Nano-silica modified with silane and fluorinated chemicals to prepare a superhydrophobic coating for enhancing self-cleaning performance Open Access

Inspired by the lotus leaf effect from nature, significant advancements have been made in developing superhydrophobic materials with biomimetic structures. In addition, with the growing demand and high criteria for superhydrophobic materials in practical application, the superhydrophobic materials designed with self-cleaning have attracted considerable attention in-depth (Afzal et al. 2014; Zhang et al. 2023). Normally, the basic principles for constructing ideal superhydrophobic materials involve creating surfaces with low surface energy and layered microstructures, which helps extend the application range of superhydrophobic materials (Rawal et al. 2016; Chen et al. 2024). Recently, superhydrophobic materials have proved promising application potentials in waterproofing, anti-icing, and anti-corrosion, as well as self-cleaning surfaces, drag reduction, and oil–water separation (Das et al. 2018; Chen et al. 2022; Liu et al. 2024). Interestingly, the fabrication of superhydrophobic surfaces is tailored to specific application scenarios. Superhydrophobic surfaces are often not prepared in the same way for various applications (Fang et al. 2021; Wang et al. 2021).

Nowadays, significant progress has been made in developing manufacturing methods for superhydrophobic materials (Zeng et al. 2021). A typical approach is applied on the material surface, indicating preparing superhydrophobic materials through changing the microscale of the material, meaning the surface is modified with low surface energy substances (Chen et al. 2023; Yang et al. 2023). After modifying the surface of the material, this approach creates new micrometer-scale or even nanometer-scale roughness, enhancing the surface's superhydrophobic properties. For example, Sharma et al. (2023) demonstrated that the superhydrophobicity of the surface was realized via the complex surface structures. In their research, the flame spraying technique was utilized to produce surface microroughness and apply a silanization transformation to aluminum coatings, imparting superhydrophobic properties to the material surface (Sharma et al. 2023). Similarly, Li et al. (2007) reported on the layered micro/nanostructure layer, where carbon nanotubes were self-assembled chemically on a single layer of polystyrene. Afterward, the material surface was modified with low surface energy chemicals, such as fluoroalkyl silane (Li et al. 2007). In contrast to the previous method, another technique involved initially modifying hydrophobic materials with low surface energy chemicals to create superhydrophobic surfaces, followed by spraying a deposition mixture and solution containing hydrophobic materials and polymers onto the surface (Ogihara et al. 2012; Celik et al. 2020). However, issues exist in this approach for preparing superhydrophobic materials that are difficult to accurately control the surface microstructure, which can compromise the maintenance of superhydrophobic properties (Zhang & Guo 2023). In addition, there will be nonnegligible health concerns when increasing the concentrations of chemicals during the spraying process. Nevertheless, there is no denying that this approach remains simple and cost-effective for preparing superhydrophobic coatings, offering potential applications across various fields. In addition, Celik et al. (2020) proposed a one-step spray deposition method of nanocomposites. They investigated the effect of solvent and spraying distance on the mechanical durability and wettability of the coatings, reporting that hydrophobic coatings exhibited excellent mechanical durability (Celik et al. 2020).

Nano-silica (nano-SiO₂) particles are commonly regarded as satisfactory functional nanoparticles for the preparation of superhydrophobic materials due to their excellent anti-aging and chemical resistance properties. However, inorganic nano-SiO₂ particles usually are hydrophilic, so one efficient strategy to convert to hydrophobicity is modification by functional alkylsilane compounds (Celik et al. 2023b, 2023c; Shen et al. 2023). In addition, N-alkyl chain compounds are also widely used as co-modifying chemicals in the preparation of environmentally friendly fluorinated materials (Gong et al. 2022). The surface free energy and wettability of fluorinated copolymer films are influenced by the hydrocarbon chain length (Gu et al. 2017). Importantly, the fluorocarbon (C–F) bond is particularly stable owing to the strong electronegativity of fluorine. Simultaneously, the fluorine-containing carbon chains can exhibit a special sawtooth structure, which can provide a spatial shielding effect, enabling the coatings prepared from fluorine-containing compounds to exhibit excellent stability and very low surface free energy (Sharma et al. 2022). Irrefutably, the durability of such coatings is often compromised due to the easy removal of SiO₂ nanoparticles from the surface (Fu et al. 2024). Moreover, both fluoride and non-fluoride chains are been widely applied in achieving low water adhesion, which provides a theoretical basis for applications in the field of superhydrophobic coatings. Hence, the modification of SiO₂ nanoparticles by N-alkylsilane and fluorosilane can successfully be used to prepare coating materials with excellent superhydrophobicity and durability properties.

