Fabrication of stable homogeneous superhydrophobic HDPE/graphene oxide surfaces on zinc substrates

Abstract

Homogeneous superhydrophobic coatings were prepared on zinc substrates using ethanol–xylene solutions of high-density polyethylene (HDPE) containing 0, 1 or 5 wt% graphene oxide (GO) at room temperature. The resulting films display a superhydrophobic character with a static water contact angle higher than 150°. The superhydrophobic films provide an effective corrosion-resistant coating for the zinc interface upon immersion in an aqueous solution of sodium chloride (3% NaCl) for up to 29 days. The corrosion process was investigated by following the change of the water contact angle over time and by electrochemical means. The presence of GO in the HDPE film was found to increase the corrosion resistance.

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1. Introduction

Recently, superhydrophobic treatments have been applied to various engineering material surfaces such as steel, copper, zinc, and aluminum, to improve their corrosion performance.¹ The development of superhydrophobic nanocomposite coatings for metals and alloys with oxide layers and the study of their protective performance represent important stages in the creation of corrosion-resistant coatings, which demonstrate consistent protective characteristics not only under atmospheric conditions, but also in aggressive media.¹

Superhydrophobic zinc surfaces have been prepared via numerous methods, including solution immersion technique,^2,3 metal replacement deposition process,^4–9 chemical oxidation,¹⁰ organic modification,¹¹ and chemical etching.^12,13

Porous polymer structures (or gels) are not new; they have been used in many contexts such as porous polymer sorbents, membranes, and filters.^14,15 Erbil et al. reported a simple and inexpensive method for the preparation of a superhydrophobic coating using isotactic polypropylene (i-PP) by a selection of solvents and temperature to control the surface roughness.¹⁶ Following this original approach, low-density polyethylene (LDPE) superhydrophobic surfaces were fabricated by controlling the polymer crystallization behavior through adjusting the crystallization time and nucleation rate.¹⁷ To extend the technique for high-density polyethylene (HDPE), Yuan et al. varied the evaporation conditions by adding ethanol in humid atmosphere at 5 °C to prepare stable porous superhydrophobic HDPE surfaces.¹⁸ Independently, Fresnais et al. applied a totally different route to generate transparent superhydrophobic polyethylene layers adopting plasma processing.¹⁹ Direct CF₄-plasma modification of LDPE or O₂-plasma treatment followed by CF₄-plasma was applied to generate superhydrophobic surfaces with high contact angles (up to 170°). Moreover, the films remain transparent to visible light when the surface roughness remains lower than 50 nm.¹⁹ In a completely different approach, Lu et al. developed a simple and reproducible method for the fabrication of high surface area biomimetic superhydrophobic surfaces on various thermoplastic polymers, including HDPE by heat- and pressure-driven polymer replication methods using etched Al surface as replication templates.^20,21 Particularly, the HDPE superhydrophilic surfaces exhibited excellent stability and durability after being exposed to various organic solvents and aqueous solutions of various pH.²¹

Recently, the outstanding properties of graphene and its derivatives such as chemical inertness, exceptional thermal and chemical stability, remarkable flexibility, optical transparency in the visible range, and impermeable to molecules make them most favorable for formation of passive layer to protect metals from oxidation and corrosion especially in severe marine environment. Research into graphene-based materials such as graphene oxide (GO) and reduced graphene oxide (rGO) has demonstrated potential for applications in material protection and anti-corrosive coatings.^22–28 Similarly, graphene-reinforced composites and graphene-embedded polymers have been investigated as anti-corrosive coatings for several metals, including copper,^29,30 cold-rolled steel,^31–33 steel,³⁴ galvanized iron,³⁵ zinc,³⁶ titanium,³⁷ and AZ91 magnesium alloy.³⁸

From the literature survey and to the best of our knowledge, there is no report on the preparation of superhydrophobic graphene-embedded polymers. In this paper, we propose to combine the good stability of superhydrophobic HDPE surfaces and the unique properties of GO for the fabrication of superhydrophobic HDPE/GO nanocomposites. The HDPE/GO coating exhibited good anticorrosion properties and durability. The technique proposed herein is simple and straightforward and easily scalable.

2. Experimental section

2.1. Reagents and materials

Zinc foil (99.9%, 0.25 mm thick), high density polyethylene (HDPE), xylene, ethanol, isopropyl alcohol, and sodium chloride (NaCl) were obtained from Aldrich and used without further purification.

