Control of sp²/sp³ Carbon Ratio and Surface Chemistry of Nanodiamond Powders by Selective Oxidation in Air

TLDR

The presence of large amounts of nondiamond carbon in detonation‑synthesized nanodiamond severely limits its applications. The study proposes a simple, environmentally friendly air‑oxidation route to selectively remove sp²‑bonded carbon from detonation‑synthesized nanodiamond. The process uses air oxidation at 375–450 °C to selectively remove sp² carbon, followed by optional treatments such as hydrogenation to tailor surface functional groups. Thermogravimetric and Raman analyses show distinct oxidation rates for sp² and sp³ carbon, with a 400–430 °C window yielding high‑purity ND; XANES reveals up to a 100‑fold increase in the sp³/sp² ratio, achieving up to 96 % sp³ content comparable to microcrystalline diamond, while TEM and FTIR confirm 5‑nm particles with oxygen‑containing surface groups, and the method avoids toxic chemicals, offering a cleaner purification route.

Abstract

The presence of large amounts of nondiamond carbon in detonation-synthesized nanodiamond (ND) severely limits applications of this exciting nanomaterial. We report on a simple and environmentally friendly route involving oxidation in air to selectively remove sp2-bonded carbon from ND. Thermogravimetric analysis and in situ Raman spectroscopy shows that sp2 and sp3 carbon species oxidize with different rates at 375−450 °C and reveals a narrow temperature range of 400−430 °C in which the oxidation of sp2-bonded carbon occurs with no or minimal loss of diamond. X-ray absorption near-edge structure spectroscopy detects an increase of up to 2 orders of magnitude in the sp3/sp2 ratio after oxidation. The content of up to 96% of sp3-bonded carbon in the oxidized samples is comparable to that found in microcrystalline diamond and is unprecedented for ND powders. Transmission electron microscopy and Fourier transform infrared spectroscopy studies show high purity 5-nm ND particles covered by oxygen-containing surface functional groups. The surface functionalization can be controlled by subsequent treatments (e.g., hydrogenization). In contrast to current purification techniques, the air oxidation process does not require the use of toxic or aggressive chemicals, catalysts, or inhibitors and opens avenues for numerous new applications of nanodiamond.