Polymer Photocatalysts for Water Splitting: Insights from Computational Modeling

Abstract

Based on insights from computational chemistry calculations, the ability of polymers to act as water splitting photocatalysts for the production of renewable hydrogen from water and sunlight is discussed. Specifically, the important role of exciton dissociation in these materials is highlighted, as well as the possible microscopic origins of the experimentally observed changes in the photocatalytic activity of a polymer with increasing chain length or changing chemical composition. The reason why water oxidation, with polymeric photocatalysts, is difficult, and which polymer properties to target when developing new polymers for water splitting photocatalysis are, finally, also discussed.

Exploring the “Goldilocks Zone” of Semiconducting Polymer Photocatalysts by Donor–Acceptor Interactions

“…We thankM µriaP iatkovµ,M artina Rimmele, andA ndrØ Sanchesf or upscalinga nd help with fluorescence measurements; BarboraB alcarova forN 2…”

“…[8] Moreover, D-A dyads with sulfur-containing donor groups and electronegative (nitrogen-and oxygen-containing) acceptor groups enable very narrow band gaps, [9] tuning of emitted colour, [10] and enhanced photoluminescence. [1,2] We explored D-A polymer photocatalysts in as ystematic fashion to identify the "Goldilocks Zone" in which the band gap is sufficiently wide to enable proton reduction, and where charge transfer is sufficiently strong to prevent undesired electron-hole recombination. Calculations suggest that D-A systems have narrow band gaps only if there are weak D-A interactions,w hile strong interactions lead to charge transfer and wide band gaps.…”

“…[1] In photocatalytic water splitting, the ideal band gap has to straddle the proton reduction and the water oxidation potentials.I np ractice,o nly one half-reaction (proton reduction) occurs preferentially,b ecause the fourhole water oxidation is sluggish in comparison to spontaneous electron-hole recombination within the catalyst. [2] Sacrificial electron donors,s uch as triethanolamine (TEOA), that have more negative potentials and require only two-hole oxidation are routinely employed, and experiments show that the optimal band gap for this redox system is 2.2-2.4 eV. [3] Synthetic strategies to narrow down the optical band gap of polymers commonly rely on sequential increases of the size of p-conjugated segments and hence,ared shift of the optical adsorption edge.…”

“…[13] Incorporating D-A motifs into polymer photocatalysts is still useful, since fast electron-hole recombination is am ajor detrimental factor for efficient photocatalysis. [1,2] We explored D-A polymer photocatalysts in as ystematic fashion to identify the "Goldilocks Zone" in which the band gap is sufficiently wide to enable proton reduction, and where charge transfer is sufficiently strong to prevent undesired electron-hole recombination. To this end, we combined C 2 and C 3 symmetric building blocks of similar sizes that contain electron-rich sulfur moieties and electron-withdrawing triazine (C 3 N 3 ) units (Scheme 1).…”

Fluorescent Sulphur‐ and Nitrogen‐Containing Porous Polymers with Tuneable Donor–Acceptor Domains for Light‐Driven Hydrogen Evolution

“…[1] For example, conjugated microporous polymers (CMPs) can be preparedf rom statistical co-polymerisation of polycyclic, aromatic sub-units of varying sizes that enablep ore-size tuning, [2] ac ontinuous spectrum of bandgaps, [3] and as ar esult also varying degrees of photocatalytic activity. [4] The chemical make-up of the overwhelming majority of CMPs is carbon-only.A saconsequence, charge-transfer is insufficient to prevent spontaneousr ecombination of photo-induced electron-hole pairs, [5] and efficient photocatalysis using CMPs strictly relies on electron migration to an oblemetal co-catalyst, usually platinum (Pt). In practice, nearly all photocatalytic water splitting studies explore one half-reaction-proton reduction-in as et-up that provides as acrificial electron donor, such as triethanolamine (TEOA) and Pt as ac ocatalyst.…”