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CuO nanopowders for water repellent and corrosion resistant coatings
Ceramics International 45 (2019) 16864–16872
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Single-step prepared hybrid ZnO/CuO nanopowders for water repellent and corrosion resistant coatings
T
Elmira Velayi, Reza Norouzbeigi∗ School of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology, P.B. 16765–163, Narmak, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid nanopowders Superhydrophobicity Spray deposition Corrosion resistant
In this study, ZnO/CuO hybrid hydrophobic nanopowders were synthesized using a common single-step chemical precipitation route without using modifiers. Influence of initial ZnO:CuO precursor concentrations and alkaline agent type on the wettability behavior of the prepared samples were investigated. Wettability properties of the prepared samples were assessed by measuring the water contact angle and contact angle hysteresis values. Fourier transform infrared spectra, scanning electron microscope micrographs and X-ray diffraction patterns were applied to identify the surface chemistry and morphological features. Scanning electron microscope images of the synthesized ZnO/CuO nanocomposites indicated flower-like morphologies containing plenty of nanoneedles, -rods, and -sheets with thicknesses lower than 90 nm. The sample prepared under the optimum conditions was superhydrophobic having water contact angle and contact angle hysteresis of 162.6° ± 1 and 2°, respectively. It was applied to coat the surface of stainless steel meshes by spray deposition method. The resultant superhydrophobic surface exhibited excellent self-cleaning (water repellency) property and a suitable stabilities under the ambient and saline solution (NaCl, 3.5%) media. Additionally, electrochemical corrosion tests confirmed that the corrosion resistance of the fabricated ZnO–CuO coating was higher than the initial bare mesh.
1. Introduction
synthesized via a simple one-step method [18]. The prepared samples showed good superhydrophobic properties after modification by stearic acid. In another research, titania-modified SiO2 nanocomposites were deposited on glass plates using dip coating method. The prepared coating at the optimal ratio of 9:1 (dip coating solution: SiO2) indicated excellent anti-adhesion properties with the lower cell adhesion rate (< 10%) and water contact angle of 152° [19]. Brassard and co-workers fabricated a nanocomposite of epoxy polymer containing stearic acid functionalized ZnO nanoparticles via spray deposition. The wetting behavior of the surface showed that the nanocomposite thin film had superhydrophobic properties with the water contact angle of 156° and contact angle hysteresis of 4° [20]. Therefore in most of the studies, the superhydrophobic nanocomposite surface has been fabricated by creation of a rough surface and modification with low surface free energy materials, respectively. 4 This paper reports synthesis of superhydrophobic ZnO/CuO nanopowders via chemical precipitation method without using any chemical modifier. The wetting characteristics of the prepared samples were investigated as a function of ZnO:CuO precursor solutions volume ratio. Additionally, effect of alkaline agent type on the morphological features and wettability properties was studied. In the next step, the
3 In recent years, creation of superhydrophobic surfaces by mimicking the lotus leaf has attracted many researchers due to the various industrial applications [1,2]. Surfaces with water contact angles (WCA) of higher than 150° and contact angle hysteresis (CAH) values less than 10° are defined as superhydrophobic surfaces which are the promising candidates in many areas such as anti-icing, self-cleaning, anticorrosion, and antifogging coatings, along with microfluidic devices and oil-water separation [3–8]. Several methods have been proposed to fabricate superhydrophobic surfaces by two main steps: creation of the rough structures and modification the rough surfaces by low surface free energy materials [1,9]. Up to now numerous semiconductor metal oxides like ZnO, CuO, and TiO2 have been extensively applied to generate uniform rough surfaces [10–15]. These surfaces usually show superhydrophobic properties just after chemical modification by low surface energy materials. However, there are few reports of construction of superhydrophobic nanocomposite materials. The nanocomposite materials exhibit better chemical and mechanical stabilities and also better wear resistance compared to the separate phases [16,17]. Hierarchical flowerlike iron-containing MnO2 particles have been
∗
Corresponding author. E-mail addresses: [email protected] (E. Velayi), [email protected] (R. Norouzbeigi).
https://doi.org/10.1016/j.ceramint.2019.05.229 Received 10 April 2019; Received in revised form 18 May 2019; Accepted 21 May 2019 Available online 22 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Ceramics International 45 (2019) 16864–16872
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Table 1 Initial different volume ratio of ZnO: CuO precursor solutions. Sample
Volume ratio (ZnO: CuO precursor solutions)
Alkaline type
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
1:0 3:1 2:2 1:3 0:1 1:0 3:1 2:2 1:3 0:1
HMTA HMTA HMTA HMTA HMTA NH3 NH3 NH3 NH3 NH3
Zn (NO3)2.6H2O
Cu (NO3)2.6H2O
Fig. 2. Photo image of the water droplet on Sample S2.
(0.025M)
(0.025M)
2. Materials and methods Mixing (different volumetric concentration)
2.1. Synthesis of superhydrophobic ZnO/CuO nanocomosits In this study, the chemical precipitation method was used to prepare the ZnO/CuO hybrid nanopowders. Zinc nitrate (Zn(NO3). 6H2O), Hexamethylenetetramine (HMTA), copper nitrate (Cu(NO3)2) and ammonia solution (25%) were used without further purification. All reagents were purchased from Merck Company. Firstly, the aqueous solutions of zinc nitrate hexahydrate (0.025 M) and copper nitrate hexahydrate (0.025) were prepared. Subsequently, the solutions were mixed with different volume ratios of ZnO: CuO, as shown in Table 1. Then a solution of HMTA (0.0375 M) or 10 ml of ammonia (25%) was added. The synthesis duration was considered 2 h for all experiments. Fig. 1 displays the flowchart of the synthesis procedure.
