Oriented CuZnAl Ternary Layered Double Hydroxide Films: In situ

Oriented dense thin films of CuZnAl–NO3 layered double hydroxide (CuZnAl–NO3 LDH) have been grown on the surface of copper foil by an in situ ...
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Oriented CuZnAl Ternary Layered Double Hydroxide Films: In situ Hydrothermal Growth and Anticorrosion Properties Xiaodong Lei,* Linna Wang, Xuhui Zhao, Zheng Chang, Meihong Jiang, Dongpeng Yan, and Xiaoming Sun State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, P.O. Box 98, Beijing 100029, China ABSTRACT: Oriented dense thin films of CuZnAl−NO3 layered double hydroxide (CuZnAl−NO3 LDH) have been grown on the surface of copper foil by an in situ hydrothermal method with Cu substrate as Cu source, boehmite sol as Al source, and Zn(NO3)2 aqueous solution as Zn source. The structure and morphology of the as-prepared CuZnAl−NO3 LDH films were studied by X-ray diffraction, elemental analysis, room temperature Fourier transform infrared spectra, and scanning electron microscopy analysis. The results showed that the [00l] direction (or ab plane) of CuZnAl−NO3 LDH platelets was parallel to the surface of the Cu substrate. By a facial immersion method, intercalation of laurate anions by ion exchange with the CuZnAl−NO3 LDH film precursors endowed hydrophobic properties on Cu substrate. Moreover, the corrosion behavior of the hydrophobic laurate intercalated LDH films was then investigated by detecting the contact angle, potentiodynamic polarization, and electrochemical impedance properties, which showed that the corrosion resistance of the pristine Cu was highly improved due to the “hydrophobic interaction” and “anion exchange” effects. Therefore, this work provides an effective way to develop a new type of ternary LDH films with potential application as hydrophobic and long-term anticorrosive materials.

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INTRODUCTION Corrosion cost estimates have been done by most industrialized countries, and it accounts for several percent of the gross domestic product (GDP).1 Metal Cu has a wide range of applications in electronics, for production of wires, sheets, tubes, and alloys, because of its excellent thermal and electrical conductivities.2 Although metal Cu is resistant toward the influence of the atmosphere and many chemicals, it is still an active metal that does not resist corrosion well. For example, solutions containing oxygen, oxidation acid (nitric acid, chromic acid, etc.), or small ions (CN−, NH4+, Cl−) can usually form complexes with Cu, which result in the severe corrosion of Cu.3 The mechanism of Cu electrodissolution in chloride media has been widely investigated by researchers.4−8 It is generally accepted that Cu anodic dissolution is influenced by chloride concentration, while is independent of pH. In such conditions, it is necessary to use copper corrosion inhibitors, since no protective passive layer can be expected.2,8 Up to now, numerous possible anticorrosion methods have been investigated and developed. For example, the use of inhibitors is very popular.2,8 In addition, the presence of heteroatoms such as nitrogen, sulfur, and phosphorus in the organic compound molecule can improve the action as Cu corrosion inhibitor. This can be explained by the presence of vacant d orbitals in Cu atom that form coordinative bonds with electronic donor.2 Moreover, from a cost-effective viewpoint, one promising approach is to create hydrophobic and/or superhydrophobic environment, which can shield the Cu surface from attack by moisture contact.9−13 For example, self-assembled monolayers (SAMs) formed by the adsorption of n-alkanethiols onto Cu provide a flexible method for producing coatings against corrosion.9 Wang et al. have explored a one-step potentiostatic electrolysis method to fabricate hydrophobic film with flower© 2013 American Chemical Society

