Uncovering the structural stabilities of the functional bismuth containing oxides: a case study of alpha-Bi2O3 nanoparticles in aqueous solutions

The majority of bismuth containing oxides have been taken as important functional materials for applications including photocatalytic water splitting and highly conductive solid electrolytes, which however suffer from property degradation owing to the presence of intermediates. This work initiated a case study on alpha-Bi2O3 nanoparticles with an aim to uncover the origin of property degradation when bismuth oxides that contain traces of secondary phase alpha-Bi2O3 were exposed in either air or aqueous solutions. First, alpha-Bi2O3 nanoparticles with spherical and rod-like morphologies were prepared by developing solution chemistries. It was found that alpha-Bi2O3 transformed partially to Bi2O2CO3 when stored in air for 6 months. Such transformation was further accelerated when alpha-Bi2O3 nanoparticles were exposed to water or methylene blue (MB) solution, probably because alpha-Bi2O3 reacts with the species HCO3- as formed by dissolution of CO2 from air. Bi2O3 surfaces had an unusual tendency to adsorb MB molecules, which in turn prevented the reaction between alpha-Bi2O3 and HCO3-. Consequently, MB solution became clear even without light irradiation. The results reported in this work may help to fully comprehend the property degradation of bismuth containing oxides when used in aqueous solutions.