HMTA (0.0375) Or NH3 (25%, 10ml)
Heating (in oil bath at 95º C for 2h)
2.2. Spray coating of the ZnO/CuO nanocomposite
Filtration
The ZnO/CuO nanopowders were deposited on the substrate in two steps. Firstly, a ZnO seed layer was formed on the stainless steel mesh via thermal decomposition of zinc acetate and secondly, ZnO/CuO nanostructures were coated on the seed layer by a simple spraying. A stainless steel mesh (2 cm × 2 cm) was used as substrate. It was washed ultrasonically with ethanol and distilled water several times and then dried. Afterward, it was immersed in an ethanolic solution of zinc acetate (20 mM) for 30 min. Lastly for seed layer formation, the substrate was dried in an oven at 100 °C for 1 h and subsequently calcined in air at 400 °C for 2 h. In the next step, 0.1 g of the ZnO/CuO nanopowders synthesized under the optimum conditions were dispersed in tetrahydrofuran (15 ml) via the ultrasonic process for 2 h.The spray deposition was performed a stainless steel nozzle as a part of airbrush using pressured air (3 bar). The distance and angle between the substrate and the airbrush were considered constant at 10 cm and 90°, respectively. The spray deposition was repeated 3 times and the coated substrate was dried between the steps.
Drying
Fig. 1. Flowchart of ZnO/CuO nanocomposite preparation process by chemical precipitation method.
Table 2 WCA values of prepared samples. Sample
Volume ratio (ZnO: CuO precursor solutions)
WCA (°)
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
1:0 3:1 2:2 1:3 0:1 1:0 3:1 2:2 1:3 0:1
148 ± 0.7.3 ± 0.7 162.6 ± 1 151.7 ± 0.8 145 ± 2.1 140.9 ± 1 150 ± 0.5 147.1 ± 1 148.7 ± 2 145.1 ± 1.8 158.2 + 1.8
2.3. Characterization
ZnO/CuO nanocomposite prepared under the optimum conditions was sprayed onto stainless steel meshes as substrate. The self-cleaning behavior, anti-corrosion properties and chemical stabilities in the air and saline solution were studied.
A digital optical microscope (DINOLITE, model AM-4113ZT, Taiwan) was used to determine the WCA and CAH values. The oscillation method was applied to measure the CAH. In this method, the measured receding angle was subtracted from the advancing one to calculate the contact angle hysteresis. Morphology of the prepared samples was evaluated by scanning electron microscopy equipped with an energy dispersive spectrometer (SEM-EDX, TESCAN VEGA). The chemical composition of the ZnO/CuO composite nanostructures was
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Table 3 Comparison of the wettability characteristics of some superhydrophobic powders. Materials
Synthesis method
WCA (°)
Modifiers
Ref
ZnO ZnO ZnO MnO2 ZnO–CuO
Hydrothermal Hydrothermal Sonochemical Hydrothermal Chemical precipitation method
> 160 160 163 156 162.6
Palmitic acid Fluoroethylene vinyl ether 1H,1H,2H,2H-perfluorooctyltriethoxysilane Stearic acid –
[22] [23] [24] [18] This research
Fig. 3. SEM images of sample: (a, b) S1, (c, d) S2, (e, f) S5, (g, h) S6, (I, j) S7, and (k, l) S10.
characterized using Fourier transform infrared (FTIR) spectrometer (ATR-FTIR, model ATR-8000, Shimadzu, Japan). The purity and crystal structure of samples were characterized by X-ray diffraction analysis (XRD). The corrosion properties of the samples were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic
polarization curve (Tafel) using an Iranian electrochemical workstation (RADSTATPS3). In this test, a standard three-electrode system was applied with an Ag/AgCl (3 M KCl) reference electrode, a platinum plate counter electrode, and the prepared sample (0.5 cm × 1 cm) as working electrode. The potential range was selected from −0.3 to
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angle), respectively. Term f in the model is the solid-liquid interface fraction. According to Eq. (1), the contact angle increases by decreasing the solid-liquid interface fraction. Table 3 presents the WCA values reported in the past researches. The WCA value obtained in this research is the highest one among the other reported hydrophobic powders. It should be noticed that the superhydrophobicity has been achieved in this research without using chemical modifiers. Fig. 3 shows the SEM images of S1, S2, S5, S6, S7, and S10. Sample S1 (Fig. 3a and b) is composed of hierarchical micro/nano branched wires with random orientation and 4–5μm length. Fig. 3c and d shows the SEM micrographs of sample S2. As obviously evidenced in this figure, several uniform microrods and flower-like architectures are grown. The flower-like structures are composed of numerous needles with the tip diameter of less than 50 nm. The SEM images of flower-like hierarchical CuO structure (sample S5) with different magnifications are presented in Fig. 3e and f. It can be seen that the regular flower-like structures are result of self-assembled accumulation of nano petals with thickness of ∼50 nm. Fig. 3g-l indicate the microstructural features of the prepared samples using ammonia as the alkaline agent. It is observed that the similar hierarchical micro/nanostructures can also be obtained using a different alkaline agent. The flower-like structures consist of needles with shorter length are observed in sample S6. Fig. 3i and j presents the uniform flower-like and microsphere structures composed of many needles and nano-sheets with diameters and thicknesses less than 100 nm, respectively. The SEM micrographs of sample S10 are shown in Fig. 3k and l. Construction of flower-like CuO micro/nanostructures, including numerous interconnected nano-sheets which are less than 50 nm in thicknesses can be seen. In general, the SEM results show that all prepared samples have hierarchical micro/nanostructures similar to the lotus leaves structure and promoting the superhydrophobic properties.
Fig. 4. FTIR spectrum of sample S2.
3.2. ATR-FTIR analysis
Fig. 5. XRD pattern of sample: (a) S1, (b) S2.
−0.1 mV to perform the potentiodynamic polarization experiments. Moreover, the scan rate was 5 mVs−1. The EIS tests were conducted in the frequency range between 5 to 5000 HZ at open circuit potential (OCP).
