Contents
2 Research status of construction of superwetting photothermal materials
2.1 Carbon-based superwetting photothermal materials
2.2 Organic-based superwetting photothermal materials
2.3 Semiconductor-based superwetting photothermal materials
2.4 Compound superwetting photothermal materials
3 Research status of application of superwetting photothermal materials
3.1 Application of anti-icing of superwetting photothermal materials
3.2 Application of superwetting photothermal materials in seawater desalination and solar evaporation
3.3 Application of superwetting photothermal materials in oil/water separation
1 Introduction
When the contact angle of the droplet on the surface of the material is close to 0 ° (super-lyophilic) or greater than or equal to 150 ° (super-lyophobic), the functional surface material with extreme wetting ability is called super-wetting material. Super-wetting materials show different special wetting behaviors for different droplets, such as super-hydrophobic, super-oleophobic, super-hydrophilic, super-oleophilic, super-amphiphobic, super-amphiphilic, super-slippery, etc. At present, super-wetting materials are mostly used in the fields of anti-fouling self-cleaning, anti-icing, oil-water separation, anti-biological adhesion, flow resistance reduction, corrosion prevention, droplet transfer, water mist collection, easy decontamination, directional transmission and so on
[1⇓~3][4⇓~6][7,8][9,10][11][12,13][14,15][16,17][18][19,20]. Su et al. Prepared a superhydrophobic surface with a contact angle of 162 ° and a rolling angle of 3 ° on a copper substrate by electrodeposition and heat treatment of (heptafluoro-1,1,2,2-tetraethylenetriethoxysilane) triethoxysilane (AC-FAS), which has good mechanical wear resistance, corrosion resistance and self-cleaning properties
[21]. Zhu et al. Coated a superhydrophobic polysiloxane layer on the surface of porous polyurethane sponge by one-step solution immersion method to obtain a superhydrophobic sponge with excellent elasticity and chemical stability, which can be used as an absorption material for oil leakage on water surface due to its special wettability and high porosity
[22]. However, with the continuous development of various industries, there is a higher demand for functional display and intelligent feedback of materials, and functional materials with only a single super-wetting characteristic no longer meet the needs of industrial development and industrial applications.Therefore, the transformation of super-wetting materials to multi-function and intellectualization has become the focus and research trend of the current academic circles.
According to the research of scientists, the synergy of super-wetting performance and photothermal effect can achieve the trend of improving quality and efficiency in related application fields. As shown in Figure 1A and Figure B, photothermal effect refers to the corresponding material after receiving light wave irradiation.Plasma resonance and photothermal electron excitation occur in it, which promotes the conversion of light energy into heat energy, resulting in the increase of temperature on the surface of the material and its surroundings under light conditions. Photothermal material is a kind of functional material with photothermal conversion ability, which mainly includes carbon-based photothermal materials, organic-based photothermal materials, semiconductor-based photothermal materials and metal-based photothermal materials. Because of its excellent light absorption ability and high photothermal conversion efficiency, it can produce significant surface heating effect when irradiated by the corresponding power and wavelength light source, so it has been widely studied and applied in photothermal targeted therapy, photothermal power generation, seawater desalination and other fields. In recent years, with the development of super-wetting materials and photothermal materials, super-wetting characteristics and photothermal effects are taken as functional elements.The super-wetting photothermal materials constructed by the binary synergistic route show outstanding application potential in industrial wastewater treatment, seawater desalination and solar evaporation (Fig. 1C), which has attracted the attention of researchers and engineers in various industries.
