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Patterned Nanobrush Nature Mimics with Unprecedented Water-Harvesting Efficiency


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Water scarcity is one of the most alarming problems of the planet. An ambient ion based method is developed to make hydrophilic‐hydrophobic patterned silver nanowires (NWs) as humidity harvesters of unprecedented efficiency. Such water harvesters are developed by two‐step surface modification of the as‐synthesized NWs (known from a report earlier) using electrospray. These patterned NWs of ≈20 µm length and ≈200 nm width grown over a relatively large area (2 × 2 cm²) exhibit atmospheric water capture (AWC) efficiency of 56.6 L m⁻² d⁻¹, the highest reported so far. The whole fabrication process of the surface is performed under ambient conditions with a home‐built nanoelectrospray ion source, without the help of any sophisticated instrumentation. The synthesized material combines and mimics two exciting examples of AWC in nature, which are cactae and Namib Desert beetles, which utilize AWC for their living. It is believed that the combination of the special features of the above two natural species helps to achieve the highest water capture efficiency reported till date. A working prototype using this surface for AWC is also fabricated.
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1800667 (1 of 7) © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Patterned Nanobrush Nature Mimics with Unprecedented
Water-Harvesting Efficiency
Depanjan Sarkar, Anindita Mahapatra, Anirban Som, Ramesh Kumar, Ankit Nagar,
Avijit Baidya, and Thalappil Pradeep*
DOI: 10.1002/admi.201800667
This is due to the fact that at any given
moment, the earth’s atmosphere contains
an astounding 37.5 million billion gallons
of water as vapor,[2] and an efficient device
to capture a fraction of this water vapor, in
a cost-effective way would help solve the
water crisis.
Over the period of human existence, it
has become apparent that biomimicking
is the most efficient way to tackle such
problems. When we look into nature,
there are organisms, which in the course
of evolution have acquired physical traits
that enabled them to capture atmos-
pheric water, even in the most arid cor-
ners of our planet. One such example
is the Stenocara beetle of Namib Desert
which capture water on its hardened fore-
wings.[3] Electron microscopic images of
these have revealed unique array of hydro-
philic regions distributed on a superhydro-
phobic background,[4] creating a surface
energy gradient, which facilitates efficient condensation and
transportation of atmospheric water. Another such example is
spider silk, which shows unique periodic spindle-knot struc-
ture when wetted and efficiently collects water from atmos-
phere through the combination of surface energy gradient and
Laplace pressure difference. Other inspirations from nature
are some cactaceae species which live in arid environments
and are extremely drought-tolerant.[5] These species are shown
to have structures with spines and trichomes which enable
them to condense humidity efficiently from the atmosphere.
Grasslands are also examples of natural atmospheric water har-
vesters. Hence, micro/nanostructuring of the water-collecting
surface plays a critical role in determining the efficiency of
water capture. With the advancement of nanotechnology, sig-
nificant efforts have been directed toward fabricating surfaces
with similar morphological features and chemical patterning to
enable efficient water capture.[6,7] However, these natural and
nature-mimicked surfaces collect atmospheric water in the
form of dew/fog and require the temperature to drop below the
dew point to cause condensation. For building a practical and
round-the-clock operating atmospheric water generator, it is
important to cool the condensing surfaces and the surrounding
air efficiently, with minimum energy input. Heat transfer effi-
ciency of the condensing surface material is as important as
the water transfer efficiency for creating a viable radiative con-
denser. Although many biomimetic, patterned surfaces have
been made for fog collection, inspired by active condensing
Water scarcity is one of the most alarming problems of the planet. An
ambient ion based method is developed to make hydrophilic-hydrophobic
patterned silver nanowires (NWs) as humidity harvesters of unprecedented
efficiency. Such water harvesters are developed by two-step surface
modification of the as-synthesized NWs (known from a report earlier) using
electrospray. These patterned NWs of 20 µm length and 200 nm width
grown over a relatively large area (2 × 2 cm2) exhibit atmospheric water
capture (AWC) efficiency of 56.6 L m2 d1, the highest reported so far.
The whole fabrication process of the surface is performed under ambient
conditions with a home-built nanoelectrospray ion source, without the help
of any sophisticated instrumentation. The synthesized material combines
and mimics two exciting examples of AWC in nature, which are cactae and
Namib Desert beetles, which utilize AWC for their living. It is believed that
the combination of the special features of the above two natural species helps
to achieve the highest water capture efficiency reported till date. A working
prototype using this surface for AWC is also fabricated.
Dr. D. Sarkar, A. Mahapatra, Dr. A. Som, R. Kumar, A. Nagar, A. Baidya,
Prof. T. Pradeep
DST Unit of Nanoscience (DST UNS) and Thematic Unit of Excellence
Department of Chemistry
Indian Institute of Technology Madras
Chennai 60036, India
The ORCID identification number(s) for the author(s) of this article
can be found under
Water Harvesting
Availability of clean fresh water is one of the measures of devel-
opment of a society. However, water resources have become
severely depleted or highly polluted due to the demands of
rapid economic growth as well as poor management in most
parts of the planet. This has resulted in water scarcity to be one
of the gravest social risks of the modern world while it was con-
sidered to be an environmental risk a few years ago.[1] Around
1.2 billion people, or almost one-fifth of the world’s popula-
tion already live in areas with water shortage, and another
500 million more people are being pushed to this situation. By the
year 2025, 1.8 billion people will be living in regions with sheer
water paucity, and two-thirds of the world’s population could
be living in water-stressed circumstances. Atmospheric water
capture is being considered as a possible solution for providing
clean water to people living in those areas, for sometime now.
