Silver nanoparticles-enabled photothermal nanofibrous membrane for

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Silver nanoparticles-enabled photothermal nanofibrous membrane for light-driven membrane distillation Haohui Ye, Xiong Li, Li Deng, Peiyun Li, Tonghui Zhang, Xuefen Wang, and Benjamin S. Hsiao Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b04708 • Publication Date (Web): 22 Jan 2019 Downloaded from http://pubs.acs.org on January 26, 2019

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Industrial & Engineering Chemistry Research

Silver nanoparticles-enabled photothermal nanofibrous membrane for light-driven membrane distillation

Haohui Yea, Xiong Lib, Li Denga, Peiyun Lia, Tonghui Zhanga, Xuefen Wanga*, Benjamin S. Hsiaoc aState

Key Lab for Modification of Chemical Fibers and Polymer Material, Donghua

University, Shanghai, 201620, P.R. China bKey

Laboratory of Oceanic and Polar Fisheries, Ministry of Agriculture, East China

Sea Fisheries Research Institute, Chinese Academy of Fishery Sciences, Shanghai 200090, PR China cDepartment

of Chemistry, Stony Brook University, Stony Brook, NY 11794, USA

ACS Paragon Plus Environment

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ABSTRACT: In this manuscript, we demonstrated a novel-innovative photothermal nanofibrous composite membrane for ultraviolet light driven membrane distillation (UVMD). In which, the photothermal nanoparticles, i.e. silver nanoparticles (Ag NPs) were embedded in porous membrane to capture light (providing by an ultraviolet LED light) without any other heating methods and complete the photothermal transformation at the interface between the surface of the membrane and the water, providing a high-efficiency (53±7 %) heating method for the MD process. Here, Ag NPs were incorporated into the hydrophobic polyvinylidene fluoride (PVDF) nanofibrous membranes by electrospinning technique. The photothermal heating effect on the desalination capability of PVDF-Ag nanofibrous membrane was investigated by a UVMD module coupled with a 50W, 400nm LED light irradiation. During 60h UVMD test period (3.5 wt % NaCl salt feed), membranes with 20 wt % Ag NPs (with respect to PVDF polymer) presented a flux of 2.5kg/ (m2·h) and low permeate conductivity (2.65 μs/cm) with no pores wetting detected. These results indicate that UVMD has prospective application for desalination with the help of novel photothermal nanofibrous membranes by efficient utilizing with renewable energy, such as solar energy. KEYWORDS: nanofibrous membrane, electrospinning, membrane distillation, plasmonic heating effect


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INTRODUCTION Fresh water, with growing demand of the past 20 years, has drawn more and more attention all over the world. In quest of more fresh water, researchers has coincidentally developed new technologies of producing potable water more efficiently. Seawater desalination is one of the important ways to solve the shortage of fresh water. As one of the membrane-based desalination technologies, reverse osmosis (RO) has developed over the past 40 years and become the dominated technology for desalination technologies.1 RO is a pressure driven process and the osmotic pressure of saline water will increase with the rising salt concentration. For this reason, the osmotic pressure of high salinity water is far beyond the maximum allowable operating pressure of existing RO technology.2 Among these, membrane distillation (MD)3 is a promising and efficient technology for highly saline water desalination due to the lower temperatures than conventional distillation (viz., boiling) and lower pressure than reverse osmosis (RO)4. Conventionally, membrane distillation and other membrane technologies differ in the driving force. To be more specific, the permeation of MD was driven by the vapor pressure difference, instead of the water pressure across the membrane. Meanwhile, the materials of MD are hydrophobic in essence.5 Thus MD has the capability to obtain pure water with 100% salt rejection theoretically and possesses low operation temperatures (40–90 °C) and at atmospheric pressure. In addition, the process has the capability to utilize solar energy (one kind of renewable energy sources), geothermal energy and industrial waste heat. On account of these evident advantages, MD has evolved as an increasing significant desalination technique, compared to the conservative RO process with inherent drawbacks, i.e. high energy consumption because of high operating pressure.4, 6


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With the advent of nanotechnology, one good example is the production of nanofibers via an electrospinning process. As one type of nanomaterials, electrospun polymer nanofibrous membranes possess high porosity, micro-nano scale diameters, interconnected open pore structure, high permeability for gases and high aspect ratio and all these excellent characteristics will meet the demands of MD applications.7 Meanwhile, they can be readily modified as the result of high specific surface area and possess remarkable mechanical properties. Electrospinning is capable of fabricating fibers with different surface morphology, which offers it a promising future for a great variety of applications, including electronic devices, membrane filters and biomedical materials.8-10 Electrospinning has also found its way to MD applications with the trend of using nanofibers in desalination and water purification processes.11-12 The possibility of fabricating membranes for MD has been shown in previous

studies,

such

as

electrospinning

of

hydrophobic

polymers,

i.e.

polyvinylidene fluoride (PVDF).13 Recently, seawater desalination with solar power employed has gained more and more attention because of the evaporation driven by photothermal materials which realize light-to-heat conversion.14-16 As an example, water evaporation driven by photothermal effect is achieved by using SiO2/Au nanoshells under light irradiation.17 Another case is that, a photothermal and hydrophobic stainless steel mesh with polypyrrole coating is proved to evaporate water effectively by photothermal effect, which broadens the application prospect for seawater desalination through direct use of sunlight.18 For a long time, it has been considered only as side-effect that heat generation in noble metal nanoparticles (NPs) caused by light-to-heat conversion. However, light-to-heat generation can be easily achieved by noble metal NPs under laser irradiation, which develops a promising new series of applications in nanotechnology, including the fields of nano-optics, nano thermodynamics and plasmonic heating19-21. The light-heat conversion effect of NPs have been believed to base on a conventional


