Porous Two-Dimensional Monolayer MetalâOrganic Framework

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Porous Two-Dimensional Monolayer Metal−Organic Framework Material and Its Use for the Size-Selective Separation of Nanoparticles Yi Jiang,† Gyeong Hee Ryu,†,§ Se Hun Joo,∥ Xiong Chen,† Sun Hwa Lee,† Xianjue Chen,† Ming Huang,†,§ Xiaozhong Wu,†,⊥ Da Luo,† Yuan Huang,† Jeong Hyeon Lee,∥ Bin Wang,† Xu Zhang,† Sang Kyu Kwak,∥ Zonghoon Lee,†,§ and Rodney S. Ruoff*,†,‡,§ †

Center for Multidimensional Carbon Materials (CMCM), Institute for Basic Science (IBS), Ulsan 44919, Republic of Korea Department of Chemistry, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea § School of Materials Science and Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ∥ School of Energy and Chemical Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan 44919, Republic of Korea ‡

S Supporting Information *

ABSTRACT: Rational bottom-up construction of two-dimensional (2D) covalent or noncovalent organic materials with precise structural control at the atomic or molecular level remains a challenge. The design and synthesis of metal− organic frameworks (MOFs) based on new building blocks is of great significance in achieving new types of 2D monolayer MOF films. Here, we demonstrate that a complexation between copper(II) ions and tri(β-diketone) ligands yields a novel 2D MOF structure, either in the form of a powder or as a monolayer film. It has been characterized by Fourier transform infrared, Raman, ultraviolet−visible, X-ray photoelectron, and electron paramagnetic resonance spectroscopies. Selected area electron diffraction and powder X-ray diffraction results show that the MOF is crystalline and has a hexagonal structure. A MOF-based membrane has been prepared by vacuum filtration of an aqueous dispersion of the MOF powder onto a porous Anodisc filter having pore size 0.02 μm. The porous MOF membrane filters gold nanoparticles with a cutoff of ∼2.4 nm. KEYWORDS: MOF, separation, planar complexation, size-selective, β-diketone

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INTRODUCTION Two-dimensional (2D) crystalline materials can be considered as being “infinite” in size in two dimensions, being freestanding, having a one atom- or monomer-unit thickness, and having periodically bonded frameworks that essentially lie in a plane.1−3 Graphene, as one of the most prototypical 2D materials, has attracted much attention due to its exceptional properties including high carrier mobility, ideal optical transparency, and wide potential application in membranes, electronics, and photonics.4−7 Other 2D materials such as protonic titanate8 and transition metal dichalcogenides9 are typically prepared by the exfoliation of the bulk layered materials available in nature.10,11 Layered covalent polymer crystals could also be synthesized and exfoliated to form monolayer films, but have the disadvantage that the as-formed films usually are nonuniform nanofilms with varying thicknesses and relatively small sizes.12−16 Chemical vapor deposition (CVD) is used to synthesize graphene and other 2D materials in a bottom-up approach;17−19 however, its drawback is that © 2017 American Chemical Society

many organic 2D materials start to decompose in such, typically, high-temperature procedures. Conventional liquidphase synthesis combined with the Langmuir−Blodgett (L−B) technique can serve as an alternative and mild bottom-up approach to synthesize 2D materials.20−22 The films formed at the water/air interface can have a large area and be easily transferred onto other substrates. To date, preparations of 2D polymer materials using the L−B technique have been reported, but these are limited to photoinduced cycloadditions of anthracene-based monomers,23,24 Schiff-base condensations,25,26 an alkyne−alkyne homocoupling reaction,27 and complexations between nickel(II) ions and bis(thiolene)s,28,29 metal ions and terpyridine ligands,20,30 or zinc(II) ions and dipyrrin ligands.31 Exploring new reactions for the synthesis of Received: July 14, 2017 Accepted: July 26, 2017 Published: August 9, 2017 28107

DOI: 10.1021/acsami.7b10228 ACS Appl. Mater. Interfaces 2017, 9, 28107−28116

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ACS Applied Materials & Interfaces

