Mechanistic insights into solvent-guided growth and structure of MoO₂ nanoparticles in solvothermal synthesis

Understanding the processes involved in the nucleation and growth of nanoparticles is essential for the development of tailored nanomaterials. Here, we investigate the solvent effects on the atomic structure and size of nanocrystalline MoO₂ obtained from a solvothermal synthesis and deduce their reaction pathways. Detailed pair distribution function (PDF) analysis reveals the formation of distinct MoO₂ structures, depending on the alcohol used. We show that the atomic structure and crystallite size of the formed materials are directly related to their formation pathway. In situ PDF analysis together with X-ray absorption spectroscopy of the reaction between MoCl₅ and an alcohol solvent allows us to see that larger nanoparticles (ca. 30 nm) with the conventional MoO₂ distorted rutile structure form when the initial Cl/O-ligand exchange is fast, but the subsequent condensation and crystallization are slowed down in the synthesis process. On the other hand, when the Cl/O exchange is slowed down, a [M^IVCl _x O _y ]-complex is formed, and we obtain very small nanoparticles (2-3 nm) with the MoO₂ high-pressure polymorph structure. The study shows how the chemistry of the reaction solvent affects the mechanistic pathways, and consequently the intermediate formed just prior to crystallization, which is directly applicable to the process of obtaining specific nanocrystalline materials.