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Journal of
Materials Chemistry A
Materials for energy and sustainability
Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: J. Wu, K. R.
Zodrow, P. B. Szemraj and Q. Li, J. Mater. Chem. A, 2017, DOI: 10.1039/C7TA04555G.
Volume 4 Number 1 7 January 2016 Pages 1–330
Journal of
Materials Chemistry A
Materials for energy and sustainability
www.rsc.org/MaterialsA
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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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Photothermal Nanocomposite Membranes for Direct Solar
Membrane Distillation
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Jinjian Wua, Katherine R. Zodrowa, Peter B. Szemraja, and Qilin Lia,b
Efficient utilization of renewable energy for desalination is critical to solving the world’s water scarcity issues. In this
manuscript, we report a novel direct solar membrane distillation (MD) process that utilizes a photothermal nanoparticle
coating to capture sunlight and convert it to heat at the membrane surface, providing the thermal driving force for the MD
process. This approach overcomes the major limitation of temperature polariztion encountered in conventional MD
processes and significantly increases the energy efficiency of MD. Membranes coated with carbon black nanoparticles or
SiO2/Au nanoshells showed up to a 33.0% increase in distillate flux when irrdiated with simulated sunlight at 1 sun unit in a
bench scale direct contact membrane distillation system, while the salt rejection and liquid entry pressure remained
unchanged. These results demonstrate the potential of photothermal membranes for direct solar MD.
 = (1 − 2 )
1 1. Introduction
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The demand for alternative water resources—such as
wastewater, seawater, and brackish groundwater—is
increasing as the global population grows and water
demand rises. These water sources require advanced water
treatment processes, such as reverse osmosis or thermal
desalination technologies to remove salts and other
dissolved contaminants. Although these technologies are
increasingly applied in treatment of alternative water
sources, they require a large amount of energy. Due to the
inferior water and energy infrastructure of the underdeveloped areas, the capability to utilize renewable energy
for water treatment is integral. It is also beneficial in
developed economies to reduce conflicts at the waterenergy nexus. Membrane distillation (MD), a desalination
and water purification technology that combines
membrane and thermal processes, has been identified as a
promising approach for solar powered water purification.1,
2
MD uses a porous hydrophobic membrane to separate
a hot feed from a cold distillate, and the partial vapor
pressure difference between each side of the membrane
drives membrane vapor flux, generating pure water upon
condensation.3 Currently the most commonly-used
membrane materials are inert hydrophobic polymers
include
poly(vinylidenefluoride)
(PVDF),
poly(tetrafluoroethylene) (PTFE), and poly(propylene)
(PP).4 Vapor flux generation can be described by the
following equation,
a. Department
of Civil and Environmental Engineering, Rice University, 6100 Main St
MS 519, Houston TX 77005
b. Corresponding author. Email: Qilin.Li@rice.edu
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(1)
Here, J is the vapor flux (m∙s-1), k is the water vapor
permeability of the membrane (m∙s-1∙Pa-1), and P1 and P2
are the vapor pressure (Pa) on at the water-air interface on
the feed and permeate side of the membrane, respectively.
MD has several advantages over other membrane and
thermal desalination processes, including reduced electrical
energy use, less requirement for membrane mechanical
properties, and low fouling propensity due to the low
pressure operation, the capability to treat feed water of a
wide range of salinity, and the ability to utilize low grade
heat. 2-5
Current solar MD designs have several limitations that
lower the energy efficiency. First, they use an external solarthermal collector to heat the feed water. Significant heat
loss occurs during water transfer from the heating unit to
the MD module.6
Furthermore,
the
inherent
phenomenon
of
temperature polarization (TP) that results from latent and
conductive heat transfer across the membrane significantly
limits the thermal efficiency of MD. Due to TP, the
temperature at the membrane surface on the feed side (T1)
may be significantly lower than the temperature of the bulk
feed water (TF), and the temperature at the membrane
surface on the distillate side (T2) may be significantly higher
than the bulk distillate (TD) (Figure 1a). This decreases the
cross-membrane vapor pressure difference and hence the
membrane flux J (Eq. 1). In some cases, the temperature
polarization coefficient, αTP, defined in Eq. 2, can be as low
as 0.3, indicating a 70% reduction in effective driving force.7
1 − 2
  =
(2)
 − 
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Journal of Materials Chemistry A Accepted Manuscript
DOI: 10.1039/C7TA04555G
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Figure 1 Schematic of (a) conventional DCMD and (b) novel photothermal
DCMD with a nanomaterial coating and localized heating at the membrane
surface.