Adv. Mater. Interfaces 2018, 1800667
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1800667 (2 of 7)
surfaces found in nature,[7,8] most of these make use of poly-
mers or polymer-hybrids, which might not be best suited for
radiative condensers due to their poor heat-transfer efficiencies.
Metallic nanostructures are of great interest in recent research
due to their excellent electrical and thermal conductivity.[9–11]
The enhanced electric field around these structures makes
them potential substrates for surface-enhanced Raman spec-
troscopy (SERS)[10,12] enabling sensing of various biological
and nonbiological compounds present in water.[13] Apart from
these, metallic nanostructures exhibit interesting mechanical
properties,[14] while maintaining high degree of porosity,[15]
ordered arrays,[16] and many more. Even though such nanostruc-
tures have potential applications in different areas, it has been
difficult to synthesize them over large areas for commercial
applications. Several methods have been developed to create pat-
terned nanostructures which include chemical[17] and physical[18]
vapor deposition, plasma chemical method,[19] spray pyrolysis,[20]
etc. They can also be synthesized using photolithography[21] or
electron beam lithography[9,22] as top-down approaches. Template
mediation[23] and self-assembly are also used for making desired
nanostructures. However, all these methods need special condi-
tions like vacuum, high temperature, templates, etc. Ambient,
solution state techniques to create well-defined metallic nano-
structures over large areas are still limited. In an earlier report
from our group, long (80 µm) nanowire (NW) arrays were made
through electrospray ionization-induced creation and deposition
of Ag nanoparticles (NPs), under ambient conditions.[24] These
wires could be grown over several cm2 areas to make a metallic
grassland like morphology. In this report, we have shown
growth of these structures over 4 cm2 area; however, they can
also be grown over several tens of cm2 area using a program-
mable moving stage.
Here, we report the creation of a hydrophilic-hydrophobic
patterned humidity-collecting surface as an efficient radiative
atmospheric water capture device, by chemical modification of
the metallic grasslands through electrospray deposition (ESD).
While the use of NW arrays maximize the available surface
area for water condensation, chemical modification of the NW
surface helps with the quick transfer of the condensed water
and regeneration of active sites for further condensation to
happen. The surface modification strategy also improved the
mechanical strength and stability of the NWs in water. In the
design of the surface, we have successfully fabricated surfaces
which mimic the hairy structures of cactae which in turn
have hydrophilic-hydrophobic patterns resembling the wings
of Stenocara beetles along with superior heat transfer capa-
bilities of metallic nanostructures. A combination of all these
effects led to high water capture efficiency (56.6 L d1 m2) at a
prototype-scale. Although we have used Ag NWs reported pre-
viously[24] as the initial building block of our patterned NWs,
the present work focuses on a method of creating hydrophilic-
hydrophobic patterned NWs using two simple steps, which
mimic the above mentioned examples from nature and com-
bine their special features to achieve highest water capture effi-
ciency reported till date.
Hydrophilic-hydrophobic patterned Ag NWs were fabricated
using ESD at room temperature in air. A schematic representa-
tion of the ESD process along with the dimensional descriptions
is shown in Figure S1 (Supporting Information). As the growth
of unpatterned NWs was described previously,[24] we present
only the essential aspects here. A detailed description of the
synthesis and characterization (electron microscopy, elemental
analysis) of the Ag NWs on an empty transmission electron
microscopy (TEM) grid, which form the core of the patterned
NWs, is given in the Supporting Information (Figures S2–S8,
Supporting Information). For all our AWC experiments, we
have selected stainless steel (SS) wire mesh as the substrate to
create a cost-effective substrate for the application. Figure S9a,b
(Supporting Information) shows scanning electron microscopy
(SEM) images of the Ag NWs grown on an empty TEM grid
and SS wire mesh, respectively, proving identical morphology
of the NWs on both the surfaces. Hence, for all our water cap-
ture experiments, we used the hydrophilic-hydrophobic pat-
terned Ag NWs grown on SS wire meshes (50 µm). Use of a
larger mesh size would reduce the active area for water conden-
sation because of the reduction of number of wires (on which
the Ag NWs are grown) in a specified area. On the other hand,
use of a lower mesh size would increase the number of wires
(on which the Ag NWs are grown) but it would also reduce
the active surface area by affecting the size of the Ag NWs.
The Ag NWs grown on a reduced mesh size SS wire mesh were
smaller in size because of the coulombic repulsion between the
growing NWs. We have also compared the growth of these NWs
on top of meshes of different shapes.
Figure 1a shows a SEM image of the as-synthesized Ag NWs
on a SS wire mesh using ESD. It shows an arrangement of
1D NWs, which upon extension will resemble a grassland-like
structure. This can be created over a larger area of the order of
Adv. Mater. Interfaces 2018, 1800667
Figure 1. Electron microscopic characterization of the hydrophilic-hydrophobic patterned Ag NWs. a) SEM image of the as-synthesized Ag NWs on
a 50 µm SS wire mesh, b) SEM image of similar NWs after fluorothiol coating. Difference in contrast is due to the organic content. c) TEM image of
the FT-coated Ag NWs, showing the intact morphology of the material, EDS spectrum (inset) showing presence of F, S, and Ag. Cu is due to the grid.
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1800667 (3 of 7)
4 cm2. Figure S10 (Supporting Information) shows a large-area
SEM image (300 µm2) of the Ag NWs on a TEM grid. These
metallic NWs were the core of the patterned NWs with specific
hydrophilic regions on a hydrophobic background. Fabrication
of the patterned NWs was designed by successive ESD pro-
cesses, as explained below.
