Durable, Transparent, and Hot Liquid Repelling Superamphiphobic

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Durable, Transparent and Hot Liquid Repelling Superamphiphobic Coatings from Polysiloxane-Modified Multiwalled Carbon Nanotubes Junping Zhang, Bo Yu, Ziqian Gao, Bucheng Li, and Xia Zhao Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.6b04213 • Publication Date (Web): 27 Dec 2016 Downloaded from http://pubs.acs.org on December 28, 2016

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Durable, Transparent and Hot Liquid Repelling Superamphiphobic Coatings from Polysiloxane-Modified Multiwalled Carbon Nanotubes Junping Zhang,†,* Bo Yu,†,‡ Ziqian Gao,† Bucheng Li,† and Xia Zhao‡ †

Key Laboratory of Clay Mineral Applied Research of Gansu Province, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, 730000, Lanzhou, P.R. China ‡

Department of Chemical Engineering, College of Petrochemical Engineering, Lanzhou University of Technology, 730050, Lanzhou, P.R. China KEYWORDS: superhydrophobic, superoleophobic, Cassie-Baxter state, silica, silanes ABSTRACT: Although encouraging progress in the field of superamphiphobic coatings have been obtained, superamphiphobic coatings with high durability, transparency and repellency to hot liquids are very rare. Here, durable, transparent and hot liquid repelling superamphiphobic coatings were successfully prepared by using polysiloxane-modified multiwalled carbon nanotubes (MWCNTs@POS) as the templates. The hydrolytic condensation of n-hexadecyltrimethoxysilane (HDTMS) and tetraethoxysilane on the surface of MWCNTs formed MWCNTs@POS, which are highly dispersible in toluene. The superamphiphobic coatings were prepared by spray-coating the homogeneous suspension of MWCNTs@POS in toluene onto glass slides, calcination in air to form the silica nanotubes (SNTs), and then modification with 1H,1H,2H,2H-perfluorodecyltrichlorosilane in dry toluene. The changes in surface microstructure, surface chemical composition and wettability were characterized by various techniques including scanning electron microscopy, transmitting electron microscopy and X-ray photoelectron spectroscopy. It was found that the microstructures of the SNTs have great influences on superamphiphobicity and transparency of the coatings, and can be regulated by the concentration of HDTMS and the diameter of MWCNTs. The SNTs with tunable wall thickness and diameter could be obtained by the method. The superamphiphobic coatings showed high contact angles and low sliding angles for various cool and hot liquids of different surface tension. The superamphiphobic coatings also exhibited high transparency and comprehensive durability.

INTRODUCTION Superhydrophobic coatings have generated extensive attention in various fields,1-5 as inspired by the self-cleaning properties of the lotus leaf.6 By searching in ISI Web of Science using the title of “superhydrophobic*” until 2016, more than 4700 records can be found and one can clearly observe an accelerated increase in the number of publications in the past decade. Now superhydrophobic coatings can be constructed simply by the combination of proper surface roughness and materials of low surface energy.7-10 However, it is still difficult to invent superamphiphobic coatings that resist wetting of both water and organic liquids because of the very low surface tension of most organic liquids compared to water.11 Owing to the unique anti-wetting property, superamphiphobic coatings have wide potential applications such as oil transportation, anti-fouling of oils and anti-creeping of lubrication oils, etc.12, 13 Thus, many methods have been developed to create superamphiphobic coatings, and encouraging results have been achieved.14-17 However, there are some crucial issues remained to be solved. First, the organic liquids with surface tension less than 27.5 mN m-1 often have high contact angles (CAs ≥ 150°) but adhere stably on most of the reported superamphiphobic coatings, i.e. lack of the unique self-cleaning

