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Fabrication of durable superhydrophobic electrodeposited tin surfaces with tremella-like structure on copper substrate
Surface & Coatings Technology 309 (2017) 590–599
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Fabrication of durable superhydrophobic electrodeposited tin surfaces with tremella-like structure on copper substrate Ge He, Shixiang Lu ⁎, Wenguo Xu, Jianying Yu, Bei Wu, Shuo Cui School of Chemistry, Beijing Institute of Technology, Beijing 100081, PR China
a r t i c l e
i n f o
Article history: Received 4 July 2016 Revised 30 November 2016 Accepted in revised form 3 December 2016 Available online 13 December 2016 Keywords: Tin Copper Electrodeposition Superhydrophobic
a b s t r a c t A durable superhydrophobic tin surface with a water contact angle of 170° and a sliding angle of b1° was fabricated on copper substrate via electrodeposition under magnetic stirring and followed by annealing treatment at 180 °C for 60 min. The resulting superhydrophobic tin surfaces shows a porous tremella-like morphology and the water drop can fully bounce as a balloon on such a surface, exhibiting excellent non-sticking property. The as-prepared superhydrophobic tin surfaces also display excellent properties of anticorrosion, self-cleaning, long-term stability and durability. The influences of magnetic stirring and electrodeposition time on the wetting properties were also investigated to understand the formation mechanism of the superhydrophobic tin surfaces. This sufficiently simple and inexpensive method may offer an effective strategy for fabricating superhydrophobic surfaces on various conductive engineering materials. © 2016 Published by Elsevier B.V.
1. Introduction As a wise mentor of humans, many naturally occurring surfaces, many plant surfaces and body parts of certain animals for example, show unique wettability properties, in which the surfaces with high water contact angles (WCAs N 150°) and low sliding angles (SAs ﹤ 5°) are called superhydrophobic surfaces (SHSs). Due to the highly low adhesion, liquid drops on these SHSs exhibit a sphere shape and can roll off spontaneously. While rolling off, the liquid drops even can carry away some dirt and dust particles, which endow these SHSs with the capabilities to survive many of nature's hazards. Many researchers studied and discussed the scientific reasons behind the superhydrophobicity of these naturally occurring surfaces in many literatures [1–3]. Based on these bio-inspirations, surfaces with remarkable superhydrophobicity and controllable wettability have attracted considerable attention in fundamental research [4–6], the products of which show a variety of interesting performances for practical applications in preventing ice/ frost, collecting water, corrosion resistance, oil-water separation, selfcleaning and drag reduction, etc. [7–14]. It is commonly accepted that fabricating SHSs on metal substrate is fairly important on account of their remarkable status in the modern industrial productions [15–17]. Besides, the possibility of obtaining SHSs on hydrophilic substrate by tuning the roughness and morphology of the surface has been confirmed by quite a few studies and experiments [18,19]. Considering the fact that, as cost-effective engineering material, copper are widely employed for the advantages of high electrical and
⁎ Corresponding author. E-mail address: [email protected] (S. Lu).
http://dx.doi.org/10.1016/j.surfcoat.2016.12.014 0257-8972/© 2016 Published by Elsevier B.V.
thermal conductivity, malleability and mechanical workability. Thence, it has become an important issue to research corrosion and protection of copper in corrosion science [20]. Recent results show that multilayer superhydrophobic coatings have become an effective and durable protection against corrosion [21–24]. Given that the excellent chemical stability, corrosion resistance as well as the utility of non-toxic, tin have shown great prospect in the modern industry of electronics, aerospace, shipbuilding and food [25]. On the other hand, it is somewhat a little difficult to fabricate uniform tin coatings by a simple method for the reason that tin compound solution shows strong hydrolization, and maybe it is why there are few reports about tin SHSs. Therefore, it would be favorable to develop a simple process to create a stable superhydrophobic tin surface without low surface energy organics modification. Electrodeposition has emerged as a competitive coating technique in view of its advantage of simplicity, low equipment cost and commercially available. So it is often utilized to coat uniform metallic surfaces with enduring superhydrophobicity regardless of the size and shape and can be easily applied to large-area surfaces [26,27]. Additionally, both morphology and chemistry of the surface can be modulated by varying the electrodeposition conditions like electrolyte, deposition method and time, allowing controllable growth of SHSs [28]. Herein, we proposed a rapid and mild electrodeposition method to produce superhydrophobic tin coatings based on hydrophilic copper substrate without any organics modification. It was attained in a mixed electrolyte composed of tin-based suspension, NH3·H2O and KCl under magnetic stirring condition and followed by thermal annealing. The resultant SHSs have the advantage of acting as anticorrosion barriers and displaying properties of self-cleaning. The morphological, structural and wetting characteristics of the tin superhydrophobic surface have been discussed. The formation mechanism, effect of magnetic
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stirring and deposition time, several good performances of the SHSs were also investigated. It is notable that this fabrication method makes it possible to fabricate a superhydrophobic surface on copper substrate in a short time (b 2 h), which would be very advantageous for the industrial large-scale production. 2. Experimental section 2.1. Raw materials Acetone (CH3COCH3, 99.5%), ethanol (C2H5OH, 99.5%), nitric acid (HNO3, 65%), two hydrated stannous chloride (SnCl2·2H2O), potassium chloride (KCl), concentrated ammonia water (NH3·H2O) were supplied by China Beijing Fine Chemical Co. Ltd. and they are of analytic grade without extra purification. Copper sheets (99.9%) (20 mm × 10 mm × 1 mm) were purchased from Beijing Nonferrous Metal Research Institute. The water used throughout the experiments was purified through a distillation system.
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Table 1 The WCA of sample surfaces with different processing conditions. Etching, Deposition and Anneal denote the samples after etching with 4 mol/L nitric acid solution for 6 min, depositing at −0.8 V for 10 min in a mix electrolyte composed of 4.5 g/L SnCl2·2H2O, 0.1 mol/L KCl and 0.03 mol/L NH3·H2O, and annealing in an oven at 180 °C for 60 min, respectively. “Y” and “N” signify conduct the corresponding process and without conducting, respectively. Sample
Etching
Deposition
Stirring
Annealing
CA(°)
1 2 3 4 5
N Y Y Y Y
N N Y Y Y
N N Y Y N
N N N Y Y
95 106 2 170 138
after etching with 4 mol/L HNO3 solution for 6 min, electrodepositing at –0.8 V for 10 min in a mix electrolyte composed of 4.5 g/L SnCl2·2H2O, 0.1 mol/L KCl and 0.03 mol/L NH3·H2O, and annealing at 180 °C for 60 min in an oven, respectively. “Y” and “N” signify conduct the corresponding operation and without conducting, separately.
2.2. Pretreatment of copper substrate Cu substrate were cleaned sequentially with a mixture of ethanol and acetone (v/v: 1/1) and deionized water in an ultrasonic cleaner for 10 min and were dried at ambient temperature subsequently. Then, they were chemically etched using 4 mol/L nitric acid solution for 6 min to offer a fresh and active surface. The etched copper were rapidly rinsed with deionized water and used in the following electrodeposition as working electrode. 2.3. Electrodeposition of superhydrophobic tin surfaces Cyclic Voltammograms were obtained using electrochemical workstation (CorrTest CHI760E, China) in a classical three-electrode cell to investigate the deposition potential before the electrodeposition of tin coatings. In this cell, the pretreated copper sheet, a Pt plate and a saturated calomel electrode (SCE) have been selected as working electrode, auxiliary electrode and reference electrode, respectively. The tin coatings were electrodeposited at a potential of −0.8 V in the electrolyte comprising 4.5 g/L SnCl2·2H2O, 0.1 mol/L KCl and 0.03 mol/L NH3·H2O. It is worth mentioning that electrolyte used in the electrodeposition process was used under ambient conditions without de-aerating. In addition, taking the suspended state of the electrolyte into consideration, magnetic stirring operation was adopted in the electrodeposition process to keep the suspended electrolyte homogeneous, which is conducive to gain a more uniform coating. KCl act as the supporting electrolyte to increase the conductivity as well as to make the crystal delicate and uniform; NH3·H2O was used as pH buffer to regulate pH of the mixed electrolyte at about 2.5. After electrodeposition, the samples were annealed at 180 °C for 60 min in the oven under atmospheric conditions. The whole preparation process was illustrated in Scheme 1. Here, five different samples were fabricated for further investigation. Table 1 lists the WCAs of sample surfaces with different processing conditions. Etching, Deposition and Annealing represent the samples
2.4. Surface characterization and tests The water contact angles (WCAs) were measured by the sessile drop method with an 8 μL water droplets using an optical contact angle meter (FAT200, Dataphysics Inc., USA) at ambient temperature. Water droplets were squeezed out lightly with a micro-syringe and deposited on to the sample surfaces randomly. WCA images were captured by the video; angles were estimated by using a tangent algorithm. The average WCA values reported were determined by measuring the same sample at five different positions. All error values of each sample are in a range of 2°, as is error bars in graph. X-ray diffraction (XRD) was performed on an X-ray diffractometer (XRD, D8 ADVANCE, Bruker, Germany) operating with Cu Kα radiation at a continuous scanning mode (40 kV, 40 mA, and λ = 0.15418 nm) and a scanning rate of 3°/min to characterize the surface composition and the crystallographic properties. The structures of the obtained tin coatings were analyzed with a scanning electron microscope (SEM) with a field emission gun (SEM, S-4800, Hitachi, Japan). All specimens were gold-sputtered before SEM scanning. The corresponding element distributions of the surface were determined by energy-dispersive X-ray spectrometry (EDX). Step profiler (Dektak 150) was applied to measure the thickness of the as-prepared samples.
