Superhydrophobic Coatings Prepared by the in Situ Growth of Silicone

Research Article Cite This: ACS Appl. Mater. Interfaces 2019, 11, 22809−22816

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Superhydrophobic Coatings Prepared by the in Situ Growth of Silicone Nanofilaments on Alkali-Activated Geopolymers Surface Zi-han Liu, Xiao-qin Pang, Kai-tuo Wang, Xue-sen Lv, and Xue-min Cui* School of Chemistry and Chemical Engineering, Guangxi Key Laboratory of Petrochemical Resource Processing and Process Intensification Technology, Guangxi University, Nanning 530004, China

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S Supporting Information *

ABSTRACT: As a highly hydrophobic and good environmental durable material, silicone nanofilaments have shown great advantages in the construction of superhydrophobic coatings. However, the synthesis of these materials has always been limited to the application of trifunctional organosilane monomers under the action of acidic catalysts. For the first time, long-chain polymeric hydrogenated siloxane-poly(methyl-hydrosiloxane) (PMHS) was used to synthesize rapidly silicone nanofilaments in situ under alkaline conditions. A dense silicone nanofilament coating was obtained by PMHS + geopolymer layer on a smooth iron sheet, and achieved by one-step brushing of PMHS on the surface of a just-solidified alkali-activated metakaolin-based geopolymer coating at 120 °C for an hour of sealed curing. This composite coating was followed by a superhydrophobic composite coating with a contact angle of approximately 161° and a rolling angle of 2°. Consistent with this, laser scanning confocal microscopy and field-emission scanning electron microscopy images show the presence of micro- and nanoscale features that enable the entrapment of air when exposed to water and endow excellent superhydrophobic properties. Because geopolymer material has good adhesion ability with metal, ceramic, or other materials, the composite superhydrophobic coating is expected to be widely used. KEYWORDS: superhydrophobic coating, surface modification, silicone nanofilament, alkali-based geopolymer, in situ growth, mechanism, all-water-borne, large scale

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INTRODUCTION Superhydrophobic phenomenon, known as the “lotus leaf effect”, refer to a water contact angle (CA) (the advancing contact angle) greater than 150° for the contact angle hysteresis and the rolling angle (SA) of less than 5−10°.1 This phenomenon can be explained by the “heterogeneous wetting regime” described by the Cassie−Baxter model.2 Due to the high mobility of water droplets on these surfaces, the droplets above present a perfect spherical structure and even a very slight tilt of the superhydrophobic object is sufficient to cause the drop to roll off. There is a consensus that the interaction between the effect of the binary collaboration of micro-nanostructures and low surface energy is considered to be the cause of the stability of the superhydrophobic phenomena.3−7 Over the years, the preparation of superhydrophobic surfaces has been devoted to the construction of multiscale roughness and the modification of surfaces by low surface free energy materials.8−15 Although the preparation methods of superhydrophobic coatings are numerous, a simple brushing process is always more conducive to meet its extensive applications due to its high efficiency and economic advantages. Furthermore, the consequent gradual substitution of water-based coatings for the existing solvent-based coatings is inevitable, as the former is irreplaceable for safety, green, and health.16−18 However, due to the incompatibility between low © 2019 American Chemical Society

surface energy materials and water medium, it is always challenging to construct absolute water-based superhydrophobic coatings. As a green inorganic binder, geopolymer is made of metakaolin or other silicon−aluminum materials with silicon, aluminum, and oxygen as essential elements and undergoes dissolution, monomer reconstruction, and polycondensation at lower temperature under the action of alkali activator.19 Up to now, this concept includes all amorphous and quasicrystalline three-dimensional (3D) network cementitious materials prepared from the mineral clay, solid wastes, or synthetic silicon−aluminum powder, which are aggregated by silica− oxygen tetrahedrons and alumina−oxygen tetrahedrons and contain abundant porous structure.20−23 They can resist high temperatures, chemicals, and friction and have characteristics of low permeability, rapid hardening, high strength, and strong bonding strength with metal, concrete, bricks, wood, and aggregate interfaces.24−29 Geopolymer coatings on metal substrates as thermal barriers and as protective coatings for concrete have been reported many times.24,28,29 However, a geopolymer is inherently hydrophilic because of the hydroxyl Received: May 9, 2019 Accepted: June 4, 2019 Published: June 4, 2019 22809

