1. Introduction

Currently, there are severe water shortages in many parts of the world. The existing form of water resources is mainly seawater. Photothermal seawater desalination is an efficient and clean way to solve water shortage [1-4]. At present, the main methods of desalination are reverse osmosis (RO) [5,6], electrodialysis [7,8], multistage flashing system [9], and membrane distillation (MD) [10,11], all of which are limited by high energy consumption. Photothermal seawater desalination is a new way of energy-saving and environmental protection, which has great potential in seawater desalination [12-15]. The key of photothermal seawater desalination is the development and utilization of advanced photothermal materials. At present, the main photothermal materials are transition metal dichalcogenides (TMDs) materials, carbon materials, and MXenes materials. Among all photothermal materials, TMDs has attracted wide attention, mainly due to its specific relatively large surface area and the outstanding performance [16-20]. Due to its excellent photothermal conversion properties, TMDs materials can effectively convert the absorbed light energy into heat energy, and produce local high temperature on the surface of the material, so that the water molecules can be transformed from liquid to gaseous state, and the water evaporation is realized [21,22]. However, some dissolved salt ions are not easy to evaporate with water vapor due to their higher boiling point and slow diffusion rate. Therefore, through the photothermal conversion effect of TMDs materials, the extraction and desalination of water in seawater can be realized [23-27]. Moreover, TMDs are mainly composed of transition metal elements (such as Mo and W) and chalcogenides (such as S and Se). These elements are relatively abundant in nature, so obtaining and refining them is relatively easy and inexpensive. Besides, a variety of methods can be used for the preparation of TMDs materials, such as solid-phase reaction method, mechanical exfoliation method, lithium-ion intercalation method, chemical vapor deposition method and hydrothermal method. These methods are relatively simple and do not require complex equipment and high-precision operation, thus reducing the cost of the preparation process [28-30]. TMDs have become a promising photothermal material for seawater desalination. At present, the application of TMDs in photothermal seawater desalination has achieved a lot of progress [31-33]. On the one hand, there are many structure regulation methods of TMDs for solar water generation has been explored, such as stacking regulation, phase engineering, and heterostructures engineering. On the other hand, some TMDs based composites in the form of gels, sols, and some three-dimensional network bulk materials also have been developed to be used as photothermal materials for solar water generation. For example, Lu et al. [34] designed a water molecular channel to change the enthalpy of evaporation of water, which exhibited an evaporation rate of 2.50 kg m⁻² h⁻¹ with an energy efficiency of 89.6% at 1 sun, making TMDs has an excellent evaporation performance. For a single material, the desalination performance can be improved mainly by controlling its morphology, stacking structure, surface properties and light absorption properties. Others mainly combine TMDs with some materials to form gels, sols, and some three-dimensional network bulk materials to improve their water transport performance and further improve their purification performance. In addition, large-scale solar thermal desalination still needs to be further explored, and there is still a lot of potential for TMDs-based photothermal materials for solar water generation [35-38]. In this review, we summarize various strategies to improve the photothermal performance of TMDs, including regulating the morphology, surface state structure and band structure of TMDs, and compounding TMDs with gels and sponges (Fig. 1). We also provide comprehensive suggestions for the development of TMDs in seawater desalination, and make a full prospect, which is conducive to promoting the research of TMDs.

2. Structure regulation of TMDs for solar water generation

Materials used for photothermal desalination need to have good light absorption efficiency, photothermal conversion performance, and water transport performance. The structure of the material determines its properties, so it is necessary to regulate the structure to improve the TMDs properties, thereby improving the performance of TMDs. Structure regulation mainly includes morphology regulation to form water channel, crystal phase regulation to create TSSs, and heterostructures regulation.

2.1. Stacking regulation to create nanoconfined water-molecule channels

To improve the performance of a single TMDs nanosheet, it is necessary to regulate the morphology. Traditional MoS₂ is prone to out-of-order stacking of sheets, which will affect its performance. The stacking of one-dimensional MoS₂ can effectively generate water channels, which is conducive to improving evaporation efficiency.

