Highly Flexible Superhydrophobic and Fire-Resistant Layered

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Highly Flexible Superhydrophobic and Fire-Resistant Layered Inorganic Paper Fei-Fei Chen,†,‡ Ying-Jie Zhu,*,†,‡ Zhi-Chao Xiong,*,†,‡ Tuan-Wei Sun,†,‡ and Yue-Qin Shen†,‡ †

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China ‡ University of Chinese Academy of Sciences, Beijing 100049, PR China S Supporting Information *

ABSTRACT: Traditional paper made from plant cellulose fibers is easily destroyed by either liquid or fire. In addition, the paper making industry consumes a large amount of natural trees and thus causes serious environmental problems including excessive deforestation and pollution. In consideration of the intrinsic flammability of organics and minimizing the effects on the environment and creatures, biocompatible ultralong hydroxyapatite nanowires are an ideal building material for inorganic fire-resistant paper. Herein, a new kind of free-standing, highly flexible, superhydrophobic, and fireresistant layered inorganic paper has been successfully prepared using ultralong hydroxyapatite nanowires as building blocks after the surface modification with sodium oleate. During the vacuum filtration, ultralong hydroxyapatite nanowires assemble into self-roughened setalike microfibers, avoiding the tedious fabrication process to construct the hierarchical structure; the self-roughened microfibers further form the inorganic paper with a nacrelike layered structure. We have demonstrated that the layered structure can significantly improve the resistance to mechanical destruction of the as-prepared superhydrophobic paper. The as-prepared superhydrophobic and fire-resistant inorganic paper shows excellent nonflammability, liquid repellency to various commercial drinks, high thermal stability, and self-cleaning property. Moreover, we have explored the potential applications of the superhydrophobic and fire-resistant inorganic paper as a highly effective adsorbent for oil/water separation, fire-shielding protector, and writing paper. KEYWORDS: hydroxyapatite, nanowires, paper, superhydrophobic, fire-resistant paper.18−22 However, as aforementioned, neither rewritable nor synthetic organic polymer paper is suitable for fireresistance due to its flammable nature, whereas inorganic materials used for papermaking can not only effectively resist the fire but also lower consumption of wood. The superhydrophobic surface, with a water contact angle greater than 150° and a sliding angle less than 10°,23 has aroused significant attention in last two decades because of its potential applications in self-cleaning,24 antifogging,25 antiicing,26 water-harvesting,27 water transportation,28 oil/water separation,29 and microfluid equipment.30 Bioinspired by the lotus effect,31−33 many superhydrophobic surfaces combine micro- and nanoscale hierarchical rough structures with lowsurface-energy compounds. However, the preparation methods for constructing hierarchical structures involving etching or multistep sequential deposition are high-cost, time-consuming, and complicated facilities are needed.34,35 Another problem of these hierarchical structures is their poor resistance to mechanical wear. These surfaces may lose their super-

1. INTRODUCTION Free-standing paperlike materials are an important integrant of our modern technological society. Their important uses include fire-shielding layers,1−4 smart filters,5 adsorbents,6,7 UV and electromagnetic shielding,8,9 and energy storage.10,11 However, traditional paper made from plant cellulose fibers is easily destroyed by either liquid or fire in daily work and life, which limits their applications in many special fields. Although waterproof paper was prepared by creating superhydrophobic surfaces,12,13 it is hard to achieve superhydrophobicity and fire-resistance simultaneously due to paper’s intrinsic flammable nature. Recently, fire-resistant composites using inorganics such as graphene and montmorillonite as raw materials were reported.1−4 It is expected that the paper with simultaneous superhydrophobicity and fire-resistance can be obtained by using superhydrophobic inorganics as building materials. Furthermore, the tremendous consumption of the natural woods in the papermaking industry causes a series of severe environmental problems including excessive deforestation and chemical contamination.14 In this regard, some innovative efforts have been reported to seek the solutions in two ways: (i) developing rewritable paper15−17 and (ii) using synthetic polymers or inorganics as raw materials to prepare the © XXXX American Chemical Society

Received: October 9, 2016 Accepted: November 23, 2016

A

DOI: 10.1021/acsami.6b12838 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces

Scheme 1. Schematic Illustration of the Preparation Process of SLIP via the Vacuum-Assisted Suction Filtration

2. EXPERIMENTAL SECTION

hydrophobicity via destruction of hierarchical structures after abrasion,36 which limits their practical applications and hinders the development of superhydrophobic materials. Designing robust or self-healing superhydrophobic materials is currently a hotspot in this research area.24,37 Recently, the unique layered structure of nacre had aroused our attention. Nacre consists of layered aragonite (CaCO3) platelets (bricks) bonded by a layer of biopolymer (mortar) in the intersheet gallery, which is called brick-and-mortar architecture.38 Such unique layered structure has been widely applied in supercapacitors11 and smart filters.5 However, the concept of layered structure has not been proposed for constructing the superhydrophobic paper before. In fact, the layered structure is intrinsically beneficial to the resistance to mechanical wear owing to their three-dimensional superhydrophobic network. The exposure of the underlying layer with the same structure and chemical composition after abrasion ensures the robust superhydrophobic performance. Therefore, we try to prepare a new kind of the inorganic paper with a layered structure which is free-standing, highly flexible, superhydrophobic, and fire-resistant. However, in some previous reports, the building blocks were two-dimensional (2D) plateletlike materials such as graphene and its derivatives,39 molybdenum disulfide,40 and montmorillonite.1 Nevertheless, the smooth surfaces of 2D building blocks decrease the roughness, which is not favorable for enhancing superhydrophobic performance. In addition, considering that it is difficult for zero-dimensional (0D) particles to construct the stable paper, we choose one-dimensional (1D) inorganic hydroxyapatite ultralong nanowires, instead of 2D sheets, to fabricate a new kind of superhydrophobic layered inorganic paper (SLIP). Herein, we report a new kind of the free-standing, highly flexible, superhydrophobic, and fire-resistant inorganic paper with a layered structure made from self-roughening ultralong hydroxyapatite nanowires (HAPNWs) with a surface modification using sodium oleate. The as-prepared SLIP exhibits excellent superhydrophobic performance, good liquid repellency to various commercial drinks, self-cleaning ability, high thermal stability, and excellent resistance to mechanical destruction. More importantly, benefiting from the unique layered structure, SLIP exhibits robust superhydrophobic performance against physical damages. In addition, we have explored potential applications of SLIP, including selective adsorbent, fire-shielding protector, water-proof and fireresistance, and writing paper.