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Finally, the low water recovery leads to large heat loss
through brine discharge. The single pass water recovery in
solar MD is typically below 5%. Thus, 95% of the heat in the
feed water is lost.8 Although brine recirculation and other
heat recovery measures can reduce such loss, they add to
capital cost, system complexity and electricity
consumption.
The energy efficiency of the thermal evaporation
process can be greatly improved by localized heating at the
liquid-vapor interface. The localized heating focuses the
energy at the boundary where evaporation occurs and
wastes little energy heating the bulk water. Several studies
have demonstrated the effectiveness of localized heating
using a photothermal film sitting atop a still bulk liquid.
Ghasemi et al. designed a double layer structure (DLS)
consisting of a thermally-insulating carbon foam supporting
a photothermal exfoliated graphite layer.9 Under 1 kW∙m-2
solar irradiation from a solar simulator, the DLS evaporated
water at a rate of 0.28 g∙m-2∙s-1 with a photothermal
efficiency of approximately 53%.9 Jiang et al. fabricated a
bilayer hybrid biofoam composed of bacterial nanocellulose
(BNC) and reduced graphene oxide (RGO) and examined
vapor generation under 10 kW∙m-2 solar beam.10 The
evaporation rate was 1.95 g∙m-2∙s-1 higher than the control
without the bilayer, and the calculated photothermal
efficiency was 83%.10 Zhang et al. coated a stainless steel
mesh with polymeric photothermal material polypyrole
(PPy), and then modified the coating with fluoroalkylsilane
to achieve hydrophobicity.11 A PPy thickness of 2.6 μm
achieved water evaporation rate of 0.25 g∙m-2∙s-1 under 1
kW∙m-2 simulated solar light irradiation and a photothermal
efficiency of 58%.11 In another study, Zhang et al. fabricated
vertically aligned graphene oxide sheets membrane (VAGSM) as solarthermal converter.12 Under 1 and 4 sun
illuminations, the VA-GSM achieved a water evaporation
rate of 0.45 and 1.74 g∙m-2∙s-1 and a photothermal
efficiency of 84.5% and 94.2%, respectively.12 These
studies, however, did not consider the condensation
process necessary to produce clean water.
Few studies have implemented photothermal
membranes in MD applications. The ideal MD membrane
needs to be flexible, low weight, low thickness, and
hydrophobic. In our previous proof-of-concept study, we
exploited the excellent photothermal and localized heating
property of SiO2/Au nanoshells (NSs) and carbon black
nanoparticles (CB NP) in MD by immobilizing them on
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commercial MD membranes.13 A recent
View Articlestudy
Online
DOI: 10.1039/C7TA04555G
demonstrated this process, termed nanophotonics-enabled
solar membrane distillation (NESMD), under natural
sunlight using a bilayer membrane structure consisting of a
25-µm electrospun photothermal nanocomposite coating
(surface oxidized CB NPs-embedded in polyvinyl alcohol
(PVA) fibers) on top of a commercial PVDF membrane.14 A
small-scale NESMD module (8.1 cm × 3.48 cm) had a flux of
0.22 kg∙m-2∙h-1 (0.06 g∙m-2∙s-1) and a solar efficiency of >20%;
and a modelled larger scale module (100 cm × 10 cm) under
normal solar intensity and 40 ˚C ambient temperature had
efficiency of 53.8%.14 Another study reported the use of a
silver nanoparticle embedded PVDF membrane with high
intensity UV irradiation (23.2 × 103 kW∙m-2), which
increased the water flux of a vacuum MD process by 11fold.15
In this paper, we report facile, scalable fabrication
methods for preparing photothermal MD membranes using
SiO2/Au NSs and unfunctionalized CB NPs, and the
performance of the membranes in a bench-scale NESMD
system operated in the direct contact mode. Using
simulated sun light at 1 sun unit, the photothermal
membranes showed significant flux increases due to the
photothermal activity of the SiO2/Au NSs and CB NPs. These
membranes and the NESMD process provide a potential
solution to provide safe water using alternative water
sources at locations that lack reliable water and power
infrastructures.
