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 specialises 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 (TiO
2
), a semiconductor. But TiO
2
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 utilise a broader range of wavelengths.
Broadening the spectrum with nitrogen doping
That is why Niederberger’s doctoral student Junggou Kwon has been looking for a new way to optimise an aerogel made of TiO
2
nanoparticles. And she had a brilliant idea: if the TiO
2
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
.
Kwon first produced the aerogel using TiO
2
nanoparticles and small amounts of the noble metal palladium, which plays a key role in the photocatalytic production of hydrogen. She then placed the aerogel in a reactor and infused it with ammonia gas. This caused individual nitrogen atoms to embed themselves in the crystal structure of the TiO
2
nanoparticles.
Modified aerogel makes reaction more efficient
To test whether an aerogel modified in this way actually increases the efficiency of a desired chemical reaction – in this case, the production of hydrogen from methanol and water – Kwon developed a special reactor into which she directly placed the aerogel monolith. She then introduced a vapour of water and methanol to the aerogel in the reactor before irradiating it with two LED lights. The gaseous mixture diffuses through the aerogel’s pores, where it is converted into the desired hydrogen on the surface of the TiO
2
and palladium nanoparticles.