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Facile fabrication of a low adhesion, stable and superhydrophobic filter paper modified with ZnO microclusters
Applied Surface Science 496 (2019) 143743
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Facile fabrication of a low adhesion, stable and superhydrophobic filter paper modified with ZnO microclusters Yanfen Wanga,b, Yin Liua, Lei Zhanga, Miao Zhangb, Gang Heb, Zhaoqi Sunb, a b
T
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School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, PR China School of Physics & Materials Science, Anhui University, Hefei, 230601, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO Superhydrophobic Paper Hierarchical structure Surface Water contact angle
A robust superhydrophobic ZnO coating on a filter paper substrate was successfully prepared for the first time by using a successive deposition process and chemical modification. The surface morphology, chemical composition, wetting property, mechanical stability, chemical stability and durability of the filter papers were examined. The as-prepared papers exhibited an improved rough surface topography compared with the uncoated paper due to the uniform ZnO microclusters decorated on the reticular fibers. More importantly, it was found that the wettability of papers was dependent on the surface roughness, which was governed by the number of ZnO microcluster deposition cycles. After the chemical modification with stearic acid, the resulting paper with 4 cycles presented the optimized superhydrophobicity with a water contact angle (WCA) of 158°, a slide angle (SA) of 3°, a low adhesion and good bounce performance. The chemical resistance of the superhydrophobic paper to corrosive liquids was studied. Moreover, the superhydrophobicity of paper was not affected after the water drops impact test or it was kept for 6 months, indicating its high impact resistance and long-term stability. The excellent superhydrophobicity of paper can be attributed to the synergistic effect of the suitable micro/ nanoscale hierarchical roughness and the low surface energy modification.
1. Introduction Filter papers, one of the most common separation materials, are used in various kinds of applications in our daily life because of their advanced technology, low cost, ready availability and environmental friendliness [1–4]. Cellulose is the main component of filter paper and is a structural polysaccharide with many hydroxyl groups, which endow filter papers with hydrophilic properties. However, this property allows it to be wetted and contaminated by liquids such as water, which is undesirable in their use as waterproof materials [5]. Therefore, great efforts have been made in the modification of papers to enhance their self-cleaning and stain-repellent properties. It has been demonstrated that the introduction of superhydrophobicity is a well-established technique for the surface of paper [6]. Superhydrophobic surfaces with a high water contact angle (WCA, > 150°) and a low slide angle (SA, < 10°) can cause water drops to roll off the surface, which can carry away dirt and realize the selfcleaning property. Thus, superhydrophobic surfaces have wide applications in many fields, such as water-repellent surfaces, the suppression of surface oxidation and the reduction of the resistance of underwater vessels [7–9]. According to previous studies, both low surface energy
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and high surface roughness with micro/nanoscale structures are needed for superhydrophobic surfaces. To date, a variety of routes have been developed to fabricate superhydrophobic surfaces [10–15], such as selfassembly processes, template synthesis, chemical etching, sol-gel methods, and electrospinning techniques. Moreover, many studies have referred to superhydrophobic coatings on different substrates, including glass materials [16], aluminum alloys [17], copper materials [18], silicon wafers [19], steel materials [20] and polymers [21]. However, studies of the superhydrophobic coating on a paper substrate are limited because the paper is easily decomposed and damaged by physical, chemical, or thermal treatments. Few approaches, such as multilayer depositions [22], plasma treatments [23], and spraying alcohol suspensions [24], have been attempted to impart superhydrophobicity to paper. For example, Deng et al. prepared superhydrophobic papers using SiO2 or CaCO3 nanoparticles modified by chemical treatment [25]. Zhang et al. used an aerogel microsphere coating and methyltrimethoxysilane (MTMS) modification to obtain superhydrophobic filter paper with selective oil-water separation [26]. Li and coworkers reported a food-safe superhydrophobic cellulose paper achieved by the formation of polymer-nanoclay hybrid multilayers and carnauba wax modification [27]. Furthermore, other inorganic nanoparticles have
Corresponding author. E-mail address: [email protected] (Z. Sun).
https://doi.org/10.1016/j.apsusc.2019.143743 Received 12 June 2019; Received in revised form 10 August 2019; Accepted 20 August 2019 Available online 20 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
Applied Surface Science 496 (2019) 143743
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Scheme 1. Formation illustration of the superhydrophobic paper.
also been reported to create a rough surface structure for superhydrophobic paper [28–32], such as TiO2, ZnO, CuO, and WO3. However, most methods often involve strict conditions, costly materials or complicated procedures. In addition, the disadvantages of superhydrophobic papers, such as high adhesion, weak mechanical stability and poor durability, have limited their practical applications. In this work, we demonstrated a simple, nontoxic and cost-effective procedure that was able to improve the water repellency and stability of the filter paper substrate. We first fabricated uniform zinc oxide (ZnO) microclusters by a simple precipitation method at low temperatures. A successive deposition process of the ZnO microcluster emulsion was used to create a suitable rough structure on the reticular fibers. After a subsequent modification step with stearic acid, the surface morphology, chemical composition and wettability of the papers were characterized. Importantly, the wettability of the paper was closely related to the chemical modification and the surface roughness, which was governed by the number of deposition cycles of ZnO microclusters. Additionally, the mechanical stability was studied by water drop impact dynamics on the superhydrophobic paper. Moreover, the chemical resistance to corrosive liquids and durability was also examined. The results showed that the resulting papers possess remarkable superhydrophobicity, low adhesion, high impact resistance and good stability.
deionized water under stirring. The mixed solution was sealed and kept static at 80 °C for 12 h. The resulting precipitate was separated by centrifuge, washed with deionized water and absolute ethanol for several times, and dried at 70 °C in an oven for 8 h.
2. Experimental
The X-ray diffraction (XRD) was performed on a Japan Shimadzu XRD-6000 equipped with CuKα radiation (λ = 1.54178 Å). Scanning electron microscopy (SEM) images were carried out on a JEOL JSM6700F. Transmission electron microscopy (TEM) images were recorded on a JEM-2100 instrument. Atomic force microscopy (AFM) was recorded on an Agilent-5500. Fourier transform infrared (FTIR) spectroscopy was performed with a Nicolet-380 FTIR spectrometer.
2.3. Preparation of superhydrophobic papers ZnO microclusters were deposited on the paper through a successive deposition method. Firstly, dry ZnO powder (0.1 g) was dissolved in 30 mL of absolute ethanol and then vigorously stirred at room temperature for 30 min, forming a homogeneous emulsion. Then, the treated filter paper was immersed in the above solution for 2 min and dried in an oven for 5 min at 100 °C, which was termed one cycle. After several cycles of the process, the as-prepared ZnO/paper was immersed in a 0.1 M stearic acid solution for 30 min, inducing the formation of a thin layer of water-insoluble stearate on the ZnO surface. After the chemical modification, the resulting papers were placed horizontally on an oven and dried at 60 °C for 5 h. To prepare the papers with controllable wettability, the deposition process of ZnO microclusters on papers was controlled to be 0, 2, 4 and 6 cycles, respectively. 2.4. Characterization
2.1. Materials The common filter papers in this work were cut to form the paper substrates with different size. The reagent-grade chemicals used include sodium hydroxide (NaOH), zinc chloride (ZnCl2), stearic acid and absolute ethanol.
