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Fabrication and theoretical explanation of the superhydrophobic CuZn coating with dandelion-like CuO microstructure
Journal of Alloys and Compounds 691 (2017) 195e205
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Fabrication and theoretical explanation of the superhydrophobic CueZn coating with dandelion-like CuO microstructure Hao Li, Sirong Yu* College of Mechanical and Electronic Engineering, China University of Petroleum (East China), Qingdao 266580, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2016 Received in revised form 23 August 2016 Accepted 26 August 2016 Available online 28 August 2016
In this work, a CueZn coating with dandelion-like CuO microstructure was fabricated on pipeline steel surface by a simple two-step combined method of electrodeposition and chemical oxidation. This coating was then coated with a thin pentadecafluorooctanoic acid to achieve superhydrophobic, with the contact angle of water about 156.81 and the sliding angle around 3 . When a water droplet contacted on the CueZn coating surface, the dandelion-like CuO microstructure was seen as the dual-scale structure that was proposed to be composed of spherical and pillared microstructures. Based on the Wenzel's and Cassie's formulas, the theoretical explanation was used to confirm the Cassie-Cassie wetted state between water droplet and the dandelion-like hierarchical superhydrophobic coating according to the geometric parameters of the surface morphology. Meanwhile, the superhydrophobic CueZn coating had the ability to repel the impinging water droplets and excellent long-term, thermal, solution immersion and chemical stability. Such robust superhydrophobic CueZn coating with hierarchical microstructures was expected to have various potential applications in our daily life. © 2016 Elsevier B.V. All rights reserved.
Keywords: Superhydrophobicity Dandelion-like structure Theoretical explanation Cassie-Cassie state
1. Introduction Pipeline steel is an important metal in the field of pipeline transportation. However, the adhesion of the aqueous mixture to the pipe wall has the negative influence on production and transportation, which results in much economic loss and waste of resources. Therefore, fabricating a superhydrophobic coating on pipeline steel surface to decline the adhesion of the aqueous mixture to the pipeline steel surface is necessary. Recently, superhydrophobic surface has become a major topic of research because of its various applications, such as self-cleaning property [1], corrosion resistance [2], anti-icing property [3], and anti-scaling potential [4], which are directly related to our daily life. Moreover, a number of creatures with excellent hydrophobic property in nature, including lotus leaves [5], peanut leaves [6], water striders [7], and nepenthes pitcher plants [8], have been observed that the combined effect of the hierarchical micro-nano structure and the low surface energy material achieves the superhydrophobicity. However, the contact angle of water droplet on the smooth solid surface with the lowest surface free energy (6.7 mJ/m2) was only
* Corresponding author. E-mail address: [email protected] (S. Yu). http://dx.doi.org/10.1016/j.jallcom.2016.08.272 0925-8388/© 2016 Elsevier B.V. All rights reserved.
119 [9]. Consequently, the hierarchical micro-nano structure plays the key role in fabricating the superhydrophobic surface [10]. Up to now, many methods have been used to fabricate hierarchical micro-nano structures for obtaining the superhydrophobic surface [11e15], and a great many of different hierarchical structured superhydrophobic surfaces have been fabricated successfully, including mushroom-like structure [16], caterpillar-like structure [17], pinecone-like structure [18], rose-like structure [19], etc. In recent years, researchers have used the simple chemical oxidation to fabricate all kinds of CuO structures for obtaining the hierarchical structures, and the prepared CuO films show excellent superhydrophobic property after being modified with fluoride [20]. Moreover, chemical oxidation method is low cost, time saving, and facile condition [21]. However, to the best of our knowledge, various superhydrophobic CuO microstructures were mainly fabricated on the copper substrate without the steel substrate [22], and it lacked the theoretical model for explaining the reason for superhydrophobic. In this paper, a superhydrophobic CueZn coating with dandelion-like CuO film was obtained on X90 pipeline steel surface. Furthermore, for the purpose of theoretical analysis, the dandelionlike microstructure was seen as the dual-scale structure that was composed of spherical and pillared microstructures. Based on the Wenzel's and Cassie's formulas, we confirmed the wetting state
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between water droplet and the dandelion-like structured superhydrophobic coating surface was the stable Cassie-Cassie state. 2. Experimental section 2.1. Materials and reagents X90 pipeline steel was obtained from TGRC of China and cut into the size of 20 50 3 mm. Brass (H62) was purchased from the common market and cut into the size of 20 50 5 mm. Absolute ethanol (AR), acetone (AR), sodium hydroxide (NaOH, AR), and sulfuric acid (H2SO4, 98%) were purchased from West Long Chemical Co., Ltd. Sodium carbonate anhydrous (Na2CO3), trisodium phosphate dodecahydrate (Na3PO4$12H2O), sodium metasilicate nonahydrate (Na2SiO3$9H2O), Copper(II) sulfate pentahydrate (CuSO4$5H2O), zinc sulfate heptahydrate (ZnSO4$7H2O), potassium hydroxide (KOH), and ammonium persulfate ((NH4)2S2O8) were all analytical grade (AR) and purchased from Sinopharm Chemical Reagent Co., Ltd. Potassium sodium tartrate tetrahydrate (C4O6H4KNa$4H2O, AR) and pentadecafluorooctanoic acid (C8HF15O2, 90%) were purchased from Aladdin. All the chemical reagents were used as received. 2.2. Fabrication of dandelion-like structured coating The processes of fabrication of a coating with dandelion-like microstructures on steel surface included the electrodeposition of CueZn coating and the in-situ growth of CuO dandelion-like microstructures via chemical oxidation, and the detailed procedures were as follows. Before electrodeposition, the steel and brass samples were polished and ultrasonically cleaned in acetone and ethanol for 5 min, respectively. After rinsed in deionized water, the steel sample was immersed in the alkaline solution containing 30 g/L of NaOH, 20 g/L of Na2CO3, 20 g/L of Na3PO4$12H2O, and 10 g/L of Na2SiO3$9H2O at 60 C for 15 min to degrease, and then rinsed with deionized water. To activate the surface, the steel sample was then immersed in a 10% (volume fraction) H2SO4 solution for 10 s at room temperature. Then, the CueZn coating was electrodeposited under direct current. The pretreated steel sample was used as the cathode in the electrodeposition process with the brass as the counter electrode. The two electrodes were immersed in the solution containing 20 g/ L of CuSO4$5H2O, 23 g/L of ZnSO4$7H2O, 100 g/L of C4O6H4KNa$4H2O and 50 g/L of NaOH at 23 C for 60 min with the current density of 2 A/dm2. Meanwhile, the distance between two electrodes maintained at 2 cm. In order to obtain CuO dandelion-like microstructure, as the preparation process reported in article [23], the steel sample with CueZn coating was immersed in a 50 mL of mixed solution including 2.5 mol/L of KOH and 0.12 mol/L of (NH4)2S2O8 at 60 C for 30 min. After chemical oxidation, the sample was ultrasonically cleaned in deionized water and anhydrous ethanol for 5 min (45 kHz), respectively, and dried in air. 2.3. Fabrication of superhydrophobic surface via fluorination The steel surface coated with dandelion-like microstructures was immersed into a beaker with 50 mL of perfluorooctanoic acid anhydrous ethanol (0.01 mol/L) at room temperature for 36 h. In order to prevent the ethanol volatilizing, the beaker was sealed with plastic wrap. Each sample was dried in air for more than one day before it was characterized and tested.
