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Superhydrophilic anti-corrosive and superhydrophobic durable TiO2 /Ti mesh for oil/water separation Xue Zhou, Sirong Yu∗, Jun Wang, Jie Zang, Zhexin Lv College of Material Science and Engineering, China University of Petroleum (East China), Qingdao 266580, China
a r t i c l e
i n f o
Article history: Received 22 July 2019 Revised 18 September 2019 Accepted 13 October 2019 Available online xxx Keywords: Oil/water separation Superhydrophilic Superhydrophobic TiO2 /Ti mesh
a b s t r a c t The effective oil/water separation is important in the treatment of oily wastewater. Recently, materials with superwetting have become research hotspots due to their high oil/water separation efficiency and wide application. Here, TiO2 /Ti mesh with both superhydrophilicity and superhydrophobicity was successfully fabricated to separate oil/water mixtures. Superhydrophilic TiO2 nanotube film was prepared on Ti mesh by anodic oxidation. The superhydrophilic TiO2 /Ti mesh showed good oil/water separation property for various oil/water mixtures. Besides, the superhydrophilic TiO2 /Ti mesh exhibited anti-corrosive separation property that had high separation efficiency for oil/strong acid/alkali/salt solution mixtures. After surface modification with lauric acid, the superhydrophilic TiO2 /Ti mesh converted to superhydrophobic. The superhydrophobic TiO2 /Ti mesh indicated good durability under different conditions that it showed thermal stability under 150 °C, anti-icing property at -20 °C and mechanical stability under abrasion and bending test. Moreover, the superhydrophobic TiO2 /Ti mesh could also separate various oil/water mixtures efficiently. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Water pollution including domestic sewage, industrial wastewater, and oil spills has posed a serious threat to nature and human life. Separating the oils and water is a key point in the water treatment process [1,2]. A variety of traditional methods have been used to separated oils and water, such as mechanical [3], chemical [4] and biological methods [5]. However, these three traditional methods have the disadvantage of low separation efficiency, the second pollution and high cost [6]. Hence, the method which is efficient, eco-friendly and cost-effective is worth being explored. Recently, the special wettability interface materials with high separation efficiency and wide selectivity have become the main research direction in the field of oil/water separation [7,8]. The special wettability interface materials always show superhydrophilic (underwater superoleophobic) or superhydrophobic (superlipophilic) [9,10]. Several materials with special wettability have been designed in oil/water separation including metal mesh [11], fabric [12], filter paper [13], foam [14]. Among them, metal mesh is widely used because that chemical reactions and modification are easy operated on its surface, and it has the advantages of high mechanical strength, low cost and high flow rate [15].
∗
Corresponding author. E-mail address: [email protected] (S. Yu).
Titanium dioxide (TiO2 ) nanomaterials often exhibit hydrophilicity due to the presence of a large amount of hydroxyl groups on their surface [16]. Besides, TiO2 nanomaterials have been used to prepare superhydrophobic surface because of their smaller solid-liquid contact area [17], which follows the Cassie-Baxter theory [18]. Li et al. [19] have fabricated underwater superoleophobic TiO2 coated mesh by spraying TiO2 nanoparticles and polyurethane mixtures onto stainless steel mesh, which exhibits high oil/water separation efficiency. Kumar and Tudu [20] have prepared both superhydrophobic steel and copper meshes modified by perfluorodecyltriethoxysilane (PFDTS) and TiO2 nanoparticles. Both two superhydrophobic metal meshes can separate not only oil-water mixtures but also oil-water emulsions efficiently. Bandara and Gunatilake [21] have built superhydrophilic TiO2 nanofibers coated stainless steel mesh, which indicates excellent oil/water separation property. The TiO2 nanofibers are hydrothermally synthesized and then sprayed on the stainless steel mesh. Compared to the above methods, the ways that nanostructured TiO2 grows directly on Ti mesh are simple and effective. Zhou et al. [22] have reported that a superhydrophilic TiO2 nanowires film grows on Ti mesh by a facile immersion method, and the obtained mesh shows good oil/water and oil/corrosive solutions separation ability. In addition, the material only exhibits superhydrophilicity or superhydrophobicity cannot meet the demands in practical applications. The superhydrophilic mesh which only allows water to penetrate is more suitable for the separation of light (ρ oil < ρ water ) oil/water and
https://doi.org/10.1016/j.jtice.2019.10.011 1876-1070/© 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: X. Zhou, S. Yu and J. Wang et al., Superhydrophilic anti-corrosive and superhydrophobic durable TiO2/Ti mesh for oil/water separation, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.011
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Fig. 1. SEM images of pristine Ti mesh (a) and (b), modified Ti mesh (c), TiO2 /Ti mesh (d) and (e), modified TiO2 /Ti mesh (f). The insets are high magnification images.
the superhydrophobic mesh which only allows oil to penetrate is more suitable for the separation of heavy (ρ oil > ρ water ) oil/water [23]. Besides, superhydrophobic surface always indicates stability in high temperature environment and anti-icing property in low temperature environment [24,25]. Therefore, preparing a material that can delivery both superhydrophilicity and superhydrophobicity would have a wider application. In this work, TiO2 nanotube film was fabricated on the Ti mesh by a facile anodic oxidation method. The TiO2 /Ti mesh exhibited superhydrophilicity after the anodic oxidation. Due to the nanotubes structure, the TiO2 /Ti mesh could obtain the superhydrophobicity easily after being modified by lauric acid. The oil/water separation property and repeatability of the superhydrophilic and superhydrophobic TiO2 /Ti mesh were both tested. For superhydrophilic TiO2 /Ti mesh, the oil/corrosive solutions separation property was measured because it always contacted aqueous solution directly. For superhydrophobic TiO2 /Ti mesh, a variety of durability tests such as anti-icing property, thermal and mechanical stability test were performed. 2. Experimental 2.1. Materials TA1 titanium mesh was purchased from KangWei materials Co. Ltd, Hengshui, China. Graphite plates were purchased from Beijing Jinglong Graphite Factory. All of the chemical regents were supplied by Sinopharm Chemical Reagent Co. Ltd, Shanghai, China. 2.2. Fabrication of superhydrophilic and superhydrophobic TiO2 /Ti mesh TiO2 was fabricated by anodizing the Ti mesh (40 mm × 25 mm) in 1 vol.% HF electrolyte. Before the anodic oxidation, Ti mesh was immersed in a solution consisting of 20 mL deionized water, 16 mL HNO3 and 4 mL HF for 1 min and ultimately cleaned by deionized water. The anodic oxidation step was conducted at 20 V for 1 h with graphite plate (40 mm × 25 mm) and Ti mesh as cathode and anode, respectively. The obtained TiO2 /Ti mesh was immediately rinsed with deionized water and dried in air. After anodic oxidation, the mesh showed superhydrophilicity. In order to obtain the superhydrophobicity, the TiO2 /Ti mesh was
