Temperature and pH endurable self-assembled camellia-like nanostructure achieved on zinc sheet with superamphiphobicity for fog harvesting

Abstract

Easy-implement and low-cost fabrication of super-hydrophobic/super-oleophobic materials is of great significance for efficient fog harvesting. Herein, we propose a simple two-step procedure based on Cu/1-octadecanethiol (ODT) self-assembled monolayer (SAM) modified zinc plates. Interestingly, the whole process mimics the blooming process of flowers in nature: the deposition of copper particle and the subsequent formation of SAM drives the surface undergo gradually stretches and finally blooms to camellia-like nanostructures. The water contact angle (WCA) and oil contact angle (OCA) reach 160 ± 1° and 159 ± 1° respectively, as a result of the formation of layered petal structure that traps air effectively, which is attributed as one of the most important factors for the superamphiphobic effect. In addition to much enhanced facility in fog collection, the materials maintain excellent performance in acid/base environments (1 ≤ pH ≤ 14), broad temperature conditions ranging from −18 °C to 240 °C, and the artificial sea environment, and exhibit capable wear resistance as well as self-cleaning property. The cooperation of all these multiple properties ensures the robustness and stability for efficient fog collection.

1. Introduction

In our marvelous planet, water is one of the most essential resources for life that nurtures lives and promotes the ecological cycle [1]. Currently, it is urgent to solve the necessary problem with the global water crisis resulted from complex factors including the water pollution in human’s manufacture and life and the climate changes. Water-saving has become a hot topic and the acquisition of collecting water is a worth exploring task [2]. It is a worthy probe and research issue to make the ideology of water collecting infiltrate every corner of life, and one of the effective ways is to gather tiny cloud droplets into water. Consequently, the study of fog collection emerged and became popular instantly.

The surface of the lotus leaf is difficult to be wetted and polluted, which is also known as the “Lotus effect” that is determined by the special micro-nano structure and the low surface energy of the lotus leaf surface, which motivated the research of superhydrophobic materials [3,4]. It is not surprising to find creatures in nature that utilizing special surface microstructure to collect fog for survival, such as Namib Desert beetles [5,6], cacti [7], Spider silk [8], Salsola crassa [9] and many others. Scientists are therefore inspired to design and fabricate materials that utilize the super-hydrophobicity for efficient fog collection [[10], [11], [12]]. The study of fog harvesting has become a hot topic, which is of great significance of environmental protection and energy sustainable development.

Materials having both the super-hydrophobic and super-oleophobic properties are also called the superamphiphobic material, which expands the application scopes compared to the traditional superhydrophobic materials that are limited to only one of the functions stated above since oils are with surface energy lower than water. The superamphiphobic materials are extensively used in the field of self-cleaning, anti-fouling, anti-adhesive as well as anti-icing. It is generally acknowledged that the basic requirements of a superamphiphobic material are that the contact angle (CA) is greater than 150° and the sliding angle (SA) is less than 10° [3,13]. The main approaches for achieving the requirements of both high contact angle and low sliding angle is by adjusting the surface roughness and reducing the surface energy. Researchers propose to use modifiers to reduce the surface energy of materials [[14], [15], [16]] and creative preparations of superamphiphobic materials can be mainly divided into two categories: preparation of coatings on substrates [17,18] and direct synthesis of chemical coatings [19,20]. Parts of the proposed methods have complicated and fine procedures or are limited to certain matrices and expensive raw materials, while some simpler methods employ the fluorinated materials to reduce surface energy, which is harmful for water resources and thus environmentally not favored.

Therefore, we aim at designing a fluorinate-free procedure to fabricate materials with low surface energy. Alkanethiols have received extensive applications in synthesizing superhydrophobic materials, especially in synthesizing conical one dimensional copper nanowires by making use of the chemical reaction between Cu(OH)₂ and thiol [17,21,22]. Considering the convenience of direct loading of Cu and the better stability of Cu for both temperature and acid/base solutions than Cu(OH)₂, we utilize the adsorption effect between Cu and alkanethiol to form the self-assembled monolayers for superamphiphobic materials.

