Synthesis and Characterization of Superhydrophobic Epoxy Resin Coating with SiO2@CuO/HDTMS for Enhanced Self-Cleaning, Photocatalytic, and Corrosion-Resistant Properties

The exceptional corrosion resistance and combined physical and chemical self-cleaning capabilities of superhydrophobic photocatalytic coatings have sparked significant interest among researchers. In this paper, we propose an economical and eco-friendly superhydrophobic epoxy resin coating that incorporates SiO2@CuO/HDTMS nanoparticles modified with Hexadecyltrimethoxysilane (HDTMS). The application of superhydrophobic coatings effectively reduces the contact area between the metal surface and corrosive media, leading to a decreased corrosion rate. Additionally, the incorporation of nanomaterials, exemplified by SiO2@CuO core–shell nanoparticles, improves the adhesion and durability of the coatings on aluminum alloy substrates. Experimental data from Tafel curve analysis and electrochemical impedance spectroscopy (EIS) confirm the superior corrosion resistance of the superhydrophobic modified aluminum alloy surface compared to untreated surfaces. Estimations indicate a significant reduction in corrosion rate after superhydrophobic treatment. Furthermore, an optical absorption spectra analysis of the core–shell nanoparticles demonstrates their suitability for photocatalytic applications, showcasing their potential contribution to enhancing the overall performance of the coated surfaces. This research underscores the promising approach of combining superhydrophobic properties with photocatalytic capabilities to develop advanced surface modification techniques for enhanced corrosion resistance and functional properties in diverse industrial settings.


Introduction
Corrosion results in economic losses, safety incidents, environmental pollution, and other detrimental effects that require urgent attention in our society [1,2].Typically, coating the substrates surface to isolate it from the surrounding environment is the most effective corrosion protection method [3,4].
Among various coatings, superhydrophobic materials exhibit a strong repellent effect towards liquids, thereby enabling oil/water separation [5], microplastics removal [6], photodegradation of dyes [7], desalination [8], heavy metal removal [9], corrosion resistance [10], and self-cleaning [11] capabilities.Generally, the wettability of a surface is influenced by its surface energy and roughness [12,13].Therefore, exposure of a superhydrophobic surface to harsh environments (acidic or alkaline solutions, organic solvents, abrasion, washing, UV radiation, and high temperatures) can lead to the deterioration of surface roughness (loss of nanoparticles or substrate abrasion) or an increase in surface energy (decomposition of hydrophobic long chains), resulting in the loss of superhydrophobic properties and a significant reduction in the service life of superhydrophobic materials [14,15].Constructing highly reliable hydrophobic and durable coatings remains a formidable challenge.To enhance the adhesion of superhydrophobic coatings effectively, the concept of "nanoparticles + binder" has been proposed to improve the durability of the coatings [16][17][18].Surface-modified SiO 2 nanoparticles, which are stable and inherently hydrophobic, are utilized in superhydrophobic coatings.Sharma et al. prepared a triethoxyoctylsilane-modified SiO 2 nanoparticle-based superhydrophobic coating by the solution method to prevent the corrosion of mild steel [19].Wang et al. reported a robust superhydrophobic SiO 2 /epoxy coating prepared by a one-step spraying method for corrosion protection of aluminum alloy [20].Luque et al. constructed a spiky SiO 2 nanoparticle supramolecular polymer superhydrophobic coatings applied to transparent oil-water separating [21].
Within the adhesive realm, epoxy resin emerges as a standout candidate due to its exceptional chemical stability, robust abrasion resistance, notable water repellency, and strong adhesion to substrates, positioning it as a favored alternative to fluoropolymers for formulating superhydrophobic coatings [22,23].Despite these advantages, a common issue with many superhydrophobic surfaces is their inherent oleophilic nature, making them susceptible to contamination by organic pollutants and leading to a gradual decline in hydrophobicity and the eventual loss of self-cleaning properties [24,25].Consequently, the fusion of superhydrophobicity with the photocatalytic decomposition of organic compounds has garnered considerable attention from the research community in recent years [26][27][28][29].Semiconductor oxides such as TiO 2 [30], SnO 2 [31], ZnO [32], NiO [33], Cu 2 O [34], and CuO [35] are extensively utilized as photocatalysts in the degradation of various pollutants, encompassing dyes, organic contaminants, natural organic substances, and pharmaceutical compounds.Notably, copper oxide (CuO), featuring a band gap within the range of 1.2~2.1 eV, stands out as a critical p-type semiconductor renowned for its catalytic, optical, antimicrobial, and cost-efficient attributes [36][37][38].Moreover, besides furnishing the requisite roughness for superhydrophobic coatings, SiO 2 can function as a core material for incorporating additional functional elements, facilitating the creation of core-shell architectures.In the realm of photocatalysis, the interplay between distinct components in composite materials enhances the efficiency of separating photo-generated electron-hole pairs, thereby prolonging the lifespan of active electrons, holes, and radicals.Consequently, SiO 2 is extensively employed as a carrier for semiconductor catalytic catalysts to fabricate core-shell configurations [39][40][41].
Herein, we present a straightforward approach for the fabrication of flower-shaped SiO 2 @CuO nanoparticles through liquid-phase reduction.A detailed analysis is conducted to elucidate the impact of physical structure and chemical composition on the performance of these nanoparticles.Subsequently, SiO 2 @CuO core-shell particles, surface-modified with HDTMS and dispersed in epoxy resin, were applied via spray coating onto an aluminum alloy substrate.This process aimed to develop long-lasting superhydrophobic coatings with the capability of photocatalytic degradation of organic compounds.The self-cleaning effectiveness and underlying principles of these coatings were thoroughly examined and discussed.