2.4. Characterizations The surface morphology was observed using a field emission scanning electron microscope (FESEM, Nova NanoSEM 450, FEI) equipped with energy disperse spectroscopy (EDS, Oxford X-MaxN, FEI). The thickness of the prepared coating was tested using a confocal microscope (DM2500 M, Leica). The crystal structure was investigated by an X-ray diffraction (XRD, X'Pert PRO MPD, PANalytical B.V.). The X-ray source was Cu target with the incident wavelength of 1.5406 Å. The operating voltage and current was 40 kV and 40 mA, respectively. The surface composition of the coating surface was recorded with an X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo). The contact angle of a water droplet (about 5 mL) was measured by a goniometer system (SL200B, USA, KINO), and the average value of the contact angle was obtained with at least three measurements of each sample. The sliding angle was the tilted angle of the measured surface once the water droplet (about 8 mL) was rolling on the sample surface. 3. Results and discussion 3.1. Fabrication and characterization of the superhydrophobic coating In order to fabricate a superhydrophobic coating on steel surface, the polished steel was first electrodeposited CueZn coating, and then immersed in the mixed solution including KOH and (NH4)2S2O8 to form CuO dandelion-like microstructure. Finally, the steel surface with dandelion-like structured coating was modified with pentadecafluorooctanoic acid anhydrous ethanol. Fig. 1 shows the surface morphology and chemical composition of the steel sample under different conditions. The surface morphology of the electrodeposited CueZn coating in Fig. 1(a) exhibited to be smooth, which was similar to the previous report [24]. In addition, the cross section of the CueZn coating was polished, and the thickness of this coating was about 20 mm by observing the metallographic microscope of the cross section in Fig. S1. The chemical composition of the CueZn coating was shown in Fig. 1(b), showing that this coating consisted of the elements of Cu and Zn, and the atomic ratio of Zn to Cu in this coating was about 1:1. There was no other impurity except little element of Fe from the steel substrate. After chemical oxidation in the KOH and (NH4)2S2O8 mixed solution, the appearance of the electrodeposited CueZn coating was transformed into black that was the type color of CuO. As shown in Fig. 1(c), the nanoflakes self-organized into dandelionlike microstructure on CueZn coating, and this morphology was the typical CuO structure [25]. Besides, the EDS analysis of this coating surface showed the existence of O element (Fig. 1(d)), which was attributed to the oxidation of this coating. Fig. 1(e) illustrates the morphology of the chemical oxidized CueZn coating after fluorination using a substance with low surface free energy (pentadecafluorooctanoic acid). There was no obvious change of the surface morphology compared to that before fluorination (Fig. 1(c)). Moreover, the EDS spectrum of the superhydrophobic coating in Fig. 1(f) exhibited the existence of F element, indicating that the pentadecafluorooctanoic acid had been successfully physical adsorbed onto the surface of this coating. The wettability of this coating showed superhydrophobic with the contact angle of water about 156.81 (Fig. 2(a)) and the sliding angle of water around 3 (Fig. 2(b)). In the present study, the non-sticking property of the superhydrophobic coating was tested according to the method reported by Liu et al. [26]. As shown in Fig. 3 and Video S1, they illustrated a water droplet of 5 mL approach, contact, and departure processes
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Fig. 1. (a, c, e) SEM images and (b, d, f) corresponding EDS of the sample under different conditions: (a, b) electrodepostion of CueZn coating, (c, d) chemical oxidation, and (e, f) fluorination.
respected to the superhydrophobic coating surface. It can be seen that the water droplet did not dip into or spread out on this coating surface when the water droplet severely contacted the coating surface. Additionally, there was not any of visual water leaving on the surface after the water droplet departing from the superhydrophobic coating [27]. This phenomenon indicated that the adhesive force between the water droplet and the
Fig. 2. (a) Photos and (b) sliding process of the water droplet on the superhydrophobic coating.
Fig. 3. Images of a water droplet (5 mL) suspending on the needle, contacting, pressing and departing from the superhydrophobic coating. The arrows represent the moving direction of the needle.
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(Fig. 5(a)). To be more specific, as shown in Fig. 5(b), it can be observed that the peaks at about 932.5 eV and 952.5 eV were the spectra of Cu2p3/2 and Cu2p1/2, respectively after chemical oxidation [35], and the peak of O1s were also exited on this coating (Fig. 5(a)). It was indicated that CuO formed on the coating surface after chemical oxidation, which was accord with the XRD analysis. In the XPS spectra of the superhydrophobic coating, a strong F1s can be easily observed, and it was located at about 689.0 eV (Fig. 5(c)) that was correlated with CeF bonds [36,37]. The highresolution spectrum of C1s (Fig. 5(d)) had peaks at 293.5 eV and 291.0 eV, which can be assigned to eCF3 and eCF2, respectively [38,39]. Therefore, it was indicated that a stable monolayer of pentadecafluorooctanoic acid had already been successfully physical adsorbed onto the superhydrophobic coating surface after fluorination. 3.2. Theoretical analysis of the superhydrophobic coating with dandelion-like microstructure
Fig. 4. XRD patterns of the sample under different conditions: (a) electrodepostion of CueZn coating, (b) chemical oxidation, and (c) fluorination (the superhydrophobic coating).