At first, as-synthesized Ag NWs were subjected to
an electrospray coating of a fluorothiol (FT), namely,
canethiol. A solution in dichloromethane (DCM) and acetoni-
trile (AcN) mixture was used for ESD of FT. This made the Ag
NWs superhydrophobic in nature by the formation of a chem-
ical bond between Ag and the thiol group, of the FT. Metal–
sulfur bond is strong and is the basis of one class of self-assem-
bled monolayers.[25] Figure 1b shows a SEM image of FT-coated
Ag NWs. The change in contrast (with the image in Figure 1a) is
due to the coating of a nonconducting layer of FT on the metal.
To prove the superhydrophobic nature of the surface, the con-
tact angle of a water droplet on it was measured. Surface energy
of the FT-coated surface was so low that a water droplet did not
stand static on the surface. Figure S11 (Supporting Informa-
tion) shows images of a water droplet rolling on the FT-coated
Ag NWs. No pinning of the droplet was observed, proving the
superhydrophobicity of the surface. Figure S12a–c (Supporting
Information) shows optical images of a water droplet bouncing
off from the FT-coated Ag brush substrate while another water
droplet is stranded on the bare stainless steel wire mesh. This
proves that the FT coating on the Ag brushes made them supe-
rhydrophobic in nature. Figure S12d (Supporting Information)
shows contact angle measurement of a water droplet on stain-
less steel wire mesh before (normal wire mesh) and after modi-
fication (FT-coated Ag NWs). Contact angle of the water droplet
was measured to be 125° in the case of normal mesh indi-
cating that the normal stainless steel surface is hydrophobic
(90° <
c < 150°). Measurement of resting contact angle was not
possible for the superhydrophobic surface as it was too slippery
for the droplet to sit on it. Hence, the contact angle was meas-
ured when the water droplet was touching the surface. A meas-
urement of advancing and receding contact angles (173° and
166°, respectively) was also performed (Figure S13, Supporting
Information). The contact angle hysteresis (173°–166° = 7°),
is within the desired 10° for good superhydrophobicity. These
measurements prove the superhydrophobic nature of the Ag
NWs after FT coating. Roughness due to the presence of NWs
also plays a significant role in acquiring superhydrophobicity.
Control experiments were performed with normal stainless
steel wire mesh and using the same sprayed with FT. It was
seen that water droplets did not bounce off from the normal
stainless steel mesh but wetted it instead, shown in Figure S14a
(Supporting Information), clearly indicating that it was not
superhydrophobic. Then FT was sprayed on the stainless steel
mesh and the same experiment was repeated, images of which
are shown in Figure S14b (Supporting Information). Here also
the droplets did not bounce off and got stuck to the surface.
However, when sprayed over by FT, the stainless steel mesh
surface becomes hydrophobic to some extent. Hence, the pres-
ence of Ag NWs, providing surface roughness in nm scale, was
essential to make it superhydrophobic. ESD is a gentle process;
hence, it does not harm the morphology of the as-synthesized
nanobrushes. This was the prime reason for us to choose ESD
of FT over Ag NWs. Other procedures of FT coating, such
as spray coating with a mechanical sprayer were not useful.
Although the spray-coated surface became superhydrophobic, it
destroyed the morphology of the NWs (Figure S15, Supporting
Information). To optimize the ESD time for FT coating, a time-
dependent ESD experiment was performed (spray time of 2, 4,
6, 8, and 10 min) on five different spots containing identical
Ag NWs. It was observed that each spot became superhydro-
phobic proving that ESD of FT for 2 min was enough to make
the surface superhydrophobic. However, these FT-coated NWs
lost their morphology when subjected to water wash. In view of
that we have enhanced the ESD time of FT to 20 min at a rate
of 20 nA deposition current. In this case, the FT-coated NWs
retained their morphology after water wash. This indicates
that ESD of FT also has a role in imparting stability to the Ag
nanobrushes by AgS bond formation (there is another aspect
how ESD of FT provides stability to the Ag NWs which will be
discussed at the end of this paragraph). Hence, for all other
experiments, the above mentioned time and rate of ESD was
taken as optimum. Figure 1c shows a TEM image of the FT-
coated Ag NWs. Inset in Figure 1c shows an energy-dispersive
spectrum (EDS) taken from these NWs showing the presence
of all the expected elements (Ag, S, and F).
From our data we know that our Ag NWs are made by an
organization of NPs. During the ESD of FT on Ag NWs, the
NPs constituting the NWs coalesce with each other, making
a solid metallic core inside. Figure S16 (Supporting Informa-
tion) shows the TEM images of the Ag NWs before and after FT
coating. From these images, we can easily compare and come to
a conclusion that the NPs in the FT-coated Ag NWs are fused,
providing a solid core on which the superhydrophobic coating
of FT is formed. This solid metallic core gives strength to the
NWs, making them more resistant toward mechanical strain.
In the second step, we have performed ESD of AgOAc
(aqueous solution, 10 × 103 m) over the superhydrophobic
NWs to create Ag NP decorated NWs. These bare Ag NPs act as
hydrophilic protrusions on the superhydrophobic NWs. In the
following paragraph, we describe the structure of our material
using electron microscopic images. The contact angle of a water
droplet was measured after creating the hydrophilic zones.
Figure S17 (Supporting Information) shows that the contact
angle of a water droplet was 152°. Reduction of the contact
angle, with respect to the superhydrophobic NWs, proves the
presence of hydrophilic protrusions.