Preparation Methods
The preparation method of superhydrophobic coating has four steps, and the process flow is shown in Figure 1.

Preparation Methods
The preparation method of superhydrophobic coating has four steps, and the process flow is shown in Figure 1.Monodisperse spherical SiO2 particles were synthesized in a semi-batch reactor using sol-precipitation, which involved modifications to the Stöber method [42].It begins by measuring 32.5 mL of absolute ethanol, 42.5 mL of deionized water, and 18 mL of ammonia water, and combining them in beaker A. Liquid A is prepared by magnetic stirring in a constant-temperature water bath set at 30 °C, with a stirring speed of 400 rpm.In beaker B, 8 mL of TEOS and 90 mL of absolute ethanol are measured and mixed to form solution B through magnetic stirring at 30 °C.The stirring speed of liquid A is then increased to 600 rpm, and within 1 min, liquid B is added to it.Subsequently, the stirring speed is reduced back to 400 rpm, and the reaction is maintained for two hours.After the mixed solution is obtained, it undergoes differential centrifugation once the Monodisperse spherical SiO 2 particles were synthesized in a semi-batch reactor using sol-precipitation, which involved modifications to the Stöber method [42].It begins by measuring 32.5 mL of absolute ethanol, 42.5 mL of deionized water, and 18 mL of ammonia water, and combining them in beaker A. Liquid A is prepared by magnetic stirring in a constant-temperature water bath set at 30 • C, with a stirring speed of 400 rpm.In beaker B, 8 mL of TEOS and 90 mL of absolute ethanol are measured and mixed to form solution B through magnetic stirring at 30 • C. The stirring speed of liquid A is then increased to 600 rpm, and within 1 min, liquid B is added to it.Subsequently, the stirring speed is reduced back to 400 rpm, and the reaction is maintained for two hours.After the mixed solution is obtained, it undergoes differential centrifugation once the ammonia water has evaporated.As a part of the process, a five-time wash with an alcohol-water mixture (v:v = 1:1) is employed to eliminate ammonia water and other impurities.The sample was subsequently dried in a constant temperature drying oven set at 60 • C for a duration of 3 h to achieve the monodisperse spherical SiO 2 core.
In the second step, 0.001 mol of copper nitrate hydrate, 0.1 g of silicon dioxide, 0.1 g of polyethylene glycol, 0.2 g of melamine, and 12 mL of water are added to a container.The mixture is magnetically stirred at 500 rpm for 6 h in a water bath held at a constant temperature of 30 • C. Subsequently, 8 mL of 30% hydrogen peroxide is added, and stirring is continued for an additional 2 h.The mixture is then transferred to a hydrothermal kettle and allowed to react at 200 • C for 12 h.The solution is washed, and centrifugation is performed to collect the SiO 2 @CuO.
In the third step, 10 mL of water, 50 mL of ethanol, and 2 mL of HDTMS were individually added to the prepared silica and SiO 2 @CuO core-shell particles.The mixture was then reacted in a constant temperature water bath at 30 • C for 24 h.Subsequently, the centrifuged precipitate was collected, washed with ethanol, and dried at 60 • C for 3 h.Finally, the SiO 2 /HDTMS and SiO 2 @CuO/HDTMS modified with HDTMS are obtained.
In the fourth step, the aluminum alloy substrate (dimensions: 3 cm × 3 cm × 1 mm) underwent a series of preparation steps.Initially, it was meticulously cleaned using deionized water and subsequently subjected to a 10-min treatment with NaOH.This treatment induced chemical corrosion on the surface, thereby imparting roughness to the substrate.The treated substrate was subsequently washed with acetone and thoroughly dried in preparation for the resin coating.A mixture consisting of 0.1 g of epoxy resin, 0.3 g of curing agent, and 10 g of xylene was carefully combined, followed by sonication for 10 min to ensure homogeneity.The resulting mixture was then evenly sprayed onto the pre-treated substrate surface.
In the fifth step, 1 g of modified powder is combined with 10 g of xylene, subjected to ultrasonic dispersion for 10 min, and subsequently sprayed onto the aluminum alloy substrate using a spray gun.The resulting coating is baked at 140 • C for 2 h to achieve a superhydrophobic coating.

Characterization
The crystal phase of the sample was determined through X-ray diffraction (XRD) analysis in the 2θ range of 20-80 • , using a Japanese Ultima IV instrument.Surface morphology was examined via scanning electron microscopy (SEM), employing a Czech TESCAN MIRA LMS instrument.The UV spectra of the samples were recorded using a Shimadzu UV-3600 spectrophotometer.The surface wettability was assessed by a contact angle meter system from GBX Scientific Instruments.Contact angle measurements were conducted to study the wettability.Methylene blue (MB) absorption was quantified using a Lambda EZ 210 UV-vis spectrophotometer.Electrochemical measurements were carried out utilizing a potentiostat/galvanostat/EIS setup from Ametek PARSTA 4000. Figure 2 presents the X-ray diffraction (XRD) results for CuO, SiO 2 , and SiO 2 @CuO.The SiO 2 diffraction pattern displays a broad peak centered at 2θ = 21 • , suggesting its amorphous nature.On the other hand, SiO 2 /CuO exhibits well-defined peaks at 2θ= 32.5  202), (020), (202), (113), (311), ( 220), (311), and (222) crystal planes of anatase SiO 2 .Additionally, in the XRD pattern of SiO 2 @CuO, a decrease in the CuO peak intensity indicates a reduction in CuO content.

