The promising solar‐powered water purification based on graphene functional architectures

Abstract

Solar‐powered water purification is able to gain freshwater from nonedible water by harnessing inexhaustible and pollution‐free sunlight energy, which is undergoing booming development to address the global water shortage. Due to the fascinating properties, numerous graphene‐based materials were devoted to the solar‐powered system from interfacial solar‐steam generation, towards solar pollutants degradation and even atmospheric water harvesting. Therefore, manipulations on graphene including chemical regulation and structural engineering were first introduced in this review, followed by the recent advancements in three main aspects of graphene‐based solar‐powered water purification concerning rapid steam generation, efficient pollutants degradation and significant atmospheric water harvesting. And the relationships between the designs of the materials with water purification performances were discussed in detail. Finally, a summary of challenges as well as the corresponding possible solutions were also suggested, aiming to provide scientific and technical inspirations to the flourishing solar‐powered water purification based on graphene functional architectures

INTRODUCTION

Water scarcity is a worldwide severe issue for human beings, and over 4 billion people are facing freshwater shortage.¹ Numerous techniques including thermal desalination,² reverse osmosis,³ electrodialysis,⁴ electrocapacitive deionization,⁵ nanofiltration,⁶ membrane distillation,⁷ and so on have been developed to obtain freshwater. For example, reverse osmosis is to use partially permeable membrane to separate ions and water molecules. By applying high pressure on seawater at one side of the membrane, freshwater can be acquired on the other side. Likewise, electrocapacitive deionization makes use of the opposite movements of positive and negative ions in an applied electric field. When the wastewater passes through the capacitor, ions will be collected in the electrodes and deionized water could be obtained. However, the huge energy consumption on fossil energy for driving above mentioned systems as well as lots of induced wastes (e.g., the deteriorated membranes and the effluents) may cause further environmental contamination and global warming.⁸ Access to fresh water in a sustainable way has been facing great challenges for a long time.

Solar-powered water purification systems for freshwater production have attracted much attention because sunlight is one of the renewable and clean energy resources which could be thought inexhaustible.² The substitution of traditional energy consumption by solar energy is a promising solution for the freshwater crisis, especially for rural and remote areas where generally the acquisition of sunlight is much easier than the large-scale power grid. In addition to traditional solar thermal distillation and photovoltaic reverse osmosis strategies of unsatisfied sunlight utilization (<20%),⁹ the exploitation of advanced materials for more effective solar water treatment with various features has been conducted for a long time. Especially, because of the high specific surface area, great mechanical properties, easily-regulated structures, and diverse functionalizations, graphene-based architectures are employed in many novel solar-powered water purification systems. For example, compared with the traditional solar steam generators, the design of uniformly vertical nanostructures on graphene film and interfacial solar-steam generation system has extremely improved the light absorbance up to 98% and the solar-steam efficiency greater than 90%.¹⁰ The graphene and TiO₂ nanoparticles' composites present great water pollutes photodegradation performance because of the proper combination of the photodegradation ability from TiO₂ nanoparticles and the excellent adsorption ability from graphene sheets.¹¹ Additionally, the modification of porous graphene foam with polyacrylate (PAA) demonstrates remarkable water absorption and outstanding solar-thermal conversion ability, which makes the quantities of water harvested from air available. Twenty-five liters clean water could be acquired under the multiple alternations between air water adsorption and solar-thermal evaporation on a 1 kg graphene/PAA foam.¹² Above all, graphene-based architectures have delivered many cutting-edge solar-powered water purification systems from efficient freshwater collection to pollutants treatment. Therefore, in this review, we will mainly summarize and discuss the recent advancement in the promising graphene-based solar water purification systems from the chemical regulation and structural engineering on graphene towards interfacial solar-steam generation, solar pollutants degradation and solar atmospheric water harvesting, which could bring new significance to the exploration of advanced materials and the development of solar-powered applications.

CHEMICAL REGULATION AND STRUCTURAL ENGINEERING ON GRAPHENE

Graphene owns many excellent theoretical properties concerning high thermal conductivity (5300 W m⁻¹ K⁻¹),¹³ high carrier mobility(2.5 × 10⁵ cm² V⁻¹ s⁻¹),¹⁴ great mechanical properties,¹⁵ and extremely high specific surface area (2600 m² g⁻¹),¹⁶ which all make it a promising candidate for advanced solar water purification systems. However, the light absorption of one-layered graphene sheet is extremely low, approximately 97.7% light can pass through it,¹⁷ making it inappropriate for solar thermal conversion. To this extent, rational chemical regulation and structural engineering on graphene is of great importance for practical applications in solar-powered systems.

