Origin of Enhanced Photocatalytic Activity in Direct Band Gap g-C3N4 Nanoribbons with Tunable Electronic Properties for Water- Splitting Reaction: A First-Principles Study

In this work, we employ density functional theory (DFT) to investigate the edge atomic structures and atomic boundaries in graphitic carbon nitride (g-C3N4) nanoribbons to explore their role on structural stability and electronic and photocatalytic properties. Interestingly, the nanoribbon structures with mirror twin boundaries (MTBs) have higher structural stability than the conventional nanoribbon structures due to the C-C bond formations at the MTB region. Irrespective of their edge atomic structure, the curved and corrugated nanoribbons with direct band gap are thermodynamically more stable than the planar nanoribbons with indirect band gap. In addition, the distinct electronic structures of nanoribbons with and without MTB are calculated to understand their influence on the band gap and band edge positions of the nanoribbons. Very importantly, unlike the other nanostructures of g-C3N4, nanoribbons are shown to possess unique electronic structures that facilitate the tunable spatial separation of valence and conduction band states. This enhances the lifetime of excited charge carriers in nanoribbon morphology. To garner deep insights into the photocatalytic properties of the g-C3N4 monolayer and nanoribbons, the Gibbs free energies (Delta G) of hydrogen and oxygen evolution reaction intermediates are studied to identify the active sites. To this end, our DFT studies predict enhanced photocatalytic activity of g-C3N4 nanoribbons over the monolayer while providing new insights into the geometry, electronic structure, and photocatalytic properties of the nanoribbons, guiding the plausible development of g-C3N4 nanoribbons.