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\begin{document}

\title{Amphipathic Janus nanofibers aerogel for efficient solar steam
generation}



\author[1]{Rui Wang}%
\author[1]{Jinshuo Deng}%
\author[1]{Ping Wu}%
\author[1]{Qianli Ma}%
\author[1]{Xiangting Dong}%
\author[1]{Wensheng Yu}%
\author[1]{Guixia Liu}%
\author[2]{进贤 王}%
\author[1]{Lei Liu}%
\affil[1]{Changchun University of Science and Technology}%
\affil[2]{Changchun university of science and technology}%


\vspace{-1em}



  
  \date{May 16, 2024}


\begingroup
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\selectlanguage{english}
\begin{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\selectlanguage{ngerman}·m-2·h-1 under 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.%
\end{abstract}\selectlanguage{ngerman}%



\sloppy


\textbf{Response to the Comments}

\textbf{Dear Editor,}

Thank you very much for your hard work in processing our manuscript and
also thanks a lot for the valuable comments and suggestions to improve
our paper. Based on these comments, we have made careful corrections on
the manuscript, and the corresponding revisions in the body text of the
manuscript are highlighted in yellow background. We submit a revised
manuscript here. The following are our point-to-point response to
reviewers concerning the comments about the manuscript.

\textbf{Reviewer \#1:}

The manuscript ''Amphipathic Janus nanofibers aerogel for efficient
solar steam generation.'' by Rui Wang et al. presents a novel
hydrophile/hydrophobe amphipathic Janus nanofibers aerogel, which can be
used as a host material for constructing the solar steam generator. The
prepared solar steam generator exhibits excellent water evaporation,
desalination and removal of organic pollutants performances. I have
several concerns as follows:

\textbf{Comment 1:} Currently, there are many solar evaporators with
Janus structure. What are the advantages of amphipathic Janus nanofibers
aerogel evaporator compared to other Janus evaporators?

\textbf{Response 1:} As the respected reviewer mentioned, many solar
evaporators with Janus structure have been reported. However, all of
these reported solar evaporators are ``macroscopically'' Janus
structure. As we described in the original manuscript: ``To date, there
have been many reports on macroscopical combination of hydrophilic and
hydrophobic materials for SSG, that is, hydrophilic and hydrophobic
materials are separated in different regions of the reported solar steam
generators, but no research on their microscopical combination can be
found'', the already reported solar evaporators with Janus structure can
fall into macroscopical combination of hydrophilic and hydrophobic
materials, in which the hydrophobic materials only provide the function
of floating. In our work, the amphipathic Janus nanofibers aerogel is
``microscopically'' Janus structure. In addition to providing
self-floating function, the hydrophobic PVB components also can limit
internal water content, suppress salt deposition and inhibit volume
expansion of swollen CA components (the discussion on inhibition of
volume expansion is newly added in the revised manuscript), and these
properties are owing to the ``microscopically'' Janus structure of
amphipathic Janus nanofibers aerogel. In order to more clearly point out
the advantages of this ``microscopically'' Janus structure, relevant
parts of the manuscript are revised.

\textbf{Comment 2:} The authors may need to evaluate the performance of
the material after 10 cycles to further show that the long-term
evaporation does not affect the material performance.

\textbf{Response 2:} As the respected reviewer suggested, the
reusability testing is extended to 20 cycles, as shown in the revised
manuscript. Due to the one-month time limit for submitting the revised
manuscript, we can only conduct 20 cycles testing at present, and longer
reusability testing can be provided when the final manuscript is
submitted.

\textbf{Comment 3:} The real photographs of CA//PVB Janus nanofibers
aerogel and the solar steam generator should be provided.

\textbf{Response 3:} As the respected reviewer's request, the
photographs of CA//PVB Janus nanofibers aerogel and the solar steam
generator are provided in the revised manuscript.

\textbf{Comment 4:} In this paper, the effects of aerogel density,
loading capacity and photothermal material composition on its
performance were studied respectively, and detailed conclusions and
comprehensive results should be given.

