A solar emulsion evaporator built with graphene oxide and carbon nanotubes adapts to oil, salt, and microbes, maintaining clean water production in harsh conditions.
(Nanowerk Spotlight) Freshwater access has shaped the growth of societies for centuries. Coastal cities, deserts, and island communities often sit close to oceans but cannot drink from them. Large-scale desalination emerged in the twentieth century as one way to bridge this gap. The earliest plants relied on thermal distillation, a process that heats seawater to create steam, which is then condensed into fresh water. The principle is simple, but the energy demand is high, as vast amounts of water must be boiled. The costs of thermal distillation has become prohibitive for most regions.
As energy efficiency gained importance, membrane-based reverse osmosis rose to dominance. Instead of boiling, this method forces seawater through dense polymer membranes under pressure, filtering out salts and impurities. Reverse osmosis now supplies drinking water in many parts of the world, from the Gulf states to southern California. Yet it is not a universal solution. The process consumes significant electricity, requires complex infrastructure, and produces concentrated brine waste that must be managed carefully. For remote communities without reliable grids, or for emergency situations where rapid deployment matters, large desalination plants are not practical.
This gap has fueled interest in smaller, decentralized systems that operate without extensive infrastructure. Solar evaporation offers such a pathway. The idea is to concentrate sunlight at the air–water boundary so only the top layer of liquid is heated. This thin heated layer evaporates into vapor that can be collected as clean water. By avoiding the need to heat the full bulk of liquid, the process cuts energy demand dramatically. The appeal is clear: a floating device powered only by sunlight that turns seawater or polluted water into safe drinking water.
The obstacle is that real-world water sources present conditions far harsher than controlled laboratory tests. Oils that spread across the sea surface block light and heat transfer. As water evaporates, salts precipitate and form crusts that shade the surface and choke the channels that supply liquid. Volatile organic compounds can escape along with vapor and contaminate the collected distillate. Microorganisms attach to surfaces, multiply, and form biofilms that limit flow and shorten device lifetimes. Most prototypes show strong performance at the beginning but quickly lose efficiency once exposed to these stresses. A further limitation is rigidity: most devices are built as sponges or solid membranes that cannot adapt to uneven or moving surfaces.
At the same time, advances in nanomaterials and photocatalysis have opened the door to new designs. Graphene and carbon nanotubes, both forms of carbon with unique nanoscale structures, absorb light strongly and interact with organic molecules in useful ways.
Photocatalysts that split charges under sunlight and generate reactive species have matured, making it possible to degrade pollutants as water evaporates. Developments in soft materials and emulsions offer new strategies to design interfaces that are less brittle and more adaptive. Together these advances create the possibility of rethinking solar evaporators not as rigid filters but as dynamic systems that use flexibility to survive harsh environments.
A Pickering emulsion is a suspension of droplets stabilized by solid particles rather than conventional surfactants. In this case, the particles are graphene oxide nanosheets supported by carboxylated carbon nanotubes.
The emulsion spreads into a soft but coherent layer that floats on water. Its dual character is central to its function. Graphene oxide carries both oxygen-containing groups that attract water and carbon regions that interact with oils. Carbon nanotubes reinforce the emulsion and add light absorption.
The result is a network of droplets whose boundaries act as channels, drawing water upward from below while trapping sunlight near the surface. Because oil resists heat transfer, when oil becomes part of the droplets it helps keep heat localized where evaporation occurs. The emulsion behaves like a viscoplastic fluid—able to flow when spread but strong enough to hold together during operation.
Schematic illustrating the liquid-like multifunctional emulsion solar evaporation system engineered for stable operation under challenging water conditions. Notably, this emulsion system can either be formed in situ on oil-contaminated surfaces or preconstructed with optimized rheological properties for flexible and scalable deployment, concurrently facilitating a) active oil remediation, b) salt resistance, c) VOCs degradation, and d) antibacterial functionality. (Image: Reprinted with permission by Wiley-VCH Verlag) (click on image to enlarge)
Performance testing under standard sunlight, equal to 1 kilowatt per square meter, shows the optimized emulsion can evaporate about 1.25 kilograms of water per square meter per hour in pure water. In seawater with 3.5 percent salt, the rate rises slightly to 1.35. This increase comes from the way salt ions alter water structure at the nanoscale. Near the graphene oxide interface, the ions weaken hydrogen bonds between water molecules, lowering the energy required for them to escape into vapor. The energy barrier, or evaporation enthalpy, falls from 1889 joules per gram in pure water to 1724 in salty water.