immersed in a 0.02 mol/L ethanol solution of lauric acid for 24 h in the dark and subsequently dried at 60 °C for 20 min. 2.3. Characterizations and tests The morphology of mesh was observed by field emission scanning electron microscope (FESEM, JEOL, JSM-7200F). The crystal structures of mesh were recorded on an X-ray diffractometer (XRD, X’Pert PRO MPD, PANalytical B.V.). The surface chemical compositions were tested by energy disperse spectroscopy (EDS, Oxford Aztec), Fourier transform infrared spectrometer (FTIR, Nexus, Thermo Nicolet) and X-ray photoelectron spectroscopy (XPS, Escalab 250Xi). The water contact angle (WCA) was measured using a contact angle meter (SL200B, USA, KINO), the volume of deionized water droplet used for the WCA measurements was 3 μL. The WCA value was the average of at least five measurements and the error was ±1°. Oil/water separation properties of the superhydrophilic and superhydrophobic TiO2 /Ti mesh were tested. The TiO2 /Ti mesh was fixed tightly between two glass tubes. A serials of oil/water mixtures (50% v/v) including hexane/water, cyclohexane/water, petroleum ether/water, diesel oil/water and dichloromethane/water were separated. The oil/water mixture was poured slowly into the upper glass tube and oil/water separation process was achieved under gravity. The separation efficiency was calculated as the following equation:
η = (ma /mb ) × 100%
(1)
where η is the separation efficiency. mb and ma is the weight of oil (superhydrophilic mesh) or water (superhydrophobic mesh) before and after separation. 3. Results and discussion 3.1. Morphology and wettability of superhydrophilic/ superhydrophobic TiO2 /Ti mesh The morphology of the pristine and modified Ti and TiO2 /Ti meshes was observed by the SEM images. It could be learnt from Fig. 1(a) that Ti mesh with a pore size of approximately 300 mesh per inch was woven together. The magnified image of the Ti mesh
Please cite this article as: X. Zhou, S. Yu and J. Wang et al., Superhydrophilic anti-corrosive and superhydrophobic durable TiO2/Ti mesh for oil/water separation, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.011
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Fig. 2. (a) WCAs on different specimens. (b) The process of water droplet touching the superhydrophilic TiO2 /Ti mesh. (c) Image of oil droplet shape on superhydrophilic TiO2 /Ti mesh underwater. (d) Image of water droplet shape on superhydrophobic TiO2 /Ti mesh under oil.
(Fig. 1(b)) indicated that there was no special structure on the surface. However, after the low surface energy modification, it appeared some white particles on the Ti mesh which was the lauric acid adsorbed on the surface (Fig. 1(c)). After anodic oxidation, TiO2 was fabricated on the Ti mesh. Compare to Fig. 1(a), there was no difference with the low magnification image in Fig. 1(d). However, it could be observed in Fig. 1(e) that TiO2 nanotubes grew on the surface neatly with 78.61 nm inner diameter and 9.12 nm wall thickness. After surface modification, although a small amount of white particles (lauric acid) adsorbed on the surface, Fig. 1(f) showed that the shape and size of the TiO2 nanotubes were almost not affected. Nanostructured TiO2 formed on Ti mesh was appropriate for achieving superhydrophobic surfaces. On this basis, lauric acid was used to modify the rough TiO2 /Ti mesh. The wetting behavior of the pristine and modified meshes was revealed by WCAs. Fig. 2(a) showed that the pristine Ti mesh showed hydrophilic with a WCA about 25.72°, and the modified Ti mesh exhibited hydrophobicity with a WCA about 126.21°. This meant the low surface energy modification played an important role in improving the hydrophobicity. After anodic oxidation, TiO2 nanotubes were fabricated but the WCA decreased to 0°, which demonstrated that the TiO2 /Ti mesh was superhydrophilic. This accorded with Wenzel’s theory [26] that the hydrophilicity of the hydrophilic surface was enhanced as the surface roughness increased. Moreover, the water droplet spread quickly on the TiO2 /Ti mesh. As shown in Fig. 2(b), by moving the TiO2 /Ti mesh up, the water droplet contacted the TiO2 /Ti mesh and then spread in an instant. For the modified TiO2 /Ti mesh, the surface exhibited superhydrophobicity with a WCA about 161.29°. It was because that the surface with low surface energy always showed water repellency. Besides, the TiO2 nanotube structure was in favor of trapping air between the mesh and water droplet, which could be explained by the Cassie–Baxter equation [18]:
cos θ c = f (cos θ0 + 1 ) − 1
(2)
In Eq. (2), f is the area fraction of the liquid-solid contact. θ 0 and θ c are the WCA of the modified Ti mesh and modified TiO2 /Ti mesh, respectively. According to Eq. (2), the value of f was calculated to be 0.1291, which meant only 12.91% of TiO2 /Ti mesh contacted with water. Thus, plenty of air was trapped between water and TiO2 nanotubes. Besides, the superhydrophilic TiO2 /Ti mesh showed underwater superoleophobicity with a contact angle about 163.94° as shown in Fig. 2(c) and Fig. S1, which had an important impact on oil/water separation applications. For superhydrophobic mesh, it maintained
superhydrophobicity under oil with a contact angle about 162.04° as shown in Fig. 2(d) and Fig. S2. Although a layer of oil film covered the surface, it did not damage the microstructure of TiO2 film and the oil film also had a lower surface energy, so the superhydrophobic surface remained its superhydrophobicity under oil. 3.2. Chemical compositions of superhydrophilic/superhydrophobic TiO2 /Ti mesh Surface chemical compositions were significant to the surface wettability and studied in detail by EDS, XRD, FTIR and XPS. As shown in Fig. 3(a), apart from C element which was originally present in the test equipment, only Ti existed in the EDS spectrum of pristine Ti mesh. After anodic oxidation, Fig. 3(b) illustrated that O and F were also checked out. Moreover, the EDS mapping images were exhibited in Fig. 3(c) showing that Ti and O almost distributed on the whole surface. The existence of O indicated that TiO2 was possible fabricated on the Ti mesh, and the F was caused by the reaction product of TiO2 and F− in electrolyte [27]. In order to prove the above analysis, the surface crystal structure was tested by the XRD. The XRD pattern of pristine Ti mesh was shown in Fig. 3(d) indicating that all peaks corresponded to Ti. After anodic oxidation, there were no other diffraction peaks appeared in the XRD pattern of TiO2 /Ti mesh. This was because that TiO2 was always amorphous and turned into anatase or even rutile at higher temperature [28]. In order to prove the existence of surface oxides, the TiO2 /Ti mesh was treated at 400 °C for 2 h under vacuum. It could be learnt from Fig. 3(d) that two more diffraction peaks appeared at 2θ = 25.20°, 48.03° which attributed to anatase. Accordingly, TiO2 was coated on the Ti mesh and was amorphous after anodic oxidation. After low energy modification, TiO2 /Ti mesh exhibited good water repellence due to the methylene groups (–CH2 –) and methyl groups (–CH3 ) on lauric acid [29]. Fig. 3(e) illustrated the FTIR spectra of superhydrophobic TiO2 /Ti mesh and lauric acid. On this two FTIR spectra, the peak at 2954.41 cm−1 (2952.48 cm−1 ) assigned to the stretching vibration of –CH3 , and the peaks at 2921.63 cm−1 (2918.20 cm−1 ) and 2852.20 cm−1 (2850.69 cm−1 ) were corresponding to –CH2 –. The peak at 1701.16 cm−1 on the lauric acid FTIR spectrum was attributed to the –COO– groups. Moreover, at 1631.48 cm−1 , the peak representing –COO– groups also appeared on the superhydrophobic TiO2 /Ti mesh FTIR spectrum. It was because that the lauric acid always combined with TiO2 by the dehydration reaction between –COO–and –OH groups, and then the peak representing –COO– groups moved to the smaller wave number [30]. Besides, some lauric acid adsorbed on
Please cite this article as: X. Zhou, S. Yu and J. Wang et al., Superhydrophilic anti-corrosive and superhydrophobic durable TiO2/Ti mesh for oil/water separation, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.011
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Fig. 3. EDS spectra of (a) pristine Ti mesh and (b) TiO2 /Ti mesh. (c) Element map of TiO2 /Ti mesh. (d) XRD patterns of pristine Ti mesh, TiO2 /Ti mesh and annealed TiO2 /Ti mesh. (e) FTIR spectra of modified TiO2 /Ti mesh and lauric acid.