In this study, N-alkyl silane vinyl triethoxysilane (VETS) and low surface energy 1H, 1H, 2H, 2H-perfluorooctanyltriethoxysilane (PFTS) were used to modify the nano-SiO₂ particles through hydrolysis and condensation reaction, resulting in the preparation of a superhydrophobic surface with a nanocomposite structure. Then, the structure and morphology of the nanoparticles, as well as the surface properties, morphology, and surface composition of the composite coating, were thoroughly investigated. In addition, the modified nano-SiO₂ and poly(vinylidene fluoride) (PVDF) were prepared and solidified to form a wear-resistant superhydrophobic coating. Simultaneously, the wettability, durability, and scale inhibition performance of the coating were tested further. This work provides a promising strategy for controlling the orientation of fluorinated groups to prepare superhydrophobic coatings with low surface free energy. Moreover, the superhydrophobic coating offers the advantages of being nontoxic, nonpolluting, and environmentally friendly, which broadens its potential applications in the fields of self-cleaning and antipollution.

Silicon dioxide (SiO₂) with a particle size range from 20 nm to 10 μm was purchased from Macklin Chemistry Company (China). Ethanol (99%) and ammonia solution (NH₃) were obtained from Guangdong Fine Chemical Industry (China). The treatment agents VTES (99%) and PFTS (99%) were supplied by Aladdin Chemical Company (China). All purchased reagents were used as received without further purification.

A suspension of modified nano-SiO₂ was synthesized using the hydrothermal method. Initially, 2.5 g of nano-SiO₂ powder with a fixed particle size of 20 nm was stirred into 50 mL of ethanol, followed by adding 10 wt.% ammonia and modifier into the ethanol suspension. The total volume of each modifier, including VETS and PFTS, was set to 10 mL. The suspension was transferred to an oven at 60 °C for 20 h. Then, the obtained products were collected through filtration and subsequently dried in an oven at 65 °C for 12 h.

About 2 g of PVDF was dissolved in 50 mL of N-methylpyrrolidone, followed by the addition of modified SiO₂ into the suspension for mixing and stirring for 6 h. After stirring completed, the obtained VETS@PFTS-SiO₂ suspension was applied onto a slide using a wire rod coater. The coated slide was then placed in an oven at 60 °C for 1 h to achieve a semi-cured coating. In addition, the prepared modified SiO₂ suspension was sprayed onto the glass sheet and aluminum sheet with spraying from top to bottom at a speed of 0.1 m s⁻¹. Finally, the glass sheet and aluminum sheet sprayed with the modified SiO₂ suspension were subsequently dried in an oven at 60 °C for 3 h to obtain the final coating. The entire preparation process is detailed in the flow chart shown in Figure S1.

VETS and PFTS were selected to modify SiO₂ with a particle size of 20 nm for evaluating the superhydrophobic performance of prepared coating. A specific amount of modified SiO₂ was added to anhydrous ethanol and then dispersed using 20 kHz ultrasonic radiation for 30 min to ensure complete dispersion. Modified SiO₂ was prepared for subsequent characterization.

The aluminum plate and glass slide coated with VETS-PFTS@SiO₂/PVDF were placed in the culture dish at the smallest possible tilt angle. The coating was then uniformly covered with sand and gravel to simulate dust accumulation. To assess the self-cleaning performance, water was sprayed onto the coating using a syringe.

The aluminum plate coated with the superhydrophobic coating was immersed in water for 6 weeks to observe the long-term stability of the material's superhydrophobic surface. In addition, the aluminum sheet with the same superhydrophobic coating was placed on a 200 mesh abrasive paper. A load of 200 g was applied, and the sheet was dragged across the sandpaper to test the wear resistance of the coating over 200 mm cycles. Furthermore, the resistance of the superhydrophobic coating to acid and alkali was evaluated using specific acid and alkali solutions.