\textbf{Response 4:} The detailed conclusions and comprehensive results
for the optimized parameters are clarified in the revised manuscript.

\textbf{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\textsuperscript{-2} h\textsuperscript{-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:

\textbf{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\selectlanguage{ngerman}·m\textsuperscript{-2}·h\textsuperscript{-1} under 1 kW
m\textsuperscript{-2} irradiation).

\textbf{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.

\textbf{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.

\textbf{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.

\textbf{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.

\textbf{Response 3:} Figure 1 has been modified as requested.

\textbf{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.

\textbf{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!

\textbf{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?

\textbf{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.

\textbf{Comment 6:} What's the purified water collection rate in the
acrylic tank?

\textbf{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\textsuperscript{-2}·h\textsuperscript{-1}under 1 kW
m\textsuperscript{-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.\textsuperscript{{[}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.\textsuperscript{{[}4-6{]}} To date, many types of solar steam
generators, such as photothermal aerogels\textsuperscript{{[}7{]}},
hydrogels\textsuperscript{{[}8{]}}, foams\textsuperscript{{[}9{]}},
membranes\textsuperscript{{[}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.\textsuperscript{{[}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\textsuperscript{{[}14{]}},
salt-resisting\textsuperscript{{[}15{]}},
self-cleaning\textsuperscript{{[}16{]}}, and cold
evaporation\textsuperscript{{[}17{]}}.

The solar steam generators based on 3D aerogels are hot spot of current
researches.\textsuperscript{{[}18-20{]}} As a commonly used method for
preparing 3D aerogels, electrospinning combined with freeze drying has
been developed for several years.\textsuperscript{{[}21-24{]}} In
addition to their applications in SSG, electrospun 3D aerogels also
exhibit other application prospects in thermal
insulation\textsuperscript{{[}25{]}}, tissue
engineering\textsuperscript{{[}26{]}},
photocatalysis\textsuperscript{{[}27{]}}, oil/water
separation\textsuperscript{{[}28{]}} and \emph{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\textsuperscript{{[}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\textsuperscript{{[}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 \textbf{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
(SiO\textsubscript{2} NPs) and polydopamine (PDA), in which CNTs play a
prominent role in photothermal conversion, SiO\textsubscript{2} 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
nanofibers\emph{via} 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.\textsuperscript{{[}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
of\emph{ca.} 40 nm, are evenly distributed on the product surface, and
SiO\textsubscript{2} NPs are attached to the CNTs. Hierarchical pores
are formed among the CNTs and SiO\textsubscript{2} NPs, which can
facilitate water transfer and steam escape.

\textbf{2.2. Hydrophilicity}

The hydrophilicities of CA nanofibers, PVB nanofibers, CA//PVB Janus
nanofibers and photothermal materials are measured, as presented
in\textbf{Figure S3} . CA nanofibers can completely absorb the water
droplet within 0.5 s, manifesting their very high hydrophilicity. On the
contrary, the water droplet shows a stable contact angle of about 130 °
on hydrophobic PVB nanofibers, which indicates that PVB nanofibers alone
are not a suitable host material for constructing solar steam generators
because water cannot permeate through PVB nanofibers. As for the CA//PVB
Janus nanofibers, the water droplet is slowly absorbed, which brings an
important benefit: the water supply rate is not too high in SSG process.
Previous studies have found that too fast water supply, normally caused
by too high hydrophilicity of host materials, results in decreased
temperature of evaporation surface and reduced water evaporation
rate.\textsuperscript{{[}41{]}} Additionally, the hydrophile/hydrophobe
amphipathic CA//PVB Janus nanofibers can float on water surface, whereas
CA nanofibers cannot. Therefore, from the perspective of hydrophilicity,
CA//PVB Janus nanofibers should be more applicable for constructing
solar steam generators compared with CA nanofibers and PVB nanofibers.
As seen from Figure S3d, the water droplet is quickly absorbed into
photothermal materials, which is because the used CNTs,
SiO\textsubscript{2} NPs and PDA are hydrophilic. Hence, in the prepared
solar steam generator, the water pumped through the CA//PVB Janus
nanofibers aerogel can quickly and uniformly spread all over the outer
photothermal materials layer owing to the high hydrophilicity of the
photothermal materials, which is conducive to water evaporation.

