Two-Dimensional Fractal Nanocrystals Templating for Substantial

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Two-dimensional fractal nanocrystals templating for SPECIAL FEATURE substantial performance enhancement of polyamide nanofiltration membrane

Yang Lua, Ruoyu Wangb, Yuzhang Zhua,1, Zhenyi Wanga, Wangxi Fanga, Shihong Linb,1, and Jian Jina,c,d,1

aInternational Laboratory for Adaptive Bio-nanotechnology, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou 215123, China; bDepartment of Civil and Environmental Engineering, Vanderbilt University, Nashville, TN 37235-1831; cCenter for Excellence in Nanoscience, Chinese Academy of Sciences, Beijing 100190, China; and dCollege of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, China

Edited by Howard A. Stone, Princeton University, Princeton, NJ, and approved December 29, 2020 (received for review September 24, 2020) In this study, we report the emergence of two-dimensional (2D) of the existing infrastructure or method of manufacturing TFC-PA branching fractal structures (BFS) in the nanoconfinement be- membranes and thus are prohibitively complex or too expensive tween the active and the support layer of a thin-film-composite to implement. A desirable approach for enhancing TFC-PA polyamide (TFC-PA) nanofiltration membrane. These BFS are crys- membrane performance should be simple, low cost, effective, and tal dendrites of NaCl formed when salts are either added to the readily integrated into the existing method of TFC-PA membrane piperazine solution during the interfacial polymerization process fabrication. or introduced to the nascently formed TFC-PA membrane before Herein, we report an elegant and highly practical method us- drying. The NaCl dosing concentration and the curing temperature ing two-dimensional (2D) fractal crystal dendrites to dramatically have an impact on the size of the BFS but not on the fractal di- increase the water permeance of the TFC-PA NF membrane ∼ mension ( 1.76). The BFS can be removed from the TFC-PA mem- while maintaining its solute rejection performance. By adding branes by simply dissolving the crystal dendrites in deionized NaCl to the aqueous PIP solution during the IP process, we water, and the resulting TFC-PA membranes have substantially observed that NaCl crystal dendrites emerged in the confinement higher water fluxes (three- to fourfold) without compromised sol-

between the PA layer and the PES support when the TFC-PA CHEMISTRY ute rejection. The flux enhancement is believed to be attributable membrane was cured by heat (Fig. 1 A–C). These spectacular to the distributed reduction in physical binding between the PA branching fractal structures (BFS) are considered to be 2D be- active layer and the support layer, caused by the exertion of crys- cause they are less thick when compared with the overall size of tallization pressure when the BFS formed. This reduced physical the BFS sprawling along the plane parallel to the membrane binding leads to an increase in the effective area for water transport, which, in turn, results in higher water flux. The BFS-templating surface. Dissolving the PES support using dimethylformamide revealed a large number of crystals adhering to the bottom of the method, which includes the interesting characteristics of 2D crystal SI Appendix dendrites, represents a facile, low-cost, and highly practical method PA film ( , Figs. S1 and S2), confirming the position of of enhancing the performance of the TFC-PA nanofiltration mem- the BFS to be between the PA active layer and the PES support. brane without having to alter the existing infrastructure of Elemental analysis using energy-dispersive X-ray spectroscopy D E membrane fabrication. (Fig. 1 and ) and crystal structure analysis using X-ray

