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Cu mesh and application in purification of oily wastewater
Materials Research Bulletin 126 (2020) 110815
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The preparation of Janus Cu(OH)2@Cu2O/Cu mesh and application in purification of oily wastewater
T
Luyang Hua,*, Zhidan Wanga, Yue Hua, Yuheng Liua, Shanmei Zhangb, Yufeng Zhouc, Yumin Zhangc, Yin Liua, Benxia Lid a
School of Materials Science and Engineering, Anhui University of Science & Technology, Huainan, 232001, China School of Mathematic and Big Data, Anhui University of Science & Technology, Huainan, 232001, China c National Key Laboratory of Science and Technology on Advanced Composites in Special Environment, Harbin, 150001, China d Department of Chemistry, College of Science, Zhejiang Sci-Tech University, Hangzhou, 310018, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Janus Cu(OH)2@Cu2O/Cu mesh Wettability Unidirectional water transport Oil-water separation Photocatalysis
A Janus Cu(OH)2@Cu2O/Cu mesh has been synthesized by UV irradiation and in situ hydrazine vapor reduction of superhydrophobic Cu(OH)2/Cu mesh prepared by electrochemical anodization of Cu mesh followed by ODS modification. The Cu2O nanoparticles derived from partial reduction of Cu(OH)2 deposit on the Cu(OH)2 to form a core-shell heterostructure, which endows the as-prepared mesh with micro/nanoscale hierarchical morphology. The wettability of different sides of the mesh can be easily controlled by tuning irradiation time and subsequent reduction. As a result, a unidirectional water penetration phenomenon can be obtained. In addition, based on the asymmetric wettability as well as the photocatalytic activity of the Cu(OH)2@Cu2O nanoneedle decorated mesh, the ultrahigh permeation flux and high separation efficiency with excellent in situ photocatalytic ability can also be achieved. This work may provide a rational and convenient strategy to construct Janus mesh with visible light photocatalysis toward in situ dual-functional water purification.
1. Introduction With the rapid development of global industry and economics, a large amount of oily wastewater from the petrochemical, food, leather, steel industries and oil spill accidents is generated [1]. The effective treatment of the polluted water has become a worldwide problem [2]. Generally, the oily wastewater often contains insoluble oil and soluble organic contaminations [2]. For insoluble parts, one of the typical treatment methods is to separate it from water. However, the separation for water and oil mixture by gravity separation, centrifugation, ultrasonic separation, air flotation, electric field or coagulation is considered as a difficult task due to some issues associated with current systems, including low separation efficiency and participation of complex separation apparatus [3,4]. As a result, the porous materials with special wettability for oil-water separation are developed [3,4]. In present, these materials can be divided into three kinds based on their surface properties, namely, “oil-removing” types, “water-removing” types, and smart controllable materials [3,4]. Although they can exhibit high separation efficiency during the separation process, most of the special wettable materials are not viable for removing soluble pollutants from water [3–5].
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Photocatalysis, as a kind of green technology, is especially suitable for degradation of soluble organic pollutants because of its potential utilization of solar energy and high removal efficiency for pollutants [5–14]. However, most of photocatalysts are in the form of powders and nanoparticles, which are difficult to post-separation and recycling and limit their industry applications [10,11]. Therefore, immobilizing catalysts on heterogeneous supports is usually as an alternative strategy to solve these problems [11]. In fact, the loading process is facilitated to the roughening of carrier surface, which is favorable for the architecture of a superwettable surface. Based on the surface microstructure modification strategy, many porous materials with the photocatalysts are also used for oil-water separation [5,6,12–19]. It is noted that once these materials are utilized to separate oil-water mixtures, more attention is paid to the separation property and less to their photocatalytic activities for the degradation of soluble pollutants in water [14–18]. One reason is that if the material works in water-removing mode, the water with soluble pollutant will pass through the substrate, which limits the photodegradation reaction [12,15–18]. In order to purify the separated water, an extra immersion operation need be employed [12,13]. The other reason is that if the material operates in oil-removing material, the hydrophobic surface will retard the
Corresponding author. E-mail address: [email protected] (L. Hu).
https://doi.org/10.1016/j.materresbull.2020.110815 Received 6 August 2019; Received in revised form 5 February 2020; Accepted 5 February 2020 Available online 06 February 2020 0025-5408/ © 2020 Elsevier Ltd. All rights reserved.
Materials Research Bulletin 126 (2020) 110815
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was fixed onto a measuring cylinder-type filter system. The effective separation area is 2.01 cm2. The experiments were carried out by pouring pure oil or oil-water mixtures onto the mesh. The pure oil flux was calculated by measuring the volume of permeate per unit time and unit area. The separation efficiency R of each oil-water mixture was characterized according to the equation as follows:
photodegradation of pollutant in water phase located on the substrate [19]. Thus, the development of a porous material for in situ separation of oil-water mixtures and highly efficient elimination of soluble pollutants is highly desired. In present, only two kinds of designs, i.e., constructing a hydrophilic-hydrophobic double-layer architecture with photocatalysis [2,20] or limiting the separation flux of hydrophilic material with photocatalysis [5,6], are explored. How to create a versatile material for successive dual-functional water purification with high efficiency is still a challenge. Benefit from the asymmetric physical structure or chemical property of Janus material [21–32], the preparation of porous Janus material seems to be a breakthrough point. Unfortunately, the present methods often only offer these materials with a single function [23,22–32]. Inspired by the design of the Janus structure [21–32] and bi-layer separation system [2,20], here we construct a multi-functional Janus Cu (OH)2@Cu2O/Cu mesh by electrochemical corrosion, hydrophobic modification, selective irradiation and in situ reduction. Such a mesh combines the characteristic of selective transport and photocatalysis, which, in addition to excellent oil-water separation behavior, can be used for the in situ degradation of soluble pollutants in water under visible light. Because the synthesis process is not complex, and the asymmetric wettability of Janus mesh can be easily controlled by tuning irradiation time, we believe this study will provide a new insight for treating oily wastewater from most industrial fields.