2.2. Preparation of graphene oxide (GO)

GO was synthesized by chemical exfoliation of flake graphite through a modified Hummers' method. The detailed experimental conditions are reported in a recently published work. A homogeneous GO aqueous suspension (0.1 mg mL⁻¹) was achieved by dispersing GO in ethanol with the aid of ultrasonication for 3 h.³⁹

2.3. Preparation of HDPE films on zinc surfaces

The zinc substrates (1 cm × 1 cm × 0.25 cm) were first cleaned ultrasonically in ethanol, isopropyl alcohol, and distilled water (5 min, two times each). The clean specimens were dried under a stream of nitrogen. Subsequently, a total of 0.3 g of HDPE was dissolved slowly in 30 mL xylene to form a uniform solution at 120 °C. Then, the HDPE xylene solution was dropped onto the zinc plates and the solvent was evaporated slowly at room temperature in a fume hood.

2.4. Preparation of superhydrophobic HDPE films on zinc surfaces

0.3 g of HDPE resin was dissolved slowly in 30 mL xylene to form a uniform solution at 120 °C, then 3 mL ethanol was added into the HDPE solution when the temperature of the HDPE solution is less than 78 °C to avoid ethanol evaporation. Some of HDPE ethanol–xylene solution was dropped onto the zinc substrate and the solvents were evaporated at room temperature in a fume hood.

2.5. Preparation of superhydrophobic HDPE/GO films on zinc surfaces

0.3 g of HDPE resin was dissolved slowly in 30 mL xylene to form a uniform solution at 120 °C, then GO (3 or 15 mg) dissolved in 3 mL ethanol under ultrasonication for 1 h was added into the HDPE solution when the temperature of the HDPE solution is less than 78 °C to avoid ethanol evaporation. Some of HDPE/GO solution was dropped onto the substrate and the solvents were evaporated at room temperature in a fume hood.

2.6. Sample characterization

The surface morphology of the samples was investigated by scanning electron microscopy (SEM, S-4800, Hitachi, Japan).

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 carbon (284.6 eV) was used as the reference.

Raman spectra were recorded using a micro-Raman system equipped with a Leica DMLM microscope and a 514.5 nm Ar-ion incident laser (LS-514 model, aserphysics) with an output power of 20 mW. The Raman spectral coverage is from a Stokes shift of 200–4000 cm⁻¹ with a resolution of about 1 cm^−1.

Contact-angle measurements were measured using a remote computer-controlled goniometer system (DIGIFROP by GBX, France) under static conditions. Water droplet (about 8 μL) was dropped carefully onto the surface. The values reported are averages of five measurements made on different positions of the sample surface. The accuracy of the CA value of each sample was ±2°, as is the error bar in the graph. All measurements were made in an ambient atmosphere at room temperature.

Linear sweep voltammetry (LSV) was performed using an Autolab potentiostat 30 (Eco Chemie, Utrecht, The Netherlands). The zinc samples were used as working electrode. A platinum sheet and a calomel electrode were used as the counter and reference electrode, respectively. LSV tests were performed by scanning the potential at a rate of 2 mV s⁻¹.

2.7. Corrosion resistance

Potentiodynamic polarization curves were used to evaluate and compare the corrosion resistance of the bare zinc and the zinc substrate coated with HDPE/GO superhydrophobic film. They were recorded in a classical three electrode cell in 3.5% wt NaCl aqueous solution at room temperature. The electrochemical corrosion tests were carried out using a three-electrode configuration with platinum as the counter electrode, saturated calomel as the reference electrode, and the samples with an exposed area of 1 cm² as the working electrode. The Tafel plots were acquired at a scan rate of 2 mV s⁻¹. The distance between working electrode and counter electrode was about 5 cm.

3. Results and discussion

3.1. Morphology of HDPE/GO films

Fig. 1 shows typical SEM images of HDPE/GO coatings on Zn substrates for different GO concentrations (0, 1 and 5 wt%) prepared at room temperature. The HDPE films appear as porous structures. The air pockets formed by the microscopic pores, on which a substantial fraction of the water drop sits led to superhydrophobic surfaces.¹⁶ It is obvious that many cracked slices stack on the surface of the samples as evidenced in the high magnification SEM images in Fig. 1a. With the addition of GO in the HDPE film, the slices become thicker. The size of each slice varies from 10 to 30 mm for HDPE sample and from 20 to 40 mm for HDPE/GO samples.

Fig. 1 SEM images of HDPE/GO surfaces deposited on zinc substrate at room temperature. HDPE/GO (0%) (a and b); HDPE/GO (1%) (c and d); HDPE/GO (5%) (e and f). The insets are the corresponding static water contact angles.

