Mechanistic Insights into the Aminolytic Decomposition of Mo(CO) 6 to Form MoC 1– x Nanoparticles

The controlled synthesis of refractory metal carbide nanoparticles under mild, solution-phase conditions remains a grand challenge in colloidal nanoparticle chemistry because the strong bonds and high formation energies of these materials make controlled nucleation and growth difficult, and, as a result, these mechanisms remain largely unexplored. Here, we provide the first mechanistic insight into the formation of α-MoC 1– x nanoparticles by investigating the aminolytic decomposition of Mo(CO) 6 in oleylamine (OAm) and N, N -dimethyloctadecylamine (DODA). Ex situ FT-IR studies reveal stepwise substitution of carbonyl ligands by the amine, with OAm yielding disubstituted molybdenum carbonyl species and DODA yielding only the monosubstituted species. Ex situ X-ray diffraction and in situ synchrotron small-angle X-ray scattering show that these precursors first convert into an isolable amorphous intermediate, which then rapidly crystallizes into α-MoC 1– x nanoparticles. Stronger precursor-ligand complexation in OAm delays decomposition, producing rapid, synchronized nucleation and smaller nanoparticles, whereas DODA promotes earlier, more gradual decomposition and the formation of larger nanoparticles. Kinetic analysis confirms that crystallization is reaction-controlled, with intraparticle densification proceeding without significant ripening. These results highlight the critical role of solvent-dependent precursor-ligand interactions in controlling the decomposition temperature, kinetics, and final size of colloidal metal carbide nanoparticles.