2.5. Electrochemical measurements The potentiodynamic polarization curves were performed in neutral 3.5 wt% NaCl solution using the classical three electrode cell above mentioned to evaluate the anticorrosion ability of the resultant superhydrophobic tin surface. The polarization curves were tested at a scanning rate of 1 mv/s from −0.1 V to −0.8 V. Every potentiodynamic polarization curve was measured three times so as to guarantee the accuracy and repeatability.
2.6. Tribological study of the superhydrophobic tin surface
Scheme 1. Schematic representation of fabricating superhydrophobic tin surface by etching in HNO3, electrodepositing in three-electrode system and annealing in the oven.
A Universal Mechanical Tester (UMT-2, Bruker, USA) was applied to investigate the bond strength of tin coating and copper substrate. The bond strength test was operated by a diamond drill sliding over the coated tin superhydrophobic surface under an incremental and progressive load, and generating damage or delamination. The experiments were operated by a load range of 2 N to 15 N with a scratch time of 5 min and scratch length of 3 mm. The friction coefficient (COF) and other relevant experimental parameters were recorded automatically using a real-time control computer and data analysis software.
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3. Result and discussion 3.1. Wettability of the as-prepared surfaces The wettability of the resultant surfaces was quantitatively characterized in detail with static WCA measurements. Fig. 1 shows the WCA test results of the sample 1 to sample 4 depending on different treatment conditions. The bare copper presents slightly hydropholic property with a WCA of 95° (Fig. 1(a)), whereas the WCA increased to 106° (Fig. 1(b)) after inched in 4 mol/L nitric acid for 6 min. Electrochemical deposition of a fresh tin coating at a constant voltage of − 0.8 V for 10 min led to a fully wetted surface and the WCA were around 2°, as shown in Fig. 1(c). However, after annealed at 180 °C for 60 min, the resultant surface is so superhydrophobic that an 8 μL water droplet exhibits an approximately perfect sphere with a WCA as high as 170° (Fig. 1(d)). This indicates the crucial role of etching and thermal annealing in rendering superhydrophobicity of the coating. 3.2. Surface morphology of the superhydrophobic tin surface As is known, the hierarchical micro/nano structure of a solid surface is one of the decisive factors to regulate its surface wettability and especially its superhydrophobicity. So, in order to gain a better understanding of the WCA changes, SEM images were applied to research the morphologies of samples 1 to sample 4. Fig. 2(a) and (b) depict the SEM images of the surfaces of sample 1 and 2, respectively. While the initial copper sheet shows a relatively smooth surface (except for some nicks), Cu etching in nitric acid created a rough surface with many convex polyhedral protrusions and numerous nanoscale mastoids on the tips. The nitric acid etched copper sheet was further modified with a tin coating through an electrodeposition process at − 0.8 V for 10 min. Fig. 2(c) and (d) denote the surface morphologies of sample 3 with different magnifications, the surface is fully covered by intertwined tremella-like structure (illustration in Fig. 2(e)). There
was not very obvious change in the surface structure after annealed at 180 °C for 60 min except that part of loose tremella-like structure altered to irregular blocky appearance, along with some exposed inter-space between the porous tremella-like structures owing to the recrystallizing and growing of the deposited crystals during annealing. Furthermore, it can be vividly seen in Fig. 2(f) that countless nanoparticles with an average diameter of several nanometers bestrew on the tremella-like crystals, which might be sufficient to take effect in increasing the porosity and surface roughness. That is, a certain degree of hierarchical structure is formed on the rough copper substrate. 3.3. Surface composition The chemical composition of tin coating before and after annealing was evaluated with XRD spectrum and pure copper substrate was also analyzed for comparison. Fig. 3 shows the XRD patterns of the corresponding samples in the 2θ scan range of 30–80°. It can be easily seen in curve A that characteristic peaks labeled with “◇”can be indexed to the face-center cubic Cu in JCPDS Card No. 04-0836. After electrodepositing at −0.8 V for 10 min in 4.5 g/L SnCl2·2H2O suspensions, several new characteristic peaks can be seen obviously in curve B. These Sn peaks at 2θ = 30.4°, 31.9°, 43.8°, 44.8°, 55.4°, 62.4°,63.8°, 64.5°, 72.2°, 73.0° and 79.5°are in accordance with Sn(2 0 0), Sn(1 0 1), Sn(2 2 0), Sn(2 1 1), Sn(3 0 1), Sn(1 1 2), Sn (4 0 0), Sn (3 2 1), Sn (4 2 0), Sn (4 1 1) and Sn (3 1 2) planes, respectively, which were labeled with “◆” in curve B and can be derived from the crystalline tetragonal Sn according to JCPDS Card No. 04-0673 [29]. No new diffraction peaks of any other phase were detected when the tin deposits were annealed at 180 °C for 60 min in the air. However, the intensities of the peaks corresponding to the Sn phase became less distinct in the patterns (curve C in Fig. 3), indicating that the preferential growth of tin over a large substrate area during the annealing process and the crystallinity decrease during the annealing process, too. This finding is also in accordance with the SEM results in Fig. 2.
Fig. 1. Surface profiles of water droplets on the different samples: (a) sample 1; (b) sample 2; (c) sample 3 and (d) sample 4.
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Fig. 2. SEM images of different sample surfaces: (a) sample 1; (b) sample 2; (c) and (d) sample 3 with different magnifications; (e) and (f) sample 4 with different magnifications.
EDX spectra were performed to further confirm the surface elemental compositions of the superhydrophobic tin surface, as shown in Fig. 4. Fig. 4(a) is the spectrum of the overall surface with low magnification,
which shows that the major elements on the superhydrophobic tin surface are Cu and Sn with an atomic percent of 0.54% and 99.46%, respectively. The presence of a small amount of copper element may be due to
Fig. 3. XRD patterns of different samples: (a) sample 1, (b) sample 3, and (c) sample 4.
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Fig. 4. (a) EDX results of the distributed elements for the overall surface of sample 4. (b) EDX spectrum of the tremella-like structure of sample 4.
the exposed inter-space between the tremella-like porous surfaces. Similarly, almost the same signals of Cu and Sn appeared in Fig. 4(b), which represent spectrum of the porous tremella-like structure. The EDX results reveal that the micro/nano tin morphology has been successfully introduced to the copper surface through the simple electrodeposition process, which is well confirmed with the XRD analysis. 3.4. The influence factors on surface wettability A series of influencing factors should be taken into account to explain the diversities in results, which include magnetic stirring and electrochemical deposition time. 3.4.1. The effect of magnetic stirring Considering that tin chloride in the aqueous solution will hydrolyze and form a suspension, thus magnetic stirring is believed to be a quite important factor in the electrodeposition process. Hence, the surface composition and morphology of the samples with and without
magnetic stirring were investigated to better understand the function of the magnetic stirring in achieving superhydrophobicity. The XRD pattern of sample 5 is displayed in Fig. 5(a). Symbols of ◆, ◇, □ and ⊕ represent the peaks of Sn (JCPDS Card No. 04-0673), Cu (JCPDS Card No. 04-0836), Cu3Sn (JCPDS card No. 01-1240) and Cu10Sn3 alloy (JCPDS card No. 26-0564), respectively. In order to further investigate the role of the magnetic stirring, corresponding SEM image of sample 5 is shown in Fig. 5(b). When the process of electrodeposition was conducted without magnetic stirring, the special tremella-like structure disappeared in the prepared surfaces. Instead, the surface morphology is composed of densely packed massive particles. And it is valuable to find out that these bulges are irregular, which result in the relatively lower porosity and a smaller WCA of 138°. Based on above results, the wetting state, surface composition and surface morphology can generate great changes when the electrodeposition process was conducted without magnetic stirring. This probably because that magnetic stirring can increase the mass transfer rate of the SnCl2 suspension and then increase the deposition current, which can be confirmed by Fig. 6(a). By comparing the deposition current curves in Fig. 6(a), we can found that the deposition current of sample 5 is much smaller than that of sample 4. Besides, the deposition current under magnetic stirring conditions gradually increased with deposition time, along with a larger fluctuation of the curves. By contrast, the curve of the deposition current without magnetic stirring changed little in the electrodeposition process unless at the very beginning. We further speculate that larger deposition current may result in differences in the thickness of the tin coatings. And this assumption was also confirmed by the average thickness obtained from a step profiler [30]. Due to the corrosive effect of nitric acid on the copper substrate in the etching procedure, there formed a large amount of deep pits on the whole surface of copper substrate. The surface roughness of the as-prepared samples can cause changes in the line roughness of the upper and lower steps. In a general way, the coating thickness obtained from samples with ideal steps is more accurate. But for the coatings with non-ideal steps, it is also possible to estimate the average thickness. From Fig. 6(b), it can be inferred that the average thickness of sample 4 and sample 5 was about 61 μm and 16 μm, respectively. Besides, as to sample 4, the larger line roughness indicates a rougher surface, which is consistent well with the SEM results in Fig. 5(b). Additionally, the magnetic stirring in the electrodeposition process may also accelerate the gases escaping produced by electrode reaction, such as hydrogen precipitation, and thereby can increase the porosity of the coating, which is in good consistence with the SEM images. 3.4.2. The effect of electrodeposition time In order to study the influence of electrodeposition time on the wettability surface morphology of tin coatings, the electrodeposition time
Fig. 5. (a) XRD spectrum of the as-prepared sample 6. ◆ Sn; ◇ Cu; □ Cu3Sn; ⊕ Cu10Sn3. (b) SEM image of the sample 5.