Characterization of SiO
The micro/nano-hierarchical structure plays a critical role in the development of superhydrophobic surfaces.As depicted in Figure 3a, the SEM image of SiO 2 showcases excellent dispersion and a smooth surface texture.Figure 3b presents the SEM image of the SiO 2 @CuO core-shell structure, formed through a standard hydrothermal reaction involving 0.1 g SiO 2 and 0.001 mol Cu(NO 3 ) 2 hydrate.The images confirm the successful deposition of CuO onto the surface of SiO 2 , with CuO evenly distributed, leading to the formation of a micro/nanoscale hierarchical structure.Given that the micro/nanohierarchical configuration is crucial for achieving superhydrophobic properties, the surface topography of the particles holds paramount significance.The micro/nano-hierarchical structure plays a critical role in the development of superhydrophobic surfaces.As depicted in Figure 3a, the SEM image of SiO2 showcases excellent dispersion and a smooth surface texture.Figure 3b presents the SEM image of the SiO2@CuO core-shell structure, formed through a standard hydrothermal reaction involving 0.1 g SiO2 and 0.001 mol Cu(NO3)2 hydrate.The images confirm the successful deposition of CuO onto the surface of SiO2, with CuO evenly distributed, leading to the formation of a micro/nanoscale hierarchical structure.Given that the micro/nanohierarchical configuration is crucial for achieving superhydrophobic properties, the surface topography of the particles holds paramount significance.
The SEM image of SiO2@CuO core-shell particles post-calcination at 400 °C for 2 h is displayed in Figure 3c.Calcination is essential to remove any residual surfactants that might be present during the synthesis process, ensuring a clean and well-defined coreshell structure free from any disrupting surfactants [43].Figure 3d-f depict the elemental mapping of oxygen (O), silicon (Si), and copper (Cu) within the structure exhibited in Figure 3c, revealing a close correlation between the distribution of these elements and the morphology of the SiO2@CuO core-shell particles.Figure 3g presents the TEM image of SiO2@CuO, demonstrating a relatively uniform loading of CuO onto the SiO2 surface to form the core-shell structure.The TEM images of SiO2@CuO/HDTMS nanoparticle was shown in Supplementary Figure S1.Compared with the SiO2@CuO, the shell layer of nanoparticles modified by HDTMS is damaged to a certain extent.
Figure 3h displays the HRTEM image of SiO2@CuO, while Figure 3i

Chemical Composition
The surface functional groups of the samples were analyzed using infrared spectroscopy.Figure 4a-c depict the FT-IR spectra of SiO2, SiO2@CuO, and SiO2@CuO/HDTMS, respectively.In all three samples, the broad peak observed at 3446 cm −1 corresponds to the -OH asymmetric absorption vibration peak of water, suggesting the presence of water adsorbed on their surfaces.The peak at 1049 cm −1 is associated −1 The SEM image of SiO 2 @CuO core-shell particles post-calcination at 400 • C for 2 h is displayed in Figure 3c.Calcination is essential to remove any residual surfactants that might be present during the synthesis process, ensuring a clean and well-defined coreshell structure free from any disrupting surfactants [43].Figure 3d-f depict the elemental mapping of oxygen (O), silicon (Si), and copper (Cu) within the structure exhibited in Figure 3c, revealing a close correlation between the distribution of these elements and the morphology of the SiO 2 @CuO core-shell particles.Figure 3g presents the TEM image of SiO 2 @CuO, demonstrating a relatively uniform loading of CuO onto the SiO 2 surface to form the core-shell structure.The TEM images of SiO 2 @CuO/HDTMS nanoparticle was shown in Supplementary Figure S1.Compared with the SiO 2 @CuO, the shell layer of nanoparticles modified by HDTMS is damaged to a certain extent.
Figure 3h displays the HRTEM image of SiO 2 @CuO, while Figure 3i offers a closer look at Figure 3h, unveiling distinct crystal lattice patterns of CuO.The HRTEM image reveals an interplanar spacing of 0.2322 nm, aligning closely with the CuO (111) interplanar spacing (PDF#89-5899).Additionally, Figure 3i presents the selected area electron diffraction (SAED) image of SiO 2 @CuO, displaying the crystal planes (002) and (311) of CuO.Moreover, the morphological features of SiO 2 @CuO are consistent with observations from SEM analysis.

