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\begin{document}

\title{Magnetic Field Induced the Assembling of Photothermal Evaporator for
Efficient Solar-driven Desalination}



\author[1]{Mingyu Zhou}%
\author[2]{Peng Han}%
\author[2]{Guicun Qi}%
\author[2]{Dali Gao}%
\author[3]{Hamdy Abdel-Ghafar}%
\author[1]{Yuchao Wang}%
\author[1]{Shengyang Tao}%
\affil[1]{Dalian University of Technology}%
\affil[2]{Sinopec Beijing Research Institute of Chemical Industry}%
\affil[3]{Central Metallurgical Research and Development Institute}%


\vspace{-1em}



  
  \date{June 06, 2024}


\begingroup
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\selectlanguage{english}
\begin{abstract}
Solar-driven interfacial evaporation is a sustainable and economical
technology for fresh water generation. Structural design of photothermal
material is an effective strategy to improve the evaporation performance
but usually bothered by complicated processes and non-adjustability.
Herein, magnetic nanoparticles assembled photothermal evaporator was
developed, which showed an adjustable spinal array surface under uniform
magnetic field induction. By regulating position in the magnetic field,
the desirable surface structures could be uniform at relatively low load
density of magnetic nanoparticles to improve light absorption via
multiple reflection. Magnetic field induced evaporator could accelerate
evaporation to over 1.39 kg m\textsuperscript{-3} h\textsuperscript{-1}
under 1-sun illumination, which was 2.8 times that of natural
evaporation. After coated by carbon layer, magnetic nanoparticles could
overcome the oxidation to realize stable evaporation in long-term
desalination. The facile strategy to optimize the surface structure via
magnetic field is appropriate for various fields with special
requirements on surface structure.%
\end{abstract}%



\sloppy


Magnetic Field Induced the Assembling of Photothermal Evaporator for
Efficient Solar-driven Desalination

Mingyu Zhou, Peng Han, Guicun Qi, Dali Gao, Hamdy Maamoun Abdel-Ghafar,
Yuchao Wang* and Shengyang Tao*

Mr. M. Zhou, Prof. Y. Wang, Prof. S. Tao

State Key Laboratory of Fine Chemicals, Frontier Science Center for
Smart Materials Oriented Chemical Engineering, School of Chemical,
Dalian University of Technology, Dalian 116024, China E-mail:
wangyuchao@dlut.edu.cn; taosy@dlut.edu.cn

Dr. P. Han, Dr. G. Qi, Dr. D. Gao SINOPEC Beijing Research Institute of
Chemical Industry, Beijing, 100013, People's Republic of China

Prof. H. M. Abdel-Ghafar

Central Metallurgical Research and Development Institute, P.O. Box: 87,
Helwan, 11421 Cairo, Egypt

Keywords: photothermal evaporation, desalination, carbon-coated magnetic
nanoparticles, magnetic field induced, adjustable surface structure

\textbf{Abstract}

Solar-driven interfacial evaporation is a sustainable and economical
technology for fresh water generation. Structural design of photothermal
material is an effective strategy to improve the evaporation performance
but usually bothered by complicated processes and non-adjustability.
Herein, magnetic nanoparticles assembled photothermal evaporator was
developed, which showed an adjustable spinal array surface under uniform
magnetic field induction. By regulating position in the magnetic field,
the desirable surface structures could be uniform at relatively low load
density of magnetic nanoparticles to improve light absorption via
multiple reflection. Magnetic field induced evaporator could accelerate
evaporation to over 1.39 kg m\textsuperscript{-3}
h\textsuperscript{-1}under 1-sun illumination, which was 2.8 times that
of natural evaporation. After coated by carbon layer, magnetic
nanoparticles could overcome the oxidation to realize stable evaporation
in long-term desalination. The facile strategy to optimize the surface
structure via magnetic field is appropriate for various fields with
special requirements on surface structure.

1. Introduction

With serious water pollution and the energy-saving demands for water
purification, solar-driven evaporation has received extensive attention
due to its potential in terms of water, energy, and environmental
sustainability,\textsuperscript{1-4} which is widely recognized as a
novel desalination technology for solving water resource scarcity
issues.\textsuperscript{5-7} To realize a desirable evaporation
efficiency, various kinds of photothermal materials, have been employed
to improve the light application thanks to their high absorption of
broad-spectrum solar radiation and light-to-heat
conversion.\textsuperscript{8-10} Besides the filtering of ideal
materials inside the range of dark matter, the logically designed
surface structure is also critical for high performance on solar
absorption.\textsuperscript{11,12} The rough photothermal surface with a
suitable structure can induce the light transfer on microscale to
dramatically reduce the diffuse reflection that is the primary type of
light loss.\textsuperscript{13,14} For the classic super-black material,
thanks to the vertically aligned array structures, carbon nanotube (CNT)
achieved approximate complete light absorption in the whole solar
spectrum with an absorbance up to 99.97\%.\textsuperscript{15}Similarly,
confining the plasmonic nanoparticles inside the sub-micron hole
channels formed a broadband absorber enabling an absorbance around 99\%
across the wavelengths from 400 nm to 10 \selectlanguage{greek}μ\selectlanguage{english}m.\textsuperscript{16}