Chemical regulation on graphene can introduce different elements or functional groups in the honeycomb carbon skeleton, leading to the spectral peak shift and the enhancement of light absorption. Meanwhile, numerous functional groups dandling on the graphene allow the composition with other functional materials possible. Different elements such as O,¹⁸ N,¹⁹ S,²⁰ B,²¹ P,²² and so on can be doped in the graphene through chemical oxidation, hydrothermal process or chemical vapor deposition (CVD) method (Figure 1A). The oxidation or doping breaks the conjugated π-bond, leading to the blue shift of the absorption band of graphene, thus enhancing the visible light absorption. At the same time, the doped atoms in graphene sheets could introduce many active sites enabling the efficient adsorption for ions,²³ and could also change the band structure, endowing graphene with good performance in photodegradation.¹¹ Meanwhile, the numerous functional groups offer great convenient connections to other functional materials, including inorganic nanoparticles, polymers, and so on. For example, the SiO₂ nanoparticles could be deposited on graphene oxide through electrostatic interactions,²⁴ while polystyrene and graphene could be uniformly combined via van de waals interactions, entanglement and π-π interactions.¹⁶ In this way, the chemically regulated graphene could play a pivotal part in the composite materials for solar-powered systems, including enhanced light absorption, anti-aggregation, more easily electron–hole separation, adsorption of effluents, and so on.

FIGURE 1. Chemical regulation and structural engineering of graphene. Chemical regulation: schematic diagrams of (A) graphene sheet doped with different elements, (B) graphene sheets composited with nanoparticles, and (C) graphene sheets composited with polymers. Structural engineering: (D) schematic diagrams of graphene architectures, including graphene foams, graphene membranes, and graphene fibers. (E) the optical image of the macroscopic ordered pillar array structure of graphene membrane. Reproduced with permission: Copyright 2018, Royal Society of Chemistry.40 (F) The microscopic ordered inner structure of graphene architectures. Reproduced with permission: Copyright 2017, American Chemical Society10

Structural engineering includes assembling graphene sheets into specific macro-assemblies and constructing highly ordered architectures. The light loss on one-layered graphene sheet mainly comes from the transmission (97.7%). Theoretically, hundreds of graphene sheets assembled together could absorb almost all the incident light with no transmission. Therefore, the studies of graphene assemblies have attracted much attention in the past few years. For example, as shown in Figure 1D, three-dimensional (3D) graphene foams with massive channels and porous structures can be obtained from graphene oxide dispersion by hydrothermal methods,^25,26 template-assisted reduction assembly,^27–29 freeze-drying process,³⁰ in situ growth on 3D metal templates,³¹ and even natural drying.^32,33 Two-dimensional (2D) graphene films can be synthesized via filtration,^34,35 evaporation induced self-assembly method,³⁶ and so on. One-dimensional (1D) graphene fibers could be fabricated through wet spinning,³⁷ or electrospinning process.³⁸ Meanwhile, the construction of order architectures on graphene assemblies like vertically aligned graphene sheets is necessary to form multiple inner reflections of light, which could be constructed via CVD,³⁹ directional freeze-drying method,¹⁰ laser ablation,⁴⁰ 3D-printing process,³¹ and so on. All of these macroscopic graphene assemblies made of a huge amount of graphene sheets ensure that the transmission of incident light is prohibited and the utilization is extremely enlarged.

Based on the rational chemical regulation on the graphene sheets and proper design of graphene assemblies, the light absorption could be increased up to 98%, which is higher than most of the nanoparticle-based and semiconductor-based photothermal materials.^9,41 Meanwhile, compared with other materials (MXene,⁴² hydrogels,⁴³ etc.) with favorable light absorption, graphene has excellent stability, great mechanical properties, easily-regulated structures and diverse functionalities as discussed above, which all make graphene-based functional materials suitable for solar water purification.

