Charge Concentration Limits the Hydrogen Evolution Rate in Organic Nanoparticle Photocatalysts

Colloidal organic nanoparticles (NPs) have proven to be a promising class of photocatalyst for performing the Hydrogen Evolution Reaction (HER) due to their dispersibility in aqueous environments, their strong absorption within the visible region, and the tunabilty of their component materials' redox potentials. Currently, there is little understanding of how charge generation and quenching mechanisms in organic semiconductors change when these materials are formed into nanoparticles that share a high interfacial area with water, nor is it known what mechanism limits the efficiency of hydrogen evolution in prior reports. Herein, we use Time-Resolved Microwave Conductivity (TRMC) to study aqueous NPs and bulk thin films composed of a blend of EH-IDTBR and PTB7-Th and examine the relationship between composition, interfacial surface area, charge carrier dynamics, and photocatalytic activity. We find that NP photocatalytic activity corresponds directly to the low-fluence estimations of the yield mobility product, and that the NPs have additional higher-order quenching processes relative to bulk samples of the same material composition. These results suggest that catalytic activity in these particles is limited by the concentration of electrons and holes in operando and not a finite number of active surface cites or the catalytic rate at the interface. This provides a clear design goal for the next generation of efficient photocatalytic particles.