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plasmon-induced hot-electron transfer (PHET) mechanism. In PHET, plasmonic metal nanostructures as light absorbers generate plasmons under light irradiation and the plasmons decay into hot electrons.22 After that, hot electrons transfer into adjacent molecules and then the plasmonic metal heat up. The heat from the plasmonic metal dissipates around and the temperature of surrounding medium increases.21 As a nanosource of heat, plasmonic NPs can be combined within polymeric membranes, which provides the possibility of light to heat conversion. Over the last several years, photothermal applications of plasmonic NPs have been further investigated by experimental and theoretical studies23-24, which preparing the way for sophisticated temperature-control nanodevices23,

25-28

and arousing interest in field of nano

thermodynamics29-30. Local heat generation of membranes induced by laser irradiation can be achieved by incorporation of noble metal NPs in the polymer matrix.18, 31-32 For instance, Ag NPs are relatively cheaper and also have a stronger plasmonic heating effect than gold NPs, and therefore they have been widely used.33 In this work, we demonstrated a new kind of hydrophobic electrospun nanofibrous membranes with membrane distillation performance driven by UV light (in order to specialize in photothermal effect and avoid thermo-effect of infrared radiation, we chose UV light), which was fabricated by electrospinning of PVDF-AgNO3 blend solutions, followed by NaBH4 reduction and n-hexadecyl mercaptan (R-SH) surface modification (as shown in Scheme 1). In specific, we extended the localized heating (photothermal effect) idea of Ag NPs and ultraviolet LED light, and Ag NPs were incorporated into the hydrophobic PVDF nanofibers via electrospinning technique. Ag NPs incorporated in PVDF nanofibers would heat up under UV light irradiation and the surrounding PVDF polymer’s temperature would gain heat from Ag NPs, thus the water contacting with light-to-heat effect nanofibrous membrane would be heated too. The nanofibrous membranes containing Ag NPs would be characterized under LED light irradiation (as shown in Scheme 2) and their UVMD performance (as shown in Figure S1) would be investigated in details. Upon light illumination, the


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photothermal effect of the nanofibrous PVDF-Ag NPs membrane was immediately activated to generate a considerable amount of heat. Consequently, water vapor generated immediately because the generated thermal energy was effectively absorbed by water. It was a remarkably efficient process that the UV light energy converted into thermal energy rapidly and the UVMD process needed no auxiliary heating, avoiding extensive energy consumption of heating bulk feed water. EXPERIMENTAL SECTION Materials. Commercial polymer Polyvinylidene Fluoride (PVDF, with a weight-average molecular weight of 573,000 g/mol) was purchased from Solvay Shanghai Co., Ltd. (China). N-hexadecyl mercaptan (n-C16H33SH (R-SH)) was purchased from sigma-Aldrich. N, N’-Dimethylformamide (DMF), silver nitrate (AgNO3), sodium borohydride (NaBH4) and absolute ethanol were supplied by Shanghai Lingfeng Chemical Reagent Co. Ltd. (China). All chemicals were of analytical grade and were used as received without further purification. Fabrication of the Photothermal Nanofibrous Membrane. 1. Solution preparation. The main dope solution for electrospinning of Photothermal membrane was 15 wt % PVDF/DMF solution, which was prepared by dissolving PVDF powder in DMF by continuous stirring in an oil bath (~80 °C) for 48h. To prepare PVDF-AgNO3/DMF solution, different amounts of AgNO3 were dissolved in 15 wt % PVDF/DMF solution in a weight ratio of 7.8, 15.7 and 31.5 wt %, of which silver content as 5, 10 and 20 wt % respectively (with respect to PVDF polymer). 2. Electrospinning of PVDF-AgNO3/PVDF double-layer nanofibrous membrane and its post-treatment. Fabrication of PVDF-AgNO3/PVDF double-layer membrane (E-PVDF-x, where x stands for the different weight percent of Ag versus PVDF) is comprised of (i) electrospinning for making pure PVDF nanofibrous membrane (PVDF ENM) and (ii) electrospinning PVDF-AgNO3 layer onto electrospun PVDF membrane. Firstly, about 5 mL of the PVDF/DMF solution was placed in a 5-mL


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syringe equipped with a blunt metal needle of 0.37 inner diameter. The syringe was set in a syringe pump that kept feeding rate of 5μL/min. The collector was a rotating (500 rpm) grounded drum covered by a piece of aluminum foil. The distance between the needle tip and collector was 15 cm, and the voltage was adjusted at 25kv. The relevant temperature and humidity were 30~35 °C and 30±5%, respectively. Then, about 5 mL of the PVDF-AgNO3/DMF solution was electrospun onto the PVDF ENM with the same conditions to form the PVDF-AgNO3 layer. The obtained electrospun nanofibrous scaffolds were then placed in a vacuum oven at 60 °C for 12h to ensure all solvents evaporated from the as-spun membranes, in order to eliminate the effect from residual solvents in the membrane. The composite membranes were cold-pressed under 2MPa pressure at room temperature for 30 seconds, which could improve the dimensional integrity with the similar membrane thickness of ~ 50 μm. 3. Membrane modification. The achieved double-layer membranes were dipped into a solution of NaBH4 (5mM) in absolute ethanol to reduce the AgNO3 to Ag NPs. Then the PVDF-Ag/PVDF composite membranes (PVDF-Ag-x, where x stands for the different weight percent of Ag versus PVDF) were rinsed in absolute ethanol to remove residual NaBH4. To obtain a hydrophobic layer onto nanofibrous membrane surfaces, the PVDF-Ag-x membranes were subsequently immersed in an ethanol solution of 10mM n-hexadecyl mercaptan. Before a series of characterizations and UVMD test, the n-hexadecyl mercaptan-coated membranes (S-PVDF-x, where x stands for the different weight percent of Ag versus PVDF) were rinsed with a mass of absolute ethanol and then dry in vacuum at 80 °C for 12h. Membrane

Characterizations.