like structure on Cu which presents excellent inhibition effect to the Cu corrosion and stability in water containing Cl−.11 Liu et al. have prepared hydrophobic films on Cu surface with a one-step immersion method for anticorrosion;12,13 the film can protect the underlying Cu effectively, and the relative long duration (5 days) affords its potential application in industry. As a result, hydrophobic surfaces are conventionally devised and fabricated by combining appropriate surface roughness with low-surface-energy materials. However, the low-surfaceenergy film material formed on rough metal surfaces with the traditional method is too thin with little affinity to be broken down, which can restrict future application. Layered double hydroxides (LDHs) are a class of anionic clays, whose structure is based on brucite-like layers and consist of stacks of positively charged, mixed-metal hydroxide layers with intercalated anionic species and solvent molecules between layers.14 LDHs have the general formula of [M1−x2+Mx3+(OH)2]x+(An−)x/n·mH2O, where M2+and M3+are divalent (Mg2+, Zn2+, Fe2+, Co2+, and Cu2+, among others) and trivalent metal cations (Al3+, Cr3+, Fe3+, and Ga3+, among others), respectively, occupying octahedral positions in the hydroxide layers, and An− is an interlayer charge-compensating anion.15 LDHs have gained significant attention because of their possible technological applications in catalysis, adsorption, pharmaceutics, photochemistry, electrochemistry, and other fields.16,17 In recent years, more and more attention has focused on the LDH applications in the field of metal anticorrosion due to their nontoxic, harmless, and anticorrosion properties. It has Received: Revised: Accepted: Published: 17934

October 3, 2013 November 29, 2013 November 29, 2013 November 29, 2013 dx.doi.org/10.1021/ie403299u | Ind. Eng. Chem. Res. 2013, 52, 17934−17940

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been reported that some studies on the use of LDH films as coatings of metal for anticorrosion. For example, mixed slurries of LDHs and polyvinyl butyral18,19 or an epoxy resin20 on some aluminum alloy, and LiAl LDH coating formed by emerging an aluminum alloy into a mixed solution of a lithium salt with some additional organic coatings can supply adequate corrosion protection.21−23 The anticorrosion properties of LDH films prepared by spin coating a MgAl-LDH sol on a magnesium alloy have been investigated by Sun et al.24 The results demonstrated that MgAl-LDH films have the potential to provide an environmentally friendly corrosion resistant coating for magnesium alloys. Kendig et al. have prepared LDH films with different anion intercalation25 and deposited them onto a Cu substrate; electrochemical studies have shown that the anions between LDH layers play an important role in inhibition of filiform corrosion. However, to achieve effective anticorrosion coatings, a densely packed LDH film structure is still required. Furthermore, in spite of the recent progress made in the study of LDH films, the development of a convenient approach for the fabrication of oriented films with controllable properties, such as film thickness and crystal orientation, remains a considerable challenge. Recently, Chen et al. have found that dense LDH films directly grown on an aluminum oxide substrate surface can be prepared by an in situ hydrothermal crystallization method, and the films showed well-defined “slippy” (Cassie−Baxter regime) superhydrophobicity after simple treatment with lauric acid.26 Zhang et al. have reported that anion exchange of laurate with a ZnAl-NO3− LDH film on a porous anodic alumina/Al substrate affords a ZnAl−laurate LDH film.27 The superhydrophobic nature of the film provides long-term corrosion protection of the coated aluminum substrate and provides an effective barrier to aggressive species. By taking advantages of the attractive features of LDH materials (such as the ability to modify the properties of the film by cointercalation of other anions), it is expected to achieve their commercial application in the corrosion protection of metal. The in situ crystallization method inspires and enlightens us to challenge the goal of fabrication of orderly LDH films on metal Cu surface for its potential corrosion protection. As well, how to rationally tune the morphology and packing manner, crystal orientations of the LDH platelets within film for optimizing the anticorrosion property of metal remains a scientific problem. In this work, we have prepared ternary CuZnAl−NO3 LDH films grown on the Cu substrate by an in situ hydrothermal method with the substrate as Cu source, and the obtained dense CuZnAl−NO3 LDH films have exhibited highly orientation with the [00l] direction of platelets parallel to the surface of the Cu substrate. Then, we describe intercalation of laurate anions with the CuZnAl−NO3 LDH film precursors by chemical bond on Cu foil leads to a hydrophobic property, which provides an effective and long-term corrosion-resistant film for the metal Cu.

