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All issues Volume 97 (2022) Eur. Phys. J. Appl. Phys., 97 (2022) 84 Full HTML
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Eur. Phys. J. Appl. Phys.
Volume 97, 2022
Special Issue on ‘Amorphous alloys and multiscale materials: Fundamental aspects and Energy applications’, edited by Zhao Zhankui, Wang Hongli and Tai Cheuk-Wai
Article Number 84
Number of page(s) 14
Section Nanomaterials and Nanotechnologies
DOI https://doi.org/10.1051/epjap/2022220179
Published online 30 November 2022

© EDP Sciences, 2022

1 Introduction

Fresh water is an enormously crucial and strategic resource for a country, which is related to human life and socio-economic development. Due to the globalization of industry and the growing population, the shortage of fresh water has become one of the major risks to the survival of human beings [1–3]. Currently, more than 1.6 billion people on the earth do not have access to clean drinking water, and most of these people live in remote and poor region [4]. By 2025, about 60% of the area in the world will face the problem of fresh water shortage, and it will be more serious in the future [5]; by 2050, more than 4 billion people in the world will be threatened by fresh water resources, and the majority of diseases will be caused by clear water and sanitation problems [6]. Faced with the problem of depletion of natural resources, people are committed to the search for drinking water and other alternative energy sources. In order to meet the demand for fresh water, huge amounts of money and effort have been invested in the development of desalination technology, which is now a reliable technology for tackling the problem of fresh water shortage [7,8]. At present, desalination technologies that have been put into market application include reverse osmosis [9], multi-stage flash distillation [10], and multi-effect desalination technologies [11]. However, these methods currently have several problems such as using expensive heat sources, emitting large amounts of greenhouse gases, and replacing freshwater resources with environmental pollution, which is a secondary harm to resources and the environment; while technologies with relatively low operating costs will have poor purification effects, scaling pollution and other problems. Therefore, from the standpoint of actual use, suitable alternative technologies must be developed. The future water purification technology should fulfill both low energy consumption, low cost, good stability, durability, easy preparation and portability [12]. For meeting the above conditions at the same time, evaporation using solar energy has become a key direction.

Solar energy is a renewable and clean energy source, and it is a sensible choice to develop the related technology. Currently, the application of solar energy includes technologies such as photovoltaic power generation [13], as well as solar water heaters that are already used in daily life [14], thus solar evaporation water purification technology has the potential to solve the shortage of fresh water resources. The solar photovoltaic evaporation process is basically similar to the water cycle process in nature. In the natural environment sunlight evaporates seawater and surface water into steam and then converts it in the form of rainwater, in the same way solar evaporators convert the energy of solar radiation into heat by evaporating seawater or wastewater in order to acquire clean freshwater resources [15]. The evaporation process in nature is carried out slowly, while the process of evaporation using solar evaporator is more rapid and efficient.

The phenomenon of solar-driven water evaporation should be traced back to 1872 [16], the traditional evaporation system in which the light-absorbing material is dispersed in the water or under the water, the sunlight needs to pass through the thick water surface and finally be absorbed by the photothermal material, which leads to the unsatisfied efficiency because solar energy needs to heat the overall water temperature before the evaporation process can occur. After reflection, convection and radiation, the heat energy actually applied to evaporate is greatly limitated, and the traditional solar evaporation system eventually leads to very low efficiency, so it was not widely used [17,18]. In 2014, due to the proposed concept of interface heating, solar evaporation technology has been rapidly enhanced and developed [19]. In this concept, the whole system is divided into three parts, which are the light absorbing layer, the heat insulation layer, and the water channel. The absorbing layer is responsible for absorbing sunlight and fixing the heat on the water surface for evaporation process; the heat insulation layer is responsible for isolating the heat diffusion from the absorbing layer to the water surface, which effectively reduces the energy loss; the water channel is responsible for sending water from the overall pump to the absorbing layer through capillary force. Compared with the traditional evaporation system, it greatly enhances the heat utilization rate and improves the photothermal conversion efficiency. Since then, more novel and innovative photothermal evaporation systems have been proposed (Fig. 1). Since 2014, the number of published research papers on “solar water evaporation” has been on the rise (Fig. 2a), with evaporation efficiencies reaching 80%, 90% or more in some papers, and even exceeding the theoretical maximum efficiency (100%) in some systems [20–22].

In this review, we present the latest research progress of solar evaporators by detailing micro- and nano-sized photothermal evaporation materials, common solar evaporator structural systems, the strategies to improve and enhance the thermal efficiency, and the practical applications of photothermal evaporation in numerous aspects. The outlook on the future development direction of evaporators is finally discussed.

thumbnail Fig. 1

Schematic of the solar evaporator material, structure and application.
thumbnail Fig. 2

(a) Number of relevant publications in recent years. By searching the Web of Science under the tag “solar evaporator” (b) solar radiation spectrum.

2 Solar evaporator materials

The most important component in the evaporator is the choice of photothermal materials. Good photothermal materials can effectively convert solar radiation into heat [23]. The wavelength range of solar radiation spectrum is in the range of 150–4000 nm (Fig. 2b), among which the wavelength less than 400 nm is the ultraviolet spectral region, which accounts for about 5% of the power; 400–760 nm range is the visible spectral region, which accounts for about 46% of the power; 700–2500 nm is the near infrared spectral region, which accounts for about 49% of the power [24,25]. Therefore, the photothermal material should capture the maximum amount of energy from the sun and further convert it into heat energy. At present, according to the different principles of photothermal conversion, photothermal materials are mainly divided into semiconductor materials [26–32], carbon-based materials [33–38], metal nanomaterials [39–44] and polymer materials [45–49], refer to Table 1.