Calcium nitrate tetrahydrate (Ca(NO₃)₂·4H₂O) reacts with sodium bicarbonate (NaHCO₃) to obtain a simulated saturated CaCO₃ solution for scaling inhibition performance testing according to the method presented in the literature (Li et al. 2016). The detailed process involved mixing 250 mL of 0.03 M Ca(NO₃)₂·4H₂O with 0.6 M NaHCO₃ to form a suspension, followed by adjusting the pH to 11 to prepare the simulated saturated CaCO₃ solution. The coated glass and aluminum plates were then immersed in this saturated solution and heated in a 60 °C water bath for 30 min. Then, the coated plates were removed from the solution, rinsed with ultrapure water, dried in an oven at 100 °C for 3 h, and finally weighed to assess the durability of the coating.

The function group of the superhydrophobic coating was identified by a Fourier transform infrared spectrometer (FTIR, Nicolet 5700, USA) with operating in the frequency range of 400–4,000 cm⁻¹. Scanning electron microscopy–energy dispersive spectrometry (SEM–EDS, INCA 250 X-Max 50, UK) was used to determine the composition and elements of the obtained coating. Water contact angle (WCA) data were measured by using a contact angle (CA) measurement device (DSA100, KRÜSS, Germany) at ambient temperature. The results of water droplet data are an average of five drops with a volume of 5 μL and placed on the surface of each sample. The chemical compositions and crystal structure of silane grafted on nano-SiO₂ coating were examined using the X-ray powder diffraction (XRD, 7000S/L, Shimadzu, Japan) technique in the range of 5–60°. The thermogravimetric data were collected via the thermal gravimetric analyzer (TGA, Discovery SDT 650, TA Instruments, USA), with a 10 °C min⁻¹ heating rate in a 20 mL min⁻¹ airflow under the temperature range of 50–800 °C. X-ray photoelectron spectroscopy (XPS) was recorded using Thermo Fisher Escalab Xi+ (USA). The surface morphology was observed by atomic force microscopy (AFM, Bruker Nano Inc. Dimension FastScan, USA).

In addition, the characteristic peak at 953 cm⁻¹ corresponding to Si–OH weakened after modification, confirming that the hydroxyl groups on the SiO₂ surface had undergone condensation with the hydrolyzed silane. For further characterizing the structure of SiO₂ nanoparticles before and after modification, both untreated and treated SiO₂ nanoparticles were examined via XRD, as shown in Figure 1(b). The XRD patterns revealed that both the unmodified and modified SiO₂ displayed a broad peak around 22°, typical of amorphous materials. The observation results showed that the crystallinity of the treated sample is very low and mainly retains an amorphous microstructure. According to the characteristic peak standard card of SiO₂ (JCPDS29-0085), SiO₂ predominantly exists in an amorphous state.

The SiO₂ materials modified by three different modifiers were extensively analyzed via XPS, with quantitative atomic percentages calculated (as seen in Table S1). The full scan spectra of the original SiO₂ and the superhydrophobic materials of the three modified methods are shown in the Figure S2. As shown in Figure S2(a), the signals of four peaks are recorded, corresponding to Si 2p, C 1s, O 1s, and F 1s, with binding energies of 103.5, 284.8, 532.7, and 688.9 eV, respectively. VETS@SiO₂ exhibited a higher C 1s peak compared to the original SiO₂. VETS-PFTS@SiO₂ and PFTS@SiO₂ displayed an F peak alongside the C peak. Notably, the relatively high oxygen content of unmodified SiO₂ is attributed to the presence of numerous hydroxyl groups on the surface coating. Figure S2 shows the C1s spectral analysis of the scanning spectrum. In Figure S2(b), the C1s spectrum of VETS @SiO₂ consists of 284.8 and 285.8 eV peaks, corresponding to C–H and C = C, respectively, indicating the presence of C = C bonds resulting from VETS hydrolysis and condensation (Zan et al. 2004). In Figure S2(c), the C1s region of PFTS@SiO₂ has several characteristic peaks near 284.8 eV (–C–C– and –C–H), 291.7 eV (–CF₂–), and 293.5 eV (–CF₃) (Cai et al. 2018). As shown in Figure S2(d), the C1s spectral region of VETS-PFTS@SiO₂ has four characteristic peaks, corresponding to –C–C–, –C = C–, –CF₂–, and –CF₃, respectively. These results demonstrate the successful co-modification of SiO₂ with VETS and PFTS, with hydrophobic groups effectively grafted onto the SiO₂ surface through reaction. In addition, in Table S1, the amount of VETS-PFTS@SiO₂ used to prepare perfluorooctyl is half of PFTS@SiO₂, indicating a reduction in the F element in VETS@SiO₂, albeit not simply halved. In other words, due to the co-modification of VETS and PFTS, the fluorinated groups are more widely distributed on the surface of VETS-PFTS@SiO₂.

