Nature-Inspired Strategy toward Superhydrophobic Fabrics for

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A Nature-Inspired Strategy toward Superhydrophobic Fabrics for Versatile Oil/Water Separation Cailong Zhou, Zhaodan Chen, Hao Yang, Kun Hou, Xinjuan Zeng, Yanfen Zheng, and Jiang Cheng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b00412 • Publication Date (Web): 21 Feb 2017 Downloaded from http://pubs.acs.org on February 22, 2017

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ACS Applied Materials & Interfaces

A Nature-Inspired Strategy toward Superhydrophobic Fabrics for Versatile Oil/Water Separation Cailong Zhou,† Zhaodan Chen,† Hao Yang,‡ Kun Hou,† Xinjuan Zeng,† Yanfen Zheng,† and Jiang Cheng*,† †

School of Chemistry and Chemical Engineering, South China University of Technology,

Guangzhou 510640, PR China ‡

Key Laboratory for Green Chemical Process of Ministry of Education, School of Chemical

Engineering and Pharmacy, Wuhan Institute of Technology, Wuhan 430205, PR China

ACS Paragon Plus Environment

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KEYWORDS. Superhydrophobic; Fabrics; Phytic acid metal complexes; PDMS; Oil/water separation

ABSTRACT. Phytic acid, a naturally occurring component, widely found in many plants, can strongly bond toxic mineral elements in human body due to its six phosphate groups. Some of the metal ions present the property of bonding with phytic acid to form insoluble coordination complexes aggregations even in room temperature. Herein, a superhydrophobic cotton fabric was prepared by a novel and facile nature-inspired strategy which introduced phytic acid metal complexes aggregations to generate rough hierarchical structures on fabric surface followed with PDMS modification. This superhydrophobic surface can be constructed not only on cotton fabric, but also on filter paper, PET fabric and sponge. AgI, FeIII, CeIII, ZrIV and SnIV are well commendatory ions in our study. Taking phytic acid-FeIII based superhydrophobic fabric as an example, it showed excellent resistance to UV irradiation, high temperature, organic solvent immersion

as

well

as

good

resistance

to

mechanical

torn

and

abrasion.

The

superhydrophobic/superoleophilic fabric was successfully used to separate oil/water mixture with separation efficiency high up to 99.5%. We envision that these superantiwetting fabrics modified with phytic acid-metal complexes and PDMS, are environmentally friendly, low cost, sustainable and easy to scale up, thus exhibit great potentials in practical applications.


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Introduction Superhydrophobic surfaces with a water contact angle (CA) larger than 150o have received extensive investigation due to their significance in potential applications such as self-cleaning,1,2 friction

reduction,3,4

anti-icing,5

oil/water

separation,6-8 anticorrosion9,10

and

droplet

manipulation.11 Inspired by lotus leaf, gecko, water strider and rose petal, etc. in nature, surfaces with superhydrophobicity can be constructed through the strategy of creating suitable surface roughness with hierarchical micro/nanostructures and chemically modifying with low surface energy materials meanwhile.12-14 Amongst the superhydrophobic materials, waterproof textiles display great potential for practical applications. The textiles such as cotton fabrics are inexpensive, ubiquitous, soft and flexible, regarded as the good candidates for treating industrial oily wastewater and the spilled oil in oceans.15 However, pristine textiles will absorb both water and oil at the same time. When endowing with stable superhydrophobicity and superoleophilicity by functional modification, the modified textiles can easily be used for absorbing oils and thus removing oils from water because of their selective wetting property. The most common techniques to fabricate superhydrophobic coatings on fabrics can be generally divided into two categories as wet chemical methods and dry physical techniques.16 Dip-coating is a versatile wet chemical method to construct a superhydrophobic coating on textile substrates, which performed by covering the fiber with a layer of micro/nano-particles, such as SiO2,17-22 TiO223 or the hybrid24 followed by hydrophobization. However, these particulate sols are prepared by acid or base catalyzed hydrolysis of corresponding precursors,25 which would consume a long time. Superhydrophobic fabrics might also be obtained using the as-grown inorganic metal oxides through a hydrothermal method and further modification. For instance, Lai and co-workers fabricated hierarchical TiO2 micro/nanoparticles onto cotton fabric


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substrates via a hydrothermal route, followed by hydrophobization with fluoroalkyl silane to obtain superantiwetting fabrics.26,27 ZnO nanostructures were also prepared on fabrics for getting superhydrophobicity.28-30 However, the hydrothermal method remains a great limitation of largescale fabrication, not suitable for practical application. As for dry physical techniques such as chemical vapour deposition (CVD),31-33 plasma etching processing34 and atomic layer deposition (ALD),35 most of these approaches suffer from the disadvantages of complex equipments and tedious fabrication processes. Therefore, to overcome the aforementioned limitations, the development of cheap, simple and large-scale strategies for preparing superhydrophobic fabrics is still challenging. Moreover, functional superhydrophobic fabrics with high stability and flexibility, as well as excellent selectivity and recyclability are worth of further studying. Recently, metal-based routes to fabricate functional materials are receiving great attention. For example, as a native biopolymer, chitosan was chosen to cross-link with transition metal ions, such as AgI, CuII, CoII, NiII, ZnII, CdII, and PdII to fabricate multistimuli-responsive hydrogels through their supramolecular complexation.36 Through a simple one-step assembly coating method, the natural polyphenol tannic acid and FeIII were employed as the organic ligand and the inorganic cross-linker to construct various films and particles based their coordination complexes.37 Feng et.al. developed a polydopamine-Cu film which could decorate on various porous substrates such as stainless steel mesh, mixed cellulose ester filter membrane, copper mesh, nylon mesh and polyurethane sponge, endowing them with superhydrophilicity and underwater superoleophobicity.38 An early report by Guo et.al. demonstrated the in-situ growth processing to prepare superhydrophobic fabrics and sponges for oil/water separation.39 The main methodology was that transition metals such as Fe, Co, Ni, Cu, and Ag could strongly bond with thiols and adhere to the fabric or sponge substrates to realize superhydrophobicity. In order to


