Graphitic carbon nitride (g-C₃N₄) has gained considerable attention being a promising visible light responsive photocatalyst and owing to its wide range of applications. Defect engineering further plays a crucial role in improving the photocatalytic activity of the material. The goal of the work is two-fold: first, to elucidate the role of dimensionality on the properties of photo-generated charge carriers, second, to analyze which defects could lead to better performance and, here, we focus on the primary aim. We carry out ab initio nonadiabatic molecular dynamics (NA-MD) combined with time-domain density functional theory (TDDFT) to investigate photo-induced excitation dynamics in bulk and monolayer g-C₃N₄. The nonradiative charge recombination is suppressed in the nanosheet as compared to the bulk, in agreement with experiment. The difference arises due to the smaller energy gap and participation of additional vibrations in the bulk system. The long carrier lifetimes are favored by small NA coupling (NAC) and rapid phonon-induced loss of quantum coherence between the excited and ground electronic states. The NAC is small due to localization of electrons and holes on different atoms while decoherence is fast (< 5 fs), because g-C₃N₄ being soft undergoes large scale vibrations. Intraband relaxation of hot electrons and holes (100-200 fs) is slightly slower at higher initial energies due to participation of fewer vibrational modes. This feature of 2D g-C₃N₄ contrasts traditional 3D semiconductors, which exhibit faster relaxation at higher energies due to increased density of states and can be used to extract hot carriers to perform useful functions. The ab initio quantum dynamics simulations would present a detailed atomistic picture of the photo-induced charge carrier dynamics in g-C₃N₄, guiding design of photovoltaic and photocatalytic devices.

Dhara Trivedi

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