Controlled Assembly of Hydrogenase-CdTe Nanocrystal Hybrids for Solar Hydrogen Production

Abstract

We present a study of the self-assembly, charge-transfer kinetics, and catalytic properties of hybrid complexes of CdTe nanocrystals (nc-CdTe) and Clostridium acetobutylicum [FeFe]-hydrogenase I (H(2)ase). Molecular assembly of nc-CdTe and H(2)ase was mediated by electrostatic interactions and resulted in stable, enzymatically active complexes. The assembly kinetics was monitored by nc-CdTe photoluminescence (PL) spectroscopy and exhibited first-order Langmuir adsorption behavior. PL was also used to monitor the transfer of photogenerated electrons from nc-CdTe to H(2)ase. The extent to which the intramolecular electron transfer (ET) contributed to the relaxation of photoexcited nc-CdTe relative to the intrinsic radiative and nonradiative (heat dissipation and surface trapping) recombination pathways was shown by steady-state PL spectroscopy to be a function of the nc-CdTe/H(2)ase molar ratio. When the H(2)ase concentration was lower than the nc-CdTe concentration during assembly, the resulting contribution of ET to PL bleaching was enhanced, which resulted in maximal rates of H(2) photoproduction. Photoproduction of H(2) was also a function of the nc-CdTe PL quantum efficiency (PLQE), with higher-PLQE nanocrystals producing higher levels of H(2), suggesting that photogenerated electrons are transferred to H(2)ase directly from core nanocrystal states rather than from surface-trap states. The duration of H(2) photoproduction was limited by the stability of nc-CdTe under the reactions conditions. A first approach to optimization with ascorbic acid present as a sacrificial donor resulted in photon-to-H(2) efficiencies of 9% under monochromatic light and 1.8% under AM 1.5 white light. In summary, nc-CdTe and H(2)ase spontaneously assemble into complexes that upon illumination transfer photogenerated electrons from core nc-CdTe states to H(2)ase, with low H(2)ase coverages promoting optimal orientations for intramolecular ET and solar H(2) production.

Efficient and Limiting Reactions in Aqueous Light-Induced Hydrogen Evolution Systems using Molecular Catalysts and Quantum Dots

“…Further examples of molecular approaches using quantum dots as photosensitizers in pure water involve the use of hydrogenases and their functional mimics. 2,9,[15][16][17][18] Yet, although the catalytic activity of some of these examples is remarkably high (up to hundreds of thousands of turnover numbers), the reported quantum yields are still below 40 % even when using monochromatic light. In this context, a deeper knowledge of the kinetics of both charge transfer and bond formation-breaking are the keys to understand the limitations of photo-driven hydrogen evolving systems based on molecular approaches.…”

Chitosan confinement enhances hydrogen photogeneration from a mimic of the diiron subsite of [FeFe]-hydrogenase

“…The formed hole remaining in the MPA-CdTe species after electron transfer, on the other hand, is subsequently regenerated by electron transfer from the sacrificial electron donor. It is known that the redox potential of H 2 A ( À 0.14 V at pH 4.5) 59 is sufficiently negative to reduce the holes photogenerated in MPA-CdTe species 25,45 , but it is too positive to directly reduce the [FeFe]-H 2 ase catalyst ( Supplementary Fig. S14).…”

“…The astonishing rates of H 2 production from the non-precious diiron catalysts via a group of enzymes under mild conditions can exceed those of platinum. However, the large-scale isolation of the enzyme from natural systems is rather difficult, hence the development of artificial [FeFe]-H 2 ase analogues capable of reproducing the enzymic activity has spurred considerable interest in both the scientific and industrial communities [7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25] . Over the past decade, a variety of mimics of the diiron subsite of [FeFe]-H 2 ase have been shown to function as catalysts for chemical reduction of protons [26][27][28][29][30][31][32][33] .…”

“…It has been clear that electron transfer, either electrochemical or photochemical, to a mimic of the active site of [FeFe]-H 2 ase is a prerequisite for H 2 evolution [10][11][12][13][14]22 . From a photochemical point of view, the electron transfer is triggered by the absorption of a photon by a photosensitizer [13][14][15][16][17][18][19][20][21][22][23][24][25] . Since the first attempt by Sun and Åkermark 34 to construct an artificial photocatalytic system for H 2 evolution in 2003, a large number of synthetic model complexes have been pursued to mimic the structure and functionality of the diiron subunit of the natural [FeFe]-H 2 ase H-cluster [35][36][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51] .…”

Multidisciplinary approaches to solar hydrogen

“…While combining QDs with hydrogenases arguably brings together a highly robust photosensitizer and an extremely efficient catalyst, results on integrated systems have been disappointing. While these systems can have high initial turnover rates [135], they generally show poor longevity [135,136] and can have low overall turnover numbers [137]. Possible weak links in the nanocrystal-hydrogenase system are a poor yield of electron injection into the hydrogenase and a loss of hydrogenase activity when modified with the nanomaterial.…”