2.1. Chemicals. Oleic acid (C18H34O2, analytical reagent (AR)), calcium chloride (CaCl2, AR), sodium hydroxide (NaOH, AR), sodium dihydrogen phosphate (NaH2PO4·2H2O, AR), cyclohexane (C6H12, AR), and methylene blue trihydrate were purchased from Sinopharm Chemical Reagent Co., Ltd. Oleate sodium (C18H33NaO2, CP) and oil red O were obtained from Aladdin Industrial Corporation. Mineral water, red tea, orange juice, milk, coffee, common commercial paper, adhesive tape, double-sided adhesive tape, and glass slides were purchased commercially. All chemical reagents were used as received without further purification. 2.2. Preparation of Ultralong Hydroxyapatite Nanowires (HAPNWs). HAPNWs were prepared by the modified calcium oleate precursor solvothermal method previously reported by this research group.21,41−43 In brief, CaCl2 (0.220 g) aqueous solution (20 mL), NaOH (1.000 g) aqueous solution (20 mL), and NaH2PO4·2H2O (0.280 g) aqueous solution (10 mL) were separately added into a mixture of oleic acid (12.000 g) and ethanol (12.000 g) dropwise under agitation. The resulting mixture was then transferred to a 100 mL Telfon-lined stainless-steel autoclave, which was sealed and solvothermally treated at 180 °C for 24 h. 2.3. Preparation of the Fire-Resistant SLIP. The as-prepared HAPNWs were stirred in water and alcohol, respectively, at 60 °C overnight to remove the impurities, and well-dispersed HAPNWs were obtained and stored in deionized water. A certain volume of the HAPNWs colloid suspension was treated by vacuum filtration to remove water and form a paper sheet. Then, the paper sheet without drying was immersed in the 0.02 mol L−1 sodium oleate aqueous solution for 1 h under magnetic agitation, followed by vacuum-assisted suction filtration. SLIP was obtained after drying at 60 °C. 2.4. Resistance to Mechanical Destruction and Self-Cleaning Tests of SLIP. Finger wipe, tape peeling, sandpaper abrasion, and knife cutting were adopted to test the resistance to mechanical destruction of SLIP. A finger was wiped across the surface of the SLIP sample in the finger-wipe test. In the tape-peeling test, a strip of common adhesive tape was attached to SLIP surface, followed by peeling it off. The SLIP sample was abraded using the sandpaper across the surface manually in the sandpaper-abrasion test. A sharp knife was used in the knife-cutting test of SLIP. The soil was placed on the tilted (about 15°) SLIP surface in the self-cleaning test. The SLIP was attached to the glass substrate using double-sided adhesive tape in order to perform the self-cleaning tests (Movie S3) and finger-wipe and knife-cutting tests (Movie S5). 2.5. Characterization. Scanning electron microscopy (SEM) micrographs were obtained on a field-emission scanning electron microscope (Hitachi S-4800, Japan). Transmission electron microscopy (TEM) micrographs were collected on a transmission electron microscope (Hitachi H-800, Japan). An X-ray diffractometer (Rigaku D/max 2550 V) was used to record powder X-ray diffraction (XRD) patterns. Thermogravimetric analysis (TGA) curves were recorded in flowing air on a simultaneous thermal analyzer (STA 409PC, Netzsch, B


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Figure 1. (a, b) SEM micrographs of the cross section of the fire-resistant unmodified layered inorganic paper (LIP), showing a layered structure. (c, d) SEM micrographs of the surface morphology of the unmodified LIP. (e) Water droplet (dyed with methylene blue) on the unmodified LIP. (f, g) SEM micrographs of the cross section of LIP with surface modification (SLIP), showing a layered structure as well. (h, i) SEM micrographs of the surface morphology of SLIP. (j) Water droplet on SLIP. Germany) with a heating rate of 10 °C min−1. Fourier transform infrared (FTIR) spectra were recorded using a FTIR spectrometer (FTIR-7600, Lambda Scientific, Australia). The zeta potentials were measured on a zeta-potential analyzer (ZetaPlus, Brookhaven Instrument Corporation). A surface area and pore size analyzer (Vsorb 2800P, Gold APP instruments) was used to measure the Brunauer−Emmett−Teller (BET) specific surface area and Barrett− Joyner−Halenda (BJH) pore size distributions. The water contact angles were measured on an optical contact angle system (Model SL200A/B/D) at ambient temperature using a 3 μL deionized water droplet at least five different locations on each paper. The thickness of the paper was measured using a digital micrometer caliper.

3. RESULTS AND DISCUSSION 3.1. Preparation and Characterization of SLIP. In the fabrication procedure of SLIP (Scheme 1), ultralong hydroxyapatite nanowires (HAPNWs) with diameters of about 20 nm (Figure S1) were synthesized using a reaction system containing CaCl2, NaOH, oleic acid, NaH2PO4·2H2O, ethanol, and deionized water by the calcium oleate precursor solvothermal method at 180 °C for 24 h. The well-dispersed HAPNWs were obtained as a stable wool-like suspension (Figure S2). After the vacuum-assisted suction filtration, HAPNWs self-assembled into a relatively thick (about 300 μm) free-standing fire-resistant layered inorganic paper (LIP) (Figure 1a,b). The layered structure can be clearly observed in Figure 1b. LIP shows high flexibility and can be rolled up or folded easily (Figure S3). Additionally, the thickness of LIP can be controlled readily by varying the concentration of HAPNWs (Figure 2 and Table S1). The light transparency of LIP may be induced by the well-aligned orientation of the layered structure.4 SEM micrographs (Figure 1c,d) show that the HAPNWs in LIP intertwined with one another to form the three-dimensional porous network with a BJH average pore size of about 20.9 nm and a specific surface area of 15.9 m2 g−1 (Figure S4). High-magnification SEM micrographs (Figure S5) show that HAPNWs in LIP self-assemble in parallel into bundles with various width scales from the nanoscale to the submicroscale. As previously reported, in order to construct nanowires-based superhydrophobic materials with micro- and nanoscale hierarchical roughness structure, introducing the particles and processing nanowires with a “beads-on-string” structure are two methods to provide a second scale of roughness in addition to the inherent scale of roughness of nanowires.44,45 However, both methods will increase the preparation cost, time, and difficulty. In this work, HAPNWs self-assemble into nanowire bundles to form microfibers

Figure 2. (a) Digital images of the as-prepared LIP with different thickness and transparency. (b) Linear relationship between thickness and weight of LIP.