Self-assembled monolayers (SAMs) are monolayers fabricated depending on the interactions between molecules and substrates, which generally exhibit specific effect after binding on the substrate surface. Currently, the substrates for self-assembly of alkanethiols (chain thiols) are metal [[23], [24], [25]], silicon [26], metallic oxides [27,28], mica [29] and others. The properties of SAMs remain unchanged after the modified material is taken out of the solution. SAMs have greater flexibility in the types of surface properties that can be modified and controlled. Because single-layer films are thin and uniform, they are often used as model surfaces in research applications, and they can be designed as material surfaces with different properties according to experimental requirements. Moreover, the surface coating is cheap and has a wide range of applications, such as controlling the proper surface wettability, biocompatibility, etc. [30,31].

Herein, we proposed a simple and non-toxic strategy by a fluorine-free two-step method to prepare the superamphiphobic material coatings based on economic and easily obtainable raw materials by using the dip-coating technology. We will firstly introduce the main points of our approach before starting the detailed illustrations. Scheme 1 depicts our method that starts with depositing copper particles on the surface of zinc surface to induce the formation of micro-/nano-scale rough structures, which is found to play an important role in achieving our main purpose. One step further, the 1-octadecanethiol is employed to form a self-assembly monolayer on the metal sheet. With these simple two-step operations, we obtain the material with excellent superamphiphobic performance, in addition to great stability across different temperatures and acid-base environments, which ensures robust applications in daily production and life application. In the following context, we will introduce the experimental procedures, characterizations and also demonstrate its efficient self-cleaning and fog collection abilities.

Scheme 1.

Schematic diagram of the preparation technology of superamphiphobic surface.

2. Experimental

2.1. Materials

All the zinc sheet (99%) are with size of 1cm × 1cm × 0.1 cm that was obtained from Hebei Weian. 1-Octadecanethiol (C₁₈H₁₈S, ODT, 96%) was purchased from Alfa Aesar Chemical Co., Ltd., China. Copper sulfate Pentahydrate (CuSO₄·5H₂O, 99%) was provided by Beijing Chemical Works Co., Ltd., China. The ethanol was bought from Beijing Yili Fine Chemicals Co., Ltd., China. And all these chemical reagents were analytical grade, without any further preparation. Ethanol was used throughout the entire experiment to clean the whole materials and used in the configuration ODT/ethanol mixed solution during the experiment.

2.2. Preparation

The zinc sheets were firstly polished by using the metallographic sandpapers (400, 800, 1200 grit) and then were cleaned in ethanol for 10 min before use. There are two steps during the experiment, as shown in Scheme 1. Firstly, to construct the rough structure on the zinc sheet, the superamphiphobic sheet (SHS) with micro/nano-structure was fabricated through the chemical deposition method. The cleaned zinc sheets were immersed with CuSO₄ solution (0.04 mol/L) in a centrifuge tube vertically for 10 min at room temperature, which allows the replacement of the zinc on the sheet surface by cupric ion, and finally resulted in material surfaces with certain roughness.

The next step is to put the samples loaded with copper micro/nano-particles vertically in centrifuge tubes, which was immerse in the 0.004 mol/L 1-octadecanethiol/ethanol mixed solution for 24 h. After the two steps illustrated above, fetch out the sheets and rinse the coated sheet surface several times with ethanol, then let the sheets dried in air at room temperature for following tests.

In order to explore the optimal experimental condition, we changed the loading time of copper sulfate and the immersion time of ODT. As shown in Table S1 and Table S2, we finally choose the loading time to be 10 min and the immersion time to be 24 h in ODT, with other conditions unchanged.

