Recent Advances in Carbon-Based Interfacial Photothermal Converters for Seawater Desalination: A Review

Abstract

Along with the rapid development of society, freshwater shortages have become a global concern. Although existing desalination technologies have alleviated this pressure to some extent, their long-term environmental impact and energy consumption are still questionable. Therefore, it is necessary to find a new effective way for seawater desalination with cleaner energy. Solar-driven interfacial water evaporation technology has the advantages of environmental protection, energy saving, high evaporation efficiency, low cost, and strong sustainability, and is considered one of the most effective technologies to relieve water resource stress. This review summarized the recent advances in carbon-based interfacial photothermal converters focused on the preparation methods of 2D and 3D photothermal absorbers, the potential ways to enhance the efficiency of photothermal conversion. Finally, this paper proposed the challenges and future trends of interfacial photothermal converters.

1. Introduction

With the continuous growth of population and the acceleration of urbanization, the shortage of freshwater resources is becoming increasingly serious. Traditional desalination technologies, such as thermal technology and membrane technology, have problems such as high energy consumption and environmental pollution; therefore, it is urgent to find a new type of clean, environmentally friendly, and highly sustainable desalination technology. As a new desalination technology, solar-driven interfacial evaporation technology is mainly driven by solar energy, offering the advantages of clean environmental protection, high conversion efficiency, strong sustainability, and low cost. In addition, it is considered one of the most effective alternatives to traditional desalination technology. Different from the traditional solar–thermal technology of heating a large amount of water, the solar-driven interfacial evaporation technology relies on the photothermal material to achieve heat localization, and reduces the diffusion of heat to the water bottom, thus effectively improving the photothermal conversion efficiency and solar energy utilization rate.

As a new desalination technology with ecological and environmental significance, solar-driven interfacial evaporation technology has great research significance in alleviating water resource stress and ecological environment protection, so it has been widely studied and applied. High conversion efficiency and evaporation rate are significant features of solar-driven interfacial evaporation. In addition to improving light absorption, the accurate coordination of the relationship between insulation, water transport, and salt resistance can also effectively improve the photothermal conversion efficiency, which requires a reasonable design of the evaporator from the preparation method. At present, the different aspects of interfacial evaporation systems have been summarized in detail in many literatures, but the preparation method of light absorber in evaporation system has not been reported. For example, He et al. [1] systematically described the concept of new materials recently used for solar-driven interface evaporation and their principle of photothermal conversion. In addition, they focus on ways to improve water evaporation rates and thermal efficiency, as well as designs to prevent salt scaling. To expand the use of seawater desalination, studies have focused on how to achieve efficient freshwater collection. Li et al. [2] reviewed various forms and structures of biomass-based materials used in seawater desalination and discussed various ways to improve evaporation performance, including light absorption regulation, thermal management, water transport optimization, salt tolerance design, and effective steam condensate recovery. However, they generalize only in terms of bio-based materials and the various structures based on them. Because of the low density of solar energy and lack of sunlight on rainy days, it is very important to provide effective methods and insights to overcome these problems. Therefore, Li et al. [3] systematically summarized various desalination technologies and focused on interfacial evaporation systems based on different photothermal materials and strategies for improving photothermal conversion performance. In addition, they conducted round-the-clock research on the evaporation system under conditions of low solar energy intensity and no light, such as cloudy days and nights. However, these review articles only focused on one or more incomplete aspects, such as how to improve the light absorption of photothermal materials, optimize structural design to achieve efficient thermal management or water transport, etc. Few studies introduced the types and selection of photothermal materials, the conversion mechanisms, the preparation of light absorbers, and the improvement of photothermal conversion efficiency through the structural design of the evaporator in a systematical and detailed manner. In addition to the photothermal conversion efficiency, the water evaporation rate was also used as a common evaluation method in the evaluation of the photothermal performance of the evaporator. However, the cost of raw materials and the complexity of the preparation process were ignored by these two evaluation methods. Therefore, in this review, we summarized and discussed the latest progress in interfacial photothermal converters for seawater desalination, focusing on the conversion mechanism of photothermal materials and the preparation methods of light absorbers. In addition, the methods to improve the photothermal conversion efficiency through the material and structure of the evaporator were discussed, taking into account the cost of the material and the difficulty of the preparation process. Finally, the future development trends of photothermal converters were explored.

2. Development of Solar-Driven Water Evaporation Technologies

Solar-driven water evaporation is a process in which light radiation is applied to water and converted into heat, turning liquid water into gaseous water. The efficiency of photothermal conversion depends on the absorption and conversion ability of materials to sunlight and the utilization of heat converted by water. Compared with the traditional method of heating water directly, the introduction of photothermal materials has greatly improved the utilization of heat, which allows the heat to be more concentrated, enabling it to quickly drive water evaporation. According to the position relationship between photothermal materials and water, the evaporation system undergoes three stages of development, which can be mainly divided into the following three types [4].

2.1. Evaporation System Based on Bottom Heating

Firstly, the photothermal material is sunk to the bottom of the water, where the surface absorbs the solar energy and generates heat, as shown in Figure 1a. However, in this case, heat and steam are generated at different locations, and the separation of heat and steam will lead to serious heat loss and a drop in temperature, which will reduce the photothermal conversion efficiency. In addition, sunlight also undergoes surface water refraction before being absorbed by the bottom photothermal material, which greatly reduces the effective use of light, and the photothermal conversion efficiency is generally less than 45% [5].

2.2. Evaporation System Based on Bulk Heating

As shown in Figure 1b, the nanofluids of photothermal materials are directly dispersed over the entire water body. When the solar rays radiate the nanoparticles, the surface temperature of the nanoparticles rises rapidly and exceeds the boiling point of liquid water, and then a thin layer of steam is formed at the particle–liquid interface [6]. The bubbles then burst, releasing steam from the surface, and the nanoparticles eventually return to the water. Evaporation-system-based bulk heating, where steam is generated by heating nanofluids, generally relies on high optical density systems to achieve large amounts of liquid heating, which increases the complexity and cost of system preparation. Moreover, when water is in direct contact with nanoparticles, heat rapidly diffuses from nanoparticles to water, resulting in low evaporation efficiency of the system. In addition, the refraction and reflection of light on the water surface both lose a certain amount of energy, while the stable dispersion of nanofluids is still a problem. Furthermore, the need for secondary recovery of these photothermal material nanofluids, which cause environmental pollution, limits their application [7].

2.3. Evaporation System Based on Interfacial Heating

The photothermal conversion system based on interfacial heating can achieve a high evaporation efficiency of more than 90% [8]. In the interfacial system, solar energy collection, photothermal conversion and steam generation are located at the water–air interface, avoiding heating the entire bulk water, which causes most of the heat to be concentrated at the interface, thus improving the utilization of heat energy, as shown in Figure 1c. Therefore, it has become one of the most widely used solar-driven seawater desalination technology. The heat confined to the surface of the absorber accelerates the evaporation of water on the surface of the absorber and reduces the diffusion of heat radiation from the surface of the absorber to the water, as well as the heat exchange with the surrounding environment, thus improving the evaporation efficiency.