Efficient solar water purification technology presents a promising solution for addressing the scarcity of potable water. In a recent study, Lu et al. have developed nano bound water molecular channels (NCWMCs) as a mean to achieve efficient solar vapor generation (SVG) even under low light conditions [34,39,40]. 1D-OMoSNSA is synthesized via a one-pot solvent-thermal method using mercaptan (M) and thiourea as precursors, with oleoamine serving as a surf convenient approach. The transmission electron microscopy (TEM) images depict the characteristic morphology of the 1D-OMoSNSA (Figs. 2a and b). The layered structure exhibits a cylindrical shape measuring approximately 140 nm in diameter and several microns in length. It comprises ultra-thin nanosheets of O-MoS_2−x with a gap of around 3.5 nm. Scanning electron microscopy (SEM that the thickness of the one-dimensional OMoSNSA layer is roughly allowing for the formation of gaps during stacking (Figs. 2c and d). As measured by UV–vis-NIR spectroscopy (Fig. 2e), 1D-OMOSSA-M has low reflectivity and almost zero transmittance in the 500–2500 nm wavelength range, having excellent light absorption performance in the whole solar spectral range. Fig. 2f shows SVG measurement setup based on 1D-OMOSSA-M. The composite film was cut into pieces, water was continuously transported from the container to 1D-OMOSSA-M with cotton, and expandable polyethylene foam was used as insulation material. The water evaporation performance and photothermal conversion performance of the device are excellent (Figs. 2g and h). MD simulation of 1D-OMOSSA-M structure based on NCWMC shows that the reason for the fast evaporation rate (Fig. 2i). Different accelerations are set to simulate the environment of water evaporation. As shown in Fig. 2j, A large number of water clusters can be observed in the gas phase of NCWMC system, which reduces the enthalpy of evaporation and is conducive to efficient evaporation [41,42]. On the other hand, the gas phase of the BW system contains a limited quantity of water clusters. Ultimately, an enumeration is conducted to determine the amount of evaporated water molecules MC and BW systems. The number of water molecules evaporated in NCWMC system was significantly higher than that in BW system. NCWMC can promote the formation of water clusters, reduce the enthalpy of evaporation, and make water vapor evaporation more effective.

2.2. Phase engineering to create topological surface states

In addition to changing the enthalpy of evaporation of intermediate water, the desalination efficiency can also be improved by changing the photothermal conversion efficiency of TMDs materials. Moreover, the absorption of the solar spectrum by traditional photothermal materials is insufficient, which will greatly affect the evaporation performance of TMD. Topological surface states (TSSs) provide locations for electrons to enhance absorbance and thus the photothermal effect.

The use of two-dimensional photothermal materials for solar photothermal water evaporation is considered to be a green and sustainable solution to the shortage of water resources. The light absorption efficiency cannot be ignored, and topological surface states (TSSs) can effectively improve the light absorption rate. Recently, Du et al. fabricated 2D WTe₂ on mixed cellulose esters (MCE) as a material for photothermal devices and suggested an approach to enhance the photothermal performance through the utilization of topological surface states (TSSs) [43-47]. WTe₂ is a two-dimensional Weyl semi-metal with special properties, including high light absorption, low thermal diffusion coefficient, high specific heat capacity and large carrier density. Fig. 3a illustrates the presence of a nanosheet structure in the sample, while the orange box highlights an area indicating a crystal spacing of 0.625 nm between planes. This corresponds to the orthogonal structure of WTe₂ (010) plane depicted in Fig. 3b. The presence of nanosheet structures in, with a highlighted region (orange box) indicating a crystal spacing the planes. This observation corresponds to the orthogonal structure of the WTe₂ depicted in Fig. 3c. Combined with the Raman spectra of WTe₂ (Fig. 3d), it can be confirmed that the prepared WTe₂ is Weyl semi-metallic material. Fig. 3e presents the constructed theoretical model of a two-dimensional structure to elucidate the mechanism. The WTe₂ material showcases a distorted lattice structure in the 1T phase known as Td-WTe₂. In the Td-phase, the W atoms deviate from their central octahedral positions, forming zigzag W-W chains that extend along the a-axis, specifically in the Γ-X direction within the Brillouin zone.

