Facile Preparation of Polyamide Thin-Film Nanocomposite

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Facile preparation of polyamide thin-film nanocomposite membranes using spray-assisted nanofiller pre-deposition Tae Hoon Lee, Inho Park, Jee Yeon Oh, Jun Kyu Jang, and Ho Bum Park Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b00029 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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Industrial & Engineering Chemistry Research

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Facile preparation of polyamide thin-film nanocomposite membranes using spray-

2

assisted nanofiller pre-deposition

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Tae Hoon Lee, Inho Park, Jee Yeon Oh, Jun Kyu Jang, and Ho Bum Park*

4

Hanyang University, Department of Energy Engineering, Seoul 04763, Republic of Korea *

5

Corresponding author. E-mail address: [email protected] (H.B. Park)

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Abstract

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Thin-film nanocomposite (TFN) membranes prepared by embedding nanofillers into ultrathin

13

polyamide layer have paved the way to developing high-performance reverse osmosis (RO) desalination

14

membranes. Scale-up production of TFN membranes is still a challenging issue, however, since previous

15

studies have merely followed the same fabrication method for conventional RO membranes. Herein, we

16

introduced a novel preparation method for TFN membranes using spray-assisted nanofiller pre-deposition

17

to circumvent the limitations in conventional method. The precise control of nanofiller (ZIF-8) loading

18

was possible by simply varying the spraying ZIF-8 concentration. Most importantly, TFN membranes

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prepared by both spray and conventional method showed similar RO performances, while spray method

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only requires ~100 times minimized amount of ZIF-8 with unprecedentedly short deposition time (< 1

21

min) ever reported. Our results revealed that spray method would be promising for the scale-up production

22

of TFN membranes in terms of cost, time, and controllability.

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ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Introduction

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Reverse osmosis (RO) using polyamide thin-film composite (TFC) membranes has been a leading

3

desalination technology for sustainable water supplies in recent decades.1-2 However, the polyamide-

4

based membranes present challenges such as a trade-off relationship between water permeability and

5

water/salt selectivity, membrane fouling, and low chlorine stability, which have retarded the development

6

of more energy-efficient desalination processes.3-4 Thus, incorporating nanoporous materials (i.e.,

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nanofiller) into the polyamide layer to fabricate thin-film nanocomposite (TFN) membranes has been

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extensively researched in order to integrate the advantages of both materials for high-performance RO

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membranes.5-7 Carbon nanotube (CNT),8-9 graphene oxide (GO),10-11 zeolite,12-13 and metal organic

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frameworks (MOFs)14-15 have been intensively researched as nanofillers that might enhance the RO

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performance of conventional polyamide TFC membranes. The roles of such nanofillers are to (1)

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physically tune the membrane morphology or thickness,10 (2) chemically change the crosslinking density

13

or free volume of the polyamide layer,14, 16 and (3) act as an additional water-permeable channel through

14

its microporosity.8,

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outstanding RO performance enhancements compared to the pristine TFC membranes.6

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The concept of TFN membranes has been demonstrated in literature with

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Scale-up of TFN membranes for practical applications, however, encounters several key challenges.

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One of the major concerns is nanofiller agglomeration inside the polyamide layer due to its high surface

18

energy, which causes undesirable defects which deteriorate the salt rejection of TFN membranes. Since

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water and aliphatic hydrocarbons (e.g., n-hexane or n-decane) are the main solvents used for the interfacial

20

polymerization of the polyamide layer, another concern is that nanofiller candidates are significantly

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restricted to ensure their stable dispersion in the conventionally used solvents.6 A third concern is the waste

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of expensive nanofillers used during interfacial polymerization has been somewhat disregarded in most

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lab-scale studies despite the importance of cost in scale-up production.18-19

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These challenges occur in the conventional preparation method of TFN membranes, designated as 2


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‘Conv’ method or ‘TFN-Conv’ (Figure 1a). In this method, a nanofiller is dispersed in one of the monomer

2

solutions (water or aliphatic hydrocarbons as solvent), and a support membrane is immersed into the

