Biomimetic Hierarchical TiO2@CuO Nanowire Arrays-Coated Copper

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Biomimetic Hierarchical TiO@CuO Nanowire ArraysCoated Copper Meshes with Superwetting and SelfCleaning Properties for Efficient Oil/Water Separation Junyi Ji, Huaqiang He, Chen Chen, Wei Jiang, Aikifa Raza, Tie-Jun Zhang, and Shaojun Yuan ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b05570 • Publication Date (Web): 18 Dec 2018 Downloaded from http://pubs.acs.org on December 20, 2018

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ACS Sustainable Chemistry & Engineering

Biomimetic

Hierarchical

TiO2@CuO

Nanowire

Arrays-Coated Copper Meshes with Superwetting and Self-Cleaning

Properties

for

Efficient

Oil/Water

Separation Junyi Ji§, Huaqiang He§, Chen Chen§, Wei Jiang§, Aikifa RazaΦ, Tie-Jun ZhangΦ, Shaojun Yuan§* §

Low-Carbon Technology & Chemical Reaction Engineering Lab, College of

Chemical Engineering, Sichuan University, Chengdu, 610065, China. Φ

Department of Mechanical and Materials Engineering, Masdar Institute,

Khalifa University, P.O. Box 54224, Abu Dhabi, United Arab Emirates

Corresponding Author *E-mail:

[email protected]

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ACS Paragon Plus Environment

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Abstract:

Creating a practical and energy-efficient method with high efficiency to separate oil–water mixtures and emulsions is essential for sustainable aquatic ecosystem and waste oil recycling. Herein, we reported a completely inorganic hierarchical TiO2@CuO nanowire array-coated copper mesh membrane

fabricated

using

chemical

oxidation

and

hydrothermal

recrystallization. The biomimetic hierarchical nano-/micro-structure can form a lotus-leaf-like interface with fine surface roughness and hydration ability, thus endows the membrane superhydrophilic and underwater superoleophobic nature. Thus, the as-synthesized TiO2@CuO nanowire array-coated copper mesh membrane demonstrated promising water flux of 87.6 kL·h-1·m-2 and ultrahigh separation efficiency with oil residue of only 12.4 mg·L-1 in the permeate. Moreover, the uniformly immobilized TiO2 nanoparticles exhibit UV-irradiated photocatalytic ability, which can be effectively regenerate the contaminated membrane after exposure to UV irradiations for 60 min. Therefore, this high efficiency, reusable, and easy-scalable membrane fabrication strategy may possess practical potential for next-generation oil/water separation application.

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Keywords: Biomimetic morphology, nano-/micro- hierarchical structure, oil/water separation, self-cleaning

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Introduction With rapid industrialization and civilization process, the environmental problems have becoming an extensive global challenge for sustainable development. Among them, oil spill during the production and transportation, as well as the oily wastewater discharge from the industrial production and sanitary wastewater, could result in serious water-body pollution.1 Effective separation of oil from oily water is thus essential for water protection and oil collection/recycling. Various methods have proposed to accomplish oil/water separation to mitigate the environmental oily wastewater damage, such as oil-absorbing materials,2, 3 air flotation,4 and flocculation5, 6 etc. However, most of these conventional processes are hindered by their high energy consumption, complicated operation process, low separation efficiency and secondary pollution for practical application. Therefore, an alternative oily wastewater separation strategy with high efficiency and low cost is essential to address the ever-increasing environmental issue.7, 8 Membrane separation is considered as an excellent separation strategy due to the high efficiency, relatively low cost and practical scalablility.9, 10 Among the separation principles, the preferential wettability of membranes towards oil or water driven by the interfacial effect can effectively distinguish and separate oil and water.11-18 Thus, the separation can spontaneously take place at the liquid/solid interface based on the surface energy and surface roughness nature 4


