Construction Of Carbon-based Photothermal Materials And Studies On Their Water Evaporation Performances

To employ solar energy as the input energy to obtain fresh water resource through the liquid-gas phase transition of water molecule is one of the most promising and eco-friendly ways to alleviate global water scarcity.Photothermal materials are the most important components of photothermal evaporation system,and concentrating heat energy at the water/air interface is the core of photothermal evaporation.Therefore,developing superior photothermal materials and and ensuring water evaporation on the water/air interface of photothermal materials become the urgent challenges to be faced in solar water evaporation.Owing to its excellent light absorption,high processability,stable chemical properties and developed pore structure,carbon materials have made remarkable achievements in solar-driven water evaporation.With this context,in this paper,the preparation of carbon-based photothermal materials and their application in solar water evaporation are closely studied.Additionally,the effects of chemical/structural engineering design on photothermal conversion and water evaporation performance of carbon materials are discussed in detail.Firstly,carbon-based photothermal conversion materials with excellent performance were obtained via chemically modifying the molecular structure of carbon materials and bonded withπ-rich electronic material polyaniline.Secondly,by constructing macroscopic directional channels,the directional transfer of appropriate water to evaporation surface was achieved,and thus the influence of water content on the evaporation performance was investigated.Finally,sphere evaporator with adjustable diameter and hydrophilicity was constructed by template-free method.The mechanism of gravity-field change driving the sphere to rotate and dissolve surface salt particles was explored.The main research contents and conclusions are as follows:（1）The design and optimization of the structure of carbon-based evaporator by surface engineering are evaluated,and the relationship between the surface morphology and light/heat management of evaporator is acquired,which provides some guidance for the design and development of high-performance solar evaporator.The surface morphology of graphene oxide（GO）was modulated by in-situ polymerization of aniline by electrostatic assembly,and herein the photothermal materials with highly foldable periodic structure was engineered.By changing the mass ratio of GO to aniline,photothermal materials with different surface morphology were prepared for solar-driven evaporation.The effect of surface topological structure on evaporation performance was surveyed by microscope observation and field simulation.The results showed that polyaniline（PANI）array on the surface of GO played three roles:（1）The PANI arrays passivated the oxygen-containing groups of GO via electrostatic interactions,thereby protecting GO from restacking or redissolving into water;（2）The periodically conical PANI configurations facilitated light management and heat management;（3）The highly-dispersed PANI nanocones enlarge the evaporative surface,thus accelerating the steam generation.Therefore,compared with the planar structure of GO,the GO modified by the PANI nanocone array enhanced light absorption,achieved thermal localization and expanded the solid-liquid contact area,demonstrating an evaporation rate of 1.42 kg m^-2 h^-1compared with planar GO.（2）A flower-like evaporator（SGS）with directional channels was created to simultaneously obtain effective light capture,water transport and thermal insulation properties.By constructing a carbon-based aerogel with macroscopic directional channels,the internal absorption of light,the directional transport of appropriate amount of water and the expansion of evaporation surface were realized.Accordingly,the influence of water content on the evaporation results was comparatively analyzed.This kind of carbon-based photothermal aerogels were prepared by using biomass sodium alginate as the matrix and graphene oxide as the photothermal material.Evaporators with variable channel diameters and channel directions were contrived by freeze-pouring method and sol-gel method.The relationship between water transport channels and specific surface area and evaporation performance was decoupled.Especially,the evaporation performance of SGS on organic dye wastewater and oil-water system was tested.The results showed that SGS with directional channels had the highest water evaporation rate of 2.55 kg m^-2 h^-1,while aerogel with microchannels（SGP）had a reduced evaporation rate,which was about 2.11 kg m^-2 h^-1.This is attributed to the fact that the small gap of SGP porous layer will adsorb a large amount of water,resulting in serious heat loss in the photothermal evaporation process.Whereas,SGS with macroscopic directional channels reduced the content of adsorbed water and greatly enlarged the evaporation area due to the three-dimensional independent structure.（3）Symmetrical spherical evaporator was constructed through sol-gel method,and the mechanism that gravity drove salt particles redissolved into water was summarized.With sodium alginate as matrix,petroleum coke as photothermal material and polyvinyl alcohol as hydrophilic additive,sodium alginate formed an"egg-shell"structure with polyvalent cations,thereby a three-dimensional sphere evaporator（SPP）was prepared by template-free method.The effects of surface hydrophilicity and sphere diameter on salt accumulation during evaporation were discussed.It demonstrated that since SPP sphere floated on the surface of saline solution,a water film gradient was generated along its perpendicular to the diameter of water surface.Water on the top surface of sphere preferentially evaporated,hence salt ions firstly nucleated and grew at the top position.As the mass of these positions gradually increased,the unevenness of gravity filed exceeded the resultant force between the sphere weight,buoyancy and the surface tension of liquid.Finally,the force balance between sphere evaporator was disrupted,deflecting the gravity center of sphere and causing the salt crystal particles to dissolve in water.Meanwhile,the side of the rotating sphere that was originally in contact with water could capture light and heat water,carrying out a new round of evaporation cycle.The sphere repeated the cycle between evaporation-rotation-re-evaporation-rotation,thus long-time evaporation without salt blocking was observed.The average evaporation rate of SPP in 20 wt%Na Cl solution reached up to 2.88 kg m^-2 h^-1.And it maintained the cycle of evaporation-crystallization-rotation-evaporation.