2 Research Status of Construction of Superwetting Photothermal Material
At present, there are two main ways to prepare super-wetting photothermal materials, namely, introducing photothermal components into the super-wetting materials or performing super-wetting treatment on the photothermal materials. Most of the existing technical means use chemical modification, chemical deposition, self-assembly, spraying, etc. To prepare super-wetting photothermal materials with different dimensions, as shown in Figure 2. Specifically, the chemical modification method is to graft the reactive group onto the material through chemical reaction, which is simple and easy to operate, but requires specific reaction sites, so it often depends on special modifiers or coupling agents. Layer-by-layer self-assembly method is a material surface and interface construction state that uses chemical or ionic bonds to combine layer by layer and finally achieve a similar stacking effect, which has good controllability for material preparation, but the prepared materials often have the problem that their structural properties are easily damaged due to physical and mechanical effects. Chemical vapor deposition (CVD) is a process in which a chemical gas is introduced into a reactor and deposited on the surface of a substrate through its chemical reaction. However, the required reaction temperature is high, which greatly limits the substrate. The spraying method usually uses air pressure to spray the blend solution to adhere or associate on the surface of the substrate, and the preparation process is simple and easy to implement, but it often requires a curing process. The reaction and molding of the template method are carried out in a reactor, and the material with the same or opposite size or shape as the reactor is prepared after the reaction. The template method can be used to precisely control the structure and shape of the material with high flexibility, but it usually depends on precision equipment, so the cost is high. The one-pot method is a multi-step chemical reaction based on simple raw materials in a reactor to obtain materials with complex structures, which can effectively avoid the separation and purification of intermediates. However, it is difficult to accurately control the composition and physical and chemical properties of the products. Table 1 compares the above methods. At present, super-wetting photothermal materials can be roughly divided into carbon-based, organic-based, semiconductor-based, metal-based and composite-based according to their substrate types.
2.1 Carbon-based super-wetting photothermal material
At present, carbon-based photothermal materials are mainly divided into traditional carbon materials and carbon-based nanomaterials, among which carbon nanotubes (CNTs) and graphene have gained good research value and prospects in many applications because of their excellent photothermal effects. Zhong et al. deposited laser-induced graphene on a surgical mask by bidirectional laser-induced forward transfer to prepare a superhydrophobic photothermal mask (Fig. 3A), whose temperature can be rapidly raised to above 80 ℃ under sunlight to play a bactericidal role (Fig. 3B)
[30]; In addition, the mask evaporation rate is 1.13 kg/(m
2·h) at 1 sun intensity, and there is no obvious salt accumulation phenomenon. Wang et al. Prepared PDMS/graphene composite superhydrophobic photothermal material by evaporation boiling method through ultrasonic and heating curing treatment, and developed a micro-robot based on the material, which can realize light drive and various forms of motion by means of its superhydrophobicity and photothermal effect
[31].
Jiang et al. Prepared CNTs/silicon carbide (SiC) particle suspension with photothermal effect by physical blending, and then sprayed it on the surface of ethylene-vinyl acetate copolymer (EVA), thus preparing super-hydrophobic SiC/CNTs coating for photothermal deicing and passive anti-icing
[25]. The coating has a contact angle of 161 ° and a roll angle of 2 °. In addition, compared with the original EVA surface, the adhesion degree of the coating to ice is reduced from 25.65 kPa to 2.65 kPa; At the same time, the temperature of the above coating jumps from 30 ℃ to 120 ℃ within 10 s under near-infrared (NIR) irradiation, thus achieving excellent photothermal deicing effect (Fig. 3C, d). Su et al. Transferred and applied multilayer carboxylated and aminated multi-walled carbon nanotubes to EVA substrate after vacuum filtration, and then modified the target substrate with n-octadecylamine (ODA) to prepare vacuum-assisted laminated superhydrophobic CNTs film with both electrothermal and photothermal effects
[32]. In addition, when the prepared ODA-(MWCNT—COOH/MWCNT—NH
2)
6-based superhydrophobic glass is irradiated by remote NIR, the prepared ODA-(MWCNT—COOH/MWCNT—NH
2)
6-based superhydrophobic glass can linearly move to a designated area, and the action mechanism is caused by a surface tension gradient generated by local photothermal action, so the film can be applied to a high-speed light-driven actuator.
Graphene and CNTs have significant photothermal conversion properties, but their high price restricts their industrial application in the field of industrial manufacturing. Li et al. Sprayed a dispersion composed of nano-carbon black, PDMS and PFDTES on the surface of different substrates treated with adhesives to prepare super-hydrophobic photothermal coatings
[33]. The coating can extend the freezing time of water droplets on the surface by 130 s, and at the same time, it can melt the ice within 9 min under sunlight by virtue of its photothermal properties (Fig. 3e, f). Zhou et al. First modified the carbon felt hydrophilically, then endowed the substrate with carbon nanowire arrays by electrodeposition and heat treatment, and then loaded the above carbon skeleton with the help of Ti
2O
3 to prepare a narrow band gap Ti
2O
3 composite multilevel carbon substrate photothermal conversion material (Ti
2O
3@CA/CF) composite photothermal material with a photothermal conversion efficiency of 71.9%
[34]. It can be seen that although nanocarbon materials have the advantage of relatively low price, their photothermal conversion ability is significantly lower than that of graphene or carbon nanotube materials. See Table 2 for specific photothermal conversion data.
