ZnO surfaces fabricated via electrodeposition on tin substrate for self-cleaning behavior and switchable wettability

Accepted Manuscript Stable superhydrophobic Zn/ZnO surfaces fabricated via electrodeposition on tin substrate for self-cleaning behavior and switchable wettability Ge He, Shixiang Lu, Wenguo Xu, Ping Ye, Guoxiao Liu, Hongtao Wang, Tanlong Dai PII:

S0925-8388(18)30976-9

DOI:

10.1016/j.jallcom.2018.03.108

Reference:

JALCOM 45338

To appear in:

Journal of Alloys and Compounds

Received Date: 3 January 2018 Revised Date:

5 March 2018

Accepted Date: 9 March 2018

Please cite this article as: G. He, S. Lu, W. Xu, P. Ye, G. Liu, H. Wang, T. Dai, Stable superhydrophobic Zn/ZnO surfaces fabricated via electrodeposition on tin substrate for self-cleaning behavior and switchable wettability, Journal of Alloys and Compounds (2018), doi: 10.1016/j.jallcom.2018.03.108. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Stable superhydrophobic Zn/ZnO surfaces fabricated via electrodeposition on tin substrate for self-cleaning behavior and switchable wettability

Tanlong Daib

School of Chemistry and Chemical Engineering, Beijing Institute of Technology,

Beijing 100081, P.R. China b

State Key Laboratory of Earth Surface Processes and Resource Ecology, Beijing

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Normal University, Beijing, 100875, China

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a

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Ge He a, Shixiang Lu a*, Wenguo Xua*, Ping Ye a, Guoxiao Liu a, Hongtao Wang a,

Abstract Superhydrophobic Zn/ZnO coatings on tin substrate with irregular polyhedral structure are grown via electrodeposition combined with anneal. The resultant surfaces deposited at −1.6 V for 10 min and annealed at 200 °C for 10 min show good superhydrophobicity, exhibiting a water contact angle of 160° and a

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sliding angle of about 1°, showing excellent rolling-off and self-cleaning properties. The influence of ultrasonic treatment on film thickness, wetting property, surface morphology and composition were systematically researched to understand the

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growth mechanism of the superhydrophobic Zn/ZnO film. The superhydrophobic stability of the Zn/ZnO surfaces were investigated by the means of exposure to the air, buried in soil, harsh mechanical bending and water jet impacting. The reversibly

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switchable wettability between superhydrophobicity and superhydrophilicity of the Zn/ZnO surface were studied by alternate UV irradiation and heating. The discoveries in this work provide an economic and simple route to fabricate versatile superhydrophobic surfaces on conductive substrates, which has excellent promise for future applications. Key words: Electrodeposition; Superhydrophobic; Tin; Self-cleaning; Stability

* Corresponding author, Email: [email protected] (Shixiang Lu); [email protected]

ACCEPTED MANUSCRIPT (Wenguo Xu)

1. Introduction Since artificial superhydrophobic (SHP) surfaces were first demonstrated in the mid-1990s, many outstanding research workers have already invested extensive deep

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study on this field over the past decades[1-3], Barthlott et al., Jiang et al. and Shirtcliffe et al. for example, and what has followed are many clever strategies to promote the preparation process of superhydrophobic surfaces, including chemical vapor deposition, electrospray process, solution-immersion method, sol−gel technique,

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electrodeposition and template synthesis, and so on [4-6]. To be noted, these methods are all based on the two typical theories (i.e. the Wenzel and Cassie–Baxter models)

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[7, 8], which pointed out that the surface wetting behavior is governed by two crucial factors, surface roughness and chemical composition, and surface roughness represents the micro/nanostructures distributed periodically or randomly. In terms of this basal principle, multifarious functional superhydrophobic surfaces have already been prepared by integrating multi-scale structures with low surface energy

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compounds like fluoroalkylsilane (FAS) and further integrated into smart devices [9, 10], and in turn, the practical applications also provide direct impetus to push the field forward rationally.

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Before going into details, it is necessary to pause to recall that the lotus leaves can achieve a water contact angle (WCA) over 160° and low rolling angle (RA) just by

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using paraffinic wax crystals comprising predominantly –CH2– radical groups, which plainly demonstrates that classical substances with low surface energy such as –CH3 groups or FAS are not a necessity to achieve non-wetting. Rather, the ability to regulate the surface morphology on micro/nano scales is crucial [11-14]. So, electrodeposition method represents a simple and suitable technique to fabricate superhydrophobic surfaces since it provides feasibility to tailor architecture in micro/nano scales by adjusting various deposition parameters [15, 16]. Moreover, the hierarchical structures and periodical configuration and orientations of surface topography not only bring out water repellence, but also endow other functional

ACCEPTED MANUSCRIPT properties to superhydrophobic surfaces [17, 18]. This decoupling of wettability from simple surface energy opens up more feasibilities for functional surfaces. With the rapid progress of preparation process and mechanism of superhydrophobic surfaces, the newly discovered unique properties have also attracted considerable

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attention of researchers, such as self-cleaning, oil-water separation, drag reduction, corrosion resistance, anti-bacterial etc [19-22]. Among these properties, self-cleaning which usually referred to it that droplets cannot stay on a surface and roll off easily is known as ‘‘lotus effect’’, showing the ability of low-adhesion [23]. Thereby, the dust

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or pollutants on the surface can also be picked up by the rolling droplets meanwhile. Then, the motivation of continuously studying self-cleaning surfaces is probably