Chemical Composition
The surface functional groups of the samples were analyzed using infrared spectroscopy.Figure 4a-c depict the FT-IR spectra of SiO 2 , SiO 2 @CuO, and SiO 2 @CuO/HDTMS, respectively.In all three samples, the broad peak observed at 3446 cm −1 corresponds to the -OH asymmetric absorption vibration peak of water, suggesting the presence of water adsorbed on their surfaces.The peak at 1049 cm −1 is associated with the asymmetric stretching vibration peak of Si-O-Si, while the peaks at 792 cm −1 and 556 cm −1 are related to the symmetric stretching and bending vibrations of Si-OH, respectively, signifying the characteristics of silica.In Figure 4c, which represents the spectrum of SiO 2 @CuO/HDTMS, the peaks at 2927 cm −1 and 2846 cm −1 are attributed to -CH 3 -and -CH 2 -groups.Additionally, the absorption peaks at 1632 cm −1 and 1460 cm −1 can be attributed to the stretching vibration peaks of the C-H bond.A comparison between Figure 4b,c reveals the presence of peaks related to -CH 3 -, -CH 2 -, and -CH, indicating the successful modification of SiO 2 @CuO particles with HDTMS.The results show that silane has successfully modified SiO 2 @CuO particles.The X-Ray Photoelectron Spectroscopy (XPS) technique was employed to conduct a comprehensive analysis of the oxidation states and chemical composition of the synthesized SiO2@CuO core-shell particles.Figure 5a presents the full-scan spectrum from XPS, demonstrating the presence of Si, Cu, and O elements in the composite particles.The distinctive binding energies of Cu 2+ and Cu + verified the presence of Cu 2+ in SiO2@CuO.The detailed XPS spectrum of Cu 2p in Figure 5b exhibits strong Cu 2+ satellite peaks at binding energies of 942.5 eV and 962.5 eV, further confirming the existence The X-Ray Photoelectron Spectroscopy (XPS) technique was employed to conduct a comprehensive analysis of the oxidation states and chemical composition of the synthesized SiO 2 @CuO core-shell particles.Figure 5a presents the full-scan spectrum from XPS, demonstrating the presence of Si, Cu, and O elements in the composite particles.The distinctive binding energies of Cu 2+ and Cu + verified the presence of Cu 2+ in SiO 2 @CuO.The detailed XPS spectrum of Cu 2p in Figure 5b exhibits strong Cu 2+ satellite peaks at binding energies of 942.5 eV and 962.5 eV, further confirming the existence of Cu 2+ in SiO 2 @CuO.Moreover, a peak at 934.3 eV also supports the presence of Cu 2+ in the composite material [44].Additionally, a subtle peak at 932.1 eV was observed, indicating the presence of Cu species on the surface of SiO 2 @CuO.During the calcination process, the leftover organic material from the hydrothermal reaction facilitates the reduction of Cu 2+ .A single broad peak at 103.54 eV was identified in the Si 2p XPS spectrum, as shown in Figure 5c.This observation is consistent with the Si valence state of +4 in the SiO 2 crystal phase [45].Figure 5d displays the deconvoluted spectrum of the O 1s peak, which exhibits two distinct peaks.Previous studies suggest that the peak at a lower binding energy (532.0 eV) can be attributed to lattice oxygen species in metal oxides, while the peak at a higher binding energy (533.2eV) is associated with oxygen in SiO 2 and/or surface hydroxyl species on the SiO 2 @CuO system [46].The presence of carbon in the spectra mainly arises from atmospheric contaminants.The deconvoluted C 1s spectra (Supplementary Figure S2

Bandgap Calculations
By analyzing the optical absorption spectra of SiO2 and SiO2@CuO core-shell particles as shown in Figure 6, we can ascertain their band structure and edge characteristics.In the ultraviolet spectral range, a substantial increase in light absorption is observed, leading to the emergence of an absorption edge corresponding to the ultraviolet bandgap energy.Plotting the relationship between the absorption coefficient and photon energy of the SiO2@CuO core-shell particles revealed band gaps of 4.59 eV for SiO2 and 2.01 eV for SiO2@CuO.These data enable the determination that the wavelength of absorbable light is equal to or less than 616 nm.

Bandgap Calculations
By analyzing the optical absorption spectra of SiO 2 and SiO 2 @CuO core-shell particles as shown in Figure 6, we can ascertain their band structure and edge characteristics.In the ultraviolet spectral range, a substantial increase in light absorption is observed, leading to the emergence of an absorption edge corresponding to the ultraviolet bandgap energy.Plotting the relationship between the absorption coefficient and photon energy of the SiO 2 @CuO core-shell particles revealed band gaps of 4.59 eV for SiO 2 and 2.01 eV for SiO 2 @CuO.These data enable the determination that the wavelength of absorbable light is equal to or less than 616 nm.

Bandgap Calculations
By analyzing the optical absorption spectra of SiO2 and SiO2@CuO core-shell particles as shown in Figure 6, we can ascertain their band structure and edge characteristics.In the ultraviolet spectral range, a substantial increase in light absorption is observed, leading to the emergence of an absorption edge corresponding to the ultraviolet bandgap energy.Plotting the relationship between the absorption coefficient and photon energy of the SiO2@CuO core-shell particles revealed band gaps of 4.59 eV for SiO2 and 2.01 eV for SiO2@CuO.These data enable the determination that the wavelength of absorbable light is equal to or less than 616 nm.

Morphology of SiO 2 @CuO/HDTMS Coating
The surface morphology and thickness of the coating play a pivotal role in determining its wettability.Figure 7a-d depict the surface morphology of the SiO 2 @CuO/HDTMS coating.As illustrated in Figure 7a, the SiO 2 @CuO/HDTMS composite particles are uniformly dispersed on the coating surface, creating a rough surface structure.Figure 7b shows that some composite particles are partially exposed and bonded in place, effectively anchoring the hydrophobic particles.Figure 7c offers an enlarged view of Figure 7b, demonstrating the intact morphology and structure of the SiO 2 @CuO/HDTMS composite.Additionally, the micro/nanoscale layered rough structure on the coating's surface acts as a foundational structure for achieving superhydrophobicity. Figure 7d presents a magnified image of Figure 7c.The study reveals that the morphology and structure of the SiO 2 @CuO composite remain unchanged after the modification and spraying processes, likely due to the formation of Si-O-Cu bonds between SiO 2 @CuO.