2.3. Characterizations

The contact angles of a liquid droplet on a solid surface through static images were measured by a CA measuring instrument (JC2000D, Shanghai Zhongchen digital technique. apparatus CO). During the whole experiment, we used the deionized water to measure the WCA for superhydrophobic tests and used the glycerol to measure OCA for super-oleophobic tests.

A scanning electron microscope (SEM, JEOL JSM-7610F) with energy dispersive spectroscopy (EDS, JEOL JSM-7610F) was used to evaluate the chemical elements and observe the micromorphology of the prepared samples.

X-ray diffractometry (XRD, D8 advanced X-ray diffraction) with Cu-kα radiation (λ = 1.5406 Å) was used to characterize the crystal structure of the prepared samples in the angle range from 5° to 90° at the scan velocity of 3°/min.

X-ray photoelectron spectroscopy (XPS, Thermo Fischer ESCALAB-250Xi) was used to measure the chemical composition and use XPS to analyze the bond energy of elements.

Fourier transform microscopic infrared imaging spectrometer (FTIR-ATR, Nicolet Nexus 410) was used to measure the surface composition of the substrate material and the transmittance spectra was recorded from 4000 to 500 cm⁻¹.

3. Results and discussion

3.1. Wettability

The change of wettability during the experimental procedure are shown in Fig. 1. The upper right corner is their values of WCAs and OCAs.

Fig. 1.

WCAs and OCAs on the surface: (a) the untreated zinc sample, (b) Zn–Cu coating, (c) superamphiphobic sample.

Fig. 1(a) is the image of static liquid that drops on the untreated zinc sheet. We put zinc sheet into CuSO₄ solution to let cupric ion replace zinc on the surface, and finally we got the Cu–Zn sheet (Fig. 1(b)). By contrast, the contact angle of static water droplets and glycerol droplets on the cleaned zinc sheets is 64° and 69° (Fig. 1(a)), which become 11° and 20° after loading the copper particles on the sheet (Fig. 1(b)). It can be observed that the contact angles were lower and the wettability was more amphiphilic than before. After the Cu–Zn sheet was immersed into ODT, the WCA and OCA became 160° and 159° (Fig. 1(c)), which implies the transformation of wettability from an amphiphilic character to a superamphiphobic behavior upon treatment by the adsorption of Cu/ODT SAM on zinc sheet.

The SHS has low adhesion so that liquid droplets can easily roll off the surface. Fig. 2 shows that a water droplet and a glycerol droplet rolling down from the SHS surface (Videos S1 and S2), when the water droplet fell down, it directly slided off the surface, and the sliding angle of water is approximately equal to 0° and the sliding angle of oil is less than 10° in general. This is a low adhesion phenomenon, as shown in Fig. S1, when a droplet (8 μL) fell down freely from a certain height, it bounced on the surface firstly, and then left the superamphiphobic surface (Video S3).

Fig. 2.

Droplet rolling down: (a) water droplet, (b) glycerol droplet.

Supplementary data related to this article can be found at https://doi.org/10.1016/j.heliyon.2023.e14775.

The following are the supplementary data related to this article:

Multimedia component 1

Water droplet rolling down from superamphiphobic surface.

Multimedia component 2

Glycerol droplet rolling down from superamphiphobic surface.

Multimedia component 3

Successive snapshots of an 8 μL droplet on superamphiphobic surface.

3.2. Surface morphology

Using SEM and EDS Mapping, we explored the microstructure and component distribution of the SHS on the micro-/nano-scale. Fig. 3 shows the SEM images at the 100 nm scale. Fig. 3(a) shows the scaly surface polished by sandpaper and Fig. 3(b) exhibits a flower-bud-like morphology when copper was loaded in the first step. The microstructure image of as-prepared Cu/ODT film on the surface of zinc sheet is shown in Fig. 3(c). After the treatment of ODT, it is interesting to observe that the flower gradually blooms and grows, which could be attributed to the interaction between ODT with long alkyl chain and copper particle, and the absorption effect by long alkyl chains leads to this phenomenon. The layered petal structure can trap air, which is an important factor that determines the superamphiphobic effect.

