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Sep 22, 2016 - Biomimetic Multi-Functional Superamphiphobic FOTS-TiO2 Particles ... micro/nano-structures of the lotus leaf surface, but it is beyond ...
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Biomimetic Multi-Functional Superamphiphobic FOTS-TiO2 Particles beyond Lotus Leaf Liwei Chen,†,‡ Zhiguang Guo,*,†,‡ and Weimin Liu‡ †

Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials and Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan 430062, People’s Republic of China ‡ State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, People’s Republic of China S Supporting Information *

ABSTRACT: It is widely known that natural examples like lotus leaves can only repel room-temperature water but cannot repel hot water and oils. Even though superamphiphobic surfaces composed of re-entrant “mushroom-like” or “T-shaped” structures are promising, they are generally regarded as substrate-dependent and difficult to fabricate, and hence, their practical use on various materials has been limited. Here, we synthesize a flower-like superamphiphobic FOTS-TiO2 powder by solvothermal process and self-assembly functionalization. These structured and functionalized submicron particles can repel the liquids with surface tension as low as 23.8 mN·m−1 (n-decane), which is the lowest among powder samples. With respect to the biomimetic aspect, the surface morphology of FOTS-TiO2 particle is similar to the hierarchical micro/nano-structures of the lotus leaf surface, but it is beyond the lotus leaf for superoleophobic capacity. The difference in the oleophobicity is suggested to be the interplay of quasi-spherical re-entrant structure and perfluorined modification. Because of superior superamphiphobicity of the powder, a facile yet versatile strategy is developed, adhesive-assisted sieve deposition fabrication (AASDF), for preparing superamphiphobic coatings on various substrates. The investigation results pertaining to the water/oil proofing, mechanical durability, self-cleaning, and antifouling performances prove that the FOTS-TiO2 coating is robust and multifunctional, which will enable more opportunities for practical applications. Apart from these general applications, we find that the superamphiphobic FOTS-TiO2 powders when coated on sponge as anti-icing surface have good ice delay and icephobic performances. Furthermore, they can be used to prepare magnetic Fe3O4&FOTS-TiO2 composite particles through liquid marbles, implying significant scientific value. KEYWORDS: biomimetic, multifunctional, superamphiphobic, lotus leaf, TiO2, anti-icing, self-cleaning

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INTRODUCTION Throughout the evolutionary history of animals and plants, the ability to protect the body from water wetting has been vitally important for their survival.1−3 For example, the lotus leaf repels water for self-cleaning.4 Water strider legs resist the water surface for ultrafast escaping.5 The wings of butterfly, cicada, and dragonfly promote the fast moving of raindrops for normal flying.6 Mosquito compound eyes repel condensed water for a clear vision.7 On the basis of interfacial wetting theories,8,9 the framework of fabricating superhydrophobic surfaces (SHPSs) has been recognizably established today, that is, combining hierarchical micro/nano-level structures with low-surface-energy chemicals.10,11 However, when the oil droplets are deposited on the natural and artificial SHPSs, they tend to collapse and wet the underlying surfaces. Thus, an interesting question arises: why these superhydrophobic surfaces, which have excellent water repellence, cannot repel oil droplets? Rather than attributing it to the lower surface energy of oils, the insufficiency of surface structures deserves to receive research. © XXXX American Chemical Society

To extend the repellent range from water to oils, the very conception of superamphiphobic surfaces (SAPSs) that can repel both water and oils emerged in response to the needs of water/oil proofing.12,13 To this end, a wealth of experimental data was accessed to study the potential wettability of various structures and a recognized condition for superamphiphobic surfaces was formed, that is, the presence of peculiarly shaped surface structures called “re-entrant”, “overhanging”, “mushroom-like”, or “T-shaped”.14,15 For instance, Liu et al. fabricated a superomniphobic surface with specific doubly re-entrant structures which can repel even perfluorohexane.16 In addition, the surface layer consisting of spherical particles or a pillar array of sintered spheres was shown to be able to realize the superamphiphobic property for re-entrant curvature.17,18 However, fabricating such superamphiphobic surfaces with fine structures usually involves Received: June 9, 2016 Accepted: September 22, 2016

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DOI: 10.1021/acsami.6b06772 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 1. Schematic of Preparing Flower-Like Superamphiphobic FOTS-TiO2 Particlesa

a

Typical solvothermal method and self-assembly functionalization were applied in the procedure.

its expanding applications in the fields of anti-icing surface, liquid marbles, and magnetic composite particles.

