ZnO Nanorod Array Modified PVDF Membrane with Superhydrophobic

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ZnO Nanorod Array Modified PVDF Membrane with Superhydrophobic Surface for Vacuum Membrane Distillation Application Manxiang Wang, Guicheng Liu, Hyunjin Yu, Sang-Hyup Lee, Lei Wang, Jianzhong Zheng, Tao Wang, Yanbin Yun, and Joong Kee Lee ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b00271 • Publication Date (Web): 04 Apr 2018 Downloaded from http://pubs.acs.org on April 4, 2018

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ACS Applied Materials & Interfaces

ZnO Nanorod Array Modified PVDF Membrane with Superhydrophobic Surface for Vacuum Membrane Distillation Application Manxiang Wang 1,2,†, Guicheng Liu 2,†,*, Hyunjin Yu 2, Sang-Hyup Lee 3,4, Lei Wang 5, Jianzhong Zheng 6, Tao Wang 1, Yanbin Yun 1*, Joong Kee Lee 2* 1.

College of Environmental Science and Engineering, Beijing Forestry University,

Beijing 100083, P.R.China 2.

Center for Energy Convergence Research, Green City Research Institute, Korea

Institute of Science and Technology (KIST), Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea 3.

Center for Water Resource Cycle Research, Korea Institute of Science and Technology

(KIST), Hwarang-ro 14-gil 5, Seongbuk-gu, Seoul 02792, Republic of Korea 4.

Green School, Korea University, 145 Anam-ro, Seongbuk-gu, Seoul 02841, Republic

of Korea 5.

Beijing Key Lab of Cryobiomedical Engineering and Key Lab of Cryogenics,

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P.R.China 6.

College of Resources and Environment, University of Chinese Academy of Sciences,

19 A Yuquan Road, Beijing 100049, P.R.China † These authors contributed equally to this work and consider to be first co-authors. *

Corresponding authors:

[email protected] (Yun), [email protected] (Liu), [email protected] (Lee).

KEYWORDS: vacuum membrane distillation, PVDF membrane, ZnO nanorods, superhydrophobic surface, anti-fouling S-1 ACS Paragon Plus Environment

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ABSTRACT

The vacuum membrane distillation (VMD) is a promising technology for lots applications. To solve the membrane fouling and wetting problems, in this paper, a novel ZnO nanorods-PDTS modified PVDF membrane with a micro/nanoscale hierarchical structured and superhydrophobic surface has been prepared and applied to the VMD process for distilling highly salty water, for the first time. Among these, a pyrolysis-adhesion method is created to obtain ZnO seeds and fasten them on PVDF substrate firmly. The novel modified membrane shows a stable superhydrophobic surface with water contact angle of 152°, easy-cleaning property, excellent thermal and mechanical stability, because of the Cassie’s state caused by pocketing much air in the hydrophobized ZnO nanorods, the low surface energy of PDTS coating, and the strong adhesion between ZnO nanorods and PVDF membrane, which has built an ideal structure for VMD application. After 8-hour-VMD of 200 g L-1 NaCl solution, compared to the virgin PVDF membrane, the novel membrane shows a similar permeate flux, but a much higher-quality permeated liquid, because of its unique anti-fouling and anti-wetting caused by the several microns gap between the feed and membrane. Due to its easy-cleaning property, the novel membrane also exhibits an excellent re-usability.

