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Journal of Membrane Science

journal homepage: www.elsevier.com/locate/memsci

3D nanoporous crystals enabled 2D channels in graphene membrane withenhanced water purification performance

Kecheng Guan, Di Zhao, Mengchen Zhang, Jie Shen, Guanyu Zhou, Gongping Liu⁎, Wanqin Jin⁎

State Key Laboratory of Materials-Oriented Chemical Engineering, Nanjing Tech University, 5 Xinmofan Road, Nanjing 210009, PR China

A R T I C L E I N F O

Keywords:GraphenePorous nanocrystalsMembranesWater purification

A B S T R A C T

Two-dimensional (2D) materials have been received increasing attention in various fields such as physics,material science, chemistry and engineering. In particular, the graphene-based membrane is an emerging subjectmainly due to the atomic thickness, simple processing and the compatibility with other materials. In the case ofwater treatment, stable graphene-based laminar structures as well as high separation performance are pursued.In this work, nanoporous crystals embedded graphene laminate membranes was reported for water purification.Reduced graphene oxide (rGO) nanosheets obtained from a solution chemical process act as building blocks toconstruct the 2D channels through a pressure-driven filtration process. By incorporating three-dimensional (3D)nanoporous crystals with sub-nano sized aperture size into the 2D graphene laminates, both the inter-layerspacing and numbers of nanofluidic channels are increased, leading to greatly-enhanced water separation per-formance. The optimized 3D/2D membranes exhibit 15 times higher water permeability than that of the rGOmembrane with similar high dye retention rate. The significance of such 3D nanoporous structure and transportmechanism through the 3D/2D membranes is systematically studied. This general approach of enhancing themolecular transport through 2D nanofluidic channels proposed here may also find application in gas separationand battery membranes.

1. Introduction

As graphene-based membranes have attracted broad interests for itsprospects to function in various processes including gas separation[1–5], pervaporation [6–9], and water treatment such as desalinationand pressurized filtration [10–15]. The distinct properties of graphenemembrane are originated from the graphene-based two-dimensional(2D) materials such as graphene oxide (GO) and chemically convertedgraphene (CCG). Particularly, in the case of applying GO membranes inan aqueous environment, chemical treatment is required due to theinherent water dispersion behavior of GO. Chemical cross-linking [10],bonding [16] or reduction of GO [17] is typically employed to enhancethe stability of GO sheets in aqueous media and to retain the laminarstructure in the meantime.

It is well proved that the main transport pathway through GO la-minates is the 2D capillary network formed by face-to-face (van derWaals’ force) and edge-to-edge (electrostatic repulsive force) alignedGO sheets [18,19]. The interlayer spacing of laminates is adjustableaccording to the oxygenated groups amount and the hydrated ratio ofGO. Chemical crosslinking or bonding of GO laminates usually expandsthe spacing for the existence of linking agent while chemical reduction

tends to narrow the transporting way due to the loss of oxygenatedgroups which contributes to the structural compaction. To optimize themembrane structure, researchers have proposed several strategies thatcan be summarized as a two-step procedure: chemical reduction andenlarging laminar interlayer space. The reduction of GO can be handledeither before or after the membrane formation via solution chemicalprocessing (e.g., hydrazine reduction [20]) and in-situ reduction of GOlayer (e.g., HI reduction [18,21]), respectively. As to overcome thepermeability decline brought by chemical reduction, various kinds ofnanomaterials are naturally considered to act as spacers through GOlaminates. zero-dimensional (0D) or One-dimensional (1D) carbonmaterials (carbon dots [22], carbon nanotubes [23–26]) and nanos-trands [27] have been reported to be assembled into graphene mem-brane to enhance the water permeability. However, these 0D or 1Dnano-spacers that only enlarge the transporting 2D channels throughgraphene laminates may not afford more transporting ways due to theirsolid property or limited-openings along the diffusion pathway.