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of the superwetting membranes.19 Two kinds of superwetting membranes, i.e. superhydrophilic and superoleophilic membranes, can specifically spread one kind of liquid and repel another in the oily wastewater.20-24 Moreover, as the densities of the oily species is always lower than that of the water, thus separation process using superhydrophilic membranes can solely be driven by gravity. Recently, to reduce the energy consumption of separation process, superhydrophilic and underwater superoleophobic membranes have attracted considerable attention for oily wastewater separation with high efficiency and low cost. To date, many attempts have been made to fabricate high performance superhydrophilic and underwater superoleophobic membranes for effective oil/water separation.25-30 However, most of them still face drawbacks of either weak environmental adaptability, poor mechanical stability, low efficiency, or complicated

fabrication

process,

which

further

hinder

their

practical

application. Therefore, the completely inorganic membrane with good mechanical stability and environmental adaptability have attract increasingly attention.31,

32

As mentioned above, the surface microstructure and hydration

ability are two important factors to achieve desirable superhydrophilic and underwater superoleophobic property.33 Besides the intrinsic water loving nature of the inorganic oxides, the controllable inorganic microstructures fabrication strategy is highly demanded. Moreover, the trace oily wastewater 5



may residue inside the microstructure, thus the antifouling or oil degradation properties of the separation membranes are also highly required.31, 34 Herein, we report an inorganic hierarchical TiO2 on CuO nanowire array (TiO2@CuO NWA)-coated copper mesh membrane fabricated using combined approaches of chemical oxidation and hydrothermal recrystallization. The as-fabricated biomimetic hierarchical nano-/micro-structure can form a lotus-leaf-like interface with fine surface roughness and hydration ability. Thus, the stable oil/water/solid three-phase interface exhibits superhydrophilic and underwater superoleophobic properties of the membrane. Moreover, the tightly immobilized TiO2 nanoparticles exhibit UV-irradiated photocatalytic ability, which can effectively remove the oil contamination. Therefore, the as-synthesized TiO2@CuO nanowire array-coated copper mesh membrane can act as the high-efficient oil/water separation membrane with self-cleaning property. The oil contamination on the membrane can be effectively removed by photodegradation under UV-light illumination within 60 min. As a result, this high efficiency, reusable, low cost, and easy-scalable membrane fabrication strategy may possess practical potential for next-generation oil/water separation application.

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Results and discussion The biomimetic hierarchical TiO2@CuO NWA-coated copper mesh membranes are fabricated using two-step method consisting of surface chemical oxidation and hydrothermal recrystallization, which is illustrated in figure 1. The clean copper mesh is firstly chemically oxidized at room temperature to form uniform and tightly anchored Cu(OH)2 nanowire array, then the TiO2 nanoparticles are immobilized uniformly on the surface of the Cu(OH)2 nanowires by hydrothermal recrystallization at 120 °C. Finally, the hierarchical TiO2@CuO NWA-coated copper mesh is obtained by annealing in air at 550 °C.35 The vertically aligned CuO nanowire arrays and the well-dispersed TiO2 nanoparticles of the dual-structured TiO2@CuO hybrids can ensure the superhydrophilic and underwater superoleophobic nature of the mesh membrane due to the surface hydration ability and hierarchical surface roughness, while the tightly immobilized TiO2 crystalline nanoparticles can endow the composite membrane with UV-irradiated photodegradation ability for self-cleaning the oil contaminations. Therefore, this rational designed hierarchical TiO2@CuO NWA- coated copper mesh structure may act as the high efficiency oil/water separation membrane with self-cleaning property.

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Figure. 1 Schematic diagram to illustrates the two-step process for fabricating hierarchical TiO2@CuO NWA on copper meshes: chemical oxidation of copper meshes to form Cu(OH)2 NWA, and loading of TiO2-P25 on the Cu(OH)2 NWA-coated copper meshes to produce TiO2@CuO NWA coatings

via hydrothermal crystallization. The surface morphology evolution of the copper mesh after chemical oxidation and TiO2 recrystallization is shown in figure 2. The copper mesh shows a relatively smooth surface before chemical oxidation (Figure S1, see Supporting Information). After the chemical oxidation of copper mesh, the surface has turned into a fluffy morphology with nanowires vertically and densely aligned on the surface (Figures 2a-c), demonstrates the successful oxidation growth of the nanowires. The Cu(OH)2 nanowires are around 6-10 m in length and with an average diameters of 300±100 nm. All the nanowires are tightly anchored on the copper mesh surface without any peeling-off after folding and crimping of the mesh. After hydrothermal recrystallization of the 8