DOI: 10.1021/acsami.9b07990 ACS Appl. Mater. Interfaces 2019, 11, 22809−22816

Research Article

ACS Applied Materials & Interfaces

composite coating was achieved on iron sheet. The morphology, composition, and wettability of the coating were characterized by Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS), fieldemission scanning electron microscopy (FESEM), energydispersive X-ray spectroscopy (EDS), transmission electron microscopy (TEM), and confocal laser scanning microscopy (CLSM) and wettability measurements (CA, SA). The growth mechanism of the SNFs on the surface of the geopolymer was analyzed in detail. This work not only extends the selection of raw materials, preparation methods, and application fields of silicone nanofilaments but also provides new insights into the formation mechanism of the filaments. It also provides a simple strategy for the study of superhydrophobic modification of alkaline materials, and opens up more possibilities for the efficient preparation of all-water-borne superhydrophobic coatings, which reveals a true potential for application.

groups on its surface. In recent years, reactive organosilicon with hydrolytic groups (commonly included −H, −Cl, and −OR) has been widely used in the modification of water-based inorganic materials for their low surface free energy, absence of fluorine, and good chemical stability.15,30,31 The grafting modification of bifunctional and trifunctional silanes to the substrate is also influenced by the reaction conditions, such as the alkyl structure and the water content, which shows a flexible micromorphology and wettability.32 From this, onedimensional soft matter, silicone nanofilaments (SNFs), with unusual wetting behavior is derived. These were first synthesized through chemical vapor deposition7 and liquid deposition by Gao and McCarthy33 using methyltrichlorosilane and vapor-phase plasma-induced polymerization34 using vinyltrichlorosilane by Rollings et al. Currently, these have been prepared on the surfaces of glass, ceramics, cotton fabric, silk, wood, nanofibrillated cellulose, etc.7,33,35−37 In addition to excellent hydrophobicity, they exhibit high transparency, environmental stability, good solvent resistance, and chemical resistance over a certain range.38−40 Up to now, the preparation technology of silicone nanofilaments is still based on vapor-phase and liquid-phase depositions mainly,34,41−45 and only a few changes or improvements on this basis have been found in the follow-up study.46,47 There is no exception that the relationship between the formation of a filamentous structure and the film water layer enriched on the surface of the substrate was emphasized in these publications. Proper control of the humid environment and reasonable dosage of silane had always occupied the core of the research on these materials.32,34,36,41,42,46 Recently, a droplet-assisted growth and shaping mechanism has been proposed to explain the in situ formation of nano- and microstructures composed of polysiloxane and other materials such as germanium oxide. It is believed that the nano- and microdroplets formed on the surface of the substrate are likely to be the starting point for the formation of the one-dimensional structure. The uniaxial extension of the filaments results from the spatial limitation of the water island.48,49 This paves the way for a new strategy for in situ chemical synthesis of nano-/microstructures. However, there are still some limitations in the synthesis of the existing silicone nanofilaments. For example, trifunctional silane has always been considered an indispensable precursor for the generation of a fibrous structure.36,39,50 Moreover, the successful synthesis of the reported SNFs is confined to acidic conditions,36,46,50,51 which cannot be used to modify metals due to rapid corrosion. Nevertheless, the method has great potential for reducing corrosion, fluid dynamic resistance, and fuel consumption in marine coatings. In view of their superhydrophobic and antireflective properties after annealing, such coatings can remain stable in nature for at least 1 year, and they also show outstanding chemical durability in organic solvents, weak acids, alkalis, and neutral aqueous solutions.38,40 Therefore, it is necessary to explore the synthesis of silicone nanofilaments in neutral and alkaline conditions. In this article, we proposed a novel method to synthesize silicone nanofilaments in situ on the surface of freshly cured alkali-activated metakaolin-based geopolymer coatings. As far as we know, this is the first report on the successful synthesis of silicone nanofilaments under strong alkaline conditions. At the same time, we chose polymeric hydrogen-containing silicone oil instead of trifunctional organosilicon small molecules as raw materials. The whole preparation did not involve the use of any volatile organic solvent. An all-water-borne superhydrophobic