R=
Vp V0
where V0 and Vp were the volume of collected filtrate and initial oil in oil-water mixture, respectively. 2.5. Photocatalytic experiment Photocatalytic activity tests for all samples were performed at room temperature. The sample with hydrophilic side upward was fixed onto a measuring cylinder-type filter system or placed on bottom of a glass bottle. 4 mL methylene blue (MB) solution (2 mg/L) was added on the mesh as a triphase or quasi-biphase system for photodegradation. Meanwhile, a glass bottle with 4 ml MB solution is used as a reference. Before testing, an absorption step was carried out in the dark for 30 min. The photocatalytic system was then illuminated by using a xenon lamp (300 W) with a 400 nm cut-off filter. The absorbance of MB was measured by UV–vis spectroscopy (Shimadzu, UV-2600) at the scheduled irradiation time. The amount of MB decomposition was determined by the linear relationship between the concentration and absorption of MB.
2. Experimental procedures 2.1. Preparation of superhydrophobic Cu(OH)2/Cu mesh
3. Results and discussion In a typical procedure, Cu mesh was first cleaned successively with ethanol, acetone, HCl and water several times to remove the surface impurities. Then, the pre-cleaning mesh was anodized in 2 M NaOH for 600 s under 10 mA/cm2 to form Cu(OH)2 nanoneedles on the mesh. Subsequently, the cleaned and dried Cu(OH)2/Cu mesh was immersed in 1 wt.% octadecyltriethoxysilane (ODS)/n-hexane solution for 12 h at room temperature. After the mesh was rinsed with n-hexane and dried, the sample was transferred to an oven operating at 110 °C for 30 min. Finally a superhydrophobic Cu(OH)2/Cu mesh was obtained.
The fabrication procedure of Janus Cu(OH)2@Cu2O/Cu mesh is depicted in Fig. 1. First, the Cu(OH)2 nanowires are electrochemically grown on the copper mesh with a constant current density. Then, the oxidized copper mesh is modified by low surface energy Octadecyltriethoxysilane (ODS) to obtain a superhydrophobic structure. Subsequently, the one side of superhydrophobic mesh is irradiated with UV light in air for certain time. After the irradiated material is exposed to N2H4 vapor for the partial reduction of Cu(OH)2, a Cu(OH)2@Cu2O/Cu mesh with asymmetric wettability is finally achieved. The typical digital photographs of samples at different preparation stages are displayed in Fig. 2a. The pristine copper mesh is purplish-red and has a twill weave structure with an average wire diameter of about 50 μm and a mesh pore size of about 74 μm (Figs. 2a and 2c). In the XRD pattern, three distinct diffraction peaks at 2θ = 43.5°, 50.4° and 74.0°, which correspond to the (111), (200) and (220) crystalline planes of Cu (JCPDS 01–1241), respectively, can be observed clearly (Fig. 2b). After the electrochemical anodization process, the color of resulting mesh is changed to blue. A high-density nanoneedle-like Cu(OH)2 (JCPDS Card No. 35-0505, Figs. 2b, 2d and 2e) are radically covered on the rough surface of copper wire (Figure S1). The diameter of nanoneedles is about 120−280 nm and their length about 13 μm (Figs. 2d and 2e). Each nanoneedle is sculptured with elaborate nanogrooves (Fig. 2e), similar to the unique structure of the setae of strider leg. Due to the hierarchical micro/nanoscale feature, the Cu(OH)2/Cu mesh exhibits a superhydrophobic property by ODS modification (Fig. 3a). The water droplet on such mesh can roll off easily with a sliding angle less than 3° (Figure S2 and Movie S1). To obtain the asymmetric wettability, a selective UV irradiation is applied on the blue superhydrophobic mesh. After treatment for 1 h, a slightly yellowish surface can be observed (Fig. 2a). SEM observation shows that the microstructure on the copper wire has no obvious change (Figure S3). All of the diffraction peaks of Cu(OH)2 are still retained (Fig. 2b). Interestingly, when the irradiated mesh is immersed in N2H4 vapor under reduced pressure, a uniform dull red mesh is gradually formed (Fig. 2a). Following the process, most of the diffraction peaks attributed to Cu(OH)2 disappear and new characteristic
2.2. Preparation of Janus Cu(OH)2@Cu2O/Cu mesh The superhydrophobic Cu(OH)2/Cu mesh was exposed to UV light of 185 nm for 0−120 min. The UV-irradiated side gradually became hydrophilic with time, while the nonirradiated side still remained hydrophobic. Then, the UV-treated mesh was placed in a vacuum desiccator together with an open glass vessels containing 3 ml hydrazine hydrate (80 %), followed by vapor-phase reduction for 10 min at reduced pressure. During this process, partial Cu(OH)2 was reduced to Cu2O, and Janus Cu(OH)2@Cu2O/Cu mesh was formed. 2.3. Characterization The micromorphology of the samples was observed by a scanning electron microscope (GeminiSEM 500) and a transmission electron microscope (TEM, Hitachi 600). XRD pattern was taken on a powder diffractometer (Rigaku TTR-III) using CuKα1 radiation. The surface compositions of sample were studied by X-ray photoelectron spectroscopy (Thermo ESCALAB 250XI) with Al Kα (1486.6 eV) as X-ray source. The contact angles of water were measured by a contact angle meter C20 (Kono). The dynamic process of water droplet on sample, oilwater separation process and photodegradation process were recorded with a digital camera or mobile phone. 2.4. Separation experiment The Janus Cu(OH)2@Cu2O/Cu mesh with hydrophilic side upward 2
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Fig. 1. Schematic illustration of the preparation processes of a Janus Cu(OH)2@Cu2O/Cu mesh. The processes include electrochemical anodization, ODS modification, UV irradiation and hydrazine vapor reduction.
