Design And Application On Solar Water Evaporation System Of Fiber-loaded Carbon Dots Hybrid Gel

In order to ameliorate the shortage of fresh water,using solar energy to drive water evaporation to achieve desalination and sewage treatment has become the focus of research.Zero-dimensional nanomaterials have attracted much attention in the design of solar water evaporation systems due to their unique properties,such as low thermal conductivity,large specific surface area,diverse morphology,and adjustable chemical composition.However,only zero-dimensional nanoparticles to construct photothermal materials are not only expensive,and the water evaporation performance of the system cannot meet actual requirements.To promote the application of solar water evaporation technology in practice,the preparation cost and practical application performance of the system should also be paid attention to.Therefore,the low-cost,high-efficiency solar water evaporation system will become the main research direction of people in the future.As a kind of zero-dimensional nanoparticles that are harmless to the environment and simple to prepare,carbon dots（CDs）exhibit excellent hydrophilicity,low thermal conductivity and good light absorption capacity.Therefore,in this paper,CDs are used as photothermal agents,and low-cost fibers are used as carriers（cotton fibers,polyacrylonitrile fibers）to propose a low-cost and universal method for designing solar-driven water evaporation systems using zero-dimensional nanoparticles.The system shows good thermal management capabilities and excellent water evaporation performance.The research work of this article mainly includes the following two aspects:（1）The cotton fibers loaded carbon dots hybrid gel system realizes high-efficiency solar water evaporation.Using cotton fibers（CFs）as the carriers,CDs are assembled on the surface of CFs as a photothermal agent using the physical adsorption of CFs;then the polyvinyl alcohol（PVA）mixed solution is used to complete the gel process on the surface of fibers;Freeze-thaw process to improve the strength and toughness of the hybrid gel.The gel process not only fixes the CDs on the CFs surface,but also adheres the CFs together to form a stable and high-strength hybrid gel system CFs@CDs.Through the characterization results of SEM,TEM and HRTEM,it can be clearly seen that the assembled CDs are supported on the surface of CFs and are wrapped and fixed by PVA gel.By comparing the solid absorption of CFs and CFs@CDs,it is found that the light absorption capacity of CFs@CDs is significantly enhanced.In the stress-strain test,the gel-treated CFs@CDs has higher mechanical strength,and the mechanical properties in the dry state are significantly improved compared to the wet state.By comparing the thermal conductivity of ungelled CFs/CDs and CFs@CDs,it is found that the thermal conductivity of CFs@CDs(0.0553 W m^-1K^-1)is slightly lower than that of CFs/CDs(0.0633W m^-1K^-1),which indicates the PVA gel layer also has the effect of heat insulation.Under the simulated sunlight of 1 k W m^-2,the sample CFs@CDs showed a high evaporation rate(2.32 kg m^-2 h^-1)and good cycle stability.CFs@CDs showed stable adaptability in tests of different concentrations of salt solutions（3.5 wt%,5 wt%,10 wt%）and acid-base environments（p H=2～10）.In the long-term（8 d）water evaporation test in an outdoor environment,CFs@CDs showed good mechanical stability and structural durability.（2）Carbonized polyacrylonitrile fibers loaded carbon dots hybrid gel system is used in seawater desalination.By high-temperature carbonization,polyacrylonitrile fibers（PANFs）are given the basic characteristics necessary for the photothermal agent carriers:more adsorption sites,high solar light absorption and good water absorption capacity.CDs were assembled on the surface by the physical adsorption of C-PANFs after carbonization,and the hybrid gel system C-PANFs@CDs was prepared by the cross-linking reaction of PVA solution.BET test analysis shows that C-PANFs carbonized at 500°C have a higher specific surface area(109 m²g^-1),which is beneficial to the adhesion of CDs.In the mechanical strength test,C-PANFs@CDs showed high mechanical stability and toughness.The solid absorption test proves that C-PANFs have good light absorption capacity.Under the simulated sunlight of 1 k W m^-2,C-PANFs@CDs showed good evaporation performance(1.9 kg m^-2 h^-1)and cycle stability;in the subsequent seawater desalination simulation experiment（3.5 wt%Na Cl solution）,C-PANFs@CDs evaporation rate is not affected and has excellent salt resistance.In addition,by optimizing the pore path of C-PANFs@CDs,it can adapt to the evaporation of high concentration（>5 wt%）brine for a long time.