The incorporating nanoporous crystals with three-dimensional (3D)channels into the graphene laminates with 2D channels was assumed tofurther promote the water permeation by introducing more nanofluidicchannels meanwhile enlarging the inter-layer spacing. The

http://dx.doi.org/10.1016/j.memsci.2017.07.055Received 13 June 2017; Received in revised form 26 July 2017; Accepted 29 July 2017

⁎ Corresponding authors.E-mail addresses: gpliu@njtech.edu.cn (G. Liu), wqjin@njtech.edu.cn (W. Jin).

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Available online 01 August 20170376-7388/ © 2017 Published by Elsevier B.V.

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incorporated crystals within graphene laminates may raise the porosity.Additionally, porous crystals are more favorable to this end for thedirect water transport through particles themselves, rendering a lesstortuous transporting path.

In this work, therefore, 3D nanoporous crystals as spacers to par-tially expand the interlayer of reduced graphene oxide (rGO) stacks wasproposed to enhance the water permeability at the same time preservethe retention rate for water pollutants. Here UiO-66 was selected as atypical spacer to be embedded in the graphene laminates. UiO-66 is anew kind of zirconium based nanoporous metal-organic frameworks(MOFs), in which each zirconium center is connected to 12 benzene-1,4-dicarboxylate linkers [28]. Recent work has demonstrated that acontinuous UiO-66 membrane has the potential for water desalinationbased on size-selective diffusion [29]. The tetrahedral cavity, octahe-dral cavity and triangular aperture (~0.9, ~1.1 and ~0.6 nm, respec-tively) of the UiO-66 are all larger than the size of a water molecule(0.32 nm). Hence water can freely pass through the 3D nanocrystalswhile solutes (e.g., organic dyes) were rejected when solution filtratedthrough main 2D channels of stacked graphene-based membranes.Another 3D nanoporous spacer demonstrated here is Prussian blue (PB),a prototype of mixed-valence transition metal hexacyanoferrates havingcentral pathways with a pore size of ~0.4 nm.

2. Experimental section

2.1. rGO preparation

Graphene oxide was synthesized by a typical modified Hummersmethod [30,31]. A certain amount of GO was dispersed into deionizedwater via sonication and centrifugation. The solution chemical routewas adopted to reduce GO through a hydrothermal process To obtainrGO (reduced graphene oxide) with the existence of aqueous ammoniasolution and L-ascorbic acid (LAA). Utilizing such method, rGO can bedispersed well in water for next step processing. A detailed typicalprocess was described as following: GO powder was dispersed intowater to obtain GO solution with the concentration of 0.1 mg/ml. Then10 ml of such solution and 0.01 g LAA were added into the autoclave.After that, ammonia solution was added to the above solution to adjustpH to the value of ~10. The autoclave was heated under 95 °C for30 min after vigorous shaking. Thus, rGO solution was obtained whenthe hydrothermal process ended.

2.2. Synthesis of UiO-66 nanocrystals

ZrCl4 (4.49 mmol, 1.06 g), Terephthalic acid (4.49 mmol, 0.67 g),5 ml formic acid were dissolved in 50 ml DMF and then heated in anautoclave under 120 °C for 24 h. Then the obtained suspension wascentrifuged at 8000 rpm for 10 min to attain white powders. At last,above powders were immersed in methanol for solvent exchange forthree days and activated under vacuum at 100 °C. Powders withoutsolvent exchange and activation were also prepared for the reference.

2.3. Synthesis of Prussian blue nanoparticles

Two solutions were prepared before the synthesis process. solutionA, 0.01 M K4[Fe(CN)6] + 0.1 M KCl + 0.1 M HCl, and solution B,0.01 M FeCl3 + 0.1 M KCl + 0.1 M HCl. An injection pump was used toinject above two solutions into a beaker with heating and stirring forhomogeneous reaction. The beaker contained distilled water before thereaction. After the reaction process, the obtained suspension was cen-trifuged to attain PB powders.

