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Advanced multi-nozzle electrospun functionalized titanium
dioxide/polyvinylidene fluoride-co-hexafluoropropylene
(TiO2/PVDF -HFP) composite membranes for direct contact
membrane distillation
Eui-Jong Lee1, Alicia Kyoungjin An1*, Pejman HADI2, Sangho Lee3, Yun Chul Woo4, and
Ho Kyong Shon4
1 School of Energy and Environment, City University of Hong Kong, Tat Chee Avenue
Kowloon, Hong Kong, China
2 New York State Center for Clean Water Technology, Stony Brook University, NY 11794, USA
3 School of Civil and Environmental Engineering, Kookmin University, Seoul, Korea
4 Centre for Technology in Water and Wastewater, School of Civil and Environmental
Engineering, University of Technology Sydney (UTS), P.O. Box 123, 15 Broadway, NSW 2007,
Australia
* Corresponding author. Tel: +(852)-3442-9626, Fax: +(852)-3442-0688, E-mail:
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Abstract
The unique capabilities of electrospinning technology are being increasingly utilized in the
fabrication of hydrophobic membranes to improve the membrane distillation (MD) process in
recent years. In this study, hydrophobic titanium dioxide (TiO2) nanoparticles functionalized by
fluorosilane were incorporated into electrospun membranes using single, coaxial, and dual
nozzles to develop novel membrane architectures for improved physico-chemical properties for
MD. By incorporating fluorosilane coated TiO2 into the PVDF-HFP solution during the
membrane synthesis and using an advanced multi-nozzle to form various hierarchical membrane
structures tuned the size and structure of the nanofibers and made them vastly superior for the
application in MD. The single and coaxial nozzle membranes showed contact angles close to
150○ and the dual-nozzle membrane assembled bead-on-string fibers achieved
superhydrophobicity (i.e., contact angle of 153.4○). To test the functionalized titanium
dioxide/polyvinylidene fluoride-co-hexafluoropropylene (TiO2/ PVDF-HFP) composite
membranes for MD performance, the membranes were subjected to long-term direct contact MD
for about two days to monitor their water vapor flux and selectivity. Compared to commercial
PVDF membranes, all electrospun F-TiO2/ PVDF-HFP membrane achieved higher water vapor
flux of 40 Lm-2h-1 (60°C feed and 20°C permeate) with a brine (7.0 wt% NaCl) as the feed
solution and also exhibited anti-wetting property while maintaining high water flux compared to
the membrane without TiO2 incorporation.
Keywords: Electrospun membrane; Coaxial & dual nozzle; Hydrophobicity; Membrane
distillation; Desalination
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1. Introduction
Membrane distillation (MD) is based on a thermal gradient applied across a hydrophobic
membrane to create a vapor pressure that results in the transportation of vapor molecule
through the membrane network [1]. This filtration technique minimizes the contact
between the membrane and high salt feed solutions to enable high-quality water
production with low-grade energy, as compared to the reverse osmosis (RO) and
conventional thermal processes, which makes it an attractive method for seawater
desalination and RO brine treatement [2,3]. However, because pure water is obtained via
vapor condensation of the water molecules individually transported through the
membrane barrier, the effectiveness of the MD process heavily depends on membrane
characteristics, such as membrane materials, pore size, porosity, and hydrophobicity.
Since commercial membranes fabricated by phase inversion or melting processes are
inherently limited to serve as suitable membranes for MD [4], numerous researches have
been conducted during the last decade to fabricate appropriate membranes for MD with
several researchers reviewing the progresses and limitations so far for full-scale
application of MD in the past year [1,2,5–8].
Electrospinning has recently been highlighted as a promising technique for fabricating
nanofibrous membranes for MD [9]. In the electrospinning process, various factors including
dope solution type, electrospinner operational parameters, and post-treatment conditions can be
controlled to tune membrane performance [10]. Nevertheless, as a relatively new approach for
membrane fabrication for MD, research has so far been limited to lab-scale MD operation and
short operation time, and most fabrications have been conducted using conventional single-
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nozzle apparatus with polyvinylidene fluoride (PVDF) as the polymer [8]. The utilization of
coaxial nozzles for the preparation of core-shell nanofibers, in which the central fiber is
surrounded by a concentric annular tube of another polymer, can further expand the application
of electrospinning by enabling the transformation of low dielectric constant polymers (such as
Teflon) into nanofibers with unique fiber surfaces [11,12]. In addition, the application of dual (or
multiple) nozzles can endow benefits from the properties of different types of materials [5] to
confer flexibility on the manipulation of the membrane framework and/or surface.
Meanwhile, the incorporation of nanomaterials into the membrane has shown promise in
enhancing membrane performance; for instance, carbon nanotube (CNT) for enhancing the
mechanical properties of the membrane [13,14] or membrane permeability due to its unique
structure [15,16], SiO2 particles for creating excellent dynamic adsorption capacity [17], and
TiO2 particles for improving the antifouling property of the membrane [18]. Occasionally, the
SiO2 and TiO2 nanoparticles have been used simultaneously for enhanced controllability and
synergy effects on membrane properties [19]. Above all, nanoparticles have been used to
increase the hydrophobicity via surface deformation in various industries [20,21]. Among
inorganic nanoparticles, TiO2 is reported to have potential to improve both the membrane
electrospinability due to its electrical conductivity [22] and functionality due to its photocatalytic
activity, in addition to its stability and non-toxicity [23]. Hence, it is possible to hypothesize that
integrating the electrospinning technique and nanomaterial incorporation will produce tailored
MD membranes through which more vapor molecules can pass at a higher rate high
hydrophobicity to prevent it from wetting. However, features such as high surface energy, nano-
size and anisotropic shape are inherently difficult to be evenly dispersed [24] and thus results in a
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defective structure (abnormal agglomeration) that may not be in favor of hydrophobic MD
process.