3. Results and discussions 3.1. Wetting characterization and morphology analysis of the synthesized powders In order to characterize the wetting properties, the synthesized ZnO/CuO hybrid (composite) nanopowders were spread out on a stainless steel surface (#304) like a dense layer and the sessile drop method was applied to measure the WCAs. Table 2 presents the measured WCA values. It can be seen that all data are in the range of 140–162°. However, the sample S2 prepared by HMTA shows the best superhydrophobic properties with WCA of 162.6° ± 1°. On the other hand, the CAH value of sample S2 is smaller than 5° which confirms the superhydrophobicity. Photo image of water droplet placed on the S2 is shown in Fig. 2. In this condition, water drops can easily roll off from the ZnO/CuO composite nanostructure which is described as a CassieBaxter state surface. The Cassie-Baxter model is expressed by Eq. (1) [21]. Cos ϴ∗ = f (cosӨ+1)-1
(1)
In this equation, Ө∗and Ө indicate the water contact angles of the ZnO/CuO composite and the flat surface (Young's equilibrium contact
Fig. 4 presents the FTIR spectrum of sample S2. The peak observed at 600 cm−1 is related to the formation of metal oxides [25]. The observed band at 2360 cm−1 is attributed to the CO2 adsorption from air which is the common impurity in IR analysis and can be neglected [26–30]. Two small peaks at around 2918 cm−1 can be assigned to the C–H stretching mode of the free hexamethylenetetramine ligand. These results show that some hexamethylenetetramine molecules have been adsorbed on the surface of ZnO/CuO particles. Similar results have been reported by other researchers in the case of ZnO, ZnS, Co3O4, and NiO [31–36]. According to the literature, these peaks can be considered as the common bands which can be observed in the FTIR spectra of different metal oxides [37–42]. However in all of the mentioned cases, the superhydrophobicity without any surface modification by low surface energy materials has not been reported. Similar to what has been observed in this study, Zhang et al. reported the appearance of weak absorption bands at 2920 and 2851 cm−1 assigned to adsorption of zinc acetate (an organic compound) on the untreated surface of a ZnO coated stainless steel mesh with hydrophilic characteristics. The cited surface has shown the superhydrophobic behavior only after the surface chemical modification by stearic acid [43]. It confirms that the superhydrophobic behavior of the fabricated ZnO/CuO hybrid nanopowders has not been created by the adsorbed hexamine molecules. The peaks showing lower intensities which are also observed at 1631 cm−1 and 3430 cm−1 correspond to the stretching and bending vibration modes of the O–H groups [44]. 3.3. XRD and EDX analyses XRD analysis was used to determine the composition and purity of the nanostructured powders (Fig. 5). Fig. 5a shows the XRD pattern of sample S1. All diffraction peaks observed in the figure are completely matched with the hexagonal wurtzite structure of ZnO according to the
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Fig. 6. EDX spectrum and mapping of sample S1.
JCPDS card No. 00-047-0956 [45]. XRD pattern of sample S2 is indicated in Fig. 5b. All the observed diffraction peaks in this XRD pattern can be indexed to the monoclinic structure of CuO (JCPDS card No. 00001-1117) along with ZnO, explaining the presence of CuO and ZnO in the prepared sample. Similar results were obtained by previous reports of ZnO/CuO composite synthesis [46–48]. Figs. 6 and 7 display the EDX spectra and elemental mapping of samples S1 and S2, respectively. The EDX spectrum displayed in Fig. 6 shows almost the presence of Zn and O without any trace of other substances. It confirms synthesis of pure ZnO particles. The elemental map of the ZnO/CuO composite reveals a uniform incorporation of both Cu and Zn in the resulted hybrid structure, confirming formation of ZnO/CuO nanocomposite. Assessments of elemental mapping and EDX spectrum of sample S7 given in Fig. S2 reveals the fabrication of ZnO–CuO composite compounds. The EDX spectra and elemental maps of the other samples are presented in Fig. S1.
3.4. Non-wettable properties and morphology analysis of the ZnO/CuO hierarchical superhydrophobic surface Sample S2 was used to create a superhydrophobic coating on the stainless steel mesh (#320) via the spray deposition method. The coated mesh exhibited high WCA of 159.4° ± 1° and low CAH of 1.2° ± 0.3°. The WCAs of the bare and seed layer deposited meshes were 100.5° ± 2° and 130.4° ± 1°, respectively. Fig. 8 shows the images of water droplets placed on the bare, seed layer coated and final
superhydrophobic meshes. The SEM images of all these surfaces with low and high magnifications are presented in Fig. 9. The SEM micrographs of the seed layer (Fig. 9b and c) confirm the regular distribution of nano-sized particles with an average diameter of ∼150 nm on the surface of the substrate. Fig. 9d–f shows the surface morphology of ZnO/CuO nanocomposite coated on the seed layer. As it can be seen, the mesh surface is covered uniformly and also densely with irregular microrods and needle-like micro-flower architectures. The micro-flower structures are composed of several needles with diameters of less than 50 nm connected to each other. Similar results can be observed for sample S2 (Fig. 3b). It can be concluded that the superhydrophobic properties are obtained in this study just by fabrication of micro/nanoscale binary roughness on the surface.
3.5. Lasting stability and corrosion resistance Maintaining the superhydrophobic properties of surfaces under different environmental conditions over time is one of the controversial issues for the practical applications. In general, the change of surface topographical features especially in its chemical composition may alter the wettability behavior of the surface with prolonged exposure time. Therefore, the lasting durability of the prepared ZnO–CuO superhydrophobic surfaces under ambient atmosphere and saline solution (3.5% wt NaCl) was investigated. The results showed that the surface wettability was not significantly changed after one-year exposure in ambient atmosphere with WCA higher than 155° and CAH less than 5°.
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Fig. 7. EDX spectrum and mapping of sample S2.
Fig. 8. Image of the water droplet on the uncoated mesh and ZnO/CuO nanostructured surface.
Fig. 9. SEM image of bare mesh (a) and ZnO/CuO superhydrophobic surface on the stainless-steel mesh at different magnifications (b–d).
