Tailoring optical and photocatalytic properties of sulfur-doped boron nitride quantum dots via ligand functionalization

Boron nitride quantum dots (BNQDs) have emerged as promising photocatalysts due to their excellent physicochemical properties. This study investigates strategies to enhance the photocatalytic performance of BNQDs through sulfur-doping (S-BNQDs) and edge-functionalization with ligands (urea, thiourea, p-phenyl-enediamine (PPD)). To analyze the geometry, electronic structure, optical absorption, charge transfer, and photocatalytic parameters of pristine and functionalized S-BNQDs, we performed density functional theory calculations. The results showed that S-doping and ligand functionalization tune the bandgap, band energies, and introduce mid-gap states to facilitate light absorption, charge separation, and optimized energetics for photocatalytic redox reactions. Notably, the PPD ligand induced the most substantial bandgap narrowing and absorption edge red-shift by over 1 electron volt (eV) compared to pristine S-BNQD, significantly expanding light harvesting. Additionally, urea and PPD functionalization increased the charge transfer length by up to 2.5 times, effectively reducing recombination. On the other hand, thiourea functionalization yielded the most favorable electron injection energetics. The energy conversion efficiency followed the order: PPD (15.0%) > thiourea (12.0%) > urea (11.0%) > pristine (10.0%). Moreover, urea functionalization maximized the first-order hyperpolarizability, enhancing light absorption. These findings provide valuable insights into tailoring S-BNQDs through strategic doping and functionalization to develop highly efficient, customized photocatalysts for sustainable applications.