2.2 Organic-based super-wetted photothermal material
Organic photothermal materials have excellent infrared absorption capacity due to the influence of photothermal electron excitation, and most organic photothermal materials are easy to biodegrade, which is in line with the development trend of green environmental protection. However, some of the existing organic-based photothermal materials are prone to photodegradation, resulting in the weakening of application durability and the continuous reduction of photothermal conversion efficiency. Therefore, it is often compounded with carbon-based photothermal materials in order to improve the photothermal conversion ability and related application performance
[35].
Liu et al. used benzene and toluene as raw materials to prepare photothermal materials based on hypercrosslinked polymer (HCPs) nanotubes. As shown in Figure 4A, after modification by polypyrrole (PPy) and sodium alginate (M), the obtained PPy-M-HCPs showed significant surface hyperhydrophilicity, strong light absorption and excellent mechanical properties
[36]. The results show that the PPy-M-HCPs have good solar absorption capacity and solar heat conversion efficiency, and the surface temperature increases with the increase of light intensity and time. Because the PPy-M-HCPs have simple synthesis route, excellent stability and high solar energy conversion efficiency, it provides a new idea for the construction of high-performance solar evaporator. Yang et al. Immersed the block melamine formaldehyde (MF) sponge in polydopamine (PDA) aqueous solution, and then immersed the sponge coated by PDA in nanoscale TiO
2 aqueous solution to form a TiO
2/PDA@MF sponge (TPMF) through multiple reciprocating immersion and deposition, and then transferred it to a Petri dish containing 1 H, 1 H, 2 H perfluorotrichlorosilane (PFDTCS) to prepare a superhydrophobic F/TiO
2/PDA@MF sponge (FTPMF) by CVD
[37]. The top and bottom of the sponge can be heated to 90.7 and 70.2 ℃ respectively under one solar test (Fig. 4 B, e). In addition, after five oil absorption cycles, the absorption capacity of the target object to gasoline, diesel and pump oil is 14.8 ~ 24.5 G/G, and the recovery rate after the oil absorption cycle test is still higher than 84.3%.
2.3 Semiconductor-based super-wetting photothermal material
Semiconductor-based photothermal materials have rich varieties and good photothermal stability. As a new type of photothermal materials, it has broad prospects for development because of its easy preparation, low cost and other advantages.Through reasonable design, the efficiency of solar evaporation can be greatly improved, so it shows interesting research potential in the fields of seawater desalination, wastewater treatment and solar evaporation
[38].
Li et al. Developed Copper oxide @ copper sulfide/polydimethylsiloxane superlipophilic array nanowires (CuO @ CuS/PDMS NWAs/CF) by in-situ self-assembly method on copper foam (CF) substrate.Due to its excellent photothermal conversion performance and super-lipophilicity, it can rapidly reduce the viscosity of crude oil due to photothermal heating under simulated sunlight (Fig. 5A, B), showing the ability to effectively and continuously collect crude oil on the surface of seawater
[39]. Xu et al. Immersed the copper mesh into the mixed aqueous solution of sodium hydroxide (NaOH) and ammonium persulfate [(NH
4)
2S
2O
8] after ultrasonic cleaning to prepare the copper hydroxide [Cu(OH)
2] mesh, and obtained the CuO nanowire mesh with super-hydrophilicity after rapid heating treatment by alcohol lamp.The CuO nanowire network was then combined with an insulating expandable polyethylene (EPE) foam, and a "CuO tree diagram" (The CuO tree system) was prepared by simulated transpiration and used to evaluate The solar evaporation performance
[40]. Under one sun illumination, the surface temperature of the "CuO tree" increased by 16.5 ° C after 40 s illumination, the evaporation rate was 1.42 kg m
2·h
-1, and the solar vapor efficiency was 84.4% (Fig. 5C – e). At the same time, due to the inherent good pollutant catalysis and bacteriostasis capabilities of the CuO nanowire mesh, the problems of bacteria and pollutants faced by materials such as seawater desalination and solar evaporation when they are in contact with and float on the water surface for a long time can be further solved. Therefore, this also provides inspiration and strategy for the expansion of semiconductor-based super-wetting photothermal materials to multi-functional applications.