\textbf{2.3. Solar Steam Generation Performance}

Firstly, the performance of solar steam generation using pure water as
analyte is evaluated to optimize the product composition. The impacts of
aerogel density, loading amount and composition of the photothermal
materials are respectively studied.

The density of an aerogel mainly affects saturation water content of the
solar steam generator. As seen from \textbf{Figure 2} a, the density of
solar steam generator becomes larger with increased mass ratio of Janus
nanofibers to dispersion medium in freeze drying process, while the
saturation water content is decreased. Figure 2b shows the water
evaporation performances of the solar steam generators. As the density
increases, the solar steam generators display increased first and then
decreased water evaporation rates. The increase in water evaporation
rates can be attributed to alleviative thermal dissipation from
evaporation surface to bulk water caused by decreased water contents in
the solar steam generators. The thermal conductivities of CA and PVB are
determined to be 0.17 W m\textsuperscript{-2}K\textsuperscript{-2} and
0.23 W m\textsuperscript{-2}K\textsuperscript{-2}, respectively, which
are substantially lower than that of water (0.59 W m\textsuperscript{-2}
K\textsuperscript{-2}). Hence, decreased water content in the solar
steam generator is beneficial to reduce heat dissipation and increase
the temperature of evaporation surface. As seen in Figure 2c, the
temperature (after one-hour irradiation) of evaporation surface
increases with increased density of the solar steam generator, whereas
that of the bulk water is reduced. However, the increased density of
solar steam generator simultaneously results in increased resistance of
water transfer, which is negative to water evaporation. Therefore,
appropriate aerogel density is an important basis to ensure
high-efficient water evaporation.

Secondly, optimized loading amount of photothermal materials is
explored, as shown in Figure 2d. It can be seen that the solar steam
generator with the photothermal materials of 0.016
g·cm\textsuperscript{-2} exhibits the best performance. Insufficient
photothermal materials cannot produce enough heat to sustain rapid water
evaporation. On the contrary, because the pores in the photothermal
materials are small (at the nanoscale), excessive photothermal materials
leads to too high resistance of water transfer and thus decreases water
evaporation rate.