75 2. Experimental
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2.1. Membrane and photothermal nanomaterials.
A hydrophobic PVDF membrane (0.22 μm nominal pore
size, 100 μm thickness, Pall Corporation, NY) was used as
the base membrane for modification and served as the
control membrane to represent current MD technologies.
The photothermal nanomaterials used were commercial CB
NPs (N115, Cabot Inc., MA) and lab synthesized SiO 2/Au
NSs. SiO2/Au NSs were kindly provided by Dr. Naomi Halas’s
research group at Rice University in a stock suspension of
4.45×109 particles/mL concentration. They were
synthesized according to a previously published
procedure.16, 17 Briefly, 120 nm diameter silica
nanoparticles (Precision Colloids, Inc) were dispersed in
ethanol
and
functionalized
with
3aminopropyltriethoxysilane (Gelest). A gold colloid
hydrosol (1-3 nm diameter) was developed according to the
method of Duff et al.18, and aged for 4-14 days at 6-8 ˚C. The
functionalized silica particles were then added to the gold
colloid hydrosol to allow the gold colloids to adsorb to the
silica surface. SiO2/Au NSs were then grown by reacting the
gold-absorbed silica particles with HAuCl4 (Sigma-Aldrich)
in the presence of formaldehyde.
Transmission electron microscope (TEM) analysis of
the nanoparticles was carried out (Figure S1 in the
Electronic Supplemental Information) to characterize the
2 | J. Name., 2012, 00, 1-3
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ARTICLE
particle size and morphology of the SiO2/Au NSs and CB
NPs. The CB NPs had an individual particle diameter of 10
nm, and existed in small aggregates. The SiO2/Au NSs had a
fairly uniform particle size of 160 nm, with a 120 nm silica
core and 20 nm thick Au coating. This translates to ~ 2.4×1014 g Au per NS. These results are consistent those reported
in previous studies that used similar materials.16,17 The
nanomaterials were stored in 4°C refrigerator prior to use.
The UV-vis adsorption spectrums of the aqueous
suspensions of the photothermal nanomaterials were
measured with an UV-2550 spectrophotometer (Shimadzu
Corporation, Kyoto, Japan) from 200 nm to 900 nm. To
facilitate the dispersion of CB NPs in water for the
measurement, the CB NPs were pre-treated using H2O2
according to a previously established method.19 The
SiO2/Au NS suspension had a concentration of 4.45×108
particles∙mL-1, and the CB suspension had a concentration
of 5 mg∙L-1. All samples were sonicated for 10 min prior to
measurement.
2.2. Membrane modification.
Membrane modification with CB NPs. Hydrophobic
interactions between CB and the PVDF base membrane
facilitated deposition of the CB using an evaporation
coating method. Unfunctionalized CB NPs were dispersed in
chloroform (> 99.8%, Sigma-Aldrich, MO) at 0.1 or 0.5
wt/vol% and sonicated for 30 min at 40 Watts using an
ultrasonicator (Vibra-cell VCX 500, Sonics & Material,
Newtown, CT). The base membrane was mounted on a flat
glass slide with a rubber gasket installed on top of the
membrane to form a reaction chamber (effective
membrane area of 8 cm × 4 cm). Then, 5 mL of the CB
suspension was poured onto the membrane surface and
allowed to contact with the membrane for 1 min. The
suspension was discarded, and the membrane was allowed
to air-dry for 30 min. Modified membranes, lCB-m and hCBm, denoting low (0.1 wt/vol %) and high (0.5 wt/vol %)
concentrations of CB in the suspension, were thoroughly
rinsed with ultrapure water (E-pure, Barnstead, Dubuque,
IA) after modification to remove any loose CB NPs.