The high conversion efficiency and evaporation rate of interfacial evaporation systems are mainly due to the excellent structure design of the interfacial evaporator, which consists of a light absorber, a heat insulation layer, and water transport channel. These three structures can be separated from each other or integrated and have different roles. The light absorber plays a decisive role, which is mainly responsible for converting incident light into heat and evaporating water. The insulator reduces the diffusion of heat from the absorber to the bottom. The water transport layer transports water to the light absorber, ensuring a steady evaporation rate of water in the evaporation system over a long period of time, so it can be designed in various aspects to improve photothermal performance.

3. Photothermal Materials and Their Conversion Mechanisms

In recent years, extensive research has been conducted on photothermal materials, including metal-bases materials [9], semiconductor materials [10], carbon-based materials [11], and organic molecular materials [12]. Light is an electromagnetic radiation that carries energy. When light illuminates the surface of a photothermal material, part or all of the heat energy is generated by the interaction between the light and the material. According to the electronic structure and band structure of materials, we divide the conversion mechanisms into three categories: the plasmon resonance effect of metals, the non-radiative relaxation of semiconductors, and the thermal vibration of molecules [13,14].

3.1. Metal-Based Materials

At present, many metal materials, such as Au, Ag, Al, Cu, Pd, Ge, Au, Ag, Al, Cu, Co, and their alloys have been widely used in photothermal conversion due to their surface plasmon resonance effects [15,16,17,18,19,20]. A surface plasmon resonance effect is formed by the interaction between photons and free electrons on a metal surface [1]. As shown in Figure 2a, when light is irradiated on metal materials, free electrons on the metal surface will oscillate with the incident light close to its natural frequency, resulting in surface plasmon resonance absorption and forming a strong local electromagnetic field around the particles, namely a localized surface plasmon resonance (LSPR) effect [21,22,23]. The frequency resonance triggers the collective excitation of electrons to become hot electrons, whose energy is redistributed through the decay of electron–electron scattering, so that the local surface of the metal rapidly heats up and then cools, and the vibration energy scattered by the lattice dissipates heat to the surrounding medium, thus increasing the local temperature of the system [24]. However, the narrow resonance band and low energy utilization of traditional metal nanoparticles limit their applications, which can be adjusted by controlling their size and dimension to adjust the absorption wavelength range, but this increases the complexity of the operation and the preparation cost. In order to solve these problems, these plasma metal materials are usually compounded by mixing them with some substrate support materials or depositing them on the surface of the substrate, including nanofiber films, wood, aerogel, and hydrogel [25,26,27,28].

3.2. Semiconductor Materials

A semiconductor material is regarded as a new type of photothermal material because of its low cost and low toxicity [29]. Semiconductor materials can be divided into two categories according to different principles of light absorption. One category is defective structural semiconductors, which mainly include copper chalcogenide and some transition metal oxides whose photothermal conversion mechanism is similar to the LSPR effect on the surface of metal nanoparticles. However, the plasmon resonance effect is dependent on the surface carrier concentration, which is basically independent of the morphology of nanoparticles, and the light stability is strong. The other category is intrinsic semiconductors with intrinsic absorption band gaps, mainly including transition metals, nitrides, carbides, etc., whose optical absorption performance mainly depends on their intrinsic absorption band gaps. As shown in Figure 2b, there is a forbidden band between the valence band and the conduction band in the semiconductor, and the forbidden bandwidths vary with different semiconductor materials. When the incident light irradiates the semiconductor material, the electrons in the valence band jump from the valence band to the conduction band after being photoexcited, resulting in electron–hole pairs with band-gap energy similar to that of the excited electrons, which release energy when they return to the ground state from the excited state. When the energy releases a large amount of heat energy in the form of phonon non-radiative relaxation, they cause the local heating of the lattice [13]. The most important factor affecting the absorption performance of semiconductor materials is band gap energy. Traditional semiconductor materials have poor absorption capacity due to their large band gaps. Thus, higher light absorption can be achieved by adjusting the band gap and the microstructure of the semiconductor material.

3.3. Organic Polymer Materials

Some organic polymer materials have a large number of π bonds and convert absorbed light energy into heat energy through lattice vibrations [15]. Most of the chemical bonds, such as C-C, C-H, O-H, and C-O, have large energy gaps between σ and σ* and cannot achieve the transition from σ to σ* under light irradiation. The electron bonding strength of the π bond is weaker than that of the σ bond, and the energy required to excite electrons from the π orbital to the π* orbital is lower. In addition, the conjugated π bond can also cause a redshift in the absorption spectrum, and the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) decreases as the number of π bonds increases [30]. When light irradiates onto the organic polymer or carbon-based materials, the electrons in the highest occupied molecular orbital (HOMO) π bond absorb energy and become excited to the lowest unoccupied molecular orbital (LUMO)π*. The excited electrons relax through electron–phonon coupling, then the absorbed light energy wis transferred from the excited electrons to the entire atomic lattice through vibration, thereby increasing the temperature of the material (Figure 2c).

3.4. Carbon-Based Materials

Carbon-based materials have attracted much attention because of their rich variety, lower cost, full spectrum of light absorption, and high efficiency of photothermal conversion. Their photothermal effect is mainly realized through the thermal motion of molecules. Carbon-based materials applied in photothermal conversion mainly include low-dimensional carbon-based materials and composite carbon-based materials such as carbon black [31], carbon nanotubes [32], graphene [33], etc. Carbon black is mainly prepared by carbonizing natural materials or synthetic precursors, such as coal, petroleum products or biomass, and has excellent light absorption capacity. Graphene, which has high thermal conductivity and excellent light absorption ability, is usually prepared by chemical vapor deposition (CVD), mechanical exfoliation or redox methods. Carbon nanotubes (CNTs) are usually prepared by chemical vapor deposition or arc discharge. They have high strength and high conductivity and are suitable for enhancing the mechanical and electrical properties of composites. Low-dimensional carbon-based materials generally have the characteristics of a large surface area, high porosity, low cost, and excellent photothermal conversion characteristics. Their structure and physical and chemical properties have excellent plasticity and are easy to combine with other materials. Composite carbon-based materials can combine the advantages of various materials, taking advantage of their developed porosity and photothermal effect to enhance the photothermal effect by limiting the trapping of incident light. When incident light illuminates the carbon-based material, the electrons are excited, transitioning from the ground state to the excited state, and finally returning to the ground state to release heat [34]. Some carbon-based composites, such as carbon nanotubes and graphene, may exhibit surface plasmon resonance effects similar to those of metal nanoparticles. Through chemical doping, surface functionalization or structural optimization, the photothermal conversion performance and stability of carbon-based materials can be further adjusted. For example, by combining carbon materials with metal nanoparticles (such as Au, Ag, and Cu), the surface plasmon resonance effect of metal particles can be used to enhance light absorption and thermal conversion efficiency. By combining carbon materials with polymers (such as polyurethane, polystyrene), the stability, hydrophilicity, and photothermal conversion efficiency of the materials are improved. Compounding with other semiconductor materials can introduce a similar mechanism to induce non-radiative relaxation. Expanded graphite is combined with carbon foam to form a composite material with excellent solar energy absorption and thermal insulation properties [34].