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actuated by the wish to fabricate such protective surfaces for outdoor facilities like photovoltaics, auto industry, satellite dishes and external architectural glass, because application of self-cleaning can greatly reduce cleanup costs [24]. Recently, functional smart surfaces with tunable wettability under external stimuli have been well studied for the potential applications in wettability driven

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microfluidics, smart on-off systems, biochip and stain resistant finishes. UV light, temperature, magnetic, pressure and pH are usually used as external stimuli, among which UV light have more widespread application prospect because it is quickly

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controllable and the tuning can be individually addressed [25, 26]. Furthermore, responsive materials including TiO2, V2O5, WO3, Fe2O3 and ZnO, have been utilized to obtain UV-induced reversible switching of the wetting properties, and heating or

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storage in dark can restore the water repellent nature [27, 28]. In comparison with some organic molecules with stimuli-responsive properties, these metallic oxides can exert their advantage of higher structural and photochemical stability to exhibit more distinct wettability variations, exhibiting promising applications. As for the reversible wettability of ZnO nanoparticles, lots of research have been done. Feng et al. [29] have fabricate an aligned ZnO nanorod surface that exhibits reversible superhydrophobicity-superhydrophilicity transition regulated by alternate UV irradiation and dark storage. Yan et al. [30] reported a photo-induced ZnO surface on steel mesh by a spray-coating approach for oil/water separation, which can realize the

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between

superhydrophobicity

and

superhydrophilicity/underwater

superoleophobicity by alternation of UV irradiation and heat, and therefore can be tuned between oil-removing to water-removing . Tin shows great prospect in the industry of shipbuilding, aerospace, food and

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electronics soldering due to the prominent advantage of non-toxic, chemical stability and corrosion resistance. Besides, coating Zn or ZnO on various substrate is easier to achieve superhydrophobicity, and superhydrophobic zinc surface have been successfully fabricated on several substrates. However, few literatures about the

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tin-based superhydrophobic surfaces have been reported at present, so, it is favorable for the preparation of superhydrophobic zinc surface on tin substrate to combine the

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superiority of the two metals to produce more advantage performance [31, 32]. Therefore, in this work, we presented an effective and economic approach for the fabrication of SHP Zn/ZnO coating on tin substrate simply by utilizing electrodeposition and anneal treatment. Diverse structures and wettability have been obtained by varying parameters like reactant concentration, deposition voltage and

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time, ultrasonic treatment, anneal temperature and time. And the surface shows excellent water repellence when electrodeposited in 0.005 mol/L Zn2+ for 10 min and annealed at 200 °C for 10 min. It is notable that the whole fabrication process can be

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completed within 1 h, which would be advantageous for mass production in industry.

2. Experimental procedure

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2.1 Materials

Commercially available tin sheets (99.9%, 1 mm thick) were obtained from

Guantai metal materials co. LTD, afterwards they were cut into sizes of (20 × 10 × 1 mm). Acetone, ethanol, hydrochloric acid (HCl, 36–38%), KCl and zinc acetate dihydrate (ZnAc2·2H2O, 98%) used with an analytic grade were supplied by China Beijing Fine Chemical Co. Ltd. Deionized water was used for preparation of all solutions and metal surface cleansing in the experiment process. 2.2 Electrodeposition experiments As the starting material, tin sheets were respectively ultrasonically cleaned in

ACCEPTED MANUSCRIPT ethanol, acetone and deionized water, and then dried in the air. Prior to electrodeposition, cyclic voltammetry was used to investigate the reduction potential of the Zn2+. Then the samples were electrodeposited in classical three-electrode system under ultrasonic conditions, in which the electrolyte was composed of 0.005

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mol/L Zn(Ac)2, 3.75 mmol/L HCl and 0.1 mol/L KCl. After electrodeposition, the coated samples were annealed at 200 °C for 10 min.

To facilitate expression and further study, five samples under different processing conditions were numbered and were listed in table 1.

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Table 1 WCAs of samples under different processing conditions. Deposition, Ultrasonic and Anneal refer to the samples after deposited at −1.6 V for 10 min in the electrolyte comprising

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0.005 mol/L Zn(Ac)2, 3.75 mmol/L HCl and 0.1 mol/L KCl, and anneal at 200 °C for 10 min. “Y” and “N” mean conducting the corresponding procedure and without conducting.

1 2 3 4 5

Deposition

Ultrasonic

N Y Y Y Y

N Y Y N N

2.3 Characterization

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Samples

Anneal N N Y N Y

CA(º) 70 10 160 63 116

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Static water contact angles (WCAs) and rolling angles (RA) were calculated by sessile-drop method with water droplets (8 µL) on the contact angle meter (FTÅ 200,

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Data physics Inc, USA) under ambient temperature. As for the reported WCA values, the measurements were repeatedly conducted for five times on different positions. The error bar of the WCA values was 2°. The morphological analyses were conducted employing a scanning electron

microscope (SEM, JSM-7500F, Hitachi, Japan) and samples were gold sputtering to improve conductivity. Equipped with the scanning electron microscope was an energy-dispersive X-ray spectrometry (EDX) which was used to analyze the surface chemical composition. X-ray diffraction (XRD, D8 discover with Cu Kα wave length 0.154 nm) and X-ray photoelectron spectra (XPS, Model PHI 5300, Physical

ACCEPTED MANUSCRIPT Electronics, USA) were further conducted to characterize the crystallographic properties and the chemical state of the coatings. The binding energy of adventitious carbon (284.6 eV) was used to calibrate all spectra. A fitting routine was adopted to analyze the XPS core level spectra, with which spectra were decomposed into

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individual mixed Gaussian–Lorentzian peaks after a Shirley background subtraction. The thickness of the deposits with and without ultrasonic treatment were measured by employing a step profiler (Dektak 150).

