Nanothermodynamics of Hydrogenated Diamond Clusters: Size-Dependent Transition Pressures and Stability

Nanodiamonds hold promise for quantum sensing, tribology, and biomedicine, but their controlled synthesis at atmospheric pressure remains limited due to poor understanding of how surface chemistry governs nanoscale sp³ carbon stability. Hydrogen adsorption offers an alternative to extreme thermobaric processes, yet the role of cluster size, face composition, and morphology on the sp²-sp³ boundary has not been quantified. Here we develop a machine learning potential trained on ab initio data via active learning to compute phase transition pressures and formation energies for hydrogenated diamond clusters (1-10 nm) with {111}, {110}, and {101̅0}/{0001} facets. The results indicate hydrogenation alone stabilizes nanodiamonds up to ∼10 nm. Notably, elongated nanorods maintain negative formation energies over a wide diameter range, avoiding convergence to the bulk diamond-graphite difference. These results map the nanothermodynamic stability of sp²-sp³ transformations and provide quantitative guidelines for synthesizing nanodiamonds and diamond nanowires from hydrogenated graphite precursors.