Materials Letters 163 (2016 247– 249

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Materials Letters

journal homepage: www.elsevier.com/locate/matlet

A photo-induced ZnO coated mesh for on-demand oil/water separation based on switchable wettability

Long Yan, Jian Li n

, Weijun Li, Fei Zha, Hua Feng, Dongcheng Hu

Key Laboratory of Eco-Environment-Related Polymer Materials, Ministry of Education of China, Key Laboratory of Gansu Polymer Materials, College of Chemistry and Chemical Engineering, Northwest Normal University, Lanzhou 730070, China

a r t i c l e i n f o

Article history:

Received 29 September 2015 Received in revised form 9 October 2015

Accepted 12 October 2015 Available online 20 October 2015Keywords:

ZnO nanoparticles

Photo-induced

Switchable wettability

Contact angle

Oil/water separation a b s t r a c t

We present a photo-induced mesh for on-demand oil/water separation based on the switchable wett-ability. This multifunctional coated surface was fabricated by spraying hydrophobic ZnO nanoparticles (NPs and waterborne polyurethane (PU mixtures on stainless steel mesh. Reversible transition between superhydrophobicity and superhydrophilicity/underwater superoleophobicity of the coated mesh can be rapidly realized by UV illumination and heat treatment alternately, which allows to change the mode of oil/water separation from oil-removing to water-removing. The oil-removing mode allows heavy oils in oil – water mixtures to permeate, while the water-removing mode letting water permeate. In addition, the high separation ef fi ciency was achieved and the driving-force was only gravity. All these features make the photo-induced ZnO coated mesh an ideal candidate for on-demand separation of various oil/water mixtures.

& 2015 Elsevier B.V. All rights reserved.

1. Introduction

With growing augment of industrial oily wastewater release and crude oil leakage, heightened technological interest in oil/ water separation has become a worldwide subject. Since oil/water separation is governed by interfacial phenomenon, the use of special wettability to design novel materials is an effective and facile approach [1]. Recently, materials with superhydrophilicity/ underwater superoleophobicity (termed as “ water-removing ” types of materials have attracted great attention in the fi eld of oil/ water separation, because they allow the water phase to penetrate through the materials easily while the oil phase to be repelled completely [2– 5]. The “ water-removing ” types of materials are widely utilized due to the fact that water commonly possesses a greater density than oils. However, such water removal materials are not a best choice for separation heavy oil from water because heavy oil accumulates to form a barrier laid on the fi lm surface and prevent water permeation. In this case, it seems an optimal choose for heavy oil/water separation to use materials with super-hydrophobic and superoleophilic properties (named as “ oil-re-moving ” types of materials. Jiang et al. for the fi rst time fabricated a mesh fi lm possessing both superhydrophobicity and super-oleophilicity to separate oil from water with the oil phase per-meated through the mesh quickly while the water phase retained

n

Corresponding author. Fax: þ86 931 7971989. E-mail address: jianli83@126.com (J. Li.

http://dx.doi.org/10.1016/j.matlet.2015.10.051 0167-577X/& 2015 Elsevier B.V. All rights reserved.

above the mesh [6]. Subsequently, lots of “ oil-removing ” types of materials have been fabricated to remove oil from water effectively [7– 11]. Although the separate water-removal and oil-removal materials have been extensively applied in the fi eld of oil/water separation, the best option will depend on the composition of the oil – water mixture and the density of the oil involved. Therefore, there is still an urgent demand for the practical separation device that should have the versatility to handle both options in one single facility.

Herein, we fabricate a photo-induced ZnO coated mesh for on-demand oil/water separation by spraying superhydrophobic ZnO nanoparticles (NPs and waterborne polyurethane (PU mixtures on stainless steel mesh. Switchable transition between super-hydrophobicity and superhydrophilicity/underwater super-oleophobicity of the coated mesh can be rapidly realized by UV illumination and heat treatment alternately, which allows to deal with both options (water-removing or oil removing in one single separation device. In addition, the driven force used for oil/water separation is only gravity and the separation effi ciency up to 99.0% for light oil and water mixtures and above 97.0% for heavy oil and water mixtures was achieved. Furthermore, the reversible transi-tion has been repeated more than 5 times with high separation effi ciency.

2. Experimental

Hydrophobic ZnO NPs were prepared according to our previous

248L. Yan et al. / Materials Letters 163 (2016 247– 249

reported work [12]. In detail, hydrophilic ZnO NPs were fi rstly added to 0.05 M stearic acid ethanol solution and stirred for 6 h to get hydrophobic ZnO NPs. Then, the mixture of 0.1 g waterborne polyurethane (PU and 0.5 g hydrophobic ZnO NPs was dispersed in 40 mL acetone and stirred to get homogeneous suspension. Subsequently, the homogeneous suspension was sprayed onto the stainless steel mesh substrate (300 mesh with 0.2 MPa com-pressed air gas. Finally, the mesh was dried at ambient tempera-ture to allow the acetone to evaporate completely.

The surface chemical composition of the as-prepared samples was analyzed on a PHI-5702 electron spectrometer using Mg Kα radiation as the excitation source and the binding energies were referenced to the C 1s at 284.80 eV. The morphological structures of the as-prepared surfaces were examined by fi eld emission scanning electron microscopy (FE-SEM, JSM-6701F. The water and oil contact angles (CAs were measured with a SL200KB apparatus at ambient temperature by injecting 5 μL of liquid droplets.