Figure 1. Synthesis of a 2D porous MOF structure based on the planar complexation of copper(II) ions and tri(β-diketone) ligands L1. Chemical structures of monomer L1 and the 2D MOF.

and tri(β-diketone) ligands L1 produces not only a porous MOF structure, but also large, porous, monolayer 2D MOF films when using the L−B technique (Figure 1). Filtration using porous membranes is relevant for the selective size separation of nanoparticles.38−41 Beyond commercial polymeric or ceramic membranes,42,43 graphene oxide paper and supramolecular membranes have been readily made by vacuum filtration.7,40 For both of these, water molecules locked in the membranes play an important role in the formation of the membranes via hydrogen bonds. MOF membranes are usually prepared using in situ growth and seed-assisted growth

2D polymer materials using the L−B technique is clearly of great interest. An important goal is to design and synthesize MOF structures based on new complexations to expand the range of MOFs.32−34 To date, 2D MOFs based on a planar complexation between copper ions and β-diketone ligands35,36 have yet to be reported, particularly a monolayer MOF film based on this type of complexation. (A 3D MOF based on the octahedral complexation of copper(II) ion with 1,3-diketone and pyridine moieties has been reported.37) We have found that a planar complexation between copper(II) ions 28108


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Figure 2. Structural characterizations of the 2D MOF powder. (a) FTIR spectra of L1 and the MOF powder. (b−d) Survey, Cu 2p, and C 1s XPS spectra of the MOF powder. (e) Raman spectrum of the MOF powder. (f) XRD patterns of the MOF powder: experimental (the black curve), Pawley refined (the red curve), their difference (the blue curve), and simulation using the staggered stacking consequence (the green curve).

methods, which are time- and energy-intensive.44 Inspired by graphene oxide paper and supramolecular membranes, we thought that a MOF membrane could be readily made by vacuum filtration of a dispersion of the MOF in the presence of water, and we have found this to be true. Here, we report the synthesis of a porous crystalline 2D MOF powder through complexation between tri(β-diketone) ligands L1 and copper(II) ions using wet chemistry. Also, a large-area monolayer MOF film was synthesized using the L−B method; the as-prepared monolayer film was 0.7 nm in thickness (as measured by AFM) and was transferred onto substrates including 300 nm SiO2-on-Si, quartz, and also holey carbon coated TEM grids. This film’s structure was characterized by Fourier transform infrared (FTIR), Raman,

ultraviolet−visible (UV−vis), X-ray photoelectron (XPS), and electron paramagnetic resonance (EPR) spectroscopies. A dispersion of the MOF powder was vacuum-filtered onto a porous Anodisc filter to produce a membrane, which was stable on the filter in the presence of a small amount of water. The porous nature of the MOF membrane allowed the size-selective filtration of gold nanoparticles. The filter cutoff for the nanoparticle diameter was determined to be ∼2.4 nm.

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RESULTS AND DISCUSSION

Synthesis and Characterizations of the 2D MOF Powder. An aqueous solution of copper(II) acetate was added to a methanol solution of monomer L1 at room temperature under ambient atmosphere, resulting in the rapid 28109


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Figure 3. Space-filling model and magnetic and porous properties of the MOF structure. (a) Space-filling model of the MOF in the staggered stacking configuration. To provide a clear view, top and bottom layers of 2D MOF are colored blue and gray, respectively. (b) EPR spectrum of the MOF powder at room temperature. (c) The isosurface of the spin density and the Mulliken spin for the MOF. The blue isosurface represents a spin density of 0.01 e/Å3, and the Mulliken spins are shown for Cu and O atoms. (d) N2 adsorption and desorption isotherm curves of the MOF powder. (e) Pore size distributions of the MOF powder.