complex techniques, such as electrospinning,19 photolithography,20,21 candle soot aggregation,22 interfacial secondary growth,23 and other combinatorial approaches.24 Moreover, most fabrication techniques for superamphiphobic layers depend much on specific substrate materials, like glass,25,26 metals,27−29 fabrics,30,31 polymers,32,33 and so on. A practical, universal, and more flexible strategy to construct stable superamphiphobic layer on various substrates is still lacking and remains a challenge. Among diverse preparation methods, dip-/spray-deposition based on micro/nanoscale building blocks is a promising way to build a new layer, which is simple, fast, and applicable to various large-area substrates.34−38 What is more, they can be repeatedly used to repair the damaged surfaces.38 Thus, the preparation of superamphiphobic building blocks is critical for the superamphiphobic coating. For instance, Chen et al.39 prepared superamphiphobic PFOA-Cu particles by chemical reduction deposition and subsequent modification, which can be coated on various materials with the assistance of double-sided tape and spay-adhesive; Ge et al.40 synthesized a transparent superamphiphobic coating by spraying a layer of stringed silica nanoparticles; Jiang et al.41 prepared fluorinated raspberry-like polymer particles for superamphiphobic coatings. In the past years, titanium dioxide (TiO2) materials continue to mature and have attracted wide scientific attention for constructing novel nanostructured surfaces due to various structures, specific photoinduced wettability, and surficial modification.42−45 To obtain TiO2 microparticles for re-entrant structures, solvothermal synthesis is usually applied. However, a typical hydrolysis of tetrabutyl titanate (TBT) produces spherical TiO2 particles with a smooth surface. At the same time, most superamphiphobic coatings based on smooth spherical particles have a limited oleophobicity, especially for the liquids with surface tension below 34.5 mN·m−1 (peanut oil), like n-hexadecane, cyclohexane (light oil), and 1, 2-dichloroethane (heavy oil). In this work, we synthesize a superamphiphobic FOTS-TiO2 powder with flower-like quasi-spherical structures and provide a universal, simple, and fast strategy to construct superamphiphobic coating on various substrates. This flower-like structure is similar to the hierarchical micronipples/nanorods structures of the lotus leaf surface, but it has quasi re-entrant structures and different geometric dimensions, for which it can not only repel water but also repel various oils. The powder sample can be firmly bonded to the substrates with commercial double-sided tape or spray adhesive by a flexible method of sieve-deposition, dip-coating, or spraying. The investigations revolve around its general applications as robust superamphiphobic coatings for oil/water-proof, self-cleaning, and antifouling performances, and

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EXPERIMENTAL SECTION

Preparation of Hierarchical Flower-Like Superamphiphobic FOTS-TiO2 Powder. The hierarchical flower-like TiO2 particles were prepared through a typical template-free solvothermal method, as illustrated in Scheme 1. In the synthetic procedure, 2 mL (5.85 mmol) of tetrabutyl titanate (TBT) was mixed with 30 mL of ethanol and 10 mL of glycerol and stirred for 5 min to form a clean mixed solution at room temperature. The mixture was then transferred into a Teflon-lined stainless steel autoclave, which was sealed and maintained at 180 °C for 24 h. After the mixture was cooled to room temperature, the white precipitates were collected by centrifugation at 7000 rpm for 5 min, washed with ethanol for four times, and fully dried at 60 °C. Lastly, the products were calcined in air at 450 °C for 2 h to convert the products into pure anatase TiO2. The prepared hierarchical flower-like TiO2 particles were modified with 1% 1H,1H,2H,2H-perfluorooctyltrichlorosilane (FOTS) in n-hexane solution for 10 min. The resultant FOTS-TiO2 particles were collected by centrifugation (7000 rpm for 5 min) and washed with n-hexane for two times. After drying, the products were treated in a vacuum oven at 100 °C for 1 h to obtain a superamphiphobic powder and harvested for subsequent experiments. Fabrication of Superamphiphobic Coatings on Various Materials. The superamphiphobic powder can be coated on hard, soft, and fluffy materials via sieve-deposition, spray-deposition, or even simply a dip-coating, combined with double-sided tapes or spray adhesives (Schematic S1). Commercial double-sided tape (3 M Scotch brand, 200C) and spray adhesive (3 M SUPER77) were purchased from 3 M China. Double-sided tape was used for the hard Zn plate and glass slide substrates, whereas spray adhesive was used for soft paper, sponge, and rubber substrates, and for fluffy cotton and fiber textile substrates. Here, the sieve-deposition method was applied with a small bottle containing FOTS-TiO2 powder. The bottle had a punched bottle cap where a copper mesh was mounted, so that the powder could be easily deposited onto the pretreated surfaces by sieving action. To improve the bonding strength, the powder-deposited samples could be gently pressed, and the excess powder was shaken off. After drying in air, the surfaces of FOTS-TiO2-coated materials became superamphiphobic. Mechanical Strength Tests. (I) For the coated sponge, it was pressed by fingers for maximum damage and then released fully. The pressing−releasing cycle was repeated for 20 times. (II) For the coated glass slide, it was abraded with sandpaper (360Cw) under the weight of 100 g. The abrasion was conducted back and forth for 10 times with an abrading distance of ∼10 cm. (III) For coated rubber, it was stretched up to twice the original length and then released to original length by hands. The stretching−releasing cycle was repeated for 40 times. The variations of water/oil contact angles (CAs) and rolling angles (RAs) with mechanical damage times were measured, and each measurement was made for three times to get an average value. B