2.5. Wetting test 2.2. Synthesis of ZnO microclusters The WCA values of the papers were measured by a C20 contact angle meter (Kono, USA) using a droplet (approximately 5 μL) of deionized water as an indicator. The SA values of the papers were evaluated by measuring the tilt values at which the water drops fall
The preparation procedure of ZnO microclusters was similar to that described in our previous study [33]. NaOH (0.4000 g) and ZnCl2 (0.2726 g) were dissolved in glass bottle (50 mL) with 30 mL of 2
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grown in a bundle, and they are properly arranged to form many flowerlike microclusters. Further, these clusters are uniform in size, and they present a typical diameter of 1–2 μm. Fig. 1(d) shows the size distribution histogram of the ZnO products. The mean size of the microclusters is 1.3 μm. Moreover, a typical TEM image of a flowerlike ZnO microcluster is shown in Fig. 1(e). It confirms a three-dimensional layered structure constructed by numerous nanosheets, and the diameter of the ZnO microcluster is approximately 1.3 μm. The HRTEM image in Fig. 1(f) shows the two kinds of lattice fringes. The lattice spacings of 0.28 nm and 0.52 nm correspond to the {0001} and {0110} planes of a wurtzite ZnO crystal, respectively. These findings suggest that the ZnO nanosheet has a good crystalline structure. The surface morphologies of the papers before and after the modification of ZnO microclusters are characterized by SEM. Fig. 2(a) and (b) show the SEM images of the pristine paper. The filter paper is composed of smooth cellulose fibers, which tightly weave together to form a neat fiber network. Fig. 2(c–f) shows the surface morphological transformation of the ZnO/papers with the increase of deposition cycles. Compared to the surface of the pristine paper, the tangled fibers are almost covered by the ordered and uniform ZnO microclusters. With an increase in the number of deposition cycles, the surface of the fibers becomes gradually rougher. After 2 deposition cycles, most of the fibers are decorated with ZnO microclusters, and a small number of the outer residuary fibers are still bare. After 4 cycles, the deposition amount of ZnO microclusters significantly increases, with a complete coating and uniform dispersion on the fiber surface. Interestingly, such a distinctive structure creates a fascinating micro/nanoscale hierarchical roughness, which is beneficial for obtaining remarkable superhydrophobicity. When increased to 6 cycles, more ZnO microclusters accumulate on the fiber surface, resulting in the formation of large agglomerates. In addition, the cross-sectional SEM images of ZnO/papers with different deposition cycles are shown in Fig. 3(a)–(c). Because the paper samples are prepared by the clipping of scissor, the partial fibers at the section
from a sample platform. The mechanical properties of the superhydrophobic paper were characterized by continuously impacting water drops onto the paper. The long-term stability was carried out by storing them in an ambient environment for 1–6 months. 3. Results and discussions 3.1. Microstructure analysis and chemical characterization In this work, superhydrophobic papers were fabricated by a successive deposition procedure, as shown in Scheme 1. The uniform flowerlike ZnO microclusters were first obtained by a simple precipitation method at low temperatures. Then, the filter paper was successively immersed into an alcohol emulsion of ZnO microclusters, which can increase the roughness of the paper surface and form a multilayer hierarchical composition. As a low-surface-energy reagent, stearic acid is an ideal candidate for obtaining water repellency because of its biocompatibility, nontoxicity and low cost. When the paper decorated by ZnO microclusters was dipped in a stearic acid solution for 30 min, the carboxyl functional groups (-COOH) in stearic acid reacted with the abundant hydroxyl groups (-OH) on the ZnO surface [34,35]. A self-assembled monolayer of water-insoluble stearate was formed on the paper surface with the nonpolar tails exposed to the surface. This strategy effectively decreased the surface free energy of the paper, leading to dramatically enhanced water repellency. Fig. 1(a) shows the XRD pattern of the ZnO microclusters, revealing the well-crystalline nature of the samples because of the sharp and strong diffraction peaks. All of the diffraction peaks can be perfectly assigned to hexagonal wurtzite ZnO, and the peaks are consistent with the values in the standard card (JCPDS no. 36-1451). The morphology of the sample is characterized by SEM, TEM and HRTEM images. From the SEM images with different magnifications in Fig. 1(b) and 1(c), it can be observed that ZnO products are composed of many nanosheets
Fig. 1. (a) XRD pattern and (b, c) SEM images of ZnO microclusters, (d) size distribution histogram of ZnO microclusters, (e) TEM image of a flowerlike ZnO microcluster, and (f) the corresponding HRTEM image. 3
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Fig. 2. SEM images of (a) pristine paper, ZnO/papers with (b, c) 2 cycles, (d) 4 cycles and (e) 6 cycles.
Fig. 3. Cross-sectional SEM images of ZnO/papers with (a) 2 cycles, (b) 4 cycles and (c) 6 cycles. 4
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Fig. 4. Phase (the top)/3D (the bottom)-AFM morphological images of (a) pristine paper and ZnO/papers: (b) 2 cycles, (c) 4 cycles, and (d) 6 cycles.
In addition to a suitable surface roughness, low surface free energy is also essential for obtaining a superhydrophobic surface. In the present case, a reasonable amount of ZnO microclusters on the tangled fibers can provide a micro/nanoscale hierarchical structure. To further decrease the surface energy, stearic acid as a surfactant is used to modify the papers deposited with ZnO microclusters. Fig. 5 shows SEM images of the ZnO/Paper surface decorated with stearic acid. From the panoramic image in Fig. 5(a), we can observe that the ZnO microclusters on the fibers are modified by a layer of stearic acid, and the uniform dense ZnO coating on the fiber surface is unchanged. It indicates that the stearic acid/ZnO microclusters composite structure has been constructed on the filter paper substrate. The magnified SEM image in Fig. 5(b) reveals that the stearic acid layer is composed of a large number of thick sheets with the size of 2–4 μm, and these sheets are randomly distributed on the ZnO surfaces. Such a structure helps to increase surface roughness, and greatly reduces the surface energy. This morphology of stearic acid molecules deposited on the ZnO is consistent with that reported in the literature [35]. The change in the chemical composition of the coatings scraped from the prepared papers is characterized by FTIR spectra, as shown in Fig. 5(c). In comparison with the ZnO coating, the stearic acid/ZnO coating not only shows the intrinsic peaks of ZnO microclusters but also exhibits other specific peaks. The peaks appearing at 2918 cm−1 and 2848 cm−1 can be attributed to the stretching vibrations of –CH3 and –CH2, respectively. The peaks at 1528 cm−1 and 1442 cm−1 belong to
are broken and deformed. As further evidence, all of SEM images show a layer of ZnO microcluters on the top and in the gaps between the fibers. The average thickness of three ZnO/papers is 95 μm. The thickness of ZnO coating on the top gradually increases with the increase of deposition cycles. After 6 deposition cycles, the highest thickness of ZnO coating is approximately 3.5 μm. The thin ZnO coating is help to extend the practical application of the paper. The roughness is a key factor in the preparation of superhydrophobic surfaces [36–38]. The surface roughness of the as-prepared papers is measured by the atomic force micrograph (AFM). Fig. 4 shows the phase images and three-dimensional (3D) AFM morphological images of the papers before and after the modification of ZnO microclusters. As shown in Fig. 4(a), the pristine filter paper exhibits a relatively smooth and flat surface topography, with a resultant rootmean-square (RMS) roughness value of 1.45 nm. Compared with bare paper, the modification of ZnO microclusters leads to a significant increase in the surface roughness. As the deposition number increases to 4 cycles, the RMS roughness reaches a maximum value with 31 nm. However, when the deposition number reaches to 6 cycles, the surface RMS roughness of ZnO/paper becomes smaller. For the hydrophobic surface, the greater the roughness is, the greater the contact angle is. It means that the high surface roughness is desirable for the excellent wettability of the surface. Therefore, the proper ZnO microclusters on the fibers create an ideal micro/nanoscale hierarchical structure, which significantly increases the surface roughness of the filter paper.
Fig. 5. (a, b) SEM images of the stearic acid/ZnO(4)/paper, (c) FTIR spectra of ZnO and stearic acid/ZnO coatings scraped from the prepared papers. 5
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Fig. 6. Static photos of a water droplet on the papers: (a) pristine paper, (b) ZnO/paper, stearic acid/ZnO/papers deposited with (c) 0 cycles, (d) 2 cycles, (e) 4 cycles, and (f) 6 cycles.
the stretching vibrations of –COO– in CH3(CH2)16COO– groups, which indicates the formation of new bonds between the CH3(CH2)16COO– groups of stearic acid and the –OH groups tethered on the ZnO surface [39]. These findings prove that stearic acid has been precisely grafted onto the surface of ZnO/paper.
Table 1 Static WCA values and SA values of the different papers. Samples
WCA
SA
Pristine paper ZnO/paper Stearic acid/paper Stearic acid/ZnO(2)/paper Stearic acid/ZnO(4)/paper Stearic acid/ZnO(6)/paper
0 38° 119° 144° 158° 152°
> 90° 10° 5° 3° 3°
3.2. Wettability behavior The wettability of the as-prepared stearic acid/ZnO/papers is investigated by the static WCA on the surface. As shown in Fig. 6(a), the WCA of pristine paper is almost 0°, showing a very high hydrophilic property. This property can be attributed to cellulose being a structural polysaccharide with many hydroxyl groups. After the deposition of ZnO
Fig. 7. Optical images of static water droplets on (a) pristine paper and (b, c) superhydrophobic paper with 4 cycles. 6
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Fig. 8. (a) Schematic and (b) sequential optical images of the bounce process of a dynamic water drop on the superhydrophobic paper with 4 cycles.
Fig. 9. (a) Evolution of WCA values of the superhydrophobic paper for various stearic acid concentrations, and (b) the reaction mechanism for ZnO/paper modified by stearic acid.