Table 1

Summary of materials and performance parameters used for solar evaporators.

2.1 Semiconductor materials

The principle of photothermal conversion in semiconductor materials is intrinsic absorption. The absorption of sufficient photon energy by valence electrons generates photogenerated carriers, and heat is generated during the transfer of photogenerated carriers to the lattice. When a semiconductor material is illuminated with light, the electron-hole pairs produce energy similar to the band gap, and the valence electrons are converted to free electrons through the band gap, and energy is released through radiative relaxation in the form of photons or non-radiative relaxation in the form of phonons [50–52].

Semiconductor materials with narrow band gaps have the opportunity to be candidates for replacing precious metals based on their relatively low manufacturing cost and their inherent potential for large-scale synthesis. However, the stability and reusability of semiconductor materials have not been well studied in response to harsh real-world environments, thus sometimes resulting in research bias. Common semiconductor materials include copper, manganese, bismuth, and titanium, etc. Yang et al. made a solar evaporator by developing a copper-zinc-selenium-tin nanoparticle and depositing the synthesized product on a hydrophilic filter membrane [53]. In narrow band gap semiconductors, p-type CZTSe has proper energy level structure, high optical absorption coefficient, good electrical conductivity and chemical stability. The local surface plasmon resonance (LSPR) effect can absorb longer wavelengths due to hole density and nanostructure oscillations. The solar evaporation rate was 1.52 kg m−2 h−1 at one sun intensity, and it was demonstrated to have stable desalination evaporation performance under a long evaporation time of 30 days (Figs. 3a–3c). Zhang's team synthesized high purity MoS2 by synthesizing molybdenum disulfide film (DMM-SA), which is very difficult to synthesize as the 1T morphology of MoS2 in sub-stable state is highly susceptible to conversion to the stable 2H phase [54]. MoS2 nanosheets have good thermal and mechanical stability, and the pores and defects on the nanosheets have both ion-selective permeability and water transport, exhibiting an evaporation efficiency of 1.68 kg m−2 h−1 and a photothermal conversion efficiency of 83.8% at one solar intensity (Figs. 3d–3f). MXenes is a novel multifunctional 2D material, and Li's team has achieved significant results by designing a thermally driven bionic 2D nanocoatings, including Ti3C2T x MXene, reduced graphene oxide (rGO) and molybdenum disulfide (MoS2) [55]. Inspired by the hierarchical ridged texture of the Bittis rhinoceros enhanced light absorption by constructing a pleated texture with a photothermal conversion efficiency of 86.7%. Since the MXene nano-coating is underpinned by an elastic substrate, it is able to reversibly unfold and restore the folded structure for a scalable solar/electric dual heater for wearable thermal management (Figs. 3g–3i).

thumbnail Fig. 3

(a) TEM image of CZTSe. (b) Top-view images of the CZTSe assembled membrane. (c) Schematic illustration of the solar-driven interfacial water evaporation process. Reproduced with permission [53], Copyright 2019, Elsevier. (d)–(f) SEM images of the flat surface of the pristine 2H-MoS2 membrane, with the partially overlapping nanosheets forming nanochannels for molecule separation. Reproduced with permission [54], Copyright 2019, Elsevier. (g)–(h) SEM images and enlarged SEM images of G1 crumpled MXene structures with areal mass loadings of MXene at 0.32 mg cm−2. (i) Enlarged SEM image of hierarchical G3-2D2D2D MXene structures. Reproduced with permission [55], Copyright 2019, Wiley.

2.2 Carbon-based materials

Carbon-based materials have a rich diversity of isomers, from zero-dimensional to three-dimensional carbon-based structures, and at the same time have a high absorption capacity for near-infrared light. The principle of the photothermal effect of carbon-based materials can be summarized as the conjugation effect. In the π–π system of carbon-based materials, the π electrons in the bonding orbitals jump to the π anti-bonding orbitals, and when the electrons return to the ground state from the highly excited state, the electrons will release heat to achieve the photothermal conversion [56–58].

Carbon materials have good solar energy absorption ability and can be prepared into a variety of excellent structures. However, the stability of some structures in actual use needs to be improved. Some solar evaporators with carbon skeleton structure will have the problem of structure collapse during long time water absorption, and also the efficiency will be greatly reduced due to the accumulation of salinity in the process of desalination. Common carbon-based materials include graphene, carbon nanotubes, and nanoporous carbon, etc. Wang's group prepared a bilayer evaporator using a hydrophobic CNT film and a hydrophilic silica substrate, and the incident light was converted almost entirely to heat [59]. The bilayer material exhibited an evaporation rate of 1.29 m−2 h−1 and a photothermal conversion efficiency of 82% (Figs. 4a–4c). Mu's group prepared aerogels by preparing carbonized conjugated microporous polymeric hollow carbon nanotubes, which could achieve a light absorption rate of 99%, while the photothermal conversion efficiency could reach 86.8% under one solar intensity irradiation (Figs. 4d–4f) [60]. Kim's team obtained a porous graphite carbon by laser-induced method and compounded it with polymer foam for a bilayer evaporator [61]. The evaporator exhibited good desalination performance and could reach a salt retention rate of 99.9% and a photothermal conversion efficiency of 83.2% (Figs. 4g–4h). In addition to common carbon materials, Yang's group also tried to use Chinese ink atomic layers deposited on wood for photothermal evaporation [62]. Chinese ink is composed of amorphous carbon nanoparticles and graphite-like nanoflakes, which have excellent photothermal properties (Fig. 4i).

thumbnail Fig. 4

(a)–(c) SEM images of the CNT−silica bilayered material with CNT layer thickness of 5.6 µm, on top of the macroporous silica substrate with thickness of 0.3 mm. Reproduced with permission [59], Copyright 2016, American Chemical Society. (d) SEM images of CMPCA-1. (e) TEM images of CMPCA-1. (f) Camera photo of CMPCA-1 on a dog's tail grass. Reproduced with permission [60], Copyright 2019, Wiley. (g) Schematic illustrations of the fabrication procedures for the MBS integrating HPGC and a porous PI film. (h) SEM images of porous structures of the upper HPGC layer. Reproduced with permission [61], Copyright 2020, Elsevier. (i) Scheme of the fabrication process for ALD/Chinese-ink-coated materials. Reproduced with permission [62], Copyright 2019, Wiley.

