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.