S-2 ACS Paragon Plus Environment

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1. INTRODUCTION Membrane distillation has been considered as an energy-saving and high-efficient separation technology for lots of applications,1-5 such as highly salty wastewater desalination, removal of heavy metal, food and beverage industry, and purification of pharmaceutical products, because multiple forms of low-quality heat sources,6-12 like solar energy, waste hot and geothermal source, can be coupled with the membrane distillation technology to heat the feed solution. Due to the location of condensation process outside the membrane module, the vacuum membrane distillation (VMD), a promising membrane distillation technology, where water vapor is extracted by a vacuum system, significantly reduces the heat conduction loss in the feed solution side.1,2,12-14 Moreover, compared to other membrane distillation technologies, the VMD can provide higher pressure gradient between two sides of membrane to achieve higher permeate flux.14 However, the decreases of permeate flux and distillation liquid quality, resulting from membrane fouling and wetting, are the major obstacles to commercial implementation of membrane distillations. Normally, polyvinylidene fluoride (PVDF) membranes as classic distillation membranes are chosen in the VMD application, owing to their hydrophobicity and microstructure. Generally, further hydrophobic treatments for the PVDF membrane are required to reduce the interaction between the feed solution and membrane surface further, and to improve the membrane fouling and wetting resistances.15 So far, the hydrophobic methods mainly include optimization of the membrane preparation,16-19 introduction the perfluorinated polymers into the membrane (by blending method,20 copolymerization

21,22

or surface grafting,23-25 and so on), and



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deposition nanoparticles (for example, SiO2 and TiO2) with fluorination on the membrane surface.26-30 Clearly, increasing roughness of hydrophobic surface and decreasing surface energy are the effective ways to enhance the hydrophobicity. Therefore, the micro/nanoscale hierarchical structured surfaces with low surface free energy are ideal on restraining the spontaneous transition of the liquid from Cassie state to Wenzel state.31,32 Compared to current nano-sized modifications, according to the Cassie theory, the hydrophobic nanorod array with the height as high as several microns grown on the membrane surface can trap more air to prevent droplets contacting with the membrane substrate, leading to much higher hydrophobicity of surface and lower heat conduction and loss, which is a more effective and promising idea on modifying membranes for the VMD application. ZnO nanorod array, as an important functional material, has been used for preparing superhydrophobic surfaces in many fields. Meanwhile, for growing metal oxide nanorods, the hydrothermal method has been believed as an effective way at low reaction temperature.33-36 Choosing stainless steel mesh as the substrate, Tian et al grew a layer of ZnO nanorod-array on the substrate via sintering treatment at 420 °C for coating ZnO seeds and hydrothermal reaction for growing ZnO nanorods, and further, put the sample under UV irradiation for 0.5h to realize a superhydrophobic surface.37 By sintering the mixture of PVDF powder and Zn(Ac)2-based crystalline powder at its melted temperature of ~270 °C to form the ZnO seeds in the PVDF material, Wen et al fabricated a superhydrophobic ZnO-nanorods-modified PVDF surface with robust antifogging and icing-delay properties.38 For coating the ZnO seeds on polymer substrate,


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Wang et al first sintered the Zn(NO3)2 solution on an aluminum model at 300 °C to form a ZnO seed layer, and then, transferred the ZnO seeds from the model to a PDMS surface by a soft lithography.39,40 Finally, they fabricated a superhydrophobic ZnO-nanorods-modified PDMS surface with robust anti-icing and de-icing properties by growing ZnO nanorods on the PDMS surface through an enhanced hydrothermal method and coating a PDTS layer on the ZnO grown PDMS substrate to decrease the surface free energy, due to the lower surface free energy of the PDTS. However, the high-temperature-sintering for forming ZnO seeds cannot be used on PVDF membranes. In fact, growing ZnO nanorods on flexible polymer substrates has long been considered a technical difficulty, because most polymer materials can’t resist the high temperature of the sintering treatment in the preparation step of ZnO seeds on substrate. Furthermore, in current studies, the superhydrophobic surfaces are mainly applied at room or low temperature, which are different from the operation temperatures (with the range of 50 to 90 °C) in the VMD process. Therefore, to date, as far as we know, nobody has realized the growth of ZnO nanorods on PVDF membranes, and also, employed the ZnO nanorods modified PVDF membrane in the VMD field. Herein, a novel ZnO nanorods-PDTS modified PVDF membrane with a micro/nanoscale hierarchical structured and superhydrophobic surface has been prepared and applied to the VMD process for distilling highly salty water, for the first time. As for the ZnO seeds preparation, a pyrolysis-adhesion method with low operation temperature of 125°C was created to obtain ZnO seeds and fasten them on PVDF substrate firmly. In detail, the preparation process, the basic properties (such as surface