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EXPERIMENTAL SECTION

Materials. Metakaolin was purchased from the Inner Mongolia Chaoyao Kaolin Co., Ltd. The chemical composition of metakaolin was SiO2, 49.13%; Al2O3, 49.15%; K2O, 0.22%; Fe2O3, 0.42%; TiO2, 0.73%; CaO, 0.15%; Na2O, 0.13%; MnO, 0.0025%; and MgO, 0.066%, as determined using X-ray fluorescence. The liquid water glass (sodium silicate solution Na2O(SiO2)x·xH2O) was obtained from the Nanning Chunxu Chemical Co., Ltd., and its mass composition is Na2O = 9.2%, SiO2 = 29.5%, and water = 62.37%. The above two materials are industrial grade. The quartz sand with an average particle size of 75 μm was provided by Xilong Chemical Co., Ltd. Guanghua Polytron Technologies Inc. in Guangdong was the supplier of the chemical grade sodium hydroxide (96% pure) and poly(methylhydrogen)siloxane (PMHS with 99% purity and molecular weight 2500−3500 Da; CAS no: 63148-57-2) with a hydrogen content of 1.6%. High-purity iron sheet (Fe ≥ 99.99%) substrates (15 cm × 7 cm × 1 mm) were used as received from Guan Tai Metal Products Co., Ltd. in Shenzhen, Guangdong. Water colored with methylene blue with 98.5% purity (0.02% (w/w) in water, CAS no: 7220-79-3; Tianjin Zhiyuan Chemical Reagent Co., Ltd.) was used for visual purposes. Deionized water was used throughout this study. All of the raw materials came from China and were used as received without further treatment. Preparation of SNFs Based on Geopolymer Coating. First, 100 g of liquid raw water glass (solid content 37.63%) with modulus of 3.31 M (M: SiO2/Na2O molar ratio = 3.31) was modified as an alkali activator by adding 33.13 g of solid NaOH with 96% purity. After stirring, ultrasonic dispersion to complete dissolution, and standing for 24 h, the clarified modified water glass with a modulus of 1.3 M was obtained; the obtained water glass was sealed and placed in a 22 °C thermostat for reserve. The SNF-covered geopolymer coatings were fabricated as demonstrated in Figure 1. The mass ratio of metakaolin, 1.3 M liquid water glass, and quartz sand was 1:1:8. Then, the geopolymer slurry was prepared by adding deionized water to adjust the liquid-to-solid ratio to 25%. A 20 g fresh geopolymer paste was coated evenly on a rectangular iron sheet of size 15 cm × 7 cm. The iron sheet was subsequently cured in an oven at 60 °C. After 20 min, the solidified slurry was removed from the oven and brushed with 0.5 g of PMHS by a brush as evenly as possible on the surface. All of the quality control processes were completed by analytical balance. It was then immediately placed in a cylindrical sealed glass container with an internal diameter of 18 cm and a height of 6 cm, and maintained at 120 °C for an hour. In this way, a geopolymerbased superhydrophobic composite coating with dense silicone nanofilaments was prepared. Characterization. A Hitachi SU8020 (Japan) field-emission scanning electron microscope (FESEM) equipped with energydispersive spectroscope (EDS) was used to analyze the structures and components in detail at acceleration voltages of 10.0 and 20.0 kV, respectively. Before the test, all samples were fixed on copper 22810


