Porous reduced graphene oxide/nickel foam for highly efficient solar steam generation

Improvement of electrothermal and photothermal properties of ultra-thin graphite film on oxygen plasma-treated polyimide substrate

Graphene and its derivatives are widely used in the field of energy conversion and management due to their excellent physical and chemical properties. In this paper, ultra-thin graphite film (GF) with thickness of 100-150 nm prepared by chemical vapor deposition was transferred to oxygen plasma-treated polyimide (PI) substrate as flexible heating film. The electrothermal and photothermal properties of GF on PI substrates with different treatment time were studied. The experimental results show that the PI substrate pretreated by oxygen plasma can change the surface morphology of GF, increase its electrical conductivity and light absorption capacity, and significantly improve the electrothermal and photothermal properties of GF heater. Under the low applied voltage of 5 V (power density of 0.81 W cm-2), the surface temperature of GF on 40 min plasma-treated PI substrate can rise to 250 °C, which is nearly 50 °C higher than that of GF on untreated PI substrate. When 100 nm thick commercial multilayer graphene film (MLG) is used, plasma-treated PI substrate can increase the electric heating temperature of MLG by 70 °C. In terms of photothermal performance, the surface temperature of GF on 50 min plasma-treated PI substrate can reach 73 °C under one Sun irradiation, which is 8 °C higher than that on untreated substrate. The experimental results are in good agreement with the simulation research. Our strategy has important implications for the development of efficient and energy-saving graphene/graphite-based heating films for advanced electrothermal and photothermal conversion devices.

Dual-Functional Solar-to-Steam Generation and SERS Detection Substrate Based on Plasmonic Nanostructure

Solar-to-steam (STS) generation based on plasmonic materials has attracted significant attention as a green method for producing fresh water. Herein, a simple in situ method is introduced to fabricate Au nanoparticles (AuNPs) on cellulose filter papers as dual-functional substrates for STS generation and surface-enhanced Raman spectroscopy (SERS) sensing. The substrates exhibit 90% of broadband solar absorption between 350 and 1800 nm and achieve an evaporation rate of 0.96 kg·m^-2·h^-1 under 1-sun illumination, room temperature of 20 °C, and relative humidity of 40%. The STS generation of the substrate is stable during 30 h continuous operation. Enriched SERS hotspots between AuNPs endow the substrates with the ability to detect chemical contamination in water with ppb limits of detection for rhodamine 6G dye and melamine. To demonstrate dual-functional properties, the contaminated water was analyzed with SERS and purified by STS. The purified water was then analyzed with SERS to confirm its purity. The developed substrate can be an improved and suitable candidate for fresh water production and qualification.

Fe3O4/Diatomite-Decorated Cotton Evaporator for Continuous Solar Steam Generation and Water Treatment

Improving the evaporation rate of solar steam generation (SSG) has always been a research hotspot to solve the shortage of water resources. Using cotton, Fe₃O₄, polyvinyl alcohol (PVA) and diatomite (DM) as raw materials, DM/PVA/Fe₃O₄@cotton composites with both firmness and hydrophilicity were prepared. Fe₃O₄ has a wide range of light absorption characteristics and good photothermal conversion performance, and is an ideal photothermal conversion material. PVA enhances the adhesion between Fe₃O₄, cotton and DM and enhances the hardness of the sample and the internal porous structure. The existence of DM greatly improves the hydrophilicity of the sample, ensuring that the water in the lower layer can be continuously transported to the surface of the sample, and DM makes the surface of the sample rough, which reduces the reflection of sunlight and improves the efficiency of light heat conversion. Under one-sun irradiation, the temperature of the sample surface increases by 52.6 °C, the evaporation rate can reach 1.32 kg m^-2 h^-1 and the evaporation efficiency is 82.9%. Using this sample as the photothermal conversion layer of the SSG device, the removal rate of salt ions in seawater is more than 98% and the removal rate of heavy metal ions in sewage is close to 100%. This work provides a new idea and design method for SSG in the field of seawater desalination and sewage treatment.

Highly stable gold nanolayer membrane for efficient solar water evaporation under a harsh environment

Interfacial solar water evaporation has attracted tremendous attention for sunlight harvesting for water purification. However, salt formation and stability of the photothermal materials (PTMs) remain a challenge that need addressing before bringing this technology to real-world applications. In this work, a nanoscale thin film of gold (Au) on a polytetrafluoroethylene (PTFE) membrane has been prepared using a magnetic sputtering technique. The fabricated membrane displays a robust mechanical strength and chemical stability arising from the adhesiveness of the thin film Au nanolayer on the PTFE membrane as well as the chemical inertness of the noble metal PTM. The Au nanolayer/PTFE membrane with cellulose sponge substrate resulted in an evaporation rate of 0.88 kg m^-2 h^-1 under 1 sun intensity. Remarkable salt ion rejection of 99.9% has been obtained, meeting the required standard for drinking water. Moreover, the membrane exhibited excellent stability and reusability in natural seawater and high salinity brine (150 g/L) and even in severe conditions (acidic, basic, and oxidized). No noticeable salt formation was observed on the evaporator surface after the tests. These findings reveal promising prospects for using a magnetron sputtering technique to fabricate a stable photothermal membrane for seawater and high salinity brine desalination.

