Preparation Of Photothermal Composite Materials And Their Performances Of Solar-driven Interface Water Evaporation

As an environment-friendly clean energy technology,solar steam generator can effectively alleviate the problem of global water shortage.Solar-driven interfacial water evaporation has been regarded as an efficient way to purify water because it can localize the solar energy heating at the air-water interface to minimize heat loss and enhance energy utilization.The water evaporation rate and efficiency of the photothermal system can be effectively regulated by the regulation of three important influencing factors:photothermal materials,thermal management of the system and water transport and supply.In recent years,various photothermal materials such as metal-based,carbon-based,semiconductor-based and polymer materials have been widely applied to the construction of photothermal interfaces.However,they also have some application limitations due to their complex preparation methods,high cost or low porosity.Meanwhile,the design of water transmission channel in photothermal system needs to balance the water transferring and heat loss in order to take unique advantages of interfacial evaporation.In addition,in practical applications,we should not only take the inherent photothermal properties of the materials into account,but also the thermal stability,mechanical stability,evaporative stability and cost.As for evaporative stability,salt ions in high salinity seawater may cause salting out and pore plugging,as well as dissolved organic matter and other bio-species present in source water may cause bio-fouling to the photothermal interfaces.Therefore,how to simultaneously achieve the high stability,reusability,evaporation efficiency and anti-fouling performance of photothermal absorber is an urgent problem to be solved in the application of solar photothermal interfacial water evaporation technology in practical seawater desalination.In this paper,the photothermal interface with high efficiency and multifunction is obtained by utilizing relatively cost-effectively carbon-based and semiconductor-based photothermal materials as building units.The works are as follows:（1）A robust porous bi-layered photothermal membrane（carbon nanotube@polyethylene imine/mixed cellulose esters,CNT@PEI/MCE）,where the top solar conversion layer of pristine CNT（the thickness was about 1.6μm）was seamlessly connected to the bottom heat transfer barrier layer of hydrophilic porous MCE via non-covalent intereaction,was facilely prepared via a simple vacuum filtration.In this solar energy interfacial water evaporation system based on carbon-based photothermal materials,with the synergistic integration of MCE-mediated heating localization,CNT-mediated solar-heating,PEI-mediated hygroscopicity and noncovalent interactions,the CNT@PEI/MCE membrane can stay at the water-air interface with Wenzel’s wetting behavior.Moreover,the efficient interface heating mode of CNT@PEI/MCE membrane was studied by optical and thermal tests.The CNT@PEI/MCE membrane can achieve a high interfacial water evaporation efficiency（72%）and a rapid evaporation rate(5.07 kg m^-2 h^-1),which is 4.23 times higher than that of pure water under a simulated solar irradiation.At the same time,the tensile test of the photothermal composite membrane showed its good mechanical stability with the breaking strength of 50 MPa.The steam generation rate of photothermal composite membrane kept unchanged after bending and folding operation to show its flexibility.Finally,the bio-fouling property of photothermal composite membrane was studied with Escherichia coli as the target strain.Meanwhile,the CNT@PEI/MCE membrane exhibited distinct antibacterial ability and good durability.This advanced membrane,together with its cost-effectiveness,provides a practical sustainable energy technique for clean water production.（2）Aiming at improving the efficient utilization of solar energy for durable interfacial water purification,in this work,a robust porous bi-layered self-floated photothermal membrane（MoS₂@PEI/MCE）,where porous MoS₂ nanoflower grafted with hygroscopic polyethylene imine（PEI）were filmed on hydrophilic mixed cellulose esters（MCE）substrate via a simple vacuum filtration,which the thickness of photothermal layer was about 1.2μm,was built for solar-driven interfacial water evaporation.During this process,a nanohydrogels（MoS₂@PEI）can be obtained by binding PEI and MoS₂ via noncovalent interaction（hydrogen bonding/electrostatic interaction）.In this membrane,PEI-grafted MoS₂ nanoflower can serve as a 3D-porous nanohydrogel network in the top layer,which integrated the enhanced light absorption via multiple reflections,the efficient water heating via the dispersed hotspots.The water evaporation performance of photothermal membrane can be regulated by changing the content of MoS₂ and PEI and the thickness of photothermal layer.In addition,the water content,the state and the evaporation of the water molecules in the nanohydrogel were studied and the results showed