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Sep 11, 2018 - These results further confirm that the as-prepared materials could be good candidates for efficient oil/water separation and wastewater...
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Durable and Recyclable Superhydrophilic−Superoleophobic Materials for Efficient Oil/Water Separation and Water-Soluble Dyes Removal Mengnan Qu,* Lili Ma, Yichen Zhou, Yu Zhao, Jiaxin Wang, Yi Zhang, Xuedan Zhu, Xiangrong Liu, and Jinmei He*

ACS Appl. Nano Mater. Downloaded from pubs.acs.org by 95.85.68.55 on 09/20/18. For personal use only.

College of Chemistry and Chemical Engineering, Xi′an University of Science and Technology, Xi′an 710054, China

ABSTRACT: Superhydrophilic/superoleophobic materials with specific wetting properties have attracted extensive attention during the recent years because of their extraordinary performance in oil/water separation. In this work, we developed a facile process to fabricate the superhydrophilic−superoleophobic material from kaolin nanoparticles. The obtained material performs universal oil repellency both in air and under water, and exhibits general applicability which can be extended to various relevant substrates, irrespective of their chemical composition. Furthermore, the as-prepared material is efficient to separate varieties of oil/water mixtures, even surfactant-stabilized oil-in-water emulsions, showing the high separation efficiency above 92%. What’s particularly attractive is that during the separation process, the water-soluble dye contaminants could be removed simultaneously, resulting in colorless and transparent filtered water. These results further confirm that the as-prepared materials could be good candidates for efficient oil/water separation and wastewater purification. Simple and scalable design of the fabrication process is also very desirable to obtain the specific wetting surfaces, making both materials and their fabrication methods attractive for scalability, which will offer variety of promising application in various fields. KEYWORDS: superhydrophilic−superoleophobic, kaolin, oil/water separation, dyes removal, wastewater purification

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marine antifouling coatings22 and reducing fluidic drag,23 which are of interest of both academia and industry. Inspired by naturally occurring underwater−oil-repellent mechanisms implemented in fish scales, short clam shells and seabirds,24−26 numerous bioinspired superoleophobic surfaces were fabricated by taking advantage of the cooperation of hydrophilic/oleophobic chemical composition and the rough surface morphology.27−31 Such novel materials are attractive for applications in microfluidic devices,32 industrial oily wastewater treatment33,34 and bioadhesion.35 However, the majority of these materials are superoleophobic under water and become superoleophilic in air.36−38 Thus, they need to be wetted prior before use to ensure a thin water layer formation between the material and oil, further protect the surface from the contamination by organic solvents or oils. Such low flexibility

INTRODUCTION Oily wastewater results from frequent oil spills,1−3 industrial production,4,5 and chemical leakage6,7 has caused serious environmental pollution and energy waste. Therefore, functional materials with special wettability that can effectively separate oil/water mixtures are in urgent demand. On the basis of advanced interface science, the bionic superhydrophobic materials have been mass produced to offer solutions for realizing oil/water mixture separations without energy consumption;8−10 however, these materials are easily polluted by oils, and hard to clean and reuse because of their typically oleophilicity.11,12 All these disadvantages considerably restrict their practical and commercial applications.13,14 Thus, researchers of the field expand their research range to develop and fabricate the more practical superoleophobic−superhydrophilic surfaces,15−18 which have oil contact angle (OCA) higher than 150° in conjunction with the low contact angle (CA) hysteresis and low water contact angle (WCA) ( 150°, revealing good oil repellency. When the material was placed in water, its surface still presented significant superoleophobicity (Figure 2f, g) with OCA ≈ 151° for the highdensity dichloromethane droplets (Figure 2f) and OCA ≈ 150° for the lower-density sunflower oil droplets (Figure 2g). Such unique property might be because of the water layer forming at the interface of the fluoroalkylsilane film, which was beneficial for oil resistance. Additionally, the as-prepared material displayed small contact angle hysteresis to oils. When a sunflower oil droplet was placed on the inclined surface with a 5° tilt angle, it could easily roll off (Figure 2h), demonstrating the super antifouling property of the fabricated surface. Unlike most previous reports, the as-prepared material is available to be applied on various substrates, such as glass, fabric, foam and sponge, etc. Notably, all these substrates hold plentiful hydroxyl groups on the surfaces, which could help to enhance the connection strength between coating materials and substrates. Therefore, the durable materials with super oilrepellency and super water-affinity were obtained, as shown in Figure 3a−d. The droplets of glycerol, sunflower oil and castor oil were all nearly spherical on the materials surfaces, whereas the blue water spread. In addition, using the coated cotton to selectively absorb water (dyed with methyl blue) from oil (dyed with methyl red) (Figure 3e−g). It can be found, once the cotton touched water in oil, it would be quickly wetted, and D

