Nucleation mechanisms of titanium oxide particles at high temperature based on cluster-assisted nucleation

Abstract

The properties of functional particles are intrinsically linked to their nucleation process. However, for such particles at high temperature, it is difficult to observe their nucleation in real time, and so the corresponding properties are rarely well understood. In the reported study, the nucleation of titanium oxide particles was systematically investigated by means of both classical nucleation theory and first principles. In high-temperature experiments, three samples (T1-T3) with differing titanium content were processed into metallographic samples and subjected to electrolytic extraction for observations via scanning electron microscopy-energy dispersive spectroscopy, X-ray microdiffraction, and transmission electron microscopy. It was determined that the titanium oxide particles in the three samples were Ti2O3, Ti3O5, and TiO2. The nucleation rate and radius of these particles were calculated using classical nucleation theory, and the results were consistent with the experimental results for the Ti2O3 particles but not the Ti3O5 ones. The theoretical and experimental values for T1–T3 decreased in the order T1>T2>T3, and there were considerable differences of as much as an order of magnitude between some of the theoretical and experimental values. The size effect on the nucleation was analysed by constructing (Ti2O3)n, (Ti3O5)n, and (TiO2)n (n = 1–6) nanoclusters, and the thermodynamic properties of the nanoclusters and nano-sized titanium oxide particles were analysed. The thermodynamic properties of the titanium oxide particles estimated using density functional theory generally coincided with those determined experimentally. Based on cluster-assisted nucleation, four nucleation pathways for titanium oxide particles were summarized as follows: (i) [Ti]atom + [O]atom → (TiO2)n → (TiO2)5 → TiO2 particle or (TiO2)5-cluster core–shell nanoparticle → (TiO2)bulk, (ii) [Ti]atom + [O]atom → (Ti3O5)n → (Ti3O5)2 → Ti3O5 particle or (Ti3O5)2-cluster core–shell nanoparticle → (Ti3O5)bulk, (iii) [Ti]atom + [O]atom → (Ti2O3)n → (Ti2O3)3 → Ti2O3 particle or (Ti2O3)3-cluster core–shell nanoparticle → (Ti2O3)bulk, and (iv) [Ti]atom + [O]atom → (TiO2)n, (Ti3O5)n, (Ti2O3)n → (Ti2O3)3 → Ti2O3 particle or (Ti2O3)3-cluster core–shell nanoparticle → (Ti2O3)bulk.

Keywords

• Titanium oxide
• Particle
• Nanocluster
• Nucleation
• First principles