4. Preparation Methods of Light Absorber

The light absorber is the most important component of the interfacial evaporator, as it determines the initial energy input of the interfacial evaporation system and ensures the high efficiency of the subsequent water evaporation. Current research has found that optical absorbers are mainly divided into two categories: planar membrane structures and 3D structures, and their preparation methods are various. The methods for preparing planar structures mainly include electrospinning [35,36], carbonization [37] and surface treatment [38,39] (including vacuum filtration, surface deposition, and magnetron sputtering), and other single methods or the combination of multiple methods. In addition, two-dimensional surface treatment and three-dimensional structure methods also include the gel method [40] (including the hydrogel method and the air-gel method) and 3D printing technology [41], as well as other single or multiple methods. All of these methods are developed to improve the heat-steam conversion efficiency of the photothermal converter and aim to be more effective and more accessible. Therefore, this paper will summarize the preparation method of optical absorbers based on their forms.

4.1. Preparation of a 2D Absorber

4.1.1. Electrospinning

Electrospinning has recently become one of the most commonly used methods to prepare nanoscale fibers. The fibers prepared by electrospinning have the advantages of a large surface area and high porosity, which can provide more attachment sites for photons and act as water transport channels, which facilitates the escape of the steam formed, thus improving the evaporation rate of water. Therefore, it is one of the commonly used methods to prepare absorbers. The steps of preparing optical absorbers by electrospinning are as follows: the photothermal material is mixed with the polymer and equipped with the mixed solution, and then spinning is carried out to obtain a disordered spinning film after setting the spinning parameters, as shown in the Figure 3a. Based on this method, Jin et al. [35] prepared a nylon/carbon black film light absorber using nylon and carbon black as raw materials and assembled it into an evaporator with a heat insulation layer and a one-dimensional water transport channel for testing experiments. At a standard solar intensity, the solar absorption rate reached 94%, and the photothermal conversion efficiency reached 83%. Ou et al. [36] prepared a fiber diameter gradient structure with a hydrophilic–hydrophobic gradient effect, i.e., a three-layer hierarchical nanofiber photothermal membrane (HNPM) with a unidirectional water transport effect, by electrostatic spinning, as depicted in Figure 3b. Under 1 sun irradiation, the designed HNPM evaporator achieved water evaporation rates of 1.44 and 1.78 kg m⁻² h⁻¹ for pure and dyed wastewater, respectively. Under outdoor solar conditions, the evaporator could evaporate 11.04 kg m⁻² in 10 h. In addition, the three-layer HNPM exhibited excellent flexibility and recyclability.

4.1.2. Surface Treatment

There are many ways to prepare absorbers by surface treatment, including vacuum filtration, surface deposition surface spraying, etc. [42]. These methods involve covering the surface of the substrate with a layer of photothermal material by physical or chemical methods, so that the substrate has photothermal properties. This is one of the most common methods for preparing interfacial optical absorbers today. For planar structured light absorbers, surface treatment is mainly applied to 2D structures such as fabric and fiber film. For example, Wang et al. [43] added pyrrole on air-laid paper by the surface deposition method, carried out oxidative polymerization at a temperature of about 0 °C, and then prepared a light absorber with air-laid paper and polypyrrole(PPy), which reached a combined conversion efficiency of 95.33% under one sun, as shown in Figure 4a. This work provides a simple method for the preparation of polymers designed to be used as solar-powered evaporators for water evaporation. Xu et al. [44] modified the surface of polyamide 6-graphene oxide (PA6-go) film with gold nanoparticles using vacuum filtration technology to prepare a light-absorbing layer for solar-driven water evaporation.

The experimental results showed that the photothermal film had a light absorption rate of 99.8%, as well as an evaporation rate and evaporation efficiency of 1.1 kg m⁻² h⁻¹ and 75.6%, respectively. In addition, it had the advantages of low cost and corrosion resistance, being suitable for seawater desalination applications. Polypyrrole has an excellent photothermal conversion performance and low thermal conductivity, so it is often deposited onto the substrate by the deposition method. Air-laid paper is one of the most commonly used C materials, and its hydrophilicity is conducive to the transport of water to the photothermal layer. Li et al. [45] prepared SiO₂/MXene/HPTFE Janus film by coating MXene nanosheets and SiO₂ with low thermal conductivity on hydrophilic polytetrafluoroethylene (HPTFE) film using a commercial spraying system (Figure 4b). The results showed that the film had excellent photothermal properties, with light absorption and conversion efficiency reaching 93% and 85.6%, respectively. In addition, it had excellent mechanical properties, salt-resistance, and self-cleaning abilities, making it easy to store and transport. Therefore, the preparation of photothermal film by spraying technology not only reduces the use of photothermal materials, but also provides an economical and energy-saving method for the preparation and application of photothermal film. In general, surface treatment has the advantages of easy operation and excellent performance, but poor performance stability.

4.1.3. Carbonizing Methods

In addition to the above methods, photothermal film can also be prepared by the carbonizing method, which is an effective method for treating samples at high temperature to impart photothermal conversion properties to them. Wan et al. [46] took basalt fiber as raw material, wove it into fabric by knitting machine, and carbonized it to prepare a basalt–fiber photothermal membrane (PTM). The evaporation rate was 1.50 kg m⁻² h⁻¹, and the photothermal conversion efficiency was 82.5% at 1 kW m⁻². In addition, the photothermal film can be washed at least 30 times without reducing efficiency, resulting in the preparation of low-cost, reusable photothermal film with high photothermal conversion efficiency (Figure 5a). Zhang et al. [47] collected discarded willow catkins and obtained a hydrophilic biochar layer through carbonization. Due to its exceptional photothermal conversion performance, they achieved a high photothermal conversion efficiency of 90.5% under solar irradiation. Additionally, the biochar exhibited a remarkable removal rate of 99.5% for heavy metal ions in simulated wastewater. The long-term stability of its evaporation performance demonstrated its potential in practical solar desalination applications, thereby offering a strategy for the reuse of various natural waste materials (Figure 5b).

4.1.4. Combining Multiple Methods

In certain cases, a single method may struggle to meet the requirements for fabricating light absorbers in specific materials, necessitating the integration of two or more approaches. For example, Liu et al. [48] employed electrospinning technology as the initial step to fabricate a substrate material, followed by in situ sulfurization of the fiber membrane to prepare a PAN@CuS fabric light absorber (Figure 6a). Under solar irradiance of 1 kW m⁻², this absorber achieved an impressive evaporation rate of 2.27 kg m⁻² h⁻¹, demonstrating a method that enables high evaporation rates while preventing the formation of salt crystals during high-salinity water evaporation.

Similarly, Ma et al. [49] utilized a combination of electrospinning technology, hydrothermal growth, and magnetron sputtering to construct a zinc oxide nanowire trapping structure within a carbon-based nanofiber membrane (Figure 6b). This innovative design facilitated broad-spectrum light absorption from multiple angles. Under solar irradiation, the absorber exhibited a remarkable evaporation rate of 1.73 kg m⁻² h⁻¹ and a high evaporation efficiency of 94%. These results underscore the successful development of a light absorber capable of absorbing light from various angles and across a wide spectral range.

By integrating different methods, researchers have overcome the limitations of individual techniques and achieved improved performance in light absorber fabrication. These advancements highlight the potential of combining diverse approaches to enhance the functionality and effectiveness of light absorbers in various applications.

