Scientists from the Swiss Federal Institute of Technology in Lausanne (EPFL) have shown that the same layer of an organic semiconductor can work as a photocathode (reducing protons to hydrogen) or as a photoanode (oxidizing water to oxygen), depending on which molecular film lies beneath it. On the basis of this effect, they built the first photocatalytic sheet on an organic semiconductor that splits water under simulated sunlight without applying an external voltage.
Photocatalytic sheets combine the hydrogen- and oxygen-evolution zones on a single conductive substrate. This makes it possible to do without external wires, as in photoelectrochemical cells, and without dissolved charge carriers, as in particle suspensions. This design is considered promising for large-scale production of solar hydrogen. Until now, however, such sheets were made from inorganic semiconductor particles, which are difficult to assemble into a working sheet.
In a study published in the journal Nature Energy, a group led by Dr. Nicolas Dirks and Dr. Jin Su Park from the LIMNO laboratory at EPFL took a different approach. They deposited a layer of a semiconducting polymer (a PBDTTTPD:PNDIHDT blend) from solution onto fluorine-doped tin oxide (FTO) substrates modified with self-assembled molecules with oppositely directed dipoles. On the carbazole contact (2PACz), the film works as a photocathode for hydrogen evolution. On the new naphthalimide contact (NI-PA), the same film acts as a photoanode for oxygen evolution.
The researchers then formed microdomains from these molecules by contact printing and added cocatalysts, platinum and iridium oxide. The resulting sheet split water without external voltage for more than two hours, releasing hydrogen and oxygen in the expected 2:1 ratio. The stabilized hydrogen evolution rate was 1.3 micromoles per hour per square centimeter, which corresponds to a modest solar-to-hydrogen conversion efficiency of about 0.1%.
The work also reveals limitations. Reducing the domain size from 1 mm to 100 um or 10 um lowered the efficiency, because charge extraction deteriorates at the boundaries between domains. A simple spraying method for patterning the bottom layer shows how the concept can be scaled up. The efficiency is still too low for practical application, but the study sets out a design strategy for organic photocatalytic sheets and points to further steps: acceptors with a narrower band gap, better molecules for selective charge transfer, and more selective deposition of cocatalysts.