Membrane modification with SiO2/Au NSs. SiO2/Au NSs
were attached to the base membrane surface using
polydopamine (PDA) as the binder. PDA is known to coat a
variety of surfaces, including chemically inert PVDF, and the
catechol groups in PDA have strong affinity for metals. 20, 21
First, one side of the membrane was contacted with 2
mg∙mL-1 dopamine hydrochloride (Sigma-Aldrich, MO) in 10
mM Tris-HCl buffer (pH 8.3) for 15 min.22 After drying under
ambient conditions, the coated surface was immediately
exposed to an aqueous suspension of SiO2/Au NSs at
8.9×108 or 4.45×109 particles∙mL-1 for 30 min. Modified
membranes, lNS-m and hNS-m to denote the low and high
concentrations of the SiO2/Au NSs in the suspension, were
dried at 60 oC for 2 hr and rinsed thoroughly with ultrapure
water.
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View Article Online
2.3. Membrane characterizations
DOI: 10.1039/C7TA04555G
Surface morphology of modified membranes was
characterized using scanning electron microscopy (SEM, FEI
Quanta 400 FEG, Hillsboro, OR). Membranes were sputtercoated (Denton Desk V, Denton Vacuum, LLC., NJ) with ~10
nm of gold prior to SEM analysis. Surface hydrophobicity
was characterized by static sessile drop contact angle of
water using a KSV CAM 200 Optical Tensiometer (KSV
instrument LTD, Helsinki, Finland). Liquid entry pressure
(LEP), the pressure at which the feed liquid starts to
penetrate the pores of the membrane, is an important
parameter that characterize the separation function of MD
membranes. LEP was measured using a custom-made
stainless steel dead-end filtration cell and ultrapure water.
Transmembrane pressure was increased at ~1 psi
increments and ~1 min intervals. The transmembrane
pressure at which water permeated through the membrane
was taken as the liquid entry pressure of the membrane.
Mass loading of the photothermal nanoparticles on the
membrane were quantified via different methods. Loadings
on the CB-m samples were quantified by measuring change
of mass before and after coating with an analytical
electronic balance (Adventurer Pro, OHAUS, NJ). Because
no significant increase in mass was observed after
modification of NS-m membranes, coating densities were
obtained using the number density of NSs observed in SEM
images.
2.4. Direct solar MD experiments
The performance of the photothermal membranes was
assessed using a bench-scale NESMD system operated in
the direct contact mode (Figure 2). The membrane module
houses a flat sheet membrane with an effective area of 28.3
cm2 (8.10 cm  3.49 cm). The feed and distillate flow
channels are both 2 mm in height. A quartz window of 6.98
cm  3.49 cm allows irradiation of the membrane surface.
Figure 2 Closed-loop bench-scale solar MD system. The membrane module
contains a quartz window through which light travels. Distilled water overflows
into a beaker on a balance.
This journal is © The Royal Society of Chemistry 20xx
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Simulated sunlight is provided by six halogen tungsten
lamps (FEIT MR16/GU10 120V 50W xenon). The light
intensity was measured to be ~1 sun unit (~1367 W∙m-2)
using a radiometer calibrated with a reference cell (91150KG5, Newport, CA).
Ultrapure water was used as the cooling fluid, and a 1%
NaCl solution was used as the feed. Both the distillate and
the feed were recirculated. Clean water produced by the
MD membrane led to a net increase in the distillate volume,
which was monitored using an overflow device, where the
excess overflowed into a beaker situated on a balance (P4002, Denver Instrument, NY) connected to a computer.
Permeate flux was calculated by least squares regression of
the cumulative distillate mass over time. Feed and distillate
entered the module counter-currently at 8.5 cm∙s-1 and
7.76 cm∙s-1 (Reynolds numbers of 470 and 309),
respectively, driven by two peristaltic pumps (Masterflex
L/S, Cole-parmer, IL). Temperature probes were placed at
the entrances and outlets of the distillate and feed
channels. A conductivity probe (PC700, Oakton, Vernon
Hills, IL) was set at the outlet of the distillate channel.
Temperatures at the inlets of the distillate and the feed
temperatures were maintained at 20°C and 35°C using a
recirculating chiller (VWR, West Chester, PA) and a water
bath (Thermo Fisher Scientific) respectively.