Thirdly, the dosages of raw materials for preparing photothermal
materials are optimized. To start with, the concentration of dopamine is
fixed (y=2.4), while the dosage of TEOS is varied (x=0.1, 0.2, 0.3, 0.4,
0.5), and relevant SSG results are shown in Figure 2e. With the
increased dosage of TEOS (i.e. enlarged content of
SiO\textsubscript{2}NPs), the water evaporation rates reveal an
increasing trend at the beginning and then decrease when
x\textgreater{}0.3. To explain this phenomenon, at first, the surface
temperatures of the solar steam generators are measured. As seen from
Figure 2f, with the gradual increase of SiO\textsubscript{2}NPs content,
the surface temperatures of the solar steam generators show a falling
trend, which can be ascribed to the fact that the SiO\textsubscript{2}
NPs, although have been modified with dark-colored PDA, possess
relatively lower photothermal conversion performance than CNTs. The
light absorption spectra of the solar steam generators shown in Figure
2g can prove the degradation of light absorption performance with
increased SiO\textsubscript{2} NPs content. Therefore, from the
perspective of surface temperature, the performance enhancement of the
products within x=0.1 to 0.3 cannot be well explained. It has been known
that there are three kinds of water in a material: bound water (BW),
intermediate water (IW) and free water
(FW).\textsuperscript{{[}42{]}}Among them, BW is the water molecule in
direct contact with the hydrophilic materials. Because strong chemical
bonds are formed between BW and the materials, BW is difficult to
evaporate. The FW is relatively far from the adsorption materials, and
its property is consistent with the bulk water. The IW is sandwiched
between the BW and FW. Due to the interaction between the adjacent BW
and adsorption materials, the hydrogen bonds between IW and BW are
weakened, leading to the lowest energy required for the evaporation of
IW. That is, IW has the lowest evaporation enthalpy among the three
kinds of water.\textsuperscript{{[}43{]}} In this study,
SiO\textsubscript{2} NPs can fill in the gaps among CNTs and thus
regulate the water state in the photothermal materials. Figure 2h
visually depicts the increased proportion of IW and reduced proportion
of FW when more SiO\textsubscript{2} NPs are introduced. In addition,
the variation of BW can be ignored because BW is in monomolecular type
and its proportion is very low among the three kinds of water. It has
been reported that the ratio of IW to FW in a material can be
quantitatively measured by Raman spectroscopy, and Gaussian function can
be used to fit the peaks at 3233 cm\textsuperscript{-1}, 3401
cm\textsuperscript{-1}, 3514 cm\textsuperscript{-1} and 3630
cm\textsuperscript{-1}.\textsuperscript{{[}44{]}} The peaks at 3233
cm\textsuperscript{-1} and 3401 cm\textsuperscript{-1} correspond to the
in-phase and out-of-phase vibration modes of O-H bonds in water that
forms four hydrogen bonds with surrounding water molecules, which
represent the existence of FW. The peaks at 3514
cm\textsuperscript{-1}and 3630 cm\textsuperscript{-1} are assigned to
the symmetric and asymmetric stretching of O-H bonds in the weakened
hydrogen bonds, which are the characteristic peaks of IW that is
relatively weakly affected by hydrogen bond. The ratio of the fitted
peak areas of the two kinds of water is the molar ratio of the two kinds
of water. The Raman spectra of the hydrated photothermal materials are
given in \textbf{Figure S4} . According to the calculation of fitted
peak areas, the molar ratios of IW to BW in these samples (x=0.1, 0.2,
0.3, 0.4, 0.5) are 0.38, 0.52, 0.67, 0.71, 0.73, respectively, which
further demonstrates increased proportion of IW when more
SiO\textsubscript{2} NPs are introduced. However, excessive
SiO\textsubscript{2} NPs (x=0.4 and 0.5) cannot substantially increase
the proportion of IW, but make the light absorption performance of the
photothermal materials continuously degrade, as revealed in Figure 2g,
resulting in the decreased water evaporation rate.

The impact of PDA is evaluated while the ratio of CNTs and
SiO\textsubscript{2} NPs is fixed (x=0.3). Here, the PDA mainly acts as
an adhesive in the photothermal materials, and also contributes to
photothermal conversion due to its dark color. In order to study the
adhesive performance of the PDA, the solar steam generators with
different PDA contents were completely immersed in water for 1 h and
then naturally dried. The mass changes of the samples before and after
soaking are given in Figure 2i. When PDA is not enough (y=0.8 and 1.6),
some photothermal materials fall off the solar steam generators and
disperse into the water, and obvious mass losses of the samples are
found. When sufficient PDA is introduced (y\selectlanguage{english}[?]2.4), the photothermal
materials can be stable on the solar steam generators. In spite of this,
excess PDA content is still detrimental because the light absorption
performance is impaired and the water evaporation rate is decreased, as
indicated in Figure 2i-l.