In general, fillers are incorporated into composite membranes either through physical
blending or the sol-gel method. Between the two, although limited in available fillers, the
sol-gel method is beneficial for preventing the agglomeration of fillers. Recent studies by
Razmjou et al. [25] and Meng et al. [26] showed that PVDF membranes coated with
functionalized TiO2 (based on the sol-gel method) had porous, multi-level structures and
superhydrophobic surfaces, which led to improved salt rejection and a substantial
reduction in pore wetting. In the case of TiO2-incorporated electrospun membranes [27],
while the particles were well-dispersed, the morphology of the fiber surface was not
improved significantly. This is because particles were synthesized during fiber formation,
resulting in the integration of the particles into the fibers. In another previous study on
electrospun membranes incorporating fillers based on the sol-gel method [28], a
hierarchical morphology of the fiber surface was observed after calcination at high
temperature. Ever-increasing interest in applying nanomaterials in many different fields
[29,30] has led to continued efforts to develop dispersion and functionalization
techniques, however very limited application of functionalized TiO2 / organic polymer
membrane was reported in MD.
In this study, we report a one-step versatile electrospinning technique for incorporating
functionalized TiO2 (referred hereafter as F-TiO2) nanoparticles into organic polymer using
different types of nozzles (single, coaxial, and dual) that control the microstructure of membrane
for MD. TiO2 functionalization using hydroxylated fluorosilane was introduced to guarantee
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proper dispersion, appropriate interfacial adhesion between the nanomaterials and polymer
matrix, and increase the hydrophobicity. This is the first attempt to fabricate F-TiO2
nanocomposite membranes by coaxial and dual nozzle electrospinning techniques to gain high
water vapor flux and stable salt rejection performances during long-term direct contact MD
(DCMD) operation. Concentrated F-TiO2 nanoparticles were directly added to i) a single-nozzle
electrospinning dope solution, ii) shells by coaxial electrospinning to see the effects of F-TiO2
particles on stable nanofiber formation, and iii) dual nozzles, where one nozzle was employed for
polymer dope solutions with a high concentration of F-TiO2 particles, while the other one was
used to fabricate pure nanofibers by electrospinning. Because adding large amounts of F-TiO2
particles increases the hydrophobicity but can lead to brittle nanofibers with beads, different
nozzles were employed to fabricate durable membranes. The properties of all membranes
produced by different nozzle types were investigated, and their performances were evaluated in a
long-term DCMD operation.
2. Materials and methods
2.1 Materials
Poly(vinylidene fluoride-co-hexafluoropropylene) (referred herein as PVDF-HFP, MW =
455,000 g/mol), lithium chloride (LiCl), N, Ndimethylformamide (DMF), and acetone
were purchased from Sigma-Aldrich and were used to make the dope solution for
electrospinning. Titanium dioxide (TiO2) powder (particle size: 21 nm) and 1H, 1H, 2H,
2Hperfluorooctyltriethoxysilane (FTES) used for the functionalization of electrospun
membranes were also acquired from the same supplier.
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2.2 Preparation of functionalized TiO2 nanoparticles and dope solutions
Separate solutions of well-dispersed TiO2 nanoparticles and hydroxylated FTES in toluene
were prepared in 50-mL bottles through sonication and vigorous stirring for 1 h,
respectively. The two solutions were mixed with deionized (DI) water (mass ratio of
TiO2, FTES, and DI water = 3:1:1.5) in a glove box for 18 h to obtain FTES-coated TiO2
particles (or F-TiO2 as shown in Figure 1), which were then washed with toluene and
completely dried in an oven prior to use.
Figure 1. Schematic representation of TiO2 functionalization using hydroxylated
fluorosilane
Two types of polymer solutions were prepared using different solvents: one with 20 wt%
PVDF-HFP: 64 DMF:16 wt% acetone mixture, and the other with 20 wt% PVDF-HFP: 80 DMF
mixture, both with a small concentration of LiCl additive (0.005 wt%) [31]. The polymer
solution of 20:64:14 was then electrospun to fabricate a base membrane (M1) using single
nozzle, while another polymer 80:20 was mixed with TiO2 to fabricate membrane using single-
nozzle (M2) and coaxial-nozzle (M3). That is because the rapid evaporation rate from acetone
solvent is unfavorable to well-positioned nanoparticles on the outside of the nanofibers [32].
Dual nozzle membrane (M4) was electrospun using two nozzles: nozzle 1 with a polymer of 15
wt% PVDF-HFP: 85 DMF mixture and 50 wt% F-TiO2 and nozzle 2 with same as M1 (20 wt%
PVDF-HFP: 64 DMF:16 wt% acetone).
Suspensions with various concentrations of F-TiO2 nanoparticles obtained by vigorous
stirring in DMF overnight were added to the a completely dissolved and clear polymer
solutions. The resulting dope solutions were stirred at 50°C for one day and were then
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gently agitated at room temperature (approximately 25°C) for 4 h prior to electrospinning.
