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Robust Superhydrophobic Polytetrafluoroethylene Nanofibrous Coating Fabricated by Self-assembly and its Application for Oil/water Separation Chaolang Chen, Chuan Du, Ding Weng, Awais Mahmood, Dong Feng, and Jiadao Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.8b00315 • Publication Date (Web): 30 May 2018 Downloaded from http://pubs.acs.org on May 30, 2018

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ACS Applied Nano Materials

Robust Superhydrophobic Polytetrafluoroethylene Nanofibrous Coating Fabricated by Self-assembly and its Application for Oil/water Separation Chaolang Chen, Chuan Du, Ding Weng, Awais Mahmood, Dong Feng, Jiadao Wang* Sate Key Laboratory of Tribology, Tsinghua University, Beijing 100084, P.R. China Abstract: A novel and facile method is developed to fabricate polytetrafluoroethylene (PTFE) nanofibrous coating on both flat surface and curved micro metal fibers via self-assembly technique. The process starts from PTFE nanoparticles as building blocks and utilizes electrostatic interaction as self-assembly driving force followed by molecular rearrangement through thermal aging treatment. The formation process of nanofibers from nanoparticles was experimentally investigated and its mechanism was discussed in detail. As-prepared PTFE nanofibrous coating presented ultra-thin morphology with high uniformity and outstanding chemical stability. Porous metal fiber sintered felt (PMFSF) coated with nanofibers exhibited superhydrophobic and superoleophilic properties and could separate various oil/water mixtures with high efficiency and good reusability. The coated PMFSF also presented excellent anti-corrosion. Overall, the self-assembling fabrication method presented in this paper is simple, cost-efficient, productive, very applicable for large-area treatment and industrialization. Keywords: polytetrafluoroethylene (PTFE) nanofibrous coating, self-assembly, anti-corrosion, superhydrophobicity, oil/water separation

1. Introduction Oil/water mixture is common contaminant in daily life and industry, which generated in food,

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textiles, petroleum, mechanical processing and so on [1-3]. The direct discharge of oil/water mixture will damage the ecological system and threat human’s healthy, and the frequent oil spill accidents also lead to serious pollution and energy waste [4-6] . Oil/water separation has become a worldwide challenge [7-9]. The traditional techniques such as oil skimmers, air flotation and centrifuge have been developed to separate oil/water mixture, however its limited by low separation efficiency, cumbersome equipment and high cost [10-12]. Recently, the porous material with superhydrophobic and superoleophilic property for selectively adsorbing or filtrating oil from oil/water mixture has attracted increasing attention due to their high selectivity, high efficiency, and simplicity [13-15]. Since the surface wettability of material could be tuned by adjusting surface energy and morphology, various material with simultaneously superhydrophobic and superoleophilic characters have been developed by modifying a micro/nanostructured surface with low surface energy composition or constructing roughness morphology on low surface energy materials [16-19]. Until now, various of low energy materials such as polytetrafluoroethylene (PTFE) [20], fluorinated alkyl silane (FAS) [21-23], stearic acid (STA) [24] and so on [25-27] have been proposed to fabricate superhydrophobic materials for oil/water separation. Among them, PTFE is an ideal candidate for construction of oil/water separation material because of their unique and superior properties, including outstanding chemical resistance, excellent thermal stability, electrical insulation and extremely low coefficient of friction [28-32]. Jiang et al. [20] fabricated a PTFE coated stainless mesh via spray and dry method, which successfully separated diesel and water mixture. Yong and co-workers developed a robust superhydrophobic PTFE porous membrane by laser and