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Fig. 6. (a) The i-t curves in the electrodeposition process: (A) sample 4; (B) sample 5. (b) Steps of tin-coated film on etched copper substrate: (A) sample 4; (B) sample 5.
varied from 5 min to 25 min were studied, as shown in Fig. 7. Fig. 7(a) shows the SEM image of the sample surface with an electrodeposition time of 5 min, in which irregular block structure was connected into a small amount of porous flocculent structures, and exhibited a WCA of 132°. Fig. 7(b) shows the SEM image of the surface after 15 min of electrodeposition, in which the intertwined porous flocculent and filiform structures replaced the irregular block morphology. This phenomenon increased the surface hydrophobic property with a WCA of 154°. Interestingly, when the time extended to 20 min, as shown in Fig. 7(c), a great number of papillae-like particles with micro and nano sizes located on the surface, which is very different from the flocculent structure displayed in Fig. 7(b). At the same time, the WCA decreased to 152°. With the electrodeposition time further prolonged to 25 min, it is clearly demonstrated in Fig. 7(d) that the surface was covered by relatively more intensive tremella-like structures compared with Fig. 7(b), leading to a smaller WCA of 133°. Thus, the deposition time of 10 min is sufficient to render superhydrophobicity and a deposition time of longer than 10 min
does not result in improvement of superhydrophobicity. With the increase of deposition time, the variation of the morphology resulted in the change of the WCA, which achieved the controllably preparation of superhydrophobic tin coatings. 4. Properties of the superhydrophobic tin coating 4.1. Anticorrosion property An attempt has been made to evaluate the anticorrosion ability of the resulting superhydrophobic tin coating by polarization test in neutral 3.5 wt% NaCl solution at a scanning rate of 1 mV/s. Corrosion potential (Ecorr), corrosion current density (Icorr) and corrosion rate (CR) were calculated from the polarization curves by using the Tafel extrapolation methods to evaluate the anticorrosion performance of the resultant surfaces, as shown in Table 2. Fig. 8 displays the polarization curves of the copper substrate and tin SHS. It is worthwhile to notice that the Icorr of the tin SHS is approximately 0.35% that of the bare copper (1.7 ×
Fig. 7. SEM images and wettability of the as-prepared tin coatings at different deposition times in the electrolyte consisted of 0.02 M tin suspensions, 0.1 M KCl and 0.03 M NH3·H2O at −0.8 V voltage, and then annealed at 180 °C for 60 min. (a) 5 min; (b) 15 min; (c) 20 min; (d) 25 min.
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Table 2 Corrosion potential (Ecorr), corrosion current (Icorr) and corrosion rate(CR) of the bare Cu and superhydrophobic surface. Sample
Ecorr(V)
Icorr(A/cm2)
CR(mm perannum)
Bare Cu Superhydrophobic surface
–0.16 –0.29
1.6 × 10−6 6.0 × 10−9
1.85 × 10−3 5.83 × 10−6
10−6) with a value of 6.0 × 10−9. Further investigation evidently reveals a noticeable lower corrosion rate (CR) of tin SHS than bare copper substrate, a surprising 1/317 of the bare copper. The lower Icorr and CR signify better corrosion resistance. We can conclude from above results that the as-prepared superhydrophobic tin surface possesses a good anticorrosion ability than the bare copper substrate, which may remove a major barrier to its practical applications [31,32]. Additionally, the superhydrophobic samples were immersed in 3.5 wt% NaCl solution at room temperature and test the WCAs at different time intervals to investigate the durability of its superhydrophobicity, and for comparison, a pure copper adopt the same procedure as described above. Fig. 9 exemplarily displays the change of the WCA values versus immersion time. It can be seen that the WCAs of the pure copper sheet gradually decreased as immersion time and the WCA value reduced to 43° when the immersion time extended to 48 h. Compared to the pure copper, it should be noted that the water droplet can bounce on the superhydrophobic tin surface until the immersion time increased to 24 h and can maintain superhydrophobicity even after immersion for 48 h (Fig. 9(a)). From the wettability point of view, we can suppose that immersion in 3.5 wt% NaCl aqueous solution for 24 h does not induce little change in the surface state of the superhydrophobic sample. In addition, from the SEM image of the copper with the immersion time of 48 h we can see that some noticeably dense and deep corrosion pits are distributed on the entire surface of copper (Fig. 9(b)). From further inspection of the SEM image we can see that the tremella-like structure became sparse compared to the structure before immersion (Fig. 9(c)). This shows that the superhydrophobic tin coating exhibited good durability to some corrosive media like NaCl and the excellent stability, which is of great significance for the practical applications of copper as engineering seawater corrosion resistant materials. 4.2. Optical property of the superhydrophobic tin coating Fig. 10(a) is the photographs of the wetting behaviors of the copper substrate and superhydrophobic tin surface. While water droplet can wet the copper substrate, the spherical water droplets (8 μL) presented
larger CA on the superhydrophobic tin surface. As the resulting superhydrophobic surface was immersed in 3.5 wt% NaCl solution, it showed interesting optical property, as shown in Fig. 10(b). When viewed at a glancing angle, a silver mirror-like appearance formed on the contact section between superhydrophobic tin surface and NaCl solution, and the surface kept completely dry when it was taken out of water. This phenomenon can be interpreted with the theory of total reflection in optics. The air can be trapped in micro/nano structures of the superhydrophobic coating, and the liquid forms a convex surface between the interface of liquid and air for the capillary, which conforms to the result explained by the Cassie-Baxter theory model [33]. Air can act as a thinner medium for water due to its lower refractive index. When light transfers from the water to the interface of air with an incidence angle larger than the critical angle, the refraction light vanishes, and the light is completely reflected. According to the total reflection phenomenon, the trapped air can serve as an effective barrier to keep corrosive media from reaching the surface and generate better corrosion protection effect [34]. 4.3. Non-sticking and self-cleaning properties of the superhydrophobic tin coating Non-sticking superhydrophobic surfaces, with droplets bounce away from them spontaneously, were initially inspired by the lotus leaf and then have received considerable attention because of their ability to remain dry [35,36]. Fig. 11 is a group of images taken from the videos of free-falling water droplet on the superhydrophobic tin surface, in which water droplet leaves the surface within0.3 s without leaving any residual traces on the surface, suggesting that the as-prepared superhydrophobic tin surfaces possess excellent water repellent property and such a good anti-adhesion to water is of great significance for the self-cleaning property. The self-cleaning property whereby droplets cannot stay on the surface and roll off immediately is referred to as the “lotus effect” and it is considered as a fascinating property for superhydrophobic materials in which high water repellence and low sliding angle are essential. The self-cleaning performance of the as-received superhydrophobic tin surface was demonstrated by sprinkling a layer of soils on the superhydrophobic tin coating and the copper substrate with a tilting angle of about 10°. Then water droplets were dropped to the soil loaded surfaces via a syringe. When a copper substrate was used (as received), water droplets adhered to the surface and could neither absorb the soils nor roll off the surface, even when the sample was placed at an angle of 90°, as shown in Fig. 12(a) and SV1. That is to say, no self-cleaning properties were observed on a copper substrate. In contrast, the rolling water droplets can clean the coating very effectively by catching hydrophilic dirt particles from the superhydrophobic tin surfaces, leaving behind a clear surface, as documented in still images in Fig. 12(b) and videos of SV2. This phenomenon can be explained by the synergy of the surface roughness and low surface energy, which led to a decrease in the contact area and adherence strength between dust particles and the surface of the superhydrophobic tin coating. More interestingly, water droplet maintained a spherical shape even after it has taken up a large amount of soils, as shown in SV2. This observation confirms that the as-prepared superhydrophobic tin surface has a highly self-cleaning efficacy, providing the possibility for industrial applications. 4.4. Tribological study of the superhydrophobic tin surface
Fig. 8. Potentiodynamic polarization curves of bare copper substrate and the as-prepared superhydrophobic tin surface in neutral 3.5 wt% NaCl solution.