The

surface

morphology

of

electrospun

nanofibrous membranes and the dispersion of the Ag NPs in the fibers were visualized by field emission scanning electron microscopy (FE-SEM, SU8000, Hitachi, Tokyo, Japan) and transmission electron microscopy (TEM, JEM-2100, JEOL, Tokyo, Japan). Ag NPs sizes and fiber diameters together with their size distribution were determined from the TEM and SEM images respectively, using an


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image analyzer, namely, ImageJ 2X software; meanwhile the sizes of a total number of 200 particles have been measured as well as the diameters of a total number of 200 fibers. The surface chemical compositions of the samples were characterized with an Axis Ultra DLDX-ray photoelectron spectroscopy (XPS) instrument (Kratos Analytical-A Shimadzu Group Company) with monochromatic Al Kα radiation as the excitation and an X-ray power of 75 W. The anti-wettability of membranes surface was detected by a dynamic contact angle testing instrument (OCA40, Dataphysics, Germany) equipped with a dynamic image capture camera. Average static water contact angle (WCA, 3 μL) was obtained by measuring the same sample at five different positions. The porosity of the membrane is defined as the volume of the pores divided by the total volume of the membrane, which was determined by a gravimetric method in this study34. A capillary flow porometer (CFP-1100A, Porous Material. Inc. (PMI)) was used to measure mean flow pore size (MFP), the pore size distribution, and the maximum pore sizes of the resultant membranes. The membranes were initially wetted with a low surface tension liquid (perfluoroether, Porefil) and then placed in a sealed chamber, followed by pressurizing with N2 gas. The change in flow rate was measured as a function of pressure for both dry and wet processes. And the results were obtained by the following equation:35

where P is the differential pressure, γ is the surface tension of the wetting liquid, θ is the contact angle of the wetting liquid (in this case zero), and D is the pore diameter. The gas permeability was yielded by the gas flow rate measured through dry membranes.35 On the basis of Darcy’s law, the fluid flow passing through porous media was in proportion to the pressure gradient causing flow.36 Liquid entry pressure of water (LEPw) is generally characterized as the capability


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of the membrane to impede liquid water penetrating into membrane pores,34 which can be used to evaluate the separation capacity of the resultant hydrophobic membranes. The dry S-PVDF-x nanofibrous samples with effective area of 0.95 cm2 were placed into the measuring cell and the reservoir was filled with deionized water. With the of a gas cylinder that was filled with nitrogen, a slight pressure was raised stepwise with 0.005 bar and each pressure was maintained for 10 min in the process of the degasification of the permeate side and the LEPw test. The minimum applied pressure that resulted in a continuous flux was regarded as the LEPw value. The measurements were carried out thrice using three different membrane samples made under the same condition. The results were averaged to obtain the final LEPw value. Photothermal direct contact membrane distillation performance. Before membrane distillation test, membranes with various amounts of Ag NPs were performed to investigate the photothermal effect under light irradiation by an infrared thermometer (Testo 865). The performance of the photothermal membrane was evaluated by a lab-scale direct contact UVMD system (Scheme 2 and Figure S1). The membrane module covers a resultant membrane with an effective area of 12 cm2 (4cm × 3cm). A quartz window of 4cm × 3cm allows irradiation of the membrane surface. The feed and distillate flow channels are both 2mm in height. UV light is supplied by UV LED light with the wavelength of 400nm (220V, 50W, 0.32 W/cm2). Ultrapure water and a 3.5 wt % NaCl aqueous solution were used as the cooling fluid and the feed fluid respectively. Both the distillate and the feed were recirculated with the flow rate of 0.6 L/min by two diaphragm laboratory pumps. The conductivity of the permeate solution was continuously monitored by conductivity meters (SevenEasy, Mettler Toledo) and the clean water produced by the MD membrane led to a net increase in the distillate volume, which was continuously monitored using an overflow device, where the excess overflowed into a beaker situated on a balance (MS8001, Mettler Toledo) connected to a computer.


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RESULTS AND DISCUSSION Morphology of PVDF-ENM and E-PVDF-x membranes. In order to combine Ag NPs and PVDF well and obtain the photothermal membrane in a facile way, AgNO3 was introduced into PVDF electrospinning solution. After that, the E-PVDF-x membranes would be reduced by NaBH4 and modified by n-hexadecyl mercaptan, which was illustrated in scheme 1. As the preliminary step in the E-PVDF-x membranes fabrication, PVDF-AgNO3 nanofibers were electrospun directly on top of the PVDF ENM. The surface morphology of PVDF ENM (Figure 1a) and E-PVDF-x membranes (Figure 1b-d) were observed by FE-SEM. The as-spun non-woven nanofibrous membrane with three-dimensional macro porous structure was formed by the random stacking nanofibers, containing interconnected passageways for water vapor throughout the whole membrane.37 As shown in Figure 1a, pure PVDF nanofiber without AgNO3 addition exhibited uniformity in fiber diameter. In the case of adding inorganic salt AgNO3, there were a few ultrathin fibers in Figure 1b, c and even spider-web structure (Figure 1c and 1d). Here, the inorganic salt AgNO3 could increase the conductivity of PVDF solution and lead to second electrospinning among the adjacent nanofibers because the AgNO3 had higher electronic density compared to the polymer matrix in the high voltage field.38-39 Preparation and characterization of Ag NPs doped PVDF nanofibrous membranes. The Ag+ ions presented in E-PVDF-x composite nanofiber membranes can be reduced to Ag NPs via two different ways i.e., NaBH4 treated and reaction with DMF. There are two different mechanisms involving the reduction which are represented as below.40-41 (1) 2Ag+ + 2BH— 4 → 2Ag0 + H2 + B2H6 (NaBH4 as reducing agent) (2) HCONMe2 + 2 Ag+ + H2O → 2 Ag0 + Me2NCOOH + 2H+ Me2NCOOH → Me2NH + CO2 (DMF as reducing agent)