DOI: 10.1021/acsanm.8b01249 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Table 1. OCAs Measurements (deg, ± 1°) of Different Kinds of Oil Droplets on the As-Prepared Materials Surfaces That Were Modified with Varying Amounts of PFOA (in air) amount of PFOA/g

0.10

0.15

0.20

0.25

0.30

0.35

0.40

0.45

castor oil sunflower oil industrial white oil olive oli glycerol hexadecane

153 152 151 148 153 148

153.5 152 153 151 154 148

154 153 151 150 154 151

154 152 150 146 153 148

154 153 143 144 153 138

155 151 151 145 153 135

154 152 143 142 150 128

152 151 145 143 141 119

Figure 4. (a) Under water OCAs of dichloromethane droplets on the as-prepared kaolin materials surfaces modified with different amounts of PFOA. (b) OCAs and SAs of various oil droplets on the materials surfaces modified with 0.20 g of PFOA.

Figure 5. (a−h) SEM images at low- and high-magnifications of (a, e) pristine kaolin and kaolin modified with (b, f) 0.10 g, (c, g) 0.20 g, and (d, h) 0.30 g of PFOA, respectively. The insets in b−d show the values of the corresponding CAs of hexadecane droplets. The scale bars are (a−d) 5 and (e−h) 1 μm. (i−k) 3D surface topography of kaolin modified with (i) 0.10 g, (j) 0.20 g, and (k) 0.30 g of PFOA.

within 20 s, water was completely absorbed, simply leaving a transparent oil layer. The specific water absorption efficiency was calculated based on the following equation:

Ea =

E

mc′ − mc 100% m0 DOI: 10.1021/acsanm.8b01249 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Figure 6. (a) FTIR spectra of (a1) the pristine kaolin material and kaolin materials modified by (a2) 0.10 g, (a3) 0.20 g, and (a4) 0.30 g of PFOA, respectively. (b) XPS survey spectra of the pristine kaolin material and PFOA-modified kaolin material.

Surface Morphology and Chemical Compositions. Herein, to further study the intrinsic influence of the amount of PFOA on the surface oleophobicity, SEM and FTIR of the pristine kaolin material, and kaolin materials that were modified with 0.10, 0.20, and 0.30 g of PFOA, respectively, have been carried out and compared. Figure 5a−h shows the SEM images of the pristine and modified kaolin materials. According to the surface morphology of pristine kaolin material (Figure 5a, e), one could find that kaolin particles just simply aggregated to larger particles and look like loosely adhered to the glass surface, which did not have any contribution to the super oil repellency. For the kaolin material that was modified with 0.10 g of PFOA (Figure 5b, f), the images revealed that there were some microstructured roughness on the surface, large numbers of protrusions and pores formed upon ethanol evaporation, contributing a macroscopically rough surface with an OCA about 148°. However, such roughness was still not sufficient for superoleophobicity and more PFOA would be needed. When the kaolin material was modified with 0.20 g of PFOA, the resulted surface became very rough (Figure 5c) with small protrusions uniformly scattered on the microsize rough features (Figure 5g). Overall, the surface modified with 0.20 g of PFOA exhibited durable micro- and nanodual structures, which substantially reduced the contact area between oil and surface, favoring the stable Cassie−Baxter state56 and resulting in a good superoleophobicity. As the PFOA amounts increased to 0.30 g, the material surface still exhibited apparent oil repellency (Figure 5d), whereas the OCA did not exceed 150°, which indicated that the excess amount of PFOA would not contribute to the roughness anymore. The corresponding SEM shown in Figure 5h demonstrates that, there were plentiful crystalline-like films wrapped on some protrusions, which could be the fluorinated salts resulted from the reaction of excessive PFOA and sodium hydroxide.57 These smooth films could significantly decrease the roughness provided by the modified kaolin particles, bringing about an unsatisfactory oleophobicity. To further analyze roughness effect on overall surface oleophobicity, 3D surface profiles of different PFOA-modified kaolin materials were obtained (Figure 5i−k). The arithmetic mean surface roughness (Ra) and root-mean-square roughness (Rq) are commonly used to describe the surface height variation. Figure 5i shows the 3D surface topography of kaolin material modified with 0.10 g of PFOA, which reveals that there were a

