A sponge robot that cleans water with sunlight

What if there were a tiny floating robot that could clean dirty water just by basking in the sun? No batteries, no fuel, just light. A group of researchers in China has just brought us one step closer to that future. A team from Jilin University and Hunan University, led by Dr. Dong-Dong Han and Professor Yong-Lai Zhang, published a paper in the journal PhotoniX that describes a “light-propelled photocatalytic evaporator”, a mouthful of a term that essentially means a sun-powered, self-moving water purifier.

At its core, this invention combines two of the biggest needs of humanity, that is, clean water and sustainable energy. With freshwater scarcity worsening worldwide due to climate change, population growth, and pollution, technologies like this could transform the way communities, especially those off the grid, access safe drinking water.

Freshwater makes up less than 3% of the water supply of our planet, and much of it is locked away in glaciers or underground. For billions of people, clean drinking water is not guaranteed. Traditional solutions like reverse osmosis and multi-stage distillation can turn seawater or wastewater into potable water, but they come at a high price, massive energy consumption, expensive equipment, and a heavy carbon footprint.

Nature, however, has already solved this puzzle. The water cycle purifies and redistributes freshwater through evaporation and rainfall. Inspired by this, scientists have been working on solar-driven water purification systems that mimic nature, using sunlight to power evaporation and filtration.

While these systems are promising, most are stationary and limited in efficiency. They either break down pollutants using light-sensitive materials (photocatalysis) or boil off water using sunlight (photothermal evaporation). Rarely do they combine both, and none can autonomously move around like a robot. That’s where the new research stands out.

The heart of the invention is a spongy, lightweight material made from three special ingredients of Graphene oxide (GO) (famous for its strength and conductivity), MXene (a futuristic 2D material known for its metallic properties), and Titanium dioxide (a classic photocatalyst already used in self-cleaning windows and sunscreens).

Through a clever freeze-drying process, the team assembled these into a porous black foam. The magic happens at the molecular level. During assembly, the materials undergo a redox reaction, forming strong Ti–O–C chemical bonds and creating tiny titanium dioxide nanoparticles within the structure. The result is a material that behaves almost like a semi-metal, excellent at absorbing sunlight, converting it to heat, and generating chemical reactions that break down pollutants. It’s like a sponge that can both sterilize and distill water at the same time.

So how does this foam purify water? First, it realizes photocatalysis. When sunlight hits the foam, it excites electrons in Titanium dioxide and MXene, creating highly reactive molecules like oxygen radicals. These attack and break apart organic pollutants, such as dyes, oils, or industrial waste, turning them into harmless carbon dioxide and water. Also, it leverages photothermal evaporation. At the same time, the foam absorbs solar heat and rapidly converts it to steam. This evaporated water can then be condensed into fresh, drinkable water, leaving salts and heavy metals behind. In experiments, the foam achieved an impressive evaporation rate of 1.72 kilograms per square meter per hour, a benchmark that makes it one of the most efficient solar purifiers to date. It even managed to reduce salt and mineral concentrations in seawater by four orders of magnitude, producing nearly pure water.

After creating the sponge, the researchers also turned it into a robot. By shining light unevenly on the foam, they discovered it could propel itself across water surfaces. This motion is powered by the Marangoni effect, a physical phenomenon where differences in surface tension cause fluids (and objects on them) to move. In simple terms, shine a laser on one side of the foam, and it heats up unevenly, creating a push that nudges the robot forward. By steering the light, the scientists could “drive” the purifier through a maze of obstacles, completely wireless and battery-free. This means future devices could navigate lakes, rivers, or even oceans, moving toward polluted hotspots, avoiding barriers, and cleaning as they go. It’s a new vision of “robotic ecology”, machines that patrol our waters like autonomous lifeguards, powered entirely by sunlight.

Imagine coastal villages where these floating purifiers could desalinate seawater on demand, or disaster zones where they could provide clean drinking water without trucks of bottled water. Remote communities could deploy fleets of them to clean ponds and reservoirs, guided by drones or lasers. Unlike massive desalination plants, which are centralized and energy-intensive, these foam robots are small, scalable, and self-sufficient. A handful could serve a family. Thousands could patrol a polluted lake.

Of course, this is still lab-scale research. The foams are prototypes, and real-world deployment will face challenges, including durability, cost of materials, and scaling up production. The concept is powerful, merging materials science, renewable energy, and robotics into one elegant solution.

When we think of robotics, we often picture humanoid machines or factory arms. The future of robotics may also lie in tiny, floating, nature-inspired helpers like these solar foams. As freshwater crises deepen, solutions that are decentralized, green, and adaptive will be crucial. This light-propelled evaporator symbolizes a shift in how we imagine technology: not as machines separate from nature, but as small, smart partners that work with the rhythms of sunlight and water.

If you want to learn more, read the original article titled "Light-propelled photocatalytic evaporator for robotic solar-driven water purification" on PhotoniX at http://dx.doi.org/10.1186/s43074-025-00169-4.