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structure and chemical composition, hydrophobicity, mechanical and thermal stabilities), and the VMD performances (including permeate flux, ionic conductivity of permeated liquid, membrane fouling and wetting degrees, and re-usability) of the novel membrane were investigated systematically. 2. EXPERIMENTAL SECTION Materials: Immobilon®-PSQ hydrophobic PVDF flat membranes with nominal 220 nm pore size, 200 µm thickness and 80% porosity (ISEQ00010, Millipore Co. Ltd., USA) were employed in this study. Sodium hydroxide and anhydrous ethyl alcohol were purchased from Daejung Chem. Co. Ltd., South Korea. Zinc acetate dehydrate, sodium chloride, 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PDTS), zinc nitrate hexahydrate, hexamethylenetetramine were from Sigma-Aldrich. Preparation of the ZnO seeds fastened PVDF membrane: After adequately dip-coated in a 70 mmol L-1 zinc acetate ethanol solution, the PVDF membrane was taken out slowly at a constant rate, and was transferred into an uncovered glass dish for drying at 125 °C in an oven for 20 min. Subsequently, the dried membrane was immersed into a 0.1 mol L-1 sodium hydroxide ethanol solution until it was fully infiltrated. Then, the membrane was pulled out from the sodium hydroxide ethanol solution slowly, and dried at 125 °C for 20 min. Finally, the ZnO seeds fastened PVDF membrane (marked as ZSP membrane) was obtained by washing the resulting membrane with several droplets of deionized water, and drying at 125 °C again. To achieve good coverage of the ZnO seeds on the membrane, the seeds coating process should be repeated for more than 3 times.


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Growth of ZnO nanorod array on the ZSP membrane: A mixed solution consisting of 50 mmol L-1 Zn(NO3)2 and 50 mmol L-1 HMTA was magnetically stirred to be uniform, as the growth solution. The ZSP membrane substrate and the growth solution were transferred into a 100 mL Teflon lined stainless steel autoclave. The sealed autoclave was heated at 90 °C in an electric oven for 18 h and then cooled down to room temperature. The ZnO nanorod array grown PVDF membrane (labeled as ZNP membrane) was got by cleaning with deionized water, and drying at 60 °C for 2 h. Hydrophobization

treatment

for

PVDF

and

ZNP

membranes:

Hydrophobization modification was performed via a fluorosilane coating method by submerging the membranes in a 1 vol% of 1H,1H,2H,2H-perfluorodecyltriethoxysilane (PDTS) ethanol solution for 24 h. The PDTS coated membranes were prepared by drying the resulting membranes at 120 °C for 2 h. The obtained PDTS coated ZNP membrane and PVDF membrane were marked as P-ZNP membrane and P-P membrane, respectively. Membrane characterization: A field emission scanning electron microscope (Hitachi S-4100) with an accelerated voltage of 15 kV and an energy dispersive X-ray (Tecnai G2, FET Co. Ltd.) was employed to record surface and section morphologies and compositions of the virgin and modified PVDF membranes. Contact angle and contact process measurements: Water contact angles of the membranes were measured by an optical contact-angle meter system with a heating stage (Dataphysics SCA40, Germany) within the temperature range of 25 ~ 85 °C. The membranes were fixed on the heating stage using copper conductive tape with double



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sided adhesive. Droplets (5 µL in volume per pure water droplet) were placed gently on to surfaces of the samples. The contact angles were tested at room temperature (aside from the specially marked ones). Each effective contact angle value is the average value of at least five measurements under the same test condition. The contact process between a 5-µL-falling droplet and the membrane surface was observed by a high-speed camera. Pore size measurement: Pore size distribution and porosity were determined by the mercury porosimetry (Auto Pore 9520, Micromeritics, America). Every membrane sample was dried in a vacuum oven at 60 °C. Liquid entry pressure (LEP) measurement: LEP points of membranes were obtained at a dead-end filtration set-up with Milli-Q water. An increasing pressure was applied on the feed side step-by-step, until the first drop of permeate was obtained on the other side. VMD performances of the membranes: The VMD performances of the PVDF and P-ZNP membranes with approximate contact area of 7 cm2 were examined in a home-made VMD device fed by 200 g L-1 sodium chloride solution (500 mL) as feed solution. Inside of membrane module, a nylon cloth was chosen, as the spacer, instead of traditional plastic gratings, to support the membrane during the VMD process, which could avoid the rigid spacer to damage microstructure of the membrane. During the VMD process, the concentration of the feed solution increased continuously over the course of test. The operation temperature, vacuum pressure, and flow velocity of the feed solution were set to 60 °C, 80 KPa, and 10 L h-1, respectively (unless otherwise