Morphology of SiO2@CuO/HDTMS Coating
The surface morphology and thickness of the coating play a pivotal role in determining its wettability.Figure 7a-d depict the surface morphology of the SiO2@CuO/HDTMS coating.As illustrated in Figure 7a, the SiO2@CuO/HDTMS composite particles are uniformly dispersed on the coating surface, creating a rough surface structure.Figure 7b shows that some composite particles are partially exposed and bonded in place, effectively anchoring the hydrophobic particles.Figure 7c offers an enlarged view of Figure 7b, demonstrating the intact morphology and structure of the SiO2@CuO/HDTMS composite.Additionally, the micro/nanoscale layered rough structure on the coating s surface acts as a foundational structure for achieving superhydrophobicity. Figure 7d presents a magnified image of Figure 7c.The study reveals that the morphology and structure of the SiO2@CuO composite remain unchanged after the modification and spraying processes, likely due to the formation of Si-O-Cu bonds between SiO2@CuO.

Liquid Repellent Properties of SiO2@CuO/HDTMS Coating
Thorough investigation was carried out to evaluate the repellent properties of the prepared coatings and glass slides against water, acid droplets (pH = 2), and alkaline droplets (pH = 11).As shown in Figure 8a, droplets placed on the SiO2@CuO/HDTMS coating surface exhibit a spherical shape and demonstrate significant repellency to these liquids.In contrast, when the same droplets are placed on the surface of the aluminum alloy sheet, as shown in Figure 8b, they spread extensively, indicating the hydrophilic

Liquid Repellent Properties of SiO 2 @CuO/HDTMS Coating
Thorough investigation was carried out to evaluate the repellent properties of the prepared coatings and glass slides against water, acid droplets (pH = 2), and alkaline droplets (pH = 11).As shown in Figure 8a, droplets placed on the SiO 2 @CuO/HDTMS coating surface exhibit a spherical shape and demonstrate significant repellency to these liquids.In contrast, when the same droplets are placed on the surface of the aluminum alloy sheet, as shown in Figure 8b, they spread extensively, indicating the hydrophilic nature of the aluminum alloy sheet surface.Table 1 presents the Water Contact Angles (WCAs) of their surfaces for different droplets.The results unequivocally show that SiO 2 @CuO/HDTMS plays a critical role in conferring superhydrophobicity to the SiO 2 @CuO/HDTMS coatings.The hydrophobic functional groups of CH 2 =CH-and -CH 3 in HDTMS are introduced into the surface of SiO 2 and SiO 2 @CuO through modification, giving the material lower surface energy.Modified SiO 2 and SiO 2 @CuO nanoparticles provided a nanoscale rough structure and lower surface energy after treatment.Nanoscale pits appeared between the materials.These pit structures can trap air, forming air pockets, causing fewer actual contact points between water and material, making it more difficult for water to spread on the surface.At this time, the contact state on the material is consistent with the Cassie model, and the water droplets easily roll off [48].A comparison was conducted to assess the contact angle between our developed coating and the commercially available superhydrophobic coating (Ultra-ever Dry ® ) produced by Ultra Tech International (as shown in Supplementary Materials Table S1).The results demonstrate that the coating in our work exhibits a comparable contact angle to commercial materials, while its practical performance is enhanced by additional functionalities such as photocatalysis.
Materials 2024, 17, x FOR PEER REVIEW 10 of 18 noscale pits appeared between the materials.These pit structures can trap air, forming air pockets, causing fewer actual contact points between water and material, making it more difficult for water to spread on the surface.At this time, the contact state on the material is consistent with the Cassie model, and the water droplets easily roll off [48].A comparison was conducted to assess the contact angle between our developed coating and the commercially available superhydrophobic coating (Ultra-ever Dry ® ) produced by Ultra Tech International (as shown in Supplementary Materials Table S1).The results demonstrate that the coating in our work exhibits a comparable contact angle to commercial materials, while its practical performance is enhanced by additional functionalities such as photocatalysis.

Self-Cleaning Property of SiO2@CuO/HDTMS Coating
The sliding angle (SA) of the SiO2@CuO/HDTMS coating was measured with a value of 4.6° (Figure 9a).The high water-repellent property of the surface with low watersliding angle helps to keep the surface clean like a lotus leaf.In the open air, many solid surfaces are contaminated by various types of dust particles.The self-cleaning efficacy of the coating was assessed through a simple simulation experiment, depicted in Figure 9b.In this setup, water droplets were deliberately dispensed from a syringe onto the coated surface.Remarkably, upon contact, the water droplets demonstrated an impressive selfcleaning capability, efficiently eliminating dirt and impurities from the surface.Consequently, the treated surface remained unsoiled and immaculate.

Self-Cleaning Property of SiO 2 @CuO/HDTMS Coating
The sliding angle (SA) of the SiO 2 @CuO/HDTMS coating was measured with a value of 4.6 • (Figure 9a).The high water-repellent property of the surface with low water-sliding angle helps to keep the surface clean like a lotus leaf.In the open air, many solid surfaces are contaminated by various types of dust particles.The self-cleaning efficacy of the coating was assessed through a simple simulation experiment, depicted in Figure 9b.In this setup, water droplets were deliberately dispensed from a syringe onto the coated surface.Remarkably, upon contact, the water droplets demonstrated an impressive self-cleaning capability, efficiently eliminating dirt and impurities from the surface.Consequently, the treated surface remained unsoiled and immaculate.

Durability of SiO2@CuO/HDTMS Coating
Coating stability is a critical property that must be thoroughly studied, particularly in outdoor environments.To assess the durability of the coatings, several samples were subjected to outdoor exposure for a three-month period.The changes in WCAs and their impact on photocatalytic degradation were analyzed.As shown in Figure 10, the initial WCA of the coating was 150.9°.Notably, after three months of exposure, there was minimal reduction in the WCA, with a measured value of 157.4° as compared to the initial reading.

