Superhydrophobic Materials With Good Oil/Water Separation And Self-Cleaning Prepared Through A Environment-Friendly And Two-Component Method

A novel, versatile, environment-friendly, and economical method was developed to fabricate functional superhydrophobic surfaces on various substrates, including wood, bamboo, cotton, lter paper, sponge, glass, textile, and copper. This method involves synthesizing a two-component modier solution consisting of SiO 2 nanoparticles combination with poly(methylhydrogen)siloxane (PMHS) modication. The superhydrophobicity of the coated surfaces was created by PMHS combined with SiO 2 nanoparticles to construct a rough hierarchical structure on the substrate surface. As a result, all superhydrophobic surfaces were maintained under an indoor environment and relative humidity (RH) of 50% for 30 days. Furthermore, the superhydrophobic surfaces were also maintained at environmental conditions of minus 20 ℃ for 24 hours. It was also conrmed that these surfaces exhibited excellent self-cleaning, oil/water separation, and elimination of underwater oil properties. The method for fabricating superhydrophobic materials proposed in this study will have great application potential in preparing large-scale superhydrophobic surfaces for use in ancient building protection.


Introduction
Cellulose-based and metal materials are easy to absorb water and moisture. Therefore, their service life is dramatically affected, which leads to a serious waste of resources. Wood plays a vital role in modern society. Its numerous advantages (light weight, high strength, easy machinability) have been widely used in varied daily applications such as indoor and outdoor decoration, wooden bridge, pavement and so on (Lu et al. 2014;Poaty et al. 2013). However, due to cellulose and hemicelluloses' hydrophilic property, wood is sensitive to humidity changes, making it dimensionally unstable, susceptible to decay and degradation (Okon et al. 2018; Han et al. 2018). Like the inherent hydrophilic properties of wood, other cellulose-based materials such as paper, cotton, fabric, and bamboo commonly seen in daily human life also have problems such as easy degradation and decay (Miao et al. 2010;Hu et al. 2009). Nowadays, metal materials have been widely used in aerospace, energy, mechanical equipment, and other elds (Mikhail et al. 2016;Qiu et al. 2014). However, the corrosion resistance of metal materials is generally weak. It is easy to be affected by the humid environment, causing signi cant performance degradation and shortening its service life.
Consequently, there is an urgent need for effective protection of metals, cellulose-based, and other materials to enhance their corrosion resistance, durability, and water repellency, thereby extending their service life and expanding their application areas. Bionic construction of hydrophobic or superhydrophobic coatings on the surface of cellulose-based or metal materials is one of the most commonly used protection methods because these coatings can effectively reduce the damage caused by water/moisture to the material ( (Daniello et al. 2009; Lee and Kim 2011), etc. Superhydrophobic surfaces are those with water contact angles (WCAs) larger than 150º and sliding angles (SAs) below 10º (Feng et al. 2002). Inspired by the lotus effect, a typical process to construct a superhydrophobic surface usually goes through two steps: (1) creation of a suitable roughness on material surfaces and (2) chemical modi cation of rough surfaces with low surface energy materials (Wang et al. 2009;Feng et al. 2002). , and so on. However, these ways face serious industrialization challenges because of light opaqucity of the coating, requiring a tedious and time-consuming process, harsh conditions and specialized equipment, and environmental pollution (toxic solvent or modi er). For example, hydrothermal and plasma treatment are only applicable to the small surfaces and chemical vapor deposition only applies to special materials. Therefore, seeking an e cient, environment-friendly, lowcost, and scalable approach is the current research focus.
This work reports a simple, inexpensive, environment-friendly, but versatile method to fabricate

Materials
SiO 2 nanoparticle (fumed silica, QS25) with a diameter of about 10 nm was purchased from Tokuyama Chemicals (Zhejiang) Co., Ltd. Ethanol (analytical grade) was purchased from Tianjin Zhiyuan Chemical Reagent Co., Ltd. PMHS with a content of active hydrogen (Si-H) of 1.5% and Kastredt catalyst (platinum-1,3-divinyl-1,1,3,3-tetramethyldisiloxane) with Pt content of 2000 ppm were purchased from Chengguang Research Institute of Chemical Industry (Chengdu, China). Inhibitor (1-ethynyl-Cyclohexanol) was purchased from Shenzhen OSBANG New Material Co., Ltd. The wood samples of Chinese Cunninghamia lanceolata for this study (obtained from Fujian, China) were cut parallel to grain direction and sawn into blocks of 20 mm × 20 mm × 12 mm. Bamboo was produced by China Resources Bamboo Co., Ltd., Zhejiang, China. Cotton, sponge, life liquids, and other substrate were purchased from the native market. All chemicals in this work were used as received without further puri cation.
Preparation of two-component modi er solution