superhydrophobic coating surface was very small. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jallcom.2016.08.272. Fig. 4 shows the XRD patterns of the sample under different conditions. After electrodeposition of CueZn coating (Fig. 4(a)), the CueZn phases were detected according to the PDF card of No. 00035-1152, showing that the simultaneous deposition of Cu and Zn in the electrodeposition process [28]. Here, the crystal lattice in Cu matrix was replaced by Zn partly, resulting in the formation of CueZn substitutional solid solution [29]. In addition, there were Fe phases (PDF card of No. 03-065-4899) that were only from the steel substrate, which was similar to the article reported by Almeida et al. [30]. After chemical oxidation, it was noticeably shown that Fig. 4(b) contained diffraction peaks of CuO corresponding to the PDF card of No. 01-089-5898, and all the peaks were consistent with the previous report [31]. Furthermore, the XRD patterns did not show any peak of Zn oxide and no obvious peaks introduced by impurities such as Cu(OH)2 and Cu2O, indicating that only CuO was formed on the CueZn coating. This could be because that the Zn in the CueZn coating was dissolved under a strong base in the chemical oxidation process [32]. In this study, the high concentration of KOH resulted in the primordially grown ZnO crystal dissolving into Zn(OH)2 4 , and the chemical reactions as follows [33,34]. Zn þ 4OH1 /ZnðOHÞ2 4 þ 2e
(1)
2 Cu þ 2OH1 þ S2 O2 8 /CuðOHÞ2 þ 2SO4
(2)
CuðOHÞ2 /CuO þ H2 O
(3)
However, after fluorination, there was no new crystal structure forming on the superhydrophobic coating surface (Fig. 4(c)), indicating that the pentadecafluorooctanoic acid as the modified material had no effect on the crystal structure of this coating. Moreover, the XPS spectra were given to confirm the obtained CuO on the surface after chemical oxidation and the existence of pentadecafluorooctanoic acid on the superhydrophobic coating (Fig. 5). Before fluorination, it reveals that C, O, and Cu exist on the coating, and it can be found C, O, Cu, and F after fluorination
According to above superhydrophobic coating with large contact angle (about 156.81 ) and small sliding angle (around 3 ), the wetting state between water droplet and this superhydrophobic coating surface may be the composite (Cassie) wetting state [40]. Fig. 6(a) is the surface morphology of the superhydrophobic coating with dandelion-like microstructures, which were more obvious in Fig. 6(b) at higher magnification. The dandelion-like microstructures were formed by the nanoflakes self-organizing into spherical structures. The geometric parameters of the surface morphology were measured by the software of Nano Measurer, and the size was measured by manually choosing dozens of points. Fig. 6(c) shows the diameter distribution histogram of the dandelion-like microstructures, which is based on statistical calculations of the SEM image in Fig. 6(a). It was found that the size from 2.4 mm to 3.2 mm almost occupied more than 80% of the dandelion-like microstructures. The dandelion-like microstructures were still uniform, and the average diameter of that was about 2.87 mm. The space among the dandelion-like microstructures based on statistical calculations of the SEM image (Fig. 6(a)) was illustrated in Fig. 6(d), and the average size of the space was around 0.94 mm. As shown in Fig. 6(e) and (f), the length and the space of the nanoflakes based on statistical calculations of the SEM image in Fig. 6(b) were about 0.15 mm and 0.22 mm, respectively. The dandelion-like microstructure can be viewed as dual-scale structure including microsphere and nanoflake. When a water droplet contacted this surface with dandelion-like hierarchical structures, there were four possible combinations of wetting states, including Cassie-Cassie, WenzelCassie, Cassie-Wenzel and Wenzel-Wenzel, which was reported in the article [41]. According to above experimental results, it was difficult to confirm that the wetting case between water droplet and the superhydrophobic coating with dandelion-like microstructures was the Cassie-Cassie state. And because of this, the calculation in the following was used to determine the reliable wetting state of the superhydrophobic coating in this work. Inspired by the formulation described by Patankar [42], the dandelion-like microstructure was considered as the dual-scale coarse structure. The first generation was a model fine scale roughness structure made of the sphere arranged in a regular array (Fig. 7), and the second generation was the pillar that had the same geometry, presenting regular arrangement (Fig. 8). Moreover, the assumptions in the following model included: (a) the gravity of the water droplet was negligible, and (b) the diameter of the water droplet was much greater than the space among the dandelion-like microstructures. In the first generation, as shown in Fig. 7, let the diameter of the sphere be D (2R), and the space among the sphere be P1. If a water
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Fig. 5. (a) XPS spectra of the sample before and after fluorination (the superhydrophobic coating), (b) Cu high-resolution spectrum of the coating before fluorination, (ced) F and C high-resolution spectrum of the superhydrophobic coating.
droplet was placed on this surface, there were two possible wetted states, including the wetted (Wenzel) state and the composite (Cassie) state. Then, the Wenzel's equation was evolved into [43]:
cos qw r ¼ r cos qe # pD2 ¼ 1þ cos qe ðD þ P1 Þ2 " # "
¼ 1þ
p
ð1 þ P1 =DÞ2
(4)
cos qe
For the Cassie's state, the water droplet penetrating the space to a certain length of H that was more less than R. Then, the contact area between the water droplet and spherical microstructure was:
S ¼ 2pRH ¼ pDH
(5)
Therefore, the contact angle of the Cassie case was given by Ref. [44]: c
cos qr ¼ ¼
pDH ðD þ P1 Þ2
pH=D ð1 þ P1 =DÞ2
ðcos qe þ 1Þ 1 (6)
ðcos qe þ 1Þ 1
here, qe was the contact angle of the polished pipeline steel surface modified with pentadecafluorooctanoic acid (99.76 ). In order to
intuitively investigate the influence of the geometric parameters of microstructure on the contact angle, Eq. (4) and Eq. (6) were converted into Fig. 9(a) and Fig. 9(b), respectively. As shown in Fig. 9(a), it can be found that the contact angle in Wenzel's state was only affected by P1/D, and it decreased with the increase of P1/D. However, as shown in Fig. 9(b), the contact angle in Cassie's state increased with increasing P1/D, and it declined with growing H/D. Moreover, when a water droplet with certain volume was static on a solid surface, the energy of the droplet (G) can be described as [45]:
G ffiffiffiffiffiffiffiffiffiffi p ¼ ð1 cos qÞ2=3 ð2 þ cos qÞ1=3 3 ð9pÞV 2=3 slv
(7)
here, slv is the surface tension of water, V is the volume of the water droplet, and q represents the contact angle in the improved Wenzel's or Cassie's state. From Eq. (7), it can be found that G increases with the increase of q, and because of this, a water droplet with smaller q has relative equilibrium state. In other words, the smaller contact angle was relative more stability. The contact angle of the Wenzel and Cassie state in spherical model changed with P1/D was shown in Fig. 10, and we set different values of H/D in the Cassie's state. It can be found that only when the contact angle of the Cassie state was equal to that of the Wenzel state, can the contact angle of the Cassie's state be the largest stable value. According to Fig. 6 (D ¼ 2.87 mm and P1 ¼ 0.94 mm), P1/D was about 0.3275, and because of this, the largest stable contact angle of Cassie's case was
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Fig. 6. (a, b) Different magnification SEM image of the superhydrophobic coating with dandelion-like microstructures; (c) the diameter and (d) the space of the dandelion-like microstructures based on the statistical calculations of the SEM image in (a); the length (e) and the space (f) of nanoflakes based on the statistical calculations of the SEM image in (b).
Fig. 7. (a) Vertical view and (b) side view of the spherical model that was arranged in a regular array.
118.14 according to Eq. (4). Then, H/D was calculated to be only about 0.20 according to Eq. (6), which indicated there was air trapped in the space among spherical microstructure. Therefore, 118.14 was the largest stable contact angle of the Cassie's case. The
value of qe was thus amplified from 99.76 to 118.14 (qe ) in the second generation (in pillar model). In the second generation, as shown in Fig. 8, let the square pillar be of size a a, the height be h, and the space among the pillar be
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Fig. 8. (a) Vertical view and (b) side view of the pillared model that was arranged in a regular array.
Fig. 9. Contact angle of (a) Wenzel (wetted) state and (b) Cassie (composite) state changed with both H/D and P1/D in spherical model.
P2. There also were two possible contact cases when a water droplet was placed on the pillar surface. The Wenzel's [43] and Cassie's [44] equations were separately evolved into:
cos qw* r ¼
¼
1þ
4ah ða þ P2 Þ2
a2
* cos qe þ 1 1
ða þ P2 Þ2 1 * cos ¼ q þ 1 1 e ð1 þ P2 =aÞ2
! cos q*e
h ! 4 * a 1þ cos qe ð1 þ P2 =aÞ2
c*
cos qr ¼
(8)
Fig. 10. The contact angle of Wenzel and Cassie changed with P1/D (set different values of H/D in Cassie's state) in spherical model.