DOI: 10.1021/acsami.6b06772 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Anti-Icing Experiments. To investigate anti-icing performance, half of the PU sponge was treated with FOTS-TiO2 powder, and the other half was not treated. For icing delay time, one water droplet (6 μL) was deposited on the uncoated half sponge, while another one was deposited on the coated half. The whole sponge was placed in the condition with a temperature of −10 °C and humidity of 80% to record the icing time of each water droplet. For icephobic performance, two water droplets (10 μL) iced on the uncoated half sponge and another two iced on the coated half were used. The sponge with iced droplets was moderately pressed by an investigator’s fingers to compare the icephobic property (that is, ice adhesion strength) of uncoated and coated sponge. Preparing Magnetic Fe3O4&FOTS-TiO2 Liquid Marbles and Composite Particles. The magnetic Fe3O4 particles were synthesized by a typical coprecipitation method, which referred to the work of Li et al.46 Next, 0.2 g of as-prepared Fe3O4 particles was dispersed in 10 mL of water through ultrasonication. The prepared Fe3O4 suspension solution was dropwise dripped onto the FOTS-TiO2 powder to form magnetic Fe3O4&FOTS-TiO2 liquid marbles (10 μL). The formed liquid marbles were transferred onto the glass surface. After fully drying at room temperature, the magnetic Fe3O4&FOTS-TiO2 composite particles were prepared. Characterization. The surface morphologies of the synthesized TiO2 powders and TiO2-coated samples were observed by a fieldemission scanning electron microscope (FESEM, JSM-6701F) both with Au-sputtered specimens. The analyses of elements and function groups were performed on X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) and Fourier transformer infrared spectra (FTIR, Thermo Scientific Nicolet iS10). The crystalline phases of the as-synthesized powders were identified by X-ray diffraction (XRD, X’Pert PRO, PANalytical Co., The Netherlands) with Cu Kα radiation, respectively. Transmission electron microscopy (TEM) measurements were carried out with a TechnaiG20 (FEI) operating at 300 kV. TG measurements were done with NETZSCH STA 449 C using a dynamic heating rate of 10 °C min−1 under the atmosphere of nitrogen. The contact angles of water and oils (WCA and OCA) on various samples were measured in air with a contact angle meter (JC2000D) at ambient temperature. All photographs were taken using a Sony camera (DSCHX200).

Scheme 2. (a) Schematic Illustration of Oil Droplet Sitting on the Top Layer of Smooth-Particle-Based Coating, Highlighting the Re-Entrant Structure and Expected LiquidVapor Interfacea; (b) Schematic Diagram for Oil Droplet Sat on Two Types of TiO2 Sphere Stacking Layers: Hierarchical Flower-Like Sphere and Smooth Sphere

a

On closer examination of eq 1, it is noted that f and rf are both positive, f is less than 1 and rf is greater than 1. Therefore, if θ < 90°, θCB r can be >150° only if f is very small. That is to say, the smooth spherical particles are very far apart according to eq 2, but it is almost impossible for practical coatings. This is why the coatings based on smooth spherical particles are shown to have the limited oleophobicity, especially when the liquid surface tension is below 27.5 mN·m−1. To minimize the value of f, another strategy is to further increase the roughness of the microspheres, such as flower-like quasi-spheres. As shown in Scheme 2b, compared to smooth submicron particles, the nanoroughened submicron particles not only retain their reentrant structures but also trap the air sublayer around them, resulting in stable CB behavior. To this end, the hierarchical flower-like superamphiphobic FOTS-TiO2 particles are prepared by a typical process of solvothermal synthesis, calcination, and final perfluorination modification (the preparation details are illustrated in Scheme 1). As is well-known, the typical hydrolysis of tetrabutyl titanate (TBT) in aqueous ethanol solution produces smooth spherical particles.48 Here, by a special TBT glycerolysis reaction in the mixture of glycerol and ethanol (1:3 v/v) at high temperature, flower-like submicron-particles (an average diameter of ∼1 μm) is obtained at first. Close-up observations reveal that the surfaces of precursor particles are actually covered with petal-like nanostructures. Numerous nanopetals radiate outward and interconnect each other, forming quasi-spheres with thorn-like tip ends (Figure 1a,d). The XRD pattern indicates that these particles are glyceric titanium (GT), because the pattern is similar to that of previously reported titanium glycerolate (Figure 1g).49,50 Indeed, the formation of such flower-like TiO2 (or GT) microspheres is greatly dependent on the presence of a supporting solvent, alcohol. The influences are embodied in two aspects: (1) Alcohol can greatly reduce the viscosity of the reaction system and therefore accelerates the nucleation of GT to obtain a small crystal nucleus due to the increased mass transfer rate; (2) Alcohol may be adsorbed on certain crystal surfaces of GT particles and thus inhibits the growth of some facets of the latter, as indicated by the change of XRD peak intensities (Figure 1g). After calcination, GT particles are converted into

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RESULTS AND DISCUSSION Fabrication of Superamphiphobic FOTS-TiO2 Powder with Hierarchical Flower-Like Structures. It has been wellknown that the main strategy for fabricating superoleophobic surfaces is to elaborate special re-entrant structures and to assemble a monolayer of perfluorinated materials on them. Therein, a simple re-entrant structure appears on the coated surfaces with randomly distributed microspheres. When a water (or oil) droplet sits on a surface composed of smooth spheres in Cassie− Baxter (CB) state, as illustrated in Scheme 2a, the apparent 9,47 contact angle on the surface (θCB r ) can be described as cos θrCB = f1 cos θ − f2 = frf cos θ − 1 + f

(1)

where θ is Young’s contact angle for a flat surface with the same composition, f1 (or f 2) is the ratio of the surface area of the liquid in contact with the solid (or the air) to the total projected area, f is the fraction of the projected area of the solid surface in contact with the liquid, and rf is the Wenzel roughness in contact with the liquid. Referring to Scheme 2a, f is given by π(R sin α)2/4 (R + d)2 and rf is 2(1 − |cos α|)/(sin α)2. According to Marmur,47 α is equal to (π − θ) when the free energy is a minimum. Substituting for f and rf in eq 1 results in cos θrCB = −

R and d are the radius and spacing of particles, respectively.