Additionally, the unique electronic band structure of WTe₂ exhibits both electron and hole pockets, resulting in its semi-metallic nature. As shown in Fig. 3f, these electron and hole pockets cross the Fermi level, contributing to the semi-metallic nature of WTe₂. This property leads to a significant carrier density in WTe₂ (Fig. 3h), which can be estimated from the slope ρ of the curve. In order to investigate the water evaporation properties of WTe₂, comparative experiments were conducted under identical conditions with 1.5 AM irradiation (100 mW/cm²). Figs. 3i and j illustrate that WTe₂@MCE exhibits excellent water evaporation capabilities. Furthermore, the evaporation rate significantly increases as the light intensity rises, as shown in Fig. 3k. However, Spin-orbit coupling (SOC) caused by W 5d and Te 5p orbits has a significant effect on the band structure which reduces photothermal conversion (Fig. 3g). The opening of the gap between electron and hole pockets in WTe₂ and the appearance of node lines caused by SOC indicate the presence of TSSs in WTe₂ [48-52]. Unoccupied TSSs can provide sites for high density free carriers, enhance the absorption spectrum, playing a leading role. Fig. 3l gives a schematic of water evaporation caused by solar energy and photothermal conversion mechanism. In the process of solar evaporation, WTe₂@MCE devices have the capability to capture photons and convert them into electrons. Subsequently, these photoexcited electrons undergo decay and interact with the lattice for efficient photothermal conversion under illumination. The TSSs generate electrons from valence to conduction bands. The photoexcited electrons then spontaneously decay from the excited state to the minimum conduction band to maintain an energy steady state. In this process, light-excited electrons convert radiant energy into thermal energy by emitting phonons to dissipate their excess energy [53-56]. Subsequently, the photoexcited electrons convert the absorbed energy into heat, which is recombined with the holes in the valence band to the ground state by non-radiative mode. In short, the excited state electrons of the two-dimensional Weyl semi-metallic WTe₂ with TSSs return to the ground state through multi-pathway phonon emission to achieve efficient photothermal. By creating TSSs through phase engineering, the band structure of TMDs can be controlled, which promotes the full absorption and utilization of light energy in photothermal conversion.

2.3. Advanced 2D-2D heterostructures engineering to enhance light absorption

In order to improve the light absorption efficiency of TMDs, heterojunctions engineering should also be considered as strategies for structural regulation of TMDs. The bandgaps of typical TMDs are between 1 eV and 2.5 eV and they appear in short-wave solar light absorption, so their light absorption and photothermal conversion efficiency are limited. Due to the exceptional ability of heterostructures to absorb electromagnetic waves and their localized surface plasma effect, they exhibit remarkable efficiency in converting light into heat.