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obtain a water harvesting fabric, they further fabricated an intriguing superhydrophobicsuperhydrophilic hybrid coating on the fabric by selectively modifying the Fe and Co nanoparticles with n-octadecyl thiol.40 In this study, the micro-nano hierarchical phytic acid metal complexes were deposited on cotton fabrics through a facile assembly process and the fabrics demonstrated excellent superhydrophobicity after PDMS modification. Phytic acid (myo-inositol 1,2,3,4,5,6hexakisphosphate, PA), a natural and nontoxic plant constituent, was widely found in legumes, cereals, oil seeds, pollens, nuts, fruits and vegetables.41 Due to the quite close six phosphate groups in PA molecule, it can strongly bond practically most of the metal ions even at room temperature.42,43 Some of the metal ions possess the character of bonding with phytic acid to form insoluble coordination complexes aggregations. Inspired by this concept, we imagine it is easy to construct micro- and nanostructures through formation of the PA-metal coordination complexes aggregations on porous supports for practical applications. Several metal ions including AgI, FeIII, CeIII, ZrIV, and SnIV were selected in our work because they can combine with PA to form insoluble coordination complexes aggregations, which might create appropriate roughness assembly on the cotton fabric, as well as some other porous supports such as filter paper, PET fabric and sponge. After hydrophobization by PDMS, the fabrics turned from superhydrophilic to superhydrophobic successfully. The superhydrophobic materials displayed excellent UV resistance, heat resistance and chemical stability as well as good mechanical durability. Meanwhile, the as-prepared materials were further applied in developing an “oil gathering package” with three-dimensional (3D) structure for effectively collecting oil, which has a cubical shape with outer layer formed by the as-prepared cotton fabric and inner part filled with the modified melamine sponge. We envision that these outstanding water-proof fabrics


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modified with PA-metal complexes and PDMS are environmentally friendly, low cost, sustainable and easy to scale-up, exhibiting great prospects in solving the serious problem of oil spill and the increasing discharge of industrial oily waste water. Experimental Section Materials. Ferric chloride hexahydrate, zinc nitrate hexahydrate, nichel nitrate hexahydrate, manganese chloride tetrahydrate, aluminium chloride crystal, cobalt nitrate hexahydrate, copper sulfate pentahydrate, calcium chloride anhydrous, magnesium sulfate, ethanol, n-hexane, isooctane, petroleum ether, dimethylbenzene, chloroform, dichloromethane, methylene blue and sudan II were purchased from Damao Chemical Reagent Co., Ltd. (Tianjin, China). Tin chloride pentahydrate, silver nitrate, cerium nitrate hexahydrate and zirconium chloride anhydrous were bought from Aladdin Industrial Corporation (Shanghai, China). All the above reagents are of analytical grade and used as purchased without further purification. Phytic acid (70 wt% in water) was purchased from Yuanye Biological Technology Co., Ltd. (Shanghai, China). PDMS prepolymer (Sylgard 184A) and the curing agent (Sylgard 184B) were received from Dow Corning Corporation (Shanghai, China). Dimethyl silicon oil was bought from Hagibis Technology Co., Ltd. (Beijing, China). Kerosene and aviation kerosene were purchased from SINOPEC (Beijing, China). Soybean oil, cotton fabric, PET fabric and melamine sponge were purchased from a local market. Fabrication of superhydrophobic PA-Mn+@PDMS coated materials. As a typical preparation, the cleaned pristine hydrophilic cotton fabric (washed sequentially with distilled water and ethanol under ultrasonication) with the size of 3 × 3 cm2 was immersed in the PA solution (0.013 mol/L, prepared by addition of 0.5 mL of the 70 wt% PA solution to 50 mL of


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deionized water) for 2 min. The fabric was then taken out and immersed into 20 ml of metal ions solution (0.01 mol/L of FeCl3·6H2O, taking FeIII as an example) for another 2 min. The multiplecycle assembly was achieved by repeating cycles of alternately dipping the fabric into the PA aqueous solution and metal ions solution. After the PA-Mn+ coated fabric was rinsed with deionized water and dried at 80 oC, the PA-Mn+@PDMS coated fabric was finally achieved by immersing the resulting dry fabric in chloroform solution containing of 2 wt% PDMS prepolymer and 0.2 wt% curing agent for 2 min and subsequent curing at 80 oC for 2 h. Besides the cotton fabric, superhydrophobic filter paper, PET fabric and melamine sponge were also successfully obtained in this work using the same strategy. Stability evaluation of the superhydrophobic cotton fabric. The superhydrophobic PAFeIII@PDMS coated cotton fabric was irradiated under a UV lamp of 40 W for 24 h to evaluate its UV resistance. During the irradiation, the CAs of the cotton fabric sample were investigated using a contact angle analyzer. The heat resistance of the superhydrophobic fabric was tested by measuring the contact angles after heated at 40 oC ~ 160 oC with an increasing interval of 20 oC respectively for 2 h. To evaluate the chemical stability, the superhydrophobic PA-FeIII@PDMS coated cotton fabrics were immersed into four different common organic solvents including ethanol, n-hexane, xylene and acetone which have different polarity parameters. The corresponding contact angles of the cotton fabric samples were monitored during the immersion. The mechanical stability of the as-prepared PA-FeIII@PDMS coated cotton fabric was analyzed via an abrasion test and a peeling test. The as-prepared superhydrophobic cotton fabric was pressed onto a sandpaper (600, 1000 and 2000 mesh) by adding a load of 100 g on it with a glass slide between them. The fabric was dragged to move in two perpendicular directions each for 10 cm in abrasion length then returned to the beginning position. This process was repeated for