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3. Results and discussion

3.1 Surface morphology and wettability of the SHP Zn/ZnO surface

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To better understand the principle of superhydrophobic formation, SEM images of samples 1 to sample 3 were employed, since micro/nano structure can regulate wettability and especially superhydrophobicity of a solid surface. Fig. 1a depicts the SEM image of sample 1, in which the initial tin sheet appears to be relatively smooth without obvious roughness, yielding a WCA of about 70°. Electrodeposition of the

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zinc-based coating at a voltage of −1.6 V for 10 min resulted in an absolutely wet surface with a WCA of around 5°, as shown in Fig. 1b, in which the surface is completely covered by intricately packed polyhedron structure with irregular shape

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(illustration in Fig. 1b). Fig.1c and 1d are the surface morphologies of the SHP Zn/ZnO surface (sample 3) in different magnifications, indicating a uniform coating. After anneal at 200°C for 10 min, the polyhedral structure reunited and countless

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nanoparticles appeared on the surface, which may because that the deposited crystals have recrystallized and grown in the heating state, leading to a WCA of 160°. It can be speculated that the electrostatic charge between the deposited particles facilitate them to attract other particles to grow into conglomerate with a larger virtual size. The deposited particles can offer necessary roughness and porosity for the surface to entrap air when the surface is submerged in water, resulting in an air belt, so that droplets can easily bounce on this surface and roll away eventually. The Cassie– Baxter equation can account for the phenomena [8]:

ACCEPTED MANUSCRIPT cosθ =ƒ1 cosθ0−

2

Here, ƒ1 and ƒ2 refer to the fraction of solid/water and air/water interface (i.e. ƒ1 + ƒ2 =1); θ and θ0 represent the WCA of rough SHP surface and smooth zinc surface. Given that the value of θ and θ0 are 160° and 66°, ƒ1 and ƒ2 can be calculated to be

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about 0.0429 and 0.9571, suggesting air occupies most of contact space, which further signify that air pockets trapped between the hierarchical structures can effectively prevent the immediate contact between liquid and SHP surfaces. Therefore, high WCA and low RA can be acquired.

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3.2 Surface composition

The relevant element distributions of the tin substrate (sample 1), electrodeposited

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zinc surface before anneal (sample 2) and SHP Zn/ZnO surface (sample 3) were firstly studied by EDS spectra, as shown in Fig. 2. To be noted, the unlabeled peak at 2.10 eV was from the spray-gold before SEM scanning for purpose of conductivity improvement. The corresponding concentrations of elements are listed in Table 2. EDS spectrum of tin substrate displays signals of Sn and O (Fig. 2a) with an atomic

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ratio of 82.13:17.87. The existence of Zn in the EDS spectrum (Fig. 2b) suggests the successful electrodeposition of zinc on tin substrate, and the atomic ratio was 12.46 (Sn): 9.28 (O): 78.26 (Zn). The spectrum of sample surface after anneal reveals the

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presence of Sn, O and Zn with a quantitative atomic ratio of 67.40 (Sn): 19.21 (O): 13.39 (Zn). The increase of O indicates the formation of ZnO and SnO2, while the

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decrease of Zn element may due to the recombination of polygonal structures in the process of anneal. At a heating temperature of 200 °C, part of tin becomes semisolid and re-solidifies upon cooling, thereby can affect the apparent concentration of Zn of the coating. In conclusion, the EDS analysis result testify the successful electrodeposition of the SHP Zn/ZnO coating. In order to discover the surface substance, XRD spectra of sample 1, 2 and 3 in the scan region of 30–90° were given in Fig. 3. It is apparent that the XRD peaks of sample 1 in Fig. 3a and 3b corresponding to 2θ = 32.0°, 43.9°, 44. 9°, 62.5°, 64. 5°, 79.4° and 89.3° are labelled as (1 0 1), (2 2 0), (2 1 1), (1 1 2), (3 2 1), (3 1 2) and (4 3

ACCEPTED MANUSCRIPT 1), respectively, which were perfectly in accord with the tetragonal Sn (JCPDS 04-0673). In addition, the peak that appeared at 2θ = 80.6° represents (2 4 1) pane of the tetragonal SnO2 (JCPDS Card No. 29-1484) [33]. After electrodeposited in the electrolyte composed of 0.005 mol/L Zn(Ac)2, 3.75 mmol/L HCl and 0.1 mol/L KCl,

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apart from the inherent peaks of tin substrate, several new peaks labeled with “ ” in curve B in Fig.3a can be derived from Zn (1 0 0), Zn (1 0 1), Zn (1 0 2), Zn (1 0 3), Zn (1 1 0), Zn (1 1 2) and Zn (1 0 1) of hexagonal Zn (JCPDS 04-0831), respectively. Meanwhile, the detected peak at 2θ = 36.3° can be ascribed to ZnO (1 0 1) (JCPDS

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36-1451). Three diffraction peaks located at 2θ = 30.6°, 59.9° and 63.8° which were marked with “*” attribute to Zn(OH)2 (JCPDS 38-0356 and JCPDS 38-0385) [34].

