High temperature transformation, O-2 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 mu m) their 532 nm absorptivity increased, and when heated they emitted blackbody-like radiation, signalling 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 O-2 etching kinetics of pre-heated silica NPs at 1200 and 1300 K. Sublimation was significant at T-NP >= 1900 K, and the NP optical properties showed further changes as the NPs sublimed. The O-2 etching kinetics for pre-heated silica NPs were similar to those previously reported for single Si NPs under identical 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. The etching and passivation kinetics at different temperatures and O-2 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.