(Figure S5). Such a hierarchical structure of the microfibers, which is analogous to the setae of a water strider’s legs,46 has been proved to be able to increase the roughness and thus improve the superhydrophobic performance.47 Therefore, selfassembled and self-roughening HAPNWs in this work can avoid the tedious fabrication process and succeed in constructing the hierarchical surface for achieving an excellent superhydrophobic performance. Without surface modification of HAPNWs, the as-obtained LIP shows intrinsic hydrophilicity. A water droplet spreads out immediately when it is on the surface of LIP (Figure 1e), showing superhydrophilicity (water contact angle ≈ 0°, Figure 3a). In the process of the suction filtration, a well-aligned layered structure is spontaneously formed (Figure 1a,b). During the filtration, HAPNWs are inclined to adhere together by the van der Waals force and hydrogen bonding between oxygen (hydrogen)-containing groups.48 At the same time, the electrostatic repulsion forces induced by negatively charged HAPNWs (Figure 3b) prevent them from completely compacting. Furthermore, the interaction among water molecules adsorbed on the oxygen-containing groups of C


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Figure 4. XRD patterns of LIP modified with 0.02 mol L−1 sodium oleate aqueous solution for different times: (a) 0 h, (b) 1 h, (c) 2 h, and (d) 3 h.

Figure 3. (a) Water contact angles of LIP modified with 0.02 mol L−1 sodium oleate for different times: 0 h (water contact angle ≈ 0°), 1 h (water contact angle = 154.40 ± 1.05°), 2 h (water contact angle = 154.55 ± 0.62°), and 3 h (water contact angle = 154.41 ± 0.86°). (b) Zeta potentials of the HAPNWs in water and in 0.02 mol L−1 sodium oleate aqueous solution. Figure 5. FTIR spectra: (a) sodium oleate; (b−e) LIP modified with 0.02 mol L−1 sodium oleate aqueous solution for different times: (b) 0 h, (c) 1 h, (d) 2 h, and (e) 3 h.

HAPNWs results in the formation of repulsive hydration forces. Therefore, the formation of nanowires-based layered structure results from the balance between attractive forces (van der Waals and hydrogen bonding) and repulsive forces (electrostatic repulsive and hydration forces).49,50 Additionally, microsized corrugated layers assembled by HAPNWs may be helpful to form such layered structure as well. SLIP is prepared using HAPNWs which are surface-modified in the sodium oleate solution. Compared with expensive, easily bioaccumulating perfluorinated compounds,51,52 sodium oleate is low-cost with good biocompatibility;53 thus, it has been selected as the grafting agent to lower the surface energy of HAPNWs. After the modification of HAPNWs with sodium oleate, a water droplet is nearly spherical on SLIP surface (Figure 1j), indicating the superhydrophobicity of SLIP (Figure 3a). The surface modification of HAPNWs for 1 h is enough to achieve the superhydrophobicity of SLIP (contact angle = 154.40 ± 1.05°), while little change is observed after extending the modification time from 1 to 3 h (Figure 3a). When the HAPNWs are immersed in the sodium oleate aqueous solution, the locations of hydroxyl groups on the HAP crystal surface (Figure S6), each connecting with two Ca2+ ions, were void at least for an instant.54 Therefore, the two positively charged Ca2+ ions constitute an adsorbing site where the negatively charged oleate group can be adsorbed, with hydrophobic longchain aliphatic groups outward to prevent water from permeating into the pores and thus sticking to the surface. As expected, there is no obvious change of the crystallinity of HAPNWs after surface modification (Figure 4). The results are further supported by FTIR (Figure 5) and TGA (Figure S7). FTIR spectra of LIP before and after the surface modification show similar characteristic peaks of the hydroxyl group (3565 and 633 cm−1),55−57 PO43− (1093, 1028, 962, 604, and 561 cm−1),57,58 and adsorbed water (3442 and 1635 cm−1).57 It is

worth noting that peaks at 2921 and 2852 cm−1, attributed to the asymmetric and symmetric C−H stretching vibration of the alkyl group of oleate group,59 were obvious in the spectra of LIP after the surface modification compared to that of LIP before the surface modification. The SEM micrographs in Figure 1f−i show that the surfacemodified LIP (SLIP) presents similar surface and crosssectional morphologies to those of unmodified LIP because HAPNWs are negatively charged as well in the sodium oleate solution (Figure 3b). The rough hierarchical structure and lowsurface-energy compound endow SLIP with a superhydrophobic performance. A jet of water from a syringe needle applied on the as-prepared SLIP bounces off the paper surface without leaving a trace (Movie S1). No blue trace is left on SLIP after SLIP is dipped and then withdrawn from water dyed with methylene blue (Figure 6a and Movie S1), exhibiting excellent superhydrophobic property of SLIP. Since paper is used in daily work and life, the biocompatibility of paper is an important issue. Although some inorganic fibers such as asbestos fibers were used to make a special kind of paper,60 the exposure of asbestos to the human being causes a major concern due to their toxicity.61,62 Hydroxyapatite is a promising biomaterial with excellent biocompatibility and has wide biomedical applications.63−65 Hydroxyapatite nanowires are very different from other inorganic fibers such as asbestos fibers which are toxic. It was reported that hydroxyapatite nanowires were biocompatible and a favorable substrate for cell adhesion, spreading, and proliferation.22,66 In consideration of reducing the potential toxicity of inorganics and avoiding pollution to the environD


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good liquid repellency performance to common commercial drinks. 3.3. Self-Cleaning Property of SLIP. The self-cleaning effect, one of the most attractive properties of superhydrophobic surfaces, is an effective tool to prevent the surfaces from the contamination with dust, soil, and other agents. As shown in Figure 7a, SLIP is attached to a tilted glass

Figure 6. Liquid repellency tests for SLIP. (a) A piece of SLIP is immersed in deionized water (dyed with methylene blue) and extracted from the water (inset). (b−f) Five common commercial drinks including (b) mineral water, (c) red tea, (d) orange juice, (e) milk, and (f) coffee are poured on SLIP.