Traditional TMDs materials have a band gap of ~1–2.5 eV, which limits their light absorption and photothermal conversion efficiency. The heterojunction formed by doping can increase the photothermal conversion efficiency [57-59]. Recently, the 2D-2D heterostructure of TMDs-NR-TMNs was investigated by Wang et al. through a partial transformation of crystals. TMDs-NR-TMNs nanosheets with excellent spectral absorption capacity have been prepared with this method, such as MoS₂-Mo₅N₆, WS₂-W₂N₃ and NbS₂-Nb₄N₅. The fabrication of the 2D-2D TMDs-NR-TMNs heterostructures is shown in Fig. 4a. Mechanical exfoliation was used to prepare the NH₂-TMD nanosheets containing MoS₂, WS₂ and NbS₂. The amino groups were successfully grafted onto the exfoliated precursor nanosheets following foliation process. Upon heating, NH₃ gas undergoes decomposition and releases N, H, or H₂. These hydrogen atoms can react with sulfur atoms located at the edge or defect region of the amino functionalized TMDs to generate H₂S. Simultaneously, the liberated nitrogen atom reacts with certain molybdenum atoms to form a heterostructure of TMDs-NR-TMNs. The absorption capacity of heterostructures composed of NH₂-TMD nanosheets (MoS₂, WS₂ and NbS₂) in infrared spectroscopy was studied. The results showed that compared with the intrinsic MoS₂ (50 mg), WS₂ (50 mg) and NbS₂ (50 mg) nanosheets, the spectral absorption capacity of the same number of heterostructures (MoS₂-Mo₅N₆, WoS₂-W₂N₃ and NbS₂-Nb₄N₅) was increased by 93%, 93% and 91%, respectively (Figs. 4b–d). Since about 50% of the solar radiant energy is distributed in the infrared region, the strong solar energy absorption capacity guarantees its application prospect in solar steam evaporation. The photothermal behavior of MoS₂/MF and MoS₂-Mo₅N₆/MF solar devices under a single solar irradiation (1 kW/m²) for 10 min was recorded by an infrared camera [60-63]. As depicted in Fig. 4e, upon reaching an irradiation duration of 2 min, the surface temperature exhibits a significant increase to approximately 84 ℃ and 103 ℃ consecutively. Furthermore, these elevated temperatures are sustained over an extended period (Fig. 4f). Compared to MoS₂/MF devices, the heating time required for MoS₂-Mo₅N₆/MF photothermal devices to reach 80 ℃ is approximately 30 s (0.5 min), which is around 72 s faster than that of MoS₂/M devices (102 s/1.7 min) (Fig. 4g). This indicates that MoS₂/MF devices have stronger solar absorption and photothermal conversion capabilities. Fig. 4h shows the change in water mass over time at 1 kW/m² solar irradiation. The results show that the mass change of MoS₂-Mo₅N₆/MF/water increases linearly with the extension of irradiation time. After 1 h of exposure to solar radiation, the MoS₂-Mo₅N₆/MF/water system exhibited a mass change rate of 2.31 kg m⁻² h⁻¹. This water, MF/MF/water systems with increases of 5.2 times, 4.7 times, and 1.4 times respectively. In addition, the MoS₂-Mo₅N₆/MF/water solar device shows high stability after 20 cycles of solar irradiation. Adjusting the solar power density from 0.5 kW/m² to 2 kW/m² resulted in an increase in the evaporation rate of MoS₂-Mo₅N₆/MF/water from 1.2 kg m⁻² h⁻¹ to 4.6 kg/m² enhancement in the evaporation efficiency from 46.4% to 186.4%. In the evaporation process, melamine foam (MF) is used to transport water and at the same time to insulate which causes the surface heat localization effect. During the reaction process, the device can continuously purify sea water and produce fresh water (Fig. 4i). Within 60 min, water vapor condenses on the sides of the cup to produce a large amount of fresh water. Through the resistance measurement, it is proved that the purification performance of the device is good (Fig. 4j). With Advanced 2D-2D heterostructures engineering, TMDs-NR-TMNs materials have broadband spectral absorption and low heat loss, which allows for better evaporation rates and efficiency.

3. TMDs-based composite building for solar water generation

The performance of a single material is limited, and building its composite can further create a new structure and improve the photothermal performance. Gels and sponges are the main existing modes of TMDs-based composites. In the process of photothermal seawater desalination, the photothermal conversion performance and stability of photothermal materials would degrade because of salt deposition. By combining TMDs with gels and sponges, salt deposition in photothermal desalination can be effectively relieved.

3.1. TMDs-based hydrogel

Due to its uncomplicated braided composition and restricted water passage within the TMDs film, both the mechanical and water supply capabilities of film fail to meet expectations [64,65]. By combining TMDs with different materials, different gels can be formed for seawater desalination. In addition, the abundant pore structure of the gel composite is helpful to improve the salt tolerance of TMDs materials, which is conducive to improving the stability of the materials.

3.1.1. Scalable MoS₂-based hydrogel

The aperture of conventional TMDs materials is difficult to be adjusted, which makes its water transport function limited. A Scalable MoS₂-based hydrogel can control its water transfer performance. In addition, due to its excellent pore structure, the salt deposition on the surface of the material to a certain extent can be solved [66-69].

