Solar Energy-driven Water Evaporation Based On Ultralong Hydroxyapatite Nanowires

In recent years,under the background of population growth,industrial development,energy crisis and environmental pollution,fresh water scarcity has become one of the most severe global challenges.Solar energy is a clean and sustainable resource.Solar energy-driven water evaporation can generate vapor by absorbing solar energy,and clean water can be obtained after condensation of vapor.Solar energy-driven water evaporation has promising applications in seawater desalination and wastewater purification.The solar energy-driven water evaporation system includes the photothermal material,supporting substrate,thermal insulating material and water transporting paths.The ideal solar energy-driven water evaporator should have excellent performances in light absorption and heat conversion,thermal localization and water transportation.To achieve high-efficient solar energy-driven water evaporation,two solar energy-driven water evaporators with novel structure have been developed based on ultralong hydroxyapatite nanowires.The preparation,properties,performance in solar energy-driven water evaporation and related mechanisms are investigated.The main research results include:?1?The bilayer photothermal aerogel composed of ultralong hydroxyapatite nanowires and carbon nanotubes has been developed.Ultralong hydroxyapatite nanowires with high aspect ratios are synthesized by the calcium oleate precursor solvothermal method.The hydrophilic ultralong hydroxyapatite nanowire aerogel is prepared by freeze drying.The spray coating method is used to deposite a layer of carbon nanotubes on the top of the ultralong hydroxyapatite nanowire aerogel.The ultralong hydroxyapatite nanowire photothermal aerogel has high light absorption?96%?,excellent photothermal conversion performance,high porosity?95.26%?,low thermal conductivity,and good water transportation.The ultralong hydroxyapatite nanowire photothermal aerogel has an evaporation rate of 1.34 kg m^-2h^-1 and evaporation efficiency of 89.4%under 1 k W m^-2.The purification of the aqueous solutions containing heavy metal ions is also carried out using the ultralong hydroxyapatite nanowire photothermal aerogel and the ion rejection percentages are close to 100%.When treating the seawater from the Huanghai Sea,the clean water collected can meet the WHO drinking water standards.Additionally,the ultralong hydroxyapatite nanowire photothermal aerogel has excellent purification performance for organic dyes in aqueous solution.The clean water production of the ultralong hydroxyapatite nanowire photothermal aerogel can reach 4.5 kg m^-2 d^-1 under the real sunlight conditions.?2?During the desalination process of the hydrophilic solar evaporator,salt crystals can gradually precipitate on the solar absorbers,which will deteriorate the water evaporation performance significantly.In this thesis,the composite photothermal paper with the Janus structure based on ultralong hydroxyapatite nanowires has been developed.The sodium oleate-modified hydrophobic ultralong hydroxyapatite nanowire paper is used as the substrate and the photothermal material black nickel oxide is deposited on the paper by the vacuum filtration process.The Janus photothermal paper with a hydrophobic substrate and a hydrophilic upper surface is constructed.The hydrophobic ultralong hydroxyapatite nanowire paper substrate can effectively prevent the salt accumulation on the surface,and the top photothermal material black nickel oxide generates heat to evaporate water at the hydrophilic-hydrophobic interface.The thin layer of air-laid paper is wrapped over the surface of the polystyrene foam as water paths to provide water supply.The as-prepared Janus photothermal paper has a solar absorption of 88.9%over a wide spectral range?300?2500 nm?,and has an evaporation efficiency of 83.5%under 1k W m^-2.Solar energy-driven desalination is performed using the 3.5 wt%sodium chloride aqueous solution to simulate the seawater.After 8 h of solar energy-driven desalination,a large amount of crystalized salt is deposited on the surface of the hydrophilic photothermal paper while the Janus evaporator exhibits no obvious salt accumulation.The desalination performance using seawater from Huanghai Sea and the purification performance using aqueous solution containing heavy metal ions are also investigated,the ion rejection percentages are close to 100%and the collected clean water can meet the WHO safe drinking water standards.The Janus evaporator can be extended to other photothermal materials such as black titanium dioxide and Ketjen black,and the as-prepared Janus evaporators also exhibit long-term stable desalination performance.