Facile preparation of chemically stable metal mesh-based superamphiphobic surface toward effective anti-adhesion

The ability of animals and plants that protect their bodies from water is vitally important for their survival in nature, and various anti-wetting surfaces have been fabricated by simulating biological surfaces in different ways.¹ Wettability is one of the most important physical and chemical properties of solid surfaces, which can be divided into superhydrophobic, superhydrophilic, underwater superoleophobic, etc.^2,3 It is worth noting that superamphiphobic materials have attracted extensive research due to their brilliant superhydrophobic and superoleophobic properties. In the past few decades, superamphiphobic materials have a wide range of applications, including pipeline transportation,^4,5 shipbuilding,^6,7 electronic devices,^8–10 self-cleaning,^11–15 and oil–water separation.^16–18 

Due to the relatively low activity of the metal surface, it is easy to be polluted and corroded.^19,20 How to explore the method to make metal materials more stable has become one of the current research hotspots. The common and highly efficient preparation strategy of metal-based superamphiphobic materials is achieved by changing the surface roughness and by loading a low surface energy compound. For example, Liu et al. described a method for the preparation of a hierarchical step-like microstructure with nanopores, which was fabricated on an Al surface through chemical etching, anodization, and fluorination and confers superamphiphobic properties.²¹ Liu et al. presented a method for preparing a robust superamphiphobic coating by using the reaction of magnesium alloy matrix with aluminum phosphate to form a layered rough structure and by coating the extremely low surface energy 1H,1H,2H,2H-perfluorooctane trichlorosilane for reducing the surface energy.²² Wu et al. proposed an industry compatible method to achieve super-repellency properties by forming alumina nanowires on the surface of sample using the electrochemical method and modifying the surface with hydrophobic materials.²³ Mevra et al. presented an approach to create a superamphiphobic TiO₂ layer on the pure titanium material by using the sol–gel method.²⁴ Nevertheless, these methods for preparing superamphiphobic materials usually have some shortcomings, such as complex synthesis process, cumbersome operation, expensive equipment or devices, and by-product formation, which restrict the large-scale application of metal-based superamphiphobic materials.

In addition, although the superamphiphobic coating can effectively separate the water and organic solvents from the metal surface, there are still some challenges in the wide practical application, such as chemical stability and high temperature stability. First, a variety of corrosive media in the environment will gradually destroy the superamphiphobic coating on the metal surface over time, eventually leading to serious metal corrosion. In addition, the brilliant superhydrophobic and superoleophobic properties can be achieved at room temperature for most natural or artificial superbiphobic surfaces. However, as temperature increases, the superamphiphobicity properties of the material surface gradually weaken, making it easier for the solution to diffuse and moisten solid surfaces.²⁵ Therefore, it is of great significance to develop a green and simple method for the preparation of metal superamphiphobic coating with excellent high temperature resistance and corrosion resistance.

Herein, we take the stainless steel metal mesh (SSM) substrates as an example. CuO nanoclusters (CuO-NCs) were formed on the surface of SSM by immersion–burning method and solution deposition method (CuO-NCs@SSM). In this process, C₁₀H₁₄CuO₄ dissolved by ethanol distributes uniformly on the SSM skeleton so as to ensure the uniform and dense distribution of CuO-NCs in situ on the surface of SSM. At the same time, CuO-NCs not only improve the surface roughness of SSM but also effectively enhance the adhesion between the superamphiphobic component and SSM. Then, silane fluoride group (SFG) was grafted on CuO-NCs@SSM to obtain the superamphiphobic sample SFG@CuO-NCs@SSM, which was characterized by field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), x-ray photoelectron spectroscopy (XPS), and x-ray diffractometer (XRD). In addition, the antifouling performance, self-cleaning ability, chemical stability, and high temperature stability of the samples were also investigated.

SSM (400 meshes, 6 × 6 cm²) was purchased from Taobao (Changzhou, China), which was ultrasonically cleaned in anhydrous ethanol for 1 h and dried at 60 °C for 4 h before using. Bis(3-trimethoxysilylpropyl)amine (BTM), cupric acetylacetonate (C₁₀H₁₄CuO₄, 99%), 3-aminopropyltrimethoxysilane (APT), and perfluorooctanoic acid (PFOA, 96%) were obtained from Macklin biochemical technology Co., Ltd. (Shanghai, China). All of these chemicals were used as received without any depuration.

C₁₀H₁₄CuO₄ (1 g) was dissolved in anhydrous ethanol (100 ml) under magnetic stirring for 1 h at room temperature. SSM was immersed in the above solution for about 10 s and then taken out to burn in air. This impregnation–combustion process was repeated 20 times. In order to remove surface residues, the prepared material was immersed in deionized water for 24 h and finally dried at 75 °C for 2 h to get CuO-NCs@SSM.