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certain times to measure the mechanical durability of the superhydrophobicity. The pealing test was conducted by using an adhesive tape. The fabric was repeatedly pressed onto an adhesive tape and then torn off. To maintain the adhesive effect, fresh adhesive tape was used after every 20 times of peeling. Oil/water separation. The oil/water separation capability was evaluated by using a filter process for the as-prepared superhydrophobic cotton fabric and a selective adsorption process for the functional fabric packed with melamine sponge inside respectively. Typically, a filter equipment, which consists of a customized Teflon joint coupled with two glass tubes on both sides, holds the fabric as the separating membrane between the two tubes firmly. A mixture of 15 mL water and 15 mL oil was poured into the filter device. Due to the excellent superhydrophobic and superoleophilic properties, oil could easily penetrate through the fabric only driven by gravity, while water was repelled at the upper tube. The efficiency of the oil/water separation was calculated by η = (ma/mb) × 100%, where ma and mb are the weight of the collected water after separation and the original water in the mixture before separation, respectively. For the superhydrophobic fabric packed with melamine sponge, the separation was realized by using the as-constructed package to selectively absorb the floating oil on the water surface. The absorbed oil in the inner sponge could be consecutively removed by an injector and the materials could therefore be reused. Characterization of surface morphology, composition and wettability. The surface morphologies and the elemental distribution of the as-prepared samples were investigated by a field-emission scanning electron microscope (FESEM, ZEISS Merlin) combined with an energy dispersive X-ray spectroscopy (EDS, Oxford Instruments Inca400). The surface roughness of the cotton fabrics was determined by an atomic force microscope (AFM, Nanoscope IIIa


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Multimode). The chemical compositions of the fabrics were further determined by a Fourier transform infrared spectrometer (FTIR, Bruker VERTEX 70) in the attenuated total reflection (ATR) mode. The wetting behavior of water on the fabric surface were tested using a contact angle analyzer (Shanghai Zhongchen Powereach JC2000C1). The static CAs and sliding angles (SAs) were obtained by measuring five different positions using 5 µL water drops and calculating the average values. Results and discussion Fabrication of superhydrophobic PA-Mn+@PDMS coated surfaces. Scheme 1 illustrates the formation mechanism of PA-Mn+@PDMS coating on the fabric substrate through an assembly method. When the cotton fabric was firstly immersed in PA solution, the PA molecules could be adsorbed and immobilized on the fabric substrate by covalent reaction between their phosphate group and the -OH group of the fabric surface.44,45 As the fabric was subsequently immersed into the metal ions solution, the phytic acid metal complexes aggregations would be further formed on the fibers to provide the appropriate surface topography or roughness for constructing superhydrophobic surfaces. Although PA can bind almost any metal ions to form PA-Mn+ complexes, not all the products are insoluble in aqueous solution. As shown in Figure S1 in Supporting Information, among thirteen metal ions performed in this work, only five ions including AgI, FeIII, CeIII, ZrIV and SnIV could react with PA to form insoluble precipitates which may be used as the roughness-maker. As is well known, the wetting behavior of a surface is mainly related to both its surface topography and chemical composition. The fabric was further modified with PDMS and thus displayed superhydrophobicity due to the abundant hydrophobic alkyl chain of PDMS.


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Scheme 1. Schematic of the preparation of superhydrophobic PA-Mn+@PDMS coated fabric. Herein, FeIII was chosen as a typical metal ion combining with PA to form PA-FeIII coordination complexes on the cotton fabric. As shown in Figure 1a, the original cotton fabric displays superhydrophilic due to the smooth fibers and the abundant -OH induced hydrophilicity. After the assembly deposition of PA-FeIII aggregations followed by PDMS modification, a compact film with lots of micro- and nanoscale protuberances could be observed on the fiber surface (Figure 1b). This hierarchical micro- and nanostructures induced roughness on the modified fabric surface is a crucial necessity to achieve superhydrophobicity. The CA of the PAFeIII@PDMS coated fabric is 151.5 ± 1.3o, showing good superhydrophobicity. The effect of PDMS is to contribute low surface energy, not to change the morphology of the surface. When adopting PDMS only to modify the cotton fabric directly, there was no obvious variation of the surface morphology but the CA decreased from 114 ± 2o to 43 ± 2.5o in 84.6 s due to the permeation of water droplet (Figure S2). The surface roughness of the pristine and PAFeIII@PDMS coated fabrics is measured using AFM. The arithmetic average (Ra) roughness of