The specific components used for the dope solutions are listed in Table 1.
Table 1. Contents of dope solutions used for different fabrication methods.
2.3 Operating conditions for the electrospinning
The electrospinning operating conditions for each membrane are summarized in Table 2.
The schematics of the different membrane fabrication processes by nozzle type (single,
coaxial, and dual) have been illustrated in Figure 2. An electrospinning machine
(ESR200R2D, NanoNC, Korea) equipped with a high-voltage generator (maximum
voltage of 30 kV), a dual syringe pump, and a drum collector was utilized to fabricate the
electrospun membranes. While the procedure of electrospinning was routine like our
previous study [32], two types of dope solution were used simultaneously in core and
shell of coaxial nozzle or nozzle 1 and nozzle 2 of dual-nozzle, respectively.
Table 2. Electrospinning operating conditions.
Figure 2. Membrane fabrication using (a) a single nozzle, (b) a coaxial nozzle, and (c) a
dual nozzle.
2.4 Membrane characterization
Fourier transform infrared spectroscopy (FTIR) spectra of the FTES, TiO2, and FTES-
functionalized TiO2 samples were acquired using an IRAffinity-1, Shimadzu. KBr pellets
were used as the solid sample in the FTIR measurements. The IR spectra of the samples
were obtained over 16 scans with 2 cm-1 of resolution in the range from 400 to 4000 cm-1.
X-ray photoelectron spectroscopy (XPS, PHI-5802, Physical Electronics) was used to
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analyze the chemical composition of FTES-functionalized TiO2 particles and prepared
membranes.
Pore size distributions (PSD) and liquid entry pressure (LEP) values were measured
using a capillary flow porometer (POROLUXTM1000, Germany). A membrane sample
containing a wetting agent (POROFIL, surface tension 16 mN/m) and a dry membrane
sample were placed in a sealed chamber and pressurized with nitrogen gas until all pores
were opened. Then, the pore size values of the two samples were determined by a bubble
point method for comparison. To measure the LEP, the membrane surface was
completely covered with distilled water after being placed in the sealed chamber. The
pressure was increased at a rate of 0.16 kPa/s until water could be transported through the
largest membrane pores, and the LEP value was determined by identifying the highest
pressure immediately before water penetration.
Contact angles were measured by a geometrical method (sessile drop) using an
EASYDROP contact angle measuring system (Kruss, Germany). A 5-µL droplet of DI
water was dropped on the membrane. A camera connected to a computer captured the
image of the water drop against a background with low illumination, and DSA1 software
was used to analyze the obtained image and calculate the corresponding contact angle
value.
To measure membrane porosity, membrane samples (3 cm × 3 cm) were thoroughly
wetted with ethanol for 30 min, and their weights were measured after removing ethanol
from the membrane. Porosity was defined as the ratio of the ethanol volume to the total
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volume, which was calculated using the material densities of ethanol and the polymer
(gravimetric method).
The mechanical strength was evaluated by measuring the tensile strength using a
materials testing machine (LS1, LloydAmetek) with a 1-kN load cell. The sample size
was 1 cm × 3 cm, and the measurements were conducted at an elongation rate of 5
mm/min at room temperature.
A field emission scanning electron microscope (FE-SEM) was used to investigate the
morphology of the membranes and to measure the fiber diameters. Gold-coated
membranes obtained by sputtering were analyzed with a FEI Quanta FEG 450 instrument
at an accelerating voltage of 20 kV. Surface roughness analysis of the membranes was
carried out using atom force microscopy (AFM, Wyko NT9300, Vecco, USA) with a
tapping mode. Transmission electron microscopy (TEM) image was obtained using
Philips CM20 (Philips, Netherlands) with an acceleration voltage of 200 kV to observe
position of TiO2 particles on the nanofibers.
2.5 Direct contact membrane distillation (DCMD) set-up
Membrane performance was evaluated using a lab-scale DCMD system for forty hours or
seven days (Figure 3). A synthetic 7.0-wt% NaCl feed solution (60°C) with a
conductivity of 115 mS/cm was circulated across the membranes (effective area: 9.8 cm 2)
at a flow rate of 450 mL/min. Permeate water (20°C) with an initial conductivity of less
than 2 µS/cm was circulated at a similar flow rate in the opposite direction. The
temperature of each solution was maintained with a hotplate/stirrer and a heat exchanger
equipped with a chiller. All tubes were covered with insulation to minimize heat loss. A
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triangular flask with the permeate solution was placed on an electronic balance connected
to a computer, and the resulting permeability was calculated from the weight changes of
the permeate container. The permeate water quality was monitored using a conductivity
meter submerged in the permeate solution.
Figure 3. Schematic diagram of the DCMD process: 1 Permeate tank, 2 Digital balance
connected to a computer, 3 Thermometers, 4 Cooling unit, 5 Pumps, 6 Flat sheet
membrane module, 7 Flow meters, 8 Feed reservoir, and 9 Hotplate.
