Currently, most hydrogen evolution photocatalysts (HEPs) are made from single-component inorganic semiconductors. These can only absorb light at ultraviolet wavelengths—constituting less than 5% of the solar spectrum—which limits their efficiency. However, photocatalysts formed from a single organic semiconductor typically suffer from inefficient intrinsic charge generation, which leads to low photocatalytic activities.

Now, a team led by Iain McCulloch from the KAUST Solar Center, in partnership with researchers from the United States and the United Kingdom, has developed HEPs made from two different semiconducting materials. They incorporated these materials into organic nanoparticles that can be tuned to absorb more of the visible light spectrum.

The resulting photocatalysts display an unprecedentedly high hydrogen evolution rate of more than 60,000 μmol h⁻¹ g⁻¹ under 350 to 800 nm illumination, and external quantum efficiencies more than 6% in the region of maximum solar photon flux. A paper on their work appears in Nature Materials.

The team first used a method called miniemulsion, in which a solution of the organic semiconductors is emulsified in water with the aid of a stabilizing surfactant. Next, they heated the emulsion to drive off the solvent, leaving behind surfactant-stabilized organic semiconductor nanoparticles.

By varying the surfactant, they were able to control the structure of the nanoparticles, transforming them from a core-shell structure to a mixed donor/acceptor structure. The blended structure allowed them to introduce a heterojunction between the layers of the donor polymer and nonfullerene acceptor.

Both structures absorb light at the same rate, but in the core-shell structure, only photogenerated holes reach the surface; however, in the mixed structure, both holes and electrons reach the surface of the nanoparticles, resulting in enhanced generation of hydrogen.

—Jan Kosco, first an co-corresponding author

The HEPs exhibited hydrogen evolution rates an order of magnitude beyond what is currently achievable with single-component inorganic HEPs.

We are currently studying the performance of nanoparticles formed from different blends of semiconductors to better understand their structure-activity relationships. We are looking to design nanoparticle photocatalysts for other photocatalytic reactions, such as oxygen evolution or carbon dioxide reduction.

—Iain McCulloch, senior and co-corresponding author

Resources

• Kosco, J., Bidwell, M, Hyojung, C., Tyler, M., Howells, C.T., Sachs, M., Anjum, D.H., Lopez, S.G., Zou, L., Zhan, L., Tellam, J., Sougrat, R., Laquai, F., Delongchamp, D.M., Durrant, J.R. & McCulloch, I. (2020) “Enhanced photocatalytic hydrogen evolution from organic semiconductor heterojunction nanoparticles.” Nature Materials doi: 10.1038/s41563-019-0591-1