ment, ultralong hydroxyapatite nanowires (HAPNWs) are the ideal building material for making biocompatible, highly flexible, and fire-resistant paper.21,22 In addition, the reported results indicated high cell viability after incubation with sodium oleate-coated Fe3O4 nanoparticles, exhibiting good biocompatibility of sodium oleate.53 Therefore, the as-prepared inorganic paper consisting of hydroxyapatite nanowires adsorbed with a small amount of sodium oleate has good biocompatibility. Caution: Long inorganic f ibers may be toxic upon inhalation. 3.2. Liquid Repellency of SLIP. We further tested the performance of SLIP including liquid repellency to commercial drinks, self-cleaning ability in air and in oil, thermal stability, and resistance to mechanical destruction. First, several common commercial drinks were employed to test the water-proof property of SLIP. As shown in Figure 6b−f and Movie S2, we poured five common commercial drinks onto the SLIP surface. In all the experiments, the liquids rolled off the horizontal SLIP surface easily, not only transparent liquids (mineral water and red tea) but also suspensions and emulsions (orange juice, milk, and coffee). There exist abundant nanopores on SLIP surface (Figure 1h,i), which can trap a lot of air. More trapped air will lower the area fraction of solid−liquid interface, which increases the contact angle. Hence, SLIP with abundant nanopores has

Figure 7. Self-cleaning tests for SLIP. (a, b) Schematic diagram of the self-cleaning tests in different media: (a) in air and (b) in oil. (c) The dirt is placed on SLIP surface before self-cleaning test. (d) The dirt is removed by water droplets from a syringe. (e) The dirt is washed away by a water flow. (f) Clean SLIP after the self-cleaning test.

slide at an angle of about 15°. The sizes of the nanopores in SLIP are small enough to prevent the dirt (soil) from penetrating into the interior space. The tiny contact area minimizes the adhesion between the dirt and SLIP surface. The contact area between the paper surface and water is reduced drastically owing to the superhydrophobicity of SLIP surface. Both effects lead to enhanced adhesion between water droplets and dirt, so that water droplets and water flow can carry and wash away dirt completely on SLIP surface, exhibiting the excellent self-cleaning performance (Figure 7c−f and Movie S3).23 Another contamination source in daily life is oil substances.24,36 The problem is that the materials usually lose their superhydrophobicity when they are contaminated with oil. We investigated the water repellency performance of SLIP E


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as-prepared SLIP shows excellent thermal stability of superhydrophobicity. 3.5. Resistance to Mechanical Destruction of SLIP. The mechanical durability of the superhydrophobic performance relates directly to the practical applications. Common physical damages in daily life may destroy paper surface severely (Figure 9) and compromise the superhydrophobic performance of the

surface in oil using cyclohexane. As shown in Figure 7b and Movie S4, water droplets can keep a nearly spherical shape when they are traveling across the oil-contaminated SLIP surface, but spread out immediately when contacting with the hydrophilic culture dish surface. The porous microstructure of SLIP surface is able to trap the oil as a lubricating liquid, forming a slippery state.24 Therefore, SLIP has the ability to maintain its water repellency in oil. More importantly, the water repellency property can be well preserved in oil even in the case of the mechanical destruction of SLIP by knife cutting (Movie S5). On account of the similar microstructure for each layer of SLIP and the three-dimensional porous network, SLIP can trap oil as a lubricating liquid (Figure 7b), leading to the good mechanical durability of the as-prepared SLIP. 3.4. Thermally Stable Superhydrophobicity of SLIP. Another challenge in the practical application is the thermal stability of superhydrophobicity.67 In this work, the thermal stability of superhydrophobicity of the as-prepared SLIP was tested under the high-temperature environment. The SLIP can keep its superhydrophobic performance at 100 °C for as long as 24 h (Figure 8a) or at a higher temperature, i.e., 200 °C, for 1 h

Figure 9. Schematic illustration of the superhydrophobic surfaces with (a) a hierarchical rough structure, and (b) a layered structure before and after abrasion.

paper. A unique characteristic of the as-prepared SLIP is the ability of resistance to physical damages. A series of mechanical destruction tests similar to the common physical damages in daily life, including finger wipe, tape peeling, knife cutting, and sandpaper abrasion, were conducted to determine mechanical durability of SLIP (Figure 10). As shown in Figure 10c1,d1, the damaged regions on SLIP surface created by the sandpaper abrasion and the knife cutting can be clearly observed. The lowmagnification SEM images (Figure 10a2,b2,c2,d2) show the damage traces by the four mechanical destruction methods. Despite severe destruction of the SLIP surface and exposure of underlying layers of SLIP, the superhydrophobic performance of SLIP can be well preserved. Figure 10a1,b1,c1,d1 shows the water droplets and corresponding contact angle measurements on SLIP after the mechanical destruction. The nearly spherical water droplets on the damaged SLIP surfaces and contact angles (all above 153°) indicate the highly stable superhydrophobicity of SLIP. This is attributed to the layered hierarchical structure and chemical properties of SLIP (Figure 9b). The results are further supported by low- and highmagnification SEM images (Figure 10), which show the same three-dimensional porous network, layered hierarchical structure, and surface roughness of the exposed underlying layers as that of the intact SLIP surface (Figure 1h,i). As shown by the representative Movie S5, the consecutive finger-wipe and knifecutting tests were operated on whole regions instead of the contact angle measurement areas. It shows that the water droplets produced by a syringe needle roll off easily on the tilted SLIP surface (about 15°) even after the mechanical destruction. Thus, benefiting from the layered structure, SLIP can withstand various common physical damages and preserve well its superhydrophobicity. 3.6. SLIP as Highly Effective Adsorbent for Oil/Water Separation. Taking the advantages discussed above, the asprepared SLIP will enable a wide range of applications. For example, SLIP also shows a superoleophilic performance (Figure 11a), which makes it promising as the highly efficient adsorbent for removal of oil from water. As shown in Figure 11b, cyclohexane is first dropped onto the water surface in a culture dish, and a piece of SLIP is then brought into contact with the cyclohexane (Figure 11c). The cyclohexane is

Figure 8. Water contact angles and the corresponding images of water droplets on SLIP thermally treated under different conditions: (a) at 100 °C for different times: 1 h (water contact angle = 153.65 ± 1.34°), 2 h (water contact angle = 153.99 ± 0.62°), 3 h (water contact angle = 154.00 ± 0.43°), 5 h (water contact angle = 153.42 ± 1.06°), 8 h (water contact angle =153.15 ± 0.63°), 12 h (water contact angle = 154.58 ± 0.96°), and 24 h (water contact angle = 153.38 ± 0.62°); (b) for 1 h at different temperatures: 100 °C (water contact angle = 153.65 ± 1.34°), 150 °C (water contact angle = 153.00 ± 0.55°), 200 °C (water contact angle = 153.33 ± 0.60°), 250 °C (water contact angle = 86.22 ± 0.99°), and 300 °C (water contact angle ≈ 0°).