3. Results and Discussion
3.1 Modification of TiO2 nanoparticles
The hydrophobicity of the membrane is one of the most critical factors in determining its
anti-wetting characteristic, fouling, and thus, efficiency. The introduction of hydrophilic
particles into the polymer matrix can readily cause a decrease in the hydrophobicity of the
membrane and facilitate its pore wetting. Therefore, it is of utmost significance to modify
the surface of the nanoparticles to diminish their hydrophilic behavior. In this regard, the
surface of the TiO2 nanoparticles was functionalized by fluorosilane moieties in order to
decrease the surface energy of the particles and thus increase their hydrophobicity. The
schematic representation of this functionalization has been depicted in Figure 1.
FTIR analysis showed the modification of the TiO2 particles by the FTES (see Figure 4).
The two convoluted peaks at 500 and 690 cm-1, represent different vibration features of the Ti-O
and TiO-O-O bonds in the TiO2 lattice [33]. Also, the very weak absorption bands, observed at
around 3400 cm-1 and 1620 cm-1, are related to the water molecules unavoidably physisorbed to
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the very hydrophilic TiO2 surface [34]. The TiO2 absorption bands between 400 and 800 cm-1 can
be attributed to the TiO and Ti-O-Ti bonds [35,36]. The FTIR spectrum of the pure FTES, on
the other hand, showed several bands at 890, 1113, 1143, 1191, and 1236 cm -1, associated with
the ν(CF2) and ν(CF3) bands, respectively [37]. F-TiO2 entailed the features of both TiO2 and
FTES, indicating that the functionalization process has been well-accomplished. The bands at
1113, 1143, 1191, and 1236 cm-1, with a lower intensity than the pure FTES, validated the
existence of distinct fluoro-moieties on the TiO2, which was in line with our hypothesis. In
addition, other small peaks of 1056 and 1210 cm-1 referring to Si-O-Si [38,39] were observed as
an evidence of well-coated TiO2 particles with FTES. The peak at 890 cm-1 resulting from the
ν(CF3) vibrations could not be verified in the F-TiO2 due to the TiOTi band overlapping.
The surface chemical composition of modified TiO2 particles was further investigated by XPS
to clarify the degree of surface modification. As shown in Fig. S1, functionalized TiO2 particles
showed two strong peaks of F1s and C1s as well as weak peak of Si2p, which was different from
pristine TiO2 particles. The presence of the strong F1s and CF2 peaks confirmed that salinized
TiO2 particles were hydrophobic. In the high resolution spectra of O1s and Si2p, two peaks of
Si-O-Ti and Si-O-Si indicated successful surface salinization of TiO2 particles.
Figure 4. FTIR transmittance spectra of (a) FTES, (b) FTES-functionalized TiO2 nanoparticles,
and (c) the pure TiO2 nanoparticles.
3.2 Morphology of electrospun membranes
The morphology of the membranes can provide an insight into the nanofiber stacking and
the effect of introducing nanoparticles to the fiber structure as well as pore network.
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Figure 5 presents the SEM surface and cross-sectional micrograph of the electrospun
membranes with and without F-TiO2 nanoparticle incorporation. In the absence of any
nanoparticles (Figure 5(a)), the electrospun membrane exhibits a randomly-organized
nanofiberous structure with relatively smooth surfaces that is distinguishably porous. As
increased surface roughness means less contact area for the solid fiber and water,
embedding nanoparticles on the fiber surface may lead to higher hydrophobicity of the
membrane [40]. The introduction of nanoparticles into the membrane network resulted in
the aggregation of the F-TiO2 particles with different intensities on the fiber surfaces.
Prolonged sonication and vigorous stirring did not prevent the formation of F-TiO2
clusters in the dope solution, and the F-TiO2 particle clusters formed on the membranes
were relatively larger than the original TiO2 particles (21 nm). Such particles were not
observed in the interior of fibers but only on their surfaces, which could be confirmed
TEM images of Figure 5(b) and cross-sectional images of fibers of Figure 5(e).
The morphological structure of fibers may be improved by a larger amount of
nanoparticles, but this may cause the membrane to become brittle due to the severe degree
of nanoparticle agglomerations. In this study, coaxial and dual nozzle techniques were
adopted to retain the mechanical strength of the membranes by increasing the loading of
F-TiO2 particles. The larger size of the F-TiO2 clusters in the coaxial electrospun
membrane compared to those in the single nozzle membrane can be attributed to the use
of the double concentration of F-TiO2 in the former. Due to the unique core-shell
structure, the numerous aggregates were well placed on the surface of fibers, rather than
integrating with the fiber as shown in Figure 5(f). Furthermore, intense F-TiO2
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aggregation or fiber thinning, which diminishes the membrane’s mechanical strength, was
not observed. When the same concentration of F-TiO2 was employed to a single nozzle,
the tensile strength decreased to under half of that of the coaxial nozzle due to the thin
and brittle fibers created, shown in Table S1 and Fig. S2. The TEM image of M3 shown
in Figure 5(c) illustrates the successful fabrication of the F-TiO2-covered nanofibers. The
core and shell solutions in membrane fabrication using the coaxial nozzle can be
potentially miscible when the same kind of polymer is used [41]. However, the
evaporation of the solvent takes place in a very short period of time, so the F-TiO2
nanoparticles do not have enough time to diffuse into the core before the core fiber
formation and thus, allowing the nanofibers to protrude.