In addition to the above photothermal materials, metal-based photothermal materials are also a major branch of photothermal conversion materials, such as heavy metals such as gold (Au), silver (Ag), platinum (Pt) and common metals such as copper (Cu), nickel (Ni) and aluminum (Al).Li and Song et al. Prepared superhydrophobic materials for oil-water separation using Cu and Ni substrates respectively, but the oil-water separation effect was mainly achieved by the superwetting performance of the materials themselves, and the good photothermal conversion performance of the metal substrate was not effectively utilized
[41⇓⇓~44][45,46][47][48]. Therefore, the combination of the excellent photothermal conversion performance and the super-wetting performance of the material is beneficial to maximizing the advantages and realizing targeted applications such as efficient oil-water separation and solar evaporation.
2.4 Composite super-wetting photothermal material
Han et al. Immersed nickel (Ni) foam substrate into Fe(acac)
3 solution for full filling, and then prepared ferric oxide/carbon nanotube/nano-nickel (Fe
2O
3/CNT/NF) composite foam by combustion treatment, as shown in Fig. 6a
[49]. The test shows that the photothermal conversion capacity is high, the hydrophilicity is strong, and the photothermal conversion performance is good, the photoheat conversion efficiency under one and three solar irradiations is 81.3% and 93.8% respectively, and the photoheat conversion efficiency can be used for collecting water in oily seawater and environmental wastewater. In addition, Wang et al. Prepared a super-hydrophobic Conductive sponge composite (CSC) strain/pressure sensor by silver precursor reduction and phase separation, which showed effective deicing behavior and excellent photothermal heating effect
[50]. In a single heating-cooling cycle with an irradiation intensity of 1 sun, the surface temperature of the original sponge increased by only 4 ° C after 2 min of irradiation, while the CSC rapidly increased to 53 ° C and reached equilibrium. Cheng et al. Based on poly { [N-methylperfluorohexane-1-sulfonamide] ethyl (meth) acrylate-r-stearoyl-r-glycidyl methacrylate) } [P(C
6SMA-r-SMA-r-GMA)] prepared by solvent polymerization,And doped with amino-functionalized modified ferroferric oxide particles (MNP@NH
2), and subjected to ultrasonic dispersion treatment after physical blending to finally form a novel multifunctional superhydrophobic magnetic composite coating, as shown in Fig. 6 B
[51]; When the amount of MNP@NH
2 added in the preparation process is 50wt%, the temperature of the obtained composite coating rises by 10 ℃ after being irradiated under a fluorescent lamp for 1 min (Fig. 6 C, d), which accelerates the melting rate of ice and shows excellent photothermal deicing performance.
The combination of different types of super-wetting photothermal materials, such as semiconductor-based/carbon-based composite, organic-based/semiconductor-based composite, etc., is beneficial to obtaining higher photothermal conversion efficiency; However, there are still considerable challenges for current researchers, such as the stability of materials after compounding and the ability to obtain better photothermal conversion performance by adjusting the proportion of different types of materials.
3 Application Research Status of Superwetted Photothermal Material
Super-wetted materials have been widely explored and applied in many fields because of their special surface wetting behavior. However, with the rapid development of high-tech industries, different industries have shown increasingly specific demands for super-wetted multifunctional materials. In view of this, super-wetting photothermal materials with synergistic combination of extreme wetting ability and light heating effect have become the preferred choice for relevant scholars to implement enhanced application in some special environments. When the super-wetted photothermal material is used for anti-icing, the photothermal performance can reduce the ice adhesion strength and delay the icing. When the super-wetting photothermal material is applied to seawater desalination, the existence of the photothermal effect can promote the seawater to achieve the purpose of efficient and continuous evaporation; When the super-wetted photothermal material is used for recovering and separating the wastewater containing the viscous oil phase, the surface of the material is significantly heated under the action of light based on the photothermal effect, so that the viscosity of the contacted viscous oil phase is reduced and the fluidity of the viscous oil phase is increased, thereby realizing the efficient oil-water separation effect on the viscous oil-phase. In addition to the above fields, scientists have also made relevant research and attempts on super-wetting photothermal materials in industrial wastewater treatment, droplet transport and other aspects, and have made different degrees of research progress.
