Controllable fabrication of stable superhydrophobic surfaces on iron substrates

Abstract

Stable superhydrophobic structures were successfully prepared on iron substrates by etching in hydrochloric acid followed by zinc coating. The zinc film was electrochemically deposited on the etched iron substrate and then annealed at 180 °C. The morphology and chemical composition of the prepared surfaces were investigated by means of scanning electron microscopy (SEM), X-ray diffraction (XRD), atomic force microscope (AFM), energy-dispersive X-ray (EDX) analysis and X-ray photoelectron spectroscopy (XPS). The wetting properties of the surfaces upon each processing step were evaluated using water contact angle (WCA) measurements. At the optimal condition, the surface displayed a superhydrophobic character with a WCA of about 163 ± 2° and a low sliding angle of about 0 ± 2°. The experimental conditions, such as electrolyte concentration, electroplating time, annealing condition, and etching time were investigated to determine their effects on the superhydrophobicity. It was also demonstrated that the as-prepared superhydrophobic surfaces exhibited anti-corrosion properties and a long-term stability. The freezing properties of the superhydrophobic surfaces were also investigated.

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Introduction

Superhydrophobicity refers to the intrinsic property of surfaces to repel water. These surfaces display water contact angles (WCA) larger than 150° and sliding angles (SA) less than 10°. With the rapid development of science and technology, not only in nature, but in real life, people are in contact with superhydrophobic surfaces due to their relevance to many areas of technology.^1,2 For instance, superhydrophobic surfaces (SHSs) have a significant impact on the development of oil/water separation,^3,4 biochemical separation,⁵ photoresponsive materials,⁶ glass and plastics with self-cleaning properties^7–10 and targeted drug delivery,¹¹ while also adding value to a wide range of antifogging coating,^12,13 tribological^14–16 and microfluidic applications.^17–22

So far, many methods have been reported to fabricate SHSs such as physical vapor deposition (PVD),²³ pulsed laser deposition (PLD),²⁴ hydrothermal technique,²⁵ polymer phase inversion,²⁶ layer-by-layer self-assembly technology,²⁷ casting method,^28,29 solution-immersion process,^30–32 sol–gel method,^33,34 chemical etching,^35,36 spin-coating,³⁷ galvanic cell reaction,³⁸ electrospinning,³⁹ sublimation⁴⁰ and so on. However, many of these methods have certain limitations such as tedious conditions, complex multi-step processing, special equipments, and poor durability which will reduce their extension to large-scale applications. Moreover, not all of the products are capable of reaching extremely high (160–170°) contact angles, and most of them in the preparation processes use organic coatings. Coating with organic materials increases the cost of the products and produces chemical waste and pollution. Hence, in this field, electrochemical route appears of great interest because the electrochemical deposition process is usually performed in aqueous medium at room temperature and in a non-vacuum environment. In addition, electrodeposition has the advantage of achieving a large area deposition with superior uniformity in composition.

Steel is a common kind of material that is extensively used in marine, aircraft, cultural heritage, machine, architecture and other areas owing to its particular properties such as good mechanical properties, perfect electrical conductivity and so on. Practically, the only factor which limits the lifetime of steel is corrosion; the corrosion of iron disables its excellent properties. Owing to the severe dependence placed upon iron, it is of great importance to seek effective approaches to prevent its corrosion. Formation of thin organic or sol gel film on the iron surface is the most common method to protect it from corrosion.^41–44 Zinc coating is one of the most investigated ways of protecting steel against corrosion processes.^45–48 To extend the lifetime of iron constructions, big efforts have been put into the optimization of zinc coating composites^49,50 and the improvement of the performance of the zinc coating. It can serve as an effective barrier to keep water, moisture, and atmospheric oxygen from reaching the base metal.

Previous studies have demonstrated that superhydrophobicity can confer corrosion inhibition capabilities to Zn.^31,51–55 Based on these results and the advantages of electrochemical deposition, we report herein on the fabrication of superhydrophobic zinc micro/nanometer hierarchical structures on iron foil by electrochemical machining in a mixed electrolyte composed of zinc acetate ((CH₃COO)₂Zn), sodium chloride (NaCl) and hydrochloric acid aqueous solution and followed by subsequent thermal annealing. The morphology, roughness, chemical composition, formation mechanism, effect factors, durability, and potential application of the SHSs were also investigated.

Experimental

Materials and reagents

Zinc acetate (Zn(CH₃COO)₂·2H₂O, 99.8%), sodium chloride (NaCl, 99.5%), hydrochloric acid (HCl, 36–38%), acetone (C₃H₆O, 99.5%) and ethanol (C₂H₅OH, 99.5%) were purchased from China Beijing Fine Chemical. Co. Ltd. Iron foil (99.9%) and zinc foil were supplied by Beijing Nonferrous Metal Research Institute. Distilled water was used throughout the experiments.

