High Temperature Transformation, O2 Etching, and Passivation of Single SiOx Nanoparticles: Kinetics and Optical Properties as Structure Probes

The surface chemistry and optical properties of silica nanoparticles (NPs) were studied using a single nanoparticle mass spectrometer. Initially, silica NPs were transparent to 532 nm excitation, but after brief exposure to a CO2 laser (10.6 μm) their 532 nm absorptivity increased, and when heated they emitted blackbody-like emissivity in the visible/near IR, signaling that their compositions had become more Si-rich. The transformation was also probed by sublimation rate measurements from 1600 K to 2350 K, and the chemistry of the transformed surface was probed by measuring the O2 etching kinetics of pre-heated silica NPs at 1200 and 1300 K. Sublimation was significant for NP temperatures at TNP ≥ 1900 K, and the NP optical properties showed further slow changes as the NPs sublimed. The O2 etching kinetics for pre-heated silica NPs were similar to those previously reported for single Si NPs under the same conditions, with etching efficiency (EEO2) initially increasing with etching time, then dropping by three orders of magnitude as the NP surface passivated by growth of a thermally stable SiO2 layer. NPs lost up to 50% of their initial masses before passivating. The etching and passivation kinetics at different temperatures and O2 pressures are compared, and comparisons are also made to the temperature and time dependence of EEO2 for pre-heated silica, Si, SiC, and carbon NPs. The comparison between pre-heated Si and silica NPs suggests that the latter retain an O-rich core, and that the core oxygen hastens the passivation process.