In Figure 2, we represent our material and compare it with
the examples available in nature. Figure 2a shows a cartoon of
a cactus with water droplets on the trichomes. Figure 2b shows
a schematic of the cactus showing that its sharp spines and tri-
chomes are responsible for water capture. Figure 2c shows a
TEM image of our material which resembles the hairy structure
of a cactus although the scales shown are different. Figure 2d
shows a cartoon of a Stenocara beetle and the hydrophobic-
hydrophilic patterning of the wings (Figure 2e). Figures 2f,g
show a schematic of the synthesized hydrophilic-hydrophobic
patterned Ag NW and a TEM image of the same with three
distinct regions: (i) the Ag-core, (ii) hydrophobic coating on the
Ag-core, and (iii) tiny hydrophilic regions (Ag NPs, see below
for its synthesis) on the hydrophobic background. Ag core gives
Adv. Mater. Interfaces 2018, 1800667
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the strength to the brushes, while the FT coating creates the
hydrophobic background. Subsequent ESD of Ag on this FT-
coated surface resulted in Ag NPs decoration all over the NWs.
These Ag NPs, embedded in the FT layer, act as hydrophilic
zones, known to be responsible for the condensation of atmos-
pheric water. A schematic illustration of atmospheric water cap-
ture by these hydrophilic-hydrophobic patterned NWs is shown
in Figure 2f.
A 2 cm × 2 cm SS wire mesh covered by these patterned
NWs was created and used for water capture experiments
described in the following sections. Evaluation of humidity
condensation was performed using this surface mounted on
a Peltier cooler, which was then videographed (Video S1, Sup-
porting Information) using a microscope. Two experiments,
one control (with normal stainless steel mesh) and another
with the prepared surface, were performed to test the effi-
ciency of the nanostructured material for atmospheric water
capture. In both the experiments, the surface being examined
was carefully mounted on a Peltier cooler, using silver paste as
glue, so that it remained in adequate contact with the cooling
stage. The entire arrangement was placed atop the viewing
stage of an inverted fluorescence microscope (Leica), in a con-
trolled room with 40% relative humidity and 28 °C tempera-
ture (the dew point at this condition 13 °C). An arrangement
was made for illumination of the surface being examined, for
the purpose of microscopy. Time-lapse optical microscopy of
the stainless steel mesh as well as the prepared surface upon
exposure to cool humid air was performed to monitor droplet
nucleation and condensation. The temperature of the surface
under examination was maintained at 12 °C, measured with a
thermocouple. Figure S18a,b (Supporting Information) shows
the optical images of a normal stainless steel wire mesh
after 60 and 120 s of collection, respectively. Condensation
of water droplets was seen on the wire mesh. Figure S18c,d
(Supporting Information) shows the optical images of con-
densation of water on the stainless steel wire mesh containing
superhydrophobic Ag NWs with hydrophilic protrusions. It is
seen that the amount of water captured in this case is much
larger in comparison to a normal wire mesh. In a time span of
2 min, a volume of 2.6 × 106 L water was captured (approxi-
mately calculated from the number of droplets considering
them to be spheres, 1.3 × 108 L of water was captured over
the normal SS wire mesh) on the brushes, even though the
humidity was less and no water vapor was forcefully passed
over the surface during the experiment. The video clearly
shows nucleation of tiny water droplets on the patterned
NWs followed by fusion of them to bigger droplets. Once the
droplet is larger in size, it rolls off the surface because of its
superhydrophobic nature.
Transport of the condensed water is a crucial criterion of any
atmospheric water capture material. Hence, we calculated the
water collection rate of the surface on a 2 × 2 cm2 area con-
sidering all the parameters like, condensation rate, transport of
the collected water, etc. Figure 3a shows an optical photograph
of the total setup where the hydrophilic-hydrophobic patterned
NWs containing SS wire mesh was firmly mounted (the black
Adv. Mater. Interfaces 2018, 1800667
Figure 2. Graphical demonstration of the natural species with atmospheric water-harvesting capabilities, illustration of their special properties and
comparison with our material. a) Graphical representation of a cactus, b) illustration of hairy structure of cactus, c) TEM image of a single Ag
NW showing similar hairy structures resembling the cactus, d) graphical representation of a Stenocara beetle collecting water from atmosphere,
e) schematic representation of hydrophobic-hydrophilic patterning of their wings, f) illustration of our superhydrophobic-hydrophilic patterned Ag NW
showing a solid Ag core, a superhydrophobic coating on the core, and hydrophilic regions (Ag NPs) on the superhydrophobic background, and g) TEM
image of a single Ag NW showing all three regions.
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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square surface) on a Peltier cooler, using a carbon tape. Carbon
tape was used in this case, instead of silver paste, for better
transport of the collected water. Inset of Figure 3a shows a
photograph of the surface. A 12 V DC fan was used for cooling
the hot side of the Peltier cooler. Temperature of the surface
was measured and kept constant (7.5 °C) during the experi-
ment. The above experiment was performed inside an air-
conditioned room where the relative humidity was 48% and the
ambient temperature was 26 °C. There was no forced airflow.
Figure 3b shows the formation of water droplets on the hydro-
philic-hydrophobic patterned nanobrushes, just after switching
on the Peltier cooler. Figure 3c shows an optical microscopic
image of the surface at 100× magnification, clearly showing the
accumulated droplets on it. In the above experiment, 3.75 mL
of water was collected in 13 h. If we convert the water collection
to L d m2 area, it comes to 7.9 L. Figure 3d shows an optical
photograph of the collected water in a 2 mL vial. Control experi-
ments were performed to compare the water capture efficiency
of the hydrophilic-hydrophobic patterned NWs with as-synthe-
sized Ag NWs and hydrophilic Ag NWs. Although the parent
surfaces without chemical functionalization were unstable
during water capture experiments, we made the structures
stable by exposing them to 4-mercaptobenzoic acid. In addi-
tion to enhanced stability during water capture, this function-
alization made the surface hydrophilic. Such surfaces showed a
water capture efficiency of 4.3 L d1 under the same conditions
as before.