Fig. 3.

SEM images: (a) untreated zinc sheet, (b) Zn–Cu coating and (c) superamphiphobic sample. And insets are the static water contact angles of corresponding surfaces.

The SEM image of the pure zinc sheet is shown in Fig. 4. Only surface scratches are observed on the surface at the 100 μm nano-meter scale (Fig. 4(a)). Similarly, the SEM image of the superamphiphobic surface coating obtained after the treatment is analyzed, as shown in Fig. 5. In the 100 μm scale, the surface was covered with a black rough morphology in the observation field (Fig. 5(a)).

Fig. 4.

The untreated zinc sheet: (a) SEM, (b) EDS spectrum and (c) EDS mapping images.

Fig. 5.

SHS sample: (a) SEM, (b) EDS spectrum and (c) EDS mapping images.

In Fig. 4(b), the EDS image shows that the surface before modification is mainly composed of zinc elements and a small amount of carbon and oxygen elements. The oxygen element comes from the surface of the zinc sheet that was previously oxidized by air.

Compared to the untreated zinc surface (Fig. 4(b,c)), SHS surface (Fig. 5(b,c)) contain five elements including zinc, copper, carbon, oxygen and sulfur, which indicates that copper element comes from the CuSO₄ solution, and carbon and sulfur elements were mainly derived from ODT, confirming the successful anchoring of ODT on the modified Cu–Zn sheets.

3.3. Surface composition

We analyzed the chemical composition and element states for further identification of the superamphiphobic sample.

The XRD experiment uses the monochromatic X-rays as the diffraction source, which can generally penetrate solids to verify its internal structure. Therefore, XRD gives information on the bulk structure of the material. Fig. 6 shows that the main structure before and after the reaction remain nearly unchanged (Fig. 6(a,b)), indicating there is no change for the internal structure, and the treatment of the superamphiphobic experiment has no effect on the crystal lattice of the metal matrix expect from the surface. The apparent difference of peak values can be found from the two XRD patterns (Figs. S2a and b) that the absorption peak belonging to the pure zinc is higher than that was modified with 1-octadecanethiol/ethanol mixed solution of modified material, and Fig. 4, Fig. 5 show the increase of carbon content, which is in accordance with previous studies the existence of surface ODT affects the absorption peak of Zn [17].

Fig. 6.

XRD spectra of the bare untreated (a, black line) and SHS sample (b, red line). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

In order to further verify the adsorption of Cu/ODT SAM, the surface chemical composition and chemical states of SHS was analyzed by XPS. Fig. 7 shows the XPS spectrum of the samples during the preparation process. The surface compositions of zinc, copper, carbon, oxygen and sulfur can be observed in Fig. 7(a). Fig. 7(b–e) displays the intensity of Zn 2p, Cu 2p, C 1s and S 2p, respectively. Fig. S3 corresponds to the XPS of the zinc material after cleaning(a) and the SHS surface(b). Fig. 7(b) exhibits two prominent peaks of zinc at 1021.8 eV and 1044.8 eV, which are the characteristic peaks of Zn 2p 3/2 and Zn 2p 1/2 states, and the intensity shows apparent decrease by comparing with the intensities of zinc in Fig. S3(b), which is in accordance with the XRD result (Fig. 6). The above analysis provides further evidence for the formation of SAM in the reaction process. The Cu 2p spectra of the modified zinc surface is shown in Fig. 7(c), The Cu 2p signal can be divided into two peaks at 952.8 eV and 932.9 eV. After immersing the cleaned Zn substrate into CuSO₄ solution, two new peaks emerge, which are attributed to copper that appeared at 952.8 eV, 932.9 eV, indicating the deposition of Cu on the surface of the Zn sheet. C 1s spectrum is divided into 284.6 eV and 285.2 eV, which indicates the presence of C1s signal (Fig. 7(d)). It can be assigned as the formation of –CH₂- and –CH₃. It is convincing since ODT has the elongated long tubular chain that is adsorbed on the surface of the material, which is conducive to the superamphiphobic properties of the products. In addition, the binding energy at 168.9 eV and 162.9 eV arise from the sulfur peaks, as shown in Fig. 7(e). We also locate the peak of R–SH at 162.9 eV [32,33]. In addition, a weak peak at 168.9 eV belongs to the peak of the metal thiolate of Metal-SH, because copper ions can directly react with mercaptans to produce black copper mercaptan, which may come from the presence of a small amount of copper sulfate on the surface by the direct reaction with stearyl mercaptan. Totally, the main peak of sulfur is 162.9 eV, indicating that ODT was adsorbed on the surface of the material surface.