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drilling process, which could separate the mixtures of oil and corrosion acid/alkali solution [33]. Qing and co-workers created a PTFE nanofibrous membrane by electrospinning, which was applied for oil/water separation and also possessed good abrasion resistance [34] . Our previous work reported PTFE nanoparticle and polystyrene coated filter paper that showed excellent durability [19]. However, the current methods used to fabricated PTFE membrane and PTFE modified material are limited by either complex processing equipment or relative high cost, and also inapplicable for modification of complex substrate. Layer-by-layer (LBL) self-assembly is an inexpensive, environmentally friendly and bottom-up approach for developing multifunctional particle coating, which makes it possible to fabricate coating with controllable thickness at nanoscale level on almost any substrate, and the thin coating will not change micro-scale morphology of substrate [35-38]. Herein, here we developed a facile method to prepare PTFE nanofibrous coating on both fat surface and porous metal fiber sintered felt (PMFSF) via LBL self-assembly followed a sintering process. The formation mechanism of NFs from NPs during sintering process was experimentally investigated and discussed. The performance of NF coating in harsh conditions like strong acidic, alkaline and salty solution was examined. In addition, the coated PMFSF also exhibited superior superhydrophobicity/superoleophilicity and was applied for the separation of various oils from water, and the results demonstrated they have potential and promising industrial application. 2. Experimental Section 2.1. Materials. All materials were purchased commercially and were used without further purification. PTFE NPs colloid solution with 60 wt% mass fraction and average 200 nm of



particle size was obtained from 3F New Material Co. Ltd., China. Sodium chloride, PDDA and ethanol and acetone were obtained from Aladdin Industrial Co., Ltd., China. Gasoline was obtained from China National Petroleum Corporation, China. China. Carbon tetrachloride, decane, silicone oil, and trichloromethane were obtained from J&K, China. H2SO4 (98%wt, extra grade) and NaOH were obtained from J&K, China. The water used in this work was deionized water. Silicon wafer was obtained from Zhejiang Dongli Co., Ltd., China. PMFSF was obtained from Hebei Huanyu-Metal Co., Ltd., China. Silicon wafer and PMFSF were cleaned by utilizing ultrasonic cleaner, in acetone, ethanol and deionized water for 15 min respectively, and then dried by using nitrogen gas before use. 2.2. Fabrication of PTFE NP coatings. Substrates were coated by NPs via electrostatic attraction self-assembly. In the beginning, substrates were modified by being immersed in PDDA (0.4 wt%) /sodium chloride (0.05 mM) composite solution for 10 min to render the substrates positively charged. Subsequently, the substrates were placed into PTFE dispersions (6 wt%) for 15 min to form PTFE NP coatings. In order to ensure the substrate coated thoroughly by NPs, an appropriate cycle times of above procedure was carried out. The samples coated by NPs were heated up to 320~420

at a rate of about 10 /min in furnace (GSL1500X). Then the samples

were kept at that constant temperature for 20~60min to form NFs. Finally, the obtained NFs were cooled down in air or furnace. 2.4. Oil/water separation. The as-prepared coated PMFSF was put on beaker and a mixture of 30 mL oil colored with methyl red and 30 mL of water was poured slowly into the upper glass tube. The separation efficiency was calculated by using the ratio of the mass of water before and


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after separation process according to following equation [19, 39].

η=

m × 100% m0

Whereη is the separation efficiency, m0 is the mass of the water before separation and m is the mass of the water after separation. 2.5. Characterization: Morphology of PTFE NF coatings was observed by FEI Quanta 200 FEG field emission scanning electron microscope. The chemical composition was analyzed by the energy dispersive spectroscopy (EDS). All measurements of WCA and WSA were conducted using an OCA 25 machine (Data-Physics, Germany) at ambient temperature. The volume of water used for measurements of WCA and WSA is 4µL and 10µL, respectively. The average value of at least three measurements performed at different positions on the same sample was adopted as the WCA or WSA. The rolling state of water droplet and the oil penetrating process were recorded by a High Speed Imaging System (AcutEye, RockeTech Technology Co., Ltd., China).

3. Results and Discussions 3.1 Preparation and Characterization of PTFE NFs Overall procedure to fabricate coating composed of PTFE NFs was schemed in Figure 1. Firstly, we fabricated NP coating on PMFSF by carrying out the self-assembly method based on electrostatic attraction, which has been successfully applied to prepare various NP coating onto different substrates [40, 41]. The substrates were modified by being immersed in poly dimethyl



diallyl ammonium chloride (PDDA)/sodium chloride composite solution to render itself positively charged, and then PTFE NPs were adsorbed onto the metal fibrous surfaces driven by electrostatic attraction between them, thus resulting in PMFSF coated by NPs. The SEM images in Figure (a-c) reveals that the stainless metal fibers including internal and external were wrapped completely and formed close-packed NPs coating. The wettability of NPs coated PMFSF was also measured. As showed in Figure 2d, the water contact angle of coated PMFSF decreased from 142° to 0° within 35 minutes, showing unstable wettability. This could be explained that NPs on PMFSF was unstable and easily destroyed by water.

