Polycationic Polymer-Regulated Assembling of 2D MOF Nanosheets

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Polycationic Polymer-Regulated Assembling of 2D MOF Nanosheets for High-Performance Nanofiltration Huixiang Ang and Liang Hong* Department of Chemical and Biomolecular Engineering, National University of Singapore, Singapore 117585, Singapore S Supporting Information *

ABSTRACT: Herein, a two-dimensional metal−organic framework (2D MOF) made of iron porphyrin complex (TCP(Fe)) interconnected with divalent metal ion (M = Zn, Co, and Cu) is used to construct a selective layer, which is explored as an ultrafast and energy-saving nanofiltration (NF) membrane for removing organic dyes from water. Among the layered 2D M-TCP(Fe) membranes, Zn-TCP(Fe) membranes display the highest water permeance, which is 3 times higher than graphene-based membranes with similar rejection. To further improve the separation performances, we utilize polycations to anchor the periphery carboxylic groups of nanosheets, regulating the assembly of 2D Zn-TCP(Fe) nanosheets to produce a new class of crack-free selective layer possessing ultrathin and highly ordered nanochannels for efficient NF. Benefiting from these structural features, our polycation-regulated 2D Zn-TCP(Fe) membranes could offer ultrahigh permeance of 4243 L m−2 h−1 bar−1 (2-fold higher than its pristine) and excellent rejection rates (over 90%) for organic dye with size larger than 0.8 × 1.1 nm. This permeance value is about 2 orders of magnitude higher than the commercial polymeric NF membrane. Additionally, the membranes demonstrate 20−40% salt rejection. KEYWORDS: two-dimensional, metal−organic framework, polycation, assembly, nanofiltration

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focused on the graphene family,14−20 while less attention has been paid to the exfoliated dichalcogenides,21−23 MXenes,24,25 and metal hydroxides.26 The application of these 2D materials for NF is often limited by low-density perforation and uneven distribution of interplane distance, while many attempts have been made in developing chemical or physical approaches to control the interlayer spacing distance of 2D nanosheets to improve the separation performances.17,27,28 Nevertheless, those methods are strenuous to obtain narrow-sized interplane distance and nanosized thick film for both excellent separation and high permeability. For instance, the intrinsic electrostatic repulsive force between the anionic oxygenated group of graphene oxide (GO) nanosheets leads to low stacking density, leaving large interlayer gaps for probe solutes to pass through the selective layer.29,30 As a result, more and more 2D nanosheets are added onto this existing selective layer to compensate for the rejection rates at the cost of water permeance. Additionally, 2D MOF material is proposed to be an alternative candidate for making efficient NF membrane because it possesses plentiful and regular in-plane nanopores, which cannot be found in the state-of-the-art, for example, graphene-based membrane. For now, 2D MOF membranes are mainly used for gas separation,31−34 but reports on water

INTRODUCTION Over the past few decades, the membrane-based separation using the NF technique has become increasingly essential to substitute the conventional thermal separation techniques (i.e., distillation and evaporation) due to their high energy efficiency and cost-effectiveness.1,2 However, there are several limitations such as the high operating pressure, poor stability, and low water permeability. For example, the commercial NF membrane made of poly(ether sulfone) suffers from slow water permeance (ca. 10 L m−2 h−1 bar−1) because it partly operates at high pressure (up to 40 bar), leading to large energy consumption and subsequently high operational cost.3,4 Typically, NF is governed by (1) physical adsorption5 or chemical interaction (i.e., cation−π interaction)6 between solute molecules and the surfaces of mass transport channels, (2) molecular sieving controlled by size exclusion,7 and (3) the Gibbs−Donnan effect (i.e., electrostatic repulsion)8 between charged solutes and charged groups anchored to the membrane. These mechanisms are interrelated, and the last two modes are more significant to NF processes.9,10 The use of 2D nanosheets as elementary blocks to construct membranes is emerging as a new-generation material for water purification by NF because their unique intrinsic structures (i.e., in-plane pores/defects and interlayer spacing) could render effective nanosieving, enabling selective transport of molecules to permeate through the nanochannels of the scaffold.11−13 Previous studies on 2D membranes for NF are primarily © 2017 American Chemical Society