A self-floating and integrated bionic mushroom for highly efficient solar steam generation

Solar desalination is considered as a promising approach to solve the shortage of fresh water resources. In this work, inspired by the transpiration of trees, a self-floating and integrated bionic mushroom solar steam generator (BMSSG) is proposed for highly efficient water evaporation. A wooden strip is used to mimic the stipe of the mushroom for water transportation, meanwhile polyvinyl alcohol (PVA) modified graphene aerogels (GA) is used to imitate the pileus of the mushroom for photothermal conversion. After optimizing compositions of the aerogel and sizes of the wooden strip, a high evaporation rate of 1.67 kg m^-2h^-1 is obtained, outcompeting most of other wood-based evaporators. Compared to traditional interfacial evaporation devices, BMSSG is an integrated structure without a thermal insulation layer and an absorbent wick, which not only increases the compactness that is good for stability and reliability, but also reduces the manufacturing cost. Moreover, the BMSSG can self-float on the water like a roly-poly. These advantages indicate that BMSSG will play a significant role in seawater desalination. The feasibility as well as stability and recyclability of the BMSSG for seawater desalination are demonstrated. This bioinspired design provides a low-cost and scalable SSG, which will have a profound impact in future practical applications.

Coffee Grounds-Doped Alginate Porous Materials for Efficient Solar Steam Generation

Solar steam generation (SSG) devices have emerged as one of the promising technologies for seawater desalination to meet the worldwide demand for clean water. Herein, we fabricated a new monolithic SSG system derived from waste coffee grounds (CG) through a simple carbonization followed by a freeze-drying process (named as CCGA). The as-prepared CCGA possesses a porous structure with superhydrophilic, abundant porosity (81.7%); low thermal conductivity (0.129 W m^-1 K^-1) in a wet state; low apparent density (25 mg cm^-3); and broad sunlight absorption in a wet state (ca. 93%). The combination of its carbon nature and abundant porous structure endowed barrier-free water transmission channels, a self-floating property, and a superb photothermal conversion performance to the SSG. The temperature of the CCGA's upper surface can reach up to 42.6 °C under 1 sun irradiation, and for pure water, the evaporation rate of CCGA can be up to 1.486 kg m^-2 h^-1, corresponding to a good photothermal conversion efficiency of 86.96%. It also exhibits an excellent desalination capability; e.g., the photothermal conversion efficiency of CCGA in NaCl (20 wt %) brine is measured to be 75.77% under 1 sun irradiation, and the fresh water obtained from artificial seawater can achieve the WHO's standard for domestic water. With the advantages of low cost and a simple preparation process, such biomass-based CCGA materials may have great potential as an efficient SSG device for seawater desalination.

Structure development of carbon-based solar-driven water evaporation systems

Pressing need goes ahead for accessing freshwater in insufficient supply countries and regions, which will become a restrictive factor for human development and production. In recent years, solar-driven water evaporation (SDWE) systems have attracted increasing attention for their specialty in no consume conventional energy, pollution-free, and the high purity of fresh water. In particular, carbon-based photothermal conversion materials are preferred light-absorbing material for SDWE systems because of their wide range of spectrum absorption and high photothermal conversion efficiency based on super-conjugate effect. Until now, many carbon-based SDWE systems have been reported, and various structures emerged and were designed to enhance light absorption, optimize heat management, and improve the efficient water transport path. In this review, we attempt to give a comprehensive summary and discussions of structure progress of the carbon-based SDWE systems and their working mechanisms, including carbon nanoparticles systems, single-layer photothermal membrane systems, bi-layer structural photothermal systems, porous carbon-based materials systems, and three dimensional (3D) systems. In these systems, the latest 3D systems can expand the light path by allowing light to be reflected multiple times in the microcavity to increase the light absorption rate, and its large heat exchange area can prompt more water to evaporate, which makes them the promising application foreground. We hope our review can spark the probing of underlying principles and inspiring design strategies of these carbon-based SDWE systems, and further guide device optimizations, eventually promoting in extensive practical applications in the future.

