Inside- and Outside-Coated PANI and/or PIN-TiO2 Nanotubes for Enhanced Photocatalytic Degradation of 4-Nitrophenol in Wastewater

We present a novel approach for enhancing photocatalytic efficiency by developing polyaniline (PANI) and polyindole (PIN)-coated TiO2 nanotubes (TNT) through a combination of chemical oxidation and hydrothermal processes. The PANI–PIN coating was systematically applied to both the internal and external surfaces of the nanotubes to enhance the photocatalytic active sites and optimize pollutant adsorption. The dual-coated structure enhances the interaction with pollutants, facilitating a more efficient degradation of 4-nitrophenol (4-NP) when exposed to visible light. Thorough characterization through X-ray diffraction (XRD), Fourier-transform infrared (FTIR), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), N2-physisorption, transient photocurrent, diffuse reflectance spectroscopy (DRS), and photoluminescence (PL) validated the exceptional structural and optical properties of the composite. The PANI/PIN polymer coating effectively inhibited electron–hole recombination, leading to a notable enhancement in photocatalytic performance. Among the tested composites, the formulation consisting of 75% PANI and 25% PIN demonstrated remarkable performance, achieving a degradation rate of 99.46% for 4-NP in only 120 min of exposure to visible light. The impressive efficiency stems from its extensive surface area (255.3 m2/g), efficient charge separation, minimized band gap (2.77 eV), and improved light absorption. Moreover, the composite demonstrated remarkable recyclability, preserving its catalytic activity across five cycles without any decline in performance. These results demonstrate the strong potential of 75%PPTN as a promising photocatalyst for environmental remediation.