  The heptazine-based polymer melon (also known as graphitic carbon nitride,
g-C3N4), is a promising photocatalyst for hydrogen evolution. Nonetheless,
attempts to improve its inherently low activity are rarely based on rational
approaches due to a lack of fundamental understanding of its mechanistic
operation. Here, we employ molecular heptazine-based model catalysts to
identify the cyanamide moiety as a photocatalytically relevant "defect". We
exploit this knowledge for the rational design of a carbon nitride polymer
populated with cyanamide groups, yielding a material with 12- and 16-times the
hydrogen evolution rate and apparent quantum efficiency (400 nm), respectively,
compared to the benchmark melon. Computational modelling and material
characterization suggest this moiety improves co-ordination (and, in turn,
charge transfer kinetics) to the platinum co-catalyst and enhances the
separation of the photo-generated charge carriers. The demonstrated knowledge
transfer for rational catalyst design presented here provides the conceptual
framework for engineering high performance heptazine-based photocatalysts.