Where mo is the water weight in the original oil/water mixture, mc and mc′ are the weights of coated cotton before and after absorption, respectively, and mc′ − mc is the weight of the collected water after separation. Water absorption efficiency of the coated cotton was about 95.7%, which is very promising for applications in purification of water-containing oils. Effects of the amount of PFOA on Surface Wettability. Generally, surface morphology and chemical composition are certified as two crucial factors to construct the superoleophobicity on a solid surface.54,55 Herein, the surface morphology and chemistry of the material were governed by the amount of PFOA, which not only defined concentration of long fluorinated chains on the modified kaolin, but also influenced distribution of these modified kaolin particles on the substrate surface, resulting the changed wettability. In this respect, we studied how superwettability of the PFOA-modified kaolin materials varied as a function of the PFOA amount. Eight equal sets of dry kaolin nanoparticles 1.5 g in weight were prepared and modified with different PFOA amounts ranging from 0.10 to 0.45 g. It was found that hydrophilicity of the PFOA-modified materials did not exhibit any significant change and water droplets still spread rapidly on these materials surfaces. However, oleophobicity of these PFOA-modified kaolin samples have changed to some extent. Table 1 summarizes OCA values of all eight sets. As the PFOA amount increased, OCAs of various oils first increased but then decreased. OCAs of some low-viscosity oils, such as olive oil and hexadecane, were lower than 150°. When PFOA approximated to 0.20 g, the material showed excellent oil repellency to various tested oils. Additionally, the as-prepared material was also oleophobic under water. To investigate the under-water oleophobicity of the eight sets of modified kaolin samples, dichloromethane served as the test oil, and its OCAs were shown in Figure 4a. Obviously, CAs of oil droplets under-water became greater as the amount of PFOA increased, especially up to 0.20−0.25 g of PFOA, the relevant OCAs were greater than 150°. On the basis of the above results, it could be concluded that when the amount of PFOA was about 0.20 g, the oleophobic property of the as-prepared material was optimal. The as-prepared material displayed superior oil repellency both in air and under water (Figure 4b), even for some organic liquids, such as glycerol and hexadecane, their CAs were still able to attain 150°, meanwhile the SAs were lower than 10°. F

DOI: 10.1021/acsanm.8b01249 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 7. (a−d) Gravity-driven separation process of different types of oil and water mixtures (vo:vw ≈ 1:1). Chromatographic column was filled with the as-prepared superhydrophilic-superoleophobic kaolin particles. (a) The sunflower oil (dyed with methyl red) and water (dyed with methyl blue) mixture before (a1) and after (a2) separation. (b) The hexadecane (dyed with methyl red) and water (dyed with congo red) mixture before (b1) and after (b2) separation. (c) Soybean oil and water (dyed with rosolic acid) mixture before (c1) and after (c2) separation. (d) Toluene and water (dyed with cresol red) mixture before (d1) and after (d2) separation. (e) Separation process of the methyl blue solution with the pristine kaolin particles. (e1, e2) Dye solution before and after separation, respectively. (f) UV−vis spectra of the methyl blue solution (f1) before and (f2) after separately filtering through the pristine kaolin materials and the as-prepared superhydrophilic−superoleophobic kaolin particles (f3); the inset in f shows photographs of (f1), (f2), and (f3).

