Facile Construction and Fabrication of a Superhydrophobic and Super Oleophilic Stainless Steel Mesh for Separation of Water and Oil

The fluoride-free fabrication of superhydrophobic materials for the separation of oil/water mixtures has received widespread attention because of frequent offshore oil exploration and chemical leakage. In recent years, oil/water separation materials, based on metal meshes, have drawn much attention, with significant advantages in terms of their high mechanical strength, easy availability, and long durability. However, it is still challenging to prepare superhydrophobic metal meshes with high-separation capacity, low costs, and high recyclability for dealing with oil–water separation. In this work, a superhydrophobic and super oleophilic stainless steel mesh (SSM) was successfully prepared by anchoring Fe2O3 nanoclusters (Fe2O3-NCs) on SSM via the in-situ flame synthesis method and followed by further modification with octadecyltrimethoxysilane (OTS). The as-prepared SSM with Fe2O3-NCs and OTS (OTS@Fe2O3-NCs@SSM) was confirmed by a field emission scanning electron microscope (FESEM), transmission electron microscope (TEM), energy dispersive spectrometer (EDS), X-ray photoelectron spectrometer (XPS), and X-ray diffractometer (XRD). The oil–water separation capacity of the sample was also measured. The results show that the interlaced and dense Fe2O3-NCs, composed of Fe2O3 nanoparticles, were uniformly coated on the surface of the SSM after the immerging-burning process. Additionally, a compact self-assembled OTS layer with low surface energy is coated on the surface of Fe2O3-NCs@SSM, leading to the formation of OTS@Fe2O3-NCs@SSM. The prepared OTS@Fe2O3-NCs@SSM shows excellent superhydrophobicity, with a water static contact angle of 151.3°. The separation efficiencies of OTS@Fe2O3-NCs@SSM for the mixtures of oil/water are all above 98.5%, except for corn oil/water (97.5%) because of its high viscosity. Moreover, the modified SSM exhibits excellent stability and recyclability. This work provides a facile approach for the preparation of superhydrophobic and super oleophilic metal meshes, which will lead to advancements in their large-scale applications on separating oil/water mixtures.


Introduction
The cost-effective separation strategies of oil/water mixtures have become a hot topic due to the frequent occurrences of water pollution issues, such as organic wastewater emissions, chemical leaks, and crude oil spills [1][2][3][4]. To date, a series of materials, including carbon aerogels [5][6][7][8], cotton fabrics [9][10][11][12], foams [13][14][15], and sponges [16][17][18][19], have been explored, owing to the excellent hydrophobicity and high separation capacity. However, some materials are difficult to use in large-scale applications because of the complicated and high-cost separation process, secondary contamination, or poor recyclability. From the perspective of application, it is necessary to develop a more efficient, facile, and low-cost method to achieve the separation of water and oil.
Over the past few years, oil/water separation materials, based on metal meshes [20][21][22], especially stainless steel meshes (SSM) [23][24][25][26], have drawn much attention, with significant advantages in terms of their high mechanical strength, easy availability, and long durability. The preparation of superhydrophobic mesh is mainly based on surface modification by changing the chemical composition or the microscopic geometry of the surface [27][28][29][30]. For example, Jiang et al. first prepared a superhydrophobic mesh with high oil-water separation efficiency via surface modification with polytetrafluoroethylene [31]. Chen et al. provided a facile approach for the preparation of oil-water separation material via the coating of the reduced graphene oxide on mesh [32]. Fu and co-workers prepared a robust superhydrophobic and super oleophilic material by loading hierarchical-layer nanospheres on the surface of metal mesh [33]. Liang et al. developed a co-precipitation strategy to prepare superhydrophobic materials by growing hierarchical micro-nanostructures of CeO 2 on the Cu mesh, followed by further modification with stearic acid [21]. Wang et al. reported that a superhydrophobic mesh was prepared via the coating of micro/nanostructured Co(OH) 2 on the mesh, followed by modification with hexadecyltrimethoxysilane [27]. Based on the aforementioned previous research, the rough surface caused by the loading of nanoparticles can greatly increase the surface area of the substrate. Furthermore, the superhydrophobic surface is usually achieved by surface modification with a low-surface-energy material. Nevertheless, most methods of preparing the oil/water separation material are usually costly, such as expensive equipment or devices, complex synthesis steps, toxic reagents, or by-products. Therefore, it is necessary to develop a facile strategy to prepare superhydrophobic SSM with high separation capacity, low costs, and high recyclability for dealing with oil/water separation.
In this work, an SSM with a superhydrophobic and super oleophilic surface was successfully prepared by anchoring Fe 2 O 3 nanoclusters (Fe 2 O 3 -NCs) by the in-situ flame synthesis method and then modified with octadecyltrimethoxysilane (OTS). The SSM with OTS and Fe 2 O 3 -NCs (OTS@Fe 2 O 3 -NCs@SSM) was characterized with a field emission scanning electron microscope (FESEM), energy dispersive X-ray spectrometer (EDS), X-ray photoelectron spectrometer (XPS), and X-ray diffractometer (XRD). Additionally, the oil-water separation capacity and recyclability of the samples were also examined.