2.4. Membrane fabrication

rGO membranes and UiO-66-rGO membranes were prepared bypressurized filtration method. A typical membrane preparation process

was described as following: 1.5 g as-prepared rGO solution was mixedwith a certain amount of UiO-66 nanocrystal aqueous suspension. Thenthis suspension was diluted by deionized water and after stirring andsonicating, homogeneous suspension was obtained. Polycarbonate filtermembrane (Millipore, pore size of 200 nm) was used as substrate tofiltrate suspension under the pressure of 1 bar. Notably, it was not en-sured that the nanocrystals were embedded into the every interlayer ofgraphene laminates although stable and homogeneous suspensions usedhere were prerequisites. Nevertheless, this did not affect the study ofthe filtration performances by graphene membranes embedded withvaried nanocrystal amount. Preparation of PB-rGO membrane, GO orrGO membrane was similar to the procedure above. PC filter membranewas selected as the support for graphene membrane due to its smoothsurface and low transport resistance.

Crosslinked rGO or UiO-66-rGO membrane was fabricated by fil-trating diamine (ethylenediamine) aqueous solution and rGO or UiO-66-rGO suspension simultaneously (for example, an aqueous suspensioncontaining 2.5 μl diamine, 0.15 mg GO and 0.15 mg UiO-66 nanocrys-tals was prepared to fabricate crosslinked UiO-66-rGO precursormembrane with embedding amount ratio of 1) through the PC filter.After the filtration process, the fresh membrane sample was thermallytreated at 80 °C for 1 h to carry out the chemical crosslinking process.The as-prepared membrane sample was stored in a desiccator at roomtemperature. Cross-linked membranes were evaluated by cross-flowfiltration.

2.5. Measurement of separation performance

All feed dye aqueous solutions were with the concentration of10 ppm. A home-made dead-end stirring cell or a cross-flow cell wasused to evaluate the retention performance of various membranesamples. Data collecting procedure was carried out one hour after thebeginning of membrane evaluation to exclude dye-adsorbing and waitfor the permeability compaction [25]. Pure water flux was calculatedwith the equation below:

=× ×

J VA p t (1)

where V (L) is collected water volume in the permeate side, A (m2) iseffective membrane area, p (bar) is applied pressure and t (h) is op-eration time. Thus, water flux J can be calculated.

The concentration of due solutions on the feed side and permeateside are characterized by UV–vis spectra. Retention rate is calculatedusing the equation below:

= − ×Rcc

1 100%p

f (2)

where cp and cf is the dye concentration on the permeate side and feedside, respectively. Thus, the retention rate R is calculated accordingly.

2.6. Characterizations

The morphologies and energy dispersive spectroscopy (EDS) ana-lysis of the membrane or nanocrystal samples were revealed by fieldemission scanning electron microscope (FESEM, S4800, Hitachi,Japan). Transmission electron microscope (JEM-2100F, Japan ElectronOptics Laboratory Co., LTD, Japan) was used to characterize the na-nocrystals. The roughness data of membrane surface at the range of 10× 10 µm and the height profiles of rGO nansoheets were obtained fromatomic force microscope (AFM, Bruker Dimension Icon, Germany andXE-100, Park SYSTEMS, Korea) characterizations. The quartz crystalmicrobalance technique (QCM200 Quartz Crystal Microbalance,Stanford Research Systems, Inc.) was used to measure the sorptionability. Water contact angles were examined by a contact angle drop-meter (A100P, MAIST Vision Inspection &Measurement Co., Ltd.). X-ray diffraction (XRD, Bruker D8 Advance, Germany) was performed at