Sample preparation

Iron substrates 2 cm × 1 cm × 1 cm were sequentially washed with acetone, ethanol for approximately 15 min each under ultrasonication to remove organic contaminants on the surface. After that, the specimens were dried at ambient temperature. Subsequently, in the acid etching experiment, the cleaned iron substrates were immersed into a hydrochloric acid aqueous solution (1 M) for 4 min at room temperature. It was worthy of attention that the iron substrates were set perpendicularly in unplasticized poly vinyl chloride (UPVC) tubes without bottom in this process of etching as shown in our previous report.⁵⁶ After etching, one iron substrate was immediately used as the working electrode.

The preparation method of the superhydrophobic surface by zinc electrodeposition on iron substrate is depicted in Scheme 1.

Scheme 1 Schematic illustration of fabricating superhydrophobic surface on iron substrate.

These experiments were conducted in a three-electrode cell with iron substrate as the working electrode, a platinum electrode as the counter electrode, and Standard Calomel Electrode (SCE) as the reference electrode. The distance between the iron substrate and the platinum electrode was about 10 mm. The electrolyte is a mixture of 0.01 M Zn(CH₃COO)₂, 0.1 M NaCl and 0.1 M HCl aqueous solution. Zn films were electrochemically grown at a constant voltage of −1.8 V for 1100 s as confirmed by the cyclic voltammograms reduction peaks (Fig. S1 and S2†). After the samples were taken out and dried with the filter paper, the samples were placed in a Petri dish covered with filter papers and annealed at 180 °C for 70 min. To verify the repeatability of the results, each test was repeated three times.

The open circuit potential (E_oc)–time (t) curves and electrochemical impedance measurements were acquired by using a computer-controlled electrochemical system (CHI 604D, CH Instruments Inc.) in 3.0 wt% NaCl aqueous solution at ambient temperature.

Surface characterization

Water contact angle (WCA) measurements were performed using a dynamic contact angle testing instrument (FTÅ 200, Data physics Inc, USA). Water droplets (about 8 μL) were gently dropped onto the surfaces with automatic syringe controlled by the computer. The accuracy of WCA values was ±2°. WCA images were captured by the video; angles were estimated by using a tangent algorithm, which had been analyzed in detail in our previous paper.⁵⁶ The average WCA values were obtained by measuring at five different positions for each sample.

The surface morphology of the samples was investigated by scanning electron microscopy (SEM, S-4800, Hitachi, Japan). The nano-textured surfaces were characterized by atomic force microscope (AFM, D3100, Veeco, USA) with a conventional rectangular cantilever in tapping mode with the scanning rate of 0.1 Hz. The microstructures were analyzed by an X-ray diffractometer (XRD, D8 ADVANCE, Bruker, Germany) operating with Cu Kα radiation at a continuous scanning mode (40 kV, 40 mA, and λ = 0.15418 nm) and scanning rate of 4° min⁻¹. The corresponding element distributions of the surface were determined by energy-dispersive X-ray spectrometry (EDX). The chemical compositional analysis of the surface was obtained by X-ray photoelectron spectrometer (XPS, Model PHI 5300, Physical Electronics, USA), using 250 W Mg Kα (λ = 0.9891 nm) X-ray as the excitation source. The XPS spectra were collected in a constant analyzer energy mode at a chamber pressure of 10⁻⁷ Pa and pass energy of 44.75 at 0.1 eV per step. The binding energy of contaminated carbon (284.6 eV) was used as the reference.

Results

Wettability of the sample surfaces

Fig. 1 exhibits the images of the WCAs on the different sample surfaces, which clearly indicated the changes of the wetting properties depending on the treatment conditions. The WCA of untreated iron sample was 94°, confirming the hydrophobic nature of the iron surface (Fig. 1a). After chemical etching in a hydrochloric acid aqueous solution (1 M) for 4 min, the WCA of the sample decreased to 36° (Fig. 1b). Electrochemical deposition of a zinc film at a constant voltage of −1.8 V for 1100 s led to a fully wetted surface; the water droplet spreads rapidly on the plate surface as shown in Fig. 1c. However, after annealing the same surface at 180 °C for 70 min, the surface WCA increased greatly to 163°, suggesting a superhydrophobic character of the surface (Fig. 1d). It is worth mentioning that the superhydrophobicity is only attained by the combination of chemical etching, electrochemical zinc deposition and thermal annealing. This means that etching, electrochemical deposition and annealing all play decisive roles in the formation of SHS.

Fig. 1 Images of the WCAs on the sample surfaces: untreated iron surface (a), etched iron surface in 1 M HCl aqueous solution for 4 min at room temperature (b), treated surface via electrochemical machining at a constant voltage of −1.8 V for 1100 s before (c) and after annealing at 180 °C for 70 min with filter papers.