precipitation of a blue MOF solid. The MOF powder was collected by filtration, washed with water and methanol, and dried under vacuum. Thermal gravimetric analysis (TGA) under N2(g) shows that the powder remains stable up to ∼200 °C (Figure S1). FTIR analysis of the monomer and the MOF powder was then performed. As shown in Figure 2a, the absorption bands of the carbonyl groups shift from ∼1600 cm−1 for L1 to ∼1560 cm−1 for the MOF, suggesting the formation of a copper(II) β-diketonate framework.45 XPS was conducted to study the surface elemental composition of the MOF powder. The survey spectrum reveals the presence of peaks for the C 1s, Cu 2p, and O 1s core levels (Figure 2b). The appearance of a Cu signal in the powder indicates that the addition of Cu(II) salts induces the

complexation of L1 ligands and copper(II) ions. The highresolution Cu 2p spectrum shows two peaks (Figure 2c) with binding energies of ∼934.5 and ∼954.5 eV, which correspond to the 2p3/2 and 2p1/2 levels, respectively. This result indicates that only one type of Cu atom occurs in the film in the absence of excess Cu atoms. For the C 1s, a COCu peak with a binding energy of ∼286.5 eV was observed, further proving the formation of a copper(II) β-diketonate framework (Figure 2d).46 In the Raman spectrum of the MOF powder (Figure 2e), the peaks at 461 cm−1 for CuO and 1597 cm−1 for COCu and phenyl groups were observed, highlighting the formation of copper(II) β-diketonate bonds.47 X-ray diffraction (XRD) analysis (Figure 2f) shows the crystalline nature of the MOF powder. Peaks located at 9.1°, 28110


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Figure 4. Morphology and spectroscopic characterizations of the monolayer MOF film. (a) AFM image of the 2D monolayer MOF film deposited on SiO2 (300 nm)/Si. (b) SEM image of the 2D monolayer MOF film suspended over a Quantifoil TEM grid. (c) TEM image of a multilayer MOF film. The insert is a SAED pattern of a multilayer film. (d) SERS spectrum of the MOF film. (e) Near-field intensity image of a MOF film obtained by nano-FTIR. (f) Nano-FTIR spectra of selected areas in spots 1−4. Scale bars: (a) 2, (b) 5, (c) 0.2, (e) 0.2 μm.

9.5°, 10.2°, 12.1°, 13.6°, 16.8°, and 23.8°, with d-spacings of 9.7, 9.3, 8.7, 7.3, 6.3, 5.3, and 3.7 Å, were observed which can be assigned to the (200), (22̅0), (020), (31̅0), (21̅1), (31̅1), and (51̅1) planes, respectively. In order to elucidate the periodic structure of the 2D MOF, density functional theory (DFT) calculations were carried out to obtain the minimum-energy structure of the 2D MOF. The XRD pattern (Figure 2f, green curve) calculated from a staggered stacking configuration is in agreement with the experimental result (Figure 2f, black curve). The negligible difference (Figure 2f, blue curve) between the simulated (Figure 2f, red curve) and experimental (Figure 2f, black curve) patterns indicates that the Pawley refinement

result is a good match to the experimental XRD result. The Pawley refinement suggests a hexagonal crystal system with a unit cell of a = 21.98 Å, b = 20.21 Å, c = 8.06 Å, α = 84.82°, β = 91.99°, and γ = 117.66°. The pseudo-Voigt profile function was used for the whole profile fitting, and the Berrar−Baldinozzi function was applied for the asymmetry correction during the refinement process. The values of the final residual factors RP and R WP were 0.71% and 1.30%, respectively. These observations suggest that this MOF has a 2D porous, rigid, hexagonal periodic structure, and has a staggered stacking configuration with a pore size of ∼0.75 nm as shown in Figure 3a. 28111