2.2. Methods. The preparation of boehmite (AlOOH) sol: 0.03 mol of aluminum isopropoxide was dissolved in 400 mL 0.05 mol/L HNO3 aqueous solution. The solution was stirred for 10 min in air at room temperature. Then heated rapidly to 90 °C and kept for 6 h under reflux conditions. At last, AlOOH sol was formed. The AlOOH sol was dried at 60 °C for testing the formation of boehmite. Preparation of CuZnAl LDH Thin Films. A 40 mL AlOOH sol and 40 mL 0.6 mol/L Zn(NO3)2 aqueous solution were placed in 100 mL polytetrafluoroethylene vessels. The 0.01 mol/L of ammonia aqueous solution was added dropwise into the above mixture until the pH reached 7.5 under room temperature. The copper foils were suspended in the mixture. The vessels were sealed and maintained at 60 °C for 3 days. After cooling, the foils were washed with deionized water, and then dried at 60 °C for 6 h, and the CuZnAl LDH thin films can be obtained. The resulting films were immersed in 100 mL 0.05 mol/L of sodium laurate (n-dodecanoate) aqueous solution at 30 °C for 3 h. After immersion, the fabricated films were rinsed with ethanol and dried at room temperature, and donated as CuZnAl−La LDH film. 2.3. Characterization. Powder X-ray diffraction (XRD) data were collected on a Rigaku XRD-6000 powder diffractometer, using Cu Ka radiation (40 kV, 30 mA, and λ = 1.542 Å) between 1° and 70° with a scanning rate of 5° min−1. Room-temperature Fourier transform infrared (FT-IR) spectroscopy spectra were recorded in the range 400−4000 cm−1 with a resolution of 2 cm−1 on a Bruker Vector-22 Fourier transform spectrometer using the KBr pellet technique (1 mg of powder scrapped from film samples in 100 mg of KBr). The morphology of the CuZnAl LDH films was investigated by using a scanning electron microscope (SEM) with an energy dispersive X-ray spectroscopy (EDX) attachment (SEM Hitachi S-3500N, EDX Oxford Instrument Isis 300). The accelerating voltage applied was 20 kV. All SEM samples were sputtered with gold. Elemental analyses for Cu, Zn, and Al were performed with a Shimadzu ICPS-7500 inductively coupled plasma spectroscopy (ICP) instrument on solutions prepared by dissolving the powders scraped from the Cu substrate in dilute HNO3 (1:1). Static water contact angles (CAs) were measured with a sessile drop at three different points of each sample using a commercial drop-shape analysis system (DSA100, KrüSS GmbH, Germany) at ambient temperature. The equilibrium water CA was measured with a fixed needle supplying a water drop (volume for 10 μL) while the drop-shape analysis system determined the CA. Three different points on each sample were investigated, and the average value determined. Polarization curves (DCs) were obtained by using a Cypress Systems CS-350 electrochemical analysis system at room temperature. A three-electrode configuration was employed in the circuit with the sample as working electrode, a platinum counter electrode, and a saturated calomel electrode as reference electrode. A 3.5% aqueous solution of sodium chloride was used as electrolyte. The sweep rate was set at 10 mV·s−1. Electrochemical impedance spectroscopy (EIS) was performed by IM6 electrochemical workstation (Zahner-Elektrik Company, Germany) under open circuit conditions. The experimental temperature was kept at 25 °C. EIS measurements were performed in the frequency range between 10 mHz and 100 kHz with a single wave amplitude of 10 mV. The

2. EXPERIMENTAL SECTION 2.1. Materials. All of the chemicals are analytical grade and used without further purification. Copper foils (Hanbo environmental equipment Co. Ltd., purity > 99.9%, thickness 1 mm) with a size of ca. 20 mm × 20 mm were degreased with ethanol and Na2CO3 solution (2 wt %) for 5 min, respectively. The degreased copper foils etched with HCl solution (1 wt %) for 15 s, washed with deionized water, and then dried at 343 K overnight. 17935

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low-angle reflections at 8.71 and 4.44 Å (Figure 2b), which can be assigned to the [003] and [006] reflections of a 3R type LDH phase, consistent with the reported CuZnAl−NO3 LDH powder systems.28−30 The presence of NO3− in the interlayer galleries of the LDH film was further confirmed by the characteristic peak at 1384 cm−1 in the FT-IR spectrum (Figure 3a). In the usual powder form of LDHs, NO3− anions can be

experimental EIS spectra were interpreted on the basis of equivalent electrical analogs using the program Zview2.0 to obtain the fitting parameters.

