1 Introduction
At present, about 2/3 of the world's population is facing the problem of water scarcity to varying degrees. The shortage of clean freshwater resources has seriously restricted the development of some countries and regions in the world, and affected the living environment and water security of human beings. Interfacial Solar Steam Generation (ISSG), which has been developed in recent years, uses green Solar energy as a heat source.ISSG is an efficient, green and low-cost method for seawater desalination and wastewater treatment, which has attracted wide attention from scientific research and industry. The core technology of ISSG is to design and construct photothermal conversion materials with strong light absorption, and a variety of photothermal conversion materials based on carbon materials, graphene, conductive polymers, noble metal nanoparticles and Ti3C2-MXene have been developed.Among them, Ti3C2-MXene is a new type of two-dimensional titanium carbide material with large specific surface area, good water dispersibility and high photothermal conversion efficiency, which shows great potential in the field of ISSG and is also a research hotspot in the field of solar water vapor conversion in recent years. However, there are few review papers on Ti3C2-MXene in the field of ISSG.
In this review, the technology of ISSG is introduced firstly, and the design principles of photothermal conversion materials in ISSG are summarized, and the specific applications of the design principles of photothermal conversion materials in ISSG are analyzed through examples. Then the MXene materials are introduced and the application of MXene in ISSG is discussed. The research progress of Ti3C2-MXene composites in the field of ISSG is reviewed, including two-dimensional MXene films, three-dimensional MXene aerogels and hydrogels, and the fabrication and properties of biomass-MXene composites. Finally, the challenges and prospects of Ti3C2-MXene are analyzed.
1.1 Brief Introduction of Water Vapor Conversion at Solar Energy Interface
Water is the most common substance on the earth, and it is also the basis of life. According to statistics, 70.8% of the earth's surface area is covered by water, but 97% of it is seawater that can not be directly used, and only less than 3% of the water is fresh water urgently needed by human beings
[1]. Both agricultural irrigation and industrial development are inseparable from freshwater resources, coupled with the uneven distribution of water resources, about two-thirds of the world's population is facing the problem of freshwater resources shortage to varying degrees
[2,3][4,5]. With the development of industrialization and urbanization, people's demand for freshwater resources is increasing. The shortage of freshwater resources seriously restricts the development of some areas, and it is also one of the important issues that have to be considered in the process of industrialization and urbanization
[6,7]. Therefore, how to effectively extract fresh water that meets the use standards from endless seawater and wastewater is one of the important ways to solve the shortage of fresh water resources
[8]. Traditional fresh water extraction methods include membrane method and thermal method. Membrane method is a method of using membrane for filtration or reverse osmosis, and thermal method is a method of heating distillation. However, these two methods inevitably require a large amount of fossil energy, high energy consumption, environmentally unfriendly, and limited actual efficiency
[9⇓⇓⇓~13]. Therefore, it is urgent to develop a green, efficient and low-cost desalination and wastewater treatment technology.
Solar water vapor conversion technology is a green water treatment technology, which uses solar energy to heat and promote water evaporation, and then condenses and collects water vapor, so as to realize seawater desalination or wastewater treatment, which can replace the traditional membrane and thermal methods with high energy consumption
[14⇓~16]. There are two traditional solar desalination methods: one is direct distillation, which uses sunlight to directly irradiate and heat seawater to promote evaporation of seawater, and then condenses and collects steam to obtain fresh water
[14]; The other is to use solar collectors or heating tubes to heat seawater to a certain temperature and then put it into flash distillation or low-temperature multi-effect distillation system to obtain fresh water
[15]. Solar water vapor conversion technology is the extension of thermal technology, which uses solar energy as a heat source, and has the advantages of low cost and green environmental protection. However, the energy density of solar energy on the earth's surface is limited. The traditional solar water vapor conversion technology is not ideal for heating the whole water body, and it often needs to add expensive high-rate concentrators to ensure efficiency
[17⇓⇓⇓⇓⇓~23]. ISSG technology developed in recent years is one of the most promising seawater desalination and wastewater treatment technologies by designing photothermal materials to confine the heat of solar energy conversion to the water-air interface, mainly heating the water at the water-air interface, reducing heat loss and greatly improving the efficiency of solar water vapor conversion
[24⇓⇓⇓⇓⇓~30].
1.2 Design idea and example analysis of material system in ISSG
The design and construction of photothermal conversion materials are the core of ISSG technology. The efficient ISSG material system should have the following five characteristics: (1) strong light absorption and photothermal conversion ability
[31⇓⇓~34]; (2) The water can be transported to the high temperature area by capillary force to ensure the supply of water; (3) able to float on water; (4) low thermal conductivity to reduce heat loss; (5) Effective release and escape of steam. Therefore, an ISSG material system generally comprises two parts: a main light absorption layer and an auxiliary function layer. The main body light absorption layer absorbs sunlight and converts it into heat, and has a high temperature, so the layer is also called a light-heat conversion layer or a high temperature layer. The auxiliary functional layer refers to the thermal insulation foam and water transport medium placed under the light absorption layer in order to improve the efficiency of solar water vapor conversion.
The core of ISSG technology is to fix the material with strong light absorption on the surface of water and form a high temperature layer, so the key of this technology lies in the design and construction of ISSG material system, including the composition and morphology of the material system. The composition of the light absorption layer directly affects the efficiency of solar water vapor conversion.At present, the commonly used photothermal conversion materials include carbon black, carbon nanotube (CNT), graphene, graphene oxide (GO), organic polymer material polypyrrole (PPy), inorganic non-metallic nanomaterials, noble metal nanoparticles with local plasmon resonance (LSPR) effect, and two-dimensional titanium nitride or titanium carbide (MXene) materials which have just emerged in recent years
[35,36][37,38][39,40][41,42][43⇓~45][46,47][48].
The morphology and structure of ISSG material system also have a direct impact on the conversion efficiency of light, heat and water vapor. For the bulk absorber layer: (1) the absorber layer may be a two-dimensional film or a three-dimensional porous material, such as aerogel and hydrogel, but the compact structure of the two-dimensional film may hinder the transport of water to the interface, resulting in a lower water vapor conversion efficiency
[45,46][39,40][49,50]; (2) Compared with two-dimensional materials, three-dimensional porous materials usually have higher water vapor conversion efficiency, because they have abundant pore structure, which enhances the multiple reflection and absorption of sunlight in the pores of the light absorption layer, and is also more conducive to the transport of water to the interface
[51]; (3) However, three-dimensional porous aerogels and hydrogels usually present irregular network structure and uneven pore size distribution, which is not conducive to water transport through capillary action.Through the vertical qualitative growth of the ice crystal, a vertical pore channel can be constructed after vacuum freeze drying, and the vertical pore channel can greatly shorten the distance of water transportation, thereby improving the transportation efficiency of water and being beneficial to the escape of steam
[49,52,53]; (4) In order to ensure that water can be effectively transported to the water-air interface, the light absorption layer should have the ability to transport water to the surface of the water body through capillary force, so the light absorption layer is generally a hydrophilic structure
[37,41,43,47,48]; (5) However, when the light absorption layer with a hydrophilic structure is used for seawater desalination, salt crystals are easily separated out from the surface of the material, so that the pore channels of the material are blocked, and light absorption and water transportation are affected; and the light absorption layer structure with a hydrophobic upper layer and a hydrophilic lower layer is constructed, so that the salt barrier capacity can be effectively improved
[53,54]. For the auxiliary functional layer: (1) a layer of hydrophobic foam material is usually introduced below the light absorption layer, on the one hand, the light absorption layer can always float on the water surface, on the other hand, the foam material has low thermal conductivity,It can effectively reduce the heat conduction from the high temperature layer to the low temperature water body. The commonly used hydrophobic thermal insulation foams are polystyrene foam (PS) and expanded polyethylene foam (EPE)
[55][56]. (2) However, the existence of hydrophobic foam will affect the transport of water, so it is necessary to introduce materials with strong water absorption capacity in the middle of hydrophobic foam materials as water transport media, such as non-woven fabrics and cotton threads, so as to promote the transport of water to photothermal materials
[51,54][57]; (3) In addition, there are a large number of bacteria in the water. During water treatment, bacteria and their metabolic biomacromolecules will inevitably lead to blockage of material pores. The introduction of antibacterial materials into the water transport bracket below the light absorption layer can effectively improve the antibacterial performance and service life of the material system
[58].