2.3 Metallic nanomaterials

The principle of photothermal conversion by metal nanoparticles is local surface plasmon resonance (LSPR). When the frequency of the incident light matches the resonant frequency of the free electrons inside the metal nanoparticles, a strong extinction effect is produced and leads to plasmon resonance. When the energy of the photons is absorbed by the metal nanoparticles, it is mainly released in the form of heat [63–65].

Although plasma metals can show excellent photothermal conversion efficiency, they still have some drawbacks to be solved. For example, they are mostly made of precious metals, which are not only relatively complicated to prepare, but also easy to cause high cost budget for large-scale preparation. There are common metallic nanoparticles such as gold particles, as well as relatively inexpensive silver nanoparticles and copper nanoparticles. Zhang's team deposited carbon nanotubes and gold nanoparticles alternately on cotton fabrics [66]. While realizing the function of solar evaporation, active pollutants can be monitored by surface-enhanced Raman scattering (SERS). And by expanding the area of the photothermal fabric, the planar state was changed to a wave state, and the photothermal conversion rate was enhanced by expanding the absorption area with an optimized rate of 2.18 kg m−2 h−1. Copper-based materials are of great interest for photothermal conversion due to their plasma effect, but copper particles are susceptible to oxidation (Figs. 5a and 5b). Ren's team enhanced the photothermal conversion rate by wrapping copper nanoparticles in a layer of carbonized lucerne and graphene, a vapor rate of 1.54 kg m−2 h−1 at solar intensity with a conversion efficiency of 90.2% (Figs. 5c and 5d) [67]. Sun's team achieved an ultra-high vapor rate of 2.10 kg m−2 h−1 at one sun intensity through the synergistic effect of two layers of agarose gels modified with silver nanoparticles and polystyrene sodium sulfonate (PSS), while achieving a conversion efficiency of 92.8% (Figs. 5e and 5f) [68].

thumbnail Fig. 5

(a) SEM images of Au film transferred onto CNT-modified cotton cloth. (b) Sketch of the Au-CNTs-cloth hybrid on the water surface for SERS application. Reproduced with permission [66], Copyright 2019, Wiley. (c) HRTEM images of Cu@C/CLS. (d) Schematic illustration for the synthesis of a Cu@C/CLS structure. Reproduced with permission [67], Copyright 2021, American Chemical Society. (e) SEM image of PSS-AG sample. (f) High resolution TEM image of silver nanoparticles (Ag NPs) embedded in PSS-AG skeleton. Reproduced with permission [68], Copyright 2021, Wiley. (g)–(i) SEM images of top surface of rGO@PSf membrane,top surface of CB/rGO/PS@PSf membrane with various magnifications. Reproduced with permission [72], Copyright 2021, Elsevier.

2.4 Polymeric materials

The photothermal conversion mechanism of polymers is due to the release of heat from the excited electrons as they return from the lowest unoccupied molecular orbital (LUMO) to the highest occupied molecular orbital (HOMO). This principle is very similar to that of carbon-based materials [69–71].

Polymeric materials have the advantage of simple synthesis process and can be prepared with a variety of materials, but the subsequently effective treatment of polymers after using still need further research. The development of degradable and environmentally friendly polymer materials is one of the current research directions. Fan's team proposes to combine hydrophobic polysulfone (PSf) membrane substrates with reduced graphene oxide (rGO) and polystyrene (PS) microspheres [72]. Relying on the multiple scattering effect of the rGO/PS layer showed broadband light absorption with an evaporation rate of 1.06 kg m−2 h−1 and an evaporation efficiency of 69.58% at one solar intensity. By adding carbon black (CB) to the membrane and splitting the membrane into several pieces, the solar evaporation rate and efficiency were further improved up to 1.86 kg m−2 h−1 and 124.14%, respectively (Figs. 5g–5i).

2.5 Perspective of the current materials

The photothermal absorber layer is the most critical component in a solar evaporator, therefore the selection of appropriate photothermal materials is particularly important. Among the four types of photothermal materials mentioned above, metallic nanoparticles have the most rapid warming response after absorbed solar energy. Although the plasma resonance effect has an important impact on enhancing the photothermal conversion efficiency, metallic nanoparticles incur a higher budget in the preparation process, which is not quite in line with the original intention of solar evaporators to be cost effective in this respect, but metallic nanoparticles show a more suitable application in photothermal therapy. Semiconductor materials make up for the lack of metallic nanoparticles in terms of cost, but semiconductor materials are complex to synthesize in practice and do not perform well in terms of stability and reusability. Polymeric materials still need further research on the effective treatment of evaporator use afterwards, and environmental protection and degradability should be the focus of research. Polymeric materials are more widely used in medical applications because of their good biocompatibility. Carbon-based materials can absorb the full spectrum of solar wavelengths, inexpensive and diverse, and may be further enhanced in photothermal efficiency in the future if more structurally robust porous materials can be prepared to overcome the problems of structural collapse and stomatal blockage that occur during evaporation. Although there has been deep research on the selection of light-absorbing materials for solar evaporators, we still expect that we can find more efficient photothermal conversion materials in the future that combine low cost, simple process, green, stable as well as durable, and pollution-free subsequent processing.

