Preparation Of Hollow Glass Microspheres Based Photothermal Conversion Materials And Their Solar-driven Interface Evaporation Performance

The shortage of freshwater resources is becoming more and more serious.Solar interface evaporation technology,as a new technology to obtain fresh water from seawater or wastewater,has made rapid development in recent years,various solar evaporators with high evaporation rate and e nergy conversion efficiency have been developed.However,many evaporators have problems such as high production cost,complex preparation process,poor mechanical prop erties,and need to be supported to float on the water surface.In addition,some evaporators may take the risk of secondary pollution to water body during use,and the problem of salt deposition on the evaporator surface remains a challenge during long-term evaporation or high-concentration brine treatment.Hollow glass microspheres（HGM）are a kind of spherical micron-scale inorganic non-metallic material with hollow structure,which have many advantages such as low thermal conductivity,low price,strong mechanical strength and eco-friendly,which make HGM quite attractive in the preparation of photothermal conversion materials.However,HGM are rarely reported to be applied in this field.Based on the problems faced by solar evaporator,in this paper,spherical,2D and 3D photothermal conversion materials based on HGM were designed and prepared,and their basic properties,interface evaporation performance and salt resistance were studied.Our main aim is to use low-cost materials to prepare high-efficiency solar evaporator with strong mechanical property,great potential for large-scale application and long-term self-floating ability,to provide reference for practical seawater desalination and wastewater treatment applications.The main research contents and conclusions of th is paper are as follows:（1）A novel photothermal conversion material（MHGM/Fe₃O₄/Ti O₂-Ag/PPy）was prepared by coating Fe₃O₄,Ti O₂and Ag,as well as fluorosilane doped polypyrrole（PPy）on（3-aminopropyl）triethoxysilane modified HGM（MHGM）,respectively,and a certain amount of such hollow microspheres self-float on the water surface as a solar evaporator for efficient solar interface evaporation.Nano pores are distributed on the surface of MHGM/Fe₃O₄/Ti O₂-Ag/PPy,and it exhibits broadband light absorption,low thermal conductivity,and hydrophobic wettability.The gaps between freely movable self-floating microspheres provide channels for water transportation.The prepared solar evaporator shows a high evaporation rate of 1.5516 kg m^-2 h^-1 under 1k W m^-2（1 sun）simulated solar irradiation,and excellent long-term salt resistance.In addition,the evaporator has high photocatalytic degradation performance for methylene blue（MB）solution and obvious antibacterial performance for Pseudomonas luteum and Pseudomonas giganteum.More importantly,the photothermal conversion material has significant magnetism and can be magnetically recovered.（2）Polyamic acid（PAA）was synthesized by 4,4’-diaminodiphenyl ether and pyromellitic dianhydride.Then an ultra-flexible hydrophilic 2D porous hybrid membrane（PI-CB-MHGM）as the upper layer of the solar evaporator was prepared by using PAA,functional fillers of carbon black（CB）and MHGM,as well as pore-forming agent sodium chloride,and a self-floating polyurethane sponge is selected as the lower layer of the as-prepare double-layer solar evaporator.Take advantage of PI-CB-MHGM with micron pores,super flexibility,strong mechanical strength,low thermal conductivity(0.0765 W m^-1 K^-1),broadband light absorption（about 90%）and superhydrophilicity,the as-prepared PI-CB-MHGM evaporator shows high evaporation rate(1.4904 kg m^-2 h^-1)under 1 sun irradiation,as well as excellent evaporation stability and salt resistance,e.g.,the evaporation rate is 1.3233kg m^-2?h^-1 in 15%Na Cl solution.（3）A 3D composite aerogel（PI-MWCNT-MHGM）with vertically aligned channels was prepared by using water-soluble polyamide,carboxylated multi-walled carbon nanotubes（MWCNT）and MHGM though directional freezing,freeze drying and thermal imidization methods.PI-MWCNT-MHGM aerogel exhibits high porosity,low thermal conductivity(0.0903 W m^-1 K^-1)in the wet state,strong mechanical property,excellent thermal insulation and broadband light absorption（about 94%）.As expected,the as-prepared PI-MWCNT-MHGM evaporator shows a high evaporation rate of 1.5060 kg m^-2 h^-1 and excellent salt resistance.（4）The reduced graphene oxide/MHGM composite hydrogel was prepared by hydrothermal method with reducing agent of ethylenediamine,and then the hydrogel was dried by two drying methods.Aerogel（GAHAF）with vertical orientation channels were prepared by freeze drying,and aerogel（GAHAS）with smaller pore was prepared by supercritical CO₂ drying.Due to the different pore properties,the thermal conductivity of GAHAS(0.0823 W m^-1 K^-1)is significantly lower than that of GAHAF(0.2317 W m^-1 K^-1).In addition,the light absorption rate of GAHAS（about93%）is higher than that of GAHAF（about 85%）.The evaporation rate of GAHAS is1.4856 kg m^-2 h^-1,which is higher than that of GAHAF(1.2811 kg m^-2 h^-1).The average evaporation rate of GAHAS in simulated seawater is 0.7356 kg m^-2 h^-1 under natural sunlight with average optical power density of about 0.678 k W m^-2.Most importantly,the concentration of main ions in purified water fully meets t he drinking water standard.（5）A kind of robust inorganic photothermal conversion material（PPy-HGMAM）was prepared through binding the building block of HGM and pore-forming material of melamine sponge by aluminum dihydrogen phosphate adhesive followed by coating of PPy as light absorption layer.HGMAM exhibits interconnection network porous structure with the density and thermal conductivity of 0.32 g cm^-2 and 0.056 W m^-1K^-1,respectively.In addition,PPy-HGMAM shows a compressive stress of 957 k Pa at15%strain in the wet state,broadband light absorption（about 92%）and superhydrophilic wettability.PPy-HGMAM shows a high evaporation rate of 1.5624kg m^-2 h^-1 and excellent salt resistance.Through outdoor experiments,it was found that under the strongest sunlight(about 0.84 k W m^-2),the evaporation rate of PPy-HGMAM is 1.14 kg m^-2 h^-1.