4.2. Fabrication of 3D Absorbers

In general, thin planar structures exhibit higher thermal conductivity, facilitating efficient heat exchange with the environment. Consequently, they retain less heat. However, due to their structural characteristics, these planar structures typically possess either photothermal conversion performance alone or a combination of photothermal and water transport capabilities, while lacking thermal insulation properties.

Conversely, 3D structures, leveraging their thickness advantages, can simultaneously achieve photothermal conversion, water transport, and thermal insulation capabilities. The fabrication methods for 3D evaporators primarily include surface treatment, gelation, carbonization, and 3D printing techniques, either as individual methods or through their integration.

4.2.1. Carbonization Methods

In the realm of 3D evaporators, carbonization is primarily applied to biomass materials. This is because biomass materials possess rough surfaces, high porosity, and inherent water transport channels. As a result, the treated absorbers exhibit excellent light absorption, photothermal conversion performance, and demonstrate remarkable capabilities in light absorption, efficient water transport, and heat management [50].

Biochars such as carbonized wood [51], loofah sponge [52], mushrooms [53], pine nut shells [54], and sunflower [55] are used in evaporator manufacturing due to their wide availability, low cost, and environmental friendliness.

Xue et al. [37] prepared a light absorber by treating the surface of natural wood by the carbonization method, as shown in Figure 7a. The carbonized wood surface looks dark and facilitates more efficient absorption of solar light. Utilizing the inherent advantages of wood, including high porosity, low thermal conductivity, and good hydrophilicity, 99% solar light absorption and 72% photothermal conversion efficiency were achieved under solar irradiation. These results validate the successful development of a cost-effective, scalable, and renewable evaporator.

Li et al. [56] investigated the evaporation performance of carbonized balsam wood evaporators (CBWs), carbonized basswood evaporators (CBSWs), and carbonized pine wood evaporators (CPWs) at different degrees of carbonization, shown in Figure 7b. The best evaporation performance was obtained for wood evaporators of different wood types at different degrees of carbonization, and it was found that the best evaporation performance of wood evaporators was not always achieved at the highest solar energy absorption. Among the studied evaporators explored to exhibit the best evaporation performance, the CBW-20%, CBSW-15%, and CPW-10% evaporators with a height of 15 mm produced high evaporation rates of 2.13 kg m⁻² h⁻¹, 2.067 kg m⁻² h⁻¹, and 1.979 kg m⁻² h⁻¹, with evaporation efficiencies of 90.56%, 88.79%, and 90.42%, respectively.

4.2.2. Surface Modification Methods

In 3D structures, surface modification methods, including surface deposition, coating, and spraying, are employed to enhance the light-to-heat conversion properties of absorbers. The focus of research lies primarily on structures with porosity and surface roughness, which facilitate the improved adhesion of added materials, ensuring their stability. Natural materials such as wood are commonly used due to their economic viability, wide availability, and ease of handling. For example, Yang et al. [57] fabricated a dual-layered light absorber by depositing dopamine and silver nanowires onto natural wood (Figure 8a). The synergistic photothermal effect between PDA and AgNPs on the top layer resulted in a light absorption rate exceeding 96% under solar irradiation and rapid temperature response (top temperature of 45.1 °C). The porous channels in the wood substrate, on the other hand, exhibited low thermal conductivity and served as pathways for water transport, enabling water evaporation from the top layer. The calculated water evaporation rate and efficiency under 1 kW m⁻² were found to be 1.58 kg m⁻² h⁻¹ and 88.6%, respectively. Moreover, Ag-PDA@wood demonstrated strong antibacterial activity, effectively eliminating harmful bacteria. These results highlight the high potential of Ag-PDA@wood in seawater desalination and wastewater purification, offering valuable insights for addressing freshwater scarcity. Chen et al. [58] coated wood surfaces with aluminum phosphate (Wood@AlP) using a brushing method and then heated them at 130 °C for 30 min, resulting in a black-colored surface (Figure 8b). The black top layer was responsible for photothermal conversion. Additionally, the natural hydrophilicity, water transport properties, and low thermal conductivity of wood limited heat diffusion towards the water, leading to a solar–thermal efficiency of 90.8% and a net evaporation rate of 1.423 kg m⁻² h⁻¹ under 1 kW m⁻². This approach provides a simple, cost-effective, and scalable method for light absorber fabrication. Li et al. [59] utilized natural wood as a framework and sprayed graphene onto its surface to create a light absorber. This surface-sprayed graphene was used to prepare light absorbers. In this case, the inherent open tubular lumen channels of natural wood provide access for water transportation and also serve to reduce thermal conductivity (Figure 8c). Tests showed that 80% water evaporation efficiency was achieved at one solar intensity. Mu et al. [60] successfully prepared a novel multifunctional MXene/PPy-coated melamine foam by simple impregnation and in situ polymerization (Figure 8d). It has excellent light absorption capacity (about 94%), low thermal conductivity (0.1047 Wm ± 1 K ± 1), and exhibits excellent performance in solar desalination, wastewater purification, and the photodegradation of organic dyes. Under 1 kW m⁻² illumination, the solar energy conversion rate and efficiency of MF MXene/PPy are as high as 1.5174 kg m⁻² h⁻¹ and 91.24%.

4.2.3. Aerogel Method

Aerogel, a new type of porous solid with a 3D network structure filled with gas, is currently the least dense solid material in the world. It has been reported that aerogel has low density, high porosity, low thermal conductivity, and high-temperature resistance [61,62]. As an integrated 3D interfacial photothermal converter, the porous structure of aerogel can both facilitate water transportation and help reduce thermal conductivity. Moreover, the light weight of aerogel could allow it to float on the water surface and accelerate water evaporation [63]. The preparation methods for aerogels are diverse, but they generally involve freeze-drying to achieve a porous structure through the sublimation of water. Although aerogels as photothermal converters have the potential to minimize heat loss and enhance photothermal conversion efficiency, the complexity of their fabrication process poses challenges. Xiao et al. [27] employed a combination of hydrothermal reduction and freeze-drying to synthesize a composite aerogel consisting of silver nanoparticles (AgNPs) and reduced graphene oxide (RGO), denoted as RGOA/Ag (Figure 9a). Initially, the hydrothermal reduction process was utilized to eliminate the oxygen functional groups from graphene oxide, thereby increasing the hydrophobicity of graphene and promoting the π-π stacking effect between its layers. Under hydrothermal conditions, the graphene sheets formed an aerogel structure through π-π conjugation and hydrophobic interactions. Concurrently, glucose acted as a reducing agent, converting Ag⁺ ions into Ag nanoparticles that were subsequently deposited onto the surface of the reduced graphene oxide sheets, resulting in the formation of RGOA/Ag composite. The hierarchical porous structure and organized water channels of the RGOA/Ag aerogel facilitated efficient water transport and reduced heat transfer. Consequently, under solar irradiation, the RGOA/Ag aerogel demonstrated a remarkable evaporation rate of 1.56 kg m⁻² h⁻¹ and achieved a photothermal conversion efficiency of 97.9%. These exceptional water evaporation properties can be attributed to the unique structure of the RGOA/Ag aerogel, providing valuable insights into the development of highly efficient solar-driven water evaporation systems. Electrospun nanofiber aerogels have gained significant attention due to their distinctive three-dimensional porous structure, lightweight nature, efficient solar absorption capability, and thermal insulation properties.