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100 contact angle instrument (KRUSS, Germany). Five different locations of each surface were selected to test the static contact angle (CA) and the rolling angle (SA) at ambient temperature. Their averages were reported. Five μmicroliters of aliquot of deionized water was dropped onto the coatings. The droplet shape was imaged with a video camera and the contact angle fitted by Laplace−Young method. Sliding angle also showed the heterogeneity of the surface and was measured by injecting a 10 μL droplet on a surface and tilting the surface to find the angle of the surface to the horizontal plain when the droplet starts to slide. The BS224S analytic balance produced by Beijing Sedoris Instrument Co., Ltd. was used for weighing the samples.

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RESULTS AND DISCUSSION Component Analysis of Composite Coatings. As Figure 2A shows, the curves in (b) and (c) show absorptions at 2966 and 2904 cm−1 assigned to the stretching vibrations of the C−H bonds of the −CH3 groups, whereas another band, related to the bending deformation of Si−CH3 groups, is also observed at nearly 1260 cm−1.52 However, these should exist only in PMHS. The Si−H stretching vibrations of PMHS in (c) at 2166 cm−1 are a very strong indication of significant amounts of active hydrogen in PMHS. However, the Si−H bond stretching vibration of the composite coating vanishes, as there is no active hydrogen in the Si−O−Si main chain of the silicone nanofilaments, which can be attributed to the Si−H bond undergoing a hydrolytic reaction from the catalysis of the strong alkali, which are then convert to Si−OH groups, as shown in curve (b). The stronger peak at 3585 cm−1 is associated with the stretching vibrations of the −OH groups.52 These results suggest that PMHS has been successfully grafted onto the surface of the geopolymer by a simple coating. In addition, from the broad spectra scan of the composite coating (Figure 2B), we see that the peaks are mainly attributable to Si, O, and C elements, meaning that the fibrous structure grafted on the surface is indeed derived from PMHS. Morphology and Wettability of Coating Surfaces. As shown in Figure 3a,b, the surface of the modified geopolymer coating is covered by large number of silicone nanofilaments with different aspect ratios. The contact angle of the coating increased from 22 to 161°, and the rolling angle was only 2°. It is difficult for a 3 μL droplet to adhere to the coating surface due to excellent superhydrophobic properties of the coating, as illustrated in Figure 3i. Figure 3b shows the contact angle of the modified coating obtained by increasing the volume of the

Figure 1. Schematic diagram of the preparation of superhydrophobic composite coatings. conductive adhesive and coated with gold. To study the change of average surface roughness of coatings before and after modification, the laser confocal scanning microscopy (CLSM, LSM 800Carl Zeiss, Germany) was used to study the three-dimensional microstructures of the coatings at three different locations on each surface, and then the average value was obtained. After initial scans, a robust Gaussian filter with a 4 μm cutoff was applied to the surface topography using the Mountains Map topography software (Digital Surf). Transmission electron microscopy (TEM, FEI TITAN ETEM G2 200 kV) was used to observe the fine morphology of the silicone nanofilaments at acceleration voltages of 300 kV. The powder obtained by scraping the surface of the coating with a stainless steel blade was dissolved in ethanol and dispersed on a copper grid after ultrasound for TEM observation. The coating sample of size 2 cm × 2 cm was cut by a cutter for X-ray photoelectron spectroscopy (XPS) test. This measurement was carried out on an ESCALAB 250XI (Thermo Fisher Scientific) electron spectrometer. Fourier transform infrared spectroscopy (FTIR) test was carried out using a NICOLET iS10Thermo scientific instrument for wavenumbers ranging from 4000 to 400 cm−1. Before testing, the coatings were purified in turn by acetone, ethanol, and a large amount of deionized water; they were then dried in a vacuum drying box. The specimens were prepared by mixing 1 mg of nanocomposite coating/geopolymer coating prepared by scraping the coating surface with 100 mg of KBr with a blade and then pressed into thin slices using a tablet press. The wettability of the coatings before and after modification was achieved by using a DSA