Figure 1. Schematic diagrams of the preparation process of PTFE nanofibrous coatings

Figure 2. (a-c) SEM images of PTFE NP coated PMFSF. (d) The relationship betwwen


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WCA and time. Then NP coatings were sintered to obtain PTFE nanofibrous coating. As shown in Figure 1, the sintering process can be divided into three stages. Firstly, the NP coating was heated up to a suitable temperature that is usually higher than its melting point, thus facilitating NPs to transform from crystalline phase to amorphous state. The thermal movement of PTFE macromolecules was greatly accelerated with the increase of temperature. Secondly, the coating was kept at constant temperature for sufficient time to make the NPs thoroughly merged and formed larger PTFE aggregates. Finally, the PTFE NFs were created during cooling process. Furthermore, to investigate the formation mechanism of NFs from NPs, the morphologies of NPs were examined after sintered at 380

for 0 min, 15min, 20 min and 40 min. As shown in

Figure , the SEM images and EDS mappings reveal the evolution process of NPs, with the sintering time increased, the NPs gradually melted and rearranged to form fibers. Additionally, the change of in situ NPs was observed before and after sintered 380 Figure

for 40 min. As shown in

(e1, e2), it can be seen that PTFE NPs were randomly deposited on substrate before

sintering process. Nevertheless, after heat treatment, some NPs (for example Region A1) moved and merged with nearby NPs, and only left some trace (Region A2), which could be contributed to the collision of separate particles with surrounding particles driving by thermal movement. Compared with Region B1and B2, the NPs were transformed to willow-leaf-like fibers. As shown in Region C1 and C2, It can be also seen that longer fiber could be obtained from closed-packed NPs. Additionally, the sintering temperature played a critical role in the formation of NFs. As



shown in Figure S1, when the sintering temperature was below 360

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, the NPs melted but no

fiber was formed. As temperature increased further, willow-leaf-like fibers began to appear and the obtained nanofibers initiated cross-linking each other, resulting in the formation of nano-scale pores and apertures (Figure S1c). PTFE molecule chain is helical shape and all the fluorine atoms are in the outer layer [29]. When being heat-treated, the PTFE macromolecule and CF2 group would reorient along the longitudinal axis [42]. As shown in Supporting Information (Scheme S1), with the temperature increasing, the PTFE began to melt and form isotropic solution. Moreover, the higher temperature enhanced the thermal movement of PTFE macromolecules and increased the probability of collision between adjacent particles, so forming lager PTFE aggregates (Scheme S1b). Finally, the PTFE macromolecules stretched and entangled with each other to form disordered willow-leaf-like nanofibers during the crystallization in air (Scheme S1c) [43-45]. That is, the formation of these nanofibers is proposed to occur by means of a liquid-crystal ‘templating’ mechanism [46-48].

Figure 3. SEM images and corresponding to EDS mappings of PTFE NPs after sintered at 380 for different time of 0min (a1, a2), 15min (b1, b2), 20min (c1, c2) and 40min (d1, d2).