(Figure 8b). The TGA curves of SLIP show that the oleate group starts to decompose at about 200 °C (Figure S7), and the superhydrophobic performance of SLIP can be maintained below this temperature. At 250 °C, SLIP becomes hydrophilic after heating for 1 h. The conversion of superhydrophobicity to superhydrophilicity occurs after heating at 300 °C for 1 h. Considering the actual working environment in daily life, the F


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Figure 10. Mechanical durability tests. Digital images of water droplets on SLIP after mechanical destruction: (a1) finger wipe, (b1) tape peeling, (c1) sandpaper abrasion, and (d1) knife cutting. The scale bars are 2 mm. Insets: water droplets (3 μL) on the damaged SLIP surfaces. Contact angles = 153.83 ± 0.55, 153.88 ± 0.58, 154.05 ± 0.15, and 154.20 ± 1.13°, respectively. (a2, b2, c2, d2) Low-magnification and (a3, b3, c3, d3) highmagnification SEM micrographs of SLIP surfaces after corresponding mechanical destruction.

capillary action, and excellent superhydrophobic performance of SLIP. Thus, the superoleophilic SLIP can be used for the treatment of water pollution resulting from oil spillage and industrial discharge of organic pollutants. 3.7. SLIP as Fire-Shielding Protector. It is well-known that inorganic HAP materials possess excellent fire-resistant property owing to their intrinsic nonflammability and high thermal stability. Hence, SLIP can be used as the fire-shielding layer to protect flammable objects. In a control group, the common commercial paper (CCP) made of flammable plant cellulose fibers is burnt to ashes in five seconds without the protection of SLIP (Figure 12a). However, no obvious change is observed for CCP with SLIP as the protective layer after heating for 5 min (Figure 12b). Consequently, SLIP can be used for the protection of flammable objects and substances from fire destruction, such as important books, paper documents, archives, and biological materials. 3.8. SLIP as Writing and Printing Paper. Another unique feature of the as-prepared SLIP is that it possesses excellent performance in writing and printing without compromising its superhydrophobic and fire-resistant properties. Figure 13 and Movie S7 show the water-proof and fire-resistant tests on the CCP and SLIP with written characters. As shown in Figure 13b, a jet of water applied on the CCP spreads out on the paper surface, whereas it bounces off SLIP surface easily (Figure 13c). No residue is observed on SLIP surface, and the appearance of SLIP has no obvious change (Figure 13d). However, as a common phenomenon in daily life, CCP has a blue stain on the surface and becomes wrinkled after the adsorption of water dyed with methylene blue (Figure 13d). Therefore, the writing has no obvious effect on the superhydrophobic performance of SLIP. Moreover, as discussed above, the CCP is burnt to ashes quickly once it is exposed to flame (Figure 13f), whereas SLIP

Figure 11. Oil/water separation. (a) Cyclohexane droplet (dyed with oil red) on SLIP surface. (b) Cyclohexane floating on the water surface. (c) Cyclohexane is completely adsorbed by SLIP. (d) Cyclohexane is completely and rapidly removed from the water surface, and oil/water separation is achieved.

completely adsorbed by SLIP quickly, and there is no residual cyclohexane in water after the oil/water separation (Figure 11d and Movie S6). The rapid and highly efficient oil/water separation can be attributed to the abundant nanopores, good G


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Figure 12. Fire-resistance tests. Combustion processes and change of the common commercial paper (a) without and (b) with SLIP as the protective layer.

Figure 13. (a, e) Common commercial paper (CCP) and SLIP before water-proof and fire-resistant tests. (b, c) Jet of water (dyed with methylene blue) is applied on the CCP and SLIP. (d, h) CCP and SLIP after water-proof and fire-resistant tests. (f, g) CCP and SLIP are exposed to the fire. The scale bars are 2 cm.

can survive even after exposure to the flame for a prolonged time (Figure 13g and Movie S7). The characters on SLIP are clearly visible after the fire-resistant test (Figure 13h).

4. CONCLUSIONS Inspired by the water strider’s setae and nacre, a new kind of the free-standing, highly flexible fire-resistant superhydrophobic layered inorganic paper (SLIP), made from nonflammable selfroughening ultralong hydroxyapatite nanowires after surface modification with sodium oleate, has been fabricated by simple vacuum-assisted suction filtration. The layered structure of SLIP provides high resistance to various mechanical destructions of the superhydrophobicity, leading to excellent mechanical durability against physical damages. Additionally, combining fire-resistance, superhydrophobicity, excellent liquid repellency, self-cleaning property in air and in oil, and high thermal stability, the as-prepared SLIP is promising for applications as selective adsorbent toward oil/water separation, fire-shielding protector, and writing/printing paper with simultaneous superhydrophobicity and fire-resistance.

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nitrogen adsorption−desorption isotherm, BJH-desorption pore size distribution curve, and SEM images of the as-prepared samples (PDF) Movie S1: superhydrophobicity of the as-prepared SLIP (AVI) Movie S2: liquid repellency tests including mineral water, red tea, orange juice, milk and coffee (AVI) Movie S3: the self-cleaning tests (AVI) Movie S4: the superhydrophobicity of the as-prepared SLIP in oil (AVI) Movie S5 shows the mechanical destruction tests on the as-prepared SLIP (AVI) Movie S6 shows the oil/water separation test (AVI) Movie S7 shows the water-proof and fire-resistant tests on the as-prepared SLIP with written characters (AVI)

AUTHOR INFORMATION

Corresponding Authors

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

ASSOCIATED CONTENT

S Supporting Information *

ORCID

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b12838. Digital images of the hydroxyapatite nanowire suspension and the as-prepared paper, TEM image, TG curves,