Power-Free and Self-Cleaning Solar Light Detector Based on the Temperature-Sensitive Structural Color and Photothermal Effect

In this work, photothermal materials are integrated with a temperature-sensitive hydrogel and structural color for visually detecting solar intensity. Inspired by the functional performance of beetles, the photothermal layer is constructed by depositing candle soot on a film of Cu nanoparticles, while the temperature-sensitive colored hydrogel is fabricated by self-assembling colloidal photonic crystals on poly(N-isopropylacrylamide) (PNiPAM). The deposition of candle soot not only improves the photothermal performance but also leads to a superhydrophobic surface with a self-cleaning function. The photothermal layer absorbs sunlight and converts it into heat, which is then transferred to the hydrogel. The structural color of the hydrogel changes due to the heat-induced volume shrinkage. As the solar intensity increases from 0.62 to 1.27 kW/m², the structural color conspicuously changes from red to orange, yellow, green, cyan, and blue, with reflection peaks shifting from 640 to 460 nm accordingly. The color change is highly apparent, which can be easily observed by the naked eye, suggesting that the solar intensity can be easily detected by reading out the structural color. This power-free and self-cleaning solar sensor can work for a long period without maintenance, which is suitable for a wide application prospect, such as smart home and agriculture.

Reusable Self-Sterilization Masks Based on Electrothermal Graphene Filters

Surgical mask is recommended by the World Health Organization for personal protection against disease transmission. However, most of the surgical masks on the market are disposable that cannot be self-sterilized for reuse. Thus, when confronting the global public health crisis, a severe shortage of mask resource is inevitable. In this paper, a novel low-cost electrothermal mask with excellent self-sterilization performance and portability is reported to overcome this shortage. First, a flexible, ventilated, and conductive cloth tape is patterned and adhered to the surface of a filter layer made of melt-blown nonwoven fabrics (MNF), which functions as interdigital electrodes. Then, a graphene layer with premier electric and thermal conductivity is coated onto the MNF. Operating under a low voltage of 3 V, the graphene-modified MNF (mod-MNF) can quickly generate large amounts of heat to achieve a high temperature above 80 °C, which can kill the majority of known viruses attached to the filter layer and the mask surface. Finally, the optimized graphene-modified masks based on the mod-MNF filter retain a relatively high particulate matter (PM) removal efficiency and a low-pressure drop. Moreover, the electrothermal masks can maintain almost the same PM removal efficiency over 10 times of electrifying, suggesting its outstanding reusability.

CoWO4-x -Based Photothermal Membranes for Solar-Driven Water Evaporation and Eutrophic Lake Water Purification

Solar-driven water evaporation has been proven to be a promising and efficient method for the energy crisis and clean water shortage issues. Herein, we strategically design and fabricate a novel nonstoichiometric CoWO_4-x -deposited foam nickel (NF) membrane (CoWO_4-x @NF) that possesses all the desirable optical, thermal, and wetting properties for efficient water evaporation and purification. The broadband absorption of CoWO_4-x nanoparticles (NPs) obtained by hydrogen reduction contributes to light-to-heat conversion, while NF with a three-dimensional porous structure can support CoWO_4-x NPs and ensure the rapid flow of water molecules during the water evaporation process. We systematically explore and compare the outdoor water evaporation performance of the pure water group, NF group, and CoWO_4-x @NF group, and the results show that CoWO_4-x @NF performs well under natural sunlight irradiation (water evaporation: 2.91 kg m^-2). Significantly, under solar irradiation, the remarkable reduction of Cyanophyta and Euglenophyta in lake water is achieved in the CoWO_4-x @NF membrane-administered group, and these two algae are the main factors for eutrophication of the lake water. Our work highlights the great potentials of the CoWO_4-x @NF membrane as a device for realizing outdoor solar energy-driven water evaporation and proposes a new strategy for purifying the eutrophication of the lake water.

Facile Deflagration Synthesis of Hollow Carbon Nanospheres with Efficient Performance for Solar Water Evaporation

Solar water evaporation is a promising and environment-friendly approach to relieve global water scarcity issues. Currently, many reports show that the voids and porous structure are beneficial to the absorption of solar energy to generate water steam. Herein, carbon nanospheres with central cavity structures are rationally designed by the one-step NaN₃/fluorinated graphite deflagration method. The Na clusters derived from NaN₃ deflagration are not only provided as the hollow templates but also react with fluorinated graphite to release heat, further boosting the formation of hollow carbon nanospheres (HCSs). Benefiting from the diversity of carbon nanomaterials, rough surface, unique hollow structures, and numerous micron/submicron holes, the light absorption ability, heat localization, and water supply capacity of HCSs have been significantly enhanced. Because of these advantages, the HCS-3 exhibits an excellent water evaporation efficiency of 92.7% at 1 kW m^-2, which is much higher than that of carbon nanospheres, graphene oxide, and even most of the previous carbon materials. In addition, we demonstrated that the HCSs have a long-term stability and high efficiency of production of drinkable water and purifying various types of wastewater, including seawater, strong acid/alkaline water, and water containing dyes. To sum up, the deflagration synthetic technology as a facile and ultrafast process can be a new insight for future photothermal material design.