For each membrane sample, flux was first measured
without any irradiation and then under lit conditions. Flux
was calculated using the slope of the liner regression of
permeate mass change over time, divided by membrane
area. Under each condition, flux data was collected for at
least 1 hr after it stabilized.
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spectrum of the SiO2/Au NSs exhibited a maximum
at ~760
View Article
Online
DOI: 10.1039/C7TA04555G
nm, similar to previously reported results
obtained with NSs
of the same structure.16 The thicker gold shell shifted the
absorption maximum to lower wavelength and matched
more closely with the solar irradiation spectrum in the
visible range.17 The CB NPs absorb over a wide range of
wavelengths with an absorption maximum at
B
A
approximately
260 nm.
Surface morphologies and mass loadings. SEM images show
successful coating of the CB NPs and SiO2/Au NSs on the
membrane surface (Figure 4). The low concentration CBcoated membrane, lCB-m, had a fairly high and uniform
surface coverage of CB NPs, mostly in the form of
aggregates. As the CB concentration increased to 0.5 wt%
in the
C coating solution, theD coating density on the
membrane (hCB-m) increased drastically, and a denselypacked nanoparticle cake layer was observed. Many of the
base membrane’s pores were blocked by the CB NPs, which
resulted in a decrease in vapor permeability (discussed
below). Mass measurements (Figure 5) yielded an observed
CB coating density of 0.14 ± 0.05 g∙m-2 and 1.17 ± 0.12 g∙m2 for lCB-m and hCB-m, respectively. Although the CB
concentration used for hCB-m was 5 times of that for lCBm, the resulting mass loading was 10 times higher. This is
attributed to the favorable interactions among the CB
nanoparticles.
Surface coverage observed on lNS-m and hNS-m was
much lower. The SiO2/Au NSs dispersed well in water, and
32 3. Results and Discussions
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3.1. Nanomaterial and membrane characterizations
E
F
Absorption spectrum of the photothermal nanomaterials.
The light absorption behaviors of the CB NPs and the
SiO2/Au NSs were very different (Figure 3). The absorption
2.0
solar spectrum
CB
NS
0.6
1.5
0.4
1.0
0.2
0.5
0
0.0
220
390
560
730
Wavelentgh (nm)
Solar Irradiance (W m -2 nm-1)
0.8
900
Figure 3 Adsorption spectrum of CB and NS as compared to the solar irradiance
(AM 1.5 G).
Figure 4 SEM surface images of (A) unmodified PVDF membrane; (B) hCBm; (C) lCB-m; (D) lCB-m under a higher magnification; (E) hNS-m; (F) lNS-m.
4 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A Accepted Manuscript
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Light absorbance (a.u.)
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0.5
1
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0
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* *
120
*
90
60
B
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the resulting coating consisted of individual, rather than
agglomerated, NSs on the PVDF membrane surface. The
particle loading on hNS-m and lNS-m were 3.18×103 ± 180
particles∙mm-2 and 1.23×103 ± 350 particles∙mm-2,
respectively (Figure 5). SiO2/Au NS loading is easily tuned by
modifying the coating solution concentration.
Membrane hydrophobicity and liquid entry pressure
(LEP). In MD operation, high membrane hydrophobicity is
necessary to provide a barrier to liquid water passage and
promote vapor transport. Figure 6(A) summarizes the water
contact angles of the control and modified membranes. The
control membrane is very hydrophobic, with a contact
angle of 129.1 ± 2.90°. After coating with CB, the contact
angle did not change significantly. The hCB-m and lCB-m
have contact angles of 130.6 ± 0.90° and 131.5 ± 3.39°
respectively. Because PDA and SiO2/Au NSs are highly
hydrophilic, the SiO2/Au NS coatings reduced the surface
hydrophobicity, resulting in contact angles of 112.6 ± 3.34°
and 93.4 ± 6.38° for lNS-m and hNS-m, respectively.
Interestingly, the LEP (Figure 6(B)) of the modified
membranes did not differ significantly from the control,
even though there was a significant decrease in water
contact angle on the NS-m samples. This suggests that the
SiO2/Au NS coating occurred mainly at the top surface of
the membrane, and penetration into the membrane pores
is minimal. The modified membranes maintain their
separation function, acting as a barrier to liquid water
passage.
