A self-cleaning underwater superoleophobic mesh for oil-water separation

Oil–water separation has recently become a global challenging task because of the frequent occurrence of oil spill accidents due to the offshore oil production and transportation, and there is an increasing demand for the development of effective and inexpensive approaches for the cleaning-up of the oily pollution in water system. In this study, a self-cleaning underwater superoleophobic mesh that can be used for oil-water separation is prepared by the layer-by-layer (LbL) assembly of sodium silicate and TiO2 nanoparticles on the stainless steel mesh. The integration of the self-cleaning property into the all-inorganic separation mesh by using TiO2 enables the convenient removal of the contaminants by ultraviolet (UV) illumination, and allows for the facile recovery of the separation ability of the contaminated mesh, making it promising for practial oil-water separation applications.

Layer-by-layer (LbL) assembly, which involves alternate deposition of species (building blocks) with complementary interactions to prepare composite coatings, is a versatile platform for fabricating various kinds of coatings with well-tailored chemical compositions and architectures on almost any substrate [26][27][28][29][30][31][32][33][34] . We believe LbL assembly holds a great potential for functionalizing base materials towards solving the aforementioned problems that occur with the previous oil-water separation materials based on the following considerations: (1) With LbL assembly, building block species are deposited in a rationally predesigned fashion, which enables judicially targeted functionalities and even multi-functionalities to be precisely integrated in a single coating and thus holds promise of imparting self-cleaning and/or anti-fouling functionalities into thus-prepared oil-water separating membranes. (2) The LbL assembly enables easy adjustment of the surface micro-nano structure of the coating, which is required for special wetting behaviors. (3) The LbL technique allows for the large-scale deposition of functional coatings on nonflat substrates with irregular and complicated morphology, which makes it scalable, versatile, and thus low-cost for oil-water separation applications.
Herein, we for the first time demonstrate the ease and utility of LbL assembly for the preparation of all-inorganic-coating-based oilwater separation materials. A proof-of-concept is provided by LbL assembly of sodium silicate and TiO 2 nanoparticles on a stainless steel mesh to fabricate an underwater superoleophobic separation membrane with self-cleaning ability. The thus-prepared mesh could effectively separate water from oil-water mixture and the UV illumination provides convenient way to self-clean the contaminated mesh. The current study contributes to the development of advanced oil-water separation materials for practical applications.