property.15, 16, 18 In order to create superamphiphobic coatings on which the organic liquids could roll off at low sliding angles (SAs), the liquid-solid interaction should be very weak.19 Until now only a few studies have reported such superamphiphobic surfaces by designing some special microstructures, e.g., overhang structures,20, 21 re-entrant curvatures22-24 and silicone nanofilaments25 or by using inherently textured textiles.26, 27 Even though the superamphiphobic coatings with low SAs have been obtained, they generally face the problem of poor durability as mentioned by Chu and Seeger in their recent review paper about superamphiphobic surfaces.11 Moreover, the methods are often very complicated, expensive, and limited to specific substrates, which seriously restrict their applications.28 Meanwhile, high transparency of superamphiphobic coatings is essential for applications on glass surfaces such as windows, goggles and optical equipment.29 However, superamphiphobic coatings with low SAs and high transparency are extremely rare, as superamphiphobic coatings have high requirement for surface roughness. 25 A higher surface roughness in the micro-/nanoscale is helpful for the fabrication of superamphiphobic coatings, but often results in light scattering and thus low transparency.30 It was suggested that light scattering is very low in the visible wavelength when the surface roughness is less than 100 nm.31 Zhang and Seeger reported transparent superamphiphobic

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coatings with a transmittance of 82% at 600 nm by controlling the microstructure of silicone nanofilaments.25 Vollmer et al. prepared a transparent robust superamphiphobic coating with a transmittance of 80% at 600 nm by using candle soot as a reported transparent and template.29 Yang et al. superamphiphobic surfaces with a transmittance of >90% at 600 nm from spray-coating of stringed silica nanoparticle/sol solutions.31 Further, most superhydrophobic surfaces lose superhydrophobicity once exposed to hot water, and there are few superhydrophobic surfaces repelling hot water (≥ 50 °C). 32, 33, 34 The condensation of water vapor between the superhydrophobic surface and the hot water due to their temperature difference results in the increase in the liquid-solid contact area.32 Also, the surface tension of hot water is lower than that at room temperature.33 Consequently, the hot water droplets adhered stably on the coatings. Similarly, it is more challenging but meaningful to design superamphiphobic coatings with high repellency to hot water and organic liquids, as the surface microstructure should be precisely regulated and such coatings have wider potential applications in many fields. We have reported the first superamphiphobic coating repellent even to hot liquids by the combination of rodlike palygorskite and organosilanes via spray-coating, however, the coating is not transparent.28 Here, we report design of durable and transparent superamphiphobic coatings repelling both cool and hot liquids by using polysiloxane (POS)-modified multiwalled carbon nanotubes (MWCNTs) as the templates (MWCNTs@POS). First, silica nanotubes (SNTs) were constructed on glass slides by spray-coating with a homogeneous suspension of MWCNTs@POS followed by calcination to remove the carbonaceous materials. Then, the SNTs coatings were modified with 1H,1H,2H,2H-perfluorodecyltrichlorosilane (PFDTCS) to create the superamphiphobic SNTs@PFDTCS coatings. The diameter and wall thickness of the SNTs can be regulated by the diameter of MWCNTs and the concentration of n-hexadecyltrimethoxysilane (HDTMS), which have great influences on the superamphiphobicity and transparency of the coatings. The SNTs@PFDTCS coatings show high CAs and low SAs for various cool and hot liquids, high transparency and comprehensive durability.