2 2 2 1⎞ 3π ⎛ R ⎞ ⎛ π⎛ R ⎞ ⎜ ⎟ ⎜|cos θ | − ⎟ + ⎜ ⎟ − 1 3⎠ 3⎝R + d⎠ 4 ⎝R + d⎠ ⎝ (2)

C

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Figure 1. Characterizations of as-prepared particles. (a) SEM image, (d) TEM image, (g) XRD spectrum, and (j) XPS spectra of the particles prepared by solvothermal method. By a typical glycerolysis reaction of tetrabutyl titanate, flower-like glyceric titanium submicron-particles is obtained at first. (b) SEM image, (e) TEM image, (h) XRD spectrum, and (k) XPS spectra of the particles after calcination. By calcination, glyceric titanium particles then transform into pure TiO2 particles. (c) SEM image, (f) TEM image, (i) XRD spectrum, and (l) XPS spectra of the resulting particles after calcination and perfluorination. The hierarchical flower-like structures of particles are retained and modified with a monolayer of low-surface-energy chemicals, referred to as FOTS-TiO2.

liquid repellence with respect to water, cooking oils, and lowsurface-tension organic solvents. To check its liquid repellency, four representative liquids are chosen, namely, water, glycerol (viscous oil), colza oil (cooking oil), cyclohexane (light oil), and 1,2-dichloroethane (heavy oil). As shown in Figure 2, when the four testing liquids dripped onto the loose powder, all of the droplets keep perfectly spherical shapes or/and form stable liquid marbles on it without any wetting or collapse. Went a step further to quantify its nonwettability, the loose powder is compacted into block for contact angle (CA) and rolling angle (RA) measurement. The optical photographs and CA images show that all testing liquid droplets on the pressed powder block are extremely repelled with high CAs above 150°, even for high-viscosity glycerol and low-surface-tension cyclohexane (Figure 2). To clearly know the liquid-repellent range, the liquids with different surface tensions that can be strongly repelled by the powder are listed in Table S1, as well as their CAs and RAs measured on a pressed powder block. The superamphiphobicity of pressed powder is registered with respect to waters (hot, acid, and alkaline), drinks (milk, coffee, and coca-cola), cooking oils (colza and peanut oil), and organic solvents (n-decane, cyclohexane, etc.). One can clearly see that the CAs and RAs of all registered liquids are above 150° and below 10°, respectively, showing superior superhydrophobicity and superoleophobicity. In particular, the ultralow-surface-tension cyclohexane (γLA = 24.95 mN·m−1) and n-decane (γLA = 23.8 mN·m−1) are extremely repelled. As far as we know, this is the best performance among powder samples, which is far beyond general fluoridated spherical particles.39 In addition, the FOTS-TiO2 powder is so stable that it is still superamphiphobic even after 1 year of sealed dry preservation, showing a long shelf life.

pure anatase TiO2 particles because all diffraction peaks of the calcined particles are perfectly matched to that of anatase TiO2 (Figure 1h),51 and the hierarchical flower-like structures are somewhat retained but the nanopetals with thorn-like tip ends become nanorod clusters (an average diameter of ∼50 nm) (Figure 1b,e). Such kind of flower-like nano/submicron particle has much higher roughness than smooth submicron particle, which is exactly what particle-based coating requires for higher oleophobicity as discussed above. A second and indispensable factor for superoleophobic surfaces is their coating with perfluorinated materials. Even if perfluorinated materials are intrinsically oleophilic, they also have the highest oleophobicity as compared to hydrocarbon or silicone materials that are usually used for superhydrophobicity. Here, 1H,1H,2H,2H-perfluorooctyltrichlorosilane (FOTS) is chosen as the surface modifier because it can rapidly self-assemble into a monolayer on the surface of TiO2 particles to do reduce the surface free energy but do not change the surface morphology (Figure 1c,f), which is vital for the final superamphiphobicity formation. The XPS and FT-IR spectra further provide the information on the successive self-assembly of low-surface-energy FOTS modifier on TiO2 particles. The characteristic peaks of titanium (Ti) and fluorine (F) elements appear at 459.0 and 688.7 eV (Figure 1l). Typical functional group −CF2− of the FOTS molecule can be observed at the wavenumber of 1145 and 1238 cm−1 (Figure S1). Note that these nanorods and the central core are one, not individuals, which are confirmed by TEM images (Figure 1d−f) and ultrasonic treatment. Superamphiphobicity of the Flower-Like FOTS-TiO2 Powder. With flower-like quasi-spherical structure and perfluorinated modification, the FOTS-TiO2 powder exhibits strong D