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DMF is a relatively weak reducing agent with regard to Ag+ ions and a good solvent for the PVDF, which resulted in the in-situ formation of small amount of metallic Ag NPs slowly. Furthermore, very few Ag NPs and silver nitrate were trapped in the PVDF nanofibers during the electrospinning process.42 When the nascent E-PVDF-x membranes were dipped in a NaBH4 solution, the membranes color changed from white to yellow which was the characteristic phenomenon of Ag NPs, indicating that the NPs were formed in the PVDF nanofibers. The few Ag NPs reduced by DMF before electrospinning and trapped inside the nanofibers would act as the nucleation sites and lead to the formation of well dispersed Ag NPs in the nanofibers during the reduction process caused by NaBH4. The mechanism of Ag NPs formation was ascribed to comprise the reduction of Ag+ ions, which would experience the nucleating process, and finally these nuclei serve as seeds for further growth.42 Ultraviolet-visible (UV-vis) spectra shown in Figure S2

indicated that

PVDF-Ag-x membranes containing different amounts of Ag NPs have maximum absorbance intensity around 390nm, corresponding to the wavelength of the Ag NPs’ plasmon resonance.43 The position of maximum absorbance was not shifted and it turned out that the average size of the formed Ag nanoparticles was close in nanofibers with different silver contents.44 In order to observe the morphology, distribution situation and size of Ag NPs in electrospun nanofibers clearly, the typical TEM images of the PVDF-Ag-x nanofibers with different Ag NPs contents as 5, 10 and 20 wt % (calculated from AgNO3 content with respect to PVDF polymer) were represented in Figure 2a-c. Spherical Ag NP aggregates were embedded at the surface and inside of the nanofibers, and they were polydisperse with the size in the range of 5-40 nm as measured from the TEM micrographs (as shown in Figure 2d-f). With increasing of Ag NPs contents, the Ag NPs agglomeration was formed along the fiber axis. It is clearly shown that only a small number of Ag nanocluster (5-10 nm, Figure 2d) were located at the nanofiber surfaces when the content of Ag NPs were 5 wt % (Figure 2a). After increasing the


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content of Ag NPs to 10 wt %, the larger Ag nanocluster (8-18 nm, Figure 2e) was formed in the nanofibers, as shown in Figure 2b. With further increasing of Ag NPs to 20 wt %, the density of the Ag nanocluster (16-40 nm) increased and an extraordinarily compact enveloping morphology was observed (Figure 2c). In addition, the well dispersed Ag nanoparticles in PVDF nanofibers would also contribute to increase the hydrophilicity of membranes and thereby, aggravate the weak anti-wettability of the PVDF-Ag-x membrane.45 Surface modification of PVDF-Ag nanofibrous scaffold. It is instructive that the particular anti-wettability and unique multiscale surface structure of creatures from natural world. 46 Herein, an appropriate multiple scale roughness could be established by a bio-inspired way, just via the in-situ formation of Ag nanoclusters in the PVDF fibers and then the surface modification of hydrophobic thiols with long alkyl chains. The FE-SEM images and fiber diameter distributions of S-PVDF-x composite nanofibrous membranes after R-SH modification were shown in Figure 3. Compared with E-PVDF-x membranes, the variation trend of surface morphology and roughness could be clearly observed from Figure 3. For the S-PVDF nanofibers with a low Ag NPs content (5 wt %, Figure 3a and b), the fibers had not changed much in appearance (Figure 1b). After changing the content of Ag NPs to 10 wt %, there were many thiol clusters on the fibers, as shown in Figure 3e, because of the growing number of reactive sites (Ag NPs). When there were even more Ag NPs (20 wt %), as shown in Figure 3g and h, the thiols agglomerated and formed some conglomerations on the fibers’ surfaces achieved by a great deal of Ag nanoclusters assembling (Figure 2c), which would result in the decrease of the membranes’ pore size in turn. Unfortunately, too much thiols agglomeration and the congested structure would obstruct the membrane pores to a certain extent, thereby make the membrane porosity decreased and further reduce the gas permeability.47 The electrospun membrane possessed unique structure, which appeared highly porous with interconnected pores achieved by the lap joint between the nanofibers, leading to the difficulty of water