The purification of seawater with an interfaced photo-vapor process was proposed to be cost-effectively and sustainably. However, salt deposition can seriously affect evaporation performance. Photothermal devices utilizing hydrogel exhibit exceptional porosity, ensuring a continuous supply of water to the device's upper surface while demonstrating excellent resistance to salt. Recently, Liu et al. developed a crosslinked foam polymerization technique to produce a highly stable and salt-resistant hydrogel called SMoS₂-PH, which incorporates scalable MoS₂-based multi-pores [30]. Firstly, wet milling reduced the size distribution of commercially available powdered molybdenum disulfide. Two types of cross-linking comonomers, acrylamide (AM) and N, N-methylenebisacrylamide (MBA), respectively was then introduced to the MoS₂ dispersions. Porous processes were developed in SDS-treated systems to make the cross-linked polymer networks porous (Fig. 5a). The SMoS₂-PH nanoparticles were freeze-dried into an extremely porous lightweight aerogel after drying with liquid nitrogen, resulting in pores with diverse sizes that facilitate water migration from SMoS₂-PH towards its exterior while enhancing light absorbency. High resolution SEM images show that MoS₂ forms flat nanosheets within the hydrogel matrix which are homogeneously coated (Fig. 5b). The water absorption characteristics of SMoS₂-PH were then investigated; it took approximately 40 ms for drops to be entirely imbibed into the film. To study how functional group effects influenced photo hydrophobic properties a comparative infrared spectrum for BH and SMoS₂-PH were carried out showing clear Polyacrylamide characteristic peaks in the major BH band seen in Fig. 5c. Amino acids are polar groups easily interacting via hydrogen bonding towards the absorbed H₂O present on SMoS₂-PH surface thus giving good hydrophile character. Subsequently, absorption ratio of UV/vis-NIR spectra of SMoS₂-PH and BH was investigated (Fig. 5d), and it was found that the performance of SMoS₂-PH was excellent. Moreover, the SMoS₂-PH film could efficiently evaporate water (1 sun irradiation) (Fig. 5e) showing a high photothermal conversion efficiency. To investigate the salt tolerance of SMoS₂-PH, individual crystals of 1.5-SMoS₂-PH were exposed in an aqueous solution containing 3.5% weight sodium chloride as the mimicking seawater conditions followed by depositing of 0.5 g NaCl powder onto SMoS₂-PH surfaces. Under 1-sun illumination, NaCl powder gradually dissolved and completely disappeared after 90 min in 3.5 wt% salt water (Fig. 5f). The rapid dissolution of NaCl powder on the SMoS₂-PH surface may be related to the uniform distribution of SMoS₂-PH intermediary pores and micropores. Due to the trapping effect of mesopores, salt ions do not accumulate on the surface of SMoS₂-PH. In this paper, the effect of SMoS₂-PH thickness on the photothermal properties was investigated. It was observed that the photothermal efficiency exhibited an increase with increasing thickness. However, it reached a maximum value at 1.5 cm (Fig. 5g). With the increase in thickness, the refractive index of light within the pores increases, thereby enhancing the light absorption efficiency. The saturation point of light absorption is reached at a thickness of 1.5 cm. The principle of water transport is shown in Fig. 5h. Each meniscus establishes a hydrogen bond with the amide group in SMoS₂-PH, facilitating the ad liquid water to the pore surface of SMoS₂-PH. Continuously, water is introduced onto the surface of SMoS₂-PH, causing the surface to gradually migrate downwards due to concentration gradients, thereby impeding salt deposition. Therefore, SMoS₂-PH not only has excellent photothermal conversion performance, but also has good salt resistance, which is difficult to realize by single photothermal materials.

3.1.2. N doped graphene aerogels decorated by MoS₂

In desalination, the hydrophilicity of the material is taken into account. The regulation of surface hydrophilicity is achieved through the introduction of nanoflower-like MoS₂ decoration and nitrogen doping, which can promote moisture transport within the material and prevent salt deposition. In addition, the gel void contains a large amount of intermediate water. It has a low enthalpy of evaporation, which is conducive to increasing the evaporation rate of water [70,71].

