AbstractThis paper reports a simple, biogenic and green approach to obtain narrow band gap and visible light-active TiO₂ nanoparticles. Commercial white TiO₂ (w-TiO₂) was treated in the cathode chamber of a Microbial Fuel Cell (MFC), which produced modified light gray TiO₂ (g-TiO₂) nanoparticles. The DRS, PL, XRD, EPR, HR-TEM, and XPS were performed to understand the band gap decline of g-TiO₂. The optical study revealed a significant decrease in the band gap of the g-TiO₂ (E _g = 2.80 eV) compared to the w-TiO₂ (E _g = 3.10 eV). The XPS revealed variations in the surface states, composition, Ti⁴⁺ to Ti³⁺ ratio, and oxygen vacancies in the g-TiO₂. The Ti³⁺ and oxygen vacancy-induced enhanced visible light photocatalytic activity of g-TiO₂ was confirmed by degrading different model dyes. The enhanced photoelectrochemical response under visible light irradiation further supported the improved performance of the g-TiO₂ owing to a decrease in the electron transfer resistance and an increase in charge transfer rate. During the TiO₂ treatment process, electricity generation in MFC was also observed, which was ~0.3979 V corresponding to a power density of 70.39 mW/m². This study confirms narrow band gap TiO₂ can be easily obtained and used effectively as photocatalysts and photoelectrode material.