Photocatalytically Stable Superhydrophobic and Translucent Coatings

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Interface Components: Nanoparticles, Colloids, Emulsions, Surfactants, Proteins, Polymers

Photocatalytically Stable Superhydrophobic and Translucent Coatings Generated from PDMS-Grafted-SiO2/TiO2@PDMS with Multiple Applications Shan Peng, Weihua Meng, Junxiang Guo, Bo Wang, Zhenguang Wang, Na Xu, Xiaolin Li, Jian Wang, and Jianzhong Xu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04247 • Publication Date (Web): 24 Jan 2019 Downloaded from http://pubs.acs.org on January 24, 2019

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Photocatalytically Stable Superhydrophobic and Translucent Coatings Generated from PDMS-Grafted-SiO2/TiO2@PDMS with Multiple Applications Shan Peng1*, Weihua Meng1, JunXiang Guo2, Bo Wang1, Zhenguang Wang1, Na Xu1, Xiaolin Li1, Jian Wang1, Jianzhong Xu1* 1 College of Chemistry and Environmental Science, Hebei University, Baoding 071002, Hebei, China 2 Research Center for Process Pollution Control, National Engineering Laboratory for Hydrometallurgical Cleaner Production Technology, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China E-mail: [email protected]

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ABSTRACT: In this paper, we presented a highly efficient, cost-effective, and wide-applicable functionalized SiO2/TiO2-polymer based coating to fabricate a translucent, fluorinefree, chemically stable, photocatalytic active, self-healable superhydrophobic coating, which was consisted of two mixed functionalized particles (MFP) and PDMS in a proper ratio. Both SiO2 and TiO2 powders were functionalized with PDMS brushes to achieve superhydrophobicity. In order to maximally optimize properties, include superhydrophobicity, transparency, and photocatalytic activity, ratios between MFP with PDMS were carefully studied and optimized. Glass slides coated with this mixed coating (MC) showed translucence with transparency of 75%. It also presented superior photocatalytic activity and strong UV resistance that could repeatedly degrade organic oil pollutants for as many as 50 times, while still reserved superhydrophobicity even exposure to UV light with high intensity of 80 mW/cm2 for as long as 36 h. When low-surface-tension oils such as dodecane wetted the MC surface, it showed excellent slippery performance, and could quickly repel strong acid/alkali/hot water, and even very corrosive liquids aqua regia. MC achieved extremely stable underoil superhydrophobicity (towards liquids include water, strong acid and base, hot water, etc.) and self-cleaning properties, not only in oils at room temperature, but also in scalded oil environment. Moreover, MC showed self-healable performance after recycled plasma treatment. The stainless steel mesh coated with MC was also used to highly efficiently separate oil-water mixtures. Moreover, harsher liquids include strong acid/alkali solutions/hot water/ice water-oil mixtures could be also successfully separated by the coated mesh. This coating was believed to largely broaden both indoor and outdoor applications for superhydrophobic surfaces. Keywords: Photocatalytic stable; Translucent; PDMS; Graft reaction; Self-healing; Oil-water separation;


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INTRODUCTION Lotus-inspired superhydrophobic surfaces have aroused much attention in recent years because of their peculiar performances and potential applications in selfcleaning,1-4 oil-water separation,5-9 anti-frost/icing,10-11 drag-reduction,12-13 etc. Mimicking micro-nano scale hybrid structure of lotus has led to the fast development of large amounts of artificial superhydrophobic materials. Superhydrophobic surfaces are always expected to have practical use in both indoor and outdoor conditions. In the case of indoor applications, the surfaces are hopeful to show liquid-repellency (liquids include water, hot water, etc.), self-cleaning properties, and transparency or semi-transparency sometimes. While superhydrophobic surfaces with good chemical stability towards various corrosive liquids such as strong acid/alkali, UV resistance, and mechanical durability are often indispensable in terms of outdoor applications. Therefore, superhydrophobic surfaces with single water-repellence are hard to fulfill the requirements of society. Multifunctional materials with various fascinating performances that can maximally satisfy people’s expectation and requirement, therefore have become the mainstream development trend. There is no doubt that superhydrophobic coating is versatile and transferable to various hard or soft substrates,14-17 which thus is an ideal method to adjust the substrates’ wettability, without considering the substrates’ roughness, shape, and size. Superhydrophobic coating can build a functional or protective new layer that quite different from the original substrate. Coatings are normally consisted of multicomponent, and each component plays its special role when applied, which provide an effective way to fabricate multifunctional materials. Moreover, it is believed that integrating the particular properties of coatings and substrates will produce more functional and superior performances, which is also beneficial to develop multifunctional materials. Hybrid organic-inorganic composite coatings are very popularly used in fabricating superhydrophobic surfaces since they combine advantages of both organic and inorganic materials.18-20 Inorganic particles are often used to create and adjust nano- or microscale structure on polymer coatings to fabricate composite superhydrophobic coatings. However, it is common that inorganic particles normally aggregate to bulks when blend with polymer solution, thus resulting in phase separation during the fabrication process. In order to solve this problem, modifiers such as surfactants, polymer coatings, organic tethers, etc. are often used to functionalize the particles to achieve hydrophobicity.21-22 Unfortunately, most


