Construction of Silane-Modified Diatomite-Magnetic Nanocomposite Superhydrophobic Coatings Using Multi-Scale Composite Principle

Abstract

To address the challenges of cotton cellulose materials being susceptible to environmental humidity and pollutant erosion, a strategy for constructing superhydrophobic functional coatings with biomimetic micro–nano composite structures was proposed. Through surface silanization modification, diatomite (DEM) and Fe₃O₄ nanoparticles were functionalized with octyltriethoxysilane (OTS) to prepare superhydrophobic diatomite flakes (ODEM) and OFe₃O₄ nanoparticles. Following the multi-scale composite principle, ODEM and OFe₃O₄ nanoparticles were blended and crosslinked via the hydroxyl-initiated ring-opening polymerization of epoxy resin (EP), resulting in an EP/ODEM@OFe₃O₄ composite coating with hierarchical roughness. Microstructural characterization revealed that the micrometer-scale porous structure of ODEM and the nanoscale protrusions of OFe₃O₄ form a hierarchical micro–nano topography. The special topography combined with the low surface energy property leads to a contact angle of 158°. Additionally, the narrow bandgap semiconductor characteristic of OFe₃O₄ induces the localized surface plasmon resonance effect. This enables the coating to attain 80% light absorption across the 350–2500 nm spectrum, and rapidly heat to 45.8 °C within 60 s under 0.5 sun, thereby demonstrating excellent deicing performance. This work provides a theoretical foundation for developing environmentally tolerant superhydrophobic photothermal coatings, which exhibit significant application potential in the field of anti-icing and anti-fouling.

1. Introduction

Through billions of years of evolution, the lotus leaf has developed a dual-scale rough structure composed of micrometer-scale papillae and nanometer-scale wax crystals, combined with low surface energy chemical components such as long-chain alkanes and fatty acid esters. This synergy results in a superhydrophobic mechanism with a water contact angle exceeding 160° and a rolling angle below 5°. Inspired by this dual strategy of physical roughening and chemical hydrophobization, researchers have engineered artificial superhydrophobic coatings with contact angles > 150° and rolling angles < 10° [1,2,3,4,5]. By minimizing water contact area and enabling self-cleaning effects, these coatings effectively mitigate material corrosion in humid environments [6,7]. Widely applied in daily use and industrial products, the biomimetic technology ensures water droplets form near-spherical shapes and roll off effortlessly, significantly enhancing waterproofing durability [8,9,10].

Despite the established role of a nanoparticle–low surface energy substance synergy as the cornerstone for stable superhydrophobicity [11,12], prevailing strategies still exhibit limitations and require further improvement. The central role of nanoparticles is to build multi-scale rough structures. By dispersing nanomaterials, such as fluorinated nanoparticle powder [13], SiO₂ nanoparticles [14,15,16], SiC nanoparticles [17], and silver nanoparticles [18,19,20] in the coating system, a hierarchical protruding structure of a micron/nanometer scale can be formed on the surface of the substrate. Current nanostructuring approaches—including sol–gel-derived porous networks and stacked 1D nanomaterial assemblies (“bird’s nest-like” structures)—effectively modulate solid–air interface fractions to enhance apparent contact angles as per the Cassie–Baxter theory [21], yet universally suffer from mechanical fragility. Rough microarchitectures degrade under tribological stress, inducing hydrophobic failure. While Zhu et al.’s biomimetic golden spherical cactus design (using microsphere–nanospine composites) improves shear resistance [22], its dependence on an equipment-intensive electrostatic powder coating restricts practical deployment. Conversely, Miyagi et al.’s equipmentless in situ polymerization method enables tunable hydrophobic coatings [23], but lacks validation for environmental resilience.

Fundamental deficiencies also plague traditional low-energy surfaces. Fluorinated compounds (such as heptafluorodecyl trimethoxysilane) can achieve ultralow surface energy (<20 mN/m) [24,25,26,27], but introduce environmental responsibility, while silicone substitutes such as PDMS [28,29,30] show ecological compatibility. Moreover, functional integration has not been explored—although photothermal materials have deicing potential [31,32], the morphological dispersion of nanoparticles (film-forming spheres and interlocking rods) and the uncertain synergy between photothermal units limit the efficiency of solar energy collection, which is worth further exploration.