Based on the above, the optimized parameters for preparing the solar
steam generator are as follows: the mass ratio of nanofibers to water is
0.0500:1 in the dispersion medium for freeze drying; the dosages of TEOS
(x) and dopamine (y) are respectively 0.3 and 2.4 for preparing the
photothermal materials; the loading amounts of the photothermal
materials is 0.016 g*cm\textsuperscript{-2}. The optimized solar steam
generator can provide a state-of-the-art water evaporation rate (2.944
kg m\textsuperscript{-2} h\textsuperscript{-1}) under 1 sun irradiation.
Next, the impact of light irradiation intensity on water evaporation
rate and energy efficiency is investigated. Light irradiation intensity
is one of the important factors affecting the water evaporation rate. In
order to further explore the water evaporation performance of the
optimized solar steam generator under different light irradiation
intensities, 0.5, 1, 1.5 and 2 suns are respectively adopted. As shown
in \textbf{Figure 3} a and b, water evaporation rates of 1.486
kg*m\textsuperscript{-2}*h\textsuperscript{-1}, 2.944
kg*m\textsuperscript{-2}*h\textsuperscript{-1}, 4.469
kg*m\textsuperscript{-2}*h\textsuperscript{-1} and 5.899
kg*m\textsuperscript{-2}*h\textsuperscript{-1} are achieved when the
irradiation intensities are 0.5, 1, 1.5 and 2 suns, while the energy
efficiencies are 88.74 \%, 91.05 \%, 91.94 \% and 92.83 \%,
respectively. In most reports on solar steam generators, energy
efficiency generally increases with the increase of irradiation
intensity.\textsuperscript{{[}10,32,45{]}} Of the solar steam generator
prepared in this work, the evaporation surface temperature significantly
rises with increased irradiation intensity, whereas the temperature of
bulk water varies little, which indicates excellent thermal insulation
performance of the prepared solar steam generator. The experimental
results show that the product possesses superior SSG performance under
different irradiation intensities, suggesting its application potential
in the natural environment where sunlight intensity changes frequently
and the conditions with concentrated sunlight by using optical
concentrators such as convex lens and heliostats.

The stability of solar steam generators is also a key factor affecting
their practical application. Hence, the real-time water content,
continuous working stability and reusability of the prepared solar steam
generator are studied. The test method of real-time water content is as
following: the solar steam generator is taken out from bulk water every
20 min during SSG process, the water drops on the surface of solar steam
generator are wiped off by using filter papers, and then the weight of
the waterlogged solar steam generator is measured and compared with that
of the dry solar steam generator. Continuous working stability is
evaluated by ten-hour uninterrupted irradiation, and the mass changes of
water are recorded per hour to calculate hourly water evaporation rates.
The reusability is studied by twenty-cycle SSG processes. Continuous
ten-hour irradiation is employed for each cycle, and the solar steam
generator is dried after each cycle.

The test results are shown in \textbf{Figure 4} a-c. The water content
of the solar steam generator is basically constant in SSG process,
indicating continuous and stable water pumping. The water evaporation
rates remain steady within both ten-hour continuous SSG process and
twenty -cycle reusing. These results manifest that the product possesses
excellent stability and has great application potential.

An unavoidable problem with solar steam generators in practical outdoor
applications is the flip of the generators due to the effects of wind,
water wave and \emph{etc} . In order to accommodate this situation, the
prepared cubic solar steam generator is fully covered with photothermal
materials on its six surfaces, so that its all surfaces can be used for
water evaporation, as depicted in Figure 4d. The performance recovery of
the solar steam generator after flipping is investigated, and the
specific experimental methods are as follows: in a sixty-minute SSG
experiment, the solar steam generator is randomly flipped every 10 min
to switch the evaporation surface. The temperature of evaporation
surface (Figure 4e) and water mass loss are recorded after flipping. In
order to meticulously investigate the effect of flipping on water
evaporation rate, the concept of ``interval mean evaporation rate'' is
proposed. As seen from the red dots in Figure 4f, every dot is also
calculated by Formula S1, and the irradiation time (\emph{T} ) and
initial mass of water are reset at the initial point of each interval.
The results reveal that temperature of evaporation surface can quickly
rise to more than 90 \% of the maximum temperature within 2 min, and
concurrently, the interval mean evaporation rate is recovered to near
maximum. The black line in Figure 4f presents the water mass change
(2.740 kg*m\textsuperscript{-2} h\textsuperscript{-1} in total) over the
course of sixty-minute SSG experiment with quintuplicate flipping of the
solar steam generator. It can be seen that the flipping of the solar
steam generator does not have a great influence on the total water
evaporation rate, meaning that the prepared solar steam generator is
adequate for dealing with dynamic water environment.