Flexible and Mildew-Resistant Wood-Derived Aerogel for Stable and Efficient Solar Desalination

Interfacial solar-driven evaporation is an effective and sustainable approach to convert solar energy to heat for desalination. However, maintaining a rapid evaporation rate and long-term stability is a critical challenge that needs to be addressed urgently to facilitate practical applications of this technology. Here, taking advantage of the inherent porosity, wettability, and low thermal conductivity of wood, we develop a flexible and mildew-resistant aerogel derived from natural wood as the substrate of evaporators for stable and efficient solar desalination. The wood-derived aerogel not only possesses good hydrophilicity, low thermal conductivity, and light weight, which are favorable to realize heat localization and efficient water evaporation, but also avoids fragility and mildew problems that seriously restricted long-term stability of wood-based evaporators. With high absorbance (>95%) of Au-rGO coating, the evaporator showed a high solar to vapor efficiency of 90.1% and an evaporation rate of 1.394 kg m^-2 h^-1 under 1 sun. During 120 h desalination, the evaporator transports brine for evaporation in the day and achieves dissolution of salt residues in the night, exhibiting a repetitive self-cleaning behavior, which enables the recovery of desalination stability. The multiple functionalities of the wood-derived aerogel make the evaporator promising as an attractive device for stable and continuous solar desalination.

Building Polyoxometalate "Nano-Walls" on 3D Porous Carbon Paper: A Solar Steam Generation System for Water Purification

As promising fresh-water purification devices, solar steam generation systems have attracted significant attention recently. However, in practice, the approach often suffers from a poor solar energy conversion efficiency and a low water production rate due to poor material selection and inefficient microscopic structure design. Here, we fabricate an efficient solar steam generation system by "building" polyoxometalate "nano-walls" on rice paper-derived three-dimensional porous carbon paper. In this solar steam generation system, the height of the vertically aligned CoP₄ Mo₆ "nano-walls" range from 100 to 150 nm with thicknesses about 15 to 25 nm. Under 1 sun irradiation (1 sun = 1 kW m^-2 ), the surface temperature increases from 29 to 50 °C in a short time with a solar thermal conversion efficiency achieving 92.8 %. The stability and durability of this solar steam generation system, which withstands fifteen cycle continuous tests, also offer good prospects. Its attractive solar energy conversion performance originates from the intense sunlight absorption and high conversion ability of the CoP₄ Mo₆ "nano-walls", as well as extremely promising heat localization and water transportation properties of the three-dimensional porous carbon paper. This solar steam generation system, which has produced some inspiring results, is employed for seawater desalination and for purification of water polluted with organic dyes.

Facile preparation of magnetic porous carbon monolith from waste corrugated cardboard box for solar steam generation and adsorption

Porous carbon monoliths (PCMs) were prepared from waste corrugated cardboard box (WCCB) via slurrying in FeCl₃ solution followed by molding and thermal treatment. The thermal process was analyzed by a thermogravimetric analyzer coupled with a Fourier transform infrared spectrometer. The evolution of physicochemical characteristics of PCMs was studied. The photothermal conversion and solar steam generation performances of the optimal sample (PCM_Fe/600) were evaluated. The adsorption properties of PCM_Fe/600 for methylene blue (MB) were investigated. Results showed that Fe³⁺ promoted the breaking of cellulose chains in WCCB, leading to the occurrence of pyrolysis of WCCB at lower temperatures and the reduction of activation energy by 76.63 kJ mol^-1. Char yield raised because volatile radicals were captured by FeCl₃-derived amorphous Fe(III) species, then involved in char formation. Amorphous Fe(III) continuously converted into Fe₃O₄ crystallites with carbonization temperature increasing from 400 to 700 °C, then α-Fe was formed at 800 °C via the carbothermal reduction of Fe₃O₄. FeCl₃ was favorable to the formation of a developed microporous structure. Surface area significantly increased with carbonization temperature increasing from 400 to 600 °C due to the removal of volatiles. The etching of carbon by Fe₃O₄ above 700 °C also led to the increase of surface area. PCM_Fe/600 exhibited higher optical absorption than other samples due to its high graphite degree and porosity. It also had excellent photothermal performance; thus, solar steam yield was 1.46 times that of the pure water with the assistance of PCM_Fe/600. PCM_Fe/600 in floating state was effective in adsorption of MB from water. Besides, the adsorption behavior fitted Langmuir model with a monolayer adsorption capacity reached up to 70.9 mg g^-1.