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After annealed at 200°C for 10 min in the air, it is clear in curve C in Fig. 3b that if the peaks of tin substrate were excluded, only two peaks located at 2θ = 36.3° and 54.3° were detected, which can be identified as ZnO (1 0 1) and Zn (1 0 1) [35]. Compared the peaks of sample 2 with that of sample 3, it can be easily found that Zn(OH)2 peaks and part of Sn and Zn peaks disappeared after anneal, which may

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because of the dissolution and cooling of tin during anneal. The results indicate that Zn/ZnO coatings were successfully prepared on tin substrate by electrodeposition. XPS spectra are investigated to verify the chemical state of the Zn/ZnO coating, as

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are shown in Fig. 4. The binding energies (BEs) are calibrated by using that of C 1s (284.6 eV). In the whole XPS patterns of the coating surface before (A) and after (B) anneal, there exist signals of C, Sn, O and Zn (Fig. 4a). And individual binding energy

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region of these elements were also scanned to acquire further information about the surface electronic structure. The BEs of the Sn3d peak for sample 2 (curve A) and 3 (curve B) are depicted in Fig. 4b. For the sample before anneal, two peaks appeared at 485.8 eV and 498.5 eV are the BEs of Sn 3d5/2 and Sn 3d3/2. However, after anneal, both Sn 3d5/2 and Sn 3d3/2 clearly show two groups of Sn chemical bond energies for Sn0 (483.4 eV and 494.2 eV) and Sn4 + (485.5 eV and 498.8 eV) [36, 37], which are corresponding to metallic Sn and SnO2 phases, respectively. These results suggest some transformation of SnO2 from Sn occurred in the process of anneal. The O1s regions before (A) and after (B) anneal are shown in Fig. 4c, the

ACCEPTED MANUSCRIPT illustration in Fig. 4c are the fitting curves. The O1s peak before anneal was the result of overlapping three peaks situated in 530.1 eV, 531.4 eV and 532 eV, which are the attribution of O2- in the Zn–O, O2- ions in the ZnO(OH) and O2- in the SnO2, respectively [38, 39]. After anneal, the O1s XPS spectrum consists of a main peak and

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a shoulder peak, which can be resolved into three bands at 530 eV, 531.3 eV and 532 eV, corresponding to the chemical bonds of O-Zn, O-H and O-Sn4+, respectively. By comparing the fitted curves of O1s, it suggests an evident increase in SnO2 components and the ratio of ZnO and ZnO(OH) increased from 1.14 to 5.5 after

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anneal. These results indicate that the anneal process facilitates the generation of oxides, yielding a more stable surface.

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Fig. 4d shows strong Zn 2p3/2 and Zn 2p1/2 spectra with corresponding BEs of 1021.7 eV and 1044.8 eV before and after anneal. In the fitted curve of the Zn2p after anneal, the position of Zn 2p3/2 and 2p1/2 located at 1021.1 (1022.3) and 1044.3(1045.5) eV respectively were for Zn (ZnO), which were accompanied by a very small difference (∼ 1 eV) and a standard peak energy separation (23.2 eV).

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These data confirm the presence of Zn and ZnO in the superhydrophobic Zn/ZnO coating [40, 41].

3.3 The influencing factors on superhydrophobicity

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To understand the diversities of superhydrophobicity, a sequence of factors should be

considered,

including

ultrasonic

treatment,

electrolyte

concentration,

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electrodeposition time, anneal temperature and anneal time. In view of the factors discussed in previous articles, here ultrasonic treatment was considered as the main influence factor.

The employ of ultrasonic treatment during electrodeposition can expedite the

kinematic velocity of the Zn2+ ions as well as gas escape. So ultrasonic treatment is identified as a quite important element. The electrodeposition curve, surface morphology and composition of the deposit with and without ultrasonic treatment were

studied

to

superhydrophobicity.

intensify

the

cognition

of

its

function

in

achieving

ACCEPTED MANUSCRIPT Since the deposition current is closely linked with the mass transfer rate of metal ions in electrolyte, the electroplating curves of samples with and without ultrasonic treatment are investigated, which are shown in Fig. 5a. As is evident in comparing with i-t curves of sample 3 and sample 5 that sample 3 possesses a larger deposition

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current than sample 5. Moreover, as deposition time went on, the deposition current of sample 3 grows larger from 0.022 A to 0.032 A along with relatively larger fluctuation, while that of sample 5 mainly maintained at 0.002 A in the electrodeposition process unless a sudden increase at the beginning. It can be speculated that changes in

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deposition current will inevitably lead to difference in coating thickness, which was confirmed by Fig. 5b that the average thickness of sample 3 (17 µm) was much larger

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than that of sample 5 (2.5 µm).

Surface morphology and element distributions of samples electrodeposited without ultrasonic treatment before anneal (sample 4) and after anneal (sample 5) were further investigated to understand the role of ultrasonic treatment, as shown in Fig. 6. The surface of sample 4 was roughened by the nanometer-scale nubble-protrusions

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(200-300 nm) with cavities as well as island structures, which distributed uniformly across the surface (Fig. 6a). At upper right of Fig. 6a is the photograph of a water droplet on sample 4, exhibiting a WCA of 63°. Fig. 6b is the EDS patterns of sample 4,

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which displays the existence of Sn and O. After anneal, the nubble-protrusions transformed into numerous polyhedral nanoparticle ( 200 nm) which scattered on the protuberant parts of the surface, as displayed in Fig. 6c, with the value of WCA

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increased to 116°. Moreover, by comparing the atomic percentage of elements of sample 4 and sample 5, a slightly decrease in Zn and O and increase in Sn can be noticed, which may due to the low melting point of Sn. As another evidence to better understand the function of ultrasonic treatment in the

process of electrodeposition, EDX mappings of samples prepared with (sample 3) and without (sample 5) ultrasonic treatment were conducted. Group a and b in Fig. 7 show the distribution of Sn, O and Zn on sample 3 and sample 5, respectively, in which the surface of sample 5 reveals a relatively sparser distribution of Zn and O than sample 3, while the distribution of Sn present similar results. To be noted, the three elements are

ACCEPTED MANUSCRIPT evenly arranged on the surface whether on the surface of sample 3 or sample 5, indicating uniform coatings was prepared via electrodeposition. These results are well corresponding to the SEM images.