(9)
here, qe was 118.14 according to the result in the first generation (in spherical model). The contact angles in Eq. (8) and Eq. (9) changed with both h/a and P2/a in pillar model were given in Fig. 11(a) and Fig. 11(b), respectively. As shown in Fig. 11(a), the contact angle of Wenzel's case increased with increasing h/a, and it decreased with the growth of P2/a. Nonetheless, the contact angle of Cassie's state increased with the increase of P2/a, and it had nothing to do with h/a (Fig. 11(b)). Furthermore, the smaller contact angle was more stable according to Eq. (7), and the contact angle of Wenzel and Cassie state in pillared model changed with P2/a was shown in Fig. 12 (set different values of h/a in Wenzel's state). It can also be seen that the contact angle of Cassie's case was the largest stable value when the contact angle of Cassie's case was equal to that of Wenzel's case. The value of P2/a was about 1.47 according to Fig. 6 (a ¼ 0.15 mm and P2 ¼ 0.22 mm) and the value of qe was 118.14 , therefore, the theoretical value of the contact angle on the superhydrophobic coating with dandelion-like CuO microstructure was around 156.00 based on Eq. (9). This value was conformed to the experimental contact angle (156.81 ), which indicated that the wetting contact sate was Cassie sate. Consequently, the wetting state of the superhydrophobic coating was Cassie-Cassie state according to the theoretical explanation of the dual-scale structure model.
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Fig. 11. Contact angle of (a) Wenzel (wetted) state and (b) Cassie (composite) state changed with both h/a and P2/a in pillared model.
Fig. 12. Contact angle of Wenzel and Cassie changed with P2/D (set different values of h/a in Wenzel's state) in pillared model.
3.3. The robust property of the superhydrophobic coating The ability of the superhydrophobic coating to repel the impinging water droplets is also important for practical application [46]. And because of this, in this study, the ability of the superhydrophobic coating to repel incoming water droplets was also investigated. In addition, the sample was fixed with about 20 of
inclination to facilitate gravity-induced water removal [47]. As shown in Fig. 13, a water droplet was vertically released from the height of about 4 cm to impact the superhydrophobic coating surface, and it was found that the droplet could completely rebound on the surface. Besides, continuous water droplets were also respected to superhydrophobic coating, as shown in Video S2, and all the water droplets could also bounce off easily from the surface. This was because the lower surface energy and the air pocket trapped in the dandelion-like microstructures, which did not allow the impacting water to wet the superhydrophobic surface [48]. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jallcom.2016.08.272. Fig. 14(a) shows the long-term, thermal, and solution immersion stability of the superhydrophobic coating. The long-term stability of the superhydrophobic coating was estimated by exposing sample in air of the inner ambient temperature for 6 months, and the contact angle of water on this superhydrophobic sample remained about 156.92 , showing good long-term stability in atmospheric environment. As reported in article [49], the thermal stability of the superhydrophobic coating was tested by treating at 200 C for 7 h, and the contact angle was still about 156.57, indicating excellent thermal stability. After immersing in water for 4 h or in 3.5 wt% NaCl solution for 1 h, there was no obvious change in contact angle of the superhydrophobic coating. Besides, the mechanical stability
Fig. 13. Process of the water droplet completely rebounded by the superhydrophobic coating.
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Fig. 14. (a) The long-term, thermal, solution immersion stability of the superhydrophobic coating, (b) Influence of the adhesive times of the 3M610 adhesive tape on contact angles.
Fig. 15. (a) Contact angle of droplets with different pH on the superhydrophobic surface; surface morphology of the superhydrophobic coating before (b) and after (c) immersed in the alkaline solution (pH ¼ 13) for 60 s.
of the superhydrophobic coating was tested by the 3M610 type adhesive tape [50]. The 3M610 type adhesive tape was first clung to the coating surface, and then manually torn. As shown in Fig. 14(b), after 6 times adhesive tape, the contact angle was still larger than
150 , which confirmed the as-prepared superhydrophobic coating had good mechanical stability. In addition, water droplet on above all tested coating surface was easy to roll, showing the superhydrophobic coating with good stability.
Fig. 16. EDS spectra of the superhydrophobic surface under different pH: (a) untreated; (b) pH ¼ 1; (c) pH ¼ 13.
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The chemical stability of the superhydrophobic coating was tested by measuring the contact angle with pH ranging from 1 to 13, as shown in Fig. 15(a), and the measurement was done immediately when the droplet was placed on the sample surface. The contact angles were larger than 150 except at pH 11, pH 12 and pH 13. This showed that the superhydrophobic property of the coating could not bear a strongly alkaline environment. This may be because the pentadecafluorooctanoic acid adsorbed onto the superhydrophobic coating reacted with alkali. In order to confirm this reason, the surface morphology of the superhydrophobic coating before and after being immersed in the alkaline solution (pH ¼ 13) for 60 s was illustrated in Fig. 15(b) and (c), respectively. It can be found that the alkaline solution did not damage the dandelion-like microstructures of the coating surface. Moreover, Fig. 16 shows the chemical composition of the superhydrophobic coating surface processed by different pH solutions for 60 s. It can be found that the prepared superhydrophobic coating surface had the elements of Cu, Zn, O, C and F (Fig. 16(a)), and these elements did not change in Fig. 16(b) after the surface immersed into the solution (pH ¼ 1) for 60 s. However, the elements of C and F disappeared in Fig. 16(c) after the surface immersed into the solution (pH ¼ 13) for 60 s. Finally, the superhydrophobic property was regained after immersing into 0.01 mol/L of pentadecafluorooctanoic acid anhydrous ethanol for 36 h. It confirmed that the pentadecafluorooctanoic acid adsorbed onto the superhydrophobic coating can react with alkali, resulting in the decrease of the contact. All above experimental results indicated that the superhydrophobic coating had remarkable stability. Therefore, this superhydrophobic coating can provide a robust anti-wetting performance, especially in the field of engineering materials. 4. Conclusion In conclusion, a superhydrophobic CueZn coating with dandelion-like CuO microstructure was successfully prepared on X90 pipeline steel surface via a simple method including electrodeposition, chemical oxidation and fluorination. The dandelion-like microstructure was seen as the dual-scale structure composed of spherical and pillared microstructures. According to the theoretical explanation, it was confirmed that the wetting state between water droplet and the superhydrophobic coating with dandelion-like microstructure was Cassie-Cassie state, which was conformed to the experimental result. Moreover, this superhydrophobic coating had the ability to repel the impinging water droplets and excellent long-term, thermal, solution immersion and chemical stability. Therefore, such robust superhydrophobic coating with hierarchical microstructure was very promising for applications requiring superhydrophobic materials and provided novel insights into electrodeposited metal or alloy coating with superhydrophobic property. Acknowledgements The authors acknowledge the financial support of the National Natural Science Foundation of China (No. 51075184), the Fundamental Research Funds for the Central Universities (No. 15CX06059A), the Postgraduate Innovation Project of China University of Petroleum (East China) (No. YCXJ2016036). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.08.272.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Fabrication and theoretical explanation of the superhydrophobic CueZn coating with dandelion-like CuO microstructure Hao Li, Sirong Yu* College of Mechanical and Electronic Engineering, China University of Petroleum (East China), Qingdao 266580, China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 24 June 2016 Received in revised form 23 August 2016 Accepted 26 August 2016 Available online 28 August 2016
In this work, a CueZn coating with dandelion-like CuO microstructure was fabricated on pipeline steel surface by a simple two-step combined method of electrodeposition and chemical oxidation. This coating was then coated with a thin pentadecafluorooctanoic acid to achieve superhydrophobic, with the contact angle of water about 156.81 and the sliding angle around 3 . When a water droplet contacted on the CueZn coating surface, the dandelion-like CuO microstructure was seen as the dual-scale structure that was proposed to be composed of spherical and pillared microstructures. Based on the Wenzel's and Cassie's formulas, the theoretical explanation was used to confirm the Cassie-Cassie wetted state between water droplet and the dandelion-like hierarchical superhydrophobic coating according to the geometric parameters of the surface morphology. Meanwhile, the superhydrophobic CueZn coating had the ability to repel the impinging water droplets and excellent long-term, thermal, solution immersion and chemical stability. Such robust superhydrophobic CueZn coating with hierarchical microstructures was expected to have various potential applications in our daily life. © 2016 Elsevier B.V. All rights reserved.
