Journal of Sol-Gel Science and Technology https://doi.org/10.1007/s10971-022-05749-5 ORIGINAL PAPER: SOL-GEL AND HYBRID MATERIALS WITH SURFACE MODIFICATION FOR APPLICATIONS Superhydrophobic ZnO thin film modified by stearic acid on copper substrate for corrosion and fouling protections Milad Abdolahzadeh Saffar1 Akbar Eshaghi ● 1 ● Mohammad Reza Dehnavi1 1234567890();,: 1234567890();,: Received: 23 November 2021 / Accepted: 18 February 2022 © The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature 2022 Abstract In the present study, a two-stage process was used to create hydrophobic and anti-fouling thin films on copper substrates. Initially, zinc oxide (ZnO) thin film was deposited on the copper substrate via a sol-gel method. Then, the film was modified with stearic acid. The structure and morphology of the thin films were characterized through X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and Fourier transform infrared (FTIR) spectroscopy. The water contact angle on the film surface was investigated by a water contact angle analyzer. Potentiodynamic polarization and salt spray tests were used to study the corrosion behavior of the copper and coated copper samples. Also, the antifouling properties of the thin films were investigated. Based on the FE-SEM result, the zinc oxide nanoparticle size was obtained as 60 nm. The water contact angle on the copper and coated copper samples increased from 39° to 155°. The corrosion current densities of the copper and coated copper samples were reduced from 1.31 μA/cm2 to 2.7 × 10−3 μA/cm2. In addition, the thin film showed the antifouling effect. * Akbar Eshaghi Eshaghi.akbar@gmail.com 1 Department of Materials Engineering, Malek Ashtar University of Technology, Shiraz, Iran Journal of Sol-Gel Science and Technology Graphical abstract Keywords Zinc oxide Superhydrophobic Anti-fouling Corrosion Sol–gel Stearic acid ● ● ● 1 Introduction When unprotected materials are immersed in a water environment, seawater, or freshwater, unwanted biological deposits, containing algae and marine animals, are formed on their surfaces [1, 2]. Accumulation and growth of these biological deposits on the hull surface increase the surface roughness, friction, and corrosion rate in marine systems thus incurring unnecessary costs [3]. Also, with microorganisms attaching onto the ship hull, these organisms may be transferred to another area which they do not belong [4]. This leads to ecological damage and, thus endangers the ecosystem balance [5, 6]. To prevent this problem, the marine structure surface could be covered with wax, bitumen, and lead sheets [7]. Antifouling paints were developed in the late of eighteenth and early nineteenth centuries [8]. Antifouling paints have always been considered as a significant solution to prevent marine fouling [9]. The use of tributyltin began in the middle of the ● ● twentieth century [10]; however, it has detrimental effects on moss larvae, causing the larvae to detach from the hull surface [11]. In addition, the toxicity of tributyltin on marine animals has been investigated [12]. These studies have reported the presence of tributyltin in the bodies of most marine animals including fish, seabirds, and whales [10]. Then, researchers began to replace the tributyltin with new materials [13–15]. The use of nanotechnology, or in other words, surface engineering technology at the atomic and molecular scales, provides unique opportunities for establishing antifouling effects [16–20]. Eshaghi et al. [21] developed superhydrophobic nano-coatings with antifouling properties. The superhydrophobic coatings prevents marine organism adhesion on the marine structures and equipment [22–24]. In this study, zinc oxide (ZnO) layer was deposited on the copper substrate. Then, to reduce the surface energy, the coated samples were deposited by stearic acid (SA). Superhydrophobic, antifouling, and corrosion behavior of the pieces were investigated. Journal of Sol-Gel Science and Technology 2 Experimental procedure 2.1 Materials High purity and laboratory-grade materials, including ethanolamine (C2H7NO, Merck, 99%), acetic acid (CH3COOH, Merck, 96%), stearic acid (C18H36O2, Merck, 98%), and diethanolamine (C4H11NO2, Merck, 97%) were used to synthesis the solutions. Sulfuric acid (H2SO4, Merck, 60%) and hydrochloric acid (HCl, Merck, 37%) were also applied to wash and prepare the substrates. 2.2 Preparation of the samples The adhesion of the ZnO film on the copper substrate is critical. Before ZnO film deposition, Copper substrates were polished through mechanical polishing (SiC sandpaper) to create suitable surface roughness. The adhesion is obtained by mechanical interlocking. The samples were also washed with soap and water to remove primary contaminants and soaked in 60% sulfuric acid for 15 min to remove residual grease. In the next step, the samples were immersed in a hydrochloric acid solution (50 wt.