Fig. 7.

XPS spectra of SHS surface: (a) the survey spectra, (b) Zn 2p region, (c) Cu 2p region, (d) C 1s region, and (e) S 2p region.

The FTIR-ATR of SHS sample is shown in Fig. 8. In the region of 2000–2500 cm⁻¹, a broad peak matches the stretching vibration of the carbon dioxide interference peak, bands at 2915 and 2844 cm⁻¹ are assigned to the symmetric and asymmetric stretching modes of –CH₂-, respectively. The peak of 2955 cm⁻¹ belongs to the stretching vibration of –CH₃ [34]. The band at 757 cm⁻¹ is the in-plane rocking vibration of the methylene group, and the number of methylene groups is greater than four. The adsorption peak at 1469 cm⁻¹ is the asymmetrical deformation vibration of the methylene group, and corresponding to the asymmetric deformation vibration absorption peak of S–CH₂–, which are mutual confirmation with XPS analysis.

Fig. 8.

FTIR-ATR spectra of superamphiphobic sample surface.

Based on previous investigations, we find the significant difference of zinc absorption peaks between pre- and post-experiment in XRD and XPS tests. The peak value of zinc is higher because it is covered by 1-octadecanethiol which in turn affects the absorption (Fig. S2). The EDS proves the existence of copper and sulfur elements (Fig. 5), and these characterizations proved the successful loading of ODT on the surface of copper particles.

3.4. Mechanism analysis of surface wettability

Based on the above analysis, it is clearly shown that the Zn sheets are effectively modified by copper, and finally the ODT is adsorbed on the surface of copper particles.

We propose the self-assembly mechanism of Cu/ODT SAMs on the zinc sheet surface [35], and the reaction equation is shown in Scheme 2. We give the possible mechanism of the superamphiphobic experiment. Wenzel recognized the relationship between the surface roughness factor and contact angle as follows [36]:

$$\cos {\theta }^{*}=r\cos \theta$$ (1)

where θ* is the contact angle between gas and liquid on the surface, θ represents the contact angle on the smooth surface, and r is the surface roughness, which shows the ratio between the real superficial area and the apparent superficial area (r > 1). According to the experimental data, the CA on the rough surface (θ*) is 11° and on the bare zinc sheet (θ) is 64°. The result of r is 2.24, which confirms Wenzel’s conclusion.

Scheme 2.

Schematic shows equations during the reaction process.

The Cassie-Baxter model deemed this process as a composite state by assuming the air layer exists between the water and the rough surface, thus water droplets cannot be completely trapped in the rough surface with nano/micro-structure [37]. The Cassie-Baxter equation is:

$$\cos {\theta }^{*}=f_1\cos \theta -f_2$$ (2)

where $f_1$ represent the area fraction of solids on the composite interface and $f_2$ is the area fraction between air and water on the same surface. $f_1$, $f_2$ are both less than 1, and they have the relation $f_2=1-f_1$, so the equation can be converted to a new equation:

$$\cos {\theta }^{*}=f_1\cos \theta -\left(1-f_1\right)=f_1\cos \theta +f_1-1$$ (3)

When water drops on the surface of the SHSs, θ* = 160° and θ = 64°. Then we can obtain $f_1$ = 4.19% by the formula, which means that 95.81% of the area of water droplets is in the air, and the rest 4.19% is in contact with solids surface. When the droplet is replaced with glycerol, θ* = 159° and θ = 69°, and $f_1$ becomes 4.89%, which implies that 95.11% of the area of glycerol droplets is in the air, and 4.89% is in contact with the solid surface.