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room temperature in the range of 5° ≤ 2θ ≤ 40° with the increment of0.02° per second. X-ray photoelectron spectroscopy (XPS, ThermoESCALAB 250, USA) was recorded to analyze the containing elementsin samples. Raman spectroscopy (LabRAM HR, HORIBA, France) wascarried out with 514 nm wavelength incident laser light in the range of1000–2000 cm−1. UV–Visible Spectrophotometer (UV–vis, PerkinElma, Lambda 950, USA) were characterized to analyze suspensionproperties and dye concentration. Adsorption experiments of the UiO-66 nanocrystals were carried out by ASAP 2460 with CO2 (298 K) andN2 (77 K). BET surface areas were calculated from the N2 isotherms at77 K. Zeta potential analysis (Zetasizer Nano ZS90, Malvern, UK) iscarried to characterize the nanocrystal suspension (0.1 mg/ml).Electrochemical impedance spectroscopy (EIS) measurements wereperformed in the presence of a stationary 5 mM K3[Fe(CN)6]/K4[Fe(CN)6] (1:1) mixture as a redox probe in 0.1 M KCl solution with thefrequency changed from 0.1 Hz to 1000 kHz and signal amplitude of5 mV.

3. Results and discussion

3.1. Pure GO and rGO Membrane

A schematic of water transport pathways through the proposedgraphene membrane is shown in Fig. 1 by using UiO-66 as a typicalspacer. Firstly the pristine GO and rGO membranes were studied for theacquirements of basic properties and some fabrication parameters. AsGO sheets are hydrophilic, the problem of GO membrane swelling andredispersion in aqueous solution cannot be overlooked. The rejection ofa typical kind of dye molecule, rhodamine B (RhB), was low for GOmembranes. Other works also reported this phenomenon and themoderate retention performance (roughly 50~80%) of GO membrane isreasonable. With the purpose of stability strengthening as well as per-formance enhancement, the reduction of GO thus was carried out as anessential step for the following study. Here, GO was reduced to obtainrGO based on a solution chemical process [32,33], after which processrGO was dispersed much well in such aqueous solution. The dispersingbehavior is of great significance since the desired membrane structurewas directly formed by a filtration method. rGO suspension turned to beblack, differed from the brown color of GO suspension.

To characterize the properties and to validate the successful re-duction, a series of characterizations were carried out including UV–visspectroscopy, Raman spectroscopy, XRD and XPS analysis (Fig. 2). InUV–vis spectra, the absorption peak in ~230 nm of GO was red-shifted

to ~260 nm for the restored electronic conjugation. Also, the overallstronger absorption intensity of rGO also implied that the aromaticstructure of graphene is recovered. The typical D and G band of carbonmaterials in Raman spectra also can reveal the variation of electronicconjunction state. rGO possessed a higher D/G intensity value, which isconsistent with the other common effective reduction results. In theXRD patterns, rGO showed up a broad peak around 2θ = 25° while GOshowed a sharp peak at the position of 2θ = 11°. According to Bragg'slaw, rGO thus owned a narrowed interlayer spacing compared to GO.These results agreed well with literature [20,32] which have beenproved to be a thorough reduction. Although rGO prepared by con-ventional process is not able to suspend in water, in this solution-basedmethod, the attachment of reduction agent on graphene plane can helpto keep rGO sheets repulsive to each other. Therefore, the as-preparedrGO is ready to be applied in following processing and laminar mem-brane construction via the flow filtration method.

The morphological changes was firstly observed between the as-prepared GO and rGO membranes. rGO membrane reveals a flattenedsurface morphology in comparison with GO membrane as seen fromSEM images in Fig. 3a, b. More apparent surficial fluctuations on GOmembranes were observed mostly attributed to the nanosheets foldingeffect during filtration assembly, and abundant oxygenated groups ofGO as well as the accordingly larger interlayer spacing. The XPS spectra(Fig. 2e, f) show a massive loss of C-O groups which were originatedfrom the hydroxyl and epoxy groups at the plane of GO sheets. Anapparent morphological change was characterized by atomic forcemicroscope in AFM images (Fig. 3c, d). The depth profiles of AFMimages indicate the weakened wrinkles on rGO membrane. Moreover,the overall surface roughness of rGO membrane also decreased in a halfwhich indicated a smoother morphology with less edge-to-edge wrin-kles.