Results
The choice of the steel mesh here as the base material is due to its inherent porous structure, which is suitable for separation applications, its good mechanical and chemical stability, as well as its easy availability and low cost. The self-cleaning property is imparted into the mesh-membrane by using photocatalytic TiO 2 as one of the LbL building blocks, which provides a convenient solution to the contamination problem [35][36][37] . After 20 cycles of LbL assembly of sodium silicate and TiO 2 , which is driven by electrostatic interaction between the negatively charged sodium silicate and positively charged TiO 2 , a nanostructured composite coating of hydrophilic silica and selfcleaning TiO 2 (denoted as (silicate/TiO 2 )*20), was formed uniformly on the surface of the stainless steel mesh. The LbL assembly of sodium silicate and TiO 2 in this work is conducted by a nondrying LbL assembly process, in which no intermittent drying steps are involved during the deposition procedure 38 . As has been reported, the deposition of the rigid building blocks of sodium silicate and TiO 2 onto the substrates is in their aggregated forms, and the non-drying LbL assembly process preserves the aggregates on the surface, leading to the loose stack of the building blocks and thus producing the nanostructured coating 38 , which along with microscale mesh wires is essential for the extreme wetting behaviors. Figure 1 shows the scanning electronic microscopy (SEM) images of the original stainless steel mesh and the one after the LbL assembly of (silicate/TiO 2 )*20 coating. The original mesh has an average pore diameter of , 190 mm, and the knitting wires have a diameter of , 110 mm. The magnified view in the inset of Fig. 1a reveals that the original wires have smooth surface. After the LbL assembly of (silicate/TiO 2 )*20 coating, the macroscopic morphology of the mesh did not show any significant change (Fig. 1b), and a layer of nanoaggregates with the size from several tens to several hundred nanometers was uniformly formed on the surface of the wires (inset of Fig. 1b). The energy-dispersive X-ray spectroscopy (EDS) measurement reveals the presence of Ti and Si on the surface (Fig. 1c), and the SEM-EDS elemental mapping results indicate a uniform distribution of the Ti and Si on the surface of the wires (insets of Fig. 1c). We found that 20 cycles of the LbL assembly of sodium silicate and TiO 2 resulted in a sufficient coverage of the nano-aggregates on the mesh surface while when fewer cycles (e.g., 10 cycles) were assembled, only island-like aggregates were discretely distributed on the wire surface, with the underneath stainless steel exposed (see Supplementary Fig. S1, and Fig. S2 online), which is undesirable.
The wettability of water and oil on the (silicate/TiO 2 )*20 coated mesh was evaluated by the contact angle measurements. In air, the water wetting behavior on the (silicate/TiO 2 )*20 coated mesh was greatly enhanced compared with the original uncoated mesh, which had a water contact angle of 127.5u. As shown in Fig. 2a, when the water droplet contacted with the surface of the (silicate/TiO 2 )*20 coated mesh, it quickly spread and penetrated the mesh (within 16 ms), with a contact angle of , 21.9u above the mesh. When more water droplets were added on the mesh surface, the water could easily drip down, indicating the good hydrophilicity and permeability of the coated mesh to water, which was a combined effect of the hydrophilic nature of the LbL assembled coating and the surface micronano hierarchical structures generated in the non-drying LbL assembly process. The coated mesh also exhibited oleophilic property in air with a hexadecane contact angle of , 18.9u above the mesh surface (Fig. 2c). Meanwhile, the current LbL assembly method enables the silicate/TiO 2 coatings to be readily deposited on stainless steel meshes with different sizes, and after coating these meshes all exhibited good hydrophilicity and water permeable property (see Supplementary Fig. S3 online).
The underwater oil wettability of the mesh was evaluated by immersing the (silicate/TiO 2 )*20 coated mesh in aqueous media. Figure 2d shows the contact angles of a series of typical oil droplets on the coated mesh in aqueous media, and the shapes of these oil droplets were also presented as the insets. Without any exception, all of the oil contact angles were larger than 150u on the coated mesh, confirming its underwater superoleophobic property. Without the coating, the original stainless steel mesh exhibited oleophilic property (see Supplementary Fig. S4 online). Furthermore, we found that these oil droplets were quite unstable on the (silicate/TiO 2 )*20 coated mesh surface and they could easily detach from the surface by gentle disturbance, suggesting a low adhesion of the surface to the oil droplets in the aqueous medium. As previously shown, the (silicate/TiO 2 )*20 coated mesh exhibited a micro-nano hierarchical surface structure and hydrophilic nature, so water could be trapped in these micro-nano structures when the mesh was immersed in aqueous medium. Because of the high repellency between polar mesh surface (water and silica) and non-polar oil phase, the mesh surface exhibits oleophobicity, which is further amplified by the surface micro-nano hierarchical structures (i.e., surface roughness), leading to an underwater superoleophobic surface [14][15][16][17][18][19][20][21][22] .
Having demonstrated the underwater superoleophobicity, as well as the hydrophilicity and the water permeable properties of the coated mesh, the coated mesh was then used for the separation of oil and water mixture. As shown in Fig. 3a, the stainless steel mesh with the (silicate/TiO 2 )*20 coating was fixed between two glass tubes, and then a mixture of commercial No.95 gasoline and water (151, v5v) was poured into the upper glass tube (see Supplementary Movie S1 online). Because of the underwater superoleophobic property and the higher density of water than gasoline, the water in the mixture passed through the mesh quickly, and no visible oil was observed in the collected water. As shown in Fig. 3b, a complete separation was achieved for the oil-water mixture. Gravity was the only available force and no other external force was used during the separation.