The electrospun membrane fabricated using the dual nozzle showed a slightly different
structure. As shown in Figure 5(d), beside the nanofibers with similar diameters as those in M1,
bead-on-string nanofibers with much smaller diameters and higher F-TiO2 aggregate loading was
found. Typically, large diameter nanofibers embark higher mechanical strength to the membrane
and highly protruded nanofibers with small diameters lead to enhanced hydrophobicity which are
unfavorable to membrane durability. However, the composite structure created as shown in
Figure 5(d) formed a more hydrophobic membrane without sacrificing its mechanical properties,
as relatively thick fibers intertwined with the finer fibers, rendering the desired mechanical
strength.
The water contact angle of hydrophobic surface increases with surface roughness. In this
study, the deposition of TiO2 nanoparticles on the PVDF-HPF nanofibers increases the
membrane surface roughness as shown in the AFM image (Fig S3.). It was observed that the
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addition of TiO2 particles increased the surface roughness from 1.35 (M1) to 3.91 μm (M4). The
significant increase of surface roughness for dual nozzle (M4) could be attributed to bead-on-
string nanofibers. The colors corresponding to the surface height were visualized by an order of
red > green > blue.
Figure 5. SEM surface and cross-sectional images of (a, e) a single-nozzle membrane
without F-TiO2 (M1), (b, f) a single-nozzle membrane containing 10 wt% F-TiO2 (M2),
(c, g) a coaxial nozzle membrane containing 20 wt% F-TiO2 in the shell (including a TEM
image) (M3), and (d, h) a dual-nozzle membrane containing 50 wt% F-TiO2 (M4)
3.3 Contact angle measurements
Figure 6 presents the contact angle values calculated using the Wenzel equation [42], which
takes into account the surface roughness and the penetration of the liquid into the grooves.
Compared to M1, which had a contact angle of 143.5° , M2 and M3 showed higher contact angles
(149.0° and 149.8°, respectively) although the values were still lower than 150°
(superhydrophobic surfaces). However, the membrane fabricated by the dual nozzle method with
the beads-on-string structure exhibited superhydrophobicity with a contact angle of 153.4°. The
presence of beads on the fibers contributes to higher water contact angle to make the bead-on-
string structure a favorable one for increasing the hydrophobicity [43,44]. In addition, because
the beads were formed by agglomerated hydrophobic nanoparticles, the surface of the beads also
had many small hills and valleys, contributing to the increased hydrophobicity of the membrane
network. Moreover, Fig. S4 reveals that the TiO2 particles embedded membranes showed slightly
higher peak of F1s with two new peaks of Ti2p and Si2p attributable to fluorosilane coated TiO2
particles. In high resolution XPS spectra of Ti2p, two peaks of 458.9 and 464.6 eV assigned to Ti
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2p3/2 and 2p1/2 [45] were observed in all membranes containing TiO2 particles, however, the dual
nozzle possesses the strongest peaks. Furthermore, in the high resolution XPS spectra of O1s and
Si2p that can be divided into three peaks of Ti-O-Ti (530.2 eV), Ti-O-Si (531.9 eV) and Si-O-Si
(532.9 eV) [46] or two peaks of Ti-O-Si (102.1 eV) and Si-O-Si (103.6 eV) [47], the dual nozzle
possesses the strongest peaks due to high concentration of TiO2 particles and also the Si-O-Si
linkage was more dominant than Ti-O-Si for dual nozzle as compared to single and coaxial
nozzles. As aforementioned, both surface morphology and chemical composition greatly
enhanced the membrane hydrophobicity.
Figure 6. Contact angles and droplet images of the neat, single-nozzle, coaxial-nozzle and dual-
nozzle electrospun membranes.
Table 3. Characteristics of the F-TiO2-containing PH electrospun membranes.
3.4 Pore size distribution (PSD) and porosity
The mean pore size and PSD are crucial parameters for the water vapor flux performance
of MD. The degree of the polydispersity (width of the PSD) strongly affects the vapor
flux and the rejection performance in MD, where better MD performance is achieved
when PSD is narrow [48]. An optimum pore size is necessary for a balance between
permeate flux and pore wetting resistance. Large pore sizes allow a higher permeate flux;
however, smaller pore sizes are also needed to avoid pore wetting and achieve high
rejection. As shown in Figure 7, the PSD did not change significantly with the addition
of F-TiO2 nanoparticles when using single or coaxial nozzles. M4 however, showed much
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larger pore sizes and wider PSD. It is believed that the larger F-TiO2 particle aggregations
in the fibers prevented the formation of interconnected fiber structures. This
polydispersity is in line with the explicitly different fiber sizes in the M4 SEM
micrograph (see Figure 5(d, h)). The maximum pore size of M4 was larger than M1, M2,
and M3, but its mean pore size showed only a little difference because the fine fibers
could partially fill the space formed by the thick fibers or beads.
Porosity, the void volume fraction of the membrane, is another factor that affects the MD flux
performance. High porosity translates into a greater evaporation surface area or more pore
channels for diffusion, increases the heat transfer coefficient, and reduces heat transfer
conduction, thus leading to a higher permeate flux [49]. Incorporation of the F-TiO2 slightly
enhanced the porosity of the M2 and M3 membranes from 88.6% (M1) to 91.6% and 90.4%,
respectively (Table 3). However, the increase in the porosity was less pronounced in M4. The
slightly higher porosities of M2 and M3 are most likely caused by small spaces generated
between the nanofibers by the F-TiO2 particles embedded on their surfaces. In the case of M4,
the effect of F-TiO2 particles was somewhat offset by the two differently sized fibers.
