Amorphous-to-Crystalline Transition during Sintering of Nascent TiO2 Nanoparticles in Gas-Phase Synthesis: A Molecular Dynamics Study

A novel amorphous-to-crystalline transition during the sintering of nascent TiO2 nanoparticles in gas-phase synthesis is reported and investigated by molecular dynamics simulations. The potential energy and the coordination number of a single nanoparticle reveal that the nascent nanoparticle remains amorphous below a critical size. The size-dependent amorphous–crystalline transition is summarized in a size–temperature phase diagram. Among the three common polymorphs of TiO2, rutile has the smallest critical size due to its dense crystal structure. Considering the size-dependent particle structure, the initial nanocrystal could be first generated in the sintering between two amorphous nanoparticles in the bottom-up synthesis processes. Our molecular dynamics simulation results indicate that the coalesced nanoparticle remains amorphous at the initial stage of sintering and then crystallizes to a nanocrystal. Crystallization is observed only when the initial particle size is larger than 1.85 nm and the temperature is lower than 1650 K. For the sintering of two equal-sized 2 nm nanoparticles, the local Lindemann index distribution shows that crystallization originates at the particle surface with some atoms aligned in orders and then quickly expands to the entire nanoparticle within 0.5 ns. The sintered nanoparticle crystallizes faster at higher temperatures, where the time scale obeys Arrhenius behavior with a fitted activation energy of 147.5 kJ mol–1. Crystallization is faster for larger nanoparticles due to their higher extent of "supercooling" in the size–temperature phase diagram. In general, the nucleated crystal lattice is dominated by rutile and occasionally forms brookite.