3 Structure of solar evaporator

Conventional solar driven vapor generation is affected by the sunlight intensity, environment and other factors that limit its practical use. In order to improve the efficiency of photothermal conversion, there are three categories according to the location of solar light-absorbing materials in the water. When the solar light-absorbing material is dispersed in water, it is called a volumetric system [73–76]; when the solar light-absorbing material is placed on the surface of the water body, it is called an interfacial system [77–85]; and when the solar light-absorbing material is separated from the water body, it is called an isolation system [86–89].

3.1 Volumetric system

Volumetric system is a direct heating of liquid by nanostructured particles. One situation occurs when the nanoparticles are not evenly dispersed in water, the nanoparticles absorb solar energy and the temperature rises rapidly, resulting in rapid heating of the surrounding water and conversion into steam bubbles (“nano bubbles”); another situation occurs when the nanoparticles are evenly dispersed in water, the nanoparticles absorb solar energy, generating a large amount of micro-heat sources that increase the overall temperature of the water.

Halas' team demonstrated that nanoparticles in a solute can be used directly for solar vapor generation without heating the liquid to the boiling point (Figs. 6a–6b) [18]. Submicron particles dispersed in water can absorb light from the solar spectrum and generate vapor in a very short time. In thermodynamics it is shown that 80% of the sunlight is converted into water vapor. Since volumetric systems mainly lead to a large amount of water being heated, they are strongly influenced by thermal radiation, thermal convection and thermal conduction, which can also lead to a reduction in evaporation efficiency. Later Halas' team analyzed for the first time the evolution of nanoparticle-induced vapor and found that the vapor formation process starts with the formation of nanoscale vapor around individual nanoparticles, after which the vapor merges to form micron-sized bubbles (Fig. 6c) [90].

thumbnail Fig. 6

(a) Schematic of nanoparticle-enabled solar steam generation. (b) SiO2/Au nanoshells dispersed in water. Reproduced with permission [18], Copyright 2012, American Chemical Society. (c) Simulated heat-source density for an illuminated 100 nm diameter Au nanoparticle immersed in water. Reproduced with permission [90], Copyright 2013, American Chemical Society. (d) Schematic drawing of BHE. Reproduced with permission [91], Copyright 2020, Elsevier. (e) The image shows the mesostructure of the black wood which contains many channels in the wood and the NPs were coated on the surface of the wood. Reproduced with permission [92], Copyright 2018, Royal Society of Chemistry. (f) Illustration of the design route of the biomass-based solarsteam-generation device. Reproduced with permission [93], Copyright 2019, American Chemical Society. (g) The digital photograph of solar steam generator for measuring water evaporation rate. (h) Digital photographs of the solar steam generator at 600 h of continuous operation. Reproduced with permission [94], Copyright 2019, Royal Society of Chemistry. (i) Digital image of self-contained monolithic carbon sponge in sunlight. Reproduced with permission [95], Copyright 2018, Wiley.

3.2 Interface systems

In recent years, researchers have studied interfacial systems extensively. Since the location of the light-absorbing material is above the water surface, the heat generated by solar energy only needs to heat the water at the water-air interface to complete the evaporation process, thus increasing the conversion efficiency. With an interfacial system for evaporation, the overall conversion efficiency can reach over 90%. In the interfacial system, one case is usually using lightweight absorbers, graphical layers or three-dimensional aerogel and nano-foam structures on the surface of hydrophobic materials; in the other case, two or more layers of structures are prepared, with the light-absorbing material on the top layer responsible for absorbing sunlight and the material on the bottom layer responsible for moisture transfer and thermal insulation.

Liu's team developed a CuFeSe2 nanoparticle-modified semiconductor film for solar evaporation by using CuFeSe2 nanoparticles as a light-absorbing material with narrow forbidden band width and wood itself with multiple advantages such as porous [91], low density and low thermal conductivity, showing 86.2% vapor efficiency at 5 solar intensities (Fig. 6d). Zhang's team [92], inspired by ficus, used activated carbon cotton fabric as a photothermal layer, commercial polyester columns and expandable polyethylene foam as pillars to prepare a layered evaporator that exhibited an evaporation rate of 1.95 kg m−2 h−1 at one solar intensity, and the layered evaporator was able to increase the evaporation area while preventing heat loss to the water column (Fig. 6e). Inspired by the rice plant, Fang's team has developed an upper light-absorbing layer consisting of carbonized rice straw leaves and a lower stalk designed as a water transport channel, showing a conversion efficiency of 75.8% at a solar intensity of 1.2 kg m−2 h−1 [93]. This solar evaporator provides an emergency idea for survival in the wild (Fig. 6f).

3.3 Isolation system

Isolation system can further reduce the heat loss and achieve the separation of solar absorber and a large amount of water body in order to improve the photothermal conversion efficiency. In the current research, the isolation pathway is divided into two categories, one is with reference to the transpiration of trees, and water is absorbed through hydrophilic materials such as air laid paper or cotton thread for transmission; the other is to use materials such as sponges to fully absorb water at once, followed by in-situ evaporation and complete isolation from large amounts of water.

Xia's team designed a structure with a horizontal evaporation tray and vertical absorption lines [94]. By adjusting the salinity gradient, salt crystallizes to the edge of the evaporation tray, and then the salt crystals fall off automatically under the action of gravity and can be collected. Excellent evaporation performance and salt collection capability were demonstrated in the test after 600 h of continuous testing (Figs. 6g and 6h). Zhu's team reports an isolated thermal evaporation utilizing an inexpensive porous carbon sponge that increases the steam conversion efficiency by a factor of 2.5 by sequestering large amounts of water, confining water to permanent hot spots, which can eliminate the non-evaporative properties that limit evaporation efficiency Excessive heat loss from bulk water [95]. What's more, the sponge also showed induced potential (Fig. 6i).

