Vacancy Defect-Assisted Enhanced Nitrogen Fixation in Metal-Free 2D Orthorhombic Boron Nitride: Insights from Nonadiabatic Molecular Dynamics Simulations

The photocatalytic nitrogen reduction reaction (NRR) is a promising approach for green and sustainable ammonia (NH₃) production under ambient conditions. However, the design of highly efficient photocatalysts remains a significant challenge owing to the inertness of N₂ molecules, complex reaction kinetics, and substantial energy barriers. In this work, we investigate the catalytic mechanism and real-time photocarrier dynamics of NRR on pristine and defect-engineered orthorhombic boron nitride (o-B₂N₂), a metal-free and environmentally friendly 2D semiconductor. Using density functional theory (DFT) and time-dependent ab initio nonadiabatic molecular dynamics (NAMD) simulations, we systematically studied the electronic structure, optical absorption, free-energy profile, and dynamics of charge carrier recombination of both pristine and vacancy-defective o-B₂N₂. Our results demonstrate that the introduction of a double vacancy (DV-BN) into o-B₂N₂ significantly widens the band gap, prolongs the electron–hole recombination time from 2.18 ns (pristine) to 3.15 ns, and markedly improves the photocatalytic activity toward NRR. These findings demonstrate the potential of o-B₂N₂ as an efficient, low-cost photocatalyst for sustainable ammonia production.