Keywords: Superhydrophobicity Dandelion-like structure Theoretical explanation Cassie-Cassie state
1. Introduction Pipeline steel is an important metal in the field of pipeline transportation. However, the adhesion of the aqueous mixture to the pipe wall has the negative influence on production and transportation, which results in much economic loss and waste of resources. Therefore, fabricating a superhydrophobic coating on pipeline steel surface to decline the adhesion of the aqueous mixture to the pipeline steel surface is necessary. Recently, superhydrophobic surface has become a major topic of research because of its various applications, such as self-cleaning property [1], corrosion resistance [2], anti-icing property [3], and anti-scaling potential [4], which are directly related to our daily life. Moreover, a number of creatures with excellent hydrophobic property in nature, including lotus leaves [5], peanut leaves [6], water striders [7], and nepenthes pitcher plants [8], have been observed that the combined effect of the hierarchical micro-nano structure and the low surface energy material achieves the superhydrophobicity. However, the contact angle of water droplet on the smooth solid surface with the lowest surface free energy (6.7 mJ/m2) was only
* Corresponding author. E-mail address: [email protected] (S. Yu). http://dx.doi.org/10.1016/j.jallcom.2016.08.272 0925-8388/© 2016 Elsevier B.V. All rights reserved.
119 [9]. Consequently, the hierarchical micro-nano structure plays the key role in fabricating the superhydrophobic surface [10]. Up to now, many methods have been used to fabricate hierarchical micro-nano structures for obtaining the superhydrophobic surface [11e15], and a great many of different hierarchical structured superhydrophobic surfaces have been fabricated successfully, including mushroom-like structure [16], caterpillar-like structure [17], pinecone-like structure [18], rose-like structure [19], etc. In recent years, researchers have used the simple chemical oxidation to fabricate all kinds of CuO structures for obtaining the hierarchical structures, and the prepared CuO films show excellent superhydrophobic property after being modified with fluoride [20]. Moreover, chemical oxidation method is low cost, time saving, and facile condition [21]. However, to the best of our knowledge, various superhydrophobic CuO microstructures were mainly fabricated on the copper substrate without the steel substrate [22], and it lacked the theoretical model for explaining the reason for superhydrophobic. In this paper, a superhydrophobic CueZn coating with dandelion-like CuO film was obtained on X90 pipeline steel surface. Furthermore, for the purpose of theoretical analysis, the dandelionlike microstructure was seen as the dual-scale structure that was composed of spherical and pillared microstructures. Based on the Wenzel's and Cassie's formulas, we confirmed the wetting state
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between water droplet and the dandelion-like structured superhydrophobic coating surface was the stable Cassie-Cassie state. 2. Experimental section 2.1. Materials and reagents X90 pipeline steel was obtained from TGRC of China and cut into the size of 20 50 3 mm. Brass (H62) was purchased from the common market and cut into the size of 20 50 5 mm. Absolute ethanol (AR), acetone (AR), sodium hydroxide (NaOH, AR), and sulfuric acid (H2SO4, 98%) were purchased from West Long Chemical Co., Ltd. Sodium carbonate anhydrous (Na2CO3), trisodium phosphate dodecahydrate (Na3PO4$12H2O), sodium metasilicate nonahydrate (Na2SiO3$9H2O), Copper(II) sulfate pentahydrate (CuSO4$5H2O), zinc sulfate heptahydrate (ZnSO4$7H2O), potassium hydroxide (KOH), and ammonium persulfate ((NH4)2S2O8) were all analytical grade (AR) and purchased from Sinopharm Chemical Reagent Co., Ltd. Potassium sodium tartrate tetrahydrate (C4O6H4KNa$4H2O, AR) and pentadecafluorooctanoic acid (C8HF15O2, 90%) were purchased from Aladdin. All the chemical reagents were used as received. 2.2. Fabrication of dandelion-like structured coating The processes of fabrication of a coating with dandelion-like microstructures on steel surface included the electrodeposition of CueZn coating and the in-situ growth of CuO dandelion-like microstructures via chemical oxidation, and the detailed procedures were as follows. Before electrodeposition, the steel and brass samples were polished and ultrasonically cleaned in acetone and ethanol for 5 min, respectively. After rinsed in deionized water, the steel sample was immersed in the alkaline solution containing 30 g/L of NaOH, 20 g/L of Na2CO3, 20 g/L of Na3PO4$12H2O, and 10 g/L of Na2SiO3$9H2O at 60 C for 15 min to degrease, and then rinsed with deionized water. To activate the surface, the steel sample was then immersed in a 10% (volume fraction) H2SO4 solution for 10 s at room temperature. Then, the CueZn coating was electrodeposited under direct current. The pretreated steel sample was used as the cathode in the electrodeposition process with the brass as the counter electrode. The two electrodes were immersed in the solution containing 20 g/ L of CuSO4$5H2O, 23 g/L of ZnSO4$7H2O, 100 g/L of C4O6H4KNa$4H2O and 50 g/L of NaOH at 23 C for 60 min with the current density of 2 A/dm2. Meanwhile, the distance between two electrodes maintained at 2 cm. In order to obtain CuO dandelion-like microstructure, as the preparation process reported in article [23], the steel sample with CueZn coating was immersed in a 50 mL of mixed solution including 2.5 mol/L of KOH and 0.12 mol/L of (NH4)2S2O8 at 60 C for 30 min. After chemical oxidation, the sample was ultrasonically cleaned in deionized water and anhydrous ethanol for 5 min (45 kHz), respectively, and dried in air. 2.3. Fabrication of superhydrophobic surface via fluorination The steel surface coated with dandelion-like microstructures was immersed into a beaker with 50 mL of perfluorooctanoic acid anhydrous ethanol (0.01 mol/L) at room temperature for 36 h. In order to prevent the ethanol volatilizing, the beaker was sealed with plastic wrap. Each sample was dried in air for more than one day before it was characterized and tested.
