Reviewer #2:
In this manuscript, the authors described a hydrophile/hydrophobe
amphipathic Janus nanofibers aerogel for constructing the solar steam
generator. The water evaporation rate can reach 2.944 kg
m-2 h-1 with an energy efficiency of
91.05% under 1 sun. The flipping issue is also involved, resulting in a
quick recovery of the water evaporation rate. Overall, this article is
well-organized but the novelty and some crucial point of this work needs
further refinement. Thus, I do not think that this work qualified enough
to be published on Advanced Functional Material:
Comment 1: The authors mentioned that: ”hydrophobic or
hydrophilic material alone hardly achieves adequate property for
constructing solar steam generators.” However, many works using
hydrophilic materials exhibit completive, even higher evaporation rates
compared to that proposed in this manuscript (2.944
kg·m-2·h-1 under 1 kW
m-2 irradiation).
Response 1: We acknowledge the respected reviewer’s observation
that hydrophilic materials can provide high water evaporation rates.
Nevertheless, the solar steam generators consisting only of hydrophilic
materials hardly independently float on water surface, and additional
supporting materials are necessary. Besides, hydrophilic materials are
not effective in preventing the shuttle of salt ions. As for the
sentence mentioned by the respected reviewer, what we wanted to express
is “hydrophilic or hydrophobic materials alone are insufficient in
achieving necessary characteristics for constructing high-quality solar
steam generators with good comprehensive properties”, high water
evaporation rate is just one of important characteristics for solar
steam generators, and the properties such as self-floating, salt
resistance, and stability are also crucial factors. In order to avoid
misunderstanding, we have modified the relevant statement.
Comment 2: According to the SEM images (Figure 1) of nanofibers
aerogel and solar steam generator, the surface and the pores of aerogel
are filled by photothermal materials. The hydrophilicity/hydrophobicity
of nanofiber still plays a key role in water evaporation. The floating
feature is due to the hydrophobic/hydrophilic Janus nanofiber or just
because of the density change? Will the blended single nanofiber show a
similar property? Blended electrospinning instead of parallel spinning
is suggested to fabricate non-Janus structural nanofiber for comparison.
The merits of hydrophile/hydrophobe amphipathic Janus nanofiber
structure should be highlighted and completely clarified since it is the
prominent description in the Title.
Response 2: Thanks a lot for the respected reviewer’s good
suggestions! Accordingly, CA-PVB blended nanofibers aerogel is
fabricated by single-nozzle electrospinning for comparison. It is found
that the solar steam generator based on CA-PVB blended nanofibers
aerogel shows a very poor water evaporation rate, and relevant
discussion is provided in the revised manuscript. Besides, the cycling
performance of the solar steam generator based on CA nanofibers aerogel
is added in the revised manuscript. The merits of the amphipathic Janus
nanofibers aerogel can be highlighted and completely clarified based on
these supplemental experimental results.
Comment 3: All figures should be mentioned in the text in
numerical or alphabetical order. Namely, Figure 1a should be organized
and mentioned before Figure 1b-e.
Response 3: Figure 1 has been modified as requested.
Comment 4: The authors investigated the influence of water
states (BW, IW and FW) on evaporation rate due to the silica NPs in
detail. But the nanofibers aerogel evaporator is a 3D evaporator, the
environmental enhancement on evaporation should be considered as well.
Response 4: There are indeed many other reports on 3D
evaporators that have investigated environmental enhancement effects
such as airflow, irradiation angle, and cold evaporation. However, these
environmental enhancement effects are significant only when the
evaporators are exposed above the water surface at considerable heights.
That is, the exposed side area is large enough. Although our products
are also 3D evaporators, their height above the water surface is less
than 1 cm and can be considered negligible in terms of environmental
enhancement. In addition, because there is no unified standard of
experimental parameters for environmental enhancement in this field, it
is hard to select widely approved experimental conditions. Based on the
above reasons, the ambient temperature, humidity and irradiation angle
are constant in this work, and no airflow is engaged. Thanks a lot for
your understanding in advance!
Comment 5: The authors claimed that ”the flipping recovery of
the solar steam generator is an important issue in practical outdoor
applications.” and flipping experiments of a cubic solar steam generator
were carefully conducted. On the other hand, purified water collection
is still a key process for solar desalination from the practical view,
which the authors also present in the manuscript. So, is the flipping
recovery experiments carried out in the evaporation/collection system
more meaningful rather than the cubic evaporator?
Response 5: Due to the lack of a standardized preparation
protocol for water collection systems in this field, the performance of
solar steam generators in different collection systems is influenced by
factors such as material, shape, and size of collection systems. To
eliminate these factors on experimental results, it is common practice
in this industry to evaluate the performance of solar steam generators
outside of the collection system whenever possible, and only
desalination and contaminant removal properties are tested within the
collection system. We conducted experimental design in accordance with
the methodology employed in relevant literature.