spectroscopy (XPS). As shown in Figure S4a, whether irradiation or reduction, the Cu, O, C, and Si elements are all present on the mesh surface. For the high-resolution XPS spectrum of Cu 2p (Figure S4b), the initial superhydrophobic Cu(OH)2/Cu mesh exhibits two distinct peaks at 934.6 and 954.5 eV, which are assigned to Cu 2p3/2 and Cu 2p1/2 of Cu2+, respectively [33,34]. Meanwhile, two shake-up lines located at 943.2 and 962.4 eV are clearly observed, consistent with the paramagnetic chemical state of Cu2+ in Cu(OH)2 [35]. When the superhydrophobic Cu(OH)2/Cu mesh is irradiated, the new peak corresponding to Cu+ appears and the intensity of the peak increases with increasing time. After the irradiated meshes are reduced, it is found that a large amount of Cu2+ species are converted to Cu+ and the ratio of Cu+ to Cu2+ remains almost 2.1: 1 regardless of irradiation time. As for the C 1s XPS spectra (Fig. 3c), because of the UV dissociation of photogenerated ozone [36,37], the alkylsiloxane monolayer on the ODS modified Cu(OH)2/Cu mesh is slowly decomposed, which results in a weakening of CeC bonds (284.8 eV) but a strengthening of CeO bonds (286.4 eV) and OeC = O bonds (288.9 eV) (Fig. 3c). As a consequence, the hydrophilicity of irradiated surface is gradually improved (Fig. 3a) due to the advancement of polar surface energy [38]. It is worth noting that the content of CeO and OeC]O bonds in composite mesh can be further increased by in situ reduction (Figs. 3c and 3d), and make the surface water contact angle declined finally to 8.2° (Fig. 3b). Based on the result of XPS and contact angle, we demonstrate the feasibility to adjust the surface tension gradient across the mesh thickness. Here, a Janus Cu(OH)2@Cu2O/Cu mesh by 60 min irradiation period is selected as an example to illustrate its application in oilwater separation and subsequent degradation of pollutant. Before the dual-functional water purification, the water breakthrough pressure is first evaluated by measuring the maximum water column height which can be retained by the mesh. The height of water on superhydrophobic Cu(OH)2/Cu mesh and corresponding superhydrophobic Cu (OH)2@Cu2O/Cu mesh is also measured as a comparison, as shown in Figure S5. According to the equation: Pexp = ρghmax, where ρ is the density of water, g is the acceleration of gravity, and hmax is the maximum height of a water column supported by the mesh, the experimental breakthrough pressure (Pexp) for superhydrophobic Cu(OH)2/Cu mesh and superhydrophobic Cu(OH)2@Cu2O/Cu mesh are 3.68 kPa and 3.53 kPa, respectively. Although the Janus mesh has the same microstructure as the superhydrophobic Cu(OH)2@Cu2O/Cu mesh, the intrusion pressure on the I-side is a half of the latter. The decrease of Pexp can be attributed to its wettability gradient (Fig. 3b) and reduced thickness of hydrophobic layer [28,29,39]. Because the intruding pressures of Janus mesh is still comparable with that of the surfacemodificated mesh [40], the as-prepared mesh is capable of separating a large amount of oil-water mixtures.
peaks of Cu2O appear (JCPDS Card No. 34-1354, Fig. 2b). The appeared Cu2O, derive from in situ reduction of Cu(OH)2, in the form of nanoparticle deposits on the surfaces of nanoneedle (Figs. 2f and 2 g), which presents a core-shell morphology (Fig. 2h). The diameter of Cu2O particles is 30−120 nm. Three types of lattice fringes from the HRTEM image of heterostructure can be observed (Fig. 2h). One set of the fringes spacing is ca. 0.250 nm, corresponding to the (111) plane of Cu (OH)2. Another set of the fringes spacing measures ca. 0.243 and 0.298 nm, which are assigned to be (111) and (110) planes of Cu2O, respectively, consistent with the XRD result where the diffraction peak appears at 2θ = 30.0° and 37.0° (Fig. 2b). It is well known that the control of surface wettability plays a very important role in oil-water separation and photodegradation of pollutant [2,7,20,23–27]. The appropriate surface wettability for Janus material is necessary to realize in situ dual-functional water purification. Thus, the influence of UV irradiation and N2H4 reduction processes on wettability of both sides of composite mesh is investigated, as shown in Fig. 3. For the irradiated samples (Fig. 3a), it is seen that the water contact angle of irradiated side (I-side) decreases apparently with increasing of the irradiation time and becomes less than 90° after irradiation for 120 min, while the hydrophobicity of unirradiated surface (N-side) with respect to the surperhydrophobic Cu(OH)2/Cu mesh remains only a small change. Because the surface wettability is primarily governed by the surface topography and chemical composition, when the nanoparticles are created on nanoneedles of the irradiated mesh by in situ reduction, the difference of wettability on both sides is further enhanced (Fig. 3b). On the I-side, a sharp transition from hydrophobic to hydrophilic state occurred after 60 min irradiation, when the water dropped can wet the Cu(OH)2@Cu2O/Cu mesh surface with a water contact angle of 29.4°. Prolonging the irradiation time will lead to a smaller contact angle. On the N-side, when the irradiation time is 60 min, it is found that water droplets can still roll off from the tilt mesh (Movie S2). As the time was increased to 90 min the mesh displays a unidirectional transport behavior, that is, water can permeate from the hydrophobic side to the hydrophilic side, whereas when the Janus mesh is turned over, water just spread on the hydrophilic side without penetration through the mesh (Movie S3). With a further increase in irradiation time to 120 min, the obtained mesh presents a more remarkable unidirectional transport ability and water droplet penetration time changes from 20 s to 0.094 s (Fig. 3b, Movies S3 and S4), which may suggest that a thinner hydrophobic layer is formed on the Janus mesh [28,29]. The combination of photochemical reaction and reduction reaction promotes the transition of I-side from superhydrophobic to hydrophilic surface. To elucidate the wettability evolution, the surface compositions at different reaction stages are analyzed by X-ray photoelectron 3
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Fig. 2. (a) Photographs and (b) XRD patterns of the samples. SEM images of (c) Cu mesh, (d) and (e) Cu(OH)2/Cu mesh, (f) Janus Cu(OH)2@Cu2O/Cu mesh. (g) TEM image and (h) HRTEM image of Cu(OH)2@Cu2O nanoneedle.