2.4. Characterizations The surface morphology was observed using a field emission scanning electron microscope (FESEM, Nova NanoSEM 450, FEI) equipped with energy disperse spectroscopy (EDS, Oxford X-MaxN, FEI). The thickness of the prepared coating was tested using a confocal microscope (DM2500 M, Leica). The crystal structure was investigated by an X-ray diffraction (XRD, X'Pert PRO MPD, PANalytical B.V.). The X-ray source was Cu target with the incident wavelength of 1.5406 Å. The operating voltage and current was 40 kV and 40 mA, respectively. The surface composition of the coating surface was recorded with an X-ray photoelectron spectroscopy (XPS, Escalab 250Xi, Thermo). The contact angle of a water droplet (about 5 mL) was measured by a goniometer system (SL200B, USA, KINO), and the average value of the contact angle was obtained with at least three measurements of each sample. The sliding angle was the tilted angle of the measured surface once the water droplet (about 8 mL) was rolling on the sample surface. 3. Results and discussion 3.1. Fabrication and characterization of the superhydrophobic coating In order to fabricate a superhydrophobic coating on steel surface, the polished steel was first electrodeposited CueZn coating, and then immersed in the mixed solution including KOH and (NH4)2S2O8 to form CuO dandelion-like microstructure. Finally, the steel surface with dandelion-like structured coating was modified with pentadecafluorooctanoic acid anhydrous ethanol. Fig. 1 shows the surface morphology and chemical composition of the steel sample under different conditions. The surface morphology of the electrodeposited CueZn coating in Fig. 1(a) exhibited to be smooth, which was similar to the previous report [24]. In addition, the cross section of the CueZn coating was polished, and the thickness of this coating was about 20 mm by observing the metallographic microscope of the cross section in Fig. S1. The chemical composition of the CueZn coating was shown in Fig. 1(b), showing that this coating consisted of the elements of Cu and Zn, and the atomic ratio of Zn to Cu in this coating was about 1:1. There was no other impurity except little element of Fe from the steel substrate. After chemical oxidation in the KOH and (NH4)2S2O8 mixed solution, the appearance of the electrodeposited CueZn coating was transformed into black that was the type color of CuO. As shown in Fig. 1(c), the nanoflakes self-organized into dandelionlike microstructure on CueZn coating, and this morphology was the typical CuO structure [25]. Besides, the EDS analysis of this coating surface showed the existence of O element (Fig. 1(d)), which was attributed to the oxidation of this coating. Fig. 1(e) illustrates the morphology of the chemical oxidized CueZn coating after fluorination using a substance with low surface free energy (pentadecafluorooctanoic acid). There was no obvious change of the surface morphology compared to that before fluorination (Fig. 1(c)). Moreover, the EDS spectrum of the superhydrophobic coating in Fig. 1(f) exhibited the existence of F element, indicating that the pentadecafluorooctanoic acid had been successfully physical adsorbed onto the surface of this coating. The wettability of this coating showed superhydrophobic with the contact angle of water about 156.81 (Fig. 2(a)) and the sliding angle of water around 3 (Fig. 2(b)). In the present study, the non-sticking property of the superhydrophobic coating was tested according to the method reported by Liu et al. [26]. As shown in Fig. 3 and Video S1, they illustrated a water droplet of 5 mL approach, contact, and departure processes
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Fig. 1. (a, c, e) SEM images and (b, d, f) corresponding EDS of the sample under different conditions: (a, b) electrodepostion of CueZn coating, (c, d) chemical oxidation, and (e, f) fluorination.
respected to the superhydrophobic coating surface. It can be seen that the water droplet did not dip into or spread out on this coating surface when the water droplet severely contacted the coating surface. Additionally, there was not any of visual water leaving on the surface after the water droplet departing from the superhydrophobic coating [27]. This phenomenon indicated that the adhesive force between the water droplet and the
Fig. 2. (a) Photos and (b) sliding process of the water droplet on the superhydrophobic coating.
Fig. 3. Images of a water droplet (5 mL) suspending on the needle, contacting, pressing and departing from the superhydrophobic coating. The arrows represent the moving direction of the needle.
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(Fig. 5(a)). To be more specific, as shown in Fig. 5(b), it can be observed that the peaks at about 932.5 eV and 952.5 eV were the spectra of Cu2p3/2 and Cu2p1/2, respectively after chemical oxidation [35], and the peak of O1s were also exited on this coating (Fig. 5(a)). It was indicated that CuO formed on the coating surface after chemical oxidation, which was accord with the XRD analysis. In the XPS spectra of the superhydrophobic coating, a strong F1s can be easily observed, and it was located at about 689.0 eV (Fig. 5(c)) that was correlated with CeF bonds [36,37]. The highresolution spectrum of C1s (Fig. 5(d)) had peaks at 293.5 eV and 291.0 eV, which can be assigned to eCF3 and eCF2, respectively [38,39]. Therefore, it was indicated that a stable monolayer of pentadecafluorooctanoic acid had already been successfully physical adsorbed onto the superhydrophobic coating surface after fluorination. 3.2. Theoretical analysis of the superhydrophobic coating with dandelion-like microstructure
Fig. 4. XRD patterns of the sample under different conditions: (a) electrodepostion of CueZn coating, (b) chemical oxidation, and (c) fluorination (the superhydrophobic coating).