3. Results and discussion

The mixture of hydrophobic ZnO NPs and waterborne PU was sprayed onto the stainless steel mesh to get superhydrophobic ZnO coated mesh. As shown in Fig. 1, the surface morphologies of the pristine and the coated meshes were investigated by FE-SEM. Fig. 1a reveals the FE-SEM images of the original stainless steel mesh surface was smooth with the average pore size about 50 μm. After coated with hydrophobic ZnO NPs and PU nanocomposites, it can be seen from low magnifi cation FE-SEM image that a mass of hydrophobic ZnO NPs have aggregated on the wires surface causing obvious decrease of pore size ( Fig. 1b. From the high magni fi cation FE-SEM image ( Fig. 1c, it reveals that hierarchical structures existed due to aggregation of the hydrophobic ZnO NPs and PU nanocomposites. This hierarchical micro- and nanoscale roughness of the coated mesh surface is vitally essential for the superhydrophobicity.

In our study, the photo-induced ZnO coated mesh is desirable for on-demand oil/water separation based on switchable wett-ability. The mesh was fi xed between two Tefl on fi xtures, both of which were fi tted with glass tubes. The as-prepared or heated mesh possessing superhydrophobic and superoleophilic property (“ oil-removing ” type is an optimal choice for heavy oil/water se-paration. As shown in Fig. 2a, when a mixture of equivoluminal chloroform (dyed with the Oil Red O and water (dyed with the methylene blue was poured onto the mesh in “ oil-removing ” state, chloroform rapidly permeate the mesh with driving force of gravity while the water was repelled on mesh (Supporting mate-rials, video1. After being irradiated by UV light for a certain time, the mesh can rapidly switch to “ water-removing ” mode as an ideal candidate for light oil/water separation. Wetting the mesh with water in advance, a mixture of equivoluminal hexane (dyed with the Oil Red O and water (dyed with the methylene blue was operated in the same process ( Fig. 2b and video2. Then the mesh was just rinsed by ethanol and heated for 2 h at 100 °C, the su-perhydrophobicity of the mesh recovered. As shown in Fig. 2c, reversible transition between superhydrophilicity/underwater su-peroleophobicity and superhydrophobicity of the mesh can be rapidly realized by UV illumination and heat treatment alternately. The water CAs were all above 150° when the mesh was in “ oil-removing ” state. On the contrary, the mesh in “ water-removing ” state possessed superhydrophilicity/underwater super-oleophobicity with underwater oil CAs above 150°. In addition, the reversible process was repeated more than 5 times. The separation ef fi ciency was calculated according to η¼(m1/m0 _ 100, where m0 and m1 are the mass of the rejected liquid phase before and after the separation process, respectively. The separation ef fi ciency of the mesh was as high as 99.0% for the chloroform/water mixture and above 98.0% for hexane/water mixture. Furthermore, the coated mesh still retained high separation ef fi ciency up to 97.0% after 5 times reversible transition ( Fig. 2d.

Supplementary material related to this article can be found online at http://dx.doi.org/10.1016/j.matlet.2015.10.051.

Fig. 3a shows the survey XPS spectra of the hydrophobic ZnO coated mesh surfaces before and after UV irradiation. It can be seen that the C 1s, O 1s and Zn 2p peaks are detected for both the surfaces. The amount of relative of oxygen increases after UV ir-radiation treatment. Fig. 3b exhibits the multi-element spectra of O 1s peak for the surface before and after UV irradiation, which was fi tted to Zn– O – C (532.4 eV, Zn– OH (531.2 eV and Zn – O – Zn (530.1 eV, respectively [13]. After UV irradiation for 2 h, it was veri fi ed that relative amount of oxygen in hydroxyl groups (Zn– OH increased from 35.7% to 41.6%. Thus, the wettability of the ZnO coated mesh surface changed from superhydrophobicity to su-perhydrophilicity/underwater superoleophobicity after UV irra-diation. However, the original superhydrophobic property was recovered by heat treatment. As a result, the wettability of the coated surface could be tuned between the superhydrophobicity and the superhydrophilicity/underwater superoleophobicity through alternate UV irradiation and heat treatment. 4. Conclusions

In summary, a photo-induced ZnO coated mesh for on-demand oil/water separation was fabricated by a facile spray-coating method. The mode of oil/water separation was tuned between the oil-removing to the water-removing in one single device, which was ascribed to the reversible wettability change between the superhydrophobicity and the superhydrophilicity/underwater su-peroleophobicity through alternate UV irradiation and heat treatment. In addition, the high separation effi ciency was achieved and the driving-force was only gravity. This study would open up a

Fig. 1. FE-SEM of original (a and (b ZnO coated mesh in low and high magnifi cation, respectively.

L. Yan et al. / Materials Letters 163 (2016 247– 249

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Fig. 2. (a, b Photograph of separation process; (c water and oil CAs and (d separation effi ciency in reversible transition.

Fig. 3. Survey (a and O1s (b XPS spectra of the ZnO surfaces before and after UV irradiation, respectively.

good future towards rational designing and developing functional materials for on-demand oil/water separation. Acknowledgment

The National Natural Science Foundation of China (Grant no. 21301141, the Nature Science Foundation of Gansu Province, China (145RJYA241, and the Scienti fi c and Technical Innovation Project of Northwest Normal University (NWNU-LKQN-12-6 are fi nancially supporting this work.

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