Materials
Ferric acetylacetonate (C 15 H 21 FeO 6 , 99%), absolute ethanol, and OTS were purchased from Macklin biochemical technology Co., Ltd., China (Shanghai, China). The commercial SSM of 400 meshes was purchased from Taobao (Changzhou, China). The SSM with a size of 5 × 5 cm was ultrasonically cleaned in ethanol for 1 h and then dried at 80 • C in air for 5 h. Distilled water was used in all the experiments.

Preparation of OTS@Fe 2 O 3 -NCs@SSM
Firstly, SSM was dipped into a solution containing 100 mL of ethanol and 2.2 g of C 15 H 21 FeO 6 for 5 s to get SSM filled with the solution and followed by burning in air. This process was repeated 40 times to ensure the loading of Fe 2 O 3 -NCs on SSM (Fe 2 O 3 -NCs@SSM). The as-obtained Fe 2 O 3 -NCs@SSM was cleaned with deionized water and dried at 80 • C in air for 3 h. Secondly, 0.5 mL of OTS was ultrasonically dissolved in 400 mL of distilled water for 1 h. Subsequently, Fe 2 O 3 -NCs@SSM was added into the aforementioned aqueous emulsion and left for 24 h. Finally, the as-obtained OTS@Fe 2 O 3 -NCs@SSM was cleaned with deionized water and dried at 80 • C in air for 5 h.

Oil-Water Separation and Recyclability Tests
In the oil/water separation test, the immiscible mixtures of water and oil or organic solvents in a 1:1 volume ratio were used. The mixtures were poured onto OTS@Fe 2 O 3 -NCs@SSM, which was fixed in the homemade device. The oil-water separation efficiency T was calculated by (V 2 /V 1 ) × 100, where V 1 and V 2 were the volume of water before and after separation, respectively. In the recyclability test, the separation process of the samples was repeated. After each cycle, the used samples were cleaned with ethanol and dried at 80 • C in air for 3 h. Figure 1 presents the schematic illustration of the controllable constructing process of OTS@Fe 2 O 3 -NCs@SSM via a simple and facile two-step strategy. Firstly, SSM wetted by the solution containing ethanol and C 15 H 21 FeO 6 was taken out and burned in air. In this immerging-burning process, absolute ethanol was not only used as a solvent to dissolve C 15 H 21 FeO 6 , but also made C 15 H 21 FeO 6 sufficient for burning in air. In the meantime, hydrocarbon moieties and Fe atoms decomposed by C 15 H 21 FeO 6 at high temperatures acted as a source of carbon atoms and catalyst, respectively [34,35]. The hydrocarbon moieties were coated on Fe atoms to form an Fe-C bond [36]. Fe 2 O 3 -NCs nanoparticles were constantly deposited on the carbon precipitates to grow Fe 2 O 3 -NCs on the surface of SSM after repeating 40 times for the immerging-burning process. The immobilization of Fe 2 O 3 -NCs on the surface of SSM is not only beneficial to enhance the surface roughness of SSM, but can also effectively improve the adhesion between hydrophobic components and SSM. Secondly, Fe 2 O 3 -NCs@SSM was added into the OTS aqueous emulsion, leading to the formation of OTS@Fe 2 O 3 -NCs@SSM. In particular, it needs to be mentioned that the growing of Fe 2 O 3 -NCs on SSM after immerging-burning treatment is beneficial to the coating of OTS on the surface of Fe 2 O 3 -NCs@SSM, which will enhance the superhydrophobic property of the material. Additionally, OTS can be used to grow a compact and self-assembled monolayer with low surface energy on the surface of Fe 2 O 3 -NCs@SSM, leading to an improvement in the hydrophobicity of the sample [37,38].