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homogeneous film several hundreds of micrometers wide lying on the surface of the 300 nm Si/SiO2 substrate. To determine whether the 2D MOF film was a monolayer, height profiling by AFM was performed in the tapping-mode. The AFM image (Figure 4a) was acquired over an area of 16.6 μm × 16.6 μm. The height profiling shows that the film has a smooth surface and is ∼0.7 nm thick, indicating that it is a monolayer film on the 300 nm Si/SiO2 substrate.25,26,28 We note that it is critical to control the surface pressure in the range ∼0.7−1.0 mN m−1 in the preparation of this film. The surface pressure should not be too high, because the monomers require sufficient space to undergo substantial structural rearrangement at the water/air interface; however, the surface pressure must be high enough to allow the structure to form at the air−water interface. As demonstrated in Figure S6, when the surface pressure was increased to be 4 mN m−1, films consisting of a few layers with thicknesses of ∼2 nm were observed. For electron microscope analysis, the as-formed monolayer film was transferred onto a Quantifoil TEM grid. Scanning electron microscopy (SEM) (Figure 4b) suggests that one homogeneous film is stable enough to be suspended over the 1.5 μm in diameter holes in the support film on the metal grid. Moreover, the film has many wrinkles possibly formed during the transfer and/or drying processes. In order to study the structure of the film, a selected area electron diffraction (SAED; in TEM) pattern was obtained from the film. Although a cryoholder was used to analyze a monolayer film at a temperature of −185 °C (Figure S7), the monolayer film was converted to an amorphous film under electron beam irradiation at 80 keV incident electron energy. However, multilayer films were observed to remain stable under electron beam irradiation (Figure 4c). The diffraction spots of the SAED at 0.61 and 0.38 nm match well with the minimum-energy structure as shown in Figure 3, and are assigned to the (21̅1) and (51̅1) planes. The SAED pattern of the multilayer film in conjunction with the XRD data and the simulation results clearly prove a hexagonal structure for the MOF. To attempt to obtain information on the chemical structure of the monolayer film, UV−vis, XPS, surface enhanced Raman scattering (SERS), and nano-Fourier transform infrared (nanoFTIR) spectra were recorded. As shown in Figure S8, the UV− vis spectrum of the MOF film shows a very strong absorption band centered at ∼300 nm, predominantly due to an intraligand charge transfer (ILCT). Compared with the corresponding peak of L1, this peak shows a red-shift of ∼30 nm, indicating the existence of a conjugated structure in the monolayer film. One absorption band appears in the region 400−550 nm, which is attributed to the ligand to ligand charge transfer (LLCT) and metal−ligand charge transfer (MLCT). A peak ranging from 550 to 650 nm is also observed, possibly due to the d−d transition of copper ions. These results indicate the formation of a conjugated MOF structure after the complexation of copper(II) ions and L1. The survey XPS spectrum of the MOF film shows C 1s, Cu 2p, Si 2p, and O 1s peaks (Figure S9). The intensity of the O 1s peaks is much stronger, and this is due to the Si/SiO2 substrate. For the Cu 2p region, two peaks corresponding to the 2p3/2 and 2p1/2 levels along with four satellite peaks were observed, demonstrating the absence of copper (0) or copper(I). These results are consistent with the XPS results on the powder samples, suggesting the similarity of the chemical compositions of the film and powder samples.