For the next set of experiments, we designed a prototype
to evaluate the water capture efficiency of the surface in pres-
ence of air flow. Figure 4a shows a schematic of the prototype.
In these experiments, a 12V DC (air flow speed of 2.55 m s1
volume 103–105 cfm) fan was used to blow atmospheric air
toward the cold hydrophilic-hydrophobic patterned surface.
Two sets of experiments were performed with this proto-
type: (i) inside an air-conditioned room (at lower humidity)
(Figure 4b shows an optical photograph of the setup), and
(ii) outside the lab, under ambient conditions (Figure 4c shows
a photograph). Conditions and data collected from these experi-
ments are described here. In the first set of experiments, sur-
face temperature and average relative humidity were 8 °C
and 58%, respectively. The water collection efficiency was
26.8 L d1 m2. In the second set of experiments, the temper-
ature of the surface was kept the same and average relative
humidity was 87%. In this case, the water collection efficiency
was 56.5 L d1 m2. This is the highest water collection reported
till date by any surface. The high efficiency of water collection
can be attributed to the combination of cactus effect as well as
hydrophilic-hydrophobic patterning. Water droplets start nucle-
ating on the hydrophilic regions and as they grow larger in size,
they start to roll off from the surface, generating fresh surface
for collection.
For long-term use of the substrate, stability of the brushes
is an important aspect. Reusability and the morphology of
the surface were checked after the water capture experiment.
Figures S19a,b (Supporting Information) show SEM images
of the hydrophilic-hydrophobic patterned Ag nanobrushes
after five cycles of water capture. The images show that the
morphology of the brushes is almost intact after the experi-
ment. Figures S19c,d (Supporting Information), respectively,
show contact angle measurements of a water droplet on the
substrate before and after the water capture experiment. The
contact angle was found to be the same in both the cases,
proving that the nature of the surface is the same after dew
An ambient solution-state method of creating hydrophilic-
hydrophobic patterned 1D Ag NWs, using electrospray deposi-
tion in three simple steps, is presented. The whole synthesis
including the Ag NWs (the core of the patterned NWs) was per-
formed at room temperature without the help of any sophisti-
cated instrumentation. This method of synthesis enabled us to
create a material that could mimic two efficient natural atmos-
pheric water harvesters. Combination of hydrophilic patterns
on a superhydrophobic background along with hairy struc-
tures of the Ag NWs led to highest water capture efficiency,
reported till date. We also built a prototype and tested its water
capture efficiency under various conditions. At 87% relative
humidity, our material showed a water capture efficiency of
56.5 L d1 m2.
Experimental Section
For all experiments, nanospray emitters were made using a micropipette
puller (P-97) purchased from Sutter Instrument, USA. To confirm the
Adv. Mater. Interfaces 2018, 1800667
Figure 3. Optical and microscopic photographs of the condensed water
droplets on a 2 × 2 cm2 area. a) Optical photograph of the water collection
setup. Inset shows a photograph of the hydrophilic-hydrophobic patterned
surface over a 2 × 2 cm2 area. Each ESD results in a 5 mm dia dark
colored disk indicating the growth of NWs. This process was repeated
multiple times to cover the 2 × 2 cm2 area. b) Zoomed optical photograph
of the surface after switching on the Peltier cooler showing condensed
water droplets on it and dripping of water at the bottom of the surface.
c) Optical photograph (at a 100× magnification) of the Ag NWs with
condensed water droplets. d) Optical photograph of the collected water
in a 2 mL vial.
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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presence of solvated Ag+ ions, mass spectra were collected using an
ion trap LTQ XL (Thermo Scientific, San Jose, CA) mass spectrometer.
Indium tin oxide-coated glass slides (Toshniwal brothers (SR) Pvt. Ltd.,
India) was the usual deposition substrate. Locally available stainless
steel wire mesh was used as a surface to grow the Ag NWs. All TEM
measurements were performed using a JEOL 3010 (JEOL Japan)
transmission electron microscope. Copper TEM grids were purchased
from SPI Supplies. A FEI Quanta 100 instrument with tungsten filament
source was used for SEM imaging. EDS analyses were performed with
an attachment on the SEM instrument. Contact angle and contact angle
hysteresis (CAH) of water droplet on the different coated substrates were
measured using a Holmarc contact angle meter. Optical photographs of
water droplets condensed over the hydrophilic-hydrophobic patterned Ag
NWs and normal SS mesh was performed using a 100× objective fitted
to a Leica DFC365 FX microscope. Optical photographs of the surface,
prototype, and water droplets bouncing off the surface were captured
using a Nikon D5100 camera using an 18–55 mm lens. Videos were also
captured using the same camera.
Supporting Information
Supporting Information is available from the Wiley Online Library or
from the author.
T.P. acknowledges financial support from the Department of Science
and Technology, Government of India for his research program on
nanomaterials. D.S. thanks the University Grants Commission for a
research fellowship. A.S. thanks Council of Scientific and Industrial
Research for a research fellowship.
Conflict of Interest
The authors declare no conflict of interest.
nature mimics, prototype, superhydrophobic-hydrophilic patterned
NWs, water harvesting
Received: May 2, 2018
Revised: June 17, 2018
Published online:
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Figure 4. Prototype of the atmospheric water capture experiment. a) Schematic and b,c) photographs of water capture experiments inside and outside
the laboratory. The photograph in (c) shows that the air inlet of the prototype is kept outside the laboratory window.