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indicated). The resulting permeate flux of the membrane and the ionic conductivity of the permeated liquid were recorded. 3. RESULTS AND DISCUSSION Briefly, the modification procedure on PVDF membrane mainly involves three steps, as shown in Figure 1, as follow: first, producing and fastening ZnO seeds on surface of PVDF membrane by the pyrolysis-adhesion method at 125 °C; second, growing ZnO nanorods via a classical hydrothermal reaction; finally, fluorosilane hydrophobization treatment by assembling PDTS thin layer. Among these, the ZnO seeds-preparation on polymer substrates is a technological difficulty.41 In this paper, during the ZnO seeds-preparation step, the ZnO seeds were obtained via two effective in situ reactions: a neutralization reaction of Zn(CH3COO)2 + 2NaOH Zn(OH)2↓+ 2NaCH3COO at room temperature and a pyrolysis reaction of Zn(OH)2↓ ZnO↓ + H2O↑ at 125 °C. Moreover, the seeds were fastened on PVDF membrane through the adhesion effect of the softened PVDF surface at 125 °C. 3.1 Basic properties of the PVDF membranes Figure 2 presents the surface morphologies of the PVDF membranes after each modification step. As is known to us all, the hydrophobicity of virgin PVDF membrane consisting of lots of fiber-structures mainly comes from the –CF2 hydrophobic group and the microscale roughness caused by micropores (Figure 2a,b). After the pyrolysis-adhesion process, as shown in the EDX data (Figure S1), ZnO seeds are coated evenly and fastened firmly on the surface of PVDF membrane substrate. Meanwhile, Figure 2c,d show that many bulges with diameter of ~180 nm distributed on



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the surface of the ZnO seeds fastened PVDF membrane (ZSP membrane) are from the deformation of the PVDF substrate, because the treatment temperature of 125 °C is in the thermal deformation temperature range of PVDF materials and leads to liquation of unstable ending-structures, for example, surface, of the PVDF fibers. Meanwhile, the liquated PVDF membrane surface can fasten the ZnO seeds firmly, which is of critical importance for growing stable ZnO nanorod array on the membrane.42 Though the hydrothermal reaction, as described in Figure 2e,f and Figure S2, ZnO nanorods with diameter of ~120 nm and height of ~1.5 µm are grown on the PVDF membrane uniformly, and the resulting membrane is marked as ZNP membrane. Finally, the PDTS coated ZNP membrane is named as P-ZNP membrane. Figure 2g,h and Figure S3 indicate that, the hydrophobization treatment almost has no effect on the morphology of the ZNP membrane, which has been verified by our previous reports.40,42,43 Pore distribution and porosity of the PVDF and P-ZNP membranes were clearly confirmed by mercury porosimetry measurements, shown in Figure 3 and Table 1. Two main peaks were observed for both the PVDF and P-ZNP membranes. Compared to the PVDF membrane, the P-ZNP membrane exhibits slightly lower porosity and pore size, which might be caused by the ZnO nanorods coating. Overall, as shown in Figures S4 and S5, after growing ZnO nanorods and coating PDTS, the internal structures of the PVDF membrane, including the thickness, micropores, and so on, are still remained on the P-ZNP membrane, which provides a micro/nanoscale hierarchical-structure for membrane distillation applications. To meet the requirement for membrane distillation process, the liquid entry pressure of membranes has been measured. The P-ZNP


