Phenyl-triazine oligomers for light-driven hydrogen evolution. Issue 11 (28th September 2015)

• Record Type: Journal Article
• Authors: Schwinghammer, K.; Hug, S.; Mesch, M. B.; Senker, J.; Lotsch, B. V.
• Is Part Of: Energy & environmental science. Volume 8:Issue 11(2015)
• Page Start: 3345
• Page End: 3353
• Publication Date: 2015-09-28
• Subjects: Energy conversion -- Periodicals; Fuel switching -- Periodicals; Environmental sciences -- Periodicals; Environmental chemistry -- Periodicals; 333.79
• DOI: 10.1039/c5ee02574e
• Languages: English
• ISSNs: 1754-5692

Abstract

Abstract : Carbon nitride oligomers derived from the covalent triazine framework CTF-1 constitute a new generation of tunable hydrogen evolution photocatalysts, which show moderate activities even without the addition of a co-catalyst. 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.