© 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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... From our previous report, the spray current measured between the tip and the substrate was 40 nA. 32 The obtained film was carefully transferred to a suitable substrate (glass slide or aluminum foil) for further studies. An optical micrograph of the film (shown in Figure S13) demonstrates its transparent nature. ...
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Two-dimensional nanostructures with atomically precise building blocks have potential applications in catalysis and sensing. However, structural instability and surface reactivity limit their practical use. In this work, we demonstrate the formation of vertically aligned nanoplates of the [Co6S8(DPPE)6Cl6] cluster (Co6 in short), protected by 1,2-bis(diphenylphosphino)ethane, using ambient electrospray deposition (ESD). Charged microdroplets of Co6 formed by ESD on a water surface created such nanostructures. Preferential arrangement of clusters in the nanoplates with enhanced surface area results in sensitive and selective electrochemical response toward arsenite down to 5 parts per billion, in tap water. Density functional theory calculations reveal the preferential binding of arsenite with Co6. Our work points to a practical application of atomically precise clusters of large societal relevance.
... Owing to the high number of reactive groups (i.e., hydroxyl groups]. Silver nanobrushes were able to conduct the process of atmospheric water harvesting successfully [Sarkar D. et al., 2018]. Ferrihydrite was used for heavy-metal elimination . ...
Here we report the comparison study of the DFT and Zindo of new coumarin derivative dye namely, 1-((4-methoxyphenoxy) methyl)-3Hbenzo[ f]chromen-3-one [4-MPMBC] in the gas phase along within solvent phase using Gaussian 16W program. From these studies, we estimated the ground and excited-state dipole moments. Total energies, transition energies, oscillator strength of the coumarin dyes were estimated by the optimization process. HOMO-LUMO energies, energy gap, and chemical hardness were studied in the gas phase along with the solvent phase using TD-DFT/IEFPCM (Time-Dependent Density Functional Theory/ Integral Equation Formalism Polarizable Continuum Model). UV-Visible spectra were obtained in both DFT, Zindo methods and were compared. Keywords: Dipole moment, Transition energy, DFT, Zindo, Coumarin derivative.
... The chemistry of microdroplets is characterized by the occurrence of accelerated organic and materials synthesis reactions. [1][2][3][4][5][6][7][8] Electrospray deposition (ESD) of microdroplets under ambient conditions is emerging as a new synthetic method for a range of nanomaterials including nanoparticles (NPs), [9] nanobrushes, [10] nanowires, [11,12] nanoclusters, [13,14] nanoplatelets or nanopyramids, [15] nanosheets, [16,17] monodispersed nanoparticles, [18] and nanosized onion-like carbon (OLC). [19] The properties and structures of these nanomaterials are highly dependent on the choice of collecting surfaces which can be flat metal surfaces, [15] Indium Tin Oxide (ITO) plates, [18,19] water films, [17] MoS 2 nanosheets, [16] Te nanowires, [20] or TEM grids. ...
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Electrospray deposition of copper salt‐containing microdroplets onto the liquid surface of an electrically grounded reaction mixture leads to the formation of Cu nanoclusters, which then catalyze the azide‐alkyne cycloaddition (AAC) reaction to form triazoles. This method of in situ nanocatalyst preparation provided 17 times higher catalytic activity compared to that in the conventional catalytic reaction. The gentle landing of the Cu‐containing droplets onto the liquid surface forms a thin film of catalyst which promotes the heterogeneous AAC reaction while showing diffusion‐controlled kinetics. UV‐vis spectral characterization confirms that the catalyst is comprised of Cu nanoclusters. This unique catalytic strategy was validated using several substrates and the corresponding products were confirmed by tandem mass spectrometry (MS/MS) analysis.
... In order to improve the AWH capability and directly obtain the dew, the active contact cooling devices based on thermal conversion and conduction such as Peltier cells are adopted in the vast majority of reported studies so far. 30,35,37,53 In this process, a huge portion of input electrical energy is consumed for the purpose of cooling other components of surrounding air except vapor, leading to a relatively low efficiency. 54,55 From the perspective of cooling efficiency, the sorbent-assisted vapor capture can be seen as a type of pretreatment of condensation that concentrates the vapor and makes the subsequent cooling and condensation more efficient. ...
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In the context of global water scarcity, water vapor available in air is a non-negligible supplementary fresh water resource. Current and potential energetically passive procedures for improving atmospheric water harvesting (AWH) capabilities involve different strategies and dedicated materials, which are reviewed in this paper, from the perspective of morphology and wettability optimization, substrate cooling, and sorbent assistance. The advantages and limitations of different AWH strategies are respectively discussed, as well as their water harvesting performance. The various applications based on advanced AWH technologies are also demonstrated. A prospective concept of multifunctional water vapor harvesting panel based on promising cooling material, inspired by silicon-based solar energy panels, is finally proposed with a brief outlook of its advantages and challenges.
... Namib desert beetle, cactus, spider silk, and nepenthes are four kinds of typical prototypes for the biomimetic materials [43]. Recently, Chen el al. conducted an excellent article summing up water harvesting systems with biomimetic materials from spider silk, cactus, Namib desert beetle and introduced the preparation method and their principle in water adsorption [44]. Inspired by Nepenthes pitcher plants, Wong et al. developed the structure of slippery liquid-infused surfaces (SLIS) with self-healing liquid-repellency, sliding characteristics and great droplet mobility, which gained high cooling efficiency and plenty water collection [45]. ...
