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

This study investigated the formation of bismuth oxide (Bi2O3) crystals by an aqueous precipitation method using bismuth nitrate pentahydrate [Bi(NO3)(3)center dot 5H(2)O] 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 Bi2O3 crystal strongly depended on precipitation conditions. The NO3- ions favored the crystal growth along the [001] direction to obtain needle-like Bi2O3 crystals. 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, Bi2O3 crystals 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. (C) 2013 Elsevier Ltd. All rights reserved.