HPHT Synthesis and Compressibility of Hydrogen-Terminated Nanodiamonds

Hydrogen-terminated nanodiamonds are new functional nanodiamonds that have many applications, particularly in the field of catalysis. However, their elastic properties have not yet been investigated. In this study, hydrogen-terminated nanodiamonds were synthesized at 15.5 GPa and 1200-1600 °C using <i>trans</i>-stilbene (C₁₄H₁₂) as the starting material in a relatively closed carbon-hydrogen system by using a gold capsule. Raman spectroscopy revealed characteristic diamond peaks, with a set of peaks around 2900 cm^-1 indicating hydrogen termination. The extent of hydrogenation and unit-cell volume are negatively correlated to the synthetic temperature, while the grain size is positively correlated to it. The size of the synthesized hydrogen-terminated nanodiamonds at 15.5 GPa and 1600 °C was confirmed to be ∼6-15 nm by synchrotron-based X-ray diffraction and transmission electron microscopy. Their equation of state was determined by synchrotron-based X-ray diffraction combined with the diamond anvil cell up to 33.6 GPa. The bulk modulus was determined to be 424(12) GPa, lower than the bulk moduli of the bulk diamond and conventional nanodiamonds. It indicates that hydrogen termination softens nanodiamonds and may play a vital role in modifying their elastic behavior.