that excellent water evaporation of nanohydrogel via localizing a small water cluster with low vaporization enthalpy（1462 J/g）.While in the bottom layer,the MCE acted as a flexible substrate with low thermal conductivity,which synergistically guaranteed the good mechanical stability,the durative capillary water replenishment and the minimal heating loss.Meanwhile,the bio-fouling property of photothermal membrane was studied with Escherichia coli,as the target strain,indicating that this membrane can achieve good anti-bacterial duo to the synergetic effect of MoS₂ and numerous cationic PEI.As a result,an enhanced interfacial water evaporation(the evaporation capacity of 2.7 kg m^-2 under 3.7 kWm^-2 solar irradiation for 30 min and the calculated water evaporation efficiency up to92%.In addition,the water evaporation efficiency can also reach about 83% under the intensity of natural sunlight 1 kWm^-2)with obvious antibacterial ability,good operability and durability of MoS₂@PEI/MCE photothermal membrane was achieved,which is an attractive system for clean water production.（3）A porous photothermal composite hydrogel,namely molybdenum disulfide intercalated graphene hydrogel MGH,was fabricated by embedding MoS₂ nanoflowers into graphene using a one-step chemical reduction method.A solar energy-driven interfacial water evaporation system was constructed combined with an artificial evaporation device.The morphology and physicochemical properties of the material were tested by scanning electron microscopy,transmission electron microscopy,X-ray powder diffractometer,UV-Vis-NIR spectrophotometer and infrared thermography.The composite hydrogel MGH was subjected to simulated solar light for water evaporation and desalination tests.The results show that MoS₂ can be stably dispersed in graphene nanosheets and the combination of the two contributes to the enhanced photothermal properties of the material.Combined with the special thermal positioning of artificial transpiration device and the advantage of rapid water replenishment,the photothermal system can achieve rapid water evaporation.The water evaporation rate(15.6 kg m^-2 h^-1)under 3.6 kWm^-2 light is about 33 times faster than that of natural evaporation.The composite hydrogel also has good stability and can achieve the sea water desalination,which has the application prospect of clean water production.（4）To synergistically increase the solar adsorption/conversion,regulate the water transfer and activation,improve the ability of photothermal materials to resist the fouling（salting out fouling and biofouling）,as well as enhance the solar-powered interfacial water evaporation performance,a smart3D porous photothermal composite hydrogel MoS₂@GH was prepared.Firstly,graphene oxide and molybdenum disulfide nanoflowers were fabricated according to the improved Hummers method and a one-step hydrothermal method,respectively.Subsequently,L-ascorbic acid as a reducing agent was used to intercalate antibacterial porous molybdenum disulfide（MoS₂）nanoparticles between graphene to form composite hydrogels via a simple chemical reduction.It has a large specific surface aera of about 77 m²/g and the water content of the hydrogel can be regulated through changing the mass ratio of molybdenum disulfide and graphene.This cooperated with a controllable self-pumping system of salt concentration differences and liquid level height differences-driven capillary water transfer/transpiration（CHTS）can be used a solar steam generator.In this generator,the porous hydrogel not only contributed the light trapping to enhance solar absorption（the absorbance up to about 99%） but also generated more dispersed hotspots to access and activate water.The study of the state and vaporization enthalpy of water molecules indicated that the composite hydrogel contained vast intermediate water,and thus had lower vaporization enthalpy,that is,the lower energy consumption can make it evaporate.Simultaneously,the CHTS system can regulate an appropriate water replenishment to match the evaporation of hydrogel.The hydrogel was not directly immersed into the bulk water,and its separation from the bulk water can effectively reduce the loss of conduction heat.As a result,this system can achieve a super-efficient water evaporation rate of 3.2 kg m^-2 h^-1 under 0.9 sun.Furthermore,CHTS-mediated water evaporation system can effectively alleviate salting out fouling under the conditions of high concentrations of saline and relatively high light intensity.Finally,E.coli was used as the target strain to study the anti-biological contamination properties of the composite hydrogel,indicating that the composite hydrogel had good antibacterial properties due to the synergistic effect of MoS₂ and graphene.This pure inorganic composite hydrogel had good thermal and chemical stability,it also can delay salting out fouling and resist the bacterial biofouling,which is expected to achieve an efficient water desalination and thus providing a potential technical solution for the sustainable production of clean water.