With the advantages of high efficiency, clean and renewable energy, the technology of sorption/desorption-based atmospheric water harvesting powered by solar energy has gradually attracted the increasing attention of researchers. In this review paper, through analyzing and summarizing the principles, the performance indexes, the measurements of parameters, the material types, the morphology modification, and the system design and devices of this technology, it was found that the existing research mainly focused on adsorbents, while there are still some imperfections, such as limited application circumstances, instability, discontinuous working and long cycle. Accordingly, the direction of future research is expected to redesign adsorbent materials for higher adsorption-desorption rate and wider range of applications, add heat accumulator and air-driving equipment to make system controllable and flexible, miniaturize devices for convenient utilization, and optimize systems to shorten the water collection cycle and to achieve rapid and continuous working. To follow the direction above, there are also some suggestions for adsorbents and system design.
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With the decrease in fresh water resources, fog collection has become a promising way to obtain fresh water. This paper presents an efficient fog collector that combines wettability and spokes geometry. Water droplets move along a fixed route on the mist collector and can easily overcome the pinning effect at the end of the structure to detach from the surface and then be collected. Its fog collecting efficiency can be 1.82 times that of a planar zinc sheet, which provides a new idea for the design of subsequent fog collectors. In this communication, based on the lack of fresh water resources, the method of combining geometric structure with wettability to prepare a fog collector is proposed. And the high efficiency of fog collection is realized, which provides a new idea for the design of fog collector.
A wide variety of pollutants can be currently found in water that are extremely difficult to remove due to their chemical composition and properties. A lot of effort has been made to tackle this issue that directly affects the environment. In this scenario, superhydrophobic surfaces, which have a water contact angle >150°, have emerged as an innovative technology that could be applied in different ways. Their environmental applications show promise in removing emerging pollutants from water. While the number of publications on superhydrophobic materials has remained largely unchanged since 2019, the number of articles on the environmental applications of superhydrophobic surfaces is still rising, corroborating the interest in this area. Herein, we briefly present the basis of superhydrophobicity and show the different materials that have been used to remove pollutants from water. We have identified five types of emerging pollutants that are efficiently removed by superhydrophobic materials: oils, microplastics, dyes, heavy metals, and ethanol. Finally, the future challenges of these applications are also discussed, considering the state of the art of the environmental applications of superhydrophobic materials.
The potential applications of textile materials in fog harvesting have long been demonstrated. This work designed novel fog harvesters according to the distinct features of elastic textile threads (ETTs) to enhance droplet capture, large-droplet growth, and droplet pouring and improve fog harvesting efficiency. We prepared m@ETTs (modified ETTs) using three novel chemical and physical methods. First, we prepared spandex elastic threads with a non-uniform rough surface containing silica nanoparticles and titanium particles through the sol-gel triethoxymethylsilane method. Second, we prepared a rubber/polyester thread with a rough surface by breaking the thread shell with toluene solution, creating knots on the surface of the rubber core. Third, we prepared a polyurethane thread with a bumpy superhydrophobic surface by spraying a tetrafluoroethylene adhesive and silica nanoparticles on the thread. Furthermore, we connected ETTs to an automatic stretching-recovery system to obtain auto-ETTs as another group of harvesters. We obtained auto-i@ETTs by introducing elastic bumps/knots onto the auto-ETT surface. The fog harvesting efficiencies of m@ETTs were approximately 60-120% greater than those of the ETTs. The water harvesting rate of the auto-i@ETT was 2.5 times that of the ETT, with the highest water harvesting rate of auto-i@ETT reaching 3.35 g/h/cm2. Moreover, several novel principles of droplet behavior and thread elasticity were revealed. The elastic elongation level of the ETTs was proportional to their water harvesting efficiency. The stretching-recovery state of the elastic thread did not influence the water contact angle but affected the droplet state on the thread surface. The temporary slack/stick state of adjacent elastic threads on auto-ETTs contributed to droplet convergence and pouring. Overall, this novel approach demonstrates the significant potential of elastic threads in fog harvesting applications.
Atmosphere water harvest offers a creative and straightforward way to tackle the global-range freshwater shortage, especially in remote areas and arid deserts. Inspired by the natural species, significant progress has been made to developing water harvesting materials and devices through the rational design of the structure and composition. In this review, we address mainly three questions. What is the design principle of a water harvesting material? How to improve the water collection efficiency of a material in an arid region? How to transform the materials into high-efficient water harvesting systems/devices? With these questions, we present a systematic interpretation of in-air water harvesting, from the materials to the devices, aiming to help find ever-advanced water harvesting systems. We summarize the important achievements towards their applications in the sustainable water supply at water-deficient regions, and for the agricultural irrigation system. The challenges and barriers that retard their applications and future research orientations in this field are also discussed.
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Inspired by the water-collecting mechanism of the Stenocara beetle's back structure, we prepared a superhydrophilic bumps-superhydrophobic/superoleophilic stainless steel mesh (SBS-SSM) filter via a facile and environmentally friendly method. Specifically, hydrophilic silica microparticles are assembled on the as-cleaned stainless steel mesh surface, followed by further spin-coating with a fluoropolymer/SiO2 nanoparticle solution. On the special surface of SBS-SSM, attributed to the steep surface energy gradient, the superhydrophilic bumps (hydrophilic silica microparticles) are able to capture emulsified water droplets and collect water from the emulsion even when their size is smaller than the pore size of the stainless steel mesh. The oil portion of the water-in-oil emulsion therefore permeates through pores of the superhydrophobic/superoleophilic mesh coating freely and gets purified. We demonstrated an oil recovery purity up to 99.95 wt % for surfactant-stabilized water-in-oil emulsions on the biomimetic SBS-SSM filter, which is superior to that of the traditional superhydrophobic/superoleophilic stainless steel mesh (S-SSM) filter lacking the superhydrophilic bump structure. Together with a facile and environmentally friendly coating strategy, this tool shows great application potential for water-in-oil emulsion separation and oil purification.
