Abstract
Semiconducting polymers are versatile materials for solar energy conversion and have gained popularity as photocatalysts for sunlight-driven hydrogen production. Organic polymers often contain residual metal impurities such as palladium (Pd) clusters that are formed during the polymerization reaction, and there is increasing evidence for a catalytic role of such metal clusters in polymer photocatalysts. Using transient and operando optical spectroscopy on nanoparticles of F8BT, P3HT, and the dibenzo[b,d]thiophene sulfone homopolymer P10, we demonstrate how differences in the time scale of electron transfer to Pd clusters translate into hydrogen evolution activity optima at different residual Pd concentrations. For F8BT nanoparticles with common Pd concentrations of >1000 ppm (>0.1 wt %), we find that residual Pd clusters quench photogenerated excitons via energy and electron transfer on the femto-nanosecond time scale, thus outcompeting reductive quenching. We spectroscopically identify reduced Pd clusters in our F8BT nanoparticles from the microsecond time scale onward and show that the predominant location of long-lived electrons gradually shifts to the F8BT polymer when the Pd content is lowered. While a low yield of long-lived electrons limits the hydrogen evolution activity of F8BT, P10 exhibits a substantially higher hydrogen evolution activity, which we demonstrate results from higher yields of long-lived electrons due to more efficient reductive quenching. Surprisingly, and despite the higher performance of P10, long-lived electrons reside on the P10 polymer rather than on the Pd clusters in P10 particles, even at very high Pd concentrations of 27000 ppm (2.7 wt %). In contrast, long-lived electrons in F8BT already reside on Pd clusters before the typical time scale of hydrogen evolution. This comparison shows that P10 exhibits efficient reductive quenching but slow electron transfer to residual Pd clusters, whereas the opposite is the case for F8BT. These findings suggest that the development of even more efficient polymer photocatalysts must target materials that combine both rapid reductive quenching and rapid charge transfer to a metal-based cocatalyst.
Highlights
Solar hydrogen production via photocatalysis provides a pathway to generate hydrogen as a carbon-free energy carrier in a clean and renewable way
Absorbance and photoluminescence spectra are in good agreement with literature reports for all three polymers, suggesting that the F8BT and P3HT nanoparticles used here are comparable to thin films of these polymers used for other applications such as organic photovoltaics or organic lightemitting diodes.[35−37] P10 is primarily a photocatalyst, and its optical properties are in good agreement with those reported in our earlier study.[20]
We previously demonstrated that residual Pd can be efficiently removed from F8BT using a combination of gel permeation chromatography (GPC) and washing with sodium N,N-diethyldithiocarbamate (DTC),[3] which yielded a series of F8BT polymer batches with Pd concentrations of 1170, 195, 36, and
Summary
Solar hydrogen production via photocatalysis provides a pathway to generate hydrogen as a carbon-free energy carrier in a clean and renewable way. The key requirement is a highly active and stable photocatalyst that can act as a light absorber and catalyze the desired chemical reactions for example, direct hydrogen production from water. Rather than performing overall water splitting, most such organic photocatalysts are currently employed to couple the hydrogen evolution reaction to a sacrificial organic oxidation.[1,2] To this end, the photocatalyst is suspended in a mixture of water and an organic electron donor, which serve as the proton and electron sources, respectively. The detailed catalytic mechanisms of such organic photocatalysts are still mostly unexplored, and a growing body of evidence suggests that residual metal impurities can act as cocatalysts for the hydrogen evolution reaction in polymeric materials.[3−5]
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