Bonding strength between coating and substrate is quite an important mechanical property which can affect its service life. Hereon, the scratch test was adopted to research the binding strength between the superhydrophobic tin coating and copper substrate, in which variable loads of 2–15 N was employed. The bond strength was evaluated by measuring the critical load at which the coated bonding between tin SHS and copper substrate failed in the scratch processes, meanwhile,
G. He et al. / Surface & Coatings Technology 309 (2017) 590–599 200 180
(a)
597
(b)
Contact angle/°
160 140
Sample 4
120 100 80
(c)
Sample 1
60 40 20
0
10
20
30
40
50
60
70
80
Immersion time/h Fig. 9. (a) WCAs of sample 1 and sample 4 immersed in the3.5 wt% NaCl solution for different time at room temperature in air. (b) SEM image of pure copper substrate after immersion in 3.5 wt% NaCl solution for 48 h. (c) SEM image of superhydrophobic tin surface after immersion in 3.5 wt% NaCl solution for 48 h.
the friction factor would appear a sudden change, bigger or smaller. Relevant research shows that the superhydrophobic coatings were completely peeled off from the substrate surface at the critical load. The curves of the coefficient of friction (COF) and load with changes of the loading time are displayed in Fig. 13. It was valuable to find that the COF abrupt changed at the time of 100 s and afterward it tends to relatively balance, therefore, compared with Fig. 13(b), it can be inferred that the critical load is about 7.4 N, which exhibits good bonding ability. Furthermore, several experiments were conducted to verify the reliability of the experimental results and the as-prepared data presented good parallelism [37]. It should be noted that the good bonding ability between superhydrophobic tin coating and copper substrate may profit from the etching and annealing treatment. Chemical etching can improve the bonding force lies in that it can roughen the substrate surface and thereby increase the contact area of the substrate and the coating, which is helpful to improve the tin nucleation density on the heterogeneous substrate and enhance the adhesion between the coating and substrate. Additionally, during annealing process, mutual diffusion may occur between the substrate and the tin coating and thus can increase the contact area between the tin coating and the copper substrate, which can also improve the bonding state of copper substrate and the superhydrophobic tin coating. 4.5. Long-term durability and chemical stability of the superhydrophobic tin coating Generally speaking, the environmental stability and durability of the prepared tin SHSs are always a very important factor that will
determine the feasibility of the proposed method in industrial applications. Therefore, the environmental stability and durability of as-prepared superhydrophobic samples were verified by further testing the WCAs at different time intervals upon storage. As a result, water droplets can still bounce on the tin SHSs and slide off eventually even after a year's exposure in air (SV3). That is to say, from the perspective of wettability, it can be speculated that exposure time in air induce no change in the surface state of the superhydrophobic samples in a period of time, which indicate that superhydrophobicity of the hierarchical structures and chemical components were stable despite the seasonal temperature changes and thus exhibit long-term stability and durability. 5. Theoretical mechanism for the superhydrophobicity Making a general survey of the surface structure and composition characteristics described above, the corresponding formation mechanism of the as-prepared superhydrophobic surface can be considered as the following process. Firstly, there was an oxidation reduction reaction on the surface of copper substrate in 4 mol/L HNO3 solution as shown in Scheme 2 (1). By etching in 4 mol/L HNO3 solution, many nubble-protrusions and island structures were obtained on copper substrate, as shown in Fig. 3(b). The protuberances caused by preferentially deposited units produce the “tip effect” [38], which can cause different deposition rate and eventually led to the porous surface morphology. Secondly, it is well known that there is some Sn(OH)Cl precipitation in SnCl2 suspensions due to the hydrolysis of SnCl2 as shown in Scheme 2 (2). Thirdly, the addition of ammonia is to form a buffer solution, and to maintain the pH of the reaction liquid in a certain range.
Fig. 10. (a) water droplets on copper substrate and superhydrophobic tin surface; (b) Mirror effectof superhydrophobic tin coating being immersed in 3.5 wt% NaCl solution.
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Fig. 11. The intercepted photographs from the rollingprocess of an 8 μL water dropleton the horizontal tin superhydrophobic surfaces.
There is an equilibrium precipitation and dissolution of Sn(OH)Cl in water as shown in Scheme 2 (3), so the Sn2 + ion concentration was kept constant with the buffer solution. When the copper electrodes were immersed in the electrolyte solution with the application of voltage, some Sn2+ ions in the suspensions captured electrons and were deoxidized to pure Sn nuclei as shown in Scheme 2 (4), which could enhance the dissolution of Sn(OH)Cl and form deposited tin coatings on copper substrate. The reaction processes can be formulated as follows: Then the deposited tin coatings were annealed at 180 °C for 60 min so that the crystals rearranged to achieve superhydrophobicity with appropriate structure and roughness. Besides, research shows that compressive mechanical stresses like residual stresses caused by electroplating may result in the growth of tin whiskers and heat annealing can reduce the whisker growth [39], which is well consistent with the SEM results. With the help of these tin whiskers, porous tremellalike structure formed on the tin coating, which contributed to the formation of the superhydrophobicity. It is notable that magnetic stirring was a critical factor for the superhydrophobicity since magnetic stirring can increase the mass transfer rate of the suspended electrolyte to change the deposition current (Fig. 6(a)). Moreover, other processing
conditions like etching, deposition time, annealing temperature and annealing time can also affect the superhydrophobicity of the tin coating. In general, two models can account theoretically for the wetting properties. One is called the Wenzel model [40], in which water drops will penetrate the grooves of the rough surface. The other model is the Cassie–Baxter model, where air pockets act as a porous medium between the superhydrophobic surface and water. Besides, surfaces featured with hierarchical micro/nanostructures favors acquiring of the superhydrophobicity. The tremella-like hierarchical structures can generate numerous grooves in which an air cushion can formed. Therefore, the water droplet can be kept away from the surface and easily bounce on this surface and finally roll off according to the Cassie–Baxter equation [33]. cosθ ¼ ƒs cosθ0 −ð1‐ƒs Þ
ð5Þ
In which θ indicates the WCA on a rough surface, while θ0 is the intrinsic WCA on a flat surface. ƒs refers to the fraction of the solid/water interface, while (1 – ƒs) represents that of the air/water interface. The value of ƒs of the superhydrophobic tin surface is estimated to be
Fig. 12. Self-cleaning property test: (a) water droplets stick to the soils loaded untreated copper surface even when put the copper substrate vertically. (b) Soils loaded superhydrophobic tin surface was cleaned by moving water droplets. The title angle was about 10°.
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Scheme 2. Reactions in the preparation process.
Acknowledgements We gratefully acknowledge the National Natural Science Foundation of China (No. 21271027) for the support of this work. References
Fig. 13. Curves of COF (a) and the load (b) with the scratch time for sample 4.
0.013, which means that the air occupies about 98.87% of the contact area between the water droplet and the superhydrophobic tin surface. From Eq. (5), we can infer that when a rough surface comes into contact with water, there will appear an air trapping in the trough area, which would contribute greatly to the increase of hydrophobicity. This means that the air pockets trapped in the grooves can produce large CA and small SA, which is effective in preventing the penetration of water droplets into the tin SHS.
6. Conclusion In summary, the micro/nano non-wetting tin surface on copper substrate has been successfully fabricated through a simple and effective electrodeposition method combined with annealing without low surface energy organics modification. The needed preparing time is just b 2 h. The resulting tin surfaces exhibit porous tremella-like architectures and remarkable superhydrophobicity with a WCA of 170° and a SA of b1°, showing excellent non-sticking property and self-cleaning performance. The as-prepared tin SHSs exhibit a much lower corrosion rate than the bare copper. Besides, the superhydrophobicity of tin coatings can maintain after one-year storage in air with good long-time chemical stability and durability. The bonding strength between the tin coating and copper substrate is about 7.4 N, exhibiting firm mechanical property. Therefore, the convenient preparation method and excellent properties of the resulting tin SHSs have promising industrial applications in various fields and can offer an effective strategy for fabricating versatile SHSs on various conductive engineering materials. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.surfcoat.2016.12.014.