3.2. Wetting properties

When the HDPE xylene solution was dropped onto the zinc plates and the solvent was evaporated slowly at room temperature, a smooth transparent film was formed without any superhydrophobicity.¹⁸ Fig. 2 shows the static contact angle (SWCA) of a water drop on the HDPE coated zinc substrate. The resulting surface displays a SWCA of 132 ± 2°. This is likely related to the smoothness of the HDPE film deposited at room temperature, in agreement with previous work.¹⁸

Fig. 2 Static water contact angle of pure HDPE sample deposited on zinc substrate at room temperature.

The investigation of Erbil et al. suggested that lower drying temperature for polypropylene solution increased the surface roughness of polypropylene coating, and the corresponding water contact angle was increased.¹⁶ Yuan et al. showed that by addition of ethanol and lowering the evaporation temperature of HDPE–xylene solutions, superhydrophobic surfaces can be generated.¹⁸ We have undertaken a similar approach to prepare superhydrophobic HDPE/GO surfaces at room temperature in xylene–ethanol solution. Fig. 3 displays the SWCAs of bare zinc substrate before and after coating with different HDPE/GO films. The unmodified Zn substrate exhibits a WCA of 90 ± 2°. A superhydrophobic behavior was reached with WCAs > 150° for the Zn substrates coated with HDPE/GO (0, 1, 5 wt%).

Fig. 3 WCAs of bare zinc substrate before (a) and after coating with HDPE/GO (0%), (b) HDPE/GO (1%) (c) and HDPE/GO (5%) (d).

3.3. Anticorrosion properties

The corrosion behavior of the bare zinc substrate and the as-prepared superhydrophobic zinc surfaces was investigated by potentiodynamic polarization tests in 3.5 wt% NaCl aqueous solutions at a scanning rate of 2 mV s⁻¹ as shown in Fig. 4. This process was repeated at least three times for every sample. The bare zinc sample exhibited a corrosion current density (j_corr) of 2.54 × 10⁻⁴ A cm⁻² and an associated corrosion potential (E_corr) of −1.048 V. Coating the Zn substrate with superhydrophobic HDPE and HDPE/GO (1%) did not impact the E_corr although a decrease of j_corr was observed (Table 1). Interestingly, coating Zn substrate with superhydrophobic HDPE/GO (5%) film led to an anodic shift of the E_corr to −0.989 V and a decrease of j_corr to 7.32 × 10⁻⁶ A cm⁻² due to a restricted supply of oxygen and water-limiting oxygen and water reduction (eqn (1) and (2)).²

O₂ + H₂O + 2e → HO₂⁻ + OH⁻ (1)

HO₂⁻ + H₂O + 2e → 3OH⁻

2H₂O + 2e → 2OH⁻ + H₂ (2)

Fig. 4 Potentiodynamic polarization curves of unmodified zinc substrate (cyan line), and zinc samples coated with superhydrophobic HDPE/GO (0%) (blue line), HDPE/GO (1%) (black line), and HDPE/GO (5%) (red line).

Table 1 The corrosion potential (E_corr/V) and the corrosion current intensity (I_corr/A) of zinc samples before and after coating with HDPE/GO at different GO concentrations

Sample	Ecorr (V)	jcorr (A cm^−2)
Zinc substrate	−1.048 ± 0.0005	2.54 ± 0.005 × 10^−4
HDPE	−1.045 ± 0.0005	2.86 ± 0.005 × 10^−5
HDPE/GO (1%)	−1.045 ± 0.0005	2.34 ± 0.005 × 10^−5
HDPE/GO (5%)	−0.989 ± 0.0005	7.326 ± 0.005 × 10^−6

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The lower corrosion current intensities correspond to better corrosion resistance. The HDPE/GO (5%) superhydrophobic surface had better anticorrosive property than the other coatings.

3.4. Chemical stability and durability

3.4.1. Storage in ambient atmosphere. With a view to real applications, environmental stability and durability were still the primary factors that limit the applications of superhydrophobic surfaces. The superhydrophobicity of the sample is further tested at different time intervals upon storage in an ambient atmosphere to verify its environmental stability and durability. After eight months of storage in air, the values of the WCA remained essentially constant, indicating that the superhydrophobicity of the hierarchical structures and chemical components were stable in air and thus superhydrophobic surfaces exhibit long-term stability and durability.