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membrane (277 kPa) possesses higher liquid entry pressure than the virgin one (238 kPa). To introduce the unique advantages of the ZnO nanorods-PDTS modified PVDF membrane directly, the degree and stability of the hydrophobicity at various temperatures, and mechanical stability of the modified PVDF membrane were tested. The hydrophobic degree of the membranes was investigated by measuring water contact angle. As exhibited in Figure 4a, the ZNP membrane shows a hydrophilic surface with the water contact angle of 16°, due to the hydrophilicity of ZnO nanorods; the ZnO nanorods-PDTS modification improves the water contact angle from 126° (the PVDF membrane) to 152° (the P-ZNP membrane), indicating that the P-ZNP membrane is superhydrophobic. The enhanced hydrophobicity comes from three aspects: first, the PDTS coating with the fluorinated methyl (–CF3) group possesses lower surface energy than the PVDF with –CF2 group;44 second, according to the Cassie-Baxter theory,45 it’s an important property for the superhydrophobic surface that the micro/nanoscale hierarchical-structure of the P-ZNP membrane surface which can capture much air pockets in the troughs between individual nanorods; Moreover, the high aspect ratio of the nanorods drastically reduces the contact area and the adhesion force between droplets and the surface. To highlight the advantage of the ZnO nanorod array in the P-ZNP membrane, a PDTS coated PVDF membrane sample (Figure S5, called as P-P membrane) was prepared to compare with PVDF and P-ZNP membranes. In Figure 4a, it’s clear that the water contact angle of the P-P membrane (137°) is a little higher than that of the virgin one due to the low surface energy of the PDTS coating, and obviously



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lower than that of the P-ZNP membrane because of lack of the micro/nanoscale hierarchical-structure formed by the ZnO nanorod array. To demonstrate the powerful hydrophobicity of the P-ZNP membrane, two comparison videos between the PVDF (Video 1) and the P-ZNP (Video 2) membranes were recorded for observing dropping processes of water droplets on their surfaces. Figure 4b shows, when a water droplet drops on the surface of the PVDF membrane, the impinging droplet was excluded, but didn’t separate from the surface. In contrast, when a water droplet falls onto the P-ZNP membrane surface (Figure 4c), the water droplet bounced completely off the surface. This result indicates the P-ZNP membrane possesses much better hydrophobicity and lower adhesion force between water droplet and the membrane than the PVDF membrane, resulting in an easy-cleaning property of the P-ZNP membrane. In the VMD application, the membrane should possess good hydrophobicity in the whole running temperature range and excellent mechanical and thermal stabilities to adapt to various working conditions, such as strong liquid flow rate and high operation temperature. Figure 5a shows, as the heating stage in the optical contact-angle meter system was heated at a rate of 1 °C /min from 25 °C to 85 °C, the contact angles of both PVDF and P-ZNP membranes have no significantly decrease with slight drops from 130° (PVDF membrane) and 149° (P-ZNP membrane) to 124° and 145°, respectively, indicating that during various operation temperatures, the P-ZNP membrane surface can maintain an excellent superhydrophobic property. To simulate the real work environment, placing membrane into hot solution and ultrasonic-treating membrane in solution are used to imitate the hot feed liquid and the


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scour of feed liquid to membrane, respectively.46-48 The contact angle on the membrane will be employed as a direct characterization to show the hydrophobicity of the membrane.47 For evaluating the mechanical stability of the modification coatings, the contact angles of the membranes were measured with the frequency of 1 time/10 min after ultrasonic processing membranes in deionized water at room temperature for 1 hour. Figure 5b presents that, similar with the PVDF membrane,49 the P-ZNP membrane shows an approximately contact angle value of ~146° from the 10th min to the 60th min, indicating that the mechanical stability of the P-ZNP membrane is good enough. There are two main reasons to explain why the mechanical stability of the modification coatings is strong. One, ZnO nanorod array has been fastened on the PVDF substrate firmly owing to the strong adhesion between ZnO seeds and PVDF substrate. The other one, based on the chemical bond between PDTS and the ZnO nanorod (the covalent bond of Si-O-Zn, as shown in insert of Figure 1), the P-ZNP membrane surface exhibits a superhydrophobicity because of the strong hydrophobic PDTS coating. Therefore, water is difficult to enter into the gaps among ZnO nanorods, resulting in weakening the effect of ultrasonic vibration on the ZnO nanorods. In addition, the P-ZNP membrane also shows a good thermal stability. Thermal stability of the microstructure and coating in the P-ZNP membrane was analyzed by immersing the membrane in 90 °C of deionized water for 1 hour. As shown in Figure 5c, the contact angles of the PVDF and the P-ZNP membranes almost don’t show any change at ~126° and ~149°, respectively. 3.2 Vacuum membrane distillation performance of the modified membrane



