In situ micro-Raman studies of laser-induced bismuth oxidation reveals metastability of β-Bi2O3 microislands

We report the laser irradiation-induced oxidation of bismuth metal investigated in situ by micro-Raman spectroscopy as a function of irradiation power and time. The purely optical synthesis and characterization of beta-Bi2O3 oxide microislands on metallic Bi surfaces is shown to be stable over time, even at room-temperature. By closely examining possible reactions on simple Bi morphologies it is revealed for the first time that the ensuing oxide phase is critically dependent on the final oxide volume and follows a fixed kinetic transformation sequence: 3/2O(2)(g) + 2Bi (l) -> beta-Bi2O3(s) -> alpha-Bi2O3(s). These findings are unusual within the framework of traditional Bi2O3 thermal transformation relations. An electrostatic mechanism involving a changing Bi2O3 surface-to-volume ratio is proposed to explain the room-temperature metastability of small beta-Bi2O3 volumes and the subsequent transformation sequence, as well as unifying the results of previous studies. (C) 2014 Optical Society of America