EXPERIMENTAL SECTION

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MWCNTs (50 mg) were charged into the mixture of ethanol (44 mL), ammonia (1 mL) and deionized water (5 mL) in a 100 mL conical flask. The suspension was magnetically stirred for 10 min followed by ultrasonication for 30 min at room temperature. Then, proper amounts of HDTMS and TEOS with a molar ratio of 1.13 were added. After reacting at room temperature for 24 h under vigorous stirring, the as-prepared MWCNTs@POS suspension was centrifugated at 10000 rpm for 30 min. The collected MWCNTs@POS was re-dispersed in toluene by magnetically stirring for 10 min and ultrasonication for 30 min at room temperature. Preparation of MWCNTs@POS Coatings. The glass slides were cleaned by washing in turn with ethanol, acetone and distilled water, and then dried under a N2 flow. The MWCNTs@POS coating was prepared by spray-coating the homogeneous MWCNTs@POS suspension (10 mL) in toluene onto the glass slide using an airbrush (INFINITY 2 in 1, Harder & Steenbeck, Germany) with a spray-coating pressure of 0.2 MPa N2 and a spray-coating distance of 10 cm. Preparation of Superamphiphobic SNTs@PFDTCS Coatings. The MWCNTs@POS coating was calcined in air in a muffle furnace at 500 °C for 2 h to remove the carbonaceous materials including MWCNTs and the hexadecyl groups of POS. Subsequently, the resulting SNTs coating was immersed in dry toluene (60 mL) containing PFDTCS (14 µL) at room temperature for 24 h. The water concentration in toluene is 10-15 ppm. Finally, the SNTs@PFDTCS coating on the glass slide was washed with 30 mL of dry toluene and dried under a N2 flow. Measurements of CAs and SAs. Measurements of CAs and SAs were performed with a Contact Angle System OCA20 (Dataphysics, Germany) equipped with a tilting table. The syringe was positioned in a way that the droplets (5 µL) could contact surface of the samples before leaving the needle. Tilting angle of the table was adjustable (0 ~ 70°) and allowed the subsequent measurement of the SAs at the same position on the sample. Because the temperature of droplets decreases quickly, it is difficult to get the real temperature of hot droplets on the coatings. The temperature of hot droplets reported in this paper is the temperature of liquids in the syringe. The temperature was measured by an IR sensor. In order to reduce thermal loss from the hot droplets, measurements of CAs and SAs were carried out within 5 s of placing the droplets on the coatings. A minimum of six readings were recorded for each sample.

Characterization. The micrographs of the samples were taken using a field emission scanning electron microscope (SEM, JSM-6701F, JEOL) and a transmitting electron microscope (TEM, TECNAI-G2-F30, FEI). Before SEM observation, all samples were fixed on copper stubs and coated with a layer of gold film (~ 7 nm). For TEM observation, the samples were prepared as follows. A drop of the ethanol solution of the sample was put on a copper grid and dried in the open atmosphere. The Fourier transform infrared (FTIR) spectra of samples were collected on a Thermo Nicolet NEXUS TM spectrophotometer (Thermo, Madison, USA) in the range of 4000-400 cm-1 using KBr pellets. The samples used for TEM observation and Preparation of Homogeneous MWCNTs@POS Suspension. recording the FTIR spectra were collected from the coated glass MWCNTs (1.0 g) were treated in 100 mL of concentrated H2SO4 slides using a scalpel. The surface chemical composition of the and HNO3 (3:1, v/v) at 60 °C for 1 h. The acid activated samples was analyzed via X-ray photoelectron spectroscopy

Materials. MWCNTs 10-20 nm, 20-40 nm, 40-60 nm or 60-100 nm in diameter and < 5 µm in length were purchased from Shenzhen Nanotech Port Co., Ltd. Tetraethoxysilane (TEOS, 99.9%), HDTMS (95%) and PFDTCS (97%) were purchased from Gelest. Glass slides (24 mm × 50 mm, Menzel, Braunschweig, Germany) were used as the substrates. H2SO4, HNO3, anhydrous ethanol, ammonia, diiodomethane, n-hexadecane, n-dodecane, n-decane and toluene were purchased from China National Medicines Co. Ltd. Other reagents used were all of analytical grade. All chemicals were used as received without further purification.

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(XPS) using a VG ESCALAB 250 Xi spectrometer equipped with a Monochromated AlKa X-ray radiation source and a hemispherical electron analyzer. The spectra were recorded in the constant pass energy mode with a value of 100 eV, and all binding energies were calibrated using the C1s peak at 284.6 eV as the reference. The transmittance of the samples was measured using a UV-Vis spectrophotometer (Specord 200, Analytik Jena AG).

and then re-dispersed in toluene (Figure 1). Compared to ethanol, toluene is a better solvent to form a homogeneous MWCNTs@POS suspension. The POS layer on the surface of MWCNTs could not only help MWCNTs disperse very well in toluene, but also form the SNTs after removal of the carbonaceous materials.