Actually, the superior photothermal performance tends to be revealed by
a delicate surface structure that is constructed via complicated
processing and rigorous preparation conditions, no matter based on
top-down etching or bottom-up assembly.\textsuperscript{17-19} Besides
the high production costs, fragility of the fine structure is another
factor to limit the application of efficient light absorbers in
photothermal evaporation. To simplify the structure construction,
external field effects are introduced to induce the controllable
assembly of nanoparticles to form the proposed
structure.\textsuperscript{20-24} In particular, magnetic field is
considered an ideal tool for applications in structure
management,\textsuperscript{25-27} especially in the construction of
functional structures with reconfigurable, recoverable and dynamically
adjustable.\textsuperscript{28-30} In Jeong's work,
Fe\textsubscript{3}O\textsubscript{4} nanoparticles were sprayed onto a
Si substrate with patterned nickel islands, and attracted by the
external magnetic field to form magnetic
micropillars.\textsuperscript{31} Besides, some conical micro-tip
patterns could be molded with the aid of a magnetic
field.\textsuperscript{32} In Song's work, the mixture of
polydimethylsiloxane (PDMS) polymer with magnetic particles was coated
onto the substrate and prepared Janus membranes with cacti-spine arrays
by magnetic field-assisted molding.\textsuperscript{33}

In our previous work, magnetic
Fe\textsubscript{3}O\textsubscript{4}nanoparticles were directly used to
assemble the spine arrays as photothermal surface following the effect
of magnetic field, which revealed an obvious improvement in light
harvesting.\textsuperscript{34}However,
Fe\textsubscript{3}O\textsubscript{4} tends to be oxidized in high
temperature and high humidity conditions during evaporation. The
generated Fe\textsubscript{2}O\textsubscript{3} shows a wider band gap
indicating the weakening of photothermal
performance.\textsuperscript{35} Additionally, non-uniform magnetic
field distribution on the magnet surface induced the formation of an
uneven spiny structure, meanwhile, resulting in a relatively high
loading density of magnetic nanoparticles to ensure an integrity
photothermal layer. Therefore, it is necessary to alleviate the
performance degradation caused by the oxidation of
Fe\textsubscript{3}O\textsubscript{4} and explore the assembly behavior
of magnetic nanoparticles at various relative distances.

Based on the above discussion, in this work, we fabricated a magnetic
field induced photothermal evaporator assembled by carbon-coated
Fe\textsubscript{3}O\textsubscript{4}(Fe\textsubscript{3}O\textsubscript{4}@C)
nanoparticles, which revealed the adjustable surface structure
corresponding to the relative distance to magnet. To improve the
stability of magnetic nanoparticles, we synthesized
Fe\textsubscript{3}O\textsubscript{4}@C by a simple one-step method of
coating Fe\textsubscript{3}O\textsubscript{4} nanoparticles with a layer
of carbon using glucose as the carbon source (Figure 1). The outer
carbon layer of Fe\textsubscript{3}O\textsubscript{4}@C nanoparticles
could provide effective protection for the
Fe\textsubscript{3}O\textsubscript{4} and maintain a stable photothermal
performance after the oxidation of magnetic core. At an appropriate
relative distance to the magnet, Fe\textsubscript{3}O\textsubscript{4}@C
nanoparticles assembled into a complete and well-distributed
photothermal layer with spine arrays at a low load density of 1 mg
mm\textsuperscript{-2} (which was 3 mg mm\textsuperscript{-2} in our
previous work) (Figure S1). With the optimized surface structure, the
Fe\textsubscript{3}O\textsubscript{4}@C assembled evaporator obviously
improved the evaporation rate up to 1.39 kg
m\textsuperscript{-2}h\textsuperscript{-1} for water and 1.31 kg
m\textsuperscript{-2}h\textsuperscript{-1} for seawater under 1-sun
illumination. During 8 hours of continuous evaporation for seawater and
4 times regeneration, there was less than 4\% performance degradation
for Fe\textsubscript{3}O\textsubscript{4}@C assembled evaporator
indicating desirable stability. Magnetic field induced adjustable
surface structure with spiny arrays is suitable for efficient
solar-driven desalination, as well as a universal method for the
structure control to satisfy the requirements of other applications in
water treatment.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{FIGURE 1} Schematic diagram of the synthesis of
Fe\textsubscript{3}O\textsubscript{4}@C and adjustment of the surface
structure by controlling relative distance to the magnet. The inserted
image shows light reflection inside the spine arrays.