triggered a great decrease of the surface energy, resulting in an obvious oleophobicity. Furthermore, the additional absorption peaks of the modified kaolin materials appeared at 650, 2790, and 2830 cm−1, respectively (Figures 6a2, a3, and a4) were mainly associated with the N−H, −CH3, and −CH2− groups of bis(3-trimethoxysilylpropyl)amine. Noticeably, there were two peaks with broad band at 3150 and 3440 cm−1 corresponding to the associated −OH groups in carboxylic acids and free −OH groups potentially generated by the reversible hydrolysis of the fluorinated ester groups. Therefore, the oleophobic material exhibited simul-hydrophilicity. It is worth mentioning that the peaks at 3440 cm−1 did not emerge in the spectrum of the kaolin material modified with 0.20 g PFOA (Figure 6a3), demonstrating the formed ester groups with long fluorinated chains hydrolyzed less in this situation, which was beneficial to reducing the surface energy and adhesion between the oil/solid surface. To clarify the specific wetting behavior of the modified materials, we have also rerecorded and depicted XPS spectra (Figure 6b). All the material samples were composed of Al, Si, O, and C elements. However, in comparison with the pristine kaolin material, a new signal peaks at 688.5 eV emerged on the surface of PFOA-modified kaolin material, which was assigned to F 1s, confirming the long fluorinated chains introduced onto the modified surface successfully. Besides, the intensity of F 1s was higher than that of other elements’ signals, proving the high concentration of fluoride segments on the modified surface, which is consistent with the results of FTIR. In light of the above results and analysis, it can be concluded that oil-repellent property of the as-prepared materials is closely associated with the amount of PFOA. Considering 1.5 g kaolin

lot of micro- and nano- scaled peaks and valleys on the modified surface, and the corresponding values of Ra and Rq were 2.211 and 2.814 μm, repectively. The surface roughness of kaolin material modified with 0.20 g of PFOA increased significantly (Figure 5j). Corresponding Ra and Rq values for this modified surface were 2.699 and 3.536 μm, respectively, larger than that of the kaolin material modified with 0.10 g of PFOA, indicating more superoleophobic property. When increased the amount of PFOA to 0.30 g, as we could see in Figure 5k, the modified surface became relatively smooth, and the corresponding values of Ra and Rq were 0.872 and 1.156 μm, respectively. These values were lower than that of the kaolin material modified with 0.20 g of PFOA, probably because the fluorinated salts greatly shielded the surface roughness, leading to the decreased oleophobicity, which is well in accordance with the results of SEM. Different chemical surface groups with specific properties greatly impact surface energy and interface properties. To confirm the inference, we have obtained the FTIR spectra of the pristine and modified kaolin materials, as noted in Figure 6a. The peak at 3440 cm−1 from the spectrum of the pristine kaolin material (Figure 6a1) was ascribed to the free −OH groups of kaolin, indicating the material was intrinsically hydrophilic. After the modification with PFOA, two additional peaks at 1440 and 1370 cm−1 appeared (Figures 6a2, a3, and a4), which were separately assigned to the −CF3 and −CF2 groups of PFOA. Besides, the new strong absorption peaks at 1700 and 1560 cm−1 were corresponded to the stretching vibration of CO for the ester carbonyl groups and carboxyl groups, respectively. All these results verify that the fluorinated ester groups were successfully grafted onto the modified kaolin surfaces, which G

DOI: 10.1021/acsanm.8b01249 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 8. (a, b) Photographs of oil-in-water emulsion (dyed with methyl blue, vo:vw ≈ 1:60) (a) before and (b) after filtering through the as-prepared superhydrophilic-superoleophobic kaolin particles. (c) UV−vis spectra of methyl blue dyed oil/water emulsion before and after filtering through the layer of as-prepared superhydrophilic-superoleophobic kaolin particles. (d−g) Optical microscopic images (d, g) and digital photos (e, f) of the dyed oil/water emulsion before and after separation, respectively.