3.4 Improving photothermal conversion efficiency through structure

In addition to the known solar evaporation system, enhancing the evaporation rate by changing the appearance shape of the evaporator has also been widely studied in recent years [96–98]. Wu's group also applied the spherical structure and constructed a self-rotating spherical evaporator with dual evaporation zones, which will rotate according to the weight of salt crystals, thus realizing two zones of high temperature and low temperature [99]. The advantage of the spherical evaporator is that it can maximize the advantage of the spherical evaporator. When the sunlight shines on the 2D evaporator at an angle, the evaporation rate will be reduced, this is improved in the 3D spherical evaporator, the evaporation rate will not change with the angle of light. Thanks to the double evaporation zone, a high evaporation rate of 2.6 kg m−2 h−1 is achieved (Figs. 7a–7c).

There are also natural structures that can increase the evaporation rate, and Xu's team found mushrooms with umbrella-like black caps [100], porous structures, and tiny fiber channels. These natural structures can concentrate the advantages of light absorption, water channels, steam channels and suppression of heat loss in the mushroom itself. Natural mushrooms exhibit a conversion efficiency of 62% at one sun intensity, and after carbonization, the photothermal conversion efficiency is increased to 78%. This is the first time that an organism's natural plant has been used for solar photothermal evaporation (Figs. 7d–7f).

Sun's group combined carbonized grapefruit peel, chitosan and bamboo fiber to make a sponge evaporator, where the carbonized peel is an excellent light-absorbing material and the bamboo fiber can form large pores in the chitosan [101]. This solar evaporator exhibited a high evaporation rate of 2.31 kg m−2 h−1 at one sun intensity, achieving a photothermal efficiency of 89.23%. This evaporator can not only be used in various harsh environments, but more importantly the used evaporator can be placed in the land to be reconverted to soil. This low-cost evaporator not only helps a lot to purify water, but also is making a great contribution to the environment (Figs. 7g–7i).

thumbnail Fig. 7

(a)–(c) Schematic illustration of the rotation mechanism of the photothermal sphere with salt deposition. Reproduced with permission [99], Copyright 2021, Wiley. (d) SEM images of the pileus of a mushroom after carbonization. (e) Schematic of a mushroom-based solar steam-generation device. (f) Physical picture of a shiitake mushroom and after carbonization. Reproduced with permission [100], Copyright 2020, Wiley. (g) SEM images of CS/BFs/CPPs sponge. (h) Schematic of the growth environment of seeds (wheat, lettuce, and string beans). (i) Three types of seeds germinated gradually over 7 days in CBC-BU. Reproduced with permission [101], Copyright 2021, Royal Society of Chemistry.

4 Improved heat utilization

In order to achieve high evaporation performance, it is important to absorb as much sunlight as possible while reducing heat loss in the form of radiation or convection.

4.1 Suppressing convection

Chen's group analyzed radiation and convection as the two main heat loss pathways in the evaporation system [102]. The data obtained from the calculations showed that the ambient radiation heat loss was 680 W m−2 and the convection heat loss was 800 W m−2. In order to reduce the heat loss caused by convection and radiation. They chose to cover the evaporator surface with a layer of bubble plastic, which is effective in reducing the convective heat loss even though this layer of plastic bubble will reduce the light absorption. At the same time, they chose insulation to separate the light absorbing layer from the water surface, suppressing radiation and heat conduction losses. In nature, the temperature in the valley folds is generally lower than that in the hill folds. This phenomenon arises mainly because heat will flow from the valleys to the hills, and the heat consumed by the valleys will be reabsorbed by the lower temperature mountains. Based on this principle, Wang's team has designed a 3D evaporation system with periodic origami folds that can achieve almost 100% evaporation efficiency [103].

4.2 Reducing the enthalpy of evaporation of water

The phase change of water to water vapor usually requires a lot of energy, and reducing the enthalpy of evaporation of water can effectively increase the evaporation rate. Yu's group prepared a PVA hydrogel for photothermal evaporation [104]. Since the hydrogel contains a large number of hydrophilic groups. These groups work with water molecules to produce water-bound small molecules, which results in the action of water on the hydrogel and reduces the enthalpy of water evaporation. The hydrogel formed by PVA and PPy reached a evaporation rate of 3.2 kg m−2 h−1, and in further studies, after adjusting the proportion of hydrophilic groups in the PVA, the evaporative water vapor was gradually reduced to 50% initial water, and finally reached a record evaporation rate of 3.6 kg m−2 h−1.

4.3 Additional energy sources

The heat converted by the material through the absorption of sunlight is relatively limited, so the evaporation rate can be increased by other energy sources. Qu's team prepared an evaporation using a graphene sponge as a light-absorbing layer and a graphene foil at the bottom, and the evaporation is carried out by solar power panels with photo-thermal synergistic electric heat [105]. Solar panels can assist evaporation on sunny days, and can continuously provide electric power at night to achieve all-weather evaporation.

The evaporation process is a natural process of absorbing heat, if the evaporative temperature is lower than the ambient temperature, the visible connector will absorb energy from the surface to evaporate. If the temperature of the evaporating surface is higher than the ambient temperature, the interface will release energy outward in the form of convection and radiation. Applying this feature, Li's group worked on a cylindrical evaporator coated with carbon black cellulose and a cotton core [106]. The top surface of the cylinder will absorb sunlight causing the temperature to rise, while the side walls of the cylinder will have a temperature lower than the ambient temperature because they do not absorb sunlight, at which point the side walls will absorb energy from the environment for evaporation, which will greatly increase the rate of evaporation.