INTERFACIAL SOLAR-STEAM GENERATION

Compared with traditional thermal distillation, solar desalination harnesses sunlight to heat the saline water to water vapor, and fresh water can be acquired after condensation. In spite of the long history of research on solar desalination, how to realize fast evaporation is a question of fundamental importance. Recently, the interfacial solar-steam generation system has been developed, which greatly improved the solar-steam efficiency (energy needed to convert liquid water to water vapor divided by all input solar power) from about 24% to nearly 100% for single-stage evaporation.⁹ As shown in Figure 2A, the interfacial solar-steam generation system is to localize the light induced heat on steam generation materials, thus decreasing the heat losses towards the bulk water.⁴⁴ Owing to excellent properties concerning high light absorption, large specific surface area, adjustable heat conductivity, and tunable hydrophilicity, graphene architectures have been proved to be excellent for interfacial solar-steam generation.^43,45–47

FIGURE 2. Schematic diagram of graphene-based ISSG, and the corresponding architectural designs for evaporation enhancement. (A) The schematic diagram of the graphene based ISSG system and the factors affecting the evaporation performance. (B–E) Engineering of graphene absorbers to enhance sunlight absorption. (B) vertical graphene sheets anchored on the graphene framework, the inset scale bar is of 200 nm. Reproduced with permission: Copyright 2017, John Wiley and Sons.39 (C) The vertically aligned graphene foam. Reproduced with permission: Copyright 2017, American Chemical Society.10 (D) Vertically aligned graphene pillars. Reproduced with permission: Copyright 2018, Royal Society of Chemistry.40 (E) Macroscopic cone shape graphene foam for omnidirectional light absorption. Reproduced with permission: Copyright 2017, Oxford University Press.50 (F–I) Engineering of the thermal insulator and water paths to reduce the heat losses. (F) Two dimensional water path. Reproduced with permission: Copyright 2017, PNAS.54 (G) One dimensional water path. Reproduced with permission: Copyright 2017, Elsevier.55 (H) Fabrication of the hydrophilicity of graphene aerogels. Reproduced with permission: Copyright 2017, American Chemical Society.56 (I) Controlling the water content by the injection control system. Reproduced with permission: Copyright 2019, John Wiley and Sons.57 (J, K) Synergistic effects to improve water yield. (J) Use of photothermal effect. Reproduced with permission: Copyright 2017, John Wiley and Sons.59 (K) Use of chemical phase change effect. Reproduced with permission: Copyright 2019, Springer Nature.61 (L, M) Factors to support practical applications. (L) Combined with polymers to increase reusable ability. Reproduced with permission: Copyright 2017, Elsevier.68 (M) Adapted to manufacture via 3D printing technique. Reproduced with permission: Copyright 2017, John Wiley and Sons69

Dating back to 2014, Ghasemi⁴⁸ and coworkers introduced exfoliated graphite as an interfacial solar-steam generator (ISSG), in which evaporation rate is approximately 1 kg m⁻² h⁻¹ under the sunlight irradiation of 1 kW m⁻² (i.e., one sun). Then, by using CVD method, Ito⁴⁹ and coworkers constructed a 3D graphene ISSG. The 3D graphene ISSG is grown on the Ni foam template, which has a porous structure and large specific surface area. Based on this ISSG, the evaporation rate was raised up to 1.5 kg m⁻² h⁻¹, and the solar-steam efficiency is up to 80%. However, due to the lack of rational and systematic structure design, lots of energy losses caused the unsatisfied evaporation rate. Therefore, many works have been further conducted to enhance the solar absorption, heat management and multi-factors coordination for graphene-based ISSG to improve the solar evaporation performance.