First of all, APT (200 μl) and BTM (200 μl) were added to anhydrous ethanol (30 ml) and stirred for 30 min at room temperature, which was recorded as solution A. In addition, PFOA (1 g) was dissolved into anhydrous ethanol (15 ml) and stirred for 30 min at room temperature to acquire solution B. Second, solution A was added dropwise into solution B and stirred for 2 h at 300 rpm, forming the APT–BTM–PFOA emulsion. Finally, CuO-NCs@SSM was put in the above emulsion and left for 24 h; after that, it was washed three times with deionized water and dried at 75 °C to obtain SFG@CuO-NCs@SSM for further characterization.

The microscopic morphology of the sample was visualized by FESEM (Gemini 500, Zeiss) and TEM (Tecnai G² F20, FEI, USA). XPS (ESCALAB 250Xi, Thermo Fisher, Waltham, MA, USA) and EDS (X-MaxN 80, Oxford, UK) were used to measure the surface chemical composition of the samples. XRD with a Cu K radiation source was used to obtain x-ray diffraction patterns (D8 Advance, Bruker, Germany). The water static contact angle and oil contact angle were measured using the contact angle goniometer (DSA100, KRUSS, Germany) at room temperature.

The self-cleaning stability of SFG@CuO-NCs@SSM was examined by coating adhesion test. The process of the TiO₂ powder on the sample being taken off by the falling water was recorded, and the surface adhesion phenomenon of the sample being immersed in liquid paraffin and muddy water and then being taken out was observed. In addition, the high temperature stability and chemical stability of SFG@CuO-NCs@SSM were studied. The high temperature resistance test was carried out by exposing the samples in the temperature range of 25–200 °C for 24 h, and the acid/alkaline resistance test was evaluated by putting the samples into the acid/alkaline solutions with different pH values (pH = 1–14) for 30 min.

Figure 1 presents the schematic diagram of the preparation of SFG@CuO-NCs@SSM by a simple immersion–burning method and solution deposition method. First, a piece of SSM was dipped into the ethanol solution of C₁₀H₁₄CuO₄ to get SSM filled with the solution and then taken out for burning in air. In this dipping–burning process, ethanol can not only dissolve C₁₀H₁₄CuO₄ and deposit it on the SSM skeleton but can also be used as a fuel to make C₁₀H₁₄CuO₄ fully burned. At the same time, C₁₀H₁₄CuO₄ was decomposed steadily at high temperatures to form CuO nanoparticles.^26,27 After repeating 20 times, CuO nanoparticles were continuously deposited to grow CuO-NCs on the SSM surface, which not only improved the surface roughness of SSM but also effectively enhanced the adhesion between superamphiphobic components and SSM.²⁸ Second, CuO-NCs@SSM was put into the APT–BTM–PFOA emulsion to form SFG@CuO-NCs@SSM. CuO would be dissociated to form hydroxyl groups (–OH) when the surface was submerged in water due to the greater polarity of Cu–O bonds of CuO nanoparticles.²⁹ In addition, APT and BTM would hydrolyze and form hydroxyl groups under alkaline conditions (–OH).^30,31 The connection of BTM and CuO-NCs was achieved by condensation reaction.³² BTM possesses high activity, which can be subjected to amidation reaction with the carboxyl group of oleophobic factors (PFOA) to form a host of long-chain perfluoropolymers on the surface.^28,32 In virtue of the C–F group of perfluorinated chains, the low surface tension can greatly reduce surface energy, which makes the modified SFG@CuO-NCs@SSM possess excellent oleophobic properties.³³ Moreover, hydrophobic factors (APT) are grafted onto BTM by condensation reaction, which grows dense single molecules with low surface energy on the surface. It can prevent droplet diffusion on the surface, which exhibits superhydrophobic characteristics.²⁹ It is worth noting that more air is retained in the surface rough structure, forming a layer of air cushion at the micro- and nano-level, which can greatly reduce the contact area between the liquid and the sample surface to realize the nonwettability of the liquid.³⁴ In summary, SFG@CuO-NCs@SSM possesses the dual properties of superhydrophobic and superoleophobic through the changing of surface microstructure and the chemical modification.

The surface morphology of the samples was analyzed by FESEM and TEM. The uniform shapes and flat surfaces of the typical FESEM images of an unprocessed SSM are shown in Figs. 2(a) and 2(d). However, the smooth and flat surface of SSM is not conducive to the formation of superamphiphobic coating on its surface, so it is necessary to further improve its surface roughness. As shown in Figs. 2(b) and 2(e), CuO-NCs@SSM obtained by the immersion–burning method is coated with uniform and dense CuO-NCs. CuO nanoparticles were continuously deposited to grow CuO-NCs on the SSM surface, which is consistent with the TEM result (Fig. 3). The formation of CuO-NCs on the surface of SSM by in situ synthesis can not only effectively improve the surface roughness of SSM but also significantly enhance the bonding between superamphiphobic components and SSM. Compared with CuO-NCs@SSM, the surface morphology of SFG@CuO-NCs@SSM was obviously changed after the solution deposition process [Figs. 2(c) and 2(f)], which can indicate that hydrophobic factors and oleophobic factors were successfully loaded on the SSM surface.