Figure 7. Pore-size distributions of membranes fabricated with (a) a single nozzle without TiO2
(M1), (b) a single nozzle with TiO2 (M2), (c) a coaxial nozzle (M3), and (d) a dual nozzle (M4).
3.5 Liquid entry pressure
LEP is a measure of pore wetting in hydrophobic membranes and is mainly affected by
the hydrophobicity of the membrane, and its pore size and shape. A material with low
surface energy (i.e., a hydrophobic material) and small maximum pore size yields high
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LEP values that are appropriate for MD applications [50]. It should be noted that if the
pore size is too small, the permeability of the membrane is negatively affected. As
presented in Table 3, M2 and M3 exhibited higher LEP values probably owing to the
higher hydrophobicity conferred to the membranes as a result of the F-TiO2 particle
embedding on the surface of the nanofibers. Since the pore sizes and PSDs of these two
membranes were close to those of M1, it is believed that the addition of F-TiO 2 particles
significantly increased the water-repelling properties of the membranes. On the other
hand, while the dual-nozzle membrane resulted in the highest contact angles, its LEP
value was lower compared to those of the single or coaxial-nozzle membranes containing
F-TiO2 particles. This can be ascribed to the larger pore size of the M4 membrane which
facilitates the pore-wetting phenomenon. However, the difference in the LEP values
between the dual-nozzle membrane and the other membranes with F-TiO2 was
comparatively small considering the difference in their pore sizes. Moreover, the LEP of
M4 was higher than that of the membrane without any F-TiO2 incorporation (M1)
although it had a larger maximum pore size than M1. Thus, it can be concluded that
intense hydrophobicity of the membrane affects the LEP to a higher extent than the pore
size, which explains the improvement in the LEP values with TiO2 incorporation.
3.6 Nanofiber diameters and mechanical strength
In electrospinning, fiber diameters are generally determined by the viscosity of the
solution and the molecular weight of the polymer used [51]. Crespy et al. reported that
fiber diameters increased with the addition of clay nanoparticles to the dope solution
which they related to the increase in the viscosity of the solution by nanoclay
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incorporation [52]. However, both M2 and M3 showed a slight decrease in nanofiber
diameters compared to M1. This contradictory result can be attributed to the fact that
inorganic nanoparticles (such as TiO2 particles) possess higher electronic density than
polymer templates [53] which can induce crystalline nanofibers [54]. As shown in Figure
8, M4 had both fibers with similar diameters as those of M1 as well as a considerable
number of fibers with diameters less than 130 nm.
Furthermore, when F-TiO2 particles were incorporated into the electrospun
membranes, there was no significant change in tensile strength compared to the
electrospun membranes without F-TiO2. However, as will be shown in Sec. 3.7, this did
not affect the DCMD performance; there was no leakage to the permeate side across the
membrane during the two days of DCMD operation with high-salt feed water.
Figure 8. Fiber diameter distributions of (a) a single nozzle without TiO2 (M1), (b) a single
nozzle with TiO2 (M2), (c) a coaxial nozzle (M3), and (d) a dual nozzle (M4).
3.7 DCMD performance
The performances of the fabricated membranes were investigated and compared with that
of a commercial PVDF membrane with a pore size of 0.45 μm. The fluxes of the
electrospun membranes (M1, M2, M3, and M4), as illustrated in Figure 9(a), were better
than that of the commercial PVDF membrane because of the unique features of the
electrospun membranes, such as high porosity and interconnected pore structure. In case
of the commercial membrane (Figure S2), after an initial flux of 25 Lm-2h-1, a significant
decline in permeate flux was monitored due to the high salinity of the feed (i.e. 7.0 %
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NaCl content) and pore blockage, while M1~4 membrane had an initial flux of 40 Lm-2h-1.
Also, permeate water rejection in the commercial membrane deteriorated due to
membrane wetting, whereas the electrospun membranes exhibited enhanced anti-wetting
properties due to their higher hydrophobicity.
Among the electrospun membranes, M1 and M4 started at a slightly higher initial flux
because of M1’s relatively lower thickness and M4’s larger pore sizes. The low
conductivity of the permeate was maintained in M4 despite its larger pore size because
the beads-on-string structure formed by F-TiO2 aggregation improved the water-repellent
properties of not only the membrane surface but also the interior of the membrane.
Similarly, in previous studies, superhydrophobic electrospun membranes with a large
mean pore size of 2.7 μm and 1.15 μm presented a high water vapor flux of 85 Lm -2h-1
and 51 Lm-2h-1 without wetting at feed water of 80°C and 50°C, respectively [55,56].
Moreover, although the presence of beads on string can weaken the mechanical properties
of the membrane, its mechanical strength was not highly altered due to the presence of
appropriately-sized fibers with no beads. M2 and M3 with contact angles of 149.0 ± 2.8°
and 149.8 ± 1.1°, respectively, showed stable performance in terms of permeability and
salt rejection, because hydrophobic F-TiO2 particles covered the fibers. At low flux, M3
showed a slight decrease in the flux, which is likely due to its slightly lower porosity
compared to M2 and which can be attributed to its comparatively large maximum pore
size (0.785 μm). Large pores can be blocked by surface or partial wetting so pore size
should be small enough to avoid membrane wetting but not too small to conflict with
required MD permeability [57]. Because the PSD of the membranes did not differ
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significantly with the addition of F-TiO2 (Figure 5), the better MD performances of M2
and M3 compared to M1 indicate that the F-TiO2 reduced the risk of pore wettability of
the membrane.