3. RESULTS AND DISCUSSION 3.1. Preparation of CuZnAl LDH Films over Cu Foil. The Cu foil was first coated with a layer of AlOOH sol and subsequently treated with an alkaline NH3·H2O solution of Zn(NO3)2 in the presence of an excess of nitrate anions. The phase of AlOOH sol was examined by XRD measurement with the dried sol (Figure 1). The diffraction peaks of the as-

Figure 3. FT-IR spectra of CuZnAl−NO3 LDH powder scrapped from film samples (a), sodium laurate (b), and CuZnAl−La LDH powder scrapped from film samples (c). Figure 1. XRD pattern of dried AlOOH sol.

exchanged with little hindrance by other anions.27,31,32 Therefore, CuZnAl−NO3 LDH film is convenient for laurateintercalation. Figure 2c shows the XRD pattern of the CuZnAl−La LDH film after ion-exchange intercalation with a solution of sodium laurate. Comparison of the XRD patterns of the literatures 33 and 34 shows that the low angle peaks correspond to the basal reflections of LDH. These peaks can be indexed as the [003], [006], [0012], and [0015] reflections of an LDH phase (Figure 2c). The basal spacing of [003] is about 33.9 Å that is a little lower than the value expected for a bilayer of laurate anions arranged in a tilted orientation within the interlayer galleries of LDHs (about 37.3 Å).33,34 Although the metal elements in the layer of laurate anion intercalated LDHs in references are different from the CuZnAl−La LDH film, the basal spacings of [003] can be used to estimate the packing mode of laurate anions. According to ref 33, it appears that in the CuZnAl−La LDH film the laurate anions were packed in a tilted bimolecular layer with a little disorder. The FT-IR spectrum of the powders scraped from the CuZnAl−La LDH film, which showed two strong absorption bands at about 1410 and 1556 cm−1 (Figure 3c), identified as the symmetric and asymmetric stretching bands of the COO− group, similar to the corresponding peaks in the spectrum of sodium laurate (Figure 3b). In addition, the shoulder peak at 1384 cm−1 in Figure 3c indicates that there are a part of NO3− anions cointercalated within the CuZnAl−La LDH crystal. Among angular platelets ranging from 100 to 200 nm, the presence of the CuZnAl−NO3 LDH stack of pancakes was revealed by camera and scanning electron microscopy (SEM). A transparent and smooth film (Figure 4b) is shown to be fabricated on the surface of Cu foil (Figure 4a) with highly ordered orientation (Figure 5a and b), in which all flat surfaces of the hexagonal platelike platelets in the film are parallel to the surface of Cu foil. As is shown in Figure 5c, the EDX pattern shows the molar ratio of Cu:Zn:Al was about 2.21:4.41:5.52, in agreement with the elemental analysis by ICP (2.27:4.35:5.43).

prepared product can be perfectly assigned to the standard value of boehmite (JCPDS card no. 21-1307). The addition of NH3·H2O in synthesis process was believed to be a key point: besides the basic properties, NH3·H2O species can also display a strong affinity with Cu2+ ion to form a complex. A complexation followed by complex removal route from the surface of Cu could also provide a homogeneous precipitation in AlOOH sol with Zn2+. In our experiments, if no NH3·H2O was added in the synthesis process, there was no Cu element tested in the film by the elemental analysis of ICP. As well, a complexing base leads to CuZnAl LDH “stack of pancakes” morphology. XRD measurement was first performed on the asprepared LDH films. In addition to the peaks of the Cu substrate (Figure 2a), the XRD pattern of the film shows two

Figure 2. XRD patterns of Cu foil (a), CuZnAl−NO3 LDH film (b), and CuZnAl−La LDH film on the Cu substrate (c). The * indicates the reflection peaks from the Cu substrate. 17936

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Figure 6. Top-view (a) and edge-view (b) SEM micrographs of the CuZnAl−La LDH film.

Figure 4. Photographs of the surface of Cu foil (a), CuZnAl−NO3 LDH film (b), and CuZnAl−La LDH film (c), respectively.

should expanded to about 3 times compared with the pristine CuZnAl−NO3 LDH based on the above XRD observation (Figure 2b and c). Herein, the SEM thickness of CuZnAl−La LDH film is enlarged only about 1.27 times compared with the CuZnAl−NO3 LDH film. Therefore, a layer of CuZnAl−NO3 LDH crystallites may appear between CuZnAl−La LDH crystallite layer and Cu substrate, in agreement with the FTIR measurement (Figure 3c). 3.2. Corrosion Resistance of CuZnAl LDH Films. The corrosion resistance of the LDH films was investigated by analyzing polarization curves and electrochemical impedance spectroscopy (EIS). Before the test, samples were immersed in a corrosive medium (3.5% aqueous sodium chloride solution) for 1 day. Figure 7 presents the polarization curves of the Cu

Figure 5. Top-view SEM micrographs at low magnification (a) and high magnification (b), EDX pattern (c), and edge-view SEM micrograph (d) of the CuZnAl−NO3 LDH film, respectively.

This confirms that the addition of NH3·H2O in a synthesis process introduces Cu2+ into to the LDH phase. By observing the side-view SEM image (Figure 5d), the thickness of the CuZnAl−NO3 LDH film can be estimated as ca. 36.80 μm, and the thickness may be controlled by varying reaction conditions; for example, it increased with the reaction time, temperature, and concentration of Zn2+ ion and NH3·H2O. The preferred orientation of the CuZnAl−NO3 LDH film was also consistent with the strongly enhanced X-ray diffraction [00l] peaks (Figure 2b). The intense [00l] reflections and the absence of any in-plane reflections (h, k ≠ 0) for the transparent film are evidence for an extremely well oriented assembly of LDH platelets in the film.35,36 The dense film with the [00l] direction (ab plane) of the CuZnAl−NO3 LDH crystallites parallel to the surface of Cu substrates suggests that the hydrothermal treatment seems favoring the emergence of the layered stacking arrangement (Figure 5d). After treatment with sodium laurate, the morphology of the surface of LDH film changes significantly. The modified surface is covered with relatively weak orientational CuZnAl−La LDH crystallites with hexagonal platelike morphology (Figure 4c and Figure 6a). The thickness of LDH film is enlarged from 36.80 μm (Figure 5d) to about 46.67 μm (Figure 6b). The expansion in crystallite thickness associated with intercalation of the laurate anions possibly induces considerable stress in the film, which is relieved by the formation of randomly distributed platelets of the CuZnAl−La LDH crystallites. The intercalation and affinity between laurate anions and CuZnAl−La LDHs resulted in the transformation of LDH platelets from parallel to the surface to disorderly arrange on the surface of Cu foil. Theoretically, the height of a pure CuZnAl−La LDH crystallite

Figure 7. Polarization curves (vsSCE) of samples immersed in 3.5% NaCl solution at room temperature for 1 day: (a) bare Cu foil, (b) CuZnAl−NO3 LDH film, and (c) CuZnAl−La LDH film. (d) CuZnAl−La LDH film after immersion for 30 days.

foil and the foil covered by CuZnAl−NO3 and CuZnAl−La LDH films, respectively. Compared with the naked Cu foil (Figure 7a), the polarization current is reduced and corrosion potential values shifted in the positive direction after in situ growth of CuZnAl−NO3 LDH film (Figure 7b). Corrosion potential could, to a certain extent, reflect the corrosion susceptibility of a material, and the more positive corrosion potential implied the lower corrosion susceptibility. However, for the copper foil covered by CuZnAl−La LDH film (Figure 7c), a further shift in the positive direction occurs and the corrosion current is obvious reduced. It is should be noted that, after being placed in 3.5% NaCl solution at room temperature for 30 days, the polarization curve of CuZnAl−La LDH film does not evidently change (Figure 7d). Therefore, the corrosion resistant of the CuZnAl−La LDH film for future long-term application can be confirmed. Figure 8 shows the EIS of the Cu foil, and the foils covered by CuZnAl−NO3 LDH film and CuZnAl−La LDH film, 17937

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Figure 8. Nyquist plots of samples immersed in 3.5% NaCl solution at room temperature for 1 day: (a) bare Cu foil, (b) CuZnAl−NO3 LDH film, and (c) CuZnAl−La LDH film.

respectively. It can be observed that the capacitive loops for the samples are not perfect semicircles, and this phenomenon is known as the dispersing effect.37 Due to the fact that the double-layer does not behave as an ideal capacitor in the presence of the dispersing effect, a constant phase element (CPE) is often used as a substitute for the capacitor in the equivalent circuit to meet the accuracy of impedance behavior of the electrical double layer. The reciprocal of the chargetransfer resistance, Rt, corresponds to the corrosion rate of a metal in corrosive solutions. A smaller Rt corresponds to a faster corrosion rate. From Figure 8, it can be seen that the diameter of capacitive loops of the Cu foil, the Cu foil covered by CuZnAl−NO3 LDH film, and the Cu foil covered by CuZnAl−La LDH film gradually enlarges, implying that their charge-transfer resistance, Rt, gradually increases. Such a result agrees with those of polarization measurements. Upon formation of the LDH films, the impedance is increased in all frequency range, and the corrosion current is decreased evidently. It indicates that the LDH film can improve the anticorrosion of Cu in 3.5% NaCl solution effectively, and the CuZnAl−La LDH film had a better corrosion resistance than the Cu foil and CuZnAl−NO3 LDH film. 3.3. Anticorrosion Mechanisms. The hydrophobic surface composed of hills (solid portion of the surface) can easily trap gas within the “valleys” between the hills.38 Therefore, the Cl− can hardly reach the bare Cu surface for the obstructive effect of “air valleys”. The results of hydrophobicity for modified Cu foil are shown in Figure 9a−c. Static water contact angles (CAs) of the surface of bare Cu foil, CuZnAl− NO3 LDH film, and CuZnAl−La LDH film are about 94.1°, 71.5°, and 139.1°, respectively. The random distribution of CuZnAl−La LDH platelets on Cu foil surface observed in Figure 6a is formed by facial immersing CuZnAl−NO3 LDH film in sodium laurate solution. The structure with intercalated laurate anion can trap a large amount of air, leading to CA of 139.1° (Figure 9c). Furthermore, after being placed in 3.5% NaCl solution at room temperature for 30 days, the hydrophobic film still remains intact and the CA does not change. As well, the surface of CuZnAl−La LDH film remains unchanged, and this confirms the superior barrier properties of the hydrophobic film. Obviously, because of “hydrophobic interaction”, the Cl− species can hardly reach the surface of Cu, resulting in high corrosion resistance of the CuZnAl−La LDH film. The XRD patterns of the fresh hydrophobic CuZnAl−La LDH film (Figure 2c) and the same film after exposure to 3.5%

Figure 9. Profile of a water droplet on the surface of bare Cu foil (a), CuZnAl−NO3 LDH film (b), and CuZnAl−La LDH film (c).

NaCl solution at room temperature for 30 days (Figure 10a) were essentially identical, suggesting that no evidence of intercalation of chloride anions from the solution or of carbonate anions by interaction with atmospheric carbon dioxide, as is observed for other LDHs.15,31,32 This confirms that CuZnAl−La LDH film exhibits good stability during longterm immersion, and by virtue of retaining their structural integrity by chemical bonding and hydrophobic properties they are able to provide long-term corrosion protection. A transparent and smooth CuZnAl−NO3 LDH film (Figure 4b) with highly ordered orientation that is all flat surfaces of the hexagonal platelike platelets in the film are parallel to the surface of Cu foil, as is shown in Figure 5a and b. In the case of the structure without organic molecular intercalation, the air 17938

dx.doi.org/10.1021/ie403299u | Ind. Eng. Chem. Res. 2013, 52, 17934−17940

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Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the NSFC, the 863 Program (grant no. 2012AA03A609), and the Program for Changjiang Scholars and Innovative Research Team in University (no. IRT1205) of P. R. China.

■ Figure 10. XRD patterns of CuZnAl−NO3 LDH (a) and CuZnAl−La LDH films (b) after exposure to 3.5% NaCl solution at room temperature for 30 d. The * indicates the reflection peaks from the Cu substrate.

cannot be trapped stably in the pores and water spreads on the surface easily, resulting in a low CA (