the dynamic behavior of the water droplet on the paper surface, the water SA of the different papers is presented in Table 1. For the superhydrophobic paper with 4 cycles, the SA value is measured to be 3°. The result indicates that the paper has a small water contact hysteresis and low adhesion. Fig. 7 shows the optical images of static water droplets and dynamic water droplets on the different papers. When a water drop contacts the pristine paper (Fig. 7a), it instantaneously spreads on the surface, finally leaving a large water spot on the paper. However, when a water drop contacts the superhydrophobic paper (Fig. 7b), it does not spread out but forms a drying bead that rests on the paper. If the paper is slightly tilted (Fig. 7c), the water droplet quickly rolls down the paper, indicating its small contact hysteresis. Furthermore, Fig. 8(a) shows that a water drop free falling from a 6 cm height, impinging to the superhydrophobic paper with a velocity of 0.84 m/s. The bounce process of the dynamic water drop is shown in Fig. 8(b). It can be observed that the water drop does not spread out on the paper but rebounds three before it undergoes damped oscillations. Finally, the drop rests on the surface of the superhydrophobic paper. The result indicates that the superhydrophobic paper has good bounce performance. Thus, superhydrophobic paper with low adhesion and good bounce performance is expected to effectively remove contamination on the surface similar to the self-cleaning property of a lotus leaf. In addition to the surface morphology, the chemical modification of stearic acid has an important effect on the wettability of the paper. Fig. 9(a) shows the effect of stearic acid concentration on the WCA of the papers. It can be observed that the WCA of the paper becomes larger with increasing concentrations of stearic acid. The wettability of the paper changes from hydrophilic to superhydrophobic. When the concentration is higher than 0.1 M, the WCA of the paper is almost unchanged. This result is because stearic acid with too low of a concentration cannot adequately react with the groups in the ZnO/paper, leading to inadequate protection and weak hydrophobicity. Hence, the creation of a micro/nanoscale hierarchical roughness and the
microclusters (Fig. 6b), the WCA of the resulting paper is 38°, which may be due to the hydrophilic nature of ZnO microclusters. However, the WCA on the paper surface is dramatically changed after the chemical modification with stearic acid. Fig. 6(c-f) shows the WCA images of the ZnO/papers modified by stearic acid. WCA values of 119°, 144°, 158° and 152° are obtained for the stearic acid/paper and the stearic acid/ZnO papers with 2, 4 and 6 cycles, respectively. The WCA value gradually increases with increasing cycles of ZnO microclusters, indicating that the wettability of the paper is closely related to the number of deposition cycles. After 4 deposition cycles, the stearic acid/ ZnO paper presents the best superhydrophobic property with a WCA as high as 158°. This result may be due to the ZnO microclusters decorating the tangled fibers and creating a reasonable micro/nanoscale hierarchical roughness, leading to more interspaces or cavities present on the surface. This structure can effectively capture a large amount of air into the coating, and the air/water interface greatly increases. Because the contact angle of the air-water is 180°, this structure can effectively prevent the penetration of water droplets into the grooves, leading to the water droplet being suspended on the rough surface. It has been noted that the greater roughness of the hydrophobic surface can cause a larger surface contact angle. As seen from the above SEM, a rougher surface is formed on the paper with an increase in deposition cycles. Combined with the chemical modification with stearic acid, the paper successfully achieves superhydrophobicity. Therefore, the remarkable superhydrophobic behavior can be attributed to the synergistic effect of the hierarchical micro/nanoscale roughness and the low surface energy composition. However, after 6 deposition cycles, the formation of ZnO agglomerates may reduce the surface roughness, resulting in a decrease in the WCA of the paper. The static contact angle cannot fully demonstrate the wetting behavior of water droplets on the paper surface. The difficulty displayed by the water droplet as it moves over the surface should be taken into account when the assessing the hydrophobic effect of films. To describe 7
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Fig. 10. (a–i) Sequential optical images of water drops impacted continuously onto the superhydrophobic paper with 4 cycles, and (j) the schematic of the impact process.
superhydrophobic paper is not destroyed due to its good impact resistance to water droplets. The as-prepared paper with 4 cycles exhibits excellent superhydrophobic properties not only for pure water but also for corrosive water, including under alkaline and acidic conditions. Fig. 11 shows the static WCAs measured under different pH values. The paper remains superhydrophobic when the pH is varied from 2 to 12. When the pH is < 2 or > 12, a drastic conversion from superhydrophobicity to hydrophilicity can be observed for the paper. This can be due to the zinc stearate monolayer that is formed on the paper surface being destroyed under an extremely corrosive water drop, resulting in weak hydrophobicity [40,41]. Although the superhydrophobicity on the paper surface is unstable under strong acid/alkali conditions, it can maintain good chemical stability for ordinary corrosive water.
appropriate modification with stearic acid play key roles in superhydrophobic paper. The reaction mechanism for ZnO/paper modified by stearic acid is shown in Fig. 9(b). 3.3. Stability study The mechanical property of the superhydrophobic paper is evaluated by a water droplet impact test to extend the outdoor applications. As shown in Fig. 10(a–i), the water droplets from a pipet are dropped on the slightly tilted superhydrophobic paper. Clearly, the water droplets do not penetrate into the grooves but constantly roll off the paper. After the drop test, the paper is spotless and retains its good superhydrophobicity. The scheme of the whole process is shown in Fig. 10(j). This finding suggests that the rough surface structure of the 8
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the paper are 153° and 3°, respectively, as shown in Fig. 12(b, c). The water droplet roll-off can also remove some dust and dirt deposited over time on the paper surface. Moreover, the optical images of the colored water drops on the superhydrophobic paper are shown in Fig. 12(d). These results prove that the good impact resistance, long-term stability and chemical stability of the superhydrophobic paper are beneficial for potential outdoor applications. 3.4. Superhydrophobic mechanism This superhydrophobic phenomenon can be explained by the CassieBaxter theory [40], as shown in Scheme 2. When the water droplet comes into contact with the surface of the modified paper, the rough structure of the surface captures a large amount of air, preventing the water droplet from penetrating into the paper. Therefore, the water droplet is actually in contact with a composite surface composed of stearic acid/ZnO hierarchical structure and air. The WCA of the composite surface can be expressed by the Cassie-Baxter equation: Fig. 11. Static WCA values measured on the condition of different pH values.
cos θr = f1 cos θ1 − f2 In addition, the durability of a superhydrophobic material is crucial in outdoor applications. Therefore, the WCA and SA of the superhydrophobic paper with 4 cycles are measured again after being stored for 1–6 months in an ambient environment. As shown in Fig. 12(a), the wettability of the superhydrophobic paper has no significant change during this period. After it was stored for 6 months, the WCA and SA of
(1)
where θγ and θ1 are the apparent WCA of the rough surface and smooth surface with the same composition, respectively; f1 is the unit for the apparent area fraction of the contact area between the water droplet and film; and f2 is the unit for the apparent area fraction of the contact area between the water droplet and air (f1+f2=1). Based on the above formula, it is found that the WCA (θγ) of the
Fig. 12. (a) WCA values of the superhydrophobic paper after placing it for 1–6 months, (b) static and (c) dynamic images of a water droplet on the superhydrophobic paper placed for 6 months, (d) an optical image of colored water drops on the paper. 9
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Scheme 2. Scheme illustration of the superhydrophobic paper.
rough surface is positively correlated with the proportion (f2) of air on the surface. The larger f2 is, the larger θγ is, and the stronger the hydrophobic property is. In our reaction system, when the paper modified with ZnO microclusters is immersed in the ethanol solution of stearic acid, the -COOH groups in the stearic acid can react with a large number of -OH groups on the surface of the ZnO microclusters, forming a self-assembled monolayer of zinc stearate [42,43]. On the one hand, the reasonable micro/nanoscale hierarchical roughness constructed by ZnO microclusters provides a structural groundwork for superhydrophobic properties. On the other hand, the hydrophobic tails of the self-assembled zinc stearate monolayer are oriented perpendicular to the surface, which effectively decreases the surface free energy of the paper. This structure enables the paper surface to easily absorb and deposit air and prevents water droplets from penetrating into the pores, thus inducing an increased f2 and a decreased θr. According to the Cassie-Baxter equation, the surface of the paper finally exhibits a remarkable superhydrophobic performance. Therefore, the excellent superhydrophobicity of paper can be attributed to the synergistic effect of the micro/nanoscale hierarchical roughness of a reasonable amount of ZnO microclusters and the low surface energy modification with stearic acid.
