High temperature transformation, O₂ etching, and passivation of single SiO_x nanoparticles: kinetics and optical properties as structure probes

Abstract

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 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 O₂ 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 O₂ etching kinetics for pre-heated silica NPs were similar to those previously reported for single Si NPs under identical conditions, with etching efficiency (EE_O2) 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 O₂ pressures are compared, and comparisons are also made to the temperature and time dependence of EE_O2 for pre-heated silica, Si, SiC, and carbon NPs.

GRAPHICAL ABSTRACT

KEYWORDS:

• Nanoparticles
• mass spectrometry
• silicon
• oxidation
• passivation

Acknowledgements

We are grateful to Dieter Gerlich for providing the NP trap design from his group, and for many enjoyable discussions and arguments that seeded our ideas for implementing single nanoparticle mass spectrometry as a kinetics tool.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by the Department of Energy, Office of Science, Basic Energy Sciences, Gas Phase Chemical Physics Program under grant DE-SC-0018049. SLA acknowledges support from the Henry Eyring Endowed Chair; University of Utah Eyring Chair Funds.