2. Results and Discussion

2.1. Characterization of carbon-coated nanoparticles

The carbon-coated nanoparticles were synthesized by a typical
hydrothermal method. After coating with a carbon layer, the
carbon-coated nanoparticles remain granular and have a certain
dispersion (Figure S2a-b). In the experiment, the thickness of carbon
layer was changed by adjusting the amount of glucose as carbon source.
As TEM images shown in Figure 2a-c,
Fe\textsubscript{3}O\textsubscript{4}@C had a carbon layer at the
thickness of \textasciitilde{}15 nm. While, as 2 g of glucose was used
in preparation, the thickness of carbon layer of
Fe\textsubscript{3}O\textsubscript{4}@tC was up to \textasciitilde{}28
nm. Following the particle size distribution in Figure 2d, the major
particle sizes of Fe\textsubscript{3}O\textsubscript{4},
Fe\textsubscript{3}O\textsubscript{4}@C and
Fe\textsubscript{3}O\textsubscript{4}@tC were about 666 nm, 691 nm and
715 nm, respectively. In comparison to the pristine magnetic
nanoparticle, Fe\textsubscript{3}O\textsubscript{4}@C and
Fe\textsubscript{3}O\textsubscript{4}@tC revealed increases in diameter
of 25 nm and 49 nm, which were generally consistent with the results in
TEM images.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image2/image2}

\end{center}
\end{figure}

\textbf{FIGURE 2} TEM images (a-c), particle sizes distributions (d),
Raman spectra (e) and XRD spectra (f) of
Fe\textsubscript{3}O\textsubscript{4},
Fe\textsubscript{3}O\textsubscript{4}@C and
Fe\textsubscript{3}O\textsubscript{4}@tC.

Raman spectra of as-prepared samples were investigated to analysis the
carbon layer (Figure 2e). There were two typical D band and G band at
1350 and 1580 cm-1 respectively confirmed in both
Fe\textsubscript{3}O\textsubscript{4}@C and
Fe\textsubscript{3}O\textsubscript{4}@tC, which indicated the relatively
low graphitization of the carbon layer.\textsuperscript{36} In XRD
patterns, pristine nanoparticles showed the diffraction peaks at 30, 35,
43, 53, 57 and 63\selectlanguage{ngerman}° corresponding to the (220), (311), (400), (422),
(511) and (440) of Fe\textsubscript{3}O\textsubscript{4} crystal.
Fe\textsubscript{3}O\textsubscript{4}@C and
Fe\textsubscript{3}O\textsubscript{4}@tC showed similar patterns without
a noticeable change in these characteristic peaks, indicating no
oxidation during the hydrothermal synthesis (Figure 2f).

2.2. Characterization of magnetic photothermal evaporator

The morphology assembled by the magnetic nanoparticles was controlled by
the magnetic field. Therefore, the numerical simulation based on
finite-element analysis was applied for the analysis of the magnetic
field distribution (Figure 3 and Figure S4).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image3/image3}

\end{center}
\end{figure}

\textbf{FIGURE 3} The distribution of magnetic field strength at
different distances from the center of the magnet in vertical direction
(a). The red line and gray line represent the actual measured data and
the simulated data respectively. The inserted image is schematic diagram
of the magnet with a diameter of 30 mm. Optical photographs of surface
morphology on magnetic evaporator before and after wetted by water with
Fe\textsubscript{3}O\textsubscript{4} (b),
Fe\textsubscript{3}O\textsubscript{4}@C (c) and
Fe\textsubscript{3}O\textsubscript{4}@tC (d) as photothermal materials,
respectively. The scale bar was 5 mm. Simulated magnetic field
distribution at various distances with the corresponding magnetic field
strength (e, f). Spiny morphology of the photothermal layers formed by
Fe\textsubscript{3}O\textsubscript{4}@C at various distances (g-j). The
scale bar was 6 mm.

As shown in Figure 3a, the simulated magnetic strength of the magnet
edge could reach over 150 mT. While the strength gradually decreased
from edge to center down to just \textasciitilde{}20 mT. Such result was
generally consistent with the data from actual measurements (red line in
Figure 3a and Figure S5). Because of the non-uniform magnetic field
distribution, as the distance between evaporator surface and the magnet
below was close (0.1 cm),
Fe\textsubscript{3}O\textsubscript{4}nanoparticles were preferentially
distributed on the edge with stronger magnetic fields. Comparatively,
the central region with weaker magnetic field strength exposed the
underlying air-laid paper due to fewer magnetic nanoparticles as the
loading density of just 1 mg mm\textsuperscript{-2} (Figure 3b).

As Fe\textsubscript{3}O\textsubscript{4} was replaced by
Fe\textsubscript{3}O\textsubscript{4}@C, spines of the
Fe\textsubscript{3}O\textsubscript{4}@C turned denser and smaller, and
tended to form a more well-distributed spiny structure. The exposed
region in the center of the magnet was relatively minimized (Figure 3c).
It was speculated that the change in the distribution of carbon-coated
nanoparticles was attributed to the decrease in magnetic intensity of
Fe\textsubscript{3}O\textsubscript{4}@C, which resulted in deviation of
the magnetic particle arrangement from the magnetic field (Figure S3).
Owing to the weaker magnetic property,
Fe\textsubscript{3}O\textsubscript{4}@tC assembled into fewer spines and
covered the magnet surface completely.

During practical solar-driven evaporation, photothermal layer would be
wetted by water, and it was necessary to investigate morphology in the
wet state. After wetting, the exposed regions of the spiny surfaces
composited by Fe\textsubscript{3}O\textsubscript{4} and
Fe\textsubscript{3}O\textsubscript{4}@C tended to be larger, and some
small spines gathered into larger ones driven by the surface tension of
water (Figure 3b-c). However, for
Fe\textsubscript{3}O\textsubscript{4}@tC, there was no obvious change in
its spiny morphology after wetting (Figure 3d).