Soc. A 374 (2016) 1–40. [2] N.M. Oliveira, S. Vilabril, M.B. Oliveira, R.L. Reis, J.F. Mano, Recent advances on open fluidic systems for biomedical applications: a review, Mat. Sci. Eng. C 97 (2019) 851–863. [3] K.R. Xu, X.P. Wang, R.M. Ford, J.P. Landeres, Self-partitioned droplet array on laser-patterned superhydrophilic glass surface for wall-less cell arrays, Anal. Chem. 88 (2016) 2652–2658. [4] Q.Z. Wang, Y.N. Liu, Y.W. Bai, S.Y. Yao, Z.J. Wei, M. Zhang, Superhydrophobic SERS substrates based on silver dendrite-decorated filter paper for trace detection of nitenpyram, Anal. Chim. Acta 1049 (2019) 170–178. [5] P.E. Oomen, J. Mulder, E. Verpoorte, R.D. Oleschuk, Controlled, synchronized actuation of microdroplets by gravity in a superhydrophobic, 3D-printed device, Anal. Chim. Acta 988 (2017) 50–57. [6] M. Zhang, C.Y. Wang, S.L. Wang, Y.L. Shi, J. Li, Fabrication of coral-like superhydrophobic coating on filter paper for water-oil separation, Appl. Surf. Sci. 261 (2012) 764–769. [7] X.X. Huang, X.F. Wen, J. Cheng, Z.R. Yang, Sticky superhydrophobic filter paper developed by dip-coating of fluorinated waterborne epoxy emulsion, Appl. Surf. Sci. 258 (2012) 8739–8746. [8] F. Zhang, H. Ren, L.L. Shen, G.L. Tong, Y.L. Deng, Micro-nano structural engineering of filter paper surface for high selective oil-water separation, Cellulose 24 (2017) 1–12. [9] M. Satapathy, P. Varshney, D. Nanda, A. Panda, S.S. Mohapatra, A. Kumar, Fabrication of superhydrophobic and superoleophilic polymer composite coatings on cellulosic filter paper for oil-water separation, Cellulose 24 (2017) 1–14. [10] X. Deng, L. Mammen, H.J. Butt, D. Vollmer, Candle soot as a template for a transparent robust superamphiphobic coating, Science 335 (2012) 67–70. [11] H.M. Kim, J.W. Choi, J.S. Kwon, C.H. Lee, B. Kim, Super-hydrophobic properties of aluminum surfaces synthesized by a two-step chemical etching process, J. Nanosci. Nanotechno. 19 (2019) 6452–6457. [12] X.G. Zhang, Z.J. Liu, Y. Li, C.J. Wang, Y.J. Zhu, H.Y. Wang, J.T. Wang, Robust superhydrophobic epoxy composite coating prepared by dual interfacial enhancement, Chem. Eng. J. 371 (2019) 276–285. [13] K.L. Wang, X.R. Liu, Y. Tan, W. Zhang, S.F. Zhang, J.Z. Li, Two-dimensional membrane and three-dimensional bulk aerogel materials via top-down wood nanotechnology for multibehavioral and reusable oil/water separation, Chem. Eng. J. 371 (2019) 769–780. [14] Y.F. Zhang, L.Q. Zhang, Z. Xiao, S.L. Wang, X.Q. Yu, A fabrication of robust and repairable superhydrophobic coatings by an immersion method, Chem. Eng. J. 369 (2019) 1–7. [15] S. Wang, X. Yu, Y. Zhang, Large-scale fabrication of translucent, stretchable and durable superhydrophobic composite films, J. Mater. Chem. A 5 (2017) 23489–23496. [16] Y.F. Wang, B.X. Li, T.X. Liu, C.Y. Xu, Z.W. Ge, Controllable fabrication of superhydrophobic TiO2 coating with improved transparency and thermostability, Colloid. Surface. A 441 (2014) 298–305. [17] Y.F. Zhang, T.G. Lin, Influence of duty cycle on properties of the superhydrophobic coating on an anodized magnesium alloy fabricated by pulse electrodeposition, Colloid. Surface. A 568 (2019) 43–50. [18] G. Ren, Y. Song, X. Li, Y. Zhou, Z. Zhang, X. Zhu, A superhydrophobic copper mesh as an advanced platform for oil-water separation, Appl. Surf. Sci. 428 (2018) 520–525. [19] L. Schneider, M. Laustsen, N. Mandsberg, R. Taboryski, The influence of structure heights and opening angles of micro- and nanocones on the macroscopic surface wetting properties, Sci. Rep-Uk 6 (2016) 1–9. [20] T. Movahedi, R. Norouzbeigi, Synthesis of flower-like micro/nano ZnO superhydrophobic surfaces: additive effect optimization via designed experiments, J. Alloy. Compd. 795 (2019) 483–492. [21] L. Mérai, N. Varga, Á. Deák, D. Sebők, I. Szenti, Á. Kukovecz, Z. Kónya, I. Dékány, L. Janovák, Preparation of photocatalytic thin films with composition dependent wetting properties and self-healing ability, Catal. Today 328 (2019) 85–90. [22] L. Guo, W. Yuan, J.P. Li, Z.J. Zhang, Z.M. Xie, Stable superhydrophobic surfaces over a wide pH range, Appl. Surf. Sci. 254 (2008) 2158–2161. [23] M. Satapathy, P. Varshney, D. Nanda, A. Panda, S.S. Mohapatra, A. Kumar, Fabrication of superhydrophobic and superoleophilic polymer composite coatings on cellulosic filter paper for oil–water separation, Cellulose 24 (2017) 4405–4418. [24] K. Tsougeni, A. Tserepi, V. Constantoudis, E. Gogolides, P.S. Petrou, S.E. Kakabakos, Plasma nanotextured PMMA surfaces for protein arrays: increased protein binding and enhanced detection sensitivity, Langmuir 26 (2010) 13883–13891.
4. Conclusions In summary, we developed a simple method to fabricate robust superhydrophobic paper via a successive deposition of ZnO microclusters and a subsequent modification. The wettability of the superhydrophobic paper is dependent on the surface roughness, which is governed by the numbers of deposition cycles of the ZnO microclusters. The paper with 4 cycles displayed the best superhydrophobicity with a high WCA of 158° and low SA of 3° and good bounce performance, which can be attributed to the synergistic effect of the micro/nanoscale hierarchical structure and chemical modification. Importantly, the superhydrophobic paper exhibited a low adhesion, high impact resistance for water drops, good chemical stability and long-term durability. This study described a facile and popular strategy to fabricate bionic superhydrophobic papers with good bounce performance, low adhesion, and good stability, and this strategy is expected to be used for other cellulose fiber substrates to broaden their outdoor application in selfcleaning materials. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 51772003, 51472003, 51701001), Provincial Natural Science Foundation of Anhui Higher Education Institution of China (No. KJ2019A0122, No. KJ2017A002), and Science Research Foundation for Young Teachers (No. QN201403). References [1] P.S. Brown, B. Bhusan, Bioinspired materials for water supply and management: water collection, water purification and separation of water from oil, Phil. Trans. R.
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Y. Wang, et al.
stearic acid modified ZnO nanotowers, Appl. Surf. Sci. 254 (2008) 2690–2695. [35] J. Zhang, Z.H. Liu, J.Q. Liu, L. E, Z.F. Liu, Effects of seed layers on controlling of the morphology of ZnO nanostructures and superhydrophobicity of ZnO nanostructure/ stearic acid composite films, Mater. Chem. Phys. 183 (2016) 306–314. [36] A.J. Patil, Y. Zhao, X. Liu, X. Wang, Durable superhydrophobic and antimicrobial cotton fabrics prepared by electrostatic assembly of polyhexamethylene biguanide and subsequent hydrophobization, Text. Res. J. 88 (2017) 1788–1799. [37] B. Raeesi, N.R. Morrow, G. Mason, Effect of surface roughness on wettability and displacement curvature in tubes of uniform cross-section, Colloid. Surface. A 436 (2013) 392–401. [38] K.S. Liu, X. Yao, L. Jiang, Recent developments in bio-inspired special wettability, Chem. Soc. Rev. 39 (2010) 3240–3255. [39] A.B. Gurav, S.S. Latthe, R.S. Vhatkar, J.G. Lee, D.Y. Kim, J.J. Park, S.S. Yon, Superhydrophobic surface decorated with vertical ZnO nanorods modified by stearic acid, Ceram. Int. 40 (2014) 7151–7160. [40] A. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc 40 (1944) 546–551. [41] E.C. Griffith, E.M. Adams, H.C. Allen, V. Vaida, Hydrophobic collapse of a stearic acid film by adsorbed l-phenylalanine at the air-water interface, J. Phys. Chem. B 116 (2012) 7849–7857. [42] Q.Y. Cheng, X.P. An, Y.D. Li, C.L. Huang, J.B. Zeng, Sustainable and biodegradable superhydrophobic coating from epoxidizedsoybean oil and ZnO nanoparticles on cellulosic substrates for efficient oil/water separation, ACS Sustain. Chem. Eng. 5 (2017) 11440–11450. [43] M. Lee, G. Kwak, K. Yong, Wettability control of ZnO nanoparticles for universal applications, ACS Appl. Mater. Interfaces 3 (2011) 3350–3356.
[25] P. Reokrungruang, I. Chatnuntawech, T. Dharakul, S. Bamrungsap, A simple paperbased surface enhanced Raman scattering (SERS) platform and magnetic separation for cancer screening, Sensor. Actuat. B-Chem. 285 (2019) 462–469. [26] D.W. Li, H.Y. Wang, Y. Liu, D.S. Wei, Z.X. Zhao, Large-scale fabrication of durable and robust super-hydrophobic spray coatings with excellent repairable and anticorrosion performance, Chem. Eng. J. 367 (2019) 169–179. [27] H. Li, Y.Q. He, J. Yang, X. Wang, T.Q. Lan, L.C. Peng, Fabrication of food-safe superhydrophobic cellulose paper with improved moisture and air barrier properties, Carbohyd. Polym. 211 (2019) 22–30. [28] V. Anand Ganesh, S.S. Dinachali, A. Sreekumaran Nair, S. Ramakrishna, Robust superamphiphobic film from electrospun TiO2 nanostructures, ACS Appl. Mater. Interfaces 5 (2013) 1527–1532. [29] X. Zhang, Y. Si, J. Mo, Z. Guo, Robust micro-nanoscale flowerlike ZnO/epoxy resin superhydrophobic coating with rapid healing ability, Chem. Eng. J. 313 (2017) 1152–1159. [30] A. Siddaramanna, N. Saleema, D.K. Sarkar, A versatile cost-effective and one step process to engineer ZnO superhydrophobic surfaces on Al substrate, Appl. Surf. Sci. 311 (2014) 182–188. [31] H. Li, S. Yu, X. Han, Fabrication of CuO hierarchical flower-like structures with biomimetic superamphiphobic, self-cleaning and corrosion resistance properties, Chem. Eng. J. 283 (2016) 1443–1454. [32] N. Shafaei, M. Jahanshahi, M. Peyravi, Q. Najafpour, Self-cleaning behavior of nanocomposite membrane induced by photocatalytic WO3 nanoparticles for landfill leachate treatment, Korean J. Chem. Eng. 33 (2016) 2968–2981. [33] Y.F. Wang, B.X. Li, C.Y. Xu, Fabrication of superhydrophobic surface of hierarchical ZnO thin films by using stearic acid, Superlattice. Microst. 51 (2012) 128–134. [34] N. Saleema, M. Farzaneh, Thermal effect on superhydrophobic performance of
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Full length article
Facile fabrication of a low adhesion, stable and superhydrophobic filter paper modified with ZnO microclusters Yanfen Wanga,b, Yin Liua, Lei Zhanga, Miao Zhangb, Gang Heb, Zhaoqi Sunb, a b
T
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School of Materials Science and Engineering, Anhui University of Science and Technology, Huainan 232001, PR China School of Physics & Materials Science, Anhui University, Hefei, 230601, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: ZnO Superhydrophobic Paper Hierarchical structure Surface Water contact angle
A robust superhydrophobic ZnO coating on a filter paper substrate was successfully prepared for the first time by using a successive deposition process and chemical modification. The surface morphology, chemical composition, wetting property, mechanical stability, chemical stability and durability of the filter papers were examined. The as-prepared papers exhibited an improved rough surface topography compared with the uncoated paper due to the uniform ZnO microclusters decorated on the reticular fibers. More importantly, it was found that the wettability of papers was dependent on the surface roughness, which was governed by the number of ZnO microcluster deposition cycles. After the chemical modification with stearic acid, the resulting paper with 4 cycles presented the optimized superhydrophobicity with a water contact angle (WCA) of 158°, a slide angle (SA) of 3°, a low adhesion and good bounce performance. The chemical resistance of the superhydrophobic paper to corrosive liquids was studied. Moreover, the superhydrophobicity of paper was not affected after the water drops impact test or it was kept for 6 months, indicating its high impact resistance and long-term stability. The excellent superhydrophobicity of paper can be attributed to the synergistic effect of the suitable micro/ nanoscale hierarchical roughness and the low surface energy modification.