nanofiltration membrane | high flux | interfacial polymerization | Significance fractal structure Fractal structures and phenomena have existed in nature for hin-film composite polyamide (TFC-PA) membranes are hundreds of millions of years. Developing their practical ap- Twidely used in reverse osmosis and nanofiltration (NF), plications in material design is of fundamental importance, but which have extensive and continuously growing applications in this goal has not yet been reached. In this work, NaCl crystals water treatment, desalination, and wastewater reuse (1–4). Typical with a fractal structure are formed between the polyamide TFC-PA membranes are fabricated using interfacial polymeriza- active layer and the support during an interfacial polymerization (IP), which involves a polymerizing reaction between tion process. The branching of fractal NaCl nanocrystals creates amine and acid chloride precursors at the water–oil interface numerous tiny interworking water channels that enable water (5–9). In a typical IP for producing TFC-PA NF membranes, a transport, maximizing the effective permeating area of the polyether sulfone (PES) ultrafiltration membrane is first impreg- polyamide nanofiltration (NF) membrane. The fractal NaCl – nated with an aqueous solution of piperazine (PIP) and then nanocrystals templated polyamide NF membrane exhibits an placed into contact with a hexane solution of trimesoyl chloride improved desalination performance with a three to four times (TMC). The PIP monomers diffuse across the water–hexane increase in permeance. Applying fractal structure successfully interface and react with the TMC to form a cross-linked dense to the design of artificial materials improves performance. PA film that serves as the active layer for water–salt separation Author contributions: Y.Z. and J.J. designed research; Y.L., R.W., and J.J. performed re- (3, 8, 10). This PA film is tightly bound to the underlying PES search; Y.L., R.W., Y.Z., Z.W., W.F., S.L., and J.J. analyzed data; and Y.L., Y.Z., S.L., and J.J. support layer, and the way they bind to each has a strong impact wrote the paper. on the water flux of the resulting TFC-PA membrane (11–15). The authors declare no competing interest. Enhancing the water flux of an NF membrane without com- This article is a PNAS Direct Submission. promising its solute rejection can potentially lead to substantial Published under the PNAS license. savings in treatment cost and has sizable practical impacts due to 1To whom correspondence may be addressed. Email: [email protected], shihong. the broad application of TFC-PA membranes. While extensive lin@vanderbilt.edu, or [email protected]. research (9, 10, 16–25) has been performed with the goal of This article contains supporting information online at https://www.pnas.org/lookup/suppl/ performance enhancement, many promising approaches (10, doi:10.1073/pnas.2019891118/-/DCSupplemental. 16–25) reported in the literature require significant modifications Published September 7, 2021.

PNAS 2021 Vol. 118 No. 37 e2019891118 https://doi.org/10.1073/pnas.2019891118 | 1of7 Downloaded by guest on September 26, 2021 Fig. 1. Formation process and surface morphology of BFS-templated TFC-PA membrane. (A) Schematic illustration of the process for preparing a BFS- templated TFC-PA membrane via interfacial polymerization. (B) Surface morphology of the BFS-templated TFC-PA membrane. (C) Close-up surface morphology of the BFS-templated TFC-PA membrane. (D and E) Elemental mapping images of Na (D) and Cl (E) on the surface of a BFS-templated TFC-PA − membrane. (NaCl concentration in PIP solution: 8 g·L 1; curing temperature: 60 °C).