The EPR spectrum of the MOF powder at room temperature (Figure 3b) yields a relatively low gyromagnetic factor (g) value of ∼2.101, which is attributed to a delocalization of the unpaired electrons in the MOF structure. In order to further evaluate the observed magnetic property of the MOF, we computed the g values (i.e., dimensionless magnetic moment), spin density, and Mulliken spin for the Cu(II)-based MOF as well as an isolated Cu(II) acetylacetonate complex (Figure 3c, Supporting Information Section 3). The Cu(II) acetylacetonate complex, which is the smallest molecule containing a planar Cu−O4 square, shows g values of gxx = 2.033, gyy = 2.037, and gzz = 2.123, in good agreement with the reported experimental results gxx = gyy = 2.050 and gzz = 2.216.48 The two distinct g values are ascribed to the axial symmetry around the Cu(II) atom. It is worth noting that the g values of the Cu(II) acetylacetonate are relatively low because the unpaired electron of the Cu(II) ion is shared by four adjacent oxygen atoms, as shown in the spin density on the planar Cu−O4 square (Figure S3). More specifically, it is found that 48% of the spin is located at the Cu(II) atom and 12% of the spin is located at each O atom. In case of the MOF, each unpaired electron of the Cu(II) ion is also shared by four adjacent oxygen atoms, which is responsible for its relatively low g value (Figure 3c). The g value was calculated to be 2.069 which agrees well with the experimental results (g = 2.101). The porosity of the 2D MOF powder was investigated by measuring the nitrogen sorption of the activated MOF powder at 77 K (Figure 3d,e). A sharp uptake at low relative pressures (P/P0 < 0.1) was observed, indicating a microporous structure. The Brunauer−Emmett−Teller (BET) surface area and the total pore volume are calculated to be 171 m2 g−1 and 0.76 cm3 g−1 (P/P0 = 0.996), respectively. The pore size distribution was calculated using the quenched solid density functional theory (QSDFT) model and shows a major peak centered at 0.72 nm. This result is very close to the pore size of 0.75 nm calculated from the energy-minimum MOF structure as shown in Figure 3, further supporting the assigned structure of the 2D MOF. Given the existence of micropores with sizes of 0.72 nm, smaller than 1 nm, the CO2 sorption isotherm of the activated MOF powder was recorded at 273 K (Figure S4). The micropore surface area and the micropore volume were determined to be 438 m2 g−1 and 0.16 cm3 g−1, respectively. Synthesis and Characterizations of the Monolayer 2D MOF Film. Because the MOF consists of 2D periodic rigid films with the staggered stacking illustrated in Figure 3a, a corresponding monolayer film could be synthesized using the L−B method. A chloroform solution of L1 was slowly injected and spread over the water surface in an L−B trough using a microsyringe. After complete evaporation of the chloroform, both L−B trough barriers were compressed at a rate of 2 mm min−1 to a surface pressure of 0.8 mN m−1. In this condition, L1 was close packed into a dense film. An aqueous solution of copper(II) acetate was then very slowly injected into the water phase at a rate of ∼0.5 mL min−1. The diffusion of copper(II) ions from the water phase to the water/air interface produced 2D complexation of copper(II) ions with L1 at the interface. The reaction was completed after ∼7 h and resulted in the formation of a large-area monolayer film that was transferred onto substrates including 300 nm Si/SiO2 and quartz substrates. Optical microscopy (OM) and atomic force microscopy (AFM) were used to investigate the morphology of the asprepared MOF film. The OM image (Figure S5) shows a 28112


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Figure 5. Use of the MOF membrane for the size-selective separation of nanoparticles. (a, b) UV−vis spectra of aqueous solutions of (a) Au NP1 and (b) Au NP2 before and after filtration. The inserts in parts a and b are photographs of aqueous solutions of nanoparticles Au NP1 and Au NP2 before and after filtration. (c, d) TEM images of Au NP2 deposited on Pelco single layer graphene on holey silicon nitride (c) before and (d) after filtration. (e, f) The corresponding particle size histograms of parts c and d. Scale bars in c and d: 20 nm.

(1−3) on the film and one spot (4) on the substrate (measurement area for each spot: 100 nm × 100 nm) were chosen to record the FTIR spectra (Figure 4f). In comparison with the spectrum taken from spot 4, a significantly different peak centered around 1570 cm−1 was observed for spots 1−3. This peak is attributed to the formation of a copper(II) βdiketonate framework in this monolayer 2D MOF film; this peak was also observed by FTIR on a KBr pellet for the MOF powder (Figure 2a). Size-Selective Separation of Gold Nanoparticles Using the 2D MOF Membrane. A MOF powder dispersion in water/methanol (V/V = 1/1) was vacuum-filtered over a supporting Anodisc filter (pore size, 0.02 μm; efficient filtration

Raman spectroscopy has shown a sensitivity down to monolayer films and has been a powerful technique for characterizing 2D polymers.25,26 We performed SERS measurements to attempt to gain further insight into the chemical structure of the MOF monolayer film. In the SERS spectrum of the MOF monolayer film (Figure 4d; SERS was done by depositing the film onto the Si substrate “decorated” by Au nanoparticles per ref 49), a peak corresponding to COCu and phenyl groups appears at 1596 cm−1, indicating the existence of copper(II) β-diketonate bonds.47 As shown in Figure 4e,f, nanoscale-FTIR was used to investigate the MOF film. A near-field intensity image (Figure 4e) shows a MOF film lying on the Si/SiO2 substrate with some wrinkles. Three spots 28113