4. Properties of the superhydrophobic Zn/ZnO coating

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4.1 Durability and stability of the SHP Zn/ZnO coating With an eye to the practical applications of SHP materials, superhydrophobic stability was still a major factor that can’t be ignored. The SHP sample were stored in

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the air at room temperature, and then tested their WCA values at certain time intervals investigate their stability. One year’s later, both the WCA and RA basically remained

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unchanged. In other words, although differences exist in the temperature and humidity as the exchange of four seasons, WCAs of the SHP sample remained almost unchanged with the extension of storage time, exhibiting long-term stability and durability. In addition, the sample can keep superhydrophobic with the WCA over 160º even after buried in soil for six months, suggesting good anticorrosion to soil,

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which provides more possibilities for their real applications.

For expanding practical applications, wetting stability under harsh mechanical bending was investigated, as displayed in Fig.8. Fig. 8a denotes water droplets on the SHP Zn/ZnO surface after limited bending. It can be observed that the droplets didn’t

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pin around the kink regions and still kept spherical shape, and were prone to roll away readily from the tilted surface. The image of droplets on the SHP Zn/ZnO surface

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after limited bending and reverted to initial position was shown in Fig. 8b. It should be pointed out that the surfaces couldn’t return to perfectly flat and still left some wrinkles. However, spherical shape of the droplets was still observed and they can slide off effortlessly when affected by little mechanical disturbance, confirming no obvious variation in wettability under rigorous mechanical bending. Generally speaking, the microstructure or surface chemical groups of a rough surface is liable to be damaged by mechanical wear, hence, an impacting water jet may unavoidably damage the water-repellent properties since it can cause elastic collision to the surface. Therefore, for the SHP Zn/ZnO surface, the wetting stability

ACCEPTED MANUSCRIPT under water jet impact was also studied, and a tin substrate was used as comparison. The water jets were produced at a syringe filled with purified water. As shown in Fig. 8c, when a water jet hit the tin substrate, the droplets spread out on the surface promptly, which may owe to the smooth surface and hydrophily of tin substrate. To

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the contrary, when a water jet hit against the SHP Zn/ZnO surface, it could bounce off in the opposite direction without leaving any trail (Fig. 8d), which result from the existence of the air belt formed by the entrapped air between the rough structure and liquid. Additionally, the water jets can bounce off the SHP Zn/ZnO surface

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continuously after targeted at the same place for about 2 min, confirming strong superhydrophobic stability under dynamic wettability.

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4.2 Sliding behavior and self-cleaning property of the SHP Zn/ZnO surface Usually, SHP surfaces in Cassie-Baxter state reveal sliding behavior, where water drops can bounce off from a slightly tilted or even a horizontal surface if dropped from a height. Fig. 9a shows a set of photographs intercepted from a video of a water drop falling freely on the SHP Zn/ZnO surface. When the water drop contacted the

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SHP Zn/ZnO surface, they would deform due to the impact force and gravity, and then the impacting kinetic energy turned into elastic potential energy, leading to rebounding of the water drops, and eventually left the surface. As for self-cleaning

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performance, the non-sticking property mention above count for much. Self-cleaning effect is a crucial characteristic necessary for the applications of

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superhydrophobic surfaces. Accordingly, the evaluation process of self-cleaning effect was shown in Fig. 9b. Soil, as typical contaminant particles, were willfully spread on the sample surface, which are much more than those under normal circumstances. Then water drops were dropped on the Zn/ZnO surfaces loaded with soil. It can be seen that once soil met the water droplet, they were immediately adsorbed on the surface of the water droplet. After several sliding processes, spherical rolling droplets could effortlessly carry away most of the soil particles from the dusted SHP sample, leaving behind a clean surface (b1-b5), and they still maintained a perfectly spherical shape even if took up a lot of particles. However, the droplet on

ACCEPTED MANUSCRIPT the other half part of uncoated tin substrate adhered to the surface even if tilted the sample at certain angle. The synergistic effect of surfaceness and low-surface-energy can account for this phenomenon, which can reduce the contact region and adhesive intension of dirt particles and the SHP Zn/ZnO surface. These observations confirm

possibilities for the industrial applications. 4.3 UV-induced reversible wettability of the SHP surface

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that the as-prepared SHP Zn/ZnO surface possess good self-cleaning effect, providing

Serial alternating of UV irradiation and heating were conducted to study the

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reversible wettability of the SHP Zn/ZnO surface. A UV lamp (15 W) was used in the exposure experiment which firsthand illuminated on the SHP Zn/ZnO surface at a

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working distance of 20 cm under ambient environment (∼25 °C). The changes in WCA with UV irradiation time were monitored to investigate the interaction of coating particles and UV light, as shown in Fig. 10a. It is clearly that the WCA of the SHP Zn/ZnO surface decreased gradually with the extension of UV irradiation time, and the value varied from 160° to 5° after 2.5 h of UV irradiation. It is interesting that