3.4.2. Immersion of the coated interface in 3% NaCl aqueous solution. The stability of the HDPE/GO (0, 1, 5 wt%) superhydrophobic films was also tested by immersion of the coated Zn interface for 29 days in a 3% NaCl aqueous solution. The change of the WCAs over time of the samples upon immersion 3% NaCl aqueous solution is shown in Fig. 5. A small increase of the WCAs to 160° was observed after 1 week exposure to 3% NaCl aqueous solution and then remained stable over the tested period i.e. 29 days. The results indicated that the superhydrophobic HDPE/GO films were very stable in corrosive media. The WCAs of the HDPE/GO (0, 1, 5 wt%) superhydrophobic films are all near 160° after immersion in a 3% NaCl for 29 days.

Fig. 5 Change of CA of HDPE/GO coated Zn samples dipped in 3% NaCl aqueous solution over time.

3.4.3. Chemical composition of the superhydrophobic surfaces after immersion in 3% NaCl aqueous solution. Fig. 6 depicts the XRD patterns of bare Zn substrates before and after immersion in 3% NaCl aqueous solution for 29 days and samples coated with HDPE and HDPE/GO on Zn substrates after immersion in 3% NaCl aqueous solution for 29 days. The peaks marked by “*” are derived from the zinc substrate (JCPDS PDF#65-3358), and the peaks labelled with “#” are attributed to crystalline ZnO (JCPDS PDF#65-3411) in the samples. The peaks marked with “Δ” and “φ” are the peaks of Zn₅(OH)₈Cl₂·H₂O (JCPDS PDF#07-0155) and Zn₅(CO₃)₂(OH)₆ (JCPDS PDF#19-1458), respectively. For comparison, the XRD patterns of the bare zinc substrate before and after immersion in 3% NaCl aqueous solution for 29 days were recorded in Fig. 6A. The peaks from Zn were observed for the bare zinc substrates before and after immersion in 3% NaCl aqueous solution for 29 days. The peaks from ZnO, Zn₅(OH)₈Cl₂·H₂O and Zn₅(CO₃)₂(OH)₆ were observed after immersion in 3% NaCl aqueous solution for 29 days. The XRD patterns of the bare zinc substrate (curve a), samples coated with superhydrophobic HDPE (curve b) and HDPE/GO (5%) (curve c) on zinc substrate after immersion in 3% NaCl aqueous solution for 29 days were also recorded in Fig. 6B, respectively. The peaks from ZnO, Zn₅(OH)₈Cl₂·H₂O and Zn₅(CO₃)₂(OH)₆ were observed on curves a, b and c in addition to the peaks from Zn, but the crystal sizes and growth directions of ZnO, Zn₅(OH)₈Cl₂·H₂O and Zn₅(CO₃)₂(OH)₆ were different.

Fig. 6 XRD patterns of samples: (A) XRD patterns of bare Zn before (black) and after (red) immersion in 3% NaCl aqueous solution; (B) bare Zn substrate (a), samples coated with HDPE (b) and HDPE/GO (5 wt%) (c) on Zn substrates after immersion in 3% NaCl solution for 29 days. The peaks marked by “*” are derived from the zinc substrate, and the peaks labeled with “#” are attributed to crystalline ZnO in the samples. The peaks marked with “Δ” and “φ” are the peaks of Zn₅(OH)₈Cl₂·H₂O and Zn₅(CO₃)₂(OH)₆, respectively.

XPS analysis was carried out to further examine the surface composition of the samples coated with HDPE and HDPE/GO (5%) on Zn substrates before and after immersion in 3% NaCl aqueous solution for 29 days. Fig. 7 shows XPS spectra of samples coated with HDPE (a) and HDPE/GO (5%) (b) before immersion in 3% NaCl aqueous solution (Fig. 7A) and after immersion in 3% NaCl aqueous solution for 29 days (Fig. 7B–E) on Zn substrate. Fig. 7A displays the XPS survey spectra of Zn substrates coated with HDPE (a) and HDPE/GO (5%) (b) before immersion in 3% NaCl aqueous solution. It shows the presence of only carbon on the surface, in good agreement with the chemical composition of the coating. Fig. 7B exhibits the XPS survey spectra of samples coated with HDPE (a) and HDPE/GO (5%) (b) on Zn substrates after immersion in 3% NaCl aqueous solution for 29 days. Immersion in 3% NaCl aqueous solution led to the appearance of oxygen and zinc peaks along with carbon, in good agreement with XRD results. Fig. 7C depicts the high resolution XPS spectra of the C1s of the samples coated with HDPE (a) and HDPE/GO (5%) (b) on Zn substrates after immersion in 3% NaCl aqueous solution. It can be fitted with two main peaks at 284.6 and 287.2 eV attributed to C–C (sp²) and C=O bonds for curve a, respectively, indicating the sample was rich in carbonate on the surface. The carbonate may be originate from Zn₅(CO₃)₂(OH)₆. A high-resolution spectrum of C1s for sample coated with HDPE/GO (5%) on Zn substrate confirmed the presence of C=C (284.8 eV), C–O (286.8 eV), and COOH (289.0 eV) bonds,^40,41 as shown in the inset of Fig. 7C (curve b), indicating the sample was rich in hydroxyl and carboxylic acid groups of GO on the surface.