the TiO2 /Ti mesh surface except for dehydration reaction, which could be observed in the lower magnification image of Fig. 1(f). Moreover, it also could be learnt from the XPS survey spectra (Fig. 4(a)) of the TiO2 /Ti mesh that the content of C element increased from 25.31 at.% to 38.88 at.% after the modification, which again proved the successful combination between lauric acid and TiO2 /Ti mesh. It could be learnt from the superhydrophobic TiO2 /Ti mesh FTIR spectrum that there were hydroxyl groups at 3424.96 cm−1 . In addition, in the high-resolution XPS spectrum of O 1 s (Fig. 4(b)), Ti-OH groups existed at 531.53 eV, and the area fraction of Ti-OH groups decreased from 33.83% to 19.94% after modification. This was because that lots of hydroxyl groups formed on the surface during the anodizing process so that TiO2 /Ti mesh showed superhydrophilic after anodic oxidation [31]. After the modification, a small amount of hydroxyl groups did not react with the lauric acid [32]. Fig. 4(c) showed that Ti 2p (3/2) and Ti 2p (1/2) peak
appeared at 458.62 eV and 464.32 eV respectively in the highresolution XPS spectrum of Ti 2p indicating TiO2 grew on the Ti mesh. The peak at 459.84 eV was attributed to [TiF6 ]2− , which corresponded to the above EDS analysis results. In summary, amorphous TiO2 was fabricated on the Ti mesh and lauric acid successfully combined with the TiO2 /Ti mesh after modification. 3.3. Oil/water separation property of superhydrophilic TiO2 /Ti mesh The oil/water separation property of the superhydrophilic TiO2 /Ti mesh was carried out. The superhydrophilic TiO2 /Ti mesh was wetted by water before the separation process. As shown in Fig. 5(a-c), the mixture of hexane (dyed by sudan III) and water was poured into the upper tubes slowly, then the oil was blocked over the tube and water flowed down smoothly (video S3). The separation efficiency of the superhydrophilic TiO2 /Ti mesh for hexane/water mixture was up to 99.45%. Moreover, various mix-
Please cite this article as: X. Zhou, S. Yu and J. Wang et al., Superhydrophilic anti-corrosive and superhydrophobic durable TiO2/Ti mesh for oil/water separation, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.011
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Fig. 4. XPS spectra of the unmodified and modified TiO2 /Ti mesh: (a) survey spectrum, (b) O 1 s and (c) Ti 2p.
Fig. 5. (a-c) Oil/water separation processes on superhydrophilic TiO2 /Ti mesh. (d) The separation efficiency for various oil/water mixtures. (e) Repeatability test of oil/water separation.
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Fig. 6. Separation processes (a)–(c) and separation efficiency (d) of oil and various corrosive solutions.
Fig. 7. (a) WCAs on superhydrophobic TiO2 /Ti mesh annealed at different temperature for 3 h. (b) and (c) The change of water droplet shape and contact angle with time on superhydrophobic TiO2 /Ti mesh annealed at 150 °C and 180 °C, respectively.
tures of oil/water were separated successfully by the superhydrophilic TiO2 /Ti mesh. Fig. 5(d) illustrated that the separation efficiency of the other oil/water mixtures was more than 98%, which revealed that the superhydrophilic TiO2 /Ti mesh had a wide range of oil/water separation applications. In order to test the repeatability of the superhydrophilic TiO2 /Ti mesh, the hexane/water separation process was repeated for 10 times. The superhydrophilic TiO2 /Ti mesh was dried after each oil/water separation. Fig. 5(e) indicated that the separation efficiency was above 99.3% after 10 separation cycles. The result showed the superhydrophilic TiO2 /Ti mesh was stable in oil/water separation. 3.4. Anti-corrosive separation property of superhydrophilic TiO2 /Ti mesh Sometimes, oily sewage is often in corrosive condition. Especially for superhydrophilic mesh where water contacted with the surface directly during oil/water separation process, the separation property in the corrosive condition was worthy of attention. Therefore, the mixtures of hexane/1 M H2 SO4 , hexane/10 wt.% NaCl and hexane/1 M NaOH solutions were separated by the superhydrophilic TiO2 /Ti mesh. The hexane was dyed by sudan III and pH indicator paper was putted into the collected solutions to testify the acidity and alkalinity. As shown in Fig. 6(a–c), oil was blocked over the upper tube, and the acidic, alkaline, and saline solutions were flowed down and collected. Moreover, Fig. 6(d) showed that the separation efficiency was all higher than 99.0% for hex-
ane/corrosive solutions. Thus, the superhydrophilic TiO2 /Ti mesh showed outstanding oil/water separation property in corrosive condition. 3.5. Durability of superhydrophobic TiO2 /Ti mesh Low-energy modifiers on the surface of superhydrophobic materials are susceptible to temperature and thus affect surface hydrophobicity. Therefore, the stability of superhydrophobic surfaces at different temperatures is worthy of attention. Fig. 7(a) indicated the WCAs on superhydrophobic TiO2 /Ti mesh annealed at different temperatures for 3 h. The WCA was basically unchanged when the annealed temperature was lower than 150 °C. However, the surface lost superhydrophobicity at 180 °C that the WCA was less than 150°. Besides, Fig. 7(b) and (c) illustrated the change of WCAs on the superhydrophobic TiO2 /Ti mesh with time. The superhydrophobicity was almost unaffected when the annealed time was 6 h at 150 °C. But when the annealed temperature was 180 °C, the WCA decreased with the time and TiO2 /Ti mesh lost superhydrophobicity at 3 h with the WCA about 145.88°. It might be affected by the decomposition lauric acid combined on the surface [32]. Fig. 8 showed the thermogravimetry (TG) and derivative thermogravimetry (DTG) curves of lauric acid. It could be learnt from Fig. 8(a) that the weight of lauric acid lost with the increase of temperature. Moreover, the rate of weight loss increased with the increasing temperature (Fig. 8(b)). When the temperature was 150 °C, the rate of weight loss was about 0.093%/ °C. But the rate of weight loss reached up to 0.441%/ °C at 180 °C, which caused the rapid
Please cite this article as: X. Zhou, S. Yu and J. Wang et al., Superhydrophilic anti-corrosive and superhydrophobic durable TiO2/Ti mesh for oil/water separation, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.011
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Fig. 8. TG (a) and DTG (b) curves of lauric acid.