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the surface recovered its superhydrophobicity when it was heated in an oven at given temperatures, and the WCA increased as the heating time went on. Additionally, higher heating temperature contributes to shorter transition time, as shown in Fig. 10b,

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the transition time decreased from 3 h to 1 h when the heating temperature increased from 120 °C to 180 °C. To precisely illustrate the wetting behaviors of the switchable

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surface, wetting switching cycles were repeated for six times, and 98% of the wettability (157°) was regained in the first cycle. Moreover, the wettability transition can be generally reversible in the subsequent cycles (Fig. 10c), indicating remarkable reversible conversion between superhydrophobicity and superhydrophilicity. The following mechanism may explain this conversion: once the SHP Zn/ZnO surface was exposed to UV light, electron–hole pairs formed in the space lattice would react with lattice oxygen to produce oxygen vacancies, and H2O and O2 competed to be adsorbed on them. Next, these surface oxygen vacancies can further interact with Zn2+ in the lattice, generating Zn+ defective sites. It should be noted that

ACCEPTED MANUSCRIPT the adsorbability between Zn+ defective sites and hydroxyl groups is larger than that of Zn+ defective sites and oxygen, so that the surface was mainly occupied by hydroxyl groups with the extension of UV exposure time, producing a more hydrophilic surface. However, the surface would change into an energetically unstable

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state when adsorbed more hydroxyl groups because oxygen would bond preferentially to the defect sites rather than the hydroxyl groups. Interestingly, heating the irradiated sample at given temperature (ie. 160 °C) is beneficial to the combination of oxygen and hydroxyl groups, which means the elimination of hydroxyl groups, because

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oxygen adsorption is thermodynamically favored. Therefore, when heated the irradiated sample at 160 °C for 1.5 h, the superhydrophilic surface reverts back to

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superhydrophobicity [42, 43]. Furthermore, heating the irradiated surface at 120 °C, 140 °C, 160 °C and 180 °C for different times was conducted to investigate elimination rate of hydroxyl groups under different heating temperatures. It turned out that the higher the heating temperature was, the shorter the recovery time needed, which was proved by Fig. 10b.

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To confirm the mechanism above, SEM and XPS measurements were carried out. Fig. 11 shows the surface morphologies of samples after UV irradiation and after UV irradiation and then heating at 160 °C for 1.5 h, which shows intricately packed

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polyhedral structure with irregular shape, exhibiting no clearly difference. Thus, the wettability switching is believed to be more related to the chemical composition of the sample surfaces. XPS spectra were used to clearly detect the chemical state causing

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the reversible wettability switching, as shown in Fig. 12a and 12b. Four O1s signals were observed at 529.7 eV, 530.2 eV, 531.3 eV and 532 eV (Fig. 12a), which are attributed to SnO, ZnO, ZnO(OH) and SnO2. To be noted, the ratio of ZnO/ZnO(OH) was calculated to be 1.1 for the sample after UV irradiation, while the proportion increased to 2.7 when the sample was irradiated by UV light and then heating. Moreover, the SnO2/SnO ratio increased from 0.4 to 0.9 when the sample experienced UV irradiation and then heating, which may because that SnO2 is more stable than SnO under heating condition. These results indicate that heating can restore hydrophobicity of the surface by accelerating elimination of hydroxyl groups, which

ACCEPTED MANUSCRIPT further confirmed the mechanism of reversible wettability transform between superhydrophobicity and superhydrophilicity of the Zn/ZnO coating.

5. Theoretical growth mechanism

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A possible forming mechanism of the SHP Zn/ZnO surface was identified as follows: First, acetate ion dissociated from zinc acetate can combine with hydrogen ions to produce acetic acid, and the pH of the solution changes because of the faintly acid of acetic acid. Second, when voltage acted on the three-electrode system, Zn2+

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ions will get electrons rapidly and were reduced to pure zinc accompanied by discharge. In addition, due to differences in discharge induction, a changeable

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thickness of metal–ion deficient layer (MIDL) formed around the tin substrate and the local environment changed due to the existence of electric field, and the coating surface was roughened at the same time. Results show that the surfaceness and interface energy as well as the superhydrophobic stability can influence the spontaneous motion of the water droplets. And the nanostructures gave rise to less

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pinning of liquid droplets based on the advantage of narrow enough spacing, higher perpendicularity and lower surface-energy, contributing to the enhanced drop mobility and low RA. At the same time, free H+ ions around the cathode can get electrons and

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formed H2. Meanwhile, generated Zn near the anode can react with O2 and H2O to produce Zn(OH)2. It is noteworthy that generation of H2 stirred the electrolyte and promoted the reactions in the vicinities of tin substrate, which contributed to the

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formation of the micro/nanostructure on the cathodic surface. Besides, ultrasonic treatment was an important step on the way to achieving superhydrophobicity because it can increase the deposition current by accelerating mass transfer rate of deposited metal ions. The preparation process is considered to contain the following reaction equations: CH3COO¯ ～+ H+ = CH3COOH

(1)

Zn2+ + 2e¯ = Zn

(2)

2H+ + 2e¯ = H2

(3)

ACCEPTED MANUSCRIPT 4Zn + O2 + 2H2O–4 e¯ = 4Zn(OH)2

(4)

2Zn + O2 = 2ZnO

(5)

Zn(OH)2 = ZnO + H2O

(6)

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6. Conclusion In summary, superhydrophobic Zn/ZnO surfaces with irregular polyhedron structure on tin substrate can be prepared via simple electrodeposition method combined with anneal. Investigations into the wettability of the Zn/ZnO surface, a

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water contact angle of 160° and rolling angle about 1° were obtained, exhibiting excellent rolling-off and self-cleaning properties. The resultant Zn/ZnO surfaces were

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able to sustain their superhydrophobicity after exposure to the air and buried in soil for one year and they also exhibit strong superhydrophobic stability under harsh mechanical

bending

and

water

jet

impacting.

