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Issue 11, 2015
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Phenyl-triazine oligomers for light-driven hydrogen evolution

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Abstract

The design of stable, yet highly tunable organic photocatalysts which orchestrate multi-step electron transfer reactions is at the heart of the newly emerging field of polymer photocatalysis. Covalent triazine frameworks such as the archetypal CTF-1 have been theorized to constitute a new class of photocatalytically active polymers for light-driven water splitting. Here, we revisit the ionothermal synthesis of CTF-1 by trimerization of 1,4-dicyanobenzene catalyzed by the Lewis acid zinc chloride and demonstrate that the microporous black polymer CTF-1 is essentially inactive for hydrogen evolution. Instead, highly photoactive phenyl-triazine oligomers (PTOs) with higher crystallinity as compared to CTF-1 are obtained by lowering the reaction temperature to 300 °C and prolonging the reaction time to >150 hours. The low reaction temperature of the PTOs largely prevents incipient carbonization and thus results in a carbon-to-nitrogen weight ratio close to the theoretical value of 3.43. The oligomers were characterized by MALDI-TOF and quantitative solid-state NMR spectroscopy, revealing variations in size, connectivity and thus nitrile-to-triazine ratios depending on the initial precursor dilution. The most active PTO samples efficiently and stably reduce water to hydrogen with an average rate of 1076 (±278) μmol h−1 g−1 under simulated sunlight illumination, which is competitive with the best carbon nitride-based and purely organic photocatalysts. The photocatalytic activity of the PTOs is found to sensitively depend on the polymerization degree, thus suggesting a prominent role of the unreacted nitrile moieties in the photocatalytic process. Notably, PTOs even show moderate hydrogen production without the addition of any co-catalyst.

Graphical abstract: Phenyl-triazine oligomers for light-driven hydrogen evolution

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Publication details

The article was received on 19 Aug 2015, accepted on 07 Sep 2015 and first published on 15 Sep 2015


Article type: Paper
DOI: 10.1039/C5EE02574E
Energy Environ. Sci., 2015,8, 3345-3353
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    Phenyl-triazine oligomers for light-driven hydrogen evolution

    K. Schwinghammer, S. Hug, M. B. Mesch, J. Senker and B. V. Lotsch, Energy Environ. Sci., 2015, 8, 3345
    DOI: 10.1039/C5EE02574E

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