Comment 6: What’s the purified water collection rate in the
acrylic tank?
Response 6: The purified water collection rate in the used
acrylic tank is provided in the revised manuscript. Please note that the
water collection rate is influenced not only by the performance of the
solar steam generator itself, but also by the material, shape and size
of the collection system. Therefore, this value should be used as a
reference only.
We have tried our best to modify the manuscript in order to meet the
requirements of reviewers, and we sincerely look forward to receiving
your positive responses!
Thanks a lot for the time and efforts you have spent on our paper!
Sincerely yours,
Prof. Qianli Ma (Corresponding author)
Article category: Full Paper
Subcategory: Solar steam generation
Amphipathic Janus nanofibers aerogel for efficient solar steam
generation
Rui Wang, Jinshuo Deng, Ping Wu, Qianli Ma*, Xiangting Dong*, Wensheng
Yu, Guixia Liu, Jinxian Wang, and Lei Liu
Rui Wang, Jinshuo Deng, Ping Wu, Qianli Ma*
Chongqing Research Institute, Changchun University of Science and
Technology, Chongqing 401135, China
E-mail: maqianli@cust.edu.cn
Xiangting Dong*, Wensheng Yu, Guixia Liu, Jinxian Wang, Lei Liu
Key Laboratory of Applied Chemistry and Nanotechnology at Universities
of Jilin Province, Changchun University of Science and Technology,
Changchun 130022, China
E-mail: xtdong@cust.edu.cn
Keywords: solar steam generation, photothermal materials,
electrospinning, aerogels, solar desalination
Abstract: Solar steam generation is a promising water purification
technology due to its low-cost and environmentally friendly applications
in water purification and desalination. However, hydrophilic or
hydrophobic materials alone are insufficient in achieving necessary
characteristics for constructing high-quality solar steam generators
with good comprehensive properties. Herein, novel hydrophile/hydrophobe
amphipathic Janus nanofibers aerogel is designed and used as a host
material for preparing solar steam generators. The product consists of
an internal cubic aerogel and an external layer of photothermal
materials. The internal aerogel is composed of electrospun amphipathic
Janus nanofibers. Owing to the unique composition and structure, the
prepared solar steam generator integrates the features of high water
evaporation rate (2.944 kg·m-2·h-1under 1 kW m-2 irradiation), self-floating,
salt-resisting, and fast performance recovery after flipping. Moreover,
the product also exhibits excellent properties on desalination and
removal of organic pollutants. Compared with traditional hydrophilic
aerogel host material, the amphipathic Janus nanofibers aerogel brings
much higher water evaporation rate and salt resistance.
1. Introduction
Solar steam generation (SSG) is an
emerging water purification technology and has become a research hotspot
in the field of water treatment in recent
years.[1-3] Its working principle is to utilize
solar energy to convert water from waste water or sea water into water
steam which is then collected and condensed into fresh water. This
technique can separate water from contaminants and other impurities in a
process similar to the water cycle in the natural environment. Hence, it
is an eco-friendly and low-cost water purification strategy that does
not require extra energy.
However, due to the low light absorption and photothermal conversion
efficiency of water, an efficient SSG process requires placing the
materials with the function of photothermal conversion on the surface of
water to realize interfacial heating, and these materials are usually
called solar steam generators or solar
absorbers.[4-6] To date, many types of solar steam
generators, such as photothermal aerogels[7],
hydrogels[8], foams[9],
membranes[10], have been fabricated.
Macroscopically, solar steam generators can be classified into
one-dimensional (1D), 2D and 3D materials. Among them, 3D solar steam
generators are considered to be the most promising materials owing to
their high water evaporation rates and
versatility.[11-13] In general, 3D solar steam
generators have large thicknesses, which is conductive to reduce thermal
dissipation from evaporation surface to bulk water and also enhance the
energy efficiency by allowing multiple reflections of light inside the
3D solar steam generators. Moreover, the 3D structure provides more
feasibility for structural design. By far, many different shapes of 3D
solar steam generators have been prepared to endow the materials with
the features such as self-floating[14],
salt-resisting[15],
self-cleaning[16], and cold
evaporation[17].