Among F-TiO2 nanocomposite membranes, dual-nozzle (M4) exhibited most stable
flux and the lowest conductivity (rejection higher than 99.99%) for seven days (Figure
9b). To compare our study with the existing literature, Table 4 summarize the
performance of MD and properties of nanocomposite electrospun membranes. Evidently,
a significant improvement in the water flux was witnessed in this present study. Although
Li et al., reported the highest flux of 41.1 kg/m2/h [61], however, their operation time was
only 24 hours and the membrane contained higher concentration of nanoparticles
compared to M4.
Overall, it is possible to achieve prolonged superior DCMD operation using the F-TiO2-doped
electrospun membranes. In particular, the hydrophobic layer created by the integration of beads-
on-string fibers preserved the performance of the membrane by preventing it from wetting.
Figure 9. Patterns of the permeate flux and conductivity in DCMD of (a) Neat and TiO2
nanocomposite membrane and (b) dual nozzle for about seven days. Feed water was 7.0 wt%
NaCl solution (conductivity = 115 mS/cm). Feed and permeate temperatures were 60°C and
20°C, respectively. Feed and permeate flow rates were both maintained at 450 mL/min.
Table 4. Properties and DCMD performance using nanocomposite electrospun membrane with
adding it into dope solution for electrospinning. Feed/permeate temperature: 60/20°C. 3.5 wt%
NaCl feed solutions except for our study (7.0 wt%)
4. Conclusions22
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Nanocomposites are versatile materials that enhance the performance of MD membranes
fabricated by electrospinning. In this study, F-TiO2 nanoparticles were incorporated into
electrospun membranes fabricated with different nozzles (including single, coaxial, and
dual nozzles). This study showed a simple and effective functionalization method to
electrospun membranes for MD. The addition of F-TiO2 particles to the dope solution
increased the contact angle of the membranes produced by using different nozzles (i.e.,
single and coaxial nozzles to 149.0° and 149.8°, respectively). Very high F-TiO2
concentrations (20 wt%) hindered the formation of high-quality nanofibers, whereas
coaxial nozzles enabled the incorporation of higher F-TiO2 concentrations in the
nanofibers. However, hydrophobicity was only slightly improved comparing to single
nozzle owing to intense agglomeration of F-TiO2 particles. The combination of beads-on-
a-string fibers and well-formed fibers by dual nozzles, which showed
superhydrophobicity, was more effective in preventing wetting of the membranes, even
though the pore size was larger than those of the other membranes. The dual nozzle
electrospun membranes’s mechanical properties were also sufficiently durable to maintain
membrane integrity during MD. Moreover, F-TiO2 particles deposited on the nanofiber
surfaces resulted in a higher membrane porosity compared to the neat electrospun
membranes without F-TiO2 particles. At the same time, using a dual nozzle to fabricate
differently sized fibers reduced the membrane porosity because fine fibers could fill the
space formed by thick fibers. Addition of fluorosilane coated hydrophobic nanoparticles
to the dope solution for electrospinning led to greater hydrophobicity of not only
membrane surface but also the inner region of the membrane. The dual-nozzle method
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was effective in increasing the amount of particles without brittleness of the membranes.
Among F-TiO2 embedded electrospun membranes, all of which showed better MD
performance than the pure electrospun membrane, the dual nozzle fabricated membrane
showed the highest and most stable flux due to its large pore size as well thorough
hydrophobicity. Therefore, this study suggests that F-TiO2 embedded electrospun
membranes has great potential for seawater desalination and RO brine treatment
applications.
Acknowledgement
This study receives fanatical support from City University of Hong Kong under its
Strategic Research Grant (SRG-fd) (Grant No. 7004521) and the Research Grants Council of
Hong Kong for Early Career Scheme (Project number: 9048074). We acknowledge the help on TEM
and XPS analysis by Prof Robert K Y Li at the department of physics and materials science
and AFM by Prof. Juankai Wang at the department of mechanical and biomedical
engineering, the City University of Hong Kong
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Figure 1. Schematic representation of TiO2 functionalization using hydroxylated fluorosilane
33
(a) (b) (c)
655
656
657
658
659
660
661
662
663
664
665
666
667
668669670671
Figure 2. Membrane fabrication using (a) a single nozzle, (b) a coaxial nozzle, and (c) a dual nozzle.
Figure 3. Schematic diagram of the DCMD process: 1 Permeate tank, 2 Digital balance connected to a computer, 3 Thermometers, 4 Cooling unit, 5 Pumps, 6 Flat sheet membrane module, 7 Flow meters, 8 Feed reservoir, and 9 Hotplate.
34
672673674675676
677678679680681
682
Figure 4. FTIR transmittance spectra of (a) FTES, (b) FTES-functionalized TiO2 nanoparticles, and (c) the pure TiO2 nanoparticles.