diesel, dichloromethane and carbon tetrachloride are higher than 98 % (Fig. 4a), indicating that the Janus mesh to be a good candidate for liquid separation. Generally, the bottom of water supported on mesh is in a state of gas-liquid-solid coexistence after the oil-water separation [42]. In order to evaluate the photocatalytic activity of Janus mesh, photocatalysis under visible light is carried out at such triphase system. The self-degradation of MB and the photocatalytic performance of Janus mesh immersed in the MB solution (quasi-biphase system) are also investigated (Figure S6). Because sufficient oxygen can be rapidly transport from air to reaction interface rather than by diffusion through the water phase [42], and as a result, MB is almost completely degraded
As other important parameters of Janus mesh for the practical applications, the permeate flux and separation efficiency are also measured. To test the flux, the filtration of pure insoluble liquid by the Janus mesh is implemented, and the result is shown in Fig. 4a. It can be seen that the mesh is highly permeable. All fluxes present a constant ratio to the ratio of density and viscosity of oil, which is consistent with the Hagen-Poiseuille equation [41]. To obtain the separation efficiency, two separation modes are adopted, i.e., tilting separation and vertical separation (Movies S5 and S6). The mass ratio of oil after and before the separation process is calculated. The separation efficiencies for different types of oil-water mixtures, including hexane, toluene, dodecane, 4
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Fig. 3. Water contact angles evolution with irradiation time for both sides of the Janus mesh (a) before and (b) after in situ reduction. The green asterisk represents the initial contact angle. The insert shows unidirectional water transport through the Janus mesh that is irradiated for 120 min within 0.094 s. (c) C 1s spectra on irradiated side before (left) and after (right) in situ reduction. The irradiation time for sample I, I′, II, II′, III and III′ are 0 min, 0 min, 60 min, 60 min, 120 min, and 120 min, respectively.(d) Hydrophilic group content of different samples in (c) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 4. (a) The pure insoluble liquid flux and separating efficiency for different oil-water mixtures. (b) Photodegradation of MB under visible light (λ > 400 nm). The insert shows corresponding kinetic linear simulation curves. (c) Cycling runs in the photodegradation of MB under visible light irradiation (λ > 400 nm).
the oily wastewater purification experiment is designed as shown in Fig. 5. One piece Janus Cu(OH)2@Cu2O/Cu mesh is fixed into the separation apparatus for oil permeation and visible light illumination. The mixture of dichloromethane and MB contaminated water is poured onto the mesh. It is clearly observed that dichloromethane can quickly penetrate the mesh, whereas MB aqueous is blocked. After photodegradation of MB under visible light, the original light blue solution turns transparent and is still supported on the mesh. The result demonstrates that the Janus mesh offers broad prospects in the field of
with the triphase system after 40 min exposure whereas 92.4 % and 84.4 % reduction are observed using the quasi-diphase system and blank system, respectively (Fig. 4b). The efficiency of the cycling photodegradation of MB is also tested under the triphase system reaction conditions, as shown Fig. 4c. It can be seen that the Janus Cu (OH)2@Cu2O/Cu mesh still maintain relatively consistent photocatalytic activity after five repetitive cycles, indicating as-prepared material good stability under visible light irradiation. Considering the multi-functional properties of as-prepared mesh,
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Fig. 5. Oil-water separation and in situ visible-light photocatalytic degradation of MB using the Janus Cu(OH)2@Cu2O/Cu mesh.
water purification due to in situ separation of oil-water mixture and elimination of soluble pollutant.
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4. Conclusions In summary, a Janus Cu(OH)2@Cu2O/Cu mesh with micro/nanoscale hierarchical architecture has been successfully prepared by combining electrochemical anodization, ODS modification, UV irradiation and hydrazine vapor reduction. The asymmetric wettability of mesh can be easily adjusted by controlling the hydrophilic group content. Increasing the irradiated time during the fabrication, the obtained mesh can show smart unidirectional water penetration capability. It is noted that the selective oil permeating behavior of Janus mesh can be also employed for the separation of oil-water mixtures with high efficiency. Combing the photocatalytic activity of the Cu(OH)2@Cu2O nanoneedle under visible light, the blocked water with soluble organic contamination by the mesh can be in situ degraded within a short time, which endow the Janus Cu(OH)2@Cu2O/Cu mesh with noticeable achievement in dual water remediation. We believe that the work open a new insight into the synthesis of versatile material for great potential applications in water purification. CRediT authorship contribution statement Luyang Hu: Conceptualization, Methodology, Data curation, Writing - original draft, Project administration. Zhidan Wang: Investigation, Writing - review & editing. Yue Hu: Methodology, Visualization, Investigation, Resources. Yuheng Liu: Visualization. Shanmei Zhang: Methodology, Formal analysis. Yufeng Zhou: Investigation. Yumin Zhang: Investigation. Yin Liu: Investigation. Benxia Li: Investigation. Declaration of Competing Interest None. Acknowledgement This work was supported by the financial support from the Outstanding Top-notch Talent Cultivation Projection in Anhui Province of China (No. gxgwfx2019014). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.materresbull.2020. 110815. References [1] W. Wu, R. Huang, W. Qi, R. Su, Z. He, Bioinspired peptide-coated superhydrophilic poly(vinylidene fluoride) membrane for oil/water emulsion separation, Langmuir 34 (2018) 6621–6627, https://doi.org/10.1021/acs.langmuir.8b01017.