To address structural vulnerability and functional fragmentation, this work pioneers an integrated approach combining mechanically robust, environmentally tolerant micro–nano architectures with sustainable surface energy regulation. We overcome substrate incompatibility and scalability limitations and establish nanostructure–photothermal synergy to mitigate UV/salt spray degradation—a critical industrial barrier. Through the facile dip-coating of cotton fabric, simultaneous superhydrophobicity (WCA: 158°) and efficient photothermal conversion were achieved. The synergistic mechanism combines hierarchical roughness from surface-modified Fe₃O₄ nanoparticles embedded in diatomite micropores (modified with low surface energy n-octyltriethoxysilane) with chemical hydrophobization, forming a “physical skeleton–chemical cloak” architecture. Epoxy resin covalent bonding ensures interfacial stability, resisting delamination under harsh conditions (7-day water immersion, 20 h UV exposure, 3.5% saline corrosion). Robust mechanical durability is validated through sand/water impact and sandpaper abrasion tests, while self-cleaning efficacy is evidenced by contaminant removal via water droplets and resistance to mud/dye contamination. Notably, the coating exhibits 80% solar absorption (350–2500 nm), enabling efficient solar–thermal conversion for de-icing applications. This work provides a scientifically grounded blueprint for the design of superhydrophobic materials with extended service life and environmental adaptability.

2. Materials and Methods

2.1. Materials

Ethanol, ammonia water, butyl acetate, and sodium hydroxide were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China); diatomaceous earth (DEM 7 μm), n-octyltriethoxysilane (OTS), and Ferric oxide particles (Fe₃O₄ 20 nm) were purchased from Macklin Reagent Co., Ltd. (Shanghai, China). All drugs were used directly and did not require further purification.

2.2. Preparation of ODEM and OFe₃O₄

In total, 3.6 g of diatomaceous earth was dispersed in a mixture of 80 mL of ethanol, 8 mL of deionized water, and 4 mL of ammonia water, followed by stirring for 5 min. Subsequently, 0.48 mL of OTS was added, and the mixture was stirred continuously at 50 °C for 24 h. The product was separated by filtration by pumping and was dried in an electric blast drying oven at 70 °C for 24 h. The OFe₃O₄ was prepared following the same procedure. The final products, ODEM and OFe₃O₄, were obtained.

2.3. Preparation of EP/ODEM@OFe₃O₄

In total, 0.6 g of OFe₃O₄ and 2 g of ODEM were added to 15 g of butyl acetate, and stirred for 10 min to make a homogeneous mixture. Subsequently, 6.8 g of EP was added with a resin-to-curing agent ratio of 1:1, and EP/ODEM@OFe₃O₄ was obtained.

2.4. Preparation of Superhydrophobic Cotton Fabrics

The cotton fabrics were ultrasonicated in a 2% NaOH solution for 30 min, followed by ultrapure water for another 30 min. The pretreated cotton fabrics were immersed in EP/ODEM@OFe₃O, removed, and dried in an electric blower drying oven at 70 °C for 24 h, and superhydrophobic cotton fabrics (SCF) were obtained.

2.5. Characterization

The morphology and structure of the material were measured by scanning electron microscopy (SEM, ZEISS, Sigma 500, Oberkochen, Germany). The element distribution was measured by an energy-dispersive X-ray spectrometer (EDS, Oxford Ultim Max 40, Oxford Instruments, Abingdon, UK) equipped for SEM. XRD was tested on an X-ray diffractometer (MAP18AHF). X-ray photoelectron spectroscopy (XPS) was characterized by a DX-2700 X-Ray Diffractometer. The chemical structure of the coating was measured by Fourier transform infrared spectroscopy (FT-IR IRTracer-100, Tokyo, Japan, and FT-IR Nicolet iS50, Tokyo, Japan). The contact angle was recorded by a dynamic contact angle tester (R1R100H-0-60, China). Reflection (R) and absorber (A) spectra were detected by a UV–vis-NIR spectrophotometer (PerkinElmer, Lambda 1050, Waltham, MA, USA). The absorption was calculated using the equation A = 1 − T − R (where T is the transmittance, ≈0). The infrared thermal images and the corresponding temperatures were captured by an infrared thermal camera (Ti32, FLUKE, Everett, WA, USA).

3. Results

3.1. Physicochemical Characterization of ODEM and OFe₃O₄ Flakes

The intrinsic properties of ODEM and OFe₃O₄, such as morphology, chemical structure, and crystal structure, were investigated. As shown in Figure 1a, we obtained superhydrophobic particles by modifying diatomaceous earth and iron tetraoxide, which were immobilized on the fabric by epoxy resin to form a special micro- and nanostructure. Figure 1b demonstrates the SEM image of the superhydrophobic fabric, which has uneven particles attached to the surface of the superhydrophobic fabric and a large number of particles piled up between the fibers, which are a special micro/-nanostructure that greatly increases the roughness of the coating surface and improves the superhydrophobic performance of the coating. As shown in Figure 1c, EDS analysis of the coating shows that the Si and Fe elements are uniformly distributed, which proves the successful composite of ODEM and OFe₃O₄ in the coating.