Received: June 12, 2017 Accepted: July 28, 2017 Published: July 28, 2017 28079

DOI: 10.1021/acsami.7b08383 ACS Appl. Mater. Interfaces 2017, 9, 28079−28088

Research Article

ACS Applied Materials & Interfaces

sheets. Subsequently, the 2D Zn-TCP(Fe) nanosheets suspension was added into the polycationic polymer solutions, where the concentrations of PEI and PDDA (0.1 wt %) are well below the reported overlap concentrations of polymer coils of these two polymers in aqueous medium,39,40 and hence both polymer molecules are presented as independent coils that have the radius of gyration (Rg) for molecular mass of 105: PEI = 16 nm and PDDA = 24 nm. Besides, the 2D Zn-TCP(Fe) sheet has roughly an average lateral length of 1.2 μm. As this is substantially larger than Rg of either polycationic polymer, stacking interactions drive regulated assembly of 2D nanosheets must be the sole manner. In this polycation-regulated assembly of nanosheets, the peripheral edge of each nanosheet, bearing free carboxylic groups, is ionically cross-linked by polycation coils (i.e., PEI or PDDA). We propose that the assembly of 2D Zn-TCP(Fe) nanosheets is driven by a combination of molecular attractive forces (i.e., polycation interacts with anionic 2D MOF nanosheets) and compressive force (i.e., pressure drop via vacuum filtration), as depicted in Figure S1 (Supporting Information). As a result, the polycation-regulated 2D Zn-TCP(Fe) assemblies were deposited on ultrafiltration support (i.e., nylon) to form a selective layer for molecular separation. For comparison purposes, similar 2D M-TCP(Fe) (M = Cu and Co) nanosheets were synthesized by replacing the Zn node with Cu and Co nodes, respectively (see the Experimental Section for details). The obtained 2D MTCP(Fe) (M = Zn, Cu, and Co) MOFs were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), transmission electron microscopy (TEM) image, and energy-dispersive X-ray spectroscopy (EDX). The XRD pattern (Figure 2a) recorded from

separation are truly rare, for the reason that most of the MOF materials are water-unstable. Recent studies have demonstrated that MOF-like membranes constructed from the zeolitic imidazolate framework (i.e., ZIF-8) are water-stable, and thus they are intensively scrutinized for water purification via NF. However, the formation of these ZIF-8 crystallites is often randomly oriented such that a thick selective layer of about 1 μm is required to achieve desirable rejection rates.35,36 In this study, we select the TCP(Fe) complex to construct nanosized thick 2D M-TCP(Fe) (M = Zn, Co, and Cu) selective layers for NF of aqueous solution because it possesses superior structural stability over most of the metalloporphyrin complexes, and the resulting M-TCP(Fe) nanosheets exhibit strong stacking ability.37,38 Our preliminary results show that various metal nodes in the 2D M-TCP(Fe) selective layers have different effects on the water permeance, and the one that contains Zn node exhibits the highest water permeability. Further enhancement of separation performances can be achieved when polycations such as polyethylenimine (PEI) and poly(diallyldimethylammonium chloride) (PDDA) are used to cross-link the periphery carboxylic groups of 2D ZnTCP(Fe) nanosheets, which regulate the assembly of 2D MOF nanosheets, resulting in the formation of ultrathin (thickness: ca. 48 nm), highly ordered, and continuous film. The asprepared polycation-regulated 2D Zn-TCP(Fe) membrane presents an ultrafast water permeance up to 4243 L m−2 h−1 bar−1 (2-fold higher than its pristine) with a rejection rate over 90% for small organic dyes (i.e., 0.8 × 1.1 nm) and 20−40% salt rejection at a low operating pressure of 0.01 bar. For the first time, such superior nanofiltration performances at such a low operating pressure are obtained as compared to the literature (Table S1, Supporting Information).

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RESULTS AND DISCUSSION The fabrication of polycation-regulated 2D Zn-TCP(Fe) membrane is illustrated in Figure 1. To illustrate our synthesis procedure, the Zn-TCP(Fe) MOF is used as an example. First, 2D Zn-TCP(Fe) nanosheets were prepared by using the softtemplate assisted methods.37 The ultrathin 2D Zn-TCP(Fe) nanosheet is made up of TCP(Fe) complex interconnected with the Zn2+ ions, forming a periodic porous framework with anionic carboxylic pendant group at the periphery of nano-

Figure 2. (a) XRD pattern. (b) XPS C 1s spectrum. (c) XPS N 1s spectrum. (d) XPS Fe 2p spectrum. (e) XPS O 1s spectrum. (f) XPS Zn 2p spectrum of the 2D Zn-TCP(Fe) nanosheets.