The design principles and construction methods of ISSG material system are described and analyzed from several examples below. Both reduced graphene oxide (rGO) and multi-walled carbon nanotubes (MWCNTs) have good light absorption properties. As shown in Fig. 1A, Jiangheqing et al. prepared the rGO-MWCNTs composite film as the light absorption layer, and adjusted and optimized the surface roughness of the rGO-MWCNTs light absorption layer to improve the solar radiation absorption by reducing the reflection of light
[59]. The interstices formed inside the rGO-MWCNTs light absorption layer are beneficial to the water transport caused by capillarity, and the photothermal water vapor conversion efficiency of the rGO-MWCNTs composite film can reach 80.4% without the insulating foam. Lu Yang et al. Introduced zinc oxide nanocrystals with antibacterial effect into graphene oxide (GO) aerogel scaffolds, which made the material system have the effect of inhibiting bacteria, thus improving the stability of the material system
[58].
Jiang Heqing et al. Designed an ISSG material system with sandwich structure
[60]. As shown in fig. 1b, C, a polypyrrole (PPy) film prepared by chemical vapor deposition polymerization (CVPD) is used as the upper light absorbing layer, the middle layer is a highly hydrophilic absorbent paper (to increase water transport), the bottom layer is an expanded polyethylene foam (EPE), and then the absorbent paper and water are connected by a cotton thread. This structure not only ensures the supply of water, but also reduces the heat loss and greatly improves the efficiency of water vapor conversion. With polyvinyl alcohol (PVA) and chitosan as the hydrated skeleton and PPy as the light-absorbing material, Guihua Yu et al. Prepared a hydrogel with hydration and strong light-heat conversion ability, and the water evaporation rate of the hydrogel was as high as 3.6 kg·m
-2·h
-1 under the light irradiation of one sunlight intensity (1 Sun=1 kW·m
-2)
[61].
Zhu Jia et al. Constructed a novel ISSG material using mushrooms, which are ubiquitous in nature
[62]. As shown in Fig. 1D, the mushroom has an umbrella-shaped black cap, a porous context, and a fibrous stalk. The black cap can effectively reduce the loss of heat. The porous context is conducive to the escape of water vapor, and the fibrous stalk is conducive to water transport. Therefore, the mushroom has the conditions required for ISSG. However, the light absorption performance of the mushroom itself is limited, so they carbonized it at 500 ℃ for 12 H in argon atmosphere to improve its light absorption ability. After carbonization, the surface of the mushroom is rougher, and the light absorption performance is greatly improved, thus improving the water vapor conversion efficiency.
1.3 Introduction to Ti3C2-MXene.
There are still some problems in the application of ISSG, such as carbon black, carbon nanotubes, graphene, graphene oxide, polypyrrole, inorganic non-metallic nanomaterials and noble metal nanoparticles with local plasmon resonance effect. For example, the photothermal conversion efficiency of carbon materials is not high; The preparation cost of the graphene is high, and the graphene cannot be dispersed in water
[63]; Graphene oxide (GO), a derivative of graphene, can be dispersed in water, but its light absorption ability is not as good as that of graphene due to the introduction of a large number of oxygen-containing groups
[64]. In addition, hydrophilic materials such as graphene oxide are easy to swell due to water absorption, which affects their mechanical properties and desalination ability, and usually need to be modified by chemical crosslinking to be suitable for water treatment
[65][66,67]. Although the light absorption capacity and stability can be improved by reducing GO to reduced graphene oxide (rGO), the mechanical properties and conductivity of rGO are poor, and the inherent hydrophobicity of rGO seriously hinders the transport of water to the evaporation surface
[68]. When noble metal nanoparticles with LSPR effect are used as light absorption layers, fusion will occur under long time illumination, which weakens the electron oscillation on the surface of the particles
[69]. Moreover, the absorption bandwidth of noble metal nanoparticles depends on their size and shape, and the control of nanoparticle morphology increases the complexity and cost of materials
[70,71]. Therefore, in order to build a more suitable material system for ISSG, it is urgent to develop a photothermal conversion material with better performance. Two-dimensional titanium nitride or titanium carbide (MXene) materials developed in recent years have shown great potential in the field of ISSG due to their excellent water dispersibility, photothermal conversion efficiency, salt resistance and antibacterial properties.
MXene is a two-dimensional titanium nitride or titanium carbide nanomaterial obtained by etching the A layer from the MAX phase. A MAX phase is a generic name for a class of metal carbides or nitrides, which derives its name from its general chemical formula M
n+1AX
n, where n = 1, 2, or 3, M is a transition metal (e.g., Ti, V, Cr, etc.), a is a metal element of the groups C, N, O (e.g., Al, Si, etc.), and X is an atom of C or/and N
[72]. As shown in fig. 2, although the crystal structure of the MAX phase is three-dimensional, it is formed by the mutual "adhesion" accumulation of two-dimensional layered structures
[73]. There are deformed octahedra composed of [XM
6] in the MAX phase regardless of the value of n, and these deformed octahedra extend in the same layer to form M-X layers by sharing edges, and the adjacent M in each layer is connected by metallic bonds. A lay is positioned on that upper side and the low side of the M-X layer and is connected with the M in the adjacent M-X layer by a metal bond. Therefore, the structure of MAX phase is equivalent to interspersing M metal layer and a metal layer in X layer, which makes it have excellent electrical and thermal conductivity. Due to the good conductivity and two-dimensional layered structure, MAX phase can be used as intercalation electrode material for Li
+[74]. However, the MAX phase itself has a limited ability to store Li
+, so it is necessary to etch and strip the a layer in the MAX phase to open its layered structure and prepare two-dimensional transition metal carbide or nitride MXene, which is more suitable for intercalation electrode materials of Li
+.
MXene is a new two-dimensional transition metal carbide or carbonitride nano-layered material.Since the first successful preparation of MXene by Gogotsi et al in 2011, it has attracted wide attention and has been used in lubrication materials, adsorption materials, batteries, sensors, supercapacitors, molecular separation, heterojunctions, catalysts, electromagnetic absorption and shielding and other fields
[75][76,77][78,79][80,81][82,83][84,85][86,87][88,89][90,91][92,93]. The structural formula of MXene is M
n+1X
nT
x, where M is a transition metal, X is a C or/and N atom, and T
x is a surface functional group (e.g., = O, — F, — OH, etc., n = 1, 2, or 3). Among the large family of MXene materials, the most widely studied is the titanium carbide Ti
3C
2-MXene, which is produced by etching the Al atomic layer from the MAX phase Ti
3AlC
2[94].
There are six methods to etch the Al atomic layer in the Ti
3AlC
2 to prepare Ti
3C
2-MXene: (1) hydrofluoric acid (HF) etching
[95][96]; (2) etching with LiF and HCl
[97]; (3) etch of hydrogen fluoride compounds (NaHF
2, KHF
2, and NH
4HF
2)
[98]; (4) etching with concentrated alkali (sodium hydroxide, NaOH)
[99]; (5) etching with sodium hydroxide (NaOH) and sulfuric acid (H
2SO
4)
[100]; And (6) electrochemical etch, wherein that electrolyte is a binary aqueous solution of NH
4Cl and tetramethylammonium hydroxide (TMAOH)
[101]. The Ti
3C
2-MXene prepared by HF etching has a relatively complete structure and clear lamellae, and is suitable for being used as a substrate material (as shown in figs. 3A and 3B). The layered structure of the Ti
3C
2-MXene prepared by LiF and HCl etching is relatively vague, and the surface is attached with detached Ti
3C
2-MXene chips, which is more suitable for the preparation of layered Ti
3C
2-MXene (as shown in fig. 3C, d). The Ti
3C
2-MXene layer prepared by hydrogen fluoride compound etching is distinct from the layer, and the structure is complete (as shown in Figure 3e-h).