RESULTS AND DISCUSSION Preparation of Homogeneous MWCNTs@POS Suspension. As is well known, MWCNTs have poor dispersibility in various liquids. In order to obtain uniform coatings by using MWCNTs as the templates, the homogeneous MWCNTs@POS suspension Figure 1. Preparation of the homogeneous MWCNTs@POS was prepared first by hydrolytic condensation of HDTMS and suspension in toluene. TEOS onto the surface of acid activated MWCNTs in ethanol,35

Figure 2. (a) Schematic illustration for preparation of superamphiphobic SNTs@PFDTCS coatings. Digital, SEM and TEM images of (b) MWCNTs@POS, (c) SNTs and (d) SNTs@PFDTCS coatings. The concentration of HDTMS is 1.76 mM and the diameter of MWCNTs is 40-60 nm. Preparation of Superamphiphobic SNTs@PFDTCS Coatings. As shown in Figure 2a, the durable and transparent superamphiphobic coatings were prepared by spray-coating the homogeneous MWCNTs@POS suspension in toluene onto glass slides, and then formation of the SNTs coatings via calcination in air followed by modification with PFDTCS in dry toluene. The uniform black coatings were formed by spray-coating the homogeneous MWCNTs@POS suspension in toluene onto glass slides (Figure 2b). The MWCNTs were cover with a very thin layer of POS according to the SEM and TEM images. The randomly deposited MWCNTs@POS in the spray-coating process generated a crosslinked network with a rough topography at the surface. The transparent SNTs coatings were obtained simply by calcination of the MWCNTs@POS coatings in air in a muffle furnace at 500 °C for 2 h. In the calcination process, the carbonaceous materials in the MWCNTs@POS

coatings were completely decomposed, and the POS layers on the surface of MWCNTs were converted into SNTs. The microstructure of the SNTs coating is different from that of the MWCNTs@POS coating (Figure 2c). On one hand, the tubular structure was partly retained, but the diameter of the newly generated SNTs was smaller than that of MWCNTs. This is because the decomposition of the carbonaceous materials caused shrinkage of the SNTs. On the other hand, the SNTs adhered together with some crosslinking points, which is owing to the POS layers among the MWCNTs in the MWCNTs@POS coating. Modification of the SNTs coatings with PFDTCS in dry toluene did not cause any evident change in the surface microstructure (Figure 2c). This is because the polycondensation of the hydrolyzed PFDTCS molecules into large particles on the surface of the SNTs was effectively restrained at a such low water concentration.

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Figure 3. (a) FTIR spectra and (b) XPS survey spectra of MWCNTs@POS, SNTs and SNTs@PFDTCS coatings. The concentration of HDTMS is 1.76 mM and the diameter of MWCNTs is 40-60 nm. The chemical composition of MWCNTs@POS, SNTs and SNTs@PFDTCS was analyzed by FTIR spectra (Figure 3a). In the spectrum of MWCNTs@POS, the characteristic absorption bands of acid activated MWCNTs and POS were detected. The band corresponding to the -COOH group (1718 cm-1) is attributed to acid activated MWCNTs.36 The bands corresponding to the -CH2 (2922 and 2853 cm-1), Si-O and Si-OH (1048 cm-1) groups are attributed to POS.37 After calcination in air, all these bands disappeared and three new bands at 1100, 793 and 469 cm-1 corresponding to the Si-O group appeared in the spectrum of SNTs. After further modification with PFDTCS, the C-F band at 1209 cm-1 was easily recognized in the spectrum of [email protected] The band at 1150 cm-1 was attributed to the silsesquioxane bands stemming from the polycondensation of hydrolyzed PFDTCS. The weak bands corresponding to the -CH2 group (2922 and 2852 cm-1) were also detected. The surface chemical composition of the MWCNTs@POS, SNTs and SNTs@PFDTCS coatings was analyzed using XPS (Figure 3b). The C 1s (284.8 eV), O 1s (532.8 eV) and Si 2p (103.2 eV) peaks were detected on the surface of the