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molecules transportation,12 thus fiber diameter has a direct influence on the pore size distribution of the membrane. Obviously, the diameter of fibers was uniform with small range of 100-580 nm and averaged on 0.26 μm (Figure 3c) and when the content of Ag NPs increased to 10 wt %, it ranged from 150 to 750 nm with the mean of 0.38 μm (Figure 3f). After further increasing of the content of Ag NPs to 20 wt %, the diameter range of fibers widened to 170-670 nm (Figure 3i) compared to S-PVDF-5, however, the average value dropped to 0.34 μm. From Figure 3g and h, there are more ultrafine fibers (less than 100 nm) appeared, which could be also ascribed to the second electrospinning38, thus membrane pore size would be much smaller. XPS was employed to confirm the surface chemical compositions of PVDF ENM, PVDF-Ag-20 and S-PVDF-20 membranes. The wide-scan spectrum of PVDF ENM and the C 1s core-level spectrum are shown in Figure S3a and b. Silver nitrate in the PVDF fibers was reduced to be Ag NPs, which can be proved in Figure S3c and d. The strong signal of Ag at binding energy of about 370eV at Figure S3c indicates that Ag NPs form on and within PVDF fibers. In Figure S3d, the Ag 3d core-level spectrum can be curved into two peaks which are Ag 3d 1/2 peak and Ag 3d 3/2 peak at binding energy of 367.5 eV and 373.5 eV respectively.48 The two peaks further indicated that the Ag NPs were in the zero valent state. The wide-scan spectrum of n-hexadecyl mercaptan-coated PVDF (S-PVDF) and the C 1s core-level spectrum are shown in Figure S3e and f. The wide-scan spectrum only have strong peaks for C 1s, Ag 3d and 3p, indicating that after R-SH modification, there are mainly carbon and silver on membrane surfaces. Furthermore, the C 1s core-level spectrum can be curve-fitted into only one peak at binding energy of 284.6 eV for CHx, which is the chemical composition in the coated thiol layer (n-hexadecyl mercaptan). In addition, the S 2p core-level spectrum (Figure S3g) also indicates the presence thiol layer of S-PVDF and the curve-fitted S 2p core-level spectrum exhibited two overlapping peaks centered at 162.3 eV, corresponding to the chemical bond of sulfur-silver. The


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XPS data fully demonstrated that the reduction and modification shown in Scheme 1 have been accomplished on the fiber surface.48 Although few Ag NPs couldn’t be observed clearly on the S-PVDF-5 fiber surface as shown in Figure 3b, it is easy to find that there are different degrees of rough structure on S-PVDF-10 and 20 nanofiber surface when the content of Ag was enough to form thiols aggregations (Figure 3e and h). The presence of Ag and S elements on the surface of the S-PVDF-x nanofiber membranes proved by XPS analysis demonstrates that the thiol agglomerations were successfully formed on the Ag NPs encased in nanofibers. Surface wettability. It is indispensable that membranes possess high hydrophobicity in MD, which can not only obstruct liquid water but also accelerate vapor transport.3 Therefore, we investigated the anti-wettability of the fabricated membranes by measuring the water contact angles (WCAs, in Figure 4). On account of the inherent hydrophobicity of PVDF and the porous structure of nanofibers, the PVDF ENM membrane exhibited a relatively high water contact angle of 137±1.2°. With the contents of Ag NPs in nanofibers increasing, WCAs of PVDF-Ag-x membranes (unmodified membranes) gradually decreased, which can be proved in Figure 4 that the WCAs were 133±2.4°, 128±1.8° and 125±1.9°, respectively. The R-SH modified membranes (S-PVDF-x membranes), on the other hand, exhibited higher hydrophobicity as the Ag NPs content increases, with a water contact angle of 138±1.9°, 142±2.5° and 148±2.1°. We attributed this opposite trend of WCAs to two combined factors: (1) the multi-scale structure caused by the Ag NPs and thiols agglomerations on the PVDF nanofibrous membranes and (2) the low surface energy achieved by R-SH modification. The Ag NPs followed by thiols modification created the second level structure on the columniform fibers, which served as an additional barrier to surface wetting as long chain thiols were bound to Ag nanoclusters firmly through Ag-S bonds.49 In addition, the Ag NPs combined with R-SH, which notably reduced the surface energy, resulted in the WCAs’ increasing, i.e. surface hydrophobicity.50-52


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With the increase of Ag content, the WCAs of PVDF-Ag-x membranes gradually decreased, because Ag NPs were hydrophilic. However, after the grafting of thiol on the surface of Ag NPs, the more silver content made more hydrophobic substances covered by the fiber surface, which would increase the WCAs and improve the hydrophobicity. The hydrophobicity surface of S-PVDF-20 membrane with large amount of multi-scale roughness would reduce the contact area the water droplet and the surface due to the decreasing air fraction of liquid-solid interfaces beneath the water droplet. Structural attributes of membranes. Before the UVMD evaluation, the structural properties of S-PVDF-x membranes were characterized, including the pore size distribution, MFP, porosity and LEPw. Broadly speaking, a novel electrospun nanofibrous membrane with high filtration efficiency was designed based on the relationship of the pore size distribution and the fiber diameter of a nonwoven layer structure.53 Above all, it is of great importance that the MD membrane has proper pore size distribution and MFP which can prevent pores wetting appearance;54 in other words, a suitable nanofiber diameter distribution should match the pore size for MD application (permeability and selectivity). As can be seen from Figure 5 and Table 1, the electrospun nanofibrous membrane showed a relatively broad pore size distribution, which ranged from 0.14 to 0.73 μm with MFP of 0.33 μm, from 0.17 to 1.00 μm with MFP of 0.43 μm, and from 0.10 to 0.75 μm with MFP of 0.31 μm, corresponding to the S-PVDF-5, S-PVDF-10, and S-PVDF-20 membranes, respectively. The S-PVDF-x samples showed the trend of climbing up and then decline, of which the MFP value toward the increment of Ag NPs, which resulted from the notable appearance of ultrathin fibers and the partial obstruction Ag NPs and the thiols agglomeration on it. The pore size distribution and the MFP of a nonwoven structure are matched with the fiber diameter and porosity; in other words, the larger pore size will result from larger fiber diameter and vice versa.53 Thus when the diameter of nanofibers became larger with the content of Ag NPs from 5 wt % to 10