One of the most important prerequisites for an ISSG is high solar absorption. Although graphene assemblies have a broad absorption band and high absorbance, there are still some inevitable light losses such as reflection and the direction changing of sunlight irradiation. To deal with these issues, a hierarchical graphene foam was developed via a plasma-enhanced CVD method.³⁹ As shown in Figure 2B, compared with the former graphene foam ISSG, a secondary structure was introduced onto the framework, where the graphene sheets at the surface tend to stand vertically other than attaching to the framework. Due to the special structural design, when the light comes into the ISSG, it will be trapped and reflected between the graphene sheets several times until being fully absorbed. Thus, the solar-thermal efficiency could be increased to 93.4%, and the solar-steam efficiency was up to 90%. Apart from the modification of graphene sheets, another strategy is to control the framework of graphene assembly. a vertically aligned graphene sheets membrane was constructed via using freeze casting and freeze-drying method, orderly channels were constructed inside the graphene membrane (Figure 2C).¹⁰ The channels in the ISSG are densely distributed from the top view and have a long length from the side view, thus endowing it great light absorbance with low reflectance. When the light comes to the surface of the ISSG, it will not only run into the densely distributed inlets of channels but also keep reflecting between the side walls until being fully absorbed by the ISSG. Additionally, the long-range channels in the ISSG could also work as the water path for evaporation through capillary force to ensure adequate water supply. Benefiting from the rational design of graphene architecture, the solar-thermal efficiency is up to 94.2%, and a high evaporation rate of up to 1.62 kg m⁻² h⁻¹ was achieved. In addition, to address the continuous direction changing of the sunlight induced the inadequate absorption, a well-defined hierarchical 3D graphene solar ISSG was presented.⁴⁰ The porous graphene aerogel was shaped in a vertically ordered pillars array (Figure 2D) using laser ablation, which owns not only high light absorption at the microscale but also omnidirectional absorption at the macroscale. As proved by Infrared images, there are only nuances of the temperatures of the ISSG surfaces with different incident light angles, which manifested the fact that the hierarchical architecture design is a powerful way to enhance light absorption. To a more macroscopic view, inspired by natural plants, a cone-shaped graphene ISSG was designed to absorb sunlight from all directions (Figure 2E).⁵⁰ Because the projections of the cone at different angles are similar, the sunlight from different directions has few influences on the net sunlight illumination. Therefore, the tree-like graphene ISSG owns high light absorption and large surface area at the same time, thus the solar-steam efficiency was boosted to 85% at different incident light angles.

Another significant factor is heat management,^51,52 because much heat could be lost during heat transfer from the solar absorber to bulk water. It's well-known that pure graphene has high heat conductivity, thus making it essential to devise rational architectures with excellent heat isolation. For example, the reduced graphene oxide foam was combined with bacterial nanocellulose to cut down heat losses.⁵³ Benefiting from the low thermal conductivity and hydrophilicity of bacterial nanocellulose, heat and finite water could be localized in a small area beneath the reduced graphene oxide layer, thus high solar-steam efficiency could be achieved. Rationally controlling the contact interface between raw water and ISSG is another efficient way to reduce heat loss. Polystyrene foam was employed as an isolation layer for the graphene oxide absorber and a 2D water path was enabled by a thin layer of cellulose fiber wrapped over the polystyrene foam (Figure 2F).⁵⁴ The direct contact between graphene oxide absorber and raw water as well as the heat loss was largely restricted because the water pathway was replaced. Further on, 1D vertically graphene oxide pillars were constructed through 3D printing method as efficient water channels(Figure 2G).⁵⁵ Energy loss was reduced because of the decline of the amount of water contained in the water paths. In this way, the solar-steam efficiency could be up to 87.5%. Owing to the demand for higher sunlight absorption, the ISSGs are designed from a 2D membrane to a 3D porous sponge, resulting in more water contained in the body of the ISSGs. Therefore, only controlling the water amount in water paths is not enough. To reduce water content in the ISSG, investigations on the effect of hydrophilicity of graphene foams were conducted.⁵⁶ The results showed that hydrophilic surface tends to have a thinner water layer than the hydrophobic one (Figure 2H). By controlling the hydrophilicity rationally, adequate water supply and heat localization could be achieved at the same time. The overall solar-to-water efficiency was increased from 38% to 48%. Although the hydrophilic surface could contain less water, there is much bulk water existing in the graphene sponge network because of capillary force. In a recent study, an injection control technique was introduced trying to solve this problem (Figure 2I).⁵⁷ With precisely controlling the supply water amount by peristaltic pump to ensure that the evaporation rate equals to the supply rate, there are much fewer pores blocked during evaporation, thus enlarging the specific surface area as well as reducing the heat loss. By using the controllable water input strategy with injection technique, the solar-steam efficiency was boosted up to 100%, and it's also a general method for nearly all evaporation systems.