Figure 2. (A) FTIR spectra of the (a) geopolymer, (b) composite coating, and (c) PMHS. (B) XPS spectra of the SNFs-covered geopolymer coating. 22811


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interface with a micro-nanohierarchical roughness is formed. We can observe the obvious silver mirror effect by immersing the modified geopolymer coating in an aqueous medium (Figure 3g), which is due to the total reflection of light in the air layer captured by the superhydrophobic interface, and the integrity of the plastron can verify the full coverage of the filament structure on the surface of the coating. Figure 3h shows that the water column falls on the surface of the composite coating to form water droplets and easily remove the surface silica fume, indicating that the coating has outstanding self-cleaning properties. Formation Mechanism for Silicone Nanofilaments on the Surface of a Geopolymer. At present, the most commonly accepted theory on the reaction mechanism for geopolymers is the processes of depolymerization, reconstruction, and condensation proposed by Davidovits.19,53−55 The main steps include the breakage of Si−O and Al−O bonds in aluminosilicate minerals under alkaline conditions, the formation of silicon−aluminum complexes after the recombination of [Al(OH)4]− and [OSi(OH)3]− monomers, and a gel phase obtained by the polymerization of the reconfigurable bodies under the presence of an alkali silicate solution. At the beginning of the reaction, the free water content decreased obviously, as it promoted the migration and transportation of ions and formed hydrates. However, at the polymerization stage, the free water was released gradually after the collision of the monomers to achieve the polycondensation reaction. Therefore, water can be deemed to play the main role in the transmission medium during the polymer reaction.20,21 With the gradual release of water, the system forms structures with abundant pores. According to the in situ observation of the micromorphology after curing for 10 min to 1.5 h by environmental scanning electron microscopy by Zhang et al.,56 at the initial stage of hydration, metakaolin particles were scattered, and there was a wide pore diameter distribution in the system. Some of the voids would be filled gradually with ageing. In the later stage, the colloids wrapped the particles thickly and the matrix became dense. In addition, high alkali concentrations are used as activators to break the Si−O and Al−O bonds and release monomers in the preparation of geopolymers. However, only a few metal cations (Na+) are responsible for the construction of the geopolymer structure, which is designed to balance the negative charge generated by the four-coordinated [AlO4]− in the later stage of the reaction, and the bases actually act as catalysts. Hence, excessive alkali exists in the system and throughout the reaction.57,58 The holes of the newly cured geopolymer coating are filled with strong alkaline aqueous solution. The tip of each channel for storing

Figure 3. FESEM images of the geopolymer-coated iron sheets before (a) and after (b) the PMHS treatment. The insets depict a water droplet (5 μL) on each of the surfaces. Confocal laser scanning microscopy images (c) and (d) correspond to the 3D surface topography of the coatings before and after modification, respectively. FESEM images (e, f) showing the silicone nanofilaments grown on the rough geopolymer surface. (g) A photograph of the composite coating submerged in water. (h) Self-cleaning was demonstrated as passing water droplets were easily able to carry away fouling powder spread over the monolith. (i) Photographs of the contact, deformation, and departure processes for a 3 μL water droplet with respect to the fabricated composite coating surface and a 10 μL water drop on a 2° tilted SNFs-covered geopolymer coating.