Characterization
The surface morphology of samples was demonstrated by FESEM and TEM images. As shown in Figure 2a,d, the typical FESEM images of the untreated SSM exhibit a smooth surface, with a diameter of ca. 34 µm, which is used as the substrate to adsorb C 15 H 21 FeO 6 . After the dipping-burning process, the interlaced and dense The surface morphology of samples was demonstrated by FESEM and TEM images. As shown in Figure 2a,d, the typical FESEM images of the untreated SSM exhibit a smooth surface, with a diameter of ca. 34 μm, which is used as the substrate to adsorb C15H21FeO6. After the dipping-burning process, the interlaced and dense Fe2O3-NCs are uniformly coated on the surface of the SSM, with a diameter of ca. 40 μm (Figure 2b,e). The thickness of the Fe2O3-NCs layer was found to be ca. 3 μm, based on the difference in diameter between SSM and Fe2O3-NCs@SSM. The TEM image ( Figure 3) clearly confirms that Fe2O3-NCs are composed of Fe2O3 nanoparticles. It is notable that Fe2O3-NCs not only provides the larger surface area for the loading of OTS, but also effectively enhances the adhesion between OTS and SSM. After modification, a dense self-assembled OTS layer is coated on the surface of Fe2O3-NCs@SSM, leading to the formation of OTS@Fe2O3-NCs@SSM ( Figure  2c,f).     The chemical compositions of samples were tested by EDS and XPS. The SEM-based EDS elemental analysis results (Figures S1-S3) revealed that the elemental content of O on Fe2O3-NCs@SSM was obviously increased, compared with SSM, due to the formation of Fe2O3-NCs on the surface of SSM after the immerging-burning process. It is worth not-  The chemical compositions of samples were tested by EDS and XPS. The SEM-based EDS elemental analysis results (Figures S1-S3) revealed that the elemental content of O on Fe 2 O 3 -NCs@SSM was obviously increased, compared with SSM, due to the formation of Fe 2 O 3 -NCs on the surface of SSM after the immerging-burning process. It is worth noting that the proportion of Si element on SSM and Fe 2 O 3 -NCs@SSM are 0.31 and 0%, respectively. These results suggest that the Si element was undetected on the surface of Fe 2 O 3 -NCs@SSM because of the interlaced and dense Fe 2 O 3 -NCs coated on SSM. As described in Figure S3 and Figure 4, the elemental distribution of Si components on OTS@Fe 2 O 3 -NCs@SSM can verify that the hydrophobic components of OTS are successfully coated on the substrate. As shown in Figure 5a, the surface XPS results of SSM verify the existence of C (284.6 eV), O (530.6 eV), and Fe (711.2 eV) elements [30]. In addition, the Si 2s and Si 2p signal peaks are detected on the surface of SSM, which is consistent with the EDS results ( Figure S1). The absence of Si 2s peak in the XPS results of Fe 2 O 3 -NCs@SSM may be caused by the coating of Fe 2 O 3 -NCs on SSM, which agrees well with the EDS results shown in Figure S2. The Si 2s and Si 2p peaks are present in the XPS results of OTS@Fe 2 O 3 -NCs@SSM, which originated from OTS [39]. These XPS results can further confirm the successful preparation of Fe 2 O 3 -NCs and OTS on the surface of SSM. The chemical compositions of samples were tested by EDS and XPS. The SEM-based EDS elemental analysis results (Figures S1-S3) revealed that the elemental content of O on Fe2O3-NCs@SSM was obviously increased, compared with SSM, due to the formation of Fe2O3-NCs on the surface of SSM after the immerging-burning process. It is worth noting that the proportion of Si element on SSM and Fe2O3-NCs@SSM are 0.31 and 0%, respectively. These results suggest that the Si element was undetected on the surface of Fe2O3-NCs@SSM because of the interlaced and dense Fe2O3-NCs coated on SSM. As described in Figure S3 and Figure 4, the elemental distribution of Si components on OTS@Fe2O3-NCs@SSM can verify that the hydrophobic components of OTS are successfully coated on the substrate. As shown in Figure 5a, the surface XPS results of SSM verify the existence of C (284.6 eV), O (530.6 eV), and Fe (711.2 eV) elements [30]. In addition, the Si 2s and Si 2p signal peaks are detected on the surface of SSM, which is consistent with the EDS results ( Figure S1). The absence of Si 2s peak in the XPS results of Fe2O3-NCs@SSM may be caused by the coating of Fe2O3-NCs on SSM, which agrees well with the EDS results shown in Figure S2. The Si 2s and Si 2p peaks are present in the XPS results of OTS@Fe2O3-NCs@SSM, which originated from OTS [39]. These XPS results can further confirm the successful preparation of Fe2O3-NCs and OTS on the surface of SSM.   [37]. This result can further confirm the successful preparation of Fe 2 O 3 -NCs on the surface of SSM. No obvious characteristic diffraction peaks for OTS can be detected due to its poor crystallinity. As can be seen in Table S1, the zeta potentials of the samples were measured. The charge of SSM is −63.25 mV in 1 mM KCl aqueous solution at pH = 7, which is consistent with previous studies [40,41]. After the immerging-burning process, the surface charge of further confirm the successful preparation of Fe2O3-NCs on the surface of SSM. No obvious characteristic diffraction peaks for OTS can be detected due to its poor crystallinity. As can be seen in Table S1, the zeta potentials of the samples were measured. The charge of SSM is −63.25 mV in 1 mM KCl aqueous solution at pH = 7, which is consistent with previous studies [40,41]. After the immerging-burning process, the surface charge of Fe2O3-NCs@SSM increases to −45.09 mV due to the coating of Fe2O3-NCs with a zeta potential value of −14.17 mV. Moreover, the zeta potential of OTS@Fe2O3-NCs@SSM is still negatively charged with a value of −27.83 mV, which can be attributed to the further loading of OTS, with a zeta potential value of −26.10 mV. These results suggest that Fe2O3-NCs and OTS are successfully immobilized on the surface of SSM.