Besides the conventional design of solar evaporation, many additional synergistic effects were explored to improve the evaporation rate further.^58,59 As depicted in Figure 2J, the photovoltaic effect was combined with solar-thermal effect, a solar cell was attached to the system to store solar energy and convert it to Joule heat.⁶⁰ Taking advantage of the Joule heating effect of graphene sponge as well as the excellent evaporation performance, the unique graphene ISSG reached a record high water production rate of 2.01–2.61 kg m⁻² h⁻¹. Apart from the assistance of a solar cell, the chemical phase change effect can also contribute to the evaporation performance.⁶¹ A graphene composite ISSG was established by combining a graphene sponge with poly (N-iso-propyl acrylamide) hydrogel (PN), which exhibits phase change ability when being heated hotter than the lower critical solution temperature (LCST) (Figure 2H). Benefiting from this, fresh water could be obtained both from the evaporation process and guttation of the hydrogel. The overall freshwater acquisition rate could be boosted to 4.2 kg m⁻² h⁻¹.

Although much progress has been made to the evaporation performance enhancement, many problems are remaining before practical application including the durability of graphene architectures,^62,63 the difficulty of the operation,^64,65 and the cost of evaporation system,^66,67 etc. As for the durability of graphene-based ISSGs, the mechanical properties of pure graphene assemblies are not good enough to sustain during multiple recycling desalination. The graphene-polymer composite may be a good choice, by using high-strength polymers as additives, a reusable graphene oxide based ISSG was established.⁶⁸ As shown in Figure 2K, polyethylenimine and polyurethane were combined with graphene sheets, with hardly sacrificing evaporation performance of composite ISSG, the cyclic utilization more than 15 times was achieved. While multiple utilization is one of the solutions, reducing the difficulty of manufacturing may be another option. Li and coworkers⁶⁹ introduced the 3D-printing method to graphene ISSG construction, and an all-in-one ISSG was established with rational design (Figure 2L). All the absorber, water path, and substrate were constructed by 3D printing. Through easy configuration, it can be directly used for desalination, and the solar-steam efficiency of 85.6% under one sun illumination was achieved.

Above all, thanks to the adjustable structures and excellent properties of graphene, many great processes were made in the field of interfacial solar steam generation, concerning the enhancement of light absorption, the regulation of heat losses, the employment of synergistic effects, and the improvements in practical applications.

SOLAR POLLUTANTS DEGRADATION

Traditional distillation could effectively collect freshwater which is important for human beings. However, the pollutants remaining in the effluent may cause environmental deterioration. As shown in Figure 3A, graphene-based photodegradation system is another promising alternative to address residual effluents, which could break down the contaminants in water only using sunlight. The selectively doped graphene demonstrated good photodegradation rate itself.^70–72 Besides, by compositing with catalysts, graphene-based functional systems have shown excellent degradation performance on organic compounds (e.g., methylene orange,^73,74 methylene blue,^75,76 bisphenol A,⁷⁷ rhodamine B,⁷⁸ 2,4-dichlorophenol⁷⁹) and the reduction of high valent metal ions owing to the high carrier mobility, large specific area, high adsorptivity and so on.

FIGURE 3. Schematic diagram of graphene-based photocatalytic degradation system and structural designs to enhance catalytic performance. (A) the schematic diagram of graphene-based photocatalytic degradation system. (B) Transmission electron microscopy (TEM) image of graphene supported photocatalysts, and (C) the absorption spectrum with different graphene content. Reproduced with permission: Copyright 2011, Elsevier.95 (D) Extending light absorption range through the introduction of n-type graphene oxide as photosensitizer, and (E) the transient photocurrents of composite photocatalyst under visible light. Reproduced with permission: Copyright 2010, American Chemical Society.100 (F) Tertiary composite photocatalyst using graphene as a Z-scheme substate to broaden the absorption edge, and (G) the absorption spectrum compared with single components. Reproduced with permission: Copyright 2019, Elsevier.106 (H) Endowing photocatalysts with reusability via magnetic control. Reproduced with permission: Copyright 2011, American Chemical Society.107 (I) Recycling the graphene-based photocatalytic aerogel just using tweezers. Reproduced with permission: Copyright 2013, American Chemical Society108

