High-Temperature Oxidation of Single Carbon Nanoparticles: Dependence on the Surface Structure and Probing Real-Time Structural Evolution via Kinetics

O2 oxidation and sublimation
kinetics for >30 individual
nanoparticles (NPs) of five different feedstocks (graphite, graphene
oxide, carbon black, diamond, and nano-onion) were measured using
single-NP mass spectrometry at temperatures (TNP) in the 1100–2900 K range. It was found that oxidation,
studied in the 1200–1600 K range, is highly sensitive to the
NP surface structure, with etching efficiencies (EEO2) varying by up to 4 orders of magnitude, whereas sublimation
rates, significant only for TNP ≥
∼1700 K, varied by only a factor of ∼3. Its sensitivity
to the NP surface structure makes O2 etching a good real-time
structure probe, which was used to follow the evolution of the NP
surface structures over time as they were either etched or annealed
at high TNP. All types of carbon NPs were
found to have initial EEO2 values in the range
near 10–3 Da/O2 collision, and all eventually
evolved to become essentially inert to O2 (EEO2 < 10–6 Da/O2 collision);
however, the dependence of EEO2 on time and
mass loss was very different for NPs from different feedstocks. For
example, diamond NPs evolved rapidly and monotonically toward inertness,
and evolution occurred in both oxidizing and inert atmospheres. In
contrast, graphite NPs evolved only under oxidizing conditions and
were etched with complex time dependence, with multiple waves of fast
but non-monotonic etching separated by periods of near-inertness.
Possible mechanisms to account for the complex etching behavior are
proposed.