5 Micro- and nano-sized materials for photothermal evaporation applications

As the types of photothermal materials continue to be developed and studied, people are applying such materials to solar energy distillation. Due to the energy sustainability of solar energy, micro and nano size based photothermal materials are used to desalinate seawater into clean water through the distillation process. In recent years, various emerging technologies and solar water evaporation technologies have been combined with each other, and their applications are not only limited to desalination. Photocatalysis, wastewater treatment, and hydropower co-generation applications have also been widely investigated. In this chapter, we will discuss in detail the challenges faced by solar thermal evaporation systems in various production aspects.

5.1 Desalination of seawater

Since the total amount of fresh water resources on earth is a small percentage of the total water resources, desalination of seawater into fresh water is one of the main ways to increase the supply of fresh water, and desalination technology driven by solar energy is the most widely researched method [107–110]. By designing a freshwater collection device outside the solar water evaporation unit, it is possible to condense the vapor into freshwater and collect it. The performance of evaporators in practical applications is limited by the ability of various materials to absorb sunlight, heat loss due to different structures, and other issues. Therefore, both improving the absorption of solar energy by the material and reducing the diffusion of heat into the water body can greatly improve the conversion efficiency.

While improving material absorption of solar energy and reducing energy losses can improve the efficiency of vapor generation, the design and material selection of salt crystal deposition and water collection devices are equally challenging for performance in desalination processes. During the desalination process, salt crystals or other impurities can be deposited on the surface or inside the evaporator, which can reduce the transport of water channels and reduce the cycling stability of the system. Factors such as robustness, transparency, low cost and collapsible shrinkage are ideal for the selection of water collection devices, while additional driving forces such as enhanced airflow in the cavity and reduced air pressure in the cavity can also improve the overall collection rate.

Liu's team has developed a self-driven salt-tolerant material by depositing black gold nanoparticles on a sponge [111]. Since the melamine resin sponge is highly absorbent, the salt solution concentration in the sponge never reaches saturation under the combined effect of gravity and capillary forces, and therefore no salt crystals precipitate out. This evaporator can be used continuously for more than 11 h without salt crystal precipitation even under 10 times sunlight intensity (Fig. 8a). Guo's team has prepared a sponge-like hydrogel that, through synergistic energy nano-limitation and water activation, reduces the enthalpy of evaporation of water and achieves an evaporation rate of 3.6 kg m−2 h−1 at one solar intensity [112]. It can effectively remove more than 99.99% of salt ions in seawater and achieve a good desalination effect (Figs. 8b and 8c).

thumbnail Fig. 8

(a) Temperature changes of the evaporation surface and bulk water. Reproduced with permission [111], Copyright 2019, Royal Society of Chemistry. (b) Photograph of a solar water distillation system based on LASH for salty water purification. (c) Measured concentration of four primary ions in seawater and the as-produced water after desalination. Reproduced with permission [112], Copyright 2019, American Chemical Society. (d) Scheme showing the possible photodegradation mechanism. Reproduced with permission [118], Copyright 2018, American Chemical Society. (e)–(f) Optical photographs of bacteria colonies in sewage and treated water. Reproduced with permission [119], Copyright 2019, Elsevier. (g) Schematic of the hybrid system for solar desalination and salinity power extraction. Reproduced with permission [124], Copyright 2017, Royal Society of Chemistry. (h)–(i) Schematic of the Condensation Processes of Interfacial Solar Steam Generation. Reproduced with permission [125], Copyright 2018, Elsevier.

5.2 Wastewater treatment

The photothermal effect of the micro and nano-sized materials can accelerate the evaporation process, which is still going on even when the temperature does not reach the boiling point. The water that has been condensed and recollected is often pure, while heavy metals, microorganisms or other contaminants remain in the remaining water. Since steam sterilization is one of the most widely used and reliable sterilization methods in medical treatment, the microorganisms in the water after distillation are basically eliminated and disinfection is achieved. Two different mechanisms are mainly reported in solar evaporation systems at this stage, one is the conventional evaporation and condensation function to treat non-volatile organic wastewater, and the other is the treatment of organic dyes, radioactive wastewater and oily wastewater by physical adsorption and photocatalysis [113–117].

Hao's team achieved both photothermal evaporation and photocatalysis by in situ polymerization of pyrrole on cotton followed by deposition of titanium dioxide nanoparticles with a solar evaporation rate of 1.55 kg m−2 h−1 at one sun intensity (Fig. 8d) [118]. Due to the separation of photogenerated electron-hole pairs and reduced charge complexation capable of enhancing Wilson's team made a floating solar evaporator by using a coated sponge made of candle ash, which showed good performance in the application of seawater and wastewater treatment [119]. Bacterial colony tests on the evaporated purified water showed no colony in the purified water, indicating the high purity of the purification (Figs. 8e and 8f).

Compared with the composition of seawater, wastewater is usually more acidic or alkaline, and the corresponding composition is more complex, so the development of more acid and alkali resistant micro and nano-sized photothermal materials is especially important to improve the stability of wastewater treatment. At the same time, the removal effect for volatile organic compounds with high concentration at this stage still needs to be improved.

5.3 Power generation

Solar energy can generate electricity through photovoltaic and photochemical processes. While the photothermal evaporation process will produce local high temperature and salinity differences, these phenomena provide research space for harvesting electricity. Combining the photothermal evaporation process and energy generation can simultaneously solve the shortage of fresh water resources and energy shortage of two problems plaguing global development [120–123].