For SHS, the sliding angle (SA) gives the adhesive behavior of water droplets on the surface. Fig. 2 is a collection of images from the videos of free-falling water droplet on the superhydrophobic sample (from Fig. 1d). The images suggest that the as-prepared SHS exhibits excellent non-sticking behavior; the water droplet rebounds upward elastically without leaving any residual traces on the surface and eventually leaves the surface within 1.6 s with a SA of almost 0°.

Fig. 2 Successive snapshots obtained from the full rebound of a 8.0 μL droplet on the zinc coated iron superhydrophobic surface (from Fig. 1d).

Morphology and elemental composition

To get a better understanding of the relationship between surface wettability and the fine micro/nano-structures, SEM was utilized to capture the surface images at different magnifications (Fig. 3). Fig. 3a displays the surface morphology of bare iron sheet with some scratches. Fig. 3b shows that the iron surface appears rough after etching 4 min in 1 M HCl aqueous solution. The whole surface exhibits the corroded state, and the surface consists of some scattered micrometer-scale papillae 1.7 to 5 μm in diameter. Fig. 3c corresponds to an SEM image of the iron surface immersed in 0.01 M Zn(CH₃COO)₂, 0.1 M NaCl and 0.1 M HCl aqueous solution after application of a DC voltage of −1.8 V for 1100 s. Numerous dendritic-like papilla are distributed on the surface in a random pattern with lengths ranging from 4 to 19 μm. Each of the dendritic-like papilla is composed of dozens of irregular micro/nano-maple leaves with uniform diameters of 2.5 to 3.25 μm as shown in Fig. 3e and g. The maple leaves grow along the substrate and almost parallel to the substrate. However, this roughness did not provide superhydrophobicity to the iron surface. We found that annealing step was necessary to reach a superhydrophobic character. The SEM images of the zinc coated iron surfaces after thermal annealing at 180 °C for 70 min are displayed in Fig. 3d, f and h. Compared with Fig. 3c, more dendritic structures cover the iron surface, while the macroscopic appearance has not changed after thermal annealing (Fig. 3d). The shape of the fine structures of the dendritic-like papilla however change from the thicker maple leave-like structures to fully thin hexagonal lamellar structures (Fig. 3g and h). It can be seen that a large number of nanoplates can spontaneously self-assemble into side-by-side packing structures with a higher density over a large area. The average thickness of the nanoplates is about 14.3 nm with an average diameter of about 340 nm (Fig. 3h).

Fig. 3 Typical SEM images of the investigated surfaces: pure iron substrate (a), iron substrate after etching in 1 M HCl for 4 min, iron substrate coated with zinc via electrochemical machining at a constant voltage of −1.8 V for 1100 s before (c, e and g) and after thermal annealing at 180 °C for 70 min with filter paper (d, f and h).

Further analysis of the surfaces was carried out using X-ray diffraction as shown in Fig. 4. For comparison, the XRD pattern of the bare iron substrate is recorded in Fig. 4A. Several strong peaks which are derived from the iron substrate can be seen obviously (JCPDS card#06-0696). Fig. 4B is the XRD pattern of zinc film grown on the iron substrate by electrochemical deposition from an aqueous solution of Zn(CH₃COO)₂ and NaCl for 1100 s at −1.8 V. The peak at 43.2° is due to (101) zinc peak, in accordance with previous results reported on electrodeposited zinc electrodes.⁵⁷ After annealing at 180 °C for 70 min, the diffraction peak assigned to the Zn (101) plane becomes much stronger, indicating the preferential growth of the zinc nanostructures in the (101) orientation (Fig. 4C). Interestingly, diffraction peaks at 36.3° (002), 39.0° (100), 54.3° (102) and 70.6° (110) appeared after thermal annealing. These reflections are in good agreement with the peak positions usually reported for crystalline Zn (JCPDS card#04-0831), confirming the pure crystal Zn nanostructures electrodeposition.

Fig. 4 XRD patterns of the iron substrate (A), the iron substrate after coating with zinc via electrochemical machining at a constant voltage of −1.8 V for 1100 s before (B) and after thermal annealing at 180 °C for 70 min with filter paper (C).

The iron surface was covered with a zinc layer, Zn is a very active metal, so we inferred that zinc oxide was produced upon annealing at high temperature.

EDX spectra of the zinc coated iron samples after thermal annealing at 180 °C for 70 min shows the presence of three distinct elements, namely, zinc (1.0 keV), oxygen (0.53 keV) and iron (0.70 keV) as shown in Fig. 5. The peak at about 2.10 eV is Au, which is used to improve contrast with the operation of the SEM. The atomic weight fraction of zinc, oxygen and iron are 82.77%, 6.07% and 11.16%, respectively. The EDX result supports the idea that the micro/nano Fe–Zn–ZnO particles have been successfully introduced to the metal surface through the simple replacement deposition process.

