Thermal stability and oxidation resistance of single-digit boron-doped nanodiamonds

Single-digit boron-doped nanodiamonds (SD-BND) are potentially promising for use in various applications, including medicine, energy storage, and electronic devices. However, thermal stability and oxidation resistance of SD-BND remains unexplored, hindering their practical application and development of production methods. In this study, we investigate thermal stability in vacuum of SD-BND in comparison with larger BND, both synthesized by pyrolysis of 9-borabicyclo(3.3.1)nonane under high pressures, and with detonation nanodiamonds (DND). In situ X-ray phase analysis shows that the thermal stability of 4.5 nm SD-BND is comparable to that of 4.5 nm DND, since graphitization is observed at 1000 °C in both cases, and inferior to the stability of 30 nm BND. The oxidation resistance of SD-BND was studied in comparison with DND. In contrast to the widely accepted assumption that boron doping increases the oxidative resistance of diamonds, the oxidation of the 4.5 nm SD-BND in air begins ∼100 °C earlier than 4.5 nm DND. Density functional theory (DFT) calculations indicate that the presence of a boron impurity promotes nanodiamond oxidation. As detected by Raman spectroscopy, the boron impurity state remains largely unchanged during heat treatment at 550 °C in air despite the instability of SD-BND in an oxidizing atmosphere. This study delineates field of SD-BND stability upon heating in vacuum and in air, which is important for practical applications.