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wt %, the MFP of the membrane rose up and the pore size distribution widened. However, as the content of Ag NPs was 20 wt %, the ultrafine fibers resulted by second electrospinning would narrow the pores which were caused by fibers stacking. Therefore the pore size distribution of S-PVDF-20 was broader than the S-PVDF-10’s and the MFP of the former declined compared to the latter. It is widely acknowledge that higher membrane bulk porosity matches with higher permeate vapor flux, and the MD membrane porosity can be ranged from 30 to 85% as reported so far.55 As can be seen from Table 1, although the partial obstruction of Ag NPs and thiols agglomeration would result in the decrease of porosity, the S-PVDF-x membranes still had enough porosity for MD application, which was 78.6±1.9, 72.2±1.1 and 67.5±1.8%, respectively. Although the bulk porosity of S-PVDF-20 membrane declined to 67.5±1.8%, the S-PVDF-20 membrane still could keep enough water vapor through and would have excellent UVMD performance by reason of the abundant Ag NPs’ immobilization for light drive. LEPw is one of the key structural characteristics to ensure the durability of membrane MD performance in the long term operation. Generally, the membrane’s LEPw is the maximum pressure before deionized water invades and passes through a nonwetting membrane; that is to say, the water droplet won’t penetrate into the membrane below this pressure. The electrospun nanofibrous membrane generally exhibited a relatively lower LEPw value. Nevertheless, a reasonable LEPw value of a membrane hinges on the structural attributes (pore size and porosity) and hydrophobicity of the membrane. Therefore, with WCAs increasing, the LEPw values of S-PVDF-x membranes trended to increase obviously, as shown in Table 1, which was 1.21±0.05, 1.37±0.07 and 1.55±0.04 bar, respectively. All the LEPw values of S-PVDF-x membranes provided guarantees for the subsequent UVMD application. Water vapors can easily pass through the electrospun nanofibrous membrane, which benefits from the porous structure with fully interconnected pores, thus the


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membrane has the characteristic of breathability. In the process of UVMD operation, water vapors penetrated the membrane and then they were condensed into pure water by a circulated chiller system. Hence, the gas permeability or breathability of the membranes is another critical characteristic, which can estimate the vapor permeability across the membrane.56 Light to heat conversion effect. The photothermal property of the S-PVDF-x membranes was then investigated. A 50W ultraviolet lamp with light wavelength of 400nm was shined on S-PVDF-x membranes and an infrared (IR) camera was utilized to detect the membranes temperature. Under continuous UV light irradiation, a stable temperature of the membranes could be detected, because the heat generation caused by light irradiation and heat dissipation because of heat radiation is at equilibrium.18 Figure 6 presented the different temperature of S-PVDF-x membranes with various contents of Ag NPs under UV light irradiation. Under continuous UV light irradiation for 60 seconds, the S-PVDF-5, 10, 20 membranes heated up rapidly to the stable temperature of 51.8 °C, 66.4 °C and 92.3 °C, respectively. This result obviously indicated the effective photothermal conversion property of the membranes, which was imparted by the Ag NPs. Figure 7 showed the temperature attenuation of the S-PVDF-20 membrane after the removal of UV-light illumination as time goes on (within 60 seconds). As can be seen, with the ambient temperature being constant at 22.0 °C, the highest temperature of S-PVDF-20 membrane was first at 93.9 °C under 1 min continuous illumination of UV light and it decayed gradually after turning off the light. A minute later, it still kept warm with temperature of 37.9 °C. UV-membrane distillation performance. After confirming the photothermal effect, further investigation of water evaporation performance of membranes was conducted with a simulated UVMD model (Figure 8). As illustrated in Figure 8, each membrane was floated on water surface with a watch glass covering on the membrane. After 1 min illumination of UV light, it was clearly observed that the condensing


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water droplets were formed on the inner wall of the watch glass surface with S-PVDF-20 membrane (Figure 8a, b and c), on the contrary, there were no droplets on the watch glass with the PVDF ENM membrane after UV illumination (Figure 8d, e and f), successfully demonstrating that the photothermal S-PVDF-20 membrane was heated under light irradiation and then water vapors easily passed through the hydrophobic nanofibrous membrane and condensed to water droplets on the inner wall of watch glass, indicating a straightforward illustration of the basic separation principle of UVMD. The continuous UVMD tests of the S-PVDF-x were performed at least 20 h operation by using a 3.5 wt % NaCl aqueous solution as feed under a temperature of 20 °C, and the permeate side was maintained at 20 °C too. As shown in Figure 9, the mean fluxes of S-PVDF-5, 10 and 20 membranes were 0.3, 1.6 and 2.1 kg/ (m2·h) respectively and it is clearly to find that the increased permeate flux was corresponding to higher Ag NPs content. The S-PVDF-20 membrane had the highest permeate flux of 2.5 kg/ (m2·h) among all samples and low permeate conductivity of 2.65 μs/cm during the long-term UVMD operation. Such good performance of considerable flux and high rejection (low permeate conductivity) benefited from the hierarchical nanofibrous structure achieved by R-SH modified electrospun PVDF nanofibers containing Ag NPs. To be more specific, the nanofibrous structure led to high porosity for water vapor transmission and R-SH modification enhanced the anti-wettability of whole membrane. Besides, abundant Ag NPs resulted in effective light-to-heat conversion and efficient generation of water vapor. The Ag NPs have realized the effective use of light energy and they resulted in a remarkable increase in flux under UV light irradiation compliance with a stronger capability of converting light to heat at the higher Ag NPs content. After UVMD tests, we characterized the WCAs and morphology of S-PVDF-x membranes to evaluate their structure stability and surface wettability. The