superhydrophobic coating surface was very small. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jallcom.2016.08.272. Fig. 4 shows the XRD patterns of the sample under different conditions. After electrodeposition of CueZn coating (Fig. 4(a)), the CueZn phases were detected according to the PDF card of No. 00035-1152, showing that the simultaneous deposition of Cu and Zn in the electrodeposition process [28]. Here, the crystal lattice in Cu matrix was replaced by Zn partly, resulting in the formation of CueZn substitutional solid solution [29]. In addition, there were Fe phases (PDF card of No. 03-065-4899) that were only from the steel substrate, which was similar to the article reported by Almeida et al. [30]. After chemical oxidation, it was noticeably shown that Fig. 4(b) contained diffraction peaks of CuO corresponding to the PDF card of No. 01-089-5898, and all the peaks were consistent with the previous report [31]. Furthermore, the XRD patterns did not show any peak of Zn oxide and no obvious peaks introduced by impurities such as Cu(OH)2 and Cu2O, indicating that only CuO was formed on the CueZn coating. This could be because that the Zn in the CueZn coating was dissolved under a strong base in the chemical oxidation process [32]. In this study, the high concentration of KOH resulted in the primordially grown ZnO crystal dissolving into Zn(OH)2 4 , and the chemical reactions as follows [33,34]. Zn þ 4OH1 /ZnðOHÞ2 4 þ 2e
(1)
2 Cu þ 2OH1 þ S2 O2 8 /CuðOHÞ2 þ 2SO4
(2)
CuðOHÞ2 /CuO þ H2 O
(3)
However, after fluorination, there was no new crystal structure forming on the superhydrophobic coating surface (Fig. 4(c)), indicating that the pentadecafluorooctanoic acid as the modified material had no effect on the crystal structure of this coating. Moreover, the XPS spectra were given to confirm the obtained CuO on the surface after chemical oxidation and the existence of pentadecafluorooctanoic acid on the superhydrophobic coating (Fig. 5). Before fluorination, it reveals that C, O, and Cu exist on the coating, and it can be found C, O, Cu, and F after fluorination
According to above superhydrophobic coating with large contact angle (about 156.81 ) and small sliding angle (around 3 ), the wetting state between water droplet and this superhydrophobic coating surface may be the composite (Cassie) wetting state [40]. Fig. 6(a) is the surface morphology of the superhydrophobic coating with dandelion-like microstructures, which were more obvious in Fig. 6(b) at higher magnification. The dandelion-like microstructures were formed by the nanoflakes self-organizing into spherical structures. The geometric parameters of the surface morphology were measured by the software of Nano Measurer, and the size was measured by manually choosing dozens of points. Fig. 6(c) shows the diameter distribution histogram of the dandelion-like microstructures, which is based on statistical calculations of the SEM image in Fig. 6(a). It was found that the size from 2.4 mm to 3.2 mm almost occupied more than 80% of the dandelion-like microstructures. The dandelion-like microstructures were still uniform, and the average diameter of that was about 2.87 mm. The space among the dandelion-like microstructures based on statistical calculations of the SEM image (Fig. 6(a)) was illustrated in Fig. 6(d), and the average size of the space was around 0.94 mm. As shown in Fig. 6(e) and (f), the length and the space of the nanoflakes based on statistical calculations of the SEM image in Fig. 6(b) were about 0.15 mm and 0.22 mm, respectively. The dandelion-like microstructure can be viewed as dual-scale structure including microsphere and nanoflake. When a water droplet contacted this surface with dandelion-like hierarchical structures, there were four possible combinations of wetting states, including Cassie-Cassie, WenzelCassie, Cassie-Wenzel and Wenzel-Wenzel, which was reported in the article [41]. According to above experimental results, it was difficult to confirm that the wetting case between water droplet and the superhydrophobic coating with dandelion-like microstructures was the Cassie-Cassie state. And because of this, the calculation in the following was used to determine the reliable wetting state of the superhydrophobic coating in this work. Inspired by the formulation described by Patankar [42], the dandelion-like microstructure was considered as the dual-scale coarse structure. The first generation was a model fine scale roughness structure made of the sphere arranged in a regular array (Fig. 7), and the second generation was the pillar that had the same geometry, presenting regular arrangement (Fig. 8). Moreover, the assumptions in the following model included: (a) the gravity of the water droplet was negligible, and (b) the diameter of the water droplet was much greater than the space among the dandelion-like microstructures. In the first generation, as shown in Fig. 7, let the diameter of the sphere be D (2R), and the space among the sphere be P1. If a water
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199
Fig. 5. (a) XPS spectra of the sample before and after fluorination (the superhydrophobic coating), (b) Cu high-resolution spectrum of the coating before fluorination, (ced) F and C high-resolution spectrum of the superhydrophobic coating.
droplet was placed on this surface, there were two possible wetted states, including the wetted (Wenzel) state and the composite (Cassie) state. Then, the Wenzel's equation was evolved into [43]:
cos qw r ¼ r cos qe # pD2 ¼ 1þ cos qe ðD þ P1 Þ2 " # "
¼ 1þ
p
ð1 þ P1 =DÞ2
(4)
cos qe
For the Cassie's state, the water droplet penetrating the space to a certain length of H that was more less than R. Then, the contact area between the water droplet and spherical microstructure was:
S ¼ 2pRH ¼ pDH
(5)
Therefore, the contact angle of the Cassie case was given by Ref. [44]: c
cos qr ¼ ¼
pDH ðD þ P1 Þ2
pH=D ð1 þ P1 =DÞ2
ðcos qe þ 1Þ 1 (6)
ðcos qe þ 1Þ 1
here, qe was the contact angle of the polished pipeline steel surface modified with pentadecafluorooctanoic acid (99.76 ). In order to
intuitively investigate the influence of the geometric parameters of microstructure on the contact angle, Eq. (4) and Eq. (6) were converted into Fig. 9(a) and Fig. 9(b), respectively. As shown in Fig. 9(a), it can be found that the contact angle in Wenzel's state was only affected by P1/D, and it decreased with the increase of P1/D. However, as shown in Fig. 9(b), the contact angle in Cassie's state increased with increasing P1/D, and it declined with growing H/D. Moreover, when a water droplet with certain volume was static on a solid surface, the energy of the droplet (G) can be described as [45]:
G ffiffiffiffiffiffiffiffiffiffi p ¼ ð1 cos qÞ2=3 ð2 þ cos qÞ1=3 3 ð9pÞV 2=3 slv
(7)
here, slv is the surface tension of water, V is the volume of the water droplet, and q represents the contact angle in the improved Wenzel's or Cassie's state. From Eq. (7), it can be found that G increases with the increase of q, and because of this, a water droplet with smaller q has relative equilibrium state. In other words, the smaller contact angle was relative more stability. The contact angle of the Wenzel and Cassie state in spherical model changed with P1/D was shown in Fig. 10, and we set different values of H/D in the Cassie's state. It can be found that only when the contact angle of the Cassie state was equal to that of the Wenzel state, can the contact angle of the Cassie's state be the largest stable value. According to Fig. 6 (D ¼ 2.87 mm and P1 ¼ 0.94 mm), P1/D was about 0.3275, and because of this, the largest stable contact angle of Cassie's case was
200
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Fig. 6. (a, b) Different magnification SEM image of the superhydrophobic coating with dandelion-like microstructures; (c) the diameter and (d) the space of the dandelion-like microstructures based on the statistical calculations of the SEM image in (a); the length (e) and the space (f) of nanoflakes based on the statistical calculations of the SEM image in (b).
Fig. 7. (a) Vertical view and (b) side view of the spherical model that was arranged in a regular array.
118.14 according to Eq. (4). Then, H/D was calculated to be only about 0.20 according to Eq. (6), which indicated there was air trapped in the space among spherical microstructure. Therefore, 118.14 was the largest stable contact angle of the Cassie's case. The
value of qe was thus amplified from 99.76 to 118.14 (qe ) in the second generation (in pillar model). In the second generation, as shown in Fig. 8, let the square pillar be of size a a, the height be h, and the space among the pillar be
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Fig. 8. (a) Vertical view and (b) side view of the pillared model that was arranged in a regular array.
Fig. 9. Contact angle of (a) Wenzel (wetted) state and (b) Cassie (composite) state changed with both H/D and P1/D in spherical model.
P2. There also were two possible contact cases when a water droplet was placed on the pillar surface. The Wenzel's [43] and Cassie's [44] equations were separately evolved into:
cos qw* r ¼
¼
1þ
4ah ða þ P2 Þ2
a2
* cos qe þ 1 1
ða þ P2 Þ2 1 * cos ¼ q þ 1 1 e ð1 þ P2 =aÞ2
! cos q*e
h ! 4 * a 1þ cos qe ð1 þ P2 =aÞ2
c*
cos qr ¼
(8)
Fig. 10. The contact angle of Wenzel and Cassie changed with P1/D (set different values of H/D in Cassie's state) in spherical model.
