Hole scavenger-free nitrogen photofixation in pure water with non-metal B-doped carbon nitride: implicative importance of B species for N₂ activation

Being the energy glutton and top ranker of greenhouse gas emission, Haber-Bosch is undoubtedly a grand challenge to reaching sustainability and carbon neutrality. Solar-driven nitrogen (N₂) fixation under mild conditions is highly attractive and promising for sustainable ammonia (NH₃) production. However, low conversion efficiency remains as a serious problem yet to be solved due to the difficult activation of strongly non-polar N[tbnd]N bond. In this study, defective and atomically thin g-C₃N₄ with boron sites was synthesized via a facile decomposition-thermal polymerization technique. The optimum photocatalyst displayed an NH₃ yield of 213.59 μmol g_cat⁻¹ h⁻¹ under visible light irradiation (λ > 400 nm) without any hole scavenger, bestowing high suitability for N₂ fixation in pure water. The impressive performance is largely ascribed to the introduction of B species, which could act as the active sites for N₂ adsorption, activation and reduction. Additionally, the atomically thin layer (∼3.25 nm) shortens charge carrier transport pathway while the B atoms and defects suppress quantum size effect as well as extends photo-response through bandgap reduction. These attributes synergistically lead to suppressed electron-hole recombination, efficacious charge transport, enhanced light utilization and enriched catalytic sites for N₂ fixation. As a whole, this work presents an easy strategy to realize efficient N₂ photofixation over a metal-free, low-cost and non-toxic photocatalyst in pure water as a continuous effort in advancing the burgeoning field of photocatalytic N₂ fixation.