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TiO2 nanoparticles and annealing, the nanowire structure of the hierarchical TiO2@CuO NWA remains unchanged (Figures 2d-l), further indicates the robust structural stability of the membranes. Moreover, the loading amount of the TiO2 nanoparticles increases with the increase of the hydrothermal period. Upon the 8 h growth of TiO2@CuO NWA, the TiO2 nanoparticles are uniformly immobilized on the surface of the CuO surface, indicating the recrystallization process firstly occurs at the contact interface. Furthermore, when the growth time increase to 16 h, more TiO2 nanoparticles are stacked on the surface of the TiO2@CuO NWA, thus small papillary hills are spotted over the CuO NWA. Further increase the growth time to 24 h, obvious TiO2 aggregation are densely covered on the CuO NWA surface, thus constructed a distinct hierarchical micro-/nano-dual structures on the copper mesh. This result demonstrates the TiO2 loading is positively correlated with the hydrothermal recrystallization time, thus the loading amount of the TiO2 can be further engineered. The transmission electron microscopy (TEM) images show that the TiO2 individual nanoparticles on the CuO nanowires are circa 20-30 nm in diameter (Figure S2, see Supporting Information), which is in line with the size of the P25 nanoparticles. Moreover, the high temperature annealing process can form partially alloying at the contact interface between the TiO2 nanoparticles and the TiO2/CuO hybrids (shown by red dash in Figure S2d). This result further strengthens the structural stability of the composite membrane. The 9



controllable deposition as well as the fine size distribution of the TiO2 nanoparticles can effectively adjust the surface roughness, thus can further influence the wettability of the mesh membranes.

Figure. 2 Representative SEM images at different magnifications for the (a-c) Cu(OH)2 NWA-coated copper meshes fabricated after chemical oxidation for 30 min, (d-f) 8*TiO2@CuO, (g-i) 16*TiO2@CuO, and (j-l) 24*TiO2@CuO NWA-coated copper meshes obtained after hydrothermal crystallization of TiO2 for 8, 16 and 24 h, respectively. The scale bar: (a, d, g, j) 50 m, (b, e, h, k) 10 m, and (c, f, i, l) 1 m. To further understand the dependence of the TiO2 loading amount on the hydrothermal recrystallization time, the SEM images and the corresponding EDS spectrum of the 8*TiO2@CuO NWA, 16*TiO2@CuO NWA and 24*TiO2@CuO NWA-coated copper mesh membranes are shown in figures 3a-c. The presence of the Ti elemental peaks in the EDS spectra reveals the successful immobilization of the 10


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TiO2 nanoparticles on the CuO nanowires. The uniformly dispersed Cu, Ti, O elements are well inherited the morphology of the Cu mesh (Figure 3d), demonstrating the uniform coating layer of the TiO2@CuO NWA on the surface. The atomic percentage of Ti element is 4.69%, 7.32% and 9.87% for the 8*TiO2@CuO NWA, 16*TiO2@CuO NWA and 24*TiO2@CuO NWA-coated copper meshes, respectively. Moreover, as illustrated in the inset figures, the cross-sectional view of the TiO2@CuO NWA for the composites also reveals a gradually increase of the nanowire thickness, further demonstrate the positively correlated of the Ti loading amount and the hydrothermal recrystallization time. As the photocatalytic degradation ability of the TiO2@CuO NWA composite is highly related to the TiO2 loading amount, the controllable and adjustable TiO2 loading amount may also be beneficial for the self-cleaning ability.