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The preparation of Janus Cu(OH)2@Cu2O/Cu mesh and application in purification of oily wastewater
T
Luyang Hua,*, Zhidan Wanga, Yue Hua, Yuheng Liua, Shanmei Zhangb, Yufeng Zhouc, Yumin Zhangc, Yin Liua, Benxia Lid a
School of Materials Science and Engineering, Anhui University of Science & Technology, Huainan, 232001, China School of Mathematic and Big Data, Anhui University of Science & Technology, Huainan, 232001, China c National Key Laboratory of Science and Technology on Advanced Composites in Special Environment, Harbin, 150001, China d Department of Chemistry, College of Science, Zhejiang Sci-Tech University, Hangzhou, 310018, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: Janus Cu(OH)2@Cu2O/Cu mesh Wettability Unidirectional water transport Oil-water separation Photocatalysis
A Janus Cu(OH)2@Cu2O/Cu mesh has been synthesized by UV irradiation and in situ hydrazine vapor reduction of superhydrophobic Cu(OH)2/Cu mesh prepared by electrochemical anodization of Cu mesh followed by ODS modification. The Cu2O nanoparticles derived from partial reduction of Cu(OH)2 deposit on the Cu(OH)2 to form a core-shell heterostructure, which endows the as-prepared mesh with micro/nanoscale hierarchical morphology. The wettability of different sides of the mesh can be easily controlled by tuning irradiation time and subsequent reduction. As a result, a unidirectional water penetration phenomenon can be obtained. In addition, based on the asymmetric wettability as well as the photocatalytic activity of the Cu(OH)2@Cu2O nanoneedle decorated mesh, the ultrahigh permeation flux and high separation efficiency with excellent in situ photocatalytic ability can also be achieved. This work may provide a rational and convenient strategy to construct Janus mesh with visible light photocatalysis toward in situ dual-functional water purification.
1. Introduction With the rapid development of global industry and economics, a large amount of oily wastewater from the petrochemical, food, leather, steel industries and oil spill accidents is generated [1]. The effective treatment of the polluted water has become a worldwide problem [2]. Generally, the oily wastewater often contains insoluble oil and soluble organic contaminations [2]. For insoluble parts, one of the typical treatment methods is to separate it from water. However, the separation for water and oil mixture by gravity separation, centrifugation, ultrasonic separation, air flotation, electric field or coagulation is considered as a difficult task due to some issues associated with current systems, including low separation efficiency and participation of complex separation apparatus [3,4]. As a result, the porous materials with special wettability for oil-water separation are developed [3,4]. In present, these materials can be divided into three kinds based on their surface properties, namely, “oil-removing” types, “water-removing” types, and smart controllable materials [3,4]. Although they can exhibit high separation efficiency during the separation process, most of the special wettable materials are not viable for removing soluble pollutants from water [3–5].
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Photocatalysis, as a kind of green technology, is especially suitable for degradation of soluble organic pollutants because of its potential utilization of solar energy and high removal efficiency for pollutants [5–14]. However, most of photocatalysts are in the form of powders and nanoparticles, which are difficult to post-separation and recycling and limit their industry applications [10,11]. Therefore, immobilizing catalysts on heterogeneous supports is usually as an alternative strategy to solve these problems [11]. In fact, the loading process is facilitated to the roughening of carrier surface, which is favorable for the architecture of a superwettable surface. Based on the surface microstructure modification strategy, many porous materials with the photocatalysts are also used for oil-water separation [5,6,12–19]. It is noted that once these materials are utilized to separate oil-water mixtures, more attention is paid to the separation property and less to their photocatalytic activities for the degradation of soluble pollutants in water [14–18]. One reason is that if the material works in water-removing mode, the water with soluble pollutant will pass through the substrate, which limits the photodegradation reaction [12,15–18]. In order to purify the separated water, an extra immersion operation need be employed [12,13]. The other reason is that if the material operates in oil-removing material, the hydrophobic surface will retard the
Corresponding author. E-mail address: [email protected] (L. Hu).
https://doi.org/10.1016/j.materresbull.2020.110815 Received 6 August 2019; Received in revised form 5 February 2020; Accepted 5 February 2020 Available online 06 February 2020 0025-5408/ © 2020 Elsevier Ltd. All rights reserved.
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was fixed onto a measuring cylinder-type filter system. The effective separation area is 2.01 cm2. The experiments were carried out by pouring pure oil or oil-water mixtures onto the mesh. The pure oil flux was calculated by measuring the volume of permeate per unit time and unit area. The separation efficiency R of each oil-water mixture was characterized according to the equation as follows:
photodegradation of pollutant in water phase located on the substrate [19]. Thus, the development of a porous material for in situ separation of oil-water mixtures and highly efficient elimination of soluble pollutants is highly desired. In present, only two kinds of designs, i.e., constructing a hydrophilic-hydrophobic double-layer architecture with photocatalysis [2,20] or limiting the separation flux of hydrophilic material with photocatalysis [5,6], are explored. How to create a versatile material for successive dual-functional water purification with high efficiency is still a challenge. Benefit from the asymmetric physical structure or chemical property of Janus material [21–32], the preparation of porous Janus material seems to be a breakthrough point. Unfortunately, the present methods often only offer these materials with a single function [23,22–32]. Inspired by the design of the Janus structure [21–32] and bi-layer separation system [2,20], here we construct a multi-functional Janus Cu (OH)2@Cu2O/Cu mesh by electrochemical corrosion, hydrophobic modification, selective irradiation and in situ reduction. Such a mesh combines the characteristic of selective transport and photocatalysis, which, in addition to excellent oil-water separation behavior, can be used for the in situ degradation of soluble pollutants in water under visible light. Because the synthesis process is not complex, and the asymmetric wettability of Janus mesh can be easily controlled by tuning irradiation time, we believe this study will provide a new insight for treating oily wastewater from most industrial fields.