Liu et al. [64] utilized PAN/CNTs nanofibers as the substrate material and subsequently deposited polydopamine (PDA) onto the surface of PAN/CNTs nanofibers through a self-polymerization reaction under mild conditions. This resulted in the formation of a composite material called PC@PDA NFM. A chitosan solution was then poured into the well-stirred dispersion of PC@PDA NFM, enabling the crosslinking between the fibers and PDA molecules (Figure 9b). The structural advantages of the nanofiber aerogel, combined with the photothermal synergistic effect between carbon nanotubes (CNTs) and polydopamine, enabled the resulting PC@PDAC aerogel to achieve a remarkable evaporation rate of 2.13 kg m⁻² h⁻¹ and a solar–thermal conversion efficiency of 94.5% under solar irradiation. These performance metrics surpass those of most electrospun fiber-based evaporators, providing an effective and direct method for the efficient production of clean water. Finally, the mixture underwent freeze-drying to generate a porous PC@PDAC aerogel.

Inspired by natural trees with fast water transport and high photothermal conversion efficiency, Ma et al. [65] successfully developed a bionic solar-driven interface evaporator micro-nano light absorption layer through vertically arranged hydrophilic sodium alginate (SA) aerogels and hierarchically assembled MXene interwoven carbon nanotube (CNT) networks (Figure 9c). The improvement of photothermal conversion efficiency and the optimization of water transport were realized.

Wang et al. [66] developed a kind of bionic aerogel with vertically ordered channels and low water evaporation enthalpy, which was inspired by the long-range ordered structure and water transport capacity of the lotus root. It exhibits a high water evaporation rate (2.62 kg m⁻² h⁻¹) and energy efficiency (93.6%) under single solar irradiation.

The integration of multiple methods in absorber fabrication can simplify the process to a certain extent. For instance, Liu et al. [67] employed a combination of aerogel synthesis and carbonization methods for evaporator fabrication. Chitosan powder was directly dissolved in an acetic acid solution and then subjected to steps such as freeze-drying to directly produce chitosan aerogel. Subsequently, the top layer of the absorber was carbonized by directly heating it to 300 °C on a hot plate. Liu et al. [68] constructed semiconductor nanofiber aerogels with narrow band gaps, vertically aligned channels and conical structures through multi-scale collaborative engineering strategies, including atomic-scale band gap engineering and nanoscale structural engineering. As a proof of concept, co-doped MoS₂ nanofiber aerogels were synthesized. By combining different methods, researchers have successfully developed absorbers with improved performance. These approaches offer potential for simplifying the fabrication process and enhancing the efficiency of solar-driven evaporation.

4.2.4. Hydrogel Methods

Hydrogels are three-dimensional network gels with crosslinked polymers. They are soft, hold a specific shape, and have excellent water absorption. Hydrogel could be used to prepare an integrated 3D interfacial photothermal converter by adding photothermal materials into it during the preparation process [69,70]. The hydrogel photothermal converter may benefit from its strong water absorption to continuously transport water to the evaporation layer and its internal porous structure to reduce heat loss [71]. However, its mechanical properties were poor [72]. For example, Li et al. [73] proposed a simple method to prepare a SnSe hydrogel evaporator by encapsulating SnSe nanosheets within a poly(2-hydroxyethyl methacrylate) hydrogel. Under solar irradiation, this evaporator achieved an evaporation rate of 2.20 kg m⁻² h⁻¹ and an evaporation efficiency of 91.70%. Furthermore, the evaporator exhibited excellent long-term performance stability and demonstrated outstanding ion rejection rates in seawater desalination experiments, suggesting its potential application in sustainable solar-powered seawater desalination during long sea voyages.

Moreover, hydrogel evaporators can also be fabricated by combining with other methods. As shown in Figure 10a, Guo et al. [74] prepared a carbon black-deposited nonwoven fabric (CBn@NF) by vacuum filtering a dispersion of carbon black (CB) and carboxylated cellulose nanofibers (CNFs) onto a sticky nonwoven fabric. Subsequently, a mixture of polyvinyl alcohol (PVA) and starch solution was poured into a mold, frozen at −12 °C for 24 h, and then thawed at 25 °C for 12 h, resulting in a PVA/starch hydrogel. CBn@NF was then overlaid on the PVA/starch hydrogel, subjected to three freeze–thaw cycles, and the corresponding bilayer composite structure was obtained. Finally, polyethylene (PE) foam was cut into small pieces, added to the PVA/starch mixture. After vacuum degassing, it was covered with CB@NF and subjected to freeze–thaw treatment to obtain a self-floating bilayer evaporator. This evaporator achieved an evaporation rate of 1.08 kg m⁻² h⁻¹ under 1 kW m⁻² solar irradiation.

Li et al. prepared an anionic polyelectrolyte-based hydrogel (APH) as an “all-in-one” evaporator, which has both photothermal properties (evaporation of seawater) and electrostatic repulsion properties (avoiding solid salt crystallization) [75]. APH with polyvinyl alcohol as skeleton and poly (3,4-ethylenedioxythiophene): poly (sodium p-styrenesulfonate) as solar absorber was prepared by the freeze–thaw method. The prepared material showed a porous structure with a solar energy absorption efficiency of 95.5% in the range of 380–2500 nm. Under simulated sunlight (1.0 kW m⁻²) irradiation, the evaporation rate of APH is as high as 2.5 kg m⁻² h⁻¹, and the solar evaporation efficiency reaches 90.7%. The SO₃-group can effectively separate anions from cations in the evaporator, avoiding the formation of solid salt crystals.

4.2.5. 3D Printing

Three-dimensional printing is a promising technology that could quickly and accurately produce the most complex patterns, so it has extensive applications in many fields [76]. So far, except for the standard methods of preparing photothermal converters, 3D printing has gradually become a standard method because of its simple preparation method and ability to design various shapes. For example, Li et al. [77] prepared an integrated photothermal converter based on 3D printing technology. Its fabrication process is shown in Figure 11a. Overall, it consisted of a CNT/GO layer, a graphene oxide/nanofibrillated cellulose (GO/NFC) layer, and a GO/NFC wall, layered in order from top to bottom (shown in Figure 11b). The CNT/GO layer had strong solar energy absorption ability and could effectively convert light into heat. The middle CNT/GO layer was thin and had high porosity, which helped to suppress heat loss and steam escape, and could also act as water transport channels that transport water up to the continuous porous CNT/GO layer. In addition, the GO/NFC wall could also be used as a support table to support the GO/NFC layer and the CNT/GO layer above, and the whole integrated structure was light enough to be placed on branches without falling off (Figure 11c), which satisfied the requirements for photothermal interface converters. The GO/NFC layer and the CNT/GO layer were connected to form a bottom-to-top continuous water delivery channel. Moreover, the air surrounded by the GO/NFC walls could effectively insulate heat, reducing the heat flow to the water to some extent, resulting in higher photothermal conversion efficiency.