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solution during interfacial polymerization. Only a small amount of nanofiller is incorporated into the

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polyamide layer by “convective assembly” of nanofiller deposition, which limits the amount of nanofiller

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that can be incorporated and requires a large volume of solution to completely wet and cover the support

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membrane surface.18, 20 In addition, the geometry of the nanofiller (e.g., particle size) can also influence

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the amount of nanofiller loaded inside the polyamide layer. It is extremely difficult to precisely control the

8

amount of nanofiller even though this is essential information for the fundamental studies on water and

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ion transport mechanism of TFN membranes.21

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In this respect, the pre-deposition of the nanofiller prior to the interfacial polymerization process has

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recently drawn attention as a modified preparation method for the cost-effective scale-up with improved

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nanofiller loading control. There are many types of assembly methods to deposit nanomaterials onto a

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support layer.22 Several methods, such as vacuum filtration,23-25 layer-by-layer (LBL) assembly,26

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evaporation-controlled filler positioning (EFP),18, 27 the Langmuir-Schaefer (LS) technique,19 and dip-

15

coating,28 have been introduced for preparing high-performance and cost-effective TFN membranes.

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These methods could overcome the limitations in Conv method, and detailed information on the reported

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different fabrication methods including the required amount of nanofiller and deposition time is

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summarized in Table S1. These approaches, however, are still too time-consuming to be practical for the

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roll-to-roll TFN membrane production process, which only allows a short time (< 1 min) for nanofiller

20

deposition.29

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Nanoparticle spray coating is a simple, fast, continuous, and controllable deposition method,22 and is

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currently used in the roll-to-roll fabrication process for nanofilm-based applications such as capacitors,30

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solar cells,31 and filed-effect transistors.32 Recently, Zhou et al. also reported a TFC membrane consisting

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of an ultrathin spray-coated CNT interlayer, but its implication on the scale-up production of TFN 3



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membranes was not clearly demonstrated yet.33 Herein, we firstly introduce a spray-assisted pre-

2

deposition of nanofiller for TFN membrane preparation, designated as the Spray method or TFN-Spray,

3

to circumvent the disadvantages in previously reported preparation methods. Zeolite imidazolate

4

framework-8 (ZIF-8), which has been widely investigated as a promising nanomaterial for desalination

5

due to its high water permeability (~5.0 L∙µm/m2∙h∙bar), narrow pore aperture (3.4 Å) to discriminate

6

water and ions, and good stability in water,14, 34 was chosen as a representative nanofiller to demonstrate

7

the validity of the Spray method (Figure 1b). This preparation method not only reduces the waste of

8

expensive nanofiller to more than 100 times compared to that of the Conv method, but only requires a

9

very short deposition time of less than 1 min. In addition, precise control of the nanofiller deposition is

10

possible by simply changing the concentration of the ZIF-8 spray solution, which offers potential

11

opportunities for investigating fundamental water and ion transport on TFN membranes in detail.

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(a) Conventional TFN preparation (TFN‐Conv)

Excess solution removal ZIF‐8 + MPD solution

(b) Spray‐assisted pre‐ deposition of nanofiller (TFN‐Spray)

13 14 15 16 17

TMC solution Spraying ZIF‐8 solution

ZIF‐8/PA TFN membrane

MPD solution

Figure 1. Schematic illustration comparing (a) TFN-Conv and (b) TFN-Spray used in this study for TFN membrane preparation. Note that ZIF-8 was added in aqueous MPD solution for TFN-Conv in this figure due to its stable dispersion in water.21

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Experimental Section

2 3

Materials. Zinc nitrate hexahydrate (Zn(NO3)2∙6H2O, 99%), 2-methylimidazole (Hmim, 99%), and

4

1,3,5-benzenetricarbonyl trichloride (TMC, 98%) were obtained from Sigma Aldrich (St. Louis, MO,

5

USA). 1,3-Phenylenediamine (MPD, 98%) was purchased from Tokyo Chemical Industry (TCI, Tokyo,

6

Japan). n-Decane (99%, extra pure), n-hexane (95%, extra pure), methanol (99.5%, extra pure), ethanol

7

(99.5%, extra pure), and sodium chloride (NaCl, 99%, extra pure) were purchased from Daejung

8

Chemicals (South Korea). The porous polysulfone (PSf) support membranes were provided by Toray

9

Chemical Korea (South Korea). De-ionized (DI) water purified by a Merck Millipore MilliQ system

10

(Darmstadt, Germany) was used throughout this study.