DOI: 10.1021/acsami.6b06772 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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salt shaker. In this work, such proof-of-concept of the superamphiphobic FOTS-TiO2 coating is carried out on several typical substrates, including glass slide, metal plate, filter paper, sponge, rubber, cotton wool, and textile. As for the coated hard substrates (i.e., glass and metal substrates), what surprised us most is their lotus-leaf-like surface morphology. To demonstrate its similarity to the lotus leaf in surface morphology, the SEM images of lotus leaf surface and coated glass are presented and compared, as shown in Figure 3. The top-view SEM images of coated glass slide surface show that the surface is fully covered by close-packed flower-like FOTSTiO2 particles (Figure 3b). First, many flower-like FOTS-TiO2 microparticles are stacked together for microlevel aggregates (Figure 3b2), which is similar to the micronipples of lotus leaf (Figure 3a2). This also indicates that the thickness of the coating is approximately equal to the height of five particles piled up, that is, about 5 μm of thickness. Second, the individual particles form the submicron-level structures (Figure 3b3). Third, the nanolevel rods on particles (Figure 3b4) correspond to the nanocrystals on the nipples of the lotus leaf (Figure 3a4). The coated Zn plate (a representative of metal materials) prepared in the same manner shows similar surface morphology (Figure S3c). Figure 3, Figure 4a,b schematically illustrate the similarities and differences between the lotus leaf and flower-like FOTS-TiO2 coating in surface features. One can clearly see that they are similar in shape but different in geometrical dimensions at all levels and surface chemical component, which are critically discussed here. In geometrical dimensions, the diameters of micronipples and waxy nanorods on the lotus leaf surface are ∼5 μm and ∼500 nm, respectively, and the hierarchical structural units are evenly distributed with spacing of ∼10 μm (Figure 4a). However, the diameters of microparticles and nanorods on FOTS-TiO2 coating are ∼1 μm and ∼50 nm, separately, and the multilevel structural units are close-packed without obvious spacing (Figure 4b). This fine configuration increases the roughness of the coating based on re-entrant quasi-spherical particles. In the surface chemical component, the lotus leaf surface is functioned by wax, namely, hydrocarbon, whereas the FOTS-TiO2 coating is modified with perfluorinated material. It is the specialty in the reentrant quasi-spherical structure and perfluorinated material that decides its superiority in repelling oils. Even though making a lotus-leaf-like structure is not unreasonably difficult any more, lotus-leaf-like superoleophobic surfaces

Figure 2. Antiwettability of superamphiphobic FOTS-TiO2 powder. The droplets of water and oil (glycerol, cyclohexane, and 1,2dichloroethane) can form liquid marbles on the superamphiphobic FOTS-TiO2 powder. The test droplets on the compacted powder showed high contact angles above 150°.

What is more, thermal gravimetric analysis (TGA) shows that the curve appears to decline at 248.4 °C (Figure S2), implying that the decomposition of FOTS-TiO2 powder will not occur up to this temperature. Therefore, we have every reason to believe that the FOTS-TiO2 powder and coating can maintain its excellent performance under this temperature. Universal Applicability of FOTS-TiO2 Powder to Various Substrates for Preparing Superamphiphobic Coating. Because the superamphiphobic powder is substrateindependent, and thus, it can be practically applied to various substrates regardless of their morphologies through a flexible method of sieve deposition, spraying, or dip-coating. Here, a facile yet versatile strategy is developed, that is, adhesive-assisted sieve deposition fabrication (AASDF). In the presence of commercial double-sided tape or spray adhesive, the powder samples are firmly bonded onto hard, soft, and fluffy materials by sieve deposition using a specially designed bottle which is similar to a

Figure 3. Surface morphologies of natural and artificial lotus leaf. (a) Top-view SEM images of natural lotus leaf surface, showing hierarchical micronipples/nanorods structures. (b) Top-view SEM images of coated glass slide, showing multilevel microaggregates/submicro-particles/nanorods structures. E

DOI: 10.1021/acsami.6b06772 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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the coated Zn plate displays great water and oil repellency also (see Figure S3a,b and Movie S2). In addition to hard substrates, this superamphiphobic FOTSTiO2 powder is also applicable to soft and fluffy substrates in the presence of spray adhesive (which is inherently hydrophobic). Figure 5 shows the surface wettability and SEM images of coated filter paper, sponge, cotton, and textile. For example, uncoated filter paper (the representative of paper materials) is wetted by the water and oils due to its own amphiphilicity (Figure 5a), while coated filter paper can hold the water and oils in perfectly spherical shapes (Figure 5b) and promote the rolling behavior readily (see Movie S3). SEM images of coated paper surface show that the messy but relatively smooth dendritic fibers are fully covered with plenty of flower-like FOTS-TiO2 particles (Figure 5c and Figure S5). Similarly, attributed to the complete coverage of superamphiphobic powder on the smooth network frame (Figure 5f and Figure S6), the coated PU sponge also manifests excellent water and oil repellency (see Figure 5d,e and Movie S4). In addition, the universality and feasibility of FOTSTiO2 coating are further confirmed in the cases of host substrates of cotton wool and textile. The photos in Figure 5g,h show that the water and cooking oil droplets are extremely repelled by the coated materials, which is of great significance for practical applications in fabricating nonwetting and antifouling clothes. Substantially, in the SEM images of uncoated and coated cotton wools, one can see that the originally smooth branched structures are fully covered by a dense layer of FOTS-TiO2 particles (Figure 5i and Figure S7), which should be responsible for the superamphiphobic property. It is worth noting that the used double-sided tape and spray adhesive are slightly hydrophobic inherently (Figure S4). These manifestations sufficiently verify the universal application of superamphiphobic FOTS-TiO2 coating on various substrates.