the as-prepared Zn-TCP(Fe) nanosheets resembles that of the previously reported by Zhang et al.,37 indicating that we have successfully synthesized Zn-TCP(Fe) MOF. The chemical structures of the Zn-TCP(Fe) nanosheets were further examined by XPS analysis (Figure 2b−f). The XPS C 1s spectrum shows the peaks at binding energies of 284.3, 285.3, 286.3, and 287.7 eV, which are attributed to the CC, C−C/ C−H, C−N/C−O, and CN/CO bonds, respectively. This series of carbon species matches the carboxylic and pyrrolic functional groups in Zn-TCP(Fe) nanosheets (Figure 2b). The XPS spectra of the N 1s and Fe 2p regions reveal that the signals of the Fe−N (399.6 eV), pyrrolic (400.3 eV), Fe0 2p3/2

Figure 1. Schematic illustration for the fabrication of polycationregulated 2D Zn-TCP(Fe) membrane. 28080

DOI: 10.1021/acsami.7b08383 ACS Appl. Mater. Interfaces 2017, 9, 28079−28088

Research Article

ACS Applied Materials & Interfaces (707.0 eV), Fe−N 2p3/2 (711.9 eV), Fe0 2p1/2 (720.1 eV), and Fe−N 2p1/2 (724.3 eV) are the main features ascribed to the TCP(Fe) complex (Figure 2c,d). The XPS spectra of O 1s and Zn 2p core levels reveal the signals of the Zn−O (530.1 eV), Zn 2p3/2 (1021.8 eV), and Zn 2p1/2 (1044.8 eV) corresponding to Zn2(COO)4 nodes (Figure 2e,f). In addition, the signal of the CO (531.1 eV) is larger than that of the C−O/O−H signal (532.0 eV), indicating the successful coordination of Zn2+ ion with the carboxylic group on TCP(Fe) complex to build Zn-TCP(Fe) MOF (Figure 2e). The FESEM, AFM, and TEM images prove the obtained Zn-TCP(Fe) nanosheets possess a lateral size in micrometer scale (Figure 3a−c). The

the as-synthesized Zn-TCP(Fe) nanosheets. In addition, the SEM and AFM images reveal that the sheet-like Cu-TCP(Fe) and Co-TCP(Fe) samples with lateral dimensions in micrometer scale with average thicknesses of 2.2 ± 0.2 and 2.0 ± 0.1 nm, respectively (Figure S6). Moreover, the XPS measurements (Figure S7) and inductively coupled plasma optical emission spectroscopy (ICP-OES) confirmed that the molar ratios of M (M = Zn, Cu, and Co) to Fe in the respective MTCP(Fe) MOFs are comparable, and they are estimated to be 2:1. All the above results suggest that we have successfully synthesized M-TCP(Fe) MOFs (M = Zn, Cu, and Co) nanosheets. Based on the nonlocal density functional theory (NLDFT), the cumulative pore volume plots (Figure 4a and Figure S8a,b) indicate the presence of microporosity in 2D MTCP(Fe) nanosheets, and the pore size distribution plots (Figure 4b and Figure S8c,d) show a dominant pore size range of 1.20−1.24 nm, which is consistent with the value of 1.18 nm based on crystallographic data (Figure S9a). The 2D MOF membranes comprises of M-TCP(Fe) selective layers prepared on porous nylon support with pore size ca. 0.2 μm (Figure S10). Briefly, 0.4 mg mL−1 of the diluted 2D M-TCP(Fe) dispersion was used to form a selective layer on nylon support through vacuum filtration. The SEM image of Zn-TCP(Fe) selective layer shows that the surface morphology of the selective layer exhibits a few wrinkles around the surface (indicated by white arrows in Figure 5a), while the photographs (Figure S11) show that the resultant Zn-TCP(Fe) layer (ca. 3.5 cm in diameter) is evenly coated on the nylon support (4.7 cm in diameter). The average thickness of the Zn-TCP(Fe) selective layer with mass loading of 80 mg m−2 was measured to be about 70 nm based on the cross-sectional SEM image (Figure 5b). The EDX mapping images of the Zn-TCP(Fe) selective layer show a uniform compositional distribution of Zn, Fe, N, O, and C elements on the selective layer (Figure 5c). The AFM analysis (Figure 5d,e) shows that the thickness of the Zn-TCP(Fe) layer is 67 ± 2.5 nm, which is in reasonably good agreement with the cross-sectional SEM image (Figure 5b). For comparison, the surface morphologies and elemental distributions of Cu-TCP(Fe) and Co-TCP(Fe) selective layers of the membranes were also examined using SEM and EDX analyses (Figure S12a,b). The separation performances of the pristine Zn-TCP(Fe), Cu-TCP(Fe), and Co-TCP(Fe) membranes were then evaluated by using a stirred vacuum filtration setup (for low pressure