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ZIF-8 nanoparticle synthesis. ZIF-8 nanoparticles with an average size of 60 nm were synthesized by

13

following the same procedure outlined in previous literature.21 In brief, 5.87 g Zn(NO3)2∙6H2O was

14

dissolved in 300 ml methanol and 12.98 g Hmim was dissolved in another 300 ml methanol. Subsequently,

15

the Hmim solution was gently poured into the former solution, and the mixture was stirred for 2 h at room

16

temperature. After the reaction, a white precipitate of ZIF-8 nanoparticles was obtained by repeated

17

washing with methanol using a centrifuge at 10,000 rpm for 10 min at 5 oC. The final product was

18

completely dried in a vacuum oven at 60 oC for 24 h.

19 20

Spray coating of ZIF-8 nanoparticles onto PSf support. Pre-deposition of the ZIF-8 nanoparticles was

21

performed using a robotic spray coater (SRC-2300RA, E-flex, South Korea). The spraying nozzle (AM12,

22

ATOMAX, Japan) was an air-atomizing system driven by compressed air at 0.6 MPa. ZIF-8 nanoparticles

23

were dispersed in ethanol using a bath sonicator (CPX8800H-E, Branson, USA) for 1 h. A syringe pump

24

containing the ZIF-8 dispersion was connected to the spraying nozzle, and a 9×14-cm2 PSf support was 5



1

mounted 3 cm under the nozzle. Note that the short distance between the PSf support and nozzle was

2

determined to minimize the loss of ZIF-8 during spray coating. Finally, a robot-assisted spray coating was

3

applied with a speed of 10 cm/s following the zigzag-like coating trajectory as represented in Figure 2d.

4

The required coating time for one PSf support sample was approximately 45 s. The amount of ZIF-8

5

deposition was readily controlled by varying the ZIF-8 concentration in the spray solution (Table 1).

6 7

Preparation of TFC and TFN membranes. TFC, TFN-Conv, and TFN-Spray polyamide membranes

8

were prepared by interfacial polymerization between two monomers (MPD and TMC) on a PSf support

9

membrane. 20 ml of 2 wt.% aqueous MPD solution was used to immerse a 9×14-cm2 piece of PSf support

10

for 2 min. The residual water droplets were removed using a rubber roller. 10 ml of 0.1 wt./vol.% TMC

11

in n-decane solution was immediately poured onto the MPD-immersed PSf support, and interfacial

12

polymerization was conducted at room temperature for 1 min. After the reaction, the excess TMC solution

13

was rinsed with n-hexane. The prepared TFC membranes were also washed several times with DI water

14

and stored in a DI water bath before characterizations. For TFN-Conv membrane preparation, 0.2 wt./vol.%

15

of ZIF-8 nanoparticles were dispersed in the aqueous MPD solution using a bath sonicator (CPX8800H-

16

E, Branson, USA) for 1 h, following the same procedure for the preparation of TFC membranes. The ZIF-

17

8 concentration used for TFN-Conv membranes was the optimum concentration as determined in our

18

previous report.21 The TFN-Spray membranes were prepared using the ZIF-8 pre-deposited PSf support

19

with the spray coating as shown in Table 1, and the same procedure for TFC membrane preparation was

20

followed. Note that the volume of aqueous MPD solution per specific area (0.16 ml/cm2) for Conv method

21

is a reasonable amount to completely wet the whole area of PSf support. A similar volume (0.13 ml/cm2)