Fig. 9. (a) and (b) Water droplet status on modified Ti mesh and superhydrophobic TiO2 /Ti mesh before the anti-icing test. (c) and (d) At −20 °C, icing delay times evaluation of the two specimens. (e) The photos of two specimens after the anti-icing test.
decomposition of lauric acid and ultimately the weaker hydrophobicity with time. The result indicated that the TiO2 /Ti mesh could maintain its superhydrophobicity when the temperature was under 150 °C. Anti-icing property of the superhydrophobic TiO2 /Ti mesh was tested as shown in Fig. 9. As shown in Fig. 9(a) and (b), at −20 °C, 10 μL water droplet was dripped on superhydrophobic TiO2 /Ti mesh and modified Ti mesh with the WCA about 161.29° and 126.21°, respectively. Fig. 9(c) illustrated that the water droplet on the modified Ti mesh froze firstly after 1016s. After 2010s, the water droplet on the superhydrophobic TiO2 /Ti mesh froze to ice as shown in Fig. 9(d). Moreover, the frozen water droplet had low adhesion with the superhydrophobic TiO2 /Ti mesh so that it was easy peeled off without any visible residue. However, for the modified Ti mesh, the frozen water droplet was hard to be peeled off and part of the ice stuck to the surface as shown in Fig. 9(e). Thus above phenomenon revealed that the superhydrophobic TiO2 /Ti mesh could delay the icing time of the water
droplet. This was because that the water-solid contact area of superhydrophobic TiO2 /Ti mesh was smaller than that of modified Ti mesh, which could effectively reduce the heat transfer rate and contribute to slower icing process. Besides, the trapped air between water and TiO2 /Ti mesh could also retard the icing process via its heat-isolating effect. Consequently, the superhydrophobic TiO2 /Ti mesh showed good anti-icing property. Mechanical stability of the superhydrophobic TiO2 /Ti mesh was evaluated by abrasion and bending test. For the abrasion test, the superhydrophobic TiO2 /Ti mesh was rubbed with sandpaper (2000#) and 100 g weight was placed on it as shown in the insert image of Fig. 10(a). Fig. 10(a) showed that TiO2 /Ti mesh still maintained its excellent superhydrophobicity when the abrasion distance was less than 300 cm. Afterward, the WCA decreased. During the abrasion process, Destruction of surface energy and microstructure of the TiO2 /Ti mesh lead to the reduced water contact angle. However, for the TiO2 /Ti mesh, the WCA was still higher than 150° at 600 cm indicating that the superhydrophobic TiO2 /Ti
Please cite this article as: X. Zhou, S. Yu and J. Wang et al., Superhydrophilic anti-corrosive and superhydrophobic durable TiO2/Ti mesh for oil/water separation, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.011
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Fig. 10. (a) Change in WCA on superhydrophobic TiO2 /Ti mesh as the increasing abrasion distance. Insert image was the abrasion test process. (b) and (c) The initial and final state of water droplets in the bending test, respectively.
mesh had a good stability under abrasion. Bending test was designed that the superhydrophobic TiO2 /Ti mesh was folded and unfolded repeatedly. After 68 cycles, the TiO2 /Ti mesh became loose but still had good superhydrophobicity as the same as the initial state (Fig. 10(b-c)). To sum up, the superhydrophobic TiO2 /Ti mesh exhibited good mechanical stability. 3.6. Oil/water separation property of superhydrophobic TiO2 /Ti mesh The oil/water separation property of the superhydrophobic TiO2 /Ti mesh was evaluated as shown in Fig. 11(a–c). The water was dyed by MB to distinguish it from oil. The hexane/water mixture was poured into the upper tubes slowly. Due to the excellent superhydrophobicity, the water was unable to penetrate the TiO2 /Ti mesh but the hexane flowed down smoothly (video S4). The blocked water was stable in the upper tube with time and no blue water appeared in the collected oil. Besides, Fig. 11(d) illustrated the separation efficiency for various oil/water mixtures was more than 98% indicating that the superhydrophobic TiO2 /Ti mesh had good oil/water separation property. The repeatability of the superhydrophobic TiO2 /Ti mesh was also measured by separating the hexane/water mixture. After 10 times of oil/water separation, the separation efficiency was above 99% indicating that the superhydrophobic TiO2 /Ti mesh could be used in oil/water separation repeatedly. In conclusion, the superhydrophilic and superhydrophobic TiO2 /Ti mesh both have good oil/water separation property. In the experiment, in order to test the oil/water separation property of the superhydrophilic and superhydrophobic TiO2 /Ti mesh, the mixtures of oil/water including light (ρ oil < ρ water ) oil/water and heavy (ρ oil > ρ water ) oil/water were all poured into the upper tubes. However, in application, superhydrophilic TiO2 /Ti mesh might be more suitable for separating light (ρ oil < ρ water ) oil/water and superhydrophobic TiO2 /Ti mesh was suitable for heavy (ρ oil > ρ water ) oil/water mixture. Therefore, the TiO2 /Ti mesh with both
superhydrophilicity and superhydrophobicity has wider application in oil/water separation. 4. Conclusions In this work, superhydrophilic and superhydrophobic TiO2 /Ti mesh is prepared for oil/water separation. During the anodic oxidation, Ti mesh can be well coated by TiO2 nanotube film which shows superhydrophilicity. After being modified by lauric acid, the TiO2 /Ti mesh exhibits superhydrophobicity. The superhydrophobic TiO2 /Ti mesh has the same microstructure with superhydrophilic TiO2 /Ti mesh that TiO2 nanotubes grew on the surface neatly with 78.61 nm inner diameter and 9.12 nm wall thickness. Besides, both the superhydrophilic and superhydrophobic TiO2 /Ti mesh show high separation efficiency for various oil/water mixtures and exhibit good repeatability. Moreover, the superhydrophilic TiO2 /Ti mesh maintains good oil/water separation property for oil/corrosive solution mixtures. For superhydrophobic TiO2 /Ti mesh, it displays thermal stability under 150 °C, and it also has an excellent anti-icing property at −20 °C. The superhydrophobic TiO2 /Ti mesh indicates mechanical stability in the abrasion and bending test. We believe that TiO2 /Ti mesh with both superhydrophilicity and superhydrophobicity is considered to have great applications in oil/water separation and recovery of oil. Acknowledgements This work was supported by the Natural Science Foundation of Shandong Province (No. ZR2019MEM020), China. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2019.10.011.