However, the byproducts Na3AlF6, K3AlF6 and Ti3C2-MXene, which are difficult to remove, may hinder the further application of Ti3C2-MXene. The method adopts concentrated alkali, NaOH and H2SO4 or electrochemical etching, which has the advantage that fluorine-containing groups are not introduced, and the existence of fluorine-containing functional groups can reduce the adsorption and energy storage performance of the Ti3C2-MXene, so the electrochemical catalytic activity and the service life of the Ti3C2-MXene prepared by the three methods are obviously improved. Wherein the single or double molecular layer yield of the Ti3C2-MXene prepared by the electrochemical etching method is more than 90%. However, these three preparation methods also have obvious disadvantages. The concentrated alkali etching method has poor etching effect and no obvious delamination (as shown in Figure 3 I, J), and has high requirements for etching reagents, so it needs to be carried out at high temperature. However, high temperature etching will increase the possibility of Ti3C2-MXene being oxidized and deteriorated. NaOH and H2SO4 etching methods are time-consuming and labor-intensive, and the etching effect is poor (as shown in Figure 3K, l), so the application range is limited. Although the electrochemical method has obvious delamination after etching (as shown in Figure 3m, n), it also has the problems of time-consuming and labor-intensive, narrow application range and non-environmental protection.
1.4 Application of Ti3C2-MXene in the conversion of water vapor at the interface of solar energy
Because of its unique structure and performance, MXene has shown outstanding advantages and great potential in the field of ISSG applications.
First of all, the surface of Ti
3C
2-MXene has a large number of = O, — F, — OH and other groups, so different from graphene or reduced graphene oxide, the unique surface chemical properties of Ti
3C
2-MXene make it have good hydrophilicity, excellent water dispersibility and easy surface chemical modification, which makes Ti
3C
2-MXene a multi-functional two-dimensional nanomaterial with adjustable properties
[102].
Secondly, MXene has excellent photothermal conversion ability. Metal nanoparticles usually have a unique localized surface plasmon resonance (LSPR) effect, which makes them have outstanding solar absorption ability
[103,104]. This is because when the photon frequency is consistent with the natural frequency of the surface electrons of metal nanoparticles, the resonant photons of the charges at the metal-dielectric interface will induce coherent oscillations, which will promote the generation of photothermal electrons and facilitate the light absorption in the near-infrared region. The absorbed solar energy is further converted into heat, which is dissipated into the surrounding medium through vibrations scattered by the lattice, thereby raising the surrounding temperature. As one of the metal nanoparticles, Ti
3C
2-MXene also has a strong LSPR effect
[105]. Moreover, the special composition structure also makes the Ti
3C
2-MXene have excellent conductivity. According to the solution of Maxwell's equation, the higher the conductivity of the material, the higher the extinction coefficient, the better the electromagnetic wave absorption, that is, the stronger the light absorption ability, so the Ti
3C
2-MXene with excellent conductivity has outstanding light absorption ability
[106]. Li et al. 'S optical absorption and thermal measurement system based on droplets confirmed that Ti
3C
2-MXene has excellent photothermal conversion ability, and the photothermal conversion efficiency can reach almost 100%
[107]. The reason why the conversion efficiency of Ti
3C
2-MXene is so outstanding has not yet been fully revealed. One of the more authoritative explanations is that the electromagnetic interference shielding effect of MXene makes the unreflected electromagnetic waves pass through the lattice structure of MXene, reflect internally between layers, and finally be absorbed into the structure with negligible emission
[105].
The study shows that Ti
3C
2-MXene has the ability of desalination and salt barrier
[50]. In the application of ISSG, after the evaporation of water, the salt in the seawater will crystallize out on the surface of the material and block the pores, which will significantly reduce the water supply and vapor pressure, resulting in a significant reduction in the evaporation rate and damage to the material
[51]. The surface of the Ti
3C
2-MXene is provided with a large amount of groups such as -O, -F, -OH and the like, and can be subjected to hydrophobic modification to prevent salt from being separated out on the surface of the photothermal material in the desalination process, improve the salt barrier performance of the material, and prolong the stability and the service life of the material
[50].
Finally, the Ti
3C
2-MXene also has good antibacterial properties. During water treatment, there are a large number of bacteria in seawater, wastewater or lake water. These bacteria and their metabolic biomacromolecules will concentrate on the ISSG material to form a biofilm, and the proliferation of microbial cells will also accumulate on the material channel, resulting in the blockage of the channel
[57]. The growth inhibition rate of Ti
3C
2-MXene on bacteria is close to 100%, which can prevent bacteria from forming biofilm on photothermal materials
[108]. The antibacterial mechanism of Ti
3C
2-MXene is still in the research stage, and there are several possible explanations at this stage: (1) bacterial cells exposed to the sharp edge of MXene may cause cell membrane damage
[108]; (2) strong reduction activity of MXene and its active surface; (3) Smaller MXene nanosheets can penetrate into microbial cells through direct physical penetration or endocytosis; (4) MXene reacts with some molecules in the cell wall and cytoplasm of microorganisms, destroying the cell structure and causing the death of microorganisms.
Compared with graphene and reduced graphene oxide with poor water dispersibility, as well as graphene oxide and other carbon-based solar absorbers which can be dispersed in water but have reduced light absorption performance,Ti3C2-MXene is a more potential material for solar thermal conversion because of its good water dispersibility, easy surface chemical modification, high photothermal conversion efficiency, desalination, salt barrier and antibacterial properties, which makes the design and construction of efficient solar interface water vapor conversion system using MXene become a research hotspot in recent years.
2 Research Progress of Water Vapor Conversion by Ti3C2 at the Solar Interface
In ISSG, the light-heat-water vapor conversion efficiency and water evaporation rate are two important indicators to evaluate the efficiency of a material system. Table 1 summarizes the partial conditions and the photothermal water vapor conversion efficiencies of the partially Ti3C2-MXene composites at the ISSG.
At present, there are three main applications of Ti3C2-MXene in the field of ISSG: two-dimensional films, three-dimensional aerogels or hydrogels, and bio-based materials. Two-dimensional films are usually based on commercial matrix films, and the preparation process is relatively simple, but the water vapor conversion efficiency is not as good as that of three-dimensional aerogels or hydrogels due to the structural differences. Three-dimensional aerogels or hydrogels have outstanding performance in water transport and light absorption due to their porous structure, especially hydrogels can produce intermediate water with low enthalpy of vaporization, which can achieve faster water vapor conversion. However, the preparation of three-dimensional materials will inevitably consume a lot of MXene materials, and the cost is high. Bio-based or biomimetic MXene composites refer to the method of combining MXene with bio-based or deforming to imitate the structure of bio-based materials. Bio-based or biomimetic MXene composites can play a role in promoting water vapor conversion efficiency, and bio-based composites also have the advantages of environmental protection, renewable, degradable, reducing the amount of MXene and sustainable development. Therefore, the bio-based or biomimetic MXene composites can effectively reduce the amount of MXene and ensure the efficiency of water vapor conversion.
2.1 Two-dimensional MXene thin film
The preparation methods of two-dimensional thin films can be roughly divided into dip-coating drying method, filtration method and other film-forming methods
[44,45][109,110][35,46,111]. The dip-coating and drying method requires two steps. First, the substrate film, such as cellulose film, polyvinylidene fluoride film (PVDF) or dust-free paper, is immersed in the solution or dispersion of the photothermal conversion material, and then it is taken out after a period of time and dried in vacuum at a certain temperature (to avoid air damage). The dip-coating drying method usually needs to repeat the above steps several times to obtain the formed photothermal conversion film. The filtration method is typically accomplished by means of a mobile phase filter that deposits the photothermal conversion material on a substrate film by suction filtration under vacuum. In addition, there are some other methods to construct MXene thin films.