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3.2. Membrane performance in DCMD
Membrane transport properties. DCMD experiments
without light showed that the flux of the lCB-m, lNS-m, and
hNS-m were not significantly different (p < 0.05) from the
control membrane (Figure 7), suggesting minimal
interference of the coating with water vapor transport. The
LEP (psi)
Figure 5 Mass and particle loading density of modified membranes. Mass
loading for CB-m was determined using an analytical balance. Particle loading
on NS-m was determined by scanning electron micrograph analysis. Error
bars indicate the standard deviation of three different samples.
20
0
Figure 6 (A) Contact angle and (B) LEP measurements of membrane samples.
Error bars in (A) are standard deviations from five measurements on one
sample. Error bars in (B) are the maximum possible error from the precision
of the pressure gauge (2.5 psi). * signifies significant difference (p<0.05) from
the control.
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only notable change in flux was observed on hCB-m. This
membrane had a flux of 1.28 ± 0.088 g∙m-2∙s-1, 12.9% lower
than the control (1.47 ± 0.096 g∙m-2∙s-1). This is attributed to
the vapor transport resistance of the thick CB layer (Figure
4) and blockage of some membrane pores by the CB NPs. In
all experiments, the permeate conductivity remained very
low (less than 5 ± 1.0 µS∙cm-1).
2
1.5
*
*
no light
light
*
*
1
0.5
0
Figure 7 Permeate flux of the control and modified membranes with and
without light irradiance by a solar simulator at 1 sun unit. * signifies significant
difference (p<0.05) from the control.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2017, 00, 1-3 | 5
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Journal of Materials Chemistry A Accepted Manuscript
1
Contact Angle (°)
3
Permeate flux (g∙m-2∙s-1)
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1.5
Particle loading (×103 ∙mm-2)
Mass loading (g∙m-2)
View Article Online
DOI: 10.1039/C7TA04555G
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Energy & Environmental Science
Membrane performance under solar irradiation.
Increased permeate flux was observed on most modified
membranes when irradiated with simulated solar light. The
lCB-m sample had the highest permeate flux of 1.71 ± 0.087
g∙m-2∙s-1 among all samples, a 15.0% (0.22 g∙m-2∙s-1) increase
from the unlit condition. The hCB-m achieved a much higher
increase in flux upon irradiation, 33.0% (0.42 g∙m-2∙s-1)
higher than when unlit, consistent with a stronger
photothermal effect at higher CB concentration. However,
because the unlit flux of the hCB-m was lower as discussed
previously, it only had a comparable flux of 1.70 ± 0.057
g∙m-2∙s-1 as the lCB-m under lit condition.
The hNS-m sample had a flux increase of 0.25 g∙m-2∙s-1
(17.8%) compared with the unlit condition, while lNS-m had
no significant flux increase suggesting insufficient heating
due to the very low loading density. The increase in flux
when the NS-m samples were irradiated was less than that
of the CB-m samples. This is attributed to the much lower
surface coverage of the NS compared to the CB.
All modified membranes showed excellent stability in
performance as suggested by the strong linear relationship
between cumulative permeate mass and operation time.
i.e., constant permeate flux. Representative permeate mass
data are presented in Figure S2. Opposite to traditional MD,
the photothermal MD process benefits from low feed
crossflow velocity in order to minimize the heat transfer
from the photothermal layer to the bulk feed flow.
Therefore, no strong shear stress is expected in practical
application. Our experimental results, obtained at crossflow
velocity significantly higher than that expected in practical
operation, suggests that the photothermal coatings would
be stable against the expected hydraulic shear. Long-term
experiments, however, are needed to evaluate the
durability of the coating.
The increase in flux for the hCB-m sample with and
without light was among the highest reported under similar
conditions.9, 11, 12, 14 Additionally, the coating method
retained the membrane hydrophobicity necessary for MD
applications, and these coatings are cheap and easy to
scale-up.
Temperature polarization and energy efficiency. The
photothermal coating on the membrane surface provides
heating at the water-air interface where evaporation
occurs. This surface heating provides a higher energy
efficiency than the conventional approach of heating the
bulk feed water. Assuming all photons in the incident light
were converted to heat and used to heat the bulk feed
water, a mass/energy calculation (Eq. 3) shows that the
feed temperature would only increase by 0.13°C.