The incorporation of OFe₃O₄ nanoparticles not only improves the superhydrophobicity of the coating, but also confers excellent photothermal conversion properties. As shown in Figure 1d, the cotton fabric exhibits low sunlight absorption in the range of 350–2500 nm, which is around 30%, while the absorption of sunlight by the coating remains around 80%. This is due to the incorporation of black OFe₃O₄ nanoparticles, which enhances the absorption of light by the superhydrophobic coating. X-ray diffraction spectroscopy (XRD) was carried out to characterize ODEM@OFe₃O₄, as shown in Figure 1e; based on the comparison with the standard cards of DEM and Fe₃O₄, the peaks are similar in position and intensity, which indicates that the silane modification of DEM and Fe₃O₄ did not destroy the original crystalline phase [33,34]. Figure 1f shows the XPS spectra of ODEM@OFe₃O₄, ODEM, and OFe₃O₄. The analysis showed that Si 2p and Fe 2p peaks appeared in ODEM@OFe₃O₄, confirming the successful encapsulation of OFe₃O₄ in the ODME in the micropores. The chemical compositions of ODEM@OFe₃O₄, OFe₃O₄, and ODEM were further analyzed by Fourier transform infrared spectroscopy (FTIR), as shown in Figure 1g. The range of 2850–2960 cm⁻¹ indicates the stretching vibration of C-H, which is a typical peak of the ortho-n-octyl long chain (a strong peak); 1550–1700 cm⁻¹ for the bending vibration of C-H proves the successful modification of Fe₃O₄ and DEM. For OFe₃O₄, 590–650 and 3480–3350 cm⁻¹ are the peaks of Fe-O and O-H, respectively, which tend to weaken. For ODEM, 1010–1200 cm-1 is the asymmetric stretching vibration of the Si-O-Si skeleton. The range of 790–870 cm⁻¹ indicates the symmetric stretching vibration of Si-O-Si. FTIR further proves that we successfully created an ODEM and OFe₃O₄ composite.

3.2. Surface Wettability of the Coatings

As shown in Figure 2a,b, before modification, both DEM and Fe₃O₄ are hydrophilic, with a contact angle of 0° with water, and after modifying both with the low surface energy substance octyltriethoxysilane, the contact angle can reach more than 150°, achieving a superhydrophobic state. As shown in Figure 2c,d, the contact angle of the untreated cotton fabric with water is 0°, which is hydrophilic, and by compounding these modified OFe₃O₄ nanoparticles and ODEM particles together and adhering them to the cotton fabric, the contact angle reaches 158°, which achieves the superhydrophobic state. Here, we designed a reasonable concave–convex structure and added special chemicals, which greatly improved the superhydrophobic performance of the coating under the synergistic effect of the two.

3.3. Self-Cleaning Test of Superhydrophobic Coatings

Self-cleaning performance is a distinctive feature of superhydrophobicity and an important indicator for evaluating the superhydrophobic performance of coatings. As shown in Figure 3a, the superhydrophobic fabric was laid flat on a glass sheet, tilted about 6°, and a syringe was used to slowly inject water droplets onto the superhydrophobic fabric with dust, which was carried away with the rolling water droplets. In addition to this, we completely immersed the superhydrophobic fabric in muddy and dye water, and upon observing the superhydrophobic fabric before and after immersion, the surface of the fabric was not contaminated (Figure 3b,c). This shows that the superhydrophobic coating has excellent self-cleaning performance.

3.4. Stability and Feasibility Analysis of Superhydrophobic Coatings

Good stability is also a necessary property of superhydrophobic coatings. As shown in Figure 4a, we placed the superhydrophobic fabric into a 3.5% brine solution and monitored the contact angle once an hour, and the contact angle of the superhydrophobic coating could still reach about 120° after 7 h of immersion. The superhydrophobic coating was tested for UV aging resistance, and the contact angle remained at 118° after 20 h of UV exposure (Figure 4b). As shown in Figure 4c, the superhydrophobic fabric was placed in tap water for seven days, and the contact angle was 120°. As shown in Figure 4d, the superhydrophobic fabric was placed in an electric blast drying oven, and the contact angle gradually decreased with the increase in temperature, and the contact angle decreased to 117° at a temperature of 160 °C. Although superhydrophobic coatings are affected by various external factors, they can still achieve a hydrophobic state.