Please cite this article as: X. Zhou, S. Yu and J. Wang et al., Superhydrophilic anti-corrosive and superhydrophobic durable TiO2/Ti mesh for oil/water separation, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.011
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Fig. 11. (a–c) Oil/water separation processes on superhydrophobic TiO2 /Ti mesh. (b) The separation efficiency for various oil/water mixtures. (d) Repeatability test of oil/water separation.
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Superhydrophilic anti-corrosive and superhydrophobic durable TiO2 /Ti mesh for oil/water separation Xue Zhou, Sirong Yu∗, Jun Wang, Jie Zang, Zhexin Lv College of Material Science and Engineering, China University of Petroleum (East China), Qingdao 266580, China
a r t i c l e
i n f o
Article history: Received 22 July 2019 Revised 18 September 2019 Accepted 13 October 2019 Available online xxx Keywords: Oil/water separation Superhydrophilic Superhydrophobic TiO2 /Ti mesh
a b s t r a c t The effective oil/water separation is important in the treatment of oily wastewater. Recently, materials with superwetting have become research hotspots due to their high oil/water separation efficiency and wide application. Here, TiO2 /Ti mesh with both superhydrophilicity and superhydrophobicity was successfully fabricated to separate oil/water mixtures. Superhydrophilic TiO2 nanotube film was prepared on Ti mesh by anodic oxidation. The superhydrophilic TiO2 /Ti mesh showed good oil/water separation property for various oil/water mixtures. Besides, the superhydrophilic TiO2 /Ti mesh exhibited anti-corrosive separation property that had high separation efficiency for oil/strong acid/alkali/salt solution mixtures. After surface modification with lauric acid, the superhydrophilic TiO2 /Ti mesh converted to superhydrophobic. The superhydrophobic TiO2 /Ti mesh indicated good durability under different conditions that it showed thermal stability under 150 °C, anti-icing property at -20 °C and mechanical stability under abrasion and bending test. Moreover, the superhydrophobic TiO2 /Ti mesh could also separate various oil/water mixtures efficiently. © 2019 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction Water pollution including domestic sewage, industrial wastewater, and oil spills has posed a serious threat to nature and human life. Separating the oils and water is a key point in the water treatment process [1,2]. A variety of traditional methods have been used to separated oils and water, such as mechanical [3], chemical [4] and biological methods [5]. However, these three traditional methods have the disadvantage of low separation efficiency, the second pollution and high cost [6]. Hence, the method which is efficient, eco-friendly and cost-effective is worth being explored. Recently, the special wettability interface materials with high separation efficiency and wide selectivity have become the main research direction in the field of oil/water separation [7,8]. The special wettability interface materials always show superhydrophilic (underwater superoleophobic) or superhydrophobic (superlipophilic) [9,10]. Several materials with special wettability have been designed in oil/water separation including metal mesh [11], fabric [12], filter paper [13], foam [14]. Among them, metal mesh is widely used because that chemical reactions and modification are easy operated on its surface, and it has the advantages of high mechanical strength, low cost and high flow rate [15].
∗
Corresponding author. E-mail address: [email protected] (S. Yu).
Titanium dioxide (TiO2 ) nanomaterials often exhibit hydrophilicity due to the presence of a large amount of hydroxyl groups on their surface [16]. Besides, TiO2 nanomaterials have been used to prepare superhydrophobic surface because of their smaller solid-liquid contact area [17], which follows the Cassie-Baxter theory [18]. Li et al. [19] have fabricated underwater superoleophobic TiO2 coated mesh by spraying TiO2 nanoparticles and polyurethane mixtures onto stainless steel mesh, which exhibits high oil/water separation efficiency. Kumar and Tudu [20] have prepared both superhydrophobic steel and copper meshes modified by perfluorodecyltriethoxysilane (PFDTS) and TiO2 nanoparticles. Both two superhydrophobic metal meshes can separate not only oil-water mixtures but also oil-water emulsions efficiently. Bandara and Gunatilake [21] have built superhydrophilic TiO2 nanofibers coated stainless steel mesh, which indicates excellent oil/water separation property. The TiO2 nanofibers are hydrothermally synthesized and then sprayed on the stainless steel mesh. Compared to the above methods, the ways that nanostructured TiO2 grows directly on Ti mesh are simple and effective. Zhou et al. [22] have reported that a superhydrophilic TiO2 nanowires film grows on Ti mesh by a facile immersion method, and the obtained mesh shows good oil/water and oil/corrosive solutions separation ability. In addition, the material only exhibits superhydrophilicity or superhydrophobicity cannot meet the demands in practical applications. The superhydrophilic mesh which only allows water to penetrate is more suitable for the separation of light (ρ oil < ρ water ) oil/water and
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Fig. 1. SEM images of pristine Ti mesh (a) and (b), modified Ti mesh (c), TiO2 /Ti mesh (d) and (e), modified TiO2 /Ti mesh (f). The insets are high magnification images.