In 2009, Zhang¹¹ and coworkers reported a composite photocatalyst by loading TiO₂ nanoparticles (P25) on graphene, which demonstrated significant photodegradation of methylene blue (MB) both under UV and visible light. Over 85% MB was eliminated by P25-graphene photocatalyst within 60 min, which is about 3.5 times faster than that of the bare P25 nanoparticles. The enhancement of photodegradation performance induced by graphene substrate inspired many research works to further illustrate the underlying mechanisms. The proposed mechanisms could be summarized as follows: Firstly, due to π-π interaction, the adsorption of pollutants was increased after the introduction of graphene, which ensures the adequate supply of pollutants to catalysts. Secondly, the interaction between TiO₂ and graphene changed the electronic band structure, thus extending light adsorption. Thirdly, the charge separation was enhanced resulting from the favorable electron transportation from TiO₂ to graphene sheets after excitation.^80–82 Besides, the enhanced mass transfer may be another reason,⁸³ compared to the dense agglomeration of bare TiO₂, the pores and channels constructed by the graphene substrate could facilitate rapid pollutants diffusion from effluent to catalytic active sites. Meanwhile, the anti-aggregating effect of graphene frameworks is another important factor, which was confirmed in the system of graphene/MXene composite photocatalyst.⁸⁴ Due to the steric hindrance of graphene, the sheets of MXene could not effectively contact each other, thus avoiding the irreversible stacking and ensuring the maintenance of photodegradation performance. Additionally, the adjustable functional groups and sheet size may also have influences on the photodegradation process. It's reported that graphene with oxygen functional groups could adsorb oxygen and convert it to hydroxyl radicals which further enhances the degradation rate.⁸⁵ Besides, when the size is confined into quantum dots by ultrasonication, graphene sheet was discovered to show an excitation-independent down-conversion and up-conversion behavior which may also contribute to the improvement of photodegradation performance.⁸⁶ Therefore, it is understandable that the graphene composite photocatalysts show excellent photodegradation performance, while there are still some issues remaining to further increase the photodegradation performance, e.g., the low absorption edge leads to unsatisfied utilization of visible light and the powder-like photocatalysts are hard to operate during practical utilization.

To broaden the absorption edge, the photothermal effect of graphene was employed to make use of visible light. It's reported that the temperature increasement caused by photothermal effect from graphene contributed to the enhancement of photodegradation rate, which accounts for 38%.⁸⁷ Another solution is to use semiconductors with a narrower bandgap. Many other kinds of semiconductors are deposited onto the graphene such as SnO₂,⁸⁸ AgX,^89,90 InNbO₄,⁹¹ Ag₃PO₄,⁹² ZnO,^93,94 Bi₂WO₆,^95,96 CuS,⁹⁷ ZnS,⁹⁸ CdS,⁹⁹ etc. For example, γ-Bi₂WO₆ deposited graphene composite showed excellent photodegradation performance under visible light (Figure 3B,C), after 2 h reaction, over 80% MB was degraded.⁹⁵ Further, many efforts have been made on the construction of the heterojunction in photocatalysts. By tuning the chemical state of graphene oxide, the graphene substrate became a p-type semiconductor, and the p/n heterojunction was constructed by combining with TiO₂. The p-type graphene oxide was confirmed to act as a sensitizer, which enhanced the visible light photocatalytic performance (Figure 3D,E).¹⁰⁰ Inspired by the heterojunction design, many binary composite catalytic systems were developed including C₃N₄-TiO₂,¹⁰¹ C₃N₄-Cu₂O,¹⁰² CeSO₄-BiVO₄,¹⁰³ MoS₂-TiO₂,¹⁰⁴ Ag/AgBr-TiO₂,¹⁰⁵ C₃N₄-Ag/AgBr,¹⁰⁶ etc. For example, a Z-scheme Ag/AgBr/C₃N₄ graphene composite photocatalyst was developed, in which AgBr and C₃N₄ both adsorb visible light generating electron–hole pairs (Figure 3F,G).¹⁰⁶ The graphene substrate between two components acts as a linker to effectively transfer the generated charges. Despite the narrow bandgap of the above two semiconductors, the potential of the conduction band of C₃N₄ is low enough to reduce oxygen to form peroxy radicals, while the potential of the valence band of AgBr is high enough to oxidate hydroxide to form hydroxyl radicals. Therefore, the rational combination led to a satisfying photodegradation rate under visible light, where 96% methylene orange was degraded within 30 min and approximately 6 log inactivation of Escherichia coli within 60 min.