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MWCNTs@POS coating. In the spectrum of the SNTs coating, the C 1s peak became very weak and the O 1s peak became very strong. In addition, the Si 2p peak also became stronger compared to that in the spectrum of the MWCNTs@POS coating. The C/O/Si atomic ratio changed from 1:0.26:0.14 to 1:6.70:3.56 after calcination. In the XPS spectrum of the SNTs@PFDTCS coating, an intensive F 1s peak appeared at 688.1 eV. The F atomic ratio is as high as 63.71%, which evidently decreased the surface energy and is of great important to form superamphiphobic coatings with low SAs. The changes in the FTIR spectra and the XPS spectra confirmed the complete removal of the carbonaceous materials and formation of the SNTs in the calcination process, and the successful modification of the SNTs with PFDTCS in dry toluene. The above variations in the surface microstructure and chemical composition resulted in extensive changes in the surface wettability of the coatings. The rough MWCNTs@POS coatings were superhydrophobic (CAwater = 159.6° and SAwater = 2.7°), as the hexadecyl groups of the POS layer evidently decreased the surface energy. However, the MWCNTs@POS coatings could be completely wetted by organic liquids (CAn-decane = 0°) because the surface energies of the coatings and the organic liquids are comparable with each other. Different from the MWCNTs@POS coatings, the SNTs coatings are superamphiphilic (CAwater = 0° and CAn-decane = 0°), as the hexadecyl groups have been decomposed and a large number of silanols have been generated in the calcination process. After modified with PFDTCS in dry toluene, the superamphiphobic SNTs@PFDTCS coatings (CAwater = 162.2°, SAwater = 0.7°, CAn-decane = 154.8° and SAn-decane = 7.2°) were successfully obtained. This is because of polycondensation of the hydrolysed PFDTCS molecules onto the silanols on the surface of SNTs. Effects of Concentration of HDTMS and Diameter of MWCNTs on Wettability and Transparency. The superamphiphobicity and transparency of a superamphiphobic coating are highly dependent on the microstructure of the coating. The microstructure of the SNTs@PFDTCS coating was constructed by using the MWCNTs@POS as the template. Thus, the microstructure of the SNTs@PFDTCS coatings can be regulated by the concentration of HDTMS and the diameter of MWCNTs. The effects of the concentration of HDTMS on the superamphiphobicity and transparency of the SNTs@PFDTCS coatings are shown in Figure 4a-b. When water was used as the probe to study the influence of the concentration of HDTMS on the surface wettability, we could not find any difference between the coatings, as all the coatings are perfectly superhydrophobic with a very high CAwater and a pretty low SAwater. Thus, we used n-decane as the probe to investigate the interesting influence of the concentration of HDTMS on the superamphiphobicity. As can be seen from Figure 4a, the coatings are oleophobic when the concentration of HDTMS was low (1.06 and 1.41 mM) and the CAn-decane increased quickly with increasing the concentration of HDTMS. The coating with a concentration of HDTMS of 1.76 mM showed a CAn-decane of 154.8° and a SAn-decane of 7.2°, indicating very good superamphiphobicity. The further increase

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of the concentration of HDTMS to 3.17 mM slightly increased the CAn-decane to 157.3° and reduced the SAn-decane to 5.0°. The concentration of HDTMS also has an evident influence on the transparency of the coatings (Figure 4b and Table S1). In the wavelength of 320-850 nm, the SNTs-coated glass slides with a concentration of HDTMS of 1.06-2.12 mM have similar transmittance (86.09%-88.87% at 600 nm), which is only slightly lower than that of the bare glass slide (91.5% at 600 nm). A

higher concentration of HDTMS of 3.17 mM reduced the transmittance (82.38% at 600 nm) of the SNTs-coated glass slide. After further modification with PFDTCS, the transmittance of the SNTs@PFDTCS coatings was further reduced although no obvious change in the surface microstructure was observed (Figure 2c-d). The coatings prepared with a concentration of HDTMS of 1.76-2.12 mM showed excellent superamphiphobicity and good transparency.

Figure 4. Variation of (a) CAn-decane and SAn-decane, and (b) transmittance of the SNTs@PFDTCS-coated glass slides with the concentration of HDTMS. SEM and TEM images of the SNTs@PFDTCS coatings and schematic illustration of transition from MWCNTs@POS to SNTs@PFDTCS with a concentration of HDTMS of (c) 1.41, (d) 1.76 and (e) 3.17 mM. The diameter of MWCNTs is 40-60 nm. It was found that the evident influences of the concentration of HDTMS on the superamphiphobicity and transparency of the SNTs@PFDTCS coatings were owing to the changes in the surface microstructure (Figures 4c-e and S1). This is because the SNTs were prepared by using MWCNTs@POS as the template, which was formed by condensation of HDTMS and TEOS on the surface of MWCNTs. At a low concentration of HDTMS of 1.41 mM, the MWCNTs were coated with a very thin POS layer and seriously deformed SNTs were formed in the calcination process (Figures 4c and S1a). This is because the wall thickness of the SNTs is too small to effectively support the tubular structure. With the increase of the concentration of HDTMS to 1.76 mM, the wall thickness of the SNTs increased. Correspondingly, a larger fraction of the tubular structure was retained and the surface roughness increased (Figures 4d and S1b). Thus, the coating became superamphiphobic with a bit lower transparency compared to the coating prepared with a concentration of

