Photocatalytic and Antimicrobial Properties of Cobalt-Doped Ag₃PO₄ Synthesized by Hydrothermal Method

Pristine and cobalt-doped silver phosphate (Ag₃PO₄) containing nominal Co concentrations of 0.5, 1, 3, and 5 mol % were synthesized by a hydrothermal method to investigate the influence of Co incorporation on structural, optical, photocatalytic, antimicrobial, and antiphotocorrosion properties. X-ray diffraction and Rietveld refinement confirmed preservation of the cubic Ag₃PO₄ structure, while variations in lattice parameters, Raman modes, particle morphology, and surface chemical states indicated Co-induced local distortion and defect formation. EDX and ICP-OES confirmed the presence of cobalt, whereas XPS revealed modifications in the Ag and O surface environments. The fundamental optical bandgaps ranged from 2.37 to 2.46 eV, with additional sub-bandgap absorptions at 1.68 and 1.65 eV for AgP:3Co and AgP:5Co, respectively. Progressive photoluminescence quenching demonstrated that Co strongly modifies charge-relaxation pathways. Despite their low BET surface areas (0.953-5.608 m² g^-1), the doped materials displayed enhanced photocatalytic activity, showing that performance was not controlled by surface area alone. AgP:3Co exhibited the highest methylene-blue removal under simulated solar irradiation, reaching 93.26% after 20 min, with 120.26 × 10^-3 min^-1 and t _1/2 = 5.76 min. Scavenger experiments identified photogenerated holes and hydroxyl radicals as the predominant oxidative species. After five reuse cycles, AgP:3Co retained approximately 87% activity and showed greater structural and morphological stability than pristine Ag₃PO₄. All samples also exhibited strain-dependent antimicrobial activity against reference strains and clinical isolates. Overall, 3 mol % Co provided the most favorable balance among defect chemistry, charge utilization, photocatalytic efficiency, antimicrobial activity, and resistance to photocorrosion.