Fabrication of superhydrophobic surfaces on various substrates
The superhydrophobic surfaces can be prepared on porous, nonporous, and homogeneous materials by simply brushing, spraying, or dipping into the two-component modi er solution. In this work, the various substrate (wood, bamboo, cotton, lter paper, sponge, glass, copper, textile) were dipped into the twocomponent modi er solution for 5 minutes, and then air-dried for 1 minute. This procedure was repeated for 3 times. Finally, the modi ed substrate was dried at 80°C for 1 hour.

Characterization
The surface morphology and microstructure of the samples were characterized using a scanning electron microscope (SEM, ZEISS Z500). Prior to SEM observation, the conductivity of the samples was improved by sputtering a thin layer of gold lm on all samples. The static contact angle (CA) and sliding angle (SA) of water and other liquids (coffee, milk, soy sauce, juice, and beer) were measured on a commercial contact angle meter (HARKE-SPCA-1, Beijing) at room temperature. The volume of all liquids was 5 µl when the contact angles were measured. The SA was measured by recording the tilt angle of the sample platform at which a droplet of liquids (10 µl) starts to roll off the surface. The nal CA and SA were calculated by averaging ve different positions on each sample.

Results And Discussion
Surface morphology and wettability SEM investigations allowed us to determine the microstructural morphology of substrates before and after surface modi cation. The morphology of the superhydrophobic surfaces of wood, glass, metal, and other cellulose-based materials is shown in Fig. 1. The original wood and bamboo surface was smooth and contained a few vessels. The unmodi ed lter paper and cotton was composed of smooth crosslinked bers. The SEM image shows that the untreated glass and copper surfaces were smooth without any feature. On the contrary, after modi cation by the two-component modi er solution, almost all treated substrate surfaces possess a rough structure characterized by a random distribution of particles having various sizes and shapes.
Moreover, after coating, all treated substrate surfaces had a WCA greater than 150º (spherical water droplets were standing on the treated substrate surfaces) and SA less than 10º (as shown in Table 1). It was indicated that superhydrophobicity was successfully obtained. Based on the analysis mentioned above, it can be con rmed that the two-component modi er solution can create similar hierarchical structures with a high WCA on different types of substrates regardless of their morphologies or sizes.
Also, the treated substrate surfaces exhibited superhydrophobicity against water and against common liquids, including milk, coffee, juice, soy sauce, and beer. Figure 2 shows the relation between the liquids and liquid repellency of coatings on various substrates (The substrates included wood, bamboo, cotton, lter paper, sponge, glass, textile, and copper). The coatings' contact angles on different substrates were larger than 150º for milk, coffee, juice, soy sauce, and beer. All the liquid droplets had a spherical shape and were well supported by the substrate surface treated with the two-component modi er solution, indicating good repellency towards these common liquids. The repellency towards common liquids implies the excellent anti-fouling potential of these superhydrophobic materials.