To analyze the chemical compositions of the samples, XPS measurements were performed. As described in Fig. 4, XPS was used to measure chemical bond states in SSM, CuO-NCs@SSM, and SFG@CuO-NCs@SSM. XPS results demonstrate the existence of C (284.6 eV), O (531.6 eV), and Fe (712.2 eV) elements on the surface of SSM.³⁰ Compared with SSM, the Cu 2p signal peak of CuO-NCs@SSM was significantly detected and the Fe 2p signal peak was relatively weakened, confirming the existence of CuO in the coating.³⁵ In addition, the distribution of Si 2p, F 1s, and N 1s peaks in the XPS spectrum of the SFG@CuO-NCs@SSM surface confirms that SFG was loaded on the surface of CuO-NCs@SSM. The chemical composition of the SFG@CuO-NCs@SSM surface elements was further measured by using EDS (Fig. 5). The SEM-based EDS elemental mappings revealed that CuO-NCs and SFG are successfully coated on the SSM surface, which corresponds to the XPS results.

In order to clarify the internal crystal structure, the XRD pattern of SSM, CuO-NCs@SSM, and SFG@CuO-NCs@SSM was analyzed, as shown in Fig. 6. There are three obvious diffraction peaks in the (110), (200), and (220) lattice planes belonging to SSM at 2θ = 43.9°, 51.1°, and 75.1°, respectively.³⁶ After the immersion–burning process, the diffraction pattern of CuO-NCs@SSM shows multiple peaks at 2θ = 35.9°, 49.3°, 53.7°, 58.6°, 61.7°, 66.6°, 74.7°, and 75.2°, corresponding to CuO replication (002), (202), (020), (202), (113), (022), (113), and (311) crystal surfaces.^37–39 It is worth noting that the diffraction peak intensity of SSM is significantly reduced after the loading of CuO-NCs on the surface of SSM, which can be attributed to the homogeneous and dense CuO-NCs layer on the substrate.³² Due to the poor crystallinity of SFG, no significant characteristic diffraction peaks of SFG@CuO-NCs@SSM were detected.

The superamphiphobicity of the sample plays an important role in supporting potential practical applications. As shown in Fig. 7(a), water, diesel oil, glycerin, corn oil, and liquid paraffin all keep nearly perfect spherical shapes on the surface of SFG@CuO-NCs@SSM, indicating an excellent superamphiphobic property. To quantify the superamphiphobic performance of the as-prepared SFG@CuO-NCs@SSM surface, the water static contact angle and oil contact angle were also observed [Fig. 7(b)]. The water static contact angle is about 153°, exhibiting excellent hydrophobic properties. The static contact angles of various oil droplets were measured, which are about 152°, 153°, 152°, and 151° for diesel oil, glycerin, corn oil, and liquid paraffin, respectively. The above results show that CuO-NCs and SFG coating on SSM can successfully transform the modified SFG@CuO-NCs@SSM into a superamphiphobicity surface due to the low surface energy and air cushion effect.

Metal materials surfaces will inevitably come into contact with different kinds of dust and pollutants to cause pollution or corrosion during the usage or outdoor exposure in real world applications. Therefore, it is crucial to give the coating the ability for resisting the surface contamination. In this paper, TiO₂ powder, glycerin, and muddy water were regarded as target pollutants, respectively. The self-cleaning and antifouling performance of the modified SFG@CuO-NCs@SSM were tested (Fig. 8). As can be seen from Fig. 8(a), the liquid rolls off the top of the sample covered with TiO₂ powder, and the liquid carries the powder away. Eventually, the surface of the sample becomes clean again due to the synergistic effect of low surface energy and micro-/nano-structure of SFG@CuO-NCs@SSM. The loading of a low surface energy material effectively prevents the diffusion of droplets on the surface of the material, and the contact area between the droplets and the sample surface can be greatly reduced due to the air cushion formed by the micro-/nano-structure. In addition, as displayed in Fig. 8(b), the modified SFG@CuO-NCs@SSM was soaked in glycerin to show a mirror-like phenomenon.³⁴ The surface of the sample remained dry after taking it out from the solutions of glycerin or muddy water [Figs. 8(b) and 8(c)]. The superamphiphobic properties of the sample surface can effectively protect the material from liquid penetration and contamination, giving SFG@CuO-NCs@SSM excellent self-cleaning properties and antifouling ability. Metal materials treated by this simple and green experimental method can be widely used in the field of construction or transportation, not only to ensure cleanliness but also to effectively extend the service time.