particles as the raw materials, when the PFOA amount was about 0.10 g, the kaolin surfaces were modified unevenly. As a result, the surface roughness and surface energy of the modified kaolin particles were unable to provide the as-prepared coating materials with superoleophobicity. On the other hand, when the PFOA amount was more than 0.30 g, it is probably a little bit excessive, the redundant PFOA could react with sodium hydroxide, forming the smooth fluoride salts layers on the material surface. These layers might have leveled the roughness of the modified material surface, leading to the poor oleophobicity. However, when the amount of PFOA was about 0.20 g, it would be appropriate, the as-prepared material surface was modified relatively uniform, providing the superior superwettability. The superhydrophilic−superoleophobic materials for all further tests were prepared according to the best ratio of PFOA (0.20 g) and kaolin material (1.5 g). Applications for Oil/Water Separation and WaterSoluble Dyes Removal. Nowadays, materials with superhydrophilicity-superoleophobicity have drawn widespread attention because of their superior performance in selective oil/water separation.58−61 In this work, the as-prepared materials also have been extend to separate a series oil/water mixtures by virtue of their outstanding wetting property, and the specific process was conducted as shown in Figure 7. The superhydrophilic−superoleophobic kaolin particles were compactly filled in the column chromatography (4 cm in diameter) with a layer of silica wool serving as the support base. A beaker was placed below the valve to collect the filtered water. All oil/ water mixtures that used in the gravity-driven separation experiments were about 1:1 volume ratio poured onto the layer of the modified kaolin materials, and separated into two immiscible layers (Figure 7 a1−d1).

As Figure 7a1 shows, when pouring the mixture of sunflower oil (dyed with methyl red) and water (dyed with methyl blue) (50% v/v) onto the layer of the as-prepared kaolin particles, water in the mixture was allowed to gently permeate through the layer because of the excellent water affinity of the modified kaolin particles, whereas the upper sunflower oil was blocked and collected in the column chromatography because of the extreme oil repellency of the modified kaolin materials. After the separation, the filtered predyed water in the beaker became colorless and transparent. Meanwhile, no oil was visible in the collected water (Figure 7a2). Such similar separation phenomenon was observed for all other oil/water mixtures (Figure 7b− d), the filtered predyed water in mixtures could still be decontaminated, and turned into colorless (Figure 7b2, c2, and d2), indicating the as-prepared materials had a great adsorption capacity to those water-soluble dyes. For comparison, the pristine kaolin particles were also tested for their dye-removal efficiency, as shown in Figure 7e. Methyl blue was used as the dye for its high solubility in water and obvious color, which was convenient for directly observing its concentration change in solution. After applying the methyl blue solution onto the pristine kaolin materials, it was clear that the filtered water solution still appeared blue (Figure 7e2), revealing an incompetently water remove property of the pristine kaolin materials. To further confirm these results, we analyzed the change of methyl blue concentration before and after separately filtering through the layer of pristine kaolin particles and as-prepared superhydrophilic-superoleophobic kaolin particles using UV− vis spectroscopy. As illustrated in Figure 7f, the UV−vis spectra of the methyl blue contaminated water before filtration showed a strong peak at 600 nm (Figure 7f1). After filtration with pristine H

DOI: 10.1021/acsanm.8b01249 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 9. (a) Schematic of the abrasion test for the as-prepared material at 200 g loading on the 800 grit sandpaper. (b) Variation in OCAs of sunflower oil measured with respect to length of the as-prepared material abraded on top of the sand paper. (c) Wetting behavior of the prepared coating material after the 150 cm abrasion test (sunflower oil droplets dyed with methyl red). The inseted rectangular region with red border in c shows the average value of CA of sunflower oil droplets on the abraded surface.

