Coulomb Barrier for Sequential Two-Electron Transfer in a Nanoengineered Photocatalyst.

Multielectron photocatalysis requires sequential, multiple charge transfer from the light absorber to the catalytic site. As a result, many-body effects induced by charge accumulation play a fundamental role in these reactions, especially when photocatalysts are miniaturized to the nanoscale. Here, we study sequential two-electron transfer in a state-of-the-art nanophotocatalyst, CdSe@CdS dot-in-rod (DIR) decorated with Pt tips, using pump-pump-probe transient absorption spectroscopy. Following the first electron transfer (ET) from DIR to the Pt tip, the second ET needs to not only compete with Auger recombination of a positively charged exciton but also experience a large Coulomb barrier exerted by two holes. As a result, both the ET rate and efficiency decrease by an order of magnitude. Analysis using a dissociation-limited long-range charge transfer model reveals that the Coulomb barrier of the second ET is ∼60 meV higher than that of the first one. This study not only uncovers the mechanism and efficiency bottleneck of a real multielectron photocatalyst but also provides general guidelines for the design of multielectron photocatalytic systems.