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Figure. 3 SEM images (at a magnification of 5000×) and the corresponding EDS spectra for (a) 8*TiO2@CuO, (b) 16*TiO2@CuO, (c) 24*TiO2@CuO NWA-coated copper meshes obtained after hydrothermal reaction for 8, 16 and 24 h, respectively, (d) SEM image and the corresponding EDS mapping of the Cu, O, Ti elements of the 24*TiO2@CuO NWA-coated copper mesh. The crystalline phase profile of the TiO2@CuO NWA mesh membranes is evaluated by the XRD pattern. As the anatase phase TiO2 is considered to exhibit better photocatalyst than rutileTiO2, and the phase transfer temperature from anatase to rutile is around 600-700 °C,36 hence the annealing temperature is chosen to be 550 oC. The respective XRD patterns of the 8*TiO2@CuO NWA, 16*TiO2@CuO NWA and 24*TiO2@CuO NWA-coated copper meshes are shown in figure 4. The six diffraction peaks located at 25.3°, 36.9°, 48.2°,

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53.9°, 62.1° and 68.8° are related to the (101), (103), (200), (105), (213) and (116) crystalline phase of the anatase TiO2 (PDF card No.21-1272), demonstrating the successful conversion of the anatase TiO2 phase. Moreover, the gradual increase of the relative intensity of the anatase diffraction peaks indicates the increase of the loading amount of the TiO2 crystals, which is in line with the EDS result. Furthermore, the Cu(OH)2 phase nanowire array is also converted to CuO crystalline after the high-temperature annealing, as compared to the XRD patterns of the Cu(OH)2 NWA (Figure S3, see Supporting Information), indicating the almost completely phase transfer of the nanowire array.24 It is worthwhile to mention that the relatively weak diffraction peaks of the pure Cu mesh located at 43.4 (111), 50.6 (200) and 74.3 (220) reveals the thick wrapping layer of the TiO2@CuO NWA outside the copper mesh, which further demonstrates the uniform coverage of the micro-/nano-hierarchical structure on the mesh surface.

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Figure. 4 XRD patterns of (a) 8*TiO2@CuO, (b) 16*TiO2@CuO, (c) 24*TiO2@CuO NWA-coated copper meshes obtained after hydrothermal reaction for 8, 16 and 24 h, respectively. The surface chemical compositions and their valance state of the 24*TiO2@CuO NWA-coated copper mesh membrane is further determined by XPS (Figure 5). In the wide scan survey of the TiO2@CuO NWA (Figure 5a), the presence of the Cu, Ti and O peaks reveals the existence of these elements. The deconvolution of the Cu 2p peak reveals two valance state as Cu(0) and Cu(II) located at 933.1 eV and 934.9 eV, respectively, corresponding to the copper mesh and CuO (Figure 5c). The curve-fitted O 1s core-level spectrum exhibits a predominant peaks of crystalline phase oxide (530.2 eV), indicating the presence of the CuO crystals (Figure 5b). On the other hand, the minor peaks centred at 531.8 eV and 532.9 eV in the curve-fitted O 1s core-level spectrum correspond to the hydroxide and H2O, respectively, which may be ascribed to the trace absorbed water molecules or surface transformation on the TiO2@CuO NWA surface. Furthermore, the binding energy gap between the Ti 2p3/2 and Ti 2p1/2 is ~5.7 eV (Figure 5d), corresponding to an average Ti oxidation state of +4, which further confirms the successful coating of the TiO2 crystals. On the other hand, the XPS spectrum of the Cu(OH)2 NWA reveals a combination of the Cu and Cu(OH)2 peak components (Figure S4, see Supporting Information), which is in line with the XRD results. The polar 14


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TiO2@CuO NWA coating surface and the surface absorbed hydroxide and H2O may facilitate the water molecular infiltration into the as-prepared mesh, thus exhibits a superhydrophilic nature of the mesh membrane.