The analysis results of AFM showed that the modified SiO₂ particles are randomly arranged, with numerous micropores and microgrooves formed between the convex structures, thus enhancing the roughness of the modified materials. In addition, these microgrooves capture air, significantly reducing the actual contact area between the materials and water.

The surface roughness of each coating was calculated using NanoScope Analysis 1.7 software. It can be seen that the surface roughness of the modified material increases significantly with the decrease of particle size. It is confirmed that the variation in particle size results in differences in the roughness of the modified material, subsequently affecting its wettability.

The wettability performance of prepared superhydrophobic coating of VETS-PFTS@SiO₂/PVDF was evaluated by the CA. The static contact of an uncoated glass sheet was hydrophilic with a static CA of 62°. However, the surface of the glass sheet with a superhydrophobic coating shows excellent superhydrophobic performance, and the water droplets on the coating surface are close to spherical. The static CA of the coating is 159.2° (Figure S3). In SiO₂ modification reaction, silane coupling agent, SiO₂ particle size, and reaction temperature are essential factors that influence the hydrophobic properties. To determine the best hydrophobicity of the modified superhydrophobic material of VETS-PFTS@SiO₂, according to Wenzel's model, higher surface roughness and lower surface energy were required to achieve a higher WCA. To achieve low surface energy, fluoro-based surface modifiers were introduced. To create high surface roughness, a binary hierarchy of nanocomposite particles (micro–nanostructures consisting of only one size of nanoparticles) was utilized. The single factor influence experiment was conducted on the hydrothermal synthesis of superhydrophobic materials. The results of the main influencing factors, including the modifier addition ratio, the proportion of the synergistic modifier, the SiO₂ particle size, and the reaction temperature, are shown in Figure 5. As shown in Figure 5(a), when the solvent ratio of the modifier added was 4%, the material achieved a maximum CA of 157.6° and a rolling angle of 3°. When the dosage increased to 1%, the hydrophobicity of the material transitioned from hydrophobic to superhydrophobic. The CA gradually increased from 143° to 157.6°, due to the increasing amount of modifiers grafted onto the SiO₂ surface, which reduced the surface energy and enhanced hydrophobicity. However, when the dosage reached 5%, the CA decreased because the excess modifier caused the nano-SiO₂ particles to agglomerate, resulting in reduced hydrophobicity. Figure 5(b) shows the influence of the proportion of synergistic modifier on the wettability. When the proportion of PFTS to VETS reached 2:1, superhydrophobicity is the most obvious. The single modification of SiO₂ was less effective than the synergistic modification of both modifiers. The short carbon chain of VETS embedded in the long carbon chain of PFTS, which made the surface modification of silicon dioxide by modifier fuller maximizing its specific surface area. In addition, Figure 5(c) shows the effect of different SiO₂ particle sizes on the surface wettability of materials. The CAs of modified SiO₂ of 5 and 2 μm were 143.3 and 146.5°, respectively, which have not reached the superhydrophobic state. It can be seen that he modified material still had strong adhesion to water, causing the water droplets to need a large inclination angle to roll off from the material. When the particle size of SiO₂ was 500 nm, the CA of the modified material could reach 151.8°, achieving a superhydrophobic state, with weak adhesion to water droplets and a rolling angle decreased to 10°. Moreover, the CA after 100 nm SiO₂ modification is 153.1°, which showed a strong water repellent ability. At this time, only 5° inclined angle of water drops will roll off. However, the CA of the 20 nm SiO₂-modified material reached 158.0°, with the rolling angle less than 3°, and the water droplets fell on the material surface and rebounded. With the particle size increases, the number of SiO₂ particles per unit amount decreases. Thus, nanoparticle size tends to have an upper limit, and once this limit is exceeded, it is difficult to achieve long-term stabilization in superhydrophobic coatings (Celik et al. 2023a). Figure 5(d) illustrated the influence of different temperatures on the modification of SiO₂. The best reaction temperature is 328 K, where PFTS and VETS exhibited the best synergistic modification effect on SiO₂. The surface CA of the material at this temperature is 155.2°, and the rolling angle is 6°. Based on the impact of the four experimental factors, the selected conditions for the subsequent experiments and determination are the 328 K temperature, 20 nm SiO₂ particle size, 4% modifier dosage ratio, and a 2:1 synergistic modifier ratio. Under these conditions, the material achieves its best wettability, with a CA of 159.2°. According to the Cassie-Baxter law, the exceptional superhydrophobicity arises from the microstructure of the surface of superhydrophobic coating when in contact with water. This structure retains a layer of air between the surface and the water, allowing only a small portion of the liquid to directly contact the solid surface. The majority of the liquid is lifted by the air film, facilitating the easy shedding of water droplets from the superhydrophobic surface (Cai et al. 2023).