Durability of the coated superhydrophobic surfaces
To be used in real life, the superhydrophobic surface should withstand a certain degree of damage. In this study, we conducted a comprehensive and systematic study on the prepared superhydrophobic surfaces' mechanical stability and durability.
In the past, most of the evaluation of the durability of superhydrophobic surfaces was carried out by immersing superhydrophobic materials in water (Lin et al. 2020). Nevertheless, few papers have explored the durability of superhydrophobic surfaces against moisture. Liquid water is larger than the pores formed in the rough structure, whereas moisture is small enough to penetrate the pores and attack the rough structure ). In other words, the superhydrophobic surface only shows high hydrophobicity to liquid water, but cannot prevent the penetration of moisture. To assess the superhydrophobic surfaces' durability to moisture, eight types of superhydrophobic materials were investigated under an indoor environment, and a relative humidity (RH) of 50% for 30 days. Figure 3 shows the variation of the WCA and SA in an indoor environment. The WCA and SA almost retained their original states, indicating long-term durability in an indoor environment. Therefore, these superhydrophobic surfaces on various substrates exhibit long-term durability in an indoor environment. Hence, these superhydrophobic materials can be well applied in indoor environments.
To evaluate the mechanical stability of the superhydrophobic surfaces, a sandpaper abrasion test was performed on the superhydrophobic surface according to the previously reported methods (Chang et al. Fig. 4, eight types of superhydrophobic materials were rubbed with sandpaper (1500 mesh) by loading 10 g weights on the sample. The WCA and SA value of the resultant surfaces were measured at every 20 cm of abrasion length. Figure 4 shows the waterrepellent ability of various superhydrophobic materials after 140 cm of abrasion. The results are as follows. Superhydrophobic wood after severe abrasion, remains superhydrophobic with a WCA greater than 150º and an SA less than 10º, indicating excellent mechanical robustness against sandpaper abrasion. The good mechanical stability of the superhydrophobic wood surface can be attributed to the su cient thickness of the superhydrophobic coating to withstand the abrasion and the hydroxyl group on the surface of the wood, which forms a covalent bond with the -Si-H on the PMHS chains. Similar to wood substrates, the superhydrophobic coating on the surface of bamboo also had good mechanical stability. However, superhydrophobic materials, including cotton, lter paper, sponge, glass, textile and copper, could not maintain their original superhydrophobicity with decreased CAs of ~ 115º and SAs of ~ 60º, which may result from their low speci c surface area and inability to form a covalent bond.

2015; Tu et al. 2018; Jia et al. 2018). As shown in
Apart from the sandpaper abrasion test, water immersion test and anti-freezing experiments were also conducted on superhydrophobic materials. Taking wood as the test sample, a anti-freezing test and water immersion test were carried out to further assess superhydrophobic surfaces (as shown in Fig. 5a-b). Brie y, Place the superhydrophobic wood in a low-temperature (minus 20 ℃) environment or immersion the superhydrophobic wood in water for a certain period of time (0 ~ 24 hours) to characterize the change in hydrophobicity. WCA and SA were measured every 3 hours. Figure 5a-b shows the variation of the WCA and SA after water immersion and low-temperature treatment. After 24 hours of water immersion and freezing, the WCA and SA almost retained their original states, indicating long-term durability under water or low-temperature environments. In addition, because many superhydrophobic surfaces easily lose their superhydrophobic properties in hot water (Li et al. 2016), water droplets with different temperature were used to study the hot water superhydrophobicity of the as-prepared surface (Fig. 5c). It can be clearly seen that even if water drops with a temperature close to 100 ℃ are dropped on the surface of the modi ed wood, it still has good water-repellency, and the water drops are spherical. In short, these superhydrophobic materials can be well applied in extreme environments.