R=
Vp V0
where V0 and Vp were the volume of collected filtrate and initial oil in oil-water mixture, respectively. 2.5. Photocatalytic experiment Photocatalytic activity tests for all samples were performed at room temperature. The sample with hydrophilic side upward was fixed onto a measuring cylinder-type filter system or placed on bottom of a glass bottle. 4 mL methylene blue (MB) solution (2 mg/L) was added on the mesh as a triphase or quasi-biphase system for photodegradation. Meanwhile, a glass bottle with 4 ml MB solution is used as a reference. Before testing, an absorption step was carried out in the dark for 30 min. The photocatalytic system was then illuminated by using a xenon lamp (300 W) with a 400 nm cut-off filter. The absorbance of MB was measured by UV–vis spectroscopy (Shimadzu, UV-2600) at the scheduled irradiation time. The amount of MB decomposition was determined by the linear relationship between the concentration and absorption of MB.
2. Experimental procedures 2.1. Preparation of superhydrophobic Cu(OH)2/Cu mesh
3. Results and discussion In a typical procedure, Cu mesh was first cleaned successively with ethanol, acetone, HCl and water several times to remove the surface impurities. Then, the pre-cleaning mesh was anodized in 2 M NaOH for 600 s under 10 mA/cm2 to form Cu(OH)2 nanoneedles on the mesh. Subsequently, the cleaned and dried Cu(OH)2/Cu mesh was immersed in 1 wt.% octadecyltriethoxysilane (ODS)/n-hexane solution for 12 h at room temperature. After the mesh was rinsed with n-hexane and dried, the sample was transferred to an oven operating at 110 °C for 30 min. Finally a superhydrophobic Cu(OH)2/Cu mesh was obtained.
The fabrication procedure of Janus Cu(OH)2@Cu2O/Cu mesh is depicted in Fig. 1. First, the Cu(OH)2 nanowires are electrochemically grown on the copper mesh with a constant current density. Then, the oxidized copper mesh is modified by low surface energy Octadecyltriethoxysilane (ODS) to obtain a superhydrophobic structure. Subsequently, the one side of superhydrophobic mesh is irradiated with UV light in air for certain time. After the irradiated material is exposed to N2H4 vapor for the partial reduction of Cu(OH)2, a Cu(OH)2@Cu2O/Cu mesh with asymmetric wettability is finally achieved. The typical digital photographs of samples at different preparation stages are displayed in Fig. 2a. The pristine copper mesh is purplish-red and has a twill weave structure with an average wire diameter of about 50 μm and a mesh pore size of about 74 μm (Figs. 2a and 2c). In the XRD pattern, three distinct diffraction peaks at 2θ = 43.5°, 50.4° and 74.0°, which correspond to the (111), (200) and (220) crystalline planes of Cu (JCPDS 01–1241), respectively, can be observed clearly (Fig. 2b). After the electrochemical anodization process, the color of resulting mesh is changed to blue. A high-density nanoneedle-like Cu(OH)2 (JCPDS Card No. 35-0505, Figs. 2b, 2d and 2e) are radically covered on the rough surface of copper wire (Figure S1). The diameter of nanoneedles is about 120−280 nm and their length about 13 μm (Figs. 2d and 2e). Each nanoneedle is sculptured with elaborate nanogrooves (Fig. 2e), similar to the unique structure of the setae of strider leg. Due to the hierarchical micro/nanoscale feature, the Cu(OH)2/Cu mesh exhibits a superhydrophobic property by ODS modification (Fig. 3a). The water droplet on such mesh can roll off easily with a sliding angle less than 3° (Figure S2 and Movie S1). To obtain the asymmetric wettability, a selective UV irradiation is applied on the blue superhydrophobic mesh. After treatment for 1 h, a slightly yellowish surface can be observed (Fig. 2a). SEM observation shows that the microstructure on the copper wire has no obvious change (Figure S3). All of the diffraction peaks of Cu(OH)2 are still retained (Fig. 2b). Interestingly, when the irradiated mesh is immersed in N2H4 vapor under reduced pressure, a uniform dull red mesh is gradually formed (Fig. 2a). Following the process, most of the diffraction peaks attributed to Cu(OH)2 disappear and new characteristic
2.2. Preparation of Janus Cu(OH)2@Cu2O/Cu mesh The superhydrophobic Cu(OH)2/Cu mesh was exposed to UV light of 185 nm for 0−120 min. The UV-irradiated side gradually became hydrophilic with time, while the nonirradiated side still remained hydrophobic. Then, the UV-treated mesh was placed in a vacuum desiccator together with an open glass vessels containing 3 ml hydrazine hydrate (80 %), followed by vapor-phase reduction for 10 min at reduced pressure. During this process, partial Cu(OH)2 was reduced to Cu2O, and Janus Cu(OH)2@Cu2O/Cu mesh was formed. 2.3. Characterization The micromorphology of the samples was observed by a scanning electron microscope (GeminiSEM 500) and a transmission electron microscope (TEM, Hitachi 600). XRD pattern was taken on a powder diffractometer (Rigaku TTR-III) using CuKα1 radiation. The surface compositions of sample were studied by X-ray photoelectron spectroscopy (Thermo ESCALAB 250XI) with Al Kα (1486.6 eV) as X-ray source. The contact angles of water were measured by a contact angle meter C20 (Kono). The dynamic process of water droplet on sample, oilwater separation process and photodegradation process were recorded with a digital camera or mobile phone. 2.4. Separation experiment The Janus Cu(OH)2@Cu2O/Cu mesh with hydrophilic side upward 2
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Fig. 1. Schematic illustration of the preparation processes of a Janus Cu(OH)2@Cu2O/Cu mesh. The processes include electrochemical anodization, ODS modification, UV irradiation and hydrazine vapor reduction.