∆ =
50
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∙
 ∙
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This calculated bulk feed temperature increase
wasOnline
very
View Article
DOI: 10.1039/C7TA04555G
small and would not result in detectable
flux increase. A
conservative estimation, assuming the membrane vapor
permeability k stayed constant and bulk temperatures
equal to the membrane surface temperatures, would reveal
that the 0.13°C increase in feed bulk temperature could
increase flux by 0.01 g∙m-2∙s-1 at best. The notable increase
in flux observed in our experiments using the modified
membranes suggests that the heating by the photothermal
coating is highly localized, at the heat transfer boundary
layer. This effect can be estimated by the change in the
overall temperature polarization coefficient, αTP, with and
without light,
 ′ −

(1 ′ −2 ′ )/( − )−(1 −2 )/( − )
(1 −2 )/( − )
=
(1 ′ −2 ′ )−(1 −2 )
(4)
(1 −2 )
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where αTP’, T1' and T2' are the TP coefficient and the watermembrane interface temperatures (˚C) under lit conditions.
The precise calculation of T1, T2, T1' and T2' requires
sophisticated modelling tools and is thus not explored in
this paper.4, 23 Under the narrow temperature ranges
employed in this study, the vapor pressure difference is
approximately proportional to the temperature difference.
Eq. (4) can thus be estimated by
  ′ −   (1′ − 2′ ) − (1 − 2 ) ′ − 
≈
=
× 100%
 
(1 − 2 )

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88
 =
89
90
91
(5)
where J’ and J are the permeate flux of the membrane
samples with and without light irradiance (g∙m-2∙s-1).
This calculation shows that, except for the lNS-m, all
other samples achieved increases in αTP equal to or above
15%. The lCB-m and hNS-m samples had similar increases
(15.0% and 17.8%), while the change of TPC for lNS is
negligible (1.71%). The hCB-m sample had a significantly
greater increase of 33.0%.
The energy efficiency of the photothermal MD process
was evaluated using two parameters: photo-energy
efficiency and overall system energy efficiency.
Eq. (6) calculates the utilization efficiency of the incident
photo-energy ηI, i.e., the percentage of incident light energy
used to generate water vapor.
(′ − )∆

(6)
Here ∆H is the latent heat of evaporation (2524 J∙°C-1∙gwater at 35 °C).
Eq. (7) calculates the overall system energy efficiency ηt.
1 for
(3)
Here, ∆TF is the bulk feed temperature increase (°C); I is the
incident light intensity (J∙m-2∙s-1); A is the irradiation area
(m2); QF is the mass feed flow rate (g∙s-1); and C is the heat
capacity of water (4.178 J∙g-1∙°C-1).
=
 =
′ ∆
+ℎ
=
′ ∆
+ ( − )
(7)
92
Here, Hh is the thermal energy (J∙s-1) used to heat the
93 feed water from ambient temperature 20 °C to the bulk
94 feed temperature of 35 °C.
6 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A Accepted Manuscript
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ARTICLE
Journal
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ARTICLE
Figure 8 presents the calculated ηI and ηt of different
samples. For lCB-m and hNS-m samples, approximately 40
– 43% of the light energy was used to generate vapor. The
hCB-m was able to utilize 74.6% of incident light energy,
which is among the higher performances reported by
studies under similar conditions (Table 1).9-12, 14
The overall energy efficiency was very low for the
control system, 2.4% and 2.3% under lit and unlit
conditions. All photothermal membranes improved the
overall energy efficiency under lit conditions (ηt = 2.6 –
2.7%), but the absolute values remain very low. This is
because the light energy input, which is utilized by the
photothermal coating, is a small fraction (~1%) of the total
energy input. These results suggest that greater
improvement of the overall system energy efficiency can be
achieved with higher incident light intensity, e.g., via solar
concentration.
18 4. Conclusions
19
20
21
22
The study explored the fabrication of novel photothermal
MD membranes and their performance in a lab-scale DCMD
system with simulated sun light. Using facile coating
methods, CB NPs and SiO2/Au NSs could be securely
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compromising the liquid entry pressure of theView
membrane.