2.1.1 Dip coating and drying method
As shown in fig. 4A, Yang Mingbo et al. prepared a Ti
3C
2-MXene/ cellulose film with antibacterial properties by using a dip-coating and drying method
[112]. The preparation method is as follows: firstly, the MXene dispersion is prepared by a HCl/LiF etching method, and then the pure cellulose film is soaked in the Ti
3C
2-MXene dispersion at room temperature for 3 s, and then vacuum dried at 60 ℃ for 10 min. After repeating the above steps for several times, the Ti
3C
2-MXene/ cellulose film is formed. The film has a good light absorption rate, which can reach 94. 1% in the wavelength range of 300 ~ 1 500 nm.However, the Ti
3C
2-MXene/ cellulose film can not float on water. In order to solve this problem, as shown in Figure 4B, polystyrene (PS) foam is introduced under the film, and melamine foam (MF) is wrapped around the polystyrene foam as a water transport medium. This structure not only ensures that the material system can float on the water surface, but also reduces the heat conduction from the high temperature area to the water body, effectively improving the water vapor conversion efficiency.
In addition, reduced graphene oxide (rGO) has good photothermal conversion ability and antibacterial properties, so they used the same method to prepare rGO/cellulose film and compared the performance with Ti3C2-MXene/ cellulose film. The results show that the Ti3C2-MXene has stronger photothermal conversion ability and antibacterial properties than rGO.Therefore, it is more suitable for ISSG. Under one solar illumination intensity (1 Sun=1 kW·m-2), the water evaporation rate of Ti3C2-MXene/ cellulose film is 1.44 kg·m-2·h-1, and the photothermal water vapor conversion efficiency reaches 85.8%, while the photothermal water vapor conversion efficiency of rGO/cellulose film is only 53.7% (as shown in Figure 4 C, d). As shown in Fig. 4 e ~ H, the Ti3C2-MXene/ cellulose film has good antibacterial effect on both Gram-positive and Gram-negative bacilli, and the antibacterial efficiency can reach 99.99% and 99.98% after contacting with Escherichia coli and Staphylococcus aureus for 24 H, which is higher than 90.57% and 97.89% of rGO/cellulose film.
2.1.2 Filtration method
Wang Peng et al. Reported a Ti
3C
2-MXene film with hydrophobic surface
[107]. Ti
3C
2-MXene aqueous dispersions were prepared by HF etching, and then deposited onto PVDF films by suction filtration through a mobile phase filter. In order to make the film float on the water surface, the surface of the Ti
3C
2-MXene film was modified with polydimethylsiloxane (PDMS), which is highly transparent and has no effect on the light absorption of the Ti
3C
2-MXene. The results show that the photothermal water vapor conversion efficiency of the film is 74% under 1 Sun irradiation, while it increases to 84% after the introduction of polystyrene foam as a thermal insulation layer.
In the process of interfacial water vapor conversion, salt crystallizes continuously on the membrane surface, which will block the pores and damage the membrane, resulting in a significant decrease in water supply and vapor pressure, resulting in a significant decrease in evaporation rate. In order to prepare a Ti
3C
2-MXene photothermal conversion film with salt barrier capability, as shown in Fig. 5A, Que Wenxiu et al. prepared a delaminated d-Ti
3C
2 aqueous dispersion through HCl/LiF etching and ultrasound, then modified the d-Ti
3C
2 Hydrophobically by using Trimethoxy (1H, 1H, 2H, 2H-perfluorodecyl) silane (PFDTMS), and finally deposited on a cellulose ester filter membrane through suction filtration by a mobile phase filter to obtain a hydrophobic film which can float on the water surface by itself
[54]. This hydrophobic surface modification makes it difficult for water to be transported to the membrane surface, so after a period of seawater evaporation, the salt crystallization on the surface of the hydrophobic d-Ti
3C
2 membrane is greatly reduced compared to the hydrophilic d-Ti
3C
2 membrane (as shown in Fig. 5C ~ H), which proves that the hydrophobic modification improves the salt barrier capacity of the MXene membrane. The improvement of salt barrier ability also improved the stability of the membrane, and it was found that the hydrophobic d-Ti
3C
2 membrane could work stably for more than 200 H under high salinity conditions. In addition, as shown in Fig. 5B, in this study, polystyrene foam was introduced and embedded in a non-woven fabric as a water transport medium. Finally, the photothermal water vapor conversion efficiency of the film was 71% and the water evaporation rate was 1.31 kg·m
-2·h
-1 under 1 Sun irradiation, and the purification rate of the film for four main ions (Na
+, Mg
2+, K
+, Ca
2+) in seawater was more than 99.5%, and the purification rate for organic dyes and heavy metals was close to 100% (as shown in Figure 5 I, J).
Yang Wei et al. (As shown in Figure 6a) developed a photothermal conversion material with Ti
3C
2-MXene coated polydopamine (PDA) microsphere structure, and prepared a PDA @ MXene photothermal conversion film with good mechanical properties through vacuum filtration
[113]. PDA microspheres were obtained by oxidative self-polymerization in a mixture containing water, ethanol and ammonia at room temperature. Because of the catechol groups on the surface of PDA microspheres, there are a lot of — O, — F, — OH groups on the surface of Ti
3C
2-MXene. When they are mixed in a certain proportion for 30 min, they will form hydrogen bonds and closely cross-linked together to form the structure of Ti
3C
2-MXene coated PDA microspheres. And then that PDA @ MXene microsphere are filtered and deposite on the surface of the PVDF film through a mobile phase filter under vacuum to form the PDA @ MXene PVDF photothermal conversion film. Ti
3C
2-MXene and PDA can play a synergistic role in light absorption, and the spherical structure of PDA makes the surface of the film rough, which enhances the multiple reflection and absorption of sunlight in the light absorption layer, so the light absorption rate of PDA @ MXene PVDF film can reach about 96% in the wavelength range of 250 ~ 1500 nm. In addition, due to the hydrophilicity of Ti
3C
2-MXene and PDA and the large number of gaps between microspheres, the film can achieve rapid water transport and vapor escape. However, the PDA @ MXene PVDF photothermal conversion film cannot float on the water surface. To solve this problem, as shown in Fig. 6B, round wood chips with low thermal conductivity were introduced as the thermal insulation foam. The round wood sheet not only has the property of low thermal conductivity, but also has a vertical structure of internal pores, which enables the round wood sheet to quickly transport water to the film through capillary force. The results show that, benefiting from the synergistic effect of Ti
3C
2-MXene and PDA, the water evaporation rate of PDA @ MXene PVDF photothermal conversion film is 1.276 kg·m
-2·h
-1 under 1 Sun light intensity, and the photothermal water vapor conversion efficiency can reach 85.2% (as shown in Figure 6 C ~ e).
2.1.3 Other
Due to the limited intensity of natural sunlight, ISSG materials with two-dimensional film structure can not achieve high conversion efficiency of light, heat and water vapor (> 85%). In order to break through this limitation, as shown in Fig. 7A, Li et al. dispersed the Ti
3C
2-MXene in the polymerization network of cellulose acetate to crosslink the two, and obtained a photothermal conversion film (CAM) with a photothermal water vapor conversion efficiency of 92.1%
[114]. During the crosslinking process, the Ti
3C
2-MXene is uniformly dispersed and encapsulated in the polymeric network of cellulose acetate, which not only improves the mechanical strength of the film, but also reduces the possibility of Ti
3C
2-MXene contacting with water and oxygen. Ti
3C
2-MXene are easily oxidized when in contact with water and dissolved oxygen, affecting the stability of the material, so CAMs can form a stable and durable membrane system. After the test, the CAM sample can maintain its structural stability well whether it is immersed in 100 ℃ boiling water, acidic solution (H
3PO
4,pH=3) or alkaline solution (KOH, pH = 13) for 1 H.