As shown in Fig. 3e-f, reduced water permeability of rGO mem-branes compared with GO membranes was observed due to the morecompact structure induced by the loss of oxygenated groups and hy-drophilicity. Membranes with increased rGO loadings resulted in acontinuous drop of water permeability but possessed higher retentionrate than GO membranes. It was hypothesized that the introduction of3D nanoporous crystals would effectively enhance the water perme-ability through graphene-based laminates. To determine the properloading amount for retention test, RhB was selected as the model pol-lutant for the sake of its relatively small molecular dimension (less than2 nm) and electroneutral property to exclude electrostatic effect duringfiltration. According to our tests, rGO membranes with loading amount

Fig. 1. Schematics of water transport through (a) nanocrystalsembedded rGO laminates, (b) stacked rGO sheets and (c) nano-porous crystal.

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larger than 0.5 g/m2 shows distinct high retention rate (> 95%). Herethe loading amount of 0.75 g/m2 is selected in this work to guaranteethe high pollutants-removal ability.

3.2. Nanocrystals embedded rGO membranes

The stable membrane precursor solutions including rGO suspensionand UiO-66 suspension are significant for fabricating integrated laminargraphene-based membranes. rGO nanosheets revealed a typical lateralsize of 1–2 µm and UiO-66 was synthesized with the particle size ofaround 20–30 nm (Fig. 4a, b) via a typical procedure in previous work[34]. Due to the atomic-thin thickness and much larger lateral size

property of graphene nanosheets, layered structure of graphene lami-nates could not be disrupted by much smaller nanoparticles throughfiltration directed assembly process. rGO nanosheets tend to stackhorizontally sheet by sheet following the direction of filtration and alsocover some nanocrystals during this process. Compared with other solidnanomaterials, the nanoporous structure of UiO-66 is expected to affordadditional transport channels as it exhibited excellent transport prop-erties for either gas or water molecules [35,36].

As to stimulate the water transport through rGO membranes, UiO-66 embedded rGO membranes were fabricated with a typical layerthickness of less than 300 nm (Fig. 4c). The surface view of membranewas shown in Fig. 4d. The XRD characteristic peak of rGO in UiO-66-

Fig. 2. (a) Optical images of GO and rGO aqueous suspension; (b) UV–vis spectra of GO and rGO aqueous suspension; (c) Raman spectra of GO and rGO; (d) XRD spectra of GO and rGO;XPS spectra of (e) GO and (f) rGO.

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rGO membrane sample shows a slight shift to lower two theta values(Fig. 4e), which may be possibly attributed to the enlarged interlayerspace of graphene laminates. Zr3d scan of membrane sample in XPSspectra also revealed the existence of UiO-66-rGO nanocrystals in rGOlaminates (Fig. 4f). Meanwhile, elements distribution was also revealedby EDS. Zr, C and O were distributed uniformly in the tested area asseen from Fig. 5.

3.3. Transport properties of nanocrystal-rGO membranes

The intercalation amount of UiO-66 was evaluated to observe itseffects on membrane separation performance. The membrane sampleswere designated as UiO-66-rGO-x, in which ‘x’ means the weight ratioof UiO-66 to rGO. A series of experiments were carried out with the ‘x’values of 0.3, 0.7, 1 and 1.3. Water permeability was continuouslyraised up as more UiO-66 nanocrystals was embedded between rGO

stacks (Fig. 6a). For one reason, UiO-66 was successfully embedded intothe interlayer of rGO stacks to expand the channel and reduce the watertransporting resistance. XRD analysis (Fig. 4e) suggests that the inter-layer spacing of rGO membrane was enlarged by incorporating UiO-66nanocrystals into the graphene laminates, while the inter-layer spacingof UiO-66-rGO membranes was not further enlarged by increasing theUiO-66 loading. It reveals that the increase of UiO-66 loading improvedthe embedding density of UiO-66 nanocrystals on the graphene na-nosheets rather than further enlarging the interlayer spacing. For an-other reason, porous UiO-66 nanocrystals may provide faster watertransport channels, leading to the higher permeability. Typical UV–visspectra of feed RhB solution and collected permeates by UiO-66-rGO-0.7 membrane are shown in Fig. 6b. The RhB retention rates wereall> 90% (except the ~89% retention rate for UiO-66-rGO-1.3 mem-brane) for these membrane samples with varied embedding amount.