Durability of SiO 2 @CuO/HDTMS Coating
Coating stability is a critical property that must be thoroughly studied, particularly in outdoor environments.To assess the durability of the coatings, several samples were subjected to outdoor exposure for a three-month period.The changes in WCAs and their impact on photocatalytic degradation were analyzed.As shown in Figure 10, the initial WCA of the coating was 157.4 • .Notably, after three months of exposure, there was minimal reduction in the WCA, with a measured value of 150.9 • as compared to the initial reading.

Durability of SiO2@CuO/HDTMS Coating
Coating stability is a critical property that must be thoroughly studied, particularly in outdoor environments.To assess the durability of the coatings, several samples were subjected to outdoor exposure for a three-month period.The changes in WCAs and their impact on photocatalytic degradation were analyzed.As shown in Figure 10, the initial WCA of the coating was 150.9°.Notably, after three months of exposure, there was minimal reduction in the WCA, with a measured value of 157.4° as compared to the initial reading.

Corrosion Resistance of the Coating
Corrosion resistance was evaluated by generating Tafel plots for both the untreated aluminum alloy surface and the superhydrophobic aluminum alloy surface, as shown in Figure 11a.Upon reaching a stable Open Circuit Potential (OCP) after immersion in a 3.5 wt% NaCl solution for at least one hour, the corrosion potential (E corr ) and corrosion current density (I corr ) were determined using the extrapolation method.The Ecorr and Icorr values for the untreated aluminum alloy surface presented in Table 1 were approximately and 1.9 × 10 −5 A/cm 2 , respectively, while the superhydrophobic aluminum alloy surface showed values of −0.94 V and 6.9 × 10 −7 A/cm 2 .A more positive Ecorr value indicates a lower risk of corrosion, while Icorr represents the corrosion rate.These results demonstrate that the superhydrophobic modification significantly reduces the likelihood and rate of corrosion on the aluminum alloy surface.

Corrosion Resistance of the Coating
Corrosion resistance was evaluated by generating Tafel plots for both the untreated aluminum alloy surface and the superhydrophobic aluminum alloy surface, as shown in Figure 11a.Upon reaching a stable Open Circuit Potential (OCP) after immersion in a 3.5 wt% NaCl solution for at least one hour, the corrosion potential (Ecorr) and corrosion current density (Icorr) were determined using the extrapolation method.The Ecorr and Icorr values for the untreated aluminum alloy surface presented in Table 1 were approximately −0.65 V and 1.9 × 10 −5 A/cm 2 , respectively, while the superhydrophobic aluminum alloy surface showed values of −0.94 V and 6.9 × 10 −7 A/cm 2 .A more positive Ecorr value indicates a lower risk of corrosion, while Icorr represents the corrosion rate.These results demonstrate that the superhydrophobic modification significantly reduces the likelihood and rate of corrosion on the aluminum alloy surface.Electrochemical impedance spectroscopy (EIS) was employed for a comprehensive assessment of the corrosion performance.Equivalent circuit models were utilized to analyze the EIS data, offering detailed insights into the corrosion mechanisms.Figure 11b displays the EIS spectra obtained from the untreated aluminum alloy surface and the superhydrophobic aluminum alloy surface in the NaCl solution.The larger Nyquist curve associated with the superhydrophobic aluminum surface indicates a reduced corrosion rate compared to the conventional aluminum surface.This notable difference highlights a substantial enhancement in the corrosion resistance of the aluminum surface after the superhydrophobic treatment.
Based on the obtained Icorr (µA/cm 2 ), the corrosion rate (CR) can be estimated using the following formula [49]: where M represents the metal s relative atomic mass (in g/mol), d signifies the metal s density (in g/cm 3 ), and n represents the number of electrons required to oxidize an atom of the element during the corrosion process, corresponding to the metal s valence state.
The Corrosion Inhibition Efficiency (CIE) can be calculated using the following formula [49]: where Icorr and I corr represent the corrosion current densities of the aluminum alloy surface before and after superhydrophobic modification, respectively.The computed values for the corrosion rate (CR) and Corrosion Inhibition Efficiency (CIE) are also presented in Table 2.These electrochemical parameters provide clear evidence that the superhydrophobic modification serves as an effective protective barrier for the aluminum sur- Electrochemical impedance spectroscopy (EIS) was employed for a comprehensive assessment of the corrosion performance.Equivalent circuit models were utilized to analyze the EIS data, offering detailed insights into the corrosion mechanisms.Figure 11b displays the EIS spectra obtained from the untreated aluminum alloy surface and the superhydrophobic aluminum alloy surface in the NaCl solution.The larger Nyquist curve associated with the superhydrophobic aluminum surface indicates a reduced corrosion rate compared to the conventional aluminum surface.This notable difference highlights a substantial enhancement in the corrosion resistance of the aluminum surface after the superhydrophobic treatment.
Based on the obtained I corr (µA/cm 2 ), the corrosion rate (CR) can be estimated using the following formula [49]: where M represents the metal's relative atomic mass (in g/mol), d signifies the metal's density (in g/cm 3 ), and n represents the number of electrons required to oxidize an atom of the element during the corrosion process, corresponding to the metal's valence state.The Corrosion Inhibition Efficiency (CIE) can be calculated using the following formula [49]: where I corr and I' corr represent the corrosion current densities of the aluminum alloy surface before and after superhydrophobic modification, respectively.The computed values for the corrosion rate (CR) and Corrosion Inhibition Efficiency (CIE) are also presented in Table 2.These electrochemical parameters provide clear evidence that the superhydrophobic modification serves as an effective protective barrier for the aluminum surface, resulting in a remarkable 96.5% reduction in CR following the application of the superhydrophobic treatment.The presence of the entrapped air layer on the superhydrophobic surface restricts water contact with the interfacial matrix phase, thus effectively preventing corrosion ions from accessing the aluminum surface and corroding the metal barrier.Additionally, the presence of entrapped air within the layered micro/nanostructures creates a repelling effect on corrosion ions, attributed to the Laplace pressure [50].The superhydrophobic modification noticeably delays the corrosion or degradation process of the aluminum surface.

