Structure‐Property Relationship in Lightly and Heavily Gas‐Phase N‐Doped TiO ₂ Nanoparticles for Solar‐Driven Photocatalysis

Lightly and heavily nitrogen (N)-doped TiO₂ systems (LD-TO and HD-TO) were synthesized via ammonia (NH₃) gas-assisted annealing at 450 and 750 °C. XRD patterns showed low- and high-angle shifts for LD-TO and HD-TO, indicating lattice expansion (LD-TO) and contraction in HD-TO, with a Ti-oxynitride (TiON) peak in HD-TO indicating secondary phase segregation. FESEM analysis shows spherical particles (∼30 nm for TO) growing to ∼40-50 nm with a layered structure in doped samples. BET surface areas were 37.8, 41.8, and 27.1 m²/g for TO, LD-TO, and HD-TO, respectively. XPS confirmed Ti⁴⁺, O^2-, and N doping of ∼5.7% (LD-TO) and 10.4% (HD-TO). Optical analysis showed reduced band gaps (3.12 eV for LD-TO, 3.02 eV for HD-TO versus 3.21 eV for TO) and improved light absorption. Urbach energies (0.603 meV for LD-TO; 0.926 meV for HD-TO) reflected moderate defect density in LD-TO. Electrochemical measurements indicated enhanced charge transfer, with LD-TO outperforming HD-TO in photocurrent response. LD-TO degraded ∼98% rhodamine B in 150 min and produced H₂ at 935.2 µmol g^-1 h^-1 under sunlight, compared to 86% degradation and 765.4 µmol g^-1 h^-1 for HD-TO. These results highlight that optimal N doping enhances TiO₂'s electronic, structural, and photocatalytic properties, while excessive doping leads to secondary phase formation and reduced performance.