As mentioned above, the spiny morphology of photothermal layer was
adjustable via controlling the distance between magnetic nanoparticles
and magnet. To investigate the effect of distance, the numerical
simulation was used to determine the distribution of magnetic fields at
four different distances of 0.1 cm, 0.6 cm, 1.1 cm, and 1.6 cm (Figure
3e). The corresponding magnetic field strengths at these distances were
shown in Figure 3f. Enlarging the distance to magnet, the magnetic field
strength gradually decreased especially at the edge. The magnetic
induction intensity of the edge declined from 44 to 11, 6 and 3 mT with
the distance increasing from 0.1 to 0.6, 1.1 and 1.6 cm. In comparison,
there was a less decline in the center of magnet from 15 to 11, 7 and 4
mT. When the distance from the magnet was 0.6 cm, the magnetic field
strength tended to be uniform.

For observing the spine structure conveniently, following the loading
density used in this work, a quantity of
Fe\textsubscript{3}O\textsubscript{4}@C nanoparticles was confined in a
narrow gap between two glass sheets above the holder with magnet (Figure
S6a). As shown in Figure 3g, non-uniform magnetic field distribution
caused a denser distribution of Fe\textsubscript{3}O\textsubscript{4}@C
spines at the edge and a relatively sparser spiny structure in the
center in both dry and wet states. As the distance increased to 0.6 cm,
following the uniform magnetic field, a well-distributed
Fe\textsubscript{3}O\textsubscript{4}@C spines was formed (Figure 3h).
However, as the distance got larger to 1.1 and 1.6 cm (Figure 3i-j), the
corresponding magnetic field strength became weaker which failed to
attract all the Fe\textsubscript{3}O\textsubscript{4}@C effectively,
which caused the less obvious spiny morphology. Similarly, the same
trend also existed in the structure composited by
Fe\textsubscript{3}O\textsubscript{4} (Figure S6b-c).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image4/image4}

\end{center}
\end{figure}

\textbf{FIGURE 4} Water contact angle images of
Fe\textsubscript{3}O\textsubscript{4} (a),
Fe\textsubscript{3}O\textsubscript{4}@C (b) and
Fe\textsubscript{3}O\textsubscript{4}@tC (c) after touching 4 \selectlanguage{greek}μ\selectlanguage{english}l of
water droplet. (d) Microstructure of the photothermal layer formed by
Fe\textsubscript{3}O\textsubscript{4}@C. Schematic images of the spiny
structures of Fe\textsubscript{3}O\textsubscript{4} (e),
Fe\textsubscript{3}O\textsubscript{4}@C (f) and
Fe\textsubscript{3}O\textsubscript{4}@tC (g) contacted with 10 \selectlanguage{greek}μ\selectlanguage{english}l of
water, the spiny structures could capture water quickly by capillary
action. An air-laid paper was laid on the surface of photothermal layer
formed by Fe\textsubscript{3}O\textsubscript{4}@C, and tested the water
transmission capacity along the photothermal layer (h).

The commercial Fe\textsubscript{3}O\textsubscript{4} nanoparticles were
hydrophilic, and the hydrophilicity gradually decreased as the carbon
layer outside the Fe\textsubscript{3}O\textsubscript{4} nanoparticles
became thickened (Figure 4a-c). The instantaneous water contact angles
of flat surfaces of Fe\textsubscript{3}O\textsubscript{4}@C and
Fe\textsubscript{3}O\textsubscript{4}@tC were 107.1 and 122.2\selectlanguage{ngerman}°,
respectively. However, with the spiny structure, all three photothermal
surfaces composited by Fe\textsubscript{3}O\textsubscript{4} and
carbon-coated nanoparticles enabled to capture water in a short time
(Figure 4e-g). Figure 4d shows morphology of the photothermal layer
formed by Fe\textsubscript{3}O\textsubscript{4}@C that featured a
multi-gap structure with spins. Such structure had capillary channels
all over the interior, which could provide excellent paths for efficient
water transport,\textsuperscript{23} and water could efficiently
transport from the bottom to the top in a short period (about 140 s) by
capillary force alone (Figure 4h).

2.3. Light absorptance of magnetic photothermal evaporator

As a fine semiconductor, the band gap of black
Fe\textsubscript{3}O\textsubscript{4} was just 0.1 eV, and such a quite
narrow band gap ensured the nearly full spectrum of solar
absorption.\textsuperscript{37} Under the evaporation environment with
high temperature and humidity, Fe\textsubscript{3}O\textsubscript{4} is
gradually oxidized into Fe\textsubscript{2}O\textsubscript{3} which has
lower solar-light absorption due to the increase in
bandgap.\textsuperscript{35} In response to this issue, a carbon layer
as good light absorber was coated around the magnetic particles to
maintain the performance on light harvesting for stable photothermal
evaporation. The light harvesting in solar spectrum was carefully
investigated based on the UV--vis--NIR spectra in the range of 280--2500
nm.

