Surface-engineered biogenic TiO2 nanoparticles with tailored isoelectric point: interfacial electrical double layer interactions, solar mineralisation, and DFT-resolved regioselectivity of azo dye degradation

Biogenically tailored anatase TiO2 nanoparticles (S3, 13–15 nm) were engineered via sequential mild thermal pre-treatment of an Eucalyptus globulus leaf extract-derived precursor, governing crystal nucleation-growth kinetics. This controlled thermal pre-treatment yielded a photocatalyst with enhanced crystallinity, suppressed aggregation, a shifted isoelectric point (IEP = 5.5–6.3), and a functionalised surface bearing residual biogenic carbonaceous species (5.33 wt% C). The S3 catalyst matched or outperformed commercial Degussa P25 in the degradation and mineralisation of three anionic azo dyes, Methyl Orange (MO), Acid Orange 7 (AO), and Orange G (OG), under both UV and natural solar irradiation, demonstrating its practical photocatalytic relevance. Dark adsorption assays (<4.5 %) confirmed that degradation is strictly governed by dynamic interactions within the electrical double layer (EDL) rather than direct hole transfer. Photocatalytic efficiency exhibited strong pH dependence relative to the catalyst’s shifted IEP: under acidic conditions (pH 3), surface protonation (Ti–OH2 +) compressed the EDL and drove Coulombic pre-concentration of anionic pollutants, inducing a kinetic inversion to OG > AO > MO, governed by the higher structural charge of the divalent OG molecule and stabilisation of reactive hydrazone tautomers. Conversely, alkaline regimes induced electrostatic repulsion (Ti–O−), shifting the degradation hierarchy to AO > OG > MO. Total organic carbon (TOC) analysis confirmed robust mineralisation with high carbon removal efficiency across all tested conditions. Complementary DFT calculations (M06-2X/6-31+G(d,p)) revealed that electrophilic •OH attack proceeds preferentially via exergonic C–N bond addition rather than N–N cleavage, with azo-hydrazone tautomerism serving as the key regioselectivity-governing factor.