In order to reduce the heat transfer from the high temperature area to the water body, as shown in Fig. 7B, polymethyl methacrylate foam (PMMA) is introduced as the thermal insulation foam, and a T-shaped water channel is constructed to ensure the water transportation. This structure system can prevent the transfer of heat from the photothermal conversion in the CAM film to the water body, thus effectively accelerating the water vapor conversion. The results show that, due to the promotion of the material system structure, the water evaporation rate of CAM reaches the 1.47 kg·m-2·h-1 under 1 Sun light intensity, and the light-heat-water vapor conversion efficiency is 92. 1%. Moreover, as shown in Figure 7 C, the purification rate of high-salinity seawater (initial concentration of Na+ is 18 000 mg·L-1) by CAM is as high as 97.7%, which can produce drinking water meeting the World Health Organization (WHO) standard.
2.2 3D MXene composites
There are three kinds of 3D materials used in ISSG. (1) Hydrogel: Hydrogel is a three-dimensional cross-linked hydrophilic polymer network polymer material, which is widely used in flexible electronics, biomedical technology and environmental sensing due to its tunable physical and chemical properties
[115,116][117,118][119,120]. In addition, the hydration of the polymer network makes the water molecules in the hydrogel exist in different States: free water, intermediate water and bound water (as shown in fig. 8)
[50,61,121⇓⇓~124]. The water molecules in free water behave the same as those in bulk water, showing high enthalpy of vaporization, while the bound water and intermediate water show different enthalpy of vaporization from bulk water due to their strong and weak interactions with the polymer network. The polar functional groups on the hydrophilic polymer chain in the hydrogel can capture water molecules through hydrogen bonds, which are stronger than the hydrogen bonds between water molecules, thus generating bound water. Water molecules around the bound water interact with fewer than four water molecules to form intermediate water. Thus, less energy is required for the intermediate water in the polymer network to break hydrogen bonds and escape from the liquid surface than for bulk water. It can be seen that the hydrophilic polymer molecular network in the hydrogel can effectively reduce the enthalpy of water evaporation and achieve faster water vapor conversion. (2) Aerogel: Aerogel is an ultra-light material with extremely high porosity (90% ~ 99%).
Due to its highly interconnected nano-porous structure, it has ultra-low thermal conductivity, refractive index, dielectric constant, sound velocity and ultra-wide adjustable density
[125,126]. The aerogel with light-heat conversion capability can be used as a light-heat conversion layer and a thermal insulation layer, and can be directly applied to the ISSG. (3) Other three-dimensional materials, such as melamine foam, which has the characteristics of interconnected three-dimensional network, high pore structure and low thermal conductivity, can promote the transport of water molecules and improve the efficiency of heat utilization
[43,51,127]. Therefore, commercial melamine and other foams can also be used as substrates to coat various photothermal materials on their surfaces so as to realize the application of ISSG
[40,127].
2.2.1 Hydrogel
Trees in nature have a vertical pore structure, as shown in Figure 9a, which enables them to transport water to high places quickly by capillary force, while also ensuring the rapid escape of steam. Inspired by this, Wu Peiyi et al. prepared a hydrogel with vertically arranged pores (TIH) by using directional freezing technology, in which polyvinyl alcohol (PVA) was used as the hydrated framework and two-dimensional Ti
3C
2-MXene nanosheets were used as light-absorbing materials
[49]. The preparation method is shown in Figure 9 B. First, the Ti
3C
2-MXene nanosheets were dispersed in the PVA solution, and then the mixture was poured into a PTFE mold, which was laid on the upper surface of a copper cylinder partially immersed in a liquid nitrogen bath. Due to the temperature gradient, the ice crystals grow vertically during the freezing process. Finally, the sample was freeze-dried and then soaked in deionized water at room temperature for 3 days to reach the equilibrium swelling state, and the light-absorbing hydrogel TIH with hydration was obtained. As shown in fig. 9c, because PVA has a large number of hydrophilic hydroxyl groups (— OH), the water molecules close to PVA will exist in the form of bound water, the water molecules far from PVA will be free water, and the rest will be intermediate water. The higher the proportion of intermediate water, the better the effect of solar water vapor conversion. Benefiting from the intermediate water with low enthalpy of vaporization, the vertically aligned pores, and the outstanding light-to-heat conversion capability of Ti
3C
2-MXene, TIH can achieve a maximum water evaporation rate of 2.71 kg·m
-2·h
-1 and a light-to-heat water-vapor conversion efficiency of 90.7% under 1 Sun irradiation. In addition, in order to verify the effect of TIH vertical pore structure on water vapor conversion and compare the photothermal conversion ability of Ti
3C
2-MXene and rGO, they prepared Ti
3C
2-MXene hydrogels with random pore distribution (n-TIH) and TIH-rGO hydrogels by random freezing. The results show that the water evaporation rate of TIHs-rGO is 2.33 kg·m
-2·h
-1,n-TIH (2.42 kg·m
-2·h
-1), which is slightly higher than that of TIHs-rGO and much lower than that of TIH hydrogel with vertically aligned pores. Moreover, TIH has good desalination and sewage treatment capabilities. As shown in Fig. 9d and e, after TIH water treatment, the ion concentration of seawater is reduced by nearly three orders of magnitude, and the strong characteristic absorption peaks of two organic pollutants, brilliant blue (BB) and rhodamine B (RhB), near the wavelengths of 630 and 554 nm, respectively, disappear.
2.2.2 Aerogel
Chen Shuo et al. (As shown in Fig. 10a) used the directional freezing technology to construct an aerogel composed of vertically arranged Ti
3C
2-MXene nanosheets
[53]. Firstly, placing the etched Ti
3C
2-MXene dispersion liquid in a polytetrafluoroethylene mold with a titanium plate as a bottom, and freezing the polytetrafluoroethylene mold in liquid nitrogen.
During the freezing process, due to the temperature gradient, ice crystals will grow from bottom to top on the titanium plate, and Ti3C2-MXene nanosheets will aggregate at the interface of ice crystals to a certain extent. VA-MXA aerogel (Vertically aligned MXene aerogel) is formed after vacuum freeze-drying to remove ice crystals.
As shown in figs. 10B to 10e, the pore channels of the prepared aerogels are of a vertical structure, and the pore diameters are basically uniformly distributed within a certain concentration range. The vertical channels facilitate the water transport, and at the same time, the dissolution of salt crystals in the channels is accelerated. However, in practical applications, after a period of desalination, salt will crystallize on the surface of aerogel, affecting the efficiency of water vapor conversion. Therefore, they further placed VA-MXA aerogel in a ring-shaped sponge mold, floated the sponge on fluoroalkyl silane under vacuum conditions, modified one side of VA-MXA aerogel Hydrophobically, and constructed Janus aerogel with hydrophobic upper layer and hydrophilic lower layer. The results showed that the salt barrier ability of Janu VA-MXA was greatly increased, which improved the life and stability of Janu VA-MXa in seawater desalination. Under 1 Sun irradiation, Janus VA-MXA can operate stably for more than 15 days, and the water evaporation rate can reach 1.46 kg·m-2·h-1, and the light-heating-water vapor conversion efficiency is about 87%.