Mechanical properties are also criteria for evaluating stability. As shown in Figure 5a, about 50 g of quicksand was allowed to fall naturally from a height of 60 cm from the coating to continuously wash the coating, simulating the impact of wind and sand, and the contact angle of the coating still remained above 150°, and about 10,000 water drops (about 200 mL) were allowed to fall continuously from the same height to simulate the rainfall process, and the contact angle of the coating was still up to above 150°. This means that the superhydrophobic coating was not damaged by the falling quicksand and water drops. In addition, a sandpaper abrasion test was also carried out. As shown in Figure 5b, the surface of the superhydrophobic coating adhered to the glass plate was orientated towards the 600-grit sandpaper, and the glass plate was pushed forward by applying a pressure of approximately 3.2 kPa. The glass plate was moved horizontally by 10 cm and then vertically by 10 cm for one cycle, and was subjected to seven cycles of cyclic testing. Figure 5c demonstrates through standardized tape peel testing that the coating maintains a stable contact angle of 123° (initial 151°) after seven cycles, preserving excellent surface wettability. When compared with literature data (e.g., contact angle remaining above 150° after 50 cycles, only decreasing to 152.4° after 30 cycles, and complete hydrophobicity loss took place after 240 cm of abrasion distance [35]), these differences may originate from variations in micro–nano structural stability [36,37,38]. The present work identifies critical factors influencing structural stability under dynamic friction conditions, establishing a foundation for future breakthroughs in durability limits through micro–nano composite structural optimization.

Good stability performance in terms of stability endows the coating with broad application prospects in fields such as architecture and marine engineering. Simultaneously, the coating demonstrates significant advantages in experimental conditions, cost-efficiency, and stability. The fabrication process is mild, controllable, and straightforward, requiring no specialized equipment, and the materials for its preparation are cost-effective. In 30 independent experiments conducted by undergraduate teams, the coatings exhibited excellent operational reproducibility (contact angles > 150°, relative standard deviation < 5%), fully validating the reliability and feasibility of the protocol. These outstanding characteristics make the coating suitable for material chemistry experiment courses, laying a technical foundation for industrial applications.

3.5. The Photothermal Properties of Superhydrophobic Coatings

As shown in Figure 6, cotton fabric and superhydrophobic fabric were placed on polyethylene foam, respectively, and a xenon lamp was used to simulate irradiation by solar energy. When 0.5 g of ice was placed on a cotton fabric and a superhydrophobic fabric under 0.5 sun, for 20 s, the ice on the superhydrophobic fabric was almost completely melted, while the ice on the cotton fabric showed no obvious change, and the melted water was absorbed by the cotton fabric. At 60 s, the ice on the superhydrophobic fabric was completely melted, while the ice on the cotton fabric still remained. Meanwhile, the ice on the superhydrophobic fabric melts at a rate obviously higher than that of the ice on the cotton fabric, which is attributed to the excellent photothermal conversion performance of the black superhydrophobic coating, and is based on its superhydrophobicity; the water droplets of the melted ice stay on the surface of the coating; the surface temperature of the cotton fabric stabilizes around 24 °C within 60 s in 0.5 sun. Under 1 sun, the surface temperature of the cotton fabric stabilizes at around 24 °C, but the surface temperature of the superhydrophobic fabric increases rapidly under sunlight, from 25.3 °C to 45.8 °C in 60 s. The surface temperature of the superhydrophobic fabric increases from 25.3 °C to 45.8 °C in 60 s. This property greatly expands the application scope of superhydrophobic coatings, especially in de-icing and anti-icing scenarios, and is expected to play a great role in providing innovative solutions for the relevant scenarios by effectively absorbing light energy and converting it into heat energy to rapidly melt ice and inhibit ice condensation.

4. Conclusions

In conclusion, a superhydrophobic coating was successfully prepared by incorporating modified ODEM and OFe₃O₄ into epoxy resin. A micro–nano structure with a rough surface was constructed by compositing micron-sized ODEM and nano-sized OFe₃O₄. The surfaces of the two particles were modified by the addition of n-octyltriethoxysilane, resulting in the low surface energy of the coating. This unique design effectively improves the superhydrophobicity of the coating with a contact angle of up to 158°, preventing the adhesion of water droplets and the diffusion of liquid droplets. In addition, the introduction of OFe₃O₄ nanoparticles endows the coating with significant photothermal conversion capability. Under light irradiation, the coating is able to efficiently convert light energy into heat energy, and it exhibits a high solar light absorption of about 80% in the 350–2500 nm range, enabling it to be applied in photothermal de-icing scenarios. This superhydrophobic coating not only exhibits excellent waterproofing properties, but also has potential applications in photothermal de-icing, providing a new solution for improving the durability and functionality of materials in harsh environments.