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Fabrication of durable superhydrophobic electrodeposited tin surfaces with tremella-like structure on copper substrate Ge He, Shixiang Lu ⁎, Wenguo Xu, Jianying Yu, Bei Wu, Shuo Cui School of Chemistry, Beijing Institute of Technology, Beijing 100081, PR China
a r t i c l e
i n f o
Article history: Received 4 July 2016 Revised 30 November 2016 Accepted in revised form 3 December 2016 Available online 13 December 2016 Keywords: Tin Copper Electrodeposition Superhydrophobic
a b s t r a c t A durable superhydrophobic tin surface with a water contact angle of 170° and a sliding angle of b1° was fabricated on copper substrate via electrodeposition under magnetic stirring and followed by annealing treatment at 180 °C for 60 min. The resulting superhydrophobic tin surfaces shows a porous tremella-like morphology and the water drop can fully bounce as a balloon on such a surface, exhibiting excellent non-sticking property. The as-prepared superhydrophobic tin surfaces also display excellent properties of anticorrosion, self-cleaning, long-term stability and durability. The influences of magnetic stirring and electrodeposition time on the wetting properties were also investigated to understand the formation mechanism of the superhydrophobic tin surfaces. This sufficiently simple and inexpensive method may offer an effective strategy for fabricating superhydrophobic surfaces on various conductive engineering materials. © 2016 Published by Elsevier B.V.
1. Introduction As a wise mentor of humans, many naturally occurring surfaces, many plant surfaces and body parts of certain animals for example, show unique wettability properties, in which the surfaces with high water contact angles (WCAs N 150°) and low sliding angles (SAs ﹤ 5°) are called superhydrophobic surfaces (SHSs). Due to the highly low adhesion, liquid drops on these SHSs exhibit a sphere shape and can roll off spontaneously. While rolling off, the liquid drops even can carry away some dirt and dust particles, which endow these SHSs with the capabilities to survive many of nature's hazards. Many researchers studied and discussed the scientific reasons behind the superhydrophobicity of these naturally occurring surfaces in many literatures [1–3]. Based on these bio-inspirations, surfaces with remarkable superhydrophobicity and controllable wettability have attracted considerable attention in fundamental research [4–6], the products of which show a variety of interesting performances for practical applications in preventing ice/ frost, collecting water, corrosion resistance, oil-water separation, selfcleaning and drag reduction, etc. [7–14]. It is commonly accepted that fabricating SHSs on metal substrate is fairly important on account of their remarkable status in the modern industrial productions [15–17]. Besides, the possibility of obtaining SHSs on hydrophilic substrate by tuning the roughness and morphology of the surface has been confirmed by quite a few studies and experiments [18,19]. Considering the fact that, as cost-effective engineering material, copper are widely employed for the advantages of high electrical and
⁎ Corresponding author. E-mail address: [email protected] (S. Lu).
http://dx.doi.org/10.1016/j.surfcoat.2016.12.014 0257-8972/© 2016 Published by Elsevier B.V.
thermal conductivity, malleability and mechanical workability. Thence, it has become an important issue to research corrosion and protection of copper in corrosion science [20]. Recent results show that multilayer superhydrophobic coatings have become an effective and durable protection against corrosion [21–24]. Given that the excellent chemical stability, corrosion resistance as well as the utility of non-toxic, tin have shown great prospect in the modern industry of electronics, aerospace, shipbuilding and food [25]. On the other hand, it is somewhat a little difficult to fabricate uniform tin coatings by a simple method for the reason that tin compound solution shows strong hydrolization, and maybe it is why there are few reports about tin SHSs. Therefore, it would be favorable to develop a simple process to create a stable superhydrophobic tin surface without low surface energy organics modification. Electrodeposition has emerged as a competitive coating technique in view of its advantage of simplicity, low equipment cost and commercially available. So it is often utilized to coat uniform metallic surfaces with enduring superhydrophobicity regardless of the size and shape and can be easily applied to large-area surfaces [26,27]. Additionally, both morphology and chemistry of the surface can be modulated by varying the electrodeposition conditions like electrolyte, deposition method and time, allowing controllable growth of SHSs [28]. Herein, we proposed a rapid and mild electrodeposition method to produce superhydrophobic tin coatings based on hydrophilic copper substrate without any organics modification. It was attained in a mixed electrolyte composed of tin-based suspension, NH3·H2O and KCl under magnetic stirring condition and followed by thermal annealing. The resultant SHSs have the advantage of acting as anticorrosion barriers and displaying properties of self-cleaning. The morphological, structural and wetting characteristics of the tin superhydrophobic surface have been discussed. The formation mechanism, effect of magnetic
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stirring and deposition time, several good performances of the SHSs were also investigated. It is notable that this fabrication method makes it possible to fabricate a superhydrophobic surface on copper substrate in a short time (b 2 h), which would be very advantageous for the industrial large-scale production. 2. Experimental section 2.1. Raw materials Acetone (CH3COCH3, 99.5%), ethanol (C2H5OH, 99.5%), nitric acid (HNO3, 65%), two hydrated stannous chloride (SnCl2·2H2O), potassium chloride (KCl), concentrated ammonia water (NH3·H2O) were supplied by China Beijing Fine Chemical Co. Ltd. and they are of analytic grade without extra purification. Copper sheets (99.9%) (20 mm × 10 mm × 1 mm) were purchased from Beijing Nonferrous Metal Research Institute. The water used throughout the experiments was purified through a distillation system.
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Table 1 The WCA of sample surfaces with different processing conditions. Etching, Deposition and Anneal denote the samples after etching with 4 mol/L nitric acid solution for 6 min, depositing at −0.8 V for 10 min in a mix electrolyte composed of 4.5 g/L SnCl2·2H2O, 0.1 mol/L KCl and 0.03 mol/L NH3·H2O, and annealing in an oven at 180 °C for 60 min, respectively. “Y” and “N” signify conduct the corresponding process and without conducting, respectively. Sample
Etching
Deposition
Stirring
Annealing
CA(°)
1 2 3 4 5
N Y Y Y Y
N N Y Y Y
N N Y Y N
N N N Y Y
95 106 2 170 138
after etching with 4 mol/L HNO3 solution for 6 min, electrodepositing at –0.8 V for 10 min in a mix electrolyte composed of 4.5 g/L SnCl2·2H2O, 0.1 mol/L KCl and 0.03 mol/L NH3·H2O, and annealing at 180 °C for 60 min in an oven, respectively. “Y” and “N” signify conduct the corresponding operation and without conducting, separately.
2.2. Pretreatment of copper substrate Cu substrate were cleaned sequentially with a mixture of ethanol and acetone (v/v: 1/1) and deionized water in an ultrasonic cleaner for 10 min and were dried at ambient temperature subsequently. Then, they were chemically etched using 4 mol/L nitric acid solution for 6 min to offer a fresh and active surface. The etched copper were rapidly rinsed with deionized water and used in the following electrodeposition as working electrode. 2.3. Electrodeposition of superhydrophobic tin surfaces Cyclic Voltammograms were obtained using electrochemical workstation (CorrTest CHI760E, China) in a classical three-electrode cell to investigate the deposition potential before the electrodeposition of tin coatings. In this cell, the pretreated copper sheet, a Pt plate and a saturated calomel electrode (SCE) have been selected as working electrode, auxiliary electrode and reference electrode, respectively. The tin coatings were electrodeposited at a potential of −0.8 V in the electrolyte comprising 4.5 g/L SnCl2·2H2O, 0.1 mol/L KCl and 0.03 mol/L NH3·H2O. It is worth mentioning that electrolyte used in the electrodeposition process was used under ambient conditions without de-aerating. In addition, taking the suspended state of the electrolyte into consideration, magnetic stirring operation was adopted in the electrodeposition process to keep the suspended electrolyte homogeneous, which is conducive to gain a more uniform coating. KCl act as the supporting electrolyte to increase the conductivity as well as to make the crystal delicate and uniform; NH3·H2O was used as pH buffer to regulate pH of the mixed electrolyte at about 2.5. After electrodeposition, the samples were annealed at 180 °C for 60 min in the oven under atmospheric conditions. The whole preparation process was illustrated in Scheme 1. Here, five different samples were fabricated for further investigation. Table 1 lists the WCAs of sample surfaces with different processing conditions. Etching, Deposition and Annealing represent the samples
2.4. Surface characterization and tests The water contact angles (WCAs) were measured by the sessile drop method with an 8 μL water droplets using an optical contact angle meter (FAT200, Dataphysics Inc., USA) at ambient temperature. Water droplets were squeezed out lightly with a micro-syringe and deposited on to the sample surfaces randomly. WCA images were captured by the video; angles were estimated by using a tangent algorithm. The average WCA values reported were determined by measuring the same sample at five different positions. All error values of each sample are in a range of 2°, as is error bars in graph. X-ray diffraction (XRD) was performed on an X-ray diffractometer (XRD, D8 ADVANCE, Bruker, Germany) operating with Cu Kα radiation at a continuous scanning mode (40 kV, 40 mA, and λ = 0.15418 nm) and a scanning rate of 3°/min to characterize the surface composition and the crystallographic properties. The structures of the obtained tin coatings were analyzed with a scanning electron microscope (SEM) with a field emission gun (SEM, S-4800, Hitachi, Japan). All specimens were gold-sputtered before SEM scanning. The corresponding element distributions of the surface were determined by energy-dispersive X-ray spectrometry (EDX). Step profiler (Dektak 150) was applied to measure the thickness of the as-prepared samples.
