Detonation nanodiamonds with ultra-pure surfaces thanks to a combination of chlorine and fluorine gas treatment

Abstract

Achieving highly-purified surface of nanodiamonds of calibrated size is of great interest considering the wide range of applications requiring a perfect control for maximizing their properties and resulting performances. However, the existing purification methods involving strong acids and oxidants suffer from a lack of selectivity since they can attack the nanodiamonds and add new contaminants to the sample. In this work, a highly-selective gas-phase approach which combines two halogen gases (chlorine and fluorine) is successfully developed. First, a treatment under elemental chlorine at 650 °C for 1 h is able to remove all the inorganic impurities except silica. The second step consists of a thermal treatment under elemental fluorine (at 520 °C for 12 h) which is shown to eliminate both silica and surface impurities (disorganized sp2 layers, the oxygen- and hydrogen-containing functional groups) of size-calibrated detonation nanodiamonds. The resulting cleaned diamond surface is covered by carbon-fluorine bonds, only. An in-depth investigation by several complementary techniques including in-situ characterizations allows determining the chemical reactions playing the major role for the manifest highly efficient impurity removal. Formation of fluorinated gases such as HF, CF4, C2F6 is evidenced along with that of silicon fluorides such as SiF4. The role of fluorine as stabilizing (“passivating”) agent of the surface is proposed by analogy with fluorinated diamane. These detonation nanodiamonds with an ultra-pure surface show a remarkable thermal stability with a combustion temperature upshifted up to 225 °C compared to the raw detonation nanodiamonds. These findings open the way to extend broadly the use of detonation nanodiamonds for applications in harsh reaction conditions for which most of the nanomaterials are strongly limited.