Fig. 5 EDX spectra of the zinc coated iron substrate via electrochemical machining at a constant voltage of −1.8 V for 1100 s after thermal annealing at 180 °C for 70 min with filter paper.

We performed XPS analysis not only to affirm the purity of the prepared Zn micro/nano-structures, but more crucially to recognize the chemical states of O_1s and Zn_2p and finally ascertain the types of defects existing in the prepared samples. The XPS survey and core level spectra of O_1s and Zn_2p of the samples are depicted in Fig. 6. The curve labeled with (A) in Fig. 6 corresponds to the sample before thermal annealing, the curve labeled with (B) corresponds to the sample after thermal annealing with filter paper. The wide scan in Fig. 6a indicates that the surface is mainly composed of zinc and oxygen whether the sample is annealed or not. A small carbon peak is also detected at 284.6 eV due most likely to surface contamination (used as the reference). The presence of O_1s in the XPS spectrum is associated with the presence of a thin oxide layer on the surface.

Fig. 6 (a) Wide scan XPS spectra of the zinc coated iron surface via electrochemical machining at a constant voltage of −1.8 V for 1100 s; (b) Zn_2p1/2 and Zn_2p3/2 core level scans; (c) XPS core level scan of O_1s; the insets of (c) are the fitted curve of A and B, respectively. (A) before and (B) after annealing at 180 °C for 70 min with filter paper.

Fig. 6b exhibits the XPS core level spectra of Zn_2p of the zinc coated iron surface via electrochemical machining at a constant voltage of −1.8 V for 1100 s before (curve A) and after thermal annealing (curve B). The Zn_2p spectrum of the curve A presents a single spin orbital pair with Zn_2p3/2 and Zn_2p1/2 at 1021.6 and 1044.7 eV, respectively. After annealing with filter paper, those peaks shift to 1021.4 eV (Zn_2p3/2) and 1044.5 eV (Zn_2p1/2), respectively. The doublet peak separation energy are both 23.1 eV, in agreement with the published data.⁵⁸ The changeless shift at high binding energy is indicative of the presence of Zn metal. It was previously reported that the splitting of the Zn_2p peak is due to strong spin–orbit coupling.⁵⁹

It has been shown that O_1s line is more sensitive than Zn⁶⁰ to the chemical environment. Fig. 6c shows the XPS high resolution of O_1s peak, which can be fitted with two components in A and B curves. In the curves A and B, they both have a double peak. Therefore, to obtain more detailed information, the curves were fitted against the dataset, respectively, as shown in the insets of Fig. 6c. The typical O_1s peak in the surface can be consistently fitted by three nearly Gaussian, centered at around 530.15 ± 0.15, 531.25 ± 0.20 and 532.40 ± 0.15 eV, respectively. The high energy peak of the O_1s spectrum is usually attributed to the presence of loosely bound oxygen on the surface.⁶¹ The low energy peak is attributed to O²⁻ ions on wurtzite structures of hexagonal Zn²⁺ ion array, surrounded by Zn atoms with their full complement of O ions.^62–64 The medium energy peak is associated with O²⁻ ions in the oxygen deficient regions.⁶⁵ Changes in the intensity of this component may be connected in part to the variations in the concentration of oxygen vacancies. The single broad band in the curve A is deconvoluted with two components with maximum intensities at 530.30 and 531.43 eV; in the curve B, the two peaks are at 530.22 and 531.45 eV. It can be seen that the intensity of the peak located at 530.30 eV is very small, but grows vigorously after annealing, which suggests that more ZnO appeared at the Zn/Fe interface, and the intensity of medium component O_1s of annealed film decreases more than the as-deposited film, indicating that more oxygen vacancies remain in the surface layer of annealed film than in the as-deposited film.

Superhydrophobicity is commonly reached through the combination of surface roughness and modification with organic coatings of low surface energy.³⁹ So both micro/nano structuration and chemical composition play very important roles in the formation of superhydrophobic surfaces.

And it can decrease the surface energy. Interestingly, formation of nanostructured zinc oxide (ZnO) films showing superhydrophobicity without any chemical modification was demonstrated in several reports,^66–68 So, we do believe that the presence of ZnO nanostructures in our films accounts for the observed superhydrophobicity.

Fe_2p3/2 high resolution XPS spectra in the range of 700–740 eV of the as-prepared sample before and after annealing with filter paper are displayed in Fig. S3.† The signal for curve B is so weak that it cannot be recognized; this is likely due to the presence of thick zinc coating, which is beyond the detection thickness of the XPS.