Catalytic Degradation Performance
The photocatalytic performance of SiO 2 @CuO core-shell particles directly impact the SiO 2 @CuO/HDTMS coating.Figure 7 illustrates the results of diverse samples' photocatalytic degradation performance assessments.During the initial dark reaction phase, the C/C 0 values of various pollutant suspensions slightly decreased post-treatment with SiO 2 @CuO core-shell particles.30 min later, the SiO 2 group in the blank test reached adsorption equilibrium, indicating a certain level of pollutant adsorption by the samples.However, notable variations in degradation efficiency were observed among samples when exposed to different pollutant solutions in the photoreaction phase.After 300 min of UV irradiation, suspensions with SiO 2 and the control group exhibited nearly identical C/C 0 values, indicating that SiO 2 lacked photocatalytic properties but retained some adsorption ability.In contrast, the SiO 2 @CuO composites showed effective photocatalytic performance.It is crucial to highlight that photolysis, conducted solely on the irradiated dye, helped evaluate the impact of UV light on photodegradation.A lower photolysis rate constant implies that degradation results from the photocatalyst rather than light exposure alone.The relatively lower photocatalytic degradation rate of CuO can be attributed to its narrow bandgap, causing rapid electron-hole pair recombination.When CuO interacts with photons exceeding its band gap energy, electron-hole pairs are generated.The resulting holes react with water molecules to yield hydroxyl radicals, while electrons convert dissolved oxygen into superoxide radicals.Protonation of superoxide ions leads to further hydroxyl radical formation.These highly reactive hydroxyl radicals, generated through light-assisted mechanisms, attack dye molecules, facilitating their degradation into water and carbon dioxide [51].The superhydrophobic performance owing to the WCA and SA of the SiO2@CuO/HDTMS coatin developed in this work is superior to those observed for other similar coating, as shown in Supporting Materials Table S2.
Figure 12a shows the 100 mg/L rhodamine b solution, and Figure 12b shows the 100 mg/L methylene blue solution.In the first 60 min, the SiO 2 @CuO composite particles decompose the two solutions at approximately the same rate.However, the subsequent rate of decomposition of rhodamine b by particles is higher than that of methylene blue, and the C/C 0 value of rhodamine b after 300 min is 0.15, which is lower than that of methylene blue, 0.24.

Superhydrophobic and Photocatalytic Mechanism
The variations in wettability and adhesion of the SiO2@CuO/HDTMS coating were analyzed by considering the obtained results alongside a theoretical model from the existing literature.In Figure 13a, the interaction process between HDTMS and SiO2@CuO is depicted.Both SiO2 and CuO exhibit hydrophilic characteristics.Upon hydrolysis of HDTMS, hexadecylsilanol containing an -OH group is produced.This -OH group then undergoes condensation with the -OH groups existing on the surfaces of SiO2 and CuO.Consequently, this process converts the SiO2@CuO core-shell particles into a superhydrophobic state post-HDTMS modification [52,53].The findings indicate that the incorporation of Si-(CH2)15-CH3 onto the SiO2@CuO composite surface reduces the composite s surface energy, thereby enhancing its superhydrophobic properties.The photocatalytic mechanism is detailed in Figure 13b.CuO, as a photosensitive material, produces electron-hole pairs when exposed to ultraviolet radiation.the accumulated e-in the conduction band (CB) of CuO, which possesses high potential energy, is capable of reducing oxygen to produce the superoxide radical •O 2-.While the holes on the valence band (VB) of CuO can react with H2O to form •OH radicals.During the process of photo-oxidation, these free radicals efficiently degrade the dye, resulting in the production of harmless CO2 and H2O substances.

Superhydrophobic and Photocatalytic Mechanism
The variations in wettability and adhesion of the SiO 2 @CuO/HDTMS coating were analyzed by considering the obtained results alongside a theoretical model from the existing literature.In Figure 13a, the interaction process between HDTMS and SiO 2 @CuO is depicted.Both SiO 2 and CuO exhibit hydrophilic characteristics.Upon hydrolysis of HDTMS, hexadecylsilanol containing an -OH group is produced.This -OH group then undergoes condensation with the -OH groups existing on the surfaces of SiO 2 and CuO.Consequently, this process converts the SiO 2 @CuO core-shell particles into a superhydrophobic state post-HDTMS modification [52,53].The findings indicate that the incorporation of Si-(CH 2 ) 15 -CH 3 onto the SiO 2 @CuO composite surface reduces the composite's surface energy, thereby enhancing its superhydrophobic properties.The photocatalytic mechanism is detailed in Figure 13b.CuO, as a photosensitive material, produces electron-hole pairs when exposed to ultraviolet radiation.the accumulated e-in the conduction band (CB) of CuO, which possesses high potential energy, is capable of reducing oxygen to produce the superoxide radical •O 2− .While the holes on the valence band (VB) of CuO can react with H 2 O to form •OH radicals.During the process of photo-oxidation, these free radicals efficiently degrade the dye, resulting in the production of harmless CO 2 and H 2 O substances.

