Why Do Sulfone-Containing Polymer Photocatalysts Work So Well for Sacrificial Hydrogen Evolution from Water?

Abstract

Many of the highest-performing polymer photocatalysts for sacrificial hydrogen evolution from water have contained dibenzo [b,d]thiophene sulfone units in their polymer backbones. However, the reasons behind the dominance of this building block are not well understood. We study films, dispersions, and solutions of a new set of solution-processable materials, where the sulfone content is systematically controlled, to understand how the sulfone unit affects the three key processes involved in photocatalytic hydrogen generation in this system: light absorption; transfer of the photogenerated hole to the hole scavenger triethylamine (TEA); and transfer of the photogenerated electron to the palladium metal co-catalyst that remains in the polymer from synthesis. Transient absorption spectroscopy and electrochemical measurements, combined with molecular dynamics and density functional theory simulations, show that the sulfone unit has two primary effects. On the picosecond timescale, it dictates the thermodynamics of hole transfer out of the polymer. The sulfone unit attracts water molecules such that the average permittivity experienced by the solvated polymer is increased. We show that TEA oxidation is only thermodynamically favorable above a certain permittivity threshold. On the microsecond timescale, we present experimental evidence that the sulfone unit acts as the electron transfer site out of the polymer, with the kinetics of electron extraction to palladium dictated by the ratio of photogenerated electrons to the number of sulfone units. For the highest-performing, sulfone-rich material, hydrogen evolution seems to be limited by the photogeneration rate of electrons rather than their extraction from the polymer.

Impact of Interfaces, and Nanostructure on the Performance of Conjugated Polymer Photocatalysts for Hydrogen Production from Water

“…The use of organic scavengers allows the access to a range of different electrochemical oxidation potentials [ 49 ], however their impact on dispersion and dielectric constant of the interface also needs to be considered [ 95 ]. Generally speaking, the use of organic scavengers will be aiding the dispersion of polymeric organic photocatalysts, but these can also lead to enrichment at the interface with the water/scavenger mixture to the point where only a few water molecules are able to access the surface [ 67 ]. Inert solvents can also be used as co-solvents that do not participate in the photocatalytic reaction itself but simply aid dispersion of the photocatalyst and wetting of the surface.…”

“…A series of solution-processible fluorene co-polymers with increasing dibenzo[ b , d ]thiophene sulfone content have shown a similar trend, with polyfluorene being nearly inactive and the activity for sacrificial hydrogen production under visible light increasing from FS1 (EQE at 420 nm = 0.04%, 3% dibenzo[ b , d ]thiophene sulfone content) to FS5 (EQE at 420 nm = 2.07%, 50% dibenzo[ b , d ]thiophene sulfone content). Furthermore, it was found that the sulfone group does not only influence the interaction with water but also assists the formation of polarons and the charge transfer to palladium where proton reduction occurs [ 67 ]. Other studies have also shown that a complex interplay of different factors is important.…”

“…When studying water oxidation typically only silver nitride is used as the electron scavenger, thus the material has to be relatively water compatible to disperse well and function as an efficient photocatalyst. It is therefore fully rational that polar materials have been reported as photocatalysts for water oxidation: Triazine-containing CTFs are sufficiently wettable to disperse in silver nitrate solution and facilitate water oxidation on their surface [ 21 , 67 , 87 ]. For example, RuO 2 -loaded CTF-T1 which has phenylene-linkers between the triazine units produces >10 µmol g −1 h −1 under visible light irradiation (λ > 420 nm, 300 W Xe light source) [ 62 ], and cobalt-loaded bipyridine containing Bpy-CTF–Co-3 had an oxygen evolution rate of approx.…”

“…A higher ratio results in a lower electron population on the photocatalyst and thus more efficient charge extraction. The study suggests that the sulfone unit acts as the electron transfer site for the electron polaron to the palladium co-catalyst, and that hydrogen evolution appears to be limited by the photogeneration rate of electrons rather than their extraction from the polymer to the co-catalyst [ 67 ].…”

Synergistic Contribution of Oligo(ethylene glycol) and Fluorine Substitution of Conjugated Polymer Photocatalysts toward Solar Driven Sacrificial Hydrogen Evolution

“…For the higher performance of polymer photocatalysts, the higher thermodynamic driving force for oxidation process of hole scavengers leads to the faster electron extraction rates for palladium reduction. [23] In this regard, increasing the hydrophilicity of organic semiconductors is considered an efficient strategy to induce more intact interactions between the polymer photocatalysts and water, resulting in a boosted HER. [24,25] For enhancing polymer hydrophilicity, polar groups can be incorporated into the polymer backbone, thereby improving the hydrogen generation activity.…”