Yang's team focused on the high concentration difference of brine under the interface after evaporation of water from the evaporator surface, and applied this feature to a system combining carbon nanotube-modified filter paper and Nafion membrane [124]. It was able to achieve a conversion efficiency of 75% at one solar intensity of evaporation while generating a point output power of 1 W m−2 (Fig. 8g). Li's team was able to achieve a conversion efficiency of 72.2% at one solar intensity while generating additional power with an efficiency of 1.23% by selecting a graphite/nonwoven film as the interfacial evaporator and by storing and recovering the vapor enthalpy during evaporation (Figs. 8h and 8i) [125].

6 Rationalization of water supply and prevention of salt crystallization

As it is well known, the solar evaporator which can realize the interface evaporation mainly depends on continuous transport of the water to the evaporation interface by hydrophilic medium. At present, numerous hydrophilic materials such as non-woven fabric, cotton, and wood, etc. can realize the continuity of water supply through capillary effect. Since the water pathway will diffuse the solar heat into the bulk water, constructing a suitable water pathway is necessary to reduce the heat loss. In addition, the salinity problem in seawater can seriously affect the solar evaporation rate. When the salinity is saturated, salt crystals will precipitate and block the evaporation pores, from which it is difficult for the vapor to escape into the air to constitute fresh water. The blocked pores will also affect the evaporator's ability to absorb light. Therefore, rationalization of water supply and prevention of salt crystallization play a key role in the evaporation rate.

6.1 Rationalization of water supply

The most direct way to improve the performance of solar evaporator is to avoid direct contact between heating zone and bulk water, which can significantly improve the overall evaporation performance by suppressing the heat loss caused by heat conduction. The advantage of the interface system that has been widely studied in recent years lies in the fact that by constructing a two-dimensional water pathway, heat conduction losses can be effectively reduced and heat loss from the local hot zone to a large amount of water can be effectively avoided. Another isolation system, by building a one-dimensional water path or completely isolate the evaporator from the body of water can also improve performance. In a one-dimensional water path, hydrophilic materials for example cotton thread in the capillary force can carry water from the bottom to the top continuously, and minimize heat transfer.

6.2 Anti-salt crystallization

Salt crystallization caused by high local salinity due to evaporation can be mitigated by reasonable construction of hydrophilic interface and proper water supply channels. When the salinity at the top is high, the water continuously delivered at the bottom will redilute the salinity at the top and rediffuse the salt back to the bulk water by horizontal flow. At the same time the large pores with low curvature enhance the convective exchange between different concentrations of salt solutions and avoid the occurrence of localized areas of salinity oversaturation. Another effective way to prevent salt crystallization is to prevent salt particles from being transported to the light-absorbing layer. To solve this problem, it is important to construct a hydrophobic surface. Hydrophobic modification of the light-absorbing surface can effectively prevent the absorption of brine and fundamentally avoid the formation and precipitation of salt.

7 Conclusion and outlook

In this review, we have reviewed in detail the progress of research on micro- and nano-sized solar absorbing materials and structural design of evaporation systems. The technology of solar evaporator is a relatively more promising approach to solve the water scarcity problem at present. It is a simple and efficient way to generate steam and condense it into pure water by using solar energy, a renewable and environmentally friendly resource, through the photothermal conversion principle. Although this technology has made many breakthroughs and has started to be applied gradually, it still faces many challenges in practical application.

Stability is the key to the long-term operation of solar evaporators in use. At present, as the environment slowly deteriorates, the composition of rivers and seawater becomes more and more complex and dangerous, which induces that the corrosion and consumption of sewage on the evaporator will accelerate. The effect of the evaporator damage includes the decrease of purification rate, the possible secondary pollution due to decomposition of the micro and nano size materials, the cost for repairing and maintenance of the evaporator. Subsequent research work should be more inclined to manufacture evaporators with better chemical and mechanical stability.

For the cost of evaporator, first of all, we should be clear that the real use scenario of solar evaporator should be poor, remote and some backward areas, which restricts the cost of evaporator must be low and does not need complex synthesis process to achieve. The cost should at least consider the material cost, process cost and maintenance cost, only when the cost of these three aspects meet the price is low enough, it can be really applied in real life on a large scale. The future research direction should pay more attention to the price of raw materials and the cost of production and preparation process.

In the real use process, the environment also has a very big influence on the evaporator. Evaporators under outdoor conditions are subject to a combination of wind speed, humidity, temperature, hours of sunlight, and air pressure. These conditions and study variables are difficult to be taken into account simultaneously in the laboratory, so the effect of different conditions on evaporation rate should be evaluated objectively in depth. The experimental parameters and performance evaluation conditions in the laboratory should also be standardized to obtain standard test results.

Current evaporators perform well for small-area applications, but large-area and enterprise-wide preparations are still difficult to achieve. While focusing on the development of new materials and structures, researchers should also focus on the realities of practical use and translate laboratory results to industry for commercial use.

Despite the challenges in advancing to practical applications, the inherent advantages of solar-powered evaporation make it a promising solution to the global water scarcity problem. Continued innovation and advancements in material selection and construction remain critical as we strive to take these technologies from laboratory scale to large scale practical applications.

This research was supported by grants from the National Natural Science Foundation of China (nos. 62004015 and 62004014), Department of Science and technology of Jilin Province (20210101077JC).

Author contribution statement

The writing of the manuscript was done by Jialun Li. The experimental protocols, the data analysis and the interpretation of the results were performed by Jialun Li, Fei Yu and Bin Cai. Liying Wang, Xijia Yang and Wei Lü contributed to the extensive revising of the manuscript. All authors discussed the results and commented the manuscript. The authors declare no competing financial interests.

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Cite this article as: Jialun Li, Fei Yu, Bin Cai, Liying Wang, Xijia Yang, Wei Lü, Micro- and nano-sized materials for solar evaporators: a review, Eur. Phys. J. Appl. Phys. 97, 84 (2022)

All Tables

Table 1

Summary of materials and performance parameters used for solar evaporators.

