Surface-Ligand Liquid to Crystalline Phase Transition Modulates the Solar H2 Production Quantum Efficiency of CdS Nanorod/Mediator/Hydrogenase Assemblies

This study reports how the length of capping ligands on a nanocrystal surface affects its interfacial electron transfer (ET) with surrounding molecular electron acceptors, and consequently, impact the H-2 production of a biotic-abiotic hybrid artificial photosynthetic system. Specifically, we study how the H-2 production efficiency of a hybrid system, combining CdS nanorods (NRs), [NiFe] hydrogenase, and redox mediators (propyl-bridged 2,2'-bipyridinium, PDQ(2+)), depends on the alkyl chain length of mercaptocarboxylate ligands on the NR surface. We observe a minor decrease of the quantum yield for H-2 production from 54 +/- 6 to 43 +/- 2% when varying the number of methylene units in the ligands from 2 to 7. In contrast, an abrupt decrease of the yield was observed from 43 +/- 2 to 4 +/- 1% when further increasing n from 7 to 11. ET studies reveal that the intrinsic ET rates from the NRs to the electron acceptor PDQ(2+) are all within 10(8) -10(9) s(-1) regardless of the length of the capping ligands. However, the number of adsorbed PDQ(2+) molecules on NR surfaces decreases dramatically when n >= 10, with the saturating number changing from 45 +/- 5 to 0.3 +/- 0.1 for n = 2 and 11, respectively. These results are not consistent with the commonly perceived exponential dependence of ET rates on the ligand length. Instead, they can be explained by the change of the accessibility of NR surfaces to electron acceptors from a disordered liquid phase at n < 7 to a more ordered crystalline phases at n > similar to 7. These results highlight that the order of capping ligands is an important design parameter for further constructing nanocrystal/molecular assemblies in broad nanocrystal-based applications.