1. Introduction Filter papers, one of the most common separation materials, are used in various kinds of applications in our daily life because of their advanced technology, low cost, ready availability and environmental friendliness [1–4]. Cellulose is the main component of filter paper and is a structural polysaccharide with many hydroxyl groups, which endow filter papers with hydrophilic properties. However, this property allows it to be wetted and contaminated by liquids such as water, which is undesirable in their use as waterproof materials [5]. Therefore, great efforts have been made in the modification of papers to enhance their self-cleaning and stain-repellent properties. It has been demonstrated that the introduction of superhydrophobicity is a well-established technique for the surface of paper [6]. Superhydrophobic surfaces with a high water contact angle (WCA, > 150°) and a low slide angle (SA, < 10°) can cause water drops to roll off the surface, which can carry away dirt and realize the selfcleaning property. Thus, superhydrophobic surfaces have wide applications in many fields, such as water-repellent surfaces, the suppression of surface oxidation and the reduction of the resistance of underwater vessels [7–9]. According to previous studies, both low surface energy
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and high surface roughness with micro/nanoscale structures are needed for superhydrophobic surfaces. To date, a variety of routes have been developed to fabricate superhydrophobic surfaces [10–15], such as selfassembly processes, template synthesis, chemical etching, sol-gel methods, and electrospinning techniques. Moreover, many studies have referred to superhydrophobic coatings on different substrates, including glass materials [16], aluminum alloys [17], copper materials [18], silicon wafers [19], steel materials [20] and polymers [21]. However, studies of the superhydrophobic coating on a paper substrate are limited because the paper is easily decomposed and damaged by physical, chemical, or thermal treatments. Few approaches, such as multilayer depositions [22], plasma treatments [23], and spraying alcohol suspensions [24], have been attempted to impart superhydrophobicity to paper. For example, Deng et al. prepared superhydrophobic papers using SiO2 or CaCO3 nanoparticles modified by chemical treatment [25]. Zhang et al. used an aerogel microsphere coating and methyltrimethoxysilane (MTMS) modification to obtain superhydrophobic filter paper with selective oil-water separation [26]. Li and coworkers reported a food-safe superhydrophobic cellulose paper achieved by the formation of polymer-nanoclay hybrid multilayers and carnauba wax modification [27]. Furthermore, other inorganic nanoparticles have
Corresponding author. E-mail address: [email protected] (Z. Sun).
https://doi.org/10.1016/j.apsusc.2019.143743 Received 12 June 2019; Received in revised form 10 August 2019; Accepted 20 August 2019 Available online 20 August 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.
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Scheme 1. Formation illustration of the superhydrophobic paper.
also been reported to create a rough surface structure for superhydrophobic paper [28–32], such as TiO2, ZnO, CuO, and WO3. However, most methods often involve strict conditions, costly materials or complicated procedures. In addition, the disadvantages of superhydrophobic papers, such as high adhesion, weak mechanical stability and poor durability, have limited their practical applications. In this work, we demonstrated a simple, nontoxic and cost-effective procedure that was able to improve the water repellency and stability of the filter paper substrate. We first fabricated uniform zinc oxide (ZnO) microclusters by a simple precipitation method at low temperatures. A successive deposition process of the ZnO microcluster emulsion was used to create a suitable rough structure on the reticular fibers. After a subsequent modification step with stearic acid, the surface morphology, chemical composition and wettability of the papers were characterized. Importantly, the wettability of the paper was closely related to the chemical modification and the surface roughness, which was governed by the number of deposition cycles of ZnO microclusters. Additionally, the mechanical stability was studied by water drop impact dynamics on the superhydrophobic paper. Moreover, the chemical resistance to corrosive liquids and durability was also examined. The results showed that the resulting papers possess remarkable superhydrophobicity, low adhesion, high impact resistance and good stability.
deionized water under stirring. The mixed solution was sealed and kept static at 80 °C for 12 h. The resulting precipitate was separated by centrifuge, washed with deionized water and absolute ethanol for several times, and dried at 70 °C in an oven for 8 h.
2. Experimental
The X-ray diffraction (XRD) was performed on a Japan Shimadzu XRD-6000 equipped with CuKα radiation (λ = 1.54178 Å). Scanning electron microscopy (SEM) images were carried out on a JEOL JSM6700F. Transmission electron microscopy (TEM) images were recorded on a JEM-2100 instrument. Atomic force microscopy (AFM) was recorded on an Agilent-5500. Fourier transform infrared (FTIR) spectroscopy was performed with a Nicolet-380 FTIR spectrometer.
2.3. Preparation of superhydrophobic papers ZnO microclusters were deposited on the paper through a successive deposition method. Firstly, dry ZnO powder (0.1 g) was dissolved in 30 mL of absolute ethanol and then vigorously stirred at room temperature for 30 min, forming a homogeneous emulsion. Then, the treated filter paper was immersed in the above solution for 2 min and dried in an oven for 5 min at 100 °C, which was termed one cycle. After several cycles of the process, the as-prepared ZnO/paper was immersed in a 0.1 M stearic acid solution for 30 min, inducing the formation of a thin layer of water-insoluble stearate on the ZnO surface. After the chemical modification, the resulting papers were placed horizontally on an oven and dried at 60 °C for 5 h. To prepare the papers with controllable wettability, the deposition process of ZnO microclusters on papers was controlled to be 0, 2, 4 and 6 cycles, respectively. 2.4. Characterization
2.1. Materials The common filter papers in this work were cut to form the paper substrates with different size. The reagent-grade chemicals used include sodium hydroxide (NaOH), zinc chloride (ZnCl2), stearic acid and absolute ethanol.