Fig. 7 XPS spectra of samples coated with HDPE (a) and HDPE/GO (5%) (b) on Zn substrates before immersion in 3% NaCl aqueous solution (A) and after immersion in 3% NaCl aqueous solution for 29 days (B–E). The inset of (C) is the fitted curve of C1s peak of curve (b) (sample coated with HDPE/GO (5%) after immersion in 3% NaCl aqueous solution for 29 days); the inset of (E) is the fitted curve of O1s peak of a (sample coated with HDPE after immersion in 3% NaCl aqueous solution for 29 days).

The Zn2p high resolution XPS spectra of the surfaces on zinc substrate are shown in Fig. 7D. The doublet peak energy separation in both cases is 23.1 eV, which is consistent with the standard separation of 23.1 eV corresponding to zinc element.^42,43 The bands at 1022.4 eV (Zn2p3/2) and 1045.5 eV (Zn2p1/2) split into two peaks respectively as shown in Fig. 7D. The band at 1022.4 eV split into 1021.4 eV and 1023.5 eV and the band at 1045.5 eV split into 1044.5 eV and 1046.5 eV. The bands at 1021.3 eV and 1044.5 eV correspond to the Zn–O bond, while bands at 1023.5 eV and 1046.5 eV might be related to the Zn²⁺ ions in oxygen-deficient regions.⁴⁴

The O1s XPS spectra of the as-prepared surface consists of a single and asymmetric component, which can be typically deconvoluted into three bands at 530.2, 531.4 and 532.6 eV as shown in the inset of Fig. 7E. The peak at 530.2 eV on the low binding side of the O1s spectrum in the curve a (sample coated with HDPE after immersion in 3% NaCl aqueous solution for 29 days) is attributed to O₂⁻ in the Zn–O. The band at the medium binding energy (531.4 eV) of the O1s peak in the curve a is associated with oxygen-deficient regions in the zinc oxyhydroxide species Zn₅(OH)₈Cl₂·H₂O and Zn₅(CO₃)₂(OH)₆. The component located at 532.6 eV in the curve a is usually attributed to the presence of loosely bound oxygen on the surface such as adsorbed O₂ or adsorbed H₂O.^45,46 An O1s peak at 532.6 eV is attributed to GO and adsorbed O₂ or adsorbed H₂O as shown in curve b (sample coated with HDPE/GO (5%) after immersion in 3% NaCl aqueous solution for 29 days) of Fig. 7E.⁴⁰

3.4.4. Morphology of the sample surfaces after immersion in 3% NaCl solution. SEM analysis was performed to check the morphological changes that occurred on the Zn surfaces upon immersion in 3% NaCl aqueous solution for 29 days as shown in Fig. 8. The morphology varies greatly before and after immersion in 3% NaCl solution comparing with Fig. 1. After immersion in 3% NaCl solution, zinc substrate surface has been corroded seriously as shown in Fig. 8a and b. The morphology of the HDPE/GO (5%) surfaces also changes before and after immersion in 3% NaCl solution (Fig. 8c and d). Although there was a significant morphological change upon immersion in 3% NaCl solution, the change of the SWCAs of the HDPE/GO (5%) surface is little.

Fig. 8 SEM images of different surfaces after immersion in 3% NaCl aqueous solution for 29 days: zinc substrate (a and b); HDPE/GO (5%) (c and d).

4. Conclusion

In conclusion, stable homogeneous porous superhydrophobic high-density polyethylene (HDPE)/graphene oxide (GO) coatings on zinc substrates were successfully prepared using ethanol-xylene solution of HDPE containing 0, 1% or 5 wt% of GO. Incorporation of a small amount of GO (1 wt%) does not seem to affect the wetting properties of HDPE. In contrast, increasing the amount of GO (5 wt%) in the HDPE film led to the formation superhydrophobic surfaces with higher corrosion resistance. Compared with other synthetic methods of superhydrophobic surfaces, the present method is much simpler and can be readily scaled to create large-area uniform superhydrophobic surfaces that can be applied in commercial manufacturing.

Acknowledgements

The authors thank the Centre National de la Recherche Scientifique (CNRS), The Lille1 University, the Nord Pas de Calais region, and the National Natural Science Foundation of China (No. 21271027) for financial support.

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