The solar steam generators based on 3D aerogels are hot spot of current
researches.[18-20] As a commonly used method for
preparing 3D aerogels, electrospinning combined with freeze drying has
been developed for several years.[21-24] In
addition to their applications in SSG, electrospun 3D aerogels also
exhibit other application prospects in thermal
insulation[25], tissue
engineering[26],
photocatalysis[27], oil/water
separation[28] and etc . However in the
existing reports, without taking into account the post-processing
procedures, the aerogels prepared by direct electrospinning are either
completely hydrophilic or hydrophobic, which causes some issues in the
application of SSG. Although hydrophilic aerogels can pump water to the
evaporation surface, the excessive water inside the aerogels acts as a
”thermal bridge” which results in serious thermal dissipation from
evaporation surface to bulk water. For another, because hydrophilic
aerogels cannot float on the water surface, additional supporting
materials are usually necessary. By contrast, completely hydrophobic
aerogels are hardly suitable for preparing solar steam generators due to
the incapability of pumping water to the evaporation surface. Therefore,
it is rational that proper combination of hydrophilic and hydrophobic
materials should be an ideal solution for constructing solar steam
generators. To date, there have been many reports on macroscopical
combination of hydrophilic and hydrophobic materials for
SSG[29-32], that is, hydrophilic and hydrophobic
materials are separated in different regions of the reported solar steam
generators, and some reported solar steam generators possess Janus
structure[32-37]. The term “Janus structure” in
these works refers to bilayer structure at the macro level, where the
hydrophilic materials acts as a water pump, while the hydrophobic
materials enable the solar steam generators to float on water surface.
However, no research on microscopical combination of hydrophilic and
hydrophobic materials can be found. Therefore, it is meaningful to
construct a solar steam generator composed of microscopically bonded
hydrophilic and hydrophobic materials and discover the advantages of
this design philosophy.
Herein, novel hydrophile/hydrophobe amphipathic Janus nanofibers aerogel
is first proposed and used as a host material for constructing the solar
steam generator. Every amphipathic Janus nanofiber in the aerogel is
composed of a hydrophilic cellulose acetate (CA) side and a hydrophobic
polyvinyl butyral (PVB) side, forming a “microscopically” Janus
structure, as depicted in Figure 1 a. Such peculiar architecture
and composition endow the aerogel with water-pumping, self-floating,
heat-insulating, salt-resisting properties and excellent continuous
working stability which are crucial characteristics for solar steam
generators. To obtain the solar steam generator, the amphipathic Janus
nanofibers aerogel is coated by photothermal materials consisted of
carbon nanotubes (CNTs), silicon dioxide nanoparticles
(SiO2 NPs) and polydopamine (PDA), in which CNTs play a
prominent role in photothermal conversion, SiO2 NPs can
regulate the water state, and PDA is the adhesive and contributes to
photothermal conversion as well. The prepared solar steam generator
exhibits excellent water evaporation, desalination and removal of
organic pollutants performances. Another advantage of the solar steam
generator is that its water evaporation performance can quickly recover
after it is flipped on the water surface, which is a meaningful feature
for dealing with dynamic water environment.
2. Results and Discussion
2.1. Structural Characterization
The microstructures of CA nanofibers, PVB nanofibers, CA//PVB Janus
nanofibers, CA//PVB Janus nanofibers aerogel and the surface of the
solar steam generator are determined by SEM observation. As shown in
Figure 1b and c, the prepared CA nanofibers and PVB nanofibers have
similar morphologies, and their diameters are about 600 nm. Figure 1d
reveals that every CA//PVB Janus nanofiber is composed of two tightly
adjacent nanofibers whose diameters are both about 600 nm, indicating a
typical Janus nanofiber structure. Although it is hard to determine the
difference in the chemical compositions of the two adjacent nanofibers
by existing material characterization techniques because CA and PVB both
consist of elemental C, H and O, it is still safe to conclude that the
two adjacent nanofibers are respectively comprised of CA and PVB
according to the existing reports on fabrications of Janus nanofibersvia parallel electrospinning. In these reports, it has been
proved that the two adjacent nanofibers of a Janus nanofiber are
respectively derived from two spinning solutions for parallel
electrospinning.[38-40] Figure 1e manifests that
the Janus nanofibers in the aerogel are successfully crosslinked, which
reinforces the shape stability of the aerogel. The crosslinked Janus
nanofibers have slightly larger sizes than those before crosslinking,
which can be due to the swelling of polymer nanofibers in the solution
containing crosslinking agent and volume expansion during thermal
crosslinking. Furthermore, it can be noticed that micron-sized irregular
channels exist among the Janus nanofibers and can allow water transfer.
Figure 1f-h are SEM imagery of the surface of solar steam generator at
different levels of magnification. The CNTs, with the diameters ofca. 40 nm, are evenly distributed on the product surface, and
SiO2 NPs are attached to the CNTs. Hierarchical pores
are formed among the CNTs and SiO2 NPs, which can
facilitate water transfer and steam escape.