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modified compounds are either expensive or environmentally harmful (fluorinecontaining reagents are typical example). In addition, synthesis processes for some of them are complex and time-consuming, which are not wide-applicable for most of inorganic particles. Superhydrophobic coatings with transparency facilitate a broad application scope of superhydrophobic surfaces since they can provide transparency for aircraft canopies, dust-free windows, engineering shield, and solar cell systems.23-25 However, it is quiet challenging to strictly control the surface roughness to simultaneously realize superhydrophobicity and transparency. Wang et al.24 have fabricated a transparent superhydrophobic coating by using fluoroalkylsilane treated silica particles and PDMS. Yao et al.26 have reported transparent superhydrophobic coatings prepared by using SiO2 particles and fluorinated multi-walled carbon nanotubes. Wang et al.27 have prepared transparent superhydrophobic solar glass through picoseconds laser pulses followed by fluoroalkylsilane modification. Although these reports have successfully fabricated transparent superhydrophobic surfaces, fluorine reagents were used in the preparation steps under all conditions. Photocatalyst metal-oxides such as TiO2, ZnO have often been used to fabricate superhydrophobic surfaces because their photocatalytic activity can effectively decompose most organic substances.28-33 However, in general, there are a conflict between hydrophobicity and photocatalytic property since when photocatalysts degrade oil contaminants, most modifiers or organic coatings itself that used to achieve superhydrophobicity have been also decomposed by photocatalysts, which thus resulting in surface hydrophilicity. There are some examples have been reported about photocatalytically stable superhydrophobic surfaces. Xu et al.34 have reported a UV-resistant and water-proof breathable membrane by electrospinning and doctorblading coating method. Gao et al.35 have fabricated highly transparent and UVresistant superhydrophobic arrays of SiO2-coated ZnO nanorods followed by fluorinecompound modification. However, they either have used UV absorber or fluorine reagent, which is expensive and environmentally harmful. Some other researches have reported photocatalytically stable superhydrophobic coatings based on TiO2 and other kind of particles. For example, Wu et al.36 have reported an all-water-based method to fabricate self-repairing superhydrophobic coatings based on UV-responsive microcapsules, which was synthesized by Pickering emulsion polymerization using TiO2 and SiO2 as the Pickering agents. The resultant coatings achieve


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superhydrophobicity after UV illumination. Wu et al.37 have prepared a self-repairing robust superhydrophobic coating by polystyrene, fluorine compound, fluorine compound-modified SiO2 and TiO2 particles. Zhao et al.38 have fabricated multifunctional coatings by using mixed TiO2 and graphite particles and fluorine compounds, which show good conductivity and photocatalysis. The superhydrophobic coating prepared in this work all show anti-UV irradiation. However, different kinds of fluorine compounds have been all used in their preparation steps. And some of their preparation steps were complex, expensive, and time-consuming. Besides, these experiments are all exposed to UV light with low strength to evaluate the stability. Recently, Butt et al.39 have used non-fluorine method to graft mesoporous TiO2 surfaces by PDMS, which simultaneously remain both photocatalytic activity and stable hydrophobicity when exposed to UV light for as long as 35 h, because PDMS are strongly covalently bonded with TiO2 surface. However, their experiments are also exposed to UV light with low strength of 5 ± 0.5 mW cm-2, in the case of strong UV light (such as more than 50 mW cm-2), no experiments are involved. From a practical view, it is very desirable to develop a cost-effective, easy-implemental, environmentally

friendly

method

to

fabricate

photocatalytically

active

superhydrophobic materials that can simultaneously keep superhydrophobicity and photocatalytic activity even after a long-time light irradiation. Also, it is important to fully evaluate the stability of the coating under high-strength UV light. Waterproof sponge/cotton/textile exhibit great practical application potential. For example, textiles are normally soft, flexible, and inexpensive, and are ideal candidates to treat oily wastewater and spilled oil.40-42 However, the original textiles would adsorb both oil and water due to their hydrophilicity. When functionalized the textile with superhydrophobicity and superoleophilicity, it can easily adsorb waste oils from water surfaces. However, the absorbed oil pollutants on the textile surface would decrease or even break the surface superhydrophobicity, which lead to malfunction of the textile. Therefore, it is very desirable to efficiently degrade these oil pollutants by photocatalysts, and make the surface wettability recovered. The textile thus can be reused to absorb oils in this case. Herein, considering the above-mentioned urgent problems, in this paper, we developed a highly-efficient, fluorine-free, wide-applicable, and easy way to prepare superhydrophobic coatings with multiply superior properties. The original hydrophilic powders of SiO2 and TiO2 all became superhydrophobic after covalently bonded with


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PDMS brushes. The superhydrophobic coatings were consisted of mixed powder of PDMS-grafted-SiO2 and PDMS-grafted-TiO2, PDMS, and its curing agent. The coatings were semi-transparent when spin-coated onto glass slide (GS) surface. The coatings achieved excellent photocatalytic activity and could keep stable superhydrophobicity without change even when exposed to UV light with high intensity

of

80

mW

cm-2

for

long

time.

