Design Of Photothermal Evaporators Based On Two-dimensional Nanomaterials And Studies On Their Water Purification Performances

Although many water purification technologies have been developed to produce clean water from seawater or polluted water sources,most of these technologies rely on high energy consumption and complex centralized equipment and facilities,limiting their practical application in remote and undeveloped areas and offshore islands.Solar-driven water evaporation is a green,sustainable,convenient,and cheap water purification technology with solar energy as the driving energy,which is an important potential means to solve the shortage of freshwater resources.Although various photothermal conversion materials have been developed for solar-driven water evaporation,the problems such as how to further utilize the solar energy per unit area for improving evaporation performances,how to construct re-shapeable and surface-patternable photothermal conversion materials,how to integrate photothermal conversion and photocatalytic degradation（solar-driven water evaporation while photocatalytic degradation of harmful substances in water）have not yet been resolved,which hinders the comprehensive development of solar-driven water evaporation technology.In view of the above problems,based on the wide spectrum absorption,high photothermal conversion efficiency,and relatively low-cost of two-dimensional Ti₃C₂T_xMXene and graphene oxide nanosheets,this study adopts the internal structure regulation,hybrid composite of different materials,spatial dimension expansion of evaporators,and multifunctional material coordination,enhancing the light absorption capacity of solar driven water evaporators,promoting water transport,improving environmental adaptability,maximizing the utilize of solar energy per unit area and ensuring its stable performance,which provides new ideas for promoting the large-scale application of solar-driven water evaporation technology.The main research contents and results are given as follows:（1）Preparation of MXene aerogel film by ferrous ion crosslinking method and its high-performance water evaporator:Ti₃C₂T_xMXene has excellent photothermal conversion capability.Although various solar-driven water evaporators with MXene as the photothermal conversion material are reported,how to improve the evaporation performances of such evaporators is still an issue to be solved.In this work,Fe Cl₂is used as both flocculant and crosslinking agent of aqueous MXene dispersion.Fe²⁺can form covalent bonds with-OH on the surface of MXene nanosheets and destroy the original charge balance,facilitating the formation of MXene microgel clusters.After vacuum filtration molding and freeze-drying,a MXene aerogel film（MAF）with rough surface,loose and porous interior is obtained.The vacuum filtration makes the MXene microgel clusters close to each other and form an overall continuous structure with Fe²⁺crosslinking;while the freeze-drying sublimates the ices in the MXene microgel clusters to form the MAF with porous interior and rough surface.The rough surface of the MAF is conducive to absorbing more solar energy and improving the light-to-heat conversion efficiency.The internal porous structure enables water to be transported quickly from the bottom to the top evaporation surface,ensuring sufficient water supply at the evaporation surface.The cross-linking effect of Fe²⁺enhances the stability of the MAF,enabling the MAF to stay stably in water for 15 days.Because of the above advantages,the average evaporation rate and evaporation efficiency of an optimized MAF photothermal evaporation film can reach 1.67 kg m^-2h^-1and 94.3%,respectively,under the solar light irradiation of 1 k W m^-2.The average evaporation rate and evaporation efficiency under the light irradiation of 10 k W m^-2reach 14.52 kg m^-2h^-1and91.7%,respectively.In addition,the MAF can also efficiently produce clean water from natural seawater and simulated organic wastewater with close to100%rejection of salt ions and organic pollutants.（2）Preparation of MXene/graphene oxide/polyaniline hybrid and its re-shapeable photothermal water evaporator:Although various solar-driven water evaporators have been developed for seawater desalination or wastewater purification,most of the solar-driven water evaporation materials cannot be used to repeatedly construct solar-driven evaporators with variable shapes and patterned surfaces.In this work,a re-shapeable and surface patternable MXene/GO/PANI hybrid（MGP）is prepared via polyaniline（PANI）assisted self-assembly of Ti₃C₂T_xMXene and GO for efficient solar-driven desalination and wastewater purification.Aniline monomers self-polymerize on the surface of MXene in a weakly acidic environment to form short-chain PANI,which is tightly combined with GO and MXene through hydrogen bonding,π-πconjugation interaction,and van der Waals forces,making the MGP have variable shapes,patternable surfaces and reusable properties.The existence of various oxygen-containing functional groups in the MGP reduces its internal water evaporation enthalpy.The relative contents of free water,bound water,and weakly bound water in the MGP were determined by low-field NMR technique.The MGP exhibits excellent solar-driven water evaporation performances.Under the solar light irradiation of 1 k W m^-2,the average water evaporation rate of the flat MGP evaporator is 2.89 kg m^-2h^-1,while that of the MGP evaporator with the surface concave pyramid structure becomes 3.30 kg m^-2h^-1.When the MGP is molded into a flower-like evaporator,an excellent interfacial evaporation rate of about 3.94 kg m^-2h^-1and an ultrahigh evaporation efficiency of about135.6%are achieved under the light irradiation of 1 k W m^-2.The re-shapeable MGP evaporator can efficiently produce clean water from seawater and wastewater,and the rejection rates of salt ions and harmful heavy metal ions are close to 100%.（3）Design of“tree-like”evaporator and its interfacial water evaporation and photothermal catalytic degradation performances:In this work,solar-driven water evaporation and photocatalytic degradation are combined to mimic the structure of"tree"in nature.A photothermal and photocatalytic synergistic dual-function evaporator is designed,which is composed of photothermal GO/NPDI/PVA（GNP）hydrogel（"canopy"）and photocatalytic degradation r GO/PU/NPDI（r PN）composite foam（"trunk"）.It can be used for evaporation purification and catalytic degradation of polluted water with ciprofloxacin hydrochloride（CIP）.The perylene imide nanowires（NPDI）are deposited on the surface of GO nanosheets throughπ-πconjugation interaction between GO and NPDI,and then mixed with polyvinyl alcohol to form the GNP hydrogel by a freeze-thaw method.The use of GNP hydrogel for solar-driven interfacial water evaporation not only has a low enthalpy of water evaporation,but also can catalytically degrade CIP in bulk water.GO is coated on a commercial polyurethane foam（PU）skeleton by a vacuum impregnation method,and chemically reduced with hydrazine hydrate.The NPDI nanowires are deposited on the surface of r GO/PU skeleton by vacuum impregnation again to form an r PN composite foam.Finally,the GNP photothermal hydrogel and r PN composite foam are assembled in a"tree"-like structure,forming a GNP-r PN photothermal evaporation-photocatalytic degradation system.The advantages of the GNP-r PN system mainly include three aspects:Firstly,GO and r GO broadens the solar light absorption range of the NPDI.Secondly,the huge specific surface area of GO and r GO can absorb more pollutants for catalytic degradation by NPDI under light irradiation.Thirdly,the excellent photothermal conversion capacity of GO and r GO can convert solar energy into heat energy,which promotes the catalytic degradation of CIP by NPDI under the light irradiation.The evaporation rate and evaporation efficiency of GNP-r PN photothermal and photocatalytic synergistic dual-function evaporator can reach 2.84 kg m^-2h^-1and 95.2%,respectively,and the interception rates of salt and harmful ions in seawater and industrial wastewater are more than 99.9%.At the same time,the optimal degradation efficiency of the GNP-r PN system for the CIP solution(initial concentration of 10 mg L^-1)reaches 74.04%within 1 h.After 4 h solar-driven photocatalytic degradation,the optimal degradation efficiency of CIP in bulk water reaches 96.37%.The GNP-r PN system achieves photocatalytic degradation while evaporating water at the photothermal interface,which greatly improves the solar energy utilization per unit area.