Figure 4. Superamphiphobicity of coated glass slide. (a,b) Schematic illustrating the similarities and differences between the surface structures of (a) lotus leaf and (b) flower-like FOTS-TiO2 coating. (c) Surface wettability of uncoated and coated glass slide with water, glycerol, colza oil, and 1,2-dichloroethane droplets.

are rarely reported still. To exhibit the superamphiphobicity of coated glass surface, water, glycerol, colza oil, and 1,2dichloroethane are used as the testing droplets, and an uncoated glass slide is taken as control group. The optical photograph in Figure 4c shows that the testing liquids wet the uncoated glass slide for inherent amphiphilicity, whereas they are repelled by the coated surface. The contact angles of water, glycerol, colza oil, and 1,2-dichloroethane droplets are measured to be 159.5 ± 1.2°, 153.6 ± 0.9°, 152.3 ± 0.6°, and 152.8 ± 1.4°, respectively. Moreover, the water and oil droplets can easily roll down from the coated glass surface without any trace of residues (see Movie S1), implying low rolling angles and low adhesion. As another exemplification,

Figure 5. Superamphiphobic property and surface topography of FOTS-TiO2-coated soft and fluffy materials. Wetting states of water and oil droplets on (a) initial filter paper and FOST-TiO2 powder coated (b) filter paper, (d) roughness nylon sponge, (e) fine PU sponge, (g) cotton and (h) textile, and SEM images of coated (c) filter paper, (f) PU sponge, and (i) cotton. The blue droplet is water, and the yellow droplets are glycerol, peanut oil, and colza oil, respectively. The insets show the water-repellent property of spray adhesive and double-sided tape coated substrates (red wireframe) and the enlarged view of states of water and oils (white wireframe). F

DOI: 10.1021/acsami.6b06772 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces Mechanical Durability of the Superamphiphobic FOTS-TiO2 Particle-Based Coatings. A well-known destructive shortcoming of particle-based coatings is their mechanical durability, which is why few works have reported this property, especially for the abrasion test by sandpaper.34,35,39 In practical applications, the mechanical strength is an important factor. For this reason, double-sided tape or spray adhesive is utilized to firmly bond the superamphiphobic FOTS-TiO2 powders onto substrates. Obviously, the binding force is mainly the physical adhesive force of binding agent. To check the mechanical durability and stability, three typical coated samples including hard glass, squeezable sponge, and stretchable rubber are chosen as the test objects. In particular, a standard abrasion test is implemented for a coated glass slide using the sandpaper (360 Cw) under the load of 0.98 N (inset in Figure 6a). During the

ORA. Significantly, almost no powder fell off during the whole process, and the droplets of water and oil can be readily shaded off without residuals even after 20 cycles (see Movie S4). Lastly, as for coated rubber, its stretch-resistant capacity comes into notice for its elastic essential. Thus, a stretching test is conducted by using hand to repeatedly stretch the coated rubber up to twice its original length (Figure 6d). After 20 times of stretching-releasing cycles, the coated rubber is still shown to be superhydrophobic and superoleophobic (Figure 6e). The droplets of water, colza oil, peanut oil, glycerol, and n-hexadecane can keep almost spherical shapes (Figure 6e) and smoothly roll down the surface after 20 stretching cycles (see Movie S6). Basically, the good durability of the FOTS-TiO2 coating is ascribed to three aspects: gear-like interlocking effect between flower-like particles, adhesive bonding strength, and embeddedness of the nanorod structures in binding layer. The reliable mechanical durability can help the superamphiphobic powder win over more opportunity for practical applications. Self-Cleaning and Antifouling Properties of the Coated Materials. To reflect the feasibility of the superamphiphobic FOTS-TiO2 coating in practical applications, the coated materials are tested with respect to the self-cleaning and antifouling properties. Here, the ultrathin carbon black is specially used as the dirt to clean here. It should be particularly emphasized that the ultrathin carbon black is used as the object to be cleaned, which is harder to be cleaned than general dusts. As shown in Figure 7a, when water droplets are dripped on coated glass, they can smoothly roll down the surface and take away the deposited carbon black at the same time, resulting in a clean surface (see Movie S7). In addition to self-cleaning property that is commonplace for every superhydrophobic surface, a reliable antifouling property is also possible with the FOTS-TiO2 coating. First, with respect to oil-fouling resistance, the coated glass slide is dipped into the peanut oil, slightly agitated, and then taken out of the oily environment. It is clear that an air layer is entrapped between the oil phase and surface. As a result, the coated glass slide can be taken out from the peanut oil without any residual oil, displaying excellent antioil-fouling performance (see Figure 7b and Movie S8). Another special antifouling test is performed with contaminated snow. As shown in Figure S8, the glass plate surface is coated with the powder at specific locations, and the whole glass is covered with contaminated snow. It is observed that, during melting at room temperature (∼25 °C), the melted liquid water with the dark contaminant spontaneously come together to the untreated hydrophilic areas, but the coated areas remain clean. Apart from the solid substrate, the soft porous material (i.e., cotton wool) with FOTS-TiO2 coating also has great antifouling property. To clearly demonstrate this property, a contrast test is done to shown the antifouling capacity of coated cotton. As shown in Figure 7c,d, the naked and coated cottons are simultaneously dipped into the methyl-blue-dyed water solution. A sharp contrast is presented in which the untreated cotton wool is completely drenched and dyed into blue, whereas the treated cotton wool is not dyed and still keeps white after immersing (see Movie S9). The excellent antiwater-fouling capacity is substantially benefited from the superamphiphobic FOTS-TiO2 layer, which forms a stable air protective layer between the liquid and the cotton surface. These properties are significant for practical applications, like oil pipeline, kitchenware, clothing, and so on. Potential Applications in the Fields of Anti-Icing Surface, Liquid Marble, and Magnetic Particle Preparation. In addition to general applications as superamphiphobic,