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SEM images of in situ PTFE NPs before (e1) and after (e2) sintering. 3.2 . Properties and Oil/water Separation Application of PMFSF Coated by PTFE NFs. 3.2.1. Wettability and Anti-Corrosion Properties of Coated PMFSF The SEM images of the PMFSF before and after coated with PTFE NFs are shown in Figure 4. It can be seen that pristine PMFSF is consisted of cross-connected metal microfibers with smooth surface and has an average diameter of 31µm (Figure 4(a, b)). After PTFE colloid and molecular self-assembly process, all the metal microfibers were wrapped by a dense layer of interconnected PTFE NFs, showing hierarchical rough structures (Figure 4(d, e)). The surface chemical composition of uncoated and coated PMFSF was also examined by energy dispersive spectroscopy (EDS) (Figure 4(c, f)). The spectrum of pristine PMFSF shows peaks for ferrum (66.88%, relative atom percentage by element), carbon (0.46%) and chromium (17.27%), and no fluorine was detected. An EDS scan of coated PMFSF yields ferrum (33.62%), carbon (9.29%), chromium (9.1%), and fluorine (37.07%). The appearance of fluorine peak and the increase of carbon are attributed to the PTFE. The difference in surface chemistry and geometrical roughness is reflected in the surface wettability of samples. The water contact angle (WCA) images of uncoated and coated PMFSF are shown in Figure 5. The pristine PMFSF exhibited intrinsic hydrophobic property with contact angle of 130° but high adhesion with water, thus the water cannot roll off even when PMFSF was turned upside down Figure 5b. After coated with PTFE NFs, the PMFSF exhibited superhydrophobicity with an average WCA of 156° and the water droplets could easily roll off



from the slightly tilted surface (water sliding angle less than 6°), as shown in Figure 5(d, e). A jet of water from a pipet would bounce off the surface of coated PMFSF without leaving a trace (Figure S2). However, coated PMFSF can be readily wetted by oil not only in air but also underwater (Figure 5f and see details in Movie S1 in Supporting Information), showing superoleophilicity. The surface wettability transformation from hydrophobic to superhydrophobic can be attributed to the combination of multi-scale roughness structures and low surface energy of PTFE NF. The effects of cycle times of LBL on the wettability and intruding pressure of as-prepared PMFSF were investigated. As shown in Figure S3a, with the increase of cycle time, the WCA increased and WSA decreased, which can be contributed to the increase of the coverage of NFs. Additionally, the intruding pressure is another important parameter to test the property of oil/water separation material, which was examined by maximum supporting height of water [49]. The measured intruding pressure of as-prepared PMFSF increased with the increase of cycle time (see details in Figure S3b), which can be explained by the increase of porosity and hydrophobicity [50]. After 5 times of cycle, the WCA of coated PMFSF reached 154° with WSA less than 6°, exhibiting superhydrophobic property.


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Figure 4. SEM images and EDS spectra of the pristine (a-c) and the coated (d-f) PMFSF.

Figure 5. (a) WCA measurement on pristine PMFSF. (b) Water droplet adhered to the surface of pristine PMFSF. (c) Oil and water droplets of different PH on uncoated PMFSF. (d) WCA measurement on coated PMFSF. (e) Water droplet rolled off from the tilted sample (less than 6°). (f) Oil and water droplets of different PH on coated PMFSF. Moreover, the coated PMFSF also shows superior repellency towards acidic, alkaline, and salt aqueous solution (3.5wt% NaCl solution). All these aqueous solution droplets keep spherical



shape on the coated PMFSF, as shown in Figure 5f. To investigate the anti-corrosion ability, the as-prepared PMFSFs were immersed in aqueous solution of various PH values for 60 h before measuring its WCA (the acidic solutions were prepared by H2SO4 , the alkaline solutions were prepared by NaOH and the PH7 solution was 3.5wt% NaCl solution). Figure 6 indicates the relationship between PH value and WCA of coated PMFSF. All the WCAs are in the 148-155° range with WSA less than 10°, thus indicating that aqueous solutions of various PH have little effects on the wettability of PTFE NF coating. Furthermore, the pristine PMFSF and PTFE NFs coated PMFSF were immersed in H2SO4 aqueous solution with 70% volume ratio for 12h. As shown in Figure 7, the SEM results show that the pristine PMFSF was damaged seriously and the WCA decreased from 130° to 0°, whereas, the PMFSF coated by PTFE NFs did not has obvious change and still keep superhydrophobic. The excellent corrosion resistance of PTFE NFs coated PMFSF can be attributed to two aspects. On the one hand, the mechanical strength was enhanced greatly due to the cross-connected structure between PTFE NFs. On the other hand, the metal fibers were wrapped thoroughly by PTFE NFs, thus inhibiting metal fibers from exposing to acidic solution. These results indicate that the PTFE NFs coated PMFSF possesses excellent chemical durability and anti-corrosion property, having potential industrial application.