Based on the XPS, XRD and SEM results, it can be concluded that after annealing, a few Zn atoms are in the +2 oxidation state at the surface of the sample and that the grown Zn micro/nano-particles are well crystallized, which can reduce the surface energy and increase the WCAs.

Influencing factors of the wetting behavior

A series of influencing factors should be considered to prepare optimized SHSs. These include annealing condition, the concentration of zinc ion (CZI), electrochemical deposition time, and etching time.

Annealing conditions. Functionalization of structured Zn surface with organic coatings is useful to lower its surface energy and thus makes the surface superhydrophobic. For example, Melot et al. prepared superhydrophobic zinc coatings on steel by electrodeposition process and further functionalization with ultra-thin films of commercial silicone rubber.⁶⁹ However, others reports demonstrated that ZnO sometimes can show superhydrophobicity without any surface modification with organic coatings. Huang et al. reported the formation of a stable superhydrophobic surface via coating aligned carbon nanotubes (CNTs) with a zinc oxide (ZnO) thin film.⁶⁶ In this work, no additional organic coating was used to reach superhydrophobicity. Similarly, Feng et al. fabricated aligned ZnO nanorod films with controllable wettability without any surface modification.⁶⁷ The ZnO films show superhydrophobicity and superhydrophilicity at different conditions, and the wettability can be reversibly switched by alternation of ultraviolet (UV) irradiation and dark storage. Here we used thermal annealing instead of organic modification to obtain SHSs micro/nanostructured Fe–Zn–ZnO surfaces. The annealing conditions include annealing temperature, annealing time, and the presence or not of filter paper in the process of annealing. The annealing device is depicted in Fig. S4.†

Fig. 7 shows the SEM patterns of the samples after electrodeposition and annealing at 180 °C for 70 min in a Petri dish without filter paper. Compared with Fig. 3d and h respectively, there are fewer large dendritic structures in Fig. 7a, and irregular lamellar structure pile up together disorderly with the average diameter of 600–850 nm as shown in Fig. 7b.

Fig. 7 SEM images under different magnifications of the iron substrate after coating with zinc via electrochemical machining at a constant voltage of −1.8 V for 1100 s followed by thermal annealing at 180 °C for 70 min without filter papers.

The XRD pattern of the samples after zinc electrodeposition and annealing at 180 °C for 70 min in a Petri dish without filter paper are shown in Fig. 8; the biggest difference between A (without filter paper) and B (with filter paper) lies in the Zn (002) and Zn (101) peaks. The diffraction peak assigned to the Zn (002) plane becomes much stronger in A, indicating the preferential growth of the zinc nanostructures in the (002) orientation when the sample was annealed without filter paper. The diffraction peak assigned to the Zn (101) plane becomes much stronger in B, indicating the preferential growth of the zinc nanostructures in the (101) orientation when the sample was annealed with filter paper.

Fig. 8 XRD patterns of the iron substrate after coating with zinc via electrochemical machining at a constant voltage of −1.8 V for 1100 s after thermal annealing at 180 °C for 70 min in a Petri dish without filter paper (A) and with filter paper (B).

Superhydrophobicity is a phenomenon which may arise from combining hydrophobicity with roughness.⁷⁰ Fig. 9 displays the three-dimensionally (3D) AFM images of the zinc coated iron surface via electrochemical machining at a constant voltage of −1.8 V for 1100 s before annealing (Fig. 9a), after annealing at 180 °C for 70 min without filter paper (Fig. 9b) and with filter paper (Fig. 9c). The distribution of the nanostructures on the surfaces is random. The heights of the most nano-textures on the surfaces in Fig. 9a–c are about 315.7 nm, 657.3 nm, and 1 μm, respectively. It is clear that the surface of the sample annealed at 180 °C for 70 min with filter paper (Fig. 9c) gradually becomes rougher than others.

Fig. 9 AFM morphologies (50 μm × 50 μm) of the zinc coated iron surface via electrochemical machining at a constant voltage of −1.8 V for 1100 s before annealing (a), after annealing at 180 °C for 70 min without filter paper (b) and with filter paper (c).

The XPS survey and core level spectra of O_1s and Zn_2p of the samples are depicted in Fig. 10. The wide scan of the samples in the inset on the left indicates that the surface is mainly composed of zinc and oxygen too, and contrasts with that of Fig. 6a; the presence of filter paper does not introduce additional elements on the surface. The inset on the right exhibits the XPS core level spectra of Zn_2p of the zinc coated iron surface via electrochemical machining at a constant voltage of −1.8 V for 1100 s after thermal annealing without filter paper. The Zn_2p spectrum of the curve A exhibits a single spin orbital pair with Zn_2p3/2 and Zn_2p1/2 at 1021.4 eV and 1044.5 eV, respectively. The single broad band of O_1s peak is deconvoluted with two components with maximum intensities at 530.30 and 531.43 eV, indicating that the number of oxygen vacancies remain in the surface layer of annealed film without filter paper is somewhere between the as-deposited film and the SHSs, compared with Fig. 6c. The number of oxygen vacancies must be important to some extent for SHSs, which results from the direction of the growth and the size of the crystals.

