Sintering-Induced Phase Transformation of Nanoparticles: A Molecular Dynamics Study

Sintering-induced phase transformation of TiO₂ nanoparticles is investigated systematically via molecular dynamics simulation. Upon defining a coordination number and bond angle distribution criteria, local phase information is identified for each individual Ti atom originating from amorphous or crystal structure as well as three TiO₂ polymorphs, anatase, brookite, and rutile. Size-dependent structures of nanoparticles lead to different dynamics of the sintering-induced phase transformation. Grain boundaries that form between nanoparticles during sintering trigger the nucleation and growth of new phases. During the sintering of two equal-sized core-shell anatase nanoparticles, crystal core regions melt with the temperature increase and the surface energy decrease in the microcanonical (NVE) ensemble. The new phase that develops from the grain boundary spreads into the destroyed core regions in stages, forming a new larger spherical nanoparticle with an ordered atomic arrangement. During the sintering of two unequal-sized nanoparticles (amorphous and core-shell anatase), atoms from the amorphous nanoparticle first nucleate to form crystal anatase in the contact region, and a grain boundary is then developed between the original core region and the newly formed anatase crystal. After that, phase transformation follows much the same route as the equal-sized case from anatase to brookite.