Figure. 5 The (a) Wide scan, (b) O 1s, (c) Cu 2p and (d) Ti 2p core-level XPS spectra of 24*TiO2@CuO NWA-coated copper mesh obtained after hydrothermal crystallization for 24 h. The water and oil wetting behaviors of the as-fabricated copper mesh membranes are measured by the static contact angle. The wetting behavior of water on the Cu(OH)2 NWA-coated copper mesh is substantially enhanced, the contact angle is decreased from 104º to 0º as compared to the pristine Cu mesh (Figure S5, see Supporting Information), demonstrating the superhydrophilic nature upon the growth of Cu(OH)2 NWA on the copper mesh. The dynamic time-resolved optical snapshots of the water droplet on Cu(OH)2 NWA-coated copper mesh membrane revealed an ultrafast spreading and penetration of the water droplet within 49.98 ms after contacting the surface (Figure S6, see Supporting Information). After deposition of 15



TiO2, similar strong wetting behavior of water on 24*TiO2@CuO NWA-coated copper mesh is observed, with slight increase in water penetration time to 83.30 ms (Figure 6). This may attribute to the increased osmotic resistance with the immobilization of the thick TiO2 layer, as well as the decreased density of hydroxyl groups (-OH) on the surface of the membrane after annealing.

Figure. 6 Time-resolved snapshots for measuring dynamic water contact angles (WCA) of a water droplet in air on the 24*TiO2@CuO NWA-coated copper meshes obtained after hydrothermal crystallization for 24 h. The high-speed camera was used to capture the photographs with 120 frames/s. Moreover, the underwater oil wettability of the 24*TiO2@CuO NWA-coated copper mesh is detected by submerging the mesh membrane in water, and placing the oil droplets onto the upper/lower surface of the mesh. As shown in the figure 7, oil droplets with different surface tensions are contact on the surface of the as-prepared copper mesh, and clear oil contact angles can be observed. The 24*TiO2@CuO 16


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NWA-coated copper mesh membrane exhibits high underwater superoleophobic property, since all the oil contact angles at the oil/water/mesh interface are larger than 150º. The oil contact angles toward dichloromethane, n-hexane, cooking oil, crude oil and kerosene are detected to be 153º, 156º, 161º, 153º and 159º, respectively. Moreover, the oil droplets reveal relatively low adhesion to the mesh surface, as the droplets can readily slide on or detach from the mesh surface by gentle disturbance. This low adhesion force may due to the reduced oil/solid (mesh membrane) contact interface. As the superhydrophilicity and designed hierarchical nanostructures of the TiO2@CuO NWA-coated copper mesh, the water can be easily trapped into the nanostructures once the as-prepared membranes are immersed in water. As a result, the trapped water molecules act as the buffer layer and greatly reduce the contact area between the oil/solid interface. The oil/water contact area for kerosene is calculated to be 91.2% based on the Cassie-Baxter equations compared to the real interface contact area.37 The strong repulsive force between the water and oil, as well as the discontinuous oil/solid contact interface may attribute to the superoleophobic nature and low adhesion force of the membrane.

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Figure. 7 Contact angles of different oil droplets and the corresponding snapshots on the 24*TiO2@CuO NWA-coated copper meshes obtained after hydrothermal crystallization for 24 h. The optical property of the 24*TiO2@CuO NWA-coated copper mesh is further verified by the diffused reflectance UV–visible spectrum to understand the photodegradation capability. As shown in figure 8, an intense absorbance peak located around 200-380 nm can be observed, which indicates the strong UV absorption properties of TiO2.38,

39

The absorbance coefficient of the

24*TiO2@CuO NWA composites is calculated using Tauc plot. The extrapolation of the linear region reveals two absorption bands located at 2.1 eV and 2.6 eV,40 respectively, which are attributed to the CuO and TiO2 crystalline, respectively. As the theoretical bandgap of the TiO2 is 3.2 eV, the red shift of the bandgap may due to the dopant or alloying between the TiO2 and the CuO crystalline.41 The photoelectrons and vacancies generated by the 18


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UV-light illumination can react with water and oxygen to generate high activity superoxide anions and hydroxyl radicals, which can further decompose organic contaminants adsorbed inside the separation meshes.38,