Fig. 10 The fitted curve of O_1s of the zinc coated iron surface via electrochemical machining at a constant voltage of −1.8 V for 1100 s after annealing at 180 °C for 70 min without filter paper. The inset on the left is the wide scan XPS spectrum of the same sample; the inset on the right is the Zn_2p core level scan.

On the sample annealed without the filter paper, the water droplet spreads rapidly on the plate surface, while the sample annealed with the filter paper, the surface WCA increased greatly to 163°. Annealing with the filter paper changes the morphology, elemental composition of the surfaces, the growth direction and size of the crystals, and thus affect the surface hydrophobicity.

To investigate the effect of annealing temperature, the anneal temperature is varied from 120 °C to 200 °C, while electrochemical deposition voltage (−1.8 V), electrochemical deposition time (1100 s), annealing time (70 min), etching time (4 min), CZI (0.010 M), NaCl concentration (0.1 M) and hydrochloric acid concentration (0.1 M) are kept constant. It can be seen from Fig. S5a† that the sample annealed at 180 °C is the most superhydrophobic one. When the annealing temperature is 120 °C, water droplets completely spread on the sample surface, which implies that this temperature is too low to form good crystalline structures quickly. When the annealing temperature is 180 °C, the highest WCA of 163° is obtained. When the annealing temperature is 200 °C, the WCA decreases to 81°. To get a better understanding of the temperature effects on morphology and crystal structure, SEM and XRD were performed on the annealed samples. The SEM images of the iron substrates annealed at 120 °C, 180 °C and 200 °C are displayed in Fig. 11. These images qualitatively show that the particle shapes become more different by increasing the annealing temperature. In Fig. 11a, the regular lamellar structures do not appear at 120 °C, while after annealing at 200 °C, the iron substrate is covered with some evenly distributed sporadic flower-like aggregates. The loose sheet-like morphology is destroyed and the superhydrophobicity disappears. The XRD patterns of the iron substrates annealed at 120 °C, 180 °C and 200 °C are exhibited in Fig. 12. Upon annealing at 120 °C for 70 min, the diffraction peaks assigned to the Zn (002) and Zn (101) planes become strong, indicating the preferential growth of the zinc nanostructures in the (002) and (101) orientations (Fig. 12A). The peaks of Zn (002) and Zn (101) decrease gradually as the temperature increases. From the SEM and XRD results above, we can conclude that only with the specific crystal growth direction (101) and appropriate elemental composition, the SHSs can be obtained.

Fig. 11 The typical SEM images of the sample surfaces annealed at different temperatures: (a) 120 °C; (b) 180 °C; (c) 200 °C. The annealing time is 70 min.

Fig. 12 The typical XRD patterns of the sample surfaces upon annealing at different temperatures: (A) 120 °C; (B) 180 °C; (C) 200 °C. The annealing time is 70 min.

To confirm the effect of annealing time, the anneal time was varied from 40 to 100 min, while electrochemical deposition voltage (−1.8 V), electrochemical deposition time (1100 s), anneal temperature (180 °C), etching time (4 min), CZI (0.010 M), NaCl concentration (0.1 M) and hydrochloric acid concentration (0.1 M) are kept constant. As shown in Fig. S5b,† the WCA of the surface first increases with the increment of anneal time. When the time is 40 min, the highest WCA is up to 129°; further increment of anneal time to 70 min led to WCA increase to 163°. This indicates that increasing the annealing time could further improve the hydrophobicity of the surface. On the contrary, the WCA decreases a little bit with further increment of the annealing time, but the sample retains its superhydrophobicity.

The concentration of zinc ion (CZI). Fig. S5c† shows that the WCAs on the resulted surfaces varied significantly under different CZI, while electrochemical deposition voltage (−1.8 V), electrochemical deposition time (1100 s), annealing temperature (180 °C), annealing time (70 min), etching time (4 min), NaCl concentration (0.1 M) and hydrochloric acid concentration (0.1 M) were all kept constant. It can be seen that with the increase of the CZI, the WCA first increases up to the maximum and then decreases. When the CZI is 0.010 M, the highest WCA of 163° is obtained. Further increase of the CZI resulted in WCA decrease to 141°. The result reveals that the CZI has a large effect on the wettability. SEM images of the samples prepared at different CZI (0.005, 0.010 and 0.020 M) are displayed in Fig. 13. These images qualitatively show that the particle shape becomes more diverse with the increase of CZI. For the lowest value of CZI (0.005 M, WCA = 124°), the particle shape is overall thick block-shaped hexagonal prism with an average diameter of about 50 nm (Fig. 13a). However, when the CZI increased to 0.010 M, it can be observed that the microstructures on the surface are quite uniform without any evident defects (Fig. 13b). The homogeneous side-by-side sheet structures resulted in pores formation, which can trap a lot of air (Fig. 13e). When the CZI reached 0.020 M (WCA = 146°), the sample is covered more densely, the sheet structures become thicker, and the surface does not have uniform and enough porous structures and interspaces as shown in Fig. 13c and f. Thus, the air trapping within such rough surface would be less.