Ying-Jie Zhu: 0000-0002-8428-4586 Notes

The authors declare no competing financial interest. H


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(18) Corrêa, A. C.; de Santi, C. R.; Manrich, S. Synthetic Paper from Plastic Waste: The Effect of CaCO3 on Physical, Surface Properties and Printability. Macromol. Symp. 2006, 245−246 (1), 611−620. (19) Yuan, J.; Laubernds, K.; Villegas, J.; Gomez, S.; Suib, S. L. Spontaneous Formation of Inorganic Paper-Like Materials. Adv. Mater. 2004, 16 (19), 1729−1732. (20) Tian, Y.; Cao, Y.; Wang, Y.; Yang, W.; Feng, J. Realizing Ultrahigh Modulus and High Strength of Macroscopic Graphene Oxide Papers through Crosslinking of Mussel-Inspired Polymers. Adv. Mater. 2013, 25 (21), 2980−2983. (21) Lu, B. Q.; Zhu, Y. J.; Chen, F. Highly Flexible and Nonflammable Inorganic Hydroxyapatite Paper. Chem. - Eur. J. 2014, 20 (5), 1242−1246. (22) Xiong, Z.-C.; Zhu, Y.-J.; Chen, F.-F.; Sun, T.-W.; Shen, Y.-Q. One-Step Synthesis of Silver Nanoparticle-Decorated Hydroxyapatite Nanowires for the Construction of Highly Flexible Free-Standing Paper with High Antibacterial Activity. Chem. - Eur. J. 2016, 22 (32), 11224−11231. (23) Li, X.-M.; Reinhoudt, D.; Crego-Calama, M. What Do We Need for a Superhydrophobic Surface? A Review on the Recent Progress in the Preparation of Superhydrophobic Surfaces. Chem. Soc. Rev. 2007, 36 (8), 1350−1368. (24) Lu, Y.; Sathasivam, S.; Song, J.; Crick, C. R.; Carmalt, C. J.; Parkin, I. P. Robust Self-Cleaning Surfaces That Function When Exposed to Either Air or Oil. Science 2015, 347 (6226), 1132−1135. (25) Sun, Z.; Liao, T.; Liu, K.; Jiang, L.; Kim, J. H.; Dou, S. X. FlyEye Inspired Superhydrophobic Anti-Fogging Inorganic Nanostructures. Small 2014, 10 (15), 3001−3006. (26) Guo, P.; Zheng, Y.; Wen, M.; Song, C.; Lin, Y.; Jiang, L. Icephobic/Anti-Icing Properties of Micro/Nanostructured Surfaces. Adv. Mater. 2012, 24 (19), 2642−2648. (27) Bai, H.; Wang, L.; Ju, J.; Sun, R.; Zheng, Y.; Jiang, L. Efficient Water Collection on Integrative Bioinspired Surfaces with Star-Shaped Wettability Patterns. Adv. Mater. 2014, 26 (29), 5025−5030. (28) Liu, Y.; Wang, X.; Fei, B.; Hu, H.; Lai, C.; Xin, J. H. Bioinspired, Stimuli-Responsive, Multifunctional Superhydrophobic Surface with Directional Wetting, Adhesion, and Transport of Water. Adv. Funct. Mater. 2015, 25 (31), 5047−5056. (29) Cao, C.; Ge, M.; Huang, J.; Li, S.; Deng, S.; Zhang, S.; Chen, Z.; Zhang, K.; Al-Deyab, S. S.; Lai, Y. Robust Fluorine-Free Superhydrophobic PDMS-Ormosil@Fabrics for Highly Effective SelfCleaning and Efficient Oil-Water Separation. J. Mater. Chem. A 2016, 4 (31), 12179−12187. (30) Leslie, D. C.; Waterhouse, A.; Berthet, J. B.; Valentin, T. M.; Watters, A. L.; Jain, A.; Kim, P.; Hatton, B. D.; Nedder, A.; Donovan, K.; Super, E. H.; Howell, C.; Johnson, C. P.; Vu, T. L.; Bolgen, D. E.; Rifai, S.; Hansen, A. R.; Aizenberg, M.; Super, M.; Aizenberg, J.; Ingber, D. E. A Bioinspired Omniphobic Surface Coating on Medical Devices Prevents Thrombosis and Biofouling. Nat. Biotechnol. 2014, 32 (11), 1134−1140. (31) Blossey, R. Self-Cleaning Surfaces - Virtual Realities. Nat. Mater. 2003, 2 (5), 301−306. (32) Feng, L.; Li, S. H.; Li, Y. S.; Li, H. J.; Zhang, L. J.; Zhai, J.; Song, Y. L.; Liu, B. Q.; Jiang, L.; Zhu, D. B. Super-Hydrophobic Surfaces: From Natural to Artificial. Adv. Mater. 2002, 14 (24), 1857−1860. (33) Barthlott, W.; Neinhuis, C. Purity of the Sacred Lotus, or Escape from Contamination in Biological Surfaces. Planta 1997, 202 (1), 1−8. (34) Liu, Y.; Moevius, L.; Xu, X.; Qian, T.; Yeomans, J. M.; Wang, Z. Pancake Bouncing on Superhydrophobic Surfaces. Nat. Phys. 2014, 10 (7), 515−519. (35) Wu, M.; Ma, B.; Pan, T.; Chen, S.; Sun, J. Silver-NanoparticleColored Cotton Fabrics with Tunable Colors and Durable Antibacterial and Self-Healing Superhydrophobic Properties. Adv. Funct. Mater. 2016, 26 (4), 569−576. (36) Verho, T.; Bower, C.; Andrew, P.; Franssila, S.; Ikkala, O.; Ras, R. H. A. Mechanically Durable Superhydrophobic Surfaces. Adv. Mater. 2011, 23 (5), 673−678. (37) Manna, U.; Lynn, D. M. Restoration of Superhydrophobicity in Crushed Polymer Films by Treatment with Water: Self-Healing and

ACKNOWLEDGMENTS We thank for the financial support from the Science and Technology Commision of Shanghai (15JC1491001), the National Natural Science Foundation of China (21601199) and the Shanghai Sailing Program (16YF1413000).