the superhydrophobic mesh which only allows oil to penetrate is more suitable for the separation of heavy (ρ oil > ρ water ) oil/water [23]. Besides, superhydrophobic surface always indicates stability in high temperature environment and anti-icing property in low temperature environment [24,25]. Therefore, preparing a material that can delivery both superhydrophilicity and superhydrophobicity would have a wider application. In this work, TiO2 nanotube film was fabricated on the Ti mesh by a facile anodic oxidation method. The TiO2 /Ti mesh exhibited superhydrophilicity after the anodic oxidation. Due to the nanotubes structure, the TiO2 /Ti mesh could obtain the superhydrophobicity easily after being modified by lauric acid. The oil/water separation property and repeatability of the superhydrophilic and superhydrophobic TiO2 /Ti mesh were both tested. For superhydrophilic TiO2 /Ti mesh, the oil/corrosive solutions separation property was measured because it always contacted aqueous solution directly. For superhydrophobic TiO2 /Ti mesh, a variety of durability tests such as anti-icing property, thermal and mechanical stability test were performed. 2. Experimental 2.1. Materials TA1 titanium mesh was purchased from KangWei materials Co. Ltd, Hengshui, China. Graphite plates were purchased from Beijing Jinglong Graphite Factory. All of the chemical regents were supplied by Sinopharm Chemical Reagent Co. Ltd, Shanghai, China. 2.2. Fabrication of superhydrophilic and superhydrophobic TiO2 /Ti mesh TiO2 was fabricated by anodizing the Ti mesh (40 mm × 25 mm) in 1 vol.% HF electrolyte. Before the anodic oxidation, Ti mesh was immersed in a solution consisting of 20 mL deionized water, 16 mL HNO3 and 4 mL HF for 1 min and ultimately cleaned by deionized water. The anodic oxidation step was conducted at 20 V for 1 h with graphite plate (40 mm × 25 mm) and Ti mesh as cathode and anode, respectively. The obtained TiO2 /Ti mesh was immediately rinsed with deionized water and dried in air. After anodic oxidation, the mesh showed superhydrophilicity. In order to obtain the superhydrophobicity, the TiO2 /Ti mesh was
immersed in a 0.02 mol/L ethanol solution of lauric acid for 24 h in the dark and subsequently dried at 60 °C for 20 min. 2.3. Characterizations and tests The morphology of mesh was observed by field emission scanning electron microscope (FESEM, JEOL, JSM-7200F). The crystal structures of mesh were recorded on an X-ray diffractometer (XRD, X’Pert PRO MPD, PANalytical B.V.). The surface chemical compositions were tested by energy disperse spectroscopy (EDS, Oxford Aztec), Fourier transform infrared spectrometer (FTIR, Nexus, Thermo Nicolet) and X-ray photoelectron spectroscopy (XPS, Escalab 250Xi). The water contact angle (WCA) was measured using a contact angle meter (SL200B, USA, KINO), the volume of deionized water droplet used for the WCA measurements was 3 μL. The WCA value was the average of at least five measurements and the error was ±1°. Oil/water separation properties of the superhydrophilic and superhydrophobic TiO2 /Ti mesh were tested. The TiO2 /Ti mesh was fixed tightly between two glass tubes. A serials of oil/water mixtures (50% v/v) including hexane/water, cyclohexane/water, petroleum ether/water, diesel oil/water and dichloromethane/water were separated. The oil/water mixture was poured slowly into the upper glass tube and oil/water separation process was achieved under gravity. The separation efficiency was calculated as the following equation:
η = (ma /mb ) × 100%
(1)
where η is the separation efficiency. mb and ma is the weight of oil (superhydrophilic mesh) or water (superhydrophobic mesh) before and after separation. 3. Results and discussion 3.1. Morphology and wettability of superhydrophilic/ superhydrophobic TiO2 /Ti mesh The morphology of the pristine and modified Ti and TiO2 /Ti meshes was observed by the SEM images. It could be learnt from Fig. 1(a) that Ti mesh with a pore size of approximately 300 mesh per inch was woven together. The magnified image of the Ti mesh
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Fig. 2. (a) WCAs on different specimens. (b) The process of water droplet touching the superhydrophilic TiO2 /Ti mesh. (c) Image of oil droplet shape on superhydrophilic TiO2 /Ti mesh underwater. (d) Image of water droplet shape on superhydrophobic TiO2 /Ti mesh under oil.
(Fig. 1(b)) indicated that there was no special structure on the surface. However, after the low surface energy modification, it appeared some white particles on the Ti mesh which was the lauric acid adsorbed on the surface (Fig. 1(c)). After anodic oxidation, TiO2 was fabricated on the Ti mesh. Compare to Fig. 1(a), there was no difference with the low magnification image in Fig. 1(d). However, it could be observed in Fig. 1(e) that TiO2 nanotubes grew on the surface neatly with 78.61 nm inner diameter and 9.12 nm wall thickness. After surface modification, although a small amount of white particles (lauric acid) adsorbed on the surface, Fig. 1(f) showed that the shape and size of the TiO2 nanotubes were almost not affected. Nanostructured TiO2 formed on Ti mesh was appropriate for achieving superhydrophobic surfaces. On this basis, lauric acid was used to modify the rough TiO2 /Ti mesh. The wetting behavior of the pristine and modified meshes was revealed by WCAs. Fig. 2(a) showed that the pristine Ti mesh showed hydrophilic with a WCA about 25.72°, and the modified Ti mesh exhibited hydrophobicity with a WCA about 126.21°. This meant the low surface energy modification played an important role in improving the hydrophobicity. After anodic oxidation, TiO2 nanotubes were fabricated but the WCA decreased to 0°, which demonstrated that the TiO2 /Ti mesh was superhydrophilic. This accorded with Wenzel’s theory [26] that the hydrophilicity of the hydrophilic surface was enhanced as the surface roughness increased. Moreover, the water droplet spread quickly on the TiO2 /Ti mesh. As shown in Fig. 2(b), by moving the TiO2 /Ti mesh up, the water droplet contacted the TiO2 /Ti mesh and then spread in an instant. For the modified TiO2 /Ti mesh, the surface exhibited superhydrophobicity with a WCA about 161.29°. It was because that the surface with low surface energy always showed water repellency. Besides, the TiO2 nanotube structure was in favor of trapping air between the mesh and water droplet, which could be explained by the Cassie–Baxter equation [18]:
cos θ c = f (cos θ0 + 1 ) − 1
(2)
In Eq. (2), f is the area fraction of the liquid-solid contact. θ 0 and θ c are the WCA of the modified Ti mesh and modified TiO2 /Ti mesh, respectively. According to Eq. (2), the value of f was calculated to be 0.1291, which meant only 12.91% of TiO2 /Ti mesh contacted with water. Thus, plenty of air was trapped between water and TiO2 nanotubes. Besides, the superhydrophilic TiO2 /Ti mesh showed underwater superoleophobicity with a contact angle about 163.94° as shown in Fig. 2(c) and Fig. S1, which had an important impact on oil/water separation applications. For superhydrophobic mesh, it maintained
superhydrophobicity under oil with a contact angle about 162.04° as shown in Fig. 2(d) and Fig. S2. Although a layer of oil film covered the surface, it did not damage the microstructure of TiO2 film and the oil film also had a lower surface energy, so the superhydrophobic surface remained its superhydrophobicity under oil. 3.2. Chemical compositions of superhydrophilic/superhydrophobic TiO2 /Ti mesh Surface chemical compositions were significant to the surface wettability and studied in detail by EDS, XRD, FTIR and XPS. As shown in Fig. 3(a), apart from C element which was originally present in the test equipment, only Ti existed in the EDS spectrum of pristine Ti mesh. After anodic oxidation, Fig. 3(b) illustrated that O and F were also checked out. Moreover, the EDS mapping images were exhibited in Fig. 3(c) showing that Ti and O almost distributed on the whole surface. The existence of O indicated that TiO2 was possible fabricated on the Ti mesh, and the F was caused by the reaction product of TiO2 and F− in electrolyte [27]. In order to prove the above analysis, the surface crystal structure was tested by the XRD. The XRD pattern of pristine Ti mesh was shown in Fig. 3(d) indicating that all peaks corresponded to Ti. After anodic oxidation, there were no other diffraction peaks appeared in the XRD pattern of TiO2 /Ti mesh. This was because that TiO2 was always amorphous and turned into anatase or even rutile at higher temperature [28]. In order to prove the existence of surface oxides, the TiO2 /Ti mesh was treated at 400 °C for 2 h under vacuum. It could be learnt from Fig. 3(d) that two more diffraction peaks appeared at 2θ = 25.20°, 48.03° which attributed to anatase. Accordingly, TiO2 was coated on the Ti mesh and was amorphous after anodic oxidation. After low energy modification, TiO2 /Ti mesh exhibited good water repellence due to the methylene groups (–CH2 –) and methyl groups (–CH3 ) on lauric acid [29]. Fig. 3(e) illustrated the FTIR spectra of superhydrophobic TiO2 /Ti mesh and lauric acid. On this two FTIR spectra, the peak at 2954.41 cm−1 (2952.48 cm−1 ) assigned to the stretching vibration of –CH3 , and the peaks at 2921.63 cm−1 (2918.20 cm−1 ) and 2852.20 cm−1 (2850.69 cm−1 ) were corresponding to –CH2 –. The peak at 1701.16 cm−1 on the lauric acid FTIR spectrum was attributed to the –COO– groups. Moreover, at 1631.48 cm−1 , the peak representing –COO– groups also appeared on the superhydrophobic TiO2 /Ti mesh FTIR spectrum. It was because that the lauric acid always combined with TiO2 by the dehydration reaction between –COO–and –OH groups, and then the peak representing –COO– groups moved to the smaller wave number [30]. Besides, some lauric acid adsorbed on
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Fig. 3. EDS spectra of (a) pristine Ti mesh and (b) TiO2 /Ti mesh. (c) Element map of TiO2 /Ti mesh. (d) XRD patterns of pristine Ti mesh, TiO2 /Ti mesh and annealed TiO2 /Ti mesh. (e) FTIR spectra of modified TiO2 /Ti mesh and lauric acid.