In order to clarify the superiority of using hydrophile/hydrophobe
amphipathic Janus nanofibers aerogels (CA//PVB-based product for short)
as host materials for solar steam generators, completely hydrophilic CA
nanofibers aerogel (CA-based product), hydrophobic PVB nanofibers
aerogel (PVB-based product) and CA-PVB blended nanofibers aerogel
(CA-PVB-based product) are also respectively fabricated and coated with
photothermal materials through the same freeze drying and air spraying
processes as those for preparing amphipathic Janus nanofibers aerogel.
The PVB-based product cannot be used as a solar steam generator at all
due to its inability to pump water. The CA-based product can pump water
but cannot float on water surface. Therefore, a scaffold has to be used
to immobilize the CA-based product on the water surface. CA-PVB-based
product also can float on water surface by itself. The water evaporation
performances of CA//PVB-based product, CA-PVB-based product and CA-based
product as well as the water without using any solar steam generators
(blank water), are compared. The results of water evaporation rates are
shown in \textbf{Figure 5} a. The CA//PVB-based product exhibits much
higher water evaporation rate than CA-PVB-based product and CA-based
product (2.944 kg m\textsuperscript{-2} h\textsuperscript{-1} \emph{vs}
. 0.771 kg m\textsuperscript{-2} h\textsuperscript{-1} and 2.125 kg
m\textsuperscript{-2} h\textsuperscript{-1}), and the evaporation rate
of blank water is only 0.414 kg
m\textsuperscript{-2}h\textsuperscript{-1}. Compared with CA//PVB-based
product, CA-PVB-based product shows a much lower water evaporation
performance, even though they have the same composition in raw
materials. Through carefully observing the evaporation surface of
CA-PVB-based product during SSG process, it can be found that almost no
water exists on the evaporation surface, proving a poor water-pumping
ability of CA-PVB-based product. As illustrated in Figure 5b, the CA
components of CA//PVB Janus nanofibers facilitate rapid upward pumping
of water molecules due to the closely situated hydrophilic groups on CA.
As for CA-PVB blended nanofibers, the presence of PVB molecular chains
results in an increased distance between hydrophilic groups, leading to
a much weaker water-pumping ability. Thus, a simple blending of
hydrophilic and hydrophobic materials is not applicable for fabricating
solar steam generators. The relatively lower water evaporation
performance of the CA-based product can be attributed to its excessive
internal water content (\textasciitilde{} 0.95 g
cm\textsuperscript{-3}), which aggravates heat dissipation, as revealed
in Figure 5c and d. In addition, it is also found that the CA-based
product exhibits poor reusability, as shown in \textbf{Figure S5} . In
order to explain this result, the physical photos of CA//PVB Janus
nanofibers aerogel, CA nanofibers aerogel, CA//PVB-based product and
CA-based product before and after water soaking are provided in
\textbf{Figure S6} . After 10 cycles, the photothermal materials are
seriously peeled off the CA-based product. This phenomenon is attributed
to the swelling of CA nanofibers aerogel in water. After water soaking,
CA-based product experiences a noticeable expansion in volume, which
creates internal stress between photothermal materials and the swollen
CA nanofibers aerogel and thus leads to the detachment of photothermal
materials. As for the CA//PVB-based product, the hydrophobic PVB
components suppresses the volume expansion of the CA components, and the
entire aerogel retains almost the same morphology before and after being
placed in water. Thus, photothermal materials can be stable on CA//PVB
Janus nanofibers aerogel during SSG process, thereby achieving excellent
reusability. From the above results, it is obvious that the
hydrophile/hydrophobe amphipathic Janus nanofibers aerogels are superior
for preparing solar steam generators in view of their high performance,
reusability and ease of use.