Yang Wei et al. (As shown in Figure 11b) used ethylenediamine (EDA) as a crosslinking agent to combine two-dimensional Crosslinked MXene Aerogels nanosheets with a small amount of graphene oxide to assemble a three-dimensional porous aerogel (Crosslinked MXene Aerogels, CMA) with high conductivity, light absorption, low thermal conductivity and hydrophilicity
[128]. Another highlight of this research is the introduction of solar cell modules (SC-B) to achieve all-weather water vapor conversion (as shown in Figure 11A), which uses CMA's alternative method of photothermal and electrothermal conversion to continuously generate steam. Due to the excellent light absorption and electrical conductivity of CMA, the ISSG material system can not only convert sunlight into heat energy for water vapor conversion in sunny days, but also continuously use the electricity converted from solar energy to heat CMA for water vapor conversion in weak light or darkness, thus avoiding additional electric energy input. The ingenious introduction of SC-B into the CMA system can not only realize all-weather water vapor conversion by using solar power generation, but also improve the efficiency of water vapor conversion due to the synergistic effect, because: (1) the input electric energy increases the surface temperature of the sample; (2) Solar illumination reduces the humidity of the system above the sample. The synergistic effect promotes the conversion of water vapor and the escape of vapor in CMA, which greatly improves the efficiency of water vapor conversion. Under the illumination of 0.5 Sun and the supply voltage of 2.5 V, the water evaporation rate of CMA is 1.624 kg·m
-2·h
-1, which is much higher than that of pure CMA (1.337 kg·m
-2·h
-1) under the illumination of 1 Sun. In addition, under the combination of 0.5 Sun light intensity irradiation and 5 V supply voltage, CMA has a stable evaporation rate of more than 20 cycles (1 H/cycle), and the purification rate of four major ions (Na
+, Mg
2+, K
+, Ca
2+) in seawater is more than 99% (as shown in Figure 11 C ~ H).
Xianbao Wang et al. Reported a three-dimensional porous aerogel (GMA) assembled by GO-assisted Ti
3C
2-MXene, which showed very high photothermal conversion capacity and mechanical properties, and could achieve efficient purification of wastewater contaminated by inorganic ions, organic dyes, bacteria and oils
[129]. The preparation method is shown in figs. 12 a-d. First, a certain amount of Ti
3C
2-MXene, GO nanosheets, and sodium alginate (SA) are mixed, and the mixture is freeze-cast using a directional freezing technology. During the freezing process, nanosheets and SA molecules will self-assemble into ice crystal boundary walls through π-π conjugation and hydrogen bonding between polar functional groups. Then, in order to improve the mechanical properties of the material, cationic crosslinking was used, and the frozen product was soaked in Ca
2+ aqueous solution to produce ionic crosslinking with alginate chains. In water environment, graphene (G block) in different polymer chains can produce ionic crosslinking with multivalent cations, forming an "Egg-box" molecular structure and forming a gelatinous network in water. Finally, the ice crystals are removed by freeze-drying to obtain GMA. GO and polymer SA chains can be used as a combination "bridge" to make the Ti
3C
2-MXene nanosheets more easily form a three-dimensional structure, which, together with appropriate chemical crosslinking, greatly improves the stability of the material. Whether GMA is immersed in high temperature (> 60 ℃), acid and alkali environment or ultrasonic treatment, it can maintain the mechanical properties of its own structure well, so it can adapt to various environments in practical application. In addition, the GO/Ti
3C
2-MXene composite system can achieve synergistic self-enhanced photothermal conversion, because Ti
3C
2-MXene has certain reducibility, which can reduce GO to rGO, and rGO has better light absorption and photothermal conversion ability than GO. Benefiting from the synergistic effect of the GO/Ti
3C
2-MXene composite system, the GMA water evaporation rate was 1.27 kg·m
-2·h
-1 and the photothermal water vapor conversion efficiency reached 90.7% under 1 Sun irradiation. And as shown in Figure 13a-j, GMA can operate stably for more than 30 cycles (30 min/cycle).Five ions in seawater (Na
+, Mg
2+, K
+,Ca
2+, B
3+) and seven heavy metal ions (Mn
6+,Cd
2+ 、Pb
2+ 、Cu
2+ 、Ni
2+ 、The purification rate of Zn
2+ and U Fe
2+ is as high as 99.9%. At the same time, GMA also showed very high purification efficiency and stability for emulsion, E. coli aqueous solution or lake water.
2.2.3 Other
In order to prove that three-dimensional porous materials are more suitable for water vapor transfer than two-dimensional films, Que Wenxiu et al. Prepared a three-dimensional MXene ISSG material (3D CMF@d-Ti
3C
2) with carbonized melamine foam as the matrix and delaminated d-Ti
3C
2 as the light-absorbing material
[51]. The preparation method is shown in Figure 14 a. First, the melamine foam (MF) is carbonized at 800 ℃ in argon atmosphere for 1 H to obtain the carbonized melamine foam (CMF). Then the delaminated d-Ti
3C
2 dispersion liquid prepared by the HCl/LiF etching method is dropped on the surface of the CMF, and finally the 3D CMF@d-Ti
3C
2 is obtained by vacuum drying at 60 deg C.
The carbonized melamine foam and the delaminated d-Ti
3C
2 can synergistically enhance the light absorption, which is beneficial to the water vapor conversion. In addition, as shown in fig. 14b, in order to reduce the heat conduction from the high temperature area to the water body, polystyrene foam is introduced below the 3D CMF@d-Ti
3C
2, and non-woven fabric is used as a water transport medium. The results show that the 3D CMF@d-Ti
3C
2 water evaporation rate reaches the 1.60 kg·m
-2·h
-1, and the light-heating-water vapor conversion efficiency is 84. 6% under 1 Sun irradiation. However, the water evaporation rate of the two-dimensional MXene film composed of the delaminated d-Ti
3C
2 dispersion deposited on the mixed cellulose ester organic filter membrane by suction filtration through the mobile phase filter is only 1.41 kg·m
-2·h
-1, and the photothermal water vapor conversion efficiency is less than 70% (as shown in Fig. 14 C, d). Although MF has the characteristics of interconnected three-dimensional network, high pore structure and low thermal conductivity, when the light-absorbing material is coated on the MF, the shedding of the light-absorbing material inevitably occurs. In order to solve this problem, Yang Wei and others developed an ISSG material (3DMAs) with a stable three-dimensional MXene structure
[130]. The preparation method is shown in fig. 15A and fig. 15b. First, a MF matrix frame with a specific shape and size is immersed in a PVA solution, and then the MF/PVA composite foam is obtained after vacuum drying. Then, the composite foam is immersed in a Ti
3C
2-MXene dispersion, and then the 3DMAs is obtained after vacuum drying at 70 ° C overnight. The main role of PVA is to improve the adhesion between the Ti
3C
2-MXene nanosheets and the melamine foam matrix frame. Without the role of PVA, the Ti
3C
2-MXene can not stably adhere to the melamine foam. However, in the presence of PVA, there was no obvious Ti
3C
2-MXene shedding even after sonication of 3DMAs in water for 5 min. In addition, as shown in fig. 15C, in order to improve the water vapor conversion efficiency, expanded polyethylene foam (EPE) is embedded in the bottom of 3DMAs, which has the performance of both heat barrier and water transport. The results show that 3DMAs exhibit a very high optical absorption efficiency of about 98% in the wavelength range of 300 ~ 1 500 nm, which makes the water evaporation rate of 3DMAs reach 1.309 kg·m
-2·h
-1 without EPE under 1 Sun irradiation, and the light-heating-water vapor conversion efficiency is 82. 4%. With the addition of EPE, the water evaporation rate of 3DMAs rises to the 1.409 kg·m
-2·h
-1, and the light-heating-water vapor conversion efficiency is 88. 7%. As shown in Fig. 15 d ~ G, the purification rate of 4major ions (Na
+, Mg
2+, K
+, Ca
2+) in seawater by 3DMAs is more than 99.7%, and the purification rate of methyl blue and methyl orange solution is close to 100%.