2.5. Wetting test 2.2. Synthesis of ZnO microclusters The WCA values of the papers were measured by a C20 contact angle meter (Kono, USA) using a droplet (approximately 5 μL) of deionized water as an indicator. The SA values of the papers were evaluated by measuring the tilt values at which the water drops fall
The preparation procedure of ZnO microclusters was similar to that described in our previous study [33]. NaOH (0.4000 g) and ZnCl2 (0.2726 g) were dissolved in glass bottle (50 mL) with 30 mL of 2
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grown in a bundle, and they are properly arranged to form many flowerlike microclusters. Further, these clusters are uniform in size, and they present a typical diameter of 1–2 μm. Fig. 1(d) shows the size distribution histogram of the ZnO products. The mean size of the microclusters is 1.3 μm. Moreover, a typical TEM image of a flowerlike ZnO microcluster is shown in Fig. 1(e). It confirms a three-dimensional layered structure constructed by numerous nanosheets, and the diameter of the ZnO microcluster is approximately 1.3 μm. The HRTEM image in Fig. 1(f) shows the two kinds of lattice fringes. The lattice spacings of 0.28 nm and 0.52 nm correspond to the {0001} and {0110} planes of a wurtzite ZnO crystal, respectively. These findings suggest that the ZnO nanosheet has a good crystalline structure. The surface morphologies of the papers before and after the modification of ZnO microclusters are characterized by SEM. Fig. 2(a) and (b) show the SEM images of the pristine paper. The filter paper is composed of smooth cellulose fibers, which tightly weave together to form a neat fiber network. Fig. 2(c–f) shows the surface morphological transformation of the ZnO/papers with the increase of deposition cycles. Compared to the surface of the pristine paper, the tangled fibers are almost covered by the ordered and uniform ZnO microclusters. With an increase in the number of deposition cycles, the surface of the fibers becomes gradually rougher. After 2 deposition cycles, most of the fibers are decorated with ZnO microclusters, and a small number of the outer residuary fibers are still bare. After 4 cycles, the deposition amount of ZnO microclusters significantly increases, with a complete coating and uniform dispersion on the fiber surface. Interestingly, such a distinctive structure creates a fascinating micro/nanoscale hierarchical roughness, which is beneficial for obtaining remarkable superhydrophobicity. When increased to 6 cycles, more ZnO microclusters accumulate on the fiber surface, resulting in the formation of large agglomerates. In addition, the cross-sectional SEM images of ZnO/papers with different deposition cycles are shown in Fig. 3(a)–(c). Because the paper samples are prepared by the clipping of scissor, the partial fibers at the section
from a sample platform. The mechanical properties of the superhydrophobic paper were characterized by continuously impacting water drops onto the paper. The long-term stability was carried out by storing them in an ambient environment for 1–6 months. 3. Results and discussions 3.1. Microstructure analysis and chemical characterization In this work, superhydrophobic papers were fabricated by a successive deposition procedure, as shown in Scheme 1. The uniform flowerlike ZnO microclusters were first obtained by a simple precipitation method at low temperatures. Then, the filter paper was successively immersed into an alcohol emulsion of ZnO microclusters, which can increase the roughness of the paper surface and form a multilayer hierarchical composition. As a low-surface-energy reagent, stearic acid is an ideal candidate for obtaining water repellency because of its biocompatibility, nontoxicity and low cost. When the paper decorated by ZnO microclusters was dipped in a stearic acid solution for 30 min, the carboxyl functional groups (-COOH) in stearic acid reacted with the abundant hydroxyl groups (-OH) on the ZnO surface [34,35]. A self-assembled monolayer of water-insoluble stearate was formed on the paper surface with the nonpolar tails exposed to the surface. This strategy effectively decreased the surface free energy of the paper, leading to dramatically enhanced water repellency. Fig. 1(a) shows the XRD pattern of the ZnO microclusters, revealing the well-crystalline nature of the samples because of the sharp and strong diffraction peaks. All of the diffraction peaks can be perfectly assigned to hexagonal wurtzite ZnO, and the peaks are consistent with the values in the standard card (JCPDS no. 36-1451). The morphology of the sample is characterized by SEM, TEM and HRTEM images. From the SEM images with different magnifications in Fig. 1(b) and 1(c), it can be observed that ZnO products are composed of many nanosheets
Fig. 1. (a) XRD pattern and (b, c) SEM images of ZnO microclusters, (d) size distribution histogram of ZnO microclusters, (e) TEM image of a flowerlike ZnO microcluster, and (f) the corresponding HRTEM image. 3
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Fig. 2. SEM images of (a) pristine paper, ZnO/papers with (b, c) 2 cycles, (d) 4 cycles and (e) 6 cycles.
Fig. 3. Cross-sectional SEM images of ZnO/papers with (a) 2 cycles, (b) 4 cycles and (c) 6 cycles. 4
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Fig. 4. Phase (the top)/3D (the bottom)-AFM morphological images of (a) pristine paper and ZnO/papers: (b) 2 cycles, (c) 4 cycles, and (d) 6 cycles.
In addition to a suitable surface roughness, low surface free energy is also essential for obtaining a superhydrophobic surface. In the present case, a reasonable amount of ZnO microclusters on the tangled fibers can provide a micro/nanoscale hierarchical structure. To further decrease the surface energy, stearic acid as a surfactant is used to modify the papers deposited with ZnO microclusters. Fig. 5 shows SEM images of the ZnO/Paper surface decorated with stearic acid. From the panoramic image in Fig. 5(a), we can observe that the ZnO microclusters on the fibers are modified by a layer of stearic acid, and the uniform dense ZnO coating on the fiber surface is unchanged. It indicates that the stearic acid/ZnO microclusters composite structure has been constructed on the filter paper substrate. The magnified SEM image in Fig. 5(b) reveals that the stearic acid layer is composed of a large number of thick sheets with the size of 2–4 μm, and these sheets are randomly distributed on the ZnO surfaces. Such a structure helps to increase surface roughness, and greatly reduces the surface energy. This morphology of stearic acid molecules deposited on the ZnO is consistent with that reported in the literature [35]. The change in the chemical composition of the coatings scraped from the prepared papers is characterized by FTIR spectra, as shown in Fig. 5(c). In comparison with the ZnO coating, the stearic acid/ZnO coating not only shows the intrinsic peaks of ZnO microclusters but also exhibits other specific peaks. The peaks appearing at 2918 cm−1 and 2848 cm−1 can be attributed to the stretching vibrations of –CH3 and –CH2, respectively. The peaks at 1528 cm−1 and 1442 cm−1 belong to
are broken and deformed. As further evidence, all of SEM images show a layer of ZnO microcluters on the top and in the gaps between the fibers. The average thickness of three ZnO/papers is 95 μm. The thickness of ZnO coating on the top gradually increases with the increase of deposition cycles. After 6 deposition cycles, the highest thickness of ZnO coating is approximately 3.5 μm. The thin ZnO coating is help to extend the practical application of the paper. The roughness is a key factor in the preparation of superhydrophobic surfaces [36–38]. The surface roughness of the as-prepared papers is measured by the atomic force micrograph (AFM). Fig. 4 shows the phase images and three-dimensional (3D) AFM morphological images of the papers before and after the modification of ZnO microclusters. As shown in Fig. 4(a), the pristine filter paper exhibits a relatively smooth and flat surface topography, with a resultant rootmean-square (RMS) roughness value of 1.45 nm. Compared with bare paper, the modification of ZnO microclusters leads to a significant increase in the surface roughness. As the deposition number increases to 4 cycles, the RMS roughness reaches a maximum value with 31 nm. However, when the deposition number reaches to 6 cycles, the surface RMS roughness of ZnO/paper becomes smaller. For the hydrophobic surface, the greater the roughness is, the greater the contact angle is. It means that the high surface roughness is desirable for the excellent wettability of the surface. Therefore, the proper ZnO microclusters on the fibers create an ideal micro/nanoscale hierarchical structure, which significantly increases the surface roughness of the filter paper.
Fig. 5. (a, b) SEM images of the stearic acid/ZnO(4)/paper, (c) FTIR spectra of ZnO and stearic acid/ZnO coatings scraped from the prepared papers. 5
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Fig. 6. Static photos of a water droplet on the papers: (a) pristine paper, (b) ZnO/paper, stearic acid/ZnO/papers deposited with (c) 0 cycles, (d) 2 cycles, (e) 4 cycles, and (f) 6 cycles.
the stretching vibrations of –COO– in CH3(CH2)16COO– groups, which indicates the formation of new bonds between the CH3(CH2)16COO– groups of stearic acid and the –OH groups tethered on the ZnO surface [39]. These findings prove that stearic acid has been precisely grafted onto the surface of ZnO/paper.
Table 1 Static WCA values and SA values of the different papers. Samples
WCA
SA
Pristine paper ZnO/paper Stearic acid/paper Stearic acid/ZnO(2)/paper Stearic acid/ZnO(4)/paper Stearic acid/ZnO(6)/paper
0 38° 119° 144° 158° 152°
> 90° 10° 5° 3° 3°
3.2. Wettability behavior The wettability of the as-prepared stearic acid/ZnO/papers is investigated by the static WCA on the surface. As shown in Fig. 6(a), the WCA of pristine paper is almost 0°, showing a very high hydrophilic property. This property can be attributed to cellulose being a structural polysaccharide with many hydroxyl groups. After the deposition of ZnO
Fig. 7. Optical images of static water droplets on (a) pristine paper and (b, c) superhydrophobic paper with 4 cycles. 6
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Fig. 8. (a) Schematic and (b) sequential optical images of the bounce process of a dynamic water drop on the superhydrophobic paper with 4 cycles.
Fig. 9. (a) Evolution of WCA values of the superhydrophobic paper for various stearic acid concentrations, and (b) the reaction mechanism for ZnO/paper modified by stearic acid.