\textbf{2.4 Desalination and Pollutants Removal Performances}

Desalination and pollutants removal performances are important
application aspects of solar steam generators. Fast evaporation rate of
saline water and effective inhibition on salt crystallization are
crucial for desalination applications. The water evaporation
performances and salt-resisting properties of CA//PVB-based product and
CA-based product are comparatively studied. The 3.5 \% NaCl aqueous
solution, which is close to the average salinity of seawater, is adopted
as a model substance. As seen from \textbf{Figure 6} a, in the
first-hour SSG experiment, the water evaporation rates with
CA//PVB-based product and CA-based product are 2.909 kg
m\textsuperscript{-2}h\textsuperscript{-1} and 1.916 kg
m\textsuperscript{-2}h\textsuperscript{-1}, respectively. Compared with
the evaporation rates of pure water when using the two kinds of solar
steam generators, the evaporation rates of 3.5 \% NaCl solution decrease
1.19 \% and 9.84 \%, respectively. From careful observation on the red
curve in Figure 6a, the slope of the curve decreases with the extension
of evaporation time, which means CA-based product occurs obvious
performance attenuation during the process of treating saline water. In
order to amplify the effect of saline water evaporation on the two solar
steam generators, five-hour evaporation experiments are carried out, and
the photographs of the two post-use solar steam generators are provided
in Figure 6b and c. It is obviously seen that a lot of salt crystals
deposit on CA-based product, whereas the situation of CA//PVB-based
product is much better, demonstrating excellent salt-resisting property
of CA//PVB Janus nanofibers aerogel. On the one hand, salt crystals
block water transfer channels, and on the other hand, they reflect
light. Both of these effects negatively impact on water evaporation.
According to the references\textsuperscript{{[}32,46{]}}, it has been
proved that hydrophobic materials can inhibit the shuttle of salt ions.
Therefore, the excellent salt-resisting property of CA//PVB Janus
nanofibers aerogel can be owing to the hydrophobic PVB components. To
sum up, in terms of desalination property, the amphipathic Janus
nanofibers aerogel also shows its superiority over the hydrophilic
substrate.

The evaporation rates of saline water with different salinities when
using the CA//PVB-based product are also studied. The NaCl solutions
with different concentrations (1 \%, 3.5 \%, 5 \%, 10 \%, 20 \%) are
prepared as analytes, and the results are shown in Figure 6d. The
evaporation rate decreases with the increase of salinity, but the
difference is not great, which proves that the product can be able to
effectively treat high-salinity water. The desalination performances of
the CA//PVB-based product on saline water with different salinities are
revealed in Figure 6f, and the desalination results on actual seawater
(sampled from Bohai Sea, geological coordinate of sampling site:
40deg53\selectlanguage{english}'23.90\selectlanguage{english}"N, 121deg12\selectlanguage{english}'26.72\selectlanguage{english}"E) are given in Figure 6g. As revealed
in\textbf{Figure S7} , the SSG process is performed in a lab-made
acrylic tank (18 cm x 9 cm x 12 cm) in which the water vapor is
condensed and collected at a rate of 2.227 kg
m\textsuperscript{-2}h\textsuperscript{-1} (this value is obtained when
pure water is treated with the amphipathic Janus nanofibers aerogel
under 1 sun). The results demonstrate that the concentrations of salt
ions in the collected liquids greatly decrease compared with those of
the original saline water and actual seawater and are much lower than
the standard of the World Health Organization (WHO) and the United
States Environmental Protection Agency (EPA) for salt ion concentration
in drinking water. Moreover, it is also found that the prepared solar
steam generator is self-cleaning in the dark. As seen from Figure 6e,
after ten-hour light irradiation (close to the total energy of sunlight
in a day), some visible salt crystals deposit on the surface of product,
and these salt crystals disappear automatically within 4 h in the dark.
This character may be owing to the large pores in the aerogel that are
conducive to the rapid downward diffusion of salt ions under the driving
of concentration gradient between evaporation surface and bulk water.
This self-cleaning property is beneficial for performance recovery of
the product with day/night changes in outdoor applications. In addition,
a faster way to eliminate the influence of salt deposition is to flip
the solar steam generator. That is, after the evaporation surface is
altered, the water evaporation rate can be recovered quickly.

The treatment on wastewater containing antibiotics or organic dyes is
another essential application of SSG. To evaluate the related
performance of CA//PVB-based product, TCH, RhB and MB solutions, with
the concentrations of 0.01 g L\textsuperscript{-1}, are selected as
model substances. As reveal from Figure 6h and \textbf{Figure S8} , the
removal rates of these organic contaminants are all above 99 \% in the
condensed water.