Various

substrates

include

textile/cotton/sponge all achieved superhydrophobicity after coated with MC. Besides, these coated surfaces could highly efficiently adsorb oil pollutants, and the coating on the surface effectively degraded these oil contaminants under UV light, thus making surface superhydrophobicity recovered, which allowed their recycle use for absorbing oil wastes. Very slippery surfaces with superior liquid-repellency and self-cleaning were achieved when MC was coated with a layer of low-surface-tension oil. Moreover, MC showed self-healable superhydrophobicity after recycled plasma treatment. It was also found that the coating show extremely stable underoilsuperhydrophobicity. The MC coated stainless steel (SS) mesh was proved to not only highly efficiently separate oil-water mixtures at room temperature, but also harsher liquids (including strong acids/alkali/hot water/ice water)-oil mixtures. Most importantly, such PDMS grafting reaction was reported to be wide-applicable for various kinds of inorganic particles.43 EXPERIMENTAL SECTION Materials PDMS ((C2H6OSi)n, Sylgard 184) was purchased from Dow Corning in USA. P25 (TiO2) were purchased from Deguess. SiO2 powders were obtained from Aladdin. Stainless Steel (SS) mesh with 1000 mesh, glass slide (GS), cotton, sponge, and textile were purchased from local store. All the solvents used in this work were analytical grade and used as received. Methods The pretreatment of TiO2 and SiO2 powders In this part, PDMS acted as modifier, which helped the particles to achieve superhydrophobicity. The grafting reactions between PDMS and TiO2 or SiO2 powder were described as follows. Briefly, for TiO2, the TiO2 powders (50 mg) were first dispersed in THF (5 ml) by sonication for 30 min. Then 6 g PDMS precursor was added into the above dispersion and stirred until THF was evaporated thoroughly. The


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mixtures were then UV illuminated for 2 h (intensity of 20 mW/cm2). After the reaction was finished, the mixtures were rinsed with copious amounts of toluene solvent to remove the residual unreacted PDMS by centrifugation for 20 min, and this step was repeated for 10 times. Finally, the PDMS-Grafted-TiO2 (PDMS-G-TiO2) could be achieved. Similar with TiO2, after the solvent evaporated, the mixture of SiO2 and PDMS were heated in an oven at 180 °C for 12 h. Then after the unreacted PDMS were further removed, the PDMS-Grafted-SiO2 (PDMS-G-SiO2) particles could be obtained. Preparation of the semi-transparent coating The original TiO2 and SiO2 powders became superhydrophobic after grafted with PDMS brush, and they all dispersed well in non-polar solvents such as hexane. The preparation procedure for semi-transparent coating was described as follows: Mixtures including 0.1 g PDMS-G-SiO2, 0.005 g PDMS-G-TiO2, 0.06 g PDMS (acted as binder), and 0.006 g PDMS curing agent were dispersed into 15 mL hexane (the dispersion solution was labeled as MC), the ratio between the mixed functionalized particle (MFP) and PDMS was optimized in this paper. The dispersion was then ultrasonically treated for 20 min, and next stirred constantly for another 30 min. The obtained suspension was next spin-coated onto glass slide substrates. For other substrates such as SS mesh, sponge, textile, and cotton, spray-coating method (0.2 Mpa compressed air gas) was used. Finally, the coated substrates were dried at 60 °C for 1 h to evaporate the solvent completely. For other coatings, 0.1 g PDMS-GSiO2 particles were dispersed into 15 mL hexane, and were further coated onto substrates (labeled as PDMS-G-SiO2 coated surfaces). 0.1 g PDMS-G-SiO2 and 0.005 g PDMS-G-TiO2 were dispersed into 15 mL hexane, and were coated onto substrates (labeled as MFP coated surfaces). Characterization The surface morphology of all the samples were carefully examined by scanning electron microscopy (S4800, Hitachi, Japan) and TEM (FEI F20). The EDS composition spectra were obtained by an energy dispersive spectroscopy (EDS) appurtenance. The surface compositions were further carefully analyzed using an Xray photoelectron spectrum (XPS) experiment (250XI, Shimadzu, PHI). The contact angels (CAs) and sliding angles (SAs) in both air and under oil were measured by utilizing OCA 35 (DataPhysics, Germany) with a high speed video camera at room temperature (~28-30 ºC). The CAs and SAs were determined by measuring each


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sample at five diverse positions. Photodegradation processes of oil pollutants and organic dyes were conducted under a UV light (PLS SXE 300 with xenon lamp, Perfectlight). The distance between the light source and the sample was adjusted by the required intensity. The photographs and videos were captured by using a digital camera (Nikon D7200, Japan). The plasma experiments were treated by oxygen plasma instrument (DT-03, Suzhou OPS oxygen plasma technology) at a power of 60 W for 20 s. RESULTS AND DISCUSSION Fabrication strategy of MC and pretreatment of SiO2 and TiO2 particles In this paper, we used a fluorine-free, simple, and highly efficient method to modify inorganic oxide powders to realize superhydrophobicity. As shown in Figure 1, TiO2 particles mixed with PDMS prepolymer were UV illuminated for 2 h, PDMS brushes could be successfully covalently bonded with TiO2 particles. Therefore, the original hydrophilic TiO2 particles became superhydrophobic because of these around grafted PDMS brushes. The grafting mechanism can be briefly understood as follows: TiO2 particles produce electron-hole pairs under UV irradiation, which excite generation of hydroxyl groups and water molecules on the surface. The activated molecules partially cleave siloxane bonds of PDMS. These segmented siloxane-based chains form a covalent bond with TiO2 via a Ti-O-Si bond, therefore lead to the surround grafted PDMS brush (Figure S1).39 Since SiO2 powder could not show photoactivity to UV light, thermal treatment was used here instead. When mixture of SiO2 powder and PDMS prepolymer were heated at 180 °C for 12 h, PDMS grafted SiO2 (PDMS-G-SiO2) powder was obtained. Water molecules on SiO2 surfaces react

Figure 1: Schematic illustration for fabricating the superhydrophobic MC. TiO2 and SiO2 powders were first covalently grafted with PDMS brush by UV illumination and heating treatment, respectively. They became superhydrophobic after the reactions finished. Then the two kinds of superhydrophobic powders mixed together with PDMS with a proper ratio and its curing agent to form the coating. The coating was further spin-coated or spray-coated onto the substrates to prepare superhdyrophobic surfaces with multifunctions.