22

was used in other literature.35

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6


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Table 1 Specific information for the TFN membranes prepared using the different methods in this study. TFC or TFN Concentration of spraying Required ZIF-8 weight per membranes ZIF-8 solution (wt./vol.%) specific area (mg/m2) Conv PSf TFC Conv PSf-Conv TFN-Conv 0.2a 3175 Spray PSf-6 TFN-6 0.01 6 Spray PSf-15 TFN-15 0.025 15 Spray PSf-30 TFN-30 0.05 30 Spray PSf-60 TFN-60 0.1 60 Spray PSf-150 TFN-150 0.25 150 Spray PSf-300 TFN-300 0.5 300 a Concentration of the ZIF-8 nanofiller in aqueous MPD solution immersing the PSf support during Conv method. Method

2 3 4

Support

5

Characterizations. A scanning electron microscope (SEM, JSM-700F, JEOL, Japan) was used to

6

observe the surface morphology of the membranes. For cross-sectional SEM images, a membrane coupon

7

was cut after completely freezing the membrane in liquid N2. The membranes’ surface roughness were

8

characterized using atomic force microscope (AFM, XE-100, Park Systems, South Korea) over a 10×10-

9

μm2-sized area. A contact angle analyzer (Phoenix 300, SEO, South Korea) was used to evaluate the

10

hydrophilicity of the supports using at least 6 water droplets. The crystalline structure of the ZIF-8

11

nanoparticles was identified using an X-ray diffractometer (XRD, Rigaku Model SmartLab, Rigaku,

12

Japan) with focused monochromatic Cu Kα radiation at a wavelength of 1.5418 Å and operating in the

13

2θ range 5o–40o at a scan rate of 10o/min. The elemental contents of the membranes were analyzed using

14

X-ray photoelectron spectroscopy (XPS, Axis Supra, Kratos, UK) with Al-Kα radiation as the X-ray

15

source.

16 17

RO performance evaluation of TFC and TFN membranes. A cross-flow filtration apparatus (Test cell

18

system, Sepratek, South Korea) was operated to test the RO performance of the prepared TFC and TFN

19

membranes. The effective filtration area of the membrane was 4×8 cm2. The brackish feed solution (2 g/L

20

in DI water, pH = 6–7) filled the apparatus’s feed tank, and the RO performance was evaluated at a cross7



Page 8 of 21

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flow rate of 2 L/min, 15.5 bar (225 psi), and 25 oC after a 30 min stabilization time. Water permeance (Jw,

2

L m-2 h-1 bar-1, LMH/bar) of the membranes was calculated from equation (1):

3

4

Jw 

V At p

(1)

5 6

where ∆𝑉 is the permeate volume (L), A is the effective membrane area (m2), ∆𝑡 is the measuring time

7

(h), and ∆𝑝 is the transmembrane pressure (bar). A conductivity meter (inoLab 740, WTW, Germany)

8

was used to calculate NaCl rejection by comparing the conductivity of feed and permeate solution using

9

the following equation (2):

10 11

 Cp  R  1    100 %  C f  

(2)

12 13

where 𝐶 is the NaCl concentration of the permeate solution, and 𝐶 is the NaCl concentration of the

14

feed solution. The RO tests were repeated at least three times and the average and standard deviation

15

values are reported in this report.

16 17

Results and Discussion

18

ZIF-8 nanoparticles were synthesized by mixing two precursor solutions at room temperature.36 The

19

crystalline structure of ZIF-8 nanoparticles with a size of 60 nm was confirmed by XRD and SEM

20

analysis, which is consistent with our previous report (Figure S1a and b).21 To control the amount of ZIF-

21

8 deposition, concentrations ranging from 0.01 wt./vol.% to 0.5 wt./vol.% of ZIF-8 nanoparticles were

22

dispersed in ethanol. Each solution was sprayed onto the polysulfone (PSf) support using an automatic 8


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1

spray-coater as described in Figure 2a-d. The ZIF-8-deposited PSf supports were designated as PSf-xx,

2

where xx denotes the weight of the ZIF-8 deposition per membrane area (mg/m2), from 6 to 300. A

3

uniform coating was achieved, and no significant ZIF-8 aggregation was observed even at the lower SEM

4

image magnification (Figure 2e). The required deposition time (45 s) for a 9x14 cm2 support membrane

5

is the shortest time ever reported for preparing TFN membranes (Table S1). This emphasizes the Spray

6

method’s effectiveness for the roll-to-roll production of TFN membranes, which requires a fast nanofiller

7

deposition for the continuous process.

