameter, tailor-made 2DOM films are available for screening sizetargeted particles.



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On the molecular level, MOFs have shown great potential toward efficient separation due to their attractive properties of adsorption and molecular-sieving. In this respect, tremendous efforts have been put on growing MOF thin films on solid porous membranes or fabricating mixed matrix membranes using MOF nanoparticles as fillers for separation in gaseous or liquid systems.70-76 Because our 2DOM ZIF-8 thin films are highly transferable and can be conveniently deposited onto arbitrary substrates, the fabrication of asymmetric membranes, where thin selective layers provide high selectivity without sacrificing flux, is much easier to achieve. In this work, we loaded the 2DOM ZIF-8 thin films onto PVDF membrane filters and evaluated their utility in the direct removal of dyes from aqueous solutions. In order to examine the possible effect of superstructure on separation, we also deposited unstructured ZIF-8 thin films by spin-coating ZIF-8 nanoparticles of an equivalent weight to the 2DOM ZIF-8 thin film (6 µg) onto the PVDF membrane for filtration experiments. The morphology of both kinds of ZIF-8-PVDF membranes was carefully examined with SEM and compared (Figure S8). The superstructure of 2DOM ZIF-8 thin film was well-retained after transferring onto the PVDF membranes except for occasional cracks due to the unevenness of the PVDF surface. In contrast, ZIF-8 nanoparticles filled in the the pores of the PVDF membranes and the as-formed thin film was not as uniform as that on a smooth substrate. The


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retention of dyes for respective membranes was analyzed based on UV/Vis absorption spectra of the filtrates (Figure S9). As listed in Table 1, the retention rates for methyl blue (MB) and methyl orange (MO) are 84.0% and 34.4%, respectively, using the 2DOM ZIF-8 thin film-loaded PVDF membrane, which are higher than that of using the pristine PVDF membrane (53.8% and 24.5%) and the unstructured ZIF-8 thin film-loaded PVDF membrane (70.8% and 29.0%). This result indicates that the loading of ZIF-8 has effectively improved the efficacy of PVDF filters for dye removal. Moreover, the theoretical separation factor of the two dyes, i.e. the ratio of the individual retention rate, increased from 2.19 to 2.44 upon ZIF-8 loading. Imidazole rings of ZIF-8 can be considered as aromatic compounds that are capable of interacting with dyes via π-π stacking.77 Due to a higher content of aromatic rings, MB is supposed to be more strongly adsorbed by ZIF-8 than MO is (Figure S10), resulting in an increase in the theoretical separation factor as compared to the pristine PVDF. It is worth noting that the 2DOM ZIF-8-PVDF membrane has shown a higher retention rate for both dyes than the unstructured ZIF-8-PVDF membrane, though its specific surface area (8.5 m2 g-1 ) is smaller than that of the unstructured counterpart (19.6 m2 g-1 ) according to nitrogen adsorption-desorption isotherms (Figure S11). This suggests that the morphology of MOF superstructures does influence the



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efficiency of dye removal, probably by affecting the pathways of dyes during adsorptive separation. Next, we evaluated the actual separation efficacy towards MB and MO by driving their 1:1 mixture solution through respective membranes. As listed in Table 1, the retention of individual dyes decreased for all the membranes. Due to a largely reduced retention rate of MB from 53.5% to 39.2%, the evaluated separation factor decreased sharply from 2.18 to 1.64 for the pristine PVDF membrane. In comparison, both kinds of ZIF-8 loaded PVDF membranes showed an increase in the separation factor. Compared to the PVDF membrane loaded with unstructured ZIF-8 thin films, the one loaded with 2DOM ZIF-8 thin films exhibited superiority in both dye retention rate (65.4% vs. 52.3% for MB and 19.6% vs. 17.4% for MO) and separation factor (3.33 vs. 3.01). This result is mainly due to the maintenance of a high retention rate for MB by the macroporous ZIF-8 thin film, which probably relates to its well-organized superstructure that facilitates a more efficient adsorption of dyes. In combination with aforementioned results, the 2DOM ZIF-8 thin film may open up a unique opportunity for separation to occur on multiple length scales simultaneously. 4. CONCLUSIONS In conclusion, we have presented a novel process to prepare highly transferable MOF thin films with well-organized superstructures in a large area by asymmetric