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Fig. 10. Variation of WCA and CAH values vs. immersion time in saline solution.
Fig. 10 displays the change of surface wettability vs. the immersion time in the saline solution. The results showed that the superhydrophobic properties were preserved after 6-day immersion with the WCA above 150° and CAH lower than 5°. However, it was decreased with further increasing of the time. The CAH rose to about 11° and WCA decreased to 138° after a 7-day duration immersion mode. Generally, the superhydropbobic characteristic reduces after a long-time immersion in corrosive media due to decline of coating as well as destruction of the hierarchical micro-nano structures. In order to investigate the corrosion resistance of the prepared superhydrophobic ZnO/CuO coating, the potentiodynamic polarization and Bode impedance curves in NaCl solution (3.5 wt%) were measured. Fig. 11 shows the Tafel curves of the bare stainless steel mesh and ZnO/ CuO superhydrophobic coating. The corrosion potential (Ecorr) and corrosion current density (icorr) derived from Fig. 11 by extrapolation of Tafel curves are presented in Table 4. According to the obtained results, the corrosion resistance of the coated mesh was significantly improved due to the higher Ecorr (−145 mV) and lower Icorr (4.58 × 10−9 A cm−2) compared to the bare stainless steel mesh (Ecorr = −190 mV, Icorr = 3 × 10−8). In general, increasing of the corrosion potential and decreasing of the current corrosion density exhibit the best corrosion protection [49–52]. These results clearly reveal that coating of the stainless steel mesh with a hydrophobic film can effectively protect it from the corrosion. The obtained anticorrosion characteristics can be explained by the air pockets trapped in the hierarchical micro/nanostructure that prevent the penetration of any dissolved corrosive ions in water (like chloride) to the surface. This phenomenon is the most important reason for the corrosion resistance improvement of the superhydrophobic surfaces. The EIS experiments were carried out for further evaluation of the corrosion resistance. Fig. 12 displays the Bode plot of as-prepared superhydrophobic coating and the substrate. Results confirm that the ǀZǀ values of superhydrophobic surface in the high-frequent and low-frequent parts increase noticeably compared to the substrate which proves that the surface hydrophobization can rapidly improve its corrosion protection performance.
Fig. 11. Potentiodynamic polarization curves of bare stainless steel mesh and ZnO/CuO superhydrophobic coating in 3.5 wt% NaCl aqueous solution.
3.6. Self-cleaning performance Table 4 Corrosion potential and corrosion current density of ZnO/CuO superhydrophobic coated and bare meshes. sample
Ecorr (mV)
Icorr (A cm−2)
Bare stainless steel mesh ZnO/CuO superhydrophobic coated mesh
−145 −190
4.58 × 10−9 3 × 10−8
Self-cleaning property is the critical feature of the surface to stop the fouling in many industrial processes. Using surfaces having these characteristics can reduce cleaning or de-fouling costs. Usually cleaning procedure is carried out using detergents, along with scrubbing and high-pressured water jets. To investigate the self-cleaning performance of the prepared surfaces, carbon powders (soot) were spread on the resulted coating (Fig. 13a). According to Fig. 13b and c, the water droplets easily rolled off the surface as soon as contact with it. Simultaneous attachment of the contaminations to the water drops made the surface clean. Fig. 13d presents an excellent self-cleaning ability of the prepared ZnO–CuO coating. 4. Conclusion
Fig. 12. Bode impedance curve of bare stainless steel mesh and ZnO/CuO superhydrophobic coating in 3.5 wt% NaCl aqueous solution.
6 ZnO–CuO composite nanostructures were synthesized via chemical precipitation method. The wettability properties of the prepared samples were investigated as a function of the ZnO:CuO precursor solutions volume ratios and alkaline agent type. All samples exhibited good hydrophobic properties with WCA values larger than 140°. The resulted composite under the optimum conditions (volume ratio of 3:1 and HMTA as alkaline agent) indicated excellent superhydrophobic properties with WCA of 162.6° ± 1° and CAH of < 5°. The obtained coated stainless steel mesh with the optimum sample showed unique self-cleaning and anti-corrosion properties along with excellent lasting stability, especially under the ambient atmosphere condition.
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Fig. 13. Self-cleaning performance of ZnO/ CuO coating (a–d).