3.5. The temperature stability of SHS

We put SHSs in an oven and investigate the stability of the surface coating of the superamphiphobic material under a very high temperature (for daily life) and the metal sheets are placed in glassware. A temperature experiment with a control time of up to eight days was conducted. In Fig. 9(a), it can be observed that both WCA and OCA of the material surface maintained excellently within the normal error during the eight days' placement at 60 °C, and the values have almost no obvious changes. On the eighth day, the average WCA is maintained at 153° and the OCA is at 154°, confirming the stability of the superamphiphobic coating at high temperatures in our daily lives.

Fig. 9.

WCAs and OCAs on the superamphiphobic surface at different temperatures: (a) 60 °C, (b) −18 °C for different days.

The temperature stability is tested by refrigerating materials at −18 °C from 1 day to 8 days. After refrigeration in corresponding conditions, the materials were taken out to measure WCAs and OCAs by deionized water and glycerol droplets. CAs were recorded to study the stability at low temperatures relative to daily life. According to Fig. 9(b), after eight days' place of these materials at low temperature, both OCA and WCA fluctuate within a very small range, which proves the stability at low temperature. The water droplets slip off the surface after falling, and the surface does not accumulate water at all, which proves the good anti-icing function.

For further knowledge about the tolerance of SHSs at high temperature range, we put SHSs in an oven and keep it at a certain gradient for 1 h to evaluate the stability of the superamphiphobic surface at different temperatures. By excluding the influence of acceptable errors, Fig. S4 shows the WCAs and OCAs on the surface of the material when the material is placed at 30 °C, 60 °C, 90 °C, 120 °C, 150 °C, 180 °C, 210 °C, 240 °C and 270 °C. According to the graph, the superamphiphobic material still shows good super-hydrophobic and super-oleophobic properties even before 240 °C. When the temperature reaches as high as 270 °C. The contact angle has a significant downward trend, contact angles of the water droplet changes from 154° to 99°, and contact angles of glycerol were weakened from 150° to 120°, but it still remains in a hydrophobic and oleophobic state.

3.6. Self-cleaning performance

One of the useful features of the hydrophobic coated material is the self-cleaning property, and the other one is the antifouling property that the falling liquid can automatically take away the dirt on the surface. These are also necessary features in the field of hydrophobic materials.

The self-cleaning and antifouling features are tested using fine dust powder. According to Fig. 10, the surface initially covered with layers of dust powder becomes clean again after the liquid rolls off the surface of the material, and this is due to the synergistic effect of the micro/nano structure and the low surface energy. On the one hand, a layer of air cushion is formed as more air can be retained in the rough structure in the micro/nano level to drastically decrease the contact area between dust and material surface. On the other hand, the effect of low surface energy prevents the diffusion of liquid droplets on the surface (Videos S4 and S5).

Fig. 10.

Self-cleaning performance: (a) untreated zinc sheet, (b) superamphiphobic sample.

Supplementary data related to this article can be found at https://doi.org/10.1016/j.heliyon.2023.e14775.

The following are the supplementary data related to this article:

Multimedia component 4

Smudginess Phenomenon on untreated zinc sheet.

Multimedia component 5

Self-cleaning on superamphiphobic surface.

Moreover, it is concluded that preventing the liquid from stagnating on the surface is important to increase the durability of material itself. Therefore, we also conducted experiments and further discussions on the durability and stability of materials.