Fig. 13 SEM images of the prepared samples obtained at various concentrations of Zn(CH₃COO)₂: (a and d) 0.005 M; (b and e) 0.010 M; and (c and f) 0.020 M, while other conditions remain constant.

Electrochemical deposition time. Fig. S5d† shows the WCAs of the surfaces prepared under different electrochemical deposition times, while electrochemical deposition voltage (−1.8 V), annealing temperature (180 °C), annealing time (70 min), etching time (4 min), CZI (0.010 M), NaCl concentration (0.1 M) and hydrochloric acid concentration (0.1 M) are all kept constant. No obvious fluctuation in the WCAs, within the experimental errors, is observed. All static WCAs are in the range of 147° to 163°, indicating that the WCA is the largest for an electrochemical deposition time of 1100 s.

Etching time. The WCA value of the polishing surface is 36°, while the WCA value of the initial surface without polishing is 94°. To evaluate the effect of the etching time on the surface, the other factors, electrochemical deposition voltage (−1.8 V), electrochemical deposition time (1100 s), anneal temperature (180 °C), anneal time (70 min), CZI (0.010 M), NaCl concentration (0.1 M) and hydrochloric acid concentration (0.1 M) are kept constant. As shown in Fig. S5e,† the WCA firstly increases from 136° to 163° with the increase of etching time and finally decreases to 143°. The result reveals that the etching time has a certain effect on the surface wettability.

Theoretical growth mechanism for superhydrophobicity

Based on the characteristics of surface morphologies and compositions above, the corresponding mechanism is believed to comprise the following steps: it is known that iron is an active metal, the dissolution reaction readily occurs when it is put into acid media such as hydrochloric acid solution. By acid etching, a rough surface is obtained. The corresponding mechanism is thought that when the DC voltage is applied to the three electrodes, a few of Zn²⁺ ions in the solution capture electrons rapidly, and then become pure zinc nuclei and will come into the crystal lattices of iron or interstices among them, causing crystal iron defects. The morphology and elemental composition of the surfaces, and the growth direction and size of the crystals change with the change of etching time, CZI, electrochemical deposition time, and annealing conditions. When the sample is annealed, the Zn atoms at active lattice defects on the surface are oxidized by O₂ in air to ZnO. In conclusion, the annealing treatment leads to fabrication of the loose sheet-like morphology with superhydrophobicity. The reaction processes can be formulated as follows:

2H⁺ + Fe → H₂↑ + Fe²⁺ (etching reaction) (1)

Zn²⁺ + 2e⁻ → Zn (cathode reaction) (2)

2H₂O → O₂↑ + 4H⁺ + 4e⁻ (anode reaction) (3)

Zn + Fe → Fe–Zn alloy (4)

2Zn + O₂ → 2ZnO (surface reaction) (5)

Theoretical explanation of the superhydrophobicity

In order to discuss the link between the contact angle and the surface morphology, let us recall that two regimes can underlie the wetting behavior: the homogeneous and the heterogeneous regimes. In the homogeneous regime, the water drop will penetrate to the grooves of the rough surface in the Wenzel state. Whereas, in the heterogeneous regime, the water droplet is suspended on the micro/nanostructures of the surface, because the air layer trapped in the porous structures restricts the contact between water and surface in the Cassie–Baxter state.⁷¹ The contact angle in the Cassie–Baxter state is described as follows:

cos θ* = f₁ cos θ − f₂ (6)

where f₁ and f₂ are ratios of solid surface and air in contact with liquid, respectively, θ* and θ represent the contact angles on the rough and smooth surfaces, respectively. Given that f₁ + f₂ = 1, θ* = 163°, and θ = 94°, f₁ and f₂ are estimated at 0.047 and 0.953, respectively. These data indicate that when a water droplet is placed on the SHS, approximately 4.7% of the surface serves as the contact area of the water droplet and the solid surface, and the remaining 95.3% serves as the contact area of the water droplet and air.