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area, 2.5 × 2.5 cm2) to produce a membrane with a thickness of about 50 μm determined by SEM. Water was then used to wash the membrane. The assembly was kept wet, and the MOF membrane was used directly for filtration experiments. The presence of water seemed to play a key role in stabilizing the membrane. If the membrane was completely dried, it became fragile and cracked. Given the 2D porous nature of the MOF, we investigated its use for the size-selective separation of gold nanoparticles. First, gold nanoparticles (Au NP1) with diameters of 4.4 ± 0.3 nm were vacuum-filtered through the MOF membrane, and the filtrate was collected. The filtrate was colorless as shown in the inset in Figure 5a (as compared to the deep red color of the original Au NP1 sample). UV−vis spectra (Figure 5a) were then recorded of the Au NP1 solutions before and after filtration. The peak located at ∼530 nm and assigned to the absorption band of Au NP1 disappears after filtration. These results suggest that Au NP1 species were mostly (or entirely) removed from the solution by vacuum filtration through the MOF membrane. In another set of experiments, gold nanoparticles (Au NP2) with smaller diameters of 1.4 ± 0.8 nm were filtered through a freshly prepared MOF membrane. These had a more polydispersed particle size distribution compared with Au NP1 (Figure S10), and during their filtration, it was observed that nanoparticles with relatively small diameters passed through the MOF membrane, whereas particles with relative big diameters were rejected. As a result, the color of the filtrate did not show an obvious change if compared to that of the solution before filtration (insert, Figure 5b). However, the light absorption of the filtrate decreased compared with that of the Au NP2 solution before filtration (Figure 5b). TEM images and their corresponding particle size histograms (Figure 5c−f) show that gold nanoparticles with diameters >2.4 nm are separated from the solution, and diameters of the nanoparticles in the filtrate are reduced from 1.4 ± 0.8 to 1.2 ± 0.4 nm (here, a range of measured diameters, rather than the error in measurement of the mean particle size, is being given). The filter cutoff was thus determined to be about 2.4 nm. These results demonstrate that the as-prepared 2D MOF membrane could be used to separate gold nanoparticles according to their sizes.