the dynamic behavior of the water droplet on the paper surface, the water SA of the different papers is presented in Table 1. For the superhydrophobic paper with 4 cycles, the SA value is measured to be 3°. The result indicates that the paper has a small water contact hysteresis and low adhesion. Fig. 7 shows the optical images of static water droplets and dynamic water droplets on the different papers. When a water drop contacts the pristine paper (Fig. 7a), it instantaneously spreads on the surface, finally leaving a large water spot on the paper. However, when a water drop contacts the superhydrophobic paper (Fig. 7b), it does not spread out but forms a drying bead that rests on the paper. If the paper is slightly tilted (Fig. 7c), the water droplet quickly rolls down the paper, indicating its small contact hysteresis. Furthermore, Fig. 8(a) shows that a water drop free falling from a 6 cm height, impinging to the superhydrophobic paper with a velocity of 0.84 m/s. The bounce process of the dynamic water drop is shown in Fig. 8(b). It can be observed that the water drop does not spread out on the paper but rebounds three before it undergoes damped oscillations. Finally, the drop rests on the surface of the superhydrophobic paper. The result indicates that the superhydrophobic paper has good bounce performance. Thus, superhydrophobic paper with low adhesion and good bounce performance is expected to effectively remove contamination on the surface similar to the self-cleaning property of a lotus leaf. In addition to the surface morphology, the chemical modification of stearic acid has an important effect on the wettability of the paper. Fig. 9(a) shows the effect of stearic acid concentration on the WCA of the papers. It can be observed that the WCA of the paper becomes larger with increasing concentrations of stearic acid. The wettability of the paper changes from hydrophilic to superhydrophobic. When the concentration is higher than 0.1 M, the WCA of the paper is almost unchanged. This result is because stearic acid with too low of a concentration cannot adequately react with the groups in the ZnO/paper, leading to inadequate protection and weak hydrophobicity. Hence, the creation of a micro/nanoscale hierarchical roughness and the
microclusters (Fig. 6b), the WCA of the resulting paper is 38°, which may be due to the hydrophilic nature of ZnO microclusters. However, the WCA on the paper surface is dramatically changed after the chemical modification with stearic acid. Fig. 6(c-f) shows the WCA images of the ZnO/papers modified by stearic acid. WCA values of 119°, 144°, 158° and 152° are obtained for the stearic acid/paper and the stearic acid/ZnO papers with 2, 4 and 6 cycles, respectively. The WCA value gradually increases with increasing cycles of ZnO microclusters, indicating that the wettability of the paper is closely related to the number of deposition cycles. After 4 deposition cycles, the stearic acid/ ZnO paper presents the best superhydrophobic property with a WCA as high as 158°. This result may be due to the ZnO microclusters decorating the tangled fibers and creating a reasonable micro/nanoscale hierarchical roughness, leading to more interspaces or cavities present on the surface. This structure can effectively capture a large amount of air into the coating, and the air/water interface greatly increases. Because the contact angle of the air-water is 180°, this structure can effectively prevent the penetration of water droplets into the grooves, leading to the water droplet being suspended on the rough surface. It has been noted that the greater roughness of the hydrophobic surface can cause a larger surface contact angle. As seen from the above SEM, a rougher surface is formed on the paper with an increase in deposition cycles. Combined with the chemical modification with stearic acid, the paper successfully achieves superhydrophobicity. Therefore, the remarkable superhydrophobic behavior can be attributed to the synergistic effect of the hierarchical micro/nanoscale roughness and the low surface energy composition. However, after 6 deposition cycles, the formation of ZnO agglomerates may reduce the surface roughness, resulting in a decrease in the WCA of the paper. The static contact angle cannot fully demonstrate the wetting behavior of water droplets on the paper surface. The difficulty displayed by the water droplet as it moves over the surface should be taken into account when the assessing the hydrophobic effect of films. To describe 7
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Fig. 10. (a–i) Sequential optical images of water drops impacted continuously onto the superhydrophobic paper with 4 cycles, and (j) the schematic of the impact process.
superhydrophobic paper is not destroyed due to its good impact resistance to water droplets. The as-prepared paper with 4 cycles exhibits excellent superhydrophobic properties not only for pure water but also for corrosive water, including under alkaline and acidic conditions. Fig. 11 shows the static WCAs measured under different pH values. The paper remains superhydrophobic when the pH is varied from 2 to 12. When the pH is < 2 or > 12, a drastic conversion from superhydrophobicity to hydrophilicity can be observed for the paper. This can be due to the zinc stearate monolayer that is formed on the paper surface being destroyed under an extremely corrosive water drop, resulting in weak hydrophobicity [40,41]. Although the superhydrophobicity on the paper surface is unstable under strong acid/alkali conditions, it can maintain good chemical stability for ordinary corrosive water.
appropriate modification with stearic acid play key roles in superhydrophobic paper. The reaction mechanism for ZnO/paper modified by stearic acid is shown in Fig. 9(b). 3.3. Stability study The mechanical property of the superhydrophobic paper is evaluated by a water droplet impact test to extend the outdoor applications. As shown in Fig. 10(a–i), the water droplets from a pipet are dropped on the slightly tilted superhydrophobic paper. Clearly, the water droplets do not penetrate into the grooves but constantly roll off the paper. After the drop test, the paper is spotless and retains its good superhydrophobicity. The scheme of the whole process is shown in Fig. 10(j). This finding suggests that the rough surface structure of the 8
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the paper are 153° and 3°, respectively, as shown in Fig. 12(b, c). The water droplet roll-off can also remove some dust and dirt deposited over time on the paper surface. Moreover, the optical images of the colored water drops on the superhydrophobic paper are shown in Fig. 12(d). These results prove that the good impact resistance, long-term stability and chemical stability of the superhydrophobic paper are beneficial for potential outdoor applications. 3.4. Superhydrophobic mechanism This superhydrophobic phenomenon can be explained by the CassieBaxter theory [40], as shown in Scheme 2. When the water droplet comes into contact with the surface of the modified paper, the rough structure of the surface captures a large amount of air, preventing the water droplet from penetrating into the paper. Therefore, the water droplet is actually in contact with a composite surface composed of stearic acid/ZnO hierarchical structure and air. The WCA of the composite surface can be expressed by the Cassie-Baxter equation: Fig. 11. Static WCA values measured on the condition of different pH values.
cos θr = f1 cos θ1 − f2 In addition, the durability of a superhydrophobic material is crucial in outdoor applications. Therefore, the WCA and SA of the superhydrophobic paper with 4 cycles are measured again after being stored for 1–6 months in an ambient environment. As shown in Fig. 12(a), the wettability of the superhydrophobic paper has no significant change during this period. After it was stored for 6 months, the WCA and SA of
(1)
where θγ and θ1 are the apparent WCA of the rough surface and smooth surface with the same composition, respectively; f1 is the unit for the apparent area fraction of the contact area between the water droplet and film; and f2 is the unit for the apparent area fraction of the contact area between the water droplet and air (f1+f2=1). Based on the above formula, it is found that the WCA (θγ) of the
Fig. 12. (a) WCA values of the superhydrophobic paper after placing it for 1–6 months, (b) static and (c) dynamic images of a water droplet on the superhydrophobic paper placed for 6 months, (d) an optical image of colored water drops on the paper. 9
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Scheme 2. Scheme illustration of the superhydrophobic paper.
rough surface is positively correlated with the proportion (f2) of air on the surface. The larger f2 is, the larger θγ is, and the stronger the hydrophobic property is. In our reaction system, when the paper modified with ZnO microclusters is immersed in the ethanol solution of stearic acid, the -COOH groups in the stearic acid can react with a large number of -OH groups on the surface of the ZnO microclusters, forming a self-assembled monolayer of zinc stearate [42,43]. On the one hand, the reasonable micro/nanoscale hierarchical roughness constructed by ZnO microclusters provides a structural groundwork for superhydrophobic properties. On the other hand, the hydrophobic tails of the self-assembled zinc stearate monolayer are oriented perpendicular to the surface, which effectively decreases the surface free energy of the paper. This structure enables the paper surface to easily absorb and deposit air and prevents water droplets from penetrating into the pores, thus inducing an increased f2 and a decreased θr. According to the Cassie-Baxter equation, the surface of the paper finally exhibits a remarkable superhydrophobic performance. Therefore, the excellent superhydrophobicity of paper can be attributed to the synergistic effect of the micro/nanoscale hierarchical roughness of a reasonable amount of ZnO microclusters and the low surface energy modification with stearic acid.