the TiO2 /Ti mesh surface except for dehydration reaction, which could be observed in the lower magnification image of Fig. 1(f). Moreover, it also could be learnt from the XPS survey spectra (Fig. 4(a)) of the TiO2 /Ti mesh that the content of C element increased from 25.31 at.% to 38.88 at.% after the modification, which again proved the successful combination between lauric acid and TiO2 /Ti mesh. It could be learnt from the superhydrophobic TiO2 /Ti mesh FTIR spectrum that there were hydroxyl groups at 3424.96 cm−1 . In addition, in the high-resolution XPS spectrum of O 1 s (Fig. 4(b)), Ti-OH groups existed at 531.53 eV, and the area fraction of Ti-OH groups decreased from 33.83% to 19.94% after modification. This was because that lots of hydroxyl groups formed on the surface during the anodizing process so that TiO2 /Ti mesh showed superhydrophilic after anodic oxidation [31]. After the modification, a small amount of hydroxyl groups did not react with the lauric acid [32]. Fig. 4(c) showed that Ti 2p (3/2) and Ti 2p (1/2) peak
appeared at 458.62 eV and 464.32 eV respectively in the highresolution XPS spectrum of Ti 2p indicating TiO2 grew on the Ti mesh. The peak at 459.84 eV was attributed to [TiF6 ]2− , which corresponded to the above EDS analysis results. In summary, amorphous TiO2 was fabricated on the Ti mesh and lauric acid successfully combined with the TiO2 /Ti mesh after modification. 3.3. Oil/water separation property of superhydrophilic TiO2 /Ti mesh The oil/water separation property of the superhydrophilic TiO2 /Ti mesh was carried out. The superhydrophilic TiO2 /Ti mesh was wetted by water before the separation process. As shown in Fig. 5(a-c), the mixture of hexane (dyed by sudan III) and water was poured into the upper tubes slowly, then the oil was blocked over the tube and water flowed down smoothly (video S3). The separation efficiency of the superhydrophilic TiO2 /Ti mesh for hexane/water mixture was up to 99.45%. Moreover, various mix-
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Fig. 4. XPS spectra of the unmodified and modified TiO2 /Ti mesh: (a) survey spectrum, (b) O 1 s and (c) Ti 2p.
Fig. 5. (a-c) Oil/water separation processes on superhydrophilic TiO2 /Ti mesh. (d) The separation efficiency for various oil/water mixtures. (e) Repeatability test of oil/water separation.
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Fig. 6. Separation processes (a)–(c) and separation efficiency (d) of oil and various corrosive solutions.
Fig. 7. (a) WCAs on superhydrophobic TiO2 /Ti mesh annealed at different temperature for 3 h. (b) and (c) The change of water droplet shape and contact angle with time on superhydrophobic TiO2 /Ti mesh annealed at 150 °C and 180 °C, respectively.
tures of oil/water were separated successfully by the superhydrophilic TiO2 /Ti mesh. Fig. 5(d) illustrated that the separation efficiency of the other oil/water mixtures was more than 98%, which revealed that the superhydrophilic TiO2 /Ti mesh had a wide range of oil/water separation applications. In order to test the repeatability of the superhydrophilic TiO2 /Ti mesh, the hexane/water separation process was repeated for 10 times. The superhydrophilic TiO2 /Ti mesh was dried after each oil/water separation. Fig. 5(e) indicated that the separation efficiency was above 99.3% after 10 separation cycles. The result showed the superhydrophilic TiO2 /Ti mesh was stable in oil/water separation. 3.4. Anti-corrosive separation property of superhydrophilic TiO2 /Ti mesh Sometimes, oily sewage is often in corrosive condition. Especially for superhydrophilic mesh where water contacted with the surface directly during oil/water separation process, the separation property in the corrosive condition was worthy of attention. Therefore, the mixtures of hexane/1 M H2 SO4 , hexane/10 wt.% NaCl and hexane/1 M NaOH solutions were separated by the superhydrophilic TiO2 /Ti mesh. The hexane was dyed by sudan III and pH indicator paper was putted into the collected solutions to testify the acidity and alkalinity. As shown in Fig. 6(a–c), oil was blocked over the upper tube, and the acidic, alkaline, and saline solutions were flowed down and collected. Moreover, Fig. 6(d) showed that the separation efficiency was all higher than 99.0% for hex-
ane/corrosive solutions. Thus, the superhydrophilic TiO2 /Ti mesh showed outstanding oil/water separation property in corrosive condition. 3.5. Durability of superhydrophobic TiO2 /Ti mesh Low-energy modifiers on the surface of superhydrophobic materials are susceptible to temperature and thus affect surface hydrophobicity. Therefore, the stability of superhydrophobic surfaces at different temperatures is worthy of attention. Fig. 7(a) indicated the WCAs on superhydrophobic TiO2 /Ti mesh annealed at different temperatures for 3 h. The WCA was basically unchanged when the annealed temperature was lower than 150 °C. However, the surface lost superhydrophobicity at 180 °C that the WCA was less than 150°. Besides, Fig. 7(b) and (c) illustrated the change of WCAs on the superhydrophobic TiO2 /Ti mesh with time. The superhydrophobicity was almost unaffected when the annealed time was 6 h at 150 °C. But when the annealed temperature was 180 °C, the WCA decreased with the time and TiO2 /Ti mesh lost superhydrophobicity at 3 h with the WCA about 145.88°. It might be affected by the decomposition lauric acid combined on the surface [32]. Fig. 8 showed the thermogravimetry (TG) and derivative thermogravimetry (DTG) curves of lauric acid. It could be learnt from Fig. 8(a) that the weight of lauric acid lost with the increase of temperature. Moreover, the rate of weight loss increased with the increasing temperature (Fig. 8(b)). When the temperature was 150 °C, the rate of weight loss was about 0.093%/ °C. But the rate of weight loss reached up to 0.441%/ °C at 180 °C, which caused the rapid
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Fig. 8. TG (a) and DTG (b) curves of lauric acid.