In addition, as the photocatalyst is in powder form, it is difficult to collect and recycle, and may cause secondary pollution. Efforts have been made to improve the operability. Firstly, ZnFe₂O₄ was introduced as a semiconductor to the catalytic system.¹⁰⁷ Due to the magnetic property of ZnFe₂O₄ and the protection of graphene substrate, as shown in Figure 3H, the photocatalysts could be retrieved after photodegradation without aggregation, thus showing stable catalytic activity for 10 cycles. However, magnetic collection only works well for small systems, a more universal collection technique is required for massive photodegradation, constructing graphene substrate into 3D aerogel may be another solution.¹⁰⁸ AgBr nanoparticles were deposited on the surface of graphene aerogel through the hydrothermal method.¹⁰⁹ Thanks to the great mechanical properties of graphene aerogel, the recycling process can be easily conducted just with tweezers (Figure 3I), and the photodegradation performance is maintained at least after eight times recycling, which shows excellent practical prospect.

In summary, graphene-based photodegradation systems have been developed to enhance visible light utilization and recyclability, which would shine lights on the solar-powered decomposition of water pollutants in the future.

SOLAR ATMOSPHERIC WATER HARVESTING

It's reported that approximately 13 000 trillion liters of water exist in the atmosphere in the form of vapor and droplets,¹¹⁰ which could provide alternative freshwater sources for human beings. Fog collection^111–116 and dewing^117–121 systems were developed to gain fresh water under high humid conditions. With the development of hygroscopic materials with strong and effective water adsorption capacity like hydrogels,^122–125 salts,^126,127 and MOF,^128–132 adsorption-based atmospheric water harvesting become universal and applicable even in a desert. However, the water desorption and collection process are difficult and have to be assisted by additional equipment with high energy costs, which restricted the practical applications. Recently, graphene-based materials have shown excellent solar thermal conversion and induced fast water release ability, constructing a series of solar atmospheric water harvesters (SAWHs). For example, Wang¹³³ and coworkers developed the graphene aerogel/CaCl₂ solution system, in which CaCl₂ solution adsorbs water from the air and graphene aerogel generates heat to release them to the container under sunlight illumination. Then, liquid water is obtained on the wall of the container (Figure 4A). Owing to the high solar energy conversion rate of graphene, the efficiency of desorption is as high as 66.9%, and the whole system could acquire 2.89 kg water per square meter per day.

FIGURE 4. Graphene-based atmospheric water harvesters and structural designs for increasing water yield. (A) Schematic diagrams for the structure of the graphene-based SAWH and the process of atmospheric water harvesting. Reproduced with permission: Copyright 2019, John Wiley and Sons.133 (B) speeding up the absorption and desorption kinetics by nanosizing the absorbent, and (C) the corresponding scanning electron microscopy (SEM) image of carbon wrapped LiCl nanoparticles. Reproduced with permission: Copyright 2020, Elsevier.134 (D) Increasing adsorption capacity through the combination of polymers and graphene aerogels. Reproduced with permission: Copyright 2019, John Wiley and Sons.12 (E) Device for simultaneous generation of atmospheric water and electricity. Reproduced with permission: Copyright 2019, American Chemical Society.135 (F) Compact and portable design for graphene-based AWH and (G) the schematic diagram of its structure. Reproduced with permission: Copyright 2020, Elsevier136

The adsorption and desorption rate control are important in order to further improve the freshwater yield. The more harvesting cycles are conducted, the more water is gained per day. Meanwhile, the water adsorption capacity of absorbent should be as high as possible such that more water could be gained in one adsorption desorption cycle. To increase the adsorption and desorption rate, an alternative solution is to nanosize the absorbent, LiCl nanoparticles were wrapped in a graphitized carbon shell (Figure 4B,C),¹³⁴ the nanosized LiCl nanoparticles have higher specific areas, which enhanced the mass transfer and adsorption kinetics. The carbon shell keeps the LiCl nanoparticles from aggregation during harvesting cycles. Consequently, the composite absorbent was able to conduct three sorption/desorption cycles within 10 h and could produce 1.6 g water using 1 g absorbent within 1 day. To increase the capacity of the absorbent, the rational design of absorbent is of great importance. By combining polyacrylate (PAA) with reduced graphene oxide (rGO), water adsorption capacity could be boosted to 5.2 g/g at the humidity of 100%, and the composite absorbent also showed great adsorption performance at broad humidity values with tolerance for contaminated air.¹² Under one sun illumination, the desorption process only cost 420 s, enabling several sorption/desorption cycles during 1 day. From the field experiment for contaminated air, the composite SAWH could produce as much as 25 L freshwater per kilogram of PGF per day and the rejection rate of impurities is up to 97% for the collected clean water (Figure 4D).