Soc. A 374 (2016) 1–40. [2] N.M. Oliveira, S. Vilabril, M.B. Oliveira, R.L. Reis, J.F. Mano, Recent advances on open fluidic systems for biomedical applications: a review, Mat. Sci. Eng. C 97 (2019) 851–863. [3] K.R. Xu, X.P. Wang, R.M. Ford, J.P. Landeres, Self-partitioned droplet array on laser-patterned superhydrophilic glass surface for wall-less cell arrays, Anal. Chem. 88 (2016) 2652–2658. [4] Q.Z. Wang, Y.N. Liu, Y.W. Bai, S.Y. Yao, Z.J. Wei, M. Zhang, Superhydrophobic SERS substrates based on silver dendrite-decorated filter paper for trace detection of nitenpyram, Anal. Chim. Acta 1049 (2019) 170–178. [5] P.E. Oomen, J. Mulder, E. Verpoorte, R.D. Oleschuk, Controlled, synchronized actuation of microdroplets by gravity in a superhydrophobic, 3D-printed device, Anal. Chim. Acta 988 (2017) 50–57. [6] M. Zhang, C.Y. Wang, S.L. Wang, Y.L. Shi, J. Li, Fabrication of coral-like superhydrophobic coating on filter paper for water-oil separation, Appl. Surf. Sci. 261 (2012) 764–769. [7] X.X. Huang, X.F. Wen, J. Cheng, Z.R. Yang, Sticky superhydrophobic filter paper developed by dip-coating of fluorinated waterborne epoxy emulsion, Appl. Surf. Sci. 258 (2012) 8739–8746. [8] F. Zhang, H. Ren, L.L. Shen, G.L. Tong, Y.L. Deng, Micro-nano structural engineering of filter paper surface for high selective oil-water separation, Cellulose 24 (2017) 1–12. [9] M. Satapathy, P. Varshney, D. Nanda, A. Panda, S.S. Mohapatra, A. Kumar, Fabrication of superhydrophobic and superoleophilic polymer composite coatings on cellulosic filter paper for oil-water separation, Cellulose 24 (2017) 1–14. [10] X. Deng, L. Mammen, H.J. Butt, D. Vollmer, Candle soot as a template for a transparent robust superamphiphobic coating, Science 335 (2012) 67–70. [11] H.M. Kim, J.W. Choi, J.S. Kwon, C.H. Lee, B. Kim, Super-hydrophobic properties of aluminum surfaces synthesized by a two-step chemical etching process, J. Nanosci. Nanotechno. 19 (2019) 6452–6457. [12] X.G. Zhang, Z.J. Liu, Y. Li, C.J. Wang, Y.J. Zhu, H.Y. Wang, J.T. Wang, Robust superhydrophobic epoxy composite coating prepared by dual interfacial enhancement, Chem. Eng. J. 371 (2019) 276–285. [13] K.L. Wang, X.R. Liu, Y. Tan, W. Zhang, S.F. Zhang, J.Z. Li, Two-dimensional membrane and three-dimensional bulk aerogel materials via top-down wood nanotechnology for multibehavioral and reusable oil/water separation, Chem. Eng. J. 371 (2019) 769–780. [14] Y.F. Zhang, L.Q. Zhang, Z. Xiao, S.L. Wang, X.Q. Yu, A fabrication of robust and repairable superhydrophobic coatings by an immersion method, Chem. Eng. J. 369 (2019) 1–7. [15] S. Wang, X. Yu, Y. Zhang, Large-scale fabrication of translucent, stretchable and durable superhydrophobic composite films, J. Mater. Chem. A 5 (2017) 23489–23496. [16] Y.F. Wang, B.X. Li, T.X. Liu, C.Y. Xu, Z.W. Ge, Controllable fabrication of superhydrophobic TiO2 coating with improved transparency and thermostability, Colloid. Surface. A 441 (2014) 298–305. [17] Y.F. Zhang, T.G. Lin, Influence of duty cycle on properties of the superhydrophobic coating on an anodized magnesium alloy fabricated by pulse electrodeposition, Colloid. Surface. A 568 (2019) 43–50. [18] G. Ren, Y. Song, X. Li, Y. Zhou, Z. Zhang, X. Zhu, A superhydrophobic copper mesh as an advanced platform for oil-water separation, Appl. Surf. Sci. 428 (2018) 520–525. [19] L. Schneider, M. Laustsen, N. Mandsberg, R. Taboryski, The influence of structure heights and opening angles of micro- and nanocones on the macroscopic surface wetting properties, Sci. Rep-Uk 6 (2016) 1–9. [20] T. Movahedi, R. Norouzbeigi, Synthesis of flower-like micro/nano ZnO superhydrophobic surfaces: additive effect optimization via designed experiments, J. Alloy. Compd. 795 (2019) 483–492. [21] L. Mérai, N. Varga, Á. Deák, D. Sebők, I. Szenti, Á. Kukovecz, Z. Kónya, I. Dékány, L. Janovák, Preparation of photocatalytic thin films with composition dependent wetting properties and self-healing ability, Catal. Today 328 (2019) 85–90. [22] L. Guo, W. Yuan, J.P. Li, Z.J. Zhang, Z.M. Xie, Stable superhydrophobic surfaces over a wide pH range, Appl. Surf. Sci. 254 (2008) 2158–2161. [23] M. Satapathy, P. Varshney, D. Nanda, A. Panda, S.S. Mohapatra, A. Kumar, Fabrication of superhydrophobic and superoleophilic polymer composite coatings on cellulosic filter paper for oil–water separation, Cellulose 24 (2017) 4405–4418. [24] K. Tsougeni, A. Tserepi, V. Constantoudis, E. Gogolides, P.S. Petrou, S.E. Kakabakos, Plasma nanotextured PMMA surfaces for protein arrays: increased protein binding and enhanced detection sensitivity, Langmuir 26 (2010) 13883–13891.
4. Conclusions In summary, we developed a simple method to fabricate robust superhydrophobic paper via a successive deposition of ZnO microclusters and a subsequent modification. The wettability of the superhydrophobic paper is dependent on the surface roughness, which is governed by the numbers of deposition cycles of the ZnO microclusters. The paper with 4 cycles displayed the best superhydrophobicity with a high WCA of 158° and low SA of 3° and good bounce performance, which can be attributed to the synergistic effect of the micro/nanoscale hierarchical structure and chemical modification. Importantly, the superhydrophobic paper exhibited a low adhesion, high impact resistance for water drops, good chemical stability and long-term durability. This study described a facile and popular strategy to fabricate bionic superhydrophobic papers with good bounce performance, low adhesion, and good stability, and this strategy is expected to be used for other cellulose fiber substrates to broaden their outdoor application in selfcleaning materials. Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Nos. 51772003, 51472003, 51701001), Provincial Natural Science Foundation of Anhui Higher Education Institution of China (No. KJ2019A0122, No. KJ2017A002), and Science Research Foundation for Young Teachers (No. QN201403). References [1] P.S. Brown, B. Bhusan, Bioinspired materials for water supply and management: water collection, water purification and separation of water from oil, Phil. Trans. R.
10
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Y. Wang, et al.
stearic acid modified ZnO nanotowers, Appl. Surf. Sci. 254 (2008) 2690–2695. [35] J. Zhang, Z.H. Liu, J.Q. Liu, L. E, Z.F. Liu, Effects of seed layers on controlling of the morphology of ZnO nanostructures and superhydrophobicity of ZnO nanostructure/ stearic acid composite films, Mater. Chem. Phys. 183 (2016) 306–314. [36] A.J. Patil, Y. Zhao, X. Liu, X. Wang, Durable superhydrophobic and antimicrobial cotton fabrics prepared by electrostatic assembly of polyhexamethylene biguanide and subsequent hydrophobization, Text. Res. J. 88 (2017) 1788–1799. [37] B. Raeesi, N.R. Morrow, G. Mason, Effect of surface roughness on wettability and displacement curvature in tubes of uniform cross-section, Colloid. Surface. A 436 (2013) 392–401. [38] K.S. Liu, X. Yao, L. Jiang, Recent developments in bio-inspired special wettability, Chem. Soc. Rev. 39 (2010) 3240–3255. [39] A.B. Gurav, S.S. Latthe, R.S. Vhatkar, J.G. Lee, D.Y. Kim, J.J. Park, S.S. Yon, Superhydrophobic surface decorated with vertical ZnO nanorods modified by stearic acid, Ceram. Int. 40 (2014) 7151–7160. [40] A. Cassie, S. Baxter, Wettability of porous surfaces, Trans. Faraday Soc 40 (1944) 546–551. [41] E.C. Griffith, E.M. Adams, H.C. Allen, V. Vaida, Hydrophobic collapse of a stearic acid film by adsorbed l-phenylalanine at the air-water interface, J. Phys. Chem. B 116 (2012) 7849–7857. [42] Q.Y. Cheng, X.P. An, Y.D. Li, C.L. Huang, J.B. Zeng, Sustainable and biodegradable superhydrophobic coating from epoxidizedsoybean oil and ZnO nanoparticles on cellulosic substrates for efficient oil/water separation, ACS Sustain. Chem. Eng. 5 (2017) 11440–11450. [43] M. Lee, G. Kwak, K. Yong, Wettability control of ZnO nanoparticles for universal applications, ACS Appl. Mater. Interfaces 3 (2011) 3350–3356.
[25] P. Reokrungruang, I. Chatnuntawech, T. Dharakul, S. Bamrungsap, A simple paperbased surface enhanced Raman scattering (SERS) platform and magnetic separation for cancer screening, Sensor. Actuat. B-Chem. 285 (2019) 462–469. [26] D.W. Li, H.Y. Wang, Y. Liu, D.S. Wei, Z.X. Zhao, Large-scale fabrication of durable and robust super-hydrophobic spray coatings with excellent repairable and anticorrosion performance, Chem. Eng. J. 367 (2019) 169–179. [27] H. Li, Y.Q. He, J. Yang, X. Wang, T.Q. Lan, L.C. Peng, Fabrication of food-safe superhydrophobic cellulose paper with improved moisture and air barrier properties, Carbohyd. Polym. 211 (2019) 22–30. [28] V. Anand Ganesh, S.S. Dinachali, A. Sreekumaran Nair, S. Ramakrishna, Robust superamphiphobic film from electrospun TiO2 nanostructures, ACS Appl. Mater. Interfaces 5 (2013) 1527–1532. [29] X. Zhang, Y. Si, J. Mo, Z. Guo, Robust micro-nanoscale flowerlike ZnO/epoxy resin superhydrophobic coating with rapid healing ability, Chem. Eng. J. 313 (2017) 1152–1159. [30] A. Siddaramanna, N. Saleema, D.K. Sarkar, A versatile cost-effective and one step process to engineer ZnO superhydrophobic surfaces on Al substrate, Appl. Surf. Sci. 311 (2014) 182–188. [31] H. Li, S. Yu, X. Han, Fabrication of CuO hierarchical flower-like structures with biomimetic superamphiphobic, self-cleaning and corrosion resistance properties, Chem. Eng. J. 283 (2016) 1443–1454. [32] N. Shafaei, M. Jahanshahi, M. Peyravi, Q. Najafpour, Self-cleaning behavior of nanocomposite membrane induced by photocatalytic WO3 nanoparticles for landfill leachate treatment, Korean J. Chem. Eng. 33 (2016) 2968–2981. [33] Y.F. Wang, B.X. Li, C.Y. Xu, Fabrication of superhydrophobic surface of hierarchical ZnO thin films by using stearic acid, Superlattice. Microst. 51 (2012) 128–134. [34] N. Saleema, M. Farzaneh, Thermal effect on superhydrophobic performance of
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