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Figure 6. The effect of solution with different PH values on the wettability of coated PMFSF

Figure 7. SEM images of the pristine (a1, a2) and PTFE NFs coated (b1, b2) PMFSF after immersed in the H2SO4 (70%) for 12 h. 3.2.2. Oil/Water Separation The as-prepared PMFSF simultaneously possessing superhydrophobicity and superoleophilicity must be an excellent material for separation of oil/water mixture. To confirm the feasibility of the coated PMFSF in practice, the separation experimental procedure was carried out as shown in Figure 8 and Movie S2 in the Supporting Information. When oil (dyed with methyl red for easy observation) and water mixture was slowly poured onto the superhydrophobic and superoleophilic PMFSF, the oil quickly permeated through the coated PMFSF and arrived to the bottom beaker. Meanwhile, more and more water was retained thoroughly on the



superhydrophobic PMFSF. Figure 9a reveals the separation efficiency of different oil/water mixture is above 98%, demonstrating the coated PMFSF exhibited high efficiency for a variety of oil/water mixture, including carbon tetrachloride, decane, silicone oil, gasoline and trichloromethane. Besides, the coated PMFSF possessed excellent repeatability. The decane-water mixture was applied in the repeatability experiment. After each cycle experiment, the as-prepared PMFSF was ultrasonically cleaned with ethanol and water for 1min respectively to remove the residual oil and then dried in an oven at 60

for 30min. Subsequently, the dried PMFSF was used for next cycle. As shown in

Figure 9b, it can be seen that the WCA of as-prepared PMFSF decreased from 155° to 151° after 30 times of cycle, which maybe resulted from the residual oil in PMFSF. However, the separation efficiency of as-prepared PMFSF had no obvious change and still keep higher than 98%. All this results demonstrated that the coated PMFSF possessed excellent stability and repeatability.

Figure 8. (a-d) The oil/water separation process using as-prepared PMFSF. (a) Before separation. (b) Water was repelled by coated PMFSF. (c) Oil permeated through the coated PMFSF. (d) After


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separation.

Figure 9. (a) The separation efficiency of as-prepared PMFSF for various of oil/water mixtures. (b) Effect of cycle times on separation efficiency and water contact angle. 4. Conclusions In summary, a simple self-assembly method was developed for the large-area fabrication of PTFE nanofibrous coating on complex substrate. The formation of NFs from NPs during heat-treatment could be attributed to the molecular self-assembly of PTFE. However, the formation mechanism still needs further exploration. Moreover, the PMFSF coated with naonofibers exhibited simultaneous superhydrophobicity and superoleophilicity, and had been proved robust to resist strong acidic, alkaline and salty solution. This coated PMFSF can also separate a variety of oil/water mixtures, with an efficiency of more than 98%. Additionally, the as-prepared PMFSF could separate oil/water mixtures over 30 times of cycle with no obvious loss of superhydrophobicity and separation efficiency, indicating excellent repeatability. Besides, this PTFE nanofibrous coating fabrication method maybe have significant potential application in field of surface protective engineering of micro device.



ASSOCIATED CONTENT

Supporting information is available free of charge on the ACS Publications website or from the author. SEM images of obtained PTFE NF coating at different sintering temperature; The effects of PH solution on the WSA, WSA and intruding pressure of PTFE NFs coated PMFSF(PDF); Movies showing the wettability of oil on PMFSF and the separation process of oil/water mixture (.WMV). AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]. Notes The authors declare no competing financial interest. Acknowledgments We thank the funding support from National Natural Science Foundation of China Project under grant nos. 51375253 and 51775296. We also acknowledge the support of this work from the Tsinghua National Laboratory for Information Science and Technology, China. References [1] Yu, Y.; Chen, H.; Liu, Y.; Craig, V. S. J.; Wang, C.; Li, L. H.; Chen, Y. Superhydrophobic and Superoleophilic Porous Boron Nitride Nanosheet/Polyvinylidene Fluoride Composite Material for Oil-Polluted Water Cleanup. Adv. Mater. Interfaces. 2015, 2, 1400267.


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