2.5. Electrochemical measurements The potentiodynamic polarization curves were performed in neutral 3.5 wt% NaCl solution using the classical three electrode cell above mentioned to evaluate the anticorrosion ability of the resultant superhydrophobic tin surface. The polarization curves were tested at a scanning rate of 1 mv/s from −0.1 V to −0.8 V. Every potentiodynamic polarization curve was measured three times so as to guarantee the accuracy and repeatability.
2.6. Tribological study of the superhydrophobic tin surface
Scheme 1. Schematic representation of fabricating superhydrophobic tin surface by etching in HNO3, electrodepositing in three-electrode system and annealing in the oven.
A Universal Mechanical Tester (UMT-2, Bruker, USA) was applied to investigate the bond strength of tin coating and copper substrate. The bond strength test was operated by a diamond drill sliding over the coated tin superhydrophobic surface under an incremental and progressive load, and generating damage or delamination. The experiments were operated by a load range of 2 N to 15 N with a scratch time of 5 min and scratch length of 3 mm. The friction coefficient (COF) and other relevant experimental parameters were recorded automatically using a real-time control computer and data analysis software.
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3. Result and discussion 3.1. Wettability of the as-prepared surfaces The wettability of the resultant surfaces was quantitatively characterized in detail with static WCA measurements. Fig. 1 shows the WCA test results of the sample 1 to sample 4 depending on different treatment conditions. The bare copper presents slightly hydropholic property with a WCA of 95° (Fig. 1(a)), whereas the WCA increased to 106° (Fig. 1(b)) after inched in 4 mol/L nitric acid for 6 min. Electrochemical deposition of a fresh tin coating at a constant voltage of − 0.8 V for 10 min led to a fully wetted surface and the WCA were around 2°, as shown in Fig. 1(c). However, after annealed at 180 °C for 60 min, the resultant surface is so superhydrophobic that an 8 μL water droplet exhibits an approximately perfect sphere with a WCA as high as 170° (Fig. 1(d)). This indicates the crucial role of etching and thermal annealing in rendering superhydrophobicity of the coating. 3.2. Surface morphology of the superhydrophobic tin surface As is known, the hierarchical micro/nano structure of a solid surface is one of the decisive factors to regulate its surface wettability and especially its superhydrophobicity. So, in order to gain a better understanding of the WCA changes, SEM images were applied to research the morphologies of samples 1 to sample 4. Fig. 2(a) and (b) depict the SEM images of the surfaces of sample 1 and 2, respectively. While the initial copper sheet shows a relatively smooth surface (except for some nicks), Cu etching in nitric acid created a rough surface with many convex polyhedral protrusions and numerous nanoscale mastoids on the tips. The nitric acid etched copper sheet was further modified with a tin coating through an electrodeposition process at − 0.8 V for 10 min. Fig. 2(c) and (d) denote the surface morphologies of sample 3 with different magnifications, the surface is fully covered by intertwined tremella-like structure (illustration in Fig. 2(e)). There
was not very obvious change in the surface structure after annealed at 180 °C for 60 min except that part of loose tremella-like structure altered to irregular blocky appearance, along with some exposed inter-space between the porous tremella-like structures owing to the recrystallizing and growing of the deposited crystals during annealing. Furthermore, it can be vividly seen in Fig. 2(f) that countless nanoparticles with an average diameter of several nanometers bestrew on the tremella-like crystals, which might be sufficient to take effect in increasing the porosity and surface roughness. That is, a certain degree of hierarchical structure is formed on the rough copper substrate. 3.3. Surface composition The chemical composition of tin coating before and after annealing was evaluated with XRD spectrum and pure copper substrate was also analyzed for comparison. Fig. 3 shows the XRD patterns of the corresponding samples in the 2θ scan range of 30–80°. It can be easily seen in curve A that characteristic peaks labeled with “◇”can be indexed to the face-center cubic Cu in JCPDS Card No. 04-0836. After electrodepositing at −0.8 V for 10 min in 4.5 g/L SnCl2·2H2O suspensions, several new characteristic peaks can be seen obviously in curve B. These Sn peaks at 2θ = 30.4°, 31.9°, 43.8°, 44.8°, 55.4°, 62.4°,63.8°, 64.5°, 72.2°, 73.0° and 79.5°are in accordance with Sn(2 0 0), Sn(1 0 1), Sn(2 2 0), Sn(2 1 1), Sn(3 0 1), Sn(1 1 2), Sn (4 0 0), Sn (3 2 1), Sn (4 2 0), Sn (4 1 1) and Sn (3 1 2) planes, respectively, which were labeled with “◆” in curve B and can be derived from the crystalline tetragonal Sn according to JCPDS Card No. 04-0673 [29]. No new diffraction peaks of any other phase were detected when the tin deposits were annealed at 180 °C for 60 min in the air. However, the intensities of the peaks corresponding to the Sn phase became less distinct in the patterns (curve C in Fig. 3), indicating that the preferential growth of tin over a large substrate area during the annealing process and the crystallinity decrease during the annealing process, too. This finding is also in accordance with the SEM results in Fig. 2.
Fig. 1. Surface profiles of water droplets on the different samples: (a) sample 1; (b) sample 2; (c) sample 3 and (d) sample 4.
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Fig. 2. SEM images of different sample surfaces: (a) sample 1; (b) sample 2; (c) and (d) sample 3 with different magnifications; (e) and (f) sample 4 with different magnifications.
EDX spectra were performed to further confirm the surface elemental compositions of the superhydrophobic tin surface, as shown in Fig. 4. Fig. 4(a) is the spectrum of the overall surface with low magnification,
which shows that the major elements on the superhydrophobic tin surface are Cu and Sn with an atomic percent of 0.54% and 99.46%, respectively. The presence of a small amount of copper element may be due to
Fig. 3. XRD patterns of different samples: (a) sample 1, (b) sample 3, and (c) sample 4.
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Fig. 4. (a) EDX results of the distributed elements for the overall surface of sample 4. (b) EDX spectrum of the tremella-like structure of sample 4.
the exposed inter-space between the tremella-like porous surfaces. Similarly, almost the same signals of Cu and Sn appeared in Fig. 4(b), which represent spectrum of the porous tremella-like structure. The EDX results reveal that the micro/nano tin morphology has been successfully introduced to the copper surface through the simple electrodeposition process, which is well confirmed with the XRD analysis. 3.4. The influence factors on surface wettability A series of influencing factors should be taken into account to explain the diversities in results, which include magnetic stirring and electrochemical deposition time. 3.4.1. The effect of magnetic stirring Considering that tin chloride in the aqueous solution will hydrolyze and form a suspension, thus magnetic stirring is believed to be a quite important factor in the electrodeposition process. Hence, the surface composition and morphology of the samples with and without
magnetic stirring were investigated to better understand the function of the magnetic stirring in achieving superhydrophobicity. The XRD pattern of sample 5 is displayed in Fig. 5(a). Symbols of ◆, ◇, □ and ⊕ represent the peaks of Sn (JCPDS Card No. 04-0673), Cu (JCPDS Card No. 04-0836), Cu3Sn (JCPDS card No. 01-1240) and Cu10Sn3 alloy (JCPDS card No. 26-0564), respectively. In order to further investigate the role of the magnetic stirring, corresponding SEM image of sample 5 is shown in Fig. 5(b). When the process of electrodeposition was conducted without magnetic stirring, the special tremella-like structure disappeared in the prepared surfaces. Instead, the surface morphology is composed of densely packed massive particles. And it is valuable to find out that these bulges are irregular, which result in the relatively lower porosity and a smaller WCA of 138°. Based on above results, the wetting state, surface composition and surface morphology can generate great changes when the electrodeposition process was conducted without magnetic stirring. This probably because that magnetic stirring can increase the mass transfer rate of the SnCl2 suspension and then increase the deposition current, which can be confirmed by Fig. 6(a). By comparing the deposition current curves in Fig. 6(a), we can found that the deposition current of sample 5 is much smaller than that of sample 4. Besides, the deposition current under magnetic stirring conditions gradually increased with deposition time, along with a larger fluctuation of the curves. By contrast, the curve of the deposition current without magnetic stirring changed little in the electrodeposition process unless at the very beginning. We further speculate that larger deposition current may result in differences in the thickness of the tin coatings. And this assumption was also confirmed by the average thickness obtained from a step profiler [30]. Due to the corrosive effect of nitric acid on the copper substrate in the etching procedure, there formed a large amount of deep pits on the whole surface of copper substrate. The surface roughness of the as-prepared samples can cause changes in the line roughness of the upper and lower steps. In a general way, the coating thickness obtained from samples with ideal steps is more accurate. But for the coatings with non-ideal steps, it is also possible to estimate the average thickness. From Fig. 6(b), it can be inferred that the average thickness of sample 4 and sample 5 was about 61 μm and 16 μm, respectively. Besides, as to sample 4, the larger line roughness indicates a rougher surface, which is consistent well with the SEM results in Fig. 5(b). Additionally, the magnetic stirring in the electrodeposition process may also accelerate the gases escaping produced by electrode reaction, such as hydrogen precipitation, and thereby can increase the porosity of the coating, which is in good consistence with the SEM images. 3.4.2. The effect of electrodeposition time In order to study the influence of electrodeposition time on the wettability surface morphology of tin coatings, the electrodeposition time
Fig. 5. (a) XRD spectrum of the as-prepared sample 6. ◆ Sn; ◇ Cu; □ Cu3Sn; ⊕ Cu10Sn3. (b) SEM image of the sample 5.