Figure 6. Mechanical durability measurements. (a) The variation of water and oil (peanut oil) contact angles and rolling angles with abrasion times. Inset shows the abrasion test of coated glass slide by a sandpaper (360Cw) under the pressure of 50 g. (b) SEM image of coated glass surface after mechanical damage. Inset 1 shows the damaged area, and inset 2 shows the undamaged area. (c) The variation of water and oil (peanut oil) contact angles and rolling angles with squeezing times. Inset shows the squeezing test. (d) Test of coated rubber under stretching. (e) Surface wettability of coated rubber after 20 times stretching with respect to water, colza oil, peanut oil, glycerol, and n-hexadecane, respectively.

abrasion, very little powder is scraped off and this micro/nanocoating can withstand more than 10 cycles (a total distance of 2 m) without the loss of the superamphiphobicity. The variation of water and oil contact angles and rolling angles (W/OAC and W/ORA) with abrasion times is shown in Figure 6a. After a total abrasion distance of 2 m, the WCA changes slightly from 160.2 ± 0.8° to 157.0 ± 2.0°, OCA (peanut oil) changes from 152.3 ± 0.3° to 148.6 ± 1.9°, and the WRA and ORA hold relatively steady below 6° (Figure 6a). Furthermore, the water and oil droplets can promptly escape from the abraded surface (see Movie S5). From the SEM image after abrasion, one can clearly see that the superamphiphobic particles are well-fixed on the surface even though few areas suffer minor damage (Figure 6b). In addition, a squeezing test is adopted for the coated PU sponge by repeated squeezing and releasing action using fingers (inset in Figure 6c). The plot of the CAs and RAs of water and oil (peanut oil), which varies with the squeezing cycles, is shown in Figure 6c. It is obvious that, even after 20 cycles, small fluctuations are taken place in the WCA and OCA (peanut oil), as well as the WRA and G

DOI: 10.1021/acsami.6b06772 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces

Figure 7. Self-cleaning and antifouling property of coated glass slide and cotton wool. (a) Water droplets deposited on the slanted coated glass slide can quickly roll down and bring away the ultrathin carbon black dust, resulting in a clean surface. (b) Coated glass slide was dipped into the peanut oil and brought out without any oil. (c) Nontreated cotton was completely dyed into blue in the methyl-blue-dyed water. (d) Treated cotton was not dyed and still remains white after immersing.

Figure 8. Extended applications of FOTS-TiO2 powder in the field of anti-icing, liquid marble research, and composite particle preparation. (A) Antiicing application. The powder-coated sponge is capable of (a) delaying ice formation and (b) reducing ice adhesion. (B) Liquid marble research. (a) Water, cooking oils, and some organic solvents could form stable liquid marbles with the powder. (b) The water marbles were so stable that they could float on the water surface for a layer of water marbles. (C) Preparation and magnetic control of magnetic compound particles. (a) Prepared Fe3O4 aqueous solution. (b) Preparation of magnetic particles by drying the Fe3O4&FOTS-TiO2 liquid marbles. (c,d) Magnetic control of Fe3O4&FOTS-TiO2 liquid marbles and particles, which can be moved by a magnet.

using such flower-like superamphiphobic powder to build an anti-icing surface. For this purpose, the PU sponge as the substrate is specially treated, in which one-half is coated with the FOTS-TiO2 powder and the other half is not coated. The icedelay performance is tested in the condition with temperature

self-cleaning, and antifouling coatings, we also try to find out other application potentials of the superamphiphobic FOTSTiO2 powders. The expanded applications in the field of antiicing, liquid marble, and magnetic composite particle are heuristically introduced in this work (Figure 8). The first aspect is H

DOI: 10.1021/acsami.6b06772 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Research Article