3.1 Anti-icing application of super-wetted photothermal material
Icing is a common natural phenomenon, which usually adversely affects road systems such as highways and public facilities such as communication lines.However, the traditional deicing methods mainly include manual removal of ice and snow, mechanical deicing with auxiliary tools such as pneumatic flow, ultrasonic or chain, accelerating the melting process of ice with the help of electric heating equipment, and adding snow-melting agent to remove ice.It has the disadvantages of difficult operation, low efficiency, high energy consumption and easy damage to pavement and other surfaces, so researchers have carried out a series of studies on the mechanism of deicing. Superhydrophobic materials in superwetting materials are widely used in anti-icing research because of their rough surface structure and low surface energy. The main reasons include the following two aspects: (1) Due to the influence of the rough structure of the low surface energy micro-nano layer of superhydrophobic surface, the solid-liquid contact area is reduced.At the same time, according to the Cassie-Baxter wetting model, when there is a semi-stable stagnant air layer between the micro-rough structures of the liquid contact surface, the thermal resistance of heat conduction can be increased to prolong the nucleation time of ice crystals on the surface of the material, thus promoting the freezing time of water droplets
[52]. (2) The droplet presents Cassie state on the superhydrophobic surface with high surface roughness, and the solid-liquid contact area reaches the minimum, thus reducing the adhesion strength of the ice layer
[53]. Therefore, for deicing, there are mainly two ways: active deicing and passive anti-icing, that is, to reduce the adhesion strength of ice or to prevent water droplets from adhering to the surface of materials
[54]. However, in extreme cold weather, the condensation of ice crystals in the internal structure of the material will increase the surface wettability, promote the formation of ice layer, and then affect the moisture resistance of the material, so the preparation of super-wetting photothermal materials by chemical modification, spraying, liquid immersion surface (LIPS), bionic and other methods has become a hot trend in the research of anti-icing.
Yang et al. Prepared a PB/FHPU superhydrophobic composite coating by mixing and ultrasonic dispersion of Prussian blue (PB) with photothermal properties and perfluorooctanol modified hyperbranched polyurethane (FHPU)
[55]. When the PB content is adjusted to 13%, the deep contact angle is 157 °, the rolling angle is 1.8 °, the moisture resistance is obvious, and the contact droplet is very easy to roll off, so the ice crystal is difficult to adhere to the surface of the coating, so the anti-icing effect is better; After near-infrared light irradiation, it is found that the surface temperature of PB/FHPU composite coating can increase by 78.1 ℃ within 10 s, and the highest temperature can reach 148.7 ℃, as shown in Figure 7A. However, the superhydrophobic property of this composite coating mainly depends on the low surface energy treatment of perfluorination, which is bound to cause harm and bioaccumulation risk to organisms and the environment, so the application of this kind of material is generally restricted. Zhu et al. Prepared magnetic fluorine-free superhydrophobic ferroferric oxide @ silica/hexamethyldisilane particles (Fe
3O
4@SiO
2/HMDS) by one-pot method, as shown in Fig. 7 B, and the product particles were sprayed onto the surface of different fabrics with PDMS as the binder.When the obtained super-hydrophobic organic-inorganic hybrid multifunctional particle coated fabric is irradiated by near-infrared light, the coated fabric can quickly achieve a photothermal heating effect compared with the original fabric, which provides a possibility for potential deicing applications
[29]. After adsorbing I
2 vapor on the surface of vinyl acetate (EVA) substrate, Hu et al. Sprayed micro-nanoscale SiC and SiO
2 on the surface of the treated substrate with tetraethyl orthosilicate (TEOS) as binder, and obtained a particle coating with superhydrophobicity, as shown in Fig. 7c
[54]. The ice adhesion strength of the coating is significantly reduced, the freezing time of water droplets is effectively prolonged, and the prepared coating can be rapidly heated under the irradiation of near-infrared light to achieve the purpose of waterproof and deicing. Xie et al. Poured PDMS/Ti
2O
3 solvent on a polytetrafluoroethylene (PTFE) template with a regular convex array structure, and after thermal curing, peeled off to prepare a super-hydrophobic photothermal surface with an opposite array structure to the template
[27]. Under the irradiation of simulated solar light source, the droplets on the surface of the photothermal material remain unfrozen within 6 H, and the frozen droplets can be melted within about 197. 38 s, showing obvious photothermal performance and anti-icing effect.
