Display omitted •α, β, α/β Bi2O3 heterojunction-catalysts were synthesized by hydrothermal method.•α/β Bi2O3 porous nano flake shows synergistic degradation of RhB under sunlight.•Various operational parameters for photocatalysis were investigated.•Plausible mechanism of degradation of RhB was drawn.•99.6% removal of RhB is achieved in 180 min using 0.5 g/L photocatalyst at pH 3. Nano particles of a few α/β Bi2O3 hetero-junctions of various compositions synthesized by one- pot hydrothermal method, exhibit exceptional and synergistic photo-catalytic activity for the degradation of Rhodamine-B in aqueous solution under natural sunlight. Pure α and pure β Bi2O3 are also synthesized by control post heating of synthesized hetero-junction. The nano-materials were characterized by diffraction (XRD), microscopic and spectroscopic techniques. The XRD reveals α-β phase hetero-junctions of Bi2O3 are made of α-Bi2O3 and β-Bi2O3 with average dimensions within 13–113 and 5–71 nm respectively and having band gap range of 2.4– 2.9 eV. The spectrophotometrically determined % degradation of the dye and associated rate constant on the best hetero-junction are increased by 4.5(/2.1) and 3.3(/1.2) times than these on pure α (/β). The effects of operational parameters and trapping agents have been analyzed. The maximum removal of the dye was achieved up to 99.6% in 3 h using 0.5 g/L photo-catalyst at pH 3. The reusability test shows that the photo-catalytic activity is retained excellently due to change in chemical nature of the catalyst from α -Bi2O3 to β-Bi2O3, Bi2O2CO3 and BiOCl. A suitable mechanism is proposed.