8 9 10 11 12

Figure 2. Photo images of (a) spray coating apparatus, (b) spraying nozzle and PSf support (c) automatic screen panel which controls the flow rate and spraying time of spraying solution, and (d) zigzag-like coating trajectory used in this study. (e) Low magnification SEM image of PSf-30 (scale bar = 2 µm).

13

Figure 3 represents the SEM images of the ZIF-8 pre-deposited supports after the spray coating

14

compared to supports prepared using the Conv method (PSf-Conv). As expected, the ZIF-8 nanoparticles

15

covering PSf support surface increased with the increasing ZIF-8 spraying solution concentration. Notably,

16

the surface of ZIF-8 coating was almost monolayer until PSf-30, whose morphology is similar to that of

17

PSf-Conv. A multilayer of ZIF-8 deposition with a rough morphology was observed from PSf-60 to PSf9



1

300. Recent work indicates that it is difficult to obtain a multilayer of ZIF-8 using the Conv method due

2

to the stable ZIF-8 dispersion and electrostatic interactions between ZIF-8 nanoparticles and the PSf

3

support. This implies that the precise control of nanofiller deposition using the Conv method would be

4

affected by multiple factors.21 In sharp contrast, the nanofiller loading could be readily controlled by

5

varying the concentration of ZIF-8 solution using the Spray method. Additionally, the amount of ZIF-8

6

deposition can be precisely calculated using the Spray method by multiplying the concentration of the

7

spray solution by the volume (which is the product of the spraying flow rate (1 ml/min) and the total

8

deposition time (45 s)). Such precise control of the deposition amount is advantageous for an in-depth

9

study on the roles of nanofillers inside the polyamide TFN membranes since it is generally challenging to

10

precisely determine the amount of incorporated nanofiller using previously reported methods for TFN

11

membrane preparation. For example, the LBL assembly method could roughly control the amount of

12

ZIF-8 by repeating the assembly procedure, but the precise ZIF-8 loading amount per membrane area was

13

not reported.26

14 15 16 17

Figure 3. SEM surface images of ZIF-8 pre-deposition on PSf support. (a) pristine PSf, (b) PSf-6, (c) PSf15, (d) PSf-30, (e) PSf-60, (f) PSf-150, (g) PSf-300, and (h) PSf-Conv (Scale bar = 200 nm).

18

Subsequently, TFC, TFN-Spray, and TFN-Conv membranes were fabricated by interfacial

19

polymerization of the polyamide layer onto the pristine or ZIF-8-deposited PSf supports. No protruded or 10


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1

leaked ZIF-8 was observed, demonstrating that the ZIF-8-deposited nanofillers were fully encapsulated

2

by the polyamide coating layer (Figure 4).13 TFN-Spray membranes with less than 30 mg/cm2 of ZIF-8

3

loading showed a typical “ridge-and-valley” morphology on the polyamide layer, which is nearly identical

4

to the pristine TFC and TFN-Conv membranes. For highly-loaded TFN-Spray membranes more than 60

5

mg/cm2, however, the polyamide layer represented non-uniform morphologies with flat and dark spots

6

that became more pronounced as the ZIF-8 loading amount increased. Cross-sectional SEM images of

7

each polyamide layer also support the theory that TFN-300 possessed an immature and undistinguishable

8

polyamide layer compared to TFC, TFN-30, and TFN-Conv, which all share similar polyamide layer

9

thicknesses (~250 nm) (Figure S2).

10

11 12 13 14

Figure 4. SEM surface images of (a) TFC, (b) TFN-6, (c) TFN-15, (d) TFN-30, (e) TFN-60, (f) TFN150, (g) TFN-300, and (h) TFN-Conv (Scale bar = 1 µm).