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growth of MOF thin films on 2-D CCA template floating at the air-solution interface. Due to the versatility of this method, not only ZIF-8 monolayer thin film but also their layered superstructures and periodically vertically spaced ZIF-8/polymeric gel hybrids were obtained, which allows for the facile fabrication of multi-component devices and can be easily extended to other combinations of templates and MOFs. The ZIF-8 grown 2-D CCAs have shown tunable photonic properties and are capable of behaving as sensors for selected vapors, affording a new optical motif with ease of fabrication. The 2DOM ZIF-8 thin film has demonstrated a different wettability from the unstructured thin film, referring to a unique chance to modify their surfaces by morphological engineering. With a highly uniform hierarchical porosity, the 2DOM ZIF-8 thin films have shown intriguing potential in separation processes at multilength scales by integrating the capability of size screening by macropores with the intrinsic adsorptive properties of MOFs. The combined results illustrate an efficient way of creating multifunctional MOF thin-film devices and the possibility of enriching the material performance of MOFs by shaping their physical forms. Consisting of uniform particles with two sides of distinct surface chemistry and morphology, the MOF-containing Janus films also likely play interesting roles in catalysis, absorption and bioreactors, which is under our further investigation.



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Scheme 1. Schematic illustration of the asymmetric growth of MOF thin films on 2-D CCAs anchored at the air-solution interface and the fabrication of 2-D ordered macroporous (2DOM) MOF thin films.

Figure 1. a-e) SEM images of 2-D CCAs after asymmetric growth of ZIF-8 at the airsolution interface: the side facing the air (a, b), the side facing the solution (c, d), and the cross section (e). b, d are corresponding magnified views of a, c. f) XRD patterns of ZIF-8 grown 2-D CCAs (middle) and the 2DOM ZIF-8 thin film (top) as compared to the simulated data (bottom).


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Figure 2. SEM images of 2DOM ZIF-8 thin films obtained after removal of 2-D CCAs: the side originally facing the air at low and high magnification (a, b), oblique view of an edge at low and high magnification (c, d), and cross-sectional view of a fracture surface (e).



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Figure 3. Schematic and corresponding fracture SEM images of vertically layered architectures based on transferable MOF superstructures: homogeneous (a) and heterogeneous (b) macroporous multilayers, and a hybrid film of 2DOM MOF and P2VP gel (c). Inset is a magnified image with a scale bar of 300 nm.

Figure 4. Transmission spectra (a) and scaled (λ/D) transmission spectra (b) of ZIF-8 grown 2-D CCAs prepared with 590 (black line) and 520 nm (red line) PS spheres. c, d) Optical response of the ZIF-8 grown 2-D CCAs prepared with 590 nm PS spheres


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to various vapors: transmission spectra (c) and corresponding shift of the transmission dip (originally at 607 nm) (d).

Table 1. Dye removal and separation performance of the 2DOM ZIF-8 thin filmloaded PVDF membrane as compared to the unstructured ZIF-8 thin film-loaded and pristine PVDF membranes.

Membrane

Retention rate (%) Methyl blue Methyl orange Single 1:1 Single 1:1 solution mixture# solution mixture#

Separation factor Theoretical

Evaluated

pristine PVDF

53.5

39.2

24.5

23.9

2.18

1.64

unstructured ZIF-8 thin film loaded PVDF

70.8

52.3

29.0

17.4

2.44

3.01

2DOM ZIF-8 thin film loaded PVDF

83.9

65.4

34.4

19.6

2.44

3.33

# a 1:1 mixture of methyl blue and methyl orange

ASSOCIATED CONTENT

Supporting Information. SEM images, nitrogen adsorption-desorption isotherms, optical properties of 2DOM ZIF-8 thin films, contact angle results, separation of particles, UV/Vis absorption spectra, and adsorptive mechanism of dyes. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author



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*(C. L.) Email: [email protected] *(D. C.) Email: [email protected]

Funding Sources This work was supported by the National Natural Science Foundation of China (NSFC, Grant 21303095), the Independent Innovation Foundation of Shandong University (IIFSDU, Grant 2013TB001), and Taishan Scholars Climbing Program of Shandong Province (Grant tspd20150201).