2.3 Bio-based or biomimetic MXene composites
Due to the pressure of environmental pollution and the demand of sustainable development, it is becoming a major trend to replace non-degradable fossil-based materials with green, renewable and degradable bio-based materials. However, even though there are many organisms in nature that can use their special biological structure to promote light absorption or water transport efficiency, not all bio-based materials can be directly used in ISSG, such as transpiration of leaves and multi-layer hierarchical structure of black scales of West African Gabonese viper. However, the solar water vapor conversion efficiency of ISSG materials can be improved by imitating the special structure of bio-based materials. Therefore, in recent years, bio-based or biomimetic MXene composites have also become a research hotspot in this field. Although biobased or biomimetic MXene composites are also shown in the form of two-dimensional films or three-dimensional composites in photothermal conversion applications, biobased or biomimetic MXene composites are discussed as a separate chapter in this paper due to their great potential. Bio-matrix-MXene composites for photothermal conversion utilize the special structure of bio-matrix materials to combine bio-matrix materials with materials with better photothermal conversion performance, which can improve the efficiency of photothermal conversion or water transport, such as PPy, noble metal nanoparticles or GO coated on round wood sheets with vertically arranged pores
[131⇓⇓~134]. Inspired by the unique structure and properties of bio-based materials, MXene composites with biomimetic structure also show excellent performance in the field of ISSG. There are many bio-based materials in nature that can enhance their own light absorption or water transport efficiency, and the solar water vapor conversion efficiency can be effectively improved by imitating the structure of bio-based materials.
2.3.1 Round wood chip bio-based composite material
Round wood chips are processed from logs and have the same vertically aligned pores as trees, so they are often used as the matrix framework for three-dimensional ISSG materials. However, when the light absorbing material is coated on the round wood pieces, the shedding of the light absorbing material inevitably occurs, and even if a light absorbing material having an extremely high light-to-heat conversion efficiency is used, the energy loss is still large. Therefore, as shown in Figure 16A, Sun Junliang et al. hydroxylated Ti
3C
2-MXene into Ti
3C
2-OH and introduced small molecular binder isocyanate to successfully prepare a three-dimensional bio-based ISSG material Ti
3C
2-Wood with high heat and water vapor conversion efficiency and high stability
[135]. Hydroxylated Ti
3C
2-MXene can not only improve the ultraviolet/visible light absorption ability of the material (the effective light absorption rate of Ti
3C
2-OH for ultraviolet/visible spectrum can reach 95%), but also help to enhance the interaction between Ti
3C
2-MXene and wood. In addition, under the irradiation of ultraviolet light, the introduction of small molecular binder isocyanate can initiate the condensation reaction, so that covalent bonds are formed between wood and Ti
3C
2-MXene to firmly crosslink together. To test the in-use stability of the Ti
3C
2-Wood, the Ti
3C
2-Wood was immersed in Pacific water at 50.d egree. C. for 8 weeks. As a result, as shown in fig. 16b, C, the storage modulus (E ') of the Ti
3C
2-Wood was still higher than that of ordinary wood after a long time of soaking, which proved that the stability of the Ti
3C
2-Wood was greatly improved after the crosslinking treatment. Benefiting from the synergistic effect of the strong light-to-heat conversion ability of Ti
3C
2-MXene, the hierarchical porous structure of wood, and the low thermal conductivity (only 0.082 W·m
-1·K
-1 in the wetted state), the water evaporation rate of Ti
3C
2-Wood reached 1.465 kg·m
-2·h
-1 under 1 Sun light intensity, and the light-to-heat water vapor conversion efficiency was as high as 96%.
2.3.2 Bionic composite material
Chen et al. simulated the multi-layer hierarchical structure of the black scales of the Gabon viper in West Africa, as shown in Fig. 17a-e, and built a bionic MXene nano-coated ISSG material
[136]. The preparation process is as follows: firstly, carrying out oxygen plasma pretreatment (increasing the hydrophilic interaction and the hydrogen bond between the substrate film and the two-dimensional photothermal material) on the low-heat-conductivity substrate PS film, and dropwise coating reagent Ti
3C
2-MXene dispersion liquid on the PS film. Then, the first thermal deformation (denoted as G1) was performed by continuous thermal driving to deform the planar nano-coating into an isotropic curved structure, simulating the micro-crown structure of the black scales of the viper. The thermal deformation can be performed multiple times to obtain the biomimetic nanocoatings G2 and G3 with higher light absorption efficiency. As shown in Fig. 17f, due to the strong scattering and multiple reflection of incident light in the biomimetic MXene nano-coating, the light absorption and photothermal conversion efficiency of the material are greatly improved, so that the broadband light absorption rate of the biomimetic MXene nano-coating in the wavelength range of 300 ~ 2500 nm is as high as 93.2%. In addition, the PS layer becomes thick due to thermal contraction, and G1 can be directly used as a support substrate and a thermal insulator. In order to improve the water transport efficiency, two small water channels (diameter ≈ 1.5 mm) were drilled in the Ti
3C
2-MXene/PS bilayer material with a distance of about 1.5 cm from each other, as shown in Fig. 17 G. The GO dispersion was then injected into the waterway with a syringe, and after air drying at room temperature, a GO coating was formed on the cylindrical surface of the waterway. The results show that the water evaporation rate of the biomimetic MXene nano-coating is 1.33 kg·m
-2·h
-1 under 1 Sun light intensity, and the light-heat-water vapor conversion efficiency can reach 86. 7%.
The incident angle of sunlight is constantly changing, which is not conducive to the stability of the illumination area of the light absorption layer and the photothermal conversion efficiency, and affects the water vapor conversion efficiency. To this end, as shown in Fig. 18a, Yang Xiaofei et al., inspired by the fact that trees transport water to high places through their roots and produce water vapor through the transpiration of their leaves, prepared a bionic ISSG material that is almost unaffected by the incident angle of solar rays
[137]. The structure of the material system is a three-dimensional spherical structure, the outer layer is a light absorption layer constructed by taking a Co
3O
4/Ti
3C
2-MXene composite material as a photothermal material, and the inner part is introduced with spherical PS foam to ensure the floating structure and reduce the energy loss caused by heat conduction from a high-temperature layer to water.
In addition, in order to ensure water transportation, the spherical PS foam is coated with cotton, and the cotton tail is immersed in the water body by imitating the structure of tree roots. Ti3C2-MXene are easy to deteriorate when they contact with water and dissolved oxygen, which affects the stability of the material. However, there are abundant active sites on the surface, which can be used as a good platform for in-situ growth of metal oxide nanoparticles. As shown in fig. 18b, the Co2+ is introduced under the protection of nitrogen, and the Co3O4/Ti3C2-MXene composite is obtained by hydrothermal treatment. The combination of the Co3O4 and the Ti3C2-MXene can effectively prevent Ti in the Ti3C2-MXene from contacting with water and oxygen, thereby avoiding the decomposition of the Ti3C2-MXene. Moreover, the heterostructure combined with nanoparticles and two-dimensional nanosheets can promote the optical perturbation of incident light and synergistically enhance the absorption of sunlight. At the same time, the metal oxide/MXene composite material also has good photocatalytic activity for the degradation of organic pollutants. Therefore, compared with the pure Ti3C2-MXene,Co3O4/Ti3C2-MXene composite material, the chemical stability, the photo-thermal conversion and the photocatalytic degradation performance are all improved. Then the Co3O4/Ti3C2-MXene composite material is sprayed on the non-woven fabric by a spraying method, and the Co3O4/Ti3C2-MXene light absorption layer can be obtained after vacuum drying.
Compared with the two-dimensional photothermal structure, the three-dimensional spherical structure can obtain energy from the environment and increase the evaporation area, which is outstanding in the conversion of solar water vapor. This is because when the sun shines, the temperature of the evaporation surface receiving the light is higher than the temperature of the surrounding environment, which will cause energy loss. But at the same time, due to the cooling effect of evaporation, the temperature of the surface not exposed to light is lower than that of the surrounding environment. The temperature gradient generated by the evaporative cooling effect enables the evaporating surface to acquire energy from the environment and compensate for the loss of thermal radiation and thermal conversion energy in the high temperature region, thereby improving the water vapor conversion efficiency. As shown in Fig. 18c, d, under the irradiation of 1 Sun, the water evaporation rate of the three-dimensional spherical material system is as high as 1.89 kg·m-2·h-1, and the photothermal water vapor conversion efficiency is 130.4%, while the photothermal water vapor conversion efficiency of the two-dimensional photothermal structure is only 75.1%. More importantly, the water vapor conversion efficiency of the material system is almost independent of the incident angle of sunlight, and the water vapor conversion efficiency is basically unchanged at different incident angles of sunlight. As shown in Fig. 18e ~ H, the three-dimensional spherical material system can work stably for more than 10 hours, and shows very high purification efficiency for artificial seawater with different salinities, heavy metal ions, methyl orange, methyl blue and other pollutants.