41

Therefore, the

rationally-designed hierarchical 24*TiO2@CuO NWA-coated copper mesh membrane may act as a high performance oil/water separation material based on the following facts: i) the CuO nanowires and TiO2 nanoparticles can construct a micro-/nano-dual hierarchical and roughness structure with hydrophilic surface nature, which can ensure the superhydrophilic and underwater superoleophobic property to facilitate the fast oil/water separation; ii) the well-designed UV photocatalytic activity of the uniformly-distributed layer of TiO2 nanoparticles can endow the mesh composite with desired photodegradation ability, hence the oil contaminant adsorbed on the mesh can be quickly removed without damage the composite structure.

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Figure. 8 (a) UV-visible spectra and (b) the corresponding Tauc plot for measuring the bandgap of the 24*TiO2@CuO NWA-coated copper meshes obtained after hydrothermal crystallization for 24 h. The oil/water separation property of the TiO2@CuO NWA-coated copper mesh membranes are ascertained by a series proof-of-concept demonstration. The mesh membrane is tailored to a round disk with the diameter of 2 cm, and then fixed between two custom-made glass vessels. After pre-wetting the mesh by water, the oil/water mixture of kerosene (red) and water (1:1 in volume) is directly added into the upper end of the vessel. As shown in movie S1 (see Supporting Information), the kerosene is firstly added into the vessel and is repelled on the mesh due to the superoleophobicity of the mesh. Meanwhile, 20


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when the water is added, the water content is rapidly penetrated though the mesh. Finally, the oil and water are completely separated in the upper/lower side of the mesh, respectively (Figures 9a, c), and the whole separation process is only driven by the gravity. Moreover, this advanced mesh membrane can also be applied to the separation of the surfactant stabilized emulsions (oilfield wastewater with surfactant). As shown in figures 9b, d and movie S2 (see Supporting Information), the oil droplets inside the oilfield wastewater emulsion can be successfully separated, no obvious oil droplets in the water can be found in the optical microscope image (Figure 9b inset), and the oil phase is also highly condensed after separation. The surfactant stabilized oil/water emulsion separation are highly demanded in the practical application in the field of industrial and domestic wastewater, further demonstrate the potential application of this robust hierarchical micro-/nano-dual mesh membrane.

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Figure. 9 The photographs of separating (a) kerosene/water mixture and (b) oilfield wastewater where water selectively permeates through 24*TiO2@CuO NWA-coated copper meshes (insets in figure 9b are the optical microscope images of the water before and after separation). Optical photos of (c) the separated kerosene (stained with oil red) and collected water after separation of the kerosene/water mixture, (d) oilfield wastewater and collected water after separation. To seek deep insight into the effect of the hierarchical nanostructures on the oil/water separation efficiency, the separation experiments are carried out by using Cu(OH)2 NWA and the TiO2@CuO NWA membranes obtained from different hydrothermal time. The water flux and oil residue content in water are calculated to quantitatively compare the performance difference of these 22


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meshes (Figure 10a). All the meshes exhibit high water flux under the sole driving-force of gravity, indicate the advantageous of the well-designed mesh structure. Moreover, the water flux gradually decreases with the increase of the TiO2 deposition time from 101.9 kL h-1 m-2 for Cu(OH)2 NWA to 87.6 kL h-1 m-2 for 24*TiO2@CuO NWA. These results are in accordance with the delayed complete penetration time of the mesh owing to the increased osmotic resistance and mesh thickness. On contrary, the oil residue content in the water is also gradually decrease with the increase of the TiO2 deposition time (from 27.9 to 12.4 mg L-1), further indicating that the increased TiO2 nanoparticles deposition could form a larger liquid/solid contact interface, thus dramatically decrease the spread oil contents.