ACS Applied Materials & Interfaces of −10 °C and humidity of 80%. The result shows that the water droplet deposited on the uncoated half freezes after ∼19 min, whereas the water droplet deposited on the coated half freezes after a long time of ∼1 h (Figure 8A-a and Movie S10). In particular, the icing behavior happens in a flash, and the long icedelay time is dependent on the precooling process. Such great performance in ice delay is suggested to be the result of small solid−liquid contact area and low thermal conductivity of sponge, as well as the small size of nanorods. Based on a classical heterogeneous nucleation theory, when the size is comparable to the critical nucleation radius, the water freezing will be greatly delayed, which has been explained in the work of Cao et al.52 Additionally, the coated sponge surface shows great icephobicity. Through simple squeezing action, the iced droplets on the coated half of the sponge are easily shed off, while the iced droplets on the uncoated half are still attached on the sponge (Figure 8A-b and Movie S11). The low ice adhesion is owing to the few anchoring points for superhydrophobicity. As can be seen, such FOTS-TiO2 powder can be used to other substrates as excellent anti-icing coatings. In other aspects, it is found that the water, colza oil, and glycerol droplets can be held in spherical shapes by a thin powder layer that is supported by the water surface (Figure S10a). Thus, if the liquid droplets are used as supporting substrates, one can envision that various liquid marbles can be formed by wrapping a layer of powder on the droplet surfaces because of the superamphiphocity of FOTS-TiO2 powder. For this reason, various types of liquid marble are formed, including glycerol, colza oil, cyclohexane, and 1,2-dichloroehane marbles (Figure 8B-a, and Figure S10b). In particular, the water marble is so stable that more than 50 water marbles (5−20 μL) can simultaneously float on the water surface to form a close-packed marble layer without rupture (Figure 8B-b, and Figure S9). The liquid marbles are further investigated to some extent. We observed the contact angle and shape of glycerol marble changing with the liquid volume (20, 40, and 60 μL). With the increase of droplet volume, the marble radii in horizontal direction (RO) and vertical direction (RV) are spontaneously increased, as well as the ratio of RO:RV, but the marble contact angles have no significant change (Figure S10c−d). Inspired from above liquid marbles, a kind of magnetic liquid marble (Fe3O4&FOTS-TiO2) is prepared through wrapping the Fe3O4 aqueous droplets with FOTS-TiO2 powder. Through natural drying, the magnetic marbles shrank from the top for faster water evaporation and resulted in wrinkled magnetic Fe3O4&FOTS-TiO2 particles (Figure 8C-b and Figure S11). Because the Fe3O4&FOTS-TiO2 liquid marbles and dried particles are magnetic, they can be controlled and moved with a magnet (Figure 8C-c−d, Figure S12, and Movie S12). This work provides a strategy to prepare magnetcontrollable, superamphiphobic composite particles that might be interesting to explore for other applications.

(AASDF) is reported, which can create robust superamphiphobic coatings against mechanical damages. Furthermore, multiple functions including self-cleaning, antifouling, and anti-icing properties reported here provide more opportunities and fast access to practical applications in surface, environment, and energy engineering. Lastly, integrating magnetic fluid into the superamphiphobic FOTS-TiO2 powder and forming magnetic superamphiphobic liquid marbles or dried composite particles represent some directions that might be interesting to explore.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b06772. Fourier transform infrared spectroscopy (FT-IR) and thermal gravimetric analysis (TGA) of the as prepared FOTS-TiO2 particles, contact angle (CA) and rolling angle (RA) of the liquids repelled by the superamphiphobic FOTS-TiO2 powder, SEM images of various coated substrates, self-cleaning and antifouling performance of the coated surface with dusty snow, optical photographs of water marbles floating on water surface, magnetic controllable property of prepared Fe3O4&FOTS-TiO2 liquid marbles and dried composite particles (PDF) Superamiphobicity of coated Zn plate substrate; movie S1 (AVI) Superamiphobicity of coated Zn plate substrate; movie S2 (AVI) Superamiphobicity of coated Zn plate substrate; movie S3 (AVI) Mechanical property test 1; movie S4 (AVI) Mechanical property test 2; movie S5 (AVI) Mechanical property test 3; movie S6 (AVI) Self-cleaning test; movie S7 (AVI) Anti-fouling test 1; movie S8 (AVI) Anti-fouling test 2; movie S9 (AVI) Anti-icing test; movie S10 (AVI) Icephobicity test; movie S11 (AVI) Magnetic control; movie S12 (AVI)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: 0086-931-4968105. Fax: 0086931-8277088. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Nature Science Foundation of China (Nos. 51522510 and 51675513), and the “Top Hundred Talents” Program of Chinese Academy of Sciences.

CONCLUSIONS In this work, a universal, versatile superamphiphobic powder based on flower-like FOTS-TiO2 particles is synthesized by a simple process of solvothermal reaction and subsequent selfassembly functionalization. These structured and functionalized submicron particles can repel the liquids with surface tension as low as 23.8 mN·m−1 (n-decane). Compared with other superamphiphobic surfaces, the biggest advantage is its own superamphiphobicity as powder sample, which is substrate-independent and can be coated onto various substrates. Here, a universal coating strategy of adhesive-assisted sieve deposition fabrication

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