The Design And Application Of Bi-layered Functionalization Of Polyvinyl Alcohol Based Composite Gels

With the worsening pollution of fresh water resources and the increasing the global population,human society is facing a serious crisis of water shortage.It is estimated that by2040,nearly two-thirds of the world’s population will face a freshwater crisis.In order to alleviate the global freshwater crisis brought by the pressure of population growth,desalination has become an important research interests to solve this problem.Different from the traditional indirect desalination method that requires the consumption of petroleum resources,solar photothermal steam evaporation could directly act on the photothermal absorption material.With the high light absorption capacity of the absorption material and good heat insulation capacity,it can reduce the heat energy loss during the water evaporation process.Polyvinyl alcohol hydrogel,as a photothermal absorbing material skeleton,is highly hydrophilic,biocompatible,low pollution,and can be combined with other photothermal absorbing materials.Meanwhile,the three-dimensional network structure of gel reduces the latent heat of evaporation.The water molecular transport channel,the low thermal conductivity of the porous structure,and the effective reduction of heat diffusion during the water evaporation process have caused researchers to attach importance to polyvinyl alcohol-based gel materials in the field of solar steam evaporation.In this paper,a polyvinyl alcohol-based hydrogel was prepared by a simple freeze-thaw method,combined with photothermal materials such as carbon nanotubes and silver nanoparticles.With the adjustable hydrophilicity and high water absorption rate of the composite structure,the gel structure was optimized to investigate its properties and efficiency when used as a solar vapor evaporation device.The specific research contents are as follows:Firstly,a hydrogel with adjustable hydrophilicity was prepared by a freeze-thaw method with the aid of a glutaraldehyde cross-linking agent,and by adjusting the concentration of polyvinyl alcohol with carbon nanotubes.By optimizing the parameters,a highly hydrophilic photothermal evaporation material with a three-dimensional porous network gel structure was obtained.Based on optical,thermal and water evaporation test analysis,it was found that the photothermal evaporation properties of carbon nanotubes/polyvinyl alcohol gels were favored with a higher water evaporation rate and excellent salt-resistance capacity.Secondly,the functionalized carbon nanotubes and the silver nitrate solution were uniformly mixed and dried,and then the carbon nanotubes grafted with silver nanoparticles were obtained by annealing in H₂ atmosphere.Using the Localized Surface Plasmon Resonance effect of silver nanoparticles and the thermal effect of carbon nanotubes,combined with the results of optical and thermal analysis,it can be seen that the introduction of silver nanoparticles makes the composite material have a higher heating effect.Finally,based on the research results of the previous two chapters,the hydrophilic capacity and the excellent thermal properties of carbon nanotubes/Ag can be adjusted by using the content of polyvinyl alcohol.The carbon nanotube/Ag is used as the light-absorbing material,and the crosslinking agent is supplemented by the freeze-thaw method.The carbon nanotube/Ag/polyvinyl alcohol double-layer gels with different hydrophilicity on the upper and lower layers were prepared.The double-layer structure was used to manage the water transport capacity and the excellent heat conversion ability of the light-absorbing material,which was achieved under a sunlight intensity.2.39 kg m^-2 h^-1 of water evaporation rate,while maintaining the gel’s excellent resistance to salt formation.The research results provide an efficient light absorbing material solution for the field of solar light and hot water evaporation.