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Fig. 10 (a) The change in water flux and oil residual concentration (COD value) in permeated water after separation of (a) kerosene/water mixture as a function of TiO2 loading amount and (b) oil types in the mixture for the 24*TiO2@CuO NWA-coated copper meshes. The general oil separation ability of the 24*TiO2@CuO NWA-coated copper mesh membrane is investigated by several kind of oils. The water flux and the oil residue content of the oil/water mixture of decane, cooking oil, kerosene and oilfield wastewater are shown in figure 10b. All types of mixture reveal similar permeate water flux of around 100 kL h-1 m-2, indicating that the general separation flux of the mesh membrane is not related to the oil species. Similarly, the oil residue content of these mixtures also shows a high separation 24


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efficiency. The oil residue of the decane, cooking oil and kerosene from permeated water is 5.1, 10.9 and 13.2 mg L-1, respectively, which is in line with their purity and water-soluble objects. Moreover, the oilfield wastewater reveals a highest oil residue of 25.3 mg L-1 (the COD of the initial oilfield wastewater is circa 2000-3000 mg L-1 with surfactant inside), which may due to the soluble surfactant additives in the oilfield wastewater. Taken together, the well-designed

hierarchical

24*TiO2@CuO

NWA-coated

copper

mesh

membrane demonstrates a general oil/water mixture separation ability with high separation efficiency, which shows great potential in the practical applications. During the oil/water separation process, trace amount of oil residue may be trapped in the micro-/nano-structure of the mesh membrane, thus the durability and reuse ability of the separation membrane is of great important in the practical application. As the TiO2 nanoparticles anchored on the CuO nanowire can generate photoelectrons and holes under UV-light illumination, and subsequently form hydroxyl radicals or superoxide radicals to decompose the organic contaminants. Therefore, the self-cleaning ability of the 24*TiO2@CuO NWA-coated copper mesh composite is also investigated by exposing the oil-contaminated mesh under UV irradiation to photodegradation the contaminants. Figure 11 shows the evolution profile in static water contact angles with the exposure time. After continuously separation of kerosene/water mixture, the dramatic increase in the oil residue is observed. This also result in 25



increase of water contact angle to 148.3º from 0º, implying the loss of the superhydrophilicity of the membrane. When exposed under UV irradiation, the water contact angle of the mesh gradually decreases, which indicates the gradual degradation of the organic contaminants by photocatalysis. The water contact angle of the mesh composite reach to 0º after 60 min of UV irradiation exposure, which illustrate the totally photodegradation of the oil residue. The approximately linear decrease in water contact angle reveals the stable organic residue degradation ratio, and the optimal UV illumination time is around 60 min. As a result, the 24*TiO2@CuO NWA-coated copper mesh can act as the high-performance oil/water separation mesh with high self-cleaning ability.

Figure. 11 The plot of water contact angle of the oil-contaminated 24*TiO2@CuO NWA-coated copper meshes after oil/water separation as a function of different UV irradiation time. Insets correspond to the optical photos of static water contact angle of a droplet on the copper meshes.

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To further evaluate the self-cleaning recycle stability of the 24*TiO2@CuO NWA-coated copper mesh membrane, the cycling experiment of the oil/water separation is carried out. The mesh composite is used to continuously separate the oil/water mixture for 4 times (5 L each time), and the oil residue in water after each separation process is recorded. After that, the contaminated membrane is irradiation under UV illumination to photodegradation the organics, and the water contact angle before and after degradation is also recorded. As shown in figure 12a, the initial oil residue is about 12.0 mg L-1, and gradually increase to 56.6 mg L-1 after 4 cycles of separation. After UV irradiation, the oil residue reduced to 13.6 mg L-1 in the next cycle, indicating the successful regeneration of the mesh properties. Similar results are repeated in the next four cycles, indicates the good reuse ability of the mesh. Moreover, the water contact angle also confirms the UV irradiation recycle ability of the mesh membrane. The water contact angle increases to 125º after 4 cycles of separation, which can be restored to 0o after UV irradiation, indicating the recovery of the superhydrophilic and underwater superoleophobic ability. The regeneration of the mesh membrane with 0º water contact angle can be repeated for five times, illustrating a good recycle ability and stable structural stability of the composite.