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[36] R. Gopikrishnan, K. Zhang, P. Ravichandran, S. Baluchamy, V. Ramesh, S. Biradar, P. Ramesh, Synthesis, characterization and biocompatibility studies of zinc oxide (ZnO) nanorods for biomedical application, Nano-Micro Lett. 2 (2010) 31–36 https://doi.org/10.1007/BF03353614. [37] S.M. Aydoghmish, S.A. Hassanzadeh-Tabrizi, A. Saffar-Teluri, Facile synthesis and investigation of NiO–ZnO–Ag nanocomposites as efficient photocatalysts for degradation of methylene blue dye, Ceram. Int. (2019), https://doi.org/10.1016/j. ceramint.2019.04.229. [38] R. Joshi, Facile photochemical synthesis of ZnO nanoparticles in aqueous solution without capping agents, Materialia 2 (2018) 104–110 https://doi.org/10.1016/j. mtla.2018.07.001. [39] S. Sharma, V.S. Dalal, V. Mahajan, Synthesis of Zinc oxide nano flower for photovoltaic applications, Mater. Today 3 (2016) 1359–1362 https://doi.org/10.1016/j. matpr.2016.04.015. [40] S. Li, Z. Sun, R. Li, M. Dong, L. Zhang, W. Qi, X. Zhang, H. 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[41] T. Jiang, Z. Guo, Robust superhydrophobic tungsten oxide coatings with photochromism and UV durability properties, Appl. Surf. Sci. 387 (2016) 412–418 https://doi.org/10.1016/j.apsusc.2016.06.125. [42] Y. Wang, B. Li, C. Xu, Fabrication of Superhydrophobic Surface of Hierarchical ZnO Thin Films by Using Stearic Acid, Superlattice, Microst., 2012, pp. 128–134. [43] Y. Zhang, X. Wang, C. Wang, J. Liu, H. Zhai, B. Liu, X. Zhao, D. Fang, Facile fabrication of zinc oxide coated superhydrophobic and superoleophilic meshes for efficient oil/water separation, RSC Adv. 8 (2018) 35150–35156. [44] I. Gromyko, M. Krunks, T. Dedova, A. Katerski, D. Klauson, I. Oja Acik, Surface properties of sprayed and electrodeposited ZnO rod layers, Appl. Surf. Sci. 405 (2017) 521–528 https://doi.org/10.1016/j.apsusc.2017.02.065. [45] H. Wang, C. Xie, Controlled fabrication of nanostructured ZnO particles and porous thin films via a modified chemical bath deposition method, J. Cryst. Growth 291 (2006) 187–195 https://doi.org/10.1016/j.jcrysgro.2006.02.043. [46] S. Harish, J. Archana, M. Sabarinathan, M. Navaneethan, K.D. Nisha, S. Ponnusamy, C. Muthamizhchelvan, H. Ikeda, D.K. Aswal, Y. Hayakawa, Controlled structural and compositional characteristic of visible light active ZnO/CuO photocatalyst for the degradation of organic pollutant, Appl. Surf. Sci. 418 (2017) 103–112 https:// doi.org/10.1016/j.apsusc.2016.12.082. [47] A. Wei, L. Xiong, L. Sun, Y.-J. Liu, W.-W. Li, Chinese. CuO nanoparticle modified ZnO nanorods with improved photocatalytic activity, Chin. Phys. Lett. 30 (2013)
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Single-step prepared hybrid ZnO/CuO nanopowders for water repellent and corrosion resistant coatings
T
Elmira Velayi, Reza Norouzbeigi∗ School of Chemical, Petroleum and Gas Engineering, Iran University of Science and Technology, P.B. 16765–163, Narmak, Tehran, Iran
A R T I C LE I N FO
A B S T R A C T
Keywords: Hybrid nanopowders Superhydrophobicity Spray deposition Corrosion resistant
In this study, ZnO/CuO hybrid hydrophobic nanopowders were synthesized using a common single-step chemical precipitation route without using modifiers. Influence of initial ZnO:CuO precursor concentrations and alkaline agent type on the wettability behavior of the prepared samples were investigated. Wettability properties of the prepared samples were assessed by measuring the water contact angle and contact angle hysteresis values. Fourier transform infrared spectra, scanning electron microscope micrographs and X-ray diffraction patterns were applied to identify the surface chemistry and morphological features. Scanning electron microscope images of the synthesized ZnO/CuO nanocomposites indicated flower-like morphologies containing plenty of nanoneedles, -rods, and -sheets with thicknesses lower than 90 nm. The sample prepared under the optimum conditions was superhydrophobic having water contact angle and contact angle hysteresis of 162.6° ± 1 and 2°, respectively. It was applied to coat the surface of stainless steel meshes by spray deposition method. The resultant superhydrophobic surface exhibited excellent self-cleaning (water repellency) property and a suitable stabilities under the ambient and saline solution (NaCl, 3.5%) media. Additionally, electrochemical corrosion tests confirmed that the corrosion resistance of the fabricated ZnO–CuO coating was higher than the initial bare mesh.
1. Introduction
synthesized via a simple one-step method [18]. The prepared samples showed good superhydrophobic properties after modification by stearic acid. In another research, titania-modified SiO2 nanocomposites were deposited on glass plates using dip coating method. The prepared coating at the optimal ratio of 9:1 (dip coating solution: SiO2) indicated excellent anti-adhesion properties with the lower cell adhesion rate (< 10%) and water contact angle of 152° [19]. Brassard and co-workers fabricated a nanocomposite of epoxy polymer containing stearic acid functionalized ZnO nanoparticles via spray deposition. The wetting behavior of the surface showed that the nanocomposite thin film had superhydrophobic properties with the water contact angle of 156° and contact angle hysteresis of 4° [20]. Therefore in most of the studies, the superhydrophobic nanocomposite surface has been fabricated by creation of a rough surface and modification with low surface free energy materials, respectively. 4 This paper reports synthesis of superhydrophobic ZnO/CuO nanopowders via chemical precipitation method without using any chemical modifier. The wetting characteristics of the prepared samples were investigated as a function of ZnO:CuO precursor solutions volume ratio. Additionally, effect of alkaline agent type on the morphological features and wettability properties was studied. In the next step, the
3 In recent years, creation of superhydrophobic surfaces by mimicking the lotus leaf has attracted many researchers due to the various industrial applications [1,2]. Surfaces with water contact angles (WCA) of higher than 150° and contact angle hysteresis (CAH) values less than 10° are defined as superhydrophobic surfaces which are the promising candidates in many areas such as anti-icing, self-cleaning, anticorrosion, and antifogging coatings, along with microfluidic devices and oil-water separation [3–8]. Several methods have been proposed to fabricate superhydrophobic surfaces by two main steps: creation of the rough structures and modification the rough surfaces by low surface free energy materials [1,9]. Up to now numerous semiconductor metal oxides like ZnO, CuO, and TiO2 have been extensively applied to generate uniform rough surfaces [10–15]. These surfaces usually show superhydrophobic properties just after chemical modification by low surface energy materials. However, there are few reports of construction of superhydrophobic nanocomposite materials. The nanocomposite materials exhibit better chemical and mechanical stabilities and also better wear resistance compared to the separate phases [16,17]. Hierarchical flowerlike iron-containing MnO2 particles have been
∗
Corresponding author. E-mail addresses: [email protected] (E. Velayi), [email protected] (R. Norouzbeigi).
https://doi.org/10.1016/j.ceramint.2019.05.229 Received 10 April 2019; Received in revised form 18 May 2019; Accepted 21 May 2019 Available online 22 May 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Table 1 Initial different volume ratio of ZnO: CuO precursor solutions. Sample
Volume ratio (ZnO: CuO precursor solutions)
Alkaline type
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
1:0 3:1 2:2 1:3 0:1 1:0 3:1 2:2 1:3 0:1
HMTA HMTA HMTA HMTA HMTA NH3 NH3 NH3 NH3 NH3
Zn (NO3)2.6H2O
Cu (NO3)2.6H2O
Fig. 2. Photo image of the water droplet on Sample S2.