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REFERENCES

(1) Ming, P.; Song, Z.; Gong, S.; Zhang, Y.; Duan, J.; Zhang, Q.; Jiang, L.; Cheng, Q. Nacre-Inspired Integrated Nanocomposites with Fire Retardant Properties by Graphene Oxide and Montmorillonite. J. Mater. Chem. A 2015, 3 (42), 21194−21200. (2) Yao, H.-B.; Tan, Z.-H.; Fang, H.-Y.; Yu, S.-H. Artificial NacreLike Bionanocomposite Films from the Self-Assembly of ChitosanMontmorillonite Hybrid Building Blocks. Angew. Chem., Int. Ed. 2010, 49 (52), 10127−10131. (3) Walther, A.; Bjurhager, I.; Malho, J.-M.; Ruokolainen, J.; Berglund, L.; Ikkala, O. Supramolecular Control of Stiffness and Strength in Lightweight High-Performance Nacre-Mimetic Paper with Fire-Shielding Properties. Angew. Chem., Int. Ed. 2010, 49 (36), 6448− 6453. (4) Walther, A.; Bjurhager, I.; Malho, J.-M.; Pere, J.; Ruokolainen, J.; Berglund, L. A.; Ikkala, O. Large-Area, Lightweight and Thick Biomimetic Composites with Superior Material Properties Via Fast, Economic, and Green Pathways. Nano Lett. 2010, 10 (8), 2742−2748. (5) Nair, R. R.; Wu, H. A.; Jayaram, P. N.; Grigorieva, I. V.; Geim, A. K. Unimpeded Permeation of Water through Helium-Leak-Tight Graphene-Based Membranes. Science 2012, 335 (6067), 442−444. (6) Yuan, J.; Liu, X.; Akbulut, O.; Hu, J.; Suib, S. L.; Kong, J.; Stellacci, F. Superwetting Nanowire Membranes for Selective Absorption. Nat. Nanotechnol. 2008, 3 (6), 332−336. (7) Zhang, W.; Zhu, Y.; Liu, X.; Wang, D.; Li, J.; Jiang, L.; Jin, J. SaltInduced Fabrication of Superhydrophilic and Underwater Superoleophobic PAA-g-PVDF Membranes for Effective Separation of Oilin-Water Emulsions. Angew. Chem., Int. Ed. 2014, 53 (3), 856−860. (8) Chen, C.; Yang, Q.-H.; Yang, Y.; Lv, W.; Wen, Y.; Hou, P.-X.; Wang, M.; Cheng, H.-M. Self-Assembled Free-Standing Graphite Oxide Membrane. Adv. Mater. 2009, 21 (29), 3007−3011. (9) Song, W.-L.; Guan, X.-T.; Fan, L.-Z.; Cao, W.-Q.; Wang, C.-Y.; Zhao, Q.-L.; Cao, M.-S. Magnetic and Conductive Graphene Papers toward Thin Layers of Effective Electromagnetic Shielding. J. Mater. Chem. A 2015, 3 (5), 2097−2107. (10) Chen, H.; Kang, Y.; Cai, F.; Zeng, S.; Li, W.; Chen, M.; Li, Q. Electrochemical Conversion of Ni2(OH)2CO3 into Ni(OH)2 Hierarchical Nanostructures Loaded on a Carbon Nanotube Paper with High Electrochemical Energy Storage Performance. J. Mater. Chem. A 2015, 3 (5), 1875−1878. (11) Shu, K.; Wang, C.; Li, S.; Zhao, C.; Yang, Y.; Liu, H.; Wallace, G. Flexible Free-Standing Graphene Paper with Interconnected Porous Structure for Energy Storage. J. Mater. Chem. A 2015, 3 (8), 4428−4434. (12) Li, L.; Breedveld, V.; Hess, D. W. Design and Fabrication of Superamphiphobic Paper Surfaces. ACS Appl. Mater. Interfaces 2013, 5 (11), 5381−5386. (13) Wang, S.; Li, M.; Lu, Q. Filter Paper with Selective Absorption and Separation of Liquids That Differ in Surface Tension. ACS Appl. Mater. Interfaces 2010, 2 (3), 677−683. (14) Hermy, M.; Honnay, O.; Firbank, L.; Grashof-Bokdam, C.; Lawesson, J. E. An Ecological Comparison between Ancient and Other Forest Plant Species of Europe, and the Implications for Forest Conservation. Biol. Cons. 1999, 91 (1), 9−22. (15) Wang, W.; Xie, N.; He, L.; Yin, Y. Photocatalytic Colour Switching of Redox Dyes for Ink-Free Light-Printable Rewritable Paper. Nat. Commun. 2014, 5, 5459. (16) Sheng, L.; Li, M.; Zhu, S.; Li, H.; Xi, G.; Li, Y.-G.; Wang, Y.; Li, Q.; Liang, S.; Zhong, K.; Zhang, S. X.-A. Hydrochromic Molecular Switches for Water-Jet Rewritable Paper. Nat. Commun. 2014, 5, 3044. (17) Sun, H.; Gao, N.; Ren, J.; Qu, X. Polyoxometalate-Based Rewritable Paper. Chem. Mater. 2015, 27 (22), 7573−7576. I


Research Article


and Promising Carrier for Gene Transfection. J. Colloid Interface Sci. 2010, 345 (2), 427−432. (57) Jevtic, M.; Mitric, M.; Skapin, S.; Jancar, B.; Ignjatovic, N.; Uskokovic, D. Crystal Structure of Hydroxyapatite Nanorods Synthesized by Sonochemical Homogeneous Precipitation. Cryst. Growth Des. 2008, 8 (7), 2217−2222. (58) Ng, S.; Guo, J.; Ma, J.; Loo, S. C. J. Synthesis of High Surface Area Mesostructured Calcium Phosphate Particles. Acta Biomater. 2010, 6 (9), 3772−3781. (59) Liu, X.; Ge, L.; Li, W.; Wang, X.; Li, F. Layered Double Hydroxide Functionalized Textile for Effective Oil/Water Separation and Selective Oil Adsorption. ACS Appl. Mater. Interfaces 2015, 7 (1), 791−800. (60) Shen, D.; Yang, S.; Luo, J.; Zhao, Y.; Yu, F. Study on MgO Modified Asbestos Paper Separator for Thermal Batteries. J. Funct. Mater. 2015, 46 (22), 22054−22057. (61) Light, W. G.; Wei, E. T. Surface Charge and Asbestos Toxicity. Nature 1977, 265 (5594), 537−539. (62) Boulanger, G.; Andujar, P.; Pairon, J.-C.; Billon-Galland, M.-A.; Dion, C.; Dumortier, P.; Brochard, P.; Sobaszek, A.; Bartsch, P.; Paris, C.; Jaurand, M.-C. Quantification of Short and Long Asbestos Fibers to Assess Asbestos Exposure: A Review of Fiber Size Toxicity. Environ. Health 2014, 13 (1), 59. (63) Chen, F.; Huang, P.; Zhu, Y.-J.; Wu, J.; Zhang, C.-L.; Cui, D.-X. The Photoluminescence, Drug Delivery and Imaging Properties of Multifunctional Eu3+/Gd3+ Dual-Doped Hydroxyapatite Nanorods. Biomaterials 2011, 32 (34), 9031−9039. (64) Qi, C.; Zhu, Y.-J.; Lu, B.-Q.; Zhao, X.-Y.; Zhao, J.; Chen, F. Hydroxyapatite Nanosheet-Assembled Porous Hollow Microspheres: DNA-Templated Hydrothermal Synthesis, Drug Delivery and Protein Adsorption. J. Mater. Chem. 2012, 22 (42), 22642−22650. (65) Chen, F.; Zhu, Y.-J. Multifunctional Calcium Phosphate Nanostructured Materials and Biomedical Applications. Curr. Nanosci. 2014, 10 (4), 465−485. (66) Zhang, Y.-G.; Zhu, Y.-J.; Chen, F.; Sun, T.-W.; Jiang, Y.-Y. Highly Porous Ceramics Based on Ultralong Hydroxyapatite Nanowires. RSC Adv. 2016, 6 (104), 102003−102009. (67) Lim, H. S.; Baek, J. H.; Park, K.; Shin, H. S.; Kim, J.; Cho, J. H. Multifunctional Hybrid Fabrics with Thermally Stable Superhydrophobicity. Adv. Mater. 2010, 22 (19), 2138−2141.