HDTMS of 1.41 mM. It should be noted that the tubular structure was not clear with a concentration of HDTMS of 1.41-1.76 mM according to the TEM images (Figure 4c-d). This is because the samples used for TEM observation were collected from the coated glass slides using a scalpel, and then dispersed in ethanol via ultrasonication. These processes may have partly damaged the SNTs. The further increase of the concentration of HDTMS to 3.17 mM perfectly retained the tubular structure, as the wall thickness had been increased to 9-15 nm (Figure 4e). The increased wall thickness further increased the surface roughness, and slightly improved the superamphiphobicity. Meanwhile, the increased wall thickness also caused stronger light scattering, which reduced the transmittance to 62.96%. We can imagine that the wall thickness of the SNTs could be further increased by increasing the concentration of HDTMS. Thus, the method can be used to prepare SNTs with tunable wall thickness simply by controlling the concentration of HDTMS.

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Figure 5. Variation of (a) CAn-decane and SAn-decane, and (b) transmittance of the SNTs@PFDTCS-coated glass slides with the diameter of MWCNTs. SEM images of the SNTs@PFDTCS coatings and schematic illustration of transition from MWCNTs@POS to SNTs@PFDTCS with a diameter of MWCNTs of (c) 10-20, (d) 20-40, (e) 40-60 and (f) 60-100 nm. The concentration of HDTMS is 1.76 mM. The effects of the diameter of MWCNTs on the superamphiphobicity and transparency of the SNTs@PFDTCS coatings are shown in Figure 5a-b. As can be seen from Figure 5a, both CAn-decane and SAn-decane are strongly dependent on the diameter of MWCNTs. The SNTs@PFDTCS coatings could be wetted by the n-decane droplets when the diameter of MWCNTs was small (10-20 and 20-40 nm). The CAn-decane increased greatly from 78.9° to 153.1° with increasing the diameter of MWCNTs from 10-20 nm to 40-60 nm. Also, the n-decane droplets could roll off the coating with a SAn-decane of 6.5°, indicating that the n-decane droplets are in the Cassie-Baxter state. With further increasing the diameter of MWCNTs to 60-100 nm, no obvious change in the CAn-decane was detected, whereas the n-decane droplets adhered strongly on the surface even when the coating was turned upside down. This means the n-decane droplets are in the Wenzel state, which is the frequently observed phenomenon for most previously reported superamphiphobic coatings.15, 16, 18 The diameter of MWCNTs also has some influence on the transparency of the coatings (Figure 5b). The SNTs-coated glass slides have similar transmittance (85.61%-87.81% at 600 nm) regardless of the diameter of MWCNTs, whereas the larger diameter of MWCNTs evidently reduced the transmittance of the SNTs@PFDTCS-coated glass slides. The variations of the superamphiphobicity and transparency with the diameter of MWCNTs are also owing to the changes in the surface microstructure (Figure 5c-f). No SNTs could be found and the surface roughness was low when the diameter of MWCNTs was 10-20 nm (Figure 5c). This is because the POS layer on the surface of the MWCNTs collapsed and sintered together in the calcination process when the diameter of MWCNTs was small. With increasing the diameter of

MWCNTs to 20-40 nm, the deformed SNTs were formed and the surface roughness was enhanced, however, the coating still could be wetted by the n-decane droplets. With further increasing the diameter of MWCNTs, the tubular structure of MWCNTs was better retained, which made the coating superamphiphobic with the n-decane droplets in the Cassie-Baxter state but reduced the transmittance, as the surface roughness was enhanced. Table 1. CAs and SAs of liquids of different surface tension on the SNTs@PFDTCS coating at 25 °C. The concentration of HDTMS is 1.76 mM and the diameter of MWCNTs is 40-60 nm. Liquids

