nanofibers and ZnO nanowires

nanofibers and ZnO nanowires

Materials and Design 162 (2019) 246–248 Contents lists available at ScienceDirect Materials and Design journal homepage: www.elsevier.com/locate/mat...

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Materials and Design 162 (2019) 246–248

Contents lists available at ScienceDirect

Materials and Design journal homepage: www.elsevier.com/locate/matdes

A lotus effect-inspired flexible and breathable membrane with hierarchical electrospinning micro/nanofibers and ZnO nanowires Rouxi Chen a,b,c, Yuqin Wan c, Weiwei Wu f, Cao Yang a, Ji-Huan He e,⁎, Jianhua Cheng a,b,⁎⁎, Reinhard Jetter d, Fank K. Ko c, Yuancai Chen a a

College of Environment and Energy, South China University of Technology, Guangzhou, China South China Institute of Collaborative Innovation, South China University of Technology, Dongguan, China Department of Material Engineering, University of British Colombia, Vancouver, Canada d Departments of Botany and Chemistry, University of British Colombia, Vancouver, Canada e National Engineering Laboratory for Modern Silk, College of Textile and Clothing Engineering, Soochow University, Suzhou, China f School of Advanced Materials and Nanotechnology, Xidian University, Shaanxi 710126, China b c

G R A P H I C A L

a r t i c l e

A B S T R A C T

i n f o

Article history: Received 21 June 2018 Received in revised form 9 November 2018 Accepted 18 November 2018 Available online 19 November 2018 Keywords: Lotus effect Bionics design Electrospinning ZnO nanowires Self-cleaning

a b s t r a c t Hierarchical structures such as the leaf of lotus are promising models for self-cleaning surfaces. A biomimetic structure that contains PVDF microfibers (ZnO nanowires covered with oleic acid) was prepared here to illustrate the biomimetic lotus effect concept. This was prepared via electrospinning, hydrothermal synthesis, and dip coating. The hierarchical structure and oleic acid coating was shown to contribute to super hydrophobicity with a water contact angle (WCA) N 150°. The super hydrophobic flexible membrane not only exhibited water droplet bouncing and rolling behaviors, but also demonstrated promising self-cleaning properties, water resistance, and permeability of air and water vapor. These characteristics have far reaching implications in broadening the application of the self-cleaning textiles, waterproof breathable membrane, medical devices, and surgical plants such as artificial blood vessels. © 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/).

⁎ Corresponding author. ⁎⁎ Correspondence to: College of Environment and Energy, South China University of Technology, Guangzhou, China. E-mail addresses: [email protected] (R. Chen), [email protected] (J.-H. He), [email protected] (J. Cheng).

https://doi.org/10.1016/j.matdes.2018.11.041 0264-1275/© 2018 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

R. Chen et al. / Materials and Design 162 (2019) 246–248

Various biological structures exhibit amazing properties for low adhesive surfaces including the legs [1] or wings [2] of insects and the leaves of plants [3]. One classic example is lotus leaf, its typical surface is structured with micron-scale pillars and nanotubes [5], which exhibits extreme water repellency and self-cleaning properties known as the ‘lotus effect’ [4]. Materials with excellent hydrophobicity and vapor-permeability, such as the Gore-tex fabrics, heavily rely on the intrinsic property of the materials themselves and remains high in cost. Although a number of other works were developed to fabricate “lotus leaf effect” materials by mimicking the nano-structured morphology, it remains a challenge to prepare such materials with both the same structure as lotus leaf and the flexibility and permeability. Electrospinning is an effective technology for the preparation of flexible and permeable membranes. Recently, super hydrophobic surfaces were successfully prepared using different polymers by electrospinning [6,7]. However, these works are generally associated with low-surfaceenergy polymer coating and nanoparticle decoration on the surface of obtained electrospun nanofibers [8]. Various artificial functional materials including ZnO, TiO2, carbon nanotubes, and silica have been modified on different surfaces to increase the roughness [9]. Of these, nanostructured ZnO has received considerable interest due to its remarkable morphological options including nanowires, nanocombs, nanorings, and nanoloops [10]. Furthermore, the nanowire structure of ZnO closely resembles the randomly oriented nanoscale tubular crystalloids on lotus leaf. Herein, we present a novel approach for the fabrication of a flexible self-cleaning nanofiber membrane with hierarchical lotus-leaf-like micro/nano structures using oleic acid coated nanowire structure of ZnO to closely resemble the randomly oriented nanoscale tubular crystalloids on lotus leaf. PVDF/ZnO composite microfibers were fabricated by electrospinning of the mixture of PVDF solution and ZnO nanoparticles. Subsequent ZnO nanowire arrays were grown onto the surface of the composite nanofibers through hydrothermal synthesis in a growth solution (0.1 mol/L zinc nitrate hexahydrate, 0.1 mol/L hexamethylenetetramine (HMTA) and 5 mL ammonia in 100 ml deionized water) at 88–95 °C for 3 h and 12 h, respectively for needle-shaped and rodshaped ZnO nanowires. The resultant hierarchical structure was then coated with 0.5% oleic acid, a low surface energy material. The PVDF/ ZnO composite microfibers replicate the micro/nano scale convex papillae and tubular crystalloids structure on the lotus leaf. The oleic acid coated ZnO nanowires imitate the low surface energy tubular crystalloids on lotus leaf. A typical top-view and cross-section image of the hierarchical PVDF/ ZnO nanowires fiber membrane are presented in Figure. The PVDF/ZnO fiber membrane exhibited high surface area and roughness at the micro and nano scales. The microscale roughness was fabricated with the consecutive PVDF microfibers, and the nanoscale roughness was achieved with ZnO nanowires arrays. SEM images also show the multilayer hexagonal structure at the tip of the ZnO nanowires. The ZnO nanowires grew from the inside of the PVDF microfiber surrounding the outside surface of the PVDF microfibers. At 12 h of hydrothermal synthesis, the ZnO nanowires were rod-like but needle-like at only 3 h. The FTIR results verified the presence of the \\CH2\\ and \\CH3 groups introduced by the oleic acid. Alkylation occurred when the oleic acid solution was subjected to a hierarchical fiber membrane surface. All samples had high WCAs (N150°), and the WCA of needle-shaped ZnO structures was relatively higher than the rod-shaped structures. The bouncing diagram shows the bouncing process of a 10 μL water droplet: When the droplet fell onto the membrane, it did not penetrate into the nanofiber membrane but rather rebound five times as its potential energy transformed into kinetic energy. The droplet then underwent damped oscillations and finally rested on the surface. The rested droplet rolled off along the membrane within 120 ms when the surface of the membrane was inclined 15° to the horizontal. Moreover, the membrane retained its super hydrophobicity even after the impingement of many water droplets from a height of 15 cm. The water