spectroscopy (XPS). As shown in Figure S4a, whether irradiation or reduction, the Cu, O, C, and Si elements are all present on the mesh surface. For the high-resolution XPS spectrum of Cu 2p (Figure S4b), the initial superhydrophobic Cu(OH)2/Cu mesh exhibits two distinct peaks at 934.6 and 954.5 eV, which are assigned to Cu 2p3/2 and Cu 2p1/2 of Cu2+, respectively [33,34]. Meanwhile, two shake-up lines located at 943.2 and 962.4 eV are clearly observed, consistent with the paramagnetic chemical state of Cu2+ in Cu(OH)2 [35]. When the superhydrophobic Cu(OH)2/Cu mesh is irradiated, the new peak corresponding to Cu+ appears and the intensity of the peak increases with increasing time. After the irradiated meshes are reduced, it is found that a large amount of Cu2+ species are converted to Cu+ and the ratio of Cu+ to Cu2+ remains almost 2.1: 1 regardless of irradiation time. As for the C 1s XPS spectra (Fig. 3c), because of the UV dissociation of photogenerated ozone [36,37], the alkylsiloxane monolayer on the ODS modified Cu(OH)2/Cu mesh is slowly decomposed, which results in a weakening of CeC bonds (284.8 eV) but a strengthening of CeO bonds (286.4 eV) and OeC = O bonds (288.9 eV) (Fig. 3c). As a consequence, the hydrophilicity of irradiated surface is gradually improved (Fig. 3a) due to the advancement of polar surface energy [38]. It is worth noting that the content of CeO and OeC]O bonds in composite mesh can be further increased by in situ reduction (Figs. 3c and 3d), and make the surface water contact angle declined finally to 8.2° (Fig. 3b). Based on the result of XPS and contact angle, we demonstrate the feasibility to adjust the surface tension gradient across the mesh thickness. Here, a Janus Cu(OH)2@Cu2O/Cu mesh by 60 min irradiation period is selected as an example to illustrate its application in oilwater separation and subsequent degradation of pollutant. Before the dual-functional water purification, the water breakthrough pressure is first evaluated by measuring the maximum water column height which can be retained by the mesh. The height of water on superhydrophobic Cu(OH)2/Cu mesh and corresponding superhydrophobic Cu (OH)2@Cu2O/Cu mesh is also measured as a comparison, as shown in Figure S5. According to the equation: Pexp = ρghmax, where ρ is the density of water, g is the acceleration of gravity, and hmax is the maximum height of a water column supported by the mesh, the experimental breakthrough pressure (Pexp) for superhydrophobic Cu(OH)2/Cu mesh and superhydrophobic Cu(OH)2@Cu2O/Cu mesh are 3.68 kPa and 3.53 kPa, respectively. Although the Janus mesh has the same microstructure as the superhydrophobic Cu(OH)2@Cu2O/Cu mesh, the intrusion pressure on the I-side is a half of the latter. The decrease of Pexp can be attributed to its wettability gradient (Fig. 3b) and reduced thickness of hydrophobic layer [28,29,39]. Because the intruding pressures of Janus mesh is still comparable with that of the surfacemodificated mesh [40], the as-prepared mesh is capable of separating a large amount of oil-water mixtures.
peaks of Cu2O appear (JCPDS Card No. 34-1354, Fig. 2b). The appeared Cu2O, derive from in situ reduction of Cu(OH)2, in the form of nanoparticle deposits on the surfaces of nanoneedle (Figs. 2f and 2 g), which presents a core-shell morphology (Fig. 2h). The diameter of Cu2O particles is 30−120 nm. Three types of lattice fringes from the HRTEM image of heterostructure can be observed (Fig. 2h). One set of the fringes spacing is ca. 0.250 nm, corresponding to the (111) plane of Cu (OH)2. Another set of the fringes spacing measures ca. 0.243 and 0.298 nm, which are assigned to be (111) and (110) planes of Cu2O, respectively, consistent with the XRD result where the diffraction peak appears at 2θ = 30.0° and 37.0° (Fig. 2b). It is well known that the control of surface wettability plays a very important role in oil-water separation and photodegradation of pollutant [2,7,20,23–27]. The appropriate surface wettability for Janus material is necessary to realize in situ dual-functional water purification. Thus, the influence of UV irradiation and N2H4 reduction processes on wettability of both sides of composite mesh is investigated, as shown in Fig. 3. For the irradiated samples (Fig. 3a), it is seen that the water contact angle of irradiated side (I-side) decreases apparently with increasing of the irradiation time and becomes less than 90° after irradiation for 120 min, while the hydrophobicity of unirradiated surface (N-side) with respect to the surperhydrophobic Cu(OH)2/Cu mesh remains only a small change. Because the surface wettability is primarily governed by the surface topography and chemical composition, when the nanoparticles are created on nanoneedles of the irradiated mesh by in situ reduction, the difference of wettability on both sides is further enhanced (Fig. 3b). On the I-side, a sharp transition from hydrophobic to hydrophilic state occurred after 60 min irradiation, when the water dropped can wet the Cu(OH)2@Cu2O/Cu mesh surface with a water contact angle of 29.4°. Prolonging the irradiation time will lead to a smaller contact angle. On the N-side, when the irradiation time is 60 min, it is found that water droplets can still roll off from the tilt mesh (Movie S2). As the time was increased to 90 min the mesh displays a unidirectional transport behavior, that is, water can permeate from the hydrophobic side to the hydrophilic side, whereas when the Janus mesh is turned over, water just spread on the hydrophilic side without penetration through the mesh (Movie S3). With a further increase in irradiation time to 120 min, the obtained mesh presents a more remarkable unidirectional transport ability and water droplet penetration time changes from 20 s to 0.094 s (Fig. 3b, Movies S3 and S4), which may suggest that a thinner hydrophobic layer is formed on the Janus mesh [28,29]. The combination of photochemical reaction and reduction reaction promotes the transition of I-side from superhydrophobic to hydrophilic surface. To elucidate the wettability evolution, the surface compositions at different reaction stages are analyzed by X-ray photoelectron 3
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Fig. 2. (a) Photographs and (b) XRD patterns of the samples. SEM images of (c) Cu mesh, (d) and (e) Cu(OH)2/Cu mesh, (f) Janus Cu(OH)2@Cu2O/Cu mesh. (g) TEM image and (h) HRTEM image of Cu(OH)2@Cu2O nanoneedle.
