Strain-Engineered s-C ₃ N ₆ Monolayer for Efficient Water Splitting: A First-Principles Study

Photocatalytic water splitting offers a sustainable route for solar-to-hydrogen energy conversion. However, identifying metal-free semiconductors that simultaneously satisfy stability, electronic, optical, and band-alignment requirements remains challenging. Here, we investigate the structural, mechanical, electronic, optical, and photocatalytic properties of the recently proposed two-dimensional s-C3N6 monolayer using first-principles density functional theory. Ab initio molecular dynamics simulations confirm its thermal stability at ambient and elevated temperatures, while elastic constant analysis demonstrates mechanical robustness. Hybrid HSE06 calculations reveal that the pristine s-C3N6 monolayer is a direct-band-gap semiconductor with a gap of 2.62 eV. However, its conduction-band minimum lies below the hydrogen reduction potential, preventing spontaneous hydrogen evolution. To overcome this limitation, we employ strain engineering to modulate its electronic structure and band-edge alignment. Both biaxial and uniaxial strains in the range of −10 to +10% are systematically explored. We find that compressive biaxial strains of −8 and −10% uniquely tune the band edges to simultaneously satisfy the thermodynamic requirements for hydrogen and oxygen evolution reactions, enabling spontaneous overall water splitting. These photocatalytically active strain (−8 and −10% biaxial strains) states are further shown to be mechanically and thermally stable, underlining their experimental feasibility. The calculations of the optical transitions both in the pristine and photocatalytically active strained structures show that the fundamental electronic gap is optically dark due to symmetry-forbidden transitions, with the first absorption peak in the UV region, arising from deeper valence states. Crucially, a strain-induced mobility mismatch between electrons and holes, arising from divergent effective mass trends, facilitates efficient charge separation and carrier protection. However, thermodynamic modeling of surface kinetics reveals that the s-C3N6 surface binds intermediates too strongly, necessitating a co-catalyst to overcome the resulting kinetic barriers. Our results establish strain engineering as an effective strategy to simultaneously tailor band-edge alignment, carrier dynamics, and optical transitions in the s-C3N6 monolayer, highlighting its potential for stable charge generation for 2D photocatalytic water splitting.