CA / °

SA / °

Surface tension (mN m-1, 20 °C)

water

162.2±1.0

0.7±0.6

72.8

diiodomethane

159.8±0.7

1.0±0.0

50.8

N-methyl-2-pyrrolidone

164.0±0.2

4.8±1.0

40.8

1,2-dichloroethane

160.3±1.0

5.5±1.0

33.3

n-hexadecane

159.6±1.0

5.8±0.8

27.5

n-decane

154.8±1.0

7.2±0.3

23.8

Superamphiphobicity and Transparency. Superamphiphobicity of the SNTs@PFDTCS coating was tested by measuring the CAs and the SAs of liquids of different surface tension at room temperature, and the results

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are listed in Table 1. All the investigated liquids are spherical on the coating with the CAs higher than 154.8° (Figure 6a). The SAs increased slightly with the decrease of the surface tension to 23.8 mN m-1 (n-decane) and remained less than 7.2°. In addition, a jet of n-hexadecane could continuously bounce off the coating without leaving a trace (Figure 6b). These phenomena indicate excellent superamphiphobicity of the coating and the droplets are in the Cassie-Baxter state. This is owing to the air cushions between the coating and various liquids, which significantly reduced the solid-liquid interaction. Interestingly, the SNTs@PFDTCS coating also showed very good superamphiphobicity for hot liquids, which made it superior to most previous superhydrophobic or superamphiphobic coatings. Most natural and artificial superhydrophobic surfaces lose superhydrophobicity once exposed to hot liquids.32 For example, 55 °C water only showed a CAwater of 40° on the lotus leaf.33 It is very interesting to develop coatings repelling towards hot water or even boiling water.39 As can be seen from Figure 6c, although the CAs decreased and the SAs increased gradually with increasing the temperature of water and n-hexadecane to above 80 °C, the coating still retained very good

superamphiphobicity. Both the 85 °C water and the 81 °C n-hexadecane showed high CAs (CAwater = 152.3° and CAn-hexadecane = 155.9°) and could roll off the coating (SAwater = 11.5° and SAn-hexadecane = 14.7°). This means the hot droplets are also in the Cassie-Baxter state on the coating. The changes in superamphiphobicity of the coating with increasing the liquid temperature are owing to the following reasons. First, the hot liquid vapor and water vapor in the air between the coating and the hot droplets quickly condensed once the hot droplets contacted with the coating owing to their temperature difference, which increased the solid-liquid contact area (Figure S3). The quick condensation of the hot liquid vapor and water vapor in the air at the solid-liquid interface was confirmed by the appearance of the white marks left on the coating after hot water (78 °C) and hot n-hexadecane (80 °C) rolled off (Figure 6d-e and Movie S1). For hot water, the white marks could be easily recognized and disappeared quickly. However, in the case of hot n-hexadecane, the marks were weak and lasted for a longer period of time. This is owing to the difference in saturation vapor pressure between hot water and hot n-hexadecane. In addition, the lower surface tension of liquids at high temperature should also be responsible for the changes in superamphiphobicity.33

Figure 6. SNTs@PFDTCS-coated glass slides with (a) droplets of different surface tension and (b) a jet of n-hexadecane bouncing off. (c) Variation of CAs and SAs of water and n-hexadecane with their temperature on the SNTs@PFDTCS coating. (d) Schematic illustration of the difference for the interaction of a superamphiphobic coating at room temperature with cool liquid or hot liquid. White marks remained on the SNTs@PFDTCS coating after (e) hot water and (f) hot n-hexadecane rolling off. The concentration of HDTMS is 1.76 mM and the diameter of MWCNTs is 40-60 nm. Figure 7 summarized the transparency of the SNTs- and SNTs@PFDTCS-coated glass slides with the bare glass slide for comparison. In order to form the SNTs coating most suitable for the preparation of superamphiphobic coating, the concentration of HDTMS is 1.76 mM and the diameter of MWCNTs is 40-60 nm. The as-prepared uniform SNTs layer only caused a small increase in light scattering, and the transmittance was slightly reduced from 91.5% (bare glass

slide) to 86.09%. Further modification of the SNTs with PFDTCS caused a big decrease in the transmittance to 66.94% at 600 nm because of polycondensation of the hydrolyzed PFDTCS molecules. The clover behind the SNTs@PFDTCS-coated glass slide can be seen clearly. This is a precious property as superamphiphobic coatings with high transparency are rare.25, 29, 40