diffraction (SI Appendix,Fig.S3) confirmed that these 2D BFS be explained merely by the increase in the specific surface area, as were indeed NaCl crystals. the specific surface area of the BFS-templated TFC-PA membrane (after dissolving NaCl) was very similar to that of the con- Results ventional TFC-PA membrane (SI Appendix,Fig.S7). Instead, this The presence of NaCl crystal dendrites significantly increased flux enhancement is caused by the increase in the effective area of the surface roughness of the TFC-PA membrane (Fig. 2 A and the active layer for water transport (Fig. 2I). The PES support B). The thickness of the BFS is estimated to be 78 ± 10 nm based layer has a surface porosity of ∼15%, meaning that the PA active on an analysis of the surface topography measured using atomic layer is in direct contact with ∼85% of the underlying PES sup- force microscopy (AFM). While the NaCl crystals can be removed port. This fraction of the PA layer area either is not available or is simply by dissolving them in deionized water, the BFS formation ineffective for water and solute transport. has lasting impacts on the binding between the PA layer and the Such a theory is corroborated by the observation in previous underlying PES support. Unlike the tightly integrated interface studies (16–18, 20, 26) that a porous interlayer (between the between the support and active layers in conventional TFC-PA active and the support layer) with a very high porosity can sub- NF membranes (Fig. 2C), voids were created where the BFS stantially enhance the water flux. In the case of a BFS-templated existed before NaCl dissolution (Fig. 2D). TFC-PA membrane, the emergence and subsequent removal of Interestingly, the addition of NaCl and the formation of the the BFS reduced the area of direct adhesion between the PA BFS did not seem to affect the properties of the PA active layer active layer and the PES substrate, thereby increasing the effective itself. Both the reference TFC-PA membrane (without BFS area of the PA active layer for water transport (Fig. 2I). This formation) and the TFC-PA membrane (with BFS dissolved after argument is also well supported by an approximate quantitative formation) had practically indistinguishable properties, including analysis that investigated the porosity of the support layer zeta potential (Fig. 2E), water contact angle (SI Appendix,Fig. and the surface coverage of the NaCl dendrite (SI Appendix, S4), elemental composition (SI Appendix,Fig.S5andTable section 2.7). S1), distribution of functional groups (SI Appendix,Fig.S6), The morphology of the BFS depends on the NaCl concen- pore size distribution (Fig. 2F), and rejection of several tested tration, as BFS with “wider branches” formed with a high con- common salts (Fig. 2G). In other words, the addition of NaCl centration of NaCl dosed in the PIP solution (Fig. 3 A–E ). However, had no impact on the formation of the PA layer via interfacial regardless of the NaCl concentration, the fractal dimension of the polymerization. BFS was consistently ∼1.76 as determined by the box-counting While the addition of NaCl did not affect the properties of the method (Fig. 3F and SI Appendix, Figs. S8 and S9), implying that PA active layer, it substantially improved the overall perfor- the BFS may form from the nucleation limited aggregation mance of the TFC-PA membrane. The formation of the BFS (NLA) model (27, 28). While the branches in the BFS formed resulted in a remarkable enhancement of the water permeance with a higher NaCl concentration appeared to be wider, we of the membrane. Specifically, a three- to fourfold increase in cannot measure the width of the branches due to the fractal water flux was measured, depending on the type of solution nature of the BFS (i.e., width varies depending on the scale). tested (Fig. 2H). The substantial improvement in water flux cannot Instead, we performed a quantitative analysis of the branch size

2of7 | PNAS Lu et al. https://doi.org/10.1073/pnas.2019891118 Two-dimensional fractal nanocrystals templating for substantial performance enhancement of polyamide nanofiltration membrane Downloaded by guest on September 26, 2021 SPECIAL FEATURE CHEMISTRY

Fig. 2. Properties of conventional and BFS-templated TFC-PA membranes. (A and B) AFM images and corresponding height profiles of conventional (A)and BFS-templated (B) TFC-PA membranes. Here, the BFS-templated TFC-PA membrane was not washed with water to remove the NaCl crystal dendrites. (C and D) Cross-sectional transmission electron microscopy (TEM) images of conventional (C) and BFS-templated (D) TFC-PA membranes. Here, the BFS-templated TFC- PA membrane was washed with water to remove the NaCl crystal dendrites. (E) Zeta potential of conventional and BFS-templated TFC-PA membranes. (F) Rejection of neutral organic solutes (200 ppm) including raffinose, sucrose, and glucose by the conventional and BFS-templated TFC-PA membranes (applied − pressure: 4 bar; cross-flow rate: 60 L·h 1). (Inset) The corresponding pore size distributions of TFC-PA membranes derived from the rejection are presented. (G) − Rejections of various salt solutions (1,000 ppm) by conventional and BFS-templated TFC-PA membranes (applied pressure: 6 bar; cross-flow rate: 40 L·h 1). (H) Fluxes of conventional and BFS-templated TFC-PA membranes with different salt solutions (1,000 ppm) as the feed solutions (applied pressure: 6 bar; cross- flow rate: 40 L·h−1). (I) Illustration of the increased effective area of the active layer by BFS templating. (NaCl concentration in PIP solution: 8 g·L−1; curing temperature: 60 °C).