Figure 5. SEM surface and cross-sectional images of (a) a single-nozzle membrane without F-TiO2 (M1), (b, e) a single-nozzle membrane containing 10 wt% F-TiO2 (M2) (including a TEM image), (c, f) a coaxial nozzle membrane containing 20 wt% F-TiO2 in the shell (including a TEM image) (M3), and (d) a dual-nozzle membrane containing 50 wt% F-TiO2 (M4)
35
683684685
686
687
688
689690
691692693694695
Figure 6. Contact angles and droplet images of the neat, single-nozzle, coaxial-nozzle and dual-nozzle electrospun membranes.
Figure 7. Pore-size distributions of membranes fabricated with (a) a single nozzle without TiO2
(M1), (b) a single nozzle with TiO2 (M2), (c) a coaxial nozzle (M3), and (d) a dual nozzle (M4).
36
696
697698699
700
701702703
704
Figure 8. Fiber diameter distributions of (a) a single nozzle without TiO2 (M1), (b) a single nozzle with TiO2 (M2), (c) a coaxial nozzle (M3), and (d) a dual nozzle (M4).
Figure 9. Patterns of the permeate flux and conductivity in DCMD of (a) Neat and TiO2
nanocomposite membrane and (b) dual nozzle for about seven days. Feed water was 7.0 wt% NaCl solution (conductivity = 115 mS/cm). Feed and permeate temperatures were 60°C and 20°C, respectively. Feed and permeate flow rates were both maintained at 450 mL/min.
37
(a) (b)
(c) (d)
705
706707708
709
710711712713714
715
Table 1. Contents of dope solutions used for different fabrication methods.
Electrospinning nozzlePVDF-HFP
(wt%)DMF (wt
%)Acetone (wt%)
TiO2 (wt%)*
M1 Single without TiO2 20 64 16
M2 Single with TiO2 20 80 10
M3 CoaxialCore 20 80
Shell 20 80 20
M4 DualNozzle 1 15 85 50Nozzle 2 20 64 16
*Concentration of TiO2 was based on weight of PVDF-HFP.
Table 2. Electrospinning operating conditions.
Nozzle Single Coaxial DualVoltage (kV) 18 14 18
Flow rate (mL/h) 1.00.4 (core)0.5 (shell)
0.8 (PVDF-HFP 15 with TiO2)1.0 (PVDF-HFP 20)
Tip-to-collector distance (cm)
15 15 15
Drum collector speed (rpm) 700Temperature 25°CHumidity 55%
Table 3. Characteristics of the F-TiO2-containing PH electrospun membranes.
Classification M1 M2 M3 M4Mean pore size (μm) 0.61 ± 0.02 0.61 ± 0.01 0.60 ± 0.02 0.76 ± 0.05Maximum pore size (μm) 0.73 ± 0.02 0.73 ± 0.06 0.78 ± 0.05 1.14 ± 0.07Thickness (μm) 87 ± 4 100 ± 8 99 ± 6 99 ± 8Porosity (%) 88.6 ± 0.9 91.6 ± 1.4 90.4 ± 2.1 89.8 ± 1.9LEP (kPa) 65.8 ± 3.2 96.3 ± 1.7 95.1 ± 0.5 90.5 ± 3.6Fiber diameter (nm) 368 ± 78 296 ± 74 300 ± 91 231 ± 139
38
716
717
718
719
720
721
Tensile strength (MPa) 9.1 ± 0.5 7.0 ± 0.2 6.5 ± 0.3 6.0 ± 0.2
39
722
723
Table 4. Properties and DCMD performance using nanocomposite electrospun membrane with adding it into dope solution for electrospinning. Feed/permeate temperature: 60/20°C. 3.5 wt% NaCl feed solution`ns except for our study (7.0 wt%)
Material
MeanPore siz
μm¿
Porosity(%
Thickness(μm¿
Contact angle (°)
Final flux (kg/m2/h)
Operation time (h)
Sal
rejection(%
a67%Clay-PVDF [54]
0.6 81. 300
154.2
5.5
8 99.97
b10%nanocrystalline cellulose-PVDF-HFP
[60]0.3 78. 2
10
132.2
11.5
3.5
99
c80%PTFE power-PVDF [4]
0.4 69. 95
152.2
18.5
15
99.99
222%SiO2-PVDF [61]
0.6 79. 100
152.3
41.1
24
99.99
c27%SiO2-PVDF [62]
0.2 79. 98
151.9
32.5
15
99.99
50%TiO2 -PVDF-HFP
(this study)
0.7 89. 99
153.4
37.6
167
99.99
a Feed/permeate temperature: 80/17°C40
724725726
727
b Feed/permeate temperature: 60/40°C. 1wt% NaCl feed solution.c Vacuum membrane distillation (9kPa)* Concentration of particles was based on weight of polymer.
41
728729730
731
732
42
(a) (b)
(c) (d)
(e) (f)
(g)
733
734
735
736
Figure S4. XPS spectra of (a) a single nozzle without TiO2 (M1), (b) a single nozzle with TiO2 (M2), (c) a coaxial nozzle (M3), and (d) a dual nozzle (M4). High resolution XPS spectra of (e) O1s, (f) Si2p, and (g) Ti2p
Figure S5. Patterns of the permeate flux and conductivity in DCMD using commercial PVDF
membrane (0.45 μm). Feed water was 7.0 wt% NaCl solution (conductivity = 115 mS/cm). Feed
and permeate temperatures were 60°C and 20°C, respectively. Feed and permeate flow rates
were both maintained at 450 mL/min.
43
737738739
740
741
742
743
744
745
746
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