3.7. Corrosion resistance

Fig. 11 shows the silver light appearing on the surface when the superamphiphobic sample was immersed into the 3.5 wt% solution, which is known as the mirror-like phenomenon [38,39]. This is due to the total reflection phenomenon formed by the air layer trapped by the micro-nano structure on the surface of the superamphiphobic sample. The mirror-like phenomenon prevents the material surface from contacting the solution, so it is an important factor for good corrosion resistance.

Fig. 11.

Mirror-like phenomenon of superamphiphobic sample inundated in 3.5 wt % NaCl solution.

The corrosion potential (E_corr) and current density (I_corr) of the polarized Tafel curve characterize the corrosion resistance of materials, as shown in Fig. 11. In the corrosion experiment, both the bare material and the superhydrophobic material were immersed in NaCl solution (3.5 wt%). The Tafel polarization curve of untreated zinc sheet and superamphiphobic sample is shown in Fig. 12. E_corr and I_corr are −0.973 V, 5.508 × 10⁻⁶ A/cm² for untreated Zn sheet and −0.799 V, 1.104 × 10⁻⁷ A/cm² for superamphiphobic sample surface respectively, and the I_corr of the superhydrophobic material is much smaller, indicating the better corrosion resistance.

Fig. 12.

Potentiodynamic Tafel polarization curves: (a) untreated zinc sheet (black line); (b) superamphiphobic sample (red line) in 3.5 wt% NaCl solution. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

The samples were immersed in a solution with a wide pH values ranging from 1 to 14 for 24 h, and the WCAs and OCAs of the sample surface were measured. The contact angles of the material surface fluctuating within 10° were observed in Fig. 13, which reflects a good corrosion resistance in acidic and alkaline solutions.

Fig. 13.

Stability on superamphiphobic surface in different gradient concentration solutions of pHs.

3.8. Wear resistance

To test the mechanical stability of SHS, we conducted experiments for abrasion resistance. The surface of SHSs material is abraded by 1200 CW sandpaper, and the operation method is shown in Fig. 14(a). The load on the top of the material is 5 g, and the sliding distance is 5–100 cm. The contact angle is measured after the abrasion operation. Fig. 14(b) shows the WCA and OCA respectively. It shows that: (1) under the same sliding distance, the contact angle decreases with the increase of sandpaper roughness; (2) under the same sandpaper roughness, the contact angle decreases with the increase of sliding distance. It can be clearly observed that both WCA and OCA remain above 120°, namely, the hydrophobicity and oleophobicity are maintained. Therefore, the superamphiphobic material prepared in this experiment possesses excellent mechanical durability and abrasion resistance.

Fig. 14.

Wear resistance of superamphiphobic sample: (a) a diagrammatize illustration of wear test; (b) the water contact angles and oil contact angles of superamphiphobic sample with increasing sliding distance on 1200 CW SiC sandpaper with a load of 5 g weight.

3.9. Performance of fog collection

Precisely because the rough structure and low interface energy of the modified zinc sheet, the surface was equipped with the ability of superamphiphobicity. We designed the roughness structure with superamphiphobic property on the primitive hydrophilic basement for the fog collecting (Fig. 15(a)).

Fig. 15.

(a) A schematic illustration of fog harvesting measurement; (b) The difference in water collecting between untreated zinc sheet and superamphiphobic sample; (c) The process of fog harvesting on untreated zinc sheet and superamphiphobic sample.

Zhong et al. utilized liquidus on the surface of the material to set up a series of JMs, and found that firstly the small droplets dropped on the JMs having different liquidus and then the water was collected by the modified materials [17]. With different proportion of Cu₂O microparticles and ZrO₂ nanoparticles, Feng et al. proposed an efficient system for fog harvesting [19].

In our experiment, we observed the whole stage of the fog collection behavior in Fig. 15(c): the fog gradually condenses on the surface of the hydrophilic metal substrate, as shown by the green line circled area. As the moisture droplets accumulate, the density of the small droplets gradually decreases (the yellow line squared area), and the small droplets finally condense into a large droplet, which leaves the surface after about 887 s (Fig. 15(b)). The pure zinc sheet collects fog by employing the aggregation of small fog droplets into bigger ones that have enough weight to overcome the anchoring force and finally slide off.