3 Development prospects and challenges
With the development of industrialization and urbanization, the shortage of freshwater resources is becoming increasingly serious, which has become one of the important factors limiting regional development. As a national strategic project, the South-to-North Water Diversion Project is the largest water conservancy project in China and the world so far, with a total investment of more than 500 billion yuan, benefiting 438 million people. Its main purpose is to solve the problem of lack of freshwater resources in northern China. Many countries and regions in the world are facing the problem of water scarcity in varying degrees. Interfacial Solar Steam Generation (ISSG) technology provides a sustainable, efficient and low-cost method for seawater desalination and sewage treatment. As a new 2D nanomaterial, Ti3C2-MXene has attracted much attention in recent years due to its outstanding photothermal conversion effect, which has great potential in the field of ISSG, and many inspiring results have been achieved. However, Ti3C2-MXenestill face some challenges in the field of ISSG.
(1)Ti
3C
2-MXene 2D films usually have limited mechanical properties, which greatly limits the application of Ti
3C
2-MXene 2D films, such as the deformation of biomimetic structures
[54,104,109]. At present, the two-dimensional film of the Ti
3C
2-MXene is usually a composite film prepared on a substrate film (a cellulose film or a PVDF film and other commercial films) by a dip-coating drying method or a filtration method, and the mechanical performance of the Ti
3C
2-MXene composite film is limited due to the limited mechanical performance of the commercial substrate film. There are abundant chemical groups (— O, — F, — OH and so on) on the surface of Ti
3C
2-MXene, which can form Ti
3C
2-MXene composite membranes with high structural stability and good mechanical properties through the physical or chemical covalent bond interaction between Ti
3C
2-MXene and other substrates
[138,139].
The (2)Ti
3C
2-Mxene has poor stability and is easy to be oxidized
[140,141]. Water is a polar solution and can dissolve large amounts of oxygen. After etching to prepare the Ti
3C
2-Mxene dispersion, in the presence of water and dissolved oxygen, oxygen will react with the Ti
3C
2-Mxene as an oxidant to consume Ti and C to produce CO
2 and anatase (TiO
2). The reaction speed is related to the ambient temperature and the size of the Ti
3C
2-MXene itself. The higher the ambient temperature is, the faster the reaction speed is, and the smaller the size is, the more unstable it is. This indicates that the aqueous solution is not a good medium for storing the Ti
3C
2-Mxene. In contrast, powdered Ti
3C
2-Mxene and their composites are stable at room temperature and under air or inert gas conditions, and chemical reactions occur only at high temperatures. Therefore, in the process of preparing the Ti
3C
2-MXene composite material, inert gas (such as Ar gas) can be injected to remove the dissolved oxygen in water or the temperature can be reduced to slow down the oxidation speed of the Ti
3C
2-MXene and improve the stability of the material.
(3) During the preparation of ISSG materials, due to the inherent high surface energy of Ti
3C
2-MXene and the Van der Waals force effect of the interface, the etching delamination of Ti
3C
2-MXene nanosheets will inevitably occur irreversible re-stacking
[142,143]. When the delaminated Ti
3C
2-MXene are restacked, the specific surface area of the material is reduced, so that the light absorption performance of the material is limited, and water transportation is not facilitated. Constructing MXene materials with three-dimensional macrostructure is one of the feasible solutions to this problem. For example, Wang Xianbao used GO to assist Ti
3C
2-MXene to assemble three-dimensional porous aerogels. GO not only gels Ti
3C
2-MXene, but also isolates delaminated Ti
3C
2-MXene to prevent their re-accumulation
[129]. However, like Ti
3C
2-MXene, GOs are relatively expensive.
When the (4)Ti
3C
2-MXene meets water, it will expand in water, and the interlayer spacing of the Ti
3C
2-MXene will inevitably increase after expansion, which greatly weakens its ability to block ion penetration and salt barrier, and damages the deionization performance of the material
[144,145]. One of the effective ways to solve this problem is to control the interlayer spacing of Ti
3C
2-MXene by modifying the ISSG material.
The intermediate water content of (5)Ti
3C
2-MXene hydrogel needs to be improved, and how to effectively improve the intermediate water content of Ti
3C
2-MXene hydrogel is also a challenge
[49,50,61]. At present, hydrogels are the most effective way to achieve water vapor conversion because the hydrophilic polymer molecular network in hydrogels can be hydrated to form intermediate water with low enthalpy of vaporization, which can achieve faster water vapor conversion. The content of water in the hydrogel can be highly regulated by functional additives, cross-linking density, polymer or monomer functional groups, among which hydrophilic functional groups such as hydroxyl (— OH), amino (—NH
2), carboxylic acid (— COOH) and sulfonic acid (—SO
3H) show outstanding ability to capture water molecules.
(6) Although there are many methods to prepare Ti
3C
2-MXene by etching, at present, Ti
3C
2-MXene are mainly produced in laboratory scale, and mass production has not been achieved. In addition, the preparation cost of Ti
3C
2-MXene is high. These limitations undoubtedly hinder the commercialization of ISSG materials prepared by Ti
3C
2-MXene
[146,147]. On the premise of ensuring the efficiency of water vapor conversion, it is one of the important ways to reduce the cost of materials by combining with other matrix materials to prepare Ti
3C
2-MXene composites and reducing the amount of Ti
3C
2-MXene. Among them, the construction of high-performance bio-based Ti
3C
2-MXene composites can not only reduce the dosage and cost of Ti
3C
2-MXene, but also endow Ti
3C
2-MXene composites with the advantages of sustainable development and biodegradability.
4 Conclusion and prospect
As a new method of seawater desalination and sewage treatment, solar interface water vapor conversion technology uses widely distributed and clean solar energy as energy source, and limits the heat converted from solar energy to the water interface through the photothermal conversion of light absorbing materials, which greatly reduces the scattering of heat and significantly improves the efficiency of photothermal water vapor conversion. ISSG technology is a low-cost, efficient and clean method for seawater desalination and wastewater treatment, which is expected to alleviate the pressure of freshwater shortage, so it has attracted wide attention from scientific research and industry in recent years. Among them, the design and construction of photothermal material system is the core and key of ISSG technology, and two-dimensional films or three-dimensional composites based on metal nanoparticles, graphene oxide, conductive polymers and Ti3C2-MXene have been developed. Compared with other photothermal conversion materials, Ti3C2-MXene has a two-dimensional nanosheet structure similar to graphene, and has excellent physical and chemical properties such as high photothermal conversion efficiency, good conductivity and easy surface modification, which has a high application prospect in the field of ISSG. A variety of MXene composites have been developed and applied to ISSG, including two-dimensional MXene composite membranes, three-dimensional MXene aerogels and hydrogels, bio-based MXene composites and biomimetic MXene composites.Ti3C2-MXene composites are one of the materials with the highest photothermal water vapor conversion efficiency reported so far. However, the application of Ti3C2-MXene materials in the field of ISSG still has a very large development space, such as using the rich surface chemical properties of Ti3C2-MXene materials for chemical modification to solve the problems of poor stability, intermediate water content to be further improved and easy agglomeration, or combining with bio-based materials to cope with the high cost constraints, and ultimately realize the commercialization of Ti3C2-MXene materials in the field of ISSG.
Natural Science Foundation of Guangdong Province(1914050005542)
Department of Science and Technology of Guangdong Province(191900014)
Program for Chang Jiang Scholars and Innovative Research Teams in Universities(IRT_17R40)
Science and Technology Program of Guangzhou(2019050001)
Guangdong Provincial Key Laboratory of Optical Information Materials and Technology(2017B030301007)
111 Project and Yunnan expert workstation and ScienceK Ltd.
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