Fig. 3. SEM, AFM images and filtration performance of (a, c, e) GO and (b, d, f) rGO membranes, respectively.

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3.4. Role of 3D porous nanocrystal

To overcome the redispersion of GO in water solution, GO mem-brane was reduced. The resulting rGO membrane became much morehydrophobic than GO membrane, as indicated by the surface contactangle increasing from 32° to 93°. Nevertheless, the addition of nano-crystals overcome this reduction-induced hydrophobicity. As shown inFig. 7a, the contact angle of rGO membrane surface was graduallydeclined from 93° to 55° with increasing the UiO-66 contents in themembrane. QCM (Quartz Crystal Microbalance) was utilized to monitorthe water sorption behavior of rGO and UiO-66-rGO (Fig. 7b). Betterhydrophilicity of UiO-66-rGO was further confirmed.

To reveal the role of 3D porous nanocrystal in water transport,

unactivated UiO-66 (labelled as unUiO-66) crystals was prepared wherethe guest solvent in synthesis, N, N-Dimethylformamide (DMF) mole-cules were not removed from the cavity of crystals, leading to a muchless porous structure indicated by BET analysis (Fig. 7c). These crystalswere used to prepare membrane samples through a similar procedure asto validate the contribution of 3D nanopores of crystals in the watertransport. These unUiO-66 crystals were embedded into rGO laminates(unUiO-66-rGO-1) and the water permeability was only the half of UiO-66-rGO-1 membrane with same embedding amount (but still almost 6times higher than rGO membrane, Fig. 7d; RhB retention rate of theunUiO-66-rGO-1 membrane was around 91.50%). The result clearlyindicates that the 3D nanoporous structure of crystal could further fa-cilitate the water transport through graphene laminates rather than

Fig. 4. (a) Optical image of rGO suspension and AFM image of rGO sheets; (b) optical image of UiO-66 suspension and TEM image of UiO-66 nanocrystals; (c) cross-section SEM image ofUiO-66-rGO membrane; (d) SEM morphological image of UiO-66 embedded graphene membrane (inset image is a curved membrane sample picked by a tweezer); (e) XRD spectra of rGOand UiO-66-rGO membranes; (f) XPS Zr3d scan of UiO-66-rGO membrane.

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only enlarging the stacked interlayers. To prove the great permeabilitycontribution brought by 3D nanoporous crystals, the water sorptionproperty of unUiO-66-rGO and UiO-66-rGO was compared (Fig. 7b).UiO-66-rGO possessed roughly 1.7 times higher sorption than unUiO-66-rGO seen from the QCM sorption amount at 10,000 s. The higherwater permeability in UiO-66-rGO can be attributed to the additionalwater transport pathways of 3D porous nanocrystals contributing tofaster mass transfer than the pristine interlayer channels of rGO.

3.5. Dye retention performance

A specific dye retention mechanism had been studied in literaturewhich generally concluded that the molecular sieving rather than ad-sorption behavior of GO membrane was the dominant effect during thefiltration process [17,24]. Similarly, from our experimental results,higher intercalation amount of UiO-66 was not helpful for the en-hancement of retention rate. In contrast, taking UiO-66-rGO-0.7 and

Fig. 5. SEM image and relevant EDX images of UiO-66-rGO membrane.

