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-3 h-1under 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,1-4 which is widely recognized as a
novel desalination technology for solving water resource scarcity
issues.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.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.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.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%.15Similarly, 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 μm.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.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.20-24 In particular, magnetic field is
considered an ideal tool for applications in structure
management,25-27 especially in the construction of
functional structures with reconfigurable, recoverable and dynamically
adjustable.28-30 In Jeong’s work,
Fe3O4 nanoparticles were sprayed onto a
Si substrate with patterned nickel islands, and attracted by the
external magnetic field to form magnetic
micropillars.31 Besides, some conical micro-tip
patterns could be molded with the aid of a magnetic
field.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.33
In our previous work, magnetic Fe3O4nanoparticles 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.34However, Fe3O4 tends to be oxidized in
high temperature and high humidity conditions during evaporation. The
generated Fe2O3 shows a wider band gap
indicating the weakening of photothermal
performance.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
Fe3O4 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
Fe3O4(Fe3O4@C) nanoparticles, which revealed
the adjustable surface structure corresponding to the relative distance
to magnet. To improve the stability of magnetic nanoparticles, we
synthesized Fe3O4@C by a simple one-step
method of coating Fe3O4 nanoparticles
with a layer of carbon using glucose as the carbon source (Figure 1).
The outer carbon layer of Fe3O4@C
nanoparticles could provide effective protection for the
Fe3O4 and maintain a stable photothermal
performance after the oxidation of magnetic core. At an appropriate
relative distance to the magnet, Fe3O4@C
nanoparticles assembled into a complete and well-distributed
photothermal layer with spine arrays at a low load density of 1 mg
mm-2 (which was 3 mg mm-2 in our
previous work) (Figure S1). With the optimized surface structure, the
Fe3O4@C assembled evaporator obviously
improved the evaporation rate up to 1.39 kg m-2h-1 for water and 1.31 kg m-2h-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
Fe3O4@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.