Shi et al. [78] used 3D printing technology to design a 3D solar evaporator. First, the delignification of waste coffee grounds (D-SCG) was modified with in situ grown polypyrrole (PPy), and then its thermoplastic composite filament polylactic acid was combined. The water evaporation rate of the 3D-printed solar evaporator was 1.81 kg m⁻² h⁻¹, and the photothermal conversion efficiency was 92.7%.

5. Methods to Improve the Efficiency of Interfacial Photothermal Conversion

The basic working principle of an interfacial photothermal converter is that the solar absorber absorbs the sunlight and then generates heat. The generated heat energy heats the water and produces steam. During this process, come heat might be exchanged with the environment. According to the structures and principles of the interfacial photothermal converter, the factors that affect photothermal conversion efficiency could be mainly attributed to the light absorption capacity of the solar absorber, water transportation efficiency, thermal management, as well as salt deposition and removal [47,79,80,81]. To improve the efficiency of the interfacial photothermal conversion, one must design the converter with these aspects in mind, as described in Figure 12.

5.1. Increase Light Absorption

5.1.1. Material Selection

Different materials exhibit different light absorptivity due to their selective absorption of photons in different wavelength bands. An ideal photothermal material should have a full spectrum and high-intensity sunlight light absorption. Therefore, selecting materials with different light absorption ranges and intensities is a valuable way to improve photothermal conversion efficiency. Single types of photothermal materials often struggle to achieve broad-spectrum and high-intensity light absorption. To overcome this limitation, the hybridization of multiple photothermal materials can be employed. The optical properties of semiconductors are determined by the energy bandgap between the conduction band and the valence band, and some exhibit selective spectral absorption properties, which prevent them from achieving full-spectrum sunlight absorption. However, their light absorption capability can be enhanced through composites with other materials. Black TiO₂ has attracted much attention due to its excellent solar absorbability and photocatalytic activity. Therefore, Wang et al. [82] synthesized hydrogen-doped black titanium dioxide (TiO₂@TiO_2-x H_x) with a core-shell structure under the action of hydrogen plasma. Just as shown in Figure 13a, this hydrogen-doped black titanium dioxide has a robust light absorption capacity that is much higher than those of high-pressure hydrogenated black titanium dioxide and ordinary black titanium dioxide. The combination of carbon-based materials with organic polymer photothermal materials can also enhance light absorption. Neelgund et al. [83] doped polyamidoamine (PAMAM), PAMAM-Ag₂S into carbon nanotubes. The solar light absorption spectrum indicated that the addition of different substances had different spectral absorption ranges and capabilities, as shown in Figure 13b. Therefore, doping certain materials is a helpful way to increase light absorption. It is believed that dark-colored materials, such as black, have stronger broadband absorption compared to light-colored materials. Indeed, certain materials exhibit varying levels of light absorption intensity under different dry and wet states. Jin et al. [35] prepared a nylon/carbon cloth through electrospinning technology showing unique reflection and absorption characteristics to sunlight in both wet and dry conditions. As demonstrated in the inset image of Figure 13c, the color of the nylon/carbon composite film can be changed from dark gray (dry state) to dark black (wet state). Accordingly, sunlight absorption increased significantly as the color changed, which suggests that we might have to take into account the color of the designed interfacial photothermal converter.

Zhu et al. [84] proposed a photothermal quality factor (Q) as a comprehensive index, which is mainly related to photothermal conversion efficiency and material cost, and is used to comprehensively evaluate the photothermal performance of the evaporator. The higher the photothermal quality factor, the better the comprehensive performance of the evaporator in terms of cost and efficiency. The photothermal properties and cost of several other materials were compared. (

Table 1. Comparison of photothermal performance and cost of materials under one sun (1 kW/m²) [84]. © 2023 Elsevier Ltd.

Ref	Materials	Efficiency (%)	Cost ($/m2)	Q (m2/$)	Characteristic
[67]	Carbonized chitosan aerogel	91	0.79	≈5.71	Low cost and mechanical properties, high efficiency
[51]	Carbonized wood	57.3	<1	<4.048	Low cost and efficiency
[84]	Carbon black, rPET	93.91	1.19	3.81	Low cost and high efficiency
[59]	Graphite wood block	80	<3	1.4607	Low cost and efficiency
[85]	Fe-MOF	90	14.9	0.302	Complicated synthesis process, excellent photothermal performance
[86]	Graphene oxide, carbon nanotube, porous cellulose filter paper	≈100	36.97	0.125	High cost, excellent photothermal performance
[87]	Cu2SnSe3, Cu2ZnSnSe4 and hydrophilic filter membrane	86.6	41.5	0.1075	Good anti-salt deposition effect, high cost
[88]	Ti2O3, PVA	90	293.21	0.0153	High cost, excellent photothermal performance
[89]	Graphene, commercial Ni foams	91.4	124.07	0.0364	High cost, excellent photothermal performance

) The quality factor of the other three groups of raw materials using carbonized chitosan aerogel, carbonized wood, carbon black/rPET was the highest, indicating that the carbonization method and the use of carbon-based raw materials have significant cost advantages. Various materials have their own advantages and limitations in photothermal conversion applications. The selection of appropriate materials needs to be considered comprehensively, according to specific application requirements and environmental conditions. For example, carbon-based materials may be a better choice if low-cost and full-spectrum absorption capabilities are required; if adjustable light absorption characteristics and versatility are needed, semiconductor materials may have more advantages.

5.1.2. Solar Absorber Structure Design

In addition to choosing different photothermal materials, the design of the evaporator structure can also enhance light absorption to a large extent. In the structural design of the absorber, the structural modification at both micro and macro levels has a significant impact on the absorption of sunlight. The structural modification at the microscopic level is mainly to change the surface roughness of the absorber, which is because a smooth surface has a strong ability to reflect incident light, leading to low absorption of sunlight [90]. It has been reported that rough surfaces increase the number of light reflections on the surface. For example, Wang et al. [43] added pyrrole drops to a porous air-laid paper substrate, and pyrrole was oxidized to form polypyrrole (PPy). Finally, PPy-modified air-laid paper with high roughness was obtained (Figure 14a). It has been shown that the rough surface of multilayer nanosheets can effectively retain and redistribute incident light on the surface compared to flat structures, capturing incident light by using the restricted space between layers. This increases the reflection of light and achieves more light absorption, which can be verified in the reflection diagram of Figure 14b. Wang et al. [91] also increased the roughness of the evaporator surface by designing a “pin screen” photothermal hollow fiber array, which resulted in enhanced light absorption, achieving a photothermal conversion performance of more than 90% at one sun intensity, which is much higher than that of a planar structure (55%). A rough surface offers higher light absorption quality compared to a relatively smooth surface.