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the raw fabric is 28.82 nm based on the AFM image (Figure 1c), while the Ra of the superhydrophobic PA-FeIII@PDMS coated fiber reaches 166.74 nm (Figure 1d), suggesting that the nano-scale roughness combined with the micro-scale roughness on the fiber surface successfully and the roughness of the fiber indeed increased after decorated with PAFeIII@PDMS coating. Energy dispersive spectroscopy (EDS) analysis was carried out to observe the chemical composition of the coated cotton fabric. Figure 1e implies that the fabric surface contains of Fe, Si, C, O, and P elements. The appearance of the Si and P peaks is attributed to PDMS and PA, respectively. The SEM-EDS elemental mapping of C, O, Fe, P, and Si is shown in Figure 1f, all the five elements are distributed on the fabric surface uniformly, suggesting the well-distributed coverage of PA-FeIII@PDMS film on the coated fabric. The chemical components on the surface of cotton fabrics were further verified by the ATR-FTIR spectra in Figure S3.


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Figure 1. SEM images and AFM topography images of (a, c) pristine cotton fabric and (b, d) superhydrophobic PA-FeIII@PDMS coated fabric prepared by 5 times of assembly. The insets of (a) and (b) are the corresponding low magnification SEM images and photographs of water droplets on the pristine fabric and the superhydrophobic fabric, respectively. (e) EDS spectra of the PA-FeIII@PDMS coated fabric surface. (f) SEM-EDS mapping of the PA-FeIII@PDMS coated fabric. The superantiwetting stability of the PA-FeIII@PDMS coated cotton fabric was demonstrated in Figure 2a. A sequence of photographs recorded a water droplet of about 5 µL staying and gradually evaporating on the coated fabric in 100 min at a temperature of 28 oC and a relative


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humidity of 65%. The droplet could maintain the spherical shape on the coated cotton fabric for a long period of testing time, which suggests the excellent stability of superhydrophobicity for the as-prepared fabric. Figure 2b shows oil (n-hexane, dyed red) and water droplets (dyed blue) on pristine fabric (A) and the PA-FeIII@PDMS coated fabric (B), respectively. The pristine cotton fabric is superamphiphilic owing to the capillary effect caused by hollow region and a great deal of hydroxyl groups within it, water and oil permeated it instantly when touching the surface

(Figure

2b(A)).

After

coated

with

PA-FeIII@PDMS,

the

fabric

displays

superhydrophobic and superoleophilic. Oil droplet could quickly be absorbed and disappear on the surface of the coated fabric, while water droplets were repelled on it as a sphere (Figure 2b(B)). This phenomenon can be explained as the surface tension of oil (18.43 mN/m, taking nhexane as an example) is universally much smaller than that of water. If the surface tension of the fabric locates between those of water and oil, hydrophobic and oleophilic appearance could be found. Even when submerged the fabric into water by a force, its surface exhibits like a silver mirror because of the presence of air layer trapped on the surface (Figure 2c and Video S1). The established composite solid-liquid-air interface underwater can effectively hamper permeation on the fabric surface and decrease the interaction between water and the surface.46 After the fabric was withdrawn from the water, its surface maintained entirely dry. Furthermore, a jet of water can easily bounce on the superhydrophobic fabric due to the presence of an air cushion between the water and fabric (Figure 2d). The SA was approximate 13o, water droplet could easily roll off from a slightly tilted surface, showing a low contact angle hysteresis (Figure 2e).


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Figure 2. (a) Profiles of a water droplet placed on the PA-FeIII@PDMS coated cotton fabric displaying stable superhydrophobicity. The environmental temperature and relative humidity are 28 oC and 65%, respectively. (b) Photographs of water and oil droplets on (A) the pristine cotton fabric and (B) the PA-FeIII@PDMS coated fabric. (c) The fabric immersed in water by an external force, exhibiting a silver mirror-like phenomenon. (d) A jet of water (dyed red) bouncing off the fabric surface. (e) Consecutive captures taken from a video to show a water droplet (~ 5 µL) sliding off a 13o-tilted PA-FeIII@PDMS coated fabric. The surface morphologies of the fabric after different cycles of assembly with PA-FeIII film were investigated using SEM. As shown in Figure 3a, the PA-FeIII@PDMS coating is relatively exiguous on the fabric surface when adopted only one time of assembly. With the assembly number increased, more compact PA-FeIII@PDMS aggregations could be observed on the surface (Figure 3b-e), providing a better hierarchical roughness in the micro- and nanoscales to further enhance the surface hydrophobicity. As shown in Figure 3f, the CAs increased from 142.1 ± 2.5o to 152.9 ± 1.8o as the assembly times increased from one to eight successively. However, when the assembly numbers were less than four, droplets could not roll off the fabric surface even when the fabric was vertically tilted. It is mainly because water droplet on such a hydrophobic surface exhibits a typical liquid-solid Wenzel state. With the number of assembly


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increased to four, more particles gathered on the fibers to form more compact structures, the CA achieved 150.4 ± 1.7o and the SA sharply decreased to about 24o. By further increasing the number of assembly to eight times, the SA could further decrease to approximate 7.5o as demonstrated in Figure 3f. Further information on the different molar ratio of PA to FeIII for preparing superhydrophibic fabrics can be seen in Figure S4 and Table S1.

