Facile fabrication of superhydrophobic surfaces with hierarchical structures

Hierarchical structures were fabricated on the surfaces of SUS304 plates using a one-step process of direct microwave irradiation under a carbon dioxide atmosphere. The surface nanostructures were composed of chrome-doped hematite single crystals. Superhydrophobic surfaces with a water contact angle up to 169° were obtained by chemical modification of the hierarchical structures. The samples maintained superhydrophobicity under NaCl solution up to 2 weeks.

Hierarchical structures consisting of both 10~100 μm-scale protrusions (Fig. 2b) and much smaller nanostructures (Fig. 2c) were formed after the microwave irradiation. The nanostructures were 200~500 nm in height and 50~200 nm in width. The individual nanostructures were observed by TEM, and the TEM image of a typical nanostructure is shown in Fig. 2d. This nanostructure is single-crystalline as evidenced by the high-resolution TEM image (Fig. 2e) and the fast Fourier transform (FFT) image (Fig. 2f). The EDS analysis (Fig. 2g) revealed that the composition of the nanostructure is Fe:Cr:O = 33:4:63. It is interesting to note that Ni was not detected after the microwave irradiation even though SUS304 is an alloy of Fe, Cr, and Ni. A plausible explanation is that the heat of formation of NiO (−58.1 kcal/mol) is higher than that of Cr 2 O 3 (−252.9 kcal/mol) or Fe 2 O 3 (−197.0 kcal/mol) 45 . Therefore, it is believed that Ni is harder to be oxidized and does not appear on the surface structure. However, further studies are required to elucidate this result. Figure 2h shows the XRD data of the SUS304 plate before and after the microwave irradiation. The XRD pattern of the pristine SUS304 plate showed peaks corresponding to the γ-phase and the α-phase. After the microwave irradiation, several peaks corresponding to α-Fe 2 O 3 were observed. This suggests that the fabricated nanostructures are composed of mainly α-Fe 2 O 3 , containing small amounts of Cr inside the crystal. It is considered that some of the Fe sites were replaced by Cr atoms; since Fe 3+ (0.785 Å) and Cr 3+ (0.755 Å) ions are similar in size, the crystal units will remain almost the same, and the XRD peaks will be observed at approximately the same angles. Interestingly, peaks of other iron oxides, such as FeO and Fe 3 O 4 , were not observed in the XRD data. It has been reported that a 3-layer structure was synthesized by thermal oxidation of an iron sheet, due to oxygen diffusion 37,40,46 . However, in the present study, it is likely that only the Fe 2 O 3 layer, which consists of the highest concentration of oxygen among the different iron oxides, was formed because of the high mobility of the O radicals inside the microwave plasma.
It is to be mentioned that carbon was not detected on the surfaces of the samples after the microwave irradiation. Dissociation and ionization processes are known to occur simultaneously inside the CO 2 plasma, and dissociation is the predominant process. When irradiated by a microwave power of 700 W at 0.5 Torr, less than 10% of carbon dioxide is decomposed to form O radicals, which react with the surface to form oxide structures. CO 2 → CO + O is the dominant reaction, and the CO radical is hardly decomposed 47 . As carbon radicals were not formed during the reaction, carbon was not detected on the surface. Figure 3 shows the variations in the surface morphology of the SUS304 plate irradiated at different values of microwave power and irradiation time. The surface of the pristine sample transforms progressively into rolling hills with increase in the power and the time; however, the large protrusions and troughs are retained. The higher magnification images of these rolling hills reveal that columnar structures of several tens of nanometers in diameter covered the surface. Furthermore, nanostructures were not formed on the surface until 10 minutes of 500 W microwave irradiation. However, after 15 minutes of irradiation, a dense formation of the nanostructures was observed. When microwave is irradiated to the metal substrate inside the reactor, an arc discharge occurs due to the short penetration depth of SUS304, and this leads to the generation of plasma and local surface heating. Oxidizing species such as O radical, O − , O + , etc. are generated from inside the plasma, and these oxidizing species causes the ion bombardment onto the substrate surface 48 . These harsh conditions lead to fast oxidation of the SUS304 sample surface, and the volume expansion caused from the oxidation become the driving force of the growth of nanostructures 49 .
It is noteworthy that the nanostructures fabricated using a higher power for the same duration of irradiation exhibited a larger average size and a lower number density. Owing to the antenna effect, the microwave power is concentrated on the local hillocks, and hot spots are created at the points, which make these spots to become molten and lumped. The nucleation starts at the molten hot spots 50 , and therefore, at higher power, the temperature of the hot spots increases, and the nuclei become larger. As a result, bigger nanostructures with a lower number density are formed at higher values of microwave power.
The hierarchical structures could also be prepared on uncleaned samples (without acid etching, see Supplementary Fig. S2), indicating that acid etching during the sample preparation is not the cause for the formation of hierarchical structures. The pristine SUS304 plate exhibited striped patterns on the surface originating from the continuous production process of the SUS 304 sheet. These striped patterns are the protruded regions that functioned as antennae to receive the microwave energy more efficiently. Therefore, these regions heated up more quickly and the hierarchical structures formed more quickly in these regions. However, as the microwave irradiation progressed, the hierarchical structures spread and eventually covered the whole surface of the SUS304 plate. Figure 4 shows that a superhydrophobic surface can be obtained by simply coating the SUS304 plate with the hierarchical structure with hydrophobic ODTS. The sample irradiated using a microwave power of 500 W for durations up to 10 min exhibited low contact angles (<150°), since the nanostructures were hardly formed (which is evident from the SEM images in Fig. 3). On the other hand, very high contact angles (162~169°) with a very low hysteresis of up to 4° were observed for the surfaces with the hierarchical structures. It is to be noted that a superhydrophobic surface cannot be obtained by coating a flat surface (without the hierarchical structures) with ODTS or vice versa. For example, pristine SUS304 with a flat surface (except the large intrusions and troughs) exhibited a water contact angle of approximately 120° even after coating with hydrophobic ODTS. Likewise, microwave irradiation generates a hierarchical surface, but it is covered with oxide nanostructures that make the surface hydrophilic.
Corrosion test was performed in 5 wt% NaCl aqueous solution with 1500 W/15 min samples which showed the highest contact angle and lowest hysteresis among the samples. Figure 5 shows the water contact angle and hysteresis of the sample change by days in NaCl solution. The surface maintained superhydrophobicity after 14 days immersion to the solution, though contact angle was slightly decreased to 165° from 169° and hysteresis was slightly increased to 5° from 2°. That is to say, the surface can maintain superhydrophobicity in the electrolyte solution for more than 2 weeks.