Apart from pursuing high production, some other properties of SAWHs like durability and easy operating are also of great importance. To make SAWH durable and portable for practical application, a superelastic graphene nanocomposite was developed (Figure 4F,G),¹³⁵ LiCl was mixed as absorbent into the framework of graphene aerogel, the porous structure ensures the efficient mass transfer of water vapor from the atmosphere, and the good mechanical properties of graphene aerogel make it stable. No performance decay was found after 10 testing cycles. Owing to the rational design, the composite SAWH is as little as a tablet and can be compressed to further reduce volume, making it compact and portable to carry out for emergency use. In addition, the energy released by adsorption and desorption processes can be further exploited. As depicted in Figure 4E, a composite SAWH that could simultaneously generate atmospheric water and electricity was established via introducing a thermoelectric module into the SAWH system.¹³⁶ During adsorption, water vapor was bonded with the framework of graphene oxide and released heat, inducing heat gradient from the SAWH to the thermoelectric module, thus leading to electricity generation with a power density of 6.6 mW m⁻². Similarly, the vapor releasing process made the surface temperature of SAWH lower than that of the thermoelectric module, resulting in an output power density of 520 mW m⁻². At the same time, approximately 4.25 L fresh water could be obtained per square meter per day.

Above all, the graphene-based SAWHs are new-born water purification systems, which currently attract much interest and many researches to further improve its water yield and the feasibility of practical utilization.

OUTLOOK AND CONCLUSION

The past few years have witnessed the fast development of graphene-based materials in the applications of solar water purification. In this review, three main aspects including solar desalination, solar pollutants photodegradation, and solar atmospheric water harvesting are summarized and discussed. Regarding the problems lying in different systems, the introduction of graphene and the rational structural design have contributed to the improvement of the purification performances to some extent, paving a promising way for practical applications in the future. Meanwhile, there are still some challenges needing to be overcome before real industrialization.

• The cost of graphene is a considerable factor, including economic costs and ecological costs.^137–139 A lot of money is spent on the preparation of graphene oxide because of the complicated procedures. At the same time, the pollution to the environment from the synthesis cannot be neglected. Therefore, new processing techniques such as laser ablation, 3D printing, and so on should be employed to simplify the processing procedure and promote the mass production of the graphene-based solar purifier.
• It's necessary to improve mechanical properties and the durability of the solar water purifier. Many graphene-based composite materials used in solar water purification systems are 3D graphene aerogels or hydrogels. As graphene sheets in the framework are loosely distributed, which could not afford longtime use or cycling. Other materials like carbon nanotubes, polymers could be composited together with graphene framework to enhance its mechanical properties. Meanwhile, it is important to make a robust structural design for the solar water purification equipment to evade the risks such as erosion, oxidation or artificial damage.
• A comprehensive and systematic design should be made for the holistic purification system. For example, the evaporation rate is currently the main evaluation index for solar ISSGs, while the condensation of the water vapor is more important because liquid water yielding is the point of solar desalination. The same problem also lies in the field of the solar atmospheric water harvesting systems, hence much more attention should be paid to seeking feasible solutions. Likewise, for the photocatalysts in the field of photodegradation, the universality of photodegradation should be broadened as various pollutants like drugs, pesticides, foods are all existing in the effluents. Meanwhile, secondary contamination should also be taken into consideration. Hence, the ensuing research works should be conducted on a more comprehensive view, thus paving the way to the future large-scale industrial applications and addressing the upcoming water scarcity for all human beings.

In summary, the rapid development of graphene-based solar water purification systems in laboratory has shown fascinating value in sustainable application. And with continuous efforts on chemistry, materials, engineering technology and so on for solar-powered systems, a bright future of solving worldwide water scarcity could be coming soon.

ACKNOWLEDGMENTS

This work was supported by the financial support from the National Science Foundation of China (Nos. 52022051, 52073159, 22035005, 22075165, 52090032), NSFC-STINT (21911530143), State Key Laboratory of Tribology (SKLT2021B03), Tsinghua-Foshan Innovation Special Fund (2020THFS0501).

AUTHOR CONTRIBUTIONS

Y.H., H.C., and L.Q. proposed the theme of the review. Y.H. and H.C. put forward the specific contents of the review. Y.H. and H.C. wrote the paper. L.Q., H.Y., T.L., and Q.L. checked and modified the paper. All authors agreed on the submission of the manuscript.

CONFLICT OF INTEREST

The authors declare no conflict of interest.