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Fig. 6. (a) The i-t curves in the electrodeposition process: (A) sample 4; (B) sample 5. (b) Steps of tin-coated film on etched copper substrate: (A) sample 4; (B) sample 5.
varied from 5 min to 25 min were studied, as shown in Fig. 7. Fig. 7(a) shows the SEM image of the sample surface with an electrodeposition time of 5 min, in which irregular block structure was connected into a small amount of porous flocculent structures, and exhibited a WCA of 132°. Fig. 7(b) shows the SEM image of the surface after 15 min of electrodeposition, in which the intertwined porous flocculent and filiform structures replaced the irregular block morphology. This phenomenon increased the surface hydrophobic property with a WCA of 154°. Interestingly, when the time extended to 20 min, as shown in Fig. 7(c), a great number of papillae-like particles with micro and nano sizes located on the surface, which is very different from the flocculent structure displayed in Fig. 7(b). At the same time, the WCA decreased to 152°. With the electrodeposition time further prolonged to 25 min, it is clearly demonstrated in Fig. 7(d) that the surface was covered by relatively more intensive tremella-like structures compared with Fig. 7(b), leading to a smaller WCA of 133°. Thus, the deposition time of 10 min is sufficient to render superhydrophobicity and a deposition time of longer than 10 min
does not result in improvement of superhydrophobicity. With the increase of deposition time, the variation of the morphology resulted in the change of the WCA, which achieved the controllably preparation of superhydrophobic tin coatings. 4. Properties of the superhydrophobic tin coating 4.1. Anticorrosion property An attempt has been made to evaluate the anticorrosion ability of the resulting superhydrophobic tin coating by polarization test in neutral 3.5 wt% NaCl solution at a scanning rate of 1 mV/s. Corrosion potential (Ecorr), corrosion current density (Icorr) and corrosion rate (CR) were calculated from the polarization curves by using the Tafel extrapolation methods to evaluate the anticorrosion performance of the resultant surfaces, as shown in Table 2. Fig. 8 displays the polarization curves of the copper substrate and tin SHS. It is worthwhile to notice that the Icorr of the tin SHS is approximately 0.35% that of the bare copper (1.7 ×
Fig. 7. SEM images and wettability of the as-prepared tin coatings at different deposition times in the electrolyte consisted of 0.02 M tin suspensions, 0.1 M KCl and 0.03 M NH3·H2O at −0.8 V voltage, and then annealed at 180 °C for 60 min. (a) 5 min; (b) 15 min; (c) 20 min; (d) 25 min.
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Table 2 Corrosion potential (Ecorr), corrosion current (Icorr) and corrosion rate(CR) of the bare Cu and superhydrophobic surface. Sample
Ecorr(V)
Icorr(A/cm2)
CR(mm perannum)
Bare Cu Superhydrophobic surface
–0.16 –0.29
1.6 × 10−6 6.0 × 10−9
1.85 × 10−3 5.83 × 10−6
10−6) with a value of 6.0 × 10−9. Further investigation evidently reveals a noticeable lower corrosion rate (CR) of tin SHS than bare copper substrate, a surprising 1/317 of the bare copper. The lower Icorr and CR signify better corrosion resistance. We can conclude from above results that the as-prepared superhydrophobic tin surface possesses a good anticorrosion ability than the bare copper substrate, which may remove a major barrier to its practical applications [31,32]. Additionally, the superhydrophobic samples were immersed in 3.5 wt% NaCl solution at room temperature and test the WCAs at different time intervals to investigate the durability of its superhydrophobicity, and for comparison, a pure copper adopt the same procedure as described above. Fig. 9 exemplarily displays the change of the WCA values versus immersion time. It can be seen that the WCAs of the pure copper sheet gradually decreased as immersion time and the WCA value reduced to 43° when the immersion time extended to 48 h. Compared to the pure copper, it should be noted that the water droplet can bounce on the superhydrophobic tin surface until the immersion time increased to 24 h and can maintain superhydrophobicity even after immersion for 48 h (Fig. 9(a)). From the wettability point of view, we can suppose that immersion in 3.5 wt% NaCl aqueous solution for 24 h does not induce little change in the surface state of the superhydrophobic sample. In addition, from the SEM image of the copper with the immersion time of 48 h we can see that some noticeably dense and deep corrosion pits are distributed on the entire surface of copper (Fig. 9(b)). From further inspection of the SEM image we can see that the tremella-like structure became sparse compared to the structure before immersion (Fig. 9(c)). This shows that the superhydrophobic tin coating exhibited good durability to some corrosive media like NaCl and the excellent stability, which is of great significance for the practical applications of copper as engineering seawater corrosion resistant materials. 4.2. Optical property of the superhydrophobic tin coating Fig. 10(a) is the photographs of the wetting behaviors of the copper substrate and superhydrophobic tin surface. While water droplet can wet the copper substrate, the spherical water droplets (8 μL) presented
larger CA on the superhydrophobic tin surface. As the resulting superhydrophobic surface was immersed in 3.5 wt% NaCl solution, it showed interesting optical property, as shown in Fig. 10(b). When viewed at a glancing angle, a silver mirror-like appearance formed on the contact section between superhydrophobic tin surface and NaCl solution, and the surface kept completely dry when it was taken out of water. This phenomenon can be interpreted with the theory of total reflection in optics. The air can be trapped in micro/nano structures of the superhydrophobic coating, and the liquid forms a convex surface between the interface of liquid and air for the capillary, which conforms to the result explained by the Cassie-Baxter theory model [33]. Air can act as a thinner medium for water due to its lower refractive index. When light transfers from the water to the interface of air with an incidence angle larger than the critical angle, the refraction light vanishes, and the light is completely reflected. According to the total reflection phenomenon, the trapped air can serve as an effective barrier to keep corrosive media from reaching the surface and generate better corrosion protection effect [34]. 4.3. Non-sticking and self-cleaning properties of the superhydrophobic tin coating Non-sticking superhydrophobic surfaces, with droplets bounce away from them spontaneously, were initially inspired by the lotus leaf and then have received considerable attention because of their ability to remain dry [35,36]. Fig. 11 is a group of images taken from the videos of free-falling water droplet on the superhydrophobic tin surface, in which water droplet leaves the surface within0.3 s without leaving any residual traces on the surface, suggesting that the as-prepared superhydrophobic tin surfaces possess excellent water repellent property and such a good anti-adhesion to water is of great significance for the self-cleaning property. The self-cleaning property whereby droplets cannot stay on the surface and roll off immediately is referred to as the “lotus effect” and it is considered as a fascinating property for superhydrophobic materials in which high water repellence and low sliding angle are essential. The self-cleaning performance of the as-received superhydrophobic tin surface was demonstrated by sprinkling a layer of soils on the superhydrophobic tin coating and the copper substrate with a tilting angle of about 10°. Then water droplets were dropped to the soil loaded surfaces via a syringe. When a copper substrate was used (as received), water droplets adhered to the surface and could neither absorb the soils nor roll off the surface, even when the sample was placed at an angle of 90°, as shown in Fig. 12(a) and SV1. That is to say, no self-cleaning properties were observed on a copper substrate. In contrast, the rolling water droplets can clean the coating very effectively by catching hydrophilic dirt particles from the superhydrophobic tin surfaces, leaving behind a clear surface, as documented in still images in Fig. 12(b) and videos of SV2. This phenomenon can be explained by the synergy of the surface roughness and low surface energy, which led to a decrease in the contact area and adherence strength between dust particles and the surface of the superhydrophobic tin coating. More interestingly, water droplet maintained a spherical shape even after it has taken up a large amount of soils, as shown in SV2. This observation confirms that the as-prepared superhydrophobic tin surface has a highly self-cleaning efficacy, providing the possibility for industrial applications. 4.4. Tribological study of the superhydrophobic tin surface
Fig. 8. Potentiodynamic polarization curves of bare copper substrate and the as-prepared superhydrophobic tin surface in neutral 3.5 wt% NaCl solution.