15

It is expected that the ZIF-8-deposited nanofillers changed the surface characteristics of the original

16

PSf support and thereby influenced the formation of the polyamide layer during interfacial polymerization.

17

Surface characteristics such as hydrophilicity and surface roughness were investigated to try to explain

18

the defect formation at the high ZIF-8 loading amount (Figure 5). Average surface roughness (Ra) and

19

surface area difference (SAD) calculated from AFM image measurements (Figure S3) significantly 11



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1

increased as the amount of ZIF-8 deposition increased, consistent with the SEM image observations.

2

Subsequently, surface hydrophilicity of ZIF-8-deposited PSf supports were evaluated from the contact

3

angle measurement. The “surface roughness-corrected” solid-liquid interfacial free energy ( ∆𝐺 ) was

4

calculated to better represent the surface hydrophilicity considering the contribution of the rough surface

5

as described in the previous report (Figure 5c and d).37 The decrease in

6

surface became more hydrophobic as the amount of ZIF-8 deposition increased, possibly due to the

7

intrinsic hydrophobicity of ZIF-8.14 Although it is reported that a proper hydrophobicity of the support

8

membrane is desired to avoid the intrusion of polyamide layer,37-38 Wang et al. reported that ZIF-8 pre-

9

deposited polyethersulfone (PES) support showed increased hydrophobicity and limited the wettability

10

with aqueous monomer solution, ultimately deteriorating the quality of the polyamide layer.24 Considering

11

such an inconsistent results on hydrophilicity and the similar

12

supports more than 60 mg/cm2, we attributed the defect generations in polyamide layer with more than

13

60 mg/cm2 primarily to too rough surface of ZIF-8 multilayer.

∆𝐺

12


∆𝐺

indicates the PSf support

values of ZIF-8-deposited PSf

80

Average Roughness (Ra, nm)

(a)

(b)

70 60 50 40 30 20 10 0

0

50

100

150

200

250

50

75

1

20

10

0

50

100

150

200

250

300

110

(d)

105

-ΔGsl (mJ/m2)

70

65

60

55

30

Amount of ZIF-8 Deposition (mg/m2)

Amount of ZIF-8 Deposition (mg/m )

(c)

40

0

300 2

Contact Angle (o)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Surface Area Difference (SAD, %)

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100

95

0

50

100

150

200

250

90

300

0

50

100

150

200

250

300



2 3 4 5

Figure 5. Surface characteristics of ZIF-8-deposited PSf support using the Spray protocol. (a) Average surface roughness (Ra), (b) surface area difference (SAD), (c) contact angle, and (d) solid-liquid interfacial free energy ( ∆𝐺 ).

6

XPS analysis was performed to investigate the differences in the chemical compositions of the

7

prepared membranes (Table 2). TFN-30 and TFN-Conv showed similar chemical compositions, which is

8

consistent with the similar amount of ZIF-8 deposition and polyamide morphologies as shown in Figure

9

3 and 4. Comparisons of TFN-300 significantly deviate from other RO membranes. To investigate the

10

crosslinking degree of the polyamide layer, the N/O ratio from XPS analysis was calculated for each

11

membrane. Considering the chemical structure of the fully cross-linked polyamide (C18H12N3O3, N/O

12

ratio = 1) and the fully linear polyamide (C15H10N2O4, N/O ratio = 0.5), the higher N/O ratio implies the 13



1

polyamide layer is more crosslinked compared to the fully linear one.39 Note that the nitrogen (N)

2

concentration from the ZIF-8 composition (C8H12N4Zn)n was subtracted for TFN membranes to obtain

3

the corrected nitrogen (N*) concentration.14 TFN-30 and TFN-Conv had similar N/O ratio values, but

4

were lower than for TFC, implying that ZIF-8 incorporation led to the loosened polyamide layer, which

5

is expected to exhibit higher water permeance since a lower N/O ratio indicates a lower crosslinking

6

degree.40 However, such corrections were not applicable for TFN-300 due to too high Zn concentration

7

which overwhelms the contribution of the actual polyamide layer. In addition, a decrease in N

8

concentration and an increase in oxygen (O) concentration were also measured for TFN-300, which was

9

unexpected result considering the elemental composition of ZIF-8 and polyamide. Those observations are

10

attributed to the immature polyamide layers which were observed in Figure 4e-g and Figure S2c, possibly

11

providing more chance to detect elemental composition of PSf support. Thus, the results further

12

substantiate the defect formation in the polyamide layer due to excessive ZIF-8 deposition.