Discussion
As discussed earlier, self-cleaning property of a separation mesh is very desirable. It is well known that under ultraviolet (UV) light illumination TiO 2 materials can generate photo-electrons and holes, which then react with oxygen and water to produce highly reactive species of superoxide anions and hydroxyl radicals [35][36][37] . The highly reactive species can then decompose and thus remove organic contaminants and fouling species that are absorbed on the surface of separation membrane. In order to evaluate the self-cleaning capability of the silicate/TiO 2 coated mesh, the mesh was first contaminated with a model contaminant of oleic acid. Once contaminated by oleic acid, the surface of the (silicate/TiO 2 )*20 coated mesh lost its hydrophilicity and the underwater superoleophobic property, and showed a hydrophobic property with a water contact angle of , 105u in air, which undesirably led to its inability to separate oil and water (see Supplementary Fig. S5, and Fig. S6 online). With the oleic acid contamination, both water and oil passed through the (silicate/TiO 2 )*20 coated mesh indiscriminately. The contaminated mesh was then subject to UV light illumination (wavelengths centered at 360 nm) and under UV exposure for about 30 minutes, the mesh restored its hydrophilicity, with water contact angle returning to its original value of , 20u (Fig. 4a). The recovery of the mesh's hydrophilicity in air was a result of the removal of the surface-absorbed oleic acid.  www.nature.com/scientificreports As a comparison, the oleic-acid-contaminated (1) original mesh (i.e., unmodified with the LbL assembled coating) and (2) mesh coated with hydrophilic PDDA/silicate multilayer without TiO 2 (see Supplementary experimental details online), both did not show any significant change in their water contact angles (larger than 90u) before and after the UV exposure (see Supplementary Fig. S7 online). These results demonstrate that the self-cleaning property of the (silicate/TiO 2 )*20 coated mesh is indeed from the TiO 2 component in the LbL assembled coating. Furthermore, after five cycles of oleic acid contamination and UV illumination recovery, the (silicate/TiO 2 )*20 coated mesh still exhibited hydrophilic property as well as the underwater superoleophobicity similar to the uncontaminated (silicate/ TiO 2 )*20 mesh (Fig. 4b, and Supplementary Fig. S8 online), indicating the good reproducibility and stability of the silicate/TiO 2 composite coating. With the UV-illumination based self-cleaning, the mesh could be used over again for the same oil-water separation with the same performance as before the contamination. The fact that the LbL assembled silicate/TiO 2 coating is entirely inorganic guarantees the integrity of the coating during the UV-based self-cleaning treatment.
In conclusion, we showed an underwater superoleophobic mesh with the self-cleaning ability could be readily prepared by a facile LbL assembly of sodium silicate and TiO 2 nanoparticles on stainless steel mesh. The thus-prepared mesh could effectively separate water from the oil-water mixture and the UV illumination is a convenient approach to the self-cleaning of the contaminated separation mesh. The current study contributes to the development of advanced oilwater separation materials for practical applications.

Methods
Materials and chemicals. Stainless steel mesh (80 mesh) was purchased from Alfa Aesar. Sodium silicate solution, titanium isopropoxide, poly(diallyldimethylammonium chloride) (PDDA, 20 wt%, Mw ca. 100 0002200 000), silicone oil, 1,2dichloroethane, diiodomethane, hexadecane, hexane, 1,3,5-trimethylbenzene, oleic acid, and mineral oil were all purchased from Sigma-Aldrich and used as received. Water purified in a Milli-Q (MilliPore) system was used during all the experiments. The colloidal TiO 2 suspension was prepared by the controlled hydrolysis of titanium isopropoxide according to a previous reported method 39 . No. 95 gasoline was purchased from a local gas station.
Fabrication of silicate/TiO 2 coatings on stainless steel mesh. The LbL assembly of silicate/TiO 2 coatings on the stainless steel meshes was conducted automatically by a programmable dipping robot (Dipping Robot DR-3, Riegler&Kirstein GmbH) at room temperature. A pre-cleaned stainless steel mesh was first immersed in aqueous PDDA solution (1.0 mg mL 21 ) for 20 min to render its surface positively charged, followed by rinsing with water and drying with N 2 flow. Then sodium silicate and TiO 2 were alternately deposited on the PDDA-modified mesh surface 38 . The mesh was first immersed in a solution of sodium silicate (0.15 M, pH 5 11.6) for 10 min, followed by rinsing in three water baths for 1 min each. Then the mesh was immediately transferred to and stay in a TiO 2 colloidal suspension (1.3 mg mL 21 pH 5 2.5) for 10 min, followed by rinsing in three water baths for 1 min each. By repeating the above deposition process in a cyclic fashion, silicate/TiO 2 composite coating was prepared. The assembly of silicate and TiO 2 was repeated until the desired cycle number was reached. No drying step was used in the deposition procedure unless it was in the last layer. The LbL assembled silicate/TiO 2 coatings with n cycle deposition are denoted as (silicate/TiO 2 )*n.