Surface-engineered biogenic TiO 2 nanoparticles with tailored isoelectric point: interfacial electrical double layer interactions, solar mineralisation, and DFT-resolved regioselectivity of azo dye degradation

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Abstract
Biogenically tailored anatase TiO 2 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 ( 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.

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