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Figure 2: (a, b) Wettability variation of TiO2 powder before and after grafted with PDMS brush. (c, d) Wettability variation of SiO2 powder before and after grafted with PDMS brush. Photographs for dispersions of mixed SiO2 and TiO2 before (e) and after grafting with PDMS (f) in hexane, and mixture of hexane and PDMS solutions. As can be seen, no matter in hexane or mixture of hexane and PDMS solutions, the untreated powder precipitated immediately, while the MFP well dispersed in both solutions. (g, h) TEM images for SiO2 and PDMS-G-SiO2 particle, respectively. (i, j) TEM images for TiO2 and PDMS-G-TiO2 particle, respectively. The surround PDMS layer for both the two kinds of particles can be clearly observed from the TEM images, indicating that PDMS brushes were successfully grafted onto powders’ surfaces.

with PDMS, and PDMS are further hydrolysis to form silanol groups, which condensate with surface silanol groups positioned on SiO2 surface, thus forming Si-OSi bonds covalently linked with PDMS and SiO2 particles (Figure S2).43 The two kinds of superhydrophobic particles were then further mixed with PDMS and its curing agent with a proper ratio to fabricate a coating solution, which could be coated onto various substrates. PDMS-G-SiO2 act as a transparent role, while PDMS-G-TiO2 help the coating photodecompose organic pollutants. In the MC, more PDMS-G-TiO2 powder would lead to decreased transparency since PDMS-G-TiO2 showed worse transparency than PDMS-G-SiO2. However, too few PDMS-G-TiO2 was believed to lead to bad photocatalytic effect. In order to keep transparency of the coating as much as possible, and simultaneously maintained the photocatalytic activity, we used 0.1 g PDMS-G-SiO2 and 0.005 g PDMS-G-TiO2 as the MFP. Both the

TiO2 and

SiO2 powders

were

originally

hydrophilic,

and

became

superhydrophobic after grafted with PDMS (Figure 2a-d). No matter in hexane, or mixture of hexane and PDMS solutions, the unmodified mixed particle (0.1 g SiO2


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and 0.005 g TiO2, labeled as MP) precipitated into bottom quickly (Figure 2e). However, the MFP well dispersed in both solutions (Figure 2f). The original SiO2 particle size was around 10~20 nm. There are an apparent PDMS brush layer appeared around PDMS-G-SiO2 particle, as demonstrated by TEM images shown in Figure 2g, h. In comparison with TEM images of TiO2 particles before and after grafted with PDMS (Figure 2i, j), an obvious PDMS brush layer with several nanometers can be also clearly observed from the high-magnified TEM image (Figure 2j). Besides, the XPS analyses (Figure S3b) also proved the new occurrence of Ti-OSi bonds after grafting reaction between PDMS and TiO2 particle. EDS data (Figure S3c, d) demonstrated the new occurrence of elements C for PDMS-G-SiO2, and C, Si for PDMS-G-TiO2. The above phenomena powerfully proved that the PDMS brush layer was successfully covalently linked with SiO2 and TiO2 particles. Fabrication of semi-transparent surfaces coated with MC

Figure 3: (a) Pictures for the coated and uncoated GS surfaces. The MC coated GS surfaces presented semi-transparency. (b, c) Low- and high-magnification SEM images for MC coated GS surface. (d) Relationship between CAs and SAs and the mass ratio of MFP and PDMS. Their mass ratios were optimized for achieving both high transparency and superhydrophobicity. (e) Transmittance spectra of coated GS surfaces by using various coatings.

Figure 3a was photographs of uncoated and MC coated GS surfaces. Compared with the hydrophilic transparent GS surface, the MC coated surface exhibited semitransparency and superhydrophobicity with water contact angle (WCA) of 156° and water sliding angle (WSA) of 4°. Figure 3b, c is low- and high-magnification SEM images for the MC coated GS surface. The coatings were homogenously deposited onto the coated GS surface. It is well known that PDMS has good chemical stability,


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the coated surfaces were demonstrated to not only show superior repellency to water drops, but also to HCl and NaOH solutions drops (all dyed with methyl blue) (Figure S4), and their CAs were 154° and 153°, respectively. Figure 3d is WCAs and WSAs versus mass ratio of MFP and PDMS (the MFP was set as a constant of 0.105 g here). It can be seen that when the mass ratio was less than 1.5, superhydrophobicity was not achieved. When the ratio is 1.5, although superhydrophobicity could be obtained, their WSAs were larger than 10°. Only when the mass ratio was around 1.7 or above, superhydrophobic surfaces with WSAs less than 10° were realized. The excessive PDMS composition would cover superhydrophobicity of the powders, thus resulting in hydrophobicity rather than superhydrophobicity. However, too less PDMS would lead to bad combination between particles and the substrates. In order to simultaneously