Under stronger light, output scales proportionally: nearly 2 kilograms per square meter per hour at two suns, over 7 at nearly seven suns, and as much as 12.5 in concentrated brine. The key finding is not just high rates but stable performance under salt and oil stress.
Oil contamination shows how adaptability matters. In a rigid sponge coated with carbon nanomaterials, oil clogs pores and reduces evaporation permanently. In the emulsion, oil at first lowers performance, but over time droplets absorb it into their structure. Their size grows, the surface reopens, and the original rate returns. The emulsion can incorporate oil up to about 43 percent of its internal volume before flow declines, meaning it can handle real polluted waters where oil films are common.
Salt management is equally critical. Most solar evaporators fail because salt crusts form on their surface. The emulsion directs crystallization away from the active area. In a hydrophilic frame, salt deposits along the edges instead of across the center. Adding hydrophilic wooden strips gives crystals a preferred site where they grow as discrete blocks that can be removed.
In another configuration, when salt does form on the emulsion surface, the softness of the layer allows heavy crystals to sink through into the bulk water. The emulsion flows back upward to reseal the surface, keeping evaporation channels clear. This adaptability is a sharp contrast to rigid devices, which once blocked remain blocked.
The material also tolerates a wide range of chemical environments. It holds stable evaporation rates in highly acidic water with a pH of 1 and in alkaline water with a pH up to 13 for short periods. Long-term tests confirm resilience in acidic and neutral conditions, though strongly alkaline solutions eventually degrade the graphene oxide. This tolerance suggests use not just in seawater but also in industrial or agricultural effluents where acidity is common.
A further challenge is ensuring the purity of the collected water. Some organic pollutants, known as volatile organics, can evaporate with water and contaminate the distillate. To address this, the researchers incorporated a photocatalyst into the emulsion, a bismuth-based compound called BiOCl@Bi. Under sunlight, it generates reactive species that oxidize and break down organic molecules.
In tests with phenol, a common pollutant, over 90 percent of the compound was removed from the water collected, and with the help of a small dose of hydrogen peroxide nearly full removal was achieved. The carbon nanostructure also draws aromatic molecules like phenol toward the catalyst, increasing efficiency. Importantly, the photocatalytic process did not lower evaporation performance.
Biological contamination is another real-world barrier. Surfaces in contact with natural water quickly accumulate bacteria and biofilms. Graphene derivatives and nanotubes already show antibacterial effects, and the fluid nature of the emulsion denies organisms a stable foothold.
In sewage-like test water, the emulsion reduced viable bacteria by more than 90 percent compared to controls. While rigid sponges became colonized, the emulsion stayed clean. Under sunlight, the combination of thermal and catalytic effects is likely to enhance disinfection further.
The emulsion is not single-use. It can be transformed into a gel with ascorbic acid, making it easier to handle. The gel maintains high evaporation rates and can perform even better when combined with insulation. After absorbing pollutants, the gel can be burned into a porous carbon aerogel, which serves as a reusable sorbent. These transitions show that the material can be recovered, recycled, and adapted rather than discarded.
Outdoor tests bring the laboratory findings closer to application. Under natural sunlight, a device based on the emulsion produced more than 6 kilograms of water per square meter over a ten-hour day. The emulsion stayed intact and productive through a week of continuous use. While still at the research stage, this performance in uncontrolled conditions points to real-world potential.
The work demonstrates a shift in how solar evaporators can be designed. Instead of building rigid barriers against contamination, the liquid-like emulsion adapts to it. Oils are absorbed rather than resisted, salts are steered away or allowed to sink, and microbes cannot anchor. The addition of photocatalysis ensures safe distillate, while recyclability links function to material lifecycle. Challenges remain in scaling and system integration, but the concept shows how adaptability can overcome the key obstacles that have kept solar evaporation from practical use.
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Xiaofeng Li (Beijing University of Chemical Technology)
, 0000-0002-5624-2733 corresponding author
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