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bouncing & rolling behaviors and artificial rain test showed the favorable water-proofing properties of the coated PVDF/ZnO nanowires fiber membrane. A dirt removal test was carried out to further investigate the selfcleaning property of the coated PVDF/ZnO nanowires nanofiber membrane. Some indoor dust was put on the fiber membrane, which was then flushed away by water droplets as shown in Figure. The droplets immediately took the indoor dust away, and the surfaces along the water path were kept clean then the membrane was left dry and clean. Breathability is critical for the comfort of self-cleaning wearable textiles. Thus, Water vapor permeation test was conducted to investigate the breathability of this hierarchical composite membrane. An Erlenmeyer flask containing water was placed on a hot plate and was capped with a piece of the membrane. The composite nanowire/microfiber membrane was flexible enough to be wrapped around the brim of the flask. As the water was gradually heated to boiling, visible accumulation of water vapor was observed in the air above the top surface of the membrane. The water droplets appeared on the surface of the nanofiber membrane as the permeated vapor gradually condensed and rested on the cold top surface. In conclusion, we present an effective approach to mimic the selfcleaning hierarchical structure of lotus by fabricating flexible hybrid inorganic ZnO nanowires decorated PVDF microfibers. Oleic acid was used to render hydrophobicity to the surface of the hybrid structure. With the combined hierarchical structure and low surface energy, the flexible composite membrane exhibited super hydrophobicity (WCA N 150°), exerted bouncing and rolling behaviors and resulted in favorable selfcleaning properties with robust water resistance, and high permeability of air and water vapor. This self-cleaning surface is of interest for various applications such as solar cells, textiles, anti-freezing and anti-snow surfaces, filters, micro-fluidics, medical devices, blood vessel replacements, etc. Author contributions Rouxi Chen, Yuqin Wan, Ji-Huan He and Jianhua Cheng conceived the concept. Rouxi Chen and Yuqin Wan designed the experiments, fabricated the hierarchical electrospinning nanofibers membrane, and did the SEM, WCA characterization and the self-cleaning performances. Weiwei Wu contributed the hydrothermal synthesis of ZnO nanowires and lipid coating. Cao Yang did the electrospinning experiments. Reinhard Jetterd and Fank K Ko contributed to the design of lotus effect and structure. Rouxi Chen, Yuqin Wan, Reinhard Jetterd and Fank K. Ko analysed the data and cowrote the paper. Rouxi Chen and Yuancai Chen conducted the graphical abstract drawing. All the authors discussed the whole paper. Rouxi Chen and Yuqin Wan contributed equally to this work. Notes The authors declare no competing financial interest. Acknowledgment The authors acknowledgment support from China Scholarship Council (CSC), AFML (Advanced Fiber Material Lab) of UBC, China Postdoctoral Science Foundation Grant (2018M630949), The Central University Scientific Research Project (Grant No. 2017BQ051). We acknowledge Yingjie Li and Caidan Zhang for TEM/FTIR tests. We thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi. org/10.1016/j.matdes.2018.11.041.

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References [1] Xuefeng Gao, Lei Jiang, Biophysics: water-repellent legs of water striders, Nature 432 (7013) (2004) 36-36. [2] Guoming Zhang, et al., Cicada wings: a stamp from nature for nanoimprint lithography, Small 2 (12) (2006) 1440–1443. [3] Lin Feng, et al., Super-hydrophobic surfaces: from natural to artificial, Adv. Mater. 14 (24) (2002) 1857–1860. [4] Andreas Solga, et al., The dream of staying clean: lotus and biomimetic surfaces, Bioinspiration Biomimetics 2 (4) (2007) S126. [5] Bharat Bhushan, et al., Lotus-like biomimetic hierarchical structures developed by the self-assembly of tubular plant waxes, Langmuir 25 (3) (2009) 1659–1666.

[6] Dan Li, Younan Xia, Electrospinning of nanofibers: reinventing the wheel? Adv. Mater. 16 (14) (2004) 1151–1170. [7] Rou-Xi Chen, Ya Li, Ji-Huan He, Mini-review on Bubbfil spinning process for massproduction of nanofibers, Matéria 19 (4) (2014) 325–343. [8] Iurii Sas, et al., Literature review on superhydrophobic self-cleaning surfaces produced by electrospinning, J. Polym. Sci. B Polym. Phys. 50 (12) (2012) 824–845. [9] Zhong Lin Wang, Zinc oxide nanostructures: growth, properties and applications, J. Phys. Condens. Matter 16 (25) (2004) R829. [10] Sunandan Baruah, Joydeep Dutta, Hydrothermal growth of ZnO nanostructures, Sci. Technol. Adv. Mater. 10 (1) (2009) 013001.