An Abrupt Change of Configuration of Surface Ligands Affects the H2 Production Efficiency of Mediator-Based CdS Nanorod/ Hydrogenase Assemblies

Abstract

Assemblies of inorganic nanocrystals and natural enzymes are promising systems that combine both the unique and tunable photophysical properties of nanocrystals as well as the high catalytic efficiency and selectivity of bioreaction centers. The key challenge in design of these assemblies is to favor the electron/energy transfer (ET) between the abiotic and biotic materials while retarding the undesired charge recombination. The solution to this challenge relies on both rational design of materials with proper energetic and other physical properties, and, equally important if not more so, the surface condition of each component and the configuration of their assemblies. In this study, we investigate a recently developed nanorod and NiFe hydrogenase (H2ase) assemblies which utilize a redox-mediated approach to shuttle the electron transfer between the nanorods and the H2ase enzymes with an impressive H2 production quantum yield up to 77 %.1 We study the impact of alkyl chain lengths of a common type of capping ligand for nanocrystals, mercaptoalkylcarboxylate, on the H2 production quantum yield of the system and elaborate the ligand’s impact on the underlying ET transfer between NRs and the electron acceptors. We observed the existence of an abrupt decrease of the quantum yield for H2 production of the system when increasing the alkyl chain length of the ligands from n= 7 to 10 (35 % and 12 %, respectively), whereas only a minor performance decrease is observed when n is below 7 (35 % and 42 % for n=7 and 2, respectively). These results are further shown in good agreement with the sudden decrease of the yield of the reduced mediator, propyl-bridged 2-2’-bipyridinium (PDQ2+), during the steady-state photoreduction experiments, suggesting that generation efficiency of the redox equivalents control the overall efficiency of the current redox-mediated systems. Further transient spectroscopic measurements revealed that the intrinsic ET transfer rates from the NR to the mediator PDQ are all on the order of 108 s-1 regardless of the length of the capping ligands. Instead, the amount of the average surface attached PDQ2+ molecules decreases dramatically when increasing the length of n above 7, with a saturated surface coverage of σ=40 to σ =0.5 for n=2 and n= 7, respectively. These results cannot be explained by the commonly perceived ligand length dependent ET transfer by tunneling through a barrier: k = exp (- ) on the nanorod surface, with the distance dependent constants, , reported to between 0.3 to 0.8 Å-1.2,3 Instead, these results are rationalized by the accessibility of CdS surface to electron acceptors, probably due to the change of the ligand configuration from the disordered to ordered phases. These results characterize quantitively the efficiency limiting step in the mediator based CdS nanorod/Hydrogenases assemblies, demonstrate impact of configurational arrangements of ligands on NR surface on its ET behavior, and therefore are important for the design of biotic and abiotic systems for various applications. Acknowledgment Wenxing Yang acknowledges the financial support from the Swedish Research Council for an International Postdoc Fellowship. Reference (1) Chica, B.; Wu, C. H.; Liu, Y.; Adams, M. W. W.; Lian, T.; Dyer, R. B. Balancing Electron Transfer Rate and Driving Force for Efficient Photocatalytic Hydrogen Production in CdSe/CdS Nanorod-[NiFe] Hydrogenase Assemblies. Energy Environ. Sci. 2017, 10 (10), 2245–2255. (2) Tagliazucchi, M.; Tice, D. B.; Sweeney, C. M.; Morris-Cohen, A. J.; Weiss, E. A. Ligand-Controlled Rates of Photoinduced Electron Transfer in Hybrid CdSe Nanocrystal/Poly(Viologen) Films. ACS Nano 2011, 5 (12), 9907–9917. (3) Wilker, M. B.; Utterback, J. K.; Greene, S.; Brown, K. A.; Mulder, D. W.; King, P. W.; Dukovic, G. Role of Surface-Capping Ligands in Photoexcited Electron Transfer between CdS Nanorods and [FeFe] Hydrogenase and the Subsequent H 2 Generation. J. Phys. Chem. C 2018, 122 (1), 741–750. Figure 1