%) for 5 min. Copper substrates were then placed for 10 min in acetone in an ultrasonic bath for final cleaning. Finally, the samples were washed with deionized water for 2 min and dried in the oven at 50 °C for 60 min. 2.3 Preparation of the zinc oxide (ZnO) film To prepare 0.2 M zinc oxide solution, first 4.38 g of the zinc acetate was added to 50 ml solution of water and Fig. 1 Schematic diagram of the thin film fabrication process ethanol (water to ethanol ratio 3): (1) under magnetic stirring for 30 min. To obtain a clear and stable solution, a few drops of the acetic acid were added. The volume of the solution increased to 100 ml by adding ethanol. It was then placed inside a water bath at 60 °C and stirred for 60 min. The solution was then slowly cooled to an ambient temperature, whereby finally zinc oxide sol was obtained. The ZnO thin film was deposited on the copper substrate via a dip-coating method. The prepared film was calcinated at 350 °C for 5 min (heating rate of 4 °C/min). The thickness of the ZnO films was increased by repeating the cycles from withdrawing to heating. This procedure was repeated 20 times. The thickness of the ZnO film was about 500 nm. 2.4 Surface modification After deposition of the zinc oxide thin film to obtain an appropriate surface morphology, the film surface should be chemically modified with a low surface energy material. Stearic acid was used for this purpose. The stearic acid powder was dissolved in 100 ml of acetone and subjected to magnetic stirring at 40 °C for 120 min. For the stability of the solution, 3 ml of diethanolamine was added to the solution and putted in the ultrasonic bath at 50 kHz for 10 min. The spray coating method was employed to deposit the solution on the zinc oxide film surface (air pressure was adjusted to 1.5 bar, and the distance was 10 cm). Next, the samples were dried in an oven at 80 °C (2 °C/min) for 120 min. The step of the modified zinc oxide thin film deposition on the copper substrate is schematically shown in Fig. 1. Journal of Sol-Gel Science and Technology 2.5 Thin film characterization The morphology of the film surface was investigated using a field emission scanning electron microscope (FE-SEM, voltage of 15 kV). Surface and chemical analyses of the film surface were performed via Fourier transform infrared spectroscopy (FTIR) and EDS analysis methods. The crystalline structure of the thin film was investigated through the grazing X-ray diffraction method. Water contact angles (WCAs) were measured using a video and photo-based contact angle measurement system (Dino-lite, AM-4515T8-Edge). To evaluate the corrosion behavior of the thin film, potentiodynamic polarization tests were performed using a galvanostat/potentiostat (EG&G, A273) in a standard corrosion cell with Pt electrode and an Ag/AgCl reference electrode saturated with KCl (SSE). NaCl solution (3.5 wt%) was used as a corrosive medium. The potentiodynamic polarization curve was obtained from −50 to + 250 mV relative to the open-circuit potential at a sweep rate of 1 mV/s. A salt spray test was also utilized to measure the corrosion resistance of the samples. The samples were placed in a salt spray chamber for 120 h at Fig. 2 FE-SEM images of the uncoated copper surface (a), ZnO film with ×50,000 magnification with water droplet placement (b), ZnO film with ×100,000 magnification (c), and SA film with ×100,000 magnification with water droplet (d) 40 ± 5 °C (sodium chloride solution 5 wt%, pH = 6.5–7.2). Finally, the antifouling test was performed in a glass container with a dimension of 30 × 30 cm2. An electric motor was used to supply air. Seawater containing fouling was obtained from the Caspian Sea, and high-power LED lamps were used to accelerate the fouling growth. 3 Results and discussion FE-SEM was used to study the nanoparticle size distribution and surface morphology. According to Fig. 2a, the surface of the copper substrate is completely heterogeneous and porous. Figure 2b, c displays the surface morphology of the zinc oxide thin film. As can be seen, the shape of the particles is quasi-spherical, and the average size is 60 nm. Figure 2d reveals the morphology of the modified surface of the zinc oxide film with stearic acid. As can be seen from Fig. 3, micro-nano structures have been formed on the surface of the film surface. The existence of such structures is one of the main factors for the formation Journal of Sol-Gel Science and Technology Fig. 3 Schematic diagram of the thin film structure and Cassie model of superhydrophobicity [25]. Indeed, according to the Cassie model [26], these structures reduce the contact surface of the droplet on the solid surface through trapping air packages, thus facilitating the slip of the droplet from the surface and increasing