However, there are still limitations to achieve the anti-icing effect by prolonging the nucleation time or reducing the ice adhesion strength. Therefore, some studies have shown that when a droplet impacts a superhydrophobic surface, its original motion path and final product are changed, and the droplet is accelerated to fall off from the surface by shortening the contact time, thus achieving anti-icing
[56].
3.2 Application of super-wetting photothermal materials in seawater desalination and solar evaporation
Freshwater shortage has always been one of the urgent problems to be solved in the process of sustainable development of today's society. Water crisis is even regarded as the biggest risk in the world by the World Economic Forum, and seawater desalination is an important way to deal with the scarcity of freshwater resources
[57]. However, the traditional desalination technology mainly relies on some professional large-scale equipment, so there is a huge energy consumption, at the same time, often accompanied by a variety of pollution problems, which brings a burden to the environment. In recent years, as people pay more and more attention to environmental protection, how to desalinate seawater with high efficiency and low energy consumption has become a hot research topic in academia and an urgent need for industrial development. With the continuous advancement of science and technology, scholars have found that super-wetting photothermal materials can achieve efficient evaporation behavior under specific conditions, so relevant researchers have high hopes for their effectiveness in the field of seawater desalination.
Hu et al. prepared a superhydrophobic silica gel sponge, coated its top surface with carbon nanotubes, and then modified it superhydrophilically by oxygen atmosphere plasma, thus preparing an egg-yolk eggshell-type superhydrophobic/superhydrophilic solar evaporator (Fig. 8 a-c)
[58]. The evaporator can perform continuous seawater desalination for 200 H under one solar irradiation, and shows a stable evaporation rate, which can collect 11.8 kg·m
-2 of fresh water. Zhao et al. Utilized two-dimensional Ti
3C
2(MXene) nanosheets (PFDTMS-Ti
3C
2) modified by filtration triethoxysilane (PFDTMS) as salt blockers, vapor evaporators, and light absorbers in the filter membrane to successfully fabricate hydrophobic diene membranes for commercial filtration (Fig. 8d, e)
[23]. Hydrophobicity makes it have high light utilization efficiency, abundant evaporation channels and high stability. The water evaporation rate of the hydrophobic membrane loaded with 10 mg d-Ti
3C
2 is 71% under 1.31 kg·m
-2·h
-1,1 solar irradiation, and the high salinity stability is more than 200 H. At the same time, the hydrophobic membrane has excellent purification effect on wastewater containing organic dyes (RhB and Mo), heavy metal ions (Cu
2+ and Cr
6+) or volatile organic compounds (acetone and benzene).The hydrophobic membrane has good seawater purification ability and efficient and stable photothermal conduction process, so it provides a new strategy for targeted desalination and purification of environmental wastewater driven by solar energy. Liu et al. First polymerized formaldehyde, phenol and hydroquinone to produce Phenolic aldehyde foams (PAFs) high-efficiency photothermal materials, and then deposited carbon soot formed by kerosene lamp combustion on the surface of PAFs to make double-layer photothermal materials (CPAFs) with excellent salt resistance, which can be used in solar evaporation engineering (Fig. 8f, G)
[59].
