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

Abstract

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 (T NP) 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 T NP ≥ ∼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 T NP. 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.

O2 Oxidation and Sublimation Kinetics of Single Silicon Nanoparticles at 1200–2050 K: Variation of Reaction Rates, Evolution of Structural and Optical Properties, and the Active-to-Passive Transition

“…Preheating had no clear effect on EE O2 at 1500, 1750, or 1800 K. The NP-to-NP variations are expected, particularly for non-preheated NPs, because there would have been NP-to-NP variations in initial shapes and structure (i.e., primary vs aggregate NPs). The variations in initial EE O2 values for preheated NPs was significantly smaller and also much smaller than the variations found for carbon NPs, , where melting is not possible, and the NP feedstocks have very different structures (diamond, graphitic, etc. ).…”

“…The Si(100) studies also showed that O ads should have mostly desorbed as SiO in our T NP range, maintaining low steady-state oxygen coverage. Our typical P O2 corresponds to ∼154 O 2 collisions/s/nm 2 ; thus, 0.1% sticking would have resulted in adsorption of ∼0.3 O ads /s/nm 2 . If all the O ads desorbed as SiO (g) , the initial mass loss rate would have been ∼8.6 Da/s/nm 2 , corresponding to EE O2 ≈ 5.6 × 10 −2 Da/O 2 collision.…”

“…The as-trapped NPs would also have had carbonaceous adsorbates (e.g., methanol, ammonium acetate, and adventitious carbon), most of which would have desorbed as the NPs were initially heated, but it is possible that pyrolysis might have left elemental carbon on the NP surface, or possibly formed a silicon carbide layer. As discussed in the SI, we have studied O 2 etching of elemental carbon NPs , under conditions identical to those used here, and for 1200 K oxidation, the initial EE O2 values ranged from 2 × 10 –2 Da/O 2 collision for graphite to 3 × 10 –3 Da/collision for diamond, i.e., the carbon NPs etch with initial rates in the same range as those for Si NPs. The SI presents an etching experiment for a SiC NP (Figure S14) under conditions identical to those used in Figure , and again, the initial EE O2 values are similar to those for Si NPs.…”

“…The instrument used has been described previously, along with the methods for single NP injection, detection, mass analysis, spectroscopic T NP determination, , and measurement of surface reaction kinetics. − Briefly, NPs are injected into the gas phase using an electrospray ionization source, pass through several stages of differential pumping, are prefiltered by a quadrupole ion guide, and then are injected into a split-ring electrode quadrupole trap, , where they are detected optically. In addition to NP injection, the trap allows high solid angle optical collection along two axes and allows lasers to be focused through the trap along two other axes to excite emission from the NP.…”

“…Furthermore, because surface reactions can alter the surface site distributions, the reactivity for individual NPs tends to evolve over time under reaction conditions. For example, in experiments using single NP mass spectrometry to study kinetics for individual carbon NPs in the 1200–2100 K range, we found that both sublimation and O 2 oxidative etching , kinetics varied substantially for NPs from different feedstocks (graphitic, graphene oxide, carbon black, diamond, and nano-onion), but that even for NPs from the same feedstock, the initial rates varied by up to an order of magnitude. More surprisingly, the etching rates varied over time, with rates oscillating by orders of magnitude.…”