UV-driven

superhydrophobic-superhydrophilic conversion were also observed for the Zn/ZnO coating. It took 2.5 h for the surface to convert from superhydrophobicity to

heating

at

given

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superhydrophilicity through UV irradiation and could be reversibly switched by temperatures.

Therefore,

the

convenient

and

efficient

electrodeposition method to obtain superhydrophobic Zn/ZnO surfaces afford

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researchers an effective strategy to fabricate versatile superhydrophobic surfaces on

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various conductive engineering materials, exhibiting prospective applications.

Acknowledgements

We gratefully acknowledge the National Natural Science Foundation of China (No. 21271027) for the support of this work. Reference [1] F. Xia, L. Jiang, Bio-Inspired, Smart, Multiscale Interfacial Materials, Adv. Mater., 20 (2008) 2842-2858. [2] P. Roach, N.J. Shirtcliffe, M.I. Newton, Progess in superhydrophobic surface development, Soft Matter, 4 (2008) 224-240. [3] C. Neinhuis, W. Barthlott, Characterization and distribution of water-repellent, self-cleaning plant surfaces, Ann. Bot., 79 (1997) 667-677.

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Hybrid

Organic/Inorganic

Surfaces

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Micro-/Nanoscale Roughness, Adv. Funct. Mater., 19 (2009) 1149-1157. [26] Y. Liu, W. Yao, G. Wang, Y. Wang, A.S. Moita, Z. Han, L. Ren, Reversibly switchable wettability on

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aluminum alloy substrate corresponding to different pH droplet and its corrosion resistance, Chem. Eng. J., 303 (2016) 565-574.

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nanostructures: new simple fabrication and investigation of their photocatalytic properties, J. Mater. [29] X. Feng, L. Feng, M. Jin, J. Zhai, L. Jiang, D. Zhu, Reversible super-hydrophobicity to

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super-hydrophilicity transition of aligned ZnO nanorod films, J. Am. Chem. Soc., 126 (2004) 62-63. [30] L. Yan, J. Li, W. Li, F. Zha, H. Feng, D. Hu, A photo-induced ZnO coated mesh for on-demand oil/water separation based on switchable wettability, Mater. Lett., 163 (2016) 247-249. [31] F. Beshkar, O. Amiri, Z. Salehi, Synthesis of ZnSnO3 nanostructures by using novel gelling agents and their application in degradation of textile dye, Sep. Purif. Technol., 184 (2017) 66-71. [32] H. Najafian, F. Manteghi, F. Beshkar, M. Salavati-Niasari, Efficient degradation of azo dye pollutants on ZnBi 38 O 58 nanostructures under visible-light irradiation, Sep. Purif. Technol., 195

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of phase transformation, CrystEngComm, 13 (2011) 6024-6026. [36] J. Zhang, Z. Ma, W. Jiang, Y. Zou, Y. Wang, C. Lu, Sandwich-like CNTs@SnO2/SnO/Sn anodes on three-dimensional Ni foam substrate for lithium ion batteries, J. Electroanal. Chem., 767 (2016) 49-55. [37] R. Kumar, N. Kushwaha, J. Mittal, Superior, rapid and reversible sensing activity of graphene-SnO hybrid film for low concentration of ammonia at room temperature, Sensors and Actuators B: Chemical, 244 (2017) 243-251. [38] G. He, S. Lu, W. Xu, S. Szunerits, R. Boukherroub, H. Zhang, Controllable growth of durable superhydrophobic coatings on a copper substrate via electrodeposition, Phys. Chem. Chem. Phys., 17 (2015) 10871-10880. [39] J.H. Kim, K.M. Jeon, J.-S. Park, Y.C. Kang, Excellent Li-ion storage performances of hierarchical SnO-SnO 2 composite powders and SnO nanoplates prepared by one-pot spray pyrolysis, J. Power Sources, 359 (2017) 363-370. [40] Z.-W. Wu, S.-L. Tyan, H.-H. Chen, J.-C.-A. Huang, Y.-C. Huang, C.-R. Lee, T.-S. Mo,

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[43] T. Zhang, L.Y.L. Wu, Z. Wang, Smart UV/Visible light responsive polymer surface switching

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reversibly between superhydrophobic and superhydrophilic, Surf. Coat. Technol., 320 (2017) 304-310.

b

c

d

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Fig. 1 SEM images of surface morphology and of samples prepared in different conditions: (a) untreated tin substrate; (b) Deposited at -1.6 V for 10 min in 0.005 mol/L Zn2+ without anneal; (c) and (d) High-magnified and low-magnified SEM images of the deposited coating after anneal, respectively.

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a～

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Fig. 2 EDS spectra of different surfaces: (a) sample 1, (b) sample 2 and (c) sample 3.