maximally

realize

multiple

superior

properties

such

as

superhydrophobicity, transparency, and photocatalysis of the coating, mass ratio between MFP and PDMS should be controlled. Figure 3e are the transmission spectra for the coated GS surfaces by using various coatings. Single PDMS-G-SiO2 or MFP coated GS surfaces achieved superior transparency almost the same as the pure glass. While the MC coated GS glass showed a decreased transparency of 75%, as shown in blue line in Figure 3e. It was demonstrated that the increasing PDMS concentration decreased the transparency of the GS surface (Figure S5). Characterization of photocatalytically stable superhydrophobic MC and its photocatalytical ability For superhydrophobic surfaces, photocatalytical function is very desirable since they induce oxidation or degradation of most organic molecules. However, for many photocatalytically

active

hydrophobic

materials,

when

the

photocatalysts

photodegraded the pollutants, the organic modifiers that used to realize hydrophobicity were often decomposed by light at the same time, which quickly resulted in losing hydrophobicity. Therefore, we tested the stability of the photocatalytic active superhydrophobic coatings towards strong UV light with an intensity of around 80 mW cm-2. As shown by pictures in Figure 4a, the MFP coated GS surface were original superhydrophobic with a WCA of 155°, and it kept stable WCA of 154° even UV irradiated for 36 h. This phenomenon demonstrated the robust anti-UV irradiation ability of the two PDMS grafted particles. Because PDMS was covalently bonded with particle, rendering the surface good stability towards UV


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Figure 4: (a) The MFP coated GS surface was original superhydrophobic with WCA of 154°, and the surface still maintain superhydrophobicity with WCA of 155° after UV illumination with strong intensity of 80 mW/cm2 for as long as 36 h. Insets in the picture were their corresponding WCAs. (b) The MC coated GS surfaces could also keep stable superhydrophobicity without change under the same UV illumination condition even as long as 36 h. The WCA has no big change (insets in pictures). (c) Relationship between WCAs of the MC coated GS surface and UV illumination time. The surface kept stable WCA value more than 150° even after UV irradiation for 36 h. The inset also indicated that the MFP coated GS surfaces retained stable superhydrophobicity after UV illumination for various time as long as 36 h. (d) The IR spectra for MFP coating before and after UV illumination for 36 h. (e) IR spectra of the MC before and after UV illumination for 36 h. The spectra clearly proved that no big change occurred for MFP and MC after the long-time UV illumination. The UV intensity was all 80 mW/cm2 in the above conditions.

irradiation. It was also demonstrated that the MC coated GS surface remained stable superhydrophobicity even after exposure to UV light (also intensity of around 80 mW cm-2) for as long as 36 h. Figure 4c are relationship between WCAs for MC and UV irradiation time, while its inset is that of the MFP coated surface, which further proved that the two coatings were always maintained stable superhydrophobicity during various UV irradiation time. We then examined IR spectra of MFP coating before and after UV exposure for 36 h, and the results were shown in Figure 4d. As can be seen, even after 36 h UV exposure, the strength of organic groups of -CH3 peaked at around 2860 cm-1 and 2921 cm-1 did not change a lot, suggesting that the PDMS-G-TiO2 particle in the MFP or MC coating did not photodegrade these organic groups of PDMS brushes, which was the main groups contributed to superhydrophobicity. This could account for stable superhydrophobicity of MC after


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UV illumination. IR spectra for MC before and after UV illumination for 36 h were presented in Figure 4e. It can be seen that those organic groups were also still remained, and no obvious weak tendency can be observed, which was also the main reason for superior stable superhydrophobicity of the coating. Pictures of Figure S6, S7 further demonstrated the stable superhydrophobicity of the MFP and MC coated GS surfaces under various exposure time. In addition, we found that the MC and MFP presented photocatalytically stable superhydrophobicity even when exposed to UV light for as long as 120 h (Figure S8). The MC coated GS surfaces were found to show very excellent anti-oil pollutant ability. As presented in Figure 5a, the surface changed from superhydrophobicity with WCA 156° to hydrophobicity of 106° after spin-coated by a layer of dodecane.

Figure 5: (a) Photodecomposition process of oil pollutants onto the MC coated GS surfaces. The original surface became hydrophilic after spin-coated with dodecane layer. However, the surfaces all recovered superhydrophobicity quickly after UV illumination with an intensity of 80 mW/cm2. One of the compositions PDMS-G-TiO2 in the MC effectively degraded these oil pollutants, and thus regained superhydrophobicity. (b1-b3) Corresponding pictures for each process of hexadecane polluted MC coated GS surface. (c1-c3) Similar phenomenon occurred for the hexadecane polluted surface. (d) IR spectra for the original MC, the polluted coating by dodecane, and the recovered coating after UV illumination. (e) Variations of CAs on MC coated GS surface via dodecane pollution and UV illumination for as long as 32 cycles. The surface all recovered superhydrophobicity after recycled polluted the surface and UV illumination.


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However, it recovered its superhydrophobicity with WCA of 155° after UV illuminated for only 30 min (the intensity was 80 mW/cm2 here). Figure 5b1-b3 were the corresponding pictures for each process of Figure 5a. Similar results occurred for hexadecane pollution (Figure 5c1-c3). IR spectra for each step were measured and the results were presented in Figure 5d. As can be seen, groups of -CH2, -CH3 peaked at 2924 cm-1, 2860 cm-1 obviously became very strong after coated by dodecane, and these groups greatly decreased to original after UV irradiation. PDMS-G-TiO2 existed in MC highly efficiently degraded the oil pollutants, and thus making superhydrophobicity recovered. Such process could be recycled for at least 32 times, as presented in Figure 5e. All the recovered WCA values almost have no big change in comparison with the original one.

