Degradation mechanism and oxidation kinetics of C60 fullerene

Abstract

Despite a great deal of research on the basic properties of C60 to the present, no detailed and comprehensive reports are available concerning its degradation mechanism and oxidation kinetics. However, a deeper understanding of the oxidation mechanism of the most common fullerene broadens the horizons of potential applications, especially at high temperatures. Therefore, this study attempted to investigate the fullerene oxidation process using kinetic analysis under non-isothermal conditions. Initially, the powder was structurally and morphologically characterized accurately via XRD, Raman spectroscopy, FESEM, and TEM analyses. This was followed by simultaneous thermal analysis (STA) (i.e., TG and DSC) at four different heating rates (2.5, 5, 10, and 15 °C min−1) in airflow, and kinetic analysis was subsequently conducted to evaluate the thermal oxidation of C60. The activation energy as evaluated by model-free methods turned out to be almost invariant within the specified range of reaction fraction (α). Model-fitting methods were also employed to identify the oxidation mechanism involved, from which emerged two models (i.e., R3 (g(α) = 1-(1-α)1/3) and F1 (g(α) = −ln(1-α))) as possible reaction mechanisms. However, further investigation using the Z(α) master plot method revealed that the kinetic model R3, a phase-boundary process, is the controlling mechanism of C60 oxidation with E ≈ 96 kJ mol−1 and lnA ≈ 9.1 min−1. Further examinations were conducted in the neutral atmosphere, and the partial oxidation process based on TEM and FESEM images revealed that oxygen and heat applied during the process play a central role in the deagglomeration of C60 particles and the consequent oxidation. In fact, considering the spherical morphology and the agglomerates present in the C60 powder, the contracting sphere kinetic model (R3) was found to capture the oxidation process due to attacks by the oxygen molecules at the interface of C60 powder agglomerates.