Without magnetic field, magnetic particles with and without coating were
respectively fixed on glass slices to form flat surfaces. As shown in
Figure 5a, the diffuse reflectance of
Fe\textsubscript{3}O\textsubscript{4}@C and
Fe\textsubscript{3}O\textsubscript{4}@tC were almost near the result of
pristine Fe\textsubscript{3}O\textsubscript{4} but with a slight
decrease at visible and infrared regions. It indicated that the carbon
layer displayed a comparable ability in light absorption to
Fe\textsubscript{3}O\textsubscript{4}. The diffuse reflectance of
Fe\textsubscript{3}O\textsubscript{4}@C was \textasciitilde{}4.4\% in UV
region (280--400 nm), \textasciitilde{}10.1\% in visible region
(400--780 nm), and \textasciitilde{}11.6\% in near-infrared region
(780--2500 nm). As the nanoparticles assembled into a spiny morphology
in magnetic field, there was a significant reduction in diffuse
reflection. Fixing the distance to magnet at 0.6 cm, the diffuse
reflectance of spiny surface composited by
Fe\textsubscript{3}O\textsubscript{4}@C was down to
\textasciitilde{}1.6\% in UV region, \textasciitilde{}5.6\% in visible
region, and \textasciitilde{}6.6\% in near-infrared region (Figure 5c).
The decrease of diffuse reflectance was attributed to the specific
surface microstructure caused by magnetic field. In the gaps of spiny
surface, part of the reflected light was re-captured by the
Fe\textsubscript{3}O\textsubscript{4}@C spines during multiple
reflections.\textsuperscript{34}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image5/image5}

\end{center}
\end{figure}

\textbf{FIGURE 5} Diffuse reflection spectra of magnetic samples in the
wavelength range of 280-2500 nm (a). AM 1.5 G solar spectrum and
UV-Vis-NIR absorption spectra (b). Diffuse reflection spectra of the
Fe\textsubscript{3}O\textsubscript{4}@C flat and photothermal layer
formed by Fe\textsubscript{3}O\textsubscript{4}@C at 0.6 cm in the
wavelength range of 280-2500 nm (c). AM 1.5 G solar spectrum and
UV-Vis-NIR absorption spectra (d). The IR thermal images of magnetic
evaporator under 1-sun illumination (e).

The low diffuse reflectance of photothermal layer corresponded to the
higher light absorption with negligible transmittance. Sunlight
absorptance in the range of 280--2500 nm was used to evaluate the
photothermal evaporator, which was calculated as below:

\(\text{Sunlig}ht\ \text{absorptance}=\selectlanguage{greek}\selectlanguage{english}\frac{\int_{280}^{2500}{I(\lambda)\times A(\lambda)\text{dλ}}}{\int_{280}^{2500}{I(\lambda)\text{dλ}}}\selectlanguage{greek}\selectlanguage{english}\)(1)

where I(\selectlanguage{greek}λ\selectlanguage{english}) is the sunlight intensity function at the wavelength (\selectlanguage{greek}λ\selectlanguage{english}) from
280 to 2500 nm, A(\selectlanguage{greek}λ\selectlanguage{english}) is the absorptance of samples at the corresponding
wavelength (Figure 5b-d). The sunlight absorptance of flat
Fe\textsubscript{3}O\textsubscript{4}@C surface was around 89.6\%. In
comparison, the absorptance of spiny surface could be enhanced over
94.6\% as the distance was 0.6 cm. Thanks to the high absorptance of
sunlight, the photothermal layer formed by
Fe\textsubscript{3}O\textsubscript{4}@C at a distance of 0.6 cm from
magnet revealed high performance on the light-to-thermal conversion.
After being exposed to the simulated solar light (\textasciitilde{}1000
W m\textsuperscript{-2}) for 30 mins, spiny surface exhibited a higher
surface temperature of surface up to 80.1 \selectlanguage{ngerman}°C on average than the flat
surface of just 73.7 °C (Figure 5e).

For testing convenience, the data was mainly collected from the center
of the spiny surface. Adjusting the distance to the magnet, spiny
surface showed changes in light harvesting. As the distances were 0.1 cm
and 1.1 cm, the diffuse reflectances were similar to the result at 0.6
cm. While, as the distance enlarged to 1.6 cm, there was an obvious
increase in diffuse reflectance, which was as high as
\textasciitilde{}12 \% in visible region (Figure S7a-b).

During photothermal evaporation, the evaporator was wetted by water, and
it was necessary to investigate the light absorption of photothermal
layer in the wet state. After being wetted by water, the diffuse
reflectances of all photothermal layers were further decreased (Figure
S7c-d). This could be attributed to the presence of a thin water layer
around the wettable Fe\textsubscript{3}O\textsubscript{4}@C that
optimized the optical path following the Fresnel
equation.\textsuperscript{34,38}

2.4. Photothermal evaporation performance of magnetic photothermal
evaporator

The photothermal evaporator for measurement was assembled as the
inserted image in Figure 6a shown. Magnetic nanoparticles were located
on a 3D-printed holder to form the photothermal layer. Between them,
there was a piece of air-laid paper for water supply. To adjust the
location of photothermal layer in magnetic field, a disc magnet was
placed on the top of screw plug, which could be rotated to change the
distance between the nanoparticles and magnet.\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image6/image6}

\end{center}
\end{figure}

\textbf{FIGURE 6} Schematic illustration of the fabrication of magnetic
photothermal evaporator (left) and the evaporation test system (right)
(a). The water evaporation rates of
Fe\textsubscript{3}O\textsubscript{4},
Fe\textsubscript{3}O\textsubscript{4}@C and
Fe\textsubscript{3}O\textsubscript{4}@tC evaporators under dark and
light conditions respectively (b). Water evaporation rates of
photothermal evaporators at various distances between magnet to
nanoparticles (c). The evaporation rate and the durability of magnetic
evaporator (d). The images right showed the surface morphologies before
and after five cycling experiments.