To enhance light absorption at the macroscopic level, a common approach is to fold the absorber into a conical shape, which restricts and maximizes the interaction of light. In contrast, planar structures exhibit higher light reflection, resulting in significant light loss. Researchers have explored various folded three-dimensional (3D) structures as absorbers, allowing incident light to undergo multiple reflections within the folded structure’s concave cavities, thereby achieving maximum light absorption [92]. Following a similar principle, Zhang et al. [93] fabricated a conical solar absorber to increase light absorption and improve evaporation rate. This structure exhibited superior photothermal conversion performance compared to planar structures, achieving a water evaporation efficiency of 80.70%, which is higher than the 69.46% of the 2D planar structure. Wang et al. [94] also developed a 3D folded structure to minimize light reflection. Under solar irradiation, the well-designed folded structure demonstrated a conversion efficiency of 83.9%, surpassing the performance of planar structures (61.9%). Additionally, Ni et al. [95] developed a method for the arbitrary transformation between 2D and 3D evaporators to adapt to different environments. Compared to planar structures, the 3D structures exhibited a larger effective evaporation area and superior photothermal conversion performance, achieving an evaporation rate of up to 2.99 kg m⁻² h⁻¹. Zhu et al. [84] developed an absorber that can be folded from a planar structure into a 3D configuration, illustrating the primary interaction between the planar and folded 3D structures with light. The evaporator assembly is shown in Figure 14c,d with hydrophilic strips and polystyrene (PS) foam. Tested under solar irradiation, it achieved a conversion efficiency of 93.91%, which is significantly higher than that of the planar structure (55.9%).

5.2. Thermal Management

In addition to maximizing light absorption, effective thermal management can reduce the heat exchange with the environment, thus maximizing the conversion of heat to vapor. The heat loss in the thermal insulator solar evaporator occurs through the following processes: thermal radiation from the photothermal layer to the surrounding environment; heat conduction; and heat convection between the evaporator and a large amount of water [96]. The methods for effective thermal management primarily involve reducing heat loss at both the microscale design and macrostructural levels. At the microscale design, modifications of photothermal materials are commonly employed, while at the macrostructural level, the design of the photothermal system is crucial.

Research has shown that the temperature of photothermal conversion is greatly influenced by the geometric shape of the materials. The rate of heat transfer is strongly dependent on the material’s specific surface area and thermal conductivity. A larger specific surface area and higher thermal conductivity facilitate efficient heat transfer to water for evaporation. A commonly used approach to improve the thermal management of materials is to design them with a microporous structure. Microporous materials utilize air as a thermal barrier, reducing thermal conductivity and confining heat within specific regions. This guides heat towards efficient water evaporation while minimizing heat exchange with the surrounding environment. Based on this principle, Li et al. [97] developed nitrogen-doped porous graphene sheets with a low thermal conductivity of 9.0 ± 1.2 W m⁻¹ K⁻¹ (as shown in Figure 15a,b). Furthermore, the introduction of insulation structures can reduce unnecessary heat loss. Ghasemi et al. [98] introduced a foam carbon layer as an insulation structure and proposed a bilayer structure consisting of a detached graphene layer supported by a foam carbon layer, as shown in Figure 15c. The top layer is the detached graphene layer, which absorbs sunlight and converts it into heat. The detached graphene layer is also hydrophilic, with a large liquid supply hole inside to transport water from the bottom layer to the surface. The bottom layer is the carbon foam layer, which not only supports the solar absorber but also acts as insulation and facilitates water transport. Figure 15d–g display the thermal conductivity of the graphene layer and carbon foam layer in air (Figure 15d,e) and water (Figure 15f,g), respectively, with infrared microscopy images illustrating the temperature gradient of the entire sample. Both the graphene layer and the carbon foam layer exhibit low thermal conductivity due to their porous structure and the presence of trapped air within the pores (Figure 15d,e). When the graphene layer is filled with water, its thermal conductivity significantly increases and surpasses that of water, which is unfavorable for localized heating. However, even when the carbon foam layer is in full contact with water, it still exhibits low thermal conductivity due to the presence of enclosed pores, which restrict water flow through the pores and carry away some heat, thereby greatly reducing thermal conductivity. This demonstrates the superior insulation properties of the carbon foam layer, which achieves an optical-to-thermal conversion efficiency of 85% under a solar intensity of 10 kW m⁻². However, the insulation performance of this structure is still compromised, as water entering the pores weakens its thermal barrier effect.

Due to the limitations of porous insulation, researchers have proposed designing the macrostructure of the evaporation system. For example, Chen et al. [99] added additional insulation materials and designed a highly efficient floating foam-flamed wood (F-F-wood) solar–thermal evaporator. As shown in Figure 16a, this structure consists of a top solar–thermal layer, a water-absorbing structure within the wood, and an internally filled polystyrene insulation foam layer. Due to the low density of the wood and foam, the entire evaporator can float on the water surface. The microchannels in the wood transport water from the side to the top, enabling efficient water evaporation in the solar–thermal layer. The polystyrene foam filled inside the wood optimizes the insulation performance of the solar–thermal evaporator, greatly reducing heat loss. The evaporator achieved a water evaporation rate of 3.92 kg m⁻² h⁻¹ under one sun, with a surface temperature of the solar–thermal film reaching 72.5 °C. Additionally, Xu et al. [53] used carbonized mushrooms as the absorber and water transport channels and prepared an evaporative system by adding an insulation layer. The experimental results showed that the prepared evaporative system achieved an energy conversion efficiency of 78% under 1 kW m⁻², with thermal conduction loss as low as 0.2% (Figure 16b). Therefore, in summary, both designing materials with a microporous structure and incorporating additional insulation devices can achieve higher water evaporation to some extent.

5.3. Efficient Water Transportation

Continuous water supply on the surface of a solar–thermal evaporator is crucial for effective steam generation. Traditional water transport channels typically rely on the capillary effect of 3D randomly arranged pores. However, the water within these channels increases thermal conductivity during transportation, resulting in heat loss [100] (Figure 17a). To reduce the heat loss caused by direct water contact, some interface evaporators require additional water transport pathways to achieve efficient solar–thermal conversion. Researchers have thus designed water evaporation devices combining insulating foam with water transport channels. Li et al. [39] devised a two-dimensional (2D) water channel structure that simultaneously enables efficient water supply and suppresses heat loss. As shown in Figure 17b, the insulating layer is enclosed within a hydrophilic cellulose paper, providing a 2D pathway for water transport to the solar–thermal layer through capillary action. The graphene oxide membrane, serving as the light absorber, is isolated from the water by the insulating device, preventing direct contact and significantly reducing heat loss. Compared to traditional “large-scale water supply” designs, the 2D water transport structure reduces the dimensions and volume of the evaporator’s water transport, thereby greatly minimizing heat loss through thermal conduction into the water. Additionally, one-dimensional (1D) water transport also offers significant advantages in suppressing heat loss. As illustrated in Figure 17c, the combination of 1D hydrophilic cotton yarn (CY) with the insulating layer and the solar–thermal membrane, with the solar–thermal membrane placed on highly absorbent filter paper, effectively prevents heat transfer to the water and achieves a conversion efficiency of 82.52%.

5.4. Improve Salt Rejection

The issue of salt deposition has become a significant bottleneck in the sustainable desalination process of evaporators. Currently, although the conversion efficiency of evaporators can exceed 90%, it gradually decreases over time. Therefore, while pursuing high solar–thermal conversion efficiency, attention should also be given to the sustainability of solar–thermal evaporators, with the most important aspect being their salt tolerance. When water evaporates in the evaporator, salt crystals form and accumulate on the surface of the light-absorbing material, reducing the light absorption area and efficiency, and even blocking the water transport channels, thereby decreasing the solar–thermal conversion efficiency [102]. Therefore, to maintain the stability and sustainability of the solar–thermal conversion efficiency, effective methods must be employed to remove deposited salt. The main methods for addressing salt deposition issues include (1) water transport channel design, (2) Janus structure design, (3) localized salt crystallization design, and (4) hydrophobic treatment [103]. These methods have reduced the adverse effects caused by salt deposition to a certain extent, greatly improving the long-term performance of solar evaporators.