Conclusions
Hierarchical structures were fabricated on the surfaces of SUS304 plates by one-step direct microwave irradiation, and superhydrophobic surfaces (maximum contact angle of approximately 170°) were obtained after coating the surfaces with ODTS. The fabrication process is simple, fast, and clean, and the fabricated surface maintains superhydrophobicity even in electrolyte soution. The scaling-up of this method to a large surface is straightforward; a scanning microwave device can be used, which is readily available.

Methods
Sample preparation. SUS304 plates of dimensions 15 mm × 15 mm × 0.5 mm and 7.5 mm × 7.5 mm × 0.5 mm were cleaned by ultrasonication with ethanol followed by water. Subsequently, the samples were immersed in 50 ml of 1 M HCl solution for 3 h at 25 °C to remove the oxides on the surface. After the cleaning, the samples were washed with deionized water and fully dried in a vacuum oven at 120 °C for 1 h.
Microwave irradiation. The details of the microwave irradiation system have been described previously 51 .
The system consists of a magnetron, isolator, directional coupler, autotuner, quartz reactor, and gas feeders. The total pressure was maintained at 0.5 Torr during the microwave irradiation. Each sample was placed on the quartz plate at the center of the quartz reactor (30 cm long, 2.54 cm in diameter). After 10 minutes of purging with CO 2 gas, the SUS304 plate was irradiated using microwave power (2.45 GHz, single mode) of 500 to 1500 W under the flow of CO 2 (50 sccm) gas for 5 to 15 min.  Hydrophobic coating. After the microwave irradiation, the sample was immersed in 10 ml of 0.1 mMol (octadecyl-tetrachlorosilane (ODTS) -toluene solution for 5 h and dried at 120 °C for 1 h to coat the surface with a hydrophobic self-assembly monolayer.
Characterization. Scanning electron microscopy (SEM) measurements were carried out using XL30S FEG of Philips Electron Optics B.V. operated at 5 kV, and X-ray diffraction (XRD) measurements were performed using Rigaku D/MAX 2500. Transmission electron microscopy (TEM) and energy dispersive spectroscopy (EDS) analyses were carried out using JEOL JEM-2200FS with an energy-dispersive X-ray spectrometer operated at 200 kV. The water contact angle was measured with SmartDrop Lab using water drops of 5 µm diameter. To check the corrosion resistivity, the samples were immersed into 5 wt% NaCl aqueous solution for 1~14 days, and dried with vacuum oven to measure contact angle.