Figure 6: Photodegraded process for various MC coated substrates. (a1-a4) The MC coated GS surface recovered its superhydrophobicity after contaminated by the dyed hexane. (the hexane solvent was dyed by Sudan). The insets for the original and recovered CAs demonstrated no big change. (b1-b4) The MC coated sponge became red after absorbed many dyed oils. These dyed oils were photodecomposed quickly, and the surface recovered white and could reabsorb oils from water again. (c1-c4) The MC coated cotton surfaces became red after absorbed many dyed oils. These dyed oils were photodecomposed quickly, and the MC coated cotton surface recovered white and could reabsorb dyed oils from water again. (d1-d4) Similar with the MC coated cotton surface, MC coated textile also presented recovered superhydrophobicity and reabsorbed oil dyes.


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In order to further evaluate the photodegradation capability of the MC, several severer tests were applied. As shown in Figure 6, MC was coated onto various kinds of substrates such as GS, sponge, cotton, and textile. The coated sponge, cotton, textile, all became superhydrophobicity (Figure S9). The structure of the untreated sponge and textile were very smooth; however, they became very rough after coated with MC, which thus rendering the surfaces superhydrophobicity (Figure S9). The coated GS surface was purposely contaminated by dyed hexane (dyed by sudan), these sudan dyes left on surface, and the surface became red in color. The sudan dyes were demonstrated to be gradually faded with increasing UV illumination time (Figure 6a1-a4), and finally the surface turned its original color again, and superhydrophobicity was totally recovered. The MC coated sponge was proved to effectively and quickly adsorb the dyed hexane from water due to its superoleophilicity. Hexane evaporated quickly, and large sudan dyes left on sponge. Therefore, the surface color changed to red. It was expected that the coated sponge surface quickly regained its white color after UV illumination (Figure 6b1-b4), and superhydrophobicity were still be reserved. The sponge surface could be recycled used to adsorb oil pollutants (Figure 6b4). The coated cotton surface was also degraded to turn white after absorbed many dyes (Figure 6c1-c4). Textile coated with MC also presented similar process. The sudan dyes on surface were photodecomposed while superhydrophobicity was still be retained (Figure 6d1-d4). All the above phenomena strongly proved that the coating could simultaneously possess photocatalytic activity and stable superhydrophobicity. PDMS-G-TiO2 in MC efficiently degraded dyes of these contaminated surfaces, which allowed their recycle use. Formation of slippery surfaces and their properties When dodecane was spin-coated onto the MC coated GS surface, the surface became very transparent and extremely slippery. As indicated in Figure 7a-d and Video 1, the slippery surfaces not only repelled water drops at room temperature quickly, but also strong acid, strong alkali solutions, and very hot water. More importantly, the surface even could repel very corrosive liquids of aqua regia (Video 2). The surface also showed good self-cleaning property as well as superhydrophobic surfaces (Figure 7e-h and Video 3). CAs for the above-mentioned liquids onto the slippery surfaces were shown in Figure S10, and all CAs were less than 150°. The


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Figure 7: The surface was very slippery after coated with dodecane, which show superior repellency towards various liquids include hot water (a), HCl (b), NaOH (c), and very corrosive liquids of aqua regia (d). (e-h) Self-cleaning process for the slippery surface. Numerous dirts were quickly brought by water drops, left a clean surface.

above phenomena and results suggested that the slippery surfaces showed superior liquid repellency, which was believed to broaden application scope of superhydrophobic surfaces and provide inspiration for developing more novel functional materials. Self-healing and underoil superhydrophobic performances of MC Functional materials with self-healing superhydrophobicity are very desired since this property can prolong the lifetime of superhydrophobic surfaces. The MC coated surface in this paper showed such performance. As demonstrated in Figure 8a, the superhydrophobic MC turned into superhydrophilicity with WCA around 8° after oxygen plasma treatment. The introduction of polar groups by plasma treatment leaded to surface hydrophilicity. However, after the treated samples were heated at 80 °C for 20 min, its wettability was self-healed to superhydrophobicity. Such process could be repeated at least 8 times without reducing the superhydrophobicity (Figure 8b), which proved the strong self-healed ability of the MC. Previous articles have reported the self-recovery hydrophobicity for PDMS.44-45 Therefore, this may be due to the rotation and self-recovery of PDMS brush.


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Figure 8: Self-healing process of MC after oxygen plasma treated. (a) Superhydrophobic MC was changed to superhydrophilicity with WCA around 8° after plasma treated. The treated surface selfrecovered its superhydrophobicity after heated at 80 °C for only 20 min. (b) Variation of WCAs of water drops on MC coated GS surfaces through oxygen plasma treatment and heated treatment for at least 8 cycles.

Underoil

superhydrophobicity

is

another

attracting

property

for

superhydrophobic surfaces. Such surfaces show anti-resistance to water both in air and oil environment. In this paper, the MC also exhibited fascinating and stable underoil superhydrophobicity. Various oils were used to examine such performance. As indicated in Figure S11, the underoil (oils include chloroform, dichloromethane, dodecane, hexane, petroleum ether, isooctane) WCAs were all larger than 150°, with a highest underoil WCA of 162°. As presented in Video 4, water drops slid the MC surface from air to oil, which always kept marble shape during the whole process. The surface also presented excellent underoil self-cleaning performance (Video 5). Large amounts of dirt were purposely putted onto part of the surface that positioned underoil. The continuous water could bring this dirt quickly, finally left a very clean surface. The underoil superhydrophobicity of the surface was tested under severer conditions. As shown in Video 6, the surface even presented superior self-cleaning process under very hot dodecane environment. The temperature of the hot plate was continuous increasing, which made the dodecane very hot. Moreover, it could also repel strong acid, strong alkali solutions under very hot dodecane (Video 7). The above evaluation tests powerfully indicated that the surface own very stable underoil superhydrophobicity. In order to eliminate the effect of PDMS role in MC, we tested the underoil superhydrophobicity of MFP coated surfaces. As shown in Figure S12, the MFP coated GS surface also showed excellent underoil superhydrophobicity.


