Construction Of Hierarchical Pore-gradient Silica Aerogel For Solar-driven Water Evaporation Applicaiton

The scarcity of fresh water supplies is a critical problem that humanity is now confronting.To extract steam from sea/waste water using renewable and clean solar energy is recognized as one of the most essential solutions to tackle this problem,since the solar water evaporation technology is environmentally friendly and produces no secondary pollutants.Solar-powered interfacial water evaporation can locate the heat energy generated by photothermal materials at the evaporation interface which could avoid heating the bulk water and reduce the energy loss,thus considerably improving photothermal evaporation efficiency.However,solar-driven interface water evaporation covers many factors including light absorption,heat and water management,as well as issues of salt deposition and pollution during the evaporation process.Therefore,more research and breakthrough are needed.In this work,to synchronously achieve desired heat and water management,silica was selected as the support for photothemal materials because of its excellent physicochemical stability and superior thermal insulation,the porous structure of silica aerogel was tailored through the design of synthesis process.The purpose of the work was to provide an effective support for solar-driven water evaporation application so as to realize highly efficient utilization of solar energy.First,a hierarchical pore-gradient silica aerogel(HPSA)was successfully constructed by introducing ethanol with low boiling point during the formation of silica aerogel,synchronously balancing heat and water management.By virtue of the introduced ethanol that can work as a foaming agent,the obtained silica aerogel consisted of large pores and some randomly distributed super-large pores at top regions,small pores at bottom regions,and porous wall assembled by mesoporous silica nanoparticles,showing desired hierarchical pore-gradient structure.Due to the unique porous structure and the advantages of low thermal conductivity and abundant hydroxyl groups on the surface of silica itself,the obtained HPSA exhibited enhanced light harvesting,efficient energy localization,fast water transport and capability in significantly reducing water evaporation enthalpy.Therefore,HPSA supported carbon dots/polydopamine photothermal material evaporator(HPSA-E)can directly float the water surface to perform water evaporation without extra thermal insulation device.And HPSA-E exhibited significant superiority in promoting solar-driven water evaporation efficiency compared to balsa wood and melamine sponge evaporator.A high evaporation rate of 1.95 kg m^-2 h^-1 under 1 sun irradiation was achieved for HPSA-E.In addition,HPSA-E not only exhibited similar water evaporation performance in high-salinity brine(20 wt%)as that in pure water,but also presented fairly stable water evaporation performance under continuous natural sunlight irradiation from 9:00 to18:00 in 15 wt%brine without salt accumulation on surface,showing excellent salt resistance capacity.The developed low cost and eco-friendly HPSA-E with simplicity and scalability of synthesis holds great potential in practical solar-driven water evaporation applications.Second,a multifunctional evaporators(HPSA-DE)with photothermal and photocatalytic characteristics was designed based on the merits of the obtained HPSA.Specifically,during the synthesis of the carbon dots/dopamine photothermal agent,certain amount of copper sulfate was introduced.HPSA was then submerged in the mixed photothermal agent solution for a certain time and taken out.After washing and drying,HPSA-DE was obtained.Under 1 sun irradiation,the water evaporation rate of HPSA-DE can reach 1.70 kg m^-2h^-1.And HPSA-DE showed no obvious decrease of water evaporation ability after repeating test for 5 cycles.Additionally,HPSA-DE exhibited good photocatalytic degradation of Rhodamine B by photothermal synergistic effect.