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Figure 7. Transmittance of the bare, SNTs- and SNTs@PFDTCS-coated glass slides. The inset is the image of the SNTs@PFDTCS-coated glass slide with clover as the background. The concentration of HDTMS is 1.76 mM and the diameter of MWCNTs is 40-60 nm.

Figure 8 Variation of CAn-decane and SAn-decane of the SNTs@PFDTCS coating with water jetting time at 10 and 25 kPa. The concentration of HDTMS is 1.76 mM and the diameter of MWCNTs is 40-60 nm.

Durability. To evaluate the durability of the SNTs@PFDTCS coating, we measured the CAn-decane and the SAn-decane of the coating after the water jetting test in which a jet of water scoured the 45° tilted coating at certain pressure.28 The SNTs@PFDTCS coating showed high mechanical durability according to the water jetting test. Variation of the CAn-decane and the SAn-decane of the coating with water jetting time at 10 and 25 kPa was shown in Figure 8. The CAn-decane decreased to 150.8° and the SAn-decane increased to 31.0° with increasing the water jetting time to 10 min at 10 kPa. The increase in the water jetting pressure to 25 kPa had no obvious influence on the CAn-decane, but resulted in greater increase in the SAn-decane. At a pressure of 25 kPa, the water scoured the coating at a speed of 7.1 m s-1, which is comparable to the heavy rain (8-9 m s-1) and much higher than that previously reported. The n-decane droplet still could roll off the 24.0° tilted coating after water jetting at 25 kPa for 5 min, indicating good mechanical durability. With the extension of water jetting time to 10 min, the n-decane droplet adhered stably on the coating although the CAn-decane was still as high as 150.8°. This means the n-decane droplet has transferred from the Cassie-Baxter state to the Wenzel state, as the coating has been slightly damaged.

CONCLUSIONS

The SNTs@PFDTCS coating also showed high chemical and environmental durability as shown in Table S3. The coating is stable against immersion in various liquids (water, ethanol, toluene and 1 M HCl) and strong UV light. No obvious change in the CAn-decane and the SAn-decane was detected after these treatments. An increase in the SAn-decane to 11.5° was observed after the coating was kept at 180 °C for 24 h. Also, a slight decrease of the CAn-decane to 152.9° and a small increase of the SAn-decane to 12.5° were recorded after immersion in the 1 M NaOH aqueous solution for 1 h.

In summary, superamphiphobic coatings with high repellency to both cool and hot liquids, high transparency and comprehensive durability have been prepared by using MWCNTs@POS as the template. Modification of MWCNTs with HDTMS and TEOS forms MWCNTs@POS, which could form a homogeneous suspension in toluene. Calcination of MWCNTs@POS in air generates SNTs, which act as the skeleton of the superamphiphobic coatings. The microstructures of the SNTs, i.e. the wall thickness and the diameter, have great influences on superamphiphobicity and transparency of the coatings, and can be regulated by the concentration of HDTMS and the diameter of MWCNTs. The method reported here could be used to prepare SNTs with tunable wall thickness and diameter. The superamphiphobic coatings may find applications in various fields, as the coatings feature unique superamphiphobicity to both cool and hot liquids, high transparency and durability.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Transmittance, SEM images and wettability of the samples and videos. Movie S1 showing hot liquid repelling property of the coating.

AUTHOR INFORMATION Corresponding Author * E-mail: [email protected], Phone: +86 931 4968251

Author Contributions The manuscript was written through contributions of all authors. / All authors have given approval to the final version of the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS ACS Paragon Plus Environment

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We are grateful for financial support of the “Hundred Talents Program” of the Chinese Academy of Sciences.

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