Abstract
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 β-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: 32O2(g)+2Bi(l)→β–Bi2O3(s)→α–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 β-Bi2O3 volumes and the subsequent transformation sequence, as well as unifying the results of previous studies.
Highlights
Bismuth trioxide (Bi2O3; known as bismuth oxide and bismuthsesquioxide) polymorphs are fascinating optical materials with wide bandgap, high refractive index, large dielectric permittivity, remarkable photoconductivity, and ionic conductivity
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
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 2
Summary
Bismuth trioxide (Bi2O3; known as bismuth oxide and bismuthsesquioxide) polymorphs are fascinating optical materials with wide bandgap, high refractive index, large dielectric permittivity, remarkable photoconductivity, and ionic conductivity. Attempts to better understand phase transformation relations among Bi2O3 polymorphs are currently underway [17], a comprehensive model of the transient stages of oxidation is yet to fully explain the varied and seemingly anomalous oxidation phase results reported in the literature [11, 18,19,20,21,22,23,24,25,26,27,28,29]. In most studies of bismuth oxidation to date, the only real-time in situ diagnostics have been through thermogravimetric analysis or X-ray diffraction (XRD) measurements. Neither of these techniques is versatile enough to provide information suitable for process control or for elucidating transformation dynamics [30,31,32,33]. An electrostatic mechanism − based on a changing Bi2O3 surface-to-volume ratio − is invoked to explain how small volumes of undoped β -Bi2O3 acquire room-temperature metastablity, as well as permitting the interpretation of previous studies
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