More importantly, the modified kaolin materials could also separate various stabilized oil-in-water emulsions directly without any pretreatment. As Figure 8a, b shows, When 40 mL of hexadecane-in-water emulsion dyed with methyl blue was poured onto the layer of the modified kaolin particles (Figure 8a), both hexadecane and methyl blue were efficiently removed from water, resulting the transparent and clear filtered water in beaker (Figure 8b). Further observing the UV−vis spectrum in Figure 8c, the oil-in-water emulsion before separation showed strong absorption band at 600 nm (methyl blue), whereas, after filtering through the layer of modified kaolin particles, the band of methyl blue disappeared, indicating the high water-soluble dye contaminants removal ability of the modified kaolin materials. Additionally, as shown in Figure 8d−f, it can be found the original blue milky emulsion (Figure 8e) became transparent and colorless after the separation (Figure 8f). The result could also be confirmed by the corresponding optical microscopic images, which revealed that there were numerous microscaled oil droplets and methyl blue particles presented in the entire view before the separation (Figure 8d). However, neither oil droplets nor methyl blue particles could be observed in the filtrate (Figure 8g). This is mainly due to the excellent superhydrophilic−superoleophobic property and size-sieving effect of the modified kaolin particles layer, which could effectively intercept the oil droplets and methyl blue in the emulsion. We also evaluated the performance of the modified kaolin materials in separating other kinds of surfactant-stabilized oil-in-water emulsions, for example sunflower oil-in-water emulsion, dichloromethane-in-water emulsion, and toluene-inwater emulsion. The corresponding separation efficiency for the different emulsions was greater than 92% despite the existence of surfactant. All these results indicate that the as-prepared material

kaolin particles (Figure 7f2), the corresponding peak did not have obvious change, demonstrating a minimal removal efficiency for methyl blue in water. However, when the methyl blue dyed water filtered through the modified kaolin materials, there was a sharp decrease for methyl blue concentration, indicating the substantial absorption of methyl blue onto the modified kaolin particles. On the basis of these results, we assume that the unique water-soluble dye absorption property of the as-prepared kaolin materials is closely associated with a high concentration of hydroxyl groups on the modified kaolin surface, which can form hydrogen bonds with the N atoms of the dyes in water and absorb them onto the surface.62,63 On the other hand, the modified kaolin particles are packed tightly in the column chromatography, resulting plentiful thin and long crevices among the packed particles, which can capture dye contaminations in filtered water. Such significant performance differentiates the as-prepared material from those that have been reported in a previous work,64 which can be used for simultaneous removal insoluble oils and water-soluble dyes, exhibiting a promising prospect in wastewater purification. Furthermore, it worth to be emphasized that all these oil/ water mixtures (Figure 7a1−d1) were successfully separated. Separation efficiency for each type of oil/water mixture was calculated according to the following equation: E=

mc 100% mo

Where mo and mc are the water weight in the original oil/water mixture and the collected water weight after separation. According to the results in Figure 7a−d, the separation efficiency of sunflower oil/water, hexadecane/water, soybean/water, and toluene/water were 92.8, 92.3, 92.1, and 93.2%, respectively. I

DOI: 10.1021/acsanm.8b01249 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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Figure 10. (a−d) Self-cleaning tests of the as-prepared materials surfaces. (a, c) Proccess of removing methyl red powders by hexadecane droplets with low viscosity. (b, d) Proccess of removing methyl red powders by sunflower oil droplets with high viscosity. (e, f) Wetting performance of the recycled materials. (e) Image of water droplets (dyed with methyle blue) and sunflower oil droplets (dyed with methyl red) on the recycled material surface. The inserted rectangular region with a red border in (e) shows the CAs of sunflower oil droplets on the recycled material surface are 152 ± 1°. (f) OCAs measurements of various oil liquids on the recycled material surface.

could efficiently separate and collect water from various oil/ water mixtures, showing the robust chemical stability, which is very promising for the on-demand oil/water separation and wastewater purification. Mechanical Durability of the Superhydrophilic− Superoleophobic Material. Apart from the excellent wetting performance, mechanical durability is another essential part for the functioned materials necessarily for practical applications.65,66 To estimate the mechanical durability of the asprepared material, abrasion tests were carried out with 800 # sandpapers served as abrasive surface, which was shown in Figure 9a. The surface to be tested was faced with the rough side of the sandpaper under a 200 g load, and then manually moved the surface on the sandpaper back and forth multiple times. This process was repeated several times until the superoleophobicity

was lost, after which sunflower oil droplets were placed on the abraded surface for visual inspection of the oil-repellent property. The CA measurements of the sunflower oil are carried out in Figure 9b. It is intuitively that the as-prepared material could abrade on the 800 # sandpaper for about 150 cm, and the OCA values changed slightly within such a long distance abrasion. After the substantial physical damage, the material still presented good superoleophobicity (OCA of sunflower oil is about 150°, Figure 9c), revealing a robust mechanical property, which is very meaningful in industry applications. Self-Cleaning Property and Recyclability of the Superhydrophilic−Superoleophobic Material. Many superoleophobic materials, especially some for oil/water separation purposes, tending to lose their oil repellency once contaminated by oil, and further inducing a significant decrease of the J

