Abstract
Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. However, beyond basic considerations such as spectral absorption, the factors that dictate their photocatalytic activity are poorly understood. Here we investigate a series of linear conjugated polymers with external quantum efficiencies for hydrogen production between 0.4 and 11.6%. We monitor the generation of the photoactive species from femtoseconds to seconds after light absorption using transient spectroscopy and correlate their yield with the measured photocatalytic activity. Experiments coupled with modeling suggest that the localization of water around the polymer chain due to the incorporation of sulfone groups into an otherwise hydrophobic backbone is crucial for charge generation. Calculations of solution redox potentials and charge transfer free energies demonstrate that electron transfer from the sacrificial donor becomes thermodynamically favored as a result of the more polar local environment, leading to the production of long-lived electrons in these amphiphilic polymers.
Highlights
Conjugated polymers have sparked much interest as photocatalysts for hydrogen production
In the presence of a sacrificial electron donor (SED) the reaction sequence for hydrogen evolution is believed to involve photon absorption resulting in the formation of an excited electron–hole pair, exciton diffusion, hole transfer to the SED, and electron transfer to a proton, but there are no direct studies of this reaction sequence for polymer photocatalysts
Solid-state carbon-13 nuclear magnetic resonance (13C NMR) (Supplementary Figure 1, Supplementary Table 1) confirmed that polymerization had occurred and scanning electron microscopy (Supplementary Figure 2c) revealed flake-like primary particles ranging from 50 to 500 nm that aggregate into bigger particles ranging from 5 to 10 μm
Summary
Conjugated polymers have sparked much interest as photocatalysts for hydrogen production. In the presence of a SED the reaction sequence for hydrogen evolution is believed to involve photon absorption resulting in the formation of an excited electron–hole pair (exciton), exciton diffusion, hole transfer to the SED, and electron transfer to a proton, but there are no direct studies of this reaction sequence for polymer photocatalysts This sequence is analogous to photoinduced charge transfer in organic photovoltaics, where photoexcitation of a polymer generates an exciton that dissociates after diffusion to the interface with a second material of different electron affinity (EA) or ionization potential (IP)[37]. Due to the low dielectric permittivity in organic semiconductors, the binding energy for these photogenerated carriers is normally too large for spontaneous dissociation at room temperature, but exciton diffusion to the polymer–water interface may allow photogenerated carriers to reach a reaction site for charge transfer to an electron or hole acceptor in the reaction medium. We rationalize the differences in photocatalytic activity between the polymers in terms of differences in their light absorption, thermodynamic driving forces, excited state lifetimes, electronic structure, microstructure, and interfacial interactions
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