Colloidal Behavior of Hydrogenated Milled Nanodiamonds in Water

Two main methods exist for synthesizing nanodiamonds (NDs): (i) detonation process (DND) which creates approximately 5 nm quasi-monodisperse but extremely defective nanoparticles, and (ii) milling of bulk diamond (whether natural or synthetic) which forms polydispersed nanoparticles (MND) of higher crystalline quality. MND are capable of hosting NV or SiV color centers and are currently under investigation for nanomedicine or quantum applications [1]. They also demonstrate comparable electronic properties to bulk material, including similar band gap and equivalent position of conductive and valence bands, according to their surface chemistry [2]. This aspect renders them appealing for energy-oriented applications including photo(electro)catalysis. However, these applications require a fine control of MND colloidal properties. Although the stability of oxidized MND colloids is well understood, further comprehension is still needed to explain the behavior of their hydrogenated counterparts (H-MND).<br/><br/>In the present study, we will correlate the surface chemistry and colloidal behavior of MND hydrogenated under different conditions (annealing under atmospheric pressure of H₂ at temperatures ranging from 600°C to 750°C) [3]. Through an original semi-quantitative IR spectroscopy approach, we will show that the maximum amount of C-H bonds that can be imparted to MNDs by annealing is reached as early as 650°C. The latter temperature is not sufficient to get rid of all oxidized terminations, which is completed only above 700°C, as evidenced by additional FTIR and XPS measurements. Consequently, H-MND annealed below and above 700°C evidence different colloidal behaviors in terms of sonication time needed to suspend them and resulting concentration, which sheds light on the mechanisms that underline their stability. Moreover, at annealing temperatures of 650°C and above, we evidenced the systematic presence of a dip at 1330 cm^-1 in the IR absorption spectrum, caused by a Fano-type interference, which betrays the presence of a surface conductive layer for stable nanoparticles. It is absent for the sedimented H-MND particles. A link between surface conductivity and colloidal stability of H-MND will thus be discussed. Finally, we will also reveal that successive sonications of unstable (sedimented) H-MND can lead to counter-intuitive colloidal properties, which provides new insights on the origin of their stability in water.<br/><br/><br/><br/><b>References</b><br/>[1] N. Nunn <i>et al.</i>, <i>Nanoscale</i>, 11, 11584–11595 (2019)<br/>[2] D. Miliaieva et al., <i>Nanoscale Adv.</i>, 5, 4402-4414 (2023)<br/>[3] L. Saoudi et al., <i>Carbon</i>, 202, 438-449 (2023)