The VETS-PFTS@SiO₂/PVDF coating was prepared on the slide using the above method. We carried out wear experiments on the coating surface and measured the static WCA before and after the coating. Under the load of 200 g, the aluminum plate was dragged onto the abrasive paper. The abrasion resistance of the superhydrophobic coating was tested in 200 mm cycles. Figure S4(a) shows the schematic diagram of the coating wear test under the condition of a friction cycle distance of 100 mm and a load of 200 g. In addition, Figure S4(b) shows the surface CA change of the coating under wear cycle. The VETS-PFTS@SiO₂/PVDF coating continues to show excellent water repellency after repeated wear cycles. Superhydrophobicity is lost after 55 wear cycles, and the CA of the coating surface is less than 150°.

The comprehensive properties of the VETS-PFTS@SiO₂/PVDF coating prepared by hydrothermal technology were compared with the published information about the superhydrophobic coating (Table 1). The hydrophobicity of most reported superhydrophobic coatings is about 160°, and the average surface CA is about 160°. However, these materials have insufficient wear resistance. In conclusion, the prepared VETS-PFTS@SiO₂/PVDF coating shows better wettability and better durability (WCA is 159°) than the published work. The durability results of immersion test exceeded 1 month, indicating that the prepared VETS-PFTS@SiO₂/PVDF coating has excellent superhydrophobic property.

In summary, VETS and PFTS used to treat SiO₂ could help improve the performance, and PVDF is conducive to prepare a superhydrophobic coating with excellent wettability, self-cleaning ability, and durability. Under the temperature of 328 K, the SiO₂ particle size of 20 nm, the modifier dosage ratio of 4%, and the synergistic modifier ratio of 2:1, the obtained coating surface has the optimal hydrophobicity coating with the CA of 159.2°. Compared with conventional materials, the treatment process could greatly promote the superhydrophobicity of SiO₂. In addition, the substrate coated with SiO₂ showed excellent self-cleaning performance. In the durability test, after 3 months in the air, there is no significant deterioration in the self-cleaning performance of the coating. Moreover, the coating still maintains its self-cleaning performance after 5 weeks in water. When sanded with abrasive paper under 200 g pressure, the number of damage cycles can reach 50, while the superhydrophobic property of the coating can still be maintained. In addition, this superhydrophobic coating also has good scale inhibition and anti-scaling effect, which can be applied in industrial production to solve the problem of pipeline scaling. In general, this material has significant advantages of outstanding durability and mechanical stability as well as excellent superhydrophobic performance in terms of preparation cost, industrial production, and environmental friendliness.

The authors acknowledge the research boards of the Nanchang Hangkong University, Xiamen University, and Shandong Jiao Tong University for the provision of the research facilities used in this work. GZ would also like to acknowledge funding in part from the Office of Jiangxi Province under Grant No. 20224ACB203013 and CK202002472.

Data cannot be made publicly available; readers should contact the corresponding author for details.

The authors declare there is no conflict.