diesel, dichloromethane and carbon tetrachloride are higher than 98 % (Fig. 4a), indicating that the Janus mesh to be a good candidate for liquid separation. Generally, the bottom of water supported on mesh is in a state of gas-liquid-solid coexistence after the oil-water separation [42]. In order to evaluate the photocatalytic activity of Janus mesh, photocatalysis under visible light is carried out at such triphase system. The self-degradation of MB and the photocatalytic performance of Janus mesh immersed in the MB solution (quasi-biphase system) are also investigated (Figure S6). Because sufficient oxygen can be rapidly transport from air to reaction interface rather than by diffusion through the water phase [42], and as a result, MB is almost completely degraded
As other important parameters of Janus mesh for the practical applications, the permeate flux and separation efficiency are also measured. To test the flux, the filtration of pure insoluble liquid by the Janus mesh is implemented, and the result is shown in Fig. 4a. It can be seen that the mesh is highly permeable. All fluxes present a constant ratio to the ratio of density and viscosity of oil, which is consistent with the Hagen-Poiseuille equation [41]. To obtain the separation efficiency, two separation modes are adopted, i.e., tilting separation and vertical separation (Movies S5 and S6). The mass ratio of oil after and before the separation process is calculated. The separation efficiencies for different types of oil-water mixtures, including hexane, toluene, dodecane, 4
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Fig. 3. Water contact angles evolution with irradiation time for both sides of the Janus mesh (a) before and (b) after in situ reduction. The green asterisk represents the initial contact angle. The insert shows unidirectional water transport through the Janus mesh that is irradiated for 120 min within 0.094 s. (c) C 1s spectra on irradiated side before (left) and after (right) in situ reduction. The irradiation time for sample I, I′, II, II′, III and III′ are 0 min, 0 min, 60 min, 60 min, 120 min, and 120 min, respectively.(d) Hydrophilic group content of different samples in (c) (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Fig. 4. (a) The pure insoluble liquid flux and separating efficiency for different oil-water mixtures. (b) Photodegradation of MB under visible light (λ > 400 nm). The insert shows corresponding kinetic linear simulation curves. (c) Cycling runs in the photodegradation of MB under visible light irradiation (λ > 400 nm).
the oily wastewater purification experiment is designed as shown in Fig. 5. One piece Janus Cu(OH)2@Cu2O/Cu mesh is fixed into the separation apparatus for oil permeation and visible light illumination. The mixture of dichloromethane and MB contaminated water is poured onto the mesh. It is clearly observed that dichloromethane can quickly penetrate the mesh, whereas MB aqueous is blocked. After photodegradation of MB under visible light, the original light blue solution turns transparent and is still supported on the mesh. The result demonstrates that the Janus mesh offers broad prospects in the field of
with the triphase system after 40 min exposure whereas 92.4 % and 84.4 % reduction are observed using the quasi-diphase system and blank system, respectively (Fig. 4b). The efficiency of the cycling photodegradation of MB is also tested under the triphase system reaction conditions, as shown Fig. 4c. It can be seen that the Janus Cu (OH)2@Cu2O/Cu mesh still maintain relatively consistent photocatalytic activity after five repetitive cycles, indicating as-prepared material good stability under visible light irradiation. Considering the multi-functional properties of as-prepared mesh,
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Fig. 5. Oil-water separation and in situ visible-light photocatalytic degradation of MB using the Janus Cu(OH)2@Cu2O/Cu mesh.
water purification due to in situ separation of oil-water mixture and elimination of soluble pollutant.
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4. Conclusions In summary, a Janus Cu(OH)2@Cu2O/Cu mesh with micro/nanoscale hierarchical architecture has been successfully prepared by combining electrochemical anodization, ODS modification, UV irradiation and hydrazine vapor reduction. The asymmetric wettability of mesh can be easily adjusted by controlling the hydrophilic group content. Increasing the irradiated time during the fabrication, the obtained mesh can show smart unidirectional water penetration capability. It is noted that the selective oil permeating behavior of Janus mesh can be also employed for the separation of oil-water mixtures with high efficiency. Combing the photocatalytic activity of the Cu(OH)2@Cu2O nanoneedle under visible light, the blocked water with soluble organic contamination by the mesh can be in situ degraded within a short time, which endow the Janus Cu(OH)2@Cu2O/Cu mesh with noticeable achievement in dual water remediation. We believe that the work open a new insight into the synthesis of versatile material for great potential applications in water purification. CRediT authorship contribution statement Luyang Hu: Conceptualization, Methodology, Data curation, Writing - original draft, Project administration. Zhidan Wang: Investigation, Writing - review & editing. Yue Hu: Methodology, Visualization, Investigation, Resources. Yuheng Liu: Visualization. Shanmei Zhang: Methodology, Formal analysis. Yufeng Zhou: Investigation. Yumin Zhang: Investigation. Yin Liu: Investigation. Benxia Li: Investigation. Declaration of Competing Interest None. Acknowledgement This work was supported by the financial support from the Outstanding Top-notch Talent Cultivation Projection in Anhui Province of China (No. gxgwfx2019014). Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.materresbull.2020. 110815. References [1] W. Wu, R. Huang, W. Qi, R. Su, Z. He, Bioinspired peptide-coated superhydrophilic poly(vinylidene fluoride) membrane for oil/water emulsion separation, Langmuir 34 (2018) 6621–6627, https://doi.org/10.1021/acs.langmuir.8b01017.
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