Figure 3. SEM images of the PA-FeIII@PDMS coated fabrics with different numbers of assembly: (a) 1, (b) 2, (c) 3, (d) 4, and (e) 8. The insets are the photographs of static water contact state on the corresponding fabrics. (f) CAs and SAs of the fabrics prepared with different assembly times. As mentioned above, not only FeIII can bond with PA and form sedimentary phytic acid metal complexes, but some other metal ions, such as AgI, FeIII, CeIII, ZrIV and SnIV have the similar


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property. Figure 4 shows the SEM images of the as-prepared PA-Mn+@PDMS coated cotton fabrics. The morphologies of the as-prepared PA-Mn+@PDMS coated cotton fabrics can distinctly be divided into three types, which were possibly controlled by the valence of the metals. Rare distribution of the PA-AgI aggregations could be found on the fabric surface when using AgI (mono- metal cation) concentration of 0.01 mol/L (Figure S5). While increased the concentration of AgI to 0.3 mol/L, the PA-AgI aggregations with granule-like structure could uniformly coated on the fiber (Figure 4a,b), leading to a CA of 148.8 ± 2.4o on the fabric surface even with only once of assembly. As a trivalent metal cation, CeIII combined with PA to form a similar morphology on the fabric surface as FeIII did when using a concentration of 0.01 mol/L (Figure 4c,d). Quadrivalent metal cations, such as ZrIV and SnIV, could bond with PA to form a considerably thick layer on the fabric surface (Figure 4e,f and Figure 4g,h, respectively). After five cycles of assembly, all the CAs of the PA-Mn+@PDMS (including PA-CeIII@PDMS, PAZrIV@PDMS, and PA-SnIV@PDMS) coated cotton fabrics are above 150o. More details (SEM images, EDS spectra and SEM-EDS mappings) about the PA-Mn+@PDMS coated cotton fabrics can be seen in Figure S5-S12.


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Figure 4. SEM images of various PA-Mn+@PDMS coated cotton fabrics: Mn+ = (a, b) Ag+, (c, d) Ce3+, (e, f) Zr4+, and (g, h) Sn4+. The insets of (a), (c), (e), and (g) are the photographs of water droplets on the fabrics, respectively. Besides cotton fabric, superhydrophobic PA-FeIII@PDMS film coated filter paper, PET fabric and melamine sponge were also fabricated successfully using this facile method. As shown in Figure 5a’-f’, the pristine superhydrophilic filter paper, PET fabric and hydrophobic melamine sponge turn to be superhydrophobic after coated with PA-FeIII aggregations followed by PDMS modification. Water droplets stayed as spherical shape on the surfaces of the treated porous materials and all the WCAs were larger than 150o. Comparison of SEM images between the


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pristine and as-prepared superhydrophobic materials (Figure 5a-f and the insets) clearly displays that PA-FeIII@PDMS aggregations packed on the skeletons of the porous supports homogenously and increased the roughness of the skeleton surfaces. Due to the micro- and nanoscale structures as well as the intrinsic low surface energy of PDMS, the porous materials can easily obtain superhydrophobicity. Results suggested that this simple method of constructing superhydrophobic surface may be realized on more other organic porous substrates besides cotton fabric.

Figure 5. (a), (c) and (e) SEM images of the pristine filter paper, PET fabric and melamine sponge, respectively. (a’), (c’) and (e’) are photographs of the corresponding materials. (b), (d) and (f) SEM images of the PA-FeIII@PDMS coated filter paper, PET fabric and melamine sponge, respectively. (b’), (d’) and (f’) are photographs of the corresponding materials. The insets are the corresponding water contact angles.


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Stability of the superhydrophobic PA-FeIII@PDMS coated cotton fabric. In general, superhydrophobic surface could easily lose its superhydrophobicity in severe conditions such as high temperature, corrosive solution, organic solvent, and mechanical abrasion.47 Herein, the UV resistance, heat resistance, chemical stability and mechanical durability of the as-prepared superhydrophobic fabric were measured. UV irradiation is commonly from the sunlight thus critically affecting the long-term stability of the fabric. Figure 6a shows the effect of UV irradiation time on the CAs of PA-FeIII@PDMS coated fabric. Results displayed that the CA displayed no obvious change when prolonged irradiation of UV light for 24 h, indicating excellent resistance of the coated fabric to UV light. In addition, stability measurements for different temperatures were also performed. It was found that the CA increased slightly as the temperature went up (Figure 6b), which might be due to the decrease of -OH on the fabric surface under high temperature. Meanwhile, the PA-FeIII@PDMS coated fabrics were immersed in organic solvents including ethanol, n-hexane, xylene and acetone for 7 days to examine the chemical stability. The fabric samples were withdrawn in certain immersing durations and dried for the CA measurement. As shown in Figure 6c, almost all the samples maintained the CA above 150o during the soaking process. The outstanding solvent resistance of the superhydrophobicity could be significantly attributed to the excellent adhesion and satisfactory solvent resistance of PDMS. The good adhesive force of PDMS onto the fabric substrate can be ascribed to the much strength of covalent bonding between the polymer and hydroxyl groups of the fabric.24


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Figure 6. The change of water contact angles of the superhydrophobic PA-FeIII@PDMS coated cotton fabrics after (a) ultraviolet light irradiation for diverse periods of time, (b) heating treatment under different temperatures for 2 h, and (c) immersion in organic solvents for different periods of time. The mechanical stability of the superhydrophobic fabrics was also qualitatively evaluated through an abrasion test and a peeling test. Figure 7a displays the methodology of the abrasion test. Sandpapers with 600, 1000, and 2000 mesh were employed as an abrasion surface to contact with the fabric closely. The fabric was driven to move along the rulers in two perpendicular dimensions for 10 cm each with a load of 100 g on it. As shown in Figure 7b, the fabric displayed much sensitive to the abrasion especially for the sandpaper with lower mesh. When adopting 600 mesh of sandpaper to perform a 30 cycles of abrasion test, the CA declined to