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morphology of S-PVDF-x didn’t change markedly before (Figure 3) and after (Figure S4) long-term UVMD tests. The WCAs of S-PVDF-x membranes after test still showed relatively high values (over 135°) indicated that they still possessed high hydrophobicity, thus the membranes exhibited excellent stability after long-term UVMD tests. All modified membranes showed excellent stability in performance as suggested by the increased MD time from 20 h to 60 h. Energy efficiency of UVMD. The photothermal membrane converts light energy to thermal energy and supplies heat to water on the membrane surface, which possesses higher energy efficiency than heating the bulk water in the conventional method.57 In order to evaluate the energy efficiency of the UVMD process, we calculated the water evaporation energy proportion of total input energy and the equation is as follow57:

where J is the permeate flux of the UVMD test, △H is the latent heat of evaporation (2453 J °C-1 g-1 for water at 20 °C), A is the irradiation area (1.2×10-3 m2) and I is the incident light intensity (3.2 kJ m-2 s-1), Hw is the auxiliary heat for the feed but it is zero in this work because of the same temperature of feed and permeate side. Figure 10 presented the calculated η of the S-PVDF-x membranes. When the Ag NPs contents were 5 and 10 wt %, the efficiency of S-PVDF-5 and S-PVDF-10 membranes were 11±2 % and 32±9 % respectively. The efficiency increased with more Ag content and the S-PVDF-20 membrane possessed the highest efficiency in UVMD as 53±7 %. Such increasing UVMD energy efficiency of S-PVDF-x membranes was corresponding to the tendency of UVMD mean permeate flux and thus the content of Ag NPs was the dominant factor of UVMD performance. In order to specifically demonstrate the UVMD performance of the prepared silver nanoparticles enabled photothermal nanofibrous membranes in this study, Table 2 summarized the properties and light driven MD performances of different


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membranes. For Ag NPs embedded phase inversion PVDF membrane16 which also used UV light as energy source, the feed temperature was 30 °C achieved by auxiliary heating and the MD process was vacuum MD with the help of a vacuum pump. Therefore, the permeate flux of Ag NPs embedded phase inversion PVDF membrane with the help of much higher extra energy consumption could be obviously higher than the flux value achieved just by UV light driven in this study. Also, there was auxiliary heating (the feed temperature was 35 °C) in the MD process of commercial PVDF membrane with carbon black NPs or SiO2/Au nanoshells57 coating and the simulated sunlight contained multiple wavelengths of light, not just UV light. Thus the permeate flux was higher than that in our work which should be attributed to the help of auxiliary heating and more light energy input. Even though, the concentration of NaCl solution in the feed side they used was only 1.0 wt %. For another commercial PVDF membrane with carbon black embedded-polyvinyl alcohol (PVA) coating58 driven by sunlight, the energy efficiency was 53.8% in the solar driven MD process and the permeation flux was relative low (0.5 kg/m2•h). Compared with these similar light-driven MD performances from different membranes reported by other groups, the UVMD process of S-PVDF-20 membranes had comparable performances with the competitive efficiency of 53±7 %. Especially, the UVMD process here was only driven by UV light without any other energy source for auxiliary heating the feed.

CONCLUSIONS In general, we have demonstrated that multiscale structure and efficient light to heat conversion with excellent durability were successfully constructed on PVDF nanofibrous membranes by electrospinning of PVDF-AgNO3 blending solution followed by reduction treatment and surface hydrophobic modification. After R-SH modification, the S-PVDF-20 membrane exhibited higher hydrophobicity with WCA


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increasing to 148±2.1° as the Ag NPs content was 20 wt % (with respect to PVDF polymer). Meanwhile the S-PVDF-20 membrane had the excellent photothermal performance which could reach a remarkable temperature of 92.3 °C under 60s UV-light irradiation. The photothermal membrane also have considerable UVMD capability—specifically, the desalination evaluation results indicated that S-PVDF-20 membrane presented outstanding performance in NaCl solution system and superior durability with the continuous UVMD working time of 60 hours. In a word, this study explored the fabrication of novel photothermal MD membranes and their performance in a lab-scale UVMD system under UV light illumination. By means of electrospinning, Ag NPs could be facilely and sufficiently combined with the PVDF nanofibrous membranes. From this study, the results indicate that the photothermal membrane achieved by photothermal nano-materials has the capability to directly convert light energy to heat energy, and produce pure water in a direct UVMD process with the highest energy efficiency of 53±7 %. Thus the photothermal materials hold great potential for water purification and desalination applications. ASSOCIATED CONTENT Supporting Information Schematic

diagram

of

UVMD

lab-scale

setup

(Figure

S1),

UV-vis

characterization of PVDF membranes containing different Ag NPs loading (Figure S2), The XPS wide-scan of (a) PVDF ENM, (c) PVDF-Ag-20 membrane and (e) S-PVDF-20 membrane; C 1s core-level spectra of (b) PVDF ENM and (f) S-PVDF-20 membrane; Ag 3d core-level spectra of (d) PVDF-Ag-20; S 2p core-level spectra of S-PVDF-20 (Figure S3), the SEM images and WCAs of S-PVDF-x membranes after UVMD tests (a-c: 5, 10 and 20) (Figure S4) AUTHOR INFORMATION


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Corresponding Author

*Tel.: +86-21-67792860. Fax: +86-21-67792855. E-mail: [email protected].

