Effect of size-dependent grain structures on the dynamics of nanoparticle coalescence

Abstract

The effect of grain structure on the coalescence dynamics of anatase TiO2 nanoparticles at different temperatures is investigated using classical molecular dynamics (MD) simulation. Examination of local-lattice-orientation distributions reveals that the grain morphology of particles is highly dependent on size. For a single anatase nanoparticle below the melting temperature, an amorphous-to-crystalline transition occurs for diameters ranging from 2 to 2.5 nm as temperature increases. Below the transition diameter (for a given temperature), the entire nanoparticle is amorphous. Above the transition diameter, the nanoparticle consists of a crystalline core and an amorphous shell (4–6 Å). Considering that such grain-structure characteristics may lead to different dynamic behaviors, the coalescence between pairs of 2 nm–2 nm, 3 nm–3 nm, and 2 nm–3 nm nanoparticles is investigated. For 2 nm–2 nm nanoparticle coalescence, the process is independent of initial temperature and is seemingly viscosity-controlled with a dynamic temperature rise due to energy transfer from surface to internal kinetic (thermal). For 3 nm–3 nm nanoparticle coalescence, the process is sensitive to initial temperature. Above the melting temperature, the dynamics are similar to the 2 nm–2 nm amorphous case. Just below the melting point, coalescence consists of melting of the crystalline cores with subsequent large increase in temperature due to recrystallization. For 2 nm–3 nm nanoparticle coalescence, recrystallization of the 2 nm particle significantly increases the total temperature compared to the 2 nm–2 nm case.