Biopolymer chitosan-capped yttrium and cobalt dual-doped SnO₂ as an advanced biodegradable photocatalyst for efficient organic pollutant degradation

The improper handling and uncontrolled discharge of toxic organic dyes result in significant adverse effects on both human health and the environment. This study investigates the fabrication of SnO₂, yttrium and cobalt dual-doped SnO₂ (YCSn), chitosan-capped SnO₂ (CS*Sn), and chitosan-capped yttrium and cobalt dual-doped SnO₂ (CS*YCSn) nanoparticles using a one-step coprecipitation method for the photocatalytic degradation of methylene blue (MB) under visible light irradiation. Characterization techniques including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), high-resolution transmission electron microscopy (HRTEM), and ultraviolet-visible (UV-Vis) spectrophotometry confirm the successful synthesis of biodegradable CS*YCSn nanoparticles. These nanoparticles, capped with biopolymer chitosan, exhibit advanced functionalities, eco-friendliness, and cost-effectiveness. HRTEM images reveal the nanosphere-like morphology of CS*YCSn with prominent lattice fringes (0.33 nm), confirming the successful preparation of the dual-doped SnO₂ nanostructures. XPS analysis verifies the substitution of Sn⁴⁺ in SnO₂ with dual dopants Y³⁺ and Co²⁺. The average particle size of CS*YCSn is 19.2 nm, with a band gap of 2.1 eV. CS*YCSn demonstrates a degradation efficiency of 96.7 % within 90 min, outperforming SnO₂, CS*Sn, and YCSn. The optimal conditions for dye removal were found to be a pH of 8.0, a catalyst dose of 10 mg, and an irradiation time of 90 min. The degradation kinetics follow pseudo-first-order reaction rates, with a fitted rate constant of 0.03358 min^-1 for CS*YCSn. A biodegradability test confirms the potential for biodegradation of CS*YCSn, as evidenced by a reduction in particle size to 15 nm. Additionally, electron paramagnetic resonance (EPR) analysis highlights the influence of doping and chitosan capping on the electronic and magnetic properties of the material, with the highest EPR signal intensity observed for CS*YCSn, suggesting enhanced charge carrier dynamics and improved photocatalytic efficiency. The synthesized biodegradable photocatalysts facilitate dye mineralization by lowering the activation energy barrier. Photocatalytic studies indicate that CS*YCSn is an efficient photocatalyst for the removal of pollutants from contaminated water.