\textbf{3. Conclusion}

In summary, hydrophile/hydrophobe amphipathic Janus nanofibers aerogel
is fabricated by the combination of parallel electrospinning technology
and freeze drying. Photothermal materials are covered on the six
surfaces of the cubic aerogel to obtain a 3D self-floating solar steam
generator. The light absorption rate of the product is above 95 \% in
the full spectrum range. Under light irradiation, the surface
temperature of the product rapidly rises to meet the requirement of fast
water evaporation within 2 min. The water evaporation rate can reach up
to 2.944 kg*m\textsuperscript{-2}*h\textsuperscript{-1} with the energy
efficiency of 91.05 \% under 1 sun irradiation, and after the product is
flipped, the water evaporation rate can be quickly recovered. Owing to
the salt-resisting property, the formation of salt deposition can be
effectively slowed down, so that the performance attenuation in the
evaporation of saline water is suppressed. The product possesses
excellent ability of solar desalination and removing organic pollutants
to acquire drinkable water.

\textbf{Conflict of Interest}

The authors have no conflict of interest to declare.

\textbf{Supporting Information}

Supporting Information is available from the Wiley Online Library or
from the author.

Acknowledgements

This work was financially supported by Natural Science Foundation of
Chongqing, China (cstc2021jcyj-msxmX0898), Natural Science Foundation of
Jilin Province (20210101080JC), National Natural Science Foundation of
China (51803012, 51573023).

Received: ((will be filled in by the editorial staff)) Revised: ((will
be filled in by the editorial staff)) Published online: ((will be filled
in by the editorial staff))

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\textbf{Figure Captions}

\textbf{Figure 1.} (a) Structure diagram of the prepared solar steam
generator; SEM imagery of (b) CA nanofibers, (c) PVB nanofibers, (d)
CA//PVB Janus nanofibers, (e) CA//PVB Janus nanofibers aerogel, and
(f-h) surface of the solar steam generator

\textbf{Figure 2.} (a) Densities and saturation water contents of solar
steam generators; (b) water mass change curves and (c) temperature
variation when using solar steam generators with different densities;
(d) water mass change curves when using solar steam generators with
different loading amounts of photothermal materials; (e) water
evaporation performances, (f) temperature variations and (g) light
absorption spectra of solar steam generators with different contents of
SiO\textsubscript{2} NPs; (h) illustration of water state in
photothermal materials; (i) mass losses of solar steam generators with
different PDA contents after water soaking; (j) water evaporation
performances, (k) temperature variations and (j) light absorption
spectra of solar steam generators with different contents of PDA

\textbf{Figure 3.} (a) Water mass change curves, (b) hourly evaporation
capacities and energy efficiencies, and (c) infrared thermal imaging
photographs of evaporation surface and bulk water under different
irradiation intensities

\textbf{Figure 4.} (a) Water content of the solar steam generator during
SSG process; (b) hourly water evaporation rates in ten-hour continuous
SSG process; (c) water evaporation rates in 20 cycles under 1 sun
irradiation; (d) illustration of flipped solar steam generator in
natural water environment; (e) temperature variation of evaporation
surface, (f) water mass change and interval mean evaporation rate with
quintuplicate flipped solar steam generator

\textbf{Figure 5.} (a) Water evaporation performances of contrast
samples; (b) illustration of water-pumping ability of CA//PVB Janus
nanofibers and CA-PVB blended nanofibers; (c) temperature variation
curves and (d) infrared thermal imaging photographs of contrast samples

\textbf{Figure 6.} (a) Mass change curves of 3.5 \% NaCl aqueous
solution with or without solar steam generators; salt deposition on (b)
CA-based product and (c) CA//PVB-based product after five-hour SSG; (d)
evaporation rates of NaCl solutions with different concentrations when
using CA//PVB-based product; (e) self-cleaning property of CA//PVB-based
product; desalination performances of CA//PVB-based product on (f) NaCl
solutions with different concentrations and (g) actual seawater; (h)
organic compounds removal performances of CA//PVB-based product\selectlanguage{english}
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