Visible light-driven
water splitting using cheap and robust photocatalysts
is one of the most exciting ways to produce clean and renewable energy
for future generations. Cutting edge research within the field focuses
on so-called “Z-scheme” systems, which are inspired
by the photosystem II–photosystem I (PSII/PSI) coupling from
natural photosynthesis. A Z-scheme system comprises two photocatalysts
and generates two sets of charge carriers, splitting water into its
constituent parts, hydrogen and oxygen, at separate locations. This
is not only more efficient than using a single photocatalyst, but
practically it could also be safer. Researchers within the field are
constantly aiming to bring systems toward industrial level efficiencies
by maximizing light absorption of the materials, engineering more
stable redox couples, and also searching for new hydrogen and oxygen
evolution cocatalysts. This review provides an in-depth survey of
relevant Z-schemes from past to present, with particular focus on
mechanistic breakthroughs, and highlights current state of the art
systems which are at the forefront of the field.
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-M 2 ) 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-M 2 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 firstprinciples calculations.
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