Bonding strength between coating and substrate is quite an important mechanical property which can affect its service life. Hereon, the scratch test was adopted to research the binding strength between the superhydrophobic tin coating and copper substrate, in which variable loads of 2–15 N was employed. The bond strength was evaluated by measuring the critical load at which the coated bonding between tin SHS and copper substrate failed in the scratch processes, meanwhile,
G. He et al. / Surface & Coatings Technology 309 (2017) 590–599 200 180
(a)
597
(b)
Contact angle/°
160 140
Sample 4
120 100 80
(c)
Sample 1
60 40 20
0
10
20
30
40
50
60
70
80
Immersion time/h Fig. 9. (a) WCAs of sample 1 and sample 4 immersed in the3.5 wt% NaCl solution for different time at room temperature in air. (b) SEM image of pure copper substrate after immersion in 3.5 wt% NaCl solution for 48 h. (c) SEM image of superhydrophobic tin surface after immersion in 3.5 wt% NaCl solution for 48 h.
the friction factor would appear a sudden change, bigger or smaller. Relevant research shows that the superhydrophobic coatings were completely peeled off from the substrate surface at the critical load. The curves of the coefficient of friction (COF) and load with changes of the loading time are displayed in Fig. 13. It was valuable to find that the COF abrupt changed at the time of 100 s and afterward it tends to relatively balance, therefore, compared with Fig. 13(b), it can be inferred that the critical load is about 7.4 N, which exhibits good bonding ability. Furthermore, several experiments were conducted to verify the reliability of the experimental results and the as-prepared data presented good parallelism [37]. It should be noted that the good bonding ability between superhydrophobic tin coating and copper substrate may profit from the etching and annealing treatment. Chemical etching can improve the bonding force lies in that it can roughen the substrate surface and thereby increase the contact area of the substrate and the coating, which is helpful to improve the tin nucleation density on the heterogeneous substrate and enhance the adhesion between the coating and substrate. Additionally, during annealing process, mutual diffusion may occur between the substrate and the tin coating and thus can increase the contact area between the tin coating and the copper substrate, which can also improve the bonding state of copper substrate and the superhydrophobic tin coating. 4.5. Long-term durability and chemical stability of the superhydrophobic tin coating Generally speaking, the environmental stability and durability of the prepared tin SHSs are always a very important factor that will
determine the feasibility of the proposed method in industrial applications. Therefore, the environmental stability and durability of as-prepared superhydrophobic samples were verified by further testing the WCAs at different time intervals upon storage. As a result, water droplets can still bounce on the tin SHSs and slide off eventually even after a year's exposure in air (SV3). That is to say, from the perspective of wettability, it can be speculated that exposure time in air induce no change in the surface state of the superhydrophobic samples in a period of time, which indicate that superhydrophobicity of the hierarchical structures and chemical components were stable despite the seasonal temperature changes and thus exhibit long-term stability and durability. 5. Theoretical mechanism for the superhydrophobicity Making a general survey of the surface structure and composition characteristics described above, the corresponding formation mechanism of the as-prepared superhydrophobic surface can be considered as the following process. Firstly, there was an oxidation reduction reaction on the surface of copper substrate in 4 mol/L HNO3 solution as shown in Scheme 2 (1). By etching in 4 mol/L HNO3 solution, many nubble-protrusions and island structures were obtained on copper substrate, as shown in Fig. 3(b). The protuberances caused by preferentially deposited units produce the “tip effect” [38], which can cause different deposition rate and eventually led to the porous surface morphology. Secondly, it is well known that there is some Sn(OH)Cl precipitation in SnCl2 suspensions due to the hydrolysis of SnCl2 as shown in Scheme 2 (2). Thirdly, the addition of ammonia is to form a buffer solution, and to maintain the pH of the reaction liquid in a certain range.
Fig. 10. (a) water droplets on copper substrate and superhydrophobic tin surface; (b) Mirror effectof superhydrophobic tin coating being immersed in 3.5 wt% NaCl solution.
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Fig. 11. The intercepted photographs from the rollingprocess of an 8 μL water dropleton the horizontal tin superhydrophobic surfaces.
There is an equilibrium precipitation and dissolution of Sn(OH)Cl in water as shown in Scheme 2 (3), so the Sn2 + ion concentration was kept constant with the buffer solution. When the copper electrodes were immersed in the electrolyte solution with the application of voltage, some Sn2+ ions in the suspensions captured electrons and were deoxidized to pure Sn nuclei as shown in Scheme 2 (4), which could enhance the dissolution of Sn(OH)Cl and form deposited tin coatings on copper substrate. The reaction processes can be formulated as follows: Then the deposited tin coatings were annealed at 180 °C for 60 min so that the crystals rearranged to achieve superhydrophobicity with appropriate structure and roughness. Besides, research shows that compressive mechanical stresses like residual stresses caused by electroplating may result in the growth of tin whiskers and heat annealing can reduce the whisker growth [39], which is well consistent with the SEM results. With the help of these tin whiskers, porous tremellalike structure formed on the tin coating, which contributed to the formation of the superhydrophobicity. It is notable that magnetic stirring was a critical factor for the superhydrophobicity since magnetic stirring can increase the mass transfer rate of the suspended electrolyte to change the deposition current (Fig. 6(a)). Moreover, other processing
conditions like etching, deposition time, annealing temperature and annealing time can also affect the superhydrophobicity of the tin coating. In general, two models can account theoretically for the wetting properties. One is called the Wenzel model [40], in which water drops will penetrate the grooves of the rough surface. The other model is the Cassie–Baxter model, where air pockets act as a porous medium between the superhydrophobic surface and water. Besides, surfaces featured with hierarchical micro/nanostructures favors acquiring of the superhydrophobicity. The tremella-like hierarchical structures can generate numerous grooves in which an air cushion can formed. Therefore, the water droplet can be kept away from the surface and easily bounce on this surface and finally roll off according to the Cassie–Baxter equation [33]. cosθ ¼ ƒs cosθ0 −ð1‐ƒs Þ
ð5Þ
In which θ indicates the WCA on a rough surface, while θ0 is the intrinsic WCA on a flat surface. ƒs refers to the fraction of the solid/water interface, while (1 – ƒs) represents that of the air/water interface. The value of ƒs of the superhydrophobic tin surface is estimated to be
Fig. 12. Self-cleaning property test: (a) water droplets stick to the soils loaded untreated copper surface even when put the copper substrate vertically. (b) Soils loaded superhydrophobic tin surface was cleaned by moving water droplets. The title angle was about 10°.
G. He et al. / Surface & Coatings Technology 309 (2017) 590–599
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Scheme 2. Reactions in the preparation process.
Acknowledgements We gratefully acknowledge the National Natural Science Foundation of China (No. 21271027) for the support of this work. References
Fig. 13. Curves of COF (a) and the load (b) with the scratch time for sample 4.
0.013, which means that the air occupies about 98.87% of the contact area between the water droplet and the superhydrophobic tin surface. From Eq. (5), we can infer that when a rough surface comes into contact with water, there will appear an air trapping in the trough area, which would contribute greatly to the increase of hydrophobicity. This means that the air pockets trapped in the grooves can produce large CA and small SA, which is effective in preventing the penetration of water droplets into the tin SHS.
6. Conclusion In summary, the micro/nano non-wetting tin surface on copper substrate has been successfully fabricated through a simple and effective electrodeposition method combined with annealing without low surface energy organics modification. The needed preparing time is just b 2 h. The resulting tin surfaces exhibit porous tremella-like architectures and remarkable superhydrophobicity with a WCA of 170° and a SA of b1°, showing excellent non-sticking property and self-cleaning performance. The as-prepared tin SHSs exhibit a much lower corrosion rate than the bare copper. Besides, the superhydrophobicity of tin coatings can maintain after one-year storage in air with good long-time chemical stability and durability. The bonding strength between the tin coating and copper substrate is about 7.4 N, exhibiting firm mechanical property. Therefore, the convenient preparation method and excellent properties of the resulting tin SHSs have promising industrial applications in various fields and can offer an effective strategy for fabricating versatile SHSs on various conductive engineering materials. Supplementary data to this article can be found online at http://dx. doi.org/10.1016/j.surfcoat.2016.12.014.
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