using a metric called characteristic length, defined as the ratio formation of additional crystals. The simulated temporal evolu- between the area and the perimeter of the BFS and determined tion of an NaCl crystal dendrite resembles that observed using an by image analysis of the scanning electron microscopy (SEM) optical microscope (Fig. 4B). We speculate the formation mecha- images of the BFS (SI Appendix, section 2.9). As the dosing NaCl nism of BFS to be as follows (SI Appendix,Figs.S11–S19): a na- concentration increased, the characteristic length increased ac- scent NaCl nucleus is first attached to multiple carboxyl groups cordingly (Fig. 3F), which is semiquantitatively consistent with the of the PA layer and then grows larger with further crystallization. increase in BFS thickness of the BFS-templated PA active layer as NLA occurs locally around an immobilized nucleus along a thin measured using AFM (Fig. 3F and SI Appendix,Fig.S10). film of water sandwiched between the PA and the PES support To better understand how BFS form, we performed a Monte layer. The local growth of nanocrystal particles exerts a crystal- Carlo simulation following NLA (SI Appendix, section 2.11)to lization pressure (29–31) that further enlarges the gap between emulate the formation of the 2D BFS (Fig. 4A) and obtained the PA and the support layer, which facilitates the further ex- structures similar to those shown in Figs. 1B and 3 A–E. The pansion of the BFS. NLA assumes the presence of a central seed (i.e., the nucleus) Interestingly, if we soak in an NaCl solution a nascent (and from which the crystal grows radially, and that existing crystal still wet) TFC-PA membrane formed without dosing NaCl in the particles on the fractal structure can serve as new nuclei for the aqueous PIP solution, fractal NaCl dendrites also form with a

Lu et al. PNAS | 3of7 Two-dimensional fractal nanocrystals templating for substantial performance enhancement https://doi.org/10.1073/pnas.2019891118 of polyamide nanofiltration membrane Downloaded by guest on September 26, 2021 Fig. 3. The effect of NaCl concentration on BFS morphology. (A–E) Surface morphology of BFS-templated TFC-PA membranes prepared with different NaCl concentrations in PIP solutions. (F) Variation of fractal dimension, characteristic length, and thickness of BFS formed with respect to NaCl concentrations.

high degree of similarity to those formed by adding NaCl to the of the PES support increases to the submicrometer range, NaCl aqueous PIP solution during interfacial polymerization (SI Ap- crystals form randomly without distinct fractal characteristics (SI pendix, Fig. S20A). In addition, the water permeance of a TFC- Appendix, Figs. S21–S24). PA membrane modified via such an approach of postfabrication BFS templating was also substantially enhanced (SI Appendix, Discussion Table S2). However, if we dry a conventional TFC-PA mem- The impact of the NaCl dosing concentration (in the aqueous brane first, and soak the dried membrane in an NaCl solution, PIP solution) on the BFS morphology can influence NF per- neither BFS formation (SI Appendix, Fig. S20B) nor enhance- formance (SI Appendix, Table S3 and Fig. S25). Using an ment in water flux is observed (SI Appendix, Table S2). The Na2SO4 solution, for example, the flux increased from ∼48 to − − comparison between these two experiments reaffirms the im- ∼198 L·m 2·h 1 when the NaCl dosing concentration increased − portance of the presence of a thin film of water in the formation from 0 to 32 g·L 1, which correlates well with the extra effective of the 2D BFS. Another key factor that determines whether BFS area created by the BFS templating (Fig. 5A). The Na2SO4 re- can form is the pore size of the PES support. When the pore size jection was barely affected by the NaCl dosing concentration up

Fig. 4. The Monte Carlo simulation and growth process of BFS. (A) Monte Carlo simulation of the formation process of BFS based on the NLA model. (B)The growth process of BFS as recorded by optical microscopy. (NaCl concentration in PIP solution: 32 g·L−1; curing temperature: 60 °C).