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EXPERIMENTAL SECTION

Methods. A Zeiss optical microscope (AxioCam MRc5) was used to characterize the morphology of the transferred monolayer film on a 300 nm SiO 2 -on-Si substrate. A Cary series UV−vis−NIR spectrophotometer (Agilent Technologies) was used to record the spectra of the ligand L1 and the MOF monolayer film on a quartz plate. A Bruker Dimension Icon AFM instrument was used to measure the thicknesses of the films on 300 nm SiO2/Si substrates. XPS data were collected using an ESCALAB 250Xi XPS (Thermo Fisher Scientific). TGA was conducted under N2 by heating to 800 °C at a rate of 5 °C min−1 on a TA Instrument Q 500 analyzer. Nitrogen and CO2 sorption analyses were carried out using a surface area and porosity analyzer (Micromeritics ASAP2020). The samples were degassed under vacuum at 100 °C for 10 h before sorption measurements. XRD was performed on a Rigaku SmartLab powder X-ray diffractometer. Nano-FTIR was performed on a neaSNOM microscope. EPR was analyzed at room temperature at 9.4 GHz on a Bruker ER075. TEM was carried out on an Advanced TEM (FEI Titan3 G2 60-300) at 80 kV. Raman spectroscopy (WITec micro Raman) was done with a 532 nm laser under ambient conditions. For SERS measurements, a Si substrate decorated with Au nanoparticles was prepared by a previously reported method.49 Au NP1 was purchased from Sigma-Aldrich. L150 and Au NP240 were synthesized as reported earlier. Synthesis of the 2D MOF Powder. A solution of copper(II) acetate (91 mg, 0.5 mmol) in 10 mL of deionized (DI) water was added to a solution of L1 (123 mg, 0.33 mmol) in methanol (20 mL) at room temperature in air while stirring. After the mixture was stirred for 24 h, a blue-green precipitate was formed and was collected by filtration. The precipitate was sequentially washed with DI water and methanol, and dried in vacuum at 120 °C for 24 h to give the 2D MOF powder with a yield of ∼96%. We note that, for nitrogen and CO2 sorption analysis, the sample was “processed” by washing with methanol and acetone several times, and was then dried under vacuum at 120 °C for 24 h. Synthesis of the 2D MOF Monolayer Film. A Langmuir− Blodgett trough (KSV 2000 System 2, KSV NIMA, Finland) placed on an antivibration table in a dust-free environment was used to make the monolayer film. A typical experiment was conducted as follows. A substrate such as 300 nm Si/SiO2 was immersed in the DI water in the trough, and a solution of monomer L1 in chloroform (1.0 mg mL−1, 100 μL) was carefully spread on the water surface with a microsyringe. After the organic solvent was allowed to evaporate for 20 min, compression was applied by the barriers at a rate of 2 mm min−1 until the surface pressure reached 0.8 mN m−1. A 6 mL portion of a freshly prepared aqueous solution of Cu(OAc)2 was then injected into the underlying water phase without disturbing the self-assembled monomer layer. After 7 h, the presubmerged substrate was pulled up vertically at a constant rate of 1 mm min−1. Subsequently, the film was carefully rinsed with chloroform and water, dried at room temperature, and examined using OM, SEM, and AFM. For TEM analysis, a Quantifoil TEM grid was carefully placed horizontally on the top of the film at the water/air interface, and clean printing paper was then put on the grid. After the grid was attached to the soaked paper, the paper was carefully pulled out of the trough and dried at room temperature. Synthesis of the MOF Membrane on the Anodisc Substrate. A solution of copper(II) acetate (4.5 mg, 0.025 mmol) in 10 mL of water was added to a solution of L1 (6.2 mg, 0.017 mmol) in methanol (10 mL), and the mixture was stirred for 24 h at room temperature. The mixture was then vacuum-filtered over a supporting Anodisc filter (pore size, 0.02 μm; efficient filtration area, 2.5 × 2.5 cm2) to produce a membrane. Before it was completely dried, the membrane was washed using 10 mL of DI water three times. The membrane was kept wet and was used directly for the size-selective separation experiments of the aqueous gold nanoparticle solutions (4 mL).

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CONCLUSION We have shown that the complexation of copper(II) ions with tri(β-diketone) ligands (L1) could be used to produce not only a crystalline MOF powder composed of staggered stacked 2D layers, but also, in separate work, a 2D monolayer film produced at a water/air interface using the L−B method. XRD analysis and SAED patterns, together with the simulation results using DFT, have demonstrated its crystalline nature and 2D periodic hexagonal structure. Nitrogen sorption measurements provide information on the microporous structure of the MOF powder. For the film obtained by the L−B method, OM, SEM, and AFM results have shown a freestanding, large, monolayer film with a thickness of about 0.7 nm. The MOF powder has been used to prepare a membrane on a supporting Anodisc filter by vacuum filtration. The as-prepared MOF membrane allows the filtration of gold nanoparticles with a cutoff diameter of about 2.4 nm. We believe that, in the future, metal ions and (β-diketone)-containing “building blocks” in this complexation could be replaced to yield a variety of 2D MOFs for tuning their structures and properties. 28114


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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.7b10228. TGA, computational details, CO2 sorption, optical micrograph image, additional XPS spectra, AFM and TEM images, and UV−vis spectra (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: ruoffl[email protected]. ORCID

Yi Jiang: 0000-0003-1080-5884 Se Hun Joo: 0000-0003-4507-150X Xiong Chen: 0000-0003-2878-7522 Xianjue Chen: 0000-0002-4757-7152 Da Luo: 0000-0002-9128-6782 Bin Wang: 0000-0001-9576-2646 Xu Zhang: 0000-0001-7320-4360 Sang Kyu Kwak: 0000-0002-0332-1534 Zonghoon Lee: 0000-0003-3246-4072 Present Address ⊥

College of Chemical Engineering, China University of Petroleum, Qingdao 266580, People’s Republic of China.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by IBS-R019-D1. Computational resources were used from CMCM, UNIST-HPC, and KISTIPLSI. We thank Neaspec GmbH (Germany) for providing the nano-FTIR measurement.

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