13 14

Table 2 Elemental compositions of prepared TFC and TFN membranes from XPS results. Atomic content (at.%) Zn C N N*,a O N/O N*/O TFC 75.0 11.8 13.2 0.89 TFN-30 0.4 73.9 12.1 10.5 13.6 0.89 0.77 TFN-300 6.0 65.4 10.5 18.1 0.58 TFN-Conv 0.4 73.6 11.9 10.3 14.1 0.84 0.73 a Corrected N concentration in polyamide layer, subtracting the N concentration ZIF-8 based on chemical composition of ZIF-8 (C8H12N4Zn)n and Zn concentration from XPS 14. Sample

15 16 17 18

The RO performances of the TFC and TFN-Spray membranes were evaluated using a cross-flow

19

filtration apparatus (Figure 6a and Table S2). All TFN-Spray membranes showed improved water

20

permeance compared to the pristine TFC membrane. In particular, TFN-30 showed the highest water

21

permeance (3.72 LMH/bar) with a 30% enhancement and higher NaCl rejection compared to the bare

22

TFC membrane. Such a superior RO performance is attributed to the loosened polyamide matrix resulting 14


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1

from ZIF-8 incorporation and the additional water transport channel provided by the intrinsic porosity of

2

the ZIF-8 nanofiller which could possibly discriminate water molecules (2.76 Å) and larger hydrated ions

3

(Na+ = 7.16 Å and Cl- = 6.64 Å, respectively) with its narrow pore aperture (3.4 Å).14 On the other hand,

4

a dramatic decrease in NaCl rejection and increase in water permeance was observed for TFN-Spray

5

membranes with higher than 60 mg/m2 ZIF-8 loading. This is attributed to the defect formation in

6

polyamide layer as previously discussed. SEM images of ZIF-8 deposited PSf supports show that the ZIF-

7

8 layer formed as a multilayer covering the entire PSf support surface. Thus, it is expected that the

8

overlapped and aggregated ZIF-8 multilayer was embedded as a nanofiller aggregation inside the coated

9

polyamide matrix during the interfacial polymerization. In addition, an excessive deposition of ZIF-8 also

10

changed the surface characteristics of the PSf support, eventually aggravating the defect-free formation of

11

polyamide layer, as observed in the SEM and XPS analyses. The results suggest that the nanofiller loading

12

optimization is critical to fully exploit the effects of nanofiller incorporation, and such influences on the

13

surface properties of support membranes should be considered for high concentration TFN membrane

14

optimization.

15

It should be noted that there has been contradictory results on the water stability of ZIF-8

16

as a recent reference pointed out.41 Thus, it can be said that water stability of ZIF-8 is highly

17

dependent on various unexpected factors such as particle size, synthesis method, temperature,

18

solution pH, and ZIF-8/water ratio, and thereby it was necessary to determine the water stability

19

of ZIF-8 in our experimental conditions. Recently, Wang et al. prepared “crumpled” polyamide

20

layer by immersing ZIF-8/polyamide TFN membranes into water to etch ZIF-8 nanoparticles.24

21

Inspired from the study, we investigated if ZIF-8 was etched out during RO tests by comparing

22

AFM images of tested TFN membranes with those of as-prepared TFN membranes. We found

23

that surface roughness and surface area difference (SAD) of all membranes were almost similar,

24

implying that the degradation of ZIF-8 nanoparticles inside the polyamide matrix was 15



1

undistinguishable even after exposed to our RO test condition (Figure S4 and Table S3). The

2

main reason why ZIF-8 was not etched after our RO test condition, which is contrast to the