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142.5 ± 2.5o maintaining good hydrophobicity, though water could not slide off the fabric easily. As a contrast, abrasion occurred when using 2000 mesh of sandpaper for 30 cycles, the CA had a relatively slight variation obtaining a value of 149 ± 1.2o, water droplet could still roll on the surface. Abrasion with 600 mesh sandpaper could destroy the textured structure of the fabric (Figure S13), making the hydrophobicity decline remarkably. The surface hierarchical microand nanostructures almost remained when adopting 2000 mesh sandpaper for the test (Figure S13), leading to a relatively lower change of wettability. Repeated tear test with an adhesive tape was further used to investigate the mechanical resistance of the fabric (Figure 7c). Significantly, after a 300-cycle peeling test with the adhesive tape, just a slight descend of the CA was observed and it keeps above 150o due to the little variation of surface morphology (Figure S14). The SA seems more sensitive than CA in measuring wettability of the samples after tear tests, increasing from 13o to 23o (Figure 7d). This can be explained as slight damage to the coated film may result in obvious increase in adhesion force between the film and water droplets.46 Results demonstrate that continually mechanical abrasion is a relatively sharp way to destroy the superhydrophobicity during the chosen robustness measurements. The fabric, however, retained high hydrophobicity after strong abrasion.


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Figure 7. (a) Photograph showing the abrasion process. (b) Correlation of contact angle with abrasion cycles using different meshes of sandpaper. (c) A piece of cotton fabric pasted onto and peeled off an adhesive tape. Water droplets on the fabric after peeling off the adhesive tape. (d) Contact angles and sliding angles of the as-prepared PA-FeIII@PDMS coated cotton fabric repeatedly torn by the adhesive tape. Oil/water separation. Due to the simultaneous superhydrophobicity and superoleophilicity, the PA-FeIII@PDMS coated cotton fabric may gain great expectation as an adsorbent material for removing oil from water. As shown in Figure 8a-c and Video S2, n-hexane (dyed red with sudan II) was dropped onto the surface of a glass of water forming a thin oil layer, then a piece of the as-prepared superhydrophobic/superoleophilic cotton fabric was carried to touch with the layer. The n-hexane was totally absorbed within only a few seconds, remaining no visible oil on the water surface. Moreover, oil with higher density than water, such as chloroform was also used to testify the absorption capacity of the coated fabric. When submerging the superhydrophobic fabric into water to close the chloroform, it could rapidly suck up the chloroform underwater due to its good superoleophilicity (Figure 8d-f and Video S2). The weight gains of the superhydrophobic fabric due to the absorption of various oils were tested, and the results were compared with those obtained from the pristine fabric as shown in Figure 8g. As a typical


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absorption test procedure, the fabric was placed into the oil, then taken out quickly after saturated absorption, and weighed rapidly to avoid evaporation. The weight gain values of the fabric were calculated as follows: weight gain (%) = (m after - m before )/m before × 100%

(1) where

mbefore and mafter were the weight of the fabric before and after adsorption, respectively. The test was taken out for three times for every oil, the fabric was washed with ethanol to clear up the absorbed oil and then dried at 60 oC after each time of measurement. The weight gains of the coated fabric were larger than 60% for all the chosen ten oils. For most types of oil, fabrics displayed a slight decrease in weight gain after coated with PA-FeIII@PDMS than the pristine ones, that’s because the as-formed PA-FeIII@PDMS layer has a relatively lower porosity than the silks of the original fabric, resulting in a weight gain decrease. The density and viscosity of the oil might be the two factors that have effect on the weight gain.48 Figure 8h demonstrates the effect of the density of oil on the oil weight gains of the fabrics. The fabrics appeared to have higher adsorption ability toward oils with great densities than those with relatively low densities. For instance, the weight gain of the modified fabric for chloroform (density = 1.49 g/cm3) is 145.99%, but for n-hexane (density = 0.66 g/cm3) is 65.63%. Viscosity could also affect the weight gain. Oils with high viscosity (especially for kerosene, soybean oil and dimethyl silicon oil) possess relative high adsorption on the fabrics. These oils could form a thick layer on the fabric surface during the absorption test due to their high viscosity values, resulting in an obvious increase of the weight gain.


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Figure 8. Photographs showing the selectively absorbed (a-c) n-hexane and (d-f) chloroform from water by superhydrophobic PA-FeIII@PDMS coated cotton fabrics, respectively. (g) Weight gains of the bare fabric and the modified cotton fabric toward ten-different types of oil. (h) Effect of the density of oil on the weight gain of the fabrics. By simply enlarging the size of raw fabric and reaction container, it is easy to perform largescale preparation of the superhydrophobic PA-FeIII@PDMS coated cotton fabric. As a proof of concept, a typical sample with a size of 20 cm × 30 cm was fabricated (Figure 9a). Water droplets exhibited typical spherical shapes on the large-scale fabric sample (Figure 9b) and the water flow could easily roll off the surface (inset of Figure 9b and Video S3), which indicates that the large-scale fabric is uniformly superhydrophobic with low adhesion to water. A miniature collector with a size about 5.5 cm × 3.2 cm × 3.0 cm (Figure 9d) which named as “oil gathering package” for the collection of oil spill was constructed. The “oil gathering package” was made by wrapping a pristine melamine sponge with the superhydrophobic/superoleophic