4of7 | PNAS Lu et al. https://doi.org/10.1073/pnas.2019891118 Two-dimensional fractal nanocrystals templating for substantial performance enhancement of polyamide nanofiltration membrane Downloaded by guest on September 26, 2021 − to 16 g·L 1 but became compromised when the NaCl dosing con- NaCl dosing concentration. Regardless of the curing temperature, centration was too high. Evaluating NF performance with four the Na2SO4 rejection was consistently high. However, the water other salts (SI Appendix, Table S3) and neutral solutes (SI Ap- permeance of the conventional TFC-PA membranes without SPECIAL FEATURE pendix, Fig. S25) also reveals qualitatively similar dependence of BFS templating dropped when the curing temperature exceeded performance on the NaCl dosing concentration. The deteriora- 40 °C due to the densification of the PA layer (32–34). Because tion in solute rejection performance is believed to be caused by of the extra effective area, BFS templating overcame such a the overstretching of the PA active layer due to the crystalliza- performance deterioration at a high curing temperature and tion pressure exerted by the relatively thick crystal dendrites maintained a very high water flux regardless of the curing tem- formed at a high NaCl dosing concentration (30). The over- perature. Balancing the effects of the extra effective area and the stretching of the active layer reduced its thickness and resulted in densification of the PA layer, the optimum curing temperature lower solute rejections. Notably, we observed no macroscopic ∼ defect from physical piercing by the forming crystals (SI Ap- was identified to be 60 °C. Our hypothesis regarding the flux enhancement by distributed pendix, Fig. S26). We also investigated the impact of the membrane curing reduction of PA adhesion to the support layer is again corrob- temperature on the BFS morphology (SI Appendix, Fig. S27) and orated by a nice linear correlation between the flux enhancement the NF performance of the BFS-templated TFC-PA membranes factor and the effective area enhancement factor in a series of (Fig. 5B). Increasing the curing temperature reduced the char- TFC-PA membranes fabricated using different NaCl concen- acteristic length (i.e., the BFS become “slimmer”) but increased trations and curing temperatures (Fig. 5C). The only outlier in the extra effective area (Fig. 5B and SI Appendix,Fig.S28); this the correlation is the membrane obtained with a curing tem- differs from the positive correlation between the characteristic perature of 80 °C (Fig. 5C). It has been reported previously that length and the extra effective area observed when changing the the pores of the support layer will collapse at a high curing CHEMISTRY

Fig. 5. Desalination performance of BFS-templated TFC-PA membranes. (A) Water flux, rejection, and extra effective area variation of BFS-templated TFC-PA − membranes with different NaCl concentrations in PIP aqueous solutions (applied pressure: 6 bar; cross-flow rate: 40 L·h 1). (B) Water flux, rejection, and extra − effective area variation of BFS-templated TFC-PA membranes with different curing temperatures (applied pressure: 6 bar; cross-flow rate: 40 L·h 1). (C) Correlation between the flux enhancement factor and the effective area enhancement factor calculated based on the measured surface porosity of 15%. Due to the uneven distribution of pores on the membrane surface, the surface porosity is biased for different samples. The flux enhancement factor is defined as the ratio between the water flux measured with a BFS-templated membrane and that measured with a referenced TFC-PA membrane, whereas the effective area enhancement factor is defined as the ratio between the effective area of a BFS-templated membrane and that of a referenced TFC-PA membrane. The R2

for correlation was calculated without using the data represented by the outlier at 80 °C. (D) Water flux and rejection of the Na2SO4 solution by the BFS- templated TFC-PA membrane under varying cross-flow rates (solid squares, applied pressure: 6 bar) and under varying applied pressures (open circles, cross- flow rate: 40 L·h−1).