(0.025M)
(0.025M)
2. Materials and methods Mixing (different volumetric concentration)
2.1. Synthesis of superhydrophobic ZnO/CuO nanocomosits In this study, the chemical precipitation method was used to prepare the ZnO/CuO hybrid nanopowders. Zinc nitrate (Zn(NO3). 6H2O), Hexamethylenetetramine (HMTA), copper nitrate (Cu(NO3)2) and ammonia solution (25%) were used without further purification. All reagents were purchased from Merck Company. Firstly, the aqueous solutions of zinc nitrate hexahydrate (0.025 M) and copper nitrate hexahydrate (0.025) were prepared. Subsequently, the solutions were mixed with different volume ratios of ZnO: CuO, as shown in Table 1. Then a solution of HMTA (0.0375 M) or 10 ml of ammonia (25%) was added. The synthesis duration was considered 2 h for all experiments. Fig. 1 displays the flowchart of the synthesis procedure.
HMTA (0.0375) Or NH3 (25%, 10ml)
Heating (in oil bath at 95º C for 2h)
2.2. Spray coating of the ZnO/CuO nanocomposite
Filtration
The ZnO/CuO nanopowders were deposited on the substrate in two steps. Firstly, a ZnO seed layer was formed on the stainless steel mesh via thermal decomposition of zinc acetate and secondly, ZnO/CuO nanostructures were coated on the seed layer by a simple spraying. A stainless steel mesh (2 cm × 2 cm) was used as substrate. It was washed ultrasonically with ethanol and distilled water several times and then dried. Afterward, it was immersed in an ethanolic solution of zinc acetate (20 mM) for 30 min. Lastly for seed layer formation, the substrate was dried in an oven at 100 °C for 1 h and subsequently calcined in air at 400 °C for 2 h. In the next step, 0.1 g of the ZnO/CuO nanopowders synthesized under the optimum conditions were dispersed in tetrahydrofuran (15 ml) via the ultrasonic process for 2 h.The spray deposition was performed a stainless steel nozzle as a part of airbrush using pressured air (3 bar). The distance and angle between the substrate and the airbrush were considered constant at 10 cm and 90°, respectively. The spray deposition was repeated 3 times and the coated substrate was dried between the steps.
Drying
Fig. 1. Flowchart of ZnO/CuO nanocomposite preparation process by chemical precipitation method.
Table 2 WCA values of prepared samples. Sample
Volume ratio (ZnO: CuO precursor solutions)
WCA (°)
S1 S2 S3 S4 S5 S6 S7 S8 S9 S10
1:0 3:1 2:2 1:3 0:1 1:0 3:1 2:2 1:3 0:1
148 ± 0.7.3 ± 0.7 162.6 ± 1 151.7 ± 0.8 145 ± 2.1 140.9 ± 1 150 ± 0.5 147.1 ± 1 148.7 ± 2 145.1 ± 1.8 158.2 + 1.8
2.3. Characterization
ZnO/CuO nanocomposite prepared under the optimum conditions was sprayed onto stainless steel meshes as substrate. The self-cleaning behavior, anti-corrosion properties and chemical stabilities in the air and saline solution were studied.
A digital optical microscope (DINOLITE, model AM-4113ZT, Taiwan) was used to determine the WCA and CAH values. The oscillation method was applied to measure the CAH. In this method, the measured receding angle was subtracted from the advancing one to calculate the contact angle hysteresis. Morphology of the prepared samples was evaluated by scanning electron microscopy equipped with an energy dispersive spectrometer (SEM-EDX, TESCAN VEGA). The chemical composition of the ZnO/CuO composite nanostructures was
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Table 3 Comparison of the wettability characteristics of some superhydrophobic powders. Materials
Synthesis method
WCA (°)
Modifiers
Ref
ZnO ZnO ZnO MnO2 ZnO–CuO
Hydrothermal Hydrothermal Sonochemical Hydrothermal Chemical precipitation method
> 160 160 163 156 162.6
Palmitic acid Fluoroethylene vinyl ether 1H,1H,2H,2H-perfluorooctyltriethoxysilane Stearic acid –
[22] [23] [24] [18] This research
Fig. 3. SEM images of sample: (a, b) S1, (c, d) S2, (e, f) S5, (g, h) S6, (I, j) S7, and (k, l) S10.
characterized using Fourier transform infrared (FTIR) spectrometer (ATR-FTIR, model ATR-8000, Shimadzu, Japan). The purity and crystal structure of samples were characterized by X-ray diffraction analysis (XRD). The corrosion properties of the samples were evaluated by electrochemical impedance spectroscopy (EIS) and potentiodynamic
polarization curve (Tafel) using an Iranian electrochemical workstation (RADSTATPS3). In this test, a standard three-electrode system was applied with an Ag/AgCl (3 M KCl) reference electrode, a platinum plate counter electrode, and the prepared sample (0.5 cm × 1 cm) as working electrode. The potential range was selected from −0.3 to
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angle), respectively. Term f in the model is the solid-liquid interface fraction. According to Eq. (1), the contact angle increases by decreasing the solid-liquid interface fraction. Table 3 presents the WCA values reported in the past researches. The WCA value obtained in this research is the highest one among the other reported hydrophobic powders. It should be noticed that the superhydrophobicity has been achieved in this research without using chemical modifiers. Fig. 3 shows the SEM images of S1, S2, S5, S6, S7, and S10. Sample S1 (Fig. 3a and b) is composed of hierarchical micro/nano branched wires with random orientation and 4–5μm length. Fig. 3c and d shows the SEM micrographs of sample S2. As obviously evidenced in this figure, several uniform microrods and flower-like architectures are grown. The flower-like structures are composed of numerous needles with the tip diameter of less than 50 nm. The SEM images of flower-like hierarchical CuO structure (sample S5) with different magnifications are presented in Fig. 3e and f. It can be seen that the regular flower-like structures are result of self-assembled accumulation of nano petals with thickness of ∼50 nm. Fig. 3g-l indicate the microstructural features of the prepared samples using ammonia as the alkaline agent. It is observed that the similar hierarchical micro/nanostructures can also be obtained using a different alkaline agent. The flower-like structures consist of needles with shorter length are observed in sample S6. Fig. 3i and j presents the uniform flower-like and microsphere structures composed of many needles and nano-sheets with diameters and thicknesses less than 100 nm, respectively. The SEM micrographs of sample S10 are shown in Fig. 3k and l. Construction of flower-like CuO micro/nanostructures, including numerous interconnected nano-sheets which are less than 50 nm in thicknesses can be seen. In general, the SEM results show that all prepared samples have hierarchical micro/nanostructures similar to the lotus leaves structure and promoting the superhydrophobic properties.