Because the contact between water and the solid surface is low, the superhydrophobic surfaces can improve the resistance of the coatings in corrosive environment and also decrease the ice adhesion over those surfaces.

The durability of the SHSs

The durability of the SHSs is a very important factor that will decide the feasibility of the proposed method in industrial applications. First, the SHSs were exposed to air at room temperature for one year. Every week, we checked their WCAs. All static WCA values are in the range of 162–166°, indicating that the SHSs have good long-term stability in air.

The freezing properties of the SHSs

The freezing properties of water droplets on sample surfaces were also investigated. Fig. 14 shows images of water droplets on pure iron (on the left) and superhydrophobic surface (on the right) at different temperatures. A water droplet of 8.0 μL at initial temperature of 20 °C is placed on the sample surface as shown in Fig. 14a. The pure iron and SHS with the water droplets were put into the refrigerator at the same time. The freezing experiments of water droplets were conducted. The water drop on the pure iron was obviously icing up at about −15 °C after 15 min, while the water droplet on SHS was still a transparent liquid droplet (Fig. 14b). About 35 min later, it had a tendency to freeze. The experimental results mentioned above demonstrate that the freezing is retarded significantly on SHSs compared with pure iron surfaces. These results are also highly significant for the application of superhydrophobic zinc coated iron surfaces as engineering materials.

Fig. 14 Images of water droplet on pure iron (on the left) and superhydrophobic surface (on the right) at different temperatures: (a) at about 20 °C (room temperature); (b) at about −15 °C.

Anticorrosion properties

The corrosion behavior of bare iron substrate, zinc foil and as-prepared zinc coated iron superhydrophobic surface was investigated by potentiodynamic polarization tests in NaCl aqueous solution (3.0 wt%) at a scanning rate of 1 mV s⁻¹ (Fig. 15). The corrosion potential (E_corr/V) and the corrosion current intensity (I_corr/A cm⁻²) of the bare iron sheet are about −0.696 V and 3.49 × 10⁻⁷ A cm⁻², respectively. The E_corr and I_corr of the iron foil coated with superhydrophobic zinc film are about −1.038 V and 2.17 × 10⁻⁵ A cm⁻², respectively. The E_corr and I_corr of the left peak of the iron substrate coated with superhydrophobic zinc film are about −0.962 V and 5.18 × 10⁻⁷ A cm⁻². The E_corr and I_corr of the right peak of the superhydrophobic film are about −0.650 V and 6.30 × 10⁻⁸ A cm⁻². As explained in the Principles of Electrochemistry of Corrosion, E_corr is the critical value at which the corrosion occurs on electrode, and more positive E_corr means the fewer degree of corrosion. The I_corr means the corrosion speed, and smaller I_corr means the better anti-corrosion property. The lower corrosion current intensity corresponds to better corrosion resistance. The corrosion potentials (E_corr) of the SHS are more positive compared respectively with the iron sheet (the right peak of the SHS) and zinc sheet (the left peak of the SHS), implying a better anticorrosion ability. The I_corr of the SHS is approximately 1/100 of the bare zinc foil. The middle peak of the SHS proves the presence of the FeZn_6.67 (E_corr = −0.739 V), FeZn_8.87 (E_corr = −0.790 V), and FeZn_10.98 (E_corr = −0.960 V). From the above, the superhydrophobic surface has better anticorrosive properties than the bare zinc foil and iron foil, which is attributed to the superhydrophobicity of the deposited coating.

Fig. 15 Tafel polarization plots of bare iron sheet (blue line), zinc sheet (red line) and the superhydrophobic surface (gray line) in 3.0 wt% NaCl aqueous solution at a scanning rate of 1 mV s⁻¹.

Conclusions

We had successfully prepared stable SHSs zinc coatings on iron substrates, consisting of Fe–Zn–ZnO micro/nano composites by etching, electrochemical deposition and annealing. The resulting SHSs showed long-term stability in air and good resistance to freezing. The optimal conditions were determined by the single factor experiments. When the etching time was 4 min, CZI was 0.010 M, electroplating time was 1100 s in acidic condition, annealing temperature was 180 °C and annealing time was 70 min, the best superhydrophobicity with a WCA of 163 ± 2° and a sliding angle of about 0 ± 2° were achieved. The simple and efficient electrodeposition technique presented in this study has great potential for the design and fabrication of efficient SHSs on other metallic substrates.

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation of China (no. 21271027) for the financial support of this work.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Fig. S1 XPS core level spectra of Fe_2p3/2 (A) before annealing, (B) after annealing at 180 °C for 70 min with filter paper and (C) without filter paper. Fig. S2 variation of the water contact angle of the sample surfaces as a function of: annealing temperature (a), annealing time (b), electrolyte concentration (c), electroplating time (d), and etching time (e). See DOI: 10.1039/c5ra02890f

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