Abstract
Covalent triazine-based frameworks (CTFs), a group of semiconductive polymers, have been identified for photocatalytic water splitting recently. Their adjustable band gap and facile processing offer great potential for discovery and development. Here, we present a series of CTF-0 materials fabricated by two different approaches, a microwave-assisted synthesis and an ionothermal method, for water splitting driven by visible-light irradiation. The material (CTF-0-M2) synthesized by microwave technology shows a high photocatalytic activity for hydrogen evolution (up to 7010 μmol h–1 g–1), which is 7 times higher than another (CTF-0-I) prepared by conventional ionothermal trimerization under identical photocatalytic conditions. This leads to a high turnover number (TON) of 726 with respect to the platinum cocatalyst after seven cycles under visible light. We attribute this to the narrowed band gap, the most negative conduction band, and the rapid photogenerated charge separation and transfer. On the other hand, the material prepared by the ionothermal method is the most efficient one for oxygen evolution. CTF-0-I initially produces ca. 6 times greater volumes of oxygen gas than CTF-0-M2 under identical experimental conditions. CTF-0-I presents an apparent quantum efficiency (AQY) of 5.2% at 420 nm for oxygen production without any cocatalyst. The activity for water oxidation exceeds that of most reported CTFs due to a large driving force for oxidation and a large number of active sites. Our findings indicate that the band positions and the interlayer stacking structures of CTF-0 were modulated by varying synthesis conditions. These modulations impact the optical and redox properties, resulting in an enhanced performance for photocatalytic hydrogen and oxygen evolution, confirmed by first-principles calculations.
Highlights
Conversion of simple molecules into chemical fuels by artificial photosynthesis via solar energy is an environmentally friendly way of producing clean sustainable power and to reduce greenhouse gas emissions.[1]
It is suggested that the Covalent triazine-based frameworks (CTFs)-0 tends to form AA stacking at high temperature and over a long period of time as in the ionothermal method and AB stacking in the low temperature and fast microwave approaches, namely, microwave irradiation targets a lower energy stacking structure with a C−N interlayer coupling, while the ionothermal method facilitates the reorganization of monomers with C−C interlayer coupling
The covalent triazine-based framework CTF-0-M2 produced by the microwave method with the most ordered interlayer structure and the highest amount of triazine units shows the best photocatalytic hydrogen evolution under both UV and visiblelight irradiation, resulting in ca. 8% apparent quantum efficiency (AQY) at 420 nm and 2% at 500 nm
Summary
Conversion of simple molecules into chemical fuels by artificial photosynthesis via solar energy is an environmentally friendly way of producing clean sustainable power and to reduce greenhouse gas emissions.[1]. Photocatalytic hydrogen generation has been widely investigated over these conjugated polymers such as graphitic carbon imide),[24] and other nitride relevant (g-C3N4),[18−23] triazine-based poly(triazine networks.[25−27] Such polymer photocatalysts possess delocalized π-bonds, which provide established pathways for charge-carrier transport and present high efficiencies for hydrogen evolution. CTF-0-I prepared by the ionothermal trimerization method possesses more AAstacking tendency and the lowest nitrogen to carbon ratio, presenting the best water oxidation performance We further confirmed this important correlation between photocatalytic activity, band alignment, and charge dynamics by diverse spectroscopies and theoretical modeling
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