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT L. L. thanks Dr. Yuguo Xia for structure simulation and Mr. Zhiming Han for technique support on chart plotting. The authors acknowledge Shandong analysis and test center for SEM measurement.

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Table of Contents Graphic and Synopsis

The

control

over

spatial

configuration,

structural

hierarchy,

and

overall

dimensionality of MOF superstructures was realized all at once by asymmetric growth of thin-film MOFs on 2-D colloidal crystal arrays (CCAs) anchored at the air-solution interfaces. The versatility of the obtained MOF superstructures were demonstrated by the construction of hybrid layered architectures, vapor sensing, and separation of nanoparticles or dyes.


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Supporting Information One-step asymmetric growth of continuous metal-organic framework thin films on 2-D colloidal crystal arrays: a facile approach towards multifunctional superstructures Limei Li, Xiuling Jiao, Dairong Chen,* and Cheng Li*

Figure S1. Low and high-magnification SEM images of 2-D CCAs before (a, b) and after asymmetric growth of ZIF-8 at the air-solution interface using 1:2 (c, d), 1:4 (e, f), and 1:8 (g, h) molecular ratios of Zn2+and MeIM.



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Figure S2. Nitrogen adsorption-desorption isotherms (77K) of 2DOM ZIF-8 thin films.

Figure S3. SEM images of ZIF-8-grown 2-D CCAs consisting of poly (styrenemethyl methacrylate-acrylic acid) (PS-MMA-AA) monodisperse spheres: the side facing the air (a), the side facing the solution (b), and the cross section (c).


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Figure S4. Transmission spectra of 2DOM ZIF-8 thin films prepared with 590 nm (black line) and 520 nm (red line) PS spheres as compared to the unstructured ZIF-8 thin film (blue line).

Figure S5. SEM images of the unstructured ZIF-8 thin film prepared by spin-coating ZIF-8 colloids on a silicon wafer: (a) top view at low magnification; (b) crosssectional view at high magnification. Inset: top view at high magnification with a scale bar of 200 nm.



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Figure S6. Static contact angle of water on the unstructured ZIF-8 thin film (a) and the 2DOM ZIF-8 thin film (b).

Figure S7. a, b) Low and high-magnification SEM image of 590 and 130 nm particles scattered on the 2DOM ZIF-8 thin film after solvent evaporation. c) Schematic illustrating the separation of particles on the 2DOM ZIF-8 thin films assisted by an adhesive tape. d, e) Low and high-magnification SEM images of the 590 nm particles on the adhesive tape.


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Figure S8. Low and high-magnification SEM images of pristine PVDF membrane (a, b), unstructured ZIF-8-loaded (c, d), and 2DOM ZIF-8-loaded PVDF membranes (e, f).



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Figure S9. UV/Vis absorption spectra of methyl blue (a), methyl orange (b) and their 1:1 mixture solution (c) before (black line) and after filtration using 2DOM ZIF-8loaded (green lines), unstructured ZIF-8-loaded (blue lines), and pristine PVDF membranes (red lines).


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Figure S10. Schematic illustration of π-π stacking interactions between imidazole rings of ZIF-8 and dye molecules.

Figure S11. Nitrogen adsorption-desorption isotherms (77 K) of 2DOM ZIF-8-PVDF (a) and unstructured ZIF-8-PVDF membranes (b).


One-Step Asymmetric Growth of Continuous Metal ... - ACS Publications

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