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Before employing the P-ZNP membrane in VMD application for distilling highly salty water, the effect of the feed temperature, vacuum pressure, and feed velocity on the permeate fluxes was examined in a homemade VMD system (Figure S6) by using pure water as the feed liquid. As shown in Figure 6, the fluxes of membranes increase as the feed temperature, vacuum pressure, and velocity increase. Because the increasing temperature, vacuum degree and velocity lead to increasement of vapor pressure of the feed liquid, enhancement of the driving force across the membrane, and reduction of the thermal boundary layer on the membrane surface, respectively. And also, compared to virgin PVDF membrane, the flux for the P-ZNP membrane in different velocities, temperatures and vacuum pressures are a little lower. It is noteworthy that, as exhibited in Figure 6c, the change of the feed velocity has only influence on the flux of the PVDF membrane, but almost not on that of the P-ZNP one, indicating that the P-ZNP membrane has stronger ability to resist the shear force on the membrane surface. By using 500 mL of 200 g L-1 of highly NaCl solution as the feed liquid, the VMD processes based on the PVDF and the P-ZNP membranes were carried out to assess the desalination performances of the membranes. Figure 7 reveals that, as the operation time goes on, both of permeate fluxes of the PVDF and the P-ZNP membranes gradually decrease, and the ionic conductivities of the permeated liquid from the two membranes gradually increase, which is in accord with current reports.50,51 Although the permeate flux of the PVDF membrane is higher than that of the modified one, the decay rate of the permeate flux of PVDF membrane is more obviously than that of the modified one, resulting in a similar flux value at the 8th hour. Meanwhile, not only the values but also


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the increasing speed of the ionic conductivity of the permeated liquid from the PVDF membrane are considerably higher than those from the modified one during the whole test period. In detail, after 8 hours test, the ionic conductivity of the permeated liquid from the PVDF membrane increases from 8.75 to 142.70 µS cm-1, and that from the modified one from 0.18 to 12.89 µS cm-1. The different VMD performances between the PVDF and the P-ZNP membranes reveal that the P-ZNP membrane has prevented the membrane fouling and wetting successfully, but the PVDF membrane hasn’t. To support this viewpoint, the tested membranes were checked by SEM images and EDX data to provide some direct evidences. As shown in Figure 8a,b and Figure S7a, lots of NaCl salt crystal diamonds with side lengths of several hundred microns and small particles with the diameter of < 1 µm are deposited on the virgin membrane surface. And also, Figure 8c and Figure S7b present that, many NaCl crystal particles are embedded into the surface pores of the PVDF membrane. More seriously, as seen from the cross-section SEM images in Figure 8d-f, some NaCl particles go deep through the surface to the interior of the PVDF membrane. The appearance of the big salt diamonds reveals that after distillation running, the hydrophobicity of the virgin PVDF membrane surface has been destroyed by the membrane fouling phenomenon. And the embedded salt particles result in membrane wetting. In contrast, Figure 8g,h and Figure S8 show that a few NaCl crystal particles with the size of several microns are deposited only on surface of the P-ZNP membrane, which verifies that the superhydrophobic surface of the P-ZNP membrane is robust enough to be remained. Notably, from Figure 8i-l, it’s clear that there are not any



