Photocatalysis for environmental cleanup is one of the potential applications of nanostructured materials with tailored optical, electrical, thermal, and structural properties. Green nanomaterial synthesis offers benefits over traditional chemical and physical methods in areas such as biocompatibility, sustainability, environmental impact, and energy consumption. Citrus sinensis leaf extract was used as a natural source of reducing, capping, and stabilizing agents in this study, which successfully reported the production of nanostructured bismuth oxide (NBO). The thermal, optoelectronic, and structural properties of the as-synthesized NBO sample were compared to those of a commercially available bulk bismuth oxide (BBO). Additionally, the morphology, composition, optical band gap, and electrical conductivity of the NBO sample were respectively examined through SEM, EDX, UV/Vis, and I–V analysis. The NBO showed exceptional photocatalytic aptitude as it mineralized the crystal violet (CV) dye and killed E. coli under visible-light irradiation. The superb photocatalytic activity of the NBO is attributed to its unique features, i.e., a large surface area (59.73 m2g-1), defective states (D-S) structure, visible-light-triggered band (2.23 eV), and good electrical conductivity (1.23 × 10−5 sm−1). These factors enhance the light-harvesting, charge-separation, electron-excitation, and charge transport properties of the NBO. Additionally, NBO retains 94.5 % photocatalytic activity after four cyclic runs. All of these feature work together to make the environmentally friendly NBO photocatalyst the leading contender for wastewater treatment applications.