The performance of magnetic evaporator on water evaporation was been
systematically evaluated based on a lab-made system that was composed of
a solar simulator with a fixed light intensity of 1000 W
m\textsuperscript{-2} and an electronic balance connected to PC (Figure
6a). Evaporation measurements were performed in a temperature and
humidity chambers to minimize the effect of conditions. The set
temperature and relative humidity were fixed at 25 \selectlanguage{ngerman}°C and 50\%,
respectively.

The evaporation performances with the assistance of magnetic
nanoparticles were shown in Figure 6b in comparison with the natural
evaporation. The evaporation rates of pure water were 0.49 kg
m\textsuperscript{-3} h\textsuperscript{-1}and 0.12 kg
m\textsuperscript{-3} h\textsuperscript{-1} with and without light. As
the distance was fixed at 0.6 cm, evaporators assembled in magnetic
field accelerated the photothermal rates to 0.26, 0.24, 0.24 kg
m\textsuperscript{-3} h\textsuperscript{-1} respectively in dark and
1.28, 1.39, 1.31 kg m\textsuperscript{-3}
h\textsuperscript{-1}respectively under illumination. The higher
evaporation rates of magnetic evaporators in dark could be attributed to
the enlargement of evaporation interface area via spiny morphology.
While, the good photothermal ability of magnetic spines resulted in
faster water evaporation with light. Among the three kinds of
nanoparticles, Fe\textsubscript{3}O\textsubscript{4}@C showed the best
performance on photothermal evaporation. Such result was consistent with
the solar light absorptance data in Figure 5 and Figure S7.

The water evaporation performance could also be affected by the distance
between the spiny surface and magnet. As discussed above, the magnetic
field tended to be uniform at an appropriate distance. A relatively
integral spiny surface could be assembled under the induction of
magnetic field, even as the loading density of nanoparticles was as low
as 1 mg mm\textsuperscript{-2}. For
Fe\textsubscript{3}O\textsubscript{4}@C, the water evaporation rates
could reach up to 1.36, 1.39, 1.29 and 1.26 kg
m\textsuperscript{-3}h\textsuperscript{-1}, as the distance was changed
from 0.1 to 0.6, 1.1 and 1.6 cm. At a narrow distance, the coverage
region with nanoparticles was incomplete under the non-uniform magnetic
field. Exposed partial sites reduced the light harvesting even in wet
state, which caused the lower evaporation rate. As the distance
increased from 1.1 to 1.6 cm, the evaporation rate declined continuously
because of the decay of magnetic field strength. A weak magnetic field
could hardly remain the spiny morphology against the strong surface
tension of water in wet state. Fe\textsubscript{3}O\textsubscript{4} and
Fe\textsubscript{3}O\textsubscript{4}@tC showed a similar tendency for
water evaporation at different distances to magnet.

Furthermore, to evaluate the stability of magnetic evaporator, the
photothermal evaporation rates were measured under 12-hour illumination
alternating with 12-hour darkness. After 5 cycles, the evaporation rate
still kept up to about 1.33 kg
m\textsuperscript{-3}h\textsuperscript{-1}, which was 96\% of the
initial rate (Figure 6d). The stable performance on long-term
photothermal evaporation could be attributed to the good stability of
the spiny morphology of Fe\textsubscript{3}O\textsubscript{4}@C.
Meanwhile, thanks to the protection of carbon layer,
Fe\textsubscript{3}O\textsubscript{4}@C remained the crystalline
structure of Fe\textsubscript{3}O\textsubscript{4} without obvious
change which was confirmed by XRD in Figure 7e. In this case, the
Fe\textsubscript{3}O\textsubscript{4}@C layer could maintain its
photothermal performance under long-term illumination for water
evaporation, thus enabling low-cost actual water desalination.

2.5. Solar-driven desalination of magnetic photothermal evaporator

To evaluate the performance of magnetic evaporator on seawater,
photothermal evaporation was measured with five concentrations of saline
solutions to simulate seawater from different seas, including the Baltic
Sea (0.8 wt\%), the world ocean (3.5 wt\%), the Red Sea (4.1 wt\%) and
the Dead Sea (30 wt\%). As shown in Figure 7a, the evaporation rate
under illumination decreased from 1.39 kg
m\textsuperscript{-3}h\textsuperscript{-1} for DI water to 1.37, 1.31,
1.26 and 0.98 m\textsuperscript{-3} h\textsuperscript{-1} for the
simulated seawater of the Baltic Sea, world ocean, the Red Sea and the
Dead Sea, respectively. Correspondingly, the evaporation rate of
evaporator decreased from 0.27 to 0.25, 0.19, 0.18 and 0.16 kg
m\textsuperscript{-3} h\textsuperscript{-1}, respectively in dark. The
decrease in evaporation rate is due to the lower saturation vapor
pressure of seawater. Higher concentration of saline solution indicated
lower saturation vapor pressure, and further caused slower evaporation.
The real seawater from Bohai Bay (obtained at 38°53\selectlanguage{english}'N, 121deg34\selectlanguage{english}'34\selectlanguage{english}'\selectlanguage{english}'E) was
also employed in measurement, and the evaporation rate with
Fe\textsubscript{3}O\textsubscript{4}@C evaporator was about 1.35 kg
m\textsuperscript{-3} h\textsuperscript{-1} (Figure 7a).\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image7/image7}