Fig. 6. (a) Filtration performance of UiO-66-rGO membranes with different embedding amount; (b) UV–vis spectra of feed and permeate samples by a UiO-66-rGO-0.7 membrane.

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UiO-66-rGO-1.3 as two representative samples (Fig. 8), the water per-meability was elevated (from ~14 to ~33 L m−2 h−1 bar−1) while dyeretention rate dropped (From ~99% to ~89%) when the intercalationamount ratio was increased from 0.7 to 1.3. The retention rate remainsalmost constant during the filtration processes. If the pure water wasmainly obtained from the dye adsorption of UiO-66 frameworks, theincreasing amount of UiO-66 would help to improve the dye retentionrate, which indirectly reveals that sieving effect of GO interlayers wasthe dominant mechanism for dye retention and the embedding of na-nocrystals accounted for the increased water permeability.

The retention of other dyes including positively charged methyleneblue (MB) and negatively charged titan yellow (TY) was also tested. TY

possessed larger molecular size than RhB while MB is smaller. For UiO-66-rGO-1 membrane, the retention rates of MB and TY are also greaterthan 90% (Table 1). These results are attributed to dye molecular size aswell as electrostatic effect between dye molecules and membranes. Acomparison with other works is listed in Table 2 for reference. Amongrecently published works, basically creating more transport channelswere able to enhance the separation performance of graphene-basedmembrane. In this work, nanoporous crystals were embedded to createmore and faster transport channels for the transport of water moleculesin graphene-based membrane. High water permeability with compar-able dye retention was achieved in the developed UiO-66-rGO mem-branes.

Amine functionalized UiO-66 crystals (UiO-66-NH2) were also syn-thesized and had been embedded into the rGO laminates to observe themembrane performance. Since the UiO-66-NH2 exhibited electrostaticattraction with rGO sheets [as proved by zeta potential (ZP) analysis,the ZP values of UiO-66 and UiO-66-NH2 suspensions are −0.03± 0.002 mV, +11.95 ± 0.15 mV, respectively], the assembled UiO-66-NH2-rGO membrane would be more compact. A membrane sample ofUiO-66-NH2-rGO-0.7 was fabricated accordingly. The RhB retentionrate (98.36%) was comparable to that of the UiO-66-rGO-0.7 sample,

Fig. 7. (a) water contact angles of different membrane samples; (b) sorption ability of different membrane samples monitored by QCM; (c) BET analysis of activated and unactivated UiO-66 nanocrystals; (d) water permeability of different membrane samples.

Fig. 8. Filtration performances of UiO-66-rGO-0.7 and −1.3 membranes during waterrecovering process.

Table 1A summary of various dye-retention performances by UiO-66-rGO-1 membrane sample.

Dye molecule Retention rate(100%)

Molecularweight (g/mol)

Moleculardimension(nm×nm)

charge

RhB 95.0 479.0 1.7×1.4 NeutralTY 98.5 695.7 3.0×1.4 NegativeMB 98.7 319.9 1.6×0.8 Positive

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and the water permeability was compromised as expected (from 14.38to 10.80 L m2 h−1 bar−1). This was attributed to the 3D nanoporouscrystals enabled channels which overcome the resistance of electro-static attraction. Although the amine modified nanocrystals possessed apositive ZP value, the major part of the nanocrystals embedded mem-branes is still comprised from negatively charged rGO (Fig. 9). There-fore, dye retention behaviors of UiO-66-NH2-rGO and UiO-66-rGOsamples were similar. From above experimental works, size effect seemsmore important than electrostatics during dye retention processthrough graphene membrane.

3.6. Universality and stability

To demonstrate the universality of the 3D nanoporous crystalsembedded 2D graphene channels approach, UiO-66 nanocrystals was

substituted with PB nanocrystals. PB owns central pathways with thediameter of ~0.4 nm, and the particle size of the as-synthesized PBnanocrystals is around 50 nm (Fig. 10a). As expected, water perme-ability of rGO membrane was greatly improved with increasing the PBnanocrystals intercalation amount in the PB-rGO membranes, whileRhB dye removal efficiency were well above 90% (Fig. 10b). It suggeststhat the membrane properties or performances can be easily regulatedby embedding versatile nanocrystals with various characteristics.

