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)3·5H2O] 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.

The variation of the starting Bi concentration and NaOH ratio allows tailoring the Bi2O3 crystals in form of needles, rectangular tubes, plates to well-crystallized polyhedra.Figure optionsDownload as PowerPoint slideHighlights
► Pure Bi2O3 crystals were only obtained from the precipitation at room temperature.
► Crystallization of Bi2O3 is a thermodynamically controlled reaction.
► The size and morphology of Bi2O3 depends on the concentration of NO3− or OH−.
► Band gap energy of Bi2O3 varied with the crystal morphology due to the size effect.