Unveiling the role of polyaniline emeraldine salt and emeraldine base in the enhanced performance of TIO2 nanotube photocatalyst towards diazinon under visible-light

This study investigates the effect of polyaniline (PANI) form modification on titanium dioxide nanotube (TNT) composite towards diazinon photodegradation. Aniline was polymerized in situ in the TNT matrix via the slow-mixing method. The concentration of dopant, i.e., hydrochloric acid (HCl) was varied (0.02, 0.10, 1.0, and 1.5 M) to obtain different forms of PANI, i.e., PANI Emeraldine Base (PANI EB) and Emeraldine Salt (PANI ES). A bluish color of the composite synthesized using an HCl concentration of 0.02 M, indicating PANI EB, while a greenish color representing PANI ES was obtained using a higher HCl concentration. The addition of PANI reduces the crystallite size and increases both the tube diameter and the surface area of the resulting composite, as confirmed by XRD and BET results. FTIR and SEM characterizations revealed the presence of PANI in all composites in the form of agglomerates. UV-Vis spectra clearly show strong visible-light absorption of the PANI/TNT composite, indicated by a low % reflectance from 70% (ES-type) to 35% (EB-type). In addition, the presence of PANI reduced the bandgap of TNT from 3.10 to 2.18 eV, due to polaronic states. PANI EB showed higher photoluminescence than others due to the absence of polaron species as a charge-separator. At 180 min of the photodegradation process under visible light, the removal efficiency of diazinon by PANI/TNT 0.02, 0.1, 1, and 1.5 M was 89.81, 88.95, 77.26, and 76.15%, respectively. The results clearly show a compelling relationship between the optical/electronic properties (i.e., visible light absorption) and the chemical structures of both PANI ES and PANI EB, with respect to their TNT-degrading capability for diazinon. • The concentration of hydrochloric acid (HCl) was varied during polymerization to obtain different forms of PANI. • Polyaniline Emeraldine Base (PANI-EB) was obtained at HCl concentration of 0.02 M, while higher HCl concentrations yielded Polyaniline Emeraldine Salt (PANI-ES). • The addition of PANI reduces the crystallite size and increases both the tube diameter and the surface area of the resulting composite. • PANI EB-based TiO 2 nanotube composite achieved the highest diazinon removal efficiency and degradation rate. • Visible-light harvesting efficiency (PANI-EB system) is a more dominant factor in driving the reaction kinetics than charge separation efficiency alone (PANI-ES system).