We spent about 30 min in observing the process of fog collection using the pure zinc sheet, and we found 5 drops of water and recorded the weight of each droplet. It is also worth noting that after the larger droplets dripped down, a water film remains on the surface, which is not beneficial for efficient fog collection because of evaporation.

For comparison, each time when the water droplet slides off the pure zinc sheet, we counted the corresponding amount of water collection by using the superamphiphobic material. In strong contrast, when the small water droplets in the fog contact the superamphiphobic surface, only the extremely fine small droplets can stay on the surface. It is within 1 min to form into the small droplets which immediately bounce off the surface. Unlike the droplets in the untreated zinc substrate that fall due to the gravity of very large droplets, the small droplets of the superhydrophobic surface have already begun to fall from the modified surface into the container below because of strong surface tension, and there are no residual droplets left on the surface. However, the first large droplets maintain the aggregated state, as shown in Fig. 15(c), the large water droplet didn’t slip off until 887 s on the untreated zinc surface, and when the droplets slipped, there was still water remaining on the plate. Although only the first 30 min are monitored for these two materials, it is convincing that the superamphiphobic one will gain much more collected water in longer time, since water becomes more easily vaporized in the plate surface [40], as compared with very small residues in the superamphiphobic surface that collects water in with much faster speed. From the above results, the superamphiphobic sample is more efficient in collecting mist.

4. Conclusions

In summary, we proposed an easy-implement and economic two-step procedure to design super-hydrophobic/super-oleophobic materials. We applied the self-assembled monolayers by utilizing the interactions between Cu and 1-octadecanethiol to the surface of zinc sheets, and fabricated the camellia-like nanostructure with a certain roughness on the metal surface and reduced interface surface energy. A layer of air cushion is formed as more air can be retained in the rough structure in the micro/nano level to drastically decrease the contact area between dust and material surface and the effect of low surface energy prevents the diffusion of liquid droplets on the surface. Compared to traditional fluorine surfactants and complicated preparation processes, the experimental procedure is designed to be low-cost, flexible manufacturing and environmentally friendly. The experiment shows the excellent performance both in super-hydrophobic and super-oleophobic with the water contact angle as 160°, the oil contact angle as 159°, and near 0° sliding angle as well as low adhesive ability. The materials exhibited good decontamination, self-cleaning performance, and abrasion resistance. We also confirmed its robustness and stability by conducting detailed tests across a wide range of temperatures, pH value (1–14) and artificial sea-water environment (3.5 wt% NaCl solution), which is of significance for practical long-term applications in human life. As a most important application, the superamphiphobic material shows excellent ability for fog harvesting in the misty air due to its lower sliding angle and low adhesive ability, which is promising for water savings and collections.

Author contribution statement

Ruishuo Li: Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Meng Zang: Performed the experiments; Analyzed and interpreted the data.

Yuanyuan Cheng: Conceived and designed the experiments; Wrote the paper.

Hongbin Qi, Yingbin Wang, Bing Sun: Contributed reagents, materials, analysis tools or data.

Funding statement

This work is supported by the “Fundamental Research Funds for the Central Universities” (No. 2652018052).

Data availability statement

Data included in article/supplementary material/referenced in article.

Declaration of interest’s statement

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Appendix A. Supplementary data

The following are the Supplementary data to this article.

Multimedia component 6

Figure S1 Successive snapshots of an 8 μL droplet on superamphiphobic surface. Figure S2 XRD pattern of the bare untreated zinc sample (a) and the superamphiphobic surface (b). Figure S3 XPS spectra of zinc sample (a) the survey spectra and Zn 2p region (b). Figure S4 WCAs and OCAs on the SHS surface at different temperatures from 30 to 270 °C. Table S1 WCAs and OCAs of different loading time of CuSO₄ solution. Table S2 WCAs and OCAs of different.