ACKNOWLEDGMENTS This work was supported by Program of Shanghai Science and Technology Innovation International Exchange and Cooperation (15230724700) and Program for Innovative Research Team in University of Ministry of Education of China (IRT_16R13).


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Table of Contents


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Graphic for manuscript Scheme 1. Illustration for the fabrication procedure of photothermal membrane. Scheme 2. UVMD cell prototype (left) and schematic diagram of lab-scale setup (right). Figure 1. FE-SEM images of PVDF ENM (a) and E-PVDF-x membranes (5: b, 10: c and 20: d) Figure 2. TEM images of Ag NPs present in the nanofibers of PVDF-Ag-x and Ag NPs size distributions: (a, d) PVDF-Ag-5, (b, e) PVDF-Ag-10, and (c, f) PVDF-Ag-20. Figure 3. FE-SEM images of S-PVDF-x membranes and their fiber diameter distributions (5: a-c; 10: d-f; 20: g-i). Figure 4. Water contact angles variation from the membranes with different contents of Ag (PVDF ENM; Ag/PVDF=5 wt %; 10 wt %; 20 wt %). Figure 5. Pore Size distribution of (a) S-PVDF-5, (b) S-PVDF-10, and (c) S-PVDF-20. Figure 6. IR thermal images of S-PVDF-x membranes under UV-light irradiation for 60 seconds: (a) S-PVDF-5, (b) S-PVDF-10, and (c) S-PVDF-20. Figure 7. IR thermal images of S-PVDF-20 membrane with UV light irradiation (a) and without UV light irradiation for 10s (b), 30s (c), 60s (d). Figure 8. Schematic diagram for the simulation of UVMD process with S-PVDF-20 (left) and PVDF ENM (right), before and after UV light irradiation for 60 seconds. Figure 9. Continuous UVMD test of S-PVDF-x membranes: (a) S-PVDF-5, (b) S-PVDF-10, and (c) S-PVDF-20. Figure 10. UVMD Energy efficiency of S-PVDF-x membranes.

Table 1. Structural attributes of S-PVDF-x membranes. Table 2. Properties and light driven MD performances of different membranes.


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①2Ag+ + 2BH— 4 ②n-Hexadecyl mercaptan

2Ag0 + H2 + B2H6 (NaBH4 as reducing agent)

Scheme 1. Illustration for the fabrication procedure of photothermal membrane.


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Scheme 2. UVMD cell prototype (left) and schematic diagram of lab-scale setup (right).


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Figure. 1. FE-SEM images of PVDF ENM (a) and E-PVDF-x membranes (5: b, 10: c and 20: d)


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Figure 2. TEM images of Ag NPs present in the nanofibers of PVDF-Ag-x and Ag NPs size distributions: (a, d) PVDF-Ag-5, (b, e) PVDF-Ag-10, and (c, f) PVDF-Ag-20.


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Figure 3. FE-SEM images of S-PVDF-x membranes and their fiber diameter distributions (5: a-c; 10: d-f; 20: g-i).


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Figure 4. Water contact angles variation from the membranes with different contents of Ag (PVDF ENM; Ag/PVDF=5 wt %; 10 wt %; 20 wt %).


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Figure 5. Pore Size distribution of (a) S-PVDF-5, (b) S-PVDF-10, and (c) S-PVDF-20.


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Figure 6. IR thermal images of S-PVDF-x membranes under UV-light irradiation for 60 seconds: (a) S-PVDF-5, (b) S-PVDF-10, and (c) S-PVDF-20.


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Figure 7. IR thermal images of S-PVDF-20 membrane with UV light irradiation (a) and without UV light irradiation for 10s (b), 30s (c), 60s (d).


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Figure 8. Schematic diagram for the simulation of UVMD process with S-PVDF-20 (a, b and c) and PVDF ENM (d, e and f), before and after UV light irradiation for 60 seconds.


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Figure 9. Continuous UVMD test of S-PVDF-x membranes: (a) S-PVDF-5, (b) S-PVDF-10, and (c) S-PVDF-20.


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Figure 10. UVMD Energy efficiency of S-PVDF-x membranes.


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Table 1. Structural attributes of S-PVDF-x membranes. Membranes

PSD (μm)

MFP (μm)

Porosity (%)

LEPw (bar)

S-PVDF-5 S-PVDF-10 S-PVDF-20

0.14-0.73 0.17-1.00 0.10-0.75

0.33 0.43 0.31

78.6±1.9 72.2±1.1 67.5±1.8

1.21±0.05 1.37±0.07 1.55±0.04


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Table 2. Properties and light driven MD performances of different membranes. Feed side

Permeate side

Permeation flux Tp (°C) (kg/m2•h)

Membranes

Input energy source

NaCl solution (wt %)

Tf (°C)

Distilled water

Phase inversion PVDF membrane with Ag NPs embedded16

UV light

3.5

30

(Vacuum)

/

8.5-25.7

Simulated sunlight

1.0

35

/

20

6.12

Solar

1.0

20

/

20

0.5

UV light

3.5

20

/

20

2.5

Commercial PVDF membrane with carbon black NPs or SiO2/Au nanoshells coating57 Commercial PVDF membrane with carbon black embedded-polyvinyl alcohol (PVA) coating58 This study

42 ACS Paragon Plus Environment

Silver nanoparticles-enabled photothermal nanofibrous membrane for

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