Conclusions
In summary, SiO2@CuO core-shell particles were synthesized using the hydrothermal method.The particles were then functionalized with HDTMS and sprayed onto an aluminum alloy surface, leading to the formation of a superhydrophobic coating with self-cleaning properties and photocatalytic functionalities.Through the application of superhydrophobic coatings, the contact area between the metal surface and corrosive media is minimized, leading to a reduction in the corrosion rate.The incorporation of nanomaterials, such as SiO2@CuO core-shell nanoparticles, further improves the adhesion and durability of superhydrophobic coatings on aluminum alloy substrates.Additionally, the utilization of copper oxide (CuO) nanoparticles for photocatalytic degradation of organic compounds demonstrates potential for enhancing the overall performance of the coated surfaces.The Tafel curve and EIS analysis showed a significant decrease in the corrosion rate of the aluminum alloy surface after superhydrophobic treatment, indicating an improved corrosion resistance compared to untreated surfaces.The combination of superhydrophobic coatings and photocatalytic degradation presents a promising approach to enhance the corrosion resistance of metal surfaces, particularly aluminum alloys.

Supplementary Materials:
The following supporting information can be downloaded at www.mdpi.com/xxx/s1, Figure S1: TEM images of SiO2@CuO/HDTMS nanoparticle; Figure S2: XPS narrow scans spectra of C1s; Figure S3: Before and after images of the degradation of (a) Rhodamine B and (b) Methylene Blue.The volumetric flask displays the color of the solution before degradation, while the centrifugal tube displays the color of the solution after degradation; Table S1: A comparison was conducted to assess the contact angle between our developed coating and the commercially available superhydrophobic coating (Ultra-ever Dry ® ) [54][55][56][57][58][59]; Table S2: Com-

Conclusions
In summary, SiO 2 @CuO core-shell particles were synthesized using the hydrothermal method.The particles were then functionalized with HDTMS and sprayed onto an aluminum alloy surface, leading to the formation of a superhydrophobic coating with self-cleaning properties and photocatalytic functionalities.Through the application of superhydrophobic coatings, the contact area between the metal surface and corrosive media is minimized, leading to a reduction in the corrosion rate.The incorporation of nanomaterials, such as SiO 2 @CuO core-shell nanoparticles, further improves the adhesion and durability of superhydrophobic coatings on aluminum alloy substrates.Additionally, the utilization of copper oxide (CuO) nanoparticles for photocatalytic degradation of organic compounds demonstrates potential for enhancing the overall performance of the coated surfaces.The Tafel curve and EIS analysis showed a significant decrease in the corrosion rate of the aluminum alloy surface after superhydrophobic treatment, indicating an improved corrosion resistance compared to untreated surfaces.The combination of superhydrophobic coatings and photocatalytic degradation presents a promising approach to enhance the corrosion resistance of metal surfaces, particularly aluminum alloys.

Supplementary Materials:
The following supporting information can be downloaded at https: //www.mdpi.com/article/10.3390/ma17081849/s1, Figure S1: TEM images of SiO 2 @CuO/HDTMS nanoparticle; Figure S2: XPS narrow scans spectra of C1s; Figure S3: Before and after images of the degradation of (a) Rhodamine B and (b) Methylene Blue.The volumetric flask displays the color of the solution before degradation, while the centrifugal tube displays the color of the solution after degradation; Table S1: A comparison was conducted to assess the contact angle between our developed coating and the commercially available superhydrophobic coating (Ultra-

Figure 1 .
Figure 1.Schematic diagram of the preparation process of superhydrophobic coating.

Figure 1 .
Figure 1.Schematic diagram of the preparation process of superhydrophobic coating.
offers a closer look at Figure 3h, unveiling distinct crystal lattice patterns of CuO.The HRTEM image reveals an interplanar spacing of 0.2322 nm, aligning closely with the CuO (111) interplanar spacing (PDF#89-5899).Additionally, Figure 3i presents the selected area electron diffraction (SAED) image of SiO2@CuO, displaying the crystal planes (002) and (3 -11) of CuO.Moreover, the morphological features of SiO2@CuO are consistent with observations from SEM analysis.

Figure 11 .
Figure 11.Tafel plots (a) and Nyquist plots (b) of the untreated aluminum alloy surface and superhydrophobic aluminum alloy surface in 3.5 wt% NaCl solution.

Figure 11 .
Figure 11.Tafel plots (a) and Nyquist plots (b) of the untreated aluminum alloy surface and superhydrophobic aluminum alloy surface in 3.5 wt% NaCl solution.

Figure 12 .
Figure 12.(a) Photocatalytic degradation effect of samples on rhodamine b.(b) Photocatalytic degradation effect of samples on methylene blue.

Figure 12 .
Figure 12.(a) Photocatalytic degradation effect of samples on rhodamine b.(b) Photocatalytic degradation effect of samples on methylene blue.

Table 1 .
WCAs of the surfaces of untreated Al slide and the SiO@CuO/HDTMS coating to different droplets.

Table 1 .
WCAs of the surfaces of untreated Al slide and the SiO@CuO/HDTMS coating to different droplets.

Table 2 .
Corrosion potential (E corr ), corrosion current density (I corr ), CR, and the CIE of the untreated aluminum alloy surface and superhydrophobic aluminum alloy surface.