In-Plane Graphene Moieties Embedded in Carbon Nitride Extend Conjugation, Narrow the Electronic Bandgap, and Generate a 10-fold Enhancement in Visible Light-Driven Photoelectrochemical Performance

We realized a <i>sp</i>² π-conjugated heterointerface comprising graphitic carbon nitride (<i>g</i>-C₃N₄) and a graphenic moiety, consisting of six-membered carbon rings (C_ring) through an <i>in situ</i> copolymerization protocol using dicyandiamide and 1,4-phenylenediamine (<i>p</i>-PDA) precursors. Advanced characterization using XANES, ssNMR, EELS, HRXPS, and UPS analysis revealed that the additional C atoms form a continuous <i>sp</i>² hybridized network sharing N and other C atoms of the heptazine moieties in a 2D-conjugated system, resulting in a narrowing of the bandgap from 2.6 to 2.13 eV. The best performing <i>p</i>-PDA modified graphitic carbon nitride (CN-C_ring) hybrid showed a 9.6 times enhancement of water-splitting photocurrent density with respect to the pristine CN under AM1.5G irradiation and a doubling of the electron drift mobility to 3.38 × 10^-3 cm² V^-1 s^-1. The narrower electronic bandgap, higher carrier mobility, nearly 4-fold increase in surface photovoltage, and improvement in water-splitting photocurrent of carbon nitride hybrids are attributed to extended π-conjugation induced delocalization of photocarriers, lower recombination, and a higher density of states in both the LUMO and HOMO of CN-C_ring. Density functional theory tight-binding (DFTB) calculations indicated an enhanced photocarrier separation and improved charge transport landscape in the hybrid that involves both interlayer and intralayer components, unlike the case of pristine carbon nitride, where charge transport is known to be dominated by interlayer pathways.