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Figure. 12 The plot of oil residual concentration (COD values) in the permeated water using 24*TiO2@CuO NWA-coated copper meshes as a function of number of cycles and UV-irradiated recycle time. The oil-polluted meshes were recovered by UV irradiation treatment with a 1000 W mercury-vapor lamp, (b) the change in water contact angles on the 24*TiO2@CuO NWA-coated copper meshes upon seven cycles of oil pollution and UV irradiation recovery. The superwetting durability and structural stability of the separation membrane is also essential for practical long-term oil/water separation 28


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application. The completely inorganic structural design of the TiO2@CuO NWA-coated copper meshes may facilitate the long-term structural and chemical stability. Therefore, the durability of the superhydrophilic and underwater superoleophobic ability of the 24*TiO2@CuO NWA-coated copper mesh was investigated by immersing the membrane into a 10 μM HCl aqueous solution (at pH 5). As shown in Figure 13a, the water contact angle (WCA, close to 0º) and underwater oil contact angle (OCA, more than 150º) of the separation membrane remains almost unchanged during a long exposure period of 5 days, and the corresponding optical microscope images also confirm the stable contact angle. Therefore, the as-prepared separation membrane reveals excellent superwetting durability within a long period, indicative of the strong chemical stability of the contact interface. On the other hand, the morphology change of the 24*TiO2@CuO NWA-coated copper mesh after continuous separation process was also investigated. No obvious change in the surface morphology can be observed from the SEM images (Figures 13b and 13c), the TiO2 nanoparticles also reveal a tight attachment on the CuO nanowires. This stable structure may be derived from the partially alloying between the TiO2 nanoparticles and TiO2/CuO interfaces during high temperature annealing, and thus resulting in a stable and continuous nanostructure. This result is confirmed by the TEM imaging (Figure S2, Supporting Information) and the bandgap shift of the composite (Figure 8). 29



Taken together, with the high oil/water separation efficiency, low oil residue in water, good self-cleaning recycle ability and high stability/durability, this TiO2@CuO NWA-coated copper mesh membrane can be used as a potential next-generation oily wastewater treatment material for practical application.

Figure 13. (a) The change in the WCA and OCA as a function of exposure time in a 10 μM HCl aqueous solution (at pH 5), together with the respective optical images, (b, c) SEM images of the 24*TiO2@CuO NWA-coated copper mesh after continuous oil/water separation process.

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Conclusion In summary, a novel biomimetic hierarchical TiO2@CuO NWA-coated copper mesh was fabricated by a two-step process of chemical oxidation and hydrothermal deposition. The well-designed micro-/nano-dual structure and TiO2

anatase

crystalline

coating

layer

ensure

the

superhydrophilic,

underwater superoleophobic and self-cleaning ability of the mesh membrane. The as-prepared completely inorganic mesh membrane has demonstrated a high oil/water mixture separation efficiency with low oil residue content of lower than 15 mg L-1, and a high water flux of up to 100 kL h-1 m-2, as well as good self-cleaning ability under UV irradiation. Moreover, this mesh membrane with well-designed hierarchical nanostructure is readily scalable for

industrial

preparation,

thus

offering

a

potential

candidate

for

next-generation separation membrane for oily wastewater treatment.

ASSOCIATED CONTENT Supporting Information. Experimental section, SEM images, TEM images, Cu leaching experiment, XRD, XPS spectrums and optical photographs of the materials are listed in the supporting information. AUTHOR INFORMATION 31



Corresponding Author *E-mail:

[email protected]

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors acknowledge the support from National Key Research and Development Program of China (2016YFB0301701) and the financial assistance of National Natural Science Foundation of China (21676169, 21776187).

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Table of Content

A biomimetic TiO2@CuO nanowire arrays-coated copper mesh with superwetting and self-cleaning properties is fabricated, the composite mesh can separate oil/water mixture with high efficiency.

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Page 40 of 40

Biomimetic Hierarchical TiO2@CuO Nanowire Arrays-Coated Copper

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