1. Introduction
The
rapid growth of the world population results in freshwater scarcity one
of the most problematic issues in the twenty-first
century.[1,2]Besides, the ever-accelerated
development of industry brings about severe environmental pollution,
especially in developing countries
or some rural regions, further making the lack of clean water a sharp
problem.[3-5] In addition, the emergence of
drug-resistant
bacteria in water resources due to antibiotics abuse has also become a
significant obstacle to the clean water
supply.[6,7]Conventional clean water production
approaches include chemical precipitation, ozone and ultraviolet
radiation, membrane filtration, and chlorine-containing disinfectants
cleaning, whereas these energy-intensive technologies may lead to high
environmental pressure and harmful by-products during the disinfection
process.[8,9]Moreover,
the water supply infrastructure
installation and maintenance in remote areas are unrealistic to some
extent ascribed to the high cost and low utilization
efficiency.[8] Thus, exploiting innovative clean
water production technologies is highly desirable to guarantee the safe
water supply.
With the advance of photothermal
technologies, the solar-driven interfacial water evaporation process, in
which the steam is generated just at the interfaces between the
photothermal materials and water, exhibits the feature of enhanced heat
utilization and evaporation efficiency, flexible portability, low cost
and feasible installation, compared with the conventional passive sea
water desalination, and thus has
emerged as an up-and-coming technology for seawater desalination and
wastewater treatment.[10-12]Except for the heat localization
concerns in the solar evaporation
process, the solar-driven steam generation efficiency largely depends on
the microstructure and components of
photothermal materials. Thus, up to
now, tremendous attention has been paid to the preparation of
plasmonic
nanoparticles,[13]conjugated
polymers,[14]semiconductors,[15,16]carbon-based materials,[17,18]aerogels,[19,20] and hydrogels[21,22] for potential solar
desalination. Among these
investigated solar steam generators, photothermal hydrogels in which
photothermal materials as “micro-heaters” are accommodated to induce
solar-powered water evaporation, enjoying the merits of low cost,
feasible fabrication and high efficiency, have shown bright prospects in
water transportation and vapor production
as they could modulate the
intriguing water molecule interaction states by forming hydrogen bonds
between water molecules and polymer chains which lowers the vaporization
enthalpy.[23] Besides,
numerous water-containing pores are
advantageous for enhancing
the
multiple reflections of incident photons, which also enhances the solar
energy utilization efficiency.[20]
On
the other side, MXenes, a developing family of 2D materials, have drawn
increased interest due to their potential in energy storage, water
desalination, light-to-heat conversion, electromagnetic shielding and
photocatalysis.[24,25] Moreover, they could be
readily integrated into hydrogel
skeletons to form various functional hybrid
nanostructures.[26,27] Besides, the layered
architecture of MXenes is also beneficial for the perturbation of
hydrogen bonding among water
molecules, and thus, lower evaporation enthalpy could be
anticipated.[28,29]Lu et al. created a 2D nanostructure-embedded hybrid hydrogel by
simultaneously interpenetrating few-layer MXenes and reduced graphene
oxide nanosheets into polymer
networks, showing a high evaporation efficiency of up to 83.5% under
one sun illumination.[21] In addition, due to its
superb photothermal performance, MXenes also
exhibit promising antibacterial
properties under light irradiation, which conduces to the pathogen-free
water supply.[24] Given that the photothermal
antibacterial effect of MXenes could only be triggered with the presence
of light irradiation and the sterilization efficiency of chitosan to
drug-resistant microbial is rather weak,
it
is envisioned that by rationally embedding the 2D-structured MXene
nanosheets loaded with strong antibacterial agents into chitosan
hydrogel matrix, optimized vaporization enthalpy and enhanced bacterial
eradiation effect could be expected.
Based
on the above analysis, in this work, we successfully prepared Ag/MXene
nanosheets with optimized mass ratios and incorporated them into
chitosan hydrogel, forming Ag/MXene@chitosan hydrogel composites as an
efficient interfacial solar steam generator. In the porous architecture,
Ag/MXene nanocomposites acted as photothermal centers for the solar
steam generation and chitosan hydrogel platform, with abundant
water-enriched pores in micrometer scale, ensured the continuous water
transportation to the evaporation sites. Therefore, an attractive
evaporation rate of 3.22 kg m−2 h−1with light energy conversion efficiency of 94.9% was achieved,
which
exceeded the majority of previously
reported photothermal evaporators.
By examining the concentrations of
main ions (Na+, Mg2+,
K+, and Ca2+) in the distilled water
samples from the Yellow Sea, China, it was found that their
concentrations were drastically reduced after desalination with the
evaporator,
meeting
the standard of drinking water according to the World Health
Organization. The hydrogel also exhibited a high capacity for
dye-contaminated water purification, showing its versatile application
potential for various water cleaning purposes. Furthermore, the
synergistic contributions from the uniformly dispersed Ag nanoparticles
(ca. 3~12nm), MXene nanosheets and chitosan chains
endowed the Ag/MXene@chitosan
hydrogel with excellent photo-induced antibacterial performances towardsS.aureus and E.coli , revealing its potential for
pathogen-free water production. Moreover,
the intrinsic bactericidal ability
of chitosan and the Ag+ ions release brought about
antibacterial self-cleaning properties to the hydrogel generator,
benefiting its durability for long-term
use. In summary, our work provides
a new strategy for designing cost-effective, highly portable and
environmentally friendly hydrogel-based solar evaporators for
bacteria-free freshwater supply.