Fig. 9. (a) and (b) Water droplet status on modified Ti mesh and superhydrophobic TiO2 /Ti mesh before the anti-icing test. (c) and (d) At −20 °C, icing delay times evaluation of the two specimens. (e) The photos of two specimens after the anti-icing test.
decomposition of lauric acid and ultimately the weaker hydrophobicity with time. The result indicated that the TiO2 /Ti mesh could maintain its superhydrophobicity when the temperature was under 150 °C. Anti-icing property of the superhydrophobic TiO2 /Ti mesh was tested as shown in Fig. 9. As shown in Fig. 9(a) and (b), at −20 °C, 10 μL water droplet was dripped on superhydrophobic TiO2 /Ti mesh and modified Ti mesh with the WCA about 161.29° and 126.21°, respectively. Fig. 9(c) illustrated that the water droplet on the modified Ti mesh froze firstly after 1016s. After 2010s, the water droplet on the superhydrophobic TiO2 /Ti mesh froze to ice as shown in Fig. 9(d). Moreover, the frozen water droplet had low adhesion with the superhydrophobic TiO2 /Ti mesh so that it was easy peeled off without any visible residue. However, for the modified Ti mesh, the frozen water droplet was hard to be peeled off and part of the ice stuck to the surface as shown in Fig. 9(e). Thus above phenomenon revealed that the superhydrophobic TiO2 /Ti mesh could delay the icing time of the water
droplet. This was because that the water-solid contact area of superhydrophobic TiO2 /Ti mesh was smaller than that of modified Ti mesh, which could effectively reduce the heat transfer rate and contribute to slower icing process. Besides, the trapped air between water and TiO2 /Ti mesh could also retard the icing process via its heat-isolating effect. Consequently, the superhydrophobic TiO2 /Ti mesh showed good anti-icing property. Mechanical stability of the superhydrophobic TiO2 /Ti mesh was evaluated by abrasion and bending test. For the abrasion test, the superhydrophobic TiO2 /Ti mesh was rubbed with sandpaper (2000#) and 100 g weight was placed on it as shown in the insert image of Fig. 10(a). Fig. 10(a) showed that TiO2 /Ti mesh still maintained its excellent superhydrophobicity when the abrasion distance was less than 300 cm. Afterward, the WCA decreased. During the abrasion process, Destruction of surface energy and microstructure of the TiO2 /Ti mesh lead to the reduced water contact angle. However, for the TiO2 /Ti mesh, the WCA was still higher than 150° at 600 cm indicating that the superhydrophobic TiO2 /Ti
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Fig. 10. (a) Change in WCA on superhydrophobic TiO2 /Ti mesh as the increasing abrasion distance. Insert image was the abrasion test process. (b) and (c) The initial and final state of water droplets in the bending test, respectively.
mesh had a good stability under abrasion. Bending test was designed that the superhydrophobic TiO2 /Ti mesh was folded and unfolded repeatedly. After 68 cycles, the TiO2 /Ti mesh became loose but still had good superhydrophobicity as the same as the initial state (Fig. 10(b-c)). To sum up, the superhydrophobic TiO2 /Ti mesh exhibited good mechanical stability. 3.6. Oil/water separation property of superhydrophobic TiO2 /Ti mesh The oil/water separation property of the superhydrophobic TiO2 /Ti mesh was evaluated as shown in Fig. 11(a–c). The water was dyed by MB to distinguish it from oil. The hexane/water mixture was poured into the upper tubes slowly. Due to the excellent superhydrophobicity, the water was unable to penetrate the TiO2 /Ti mesh but the hexane flowed down smoothly (video S4). The blocked water was stable in the upper tube with time and no blue water appeared in the collected oil. Besides, Fig. 11(d) illustrated the separation efficiency for various oil/water mixtures was more than 98% indicating that the superhydrophobic TiO2 /Ti mesh had good oil/water separation property. The repeatability of the superhydrophobic TiO2 /Ti mesh was also measured by separating the hexane/water mixture. After 10 times of oil/water separation, the separation efficiency was above 99% indicating that the superhydrophobic TiO2 /Ti mesh could be used in oil/water separation repeatedly. In conclusion, the superhydrophilic and superhydrophobic TiO2 /Ti mesh both have good oil/water separation property. In the experiment, in order to test the oil/water separation property of the superhydrophilic and superhydrophobic TiO2 /Ti mesh, the mixtures of oil/water including light (ρ oil < ρ water ) oil/water and heavy (ρ oil > ρ water ) oil/water were all poured into the upper tubes. However, in application, superhydrophilic TiO2 /Ti mesh might be more suitable for separating light (ρ oil < ρ water ) oil/water and superhydrophobic TiO2 /Ti mesh was suitable for heavy (ρ oil > ρ water ) oil/water mixture. Therefore, the TiO2 /Ti mesh with both
superhydrophilicity and superhydrophobicity has wider application in oil/water separation. 4. Conclusions In this work, superhydrophilic and superhydrophobic TiO2 /Ti mesh is prepared for oil/water separation. During the anodic oxidation, Ti mesh can be well coated by TiO2 nanotube film which shows superhydrophilicity. After being modified by lauric acid, the TiO2 /Ti mesh exhibits superhydrophobicity. The superhydrophobic TiO2 /Ti mesh has the same microstructure with superhydrophilic TiO2 /Ti mesh that TiO2 nanotubes grew on the surface neatly with 78.61 nm inner diameter and 9.12 nm wall thickness. Besides, both the superhydrophilic and superhydrophobic TiO2 /Ti mesh show high separation efficiency for various oil/water mixtures and exhibit good repeatability. Moreover, the superhydrophilic TiO2 /Ti mesh maintains good oil/water separation property for oil/corrosive solution mixtures. For superhydrophobic TiO2 /Ti mesh, it displays thermal stability under 150 °C, and it also has an excellent anti-icing property at −20 °C. The superhydrophobic TiO2 /Ti mesh indicates mechanical stability in the abrasion and bending test. We believe that TiO2 /Ti mesh with both superhydrophilicity and superhydrophobicity is considered to have great applications in oil/water separation and recovery of oil. Acknowledgements This work was supported by the Natural Science Foundation of Shandong Province (No. ZR2019MEM020), China. Supplementary material Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2019.10.011.
Please cite this article as: X. Zhou, S. Yu and J. Wang et al., Superhydrophilic anti-corrosive and superhydrophobic durable TiO2/Ti mesh for oil/water separation, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.011
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Fig. 11. (a–c) Oil/water separation processes on superhydrophobic TiO2 /Ti mesh. (b) The separation efficiency for various oil/water mixtures. (d) Repeatability test of oil/water separation.
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Please cite this article as: X. Zhou, S. Yu and J. Wang et al., Superhydrophilic anti-corrosive and superhydrophobic durable TiO2/Ti mesh for oil/water separation, Journal of the Taiwan Institute of Chemical Engineers, https://doi.org/10.1016/j.jtice.2019.10.011