Efficient and rapid water transport is beneficial for dissolving salt quickly before crystallization occurs, thereby avoiding the degradation of solar–thermal performance caused by salt deposition. Zhang et al. [104] improved the salt deposition issue by designing a unidirectional fluid structure. As shown in Figure 18a, a poly(aniline)/cellulose dual-layer structure was obtained by depositing polyaniline on the cellulose aerogel surface. The top layer of polyaniline serves as the light absorber, while the cellulose aerogel exhibits strong hydrophilicity and downward thermal insulation, preventing heat diffusion into the water. Figure 18a illustrates the structural schematic of the unidirectional fluid evaporation system and the salt removal principle. In the schematic of the evaporation system structure, the middle part represents the solar–thermal evaporator, the bottom part is the insulation layer for preventing downward heat diffusion, and the two ends of the inlet and outlet are connected to hydrophilic fabrics attached to the ends of the poly(aniline)/cellulose structure. However, only one end of the inlet is in contact with the seawater solution containing salt, while the other end is responsible for transferring the solution containing dissolved salt crystals. Benefiting from the excellent wettability and water conductivity of the capillary wicking fabric and cellulose aerogel, a low-resistance unidirectional water flow is achieved, which allows the low salinity seawater introduced at the outlet to become high salinity seawater in the direction of the unidirectional flow, avoiding seawater deposition on the evaporator surface, and achieving an outstanding conversion efficiency of 92% under sunlight.

Apart from using hydrophilicity to dissolve salt in water and remove it along the water flow direction to improve salt deposition issues, the hydrophobicity of the surface of solar–thermal evaporators can also prevent salt precipitation. Zhang et al. [105] fabricated a Janus porphyrin Ti₃C₂T_x MXene membrane (Janus PMX membrane) for solar–thermal desalination using a vacuum filtration process. As shown in Figure 18b, the deposition of porphyrin on the Ti₃C₂T_x MXene membrane not only created a hydrophobic surface but also achieved efficient solar–thermal conversion, reaching an efficiency of 86.4% under sunlight. To validate the salt-resistant properties of the hydrophobic surface, a comparison was made between the pure MXene membrane and the Janus PMX membrane. Due to its hydrophilicity, the pure MXene membrane sank to the bottom of the water, while the Janus PMX membrane with a hydrophobic surface floated on the water surface. As shown in Figure 18c, salt deposits were observed on the surface of the pure MXene membrane, while no salt particles appeared on the Janus PMX membrane, confirming the excellent desalination performance of the Janus PMX membrane, in addition to achieving high solar–thermal conversion efficiency. This work provides a simple and effective strategy for efficient solar–thermal desalination.

Traditional approaches to addressing salt deposition issues in solar–thermal evaporators typically involve reducing the concentration of salt within the evaporator system to maintain stable water evaporation rates over an extended period. In contrast, localized salt deposition offers a new approach that enables the collection of salt solids from seawater while maintaining high and stable evaporation rates. Lei et al. [106] designed a disk-shaped solar evaporator that achieves both steam generation and salt crystal collection. As shown in Figure 18d, assisted by a one-dimensional water channel at the center of the evaporator, the transport of salt solution within the evaporator creates a radial concentration gradient from the center to the edge, spatially isolating salt crystals from the evaporator surface. This ensures the long-term stability and salt harvesting capability of the evaporator, with salt crystals crystallizing at the periphery even in a 21% brine solution, enabling complete separation of water/solutes and efficient salt harvesting. This study provides a pathway for the scalable manufacturing of high-performance solar steam generators.

Additionally, there is a simpler method that involves removing salt particles from the evaporator surface through washing, without compromising surface properties. For example, Kou et al. [102] developed a low-cost and washable solar–thermal fabric that achieved a water evaporation rate of 1.59 kg m⁻² h⁻¹ under 1 kW m⁻² irradiation. The simple fabrication of the solar–thermal evaporator not only yields high conversion efficiency but also allows for the removal of deposited salt through hand washing, thereby improving the recovery rate of the solar–thermal evaporator. This provides a new approach for the development of low-cost, efficient, and sustainable solar–thermal evaporators.

6. Summary and Outlook

Solar-driven interfacial water evaporation is considered one of the most promising alternatives to traditional seawater desalination techniques due to its high conversion efficiency, environmental friendliness, low cost, and high-performance stability. This article provides a detailed overview of the latest research advances in solar-driven interfacial water evaporation. It primarily focuses on the classification and conversion mechanisms of photothermal materials. Additionally, it systematically presents the fabrication methods of light absorbers and summarizes effective approaches to enhance the photothermal conversion efficiency of evaporators. Finally, it summarizes and discusses the challenges and prospects that may be encountered in future research on solar-driven interfacial water evaporation.

Despite the remarkable progress achieved in solar-driven interfacial water evaporation, several challenges and issues still need to be addressed. Firstly, there is a need to develop low-cost and environmentally friendly materials. Current research in solar-driven interfacial water evaporation mainly focuses on improving the photothermal conversion efficiency and water evaporation rate, while overlooking costs and environmental considerations. However, as the photothermal conversion efficiency and evaporation rate have nearly reached their limits, future research should shift the focus toward the economic and environmental benefits of materials. Among many photothermal materials, carbon-based materials are particularly interesting due to their rich variety, low cost, full-spectrum light absorption and high photothermal conversion efficiency. Carbon-based materials include carbon black, carbon nanotubes, and graphene, which usually have large surface areas, high porosity, and excellent photothermal conversion characteristics. In addition, semiconductor materials have also attracted much attention due to their low cost and low toxicity. By adjusting the band gap and microstructure, semiconductor materials can achieve higher light absorption.

Secondly, innovative fabrication methods are required. To enhance photothermal performance, numerous fabrication methods for light absorbers have been developed to cater to the advancements in the field of interfacial water evaporation. However, most of these methods have drawbacks such as complex processes or high energy consumption. Therefore, future research should emphasize the simplicity of the fabrication methods and energy-saving approaches.

Thirdly, it is crucial to achieve precise matching among water transport, heat management, and salt tolerance. Solar-driven interfacial water evaporation for seawater desalination is a complex process that requires an adequate water supply while preventing salt deposition and minimizing heat loss. Thus, further research is needed to identify the optimal balance among water transport, heat management, and salt tolerance.

Lastly, most of the current research on solar-driven interfacial seawater desalination techniques is still in the laboratory experimental stage. The seawater solutions required are primarily prepared in laboratory conditions. Transitioning from the laboratory stage to practical applications still faces numerous challenges, particularly regarding the scalability of interfacial evaporation seawater desalination devices and their adaptability to real seawater environments.

We firmly believe that with the continuous advancement of solar-driven seawater desalination research and interdisciplinary collaborations, the aforementioned challenges and shortcomings will be overcome. Ultimately, the gap between ideal solar-driven seawater desalination and practical applications will be narrowed, striving to achieve the ideal of quenching the world’s thirst with a clean and sustainable freshwater solution.