the WCA. EDS elemental analysis was performed to confirm the successful formation of the ZnO and SA films. As displayed in Fig. 4a, the elements Cu, O, and Zn are present within the film structure, confirming the formation of the ZnO film due to strong Zn and O peaks. The strong Cu peak is also related to the copper substrate. The EDS results indicate the successful formation of the ZnO film on the copper substrate. According to Fig. 4b, the presence of the two strong peaks, O and C elements, confirms the formation of the stearic acid film. Figure 5 indicates the X-ray diffraction pattern of the ZnO thin film. Considering this pattern and comparing it with standard zinc oxide patterns (JCPDS card number, 1451-351), it was found that the sample has a hexagonal wurtzite crystal structure. It can be seen that the X-ray diffraction angles associated with extreme peaks are slightly increased compared to the diffraction angles of the standard JCPDS card (145136). This is due to the slight deviation of the hexagonal wurtzite lattice from its equilibrium structure due to inherent point defects within the zinc oxide lattice, including oxygen voids or interstitial Zn2 + ions. It can also be seen that the three peaks of (100), (002), and (101) hexagonal wurtzite crystals are more intense than the other peaks. The intensity of these three peaks is close to each other. Thus, the crystallites in all three samples have been oriented in three main directions <100>, <002>, and <101>. Indeed, their orientation has been random, though the intensity of the (101) peak has been more intense than that of the other two peaks. This result is consistent with other results produced via a sol–gel method [27]. Figure 6 depicts the FTIR spectrum of the stearic acidcoated samples. In this spectrum, two strong peaks are observed at the wavenumbers of 2912 and 2847 cm−1, which correspond to the symmetric and asymmetric stretching vibrations of the CH in the –CH2 groups [28]. As also, the peak corresponding to 2959 cm−1 is assigned to the CH3 group [29]. The absorption band at 1697 cm−1 is associated with the carbonyl group (C=O) [28], while the absorption bands at 1296 and 1296 cm−1 are related to the unstable vibration of the CO and OH groups in the carboxyl group (–COOH) [30]. Also, the peaks at 1469 and 563 cm−1 belong to the C–H bending vibration and C–C carbon chain [31]. The presence of the CH3, –CH2, and COO bonds in the stearic acid can undoubtedly reduce the surface energy and enhance the hydrophobic property [29]. Figure 7 shows the static contact angle of the water droplets on the pristine copper surface, copper surface prepared with sandpaper (numbers 600 and 1200), ZnO coated copper surface, and ZnO coated copper surface modified with different concentrations of the stearic acid. It is well known that to obtain a superhydrophobic surface, both roughness and low surface energy parameters need to be present at the same time [32]. It is observed that the contact angle obtained on the ZnO coated copper surface is 74 ± 3° and the optimal concentration of the stearic acid for the highest contact angle (155 ± 3°) is 6 g/l. Stearic acid creates the minimum surface energy on the surface due to the formation of the CH3 groups. In this way, water and fouling repellent properties can be achieved. When stearic acid is applied to the surface, it is created a dense outer CH3 surface for the maximum repellent effect. The proposed mechanism of the zinc oxide with an applied stearic acid coating is illustrated in Fig. 8. The potentiodynamic polarization diagrams of the samples after 0.5 h immersion in 3.5 wt% sodium chloride solution are shown in Fig. 9. In the bare copper diagram, a decline in the current density at a potential of 60 mV is observed in the anode branch. At this potential, as the current density diminishes, the diagram enters the quasiinactive region, which is similar to the results obtained by other researchers [33, 34]. Indeed, the reduction in the current density in the anodic branch is due to forming a CuCl2 film on the copper surface. The film is formed through an anodic dissolution reaction of the copper in the active region of the diagram [35]. The corrosion current Journal of Sol-Gel Science and Technology Fig. 4 EDS spectra of the ZnO (a) and SA (b) films Fig. 5 X-ray diffraction pattern of the ZnO thin film Journal of Sol-Gel Science and Technology Fig. 6 FTIR spectrum of the stearic acid-coated sample Fig. 7 Water contact angle images on the uncoated copper surface (a), after sandpaper preparation 600 (b) and 1200 (c), ZnO coating (d), after modification by stearic acid at different concentrations; 5 g/l (e), 5.5 g/l (f), 6 g/l (g) and 6.5 g/l (h) Fig. 8 Reaction mechanism for ZnO and SA [36] Journal of Sol-Gel Science and Technology density (icorr) and the corrosion potential (Ecorr) in Fig. 