Fig. 4. FTIR spectrum of sample S2.
3.2. ATR-FTIR analysis
Fig. 5. XRD pattern of sample: (a) S1, (b) S2.
−0.1 mV to perform the potentiodynamic polarization experiments. Moreover, the scan rate was 5 mVs−1. The EIS tests were conducted in the frequency range between 5 to 5000 HZ at open circuit potential (OCP).
3. Results and discussions 3.1. Wetting characterization and morphology analysis of the synthesized powders In order to characterize the wetting properties, the synthesized ZnO/CuO hybrid (composite) nanopowders were spread out on a stainless steel surface (#304) like a dense layer and the sessile drop method was applied to measure the WCAs. Table 2 presents the measured WCA values. It can be seen that all data are in the range of 140–162°. However, the sample S2 prepared by HMTA shows the best superhydrophobic properties with WCA of 162.6° ± 1°. On the other hand, the CAH value of sample S2 is smaller than 5° which confirms the superhydrophobicity. Photo image of water droplet placed on the S2 is shown in Fig. 2. In this condition, water drops can easily roll off from the ZnO/CuO composite nanostructure which is described as a CassieBaxter state surface. The Cassie-Baxter model is expressed by Eq. (1) [21]. Cos ϴ∗ = f (cosӨ+1)-1
(1)
In this equation, Ө∗and Ө indicate the water contact angles of the ZnO/CuO composite and the flat surface (Young's equilibrium contact
Fig. 4 presents the FTIR spectrum of sample S2. The peak observed at 600 cm−1 is related to the formation of metal oxides [25]. The observed band at 2360 cm−1 is attributed to the CO2 adsorption from air which is the common impurity in IR analysis and can be neglected [26–30]. Two small peaks at around 2918 cm−1 can be assigned to the C–H stretching mode of the free hexamethylenetetramine ligand. These results show that some hexamethylenetetramine molecules have been adsorbed on the surface of ZnO/CuO particles. Similar results have been reported by other researchers in the case of ZnO, ZnS, Co3O4, and NiO [31–36]. According to the literature, these peaks can be considered as the common bands which can be observed in the FTIR spectra of different metal oxides [37–42]. However in all of the mentioned cases, the superhydrophobicity without any surface modification by low surface energy materials has not been reported. Similar to what has been observed in this study, Zhang et al. reported the appearance of weak absorption bands at 2920 and 2851 cm−1 assigned to adsorption of zinc acetate (an organic compound) on the untreated surface of a ZnO coated stainless steel mesh with hydrophilic characteristics. The cited surface has shown the superhydrophobic behavior only after the surface chemical modification by stearic acid [43]. It confirms that the superhydrophobic behavior of the fabricated ZnO/CuO hybrid nanopowders has not been created by the adsorbed hexamine molecules. The peaks showing lower intensities which are also observed at 1631 cm−1 and 3430 cm−1 correspond to the stretching and bending vibration modes of the O–H groups [44]. 3.3. XRD and EDX analyses XRD analysis was used to determine the composition and purity of the nanostructured powders (Fig. 5). Fig. 5a shows the XRD pattern of sample S1. All diffraction peaks observed in the figure are completely matched with the hexagonal wurtzite structure of ZnO according to the
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Fig. 6. EDX spectrum and mapping of sample S1.
JCPDS card No. 00-047-0956 [45]. XRD pattern of sample S2 is indicated in Fig. 5b. All the observed diffraction peaks in this XRD pattern can be indexed to the monoclinic structure of CuO (JCPDS card No. 00001-1117) along with ZnO, explaining the presence of CuO and ZnO in the prepared sample. Similar results were obtained by previous reports of ZnO/CuO composite synthesis [46–48]. Figs. 6 and 7 display the EDX spectra and elemental mapping of samples S1 and S2, respectively. The EDX spectrum displayed in Fig. 6 shows almost the presence of Zn and O without any trace of other substances. It confirms synthesis of pure ZnO particles. The elemental map of the ZnO/CuO composite reveals a uniform incorporation of both Cu and Zn in the resulted hybrid structure, confirming formation of ZnO/CuO nanocomposite. Assessments of elemental mapping and EDX spectrum of sample S7 given in Fig. S2 reveals the fabrication of ZnO–CuO composite compounds. The EDX spectra and elemental maps of the other samples are presented in Fig. S1.