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Those grafted PDMS brush existed on the particles’ surface could also make the surface achieve underoil superhydrophobicity. Oil-water separation by MC coated SS mesh surfaces

Figure 9: (a1-a3) Separation of water (dyed by methyl blue) and chloroform (dyed by sudan) mixtures by MC coated SS mesh. (b1-b3) Separation of NaOH solution (dyed by methyl blue) and chloroform (dyed by sudan) mixtures. (c1-c3) Separation of HCl solution (dyed by methyl blue) and chloroform (dyed by sudan) mixtures. (d1-d3) Separation of ice water and chloroform (dyed by sudan) mixtures.

Figure 10: (a) The oil/water separation efficiency as a function of the recycled used numbers. Taking chloroform-water mixture as an example here. The coated mesh achieved high separation efficiency, and even higher than 98% when it was repeatedly used for 50 times. (b) Underoil (chloroform) CAs for various liquids on MC coated surface. (c) Separation efficiency towards various harsh liquids/chloroform mixtures.

As

above

mentioned,

the

MC

presented

superhydrophobicity

and

superoleophilicity. Based on such wettability difference, we can use the MC coated SS mesh to separate oil-water mixtures. As shown in Figure 9a1-a3 and Video 8, when the mixture of chloroform (dyed by sudan)-water (dyed by methyl blue) mixtures were poured onto the coated SS mesh, chloroform quickly infiltrated through the


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coated mesh and was collected in the bottom beaker, while water was blocked by the mesh and stayed in the upper glass tube. No water was visible in the bottom oil beaker. Therefore, oil-water mixture was successfully and quickly separated. The coated SS mesh could be repeatedly used for separating oil-water mixtures. Besides water under room temperature, other harsher liquids-oil mixtures could be also separated by the MC coated SS mesh surfaces. Figure 9b1-d3 are NaOH (dyed by methyl blue)chloroform (dyed by sudan) mixture (Figure 9b1-b3), HCl (dyed by methyl blue)chloroform (dyed by sudan) mixture (Figure 9c1-c3), ice water-chloroform (dyed by sudan) mixture (Figure 9d1-d3) separation processes in sequence, and their corresponding videos were also recorded (Videos 9-11). It can be clearly observed that all the separation processes were fast. No HCl, NaOH, or ice water drops were visible in their bottom collected chloroform, indicating the high separation efficiency of the coated SS mesh. Figure 10a was the relationship between separation efficiency and number of cycles. The separation efficiency was reached higher than 98% even repeatedly used for as many as 50 times, which indicated that the surface owned stable wettability. Figure 10b was underoil (chloroform) wettability towards various liquids. As can be seen, NaOH, HCl, NaCl, ice water, and hot water all obtained CAs higher than 150° under chloroform environment. The separation efficiency to various liquids were summarized in Figure 10c. Each liquid achieved very high separation efficiency. The MC coated SS mesh could be also recycled used to separate these harsher liquids-oil mixtures. The PDMS strongly bind the nanoparticles onto the mesh, which gives rise to its high efficiency and reused performances. CONCLUSIONS In conclusions, this paper fabricated multifunctional coatings by using a fluorinefree, cost-effective, wide-applicable, and easy-implemental method. Both the TiO2 and SiO2 particles were grafted with PDMS brush, and became superhydrophobicity. The PDMS grafted particles well dispersed into non-polar solvents. The glass coated with the coating was translucent with transmission of 75%. It showed photocatalytically stable superhydrophobicity when exposed to UV light even with strong intensity of 80 mW/cm2 for as long as 36 h. Taking advantage of the photocatalytic activity of PDMS grafted TiO2, the MC coated surface highly efficiently photodegraded the oil layer and organic dyes polluted the surface, while its superhydrophobicity was still be reserved, which made the coated surfaces repeatedly


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used. The coating coated surface formed very slippery surfaces when coated with a layer of low-surface-tension oil, and showed superior liquid-repellency towards strong acid/alkali/hot water, and self-cleaning process. In addition, MC showed selfrecovered superhydrophobicity after repeatedly oxygen plasma treatments. Moreover, MC also exhibited extremely stable underoil superhydrophobicity to various liquids include strong acid/alkali/hot water and self-cleaning properties even under scalding oil environment. SS mesh coated with MC could be used to highly efficiently separate oil-liquids (including water at room temperature, strong acid, strong alkali, ice water, and hot water) mixtures. This investigation indicated that this PDMS grafted SiO2/TiO2@PDMS coating with multiple superior properties is believed to have very promising commercial applications. ASSOCIATED CONTENT Supporting Information Figures showing grafting mechanism between PDMS and TiO2, SiO2 particles; XPS and EDS analyses of PDMS-G-TiO2 and PDMS-G-SiO2; Repellency to acid and alkali solution of MC; The effect of ratio of PDMS and MFP on transparency of GS; Photographs of MFP and MC showing water repellency when UV-irradiated for various time; Relationship between WCAs for MC and MFP coating and various UV irradiation time; Various substrates coated with MC presented superhydrophobicity; CAs for various liquids positioned onto slippery surfaces; Underoil CAs for MC coated GS surfaces under various oils; Underoil CAs for MFP coated GS surfaces under various oils; Video S1 recorded the repellence towards various liquids of the slippery surface; Video S2 recorded the repellence to aqua regia by the slippery surface; Video S3 recorded the self-cleaning process of the slippery surface; Video S4 recorded the underoil superhydrophobicity of MC at room temperature; Video S5 recorded the underoil self-cleaning process of MC; Video S6 recorded the underoil self-cleaning process of MC at high temperature; Video S7 recorded the underoil superhydrophobicity at high temperature; Video S8 recorded the separation of water and chloroform process; Video S9 recorded the separation of strong acid solution and chloroform process; Video S10 recorded the separation of strong alkali solution and chloroform process; Video S11 recorded the separation of ice water and chloroform process.