Content

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Table 2 Chemical composition of sample 1, sample 2 and sample 3. Element

O

Zn

at. %

82.13

17.87

----

at. % at. %

12.46 67.40

9.28 19.21

78.26 13.39

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Sample Sample 1 Sample 2

Sn

Sample 3

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♠ Sn ♦ Zn ♥ ZnO ∗ Zn(OH)2

♠

♦ ♥

♠

♠

♦ ♦

30

♠ 40

∗ ♠

♠♠ 50

60

∇

♦ ♦♠

♦ ♦ B ∇ ♠

♠ 70

80

♠

♠

∇ SnO2

♦

♠

♠

A

90

2θ/°

♥

♠

♦

∇

♠

C

♠ 30

♠ 40

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Intensity(a.u.)

♠

♠ Sn ♦ Zn ♥ ZnO ∇ SnO2

(b)

Intensity(a.u.)

(a)

♠ ∇ ♠A

♠♠

50

60

70

80

90

2θ/°

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Fig. 3 (a) XRD patterns of sample 1 (A) and sample 2 (B). (b) XRD patterns of sample 1 (A) and sample 3 (C).

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0

C1s

Zn3d

Zn3p Zn3s

C/S

Sn3d5/2

O1s

Zn2p3/2

a

Zn2p1/2

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200

400

600

800

1000

1200

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Binding energy/eV

b

Sn 3d

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SnO2

3d3/2

Sn

3d3/2

C/S

SnO2 3d5/2

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Sn 3d5/2

480

485

490

495

B A

500

Binding energy/eV

c

SnO2

ZnO(OH)

ZnO

SnO2

ZnO(OH)

ZnO

C/S

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O1s

526

528

530

532

534

536

Binding energy/eV

526

528

530

532

534

536

Binding energy/eV

A B 515

520

525

530

535

Binding energy/eV

540

545

550

ACCEPTED MANUSCRIPT

d

Zn2p

Zn2p3/2

Zn2p

Zn2p1/2

Zn ZnO Zn

C/S

ZnO

1020

1030

1040

1050

B A

1010

1020

1030

1040

1050

1060

1070

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Binding energy/eV

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Binding energy/eV

Fig. 4 XPS spectra of the sample 3 before (A) and after (B) anneal at 200 °C for 10 min: (a)

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fitted curves of O1s and Zn2p.

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survey, (b) Sn3d, (c) O1s and (d) Zn2p high resolution spectra. The insets of Fig.4c and 4d are the

ACCEPTED MANUSCRIPT

0.005 40

a B

-0.010

-0.025

A

Angstrom/µ m

i/A

-0.020

-0.030

A

20

A:sample 3 B:sample 5

-0.015

b

30

-0.005

A:sample 3 B:sample 5

10

0

-10

-0.035

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0.000

B

-20

-0.040 -0.045

-30 0

100

200

300

400

500

600

0

500

1000

1500

2000

2500

3000

Lateral/µm

Time/sec

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Fig. 5 (a) The deposition current curves of sample 3 (A) and sample 5 (B); (b) Steps of zinc-coated film on tin substrate: (A) sample 3; (B) sample 5.

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Fig. 6 Typical SEM images, WCAs and corresponding EDS patterns of sample 4 (a and b) and sample 5 (c and d).

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Fig. 7 The EDS mapping of sample 3 (a) and sample 5 (b) showing the distribution of Sn, O and Zn elements.

b

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Fig. 8 (a) Photograph of water droplets on the bendable SHP Zn/ZnO surface; (h) Photograph of water droplets on the SHP Zn/ZnO surface after bending and released back to the original position at side view; (c) and (d): Optical images of water jets impact on the tin substrate and SHP Zn/ZnO surface, respectively.

ACCEPTED MANUSCRIPT a2

a3

a4

a5～

b1

b2

b3

b4

b5 ～

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a1

Fig. 9 (a1-a5) Time lapsed photographs of a water droplet rolling off the SHP Zn/ZnO sample with

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sliding angle of 1°. (b1-b5) Soil loaded SHP Zn/ZnO surface was cleaned by rolling water droplets.

ACCEPTED MANUSCRIPT

160

a

160

120

Contact angle/°

120

Contact angle/°

b

140

140

100 80 60 40

100

20

120°C 140°C 160°C 180°C

80 60 40 20

0

0 0.0

0.5

1.0

1.5

2.0

2.5

0.0

0.5

1.0

UV time/h

c

60 40 20 0 0

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80

Heating

100

2.0

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140 120

1.5

Heating time/h

180 160

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180

2

3

4

5

6

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Fig. 10 (a) Changes in WCAs with UV irradiation time for the as-prepared Zn/ZnO surface. (b) Variations in WCAs of the Zn/ZnO surfaces with heating time at specified temperatures; (c) Reversible wettability switching by cyclic alternation of UV irradiation and heating.

ACCEPTED MANUSCRIPT b

a

O1s

b

O1s

SnO

SnO ZnO

ZnO ZnO(OH)

ZnO(OH)

SnO2

528

530

532

534

Binding energy/eV

528

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Fig. 11 SEM images of different sample surfaces: (a) after UV irradiation, (b) after UV irradiation and then heating at 160 °C for 1.5 h.

530

532

534

Binding energy/eV

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Fig. 12 High resolution scan of the O1s peak: (b) after UV irradiation, (c) after UV irradiation and then heating at 160 °C for 1.5 h.

ACCEPTED MANUSCRIPT 1. Superhydrophobic

Zn/ZnO

surfaces

were

grown

on

tin

sheets

via

electrodeposition. 2. The surface showed good wetting stability under different conditions.

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3. The surface exhibited UV-driven reversibly switchable wettability