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Nowadays, water shortage has become a severe issue all over the world, especially in some arid and undeveloped areas. Interestingly, a variety of natural creatures can collect water from fog, which it can provide a source of inspiration to develop novel and functional water-collecting materials. Recently, as an increasingly hot research topic, bioinspired materials with water collection have captured vast scientific attentions into both practical applications and fundamental researches. In this review, we summarize the mechanisms of water collection in various natural creatures and also present the fabrications, functions, applications, and new developments in recent years. The theoretical basis related to the phenomenon of water collection containing wetting behaviors and water droplets transportation is firstly described, i.e., the Young’s equation, Wenzel model, Cassie model, surface energy gradient model and Laplace pressure equation. Then, the water collecting mechanisms of three typical and widely-researched natural creatures are discussed and their corresponding bioinspired materials are simultaneously detailed, which are cactus, spider, desert beetle, respectively. This is followed by introducing other eight animals and plants (butterfly, shore birds, wheat awns, green bristlegrass bristle, cotula fallux plant, Namib grass, green tree frogs and Australian desert lizards) that are rarely reported, also exhibiting water collecting properties or similar water transportation. Finally, conclusions and outlook concerning the future development of bioinspired fog-collecting materials are presented.
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Bumps are omnipresent from human skin to the geological structures on planets, which offer distinct advantages in numerous phenomena including structural color, drag reduction, and extreme wettability. Although the topographical parameters of bumps such as radius of curvature of convex regions significantly influence various phenomena including anti-reflective structures and contact time of impacting droplets, the effect of the detailed bump topography on growth and transport of condensates have not been clearly understood. Inspired by the millimetric bumps of the Namib Desert beetle, here we report the identified role of radius of curvature and width of bumps with homogeneous surface wettability in growth rate, coalescence and transport of water droplets. Further rational design of asymmetric convex topography and synergetic combination with slippery coating simultaneously enable self-transport, leading to unseen five-fold higher growth rate and an order of magnitude faster shedding time of droplets compared to superhydrophobic surfaces. We envision that our fundamental understanding and innovative design of bumps can be applied to lead enhanced performance in various phase change applications including water harvesting.
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A review article on fundamental aspects of thiolate self-assembled monolayers (SAMs) on the (111) and (100) surfaces of the Cu and Ni groups is presented. In particular this work is focused on two important points that remain poorly understood in most of these metals: the chemistry of the S-metal interface, which strongly depends on the nature of the metallic surface, and the role of the interaction forces that not only guide the self-assembly process but also influence the surface structure of SAMs. In addition to recent experimental and theoretical data on these issues we present new density functional calculations including van der Waals forces for an important number of known thiolate surface structures as a function of the hydrocarbon chain length
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Inspired by the water-collecting strategies of desert beetles and spider silk, a novel kind of surface with star-shaped wettablity patterns has been developed. By combining both wettability and shape gradients, the as-prepared surface has gained higher efficiency in water collection compared to circle-shaped wettability patterns and uniformly superhydrophilic or superhydrophobic surfaces.
Elastomeric stamps and molds provide a great opportunity to eliminate some of the disadvantages of photolithograpy, which is currently the leading technology for fabricating small structures. In the case of "soft lithography" there is no need for complex laboratory facilities and high-energy radiation. Therefore, this process is simple, inexpensive, and accessible even to molecular chemists. The current state of development in this promising area of research is presented here.
Three water adsorption-desorption mechanisms are common in inorganic materials: chemisorption, which can lead to the modification of the first coordination sphere; simple adsorption, which is reversible; and condensation, which is irreversible. Regardless of the sorption mechanism, all known materials exhibit an isotherm in which the quantity of water adsorbed increases with an increase in relative humidity. Here, we show that carbon-based rods can adsorb water at low humidity and spontaneously expel about half of the adsorbed water when the relative humidity exceeds a 50-80% threshold. The water expulsion is reversible, and is attributed to the interfacial forces between the confined rod surfaces. At wide rod spacings, a monolayer of water can form on the surface of the carbon-based rods, which subsequently leads to condensation in the confined space between adjacent rods. As the relative humidity increases, adjacent rods (confining surfaces) in the bundles are drawn closer together via capillary forces. At high relative humidity, and once the size of the confining surfaces has decreased to a critical length, a surface-induced evaporation phenomenon known as solvent cavitation occurs and water that had condensed inside the confined area is released as a vapour.
An ambient solution-state method of making uniform nanobrushes composed of oriented 1D silver nanowires (NWs) with aspect ratios of 10(2) -10(4) is reported. These structures are grown over cm(2) areas on conducting surfaces. Assemblies of NWs form uniform nanobrush structures, which can capture micron-sized objects, such as bacteria and particulate matter. Variation in composition produces unique structures with catalytic properties.
Wetting-transparent graphene films grown in situ by chemical vapor deposition on hydrophobic (roughened) copper surfaces offer excellent resistance to copper corrosion while maintaining the intrinsic hydrophobicity of the surface, enabling superior performance for water-harvesting applications.
Ultratrace detection attracts great interest because it is still a challenge to the early diagnosis and drug testing. Enriching the targets from highly diluted solutions to the sensitive area is a promising method. Inspired by the fog-collecting structure on Stenocara beetle's back, a photonic-crystal (PC) microchip with hydrophilic-hydrophobic micropattern was fabricated by inkjet printing. This device was used to realize high-sensitive ultratrace detection of fluorescence analytes and fluorophore-based assays. Coupled with the fluorescence enhancement effect of a PC, detection down to 10(-16) mol L(-1) was achieved. This design can be combined with biophotonic devices for the detection of drugs, diseases, and pollutions of the ecosystem.