Lu et al. PNAS | 5of7 Two-dimensional fractal nanocrystals templating for substantial performance enhancement https://doi.org/10.1073/pnas.2019891118 of polyamide nanofiltration membrane Downloaded by guest on September 26, 2021 temperature (33–35), which results in a very low flux, even with found in NaCl, KCl, and RbCl dendrites (SI Appendix,Fig.S35). the reference TFC-PA membrane (without BFS templating). In all cases, 2D crystal templating can enhance the water flux of Because water flux enhancement is attributable to the in- TFC-PA membranes without compromising their salt-rejecting creased effective area of the active layer and the overall weaker performance (SI Appendix,TableS5). binding between the active layer and the PES support, a BFS- In summary, our work shows that 2D BFS can form in the templated TFC-PA membrane may face the risk of compromised confined space between the PA active layer and the porous PES mechanical integrity and even delamination of the active layer. support in a TFC-PA NF membrane. Templating the TFC-PA Fortunately, these detrimental impacts were not observed in our membrane with these 2D BFS results in interconnecting net- experiments in which the BFS-templated membranes were sub- works between the two layers and an increased effective area of ject to a wide spectrum of NF operating conditions. Specifically, PA for water and solute transport but does not alter the prop- both the water flux and the Na SO rejection were constantly 2 4 erties of the PA active layer (except when the concentration of high, regardless of the cross-flow rate (Fig. 5D and SI Appendix, the crystal-forming salt is too high). Consequently, the templated Figs. S29–S31) and applied pressure (Fig. 5D), and the performance was stable with each set of operating conditions over 90 h TFC-PA membranes have dramatically higher water flux and of experiments (SI Appendix, Fig. S32). uncompromised solute rejection. The mechanism of performance The sustained mechanical integrity of the BFS-templated enhancement using this novel BFS-templating approach is similar TFC-PA membrane is likely attributable to the distributed pres- to that using a fibrous interlayer of nanotubes or nanostrands. ence of a strong binding area between the PA and the PES sup- Compared with using a physical interlayer, however, BFS tem- port layer. In theory, the strength of binding is not affected in plating is substantially more practical due to its low cost and easy regions not occupied by the NaCl crystal branches. If the NaCl implementation by retrofitting the existing infrastructure for formed big cubic single crystals instead of 2D BFS such as those roll-to-roll manufacturing of TFC-PA membranes. we observed, not only does local delamination become more likely, but the active layer may also become nonfunctional due to Materials and Methods overstretching or even physical damage. In this regard, BFS Materials and methods for membrane preparation, membrane performance templating truly takes advantage of the characteristics of spa- evaluation and characterization, Monte Carlo simulation, a detailed de- tially distributed fractals. While it may be argued that it is the 2D scription of fractal dimension, and characteristic length calculation can be distributed structure of the BFS, not the fractality of it, that found in the SI Appendix. contributes mainly to enhancing the performance of the TFC-PA membranes without compromising their mechanical integrity, the Data Availability All study data are included in the article and/or SI Appendix. common chloride salts that we experimented with happen to form There are no data underlying this work. beautiful fractal structures that serve the same role. While we used NaCl as the dosing salt for BFS formation ACKNOWLEDGMENTS. This study was supported by grants from the Na- tional Key Research and Development Plan (Grant 2019YFC1711300), the throughout, other inorganic minerals may be used for the same National Natural Science Funds for Distinguished Young Scholars (Grant purpose. For example, KCl can also form morphologically similar 51625306), the National Natural Science Foundation of China (Grants crystal dendrites (SI Appendix,Fig.S33) with a fractal dimension 21988102 and 51873230), the Science and Technology Service Network (SI Appendix,TableS4) similar to that of NaCl dendrites. In ad- Initiative program of the Chinese Academy of Sciences (Grant KFJ-STS-QYZD- dition to NaCl and KCl, RbCl can form BFS, although the fractal 141), and the Natural Science Foundation of Jiangsu Province (Grant characteristic becomes less obvious as the structure boundary BK20180259). R.W. and S.L. acknowledge partial support from the US NSF SI Appendix via Grant CBET 1739884. We also appreciate Prof. Xing Yang at Katholieke becomes blurred at a smaller scale ( ,Fig.S34). With Universiteit Leuven for the computational fluid dynamics (CFD) simulation CsCl, another chloride salt of alkali metal, the 2D crystal still has and Ms. Yi Zhang at Suzhou Institute of Nano-Tech and Nano-Bionics for some scale-invariant similarity but not the branching structures SEM characterization.

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