Binding MoS₂ to gels facilitates porous structures and promotes water transport. Recently, Lan et al. have created graphene-based multi-layered porous ceramic foams interleaved with molybdenum disulfide nanoparticles decorated with gold that absorb over a broad-spectrum beam of radiation, tackling the issue of inadequate sunlight throughout the majority of the day. Fig. 6a shows the production process of MNGA. The morphology of loose and porous graphene sheets can be observed in scanning tunneling microscopy. MoS₂ nanosheets condense into flowers and adhere to graphene sheets (Fig. 6b). In the investigation of the hydrophilic properties of MNGA and MoS₂ nitrogen free doped GA (NGA), it can be found that the penetration time of water droplets on the MNGA surface is very short. This can be attributed to the doping effect of nitrogen, which inhibits the absorption of hydrocarbons in the air by the gel, allowing nitrogen to be doped with GA [72]. In addition, the doping of MoS₂ nanoflowers improves the roughness and hydrophilicity of the gel surface, making MNGA extremely hydrophilic. The light absorption properties of MNGA and NGA in ultraviolet-relative near-infrared (NIR) are shown (Fig. 6c). The doping of nanoflowers formed by MoS₂ clusters makes the light absorption ability of MNGA enhanced, which is significantly higher than that of NGA. This is because nanoflowers structure of MoS₂ effectively extends the light scattering distance in the aerogel, which reduces the light reflection of MNGA and improves the light absorption efficiency. In the evaluation of the water evaporation performance of MNGA, it can be found that the water evaporation of MNGA further reaches 1.483 kg/m² under the 1 sun light, which is 82% higher than that of NGA and 6 times higher than that of blank water (Fig. 6d). There are many pores in MNGA, some open and some closed. After a long period of evaporation, the continuous formation of new bubbles can be observed, which exist on the surface of the MNGA (Fig. 6e). Due to the hydrophilicity of the MNGA surface, the vapor-liquid interface exists in the open pores, which can expand the evaporation area [73]. The open and closed pores contain a large amount of intermediate water (freezable water), whose enthalpy of evaporation is lower than that of pure water. The lower evaporation enthalpy makes the heat required to evaporate water in the MNGA void lower, which results in the higher evaporation rate of MNGA. Fig. 6f shows the water evaporation of MNGA under different light intensity [74]. With the increase of light intensity, the evaporation rate is continuously increased, and the evaporation efficiency is always kept above 80%. The performance changes after different cycles are shown in Fig. 6g. The evaporation rate of the MNGA evaporator is maintained within 1.45–1.62 kg m⁻² h⁻¹, and the evaporation rate hardly decreases. The energy conversion efficiency has remained extremely high, fluctuating in the range of 94.6%–98.4%. This shows that MNGA is extremely stable and always maintains a high photothermal conversion performance [75-79]. This indicates that MNGA has a strong salt tolerance, which is because strong hydrophilicity of MNGA makes seawater automatically transfer upward during evaporation, which is conducive to the diffusion of ions and prevents salt deposition.

3.2. Sponges decorated by TMDs

General composite materials are difficult to prevent a large amount of salt deposition. While sponge has a very good pore structure, TMDs combined with sponge has very good durability, where salt is difficult to accumulate. The sponges combined with TMDs mainly include the loofah of the biological sponge, and the melamine sponge [80-82].

3.2.1. Three-dimensional bio-based sponges modified with TMDs

The loofah sponge has an excellent spatial structure, and the aperture structure is amazing. The ion density of large aperture channels and small aperture channels can greatly promote salt diffusion and prevent salt deposition.