When the photothermal material achieves the super-hydrophobic effect, water droplets are difficult to wet and attach to the surface of the material due to the existence of micro-nano synergistic phase configuration with low surface energy on the surface of the material, so that seawater can not enter the material smoothly, resulting in the difficulty of seawater desalination process. When the photothermal material as a whole tends to be super-hydrophilic, the surface of the material converts the light energy absorbed by the sun into heat energy, which promotes the temperature of the surface and surrounding areas to rise, but the salt in the seawater will precipitate and crystallize on the surface of the material due to supersaturation during evaporation, thus restricting the efficiency of solar evaporation or desalination. At present, most of the wetting asymmetric materials with photothermal effect are composed of a super-hydrophilic bottom and a super-hydrophobic top for synergistic photothermal conversion, so when exposed to sunlight,The top of the material is rapidly heated up due to photothermal conversion, and the heated area heats and evaporates the absorbed seawater, so that the water in the seawater evaporates from the pores on the super-hydrophobic surface of the material.At the same time, the salt concentration at the top of the material will continue to rise, so that the salt concentration gradient will be generated due to osmotic pressure, resulting in the diffusion of high-concentration salt from the top area to the bottom, so this kind of wetting asymmetric super-wetting photothermal material can avoid the problem of limited application efficiency caused by excessive accumulation of evaporated salt on the surface of the material.
3.3 Application of super-wetted photothermal materials in oil-water separation
With the increasing global demand for energy, the consumption of crude oil has greatly increased. Nowadays, petrochemical, metal processing and other industries generally have the environmental problems of oily wastewater discharge, and even the risk of pollution such as oil leakage. However, the conventional treatment of crude oil leakage consumes a lot of energy, manpower and time, and has different degrees of restrictions on the viscosity of the treatable oil phase and the amount of leakage, so the above methods have certain limitations in application.
In general, the super-wetted materials used for oil-water separation mainly include super-hydrophobic/super-lipophilic materials and super-hydrophilic/underwater super-oleophobic materials, of which the super-hydrophilic/underwater super-oleophobic materials are mainly used for the separation of multi-aqueous phase oil-water mixtures, which often require pre-wetting before separation, so their application scope is generally limited. In addition, the super-hydrophobic/super-lipophilic material mainly separates the multi-oil-phase oil-water mixture by absorbing the oil phase, but the problem of structural blockage caused by the residual oil phase easily exists in the recycling application process of the super-hydrophobic/super-lipophilic material, so that the material is wetted when being reused, and the long-acting and stable oil-water separation effect cannot be achieved. Based on this, the superhydrophobic/superlipophilic photothermal material with photothermal properties can increase the surface temperature under illumination to reduce the viscosity of the contact oil phase, thus achieving efficient oil-water separation for the viscous oil phase.
Guo et al. used a cubic sugar with a porous structure as a template to obtain a PDMS foam skeleton through operations such as soaking, dissolving, and peeling, and used an ultrasonic process to uniformly disperse CNFs on the surface of the skeleton, and performed PDMS modification to prepare a PDMS-modified CNFs foam photothermal composite, as shown in Figure 9a
[60]. The material can absorb oil phases with different specific gravity in water, and at the same time, it can reduce the viscosity of crude oil by means of its photothermal effect, thus realizing the rapid cleaning of crude oil (Fig. 9 B, C). Ji et al. Adsorbed the prepared dual-scale black TiO
2/ZIF-8 nanoparticles on the surface of polyurethane (PU) sponge under the action of polydopamine
[61]. Superhydrophobic composite sponges were obtained after modification with octadecylamine (ODA). Under laser irradiation, the temperature of the composite sponge increased by 55. 9 ℃ within 10 s, and the separation efficiency of the composite sponge for oil phases with different viscosities remained above 93%, and the composite sponge had a relatively stable oil absorption capacity. Niu et al. deposited PDA, copper sulfide (CuS) nanoparticles and PDMS on melamine (MF) sponge to prepare PDMS/CuS/PDA @ MF composite sponge, as shown in Fig. 9d, which has superhydrophobicity and excellent photothermal conversion performance
[62]. The sponge can rapidly heat up under sunlight to effectively reduce the viscosity of crude oil, providing a solution to the problem of oil leakage (Fig. 9e, f). Zhang et al. Added Fe
3O
4 nanoparticles into the PDMS prepolymer solution, and obtained the Fe
3O
4/PDMS suspension after ultrasonic and mechanical stirring. The MF sponge after ultrasonic treatment was immersed in the above suspension, and then the superhydrophobic Fe
3O
4/PDMS@MF sponge was prepared. The contact angle was 158 °, and the temperature of the original sponge and the Fe
3O
4/PDMS@MF sponge increased to 40 and 71 ° C within 100 s under sunlight
[63]. In addition, Fe
3O
4/PDMS@MF sponge can achieve complete absorption of high viscosity castor oil under short time illumination. Zhang et al. Modified polyurethane fibers with carbon nanotubes, and then modified them with polydimethylsiloxane (PDMS) to prepare a superhydrophobic/superhydrophilic Janus nanofiber composite membrane with photothermal conversion properties, which showed higher dichloromethane permeation flux in photothermal interaction, so it could achieve rapid oil-water separation
[64].