3.4. Non-wettable properties and morphology analysis of the ZnO/CuO hierarchical superhydrophobic surface Sample S2 was used to create a superhydrophobic coating on the stainless steel mesh (#320) via the spray deposition method. The coated mesh exhibited high WCA of 159.4° ± 1° and low CAH of 1.2° ± 0.3°. The WCAs of the bare and seed layer deposited meshes were 100.5° ± 2° and 130.4° ± 1°, respectively. Fig. 8 shows the images of water droplets placed on the bare, seed layer coated and final
superhydrophobic meshes. The SEM images of all these surfaces with low and high magnifications are presented in Fig. 9. The SEM micrographs of the seed layer (Fig. 9b and c) confirm the regular distribution of nano-sized particles with an average diameter of ∼150 nm on the surface of the substrate. Fig. 9d–f shows the surface morphology of ZnO/CuO nanocomposite coated on the seed layer. As it can be seen, the mesh surface is covered uniformly and also densely with irregular microrods and needle-like micro-flower architectures. The micro-flower structures are composed of several needles with diameters of less than 50 nm connected to each other. Similar results can be observed for sample S2 (Fig. 3b). It can be concluded that the superhydrophobic properties are obtained in this study just by fabrication of micro/nanoscale binary roughness on the surface.
3.5. Lasting stability and corrosion resistance Maintaining the superhydrophobic properties of surfaces under different environmental conditions over time is one of the controversial issues for the practical applications. In general, the change of surface topographical features especially in its chemical composition may alter the wettability behavior of the surface with prolonged exposure time. Therefore, the lasting durability of the prepared ZnO–CuO superhydrophobic surfaces under ambient atmosphere and saline solution (3.5% wt NaCl) was investigated. The results showed that the surface wettability was not significantly changed after one-year exposure in ambient atmosphere with WCA higher than 155° and CAH less than 5°.
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Fig. 7. EDX spectrum and mapping of sample S2.
Fig. 8. Image of the water droplet on the uncoated mesh and ZnO/CuO nanostructured surface.
Fig. 9. SEM image of bare mesh (a) and ZnO/CuO superhydrophobic surface on the stainless-steel mesh at different magnifications (b–d).
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Fig. 10. Variation of WCA and CAH values vs. immersion time in saline solution.
Fig. 10 displays the change of surface wettability vs. the immersion time in the saline solution. The results showed that the superhydrophobic properties were preserved after 6-day immersion with the WCA above 150° and CAH lower than 5°. However, it was decreased with further increasing of the time. The CAH rose to about 11° and WCA decreased to 138° after a 7-day duration immersion mode. Generally, the superhydropbobic characteristic reduces after a long-time immersion in corrosive media due to decline of coating as well as destruction of the hierarchical micro-nano structures. In order to investigate the corrosion resistance of the prepared superhydrophobic ZnO/CuO coating, the potentiodynamic polarization and Bode impedance curves in NaCl solution (3.5 wt%) were measured. Fig. 11 shows the Tafel curves of the bare stainless steel mesh and ZnO/ CuO superhydrophobic coating. The corrosion potential (Ecorr) and corrosion current density (icorr) derived from Fig. 11 by extrapolation of Tafel curves are presented in Table 4. According to the obtained results, the corrosion resistance of the coated mesh was significantly improved due to the higher Ecorr (−145 mV) and lower Icorr (4.58 × 10−9 A cm−2) compared to the bare stainless steel mesh (Ecorr = −190 mV, Icorr = 3 × 10−8). In general, increasing of the corrosion potential and decreasing of the current corrosion density exhibit the best corrosion protection [49–52]. These results clearly reveal that coating of the stainless steel mesh with a hydrophobic film can effectively protect it from the corrosion. The obtained anticorrosion characteristics can be explained by the air pockets trapped in the hierarchical micro/nanostructure that prevent the penetration of any dissolved corrosive ions in water (like chloride) to the surface. This phenomenon is the most important reason for the corrosion resistance improvement of the superhydrophobic surfaces. The EIS experiments were carried out for further evaluation of the corrosion resistance. Fig. 12 displays the Bode plot of as-prepared superhydrophobic coating and the substrate. Results confirm that the ǀZǀ values of superhydrophobic surface in the high-frequent and low-frequent parts increase noticeably compared to the substrate which proves that the surface hydrophobization can rapidly improve its corrosion protection performance.
Fig. 11. Potentiodynamic polarization curves of bare stainless steel mesh and ZnO/CuO superhydrophobic coating in 3.5 wt% NaCl aqueous solution.
3.6. Self-cleaning performance Table 4 Corrosion potential and corrosion current density of ZnO/CuO superhydrophobic coated and bare meshes. sample
Ecorr (mV)
Icorr (A cm−2)
Bare stainless steel mesh ZnO/CuO superhydrophobic coated mesh
−145 −190
4.58 × 10−9 3 × 10−8
Self-cleaning property is the critical feature of the surface to stop the fouling in many industrial processes. Using surfaces having these characteristics can reduce cleaning or de-fouling costs. Usually cleaning procedure is carried out using detergents, along with scrubbing and high-pressured water jets. To investigate the self-cleaning performance of the prepared surfaces, carbon powders (soot) were spread on the resulted coating (Fig. 13a). According to Fig. 13b and c, the water droplets easily rolled off the surface as soon as contact with it. Simultaneous attachment of the contaminations to the water drops made the surface clean. Fig. 13d presents an excellent self-cleaning ability of the prepared ZnO–CuO coating. 4. Conclusion
Fig. 12. Bode impedance curve of bare stainless steel mesh and ZnO/CuO superhydrophobic coating in 3.5 wt% NaCl aqueous solution.
6 ZnO–CuO composite nanostructures were synthesized via chemical precipitation method. The wettability properties of the prepared samples were investigated as a function of the ZnO:CuO precursor solutions volume ratios and alkaline agent type. All samples exhibited good hydrophobic properties with WCA values larger than 140°. The resulted composite under the optimum conditions (volume ratio of 3:1 and HMTA as alkaline agent) indicated excellent superhydrophobic properties with WCA of 162.6° ± 1° and CAH of < 5°. The obtained coated stainless steel mesh with the optimum sample showed unique self-cleaning and anti-corrosion properties along with excellent lasting stability, especially under the ambient atmosphere condition.
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Fig. 13. Self-cleaning performance of ZnO/ CuO coating (a–d).
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