14-Jun-2022 - King Abdullah University of Science and Technology (KAUST)

Organic water splitters get a boost

Nanoparticle photocatalyst can generate hydrogen fuel from water more efficiently than their standard inorganic semiconductor counterparts

Organic semiconductor-based photocatalysts conceived by a KAUST-led team could make hydrogen easier to generate from water using sunlight.

Sunlight is the most abundant source of renewable energy, but its inability to produce consistent energy levels over time means it cannot meet energy needs on demand. A promising option is to store solar energy as clean hydrogen fuel, derived from water by the so-called hydrogen evolution reaction in the presence of a light-responsive catalyst.

Most hydrogen evolution photocatalysts consist of inorganic semiconductors, such as titanium dioxide, that almost exclusively absorb ultraviolet light. But because ultraviolet light represents less than five percent of the solar spectrum, the resulting photocatalysts are not efficient enough for commercial use.

An international team led by Iain McCulloch and postdoc Jan Kosco were able to develop organic semiconductor-based photocatalysts because they could tune the semiconductor bandgaps — which define the absorption wavelength range — to absorb visible light.

“All else being equal, the more light a photocatalyst absorbs, the more efficiently it can convert solar energy into hydrogen,” Kosco explains. “So, it is important to develop photocatalysts that are active over a broad range of ultraviolet-visible-infrared wavelengths to maximize light absorption.”

When exposed to light, semiconductor-based photocatalysts generate pairs of electrons and positively charged holes, or excitons, which dissociate into free charges that subsequently can migrate to the photocatalyst surface and drive hydrogen evolution. However, excitons are tightly bound in typical single-component organic semiconductors, which restricts charge separation and photocatalytic efficiency.

The researchers combined electron donor and acceptor semiconductor materials to form nanoparticles, known as heterojunction photocatalysts, whose overall bandgap configuration promotes exciton dissociation at the semiconductor interface.

“This is analogous to the bulk heterojunction used in organic solar cells,” Kosco says. “We, therefore, generated more charges in these nanoparticles than in those composed of individual semiconductors, which improved hydrogen production.”

Unexpectedly, the heterojunction resulted in extremely long-lived photogenerated charges in the nanoparticles.

“Charges typically recombine on the microsecond timescale, but we observed charges in our nanoparticles even a few seconds after photoexcitation, which is exceptionally long for photogenerated charges in organic semiconductors,” Kosco says. This is critical for catalyst performance because it gives more time for the charges to take part in relatively slow redox reactions at the nanoparticle surface, he adds.

The team is now exploring ways to apply the new photocatalysts to water splitting Z-schemes, where hydrogen and oxygen evolution photocatalysts are coupled to simultaneously drive the production of hydrogen and oxygen. They are also developing organic semiconductor photocatalysts for oxygen evolution.

Facts, background information, dossiers
More about King Abdullah University of Science and Technology
  • News

    Smog clears on car exhaust catalyst design

    Smog-producing chemicals could be almost eliminated from the tailpipes of diesel cars and vans, using a new exhaust catalyst concept developed at KAUST. After systematically studying multiple catalyst compositions, the research team identified the ideal atomic recipe to catalytically remove ... more

    Easing oxygen's evolution

    A metal foam could underpin a low-cost method for generating carbon-free fuels, researchers from KAUST have shown. The team seamlessly coated the foam with iron and cobalt nanomaterials to create a highly active electrode for a device that splits water molecules to release oxygen and hydrog ... more

    Shrimply the best composite membranes

    Shrimp shells, plant extracts and recycled plastic have helped KAUST researchers to build a sustainable thin-film composite membrane that could replace conventional membranes whose environmental toll is greater. Thin-film composite membranes are widely used in applications such as wastewate ... more