Article Online
Different coating densities of CB NPsDOI:
or 10.1039/C7TA04555G
SiO2/Au NSs were
achieved by varying the CB NP and SiO2/Au NS
concentrations in the coating solutions. Upon irradiation by
simulated sunlight at 1 sun unit, the CB NP and SiO2/Au NS
coatings demonstrated strong photothermal effects,
resulting in notable increase in water flux, up to 33.0%
higher than the flux measured without light, in a bench
scale DCMD system. Meanwhile, while the control PVDF
membrane showed no detectable change in flux.
Theoretical calculations show that this increase in flux
cannot be achieved if all the photon energy is used to heat
the bulk feed water. It also revealed that the surface
heating provided by the photothermal coating can improve
the thermal efficiency and alleviate temperature
polarization in the MD process. Results from this study
demonstrate the application of photothermal nanomaterial
membrane coatings to directly utilize sunlight to produce
pure water in a direct solar MD process. These
photothermal membranes and the direct solar MD process
they enable hold great potential for off-grid water
purification and desalination applications.
47
Table 1 Comparison of photothermal layers from different studies
Efficiency of light energy
ηl
100%
Ref.
Photothermal layer
Irradiance
(kW∙m-2)
1
Photothermal
efficiency
53%
9
Thermally-insulating
carbon foam supporting
a photothermal
exfoliated graphite
layer
10
Bilayer hybrid biofoam
composed of bacterial
nanocellulose (BNC)
and reduced graphene
oxide (RGO)
10
83%
11
Stainless steel mesh
with polypyrole (PPy)
and fluoroalkylsilane
coating modification for
hydrophobicity
1
58%
12
Vertically aligned
graphene oxide sheets
membrane (VA-GSM)
1-4
84.5% - 94.2%
14
A 25 µm CB NPsembedded polyvinyl
alcohol (PVA) fiber
mesh coating
0.7
53.8%
Present
study
Self-assembled
photothermal CB NP
layer on hydrophobic
PVDF membrane
1.3
74.6%
80%
60%
40%
20%
0%
Efficiency of total energy ηt
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Journal Name
3.5%
3.0%
No light
Light
2.5%
2.0%
1.5%
1.0%
0.5%
0.0%
Figure 8 (A) Calculated photo-energy efficiency ηl and (B) overall system
energy efficiency ηt.
48
49
50
23 immobilized on a PVDF MD membrane without
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Journal of Materials Chemistry A Accepted Manuscript
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Energy & Environmental Science
1 Acknowledgements
2
3
4
5
6
7
8
9
Partial funding was provided by the NSF NERC on
Nanotechnology-Enabled Water Treatment (EEC-1449500),
the Energy and Environment Initiative at Rice University,
and the James A. Baker III Institute for Public Policy’s Center
for Energy Studies. We thank Dr. Wenhua Guo and Dr.
Tianxiao Wang for assisting in the TEM analysis of the
nanoparticles. We thank Dr. Oara Neumann for providing
the SiO2/Au NS material.
55 21. H. Lee, S. M. Dellatore, W. M. MillerViewand
B.
ArticleP.
Online
10.1039/C7TA04555G
56
Messersmith, Science, 2007, 318,DOI:
426-430.
57 22. D. J. Miller, P. A. Araújo, P. B. Correia, M. M. Ramsey, J.
58
C. Kruithof, M. C. M. van Loosdrecht, B. D. Freeman, D.
59
R. Paul, M. Whiteley and J. S. Vrouwenvelder, Water
60
Research, 2012, 46, 3737-3753.
61 23. M. M. A. Shirazi, A. Kargari, A. F. Ismail and T. Matsuura,
62
Desalination, 2016, 377, 73-90.
63
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10 Notes and references
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8 | J. Name., 2012, 00, 1-3
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Journal of Materials Chemistry A Accepted Manuscript
ARTICLE
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Page 9 of 9
Journal of Materials Chemistry A
View Article Online
DOI: 10.1039/C7TA04555G
Photothermal membranes that convert light to heat locally on membrane surface and
significantly improve membrane distillation flux and energy efficiency.
Journal of Materials Chemistry A Accepted Manuscript
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