9 have been obtained using the extrapolation method with the values reported in Table 1. Fig. 9 Potentiodynamic polarization diagram of the samples Table 1 Corrosion characteristics of the samples Sample IE (%) Rp (KΩ cm2) βα (mV dec−1) βc (mV dec−1) icorr(μA cm−2) Ecorr (mV) Cu – 13.9 ± 1.15 0.05 0.25 1.31 ± 0.11 −203 ± 7 Cu + SA 90 13.8 ± 21 0.065 0.12 1.405 ± 0.015 −298 ± 25 577.3 ± 196 0.06 0.29 Cu + ZnO/SA 98 Fig. 10 Light microscopic images of the samples under salt spray test; copper sample before (a) and after (b) salt spray, coated copper sample before (c) and after (d) salt spray Table 1 shows that the SA coating cannot protect the copper substrate from corrosion in the presence of chloride ions. The corrosion potential of this coating is almost equal to that of the uncoated copper. This is due to the lack of proper adhesion to the copper substrate. As outlined in Table 1, the ZnO/SA coating showed better corrosion protection than the SA coating and copper samples. This coating reduced the icorr in the anodic and cathodic branches. The effect on both anodic and cathodic branches indicates that the anodic (dissolution of the copper substrate in the anodic regions) and cathodic reactions are limited by the coating. This can be attributed to the protective properties of the coating. ZnO coating can be adhesive on the copper substrate through a Cu-Zn bonding. Indeed, this coating prevents the penetration of aggressive agents (such as chloride ions and oxygen molecules) from reaching the copper surface, thus reducing the corrosion reaction rates. Figure 10 indicates the salt spray test results of the uncoated and coated copper samples. According to Fig. 10a, b, the bare 2.71 × 10−3 ± 0.18 −154 ± 15 Journal of Sol-Gel Science and Technology copper has been completely attacked by corrosive ions, and corrosion effects can be seen on its entire surface. In contrast, corrosion is not visible on the coated surface and corrosion signs are only observed around the scratched areas in the coated copper substrate (Fig. 10c, d). As a result, there is no Fig. 11 Variation of the static contact angle on the coated copper substrate in the salt spray exposure Fig. 12 Schematic design of the anti-fouling performance Fig. 13 Fouling test on copper and coated copper surface after 1 day (a) and 30 days (b) sign of blistering, separation of the coating, or accumulation of particles in the coated sample. It indicates the appropriate resistance of the coating to the penetration of corrosive ions, and hence, its high corrosion resistance. Figure 11 reveals the WCA of the coated copper surface during the salt spray test. The WCA decreased from 162° to 114° after 60 h exposed to the salt spray test, which indicates the conversion of the superhydrophobic to the hydrophobic state. The result suggests the appropriate stability of the coating under marine environments. It can be used as a protective corrosion-resistant coating in marine environments. The simulated environment was used to evaluate the antifouling properties of the samples. The superhydrophobicity of the ZnO/SA coating prevents fouling from adhering onto the coating surface by reducing the surface energy and trapping air bubbles in the hierarchical structure [18]. A schematic of this process is shown in Fig. 12. Figures 13 and 14 depicted the fouling formation on the copper and coated copper surface. As displayed in Fig. 13a, after 30 days, fouling begins to form on the bare copper surface. It is also observed that fouling occurs less Journal of Sol-Gel Science and Technology Compliance with ethical standards Conflict of interest Akbar Eshaghi have affiliations with organizations with direct or indirect financial interest in the subject matter discussed in the manuscript. Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. References Fig. 14 Fouling test on copper and coated copper surfaces after 90 days on the coated copper than on the bare copper surface (Fig. 13b). Also, according to Fig. 14, after 90 days, the amount of fouling on the bare copper surface has increased in comparison to the coated surface. The mechanism of the fouling growth is such that initially, the primary fibers adhere to the surface, after which the process of growth and reproduction begins [21]. As a result, the coated surface has far less fouling after 90 days (approximately 60%). It indicates that the ZnO/SA superhydrophobic coating reduces the fouling formation on the solid surface and can be used as an antifouling coating in marine applications. 4 Conclusion In this study, ZnO/ SA superhydrophobic nano-coating was created on the copper substrate. The results showed: 1. The optimal concentration of the stearic acid to obtain the maximum WCA was 6 g/l. 2. The size of the ZnO nanoparticles was observed as 60 nm. 3. 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