In the text

All Figures

thumbnail Fig. 1

Schematic of the solar evaporator material, structure and application.
In the text
thumbnail Fig. 2

(a) Number of relevant publications in recent years. By searching the Web of Science under the tag “solar evaporator” (b) solar radiation spectrum.
In the text
thumbnail Fig. 3

(a) TEM image of CZTSe. (b) Top-view images of the CZTSe assembled membrane. (c) Schematic illustration of the solar-driven interfacial water evaporation process. Reproduced with permission [53], Copyright 2019, Elsevier. (d)–(f) SEM images of the flat surface of the pristine 2H-MoS2 membrane, with the partially overlapping nanosheets forming nanochannels for molecule separation. Reproduced with permission [54], Copyright 2019, Elsevier. (g)–(h) SEM images and enlarged SEM images of G1 crumpled MXene structures with areal mass loadings of MXene at 0.32 mg cm−2. (i) Enlarged SEM image of hierarchical G3-2D2D2D MXene structures. Reproduced with permission [55], Copyright 2019, Wiley.
In the text
thumbnail Fig. 4

(a)–(c) SEM images of the CNT−silica bilayered material with CNT layer thickness of 5.6 µm, on top of the macroporous silica substrate with thickness of 0.3 mm. Reproduced with permission [59], Copyright 2016, American Chemical Society. (d) SEM images of CMPCA-1. (e) TEM images of CMPCA-1. (f) Camera photo of CMPCA-1 on a dog's tail grass. Reproduced with permission [60], Copyright 2019, Wiley. (g) Schematic illustrations of the fabrication procedures for the MBS integrating HPGC and a porous PI film. (h) SEM images of porous structures of the upper HPGC layer. Reproduced with permission [61], Copyright 2020, Elsevier. (i) Scheme of the fabrication process for ALD/Chinese-ink-coated materials. Reproduced with permission [62], Copyright 2019, Wiley.
In the text
thumbnail Fig. 5

(a) SEM images of Au film transferred onto CNT-modified cotton cloth. (b) Sketch of the Au-CNTs-cloth hybrid on the water surface for SERS application. Reproduced with permission [66], Copyright 2019, Wiley. (c) HRTEM images of Cu@C/CLS. (d) Schematic illustration for the synthesis of a Cu@C/CLS structure. Reproduced with permission [67], Copyright 2021, American Chemical Society. (e) SEM image of PSS-AG sample. (f) High resolution TEM image of silver nanoparticles (Ag NPs) embedded in PSS-AG skeleton. Reproduced with permission [68], Copyright 2021, Wiley. (g)–(i) SEM images of top surface of rGO@PSf membrane,top surface of CB/rGO/PS@PSf membrane with various magnifications. Reproduced with permission [72], Copyright 2021, Elsevier.
In the text
thumbnail Fig. 6

(a) Schematic of nanoparticle-enabled solar steam generation. (b) SiO2/Au nanoshells dispersed in water. Reproduced with permission [18], Copyright 2012, American Chemical Society. (c) Simulated heat-source density for an illuminated 100 nm diameter Au nanoparticle immersed in water. Reproduced with permission [90], Copyright 2013, American Chemical Society. (d) Schematic drawing of BHE. Reproduced with permission [91], Copyright 2020, Elsevier. (e) The image shows the mesostructure of the black wood which contains many channels in the wood and the NPs were coated on the surface of the wood. Reproduced with permission [92], Copyright 2018, Royal Society of Chemistry. (f) Illustration of the design route of the biomass-based solarsteam-generation device. Reproduced with permission [93], Copyright 2019, American Chemical Society. (g) The digital photograph of solar steam generator for measuring water evaporation rate. (h) Digital photographs of the solar steam generator at 600 h of continuous operation. Reproduced with permission [94], Copyright 2019, Royal Society of Chemistry. (i) Digital image of self-contained monolithic carbon sponge in sunlight. Reproduced with permission [95], Copyright 2018, Wiley.
In the text
thumbnail Fig. 7

(a)–(c) Schematic illustration of the rotation mechanism of the photothermal sphere with salt deposition. Reproduced with permission [99], Copyright 2021, Wiley. (d) SEM images of the pileus of a mushroom after carbonization. (e) Schematic of a mushroom-based solar steam-generation device. (f) Physical picture of a shiitake mushroom and after carbonization. Reproduced with permission [100], Copyright 2020, Wiley. (g) SEM images of CS/BFs/CPPs sponge. (h) Schematic of the growth environment of seeds (wheat, lettuce, and string beans). (i) Three types of seeds germinated gradually over 7 days in CBC-BU. Reproduced with permission [101], Copyright 2021, Royal Society of Chemistry.
In the text
thumbnail Fig. 8

(a) Temperature changes of the evaporation surface and bulk water. Reproduced with permission [111], Copyright 2019, Royal Society of Chemistry. (b) Photograph of a solar water distillation system based on LASH for salty water purification. (c) Measured concentration of four primary ions in seawater and the as-produced water after desalination. Reproduced with permission [112], Copyright 2019, American Chemical Society. (d) Scheme showing the possible photodegradation mechanism. Reproduced with permission [118], Copyright 2018, American Chemical Society. (e)–(f) Optical photographs of bacteria colonies in sewage and treated water. Reproduced with permission [119], Copyright 2019, Elsevier. (g) Schematic of the hybrid system for solar desalination and salinity power extraction. Reproduced with permission [124], Copyright 2017, Royal Society of Chemistry. (h)–(i) Schematic of the Condensation Processes of Interfacial Solar Steam Generation. Reproduced with permission [125], Copyright 2018, Elsevier.
In the text

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