\end{center}
\end{figure}

\textbf{FIGURE 7} Water evaporation rates of magnetic evaporator for
saline solutions with different concentrations and seawater from Bohai
Bay under 1-sun illumination (a). The evaporation rates and the
regeneration performance of magnetic evaporator in 8-hour evaporation
for simulated seawater (b). Long-term seawater evaporation performance
of magnetic evaporator 12-hour illumination alternating with 12-hour
darkness, 3.5 wt\% of saline solution was used as simulated seawater
(c). Schematic diagram of condensate water collection device made of
PMMA for outdoor solar-driven desalination (d). The XRD spectra of the
Fe\textsubscript{3}O\textsubscript{4}@C after photothermal evaporation
in various conditions (e).

The stability of the evaporator in seawater evaporation was carefully
measured as Figure 7b and Figure 7 c shown. In Figure 7b, photothermal
evaporation with the Fe\textsubscript{3}O\textsubscript{4}@C evaporator
was sustained for 8 hours. Thereafter, the evaporator was regenerated by
washing it with a little water. During continuous evaporation, the
evaporation rate for simulated seawater (3.5 wt\%) gradually decreased
from 1.31 kg m\textsuperscript{-3} h\textsuperscript{-1} at the
beginning to 1.21 kg m\textsuperscript{-3} h\textsuperscript{-1} with
the assistance of evaporator. The precipitation of salt crystals on
spiny surface could be considered as the reason for the slight
efficiency decline. By removing salt crystals through washing them in
water, Fe\textsubscript{3}O\textsubscript{4}@C nanoparticles could be
collected and assembled into the spiny surface again for next
measurement. After four-time regeneration, evaporator still showed an
evaporation rate around 1.26 kg
m\textsuperscript{-3}h\textsuperscript{-1} at the beginning, about 96\%
of the initial result. It was demonstrated that the magnetic
field-induced method is an alternative reusable strategy to improve the
cycling performance of evaporator.

In the other measurement as shown in Figure 7c, the
Fe\textsubscript{3}O\textsubscript{4}@C evaporator was continuously used
for 12-hour evaporation under illumination and 12-hour regeneration by
itself in dark, which simulated the case in daytime and nighttime. The
evaporation rate was decreased from 1.30 kg
m\textsuperscript{-3}h\textsuperscript{-1} to 1.19 kg
m\textsuperscript{-3}h\textsuperscript{-1} with the accumulated salt
crystals after 12-hour evaporation of simulated seawater (3.5 wt\%).
During the regeneration in dark, the salt crystals were not completely
dissolved as Figure S8 shown, which might be due to the weak
hydrophilicity of Fe\textsubscript{3}O\textsubscript{4}@C. Salt ions
were difficult to diffuse rapidly into the bulk solution. Thus, the
regeneration of evaporator was not sufficient. The evaporation rate on
Day 5 was just about 1.12 kg m\textsuperscript{-3}
h\textsuperscript{-1}, which was 86\% of the initial rate after
four-time regeneration. The performance decline of evaporator could be
attributed to the accumulation of crystalline salt that influenced the
light harvesting. During the application in seawater,
Fe\textsubscript{3}O\textsubscript{4}@C still kept stable without an
observable change in the crystal structure as XRD patterns shown in
Figure 7e.

Furthermore, the outdoor experiment was conducted to investigate the
potential of magnetic photothermal evaporator for practical application.
The lab-made evaporator in above measurement had a small size which was
not appropriate for practical desalination. Consequently, a larger
magnet with a diameter of 60 mm was used to compose a larger evaporator.
A portable water collection device made of polymethyl methacrylate
(PMMA) was used for solar-driven desalination. The specific design of
device was shown in Figure 7d. The as-prepared larger evaporator was
placed inside the device and converted solar light to heat for vapor
generation. The formed vapor condensed into water droplets that flowed
along the side walls into the container located below. After actual
solar irradiation in an open field from 07:35 to 18:06 (Sunset time) on
March 14 (38\selectlanguage{ngerman}°53\selectlanguage{english}'28\selectlanguage{english}'\selectlanguage{english}'N, 121deg31\selectlanguage{english}'31\selectlanguage{english}'\selectlanguage{english}'E) (Figure S9), over 10 ml of water
was finally collected. Meanwhile, there was no visible salt
precipitation on the black Fe\textsubscript{3}O\textsubscript{4}@C
layer. Suffering from the interference elements including low
temperatures, clouds, condensed water droplets and PMMA cover, the
actual light intensity accepted by sample was far lower than 1000 W
m\selectlanguage{english}\textsuperscript{-2}. As Figure S10 shown, the concentrations of
residual Na\textsuperscript{+}, K\textsuperscript{+},
Mg\textsuperscript{2+}, Ca\textsuperscript{2+}, Cl\textsuperscript{-},
Br\textsuperscript{-} and SO\textsubscript{4}\textsuperscript{2-} in
condensate water were down to about 30.54, 3.97, 1.95, 3.459, 41.52,
0.05 and 7.02 mg L\textsuperscript{-1}, respectively, which were all
lower than the standard limits of desalinated water from WHO (World
Health Organization).\textsuperscript{39}