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AUTOR INFORMATION Corresponding author *Phone: (+86)15013038214

E-mail: [email protected]

E-mail: [email protected] ORCID Shan Peng:0000-0002-6124-4151 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS Financial supports by the Natural Science Foundation of Youth Fund Project of China (201804030) and the research project of “one province one university”: Hebei University (801260201238) are gratefully acknowledged. REFERENCES (1) Lai, Y.; Tang, Y.; Gong, J.; Gong, D.; Chi, L.; Lin, C.; Chen, Z. Transparent superhydrophobic/superhydrophilic TiO2-based coatings for self-cleaning and antifogging. J. Mater. Chem. 2012, 22, 7420-7426. (2) Liu, K. S.; Jiang, L. Bio-inspired self-cleaning surfaces. Annu. Rev. Mater. Res. 2012, 42, 231-263. (3) Min, W. L.; Jiang, B.; Jiang, P. Bioinspired self-cleaning antireflection coatings. Adv. Mater. 2008, 20, 3914-3918. (4) Xue, C. H.; Fan, Q. Q.; Guo, X. J.; An, Q. F.; Jia, S. T. Fabrication of superhydrophobic cotton fabrics by grafting of POSS-based polymers on fibers. Appl. Surf. Sci. 2019, 465, 241-248. (5) Li, J.; Kang, R.; Tang, X.; She, H.; Yang, Y.; Zha, F. Superhydrophobic meshes that can repel hot water and strong corrosive liquids used for efficient gravity-driven oil/water separation. Nanoscale 2016, 8, 7638-7645. (6) Li, J.; Xu, C.; Zhang, Y.; Wang, R.; Zha, F.; She, H. Robust superhydrophobic attapulgite coated polyurethane sponge for efficient immiscible oil/water mixture and emulsion separation. J. Mater. Chem. A 2016, 4, 15546-15553. (7) Zhou, C.; Chen, Z.; Yang, H.; Hou, K.; Zeng, X.; Zheng, Y.; Cheng, J. Natureinspired strategy toward superhydrophobic fabrics for versatile oil/water separation. ACS Appl. Mater. Interfaces 2017, 9, 9184-9194. (8) Long, M.; Peng, S.; Deng, W.; Miao, X.; Wen, N.; Zhou, Q.; Deng, W. Highly efficient separation of surfactant stabilized water-in-oil emulsion based on surface energy gradient and flame retardancy. J. Colloid Interface. Sci. 2018, 520, 1-10. (9) Ren, G.; Song, Y.; Li, X.; Zhou, Y.; Zhang, Z.; Zhu, X. A superhydrophobic copper mesh as an advanced platform for oil-water separation. Appl. Surf. Sci. 2018, 428, 520-525. (10) Wang, Y.; Xue, J.; Wang, Q.; Chen, Q.; Ding, J. Verification of icephobic/antiicing properties of a superhydrophobic surface. ACS Appl. Mater. Interfaces 2013, 5, 3370-3381.


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(45) Eddington, D. T.; Puccinelli, J. P.; Beebe, D. J. Thermal aging and reduced hydrophobic recovery of polydimethylsiloxane. Sens. Actuators, B 2006, 114, 170172.


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Table of contents


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PDMS-Grafted-SiO2/TiO2@PDMS

coating

was

Page 26 of 26

fabricated

by

mixed

two

kinds

of

PDMS-Grafted-SiO2/TiO2 particles and polymer PDMS and its curing agent. Hydrophilic SiO2/TiO2 particles were grafted with PDMS brush through heating and UV-illumination treatments, respectively. The substrates coated with this coating showed multifunctions include semi-transparency (75% transmission), superhydrophobicity, self-cleaning, photocatalytical activity, underoil superhydrophobicity, self-healable, oil-water separation. The coated surface presented extremely stable superhydrophobicity even when exposed to UV with high intensity of 80 mW/cm2 for as long as 120 h. A very slippery surface formed when the coating was coated with a layer of low-surface-tension oil. The coating showed self-healable superhydrophobicity after recycled oxygen plasma treatment. In addition, the coated stainless steel mesh surface could highly efficiently separate oil-water (also strong acid/alkali/hot water/ice water) mixtures. Most importantly, in this paper, the method used to modify inorganic particle to obtain hydrophobicity was wide-applicable, without considering the particle’s size, morphology, or crystal form, etc.


Photocatalytically Stable Superhydrophobic and Translucent Coatings

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