Recovery of Damaged Topographic Features Aided by an Unlikely Source. Adv. Mater. 2013, 25 (36), 5104−5108. (38) Bai, H.; Walsh, F.; Gludovatz, B.; Delattre, B.; Huang, C.; Chen, Y.; Tomsia, A. P.; Ritchie, R. O. Bioinspired Hydroxyapatite/ Poly(Methyl Methacrylate) Composite with a Nacre-Mimetic Architecture by a Bidirectional Freezing Method. Adv. Mater. 2016, 28 (1), 50−56. (39) Liu, G.; Jin, W.; Xu, N. Graphene-Based Membranes. Chem. Soc. Rev. 2015, 44 (15), 5016−5030. (40) Wan, S.; Li, Y.; Peng, J.; Hu, H.; Cheng, Q.; Jiang, L. Synergistic Toughening of Graphene Oxide-Molybdenum Disulfide-Thermoplastic Polyurethane Ternary Artificial Nacre. ACS Nano 2015, 9 (1), 708−714. (41) Chen, F.-F.; Zhu, Y.-J.; Xiong, Z.-C.; Sun, T.-W. Hydroxyapatite Nanowires@Metal-Organic Framework Core/Shell Nanofibers: Templated Synthesis, Peroxidase-Like Activity and Their Derived Flexible Recyclable Test Paper. Chem. - Eur. J. 2016, DOI: 10.1002/ chem.201604813. (42) Zhang, Y.-G.; Zhu, Y.-J.; Chen, F.; Wu, J. Ultralong Hydroxyapatite Nanowires Synthesized by Solvothermal Treatment Using a Series of Phosphate Sodium Salts. Mater. Lett. 2015, 144, 135−137. (43) Jiang, Y.-Y.; Zhu, Y.-J.; Chen, F.; Wu, J. Solvothermal Synthesis of Submillimeter Ultralong Hydroxyapatite Nanowires Using a Calcium Oleate Precursor in a Series of Monohydroxy Alcohols. Ceram. Int. 2015, 41 (4), 6098−6102. (44) Nuraje, N.; Khan, W. S.; Lei, Y.; Ceylan, M.; Asmatulu, R. Superhydrophobic Electrospun Nanofibers. J. Mater. Chem. A 2013, 1 (6), 1929−1946. (45) Ma, M.; Gupta, M.; Li, Z.; Zhai, L.; Gleason, K. K.; Cohen, R. E.; Rubner, M. F.; Rutledge, G. C. Decorated Electrospun Fibers Exhibiting Superhydrophobicity. Adv. Mater. 2007, 19 (2), 255−259. (46) Gao, X.; Jiang, L. Biophysics: Water-Repellent Legs of Water Striders. Nature 2004, 432 (7013), 36−36. (47) Xue, Y.; Wang, H.; Yu, D.; Feng, L.; Dai, L.; Wang, X.; Lin, T. Superhydrophobic Electrospun Poss-Pmma Copolymer Fibres with Highly Ordered Nanofibrillar and Surface Structures. Chem. Commun. 2009, No. 42, 6418−6420. (48) Liang, Y.; Hilal, N.; Langston, P.; Starov, V. Interaction Forces between Colloidal Particles in Liquid: Theory and Experiment. Adv. Colloid Interface Sci. 2007, 134−135, 151−166. (49) Yang, X.; Qiu, L.; Cheng, C.; Wu, Y.; Ma, Z.-F.; Li, D. Ordered Gelation of Chemically Converted Graphene for Next-Generation Electroconductive Hydrogel Films. Angew. Chem., Int. Ed. 2011, 50 (32), 7325−7328. (50) Yang, X.; Zhu, J.; Qiu, L.; Li, D. Bioinspired Effective Prevention of Restacking in Multilayered Graphene Films: Towards the Next Generation of High-Performance Supercapacitors. Adv. Mater. 2011, 23 (25), 2833−2838. (51) Rabnawaz, M.; Liu, G.; Hu, H. Fluorine-Free Anti-Smudge Polyurethane Coatings. Angew. Chem., Int. Ed. 2015, 54 (43), 12722− 12727. (52) Lindstrom, A. B.; Strynar, M. J.; Libelo, E. L. Polyfluorinated Compounds: Past, Present, and Future. Environ. Sci. Technol. 2011, 45 (19), 7954−7961. (53) Sun, J.; Zhou, S.; Hou, P.; Yang, Y.; Weng, J.; Li, X.; Li, M. Synthesis and Characterization of Biocompatible Fe3O4 Nanoparticles. J. Biomed. Mater. Res., Part A 2007, 80A (2), 333−341. (54) Kawasaki, T.; Niikura, M.; Kobayashi, Y. Fundamental-Study of Hydroxyapatite High-Performance Liquid-Chromatography. III. Direct Experimental Confirmation of the Existence of 2 Types of Adsorbing Surface on the Hydroxyapatite Crystal. J. Chromatogr. 1990, 515, 125− 148. (55) Ma, M.-G.; Zhu, Y.-J.; Chang, J. Monetite Formed in Mixed Solvents of Water and Ethylene Glycol and Its Transformation to Hydroxyapatite. J. Phys. Chem. B 2006, 110 (29), 14226−14230. (56) Wu, G.-J.; Zhou, L.-Z.; Wang, K.-W.; Chen, F.; Sun, Y.; Duan, Y.-R.; Zhu, Y.-J.; Gu, H.-C. Hydroxylapatite Nanorods: An Efficient J


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