The investigation of formation mechanism
Superhydrophobic surfaces were successfully synthesized via a novel method proposed in this paper. The formation mechanism of the superhydrophobic surface is described herein (illustration shown in contain a large number of hydroxyl groups, so It can combine with PMHS to improve their hydrophobic properties. However, because the dispersant (absolute ethanol) used in the experiment also contains hydroxyl groups, it will also react with the hydrophobic modi er PMHS, which affects the reaction between PMHS and SiO 2 and the base material, which will lead to the modi er solution failure. Therefore, an inhibitor was introduced in the experiment, which can well avoid the modi er solution's failure. As shown in Fig. 6, when the substrates are dried, the reaction between PMHS and SiO 2 , substrates, and the dispersant absolute ethanol is delayed due to the inhibitor's presence (as a ligand, the inhibitor will coordinate with the platinum atom in the catalyst and hinder the contact between the platinum atom and the reactant). During the drying process, the absolute ethanol gradually evaporates. After the absolute ethanol was completely evaporated and the inhibitor no longer had an inhibitory effect, the PMHS on the substrate surface reacted with SiO 2 or the substrate to form a rough hierarchical structure (low surface energy materials combined with rough nanomaterials). Therefore, a superhydrophobic coating can be constructed on the surface of various substrates.
The application of superhydrophobic surfaces Self-cleaning. Superhydrophobic surfaces with low adhesion to water will have good self-cleaning properties (Zhou et al. 2016). Water has a very low adhesion force on the superhydrophobic surface. The water droplets rolling on the superhydrophobic surface can take away the contaminants accumulated on the surface, mimicking the lotus leaf effect. Therefore, it can be considered that the contaminants accumulated on the superhydrophobic surface can be easily removed. Figure 7 shows the self-cleaning effect of lter paper and wood surface without and with superhydrophobic coating. Both lter paper and wood surfaces were contaminated by methyl blue powder. For modi ed lter paper and wood surface, as the water was dropping to the surface, the water droplets readily rolled off, removing the methyl blue powder and resulting in a dry and clean surface ( Fig. 7(a 5 -a 8 and b 5 -b 8 )). On the contrary, water droplets merged with the methyl blue powder and stuck to the original lter paper and wood surface, leading to a wet and polluted surface ( Fig. 7(a 1 -a 4 and b 1 -b 4 )). As shown in Movie S1 and Movie S2, methyl blue powder on the superhydrophobic lter paper and the wood surface can be rapidly and completely removed by spraying water onto it. In contrast, the methyl blue powder cannot be completely removed from the surface of original lter paper and wood.
Oil-water separation. The e cient separation of oil-water mixtures is a long-term problem and challenging environmental pollution such as crude oil leakage in the ocean from industrial production . Materials with special wettability, including superhydrophobic and superoleophilic properties, have been widely used in oil-water separations (Xue et al. 2011). Herein, the modi ed lter paper demonstrated superhydrophobic-super-oleophilic properties. So it was used to demonstrate potential applications for oil-water separation. As shown in Fig. 8, the oil-water separation experiment was carried out using lter equipment and superhydrophobic lter paper serving as lter elements. When the mixture of diesel and water is poured onto the superhydrophobic lter paper, the diesel rapidly diffuses and penetrates through the lter paper and falls into the beaker below. On the contrary, the water remains on the lter paper. In this manner, the effective separation of oil-water mixture was achieved. The separation e ciency was higher than 97%.
Finally, through a 20 cycles of oil-water separation test, the modi ed lter paper still has good oil-water separation performance, which also shows good durability. It also means that the as-prepared ltration material can be repeatedly used to separate oil-water mixtures.
Elimination of underwater oil. Herein, the superhydrophobic cotton was assessed for the elimination of underwater oil droplet. Figure 9 shows an oil red-dyed CCl 4 droplet resting at the bottom of the water.
When the superhydrophobic cotton was brought into contact with the water, its whole surface turned to silver-like. Furthermore, once the modi ed cotton contacted the oil droplet, part of the CCl 4 was sucked into the cotton due to the oleophilic effect. The longer the contact, the more oil is loaded. Finally, when the cotton was taken out of the beaker, the water was cleaned up with no trace of dyed oil. Consequently, it can be concluded that superhydrophobic materials can be well applied in the eld of oil-water separation, which can effectively solve the environmental pollution caused by oil-water mixing and the problem of oilcontaminated water in industrial production.

Conclusions
In summary, a novel, environment-friendly, and versatile method to fabricate superhydrophobic surfaces using inorganic nanoparticles SiO 2 combined with PMHS was developed. It was con rmed that the superhydrophobic surfaces were fabricated on a variety of substrates, including wood, bamboo, cotton, lter paper, sponge, glass, textile, and copper. All superhydrophobic surfaces were endowed with a high WCA larger than 150º and a low water SA less than 10º. The superhydrophobicity originated from its rough and hierarchical structures formed by SiO 2 combined with PMHS. In addition, these superhydrophobic surfaces exhibited good long-term durability, self-cleaning, oil/water separation properties, and elimination of underwater oil. Because this modi cation method can easily be used to prepare multifunctional superhydrophobic materials, it will have a good application prospect in the preparation of bionic materials.  Wettability behavior of superhydrophobic surfaces exposed to milk, coffee, juice, soy sauce, and beer.     Illustration of the formation mechanism from hydrophilic to superhydrophobic states on various substrates.

Figure 8
Schematic of the superhydrophobic material used to separate an oil-water mixture.

Figure 9
Elimination of underwater oil droplet (CCl4) by superhydrophobic cotton; the CCl4 is dyed red.

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