The temperature has a certain impact on the superamphiphobic properties of the sample, so the thermal stability of SFG@CuO-NCs@SSM was also studied. The results of the water contact angle and oil contact angle of the sample exposed to different temperatures for 24 h are shown in Fig. 9(a). As the temperature increases in the range from 25 to 200 °C, the static contact angle of the sample for water and glycerin is still maintained at about 150°, showing its good superamphiphobicity. Therefore, the modified SFG@CuO-NCs@SSM possesses good thermal stability, which greatly broadens the practical application fields, such as a high temperature industrial environment. In practical applications, the chemical stability of the material is essential for its usage under complex conditions, such as corrosive environments. The chemical stability test was evaluated by putting the samples into the acid/alkaline solutions with different pH values (pH = 1–14) for 30 min, and then, the static contact angle variation of the as-treated SFG@CuO-NCs@SSM for water and glycerin was studied. As shown in Fig. 9(b), the surface exhibits superhydrophobic and superoleophobic properties with the static contact angle for water and glycerin remaining unchanged at about 150° after immersion under corrosive conditions. The main reason is that SFG@CuO-NCs@SSM still possesses an excellent superhydrophobic characteristic in strong acid or alkali solutions so that the acid or alkali solutions cannot fully contact the surface of the material. Thus, the surface chemical structure and properties of SFG@CuO-NCs@SSM cannot be changed. Compared with previous literature that only measured the contact angle of the acid/alkaline droplets on the sample surface,⁴⁰ this study immersed the sample into the acid/alkaline solutions for 30 min, so the as-obtained results are more valuable for practical applications. In summary, the superamphiphobic material described in this paper shows excellent performance in terms of self-cleaning, antifouling ability, high temperature resistance, and corrosion resistance, which makes this material of great application potential in pipeline transportation, shipbuilding, oil–water separation, construction materials, and high temperature resistant materials.

In this work, we have successfully prepared a kind of superamphiphobic coating with excellent superamphiphobicity and stability by immersion–burning method and solution deposition method. The uniform and dense CuO-NCs coated on the SSM surface by immersion–burning method play a crucial role in forming the rough surface structure and the adhesion between superamphiphobic components and SSM. Meanwhile, oleophobic factors (PFOA) with long-chain perfluoropolymers and hydrophobic factors (APT) with low-surface-energy functional groups were successfully grafted onto the surface of SSM through the connection of BTM, contributing to their brilliant superhydrophobic and superoleophobic properties. The as-prepared SFG@CuO-NCs@SSM also possesses remarkable self-cleaning and antifouling properties, which are desirable in practical applications. Moreover, the samples still show excellent superamphiphobic properties after immersion in acid/alkaline solutions, demonstrating an outstanding performance of long-period anticorrosion. In view of the facile and cost-effective experimental process and superior performance, the proposed superamphiphobic coating technology on metal surface can be extended with widespread industrial applications, especially in high temperature and corrosive environments.

This work was supported by Anhui Nature Science Research Program (Grant No. 2022AH051954), Anhui Academic Leaders Cultivation Program (Grant No. DTR2023046), Huangshan University Science Research Program (Grant No. 2022xkjzd001), Anhui Offline First-Class Courses Program (2023xxkc303), and Huangshan University Educational Reform Program (Grant No. 2023JXYJ01).

The authors have no conflicts to disclose.

The authors’ contributions to the manuscript are as follows: Yinyu Sun, Changjiang Li, and Caiyun Shen conceived the study; Yinyu Sun, Zihan Yin, and Caiyun Shen designed the methodology; Zihan Yin, Yu Liu, Qi Chen, and Wei Yang performed validation; Yinyu Sun, Zihan Yin, Changjiang Li, and Caiyun Shen wrote the original draft of this manuscript; Zihan Yin, Yinyu Sun, Zhongcheng Ke, and Changjiang Li reviewed and edited the manuscript. All authors have read and agreed to the published version of the manuscript.

Yinyu Sun: Conceptualization (equal); Methodology (equal); Writing – original draft (equal); Writing – review & editing (equal). Zihan Yin: Methodology (equal); Validation (equal); Writing – original draft (equal); Writing – review & editing (equal). Caiyun Shen: Conceptualization (equal); Methodology (equal); Writing – original draft (equal). Wei Yang: Validation (equal). Qi Chen: Validation (equal). Yu Liu: Validation (equal). Zhongcheng Ke: Writing – review & editing (equal). Changjiang Li: Conceptualization (equal); Writing – original draft (equal); Writing – review & editing (equal).

The data that support the findings of this study are available from the corresponding author upon reasonable request.