Morphology-controllable Bi₂O₃ crystals through an aqueous precipitation method and their photocatalytic performance

This study investigated the formation of bismuth oxide (Bi ₂O₃) crystals by an aqueous precipitation method using bismuth nitrate pentahydrate [Bi(NO₃)₃·5H ₂O] and NaOH. The phase evolution during NaOH titration was also studied. Basic bismuth nitrates were formed during NaOH titration. However, these nitrates dissolved and recrystallized into Bi₂O₃ when the pH attained a critical range between 13 and 14. A second phase of Bi(OH)₃ was induced when the pH of the solution was over the critical pH range. The morphology of the Bi₂O₃crystal strongly depended on precipitation conditions. The NO₃^- ions favored the crystal growth along the [001] direction to obtain needle-like Bi₂O₃crystals. Conversely, NaOH tended to be absorbed on the (001) face and suppressed the preferred growth in the [001] direction because of the shielding effect. The crystal changed from needle-like to plate or polyhedral shape with increasing NaOH concentration. The optical absorption properties were also modified accordingly, resulting in the change of color from yellow to pale orange because of size and shape effects. Finally, Bi ₂O₃crystals with plate and polyhedral morphologies were found to show relatively higher photocatalytic activity than the needle-like crystals due to the better development of the active crystal faces with suppression of the preferred crystal growth habit.