By using TMDs composite materials, water evaporation performance can be greatly improved and seawater desalination rate can be increased. Recently, Wu et al. developed a three-dimensional, highly salt-tolerant solar evaporator for efficient and long-term seawater desalination by attaching molybdenum disulfide sheets and glucose derived carbon particles to a porous sponge [83]. A mature loofah sponge has a rich pore structure at the edges (Fig. 7a). Once the center is removed, the remaining sponge gourd can be opened into a flexible porous plate [84-86]. There are many gaps between the loofah fibers, making it easier for water to move (Fig. 7b). The SEM images of the cross section of the filament fibers showed that there were many tiny voids in the luffa fibers, with a diameter of 10–30 µm (Figs. 7c and d). The presence of a substantial void between the fibers and the porous micro-channels within them facilitates the natural movement of water from the lower region surface where evaporation occurs, thanks to cap forces. Additionally, it aids in maintaining a continuous flow of water and salt within the cylindrical evaporator, preventing any build-up of salt [87]. The loofah was immersed in an aqueous solution containing Na₂MoO₄·2H₂O, CS(NH₂)₂ and glucose, and then hydrothermal synthesis was performed at 180 ℃. Then, MoS₂ nanosheets and carbon particles can be attached to the surface of loofah (Fig. 7e). After undergoing thermal procedure, the MoS₂ sheets are naturally attached to the surface of the hydrothermal loofah (HL) fiber (Fig. 7f). Furthermore, as a result of glucose dehydration and dehydrogenation during this process, amorphous carbon particles are generated, leading to the formation of nodular structures in hydrothermally decorated loofah sponge with MoS₂ sheets and carbon particles (HLMC) composites (Fig. 7g). The impact of solar light absorption efficiency on the performance of solar steam power generation cannot be ignored. The UV–vision-near-infrared absorption spectra of loofah, HL, HLM, and HLMC indicate that HLMC has excellent performance (Fig. 7h). The unfolded HLMC board is rolled up to form a porous cylinder, which can construct a 3D water evaporation structure (Fig. 7i). And the volume of the middle part is tight and the surrounding area is loose, making the middle of the model have strong water absorption ability and better photothermal performance. This will concentrate water in the hot area, which will help with water evaporation. The water transfer rate at different positions of HLMC can be observed by observing the discoloration of pH test paper (Fig. 7j). The evaporation rate at the middle position of the evaporator is significantly higher than that around. In order to explore the effect of the internal structure of the evaporator on the evaporation rate, an HLMC evaporator with uniform structure (HLMC-U) and the same height was prepared for the generation of solar steam. Obviously, the evaporation rate of HLMC is higher than that of HLMC-U (Fig. 7k). In practical applications, the exposure height of the evaporator will be considered [88-90]. Fig. 7j explores the relationship between the water evaporation performance of HLMC-X evaporator and the exposure height (x, cm), and the results show that the evaporation rate of HLMC increases linearly with the exposure height within a certain range. To further explore the surface temperature change caused by the exposure height, the top surface temperature and side wall temperature of the HLMC-x evaporator during the sun-driven evaporation process were recorded by an infrared camera (Figs. 7l and m). Due to the cooling reaction of water vapor, the surface temperature of HLMC-x may be lower than the ambient temperature, which is conducive to the evaporator absorbing heat from the environment. In addition, the purification performance of HLMC is excellent (Fig. 7n). As illustrated in Fig. 7o, the evaporator's uneven structure gives rise to a non-uniform distribution of water transfer, resulting in a gradient effect. With insufficient water supply, salt particles form on the side surface of the HLMC evaporator, creating a high-salt region along the interface between its central and edge regions [91]. Moreover, due to the rapid evaporation of water on the surface, the upper layer exhibits a higher salt concentration compared to the surrounding bulk water. This disparity in salt concentration between different regions triggers the diffusion and convection of salt, causing dynamic movement. Furthermore, the microchannels of the fibers contain a significantly higher concentration of salt compared to the macropores of the sponge due to limited water availability [92-94]. Consequently, a gradient of salt concentration emerges between the macropores and microchannels, facilitating efficient exchanges of brine and preventing salt deposition on the evaporator's surface. Therefore, the HLMC evaporator benefits from an effective circulation of water and salt, effectively alleviating salt buildup.

3.2.2. Melamine foam wrapped by MoS₂-sodium alginate hydrogel

In addition to the pore structure of the sponge itself, the water supply channels on the surface of the sponge and the lack of direct contact between the sponge material and the sea water can also prevent salt deposition [95-98].