In addition to anti-icing, seawater desalination and oil-water separation applications, super-wetting materials with photothermal effect also have broad applications in photothermal driving, droplet separation and other fields. For example, Yang et al. modified ultra-long hydroxyapatite nanowires by PDA and oleylamine, and then obtained super-hydrophobic photothermal paper by vacuum filtration. Under NIR irradiation, it can float on the water surface and produce a significant temperature gradient by means of its own photothermal effect, and achieve controllable light-driven attraction by means of Marangoni effect
[65]. Table 3 summarizes the application of super-wetting materials with photothermal effect in photothermal driving and droplet sorting in recent years.
4 Summary and Prospect
To sum up, in recent years, the research on super-wetting photothermal materials has gradually become a research hotspot, based on the conversion process from light energy to thermal energy and the surface characteristics of super-wetting materials.Super-wetting photothermal materials have shown good research value and application prospects in the fields of anti-icing, seawater desalination, oil-water separation and solar evaporation, which are of great significance for alleviating energy crisis, alleviating water pollution and protecting the environment.
The combination of super-wetting materials and photothermal materials not only conforms to the development trend of multi-function in the field of materials, but also realizes complementary advantages. However, the current research and development and construction mechanism of super-wetting photothermal materials still need to be further improved to meet the transformation direction of green chemistry and intelligent manufacturing. The details are as follows:
(1) The moisture resistance of super-wetting photothermal materials, especially super-lyophobic photothermal materials, is often achieved by the combination of appropriate micro-nano hierarchical structure and low surface energy, but at present, such products are mostly made by chemical modification with long fluorocarbon chains or perfluorinated compounds.These reagents are expensive, highly carcinogenic and teratogenic, easy to bioaccumulate, and have strong resistance to degradation, so it is advocated to use chemicals containing short fluorocarbon chains or fluorine-free chemicals to achieve the effective preparation of super-wetting photothermal materials to meet the needs of the transformation of construction technology to bio-friendly and environmental protection.
(2) Although the synergy of photothermal effect and super-wetting function can be effectively realized by endowing photothermal materials with super-wetting function or introducing materials with photothermal conversion performance into super-wetting substances, it still can not meet the application requirements of today's society for multi-functional and intelligent materials.Therefore, the preparation of environmental responsive super-wetting photothermal materials by compounding super-wetting photothermal materials with environmental stimuli such as pH, voltage, temperature and humidity has become the future research and development direction of super-soaking photothermal materials.
And (3) when that Janus type material is use in the field of seawater desalination, the super-hydrophilic bottom of the material is immersed in seawater to keep a low temperature, and the super-hydrophobic top realize rapid temperature rise by means of photothermal action, thereby effectively avoiding the problem of salt deposition by means of transpiration, and realize the application of efficient and continuous solar evaporation. When the material is applied to the field of oil-water separation, the super-hydrophobic/super-lipophilic side can be used for oil-water separation application of oil removal type, and the super-hydrophilic/underwater super-lipophilic side can be used for oil-water separation application of water removal type; By mean of that wetting asymmetry, the purpose of separate different types of oil-water mixtures can be achieved, and the problem that the application of the traditional oil-water separation material is limit is avoided; In addition, based on the photothermal conversion effect of the material, the oil-water separation efficiency can be significantly improved. Therefore, Janus-type super-wetting photothermal materials have shown outstanding research value and application prospects in seawater desalination, solar evaporation, oil-water separation and other fields, and have become the research focus and development trend of current experts and scholars.
Natural Science Foundation of Xinjiang Uygur Autonomous Region(2022D01C68)
Scientific Research Program of the Higher Education Institution of Xinjiang(XJEDU2021Y007)
Tianchi Doctoral Program of Xinjiang(TCBS202011)
PhD Start-Up Fund of Xinjiang University(BS210215)
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