Superhydrophobic Performance
The surface hydrophobicity of the sample is very important to support potential practical applications. As depicted in Figure 6a, water and methyl blue solution droplets were prevented from penetrating through OTS@Fe2O3-NCs@SSM, exhibiting excellent hydrophobic properties. Interestingly, a water column ejected from a syringe and bouncing off the superhydrophobic surface of OTS@Fe2O3-NCs@SSM, indicating that the modified SSM has an ultralow contact-angle hysteresis (Figure 6b). To further evaluate the hydrophobicity of the samples, the floating test and the water static contact-angle test were carried out. As shown in Figure 6c, both SSM and Fe2O3-NCs@SSM sink to the bottom of water. However, OTS@Fe2O3-NCs@SSM can float on water and pick it up without water droplets. This result indicates that OTS@Fe2O3-NCs@SSM has better hydrophobicity than SSM and Fe2O3-NCs@SSM. As can be seen in Figure 6d-f, the water static contact angle of the samples was observed. The original SSM exhibits a hydrophilic property with a water static contact angle of ca. 116.5° (Figure 6d). The water static contact angle of Fe2O3-NCs@SSM is ca. 0° (Figure 6e). This result suggests that the hydrophilic component of Fe2O3-NCs changes the surface hydrophobicity of SSM. After modification, OTS@Fe2O3-NCs@SSM shows extreme superhydrophobic surface with a water static contact angle of