droplet to 5 μL. This can be attributed to the uniform coverage of the silicone nanofilaments on the surface of the geopolymer coating that allows more air to be stored in the network, which is made of interwoven filaments, resulting in the occurrence of the Cassie−Baxter wetting state. The dense fibrous coating shown in Figure 3e,f is similar to a “lawn” growing on the epidermal structure of the geopolymer “land”. Correspondingly, due to the combination of the packed quartz sand and the natural porous structure of the geopolymer, Figure 3c,d shows that the average surface roughness (Sa) of the local 1.277 mm × 1.277 mm range of the coatings before and after modification is 14.6 and 10.8 μm, respectively. The modification process of PMHS is helpful in improving the smoothness of the coating. This may be because the dense growth of SNFs fills in the macroporous defects partly from the geopolymer substrates. The dense fibrous coating shown in Figure 3e,f is similar to a lawn growing on the epidermal structure of the geopolymer land. A stable superhydrophobic

Figure 4. Transmission electron microscopy (TEM) images (a−c) of silicone nanofilaments. 22812


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ACS Applied Materials & Interfaces water is like a tiny reaction vessel. When the PMHS (with a ) is coated on the surface of

structure of

a geopolymer coating, only a small amount of PMHS in contact with the pore solution participated in hydrolytic polymerization in the tiny reaction vessel through adsorption. With the increase of hydrolysis products (called “PMHSO”), the PMHSO is repolymerized and precipitated. The nucleation sites of silicone nanofilaments are formed as shown in Figure 4. The reaction equation is described as follows OH−

≡Si−H + H 2O ⎯⎯⎯⎯→ ≡Si−OH + H 2↑ OH−

2≡Si−OH ⎯⎯⎯⎯→ ≡S−O−Si≡ + H 2O Figure 5. Brief mechanism map of the formation of silicone nanofilaments on the surface of a geopolymer.

OH−

≡Al−OH + HO−Si≡ ⎯⎯⎯⎯→ ≡Al−O−Si≡ + H 2O

When the coating is kept in a sealed environment at 120°, water is released rapidly from the interior of the geopolymer and water vapor can quickly reach saturation in the sealed high borosilicate glass cabin. At this point, the liquid and vapor phases are in equilibrium. The water vapor partially diffused on the surface will condense at the interface of the new phase, thus ensuring the continuous water supply in the pore. According to Seeger et al.,48 sustained supply of stable droplets is the key reason for the directed in situ shaping of nano- and microstructures during the chemical synthesis. The uninterrupted supply of water in the pore attracts more PMHS to participate in hydrolytic polymerization. Driven by vapor pressure and capillary force, new solid phases formed are constantly pushed out of the pores, leading to the growth of one-dimensional nanostructures. In other words, we believe that the natural pore in the geopolymer coating controls the size of water droplets, which has been proved to be positively correlated with the formed diameter of the filaments.48 Therefore, these holes act as templates to direct the formation of the filaments of one dimension. These different pore sizes lead to diverse water content in each pore usually. The uneven distribution of moisture in the rough surface may result in it covering a wide range of aspect ratios.48,59 As shown in the TEM images of the filaments in Figure 4, the diameters of the amorphous silicone nanofilaments are obviously inhomogeneous. The water gradually discharged from the curing of the geopolymer is the only water source in the system. Controlling the rate of moisture transfer from the substrate to the surface, or controlling the water content in the surface, is the key to the in situ growth of silicone nanofilaments by a simple coating method in an alkali-activated cementitious system. A brief summary of the formation mechanism of silicone nanofilaments is shown in Figure 5. Subsequently, this viewpoint was further confirmed by comparing the surface morphologies of the coatings prepared under unsealed and sealed environment at 25, 60, and 120°. As shown in Figure 6, the nucleation of SNFs can be observed under the sealing modification, and the formation of SNFs can hardly be seen in the open system. This is because in the unsealed system, water diffuses on the surface and then evaporates quickly; almost no more water will stay on the surface to supply the growth of silicone nanofilaments. Because water in an unsealed system diffuses from the interior to the surface and then evaporates into the air quickly, an