Solar steam conversion (STG) is considered a promising strategy for alleviating freshwater shortages. However, salt deposition in STG has become a crucial problem that restricts the efficient and continuous water production of evaporators. Recently, a low cost and high efficiency continuous salt resistant MoS₂/SA@MF solar steam generator has been developed [99,100]. The flow diagram illustrates the sequential steps involved in the production of MoS₂/SA@MF hybrid which includes impregnation, drying, and cross-linking processes (Fig. 8a). Efficient STG evaporators require light absorption, which is facilitated structure of MoS₂ and the pore channel MF. The UV–vis-NIR absorption spectrum indicates that the MoS₂/SA@MF hybrid sponge has exceptional and comprehensive sunlight absorption capabilities, a rate as high as 92.3% (Fig. 8b). Fig. 8c is a schematic of a solar vapor test facility in an open surface system. In practical applications, the size of the sponge will affect the evaporation of water, which further affects the performance of the evaporator. The weight loss distribution of an open surface system and a MoS₂/SA@MF evaporator of different sizes was evaluated in a 3.5% NaCl solution of 1 kW/m² (Fig. 8d). The results show that with the increase of evaporator size, the evaporation rate of evaporator may decrease due to the decrease of upper water transport capacity. In addition, Fig. 8e shows that the photothermal evaporation performance of MoS₂/SA@MF is excellent. In addition, evaporation rates at different light intensities (under 0.7, 2, and 4 sun) were measured and the corresponding solar heat conversion efficiency was calculated (Fig. 8f). The results show that the energy efficiency is improved due to more local heating and faster water evaporation under concentrated solar irradiation. Furthermore, the levels of the four primary ions (Na⁺, Mg²⁺, Ca²⁺ and K⁺) were discovered to be significantly lower than the safe thresholds mandated by the World Health Organization (WHO), indicating exceptional purification efficacy (Fig. 8g). In order to investigate the resistance of MoS₂/SA@MF STG evaporator against salt, we conducted measurements on its evaporation rate under varying levels of saline salinity in one sun conditions. For S1, after 48 h, a salt shell gradually precipitates from the edge to the interior. Because the edge is farthest from the channel. During evaporation, the marginal region first reaches supersaturation, resulting in crystallization [101-104]. Finally, a continuous salt crystal layer is formed, which almost completely covers the entire evaporator except for a small circular area in the center, and the solid salt is difficult to gather (Fig. 8h). Due to the drilled through holes on S2, the salt crystallizes grain by grain from edge to center, making it easier to collect, rather than a continuous salt layer. Compared with S1, evaporation rate only decreased by 16.2% after 100 h due to discontinuous salt precipitation [105]. As mentioned above, for S3 and S3*, due to the presence of multiple water channels, the water transport and ion convection process are accelerated, so that after 100 h, the upper surface of the sample is still in an unsaturated salt solution without crystallization. It should be noted that after 72 h, the salt crystals on the edges of the PS foam and the sides of S3 began to creep, which is similar to the phenomenon of S2. S3* is isolated from water and is unlikely to have no salt crystals creeping onto PS foam after 100 h and no salt precipitation on the side of S3*. In this work, TMDs were combined with MF sponges to construct MoS₂/SA@MF. By isolating the evaporator from the brine to prevent the flow of Marangoni, the salt deposition can be effectively prevented and the service life of the evaporator can be increased.

4. Conclusion and outlooks

TMDs have a significant effect in the application of solar thermal seawater desalination. This review summarized the latest progress of TMDs in solar desalination which is classified by regulatory means, including the creation of nano-water molecular channels, topological surface states, transverse heterojunctions, and recombination with gels and sponges. This review also describes the mechanism of different regulatory methods to improve the desalination efficiency, which provides a better understanding for the selection of different mechanisms of desalination. Solar desalination systems with practical application value should consider water transportation capacity, solar spectrum absorption capacity, water evaporation efficiency, salt tolerance and other aspects. Herein, we proposed the application and prospect of TMDs in solar thermal desalination, which would provide some insights for the research of solar thermal materials.

At present, there are still many shortcomings in the application of TMDs in seawater desalination. The stability of TMDs gradually decline due to its own oxidation in the process of photothermal evaporation, which will limit the application of TMDs photothermal materials in industrial scale seawater desalination. The application of TMDs for photothermal seawater desalination is still in its infancy and we anticipate that our research will provide novel insights for enhancing the performance of TMDs in solar water generation.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence that work reported in this paper.

CRediT authorship contribution statement

Chen Gu: Writing – original draft. Huacao Ji: Investigation. Keyu Xu: Investigation. Jianmei Chen: Investigation. Kang Chen: Investigation. Junan Pan: Investigation. Ning Sun: Investigation. Longlu Wang: Writing – review & editing.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (No. 51902101) and Natural Science Foundation of Jiangsu Province (No. BK20201381).