To further validate the permeability improvement brought by 3Dnanoporous crystals, UiO-66 nanocrystals were embedded into cross-linked rGO membranes that would exhibit high stability due to theinterlocked rGO sheets leading an integrated laminar structure. In thisstudy, UiO-66-rGO-1 membrane was chemically cross-linked with dia-mine [38] as proved by XPS spectra (Fig. 11a, b). Compared with thespectra of rGO (Fig. 2f), the cross-linked membrane revealed a higher C-O or C-N peak intensity, which was resulted from the nucleophilicsubstitution reaction amine opened epoxy groups and formed newcovalent bonds linking C-OH and C-N groups. The cross-linked UiO-66-rGO membrane exhibited water permeability of ~15 L m2 h−1 bar−1,which is more than 10 times higher than cross-linked rGO membrane,Fig. 11b). In addition, the cross-flow filtration was carried out toevaluate the cross-linked membranes. Both cross-linked rGO and cross-linked UiO-66-rGO membrane showed stable retention performanceduring the operation period (Fig. 11d). The lower retention rate waspossibly resulted from the diamine which further enlarged the capillarypores compared to the UiO-66-rGO membrane. Regardless, embeddingnanoporous crystals was proved to be efficient approach to enhance thewater permeability for different types of graphene-based membranes.

4. Conclusions

In summary, 3D nanoporous crystals embedded rGO membraneswith 2D channels were fabricated and employed for pressure-drivenwater filtration. The water purification performance of UiO-66-rGOmembranes and PB-rGO membranes was finely regulated by varying theintercalation amount of these nanocrystals, while excessive embeddingresulted in incompatible structure and destroyed the sieving effect ofrGO membrane. It was demonstrated that high retention rate of organicdyes and greatly-improved water permeability could be realized bynanoporous crystals intercalation. The proposed 3D nanoporous crys-tals enabling 2D graphene channels approach may be extended to othermembrane fabrication and separation processes.

Acknowledgements

This work was financially supported by the National Natural ScienceFoundation of China (Grant Nos. 21490585, 21476107, 21406107), theInnovative Research Team Program by the Ministry of Education of

Table 2A summary of graphene-based membrane performance for water purification (DY: Directyellow, EB: Evans blue). *The water permeability value was re-calculated since the ori-ginal data (279± 20 L m−2 h−1 bar−1) was obtained after considering 10% porosity ofPC substrate).

Membrane sample Feed dyesolution

Water permeability(Lm−2h−1bar−1)

Retentionrate (%)

Reference

90°C-CCG DY ~40 67 [13]Mesoporous GO EB 191 88.5 [37]*NSC-GO RhB 27.9±2 87±3 [27]TMC-GO MB 8–27.6 46–66 [10]brGO RhB 5 78 [17]MWCNT-brGO MO 11.3 96.1 [24]rGO_50MWCNT (50) RhB ~35 >95 [25]UiO−66-rGO−0.7 RhB 14.4 98.2 This workUiO−66-rGO−1 RhB 30.6 95.0

Fig. 9. Zeta potential values for rGO, UiO-66-rGO and UiO-66-rGO samples.

Fig. 10. (a) SEM image of Prussian blue nanocrystals. Inset image is the schematic of PB crystal structure; (b) Filtration performance of rGO membranes embedded with PB nanocrystals.

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China (Grant No. IRT13070) and the Topnotch Academic ProgramsProject of Jiangsu Higher Education Institutions (TAPP).

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Fig. 11. XPS (a) survey and (b) C1s scan of crosslinked UiO-66-rGO membrane; (c) Water permeability of different membrane samples; (d) Cross-flow evaluation of rGO and UiO-66-rGOmembranes.

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