uninterrupted supply of water cannot be ensured. In addition, the growth rate of silicone nanofilaments at 120 °C was significantly higher than that at 25 and 60 °C by comparing the nanostructures formed under different temperatures at sealing conditions. This can be attributed to the fact that the increase of the curing temperature in the geopolymerization not only promotes dissolution but also accelerates interprecursor polymerization, which is conducive to the rapid release and diffusion of internal water to the surface. On the other hand, high temperature accelerates the hydrolysis of silane and promotes the rapid condensation of silanol, which is more conducive to the rapid growth of silicone nanofilaments. Most notably, polymerization of the partial PMHSO with dissolved aluminosilicate monomers or segments in metakaolin can also occur during the precipitation of the PMHSO molecule in holes. As seen from the EDS characterization of the bulk composition of silicone nanofilament in Figure 7, not only are the Si, O, and C elements from the PMHS found in the filament body but also a small amount of Al and Na elements are detected regularly in metakaolin. It is logical to conclude that the condensation between Si−OH/Al−OH existing on the surface of the geopolymer and PMHSO may always occur during the self-cross-linking of the PMHS to form silicone nanofilaments. This may be different from the previous view that the growth of the silicon nanofilaments is initiated by the tip.41,44,45,47,48 The in situ growth of this silicon nanofilaments can be attributed to the root, which triggers the “mushroom like” strand to stretch upwards successively to form the final one-dimensional nanostructure.

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CONCLUSIONS In summary, silicone nanofilaments were synthesized by a simple grafting modification of organosilicon on the surface of a geopolymer based on an alkaline environment. The geopolymer layer was first prepared on the surface of an iron sheet using the metakaolin precursor, quartz sand, and liquid water glass as raw materials. Then, the silicon nanofilaments can be grown in situ quickly by coating hydrogen-containing silicone oil. A geopolymer-based superhydrophobic composite coating with dense bundles of filaments on the surface was successfully prepared. The results of FTIR and XPS confirmed that the fibrous structure originated from the hydrolytic selfcross-linking of the PMHS and was successfully grafted onto the metakaolin-based geopolymer substrates. In the meantime, 22813


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Figure 6. FESEM figures of the composite coatings with different modification temperatures and curing conditions: (a)−(f) modified 30 min for 25 °C unsealed, 60 °C unsealed, 120 °C unsealed, 25 °C sealed, 60 °C sealed, and 120 °C sealed, respectively.

Figure 7. Energy-dispersive spectroscopy (EDS) spectra of the bulk of single silicone nanofilament.

the results of the contact angle measurements, rolling angle measurements, and water droplet nonstick tests indicated jointly the excellent superhydrophobicity and self-cleaning properties of the composite coating. The EDS and TEM characterizations of the nanofilament bulk composition together demonstrated that growth of the filaments gradually stretched upward and was initiated by the root. Water and alkali released gradually during the high-temperature-sealed curing of geopolymer create conditions for the growth of silicone nanofilaments. The natural pore structures of the geopolymer act as templates, which limit the morphology of polysiloxane to one dimension. Therefore, we propose a novel, simple, and efficient method for the in situ synthesis of silicone nanofilaments. The surface superhydrophobic modification of alkali-activated cementitious materials was successfully realized. We believe that our results can provide a new strategy for designing the next generation of full-water-borne superhydrophobic coatings materials.

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Preparation process of superhydrophobic composite coating (MP4)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xue-min Cui: 0000-0003-1818-8470 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Chinese Natural Science Fund (grants 51772055 and 21566006) and the Guangxi Natural Science Fund (grant 2016GXNSFGA380003).

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ASSOCIATED CONTENT

REFERENCES

(1) Drelich, J.; Chibowski, E.; Meng, D. D.; Terpilowski, K. Hydrophilic and Superhydrophilic Surfaces and Materials. Soft Matter. 2011, 7, 9804−9828. (2) Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546−551.

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b07990. 22814


Research Article

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Superhydrophobic Coatings Prepared by the in Situ Growth of Silicone

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