3

work that Wang et al. reported, is attributed to the different polyamide chemistry we used

4

(MPD-based) compared to the report (piperazine (PIP)-based). It is expected that ZIF-8

5

embedded in PIP-based polyamide is highly susceptible to water degradation considering PIP-

6

based polyamide possesses much thinner layer (~10 nm) and loose structure compared to those

7

of MPD-based polyamide. Although a question on long-term water stability of ZIF-8 still

8

remains, we expect that further functionalization of ZIF-8 (or any other water unstable

9

nanofillers) could mitigate such a concern, which is beyond the scope of this study.42-43

10

Finally, the TFC, TFN-Spray, and TFN-Conv membranes RO performances were compared to

11

demonstrate the availability of the Spray method (Figure 6b). TFN-30 was selected as a representative

12

TFN-Spray membrane due to its optimal RO performance. Both the TFN-30 and TFN-Conv membranes

13

showed similar RO performance improvements compared to the bare TFC membrane. The TFN-Spray

14

membranes surpassed the TFN-Conv membranes in terms of cost-effectiveness. Compared to the Conv

15

method, the Spray method uses 100 times less nanofiller and a 45 s process time for nanofiller deposition.

16

Based on these results, we propose that the Spray method provides many advantages for TFN membrane

17

preparation by not only reducing the fabrication cost and time, but in realizing the precise control of the

18

nanofiller deposition amount. The Spray method also allows the use of diverse solvents, some of which

19

are better candidates for dispersing certain nanofillers than water or aliphatic hydrocarbons as long as the

20

solvent does not dissolve the support membrane used for the polyamide layer coating.

16


Page 16 of 21

Page 17 of 21

5.0

1 2 3

Water Permeance (LMH/Bar)

4.5

80

4.0 60 3.5 40 3.0 20

2.5

2.0

(b)

100

0

50

100

150

200

250

300

0

5.0

4.5

100

96.57

97.52

97.83 90

3.73

4.0

3.72

3.5

3.0

80

2.86 70

NaCl Rejection (%)

Water Permeance (LMH/Bar)

(a)

NaCl Rejection (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5

2.0

Amount of ZIF-8 Deposition (mg/cm2)

TFC

TFN-Conv

TFN-30

60

Membranes

Figure 6. (a) RO performances of TFN membranes prepared by Spray protocol depending on the amount of ZIF-8 deposition. (b) Performance comparison of RO membranes prepared by different protocols.

4 5

Conclusions

6

In this work, high-performance TFN membranes were successfully fabricated using a spray-assisted

7

nanofiller pre-deposition method (Spray method). ZIF-8 was used as a representative nanofiller to

8

overcome the challenges confronted in conventional preparation methods (Conv method). Precise control

9

of ZIF-8 deposition was achieved by altering the concentration of the ZIF-8 spray solution, which would

10

be useful for the fundamental transport studies on TFN membranes. The PSf support membrane’s surface

11

characteristics depended on the amount of ZIF-8 deposition, implying that such influences on the surface

12

properties should be considered when optimizing the nanofiller loading amount. TFN-Spray membranes

13

showed a superior RO performance compared to pristine TFC membranes until the optimal amount of

14

ZIF-8 deposition was reached. The spray method’s 30 mg/m2 of ZIF-8 deposition was enough to exhibit

15

a similar RO performance to that of TFN-Conv membranes, which require 3175 mg/m2 of ZIF-8

16

deposition. This demonstrates that the Spray method only uses 100 times minimized nanofiller within an

17

unprecedentedly short 45 s deposition time. Our results indicate that the Spray method is a promising

18

candidate for scaling-up TFN membranes in the roll-to-roll production process, which requires 17



1

consecutive nanofiller deposition, minimum use of nanofiller, and fast deposition time.

2 3

Acknowledgements

4

This work was supported by a grant from the Korea Water Resources Corporation (K-water)

5

(201700000000726).

6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42

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1 2

Table of Contents Figure:

3

4 5 6

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Facile Preparation of Polyamide Thin-Film Nanocomposite

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