(9)
here, qe was 118.14 according to the result in the first generation (in spherical model). The contact angles in Eq. (8) and Eq. (9) changed with both h/a and P2/a in pillar model were given in Fig. 11(a) and Fig. 11(b), respectively. As shown in Fig. 11(a), the contact angle of Wenzel's case increased with increasing h/a, and it decreased with the growth of P2/a. Nonetheless, the contact angle of Cassie's state increased with the increase of P2/a, and it had nothing to do with h/a (Fig. 11(b)). Furthermore, the smaller contact angle was more stable according to Eq. (7), and the contact angle of Wenzel and Cassie state in pillared model changed with P2/a was shown in Fig. 12 (set different values of h/a in Wenzel's state). It can also be seen that the contact angle of Cassie's case was the largest stable value when the contact angle of Cassie's case was equal to that of Wenzel's case. The value of P2/a was about 1.47 according to Fig. 6 (a ¼ 0.15 mm and P2 ¼ 0.22 mm) and the value of qe was 118.14 , therefore, the theoretical value of the contact angle on the superhydrophobic coating with dandelion-like CuO microstructure was around 156.00 based on Eq. (9). This value was conformed to the experimental contact angle (156.81 ), which indicated that the wetting contact sate was Cassie sate. Consequently, the wetting state of the superhydrophobic coating was Cassie-Cassie state according to the theoretical explanation of the dual-scale structure model.
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Fig. 11. Contact angle of (a) Wenzel (wetted) state and (b) Cassie (composite) state changed with both h/a and P2/a in pillared model.
Fig. 12. Contact angle of Wenzel and Cassie changed with P2/D (set different values of h/a in Wenzel's state) in pillared model.
3.3. The robust property of the superhydrophobic coating The ability of the superhydrophobic coating to repel the impinging water droplets is also important for practical application [46]. And because of this, in this study, the ability of the superhydrophobic coating to repel incoming water droplets was also investigated. In addition, the sample was fixed with about 20 of
inclination to facilitate gravity-induced water removal [47]. As shown in Fig. 13, a water droplet was vertically released from the height of about 4 cm to impact the superhydrophobic coating surface, and it was found that the droplet could completely rebound on the surface. Besides, continuous water droplets were also respected to superhydrophobic coating, as shown in Video S2, and all the water droplets could also bounce off easily from the surface. This was because the lower surface energy and the air pocket trapped in the dandelion-like microstructures, which did not allow the impacting water to wet the superhydrophobic surface [48]. Supplementary video related to this article can be found at http://dx.doi.org/10.1016/j.jallcom.2016.08.272. Fig. 14(a) shows the long-term, thermal, and solution immersion stability of the superhydrophobic coating. The long-term stability of the superhydrophobic coating was estimated by exposing sample in air of the inner ambient temperature for 6 months, and the contact angle of water on this superhydrophobic sample remained about 156.92 , showing good long-term stability in atmospheric environment. As reported in article [49], the thermal stability of the superhydrophobic coating was tested by treating at 200 C for 7 h, and the contact angle was still about 156.57, indicating excellent thermal stability. After immersing in water for 4 h or in 3.5 wt% NaCl solution for 1 h, there was no obvious change in contact angle of the superhydrophobic coating. Besides, the mechanical stability
Fig. 13. Process of the water droplet completely rebounded by the superhydrophobic coating.
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Fig. 14. (a) The long-term, thermal, solution immersion stability of the superhydrophobic coating, (b) Influence of the adhesive times of the 3M610 adhesive tape on contact angles.
Fig. 15. (a) Contact angle of droplets with different pH on the superhydrophobic surface; surface morphology of the superhydrophobic coating before (b) and after (c) immersed in the alkaline solution (pH ¼ 13) for 60 s.
of the superhydrophobic coating was tested by the 3M610 type adhesive tape [50]. The 3M610 type adhesive tape was first clung to the coating surface, and then manually torn. As shown in Fig. 14(b), after 6 times adhesive tape, the contact angle was still larger than
150 , which confirmed the as-prepared superhydrophobic coating had good mechanical stability. In addition, water droplet on above all tested coating surface was easy to roll, showing the superhydrophobic coating with good stability.
Fig. 16. EDS spectra of the superhydrophobic surface under different pH: (a) untreated; (b) pH ¼ 1; (c) pH ¼ 13.
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The chemical stability of the superhydrophobic coating was tested by measuring the contact angle with pH ranging from 1 to 13, as shown in Fig. 15(a), and the measurement was done immediately when the droplet was placed on the sample surface. The contact angles were larger than 150 except at pH 11, pH 12 and pH 13. This showed that the superhydrophobic property of the coating could not bear a strongly alkaline environment. This may be because the pentadecafluorooctanoic acid adsorbed onto the superhydrophobic coating reacted with alkali. In order to confirm this reason, the surface morphology of the superhydrophobic coating before and after being immersed in the alkaline solution (pH ¼ 13) for 60 s was illustrated in Fig. 15(b) and (c), respectively. It can be found that the alkaline solution did not damage the dandelion-like microstructures of the coating surface. Moreover, Fig. 16 shows the chemical composition of the superhydrophobic coating surface processed by different pH solutions for 60 s. It can be found that the prepared superhydrophobic coating surface had the elements of Cu, Zn, O, C and F (Fig. 16(a)), and these elements did not change in Fig. 16(b) after the surface immersed into the solution (pH ¼ 1) for 60 s. However, the elements of C and F disappeared in Fig. 16(c) after the surface immersed into the solution (pH ¼ 13) for 60 s. Finally, the superhydrophobic property was regained after immersing into 0.01 mol/L of pentadecafluorooctanoic acid anhydrous ethanol for 36 h. It confirmed that the pentadecafluorooctanoic acid adsorbed onto the superhydrophobic coating can react with alkali, resulting in the decrease of the contact. All above experimental results indicated that the superhydrophobic coating had remarkable stability. Therefore, this superhydrophobic coating can provide a robust anti-wetting performance, especially in the field of engineering materials. 4. Conclusion In conclusion, a superhydrophobic CueZn coating with dandelion-like CuO microstructure was successfully prepared on X90 pipeline steel surface via a simple method including electrodeposition, chemical oxidation and fluorination. The dandelion-like microstructure was seen as the dual-scale structure composed of spherical and pillared microstructures. According to the theoretical explanation, it was confirmed that the wetting state between water droplet and the superhydrophobic coating with dandelion-like microstructure was Cassie-Cassie state, which was conformed to the experimental result. Moreover, this superhydrophobic coating had the ability to repel the impinging water droplets and excellent long-term, thermal, solution immersion and chemical stability. Therefore, such robust superhydrophobic coating with hierarchical microstructure was very promising for applications requiring superhydrophobic materials and provided novel insights into electrodeposited metal or alloy coating with superhydrophobic property. Acknowledgements The authors acknowledge the financial support of the National Natural Science Foundation of China (No. 51075184), the Fundamental Research Funds for the Central Universities (No. 15CX06059A), the Postgraduate Innovation Project of China University of Petroleum (East China) (No. YCXJ2016036). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jallcom.2016.08.272.
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