DOI: 10.1021/acsanm.8b01249 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials separation efficiency.67,68 Such unexpected property now has turned into the bottleneck for large-scale industrial applications of the superoleophobic surfaces. Herein, self-cleaning property of the as-prepared material has been discussed. As shown in Figure 10a, b, the coated glass slides were placed on the watchglasses severally with a small inclination angle, methyl red powder was then sprinkled on the tilted surfaces. Upon contact with the rolling hexadecane droplets, the powders quickly dissolved and fell off the material surface together with the droplets creating a visibly clean path (Figure 10c). Similarly, the rolling sunflower oil droplets were also able to remove the methyl red powders on the tilted surface without any influence on their motion (Figure 10d), displaying an excellent antifouling property of the modified coating, which will be highly desired for its practicability. Materials with the special wettability are widely used in numerous fields, therefore, the preferable durability and excellent chemical stability of the materials have always been the focus of researchers’ attentions.69 However, studies on recycling such materials are rare. To satisfy current environmental requirements, recycling and using materials fabricated from recycled raw materials are necessary. In this study, we reused some oil-contaminated and surface-damaged coating materials. The concrete process was scraping coating debris from the glass substrates by a chisel, then grinded the debris uniformly and dissolved them in ethanol, the obtained mixture was ultrasonic dispersed for 30 min and stirred at ambient temperature for 2 h, finally drop-coating the result suspension onto some glass substrates and dried at 80 °C for 2 h. Therefore, the reconstructed superhydrophilic-superoleophobic material was obtained without additional PFOA added during the process. As shown in Figure 10e, the recycled material exhibited great oil repellency and high water affinity. Sunflower oil droplets (dyed with methyl red) held near-perfect spheres on the as-prepared material surface with OCAs about 152°, whereas water droplets (dyed with methyle blue) were absorbed just in a few seconds. To further investigate the superoleophobicity of the recycled material, we have tested different kinds of oils. The result was shown in Figure 10f, which indicated that the recycled material maintained good oleophobicity, and the CAs of various oils were greater than 140°. Thus, we envision that the appearance of the successful recyclable materials with specific wettability could draw widespread attention from numerous commercial applications.

materials, which well-matches the practical requirements and sustainable development. We believe that the as-prepared material holds great potential for the diversified applications in industries simultaneously providing certain fresh ideas for development of functional materials for applications related to oil/water separation.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Mengnan Qu: 0000-0002-0684-4162 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Natural Science Foundation of China (Grant 21473132) and Huyang Scholar Program of Xi’an University of Science and Technology for continuing financial support.

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REFERENCES

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CONCLUSIONS In summary, we have successfully fabricated a superhydrophilicsuperoleophobic material from kaolin nanoparticles. Durable micro- and nanostructures and numerous hydrophilic− oleophobic groups on the material surface enable it to display stable oil repellency in both air and water environments. The asprepared material is suitable to be applied on almost any substrates regardless of their chemical composition, and all exhibit universal oil repellency and water affinity. By virtue of its specific wetting property, the material is capable of separating a variety of oil/water mixtures, even the surfactant stabilized oilin-water emulsions, simultaneously removing many kinds of soluble dyes in separated water. These superior performances reveal the important practical value of the as-prepared material in oil/water separation and sewage purification. What’s more, in contrast to the traditional functional materials, our work here has provided a simple and scalable method to fabricate the antifouling, energy-saving, and easily recyclable coating K

DOI: 10.1021/acsanm.8b01249 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acsanm.8b01249 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX