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Fabrication of superhydrophobic aluminum surface by droplet etching and chemical modification
Colloids and Surfaces A 567 (2019) 205–212
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Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Fabrication of superhydrophobic aluminum surface by droplet etching and chemical modification
T
Xin Zhanga, Jing Zhaoa, , Jiliang Moa, Ruoyu Suna, Zhen Lia, Zhiguang Guob,c ⁎
a
School of Mechanical Engineering, Southwest Jiaotong University, Chengdu, 610031, PR China Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials and Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan, 430062, PR China c State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, PR China b
GRAPHICAL ABSTRACT
A simple method was used to fabricate the superhydrophobic aluminum with good superhydrophobicity, thermostability, antifouling and anticorrosion abilities.
ARTICLE INFO
ABSTRACT
Keywords: Superhydrophobic Aluminum Droplet etching Thermostability Anticorrosion Antifouling
The development of techniques to fabricate superhydrophobic aluminum surfaces is rapidly maturing, yet the problems of high loss of material and complex preparation still persist. To address this, a simple method that combines droplet etching and chemical modification was used to fabricate a superhydrophobic aluminum surface, with a contact angle of 156° and a sliding angle of 5°. Compared with the traditional immersion method, droplet etching can properly preserve the integrity of the aluminum material and also provide a rough structure on the aluminum surface. The optimal superhydrophobicity of the aluminum surface was obtained by one-step immersion in pentadecafluorooctanoic acid aqueous solution at 80 ℃. Thermostability, anticorrosion, selfcleaning, and antifouling tests were subsequently performed. The results demonstrated that the superhydrophobic aluminum surface was easily prepared and possessed thermostability, anticorrosion, self-cleaning and antifouling abilities.
1. Introduction Bionics is a discipline that implements and effectively applies biological functions to engineering [1–6]. Many bionics studies have
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advanced beyond the concept stage and have been successfully applied, such as designing and producing a crawling robot by imitating a gecko [7,8], fabricating robots that walk on water by imitating the water strider [9,10], fabricating a hopping robot by imitating the jumping of
Corresponding author. E-mail address: [email protected] (J. Zhao).
https://doi.org/10.1016/j.colsurfa.2019.01.046 Received 15 October 2018; Received in revised form 24 January 2019; Accepted 24 January 2019 Available online 28 January 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
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locusts [11], fabricating an erosion-resistant surface by imitating the unique structures produced by desert scorpions and lizards [12,13], and fabricating a superhydrophobic surface by imitating the surface structure of a lotus leaf [14–16]. In recent years, the research related to superhydrophobic surfaces has matured. There is a great interest in further examining superhydrophobic surfaces due to their excellent performance, which can be attributed to their self-cleaning [17,18], anticorrosion [19,20], and anti-icing [21,22] properties. In order to develop a superhydrophobic and self-cleaning material based on biomass, Xie et al. [17] grew Cu3(PO4)2·3H2O nanoflowers on a soy protein isolate (SPI) film surface using a moderate and simple method. Liu et al. [23] fabricated a superhydrophobic magnesium alloy surface with good anticorrosion property by an electrodeposition method. The fabrication process consisted of immersing the magnesium alloy in a mixed solution of myristic acid/cerium nitrate hexahydrate. Then, two corrosion tests were used to verify the anticorrosion property, and the results showed that the anticorrosion property of the superhydrophobic magnesium alloy surface was greatly improved. Liao et al. [21] fabricated a superhydrophobic aluminum (Al) surface that showed good superhydrophobicity and anti-icing properties. These properties could prevent the icing phenomenon from developing on the sample surface in a low-temperature environment by forcing the water to converge into large drops that then rolled off the sample surface before freezing. There are many applications for superhydrophobic surfaces because of their excellent properties [24–30]. For instance, Zhang et al. [24] demonstrated the drag-reducing property of a superhydrophobic coating by the contrast sailing test. A submarine model with a superhydrophobic coating showed good drag-reducing property relative to the model without a superhydrophobic coating under the same conditions. This experiment provided a promising application for the underwater drag reduction of a ship or submarine. Deng et al. [26] fabricated a multifunctional TiO2–SiO2@polydimethylsiloxane (PDMS) hybrid material that can be applied to fabric and used as a water-repellent ship coating, and it was discovered that superhydrophobic fabrics are wash-resistant and resistant to strong acid attack, and can be applied as a filter cloth for both oil–water separation and colorful pattern printing. Liu et al. [29] developed a transparent superhydrophobic surface through calcination of candle-soot-coated PDMS films, which can be used on the surface of windshields or building facades. Although metallic materials are commonly used in these fields, there are problems associated with their use, including rust or the adsorption of pollutants, which often appear on metal surfaces. As noted above, a superhydrophobic surface with excellent performance, such as anticorrosion and antifouling ability, can solve the problems described above. Therefore, it is very meaningful to design and fabricate superhydrophobic metal surfaces. At present, the methods utilized to prepare a superhydrophobic metal surface can be divided into two types: direct and indirect methods. The indirect method is the fabrication of a superhydrophobic coating on a metal surface [31–35]. For example, Mahadik et al. [32] produced multifunctional superhydrophobic coatings on various metal substrates with the assistance of a sol-gel spray coating method. Wang et al. [33] fabricated a superhydrophobic coating on magnesium alloy, and the fabrication method can realize the large-scale production of superhydrophobic surfaces. Gnedenkov et al. [35] fabricated superhydrophobic coatings on metal and alloy surfaces, and then studied the anticorrosion abilities of the samples with superhydrophobic coatings and other protective layers. The test results showed that the superhydrophobic coatings have the best inhibitory effect. Although these coatings can greatly improve the superhydrophobicity, corrosion resistance, and other properties of metal surfaces, they can not change the wettability of the metal material itself. An additional deficiency is that the poor adhesive strength between the sticky coating and metal substrate limits its application and service life. The direct method
involves a direct process to endow the metal surface with superhydrophobicity [36–41]. For instance, Hou et al. [37] fabricated a superhydrophobic zinc oxide surface by a simple two-step method consisting of oxidizing the sample surface, and then modifying the surface with a monolayer of n-octadecyl thiol, and the prepared superhydrophobic surface was subsequently durable and long-lasting. Ji et al. [39] prepared a superhydrophobic Al surface via grinding, acid etching, and low surface tension chemical modification, in sequence. This direct method provides satisfactory superhydrophobicity for metal materials; however, there is considerable loss of material and the process is complicated. Therefore, it is necessary to develop a convenient and facile method to fabricate a superhydrophobic metal surface. In this work, Al with excellent properties, including light weight and high strength, was selected as the sample. A simple method consisting of only two steps was used to fabricate an Al surface that can achieve superhydrophobicity in a short period of time. The method will not result in excess loss of material, compared with the traditional method. The superhydrophobic Al surface exhibited good properties, such as thermostability, anticorrosion, self-cleaning and antifouling abilities. 2. Experimental 2.1. Materials A 6061 Al plate (wt.%: Mg 0.80–1.21; Si 0.39-0.80; Cu 0.15-0.40; Mn 0.15; Zn 0.25; Cr 0.04-0.35; Ti 0.15; Fe 0.70; Al balance) was cut into small-sized plates (20 × 20 × 2 mm). Ethanol and acetone were purchased from Tianjin Lianlongbohua Pharmaceutical Chemistry Co., Ltd., China. Hydrochloric acid (HCl) (AR) was purchased from Shantou Xilong Chemical Plant Co., Ltd., China. Pentadecafluorooctanoic acid (PFOA) was purchased from Shanghai Saen Chemistry Technology Co., Ltd., China. All other chemicals were analytical grade reagents, and were used as received. Deionized water was used throughout the experiments performed for this study. 2.2. Preparation of the superhydrophobic Al surface The superhydrophobic Al surface was fabricated by a simple production process. First, the Al plates were ultrasonically cleaned in ethanol and acetone. Second, the horizontally placed Al plates were treated by droplet etching with a 2.5 M HCl solution for 4 min. After droplet etching, the Al plates were cleaned with deionized water. Finally, the Al plates were immersed in 0.01 M PFOA aqueous solution for 2 h and then dried at 80 °C for 1 h. In order to observe the effect of the immersion temperature, four types of Al surfaces were prepared at 20 °C, 40 °C, 60 °C, and 80 °C for 2 h, respectively. Fig. 1a schematically shows the production process of the superhydrophobic Al surface. Fig. 1b shows photographs of water droplets on the Al surfaces in the preparation process. It can be seen that there was a visible transformation in the wettability of the Al surfaces. 2.3. Characterization The morphological structures of the prepared Al surfaces were examined by field emission scanning electron microscopy (FESEM, JSM6701 F). The elemental analysis of the fabricated Al surface was revealed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) with Al-Kα radiation. The water contact angles (CA) and sliding angles (SA) were measured by contact angle goniometer (JC2000D) with a 5-μL distilled water droplet at ambient temperature. The CA and SA measurements represent the averaged values over five different spots on each sample. The three-dimensional (3D) surface morphologies were examined using a white light interferometer (Bruker Contour GT). Thermalstability is an important factor for determining the working temperature of the superhydrophobic samples. The anticorrosion ability of the superhydrophobic Al surface was investigated by 206
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Fig. 1. Schematic illustrating the production process of the superhydrophobic Al surface (a) and photographs of water droplets on the original, etched and superhydrophobic Al surfaces (b).
Fig. 2. CA of different Al surfaces with optical images of static water droplets (5 μL) (a) and SA of the Al surface immersed at 80 °C (b).
potentiodynamic polarization using an electrochemical workstation (V38135, Netherlands) with 3.5% NaCl aqueous solution as the corrosive liquid. In a typical process, a standard three-electrode system equipped with a reference saturated calomel electrode (SCE), Pt plate as counter electrode, and specimen as working electrode was employed. The scan rate for potentiodynamic polarization curves was 0.5 mV/s.
in PFOA aqueous solution, the CA of the Al surface gradually increases and its wettability changes. The Al surfaces immersed at 20 °C and 40 °C are hydrophobic with a CA of approximately 122.1° and 140.9°, respectively. The Al surfaces immersed at 60 °C and 80 °C are superhydrophobic with a CA of approximately 152.2° and 156.2°, respectively. It is reported that surface modification by low surface energy materials is also a key factor for constructing a superhydrophobic surface besides rough structure [43]. Therefore, the transformation of wettability may be due to the introduction of low surface energy materials after immersion in PFOA. Higher temperature is more conducive to grafting low surface energy materials on rough Al surface. Fig. 2b present the SA of the Al surface immersed at 80 °C, which is 5° and indicates that the Al samples can acquire good superhydrophobicity when the immersion temperature reaches 60 °C or higher. The optimal immersion temperature to prepare the superhydrophobic Al surface in view of the wettability was 80 °C. Clearly, the immersion temperature plays a key factor in fabricating and attaining the superhydrophobic Al surface.
3. Results and discussion 3.1. Surface wettability Fig. 2a shows the CA and the optical images of static water droplets on different Al surfaces. It can be seen that the original and etched Al surfaces are hydrophilic with a CA of 85.5° and 58.0°, respectively. According to Wenzel law, an increase in roughness for hydrophilic surfaces decreases the CA and makes the surface more hydrophilic [42]. This explains very well why the Al surface became more hydrophilic after droplet etching. With the increase in the immersion temperature 207
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3.3. Surface morphologies of the Al surface As an essential condition for determining superhydrophobicity, the surface structures of the Al samples at different process stages were investigated, as shown in Fig. 4. The original Al surface was flat with no observable microstructures (Fig. 4a, d). After droplet etching, there are many microcracks and micro-nano balls on the Al surface (Fig. 4b). The high-magnification SEM image reveals that the Al surface is totally covered with innumerable nano-sticks (Fig. 4e). After immersion at 80 °C, no microcracks can be found on the Al surface, while many micropores are observed (Fig. 4c). It is clear that a large number of nanoflakes exist on the Al surface and a portion of the nano-sticks fuses together (Fig. 4f). The existence of micropores and nano-flakes are helpful to obtain a rougher surface. Therefore, the Al samples immersed at 60 °C and 80 °C satisfied two essential conditions of rough structure and low surface energy, and thus superhydrophobicity was realized. Combined with the results of wettability and XPS spectra, these phenomena indicate that the rough structure on the Al surface immersed at 40 °C or less than 40 °C was insufficient to produce superhydrophobicity. Higher immersion temperature may further roughen the sample surface to form a micro-nano structure. It also proves that the surface micro-nano structure is the primary factor that affects the superhydrophobicity of the surface under similar surface energy conditions.
Fig. 3. XPS spectra of the Al surface after droplet etching and immersion at 40 °C, 60 °C and 80 °C.
3.2. Chemical compositions In order to further verify the chemical compositions of the Al surfaces, the XPS spectra are presented in Fig. 3. It represents the XPS spectra of the Al surfaces after droplet etching and immersion in PFOA at 40 °C, 60 °C and 80 °C, respectively. There was no peak of elemental F on the etched Al surface, but a high peak of elemental F appeared on the Al surfaces after immersion in PFOA. This phenomenon indicated that after immersion, the Al samples were modified by PFOA, which is evidence that the Al surfaces obtained low surface energy. It was reported that an optimal superhydrophobic surface must be satisfied with two essential conditions of a rough surface and low surface energy [44–46]. Clearly, the three types of Al samples after immersion satisfy the condition of low surface energy but possess different wettability. These results suggest that the difference in wettability may be mainly attributed to the different roughness levels. The lower immersion temperature (< =40 °C) was insufficient to construct sufficient roughness for superhydrophobicity on the Al surface, but the higher immersion temperature (> =60 °C) further roughened the sample surface and thus endowed the sample with superhydrophobicity. Therefore, we speculate that the different roughness levels of the sample surfaces were the reason for the different hydrophobic properties at different temperatures.
3.4. Practicability of the superhydrophobic Al surface The ultimate purpose of the studies that examined superhydrophobic surfaces is to determine the practicability. One barrier to practical application is that the sizes of metal materials are generally required to be extremely accurate, but the traditional method for creating a superhydrophobic metal surface involves immersing the entire sample in etching solution, which will severely damage the sample. Fig. 5a shows the optical images of the Al surface after immersion etching and droplet etching, with an immersion time of 4 min. The size of the original Al sample is 20 mm × 20 mm × 2 mm in this study. However, it is shown that the size of the Al sample after immersion etching decreased to 18 mm × 18 mm × 0.5 mm, and the material loss rate is approximately 79.8%. The size of the Al sample after droplet etching is almost 20 mm × 20 mm × 2 mm, which indicates that the integrity of the sample was properly maintained. Fig. 5b presents the white light interferogram of the Al samples after
Fig. 4. SEM images of the original Al surface (a, d), the etched Al surface (b, e), and the superhydrophobic Al surface (c, f).
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Fig. 5. The optical images (a) and the white light interferogram (b) of the Al surface after the immersion etching and droplet etching.
immersion etching and droplet etching. The average surface roughness is approximately 9.8 μm for the immersion etching sample and 3.5 μm for the droplet etching sample. There are many protrusions on both rough Al surfaces but with quite different sizes. Apparently, the rough structure of the Al surface after droplet etching is more uniform, and therefore, it is easy to ensure the homogeneity of superhydrophobicity on the entire sample surface. These tests prove that the droplet etching method can contribute to the preservation of the practicability of the superhydrophobic Al surface, thus ensuring the reusability of the superhydrophobic Al surface.
after heating at high temperature. Fig. 6 shows the CA of the superhydrophobic Al sample heated for 2 h at different temperatures. The results illustrate that the Al sample can maintain its superhydrophobicity with a CA of approximately 155°in the temperature range of 100–180 °C, but abruptly loses it at 200 °C. The thermostability test proves that the superhydrophobic Al surface in this study has good thermostability in the temperature range of 100–180 °C. Further analysis was used to determine the factors responsible for the loss of sample superhydrophobicity after heated at 200 °C. Fig. 7(a) shows the SEM images of the superhydrophobic Al surface after the thermostability test at 200 °C. It can be seen that micro-balls and nanoflakes with bigger size exist on the sample surface, and the rough structure was not totally destroyed by the high temperature of 200 °C. Moreover, an XPS spectrum was obtained to examine the surface and elucidate the reason for the loss of superhydrophobicity. Fig. 7(b) shows the XPS spectra of the superhydrophobic Al surface before and after the thermostability test at 200 °C. The peak height of elemental F indicates that there was no PFOA on the superhydrophobic Al surface after heated at 200 °C. It has been reported that the boiling point of PFOA is approximately 189–191 °C [47]. Therefore, the PFOA on the superhydrophobic Al surface would vaporize when the temperature reached 200 °C. Combined with the previous analysis, the vaporization of PFOA on the superhydrophobic Al surface at 200 °C increased the surface energy, and then resulted in the loss of the sample’s superhydrophobicity.
3.5. Thermostability of the superhydrophobic Al surface The superhydrophobic surface can be a candidate for widespread practical application if it can resist high temperature damage. It is significant to examine the wettability of superhydrophobic Al samples
3.6. Anticorrosion ability of the superhydrophobic Al surface Metal and alloy are the most important structural materials due to their excellent properties such as higher mechanical strength, good durability, good heat resistance, and good size stability. However, corrosion often invalidates the materials, and then reduces the safety performance of the structure. Metal corrosion is one of the most noticeable problems, and it is important to improve the anticorrosion properties of metal materials. Fig. 8 shows the potentiodynamic
Fig. 6. CA of the superhydrophobic Al surface after thermostability test. Shown in the insets are the optical images of static water droplets (5 μL). 209
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Fig. 7. SEM images (a) and XPS spectra (b) of the superhydrophobic Al surface after heated at 200 °C.
polarization curves of the etched and superhydrophobic Al samples in 3.5 wt% NaCl solution. The corrosion potential (Ecorr) and corrosion current density (Icorr) of the etched and superhydrophobic Al samples obtained by Tafel extrapolation are shown in Table 1. It can be seen that Icorr of the superhydrophobic Al sample (8.8 × 10−8 A·cm-2) is reduced by one order of magnitude as compared to that of the etched Al sample (7.2 × 10-7 A·cm-2). Additionally, Ecorr of the superhydrophobic Al sample (-1.369 V) is more positive than that of the etched Al sample (-1.451 V). In general, a higher corrosion potential and a lower corrosion current density indicate a superior corrosion resistance [48]. After droplet etching, the protective film on the Al surface was destructed by the Cl- ion in the aqueous salt. The internal metal was in direct contact
with the solution and is further corroded. The introduction of PFOA can effectively separate the internal metal and the solution and thus prevent the destruction of the protective film on the Al surface. Obviously, the anticorrosion ability of the superhydrophobic Al sample was increased as compared to the etched Al sample. 3.7. Self-cleaning and antifouling abilities of the superhydrophobic Al surface As a common and unavoidable metal used outdoors or in other mechanical environments, the surfaces composed of Al are frequently soiled by environmental pollutants. Therefore, it is necessary to study the self-cleaning and antifouling abilities of the superhydrophobic Al surface. Fig. 9 shows the evolution processes including self-cleaning and antifouling abilities of the superdydrophobic Al surface. The water droplets can easily clean off the dust, which proves a good self-cleaning ability of the superhydrophobic Al surface (Fig. 9a). Fig. 9b presents the antifouling process that the muddy water droplets dripped on the surface and rolled off immediately. The superhydrophobicity of the Al surface efficiently prevent the dirt from adhering to the surface in contact with the muddy water. All these results indicate that the superhydrophobic Al surface can protect the substrates from pollution. 4. Conclusions A superhydrophobic Al surface was successfully prepared by a novel method. The method is simple and time-saving, and sufficient to preserve the integrity of the samples. The superhydrophobic Al surface showed a water contact angle of 156 ± 2° and a sliding angle of 5°. A rough surface with micro-balls and nano-flakes was observed in SEM images. Thermostability test indicated that the superhydrophobic Al samples maintained their superhydrophobicity in the temperature range from 100 °C to 180 °C. The polarization measurement test, selfcleaning and antifouling tests also showed that the superhydrophobic Al surface possesses satisfactory anticorrosion, self-cleaning and antifouling abilities.
Fig. 8. Potentiodynamic polarization curves of the etched and superhydrophobic Al samples.
Table 1 The corrosion potential (Ecorr) and the corrosion current density (Icorr) of the etched and superhydrophobic Al samples in 3.5 wt% NaCl solution. Samples
Ecorr (V)
Icorr (A·cm−2)
Etched Al Superhydrophobic Al
−1.451 −1.369
7.2 × 10−7 8.8 × 10−8
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Fig. 9. The evolution processes including (a) self-cleaning and (b) antifouling abilities of the superdydrophobic Al surface.
Acknowledgment [12]
This work is supported by the National Natural Science Foundation of China (No. 51822508 and No. 51675448).
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Contents lists available at ScienceDirect
Colloids and Surfaces A journal homepage: www.elsevier.com/locate/colsurfa
Fabrication of superhydrophobic aluminum surface by droplet etching and chemical modification
T
Xin Zhanga, Jing Zhaoa, , Jiliang Moa, Ruoyu Suna, Zhen Lia, Zhiguang Guob,c ⁎
a
School of Mechanical Engineering, Southwest Jiaotong University, Chengdu, 610031, PR China Hubei Collaborative Innovation Centre for Advanced Organic Chemical Materials and Ministry of Education Key Laboratory for the Green Preparation and Application of Functional Materials, Hubei University, Wuhan, 430062, PR China c State Key Laboratory of Solid Lubrication, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou, 730000, PR China b
GRAPHICAL ABSTRACT
A simple method was used to fabricate the superhydrophobic aluminum with good superhydrophobicity, thermostability, antifouling and anticorrosion abilities.
ARTICLE INFO
ABSTRACT
Keywords: Superhydrophobic Aluminum Droplet etching Thermostability Anticorrosion Antifouling
The development of techniques to fabricate superhydrophobic aluminum surfaces is rapidly maturing, yet the problems of high loss of material and complex preparation still persist. To address this, a simple method that combines droplet etching and chemical modification was used to fabricate a superhydrophobic aluminum surface, with a contact angle of 156° and a sliding angle of 5°. Compared with the traditional immersion method, droplet etching can properly preserve the integrity of the aluminum material and also provide a rough structure on the aluminum surface. The optimal superhydrophobicity of the aluminum surface was obtained by one-step immersion in pentadecafluorooctanoic acid aqueous solution at 80 ℃. Thermostability, anticorrosion, selfcleaning, and antifouling tests were subsequently performed. The results demonstrated that the superhydrophobic aluminum surface was easily prepared and possessed thermostability, anticorrosion, self-cleaning and antifouling abilities.
1. Introduction Bionics is a discipline that implements and effectively applies biological functions to engineering [1–6]. Many bionics studies have
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advanced beyond the concept stage and have been successfully applied, such as designing and producing a crawling robot by imitating a gecko [7,8], fabricating robots that walk on water by imitating the water strider [9,10], fabricating a hopping robot by imitating the jumping of
Corresponding author. E-mail address: [email protected] (J. Zhao).
https://doi.org/10.1016/j.colsurfa.2019.01.046 Received 15 October 2018; Received in revised form 24 January 2019; Accepted 24 January 2019 Available online 28 January 2019 0927-7757/ © 2019 Elsevier B.V. All rights reserved.
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locusts [11], fabricating an erosion-resistant surface by imitating the unique structures produced by desert scorpions and lizards [12,13], and fabricating a superhydrophobic surface by imitating the surface structure of a lotus leaf [14–16]. In recent years, the research related to superhydrophobic surfaces has matured. There is a great interest in further examining superhydrophobic surfaces due to their excellent performance, which can be attributed to their self-cleaning [17,18], anticorrosion [19,20], and anti-icing [21,22] properties. In order to develop a superhydrophobic and self-cleaning material based on biomass, Xie et al. [17] grew Cu3(PO4)2·3H2O nanoflowers on a soy protein isolate (SPI) film surface using a moderate and simple method. Liu et al. [23] fabricated a superhydrophobic magnesium alloy surface with good anticorrosion property by an electrodeposition method. The fabrication process consisted of immersing the magnesium alloy in a mixed solution of myristic acid/cerium nitrate hexahydrate. Then, two corrosion tests were used to verify the anticorrosion property, and the results showed that the anticorrosion property of the superhydrophobic magnesium alloy surface was greatly improved. Liao et al. [21] fabricated a superhydrophobic aluminum (Al) surface that showed good superhydrophobicity and anti-icing properties. These properties could prevent the icing phenomenon from developing on the sample surface in a low-temperature environment by forcing the water to converge into large drops that then rolled off the sample surface before freezing. There are many applications for superhydrophobic surfaces because of their excellent properties [24–30]. For instance, Zhang et al. [24] demonstrated the drag-reducing property of a superhydrophobic coating by the contrast sailing test. A submarine model with a superhydrophobic coating showed good drag-reducing property relative to the model without a superhydrophobic coating under the same conditions. This experiment provided a promising application for the underwater drag reduction of a ship or submarine. Deng et al. [26] fabricated a multifunctional TiO2–SiO2@polydimethylsiloxane (PDMS) hybrid material that can be applied to fabric and used as a water-repellent ship coating, and it was discovered that superhydrophobic fabrics are wash-resistant and resistant to strong acid attack, and can be applied as a filter cloth for both oil–water separation and colorful pattern printing. Liu et al. [29] developed a transparent superhydrophobic surface through calcination of candle-soot-coated PDMS films, which can be used on the surface of windshields or building facades. Although metallic materials are commonly used in these fields, there are problems associated with their use, including rust or the adsorption of pollutants, which often appear on metal surfaces. As noted above, a superhydrophobic surface with excellent performance, such as anticorrosion and antifouling ability, can solve the problems described above. Therefore, it is very meaningful to design and fabricate superhydrophobic metal surfaces. At present, the methods utilized to prepare a superhydrophobic metal surface can be divided into two types: direct and indirect methods. The indirect method is the fabrication of a superhydrophobic coating on a metal surface [31–35]. For example, Mahadik et al. [32] produced multifunctional superhydrophobic coatings on various metal substrates with the assistance of a sol-gel spray coating method. Wang et al. [33] fabricated a superhydrophobic coating on magnesium alloy, and the fabrication method can realize the large-scale production of superhydrophobic surfaces. Gnedenkov et al. [35] fabricated superhydrophobic coatings on metal and alloy surfaces, and then studied the anticorrosion abilities of the samples with superhydrophobic coatings and other protective layers. The test results showed that the superhydrophobic coatings have the best inhibitory effect. Although these coatings can greatly improve the superhydrophobicity, corrosion resistance, and other properties of metal surfaces, they can not change the wettability of the metal material itself. An additional deficiency is that the poor adhesive strength between the sticky coating and metal substrate limits its application and service life. The direct method
involves a direct process to endow the metal surface with superhydrophobicity [36–41]. For instance, Hou et al. [37] fabricated a superhydrophobic zinc oxide surface by a simple two-step method consisting of oxidizing the sample surface, and then modifying the surface with a monolayer of n-octadecyl thiol, and the prepared superhydrophobic surface was subsequently durable and long-lasting. Ji et al. [39] prepared a superhydrophobic Al surface via grinding, acid etching, and low surface tension chemical modification, in sequence. This direct method provides satisfactory superhydrophobicity for metal materials; however, there is considerable loss of material and the process is complicated. Therefore, it is necessary to develop a convenient and facile method to fabricate a superhydrophobic metal surface. In this work, Al with excellent properties, including light weight and high strength, was selected as the sample. A simple method consisting of only two steps was used to fabricate an Al surface that can achieve superhydrophobicity in a short period of time. The method will not result in excess loss of material, compared with the traditional method. The superhydrophobic Al surface exhibited good properties, such as thermostability, anticorrosion, self-cleaning and antifouling abilities. 2. Experimental 2.1. Materials A 6061 Al plate (wt.%: Mg 0.80–1.21; Si 0.39-0.80; Cu 0.15-0.40; Mn 0.15; Zn 0.25; Cr 0.04-0.35; Ti 0.15; Fe 0.70; Al balance) was cut into small-sized plates (20 × 20 × 2 mm). Ethanol and acetone were purchased from Tianjin Lianlongbohua Pharmaceutical Chemistry Co., Ltd., China. Hydrochloric acid (HCl) (AR) was purchased from Shantou Xilong Chemical Plant Co., Ltd., China. Pentadecafluorooctanoic acid (PFOA) was purchased from Shanghai Saen Chemistry Technology Co., Ltd., China. All other chemicals were analytical grade reagents, and were used as received. Deionized water was used throughout the experiments performed for this study. 2.2. Preparation of the superhydrophobic Al surface The superhydrophobic Al surface was fabricated by a simple production process. First, the Al plates were ultrasonically cleaned in ethanol and acetone. Second, the horizontally placed Al plates were treated by droplet etching with a 2.5 M HCl solution for 4 min. After droplet etching, the Al plates were cleaned with deionized water. Finally, the Al plates were immersed in 0.01 M PFOA aqueous solution for 2 h and then dried at 80 °C for 1 h. In order to observe the effect of the immersion temperature, four types of Al surfaces were prepared at 20 °C, 40 °C, 60 °C, and 80 °C for 2 h, respectively. Fig. 1a schematically shows the production process of the superhydrophobic Al surface. Fig. 1b shows photographs of water droplets on the Al surfaces in the preparation process. It can be seen that there was a visible transformation in the wettability of the Al surfaces. 2.3. Characterization The morphological structures of the prepared Al surfaces were examined by field emission scanning electron microscopy (FESEM, JSM6701 F). The elemental analysis of the fabricated Al surface was revealed by X-ray photoelectron spectroscopy (XPS, Thermo Scientific ESCALAB 250Xi) with Al-Kα radiation. The water contact angles (CA) and sliding angles (SA) were measured by contact angle goniometer (JC2000D) with a 5-μL distilled water droplet at ambient temperature. The CA and SA measurements represent the averaged values over five different spots on each sample. The three-dimensional (3D) surface morphologies were examined using a white light interferometer (Bruker Contour GT). Thermalstability is an important factor for determining the working temperature of the superhydrophobic samples. The anticorrosion ability of the superhydrophobic Al surface was investigated by 206
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Fig. 1. Schematic illustrating the production process of the superhydrophobic Al surface (a) and photographs of water droplets on the original, etched and superhydrophobic Al surfaces (b).
Fig. 2. CA of different Al surfaces with optical images of static water droplets (5 μL) (a) and SA of the Al surface immersed at 80 °C (b).
potentiodynamic polarization using an electrochemical workstation (V38135, Netherlands) with 3.5% NaCl aqueous solution as the corrosive liquid. In a typical process, a standard three-electrode system equipped with a reference saturated calomel electrode (SCE), Pt plate as counter electrode, and specimen as working electrode was employed. The scan rate for potentiodynamic polarization curves was 0.5 mV/s.
in PFOA aqueous solution, the CA of the Al surface gradually increases and its wettability changes. The Al surfaces immersed at 20 °C and 40 °C are hydrophobic with a CA of approximately 122.1° and 140.9°, respectively. The Al surfaces immersed at 60 °C and 80 °C are superhydrophobic with a CA of approximately 152.2° and 156.2°, respectively. It is reported that surface modification by low surface energy materials is also a key factor for constructing a superhydrophobic surface besides rough structure [43]. Therefore, the transformation of wettability may be due to the introduction of low surface energy materials after immersion in PFOA. Higher temperature is more conducive to grafting low surface energy materials on rough Al surface. Fig. 2b present the SA of the Al surface immersed at 80 °C, which is 5° and indicates that the Al samples can acquire good superhydrophobicity when the immersion temperature reaches 60 °C or higher. The optimal immersion temperature to prepare the superhydrophobic Al surface in view of the wettability was 80 °C. Clearly, the immersion temperature plays a key factor in fabricating and attaining the superhydrophobic Al surface.
3. Results and discussion 3.1. Surface wettability Fig. 2a shows the CA and the optical images of static water droplets on different Al surfaces. It can be seen that the original and etched Al surfaces are hydrophilic with a CA of 85.5° and 58.0°, respectively. According to Wenzel law, an increase in roughness for hydrophilic surfaces decreases the CA and makes the surface more hydrophilic [42]. This explains very well why the Al surface became more hydrophilic after droplet etching. With the increase in the immersion temperature 207
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3.3. Surface morphologies of the Al surface As an essential condition for determining superhydrophobicity, the surface structures of the Al samples at different process stages were investigated, as shown in Fig. 4. The original Al surface was flat with no observable microstructures (Fig. 4a, d). After droplet etching, there are many microcracks and micro-nano balls on the Al surface (Fig. 4b). The high-magnification SEM image reveals that the Al surface is totally covered with innumerable nano-sticks (Fig. 4e). After immersion at 80 °C, no microcracks can be found on the Al surface, while many micropores are observed (Fig. 4c). It is clear that a large number of nanoflakes exist on the Al surface and a portion of the nano-sticks fuses together (Fig. 4f). The existence of micropores and nano-flakes are helpful to obtain a rougher surface. Therefore, the Al samples immersed at 60 °C and 80 °C satisfied two essential conditions of rough structure and low surface energy, and thus superhydrophobicity was realized. Combined with the results of wettability and XPS spectra, these phenomena indicate that the rough structure on the Al surface immersed at 40 °C or less than 40 °C was insufficient to produce superhydrophobicity. Higher immersion temperature may further roughen the sample surface to form a micro-nano structure. It also proves that the surface micro-nano structure is the primary factor that affects the superhydrophobicity of the surface under similar surface energy conditions.
Fig. 3. XPS spectra of the Al surface after droplet etching and immersion at 40 °C, 60 °C and 80 °C.
3.2. Chemical compositions In order to further verify the chemical compositions of the Al surfaces, the XPS spectra are presented in Fig. 3. It represents the XPS spectra of the Al surfaces after droplet etching and immersion in PFOA at 40 °C, 60 °C and 80 °C, respectively. There was no peak of elemental F on the etched Al surface, but a high peak of elemental F appeared on the Al surfaces after immersion in PFOA. This phenomenon indicated that after immersion, the Al samples were modified by PFOA, which is evidence that the Al surfaces obtained low surface energy. It was reported that an optimal superhydrophobic surface must be satisfied with two essential conditions of a rough surface and low surface energy [44–46]. Clearly, the three types of Al samples after immersion satisfy the condition of low surface energy but possess different wettability. These results suggest that the difference in wettability may be mainly attributed to the different roughness levels. The lower immersion temperature (< =40 °C) was insufficient to construct sufficient roughness for superhydrophobicity on the Al surface, but the higher immersion temperature (> =60 °C) further roughened the sample surface and thus endowed the sample with superhydrophobicity. Therefore, we speculate that the different roughness levels of the sample surfaces were the reason for the different hydrophobic properties at different temperatures.
3.4. Practicability of the superhydrophobic Al surface The ultimate purpose of the studies that examined superhydrophobic surfaces is to determine the practicability. One barrier to practical application is that the sizes of metal materials are generally required to be extremely accurate, but the traditional method for creating a superhydrophobic metal surface involves immersing the entire sample in etching solution, which will severely damage the sample. Fig. 5a shows the optical images of the Al surface after immersion etching and droplet etching, with an immersion time of 4 min. The size of the original Al sample is 20 mm × 20 mm × 2 mm in this study. However, it is shown that the size of the Al sample after immersion etching decreased to 18 mm × 18 mm × 0.5 mm, and the material loss rate is approximately 79.8%. The size of the Al sample after droplet etching is almost 20 mm × 20 mm × 2 mm, which indicates that the integrity of the sample was properly maintained. Fig. 5b presents the white light interferogram of the Al samples after
Fig. 4. SEM images of the original Al surface (a, d), the etched Al surface (b, e), and the superhydrophobic Al surface (c, f).
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Fig. 5. The optical images (a) and the white light interferogram (b) of the Al surface after the immersion etching and droplet etching.
immersion etching and droplet etching. The average surface roughness is approximately 9.8 μm for the immersion etching sample and 3.5 μm for the droplet etching sample. There are many protrusions on both rough Al surfaces but with quite different sizes. Apparently, the rough structure of the Al surface after droplet etching is more uniform, and therefore, it is easy to ensure the homogeneity of superhydrophobicity on the entire sample surface. These tests prove that the droplet etching method can contribute to the preservation of the practicability of the superhydrophobic Al surface, thus ensuring the reusability of the superhydrophobic Al surface.
after heating at high temperature. Fig. 6 shows the CA of the superhydrophobic Al sample heated for 2 h at different temperatures. The results illustrate that the Al sample can maintain its superhydrophobicity with a CA of approximately 155°in the temperature range of 100–180 °C, but abruptly loses it at 200 °C. The thermostability test proves that the superhydrophobic Al surface in this study has good thermostability in the temperature range of 100–180 °C. Further analysis was used to determine the factors responsible for the loss of sample superhydrophobicity after heated at 200 °C. Fig. 7(a) shows the SEM images of the superhydrophobic Al surface after the thermostability test at 200 °C. It can be seen that micro-balls and nanoflakes with bigger size exist on the sample surface, and the rough structure was not totally destroyed by the high temperature of 200 °C. Moreover, an XPS spectrum was obtained to examine the surface and elucidate the reason for the loss of superhydrophobicity. Fig. 7(b) shows the XPS spectra of the superhydrophobic Al surface before and after the thermostability test at 200 °C. The peak height of elemental F indicates that there was no PFOA on the superhydrophobic Al surface after heated at 200 °C. It has been reported that the boiling point of PFOA is approximately 189–191 °C [47]. Therefore, the PFOA on the superhydrophobic Al surface would vaporize when the temperature reached 200 °C. Combined with the previous analysis, the vaporization of PFOA on the superhydrophobic Al surface at 200 °C increased the surface energy, and then resulted in the loss of the sample’s superhydrophobicity.
3.5. Thermostability of the superhydrophobic Al surface The superhydrophobic surface can be a candidate for widespread practical application if it can resist high temperature damage. It is significant to examine the wettability of superhydrophobic Al samples
3.6. Anticorrosion ability of the superhydrophobic Al surface Metal and alloy are the most important structural materials due to their excellent properties such as higher mechanical strength, good durability, good heat resistance, and good size stability. However, corrosion often invalidates the materials, and then reduces the safety performance of the structure. Metal corrosion is one of the most noticeable problems, and it is important to improve the anticorrosion properties of metal materials. Fig. 8 shows the potentiodynamic
Fig. 6. CA of the superhydrophobic Al surface after thermostability test. Shown in the insets are the optical images of static water droplets (5 μL). 209
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Fig. 7. SEM images (a) and XPS spectra (b) of the superhydrophobic Al surface after heated at 200 °C.
polarization curves of the etched and superhydrophobic Al samples in 3.5 wt% NaCl solution. The corrosion potential (Ecorr) and corrosion current density (Icorr) of the etched and superhydrophobic Al samples obtained by Tafel extrapolation are shown in Table 1. It can be seen that Icorr of the superhydrophobic Al sample (8.8 × 10−8 A·cm-2) is reduced by one order of magnitude as compared to that of the etched Al sample (7.2 × 10-7 A·cm-2). Additionally, Ecorr of the superhydrophobic Al sample (-1.369 V) is more positive than that of the etched Al sample (-1.451 V). In general, a higher corrosion potential and a lower corrosion current density indicate a superior corrosion resistance [48]. After droplet etching, the protective film on the Al surface was destructed by the Cl- ion in the aqueous salt. The internal metal was in direct contact
with the solution and is further corroded. The introduction of PFOA can effectively separate the internal metal and the solution and thus prevent the destruction of the protective film on the Al surface. Obviously, the anticorrosion ability of the superhydrophobic Al sample was increased as compared to the etched Al sample. 3.7. Self-cleaning and antifouling abilities of the superhydrophobic Al surface As a common and unavoidable metal used outdoors or in other mechanical environments, the surfaces composed of Al are frequently soiled by environmental pollutants. Therefore, it is necessary to study the self-cleaning and antifouling abilities of the superhydrophobic Al surface. Fig. 9 shows the evolution processes including self-cleaning and antifouling abilities of the superdydrophobic Al surface. The water droplets can easily clean off the dust, which proves a good self-cleaning ability of the superhydrophobic Al surface (Fig. 9a). Fig. 9b presents the antifouling process that the muddy water droplets dripped on the surface and rolled off immediately. The superhydrophobicity of the Al surface efficiently prevent the dirt from adhering to the surface in contact with the muddy water. All these results indicate that the superhydrophobic Al surface can protect the substrates from pollution. 4. Conclusions A superhydrophobic Al surface was successfully prepared by a novel method. The method is simple and time-saving, and sufficient to preserve the integrity of the samples. The superhydrophobic Al surface showed a water contact angle of 156 ± 2° and a sliding angle of 5°. A rough surface with micro-balls and nano-flakes was observed in SEM images. Thermostability test indicated that the superhydrophobic Al samples maintained their superhydrophobicity in the temperature range from 100 °C to 180 °C. The polarization measurement test, selfcleaning and antifouling tests also showed that the superhydrophobic Al surface possesses satisfactory anticorrosion, self-cleaning and antifouling abilities.
Fig. 8. Potentiodynamic polarization curves of the etched and superhydrophobic Al samples.
Table 1 The corrosion potential (Ecorr) and the corrosion current density (Icorr) of the etched and superhydrophobic Al samples in 3.5 wt% NaCl solution. Samples
Ecorr (V)
Icorr (A·cm−2)
Etched Al Superhydrophobic Al
−1.451 −1.369
7.2 × 10−7 8.8 × 10−8
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Fig. 9. The evolution processes including (a) self-cleaning and (b) antifouling abilities of the superdydrophobic Al surface.
Acknowledgment [12]
This work is supported by the National Natural Science Foundation of China (No. 51822508 and No. 51675448).
[13]
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