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particle goes inside the P-ZNP membrane, meaning that the hydrophobized ZnO nanorod array has prevented the membrane wetting problem and made the membrane be easy to clean. The difference of working mechanisms between the PVDF and the P-ZNP membranes is easy to understand. As shown in Figure 9a, in the off-working state, water is difficult to enter into the pore-structures because of the hydrophobicity of the PVDF membrane. However, under working condition, the water will be sucked into the pores as humps with certain heights by the vacuum force from the other side of the membrane, which raises the possibility that the membrane might be wetted and fouled by the feed liquid. In contrast, the feed liquid can be kept several microns away from the surface of the modified membrane by the hydrophobized ZnO nanorod array, resulting in the feed liquid can’t touch the membrane surface directly, which is also different from the nano-sized modification for the distillation membranes reported in current literature. Even under vacuum force in working state, as described in Figure 9b, the feed liquid can’t come into the pore-structures, because the hydrophobized ZnO nanorods improve the hydrophobicity of the membrane to be a superhydrophobic surface, and also reduce the size of the pore-openings to lead to a smaller hump of the feed liquid. This working mechanism has also been discussed and supported by He team’s report.52 After the first VMD test, the fouled membranes were rinsed with deionized water to remove the deposited salt, and then dried. As shown in Figure 10, the contact angles of the cleaned PVDF and P-ZNP membranes are 116° and 148°, respectively, meaning that the hydrophobicity of the P-ZNP membrane can recover to nearly its original value owing to


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its easy-cleaning property, but the PVDF membrane can’t, because of the salt pollutant inside the membrane pores. Video 3 shows that, after cleaning and drying, the 8-hour-used P-ZNP membrane still exhibits an excellent hydrophobicity. To verify the re-usability of the membranes, the cleaned membranes were applied to the second VMD application. Figure S9 shows that, the permeate fluxes of the PVDF and the P-ZNP membranes recover to 85.97% and 98.09% of their original values, respectively. Compared to the PVDF membrane, the P-ZNP membrane exhibits a better recovery rate due to the improvement of its antifouling property. And also, the salt pollutant inside pores of the PVDF membrane couldn’t be removed cleanly. Additionally, the ionic conductivities of the permeated liquid from the PVDF membrane increases significantly, reaching up to 190.88 µS cm-1 after 8 h. The permeate conductivity of the modified one is only 10.19 µS cm-1 after 8h, indicating the modified membrane possesses a stable anti-wetting property and an excellent re-usability. 4 CONCLUSIONS Based on a pyrolysis-adhesion method for preparation the ZnO seeds on a PVDF membrane at 125°C, a novel ZnO nanorods-PDTS modified PVDF membrane with a micro/nanoscale hierarchical structured and superhydrophobic surface has been created in this paper, and applied to the VMD process for distilling highly salty water, for the first time. The hydrophobized ZnO nanorods in the novel membrane can pocket much air and induce the droplet to be Cassie’s state, leading to a superhydrophobic surface with water contact angle of 152°, a low adhesion force between water droplet and the membrane surface, and an easy-cleaning property of the membrane. The novel P-ZNP



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membrane has shown not only a stable superhydrophobicity at various temperature because of the low surface energy of the PDTS coating, but also an excellent thermal and mechanical stability owing to the strong adhesion between ZnO nanorods and PVDF membrane, which is essential for the membrane distillation application. After 8 hours of distilling 200 g L-1 of NaCl solution in a homemade VMD system at 60 °C, compared to the virgin PVDF membrane, the novel membrane shows a similar permeate flux and a much lower ionic conductivity of the permeated liquid, because of its unique anti-fouling and anti-wetting caused by the several microns gap between the feed and membrane. Due to its easy-cleaning property, the novel membrane also exhibits excellent re-usability via a simple water rinsing for the VMD application.

ASSOCIATED CONTENT: Supplementary materials Schematic diagram of the homemade VMD setup used to test desalination performance. SEM and EDX mapping images of the PVDF, ZNP, ZSP, P-ZNP, the PDTS coated PVDF, the used PVDF and the used P-ZNF membranes. Flux and ionic conductivity of the permeated liquid from the deionized water cleaned membranes.