Superhydrophobic Performance
The surface hydrophobicity of the sample is very important to support potential practical applications. As depicted in Figure 6a, water and methyl blue solution droplets were prevented from penetrating through OTS@Fe 2 O 3 -NCs@SSM, exhibiting excellent hydrophobic properties. Interestingly, a water column ejected from a syringe and bouncing off the superhydrophobic surface of OTS@Fe 2 O 3 -NCs@SSM, indicating that the modified SSM has an ultralow contact-angle hysteresis (Figure 6b). To further evaluate the hydrophobicity of the samples, the floating test and the water static contact-angle test were carried out. As shown in Figure 6c, both SSM and Fe 2 O 3 -NCs@SSM sink to the bottom of water. However, OTS@Fe 2 O 3 -NCs@SSM can float on water and pick it up without water droplets. This result indicates that OTS@Fe 2 O 3 -NCs@SSM has better hydrophobicity than SSM and Fe 2 O 3 -NCs@SSM. As can be seen in Figure 6d (Figure 6f). The preparation of superhydrophobic SSM can be explained by changing the microscopic geometry and the surface energy. Firstly, the rough surface caused by the formation of Fe 2 O 3 -NCs can retain more air beneath the water droplets, effectively reducing the solid-liquid contact area. Furthermore, OTS, a typical low-surface-energy material, is used to modify the surface property of Fe 2 O 3 -NCs@SSM, leading to enhanced hydrophobicity. To ensure a better evaluation for the superhydrophobic behavior of the samples, the oil-water separation performance was tested. As shown in Figure 7, dichloromethane and water were dyed with oil red O and methyl blue, respectively. Subsequently, the mixtures were poured into the homemade gravity-driven separation device through OTS@Fe 2 O 3 -NCs@SSM. It was found that dichloromethane can quickly permeate through OTS@Fe 2 O 3 -NCs@SSM through the driving force of gravity and water retained on the top side of the modified mesh. The prepared OTS@Fe 2 O 3 -NCs@SSM has excellent performance to separate the oil/water mixtures due to the superhydrophobic and super oleophilic surfaces.
As depicted in Figure 8a, the oil-water separation efficiency was evaluated by the homemade separation device. The separation efficiencies of OTS@Fe 2 O 3 -NCs@SSM for the mixtures of petroleum toluene/water, n-octane/water, gasoline/water, n-hexane/water, chloroform/water, dichloromethane/water, and cyclohexane/water were all above 98.5%, except for corn oil/water (97.5%), because of its high viscosity. These data suggest that the prepared superhydrophobic SSM can be used as an ideal material to efficiently separate an oil/water mixture. As can be seen from the data in Table 1, the modified SSM shows higher separation efficiency than many previously reported meshes [22,24,33,[42][43][44][45][46]. Although the separation efficiency of OTS@Fe 2 O 3 -NCs@SSM is still lower than that of those reported materials [21,27,32], the preparation method of OTS@Fe 2 O 3 -NCs@SSM is more facile, time saving, and economical. Therefore, OTS@Fe 2 O 3 -NCs@SSM is a cost-effective and promising material for oil/water separation. In addition, the oil-water separation materials should be easily cleaned and recycled in terms of the practical applications. The recycled experiments of the superhydrophobic and super oleophilic OTS@Fe 2 O 3 -NCs@SSM were also investigated (Figure 8b). The separation efficiency of the samples for the n-hexane/water mixture could be maintained above 98.6% after 10 cycles of separation, indicating the excellent reusability of the samples. In addition, data analysis of group 10 and group 0 shows that the p value is greater than 0.05, suggesting that the separation efficiency of the two groups is not significantly different. This result suggests that the immobilization of Fe 2 O 3 -NCs on the surface of SSM can effectively improve the adhesion between hydrophobic components and SSM and enhance the reusability of the material. As depicted in Figure 8a, the oil-water separation efficiency was evaluated by the homemade separation device. The separation efficiencies of OTS@Fe2O3-NCs@SSM for the mixtures of petroleum toluene/water, n-octane/water, gasoline/water, n-hexane/water, chloroform/water, dichloromethane/water, and cyclohexane/water were all above 98.5%, except for corn oil/water (97.5%), because of its high viscosity. These data suggest that the prepared superhydrophobic SSM can be used as an ideal material to efficiently separate an oil/water mixture. As can be seen from the data in Table 1, the modified SSM shows higher separation efficiency than many previously reported meshes [22,24,33,[42][43][44][45][46]. Although the separation efficiency of OTS@Fe2O3-NCs@SSM is still lower than that of those reported materials [21,27,32], the preparation method of OTS@Fe2O3-NCs@SSM is more facile, time saving, and economical. Therefore, OTS@Fe2O3-NCs@SSM is a cost-effective and promising material for oil/water separation. In addition, the oil-water separation materials should be easily cleaned and recycled in terms of the practical applications. The recycled experiments of the superhydrophobic and super oleophilic OTS@Fe2O3-NCs@SSM were also investigated (Figure 8b). The separation efficiency of the samples for the n-hexane/water mixture could be maintained above 98.6% after 10 cycles of separation, indicating the excellent reusability of the samples. In addition, data analysis of group 10 and group 0 shows that the p value is greater than 0.05, suggesting that the separation

Conclusions
In summary, a robust and stable superhydrophobic and super oleophilic SSM was fabricated successfully by anchoring Fe 2 O 3 -NCs on SSM via the in-situ flame synthesis method and followed by further modification with OTS. The characterization results indicate that the preparation of Fe 2 O 3 -NCs, composed of Fe 2 O 3 nanoparticles on the surface of SSM, is not only beneficial to enhance the surface roughness of SSM, but can also effectively improve the adhesion between hydrophobic components and SSM. In addition, the coating of OTS with low surface energy endows SSM with excellent superhydrophobicity, as well as super oleophilicity, with a water static contact angle of 151.3 • . Moreover, the prepared OTS@Fe 2 O 3 -NCs@SSM exhibits excellent oil-water separation efficiency for a series of oil and water mixtures. It is important to note that the modified SSM shows good stability and reusability after 10 cycles. Therefore, this work provides an intriguing methodology for coating nanostructures on metal substrates, which will lead to a new perspective on separating oil/water mixtures.
Author Contributions: Conceptualization, Y.S. and R.S.; methodology, Y.S.; validation, Z.K., C.S. and Q.W.; writing-original draft preparation, Y.S. and C.S.; writing-review and editing, Z.K., W.Y. and Z.Y. All authors have read and agreed to the published version of the manuscript.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.