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Aerogels are extraordinary materials that have set Guinness World Records more than a dozen times, including as the world's lightest solids.

Professor Markus Niederberger from the Laboratory for Multifunctional Materials at ETH Zurich has been working with these special materials for some time. His lab specializes in aerogels composed of crystalline semiconductor nanoparticles. "We are the only group in the world that can produce this kind of aerogel at such high quality," he says.

One use for aerogels based on nanoparticles is as photocatalysts. These are employed whenever a chemical reaction needs to be enabled or accelerated with the aid of sunlight—one example being the production of hydrogen.

The material of choice for photocatalysts is titanium dioxide (TiO2), a semiconductor. But TiO2 has a major disadvantage: it can absorb only the UV portion of sunlight—just about 5 percent of the spectrum. If photocatalysis is to be efficient and industrially useful, the catalyst must be able to utilize a broader range of wavelengths.

That is why Niederberger's doctoral student Junggou Kwon has been looking for a new way to optimize an aerogel made of TiO2 nanoparticles. And she had a brilliant idea: if the TiO2 nanoparticle aerogel is "doped" (to use the technical term) with nitrogen, such that individual oxygen atoms in the material are replaced by nitrogen atoms, the aerogel can then absorb further visible portions of the spectrum. The doping process leaves the aerogel's porous structure intact. The study on this method was recently published in the journal Applied Materials & Interfaces.

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