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cotton fabric (Figure 9c) and then fastening the package with a copper wire. Figure 9e-j and Video S4 show the clean-up processes of an oil spill using the “oil gathering package”. The package was firstly put into a mixture of n-hexane/water (Figure 9e,f), due to the superoleophicity of the outer fabric, oil permeated through the fabric and was absorbed by the inner melamine sponge forming an oil storeroom (Figure 9g). Then a injector was inserted into the package to suck the oil away (Figure 9h,i). The sucking process was persistent until the oil was absorbed totally. This approach of adsorption combining with collection could successfully remove all the floating oil on the water surface, leaving a clean water surface (Figure 9j). Hence, a schematic depiction of the collection of floating oil from an oil spill can be illustrated in Figure 9k. The proposal of the oil collecting system is equipped with the “oil gathering package” and driven by a pump. When the system is working, oil will be sucked and collected continuously, while water will be repelled, indicating a great potential of practical application.

Figure 9. (a) Photograph of a piece of PA-FeIII@PDMS coated cotton fabric with a size of 20 × 30 cm2. (b) Water droplets on the superhydrophobic PA-FeIII@PDMS coated cotton fabric. The inset is a jet of water roll off the fabric surface. Using (c) a piece of PA-FeIII@PDMS coated cotton fabric and a piece of pristine melamine sponge to fabricate (d) an “oil gathering package”. (e-j) Photographs of the oil (n-hexane, dyed red) adsorption process using the “oil gathering


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package”. (k) Schematic of the superhydrophobic/superoleophilic “oil gathering package” and its application in treating oil spill. Gravity-driven separation of oil-water mixtures is another commonly used oil/water separation approach.49 Figure 10a-c and Video S5 show the typical gravity-driven separation process using the superhydrophobic cotton fabric. A mixture of chloroform (dyed red with sudan II) and water (dyed blue with methylene blue) was poured into the upper tube of the separation device. Thanks to the good superhydrophobicity and superoleophilicity property of the fabric, chloroform could permeate through the fabric quickly with gravity acting as the driving force only, while water was repelled on the top. No visible water can be found in the bottom beaker and no visible residual oil on the water surface, suggesting the excellent oil/water separation ability of the fabric. A series of other oil/water mixtures including xylene/water, dimethyl-silicon oil/water, nhexane/water, and dichloromethane/water were successfully separated by the as-prepared fabric. The separation efficiencies were above 95.0% for all the chosen oil/water mixtures (Figure 10d). Moreover, after 20 cycles of separation on chloroform/water mixtures, the fabric possesses high efficiency above 98% persistently (Figure 10e), indicating outstanding stability. The water intrusion pressure (∆p) of the superhydrophobic fabric were measured by Eq. (2): ∆p =ρ ghmax

(2)

where ∆p is the water intrusion pressure, ρ is the density of water, g is the acceleration of gravity, and hmax is the maximum height of the water that the fabric can support. The experimental detail of testing water intrusion pressure was shown in Figure S15 and Video S6. The average maximum bearable height of the fabric was high up to 38.9 cm. The calculated intrusion pressure is 3.81 kPa, which means water could not pass through the fabric under this pressure, indicating a good stability of the separating system.


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Figure 10. (a-c) Photographs of the cotton fabric-based oil/water separation process of water and chloroform (details supported in Video S5). Water was dyed with methylene blue and chloroform was dyed with sudan II for clear observation. (d) Separation efficiency of the fabric for various oil/water mixtures. (e) Recyclable separation efficiency for 20 times over water/ chloroform mixtures on the superhydrophobic PA-FeIII@PDMS coated cotton fabric. Conclusions In summary, we have demonstrated a facile assembly strategy to fabricate superhydrophobic fabric, which was performed through the dispersion of micro-nano phytic acid metal complexes aggregations and further hydrophobization. Several metal ions, including AgI, FeIII, CeIII, ZrIV, and SnIV could act as the inorganic cross-linker to combine with phytic acid forming roughness on the substrate. The method is easy for preparing various superhydrophobic porous materials even in a large-scale. Furthermore, taking the PA-FeIII@PDMS coated fabric as an example, the superhydrophobic materials displayed excellent resistance to UV irradiation, high temperature, organic solvent immersion as well as good mechanical torn and abrasion. Importantly, the


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superhydrophobic/superoleophilic fabric was successfully used to collect oil-spill and separate the oil/water mixture with high separation efficiency. ASSOCIATED CONTENT

Supporting Information. Summary of some experimental details; ATR-FTIR spectra of the cotton fabrics; SEM images and SEM-EDS mappings of the samples; Photograph of the water intrusion pressure measurement. (PDF) Movies showing the superantiwetting property of the PA-FeIII@PDMS coated cotton fabric and its application in oil/water separation. (AVI) AUTHOR INFORMATION Corresponding Author * Corresponding author: J. Cheng; E-mail: [email protected]; phone: +86-20-87112057 Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT


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This work is supported by the National Natural Science Foundation of China (20976055), Science and Technology Planning Project of Guangdong Province (2014A010105008) and Science and Technology Planning Project of Guangzhou (2014J4100037). REFERENCES (1) Afzal, S.; Daoud, W. A.; Langford, S. J. Superhydrophobic and Photocatalytic Self-Cleaning Cotton. J. Mater. Chem. A 2014, 2, 18005-18011. (2) Wu, Y. W.; Hang, T.; Yu, Z. Y.; Xu, L.; Li, M. Lotus Leaf-Like Dual-Scale Silver Film Applied as a Superhydrophobic and Self-Cleaning Substrate. Chem. Commun. 2014, 50, 84058407. (3)

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Schematic of the preparation of superhydrophobic PA-Mn+@PDMS coated fabric. Scheme 1 231x134mm (200 x 200 DPI)



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SEM images and AFM topography images of (a, c) pristine cotton fabric and (b, d) superhydrophobic PAFeIII@PDMS coated fabric prepared by 5 times of assembly. The insets of (a) and (b) are the corresponding low magnification SEM images and photographs of water droplets on the pristine fabric and the superhydrophobic fabric, respectively. (e) EDS spectra of the PA-FeIII@PDMS coated fabric surface. (f) SEMEDS mapping of the PA-FeIII@PDMS coated fabric. Figure 1 412x390mm (150 x 150 DPI)


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(a) Profiles of a water droplet placed on the PA-FeIII@PDMS coated cotton fabric displaying stable superhydrophobicity. The environmental temperature and relative humidity are 28 oC and 65%, respectively. (b) Photographs of water and oil droplets on (A) the pristine cotton fabric and (B) the PAFeIII@PDMS coated fabric. (c) The fabric immersed in water by an external force, exhibiting a silver mirrorlike phenomenon. (d) A jet of water (dyed red) bouncing off the fabric surface. (e) Consecutive captures taken from a video to show a water droplet (~ 5 µL) sliding off a 13o-tilted PA-FeIII@PDMS coated fabric. Figure 2 165x120mm (150 x 150 DPI)



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SEM images of the PA-FeIII@PDMS coated fabrics with different numbers of assembly: (a) 1, (b) 2, (c) 3, (d) 4, and (e) 8. The insets are the photographs of static water contact state on the corresponding fabrics. (f) CAs and SAs of the fabrics prepared with different assembly times. Figure 3 277x417mm (150 x 150 DPI)


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SEM images of various PA-Mn+@PDMS coated cotton fabrics: Mn+ = (a, b) Ag+, (c, d) Ce3+, (e, f) Zr4+, and (g, h) Sn4+. The insets of (a), (c), (e), and (g) are the photographs of water droplets on the fabrics, respectively. Figure 4 308x559mm (150 x 150 DPI)



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(a), (c) and (e) SEM images of the pristine filter paper, PET fabric and melamine sponge, respectively. (a’), (c’) and (e’) are photographs of the corresponding materials. (b), (d) and (f) SEM images of the PAFeIII@PDMS coated filter paper, PET fabric and melamine sponge, respectively. (b’), (d’) and (f’) are photographs of the corresponding materials. The insets are the corresponding water contact angles. Figure 5 656x416mm (150 x 150 DPI)


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The change of water contact angles of the superhydrophobic PA-FeIII@PDMS coated cotton fabrics after (a) ultraviolet light irradiation for diverse periods of time, (b) heating treatment under different temperatures for 2 h, and (c) immersion in organic solvents for different periods of time. Figure 6 237x377mm (150 x 150 DPI)



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(a) Photograph showing the abrasion process. (b) Correlation of contact angle with abrasion cycles using different meshes of sandpaper. (c) A piece of cotton fabric pasted onto and peeled off an adhesive tape. Water droplets on the fabric after peeling off the adhesive tape. (d) Contact angles and sliding angles of the as-prepared PA-FeIII@PDMS coated cotton fabric repeatedly torn by the adhesive tape. Figure 7 486x324mm (150 x 150 DPI)


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Photographs showing the selectively absorbed (a-c) n-hexane and (d-f) chloroform from water by superhydrophobic PA-FeIII@PDMS coated cotton fabrics, respectively. (g) Weight gains of the bare fabric and the modified cotton fabric toward ten-different types of oil. (h) Effect of the density of oil on the weight gain of the fabrics. Figure 8 303x372mm (150 x 150 DPI)



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(a) Photograph of a piece of PA-FeIII@PDMS coated cotton fabric with a size of 20 × 30 cm2. (b) Water droplets on the superhydrophobic PA-FeIII@PDMS coated cotton fabric. The inset is a jet of water roll off the fabric surface. Using (c) a piece of PA-FeIII@PDMS coated cotton fabric and a piece of pristine melamine sponge to fabricate (d) an “oil gathering package”. (e-j) Photographs of the oil (n-hexane, dyed red) adsorption process using the “oil gathering package”. (k) Schematic of the superhydrophobic/superoleophilic “oil gathering package” and its application in treating oil spill. Figure 9 628x261mm (150 x 150 DPI)


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(a-c) Photographs of the cotton fabric-based oil/water separation process of water and chloroform (details supported in Video S5). Water was dyed with methylene blue and chloroform was dyed with sudan II for clear observation. (d) Separation efficiency of the fabric for various oil/water mixtures. (e) Recyclable separation efficiency for 20 times over water/ chloroform mixtures on the superhydrophobic PA-FeIII@PDMS coated cotton fabric. Figure 10 257x236mm (150 x 150 DPI)



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graphical abstract 162x136mm (200 x 200 DPI)


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Nature-Inspired Strategy toward Superhydrophobic Fabrics for

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