\textbf{3. Conclusion}

In summary, we fabricated a magnetic field induced photothermal
evaporator assembled by carbon-coated
Fe\textsubscript{3}O\textsubscript{4}(Fe\textsubscript{3}O\textsubscript{4}@C)
nanoparticles, which revealed the adjustable spiny surface structure
corresponding to the relative distance to magnet. At an appropriate
relative distance, just 1 mg mm\textsuperscript{-2} of
Fe\textsubscript{3}O\textsubscript{4}@C nanoparticles was necessary to
form a well-distributed spiny surface. With the optimized surface
structure, the solar absorbance of evaporator could reach up to 94.6\%
via multiple reflections. The Fe\textsubscript{3}O\textsubscript{4}@C
assembled evaporator obviously improved the water evaporation as high as
1.39 kg m\textsuperscript{-2}h\textsuperscript{-1} under the 1-sun
illumination, about 2.8 times higher than natural evaporation.
Meanwhile, the magnetic evaporator could also accelerate the seawater
evaporation up to 1.31 kg m\textsuperscript{-2} h\textsuperscript{-1}
with just a little performance degradation during long desalination
owing to the good stability of Fe\textsubscript{3}O\textsubscript{4}@C.
The work can be considered as a beneficial attempt to optimize the
magnetic evaporator for application. Besides, the facile adjustment of
the magnetic nanoparticles assembled surface via the position in
magnetic field has high potential in not only sustainable solar-driven
fresh water generation but more fields of water treatment.

Experimental Section

\emph{Preparation of the carbon-coated nanoparticles} : The
carbon-coated nanoparticles were synthesized by a typical hydrothermal
method. Specifically, 0.3 g of
Fe\textsubscript{3}O\textsubscript{4}nanoparticles, 1 mL of acrylic
acid, and 40 mL of H\textsubscript{2}O were mixed with a certain amount
of glucose in a 100 mL capped glass bottle. Different amounts of glucose
led to different thicknesses of carbon layers around
Fe\textsubscript{3}O\textsubscript{4}nanoparticles. The mixture was
stirred vigorously at room temperature for 4 h and transferred to a 100
mL Teflon-lined stainless-steel autoclave. The autoclave was heated at
200 \selectlanguage{english} for 12 h, then naturally cooled to room temperature at the end of
the reaction. The black products were washed with DI water and anhydrous
ethanol alternately three times. Subsequently, the products were dried
at 60 degC for 6 h. The obtained samples were named
Fe\textsubscript{3}O\textsubscript{4}@C, and
Fe\textsubscript{3}O\textsubscript{4}@tC respectively according to 1.0
g/2.0 g of glucose.

\emph{Fabrication of magnetic photothermal evaporator} : A 3D model of
the evaporator was established by SOLIDWORKS software and was converted
into object through 3D printer (raise3D Pro2 Plus) using PLA (polylactic
acid) wire. This evaporator had two parts including an upper holder to
support the magnetic nanoparticles and a lower screw plug equipped with
a NdFeB magnet. The distance between magnet and magnetic nanoparticles
could be adjusted by rotating the screw plug. The air-laid paper was cut
into a specific shape serving as water transfer channels. Magnetic
nanoparticles were deposited at the holder at the density of 1 mg
mm\textsuperscript{-2}, which could self-assemble into a spiny surface
induced by magnetic field.

\textbf{Supporting Information}

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

Acknowledgements

This work was funded by the Fundamental Research Funds for the Central
Universities (DUT22GF306, DUT22LAB607), Dalian Science and Technology
Innovation Fund (2022JJ13SN098), the Natural Science Foundation of
Liaoning Province (2022-MS-144) and Project 1912 Funds provided by the
Chinese Aeronautical Establishment.

The authors declare no potential conflict of interest.

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{The table of contents}

A magnetic field induced photothermal evaporator assembled by
carbon-coated
Fe\textsubscript{3}O\textsubscript{4}(Fe\textsubscript{3}O\textsubscript{4}@C)
nanoparticles, which revealed the adjustable surface structure
corresponding to the relative distance to magnet. The optimized
evaporator assembled by Fe\textsubscript{3}O\textsubscript{4}@C showed a
uniform spiny surface contributing to an obvious improvement on
evaporation rate under illumination and lower performance degradation in
long-term desalination.

\textbf{Keywords}

photothermal evaporation, desalination, carbon-coated magnetic
nanoparticles, magnetic field induced, adjustable surface structure

M. Zhou, P. Han, G. Qi, D. Gao, H. M. Abdel-Ghafar, Y. Wang* and S. Tao*

\textbf{Magnetic Field Induced the Assembling of Photothermal Evaporator
for Efficient Solar-driven Desalination}

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