Acknowledgements M. W. and G. L. contributed equally to this work and should be considered to be the first co-authors. This work was supported by the National Natural Science Foundation of China (Grant No. 21376030), and the research grants of NRF funded by the National Research Foundation under the Ministry of Science, ICT & Future, Korea


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(NRF-2017R1A2B2002607, and NRF-2015H1D3A1036078). The authors also thank Mr. Joo Man Woo and Mr. Un Seok Kim for the support during the preparation of this study. REFERENCES 1.

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Figures

Figure 1. Schematic illustration of the preparation process of the novel ZnO nanorods-PDTS modified PVDF membrane. Notes of the abbreviations: ZnO seeds fastened PVDF membrane (ZSP membrane), ZnO nanorod array grown PVDF membrane (ZNP membrane), and PDTS coated ZNP membrane (P-ZNP membrane).



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Figure 2. Surface SEM images of the (a,b) PVDF, (c,d) ZSP, (e,f) ZNP and (g,h) P-ZNP membranes.


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4.0 3.5

Pore volume (mL g-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.0

PVDF membrane P-ZNP membrane

2.5 2.0 1.5 1.0 0.5 0.0 10000

1000

100

10

Pore diameter (µm)

Figure 3. Pore size distributions of the PVDF and P-ZNP membranes.



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Figure 4. Hydrophobic degrees of the virgin PVDF and modified membranes. (a) Water contact angles of the membranes. Contact processes of falling droplet with (b) the PVDF and (c) the P-ZNP membrane surfaces. S-30 ACS Paragon Plus Environment

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Figure 5. Effects of the (a) test temperatures, (b) ultrasonic treatment and (c) immersion in 90 °C DI water on the hydrophobicity and structural stability of the PVDF and modified membranes. S-31 ACS Paragon Plus Environment


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Figure 6. Effects of the (a) feed temperature, (b) vacuum pressure, (c) feed velocity in the permeate side on permeate fluxes of both PVDF and P-ZNP membranes. S-32 ACS Paragon Plus Environment

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160

6

140 120 4

100 80

3

PVDF membrane

60

P-ZNP membrane

2

40

Conductivity (µS cm-1)

5

Flux (kg m-2 h-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


20

1

0 0 0

1

2

3

4

5

6

7

8

9

Time (h)

Figure 7. Varies of the flux and ionic conductivity of the permeated liquid from the membranes with running time at 60 °C. Among these, 200 g L-1 NaCl solution with 10 L h-1 of feed velocity was employed as the feed liquid, and the permeate vacuum pressure was 80 kPa.



(a)

100 µm

(d)

(b)

(c)

10 µm

1 µm

(e)

(f)

10 µm

100 µm

(g)

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(h)

1 µm

(i)

10

100 µm

(j)

100µm

10 µm

(k)

10 µm

1 µm

(l)

1 µm

Figure 8. (a-c, g-i) Surface and cross-section (d-f, j-l) SEM images of the used (a-f) PVDF and (g-l) P-ZNP membrane surfaces after 8h of vacuum membrane desalination.


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Figure 9. Schematic diagrams of the surface wetting mechanism of the (a) PVDF and (b) modified membranes in the off-working and working states (with and without vacuum force).



(a)

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(b)

Figure 10. The pictures and water contact angles of the (a) PVDF and the (b) P-ZNP membranes rinsed by deionized water after 8 hours of vacuum membrane desalination test.


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Table 1. Informations about the liquid entry pressure, pore parameter, and thickness of the virgin and modified PVDF membranes.

Liquid entry pressure Membrane /kPa

Maximum

Median pore

Median pore

Average pore

pore size

diameter (Volume)

diameter (Area)

diameter (4V/A)

Porosity

Thickness

/%

/µm /nm

/nm

/nm

/nm

PVDF

238±5

73.8433

1065

464.7

289.8

395.3

202.76±3.57

P-ZNP

277±8

73.2593

794

430.6

226.6

300.9

207.24±5.23

S-37

ACS Paragon Plus Environment


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Graphical Abstract

A novel ZnO nanorods-PDTS modified PVDF membrane with a micro/nanoscale hierarchical structured and superhydrophobic surface has been prepared and applied to the VMD process for distilling highly salty water, for the first time.


ZnO Nanorod Array Modified PVDF Membrane with Superhydrophobic

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